Electrochemical Surface Plasmon Resonance and Its Applications in Biosensing, Bioelectronics, and Material Science

Introduction

A large number of modern chemical and biological sensors are based on the surface plasmon resonance (SPR) phenomenon, and quite often research with SPR sensors is carried out under the conditions of external electric voltage applied to the metal-liquid sensor interface [1-4], which allows to expand the number of applications of SPR method. In particular, the method of electrochemical SPR (ESPR) spectroscopy was used to study electropolymerization [5, 6] and redox properties of polymers [7,8], as well as to detect cofactors [2], glucose [1, 9], hydrogen peroxide [10], and other analytes [11]. An attractive idea for SPR biosensing is to use the applied electrical potential to manipulate adsorption and interaction properties of biomolecules carrying electric charges of different sign and magnitude. It should be borne in mind that the processes affecting the response of the SPR biosensor to biomolecular analytes upon the application of electric potential occur in the nanoscale area at the metal-liquid interface and require careful consideration in order to ensure the correctness of the SPR measurements. In particular, there is a problem in the correct interpretation of the results obtained

Molecular Plasmonics: Theory and Applications Volodymyr I. Chegel and Andrii M. Lopatynskyi Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-65-5 (Hardcover), 978-0-429-29511-9 (eBook) www.jennystanford.com taking into account the effect of the electrical potential both on the object of the study and on the sensitive part of the sensor.

Studies on the influence of electrical potential applied to the metal-electrolyte interface on its optical properties via the excitation of surface plasma waves have been actively conducted over the past decades. In light electroreflection experiments, it was discovered that the optical properties of the metal electrode surface depend significantly on the potential applied to them [12, 13]. Later it was shown that the excitation energy of surface plasmon-polaritons in thin silver and gold films also depends on the magnitude of the applied potential [14]. This chapter gives a theoretical explanation for this effect, but only in the approximation of the change in the concentration of free electrons in the near-surface layer of metal when the electrical potential is applied. The studies in Refs. [15, 16] investigated the excitation of surface plasmon-polaritons on the silver-electrolyte interface, where the effect of the applied potential on the angular position of the SPR was observed. It was noted that at a constant light wavelength, the resonance was shifted toward higher values of the surface plasmon-polariton wave vector (larger angles of incidence] with an increase in the applied potential above the potential of zero charge of the electrode. In the above-mentioned conditions, an increase in the half-width of the SPR curves was also registered. In addition, a method for measuring the potential of zero charge of a metal electrode in an electrolyte solution by means of ESPR measurements was proposed in these works. An investigation of the gold-electrolyte interface using the ellipsometric method for detecting surface plasma waves was carried out in Ref. [17]. Using this method, the possibility of separating the influence of the applied voltage on the electrons in the metal and the ions in the electrolyte was shown. Additionally, the penetration depth of a static electric field into gold was demonstrated to be no more than 1 A.

At the same time, the effect of the external electric potential on the response of SPR biosensor is insufficiently investigated, especially theoretically. In the studies in Refs. [18, 19], the SPR response was studied upon the cyclic change in the potential applied to a thin gold film in electrolytes of different composition with different pH levels, and redox reactions on the gold surface were identified. The paper in Ref. [19] presents a model for calculating the SPR response that considers the structure of the electric double layer at the metal- electrolyte boundary in the Gouy-Chapman approximation and the presence of an oxide layer on gold at significant positive potentials.

It remains necessary to develop a more general theoretical model of the SPR response in the potential range corresponding to the double layer region, in which there are no electrochemical reactions present and which is most suitable for biosensor applications.

This chapter presents the results of specific studies on SPR sensor response peculiarities under the conditions of applied external electrical potential by means of ESPR technique and describes the applications of this method for the investigation of biosensor response manipulation, electropolymerization processes, redox properties, and switchable functions in various systems. The first study is focused on the experimental and theoretical analysis of ESPR sensor response and investigation of the biomolecular adsorption processes under the electrical potential applied to the gold-electrolyte interface. The second study deals with the ESPR transduction of the redox transformations in electropolymerized thin polyaniline films. The promising potential of the ESPR method is shown in the two last studies by the examples of cyclic control of hydrophilic/hydrophobic properties of Ag⁺-thiolate monolayers and biocatalytic charging of gold nanoparticles associated with self- assembled monolayers on Au surface of the SPR chip.

