Polymer Membrane Stabilized Gold Nanostructures Modified

Nov 19, 2008 - Centre for Photoelectrochemistry, School of Chemistry, Madurai Kamaraj UniVersity, Madurai-625 021, India. ReceiVed: May 15, 2008; ...
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J. Phys. Chem. C 2008, 112, 19825–19830

19825

Polymer Membrane Stabilized Gold Nanostructures Modified Electrode and Its Application in Nitric Oxide Detection Subramani Thangavel and Ramasamy Ramaraj* Centre for Photoelectrochemistry, School of Chemistry, Madurai Kamaraj UniVersity, Madurai-625 021, India ReceiVed: May 15, 2008; ReVised Manuscript ReceiVed: September 20, 2008

Nafion polymer/gold nanostructures film that can be utilized as a proficient electrochemical sensor for nitric oxide was prepared by an electrochemical process, forming the gold nanostructures (Aunano) through infiltration into a Nafion (Nf) matrix preassembled on an electrode. The formation of gold nanostructures was monitored by the in situ spectroelectrochemical method. The in situ absorption spectra of Aunano showed systematic and uniform formation of gold nanostructures at the Nafion (Nf-Aunano)-modified electrode. The electrochemically formed Nf-Aunano was characterized by UV-visible spectroscopy, X-ray diffraction, scanning electron microscope, transmission electron microscope, and electrochemical techniques. The surface plasmon absorption spectra recorded for the wet and dry Nf-Aunano composite film showed the interaction between the gold nanostructures and the swelled polymer matrix. The longitudinal surface plasmon band and the TEM images observed for the Nf-Aunano showed the formation of nanorod-like and Y-shaped gold nanostructures in the Nafion matrix. In addition to the nanoparticles, the edge-to-edge interactions lead to the formation of 1D assembly. The electrical communication between the gold nanostructures embedded in the Nafion film improved the electrocatalytic properties of the modified electrode toward NO detection. The Nf-Aunano electrode showed excellent sensitivity for NO detection with the experimental detection limit of 1 nM. The present Nf-Aunano electrode is very simple to fabricate and is stable, sensitive, and reproducible. 1. Introduction Nanostructured materials show many aspects of interesting characteristics, i.e., optical,1-4 electronic,5,6 and catalytic,7-9,17 that greatly depend on the size and shape of nanoparticles as an effect of quantum confinement of electrons.10,11 All these characteristics have turned out to be a driving force for exploring many opportunities to synthesize materials of different characteristics of interest that range from metal nanoparticles to nano-semiconductors5,10,12 to organic nanostrutures13 and for exploiting their unusual properties for use as promising functional materials in nanoelectronics,5,14 photonics,15 nano-bioelectronics,16 catalysis, and sensors.17 Gold nanoparticles are among the most widely studied and have become promising candidates in these areas.8,17 The synthesis of nanostructures via reduction of the precursor metal salts using reducing and stabilizing reagents is achieved. However, achieving control over the growth of nanostructures leading to proper dimensional confinement and the promising applications of nanostructures in electrochemical sensors is a challenging task.18,19 In the field of catalytic applications, the Campbell18 and Chen and Goodman19 studies mark an important step toward the discovery of the active site in the gold nanoparticles. Nafion consists of a hydrophobic poly(tetrafluoroethylene) component with regularly spaced short perfluorovinyl ether side chains, each terminated with a highly hydrophilic sulfonate (-SO3-) group.20-26 Nafion, the ionomer membrane, is a well-known solid proton-conducting electrolyte in electrochemical technology.20-23 Nafion membrane contains a heterogeneous fluorocarbon hydrophobic phase, interconnected -SO3- ionic clusters, and an interfacial region formed between these two. * Corresponding author. Phone: +91-452-2459084. E-mail: ramarajr@ yahoo.com.

