Spatially resolved monitoring of cellular metabolic activity with a semiconductor-based biosensor

Quantum Dot/TiO2 Nanocomposite-Based Photoelectrochemical Sensor for Enhanced H2O2 Detection Applied for Cell Monitoring and Visualization

This work exploits the possibility of using CdSe/ZnS quantum dot (QD)-electrodes to monitor the metabolism of living cells based on photoelectrochemical (PEC) measurements. To realize that, the PEC setup is improved with respect to an enhanced photocurrent signal, better stability, and an increased signal-to-noise ratio, but also for a better biocompatibility of the sensor surface on which cells have been grown. To achieve this, a QD-TiO2 heterojunction is introduced with the help of atomic layer deposition (ALD). The heterojunction reduces the charge carrier recombination inside the semiconductor nanoparticles and improves the drift behavior. The PEC performance is carefully analyzed by adjusting the TiO2 thickness and combining this strategy with multilayer immobilizations of QDs. The optimal thickness of this coating is ≈5 nm; here, photocurrent generation can be enhanced significantly (e.g., for a single QD layer electrode by more than one order of magnitude at 0 V vs Ag/AgCl). The resulting optimized electrode is used for hydrogen peroxide (H2O2) sensing with a good sensitivity down to µmolar concentrations, reusability, stability, response rate, and repeatability. Finally, the sensing system is applied to monitor the activity of cells directly grown on top of the electrode surface.

A Silicon-Based Field-Effect Biosensor for Drug-Induced Cardiac Extracellular Calcium Ion Change Detection

Calcium ions participate in the regulation of almost all biological functions of the body, especially in cardiac excitation-contraction coupling, acting as vital signaling through ion channels. Various cardiovascular drugs exert their effects via affecting the ion channels on the cell membrane. The current strategies for calcium ion monitoring are mainly based on fluorescent probes, which are commonly used for intracellular calcium ion detection (calcium imaging) and cannot achieve long-term monitoring. In this work, an all-solid-state silicone-rubber ion-sensitive membrane was fabricated on light-addressable potentiometric sensors to establish a program-controlled field-effect-based ion-sensitive light-addressable potentiometric sensor (LAPS) platform for extracellular calcium ion detection. L-type calcium channels blocker verapamil and calcium channel agonist BayK8644 were chosen to explore the effect of ion channel drugs on extracellular calcium ion concentration in HL-1 cell lines. Simultaneously, microelectrode array (MEA) chips were employed to probe the HL-1 extracellular field potential (EFP) signals. The Ca2+ concentration and EFP parameters were studied to comprehensively evaluate the efficacy of cardiovascular drugs. This platform provides more dimensional information on cardiovascular drug efficacy that can be utilized for accurate drug screening.

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.

A Microphysiometric System Based on LAPS for Real-Time Monitoring of Microbial Metabolism

Macronutrients (carbohydrates, fat and protein) are the cornerstones of daily diet, among which carbohydrates provide energy for the muscles and central nervous system during movement and exercise. The breakdown of carbohydrates starts in the oral cavity, where they are primarily hydrolyzed to glucose and then metabolized to organic acids. The end products may have an impact on the oral microenvironment, so it is necessary to monitor the process of microbial metabolism and to measure the pH change. Although a pH meter has been widely used, it is limited by its sensitivity. We then introduce a light addressable potentiometric sensor (LAPS), which has been used in extracellular acidification detection of living cells with the advantages of being objective, quantitative and highly sensitive. However, it is difficult to use in monitoring bacterial metabolism because bacteria cannot be immobilization on the LAPS chip as easily as living cells. Therefore, a microphysiometric system integrated with Transwell insert and microfluidic LAPS chip was designed and constructed to solve this problem. The decrease in pH caused by glucose fermentation in Lactobacillus rhamnosus was successfully measured by this device. This proves the feasibility of the system for metabolism detection of non-adhere targets such as microorganisms and even 3D cells and organoids.

