Plasmonic metal nanoparticles have been widely investigated as a means of enabling sensitive chemical sensing, primarily using surface-enhanced Raman scattering or refractive-index-induced shifts in plasmon resonance frequencies.  Mechanical vibrations of the nanoparticles offer another potential sensing mechanism: adsorption of molecules on the nanoparticle surfaces will reduce the vibration frequency, and the small mass of the nanoparticles means the fractional frequency change will be large. However, in order for a precise frequency measurement to be possible damping of the vibrations within the nanoparticles and by the surrounding media cannot be too strong.  This is a particular concern in the liquid environments that are relevant for biomolecular sensing.<br/> <br/>Ultrafast laser spectroscopy enables sensitive measurement of the vibrations: an incident pump laser heats the nanoparticles, leading to their expansion and the excitation of mechanical vibrations. The vibrations produce oscillations in the plasmon resonance frequency of the nanoparticles, which are monitored by measuring the change in transmission through the sample of a second, probe laser pulse.  By making such measurements on a highly monodisperse sample of bipyramidal gold nanoparticles, we were able to determine both the frequency and the decay rate of the vibrations.  Measurements on nanoparticles in different solvents made it possible to determine the portion of damping and the vibrational frequency shift that are due to coupling to the surrounding liquid environment.  In high-viscosity fluids, conventional viscous damping could not explain the observations, and it was necessary to take into account the viscoelastic response of the liquids.  This occurs because the vibrational periods are comparable to the intrinsic molecular response times of the liquids.  These measurements demonstrate the ability of vibrating metal nanoparticles to serve as a sensitive, high-frequency probe of their mechanical environment, and to reveal new phenomena on the nanoscale.<br/>

Mechanical vibrations of metal nanoparticles for sensing applications and fundamental fluid dynamics

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.