Electrochemical Surface Plasmon Resonance Research Articles

Development of graphene oxide/poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) thin film-based electrochemical surface plasmon resonance immunosensor for detection of human immunoglobulin G

An electrochemically synthesized graphene oxide (GO)/poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(styrene sulfonate) (PSS) thin film-based electrochemical surface plasmon resonance (EC-SPR) sensor chip was developed and employed for the detection of human immunoglobulin G (IgG). GO introduced the carboxylic group on the film surface, which also allowed electrochemical control, for the immobilization of the anti-IgG antibody via covalent bonding through amide coupling reaction. The SPR sensitivity of the detection was improved under the control by applying an electrochemical potential, by which the sensitivity was increased by the increment in applied potential. Among the open-circuit and different applied potentials in the range of −1.0 to 0.50 V, the EC-SPR immunosensor at an applied potential of 0.50 V exhibited the highest sensitivity of 6.08 × 10−3 mL µg−1 cm−2 and linearity in the human IgG concentration range of 1.0 to 10 µg mL−1 with a relatively low detection limit of 0.35 µg mL−1. The proposed sensor chip is promising for immunosensing at the physiological level.

Application of electrochemical surface plasmon resonance (ESPR) to the study of electroactive microbial biofilms.

Electrochemical surface plasmon resonance (ESPR) monitors faradaic processes optically by the change in refractive index that occurs with a change in redox state at the electrode surface. Here we apply ESPR to investigate the anode-grown Geobacter sulfurreducens biofilm (GSB), a model system used to study electroactive microbial biofilms (EABFs) which perform electrochemical reactions using electrodes as metabolic electron acceptors or donors. A substantial body of evidence indicates that electron transfer reactions among hemes of c-type cytochromes (c-Cyt) play major roles in the extracellular electron transfer (EET) pathways that connect intracellular metabolic processes of cells in an EABF to the electrode surface. The results reported here reveal that when the potential of the electrode is changed from relatively oxidizing (0.40 V vs. SHE) to reducing (-0.55 V vs. SHE) and then back to oxidizing, 70% of c-Cyt residing closest to the biofilm/electrode (within hundreds of nm from the electrode surface) appear to remain trapped in the reduced state, requiring as long as 12 hours to be re-oxidized. c-Cyt storing electrons cannot contribute to EET, yet turnover current resulting from cellular oxidation of acetate coupled with EET to the electrode surface is unaffected. This suggests that a relatively small fraction of c-Cyt residing closest to the biofilm/electrode interface is involved in EET while the majority store electrons. The results also reveal that biomass density at the biofilm/electrode interface increases rapidly during lag phase, reaching its maximum value at the onset of exponential biofilm growth when turnover current begins to rapidly increase.

Stepwise control of reduction of graphene oxide and quantitative real-time evaluation of residual oxygen content using EC-SPR for a label-free electrochemical immunosensor

Abstract The aim of this study was to develop an electrochemical surface plasmon resonance (EC-SPR) method for the stepwise reduction of graphene oxide (GO) by monitoring the real-time refractive index in an effort to control the residual oxygen functionality and conductivity in GO sheets, and then to enhance sensitivity to detect immunoaffinity. The EC-SPR technique acts as a real-time operating system to be used for the observation of oxygen chemical element on the surface of GO. Experimental results demonstrated that EC-SPR signals could quantitatively detect real-time changes in the refractive index of GO films during the stepwise removal of oxygen functional groups. Cyclic voltammetry (CV) in the initial cycles of the electrochemical reduction of GO showed that the oxygen content of the GO film declined by approximately 60%. The SPR angle shifts during the electrochemical reduction for 10, 50 and 100 CV cycles were 164, 218 and 223 mdeg, and the corresponding X-ray photoelectron spectroscopy (XPS) spectra carbon-to-oxygen (C/O) ratios were 17.35, 21.07 and 30.95, respectively. The obtained electrochemically-reduced graphene oxide (ERGO) film chip had a very high sensitivity to detect anti-BSA, and this electrochemical immunosensor was more sensitive than an SPR immunosensor. Our results confirm that this EC-SPR technique could be used to develop electrochemical biosensors with immediate modification of GO film surfaces.

