Conductimetric Sensor Research Articles

Single-point calibration process based integrated electrical impedance analyzer for multi-selective gas detection

Impedance analysis is a powerful technique that has become increasingly important in various applications, it represents a leap forward in the field of electronic measurments and diagnostics. In this work, we present the development of miniaturized, multiplexed, and connected platform for impedance spectroscopy. Designed for online measurements and adapted to wireless network architectures, our platform has been tested and optimized to be used for multi-selective chemical organic sensor nodes. This compact and versatile circuit is built from low cost and low power consumption (250 mW) microelectronics components that achieve long duration operability (5 days and 16 h) without compromising on sensor measurement accuracy and precision. We used the well-known impedance network analyzer AD5933 (Analog Devices, Norwood, MA, USA) chip which can measure a spectrum of impedances in the range 5 kHz to 100 kHz. The proposed system is based on ESP32-C3 Microcontroller enabling the management of the AD5933 through its I2C interface. Our system benefits from two multiplexer components CD74HC4067 allowing calibration process and the interface of 15 conductimetric sensors with real time acquisition (less than 90 ms per acquisition). The system is capable of relaying information through the network for data analysis and storage. The paper describes the microelectronics design, the impedance response over time, the measurement’s sensitivity and accuracy and the testing of the platform with embedded chemical sensors for gas classification and recognition.

High performance conductimetric sensor for detection of trace triethylamine based on the metalloporphyrin 2D-conjugated polymer assisted with a phosphating strategy

Porphyrin conjugated polymers (PorCPs) have emerged as next-generation semiconducting materials with pre-designed π-electronic skeletons and highly ordered topological structures, especially for gas sensing applications. In this study, two porphyrin conjugated polymers M-PorCPs (M = Fe and Co) with highly dispersed central metal active adsorption sites were prepared, and the band gaps of the M-PorCP were adjusted by phosphating process. Both M-PorCP and the resulted P @ M-PorCP are characterized by a wide range of methods including FT-IR, Raman, X-ray diffraction, SEM, TEM, UPS, UV–vis diffuse reflectance spectra, BET, and current–voltage (I–V) measurements. The experimental results indicate that subsequent phosphating improves electrical conductivity of the parent M-PorCPs by an order of magnitude (from ∼10−5 S cm−1 to 10−4 S cm−1) thanks to the smaller band gap of the resulted P @ M-PorCPs than their parent polymer M-PorCPs but the structures of polymer are maintained. Moreover, the sensing behavior of the both P @ M-PorCPs and M-PorCPs was investigated towards triethylamine (TEA) vapor in the 1–10 ppm range. High sensitive responses to TEA are obtained for the four sensors with the increasing degree in the order of Co-PorCP < Fe-PorCP < P @ Co-PorCP < P @ Fe-PorCP. The highest sensitivity (5.27%ppm−1) to the response of 1–10 ppm TEA was obtained for the P @ Fe-PorCP with a low limit of detection (LOD) of 0.17 ppm among the four sensors, which are attributed to larger specific surface area of its parent Fe-PorCP and importantly stronger N-Fe binding energy of TEA to FePor metal center at the air/Fe-PorCP interface relative to those of the Co-PorCP, combined with the highest electrical conductivity among the four sensors. Moreover, P @ Fe-PorCP also showed excellent selectivity, good repeatability and humidity stability. The present works not only represent the first example for TEA sensor based on porphyrin-based organic semiconductor materials, but importantly the sensing performances in terms of LOD and sensitivity for the P @ Fe-PorCP are superior to those of the TEA sensors based on inorganic semiconductor-based nanomaterials.

934-P: Continuous, Quantitative, Selective, Nonenzymatic Glucose Monitoring Using Conductimetric Analysis

