A 3D photoelectrochemical imaging system (PEIS) for investigating localized kinetics of photocatalytic water splitting in complex porous electrode structures will be presented. Porous ITO coated with hematite via electrodeposition was fabricated and used as the light addressable electrode. Photocurrents were excited with a two-photon effect using a focused, modulated 780 nm fs laser to provide depth resolution. The variation of the photocurrents collected in different depths of the photoelectrode demonstrates the potential of 3D photoelectrochemical imaging. Introduction: Electrochemical imaging techniques are powerful tools to investigate charge and catalytic activity of surfaces with high resolution 1 . Complex mesoporous structures are being considered for efficient, low-cost photocatalytic solar water splitting. However, current electrochemical imaging technologies have been limited to probing thin films leaving the pore structure effects on gas/liquid exchange rarely examined. Here, we present a novel 3D imaging technology that is expected to aid the investigation of the localized kinetics of photocatalytic water splitting in the future, using hematite as the photoelectrode which has exhibited excellent performance in both water splitting 2 and photoelectrochemical imaging 3 . Methods: In this work, an inverse opal ITO porous scaffold was fabricated on FTO glass using polystyrene beads with 30 μm diameter as the template followed by electrodeposition of hematite covering the ITO scaffold. The ITO-hematite electrode is used as the electrode for photocurrent imaging. Depth resolution based on a two-photon effect is achieved by exciting photocurrents with a 780 nm femtosecond laser (Fig. 1). Results & Discussion: The ITO-hematite porous electrode with interconnected pores and a well-connected interface with the substrate (Fig 2a and b) was fabricated successfully. Photocurrent images of the porous structure were taken on both the XY plane and the XZ plane (Fig. 2c and d). The photocurrent was significantly larger when the laser was focused on the pore wall rather than the cavities of the structure, both in the XY scan (Fig. 2c) and an XZ area scan (Fig 2d). The image reveals discernible features showing the shape and size of the observed pores. Conclusions: Porous ITO-hematite scaffold was used successfully as a photoelectrode for 3D photoelectrochemical imaging. Our photoelectrochemical imaging system is able to capture images of pores in the Z-direction, thereby demonstrating the significant potential of this technique for in-situ 3D functional imaging in 3D tissue culture and localized kinetics study of photocatalytic water splitting in the future. Acknowledgments: The authors are grateful to the China Scholarship Council for providing Jiazhe Zhao with a PhD studentship, and to EPSRC (EP/V047523/1) for funding. References: 1. Richter A.P. et al. Langmuir 2016; 32:6468-77. 2. Brillet J. et al. Nano letters 2010; 10:4155-60. 3. Zhou, B. et al. Biosensors and Bioelectronics 2021. 180: 113121. Figure 1

A light-addressable potentiometric sensor (LAPS) for a metal ion with a single self-assembled monolayer of a chelator as the sensor layer has been achieved for the first time. Immobilisation of two zinc ion chelators, DPA and Cyclam, on a 1,8-nonadiyne modified p-type silicon on sapphire (p-SOS) surface led to effective zinc ion selective LAPS. This is the first use of these chelators in potentiometric sensors as they have previously only been employed in small molecule fluorescent probes. The zinc sensor with a Cyclam monolayer showed excellent reversibility and selectivity for zinc against calcium and magnesium ions at physiological pH 7.4. The Cyclam modified device showed potential shifts indicative of negatively charged complexes [ZnX]- binding to the surface at pH 7.4, while potential shifts in the opposite direction were observed at lower pH. In contrast, the DPA-surface was insensitive to Zn2+ at pH 7.4, but showed potential shifts corresponding to positively charged Zn2+ species binding in an acidic environment. In contrast to the polymer membranes traditionally employed in ion-selective sensors, the ultrathin sensor layer used in this work exhibits a high signal-to-noise ratio and remains resistant to membrane leaching and degradation. The LAPS developed herein thus have the potential of label-free imaging of extracellular zinc ion concentrations in cell culture and organ-on-chip devices with high spatial resolution in the future.

