Environ. Sci. Technol. 2009, 43, 7531–7535
Evaluating Aggregation of Gold Nanoparticles and Humic Substances Using Fluorescence Spectroscopy V A S A N T A L . P A L L E M , †,‡ H O L L Y A . S T R E T Z , * ,† A N D M A R T H A J . M . W E L L S ‡,§ Department of Chemical Engineering, Tennessee Technological University Cookeville, Tennessee 38505, Center for the Management, Utilization, and Protection of Water Resources, Tennessee Technological University Cookeville, Tennessee 38505, and Department of Chemistry, Tennessee Technological University Cookeville, Tennessee 38505
Received April 22, 2009. Revised manuscript received July 21, 2009. Accepted July 27, 2009.
The fate and transport of diagnostic gold nanoparticles in surface waters would significantly depend on their interactions with humic substances, which are ubiquitously found in natural aquatic systems. The current study employs UV-visible absorbance and fluorescence spectroscopy to investigate the interactions of commercial humic acid (HA) with gold nanoparticles having a core size of 5 nm and coated with two different stabilizers, β-D-glucose and citrate. Humic substances (HS) are fluorescent in nature, providing a unique probe of nanometer-scale morphological changes for interactions between these natural polyelectrolytes and water-soluble gold nanoparticles. Quenching of fluorescence intensity was observed with β-D-glucose-coated gold nanoparticles, whereas an enhancement effect was noticed with the citrate-coated particles when mixed with HA having concentrations of 2 and 8 ppm (surface waters typically may contain ∼10 ppm HS). Examining the quenching and enhancement of fluorescence provides insight into the structural changes taking place at the coated gold nanoparticle-HA interface. The quenching behavior suggested ligand exchange due to nanometer-scale contact between the HA and β-D-glucose-coated gold nanoparticles, whereas the enhancement effect with citrate particles would indicate overcoating, leading to increased transfer distances for fluorescence resonance energy transfer.
Introduction In recent years, a tremendous increase in the applications of gold nanoparticles (GNPs) in biomedical imaging (1, 2), cancer therapy and diagnostics (3-5), and biological and chemical sensing (5, 6) have occurred. Simultaneously, a number of electronic, catalytic, and sensor applications are being developed and marketed (7-10). The rise in applications for GNPs results in increased potential for their widespread release into surface waters. Therefore, it is * Corresponding author phone: (931) 372-3495; fax: (931) 3726352; e-mail:
[email protected]. † Department of Chemical Engineering. ‡ Center for the Management, Utilization, and Protection of Water Resources. § Department of Chemistry. 10.1021/es901201z CCC: $40.75
Published on Web 08/12/2009
2009 American Chemical Society
imperative to gain information about the environmental impact of GNPs (11, 12). The fate and transport of GNPs in the aquatic environment will be primarily governed by their dispersibility and interactions with naturally occurring humic substances (13, 14). Humic substances form the major part of the natural organic matter present in aquatic systems and are mainly present as fulvic acid (FA), which is water-soluble at acidic and alkaline pHs, and humic acid (HA), which is insoluble at acidic pH (15). The complexation of GNPs with humic substances may result in colloid-facilitated transport (13) and form stable colloidal dispersions that can enable interactions with aquatic organisms (14, 16, 17). In this respect, studying the interactions of GNPs with humic substances is critical for understanding fate and transport of a potential emerging pollutant in surface water. Dr. Pedro Alvarez summarized this at the April 2009 International Council on Nanotechnology at Rice University (18), “The acquisition... of coatings on nanoparticles in the environment could influence mobility, behavior, and impact.” Several authors have previously studied gold nanoparticles in surface water model systems. Gold nanoparticles are typically coated with stabilizing agents such as citrates, which impart colloidal stability by electrostatic repulsion (2, 19). Several studies indicate that the stabilizer coating present on GNPs may play a significant role in their interactions with certain microorganisms (20, 21). Some independent reports (22-24) employed humic substances as reducing and stabilizing agents for the production of GNPs. These authors also indicate that gold nanoparticles having different sizes and shapes can be formed in the presence of humic substances by varying pH. Alvarez-Puebla et al. (22) further reported that humic acid-stabilized gold nanoparticles can be used to detect certain organic pollutants using surfaceenhanced Raman scattering (SERS). Baigorri et al. and Santos et al. (23, 24) have demonstrated that fulvic acid-stabilized gold nanoparticles produce a high SERS signal at low pH. Diegoli et al. (25) have characterized GNPs coated with two different stabilizers, citrate and acrylate, and their interactions in water with Suwannee River humic acid (SRHA). Results of UV-visible spectroscopy, dynamic light scattering, and transmission electron microscopy (TEM) were described. Their findings will be directly discussed in the context of the present research in Results. The current study examines the manner in which the organic coating present on GNPs affects their interactions with HAs. Fluorescence spectroscopic measurements were coupled with UV-visible spectroscopy to elucidate the role of the GNP surface coating-HA interactions for two different types of gold nanoparticle suspensions, those engineered with coatings of β-D-glucose (BG-GNPs) and with coatings of citrate (CT-GNPs). Fluorescence measurements will enable a complementary understanding of the structural changes occurring when HAs interact with GNPs because the distance scales required for quenching or enhancement reflect only what is happening at the GNP-HA interface (26). HAs are polyelectrolytes that contain organic groups showing variable aromaticity. These fluorescent structures exhibit distinct spectral signatures (27). Fluorescence emission spectra probe the possibility of interactions at the interface and yield important information about the role of the stabilizer in dispersion in a model surface water environment.