Control over the nanoparticle size, size distribution, and metal concentration in polymer membrane is a challenging task during the electrochemical formation of metal nanostructures. In this regard, the poly(perfluorosulfonic) acid membrane (Nafion-117) has received much attention as a host for a variety of metal and metal sulfide and metal oxide nanoparticles due to the following: (i) its superior chemical stability, which prevents the agglomeration and corrosion of the nanoparticles, (ii) its high optical quality, (iii) its ease in the loading of a variety of metal ions via the ion-exchange mechanism and the subsequent formation of metal nanoparticles in the membrane, and (iv) its ease of handling and for catalytic purposes.27,28 Nitric oxide (NO) has been shown to be involved in regulating neuronal excitability, synaptic transmission, functioning of neuronal networks, arousal and learning and memory mechanisms.29,30 The accurate measurement of NO is very important to unravel the action of this key compound. The polyelectrolyte-gold nanoparticles hybrid films were reported to be an electrochemical sensor for nitric oxide.31-33 The main aim of the present investigation is to improve the sensitivity of the electrochemical detection of NO by employing a simple and different modification procedure to prepare Nafion-nanostructure gold-modified electrodes. We report a simple and facile method for the preparation of nanostructured gold particles using Nafion matrix by the electrochemical method and its specific application toward the detection of NO up to 1 nM. The preparation of gold nanostructures-embedded Nafion-modified electrode was found to be very simple and direct detection of NO was observed at this electrode. 2. Experimental Section Nafion perfluorinated ion-exchange resin (equiv wt 1100) 5 wt % solution in lower aliphatic alcohols/H2O mixture (Aldrich)

10.1021/jp804310u CCC: $40.75  2008 American Chemical Society Published on Web 11/19/2008

19826 J. Phys. Chem. C, Vol. 112, No. 50, 2008 and chloroauric acid were used as received. All other chemicals were of analytical grade and were used as received. Doubly distilled water was used throughout the experiments. Indium tin oxide (ITO) electrode was fabricated using a commercially available ITO plate (CG-41IN-1507, Delta Technologies Ltd., Stillwater, MN) and used for monitoring in situ spectral changes. The glassy carbon (GC) electrode (3 mm diameter, CH Instruments, Austin, TX) was used for the electrochemical studies. The in situ spectroelectrochemical experiments were performed with the potentiostat/galvanostat model 283A from Princeton Applied Research controlled by the Echem M270 software and the Agilent 8453 diode array spectrophotometer. The 5 wt % Nafion solution in lower aliphatic alcohols/H2O was diluted to 1 wt % with ethanol and the desired amount of Nafion was casted on the cleaned ITO (ITO/Nf) or GC (GC/Nf) electrode surface, dried at room temperature for 10 min, and dipped in doubly distilled water for 10 min. Electrochemical experiments were performed using a conventional three-electrode two-compartment cell. An electrochemical cell consisting of an ITO/Nf or GC/Nf electrode as the working electrode, Pt wire as the counter electrode, and a reference electrode were used for electrochemical studies. Gold nanostructures were electrodeposited at the ITO/Nf and GC/Nf electrodes by applying -0.2 V(Ag/AgCl) for 200 s using an electrolyte solution containing 0.1 wt % of HAuCl4 in 0.5 M H2SO4. All the other electrochemical studies were performed using a saturated calomel electrode (SCE) and all potential values are reported against SCE. For in situ monitoring of the formation of gold nanostructures, we used a quartz spectroelectrochemical cell with 1 cm path length. Nitrogen gas was used for deaerating the experimental solutions and all the measurements were conducted at room temperature (25 °C). The transmission electron microscope (TEM) images for the Nafion-gold nanostructures film prepared from the dissolved Nf-Aunano in ethanol were recorded using a JEOL 3010 instrument with a lattice resolution of 0.14 nm and a point to point resolution of 0.12 nm. The scanning electron microscope (SEM) images were recorded for the gold nanostructures electrodeposited Nafion-modified ITO electrode film using an Hitachi SEM (Model S-3400). The X-ray diffraction (XRD) analysis of the resulting product was carried out using a PANalytical X’per PRO model diffractometer with Cu KR (2.2 kW Max.) source. 3. Results and Discussion 3.1. Preparation and in Situ Absorption Spectral and Surface Morphological Characterization of Nafion-Au Nanostructures. Nanostructured gold particles were deposited by the electrochemical method at the Nafion-coated ITO electrode. When HAuCl4 was electrochemically reduced at appropriate applied potential, the growth of the nanostructured Au nanoparticles at the Nafion film modified electrode was observed. The growth of nanostructured gold particles is directed by the microheterogeneous structure of the interconnected sulfonated (-SO3-) ionic cluster domains of the swelled Nafion film at the electrode. For the preparation of gold nanostructures (Aunano)-modified electrode, the Nafion-coated ITO electrode (ITO/Nf) was dipped in a solution containing deaerated 0.1% HAuCl4 and 0.5 M H2SO4 and an applied potential of -0.2 V(Ag/AgCl) was applied for 200 s. The in situ formation of Aunano at the ITO/Nf electrode was monitored by recording the in situ absorption spectral changes with respect to applied charge at a time interval of 10 s. The 3D in situ spectroelectrochemical

