The Fabrication of Stable Gold Nanoparticle-Modified Interfaces for

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The Fabrication of Stable Gold Nanoparticle-Modified Interfaces for Electrochemistry Guozhen Liu,† Erwann Luais, and J. Justin Gooding* School of Chemistry, University of New South Wales, Sydney NSW 2052, Australia

bS Supporting Information ABSTRACT: Forming stable gold nanoparticle (AuNP)-modified surface is important for a number of applications including sensing and electrocatalysis. Herein, tethering AuNPs to glassy carbon (GC) surfaces using surface bound diazonium salts is investigated as a strategy to produce stable AuNP surfaces. GC electrodes are first modified with 4-aminophenyl (GC-Ph-NH2), and then the terminal amine groups are converted to diazonium groups by incubating the GC-Ph-NH2 interface in NaNO2 and HCl solution to form a 4-phenyl diazonium chloride-modified interface (GC-Ph-N2þCl-). Subsequently AuNPs are immobilized on the interface by electrochemical reduction to give a 4-phenyl AuNP-modified interface (GC-PhAuNP). For comparison, 4-aminophenyl AuNP- and 4-thiophenol AuNP-modified GC interfaces (GC-Ph-S-AuNP and GC-Ph-NH-AuNP), in which AuNPs are tethered to the surfaces by forming S-Au and NH-Au bond, respectively, were also prepared. Cyclic voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy are used to characterize these fabricated interfaces. The AuNP on GC-Ph-AuNP surfaces demonstrate good stability under sonication in Milli-Q water, during electrochemical treatment in 0.05 M H2SO4 solution, and over several weeks. By contrast, the GC-Ph-NH-AuNP and GC-Ph-S-AuNP surfaces showed significant particle losses under equivalent conditions.

1. INTRODUCTION In recent years, the modification of electrodes with nanomaterials, in order to provide the electrode with enhanced properties, has received considerable attention for applications such as bioelectronics, catalysis, and electroanalytical chemistry.1 Metal nanoparticles, especially gold nanoparticles (AuNPs), have been extensively studied because of their attractive physicochemical characteristics, such as high surface-to-volume ratio, good biocompatibility, and being able to facilitate electron transfer between biomolecules and electrodes.2 More importantly, recent work by us3-5 and others6-9 has demonstrated that attaching AuNPs onto the ends of otherwise passivating organic layers opens up conducting pathways through which electron transfer can proceed as though the organic layer is not even present. Thus, immobilization of AuNP on bulk electrodes using covalently attached organic layers containing thiol groups and amino groups on their distal end offers a simple, fast, and versatile approach for preparing nanoparticle electrode arrays with low background capacitance. The possibility of altering AuNP size and density by controlling AuNP synthesis conditions and the density of coupling points,10 respectively, further enhances the attractiveness of AuNP-modified interfaces for sensing applications. A range of strategies based on electrostatic interactions,11,12 biomolecular recognition,13-15 block copolymer matrixes,16,17 and chemical bonding18-20 have been exploited to modify AuNP to different substrates. However, the stability of these fabricated AuNP systems is seldom discussed. r 2011 American Chemical Society

The long-term stability of the sensing interface is a crucial issue for many applications. The majority of AuNP-modified interfaces are based on the affinity between thiols and gold to form S-Au bonds or the affinity of amines for gold. The binding of amines to gold can be by the simple molecular cluster (NH2-Au) or the dehydrogenated form (NH-Au). Fagas and co-workers21 claim that forming a NH-Au bond is more likely to happen than forming a NH2-Au bond because the N-Au bond with NHAu linker has stronger covalent bond character than that with NH2-Au linker. Thus, in this paper the bonding between amines to AuNP is proposed to be a NH-Au bond. Alkanethiols have been shown to place exchange,22 oxidize,23 and dynamically move around on gold,24 and the thermal stability of S-Au can be an issue.25,26 The bond energy for a S-Au bond is 1.6 eV, corresponding to 154.4 kJ mol-1.24,27 Similarly, the bond energy for a NH-Au bond is 1.59 eV,21 corresponding to a NH-Au bond strength of 153.6 kJ mol-1. With the NH-Au bond, the AuNP can easily be displaced in the presence of competitive solvent, temperature, and pH.28 Thus both thiols and amines attached to gold surfaces may lack the robustness required for some device application.29 Attempts to increase the stability of AuNPs on fabricated interfaces have been made using multiple thiol anchoring molecules,30,31 but the dithiol-gold bond is still not resistant to the thermal treatment.32 It has recently been Received: November 2, 2010 Revised: January 12, 2011 Published: February 24, 2011 4176

