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Interparticle Interactions in Glutathione Mediated Assembly of Gold Nanoparticles I-Im S. Lim,† Derrick Mott,† Wui Ip,† Peter N. Njoki,† Yi Pan,‡ Shuiqin Zhou,‡ and Chuan-Jian Zhong*,† Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902, and Department of Chemistry, College of Staten Island, City UniVersity of New York, Staten Island, New York 10314 ReceiVed March 27, 2008. ReVised Manuscript ReceiVed May 21, 2008 The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH- groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.
Introduction The study of the interactions between biologically relevant molecules and nanoparticles has attracted increasing interest because of potential applications in sensors, biosensors, and biomedical diagnostics.1–6 In our earlier studies, we reported the detection of thiol-containing amino acids such as homocysteine and cysteine using gold nanoparticles.7,8 The reactivity involves strong affinity of the thiol functional group of the amino acids to gold and intermolecular zwitterionic interactions between the amino acids attached onto gold nanoparticles. There are several reports supporting such interparticle interactions.7–13 The findings led to the exploration of interactions and reactivities of nanoparticles with other thiol-containing amino acids. * To whom correspondence should be addressed. E-mail: cjzhong@ binghamton.edu. † State University of New York at Binghamton. ‡ College of Staten Island, City University of New York. (1) Wang, W.; Rusin, O.; Xu, X.; Kim, K. K.; Escobedo, J. O.; Fakayode, S. O.; Fletcher, K. A.; Lowry, M.; Schowalter, C. M.; Lawrence, C. M.; Fronczek, F. R.; Warner, I. M.; Strongin, R. M. J. Am. Chem. Soc. 2005, 127, 15949. (2) (a) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (b) Parak, W. J.; Gerion, D.; Pellegrino, T.; Zanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Gros, M. A. L.; Larabell, C. A.; Alivisatos, A. P. Nanotechnology 2003, 14, R15. (3) (a) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (b) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (4) Park, H. Y.; Schadt, M. J.; Wang, L. Y.; Lim, I-I. S.; Njoki, P. N.; Kim, S. H.; Jang, M. Y.; Luo, J.; Zhong, C. J. Langmuir 2007, 23, 9050. (5) (a) Ni, J.; Lipert, R. J.; Dawson, G. B.; Porter, M. D. Anal. Chem. 1999, 71, 4903. (b) Grubisha, D. S.; Lipert, R. J.; Park, H. Y.; Driskell, J.; Porter, M. D. Anal. Chem. 2003, 75, 5936. (6) Ghosh, P.; Han, G.; Erdogan, B.; Rosado, O.; Krovi, S. A.; Rotello, V. M. Chem. Biol. Drug Des. 2007, 70, 13. (7) Lim, I-I. S.; Ip, W.; Crew, E.; Njoki, P. N.; Mott, D.; Zhong, C. J.; Pan, Y.; Zhou, S. Langmuir 2007, 23, 826. (8) Zhang, F. X.; Han, L.; Israel, L. B.; Daras, J. G.; Maye, M. M.; Ly, N. K.; Zhong, C. J. Analyst 2002, 127, 462.
