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Surface-enhanced Raman spectroscopy investigations of interfacial chemistry under potential control

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Chaoxiong Ma
Abstract:
Surface-enhanced Raman scattering (SERS) spectroscopy is a sensitive tool that can be used to probe chemistry at metal-solution interfaces. To control the interfacial activity of ionic species at the metal surface, the metal-surface potential is controlled with a potentiostat, while SERS provides an in situ method to observe the interfacial chemistry. In this work, SERS studies are carried out with potential control to investigate monolayer self-assembly, acid-base chemistry, ion-pair interactions, and reduction of electroactive anions at chemically-modified silver surfaces. Adsorption and self-assembly of 11-mercaptoundecanoic acid on silver was monitored by SERS. The time-dependent profiles of Raman spectra indicate a multistep self-assembly process, which involves participation of both thiol and carboxylate groups in the adsorption process and depends on the solvent, solution pH, and surface potential. The acid-base chemistry of 2-mercaptobenzoic acid (2-MBA) immobilized on a silver surface was also investigated. The benzoate form and benzoic acid form of 2-MBA could be identified spectroscopically to determine the relative populations of the bound ligand. In addition, shifts in the carboxylate stretching mode of 2-MBA revealed interactions between the benzoate group and the silver surface, which could be displaced by other anions in solution. It was found that applied potential has significant effects on the proton dissociation equilibrium of immobilized 2-MBA, an effect arising from the changes in the interfacial pH relative to bulk solution. Adsorption of cetylpyridinium (CP+ ) and its interaction with nitrobenzenesulfonate (NB- ) on a 1-dodecanthiol (C 12 ) modified silver surface was also studied. The binding of NB - to the C12 surface relies on its ion-pairing with CP + . Adsorption of CP+ and ion-pair stability on the C12 surface can be modulated by electrolyte concentration. The results provide understanding of surfactant adsorption and ion interactions involved in ion-interaction chromatography. SERS and cyclic voltammetry were used to investigate reduction of NB- on bare and C12 -modified silver surfaces in the presence of CP+ . The reduction was identified by the disappearance of the NO2 symmetric stretching mode and frequency shifts in the ring breathing mode. Ion-pair accumulation of NB- can be observed on C12 surface, and its repeatable reduction was studied by cyclic voltammetry and SERS.

TABLE OF CONTENTS ABSTRACT………………………………………………………………………….iii

ACKNOWLEDGMENTS………………………………………………………..….vii

1.

INTRODUCTION…………………………………………………………….1 1.1

Background …………………………………………………………….....1 1.2

Overview……………………………………………………………..…..10 1.3

References………………………………………………………………..13

2.

SURFACE-ENHANCED RAMAN SCATTERING STUDY OF THE KINETICS OF SELF ASSEMBLY OF CARBOXYLATE-TERMINATED N-ALKANETHIOLS ON SILVER ……………….………………….…......17

2.1

Introduction……………………………………………………….……...17 2.2

Experimental.………………………………………………………….....20 2.3

Results and Discussion…………………………………………………..22 2.4

Conclusions………………………………………………………………46 2.5

References………………………………………………………………..46

3.

SURFACE-ENHANCED RAMAN SPECTROSCOPY INVESTIGATION OF THE POTENTIAL-DEPENDENT ACID-BASE CHEMISTRY OF SILVER-IMMOBILIZED 2-ME RCAPTOBENZOIC ACID.………………51

3.1

Introduction……………………………………………………..………..51 3.2

Experimental ……………………………………………………..….......54 3.3

Results and Discussion…………………………………………...…..….55 3.4

Conclusions…………………………………………………….………...75 3.5

References…………………………………………………….………….77

4.

SURFACE-ENHANCED RAMAN SPECTROSCOPY STUDIES OF SURFACTANT ADSORPTION A ND ION-INTERACTION ON A HYDROPHOBIC SURFACE..……………………………………………....81

4.1

Introduction….……………………………………………………….…..81 4.2

Experimental….……………………………………………………….....84 4.3

Results and Discussion…………………………………………………..85 4.4

Conclusions……………………………………………………………..103

vi 4.5

References………………………………………………………………104

5.

