Recent advances in spectroelectrochemistry

Yanling Zhai ae, Zhijun Zhu *bc, Susan Zhou *c, Chengzhou Zhu *d and Shaojun Dong *e
aDepartment of Chemistry and Chemical Engineering, Qingdao University, 308 Ningxia Road, Qingdao, Shandong 266071, China
bDepartment of Material Science and Engineering, Qingdao University, 308 Ningxia Road, Qingdao, Shandong 266071, China. E-mail: zhuzhujun001@gmail.com
cDepartment of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA. E-mail: szhou@wpi.edu
dKey Laboratory of Pesticide and Chemical Biology, Ministry of Education of the PR China and College of Chemistry, Central China Normal University, Wuhan 430079, China. E-mail: czzhu@mail.ccnu.edu.cn
eState Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China. E-mail: dongsj@ciac.ac.cn

Received 20th October 2017 , Accepted 12th January 2018

First published on 15th January 2018


The integration of two quite different techniques, conventional electrochemistry and spectroscopy, into spectroelectrochemistry (SEC) provides a complete description of chemically driven electron transfer processes and redox events for different kinds of molecules and nanoparticles. SEC possesses interdisciplinary advantages and can further expand the scopes in the fields of analysis and other applications, emphasizing the hot issues of analytical chemistry, materials science, biophysics, chemical biology, and so on. Considering the past and future development of SEC, a review on the recent progress of SEC is presented and selected examples involving surface-enhanced Raman scattering (SERS), ultraviolet-visible (UV–Vis), near-infrared (NIR), Fourier transform infrared (FTIR), fluorescence, as well as other SEC are summarized to fully demonstrate these techniques. In addition, the optically transparent electrodes and SEC cell design, and the typical applications of SEC in mechanism study, electrochromic device fabrication, sensing and protein study are fully introduced. Finally, the key issues, future perspectives and trends in the development of SEC are also discussed.


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Yanling Zhai

Yanling Zhai received her Ph.D. degree in January 2015 from Changchun Institute of Applied Chemistry under the supervision of Prof. Shaojun Dong. She then moved to Worcester Polytechnic Institute and Northeastern University for her postdoctoral training and is now a professor at Qingdao University. She has published over 20 papers with an h-index of 14. Her research interests mainly focus on spectroelectrochemistry and nanomaterial-based catalysis.

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Zhijun Zhu

Zhijun Zhu is currently an associate professor at Qingdao University. He earned his Ph.D. degree at Changchun Institute of Applied Chemistry with Prof. Zhenxin Wang in 2014. He then worked in Prof. Shaojun Dong's group as a research assistant. After that, he did his postdoctoral work with Prof. Qing-Hua Xu at the National University of Singapore, Prof. Susan Zhou at the Worcester Polytechnic Institute and Prof. Ming Su at Northeastern University. His research interests include nanomaterials for analysis, optics and catalysis.

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Susan Zhou

Prof. Susan Zhou joined WPI (Worcester Polytechnic Institute) in 2005 after a postdoctoral research position with Dr Marc Madou at University of California, Irvine. Prof. Zhou's research interests are in the general area of microfabrication and nanotechnology for biomedical, energy, and environmental applications, with emphases on microfluidics and biosensors, biomaterials for neuron regeneration, and nanomaterials for drug delivery applications.

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Chengzhou Zhu

Chengzhou Zhu received his Ph.D. degree in January 2013 at the Changchun Institute of Applied Chemistry under the supervision of Prof. Shaojun Dong. Subsequently, he did postdoctoral work with Prof. Alexander Eychmüller supported by the Alexander von Humboldt Foundation in Dresden University of Technology. Currently, he is an Assistant Research Professor at Washington State University. He has coauthored over 120 peer-reviewed publications with ∼6000 citations and an h-index of 36, according to Google Scholar. His scientific interests focus on nanomaterial-based electrochemical energy and analytical applications.

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Shaojun Dong

Shaojun Dong is a professor of the Changchun Institute of Applied Chemistry (CIAC) and the Chinese Academy of Sciences (CAS), and a Member of the Academy of Sciences for the Developing World (TWAS). She has published over 1000 papers in international journals cited over 43[thin space (1/6-em)]000 times with a h-index of 103. Her awards include: 1 International, 3 National Natural Science and 11 Advanced Science and Technology Awards from CAS and Jilin Province. She was selected as a global Highly Cited scientist for 2017 (2005–2015), 2016 (2004–2014), 2015 (2003–2013) and 2014 (2002–2012) by ISI Web of Science in the past 14 years.


1. Introduction

Electrochemistry is one of the oldest branches of chemistry, being essential for many relevant studies in the fields of biology, physics, and chemistry, and has been used for studying transformation of materials, information transfer, heterogeneous electron transfer kinetics, and large numbers of chemical reactions in the laboratory or on industrial scales.1 There are many electrochemical analysis modes, including amperometry, voltammetry, and electrochemical impedance spectroscopy (EIS). Today, electrochemistry has become a pervasive scientific discipline, and an influential and significant area of great interest and practicability for diverse related research on analysis, chemistry, physics, energy conversion and storage, and biology with applications in fuel cells, corrosion science, self-powered devices, self-assembled coatings, and so on.2–4 Different kinds of spectroscopic approaches have played a critical role in deepening the understanding of various research areas for the past decades and are continually developing and improving.5,6 The integration of electrochemistry, supplying the kinetic and thermodynamic processes, and spectroscopy, supplying the molecular vibrational process, results in a new technique, spectroelectrochemistry (SEC),7–12 which is generally defined as the application of spectroscopic methods to assay the changes initiated by an electrochemical system in an electrochemical cell.13–16 Under potential control, spectroscopic information about in situ electrogenerated species can be readily obtained, including electronic absorption, vibrational modes and frequencies, light emission and scattering, magnetic resonance and circular dichroism.17,18

