Asif I. Zia

Rapid and molecular selective electrochemical sensing of phthalates in aqueous solution

Reported research work presents real time non-invasive detection of phthalates in spiked aqueous samples by employing electrochemical impedance spectroscopy (EIS) technique incorporating a novel interdigital capacitive sensor with multiple sensing thin film gold micro-electrodes fabricated on native silicon dioxide layer grown on semiconducting single crystal silicon wafer. The sensing surface was functionalized by a self-assembled monolayer of 3-aminopropyltrietoxysilane (APTES) with embedded molecular imprinted polymer (MIP) to introduce selectivity for the di(2-ethylhexyl) phthalate (DEHP) molecule. Various concentrations (1-100 ppm) of DEHP in deionized MilliQ water were tested using the functionalized sensing surface to capture the analyte. Frequency response analyzer (FRA) algorithm was used to obtain impedance spectra so as to determine sample conductance and capacitance for evaluation of phthalate concentration in the sample solution. Spectrum analysis algorithm interpreted the experimentally obtained impedance spectra by applying complex nonlinear least square (CNLS) curve fitting in order to obtain electrochemical equivalent circuit and corresponding circuit parameters describing the kinetics of the electrochemical cell. Principal component analysis was applied to deduce the effects of surface immobilized molecular imprinted polymer layer on the evaluated circuit parameters and its electrical response. The results obtained by the testing system were validated using commercially available high performance liquid chromatography diode array detector system.

Novel Sensing Approach for LPG Leakage Detection—Part II: Effects of Particle Size, Composition, and Coating Layer Thickness

Prominent research has been going on to develop a low-cost, efficient gas sensing system. This paper presents a continuation of our earlier research work done to develop a new sensing approach for gas detection at ambient conditions. This paper exhibits the optimization of the response time of the sensor by inhabiting characteristic changes such as variation in the concentration of the dispersion medium, thickness of the coating, and the size of the dispersed medium. Different concentrations of the dispersion medium in the coated suspension were tested to determine the optimal composition required to achieve the highest sensitivity of the tin oxide (SnO2) layer toward the tested gas. The control over adsorption and desorption of the gas molecules in the coated layer was achieved by investigating the particle size of the dispersed medium. The response time of the coated sensor was encouraging and owns a promising potential to the development of a more efficient gas sensing system.

Introducing molecular selectivity in rapid impedimetric sensing of phthalates

This research article reports a real-time and non-invasive detection technique for phthalates in liquids by Electrochemical Impedance Spectroscopy (EIS), incorporating molecular imprinting technique to introduce selectivity for the phthalate molecule in the detection system. A functional polymer with Bis (2-ethylhexyl) phthalate (DEHP) template was immobilized on the sensing surface of the inter-digital (ID) capacitive sensor with sputtered gold sensing electrodes fabricated over a native layer of silicon dioxide on a single crystal silicon substrate. Various concentrations (10 to 200 ppm) of DEHP in deionized MilliQ water were exposed to the sensor surface functionalized with molecular imprinted polymer (MIP) in order to capture the analyte molecule, hence introducing molecular selectivity to the testing system. Impedance spectra were obtained using EIS in order to determine sample conductance for evaluation of phthalate concentration in the solution. Electrochemical Spectrum Analyzer algorithm was used to deduce equivalent circuit and equivalent component parameters from the experimentally obtained impedance spectra employing Randles cell model curve fitting technique. Experimental results confirmed that the immobilization of the functional polymer on sensing surface introduces selectivity for phthalates in the sensing system. The results were validated by testing the samples using High Performance Liquid Chromatography (HPLC-DAD).

Novel Sensing Approach for LPG Leakage Detection: Part I—Operating Mechanism and Preliminary Results

Gas sensing technology has been among the topical research work for quite some time. This paper showcases the research done on the detection mechanism of leakage of domestic cooking gas at ambient conditions. Micro-electro mechanical systems-based interdigital sensors were fabricated on oxidized single-crystal silicon surfaces by the maskless photolithography technique. The electrochemical impedance analysis of these sensors was done to detect liquefied petroleum gas (LPG) with and without coated particles of tin oxide (SnO2) in form of a thin layer. A thin film of SnO2 was spin-coated on the sensing surface of the interdigital sensor to induce selectivity to LPG that consists of a 60/40 mixture of propane and butane, respectively. This paper reports a novel strategy for gas detection under ambient temperature and humidity conditions. The response time of the coated sensor was encouraging and own a promising potential to the development of a complete efficient gas sensing system.

