Light-Addressable Electrochemical Sensors toward Spatially Resolved Biosensing and Imaging Applications

Direct Current Redox Imaging of Single Retinal Pigment Epithelial Cells Using Light-Addressable Electrochemistry at Semiconductor Electrodes

Light-addressable electrochemistry (LAE) enables the activation and detection of localized faradaic electrochemical processes on a flat, unconstructed semiconductor electrode through targeted light illumination, making it a promising approach for single-cell electrochemistry. Herein, we report a direct current (DC) redox imaging technique for single retinal pigment epithelial cells utilizing LAE with an α-Fe2O3 electrode activated by a constant, focused laser beam. The proposed DC-LAE method showed micron-scale resolution and was successfully used to image single cells under physiological conditions. We demonstrated that the visualization was primarily due to the hindrance of photoinduced OH- oxidation by adherent cells, resulting in a reduction of local photocurrents. Inspired by this funding, electroactive substances with high redox activities, such as hydroquinone (HQ), l-ascorbic acid (AA), and potassium ferricyanide (K4Fe(CN)6) were introduced into the culture medium. These substances significantly enhanced the DC redox imaging performance under low-intensity laser conditions without compromising cell viability. We believe that integrating pure DC-LAE with cell imaging advances quantitative analyses of cellular electrochemical behavior, offering valuable insights into cellular functions and processes.

Label-free determination of glypican-3 using PtPd@H-rGO nanocomposites decorated light-addressable potentiometric sensor

Glypican-3 (GPC3) is exclusively overexpressed in most Hepatocellular carcinoma (HCC) tissue but not in normal liver tissue, making it a promising biomarker for the precise detection of HCC. In this paper, a label-free light-addressable potentiometric sensor (LAPS) decorated by platinumpalladium-hemin-reduced graphene oxide nanocomposites (PtPd@H-rGO NCs) was constructed for determination of GPC3. The GPC3 aptamer (GPC3Apt) and PtPd@H-rGO NCs were modified on the surface of silicon-based LAPS chip to build sensitive unit of LAPS system. A readout photocurrent elicited from a modulated light source, registers the localized surface potential change. When a bias voltage is provided to the LAPS system, the GPC3-GPC3Apt complexes formed by the specific reaction between GPC3 and GPC3Apt at the sensing interface can cause the sensitive membrane surface potential to change, resulting in the photocurrent-voltage (I-V) curves generate a corresponding offset response. Therefore GPC3 concentration can be determined by monitoring the potential shifts (△V). Under optimal conditions, the potential shift is linearly related to the concentration of GPC3 in the range of 0.001-3.00 μg/mL with the limit of detection (LOD) of 0.0001 μg/mL. The LAPS has a good analytical performance with good specificity, reproducibility and stability, and can be used for the detection of GPC3 in actual serum samples, which provides a broad application prospect for the combined application of LAPS and aptamers in biooassay.

Mapping Surface Potential in DNA Aptamer-Neurochemical and Membrane-Ion Interactions on the SOS Substrate Using Terahertz Microscopy

In this study, we utilized a terahertz chemical microscope (TCM) to map surface potential changes induced by molecular interactions on silicon-on-sapphire (SOS) substrates. By functionalizing the SOS substrate with DNA aptamers and an ion-selective membrane, we successfully detected and visualized aptamer-neurochemical complexes through the terahertz amplitude. Additionally, comparative studies of DNA aptamers in PBS buffer and artificial cerebrospinal fluid (aCSF) were performed by computational structure modeling and terahertz measurements. Beyond neurochemicals, we also investigated calcium ions, measuring their concentrations in PDMS-fabricated micro-wells using minimal sample volumes. Our results highlight the capability of TCM as a powerful, label-free, and sensitive platform for the probing and mapping of surface potential arising from molecular interactions, with broad implications for biomedical diagnostics and research.