Glutathione (GSH) is a tripeptide (γ-Glu-Cys-Gly) that contains an -SH group. Glutathione is known to protect red cells from oxidative damage when it is present in sufficiently high quantities (∼5 mM) and to maintain the normal reduced state of the cell because of its antioxidant nature. Glutathione also plays an important role in the detoxification of the cell and is responsible for removing harmful organic peroxides and free radicals.14 It binds to toxins, such as heavy metals, solvents, and pesticides, and transforms them into a form that can be excreted in urine or bile.14 The unique optical properties and surface binding affinity of gold (Au) nanoparticles to thiol-containing amino acids or peptide provide an intriguing opportunity to develop nanoprobes to address some of the fundamental questions related to the role of glutathione in the biological systems. In view of the importance of this biological molecule, many studies have recently been performed to understand the reactivity of glutathione in the presence of nanoparticles.11,12,15–26 For example, the examination of the assembly11 and directional growth12a of nanorods in the presence of GSH showed potential applications in optics and biotechnology. Gold nanorods and glutathione were shown to form a necklace assembly, which was proposed to involve a cooperative two-point electrostatic interaction of the zwitterionic groups.12b The preparation of GSH(9) Lu, C.; Zu, Y. Chem. Commun. 2007, n/a, 3871. (10) Lo, C. K.; Xiao, D.; Choi, M. M. F. J. Mater. Chem. 2007, 17, 2418. (11) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516. (12) (a) Kou, X.; Zhang, S.; Yang, Z.; Tsung, C.-K.; Stucky, G. D.; Sun, L.; Wang, J.; Yan, C. J. Am. Chem. Soc. 2007, 129, 6402. (b) Zhang, S.; Kou, X.; Yang, Z.; Shi, Q.; Stucky, G. D.; Sun, L.; Wang, J.; Yan, C. Chem. Commun. 2007, n/a, 1816. (13) Aryal, S.; Bahadur, K. C. R.; Bhattarai, N.; Kim, C. K.; Kim, H. Y. J. Colloid Interface Sci. 2006, 299, 191. (14) Berg, J. M.; Tymoczko, J. L.; Stryer. L. Biochemistry, 5th ed.; Freeman & Company: New York, 2002.
10.1021/la800970p CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
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capped Ag2S semiconductor nanoparticles15 and GSH-encapsulated on gold nanoparticles16,17 has been reported. Ligand exchange between the GSH-Au nanoparticles with thiolated oligonucleotides has also been examined.18 When silver nanoparticles are capped with GSH, a new generation of circular dichroism signals is observed.19 In particular, a recent study reported that the interparticle coupling effect of GSH induced nanoparticle aggregation when nanoparticles are introduced.20 Recently, the development of a nanoparticle-based drug delivery and release system using GSH as the releasing agent has been shown to be viable.21 The self-assembly of glutathione films on gold electrode surfaces has been shown to exhibit ion-gating properties, which are useful for the selective detection of specific metal cations.24,25 While these previous studies have demonstrated the importance of interparticle interactions of glutathiones, the detailed molecular interactions between glutathione molecules in the assembly and disassembly of metal nanoparticles remain elusive. To probe such interactions, we carried out a systematic investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The reactivity, that is, the assembly between GSH molecules adsorbed on the gold nanoparticle surfaces via interparticle molecular interactions, has been characterized in terms of the particle size effect, the effect of ionic strength on the reaction kinetics, and the pH effect on the assembly-disassembly reversibility. The results have provided some new insights into the mechanistic aspects of the interparticle interactions in the assembly and disassembly processes.