MEMS based impedimetric sensing of phthalates

Phthalate esters are known ubiquitous teratogenic and carcinogenic environmental and food pollutants. Their detection and quantification is strictly laboratory based, time consuming, expensive and professionally handled procedure. Presented research work describes a real time non-invasive detection technique for phthalates detection in ethanol, water and drinks. The new type of inter-digital sensor design incorporating multiple sensing gold electrodes were fabricated on silicon substrate based on thin film micro-electromechanical system (MEMS) using semiconductor device fabrication technology. A passivation layer of Silicon Nitride (Si3N4) was used to functionalize the sensor. Various concentrations (0.1 to 20ppm) of DINP (di-isononyl phthalates) in ethanol and di (2-ethylhexyl) phthalate (DEHP) in deionized MilliQ water were subjected to the testing system by dip testing method. Electrochemical impedance spectroscopy (EIS) technique was used to obtain impedance spectra in order to determine sample conductance for evaluation of its dielectric properties. The impedance spectra so obtained showed that the sensor was able to detect the presence of phthalates in the samples distinctively. Electrochemical Spectrum Analyser was used to model the experimentally obtained impedance spectra by curve fitting technique to figure out Constant Phase Element (CPE) equivalent circuit. Locally available energy drink and juice was added with phthalates in concentrations of 2, 6 and 10ppm to observe the performance of the sensor in such products. Experimental results showed that the new sensor was able to detect different concentrations of phthalates in energy drinks.

Post Annealing Performance Evaluation of Printable Interdigital Capacitive Sensors by Principal Component Analysis

The surface roughness of thin-film gold electrodes induces instability in impedance spectroscopy measurements of capacitive interdigital printable sensors. Post-fabrication thermodynamic annealing was carried out at temperatures ranging from 30 °C to 210 °C in a vacuum oven and the variation in surface morphology of thin-film gold electrodes was observed by scanning electron microscopy. Impedance spectra obtained at different temperatures were translated into equivalent circuit models by applying complex nonlinear least square curve-fitting algorithm. Principal component analysis was applied to deduce the classification of the parameters affected due to the annealing process and to evaluate the performance stability using mathematical model. Physics of the thermodynamic annealing was discussed based on the surface activation energies. The post anneal testing of the sensors validated the achieved stability in impedance measurement.

Sensor and instrumentation for progesterone detection

The reported research work uses a real time and noninvasive method to detect progesterone hormone concentration in purified water using Electrochemical Impedance Spectroscopy (E.I.S.) technique. Planar capacitive sensor, consisting of inter-digitated microelectrodes, is designed and fabricated on silicon substrate using thin-film Microelectromechanical system (MEMS) based semiconductor device fabrication technology. The sensor in conjunction with EIS is used to evaluate conductivity, permeability and dielectric properties of reproductive hormone progesterone and its concentration quantification in purified water. Impedance spectrums are obtained with various concentrations of the hormone in purified water by using an electric circuit in order to extract sample conductance. Relationship of sample conductance with progesterone concentration level is studied in this research work. The ability of E.I.S. to detect progesterone concentration is aimed to be used in dairy farming industry in order to obtain better reproductive performance of the dairy cattle.

Practical nitrate sensor based on electrochemical impedance measurement

This paper showcases the design and development of a portable sensing system that could be used in-situ to detect the concentrations of nitrate salts present in the groundwater. Electrochemical Impedance Spectroscopy was employed to identify and display nitrate concentrations by using planar interdigital sensor immersed in the stream water samples. Calibration samples were prepared by serial dilution of the nitrate stock solution in the laboratory and test samples were collected from different surface water sources. The test samples were evaluated by the commercial equipment and designed system. Although a difference was observed between these two results, the designed system showed a good linear relationship between the measured nitrate concentrations (range from 0.1 to 0.5 mg/L) of the water solution in the real part of the impedance. The promising comparative results announced the extraordinary potential of the developed system for its in-situ nitrate contamination detection with real-time monitoring.

Smart Sensing System for the Prognostic Monitoring of Bone Health

The objective of this paper is to report a novel non-invasive, real-time, and label-free smart assay technique for the prognostic detection of bone loss by electrochemical impedance spectroscopy (EIS). The proposed system incorporated an antibody-antigen-based sensor functionalization to induce selectivity for the C-terminal telopeptide type one collagen (CTx-I) molecules—a bone loss biomarker. Streptavidin agarose was immobilized on the sensing area of a silicon substrate-based planar sensor, patterned with gold interdigital electrodes, to capture the antibody-antigen complex. Calibration experiments were conducted with various known CTx-I concentrations in a buffer solution to obtain a reference curve that was used to quantify the concentration of an analyte in the unknown serum samples. Multivariate chemometric analyses were done to determine the performance viability of the developed system. The analyses suggested that a frequency of 710 Hz is the most discriminating regarding the system sensitivity. A detection limit of 0.147 ng/mL was achieved for the proposed sensor and the corresponding reference curve was linear in the range of 0.147 ng/mL to 2.669 ng/mL. Two sheep blood samples were tested by the developed technique and the results were validated using enzyme-linked immunosorbent assay (ELISA). The results from the proposed technique match those from the ELISA.

Impedance Spectroscopy and Experimental Setup

The objective of this chapter is to introduce the measuring instruments and software programs used for the experimental setup. It provides the reader a detailed insight of, electrochemical impedance spectroscopy basics and data representation along with highlighting the methodology of measurement data collection. It explains all the hardware and software used to collect the impedance data and the models involved in analyzing the acquired information.