In situ , real - time detection of soot particles coated with NaCl using 193 nm light

We report in situ, real-time detection of soot particles coated with NaCl using excimer laser fragmentation fluorescence spectroscopy (ELFFS). Carbon atom fluorescence at 248 nm and the Na D-line at 589 nm are used as signatures of soot and NaCl, respectively. Soot particles are encapsulated with a NaCl layer in a well-controlled inverted flame burner. NaCl particles are injected into the methane-air co-flow flame to coat the soot particles. ArF laser irradiation of the coated particles in an air stream at 1.14 J/cm 2 produces fluorescence from Na, C, and CH. At 0.69 J/cm 2 , which is slightly above the fluorescence threshold, but not enough for considerable fragmentation of the particles, Na D-line persists with little carbon and no CH observed. These results suggest that the photolytic fragmentation-fluorescence using 193 nm excitation can be effectively used for in situ, real-time chemical analysis of core-shell nanoparticles. 1 Introduction Core-shell heterostructures, including coated nanoparticles, have received much attention recently for their use as catalysts, high density recording media, nanoenergetic materials, and biomarkers [1, 2]. Heterogeneous particles have received interest in the atmospheric science and environmental health fields because of their potential influence on global climate change and adverse health effects. For example, when black carbon particles in the atmosphere are coated with an inorganic species such as NaCl, the sunlight absorbed by the particles drastically increases due to a lensing or focusing effect [3]. Nanoparticles also have the potential to cause adverse health effects because they can penetrate deeply into lung tissue and may act as a carrier of toxic material coated on the particle surface [4]. The core-shell particles are created by a multitude of techniques including wet chemistry, laser ablation, and in high temperature flames. Among these methods, flame synthesis is readily scaled for industrial applications and is most suitable for generating nanoparticle/gas suspensions. In laboratory experiments, Dufaux and Ax-elbaum [5] synthesized nanoscale particles in a sodium coflow flame. Titanium particles are coated with a NaCl layer, which prevents oxidation after their formation. Supersaturation of NaCl can be easily achieved at normal flame conditions, so encapsulation using gas phase NaCl can be controlled by varying flame conditions such as the flame temperature. In addition, the NaCl shell minimizes agglomeration and oxidation of the core particled. Zachariah and co-workers [6] used a similar flame configuration to encapsulate silicon and germanium particles with NaCl. They found that spherical particles encapsulated with …