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Electrochemical attachment of diazoniumgenerated films on nanoporous gold

Christine L. Chevalier and Elizabeth C. Landis*

Department of Chemistry, College of the Holy Cross, 1 College St Box C, Worcester, MA 01610

AUTHOR EMAIL ADDRESS: [email protected]

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Abstract Nanoporous gold provides a high surface area platform for further chemistry, but the stability of the molecular linkages to the surface will limit applications. We attached aryl molecular layers to nanoporous gold electrodes through electrochemical reduction of the corresponding aryl-diazonium salt and studied the properties and stability of the resulting films in varied attachment conditions. Infrared reflection absorption spectroscopy and X-Ray photoelectron spectroscopy were used to confirm the presence of the molecular layers. X-ray photoelectron spectroscopy indicates that the molecular layer is thick and that attachment conditions can form multilayers. However, cyclic voltammetry shows that the multilayers do not block electrochemical activity at the nanoporous gold surface. The molecular layers are resistant to replacement by alkane-thiol chains and exhibit some stability with respect to applied potential. These results indicate that a thick but highly defective molecular film forms with a mixture of strongly and weakly bound molecules.

Introduction Nanoporous gold is a mesoporous material composed of a bicontinuous network of gold ligaments. It is a promising material because nanoporous gold can be easily fabricated in ambient conditions by selectively dissolving silver from a silver-gold alloy.1,2 The resulting gold surface provides a conductive, chemically inert, and biocompatible material with a large surface area and tuneable pore sizes.3

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These properties make nanoporous gold attractive for applications including energy conversion,2 catalysis, 4,5 and sensing.6,7 Molecular layers on nanoporous gold are necessary for a number of possible applications. 2,6,8 Previously, molecular layers on nanoporous gold have been formed through thiol-based self assembled monolayers (SAMs). Stability studies have shown that thiol-based SAMs on nanoporous gold9 and other nanostructured gold substrates10,11 are more stable than SAMs on planar gold surfaces. Despite the stability observed in nanostructured substrates, thiol-based SAMs are known to be unstable, particularly when exposed to liquids,12,13 so more stable binding methods for nanoporous gold are desirable. Surface functionalization by electrochemical or spontaneous reduction of aryl diazonium salts has become a popular technique for attaching molecules to carbon and metallic surfaces. 14-16 The reaction typically involves the electrochemical reduction of the diazonium functional group and formation of a radical as shown in Scheme 1. 17,18 Chemical attachment follows between the phenyl ring and the surface.19 Recent results measuring surface-enhanced Raman scattering on gold nanoparticles indicate that the attachment between the aryl group and the gold surface is a carbon-gold covalent bond.20 Electrochemically grafted diazonium-based molecular layers are particularly promising because they have demonstrated high stability on planar gold surfaces and gold nanoparticles compared to thiol-based attachments. 16,21,22 The molecular layers are notably stable when subjected to reflux conditions21 or sonication.19

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Computational studies also predict a strong bond between the aryl group and a Au(111) surface. 23,24 However, the films resulting from electrochemical reduction tend to be disordered and frequently form multilayers. 18,25,26 These dense multilayer can block electrochemical activity on planar gold surfaces. 27-29 Detailed studies have found that the electrochemical conditions during molecular layer deposition play an important role in the thickness and ordering of the molecular layer.30 In this paper we demonstrate the attachment of diazonium salt-derived nitrobenzene molecular layers on nanoporous gold. We combine surface spectroscopic analysis with electrochemical probes to investigate the molecule binding density and stability. Our results show that the diazonium-based molecular layers on nanoporous gold are stable relative to alkanethiols, but we do not observe the dense blocking layers that form on planar gold substrates.

Experimental Section Preparation of Nanoporous Gold Substrates. Nanoporous gold surfaces were prepared following a procedure described by Jennings et. al. in which gold leaf is dealloyed in nitric acid and mounted on silanized glass slides.2 Silver/gold leaf (Monarch, 12kt, patent) was cut into approximately 1 in2 pieces. Tweezers were used to briefly place the leaf in water to release the backing paper, and then to place the leaf gently in a dish of concentrated nitric acid (70%). The leaf was allowed to float at the acid-air interface for 30 minutes.

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Glass slides were hydroxylated in a solution of 5:1:1 H2O: H2O2 (30%) : NH4OH (30%) by volume for 30 minutes, then rinsed in water before use. They hydroxylated glass slides were used to lift the dealloyed gold leaf out of the nitric acid. The hydroxylated slides were then used to transfer the leaf through three dishes of water sequentially for rinsing. After rinsing, the dealloyed leaf was typically mounted on silanized glass slides. To perform the silanization reaction, microscope slides were cleaned in piranah solution (3:1 solution of concentrated H2SO4 and 30% H2O2, reacts violently with organic material), rinsed in water, and dried. The silanization was performed in approximately 5 mM 3-mercaptoproyltrimethoxysilane in hexanes at 60 °C for 60 minutes.31,32 Silanized slides were rinsed in hexanes and dried before use. Nanoporous gold samples for scanning electron microscopy (SEM) were mounted on gold-coated silicon wafers (Amsbio) with no chemical attachment. An SEM image of the nanoporous gold surface is shown in Figure 1.

