Metal Oxide Sensors Research Articles

Surface defect engineering: gigantic enhancement in the optical and gas detection ability of metal oxide sensor

By using surface defect engineering, the gigantic enhancement in UV and gas detection abilities of nanosensors can be achieved.

Low-Power Gas Sensing using Carbon Nanotubes in Wearable Devices

Air quality monitoring is gaining importance in public health due to the increasing level of the pollution in cities. In addition, people are more concerned about their personal exposure and they are interested to know the concentration levels of the pollutants, which surround them. In recent years, a wearable technology can be useful for continuous air quality monitoring when people are moving in urban and industrial environments. As wearable systems are usually battery-powered and gas sensors are power-hungry, energy-efficient design and power management are required. In this paper, we present a two-stage gas sensing concept where novel multiple-single-walled carbon nanotubes (SWCNT) are proposed as detectors for an energy-hungry metal–oxide (MOX)–semiconductor gas sensor. We simulate the system performance combining the low power consumption of SWCNT gas sensors and the more mature MOX sensor to achieve an energy-efficient wearable device able to monitor the air quality continuously while achieving long lifetime. We perform the simulations using measured power consumptions for two event-driven scenarios to evaluate the power consumption reduction and lifetime extension in a wearable mobile context. Our results show that the proposed approach prolongs node lifetimes by 30 times compared with adaptive duty-cycling with only MOX gas sensors. We also propose that the nanotube recovery time issue can be overcome by using four single nanotubes on the same chip, which results in an extension of lifetime.

Comparison and data fusion of an electronic nose and near-infrared reflectance spectroscopy for the discrimination of ginsengs

This paper reports a hybrid system consisting of a homemade electronic nose system (E-nose) with a sensor array of 16 metal-oxide sensors and near-infrared reflectance spectroscopy (NIRS) system for discriminating different kinds of ginsengs.

NH3 sensing with self-assembled ZnO-nanowire μHP sensors in isothermal and temperature-pulsed mode

Dielectrophoretic alignment is found to be a simple and efficient method to deposit the solution prepared ZnO nanowires onto micro hot plate substrates. Due to the strong surface effects, positive temperature coefficient for resistance was encountered with ZnO nanowires in the high temperature range (>250°C). The response to ammonia (NH3) was evaluated in isothermal and temperature-pulsed operation mode; the relative higher response observed in the latter case demonstrates that the use of this methodology is a good strategy to improve the performance of metal oxide sensors based on nanomaterials. Here, we evaluate the response to NH3 and qualitatively describe the sensing mechanism in temperature-pulsed mode, highlighting the main differences compared to the standard isothermal methodology.

Quantitative Ethylene Measurements with MOx Chemiresistive Sensors at Different Relative Air Humidities.

The sensitivity of two commercial metal oxide (MOx) sensors to ethylene is tested at different relative humidities. One sensor (MiCS-5914) is based on tungsten oxide, the other (MQ-3) on tin oxide. Both sensors were found to be sensitive to ethylene concentrations down to 10 ppm. Both sensors have significant response times; however, the tungsten sensor is the faster one. Sensor models are developed that predict the concentration of ethylene given the sensor output and the relative humidity. The MQ-3 sensor model achieves an accuracy of ±9.2 ppm and the MiCS-5914 sensor model predicts concentration to ±7.0 ppm. Both sensors are more accurate for concentrations below 50 ppm, achieving ±6.7 ppm (MQ-3) and 5.7 ppm (MiCS-5914).

