With the rapid growth of emerging point-of-use (POU)/point-of-care (POC) detection technologies, miniaturized sensors for the real-time detection of gases and airborne pathogens have become essential to fight pollution, emerging contaminants, and pandemics. However, the low-cost development of miniaturized gas sensors without compromising selectivity, sensitivity, and response time remains challenging. Microfluidics is a promising technology that has been exploited for decades to overcome such limitations, making it an excellent candidate for POU/POC. However, microfluidic-based gas sensors remain a nascent field. In this review, the evolution of microfluidic gas sensors from basic electronic techniques to more advanced optical techniques such as surface-enhanced Raman spectroscopy to detect analytes is documented in detail. This paper focuses on the various detection methodologies used in microfluidic-based devices for detecting gases and airborne pathogens. Non-continuous microfluidic devices such as bubble/droplet-based microfluidics technology that have been employed to detect gases and airborne pathogens are also discussed. The selectivity, sensitivity, advantages/disadvantages vis-a-vis response time, and fabrication costs for all the microfluidic sensors are tabulated. The microfluidic sensors are grouped based on the target moiety, such as air pollutants such as carbon monoxide and nitrogen oxides, and airborne pathogens such as E. coli and SARS-CoV-2. The possible application scenarios for the various microfluidic devices are critically examined.

The most well-known criterion for POC devices is ASSURED, and affordability, i.e., using low-cost instrumentation, is the most challenging one. This manuscript provides a pathway for transitioning ESSENCE, an impedance-based biosensor platform, from using an expensive benchtop analyzer—KeySight 4294A (~$50k)—to using a significantly portable and cheaper USB oscilloscope—Analog Discovery 2 (~$200) —with similar sensitivity (around 100 times price difference). To achieve this, we carried out a fundamental study of the interplay between an electrolyte like potassium chloride (KCl), and an electrolyte buffer like phosphate buffered saline (PBS) in the presence and absence of a redox buffer like ferro/ferricyanide system and ([Ru(bpy)₃]²⁺). Redox molecules in the electrolyte caused a significant change in the Nyquist curve of the impedance depending on the redox molecule type. The redox species and the background electrolyte have their own RC semicircles in the Nyquist curve, whose overlap depends on the redox concentration and electrolyte ionic strength. We found that by increasing the electrolyte ionic strength or the redox concentration, the RC semicircle moves to higher frequencies and vice versa. Importantly, the use of the buffer electrolyte, instead of KCl, led to a lower standard deviation and overall signal (lesser sensitivity). However, to achieve the best results from the biorecognition signal, we chose a buffered electrolyte like PBS with high ionic strength and lowered the redox probe concentrations to minimize the standard deviation and reduce any noise from migrating to the low-cost analyzer. Comparing the two analyzers shows similar results, with a lowered detection limit from the low-cost analyzer.

Membrane fouling remains a major challenge in water treatment, biomedical, and pharmaceutical fields, yet understanding its mechanisms at the membrane interface remains difficult. Herein, we report a 3D impedimetric microfluidic membrane-mimic (IM3) cassette that integrates a porous membrane between microfluidic channels for real-time fouling investigation via electrochemical impedance spectroscopy (EIS). System validation using fluorescein and KCl electrolytes demonstrated reproducible charge-transfer resistance (R_ct) values and confirmed cassette robustness. Colloidal fouling studies using 800 nm polystyrene latex (PS) beads demonstrated a clear concentration-dependent behavior. High particle loading (10⁵ particles/mL) caused rapid, severe impedance increases, indicating extensive pore blockage and cake layer formation. Low loading (10¹ particles/mL) showed minimal changes, suggesting negligible fouling, as confirmed by scanning electron microscopy. The distribution of relaxation times (DRT) analysis revealed a single dominant relaxation peak at 10^-2 s that grew stronger as fouling severity increased, confirming that pore blockage raised interfacial resistance through a unified charge-transfer mechanism rather than caused diffusion-limited processes. A quantitative fouling model using EIS-measured R_ct changes captured temporal and concentration dependencies through an exponential growth framework. Maximum fouling extent (F_max) increased significantly with particle concentration (30% at 10¹ to 215% at 10⁵ particles/mL), demonstrating concentration-dependent pore blockage. The fouling rate constant (k) showed weak concentration dependence and plateaued at high loadings, indicating the limitation by available deposition sites rather than particle transport. These findings show that for PS beads, fouling is mainly governed by the extent of surface deposition (driven by concentration) rather than by the particle arrival rate (controlled by kinetics). The developed modular IM3 cassette enables customization of different membranes, channel geometries, and flow configurations. These results establish a quantitative framework linking EIS data to physically meaningful fouling phenomena, offering a versatile platform for the mechanistic investigation of dynamic membrane fouling behavior under varying particulate conditions.