ADSORPTION AND ELECTROCHEMI CAL REDUCTION OF IONIC ANALYTES ON A HYDROPHOBI C SURFACE THROUGH ION INTERACTION WITH CETYLP YRIDINIUM CHLORIDE…………......108

5.1

Introduction…………………………………………………………..…108 5.2

Experimental.…………………………………………………………...110 5.3

Results and Discussion……………………………………………...….112 5.4

Conclusions………………………………………………………...…...126 5.5

References……………………………………………………...……….126

ACKNOWLEDGMENTS I would like to take this opportunity to thank all those people who helped me to finish my research and dissertation over the years. First of all, I would like to thank my advisor, Dr. Joel Harris for his guidance and support. His enthusiasm and love for science, generosity, patience and willingness to help have always impressed me. His insight and broad knowledge of chemistry have been a tremendous help in my research and have made my graduate study enjoyable. I was extremely fortunate to have him as my mentor during my graduate study and as an advisor for my future career. Secondly, I am so grateful to everybody in the Harris group for their kindness and support. Eric Peterson provided helpful instruction using the coating system. He was somebody I could count on whenever the system was down or when I encountered a problem in the lab. Emily Heider and Jennifer Ramirez are such encouraging people, giving me a lot of support in my writing. It was such a wonderful experience to have run a marathon with them. Jonathan Schaefer and Grant Myers were always open to help whenever I had a question. Doug Kriech and Jay Kitt helped machining parts and setting up the green laser. Appreciation is also given to Moussa Barhoum, Justin Cooper, Christopher Fox, Joshua Wayment, and all other group members who have been helpful, stimulating and supportive of my research. I also would like to express my thanks to Jiewen Xiong and Dr. Henry White for their help with the cyclic voltammetry measurements.

viii Finally, I would like to thank my family for their support of my career. Special thanks are given to my wife, Qili Shen, for her support, understanding and patience through all these years. This research was supported in part with funds from the U.S. Department of Energy under Grant DE-FG03-93ER14333.

CHAPTER 1 INTRODUCTION 1.1 Background

Chemistry at liquid/solid interfaces is of increasing interest in research because it is fundamental to many chemical and biochemical systems. 1-21 Adsorption, desorption, and reactions at interfaces are involved in numerous analytical methods 1-6 and heterogeneous catalysis, 12-14 which generally require immobilizing target molecules to the solid surface. Therefore, understanding the chemistry that governs surface reactions and the ability to control the chemical and structural properties of surfaces contribute to advances in chromatography, 1-4, 19, 21 chemical and biological sensing, 1-11 catalysis, and many other techniques. The structure and properties at interfacial regions differ significantly from those in bulk solution, and accordingly, exhibit unique physical processes and chemical reactions compared with the homogeneous phase. 1-5, 20

The special characteristic of the interface originates largely from the substrates that are used for the immobilization of ligands or other target molecules. Numerous substrates including metals, 1, 2, 5-8 metal oxides, 12, 22, 23 graphite, 3, 24 alumina, 25 mica, 26

silica, 4, 19, 21 and polymers 13 have been employed as supports for interfacial chemistry. The variety of the substrates also leads to huge measurement challenge for probing the chemical structure of the interface. Coinage metals, such as gold or silver, are popular materials that have been used widely not only as electrodes, 1, 7, 8, 27-29 but also as

2 substrates for surface modification by self-assembly, 1, 5-7, 15-17 providing functionalized surfaces with controllable properties such as wettability, 16, 17, 30 adhesion, 9, 23 redox activity, 7, 15, 18 and biocompatibility. 7, 9, 17

1.1.1 The metal/aqueous interface and potential modulation

Ionic and electronic processes involved at metal surfaces in contact with an aqueous solution form a charged surface, which polarizes the adjacent aqueous phase and gathers the counter ions so as to minimize the free energy of the interfacial system. 20, 27

As a direct consequence of the charged metal surface, ion accumulation, and the formation of a diffuse double-layer, the interfacial potential is one of the key factors controlling the behavior of both bound and free species at the interface. The interfacial potential, which is determined by the structure of the diffuse double-layer, decays as it propagates into solution. A well-known theory developed by Gouy and Chapman based on a statistical mechanical approach provides an expression for the interfacial potential profile φ as a function of the distance, x, from the electrode surface. 20, 27

