Monitoring Interfacial Bioelectrochemistry Using a FRET Switch

In Situ Spectroelectrochemical Investigations of Electrode-Confined Electron-Transferring Proteins and Redox Enzymes

This perspective analyzes recent advances in the spectroelectrochemical investigation of redox proteins and enzymes immobilized on biocompatible or biomimetic electrode surfaces. Specifically, the article highlights new insights obtained by surface-enhanced resonance Raman (SERR), surface-enhanced infrared absorption (SEIRA), protein film infrared electrochemistry (PFIRE), polarization modulation infrared reflection-absorption spectroscopy (PMIRRAS), Förster resonance energy transfer (FRET), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and differential electrochemical mass spectrometry (DMES)-based spectroelectrochemical methods on the structure, orientation, dynamics, and reaction mechanisms for a variety of immobilized species. This includes small heme and copper electron shuttling proteins, large respiratory complexes, hydrogenases, multicopper oxidases, alcohol dehydrogenases, endonucleases, NO-reductases, and dye decolorizing peroxidases, among other enzymes. Finally, I discuss the challenges and foreseeable future developments toward a better understanding of the functioning of these complex macromolecules and their exploitation in technological devices.

Self-Driven Multicolor Electrochromic Energy Storage Windows Powered by a "Perpetual" Rechargeable Battery

Electrochromic windows (ECWs) become an appealing concept for green buildings. However, conventional ECWs need external biases to operate causing energy consumption and are usually restricted by monotonous color. Recently, electrochromic energy storage windows (EESWs) integrating the functions of electrochromism and energy storage in one device have attracted particular attention in various fields, such as self-powered addressable displays, human-readable batteries, and most importantly energy-efficient smart windows. Herein, a color-tunable (nonemissive-red-yellow-green) self-powered EESW is initially presented utilizing Prussian blue (PB) as a controller of the fluorescent component of CdSe quantum dots. The key design feature is that without any external stimuli, the EESW can be powered by a rechargeable "perpetual" battery, which is composed of two half-cell couples of Fe/PB and Prussian white (PW)/Pt. This technique allows to achieve only by switching the connection status of the two half-cells, the fast discharging and self-charging process of the EESWs with high and sustainable charge-storage capacity. Remarkably, the fabricated self-powered EESWs exhibit quick response ("off" 7 s, "on" 50 s), large transmittance spectra contrast, and high fluorescent contrast modulation (60-86%) over a wide optical range, and great reproducibility (only 3% of the modulation ratio decreased after 30 cycles), which is comparable to ECWs powered by an electrochemical potentiostat.

In situ reversible color variation of a ready-made upconversion material using the designed component of a three-state fluorescence switching system

In recent years, upconversion materials have attracted considerable attention because of their unique physicochemical features. Numerous studies have focused on the synthesis of upconversion materials with different colors. However, an easier way to vary the upconversion colors without changing the materials' components has not been extensively studied. In this study, we realized the in situ color variation of the designed upconversion material with the help of a three-state fluorescence switching hybrid device. The device was composed of Prussian blue and upconversion materials; the former element functioned as a fluorescence resonance energy transfer acceptor and the latter acted as a donor. Smartly applying the RGB color model guaranteed multicolor of the device. Moreover, the highest fluorescence contrast of the three-state fluorescence switching system was 86% (larger than the result of a previous study), and the three-state reversibility was remarkable; this was probably owing to the unique layer-by-layer dripping/electrodepositing assembly method. To the best of our knowledge, the in situ reversible color variation of the ready-made upconversion material has been demonstrated for the first time.

Recent advances in spectroelectrochemistry

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.

