Changes In Sensor Response Research Articles

Fabrication of hydrogen gas sensor using sputtered palladium-copper composite on tapered optical fiber

Abstract In this study, we fabricated a hydrogen (H2) gas sensor based on tapered optical fiber using sputtering method. Also, as the first attempt, we explored how palladium (Pd) and palladium-copper (Pd-Cu) coatings, deposited using the sputtering method (RF and DC), affect tapered optical fibers as H2 gas sensors (ranging from 1 to 8% H2). It investigates changes in sensor output power, response and recovery times, and the influence of fiber tapering angle on output power. The investigation reveals that two main factors, including permeability and elasto-optic effect significantly impact the results. At H2 concentrations of 1 to 3%, permeability predominantly affects Pd sensors, yielding better output power changes and sensitivity than Pd-Cu tapered optical fiber sensors. Conversely, at higher H2 concentrations (4 to 8%), the dominant factors appear to be permeability as well as elasto-optic effect. These characteristics have a greater influence in the Pd-Cu layer at higher H2 concentration, resulting in smoother slope in response to H2. Due to higher permeability, Pd sensors reach saturation faster, while Pd-Cu sensors exhibit more linear changes with increasing H2 levels and do not saturate like Pd sensors very fast. Moreover, the study shows that a larger tapering angle can enhance the output power of Pd-Cu tapered optical fiber sensors.

Rapid distance estimation of odor sources by electronic nose with multi-sensor fusion based on spiking neural network

The inference of odor source locations holds great potential value in chemical leak detection, explosive searches and flammable substance detection. Sensor response and derivative changes are common indicators used to estimate source distance. However, most studies often rely on single-feature methods using a single sensor. In this study, a novel distance indicator based on a spiking neural network (SNN) that integrates multiple sensors is proposed. SNN was introduced for effective multi-sensor data fusion, which can provide accurate and rapid source distance estimation. The indicator's accuracy and quick detection capabilities were tested using a publicly available wind tunnel dataset and compared with single-sensor indicators. Additionally, an olfactory robot platform was constructed to test the method's real-time performance. Results demonstrate that this method effectively utilizes sensor arrays to estimate source distance, with an average root mean square error (RMSE) of less than 0.1 m across different wind speeds, outperforming single-sensor-based indicators. Additionally, the model exhibits good distance estimation performance within a relatively short detection time, with an average RMSE of 0.118 m. Practical testing on a mobile robot platform confirms the applicability of this distance indicator for real-time distance monitoring. In conclusion, the proposed distance indicator enables effective distance estimation for odor sources, providing a promising method for odor source localization and olfactory navigation.

Detection of DNA using gold nanoparticle-coated silica nanoparticles

We report a sensitive lateral flow assay (LFA) in which the assay colour change originated from reporter labels constructed from silica spheres (radius = 450 nm) coated with approximately 3.9 × 103 gold nanoparticles (radius = 8.5 nm). These reporter labels were modified with DNA and deposited in the conjugation area of an LFA device assembled on wax-patterned Fusion 5 paper. Test and control zones of the device were pre-loaded with capture probe formed by avidin-coated mesoporous silica nanoparticles attached with biotin-tagged DNA sequences. Proof-of-concept was demonstrated by the detection of a partial sequence of the actin gene of Colletotrichum truncatum. The DNA target could be detected with an LOD of 46 pM, which was 5 times lower than a comparative assay using gold nanoparticles alone. The assay showed good selectivity against the Colletotrichum species C. scovillei and C. gloeosporioides, as well as against DNA from the fungal genera Aspergillus niger and Alternaria alternata. There was negligible change in sensor response over storage for one month at room temperature. The LFA was used to detect PCR products following extraction from mycelium.

Miniaturized wireless sensor enables real-time monitoring of food spoilage.

