Mechanism and optimization of pH sensing using SnO2 nanobelt field effect transistors

Iridium oxide and cobalt hydroxide microfluidic-based potentiometric pH sensor

Microliter volume pH determination is of great importance in the biomedical and industrial applications. The current available pH meter and measurement techniques are hard to reach the high demand of microliter volume pH determination in a repeatable, stable, and sensitivity manner. This work aims to fill the gap of microliter volume pH measurements while maintaining good sensing performance. The electrodeposited iridium oxide and cobalt hydroxide along with gold electrode served as working, counter, and reference electrode, respectively, for 10-12 μL volume pH measurements with Nernst constant of 55.9 ± 4.4 mV/pH. The electrodeposited thin film was further characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Raman spectrometry, etc. to confirm its morphology and composition. The constructed pH sensor was used for human serum sample measurements to confirm the suitability of future applications. The results show that it has only 0.80% variation compared to a commercial pH meter with a limit of detection (LOD, or resolution) of ± 0.01 pH. It holds a great potential to be used in the future for microliter volume in situ pH measurements.

Metal-Oxide FET Biosensor for Point-of-Care Testing: Overview and Perspective

Metal-oxide semiconducting materials are promising for building high-performance field-effect transistor (FET) based biochemical sensors. The existence of well-established top-down scalable manufacturing processes enables the reliable production of cost-effective yet high-performance sensors, two key considerations toward the translation of such devices in real-life applications. Metal-oxide semiconductor FET biochemical sensors are especially well-suited to the development of Point-of-Care testing (PoCT) devices, as illustrated by the rapidly growing body of reports in the field. Yet, metal-oxide semiconductor FET sensors remain confined to date, mainly in academia. Toward accelerating the real-life translation of this exciting technology, we review the current literature and discuss the critical features underpinning the successful development of metal-oxide semiconductor FET-based PoCT devices that meet the stringent performance, manufacturing, and regulatory requirements of PoCT.

Fabrication of a Robust In2O3 Nanolines FET Device as a Biosensor Platform

Field-effect transistors (FETs) are attractive biosensor platforms for rapid and accurate detection of various analytes through surface immobilization of specific bio-receptors. Since it is difficult to maintain the electrical stability of semiconductors of sensing channel under physiological conditions for long periods, passivation by a stable metal oxide dielectric layer, such as Al₂O₃ or HfO₂, is currently used as a common method to prevent damage. However, protecting the sensing channel by passivation has the disadvantage that the distance between the target and the conductive channel increases, and the sensing signal will be degraded by Debye shielding. Even though many efforts use semiconductor materials directly as channels for biosensors, the electrical stability of semiconductors in the physiological environments has rarely been studied. In this work, an In₂O₃ nanolines FET device with high robustness in artificial physiological solution of phosphate buffered saline (PBS) was fabricated and used as a platform for biosensors without employing passivation on the sensing channel. The FET device demonstrated reproducibility with an average threshold voltage (V_TH) of 5.235 V and a standard deviation (SD) of 0.382 V. We tested the robustness of the In₂O₃ nanolines FET device in PBS solution and found that the device had a long-term electrical stability in PBS with more than 9 days’ exposure. Finally, we demonstrated its applicability as a biosensor platform by testing the biosensing performance towards miR-21 targets after immobilizing the phosphonic acid terminated DNA probes. Since the surface immobilization of multiple bioreceptors is feasible, we demonstrate that the robust In₂O₃ FET device can be an excellent biosensor platform for biosensors.

Enhanced chemiresistive sensing performance of well-defined porous CuO-doped ZnO nanobelts toward VOCs

