The effect of gold nanoparticles on the impedance of microcapsules visualized by scanning photo-induced impedance microscopy

Rapid Surface Charge Mapping Based on a Liquid Crystal Microchip

Rapid surface charge mapping of a solid surface remains a challenge. In this study, we present a novel microchip based on liquid crystals for assessing the surface charge distribution of a planar or soft surface. This chip enables rapid measurements of the local surface charge distribution of a charged surface. The chip consists of a micropillar array fabricated on a transparent indium tin oxide substrate, while the liquid crystal is used to fill in the gaps between the micropillar structures. When an object is placed on top of the chip, the local surface charge (or zeta potential) influences the orientation of the liquid crystal molecules, resulting in changes in the magnitude of transmitted light. By measuring the intensity of the transmitted light, the distribution of the surface charge can be accurately quantified. We calibrated the chip in a three-electrode configuration and demonstrated the validity of the chip for rapid surface charge mapping using a borosilicate glass slide. This chip offers noninvasive, rapid mapping of surface charges on charged surfaces, with no need for physical or chemical modifications, and has broad potential applications in biomedical research and advanced material design.

Photoelectrochemical imaging of single cardiomyocytes and monitoring of their action potentials through contact force manipulation of organoids

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.

Rapid saline-alkali sensitivity testing using hydrogel/gold nanoparticles-modified screen-printed electrodes

Rapid screening of microorganisms with good saline-alkali tolerance is of great significance for the improvement of saline-alkali land. In this study, a novel electrochemical method was developed for the rapid screening of saline-alkali-tolerant bacteria using a hydrogel/gold nanoparticles-modified screen-printed electrode. Monitoring bacterial growth using electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) yielded a new method to measure saline-alkali sensitivity. The strains were deposited on agarose hydrogel-AuNPs composite-modified electrodes with saline-alkali treatment control at a concentration of 50 mM. The electrochemical-derived growth curve of each bacterial strain was established to monitor the effect of saline-alkaline conditions on bacterial growth. The results showed that E. coli could grow on the hydrogel-AuNPs composite-modified electrodes without saline and alkali, while the growth of E. coli was inhibited after adding saline and alkali to the modified electrodes. In contrast, Paenibacillus lautus (HC_A) and Lysinibacillus fusiformis (HC_B) were able to grow on electrodes containing saline-alkali hydrogel-AuNPs composite modification. This fast growth curves of the strains derived from electrochemical analysis indicate that the possible time for salinity sensitivity results is <45 min. Compared to the traditional bacterial culture method lasting at least 1-2 days, this method has the clear advantages of rapidity, high efficiency, and low cost.

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

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.

The Light-Addressable Potentiometric Sensor and Its Application in Biomedicine towards Chemical and Biological Sensing

The light-addressable potential sensor (LAPS) was invented in 1988 and has developed into a multi-functional platform for chemical and biological sensing in recent decades. Its surface can be flexibly divided into multiple regions or pixels through light addressability, and each of them can be sensed independently. By changing sensing materials and optical systems, the LAPS can measure different ions or molecules, and has been applied to the sensing of various chemical and biological molecules and cells. In this review, we firstly describe the basic principle of LAPS and the general configuration of a LAPS measurement system. Then, we outline the most recent applications of LAPS in chemical sensing, biosensing and cell monitoring. Finally, we enumerate and analyze the development trends of LAPS from the aspects of material and optical improvement, hoping to provide a research and application perspective for chemical sensing, biosensing and imaging technology.

Mapping Surface Charge Distribution of Single-Cell via Charged Nanoparticle

Many bio-functions of cells can be regulated by their surface charge characteristics. Mapping surface charge density in a single cell's surface is vital to advance the understanding of cell behaviors. This article demonstrates a method of cell surface charge mapping via electrostatic cell-nanoparticle (NP) interactions. Fluorescent nanoparticles (NPs) were used as the marker to investigate single cells' surface charge distribution. The nanoparticles with opposite charges were electrostatically bonded to the cell surface; a stack of fluorescence distribution on a cell's surface at a series of vertical distances was imaged and analyzed. By establishing a relationship between fluorescent light intensity and number of nanoparticles, cells' surface charge distribution was quantified from the fluorescence distribution. Two types of cells, human umbilical vein endothelial cells (HUVECs) and HeLa cells, were tested. From the measured surface charge density of a group of single cells, the average zeta potentials of the two types of cells were obtained, which are in good agreement with the standard electrophoretic light scattering measurement. This method can be used for rapid surface charge mapping of single particles or cells, and can advance cell-surface-charge characterization applications in many biomedical fields.