Food spoilage results in food waste and food-borne diseases. Yet, standard laboratory tests to determine spoilage (mainly volatile biogenic amines) are not performed regularly by supply chain personnel or end customers. Here we developed a poly(styrene-co-maleic anhydride)-based, miniature (2 × 2 cm2) sensor for on-demand spoilage analysis via mobile phones. To demonstrate a real-life application, the wireless sensor was embedded into packaged chicken and beef; consecutive readings from meat samples using the sensor under various storage conditions enabled the monitoring of spoilage. While samples stored at room temperature showed an almost 700% change in sensor response on the third day, those stored in the freezer resulted in an insignificant change in sensor output. The proposed low-cost, miniature wireless sensor nodes can be integrated into packaged foods, helping consumers and suppliers detect spoilage of protein-rich foods on demand, and ultimately preventing food waste and food-borne diseases.

Highly sensitive MWCNTs/SiNWs hybrid nanostructured sensor fabricated on silicon-chip for alcohol vapors detection

Abstract The present work focuses on the fabrication of highly sensitive, stable and fast response alcohol sensor using MWCNTs/SiNWs hybrid nanostructure. Herein, vertically aligned Silicon Nanowires (SiNWs) were synthesized on Silicon (Si) chip using Metal Assisted Chemical Etching (MACE) process, which were decorated by Multiwall Carbon Nanotubes (MWCNTs) and resulted into a MWCNTs/SiNWs hybrid nanostructure. These types of MWCNTs were separately grown by using Chemical Vapor Deposition (CVD) technique. The structural analysis and characterization of as-grown materials along with fabricated hybrid nanostructure were done by using different techniques such as Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Raman spectrometer and X-Ray diffractometer. The prepared materials were observed to be dense having high yield and uniformity throughout the structure. The alcohol vapors sensing properties, such as sensitivity, selectivity, repeatability, response & recovery characteristics, of the fabricated MWCNTs/SiNWs sensor, were investigated at room temperature. The data was recorded in the form of changes in sensor response with respect to time and analyzed using the mechanism of interaction of materials with the analyte gas species. The as-fabricated sensor exhibits not only excellent response ~1.42 and selectivity for alcohol concentration of 1 ppm under atmospheric conditions at room temperature but also, it's response time (3 s) and recovery time (5 s) is quite fast along with excellent reproducible nature. Most importantly, the sensor recovered to its original state without applying any external energy source. Hence, all the measured properties indicate the better operational performance of the fabricated sensor and therefore, could be suitably employed in various applications such as alcohol industries, fuel processing and other societal problems.

Multifunctional dumbbell-shaped ZnO based temperature-dependent UV photodetection and selective H2 gas detection

In this article, a single step, chemically synthesized dumbbell-shaped ZnO microcrystal has been utilized to fabricate UV photodetectors and highly selective H2 gas sensors. The structural, morphological, chemical states and optical properties were investigated via using various characterization tools. The dumbbell-shaped morphology acquires 1.2–1.4 μm length and diameters of 300–400 nm respectively. The temperature-dependent photodetection and gas sensing characteristics has been studied in the temperature range 298.15–433.15 K respectively. The maximum relative change in photoresponse and gas sensor response has been found at 358.15 K. The rise time and fall time have been reduced from ~2.4 sec to ~1.0 sec and ~3.1 sec–~2.2 sec respectively in the applied temperature range. Such behavior is attributed to the increasing desorption rate with a higher operating temperature. The maximum sensor response towards H2 was found to be ~20% for 100 ppm concentration at an operating temperature of 358.15 K.

