Journal of Colloid and Interface Science 437 (2015) 28–34
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon dots functionalized by organosilane with double-sided anchoring for nanomolar Hg2+ detection Wentai Wang a,b,c, Tak Kim a,b, Zifeng Yan c, Guangshan Zhu a, Ivan Cole d, Nam-Trung Nguyen a, Qin Li a,b,⇑ a
Queensland Micro- and Nanotechnology Centre, Griffith University, Brisbane, QLD 4111, Australia Environmental Engineering, Griffith University, Brisbane, QLD 4111, Australia c State Key Laboratory of Heavy Oil Processing Key Laboratory of CNPC, China University of Petroleum, Qingdao, China d CSIRO Materials Science and Engineering, Victoria, Australia b
a r t i c l e
i n f o
Article history: Received 13 July 2014 Accepted 7 September 2014 Available online 16 September 2014 Keywords: Carbon dots Fluorescent nanoparticles Hg2+ detection Water quality
a b s t r a c t Surface functional groups on carbon dots (CDs) play a critical role in defining their photoluminescence properties and functionalities. A new kind of organosilane-functionalized CDs (OS-CDs) were formed by a low temperature (150 °C) solvothermal synthesis of citric acid in N-(b-aminoethyl)-c-aminopropylmethyl-dimethoxysilane (AEAPMS). Uniquely, the as-synthesized OS-CDs have dual long chain functional groups with both ANH2 and ASi(OCH3)3 as terminal moieties. Double sided anchoring of AEAPMS on CDs occurs, facilitated by the water produced (and confined at the interface between CDs and solvent) when citric acid condenses into the carbon core. The resultant OS-CDs are multi-solvent dispersible, and more significantly, they exhibit excellent selectivity and sensitivity to Hg2+ with a linear detection range of 0–50 nM and detection limit of 1.35 nM. The sensitivity and selectivity to Hg2+ is preserved in highly complex fluids with a detection limit of 1.7 nM in spiked 1 M NaCl solution and a detection limit of 50 nM in municipal wastewater effluent. The results show that the OS-CDs synthesised by the solvothermal method in AEAPMS may be used as an effective Hg2+ sensor in practical situations. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Despite being one of the most toxic heavy metal ions, mercury (Hg2+) ion is widespread and widely used in industry, and causes serious environmental and health concerns [1–4]. With stringent regulations on Hg2+ contamination in drinking water in place (e.g. the maximum contamination limit for Hg2+ is 2 ppb in the USA [5], 1 ppb in Australia and the European Union [6,7]), detection and remediation of Hg2+ in water has always been a field of high importance. The increased necessity and practice in wastewater recycling in recent years has further increased the urgency of developing facile and accurate Hg2+ detection methods. Many analytical methods for Hg2+ detection have been developed including surface-enhanced Raman scattering (SERS) technique [8], surface plasmon resonances [9], inductively coupled plasma mass spectrometry [10], fluorescence chemosensors [11] and electrochemical methods [12], etc. Most of the Hg2+ fluorescence probes are metal-based, such as gold and silver nanoparticles and nanowires [13–15]. Organic molecules and semiconductor quantum dots were also applied as fluorescence probes for Hg2+ detection ⇑ Corresponding author at: Queensland Micro- and Nanotechnology Centre, Griffith University Nathan Campus, Brisbane, Australia. Fax: +61 7 555 28226. E-mail address: qin.li@griffith.edu.au (Q. Li). http://dx.doi.org/10.1016/j.jcis.2014.09.013 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
[15,16]. However, the above fluorescence probes possess some disadvantages, which greatly limit their practical applications, such as high cost, toxicity of the probe materials, poor stability and complex synthesis procedures. Therefore, new Hg2+ fluorescent probes that can overcome the above limitations are highly desirable. Carbon dots (CDs) are a class of carbon-based nanoparticles that comprise discrete carbogenic nanoparticles with sizes below 10 nm. CDs have emerged as versatile fluorescent nanoparticles possessing unique features such as high quantum yields [17], nontoxicity, nonblinking, high photostability and vast accessibility [18–20], with strong potential to be applied in bio-imaging, sensing and optoelectronic devices [19,21–24]. CDs can be synthesized through a number of methods including laser ablation [25], electrochemical exfoliation [26], carrier-supported aqueous route [19], combustion route [27], hot injection [28], hydrothermal [29] and microwave treatment [30] etc. These methods generally result in hydrophilic CDs with abundant ACOOH and AOH groups on surface, which are amenable for further functionalization. Owing to the rich surface functional groups, CDs have been demonstrated as an effective fluorescence probe for the detection of copper ions [31], ferric ions [32], silver ions [33], as well as mercury ions [34] in water. The presence of the cation analyte quenches the CD fluorescence with the fluorescence intensity being proportional to the concentration of analytes, most likely
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due to the effect of electron transfer. In terms of Hg2+ sensing, CDs potentially offer advantages such as high sensitivity, economic and green synthesis routes, convenient detection procedures amongst others [35]. However, there is still much room for improvements in order to bring CDs closer to practical applications, such as simpler and more efficient synthesis methods, tunable emission bands, enhanced quantum yield (QY), heightened sensitivity, specificity and durability in complex fluids, such as wastewater effluent. Wastewater effluent contains large amount of organic matters, bacteria and viruses; it has high fluorescence background and is prone to interact or contaminate nanoparticle surfaces, representing a challenging sample type for fluorescence probes. Herein, we report a new kind highly photoluminescent (PL) organosilane functionalized CDs (OS-CDs) for Hg2+ detection, which were synthesized by a low temperature solvothermal treatment of citric acid in coordination solvent, N-(b-aminoethyl)-caminopropylmethyldimethoxysilane (AEAPMS), in an enclosed system. In addition to high QY and excellent stability, the resultant OS-CDs show multi-solvent dispersibility, being dispersable in water and most of the common organic solvents. Importantly, the as-prepared OS-CDs is an effective fluorescence probe with superior selectivity and sensitivity to Hg2+ with a detection limit of 1.35 nM (0.27 ppb) and a linear range of 0–50 nM. Furthermore, the OS-CDs’ sensing ability to Hg2+ in real municipal wastewater and water of high salinity is preserved, still achieving nanomolar sensitivity. This work may broaden the potential of OS-CDs in practical use for Hg2+ detection.