Although the post-doping approach as a typical and effective method has been widely employed to improve the gas sensing performance of nanostructured metal oxides, it easily breaks their porous nanostructures. Herein a facile partial cation-exchange strategy combined with thermal oxidation has been developed to prepare porous CuO-doped ZnO nanobelts. Using ZnSe·0.5N₂H₄ nanobelts as the precursor template, Cu₂Se-doped precursor nanobelts were obtained with Zn²⁺ cations partially exchanged by Cu²⁺ cations. After annealing in air, they are further oxidized into well-defined porous CuO-doped ZnO nanobelts. Through manipulating the amount of exchanged Cu²⁺ cations, the CuO-doping concentration can be precisely tuned. Based on the assembly approach and in situ thermal oxidation, a uniform and stable sensing film consisting of porous CuO-doped nanobelts was fabricated. Compared with pristine porous ZnO nanobelts, the as-prepared porous CuO-doped nanobelts with p-type CuO|n-type ZnO heterojunctions exhibited better sensing performance toward volatile organic compounds (VOCs). Especially for 3 at% CuO-doped porous ZnO nanobelts, the relative responses toward 100 ppm of ethanol, acetone and formaldehyde were greatly enhanced more than two, four and ten times, respectively. Due to the porous structure, they also displayed a fast response/recovery time. Finally, this enhanced sensing mechanism was discussed for porous CuO-doped ZnO nanobelts.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO₂ nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO₂ nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10^-3 to 10^-4 Pa. This contradicts the conventional trend of n-type SnO₂ semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

Multi-Organs-on-Chips: Towards Long-Term Biomedical Investigations

With advantageous features such as minimizing the cost, time, and sample size requirements, organ-on-a-chip (OOC) systems have garnered enormous interest from researchers for their ability for real-time monitoring of physical parameters by mimicking the in vivo microenvironment and the precise responses of xenobiotics, i.e., drug efficacy and toxicity over conventional two-dimensional (2D) and three-dimensional (3D) cell cultures, as well as animal models. Recent advancements of OOC systems have evidenced the fabrication of 'multi-organ-on-chip' (MOC) models, which connect separated organ chambers together to resemble an ideal pharmacokinetic and pharmacodynamic (PK-PD) model for monitoring the complex interactions between multiple organs and the resultant dynamic responses of multiple organs to pharmaceutical compounds. Numerous varieties of MOC systems have been proposed, mainly focusing on the construction of these multi-organ models, while there are only few studies on how to realize continual, automated, and stable testing, which still remains a significant challenge in the development process of MOCs. Herein, this review emphasizes the recent advancements in realizing long-term testing of MOCs to promote their capability for real-time monitoring of multi-organ interactions and chronic cellular reactions more accurately and steadily over the available chip models. Efforts in this field are still ongoing for better performance in the assessment of preclinical attributes for a new chemical entity. Further, we give a brief overview on the various biomedical applications of long-term testing in MOCs, including several proposed applications and their potential utilization in the future. Finally, we summarize with perspectives.

Large-Area, Ultrathin Metal-Oxide Semiconductor Nanoribbon Arrays Fabricated by Chemical Lift-Off Lithography

Nanoribbon- and nanowire-based field-effect transistors (FETs) have attracted significant attention due to their high surface-to-volume ratios, which make them effective as chemical and biological sensors. However, the conventional nanofabrication of these devices is challenging and costly, posing a major barrier to widespread use. We report a high-throughput approach for producing arrays of ultrathin (∼3 nm) In2O3 nanoribbon FETs at the wafer scale. Uniform films of semiconducting In2O3 were prepared on Si/SiO2 surfaces via a sol-gel process prior to depositing Au/Ti metal layers. Commercially available high-definition digital versatile discs were employed as low-cost, large-area templates to prepare polymeric stamps for chemical lift-off lithography, which selectively removed molecules from self-assembled monolayers functionalizing the outermost Au surfaces. Nanoscale chemical patterns, consisting of one-dimensional lines (200 nm wide and 400 nm pitch) extending over centimeter length scales, were etched into the metal layers using the remaining monolayer regions as resists. Subsequent etch processes transferred the patterns into the underlying In2O3 films before the removal of the protective organic and metal coatings, revealing large-area nanoribbon arrays. We employed nanoribbons in semiconducting FET channels, achieving current on-to-off ratios over 107 and carrier mobilities up to 13.7 cm2 V-1 s-1. Nanofabricated structures, such as In2O3 nanoribbons and others, will be useful in nanoelectronics and biosensors. The technique demonstrated here will enable these applications and expand low-cost, large-area patterning strategies to enable a variety of materials and design geometries in nanoelectronics.