Classification of North Africa for Use as an Extended Pseudo Invariant Calibration Sites (EPICS) for Radiometric Calibration and Stability Monitoring of Optical Satellite Sensors

Pseudo invariant calibration sites (PICS) have been extensively used for the radiometric calibration and temporal stability monitoring of optical satellite sensors. Due to limited knowledge about the radiometric stability of North Africa, only a limited number of sites in the region are used for this purpose. This work presents an automated approach to classify North Africa for its potential use as an extended PICS (EPICS) covering vast portions of the continent. An unsupervised classification algorithm identified 19 “clusters” representing distinct land surface types was used; three clusters were identified with spatial uncertainties within approximately 5% in the shorter wavelength bands and 3% in the longer wavelength bands. A key advantage of the cluster approach is that large numbers of pixels are aggregated into contiguous homogeneous regions sufficiently distributed across the continent to allow multiple imaging opportunities per day, as opposed to imaging a typical PICS once during the sensor’s revisit period. This potential increase in temporal resolution could result in increased sensitivity for the quicker identification of changes in sensor response.

Graphene oxide based soil moisture microsensor for in situ agriculture applications

In this paper, we have designed a robust graphene oxide (GO) based capacitive sensor which is highly sensitive to soil moisture. The sensor consists of inter-digitated electrode (IDE) with chemically synthesized GO as the sensing platform. We have used the MEMS fabrication technique to fabricate the IDE. We have performed the characterization such as AFM, RAMAN and XPS on the as-prepared GO to understand the surface properties of the GO. The sensor response changes by 340% and 370% over soil moisture changes from 1% to 55% for red and black soil, respectively. GO sensor array shows a fast response time of 100–120 seconds for the soil moisture measurements. For in situ soil moisture measurements, the diurnal temperature and salt concentration (soil conductivity) are the variable parameters, which might affect the sensor response. We observed that the sensor output changes by 6% when the ambient temperature varies from 25°C to 65°C, which leads to just 3% discrepancies for the soil moisture measurements. For the salt concentration (soil conductivity) measurements we noted that the sensor output changes by 4% when salt concentration in the soil sample varies from 0 mol to 0.35 mol (standard for the field measurement), and discrepancies in the soil moisture measurements is around 2%.

EDL Gated FET Biosensor Array for the Investigation of Ion Channels and Bioelectric Signals of Circulating Tumor Cells

This paper demonstrates the applications of novel high sensitive electrical double layer (EDL) gated FET biosensor array in detecting and counting cells, and analyzing their bioelectric signals in different test conditions. We focused on extremely rare cell type such as Circulating Tumor Cells (CTCs) (HCT-8 cell line) derived from Colorectal Cancer (CRC) cell line. CRC is one of the major forms of cancer that affects human population and CTCs have emerged as a clinical biomarker for diagnosis and prognosis of metastatic CRC. Previously, we had shown that our methodology can detect and count CTCs with very high sensitivity, with up to single cell resolution. Simultaneously, we discovered our sensor platform can be used to dynamically monitor the changes in transmembrane potential (Vm) of the cell, which is an important fundamental bioelectric signal of cell. Studies have shown Vm may be useful as a primary parameter in understanding disease progression, cell signaling mechanism and ion channel gating. We demonstrated that the EDL gated FET sensor can dynamically monitor the changes in Vm induced by high concentrations of extracellular KCl. It was also shown that the sensor response changes as extracellular divalent cation concentrations are dynamically increased towards physiological concentrations. The results display the ability of the sensing platform to monitor the cellular response to dynamically changing, very low ion concentrations as well. Furthermore, an interesting application of bioelectric signal study is demonstrated using cadmium ions to block the calcium channels present in the cell. As shown in figure, in low micro molar concentrations, cadmium acts as a permeant channel blocker (electrically, the cell is depolarized and as the Cd2+ concentration increases, it re-polarizes) and in higher concentrations, it accumulates in the intracellular region (the cell is depolarized again), causing induction of apoptotic signals. The sensor response closely follows the cellular response to permeant channel blockers such as cadmium. These results demonstrate that our sensing methodology can replace the conventional electrophysiological methods or optical potentiometric probes to study fundamental cellular mechanisms such as ion channel gating. Our method offers non-invasive means to monitor changes in Vm, with very high sensitivity for individual cells. The cell based studies can be carried out directly in physiological media with high ionic strength, without the use of extensive pre-treatments or additional reagents. Figure 1

The Effect of Pressure and Temperature on Mid-Infrared Sensing of Dissolved Hydrocarbons in Water.

