Electrolyte-insulator-semiconductor Research Articles

Gate Alignment of Liquid Water Molecules in Electric Double Layer.

The behavior of liquid water molecules near an electrified interface is important to many disciplines of science and engineering. In this study, we applied an external gate potential to the silica/water interface via an electrolyte-insulator-semiconductor (EIS) junction to control the surface charging state. Without varying the ionic composition in water, the electrical gating allowed an efficient tuning of the interfacial charge density and field. Using the sum-frequency vibrational spectroscopy, we found a drastic enhancement of interfacial OH vibrational signals at high potential in weakly acidic water, which exceeded that from conventional bulk-silica/water interfaces even in strong basic solutions. Analysis of the spectra indicated that it was due to the alignment of liquid water molecules through the electric double layer, where the screening was weak because of the low ion density. Such a combination of strong field and weak screening demonstrates the unique tuning capability of the EIS scheme, and would allow us to investigate a wealth of phenomena at charged oxide/water interfaces.

An Electrochemical Ti3C2Tx Aptasensor for Sensitive and Label-Free Detection of Marine Biological Toxins.

Saxitoxin (STX) belongs to the family of marine biological toxins, which are major contaminants in seafood. The reference methods for STX detection are mouse bioassay and chromatographic analysis, which are time-consuming, high costs, and requirement of sophisticated operation. Therefore, the development of alternative methods for STX analysis is urgent. Electrochemical analysis is a fast, low-cost, and sensitive method for biomolecules analysis. Thus, in this study, an electrolyte-insulator-semiconductor (EIS) sensor based on aptamer-modified two-dimensional layered Ti3C2Tx nanosheets was developed for STX detection. The high surface area and rich functional groups of MXene benefited the modification of aptamer, which had specific interactions with STX. Capacitance-voltage (C-V) and constant-capacitance (ConCap) measurement results indicated that the aptasensor was able to detect STX with high sensitivity and good specificity. The detection range was 1.0 nM to 200 nM and detection limit was as low as 0.03 nM. Moreover, the aptasensor was found to have a good selectivity and two-week stability. The mussel tissue extraction test suggested the potential application of this biosensor in detecting STX in real samples. This method provides a convenient approach for low-cost, rapid, and label-free detection of marine biological toxins.

Detection of plant virus particles with a capacitive field-effect sensor

Plant viruses are major contributors to crop losses and induce high economic costs worldwide. For reliable, on-site and early detection of plant viral diseases, portable biosensors are of great interest. In this study, a field-effect SiO2-gate electrolyte-insulator-semiconductor (EIS) sensor was utilized for the label-free electrostatic detection of tobacco mosaic virus (TMV) particles as a model plant pathogen. The capacitive EIS sensor has been characterized regarding its TMV sensitivity by means of constant-capacitance method. The EIS sensor was able to detect biotinylated TMV particles from a solution with a TMV concentration as low as 0.025 nM. A good correlation between the registered EIS sensor signal and the density of adsorbed TMV particles assessed from scanning electron microscopy images of the SiO2-gate chip surface was observed. Additionally, the isoelectric point of the biotinylated TMV particles was determined via zeta potential measurements and the influence of ionic strength of the measurement solution on the TMV-modified EIS sensor signal has been studied.

Silicone-Rubber Based Multiplex Ion-Sensitive Light-Addressable Potentiometric Sensor (ISLAPS) System for Physiological Ions Detection

Silicone-Rubber Based Multiplex Ion-Sensitive Light-Addressable Potentiometric Sensor (ISLAPS) System for Physiological Ions Detection

(IMCS Second Place Best Paper Award) High-Density Immobilization of Tobacco Mosaic Virus Nanotubes As Enzyme Nanocarriers Onto Field-Effect Biosensor Structures