Retuning the Catalytic Bias and Overpotential of a [NiFe]-Hydrogenase via a Single Amino Acid Exchange at the Electron Entry/Exit Site

The redox chemistry of the electron entry/exit site in Escherichia coli hydrogenase-1 is shown to play a vital role in tuning biocatalysis. Inspired by nature, we generate a HyaA-R193L variant to disrupt a proposed Arg-His cation-π interaction in the secondary coordination sphere of the outermost, "distal", iron-sulfur cluster. This rewires the enzyme, enhancing the relative rate of H₂ production and the thermodynamic efficiency of H₂ oxidation catalysis. On the basis of Fourier transformed alternating current voltammetry measurements, we relate these changes in catalysis to a shift in the distal [Fe₄S₄]^2+/1+ redox potential, a previously experimentally inaccessible parameter. Thus, metalloenzyme chemistry is shown to be tuned by the second coordination sphere of an electron transfer site distant from the catalytic center.

Tip-Enhanced Raman Voltammetry: Coverage Dependence and Quantitative Modeling

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was employed for the first time to observe nanoscale spatial variations in the formal potential, E^0', of a surface-bound redox couple. TERS cyclic voltammograms (TERS CVs) of single Nile Blue (NB) molecules were acquired at different locations spaced 5-10 nm apart on an indium tin oxide (ITO) electrode. Analysis of TERS CVs at different coverages was used to verify the observation of single-molecule electrochemistry. The resulting TERS CVs were fit to the Laviron model for surface-bound electroactive species to quantitatively extract the formal potential E^0' at each spatial location. Histograms of single-molecule E^0' at each coverage indicate that the electrochemical behavior of the cationic oxidized species is less sensitive to local environment than the neutral reduced species. This information is not accessible using purely electrochemical methods or ensemble spectroelectrochemical measurements. We anticipate that quantitative modeling and measurement of site-specific electrochemistry with EC-AFM-TERS will have a profound impact on our understanding of the role of nanoscale electrode heterogeneity in applications such as electrocatalysis, biological electron transfer, and energy production and storage.

Self-powered fluorescence display devices based on a fast self-charging/recharging battery (Mg/Prussian blue)

Stimuli-responsive (such as voltage and/or light) fluorescence display systems have attracted particular attention in their promising fields of application. However, there are few examples of self-powered fluorescence display devices. Here we designed and fabricated a self-powered fluorescence display device based on a fast-charging/recharging battery. The specially designed battery was composed of a Prussian blue (PB) cathode and a magnesium metal anode with a high theoretical redox potential difference (∼2.8 V). Moreover, smartly adding a trace amount of NaClO in the electrolyte could realize oxidizing PW to PB ∼480 times faster than when oxidizing without NaClO, leading to the fast self-charging and high power density (maximum power density of 13.34 mW cm^-2, about two to three orders of magnitude larger than previous bio-fuel cells) of the Mg/PB battery. Most importantly, PB was used as not only the cathodic catalyst but also as an electrochromic material, making it possible to construct a self-powered and rechargeable electrochromic fluorescence display with only two electrodes. Besides, fluorescent [Ru(bpy)₃]²⁺-doped silica nanoparticles (Ru@SiO₂), were selected as the fluorescence resonance energy transfer (FRET) donor to match PB (FRET acceptor). To the best of our knowledge, we demonstrated a self-powered and rechargeable electrochromic fluorescence display with only two electrodes for the first time.

Voltage-controlled fluorescence switching of a single redox protein

Heterogeneous electron transfer (ET) of the redox protein, wild-type azurin (wt-Az) from Pseudomonas aeruginosa, was monitored at the single-molecule (SM) level by fluorescence resonance energy transfer (FRET), one electron at a time. Azurin molecules were labeled with an organic fluorophore (Cy5), and the FRET-coupling between Cy5 and the redox center (copper) was used to study ET to a semi-transparent, 10nm thin gold electrode in an optical configuration. By using a confocal microscope and a bipotentiostat for control of the electrode potential, the oxidation and reduction processes of individual Az-Cy5 molecules were monitored. In the oxidized state of the redox center of the azurin molecule, the fluorescence emission of the covalently attached Cy5 was largely quenched by FRET ('off'-state), whereas the emission was recovered upon reduction ('on'-state). The work presented here, shows directly controlled single redox switching events of an individual redox protein and its thermodynamic dispersion. We show that the distribution of midpoint potentials (E0) of individual azurin molecules peaks at 45.7±0.5 mV with a full width at half maximum of 15 mV vs saturated calomel electrode (SCE).

