We present the incorporation into the chalcogenide glasses of two-dimensional periodic nanopatterns to realize the first of a kind chalcogenide-based planar optical mid-infrared tunable resonant structure. Chalcogenide (ChG) glasses are promising for infrared photonics owing to their transparency in visible to far infrared, where various biomolecules and gases have their characteristic absorption lines, arising from rotational-vibrational transitions. The region of the electromagnetic spectrum in which this absorption occurs, the amount of absorption, and the specific characteristics of the absorption curve are unique to each gas. Thus, gases can be fingerprinted using their absorption characteristics. Utilizing the mid-IR resonance feature of our nanopatterned ChG glass, an innovative approach is proposed to achieve selective gas sensing through the tuning of the sensor resonance, providing an inbuilt selectivity. As an illustration, the presented chalcogenide-based nanostructure is customized to match its resonance wavelength with the absorption band of gaseous methanol, a key plant health indicator. The highly concentrated electromagnetic field at the nanostructure surface allows highly sensitive detection of the target analyte methanol.
We present the first heater integrated nanostructured optical fiber of 200 μm diameter to realize a high-sensitivity and reusable fiber-optic gas sensor. In our guided mode resonance-enabled fiber-optic gas sensor, resonance shifts upon the adsorption of the analytes on the graphene oxide (GO) coated sensor surface. For repeated use of this sensor, a regeneration of the sensor surface is required by a complete desorption of the analyte molecules from the GO layer. In our presented design, this has been achieved by the integration of a controllable heater at the fiber tip. The heater was fabricated by embedding a helical thin nichrome wire wrapped along a cylindrical rod into a precursor solution of polydimethylsiloxane, and subsequently removing the rod from the cured elastomer and leaving the helical wire inside the elastomer. Thus, a cylindrical cavity of length 16 mm and diameter 4 mm surrounded by the helical wire was formed that then contains the fiber-tip sensor. For the ethylene gas analyte, we demonstrated the reversibility of the heater integrated fiber-tip sensor, with a tunable recovery time. Owing to the rapid heat transfer from the helical wire to the encased fiber-tip sensor, the heater integrated fiber-tip sensor responds to heating in only about 2.5 min. The high resonance sensitivity of the nanopatterned fiber-tip to its surrounding refractive index, in conjunction with excellent repeatability through integrated heating for surface regeneration, enables a practical fiber-tip based remote sensing.
An approach is presented for measuring the dynamic variations in refractive index (RI) of Graphene Oxide (GO) based optical gas‐sensors [1‐ 3] upon their exposure to gases, using thin‐film interference method . The approach is simple yet effective, and also is physically integratable into the sensing system, a required attribute for being able to measure the dynamically changing RI in response to the ongoing interactions with the analyte. The details can be found in our journal paper .
One major challenge to the usability of IoT devices is limited onboard battery lifetime. Integrating an energy harvester to scavenge the energy from ambient sources is a viable green option. In recent 5 years, Triboelectric Nanogenerators (TENG) have gained attention for harvesting ambient vibration energy from sources ranging from ocean waves to human body motion due to their flexibility in the choice of materials and fabrication processes. However, due to the high nonlinearly varying impedance (typically in mega ohms) of TENG, standard full wave rectifier based AC to DC conversion for energy extraction is unable to provide a matching impedance needed for optimized energy transfer. In the presented work, Synchronous Charge Extraction (SCE), Parallel and Series synchronized switch harvesting on inductor (P-SSHI and S-SSHI) energy extraction circuits are mathematically modeled, analyzed, simulated, and compared with the standard full wave rectifier (FWR) circuit for the first time to the best of our knowledge. While the above-mentioned extraction schemes have been studied for piezoelectric transducers, the models (and gains) are different in the case of triboelectric transducers. For TENG with an area, 12 x 8 cm2, surface charge density 8 μC=m2, and subjected to vibration with 3 mm amplitude and 1 Hz frequency, energy gains of 2.8, 14.5, 385 over FWR were realized for P-SSHI, S-SSHI and SCE for a 5V battery load respectively. The above findings were also confirmed by SPICE-based circuit simulation.
