Low-Cost Resistive Microfluidic Salinity Sensor for High-Precision Detection of Drinking Water Salt Levels.

Rapid, inexpensive, and precise water salinity testing remains indispensable in water quality monitoring applications. Despite many sensors and commercialized devices to monitor seawater salinity, salt detection and quantification at very low levels of drinking water (below 120 ppm) have been overlooked. In this paper, we report on optimization of a low-cost microfluidic sensor to measure water salinity in the range of 1-120 ppm. The proposed design employs two copper microbridge wires suspended orthogonally in a PDMS microchannel to measure salinity based on the electrical resistance between the wires. The preliminary design of the sensor microchannel with a rectangular cross-section width (w) of 900 µm and height (h) of 500 µm could measure the water salinity in the range of 1-20 ppm in less than 1 min with detection sensitivity, limit of detection (LOD), and limit of quantification (LOQ) of 17.1 ohm/ohm·cm, 0.31 ppm, and 0.37 ppm, respectively. Data from the preliminary design was used for developing and validating a numerical model which was subsequently used for parametric studies and optimization to improve the sensor's performance. The optimized design demonstrated an order of magnitude increase in sensitivity (385 ohm/ohm·cm), a 6-fold wider detection range (1-120 ppm), and a 15-fold enhancement in miniaturization of the microfluidic channel (w = 200 µm and h = 150 µm) with LOD and LOQ of 0.39 and 0.44 ppm, respectively. In the future, the sensor can be integrated into a hand-held device to remove present impediments for low-cost and ubiquitous salinity surveillance of drinking water.

Unravelling the physisorption characteristics of H2S molecule on biaxially strained single-layer MoS2.

Sensing ultra-low levels of toxic chemicals such as H2S is crucial for many technological applications. In this report, employing density functional theory (DFT) calculations, we shed light on the underlying physical phenomena involved in the adsorption and sensing of the H2S molecule on both pristine and strained single-layer molybdenum disulfide (SL-MoS2) substrates. We demonstrate that the H2S molecule is physisorbed on SL-MoS2 for all values of strain, i.e. from -8% to +8%, with a modest electron transfer, ranging from 0.023e- to 0.062e-, from the molecule to the SL-MoS2. According to our calculations, the electron-donating behaviour of the H2S molecule is halved under compressive strains. Moreover, we calculate the optical properties upon H2S adsorption and reveal the electron energy loss (EEL) spectra for various concentrations of the H2S molecule which may serve as potential probes for detecting H2S molecules in prospective sensing applications based on SL-MoS2.