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Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase Junfeng Niu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505089e • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on December 26, 2014
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Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase
Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:
Environmental Science & Technology es-2014-05089e.R1 Article 20-Dec-2014 Li, Yang; Beijing Normal University, School of Environment Niu, Junfeng; Beijing Normal University, School of Environment Shang, Enxiang; Beijing Normal University, School of Environment Crittenden, John; Georgia Institute of Technology, Brook Byers Institute for Sustainable Systems
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Synergistic Photogeneration of Reactive Oxygen
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Species by Dissolved Organic Matter and C60 in
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Aqueous Phase Yang Li1, Junfeng Niu1∗, Enxiang Shang1, and John Charles Crittenden1,2
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Beijing Normal University, Beijing 100875, People’s Republic of China
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State Key Laboratory of Water Environment Simulation, School of Environment,
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School of Civil and Environmental Engineering and the Brook Byers Institute for
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Sustainable Systems, Georgia Institute of Technology, Atlanta, GA 30332, United
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States
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Corresponding author: e-mail: [email protected], phone: +86-10-5880 7612, fax: +86-10-5880 7612. 1
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ABSTRACT: We investigated the photogeneration of reactive oxygen species (ROS)
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by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) are present.
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When C60 and dissolved organic matter (DOM) were present as a mixture, singlet
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oxygen (1O2) generation concentrations were 1.2~1.5 times higher than the sum of 1O2
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concentrations that were produced when C60 and DOM were present in water by
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themselves. When C60 and HA were present as a mixture, superoxide radical (O2•−)
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were 2.2~2.6 times larger than when C60 and HA were present in water by themselves.
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A synergistic ROS photogeneration mechanism involved in energy and electron
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transfer between DOM and C60 was proposed. Enhanced 1O2 generation in the
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mixtures was partly due to 3DOM* energy transfer to O2. However it was mostly due
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to 3DOM* energy transfer to C60 producing 3C60*. 3C60* has a prolonged lifetime (> 4
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μs) in the mixture and provides sufficient time for energy transfer to O2 which
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produces 1O2. The enhanced O2•− generation for HA/C60 mixture was because 3C60*
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mediated electron transfer from photoionized HA to O2. This study demonstrates the
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importance of considering DOM when investigating ROS production by C60.
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INTRODUCTION
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C60 fullerenes possess unique photochemical reactivity because they have strong
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absorbance in the ultraviolet-visual spectrum and have conjugated double bonds.1, 2
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When irradiated with light, the individual C60 molecule is excited to singlet state
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(1C60*) followed by conversion to triplet state (3C60*). The process has a high
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quantum yield (approximately 100%) for light that has photonic energy greater than
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2.3 eV.2-4 Subsequently, the excess energy in 3C60* is efficiently transferred to O2
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resulting in singlet oxygen (1O2) generation via a type II energy-transfer pathway.4, 5
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In addition, C60 is an excellent electron acceptor and can accept up to six electrons per
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molecule due to its delocalized π-electrons within the spherical carbon framework,
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small reorganization energy, and low reduction potential.6, 7 In the presence of an
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electron donor (ED), 3C60* can mediate electron transfer from ED to O2 and produce
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superoxide radical (O2•−) through a type I electron-transfer pathway.7-9 Concerns over
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the potential threat of C60 to human health and ecosystems have stimulated intense
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study on their photoreactivity in the natural aqueous environment.
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C60* is a key intermediate for both energy and electron transfer and its decay
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rate and lifetime governs C60’s photoreactivity in water.5 Once released into water, C60
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forms aggregates in which the lifetime of 3C60* is less than 100 ns due to their
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quenching by surrounding ground-state C60 (self-quenching) or another
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(triplet-triplet annihilation).5 This short lifetime of 3C60* does not provide sufficient
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time for energy and electron transfer and this leads to loss of intrinsic molecular
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photoreactivity of C60 in water.5 C60 will not flocculate (and remain in solution as
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stable single particles) when surfactants are adsorbed to C60 surface, such as dissolved
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organic matter (DOM) that impose electrosteric repulsion (i.e., combination of
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electrostatic and steric) between particles.10-12 The presence of individual particles in
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C60*
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solution significantly prolongs the lifetime of 3C60*, and facilitates both energy and
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electron transfer to O2 and increases their photochemical activity.5 What remains
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unclear is how DOM influences the decay rate and lifetime of 3C60*, and the
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photochemical activity of C60 in aqueous phase and our study focuses on this.
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DOM can absorb photons in the 300-500 nm range of the solar spectrum because
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they contain conjugated unsaturated bonds and free electron pairs on heteroatoms.13-15
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After light absorbance, DOM can form short-lived reactive species such as excited
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triplet states (3DOM*), hydrated electrons (e-), O2•−, and 1O2.16,
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released into natural waters, they can absorb DOM on C60 surfaces due to their high
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surface-to-volume ratios and DOM surfactant properties.18 Also, the electrons
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produced from DOM photoionization could become trapped on C60 surfaces.18-20
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DOM photoionization raises several important questions that we address in the study.
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Can a charged DOM transfer an electron to C60 and can this increase O2•− generation
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for DOM/C60 mixtures? Can DOM photosensitize C60 and enhance
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photogeneration in mixtures of C60 and DOM? Accordingly, we investigated the
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electron and energy transfer between C60 and DOM and how their interactions
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facilitated production of 1O2 and O2•−. This facilitated production is crucial for
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assessing the toxicity of C60 upon their release into natural waters.
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Once C60 were
1
O2
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Humic acid (HA) and fulvic acid (FA) are main components of DOM in surface
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waters.13-15 Their distinct physicochemical properties (e.g., chemical composition,
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molecular structure, surface charge, solubility, and molecular weight) result in
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different photochemical reactivity.21, 22 HA has a higher aromatic content and a lower
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photosensitizing ability to generate 1O2 than that of FA. Because the quantum yields
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for
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aromaticity.19, 21 In addition, HA contains more phenolic functional groups than those
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O2 generation have been shown to be inversely proportional to DOM
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of FA, which are primary chromophores responsible for DOM photoionization and
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charge-transfer reactions.22 Therefore, it is particularly important to compare the
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energy transfer and electron donation capacity of HA and FA to C60 and their impact
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on 1O2 and O2•− photogeneration in DOM/C60 mixtures.
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The objective of this work was to compare the effect of different DOM fractions
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(HA and FA) on the photoreactivity of C60. We investigated the production kinetics
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and concentrations of 1O2 and O2•− in C60 aqueous suspension, DOM solutions, or
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their mixtures under UV irradiation for a wavelength of 365 nm (UV-365), which is
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the primary wavelength of UV irradiation that reaches the earth’s surface.23 The decay
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kinetics and lifetime of 3C60* in C60 aqueous suspension with or without DOM was
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also investigated. The generation mechanism of 1O2 and O2•− in DOM/C60 mixtures
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was elucidated by the energy and electron transfer between C60 and DOM. Overall,
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this work provides insight into the photoreactivity and transformation of C60 in natural
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aqueous environment under UV light irradiation.
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MATERIALS AND METHODS
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Preparation of DOM Solutions. Chemicals used in this study are provided in
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Section S1 of Supporting Information (SI). DOM stock solutions were prepared
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following published method.24 1 g/L HA stock solution was prepared by dissolving
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HA powder in DI water, adjusting pH to 8.0 with 0.01 mol/L NaOH solution. The
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solution was then filtered through a 0.45 μm pore size membrane filter (Millipore,
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Billerica, MA, USA) to remove undissolved HA. 1 g/L FA stock solution was
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prepared by dissolving FA powder in DI water. The concentrations of DOM stock
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solution were confirmed by a total organic carbon (TOC) analyzer (HACH IL 550,
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Loveland, CO, USA). The standard redox potentials of HA or FA were measured by
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the titration method reported by Struyk et al.25
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Preparation of C60 Stock Suspension and Particle Characterization. The
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aqueous C60 stock suspension was prepared by direct sonication method.26, 27 Section
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S2 of SI provides detail about the preparation and characterization method of C60
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stock suspension. The concentration of C60 stock suspension was 10.0 mg/L and was
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measured using a TOC analyzer (pH = 5.7). The hydrodynamic sizes, particle size
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distributions (PSDs), and zeta potentials of C60 aqueous suspensions (5 mg/L) in the
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absence or in the presence of DOM (10, 20, or 50 mg/L) were characterized by
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dynamic light scattering (DLS) on a Zetasizer Nano ZS instrument (Malvern,
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Worcestershire, UK).
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ESR detection of 1O2 and O2•−. Electron spin resonance spectrometry (ESR;
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Bruker ESP-300E, Karlsruhe, Germany) was employed to determine ROS that was
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photogenerated in C60 aqueous suspensions, DOM solutions, and their mixtures. The
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setting of ESR spectrometer was as follows: (1) center field of 3480.00 G; (2)
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microwave power of 10 mW; (3) modulation frequency of 100.00 kHz; (4)
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modulation amplitude of 2.071 G; (5) sweep width of 100.00 G; and (6) sweep time
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of 41.943 sec.