The miniaturization of microfluidic systems usually comes at the cost of more difficult integration of sensors and actuators inside the channel. As an alternative, this work demonstrates the embedding of semiconductor-based sensor and actuator technologies that can be spatially and temporally controlled from outside the channel using light. The first element is a light-addressable potentiometric sensor, consisting of an Al/Si/SiO2/Si3N4 structure, that can measure pH changes at the Si3N4/electrolyte interface. The pH value is a crucial factor in biological and chemical systems, and besides measuring, it is often important to bring the system out of equilibrium or to adjust and control precisely the surrounding medium. This can be done photoelectrocatalytically by utilizing light-addressable electrodes. These consist of a glass/SnO2:F/TiO2 structure, whereby direct charge transfer between the TiO2 and the electrolyte leads to a pH change upon irradiation. To complement the advantages of both, we integrated a light-addressable sensor with a pH sensitivity of 41.5 mVpH(-1) and a light-addressable electrode into a microfluidic setup. Here, we demonstrated a simultaneous operation with the ability to generate and record pH gradients inside a channel under static and dynamic flow conditions. The results show that dependent on the light-addressable electrode (LAE)-illumination conditions, pH changes up to Delta pH of 2.75 and of 3.52 under static and dynamic conditions, respectively, were spatially monitored by the light-addressable potentiometric sensor. After flushing with fresh buffer solution, the pH returned to its initial value. Depending on the LAE illumination, pH gradients with a maximum pH change of Delta pH of 1.42 were tailored perpendicular to the flow direction. In a final experiment, synchronous LAE illumination led to a stepwise increase in the pH inside the channel.

Accurate monitoring of cardiomyocyte action potentials (APs) is essential to understand disease propagation and for trials of novel therapeutics. Patch clamp techniques offer 'gold standard' measurements in this field, but are notoriously difficult to operate and only provide measurements of a single cell. Here we propose photoelectrochemical imaging (PEI) with light-addressable potentiometric sensors (LAPS) in conjunction with a setup for controlling the contact force between the cardiomyocyte organoids and the sensor surface for measuring APs with high sensitivity. The method was validated through measuring the responses to drugs, and the results successfully visualized the expected electrophysiological changes to the APs. PEI allows for several cells to be monitored simultaneously, opening further research to the electrophysiological interactions of adjoining cells. This method expands the applications of PEI to three-dimensional geometries and provides the fields of stem cell research, drug trials and heart disease modelling with an invaluable tool to further investigate the role of APs.

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.

A photoelectrochemical imaging system (PEIS) using α-Fe 2 O 3 (hematite) thin films as the sensor substrate has been demonstrated for mapping the dynamic responses of multiple independent cells simultaneously. Herein, one-dimensional (1D) hematite nanorods (NRs) synthesized via a simple hydrothermal method are proposed as a platform for PEIS with enhanced photocurrent responses, good stability and excellent spatial resolution for applications requiring concurrent imaging of live cells and ion sensing applications. The photoelectrochemical sensing capability of hematite NRs was demonstrated by recording live cell images and by a high pH sensitivity without any surface modification. In contrast to potentiometric field-effect sensors based on silicon substrates, the deposition of a ion-sensitive PVC membrane on the highly structured hematite surface resulted in enhanced adhesion and an amperometric response to calcium ions (Ca 2+ ).

α-Fe2O3 (hematite) thin films have been shown to be a robust sensor substrate for photoelectrochemical imaging with good stability and high spatial resolution. Herein, one-dimensional (1D) hematite nanorods (NRs) synthesized via a simple hydrothermal method are proposed as a substrate which provides nanostructured surfaces with enhanced photocurrent responses compared to previously described hematite films, good stability, and excellent spatial resolution for potential imaging applications. The photoelectrochemical sensing capability of hematite NRs was demonstrated by a high pH sensitivity without modification. The modification of the hematite NRs with a thin poly(vinyl chloride) (PVC)-based ion-selective film allowed highly reversible amperometric detection of calcium ions with sensor materials traditionally employed in potentiometric devices.

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Steven Linfield opened a general discussion of the paper by Alain Walcarius: Pore diameter can be a limiting factor during metal deposition in nanopores. Did you encounter any problems filling the nanopores with the aniline solution? What percentage of pores did you observe the electropolymeris

The light-addressable electrochemical sensor (LAES) is a recently emerged bioanalysis technique combining electrochemistry with the photoelectric effect in a semiconductor. In an LAES, a semiconductor substrate is illuminated locally to generate charge carriers in a well-defined area, thereby confining the electrochemical process to a target site. Benefiting from the unique light addressability, an LAES can not only detect multiple analytes in parallel within a single sensor plate but also act as a bio(chemical) imaging sensor to visualize the two-dimensional distribution of specific analytes. An LAES usually has three working modes: a potentiometric mode using light-addressable potentiometric sensors (LAPS) and an impedance mode using scanning photoinduced impedance microscopy (SPIM), while an amperometric mode refers to light-addressable electrochemistry (LAE) and photoelectrochemical (PEC) sensing. In this review, we describe the detection principles of each mode of LAESs and the concept of light addressability. In addition, we highlight the recent progress and advance of LAESs in spatial resolution, sensor system design, multiplexed detection, and bio(chemical) imaging applications. An outlook on current research challenges and future prospects is also presented.