Facile fabrication of wire-type indium gallium zinc oxide thin-film transistors applicable to ultrasensitive flexible sensors

We fabricated wire-type indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) using a self-formed cracked template based on a lift-off process. The electrical characteristics of wire-type IGZO TFTs could be controlled by changing the width and density of IGZO wires through varying the coating conditions of template solution or multi-stacking additional layers. The fabricated wire-type devices were applied to sensors after functionalizing the surface. The wire-type pH sensor showed a sensitivity of 45.4 mV/pH, and this value was an improved sensitivity compared with that of the film-type device (27.6 mV/pH). Similarly, when the wire-type device was used as a glucose sensor, it showed more variation in electrical characteristics than the film-type device. The improved sensing properties resulted from the large surface area of the wire-type device compared with that of the film-type device. In addition, we fabricated wire-type IGZO TFTs on flexible substrates and confirmed that such structures were very resistant to mechanical stresses at a bending radius of 10 mm.

Molecular recognition by synthetic receptors: Application in field-effect transistor based chemosensing

Molecular recognition, i.e., ability of one molecule to recognize another through weak bonding interactions, is one of the bases of life. It is often implemented to sensing systems of high merits. Preferential recognition of the analyte (guest) by the receptor (host) induces changes in physicochemical properties of the sensing system. These changes are measured by using suitable signal transducers. Because of possibility of miniaturization, fast response, and high sensitivity, field-effect transistors (FETs) are more frequently being used for that purpose. A FET combined with a biological material offers the potential to overcome many challenges approached in sensing. However, low stability of biological materials under measurement conditions is a serious problem. To circumvent this problem, synthetic receptors were integrated with the gate surface of FETs to provide robust performance. In the present critical review, the approach utilized to devise chemosensors integrating synthetic receptors and FET transduction is discussed in detail. The progress in this field was summarized and important outcome was provided.

Molecular gated-AlGaN/GaN high electron mobility transistor for pH detection

A molecular gated-AlGaN/GaN high electron mobility transistor has been developed for pH detection.

Quasi-Two-Dimensional Metal Oxide Semiconductors Based Ultrasensitive Potentiometric Biosensors

Ultrasensitive field-effect transistor-based biosensors using quasi-two-dimensional metal oxide semiconductors were demonstrated. Quasi-two-dimensional low-dimensional metal oxide semiconductors were highly sensitive to electrical perturbations at the semiconductor-bio interface and showed competitive sensitivity compared with other nanomaterial-based biosensors. Also, the solution process made our platform simple and highly reproducible, which was favorable compared with other nanobioelectronics. A quasi-two-dimensional In₂O₃-based pH sensor showed a small detection limit of 0.0005 pH and detected the glucose concentration at femtomolar levels. Detailed electrical characterization unveiled how the device's parameters affect the biosensor sensitivity, and lowest detectable charge was extrapolated, which was consistent with the experimental data.

Modulation of electronic properties of tin oxide nanobelts via thermal control of surface oxygen defects

Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO₂) nanobelts (NB) synthesized by the vapor-liquid-solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 10⁵, likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.

Improved sensing characteristics of dual-gate transistor sensor using silicon nanowire arrays defined by nanoimprint lithography

This work describes the construction of a sensitive, stable, and label-free sensor based on a dual-gate field-effect transistor (DG FET), in which uniformly distributed and size-controlled silicon nanowire (SiNW) arrays by nanoimprint lithography act as conductor channels. Compared to previous DG FETs with a planar-type silicon channel layer, the constructed SiNW DG FETs exhibited superior electrical properties including a higher capacitive-coupling ratio of 18.0 and a lower off-state leakage current under high-temperature stress. In addition, while the conventional planar single-gate (SG) FET- and planar DG FET-based pH sensors showed the sensitivities of 56.7 mV/pH and 439.3 mV/pH, respectively, the SiNW DG FET-based pH sensors showed not only a higher sensitivity of 984.1 mV/pH, but also a lower drift rate of 0.8% for pH-sensitivity. This demonstrates that the SiNW DG FETs simultaneously achieve high sensitivity and stability, with significant potential for future biosensing applications.

