Spin-on nanoparticle tin oxide for microhotplate gas sensors

A colloidal suspension of tin oxide nanoparticles is used to prepare a highly sensitive gas sensing film on a microhotplate. Fabrication consists of spin-coating the solution over an array of micromachined hotplates and annealing. A thermolithographic process using a nitrocellulose coating is demonstrated. Scanning electron microscopy images show the films to be smooth and consisting of 10 nm particles. Response to methanol shows a resistance change of a factor of two between zero-grade dry air and a concentration of 10 nmole/mole (10 ppB) at an operating temperature of 350°C. Sensitivity is observed at operating temperatures as low as 150°C, with generally lower recovery times. Tests comparing the response in saturated humid air and dessicated dry air show that recovery to wet air exposures is greatly accelerated at temperatures above 300°C. Stable response with no evident poisoning of the sensor was observed in a run lasting 120 h consisting of methanol and ethanol exposures. Temperature-programmed sensing was used to create different response patterns in air, methanol, and toluene, useful for identification of a detected gas. The results suggest colloidal suspensions may be used in combination with the microhotplate platform to produce a low-power, highly sensitive, device that is fabricated using planar technology.