Light-Addressable Square Wave Voltammetry (LASWV) Based on a Field-Effect Structure for Electrochemical Sensing and Imaging

Here, we describe a new photoelectrochemical imaging method termed light-addressable square wave voltammetry (LASWV). It measures local SWV currents at an unstructured electrolyte/insulator/semiconductor (EIS) field-effect substrate by illuminating and addressing the substrate with an intensity-constant laser. Due to the continuous generation of charge carriers in the light-irradiated semiconductor, the drift and diffusion of photoinjected carriers within the semiconductor bulk would slow down the equilibrium processes of charge and discharge in one potential pulse cycle. Therefore, even though SWV is sampled at the end of the direct and reverse pulses to reject capacitive currents, in our approach, photoinduced capacitive current can still be detected as an effective sensory signal. The obtained current-potential (I-V) curve shows a typical shape corresponding to the accumulation, depletion, and inversion regions of field-effect devices. We demonstrated that LASWV can be used as a field-effect chemical sensor to measure the solution pH and monitor enzymatic reactions. More importantly, since the charge carriers are only generated in the illuminated area, the laser spot in the device can be used as a virtual probe to record local electrochemical properties such as impedance with microresolution.

A LAPS-Based Differential Sensor for Parallelized Metabolism Monitoring of Various Bacteria

Monitoring the cellular metabolism of bacteria in (bio)fermentation processes is crucial to control and steer them, and to prevent undesired disturbances linked to metabolically inactive microorganisms. In this context, cell-based biosensors can play an important role to improve the quality and increase the yield of such processes. This work describes the simultaneous analysis of the metabolic behavior of three different types of bacteria by means of a differential light-addressable potentiometric sensor (LAPS) set-up. The study includes Lactobacillus brevis, Corynebacterium glutamicum, and Escherichia coli, which are often applied in fermentation processes in bioreactors. Differential measurements were carried out to compensate undesirable influences such as sensor signal drift, and pH value variation during the measurements. Furthermore, calibration curves of the cellular metabolism were established as a function of the glucose concentration or cell number variation with all three model microorganisms. In this context, simultaneous (bio)sensing with the multi-organism LAPS-based set-up can open new possibilities for a cost-effective, rapid detection of the extracellular acidification of bacteria on a single sensor chip. It can be applied to evaluate the metabolic response of bacteria populations in a (bio)fermentation process, for instance, in the biogas fermentation process.

InGaN as a Substrate for AC Photoelectrochemical Imaging

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.

Recent Developments of High-Resolution Chemical Imaging Systems Based on Light-Addressable Potentiometric Sensors (LAPSs)

A light-addressable potentiometric sensor (LAPS) is a semiconductor electrochemical sensor based on the field-effect which detects the variation of the Nernst potential on the sensor surface, and the measurement area is defined by illumination. Thanks to its light-addressability feature, an LAPS-based chemical imaging sensor system can be developed, which can visualize the two-dimensional distribution of chemical species on the sensor surface. This sensor system has been used for the analysis of reactions and diffusions in various biochemical samples. In this review, the LAPS system set-up, including the sensor construction, sensing and substrate materials, modulated light and various measurement modes of the sensor systems are described. The recently developed technologies and the affecting factors, especially regarding the spatial resolution and temporal resolution are discussed and summarized, and the advantages and limitations of these technologies are illustrated. Finally, the further applications of LAPS-based chemical imaging sensors are discussed, where the combination with microfluidic devices is promising.

