Time Resolved Fluorescence Anisotropy measurements of nanoparticles in flames

Recently, increasing concern about the health effects of breathing fine particulate matter are motivating worldwide government agencies to pursue more stringent regulations for submicron particulate emission from combustors. As a result, there is a growing urgency to control fine particle formation in practical devices. Many difficulties arise in developing diagnostics and evaluating their performance for nanoparticles especially in complex environments, like flames or engines. Consequently, there is an increasing interest for non-intrusive diagnostics capable to real-time monitoring particles in the size range below 30 nm and to determine their physical and chemical characteristics within combustion regions, exhausts or atmosphere. A promising non invasive optical technique able to detect fast rotational diffusivity of molecules or particles is the TRFPA, which is also able to measure the size of particles selected by their spectroscopic properties. This technique has been extensively used to study liquid samples and biological systems, providing rotational properties and shape of proteins and macromolecules. More recently, this technique has been applied to the ex-situ characterization of flame-produced nanoparticles, by analysing combustion products collected in water from different flames and engine exhausts [1,2]. Only a few previous works have used TRFPA to study polyatomic species in gas phases and high temperature regime [3,4]. The high temperature environment make TRFPA measurements very difficult because rotational diffusivity increases so that fast resolution is necessary and data analysis become more complex. Furthermore, flame luminosity and low particle concentrations give rise to high signal to noise ratio. Here we present a new implementation of the TRFPA technique for the analysis of nanometric particles in situ in a controlled diffusion flame able to follow the nanoparticles growth process inside the flame. Fluorescent particles are excited by femtosecond laser pulses whereas the fluorescence intensity profiles are detected by a fast photomultiplier tube and recorded on a digital oscilloscope. The experimental set-up achieves rather short acquisition times and a good time resolution, which allows to detect particles in the size range between 5 and 30 nm with reasonable uncertainties. [1]. A .Bruno, P. Minutolo, C. de Lisio, A. D'Alessio, Combust. Flame, 151, 472-481, (2007) [2] A. Bruno, P. Minutolo, C. de Lisio, Opt. Express, 13, 5393-5408 (2005) [3] J. S. Baskin, M. Gupta, M. Chachisvilis, and A. H. Zewail, Chem. Phys. Lett. 275, 437-444 (1997) [4] A.Bruno; F. Ossler; C.de Lisio; P. Minutolo; N. Spinell; A. D'Alessio, Optics Express,16, 5623-5632 (2008)