The accurate control of the charge-transfer process provides an ideal approach towards carbon materials,19 especially the doped states of which are fully studied by SEC promptly after their discovery.20 For instance, under the ideal approach of SEC, carbon nanotubes (CNTs) were successfully developed during the past 20 years. Sgobba and Guldi reviewed the electronic, electrochemical and optical properties of carbon nanotubes using SEC.21 Recently, SEC has become a powerful tool in electrode process microenvironment examination, including the diffusion layer and the electrode surface.22 Using a small amount of solution and reducing the optical path length with an aqueous microdrop, Heineman et al. reported an aqueous microdrop technique to perform thin layer SEC with [Fe(CN)6]3+/4+ and [Ru(bpy)3]3+/2+ as absorption and emission probes.6,23 Currently, electrochromic, gasochromic, thermochromic, and photochromic materials are four classes of responsive materials used in dynamic fenestration investigation,24–29 and electrochromic materials are the most popular because of their highest efficiency in energy savings.30–33 In the meantime, the SEC properties of some electrochromic materials have been studied.34–39 A typical example is the SEC investigation of single-walled carbon nanotubes (SWNTs).40 Through the Vis–NIR, photoluminescence, Raman and electron spin resonance (ESR) SEC investigation, it was confirmed that the doped states of CNTs can be accurately controlled by charge-transfer conditions. Nanotube-based SEC is attracting growing interest from the scientific community. Pan et al. presented in detail the fluorescence and SERS SEC properties of Ag nanoparticles and nanowires.41,42

Since its inception in 1964,43 SEC has been developed into an active interdisciplinary field. Although conspicuous advances have been achieved in this field, the spectroscopies are mainly focused on electron paramagnetic resonance (EPR), near-infrared (NIR), and ultraviolet-visible (UV-Vis), with little attention given to surface-enhanced Raman scattering (SERS), Fourier transform infrared (FTIR), X-ray absorption spectroscopy (XAS), fluorescence, and other spectroscopies.44 In this modern extended overview addressed to the general chemical audience, we aim to summarize key SEC studies and highlight the most important SEC work during the past several (or six) decades. The spectroscopic methods that are included in this review consist of UV–Vis, NIR and FTIR, SERS, XAS, fluorescence and others. Coupled with the advantages of SEC, the corresponding applications in mechanism study, electrochromic devices fabrication, sensing and protein study are discussed. Finally, conclusions and perspectives on the developments of the SEC are also given.

2. Optically transparent electrodes and spectroelectrochemical cells

Optically transparent electrodes (OTEs) are needed for a wide range of applications.45 The area of SEC was broadened by the availability of OTEs for normal transmission SEC measurements to obtain electrochemical and spectroscopic responses simultaneously.46 Therefore, the search for new materials with basic requirements for an OTE, such as good transparency and conductivity, is essential. In 1964, Kuwana made the first OTEs based on Sb-doped SnO2 glass, and used them for measuring the absorption features of the electrochemical species, thus giving birth to SEC.43 OTEs have proven practical for wide applications in the exploration of electrochemical processes.47,48 Traditionally, OTEs are based on the deposition of a conductive material, such as Pt, Au, SnO2, C, or Hg-coated Pt, on a transparent conductive substrate, such as glass or plastic,49 while most commercially available OTEs rely on expensive metal oxides or contain toxic metal oxides. As an effective alternative to metal and metal oxide electrodes, further breakthrough in OTEs lies in the significant advances of carbon-based electrodes, particularly CNT networks and graphene.34,45,50,51 In an early study, our group used a minigrid electrode in an optically transparent thin layer cell with UV–Vis spectroscopy technique for the investigation of the reduction process of murexide (or ammonium purpurate).52 Recently, a CNT film synthesized using chemical vapor deposition (CVD) was used as an OTE for thin layer SEC.53 Significantly, some microelectrodes, such as CNT film,54 gold ultramicroelectrode,55 and screen-printed electrodes,56 have been also used for OTEs.

In general, the measurements on SEC are usually carried out by using a thin-layer spectroscopic cell incorporated with OTEs.59,60 In 1981, Bancroft et al. developed a SEC method based on linear potential sweep.219 As shown in Fig. 1A, a Raman SEC device coupled with the three-electrode cell is presented, which was driven by eccentric precession movement on the electrochemical cell.57 This design highly increased the signal-to-noise ratio and was used for charge transfer study on ordered C60 films. As shown in Fig. 1B, Boujtita's group demonstrated a new device based on in situ ultrafast two-dimensional (2D) nuclear magnetic resonance (NMR) SEC for the real-time monitoring of redox reactions in the NMR tube.58 Very recently, complex electrochemical processes were studied in a long optical path length configuration based on Raman response measurement using UV–Vis bare optical fibers.61


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Fig. 1 (A) An in situ Raman SEC device with an eccentric rotation of the electrochemical cell. Reprinted with permission from ref. 57. Copyright 2009 American Chemical Society. (B) Schematization of an in situ EC-2D ultrafast NMR. Reprinted from ref. 58 with permission from 2015 American Chemical Society. (C–E) Photograph of the assembled cell. Reprinted with permission from ref. 53. Copyright 2015 American Chemical Society.

It is well known that advances in the spectroscopic cell are equally important to those in the OTEs for the rapid development of SEC, ranging from transmission electrochemical cells to some unique designs including utilizing reflectance methods to reduce absorption by aqueous electrolytes. Recently, a SWNTs film OTE-based novel bidimensional UV–Vis SEC cell was developed. Typically, as the photograph and the schematic view show in Fig. 1C–E, the reproducible cell is formed by three pieces of PMMA.53 A distinct advantage of this homemade cell is that the diameter of the optical fibers and the distance between them can be tuned easily, and can be used for normal and parallel measurement.

3. Spectroelectrochemical types

3.1 Surface-enhanced Raman scattering SEC

After its discovery in 1973, SERS rapidly became a mature vibrational spectroscopic technique for wide use in material, chemistry and life sciences.63–65 The combination of SERS with electrochemistry has been proven a powerful tool in interface study.66 Tian and co-workers have been expanding the breadth and depth of the understanding of SERS SEC in surface and material science from the SERS-active substrates to methodology and theory.62 They have invented and developed diverse methods to prepare rough film electrodes for SERS SEC characterization, especially to expand SERS SEC to transition metal (VIII B) surfaces.67

Shannon et al. reported a SERS SEC analysis system based on Au bipolar electrodes and near double layer,68 while Au film-over-nanosphere electrode-based SERS SEC was adopted for response monitoring on a tetrathiafulvalene derivative self-assembled monolayer.69 However, photostability is considered very crucial in Raman for measurement of the vibrational patterns of molecules.62 Unexpectedly, the combination of electrochemistry and SERS spectroscopy turned out to be a powerful tool for in situ monitoring of chemically driven electron transfer processes and redox events with structural changes of surface adsorbates or reaction intermediates.70–72 The photon-driven charge transfer in the EC-SERS system is illustrated in Fig. 2. Since the pioneering studies in 1999–2000, in situ Raman SEC has been shown to be an important technique for charge characterization of carbon nanostructures.73,74 For example, a Raman scattering study on SWNT was presented through the defects and charge-transfer change upon electrochemical application.75


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Fig. 2 Schematic diagram of EC-SERS system showing photon-driven charge transfer from the metal electrode to the adsorbed molecule. Reprinted with permission from ref. 62. Copyright 2008 Royal Society of Chemistry.

