Single-Nanoparticle Plasmonic Spectroelectrochemistry - ACS

Chapter 4

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Single-Nanoparticle Plasmonic Spectroelectrochemistry Jun-Gang Wang, Chao Jing, and Yi-Tao Long* Key Laboratory for Advanced Materials, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China *E-mail: [email protected]

Plasmonic nanostructures hold great promise in the areas of materials science, chemical and biological sensing. They have been intensively investigated owing to their desirable structure-defined electronic and optical properties and considerable catalytic capacities. Localized surface plasmon resonance is associated with the collective oscillation of dielectrically confined conduction electrons in plasmonic nanostructures and is often excited by coupling with electromagnetic radiation, resulting in spatial confinement and localized field enhancement, which have been widely studied for use in plasmon-enhanced spectroscopy, even to a single-nanoparticle level. Notably, plasmonic nanoparticles can serve as nanoelectrodes with great potential in ultra-sensitive electrochemical detection. Thus, in this chapter, we discuss recent advances in the field of single-nanoparticle plasmonic spectroelectrochemistry. We focus on the fundamental theory as well as applications. Finally, the challenges that must be overcome in emerging key areas such as single-nanoparticle sensing and catalysis are highlighted.

© 2016 American Chemical Society Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Introduction Localized surface plasmon resonance (LSPR) is the coherent oscillation of conduction electrons in noble metal nanoparticles. When incident light irradiates noble metal nanoparticles with sizes far smaller than the wavelength of the light, the surface free electrons are excited and collectively oscillate with the incident light (1, 2). When the oscillation frequencies of the incident light and surface electrons reach resonance, LSPR occurs and creats the strong extinction light and catalysis ability owing to the abundant active surface electrons (3, 4). The electric fields near the particle surface are greatly enhanced and are confined only to the noble metal nanostructure. This enhancement creates a variety of light-matter interactions with new mechanisms, such as plasmon-enhanced Raman spectroscopy, the nanoantenna effect, plasmonic energy transfer, solar energy conversion, and plasmonic photochemical reactions (5). The near-electric-field enhancement is largest at the surface, rapidly decreases with distance, and decays evanescently into the dielectric medium. LSPR enables noble metal nanoparticles to exhibit unique scattering and absorption spectroscopy properties (6–9). The size, shape, composition and organization of plasmonic nanoparticles and changes in the dielectric constant of the medium within the plasmon electric field have considerable influences on the the resonance frequency, intensity and full width at half-maximum (FWHM) of the LSPR extinction band (4, 10, 11), which creates a foundation for sensing applications. Recently, the combination of electrochemistry and LSPR has attracted considerable attention in the emerging field of single-nanoparticle plasmonic spectroelectrochemistry (SN-PS). Electrochemistry focuses on the interrelation of electrical and chemical effects and the study of the chemical changes caused by the passage of an electric current and the production of electrical energy via chemical reactions (12). Electrochemical protocols allow manipulation of surface chemistry at the interface of metal electrodes that are classically built on conventional macroscopic electrochemical current, potential, and charge relationships (13–15). Nanoelectrodes with a critical dimension of less than 100 nm have been the primary tool used in nanoscale electrochemistry (16–18). Owing to nanoscale spaces and faster time resolution, nanoelectrodes have been widely used to investigate single nanoparticles, single cells and molecules, and fast reaction kinetics and to realize fundamental electrochemical studies and electrochemical interface imaging (19–21). However, nanoelectrode fabrication is still a time-consuming and tedious process with low yield. Nanoelectrode fabrication protocols have been reviewed, and the reader can refer to these reviews for specific details (16, 18, 22–24). The electrochemical behaviors of nanoscale electrodes are crucially influenced by processing steps and handling skills. To meet these challenges, the emerging field of SN-PS, which combines electrochemical processes and plasmonic responses at the nanoparticle interface, offers a new approach for the study of heterogeneous catalytic and electrochemical reactions on the surface of single nanoparticles. Single nanoparticles serving as the nanoelectrodes in the SN-PS technique avoided complicated the fabrication of nanoelectrodes. Owing to the enormous progress of nanotechnology, it is possible to prepare a great variety of plasmonic nanostructures, which expands 58 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the applications of SN-PS. Changes in the permittivity of medium and free electron density in the conduction band of plasmonic nanostructures leads to variation in the LSPR frequency, intensity and FWHM. Use of these spectral characteristics and correlation with the electrochemical processes that occur on single nanoparticles is the theme of SN-PS (Figure 1). The development of the SN-PS technique comprises two primary aspects. First, fundamental study of the relationship between the applied electrochemical bias and the plasmonic response pave the way toward further understanding of the electrodynamic coupling between electrons and plasmons. Second, the SN-PS technique has been used in many fields such as charge transfer and storage, electrochemical sensing and catalytic reaction monitoring. These advanced applications are critical for guiding the design of nanoparticle electrodes, electrooptical devices and biological sensors. In this chapter, both the relevant theory and applications are discussed. Although the authors have attempted to make this review as comprehensive as possible, it is impossible to cover all aspects involved in the field of plasmonic electrochemistry. We hope that the reader views this chapter as an account of the important advances made in this field.

Figure 1. Schematic of charge-discharge of a single plasmonic nanoparticle. The electrons are injected into the nanoparticle from the electrode under negatively charged (left) and extracted from the nanoparticle under positively charged (right). Fundamental Theory The Fermi Level and Redox Potential of a Single Nanoparticle When single nanoparticles serve as the nanoelectrode, their immersion into an electrolyte solution leads to the formation of an electric double layer at the metal-electrolyte interface. From an electrochemical viewpoint, a single nanoparticle can serve as a multivalent redox species with a wide range of redox states and exhibits metal-like and molecule-like charging behaviors via interaction with the environment (25–27). A further grasp of Fermi level equilibration and the associated charge transfer process offers new insight into the electrocatalytic, catalytic, and optical properties of nanosystems compared with systems composed of bulk metal. The electronic and optical properties of nanoparticles can be unambiguously studied as a function of their size because the size factor plays an important role in their optical properties and sensing abilities (28, 29). The 59 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


highest occupied band in the bulk metal is half filled, and thus electrons can be excited thermally or by an electric field to observe the metallic properties of the crystal. The Fermi level is located in the middle of the band in which the density of state is highest (30). With the decreasing the size of the nanoparticles, the energy levels are split to discrete levels. These changes in energy levels arise from systematic transformations in the density of the electronic energy levels as a function of the size, known as quantum size effects (31). The energy levels of nanocrystals lie in between the continuous bands of the extended crystalline properties and the discrete density of the electronic states of the atomic and molecular structures (32). A schematic of the energy levels that evolve from bulk metal to an isolated atom is shown in Figure 2A. For metals, the center of the band develops to follow edge development as the size of the nanoparticles increases. In metal nanoparticles, the Fermi level lies in the center of a band, with a small spacing between relevant energy levels. The Fermi level of the nanoparticle EF in solution has been evaluated according to the thermodynamic cycles (33). During the SN-PS, the single nanoparticle not only serves as the nanoelectrode to obtain the electrochemical information but also acts as the plasmonic antenna for optical detection. Thus, the redox potentials of a single metal nanoparticle play important roles in spectroelectrochemistry study. For reduction of a single metal nanoparticle in solution, the absolute standard redox potential (EZ/Z-10)abs can be given as (33):

where d is the thickness of an adsorbed layer, ε0 and εs are the relative permittivities of the adsorbed layer and dielectric medium, respectively, r0 is the distance from the center of the metallic core, e is the elementary electronic charge, and ze includes both the adsorbed ionic charges and the electrons in nanoparticle. In this work, Φb is the work function of the metal, and for bulk gold, ΦAu reaches to 5.32 eV (34). The adsorbed layer alters the work function of the metal owing to formation of the dipole layer, which affects the motion of the free electrons in the metal (35, 36). This equation considers the work function of the bulk metal, the charge density and dimension of the nanoparticle, and the dielectric properties of the adsorbed layer and surrounding medium. The adsorbed molecular or ion charges also contribute to the position of the redox potential of the nanoparticle Eparticle. Based on gasphase thermodynamic and kinetic analysis, Henglein predicted that redox potential of a single metal nanoparticle increases with the decreasing of the radius of the nanoparticle (37). However, owing to quantum mechanical effects, the redox potential for Agn clusters is predicted to increase to such extents with decreasing r for n from 1 to 3 (38). Sieradzki demonstrated that for Pt nanoparticles with diameter < 4 nm, the dissolution potential is well below the bulk metal value and displays a different dissolution mechanism (39). However, for Ag nanoparticles with diameters from 25 to 100 nm, no obvious change in the oxidation potential occurs based on diffusion only (40). Based on the increase in surface area of the nanoparticle, Plieth predicted a 1/r-dependent negative shift in the oxidation potential relative to the bulk metal for metal nanoparticles with radius r (41, 42). 60 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