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Scheme 1. Fabrication of Different AuNP-Modified GC Surfaces

reported that dithiocarbamates provide superior electrical contact and thermal stability compared to thiols on metals,29 and AuNPs have been successfully immobilized on surfaces via dithiocarbamate bond formation.33-35 However, the presence of thiol chemistry still makes the surface susceptible to oxidation in air. Thus, it is desirable to develop another AuNP-modified system with greater stability. Previous work has demonstrated that aryl diazonium salt chemistry can produce significantly more stable organic layers on gold electrodes for sensing applications than alkanethiol selfassembled monolayers (SAMs).36 It has been proposed that the improved stability originates from the formation of a goldcarbide bond, which has a calculated bond energy of 3.29 eV,37 corresponding to 317.1 kJ mol-1. Such a bond energy is significantly greater than that of S-Au (154.4 kJ mol-1) and NH-Au (153.6 kJ mol-1). The more robust C-Au bond is less prone to oxidation than the S-Au bond. It was recently reported that the C-Au bond also shows higher thermal stability than the S-Au bond.32 Thus, tethering AuNP to the electrode surface by a covalent bonding reaction such as a C-Au bond may increase the stability of electrode-organic layerAuNP constructs. Related studies have used aryl diazonium salts to cap nanoparticles38 and to form nanoparticle assemblies on surfaces.39 The purpose of this paper is to introduce a strategy to fabricate a stable AuNP-modified glassy carbon (GC) electrode interface (GC-Ph-AuNP) for sensing applications. On the basis of aryl diazonium salt chemistry (Scheme 1), GC surfaces can be first modified with 4-aminophenyl (GC-Ph-NH2). Then the terminal amine groups can be converted to diazonium groups by incubating the GC-Ph-NH2 interface in NaNO2 and HCl solution to form GC-Ph-N2þCl-. Subsequently, AuNPs are in situ immobilized on the interface by scanning cyclic voltammetry between 0.6 and -0.8 V to achieve a 4-phenyl AuNP-modified interface

(GC-Ph-AuNP). For comparison, 4-thiophenol AuNP- and 4-aminophenyl AuNP-modified GC interfaces (GC-Ph-S-AuNP and GC-Ph-NH-AuNP) are also prepared by tethering AuNP to 4-thiophenyl (GC-Ph-SH) and GC-Ph-NH2 modified surfaces through S-Au and NH-Au bonds, respectively. The density of AuNPs and the stability of these AuNP-modified GC electrodes are compared. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) are used to characterize the different fabricated interfaces.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Sodium nitrite, potassium ferricyanide, potassium chloride, hydrochloric acid, 4-phenylenediamine (H2N-Ph-NH2), 4-aminothiophenol (HS-Ph-NH2), cysteamine, hydrogen tetrachloroaurate (III), and trisodium citrate (99% purity) were purchased from Sigma-Aldrich (Sydney, Australia). AuNPs were synthesized using a 1 mM solution of HAuCl4 in Milli-Q water that was boiled prior to adding trisodium citrate with constant stirring. A color change started to occur 10 s after the addition of citrate solution according to the method of Frens.40 The resulting aquasol had a concentration of ca. 17 nM and a diameter of 27.5 ( 0.5 nm as determined by SEM. All reagents were used as received, and aqueous solutions were prepared with purified water (18 MΩ cm, Millipore, Sydney, Australia). Phosphate buffer solution used in this work contained 0.05 M KCl and 0.05 M K2HPO4/KH2PO4 adjusted to pH 7.0 with NaOH or HCl solution. 2.2. Electrode Preparation and Modification. GC electrodes were purchased as 3-mm-diameter disks from Bioanalytical Systems Inc., USA. The electrodes were polished successively with 1.0, 0.3, and 0.05 μm alumina slurries made from dry Buehler alumina and Milli-Q water on microcloth pads (Buehler, Lake Bluff, IL, USA). The electrodes were thoroughly rinsed with Milli-Q water and sonicated in Milli-Q water for 2 min after polishing. Before derivatization, the electrodes were dried 4177