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(1)
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O2 Detection. Production of
1
O2 was monitored using TEMP as
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spin-trapping agent. Experimental solution of 5 mg/L C60 aqueous suspension was
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prepared by mixing 6 μL TEMP (4 M), 150 μL C60 stock solutions (10 mg/L), and 144
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μL DI water. The mixture containing 5 mg/L C60 and 10 mg/L DOM was prepared by
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mixing 6 μL TEMP (4 M), 150 μL C60 stock solution (10 mg/L), 3 μL DOM (1000
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mg/L), and 141 μL DI water. The reaction solutions were placed into the cylindrical
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quartz cell and irradiated by a 4-W compact ultraviolet lamp (UV-365, UVP, San
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Gabriel, CA, USA). The UV lamp has an output spectrum ranging from 315 to 400
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nm with peak intensity at a wavelength of 365 nm as measured by a
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spectrophotometer (Spectropro-500, Acton, MA, USA). The light intensity in the
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reaction solution was 1.0 × 10-6 Einstein·L-1·s-1 as measured by a UVX radiometer
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(Model UVX-25, UVP, San Gabriel, CA, USA). After 30 min, the quartz cell was
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quickly subjected to ESR measurement. When 1O2 was produced, TEMP was
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oxidized by 1O2 to 4-oxo-2, 2, 6, 6-tetramethyl-1-piperdinyloxy radical (TEMPO).
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Accordingly, the ESR signals for TEMPO were used to measure 1O2 formation.
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(2) O2•− Detection. DMPO was used as spin-trapping agent for O2•− detection.
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The reaction solution of 5 mg/L C60 was prepared by mixing 150 μL C60 dispersed in
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DMSO (10 mg/L), 20 μL DMPO (0.5 M), and 130 μL DMSO. The mixture of 5 mg/L
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C60 and 10 mg/L DOM was prepared by mixing 20 μL DMPO, 150 μL C60 dispersed
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in DMSO (10 mg/L), 3 μL DOM (1000 mg/L), and 127 μL DMSO. The
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photochemical reaction was performed following the same procedure as described for
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1
O2. ESR signals for DMPO-O2•− adducts were used to measure O2•− formation.
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Detection of 1O2 and O2•− Concentrations. The molecular probe assays were
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conducted to confirm the photogeneration of ROS and measure their concentrations.
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One hundred mL of aqueous suspension containing 5 mg/L C60, DOM with different
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concentrations (10, 20, or 50 mg/L), or their mixture was placed into a beaker and
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irradiated by the same UV lamp. No buffer solutions or salts were added to the
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reaction solutions to prevent the possibility of colloidal instability for C60 during the
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photochemical experiments. The reaction temperature was maintained at (20 ± 2)oC
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by a recirculating water bath and the reaction solutions were stirred by a magnetic
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stirrer (RO 10; Staufen, Germany). Furfuryl alcohol (FFA, 0.85 mM) and XTT (200
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µM) were used as molecular probes for 1O2 and O2•−, respectively. XTT reduction by
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O2•− results in the formation of orange-colored XTT-formazan, which is water-soluble
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and has an adsorption peak at 470 nm. The UV-vis spectrophotometer (Beckman, DU
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7700, Brea, CA, USA) was used to measure the concentrations of XTT-formazan,
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which can be used to quantify the concentration of O2•−. Our previous studies have
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demonstrated that XTT-formazan was stable under UV-365 light irradiation.28-31 After
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UV illumination for several elapsed times, the reaction solutions were sampled,
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prepared, and analyzed according to the reported method.29-32 The average molar
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concentration of 1O2 and O2•− was calculated following the previous methods.29, 30
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Laser Flash Photolysis. The decay kinetics of 3C60* in C60 aqueous suspensions
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or DOM/C60 mixtures were traced by a nanosecond laser flash photolysis instrument
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(LP920, Edinburgh, England). Prior to the experiment, 3 mL of solution that contains
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5 mg/L of C60 with or without 10 mg/L of DOM was placed in a rectangular quartz
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cell, purged with ultrapure nitrogen gas for at least 30 min, and sealed from the
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atmosphere to inhibit energy transfer from 3C60* to O2. A 355 nm laser pulse (10 mJ,
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pulse width of 7 nm) was generated from a Quanta Ray Nd: YAG laser system
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(Continuum, Santa Clara, CA, USA) and was used as an excitation source. A 500 W
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xenon lamp was used as a monitoring light. Instantaneous formation of 3C60* after
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laser pulse and its decay kinetics was obtained from the time resolved data using
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absorbance at 740 nm.3, 5 The lifetime of 3C60* was determined by the least squares fit
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of a single exponential equation with offset, performed with Origin 7.5.
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RESULTS AND DISCUSSION
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Particle Size Distribution. The intensity-averaged particle size distributions
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(PSDs) of C60 dispersed in water after addition of different concentrations of HA or
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FA are presented in Figure 1. The average hydrodynamic radius of C60 with 10.0, 20.0,
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and 50.0 mg/L HA were 70.3, 61.5, and 36.6 nm, respectively, which were much
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smaller than the hydrodynamic radius of C60 without HA (108.1 nm). Similarly, the
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particle sizes of C60 decreased with increasing FA concentrations, with the average
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hydrodynamic radius of C60; i.e., 72.8, 67.0, and 43.5 nm for FA concentrations of
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10.0, 20.0, and 50.0 mg/L, respectively. Even though DOM reduced the aggregation
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degree of C60, they did not prevent their aggregation. Our experimental results are
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consistent with previous reports that DOM decreased the particle size and increased
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colloidal stability of C60 with increasing DOM concentrations.20, 33 Also, HA has been
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shown to be more effective than FA in suppressing C60 aggregation.20,
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mass-averaged PSDs of C60 in the presence of different concentrations of HA or FA
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are provided in Figure S1 of SI.
33
The
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The zeta potential measurements (Table S1 of SI) indicated that the negative
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potential of C60 increased slightly when DOM was added. The changes in particle
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sizes of C60 did not correlate well to their changes in zeta potentials, indicating that
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the electrostatic interaction was not the primary mechanism for particle stability. The
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decreased particle size and increased C60 stability in the presence of DOM was due to
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the steric force they impart as discussed below.10-12, 34 The different structural and
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conformational characteristics between HA and FA account for their distinct
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stabilization capacity for C60 aqueous suspensions.10, 34 HA has less polarity, longer
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chain hydrophobic moieties, and higher molecular weight than FA,35 and this causes
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stronger sorption of HA onto C60 surface via hydrophobic and π-π interactions
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providing more steric stability. Also HA have a thicker organic coating on C60 surface
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preventing C60 getting closer to form bigger aggregates.10, 34
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(a)
(b)
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Figure 1. Intensity-averaged PSD diagrams of C60 aqueous suspensions without or with different concentrations of (a) HA or (b) FA. 9
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ESR Detection of 1O2. As shown in Figure 2(a)-(d), three characteristic peaks of
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TEMPO spin adducts could be detected by ESR for DOM solutions with or without
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C60 when irradiated by UV light. The intensities of the TEMPO spin adducts increased
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as the irradiation time was increased. In contrast, no TEMPO signals were detected in
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C60 aqueous suspension (Figure 2(e)), indicating that C60 aggregates lost their
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photochemical reactivity with respect to 1O2 production. These are consistent with
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previous studies that 1O2 could not be detected in C60 aggregates. These studies
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hypothesize that the close proximity of the C60 molecules in an aggregate result in
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rapid decay of 3C60* and its dramatically shortened lifetime.5,
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aggregates exposed less C60 molecules to photons and O2,37, 38 potentially reducing
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active surface areas for 1O2 production.
36
In addition,
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The TEMPO signals have been detected in DOM solutions, which is consistent
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with previous studies that both HA and FA could photosensitize 1O2 generation under
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UV irradiation.39, 40 The excited triplets of phenyl ketone or aromatic quinone in the
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DOM molecule are primarily responsible for their photosensitization effects.41,
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Moreover, the TEMPO signals in FA solution are stronger than those in HA solution
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with or without C60. The higher photosensitization capacity of FA than HA is
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primarily due to the presence of more chromophores in the lower-molecular-weight
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fractions of DOM.17,
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mixtures are stronger than those in solutions that contained only DOM or C60. The
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reasons for the enhanced 1O2 generation in DOM/C60 mixtures will be discussed in the
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following “Proposed Reaction Framework” Section. No TEMPO signals have been
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detected in C60 aqueous suspension, DOM solutions, or their mixtures in the dark
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(Figure S2 of SI).
39
42
It is also noteworthy that TEMPO signals in DOM/C60
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Figure 2. ESR spectra recorded at ambient temperature for TEMP adduct with 1O2 in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).