Zetasizer Nano-ZS. Atomic force microscopy (AFM, Dimension 3000) analysis was carried out with tapping mode on a platinum coated mica substrate. FT-IR spectra were collected on a PerkinElmer Spectrum 100 with a resolution of 4 cm1 in transmission mode. A baseline correction was applied after the measurement. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Kratos Axis Ultra photoelectron spectrometer which uses Al Ka (1253.6 eV) X-rays. The UV–vis absorption and fluorescence emission were measured by a Jasco V670 UV–VIS spectrometer and a Thermal Scientific Lumina fluorescence spectrometer, respectively. The concentration of CDs was determined by a gravimetric method (details in Supporting Information).
2. Material and methods
Detection of Hg2+ in pure water was performed at room temperature. OS-CDs solution with a given concentration was prepared before measurement. 2 ml of OS-CDs solution was transferred into a quartz cuvette followed by addition of calculated amount of Hg2+ solution. After mixing uniformly and incubating for 30 min, the PL emission spectra were collected. Detection of Hg2+ in municipal wastewater effluent was tested in wastewater effluent which was collected from Redland Wastewater Treatment Plant, Brisbane, Australia, and has gone through secondary treatment. The wastewater sample was filtered with a 0.22 lm syringe filter firstly to remove large particles. A given concentration of OS-CDs in wastewater was then prepared. 2 ml of above OS-CDs in wastewater solution was transferred to a quartz cuvette for PL measurement. Hg2+ solution was then added into the vial step-wisely to increase the concentration from 1 nM. Each time after adding Hg2+ solution, a 30 min time interval was given to allow a good diffusion-driven mixing in the vial, before the PL measurement.
2.1. Materials Citric acid anhydrous was purchased from Sigma–Aldrich. N-(baminoethyl-c-aminopropylmethyldimethoxysila (AEAPMS) was purchased from Beijing Shenda Fine Chemical Co., Ltd. Sodium hydroxide, sodium chloride, potassium chloride, calcium chloride, magnesium sulfate, zinc sulfate heptahydrate, cadmium chloride hydrate, chromium trichloride and sodium hypochlorite were purchased from Chem-Supply. Hydrochloride acid, silver nitrate, mercury (II) chloride and cobaltous oxalate dehydrate, and solvents including dimethylsulfoxide (DMSO), methanol, dimethyl formamide (DMF), acetone, ethanol, tetrahydrofuran (THF), toluene and hexane were all purchased from Alfa Aesar. All chemicals and reagents were used as received without any further purification. 2.2. Synthesis of organosilane-functionalized CDs The typical procedure of solvothermal synthesis of OS-CDs is as follows: 0.5 g citric acid anhydrous was added into 10 ml AEAPMS with continuous stirring. The mixture was then transferred into an autoclave with a PTFE inner vessel and placed in oven at 150 °C for 4 h. Brownish liquid was obtained after the reaction process. The product was dispersed in Milli-Q water or other appropriate solvents, followed by purifying three times with an Al2O3 filled chromatographic column for removing the residue reactants. The collected fraction was further filtered by a 0.22 lm syringe filter to remove the large particles. Finally, the solution was centrifuged for 30 min at 12,000 rpm for further purification, and the supernatant was collected as the product. 2.3. Characterization The hydrodynamic particle size and zeta potential were measured by dynamic light scattering (DLS) on a Malvern Instrument
2.4. Multi-solvent dispersibility test 100 lL of OS-CDs was dropped into 5 ml of various solvents, such as DMSO, methanol, DMF, acetone, ethanol, THF, toluene and hexane, as well as Milli-Q water, respectively, and mixed uniformly. The samples were kept at the room temperature for 2 weeks. To observe the transfer of CDs from toluene to water, 100 lL of OS-CDs was first dispersed into 5 ml toluene, and then 5 ml of Milli-Q water was slowly added into above solution. An interface was clearly observed between water and toluene. The vial was placed under a 365 nm UV lamp to observe the movement of OS-CDs between the organic and water phases. 2.5. Procedures for Hg2+ sensing
3. Results and discussion 3.1. Synthesis and physiochemical characterization of OS-CDs The analysis of the OS-CDs morphology by transmission electron microscopy was challenging because the AEAPMS passivated OS-CDs tend to draw moisture from the air and turn into a gel-type material, similar to previous reports [28]. The AFM images of the OS-CDs in Fig. 1A provide the two-dimensional (2D) and 3D morphology. The size monodispersity of OS-CDs by counting the height of 150 particles was shown in Fig. 1B, indicating the size of OS-CDs is mainly distributed in the range of 1–2.5 nm. The DLS data (Fig. 1C) also shows a narrow size distribution in the range of 0.5–2 nm. The smaller size derived by DLS can be attributed to the better dispersion of CDs in water, whilst agglomeration may have occurred during the drying process of AFM sample preparation.
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Fig. 1. (A) AFM image of OS-CDs. (B) Histogram of OS-CDs particle size. (C) Size distribution of OS-CDs measured by DLS.
Fig. 2A shows the FT-IR spectra of OS-CDs in comparison with the reactants, namely citric acid and AEAPMS. It is clear that C@O stretching vibration of the ACOOH groups appeared at 1745 cm1 after the reaction. The broad absorption between 3200 and 3600 cm1 can be attributed to AOH or NAH. The peaks at 1630, 1565 and 1460 cm1, which belong to the C@O, NAH and CAH stretching of amide bond, respectively, suggest the formation of RAC@ONR between AEAPMS and carbon core, as illustrated by the red branches in Scheme 1. It should be highlighted that there is a distinctive, broad peak between 856 cm1 and 1140 cm1 ascribed to SiAOASi and SiAOAC peaks, suggesting the formation of siloxane groups by hydrolization, which may lead to the attachment of organosilane long chain onto the carbon core surface, as illustrated by the blue branches in Scheme 1. The vibrational fingerprints of CAN (1180, 1250 cm1) and ANH2 (3300 cm1) stretching vibration belonging to the amine-terminated long chains were both observed in the spectra of AEAPMS and OS-CDs. The XPS data shown in Fig. 2B reveals the elementary composition of OS-CDs, namely C 61.17%, N 13.93%, O 12.95% and Si 11.95%, confirming the framework of the OS-CDs are mainly constructed by carbon. High resolution spectra of C 1s, Si 2p, O1s and N 1s detailing the chemical bonding states are shown in Fig. 2C–F, respectively. The C 1s core level spectrum (Fig. 2C) can be decomposed into three contributions at 284.6 eV (46.5%), 285.6 eV(48.5%), and 287.8 eV(5.0%), which can be assigned to CAC/C@C bonds, CAN/ CAO, and C@O bonds, respectively [36]. The Si 2p spectrum (Fig. 2D) can be deconvoluted into two components, namely SiAOACCDs at 101.9 eV [37] (84.93%), suggesting the covalent attachment of silane end to the carbon core, and SiAOASi at 104.9 eV [38] (15.07%) of the silica network formed by hydrolyzation of organosilane. The O1s spectrum can be decomposed into four contributions (Fig. 2E): the peak at 531.2 eV can be ascribed to the ACOO groups (14.12%) [39], peaks at 532.2 eV and 533.0 eV are attributed to C@O (68.11%) and CAOH/CAOAC (14.34%), respectively [40]; a small peak at 535.2 eV can be assigned to SiAO (3.44%). In the N 1s spectrum (Fig. 2F), CAN bond (87.34%) at 399.1 eV and ANH2 (12.66%) at 400.0 eV can be identified. The zeta potential (f) of OS-CDs in pure water was measured to be slightly negative, around 2 mV (Fig. S2). This suggests that there may be comparable amount of positive charged (such as ANH+3) and negative charged moieties (such as ACOO, AOH and the hydrolysed organosilane terminals) on the surface of OS-CDs, as depicted in Scheme 1.