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Figure 1. SEM image of nanoporous gold at 50 kx magnification Electrochemical Deposition of the Aryl Films 4-Nitrobenzenediazonium tetrafluoroborate (>98.0%) and tetrabutyl ammonium tetrafluoroborate (98%) were used as received. Electrochemical grafting of diazonium films was performed in solutions of 4-nitrobenzenediazonium

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tetrafluoroborate and tetrabutylammonium tetrafluoroborate in acetonitrile (HPLC). The tetrabutylammonium tetrafluoroborate concentration remained constant at 0.1M, while the concentration of 4-nitrobenzene diazonium tetrafluoroborate was varied between 0.1 mM and 10 mM. Electrochemical grafting of the aryl layers was performed on a µAutolab III with FRA or a Pine WaveNow USB potentiostat with a Basi Ag/AgCl reference electrode and a platinum counter electrode. All electrochemical depositions were performed using cyclic voltammetry sweeps from 0.2 V to -0.6 V at 100 mV/s. The number of full cycles varied between 3 and 350. Surface Spectroscopic Characterization SEM images of the nanoporous gold were collected on a Zeiss Supra 55VP with a 3keV beam. X-Ray photoelectron spectroscopy (XPS) was collected on a Thermo Scientific K-Alpha XPS using monochromatic Al Ka X-rays at 1.4866 keV. All spectra were collected with a 400 µm spot size and a flood gun to avoid sample charging. Survey spectra were collected using a 1 eV/step, 10 ms dwell time, and 200 eV pass energy. High-resolution scans were performed using a 0.1 eV/step, 50 ms dwell time, and 50 eV pass energy. Spectra were collected using 50 scans for nitrogen, 20 scans for carbon, and 2 scans for gold. Peaks were fit with the Thermo Advantage program using a Shirley background correction. Peak areas were corrected using sensitivity factors. Infrared Reflection Absorption Spectroscopy (IRRAS) was collected on a Bruker Tenor 27 fourier transform infrared spectrometer with a variable angle

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reflectance accessory (VeeMaxII, Pike) purged with air. Spectra were collected using p-polarized light at an angle of 70° with 200 scans at 4 cm-1 resolution for both the background of unmodified nanoporous gold and the sample. IRRAS spectra were background corrected and offset for clarity. Electrochemical Analysis Potassium ferrocyanide (99+%) and potassium chloride were used as received. All aqueous solutions were prepared in 18 MΩ deionized H2O and degassed by bubbling with argon for 15 minutes before analysis. Electrochemical analysis was performed on a µAutolab III with FRA. Electrochemical measurements were performed using a three-electrode cell with an exposed area of 0.24 cm2. The nanoporous gold sample was used as the working electrode with a platinum wire counter electrode and a Basi Ag/AgCl junctioned reference electrode. Cyclic voltammetry measurements were collected in 1M KCl and 1 mM K3Fe(CN)6 at 100 mV/s. Impedance spectroscopy measurements were collected in 0.1M KCl and 1 mM K3Fe(CN)6. A 10 mM rms AC voltage was applied to the sample and the frequency varied from 105 Hz to 0.1 Hz. 50 data points were collected between those frequencies and spaced logarithmically.

Scheme 1. Potential reaction between 4-nitrobenzenediazonium and nanoporous gold

Results and Discussion

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Formation of nitrobenzyl layers on nanoporous gold Scheme 1 shows the proposed nitrobenzyl attachment to the nanoporous gold surface. Figure 2a shows a typical cyclic voltammogram collected during the electrochemical reduction of 0.1 mM nitrobenzenediazonium on nanoporous gold. The cycles show the characteristic reduction peak of nitrobenzenediazonium at 0.4 V. This reduction potential, which is expected to correlate with the Hammett constant of the substituent,33 is consistent with previous reports of nitrobenzenediazonium reduction on planar gold electrodes.19 The first cycle shows the largest reduction peak but subsequent cycles also show significant peaks. The first three cycles are shown in figure 2a, and the peak area slowly decays with repeated cycling. After approximately 350 cycles the peak is no longer discernable. The peak shape is consistent with a diffusion-controlled process. Higher concentrations of nitrobenzenediazonium show different grafting behavior. The cyclic voltammograms for 1 mM and 10 mM solutions of nitrobenzenediazonium are shown in Figures 2b, and 2c, respectively. For 1 mM nitrobenzenediazonium, the peak potential for the first grafting cycle is located at 0.16 V vs. Ag/AgCl, and is very broad. The second scan is significantly smaller than the first, and the peak area decreases with subsequent scans. Electrochemical blocking is apparent in later scans, and typically scans past the 40th cycle do not have significant cathodic peaks, as demonstrated by the 50th scan included in Figure 2b. The first cycle collected with a 10 mM nitrobenzenediazonium solution shows an extremely broad cathodic peak, corresponding to the diazonium reduction.

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Electrochemical blocking occurs after a single cycle, and reduction peaks are not present in the second and third cycles plotted in Figure 2c.

Figure 2. Cyclic voltammograms of nitrobenzenediazonium grafting onto nanoporous gold, (a) 0.1 mM, (b) 1.0 mM, (c) 10.0 mM nitrobenzenediazonium in 0.1M tetrabutylammonium tetrafluoroborate collected at 100 mV/s

The nitrobenzyl attachment behavior can be compared to results on planar gold surfaces. For solutions of 2 mM and 5mM nitrobenzenediazonium on planar gold, a significant cathodic peak was observed on the first cycle, but almost no

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current variation was observed on subsequent cycles.19,30 The substantial decrease in peak intensity indicated that the diazonium molecules formed a blocking monolayer on planar gold surfaces after one grafting cycle. The peak shapes for the planar gold surfaces also showed a sharp decrease in current following the peak as a blocking layer was formed, rather than the more symmetrical peak shape of a diffusion controlled process.