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ESR Detection of O2•−. As shown in Figure 3, six characteristic peaks of the
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DMPO-O2•− spin adducts could be detected using ESR under UV irradiation in HA
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solution or HA/C60 mixture. Similarly, the strengths of DMPO-O2•− spin adducts were
249
enhanced with prolonged UV irradiation time (Figure 3(a) and (c)). C60 aqueous
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suspension did not induce O2•− generation primarily due to lack of ED and short
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lifetime of 3C60*.8, 9, 43 DMPO-O2•− signals were observed in HA solution primarily
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due to electron transfer from HA photoionization to O2.44 The phenolic moieties in
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HA molecules were responsible for their photoionization and charge-transfer
254
transitions.19, 45 DMPO-O2•− signals in HA/C60 mixture were stronger than those in
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HA solution, indicating that HA could serve as ED for C60 and subsequently enhanced
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O2•− production in the mixtures. Unlike HA solution, no DMPO-O2•− adducts were
257
detected in FA solution with or without C60, demonstrating that FA could not undergo
258
photoionization and then inject electrons to O2 or C60. A positive correlation has been
259
found between the charge transfer efficiency of DOM and their aromaticity.19, 45 The
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lower electron donation capacity of FA than that of HA was probably due to the lower
261
aromaticity of FA (0.28) as compared to that of HA (0.42).46 No measurable amount
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of O2•− was detected in C60 aqueous suspension, DOM solutions, or their mixtures in
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the dark (Figure S2 of SI). It is worth mention that DMSO was used as solvent to 11
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detect O2•− due to its spontaneous dismutation in water.47 The potential effect of
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DMSO on particle size distribution and O2•− generation by C60 with or without DOM
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was discussed in Section S6 of SI.
267 268 269 270 271 272 273 274
Figure 3. ESR spectra recorded at ambient temperature for DMPO adduct with O2•− in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).
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Generation Kinetics of 1O2 and O2•−. Molecular probe assays were conducted
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to measure the photogenerated concentrations of 1O2 and O2•− in C60 aqueous
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suspensions, DOM solutions, or their mixtures under UV-365 irradiation. Basically,
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the ROS production profile that was measured by the molecular probe assays was
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similar to that detected using ESR. Previous studies have shown that FFA photolysis
280
under UV-365 irradiation was negligible.29-31 As shown in Figure 4(a), (b), we
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observed that both HA and FA solutions result in FFA photodegradation, and the
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degradation rate increases with DOM concentration. At the same mass concentration,
283
the photodegradation rate of FFA by FA is higher than that by HA. This is consistent
284
with our ESR results and previous studies that showed FA produced more 1O2 than
285
HA.16, 19 Aqueous C60 suspensions alone did not degrade FFA, which indicates that
286
1
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few sites for UV absorption and O2 exposure.5 When DOM was added to C60 aqueous
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suspensions, the photodegradation rate of FFA was significantly enhanced and the
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FFA photodegradation rate increased with DOM concentration. A comparison
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between Figure 4 (a) and (b) shows the FFA photodegradation rates in FA/C60
O2 was not generated by C60. This was primarily due to short lifetime of 3C60* and
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mixtures were much faster than those in HA/C60 mixtures at the same DOM mass
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concentrations. This indicated that the photosensitization capacity of FA for 1O2
293
generation in DOM/C60 mixture was higher than that of HA because most of
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chromophores (such as phenyl ketone and aromatic quinone) were located in the
295
low-molecular-weight fractions of DOM.16,
296
mixtures did not induce significant degradation of FFA (data not shown).
19
In the dark, C60, DOM, or their
297
Figure 4(c)-(f) shows the changes in the absorption spectrum of C60 aqueous
298
suspension, HA solutions, or their mixtures during a 48-h exposure to UV-365
299
irradiation. The absorption peak at λ = 470 nm indicates the production of O2•−. As
300
shown in Figure 4(c), no absorption peaks are detected for C60 aqueous suspension
301
under UV irradiation. This is consistent with our ESR results which have shown that
302
C60 aggregates do not produce O2•−, because of a lack of ED and short lifetime of
303
3
304
increased with increasing HA concentrations. Previous studies have also detected O2•−
305
in HA solution under simulated sunlight or UV irradiation.17, 44 Our work has shown
306
that C60 alone did not produce O2•−, and their addition to HA solutions significantly
307
enhanced O2•− production rates, and the production rate increased with HA
308
concentration. This indicated that C60 could photocatalytically promote O2•−
309
generation by HA. In marked contrast, no measurable amount of O2•− was detected in
310
FA solution with or without C60 (Figure S4 of SI). These results indicated that the
311
electron donation efficiency of HA was higher than that of FA.
C60*.8, 9, 43 HA solution generated O2•− under UV irradiation and the production rate
312
Two factors can influence the electron transfer capacity of DOM, i.e.,
313
aromaticity and phenolic group content.19, 45 Lower aromaticity and phenolic group
314
content result in lower electron transfer efficiency of DOM.19, 45 Previous studies have
315
demonstrated that the aromaticity of FA (0.28) is approximately 1.5-fold lower than
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the aromaticity of HA (0.42).26, 27 Furthermore, the phenolic moieties are responsible
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for charge transfer in FA (1.5 meq/g), which is approximately two-fold lower than that
318
in HA (2.8 meq/g).46 In the dark, no absorption peaks were detected for a C60 solution,
319
DOM solutions, or their mixtures (Figure S5). Many previous studies have
320
demonstrated that the degradation of XTT by UV-365 light was negligible. 29-31
321 322 323 324 325 326 327
(c)
(d)
10 mg/L HA
(e)
(f)
50 mg/L HA
328 329 330 331 332 333 334 335
Figure 4. 1O2 generation kinetics indicated by the degradation of 0.85 mM FFA (a-b) and O2•− generation kinetics (c-f) in C60 aqueous suspension, DOM solutions, or their mixtures as shown by the reduction of 200 μM XTT (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, and C60 of 5 mg/L).
336
ROS Concentration Photogenerated by C60, DOM Solution, or their
337
Mixtures. Table 1 summarizes the average molar concentrations of 1O2 and O2•−
338
photogenerated in C60 aqueous suspension, DOM solutions, or their mixtures. Only
339
C60 dispersed in water did not produce measurable amount of ROS, whereas DOM
340
solutions with or without C60 generated at least one type of ROS. The average molar
341
concentrations of total ROS (the sum of the concentrations of 1O2 and O2•−) generated
342
in DOM/C60 mixtures were 1.3~1.8 times higher than the sum of their individual 14
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production concentrations, indicating that DOM/C60 mixtures generated ROS in a
344
synergistic way. This was primarily because the decreased C60 aggregation by DOM
345
increased the specific surface area for light adsorption and O2 exposure. It was
346
observed that: (1) The photogenerated 1O2 concentrations in decreasing order
347
followed this trend: FA/C60 mixture > FA > HA/C60 mixture > HA at the same DOM
348
concentration and C60 concentration of 5 mg/L. The 1O2 production concentrations for
349
FA solutions were approximately 1.6-fold, 1.9-fold, and 2.3-fold more than the
350
concentrations generated in HA solutions at 10.0, 20.0, and 50.0 mg/L, respectively.
351
After addition of 5.0 mg/L C60, 1O2 production concentrations in FA solutions were
352
approximately 2.0 times higher than those generated in HA solutions at all DOM
353
concentrations. (2) No measurable amount of O2•− was detected in C60 aqueous
354
suspension, FA solution, or their mixture. The addition of C60 into HA solution
355
significantly enhanced O2•− generation concentrations, which were increased by up to
356
1.8 fold as compared with those generated in HA solution alone.
357 358
Table 1. Average molar concentrations of ROS generated by C60, DOM solutions, or their mixtures (UV-365 irradiation and light intensity of 1.0 × 10-6 Einstein·L-1·s-1). C60 (mg/L) 0
HA
0
FA
5.0
5.0
5.0 5.0 359 360
DOM (mg/L)
HA
FA
HA FA
1
O2 (μM)
O2•− (μM)
Total (μM)
10.0 20.0 50.0 10.0
49.8 ± 2.1 63.0 ± 2.5 72.8 ± 2.8 79.2 ± 0.3
18.9 ± 1.8 36.8 ± 2.0 58.2 ± 2.2 N.D.
68.7 ± 3.9 99.8 ± 4.5 131.0 ± 5.0 79.2 ± 0.3
20.0
121.6 ± 7.2
N.D.
121.6 ± 7.2
50.0 10.0
165.1 ± 8.2 60.8 ± 2.3
N.D. 49.0 ± 2.1
165.1 ± 8.2 109.8 ± 4.4
20.0
91.1 ± 4.1
83.8 ± 3.8
174.9 ± 7.9
50.0 10.0
108.7 ± 6.6 114.8 ± 6.8
127.9 ± 7.4 N.D.
236.6 ± 14.0 114.8 ± 6.8
20.0
163.4 ± 8.6
N.D.
163.4 ± 8.6
50.0 0 0
215.9 ± 13.2 N.D. N.D.