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Langmuir under a nitrogen gas stream. The GC electrode surface modification with in situ generated aryl diazonium cations in acidic media has been reported previously.41,42 Specifically, 1 mM H2N-Ph-NH2 or HS-PhNH2 was solubilized in 0.5 M aqueous HCl, to which 1 mM NaNO2 was added to generate in situ the aryl diazonium salt in the electrochemical cell. The mixture was degassed with nitrogen flow and left to react for about 10 min at 0 °C. The electrochemical reductive modification of the GC surface with in situ generated diazonium salt was carried out by scanning in a potential range between 0.6 V and -1.0 V versus Ag/AgCl for two cycles at a scan rate of 100 mV s-1. After surface derivatization with 4-aminophenyl (GC-Ph-NH2) or 4-thiophenol (GC-Ph-SH), the electrodes were rinsed with copious amounts of Milli-Q water, acetonitrile, and Milli-Q water, respectively, and finally dried under a stream of nitrogen prior to the next step. Two AuNP-modified surfaces (GC-Ph-SAuNP and GC-Ph-NH-AuNP) were prepared by immersing GC-Ph-SH and GC-Ph-NH2 surfaces in AuNP solution for 3 h at room temperature, respectively. In order to achieve another AuNP modified surface (GCPh-AuNP), the GC-Ph-NH2 surface was first immersed in 5 mM NaNO2 and 0.5 M HCl for 15 min followed by cycling between þ0.6 V and -0.8 V versus Ag/AgCl in AuNP solution for two cycles at a scan rate of 100 mV s-1. All modification steps for the preparation of the three AuNPs interfaces are displayed in Scheme 1. 2.3. Electrochemical Measurement. All electrochemical measurements were performed with a BAS-100B electrochemical analyzer (Bioanalytical System Inc., West Lafayette, IN, USA) and a conventional three-electrode system, comprising a GC working electrode, a platinum foil as the auxiliary electrode, and a Ag/AgCl 3.0 M NaCl electrode (from BAS) as reference. All potentials were reported versus the Ag/ AgCl reference electrode at room temperature. The EIS measurements were performed using a Solartron SI 1287 Electrochemical Interface coupled with an SI 1260 Frequency Response Analyzer (Solartron Analytical, Hampshire, England). The 1 mM Fe(CN)63-/Fe(CN)64(1:1) redox couple in phosphate buffer solution (pH 7.0) was used as the electrolyte solution. EIS measurements were recorded at room temperature within the frequency range of 10-1-105 Hz superimposed on a dc potential of þ0.236 V, with AC of 10 mV peak to peak amplitude, and 5 points per decade of frequencies. The Z-view software was used for complex circuit modeling. 2.4. Surface Analysis: Morphology and Chemistry. SEM was carried out using a Hitachi S-900 SEM (Berkshire, England). XPS spectra were collected from GC plates (vitreous carbon foil version 6, Goodfellow Cambridge-Limited England) on a VG EscaLab 220-IXL spectrometer with a monochromated Al KR source (1486.6 eV), hemispherical analyzer, and multichannel detector. The spectra were accumulated at a takeoff angle of 90° with a 0.79 mm2 spot size. The pressure in the analysis chamber was below 10-8 mbar. The pass energy for the survey scan is 100 eV and for the narrow scan 20 eV. The step size for the survey scan is 1.0 eV and for the narrow scan is 0.1 eV. Survey spectra were obtained, followed by high-resolution scans of C1s, O1s, N1s, and S2p regions. The spectra were calibrated on the C1s peak (285.0 eV). Atomic sensitivity factors are C1s 1.0, O1s 2.93, S2p 1.68, N1s 1.8, and Au4f7/2 9.58. Spectra were analyzed using XPSPEAK41 software.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Formation of GC-Ph-AuNP Surfaces. In order to form GC-Ph-AuNP surfaces (Scheme 1), the GC-PhNH2 surface was first formed, as shown many times previously for aryl diazonium salts.43 The GC-Ph-NH2 surface was converted into a GC-Ph-N2þCl- surface using the in situ method developed by Belanger and co-workers44 in the presence of AuNPs. The potential of the electrode surface was scanned negative to initiate electrochemical attachment of the AuNPs