)exp( )4tanh( )4tanh( 0 x kTze kTze κ φ φ −= [1.1] where z is the magnitude of the charge on ions, e is the charge of the electron, k is the Boltzmann constant, T is the absolute temperature, and κ is the reciprocal of the characteristic thickness of the diffuse layer. κ /1 is also referred to as the “Debye length.” with dimensions of distance and can be calculated by, 20, 27

2/1 22 0 1 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = ∑ i zne kT i εε κ [1.2]

3 where ε is the dielectric constant of the medium, 0 ε is the permittivity of free space, and i n is the number concentration of ion i in the bulk solution. In case of a small applied potential 0 φ ( mV) /50 0 z < φ , the expression of the potential profile (eq 1.1) can be approximated by: )exp( 0 x κ φ φ − = [1.3] For small applied potentials, the interfacial potential decays by a factor of 1/e at a distance of κ /1 from the electrode surface. The value of κ /1 is inversely proportional to the square of the electrolyte concentration and drops off very rapidly as the concentration of electrolyte increases. κ /1 also depends directly on the valency (z) of the ions involved. For a 1:1 electrolyte, κ /1 has a value of 0.3 and 30 nm for a 1.0 M and 4 101 − × M solutions. Figure 1.1 gives the potential profile calculated from eq 1.3 for a 1:1 electrolyte at several different electrolyte concentrations. It is obvious that the higher the electrolyte concentration, the faster the decay of potential from the electrode surface. The Poisson-Boltzmann equation (eq 1.4) predicts the activity of the ions ( xi a , ) at a distance x away from a charged surface depending on the interfacial potential. 20, 27

⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Φ− = RT zF aa x xi i exp 0 , [1.4] In eq 1.4, 0 i a is ion activity of the bulk solution and x Φ is the potential at the distance x, and z is the charge of the ion. The exponential relationship of this equation shows that small variations in the potential can result in large changes in the interfacial ion activity and, thereby, perturb the binding equilibria of ionic species.

4

0 5 10 15 20 25 30 0.0 0.2 0.4 0.6 0.8 1.0 φ/φ 0 x, (nm) 10 -1 M 10 -2 M 10 -3 M

Figure 1.1 The decay of interfacial potential with distance from a surface in different electrolyte concentrations

5 In addition to its effect on the interfacial ion activity, the decay of interfacial potential generates an electric field that can affect the electronic structure of immobilized ligands, and consequently, their binding affinity for species in solution. 1, 2, 8, 16 The influence of the potential and electric field on interfacial processes has been widely investigated and exploited successfully for many different applications. For example, Porter et al. have demonstrated the utility of electrochemical modulation for modifying the retention of ionic species on graphite electrode surfaces. 3, 31, 32 The applied potential altered the interaction of charged analytes with the carbon surface, changing their retention. The technique was applied successfully to manipulate separations in chromatography without the need for changes in mobile phase composition . Lahann et al. have reported control of surface properties based on the conformational transition of 16-mercapto-hexadecanoic acid immobilized on gold. 16 An applied positive potential attracts the negatively charged carboxylate groups toward the gold surface, leading to a switch in surface wettability as the hydrophobic chains undergo conformational changes. The same concept has been applied successfully to manipulate the binding and unbinding of a protein on a monolayer modified surface using applied potential for control. 11 Kelley et al. found that the orientation of self-assembled DNA duplexes on the surface undergo a dramatic morphology change as a function of potential. 8 The helices stand perpendicularly or lie flat on the metal surface depending on the applied potential. Hybridization and dehybridization of DNA immobilized on electrode surfaces modulated by electric field have been demonstrated by fluorescence, 33 surface plasmon resonance, 34

and surface-enhanced Raman spectroscopy. 35 Oklejas et al. developed a probe to measure electric fields in the diffuse double-layer based on SERS detection of the vibrational

6 Stark effect from nitrile-terminated monolayers on silver surfaces. 36, 37 They also used the electric fields to exert control over the tautomerization of the ligand immobilized on silver surface, shifting the equilibrium of metal ion complexation. 1 Burgess et al. demonstrated that protonation/deprotonation of self-assembled monolayers of carboxylic acid-terminated thiols can be driven by electric fields as investigated with electrochemical impedance spectroscopy, cyclic voltammetry, and Fourier transform infrared spectroscopy. 38, 39 The effect of the applied potential on the charge-transfer contribution to the adsorption free energy has been utilized successfully to control the adsorption and desorption of the n-alkanethiolates on Ag surface from aqueous solution. 40-42 The electrochemical and SERS measurement provide information about the chemisorption free energy and the capability to control the accumulation of molecules on the surface using applied potentials. 40-42