Experimental Section Chemicals. The chemicals included hydrogen tetracholoroaurate (HAuCl4, 99%), sodium citrate (Cit, 99%), sodium acrylate (97%), sodium chloride (NaCl, 99%), and L-glutathione (reduced) (L-GSH, 99%). All chemicals were purchased from Aldrich and used as received. Water was purified with a Millipore Milli-Q water system. Synthesis of Gold Nanoparticles. The synthesis of citrate-capped gold nanoparticles followed the reported procedure.27 Briefly, freshly prepared sodium citrate (38.8 mM dissolved in 5 mL of deionized water) is added to a boiling solution containing 1 mM of HAuCl4 (in 45 mL of deionized water). Upon the addition of sodium citrate, the color of the solution turned ruby-red, which is an indication for the formation of the gold nanoparticles. The solution is heated for an additional 30 min where the heating mantle is removed and the nanoparticle solution is allowed to cool to room temperature with (15) Brelle, M. C.; Zhang, J. Z.; Nguyen, L.; Mehra, R. K. J. Phys. Chem. A 1999, 103, 10194. (16) (a) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630. (b) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643. (17) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Am. Chem. Soc. 2004, 126, 6518. (18) Ackerson, C. J.; Sykes, M. T.; Kornberg, R. D. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 13383. (19) Li, T.; Park, H. G.; Lee, H. S.; Choi, S. H. Nanotechnology 2004, 15, S660. (20) (a) Basu, S.; Ghosh, S. K.; Kundu, S.; Panigrahi, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, T. J. Colloid Interface Sci. 2007, 313, 724. (b) Basu, S.; Pal, T. J. Nanosci. Nanotechnol. 2007, 7, 1904. (21) Hong, R.; Han, G.; Fernandez, J. M.; Kim, B. J.; Forbes, N. S.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 1078. (22) Gerdon, A. E.; Wright, D. W.; Cliffel, D. E. Anal. Chem. 2005, 77, 304. (23) Odriozola, I.; Loinaz, I.; Pomposo, J. A.; Grande, H. J. J. Mater. Chem. 2007, 17, 4843. (24) Hepel, M.; Tewksbury, E. J. Electroanal. Chem. 2003, 552, 291. (25) Ali, E. M.; Zheng, Y.; Yu, H.; Ying, J. Y. Anal. Chem. 2007, 79, 9452. (26) (a) Bieri, M.; Burgi, T. Langmuir 2005, 21, 1354. (b) Bieri, M.; Burgi, T. J. Phys. Chem. B 2005, 109, 10243. (c) Bieri, M.; Burgi, T. J. Phys. Chem. B 2005, 109, 22476. (d) Bieri, M.; Burgi, T. Phys. Chem. Chem. Phys. 2006, 8, 513. (27) Lim, I-I. S.; Goroleski, F.; Mott, D.; Kariuki, N.; Ip, W.; Luo, J.; Zhong, C. J. J. Phys. Chem. B 2006, 110, 6673.
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Figure 1. UV-vis spectral evolution for GSH-mediated assembly of Au11nm in the absence of (A) and in the presence of (B) NaCl ([Au11nm] ) 2.4 nM (red curves); [GSH] ) 1 mM; [NaCl] ) 10 mM). pH of the assemblies taken after 2 h of reaction: 4.3 (A) and 4.2 (B). The arrows indicate the direction of spectral evolution. The reactions were followed for 2 h.
continuous stirring. The particle size determined by transmission electron microscopy (TEM) was 11.4 ( 0.9 nm (Au11nm). The synthesis of 30 nm acrylate-capped gold nanoparticles (Au30nm) followed one of our previously reported procedures.28 The particle size determined by TEM was 32.6 ( 1.6 nm. Instrumentation and Measurements. UV-visible (UV-vis) spectra were acquired with an HP 8453 spectrophotometer. Spectra were collected over the range of 200-1100 nm. A plastic cuvette with a path length of 1.0 cm was utilized. Briefly, a quantitative amount of GSH (pH of stock GSH ) ∼3.1) was added to the assynthesized aqueous gold nanoparticles (pH of diluted Au11nm ) ∼6.4; Au30nm ) ∼7.9). The solution was quickly mixed for ∼2 s before the UV-vis spectra were taken. For experiments in the presence of an electrolyte (e.g., NaCl), the Aunm and NaCl solutions were first mixed for 15 min before adding a quantitative amount of GSH. Control experiments with higher concentrations of NaCl showed that no aggregation was observed in the Aunm solution when the NaCl concentration was 5 for several weeks. These results demonstrate that the GSH-mediated assembly is inhibited at high pH in the solution. To further understand the detailed mechanism of the pHdependent interparticle interaction, a quantitative amount of NaOH is added to the GSH-Au11nm assembly. Parts C and D of Figure 4 show a typical set of UV-vis data for the spectral evolution of the GSH-Au11nm assembly upon the