Experimental Section Materials. Sodium borohydride (NaBH4), hydrogen tetrachloroaurate trihydrate (HAuCl4 · 3H2O), and HA were purchased from Sigma-Aldrich (MO). β-D-glucose was purchased VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7531
from ChromaDex, Inc. (CA). CT-GNPs of size 5 nm were purchased from Ted Pella (CA). HPLC grade water and sodium citrate were obtained from Fisher Scientific (NJ). The Aldrich HA chosen for these initial studies represents a range of humic and fulvic acids in terms of diffusion coefficients, weight-average, and number-average molecular weights (polydispersity). These properties have been wellcharacterized by Wells et al. in terms of isotherm adsorption onto activated carbon, EEM fluorescence spectra, and compared favorably with IHSS Suwanee River humic and fulvic acids (28, 29). Preparation of Gold Nanoparticles Capped with β-DGlucose. The BG-GNPs were prepared using the procedure developed by Liu et al. (30). During preparation, 200 µL of 0.05 M aqueous HAuCl4 · 3H2O was added to 50 mL of 0.05 M β-D-glucose aqueous solution followed by the addition of 600 µL of freshly prepared 0.05 M aqueous NaBH4 solution, resulting in the formation of GNPs. A Varian Cary 3E UV-visible spectrophotometer was applied for measuring surface plasmon resonance absorbance spectra (see Supporting Information for details about BG-GNPs). Preparation of Humic Acid Solutions. The humic acid stock solution was prepared by dissolving 7.6 mg of HA in 130 mL of HPLC grade water and stirring for 24 h in the dark. The dissolved organic carbon (DOC) concentration of the HA sample was measured after filtering through a 0.45 µm nylon fiber filter (GE Water & Process Technologies, MA). Total organic carbon (TOC) and DOC of the HA solution were measured on a Shimadzu TOC-VCPH total organic carbon analyzer (see Supporting Information for details about the HA solution). Solutions of 2 and 8 ppm DOC of aqueous HA were prepared. For each of the HA concentrations, four different dispersions containing BG-GNPs (6 nM), CT-GNPs (16 nM), BG + NaBH4 (0.01M), and sodium citrate (0.3 mM) were prepared. The controls consisted of 2 and 8 ppm DOC HA, BG-GNPs, CT-GNPs, BG + NaBH4, and sodium citrate in water. All solutions were prepared using HPLC-grade water. The different concentrations of HA, containing BG-GNPs, CT-GNPs and respective stabilizers, were allowed to equilibrate at room temperature for a period of 4 h. A summary of the various materials prepared is listed in Table S1 of the Supporting Information. UV-Vis and Fluorescence Spectroscopy. All HA samples were subsequently analyzed for UV-visible absorbance and fluorescence. UV-visible absorbance spectra were recorded from 200 to 800 nm using a Varian Cary 3E UV-visible spectrophotometer and a quartz cuvette having a 1 cm path length. Fluorescence excitation emission spectra (EEM) were generated by collecting emission spectra over a range of excitation wavelengths (200-800 nm in increments of 3 nm) and emission wavelengths (200-850 nm in increments of 2 nm). A Varian Cary Eclipse fluorescence spectrophotometer with full spectrum xenon pulse single source lamp was employed. The excitation and emission slit width was set to 10 nm. Controls of HPLC-grade water were checked regularly. Fluorescence intensities of all samples and controls were corrected (using a MATLAB program developed in house) for primary and secondary inner filtering effects according to Tucker et al. (31). Further, the fluorescence intensity of HPLC-grade water was subtracted from the EEMs of samples and controls to correct for water Raman scattering. The corrected two-dimensional (2D) emission spectra at a constant excitation wavelength λex ) 221 nm were plotted for all samples.