Introduction Viruses are not only infectious agents, they are also known as promising functional building blocks for application in nano- and biotechnologies. The tobacco mosaic virus (TMV) was the first studied plant virus and is widely distributed; it infects vegetables, like tomato, bell pepper, beans and other members of the family Solanaceae, while it is totally harmless for mammals [1]. It is one of the most studied plant viruses and its genome is completely sequenced, whereby genetical and chemical modification is easy. TMV has a nanotube-like shape with a length of 300 nm, an outer diameter of 18 nm and an inner diameter of 4 nm. Since it possesses a high chemical and physical robustness, it can be integrated with different electronic transducers for bio- and chemical sensing applications [2].Recently, we presented a TMV-based amperometric glucose biosensor [3] and a potentiometric penicillin biosensor [4]. TMV was used as enzyme nanocarrier for the enzyme glucose oxidase and penicillinase, respectively [3,4]. The sensitivity and detection limit of these biosensors, among others, depend on the density of TMVs on the sensor surface. The surface density of the immobilized TMVs is strongly influenced by the electrostatic interactions between the charged TMVs and sensor surface as well as by the inter-TMV-nanotubes repulsion, which could be changed by varying the pH value and the ionic strength of the TMV solution. In this study, we investigated an impact of the pH value and ionic strength of the TMV solution on the surface density of TMV nanotubes immobilized onto Ta2O5-gate capacitive field-effect electrolyte-insulator-semiconductor (EIS) sensors. Materials and Methods TMV particles modified with biotin-linker molecules (TMVBio), which serve as binding sites for streptavidin-conjugated enzymes, have been immobilized onto capacitive field-effect EIS sensors with Ta2O5 as transducer layer, as shown in Fig. 1. Immobilization was performed from TMVBiosolutions with different values of pH between pH 3.0 and pH 9.5 and ionic strength between 0.1 mM and 750 mM. The sensors have been electrochemically characterized before and after TMVBio immobilization by capacitance-voltage- and constant-capacitance methods, respectively. In addition, the density of the immobilized TMVs and morphology of the sensor surface has been investigated by means of scanning electron microscopy (SEM). Results and Conclusions The TMVBio density on the Ta2O5 sensor surface was influenced by varying the pH value (as exemplarily shown in Fig. 1) and ionic strength (not shown) of the TMVBio solution. The amplitude of the field-effect sensor signal correlates well with the density of the immobilized TMVBionanotubes. Thus, optimized conditions for the high-density immobilization of TMVBio nanotubes onto the Ta2O5 surface and thereby enhanced biosensing have been found. Details of the experiments and the obtained results will be presented and discussed. Acknowledgements The authors like to thank Dr. Claudia Koch and Rebecca Hummel, Stuttgart, for scientific and technical support.

Influence of Ti content on sensing performance of LaTixOy sensing membrane based electrolyte-insulator-semiconductor pH sensor

In this article, we explored the ability of inorganic LaTixOy (LTO) sensing materials grown on bare p-type substrate to be used as a pH electrolyte-insulator-semiconductor (EIS) sensor. Herein, we fabricated the LTO EIS sensors through the sol-gel synthesis technique under the 0.2 M condition with subsequent annealing at 800 °C in O2 ambient for 20 min. In order to avoid the generation of hydration layer of the gate insulator and reduce the formation of La-silicate, we doped Ti into the La2O3. The structural properties, surface morphologies, film micromorphologies, and chemical states of the sensing films have been investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy analyses, respectively. The sol-gel derived La2Ti2O7 sensing film exhibited super Nernstian pH sensitivity of 68.17 mV/pH with a linearity of 0.999. From the reliability point of view, the La2Ti2O7 sensor showed the best long-term stability with a lower drift rate of 0.21 mV/h as well as a smaller hysteresis voltage of 0.7 mV, as compared with other combination of LTO films. This finding behavior might be due to the high Ti content into the La2Ti2O7 film reducing the silicate layer and increasing the surface sites.

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.

Highly Sensitive Magnesium-Doped ZnO Nanorod pH Sensors Based on Electrolyte-Insulator-Semiconductor (EIS) Sensors.

For highly sensitive pH sensing, an electrolyte insulator semiconductor (EIS) device, based on ZnO nanorod-sensing membrane layers doped with magnesium, was proposed. ZnO nanorod samples prepared via a hydrothermal process with different Mg molar ratios (0–5%) were characterized to explore the impact of magnesium content on the structural and optical characteristics and sensing performance by X-ray diffraction analysis (XRD), atomic force microscopy (AFM), and photoluminescence (PL). The results indicated that the ZnO nanorods doped with 3% Mg had a high hydrogen ion sensitivity (83.77 mV/pH), linearity (96.06%), hysteresis (3 mV), and drift (0.218 mV/h) due to the improved crystalline quality and the surface hydroxyl group role of ZnO. In addition, the detection characteristics varied with the doping concentration and were suitable for developing biomedical detection applications with different detection elements.

Electron Density-Change in Semiconductor by Ion-Adsorption at Solid-Liquid Interface.

