Detecting Methamphetamine in Aerosols by Electroanalysis in a Soap Bubble Wall

We present a facile method to detect methamphetamine in aerosols by trapping aerosols in a soap bubble wall for electroanalysis. A microwire was placed through a soap bubble wall as a sensing electrode along with a 1 mm diameter platinum wire as the counter/reference electrode. The resulting electrochemical cell and electrode geometry are unique and allow for reproducible electrochemistry between bubble walls. We first provide a thorough investigation of the cell and electrode geometry and an electrochemical characterization of ferrocene methanol in a soap bubble wall composed of 0.1 M KCl and 0.1% Triton X-100 (v/v). To visualize the boundary where the bubble wets the microwire (the effective electrode area) with tens of nanometer resolution, we electrodeposited platinum on carbon microwire. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed the bubble contact (i.e., cylindrical electrode height) is 157 ± 30 μm. Correlated digital microscopy suggests that the wetting reaches r ∼ 125 μm along the bubble wall laterally from the microwire. Beyond the wetting region, the bubble thickness is 18 ± 1 μm, as indicated by ultraviolet-visible spectroscopy experiments probing dissolved bis(bipyridine)ruthenium(II) chloride. We illustrate that the voltammetric character in this system is highly dependent on the bubble wetting parameters, which are tuned by changing the microwire material. We then applied this system to the collection and electrochemical detection of methamphetamine in liquid aerosols, where the bubble wall acts as a low volume collector.