N.D. N.D. N.D.
215.9 ± 13.2 0 0
N.D. indicates that ROS were not detected or were not statistically significant. Decay Kinetics and Lifetime of 3C60*. 3C60* is the key transient intermediate 15
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361
for both energy transfer (i.e., 1O2 generation) and electron injection (i.e., O2•−
362
generation) process in C60 aqueous suspensions after light irradiation.1,
363
quantitatively analyze the lifetime of 3C60*, the decay kinetics of 3C60* in the C60
364
aqueous suspension with or without DOM was fitted by the following single
365
exponential function:
2
It = I0 exp (-t/τ0) + C
366
To
(1)
367
where It is the fluorescence intensity of 3C60* after a UV irradiation time of t (μs), I0 is
368
the initial fluorescence intensity of 3C60*, τ0 is the lifetime of 3C60* (μs), and C is a
369
constant. The experimental data have been fitted by Eq 1, and the model fits are
370
shown by red lines in Figure 5. The correlation coefficients (R2), fitted values of I0, τ0,
371
and C are shown in Table S2 of SI. Apparently, the addition of HA or FA into C60
372
aqueous suspension significantly prolonged the lifetime of 3C60*.
373
As shown in Table S2, the lifetime of 3C60* in the C60 aqueous suspension is 0.09
374
± 0.01 μs. This is consistent with previous studies that showed 3C60* in the C60
375
aqueous suspension decayed within 100 ns.9, 10 As demonstrated by our DLS results
376
and previous studies,37, 38 C60 dispersed in water was comprised of closely packed C60
377
molecules. The short lifetime of 3C60* in C60 aggregates was primarily due to
378
quenching by surrounding ground-state C60 (self-quenching) or another
379
(triplet-triplet annihilation) on the aggregate surface.3, 5 Therefore, the pathway for
380
energy and electron transfer to O2 is fundamentally blocked as 3C60* becomes
381
short-lived and less available,3, 5 which could explain why no ROS was detected in C60
382
aqueous suspenion.
3
C60*
383
In contrast, when HA or FA was added into C60 aqueous suspensions, the
384
liftetime of 3C60* extended to 4.63 ± 0.05 or 4.59 ± 0.04 μs, respectively, which was
385
approximately fifty-fold longer than that in C60 aqueous suspension (Table S2 of SI).
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This was primarily because DOM decreased the aggregate size and enhanced the
387
colloidal stability of C60 aggregates. As the size of aggregates decreased, more C60
388
molecules on aggregate surface were exposed to light forming more 3C60*.37, 38 These
389
lead to less ground state C60 and subsequently decreased their quenching for 3C60*.
390
Meanwhile, the enhanced colloidal stability decreased the possibility of triplet-triplet
391
annihilation among different C60 aggregates. Therefore, the lifetime of 3C60* in the
392
mixture is sufficiently long for 3C60* to mediate both energy transfer and charge
393
injection to O2. No absorbance has been observed at 740 nm in HA or FA solution.
394
Hotze et al.48 have also demonstrated that aggregate size and structure strongly
395
affected the fraction of 3C60* within an aggregate and influenced ROS generation in
396
C60 aqueous suspension.
397 398 399 400 401 402 403
Figure 5. Absorption-time profiles of 3C60* recorded at 740 nm in C60 aqueous suspensions with or without HA/FA at different laser excitation time in N2-saturated condition (C60 of 5 mg/L, HA of 10 mg/L, and FA of 10 mg/L).
404
Proposed Reaction Framework. We propose a ROS photogeneration
405
mechanism for DOM/C60 mixtures. As shown in Figure 6, the synergistic
406
photogeneration of 1O2 in the DOM/C60 mixutres could be attributed to the energy
407
transfer between them. The 365 nm UV light can induce the generation of 3DOM*
408
because the incident photon energy (3.4 eV) is higher than the energy of 3DOM*
409
(180~250 kJ/mol, 1.9 to 2.6 eV).45, 49 As far as the photophysics is concerned, 3DOM*
410
has a much higher energy level than that of 3C60* (157~176 kJ/mol, 1.63~1.83 eV) or
411
1
O2 (94.3 kJ/mol, 0.98 eV).19, 50, 51 The fractions of light absorbed by C60 or DOM in
17
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their mixtures are calculated in Section S10 of SI. DOM absorbs much more light
413
than C60 in C60/DOM mixtures. Therefore 3DOM* can transfer energy to O2 but more
414
importantly 3DOM* can transfer energy to C60 and create 3C60*. And the prolonged
415
lifetime of 3C60* provides more opportunity for the energy transfer to O2 and
416
generates more 1O2. It is important to note that the creation of smaller C60 aggregates
417
from the adsorption and stabilization caused by DOM increases the lifetime of 3C60*
418
because larger aggregates contain several 3C60* and they can annihilate each other
419
before energy transfer to O2.
420
As shown in Figure 6, the synergistic photogeneration of O2•− in HA/C60 mixture
421
is primarily attributed to the electron donation from photoionized HA to electrophile,
422
C60. With respect to thermodynamics, the standard oxidation potential of HA was
423
+0.797 V, demonstrating that HA can reduce 3C60* which has a standard reduction
424
potential of E0(3C60*/C60•−) +1.1 V.3, 5, 36 The ground state C60 has a standard redox
425
potential of E0(C60/C60•−) -0.2 V,3, 5, 36 indicating that the electron transfer from HA is
426
more favorable to 3C60* (1.897 V) than the ground state of C60 (0.597 V), resulting in
427
C60•− formation. Subsequently, C60•− transfers electrons to O2 and produces O2•−.
428
Moreover, C60 promotes electron depletion from HA during photoionization and
429
photocatalytically promotes O2•− generation by HA. The standard oxidation potential
430
of FA is +0.507 V, which indicates that FA could reduce 3C60*. However, our
431
experimental results of fluorescence quenching of DOM by C60 showed that the
432
electron donation capacity of FA was lower than that of HA (Section S11 of SI). In
433
addition, the quantum yield for e- production by FA (7.9 × 10-6)52 was two orders of
434
magnitude lower than that by HA (1.7 × 10-3) under UV irradiation.53 Therefore, it is
435
hard for FA to loose an electron and serve as electron donors for C60. This explains
436
why no measurable amount of O2•− was detected in FA/C60 mixture.
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C60 is electron deficient and a good electron acceptor (can reversibly accept up to
438
six electrons). Thus the electron transfer from C60 to DOM is not likely to occur in
439
principle. Kong et al.19 have also demonstrated that DOM can act as electron donors
440
and transfer electron to C60 in their mixtures, and the reverse, did not take place. The
441
lower energy level of 3C60* than that of 3DOM* makes it impossible for energy
442
transfer from C60 to DOM. Therefore, C60 cannot transfer electron and energy to DOM
443
under light illumination. In conclusion, the electron and energy transfer from DOM to
444
C60 contributes to the synergistic ROS generation in the mixed system.
445 446 447 448 449 450 451 452 453
Figure 6. The reaction schema of the synergistic generation of 1O2 and O2•− by the mixture of C60 and DOM in aqueous phase under UV-365 irradiation.
454
Our work demonstrated that the energy and electron transfer between DOM and
455
C60 resulted in the synergistic generation of 1O2 and O2•− for DOM/C60 mixtures when
456
exposed with 365 nm UV light. This strongly suggests that DOM and sunlight may
457
play critical roles in the fate and photochemical activity of C60 in the natural aqueous
458
environment. After C60 are released into surface waters where DOM is ubiquitous, the
459
enhanced ROS generation concentrations of C60 could increase their possibility to
460
pose hazardous to aquatic organisms under natural solar irradiation. Thus the risk
461
assessment of C60 should consider both their inherent toxicity and their possible
462
interactions with coexisting DOM in natural waters. 19
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ASSOCIATED CONTENT
464
Supporting Information Available
465
Chemicals used in this study, preparation and characterization method of C60,
466
mass-averaged PSDs of C60 with or without DOM, zeta potential of C60, O2•−
467
production kinetics in FA solutions with or without C60, correlation coefficient for
468
3
469
Supporting Information. This material is available free of charge via the Internet at
470
http://pubs.acs.org.
471
ACKNOWLEDGMENTS
C60* decay kinetics, and fluorescence quenching of DOM by C60 are available in the
472
This study was financially supported by the Fund for Innovative Research Group
473
of the National Natural Science Foundation of China (Grant No. 51421065), the
474
National Natural Science Foundation of China (Nos. 51378065 and 21407010), and
475
China Postdoctoral Science Foundation (No. 224234).