Previous studies have demonstrated the advantages of potential control by electrochemical methods for the investigation of metal/solution interfaces. With potential control, the variations in interfacial properties that would arise from an uncontrolled surface potential are minimized and more consistent and reproducible results can be obtained. Additionally, the applied potential affords an opportunity to change the activity of ions at the interface. The interfacial activities of target analytes can be adjusted in situ

to manipulate the bulk concentration response range of an immobilized ligand. Manipulation of surface ligand electronic structure and conformation is another tool allowed by controlling the applied potential. This manipulation may shift the equilibrium to favor a desired product, or to turn a sensor on and off. 1, 11, 16, 18 By quickly and reversibly perturbing interfacial equilibria, potential control may also be used in kinetic

7 studies of interfacial ion-binding equilibria when combined together with time-resolved detection techniques. 1, 11, 16, 18 In short, potential control may be employed for analytical measurements at interfaces with improved sensitivity, dynamic range, speed, and reproducibility. 1.1.2 Surface enhanced Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) is a surface-sensitive technique that provides vibrational information about molecules adsorbed or bound to coinage metal surfaces

with significant enhancement of Raman scatting. 43-46

The overall enhancement factor is typically observed on the order of 10 4 -10 6 , and can be as high as 10 8 and 10 14 under favorable circumstances. 43-46 The SERS effect was first observed from the adsorption of pyridine on an electrochemically roughened silver electrode by Fleischmann et al. in 1974. 47

They attributed the huge boost of the Raman signal to the increase of surface area of Ag electrode by roughening process, which however could not account for magnitude of the signal enhancement. More credit for the SERS discovery 48, 49 was given later to Van Duyne and Creighton et al., who independently pointed out that an enhancement of the scattered intensity was involved in the adsorption process, and proposed two different theories to explain the enhancement effect. In 1978, Moskovits suggested that the unusually increased intensity of the Raman signals could be a consequence of the excitation of surface plasmons. 50 This idea led to a number of predictions about potentially SERS-active substrates, their relative enhancements, nanoscale structural features of substrates that are required for SERS to occur. These predictions were subsequently confirmed by experimental results. Although the mechanisms leading to the phenomenal enhancement of Raman scattering intensity are still under investigation, two mechanisms are commonly cited by

8 the researchers to describe the enhancement effect. 43-46, 48-50 The primary one that contributes to ~10 4 of enhancement is electromagnetic enhancement 43-46, 49 (EM), which involves increases in the excitation and scattering field intensities as a result of plasmon resonance excitation. When interacting with a rough metal, an electromagnetic wave can excite localized plasmons on the metal surface, resulting in amplification of the electromagnetic fields near the surface. The enhancement in the EM model is a direct consequence of the interaction of adsorbed molecules with the amplified electromagnetic fields. The intensity of the Raman scattering is proportional to the square of the amplitude of the electric field of light incident on the adsorbate. Using a simplified model, the enhancement factor E of the Raman signal is given by, 44, 45

22 )'()( ωω EEE = [1.5] where )( ω E and )'( ω E are the local electric-field enhancement factors at the incident frequency, ω , and Stockes-shifted frequency, ' ω , respectively. It comes as no surprise from the above equation that a small increase in the local field can lead to such huge enhancement in the Raman scattering. The presence of EM depends strongly on the roughness features of the metal surface. Only when the surface roughness is small in comparison to the wavelength of the incident light will excitation of the plasmon occur. In addition, visible and near-infrared radiation commonly employed for Raman spectroscopy restricts the choice of the metals to silver, copper, and gold so as to satisfy the resonance condition to provide maximal enhancement.