A microfluidic optical platform for real-time monitoring of pH and oxygen in microfluidic bioreactors and organ-on-chip devices

There is a growing interest to develop microfluidic bioreactors and organ-on-chip platforms with integrated sensors to monitor their physicochemical properties and to maintain a well-controlled microenvironment for cultured organoids. Conventional sensing devices cannot be easily integrated with microfluidic organ-on-chip systems with low-volume bioreactors for continual monitoring. This paper reports on the development of a multi-analyte optical sensing module for dynamic measurements of pH and dissolved oxygen levels in the culture medium. The sensing system was constructed using low-cost electro-optics including light-emitting diodes and silicon photodiodes. The sensing module includes an optically transparent window for measuring light intensity, and the module could be connected directly to a perfusion bioreactor without any specific modifications to the microfluidic device design. A compact, user-friendly, and low-cost electronic interface was developed to control the optical transducer and signal acquisition from photodiodes. The platform enabled convenient integration of the optical sensing module with a microfluidic bioreactor. Human dermal fibroblasts were cultivated in the bioreactor, and the values of pH and dissolved oxygen levels in the flowing culture medium were measured continuously for up to 3 days. Our integrated microfluidic system provides a new analytical platform with ease of fabrication and operation, which can be adapted for applications in various microfluidic cell culture and organ-on-chip devices.

Impedimetric blood pH sensor based on MoS2–Nafion coated microelectrode

A compact microelectrode based impedimetric pH sensor fabricated by simple and cost effective materials and techniques for point of care application.

Printable Ultrathin Metal Oxide Semiconductor-Based Conformal Biosensors

Conformal bioelectronics enable wearable, noninvasive, and health-monitoring platforms. We demonstrate a simple and straightforward method for producing thin, sensitive In2O3-based conformal biosensors based on field-effect transistors using facile solution-based processing. One-step coating via aqueous In2O3 solution resulted in ultrathin (3.5 nm), high-density, uniform films over large areas. Conformal In2O3-based biosensors on ultrathin polyimide films displayed good device performance, low mechanical stress, and highly conformal contact determined using polydimethylsiloxane artificial skin having complex curvilinear surfaces or an artificial eye. Immobilized In2O3 field-effect transistors with self-assembled monolayers of NH2-terminated silanes functioned as pH sensors. Functionalization with glucose oxidase enabled d-glucose detection at physiologically relevant levels. The conformal ultrathin field-effect transistor biosensors developed here offer new opportunities for future wearable human technologies.

A Highly Responsive Silicon Nanowire/Amplifier MOSFET Hybrid Biosensor

This study demonstrates a hybrid biosensor comprised of a silicon nanowire (SiNW) integrated with an amplifier MOSFET to improve the current response of field-effect-transistor (FET)-based biosensors. The hybrid biosensor is fabricated using conventional CMOS technology, which has the potential advantage of high density and low noise performance. The biosensor shows a current response of 5.74 decades per pH for pH detection, which is 2.5 × 10⁵ times larger than that of a single SiNW sensor. In addition, we demonstrate charged polymer detection using the biosensor, with a high current change of 4.5 × 10⁵ with a 500 nM concentration of poly(allylamine hydrochloride). In addition, we demonstrate a wide dynamic range can be obtained by adjusting the liquid gate voltage. We expect that this biosensor will be advantageous and practical for biosensor applications which requires lower noise, high speed, and high density.

A device design of an integrated CMOS poly-silicon biosensor-on-chip to enhance performance of biomolecular analytes in serum samples

For on-site clinical diagnosis of biomolecules, the detection performances of most point-of-care (POC) biosensor devices are limited by undesired cross-detection of other non-analyte proteins in patient serum samples and other complex samples. To conquer this obstacle, this work presents a fully integrated bottom-gate poly-silcion nanowire (polySi NW) biosensor system-on-chip (SoC) to enhance the detection performance of cardiac-specific troponin-I (cTnI) concentration levels in serum samples. By applying proper electrical potential at the bottom gate under polySi NW biosensor, the biosensor response to cTnI biomarker can be improved by at least 16 fold in 50% phantom serum samples. The experimental result shows its detection range is from 3.2 × 10(-13)M(mol l(-1)) to 3.2 × 10(-10)M. This enhancement can be attributed to the electrostatic interactions between target biomolecules and voltage-applied bottom gate electrodes. This is the first time that a polySi NW CMOS biosensor chip has shown feasibilities to detect specific biomarkers in serum samples. Therefore, the developed technology paves the way toward on-field applications of CMOS compatible SiNW biosensing technologies and it can be employed for future biomolecular analysis in on-site serum diagnosis applications.