One-Dimensional Tin (IV) Oxide Nanostructures as Gas-Sensing Materials

Background. Gas sensors based on SnO 2 are characterized by small size and low cost. However, its significant disadvantages are insufficient sensitivity, small selectivity and low stability. Therefore, the determination of basic parameters, a change that will create effective, highly sensitive and selective semiconductor sensors based on SnO 2 nanostructures, is extremely important. Objective. The purpose of this paper is to establish the main parameters affecting the sensitivity, selectivity and stability of semiconductor sensors. Methods. A critical review of recent scientific literature is done. Found that the usage of 1D tin (IV) oxide nanostructures (as pure and doped) will increase the sensitivity and selectivity of the metal oxide sensors due to high values of surface to volume ratio and the creation of active centers in relation to the detected gases. Results. It was determined that the creation of efficient and sensitive semiconductor sensors requires the use of 1D SnO 2 nanostructures and their directed modification by various additives. Conclusions. In terms of data presented in contemporary scientific literature, to create effective semiconductor tin (IV) oxide based sensors 3S parameters of these sensors need to be improved. From this point of view, 1D SnO 2 nanostructures deserve special attention due to the totality of their physical and chemical properties. Vapor transport method (method CVD) is enough effective for the synthesis of 1D nanostructures. It secures superior performance in conjunction with relative simplicity and availability. And this method allows us to obtain single-crystal nanostructures of controlled morphology. However, as of day there is no almost information on the impact of operational parameters of CVD synthesis on physico-chemical characteristics of obtained nanosized SnO 2 . Therefore, the advanced study of scientific bases of purposeful synthesis and systematization of approach in the selection a dopant to increase selectivity of metal oxide gas sensors is the main task.

Staining-free cell viability measurement technique using lens-free shadow imaging platform

Tests for cell viability, i.e., an assay quantifying the ratio of viable cells or tissues over the total cells or tissues within an index between 0 and 1 (or 0 and 100%), play an important role in cell or tissue culturing procedures. The viability test result, varying with several biological factors such as mechanical activity, motility, contraction, or mitotic activity of cells or tissues, is a crucial indicator in cell related research protocols including toxicity and anabolic activity assays. There are several well-established methods for evaluating cell viability, such as trypan blue assay, propidium iodide assay, 7-aminoactinomycin D assay and resazurin and formazan (MTT/XTT) assay. However, most of these methods determine viability using stained cell samples, which intern affect the cells morphology eventually making it unable to keep culturing the specimen. To address this issue, we have developed a novel shadow imaging technique to capture the diffraction patterns (shadow patterns) of micro objects without the use of any staining reagent. In this paper, we introduce a shadow imaging platform that can determine cell viability of more than 3000 human cancer cells immediately with a single digital image. Our custom-built lens-free shadow imaging platform consists of a compact, cost-effective light source, i.e., a light-emitting diode, and an optoelectronic image recording device, i.e., a complementary metal-oxide semiconductor image sensor. Three types of human cancer cell lines (Caco-2, HepG2, and MCF7) were incubated in 24-well plates, and H2O2 was added to track and compare the cell viability at each concentration tested. We obtained high correlation indices, with a minimum of 0.94, between the MTT assay and the shadow imaging platform. All these characterizations were done by custom developed automated detection algorithm. This algorithm analyzes the various elements of the diffraction pattern (shadow image), such as pixel intensity and connected pixel numbers, and counts the viable cells automatically, allowing the cell viability to be determined easily and immediately in a staining-free manner.

Miniaturized metal oxide pH sensors for bacteria detection

It is well known that the metabolic activity of some microorganisms results in changes of pH of the culture medium, a phenomenon that can be used for detection and quantification of bacteria. However, conventional glass electrodes that are commonly used for pH measurements are bulky, fragile and expensive, which hinders their application in miniaturized systems and encouraged to the search for alternatives.In this work, two types of metal oxide pH sensors have been tested to detect the metabolic activity of the bacterium Escherichia coli (E. coli). These pH sensors were produced on silicon chips with platinum metal contacts, onto which thin layers of IrOx or Ta2O5 were incorporated by two different methods (electrodeposition and e-beam sputtering, respectively). In order to facilitate measurement in small sample volumes, an Ag/AgCl pseudo-reference was also screen-printed in the chip and was assayed in parallel to an external Ag/AgCl reference electrode. As it is shown, the developed sensors generated results indistinguishable from those provided by a conventional glass pH-electrode but could be operated in significantly smaller sample volumes. After optimization of the detection conditions, the metal oxide sensors are successfully applied for detection of increasing concentrations of viable E. coli, with detection of less than 103cfumL−1 in undiluted culture medium in just 5h.