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Figure 3. (a) IRRAS of a 4-nitrobenzene diazonium film electrochemically grafted on nanoporous gold by grafting 0.1 mM 4-nitrobenzene diazonium for 50 cycles, followed by rinsing in acetonitrile (b) comparison of the electrochemically attached 4-nitrobenzene diazonium to 4-nitrobenzene diazonium dried on the nanoporous gold surface without electrochemical attachment. Figure 3a shows an IRRAS spectrum for a nanoporous gold substrate modified with diazonium salt-derived nitrobenzene layers. The fingerprint region of the spectra shows significant peaks at 1350 cm-1 corresponding to the symmetric

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NO2 stretch, 1529 cm-1 due to the asymmetric NO2 stretch, and 1597 cm-1 due to C=C aromatic stretching. These peaks are similar to previously collected spectra of diazonium salt-derived nitrobenzene layers on planar gold. 21,34 Figure 3b shows a comparison of the spectrum of nitrobenzyl layers electrochemically grafted to nanoporous gold as described and nitrobenzenediazonium dried on a nanoporous gold surface from an acetonitrile solution without an electrochemical potential applied. The peak at 2307 cm-1 is assigned to the N≡N stretch of the diazonium functional group. The higher frequency peaks in both spectra are due to residual CO2 in the sample compartment. The absence of a N≡N stretching peak in the sample in which nitrobenzenediazonium was electrochemically grafted shows that the electrochemical reduction does lead to the loss of the diazonium group and confirms that little or no nitrobenzenediazonium adsorbs to the electrode surfaces. XPS was used to characterize the species following nitrobenzyl attachment. Figure 4 shows a survey spectrum of the bare nanoporous gold and nanoporous gold functionalized with 1.0 mM nitrobenzenediazonium for 350 cycles. Both samples show the expected peaks of gold, including the Au4f5/2 and Au4f7/2 peaks at 88 ev and 84 ev respectively.

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Figure 4. XPS survey spectra of bare nanoporous gold and nanoporous gold with nitrobenzyl electrochemically grafted for 350 cycles High resolution scans show that residual silver is present from the dealloying process and Ag3d3/2 and Ag3d5/2 peaks are also visible at 374 and 368 eV. From analysis of multiple bare nanoporous gold samples we find that the silver peak area is 2.8±0.5% of the gold peak area. Analysis of the Au4f core level spectra before and after the nitrobenzyl attachment shows attenuation of the gold peaks, consistent with a thick molecular layer. The binding energies of the Au4f peaks are consistent with metallic gold, so it is unlikely that the nitrobenzyl binding is through a Au-O-C linkage.35 Similarly, the 3d peaks corresponding to residual silver on the nanoporous gold surface are consistent with metallic silver. Figure 5a shows the nitrogen region for samples grafted from 1 mM nitrobenzenediazonium with the number of grafting cycles varying between 25 and 350. The bare nanoporous gold samples show minimal signal in the nitrogen 1s region, while the diazonium grafted samples show two or three nitrogen peaks. The peak at 406.1 eV present in all diazonium-grafted samples is attributed to the NO2 group. 36,37 The absence of peaks at 403.8 eV indicates the absence of significant N2+

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species, confirming that the electrochemical grafting process does convert these groups.38 Additional nitrogen peaks are apparent at lower binding energies. These peaks indicate the presence of nitrogen in more reduced oxidation states. Samples grafted using a large number of scans, specifically 100 scans and higher show a single low binding nitrogen peak centered at 400.3 eV. Samples grafted with 75 or fewer scans show an additional small peak around 397.5 eV. These peaks have been observed previously in diazonium grafting on gold substrates and the origin of the peaks has been widely discussed in the literature, with several possible causes proposed.

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a.

b.

c.

Figure 5. (a) XPS of the nitrogen region for bare nanoporous gold and nanoporous gold with nitrobenzenediazonium attached using a varying number of cyclic voltammetry cycles, (b) XPS comparison of the nitrogen region following 50 and 100 scans collected, (c) growth of nitrogen and carbon peak areas for different number of cyclic voltammetry cycles used to attach nitrobenzenediazonium.

X-Ray induced reduction of NO2 groups has been observed for nitrophenyl groups on zinc surfaces, leading to a peak around 400 eV.39 To test for X-ray beam

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induced reduction of the NO2 groups we collected a nitrogen spectrum of the same sample following 50 scans and following 100 scans, shown in Figure 5b. Between 50 scans and 100 scans the area of the peak at 400 eV increased by 2% and the area of the peak at 406 eV attributed to the NO2 group decreased by 11%. While this indicates that some of the NO2 groups were reduced or degraded by the X-ray beam, beam induced reduction does not seem to be the primary cause of the peak at 400 eV. The consistent size of the peak at 400 eV with increasing X-ray exposure and the absence of similar peaks in previously reported 4-nitrobezenethiol spectra21 lead to our conclusion that X-Ray induced reduction may contribute but it is not the only cause of the reduced nitrogen peaks. The presence of the reduced nitrogen species has also been attributed to the formation of azo linakges in a multilayered network of molecules. Several groups attribute N1s peaks at 400 or 401 eV to N=N-azo linkages,19,21,40 and this type of structure is supported by TOF-SIMs data.41 We therefore conclude that the peak at 400 eV is due to the presence of N=N-azo linkages. We do not unambiguously observe corresponding evidence of N=N-azo linkages in our IRRAS data. These peaks would be expected between 1370 and 1463 cm-1. 42,43 We do observe a small peak at 1408 cm-1 and a shoulder on the side of the NO2 symmetric stretching peak at 1383 cm-1 that could be due to N=N-azo groups. The small peak at 397.5 eV in the 25 scan samples in Figure 5a suggests the presence of Au-N bonds. These small peaks around 397 eV are also present in samples functionalized with 50 and 75 reducing scans (Figure S1). Peaks at this

