Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring: A Technique to Optimize Enzyme Use in Bioelectrocatalysis

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

Artifact Correction of Light Induced Detuning in QCM-D Experiments

The quartz crystal microbalance with dissipation (QCM-D) has become an efficient and versatile measurement technique for investigating in situ the external stimuli responsiveness such as pH, temperature, or chemical gradients of surface-active substances at solid-liquid interfaces. However, light responsive adsorption investigation is more challenging presumably since the quartz crystal itself reacts to optical stimulation, showing frequency and dissipation shifts known as light induced detuning (LID). This yields an effective measurement artifact and makes data interpretation with respect to dynamic interactions of light responsive materials rather challenging. Here we introduce a simple guideline for correcting the artifacts of the QCM sensor response on irradiation to ensure quantitative analysis for light responsive materials via OCM-D. We also show that the LID depends on the adsorption properties of the sensor and the solvent properties (ionic concentration or viscosity), providing a guideline to minimize impact of the LID.

Interactions of Catalytic Enzymes with n-Type Polymers for High-Performance Metabolite Sensors

The tight regulation of the glucose concentration in the body is crucial for balanced physiological function. We developed an electrochemical transistor comprising an n-type conjugated polymer film in contact with a catalytic enzyme for sensitive and selective glucose detection in bodily fluids. Despite the promise of these sensors, the property of the polymer that led to such high performance has remained unknown, with charge transport being the only characteristic under focus. Here, we studied the impact of the polymer chemical structure on film surface properties and enzyme adsorption behavior using a combination of physiochemical characterization methods and correlated our findings with the resulting sensor performance. We developed five n-type polymers bearing the same backbone with side chains differing in polarity and charge. We found that the nature of the side chains modulated the film surface properties, dictating the extent of interactions between the enzyme and the polymer film. Quartz crystal microbalance with dissipation monitoring studies showed that hydrophobic surfaces retained more enzymes in a densely packed arrangement, while hydrophilic surfaces captured fewer enzymes in a flattened conformation. X-ray photoelectron spectroscopy analysis of the surfaces revealed strong interactions of the enzyme with the glycolated side chains of the polymers, which improved for linear side chains compared to those for branched ones. We probed the alterations in the enzyme structure upon adsorption using circular dichroism, which suggested protein denaturation on hydrophobic surfaces. Our study concludes that a negatively charged, smooth, and hydrophilic film surface provides the best environment for enzyme adsorption with desired mass and conformation, maximizing the sensor performance. This knowledge will guide synthetic work aiming to establish close interactions between proteins and electronic materials, which is crucial for developing high-performance enzymatic metabolite biosensors and biocatalytic charge-conversion devices.

A guide for the characterization of organic electrochemical transistors and channel materials

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO₂ reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots (a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (-NHMe₂⁺) and negative (-COO^-) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO₂ reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO₂ reduction via direct electron transfer with >90% Faradaic efficiency and -250 μA cm^-2 at -0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (-NHMe₂⁺-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO₂ reduction (turnover frequency: 3.5 × 10³ h^-1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO₂ reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.

Elucidating Film Loss and the Role of Hydrogen Bonding of Adsorbed Redox Enzymes by Electrochemical Quartz Crystal Microbalance Analysis

The immobilization of redox enzymes on electrodes enables the efficient and selective electrocatalysis of useful reactions such as the reversible interconversion of dihydrogen (H₂) to protons (H⁺) and formate to carbon dioxide (CO₂) with hydrogenase (H₂ase) and formate dehydrogenase (FDH), respectively. However, their immobilization on electrodes to produce electroactive protein films for direct electron transfer (DET) at the protein–electrode interface is not well understood, and the reasons for their activity loss remain vague, limiting their performance often to hour timescales. Here, we report the immobilization of [NiFeSe]-H₂ase and [W]-FDH from Desulfovibrio vulgaris Hildenborough on a range of charged and neutral self-assembled monolayer (SAM)-modified gold electrodes with varying hydrogen bond (H-bond) donor capabilities. The key factors dominating the activity and stability of the immobilized enzymes are determined using protein film voltammetry (PFV), chronoamperometry (CA), and electrochemical quartz crystal microbalance (E-QCM) analysis. Electrostatic and H-bonding interactions are resolved, with electrostatic interactions responsible for enzyme orientation while enzyme desorption is strongly limited when H-bonding is present at the enzyme–electrode interface. Conversely, enzyme stability is drastically reduced in the absence of H-bonding, and desorptive enzyme loss is confirmed as the main reason for activity decay by E-QCM during CA. This study provides insights into the possible reasons for the reduced activity of immobilized redox enzymes and the role of film loss, particularly H-bonding, in stabilizing bioelectrode performance, promoting avenues for future improvements in bioelectrocatalysis.