Overall, the chemical analyses have suggested that the solvothermal synthesized OS-CDs are constructed by mainly by carbon with dual long chains attached through RAC@ONR bond originated from amidation as well as SiAOAC and SiAOASi bonding due to silane hydrolization on the surface of CDs as illustrated in Scheme 1. The co-existence of both amine terminated and Si(OCH3)2CH3 terminated long chains on the surface of as-prepared OS-CDs impart them unique properties and functionalities. It is well known that even small changes of reaction parameters may lead to different properties of nanomaterials, especially in the synthesis of CDs [41]. In this work, solvothermal method was employed using citric acid as the carbon source and N-(b-aminoethyl)-c-aminopropylmethyldimethoxysilane (AEAPMS) as the coordination solvent to produce a highly photoluminescent CDs at a low temperature of 150 °C. It is worth noting that with the same carbon precursor and same solvent, previous study [28] which used an open reaction system at 240 °C resulted in hydrophobic CDs. The difference between the two synthesis strategies is that in solvothermal method the reactor, i.e. the autoclave, is enclosed, therefore, when citric acid condenses into the carbon cores, the water produced is trapped in the autoclave under pressure. A fraction of the trapped water is very likely to stay at the interface between carbon nucleates and organosilane solvent through hydrogen bonds, forming a thin water layer, which facilitates the formation and retention of these hydrophilic functional groups (AOH and ACOOH). Meanwhile, the organosilane molecules may be hydrolyzed within the thin water layer and attach onto the surface of CDs through SiAOASi or SiAOAC bonding [42]. The overall formation mechanism is illustrated in Scheme 1. 3.2. The optical properties of OS-CDs The as-synthesized OS-CDs show a distinctive absorption peak centred at 360 nm in the UV–Vis absorption spectrum and a maximum emission peak at 465 nm in the fluorescence (PL) spectra as shown in Fig. 3. In contrast to earlier reported OS-CDs [28], the asprepared OS-CDs are excitation-independent. When the excitation wavelength was varied in range of 320–420 nm, the emission maxima remained at 465 nm. Further increasing excitation wavelength to 440 nm and above, the OS-CDs was almost non-fluorescent, indicating that the OS-CDs have only one fluorescence centre. The excitation-independence may also be ascribed to the narrow particle size distribution. 360 nm was selected as the excitation wavelength in the following experiment as it induced the highest
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Fig. 2. (A) FTIR spectra of citric acid, OS-CDs and AEAPMS. (B) Surface scan of XPS spectra. (C) High resolution spectra of C1s. (D) Si 2p. (E) O1s. (F) N1s.
Scheme 1. Illustration of the possible formation process of OS-CDs from citric acid and AEAPMS in solvothermal synthesis.
fluorescent intensity. The QY of the as-prepared OS-CDs in Milli-Q water was calculated to be 51%, higher than most other reported CDs [28,43]. The fluorescence lifetime of OS-CDs in water under the excitation of 360 nm can be fitted by a multi-exponential function as shown in Fig. S3. Two fitting decay times were acquired, namely
s1 = 5.45 ns (5%) and s2 = 15.49 ns (95%) with the average lifetime of 14.99 ns. This result confirms that the fluorescence of OS-CDs was dominated by the emitting centre with lifetime of s2 = 15.49 ns. Its long fluorescence lifetime compared to the other CDs [44] is likely due to the abundant long chain surface functional groups which provide better trapping effect.
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Fig. 3. UV–Vis absorption spectrum and PL emission spectra of OS-CDs at different excitation wavelengths; the inset is a picture of OS-CDs under room light and 365 nm UV lamp.
3.3. Multi-solvent dispersibility of OS-CDs The OS-CDs can be well dispersed in both polar and apolar solvents. Fig. 4A shows that OS-CDs can be well dispersed in DMSO, methanol, DMF, acetone, ethanol, THF, toluene and hexane as well as Milli-Q water, with no sediment or layering phenomenon observed after two weeks, which shows the excellent multi-solvent solubility and stability. The multi-solvent dispersibility of OS-CDs was vividly observed by mixing OS-CDs in toluene and water mixture, as shown in Fig. 4B. Fluorescence disappeared completely in toluene phase but transferred into water phase after 1 day, indicating the better solubility of OS-CDs in water than in toluene. The strong multi-solvent dispersibility of the as-synthesized OS-CDs further confirms the dual long chain surface chemistry illustrated in Scheme 1. Furthermore, the PL intensity of as-synthesized OS-CDs (in Fig. S4) does not vary in ionic solutions when ionic strength is increased by adding NaCl up to 1 M. The excellent dispersity in solutions of high ionic strength can be ascribed to the steric effect due to the surface long chains [45]. 3.4. Hg2+ detection 3.4.1. In pure water The as-synthesized OS-CDs fluorescence sensitivity to metal cations was first assessed against a series of 100 lM aqueous solu-
