Time-of-flight photon spectroscopy for spectral analysis of particles diffusing one by one

The functionalizability and biocompatibility of gold particles make them attractive for applications in sensing and theranostics. The emission of hybrid particles obtained by grafting fluorescent molecules at the surface of the particle or inside an encapsulating organic shell extends the possibilities for studying their dynamics in situ. Under illumination, these hybrid particles can undergo radical changes in their emission such as spectral shifts and decay time shortening, which gives information on the dynamics of the studied system. We report on a method that we have recently developed to record the spectrum of emission of a particle diffusing through the excitation volume of a confocal microscope in a few milliseconds. This method, named photon time-of-flight spectroscopy (PTOFS), exploits the time-frequency conversion principle [1]. This method is especially appropriate to monitor the dynamics of the intrinsic luminescence or enhanced fluorescence of individual particles diffusing one by one through the excitation volume, simultaneously with their light scattering signal. Information on the diffusional and photophysical dynamics of the particles is derived from fluctuation correlation spectroscopy (FCS). We will illustrate the PTOFS method by discussing the example of the luminescence of bipyramids [2] and the idea that it may be a useful tool to study the fluorescence of nanohybrid particles and molecular plasmonic assemblies. [1] M. Loumaigne, P. Vasanthakumar, A. Richard, and A. Débarre, "Time of flight spectroscopy : A simple scheme to monitor simultaneously spectral and temporal fluctuations of emission on single nanoparticles", ACS Nano 6, 10512 (2012). [2] M. Loumaigne, J. R. G. Navarro, S. Parola, M. H. V. Werts and A. Débarre, "The intrinsic luminescence of individual plasmonic nanostructures in aqueous suspension by photon time-of-flight spectroscopy", Nanoscale 7, 9013 (2015).