Portable lensless wide-field microscopy imaging platform based on digital inline holography and multi-frame pixel super-resolution.

In this paper, an irregular displacement-based lensless wide-field microscopy imaging platform is presented by combining digital in-line holography and computational pixel super-resolution using multi-frame processing. The samples are illuminated by a nearly coherent illumination system, where the hologram shadows are projected into a complementary metal-oxide semiconductor-based imaging sensor. To increase the resolution, a multi-frame pixel resolution approach is employed to produce a single holographic image from multiple frame observations of the scene, with small planar displacements. Displacements are resolved by a hybrid approach: (i) alignment of the LR images by a fast feature-based registration method, and (ii) fine adjustment of the sub-pixel information using a continuous optimization approach designed to find the global optimum solution. Numerical method for phase-retrieval is applied to decode the signal and reconstruct the morphological details of the analyzed sample. The presented approach was evaluated with various biological samples including sperm and platelets, whose dimensions are in the order of a few microns. The obtained results demonstrate a spatial resolution of 1.55 µm on a field-of-view of ≈30 mm2.

Electronic nose analysis of exhaled breath to diagnose ventilator-associated pneumonia

BackgroundExhaled breath analysis is an emerging technology in respiratory disease and infection. Electronic nose devices (e-nose) are small and portable with a potential for point of care application. Ventilator-associated pneumonia (VAP) is a common nosocomial infection occurring in the intensive care unit (ICU). The current best diagnostic approach is based on clinical criteria combined with bronchoalveolar lavage (BAL) and subsequent bacterial culture analysis. BAL is invasive, laborious and time consuming. Exhaled breath analysis by e-nose is non-invasive, easy to perform and could reduce diagnostic time. Aim of this study was to explore whether an e-nose can be used as a non-invasive in vivo diagnostic tool for VAP. MethodsSeventy-two patients met the clinical diagnostic criteria of VAP and underwent BAL. In thirty-three patients BAL analysis confirmed the diagnosis of VAP [BAL+(VAP+)], in thirty-nine patients the diagnosis was rejected [BAL−]. Before BAL was performed, exhaled breath was sampled from the expiratory limb of the ventilator into sterile Tedlar bags and subsequently analysed by an e-nose with metal oxide sensors (DiagNose, C-it, Zutphen, The Netherlands). From further fifty-three patients without clinical suspicion of VAP or signs of respiratory disease exhaled breath was collected to serve as a control group [control(VAP−]). The e-nose data from exhaled breath were analysed using logistic regression. ResultsThe ROC curve comparing [BAL+(VAP+)] and [control(VAP−)] patients had an area under the curve (AUC) of 0.82 (95% CI 0.73–0.9). The sensitivity was 88% with a specificity of 66%. The comparison of [BAL+(VAP+)] and [BAL−] patients revealed an AUC of 0.69; 95% CI 0.57–0.81) with a sensitivity of 76% with a specificity of 56%. ConclusionE-nose lacked sensitivity and specificity in the diagnosis of VAP in the present study for current clinical application. Further investigation into this field is warranted to explore the diagnostic possibilities of this promising new technique.

Heterogeneously integrated optical detection platform for on-chip sensing applications

This paper introduces a resonant waveguide grating (RWG) integrated with a photodetector for on-chip sensing applications. An inverted thin film indium gallium arsenide (InGaAs) metal–semiconductor–metal photodetector is heterogeneously integrated on silicon dioxide (SiO2)/ silicon substrate using fluidic self-assembly. The inverted optical waveguide and the RWG structure are subsequently fabricated utilizing layer-by-layer post-processing techniques. Detailed electrical, optical, and optoelectronic characterization were performed to analyze the functionality of the sensing platform. This approach can open new avenues allowing the development of low-cost complementary metal-oxide semiconductor compatible integrated optical sensors.