Odd–Even Effects in Electroactive Self-Assembled Monolayers Investigated by Electrochemical Surface Plasmon Resonance and Impedance Spectroscopy

Subtle conformational-based distinctions in the supramolecular structure of self-assembled monolayers (SAMs) of ω-ferrocenylalkanethiolates on gold (FcCnSAu) have been shown to significantly impact their electrical and redox properties. We investigate the effect of differences in the intermolecular van der Waals interactions and molecular packing energy of FcCnSAu SAMs comprised of an odd (SAModd) versus even (SAMeven) number of methylenes on the electrochemical behavior for n = 6–16. Redox-induced thickness changes are investigated by electrochemical surface plasmon resonance (ESPR). Electrochemical impedance spectroscopy (EIS) under non-Faradaic conditions is used to evaluate the dielectric properties of the reduced (neutral) state of the FcCnSAu SAMs. Oxidation of the SAM-bound ferrocenes yields an effective film thickness change of 0.19 ± 0.01 nm, which is attributed to untilting of the alkyl chains. The SAMs manifest odd–even differences in the apparent redox potential, and hence in the thermodynamic...

Electrochemical Impedance Spectroscopy on Interdigitated Gold Microelectrodes for Glycosylated Human Serum Albumin Characterization.

Glycosylated albumin is considered as a potentially accurate indicator of shorter-term average glucose concentration compared with the current standard HbA1c and as such, it is attracting the interest of the scientific community as a possible diagnosis marker for diabetic patients. The purpose of this paper is to achieve a better understanding of the glycation effect of albumin on its electrochemical properties. That is done through the use of Interdigitated gold microelectrodes (IDGE) as support in a label free impedimetric immunosensor for the detection of human serum albumin detection in glycated (GA) and non-glycated (HSA) form. Anti-human serum albumin, a monoclonal antibody, was physisorbed on the surface of IDGE and used as a HSA/GA bioreceptor. Electrochemical impedance spectroscopy, cyclic voltammetry, and surface plasmon resonance imaging (SPRi) were used for the characterization of the grafted layers onto the gold surface. A detection range from 1 to 401 ng/mL of non glycated HSA antigen in phosphate buffered saline buffer was obtained with the impedance spectroscopy technique. The experiment led to the observation of a significant impedance difference between the glycated and non-glycated antigen of HSA. SPRi measurements confirmed these findings and allowed us to suggest an increase of the dielectric permittivity for human serum albumin upon glycation.

Dual-Mode Electro-Optical Techniques for Biosensing Applications: A Review.

The monitoring of biomolecular interactions is a key requirement for the study of complex biological processes and the diagnosis of disease. Technologies that are capable of providing label-free, real-time insight into these interactions are of great value for the scientific and clinical communities. Greater understanding of biomolecular interactions alongside increased detection accuracy can be achieved using technology that can provide parallel information about multiple parameters of a single biomolecular process. For example, electro-optical techniques combine optical and electrochemical information to provide more accurate and detailed measurements that provide unique insights into molecular structure and function. Here, we present a comparison of the main methods for electro-optical biosensing, namely, electrochemical surface plasmon resonance (EC-SPR), electrochemical optical waveguide lightmode spectroscopy (EC-OWLS), and the recently reported silicon-based electrophotonic approach. The comparison considers different application spaces, such as the detection of low concentrations of biomolecules, integration, the tailoring of light-matter interaction for the understanding of biomolecular processes, and 2D imaging of biointeractions on a surface.

Voltammetric, Spectroscopic, and Microscopic Investigation of the Oxidation of Solid and Solution Phases of Tetrathiafulvalene (TTF) to (TTF)2MO4 (M=Mo, W)

AbstractThe diffusion‐controlled solid−solid‐state transformation of TTF to (TTF)2MoO4 and (TTF)2WO4 is reported at TTF‐modified glassy carbon, indium tin oxide (ITO), Au, and Pt electrodes in contact with aqueous MoO42− or WO42− electrolytes. Electrocrystallization of (TTF)2MoO4 and (TTF)2WO4 onto the electrode surface also was achieved by oxidation of TTF to TTF+ in the presence of Na2MoO4 and Na2WO4 in the mixed CH3CN/H2O (90 : 10 v/v) solvent containing 0.1 M Bu4NPF6 as the supporting electrolyte. Mechanistic aspects of the solid−solid state interconversion reactions and electrocrystallization have been probed by using cyclic voltammetry, chronoamperometry, electrochemical quartz crystal microbalance (EQCM), and electrochemical surface plasmon resonance (ESPR), whereas the morphology of the electrocrystallized materials was established by using scanning electron microscopy (SEM). Vibrational (IR, Raman) and UV/Vis spectroscopic methods, as well as other techniques, were used to establish that the products of electrooxidation are (TTF)2MoO4 and (TTF)2WO4.