The unique nature of graphene gives great opportunities for wide applications in many scientific fields. One of the amazing properties of graphene is the easy adjustment of its electronic structure (Fig. 3). A typical application of in situ Raman SEC is in the investigation of multilayer graphene with isotope compositions embedded between individual layers, which can provide comprehensive knowledge about the interactions between the graphene layers, their directly adjacent environment, or their extended environment.76 Moreover, graphene and CNT can be doped electrochemically, by electrostatic gating, or by direct introduction of heteroatoms into the lattice. A direct injection of charge carriers, such as heteroatoms, into the conduction or valence bands, always causes a shifting of the Fermi level, as well as the C–C bond strength. As a result, the creation of electron–hole pairs might be quenched as the Fermi energy is shifted from the Dirac point after the heteroatom doping. Electrons tend to move towards the antibonding orbitals upon negative doping, resulting in downshifting of the G band frequency.76In situ Raman SEC was used for the structural evolution study of few-layer graphene oxide during electrochemical treatment,77 while reaction kinetics on the graphene surface were exploited using Raman SEC during Faradaic reactions.78 In addition, the charge transfer and electronic structure in doped single-walled carbon nanotubes was detected by Raman SEC. It revealed the composites of CuCl@SWNT, and the obtained results revealed a significant Kohn anomaly shift of ca. 0.7 eV corresponding to a Fermi level downshift because of the metallic redox doping.79


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Fig. 3 Sketches of different doping experiments conducted on graphene: (a) chemical doping, (b) electrochemical three-electrode setup used, (c) electrochemical gating setup, (d) electrostatic gating setup, and (e) incorporation of atoms into the graphene lattice. Reprinted with permission from ref. 76. Copyright 2015 American Chemical Society.

SERS spectroscopy has become a useful spectroscopic tool for redox species characterization at electrodes and electrode–electrolyte interfaces. As presented in Fig. 4, an in situ SERS SEC technique interrogating a three-electrode configuration with a microfluidic sample chamber of small volume was fabricated.80 An external Ag/AgCl wire was inserted into the microfluidic chamber directly as the reference electrode (RE). The nanostructured Au strips with a width of about 150 nm (Fig. 4D) worked as the WE and SERS substrate, enabling highly sensitive detection through large and reproducible SERS enhancements. The proposed SERS SEC system with very low laser power allows simultaneous SERS and electrochemical investigation, and therefore exhibited wide applicability in small volume target molecules analysis with a low detection limit.


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Fig. 4 Schematic representation of in situ SERS SEC analysis system. (A–C), and representative SEM image of Au WE (D). Reprinted with permission from ref. 80. Copyright 2015 American Chemical Society.

3.2 UV–Vis and IR SEC

The primary advantage of UV–Vis SEC is the cross-correlation of information from UV–Vis changes with the simultaneous electrochemical scan.82–85 For instance, a UV–Vis SEC approach was used for phenazine detection and exploration of their redox characteristics.86 Bachmann et al. explored 80% of the DNA reversibly hybridized using in situ UV–Vis SEC based on the fact that the altered properties of daunomycin between binding and nonbinding states are caused by its redox-state switching.87 Infrared SEC combining electrochemical studies with infrared spectroscopy has been shown to be a powerful tool.88–90 Derivative cyclic voltabsorptometry (CVA) and IR CVA SEC techniques were reported for elucidating the redox mechanism of 1,4-benzoquinone and 1,4-bis(2-ferrocenylvinyl)benzene by Jin and co-workers,91 while Griffiths et al. developed it for spectroelectrochemical analysis of adsorbed hexacyanoferrate species formation using the ferri-/ferro-cyanide redox couple.92,93 The ion dynamics in supercapacitors made of nonporous onion-like carbons (OLCs) and carbide-derived carbons (CDCs) with nanosized pores were investigated with IR SEC technique, as shown in Fig. 5. The spectra of 1-ethyl-3-methylimidazolium bis-((trifluoromethyl)sulfonyl)imide (EMIm-TFSI) in the electrodes are nearly identical in the charged and uncharged states, and there is no shift in peak position during charging (Fig. 5A and B). Based on the CV scans of the two electrodes at three different scan rates, it showed that the charge in OLCs is stored on the surface and in the pores of CDCs (Fig. 5C), and the charging mechanism is illustrated in Fig. 5D. The results showed that the ions stored in OLC electrodes can enter and exit the pores easily, and benefit for charging and discharging in the whole process.81 Very recently, UV–Vis SEC was carried out for frontier orbital energetics characterization of formamidinium lead trihalide perovskite (FA-PVSK) films, as shown in Fig. 6A.94 The normalized absorbance response of the thin films on ITO and TiO2 electrodes as a function of applied potential is given in Fig. 6B, C. In addition, the energy level diagram was constructed based on these SEC data (Fig. 6D and E). These results proved that the proposed UV–Vis SEC technique provided a unique methodology for interfacial energetic, heterogeneity, and stability characterizations, which is crucial for optoelectronic platform design.
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Fig. 5 Results for OLC electrodes (A, B and C); and schematic illustration (D) of ion dynamics around OLC particles during CV experiments. Reprinted with permission from ref. 81. Copyright 2013 American Chemical Society.

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Fig. 6 Schematic illustration (A), normal pulse voltammetry potential step method (B), and electron injection (C) of formamidinium lead trihalide perovskite (FA-PVSK), and UV–Vis SEC of thin films for ITO (D) and TiO2 (E) electrodes. Reprinted with permission from ref. 94. Copyright 2017 American Chemical Society.