9 The electromagnetic field involved in Raman scattering that has a maximum at the metal surface decays rapidly in strength with distance from the surface. As a consequence, the enhancement falls off according to: 44, 45

12 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = dr r G [1.6] where r is the radius of the spherical of the metal roughness feature and d is the distance of scattering molecules from the metal surface. Experimental results also showed that the enhancement decreases ten-fold for the analytes with a distance of 2-3 nm away from substrate surface. This distance-dependent effect of the enhancement makes SERS a surface-selective and powerful tool of for investigating on the interfacial processes with little interference from the bulk solution. A second mechanism called chemical enhancement 43-46, 48 (CE) provides an order of magnitude or two of enhancement to the Raman scattering intensity. The CE mechanism requires direct interaction of the molecules with the metal surface to allow charge transfer between the molecules and the metal substrate to occur, which leads to an increase in the Raman scattering cross-section or polarizability of the molecules and boosts the Raman intensity. This mechanism can be explained by the resonance Raman effect, 43, 51 which assumes that the Fermi level of the metal is half way between the highest occupied orbital and lowest unoccupied orbital of the adsorbate, acting as a bridge to lower the energy required for the excitation to occur. As a result, this charge transfer interaction shifts the transition of the molecules that have lowest-lying excitation at the near ultraviolet region into the visible region, offering a possibility for resonance Raman scattering to occur in typical wavelength region of Raman excitation.

10 1.2 Overview

The applications of surface-enhanced Raman spectroscopy have grown dramatically since its discovery more than 30 years ago. The overall enhancement of Raman scattering intensity in SERS measurements can range from 10 6 -10 14 , making it sensitive enough for detection of a submonolayer coverage of molecules at an interface. 29, 35-37, 43-46 In addition, only those molecules immobilized or close enough to the substrate surface experience signal enhancement; accordingly, SERS is specific for interfacial detection. 43, 45, 46 Furthermore, SERS is compatible with measurements in aqueous solution making it especially useful for analysis of biological systems. 5-7 SERS has been widely used as a molecular probe for the detection of acid/base equilibria, 5, 6, 38

metal-ions binding reactions, 1, 2 glucose accumulation at selective surface coating, 52, 53

and protein and DNA binding to immobilized biological ligands. 7, 35, 54, 55

In this research, SERS was employed to investigate molecular structure, properties, adsorption, and interactions at the metal/solution interface; and to investigate how an applied potential can be employed to control ion activity and the self-assembly kinetics on a surface. These studies may provide insight into the understanding of interfacial phenomena and lay groundwork for the development of new sensors with controlled sensitivity, selectivity, and reversibility. In Chapter 2, adsorption of 11-mercaptoundecanoic acid (MUA) on silver from both aqueous and methanol solutions was monitored in situ by surface-enhanced Raman spectroscopy (SERS). Raman spectra reveal that in addition to the thiol group, the carboxylate group of MUA also interacts with the silver surface during the self-assembly process. Several bands including the ν (C-S), ν s (COO - ) and ν (C-C) were used to describe

11 the evolution of the structure of adsorbed MUA on silver surfaces. The time-dependent profiles of these bands indicate a multistep process, which in the case of aqueous solutions is initiated by the binding of both carboxylate and thiol groups to the silver surface, producing a mixture of gauche and trans conformations. In a subsequent step, the COO-Ag is displaced by the S-Ag, a relatively stronger bond, leading to ordering of the resulting monolayer with formation of a complete SAM with all-trans conformations. This study also showed that the adsorption process depends strongly on the solvent, solution pH, and surface potential of the metal. These factors can significantly affect the participation and displacement of –COO - during the assembly process. In Chapter 3, SERS was used to investigate the potential effect on acidic/basic properties of 2-mercaptobenzoic acid (2-MBA) immobilized on silver surfaces. The COO -

bending mode of the benzoate form and the C-COOH stretching mode of the benzoic acid form of 2-MBA were employed to determine the relative deprotonated and protonated populations of the bound ligand, respectively. In addition, shifts in the symmetric carboxylate stretching mode of 2-MBA reveal interactions between the silver surface and benzoate group, which could be displaced by acetate and other buffer anions from solution. It was found that the applied potential has a significant effect on the proton dissociation equilibrium of immobilized 2-MBA. This effect arises from the surface potential governing the activity of protons at the interface, which changes the interfacial pH relative to bulk solution. The results were fit to a Poisson-Boltzmann model, corrected for potential distribution across monolayer and for interactions between adjacent immobilized ligands. The results show a significant increase in the intrinsic p K a of the immobilized ligand compared to the 2-MBA in free solution, which is likely due to