Thangavel and Ramaraj

Figure 1. Three-dimensional in situ surface plasmon absorption spectra observed during the formation of gold nanostructures at the Nafionmodified ITO electrode in 0.5 M H2SO4 containing 0.1% HAuCl4 at an applied potential of -0.2 V for 200 s. The spectra were recorded at a time interval of 10 s.

Figure 2. Surface plasmon absorbance spectra of electrochemically formed gold nanostructures at the Nafion film-coated ITO electrode (ITO/Nf-Aunano) in wet (a) and dry (b) conditions. Spectra of Nf-Aunano in ethanol solution (after the Nf/Aunano film dissolved in ethanol) (c).

graph showing the growth of the surface plasmon band (SPB) due to the formation and distribution of gold nanostructures at the Nafion-modified ITO electrode is shown in Figure 1. The absorption spectra recorded at 10 s showed a small absorption with a broadband and the absorbance increased with increasing applied charge and time (Figure 1). The spectrum features the typical surface plasmon absorption maximum around 606 nm due to the changes in the dielectric environment since the Aunano are formed at the solvent-swollen Nafion-matrix-coated electrode. The increase in the electrodeposition charge at the modified electrode lead to a continuous increase in the intensity of the SPB with increased time due to the growth of gold nanostructures at the Nafion-modified electrode (Figure 1). The increase in the intensity of the SPB shows that the primarily formed tiny gold nanostructures grow into larger nanostructures and the gold nanostructures are well-separated and stabilized by the Nafion matrix. The so-grown gold nanostructures were merely separated by a small distance and these isolated nanostructures showed the SPB at comparatively lower wavelength than the initial band since the SPB of metal nanoparticles were directly affected by the surroundings and the size of the nanostructures.17

Polymer Membrane Stabilized Gold Nanostructures

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19827

Figure 3. SEM image of ITO/Nf-Aunano electrode surface with applied charge of 37.3 mC. Figure 6. Plot of cathodic peak potential (Epc) variation against pH of the electrolyte solution for GC/Nf-Aunano-modified electrode. (Data taken from Figure S5.)

Figure 7. Cyclic voltammograms observed at GC/Nf-Aunano electrode during successive addition of 50 µM NO2- in pH ) 2.5 (0.1 M PBS). Scan rate ) 50 mV s-1. Figure 4. TEM image of dissolved Nf-Aunano.

Figure 5. Cyclic voltammograms recorded at GC (a), GC/Nf (b), and GC/Nf/Aunano (c) for 1 × 10-3 M K3[Fe(CN)6] in 0.1 M KCl solution at a scan rate of 50 mV s-1.

Figure 8. (A) Linear sweep voltammograms observed at gold-loaded Nafion-modified GC electrode during the successive addition of 50 µM NO2- in pH ) 2.5 (0.1 M PBS). (B) Calibration curve (inset) obtained during the addition of 50 µM NO2- in pH ) 2.5 (0.1 M PBS).

The electrochemical formation of gold nanostructures at the Nafion matrix mainly depends on the number of interconnected ion-conductive -SO3- cluster domains in the Nafion film.34 One can easily control the amount of gold reduced at each stage during the electrochemical reduction by controlling the applied charge at the modified electrode, Nafion film thickness, and

concentration of HAuCl4. The intensity of the nanogold SPB increased linearly with time and applied charge (Figure S1) and reached saturation and further shifts in the SPB were not noticed at the Nafion-modified electrode. After the completion of gold nanostructures formation at the Nafion-modified electrode (represented as ITO/Nf-Aunano), the

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Figure 9. Chronoamperometric curves observed at GC/Nf-Aunano electrode 0.5 mM (A) and 1 nM (B) NO2- with 45 s interval at an applied potential of 0.9 V in 0.1 M PBS (pH 2.5).