476
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Article
Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase Junfeng Niu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505089e • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on December 26, 2014
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Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase
Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:
Environmental Science & Technology es-2014-05089e.R1 Article 20-Dec-2014 Li, Yang; Beijing Normal University, School of Environment Niu, Junfeng; Beijing Normal University, School of Environment Shang, Enxiang; Beijing Normal University, School of Environment Crittenden, John; Georgia Institute of Technology, Brook Byers Institute for Sustainable Systems
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1
Synergistic Photogeneration of Reactive Oxygen
2
Species by Dissolved Organic Matter and C60 in
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Aqueous Phase Yang Li1, Junfeng Niu1∗, Enxiang Shang1, and John Charles Crittenden1,2
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5
1
Beijing Normal University, Beijing 100875, People’s Republic of China
6 7
State Key Laboratory of Water Environment Simulation, School of Environment,
2
School of Civil and Environmental Engineering and the Brook Byers Institute for
8
Sustainable Systems, Georgia Institute of Technology, Atlanta, GA 30332, United
9
States
10
11
12
13
14
15
16
17 *
Corresponding author: e-mail: [email protected], phone: +86-10-5880 7612, fax: +86-10-5880 7612. 1
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ABSTRACT: We investigated the photogeneration of reactive oxygen species (ROS)
19
by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) are present.
20
When C60 and dissolved organic matter (DOM) were present as a mixture, singlet
21
oxygen (1O2) generation concentrations were 1.2~1.5 times higher than the sum of 1O2
22
concentrations that were produced when C60 and DOM were present in water by
23
themselves. When C60 and HA were present as a mixture, superoxide radical (O2•−)
24
were 2.2~2.6 times larger than when C60 and HA were present in water by themselves.
25
A synergistic ROS photogeneration mechanism involved in energy and electron
26
transfer between DOM and C60 was proposed. Enhanced 1O2 generation in the
27
mixtures was partly due to 3DOM* energy transfer to O2. However it was mostly due
28
to 3DOM* energy transfer to C60 producing 3C60*. 3C60* has a prolonged lifetime (> 4
29
μs) in the mixture and provides sufficient time for energy transfer to O2 which
30
produces 1O2. The enhanced O2•− generation for HA/C60 mixture was because 3C60*
31
mediated electron transfer from photoionized HA to O2. This study demonstrates the
32
importance of considering DOM when investigating ROS production by C60.
33 34
35
36 37
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INTRODUCTION
39
C60 fullerenes possess unique photochemical reactivity because they have strong
40
absorbance in the ultraviolet-visual spectrum and have conjugated double bonds.1, 2
41
When irradiated with light, the individual C60 molecule is excited to singlet state
42
(1C60*) followed by conversion to triplet state (3C60*). The process has a high
43
quantum yield (approximately 100%) for light that has photonic energy greater than
44
2.3 eV.2-4 Subsequently, the excess energy in 3C60* is efficiently transferred to O2
45
resulting in singlet oxygen (1O2) generation via a type II energy-transfer pathway.4, 5
46
In addition, C60 is an excellent electron acceptor and can accept up to six electrons per
47
molecule due to its delocalized π-electrons within the spherical carbon framework,
48
small reorganization energy, and low reduction potential.6, 7 In the presence of an
49
electron donor (ED), 3C60* can mediate electron transfer from ED to O2 and produce
50
superoxide radical (O2•−) through a type I electron-transfer pathway.7-9 Concerns over
51
the potential threat of C60 to human health and ecosystems have stimulated intense
52
study on their photoreactivity in the natural aqueous environment.
53
3
C60* is a key intermediate for both energy and electron transfer and its decay
54
rate and lifetime governs C60’s photoreactivity in water.5 Once released into water, C60
55
forms aggregates in which the lifetime of 3C60* is less than 100 ns due to their
56
quenching by surrounding ground-state C60 (self-quenching) or another
57
(triplet-triplet annihilation).5 This short lifetime of 3C60* does not provide sufficient
58
time for energy and electron transfer and this leads to loss of intrinsic molecular
59
photoreactivity of C60 in water.5 C60 will not flocculate (and remain in solution as
60
stable single particles) when surfactants are adsorbed to C60 surface, such as dissolved
61
organic matter (DOM) that impose electrosteric repulsion (i.e., combination of
62
electrostatic and steric) between particles.10-12 The presence of individual particles in
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C60*
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solution significantly prolongs the lifetime of 3C60*, and facilitates both energy and
64
electron transfer to O2 and increases their photochemical activity.5 What remains
65
unclear is how DOM influences the decay rate and lifetime of 3C60*, and the
66
photochemical activity of C60 in aqueous phase and our study focuses on this.
67
DOM can absorb photons in the 300-500 nm range of the solar spectrum because
68
they contain conjugated unsaturated bonds and free electron pairs on heteroatoms.13-15
69
After light absorbance, DOM can form short-lived reactive species such as excited
70
triplet states (3DOM*), hydrated electrons (e-), O2•−, and 1O2.16,
71
released into natural waters, they can absorb DOM on C60 surfaces due to their high
72
surface-to-volume ratios and DOM surfactant properties.18 Also, the electrons
73
produced from DOM photoionization could become trapped on C60 surfaces.18-20
74
DOM photoionization raises several important questions that we address in the study.
75
Can a charged DOM transfer an electron to C60 and can this increase O2•− generation
76
for DOM/C60 mixtures? Can DOM photosensitize C60 and enhance
77
photogeneration in mixtures of C60 and DOM? Accordingly, we investigated the
78
electron and energy transfer between C60 and DOM and how their interactions
79
facilitated production of 1O2 and O2•−. This facilitated production is crucial for
80
assessing the toxicity of C60 upon their release into natural waters.
17
Once C60 were
1
O2
81
Humic acid (HA) and fulvic acid (FA) are main components of DOM in surface
82
waters.13-15 Their distinct physicochemical properties (e.g., chemical composition,
83
molecular structure, surface charge, solubility, and molecular weight) result in
84
different photochemical reactivity.21, 22 HA has a higher aromatic content and a lower
85
photosensitizing ability to generate 1O2 than that of FA. Because the quantum yields
86
for
87
aromaticity.19, 21 In addition, HA contains more phenolic functional groups than those
1
O2 generation have been shown to be inversely proportional to DOM
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of FA, which are primary chromophores responsible for DOM photoionization and
89
charge-transfer reactions.22 Therefore, it is particularly important to compare the
90
energy transfer and electron donation capacity of HA and FA to C60 and their impact
91
on 1O2 and O2•− photogeneration in DOM/C60 mixtures.
92
The objective of this work was to compare the effect of different DOM fractions
93
(HA and FA) on the photoreactivity of C60. We investigated the production kinetics
94
and concentrations of 1O2 and O2•− in C60 aqueous suspension, DOM solutions, or
95
their mixtures under UV irradiation for a wavelength of 365 nm (UV-365), which is
96
the primary wavelength of UV irradiation that reaches the earth’s surface.23 The decay
97
kinetics and lifetime of 3C60* in C60 aqueous suspension with or without DOM was
98
also investigated. The generation mechanism of 1O2 and O2•− in DOM/C60 mixtures
99
was elucidated by the energy and electron transfer between C60 and DOM. Overall,
100
this work provides insight into the photoreactivity and transformation of C60 in natural
101
aqueous environment under UV light irradiation.
102
MATERIALS AND METHODS
103
Preparation of DOM Solutions. Chemicals used in this study are provided in
104
Section S1 of Supporting Information (SI). DOM stock solutions were prepared
105
following published method.24 1 g/L HA stock solution was prepared by dissolving
106
HA powder in DI water, adjusting pH to 8.0 with 0.01 mol/L NaOH solution. The
107
solution was then filtered through a 0.45 μm pore size membrane filter (Millipore,
108
Billerica, MA, USA) to remove undissolved HA. 1 g/L FA stock solution was
109
prepared by dissolving FA powder in DI water. The concentrations of DOM stock
110
solution were confirmed by a total organic carbon (TOC) analyzer (HACH IL 550,
111
Loveland, CO, USA). The standard redox potentials of HA or FA were measured by
112
the titration method reported by Struyk et al.25
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Preparation of C60 Stock Suspension and Particle Characterization. The
114
aqueous C60 stock suspension was prepared by direct sonication method.26, 27 Section
115
S2 of SI provides detail about the preparation and characterization method of C60
116
stock suspension. The concentration of C60 stock suspension was 10.0 mg/L and was
117
measured using a TOC analyzer (pH = 5.7). The hydrodynamic sizes, particle size
118
distributions (PSDs), and zeta potentials of C60 aqueous suspensions (5 mg/L) in the
119
absence or in the presence of DOM (10, 20, or 50 mg/L) were characterized by
120
dynamic light scattering (DLS) on a Zetasizer Nano ZS instrument (Malvern,
121
Worcestershire, UK).
122
ESR detection of 1O2 and O2•−. Electron spin resonance spectrometry (ESR;
123
Bruker ESP-300E, Karlsruhe, Germany) was employed to determine ROS that was
124
photogenerated in C60 aqueous suspensions, DOM solutions, and their mixtures. The
125
setting of ESR spectrometer was as follows: (1) center field of 3480.00 G; (2)
126
microwave power of 10 mW; (3) modulation frequency of 100.00 kHz; (4)
127
modulation amplitude of 2.071 G; (5) sweep width of 100.00 G; and (6) sweep time
128
of 41.943 sec.