Results HA Interaction with BG-GNPs. The effects of the BG or CT stabilizer on interactions with HA were probed using 7532
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 19, 2009
FIGURE 1. (A) Corrected fluorescence emission spectra at excitation wavelength (λ ) 221 nm) for control HA (2 ppm), BG-GNPs + HA (2 ppm), controls BG-GNPs, and BG in HPLCgrade water. (B) Corrected fluorescence emission spectra at excitation wavelength (λ ) 221 nm) for control HA (8 ppm), BG-GNPs + HA (8 ppm), controls BG-GNPs, and BG in HPLCgrade water. fluorescence and UV-visible spectroscopy. The results of the fluorescence studies will be discussed first. Interactions of HA with BG-GNPs were determined by comparison of the corrected fluorescence emission spectra of HA/BG-GNPs mixture with certain controls. Backgroundcorrected fluorescence emission spectra (λex ) 221 nm) for individual solutions of HA, BG-GNPs, and BG in HPLC-grade water are compared to the spectrum for the combined solution of HA + BG-GNPs for HA concentrations of 2 and 8 ppm DOC in panels a and b of Figure 1, respectively. (Note, complete EEMS maps are presented in the Supporting Information.) The emission spectra of all HA-containing samples exhibited similar shapes with a peak centered at approximately λem ) 460 nm. This peak is characteristic of HA-like materials (32). However, the presence of BG-GNPs appears to modify the HA signature by causing a decrease in intensity (quenching) when they are combined. This observation leads to consideration of whether BG is interacting directly with HA (e.g., not the GNP) and therefore causing quenching. Evidence demonstrated (Figures S1a,b of the Supporting Information) that the presence of a BG stabilizer with HA (in the absence of GNPs) did not decrease the intensity of the 460 nm peak. Therefore, the BG-GNP interaction with HA was significant and cannot be explained by an interaction of HA with the stabilizer alone. The evidence suggested that the quenching observed was due to nanometer-scale contact between HA and GNPs. As the concentration of neat HA increased from 2 to 8 ppm, the intensity increased in a concentration-dependent and linear manner (Figure S7 of the Supporting Information).
FIGURE 2. (A) Corrected fluorescence emission spectra at excitation wavelength (λ ) 221 nm) for control HA (2 ppm), CT-GNPs + HA (2 ppm), controls CT-GNPs, and CT in HPLCgrade water. (B) Corrected fluorescence emission spectra at excitation wavelength (λ ) 221 nm) for control HA (8 ppm), CT-GNPs + HA (8 ppm), controls CT-GNPs, and CT in HPLCgrade water. HA-HA interactions were ruled out on the basis of this linearity and dynamic light scattering (Figure S6a,b of the Supporting Information), and quenching at even higher concentrations was observed to be due to HA interacting with BG-GNPs. Peak enhancement of the fluorophore at 300 nm was observed which is attributed not to an interaction between HA and GNPs but instead to an interaction between HA and BG (Figure S1a,b of the Supporting Information). HA Interaction with CT-GNPs. The interactions of HA with CT-GNPs were determined by comparison of corrected fluorescence emission spectra of a HA/CT-GNPs mixture with certain controls. Background-corrected fluorescence emission spectra (λex ) 221 nm) for individual solutions of HA, CT-GNPs, and CT in HPLC-grade water are compared to the spectrum for the combined solution of HA + CT-GNPs for HA concentrations of 2 and 8 ppm DOC in panels a and b of Figure 2, respectively. (Note that fluorescence emission spectra were also reviewed at excitation wavelengths of 329 and 380 nm, and similar conclusions can be drawn compared to the data now discussed for an excitation of 221 nm.) Conversely to the interaction observed between HA with BGGNPs, CT-GNPs caused an increase in the intensity (enhancement) of HA at λem ) 460 nm relative to HA controls for both concentrations. By comparison, the enhancement in fluorescence intensity may indicate little or no contact between the HA and GNP. Similar enhancement (termed metal-enhanced fluorescence) was seen by Aslan et al. (33) in Ag@SiO2 core-shell nanoballs, where the fluorophore (rhodamine) was doped on the outside of a SiO2 shell.
