Micropatterning of metal oxide nanofibers by electrohydrodynamic (EHD) printing towards highly integrated and multiplexed gas sensor applications

Integration of heterogeneous sensing materials in microelectronic devices is essential to accomplish compact and highly integrated environmental sensors. For this purpose, a micro-patterning method of electrospun metal oxide nanofibers based on electrohydrodynamic (EHD) printing process was developed in this work. Several types of metal oxide (SnO_2, In_2O_3, WO_3 and NiO) nanofibers that were produced by electrospinning, fragmented into smaller pieces by ultrasonication, and dissolved in organic solvents were utilized as inks for the printing. Constant or pulsed wave bias consisting of base and jetting voltages were applied between a nozzle and a substrate to generate a jetting of nanofiber solutions. Several parameters for EHD printing such as pulse width, inner diameter of the nozzle, distance from the nozzle to the substrate, and stage speed, were optimized for accurate micro-patterning of electrospun nanofibers. By using optimized printing parameters, microscale patterns of electrospun nanofibers with a minimum diameter less than 50 μm could be realized. Gas sensors were fabricated by EHD printing on the microelectrodes and then used for the detection of toxic gases such as NO_2, CO and H_2S. Four kinds of metal oxides could detect down to 0.1 ppm of NO_2, 1 ppm of H_2S and 20 ppm of CO gases. Also, heterogeneous nanofiber gas sensor array was fabricated by the same printing method and could detect NO_2 using the sensor array platform with microheaters. Furthermore, microscale patterns of nanofibers by EHD printing could be applied to the suspended MEMS platform without any structural damage and this sensor array could detect NO_2 and H_2S gases with 20 mW power consumption