Nanoscale methods for single-molecule electrochemistry

The development of experiments capable of probing individual molecules has led to major breakthroughs in fields ranging from molecular electronics to biophysics, allowing direct tests of knowledge derived from macroscopic measurements and enabling new assays that probe population heterogeneities and internal molecular dynamics. Although still somewhat in their infancy, such methods are also being developed for probing molecular systems in solution using electrochemical transduction mechanisms. Here we outline the present status of this emerging field, concentrating in particular on optical methods, metal-molecule-metal junctions, and electrochemical nanofluidic devices.

Chemically-induced redox switching of a metalloprotein reveals thermodynamic and kinetic heterogeneity, one molecule at a time

FRET-based detection of individual azurin–Cy5 molecules shows an on (reduction)–off (oxidation) fluorescence switching, reveals the redox parameters.

Reversible photo-chem-electrotriggered three-state luminescence switching based on core-shell nanostructures

Reversible three-state fluorescence switches triggered by light, electricity and chemical inputs based on "sponges" of Pyronin Y-doped silica nanoparticles (PYDS) and polyoxometalate K14[Na(H2O)P5W30O110] (Na-POMs) core-shell nanostructures were realized. Under one or two signal inputs, the system exhibited distinct three-state interconvertible automaton, achieving reversible "on" and "off" luminescence switches via the related luminescence quenching effect. The features of the system correspond to the equivalent circuitry of an IMPLICATION logic gate performing the Boolean operation by using potential and chemical as inputs. Such a multi-chromic device with novel structure possesses several advantages, such as relative low operation voltage, large reproducibility and reversibility, apparent fluorescence contrast, and long-time stability, which make it a suitable candidate for nonvolatile memory devices. In addition, the current protocol for the hybrid film fabrication can be easily extended from the polyoxometalate and organic dyes to other novel nanostructures matched multifunctional stimulus-responsive species and fluorescence materials in the future.

Probing redox proteins on a gold surface by single molecule fluorescence spectroscopy

The interaction between the fluorescently labeled redox protein, azurin, and a thin gold film is characterized using single-molecule fluorescence intensity and lifetime measurements. Fluorescence quenching starts at distances below 2.3 nm from the gold surface. At shorter distances the quantum yield may decrease down to fourfold for direct attachment of the protein to bare gold. Outside of the quenching range, up to fivefold enhancement of the fluorescence is observed on average with increasing roughness of the gold layer. Fluorescence-detected redox activity of individual azurin molecules, with a lifetime switching ratio of 0.4, is demonstrated for the first time close to a gold surface.

Theoretical analysis of the two-electron transfer reaction and experimental studies with surface-confined cytochrome c peroxidase using large-amplitude Fourier transformed AC voltammetry