Commercially available continuous glucose monitors (CGMs) typically use electrochemical sensing methods and depend on the activity of enzymes, which is subject to drift with time. Newly developed optical CGMs also suffer from drift due to the photobleaching of the fluorescent dyes. Accordingly, existing CGMs must be calibrated over periods of hours to days. Here we developed a new molecule that can be used to sense glucose concentration through conductivity sensing. The new recognition molecule binds glucose with a suitable affinity (Kd = 1 mM) and selectivity, and hosts negative charges in physiological conditions after binding with glucose. The glucose binding and charge change alters the charge spices which contribute to the conductivity of solution. Glucose concentration therefore determines the conductivity of solution. We use a conductimetric method based on low voltage AC (∼20 mV) impedance spectroscopy which offers the promise of long-term stability. Current conductimetric sensors shall be interpreted with selective permeable membrane systems and would be used to monitor sampled fluid soon. This sensor does not need expensive enzyme and fluorescent dyes, and the preparation of the novel molecule is easy and low cost. The sensing strategy only uses low voltage power, and thus is more stable than enzymatic and optical methods, providing the possibility of less or no calibration. Our current conductimetric sensors are compatible with the well-developed high frequency AC microchip technology, which makes it lower-powered and easy to be miniaturized. Disclosure B. Wang: None. S. Pennathur: Board Member; Self; Laxmi Therapeutic Devices. Consultant; Self; Alveo Technologies. K. Chou: None. Funding American Diabetes Association (1-17-VSN-18 to S.P.)

Fabrication of thick film sensitive RuO2-TiO2 and Ag/AgCl/KCl reference electrodes and their application for pH measurements

In this work thick film potentiometric and conductimetric sensors based on mixed metal oxides RuO2 and TiO2 were fabricated and investigated. For potentiometric sensors, pH sensitive RuO2-TiO2 electrode and a reference Ag/AgCl/KCl electrode were developed and deposited on an alumina substrate by screen printing method. For conductimetric pH measurements, the sensitive RuO2-TiO2 paste was screen printed on the surface of an interdigitated conducting electrode. The results of the potentiometric, impedance spectroscopic, SEM, EDS, XRD and XPS studies of the fabricated sensors were presented and discussed. The thick film pH sensor with both the reference and sensitive electrodes screen printed on the same alumina substrate shows a sensitivity of 56.11mV/pH, close to the theoretical value, a very fast response shorter than 15s, long lifetime and reusability. Its applicability for measurements of pH of water was confirmed. The electrochemical impedance spectroscopy method was used for characterization of the properties of the sensitive RuO2-TiO2 film in a frequency range of 10Hz–2MHz. The sensor conductance was found to show a linear dependence on the pH value in the range of 4–11. Comparison of X-ray photoelectron spectra of the sensors before and after treatment in various solutions revealed some differences in surface composition dependent on pH value. The cost of the fabricated thick film pH sensor was significantly reduced by using for the sensitive electrode besides RuO2 a much cheaper second oxide TiO2, and due to a simple method of the reference electrode preparation.

Copper Oxide Nanowires for Surface Ionization Based Gas Sensor

This work focuses on the development of surface ionization gas sensors based on copper oxide (CuO). CuO nanowires were prepared by thermal oxidation starting from a thin film of metallic copper deposited by RF magnetron sputtering. In addition to the well-known conductimetric sensors, we have developed another kind of device depending upon a different sensing mechanism, namely surface ionization (SI). SI phenomenon involves ions formed by the adsorption of gas molecules on heated solid surface, the electron transfer from the adsorbed species to the electrode surface and the extraction of the ion adsorbates into free space by an external electrical field. Our preliminary results show that humidity plays an important role in the ions formation and so in sensors performance. For this reason we have done a systematic study on the interaction mechanism of target molecules with hydroxyl groups adsorbates on CuO surface, explaining the experimental results in terms of acidity/basicity.

Milk electrical conductivity and mastitis status in dairy goats: Results from a pilot study

Mastitis in small ruminants has a major effect in reducing both yield and quality of milk leading to strong economic losses. In addition, mastitis is one the most frequently cited reason for culling. In dairy cows electrical conductivity (EC) of milk has been introduced as an indicator trait for mastitis. When the measurement of electrical conductivity (EC) of milk is based on time-series analysis of historical data, comparing values from different quarters within milking, satisfactory results in mastitis detection can be achieved. On the contrary, mastitis detection models based on EC measurement of milk within milking are not available in literature for dairy goats. Aim of the study was to evaluate the usefulness of milk EC, measured within milking from halves of each udder, to predict mastitis status in dairy goats at farm level. For this purpose, a group of 8 Saanen goats was involved in a 225-day study. Two foremilk samples were daily collected during morning milking from halves of each udder for a total of 4 samples per animal/day. Results of bacteriological analyses were used to distinguish between healthy (absence of mastitis agents) and infected (presence of contagious or environmental mastitis agents) udder halves. EC was measured in milk from halves of each udder during every morning milking at 1-s intervals using four experimental milking units equipped with conductimetric sensors. The average of the 20 highest EC values ( X 20 ) and the variation of the 20 highest EC values ( σ 20 2 ) within milking were calculated for each udder halves. In addition 4 EC traits based on X 20 and σ 20 2 were computed: the highest half udder X 20 value within goat and milking (Max_ X 20) , the highest half udder σ 20 2 value within goat and milking ( Max _ σ 20 2 ), the inter-udder halves ratio between the udder halves X 20 values within goat and milking (IHR_ X 20 ), and the inter-udder halves ratio between udder halves σ 20 2 value within goat and milking ( IHR _ σ 20 2 ). Values for the traits were calculated for every milking throughout the whole lactation. Proc Mixed of SAS ® 9.1 (SAS Institute Inc., Cary, NC, USA, 2008) was performed to test for significant differences in EC traits between healthy and infected udder halves and between healthy (absence of infected udder halves) and infected (presence of at least one infected udder half) goats. Results highlighted that the average of the 20 highest electrical conductivity half udder values recorded within milking in early lactation and the highest half udder X 20 value within goat and milking in mid lactation could be potentially used in monitoring udder health of dairy goats.