ZnO Nanostructures with Different Morphologies and Their Combinatorial Optimization with Graphene Oxide for Gas Sensor Properties

Consumer concerns on food safety and quality are increasing worldwide. Without opening the package of food products, collecting information about freshness of the product is desirable. Therefore, an electronic nose device consisting of multiple gas sensors can be utilized for rapid monitoring of food quality and freshness at low cost. Such multiple gas sensors, i.e array structure can provide selective detection and identification of various compounds by linking gas responses to signal process. Gas sensors based on chemical reaction on the surface of semiconducting metal oxides are readily available commercially. Common material of the metal oxide sensor is tin oxide doped with a small amount of a catalytic metal such as palladium or platinum. The improvement of gas sensing performance such as low operating temperature as well as selective and sensitive detection can be obtained by investigating potential oxides with controlled morphologies at the nanoscale and their composite structures. Among various semiconducting materials, ZnO is an excellent candidate for gas sensor applications. In addition, many methods can be utilized to fabricate various ZnO nanostructures. Solution process can be simple and effective bottom-up path to construct hierarchical ZnO nanostructures on various substrates. Therefore, we investigated a solution method as controlling the morphology of ZnO nanostructures by applying DC and AC bias and by selecting different precursors. ZnO nanorods structure was synthesized using zinc nitrate hexahydrate and HMT (hexamethylenetetramine) in aqueous solution. The use of zinc chloride instead of zinc nitrate hexahydrate for zinc precursor, nanoflake structure was successfully fabricated where zinc chloride serves as a capping ion to prevent isotropic growth. With applying external voltage during solution growth, ZnO nanorods became agglomerate structures and the degree of agglomerates varies depending on the range of external bias. Various ZnO nanostructures have been analyzed with SEM, XRD and UV-visible. Figure 1 shows one example of morphological control of ZnO nanostructures according to applying bias and precursors. The change of morphologies showed different degree of orientation of the structures by XRD result. Gas sensing response was measured to extract characteristic feature at different ZnO morphologies. Elevated operating temperature approaching 200 – 300°C from ZnO gas sensor can be lowered by combining graphene oxide (GO). Various combinatorial structures from ZnO and GO were explored. Detailed discussion on gas sensing properties and their mechanisms will be given. Figure 1

New Strategy for Rapid Detection of the Simulants of Persistent Organic Pollutants Using Gas Sensor Based on 3-D Porous Single-Crystalline ZnO Nanosheets

It is significant to detect the persistent organic pollutants (POPs) in an environment for both human health and environmental risk assessment. However, most of the current analytical techniques require complicated and tedious processes and are time consuming. Semiconducting metal-oxide sensors are predominant solid-state gas detecting devices for domestic, commercial, and industrial application. Here, combing both the advantages of instrument analytical methods and the metal-oxide sensors, a gas chromatography column was placed in front of a metal-oxide sensor to separate POP simulants from a mixture. The metal-oxide sensor was used to detect the separated sample. To improve the sensitivity, 3D porous single-crystalline ZnO nanosheets (PSCZNs) were synthesized by a one-pot hydrothermal method followed by an annealing treatment. Then, the as-prepared 3D PSCZN-based sensors were exploited to couple with gas chromatography column. The simulants of POPs were used to assess the as-prepared sensor and chromatography column coupled measurement method. The experiment results showed a promise for rapid detection of POPs by metal-oxide sensors.