tions containing Ag+, K+, Na+, Ca2+, Mg2+, Zn2+, Hg2+, Co2+, Cd2+, Fe3+ and Cr3+. As shown in Fig. 5A, the PL of the OS-CDs was insensitive to most of the metal cations but Hg2+ and Fe3+. Although Fe3+ also induced the fluorescence quenching of OS-CDs, Hg2+ can be selectively identified by adding sodium hexametaphosphate as the masking agent of Fe3+ ions [46] as shown in Fig. S5. To further verify the effect of the co-presence of some common mineral elements such as K+, Na+, Ca2+ and Mg2+ on OS-CDs’ sensing selectivity to Hg2+, the PL intensities of OS-CDs in pure water, and in solutions containing K+, Na+, Ca2+, Mg2+ and a mixture of all above (100 lM) with and without Hg2+ were compared as shown in Fig. 5B. The result clearly demonstrates that the presence of one or more above common metal ions has no adverse impact on the sensitivity of OS-CDs to Hg2+ ions. In addition, anions such as SO2 4 , NO3 , Cl and ClO4 were also found to have no effect on the fluorescence of OS-CDs (Fig. S6 showed data of ClO 4 ). It is worth noting that ClO 4 is highly oxidative and has been shown to have an effect on the PL of graphene quantum dots [47]; the OS-CDs appear to be stable against oxidative conditions. All these lead to the conclusion that the as-prepared OS-CDs possess excellent selectivity to Hg2+. The sensitivity of the as-prepared OS-CDs to Hg2+ concentration in the range of 0–5 lM was evaluated. As shown in Fig. 6A and B, concentration-dependent quenching was observed: with the increase of Hg2+ concentration from 0 to 5 lM, the PL intensity decreased gradually. The fluorescence quenching data can be fitted by the Stern–Volmer equation: FF0 1 ¼ K SV c, where KSV is the Stern–Volmer quenching constant, c the concentration of Hg2+, F0 and F the PL intensity of OS-CDs without Hg2+ and with different concentration of Hg2+, respectively. As shown in Fig. 6B, a good linear correlation (R2 = 0.9977) was obtained over the concentration range of 0–50 nM, with a quenching constant KSV of 6.49 103 L/ mol. The detection limit was determined to be 1.35 nM, using the equation 3r/m, where r is the relative standard deviation and m the slope of calibration curve as described in detail in Fig. S7. The as-prepared OS-CDs by solvothermal method showed a significantly better sensitivity to Hg2+ than that of the OS-CDs prepared by ‘hot injection’ method [48]. As depicted in Scheme 1, the solvothermal synthesized OS-CDs have amine-terminated alkyl chain due to the silane reaction with carbon surface with the presence of water layer, in contrast to the OS-CDs by ‘hot injection’. It is known that the binding affinity between Hg2+ and ANH2/ANH groups is stronger than that between Hg2+ and hydroxyl or carboxylate groups [49]. In addition, the spatial distribution of these terminal amino groups and the amine groups in the middle of the alkyl chain is very likely to trap Hg2+ and forms complexations, as illustrated in Scheme 2. Such a binding event would facilitate the non-radiative electron/hole recombination annihilation through an effective electron transfer process, causing fluorescence quenching. 3.4.2. In brine solutions The high ionic strength PL stability of OS-CDs is highly desirable in real applications such as water quality measurement at mining sites. Therefore, we further evaluated the sensitivity of OS-CDs to Hg2+ ions in salty solutions (with NaCl concentration of 1 M). As shown in Fig. S8, the PL of the as-prepared OS-CDs maintains excellent response to the concentration of Hg2+ in the range of 0–50 nM with a good linearity (R2 = 0.9987). The detection limit was determined as 1.7 nM, very close to the result in pure water, confirming the stability of OS-CDs in solutions of high ionic strength as a result of the steric effect.
Fig. 4. (A) The dispersion of OS-CDs in different solvents. (B) The multi-solvent dispersibility performance of OS-CDs in toluene and water phase.
3.4.3. In municipal wastewater effluent To further increase the complexity of the sample composition, wastewater after secondary treatment was employed, which con-
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Fig. 5. (A) Effect of metal ions (100 lM) on the fluorescence of OS-CDs. (B) PL intensity of OS-CDs in water, K+, Na+, Ca2+ and Mg2+ aqueous (100 lM) with and without Hg2+ ions, Mixed ions including K+, Na+, Ca2+ and Mg2+, with the concentration of 100 lM.
Fig. 6. (A) PL spectra of OS-CDs in water with different Hg2+ concentrations of 0–5 lM. (B) The linear region (0–50 nM) of Stern–Volmer plot; Inset: The relationship between F0/F1 and Hg2+ concentrations within the range of 0–1 lM. (C) PL spectra of OS-CDs in wastewater with different concentrations of Hg2+ in range of 0–40 lM. (D) The linear region (0–1 lM) of Stern–Volmer plot; Inset: The relationship between F0/F1 and Hg2+ concentrations within the range of 0–40 lM.
shown in Fig. 6C and D, the OS-CDs can still detect the Hg2+ ions from 0.2 lM to 40 lM as shown in Fig. 6C. The Stern–Volmer plot shows the linear range of 0–1 lM with R2 = 0.9978 and KSV = 0.26 L/mol. The detection limit for Hg2+ in wastewater was calculated to be 50 nM, highly sensitive for wastewater quality monitoring. The preserved excellent Hg2+ sensitivity in wastewater effluent suggests the as-synthesized OS-CDs are largely noninteractive with the microorganisms and organic molecules in wastewater, likely owing to the negative charge, long chain surface functional groups. The slight loss of Hg2+ sensitivity of OS-CDs in wastewater is likely due to the high background fluorescence caused by the organic molecules in wastewater. Scheme 2. Schematic illustration of the Hg(II) complexation on OS-CDs surface.
4. Conclusions tains many kinds of bacteria, viruses, metal ions and organic molecules. The fluorescence quenching of OS-CDs by different concentration of Hg2+ in the wastewater sample were investigated. As
In summary, a new kind of OS-CDs was formed by a low temperature (150 °C) solvothermal synthesis of citric acid in AEAPMS.