where Ebulk is the oxidation potential of the bulk metal, γ is the surface tension of nanoparticle, Z is the number of electrons, F is Faraday’s constant, r is the diameter of nanoparticle and Vm is the molar volume. Based on the Plieth equation and using voltammetry, Zamborini and co-workers were the first to show the negative shift in the oxidation potential in size-dependent oxidation of Ag nanoparticles over a size range from 8 to 50 nm and Au nanoparticles over a size range from 4 to 250 nm in diameter (43, 44). However, only the change in the Gibbs free energy with the surface area of the nanoparticle is considered, and the chemical potential contributed by the effect of charges is not involved in equation 2. If considering the size effect and the changes in the Gibbs free energy and chemical potential, the general equation that describes the oxidation potential of the metal nanoparticle can be obtained (41, 45):

where e is the elementary charge, Z is the number of charges, and k is defined by 1/(4kε), where ε is the vacuum permittivity. Owing to the predicted extraordinary reducing capability, metal nanoclusters have been tested as catalysts in the Suzuki cross-coupling of various aryl halides with phenylboronic acid, electro-catalytic CO2 conversion to CO, and the O2 reduction reaction (46–49). However, the facet structures and shapes of the nanoparticles related to Eparticle are not considered in equation 3, which requires deep consideration to elucidate a precise evaluation of the redox potential of metal nanoparticles. In traditional electrochemistry, Eparticle is evaluated by the ensemble stripping peak, which direct relates to the oxidative dissolution of the bulk metal nanoparticles and demonstrates the electrochemical activity of these nanoparticles (40, 42–44). The inherent heterogeneous size and shape of chemically prepared metal nanoparticles, even under identical growth conditions gives rise to heterogeneity in the electrochemical, catalytic activity and optical properties within nanoparticle populations (50–56). In addition, it is difficult to obtain the Eparticle in an ensemble system using traditional electrochemistry, such as linear sweep cyclic voltammetry. Therefore, to truly understand the controlling effects of catalytic events and the redox potential of metal nanoparticles, it is useful to study reactions that occur at the single-nanoparticle level. Thus, we can eliminate the average effects and better understand the factors affecting catalysis, such as shape, quantum effect, and surface functions, on the catalytic reaction and the electrochemical activity of single nanoparticles (57). When the nanoparticles make contact with conductive substrates, they can serve as a nanoelectrode at which an electrochemical reaction can occur. The Fermi levels of the nanoparticles equilibrate with those of the conductive substrates and are tunable with the change in the potential (Figure 2B) (58–60). However, the electron transfer between the polarized nanoparticle and the electrochemical reactants in the electrolyte is still unclear. It is imperative to build a theoretical model to illustrate the relative positions of the electrochemical potentials of the electrons in the nanoelectrode and those of electrochemical 61 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


reactants in the electrolyte during polarization (61). A theoretical approach to analysis of heterogeneous kinetics based on the overlap between the electronic states of the electrode and those of the reactants in solution has been established (12), and the schematic is presented graphically in Figure 2C. According to this concept, when the occupied energy state is matched in energy E with an unoccupied state between the nanoelectrode and reactant, electron transfer can occur from any occupied energy state to an unoccupied state. With the change in the potential, the Fermi level shifts toward to higher energy levels at more negative potentials and vice versa (62). For a single-nanoparticle nanoelectrode during polarization, the metal nanoparticle is charged, and thus all states are shifted by the effect of polarization. The charging process involves a change in the total electron population on the metal nanoparticle, but this change involves only a tiny fraction of the total electrons. For electrochemically active species, a redox couple can communicate with the nanoelectrode if it is located near the electrode surface. When the electrode potential changes to a more negative value, the Fermi level shifts upward, and the oxidation state begins to overlap the occupied electrode states. The electron transfer is considered in terms of tunneling of electrons between the reactants and the electrode. The probability of electron tunneling follows the expressions (12):

where d is the distance over which electron tunneling occurs, β is a factor related to the energy level and the states of the reactants, m is the mass of the electron, Φ is the function of the metal and h is the Planck constant. According to electron transfer theory, when the dissolved reactants transfer the electrons to the metal electrode, i.e., participate in a heterogeneous reaction, this reaction can be considered a distance-related reaction in which the rate constant decays exponentially with distance. The electrical double layer around the single nanoparticle can be divided into several planes. A schematic of the electrical double layer structure in aqueous solution and the potential profile across the double layer is shown in Figure 2D. The layer closest to the metal, namely, the inner Helmholtz plane (IHP), includes the solvent molecules and specifically adsorbed molecules or ions with a charge density σI. The distance d1 of the IHP is measured from the electrical center of the specifically adsorbed molecules or ions to the surface of the metal electrode. The solvated ions can only approach the metal surface at a nearest distance d2, and this layer is known as the outer Helmholtz plane (OHP). The interaction between the solvated ions and the charged metal surface involves only long-range electrostatic forces that are independent of the specific chemical properties of the ions (12). Adsorbates such as solvated ions have a significant influence on the optical electrochemical activities and the Fermi level of the metal nanoparticles. The Fermi level of the nanoparticles rises with the increasing nucleophilicity of the adsorbates (63, 64). Owing to the thermal motion of the nonspecifically adsorbed ions, these ions form the diffuse layer that covers the range from the OHP to the bulk solution. The thickness of this layer is influenced by the total 62 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


ionic strength of the solution. The interfacial potential difference φ between the electrode and solution depends on interfacial properties such as charge imbalance. The diffusion layer potential ψ indicates the electric field intensity from the OHP to the diffusion layer. The total charge density in the diffuse layer is σD. Thus, the charge density in the electrical double layer can be given as (12):

The capacitance Cd of the electrical double layer indicates the capacity of the nanoelectrode to store charge under a perturbed potential. The differential capacitance is based on the change in the charge density in the double layer produced by alteration of the applied potential, and we obtain the following:

Considering the adsorbed molecular layer as two conducting concentric spheres, its capacitance can be derived as follows (65):

where ε0 is the permittivity of the vacuum, εd is the relative permittivity of the adsorbed molecular layer, and d and r0 are the radii of the nanoparticle and the thickness of the adsorbed molecular layer, respectively. Considering the contribution from the dielectric medium in an electrostatic model, the capacitance of the adsorbed molecular layer can be obtained by evaluating its electrostatic potential. The capacitance from the adsorbed molecular layer and the dielectric medium is (66):

where εs is the relative permittivity of the medium outside the adsorbed molecular layer. This capacitance is composed of the adsorbed molecular layer capacitance CAML and the dielectric medium capacitance CDM, and they can be expressed as follows:

If we use the planar Gouy-Chapman theory and consider the diffuse layer surrounding a metal nanoparticle as that of a planar electrode and the DebyeHückel approximation, then a potential distribution in the diffuse layer and the diffuse layer capacitances can be obtained (26, 66):

