Investigation of coatings of natural organic matter on silver nanoparticles under environmentally relevant conditions by surface-enhanced Raman scattering.

STOTEN-17166; No of Pages 9 Science of the Total Environment xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Investigation of coatings of natural organic matter on silver nanoparticles under environmentally relevant conditions by surface-enhanced Raman scattering Melanie Kühn a,⁎, Natalia P. Ivleva a, Sondra Klitzke b,c, Reinhard Niessner a, Thomas Baumann a a b c

Technische Universität München, Institute of Hydrochemistry, Marchioninistr. 17, D-81377 Munich, Germany University of Freiburg, Institute of Forest Sciences, Chair of Soil Ecology, D-79085 Freiburg, Germany Technische Universität Berlin, Department of Soil Science, Ernst-Reuter-Platz 1, 10587 Berlin, Germany

H I G H L I G H T S • • • •

Natural organic coatings on engineered nanoparticles are analytically hard to access SERS represents a suitable tool to display dynamic changes of organic coatings on silver nanoparticles (Ag NP) Humic acid forms a coating around Ag NP that is still present after several steps of centrifugation and resuspension SERS bands similar to that of humic acid can be found in SERS spectra of Ag NP aged in river water and soil solution

a r t i c l e

i n f o

Article history: Received 15 October 2014 Received in revised form 5 December 2014 Accepted 5 December 2014 Available online xxxx Keywords: Engineered inorganic nanoparticles Coatings Natural organic matter Surface-enhanced Raman scattering

a b s t r a c t The widespread use of engineered inorganic nanoparticles (EINP) leads to a growing risk for an unintended release into the environment. Despite the good characterization of EINP in regard to their function scale and the application areas, there is still a gap of knowledge concerning their behaviour in the different environmental compartments. Due to their high surface to volume ratio, surface properties and existence or development of a coating are of high importance for their stability and transport behaviour. However, analytical methods to investigate organic coatings on nanoparticles in aqueous media are scarce. We used Raman microspectroscopy in combination with surface-enhanced Raman scattering (SERS) to investigate humic acid coatings on silver nanoparticles under environmentally relevant conditions and in real world samples. This setup is more challenging than previous mechanistic studies using SERS to characterize the humic acids in tailored settings where only one type of organic matter is present and the concentrations of the nanoparticles can be easily adjusted to the experimental needs. SERS offers the unique opportunity to work with little sample preparation directly with liquid samples, thus significantly reducing artefacts. SERS spectra of different natural organic matter brought into contact with silver nanoparticles indicate humic acid in close proximity to the nanoparticles. This coating was also present after several washing steps by centrifugation and resuspension in deionized water and after an increase in ionic strength. © 2014 Published by Elsevier B.V.

1. Introduction The increasing use of engineered inorganic nanoparticles (EINP) in e.g. medical applications, paints and pigments, nanofunctionalized plastics and textiles, or personal care products, represents an increasing risk

⁎ Corresponding author. E-mail addresses: [email protected] (M. Kühn), [email protected] (N.P. Ivleva), [email protected] (S. Klitzke), [email protected] (R. Niessner), [email protected] (T. Baumann).

for EINP to be released into the environment (Frimmel and Niessner, 2010; Gondikas et al., 2014; Mahapatra et al., 2013; Blaser et al., 2008; Windler et al., 2012). EINP are designed for specific purposes and are therefore well characterized to match the specific needs of their applications. EINP which are only weakly bound (e.g. textiles, coated surfaces) or unbound (e.g. sunscreen, cosmetics, paints) in their application can easily be transported into soil and surface water (Froggett et al., 2014; Kaegi et al., 2008, 2010; Johnson et al., 2011), even EINP incorporated into a matrix are released from their products over time (Benn and Westerhoff, 2008). Despite of these obvious emission scenarios there is still a lack of knowledge regarding the stability and transport

http://dx.doi.org/10.1016/j.scitotenv.2014.12.026 0048-9697/© 2014 Published by Elsevier B.V.

Please cite this article as: Kühn, M., et al., Investigation of coatings of natural organic matter on silver nanoparticles under environmentally relevant conditions by..., Sci Total Environ (2014), http://dx.doi.org/10.1016/j.scitotenv.2014.12.026