modified electrode was removed from the cell solution and the surface plasmon absorption spectra were recorded in wet and dry conditions (Figure 2) after careful washing of the electrode with doubly distilled water. The absorption spectra recorded for wet Nf-Aunano film (Figure 2a) and dry Nf-Aunano film (Figure 2b) are shown in Figure 2. When the Nf-Aunanomodified electrode was washed with water, the gold nanostructures experienced the dielectric environment provided by the water-swollen Nafion film and showed the SPB at 561 nm. When the Nf-Aunano-modified electrode was allowed to dry for 30 min at room temperature in air, the spectra showed the SPB at 580 nm (Figure 2b) with a red shift of 19 nm when compared to the wet Nf-Aunano film (Figure 2a). This observation reveals that the size of the interconnected ionic clusters decreased in the air-dried Nafion film due to dehydration and this brought about a change in the dielectric environment. It confirms that the surface plasmon resonance behavior of the gold nanostructures-loaded Nafion film is influenced by the presence of electrolyte ions/solvent in the swelled Nafion film coupled with the dielectric medium and the amount of hydration that exists in the Nafion film. It has been reported that the SPB is shifted to higher wavelength as the distance between the nanoparticles is reduced and this shift is understood as an exponential function of the gap between the two particles.17,35 It is clear that the swelling of the Nafion polymer film in electrolyte solution will bring about expansion of -SO3- ionic clusters (∼40 Å diameter) interconnected by channels (∼10 Å diameter)22 and this will

Thangavel and Ramaraj

Figure 10. Calibration plots of amperometric current against different concentration ranges of NO2- (data from Figure 9).

bring about the interaction of embedded gold nanostructures with each other. The Nf-Aunano film formed on the ITO electrode was completely dissolved in ethanol by dipping the electrode for 15 min followed by stirring. The homogeneously dispersed Nf-Aunano sol in ethanol showed pink color. The SP absorption spectrum recorded for Nafion-stabilized Aunano sol showed an intense band at 528 nm along with a low-intensity broadband at 700 nm (Figure 2c). It is clear that the electrochemical deposition of Aunano in Nafion led to the formation of not only spherical particles but also some nanorod-like structures which showed the longitudinal SP absorption at 700 nm. The edgeto-edge interaction of gold nanostructures would lead to the formation of 1D nanostructure assemblies (vide infra (Figures 4 and S3)). The amount of electrochemically formed nanogold at the Nafion film can easily be calculated quantitatively from the amount of charge consumed during the Aunano formation. The scanning electron micrograph (SEM) images recorded for the Nafion and gold nanostructure deposited with different applied charges at the Nafion-modified ITO electrode are shown in Figures 3 and S2. The SEM images provide more detailed information about the formation and distribution of Aunano at the Nf-coated ITO electrode (ITO/Nf-Aunano) and the surface morphology and homogeneity of the particles. The SEM images were recorded for the electrodeposited Au nanostructures at different applied charges of 17.21, 28.52, and 37.3 mC. The