Low voltage operating field effect transistors with composite In2O3-ZnO-ZnGa2O4 nanofiber network as active channel layer

Field effect transistors (FETs), incorporating metal-oxide nanofibers as the active conductive channel, have the potential for driving the widespread application of nanowire or nanofiber FETs-based electronics. Here we report on low voltage FETs with integrated electrospun In2O3-ZnO-ZnGa2O4 composite fiber channel layers and high-K dielectric (MgO)0.3-(Bi1.5Zn1.0Nb1.5O7)0.7 gate insulator and compare their performance against FETs utilizing conductive single phase, polycrystalline ZnO or In2O3 channel layers. The polycrystalline In2O3-ZnO-ZnGa2O4 composite fibers provide superior performance with high field effect mobility (∼7.04 cm2 V(-1) s(-1)), low subthreshold swing (390 mV/dec), and low threshold voltage (1.0 V) combined with excellent saturation, likely resulting from the effective blocking of high current-flow through the In2O3 and ZnO nanocrystallites by the insulating spinel ZnGa2O4 phase. The microstructural evolution of the individual In2O3, ZnO, and ZnGa2O4 phases in composite fibers is clearly observed by high resolution TEM. A systematic examination of channel area coverage, ranging from single fiber to over 90% coverage, demonstrates that low coverage results in relatively low current outputs and reduced reproducibility which we attribute to the difficulty in positioning fibers and fiber length control. On the other hand, those with ∼80% coverage exhibited high field effect mobility, high on/off current ratios (>10(5)), and negligible hysteresis following 15 sweep voltage cycles. A special feature of this work is the application of the FETs to modulate the properties of complex polycrystalline nanocomposite channels.

pH-triggered conduction of amine-functionalized single ZnO wire integrated on a customized nanogap electronic platform

The electrical conductance response of single ZnO microwire functionalized with amine-groups was tested upon an acid pH variation of a solution environment after integration on a customized gold electrode array chip. ZnO microwires were easily synthesized by hydrothermal route and chemically functionalized with aminopropyl groups. Single wires were deposited from the solution and then oriented through dielectrophoresis across eight nanogap gold electrodes on a platform single chip. Therefore, eight functionalized ZnO microwire-gold junctions were formed at the same time, and being integrated on an ad hoc electronic platform, they were ready for testing without any further treatment. Experimental and simulation studies confirmed the high pH-responsive behavior of the amine-modified ZnO-gold junctions, obtaining in a simple and reproducible way a ready-to-use device for pH detection in the acidic range. We also compared this performance to bare ZnO wires on the same electronic platform, showing the superiority in pH response of the amine-functionalized material.

Performance limitations for nanowire/nanoribbon biosensors

Field-effect transistor-based biosensors (bioFETs) have shown great promise in the field of fast, ultra-sensitive, label-free detection of biomolecules. Reliability and accuracy, when trying to measure small concentrations, is of paramount importance for the translation of these research devices into the clinical setting. Our knowledge and experience with these sensors has reached a stage where we are able to identify three main aspects of bioFET sensing that currently limit their applications. By considering the intrinsic device noise as a limitation to the smallest measurable signal, we show how various parameters, processing steps and surface modifications, affect the limit of detection. We also introduce the signal-to-noise ratio of bioFETs as a universal performance metric, which allows us to gain better insight into the design of more sensitive devices. Another aspect that places a limit on the performance of bioFETs is screening by the electrolyte environment, which reduces the signal that could be potentially measured. Alternative functionalization and detection schemes that could enable the use of these charge-based sensors in physiological conditions are highlighted. Finally, the binding kinetics of the receptor-analyte system are considered, both in the context of extracting information about molecular interactions using the bioFET sensor platform and as a fundamental limitation to the number of molecules that bind to the sensor surface at steady-state conditions and to the signal that is generated. Some strategies to overcome these limitations are also proposed. Taken together, these performance-limiting issues, if solved, would bring bioFET sensors closer to clinical applications.