addition of NaOH. First, the addition of GSH to the Au11nm solution causes a new SP band to evolve at a longer wavelength (∼650 nm) (arrow a). Next, the addition of NaOH to this solution leads to a reversal of the SP band evolution (arrow b). The solution changed from red to purple upon the addition of GSH and became reddish purple after the addition of NaOH. The final pH values of the solution taken after 1 h were 6.7 (C) and 9.4 (D). While the increase in pH showed a higher degree for the SP band reversal, it did not lead to a complete reversal of the SP band to the original position of Aunm (red curve). This indicates that while the increased pH is disruptive to the interparticle interaction of the GSH bound to the Au surface, it does not lead to complete inhibition of the interparticle interaction of the GSH because it does not replace the GSH from the Au surface. Citrate-Induced Disassembly. In an attempt to gain insight into the reversal of the SP band evolution, sodium citrate (Cit), a conjugate base of the weak acid, was used to adjust the pH. Figure 5 shows a set of SP band evolution for the GSH-mediated assembly of Au11nm upon adding Cit of three different concen-
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Figure 5. UV-vis spectral evolution for the GSH-mediated assembly of Au11nm (arrow a (from red to black curve) shows the direction of the spectral evolution) and its disassembly upon the addition of sodium citrate of different concentrations (arrow b (blue curves) shows the direction of reversal of the spectral evolution) [Cit]: 0 (A); 0.5 (B); 1.0 (C); and 10 mM (D). ([Au11nm] ) 2.4 nM (red curves); [GSH] ) 1.5 mM (the black curves represent the spectra taken after a 5 min reaction time.) The pH values of the GSH-Au11nm solutions in the presence of citrates taken after 2 h of reaction time are 3.7 (A), 4.6 (B), 5.4 (C), and 6.6 (D). The spectral evolution was followed for 2 h. The solutions on the right show the corresponding color of the assembly (A) and changes upon the addition of Cit (B-D). The insert in (B) is a magnified view of the spectra.
trations ([Cit]: 0 (A); 0.5 (B); 1 (C); and 10 mM (D), respectively) in solution. The concentrations of Au11nm and GSH remained identical in all experiments. The increase in the concentration of citrate also causes an increase in the solution pH. To ensure validity of the experimental data comparison, control experiments of Aunm + Cit in the absence of GSH were also performed. No changes in spectral evolution were observed. For the assembly of nanoparticles in the presence of GSH (Figure 5A), the spectral evolution of the SP band with a maximum at ∼650 nm is evident. The color of the solution changed from red to blue, and the solution has a final pH of 3.7. The spectral evolution was followed for 2 h. In parts B and C of Figure 5, the GSH-Au11nm solutions were allowed to react for 5 min (arrow a) before different concentrations of citrate were added (arrow b) and the disassembly of the solutions was monitored. When 0.5 mM citrate was added to the GSH-Au11nm assembly after 5 min of reaction, the spectral evolution of the SP band appeared to be prevented from further progressing to the longer wavelength, as evidenced by the apparent overlaying of spectra (Figure 5B). The solution color remained purple after 2 h of reaction and no precipitation was observed for several weeks at a pH of 4.6. The spectral evolution was basically “frozen” at a certain point. When 1 mM citrate was added to the GSH-Au11nm (Figure 5C), the spectra reveal an abrupt stop in the formation of the GSH-Au11nm assembly. The slow reversal of the SP band toward shorter wavelength (as indicated by the arrow b) indicates a slow disassembly of the GSH-Au11nm. The color of the solution changed from red to purple upon the addition of GSH and back to red upon the addition of citrate and remained red for several
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Figure 6. TEM images for GSH-mediated assembly of Au11nm under different sodium citrate concentrations. [Cit]: 0.5 (A) and 1.0 (B) mM. ([Au11nm] ) 2.4 nM; [GSH] ) 1.5 mM.) The samples were taken for TEM analysis after the citrate was added to the GSH-Aunm solution for 2 h of reaction time, which correspond to the last spectrum in the spectral evolution shown in parts B and C of Figure 5.