2

M. Kühn et al. / Science of the Total Environment xxx (2014) xxx–xxx

mechanisms for EINP and the consequences that they impose to human and environmental health (Reidy et al., 2013). The fate of EINP in the environment is determined by their surface properties and, closely connected, their stability in terms of dissolution and aggregation. Surface properties and stability on the other hand depend on the respective environmental conditions. EINP are often functionalized by applying various coating materials (e.g. citrate, polyvinylpyrrolidone) to protect them against aggregation and dissolution. Additionally, surface functionalization changes the physicochemical properties of the nanoparticles, which may lead to a different behaviour after they are released into the environment compared to uncoated nanoparticles. Furthermore, stability of the nanoparticle suspensions is also depending on the kind of stabilization provided by the capping agent, e.g. electrostatically, sterically, and electrosterically (Badawy et al., 2012). EINP will interact with dissolved substances in water and attach to the soil matrix (Cornelis et al., 2013; Klitzke et al., 2014-in this issue). The sorption of fulvic and humic acids (Li et al., 2010) as well as Suwannee River Humic Acid (Huynh and Chen, 2011) to Ag NP was reported to increase nanoparticle stability. Naturally occurring organic matter sorbing to nanoparticles may override the chemical properties of the initial coating, thus natural organic matter is considered the controlling factor determining nanoparticle stability (Stankus et al., 2010). Additionally, the molecular size of the sorbing organic matter controls nanoparticle stability (Louie et al., 2013). Understanding the interactions between Ag NP and their coatings is important as the coating determines fate, stability and toxicity of Ag NP in the environment (Sharma et al., 2014). The formation of a natural coating around uncoated EINP and the substitution of a surface functionalization or the development of multilayer coating are likely scenarios and might change the stability and transport behaviour of the original EINP. The current toolset for investigating inorganic coatings on nanoparticles includes scanning and transmission electron microscopy (SEM, TEM) equipped with EDX as well as X-ray absorption spectroscopy (XAS), in particular extended X-ray absorption fine structure (EXAFS) or near-edge X-ray absorption fine structure (XANES). These methods have been applied successfully to investigate sulphide coatings on silver and zinc oxide nanoparticles (Kaegi et al., 2013; Ma et al., 2013). Analytical access to organic coatings on EINP, however, is scarce, especially in aqueous media. One promising tool to identify natural organic coating and their interactions with a subclass of EINP is Raman microspectroscopy (RM). RM is based on inelastic scattering of monochromatic light by a molecule. This interaction causes a shift in the frequency of the emitted photons which is characteristic for the molecular vibrations of the analyte molecule. Infrared spectroscopy offers information on a sample that is complementary to Raman spectroscopy. In contrast to Raman spectroscopy, IR is not suitable for aqueous samples because the strong IR bands of water would interfere with the spectrum of the sample. Due to the low quantum efficiency of the Raman effect (typically 10−6–10− 8), RM suffers from the limited sensitivity. Fortunately, a strong enhancement of the Raman signal can be achieved by the adsorption of the analyte molecule on nanostructured metal surfaces (surfaceenhanced Raman scattering, SERS) (Kneipp et al., 2002; Etchegoin and Ru, 2008; Ru and Etchegoin, 2012). An enhancement factor of about 106 and higher can be obtained and is attributed to electromagnetic and chemical enhancement. Apart from, e.g. roughened metal surfaces, nanosphere lithography or soft lithography, colloidal suspensions containing silver and gold nanoparticles are commonly used as SERS media. SERS can give access to the functional groups of the coating that interact with the nanoparticles (Corrado et al., 2008; Ivleva et al., 2010). In this study, we use the SERS effect as an indirect approach for measuring the interaction of silver nanoparticles with natural organic matter present in aqueous solution. While SERS has been used as an analytical tool to characterize humic substances (Corrado et al., 2008; Carletti et al., 2010; Francioso et al., 2001, 2002; Vogel et al., 1999), the technical setup and concept in our

case differ significantly from published studies. First of all, in previously published experiments both, humic substances and SERS-active nanoparticles are available in excess quantities. Second, the measurements were performed in simplified batch systems with one single substance only. In contrast we have investigated the dynamic changes of coatings on silver nanoparticles. Here, there is no surplus of the coating substance and the concentrations of nanoparticles are significantly lower. 2. Materials & methods 2.1. Chemicals Hydrochloric acid (0.1 N, p.a.), nitric acid (0.1 N, p.a.), sodium hydroxide solution (0.1 N p.a.), sodium nitrate, and humic acid (RothHA) were purchased from Carl Roth (Karlsruhe, Germany). Suwannee River Humic Acid (SRHA) standard 2S101H was purchased from IHSS (St. Paul, MN). Hydroxylammonium chloride and silver nitrate were purchased from Merck (Darmstadt, Germany). 2.2. Nanoparticle synthesis and preparation of humic acid-coated Ag NP Experiments were run with citrate as well as hydroxylammonium chloride reduced Ag NP. Citrate reduced and stabilized nanoparticles (CN) were produced at Univ. Landau (see Metreveli et al., 2014-in this issue). Their diameter is ~30 nm and the zeta potential is −65 mV. Measurements of the diameter and zeta-potential with dynamic light scattering revealed insignificant changes over a period of three months. Hydroxylammonium chloride reduced and stabilized nanoparticles (HN) were produced at the Institute of Hydrochemistry (TU München) following a modified procedure of Leopold and Lendl (2003). 11.6 mg hydroxylammonium chloride was mixed with 96.7 mL water and 3.3 mL sodium hydroxide solution and stirred for 90 min. 9 mL of the reducing agent was transferred into 50 mL centrifuge tubes. 17 mg of AgNO3 was dissolved in 10 mL water, and an aliquot of 1 mL was added quickly to each tube. The tubes were shaken immediately after the addition to ensure complete mixing. The stability of these nanoparticles has been determined experimentally to a minimum of 3 weeks (Ivleva et al., 2010). Both types of nanoparticle suspensions were stored at 4 °C and protected against light. Humic acid stock solutions were prepared by adding 100 mg/L of humic acid to 500 mL deionized water and adjusting the pH to N10 with 0.1 M NaOH. Samples were put in an ultrasonic bath for 15 min and left to equilibrate for 24 h. Solutions were filtered over a 100 nm polycarbonate filter (Piper Filter GmbH, Bad Zwischenahn, Germany) in a vacuum filtration system (Sartorius AG, Gö ttingen, Germany). The pH was lowered to pH 7 by using 0.1 M HCl before adding the nanoparticles. Humic acid stock solutions were mixed with nanoparticle suspensions in equal parts in a 50 mL centrifuge tube and placed in an overhead shaker for 24 h at 4 °C. Samples were centrifuged afterwards for 90 min (19,430 g). After removal of the supernatant the coated particles were resuspended in deionized water while maintaining the same particle concentrations. Samples were treated in an ultrasonic bath for 15 min to disrupt aggregates formed during centrifugation. To reduce thermal damage to the humic acid, the samples were kept in a water bath filled with crushed ice. The stability of the coatings on Ag NP was tested by repeating this washing step (centrifugation followed by resuspension) up to four times. To test the influence of pH and ionic strength on the stability of the coating and the nanoparticles, washing steps were performed at different pH and ionic strength. Therefore, pH of the deionized water used for resuspension was adjusted to pH 5 and pH 9 with 0.1 M HNO3 and 0.1 M NaOH respectively. Ionic strength was adjusted by adding sodium nitrate with a final concentration of 10 mM NaNO3.