Polymer Membrane Stabilized Gold Nanostructures initiation and growth of Au nanostructures occurs inside the Nafion film and spots are clear with a hazy/blurred image (Figure S2(A)). At 28.52 mC applied charge, the growth of the Au particles are visible with bright spots (Figure S2(B)). When 37.3 mC was applied, growth of the Au spots led to the formation of larger size Au nanostructures on the surface of the Nafion film (Figure 3). Figure 4 shows the TEM image of Nf-Aunano recorded after dissolving the Nf-Aunano film in ethanol. The TEM image in Figure 4 shows the Au nanostructures and they also form 1D assembly through particle impingement and coalescence (Figure S3). The rodlike and Y-shaped nanostructures formed at the interconnected -SO3- ionic clusters present in the Nafion film are clear in Figure 4. This observation is also supported by the fact that the absorption spectra of the Nf-Aunano solution showed a broadband at 700 nm with a lower intensity (Figure 2c). The Nafion membrane consists of hydrophilic -SO3- ionic clusters interconnected by ionic channels.26-35 The swelled Nafion film containing electrolyte ions forms much wider channels and interconnected clusters acting as a template for the formation of more nanorodlike and Y-shaped particles of different dimensions than the nanorods formed in the homogeneous solution. Recently, the platinum Y-junction nanostructures were prepared using hierarchically designed alumina templates.36 These platinum Yjunction nanostructures showed enhanced electrocatalytic activity. 3.2. Electrochemical Characterizations of the Nafion-Au Nanostructures-Modified Electrode. The [Fe(CN)6]3-/4- couple was widely used as an electrochemical probe to investigate the characteristics of modified electrodes.37,38 We checked the electrochemical behavior of the [Fe(CN)6]3-/4- couple at the GC/ Nf-Aunano electrode. Figure 5a shows the cyclic voltammograms recorded for [Fe(CN)6]3- at the bare GC electrode in a 0.1 M KCl solution and showed a reversible electrochemical response for the [Fe(CN)6]3-/4- couple. After the GC electrode was modified with Nafion film, very low peak currents were observed with a large peak separation (∆E p) for the [Fe(CN)6]3-/4- couple (Figure 5b) due to the electrostatic repulsion between the negatively charged [Fe(CN)6]3- and -SO3- groups in the Nafion film, resulting in a sluggish electron-transfer kinetics at the GC/ Nf electrode.39 When Aunano was introduced at the Nafionmodified electrode (GC/Nf-Aunano), the electrochemical behavior of the [Fe(CN)6]3-/4- couple (Figure 5c) was improved when compared to that of GC/Nf electrode. The Aunano particles introduced in the Nafion film provide the conduction pathway. The cyclic voltammetric behavior of the [Fe(CN)6]3-/4- couple was not recovered at the GC/Nf-Aunano due to the presence of negatively charged -SO3- groups in the Nafion film. The anodic polarization of the bulk gold electrode leads to the formation of gold oxides on the electrode40-44 and predominant gold oxide formed on the gold electrode was Au2O3 with a characteristic cathodic shift of 73 mV/pH when compared to the theoretical value of 59.16 mV/pH.40-43 This gold oxide coverage on the electrode was found to influence the redox reactions at the electrode surface. The electrochemical redox reactions of redox couples that behave reversibly at the bulk gold electrode are inhibited at the gold oxide-covered gold electrode.43 To understand the pH influence on the gold oxide reduction potential, the electrochemical characteristics of the Nf-Aunano-modified electrode were examined at different pHs. When the working pH was changed systematically from 1 to 7, the gold oxide reduction peak potential shifted cathodically and showed a linear dependency (Figure 6). During the anodic scan, the gold oxide formation started at around 0.8 V and pHdependent peak potential showed Nernstian behavior with a

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19829 slope of 75.6 mV/pH, which is close to the reported value, and the gold oxide was confirmed as Au2O3.43 The electrochemical characteristics of the GC/Nf-Aunano electrode were studied in 0.1 M PBS (pH 7) at different potential windows. During the anodic scan, the oxide formation started at 0.7 V, and in the reverse scan, the oxide reduction potential shifted cathodically and the corresponding reduction peak current increased and reached saturation limit at around 1.3 V anodic potential window (Figure S4). This means that the formation of the oxide layer coverage increases on gold nanoparticles and reaches saturation around 1.3 V anodic potential. A similar behavior is reported for polycrystalline gold electrode.43,45 These observations reveal that the gold nanostructures behave as nanoelectrodes and the electrochemical properties of the Nf-Aunano are very similar to those of the bulk gold electrode. However, the electrocatalytic and sensor properties are found to be very interesting and different from the bulk gold electrode (vide infra). 3.3. Electrocatalytic Oxidation and Detection of NO at Nanostructured Gold-Deposited Nafion-Modified Electrode Film. Sodium nitrite (NaNO2) was used as precursor to produce NO in solution to study the electrocatalytic activity of the modified electrode.31,46 In acidic solution (pH e 4), NaNO2 can generate NO by the disproportionation reaction (eq 1).46,47 Addition of a known amount of NaNO2 into the bulk electrolyte solution at pH e 4 generates a series of concentrations of NO.

