Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods

pubs.acs.org/JPCL

Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods in Aqueous Solution Weihai Ni,* Ricardo A. Mosquera, Jorge P erez-Juste, and Luis M. Liz-Marz an* Departamento de Química Física and Unidad Asociada CSIC, Universidade de Vigo, 36310 Vigo, Spain

ABSTRACT We report experimental and theoretical demonstration regarding the hydrogen-bonding mechanism behind the end-to-end assembly of gold nanorods modified with bifunctional linking molecules. Time-dependent assembly studies were carried out for different linking molecules at both higher and lower pH values with respect to their respective pKa values and were all found to be in agreement with a hydrogen-bonding theory. The results indicate that hydrogen bonding between protonated and unprotonated linking molecules is responsible for nanorod assembly in aqueous solution. Complementary information regarding the stability of the hydrogen-bonded configurations was obtained by density functional calculations for different protonation states. SECTION Nanoparticles and Nanostructures

G

old nanoparticles assembled into various architectures are of particular significance because the surface plasmon resonances of closely spaced nanoparticles can be coupled together. This coupling can give rise to a highly enhanced electric field at the space between the neighboring nanoparticles, and hence, a variety of interesting phenomena can occur on the nanoparticle assemblies, with potential applications as optical antennas,1 subwavelength waveguides,2 or single-molecule sensors.3 The assembly of Au nanorods has been most often accomplished using biorecognition systems,4 which involves a rather high cost. Additionally, Au nanorods have also been assembled in acetonitrile-water mixtures (4:1 v/v) using cysteine (CYS), glutathione (GSH), as well as dithiols as linking molecules.5,6 However, one must be cautious as precipitation is often observed when Au nanorods are transferred from aqueous solution to the acetonitrile-water mixture, and thus, assembling Au nanorods directly in aqueous solution would be highly preferable as no phase transfer is required. The assembly is usually assisted by specific (linking) molecules, such as CYS and GSH, carrying a thiol group and a different functional group at opposite positions,7,8 and the assembly efficiency has been found to depend on solution pH for both nanorods8 and spherical nanoparticles.9 However, the mechanism of such a pH-dependent assembly in aqueous solution is still poorly understood. We present in this Letter experiments, as well as calculations, supporting a hydrogenbonding-directed assembly mechanism. Gold nanorods were prepared in cetyltrimethylammonium bromide (CTAB) aqueous solution at pH 3.5 using the seed-mediated growth method10 (see Supporting Information (SI) for synthesis details, the extinction spectrum, and transmission electron microscopy (TEM) images). The dimensions of the nanorods measured from TEM are 76.9 ( 7.6 24.0 ( 1.9 nm, with an aspect ratio of 3.2 ( 0.5, resulting in an ensemble longitudinal plasmon resonance wavelength

r 2010 American Chemical Society

centered at 765 nm. As prepared gold nanorods were centrifuged to remove silver nitrate and excess ascorbic acid. The assembly of Au nanorods was carried out by mixing freshly prepared bifunctional molecules with Au nanorod aqueous solutions in the presence of 0.1 M CTAB. The molecules used in our experiment were 4-mercaptophenol (MPh), CYS, and GSH. The mixture solutions were placed in a Cary 5000 UV-vis-NIR spectrophotometer equipped with a thermostatted cell holder. Extinction spectra were recorded after mixing and equilibration for at least 20 min. Figure 1A shows time-dependent extinction spectra of Au nanorods, acquired at a pH of 7.37 in the presence of 0.64 mM MPh. The extinction spectra were acquired every 2 min at a constant temperature of 40 °C. Over time, the longitudinal plasmon band of the nanorods gradually decreased while a shoulder peak centered at 950 nm started to appear, increase, and broaden, indicating the formation of nanorod assemblies in solution.5-8 After a sufficient time, the peaks for both isolated and assembled Au nanorods gradually decreased, indicating the precipitation of nanorod aggregates. Timedependent extinction spectra were also acquired at several other pH values, and we show in Figure 1B-D the extinction spectra at representative pH values of 8.16, 8.59, and 8.72, respectively. It is worth mentioning that the assemblies were very stable in solution at pH = 8.72, and no precipitation was observed even after storage of the samples overnight. This suggests that, at the critical pH, equilibrium is achieved in solution, so that the assemblies are not large enough to precipitate. The extinction spectra at additional pH values ranging from 2.17 to 11.86 are shown in the SI (Figure S2). In an attempt to evaluate the efficiency of the assembly, the aggregation rate was monitored at different pH values by Received Date: February 16, 2010 Accepted Date: March 16, 2010 Published on Web Date: March 22, 2010