59.2 ± 0.5 0.0029 ± 0.0003 1.985 ± 0.005 7.5 ± 0.0 0.003 ± 0.0 10.3 ± 0.1 2.363 ± 0.006 0.010 ± 0.001 1.86 ± 0.0

0.8

33.25 ± 0.25 0.020 ± 0.006 1.185 ± 0.007 3.8 ± 0.014 n. d. 2.09 ± 0.14 0.022 ± 0.009 0.034 ± 0.007 9.4 ± 1.7

0.6

Ca Fe K Mg Mn Na Zn Al Dissolved organic carbon

Absorbance, %

River water concentration [mg/L]

0.4

Soil solution concentration [mg/L]

HN CN

0.2

Element

1.0

Table 1 Composition of the soil solution (n. d.: not detected) ± one standard deviation and composition of river water (b0.1 μm) ± one standard deviation.

3

The hydrodynamic diameter of bare and coated nanoparticles was determined by Asymmetrical Flow Field Flow Fractionation (AF4), transmission electron microscopy (TEM) and dynamic light scattering (DLS). AF4 was carried out at the Department of Environmental Geosciences at the University of Vienna (Wyatt, Eclipse 3) based on size calibration by polystyrene beads and using a UV detector for the silver nanoparticles as outlined in von der Kammer et al. (2011) and Loeschner et al. (2013). TEM measurements of bare and humic acid coated Ag NP were performed to characterize size and shape of the nanoparticles. Uranyl acetate was used as contrast agent for improved representation of the coating. For UV–vis measurements nanoparticle stock dispersion were diluted 1:20 with deionized water in a disposable, semi-micro PMMA-cuvette. Deionized water was measured for reference spectrum. Absorbance spectra of the nanoparticle solution were measured for 3 cycles per sample in scan mode with a speed of 50 nm and a slit of 4 nm. Average spectra of the three cycles were calculated using the software WinASPECT PLUS. RM and SERS spectra were recorded using a LabRAM HR Raman microscope (Horiba, Jobin Yvon, Japan) employing a He–Ne laser (633 nm, 14 mW at sample). Wavelength calibration was carried out

300

400

500

600

700

800

Wavelength, nm

Fig. 2. UV–vis spectra of hydroxylammonium chloride reduced Ag NP (HN) (red line) and citrate stabilized Ag NP (CN) (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

using a silicon waver. A 50 × objective (NA = 0.75) was used with acquisition times of 1 s, 10 s, or 100 s. Coatings of humic acid on Ag NP were characterized using RM and SERS. Here, at least one washing step (see above) was applied to the nanoparticle suspensions to remove the dissolved humic material not associated with Ag NP. Samples were then transferred to a silicon wafer and dried in a petri dish in a laminar flow box before the measurements. It is known that thermal damage induced by laser illumination can lead to changes of the width and relative intensities of the D and G peaks. In this set of experiments and with the given experimental conditions we observed no changes in the spectra of RothHA and SRHA, therefore, one can safely assume that thermal decomposition of samples

b) 2.5

a)

0.0

2.3. Characterization of Ag NP

d)

1.5 0.0

0.5

c)

50 nm

1.0

50 nm

Absorbance, %

2.0

RothHA HN RothHA coated HN

300

200 nm

50 nm

Fig. 1. TEM images of bare hydroxylammonium chloride reduced Ag NP (HN) (a), RothHA coated HN (b), bare citrate stabilized Ag NP (CN) (c) and RothHA coated CN (d). All samples were stained with uranyl acetate.

400

500

600

700

800

Wavelength, nm

Fig. 3. UV–vis spectra of RothHA (brown line), hydroxylammonium chloride reduced Ag NP (red line) and RothHA coated hydroxylammonium chloride reduced Ag NP (orange line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



90

94

a

86

% Transmittance

4

4000

3000

2000

1000

0

50

55

60

b

45

% Transmittance

65

wavelength/cm−1

4000

3000

2000

1000

0

58

62

66

c

54

% Transmittance

wavelength/cm−1

4000

3000

2000

1000

0

wavelength/cm−1

Fig. 4. FTIR spectra of RothHA (brown line), hydroxylammonium chloride reduced Ag NP (lyophilized, red line), RothHA coated hydroxylammonium chloride reduced Ag NP (lyophilized, yellow line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

during the analysis is negligible. Liquid samples were measured with RM in microtiter plates. Fourier transform infrared spectroscopy (FTIR) was performed on powder samples (humic acid samples) and centrifuged and lyophilized samples (nanoparticle suspensions and coated nanoparticles). Spectra were recorded using a Nicolet 6700 FTIR (Thermo Scientific) equipped with Smart iTR and diamond plate with a resolution of 4 cm−1 and 16 iterations. Background correction was performed against air.