FT-IR spectroelectrochemical analysis has been employed for decades in the investigation of the structure and properties of redox reactants and products in biological, organic, and inorganic systems.8,97,98 In the past, a wide variety of FT-IR SEC has been designed and developed.99–101 As shown in Fig. 7A–E, an electroactive nitrospiropyran-substituted polyterthiophene was synthesized using spiropyrans and polythiophenes, which served as photochromic, electrical and optical materials, leading to the resultant device displaying many states with different colors during potential cycling.95 A spectroelectrochemical fiber-optic sensor consisting of a gold mesh cover on a multimode fiber optic was fabricated (Fig. 7F–G). This sensor was based on the change of the light attenuation that passes through the fiber-optic core accompanying the electrochemical oxidation–reduction of an analyte at the electrode. Methylene blue and ferrocyanide were used as model analytes to evaluate the performance of the proposed sensor.96 FT-IR SEC was successfully used for molecular electrocatalyst study,102–104 in which stepwise key intermediates, active species and products were tracked during electrochemical reaction by cycling potentials.105–108 Wrighton et al. used it for characterization of CO2 reduction catalytic species and products by 2,2′-bipyridyl-based Re1 and Mn1fac-tricarbonyl electrocatalysts, and they also gave a review on the IR-SEC cell requirements and addressed many mechanistic questions on these methods.83


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Fig. 7 CV of SP1 (A), SEC with CV scan from 0.9 to 0.6 V in optically transparent thin layer electrode cell (B), and electrochromic hybrids (C–E). Reprinted with permission from ref. 95. Copyright 2011 American Chemical Society. (F) Sensing based on the changes in the light attenuation that pass through the fiber-optic core accompanying electrochemical analyte oxidation or reduction. (G) Calibration curves for ferrocyanide based on absorbance change at 420 nm. Reprinted with permission from ref. 96. Copyright 2015 American Chemical Society.

3.3 Extended X-ray absorption fine structure (EXAFS) SEC

X-ray absorption spectroscopy (XAS) is used to investigate the oxidation state of an absorbing atom in addition to the numbers and types of atom close to the absorber and the corresponding distances between atoms. In combination with electrochemical techniques, the bond length change and coordination number alteration with oxidation state can be directly determined. According to the regions of the spectra, two XAS are of particular interest, EXAFS and X-ray absorption near edge structure (XANES). Heineman et al. presented a detailed review on the EXAFS SEC technique, including the properties, advantages, electrochemical cells, and applications.109

Recently, EXAFS has been reported to have been successfully used for mechanism and active site analysis of catalysts, including non-platinum group metal (non-PGM) catalysts, such as MOF-derived Fe–N–C,110,113 poly-FeNxC,114,115 Fe–NCB,116 and Fe–N/C,112 and platinum group metal (PGM) catalysts, such as PtNi3/C111 and PtxCo/C NPs.117 Typically, as shown in Fig. 8A, Fe-based non-PGM catalysts with exceptional ORR activity were prepared by utilizing MOF as the support.110 In combination with the EXAFS and other analytical methods, the results demonstrated that the ORR activity was mediated by the moieties of FeNxCyvia active site change by applying redox potential. We also reported another Fe-based non-PGM catalyst using Zn (ZIF-8) as the host for Fe-based sites, which exhibited much higher activity in acidic media, and better performance than platinum catalysts in alkaline media.112In situ synchrotron XAS at the Fe K-edge was utilized to investigate the exceptional activity of the prepared electrocatalysts. As shown in Fig. 8B, based on the edge shift of the XANES characteristic, and the increasing tendency of the Fourier transform peak with the potential increasing from 0.3 to 0.9 V, it proved the oxidation state formation of Fe. One of the great hurdles to commercializing proton exchange membrane fuel cells (PEMFCs) is the usage of scarce platinum in the cathode. To reveal the unusual activity trend, four PtNi3/C catalysts were prepared and the structural evolutions of these PEMFC-cycled catalysts were studied by implementation of an in situ XANES under operating conditions (Fig. 8C). It was claimed that the extraordinary durability of the PtNi3/C catalysts was primarily attributed to their integrated Pt overlayers that well protect the Ni in the cores against acidic pretreatment and lead to higher Ni content at both the beginning-of-life and end-of-life stages after long-term operation.111 Above all, EXAFS and XANES SEC were found to be advantageous tools for in situ structural studies.


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Fig. 8 (A) Schematic illustration of synthesis and characterizations of FePhenMOF–ArNH3 catalyst. Reprinted with permission from ref. 110 and 111. Copyright 2016 The Royal Society of Chemistry. (B) Potential-dependent normalized Fe K-edge XANES of the catalysts collected in N2 saturated 0.1 M HClO4, and their corresponding FT-EXAFS collected in O2 saturated 0.1 M HClO4. Reprinted with permission from ref. 112. Copyright 2015 Nature Publishing Group. (C) Pt L3 edge (a, c) and Ni K edge (b, d) XANES spectra of P1-NA (top) and P2-SA (bottom) representing porous and solid Pt-Ni3/C catalysts in O2-purged 0.1 M HClO4. (e) Overlaid elemental maps of Pt and Ni. (f) Scheme of the adsorption of oxygen on Pt surfaces at elevated potentials as revealed by in situ XANES. Reprinted with permission from ref. 111. Copyright 2016 American Chemical Society.

3.4 Fluorescence spectroelectrochemistry

In situ fluorescence spectroscopy electrochemistry simultaneously combining electrochemistry and fluorescence spectroscopy systems has turned out to be a powerful tool for many redox behavior characterizations.119–121 In this sense, tremendous efforts have been made towards fabricating a more stable, multifunctional and smart fluorescent switch system to meet the current requirements of the miniaturization and function integration, and even intelligent devices through the rational design of the platform of the electrochemical method and the fluorescence technology.122 A supramolecular copolymer with diblock was assembled by Yan's group with two end-decorated homopolymers, poly(styrene)-cyclodextrin (PS-CD) and poly(ethylene oxide)-ferrocene (PEO-Fc), and they also investigated their reversible voltage-responsive behavior between assembly and disassembly in detail (Fig. 9).118 It is worth noting that these mentioned electrochromic devices (ECDs) are anticipated to be of wide use in electrochemical therapeutics.
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Fig. 9 Schematic representation of PS-CD/PEO-Fc supramolecular vesicle-based voltage-responsive device. Reprinted with permission from ref. 118. Copyright 2010 American Chemical Society.