On the Ammonia Gas Sensing Performance of a RF Sputtered NiO Thin-Film Sensor

An interesting ammonia gas sensor based on a p-type NiO thin film, prepared by a radio frequency sputtering process, is studied and demonstrated. As compared with conventional n-type metal-oxide sensors, the studied device shows comparable and good sensing performance, including a high-sensing response ratio of 289%, a lower response time of 31 s, and a lower recovery time of 78 s, under an introduced 1000 ppm NH3/air gas at 250 °C and 350 °C, respectively. In addition, the studied sensor device exhibits a lower detection limit (<5 ppm NH3/air) at 250 °C. Consequently, based on these advantages and inherent benefits of low cost, chemical stability, and easy fabrication, etc., the studied NiO thin-film sensor shows the promise for high-performance ammonia gas sensing applications.

Diagnosis of bovine tuberculosis using a metal oxide-based electronic nose.

Electronic noses (e-noses) have been used for environmental monitoring, standardization of medicinal flavourings, food safety tests and diagnosis of infectious diseases based on the statistical analysis of volatile organic compounds (VOCs). Bovine tuberculosis (bTB) is officially diagnosed using the intradermal skin test (IST), which is time-consuming and labour-intensive. Therefore, a more convenient and rapid test with greater sensitivity would be advantageous as prescreening test. In this study, we used a metal oxide sensor (MOS) type e-nose to analyse VOCs in a bTB-infected (n = 11) and bTB-free (n = 10) sera, from cattle whose health status was confirmed using the IST, and pathological and bacteriological examinations. The differences in VOCs from bTB-infected and bTB-free sera detected by the e-nose were statistically analysed using principal components and discriminant factor analyses. bTB-infected and bTB-free sera could be discriminated by MOS type e-nose, and analysing time per sample was only 20 min. VOC analysis using a MOS e-nose was a rapid and automated prescreening method to diagnose bTB, and can be used to select bTB-suspect cattle for IST confirmation. Further studies are required to estimate test sensitivity and specificity. Significance and impact of the study: Bovine tuberculosis (bTB) in cattle is diagnosed using the intradermal skin test (IST); however, this method is very time-consuming and labour-intensive. We analysed volatile organic compounds that are obtained from serum using a metal oxide sensor type of electronic nose to discriminate between TB-infected and TB-free sera. This simple and automated technique will be useful to prescreen bTB-suspects and reduce the time and labour required to perform the IST.

Detection of colorectal cancer biomarkers in the presence of interfering gases

Medical studies have shown that tumor growth is accompanied by protein changes that may lead to the peroxidation of the cell membrane, with consequent emission of volatile organic compounds (VOCs). VOCs can be detected through breath or intestinal gases and are biomarkers for colorectal cancer (CRC). The analysis of VOCs represents a non-invasive and potentially inexpensive pre-screening technique. An array of chemoresistive gas sensors, based on screen-printed Metal OXide (MOX) semiconducting films, has been selected to identify gaseous compounds of oncological interest, i.e. benzene, 1-iodo-nonane and decanal, from the main interferers that can be found in the intestine. MOX sensors are able to detect concentrations down to about 10th ppb, as experimentally proven in previous works, so they can identify very slight differences in concentration among gas mixtures. In this work it has been proven that the array used is able to identify tumor markers singularly and in combination with other gases both in wet and dry conditions. Moreover, the sensors chosen can discriminate target VOCs from interferers even at low concentrations.

Atomic Layer Deposition of SnO2 for Selective Room Temperature Low Ppb Level O3 Sensing