A light-addressable potentiometric sensor-based extracellular calcium dynamic monitoring and imaging platform for cellular calcium channel drug evaluation

Disruption and dysregulation of cellular calcium channel function can lead to diseases such as ischemic stroke, heart failure, and arrhythmias. Corresponding calcium channel drugs typically require preliminary efficacy evaluations using in vitro models such as cells and simulated tissues before clinical testing. However, traditional detection and evaluation methods often encounter challenges in long-term continuous monitoring and lack calcium specificity. In this study, a dynamic monitoring system based on ion-sensitive membranes for light-addressable potentiometric sensor (LAPS) was developed to meet the demand for monitoring changes in extracellular calcium ion (Ca2+) concentration in live cells. The effects of Ca2+ channel agonists and blockers on 2D and 3D HL-1 cells were investigated, with changes in extracellular Ca2+ concentration reflecting cellular calcium metabolism, facilitating drug evaluation. Additionally, calcium imaging technology with optical addressing capability complemented the LAPS system's ability to perceive 3D cell morphology, enhancing its drug evaluation capabilities. This work provides a novel, label-free, specific, and stable technique for monitoring cellular calcium metabolism. It achieves both continuous monitoring at single points and custom sensing area calcium imaging, holding significant implications for drug screening and disease treatment related to human calcium homeostasis.

Photocurrent Polarity-Switchable Imaging of Single Living Cells by Light-Addressable Electrochemical Sensor

The light-addressable electrochemical sensor (LAES) is a powerful tool for single-cell imaging due to its label-free and probe-free advantages. In this work, we report a photocurrent polarity-switchable LAES using a single-phase photoelectrode of a BiFeO3 thin film for living cell imaging. The proposed BiFeO3 could show both p- and n-type photocurrent behavior by simply altering the external bias voltage. LAES imaging of the same individual MCF-7 cells was performed in anodic and cathodic modes. Decreases in both photocurrents were observed due to the hindering effect of the adherent cells on local photoinduced Faraday currents. Furthermore, the dynamic photocurrent changes on cells after trypsin treatment were imaged and studied at anode and cathode polarities. Both polarities showed an increase in local photocurrents on cells as the cell-substrate junction weakened, and this change displayed heterogeneous characteristics. This is the first time that LAES cell imaging was achieved in a p-type mode. Meanwhile, our photocurrent polarity-switchable imaging approach overcomes the limitations of conventional photoelectrodes, which have been confined to single-polarity operation. We believe this work has the potential to significantly broaden the application scope of LAES in single-cell visualization and analysis, offering great insights into cellular behavior and function.

Control Circuits for Potentiostatic/Galvanostatic Polarization and Simultaneous Chemical Sensing by a Light-Addressable Potentiometric Sensor

A light-addressable potentiometric sensor (LAPS) is a semiconductor-based sensor platform for sensing and imaging of various chemical species. Being a potentiometric sensor, no faradaic current flows through its sensing surface, and no electrochemical reaction takes place in the course of LAPS measurement. In this study, a four-electrode system is proposed, in which a LAPS is combined with the conventional three-electrode electrochemical system. A LAPS is included as the fourth electrode for potentiometric sensing and imaging of the target analyte in the course of an electrochemical reaction taking place on the surface of the working electrode. The integrated system will be useful for analyzing dynamic processes, where both the electrochemical process on the electrode surface and the ion distribution in the solution need to be simultaneously investigated. Different grounding modes of control circuits that can simultaneously conduct potentiostatic/galvanostatic polarization and LAPS measurement are designed, and their functionalities are tested. The interference between polarization and LAPS measurement will also be discussed.

Flexible and Dynamic Light-Guided Electrochemiluminescence for Spatiotemporal Imaging of Photoelectrochemical Processes on Hematite

Here, we report an electrochemiluminescence (ECL)-based approach for imaging of local photoelectrochemical processes on hematite in a spatially and temporally controlled manner. The local processes were guided by flexible and dynamic light illumination, not requiring any prepatterned conductive features or photomasks, with a digital micromirror device (DMD). The imaging approach was based on light-addressable electrochemical reactions on hematite, resulting in photoinduced ECL emission for spatiotemporally resolved imaging of photoelectrochemical processes selectively guided by light illumination. After clarifying the capability of hematite as a photosensitive electrode, we validated that the illuminated hematite exhibited stable light-guided ECL emission in correspondence with the illuminated area, with a spatial resolution of 0.8 μm and a temporal resolution of 1 μs, even over a long period of 6 h. More importantly, this study exemplified the simple yet effective ECL-based approach for electrochemical visualization of local photoelectrochemical processes guided by flexible and dynamic adjustment of light illumination in a spatiotemporally controlled way.