129
(1)
1
O2 Detection. Production of
1
O2 was monitored using TEMP as
130
spin-trapping agent. Experimental solution of 5 mg/L C60 aqueous suspension was
131
prepared by mixing 6 μL TEMP (4 M), 150 μL C60 stock solutions (10 mg/L), and 144
132
μL DI water. The mixture containing 5 mg/L C60 and 10 mg/L DOM was prepared by
133
mixing 6 μL TEMP (4 M), 150 μL C60 stock solution (10 mg/L), 3 μL DOM (1000
134
mg/L), and 141 μL DI water. The reaction solutions were placed into the cylindrical
135
quartz cell and irradiated by a 4-W compact ultraviolet lamp (UV-365, UVP, San
136
Gabriel, CA, USA). The UV lamp has an output spectrum ranging from 315 to 400
137
nm with peak intensity at a wavelength of 365 nm as measured by a
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spectrophotometer (Spectropro-500, Acton, MA, USA). The light intensity in the
139
reaction solution was 1.0 × 10-6 Einstein·L-1·s-1 as measured by a UVX radiometer
140
(Model UVX-25, UVP, San Gabriel, CA, USA). After 30 min, the quartz cell was
141
quickly subjected to ESR measurement. When 1O2 was produced, TEMP was
142
oxidized by 1O2 to 4-oxo-2, 2, 6, 6-tetramethyl-1-piperdinyloxy radical (TEMPO).
143
Accordingly, the ESR signals for TEMPO were used to measure 1O2 formation.
144
(2) O2•− Detection. DMPO was used as spin-trapping agent for O2•− detection.
145
The reaction solution of 5 mg/L C60 was prepared by mixing 150 μL C60 dispersed in
146
DMSO (10 mg/L), 20 μL DMPO (0.5 M), and 130 μL DMSO. The mixture of 5 mg/L
147
C60 and 10 mg/L DOM was prepared by mixing 20 μL DMPO, 150 μL C60 dispersed
148
in DMSO (10 mg/L), 3 μL DOM (1000 mg/L), and 127 μL DMSO. The
149
photochemical reaction was performed following the same procedure as described for
150
1
O2. ESR signals for DMPO-O2•− adducts were used to measure O2•− formation.
151
Detection of 1O2 and O2•− Concentrations. The molecular probe assays were
152
conducted to confirm the photogeneration of ROS and measure their concentrations.
153
One hundred mL of aqueous suspension containing 5 mg/L C60, DOM with different
154
concentrations (10, 20, or 50 mg/L), or their mixture was placed into a beaker and
155
irradiated by the same UV lamp. No buffer solutions or salts were added to the
156
reaction solutions to prevent the possibility of colloidal instability for C60 during the
157
photochemical experiments. The reaction temperature was maintained at (20 ± 2)oC
158
by a recirculating water bath and the reaction solutions were stirred by a magnetic
159
stirrer (RO 10; Staufen, Germany). Furfuryl alcohol (FFA, 0.85 mM) and XTT (200
160
µM) were used as molecular probes for 1O2 and O2•−, respectively. XTT reduction by
161
O2•− results in the formation of orange-colored XTT-formazan, which is water-soluble
162
and has an adsorption peak at 470 nm. The UV-vis spectrophotometer (Beckman, DU
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7700, Brea, CA, USA) was used to measure the concentrations of XTT-formazan,
164
which can be used to quantify the concentration of O2•−. Our previous studies have
165
demonstrated that XTT-formazan was stable under UV-365 light irradiation.28-31 After
166
UV illumination for several elapsed times, the reaction solutions were sampled,
167
prepared, and analyzed according to the reported method.29-32 The average molar
168
concentration of 1O2 and O2•− was calculated following the previous methods.29, 30
169
Laser Flash Photolysis. The decay kinetics of 3C60* in C60 aqueous suspensions
170
or DOM/C60 mixtures were traced by a nanosecond laser flash photolysis instrument
171
(LP920, Edinburgh, England). Prior to the experiment, 3 mL of solution that contains
172
5 mg/L of C60 with or without 10 mg/L of DOM was placed in a rectangular quartz
173
cell, purged with ultrapure nitrogen gas for at least 30 min, and sealed from the
174
atmosphere to inhibit energy transfer from 3C60* to O2. A 355 nm laser pulse (10 mJ,
175
pulse width of 7 nm) was generated from a Quanta Ray Nd: YAG laser system
176
(Continuum, Santa Clara, CA, USA) and was used as an excitation source. A 500 W
177
xenon lamp was used as a monitoring light. Instantaneous formation of 3C60* after
178
laser pulse and its decay kinetics was obtained from the time resolved data using
179
absorbance at 740 nm.3, 5 The lifetime of 3C60* was determined by the least squares fit
180
of a single exponential equation with offset, performed with Origin 7.5.
181
RESULTS AND DISCUSSION
182
Particle Size Distribution. The intensity-averaged particle size distributions
183
(PSDs) of C60 dispersed in water after addition of different concentrations of HA or
184
FA are presented in Figure 1. The average hydrodynamic radius of C60 with 10.0, 20.0,
185
and 50.0 mg/L HA were 70.3, 61.5, and 36.6 nm, respectively, which were much
186
smaller than the hydrodynamic radius of C60 without HA (108.1 nm). Similarly, the
187
particle sizes of C60 decreased with increasing FA concentrations, with the average
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hydrodynamic radius of C60; i.e., 72.8, 67.0, and 43.5 nm for FA concentrations of
189
10.0, 20.0, and 50.0 mg/L, respectively. Even though DOM reduced the aggregation
190
degree of C60, they did not prevent their aggregation. Our experimental results are
191
consistent with previous reports that DOM decreased the particle size and increased
192
colloidal stability of C60 with increasing DOM concentrations.20, 33 Also, HA has been
193
shown to be more effective than FA in suppressing C60 aggregation.20,
194
mass-averaged PSDs of C60 in the presence of different concentrations of HA or FA
195
are provided in Figure S1 of SI.
33
The
196
The zeta potential measurements (Table S1 of SI) indicated that the negative
197
potential of C60 increased slightly when DOM was added. The changes in particle
198
sizes of C60 did not correlate well to their changes in zeta potentials, indicating that
199
the electrostatic interaction was not the primary mechanism for particle stability. The
200
decreased particle size and increased C60 stability in the presence of DOM was due to
201
the steric force they impart as discussed below.10-12, 34 The different structural and
202
conformational characteristics between HA and FA account for their distinct
203
stabilization capacity for C60 aqueous suspensions.10, 34 HA has less polarity, longer
204
chain hydrophobic moieties, and higher molecular weight than FA,35 and this causes
205
stronger sorption of HA onto C60 surface via hydrophobic and π-π interactions
206
providing more steric stability. Also HA have a thicker organic coating on C60 surface
207
preventing C60 getting closer to form bigger aggregates.10, 34
208
(a)
(b)
209 210 211 212 213
Figure 1. Intensity-averaged PSD diagrams of C60 aqueous suspensions without or with different concentrations of (a) HA or (b) FA. 9
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ESR Detection of 1O2. As shown in Figure 2(a)-(d), three characteristic peaks of
215
TEMPO spin adducts could be detected by ESR for DOM solutions with or without
216
C60 when irradiated by UV light. The intensities of the TEMPO spin adducts increased
217
as the irradiation time was increased. In contrast, no TEMPO signals were detected in
218
C60 aqueous suspension (Figure 2(e)), indicating that C60 aggregates lost their
219
photochemical reactivity with respect to 1O2 production. These are consistent with
220
previous studies that 1O2 could not be detected in C60 aggregates. These studies
221
hypothesize that the close proximity of the C60 molecules in an aggregate result in
222
rapid decay of 3C60* and its dramatically shortened lifetime.5,
223
aggregates exposed less C60 molecules to photons and O2,37, 38 potentially reducing
224
active surface areas for 1O2 production.
36
In addition,
225
The TEMPO signals have been detected in DOM solutions, which is consistent
226
with previous studies that both HA and FA could photosensitize 1O2 generation under
227
UV irradiation.39, 40 The excited triplets of phenyl ketone or aromatic quinone in the
228
DOM molecule are primarily responsible for their photosensitization effects.41,
229
Moreover, the TEMPO signals in FA solution are stronger than those in HA solution
230
with or without C60. The higher photosensitization capacity of FA than HA is
231
primarily due to the presence of more chromophores in the lower-molecular-weight
232
fractions of DOM.17,
233
mixtures are stronger than those in solutions that contained only DOM or C60. The
234
reasons for the enhanced 1O2 generation in DOM/C60 mixtures will be discussed in the
235
following “Proposed Reaction Framework” Section. No TEMPO signals have been
236
detected in C60 aqueous suspension, DOM solutions, or their mixtures in the dark
237
(Figure S2 of SI).