12 an increase in electron density on the benzoic acid group that occurs upon binding of the thiol group to the silver surface. The study provides a clear picture for the potential effect on protonation/deprotonation of immobilized acid/base molecules. It demonstrates control of interfacial properties using applied potential. In Chapter 4, t he adsorption of cetylpyridinium (CP + ) and its interaction with nitrobenzenesulfonate (NB - ) on 1-dodecanthiol (C 12 ) modified silver surface was studied by SERS. The electrolyte effect on the adsorption equilibrium of CP + on the C 12 surface was investigated. Frumkin and Langmuir isotherms can be used to describe the adsorption process in the absence and presence of KCl, respectively. The binding of the NB - to the C 12 surface relies strongly on the presence of CP + , which is observed to form an ion-pair complex with NB - in the solution phase. The influence of several anions on the binding of NB - to the C 12 surface in the presence of CP + provided their binding affinity with CP + . The concentration effect of CP + on the adsorbed NB - showed a bell shape dependence that is commonly observed for the effect of an ion-interaction regent on the retention of ionic analytes. The plot was quantitatively fit by a dynamic ion exchange model with the presence of ion-pairs in the solution phase. The study demonstrated SERS to be a useful technique to investigate surfactant adsorption and ion interactions at the interface, and to provide information for retention mechanism in ion- interaction chromatography. In Chapter 5, ion pair interactions were used to concentrate NB - near a C 12

modified silver surface. Electrochemical reduction of surface-bound NB - was measured by both SERS and cyclic voltammetry. The decrease and then disappearance of the ν s (NO 2 )

mode , and the shift of ring breathing mode ν 12 are

two distinctive features

13 observed in the reduction process. Similar to electrochemical reduction of nitrobenzene, the reduction of NB - on C 12 surface involves complex pathway and is strongly pH dependent. Differences in reversibility were observed for the reduction on C 12 surface compared to reduction on a bare Ag electrode. For the reduction of NB - on a C 12 surface with the ion-pair protocol, recovery of the adsorbate (NB - ) was observed upon removal of the applied potential, which was not seen for the reduction of NB - on a bare Ag surface. The study showed that protection of the hydrophobic monolayer allows repeatable measurements on the same electrode surface, because the adsorption of the analyte is reversible. In addition, selective adsorption and redox reaction at the interface is possible by using ionic surfactant with strong affinity for the target analyte. 1.3 References

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Full document contains 139 pages
Abstract: Surface-enhanced Raman scattering (SERS) spectroscopy is a sensitive tool that can be used to probe chemistry at metal-solution interfaces. To control the interfacial activity of ionic species at the metal surface, the metal-surface potential is controlled with a potentiostat, while SERS provides an in situ method to observe the interfacial chemistry. In this work, SERS studies are carried out with potential control to investigate monolayer self-assembly, acid-base chemistry, ion-pair interactions, and reduction of electroactive anions at chemically-modified silver surfaces. Adsorption and self-assembly of 11-mercaptoundecanoic acid on silver was monitored by SERS. The time-dependent profiles of Raman spectra indicate a multistep self-assembly process, which involves participation of both thiol and carboxylate groups in the adsorption process and depends on the solvent, solution pH, and surface potential. The acid-base chemistry of 2-mercaptobenzoic acid (2-MBA) immobilized on a silver surface was also investigated. The benzoate form and benzoic acid form of 2-MBA could be identified spectroscopically to determine the relative populations of the bound ligand. In addition, shifts in the carboxylate stretching mode of 2-MBA revealed interactions between the benzoate group and the silver surface, which could be displaced by other anions in solution. It was found that applied potential has significant effects on the proton dissociation equilibrium of immobilized 2-MBA, an effect arising from the changes in the interfacial pH relative to bulk solution. Adsorption of cetylpyridinium (CP+ ) and its interaction with nitrobenzenesulfonate (NB- ) on a 1-dodecanthiol (C 12 ) modified silver surface was also studied. The binding of NB - to the C12 surface relies on its ion-pairing with CP + . Adsorption of CP+ and ion-pair stability on the C12 surface can be modulated by electrolyte concentration. The results provide understanding of surfactant adsorption and ion interactions involved in ion-interaction chromatography. SERS and cyclic voltammetry were used to investigate reduction of NB- on bare and C12 -modified silver surfaces in the presence of CP+ . The reduction was identified by the disappearance of the NO2 symmetric stretching mode and frequency shifts in the ring breathing mode. Ion-pair accumulation of NB- can be observed on C12 surface, and its repeatable reduction was studied by cyclic voltammetry and SERS.