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Figure 1. Cyclic voltammograms of a GC-Ph-N2þCl--modified electrode recorded in 0.5 M aqueous HCl and 1 mM NaNO2 at a scan rate of 100 mV s-1. The broad peak between 0 and -0.3 V in the first anodic scan is indicative of the reduction of an aryl diazonium salt.

via a proposed C-Au bond. Associated with the binding of nanoparticles to the surfaces is a Faradaic process, observed in cyclic voltammograms indicative of the reduction of a diazonium moiety to give a phenyl radical (Figure 1). In the absence of a modifying layer no significant AuNP adsorption onto the GC surface was observed. The proposal of the C-Au bond is based on research into aryl diazonium salts on metal surfaces such as C, Fe, Au, Pt, and Cu where strong linkages have been formed,36,45-49 although definitive evidence of a metal carbide bond has only been shown for iron surfaces.45 For such a bond to occur with the citrate-capped AuNPs first requires the removal of the citrate. It has been shown, however, that in potentials negative of the point of zero charge (pzc) the citrate will desorb from the AuNPs.50,51 As the reductive adsorption of the AuNPs to the GC-Ph-N2þClsurface requires potentials cathodic of the pzc, the citrate on AuNPs close to the electrode surface is expected to be removed during the coupling reaction. The electrochemistry of the GC electrodes during different stages of the modification was studied in solutions containing the redox species Fe(CN)63-/4- (Figure SI1, Supporting Information). The pronounced ferricyanide electrochemistry observed on bare electrodes is completely suppressed on the GC-Ph-NH2 electrode. After conversion of the GC-Ph-NH2 to GC-Ph-N2þCl- and applying a cathodic potential sweep to the surface with AuNP solution to give a GCPh-AuNP electrode, a well-defined voltammogram from Fe(CN)63-/4- was observed (red curve in Figure SI1). The good electrochemistry upon attachment of the AuNPs is consistent with previous studies showing that nanoparticles attached to passivating layers on electrodes enhances the efficiency of Faradaic electrochemistry.3,8 3.2. XPS Characterization of the GC-Ph-AuNP Surface. The important steps in the formation of the GC-Ph-AuNP surface not previously characterized, that is, the formation of the GC-Ph-N2þCl- interface and the subsequent attachment of AuNPs, were evaluated using XPS (Figure 2). The N1s narrow scan for GC-Ph-N2þCl- interface (Figure 2b), can be fitted with five peaks (located at 399.4, 400.2, 401.4, 403.7, and 405.6 eV, respectively), and provides strong evidence for the formation of a surface diazonium moiety. The N1s component at 399.4 eV is consistent with the presence of amino groups at the GC electrode surfaces. The peak at 400.2 eV is attributed to the formation of azo bridges in the film. The percentage of azo groups is estimated to be 13.7% in the N1s XPS peak, which is consistent with that observed by Belanger and co-workers.52 The peak at 401.4 eV is 4178

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Figure 2. (a) Wide scan of XPS for GC-Ph-N2þCl-; (b) N1s narrow scans for GC-Ph-N2þCl-; (c) wide scan of XPS for GC-Ph-AuNP (inset: C1s narrow scans); and (d) N1s narrow scans for GC-Ph-AuNP.