39
42
It is also noteworthy that TEMPO signals in DOM/C60
238
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243 244 245
Figure 2. ESR spectra recorded at ambient temperature for TEMP adduct with 1O2 in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).
246
ESR Detection of O2•−. As shown in Figure 3, six characteristic peaks of the
247
DMPO-O2•− spin adducts could be detected using ESR under UV irradiation in HA
248
solution or HA/C60 mixture. Similarly, the strengths of DMPO-O2•− spin adducts were
249
enhanced with prolonged UV irradiation time (Figure 3(a) and (c)). C60 aqueous
250
suspension did not induce O2•− generation primarily due to lack of ED and short
251
lifetime of 3C60*.8, 9, 43 DMPO-O2•− signals were observed in HA solution primarily
252
due to electron transfer from HA photoionization to O2.44 The phenolic moieties in
253
HA molecules were responsible for their photoionization and charge-transfer
254
transitions.19, 45 DMPO-O2•− signals in HA/C60 mixture were stronger than those in
255
HA solution, indicating that HA could serve as ED for C60 and subsequently enhanced
256
O2•− production in the mixtures. Unlike HA solution, no DMPO-O2•− adducts were
257
detected in FA solution with or without C60, demonstrating that FA could not undergo
258
photoionization and then inject electrons to O2 or C60. A positive correlation has been
259
found between the charge transfer efficiency of DOM and their aromaticity.19, 45 The
260
lower electron donation capacity of FA than that of HA was probably due to the lower
261
aromaticity of FA (0.28) as compared to that of HA (0.42).46 No measurable amount
262
of O2•− was detected in C60 aqueous suspension, DOM solutions, or their mixtures in
263
the dark (Figure S2 of SI). It is worth mention that DMSO was used as solvent to 11
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detect O2•− due to its spontaneous dismutation in water.47 The potential effect of
265
DMSO on particle size distribution and O2•− generation by C60 with or without DOM
266
was discussed in Section S6 of SI.
267 268 269 270 271 272 273 274
Figure 3. ESR spectra recorded at ambient temperature for DMPO adduct with O2•− in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).
275
Generation Kinetics of 1O2 and O2•−. Molecular probe assays were conducted
276
to measure the photogenerated concentrations of 1O2 and O2•− in C60 aqueous
277
suspensions, DOM solutions, or their mixtures under UV-365 irradiation. Basically,
278
the ROS production profile that was measured by the molecular probe assays was
279
similar to that detected using ESR. Previous studies have shown that FFA photolysis
280
under UV-365 irradiation was negligible.29-31 As shown in Figure 4(a), (b), we
281
observed that both HA and FA solutions result in FFA photodegradation, and the
282
degradation rate increases with DOM concentration. At the same mass concentration,
283
the photodegradation rate of FFA by FA is higher than that by HA. This is consistent
284
with our ESR results and previous studies that showed FA produced more 1O2 than
285
HA.16, 19 Aqueous C60 suspensions alone did not degrade FFA, which indicates that
286
1
287
few sites for UV absorption and O2 exposure.5 When DOM was added to C60 aqueous
288
suspensions, the photodegradation rate of FFA was significantly enhanced and the
289
FFA photodegradation rate increased with DOM concentration. A comparison
290
between Figure 4 (a) and (b) shows the FFA photodegradation rates in FA/C60
O2 was not generated by C60. This was primarily due to short lifetime of 3C60* and
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mixtures were much faster than those in HA/C60 mixtures at the same DOM mass
292
concentrations. This indicated that the photosensitization capacity of FA for 1O2
293
generation in DOM/C60 mixture was higher than that of HA because most of
294
chromophores (such as phenyl ketone and aromatic quinone) were located in the
295
low-molecular-weight fractions of DOM.16,
296
mixtures did not induce significant degradation of FFA (data not shown).
19
In the dark, C60, DOM, or their
297
Figure 4(c)-(f) shows the changes in the absorption spectrum of C60 aqueous
298
suspension, HA solutions, or their mixtures during a 48-h exposure to UV-365
299
irradiation. The absorption peak at λ = 470 nm indicates the production of O2•−. As
300
shown in Figure 4(c), no absorption peaks are detected for C60 aqueous suspension
301
under UV irradiation. This is consistent with our ESR results which have shown that
302
C60 aggregates do not produce O2•−, because of a lack of ED and short lifetime of
303
3
304
increased with increasing HA concentrations. Previous studies have also detected O2•−
305
in HA solution under simulated sunlight or UV irradiation.17, 44 Our work has shown
306
that C60 alone did not produce O2•−, and their addition to HA solutions significantly
307
enhanced O2•− production rates, and the production rate increased with HA
308
concentration. This indicated that C60 could photocatalytically promote O2•−
309
generation by HA. In marked contrast, no measurable amount of O2•− was detected in
310
FA solution with or without C60 (Figure S4 of SI). These results indicated that the
311
electron donation efficiency of HA was higher than that of FA.
C60*.8, 9, 43 HA solution generated O2•− under UV irradiation and the production rate
312
Two factors can influence the electron transfer capacity of DOM, i.e.,
313
aromaticity and phenolic group content.19, 45 Lower aromaticity and phenolic group
314
content result in lower electron transfer efficiency of DOM.19, 45 Previous studies have
315
demonstrated that the aromaticity of FA (0.28) is approximately 1.5-fold lower than
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the aromaticity of HA (0.42).26, 27 Furthermore, the phenolic moieties are responsible
317
for charge transfer in FA (1.5 meq/g), which is approximately two-fold lower than that
318
in HA (2.8 meq/g).46 In the dark, no absorption peaks were detected for a C60 solution,
319
DOM solutions, or their mixtures (Figure S5). Many previous studies have
320
demonstrated that the degradation of XTT by UV-365 light was negligible. 29-31
321 322 323 324 325 326 327
(c)
(d)
10 mg/L HA
(e)
(f)
50 mg/L HA
328 329 330 331 332 333 334 335
Figure 4. 1O2 generation kinetics indicated by the degradation of 0.85 mM FFA (a-b) and O2•− generation kinetics (c-f) in C60 aqueous suspension, DOM solutions, or their mixtures as shown by the reduction of 200 μM XTT (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, and C60 of 5 mg/L).
336
ROS Concentration Photogenerated by C60, DOM Solution, or their
337
Mixtures. Table 1 summarizes the average molar concentrations of 1O2 and O2•−
338
photogenerated in C60 aqueous suspension, DOM solutions, or their mixtures. Only
339
C60 dispersed in water did not produce measurable amount of ROS, whereas DOM
340
solutions with or without C60 generated at least one type of ROS. The average molar
341
concentrations of total ROS (the sum of the concentrations of 1O2 and O2•−) generated
342
in DOM/C60 mixtures were 1.3~1.8 times higher than the sum of their individual 14
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production concentrations, indicating that DOM/C60 mixtures generated ROS in a
344
synergistic way. This was primarily because the decreased C60 aggregation by DOM
345
increased the specific surface area for light adsorption and O2 exposure. It was
346
observed that: (1) The photogenerated 1O2 concentrations in decreasing order
347
followed this trend: FA/C60 mixture > FA > HA/C60 mixture > HA at the same DOM
348
concentration and C60 concentration of 5 mg/L. The 1O2 production concentrations for
349
FA solutions were approximately 1.6-fold, 1.9-fold, and 2.3-fold more than the
350
concentrations generated in HA solutions at 10.0, 20.0, and 50.0 mg/L, respectively.
351
After addition of 5.0 mg/L C60, 1O2 production concentrations in FA solutions were
352
approximately 2.0 times higher than those generated in HA solutions at all DOM
353
concentrations. (2) No measurable amount of O2•− was detected in C60 aqueous
354
suspension, FA solution, or their mixture. The addition of C60 into HA solution
355
significantly enhanced O2•− generation concentrations, which were increased by up to
356
1.8 fold as compared with those generated in HA solution alone.
357 358
Table 1. Average molar concentrations of ROS generated by C60, DOM solutions, or their mixtures (UV-365 irradiation and light intensity of 1.0 × 10-6 Einstein·L-1·s-1). C60 (mg/L) 0
HA
0
FA
5.0
5.0
5.0 5.0 359 360
DOM (mg/L)
HA
FA
HA FA
1
O2 (μM)
O2•− (μM)
Total (μM)
10.0 20.0 50.0 10.0
49.8 ± 2.1 63.0 ± 2.5 72.8 ± 2.8 79.2 ± 0.3
18.9 ± 1.8 36.8 ± 2.0 58.2 ± 2.2 N.D.
68.7 ± 3.9 99.8 ± 4.5 131.0 ± 5.0 79.2 ± 0.3
20.0
121.6 ± 7.2
N.D.
121.6 ± 7.2
50.0 10.0
165.1 ± 8.2 60.8 ± 2.3
N.D. 49.0 ± 2.1
165.1 ± 8.2 109.8 ± 4.4
20.0
91.1 ± 4.1
83.8 ± 3.8
174.9 ± 7.9
50.0 10.0
108.7 ± 6.6 114.8 ± 6.8
127.9 ± 7.4 N.D.
