Linear and non-linear optical spectroscopy of 'well-behaved' colloidal suspensions of nanocrystal assemblies

The directed self-assembly of nanocrystals in the liquid phase can yield new materials with interesting optical properties for potential applications in medical diagnostics, phototherapy, photoenergy, and the guiding of light. Development of self-assembly strategies for such materials require clear relations between the generated structures and their optical properties. It is therefore useful to study the spectroscopy of nanocrystal assemblies in their native colloidal suspension. If a suspension of nanoparticles obeys established models of how colloidal suspensions behave in terms of diffusion[1], sedimentation and dependence of aggregation state on liquid phase composition, spectroscopic parameters of the suspended nanoparticles can be determined quantitatively in real-time, giving access to their dynamic behaviour and information during their functional performance. In this presentation we will show the spectroscopic methods that we utilise for the characterisation of colloidal plasmonic nanoparticle suspensions. These consist of steady- state methods (such as resonant light scattering[2]), as well as time-correlated spectroscopies[3] that work at the single-particle level, and even allow for recording of spectra of single particles diffusing in suspension[4,5]. We will present our latest developments concerning the measurement of extinction and scattering cross sections of plasmonic nano-objects, the diffusion and sedimentation behaviour of plasmonic colloids, and the intrinsic photoluminescence of various advanced gold nanoparticle assemblies (such as DNA-linked gold nanoparticles[6]). This plasmon-mediated luminescence is observed under both monophotonic or multiphoton excitation. [1] M. H. V. Werts, V. Raimbault, R. Texier-Picard, R. Poizat, O. Français, L. Griscom, J. R. G. Navarro. Lab Chip 2012, 12, 808. [2] J. R. G. Navarro, M. H. V. Werts, Analyst 2013, 138, 583. [3] M. Loumaigne, R. Praho, D. Nutarelli, M. H. V. Werts, A. Débarre. Phys. Chem. Chem. Phys. 2010, 12, 11004. [4] M. Loumaigne, P. Vasanthakumar, A. Richard, A. Débarre. ACS Nano 2012, 6, 10512. [5] M. Loumaigne, J. R. G. Navarro, S. Parola, M. H. V. Werts, A. Débarre, Nanoscale 2015, 7, 9013. [6] P. K. Harimech, S. R. Gerrard, A. H. El-Sagheer, T. Brown, A. G. Kanaras, J. Am. Chem. Soc. 2015, 137, 9242.