Table 1. XPS Summary Data total atomic percentage from appropriate XPS core level spectra surfaces

Au

GC-Ph-SH GC-Ph-S-AuNP

3.4

GC-Ph-NH2

C

Cl

N

O

S

81.8

2.6

7.8

7.8

78.7

2.2

9.0

7.7

83.8

1.1

5.7

9.4

GC-Ph-NH-AuNP GC-Ph-N2þCl-

3.9

80.3 82.4

1.0 1.8

5.6 5.1 (N in -NtN- 0.5)

9.2 10.7

GC-Ph-AuNP

1.5

80.5

1.4

4.6

12.0

ascribed to ammonium.53 The peaks at 403.7 and 405.6 eV are characteristic of diazonium groups52,54,55 and thus demonstrate that some of the amino groups have been converted to diazonium moieties during the chemical treatment at 5 mM NaNO2 and 0.5 M HCl aqueous solution for 15 min. The component at the higher binding energy (405.6 eV) is due to the N closest to the phenyl ring. From the area of the different components in the N1s region, it is estimated that only 10% of the amine groups have been converted to diazonium moieties under this conditions (Table 1). Hence free amine groups are also still present on this surface. The conversion percentage increased to 17.3% if the GC-Ph-NH2 surface was left in 5 mM NaNO2 and 0.5 M HCl aqueous solution for overnight. The concentration of NaNO2 and HCl did not make a significant difference in the conversion efficiency. After applying cathodic potential sweeps to the GC-PhN2þCl- surface in AuNP solution to give the GC-Ph-AuNP surface, Au peaks were present in the wide scan (Figure 2 c). The Au4f core spectrum (Figure 2d, left) presents characteristic Au 4f7/2 and Au4f5/2 XPS peaks at 84 and 88 eV, respectively, with a spin-orbit splitting of 3.7 eV, indicating the presence of zerovalent gold.56 Although a significant amount of amino groups are present on GC-Ph-N2þCl- surfaces, the time used for modification of the AuNP to GC-Ph-N2þCl- surfaces by cycling between 0.6 V and -0.8 V for two cycles at the scan rate of 100 mV s-1 is

Figure 3. SEM images of (a) GC-Ph-NH-AuNP and (b) GC-Ph-AuNP.

about 1 min. During such short time intervals, the attachment of AuNP to the amino surface through NH-AuNP linkage is negligible. Compared to the GC-Ph-N2þCl- surface, the total percentage of N species on GC-Ph-AuNP decreased from 5.1% to 4.6% (Table 1), which is consistent with the disappearance of the 10% of the N that are diazonium species (centered at 403.5 and 405.8 eV) on the nitrogen core spectrum of GC-Ph-AuNP (Figure 2 d). This result suggests that applying a negative potential to GC-Ph-N2þCl- surfaces converted the diazonium moieties to radicals, with the loss of N2, and the attachment of the AuNP is assumed to be via stable C-Au bonds.36 4179

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Table 2. AuNP Densities from SEM Images AuNP density (108AuNP cm-2) surfaces

before sonication

after sonication

before electrochemistrya

after electrochemistry

GC-Ph-SH-AuNP

432 ( 24

308 ( 29

409 ( 15

292 ( 19

(100%)

(71.3%)

(100%)

(71.4%)

458 ( 21

369 ( 23

444 ( 19

384 ( 11

(100%)

(80.7%)

(100%)

(85.7%)

51 ( 3

49 ( 5

48 ( 4

47 ( 5

(100%)

(97.1%)

(100%)

(98.2%)

GC-Ph-NH-AuNP GC-Ph-AuNP a

Scanning two cycles of cyclic voltammetry in 0.05 M H2SO4 between 1.5 V and -0.3 V at a scan rate of 100 mV s-1.