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energy attributed to Au-N bonds44 have previously been reported for spontaneously grafted nitrobenzenediazonium on planar gold44,45 and one study of electrochemically grafted nitrobenzenediazonium on planar gold.42 The 397.5 eV peak may be due to direct interaction between the diazonium group and the gold surface,45 or through an Au-N=N bond.42 The peak at 397.5 eV is present only at low numbers of binding cycles and decreases with an increased number of binding cycles. Some of the decrease can be attributed to attenuation of the peak due to multilayer formation, but the decrease in peak area more substantial than the attenuation of the Au4f peaks. It is possible that Au-N bonds form at low nitrobenzyl coverages but are displaced by Au-C bonds as the surface coverage of nitrobenzyl increases with additional grafting cycles. This is consistent with mechanistic analysis indicating that molecules can be initially physisorbed during the early deposition before slower chemisorption is completed.46 We collected C1s XPS spectra to investigate the nature of the bond between nitrobenzyl and the nanoporous gold surface (Figure S2). The carbon spectra show a bulk carbon peak at 285.0 eV, with a higher binding energy component centered around 286.1 eV. These energies are expected for aromatic carbon atoms and the carbon atom bound to the nitro group, respectively.19 We would expect a peak around 283 eV for a gold-carbon bond but do not observe an unambiguous peak in this area. As a result, we are unable to assign carbon peaks to an Au-C bond. The bare nanoporous gold contains significant levels of carbon and the fragile nature of the surface precludes most methods of cleaning used on planar gold surfaces. The most viable method of removing carbononaceous material in the literature was an

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ozone treatment, which produced a large amount of oxide on the gold surface rendering it unsuitable for this work.47 In Figure 5c we have plotted the area of the nitrogen peaks at 400 eV and at 406 eV as a function of the number of grafting cycles as well as the area of the carbon peaks, all normalized to the size of the gold peaks. The peak at 406 eV, attributed to NO2 groups, grows quickly between 0 and 100 cycles and continues to increase more slowly through 350 cycles. The peak at 400 eV, on the other hand, appears to achieve a constant area around 50 cycles. These results indicate that additional cycles add more NO2 groups on the surface, but N=N-azo linkages do not increase with additional grafting cycles. The growth of the carbon peak appears to mirror the growth of the 406 eV/NO2 peak. The growth of multilayers and polymerization of the azo groups has been widely reported on flat gold surfaces. However, it is notable that on the nanoporous gold surfaces, we are able to increase the number of NO2 groups with additional grafting cycles. Density of nitrobenzyl layers on nanoporous gold The electrochemical response of Fe(CN)63-/4- is frequently used to evaluate the density of molecular layers. A pinhole-free, dense molecular layer should block electron transfer between the Fe(CN)63-/4- molecules and the nanoporous gold surface, leading to no reduction or oxidation response of the probe. Figure 6 shows the response of Fe(CN)63-/4- at a nanoporous gold electrode with 4nitrobenzene diaznoium grafted from 10 mM solutions. The solid lines shows the Fe(CN)63-/4- response after 3 grafting cycles. These plots show typical reduction and oxidation behavior of a diffusion controlled redox couple. The dashed blue plots

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show the Fe(CN)63-/4- response after 50 grafting cycles and display similar behavior to the cyclic voltammograms measured following 3 grafting cycles, although the peaks are slightly attenuated. The dotted red plot shows the Fe(CN)63-/4- response after 350 grafting cycles. The redox behavior is significantly attenuated compared to the 3 and 50 cycle grafting plots. The electron transfer kinetics are also significantly decreased for the samples functionalized with 350 cycles as evidenced by the increase in peak-to-peak splitting. However, we do not observe complete blocking of the Fe(CN)63-/4- electron transfer and there are still significant redox peaks present in all of the samples. We observe similar results while using lower concentrations of 4nitrobenzene diazonium during the electrochemical grafting. Grafting 0.1 mM or 1.0 mM 4-nitrobenzene diazonium for 3 or 50 cycles leads to little to no attenuation of the Fe(CN)63-/4- oxidation and reduction peaks while grafting either concentration for 350 cycles leads to significant attenuation of the peaks but not full blocking of electrochemical activity.

Figure 6. Cyclic voltammograms of 1 mM Fe(CN)63-/4- in 1M KCl collected at 100 mV/s on nanoporous gold samples functionalized with 10 mM 4-nitrobenzene diazonium for a varying number of grafting cycles.

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The attenuation of the cyclic voltammetry peaks and decreased electron transfer kinetics confirms that the diazonium functionalization takes place throughout the pores of the nanoporous gold substrate. The presence of electron transfer between the solution and the nanoporous gold electrode following all nitrobenzenediazonium attachment procedures tested indicates that all of the molecular layers formed either have defects or are not sufficiently thick to block electron transfer to the surface. These results are in contrast to previous reports of nitrobenzenediazonium grafting on planar gold surfaces, in which blocking molecular layers have been achieved under similar conditions to those used here.21 However, low density disordered films have been imaged by STM on carbon48 and gold surfaces. 17 STM images on graphite show the growth of polyaryl groups at low grafting concentrations and low surface coverages, indicating that radical attachment to the initially attached aryl species is kinetically favorable. 48 Preferential attachment of molecules to pregrafted aryl species may explain the absence of an electrochemically blocking layer on nanoporous gold despite the XPS evidence of multilayers. Stability of nitrobenzyl layers on nanoporous gold Tests of the stability of the nitrobenzyl molecular layers on nanoporous gold are somewhat limited by the fragile nature of the nanoporous gold surfaces. For example, refluxing conditions or sonication, which are commonly used to test molecular layer stability, both lead to destruction or delamination of the nanoporous gold itself.

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To test the stability of the molecular layers, we quantified the absorbance of the NO2 symmetric stretching peak, νs, NO2, at 1352 cm-1. To validate the use of this peak as a quantitative diagnostic, we calculated the uncertainty of the peak height by measuring the spectrum of the same sample at three different locations. The percent relative standard deviation for the νs, NO2 peak absorbance was 4%. Exposure to solvents can also cause reorientation of the film, which can lead to changes in the absorbance due to reorientation of surface dipole moments. We tested the impact of solvent by measuring the absorbance of the νs, NO2 peak before and after soaking samples in ethanol or acetonitrile. There was no measureable change in the νs, NO2 peak absorbance following exposure to either solvent. These measurements indicate that the absorbance of the νs, NO2 peak is a reproducible parameter for quantification. a.

