Picoliter drop deposition of SnO2 nanoparticles onto microsensor platforms

Abstract

Microhotplates produced by micromachining processes provide a robust substrate for miniaturized solid-state gas sensors; however, it can be challenging to locally deposit solution-suspended nanomaterials for sensing directly onto these small (≈ 100 µm), 3-dimensional platforms which are often configured in arrays to increase analytical capabilities. A picoliter drop-deposition technique based on inkjet printing is described here which has been used to achieve accurate, precise drop placement and reproducible film formation. Tin oxide nanoparticles, a well-established sensing material for use in gas sensor devices, were used to demonstrate the methodology. The nanoparticles were synthesized via a hydrothermal route to achieve crystalline materials with an average diameter of 5.5 nm. Tin oxide nanoparticle-laden suspensions were then formulated to reduce agglomeration, enable consistent first-drop formation, and prevent dispenser clogging. A priming technique was developed to accurately and repeatably perform deposition of picoliter-range drops onto the microsensor platforms. The technique was employed with a dynamic drop placement procedure to yield formation of spatially registered mesoporous oxide films on individual microhotplates. Microsensors fabricated with the drop-deposited SnO2 nanoparticles showed sensitive and rapid gas-sensing responses to two reducing gases and one oxidizing gas when tested at concentrations within the range of (5 to 100) µmol/mol. The presented results indicate that the combination of pre-formed nanoparticles and picoliter drop deposition offers a versatile approach for reproducibly manufacturing gas microsensors, which can be adapted for a range of gas sensing materials and microscale platforms.