Enhancement intensity was observed to be optimized when using a shell thickness of 11 nm. Thus, excitation-emission fluorescence data could be a sensitive indicator of the morphology of HA-GNP. Further, the morphology was demonstrated to depend significantly on the nature of the stabilizer. At λem ) 300 nm, the fluorescence intensity of the 2 ppm HA sample containing CT-GNPs was less than that of controls of CT-GNPs and CT (Figure 2a), thereby indicating that specific fluorophores of HA interact with CT molecules present on CT-GNPs, which cause a decrease in the intensity of the HA + CT-GNPs complex. However, with 8 ppm HA, the CT-GNPs did not display a major change of fluorescence intensity as compared to the CT control. Thus, opposite effects are seen between the CT fluorophore and HA as the HA concentration changes. While HA-GNP aggregation might explain this at the higher HA concentrations, such a mechanism would be contradictory to the UV-vis results and dynamic light scattering results (DLS) (Figure S6c,d of the Supporting Information) (34). This phenomena will be further investigated. Rayleigh Scattering. Raleigh scattering occurs when λex ) λem. At an excitation-emission wavelength pair equal to 221 nm, Rayleigh scattering was observed for both concentrations of HA. In the presence of BG-GNPs as well as CTGNPs, HA molecules yield bigger scattering bodies, having a scattering intensity nearly twice that of the HA controls. The solutions of BG-GNPs and CT-GNPs in water did not indicate strong scattering. However, in the presence of HA, an increase in scattering intensity was observed, which suggests that the interactions of GNPs with HA result in large complexes of x-GNPs-HA. HA-GNP aggregates, however, were not noted in the DLS size distributions based on volume percent (Figure S6b,d of the Supporting Information). We cannot discount that this Raleigh scattering might indicate the presence of HA-GNP aggregates, but DLS indicates this is not a significant volume fraction of the sample (35). UV-Vis Absorbance Spectra of BG-GNPs in Increasing Concentrations of HA. UV-vis absorbance spectra of BGGNPs in the presence of increasing concentrations of HA are displayed in Figure 3a. The absorbance spectra are presented as the difference in the spectra of the x-GNP-HA and spectra of the HA alone. The control BG-GNPs solution exhibited absorption maxima at 515 nm; however, with increasing concentrations of HA, a red shift of the peak toward 525 nm was observed. The red shift of the absorption maxima has two potential causes: the local dielectric constant surrounding the particle has changed or the nanoparticles have aggregated (6). Recalling that fluorescence quenching was also observed for HA with GNP-BG, it is most likely that the red shift results from a change in the local dielectric constant, attributed here to the proximity of HA to the gold surface. AlvarezPuebla et al. have seen an analogous blue shift for GNPs in “island films” in the presence of fulvic acids (36). This seeming discrepancy may be explained by different absorbance wavelengths for the “control”. Their neat GNP films on a surface absorbed at 702 nm, whereas the neat GNPs discussed here in free solution absorbed at 515 nm. In both cases, the modified GNPs shifted toward a wavelength intermediate between the two controls, suggesting a common dielectric constant range for humic acid- or fulvic acid-gold interactions. The BG-GNPs did not exhibit precipitation with increased concentration of HA. Therefore, HA (a natural polyelectolyte) could act as a vehicle for the transport of BG-GNPs in surface waters. UV-Vis Absorbance Spectra of CT-GNPs in Increasing Concentrations of HA. A similar study by Diegoli et al. (25) was reported for CT-GNP, but the particles in that study were 15-20 nm in diameter (by TEM), whereas, in this research, the gold cores were 5-6 nm in diameter. UV-vis VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7533
FIGURE 4. Schematic of interactions between HA and BG-GNPs and CT-GNPs (not to scale).