NBD after ODT

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NBD after ODT NBD before ODT

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Figure 7. IRRAS study of nitrobenzyl layers before and after exposure to 1octadecanethiol (ODT) (a) finger print region before and after ODT exposure, (b) CH2/CH3 region before and after ODT exposure.

To test the stability of the nitrobenzyl layers compared to alkanethiol self assembled monolayers, we exposed a nitrobenzyl layer to a solution of 1octadecanethiol. We formed a diazonium-based molecular layer by grafting 0.1 mM nitrobenzenediazonium for 50 cycles, which produces a partial molecular layer that does not cause electrochemical blocking of Fe(CN)62+/3+. We then exposed the nitrobenzyl functionalized sample to a solution of 1 mM 1-octadecanethiol in ethanol for seven days. Alkanethiols are known to form well-ordered molecular layers on gold surfaces. Long alkyl chains such as 1-octadecanethiol will also bind preferentially compared to short chains due to thermodynamic control of the molecular layer composition.49 The IRRAS in Figures 7a and 7b show the extent of 1-octadecanethiol displacement of the nitrobenzyl layer. Figure 7a shows the fingerprint region. The lower spectrum shows the sample following nitrobenzyl grafting. The sample was then exposed to 1-octadecanethiol and rinsed in ethanol to produce the upper spectrum. The νs, NO2 at 1350 cm-1 for the as-deposited nitrobenzyl film had an absorbance of 0.00175. Following the 1-octadecanethiol displacement reaction, the peak height decreased to 0.00143. These results show that approximately 80% of the NO2 groups were retained following the reaction in 1-octadecanethiol. Rinsing the sample in ethanol for the same week-long time period resulted in retention of 79±6% of the νs, NO2 intensity. These results indicate that the decrease in νs, NO2

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intensity is likely due to the loss of nonspecifically adsorbed components that are weakly bound to the surface. Decreases in peak intensity may also be due to changes in the inclination of the attached groups. According to surface selection rules, only dipole moments with components oriented perpendicular to the surface can interact with p-polarized light.50 While there is loss of material following the ethanol rinsing or 1-octadecanethiol reaction, the remaining nitrobenzyl groups indicate the presence of a strongly-bound molecular layer. Figure 7b shows the region of CH2 and CH3 stretching peaks. The lower spectrum shows the sample following nitrobenzyl grafting, and shows an absence of peaks in the CH2/CH3 region as expected. The upper spectrum shows the same sample following 1-octadecanethiol exposure. The presence of peaks in the CH2/CH3 stretching region shows that the 1-octadecanethiol molecules do bind to the nanoporous gold surface during the displacement study. The νa, CH2 peak is located at 2918 cm-1. The position and width of the νa, CH2 are indicative of the structure of the molecular layer. The peak frequency observed is consistent with the formation of a crystalline molecular layer, in which the expected peak position is 2917 cm-1, compared to 2924 cm-1 expected for a liquid state.51 The crystalline-like peak placement indicates that the 1-octadecanethiol chains are able to achieve a high degree of ordering within the nitrobenzyl layer. Following the IRRAS measurements, we measured the response of these nitrobenzyl and 1-octadecanethiol functionalized surfaces to Fe(CN)63-/4-. The resulting cyclic voltammograms show no oxidation or reduction peaks (Figure S3), indicating that the 1-octadecane thiol molecules form a dense layer throughout the

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surface and the crystallinity we observe in IRRAS is not restricted to limited domains. The electrochemical response of nitrobenzyl groups on nanoporous gold is affected by the structure of the molecular layers and the three dimensional nature of the underlying nanoporous gold. We used electrochemical impedance spectroscopy to more fully characterize the electrical response of the nitrobenzyl layers with respect to applied potential. The impedance response of the nitrobenzyl grafted nanoporous gold samples is compared to the impedance response of bare nanoporous gold in Figure 8. All impedance spectra were collected at 0 V vs. the Ag/AgCl reference electrode in a solution of 0.1 M KCl and 1 mM K3Fe(CN)6. We performed cyclic voltammetry before each impedance scan to confirm the integrity of the nanoporous gold material. Following impedance spectroscopy collection at 0 V, we applied a series of potentials to the surfaces. In each case, the noted potential was applied for 30s before collecting an impedance spectrum at 0 V. We applied increasingly positive and negative potentials in sequence, beginning with +0.2V, then -0.2V and proceeding in intervals of ±0.2V. Control experiments show that the impedance spectrum of the bare nanoporous gold does not change significantly with applied potentials within ±0.8 V. At more extreme potentials the nanoporous gold tends to delaminate from the underlying glass substrate, likely due to the reduction of the thiol attachment between the underlying glass and the nanoporous gold,52 so our experiments were limited to the ±0.8 V potential range.

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a.

b.

c.

Figure 8. Impedance spectroscopy of nitrobenzyl functionalized nanoporous gold compared to bare nanoporous gold in 1 mM Fe(CN)63-/4-, 0.1 M KCl. Nitrobenzyl functionalized samples were subjected to a series of increasing applied potentials for 30 s each following the initial spectrum. (a) Nyquist plot, (b) absolute impedance, (c) phase angle We will compare the impedance spectra of the nitrobenzyl functionalized nanoporous gold sample to the bare nanoporous gold sample before discussing the effects of applied potential. At high frequency, the behavior of both the bare nanoporous gold and the nitrobenzyl modified nanoporous gold is largely resistive.