months (pH 5.4). It is important to note that at this citrate concentration the SP band was not completely reversed to the original SP band position for Au11nm (red curve). The spectral evolution was essentially a partial reversal under this condition. However, when the citrate concentration was increased to 10 mM (Figure 5D), the SP band showed an almost immediate reversal to the shorter wavelength position corresponding to the original Au11nm (arrow b). The solution remained soluble and reddish in color for months (pH 6.6). The complete reversal of the SP band back to the original SP position is indicative of a complete disruption of the interparticle interaction of GSH, which can be explained by blocking the sites of GSHs or the participation of citrate molecules in the interfacial structure of GSH/Au particles. The latter involves -CO2- and -NH3+ ion pairing or zwitterionic interactions of the negatively charged citrates surrounding the GSH monolayer, which may cause a more effective interparticle spatial and electrostatic isolation than OHgroups. In another experiment (not shown), a GSH-Au11nm assembly, which was aged for several months, was completely reversed by simply adjusting the pH to a basic condition. Figure 6 reveals a set of TEM images for the GSH-Au11nm disassemblies. The solutions were sampled for TEM analysis after citrate was added to the GSH-Aunm solution for a 2 h reaction time, which correspond to the last spectrum in the spectral evolution shown in parts B and C of Figure 5. Upon the addition of 0.5 mM Cit (Figure 6A), a close examination of the TEM features reveals the presence of cluster domains of nanoparticle assemblies where certain domains of interparticle ordering are evident. For the sample taken from the solution after addition of 1 mM Cit (Figure 6B), well-ordered domains can be identified. The average edge-to-edge interparticle distance was estimated based on statistical analysis of the edge-to-edge distances of many particles in an ordered domain, which was found to be 1.43 ( 0.27 nm. This interparticle distance showed an increase when compared to the theoretical estimate of the edge-to-edge interparticle distance for two surface-adsorbed citrate molecules (1.28 nm).30 This result reflects the presence of GSH molecules between the particles. The average value of 1.43 ( 0.27 nm was estimated for the particles in Figure 6B only. In Figure 6A, the addition of the low-concentration citrate effectively stopped the assembly of GSH-Aunm. In another experiment (not shown), the capture of the partial disassembly of the particles along the edge of the cluster was also observed. Some of the outermost nanoparticles around the large clustered nanoparticle assembly
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seem to display features corresponding to nanoparticles at the verge of disassembly. 3. Size Evolution in the Assembly-Disassembly Processes. The assembly and disassembly processes involve size changes of the GSH-Au nanoparticle clusters. Such changes were examined in situ using the DLS technique. The plots are based on z-average (diameter by scattering intensity) from CONTIN program analysis. Figure 7A shows a representative set of DLS data for assessing the size evolution of the assembly and disassembly. The measurement was performed at a scattering angle of 30°. The measured hydrodynamic diameter Dh for gold nanoparticles, 18.9 nm (a), is somewhat larger than the size determined by TEM (11.4 nm). This is not surprising because the DLS experiment is performed under in situ conditions and measures the hydrodynamic diameters of the nanoparticles. The electrical double layer surrounding the charged surfaces of gold nanoparticles