The change in electrical properties of electrodes by adsorption or desorption at interfaces is a well-known phenomenon required for signal production in electrically transduced sensing technologies. Furthermore, in terms of electrolyte-insulator-semiconductor (EIS) structure, several studies of energy conversion techniques focused on ion-adsorption at the solid-liquid interface have suggested that the electric signal is generated by ionovoltaic phenomena. However, finding substantial clues for the ion-adsorption phenomena in the EIS structure is still a difficult task because direct evidence for carrier accumulation in semiconductors by Coulomb interactions is insufficient. Here, a sophisticated Hall measurement system is demonstrated to quantitatively analyze accumulated electron density-change inside the semiconductor depending on the ion-adsorption at the solid-liquid interface. Also, an enhanced EIS-structured device is designed in an aqueous-soaked system that works with the ionovoltaic principle to monitor the ion-dynamics in liquid electrolyte media, interestingly confirming ion-concentration dependence and ion-specificity by generated peak voltages. This newly introduced peculiar method contributes to an in-depth understanding of the ionovoltaic phenomena in terms of carrier actions in the semiconductors and ionic behaviors in the aqueous-bulk phases, providing informative analysis about interfacial adsorptions that can expand the scope of ion-sensing platforms.

Biosensor Based on Self‐Assembled Films of Graphene Oxide and Polyaniline Using a Field‐Effect Device Platform

A new functionalization method to modify capacitive electrolyte–insulator–semiconductor (EIS) structures with nanofilms is presented. Layers of polyallylamine hydrochloride (PAH) and graphene oxide (GO) with the compound polyaniline:poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid) (PANI:PAAMPSA) are deposited onto a p‐Si/SiO2 chip using the layer‐by‐layer technique (LbL). Two different enzymes (urease and penicillinase) are separately immobilized on top of a five‐bilayer stack of the PAH:GO/PANI:PAAMPSA‐modified EIS chip, forming a biosensor for detection of urea and penicillin, respectively. Electrochemical characterization is performed by constant capacitance (ConCap) measurements, and the film morphology is characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). An increase in the average sensitivity of the modified biosensors (EIS–nanofilm–enzyme) of around 15% is found in relation to sensors, only carrying the enzyme but without the nanofilm (EIS–enzyme). In this sense, the nanofilm acts as a stable bioreceptor onto the EIS chip improving the output signal in terms of sensitivity and stability.

Capacitive Field‐Effect Biosensor Applied for the Detection of Acetoin in Alcoholic Beverages and Fermentation Broths

An acetoin biosensor based on a capacitive electrolyte–insulator–semiconductor (EIS) structure modified with the enzyme acetoin reductase, also known as butane‐2,3‐diol dehydrogenase (Bacillus clausii DSM 8716T), is applied for acetoin detection in beer, red wine, and fermentation broth samples for the first time. The EIS sensor consists of an Al/p‐Si/SiO2/Ta2O5 layer structure with immobilized acetoin reductase on top of the Ta2O5 transducer layer by means of crosslinking via glutaraldehyde. The unmodified and enzyme‐modified sensors are electrochemically characterized by means of leakage current, capacitance–voltage, and constant capacitance methods, respectively.

High Sensing Performance of Nanoimprinted HfO2 Sensing Membrane for Electrode-Insulator-Semiconductor pH Sensors

In this paper, HfO2 thin films as a sensing membrane deposited on nanoimprinted Si substrates by both atomic layer deposition (ALD) and sputtering methods were investigated for an electrolyte–insulator–semiconductor (EIS) pH sensor application. X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy were performed to examine the crystalline structures, chemical compositions and surface morphologies of the HfO2 sensing films, respectively. The structural properties of these HfO2 films were correlated to their sensing performances. Compared with the sputtering method, the unpatterned HfO2 sensing film by the ALD method showed a higher pH sensitivity of 55.06 mV pH−1, a lower drift rate of 0.31 mV h−1 and a smaller hysteresis voltage of 0.90 mV. The high pH sensitivity and good stability may be attributed to the increase in surface roughness and the reduction of a silicate layer caused by the ALD method. Additionally, the ALD-HfO2 EIS sensor deposited on the nanoimprinted Si substrate demonstrated a Nernstian pH response (61.36 mV pH−1) than that on an unpatterned Si substrate. For compatibility with advanced complementary metal–oxide–semiconductor technology, the ALD-HfO2 sensing film deposited on the nanoimprinted Si substrate is a promising candidate for pH sensing applications.