Ozone sensing is particularly interesting as exposure has been shown to be very harmful to human health. Ozone exposure has been associated with an increased risk of developing respiratory disease, inflammation of airways and increased response to allergens. Individuals suffering from respiratory diseases such as asthma are particularly sensitive to ozone and both short and long term exposures can induce undesirable effects on the human condition [1-4] and ozone concentrations have increased strongly in highly populated areas over the past decade [2,5]. Key challenges in metal oxide sensors are high power consumption associated with high operating temperatures and lack of selectivity as the sensors tend to respond broadly to many different gases. In this study we demonstrate low power and selective ozone sensors using SnO2 metal oxide thin film gas sensors produced by Atomic Layer Deposition (ALD). ALD enables precise film thickness control and conformal coating tailoring of film thickness to the Debye length, enhancing depletion region effects and increasing surface to volume ratios for enhanced sensitivity at lower temperatures. SnO2 films were deposited via ALD using tetrakis dimethylamino tin precursor and ozone reactant on SiO2/Si substrates. The films were annealed in atmosphere or inert ambient at 400C for 30 minutes and were then patterned for lift off using standard photolithography. Ten nm of titanium were used for an adhesion layer followed by 150 nm of Au. The SnO2 deposition rate was characterized with variable angle spectroscopic ellipsometry and determined to be 1.1 A/cycle. X-ray photoelectron spectroscopy and X-ray diffraction were used to evaluate the film properties. Ozone testing was done using custom built testing chamber and a Teledyne T700U gas calibration system equipped with a UV ozone generator and photometer feedback for precision concentrations. Resistance measurements were made during gas exposure using a Keithley 4200 semiconductor parameter analyzer. We have characterized our ALD SnO2 process and film properties. XRD confirmed rutile phase of the film after 400C anneal for 30 minutes as shown in Figure 1. Ozone exposure resulted in increased resistance of the SnO2 films and response was demonstrated for various concentrations at room temperature resulting in ozone detection at several microwatts of power. Response to concentrations from 50-150 ppb can be seen in figure 2, and to 400 ppb in figures 3 and 4. It is found that the recovery of the SnO2 sensors is related to the film thickness. Films of 12 nm of SnO2 demonstrated extremely slow recovery while films of 6 nm recovered more quickly even at room temperature as shown in Figures 3 and 4. The recovery process can be expedited by the addition of thermal energy to aid in ozone desorption but low power applications at room temperature can take advantage of both of these modes of operation. Thicker sensors with slow recovery may be useful in measuring dose of ozone over time for asthmatics and other health conscious users while the thinner films may be better suited toward instantaneous concentration quantification. Both sensors offer faster response and recovery with increasing temperature enabling trading off of power needs with sensor response but room temperature operation offers yet another advantage. Without any added thermal energy, our films respond to tens of ppb of ozone while not responding to hundreds of ppb of NO2 or ppm of CO, typical concentration found in the environment. Response to each of these gases can be seen in Figures 5 and 6. Sensor reset can be accomplished with periodic thermal cycles to maintain low power levels and selectivity advantages of room temperature operation. In summary, we have demonstrated room temperature SnO2 gas sensor deposited by ALD method. It was observed that the ALD SnO2 gas sensor enables sensitivity to ozone in the 50-400ppb range with selectivity toward NO2 and CO, common interfering gases in the environment. It was also shown that our SnO2 sensor operates at room temperature and hence the total power consumption reduces to mW range that is feasible for wearable sensor system. Figure 1

Contribution of chicken base addition to aroma characteristics of Maillard reaction products based on gas chromatography-mass spectrometry, electronic nose, and statistical analysis

Maillard reaction products containing 0–86.67% (w/w) enzymatically hydrolyzed chicken breast (chicken base) were analyzed. Descriptive sensory analysis was conducted and the 5 sensory attributes of meaty, caramel, umami, continuity, and off-flavor were used for evaluation. A sample group containing 65.00% chicken base had high scores for meaty note. A sample group containing 86.67% had a high score for caramel note. The electronic nose was used for measurement of flavor quality changes based on a metal oxide sensor (MOS) array. GC-MS was used for identification of volatile compounds. Heterocyclic sulfur and nitrogen oxygen compounds (especially 2-methyl-3-furanthiol, 2-methyl-furan, 2-pentyl-furan, 2-acetyl-furan, bis (2-methyl-3-furyl) disulfide, furfural, 2-acetyl-pyrrole, 1-methyl-pyrrole, thiazole, and 4-methyl-thiazole) were responsible for meaty and caramel attributes. Statistical analysis was used to interpret the distribution of samples and correlations among variations.

