Letter pubs.acs.org/NanoLett
Removal of Molecular Adsorbates on Gold Nanoparticles Using Sodium Borohydride in Water Siyam M. Ansar,† Fathima S. Ameer,† Wenfang Hu,‡ Shengli Zou,‡ Charles U. Pittman, Jr.,† and Dongmao Zhang*,† †
Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States
‡
S Supporting Information *
ABSTRACT: The mechanism of sodium borohydride removal of organothiols from gold nanoparticles (AuNPs) was studied using an experimental investigation and computational modeling. Organothiols and other AuNP surface adsorbates such as thiophene, adenine, rhodamine, small anions (Br− and I−), and a polymer (PVP, poly(N-vinylpyrrolidone)) can all be rapidly and completely removed from the AuNP surfaces. A computational study showed that hydride derived from sodium borohydride has a higher binding affinity to AuNPs than organothiols. Thus, it can displace organothiols and all the other adsorbates tested from AuNPs. Sodium borohydride may be used as a hazard-free, general-purpose detergent that should find utility in a variety of AuNP applications including catalysis, biosensing, surface enhanced Raman spectroscopy, and AuNP recycle and reuse. KEYWORDS: Organothiols, hydride, gold nanoparticles, desorption, sodium borohydride
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desorption predominantly through OT displacement by hydride. Four model OTs were used including p-methylbenzenethiol (p-MBT), 2-naphthalenethiol (2-NT), p-benzenedithiol (pBDT), and homocysteine (Hcy) (Figure 1). These encompass
elf-assembly of organothiols (OTs) and thiolated biomolecules has been used extensively for AuNP surface modification to improve functionality, biocompatibilities, and target specificities.1−4 On the other hand, undesirable organothiol adsorption can be detrimental to catalytic AuNP applications.5−8 Unfortunately, there is no simple technique capable of removing OTs from AuNP surfaces. This limits the Au recycle and reuse, and also hinders our quantitative understanding of OT interactions with AuNPs. Current OT removal methods are destructive. For example, S−Au bonding is cleaved by either cyanide digestion of AuNPs or oxidizing OTs into sulfonates or sulfoxides.9−12 Recently, Yuan et al. reported that OTs on a planar gold film can be desorbed using NaBH4 in water,13 while Scott et al. showed that the thiolate in monolayer-protected gold clusters (MPC-Au) can be desorbed with a large excess of NaBH4 in tetrahydrofuran.14 This reaction may provide an environmentfriendly approach for OT removal from gold surfaces, which can be enormously important to a wide range of OT selfassembly applications. However, the mechanism of this critically important reaction has not been experimentally examined. Neither has OT desorption kinetics nor the fate of the desorbed OT. Furthermore, the applicability of this OT desorption method to AuNPs is unknown. We report herein that OTs on the AuNPs can be rapidly and completely removed by NaBH4 treatment. Our combined experimental and computational modeling studies reveal that NaBH4 induces OT © 2013 American Chemical Society
Figure 1. Molecular structure of the model OTs.
monothiols (p-MBT, 2-NT, Hcy), a dithiol (p-BDT), aromatic thiols (p-MBT, p-BDT, 2-NT), and an aliphatic thiol (Hcy). To test the generality of this ligand desorption method, common AuNP adsorbates were employed in this study including rhodamine 6G, adenine, organosulfur compound thiophene, small anions I− and Br−, and polymer poly(N-vinylpyrrolidone) (PVP). AuNPs with nominal diameters of 10, 30, and 90 nm were used in this study (Supporting Information, Figure S1). The inclusion of structurally diverse OTs and a wide range of Received: December 20, 2012 Revised: January 28, 2013 Published: February 6, 2013 1226
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AuNP sizes allows us to critically evaluate the general applicability of this OT desorption method. Kinetics studies of the OT desorption were conducted by time-resolved UV−vis measurements of aromatic OTs adsorbed onto 10 nm AuNPs (Supporting Information, Figure 2). Several observations are noteworthy. (1) OTs can be
concentration. (5) The rate of the OT readsorption is drastically enhanced by completely depleting BH4− through addition of HCl. The desorption and readsorption rates of p-BDT are very similar to those for p-MBT and 2-NT (Figure 2), suggesting that the thiol structure has no significant effect on this thiol desorption method. Hcy desorption from AuNPs was confirmed with a NaBH4 washing experiment (Supporting Information, Figure 3), which shows the Hcy SERS signal can
Figure 3. SERS spectra of the Hcy adsorbed on AuNPs (a) before and (b) after NaBH4 washing. (c) The spectrum obtained by spiking 2-NT into Hcy-containing AuNPs that had been washed with NaBH4 and (d) the spectrum obtained after washing the sample (c) with aqueous NaBH4.