increases the overall hydrodynamic diameter. Upon addition of GSH to the solution of Au nanoparticles, there was a rapid increase of the hydrodynamic diameter. After 10 min, there was a short period of insignificant change (b to c). However, continuous monitoring of the system at room temperature showed a growth in size for the GSH-Au11nm assembly. Interestingly, the narrowing in the size distribution of the GSH-Au11nm assembly suggests that the assemblies became more uniform in size with time (see graphs c to f in Figure 7A). After the size evolution was monitored for 74 h (f), sodium citrate was added to the GSH-Au11nm solution. It is evident that the trend of the size evolution was quickly reversed, indicating that disassembly occurred quickly within 60 min (see graphs f-h in Figure 7A). The final average size was largely identical to that for the initial Au11nm particles. Figure 7B shows a bar chart to illustrate the progression for the change in the average Dh at different stages of the assembly and the disassembly processes. It is evident that the average hydrodynamic diameter increases in the process of assembly (red bars) and the assembly process is completely reversed by adding citrate leading to a fast return of the hydrodynamic diameter (blue bars). 4. Mechanistic Aspects of the Interparticle Interactions. Glutathione23,26a,31,32 consists of the glutamate moiety with a pKa of 2.05 for the carboxylic group and a pKa of 9.49 for the amine group; the carboxylic group on the glycine moiety has a pKa of 3.40 and the thiol (-SH) moiety on the cysteine has a pKa of 8.72.32 Considering these parameters, the well-established Au-sulfur chemistry, and previous studies of similar systems7–13,19–21,24–26 including our own work,7,8 several possible interparticle interactions for the GSH-Aunm assembly can be considered. GSH was previously proposed11 to be immobilized on the surface of gold nanoparticles/nanorods via the thiol group from the cysteine moiety. The nanoparticles/nanorods were then assembled via electrostatic interaction between the zwitterionic groups from the glutamate moiety. In another study,20 a different mechanism for the interaction of GSH between neighboring particles was proposed. GSH was proposed to involve binding of the -SH group from the cysteine moiety on one particle and the binding of the R-amine group from the glutamate moiety onto another particle surface. In our recent work on the assembly of homocysteine and gold nanoparticles,7 the strong head-tohead zwitterionic electrostatic interactions between the amino acid groups of homocysteine bound via thiol on the nanoparticles (31) Tajc, S. G.; Tolbert, B. S.; Basavappa, R.; Miller, B. L. J. Am. Chem. Soc. 2004, 126, 10508. (32) Rabenstein, D. L. J. Am. Chem. Soc. 1973, 95, 2797.
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Figure 7. DLS data showing the size changes corresponding to the assembly (red) and the disassembly (blue) for GSH-Au11nm. (A) Plot of intensity contribution vs diameter (Dh in nm) for data collected at different times of the assembly process: 0 h (a); 0.17 h (b); 3 h (c); 38 h (d); 60 h (e); 74 h (f), and upon adding citrate to disassemble the nanoparticle assemblies at 74.3 h (g) and 75 h (h). (B) Plot of average Dh at different stages of assembly (a to f) and disassembly (f to h). The arrow represents the assembly and disassembly reaction progress. ([Au11nm] ) 2.4 nM; [GSH] ) 1.5 mM; [Cit] ) 10 mM). Scheme 1. A Schematic Illustration (not to scale) of the pH-Tuned Interparticle Interactions for the Assembly and Disassembly of GSH-Au Nanoparticlesa
a
The structural illustrations on the right: (A) GSH and (B) deprotonated GSH on Au nanoparticle surface.