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy using a polymer coated internal reflection element/waveguide is an established sensor platform for the detection of a range of organic and hydrocarbon molecules dissolved in water. The polymer coating serves two purposes: to concentrate hydrocarbons from the aqueous phase and to exclude water along with other interfering molecules from the surface of the internal reflection element. Crucial to reliable quantification and analytical performance is the calibration of the ATR-FTIR sensor which is commonly performed in water under mild ambient conditions (i.e., 25 °C and 1 atm). However, there is a pressing need to monitor environmental and industrial processes/events that may occur at high pressures and temperatures where this calibration approach is unsuitable. Using a ruggedized optical fiber probe with a diamond-based ATR, we have conducted mid-infrared sensor experiments to understand the influence of high pressure (up to 207 bar) and temperature (up to 80 °C) on the detection of toluene and naphthalene dissolved in water. Using a poly(isobutylene) film, we have shown that the IR spectroscopic response is relatively unaffected by changes in pressure; however, a diminished response was observed with increasing temperature. We reveal that changes in the refractive index of the polymer film with temperature have only a minor effect on sensitivity. A more plausible explanation for the observed significant change in sensor response with temperature is that the partitioning process is exothermic and becomes less favorable with increasing temperature. This Article shows that the sensitivity is relatively invariant to pressure; however, the thermal variations are significant and need to be considered when quantifying the concentration of hydrocarbons in water.

Design of an ozone and nitrogen dioxide sensor unit and its long-term operation within a sensor network in the city of Zurich

Abstract. This study focuses on the investigation and quantification of low-cost sensor performance in application fields such as the extension of traditional air quality monitoring networks or the replacement of diffusion tubes. For this, sensor units consisting of two boxes featuring NO2 and O3 low-cost sensors and wireless data transfer were engineered. The sensor units were initially operated at air quality monitoring sites for 3 months for performance analysis and initial calibration. Afterwards, they were relocated and operated within a sensor network consisting of six locations for more than 1 year. Our analyses show that the employed O3 and NO2 sensors can be accurate to 2–5 and 5–7 ppb, respectively, during the first 3 months of operation. This accuracy, however, could not be maintained during their operation within the sensor network related to changes in sensor behaviour. For most of the O3 sensors a decrease in sensitivity was encountered over time, clearly impacting the data quality. The NO2 low-cost sensors in our configuration exhibited better performance but did not reach the accuracy level of NO2 diffusion tubes (∼ 2 ppb for uncorrected 14-day average concentrations). Tests in the laboratory revealed that changes in relative humidity can impact the signal of the employed NO2 sensors similarly to changes in ambient NO2 concentration. All the employed low-cost sensors need to be individually calibrated. Best performance of NO2 sensors is achieved when the calibration models also include time-dependent parameters accounting for changes in sensor response over time. Accordingly, an effective procedure for continuous data control and correction is essential for obtaining meaningful data. It is demonstrated that linking the measurements from low-cost sensors to the high-quality measurements from routine air quality monitoring stations is an effective procedure for both tasks provided that time periods can be identified when pollutant concentrations can be accurately predicted at sensor locations.