Characterizing and understanding the photovoltage in n-Si/Au light-addressable electrochemical sensors

Here, we characterize the photovoltage of n-Si/Au light-addressable electrodes (LAEs) over a range of solution potentials from ca. -1 to +1 V. We find that the n-Si/Au photoelectrodes show photovoltages consistent with a semiconductor/liquid junction in contrast to a buried junction, which opposes our previous understanding of how photovoltage originates in these sensors.

One-step construction of multiplexed enzymatic biosensors using light-addressable electrochemistry on a single silicon photoelectrode

The multiplexed detection of metabolites in parallel within a single biosensor plate is sufficiently valuable but also challenging. Herein, we combine the inherent light addressability of silicon with the high selectivity of enzymes, for the construction of multiplexed photoelectrochemical enzymatic biosensors. To conduct a stable electrochemistry and reagentless biosensing on silicon, a new strategy involving the immobilization of both redox mediators and enzymes using an amide bond-based hydrogel membrane was proposed. The membrane characterization results demonstrated a covalent coupling of ferrocene mediator to hydrogel, in which the mediator acted as not only a signal generator but also a renewable sacrifice agent. By adding corresponding enzymes on different spots of hydrogel membrane modified silicon and recording local photocurrents with a moveable light pointer, this biosensor setup was used successfully to detect multiple metabolites, such as lactate, glucose, and sarcosine, with good analytical performances. The limits of detection of glucose, sarcosine and lactate were found to be 179 μM, 16 μM, and 780 μM with the linear ranges of 0.5-2.5 mM, 0.3-1.5 mM, and 1.0-3.0 mM, respectively. We believe this proof-of-concept study provides a simple and rapid one-step immobilization approach for the fabrication of reagentless enzymatic assays with silicon-based light-addressable electrochemistry.

A photoelectrochemical sensor based on In2O3/In2S3/ZnIn2S4 ternary Z-scheme heterojunction for ultrasensitive detection of dopamine in sweat

A novel ternary heterojunction material In2O3/In2S3/ZnIn2S4 was synthesized, and a photoelectrochemical sensor was fabricated for the non-invasive test of dopamine (DA) in sweat. In2O3 multihollow microtubules were synthesized and then In2S3 was formed on their surface to construct a type-I heterojunction between In2S3 and In2O3. ZnIn2S4 was further introduced to form a Z-scheme heterojunction between In2S3/ZnIn2S4. Under photoexcitation, the photogenerated holes of In2O3 transferred to the valence band of In2S3, superimposed with the holes produced by In2S3, leads to a significantly higher photocatalytic oxidation capacity of In2O3/In2S3/ZnIn2S4 ternary composites than that of In2O3/In2S3. The Z-scheme heterojunction accelerates the transfer of photogenerated electrons accumulated on the type-I heterojunction. In the presence of DA, it is rapidly oxidized into polydopamine (PDA) by In2O3/In2S3, and the benzoquinone groups of PDA compete for the photogenerated electrons to reduce the current in the external circuit, whereby DA determination is achieved. Owing to the combination of type-I and Z-scheme heterojunction, the sensor showed extremely high sensitivity, with a detection limit of 3.94 × 10-12 mol/L. It is one of the most sensitive methods for DA detection reported and has been applied to the determination of DA in human sweat.