1181

DOI: 10.1021/jz1002154 |J. Phys. Chem. Lett. 2010, 1, 1181–1185

pubs.acs.org/JPCL

Figure 1. MPh-assisted assembly of Au nanorods. (A) Time-dependent extinction spectra acquired for the assembly of Au nanorods at pH 7.37 and 0.1 M CTAB. The concentration of MPh was 0.64 mM, and the sample was maintained in a water bath at 40 °C. The spectra were acquired every 2 min. (B-D) Extinction spectra measured at pH values of 8.16, 8.59, and 8.72, respectively. (E) Extinction changes at a wavelength of 950 nm as a function of time at the different pH values used in (A-D). (F) Representative TEM image of a Au nanorod assembly (see more images in the SI, Figure S3).

Figure 2. CYS- and GSH-assisted assembly of Au nanorods. (A) Time-dependent extinction spectra acquired for the CYS-assisted assembly of Au nanorods at a pH of 1.18. The concentration of CYS was 2 mM, and the temperature was 55 °C. The spectra were acquired every 2 min. (B-D) spectral evolution at pH values of 1.35, 1.70, and 2.01, respectively. (E) Extinction intensity variations at a wavelength of 950 nm as a function of time for the pH values shown in (A-D). (F) Extinction changes of the GSH-assisted assembly at a wavelength of 930 nm as a function of time for pH values of 1.21, 1.46, 1.73, and 2.08. The concentration of GSH was 2 mM in these experiments.

plotting the intensity of the shoulder at 950 nm as a function of time. This is not meant to claim a kinetic control of the process but rather a simpler measure than absolute intensity because of the different width of the bands for nanorod assemblies formed in different experiments. Figure 1E shows the time dependence of the extinction intensity at 950 nm for the four different pH values illustrated in Figure 1A-D. From time 0 to 6 min, no changes were observed in the extinction intensity, but then, the extinction intensity drastically increased until reaching a maximum, and finally, it gradually decreased from about 14 min. The slope was found to be sharpest at pH = 8.16 and 8.59 but significantly gentler at either higher or lower pH values. Almost no assembly was found to occur at pH values differing by more than 3 units from 8.16 (Figure S2, SI). Representative TEM images of the Au nanorod assembly are shown in Figures 1F and S3 (SI), indicating that the Au nanorods preferentially assemble in an end-to-end fashion. The assembly of Au nanorods was also investigated at different pH values using CYS and GSH as molecular linkers. The time-dependent extinction spectra for CYS-assisted assembly ([CYS] = 2 mM, T = 55 °C) at four different pH values and the corresponding kinetic traces at 950 nm are shown in Figure 2, whereas for GSH, only the kinetic traces are