A nanohybrid membrane with lipid bilayer-like properties utilized as a conductimetric saccharin sensor

Since their introduction, artificial lipid bilayer membranes were used in a wide array of applications, such as sensors, biocompatible materials and study-models of the cell's outer boundary. Here, we present a nanohybrid membrane using an inorganic host and amphiphilic organic molecules with lipid bilayer-like properties. The stability of the presented mimetic membrane is significantly improved when compared to existing methods. The nanohybrid membrane exhibited two thermotropic phases corresponding to the Lα and Lβ phases that lipid bilayer membranes are known to adopt. Integration of cholesterol molecules into the nanohybrid membrane lead to the same qualitative effects as in lipid bilayers, including expansion of the bilayer spacing and decrease of the Lα to Lβ transition enthalpy. To further illustrate the similarities of the synthesized membrane with a lipid bilayer, the ability of the nanohybrid membrane to function as saccharin conductimetric sensor was evaluated. The lower limit of detection of the sensor was 6μM and the linear range of response was from 20 to 400μM.

Poly(aniline boronic acid)-based conductimetric sensor of dopamine.

Interdigitated microelectrodes derivatized with poly(aniline boronic acid) show a dopamine-sensitive response at pH 7.4.

MoO3 and WO3 based thin film conductimetric sensors for automotive applications

Un-doped semiconducting oxides suitable for automotive gas sensor applications have been studied in this work. Thin films of MoO3 and WO3 were fabricated by ion beam deposition on alumina substrates with gold interdigitated electrodes. The process pressure inside the deposition chamber was 1.6 × 10−4 Torr. The oxygen to argon ratio in the secondary plasma was maintained at 5:5 sccm. A stabilization heat treatment of 500°C for 8 h was performed for each set of films that produced nanocrystalline structures. Gas sensing tests were carried out at 450°C with nitrogen dioxide/ammonia with synthetic air background similar to those realized in diesel automotive exhausts. XRD and electron microscopy studies were performed to understand the microstructure of the thin films following the sensing tests. The MoO3 films were selective to ammonia whereas the WO3 films showed high senstitivity towards NO2 with respect to NH3. An attempt is made to correlate the structural characteristics to the sensing behavior of the materials.

Characterisation and optimisation of AC conductimetric biosensors

Urease, immobilised on interdigitated gold electrodes, is employed as a model enzyme for characterisation and optimisation of a.c. conductimetric sensors. The sensors’ response is measured over a frequency range of 20 Hz to 300 kHz and an optimum operating frequency established. The activity of the urease, both in solution and immobilised states, is investigated and K m values obtained. The effect of method of immobilisation and enzyme loading on the sensors’ performance are studied and urease electrodes are characterised as a function of temperature, pH and electrolyte concentration. An important finding, particularly for conductimetric sensors designed for clinical use, is that proper consideration of the effects of electrode polarisation must be taken into account in order to maintain high sensor sensitivity at physiological electrolyte concentrations. Measurements of urea concentration in untreated serum are described.