Characterization of Covalently Bound Anti-Human Immunoglobulins on Self-Assembled Monolayer Modified Gold Electrodes.

Bioconjugated gold surfaces constitute interesting platforms for biosensing applications. The immobilization of antibodies such as anti-immunoglobulin G and M (anti-IgG and anti-IgM) on gold electrodes via self-assembled monolayers (SAMs) is here studied as a model system for further immunoassays development. The biolayer is characterized by means of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), a dedicated thin-film transistor (TFT)-based platform and electrochemical surface plasmon resonance (EC-SPR). XPS analysis confirms the presence of all the chemical species involved in the fabrication process as well as the covalent attachment of the antibodies with high reproducibility. Visualization of the biolayer topography by AFM shows nanostructures with a thickness consistent with the actual size of the protein, which is also verified by SPR measurements. EC-SPR allows taking advantage of complementary electrochemical and optical signals during the functionalization steps. Moreover, the functionalization of gold leads to a change in the work function, which is demonstrated in an electrolyte gated thin-film transistor configuration. Such configuration enables also to evaluate the electrostatic changes occurring on the gate that are connected with the threshold voltage shifts. The data support that functional biomodified gold surfaces can be reproducibly prepared, which is a prerequisite for further biosensor development.

2D zirconium-based metal-organic framework nanosheets for highly sensitive detection of mucin 1: consistency between electrochemical and surface plasmon resonance methods

Two-dimensional (2D) zirconium-based metal-organic framework (denoted as 521-MOF) nanosheets with the thickness of 6.0–7.5 nm were prepared with the aid of polyvinyl pyrrolidone (PVP) under the mild conditions and low temperature (50 °C). Since 521-MOF nanosheets displayed good electrochemical activity, high surface area, and strong affinity interaction between the MOF and the oligonucleotides sequences, they can impel the immobilization of large amounts of aptamer strands when applied as a platform of biosensor. As a result, the developed aptasensor exhibited sensitive bio-recognition for the cancer determination marker protein, mucin 1 (MUC1). The combination of electrochemical techniques and surface plasmon resonance spectroscopy (SPR) was performed to probe the kinetic processes of the aptamer immobilization and the MUC1 detection. The consistency between different determination approaches was observed, in which the developed aptasensor based on 521-MOF nanosheets exhibits pretty high sensitivity for detecting MUC1 with a low detect limit of 0.12 and 0.65 pg·ml−1 deduced from electrochemical impedance spectroscopy and SPR, respectively, within the broad concentration range of MUC1 from 0.001 to 0.5 ng·ml−1. Simultaneously, a comparable affinity constant, Ka, was derived from EIS and SPR, which also demonstrates that this new biosensing strategy has high selectivity, stability, reproducibility, and good applicability for the MUC1 detection in the human serum. The present finding indicates that the synthesized 521-MOF nanosheets can be employed in the fields of the biosensing or biomedical diagnosis and explored for different kinds of biosensors.

In Situ Investigation of Electrochemically Mediated Surface-Initiated Atom Transfer Radical Polymerization by Electrochemical Surface Plasmon Resonance.

Electrochemically mediated atom transfer radical polymerization (eATRP) initiates/controls the controlled/living ATRP chain propagation process by electrochemically generating (regenerating) the activator (lower-oxidation-state metal complex) from deactivator (higher-oxidation-state metal complex). Despite successful demonstrations in both of the homogeneous polymerization and heterogeneous systems (namely, surface-initiated ATRP, SI-ATRP), the eATRP process itself has never been in situ investigated, and important information regarding this process remains unrevealed. In this work, we report the first investigation of the electrochemically mediated SI-ATRP (eSI-ATRP) by rationally combining the electrochemical technique with real-time surface plasmon resonance (SPR). In the experiment, the potential of a SPR gold chip modified by the self-assembled monolayer of the ATRP initiator was controlled to electrochemically reduce the deactivator to activator to initiate the SI-ATRP, and the whole process was simultaneously monitored by SPR with a high time resolution of 0.1 s. It is found that it is feasible to electrochemically trigger/control the SI-ATRP and the polymerization rate is correlated to the potential applied to the gold chip. This work reveals important kinetic information for eSI-ATRP and offers a powerful platform for in situ investigation of such complicated processes.