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.

With the emergence of stretchable/wearable devices, functions, such as sensing, energy storage/harvesting, and electrical conduction, should ideally be carried out by a single material, while retaining its ability to withstand large elastic deformations, to create compact, functionally-integrated and autonomous systems. A new class of trimodal, stretchable yarn-based transducer formed by coating commercially available Lycra® yarns with PEDOT:PSS is presented. The material developed can sense strain (first mode), and temperature (second mode) and can power itself thermoelectrically (third mode), eliminating the need for an external power-supply. The yarns were extensively characterized and obtained an ultrahigh (gauge factor ∼3.6 × 105, at 10-20% strain) and tunable (up to about 2 orders of magnitude) strain sensitivity together with a very high strain-at-break point (up to ∼1000%). These PEDOT:PSS-Lycra yarns also exhibited stable thermoelectric behavior (Seebeck coefficient of 15 μV K-1), which was exploited both for temperature sensing and self-powering (∼0.5 μW, for a 10-couple module at ΔT ∼ 95 K). The produced material has potential to be interfaced with microcontroller-based systems to create internet-enabled, internet-of-things type devices in a variety of form factors.

Conventional metal oxide semiconductor (MOS) gas sensors have been investigated for decades to protect our life and property. However, the traditional devices can hardly fulfill the requirements of our fast developing mobile society, because the high operating temperatures greatly limit their applications in battery-loaded portable systems that can only drive devices with low power consumption. As ammonia is gaining importance in the production and storage of hydrogen, there is an increasing demand for energy-efficient ammonia detectors. Hence, in this work, a Schottky diode resulting from the contact between zinc oxide nanorods and gold is designed to detect gaseous ammonia at room temperature with a power consumption of 625 μW. The Schottky diode gas sensors benefit from the change of barrier height in different gases as well as the catalytic effect of gold nanoparticles. This diode structure, fabricated without expensive interdigitated electrodes and displaying excellent performance at room temperature, provides a novel method to equip mobile devices with MOS gas sensors.

Modulated light-activated electrochemistry (MLAE) at semiconductor/liquid interfaces derived from light-addressable potentiometric sensor (LAPS) and light-activated electrochemistry (LAE) for addressable photoelectrochemical sensing has been proposed as a new sensor platform. In this system, a bias voltage is applied to create a depletion layer at the silicon/electrolyte interface. Meanwhile, intensity-modulated light illuminates the movable electrode to generate electron/hole pairs and causes a detectable local AC photocurrent. The AC measurement showed a higher signal-to-noise ratio (SNR) of photocurrents compared to the traditional DC response, while a steeper photocurrent-voltage (I-V) curve than that of LAPS with an insulating layer was obtained. Furthermore, to stabilize and functionalize the silicon substrate, metal-organic framework (MOF) nanoparticles were grown in-situ on the silicon electrode. The successful modification was validated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The AC photocurrent increased as a result of the adsorption of negatively charged DNA, which contributed to the enhancement of the cathodic reduction process at the semiconductor electrodes, indicating a different response mechanism of MLAE from LAPS. The results obtained demonstrate the potential of MOF functionalized MLAE as a robust platform for light-addressable DNA chips with high sensitivity and specificity.

Peptide cross-linked poly(ethylene glycol) hydrogel has been widely used for drug delivery and tissue engineering. However, the use of this material as a biosensor for the detection of collagenase has not been explored. Proteases play a key role in the pathology of diseases such as rheumatoid arthritis and osteoarthritis. The detection of this class of enzyme using the degradable hydrogel film format is promising as a point-of-care device for disease monitoring. In this study, a protease biosensor was developed based on the degradation of a peptide cross-linked poly(ethylene glycol) hydrogel film and demonstrated for the detection of collagenase. The hydrogel was deposited on gold-coated quartz crystals, and their degradation in the presence of collagenase was monitored using a quartz crystal microbalance (QCM). The biosensor was shown to respond to concentrations between 2 and 2000 nM in less than 10 min with a lower detection limit of 2 nM.