(Invited) Chemical Sensors Based on AlGaN/GaN Transistors

Reliable, economically accessible technology for in situ monitoring of contaminants in water has the power to transform health, industry, and society. Applications of such monitoring range from process control monitoring and optimisation for industry, to water supply quality and wastewater monitoring, to environmental monitoring for resource extraction, and beyond. Semiconductor-based technology offers high performance for in situ, real-time contaminant monitoring and can also be mass produced at low-cost with flexible functionalisation allowing for a variety of analytes. Furthermore, it offers the ability to integrate multiple sensors into one chip, along with wireless communication technology for maximum benefit of the in situ monitoring capability. AlGaN/GaN transistors have the unique combined benefits of high sensitivity to surface charge, fast response, and excellent chemical and thermal stability. AlGaN and GaN are members of the III-nitride alloy family (AlN, GaN and InN), which has become extremely technologically important for a variety of applications such as wireless base stations, solid-state lighting, automotive electronics, power conditioning and DVD information storage. The properties of AlGaN/GaN transistors are also very attractive for sensors; however, this application niche has not yet been widely explored. The strained AlGaN layer induces a region of high sheet carrier concentration, electron mobility and high saturation velocity induced by piezoelectric polarization known as a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The 2DEG responds to changes in surface charge, so ion selective field effect transistors (ISFETS) can be developed into robust and sensitive ion sensors. Importantly, in contrast to many other potentiometric sensors, including Si-based ISFETS, high sensitivity can be obtained without the need for a reference electrode. This is a significant advantage over electrochemical cell techniques, for which a reference electrode is a major impediment to in situ and robust operation. This paper will report on development of various AlGaN/GaN ISFETs for chemical sensing – in particular pH sensors, nitrate ion, mercury (II) ion and calcium ion sensors. The ability to fabricate different sensors from the same underlying technology paves the way for array-based multi-analyte sensing solutions. The ion sensors were all obtained by functionalising the transistors with a plasticised PVC membrane containing a dissolved ionophore selective to the ion of interest. Using transistor theory, we show that the apparent gate response for these sensors is near-Nenrstian under a variety of testing conditions. In each case, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to ion exposure. Some other key results are summarised below. pH sensors AlGaN/GaN transistor-based pH sensors have been investigated by several research groups. When operating without a reference electrode, researchers have reported two types of pH sensor response: linear (related to pH selectivity) and U-shape (related to anion selectivity). Previously, this distinct difference in behaviour had not been well explained. We have compared the pH response of a reference electrode-free AlGaN/GaN device with and without a GaN capping layer. From our investigations, in the absence of a reference electrode, a linear response towards pH requires a GaN capping layer. Apparently the difference in surface chemistry for GaN versus AlGaN surface affects pH/anion selectivity when there is no reference electrode controlling the surface potential. Nitrate ion sensors In a 0.1 M KH2PO4 ion buffer, a detection limit of less than 10− 6M and a linear response range of 10-6-10− 3M were achieved. The detection limit remains consistently low over multiple runs/days. In addition, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11 [1]. Mercury (II) ion sensors At pH 2.8 in a 10-2 M KNO3 ion buffer, a detection limit below 10−8 M in and a linear response range between 10-8 M-10−4 M were achieved. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg2+ solution and rinsed with deionised water. Calcium ion sensors In 0.1M KOH ion, a buffer detection limit of less than 10-8 M and a linear response range between 10-8M - 10-3M were achieved. In addition, there was negligible change in sensor response upon the addition of KOH, which increased pH level from approximately 4 to 11. [1] M. Myers, F.L.M. Khir, A. Podolska, G.A. Umana-Membreno, B. Nener, M. Baker, G. Parish, “Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode”, Sens. Actuators, B 181 301-5 (2013).

Design and In Vitro Interference Test of Microwave Noninvasive Blood Glucose Monitoring Sensor

A design of a microwave noninvasive continuous blood glucose monitoring sensor and its interference test results are presented. The novelty of the proposed sensor is that it comprises two spatially separated split-ring resonators, where one interacts with the change in glucose level of a sample under test while the other ring is used as a reference. The reference ring has a slightly different resonant frequency and is desensitized to the sample owing to its location, thus allowing changes in temperature to be calibrated out. From an oral glucose tolerance test with two additional commercially available sensors (blood strip and continuous glucose monitor) in parallel, we obtained encouraging performance for our sensor comparable with those of the commercial sensors. The effects of endogenous interferents common to all subjects, i.e., common sugars, vitamins (ascorbic acid), and metabolites (uric acid) have also been investigated by using a large Franz cell assembly. From the interference test, it is shown that the change in sensor response is dominated by changes in glucose level for concentrations relevant to blood, and the effects of interferents are negligible in comparison.

