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Poor electrochemical communication between biocatalysts and electrodes is a ubiquitous limitation to bioelectrocatalysis efficiency. An extensive library of polymers has been developed to modify biocatalyst-electrode interfaces to alleviate this limitation. As such, conducting redox polymers (CRPs) are a versatile tool with high structural and functional tunability. While charge transport in CRPs is well characterized, the understanding of charge transport mechanisms facilitated by CRPs within decisively complex photobioelectrocatalytic systems remains very limited. This study is a comprehensive analysis that dissects the complex kinetics of photobioelectrodes into fundamental blocks based on rational assumptions, providing a mechanistic overview of charge transfer during photobioelectrocatalysis. We quantitatively compare two biohybrids of metal-free unbranched CRP (polydihydroxy aniline) and photobiocatalyst (intact chloroplasts), formed utilizing two deposition strategies (“mixed” and “layered” depositions). The superior photobioelectrocatalytic performance of the “layered” biohybrid compared to the “mixed” counterpart is justified in terms of rate (Dapp), thermodynamic and kinetic barriers (H≠, Ea), frequency of molecular collisions (D0) during electron transport across depositions, and rate and resistance to heterogeneous electron transfer (k0, RCT). Our results indicate that the primary electron transfer mechanism across the biohybrids, constituting the unbranched CRP, is thermally activated intra- and inter-molecular electron hopping, as opposed to a non-thermally activated polaron transfer model typical for branched CRP- or conducting polymer (CP)-containing biohybrids in literature. This work underscores the significance of subtle interplay between CRP structure and deposition strategy in tuning the polymer-catalyst interfaces, and the branched/unbranched structural classification of CRPs in the bioelectrocatalysis context. [ABSTRACT FROM AUTHOR]

Here, we report for the first time that resonance in dynamic-mode cantilever sensors persists in hydrogels and enables the real-time characterization of hydrogel viscoelastic properties and the continuous monitoring of sol-gel phase transitions (i.e., gelation and dissolution processes). Real-time tracking of piezoelectric-excited millimeter cantilever (PEMC) sensor resonant frequency (fair = 55.4 ± 8.8 kHz; n = 5 sensors) and quality factor (Q; Qair = 23.8 ± 1.5) enabled continuous monitoring of high-frequency hydrogel shear storage and loss moduli (G′f and G″f, respectively) calculated by sensor data and fluid–structure interaction models. Changes in the sensor phase angle, quality factor, and high-frequency shear moduli obtained at the resonant frequency (G′f and G″f) correlated with low-frequency moduli obtained at 1 Hz using dynamic mechanical analysis. Characterization studies were performed using physically and chemically crosslinked hydrogel systems, including gelatin hydrogels (6–10 wt. %) and alginate hydrogels (0.25–0.75 wt. %). The sensor exhibited a dynamic range from the rheological properties of inviscid solutions to hydrogels with high-frequency moduli of 80 kPa and low-frequency moduli of 26 kPa. The sensor exhibited a limit of detection of 260 Pa and 1.9 kPa for changes in hydrogel storage modulus (E′) based on the sensor's phase angle and quality factor responses, respectively. We also show that sensor data enable quantitative characterization of gelation process dynamics using a modified Hill model. This work suggests that cantilever sensors provide a promising platform for the sensor-based characterization of hydrogels, such as quantification of viscoelastic properties and real-time monitoring of gelation processes. [ABSTRACT FROM AUTHOR]

A spectroelectrochemical cell is described that enables confocal Raman microscopy studies of electrode-supported films. The confocal probe volume (∼1 μm 3 ) was treated as a fixed-volume reservoir for the observation of potential-induced changes in chemical composition at microscopic locations within an ∼20 μm thickness layer of a redox polymer cast onto a 3 mm diameter carbon disk electrode. Using a Raman system with high collection efficiency and wavelength reproducibility, spectral subtraction achieved excellent rejection of background interferences, opening opportunities for measuring within micrometer-scale thickness redox films on widely available, low-cost, and conventional carbon disk electrodes. The cell performance and spectral difference technique are demonstrated in experiments that detect transformations of redox-active molecules exchanged into electrode-supported ionomer membranes. The in situ measurements were sensitive to changes in the film oxidation state and swelling/deswelling of the polymer framework in response to the uptake and discharge of charge-compensating electrolyte ions. The studies lay a foundation for confocal Raman microscopy as a quantitative in situ probe of processes within electrode-immobilized redox polymers under development for a range of applications, including electrosynthesis, energy conversion, and chemical sensing., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)