Rational design of sub-parts per million specific gas sensors array based on metal nanoparticles decorated nanowire enhancement-mode transistors

"One key to one lock" hybrid sensor configuration is rationally designed and demonstrated as a direct effective route for the target-gas-specific, highly sensitive, and promptly responsive chemical gas sensing for room temperature operation in a complex ambient background. The design concept is based on three criteria: (i) quasi-one-dimensional metal oxide nanostructures as the sensing platform which exhibits good electron mobility and chemical and thermal stability; (ii) deep enhancement-mode field-effect transistors (E-mode FETs) with appropriate threshold voltages to suppress the nonspecific sensitivity to all gases (decouple the selectivity and sensitivity away from nanowires); (iii) metal nanoparticle decoration onto the nanostructure surface to introduce the gas specific selectivity and sensitivity to the sensing platform. In this work, using Mg-doped In2O3 nanowire E-mode FET sensor arrays decorated with various discrete metal nanoparticles (i.e., Au, Ag, and Pt) as illustrative prototypes here further confirms the feasibility of this design. Particularly, the Au decorated sensor arrays exhibit more than 3 orders of magnitude response to the exposure of 100 ppm CO among a mixture of gases at room temperature. The corresponding response time and detection limit are as low as ∼4 s and ∼500 ppb, respectively. All of these could have important implications for this "one key to one lock" hybrid sensor configuration which potentially open up a rational avenue to the design of advanced-generation chemical sensors with unprecedented selectivity and sensitivity.

Controllable low-temperature chemical vapor deposition growth and morphology dependent field emission property of SnO2 nanocone arrays with different morphologies

Vertically aligned SnO2 nanocones with different morphologies have been directly grown on fluorine-doped tin oxide (FTO) glass substrates in a large area by heating a mixture of stannous chloride dihydrate (SnCl2·2H2O) and anhydrous zinc chloride (ZnCl2) at 600 °C in air. Control over the SnO2 nanocone arrays with different morphologies is achieved by adjusting the heat treatment time. The SnO2 nanocones are single crystalline with the tetragonal structure. A single-layer SnO2 nanoparticle film is first formed via the vapor-solid (VS) process due to the decentralization function of ZnCl2 vapor, and the SnO2 nanoparticles served as seeds and grew into nanocone arrays via the VS process. The sharp-tipped nanostructure formation may originate from a concentration gradient of reactant in the growth process. The as-obtained whiskerlike nanocone arrays exhibit enhanced field emission properties in comparison with typical nanoconelike structure arrays and other SnO2 nanostructured materials reported previously, and the turn-on field and field-enhancement factor is 1.19 V/μm and 3110, respectively. The experimental result is consistent with the Utsumi's relative figure of merit for pillar-shaped emitters.

Controllable synthesis of recyclable core-shell γ-Fe2O3@SnO2 hollow nanoparticles with enhanced photocatalytic and gas sensing properties

Composite materials containing different components with well-defined structures may cooperatively enhance their performance and extend their applications. In this work, core-shell γ-Fe2O3@SnO2 hollow nanoparticles (NPs) were synthesized by a low-cost and environmentally friendly seed-mediated hydrothermal method. Firstly, the γ-Fe2O3 hollow NPs were synthesized by a template-free method. Then they were used as the cores for the growth of SnO2 shells. The thickness of the shell can be simply tailored by controlling the reaction time. Various techniques, including SEM, XRD, TEM and HRTEM, were employed to investigate the morphology, structure and formation process of the special core-shell hollow structures. The combination of magnetic semiconductor (γ-Fe2O3) and wide band-gap semiconductor (SnO2) endowed them with great potential to be used as recyclable photocatalysts. Experiments on photo-degradation of Rhodamin B (RhB) dye in the presence of the samples showed that the hybrid structures possessed higher photocatalytic activities than the monomer structures of SnO2 and γ-Fe2O3 materials indicating a strong coupling enhancement effect between the wide and narrow band-gap semiconductors. Moreover, the gas sensing tests of the γ-Fe2O3@SnO2 hollow NPs revealed that the samples exhibited fast response and recovery rates, which enable them to be promising materials for gas sensors.