Polyoxometalates (POMs) or metal oxide clusters as a well-known class of electrochromicmaterials for electron reservoir are showing promise as intriguing and prominent candidates for ECDs fabrication. Owing to the extensive range of structures and stable redox states, a series of luminescence ECDs has been realized based on POMs.125,126 In addition, using POMs as electrochromicmaterials, multicolored electrochromic devices have also been fabricated for the construction of full-color electronic paper.127 Multicolored fluorescence switching systems were demonstrated by integrating QDs into the POMs film in Liu's group.128,129 Taking advantage of these unique optical properties of POMs, our group systematically investigated and developed electronic devices based on POMs. In the early work, assembled multilayers based on CdTe QDs and POMs were fabricated into electrochromic devices in aqueous solution. Significantly, the results indicated that the luminescence property of the device is highly dependent on the surface structure variation of QDs induced by the applied potential. Later, as shown in Fig. 10A, three-state switches devices based on the core–shell “sponge” nanostructures of Pyronin Y and POMs were realized, which can be reversibly switched by three triggers, electricity, light and chemical inputs. The proposed devices possessed many advantages, such as high reproducibility and reversibility, large fluorescence contrast, low operation voltage, and high stability.123 What is more, by taking advantage of the nonautofluorescence of upconversion nanoparticles (UCNPs), and the highly sensitive redox switch of POMs, an electricity-stimulated fluorescence switch device applying POMs as the electrochromic material was also fabricated, and it was first used for sensitive detection of antioxidants (Fig. 10B and C).124 Moreover, reversible electroswitched luminescence based on novel organic–inorganic hybrid assemblies in aqueous solutions was realized by us.130 To date, various kinds of electrochromic material-based fluorescence ECDs have been fabricated. In another report, as shown in Fig. 11, single layer electrofluorochromic devices based on high quantum yield polymer gels was reported. The fluorescence intensity of the resultant system was easily modulated in a wide visible spectral range by changing the state of the thienoviologen fluorophore, and it exhibited three states with different emission properties and a high contrast ratio.


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Fig. 10 (A) Schematic illustration of the three-state switches device based on ITO/PAH/Na-POMs@PYDS/PDDA films; (B) representation of fluorescence quenching mechanism and (C) spooled fluorescence spectra with potential cycling between 0.1 and 0.9 V. Reprinted with permission from ref. 123 and 124. Copyright 2013 Royal Society of Chemistry.

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Fig. 11 Electrofluorescence properties of the gels. Reprinted with permission from ref. 119. Copyright 2015 Wiley-VCH.

However, the mentioned studies mainly concentrated on device fabrication. Thus, we extend their potential applications to biofuel cells (BFCs), extracting green energy conversion from biochemical reactions to electricity (Fig. 12A and B).131 In detail, CNTs immobilized with glucose dehydrogenase (GDH) and bilirubin oxidase (BOD) were used as the bioanode and biocathode, respectively. Then electricity was generated by adjusting the redox state of Prussian blue (PB) because its redox potential was just in the range of the active potential of BFC. A visual photograph of the fluorescence switching process is shown in Fig. 12B. Initially, the hybrid exhibited weak fluorescence because of the quenching mechanisms by the strong absorbance from PB (Fig. 12B(a)). When connected with the bioanode, the hybrid gave bright fluorescence because of the weak absorbance of the reduced PB (Fig. 12B(b)). After being connected with the cathode, the fluorescence was “OFF” again (Fig. 12B(c)). The proposed self-powered fluorescence switch system demonstrated high power density, good reversibility and good reproducibility. Recently, for the first time, a reversible self-powered fluorescence display device with two electrodes was fabricated based on a fast charging/recharging battery. The self-powered and rechargeable device is shown in Fig. 12C and D. PB was used as both the cathodic catalyst and the electrochromic material, and magnesium metal was used for the anode. Because of the high theoretical redox potential difference of 2.8 V between the Mg and PB electrodes, just by “connecting” and “disconnecting” the two electrodes, the fluorescence could be easily switched between “on” and “off”, therefore the LED can be lit up by simply connecting the two electrodes.132


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Fig. 12 Schematic diagram of luminescence modulation by bioanode/biocathode (A) and fluorescence images (B) of the hybrid film. Reprinted with permission from ref. 131. Copyright 2013 Royal Society of Chemistry. Scheme of the self-powered electrochromic fluorescence device (C) and photographs of the LED with the two electrodes disconnected and connected (D). Reprinted with permission from ref. 132. Copyright 2016 Royal Society of Chemistry.

3.5 Other spectroscopy method-based SEC

Despite the recent advances mentioned above, less common but also well-developed SEC techniques involving electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) are rarely reported. Since its initiation in 1958,133 a large number of in situ EPR SEC techniques have been developed for characterization of organic structures. Furthermore, NMR SEC was realized in 1975 for molecular structure studies through setting a spinning cell on well-resolved NMR spectra.134 In addition, in situ X-ray diffraction was accessible to the cell construction for Bragg-type measurements and a special cell for metal monolayer studies.135 Besides all the spectroscopic methods listed above, several types of spectroscopies used in SEC are summarized in Table 1.
Table 1 Summary of typical SEC methods
Method Applications Ref.
DFS = dark-field scattering, PL = photoluminescence.
EPR Properties of formal nickel(III) complexes 136
PM-IRRA Redox of LbL PAH and PAA multilayers 137
UV–Vis–NIR Optical and redox properties of metal–organic framework 138
SERS The role of molecular orientation on the electrode surface 1
In situ analysis system with modified gold surface 80
EPR/UV–Vis–NIR Radical cation of β-oligothiophene 139
ESR/UV–Vis–NIR p-Toluenediamine anodic oxidation mechanism 140
Radical cations of dendrimer 39
ESR/Vis–NIR Origin of ESR “spikes” and reactivity of fullerene anions 141
Charged structural dependence of redox-induced dimerization 142
ATR-FTIR Structure of states in polyaniline 143
XANES Formal potentials determining 144
NMR Charge stabilization by dimer formation of thiophene tetramer 145
ESR/NMR p-Benzoquinone reaction 146
EXAFS Direct structural information 109 and 147
ATR/IR Mechanism of aqueous CO2 reduction 129
DFS-PL Single Ag nanoparticle luminescent 42


4. Applications of spectroelectrochemistry

4.1 Mechanism study

Recently, SEC has aroused great attention for mechanism study owing to its exceptional functions and wide applications.16,148–154 The emergence of carbon materials provides an excellent alternative to traditional catalysts.19 Specifically, SEC has turned out to be a powerful tool for charge-transfer mechanism study on carbon nanostructures, which highly promoted the development of carbon materials.155–157 In our group, the SERS technique was successfully used for interaction mechanism study of DNA with cytochrome c (Cyt c), which confirms that the conformational equilibrium of Cyt c was induced by redox electric field.158 Using the same method, the interaction of Cyt c with cardiolipin-containing membranes was studied by Jiang et al. with label-free surface-enhanced infrared SEC.159 In addition, in situ rapid scan time-resolved IR SEC was used for redox mechanism study of polyaniline film.160 4,4′-Dimercaptoazobenzene formation by photo-induced catalysis from 4-aminobenzenethiol was explained through SERS SEC study.161 Single molecule fluorescence SEC of cresyl violet adsorbed on a clay-modified surface was observed. The molecules exhibit on/off fluorescence switching controlled by redox potential scan,162 and electron transfer rates of the measured oxidation/reduction are in coincidence with that of the ensemble-average using electron transfer mediators.