3.3. SEM Characterization of the GC-Ph-AuNP Surface. The successful attachment of the AuNP to give the GC-PhAuNP surface was also confirmed by SEM (Figure 3b). The number of AuNPs in SEM images is counted manually. The density of AuNP on GC-Ph-AuNP is calculated to be 51 ( 5  108 AuNP cm-2 (n = 5, 95% confidence). For comparison, AuNPs on both GC-Ph-S-AuNP and GC-Ph-NH-AuNP surfaces are also characterized by SEM (typical SEM image for GC-PhNH-AuNP is shown in Figure 3a). On the GC-Ph-S-AuNP surface the density of AuNPs was 432 ( 29  108 AuNP cm-2 (n = 5, 95% confidence), and on the GC-Ph-NH-AuNP surface it was 458 ( 31  108 AuNP cm-2 (n = 5, 95% confidence). Hence the GC-Ph-AuNP surfaces have only 11% of the surface density of AuNP compared with the other two surfaces. The density of AuNP on GC-Ph-AuNP surfaces is thus consistent with the conversion efficiency of amino to diazonium (ca. 10%). Further, this result suggests all the locations where diazonium moieties were produced on the surface of the modified electrode have been converted to locations where AuNPs are attached by forming the C-Au bond. The AuNP density on the surface could be increased to 84 ( 6  108 AuNP cm-2 (n = 5, 95% confidence) by incubating the surface in 5 mM NaNO2 and 0.5 M HCl aqueous solution overnight, but this coverage is still only 17% of that seen on the GC-Ph-NH-AuNP surface. 3.4. Stability of AuNP on GC-Ph-AuNP Surfaces by Electrochemistry Treatment in 0.05 M H2SO4 Solution or by Sonication in Milli-Q Water. The electrochemistry of GC-Ph-AuNP interfaces was investigated in 0.05 M H2SO4 solution by scanning two cycles between -0.3 and 1.5 V versus Ag/AgCl at the scan rate of 100 mV s-1 to oxidize surface gold atoms and subsequently reduce the oxide formed. This is a standard method for cleaning gold surfaces and has also been employed to assess the robustness of organic layers on gold surfaces.57 This experiment aims to clarify two issues: (1) whether the AuNPs on the surface are stable with the electrochemical oxidation of gold and the subsequent removal of the gold oxide in H2SO4 solution (thus serving as a measure of the robustness of the linkage to the AuNPs and providing some information regarding the nature of the linkage) and (2) as an additional measure of the density of AuNPs on the surface, measuring the area under the peak, indicative of the reduction of gold oxide after accounting for background current. This is achieved using the conversion factor of 482 μC cm-2 as determined by Hoogvliet et al.,58 assuming only the surface layer of gold atoms is oxidized and knowing the average AuNP size. It is observed that a bare GC electrode does not show any Faradaic peaks between -0.3 and 1.5 V versus Ag/ AgCl in 0.05 M H2SO4 solution as expected (Figure SI2a). For the GC-Ph-AuNP surface, an oxidation peak, centered at 1.2 V, and a reduction peak, centered at 0.8 V, were observed (Figure