63 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


where κ is the reciprocal Debye length, and r2 is the sum of the radius of the nanoparticle and the thickness of the adsorbed molecular layer. These equations demonstrate the ionic contribution and bulk solvent contribution to the capacitance without considering the effect of the charge on the capacitance of the nanoparticle. The double-layer capacitance and the charging current can have a great influence on electrochemical experiments. Especially for low concentration of electroactive reactants, the charging current contributes to a higher proportion of the total current than the faradaic current for the electrochemical reaction. With the decrease in the surface area of the nanoparticle, the influence of the electrical double layer capacitance and the uncompensated ionic resistance of the electrolyte solution on the electrochemical reaction is alleviated. Moreover, the decrease in the working area of the nanoelectrode results in a decrease in the time constant which enables detection of kinetic constants on the nanosecond time scale (22, 67–69). However, enhancement of current signal-noise ratios for electroanalysis at low analyte concentrations encounters great difficulties in classical electrochemistry. Based on the changes in the localized surface plasmon of noble metal nanoparticles owing to charge injection and discharge, pioneering studies have reported interactions between noble metal nanoparticles and electric fields (60, 70–72). This strategy has served as an alternative approach to detection of the weak current occurring on the metal nanoparticles and has been widely used in SN-PS for electrochemical reaction monitoring and biological sensing (57). The change in the charge density on the metal nanoparticles through polarization alters the potential of metal nanoparticles and simultaneously results in change of the potential of the electrolyte. Considering the charging mechanism, electrons are pumped into and outflow from the metal nanoparticles during polarization. In addition, the chemical mechanisms contribute to the charging mechanism. The redox couple species establishes the potential equilibrium on the surface of the metal nanoparticles based on the Nernst equation. In addition, the mass transfer of the redox couple species during electrochemical reactions also leads to a change in the dielectric environment surrounding the metal nanoparticles (Figure 2E). These processes have a great influence on the plasmonic properties of single nanoparticles during spectroelectrochemical experiments. However, additional details are still required for further development of the plasmoelectronic theory and concepts, including questions of how to identify and eliminate interference from the charging current in faradaic current detection and distinguish the contribution from changes in the dielectric environment from that in the electron density, and as such, plasmon resonance spectral change is still a conundrum for scientists.



Figure 2. (A) Density of states in metal. The density of states is discrete at the band edges. Reproduced with permission from reference (32). Copyright 1996, American Chemical Society. (B) Equilibration of the Fermi level of the metal nanoparticle with a polarized electrode. Reproduced with permission from reference (26). Copyright 2015, Royal Society of Chemistry. (C) Schematic of the relationships among electronic states at a solid-liquid interface between a metal nanoelectrode and a solution containing redox species О and R (at equal concentrations). The vertical axis is absolute electron energy E. The electron state density distributions of О and R are shown on the solution side. (D) The representation of a double layer in a nanoelectrode model. The potential development in the area and in the further course of a Helmholtz double layer(d1 inner Helmholtz plane, (IHP), d2 outer Helmholtz plane (OHP), d3 Diffuse layer). (E) The schematic of the mass transfer for an electrochemical reaction occurring on the nanoelectrode.



Plasmon Resonance Wavelength and Scattering Cross-Section of Single Nanoparticles Localized surface plasmon resonance is caused by the collective oscillation of free electrons in the nanoparticle when the dimension of the particle is comparable to the wavelength of the excitation light. LSPR leads to great enhancement of the near field, and the electric field intensity decays rapidly with increasing distance from the surface of the nanoparticle. Spatial confinement of conduction electrons in noble metal nanoparticles (i.e., gold, silver, copper and aluminum particles) leads to absorption and scattering resonances, which occur at visible wavelengths for noble metal nanoparticles (73–75). The energy of the plasmon resonance for a given nanoparticle is extremely sensitive to the morphology and composition of the nanocrystal, including the size, shape, and surface roughness at the atomic level, and can be altered by changes in the free electron density in the conduction band and the refractive index of the surrounding medium. These properties constitute the sensing fundamentals for practical applications (76–79). Changes in the surface free electron density of plasmonic nanoparticles during the electrocatalytic reaction have been detected via changes in the localized surface plasmon resonance energy (80). To understand how localized surface plasmon resonance correlates with the free electron density, we turn to scattering theory and review the theoretical basis for the SN-PS. The model for the influence of free electron density on the energy of localized surface plasmon resonance was established by Mulvaney (81). Following the Drude model, the dielectric behavior of gold can be well described in the visible portion of the spectrum (82).

where ωp is the plasma frequency, γ is the bulk damping constant and ε∞ is the high-frequency dielectric constant of gold (5, 83). For bulk plasmons, the oscillations of free electrons occur at the plasmon frequency and have the energy:

where ε0 is the permittivity of free space, N is the conduction electron density, e is the elemental charge, and m is the electron effective mass. (bulk refers to materials with a dimension that is large compared with the wavelength of the excitation light). According to this equation, the complex dielectric function of the metal can be established as a function of free charge density. The ε(ω) is separated into real and imaginary components, and these two components can be obtained as follows (noting that ω >> γ for optical frequencies):



A change in the number of free electrons alters the free electron density within the nanoparticle and results in a shift in the dielectric function. Additionally, the change in the dielectric function of the nanoparticle can cause a change in the plasmon resonance frequency owing to the enhanced or decayed restoring force (81).

According to Gan’s theory, the optical cross-sections of elliptical nanoparticles can be quantitatively described when the elliptical nanoparticle is excited by polarized light parallel to the principle axes (noting that this expression considers only the dipole contribution) (4, 84, 85):

where σabs and σsca are the absorption and scattering cross-sections, respectively, λ is the wavelength corresponding to the plasma frequency of the bulk metal. εm is the dielectric function of the embedding medium, and ε2(ω) are the imaginary components of the dielectric function of the nanoparticle. Note that only the dipole contribution is considered. In addition, Li is the depolarization factor; a, b and c are the three axes of the elliptical particle with b > a = c; V0 is the particle volume; and R is the aspect ratio of the elliptical particle, R = b/a. The absorption cross-section is maximized when the denominator in equation 22 is minimized, a condition that is met if ε1(ω)+ ((1-Li)/Li))εm = 0. According to equation 19 and 22, we obtain

Converting from plasmon resonance frequency to free electron density via equation 16, the above expression becomes (81):



where c is the velocity of light, λmax is the LSPR peak wavelength and λp is the wavelength corresponding to the plasma frequency of the bulk metal. According to equation 28, the change in the scattering cross-section of the single nanoparticle depends on the free electron density. If the change in free electron density in the nanoparticle, ΔN, is small compared with the total N, i.e., ΔN/N |∂ε2(ω)/∂ω|, and the equation can be simplified as (92): 68 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Although this equation is derived for spherical particles at the quasi-static (dipole) limit, it is appropriate for any isolated resonance. If the nanoparticles are sufficiently small, the radiation damping is not significant (93). At frequencies located far away from the interband transitions, the dielectric function is dominated by the free electron contribution: |∂ε1(ω)/∂ω| ≈ 2ωp2/ω3. In this case, the plasmon linewidth can be given by (91):

where γ is the bulk damping constant, νF is the Fermi velocity, leff is the effective path length of the electrons, and A is a constant to be determined. The electron surface scattering in nanoparticles with different shapes can be calculated using a general expression for leff in terms of the volume V0 and surface area S of the nanoparticles leff = 4V0/S (94, 95). The effective path length for the electrons is influenced by the size and shape of the nanoparticles. This expression shows that for small particles, the LSPR linewidth is directly related to damping of the free electron motion by intrinsic and surface scattering processes (96). When the radiation damping for small nanoparticles is not significant and the LSPR frequencies are located far away from the interband transitions of the noble metal, expression 21 can be used as an approximation for noble metal nanoparticles. When the interband contributions to the dielectric function become important at frequencies located near the visible to near-UV regions (such as for gold and silver), then |(∂ε1/∂ω)| ≠ 2ωp2/ω3. In this case, the linewidth is obtained as follows (93):

where the bulk contribution includes effects from the interband transitions (first term) and the electron-surface scattering contribution (second term), which both contribute to the linewidth of the nanoparticle. With the increase in the volume of the nanoparticle, radiation damping becomes an important energy loss mechanism owing to coupling of the LSPR oscillation and the radiation field. The linewidth can be evaluated by adding a volume-dependent term:

where κ is a constant that characterizes the efficiency of the radiation damping and V0 is the volume of the nanoparticle. This equation can be used only when the LSPR corresponds to a single dipolar resonance and the nanoparticles are not large at sizes for which the quadrupole resonance is not significant (93, 96). Changes in the refractive index of the surroundings and a change in electron density could lead to a plasma peak shift (Δλmax) of the scattering spectra (expressions 16 and 18). In addition, the linewidth of the scattering spectrum of a single nanoparticle is sensitive to the change in the refractive index. The change in linewidth can also reflect the physical and chemical processes occurring in 69 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

proximity of the plasmonic nanostructures (97, 98). Following the Drude model, the dielectric constant is split into interband and intraband contributions (93, 96):


When different damping effects and potential (E) dependence are considered, the modified dielectric function of the plasmonic nanoparticle ε(ω) is given by (99):