Electrochemical surface plasmon resonance sensor based on two-electrode configuration

To obtain detailed information about electrochemistry reactions, a two-electrode electrochemical surface plasmon resonance (EC-SPR) sensor has been proposed. We describe the theory of potential modulation for this novel sensor and determine the factors that can change the SPR resonance angle. The reference electrode in three-electrode configuration was eliminated, and comparing with several other electrode materials, activated carbon (AC) is employed as the suitable counter electrode for its potential stability. Just like three-electrode configuration, the simpler AC two-electrode system can also obtain detailed information about the electrochemical reactions.

Using electrochemical surface plasmon resonance for in-situ kinetic investigations of solid electrolyte interphase formation in lithium ion battery

Abstract The solid electrolyte interphase (SEI) significantly affects the energy density and safety performance of lithium ion batteries. Previous studies have shown that conventional analyses cannot characterize the real-time molecule interactions of SEI formation on the surface of an electrode. In this study, a novel in situ electrochemical-surface plasmon resonance (EC-SPR) was developed for evaluating the kinetic changes of ionic dissociation, SEI formation, and Li-Au alloying reaction. The novel EC-SPR not only indicates the rates of each reaction stage but also reveals the characteristics of the passivated layers. This research demonstrates that the dissociation rates of ionic clusters are affected by several reactions on the surface of an electrode. The rate and mass of the SEI formation from the reduction of ethylene carbonate (EC) are calculated at 0.004 ngs-1 and 5.858 ng, respectively. The EC-SPR is a powerful tool for further in situ kinetic investigations of different electrolyte and electrode systems.

Electrochemical Surface Plasmon Resonance Fiber-Optic Sensor: In Situ Detection of Electroactive Biofilms.

Spectroelectrochemistry has been found to be an efficient technique for revealing extracellular electron transfer (EET) mechanism of electroactive biofilms (EABs). Herein, we propose a novel electrochemical surface plasmon resonance (EC-SPR) optical fiber sensor for monitoring EABs in situ. The sensor uses a tilted fiber Bragg grating (TFBG) imprinted in a commercial single-mode fiber and coated with nanoscale gold film for high-efficiency SPR excitation. The wavelength shift of the surface plasmon resonance (SPR) over the fiber surface clearly identifies the electrochemical activity of the surface localized (adjacent to the electrode interface) bacterial cells in EABs, which differs from the "bulk" detections of the conventional electrochemical measurements. A close relationship between the variations of redox state of the EABs and the changes of the SPR under potentiostatic conditions has been achieved, pointing to a new way to study the EET mechanism of the EABs. Benefiting from its compact size, high sensitivity, and ease of use, together with remote operation ability, the proposed sensor opens up a multitude of opportunities for monitoring EABs in various hard-to-reach environments.

Biochip for the Detection of Bacillus anthracis Lethal Factor and Therapeutic Agents against Anthrax Toxins.

Tethered lipid bilayer membranes (tBLMs) have been used in many applications, including biosensing and membrane protein structure studies. This report describes a biosensor for anthrax toxins that was fabricated through the self-assembly of a tBLM with B. anthracis protective antigen ion channels that are both the recognition element and electrochemical transducer. We characterize the sensor and its properties with electrochemical impedance spectroscopy and surface plasmon resonance. The sensor shows a sensitivity similar to ELISA and can also be used to rapidly screen for molecules that bind to the toxins and potentially inhibit their lethal effects.

Redox-dependent interactions between reduced/oxidized cytochrome c and cytochrome c oxidase evaluated by in-situ electrochemical surface plasmon resonance.