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The impedance magnitude, shown in Figure 8b, is constant at around 85 Ω, and the phase angle, shown in Figure 8c, is close to zero. The phase angle of zero indicates an absence of capacitive elements. The low, constant resistance is indicative of the uncompensated solution resistance. The exact magnitude of the impedance at high frequency depends on the distance between the surface and reference electrode, and the variations in impedance in this region show small differences in reference electrode placement between samples. With a decrease in frequency, the system becomes more capacitive for all samples, and the phase angle decreases. In this region we can observe small differences between the nitrobenzyl functionalized nanoporous gold and the bare surfaces. The minimum phase angle for the bare surfaces reaches -77°, while the nitrobenzyl functionalized nanoporous gold sample reaches a minimum of -66°. The impedance of a perfect capacitor is -90°, so this indicates that none of the samples behave as an ideal capacitor. Increased capacitive behavior of the films is accompanied by an increase in the magnitude of the impedance with decreased frequency. The increase in impedance magnitude is similar for all samples and we do not observe significant differences between the bare and nitrobenzyl functionalized samples. Following applied potentials, the Nyquist plot of nitrobenzyl functionalized nanoporous gold samples in Figure 8a approaches the appearance of the bare nanoporous gold. Small changes are noted in the spectra following the application of 0.4V, with larger changes apparent following applications of 0.6 and 0.8V. Notably, the samples to which 0.8V were applied are still substantially different than

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the bare nanoporous gold sample, suggesting that there is still a significant amount of the molecular layer remaining. We can use the plots of impedance characteristics as a function of frequency in Figures 8b and 8c to further determine the cause of the changes observed in the Nyquist plot with applied potential. While the nitrobenzyl functionalized and bare nanoporous gold samples have similar impedance magnitudes throughout the frequency range, the maximum phase angle of the nitrobenzyl functionalized samples decreases with applied potential, approaching the value of the bare nanoporous gold sample. These results indicate that the changes resulting from the applied potential are affecting the capacitive character of the molecular layers. While the deLevie model has previously been used to fit impedance spectroscopy data of nanoporous gold electrodes, the model did not converge for our data, which limits our theoretical understanding of the data. Overall we observe that the spectra for nitrobenzyl functionalized nanoporous gold approaches the spectrum for bare nanoporous gold following the application of increasing applied potentials, but some of the molecular layer does appear to resist the applied potential. Our impedance spectroscopy suggests that a portion of the diazoniumderived molecular layer is more stable than the underlying nanoporous gold substrate. Our study of 1-octadecanethiol replacement also indicates the presence of a strongly-bound molecular component. Differences in binding between planar gold electrodes and nanoporous gold likely originate from the exposed crystallographic faces and step edges in the surface. Planar gold substrates typically consist of the preferential (111)

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orientation, often with grain sizes that can reach microns in thermally annealed surfaces. Electrochemical analysis of nanoporous gold electrodes indicates that a combination of (111), (100), and (110), surfaces are exposed, with other higher index sites likely present in smaller amounts. 7 On an Au(111) surface, DFT calculations predict that attachment of a phenyl ring will be chemical in nature and the ring will be oriented upright.24 Nanoparticles modeled as an Au20 pyramidal cluster showed that nitrobenzene molecules were 44 kJ/mol more stable when bound to the vertex of the pyramid compared to the face of the pyramid.20 It is therefore likely that nitrobenzyl binds preferentially at defect and step sites on the nanoporous gold surfaces. The high density of these sites on nanoporous gold may account for the high stability of a portion of the nitrobenzyl layer. Different nanoporous gold fabrication methods will be required to fully measure the stability of the diazonium-derived layers. While the use of gold leaf is an inexpensive and accessible way to manufacture nanoporous gold electrodes, the underlying gold substrates are not stable with applied potentials above ±0.8V and are mechanically too fragile to withstand sonication or refluxing conditions. As a result, tests of the stability of the diazonium-derived layers are limited. Conclusions The grafting of diazonium-based salts has been applied to nanoporous gold electrodes. The observation of a thick molecular layer with the absence of electrochemical blocking indicates a thick and disordered layer of molecules that contains defects and openings. The resulting molecular layers are notably stable

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with respect to displacement by long alkanethiol chains, indicating that this method of functionalization should be considered for applications.

Supporting Information: Additional XPS and cyclic voltammetry figures. This information is available free of charge via the Internet at http://pubs.acs.org/

Acknowledgements The authors thank the Research Corporation for Science Advancement and the College of the Holy Cross for financial support. CL thanks Kathleen and Stephen R. Winslow for summer stipend support. SEM and XPS were performed at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award no. ECS-0335765. CNS is part of Harvard University.

References (1)

Ding, Y.; Kim, Y. J.; Erlenbacher, J. Nanoporous Gold Leaf: “Ancient Technology”/Advanced Material. Adv. Mater. 2004, 16, 1897–1900.

(2)

Ciesielski, P. N.; Scott, A. M.; Faulkner, C. J.; Berron, B. J.; Cliffel, D. E.; Jennings, G. K. Functionalized Nanoporous Gold Leaf Electrode Films for the Immobilization of Photosystem I. ACS Nano 2008, 2, 2465–2472.

(3)

Wittstock, A.; Biener, J.; Bäumer, M. Nanoporous Gold: a New Material for Catalytic and Sensor Applications. Phys. Chem. Chem. Phys. 2010, 12, 12919.

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Page 29 of 36

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(4)

Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Science 2010, 327, 319–322.

(5)

Wittstock, A.; Neumann, B.; Schaefer, A.; Dumbuya, K.; Kübel, C.; Biener, M. M.; Zielasek, V.; Steinrück, H.-P.; Gottfried, J. M.; Biener, J.; et al. Nanoporous Au: an Unsupported Pure Gold Catalyst? J. Phys. Chem. C 2009, 113, 5593– 5600.