A membrane-moderated, conductimetric sensor for the detection and measurement of specific organic solutes in aqueous solutions

A novel conductimetric sensor, capable of detecting and continuously monitoring the concentration of a specific nonionic microsolute (glucose) in a multicomponent aqueous solution (e.g., whole blood or plasma) is described, and preliminary experimental evidence supporting the validity of the sensor concept is presented. Detection is based upon the use of a glucose-selective complexing agent (a boronic acid immobilized in a hydrogel) which liberates a mobile microion (hydrogen ion) when it binds a glucose molecule. The change in ionic conductivity of the hydrogel resulting from the increase in ion concentration is thus directly related to the ambient glucose concentration. Confinement of the liberated ions within the hydrogel, and prevention of entry of extraneous electrolytes present in the test solution, is necessary to make the conductimetric measurement meaningful. This is achieved by encapsulating the hydrogel within a bipolar ion exchange membrane impermeable to ions but freely permeable to glucose (and other nonionic microsolutes). A physicochemical model of the complexation equilibrium, kinetics of solute transport through the membrane and hydrogel phase, and their impact upon the ionic conductivity of the hydrogel is presented, which supports the utility of this sensor concept as a potentially reliable and sensitive glucose monitor. Its generic utility for monitoring nonionic microsolutes in multicomponent aqueous solutions is suggested.

Conductimetric sensor for atrazine detection based on molecularly imprinted polymer membranes

Atrazine-sensitive conductimetric sensors were designed using molecularly imprinted polymer membranes. Membranes containing artificial recognition sites for atrazine were prepared by copolymerization of methacrylic acid and a cross-linker, tri(ethylene glycol) dimethacrylate, in the presence of atrazine as a template. In order to improve the flexibility and mechanical stability of the membranes, oligourethane acrylate was added to the mixture of monomers. The recognition sites complementary to atrazine were formed in the membranes after extraction of the template molecules with ethanol. Alternatively, reference polymer membranes were prepared with the same monomers but without the template. The responses of the membranes prepared with and without the template were monitored conductimetrically. The membranes prepared in the presence of atrazine showed significantly stronger responses to atrazine than to analogous compounds (triazine, simazine, prometryn). The response time was 6–15 min depending on the membrane thickness. The effect of the membrane composition and the porogen concentration on the magnitude of the conductimetric responses was also investigated. With the sensor designed here, the detection of the atrazine at concentrations down to 5 nmol dm–3 was demonstrated. During a 6 month period, the sensitivity of the molecularly imprinted membranes to atrazine was found to remain constant.

Chemiresistive sensors of electrically conducting poly(2,5-thienylene vinylene) and copolymers: their responses to nine organic vapours

An apparatus and a method are described to test the sensing capacities, in the context of an electronic nose, of conductimetric sensors for organic vapours. Such a sensor was constructed using iodine doped poly(2,5-thienylene vinylene) (PTV) and tested in detecting nine ‘representative’ vapours. Responses after 1 min exposure are high and mostly linear with respect to concentrations. Recoveries, however, using 4 min of clean air, are only partial. Yet, responses and recoveries are reproducible and characteristic for a particular vapour. Partial methanol elimination from the methoxy-precursor polymer of PTV leads to new sensing materials with different selectivities. Also discussed are: (i) influence of the humidity of the carrier gas (air), (ii) possible causes of observed baseline-drift and (iii) possibilities of regeneration of the materials after prolonged use. It is concluded that PTV and derivatives show promise as active materials in sensor arrays.

Application of Poly(thienylene vinylene) as a chemiresistor for organic vapours

Results are reported about the application of iodine doped Poly(thienylene vinylene) (PTV) as a conductimetric sensor. A description is given of the layout of the sensor, formed by spin coating the soluble precursor polymer on an alumina substrate with gold contacts. The set-up for generating continuous vapour flows over the sensor and the monitoring of the electrical resistance are briefly discussed. Responses (%Ra) of the sensor to a series of nine saturated vapours are reported. Recovery (%Rb) from the response is in general not complete and characteristic for the vapour. The partial recovery is not a poisoning effect and does not affect the reproducibility; on the contrary, graphical combination of %Ra and %Rb shows additional information.

Microfabricated conductimetric pH sensor

A miniature conductimetric pH sensor has been developed, based on the measurement of the conductivity of a pH-responsive hydrogel. The microfabricated sensor is constructed by coating planar interdigitated electrode arrays with a photolithographically patterned hydrogel membrane. The hydrogel sensing layer swells or shrinks to a hydration determined by the pH of the solution in which it is immersed. This leads to a corresponding increase or decrease in the mobility of the ions partitioned by the gel. The sensitivity of ion mobility to small changes in hydration leads to sensor resistance changes as large as 45% per pH unit at physiological pH. Since the sensor operation is based on changes in ion mobility, the sensor operates in high ionic strength solutions, whereas most conductimetric sensors to date have been limited to use in solutions with low mobile ion content. The sensor response time is approximately five minutes, and appears to be limited by the protonation/deprotonation of the polybasic hydrogel. Devices have operated for more than six weeks with a 50% loss of sensitivity over three weeks.