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The as-synthesized OS-CDs have dual long chain functional groups with both ANH2 and ASi(OCH3)3 as terminal moieties. An enclosed synthesis system such as the autoclave traps water, a by-product of carbon core formation, therefore, enables the organosilane, AEAPMS, to covalently attach on carbon surface by both ends, namely the ANH2 end by amidation and the ASi(OCH3)2CH3 end by hydrolization and SiAOAC formation. The resultant OS-CDs are amphiphilic, and stable in solutions of high ionic strength with QY of 51%. More significantly, they exhibit excellent selectivity and sensitivity to Hg2+ with a linear detection range of 0–50 nM and detection limit of 1.35 nM. The sensitivity and selectivity to Hg2+ is preserved in highly complex fluids with a detection limit of 1.7 nM in spiked 1 M NaCl solution and a detection limit of 50 nM in municipal wastewater effluent. Our results demonstrate that by tuning the arrangement of the surface functional groups, the as-prepared OS-CDs can be devised into an effective fluorescence probe for Hg2+ detection in complex fluid samples. Acknowledgments The authors thank Dr. Barry Wood at The University of Queensland for his assistance in acquiring the XPS data. We also thank Mr. Bradley Taylor at Redland City Council for providing access to wastewater samples. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.09.013. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon dots functionalized by organosilane with double-sided anchoring for nanomolar Hg2+ detection Wentai Wang a,b,c, Tak Kim a,b, Zifeng Yan c, Guangshan Zhu a, Ivan Cole d, Nam-Trung Nguyen a, Qin Li a,b,⇑ a
Queensland Micro- and Nanotechnology Centre, Griffith University, Brisbane, QLD 4111, Australia Environmental Engineering, Griffith University, Brisbane, QLD 4111, Australia c State Key Laboratory of Heavy Oil Processing Key Laboratory of CNPC, China University of Petroleum, Qingdao, China d CSIRO Materials Science and Engineering, Victoria, Australia b
a r t i c l e
i n f o
Article history: Received 13 July 2014 Accepted 7 September 2014 Available online 16 September 2014 Keywords: Carbon dots Fluorescent nanoparticles Hg2+ detection Water quality
a b s t r a c t Surface functional groups on carbon dots (CDs) play a critical role in defining their photoluminescence properties and functionalities. A new kind of organosilane-functionalized CDs (OS-CDs) were formed by a low temperature (150 °C) solvothermal synthesis of citric acid in N-(b-aminoethyl)-c-aminopropylmethyl-dimethoxysilane (AEAPMS). Uniquely, the as-synthesized OS-CDs have dual long chain functional groups with both ANH2 and ASi(OCH3)3 as terminal moieties. Double sided anchoring of AEAPMS on CDs occurs, facilitated by the water produced (and confined at the interface between CDs and solvent) when citric acid condenses into the carbon core. The resultant OS-CDs are multi-solvent dispersible, and more significantly, they exhibit excellent selectivity and sensitivity to Hg2+ with a linear detection range of 0–50 nM and detection limit of 1.35 nM. The sensitivity and selectivity to Hg2+ is preserved in highly complex fluids with a detection limit of 1.7 nM in spiked 1 M NaCl solution and a detection limit of 50 nM in municipal wastewater effluent. The results show that the OS-CDs synthesised by the solvothermal method in AEAPMS may be used as an effective Hg2+ sensor in practical situations. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Despite being one of the most toxic heavy metal ions, mercury (Hg2+) ion is widespread and widely used in industry, and causes serious environmental and health concerns [1–4]. With stringent regulations on Hg2+ contamination in drinking water in place (e.g. the maximum contamination limit for Hg2+ is 2 ppb in the USA [5], 1 ppb in Australia and the European Union [6,7]), detection and remediation of Hg2+ in water has always been a field of high importance. The increased necessity and practice in wastewater recycling in recent years has further increased the urgency of developing facile and accurate Hg2+ detection methods. Many analytical methods for Hg2+ detection have been developed including surface-enhanced Raman scattering (SERS) technique [8], surface plasmon resonances [9], inductively coupled plasma mass spectrometry [10], fluorescence chemosensors [11] and electrochemical methods [12], etc. Most of the Hg2+ fluorescence probes are metal-based, such as gold and silver nanoparticles and nanowires [13–15]. Organic molecules and semiconductor quantum dots were also applied as fluorescence probes for Hg2+ detection ⇑ Corresponding author at: Queensland Micro- and Nanotechnology Centre, Griffith University Nathan Campus, Brisbane, Australia. Fax: +61 7 555 28226. E-mail address: qin.li@griffith.edu.au (Q. Li). http://dx.doi.org/10.1016/j.jcis.2014.09.013 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
[15,16]. However, the above fluorescence probes possess some disadvantages, which greatly limit their practical applications, such as high cost, toxicity of the probe materials, poor stability and complex synthesis procedures. Therefore, new Hg2+ fluorescent probes that can overcome the above limitations are highly desirable. Carbon dots (CDs) are a class of carbon-based nanoparticles that comprise discrete carbogenic nanoparticles with sizes below 10 nm. CDs have emerged as versatile fluorescent nanoparticles possessing unique features such as high quantum yields [17], nontoxicity, nonblinking, high photostability and vast accessibility [18–20], with strong potential to be applied in bio-imaging, sensing and optoelectronic devices [19,21–24]. CDs can be synthesized through a number of methods including laser ablation [25], electrochemical exfoliation [26], carrier-supported aqueous route [19], combustion route [27], hot injection [28], hydrothermal [29] and microwave treatment [30] etc. These methods generally result in hydrophilic CDs with abundant ACOOH and AOH groups on surface, which are amenable for further functionalization. Owing to the rich surface functional groups, CDs have been demonstrated as an effective fluorescence probe for the detection of copper ions [31], ferric ions [32], silver ions [33], as well as mercury ions [34] in water. The presence of the cation analyte quenches the CD fluorescence with the fluorescence intensity being proportional to the concentration of analytes, most likely
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due to the effect of electron transfer. In terms of Hg2+ sensing, CDs potentially offer advantages such as high sensitivity, economic and green synthesis routes, convenient detection procedures amongst others [35]. However, there is still much room for improvements in order to bring CDs closer to practical applications, such as simpler and more efficient synthesis methods, tunable emission bands, enhanced quantum yield (QY), heightened sensitivity, specificity and durability in complex fluids, such as wastewater effluent. Wastewater effluent contains large amount of organic matters, bacteria and viruses; it has high fluorescence background and is prone to interact or contaminate nanoparticle surfaces, representing a challenging sample type for fluorescence probes. Herein, we report a new kind highly photoluminescent (PL) organosilane functionalized CDs (OS-CDs) for Hg2+ detection, which were synthesized by a low temperature solvothermal treatment of citric acid in coordination solvent, N-(b-aminoethyl)-caminopropylmethyldimethoxysilane (AEAPMS), in an enclosed system. In addition to high QY and excellent stability, the resultant OS-CDs show multi-solvent dispersibility, being dispersable in water and most of the common organic solvents. Importantly, the as-prepared OS-CDs is an effective fluorescence probe with superior selectivity and sensitivity to Hg2+ with a detection limit of 1.35 nM (0.27 ppb) and a linear range of 0–50 nM. Furthermore, the OS-CDs’ sensing ability to Hg2+ in real municipal wastewater and water of high salinity is preserved, still achieving nanomolar sensitivity. This work may broaden the potential of OS-CDs in practical use for Hg2+ detection.
Zetasizer Nano-ZS. Atomic force microscopy (AFM, Dimension 3000) analysis was carried out with tapping mode on a platinum coated mica substrate. FT-IR spectra were collected on a PerkinElmer Spectrum 100 with a resolution of 4 cm1 in transmission mode. A baseline correction was applied after the measurement. X-ray photoelectron spectroscopic (XPS) measurements were performed on a Kratos Axis Ultra photoelectron spectrometer which uses Al Ka (1253.6 eV) X-rays. The UV–vis absorption and fluorescence emission were measured by a Jasco V670 UV–VIS spectrometer and a Thermal Scientific Lumina fluorescence spectrometer, respectively. The concentration of CDs was determined by a gravimetric method (details in Supporting Information).