Overview of Vehicle Integrated Propulsion Research (VIPR) Testing

Maintaining safe operation of an aircraft is a major emphasis of the NASA Aviation Safety Program [1]. An Integrated Vehicle Health Management (IVHM) system aims to maintain vehicle health through detection, diagnostics, state awareness, prognostics, and lastly, mitigation of detrimental situations for each of the vehicle subsystems and throughout the vehicle as a whole [2]. Major subsystems associated with a vehicle wide IVHM system are the airframe, avionics, and propulsion systems. Each subsystem has its own challenges associated with development of IVHM technologies. One particular challenging subsystem for vehicle health management is related to Propulsion Health Management (PHM). The engine is a complex system with a range of operating components whose failure can affect passenger safety. Reliable operation of these engines is critical for aircraft safety. The harsh environment conditions within an engine often present significant challenges for the integration and application of health management systems. In parallel, diagnostic systems often have to perform evaluations with limited sensor information, while evaluating a complex system whose components include gas path, turbomachinery, hydraulics, and other components. Demonstration of Propulsion Health Management technologies is critical for their commercial acceptance. A major aspect of ongoing NASA Propulsion Health Management work is demonstration of these maturing technologies on an operational engine in series of tests called Vehicle Integrated Propulsion Research, or VIPR. This paper gives a brief overview of the VIPR series of tests, the technologies involved in this testing, and how this approach is being used to advance the commercialization and acceptance of these technologies. VIPR is a means to test and evaluate emerging health management technologies on a commercial engine, incorporating new sensors directly on the engine, providing seeded fault scenarios, and evaluating advances in engine diagnostics. This step is critical in order for IVHM technologies to mature from lab work and simulation demonstrations needed for industry acceptance. This work is in partnership with the Air Force, which has provided access to two F117 high bypass turbofan engines and time on operational planes for the ground-based, on-board engine tests. In these tests, the research engines are instrumented to achieve NASA and partner goals and mounted on a C-17 aircraft, though it is important to note that the planes are grounded and do not take flight. A series of tests are planned with overall Vehicle Systems Safety Technologies (VSST) project objectives of testing health management sensors, sensor systems and algorithms on a high bypass turbofan engine under the following conditions: (1) Normal engine operations; (2) Seeded mechanical faults; (3) Seeded gas path faults; and (4) Accelerated engine life degradation through volcanic ash ingestion testing. The first VIPR test took place in December 2011 at NASA Dryden Flight Center/Edwards Air Force Base as an on-wing ground test on a heavily instrumented high bypass turbofan engine. This test featured: 1. Emission Sensor System: Investigation of the ability of of an array of chemical sensors to detect a simulated oil leak and measure changes in sensor response with changing engine conditions.2. Self Diagnostic Accelerometer feasibility test: Demonstrate operability of self-diagnostic accelerometer in aircraft engine environment3. Validate Gas Path Diagnostics: Test analytical model predictions of engine parameters in the presence of bleed valve faults (failed, full open; schedule bias) An unique feature of this VIPR series is an upcoming test, which includes the planned ingestion of volcanic ash into an engine on-board an aircraft. The primary objectives of this work are to determine the effect on the engine of several hours of exposure to low to moderate ash concentrations and to evaluate the capability of engine health management technologies for detecting these effects. Numerous observations and measurements of the engine’s performance and degradation will be made during the course of the experiment. While not intended to be sufficient for rigorous certification of engine performance when ash is ingested, the experiment should provide useful information to aircraft manufacturers, airline operators, and military and civil regulators in their efforts to evaluate the range of risks that ash hazards pose to aviation. A research team of U.S. Government agencies and engine manufacturers are involved in this VIPR testing. This government and industry partnership is not only crucial in enabling the tests, but is also a method by which various organizations can be involved in the demonstration and maturation of technologies, facilitating potential future acceptance. A conclusion of this paper is that partnerships between developers and users of new technologies are crucial towards enabling technology commercialization and acceptance. Not only does this type of work allow the ability to mature and evaluate the technologies in environments reflective of potential application, interaction with potential technology users can provide feedback on pathways towards technology improvements and possible implementation.