Improving the fundamental understanding of the electrode interface and identifying the intermediate species are critical to fabricating a better catalyst and improving the performance of the catalyst in an electrochemical reaction. To this end, Tian and co-workers successfully observed the Raman signal of pyridine adsorbed on transition metal surfaces for the first time.62 Using a combination of in situ Raman and electrochemical techniques with density functional theory (DFT) calculation, in situ electrochemical SERS study was performed in Tian's group.163 The species of PhCH2Cl on a silver electrode under various potential was characterized (Fig. 13). The results revealed that the benzyl radical and its anionic derivate attached to the silver electrode are the key intermediates. In addition, they used electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy to in situ monitor the electrooxidation processes on the surface of a single crystal of gold.164 Chen et al. studied the electrooxidation processes of ethylene glycol (EG) and glucose on PtAu catalysts with in situ FTIR technology.165 C–C bond breaking was observed in glucose on a Pt/Au disk but not in ethylene glycol based on the production of CO2, as the evidence of C–C breaking, from the in situ FTIR results. These results show the possibility of catalysts with reducing Pt in polyols electrooxidation.


image file: c7nr07803j-f13.tif
Fig. 13 (A) CV (a) and SERS spectra (b) of 5 mM benzyl chloride on an Ag electrode with different scan rates (a) and under various potentials (b). (B) DFT calculated and experimental Raman spectra of the possible solvated reaction intermediates. Reprinted with permission from ref. 163. Copyright 2016 American Chemical Society.

It is a fact that noble metal clusters with rich and tunable physicochemical properties are promising candidates for a variety of applications. UV–Vis–NIR SEC was employed to study the transitions in discrete absorption bands of Au130 clusters upon stepwise charging. It was revealed that Au core charging with well-defined quantized double-layer and oxidizable ligands make it unique and enable core and ligand with different charge states.166

The reaction of the bis(imidazoliumyl)-substituted P1 cation [(2-ImDipp)P(4-ImDipp)]+ (10+) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated dications and methylated dications. EPR/UV–Vis–NIR SEC was used to investigate how the low-coordinated P1 cation 10+ predicted the P-centered radical dication [(2-ImDipp)P(4-ImDipp)]2+ (132+). It was revealed that the two radical species were involved by electrochemical oxidation and reduction (Fig. 14).167 It is noted that conduction and valence band energies (ECB, EVB) directly affect the photoelectrochemical efficiency of heterostructured materials. Furthermore, for photoelectrochemical reactions, Saavedra's group obtained the band edge energetics of Au-tipped CdSe nanorods using photoemission spectroscopy and waveguide SEC.168 After Au tipping, the EVB shifted close to vacuum by up to 0.4 eV, and the apparent Fermi energy shifted towards the middle of the band gap. Electron-responsive self-assembled molecular materials are currently subjected to intense research activity. Very recently, viologen-centered electron transfer was used to trigger the complete dissociation of a porphyrin-based supramolecular architecture. As shown in Fig. 15A, in the oxidized state, self-assembly is induced by iterative association of individual porphyrin-based tectons. Dissociation of the self-assembled species is actuated upon changing the redox state of the bipyridium units, which are involved in the tectons from their dicationic state to their radical cation state. The driving force of the disassembling process is the formation of an intramolecularly locked conformation partly stabilized by π-dimerization of both viologen cation radicals.169 In addition, as shown in Fig. 15B, UV–Vis–NIR and Raman SEC were employed to characterize the structural and electronic properties of the open-shell diindeno[b,i]anthracene (DIAn) derivative, an oxygen- and temperature-stable singlet biradical compound.170 UV–Vis SEC was used as a powerful method to determine the pH-dependent redox potential and mechanistic details of the glucose oxidase (GOx) from Aspergillus Niger. As expected, a cofactor of GOx changed directly from the oxidized quinone to its doubly reduced state without the formation of stable singly reduced semiquinone intermediates in the entire pH range between 4.5 and 8.5.171


image file: c7nr07803j-f14.tif
Fig. 14 In situ reflective UV–Vis–NIR spectroelectrochemical measurement of a Pt disc electrode surface of a CH2Cl2/nBu4N[OTf] (0.1 M) solution of 10[OTf] (1.3 × 10−3 M) during a cyclic voltammogram. Reprinted with permission from ref. 167. Copyright 2015 Wiley-VCH.

image file: c7nr07803j-f15.tif
Fig. 15 (A) Concept of redox-switchable metal–organic self-assembled system. Reprinted with permission from ref. 169 Copyright 2016 American Chemical Society. (B) Redox processes of DIAn. The ground state is a resonance hybrid of quinoidal and aromatic structures. R = (triisopropylsilyl)ethynyl. Reprinted with permission from ref. 170. Copyright 2016 American Chemical Society.

4.2 Electrochromic device fabrication

A fruitful area of SEC is the popular research on the fabrication of ECDs, which are emerging as a novel technology for various applications, such as commercialized smart window glasses, goggles, and autodimming rear view mirrors.172 Erochromism is a process by which the color, transparency, and reflectivity of a material is changed during electrochemical reactions, which have a significant impact on energy consumption in architectural smart windows.127,172–176 Nanomaterials with reversible electroswitchable properties have drawn much scientific and technical attention owing to their amazing potential applications in the field of ECDs.95,177–180 A large amount of nanomaterials with outstanding electrochromic properties have been developed for ECDs fabrication so far.181–187 Reynolds et al. gave a detailed review on the construction of ECDs based on π-conjugated organic polymers with color control.29 Our group also developed different multifunctional ECDs based on polyoxometalate and organic–inorganic hybrid assemblies.123,124,130,188–190 It is worth noting that besides luminescent ON/OFF switching, a number of ECDs with multicolor switching have been realized. Nanodevices that are able to perform controlled motion, molecular grippers, are of particular interest because of their controlled motion in response to external stimuli. Very recently, semiquinone-based paramagnetic molecular grippers were developed, which open the way to six-state redox switches by virtue of two conformations in three redox states generated by electrochemistry, chemistry, and photochemistry. UV–Vis SEC and EPR spectroscopy were utilized to evaluate the conformational features of resorcin[4]arene cavitands. In recent reviews, the fundamentals and recent progress in the field of ECDs have been highlighted, which focused on working mechanisms, materials, devices and performance improvements.191,192 The progress and development of ECDs with multicolor switching are introduced in detail in Fig. 16.
image file: c7nr07803j-f16.tif
Fig. 16 Fabrication (A) and mechanism (B) of ECDs and photograph images (A, down) of the device at different potential under 365 nm UV excitation. Reprinted with permission from ref. 193. Copyright 2014 American Chemical Society. (C) The chemical structures of the probe used in the device. The reversible emission color change (D) and fluorescence switching (E) of the device between multi-color states. Reprinted with permission from ref. 194 Copyright 2012 Wiley-VCH.