Su1352 Detection of Clostridium difficile-Infected Stool by Electronic-Nose Analysis of Fecal Headspace Volatile Organic Compounds

Introduction Electronic-noses (e-noses) have the ability of converting smells to digital signatures. By interacting with volatile organic compounds (VOC's) emitted by metabolic end-products of disease, e-noses create distinct patterns of aggregate VOC's to generate unique signatures that can be used to identify disease. The detection of Clostridium difficile (C. diff) by VOC's is prototypic as it has been successfully identified by scent using canines. We seek to pilot a novel portable e-nose device to profile fecal headspace to diagnose C. diff infection. Methods We are prospectively enrolling patients undergoing stool testing by C. diff PCR for infectious diarrhea for fecal headspace analysis using the Aetholab e-nose (eNose Company, Zutphen, NL). Stool specimens were obtained from the clinical microbiology lab of our institution within 7 days of ambient collection and aliquots of 10-ml of homogenized stool were transferred to disposable analysis bottles. The e-nose circulates fecal headspace gas within each analysis bottle to allow interaction with its respective sensor array. Each sensor array comprises of three metal-oxide sensors that undergo reversible redox reactions with VOC's at an electrochemical interface. Electrical resistance changes are measured by applying a 32-point thermal cycle to the sensor array generating a digital signature (Figure 1). This process has been shown to interact with a wide range of VOC's. Combined data from the 3 sensors in 7 combinations (A, B, C, AB, AC, AC, ABC) is permuted across 3 scaling factors generating a total of 21 data sets per analysis. These are then compressed through a multi-way analysis to reduce the large amount of data to avoid spurious associations. Finally each sample's compressed dataset is introduced into an artificial neural network (ANN) using a supervised approach. The 5 best models generated using a leave-one-out approach are used to obtain the reported results. Results Separation was observed using all 20 C. diff-PCR positive and 53 C. diff-PCR negative stools. The ROC curve for this model is shown in Figure 2 with an AUC = 0.85. This demonstrated 80% sensitivity, 85% specificity with 84% accuracy. The positive predictive value was 0.67 and the negative predictive value 0.92. Matthews correlation coefficient was 0.64. Conclusion Through the use of a pattern-recognition e-nose device it is possible to distinguish C. diffinfected stools. This is the first time that this type of device has been used for this diagnosis, and also the first time this e-nose has been applied for fecal headspace analysis of infectious disease. This indicates that C. diff testing can translate into a point-of-care evaluation.

Reservoir computing compensates slow response of chemosensor arrays exposed to fast varying gas concentrations in continuous monitoring

Metal oxide (MOX) gas sensors arrays are a predominant technological choice to perform fundamental tasks of chemical detection. Yet, their use has been mainly limited to relatively controlled instrument configurations where the sensor array is placed within a closed measurement chamber. Usually, the experimental protocol is defined beforehand and it includes three stages: the array is first exposed to a gas reference, then to the gas sample, and finally to the reference again to recover the initial state. Such sampling procedure requires signal acquisition during the complete experimental protocol and usually delays the output prediction until the predefined measurement duration is complete. Due to the slow time response of chemical sensors, the completion of the measurement typically requires minutes. In this paper we propose the use of reservoir computing (RC) algorithms to overcome the slow temporal dynamics of chemical sensor arrays, allowing identification and quantification of chemicals of interest continuously and reducing measurement delays. We generated two datasets to test the ability of RC algorithms to provide accurate and continuous prediction to fast varying gas concentrations in real time. Both datasets – one generated with synthetic data and the other acquired from actual gas sensors – provide time series of MOX sensors exposed to binary gas mixtures where concentration levels change randomly over time. Our results show that our approach improves the time response of the sensory system and provides accurate predictions in real time, making the system specifically suitable for online monitoring applications. Finally, the collected dataset and developed code are made publicly available to the research community for further studies.