Nitrate ion detection using AlGaN/GaN heterostructure-based devices without a reference electrode

AlGaN/GaN heterostructure-based devices can be engineered through heterostructure design to have a high transconductance near zero gate–drain voltage, potentially enabling high sensitivity, reference electrode free, ion sensing. As a proof of concept, these devices were coated with a PVC-based membrane containing a plasticizer and an ionophore to detect nitrate ions in solution. The sensor response is measured as a change in conductivity across two contacts using a Kelvin probe (or four-contact) geometry, with the current between the two outer contacts kept constant. We show that this sensor for nitrate is sensitive and stable with a rapid response time (i.e. less than 60s). The detection limit remains consistently low over multiple runs/days. In a 0.1M KH2PO4 ion buffer, a detection limit of less than 1×10−6M and a linear response range of 10−6–10−3M were achieved. Furthermore, detection limits of approximately 10−6M and 10−4M in 0.1M K2SO4 and 0.1M KCl ion buffers, respectively, were demonstrated. In a 0.1M KH2PO4 ion buffer, there was minimal change in sensor response upon addition of KOH increasing the pH from approximately 4–11. As a control, devices without a PVC membrane coating were tested under identical conditions and exhibited negligible response to nitrate ion exposure. Furthermore, using transistor theory, we show that the apparent gate response is near-Nenrstian under a variety of conditions. The success of this study paves the way for extending this technology to selectively sensing multiple ions in water through incorporation of the appropriate polymer based membranes on arrays of devices.

Biofouling of Polymer Hydrogel Materials and its Effect on Diffusion and Enzyme-Based Luminescent Glucose Sensor Functional Characteristics

Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However, biofouling and host response make in vivo sensor performance difficult to predict. We investigated changes in glucose diffusivity and sensor response of optical enzymatic glucose sensors due to biological exposure. Three hydrogel materials, poly(2-hydroxyethyl methacrylate) (pHEMA), poly(acrylamide) (pAM), and poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (p(HEMA-co-AM)), were tested for glucose diffusivity before and after exposure to serum or implantation in rats for 1 month. Luminescent sensors based on these materials were measured to compare the response to glucose before and after serum exposure. Glucose diffusivity through the pHEMA [(8.1 ± 0.38) × 10(-8) cm(2)/s] slabs was much lower than diffusivity through pAM [(2.7 ± 0.15) × 10(-6) cm(2)/s] and p(HEMA-co-AM) [(2.5 ± 0.08) × 10(-6)]. As expected from these differences, sensor response was highly dependent on material type. The pHEMA sensors had a maximum sensitivity of 2.5%/(mg/dl) and an analytical range of 4.2-356 mg/dl, while the p(HEMA-co-AM) sensors had a higher sensitivity [14.9%/(mg/dl)] and a narrower analytical range (17.6-70.5 mg/dl). After serum exposure, the pHEMA sensors were unaffected, whereas the p(HEMA-co-AM) sensors exhibited significantly decreased sensitivity and increased analytical range. Decreases in glucose diffusivity in the polymers resulting from in vitro serum exposure and residence in vivo were shown to be similar, suggesting that serum incubation was a reasonable approximation of in vivo fouling. While biofouling is expected to affect the response of flux-based sensors, we have shown that this depended on the type of sensor and matrix used. Therefore, proper design and materials selection may minimize response alterations occurring upon implantation.