2.4. Ageing of Ag NP in soil solution and river water To simulate natural conditions, SERS spectra of Ag NP aged in soil solution and river water were acquired. Here, Ag NP aged in deionized water were used as blank measurements. Soil solutions were prepared from soil samples taken from the floodplain area of the River Rhine. After sampling, the soil material, a silty clay with a carbon content of 6.2% and a C/N-ratio of 21.5, was sieved to 2 mm and stored at field-

Fig. 5. SERS spectra of RothHA and SRHA coated Ag NP. The HA show differences in the intensity ratio of the D and G peaks.



moisture in a refrigerator. The soil solution was obtained by equilibrating these soil samples with pure water at a liquid-to-solid ratio of 10:1 in an end-over-end shaker (8 rpm; GFL 3040) for 16 h. To remove soilborne colloids, the suspension was ultracentrifuged (Beckman Optima TL) at 107,528 g for 1.5 h. At an aggregate density of 1.2 g/cm3 this procedure removes any colloidal material larger than a diameter of 30 nm. The pH of the centrifuged soil solution was 7.7. Concentrations of elements in the soil solution as determined in acidified duplicate samples by ICP-OES analysis (iCAP 6000, Thermo Scientific) and by a TOC analyser (TOC — 5050 A Shimadzu) are summarized in Table 1. River water samples were collected from the River Rhine and filtered through a 0.1 μm membrane filter. The hydrochemical composition of the water is summarized in Table 1. One part of citrate-stabilized Ag NP (c = 90.6 mg/L Ag NP; initial particle size as determined by dynamic light scattering 34 nm) and 4 parts of soil solution, river water, or deionized water, respectively, were mixed and equilibrated on a horizontal shaker (130 rpm; KS501 digital, IKA Labortechnik) for 24 h. In a second step, the previously aged nanoparticles were added to surface water, which was diluted with reverse osmosis water by a factor of 5. The pH of the diluted mixture was 7 and the conductivity was 316

5

μS/cm. SERS spectra were taken from the nanoparticles equilibrated in the different solutions before and after their addition to the surface water. 3. Results & discussion 3.1. Characterization of Ag NP TEM images of hydroxylammonium chloride reduced Ag NP (HN) show regular spheres with a diameter of ~ 27 nm (see Fig. 1a). These results are in good agreement with AF4 measurements which showed a diameter of 27 nm for HN and a slightly higher diameter of 31 nm for the RothHA and SRHA coated HN after one washing step. Citrate stabilized Ag NP (CN) are a mixture of spheres and rods (see Fig. 1c). The UV–vis spectra of HN (see Figs. 2 and 3, red line) show an even peak with a maximum at a wavelength of 405 nm, which is within the region for the absorption peak of silver, and a full width at half maximum (FWHM) of 73 nm. The UV–vis spectrum of CN shows a broader peak with a FWHM of 103 nm and two shoulders at ca. 350 and 400 nm (see Fig. 2, blue line). The maximum is redshifted towards

Fig. 6. SERS spectra of humic acid coatings (RothHA) on Ag NP over a concentration range from 1 to 100 mg/L and inserted optical image of the particles being measured. The concentration of HA affects the drying behaviour of the nanoparticles. Scale bars represent 40 μm.



After addition of 100 mg/L RothHA to the Ag NP suspension, equilibration for 24 h, centrifugation and resuspension in deionized water by sonification, the SERS spectrum (Fig. 7, brown line) shows broad G and D bands at ~1575 cm−1 and ~1370 cm−1 typical for carbonaceous materials as well as additional bands in this region which can be assigned to (poly)aromatic ring stretching motions (Corrado et al., 2008; Agarwal and Reiner, 2009). After the fourth repetition of the washing and resuspension steps the relative intensity of the G and D peaks increased significantly (Fig. 7, olive-green line). This points to a closer interaction between RothHA and Ag NP. The reason for this could be a change of the coating and/or a change of the aggregation state. In the first case more of the RothHA would be in range of SERS enhancement, whereas in the latter case the nanoparticles themselves would be closer together (less stabilizing RothHA in between) causing stronger electromagnetic enhancement in hot spots. 3.3. SERS spectra of aged Ag NP

20000

The SERS spectra of CN aged in water from the river Rhine and soil suspension from a bank filtration experiment are presented in Fig. 8. The SERS spectra show modes in the regions typical for dissolved organic carbon (DOC). Important functional groups of DOC are acidic groups (carboxylic acid, phenolic OH, enolic hydrogen and quinone), basic groups (amine, amide, imines) and neutral groups (alcoholic OH, ether, ketone, aldehyde, ester, cyclic imides), where carboxylic groups are the most abundant and can be found in 90% of all DOC (McDonald et al., 2004). Again, the liquid samples of the bare Ag NP in deionized water (no coating material) only show the modes of OH-bending (1640 cm−1) and symmetric and antisymmetric OH-stretching of water between 2750 and 3900 cm− 1. After addition to the surface water, the spectrum is similar to the spectrum of the initial solution. The nanoparticles aged in soil suspension and river water show spectral features typical for carbonaceous materials. Similar peaks have been observed for aromatic ring stretching (~ 1600 cm−1), COO\ stretching (~1380 cm−1), and CH3 deformation vibrations (~1357 cm−1) in lignin (Agarwal and Reiner, 2009) as well as C\H deformation vibrations of sodium alginates (1270–1400 cm, Campos-Vallette et al., 2010). After addition to the supernatant, these bands could only be observed for the Ag NP aged in river water. For the soil-aged particles the concentration

15000

HA coated AgNP after 4 washing steps HA coated AgNP after 1 washing step 10 mg/L HA Citrate stabilized AgNP