is found to be responsible for the interparticle assembly, which is in fact in agreement with the earlier work.11 Our study showed that the disassembly could be accomplished by elevating the temperature of the solution in the presence of a high pH. The results from raising the pH of the assembled solution alone without increasing the temperature showed little or very slow reversal of the SP band. In contrast, under the pH conditions for our current study of GSH-Au nanoparticle assembly, the disassembly of the solution can be easily achieved by changing the pH or adding Cit without elevating temperature. This finding led us to believe that while some head-to-head zwitterionic interaction is possible, the hydrogen bonding of the carboxylic acid groups is
mainly responsible for the interparticle interactions in the GSH-Au nanoparticle assembly. Consider further the recent study of the adsorption of GSH on a gold surface using polarization modulation infrared reflection absorption spectroscopy and attenuated total reflection infrared spectroscopy reported by Burgi and co-workers.26a The different ionic forms (cationic or anionic) of GSH were evaluated based on the spectral evolution changes as acid and base were introduced. In a basic environment, in addition to the strong Au-S bond, one of the carboxylic acid groups (from the glycine moiety) deprotonates and forms an additional anchor to the gold surface,26 leaving the glutamate moiety in the zwitterionic form. In a slightly
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acidic environment, both of the carboxylic groups from the glycine and glutamate moieties are in protonated form, allowing hydrogen bonding among GSH molecules on the nanoparticles. Theoretically, since the COOH groups on the glutamate and glycine moieties have a pKa of 2.05 and 3.40, respectively, the addition of either sodium hydroxide or sodium citrate would increase the pH of the solution and deprotonate the acid groups. However, in the presence of gold nanoparticles, the pKa could be slightly shifted to a higher value.33 That is why a change in SP band was observed (Figure 1 and 3) when the assembly solutions were in the pH range of 4.0-4.5. When the pH was above 4.5, the assembly was hindered or not favored. On the basis of the above considerations, Scheme 1 depicts the hydrogen-bonding character of the interparticle interaction of glutathiones bound on gold nanoparticles. Both the carboxylic acid groups of GSH-capped nanoparticles can form hydrogen bonding at pH ∼4.5, the assembly is either hindered or not favored as a result of the deprotonation of the acid groups. The two surface structures of GSH were based on DFT computational data reported by Burgi and co-workers.26a While there is no direct evidence for this mechanism in our nanoparticle case, we considered it as one of the possibilities in view of the computational result. Therefore, increasing pH or adding citrate molecules changes the surface structure of GSH adsorbed on the nanoparticle surface, where the COO- group from the glycine moiety is adsorbed on the nanoparticle surface. The other possibility is that the deprotonation of both COOH groups would reduce the hydrogen-bonding formation between GSHs adsorbed on gold nanoparticles. Part of our ongoing work involves zeta potential measurement to gain an in-depth understanding. While electrostatic zwitterion interaction is possible for the adsorbed GSHs under the experimental conditions, the possibility of steric hindrance from (33) (a) Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Analyst 2000, 125, 17. (b) Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Anal. Chem. 2000, 72, 2190. (c) Luo, J.; Kariuki, N.; Han, L.; Maye, M. M.; Moussa, L. W.; Kowaleski, S. R.; Kirk, F. L.; Hepel, M.; Zhong, C. J. J. Phys. Chem. B 2002, 106, 9313.
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the glutamate moiety could have played an important role in determining the nature of the interparticle interaction. The fact that citrate addition led to a more effective disassembly than hydroxide likely reflects the participation of citrate molecules in the interfacial structure of GSH/Au particles. One possibility is that the negatively charged citrates surround the GSH monolayer via -CO2- and -NH3+ interaction, thus causing a more effective interparticle spatial and electrostatic isolation than OH- groups. A more detailed study of the structural effects on the interparticle interactions is part of our ongoing work.
Conclusions In conclusion, the hydrogen bonding between the adsorbed GSHs is believed to be responsible for the interparticle assembly of gold nanoparticles mediated by glutathione. The fact that the salt concentration range used in the experiments does not cause any aggregation of gold nanoparticles in the absence of GSH and that the assembly reactivity is dependent on pH rules out the possibility of a simple “salting-out” effect. This understanding is important for the exploitation of gold nanoparticles as nanoprobes to biological reactivities. The interparticle reactivity of glutathione with gold nanoparticles was also shown to depend on the particle size and the ionic strength of the solution. The interparticle hydrogen bonding is highly dependent on pH and additives in the solution, which determines the assembly disassembly reversibility. The new insights into the pH and chemical tunability of the interparticle hydrogen-bonding interactions of glutathiones bound on gold nanoparticles have implications to the exploitation of the glutathione-nanoparticle system as functional nanoprobes for biological detection and biosensors. Acknowledgment. This work is supported by the National Science Foundation (Grant No. CHE0349040). I-Im S. Lim acknowledges the support of the National Science Foundation Graduate Research Fellowship. LA800970P