Photocatalytic Titanium Dioxide Nanostructures for Self-Regenerating Relative Humidity Sensors

Relative humidity sensors with a nanostructured titanium dioxide sensing layer were fabricated using glancing angle deposition. Such sensors exhibit very quick response times and high sensitivity but are prone to aging. In previous work, ultraviolet irradiation has been shown to regenerate and improve the sensor performance. Here, the effect of ultraviolet wavelength and power on sensor performance, aging, and regeneration was studied. In order to quantify the change in sensor response a figure of merit (FOM) was defined as the integrated area between the sensor response curves before and after regeneration or aging. FOM values ranged from 2.1 to 61 for treatment with a mercury vapor lamp. Treated sensors aged at an average rate of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$-$</tex></formula> 0.43 FOM/day. Regeneration of sensors using commercial ultraviolet light-emitting diodes was demonstrated, resulting in FOMs significantly larger than irradiation with comparable mercury vapor lamp wavelengths.

Heteroepitaxy of Nd0.67Sr0.33MnO3 on silicon for bolometric x-ray detector application

We have recently reported the design concept and sensor fabrication for a novel bolometric x-ray detector based on a rare earth manganite material for application as a total energy monitor for the Linac Coherent Light Source (LCLS) free electron laser at the Stanford Linear Accelerator Center (SLAC). The detector employs epitaxial thin films of Nd(0.67)Sr(0.33)MnO(3) grown on Si by pulsed laser deposition. In this paper we report details of the fabrication of the actual detector, its response characteristics under photon illumination from LCLS, and improvements in the growth scheme of the sensor material on Si using a buffer/template layer scheme that employs yttria-stabilized zirconia, cerium oxide (CeO(2)), and bismuth titanate (Bi(4)Ti(3)O(12)). The thermal sensor response changes linearly with the energy of an optical calibration laser as expected, and the signals from optical and x-ray pulses at LCLS are very similar, thereby validating the design concept. To the best of our knowledge, the LCLS detector application reported here is the first practical use of colossal magnetoresistive manganite bolometers.

In Situ Measurement of pH Under High Pressure

In situ measurement of pH under high pressure is complicated by the need to remain functional under extreme pressures, electrically noisy environments, and the difficulty in inferring chemical equilibrium conditions under pressure. We describe a pH sensor wherein the current flowing at a given input voltage is directly related to hydrogen ion concentration and can be operated within extreme environments. The sensor was tested at 25 °C for in situ pH measurement of several buffer solutions under high pressure up to 784.6 MPa. We found generally an increase in the acidity of the buffer solutions with increasing pressure. In all cases, pH changes were found to be less than predicted using inferred reaction volume data from the literature. Sensor response and pH changes were found to be completely reversible upon depressurization at the same temperature.

The influence of ZnO-Sensor modification by porphyrins on to the character of sensor response to volatile organic compounds

The influence of modification of semi conductive ZnO and SnO2 sensors by porphyrins and their metal complexes on the parameters of sensor response to volatile organic compounds, ethanol, acetone and benzene, was analyzed. The concentration of volatile organic compounds (ci) varied in the range of 1–200 ppm. The sensor response was characterized by specific sensitivity γi = 100ΔRi/R0ci, where ΔRi = Ri − R0, Ri and R0 are measurable and initial resistance of the sensitive sensor layer correspondingly. It was established that the modification of sensors by porphyrins caused changes in sensor response and first of all decrease of ZnO sensor temperature at which the threshold of sensitivity is achieved (as used at this method cimin = 0.1 ppm) from 300 to 100°C. It also caused a change in sing of the parameter γ, which was of importance for creating “electronic nose” sensor systems. It was shown also, that the modification of ZnO sensor by metal complexes of porphyrin did not change sensor response parameters.