Department of Electrical and Electronics Engineering
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Item Photoplastic Microcantilever Sensor Platform for Explosive Vapor Detection(World Scientific, 2011) Rao, V. RamgopalWe present an ultrasensitive and cost effective polymeric microcantilever sensor platform for explosive vapor detection. These sensors were fabricated using SU-8 polymer composite materials with two transduction approaches namely (i) optical and (ii) electrical detection. The cantilevers have been characterized for their electrical and mechanical properties. The functionalization of these microcantilevers has been carried out using selective chemicals for most popular explosives such as TNT and RDX. Optical and electrical responses of respective microcantilevers to TNT and RDX vapors are also presented here.Item Organic Nano-Electro-Mechanical-Sensor Systems for healthcare and Environmental Applications(IEEE, 2012) Rao, V. RamgopalOrganic field effect transistors and polymer microcantilevers are two classes of organic sensors with potential applications in biochemical sensing. The organic field effect transistors (OFETs) with region regular poly3- hexylthiophene(rr-P3HT) and hexafluoro-2-propanol-substituted polysiloxane (SXFA) as an organic layer, have been used for detection of explosive vapors with excellent sensitivity of less than 70 ppt for 1,3,5-trinitro-1,3,5- triazacyclohexane (RDX) and less than 100 ppt for 2,4,6-trinitrotoluene (TNT) [1-3]. Piezo-resistive & piezoelectric polymer composite microcantilevers have been demonstrated as sensitive and cost effective platforms for bio-chemical detection [4-12]. Integration of the OFET and the polymer cantilever platform can bring orthogonality to the sensing where selectivity is a critical requirement. We have recently achieved this by integrating a strain sensitive organic field effect transistor (OFET) on the surface of a polymer nanomechanical cantilever [6]. This sensor named as the “Organic CantiFET” shows the highest deflection and surface stress sensitivity reported till date and opens up opportunities for realizing highly sensitive yet selective low cost polymer sensors for a variety of applicationsItem A highly sensitive piezoresistive cantilever based sensor platform for detection of macronutrients in soil(IEEE, 2015-07) Rao, V. RamgopalIn this paper we have demonstrated for the first time a piezoresistive microcantilever based platform for detection of one of the macronutrients in the soil in a fluid medium. Also a low cost piezoresistive microcantilever platform is introduced which is ideal for bio-sensing applications. The cantilever surface was immobilized with 18-crown-6 ether in a polyvinyl chloride and dioctyl sebacate matrix. This layer acts as a sensing layer for potassium. This platform can be extended to sense other macronutrients and micronutrients in the soil. The low cost handy tool for soil nutrient sensing is an essential device for any small-town farmer or a researcher in the agricultural laboratory.Item Vapor-phase self-assembled monolayer on SU-8 cantilever for explosive sensing(IEEE, 2016-05) Rao, V. RamgopalThis paper reports novel technique of coating SU-8 cantilevers with self-assembled monolayer of Zinc porphyrin in vapor phase. Porphyrins have been reported to be effective for explosive detection, though incorporation of its thin films on polymer cantilevers is difficult due to harsh solvents of porphyrin which damage the polymer layers like SU-8. Hence a nondamaging process which is also industry friendly and useful for mass production is required. The vapor phase technique developed here to deposit self-assembled monolayers (VPSAM) of Porphyrins is a MEMS/CMOS compatible process which opens up a whole new set of opportunities. In this paper, we discuss the process of VPSAM and show its application in explosive detection of nitro-based explosives (PETN, RDX, TNT).Item A lab-on-a-chip system for detection of multiple macronutrients in the soil(IEEE, 2016) Rao, V. RamgopalIn this work, we have demonstrated for the first time the sensing of soil macronutrients nitrate and potassium with the use of MTDAN ionophore, nitrate ionophore VI and 18crown6 ether in a PVC/DOS matrix using a highly sensitive piezoresitive silicon oxide cantilevers. The complete portable electronics system along with a liquid cell for on the field experimentation has been demonstrated.Item Experimental and theoretical analyses of effect of ZnO nanowire growth on mechanical properties of microcantilevers for dynamic sensing applications(IEEE, 2016) Rao, V. RamgopalIn this paper, we have demonstrated a hydrothermal approach to grow ZnO nanowires (NW) on top of a microcantilever of dimension 250μm × 90μm × 0.7μm and investigated the effect of these nanowires over the mechanical properties of the microcantilever using the Laser Doppler Vibrometry (LDV) and Nanoindentation studies. Experimental results show that growth of ZnO nanowires on microcantilever increases its spring constant. Furthermore, based on the experimental results of changes in resonant frequency, we have also calculated the stress developed due to the growth of ZnO nanowires on the microcantilever considering both surface stress and geometric effect independently. We found that the stress induced due to the geometric effect was an order of magnitude higher as compared to that of the surface stress. Active mass sensing using the