Herein, εbulk(ω) is the bulk dielectric function, ε0 is the vacuum permittivity, Γ0 is the bulk damping constant (namely, the electron relaxation rate) and ωp(E) is a modified plasma frequency that depends on the applied potential and can be written as:

where N(E) is the electron density change with the applied potential. Because the single noble metal nanoparticle can be considered a nanoelectrode or nanocapacitor, e is the electronic charge, m is the effective mass of the sp-band electron, and ε0 is the vacuum permittivity. The linewidth Γ can be decomposed into contributions from other damping effects, such as Γ0 (bulk damping constant), Γc (chemical interface damping), and Γd(E) (charging effect, related to the electron density change during potential scan):

Plasmon damping leads to a slow dephasing of the plasmon. The dephasing time of the plasmon in plasmonic nanoparticles has attracted extensive attention (100–102). A schematic of plasmon damping is shown in Figure 3C. The quasi-particle plasmon lifetime can be characterized by the time constant T2, and the time constant T1 of the inelastic decay of the plasmon characterizes the speed of the plasmon dephasing (86). The relationship between the different time constants is T2 = 2T1 + T*, where T* denotes the pure dephasing contribution. Using single-particle spectroscopy, the homogeneous linewidth was obtained by measuring the Γ of the single-particle resonance spectrum, and thus, the intrinsic oscillator dephasing time T2 could be investigated in the frequency domain to avoid the inhomogeneous broadening effect in ensemble measurement (103). The plasmon linewidth is connected to the dephasing times for the different processes by Γ = 2ћ/T2 (86, 104). In addition, the quality factor Q of the plasmon resonance is used to represent the oscillation magnitude of the plasmonic nanostructures, where Q can be described as Q = Ep/Γ (Ep is the energy corresponding to the resonance wavelength of the nanoparticle, λmax) (Figure 3D). The scattering spectral shift (Δλmax), changes in the scattering cross-section (σsca), linewidth (Γ) and dephasing of single gold nanorod during the electrochemical reaction offer prominent information related to the change in the surface state and charge transfer occurring on the single nanoparticle. This information can be used to infer the electrochemical processes that occur on the surface 70 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of single nanoparticles. The obtained optical characteristics are conductive to probing electrochemical processes on the surface of nanoparticles under realistic conditions in heterogeneous electrochemical catalysis and plasmonic sensing.

Figure 3. Schematic depiction of spectral characteristics obtained in single-nanoparticle plasmonic spectroelectrochemistry shown in (A) and (B). The scattering spectral shifts, Δλmax, changes in the peak full-width at half maximum (ΔΓ) and scattering intensity (ΔImax) induced by the potential polarization can serve as the readout from the scattering spectra to provide information about the electrocatalytic reactions on the single nanoparticles. The λmax and Γ obtained at open circuit serve as reference points to calcualte the Δλmax, ΔΓ and ΔImax. (C) Schematic representation of the major plasmon damping pathways in noble metal nanoparticles. (D) Comparison of the light-scattering spectra of a 60 nm gold nanosphere and gold nanorod (60 nm by 30 nm).



Single-Nanoparticle Plasmonic Spectroelectrochemistry Applications Metal nanoparticles play an important role in electrochemical reactions for related applications, such as energy conversion and biological sensing (105, 106). Owing to the excellent conductivity of metal nanoparticles, they can serve as transducers in electrochemial analysis (107–109). However, further investigation of the dependence between the charge and plasmonic properties of the nanoparticle is still a significant challenge. Full investigation of these behaviors of nanoparticles is required at the individual nanoparticle level. Consideration only of the properties of the ensemble cannot furnish the precise relationship between charge and plasmon affected by populations of nanoparticles (110), although direct electrical detection of electrochemical behaviors at the nanoscale has been realized by nanoparticle impact electrochemistry (111, 112). Owing to the weak current that flows through the nanoparticles during single-nanoparticle catalysis, traditional electrochemical equipment is not able to directly acquire these electrochemical signals. Hence, owing to limitations in electrochemical amplifier and morphology characterization of single nanoparticles supported on the electrode, alternative approaches are required to improve electrochemical study at the surface of individual nanoparticles. In this chapter, the emerging SN-PS technique is introduced, with high spatial and time resolution, and related applications such as monitoring of electrocatalytic reactions, investigation of electrolyte effects and electrodeposition on individual nanoparticles are described.

Electrochemical Reaction at the Single-Particle Level Dark-field microscopy has served as a useful side illumination technique with notably high contrast and a good signal-to-noise ratio. The dark-field condenser contains a ring-shaped mirror that forms a ring-like pathway for transfer of incident light. The illumination and detection angle must be chosen carefully to ensure that no direct light enters the objective. With a transparent substrate, the incident light continues to transfer in the incoming direction, and only the scattered light from the nanoparticle is collected by the objective lens, resulting in a dark background. Owing to the special functions of dark-field microscopy, the scattering spectra of a single plasmonic nanoparticle are readily obtained with spectrograph equipment and a light collection device (113–116). By combining the dark-field microscopy and electrochemical techniques, we can perform SN-PS experiments and quantify the optical signal at the individual nanoparticle level under electrical field control. The optical illumination configuration of the SN-PS technique is illustrated in Figure 4. The other optical technique used to monitor an electrochemical reaction at the single nanoparticle level is the plasmonic-based electrochemical current imaging method (P-ECi) (117). When electrochemical reactions occur on the surface of the nanoparticles, the local refractive index around the nanoparticles changes with these reactions, and the reactions can be monitored using surface plasmon resonance (SPR) microscopy. Tao and co-workers previously demonstrated that this technique can be used to monitor the electrochemical current via the 72 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


plasmonic signal and to record cyclic voltammograms at the single-particle level with high time resolution, high spatial resolution and high-throughput capability (118). Recently, the Tao group also reported that the spatial distribution of the surface charge and the local electric field at the electrode surface was imaged using plasmonic electrochemical impedance microscopy (P-EIM) (119). P-EIM is a method that measures the dependence of the electric field and local charge density on the distance between the metal surface and the micropipette and also indicates the possibility of integrating SPR imaging with scanning probe microscopy techniques such as scanning ion conductance microscopy (SICM).

Figure 4. Schematic of dark-field spectroelectrochemistry illumination techniques. (A) Transmission, (B) Reflected, (C) Kretshmann, (D) Total internal reflection illumination. The excitation light and scattering light is directed to the sample surface and depicted by solid and dashed arrows, respectively. Based on these methods, electrochemical processes on single nanoparticles were studied, including the electron charging, mass transfer and reaction kinetics. For instance, Mulvaney and Novo pioneered the concept that the energies of conduction electrons in noble metal nanoparticles are altered by modulating the free electron density rather than the microelectrodes (79). The dark-field setup and the thin electrochemical cell are depicted in Figure 5A. With polarization of the working electrode, the color changes of a single gold nanorod induced by the cathodic charging were substantial, as shown in Figure 5B. The scattering spectral shifts in plasmon resonance during cathodic polarization are illustrated in Figure 5C. The obvious scattering spectral blue shift was attributed to electron injection into the gold nanoparticles and not to heating effects. For gold nanorods, the color shifts from red to orange and to yellow-orange, and gold spheres appear to 73 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


change slightly to blue-green owing to a lower geometric factor than that of gold nanorods. In addition, according to scanning electron micrographs, no significant change in the morphology of gold nanoparticles induced by the charging effect was observed under an electron charging process (Figure 5D). These experiments demonstrated a useful method for modulating the optical properties of single gold nanorods using a strategy of electrochemical charge injection that forms the basis for real-time monitoring of electrochemical reactions.

