Metal-free functionalized carbon nitride as a photocatalyst driven by sunlight for acetal synthesis and selective regeneration of NAD(P)H cofactor

Nicotinamide Adenine Dinucleotide Phosphate (NAD(P)H) plays an important role in numerous biologically significant redox reactions. The photochemical restoration of its oxidized form (NAD(P)+) under physiological conditions is intriguing in the context of integrated photo and catalysis. Herein, we report the functionalized graphitic carbon-based solar light active photocatalyst by doping boron and fluorine in the native graphitic carbon nitride (GCN) (nonfunctionalized) for the regeneration of enzymatically visible light active coenzyme and in photo-acetalization reactions. The metal-free functionalized photocatalyst systems such as BFGCN-x leads to higher yield NADH and NADPH regeneration. They are also capable of catalyzing acetal reactions in the absence of any Lewis and Bronsted acids. The current research endeavor provides the advancement and the application of functionalized GCN-based photocatalysts for NADH (61.89%), NADPH (59.84%) regeneration, and photo-acetalization reactions.

Photoelectrochemical imaging system with high spatiotemporal resolution for visualizing dynamic cellular responses

Photoelectrochemical imaging has great potential in the label-free investigation of cellular processes. Herein, we report a new fast photoelectrochemical imaging system (PEIS) for DC photocurrent imaging of live cells, which combines high speed with excellent lateral resolution and high photocurrent stability, which are all crucial for studying dynamic cellular processes. An analog micromirror was adopted to raster the sensor substrate, enabling high-speed imaging. α-Fe₂O₃ (hematite) thin films synthesized via electrodeposition were used as a robust substrate with high photocurrent and good spatial resolution. The capabilities of this system were demonstrated by monitoring cell responses to permeabilization with Triton X-100. The ability to carry out dynamic functional imaging of multiple cells simultaneously provides improved confidence in the data than could be achieved with the slower electrochemical single-cell imaging techniques described previously. When monitoring pH changes, the PEIS can achieve frame rates of 8 frames per second.

Functional polymers in photoelectrochemical biosensing

Photoelectrochemical (PEC) analysis is a detection technique that has gained a wide attention in sensing applications. PEC presents the advantages of high sensitivity, low background signal, simple equipment and easy miniaturization. In PEC detection, light is used as an excitation source while current or voltage is measured as the output detection signal. The ability to couple the PEC process with specific bioreceptors gives PEC biosensing a unique advantage of being both selective and sensitive. The growing interest in PEC bioanalysis has resulted in essential progress in its analytical performance and biodetection applications. Functional polymers have different applications in the development of novel PEC biosensing platforms. Recently, the interest in polymer-based photoactive materials has emerged as they are efficient and less toxic alternatives to certain kinds of inorganic semiconductors and sensitizers. Moreover, molecularly imprinted polymers are a class of synthetic bioreceptors that are increasingly used in PEC bioanalytics. In this review, we will provide an overview on functional polymer-based PEC biosensing approaches. Novel classes of polymers as photoactive materials are reviewed and selected applications are described. Furthermore, molecularly imprinted polymers in the development of smart and sensitive PEC bioanalytical strategies are discussed.

Enhancement of Biosensors by Implementing Photoelectrochemical Processes

Research on biosensors is growing in relevance, taking benefit from groundbreaking knowledge that allows for new biosensing strategies. Electrochemical biosensors can benefit from research on semiconducting materials for energy applications. This research seeks the optimization of the semiconductor-electrode interfaces including light-harvesting materials, among other improvements. Once that knowledge is acquired, it can be implemented with biological recognition elements, which are able to transfer a chemical signal to the photoelectrochemical system, yielding photo-biosensors. This has been a matter of research as it allows both a superior suppression of background electrochemical signals and the switching ON and OFF upon illumination. Effective electrode-semiconductor interfaces and their coupling with biorecognition units are reviewed in this work.

3D hollow-out TiO2 nanowire cluster/GOx as an ultrasensitive photoelectrochemical glucose biosensor

Ultra-high sensitivity is difficult to achieve using conventional enzymatic glucose biosensors due to the lack of exposed active sites and steric-hinderance effects. Thus, in the present study, we report a photoelectrochemical (PEC) enzymatic glucose biosensor based on 3-dimensional (3D) hollow-out titanium dioxide (TiO₂) nanowire cluster (NWc)/glucose oxidase (GOx), providing more number of exposed active sites, thus constructing a sensor with a higher affinity toward glucose reaction. Excellent performance with an ultra-high sensitivity of 58.9 μA mM^-1 cm^-2 and 0-2 mM linear range with a determination limit of 8.7 μM was obtained for the detection of glucose. This study might provide a new approach to expose active sites efficiently for remarkable photoelectrochemical performances.