Measurement of tear electrolyte concentration and turnover rate using a flexible conductimetric sensor

Tear fluid conductivity was measured by a non-invasive method. A flexible 3 mm-wide conductimetric sensor was placed inside the human subjects temporal lower cul-de-sac (similar to Schirmer test strip), and used to evaluate electrolyte concentration and turnover rate in tear for normal, healthy subjects aged between 30 to 85 years in a normal, light indoor environment. The tear electrolyte concentration was calculated from tear conductivity to give a mean value of 297 mEq/l (S.D. 30 mEq/l; n = 29) which was consistently with previously reported values. Tear turnover rate was calculated by a single exponential equation of tear conductivity change, following the application to the eye-drops of 40.0 milligrams sodium chloride solution. The mean turnover rate was 44.2% per minute (S.D. = 13.3% per minute; n = 30), being in agreement with previously reported values.

Atrazine sensing by molecularly imprinted membranes

New types of polymeric membranes containing molecular recognition sites for atrazine have been prepared using the molecular imprinting approach. The membrane synthesis includes radical polymerization of diethyl aminoethyl methacrylate and ethylene glycol dimethacrylate in the presence of atrazine as template. After splitting off the template molecules, these polymers have been used as materials for conductimetric sensors, sensitive for the herbicide. Influence of polymerization conditions on membrane sensitivity and nature of sensor response is discussed.With this system atrazine in solution can be detected in the range 0.01–0.50 mg/L. Although this dynamic range is at present not large, the membranes did not show loss of sensitivity for at least 4 months. The response time for the sensor is in the order of 30 min, which might be reduced using thinner imprinted membranes.

Flexible conductometric sensor

A flexible sensor constructed in a sandwich configuration with a hdyrophilic poly(tetrafluoroethylene) membrane placed between two gold deposited layers was evaluated for use as a conductimetric sensor in biologic fluids. The conductivity was measured using the device at frequencies ranging from 100 Hz to 100 kHz, and the device was calibrated at 100 kHz against sodium chloride solutions over the range of 0.1-50.0 g/L, which includes physiologic ion concentrations

Doped (conducting) and undoped (semi-insulating) LB films: application to gas sensing

The present paper deals with the use of semi-insulating and conducting LB films as conductimetric sensors. The response of such sensors consists in conductivity changes induced by gas exposure. Examples collected in the literature are presented and show that the LB technique is very convenient for making continuous ultrathin organic materials exhibiting detection properties towards toxic gases (NO 2, PH 3,…). Very interesting results have been obtained using mixed LB films even if the conducting properties of the resulting material and the detection mechanism are not well understood. Some multifunctional materials are required exhibiting conduction properties for transduction and ‘recognition properties’ for selective detection. With that goal, the LB technique associated with organic chemistry provides undeniable advantages.

A Photopaiterned Glucose Responsive Hydrogel for Use in a Conductimetric Sensor

AbstractThe glucose responsive hydrogels developed by Horbett et al. [1, 2] are potentially useful as materials for constructing glucose sensors. The construction of microsensors presents challenges in developing processes for preparing these gels that are compatible with semiconductor fabrication technology.We have lithographically patterned a pH-sensitive copolymer (poly-(2-hydroxyethyl methacrylate-co-dimethyl aminoethyl methacrylate) 90:10 (mole ratio) (HEMA/DMA) from a prepolymer solution of monomers, crosslinker, solvent, and a viscosity agent (p(HEMA)). Varying the spin speed and p(HEMA) concentration resulted in reproducible thicknesses between 5 and 25 μm. pH sensors produced by this method showed sensitivity ranging from 20 to 50%/pH and a response time (10%–90%) of 6.8 minutes. Sensitivity dropped by a factor of two over a 24 day testing period. The limited lifetime is thought to be due to loss of adhesion of the polymer to the substrate.Derivatizing glucose oxidase with a photoactivatible crosslinker allowed it to be covalently immobilized onto the surface of the gel layer in a spatially specific manner. After immobilization, GOX specific staining showed that enzyme activity was preserved, however the amount of enzyme immobilized was insufficient to create a working glucose sensor.