displayed. Additional extinction spectra at pH values in a wider range for both CYS and GSH are shown in the SI (Figures S4 and S5). Interestingly, the slope of the kinetic traces was found to be sharpest at pH values of 1.70 and 1.73 for CYS and GSH, respectively. In order to characterize the assembly rate at different pH values, the slope of the time-dependent extinction intensity at high wavelength (950 nm, characteristic of the assembly) was evaluated. Represented by this slope, the assembly rate can be compared between different pH values. The rate of MPh, CYS, and GSH was thus determined, normalized, and plotted (Figure 3) as a function of pH. It can be observed that the rate reaches a well-defined maximum at a certain pH value for each of the three species. It can be noticed that the peak pH values (8.16 for MPh, 1.70 for CYS, and 1.73 for GSH) are very close to their respective pKa values. For the carboxylic group of CYS and GSH, the pKa has been reported to be at about 2, while the pKa of the hydroxyl group of MPh is 6.50 in water. It has also been reported that the pKa of weak acids can be strongly affected by the presence of the CTAB micellar system in solution.11 To address this issue, titration experiments were performed at various CTAB concentrations, and pKa values of MPh were measured (see SI for details) to be 6.73, 7.18, 7.55, and 7.67 at 0, 0.1, 0.3, and 0.5 M CTAB

1182


pubs.acs.org/JPCL

where Z = [HA] þ [A-] is the total amount of uncomplexed base, which is assumed to be a constant when only a small fraction of the acid is complexed, and log n = (pKa - pH). This function suggests the presence of a sharp peak located at a pH value equal to the pKa of the acid, and it drops to less than 4% of the maximum when the pH differs by 2 units from the pKa. When eq 3 is plotted using the pKa values of the three linking molecules in saturated CTAB (solid curves) and compared with the measured assembly rate (scattered symbols), as shown in Figure 3A, a good agreement is obtained for all three linker molecules, MPh, CYS, and GSH. In the equation, the pKa values were set as 8.02 and 1.80 for MPh and CYS/ GSH, respectively. The calculated peaks were normalized for comparison with the experiment. The good agreement obtained indicates that the assembly rate is closely related with the concentration of the P and U monomers and reaches a maximum at the most favorable pH for the formation of the PU complex. As evidenced by the measurements, formation of complexes for either P or U monomers with themselves is not favorable. Therefore, the assembly rate can be seen as a measure of how fast a P/U monomer can find its counterpart, where the concentration of both P and U monomers is expected to play an important role. One can imagine that, if the concentrations of P and U monomers are both high (in case pH is close to pKa), the P/U monomers are very likely to find their counterparts through random collisions, so that the assembly rate turns out to be high. On the contrary, if the concentration of either of the two monomers is low (in case pH is very different from pKa), the rate is low. In this way, the assembly rate is related to the concentration of both P and U monomers. In addition, the assembly rate also depends on temperature, the rate increasing with temperature since collisions are then more frequent. The temperature of the cell holder was set at 40 °C for MPh and 55 °C for CYS and GSH. The reason for this is that MPh possesses a higher assembly rate than CYS and GSH. By setting at a lower temperature, the assembly rate of MPh was expected to be comparable to that of CYS and GSH. The agreement between the pH dependence of the gold nanorod assembly rate and that of the PU hydrogen-bonded adduct concentration suggests that these are the most stable adducts formed by the linking molecules at the appropriate pH values. That is, when the pKa of the linking molecule is clearly acidic (like CYS), we should only observe hydrogenbonded adducts formed by one protonated monomer and a neutral one, that is, PU is representing AHAþ cations. In contrast, for linking molecules displaying higher pKa (like MPh), the most stable adduct should be composed of neutral and deprotonated monomers; therefore, PU represents AHA- anions. In order to confirm this hypothesis, we carried out B3LYP/6-31þþG(d,p) calculations with counterpoisecorrected geometry optimizations,13,14 using Gaussian-03,15 for diverse conformations of dimers of CYS and MPh, which comprise, in the case of CYS, either two protonated monomers (PP) or one protonated and one unprotonated monomer (PU), the latter displaying its zwitterionic form. For MPh, we considered dimers formed by two neutral monomers (NN) and those obtained by establishing one O-H 3 3 3 O intermolecular hydrogen bond (IHB) between the parent molecule