10000

Intensity, arb. units

410 nm. This indicates a bigger particle size of the CN (Heard et al., 1983). The lower absorption of CN compared to the spectrum of HN is partly caused by the lower particle concentration of the stock solution. Particle aggregation as indicated by the tailing at the long wavelength side of the peak can lead to an additional decrease of the absorption values (Heard et al., 1983; Fornasiero and Grieser, 1991). Furthermore, dipolar scattering and quadrupolar adsorption can cause broadening of the absorption peak for particles bigger than 30 nm (Huang et al., 1996). The UV–vis spectrum of RothHA is featureless with increasing absorbance towards lower wavelengths (see Fig. 3, orange line). The RothHA coated HN show a combination of the spectral features of humic acid and Ag NP. FTIR spectra of SRHA (Fig. 4) are dominated by two peaks at 1720 cm−1 and 1600 cm−1 which can be assigned to C_O vibrations of COOH as well as aromatic C_C vibrations, strongly H bonded C_O vibrations of quinones or H bonded and conjugated ketones respectively (Stevenson and Goh, 1971). The band at 1200 cm− 1 represents C\O stretching and O\H deformation of COOH groups and is correlated with the band at 1720 cm− 1 (Stevenson and Goh, 1971). In comparison to the FTIR spectrum of RothHA, the band at 2920 cm− 1 (aliphatic C\H absorption) is less dominant. FTIR spectra of HN are dominated by the asymmetric stretching vibration of carbon dioxide indicating a partial oxidation of the silver nanoparticles. FTIR spectra of RothHA coated HN were similar to the spectra of uncoated HN. This indicates that, although the bulk substances can be characterized quite well, the thin HA coating on the Ag NP cannot be displayed with FTIR. Raman spectra of RothHA and SRHA are dominated by two peaks at ca. 1345 cm−1 and 1564 cm−1 which represent the carbonaceous parts within the humic acids (Court et al., 2007, see Fig. 5). In carbon analysis, these peaks are referred to as G peak (graphite peak) and D peak (disordered peak) of carbonaceous materials. The G peak is caused by in-plane band-stretching motion of pairs of sp2 atoms (E2g symmetry). The D peak is linked to the breathing mode of sixfold aromatic rings (A1g symmetry, Ferrari and Robertson, 2000). Both samples display an elevation of the baseline due to unspecific fluorescence caused by the humic acid. SERS spectra are also dominated by the D (ca. 1355 cm−1) and G peaks (ca. 1575 cm− 1). The two samples show differences in the intensity ratios of the D and G peaks (see Fig. 5). The higher value of D/G ratio for RothHA indicates a higher degree of structural order of this sample (Ferrari and Robertson, 2000; Ivleva et al., 2007). The addition of Ag NP leads to an enhancement of the signal as well as considerable fluorescence quenching. In some cases, the appearance of additional narrow intense SERS bands could be observed. Humic acid coatings on silver nanoparticles were detected in a concentration range from 1 to 100 mg/L (Fig. 6). Environmental concentrations of humic substances are about 0.5–4.0 mg/L in river water and 0.5–40 mg/L in lakes (Frimmel, 2001). For RothHA, at the lowest (1 mg/L) and highest (100 mg/L) concentrations of RothHA bigger aggregates are formed whereas the particle sizes are the lowest at around 10 mg/L RothHA. At first glance this finding does not make sense, as HA should stabilize the Ag NP (Klitzke et al., 2008; Tiller and O'Melia, 1993) and should prevent the formation of larger aggregates at higher HA concentrations. Thus, the formation of heteroaggregates of HA with Ag NP during drying has to be considered.

5000

6

Fig. 7 shows the SERS spectrum of a citrate stabilized Ag NP suspension (blue line) measured in a microtiter plate. The main feature is the broad band of water (from ~ 3100 to ~ 3600 cm−1 representing symmetric and antisymmetric O\H stretching modes of H2O), the Ag NP themselves do not exhibit any distinct Raman signatures. The Raman spectrum of the RothHA suspension which was used as a coating agent (Fig. 7, orange line), while dominated by the water signal reveals additional weak specific features in the region from 1200 to 1600 cm−1.

0

3.2. Stability of the coating

0

1000

2000

3000

4000

Raman shift, cm−1

Fig. 7. Comparison of Raman spectra of dissolved humic acid and humic acid coated Ag NP after one and four washing steps (λ0 = 633 nm, 50× objective, acquisition time: black 10 s, blue 10 s, red 1 s, green 10 × 10 s). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



Millipore, supernatant

2000

3000

15000

1637.0

1637.0

1000

4000

0

1000

2000

3000

Raman shift, cm−1

Soil suspension, initial solution

Soil suspension, supernatant

4000

5000

0

1000

1637.0

3000


1378.0 1465.0 1550.0 1614.0

2000

3000

7000

Raman shift, cm−1

1000


4000

0

5000


5000 3000 1000


7000

25000

Millipore, initial solution

7

1000

2000

3000

4000

0

2000

3000

Raman shift, cm−1

Riverwater, initial solution

Riverwater, supernatant

1176.0

1356.0 1489.0 1626.0

5000

7000

4000

1000

3000


1271.0 1402.0 1512.0 1597.0

200 400 600 800


1000

Raman shift, cm−1

1200

0

0

1000

2000 Raman shift, cm−1

3000

4000

0

1000

2000

3000

4000

Raman shift, cm−1

Fig. 8. SERS spectra of the initial solution of particles aged in pure water, soil suspension, and river water (left column) and after addition to the surface water (right column).

Fig. 9. Optical image (left) and SERS spectrum of the soil-aged Ag NP after addition to the surface water. The band at 2921 cm−1 can be observed in biomass. The band at 230 cm−1 represents the Ag\N vibration.