The interactions between the redox couple of cytochrome c (Cyt c) and cytochrome c oxidase (COX) were investigated at a mimic redox-modulated interface by using an electrochemical surface plasmon resonance (EC-SPR) system. Although early studies of the binding between COX and Cyt c have been conducted using several techniques in homogeneous solutions, a problem still inherent is that ferro-cytochrome c (Cyt c red), the reduced form of Cyt c, can be easily oxidized into ferri-cytochrome c (Cyt c ox) and adversely impact the accuracy and reproducibility of the binding measurements. In order to realize reliable redox-dependent binding tests, here the Cyt c red is quantitatively electro-generated from Cyt c ox by in situ cathodic polarization in a flow cell. Then the kinetic and dissociation constants of the bindings between COX and Cyt c red/Cyt c ox can be evaluated accurately. In this study, the values of association/dissociation rate constants (k a, k d) for both COX/Cyt c red and COX/Cyt c ox were obtained. The dissociation constants, K D, were finally calculated as 3.33 × 10(-8) mol · L(-1) for COX/Cyt c red and 4.25 × 10(-5) mol · L(-1) for COX/Cyt c ox, respectively. In-situ EC-SPR is promising for better mimicking the in vivo condition that COX is embedded in the inner mitochondrial membrane and Cyt c acts as an electron shuttle in the mobile phase. It is an effective method for the investigation of redox-dependent biomolecular interactions. Graphical Abstract Schematic representation of the experimental designs using EC-SPR system. (a) the Au-Cys-COX SPR chip with SAM layers. (b) redox-modulated Cyt c and its binding onto pre-immobilized COX.

A Universal Surface for Label-Free Electrical and Optical Sensing of Disease Markers

Quantitative point-of-care (POC) diagnostic tools are crucial for next-generation human (and animal) health monitoring and disease diagnosis to rapidly infer effective treatment decisions. ELISA based disease diagnostics is often performed off-site in centralized labs, requiring long turnaround times (>72hrs), reducing the effective period for disease intervention. In this study, a gold sensing surface is shown to provide ELISA-equivalent results across multiple label-free and rapid biosensing platforms including potentiometric field-effect transistor (FET) biosensors, electrochemical impedance spectroscopy (EIS) biosensors and surface plasmon resonance (SPR) devices. Using the model pathogen Bovine Herpes Virus 1 (BHV-1) (a major source of economic loss in the agri-food industry globally 1), BHV-1 glycoprotein E (gE) was attached to sensor surfaces to detect the immunological interaction against anti-BHV-1 antibodies. The use of a gold sensing surface offers access to the well-established thiol surface functionalization and protein immobilization via amine coupling. This functionalized gold sensing surface is used to compare the response of ELISA in four different sensing platforms: (1) as the gate electrode for graphene ion-sensitive field-effect transistor (ISFET), (2) as the gate electrode for extended-gate field-effect transistor (EGFET) sensors (3) as the working electrode (WE) for EIS sensors, and (4) as the sensing surface for SPR. The results show that the sensitivity of the EGFET biosensors is equivalent to the ISFETs.2 Moreover, EGFET biosensors provide many advantages including improved reliability by separating the sensor and transducer components, low-cost and easy fabrication, disposable chip design and high compatibility with state-of-art semiconductor technologies.3-5 While the mechanism for EIS is quite different than potentiometric FET-based sensing, the dynamic response is found to be equivalent. Similarly, the same Au surface can be used in optical SPR biosensors using the same surface chemistry. The universality of a Au sensing surface allows detection using multi-platforms but with a single chip and, therefore, simplifies the sample preparation and reduces the required sample volumes for diagnostics. All three electrical sensors demonstrate performance equivalent to SPR and ELISA, with dynamic range approaching the limit of detection for sera dilutions < 10-3of anti-BHV-1 antiserum. However, the potentiometric and EIS sensors provide results much faster (~10 min.) than SPR (~2 hr.) and ELISA (> 20 hr.) Additionally, the electronic biosensors are compact in size and are promising for POC devices. In conclusion, a gold surface is shown to provide universal and equivalent sensing across multiple platforms using well-established surface chemistry. Furthermore, the use of one surface to facilitate integration of multiple sensing mechanisms can potentially improve specificity. Among all of the available biosensor technologies, electrical biosensors provide rapid, label-free and cost-efficient sensing capability in a compact size, which is promising for quantitative point-of-care devices. 1G. D. Snowder et al. Journal of Animal Science 85, 1885 (2007). 2A. Tarasov et al. 2D Materials 2, 044008 (2015). 3L.-L. Chi et al. Materials Chemistry and Physics 63 , 19 (2000). 4P. Dak et al. Device Research Conference (DRC), 2013 71st Annual, 105 (2013). 5W. Guan et al. Biosensors and Bioelectronics 51, 225 (2014).