Figure 3. (A) Comparison between the measured assembly rate and the calculated PU complex concentration. The measured assembly rates for MPh, CYS, and GSH are shown by symbols, whereas the calculated concentrations of the PU complex at the pKa values of 1.80 and 8.02 are plotted as solid curves. (B) Most stable conformer of the PU adduct for MPh. (C) Most stable conformer of the PU adduct for protonated CYS. The green ellipses in B and C represent bond critical points associated with IHBs.

concentrations, respectively. These results indicate that the pKa of MPh is significantly increased with increasing CTAB concentration, reaching a value of 8.02 at the CTAB saturation concentration, pKam = pKaw þ ΔpKamax = 6.73 þ 1.30 = 8.02 (see the Langmuir fit in the SI, Figure S6). When the MPh molecules are bound to the tips of Au nanorods, where they are closely surrounded by CTAB molecules, the local CTAB concentration can be considered to be saturated. Therefore, instead of 6.50, the pKa of MPh will be considered to be 8.02 when MPh molecules are used as linker molecules during the assembly. Oppositely, for CYS, the measured pKa value for the carboxylic group through the titration experiments (Figure S6, SI) revealed basically no change in the presence of CTAB. The pKa values of CYS and GSH at CTAB solutions were thus assumed in the following analysis to be close to those in water. A pH-controlled hydrogen-bonding theory has been reported for the pH dependence of the degree of hydrogen bonding between a protonated acid HA (P) and its unprotonated conjugate base A- (U).12 In a solvent S, equilibria involving the dissociation of the acid Ka

HAðSÞ T A - ðSÞ þ Hþ ðSÞ

ð1Þ

and the protonated-unprotonated (PU) complexation K

HAðSÞ þ A - ðSÞ T AHA - ðSÞ

ð2Þ

are provided, where Ka is the dissociation constant of HA, and K is the association constant of the PU complex adduct AHA-. Ka and K are different from acid to acid. Equations 1 and 2 yield the pH dependence of the concentration of the PU complex adduct as12 KZ 2 n ½AHA - ¼ ð3Þ ðnþ1Þ2


1183


pubs.acs.org/JPCL

and its anion obtained by deprotonating the hydroxyl group (NA). Since we assume that the SH group is bonded to a gold nanorod surface, solvation effects were taken into account performing single-point calculations with the polarizable continuum model (PCM)16 on the counterpoise-corrected optimized structures. The corresponding dissociation energies, ΔdE, were calculated including thermal corrections at 298.15 K, obtained from the vibrational analysis carried out for the most stable structures of PP, PU, NN, and NA adducts, as well as for N, A, P and U monomers. In all cases, we have taken into account the effects of the presence of gold nanorods on the conformational preferences of monomers and complexes through restriction of the arrangements to those where SH units are not involved in intramolecular hydrogen bonds as SH groups should be bonded to a gold nanorod surface. We observe that the most stable conformation of PP, where an eight-membered ring involving two O-H 3 3 3 O IHBs is present (see Figures S7 and S8 in the SI), dissociates spontaneously into its monomers both in the gas phase (ΔdE = -140 kJ mol-1) and in aqueous solution (ΔdE = -12 kJ mol-1). In contrast, the most stable conformer of PU, showing nearly coplanar N-H 3 3 3 O and C-H 3 3 3 O IHBs (Figure 3), displays positive values for ΔdE in gas (94 kJ mol-1) and in aqueous solution (13 kJ mol-1). It should be noticed that less stable conformers of PU, like that containing C-H 3 3 3 S, O-H 3 3 3 O, and two C-H 3 3 3 O IHBs (see Figure S8, SI), are stable in the gas phase (ΔdE = 73 kJ mol-1) but not in aqueous solution (ΔdE = -6 kJ mol-1). Our calculations allowed us also to conclude that PP conformers displaying nearly parallel arrangements of S-H bonds are only 1.5 kJ mol-1 higher in energy than their most stable conformer. Finally, we carried out an electron density analysis with the quantum theory of atoms in molecules (QTAIM).17-20 According to QTAIM, a critical point for the electron density presenting two negative and one positive eigenvalues for the corresponding Hessian matrix, which is called a bond critical point (BCP), should be found roughly between IHB donor and acceptor atoms. QTAIM additionally relates the electron density at the BCP, Fb, which usually has values below 0.03 au,21,22 to the strength of the corresponding IHB (the larger the Fb, the stronger the IHB).18 It has also been observed that IHBs display small positive values for the total energy density function,23 Hb, and for the Laplacian of the electron density, r2Fb.21,22 Our analysis reveals the presence of BCPs for all of the above-indicated IHBs, whose properties (Figure S9, SI) verify all of the criteria usually associated with hydrogen bonds in this theory.21,22 Moreover, usual geometry criteria for IHB24,25 are also verified. Thus, H 3 3 3 O distances and X-H 3 3 3 O angles in the most stable conformer of PU are, respectively, 1.495 Å and 177.0° (in N-H 3 3 3 O) and 2.407 Å and 135.6° (in C-H 3 3 3 O). Both geometry parameters and QTAIM properties indicate that the N-H 3 3 3 O IHB in PU is stronger than the O-H 3 3 3 O ones observed in PP (Figure S9, SI); for example, Fb is 0.035 au in PP and 0.079 au in PU. The most stable NA adduct for MPh (mainly formed when the pH approaches its pKa) displays, according to the QTAIM molecular graph (Figure S9, SI), one bifurcated IHB, where the oxygen of the anion is simultaneously involved in a very


