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Mercury Sensing with Optically Responsive Gold Nanoparticles


Mercury, a potent neurotoxin, is a global environmental problem. New mercury sensing technologies are needed to meet the expanding demands on the mercury observation network. We demonstrate the utility of gold nanoparticles as a stand-alone, inexpensive, and sensitive mercury monitor. Gold nanoparticles display a peak in the visible range of their UV-vis absorbance spectra due to localized surface plasmon resonance (LSPR). The energy of this resonance is affected by adsorption of mercury. Isolated individual gold nanorods with an average length and diameter of 60 and 20 nm saturate after adsorption of 4 attograms of elemental mercury, and produce a 3 nm blue shift in their LSPR, with the shift dependent on the surface-area-to-volume ratio and aspect ratio. A modified Gans theory model predicts the shift at saturation given the particle dimensions if saturation occurs with 45% monolayer coverage.

Nanoparticle films on a transparent substrate show potential as a practical and robust method for sensing low levels of mercury vapor. The adsorption of 15 atoms of Hg causes a 1 nm shift in the LSPR wavelength of 5 nm gold spheres. The rate of shift in the peak absorbance is linear with mercury concentrations from 1 to 825 µgHg/mair3. The speed of the sensor response is limited by diffusive mass transfer and can be enhanced by optimizing the sample flow characteristics. We modeled the diffusive mass transfer and optimized the sample delivery accordingly. Regeneration of the sensing films, done by heating to 160°C, allows for repeatable measurements on the same film.

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