Sangeeta N. Kale

Conventional complementary split-ring resonators (CSRR) exhibit good sensing performance, which has been widely reported in the recent past. The simulation studies used for such resonator designs often show higher-order modes, along with the fundamental modes, which are rarely explored for sensing applications. Here we have explored this “window of opportunity” for a 432 MHz-designed CSRR circuit at 2.682 GHz. The results reported here are for deionised (DI) water, DMSO (Dimethyl Sulfoxide), and DMF (Dimethyl Formamide), varying in the range of 0–60 µL. The comparison shows concomitant higher shifts in resonance frequencies for given quantities of test samples in the GHz operational window (~30 times) for very low volumes (0-30 µl) of and ~5 times for higher volumes (30-60 µl) of test samples. The shifts are mainly attributed to the capacitive loading on the resonant structures, which change with the dielectric constant of the sample to be tested and its test volume. Higher-order modes, therefore, show better calibration and precision for extremely small quantities of the chemical moieties. This non-intuitive approach can yield more responsive chemical sensors with a wider scale of optimizations.

Metamaterial-based devices show properties like negative magnetic permeability and negative electrical permittivity, which are not found naturally. Due to their extraordinary sensitivity to nearby perturbations, metamaterial structures have recently emerged as highly desirable and effective techniques of detection. Wide dynamic range, multi-functionality, and simple design and production are a few benefits of metamaterial-based structures. The sensitivity and limit of detection for sensors based on metamaterials can be enhanced by properly altering the device parameters. With the inclusion of microfluidics in metamaterial-based sensors, the sensitivity, selectivity, and detection limit can also be improved. In this review, we have focused on the various meta-surfaces employed for sensing purposes. We have provided a brief introduction to the principle and working of sensors based on meta-surfaces. The main content of the paper is a detailed review of metamaterial-inspired microwave sensor device applications in different fields. These fields include applications in biological sensing, chemical and fuel sensing, physical sensing (includes temperature, strain, displacement, and thickness measurement), and non-destructive techniques for structural health monitoring.

Bacterial infections have a large impact on public health. Through this study, we report on the development of complementary split-ring resonators (CSRR) supplemented by functionalized nanoparticles to detect bacteria in the aqueous medium. Iron oxide (Fe ₃ O ₄ ) nanoparticles were functionalized with amino groups using (3-aminopropyl) triethoxysilane (APTES) to form (APTES@Fe ₃ O ₄ ) nanoparticles, which have a specific affinity towards the bacterial species. This affinity was evaluated using the Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacterial species. The resonant sensor was tuned at 430 MHz and the CSRR sensor bed was further activated using APTES@Fe ₃ O ₄ nanoparticles. Bacterial detection was studied over a range of concentrations from 2.66 × 10 ⁹ cells to 2.66 × 10 ⁸ cells. The sensor actively responded to small changes in bacterial concentration, showing an overall shift in resonance frequency of ∼44 MHz (∼40 MHz/cell count) for E. coli and ∼55 MHz (50.43 MHz/cell count) for S. aureus . Dextran sulphate and Chitosan were used as the references. The magnetic character of the conjugated system exhibited strong interaction of the bacterial species with APTES@Fe ₃ O ₄ , justifying the high selectivity towards these species. This demonstrates the feasibility of a sensitive, fast, portable device, against the traditionally used time-consuming bio-assays.