strong O-H 3 3 3 O IHB and in a weak C-H 3 3 3 O one, both of them comprising the neutral monomer. Our DFT calculations indicate that the significant NA adduct stability in the gas phase (ΔdE = 48 kJ mol-1) is substantially reduced in solution (ΔdE = 4 kJ mol-1), whereas the adduct formed at acidic pH values, NN, (comprising two neutral MPh monomers) displays lower stabilities both in gas (ΔdE = 12 kJ mol-1) and solution (ΔdE = 1 kJ mol-1) phases. In this case, we observe a unique intermolecular bond path (O-H 3 3 3 O), involving an interaction that is significantly weaker than that displayed by NA, according to both QTAIM and geometry criteria. Thus, Fb is 0.104 au in the IHB of NA and 0.024 au in the corresponding IHB of NN, and the H 3 3 3 O internuclear distance is accordingly much shorter in the former (1.387 versus 1.966 Å). We also notice that the O-H donor bond lengthens as the IHB gets stronger (0.966 Å in the isolated monomer, 0.973 Å in NN, and 1.082 Å in NA). In summary, we present experiments, as well as calculations, that support a hydrogen-bonding-directed assembly mechanism of gold nanorods in the presence of bifunctional linking molecules. The experiments indicate that the assembly rate is peaked at pH values close to the pKa of the linking molecule. A pH-controlled hydrogen-bonding theory was found to properly explain the experimental observations. Furthermore, DFT calculations show that hydrogen bonding is preferentially formed between monomers bearing a different number of protons.

SUPPORTING INFORMATION AVAILABLE Experimental details, Au nanorod extinction spectra and TEM image, titration experiment, and DFT data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: niweihai@ uvigo.es (W.H.N.); [email protected] (L.M.L.-M.).

ACKNOWLEDGMENT Patricia Taladriz is acknowledged for contributing to the titration experiments. This work has been funded by the Spanish Ministerio de Ciencia e Innovaci on (MAT2007-62696) and by the EU (NANODIRECT, Grant Number CP-FP 213948-2). We thank CESGA for access to its computational resources.