High-Performance Solution-Processed La₂Ti₂O₇ Sensing Film for a Capacitive Electrolyte–Insulator–Semiconductor pH Sensor

This work presents a comparison between the La2O3 and La2Ti2O7 sensing membranes for solution-based capacitive electrolyte-insulator-semiconductor (EIS) pH sensors. The La2O3 and La2Ti2O7 sensing membranes were fabricated through a simple spin-coating method. The structural characteristics of these sensing membranes were examined by using X-ray photoelectron spectroscopy and X-ray diffraction. The La2Ti2O7 EIS sensor made using the spin-coating process demonstrated a higher sensitivity of 70.60 mV/pH, a smaller drift rate of 0.14 mV/h and a lower hysteresis voltage of 1 mV, as compared to the La2O3 EIS sensor.

Microstructural properties and sensitive performance of TbTaxOy sensing membranes for electrolyte-insulator-semiconductor pH sensors

The sensitivity and stability of sensing film material are mainly factors to impede the practical applications of ion-selective field effect transistor (ISFET) or electrolyte-insulator-semiconductor (EIS) pH sensors. Metal oxide films have demonstrated great advances on achieving high-performance pH sensors. However, some metal oxide films with low sensitivity and poor reliability still hinder the biochemical, environmental and medical applications. Therefore, choosing a proper sensing film material plays a very important role for the fabrication of highly sensitive and reliable sensors. In this article, the novel TbTaxOy sensing membrane demonstrating a super-Nernstian pH response and excellent stabililty was developed for a solid-state pH sensor. We investigated the relationship between sensing performance and microstructural property of the TbTaxOy sensing membranes deposited onto Si substrates through a reactive co-sputtering system and subsequently post-deposition annealing (PDA) at four temperatures (from 600°C to 900°C). To check the influence of PDA treatment on the microstructural properties of the TbTaxOy films, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to manifest the morphological, microstructural, and chemical features, respectively, of these films. Among these PDA temperatures, the TbTaxOy EIS device after annealing at 900 °C exhibited the best sensing performances including, the super-Nernstian sensitivity (67.04 mV/pH), the lowest drift rate (0.12 mV/h), and the smallest hysteresis voltage (1 mV). Probably, this PDA temperature can enhance the surface roughness, form the stoichiometry of TbTaO4 film, and reduce the crystal defects. The super-Nernstian response may be contributed to a variation in the oxidation state of Tb ion changing from Tb3+ to Tb2+, thereby generating two H+ ions and one electron transferred in the chemical reaction. Moreover, this TbTaxOy sensing film demonstrates better sensing performance compared to previously fabricated sensing materials available in the literature.

Internal Electrical Noises of BioFET Sensors Based on Various Architectures

The results of a comparative literature analysis of internal electrical noises and signal-to-noise ratio for nanoscale BioFET (biological field-effect transistor) and DNA (deoxyribonucleic acid) sensors based on different architectures MIS (metal-insulator-semiconductor), EIS (electrolyte-insulator-semi-conductor) and ISFET (ion-selective field-effect transistor) are presented. Main types, models and mechanisms of internal noises of bio- & chemical field-effect based sensors are analyzed, summarized and presented. For the first time, corresponding detail electrical equivalent circuits were built to calculate the spectral densities of noises generated in the active part of a solid (semiconductor, dielectric) and in an aqueous solution for MIS, EIS and ISFET structures based sensors. Complete expressions are obtained for the rms (root mean square) value of the noise current (or voltage), as well as the noise spectral densities for the architectures under study. The miniaturization of biosensors leads to a decrease in the level of the useful signal-current. For successful operation of the sensor, it is necessary to ensure a high value of the SNR (signal-to-noise ratio). In case of weak useful signals, it is necessary to reduce the level of internal electrical noise. This work is devoted to a detailed study of the types and mechanisms of internal electrical noises in specific biosensor architectures.