Figure 5. (A) Dark-field microscope with dark-field condenser, objective, CCD camera, and spectrometer. Thin cell for electrochemical charging under the dark-field microscope, comprising a steel shell with two Ag wires (one contacting the ITO to provide a working electrode, the other a quasi-reference) and an auxiliary Pt electrode. (B) Dark-field images of gold nanorods and spheres or trigonal prisms at applied potentials of -1 V and -1.6 V. (C) Normalized scattering spectra of gold nanoparticles at potentials varying from 0 V to -1.4 V and back to 0 V. Full lines are Lorentzian fits to the spectra, which are offset for clarity. Insets are plots of λmax vs potential. (D) are SEM images of the particles in (C), before and after the charging; the granular structure is the ITO. Scale bars: 100 nm. Reproduced with the permission from reference (79). Copyright 2009, American Chemical Society. Long and Jing et al. firstly demonstrated electrocatalytic oxidation of hydrogen peroxide (H2O2) on the surface of single gold nanorods monitored by the SN-PS technique (Figure 6A) (57, 120). Based on this technique, the electrocatalytic oxidation of hydrogen peroxide on a single gold nanoparticle surface was monitored in real-time with elimination of the bulk effect (Figure 6B and C). The catalytic mechanism for oxidation of H2O2 was revealed by the scattering spectral shift of a single gold nanorod. In addition, the effect of halide anion, Cl− on the catalytic activity of a single gold nanorod was investigated, and the results suggested that the catalytic activity was blocked because the formed gold chloride cannot be reduced to gold atoms by H2O2 (Figure 6B). Figure 6D shows the scattering spectra Δλmax of a single gold nanorod (40 nm × 65 nm) in the absence (III) and in the presence (I) of H2O2. The formed hydroxide/oxide film 74 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


under anodic polarization induced the scattering spectral red shift of the nanorods. In the presence of H2O2, the hydroxide/oxide film was reduced to gold atoms, which resulted in the spectral blue shift. This electrochemical reaction correlated with scattering spectral change represents a novel method that can reveal the reaction process and mechanism. Most importantly, these plasmon resonance scattering signals showed that heterogeneity in the size and shape of the individual gold nanorods caused their different activities in electrochemical catalysis of H2O2 oxidation. Based on the inherent properties of LSPR, which is sensitive to the perturbations of free electron density in nanoparticles, the problems associated with low detection limits in electrochemical reactions on single metal nanoparticles using traditional techniques such as cyclic voltammetry can be overcome with the SN-PS technique. With this strategy, better comprehension of the mechanism of various heterogeneous catalytic or electrochemical reactions can be expected, and this approach is also expected to be beneficial for high-throughput screening of novel nanoparticle electrocatalysts and dynamic monitoring of reaction processes at the single-nanoparticle level (121).

Figure 6. (A) Setup of dark-field microscopy integrated with an electrochemical workstation. (B) Scheme of electrocatalytic oxidation of H2O2 on the surface of gold nanorods in KNO3 and KCl solutions, respectively. (C) Scattering spectra of single gold nanorod (~40 nm × 65 nm) under the cyclic triangle wave scanning. (D) Scattering spectra Δλmax of two types of single gold nanorod: 40 nm × 65 nm (I, III) and 40 nm × 84 nm (II, IV) in the presence (I, II) and absence (III, IV) of 1.00 mM H2O2 in 0.10 M KNO3 solution under the applied potential (V) from −0.10 to 1.00 V. Reproduced with permission from reference (57). Copyright 2014 American Chemical Society. 75 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Recently, based on the SN-PS technique, Long and Zhou et al. monitored the electrochemical oxidation process of nicotinamide adenine dinucleotide hydrogen (NADH) at the surface of a single gold nanoparticle modified with a graphene oxide (GO) film (122). Nicotinamide adenine dinucleotide/reduced nicotinamide adenine dinucleotide (NAD+/NADH) serves as an important cofactor and plays an important role in numerous biocatalyzed processes, including energy metabolism and immunological functions (123). Gold nanorods immobilized on the indium-tin oxide (ITO) electrode for use as the plasmonic probe were covered with a thin GO film to enhance the electrochemical oxidation ability of NADH. When the oxidation of NADH electrocatalyzed by graphene underwent irreversible oxidation into NAD+, the released electrons transferred from the graphene film to the single gold nanorods owing to the higher electronic mobility in graphene than in gold (124). In the absence of NADH, the scattering spectra of the single gold nanorod displayed a weak and reversible red shift in the plasmon resonance wavelength during anodic polarization in the range from 0 to 0.7 V, which was attributed to electron ejection from the single gold nanorods. However, in the presence of NADH, the scattering peak position displayed a blue shift compared with that without NADH at the same applied potential. This observation indicated the existence of competition between anodic charging and faradaic charging. The capacitive current and scattering spectral peak shifts observed in the absence of NADH were set as background and subtracted from the results obtained with the addition of NADH. The obtained differential spectrum can be used to evaluate the effect of the faradaic reaction without interference from the charging effect. The adjustment of the faradaic reaction kinetics was also monitored, which indicated that the reduced graphene oxide (reGO) membrane showed better catalytic performance than GO and resulted in a further blue shift. This study investigating the electron accumulation and faradaic reaction on individual nanoparticles is of great importance for understanding plasmoelectronic effects and redox events on the surface of individual nanoparticles and facilitates the development of plasmoelectronic applications. Owing to the inhomogeneity of nanoparticles, random events on a single nanoparticle cannot be eliminated in single nanoparticle analysis. Hence, it is necessary to collect data with statistical significance and avoid random events. The problem of how to monitor electrochemical events on single nano-objects with high throughput is still enormously difficult for researchers to overcome. Traditional electrochemical techniques such as scanning electrochemical microscopy (SECM) and SICM are imaging techniques capable of providing chemical and topographic information (125–127). However, high-throughput analysis in dynamic processes is limited by the lower scan rate of these techniques. To detect kinetic processes with rapid dynamics and obtain fundamental insight into how these processes are correlated with each individual nanoparticle rather than an ensemble state, it is important to develop a simultaneous imaging technique with high time and spatial resolution. Recently, based on the surface plasmon resonance property, Tao and co-workers used the P-ECi technique to detect the transient electrochemical oxidation of single Ag nanoparticles during collision with a detection electrode, and the oxidation kinetics were investigated using voltammetry (128). During the oxidation of Ag nanoparticles, 76 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the plasmonics imaging intensity decayed with the decreasing size of the Ag nanoparticles. According to Faraday’s law, the charge transferred during oxidation is proportional to the volume change of the Ag nanoparticles. Hence, the local electrochemical current of individual nanoparticles could be monitored, and thus, this technique enables quantitative examination of size-dependent electrochemical activities at the single-nanoparticle level. Pan and co-workers used dark-field scattering spectroelectrochemistry to analyze the electrodeposition of individual Ag nanoparticles and subsequent oxidation at the surface of an indium tin oxide electrode (129). Based on the correlation of scattering intensity of individual nanostructures with their size (130), these researchers monitored the single Ag nanoparticle growth in a wide-field configuration and enabled the facile reconstruction of voltammetric curves for individual Ag nanoparticles, which are inaccessible through traditional electrochemical techniques. Furthermore, Pan’s group investigated the relationship of the redox potential of individual Ag nanoparticles between the dark-field scattering and photoluminescence spectroelectrochemistry (131). These studies indicated the heterogeneities in redox potentials of Ag nanostructures that are directly related to particle size. In addition, these results also demonstrated that the redox activities of Ag nanoparticles are dependent on the surrounding environment. To directly couple the individual optical and electrochemical signatures, Tessier and Kanoufi developed a 3D dark-field approach using holographic microscopy. This technique has been applied to visualize electrochemically triggered processes occurring on single Ag nanoparticles and detect landing events or detachment processes on the microelectrode (132–134). Taking advantage of the easier capture for scattering intensity vs. resonance wavelength allows determination of the intensity of individual nanoparticles with high throughput. Long and co-workers developed a novel approach to image the electrocatalytic reaction at the single-particles level based on the SN-PS technique (135). Under constant anodic polarization, gold hydroxide and oxide that formed on the surface of the gold nanorods produced a red shift of ca. 12 nm accompanied by a decay in scattering intensity (99). The scattering peak intensity and wavelength of a single gold nanorod showed reversible changes during an applied dynamic potential scan from -0.10 V to 1.00 V. Because the scattering intensity change is sensitive to the applied potential and redox reactions, it was used as a reporting signal to characterize the electrochemical process in real-time. The scattering intensity of single nanoparticles was converted into easily recognized colors by a Matlab program to achieve imaging of electrochemical reactions. This imaging approach offered a semi-quantitative analysis of nanoparticles, dramatically enhanced detection accuracy, and avoided random events. In addition, this imaging strategy also enabled the detection of redox reactions such as electrocatalytic oxidation of hydrogen peroxide on single nanoparticles with high time-spatial resolution, which has great promise in the investigation of fundamental electrochemistry at the nanoscale. Electrochromism has been widely used in energy conservation, photovoltaic cells, lightemitting diodes and their possible applications, and light-emitting and non-emissive electrochromic devices owing to their tunable optical properties within the electromagnetic spectrum under an applied voltage (136–139). 77 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Real-time monitoring of the transition of chromophores at the nanoscale can offer guidance for design of smart devices and could benefit the development of fundamental applications involving renewable sources and sensor design. In 2007, Lee discovered the phenomenon of PRET from a single nanoplasmonic particle to adsorbed biomolecules under dark-field microscopy, which enhanced the sensitivity of absorption spectroscopy by several orders of magnitude (140). A schematic of PRET is shown in Figure 7A-C. When the absorption band of chromophore molecules modified on the nanoparticle overlaps with the scattering resonance band of the nanoparticles, the energy transfers from the particles to the modified molecules, resulting in the quenching of scattering light (113). The quenching position of the scattering spectra corresponds to the absorption band of the chromophores. This concept has been applied in the design of plasmonic sensors with great sensitivity and selectivity for biological and environmental applications (141, 142). Long’ s group developed a novel method for real-time imaging of an electrochromic process at the single-particle level based on the plasmon resonance energy transfer (PRET) technique (143), and the energy transfer from nanoparticles to the chromophores during electrochromism was revealed. In this study, polymerized methylene blue (pMB) was selected as the model analyte owing to its excellent electrochemical activity. When pMB was reduced to polymerized methylene white (pMW) during cathodic polarization, its color turned from blue to colorless. The transition of the absorption band of the molecules resulted in PRET alternation. The electrochromic process of pMB under a constant potential at 0.10 V is displayed in Figure 7D and F. As the time under this potential increases, the scattering intensity of the gold nanorod decreases. The increasing amount of pMB enhanced the PRET effect and resulted in resonance energy transfer from the gold nanorods to the pMB layer. During continuous cathodic polarization at -0.70 V, the electrochemical reduction process occurred and restrained the PRET effect, which led to an increase in the scattering intensity of the gold nanorods (Figure 7E and G). Based on this strategy, the authors used the scattering intensity of the plasmonic nanoparticles to rapidly image the electrochemical reaction on the single nanoparticle surface with high spatial resolution and high throughput. Adoption of this facile and rapid opto-electronic approach has promising applications for detection of electrochromic processes at the nanoscale.



