Light-Addressing and Chemical Imaging Technologies for Electrochemical Sensing

Visualizing chemical components in a specimen is an essential technology in many branches of science and practical applications. This book deals with electrochemical imaging techniques based on semiconductor devices with capability of spatially resolved sensing. Two types of such sensing devices have been extensively studied and applied in various fields, i.e., arrayed sensors and light-addressed sensors. An ion-sensitive field-effect transistor (ISFET) array and a charge-coupled device (CCD) ion image sensor are examples of arrayed sensors. They take advantage of semiconductor microfabrication technology to integrate a large number of sensing elements on a single chip, each representing a pixel to form a chemical image. A light-addressable potentiometric sensor (LAPS), on the other hand, has no pixel structure. A chemical image is obtained by raster-scanning the sensor plate with a light beam, which can flexibly define the position and size of a pixel. This light-addressing approach is further applied in other LAPS-inspired methods. Scanning photo-induced impedance microscopy (SPIM) realized impedance mapping and light-addressable electrodes/light-activated electrochemistry (LAE) realized local activation of Faradaic processes. This book includes eight articles on state-of-the-art technologies of light-addressing/chemical imaging devices and their application to biology and materials science

Field-Effect Sensors

This Special Issue focuses on fundamental and applied research on different types of field-effect chemical sensors and biosensors. The topics include device concepts for field-effect sensors, their modeling, and theory as well as fabrication strategies. Field-effect sensors for biomedical analysis, food control, environmental monitoring, and the recording of neuronal and cell-based signals are discussed, among other factors

Label-free and Multi-parametric Monitoring of Cell-based Assays with Substrate-embedded Sensors