REFERENCES (1) (2)

(3)

(4)

1184

M€ uhlschelgel, P.; Eisler, H.-J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W. Resonant Optical Antennas. Science 2005, 308, 1607–1609. Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E. B.; Koel, E.; Requicha, A. G. Local Detection of Electromagnetic Energy Transport below the Diffraction Limit in Metal Nanoparticle Plasmon Waveguides. Nat. Mater. 2003, 2, 229–232. Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102–1106. Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. Preferential End-to-End Assembly of Gold Nanorods by Biotin-Streptavidin Connectors. J. Am. Chem. Soc. 2003, 125, 13914–13915.


pubs.acs.org/JPCL

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12) (13)

(14)

(15)

(16)

(17) (18) (19) (20) (21)

(22)

(23)

(24) (25)

Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. Uniaxial Plasmon Coupling through Longitudinal SelfAssembly of Gold Nanorods. J. Phys. Chem. B 2004, 108, 13066–13068. Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. Selective Detection of Cysteine and Glutathione Using Gold Nanorods. J. Am. Chem. Soc. 2005, 127, 6516–6517. Zhang, S. Z.; Kou, X. S.; Yang, Z.; Shi, Q. H.; Stucky, G. D.; Sun, L. D.; Wang, J. F.; Yan, C. H. Nanonecklaces Assembled from Gold Rods, Spheres, And Bipyramids. Chem. Commun. 2007, 1816–1818. Sun, Z. H.; Ni, W. H.; Yang, Z.; Kou, X. S.; Li, L.; Wang, J. F. pH-Controlled Reversible Assembly and Disassembly of Gold Nanorods. Small 2008, 4, 1287–1292. Lim, I-I. S.; Mott, D.; Ip, W.; Njoki, P. N.; Pan, Y.; Zhou, S.; Zhong, C. J. Interparticle Interactions in Glutathione Mediated Assembly of Gold Nanoparticles. Langmuir 2008, 24, 8857–8863. Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of Silver Nanorods and Nanowires of Controllable Aspect Ratio. Chem. Commun. 2001, 617–618. Gerakis, A. M.; Koupparis, M. A.; Efstathiou, C. E. Micellar Acid-Base Potentiometric Titrations of Weak Acidic and/or Insoluble Drugs. J. Pharm. Biomed. Anal. 1993, 11, 33–41. Wood, J. L. pH-Controlled Hydrogen-Bonding. Biochem. J. 1974, 143, 775–777. Simon, S.; Duran, M.; Dannenberg, J. J. How Does Basis Set Superposition Error Change the Potential Surfaces for Hydrogen-Bonded Dimers?. J. Chem. Phys. 1996, 105, 11024–11031. Boys, S. F.; Bernardi, F. Calculation of Small Molecular Interactions by Differences of Separate Total Energies ; Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553– 556. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; et al. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. Tomasi, J.; Mennuci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999– 3093. Bader, R. F. W. A Quantum Theory of Molecular Structure and Its Applications. Chem. Rev. 1991, 91, 893–898. Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University Press: New York, 1990. Bader, R. F. W. The Quantum Mechanical Basis of Conceptual Chemistry. Monatsch. Chem. 2005, 136, 819–854. Popelier, P. L. A. Atoms in Molecules: An Introduction; Prentice Hall: New York, 1999. Koch, U.; Popelier, P. L. A. Characterization of C-H-O Hydrogen Bonds on the Basis of the Charge Density. J. Phys. Chem. 1995, 99, 9747–9754. Caroll, M. T.; Bader, R. F. W. An Analysis of the HydrogenBond in Base-HF Complexes Using the Theory of Atoms in Molecules. Mol. Phys. 1988, 65, 695–722. Cremer, D.; Kraka, E. Chemical Bonds without Bonding Electron Density ; Does the Difference Electron-Density Analysis Suffice for a Description of the Chemical Bond?. Angew. Chem., Int. Ed. Engl. 1984, 23, 627–628. Grabowski, S., Ed. Hydrogen Bonding: New Insights; SpringerVerlag: New York, 2006. Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997.


1185


Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods

Recommend Documents