With the growth of flexible and bendable electronic displays, the fabrication of low-cost display materials with high mechanical tolerance has become indispensable to commercial practicability. Interestingly, a carbon dot-based light emitting diode (LED) featured with color-switchable electroluminescence has been reported. The device is made of a sandwich construction through a solution-based process: an organic hole transport layer, an organic or inorganic electron transport layer, and a carbon dot emissive layer in between. For the first time, switchable multicolor emission of blue, cyan, magenta, and white from the same carbon dots can be obtained in single emitting layer nanostructured LEDs by tuning the device structure and changing the applied voltage, which presents a wide range of possibilities for the development of colorful LEDs.195 A novel ECD based on CdS-modified TiO2 (TiO2/CdS) film with enhanced electrochromic properties was developed by Xu's group, which possessed an improved optical contrast window of about 79.7%, an enhanced coloration efficiency of 70.8% and a half shortened coloration time of 12.7 s.196

Recently, a visible to NIR-sensitive poly(3,4-ethylenedioxyselenophene) (PEDOS) derivative film for transparent photo-thermo-electric (PTE) converters was investigated.25Fig. 17 shows the scheme of the PTE harvester of the PEDOS-C6 films for thermoelectricity collection, and the fabricated films exhibited high PT and TE converting efficiency. With respect to ECDs, photovoltaics devices are also an important branch. A series of large-area 2D materials with high optical and electrical properties was developed previously, such as boron nitride (BN), graphene, molybdenum disulfide (MoS2), and gallium telluride (GaTe), most of which were synthesized using CVD.197,198 The CVD-grown graphene on high-quality BN film yields high carrier mobility, and BN on a SiO2/Si substrate can be developed for a MoS2 (WSe2) field-effect transistor.


image file: c7nr07803j-f17.tif
Fig. 17 The fabrication of the photo-thermo-electric harvester. (A) Photo images of doping-controlled PEDOS-C6 films on ITO glass. SEM image of the pristine CPP film. (B and C) Transfer of the film from ITO glass to a transparent tape. (D) Au electrode deposited thermo-electric device. (E) The measurement setting of the PTE effect, and (F) the vertical type TE harvester on an arm for generating thermoelectricity from body heat. Reprinted with permission from ref. 25. Copyright 2013 Wiley-VCH.

4.3 SEC-based sensor

In analytical chemistry, major advances often arise from the combination of orthogonal techniques to obtain more information from one experiment.58 During the past decade, with the development of the SEC analysis system, multiple sensing platforms with high selectivity and sensitivity have been designed and studied.86,199,200 Heineman and co-workers have made great contributions to the development of sensors with multimode selectivity using different new polymeric materials, such as technetium complex, ultrathin Nafion films, polymer poly(vinyl alcohol)-polyelectrolyte blend-modified OTE, and polymer-coated boron, which have greatly increased the sensitivity and selectivity toward the analyte of interest.201–206 Specifically, an SEC sensor for [Tc(dmpe)3]2+/+ consisting of a thin film of partially sulfonated block copolymer-coated OTE was developed.203 The sensor showed a linear relationship between the current intensity and the concentration of [Tc(dmpe)3]2+/+ (from 0.16 to 340.0 μM) in aqueous solution with a limit of detection of 24 nM. Electron transfer at the interface plays an important role in many chemical and biological processes. However, electron transfer at the interface is usually very complicated owing to its high dependence on the local environments.162 In this context, Chen's group proposed a thrombin-specific aptamer linked to an Au nanowire. When a positive potential was applied on the nanowire, the positive charged aptamer–thrombin complex was attracted toward the surface (Fig. 18, right), otherwise the complex was repelled from the nanowire when a negative potential was applied (Fig. 18, left). The fluorescence intensity from the probe–thrombin–aptamer complex showed a proportional relationship with the distance between the probe and the Au nanowire (Fig. 18, top).207 It is well known that in the mammalian central nervous system, dopamine (DA) is a neurotransmitter for biological function control and pathologies. A low level of DA is linked to Parkinsonism, while a high concentration of this neurotransmitter is associated with schizophrenia. As shown in Fig. 18 (bottom), screen-printed electrodes (SPEs) were used in UV–Vis SEC for DA oxidation mechanism study and detection, which highly depended on the initial concentration of DA. Different products were generated from the different concentrations of dopamine and under different potentials, which provided a comprehensive understanding of the DA oxidation mechanism and an easy way to distinguish DA from catechol. It is demonstrated that SEC represents an autovalidated technique compared with spectrophotometric and electrochemical methods.56 Very recently, an in vivo UV–Vis absorption SEC device based on SWCNTs was developed for direct ascorbic acid determination in grapefruit. The two optical fibers are fixed in a long optical path length configuration, and the three electrodes are flat on the surface and bare optical fibers in a parallel arrangement (Fig. 18D and E). This is the first SEC sensor to be inserted directly in a biological matrix for in vivo measurements.208
image file: c7nr07803j-f18.tif
Fig. 18 (A) Scheme of an electrically controlled protein assay and (B) fluorescence measured from 100 nM sample versus time. Reprinted with permission from ref. 207. Copyright 2008 American Chemical Society. (C) Screen-printed electrodes for SEC-based dopamine assay. Reprinted with permission from ref. 56. Copyright 2012 American Chemical Society. (D) Scheme of UV–Vis SEC device. (E) Device for direct ascorbic acid detection and its setup: (a) potentiostat, (b) light source, (c) spectrometer, (d) grapefruit, and (e) SEC device. Reprinted with permission from ref. 208. Copyright 2017 American Chemical Society.

