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Abstract
Thiolate-monolayer-protected gold nanoparticle (MPN) chemiresistors were studied as the sensing devices for micro-gas chromatography (µGC) systems. Because transport through chemiresistors is dominated by tunneling, they are highly sensitive. In order to improve their limit of detection, their fundamental noise was studied. Chemiresistors exhibit 1/f type noise where noise scales inversely with frequency. Chemiresistor noise was found to scale inversely with MPN film thickness. We lowered the noise prefactor of a 50×60 µm2 chemiresistor by coating a thick rather than monolayer MPN film. Electron beam induced crosslinking (EBIX) of the MPN film slightly reduced chemiresistor noise. A technique for patterning chemiresistor arrays with MPN films using EBIX was developed, and an array with four distinct MPNs was fabricated in an area ∼600 µm 2. This is the smallest chemiresistor array reported to date. Chemiresistors were exposed to vapors and provided differential sensitivities comparable to those from larger uncrosslinked chemiresistors.
Chemiresistors were studied to assess their long term stability. Chemiresistors exhibited decreases in resistance over time that is likely caused by loss of MPN ligands. Temperature dependent current-voltage measurements verified the resistance change was not due to changes in the size of the MPN core. While resistance could change by orders of magnitude, vapor sensitivity did not show significant changes. Heating increased the change in resistance, but chemiresistors remained responsive after being held at 80°C for a cumulative 400 hours.
It was unknown whether tunneling in the MPN film is through the highest unoccupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO). A new technique was explored to distinguish tunneling through the HOMO and LUMO by measuring the induced thermoelectric voltage caused by a temperature difference across the MPN film.
For integration into a µGC system, we fabricated a chemiresistor array on the surface of a 2.2×2.2 mm2readout circuitry chip creating a monolithic sensor system. A model for determining the optimal sensor size for a µGC system is presented. While noise is inversely proportional to chemiresistor volume, the amount of analyte detectable is proportional to volume making smaller chemiresistors able to detect lesser amounts of analyte.