Various approaches have been pursued on the basis of electrochemical or piezoelectric transducers, particularly of the quartz crystal microbalance (QCM), to monitor non-invasively and in real-time cellular states and reactions with substrate-embedded sensors. On the one hand, these comprised the technical development of piezoelectric sensors with multiple read-out spots and the integration of additional non-invasive (electro- and optochemical) sensor technologies on the QCM surface. On the other hand, a variety of studies and cell-based assays (CBAs) have been performed in order to test the sensor performances and to gain a deeper understanding of the sensors’ readout parameters with respect to their information content about the biophysical properties and the metabolic behavior of cells. Fig. 7–1 presents an overview of the different projects on the basis of QCM sensor disks presented in this thesis. In the first project (Fig. 7–1 A) a novel electrode layout was designed on the basis of commercial 5 MHz AT-cut quartz disks to implement two independent readout spots on the QCM surface. This also comprised the construction of new measurement chambers for the electrical actuation and sensing of quartz oscillations. These two-electrode QCM sensors (2ElQ) are also referred to as multichannel QCM (MQCM). The developed MQCM sensor spots on one quartz disk exhibited a strong interference, even though they were operated sequentially, which is in contrast to the results of previous theoretical calculations. The resonances could be successfully decoupled by coating half of the quartz surface and one sensor spot, respectively, with a thin and rigid film of photoresist. This quartz loading with mass caused a shift in the resonance spectra of the coated resonator to lower frequencies and efficient decoupling. The operation of the decoupled MQCM sensors demonstrated both, a sensitive and equal change in the oscillation characteristics of the two resonators upon loading of the quartz with medium. The Q-factor was not significantly different for the two resonators, qualifying the MQCM for its application in CBAs. Building on the preceding development of the double-electrode quartz disks, a novel electrode layout has been realized at the sensor surface, which enables the complementary electrochemical (impedance spectroscopical) characterization of the substrate-liquid interface in addition to its mechanical characterization by the piezoelectric transducers (Fig. 7–1 B). This layout was achieved by removing a small area of the insulating photoresist on the coated electrode in the photolithographic process. By this, a coplanar electrode arrangement of a small working electrode and a bigger counter electrode was created. This sensor combination on the basis of the MQCM is an improvement of the so-called 2nd generation QCM to what we call the 3rd generation QCM, and which is also referred to as QCM-ECIS. Various electrode layouts, varying in size and number of the working electrode(s), were fundamentally characterized microscopically and by profilometry regarding the geometrical properties and by means of impedance spectroscopy with respect to the sensing performances in QCM- and ECIS-mode. An optimal electrode layout was identified and defined as standard for subsequent applications in CBAs. In both QCM- and ECIS studies of cell-covered sensor surfaces significant changes in the characteristic sensing parameters with respect to the cell-free electrodes are measurable. In addition to the measurement of absolute signal changes, the transducer technologies of QCM and ECIS also enable to monitor the kinetic changes of the readout parameters with high temporal resolution. This allows to use the dual sensors for monitoring and analyzing the states of adherent cell cultures in any kind of assay, label-free, non-invasively, and in real-time. Mechanical (QCM-mode) and the dielectric (ECIS-mode) characteristics of cell adhesion were simultaneously measured for two different cell lines (MDCK II and NRK), with high reproducibility for each. The total and kinetic parameter changes in both sensing modes distinguished clearly and were specific for the cell lines under test. The signals from both QCM-mode and ECIS-mode recordings also reported on significant impacts of the presence/ absence of bivalent cations (Ca2+, Mg2+) on the attachment and spreading kinetics and behavior of MDCK II cells. Aside from cell adhesion studies, the cytomechanical and cell morphological reactions towards various stimuli were monitored and analyzed by QCM-ECIS in a multitude of cellular assays: systematic softening and stiffening of cells (using agents for disassembling the actin cytoskeleton and cross-linking protein structures, respectively), intracellular stimulation (using a second messenger analogue), as well as electrical manipulation (electroporation (ELPO) and wounding) of cell layers (applying invasive voltage pulses). The applicability of electrical actuation and the subsequent non-invasive, time-resolved, and dual sensing with the electrodes of the QCM-ECIS substrates has been successfully demonstrated. The monitoring of CBAs with the dual QCM-ECIS sensor chips developed in this thesis provides not only a multiplication of the information gain due to the complementarity of QCM and ECIS readout parameters. The simultaneous, time-resolved measurements also enable the kinetic correlation of the sensor signals in novel 2 D and 3 D diagrams, which offers the hitherto unprecedented opportunity for a more detailed view and analysis of the coherence or consecutiveness of mechanical and morphological/ dielectric changes of a cell layer under study. A third research project focused on the combination of optical-chemical sensors (OCS) with the piezoelectric (QCM) sensor technology. For this purpose, the quartz crystal surface was coated with a polymer film with embedded phosphorescent indicator dye for the target analyte. The luminescence properties were measured by means of fluorescence (phosphorescence) lifetime imaging (FLIM). By using a temperature-sensitive paint (TSP), an increase in temperature on the sensor surface upon high-amplitude oscillations was monitored and imaged this way in one project (Fig. 7–1 C). Based on this experimentally determined local heating on the QCM surface and the thereby generated temperature gradient in the liquid above the resonator, a thermophoretic convection in the fluid has been simulated. Theoretical considerations showed that the convection profile in the measurement vessel counteracts and even largely prevents the sedimentation of cells onto the sensor surface. It is suggested that the effect of thermophoresis is crucial especially in studies of biomolecular interactions on QCM surfaces at elevated shear amplitudes and driving voltages, respectively, which however has not been considered in literature to date. The phosphorescence quenching capability of oxygen was utilized in a second imaging project to monitor and image the local concentration and distribution of oxygen on the growth substrate of cells by means of a so-called pressure(/oxygen)-sensitive paint (PSP) (Fig. 7–1 D). A home-made experimental setup was constructed for sensor calibration and the imaging of subcellular oxygen, consisting of a FLIM setup coupled to an upright microscope and a temperature- and oxygen-controlled calibration and measurement chamber suitable for cellular applications. The cytocompatible sensor films have been characterized under various test conditions (in air, under medium, at different temperatures) regarding their sensitivity and response characteristics to different oxygen partial pressures. The oxygen consumption of cells adherently grown on the sensor film was successfully monitored and imaged by this setup. The time-resolved measurements demonstrated a significantly faster consumption of oxygen of a cell layer stimulated with a respiration chain decoupler compared to an unstimulated control cell layer. Taken together, various technical improvements of piezoelectric sensors (QCM) have been realized (MQCM, QCM-ECIS, ELPO-QCM-ECIS, QCM-OCS), which provide a significant information gain in cell-based applications. The sensors developed enable the high-content screening (HCS) of adherent cell lines in a wide range of assay formats and provide complementary physico-chemical information for obtaining a more complete picture of the state of cells and their reactions in contact to diverse stimuli. All sensor techniques share the characteristics of time-resolved, label-free, and non-invasive monitoring. This allows to disclose and analyze even the kinetics, delayed effects, recoveries, and fluctuations of physicochemical alterations of a studied cell layer, in addition to the absolute parameter changes, which is a valuable improvement compared to classical endpoint assays. The approach of combined, independent sensor systems also provides the novel possibility to bring parameters obtained by the different readout technologies from one cell layer in a temporal correlation, by which new insights into physiological relationships are possible