236.6 ± 14.0 114.8 ± 6.8
20.0
163.4 ± 8.6
N.D.
163.4 ± 8.6
50.0 0 0
215.9 ± 13.2 N.D. N.D.
N.D. N.D. N.D.
215.9 ± 13.2 0 0
N.D. indicates that ROS were not detected or were not statistically significant. Decay Kinetics and Lifetime of 3C60*. 3C60* is the key transient intermediate 15
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for both energy transfer (i.e., 1O2 generation) and electron injection (i.e., O2•−
362
generation) process in C60 aqueous suspensions after light irradiation.1,
363
quantitatively analyze the lifetime of 3C60*, the decay kinetics of 3C60* in the C60
364
aqueous suspension with or without DOM was fitted by the following single
365
exponential function:
2
It = I0 exp (-t/τ0) + C
366
To
(1)
367
where It is the fluorescence intensity of 3C60* after a UV irradiation time of t (μs), I0 is
368
the initial fluorescence intensity of 3C60*, τ0 is the lifetime of 3C60* (μs), and C is a
369
constant. The experimental data have been fitted by Eq 1, and the model fits are
370
shown by red lines in Figure 5. The correlation coefficients (R2), fitted values of I0, τ0,
371
and C are shown in Table S2 of SI. Apparently, the addition of HA or FA into C60
372
aqueous suspension significantly prolonged the lifetime of 3C60*.
373
As shown in Table S2, the lifetime of 3C60* in the C60 aqueous suspension is 0.09
374
± 0.01 μs. This is consistent with previous studies that showed 3C60* in the C60
375
aqueous suspension decayed within 100 ns.9, 10 As demonstrated by our DLS results
376
and previous studies,37, 38 C60 dispersed in water was comprised of closely packed C60
377
molecules. The short lifetime of 3C60* in C60 aggregates was primarily due to
378
quenching by surrounding ground-state C60 (self-quenching) or another
379
(triplet-triplet annihilation) on the aggregate surface.3, 5 Therefore, the pathway for
380
energy and electron transfer to O2 is fundamentally blocked as 3C60* becomes
381
short-lived and less available,3, 5 which could explain why no ROS was detected in C60
382
aqueous suspenion.
3
C60*
383
In contrast, when HA or FA was added into C60 aqueous suspensions, the
384
liftetime of 3C60* extended to 4.63 ± 0.05 or 4.59 ± 0.04 μs, respectively, which was
385
approximately fifty-fold longer than that in C60 aqueous suspension (Table S2 of SI).
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This was primarily because DOM decreased the aggregate size and enhanced the
387
colloidal stability of C60 aggregates. As the size of aggregates decreased, more C60
388
molecules on aggregate surface were exposed to light forming more 3C60*.37, 38 These
389
lead to less ground state C60 and subsequently decreased their quenching for 3C60*.
390
Meanwhile, the enhanced colloidal stability decreased the possibility of triplet-triplet
391
annihilation among different C60 aggregates. Therefore, the lifetime of 3C60* in the
392
mixture is sufficiently long for 3C60* to mediate both energy transfer and charge
393
injection to O2. No absorbance has been observed at 740 nm in HA or FA solution.
394
Hotze et al.48 have also demonstrated that aggregate size and structure strongly
395
affected the fraction of 3C60* within an aggregate and influenced ROS generation in
396
C60 aqueous suspension.
397 398 399 400 401 402 403
Figure 5. Absorption-time profiles of 3C60* recorded at 740 nm in C60 aqueous suspensions with or without HA/FA at different laser excitation time in N2-saturated condition (C60 of 5 mg/L, HA of 10 mg/L, and FA of 10 mg/L).
404
Proposed Reaction Framework. We propose a ROS photogeneration
405
mechanism for DOM/C60 mixtures. As shown in Figure 6, the synergistic
406
photogeneration of 1O2 in the DOM/C60 mixutres could be attributed to the energy
407
transfer between them. The 365 nm UV light can induce the generation of 3DOM*
408
because the incident photon energy (3.4 eV) is higher than the energy of 3DOM*
409
(180~250 kJ/mol, 1.9 to 2.6 eV).45, 49 As far as the photophysics is concerned, 3DOM*
410
has a much higher energy level than that of 3C60* (157~176 kJ/mol, 1.63~1.83 eV) or
411
1
O2 (94.3 kJ/mol, 0.98 eV).19, 50, 51 The fractions of light absorbed by C60 or DOM in
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their mixtures are calculated in Section S10 of SI. DOM absorbs much more light
413
than C60 in C60/DOM mixtures. Therefore 3DOM* can transfer energy to O2 but more
414
importantly 3DOM* can transfer energy to C60 and create 3C60*. And the prolonged
415
lifetime of 3C60* provides more opportunity for the energy transfer to O2 and
416
generates more 1O2. It is important to note that the creation of smaller C60 aggregates
417
from the adsorption and stabilization caused by DOM increases the lifetime of 3C60*
418
because larger aggregates contain several 3C60* and they can annihilate each other
419
before energy transfer to O2.
420
As shown in Figure 6, the synergistic photogeneration of O2•− in HA/C60 mixture
421
is primarily attributed to the electron donation from photoionized HA to electrophile,
422
C60. With respect to thermodynamics, the standard oxidation potential of HA was
423
+0.797 V, demonstrating that HA can reduce 3C60* which has a standard reduction
424
potential of E0(3C60*/C60•−) +1.1 V.3, 5, 36 The ground state C60 has a standard redox
425
potential of E0(C60/C60•−) -0.2 V,3, 5, 36 indicating that the electron transfer from HA is
426
more favorable to 3C60* (1.897 V) than the ground state of C60 (0.597 V), resulting in
427
C60•− formation. Subsequently, C60•− transfers electrons to O2 and produces O2•−.
428
Moreover, C60 promotes electron depletion from HA during photoionization and
429
photocatalytically promotes O2•− generation by HA. The standard oxidation potential
430
of FA is +0.507 V, which indicates that FA could reduce 3C60*. However, our
431
experimental results of fluorescence quenching of DOM by C60 showed that the
432
electron donation capacity of FA was lower than that of HA (Section S11 of SI). In
433
addition, the quantum yield for e- production by FA (7.9 × 10-6)52 was two orders of
434
magnitude lower than that by HA (1.7 × 10-3) under UV irradiation.53 Therefore, it is
435
hard for FA to loose an electron and serve as electron donors for C60. This explains
436
why no measurable amount of O2•− was detected in FA/C60 mixture.
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C60 is electron deficient and a good electron acceptor (can reversibly accept up to
438
six electrons). Thus the electron transfer from C60 to DOM is not likely to occur in
439
principle. Kong et al.19 have also demonstrated that DOM can act as electron donors
440
and transfer electron to C60 in their mixtures, and the reverse, did not take place. The
441
lower energy level of 3C60* than that of 3DOM* makes it impossible for energy
442
transfer from C60 to DOM. Therefore, C60 cannot transfer electron and energy to DOM
443
under light illumination. In conclusion, the electron and energy transfer from DOM to
444
C60 contributes to the synergistic ROS generation in the mixed system.
445 446 447 448 449 450 451 452 453
Figure 6. The reaction schema of the synergistic generation of 1O2 and O2•− by the mixture of C60 and DOM in aqueous phase under UV-365 irradiation.
454
Our work demonstrated that the energy and electron transfer between DOM and
455
C60 resulted in the synergistic generation of 1O2 and O2•− for DOM/C60 mixtures when
456
exposed with 365 nm UV light. This strongly suggests that DOM and sunlight may
457
play critical roles in the fate and photochemical activity of C60 in the natural aqueous
458
environment. After C60 are released into surface waters where DOM is ubiquitous, the
459
enhanced ROS generation concentrations of C60 could increase their possibility to
460
pose hazardous to aquatic organisms under natural solar irradiation. Thus the risk
461
assessment of C60 should consider both their inherent toxicity and their possible
462
interactions with coexisting DOM in natural waters. 19
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ASSOCIATED CONTENT
464
Supporting Information Available
465
Chemicals used in this study, preparation and characterization method of C60,
466
mass-averaged PSDs of C60 with or without DOM, zeta potential of C60, O2•−
467
production kinetics in FA solutions with or without C60, correlation coefficient for
468
3
469
Supporting Information. This material is available free of charge via the Internet at
470
http://pubs.acs.org.
471
ACKNOWLEDGMENTS
C60* decay kinetics, and fluorescence quenching of DOM by C60 are available in the
472
This study was financially supported by the Fund for Innovative Research Group
473
of the National Natural Science Foundation of China (Grant No. 51421065), the
474
National Natural Science Foundation of China (Nos. 51378065 and 21407010), and
475
China Postdoctoral Science Foundation (No. 224234).
476
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