3HONO f H+ + 2NO + NO3- + H2O

(1)

Nanostructured gold particles-deposited Nafion-modified electrode exhibited good electrocatalytic behavior toward the oxidation of NO. Figure 7 shows the cyclic voltammograms recorded for different concentrations of NO2- at the GC/ Nf-Aunano electrode in 0.1 M PBS (pH 2.5). Figure 7 reveals that the presence of nanostructured gold particles in the Nafion film mediates the electrochemical oxidation of NO and a large oxidation current was observed starting around 0.7 V due to the direct oxidation of NO at Aunano particles. The polycrystalline Au electrode, GC/Aunano, and GC/Nf electrode were used for NO oxidation (500 µM NO2-) under similar experimental conditions. These electrodes showed lower oxidation currents (and the peak currents were unstable and not reproducible) and the corresponding amperometry sensor experiments showed no peak current at sub-micromolar concentrations of NO. Since the Aunano deposited at the Nafion matrix are in good electrical communication with each other, an efficient electron-transfer process is observed at the Nf-Aunano-modified electrode without any other mediator in the film. The gold nanostructures provide a larger surface area with specific interaction toward the substrate, resulting in improved electron-transfer kinetics and enhancement in the oxidation of NO.31 The linear sweep voltammograms were recorded for different concentrations of NO2- at the GC/Nf-Aunano electrode in 0.1 M PBS (pH 2.5) and the curves are shown in Figure 8A. The GC/Nf-Aunano electrode showed an anodic peak around 0.8 V and the peak current increased with increasing concentration of NO2-. The plot of the peak current against the concentration of NO2- clearly showed a linear response (Figure 8B). The GC/Nf-Aunano electrode was used to construct an amperometric sensor for NO detection. To show the range of concentrations of NO that can be detected at the GC/Nf-Aunano electrode, we carried out amperometry experiments by injecting different concentrations of NO2- (starting from 500 µM to 1 nM) at pH 2.5 and the amperometric curves were recorded (Figure 9). The NO oxidation current was recorded using the GC/Nf-Aunano elec-

19830 J. Phys. Chem. C, Vol. 112, No. 50, 2008 trode at an applied potential of 0.9 V with subsequent addition of NO2-. A typical NO calibration curve based on the steadystate NO oxidation is shown in Figure 10. The amperometry experiment carried out using GC/Nf-Aunano electrode at 10 nM addition of NO2- is shown in Figure S5. A linear dependence of NO oxidation peak current on the [NO2-] was obtained with a correlation coefficient of 0.999 at a signal-to-noise ratio of 3 in the concentration ranges employed. The experimental detection limit of NO at the GC/Nf-Aunano electrode was estimated to be 1 nM. This concentration limit is much lower than that reported for NO detection at gold nanoparticles-modified electrodes.31,48,49 The amperometry experiment shows that the GC/Nf-Aunano electrode can be used to study the NO concentration in the nanomolar range. The response time was ca. 4 s, which indicates a fast electron-transfer process at this modified electrode. The electrode was stable for 1 week when stored in doubly distilled water at room temperature. In summary, we have demonstrated the preparation of Nf-Aunano-modified electrode comprising a uniform and dense packing of gold nanostructures by infiltrating the gold nanostructures into the Nafion matrix. The amount of gold nanostructures at the Nafion-modified electrode increases regularly and uniformly with applied charge. The nanorod and particlelike structures are grown in the Nafion matrix due to the interaction between the Aunano supported by the microheterogeneous structure of the Nafion film containing ionic clusters (∼40 Å diameter) and interconnected channels (∼10 Å diameter). The presence of gold nanostructures greatly improves the electrical communications and the electrocatalytic properties of Nf-Aunano-modified electrode. The catalytic properties of NfAunano-modified electrodes can be exploited for the electrochemical sensing of NO. The present study reports a very simple preparation and fabrication of Nf-Aunano-modified electrode and the applications of this modified electrode are likely to have an impact on the areas of sensing, catalysis, and optoelectronics. Acknowledgment. The financial assistance from the Council of Scientific and Industrial Research (CSIR), New Delhi, is gratefully acknowledged. Supporting Information Available: The in situ SPB maximum and SPB intensity variation; SEM images of Nf-Aunano films at applied charges of 17.21 and 25.52 mC; TEM images of 1D gold nanostructures; CV of GC/Nf-Aunano electrode at different potential windows; chronoamperometric curve for 10 nM NO2- addition at GC/Nf-Aunano electrode and the corresponding calibration plot. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (2) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Wheten, R. L. J. Phys. Chem. B 1997, 101, 3706. (3) Hornyak, G. L.; Patrissi, C. J.; Martin, C. R. J. Phys. Chem. B 1997, 101, 1548. (4) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (5) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699.

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