Effect of Counterion Binding to Swelling of Polyelectrolyte Brushes

The effect of binding strength of counterions with the polyelectrolyte chain to the swelling of polyelectrolyte brushes is studied, by investigating the swelling of both the polycation and polyanion in response to the variation of the salt concentration under the change of counterion's identity. Two polyelectrolyte brushes grafted on solid substrates are adopted: the cationic poly [2-(methacryloyloxy)ethyltrimethyl ammonium] (PMETA-X, X = F, Cl, Br, and I) and the anionic polystyrene sulfonate (M-PSS, M = Li, Na, K, and Cs). The swelling change with the salt concentration is investigated by ellipsometry, quartz crystal microbalance with dissipation, and dielectric spectroscopy. It is discovered that although the thickness of PMETA-X brushes is larger than that of M-PSS brushes of similar grafting density, the former has much less solvent incorporated than the latter. Such a difference is attributed to the weaker interaction between the PMETA⁺ chain and its halide counterions than that between the PSS^- chain and its alkali counterions, discovered by dielectric spectroscopy. This makes the original charges on the PMETA-X chain less neutralized and therefore have a higher charge density, compared with the M-PSS chain. The results demonstrate that the stronger binding of the counterions to the polyelectrolytes makes the main chains less charged, resulting in the weaker inter-chain electrostatic repulsion and less swelling of the brushes. Investigations into the effect of ion identity show the following order of binding strength: for the cationic PMETA⁺ chain, F^- < Cl^- < Br^- < I^- and for the anionic PSS^- chain, Li⁺ < Na⁺ < K⁺ < Cs⁺.

From Enzyme Stability to Enzymatic Bioelectrode Stabilization Processes

Bioelectrocatalysis using redox enzymes appears as a sustainable way for biosensing, electricity production, or biosynthesis of fine products. Despite advances in the knowledge of parameters that drive the efficiency of enzymatic electrocatalysis, the weak stability of bioelectrodes prevents large scale development of bioelectrocatalysis. In this review, starting from the understanding of the parameters that drive protein instability, we will discuss the main strategies available to improve all enzyme stability, including use of chemicals, protein engineering and immobilization. Considering in a second step the additional requirements for use of redox enzymes, we will evaluate how far these general strategies can be applied to bioelectrocatalysis.

Rational Surface Modification of Carbon Nanomaterials for Improved Direct Electron Transfer-Type Bioelectrocatalysis of Redox Enzymes

Interfacial electron transfer between redox enzymes and electrodes is a key step for enzymatic bioelectrocatalysis in various bioelectrochemical devices. Although the use of carbon nanomaterials enables an increasing number of redox enzymes to carry out bioelectrocatalysis involving direct electron transfer (DET), the role of carbon nanomaterials in interfacial electron transfer remains unclear. Based on the recent progress reported in the literature, in this mini review, the significance of carbon nanomaterials on DET-type bioelectrocatalysis is discussed. Strategies for the oriented immobilization of redox enzymes in rationally modified carbon nanomaterials are also summarized and discussed. Furthermore, techniques to probe redox enzymes in carbon nanomaterials are introduced.

Aldehyde-Mediated Protein-to-Surface Tethering via Controlled Diazonium Electrode Functionalization Using Protected Hydroxylamines

We report a diazonium electro-grafting method for the covalent modification of conducting surfaces with aldehyde-reactive hydroxylamine functionalities that facilitate the wiring of redox-active (bio)molecules to electrode surfaces. Hydroxylamine near-monolayer formation is achieved via a phthalimide-protection and hydrazine-deprotection strategy that overcomes the multilayer formation that typically complicates diazonium surface modification. This surface modification strategy is characterized using electrochemistry (electrochemical impedance spectroscopy and cyclic voltammetry), X-ray photoelectron spectroscopy, and quartz crystal microbalance with dissipation monitoring. Thus-modified glassy carbon, boron-doped diamond, and gold surfaces are all shown to ligate to small molecule aldehydes, yielding surface coverages of 150-170, 40, and 100 pmol cm^-2, respectively. Bioconjugation is demonstrated via the coupling of a dilute (50 μM) solution of periodate-oxidized horseradish peroxidase enzyme to a functionalized gold surface under biocompatible conditions (H₂O solvent, pH 4.5, 25 °C).