Electrochemical determination of lead ion with DNA oligonucleotide-based biosensor using anionic redox marker

The development of DNA-modified electrodes for the determination of lead cation is described. The performance of anionic (AQMS) and cationic (methylene blue) redox markers for this aim is compared. It is shown that the use of anionic marker results in a significantly better analytical parameters, as compared to measurements with cationic marker. The optimized sensor distinguishes itself with high selectivity towards lead cation and lower detection limit of 0.42 10−8moldm−3. The interaction between lead ion and DNA strands were studied using electrochemical impedance spectroscopy (EIS) and surface plasmon resonance with imaging (SPRi).

Using QCM and SPR for the Kinetic Evaluation of the Binding Between A New Recombinant Chimeric Protein and Specific Antibodies of the Visceral Leishmaniasis.

In the present study, the surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) techniques were employed to kinetically evaluate the binding affinity of a new recombinant chimeric protein (CP10) toward anti-Leishmania infantum antibodies for the immunodiagnostics of the visceral leishmaniasis (VL). This chimeric protein was formed by the union in a same artificial coding DNA of ten different peptides, which showed themselves reactive toward positive canine serum for VL. Using the CP10 in enzyme-linked immunosorbent assays (ELISA), it was possible to detect 80% of the asymptomatic infected dogs. After this, SPR and QCM immunosensors were constructed by the covalent immobilization of the CP10 on a self-assembled monolayer (SAM) formed by adsorption of alkanethiol on gold substrates. The thickness (6.80 nm) and the refractive index (1.475) of the protein on the SAM were simultaneously determined through SPR curves measured in different wavelengths (670 and 785 nm). Interactions between the CP10 and its specific IgGs (anti-CP10 antibodies) were characterized by the electrochemical impedance spectroscopy, SPR and QCM techniques. The equilibrium dissociation constant obtained by SPR (K(D) = 8.27 x 10(-10) mol.L(-1)) and QCM (K(D) = 2.42 x 10(- 10) mol.L(-1)) demonstrated high binding affinity of the CP10 toward anti-CP10 antibodies. In this sense, this work quantitatively proves the strong antigenic character of a new recombinant chimeric protein, giving evidence to potential contribution for the use of this protein in programs of control of the VL.

Electrochemical surface plasmon resonance sensing with absorptive redox mediator film

Surface plasmon resonance (SPR) spectroscopy has been used for the detection of reversible refractive index changes that occur in a redox mediator film during potentiostatic oxydoreduction. We investigated the theoretical and experimental parameters of electrochemical SPR (eSPR) sensing using as an example of the electroactive labels the Methylene Blue (MB) organic dye. The optimal eSPR interrogation method for MB sensing in a solution and in a uniform interfacial thin film formed by stem-loop oligonucleotides tagged with MB are evaluated. Electrochemical activation of the optical SPR response depends on the local MB concentration and can be used to design sensitive and highly selective biosensing approaches.

Electrochemical Reactions at Ionic Liquid/Gold Interface Probed By Electrochemical Surface Plasmon Resonance

Ionic liquids (ILs) are promising materials for electrochemical devices. To maximize the performance of such devices, studies on the interfacial structure, its dynamics, and the kinetics of electrochemical reactions at electrochemical IL interfaces are of crucial importance. Previously we studied the dynamics of the structure at IL|Au interface by using electrochemical surface plasmon resonance (ESPR) [1]. The transient curves of the SPR angle, which reflects the change in the structure in the electrical double layer, showed extraordinarily slow relaxation on the order of minutes during multi-potential steps, and revealed asymmetry of the relaxation time to the potential step direction [1]. In the present study, electrochemical reactions at the IL|Au interface have been studied using ESPR. We have found that redox reactions at the IL|Au interface can also be sensitively probed using ESPR when we add redox species such as ferrocene and its derivatives in ILs. Sigmoid curves of the SPR angle are observed during electrode potential scan. We have analyzed the sigmoid curves by simultaneously using a theory and a digital simulation, and have revealed specific interaction between redox species and IL ions depending on their charges. [1] N. Nishi, Y. Hirano, T. Motokawa, T. Kakiuchi, Phys. Chem. Chem. Phys. 15 (2013) 11615.