8


of Ag NP in the laser focus during the measurement was obviously below the limit of detection indicating aggregation with suspended matter in the surface water. The detection of Ag NP in the dried samples turned out to be difficult. The majority of the optically identified particles measured did not show any Raman signals apart from the signal of the silicon wafer (521 cm−1) on which the particles were deposited. This can be explained if the Ag NP were attached on particles that do not exhibit a distinct Raman signal, like clay minerals (Sobanska et al., 2012), and could therefore not be detected. On the other hand, the concentration of Ag NP in the laser spot could be too low to be detected by random measurements of individual particles. Those particles that showed Raman bands were clearly identified by the band at 230 cm− 1 which represents the Ag\N vibration (Fig. 9) (Dai et al., 2005). In samples aged in soil suspension, Ag NP seemed to preferentially adsorb on the cell walls of aquatic (micro)organisms. Optical microscope images indicate the presence of algae and SERS bands reveal vibrational modes of carbohydrates and proteins typically present in algae and bacterial biofilms (Ramya et al., 2010; Ivleva et al., 2010) as well as C\H stretching vibration at 2921 cm− 1 that can be attributed to polysaccharides, proteins, etc. present in biomass (Fig. 8). 4. Conclusion The results underline unique features and advantages of SERS for the detection of organic coatings on silver nanoparticles. SERS can be applied in aqueous solution without or with little sample treatment, thus avoiding changes of the coatings which are likely to occur, i.e., during drying when the concentration of the coating agent is increasing. SERS is also specific to molecules present in close proximity to the surface, that is with a distance of less than 10 nm. With typical SERS enhancement factors of 103-106 conventional Raman signals from humic substances in solution are negligible at typical concentrations. Both effects render SERS selective and sensitive for the detection of organic coatings. It also seems possible to detect changes of organic coatings using SERS if the Raman features are significantly different. This already seems to be the case for different humic substances. The results with silver nanoparticles easily apply to gold nanoparticles which also exhibit a strong SERS effect. Putting SERS measurements in a context with other methods to characterize the properties of nanoparticles, the results of AF4measurements indicate a thin, but rather stable coating of the nanoparticles: SERS has shown specific Raman bands for humic substances even after the nanoparticles have been centrifuged and resuspended in pure water. The diameter of those particles showed an increase of only a few nanometers. Other studies have observed an enhancement of the fluorescence signal of humic acid adsorbed to Ag NP (Manoharan et al., 2014) which requires a distance of the fluorophores of at least 15–25 nm (Manoharan et al., 2014; Lee and Meisel, 1982). While SERS would not give a signal for fluorophores at that distance, the results are in contrast to the AF4 measurements. This could be interpreted as a secondary layer of humic substances around the nanoparticle which is weakly bound, can be removed by washing, and which does not affect the hydrodynamic diameter. FTIR, while providing complementary information for bulk substances is not sensitive enough to detect the coatings directly. Acknowledgement The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within research unit FOR 1536 InterNano and its subprojects BA 1592/6-1 and LA 1398/9-1. We would like to thank Dr. G. Metreveli (Universität Koblenz-Landau, Institute of Environmental Sciences) for the preparation of citrate stabilized nanoparticles and characterization of river water. We appreciate assistance with TEM by Dr. M. Hanzlik (Technische Universität München,

