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Nanoporous Quartz Crystal Microbalance: Increasing Sensitivity 1000x and Beyond

  • Author(s): Milgie, Michael James
  • Advisor(s): Taborek, Peter
  • et al.
Abstract

We report experimental results of the adsorption and desorption kinetics of 1-octadecanethiol onto porous gold quartz crystal microbalances (QCM). Fabricated porous gold substrates range in thickness from 75 nm to 3 µm. The porous gold was fabricated using a two-step process of sputtering and etching. First, an alloy of gold and silver was sputtered onto a flat gold QCM with the desired ratio of roughly 70 atomic percent silver and 30 atomic percent gold. Second, the sputtered silver from the alloy was removed by etching in concentrated nitric acid, leaving a gold nanoporous structure on the surface. This porous gold structure on the QCM surface greatly increases the surface area of the sensor, allowing much more thiol to be adsorbed.

Experimental adsorption kinetics are compared to a Langmuir isotherm model. Thiol adsorption on porous QCM in n-Hexane is presented with concentrations ranging from 10nM (5ppb) to 3mM (900ppm). Thiol adsorption and kinetics on a nominally flat gold surface are included for comparison. Optimized sputtering conditions for porous gold substrates lead to a 1000-fold increase of thiol adsorption over flat gold QCM. It follows that the high specific area of porous gold can amplify the final sensitivity of a flat QCM by more than 3 orders of magnitude. We further present evidence that the three orders of magnitude sensitivity increase we achieve is the maximum enhancement possible when used in liquids for thiol detection. We verify our theoretical model of the QCM, describe our fabrication technique, characterize our substrates, and detail the uses of our porous gold QCM.

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