A detailed analysis of the cooperative two-electron transfer of surface-confined cytochrome c peroxidase (CcP) in contact with pH 6.0 phosphate buffer solution has been undertaken. This investigation is prompted by the prospect of achieving a richer understanding of this biologically important system via the employment of kinetically sensitive, but background devoid, higher harmonic components available in the large-amplitude Fourier transform ac voltammetric method. Data obtained from the conventional dc cyclic voltammetric method are also provided for comparison. Theoretical considerations based on both ac and dc approaches are presented for cases where reversible or quasi-reversible cooperative two-electron transfer involves variation in the separation of their reversible potentials, including potential inversion (as described previously for solution phase studies), and reversibility of the electrode processes. Comparison is also made with respect to the case of a simultaneous two-electron transfer process that is unlikely to occur in the physiological situation. Theoretical analysis confirms that the ac higher harmonic components provide greater sensitivity to the various mechanistic nuances that can arise in two-electron surface-confined processes. Experimentally, the ac perturbation with amplitude and frequency of 200 mV and 3.88 Hz, respectively, was employed to detect the electron transfer when CcP is confined to the surface of a graphite electrode. Simulations based on cooperative two-electron transfer with the employment of reversible potentials of 0.745 ± 0.010 V, heterogeneous electron transfer rate constants of between 3 and 10 s(-1) and charge transfer coefficients of 0.5 for both processes fitted experimental data for the fifth to eighth ac harmonics. Imperfections in theory-experiment comparison are consistent with kinetic and thermodynamic dispersion and other nonidealities not included in the theory used to model the voltammetry of surface-confined CcP.

Transparent gold as a platform for adsorbed protein spectroelectrochemistry: investigation of cytochrome c and azurin

The majority of protein spectroelectrochemical methods utilize a diffusing, chemical mediator to exchange electrons between the electrode and the protein. In such methods, electrochemical potential control is limited by mediator choice and its ability to interact with the protein of interest. We report an approach for unmediated, protein spectroelectrochemistry that overcomes this limitation by adsorbing protein directly to thiol self-assembled monolayer (SAM) modified, thin (10 nm), semitransparent gold. The viability of the method is demonstrated with two diverse and important redox proteins: cytochrome c and azurin. Fast, reversible electrochemical signals comparable to those previously reported for these proteins on ordinary disk gold electrodes were observed. Although the quantity of protein in a submonolayer adsorbed at an electrode is expected to be insufficient for detection of UV-vis absorption bands based on bulk extinction coefficients, excellent spectra were detected for each of the proteins in the adsorbed state. Furthermore, AFM imaging confirmed that only a single layer of protein was adsorbed to the electrode. We hypothesize that interaction of the relatively broad gold surface plasmon with the proteins' electronic transitions results in surface signal enhancement of the molecular transitions of between 8 and 112 times, allowing detection of the proteins at much lower than expected concentrations. Since many other proteins are known to interact with gold SAMs and the technical requirements for implementation of these experiments are simple, this approach is expected to be very generally applicable to exploring mechanisms of redox proteins and enzymes as well as development of sensors and other redox protein based applications.

Detection of mercury(II) by quantum dot/DNA/gold nanoparticle ensemble based nanosensor via nanometal surface energy transfer

An ultrasensitive fluorescent sensor based on the quantum dot/DNA/gold nanoparticle ensemble has been developed for detection of mercury(II). DNA hybridization occurs when Hg(II) ions are present in the aqueous solution containing the DNA-conjugated quantum dots (QDs) and Au nanoparticles. As a result, the QDs and the Au nanoparticles are brought into the close proximity, which enables the nanometal surface energy transfer (NSET) from the QDs to the Au nanoparticles, quenching the fluorescence emission of the QDs. This nanosensor exhibits a limit of detection of 0.4 and 1.2 ppb toward Hg(II) in the buffer solution and in the river water, respectively. The sensor also shows high selectivity toward the Hg(II) ions.

The enzyme mechanism of nitrite reductase studied at single-molecule level

A generic method is described for the fluorescence "readout" of the activity of single redox enzyme molecules based on Förster resonance energy transfer from a fluorescent label to the enzyme cofactor. The method is applied to the study of copper-containing nitrite reductase from Alcaligenes faecalis S-6 immobilized on a glass surface. The parameters extracted from the single-molecule fluorescence time traces can be connected to and agree with the macroscopic ensemble averaged kinetic constants. The rates of the electron transfer from the type 1 to the type 2 center and back during turnover exhibit a distribution related to disorder in the catalytic site. The described approach opens the door to single-molecule mechanistic studies of a wide range of redox enzymes and the precise investigation of their internal workings.