(6)

Scanlon, M. D.; Salaj-Kosla, U.; Belochapkine, S.; MacAodha, D.; Leech, D.; Ding, Y.; Magner, E. Characterization of Nanoporous Gold Electrodes for Bioelectrochemical Applications. Langmuir 2012, 28, 2251–2261.

(7)

Seo, B.; Kim, J. Electrooxidation of Glucose at Nanoporous Gold Surfaces: Structure Dependent Electrocatalysis and Its Application to Amperometric Detection. Electroanalysis 2010, 22, 939–945.

(8)

Wittstock, A.; Wichmann, A.; Bäumer, M. Nanoporous Gold as a Platform for a Building Block Catalyst. ACS Catal. 2012, 2, 2199–2215.

(9)

Hakamada, M.; Takahashi, M.; Furukawa, T.; Tajima, K.; Yoshimura, K.; Chino, Y.; Mabuchi, M. Electrochemical Stability of Self-Assembled Monolayers on Nanoporous Au. Phys. Chem. Chem. Phys. 2011, 13, 12277.

(10)

Cortés, E.; Rubert, A. A.; Benitez, G.; Carro, P.; Vela, M. E.; Salvarezza, R. C. Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on Nanostructured Gold Substrates. Langmuir 2009, 25, 5661–5666.

(11)

Vericat, C.; Benitez, G. A.; Grumelli, D. E.; Vela, M. E.; Salvarezza, R. C. ThiolCapped Gold: From Planar to Irregular Surfaces. J. Phys.: Condens. Matter 2008, 20, 184004.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12)

Page 30 of 36

Schlenoff, J. B.; Li, M.; Ly, H. Stability and Self-Exchange in Alkanethiol Monolayers. J. Am. Chem. Soc. 2001, 117, 12528–12536.

(13)

Noh, J.; Hara, M. Molecular-Scale Desorption Processes and the Alternating Missing-Row Phase of Alkanethiol Self-Assembled Monolayers on Au(111). Langmuir 2001, 17, 7280–7285.

(14)

Pinson, J.; Podvorica, F. Attachment of Organic Layers to Conductive or Semiconductive Surfaces by Reduction of Diazonium Salts. Chem. Soc. Rev. 2005, 34, 429.

(15)

Bélanger, D.; Pinson, J. Electrografting: a Powerful Method for Surface Modification. Chem. Soc. Rev. 2011, 40, 3995.

(16)

Liu, G.; Böcking, T.; Gooding, J. J. Diazonium Salts: Stable Monolayers on Gold Electrodes for Sensing Applications. J. Electroanal. Chem. 2007, 600, 335– 344.

(17)

Cui, B.; Gu, J.-Y.; Chen, T.; Yan, H.-J.; Wang, D.; Wan, L.-J. Solution Effect on Diazonium-Modified Au(111): Reactions and Structures. Langmuir 2013, 29, 2955–2960.

(18)

Kariuki, J. K.; McDermott, M. T. Formation of Multilayers on Glassy Carbon Electrodes via the Reduction of Diazonium Salts. Langmuir 2001, 17, 5947– 5951.

(19)

Laforgue, A.; Addou, T.; Bélanger, D. Characterization of the Deposition of Organic Molecules at the Surface of Gold by the Electrochemical Reduction of Aryldiazonium Cations. Langmuir 2005, 21, 6855–6865.

(20)

Laurentius, L.; Stoyanov, S. R.; Gusarov, S.; Kovalenko, A.; Du, R.; Lopinski, G.

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P.; McDermott, M. T. Diazonium-Derived Aryl Films on Gold Nanoparticles: Evidence for a Carbon–Gold Covalent Bond. ACS Nano 2011, 5, 4219–4227. (21)

Shewchuk, D. M.; McDermott, M. T. Comparison of Diazonium Salt Derived and Thiol Derived Nitrobenzene Layers on Gold. Langmuir 2009, 25, 4556– 4563.

(22)

Liu, G.; Luais, E.; Gooding, J. J. The Fabrication of Stable Gold NanoparticleModified Interfaces for Electrochemistry. Langmuir 2011, 27, 4176–4183.

(23)

la Llave, de, E.; Ricci, A.; Calvo, E. J.; Scherlis, D. A. Binding Between Carbon and the Au(111) Surface and What Makes It Different From the S−Au(111) Bond. J. Phys. Chem. C 2008, 112, 17611–17617.

(24)

Jiang, D.-E.; Sumpter, B. G.; Dai, S. Structure and Bonding Between an Aryl Group and Metal Surfaces. J. Am. Chem. Soc. 2006, 128, 6030–6031.

(25)

Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Time-of-Flight Secondary Ion Mass Spectroscopy Characterization of the Covalent Bonding Between a Carbon Surface and Aryl Groups. Langmuir 2005, 21, 280–286.

(26)

Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Nitrophenyl Groups in Diazonium-Generated Multilayered Films: Which Are Electrochemically Responsive? Langmuir 2010, 26, 10812–10821.

(27)

Paulik, M. G.; Brooksby, P. A.; Abell, A. D.; Downard, A. J. Grafting Aryl Diazonium Cations to Polycrystalline Gold: Insights Into Film Structure Using Gold Oxide Reduction, Redox Probe Electrochemistry, and Contact Angle Behavior. J. Phys. Chem. C 2007, 111, 7808–7815.

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Page 32 of 36

Kullapere, M.; Marandi, M.; Matisen, L.; Mirkhalaf, F.; Carvalho, A. E.; Maia, G.; Sammelselg, V.; Tammeveski, K. Blocking Properties of Gold Electrodes Modified with 4-Nitrophenyl and 4-Decylphenyl Groups. J Solid State Electrochem 2011, 16, 569–578.

(29)

Kibena, E.; Marandi, M.; Mäeorg, U.; Venarusso, L. B.; Maia, G.; Matisen, L.; Kasikov, A.; Sammelselg, V.; Tammeveski, K. Electrochemical Modification of Gold Electrodes with Azobenzene Derivatives by Diazonium Reduction. ChemPhysChem 2013, 14, 1043–1054.