SI2b). These peaks correspond to the oxidation and reduction peaks of gold on the surface, respectively, hence showing that AuNPs were successfully attached to the surface. The electrochemistry remains the same after several cycles, providing evidence for a robust linkage of the AuNPs to the surface. The stability of the particles on the GC-Ph-AuNP interfaces suggests that the bonding of the AuNPs to the surface does not involve the citrate, which desorbs at such cathodic potentials,51 but a strong bond such as the proposed Au-C linkage. On the basis of the size of the reduction peak at about 0.8 V, the total surface area of AuNPs on the GC-Ph-AuNP surface is estimated to be 2.22  10-3 cm2. The geometric area of a GC disk electrode is 7.07  10-2 cm2, and the diameter of the AuNPs determined by SEM is 27.5 ( 0.5 nm (n = 5, 95% confidence). Thus the density of AuNPs on the GC-Ph-AuNP can be estimated to be 50  108 AuNP cm-2 (or 8.30  10-15 mol cm-2). This measure of AuNP density corresponds almost exactly to the estimate from the SEM prior to electrochemical measurement 51 ( 5  108 AuNP cm-2. After the electrochemical stripping experiment, the SEM estimate of the surface coverage was barely changed (47 ( 5  108 AuNP cm-2), showing that the particles on these surfaces were able to withstand such harsh treatments. The stability of AuNP on GC-Ph-AuNP surfaces was also studied by sonication in Milli-Q water for 10 min. On the basis of SEM images, the density of AuNPs on GC-Ph-AuNP remains almost the same before (51 ( 3  108 AuNP cm-2) and after (49 ( 5  108 AuNP cm-2) sonication in Milli-Q water for 10 min (Table 2). In comparison, GC-Ph-S-AuNP and GC-Ph-NH-AuNP surfaces show significant loss of AuNPs from the surface after scanning two cycles of cyclic voltammetry in 0.05 M H2SO4 between 1.5 V and -0.3 V at the scan rate of 100 mV s-1 and after sonication in Milli-Q water for 1 min (Table 2). In the case of the GC-Ph-S-AuNP surface, XPS shows that the loss of nanoparticles is due to the breakage of the S-Au bond, rather than the loss of the organic layer attached to the GC electrode, as after sonication in Milli-Q water for 10 min or the electrochemical treatment in 0.05 M H2SO4 solution, the percentage of the different species on GC-Ph-SH surface remains virtually unchanged. From the extent of AuNP loss (Table 2), it can be concluded that the order of stability for three AuNP-modified surfaces under sonication and electrochemistry treatment in H2SO4 is GC-Ph-AuNP > GC-Ph-NH-AuNP g GC-Ph-SAuNP. This relative stability is attributed to the relative bond strengths where C-Au bond (317.1 kJ mol-1) > NH-Au bond (159 kJ mol-1)21,28 g S-Au bond (154.4 kJ mol-1).24,27 3.5. Stability of AuNP-Modified Surfaces to the Further Modification with Alkanethiols. The results above show that the GC-Ph-AuNP interface is very stable. However, in sensing 4180

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Figure 6. The relative density of AuNPs on three AuNP-modified surfaces with the change of incubation time in phosphate buffer solution (pH 7.0). Figure 4. The change of charge transfer resistance (Rct) recorded in phosphate buffer solution containing 0.05 M KCl and 1 mM Fe(CN)63-/4- for different surfaces after incubation in 1 mM cysteamine solution for different times.

Figure 5. SEM images for GC-Ph-S-AuNP after incubation in 1 mM cysteamine (a) for 2 h and (b) overnight.

applications there will often be further modification of the AuNP to provide them with selectivity for a target analyte.59 The most prevalent approach to further modification of the nanoparticles is via using alkanethiol chemistry. Here the AuNP modified surfaces are modified with cysteamine by incubation of three AuNP modified surfaces (GC-Ph-AuNP, GC-Ph-NH-AuNP, and GC-Ph-S-AuNP) in a 1 mM cysteamine ethanol solution. EIS is used to investigate the stability of cysteamine/AuNPmodified GC surfaces. Figure 4 shows the charge transfer resistance (Rct) obtained from the Nyquist plots for GCPh-AuNP surface before and after cysteamine incubation for different times (Figure SI3). For comparison, Rct for surfaces (GC-Ph-SH, GC-Ph-NH2, and GC-Ph-N2þCl-) before AuNP modification are also measured, and it is observed that these surfaces give very high Rct. However, after attachment of the AuNP, Rct for these surfaces (GC-Ph-S-AuNP, GC-Ph-NHAuNP, and GC-Ph-AuNP) decreased significantly as a result of the ability of AuNP to enhance electron transfer.3 In contrast to the other two surfaces, the GC-Ph-AuNP surfaces showed only a minor increase in Rct over the first 4 h of incubation in cysteamine. Thereafter, however, Rct did not change, even after overnight incubation in cysteamine. Furthermore, the density of AuNP, as determined by SEM, was almost unchanged (49 ( 6  108 AuNP cm-2 (n = 5, 95% confidence)). For GC-Ph-S-AuNP surfaces, Rct increased slightly after an incubation time of 2 h in cysteamine, which was attributed to the binding of the cysteamine to the AuNP forming a physical