Electron Microscopy) and AF4 measurements by Dr. F. von der Kammer (Universität Wien, Department of Environmental Geosciences). We would like to thank Albrecht Richter (Technische Universität Berlin) for help and support with the preparation of aged particles. References Agarwal, U.P., Reiner, R.S., 2009. Near-IR surface-enhanced Raman spectrum of lignin. J. Raman Spectrosc. 40, 1527–1534. Badawy, A.E., Scheckel, K.G., Suidan, M., Tolaymat, T., 2012. The impact of stabilization mechanism on the aggregation kinetics of silver nanoparticles. Sci. Total Environ. 429, 325–331. Benn, T., Westerhoff, P., 2008. Nanoparticle silver released into water from commercially available sock fabrics. Environ. Sci. Technol. 42, 4133–4139. Blaser, S., Scheringer, M., MacLeod, M., Hungerbühler, K., 2008. Estimation of cumulative aquatic exposure and risk due to silver: contribution of nano-functionalized plastics and textiles. Sci. Total Environ. 390, 396–409. Campos-Vallette, M., Chandia, N., Clavijo, E., Leal, D., Matsuhiro, B., Osorio-Roman, I., Torres, S., 2010. Characterization of sodium alginate and its block fractions by surface-enhanced Raman spectroscopy. J. Raman Spectrosc. 41, 751–763. Carletti, P., Roldan, M., Francioso, O., Nardi, S., Sanchez-Cortes, S., 2010. Structural characterization of humic-like substances with conventional and surface-enhanced spectroscopic techniques. J. Mol. Struct. 982, 169–175. Cornelis, G., Pang, L., Doolette, C., Kirby, J., McLaughlin, M., 2013. Transport of silver nanoparticles in saturated columns of natural soils. Sci. Total Environ. 463, 120–130. Corrado, G., Sanchez-Cortes, S., Francioso, O., Garcia-Ramos, J., 2008. Surface-enhanced Raman and fluorescence joint analysis of soil humic acids. Anal. Chim. Acta. 616, 69–77. Court, R., Sephton, M., Parnell, J., Gilmour, I., 2007. Raman spectroscopy of irradiated organic matter. Geochim. Cosmochim. Acta 71, 2547–2568. Dai, S., Zhang, X., Du, Z., Huang, Y., Dang, H., 2005. Structural properties and Raman spectroscopy of lipid Langmuir monolayers at the air–water interface. Colloids Surf. B: Biointerfaces 42, 21–28. Etchegoin, P., Ru, E.L., 2008. A perspective on single molecule SERS: current status and future challenges. Phys. Chem. Chem. Phys. 10, 6079–6089. Ferrari, A., Robertson, J., 2000. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 61, 14095–14107. Fornasiero, D., Grieser, F., 1991. The kinetics of electrolyte induced aggregation of Carey Lea silver colloids. J. Colloid Interface Sci. 141, 168–179. Francioso, O., Sanchez-Cortes, S., Tugnoli, V., Marzadori, C., Ciavatta, C., 2001. Spectroscopic study (DRIFT, SERS and 1H NMR) of peat, leonardite and lignite humic substances. J. Mol. Struct. 565, 481–485. Francioso, O., Sanchez-Cortes, S., Casarini, D., Garcia-Ramos, J., Ciavatta, C., Gessa, C., 2002. Spectroscopic study of humic acids fractionated by means of tangential ultrafiltration. J. Mol. Struct. 609, 137–147. Frimmel, F., 2001. Biopolymers. Lignin, Humic Substances and CoalAquatic Humic Substances vol. 1. Wiley-VCH (Ch.). Frimmel, F., Niessner, R. (Eds.), 2010. Nanoparticles in the Water Cycle: Properties, Analysis and Environmental Relevance. Springer. Froggett, S., Clancy, S., Boverhof, D., Canady, R., 2014. A review and perspective of existing research on the release of nanomaterials from solid nanocomposites. Part. Fibre Toxicol. 11. Gondikas, A., v.d. Kammer, F., Reed, R., Wagner, S., Ranville, J., Hofmann, T., 2014. Release of TiO2 nanoparticles from sunscreens into surface waters: a one-year survey at the old Danube recreational lake. Environ. Sci. Technol. 48, 5415–5422. Heard, S., Grieser, F., Barraclough, C., 1983. The characterization of Ag sols by electron microscopy, optical absorption, and electrophoresis. J. Colloid Interface Sci. 93, 545–555. Huang, H., Ni, X., Loy, G., Chew, C., Tan, K., Loh, F., Deng, J., Xu, G., 1996. Photochemical formation of silver nanoparticles in poly(n-vinylpyrrolidone). Langmuir 12, 909–912. Huynh, K., Chen, K., 2011. Aggregation kinetics of citrate and polyvinylpyrrolidone coated silver nanoparticles in monovalent and divalent electrolyte solutions. Environ. Sci. Technol. 45, 5564–5571. Ivleva, N., McKeon, U., Niessner, R., Pöschl, U., 2007. Raman microspectroscopic analysis of size-resolved atmospheric aerosol particle samples collected with an ELPI: soot, humic-like substances, and inorganic compounds. Aerosol Sci. Technol. 41, 655–671. Ivleva, N., Wagner, M., Szkola, A., Horn, H., Niessner, R., Haisch, C., 2010. Label-free in situ SERS imaging of biofilms. J. Phys. Chem. B 114, 10184–10194. Johnson, A., Bowes, M., Crossley, A., Jarvie, H., Jurkschat, K., Jürgens, M., Lawlor, A., Park, B., Rowland, P., Spurgeon, D., Svendsen, C., Thomson, I., Barnes, R., Williams, R., Xu, N., 2011. An assessment of the fate, behaviour and environmental risk associated with sunscreen TiO2 nanoparticles in UK field scenarios. Sci. Total Environ. 409, 2503–2510. Kaegi, R., Ulrich, A., Sinnet, B., Vonbank, R., Wichser, A., Zuleeg, S., Simmler, H., Brunner, S., Vonmont, H., Burkhardt, M., Moller, M., 2008. Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment. Environ. Pollut. 156, 233–239. Kaegi, R., Sinnet, B., Zuleeg, S., Hagendorfer, H., Mueller, E., Vonbank, R., Boller, M., Burkhard, M., 2010. Release of silver nanoparticles from outdoor facades. Environ. Pollut. 158, 2900–2905. Kaegi, R., Voegelin, A., Ort, C., Sinnet, B., Thalmann, B., Krismer, J., Hagendorfer, H., Elumelu, M., Mueller, E., 2013. Fate and transformation of silver nanoparticles in urban wastewater systems. Water Res. 47, 3866–3877. Klitzke, S., Lang, F., Kaupenjohann, M., 2008. Increasing pH releases colloidal lead in a highly contaminated forest soil. Eur. J. Soil Sci. 59 (2), 265–273 (URL b Go to ISIN:// 000253758500015).


M. Kühn et al. / Science of the Total Environment xxx (2014) xxx–xxx Klitzke, S., Metreveli, G., Peters, A., Schaumann, G., Lang, F., 2014. The fate of silver nanoparticles in soil solution — sorption of solutes and aggregation. Sci. Total Environ. (in this issue). Kneipp, K., Kneipp, H., Itzkan, I., Dasari, R., Feld, M., 2002. Surface-enhanced Raman scattering and biophysics. J. Phys. Condens. Matter 14, R597–R624. Lee, P.C., Meisel, D., 1982. Adsorption and surface-enhanced Raman of dyes on silver and gold sols. J. Phys. Chem. 86 (17), 3391–3395. http://dx.doi.org/10.1021/j100214a025. Leopold, N., Lendl, B., 2003. A new method for fast preparation of highly surface-enhanced Raman scattering (SERS) active silver colloids at room temperature by reduction of silver nitrate with hydroxylamine hydrochloride. J. Phys. Chem. B 107, 5723–5727. Li, X., Lenhart, J., Walker, H., 2010. Dissolution-accompanied aggregation kinetics of silver nanoparticles. Langmuir 26, 16690–16698. Loeschner, K., Navratilova, J., Legros, S., Wagner, S., Grombec, R., Snell, J., v. d. Kammer, F., Larsen, E.H., 2013. Optimization and evaluation of asymmetric flow field-flow fractionation of silver nanoparticles. J. Chromatogr. A 1272, 116–125. Louie, S., Tilton, R., Lowry, G., 2013. Effects of molecular weight distribution and chemical properties of natural organic matter on gold nanoparticle aggregation. Environ. Sci. Technol. 47, 4245–4254. Ma, R., Levard, C., Michel, F., Brown, G., Lowry, G., 2013. Sulfidation mechanism for zinc oxide nanoparticles and the effect of sulfidation on their solubility. Environ. Sci. Technol. 47, 2527–2534. Mahapatra, I., Clark, J., Dobson, P., Owen, R., Lead, J., 2013. Potential environmental implications of nano-enabled medical applications: critical review. Environ. Sci. Process. Impacts 15, 123–144. Manoharan, V., Ravindran, A., Anjali, C.H., 2014. Mechanistic insights into interaction of humic acid with silver nanoparticles. Cell Biochem. Biophys. 68 (1), 127–131 (URL http://www.ncbi.nlm.nih.gov/pubmed/23801156). McDonald, S., Bishop, A., Prenzler, P., Robards, K., 2004. Analytical chemistry of freshwater humic substances. Anal. Chim. Acta. 527, 105–124. Metreveli, G., Philippe, A., Schaumann, G., 2014. Disaggregation of silver nanoparticle homoaggregates in a river water matrix. Sci. Total Environ. (in this issue).

