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The role of 3-dimensional (3-D) geometries in facilitating surface reactions in microchannels is the subject of the present work. 3-D microfluidic immunosensor geometries inspired from the design of packed bed columns, utilizing splitting and recombination of flows in improving the transport in flow heterogeneous microfluidic immunosensors, were studied systematically. First, the effect of the passive mixing on the antigen capture was tested experimentally using prostate specific antigen (PSA) and anti-PSA and validated using numerical studies. Subsequently, numerical studies on the effects of geometrical parameters like the height and width of the microchannel, the height and width of the grooves, number of groove sets, the ratio of the heights of the grooves and the microchannel etc., on the antigen capture were conducted. The Sherwood number (Sh) and Peclet number (Pe) were used to explain the mass transfer. Finally, the optimum Peclet number (Pe∼x223C 3400) was found, having a maximum figure of merit ratio for the grooved to the ungrooved microchannels Correlations were also obtained between the average surface concentration and the geometrical parameters. The paper presents novel scientific results which are useful for designing efficient heterogeneous immunosensors.

Metal oxide-Polyaniline (PANI) nanocomposites have shown improved gas sensing characteristics that can be attributed to the formation of a p–n junction between the n-type metal oxide and the p-type PANI. The charge transport, grain boundary depletion region, and intragrain resistance are studied to understand the gas sensing mechanism of pristine metal oxide gas sensors. However, gas sensing mechanisms for metal-oxide/PANI nanocomposites have not been studied extensively. In this work, we have studied the gas sensing mechanism of SnO2/PANI nanocomposites using electrochemical impedance spectroscopy, and temperature dependent gas sensing experiments. Well-defined SnO2 nanoclusters were observed in the PANI matrix. The n-type SnO2 was covered by p-type PANI, and a depletion region was formed at the interface. The presence of the p–n junction depletion region was confirmed by impedance spectroscopy. The polarons in PANI were trapped by NH3 leading to a change in the width of the conducting path due to rearrangement of charge carriers along the depletion region. The change in the conduction path, along with the trapped polarons, enhanced the sensor response. For higher loadings of SnO2, the depletion region was deformed, and the sensor response decreased due to non-uniform boundaries. 1 wt% SnO2 with respect to aniline precursor in in situ synthesis showed the best response of 37.8% for 100 ppm NH3 at 35 °C. The response was stable for low humidity levels up to 51%RH. The response increased for higher humidity levels. The sensor response increased from 0.17 to 2.99 upon bending 1000 times at 7.8 mm diameter due to cracks in the surface. The sensor showed only 10% variation in response after 9 months.