microcantilever/microresonator platform has been extensively reported in literature due to their high sensitivity and low detection limit. Researchers have used metal oxide (MOX) nanowires to improve the sensitivity of microcantilevers. The approach presented in this report can be used for important design considerations while fabricating sensors based on this platform.Item Microcantilever Based Dual Mode Optical Biosensor for Agricultural Pathogen Detection(IEEE, 2018-10) Rao, V. RamgopalWith the aim of increasing crop yields, guiding precision farming applications and for more sustainable disease management practices development of early diagnostics sensors for pathogens in agriculture is vital. A sensitive method of diagnosis which will detect the low concentration of pathogens in the initial stages of infection will allow early diagnosis and help in saving the crop. Microcantilever based biosensors are ideal devices for such kind of applications. This paper presents development of a highly sensitive dual mode operated optical sensor for agricultural pathogen detection. Agricultural pathogen detection using a microcantilever platform in both static and dynamic mode of operation is reported here for the first time. Micromolar concentartion of Ralstonia solanacearum antigens was detected optically by using the ultrasensitive Mecwins® SCALA platform in liquid medium.Item Bio-functionalization of silicon nitride-based piezo-resistive microcantilevers(Springer, 2009-10) Rao, V. RamgopalMethods of bio-functionalize silicon nitride involve process steps to convert it into an oxynitride via plasma implantation techniques. Such methods can potentially damage microstructures such as cantilevers. In this paper, we report successful bio-functionalization of Hotwire CVD silicon nitride-based piezo-resistive cantilevers without any oxygen plasma treatment. Process to fabricate such structures and to bio-functionalize them is discussed in detail.Item Polymer microcantilever biochemical sensors with integrated polymer composites for electrical detection(Elsevier, 2009-09) Rao, V. RamgopalMicro fabricated sensors based on nanomechanical motion with piezoresistive electrical readout have become a promising biochemical sensing tool. The conventional microcantilever materials are mostly silicon-based. The sensitivity of the sensor depends on Young's modulus of the structural material, thickness of the cantilever as well as on the gauge factor of the piezoresistor. UV patternable polymers such as SU-8 have a very low Young's modulus compared to the silicon-based materials. Polymer cantilevers with a piezoresistive material having a large gauge factor and a lower Young's modulus are therefore highly suited for sensing applications. In this work, a spin coatable and photopatternable mixture of carbon black (CB) and SU-8, with proper dispersion characteristics, has been demonstrated as a piezoresistive thin film for polymer microcantilevers. Results on percolation experiments of SU-8/CB composite and fabrication of piezoresistive SU-8 microcantilevers using this composite are presented. With our controlled dispersion experiments, we could get a uniform piezoresistive thin film of thickness less than 1.2 μm and resistivity of 2.7 Ω cm using 10 wt% of CB in SU-8. The overall thickness of the SU-8/composite/SU-8 is approximately 3 μm. We further present results on the electromechanical characterization and biofunctionalization of the cantilever structures for biochemical sensing applications. These cantilevers show a deflection sensitivity of 0.55 ppm/nm. Since the surface stress sensitivity is 4.1 × 10−3 [N/m]−1, these cantilevers can well be used for detection of protein markers for pathological applications.Item Organic Sensor Platforms for Environmental and Security Applications(IOP, 2011) Rao, V. RamgopalOrganic sensors based on polymer microcantilevers and organic field effect transistors (OFETs) bring orthogonality to the sensing mechanism for different environmental and security applications. Orthogonolity is an important requirement from the point of reducing the false positives, which is of utmost importance for many applications. Development of polymer nanocomposite microcantilever based sensors for explosive vapour detection was reported. These polymer microcantilevers offer a deflection sensitivity of 1.1ppm for 1nm of deflection, which is the highest sensitivity reported till date. The sensor response to trinitrotoluene (TNT) vapours at a few parts per billion concentration levels was demonstrated. OFETs using poly 3-hexylthiophene (P3HT) and CuII tetraphenylporphyrin (CuTPP) composite as their active material were studied as sensors for detection of various nitro-based explosives. Significant changes were observed in the ON current (Ion) and transconductance (gm) of the OFET sensor after exposure to vapours of various explosive compounds. Sensor selectivity to explosive vapours over strong oxidizing agents was also demonstrated. Also, the change in conductivity for organic semiconducting material as a function of ionizing radiation was studied with an organic semiconducting material in resistor and OFET configurations. 30MΩ/Gy sensitivity for the organic resistor sensor and 28fA/Gy/1µm-width sensitivity for OFF current for the OFET sensor were observed. Moreover, changes in various other electrical parameters for an OFET sensor were also found proportional to the ionizing radiation dose.