9

Ramya, S., George, R., Rao, R.S., Dayal, R., 2010. Detection of algae and bacterial biofilms formed on titanium surfaces using micro-Raman analysis. Appl. Surf. Sci. 256, 5108–5115. Reidy, B., Haase, A., Luch, A., Dawson, K., Lynch, I., 2013. Mechanisms of silver nanoparticle release, transformation and toxicity: a critical review of current knowledge and recommendations for future studies and applications. Materials 6, 2295–2350. Ru, E.L., Etchegoin, P.G., 2012. Single-molecule surface-enhanced Raman spectroscopy. Annu. Rev. Phys. Chem. 63, 65–87. Sharma, V., Siskova, K., Zboril, R., Gardea-Torresdey, J., 2014. Organic-coated silver nanoparticles in biological and environmental conditions: fate, stability and toxicity. Adv. Colloid Interf. Sci. 204, 15–34. Sobanska, S., Hwang, H., Choel, M., Jung, H., Eom, H.-J., H, 2012. Investigation of the chemical mixing state of individual Asian dust particles by the combined use of electron probe X-ray microanalysis and Raman microspectrometry. Anal. Chem. 84, 3145–3154. Stankus, D., Lohse, S., Nason, J.A., J. H., 2010. Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents. Environ. Sci. Technol. 45, 3238–3244. Stevenson, F., Goh, K., 1971. Infrared spectra of humic acids and related substances. Geochim. Cosmochim. Acta 35, 471–483. Tiller, C.L., O'Melia, C.R., 1993. Natural organic-matter and colloidal stability — models and measurements. Colloids Surf. A Physicochem. Eng. Asp. 73, 89–102 (URL b Go to ISIN://WOS:A1993LP04600008). Vogel, E., Geßner, R., Hayes, M., Kiefer, W., 1999. Characterisation of humic acid by means of SERS. J. Mol. Struct. 482, 195–199. von der Kammer, F., Legros, S., Larse, E.H., Loeschner, K., Hofmann, T., 2011. Separation and characterization of nanoparticles in complex food and environmental samples by field-flow fractionation. Trends Anal. Chem. 30, 425–436. Windler, L., Lorenz, C., von Goetz, N., Hungerbühler, K., Amberg, M., Heuberger, M., Nowack, B., 2012. Release of titanium dioxide from textiles during washing. Environ. Sci. Technol. 46, 8181–8188.


Stability of Fe-oxide nanoparticles coated with natural organic matter under relevant environmental conditions.

Heteroaggregation of titanium dioxide nanoparticles with model natural colloids under environmentally relevant conditions.

Toxic effect of environmentally relevant concentration of silver nanoparticles on environmentally beneficial bacterium Pseudomonas putida.

Oil Recovery from Water under Environmentally Relevant Conditions Using Magnetic Nanoparticles.

Surface Enhanced Raman Scattering (SERS) Studies of Gold and Silver Nanoparticles Prepared by Laser Ablation.

Enhancement of Raman scattering for silver nanoparticles located on electrolessly roughened silicon.

Particle coating-dependent interaction of molecular weight fractionated natural organic matter: impacts on the aggregation of silver nanoparticles.

The Effect of Microplastic on the Uptake of Chemicals by the Lugworm Arenicola marina (L.) under Environmentally Relevant Exposure Conditions.

Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering.

silver nanoparticles and hyperbolic metamaterial.

Morphological evolution and reconstruction of silver nanoparticles in aquatic environments: the roles of natural organic matter and light irradiation.

Hydroxyapatite-silver nanoparticles coatings on porous polyurethane scaffold.

Templated green synthesis of plasmonic silver nanoparticles in onion epidermal cells suitable for surface-enhanced Raman and hyper-Raman scattering.

Adsorption study of antibiotics on silver nanoparticle surfaces by surface-enhanced Raman scattering spectroscopy.

Impact of TiO₂ and ZnO nanoparticles at predicted environmentally relevant concentrations on ammonia-oxidizing bacteria cultures under ammonia oxidation.

Quantifying "Softness" of Organic Coatings on Gold Nanoparticles Using Correlated Small-Angle X-ray and Neutron Scattering.

Silicon nanoparticles as Raman scattering enhancers.

Environmentally relevant approaches to assess nanoparticles ecotoxicity: a review.

Silver nanoparticle behavior, uptake, and toxicity in Caenorhabditis elegans: effects of natural organic matter.

A systematic evaluation of agglomeration of Ag and TiO2 nanoparticles under freshwater relevant conditions.

Theoretical vibrational Raman and surface-enhanced Raman scattering spectra of water interacting with silver clusters.

Surface-enhanced Raman scattering of perchlorate on cationic-modified silver nanofilms - Effect of inorganic anions.

Imaging of Liver Tumors Using Surface-Enhanced Raman Scattering Nanoparticles.

Identification of intracellular gold nanoparticles using surface-enhanced Raman scattering.