Dynamic pH mapping in microfluidic devices by integrating adaptive coatings based on polyaniline with colorimetric imaging techniques

In this paper we present a microfluidic device that has integrated pH optical sensing capabilities based on polyaniline. The optical properties of polyaniline coatings change in response to the pH of the solution that is flushed inside the microchannel offering the possibility of monitoring pH in continuous flow over a wide pH range throughout the entire channel length. This work also features an innovative detection system for spatial localisation of chemical pH gradients along microfluidic channels through the use of a low cost optical device. Specifically, the use of a microfluidic channel coated with polyaniline is shown to respond colorimetrically to pH and that effect is detected by the detection system, even when pH gradients are induced within the channel. This study explores the capability of detecting this gradient by means of imaging techniques and the mapping of the camera's response to its corresponding pH after a successful calibration process. The provision of an inherently responsive channel means that changes in the pH of a sample moving through the system can be detected dynamically using digital imaging along the entire channel length in real time, without the need to add reagents to the sample. This approach is generic and can be applied to other chemically responsive coatings immobilised on microchannels.

Nano-environment effects on the luminescence properties of Eu(3+)-doped nanocrystalline SnO2 thin films

Nanocrystalline tin (IV) oxide thin films doped with Eu(3+) ions are synthesized using a simple spin-coating method followed by postannealing in an O(2) flow at 700 °C. Transmission electron microscopy and x-ray photoelectron spectroscopy studies illustrate the incorporation of Eu(3+) ions in the films with a high atomic percentage of 2.7%-7.7%, which is found to be linearly dependent on the initial concentration of Eu(3+) in the precursor solution. Glancing incidence x-ray diffraction results show that the crystalline grain sizes decrease with increasing the Eu(3+) concentration and decreasing the postannealing temperature with the emergence of the Eu(2)Sn(2)O(7) phase at high Eu(3+) concentrations (≥5.3 at.%). Luminescence spectra of these doped samples show the characteristic narrow-band magnetic dipole emission at 593 nm and electric dipole emission at 614 nm of the Eu(3+) ions, arising from UV absorption at the SnO(2) band-edge followed by energy transfer to the emission centers. Manipulating the crystallite size, composition, and defect density of the samples greatly affects the absorption edge, energy transfer, and therefore the emission spectra. These modifications in the environment of the Eu(3+) ions allow the emission to be tuned from pure orange characteristic Eu(3+) emission to the broadband emission corresponding to the combination of strong characteristic Eu(3+) emission with the intense defect emissions.

Graphene transistor as a probe for streaming potential

We explore the dependence of electrical transport in a graphene field effect transistor (GraFET) on the flow of water/sodium chloride electrolyte within the immediate vicinity of that transistor. We find large and reproducible shifts in the charge neutrality point of GraFETs that are dependent on the liquid velocity and the ion concentration. We show that these shifts are consistent with the variation of the local electrochemical potential of the liquid next to graphene that are caused by the fluid flow (streaming potential). Furthermore, we utilize the sensitivity of electrical transport in GraFETs to the parameters of the fluid flow to demonstrate graphene-based mass flow and ionic concentration sensing. We successfully detect a flow as small as ∼70 nL/min and detect a change in the ionic concentration as small as ∼40 nM.

Microfabricated electrochemical cell-based biosensors for analysis of living cells in vitro

Cellular biochemical parameters can be used to reveal the physiological and functional information of various cells. Due to demonstrated high accuracy and non-invasiveness, electrochemical detection methods have been used for cell-based investigation. When combined with improved biosensor design and advanced measurement systems, the on-line biochemical analysis of living cells in vitro has been applied for biological mechanism study, drug screening and even environmental monitoring. In recent decades, new types of miniaturized electrochemical biosensor are emerging with the development of microfabrication technology. This review aims to give an overview of the microfabricated electrochemical cell-based biosensors, such as microelectrode arrays (MEA), the electric cell-substrate impedance sensing (ECIS) technique, and the light addressable potentiometric sensor (LAPS). The details in their working principles, measurement systems, and applications in cell monitoring are covered. Driven by the need for high throughput and multi-parameter detection proposed by biomedicine, the development trends of electrochemical cell-based biosensors are also introduced, including newly developed integrated biosensors, and the application of nanotechnology and microfluidic technology.