be completely removed by washing the Hcy-coated AuNPs with a NaBH4 aqueous solution. These cleaned AuNPs can be reused for further SERS detection (Figure 3). The disappearance of both the Hcy and 2-NT SERS features after NaBH4 washing is particularly noteworthy, as it critically attested to the effectiveness of this OT removal method. If there were any remaining Hcy and 2-NT adsorbed molecules in the NaBH4treated AuNP aggregations, they would most likely be located in the AuNP junctions. This is the area where mass transfer is the most difficult and known for its ultrahigh SERS activity.15−17 The absence of the Hcy and 2-NT SERS features indicates that all the OTs on these AuNPs, including those located in the AuNP junctions, are removed by NaBH4 washing. SERS measurements also showed that 2-NT adsorbed on AuNPs with diameters of 10, 30, and 90 nm can also be completely removed by NaBH4 washing (Supporting Information, Figure S2). This indicates that this ligand desorption method is applicable to the AuNPs of different sizes. Exposure of the OT-coated AuNPs to sodium borate or hydrogen (the products of NaBH4 with H2O) does not induce OT desorption (data not shown). These experiments indicate that the molecular species responsible for the OT desorption is either hydride or another reactive intermediate derived from NaBH4. Computational modeling (Figure 4) indicates that hydride has, by far, the highest binding energy with the AuNPs, among all the main NaBH4 derivatives. Moreover, hydride’s binding affinity to AuNPs is 48 kcal/mol higher than that of the thiolate form of p-MBT and 102 kcal/mol greater than that for the thiol, p-MBT, form on AuNPs. This theoretical prediction is consistent with the fact that borohydride can readily remove all the molecular adsorbates tested on the AuNPs. These adsorbates include rhodamine 6G, thiophene, adenine, small anions (I− and Br−), and polymer (PVP) (Supporting Information, Figures S3−S5). This hydride displacement mechanism is also consistent with the exper-
Figure 2. OT desorption and readsorption after the addition of sodium borohydride in water. (A) Percentage of p-MBT in the supernatant as a function of time after the addition of NaBH4 into the AuNP aggregates containing adsorbed p-MBT. After mixing a known amount of p-MBT with sufficiently concentrated AuNPs to allow complete p-MBT adsorption onto AuNPs, these AuNPs were left sitting overnight to allow the p-MBT-coated AuNPs to aggregate and settle (inset I). Complete p-MBT adsorption from solution is signified by the absence of p-MBT UV−vis absorbance in the 270 nm region in the supernatant of the UV−vis cuvette (bottom spectra in inset II). Immediate p-MBT desorption is observed upon NaBH4 addition, as shown by the increase in the UV−vis absorbance (λmax = 264 nm) (inset II). p-MBT is completely readsorbed onto the AuNPs with prolonged sample incubation (inset III). The cycle of p-MBT desorption and readsorption process can be repeated by adding more NaBH4 into the p-MBT readsorbed AuNP solution. (B) NaBH4 concentration dependence of the p-MBT desorption and readsorption kinetics. The inset in part B zooms in to show the time delay between OT desorption and onset of detectable OT readsorption in the first 200 min. (C) The addition of HCl, which rapidly destroys all NaBH4 and drops the pH, drastically enhances the rate of the p-MBT readsorption. (D) Comparison of the desorption and readsorption kinetics of p-BDT, 2-NT, and p-MBT. The concentrations of NaBH4 are the same for these samples.