Factor of Interfacial Electrical Potential for the SPR Sensor Response

General Theoretical Background

Electrochemical reactions may progress upon application of an external electrical voltage to the metal-electrolyte interface of the SPR sensor sensitive element [20]. If the metal electrode potential is sufficient for oxidation or reduction of the electrode, molecules, or ions on its interface, then an electrochemical reaction begins, and the current flows through the metal-electrolyte boundary. If the conditions on the interface are such that the electrochemical reactions do not take place, then the current does not flow across the interface and the electrical double layer (EDL) (Fig. 6.1) at the boundary behaves like a capacitor. The potential region where this behavior is observed is the well-known double layer region.

Figure 6.1 Model of electrical double layer on the positively charged electrode surface. Arrows indicate the dipole moments of water molecules. Reprinted from Ref. [20] under a Creative Commons Attribution-NoDerivatives 4.0 International License. Figure caption was adapted.

According to the double layer theory by Stern, the differential capacity of the EDL unit area C_EDL equals the capacity of two series- connected capacitors C_H and Q;c that correspond to the compact layer in the Helmholtz theory and the diffuse layer in the Gouy- Chapman theory [21]:

where d_H is the compact layer thickness, £_el is the relative static dielectric permittivity of the electrolyte, £₀ is the vacuum dielectric permittivity, z is the ion charge, e₀ is the electron charge, N_A is the Avogadro constant, c_el is the electrolyte concentration in mol/1, к is the Boltzmann constant, T is the electrolyte temperature, and U is the potential at the distance d_H from the metal surface. Potential U is determined from Eq. (6.2) [22]:

where U₀ is the total potential difference between the metal electrode and electrolyte. Then the surface charge density Да that appears on the electrode surface is represented by Eq. (6.3) [21]:

where U_pzc is the potential of zero charge of the electrode in the present electrolyte.

Formation of a charge on the metal electrode surface during the charging of EDL leads to a change in the free electron concentration in the surface layer of metal, which is one of the effects taking place when applying the voltage to a metal-electrolyte system. As the static electric field penetrates the metal to the distance d ~ 1 A, the change in the free electron concentration, which is defined by the equation AN = Aa/e₀d, takes place in this thin layer only [14]. This results in the change in the free electron contribution to the relative dielectric permittivity of this layer, which is represented by Eq. (6.4) [14]:

where is the free electron contribution to the relative dielectric permittivity of the metal and N is the free electron concentration in the metal. The free electron contribution to the relative dielectric permittivity of the metal is defined by Eq. (6.5) [23]:

where (O_p = Nel/s₀m* is the plasma frequency, m* is the effective mass of an electron, со is the light angular frequency, and ris the free electron relaxation time.

Since the relative dielectric permittivity of the surface layer of a metal undergoes a change when the voltage is applied to the metal- electrolyte interface according to Eq. [6.4), the value of the SPR angle will also depend on the value of the applied voltage. Therefore, the theoretical calculation of the SPR angle will allow predicting the effect of applying external voltage on the optical properties of the metal-electrolyte interface and taking this into account when carrying out experimental studies.

However, thin films of noble metals, for example gold films, used in SPR sensors are made using the method of thermal vacuum evaporation and are characterized by a rough surface, which needs to be taken into account for theoretical calculation of optical properties. To this end, the metal-electrolyte separation boundary was proposed to be modeled as a statistical heterosystem consisting of three components: a rough metal surface, a surface layer of metal of 1 A thickness, whose optical properties depend on the externally applied voltage, and the electrolyte (Fig. 6.2). Since there is no predominant component in this system that could play the role of the main matrix, the symmetrical Bruggeman approximation was used to calculate the effective complex refractive index of this heterosystem [24]:

where i =1,2,3 is the sequence number of the component,and N, are the volume fractions and refractive indices for the corresponding components of the heterosystem.

Figure 6.2 Structure of the near-surface metal layer in the SPR sensor sensitive element upon the application of external voltage.