Study on laser fabrication methods of fluorescent organic crystalline materials
supervised by Rachel Meallet
Two different methods for the laser synthesis of fluorescent (nano)crystals were studied in order to contribute to the understanding of this techniques applied to molecular organic dyes. In the first part we worked on laser induced nucleation (LIN) by femtosecond laser of rhodamine B (RhB). LIN showed to be an outstanding technique to improve the crystallization probability and the selectivity towards plate-shaped crystals in RhB aqueous supersaturated solutions. Whereas the technique showed interesting results for crystal morphology selectivity, the large amounts of solute required for LIN and our pursuit of finding different polymorph through LIN, lead us to investigate the possibility on working with smaller volumes in 96-well plates. Such a system would allow us to decrease the volume of sample required for the experiments. However, we detected inconsistent results depending on the well position, finding softer crystallization towards the center of the well plate compared to the edges. Therefore, a new sample preparation for LIN was achieved to decrease the edge effect. Nevertheless, more test should be done to find proper conditions to induce polymorphic selectivity on such system. In the second part we studied the laser fragmentation in liquids (LFL) as a clean top-down alternative for the synthesis of mesitylBODIPY (MB) nanoparticles (NP) in water. The spectroscopic analysis of the raw powder and nanosocond pulsed laser irradiation into MB micro particle suspensions in water showed that MB crystals tends to H-aggregation, having similar absorption maximum but a wider band with a decrease on the 0-0/0-1 absorption ratio.
NP suspensions in water prepared by LFL showed similar absorption spectra compared to the raw solid, being just wider (around 2400 cm-1 higher FWHM) and slightly red-shifted (from 180 to 320 cm-1). The evolution of the absorption intensity upon the increase of the laser fluence showed a two-step growth. On the first one at lower fluence (5 to 20 mJ/cm2) the absorption maximum barely increases (only 0.01 units at 20 mJ/cm2) and the nanoparticle size increases up to twice the original particle size, while after 30 mJ/cm2 the absorption increases exponentially and the nanoparticle size decreases until around one third of the original size. This might be an indication of different phenomena occurring at low and high fluence regimes, that could include bigger size nanoparticles induced by laser melting in liquids (LML) at low fluence. The addition of surfactant is known to improve the colloidal stability and reduce NP size. Therefore, experiments using Pluronic®F17, suitable surfactant for biological analysis, were carried on. The addition of this surfactant induced the formation of two new bands with absorption maxima at 550 nm and 580 nm which disappeared at higher fluence. For the moment we hypothesize that at high laser fluence the crystals get fragmented with similar structure to the raw solid powder while at low laser fluence intermediates are created by LML and evolve to the original crystal in pure water samples, but in the presence of surfactant such intermediates get stabilized leading to different aggregated systems. However, X-ray and electronic microscopy are still required to validate our hypothesis.