2. Material and methods
Detection of Hg2+ in pure water was performed at room temperature. OS-CDs solution with a given concentration was prepared before measurement. 2 ml of OS-CDs solution was transferred into a quartz cuvette followed by addition of calculated amount of Hg2+ solution. After mixing uniformly and incubating for 30 min, the PL emission spectra were collected. Detection of Hg2+ in municipal wastewater effluent was tested in wastewater effluent which was collected from Redland Wastewater Treatment Plant, Brisbane, Australia, and has gone through secondary treatment. The wastewater sample was filtered with a 0.22 lm syringe filter firstly to remove large particles. A given concentration of OS-CDs in wastewater was then prepared. 2 ml of above OS-CDs in wastewater solution was transferred to a quartz cuvette for PL measurement. Hg2+ solution was then added into the vial step-wisely to increase the concentration from 1 nM. Each time after adding Hg2+ solution, a 30 min time interval was given to allow a good diffusion-driven mixing in the vial, before the PL measurement.
2.1. Materials Citric acid anhydrous was purchased from Sigma–Aldrich. N-(baminoethyl-c-aminopropylmethyldimethoxysila (AEAPMS) was purchased from Beijing Shenda Fine Chemical Co., Ltd. Sodium hydroxide, sodium chloride, potassium chloride, calcium chloride, magnesium sulfate, zinc sulfate heptahydrate, cadmium chloride hydrate, chromium trichloride and sodium hypochlorite were purchased from Chem-Supply. Hydrochloride acid, silver nitrate, mercury (II) chloride and cobaltous oxalate dehydrate, and solvents including dimethylsulfoxide (DMSO), methanol, dimethyl formamide (DMF), acetone, ethanol, tetrahydrofuran (THF), toluene and hexane were all purchased from Alfa Aesar. All chemicals and reagents were used as received without any further purification. 2.2. Synthesis of organosilane-functionalized CDs The typical procedure of solvothermal synthesis of OS-CDs is as follows: 0.5 g citric acid anhydrous was added into 10 ml AEAPMS with continuous stirring. The mixture was then transferred into an autoclave with a PTFE inner vessel and placed in oven at 150 °C for 4 h. Brownish liquid was obtained after the reaction process. The product was dispersed in Milli-Q water or other appropriate solvents, followed by purifying three times with an Al2O3 filled chromatographic column for removing the residue reactants. The collected fraction was further filtered by a 0.22 lm syringe filter to remove the large particles. Finally, the solution was centrifuged for 30 min at 12,000 rpm for further purification, and the supernatant was collected as the product. 2.3. Characterization The hydrodynamic particle size and zeta potential were measured by dynamic light scattering (DLS) on a Malvern Instrument
2.4. Multi-solvent dispersibility test 100 lL of OS-CDs was dropped into 5 ml of various solvents, such as DMSO, methanol, DMF, acetone, ethanol, THF, toluene and hexane, as well as Milli-Q water, respectively, and mixed uniformly. The samples were kept at the room temperature for 2 weeks. To observe the transfer of CDs from toluene to water, 100 lL of OS-CDs was first dispersed into 5 ml toluene, and then 5 ml of Milli-Q water was slowly added into above solution. An interface was clearly observed between water and toluene. The vial was placed under a 365 nm UV lamp to observe the movement of OS-CDs between the organic and water phases. 2.5. Procedures for Hg2+ sensing
3. Results and discussion 3.1. Synthesis and physiochemical characterization of OS-CDs The analysis of the OS-CDs morphology by transmission electron microscopy was challenging because the AEAPMS passivated OS-CDs tend to draw moisture from the air and turn into a gel-type material, similar to previous reports [28]. The AFM images of the OS-CDs in Fig. 1A provide the two-dimensional (2D) and 3D morphology. The size monodispersity of OS-CDs by counting the height of 150 particles was shown in Fig. 1B, indicating the size of OS-CDs is mainly distributed in the range of 1–2.5 nm. The DLS data (Fig. 1C) also shows a narrow size distribution in the range of 0.5–2 nm. The smaller size derived by DLS can be attributed to the better dispersion of CDs in water, whilst agglomeration may have occurred during the drying process of AFM sample preparation.
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Fig. 1. (A) AFM image of OS-CDs. (B) Histogram of OS-CDs particle size. (C) Size distribution of OS-CDs measured by DLS.
Fig. 2A shows the FT-IR spectra of OS-CDs in comparison with the reactants, namely citric acid and AEAPMS. It is clear that C@O stretching vibration of the ACOOH groups appeared at 1745 cm1 after the reaction. The broad absorption between 3200 and 3600 cm1 can be attributed to AOH or NAH. The peaks at 1630, 1565 and 1460 cm1, which belong to the C@O, NAH and CAH stretching of amide bond, respectively, suggest the formation of RAC@ONR between AEAPMS and carbon core, as illustrated by the red branches in Scheme 1. It should be highlighted that there is a distinctive, broad peak between 856 cm1 and 1140 cm1 ascribed to SiAOASi and SiAOAC peaks, suggesting the formation of siloxane groups by hydrolization, which may lead to the attachment of organosilane long chain onto the carbon core surface, as illustrated by the blue branches in Scheme 1. The vibrational fingerprints of CAN (1180, 1250 cm1) and ANH2 (3300 cm1) stretching vibration belonging to the amine-terminated long chains were both observed in the spectra of AEAPMS and OS-CDs. The XPS data shown in Fig. 2B reveals the elementary composition of OS-CDs, namely C 61.17%, N 13.93%, O 12.95% and Si 11.95%, confirming the framework of the OS-CDs are mainly constructed by carbon. High resolution spectra of C 1s, Si 2p, O1s and N 1s detailing the chemical bonding states are shown in Fig. 2C–F, respectively. The C 1s core level spectrum (Fig. 2C) can be decomposed into three contributions at 284.6 eV (46.5%), 285.6 eV(48.5%), and 287.8 eV(5.0%), which can be assigned to CAC/C@C bonds, CAN/ CAO, and C@O bonds, respectively [36]. The Si 2p spectrum (Fig. 2D) can be deconvoluted into two components, namely SiAOACCDs at 101.9 eV [37] (84.93%), suggesting the covalent attachment of silane end to the carbon core, and SiAOASi at 104.9 eV [38] (15.07%) of the silica network formed by hydrolyzation of organosilane. The O1s spectrum can be decomposed into four contributions (Fig. 2E): the peak at 531.2 eV can be ascribed to the ACOO groups (14.12%) [39], peaks at 532.2 eV and 533.0 eV are attributed to C@O (68.11%) and CAOH/CAOAC (14.34%), respectively [40]; a small peak at 535.2 eV can be assigned to SiAO (3.44%). In the N 1s spectrum (Fig. 2F), CAN bond (87.34%) at 399.1 eV and ANH2 (12.66%) at 400.0 eV can be identified. The zeta potential (f) of OS-CDs in pure water was measured to be slightly negative, around 2 mV (Fig. S2). This suggests that there may be comparable amount of positive charged (such as ANH+3) and negative charged moieties (such as ACOO, AOH and the hydrolysed organosilane terminals) on the surface of OS-CDs, as depicted in Scheme 1.