Figure 7. (A–C) Scheme of PRET between the gold nanorod and MB/MW inducing the change of scattering light intensity. (D) Time dependent high-throughput scattering intensity change of mono-dispersed pMB/GNRs under potential of 0.10 V, vs. Pt quasi-reference, in 0.1 M PBS (pH = 7.0) solution, calculated by Matlab program. (E) Time dependent high-throughput scattering intensity change of mono-dispersed pMB/GNRs under the potential of −0.70 V calculated by Matlab program. (F) Scattering intensity change (1) of single pMB/GNR particle labeled in (D) (dotted circles) and pMB/ITO background intensity change (2) labelled in (D) (solid line circles) under the potential of 0.10 V. (G) Scattering intensity change (1) of single pMB/GNR particle labeled in (E) (dotted circles) and pMB/ITO background intensity change (2) labelled in (E) (solid line circles) under the potential of −0.70 V. Scale bars are 10 μm. Reproduced with permission from reference (143). Copyright 2016 Royal Society of Chemistry.



Electrolyte Effects at the Single-Particle Level Noble metal nanoparticles have great potential in a wide variety of applications such as nanoelectrodes, electro-optical devices, single electron capacitors, and energy conversion devices (22, 144–148). The size, shape, and surface properties of noble metal nanoparticles have significant influence on the charge transfer and storage at the interface of the nanoparticles. Knowledge of the detailed relationship between these properties and the plasmonic quality at each individual nanoparticle level is required for advanced applications. In particular, knowledge of how to obtain an integrated grasp of the interactions between the nanoparticle and the surrounding medium (such as adsorbed molecules) is needed for deep insight into the electro-optical phenomena such as PRET, SERS and plasmonic-enhanced catalysis. The plasmon resonance of nanoparticles suffers from damping owing to the dielectric immersion medium and temperature (149–152). Reversible adsorbate damping during an electronic transition and nonlinear spectral shifts under an applied potential scan have been described by Klar and co-workers (99). In plasmon resonance scattering experiments, the resonance energy and lifetime was influenced by the surrounding medium and the morphology of the nanoparticles. Using single nanoparticle spectroscopy to avoid ensemble spectral broadening, these researchers accessed the homogeneous linewidth and obtained the damping constants of plasmons. Under a double-electrode system, the scattering spectra shifted with the applied potential (Figure 8A). A scattering spectral blue shift and increased peak intensity were observed at a cathodic polarization of -1 V, and a large red shift was observed upon application of anodic polarization at +2 V, which was associated with an obvious damping process that resulted in a reduction in peak intensity and a broadening in linewidth (Figure 8B). As shown in Figure 8C, below the point of zero charge (PZC) potential (0.3 V) from -1 to 0.3 V, a linear spectral shift was obtained owing to double layer charging without spectral broadening. When the applied potential was above the PZC but below the onset of oxidation (1.0 V) from 0.3 to 1.0 V, only a nonlinear spectral shift with pronounced linewidth broadening was observed. This result was explained by adsorbate damping of plasmons attributed to hydration of the gold interface and physisorption of anions. Above the oxidation potential, a shift without additional damping was observed and was attributed to gold oxide formation on the surface of the gold nanorod, which decreased the free electron density. Nevertheless, this spectral shift was reversed when a reduction potential was applied (153). In addition, the influence of the type of anion ion (such as Cl−, ClO4− and NO3−) on the voltage-induced plasmon shift and damping was investigated. In the potential range from 0.3 to 1.1 V, no physisorption of NO3− or pronounced damping occurred, which differed from the behavior in the presence of Cl− and ClO4−. Study of the potential induced adsorbate damping on a single nanorod improved our grasp of electrochemistry and supported the use of an optical strategy to investigate the electrochemical behavior of ion adsorption and redox catalysts on the surface of single nanoparticles.



Figure 8. (A) Setup: Dark-field spectroscopy on gold nanorods in a transparent ITO capacitor geometry. The upper electrode is grounded, and potentials are applied to the bottom ITO electrode carrying the Au nanorods. (B) Scattering intensity versus wavelength at different applied potentials as indicated. (C) Plasmon resonance peak energy (a) and damping ћ(Γs+Γc(U)) (b) versus applied potential on the full potential range from −1 to +2.3 V. (c) Sketch of the positive jellium background in the AuNR (solid line) and the sp-electron density (dotted line). Three different regions can be associated: (i) Charging of double layer capacitance leads to a spectral shift but no additional damping. An interlayer of thickness d is formed where the electron spill-out repels both the solvent molecules and dissolved ions (left sketch in c). No chemical damping takes place via the solvent or dissolved ions (left sketch in d). In region (ii), potentials positive of the PZC cause a rapid nonlinear red shift (anodic scan in (a)) and substantial additional damping (region (ii) in panel b). The sp-electron spill-out retracts (reducing d) and solvent molecules and anions adsorb (center sketch in c). Chemical surface damping of the NPPR becomes possible by the excitation of either sp-electrons or adsorbate electrons into empty adsorbate states (dotted arrows in the center sketch of d). (iii) At potentials above 1.1 V, Au oxidation leads to no additional damping (b) but some further spectral red shift (a) due to the trapping of sp electrons at the oxide (right sketches in c, d). Reproduced with permission from reference (99). Copyright 2012, American Chemical Society. For a single model, such as a single nanoparticle without capping molecules in a simple electrolyte without redox active molecules, many factors must be considered, including ion adsorption and electric double layer charging (154). Owing to complexity, this topic requires additional investigation to understand these effects in an electrochemical-plasmonic multiplexed system. McIntyre 81 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