completely desorbed from AuNPs using NaBH4 at room temperature. (2) The rate of the OT desorption increases with increasing NaBH4 concentration, and complete OT desorption occurs within 10 min when the concentration of NaBH4 is higher than 25 mM. (3) The desorbed OTs can be completely readsorbed onto AuNPs upon prolonged sample incubation. This occurs because NaBH4 eventually decomposes in the aqueous medium. (4) The time delay between complete OT desorption and the onset of the detectable OT readsorption increases with increasing NaBH4 concentration. Also, the average rate of readsorption decreases with increasing NaBH4 1227
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However, after depletion of hydride or borohydride with prolonged sample incubation (which can be several hours after complete OT readsorption) or by HCl addition, the transient OT SERS spectrum became identical to that of OT adsorbed onto the AuNP without NaBH4 treatment. Importantly, the transient spectrum is the same when the OT adsorbed onto the AuNPs was treated by NaBH4 in H2O or NaBD4 in D2O (Figure 5 and Supporting Information, Figure S6), indicating that there is no direct hydride or deutride interaction with OT adsorbed onto the AuNPs. Otherwise, the transient SERS spectra obtained with the NaBH4- and NaBD4-treated samples would certainly be different due to the strong isotope effect of hydrogen/deuterium in vibrational spectroscopy.18,19 Definitive determination of the OT structures coadsorbed with hydride (or borohydride) is not presently possible. The most plausible explanation for the OT spectral change induced by NaBH4 and NaBD4 treatment is the indirect transfer of charge from hydride to the coadsorbed OT via AuNPs. Previous studies showed that OTs are adsorbed as the thiolate form onto AuNPs in water,20,21 and negative charge is spread over the conducting gold surface region associated with surface bonding of the OT thiolate to the AuNPs.22 This charge on gold, transferred during the OT adsorption, will be reversed during the OT desorption process. The fact that one can detect the transient OT SERS spectra throughout the entire OT desorption and readsorption process indicates that the charge transfer alone does not lead to OT desorption. Otherwise, the lifetime of the transient SERS spectra would not be as long as what has been observed in this work. Given the small hydride footprint (the smallest anion among all the chemical species), it is entirely possible that a significant amount of hydride is coadsorbed with OT onto the AuNPs without inducing OT desorption. This explains why a large excess of NaBH4 is required in order to induce detectable desorption of OT or other surface adsorbates from AuNPs (Figure 2). In summary, this mechanistic study reveals that hydride has a higher binding affinity to gold than OTs. NaBH4 can be used as a hazard-free, general-purpose “superdetergent” for removal of surface adsorbates including OTs, thiophene, PVP, and other molecular adsorbates from AuNPs. This finding should be highly significant for essentially all types of AuNP research and applications, including catalysis, biosensing, surface enhanced spectroscopy, drug delivery, and AuNP recycle and reuse.
Figure 4. Density functional theory calculations of the AuNP binding energies for the possible p-MBT and BH4− derivatives on AuNPs (the bp86 functional with the 6-311G(d) basis set was used for main group elements and the LANL2DZ effective core potential for a 20 atom Au cluster). Binding energies were calculated for molecules attached to the Au cluster at the vertex, edge side, or surface. The binding energies listed here are for indicated species attached to the vertex of the Au cluster.
imental observation that the desorbed OT can be readsorbed onto the AuNPs, and the readsorption rate can be drastically enhanced by adding HCl to scavenge the surface-bound hydride and destroy remaining NaBH4. The fact that desorbed OT can be readsorbed onto AuNPs before complete depletion of the hydride indicates that either hydride or borohydride and the OT can be coadsorbed onto the AuNPs. PVP desorption from AuNPs was achieved with a low concentration of NaBH4 (∼1 mM) in which the AuNPs remain totally stable (no aggregation) (Supporting Information, Figure S5). Like OTs, the desorbed PVP was readsorbed onto AuNPs upon prolonged sample incubation. Another notable finding is that the structure of the OT coadsorbed onto AuNPs with hydride or another reactive borohydride species is different from the structure of the OT that is adsorbed alone. Time-resolved SERS measurements showed that a transient OT SERS spectrum is associated with the NaBH4-treated AuNPs containing adsorbed OT, and the lifetime of this transient SERS spectrum is the same as that of the hydride in water. This OT transient SERS spectrum appears immediately upon addition of NaBH4 into AuNPs containing adsorbed OT (Figure 5 and Supporting Information, Figure S6). This transient spectrum changes only in its peak intensity but not in the peak positions during the entire OT desorption and readsorption process (Supporting Information, Figure S7).
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, UV−vis spectra and TEM images for AuNPs, desorption of 2-NT from AuNPs (30 and 90 nm), desorption of rhodamine 6G, thiophene, adenine, small anions (Br− and I−), and polymer (PVP) from AuNPs, comparison of the SERS spectra of p-MBT after NaBD4 addition, and timeresolved SERS spectra of OT/AuNPs after NaBH4 addition. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Figure 5. Comparison of the SERS spectra of OT adsorbed onto AuNPs which were treated by (c) NaBH4 in water and (d) NaBD4 in D2O. Spectra a are the SERS spectra of the OT before NaBH4 or NaBD4 addition, and spectra b are those after NaBH4 or NaBD4 depletion. The OTs in plots A and B are p-BDT and 2-NT, respectively.
*E-mail:
[email protected],
[email protected]. edu. Notes
The authors declare no competing financial interest. 1228
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Letter
ACKNOWLEDGMENTS This work was supported by an NSF CAREER Award (CHE 1151057) and NSF funds from a grant (EPS-0903787) provided to D.Z. S.Z is grateful for the support of this research from ONR, NSF, and ACS/PRF.
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