In Situ Fluorescence Tomography Enables a 3D Mapping of Enzymatic O2 Reduction at the Electrochemical Interface

Getting information about the fate of immobilized enzymes and the evolution of their environment during turnover is a mandatory step toward bioelectrode optimization for effective use in biodevices. We demonstrate here the proof-of-principle visual characterization of the reactivity at an enzymatic electrode thanks to fluorescence confocal laser scanning microscopy (FCLSM) implemented in situ during the electrochemical experiment. The enzymatic O₂ reduction involves proton-coupled electron transfers. Therefore, fluorescence variation of a pH-dependent fluorescent dye in the electrode vicinity enables reaction visualization. Simultaneous collection of electrochemical and fluorescence signals gives valuable space- and time-resolved information. Once the technical challenges of such a coupling are overcome, in situ FCLSM affords a unique way to explore reactivity at the electrode surface and in the electrolyte volume. Unexpected features are observed, especially the pH evolution of the enzyme environment, which is also indicated by a characteristic concentration profile within the diffusion layer. This coupled approach also gives access to a cartography of the electrode surface response (i.e., heterogeneity), which cannot be obtained solely by an electrochemical means.

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.

Electrochemically Induced Changes in TiO2 and Carbon Films Studied with QCM-D

Semi-solid fluid electrode-based battery (SSFB) and supercapacitor technologies are seen as very promising candidates for grid energy storage. However, unlike for traditional batteries, their performance can quickly get compromised by the formation of a poorly conducting solid-electrolyte interphase (SEI) on the particle surfaces. In this work we examine SEI film formation in relation to typical electrochemical conditions by combining cyclic voltammetry (CV) with quartz crystal microbalance dissipation monitoring (QCM-D). Sputtered layers of typical SSFB materials like titanium dioxide (TiO₂) and carbon, immersed in alkyl carbonate solvents, are cycled to potentials of relevance to both traditional and flow systems. Mass changes due to lithium intercalation and SEI formation are distinguished by measuring the electrochemical current simultaneously with the damped mechanical oscillation. Both the TiO₂ and amorphous carbon layers show a significant irreversible mass increase on continued exposure to (even mildly) reducing electrochemical conditions. Studying the small changes within individual charge-discharge cycles, TiO₂ shows mass oscillations, indicating a partial reversibility due to lithium intercalation (not found for carbon). Viscoelastic signatures in the megahertz frequency regime confirm the formation and growth of a soft layer, again with oscillations for TiO₂ but not for carbon. All these observations are consistent with irreversible SEI formation for both materials and reversible Li intercalation for TiO₂. Our results highlight the need for careful choices of the materials chemistry and a sensitive electrochemical screening for fluid electrode systems.

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO₂ reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic–inorganic model system that utilizes host–guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO₂ inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO₃ at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO₂ concentration close to the metal interface. Electrocatalytic CO₂ reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO₂ reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H₂ production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host–guest chemistry.

Advancing Techniques for Investigating the Enzyme-Electrode Interface

Enzymes are the essential catalytic components of biology and adsorbing redox-active enzymes on electrode surfaces enables the direct probing of their function. Through standard electrochemical measurements, catalytic activity, reversibility and stability, potentials of redox-active cofactors, and interfacial electron transfer rates can be readily measured. Mechanistic investigations on the high electrocatalytic rates and selectivity of enzymes may yield inspiration for the design of synthetic molecular and heterogeneous electrocatalysts. Electrochemical investigations of enzymes also aid in our understanding of their activity within their biological environment and why they evolved in their present structure and function. However, the conventional array of electrochemical techniques (e.g., voltammetry and chronoamperometry) alone offers a limited picture of the enzyme–electrode interface.

How many enzymes are loaded onto an electrode? In which orientation(s) are they bound? What fraction is active, and are single or multilayers formed? Does this static picture change over time, applied voltage, or chemical environment? How does charge transfer through various intraprotein cofactors contribute to the overall performance and catalytic bias? What is the distribution of individual enzyme activities within an ensemble of active protein films? These are central questions for the understanding of the enzyme–electrode interface, and a multidisciplinary approach is required to deliver insightful answers.