Detection of acetone using nanostructured WO3 for diabetes mellitus monitoring applications

Diabetes mellitus which is characterized by a high levels of blood glucose is a major source of mortality, morbidity and health costs worldwide. Major gaps exist in efforts to comprehend the burden nationally and globally, especially in developing nations, due to a lack of accurate, cheap and non-invasive data and devices for monitoring and surveillance. In Africa, type 2 diabetes mellitus represents 90% of diabetes cases. The disease mainly relies on management and monitoring. Although reliable blood glucose monitoring techniques and devices exist worldwide, the challenge is with the cost, invasiveness, and long sample preparation. Herein this study, the challenge was addressed by synthesizing WO3 materials for the detection of acetone in a simulated human breath. Acetone has been reliably confirmed to be the biomarker of diabetes mellitus. The Gas Chromatography-Mass Spectrometry (GC-MS) was employed to quantify acetone in type 2 diabetes mellitus. A statistically significant correlation (R=0.756) between blood glucose and breath acetone was observed, between blood acetoacetate and breath acetone (R=0,897), and between beta-hydroxybutyrate and breath acetone (R=0,821). Furthermore, we used semiconducting metal oxide (WO3) to investigate its selectivity, sensitivity, and response towards acetone. Semiconducting metal oxides sensor has the potential to detect volatile organic compound (VOCs) at low concentrations as low as 0.1 ppb. Other advantages of semiconducting metal oxides sensors include, facile and cheap device fabrication, portability, real-time analysis, and facile operating principle. We used two synthesis methods for fabrication of acetone sensors namely solvothermal method whereby solvent ratios were varied, and the sol-gel method where carbon nanospheres were used as a template and cobalt as a dopant. The sensor fabricated with 51:49 water: ethanol is found to demonstrate high response and good selectivity to 2 ppm level of acetone when compared with the one fabricated with pure ethanol, 18:92 (ethanol: water) and 92% water. Furthermore, the sensor could respond to low concentrations of acetone ranging from 0.5 to 4.5 ppm of acetone at 100 °C. For the sol-gel method, the 0.6 % Co-doped WO3 showed higher response and selectivity towards acetone gas from as low as 0.5 ppm at a very low operating temperature of 50 °C. Contrary, there was a very low response from other gases including toluene, NO2, NH3, CH4 and H2S operating at a similar temperature. This highlights the acetone selectivity of our 0.6 % Co-doped WO3 sample. Based on the two methods used for the synthesis of the acetone sensor, we can conclude that the Co-doped sensor shows better performance as compared to the as-prepared WO3. This is from the findings that the Co-doped WO3 can respond and select acetone concentration at 50 ◦C, which is a very low temperature in comparison to other platforms described in literature. An envisioned portable point of care diabetic device could therefore be operated at 50 ◦C in any point of care setting.Thesis (PhD (Biochemistry))--University of Pretoria, 2020.Council for Scientific and Industrial ResearchBiochemistryPhD (Biochemistry)Unrestricte

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