FIGURE 3. (A) UV-vis absorbance spectra of BG-GNPs in HPLC-grade water and difference in spectra of BG-GNPs + HA with respect to HA for concentrations of HA at 2 and 8 ppm. (B) UV-vis absorbance spectra of CT-GNPs in HPLC-grade water and difference in spectra of CT-GNPs + HA with respect to HA for concentrations of HA at 2 and 8 ppm. absorbance spectra of CT-GNPs in the presence of increasing concentrations of HA are presented in Figure 3b. The control CT-GNPs solution exhibited absorption maxima at 519 nm; however, no peak shift was observed with increasing HA concentrations. The lack of the red shift for this system indicated that neither aggregation nor surface contact between HA and GNP occurred for CT. This observation corroborates the fluorescence results, and similar UV-visible results were also noted by Diegoli et al. (25). The Diegoli study suggested that SRHA (Suwannee River Humic Acid) was either (i) substituted or (ii) overcoated on the citrate anions. The fluorescence data obtained in the current research favors the overcoated hypothesis. Discussion of Interactions of HA with BG-GNPs and with CT-GNPs. A schematic of the expected interactions for HA with BG-GNPs and for HA with CT-GNPs is shown in Figure 4. The HA sample containing BG-GNPs displayed quenching of fluorescence intensity; therefore, the HA molecules must be in close proximity (less than 10 nm) to the GNPs (26, 37-39). Furthermore, the UV-vis shows a red shift when HA is added to BG-GNPs (e.g., a change in dielectric constant of the GNP environment occurred), and this information coupled with fluorescence quenching would indicate that the HA is complexing with GNPs. Certainly the extent of fluorescence quenching would be distance dependent, but the present system probably is a distribution of HA layering on the surface of the GNP rather than a discrete ordered layering such as that reported by Schneider, et al. (26). This distribution of layering is postulated simply because of the polydisperse nature of natural organic materials. Therefore, the quenching observed could represent a multiplicity of states, where HA fluorophores in some cases reside 7534
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 19, 2009
on the surface (UV-vis shift) and in some cases simply reside within the 10 nm required for significant fluorescence quenching. In this case, some HA may be substituting for the BG coating present on the GNPs. A weak affinity of the β-Dglucose for the gold surface has been documented by Liu et al., where other ligands are known to displace the BG (30). However, the CT-GNPs cause an enhancement of fluorescence intensity of HA; therefore, the HA molecules may overcoat the citrate layer on GNPs (Figure 4). This layering would restrict nanometer-scale contact between the GNP surface and HA (25). The interactions between HA and GNPs coated with the two different stabilizers implicates two different morphologies for HA complexation with GNPs. Regarding the very important issue of stability of these complexes, we note that in both cases there were no visible precipitates observed. Neither evidence of UV-vis peak broadening nor development of a longitudinal plasmon band (40) were noted, again indicating lack of aggregation. Lastly, the samples were analyzed under DLS, and no aggregates were detected when HA was added to x-GNP nor did the size distribution broaden (up to 10 µm diameter was examined). (The additional DLS scans may be found in Figure S6a-d of the Supporting Information). Therefore, the complexes of GNPs and HA were in all cases considered stable for the conditions studied. This stability is important in that HA as a polyelectrolyte would act as a vehicle for transport of GNPs in surface water. The fluorescence information reported herein has imparted significant new morphological information to aide in modeling of transport of GNPs in surface waters.
Acknowledgments The authors gratefully thank the Center for the Management, Utilization, and Protection of Water Resources at Tennessee Technological University for research funding and support. The authors thank the Department of Chemistry at Tennessee Technological University for instrumental support. The authors also gratefully acknowledge long and helpful conversations with Drs. Chris Roberts and Juncheng Liu at Auburn University. The TEM samples were imaged by Dr. Jibao He at the Coordinated Instrument Facility, Tulane University. Dynamic light scattering data were kindly provided by Drs. Xin Ma and Dermont Bouchard at the Environmental Protection Agency in Athens, GA.