Overall, the chemical analyses have suggested that the solvothermal synthesized OS-CDs are constructed by mainly by carbon with dual long chains attached through RAC@ONR bond originated from amidation as well as SiAOAC and SiAOASi bonding due to silane hydrolization on the surface of CDs as illustrated in Scheme 1. The co-existence of both amine terminated and Si(OCH3)2CH3 terminated long chains on the surface of as-prepared OS-CDs impart them unique properties and functionalities. It is well known that even small changes of reaction parameters may lead to different properties of nanomaterials, especially in the synthesis of CDs [41]. In this work, solvothermal method was employed using citric acid as the carbon source and N-(b-aminoethyl)-c-aminopropylmethyldimethoxysilane (AEAPMS) as the coordination solvent to produce a highly photoluminescent CDs at a low temperature of 150 °C. It is worth noting that with the same carbon precursor and same solvent, previous study [28] which used an open reaction system at 240 °C resulted in hydrophobic CDs. The difference between the two synthesis strategies is that in solvothermal method the reactor, i.e. the autoclave, is enclosed, therefore, when citric acid condenses into the carbon cores, the water produced is trapped in the autoclave under pressure. A fraction of the trapped water is very likely to stay at the interface between carbon nucleates and organosilane solvent through hydrogen bonds, forming a thin water layer, which facilitates the formation and retention of these hydrophilic functional groups (AOH and ACOOH). Meanwhile, the organosilane molecules may be hydrolyzed within the thin water layer and attach onto the surface of CDs through SiAOASi or SiAOAC bonding [42]. The overall formation mechanism is illustrated in Scheme 1. 3.2. The optical properties of OS-CDs The as-synthesized OS-CDs show a distinctive absorption peak centred at 360 nm in the UV–Vis absorption spectrum and a maximum emission peak at 465 nm in the fluorescence (PL) spectra as shown in Fig. 3. In contrast to earlier reported OS-CDs [28], the asprepared OS-CDs are excitation-independent. When the excitation wavelength was varied in range of 320–420 nm, the emission maxima remained at 465 nm. Further increasing excitation wavelength to 440 nm and above, the OS-CDs was almost non-fluorescent, indicating that the OS-CDs have only one fluorescence centre. The excitation-independence may also be ascribed to the narrow particle size distribution. 360 nm was selected as the excitation wavelength in the following experiment as it induced the highest
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Fig. 2. (A) FTIR spectra of citric acid, OS-CDs and AEAPMS. (B) Surface scan of XPS spectra. (C) High resolution spectra of C1s. (D) Si 2p. (E) O1s. (F) N1s.
Scheme 1. Illustration of the possible formation process of OS-CDs from citric acid and AEAPMS in solvothermal synthesis.
fluorescent intensity. The QY of the as-prepared OS-CDs in Milli-Q water was calculated to be 51%, higher than most other reported CDs [28,43]. The fluorescence lifetime of OS-CDs in water under the excitation of 360 nm can be fitted by a multi-exponential function as shown in Fig. S3. Two fitting decay times were acquired, namely
s1 = 5.45 ns (5%) and s2 = 15.49 ns (95%) with the average lifetime of 14.99 ns. This result confirms that the fluorescence of OS-CDs was dominated by the emitting centre with lifetime of s2 = 15.49 ns. Its long fluorescence lifetime compared to the other CDs [44] is likely due to the abundant long chain surface functional groups which provide better trapping effect.
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Fig. 3. UV–Vis absorption spectrum and PL emission spectra of OS-CDs at different excitation wavelengths; the inset is a picture of OS-CDs under room light and 365 nm UV lamp.
3.3. Multi-solvent dispersibility of OS-CDs The OS-CDs can be well dispersed in both polar and apolar solvents. Fig. 4A shows that OS-CDs can be well dispersed in DMSO, methanol, DMF, acetone, ethanol, THF, toluene and hexane as well as Milli-Q water, with no sediment or layering phenomenon observed after two weeks, which shows the excellent multi-solvent solubility and stability. The multi-solvent dispersibility of OS-CDs was vividly observed by mixing OS-CDs in toluene and water mixture, as shown in Fig. 4B. Fluorescence disappeared completely in toluene phase but transferred into water phase after 1 day, indicating the better solubility of OS-CDs in water than in toluene. The strong multi-solvent dispersibility of the as-synthesized OS-CDs further confirms the dual long chain surface chemistry illustrated in Scheme 1. Furthermore, the PL intensity of as-synthesized OS-CDs (in Fig. S4) does not vary in ionic solutions when ionic strength is increased by adding NaCl up to 1 M. The excellent dispersity in solutions of high ionic strength can be ascribed to the steric effect due to the surface long chains [45]. 3.4. Hg2+ detection 3.4.1. In pure water The as-synthesized OS-CDs fluorescence sensitivity to metal cations was first assessed against a series of 100 lM aqueous solu-
tions containing Ag+, K+, Na+, Ca2+, Mg2+, Zn2+, Hg2+, Co2+, Cd2+, Fe3+ and Cr3+. As shown in Fig. 5A, the PL of the OS-CDs was insensitive to most of the metal cations but Hg2+ and Fe3+. Although Fe3+ also induced the fluorescence quenching of OS-CDs, Hg2+ can be selectively identified by adding sodium hexametaphosphate as the masking agent of Fe3+ ions [46] as shown in Fig. S5. To further verify the effect of the co-presence of some common mineral elements such as K+, Na+, Ca2+ and Mg2+ on OS-CDs’ sensing selectivity to Hg2+, the PL intensities of OS-CDs in pure water, and in solutions containing K+, Na+, Ca2+, Mg2+ and a mixture of all above (100 lM) with and without Hg2+ were compared as shown in Fig. 5B. The result clearly demonstrates that the presence of one or more above common metal ions has no adverse impact on the sensitivity of OS-CDs to Hg2+ ions. In addition, anions such as SO2 4 , NO3 , Cl and ClO4 were also found to have no effect on the fluorescence of OS-CDs (Fig. S6 showed data of ClO 4 ). It is worth noting that ClO 4 is highly oxidative and has been shown to have an effect on the PL of graphene quantum dots [47]; the OS-CDs appear to be stable against