Light-Addressable Potentiometric Sensors Using ZnO Nanorods as the Sensor Substrate for Bioanalytical Applications

Light-addressable potentiometric sensors (LAPS) are of great interest in bioimaging applications such as the monitoring of concentrations in microfluidic channels or the investigation of metabolic and signaling events in living cells. By measuring the photocurrents at electrolyte-insulator-semiconductor (EIS) and electrolyte-semiconductor structures, LAPS can produce spatiotemporal images of chemical or biological analytes, electrical potentials and impedance. However, its commercial applications are often restricted by their limited AC photocurrents and resolution of LAPS images. Herein, for the first time, the use of 1D semiconducting oxides in the form of ZnO nanorods for LAPS imaging is explored to solve this issue. A significantly increased AC photocurrent with enhanced image resolution has been achieved based on ZnO nanorods, with a photocurrent of 45.7 ± 0.1 nA at a light intensity of 0.05 mW, a lateral resolution as low as 3.0 μm as demonstrated by images of a PMMA dot on ZnO nanorods and a pH sensitivity of 53 mV/pH. The suitability of the device for bioanalysis and bioimaging was demonstrated by monitoring the degradation of a thin poly(ester amide) film with the enzyme α-chymotrypsin using LAPS. This simple and robust route to fabricate LAPS substrates with excellent performance would provide tremendous opportunities for bioimaging.

Light-Addressable Potentiometric Sensors for Quantitative Spatial Imaging of Chemical Species

A light-addressable potentiometric sensor (LAPS) is a semiconductor-based chemical sensor, in which a measurement site on the sensing surface is defined by illumination. This light addressability can be applied to visualize the spatial distribution of pH or the concentration of a specific chemical species, with potential applications in the fields of chemistry, materials science, biology, and medicine. In this review, the features of this chemical imaging sensor technology are compared with those of other technologies. Instrumentation, principles of operation, and various measurement modes of chemical imaging sensor systems are described. The review discusses and summarizes state-of-the-art technologies, especially with regard to the spatial resolution and measurement speed; for example, a high spatial resolution in a submicron range and a readout speed in the range of several tens of thousands of pixels per second have been achieved with the LAPS. The possibility of combining this technology with microfluidic devices and other potential future developments are discussed.

Biological imaging using light-addressable potentiometric sensors and scanning photo-induced impedance microscopy

Light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) use photocurrent measurements at electrolyte-insulator-semiconductor substrates for spatio-temporal imaging of electrical potentials and impedance. The techniques have been used for the interrogation of sensor arrays and the imaging of biological systems. Sensor applications range from the detection of different types of ions and the label-free detection of charged molecules such as DNA and proteins to enzyme-based biosensors. Imaging applications include the temporal imaging of extracellular potentials and dynamic concentration changes in microfluidic channels and the lateral imaging of cell surface charges and cell metabolism. This paper will investigate the current state of the art of the measurement technology with a focus on spatial and temporal resolution and review the biological applications, these techniques have been used for. An outlook on future developments in the field will be given.

Copper Contamination of Self-Assembled Organic Monolayer Modified Silicon Surfaces Following a "Click" Reaction Characterized with LAPS and SPIM

A copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) reaction combined with microcontact printing was used successfully to pattern alkyne-terminated self-assembled organic monolayer-modified silicon surfaces. Despite the absence of a copper peak in X-ray photoelectron spectra, copper contamination was found and visualized using light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) after the "click"-modified silicon surfaces were rinsed with hydrochloric acid (HCl) solution, which was frequently used to remove copper residues in the past. Even cleaning with an ethylenediaminetetraacetic acid (EDTA) solution did not remove the copper residue completely. Different strategies for avoiding copper contamination, including the use of bulky chelators for the copper(I) catalyst and rinsing with different reagents, were tested. Only cleaning of the silicon surfaces with an EDTA solution containing trifluoroacetic acid (TFA) after the click modification proved to be an effective method as confirmed by LAPS and SPIM results, which showed the expected potential shift due to the surface charge introduced by functional groups in the monolayer and allowed, for the first time, imaging the impedance of an organic monolayer.