4.4 Protein study

Redox proteins carry out numerous key reactions in biological processes, the underlying process of which is electron transfer. The fundamental structure of redox proteins consists of catalytic sites, which can be defined as multi-electron redox centers or clusters of single electron redox centers interacting with substrates and acting as sources of electrons.209 By using the interdisciplinary advantages of electrochemistry and spectroscopy, the electron transfer process of the proteins and signaling processes can be studied more comprehensively, which can provide a basis for further understanding of the life processes of biological system.210–213

Aartsma and co-workers simultaneously monitored the oxidation state of type-1 copper and the nitrite reductase (NiR) turnover rate based on Alcaligenes faecalis S-6 using fluorescence SEC (Fig. 19).214 The redox site of type-1 copper and the catalytic activity of NiR were investigated through fluorescence intensity changes based on electrochemical measurement. As shown in Fig. 19B, the fluorescence intensity of ATTO 565 labeled L93C Cu-NiR decreased rapidly with the addition of nitrite (black line), while the catalytic current increases (gray line) because of NO2 reduction by the enzyme at 200 mV applied potential.


image file: c7nr07803j-f19.tif
Fig. 19 An example of SEC data collected from ATTO 565 labeled L93C Cu-NiR activity titration on a 6-MH-modified gold electrode with nitrite concentrations increase at pH 5.45. Reprinted with permission from ref. 214. Copyright 2011 American Chemical Society.

Simultaneous investigation of the influence of surface structure on single or mixed component self-assembled monolayers was accomplished via in situ spectroelectrochemical fluorescence imaging technique coupled with electrochemical measurements of the complete stereographic triangle on a single crystal Au bead electrode.215 As shown in Fig. 20A, a circular defect and four large facets are created during the cooling process, which can be observed clearly from the fluorescence images of the Au bead electrode surface upon reductive desorption. The changes of fluorescence intensity as a function of potential are plotted for surface crystallography and surface energy analysis (Fig. 20B).


image file: c7nr07803j-f20.tif
Fig. 20 (A) Surface images of the Au bead electrode. Bright field image of 111 facets (a) and fluorescence images at different potentials for (b) 111, (c) 100 and (d) 110 planes. (B) Fluorescence intensity changes depending on the surface crystallography by potential shown for the 100−111, 111−110 and 110−100 zone axes. Reprinted with permission from ref. 215. Copyright 2015 American Chemical Society.

4.5 Other applications

To date, besides the above-mentioned applications of SEC in mechanism study, device fabrication, sensing and protein study, there are some other amazing potential applications. For example, Libuda and coworkers studied the solar energy storage ability of valence isomers norbornadiene (NBD) and quadricyclane (QC) on a Pt(111) single crystal working electrode with electrochemistry-infrared spectroscope (EC-IRS) at the single molecule level. The energy storage and release cycles were monitored through in situ vibrational spectroscopy. NBD can be converted to QC under UV irradiation in the presence of a photosensitizer, while an electrochemical trigger is applied, and QC is ready to convert back to NBD (Fig. 21A). The fingerprint region signal intensity of NBD and QC obtained from IRS was used to analyze the conversion efficiency of the isomers under photochemical and electrochemical stimulus (Fig. 21B), demonstrating a potential molecular photoelectrochemical energy storage system.216 The conduction band energy and the extinction coefficient of the nanoparticle semiconductor electrodes were determined by Hamann's group with absorbance change upon applied potentials.151 Inganäs et al. reported an in situ FTIR SEC method to investigate the charge storage mechanism and quinone group formation of a polypyrrole/lignin composite.217
image file: c7nr07803j-f21.tif
Fig. 21 (A) Schematic representation of the combined PEC-IR reflection absorption spectroscope. (B) Calculated concentration changes of norbornadiene (NBD) and quadricyclane (QC). Reprinted with permission from ref. 216. Copyright 2017 American Chemical Society.

A new method coupling electrochemistry and fluorescence microscopy was developed for exocytotic processes investigation in single adrenal chromaffin cells.218 As shown in Fig. 22, simultaneous electrochemical and optical detections were carried out on the ITO surface. A comprehensive view of the “life” of a secretory vesicle before and after its fusion with the cell membrane was offered and the kinetics of the released material were monitored. Recently, solution-processed films based on colloidal aliovalent niobium-doped anatase TiO2 nanocrystals were developed. The Nb-TiO2 film exhibited modulation of optical transmittance in the NIR and visible light regions, and supports localized surface plasmon resonance in the NIR, and its multimodal electrochromic properties show promise for application in dynamic optical filters or smart windows.174


image file: c7nr07803j-f22.tif
Fig. 22 (A) Schematic illustration of the ITO microelectrode. (B) Photograph of a chromaffin cell adhering on the electrode. (C) Phase contrast and fluorescence microscopy image of cell preincubated with acridine orange. Reprinted with permission from ref. 218.Copyright 2006 Wiley-VCH.

5. Conclusions and future prospects

Electrochemistry is a pervasive scientific discipline that is essential for various relevant fields in chemistry, physics and biology. Coupled with the rapid development of spectroscopic techniques, the combination of the two techniques can provide much more information during redox processes under rapid, remote and reversible electrical stimuli. Although a large variety of electrochemical techniques have been developed, there is no other way than SEC to get a qualified picture of the state-of-the-art in the detailed study of electrode reactions or complex electrode systems. Since its discovery in 1964, a great number of SEC methods have been developed, and in recent years considerable advances have been made in the field of SEC. In this review, we have summarized the state of research on SEC in the past decade and selected examples are illustrated in detail, including the potential and applicability of these techniques. These high advances have opened up new pathways to develop novel high-performance SEC techniques for potential applications.

Despite these achievements, there is still a long way to go in the studies of this field, and large challenges still exist. First, it is well known that the performance of SEC is highly dependent on the OTEs. Thus, new types of OTEs with high optical transparency, smaller size and excellent accuracy are greatly demanded via novel material utilization, improving reflectance methods to reduce absorption by aqueous electrolytes and so on. Second, in order to study more diverse and complex processes, diverse SEC techniques, especially multi-SEC methods, are required, such as ATR-FTIR and EPR/UV–Vis–NIR SEC. Extra efforts are needed to further improve corresponding equipment, including miniaturization, real-time and ultrafast detection, and sensitivity, which can ensure the accuracy and sensitivity of the proposed system. The third is the application scope of SEC. One of the key challenges is their wide applications in life sciences. Some critical problems, particularly regarding the precise control of temperature, wide redox potential window, strong anti-interference and high sensitivity, still need efficient solutions, which is crucial for both cells and living organisms. Finally, a full understanding of SEC mechanisms is still awaiting more exploration. Specific attention should be paid to studying the mechanisms of electrode reactions or complex electrode systems. Therefore, the aim of this review is not only to list the present SEC techniques but also to point out the benefits and remaining challenges in this area. It is anticipated that the future of this field should be interesting and fantastic.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21375123, 21675151) and the Ministry of Science and Technology of the People's Republic of China (no. 2013YQ170585, 2016YFA0203203).

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