Optimal signal-to-noise ratio for silicon nanowire biochemical sensors

The signal-to-noise ratio (SNR) for silicon nanowire field-effect transistors operated in an electrolyte environment is an essential figure-of-merit to characterize and compare the detection limit of such devices when used in an exposed channel configuration as biochemical sensors. We employ low frequency noise measurements to determine the regime for optimal SNR. We find that SNR is not significantly affected by the electrolyte concentration, composition, or pH, leading us to conclude that the major contributions to the SNR come from the intrinsic device quality. The results presented here show that SNR is maximized at the peak transconductance.

Nanowires assembled SnO2 nanopolyhedrons with enhanced gas sensing properties

Self-assembly of one-dimensional nanoscale building blocks into functional 2-D or 3-D complex superstructures has stimulated a great deal of interest. We report the synthesis and characterization of nanopolyhedrons assembled from ultrathin SnO(2) nanowires based on the sodium dodecyl sulfate (SDS)-assisted hydrothermal process. As-synthesized SnO(2) nanopolyhedrons have uniform diameters around 300 nm and are self-assembled by numerous ultrathin SnO(2) nanowires with diameters of 5-10 nm. The growth mechanism was also studied by investigating the samples synthesized at different reaction time. Thin films of the assembled SnO(2) nanopolyhedrons were configured as high performance sensors to detect methanol, ethanol, and acetone, which exhibited 1 ppm sensitivity, very fast response and recovery times (several seconds for different gases with concentrations of 1-200 ppm) to all the target gases and highly selective detection to acetone.

Bionanoelectronics

Every cell in a living organisms performs a complex array of functions using a vast arsenal of proteins, ion channels, pumps, motors, signaling molecules, and cargo carriers. With all the progress that humankind has made to date in the development of sophisticated machinery and computing capabilities, understanding and communicating with living systems on that level of complexity lags behind. A breakthrough in these capabilities could only come if a way is found to integrate biological components into artificial devices. The central obstacle for this vision of bionanoelectronics is the absence of a versatile interface that facilitates two-way communication between biological and electronic structures. 1D nanomaterials, such as nanotubes and nanowires, open up the possibility of constructing tight interfaces that could enable such bidirectional flow of information. This report discusses the overall progress in building such interfaces on the level of individual proteins and whole cells and focuses on the latest efforts to create device platforms that integrate membrane proteins, channels, and pumps with nanowire bioelectronics.

Degradation pattern of SnO(2) nanowire field effect transistors

The degradation pattern of SnO(2) nanowire field effect transistors (FETs) was investigated by using an individual SnO(2) nanowire that was passivated in sections by either a PMMA (polymethylmethacrylate) or an Al(2)O(3) layer. The PMMA passivated section showed the best mobility performance with a significant positive shift in the threshold voltage. The distinctive two-dimensional R(s)-μ diagram based on a serial resistor connected FET model suggested that this would be a useful tool for evaluating the efficiency for post-treatments that would improve the device performance of a single nanowire transistor.

Estimation of frequency-dependent electrokinetic forces on tin oxide nanobelts in low frequency electric fields

A novel experimental approach is used for studying the response of ethanol-suspended SnO(2) nanobelts under the influence of low frequency ac electric fields. The electrically generated forces are estimated by analyzing the angular motion of the nanobelt, induced by repulsive forces originating predominantly from negative dielectrophoresis (DEP) on planar microelectrodes. The nanobelt motion is experimentally recorded in real time in the low frequency range (<100 kHz) and the angular velocities are calculated. A simple analytical model of force balance between the electrical forces and fluidic drag for long nano-objects is developed and used to deduce estimates of the frequency-dependent DEP force and torque magnitudes from the angular velocity data. Additional experiments, performed in a parallel plate electrode configuration in a fluidic channel to investigate the effect of dc and very low frequency ac (approximately Hz) electric fields, indicate the presence of electrophoresis in the ethanol-suspended SnO(2) nanobelts. The experimentally observed nanobelt motion is analyzed using the equation of motion, and an order-of-magnitude estimate of the nanobelt surface charge density is obtained.