A platform for microplastic assessment in aquatic environments based on the protein corona-induced aggregation effect

The environmental hazards of microplastics have received widespread attention. However, the in-situ detection of microplastics, particularly in aquatic environments, has been challenged by the limitations of detection methods, the large-scale instruments, and small size. Herein, a photoelectrochemical sensor based on the protein corona-induced aggregation effect is designed for the detection of polystyrene microplastics. The sensor has advantages of high sensitivity, reproducibility, and detection capability. A linear detection range of 0.5-500 μg mL-1, a method detection limit of 0.06 μg mL-1, and a limit of quantification of 0.14 μg mL-1 are achieved. Furthermore, the relative standard deviations of intra-day and inter-day precision, ranging from 0.56% to 4.63% and 0.84%-3.36% are obtained. A digital multimeter was employed to construct a platform for the real-time detection in real water samples, streamlining the detection process and yielding clear results. We believe this sensor provides new insight for the in-situ real-time detection of microplastics and has broad applications for the analysis of microplastic pollution in aquatic environments.

Near-IR Photoinduced Electrochemiluminescence Imaging with Structured Silicon Photoanodes

Infrared (IR) imaging devices that convert IR irradiation (invisible to the human eye) to a visible signal are based on solid-state components. Here, we introduce an alternative concept based on light-addressable electrochemistry (i.e., electrochemistry spatially confined under the action of a light stimulus) that involves the use of a liquid electrolyte. In this method, the projection of a near-IR image (λexc = 850 or 840 nm) onto a photoactive Si-based photoanode, immersed into a liquid phase, triggers locally the photoinduced electrochemiluminescence (PECL) of the efficient [Ru(bpy)3]2+-TPrA system. This leads to the local conversion of near-IR light to visible (λPECL = 632 nm) light. We demonstrate that compared to planar Si photoanodes, the use of a micropillar Si array leads to a large enhancement of local light generation and considerably improves the resolution of the PECL image by preventing photogenerated minority carriers from diffusing laterally. These results are important for the design of original light conversion devices and can lead to important applications in photothermal imaging and analytical chemistry.

Numerical investigation of a graphene-on-semiconductor device for optical monitoring of cell electrophysiology

Spatially resolved sensing devices for electrostatic potentials are extremely useful for characterization of living cells, however, many current techniques lack the speed necessary to capture spatially resolved, functional information of cells in real-time. Here, an optical sensing technique is proposed based on graphene on a semiconductor stack operating in the near-infrared spectrum. By modeling coherent interference of multiply reflected beam paths within the semiconductor stack, we demonstrate how the device produces a continuous reflectivity change in response to graphene Fermi energy which is ideal for sensing changes in local electrostatic fields produced by action potentials of living cells. By coupling the device with a high-speed camera, we propose this platform will allow for high-speed imaging of action potentials over a large sensing area with micron scale resolution.

Light Addressable Potentiometric Sensors for Biochemical Imaging on Microscale: A Review on Optimization of Imaging Speed and Spatial Resolution

Light addressable potentiometric sensors (LAPS) are a competitive tool for unmarked biochemical imaging, especially imaging on microscale. It is essential to optimize the imaging speed and spatial resolution of LAPS since the imaging targets of LAPS, such as cell, microfluidic channel, etc., require LAPS to image at the micrometer level, and a fast enough imaging speed is a prerequisite for the dynamic process involved in biochemical imaging. In this study, we discuss the improvement of LAPS in terms of imaging speed and spatial resolution. The development of LAPS in imaging speed and spatial resolution is demonstrated by the latest applications of biochemistry monitoring and imaging on the microscale.

Carbonized polydopamine layer-protected silicon substrates for light-addressable electrochemical sensing and imaging

The application of silicon (Si) substrate as photoelectrode in light-addressable electrochemistry (LAE) is severely limited due to its ease of surface oxidation. The resulted silicon oxide (SiO_x) layer is electronically insulating and blocks charge transfer between the electrode and electrolyte. Keeping the Si from being oxidized is a key challenge for its practical use as a semiconductor electrode. In this work, we find that by developing a thin layer of polydopamine film on the surface of Si substrate, followed by carbonization at 550 °C, the natural oxidation of Si substrate can be successfully forestalled. When applied as an electrode, it is further found that the carbonized polydopamine (cPDA) layer can also prevent anodic oxidation of Si. The cPDA layer-modified Si substrate exhibits good photoelectrochemical performance and great stability, with no obvious signal decrease under ambient environment over 32 h. Our work here provides a new modification strategy for anti-oxidation of Si substrate and it is promising in the application of light-addressable electrochemical sensing and imaging.