Salicylic Acid (SA) is a phyto-hormone involved in the regulation of induced plant defense mechanisms, primarily against biotic stresses. Various methods have been reported for detecting SA. The electrochemical methods offer economical, portable, and accurate concentration measurement of bio-chemicals like SA. Electrochemical biosensors often require modification of the working electrode (WE) with specific materials to functionalize it with bio-molecules, needed for target analyte recognition. The proposed biosensor provides a unique methodology of selectively coating the inter-digital electrodes (IDE) and further applying the method to develop a biosensor to detect SA. The electrodes are fabricated using a novel deposition process termed as, Capillary action assisted deposition (CAAD) which consists of IDEs fabricated in the form of small finger-like channels connected to a wider main channel. The drop-casted sample automatically flows from the main channel into the fine fingers under the effect of capillary action. The sensor includes a 3-electrode system arranged in a 3-D geometry, forming an integrated microfluidic channel for analyte solution flow. The WE is selectively coated with, first, Graphene oxide (GO) and next, the bi-enzyme Salicylate Hydroxylase (SH) and Tyrosinase (TYR) recipe using the proposed CAAD process. The bi-enzyme exhibits selectivity towards SA and the proposed sensor shows the detection range of 0.5 μ𝑀 to 64 μ𝑀. The electrochemical reactions are characterized by Chrono-amperometry (CA) and shows the sensitivity of 34.4 μA cm-2 per decade change in SA concentration (in μ𝑀). To the best of our knowledge, the proposed bi-enzyme system in a microfluidic device for SA sensing is the first of its kind.
An impedance measurement based level sensor is proposed using a co-axial probe for sensing liquid level in a container. The co-axial sensing probe is made with a hollow stainless steel outer conductor enclosing an insulated inner conductor. The impedance of the co-axial probe varies with the water level in a nonlinear fashion. The supporting electronics was developed using MSP 432 microcontroller unit (MCU) platform from Texas Instruments (TI) and a newly designed Impedance Analyzer-Analog Front End (IA-AFE) developed at TI. An inverter amplifier based circuit was implemented within the IA-AFE for impedance measurement. Discrete Fourier Transform (DFT) is calculated on the MCU platform from the sampled input and output square wave voltages of the IA-AFE. The proposed sensor shows a maximum error within ±1.5 mm, for the probe of length 40 cm. The proposed system offers an accurate and economical liquid level measurement platform outperforming the state-of-art level sensors to the best of our knowledge.
This paper presents a novel approach to estimate the soil ionic concentration by way of multi-frequency impedance measurements and using the quasi-static dielectric mixing models to infer the various ionic concentrations. In our approach, the permittivity of the soil dielectric mixture is measured using impedance spectroscopy and the results are used as input parameters to dielectric mixing models, combined with the debye-type dielectric relaxation models. We observe that the dielectric mixing models work well for low RF (radio-frequency) range and help in determining the individual ionic concentration in a multi-component soil mixture. Using the fact that the permittivity of a dielectric mixture is proportional to its impedance, we validated our approach by making multi-frequency impedance measurements of a soil mixture at different concentrations of various components. The method provides a good estimate of individual components such as air, water and ions like nitrates. While the paper is written with the perspective of soil constituent concentration determination, the underlying principle of determining individual component concentration using multi-frequency impedance measurement is applicable more generally in areas such as characterizing biological systems like pathogens, quality control of pharmaceuticals etc.
Real time and accurate measurement of sub-surface soil moisture and nutrients is critical for agricultural and environmental studies. This paper presents a novel on-board solution for a robust, accurate and self-calibrating soil moisture and nutrient sensor with inbuilt wireless transmission and reception capability that makes it ideally suited to act as a node in a network spread over a large area. The sensor works on the principle of soil impedance measurement by comparing the amplitude and phase of signals incident on and reflected from the soil in proximity of the sensor. Accuracy of measurements is enhanced by considering a distributed transmission line model for the on-board connections. Presence of an inbuilt self-calibrating mechanism which operates on the standard short-open-load (SOL) technique makes the sensor independent of inaccuracies that may occur due to variations in temperature and surroundings. Moreover, to minimize errors, the parasitic impedances of the board are taken into account in the measurements. Measurements of both real and imaginary parts of soil impedance at multiple frequencies gives the sensor an ability to detect variations in ionic concentrations other than soil moisture content. A switch-controlled multiple power mode transmission and reception is provided to support highly energy efficient medium access control.1