Complementing standard electrochemical experiments with an orthogonal set of techniques has recently allowed to provide a more complete picture of enzyme–electrode systems. Within this framework, we first discuss a brief history of achievements and challenges in enzyme electrochemistry. We subsequently describe how the aforementioned challenges can be overcome by applying advanced electrochemical techniques, quartz-crystal microbalance measurements, and spectroscopic, namely, resonance Raman and infrared, analysis. For example, rotating ring disk electrochemistry permits the simultaneous determination of reaction kinetics and quantification of generated products. In addition, recording changes in frequency and dissipation in a quartz crystal microbalance allows to shed light into enzyme loading, relative orientation, clustering, and denaturation at the electrode surface. Resonance Raman spectroscopy yields information on ligation and redox state of enzyme cofactors, whereas infrared spectroscopy provides insights into active site states and the protein secondary and tertiary structure. The development of these emerging methods for the analysis of the enzyme–electrode interface is the primary focus of this Account. We also take a critical look at the remaining gaps in our understanding and challenges lying ahead toward attaining a complete mechanistic picture of the enzyme–electrode interface.

Quartz Crystal Microbalance Assay of Clinical Calcinosis Samples and Their Synthetic Models Differentiates the Efficacy of Chelation-Based Treatments

This paper sets out in vitro protocols for studying the relative effectiveness of chelators used in the dissolution-based treatment of hard calcinosis. Pulverized hard calcinosis samples from human donors or synthetic hydroxyapatite nanoparticles were deposited by electrophoretic deposition on the surface of a quartz crystal microbalance sensor. Over 150 deposits of <20 μg were dissolved over the course of 1 h by aliquots of buffered, aqueous solutions of two calcium chelators, EDTA and citrate, with the surface-limited dissolution kinetics monitored with <1 s time resolution. There was no statistically significant difference in dissolution rate between the four synthetic hydroxyapatite materials in EDTA, but the dissolution rates in citrate were lower for hydroxyapatite produced by acetate or nitrate metathesis. Hard calcinosis and synthetic hydroxyapatites showed statistically identical dissolution behavior, meaning that readily available synthetic mimics can replace the rarer samples of biological origin in the development of calcinosis treatments. EDTA dissolved the hydroxyapatite deposits more than twice as fast as citrate at pH 7.4 and 37 °C, based on a first-order kinetic analysis of the initial frequency response. EDTA chelated 6.5 times more calcium than an equivalent number of moles of citrate. Negative controls using nonchelating N,N,N',N'-tetraethylethylenediamine (TEEDA) showed no dissolution effect. Pharmaceutical dissolution testing of synthetic hydroxyapatite tablets over 6 h showed that EDTA dissolved the tablets four to nine times more quickly than citrate.

A sensitivity metric and software to guide the analysis of soft films measured by a quartz crystal microbalance

The TPM-sensitivity metric guides the analysis of viscoelastic thin films studied with a quartz crystal microbalance.

Optimizing the Mass-Specific Activity of Bilirubin Oxidase Adlayers through Combined Electrochemical Quartz Crystal Microbalance and Dual Polarization Interferometry Analyses

Two surface analysis techniques, dual polarization interferometry (DPI) and analysis by an electrochemical quartz crystal microbalance with dissipation capability (E-QCM-D), were paired to find the deposition conditions that give the highest and most stable electrocatalytic activity per adsorbed mass of enzyme. Layers were formed by adsorption from buffered solutions of bilirubin oxidase from Myrothecium verrucaria at pH 6.0 to planar surfaces, under high enzyme loading (≥1 mg mL(-1)) for contact periods of up to 2 min. Both unmodified and carboxylate-functionalized gold-coated sensors showed that a deposition solution concentration of 10-25 mg mL(-1) gave the highest activity per mass of adsorbed enzyme with an effective catalytic rate constant (k(cat)) of about 60 s(-1). The densification of adsorbed layers observed by DPI correlated with reduced bioactivity observed by parallel E-QCM-D measurements. Postadsorption changes in thickness and density observed by DPI were incorporated into Kelvin-Voigt models of the QCM-D response. The modeled response matched experimental observations when the adlayer viscosity tripled after adsorption.