Multiplexed all-solid-state ion-sensitive light-addressable potentiometric sensor (ISLAPS) system based on silicone-rubber for physiological ions detection

Light-addressable potentiometric sensor (LAPS) has been widely used in biomedical applications since its advent. As a member of the potentiometric sensors, ion-sensitive LAPS (ISLAPS) can be obtained by modifying ion selective sensing membrane on the sensor surface. Compared with the conventional ion-selective electrodes (ISEs) with liquid contact, the all-solid-state ISEs have more advantages such as easy maintenance, more convenient for miniaturization and practical applications. However, the commonly used ion-sensitive membrane (ISM) matrix like PVC has many limitations such as poor adhesion to silicone-based sensor and easy overflow of the plasticizer from the membrane. In this work, LAPS was combined with a variety of ionophore-doped all-solid-state silicone-rubber ISMs for the first time, to establish a program-controlled multiplexed ISLAPS system for physiological ions (Na⁺, K⁺, Ca²⁺ and H⁺) detection. The silicone-rubber ISMs have better adhesion to silicon-based sensors without containing plasticizers, which can avoid the plasticizer pollution and improve the long-term stability. A layer of poly(3-octylthiophene-2,5-diyl) (P3OT) was pre-modified on the sensor surface to inhibit the formation of an aqueous layer and improve the sensor lifetime. With the aid of a translation stage, the light spot automatically illuminated the detection sites in sequence, and the response of the four ions could be obtained in one measurement within 1 min. The proposed multiplexed ISLAPS has good sensitivity with micromolar limit of detection (LOD), good selectivity and long-term stability (more than 3 months). The results of the real Dulbecco's Modified Eagle Medium (DMEM) sample detection proved that the ISLAPS system can be used for the physiological ions detection, and is promising to realize a multi-parameter microphysiometer.

Introducing visible-light sensitivity into photocatalytic CeO2 nanoparticles by hybrid particle preparation exploiting plasmonic properties of gold: enhanced photoelectrocatalysis exemplified for hydrogen peroxide sensing

In this report we combine the catalytic properties of CeO₂ nanoparticles with their transduction ability for photoelectrochemical sensing. This study highlights the usage of CeO₂ providing catalytic activity towards H₂O₂, but only with a limited excitation range in the UV for the construction of a sensing system. In order to improve the photoelectrocatalysis of CeO₂ nanoparticles by extending their excitation to visible light, Au/CeO₂ core/shell hybrid nanoparticles have been synthesized. The hybrid nanoparticles are fixed on electrodes, allowing for the generation of photocurrents, the direction of which can be controlled by the electrode potential (without bias). The application of the hybrid nanoparticles results in an enhanced photocurrent amplitude under white light illumination as compared to the pure CeO₂ nanoparticles. Wavelength-dependent measurements confirm the participation of the Au core in the signal transduction. This can be explained by improved charge carrier generation within the hybrid particles. Thus, by using a plasmonic element the photoelectochemical response of a catalytic nanoparticle (i.e. CeO₂) has been spectrally extended. The effect can be exploited for sensorial hydrogen peroxide detection. Here higher photocatalytic current responses have been found for the hybrid particles fixed to gold electrodes although the catalytic reduction has been comparable for both types of nanoparticles. Thus, it can be demonstrated that Au/CeO₂ core-shell nanoparticles allow the utilization of visible light for photoelectrochemical hydrogen peroxide (H₂O₂) detection with improved sensitivity under white light illumination or application of such particles with only visible light excitation, which is not possible for pure CeO₂. With help of the layer-by-layer (LbL) technique for nanoparticle immobilization, the electrode response can be adjusted and with a 5 layers electrode a low detection limit of about 3 μM H₂O₂ with a linear detection range up to 2000 μM is obtained.

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 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.

"Click" Patterning of Self-Assembled Monolayers on Hydrogen-Terminated Silicon Surfaces and Their Characterization Using Light-Addressable Potentiometric Sensors

