Search

A device structure made from the Si substrate etched from the smooth surface by KOH wet etching is investigated for use as a light-addressable potentiometric sensor (LAPS). Stacked Si3N4/SiO2 layers were used as the sensing membrane on the top side, and a hard mask layer generated by KOH etching comprises the bottom side of the LAPS. A single-side polished P-type Si wafer with a thickness of 350~\mu \textm was reduced to 175~\mu \textm by KOH wet etching of the polished side. A high photocurrent was observed in an LAPS sample with increased surface roughness while under backside illumination. Additionally, the increased roughness of the sensing membrane on the top side generated by the unpolished Si surface in the same sample produced higher pH sensitivity than conventional polished Si surfaces. The pH sensitivity and linearity were 45.4 mV/pH and 99.7%, respectively, for pH values ranging from 2 to 12. A 4.4-fold higher photocurrent was achieved using the thin-Si LAPS with a thickness of 175~\mu \textm measured at a light-modulated frequency of 1 kHz compared with that using a Si substrate with a thickness of 550~\mu \textm . An acceptable photocurrent was obtained with a modulated illumination frequency up to 20 kHz, which is beneficial for 2-D images acquired by high-speed scanning. This simple device structure consisting of a thin-Si LAPS increases the photocurrent and pH sensitivity. The thickness of the LAPS Si substrate could be reduced in the future to optimize the process stability and sensing performance. [ABSTRACT FROM PUBLISHER]

An In–Ga–Zn oxide (IGZO) layer is applied, for the first time, as the semiconductor layer of a light-addressable potentiometric sensor (LAPS). The IGZO layer sputtered on the commercial ITO/glass substrate could be used as the active layer of an LAPS illuminated by a ultraviolet light-emitting diode (UV LED). In the operation of LAPS, an IGZO/ITO glass substrate with a higher photocurrent and less interference of the ambient light compared with a conventional n-type silicon wafer is obtained. A sensing membrane of sputtered niobium oxide directly on the IGZO layer, with a substrate temperature of 250 °C, is verified to have a pH sensitivity of 61.8 mV/pH according to photocurrent versus gate bias curves measured in various pH buffer solutions. The ac signal applied to the UV LED can be increased to 30 kHz for IGZO LAPS pH sensing. This improvement is beneficial for the sensor’s use in high-speed 2-D chemical image applications. [ABSTRACT FROM PUBLISHER]

A review of solid-state chemical and electrochemical sensors to detect metabolic activity at the extracellular, single-cell level is presented in the context of the development of lab-on-a-chip research instrumentation. Metabolic processes in cells are briefly reviewed with the goal of quantifying the role of metabolites within the cell. Sensors reviewed include both research and commercial devices that can noninvasively detect extracellular metabolites, including oxygen, carbon dioxide, and glucose. Metabolic activity can also be sensed nonselectively by measuring pH gradients. Performance metrics, such as sensitivity, sensor size, drift, time response, and sensing range, are included when available. Highly suitable sensor technologies for monitoring cellular metabolic activity include electrochemical sensors, scanning electrochemical microscopy, ion-sensitive field effect transistor sensors, and solid-state light-addressable potentiometric sensors. Other less-suitable, but still potentially viable, solid-state sensing technologies are also reviewed briefly, including resonant chemical sensors (surface acoustic wave and quartz crystal microbalance), conductivity or impedance sensors, and sensors with multiple transduction stages. Specific biological applications which benefit from detection of extracellular metabolic events at the single-cell level are discussed to provide context to the practical use of these sensor technologies; these applications include case studies of various diseases (cancer, diabetes, mitochondrial disorders. etc.), cell and tissue differentiation; cell and tissue storage; cell life cycle and basic cellular processes; and developmental biology. [ABSTRACT FROM PUBLISHER]