(30)

Haccoun, J.; Vautrin-Ul, C.; Chaussé, A.; Adenier, A. Electrochemical Grafting of Organic Coating Onto Gold Surfaces: Influence of the Electrochemical Conditions on the Grafting of Nitrobenzene Diazonium Salt. Prog. Org. Coat. 2008, 63, 18–24.

(31)

Singh, J.; Whitten, J. E. Adsorption of 3-Mercaptopropyltrimethoxysilane on Silicon Oxide Surfaces and Adsorbate Interaction with Thermally Deposited Gold. J. Phys. Chem. C 2008, 112, 19088–19096.

(32)

Halliwell, C. M.; Cass, A. E. G. A Factorial Analysis of Silanization Conditions for the Immobilization of Oligonucleotides on Glass Surfaces. Anal. Chem. 2001, 73, 2476–2483.

(33)

Andrieux, C. P.; Pinson, J. The Standard Redox Potential of the Phenyl Radical/Anion Couple. J. Am. Chem. Soc. 2003, 125, 14801–14806.

(34)

Ricci, A.; Bonazzola, C.; Calvo, E. J. An FT-IRRAS Study of Nitrophenyl Monoand Multilayers Electro-Deposited on Gold by Reduction of the Diazonium Salt. Phys. Chem. Chem. Phys. 2006, 8, 4297.

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Langmuir

(35)

Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, 1992.

(36)

Elliott, C. M.; Murray, R. W. Chemically Modified Carbon Electrodes. Anal. Chem. 1976, 48, 1247–1254.

(37)

Lui, Y.-C.; McCreery, R. L. Reactions of Organic Monolayers on Carbon Surfaces Observed with Unenhanced Raman Spectroscopy. J. Am. Chem. Soc. 1995, 117, 11254–22359.

(38)

Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. Direct Covalent Grafting of Conjugated Molecules Onto Si, GaAs, and Pd Surfaces From Aryldiazonium Salts. J. Am. Chem. Soc. 2004, 126, 370–378.

(39)

Adenier, A.; Cabet-Deliry, E.; Chaussé, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Grafting of Nitrophenyl Groups on Carbon and Metallic Surfaces Without Electrochemical Induction. Chem. Mater. 2005, 17, 491– 501.

(40)

Saby, C.; Ortiz, B.; Champagne, G. Y.; Bélanger, D. Electrochemical Modification of Glassy Carbon Electrode Using Aromatic Diazonium Salts. 1. Blocking Effect of 4-Nitrophenyl and 4-Carboxyphenyl Groups. Langmuir 1997, 13, 6805–6813.

(41)

Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Surface Modification of Conducting Substrates. Existence of Azo Bonds in the Structure of Organic Layers Obtained From Diazonium Salts. Chem. Mater. 2007, 19, 4570–4575.

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(42)

Page 34 of 36

Ricci, A. M.; Méndez De Leo, L. P.; Williams, F. J.; Calvo, E. J. Some Evidence for the Formation of an Azo Bond During the Electroreduction of Diazonium Salts on Au Substrates. ChemPhysChem 2012, 13, 2119–2127.

(43)

Jung, U.; Kuhn, S.; Cornelissen, U.; Tuczek, F.; Strunskus, T.; Zaporojtchenko, V.; Kubitschke, J.; Herges, R.; Magnussen, O. Azobenzene-Containing Triazatriangulenium Adlayers on Au(111): Structural and Spectroscopic Characterization. Langmuir 2011, 27, 5899–5908.

(44)

Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vilà, N. Electrochemical Oxidation of Aliphatic Amines and Their Attachment to Carbon and Metal Surfaces. Langmuir 2004, 20, 8243–8253.

(45)

Mesnage, A.; Lefèvre, X.; Jégou, P.; Deniau, G.; Palacin, S. Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces. Langmuir 2012, 28, 11767–11778.

(46)

Jayasundara, D. R.; Cullen, R. J.; Soldi, L.; Colavita, P. E. In Situ Studies of the Adsorption Kinetics of 4-Nitrobenzenediazonium Salt on Gold. Langmuir 2011, 27, 13029–13036.

(47)

Schaefer, A.; Ragazzon, D.; Wittstock, A.; Walle, L. E.; Borg, A.; Baumer, M.; Sandell, A. Toward Controlled Modification of Nanoporous Gold. a Detailed Surface Science Study on Cleaning and Oxidation. J. Phys. Chem. C 2012, 116, 4564–4571.

(48)

Greenwood, J.; Phan, T. H.; Fujita, Y.; Li, Z.; Ivasenko, O.; Vanderlinden, W.; Van Gorp, H.; Frederickx, W.; Lu, G.; Tahara, K.; et al. Covalent Modification of Graphene and Graphite Using Diazonium Chemistry: Tunable Grafting and

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Nanomanipulation. ACS Nano 2015, 9, 5520–5535. (49)

Bain, C. D.; Evall, J.; Whitesides, G. M. Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Head Group, Tail Group, and Solvent. J. Am. Chem. Soc. 1989, 111, 7155–7157.

(50)

Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of Intfrared Spectroscopy of Ultrathin Films; John Wiley & Sons: Hoboken, N. J., 2003.

(51)

Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. Spontaneously Organized Molecular Assemblies. 4. Structural Characterization of N-Alkyl Thiol Monolayers on Gold by Optical Ellipsometry, Infrared Spectroscopy, and Electrochemistry. J. Am. Chem. Soc. 1987, 109, 3559–3568.

(52)

Widrig, C. A.; Chung, C.; Porter, M. D. The Electrochemical Desorption of NAlkanethiol Monolayers From Polycrystalline Au and Ag Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1991, 310, 335–359.

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