oxidative conditions. All these lead to the conclusion that the as-prepared OS-CDs possess excellent selectivity to Hg2+. The sensitivity of the as-prepared OS-CDs to Hg2+ concentration in the range of 0–5 lM was evaluated. As shown in Fig. 6A and B, concentration-dependent quenching was observed: with the increase of Hg2+ concentration from 0 to 5 lM, the PL intensity decreased gradually. The fluorescence quenching data can be fitted by the Stern–Volmer equation: FF0 1 ¼ K SV c, where KSV is the Stern–Volmer quenching constant, c the concentration of Hg2+, F0 and F the PL intensity of OS-CDs without Hg2+ and with different concentration of Hg2+, respectively. As shown in Fig. 6B, a good linear correlation (R2 = 0.9977) was obtained over the concentration range of 0–50 nM, with a quenching constant KSV of 6.49 103 L/ mol. The detection limit was determined to be 1.35 nM, using the equation 3r/m, where r is the relative standard deviation and m the slope of calibration curve as described in detail in Fig. S7. The as-prepared OS-CDs by solvothermal method showed a significantly better sensitivity to Hg2+ than that of the OS-CDs prepared by ‘hot injection’ method [48]. As depicted in Scheme 1, the solvothermal synthesized OS-CDs have amine-terminated alkyl chain due to the silane reaction with carbon surface with the presence of water layer, in contrast to the OS-CDs by ‘hot injection’. It is known that the binding affinity between Hg2+ and ANH2/ANH groups is stronger than that between Hg2+ and hydroxyl or carboxylate groups [49]. In addition, the spatial distribution of these terminal amino groups and the amine groups in the middle of the alkyl chain is very likely to trap Hg2+ and forms complexations, as illustrated in Scheme 2. Such a binding event would facilitate the non-radiative electron/hole recombination annihilation through an effective electron transfer process, causing fluorescence quenching. 3.4.2. In brine solutions The high ionic strength PL stability of OS-CDs is highly desirable in real applications such as water quality measurement at mining sites. Therefore, we further evaluated the sensitivity of OS-CDs to Hg2+ ions in salty solutions (with NaCl concentration of 1 M). As shown in Fig. S8, the PL of the as-prepared OS-CDs maintains excellent response to the concentration of Hg2+ in the range of 0–50 nM with a good linearity (R2 = 0.9987). The detection limit was determined as 1.7 nM, very close to the result in pure water, confirming the stability of OS-CDs in solutions of high ionic strength as a result of the steric effect.
Fig. 4. (A) The dispersion of OS-CDs in different solvents. (B) The multi-solvent dispersibility performance of OS-CDs in toluene and water phase.
3.4.3. In municipal wastewater effluent To further increase the complexity of the sample composition, wastewater after secondary treatment was employed, which con-
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Fig. 5. (A) Effect of metal ions (100 lM) on the fluorescence of OS-CDs. (B) PL intensity of OS-CDs in water, K+, Na+, Ca2+ and Mg2+ aqueous (100 lM) with and without Hg2+ ions, Mixed ions including K+, Na+, Ca2+ and Mg2+, with the concentration of 100 lM.
Fig. 6. (A) PL spectra of OS-CDs in water with different Hg2+ concentrations of 0–5 lM. (B) The linear region (0–50 nM) of Stern–Volmer plot; Inset: The relationship between F0/F1 and Hg2+ concentrations within the range of 0–1 lM. (C) PL spectra of OS-CDs in wastewater with different concentrations of Hg2+ in range of 0–40 lM. (D) The linear region (0–1 lM) of Stern–Volmer plot; Inset: The relationship between F0/F1 and Hg2+ concentrations within the range of 0–40 lM.
shown in Fig. 6C and D, the OS-CDs can still detect the Hg2+ ions from 0.2 lM to 40 lM as shown in Fig. 6C. The Stern–Volmer plot shows the linear range of 0–1 lM with R2 = 0.9978 and KSV = 0.26 L/mol. The detection limit for Hg2+ in wastewater was calculated to be 50 nM, highly sensitive for wastewater quality monitoring. The preserved excellent Hg2+ sensitivity in wastewater effluent suggests the as-synthesized OS-CDs are largely noninteractive with the microorganisms and organic molecules in wastewater, likely owing to the negative charge, long chain surface functional groups. The slight loss of Hg2+ sensitivity of OS-CDs in wastewater is likely due to the high background fluorescence caused by the organic molecules in wastewater. Scheme 2. Schematic illustration of the Hg(II) complexation on OS-CDs surface.
4. Conclusions tains many kinds of bacteria, viruses, metal ions and organic molecules. The fluorescence quenching of OS-CDs by different concentration of Hg2+ in the wastewater sample were investigated. As
In summary, a new kind of OS-CDs was formed by a low temperature (150 °C) solvothermal synthesis of citric acid in AEAPMS.
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The as-synthesized OS-CDs have dual long chain functional groups with both ANH2 and ASi(OCH3)3 as terminal moieties. An enclosed synthesis system such as the autoclave traps water, a by-product of carbon core formation, therefore, enables the organosilane, AEAPMS, to covalently attach on carbon surface by both ends, namely the ANH2 end by amidation and the ASi(OCH3)2CH3 end by hydrolization and SiAOAC formation. The resultant OS-CDs are amphiphilic, and stable in solutions of high ionic strength with QY of 51%. More significantly, they exhibit excellent selectivity and sensitivity to Hg2+ with a linear detection range of 0–50 nM and detection limit of 1.35 nM. The sensitivity and selectivity to Hg2+ is preserved in highly complex fluids with a detection limit of 1.7 nM in spiked 1 M NaCl solution and a detection limit of 50 nM in municipal wastewater effluent. Our results demonstrate that by tuning the arrangement of the surface functional groups, the as-prepared OS-CDs can be devised into an effective fluorescence probe for Hg2+ detection in complex fluid samples. Acknowledgments The authors thank Dr. Barry Wood at The University of Queensland for his assistance in acquiring the XPS data. We also thank Mr. Bradley Taylor at Redland City Council for providing access to wastewater samples. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.09.013. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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