The surface charge of cells affects cell signaling, cell metabolic processes, adherence to surfaces, and cell proliferation. Our understanding of the role of membrane charges is limited due to the inability to observe changes without interfering, chemically or physically, with the cell or its membrane. Here, we report that a photoelectrochemical imaging system (PEIS) based on label-free ac-photocurrent measurements at indium tin oxide (ITO) coated glass substrates can be used to map the basal surface charge of single live cells under physiological conditions. Cells were cultured on the ITO substrate. Photocurrent images were generated by scanning a focused, modulated laser beam across the back of the ITO coated glass substrate under an applied bias voltage. The photocurrent was shown to be sensitive to the negative surface charge of the substrate facing, basal side of a single living cell-an area not accessible to other electrochemical or electrophysiological imaging techniques. The PEIS was used to monitor the lysis of mesenchymal stem cells.

Flexible and stretchable electronic devices have a broad range of potential uses, from biomedicine, soft robotics, and health monitoring to the internet-of-things. Unfortunately, finding a robust and reliable power source remains challenging, particularly in off-the-grid and maintenance-free applications. A sought-after development overcome this challenge is the development of autonomous, self-powered devices. A potential solution is reported exploiting a promising n-type thermoelectric compound, poly nickel-ethenetetrathiolates (Na-x(Ni-ett)(n)). Highly stretchable n-type composite films are obtained by combining Na-x(Ni-ett)(n) with commercial polyurethane (Lycra). As high as 50 wt% Na-x(Ni-ett)(n) content composite film can withstand deformations of approximate to 500% and show conductivities of approximate to 10(-2) S cm(-1) and Seebeck coefficients of approx. -40 mu V K-1. These novel materials can be easily synthesized on a large scale with continuous processes. When subjected to a small temperature difference (<20 degrees C), the films generate sufficient thermopower to be used for sensing strain (gauge factor approximate to 20) and visible light (sensitivity factor approximate to 36% (kW m(-2))(-1)), independent of humidity (sensitivity factor approximate to 0.1 (%RH)(-1)). As a proof-of-concept, a wearable self-powered sensor is demonstrated by using n-type Na-x(Ni-ett)(n)/Lycra and PEDOT:PSS/Lycra elements, connected in series by hot pressing, without employing any metal connections, hence preserving good mechanical ductility and ease of processing.

AC photoelectrochemical imaging at electrolyte-semiconductor interfaces provides spatially resolved information such as surface potentials, ion concentrations and electrical impedance. In this work, thin films of InGaN/GaN were used successfully for AC photoelectrochemical imaging, and experimentally shown to generate a considerable photocurrent under illumination with a 405 nm modulated diode laser at comparatively high frequencies and low applied DC potentials, making this a promising substrate for bioimaging applications. Linear sweep voltammetry showed negligible dark currents. The imaging capabilities of the sensor substrate were demonstrated with a model system and showed a lateral resolution of 7 microns.

Inflammatory conditions are frequently accompanied by increased levels of active proteases, and there is rising interest in methods for their detection to monitor inflammation in a point of care setting. In this work, new sensor materials for disposable single-step protease biosensors based on poly(2-oxazoline) hydrogels cross-linked with a protease-specific cleavable peptide are described. The performance of the sensor material was assessed targeting the detection of matrix metalloproteinase-9 (MMP-9), a protease that has been shown to be an indicator of inflammation in multiple sclerosis and other inflammatory conditions. Films of the hydrogel were formed on gold-coated quartz crystals using thiol-ene click chemistry, and the cross-link density was optimized. The degradation rate of the hydrogel was monitored using a quartz crystal microbalance (QCM) and showed a strong dependence on the MMP-9 concentration. A concentration range of 0-160 nM of MMP-9 was investigated, and a lower limit of detection of 10 nM MMP-9 was determined.

Light-addressable potentiometric sensors (LAPS) measure ac photocurrent at electrolyte-insulator-semiconductor (EIS) and, more recently, electrolyte-semiconductor structures to produce spatiotemporal images of chemical or biological analytes, electrical potentials and impedance. One of the most important properties for LAPS is spatial resolution, which determines the smallest features that can be resolved in LAPS images. In this work, the use of nanostructured ZnO for LAPS was explored. The effect of ZnO morphology on the spatial resolution was studied with a LAPS setup. The best resolution of 2 µm was achieved in ZnO films produced by aerosol-assisted chemical vapour deposition (AACVD).