Supporting Information Available List of materials prepared; graphical information about corrected fluorescence emission spectra for controls HA, HA + BG, and HA + CT at HA concentrations of 2 and 8 ppm; corrected fluorescence excitation emission maps of HA, HA + BG-GNPs, and HA + CT-GNPs for the above-stated HA concentrations; TEM micrograph of BG-GNPs; dynamic light scattering results for control BG-GNPs and CT-GNPs and in
the presence of 2 and 8 ppm HA; and corrected fluorescence emission spectra at 221 nm for different concentrations of HA. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Surface plasmon resonance scattering and adsorption of anti-EGFR antibody conjugated gold nanoparticles in cancer diagnostics: applications in oral cancer. Nano Lett. 2005, 5 (5), 829–834. (2) Rayavarapu, R. G.; Wilma, P.; Constantin, U.; Janine, P. N.; Ton, G.; van, L.; Srirang, M. Synthesis and bioconjugation of gold nanoparticles as potential molecular probes for light-based imaging techniques. Int. J. Biomed. Imaging. 2007, 2007, 1–10. (3) Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Immunotargeted nanoshells for integrated cancer imaging and therapy. Nano Lett. 2005, 5 (4), 709–711. (4) Loo, C.; Lin, A.; Hirsch, L. R.; Lee, M. H.; Barton, J.; Halas, N.; West, J.; Drezek, R. Nanoshell-enabled photonics-based imaging and therapy of cancer. Technol. Cancer Res. Treat. 2004, 3, 33– 40. (5) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble metals on the nanoscale: Optical and photothermal properties and some applications in imaging, sensing, biology, and medicine. Acc. Chem. Res. 2008, 41, 1578–1586. (6) Murphy, C. J.; Gole, Anand M.; Hunyadi, S. E.; Stone, J. W.; Sisco, P. N.; Alkilany, A.; E., K. B.; Hankins, P. Chemical sensing and imaging with metallic nanorods. Chem. Commun. 2008, 544–557. (7) Shipway, A.; Willner, I. Nanoparticles as structural and functional units in surface-confined architectures. Chem. Commun. 2001, 2035–2045. (8) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale forces and their uses in self-assembly. Small 2009, 9999. (9) Ghosh, S. K.; Pal, T. Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to applications. Chem. Rev. 2007, 107, 4797–4862. (10) Eustis, S.; El-Sayed, M. A. Why gold nanoparticles are more precious than pretty gold: Noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes. Chem. Soc. Rev. 2006, 35, 209–217. (11) Colvin, V. L. The potential environmental impact of engineered nanomaterials. Nat. Biotechnol. 2003, 21 (10), 1166–1170. (12) Stern, S. T.; Scott, M. E. Nanotechnology safety concerns revisited. Toxicol. Sci. 2007, 101 (1), 4–21. (13) Deshikan, S. R.; Eschenazi, E.; Kyriakos, P. D. Transport of colloids through porous beds in the presence of natural organic matter. Colloids Surf., A 1998, 145, 93–100. (14) Klaine, S. J.; Alvarez, P. J. J.; Batley, G. E.; Fernandes, T. F.; Handy, R. D.; Lyon, D. Y.; Mahendra, S.; Mclaughlin, M. J.; Lead, J. R. Nanomaterials in the environment: Behavior, fate, bioavailability, and effects. Environ. Toxicol. Chem. 2008, 27 (9), 1825– 1851. (15) Sutton, R.; Sposito, G. Molecular structure in soil humic substances: The new view. Environ. Sci. Technol. 2005, 39 (23), 9009–9015. (16) Oberdorster, E. Manufactured nanomaterials (Fullerenes, C60) induce oxidative stress in the brain of juvenile largemouth bass. Environ. Health Perspect. 2004, 112, 1058–1062. (17) Weisner, M.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 4337–4345. (18) Erickson, B. E. Sustainable Nanotech. Chem. Eng. News 2009, 87 (15), 37–40. (19) Brewer, S. H.; Glomm, W. R.; Johnson, M. C.; Knag, M. K.; Franzen, S. Probing BSA binding to citrate-coated gold nanoparticles and surfaces. Langmuir 2005, 21, 9303–9307. (20) Lewinski, N.; Colvin, V. L.; Drezek, R. Cytotoxicity of nanoparticles. Small 2008, 4 (1), 26–49.
