Microcantilever-based sensor platform has attracted a lot of attention over the time in detection of a variety of molecules due to their miniaturized dimensions. Sensitivity enhancement is an important aspect of such sensors, especially when used for point-of-care diagnostic purpose. However, the major concern while operating these sensors in deflection mode is their sensitivity which mainly relies on selective chemical modification protocols employed on these sensor surfaces. One of the ways of getting better sensitivity is through asymmetric (one side) biofunctionalization of the sensor surface. In the presented work here, we have demonstrated a novel approach of asymmetric biofunctionalization of proteins in overall sensitivity enhancement of piezoresistive silicon nitride-oxide microcantilever sensor platform inside a flow chamber. Herein, using our developed surface chemistry, asymmetrically biofunctionalized microcantilevers first exhibited a greater electrical response in terms of piezoresistance change than their symmetric counterpart in the detection of human immunoglobulins (HIgGs) protein. Finally, these microcantilevers were employed to exhibit the enhanced sensitivity towards the detection of a crucial cardiac marker protein, i.e. Troponin-I (cTnI) down to 250 ng ml−1 using asymmetric biofunctionalization process. This study shows that the developed asymmetric biofunctionalization methodology may be used as a general protocol to detect other important biomarkers of clinical applications with improved sensitivity.
In this article, we report the generation of alternating current by the application of constant and ramping DC voltages across oil–water interfaces. The work reported here can be broadly divided into two parts depending on the shapes of oil–water interfaces, i.e., flattened and curved. In the first part, an alternating current of ∼100 nA (amplitude) was generated by applying a constant DC voltage of −3 V and above across a freestanding and flattened oil–water interface. In another part, an alternating current of ∼150 nA (amplitude) was generated by applying a ramping up DC voltage starting from −5 V to 5 V, then again ramping back down to −5 V for the freestanding and curved interface. The suggested qualitative mechanism that engenders such a phenomenon includes the oil–water interface acting like a membrane. This membrane oscillates due to the electrophoretic movement of ions present in the aqueous phase by the application of a DC voltage across the interface. This electrophoretic movement of ions across oil–water interfaces causes Faraday instabilities leading to oscillations of the said interface. This method could also be used to study the stress levels in the interfacial films between two immiscible liquids. It explores the more-than-Moore’s paradigm by finding a substitute to a conventional alternator/inverter that generates alternating current upon applying a DC voltage input. This work would be of substantial interest to researchers exploring alternatives to conventional AC generators that can be used in liquid environments and in the design of novel integrated circuits that could be used for unconventional computing applications.
Dynamic sensing using microcantilevers has held the centre stage for a long time in biomolecular detection. In this work, we have reported a comparative study of dynamic responses of asymmetric (top or bottom surface) and symmetric (both top and bottom surfaces) modifications (biofunctionalization) of silicon nitride/oxide microcantilevers. For the first time, surface stress (in terms of surface energy/area) has been shown to govern the change in resonant frequency by studying the dynamic responses of asymmetrically and symmetrically modified microcantilevers using a conventional laser doppler vibrometer. The resonant frequency of asymmetrically modified microcantilever was found to be lower than that of symmetrically modified one by a magnitude of ~0.4 kHz. Also, it was observed that the amplitude of oscillation increased from symmetrically modified to asymmetrically modified microcantilever. Consequentially, we have reported (by means of mathematical calculations and nanoindentation experiments) the increase in flexural rigidity as well as surface stress from symmetric modification to the asymmetric one. Owing to the decrease in the resonant frequencies as well as downward bending of microcantilevers, we conclude that compressive surface stress is developed after surface modifications. This work will be of immense interest to the researchers working in the field of dynamic sensing using MEMS-based sensors.
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