Two potential strategies for chemically patterning alkyne-terminated self-assembled monolayers (SAMs) on oxide-free silicon or silicon-on-sapphire (SOS) substrates were investigated and compared. The patterned surfaces were validated using a light-addressable potentiometric sensor (LAPS) for the first time. The first strategy involved an integration of photolithography with "click" chemistry. Detailed surface characterization (i.e. water contact angle, ellipsometry, AFM, and XPS) and LAPS measurements showed that photoresist processing not only decreases the coverage of organic monolayers but also introduces chemically bonded contaminants on the surfaces, thus significantly reducing the quality of the SAMs and the utility of "click" surface modification. The formation of chemical contaminants in photolithography was also observed on carboxylic acid- and alkyl-terminated monolayers using LAPS. In contrast, a second approach combined microcontact printing (μCP) with "click" chemistry; that is azide (azido-oligo(ethylene glycol) (OEG)-NH2) inks were printed on alkyne-terminated SAMs on silicon or SOS through PDMS stamps. The surface characterization results for the sample printed with a flat featureless PDMS stamp demonstrated a nondestructive and efficient method of μCP to perform "click" reactions on alkyne-terminated, oxide-free silicon surfaces for the first time. For the sample printed with a featured PDMS stamp, LAPS imaging showed a good agreement with the pattern of the PDMS stamp, indicating the successful chemical patterning on non-oxidized silicon and SOS substrates and the capability of LAPS to image the molecular patterns with high sensitivity.

Field-effect devices for detecting cellular signals

The integration of living cells together with silicon field-effect devices challenges a new generation of biosensors and bioelectronic devices. Cells are representing highly organised complex systems, optimised by millions of years of evolution and offering a broad spectrum of bioanalytical receptor "tools" such as enzymes, nucleic acids proteins, etc. Their combination with semiconductor-based electronic chips allows the construction of functional hybrid systems with unique functional and electronic properties for both fundamental studies and biosensoric applications. This review article summarises recent advances and trends in research and development of cell/transistor hybrids (cell-based field-effect transistors) as well as light-addressable potentiometric sensors.

Development of an array of ion-selective microelectrodes aimed for the monitoring of extracellular ionic activities

In this study, we present the development and the characterization of a generic platform for cell culture able to monitor extracellular ionic activities (K+, NH4+) for real-time monitoring of cell-based responses, such as necrosis, apoptosis, or differentiation. The platform for cell culture is equipped with an array of 16 silicon nitride micropipet-based ion-selective microelectrodes with a diameter of either 2 or 6 microm. This array is located at the bottom of a 200-microm-wide and 350-microm-deep microwell where the cells are cultured. The characterization of the ion-selective microelectrode arrays in different standard and physiological solutions is presented. Near-Nernstian slopes were obtained for potassium- (58.6 +/- 0.8 mV/pK, n = 15) and ammonium-selective microelectrodes (59.4 +/- 3.9 mV/pNH4, n = 13). The calibration curves were highly reproducible and showed an average drift of 4.4 +/- 2.3 mV/h (n = 10). Long-term behavior and response after immersion in physiological solutions are also presented. The lifetime of the sensors was found to be extremely long with a high recovery rate.

The light-addressable potentiometric sensor for multi-ion sensing and imaging

The light-addressable potentiometric sensor (LAPS) is a semiconductor-based chemical sensor with an electrolyte-insulator-semiconductor structure. The LAPS can have many measuring points integrated on the sensing surface, which are individually accessed by a light beam. By modifying the measuring points with different materials, a single sensor plate can be used as a multi-analyte sensor. In this paper, instrumentation and application of LAPS to multi-ion sensing and imaging are described. As a new application of LAPS, potentiometric imaging of a microfluidic channel is proposed.

A microphysiometer for simultaneous measurement of changes in extracellular glucose, lactate, oxygen, and acidification rate

A microphysiometer capable of measuring changes in extracellular glucose, lactate, oxygen, and acidification rate has been developed by incorporating modified electrodes into a standard Cytosensor Microphysiometer plunger. Glucose and lactate are measured indirectly at platinum electrodes by amperometric oxidation of hydrogen peroxide, which is produced from catalysis of glucose and lactate at films containing their respective entrapped oxidase. Oxygen is measured amperometrically at a platinum electrode coated with a Nafion film, while the acidification rate is measured potentiometrically by a Cytosensor Microphysiometer. Analytical information is obtained during the Cytosensor stop-flow cycles, where the electrodes measure changes in the extracellular medium corresponding to the consumption or production of the analyte by the cells. Modification of the Cytosensor plunger for multianalyte determination is described, and the operation of the technique is illustrated by the simultaneous measurement of all four analytes during the addition of fluoride and DNP to Chinese hamster ovary cells and fluoride and antimycin A to mouse fibroblast cells. Cell metabolic recovery and dynamics after exposure to agents can also be observed in specific cases.