(21) Hoshino, A.; Fujioka, K.; Oku, T.; Suga, M.; Sasaki, Y. F.; Ohta, T.; Yasuhara, M.; Suzuki, K.; K., Y. Physicochemical properties and cellular toxicity of nanocrystal quantum dots depend on their surface modification. Nano Lett. 2004, 4 (11), 2163–2169. (22) Alvarez-Puebla, R. A.; Santos, D. S. D.; Aroca, R. F. SERS detection of environmental pollutants in humic acid-gold nanoparticle composite materials. Analyst 2007, 132, 1210–1214. (23) Baigorri, R.; Garcı´a-Mina, J. M.; Aroca, R. F.; Alvarez-Puebla, R. A. Optical enhancing properties of anisotropic gold nanoplates prepared with different fractions of a natural humic substance. Chem. Mater. 2008, 20, 1516–1521. (24) Santos, D. S. D.; Alvarez-Puebla, R. A.; Oliveira, O. N.; Aroca, R. F. Controlling the size and shape of gold nanoparticles in fulvic acid colloidal solutions and their optical characterization using SERS. J. Mater. Chem. 2005, 15, 3045–3049. (25) Diegoli, S.; Manciulea, A. L.; Begum, S.; Jones, I. P.; Lead, J. A.; Preece, J. A. Interaction between manufactured gold nanoparticles and naturally occuring organic macromolecules. Sci. Total Environ. 2008, 402, 51–61. (26) Schneider, G.; Decher, G.; Nerambourg, N.; Praho, R.; Werts, M. H. V.; Blanchard-Desce, M. Distance-dependent fluorescence quenching on gold nanoparticles ensheathed with layer-bylayer assembled polyelectrolytes. Nano Lett. 2006, 6 (3), 530– 536. (27) Mobed, J. J.; Hemmingsen, S. L.; Autry, J. L.; Mcgown, L. B. Fluorescence characterization of IHSS humic substances: Total luminescence spectra with absorbance correction. Environ. Sci. Technol. 1996, 30, 3061–3065. (28) Dycus, P. J. M.; Healy, K. D.; Stearman, G. K.; Wells, M. J. M. Diffusion coefficients and molecular weight distributions of humic and fulvic acids determined by flow field-flow fractionation. Sep. Sci. Technol. 1995, 30 (7-9), 1435–1453. (29) Schmit, K. H.; Wells, M. J. M. Preferential adsorption of fluorescing fulvic and humic acid components on activated carbon using flow field-flow fractionation analysis. J. Environ. Monit. 2002, 4 (1), 75–84. (30) Liu, J.; Anand, M.; Roberts, C. B. Synthesis and extraction of β-D-glucose-stabilized Au nanoparticles processed into lowdefect, wide-area thin films and ordered arrays using CO2expanded liquids. Langmuir 2006, 22, 3964–3971. (31) Tucker, S. A.; Amszi, V. L.; Acree, W. E. J. Primary and secondary inner filtering: Effect of K2Cr2O7 on fluorescence emission intensities of quinine sulfate. J. Chem. Educ. 1992, 69, A8–A12. (32) Yan, Y.; Hong, L.; Myrick, M. L. Fluorescence fingerprint of waters: Excitation-emission matrix spectroscopy as a tracking tool. Appl. Spectrosc. 2000, 54 (10), 1539–1542. (33) Aslan, K.; Wu, M.; Lakowicz, J. R.; Geddes, C. D. Metal enhanced fluorescence solution-based sensing platform 2: Fluorescent core-shell AG@SIO2 nanoballs. J. Fluoresc. 2007, 17, 127–131. (34) Lochmuller, C. H.; Saavedra, S. S. Conformational changes in a soil fulvic acid measured by time-dependent fluorescence depolarization. Anal. Chem. 1986, 58, 1978–1981. (35) Ma, X.; Bouchard, D. Formatation of aqueous suspensions of fullerenes. Environ. Sci. Technol. 2009, 43, 330–336. (36) Alvarez-Puebla, R.; Garrido, J. J.; F., A. R. Surface-enhanced vibrational microspectroscopy of fulvic acid micelles. Anal. Chem. 2004, 76, 7118–7125. (37) Dulkeith, E.; Ringler, M.; Klar, T. A.; Feldmann, J.; Javier, A. M.; Parak, W. J. Gold nanoparticles quench fluorescence by phase induced radiative rate suppression. Nano Lett. 2005, 5 (4), 585– 589. (38) Kim, K. C.; Kalluru, R. R.; Singh, J. P.; Fortner, A.; Griffin, J.; Darbha, G. K.; Ray, P. C. Gold-nanoparticle-based miniaturized laser-induced fluorescence probe for specific DNA hybridization detection: Studies on size-dependent optical properties. Nanotechnology 2006, 17, 3085–3093. (39) Zhao, Y.; Jiang, Y.; Fang, Y. Quenching and enhancement of fluorescence of fullerene molecules on gold particle. Chem. Phys. 2006, 323, 169–172. (40) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S.; Li, T. Anisotropic metal nanoparticles: Synthesis, assembly and optical applications. J. Phys. Chem. B 2005, 109, 13857–13870.
ES901201Z
VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7535