demonstrated that the optical properties of a metal interface in contact with the electrolyte changed with the applied potential and that the photon-assisted charge-transfer transitions between the metal-adatom complex and the conduction band of the substrate contributed to this change (155). The influence of the electron density on the plasmon resonance wavelength has been investigated in electro-plasmonic systems (72, 156). In previous studies, under a polarized potential, the scattering spectral shift and linewidth broadening were observed as a result of the formation of surface complexes, such as the gold oxide and ion-adsorbing layer (99, 153, 157). These studies revealed that chemical mechanisms occurring on the surface of nanoparticles play a significant role in the plasmonic response. Furthermore, to precisely distinguish the electrochemical tuning contribution and chemical mechanisms in an electro-plasmonic system, Link and Landes et al. adopted a methodology with the ability to examine electrochemical heterogeneity and subpopulations at the individual nanoparticle level by combining a hyperspectral imaging approach with single-particle spectroscopy, which increased the throughput for collection of spectra and captured the heterogeneous behavior across nanoparticles (Figure 9A) (56). Under a dynamic potential scan, the plasmonic response was found to be completely reversible for selected nanoparticles. However, for other nanoparticles under anodic polarization, an additional chemical mechanism appeared and indicated their charge transfer and storage capability. This approach allowed for high-throughput investigation of the electrocatalytic capability and activity of single nanoparticles, both dynamically and statistically. Landes and co-workers developed single-particle plasmon voltammetry to sense perchlorate, sulfate and acetate adsorption on a gold nanoparticle dimer (158). Anion adsorption on the single nanoparticle surface is highly important to the optoelectronic and electrochemical properties of nanoparticles. The adsorption process was detected via the change in the optical properties of single nanoparticles (159). In this work, the researchers demonstrated that the time differential of the primary plasmonic mode’s scattering intensity and the integrated scattering intensity was correlated with anion adsorption and desorption. Owing to chemical mechanisms such as ion-specific adsorption with a pronounced influence on the dielectric function of nanoparticles, the electro-adsorption and desorption of polyatomic ions such as perchlorate and acetate can be tracked dynamically using the SN-PS technique. In traditional electrochemical measurements, halide electrolytes such as chloride were often used. Because of the strong surface complexation effect on noble metals such as gold and Pt, halide ions have a remarkable inhibition effect on the electrocatalytic activity of gold or Pt nanoparticles (154, 160). Moreover, the adsorption of halide ions on the surface of the nanoparticle causes plasmon spectral shift and linewidth broadening, which disturbs the effective optical signal acquisition during spectroelectrochemical experiments. Thus, examination of how to better understand halide anion adsorption and its influence on the plasmon spectral characteristics is important for reliable single particle spectroelectrochemical measurements. Recently, Link and Landes et al. presented a primary study on the effects of halide electrolyte anion adsorption and subsequent reactions and their influence on scattering spectra (161). Under positive potentials of up to +0.25 V, which are below the gold oxidation potential, 82 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


halide anion chemisorption was expected to be anion dependent, and the plasmon spectral shifts were dominated by charge density tuning. At more positive potentials of +0.30 V, the resonance energy and linewidth changed with the increase in halide electrolyte anion reactivity. The obvious scattering spectral change for the same gold nanorod in a NaBr electrolyte was explained by the lower adsorption energy for adsorption of bromide on the gold surface than that of Cl− and F−. As illustrated in Figure 9B, when the anodic polarization reached +0.35 V, the irreversible decay in scattering intensity was due to the decrease in gold volume dissolved by a chemical reaction (90, 162). Moreover, the authors studied the kinetics of halide-mediated gold nanorod dissolution according to the irreversible changes in resonance energy and linewidth.

Figure 9. (A) Cathodic and anodic spectroelectrochemistry of single gold nanoparticles. (a) Optically transparent thin electrochemical cell for dark-field spectroscopy of single 50 nm gold spheres on an ITO working electrode (WE) under electrochemical potential with auxiliary and reference electrodes (AE, RE) composed of silver wires. (b, left) Steady-state hyperspectral imaging of many single nanoparticles under potential control (32 × 32 μm). (b, right) Normalized spectra at potential vertices for a single nanoparticle along with Lorentzian fits. (c) Non-Faradaic cathodic and anodic potential ranges with predicted charging mechanisms shown schematically. Reproduced with permission from reference (56). Copyright 2014, American Chemical Society. (B) Reversibility of change in intensity as a function of potential range. Change in intensity relative to the initial 0 V measurement for three different single AuNRs from −0.40 to +0.25 V (a), −0.40 to +0.30 V (b), and −0.40 to +0.35 V (c) in 1 mM NaF (i), NaCl (ii), and NaBr (iii) electrolyte. The applied potential cycle (solid line) is superimposed over the scattering intensity. The dotted line represents no change from the initial intensity. Reproduced with permission from reference (161). Copyright 2016, American Chemical Society. 83 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Electrodeposition at the Single-Particle Level The noble metals can serve as plasmonic reporters for biodetection and chemical reactions occurring on the nanoparticle surface (10, 73, 163). Recently, controllable deposition of metal nanoparticles on a transparent electrode surface has been widely studied using SN-PS techniques (129, 131, 164). Electrochemical deposition of noble metal nanocrystals represents an important fundamental realm in traditional electrochemistry and has been exploited for several decades (47, 165–167). Under an applied cathodic polarization, the metal salt precursor is gradually reduced and nucleated on the electrode. With the growth of the crystal nucleus, the scattering cross-section increases with the increase in the volume enlargement of the noble metal nanocrystals (83, 85). Therefore, the mechanism of electrochemical growth of nanocrystals can be proposed using the change in plasmonic spectral features. During the electrochemical deposition process, multiple factors are involved in the plasmon resonance of nanocrystals, including the electrochemical tuning mechanism between the nanoparticles and electrode, electrolyte ion adsorption, surface reactions, and nanoparticle reshaping during the interface reactions, which all contribute to the changes in the plasmon resonance wavelength, intensity and linewidth (168–174). Mulvaney and Chirea et al. reported the underpotential deposition of silver onto single gold nanocrystalys as monitored in real-time using a spectroelectrochemical approach (Figure 10A) (175). As illustrated in Figure 10B, the enhanced scattering intensity of gold nanostars after Ag deposition was observed from dark-field images. Under a dynamic cathodic polarization, the plasmon resonance wavelength as a function of the time is shown in Figure 10C. The plasmon resonance position was stable at 0 mV, which is above the reduction potential of the Ag+, and indicated that no silver deposition occurred on the surface of gold nanostars. With increasing cathodic polarization potential, silver ion reduction began to occur and resulted in a plasmon resonance wavelength blue-shift of 51 nm and a 45% increase in the scattering intensity at -40 mV. At -100 mV, the plasmon spectral blue-shifted by 122 nm with a scattering intensity that was increased 2.6 times over the original value (Figure 10D). This result was explained by continuous silver deposition and an increase in the volume of nanocrystals. When the polarized potential was scanned back to 0 V, the plasmon peak position was stable, indicating that reoxidation of silver was not observed at this potential. The authors addressed highly selective deposition of silver atoms on the tips of gold nanostars without bulk nucleation on the substrate. Their results demonstrated that the composition and morphology of single bimetallic nanocrystals can be engineered electrochemically and that their kinetic growth process can be monitored in real-time using the plasmonic spectroelectrochemical approach.



Figure 10. (A) The schematic of silver deposition on single gold nanocrystals. (B) The dark-field image of gold nanocrystals before (a) and after (b) deposition. (C) Position of the surface plasmon band peak of a single gold nanostar as a function of both time (circles) and the applied potential (solid line), measured during electrodeposition of metallic silver from 6.7 × 10–7 M AgNO3 and 0.1 M NaNO3 aqueous solution. (D) Selected Rayleigh scattering spectra of the same gold nanostar collected at various applied potentials during the deposition process. The nanostar was coated with PVP. Reproduced with permission from reference (175). Copyright 2014, American Chemical Society.