barrier and so attenuating the charge transfer. However, Rct continued to increase as the incubation time was increased. SEM observations (Figure 5) show this increase in Rct is due to the loss of AuNP (decreasing by 40% after 2 h). The loss of particles was also confirmed by XPS where a sample of the solution above the surface was deposited onto a GC plate, the solvent evaporated and the remaining material analyzed. The XPS analysis showed the presence of gold species (Figure SI4). Furthermore, the AuNPs were also observed to form clusters after the overnight incubation of GC-Ph-S-AuNP surface in cysteamine. These observations indicate that the cysteamine incubation is mobilizing the AuNPs and hence breaking the bond between the thiolated organic underlayer and the AuNPs. Similar observations were made for GC-Ph-NH-AuNP, with the density of AuNP on the surface of GC-Ph-NH-AuNP decreasing by 30% after incubation in cysteamine overnight. 3.6. The Stability of the Three AuNP-Modified GC Surfaces over Time. The long-time stability of the sensing interface is crucial for an applicable sensing interface. The three AuNPmodified sensing interfaces (GC-Ph-AuNP, GC-Ph-NH-AuNP, and GC-Ph-S-AuNP) were left in phosphate buffer solution (pH 7.0) for 1 month with periodic assessment of the surface stability. The relative density of AuNPs on the three different surfaces is displayed in Figure 6 following the incubation time in phosphate buffer. The relative density of AuNP for freshly fabricated surfaces is considered to be 1. It is observed that the density of AuNP on GC-Ph-S-AuNP started to decrease significantly after storing in phosphate buffer for 1 week, and moreover the AuNPs form clusters. The density of AuNPs on GC-Ph-NH-AuNP started to decrease significantly after 10 days of incubation in phosphate buffer. By contrast, there is almost no change in the number of AuNPs on the surface of GC-Ph-AuNP, even after 1-month storage in phosphate buffer solution at pH 7.0. These long-term stability experiments again show that the AuNPs on GC-Ph-AuNP surfaces are much more stable compared with those on GC-Ph-S-AuNP and GC-Ph-NH-AuNP surfaces.

4. CONCLUSIONS In summary, a simple, fast, and reliable technique to functionalize GC surfaces with robust attachment of AuNP has been investigated. On the basis of in situ aryl diazonium salt modification, GC electrodes can be modified with GC-Ph-NH2. XPS and electrochemistry showed that amine groups on GC-Ph-NH2 can be converted to diazonium groups to form a GC-Ph-N2þClsurface in acidic solution containing NaNO2. AuNPs can be tethered to the GC-Ph-N2þCl- surface, with a strong bond 4181

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Langmuir proposed to be a C-Au bond, by applying the reductive potential to the GC-Ph-N2þCl- surface in a solution containing AuNPs. For comparison, GC-Ph-S-AuNP and GC-Ph-NHAuNP surfaces, in which AuNPs are tethered by forming SAu and NH-Au bonds, respectively, were also prepared. It was observed that the density of AuNPs on GC-Ph-AuNP is about 11% of that on the GC-Ph-NH-AuNP surface, which provides AuNP density similar to that of the GC-Ph-S-AuNP surface. However, the GC-Ph-AuNP surface is far more stable than the other two surfaces with regards to sonication in Milli-Q water, repeated potential scanning in 0.05 M H2SO4 solution between -0.3 and 1.5 V, incubation in alkanethiol solutions, and longterm storage in aqueous solution over a month.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure SI1 shows electrochemistry recorded in potassium fericyanide at different modified electrodes; Figure SI2 contains the electrochemistry of a bare GC surface and a GC-Ph-AuNP surface in 0.05 M H2SO4 solution; Figure SI3 contains the Nyquist plots for a GC-Ph-AuNP surface before and after incubation with cysteamine for different times; and Figure SI4 contains the XPS scans for cysteamine solution after being incubated with a GC-Ph-S-AuNP surface for 2 h. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ61-2-93855384. Fax: þ61-2-93856141. E-mail: justin. [email protected]. Present Addresses †

Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, 430079, P. R. China.

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