Improved description of hematite surfaces by the SCAN functional

Controversies on the surface termination of α-Fe₂O₃ (0001) focus on its surface stoichiometry dependence on the oxygen chemical potential. Density functional theory (DFT) calculations applying the commonly accepted Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional to a strongly correlated system predict the best matching surface termination, but would produce a delocalization error, resulting in an inappropriate bandgap, and thus are not applicable for comprehensive hematite system studies. Besides, the widely applied PBE+U scheme cannot provide evidence for existence of some of the successfully synthesized stoichiometric α-Fe₂O₃ (0001) surfaces. Hence, a better scheme is needed for hematite DFT studies. This work investigates whether the strongly constrained and appropriately normed (SCAN) approximation reported by Perdew et al. could provide an improved result for the as-mentioned problem, and whether SCAN can be applied to hematite systems. By comparing the results calculated with the PBE, SCAN, PBE+U, and SCAN+U schemes, we find that SCAN and SCAN+U improves the description of the electronic structure of different stoichiometric α-Fe₂O₃ (0001) surfaces with respect to the PBE results, and that they give a consistent prediction of the surface terminations. Besides, the bulk lattice constants and the bulk density of states are also improved with the SCAN functional. This study provides a general characterization of the α-Fe₂O₃ (0001) surfaces and rationalizes how the SCAN approximation improves the results of hematite surface calculations.

Improving Photovoltaic and Enzymatic Sensing Performance by Coupling a Core-Shell Au Nanorod@TiO2 Heterostructure with the Bioinspired l-DOPA Polymer

The photoelectrochemistry (PEC) performance of TiO₂ is somewhat limited by its wide band gap and low quantum efficiency, and the innovation of its composite materials provides a promising solution for an improved performance. Herein, a composite of a Au nanorod@TiO₂ core-shell nanostructure (AuNR@TiO₂) and a melanin-like l-DOPA polymer (PD) is designed and prepared, where the outer layer PD tethered by TiO₂-hydroxyl complexation and the AuNR core can intensify the long-wavelength light harvesting, and the AuNR@TiO₂ core-shell structure can strengthen the hot-electron transfer to TiO₂. The photocurrent of PD/AuNR@TiO₂ is 8.4-fold improved versus that of commercial TiO₂, and the maximum incident photon-to-electron conversion efficiency reaches 65% in the UV-visible-near-infrared region. In addition, the novel PD/AuNR@TiO₂ photocatalyst possesses the advantages of good biocompatibility and stability, which can act as a versatile PEC biosensing platform for providing a biocompatible environment and improving detection sensitivity. Herein, a PEC enzymatic biosensor of glucose is developed on the basis of the immobilization of dual enzyme [glucose oxidase (GOx) and horseradish peroxidase (HRP)] in PD and the signaling strategy of biocatalytic precipitation. In phosphate buffer containing glucose and 4-chloro-1-naphthol, the HRP-catalyzed oxidation of 4-chloro-1-naphthol by GOx-generated H₂O₂ can form a precipitate on the electrode, by which the decrement of photocurrent intensity is proportional to the common logarithm of glucose concentration. The linear detection range is from 0.05 μM to 10.0 mM glucose, with a limit of detection of 0.01 μM (S/N = 3). Glucose in some human serum samples is analyzed with satisfactory results.

A carbon-rich nanofiber framework based on a conjugated arylacetylene polymer for photocathodic enzymatic bioanalysis

The metal-free photocathode fabricated by porous carbon-rich nanofiber framework of PTEB film realized “signal-off” photocathodic bioanalysis of glucose.

Semiconducting Synthetic Melanin-Based Organic/Inorganic Hybrid Photoanodes for Solar Water Oxidation

We report the development of semiconducting melanin-based organic/inorganic hybrid photoanodes for solar water oxidation. Synthetic melanin thin-film incorporating polyoxometalate (POM) water oxidation catalysts (WOCs) are readily deposited on the surface of various n-type inorganic semiconductors (e.g., Fe₂ O₃ , BiVO₄ , and TiO₂ ) by electropolymerization. The deposition of melanin and POM hybrid (MP) thin-film results in the remarkably improved performance of an underlying semiconductor photoanode for solar water oxidation with a significantly increased photocurrent density and decreased onset potential for water oxidation through the formation of a melanin-based p-n heterojunction structure. We believe that this study can provide insights into the design and fabrication of various melanin-based optoelectronic devices by utilizing its unique physicochemical properties.

Robust and High Spatial Resolution Light Addressable Electrochemistry Using Hematite (α-Fe2O3) Photoanodes

Light addressable/activated electrochemistry (LAE) has recently attracted attention as it can provide spatially resolved electrochemical information without using pre-patterned electrodes whose sizes and positions are unchangeable. Here, we propose hematite (α-Fe₂O₃) as the photoanode for LAE, which does not require any sort of surface modification for protection or facilitating charge transfer. As experimentally confirmed with various redox species, hematite is stable enough to be used for repetitive electroanalytical measurements. More importantly, it offers exceptionally high spatial resolution so that the "virtual electrode" is exactly as large as the light spot owing to the short diffusion length of the minority carriers. Quantitative analysis of dopamine in this study shows that the hematite-based photoanode is a promising platform for many potential LAE applications including spatially selective detection of oxidizable biomolecules.

Hematite/silica nanoparticle bilayers on mica: AFM and electrokinetic characterization

Tuning the properties of bilayers by controlled deposition of nanoparticles.

CsPbBr3 perovskite quantum dots/ZnO inverse opal electrodes: photoelectrochemical sensing for dihydronicotinamide adenine dinucleotide under visible irradiation

All-inorganic perovskite quantum dots (PQDs) have attracted tremendous attention due to their extraordinary optical properties, especially CsPbBr₃ QDs with their high stability and photoluminescence efficiency.