When two metal nanoparticles are brought into proximity, the plasmon resonances are coupled, resulting in an enhanced localized field. The near-field interparticle coupling shifts the plasmon resonance wavelength as a function of the interparticle distance (176–180). According to this strategy, the concept of a ‘plasmon ruler’ was applied to study the dynamics of DNA hybridization at the single-molecule level and to monitor the click reaction on a single nanoparticle surface in real-time (181, 182). Recently, Landes and co-workers reported electrochemical modification of individual nanoparticles, dimers and interparticle distances and achieved reversible changes in the plasmonic and electronic properties of nanostructures over a broad optical range (183). The coupling mechanism for the Au/Ag bridged dimers was fully reversibly switched between the capacitive and conductive coupling regimes by electrochemically bridging the interparticle gap. Under a cathodic polarized potential at -0.5 V, the electrical contact of two gold nanocores established through a highly conductive Ag shell resulted in an oscillating current between the nanoparticles. The new longitudinal screened bonding dipolar mode corresponding to the charge transfer plasmon mode and a weak quadrupolar polarization was revealed in simulated and experimental spectra. The plasmon modes were modulated by repeated formation and removal of the conductive layer during a dynamic potential scan. The plasmonic “drawbridge” was stable and reversible without degradation in at least 30 continuous potential scan cycles. At a negative bias of -0.5 V, the emergence of a brighter shell-dominated screened bonding dipolar and transverse mode indicated the transition from capacitive to conductive coupling. With an applied positive bias of + 0.5 V, a chlorination reaction occurred on the Ag surface and resulted in the formation of a semiconducting AgCl shell. Therefore, capacitive coupling was constructed because the conductive bridge was broken. The longitudinal screened bonding dipolar mode dominated in the plasmon modes, and the charge density on the Ag shell surface was 10 times higher than that on the Au core in the reduced state. In the oxidized state, the longitudinal bonding dipolar mode played an important role in the plasmon mode, and the charges were primarily distributed on the Au core surfaces and without current tuning between two Au cores. These results offer a deeper comprehension of the plasmonic coupling effect and electrochemical-mediated nanocrystal transition. In addition, nanoalloys have attracted considerable attention owing to their significantly different morphologies, electrocatalytic activities, and optical properties, which are accessible from elemental nanostructures (184, 185). Nanoalloys with optimized nanoplasmonic properties and nanostructures are crucial in practical applications. A general grasp of the growth mechanism of nanoalloys benefits the design and precise synthesis of nanoalloys with tailored properties and extends their applications (168, 186). Therefore, it is of fundamental significance to study the growth mechanisms of nanoalloys at the single nanoparticle level. Recently, Long and Wang et al. used dark-field spectroelectrochemistry techniques to monitor gold amalgam nanoalloy formation in real-time and stripping at the individual-nanoparticle level (Figure 11) (187). As illustrated in the time-dependent scattering spectra of a single Au nanorod during amalgamation (Figure 11A-C), blue shift and damping caused by the amalgamation could be observed. This result was explained by the inward 86 Ozaki et al.; Frontiers of Plasmon Enhanced Spectroscopy Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


diffusion behavior of Hg atoms within the gold nanorods. The linewidth of the scattering spectra decreased remarkably at the early deposition stage and reached a plateau at ca. 82.2 meV, indicating that the interfacial region of the gold nanorods was saturated by doped Hg atoms. According to Figure 11D-E, under anodic polarization, the plasmon peak red shift and linewidth narrowing contributed to the stripping of the Hg atoms doped on the interfacial area of the nanoalloys. Subsequent slight spectral blue shift and substantial narrowing of the homogeneous plasmon linewidth were observed, as explained by the more sluggish diffusion of Hg atoms from the interior of the gold amalgam nanoalloy rather than diffusion into the nanoalloy. After Hg stripping, only a ca. 1.1 nm blue shift was observed in the original scattering spectrum of a single Au nanorod, indicating no obvious change in the aspect ratio of the gold amalgam nanoalloy. After an increase in the concentration of Hg to 150 μg/mL, the LSPR spectral shift and linewidth modulated by doping and stripping of Hg atoms are shown in Figure 11G-I, which indicates that the gold amalgam nanoalloy under electrochemical modulation has promising applications in spatial light modulators and photovoltaics. The intense spectral shift after Hg deposition is attributed to electrochemical reshaping of gold nanorods during Hg deposition. The single-nanoparticle linewidth and the relative standard deviation decreased after stripping of Hg, and this electrochemical plasmonic focusing (EPF) strategy can be used to increase the high quality factors and switch the plasmon resonance wavelengths of the nanoalloys. If the plasmonic nanoparticle is modified or doped with other metals, such as Ag, Co and Pt, the oscillation electrons in the nanoparticle can be coupled with the surface states, resulting in energy loss of the oscillating electrons and damping of the plasmon (188–190). However, considerable attention has been focused on the switching of plasmon resonance energies by chemical approaches, and few reports have addressed plasmon damping modulation by an electrochemical approach. Long’s work demonstrated the controlled plasmon damping effect on a gold amalgam nanoalloy for the first time. Further detailed studies are expected to enhance comprehension of the plasmon damping effect to guide effective design and controlled fabrication of plasmonic heterostructures for use in electrical nanodevices.



Figure 11. (A) Full scattering spectra (contour plots) of the single Au nanorod exposed to the electrolyte containing 40 μg/mL Hg2+ under −0.4 V for 400 s. (B) Plasmon shift (a) and fwhm (b) as a function of the reaction period during growth of gold amalgam nanoalloy from a single Au nanorod. (C) Scattering spectra of a typical single Au nanorod after amalgamation: (a) original spectrum; (b) spectrum at 50 s and (c) spectrum at 410 s at open circuit potential. (D) Contour plots showing the evolution of the scattering spectra of the single Au nanorod in the absence of Hg2+ under 0.4 V for 400 s. (E) Plasmon shift (a) and fwhm (b) as a function of the stripping period. (F) Scattering spectra of a typical single Au nanorod after stripping of Hg: (a) original spectrum; (b) spectrum at 30 s; and (c) spectrum at 410 s at open circuit potential. (G) Representative plasmon scattering spectra of single gold nanorod: (a) before Hg deposition; (b) after Hg deposition; and (c) after Hg stripping. (H) Line width of gold nanorods (without exposure to Hg (square, a); after deposition of Hg (dots, b); and after stripping of Hg (triangle, c)) plotted against their plasmon resonance wavelength λ. Line width (fwhm) and λ were both extracted directly from the scattering spectrum of a single nanorod (inset). The data points are clustered into three regions, as indicated by the ellipses. (I) The corresponding statistical data (standard deviations and mean value) of the line width of gold nanorods versus the peak wavelength. Data comes from (H). Reproduced with permission from reference (187). Copyright 2016, American Chemical Society.



Conclusions and Outlook In this chapter, we have highlighted and discussed recent advanced applications in SN-PS. This innovative technique offers considerable promise for the study of the electrochemical reactions on individual nanoparticles and avoidance of the average effect in the bulk system. This understanding is beneficial to the acquisition of new insight into the fundamentals of electrochemical catalysis, nanoalloying, and biomolecule sensing at the surface of individual nanoparticles. However, several constraints must be considered for the development of SN-PS techniques and their versatile applications. First, the controlled synthesis of plasmonic nanostructures with specific sizes and shapes has made significant progress in recent decades. However, precise and facile nanofabrication of stable and well-defined single-crystal nanoparticles and nanoarrays still represents a great challenge for practical applications. Single-crystal nanoparticles with specific surface facets can serve as model nanocatalysts for the investigation of various electrocatalytic reactions at the nanoscale. In addition, plasmonic anisotropic nanostructures such as nanowires or nanoholes have potential for use in plasmon-electro systems owing to their distinct plasmonic and electrical properties. Hence, facile fabrication of addressable plasmonic nanostructures with specific surface facets is expected to aid in the development of the SN-PS technique. Moreover, in situ morphological characterization with high spatial resolution of nanoparticles immobilized on a supported substrate during a reaction or sensing process must be properly considered. The combination of plasmon-electro approaches and morphological characterization to study the same nano-object is expected to be conducive to applications. At the same time, increasing the optical and electrical readout speed of photoelectric devices and decreasing the detection limit for weak currents in electrochemical equipment constitute major advances for the synchronous acquisition of optoelectronic signals from nanosystems. These improvements will facilitate rapid monitoring of dynamic processes such as mass transfer and reaction dynamics near or on the surface of plasmonic transducers at the nanoscale or even at the single-atom level. From a more fundamental perspective, a comprehensive understanding and description of the electrodynamic coupling of charges and plasmon resonance will direct further applications in plasmoelectronics and nanoelectrochemistry, such as single molecule detection, energy conversion, and biological analysis via SN-PS.

Acknowledgments Financial support from the Chinese National Foundation of Natural Science Research (21327807), the National Science Fund for Creative Research Groups (21421004), and the 973 Program (2013CB733700) is gratefully acknowledged.


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