Zinc Oxide Nanorod Electrolyte–Insulator– Semiconductor Sensor for Enhanced 2-Methoxyethanol Selectivity

In this work, we demonstrate 2-methoxyethanol-sensitive ZnO nanorods based on electrolyte–insulator–semiconductor (EIS) structure. The hydrothermal method is employed to synthesize the ZnO nanorods on Si substrate. The structural, morphological, compositional, and optical features of ZnO nanorods are analyzed, confirming the formation of hexagonal structure in ZnO nanorods. We investigate the sensing performance of EIS-based ZnO nanorods for the detection of various organic solvents (2-methoxyethanol, ethanol, and methanol) at various concentrations. As a chemical sensor, these EIS-based ZnO nanorods are capable of generating a different exposure response to 2-methoxyethanol than to methanol than to ethanol. The highest sensitivity, stability, and reproducibility are with respect to 2-methoxyethanol, making the ZnO nanorods prepared here a promising sensing membrane for 2-methoxyethanol.

Capacitive Field-Effect EIS Chemical Sensors and Biosensors: A Status Report.

Electrolyte-insulator-semiconductor (EIS) field-effect sensors belong to a new generation of electronic chips for biochemical sensing, enabling a direct electronic readout. The review gives an overview on recent advances and current trends in the research and development of chemical sensors and biosensors based on the capacitive field-effect EIS structure—the simplest field-effect device, which represents a biochemically sensitive capacitor. Fundamental concepts, physicochemical phenomena underlying the transduction mechanism and application of capacitive EIS sensors for the detection of pH, ion concentrations, and enzymatic reactions, as well as the label-free detection of charged molecules (nucleic acids, proteins, and polyelectrolytes) and nanoparticles, are presented and discussed.

Multianalyte Mg-Doped InGaZnO Electrolyte-Insulator-Semiconductor Biosensors and Multiple Material Characterizations of Membrane Nanostructures

Mg-doped InGaZnO (IGZO) electrolyte-insulator-semiconductor (EIS) biosensors were fabricated with multianalyte sensing capability. H+, Na+, and K+ ion sensing behaviors were examined and urea, glucose, and creatine detection through enzymatic reaction was performed. Results indicate that Mg doping IGZO membranes with appropriate annealing had better multi-sensing performance than that of regular membranes. Moreover, multiple material characterizations of membrane nanostructures, including of atomic force microscopy (AFM) surface morphology evaluation, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were conducted. Material crystalline and chemical binding properties were also performed, including energy dispersive X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDX), and photoluminescence (PL). These detailed studies indicate that the incorporation of Mg in the membrane can form MgO crystals to enhance compactness, crystallization and grainization, which can reinforce the surface sites on the membrane. Mg-doped IGZO EIS multianalyte biosensors show promise for future portable and wearable biosensing applications.

Effects of CF4 Plasma Treatment on Indium Gallium Oxide and Ti-doped Indium Gallium Oxide Sensing Membranes in Electrolyte–Insulator–Semiconductors

Electrolyte–insulator–semiconductor (EIS) sensors, used in applications such as pH sensing and sodium ion sensing, are the most basic type of ion-sensitive field-effect transistor (ISFET) membranes. Currently, some of the most popular techniques for synthesizing such sensors are chemical vapor deposition, reactive sputtering and sol-gel deposition. However, there are certain limitations on such techniques, such as reliability concerns and isolation problems. In this research, a novel design of an EIS membrane consisting of an optical material of indium gallium oxide (IGO) was demonstrated. Compared with conventional treatment such as annealing, Ti doping and CF4 plasma treatment were incorporated in the fabrication of the film. Because of the effective treatment of doping and plasma treatment, the defects were mitigated and the membrane capacitance was boosted. Therefore, the pH sensitivity can be increased up to 60.8 mV/pH. In addition, the hysteresis voltage can be improved down to 2.1 mV, and the drift voltage can be suppressed to as low as 0.23 mV/h. IGO-based membranes are promising for future high-sensitivity and -stability devices integrated with optical applications.

Characterizations of Electrolyte–Insulator–Semiconductor Sensors With Array Wells and a Stack-Sensing Membrane

This study addressed the characteristics of electrolyte-insulator-semiconductor (EIS) pH sensors with array wells and sensing membrane stacks of 3-aminopropyltriethoxysilane (APTES)/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The arrays of square wells were fabricated by nanoimprint lithography (NIL) and investigated using an atomic force microscope (AFM). The surface chemical properties of molecular APTES on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were investigated by water contact angle (WCA) studies and X-ray photoelectron spectroscopy (XPS). The sensitivity, hysteresis, and drift of EIS sensors were studied. All of the sensing properties were greatly enhanced by using the techniques we propose. With these improvements, high-performance sensors can be fabricated for future biochemical applications.