Laser fabrication and spectroscopy of organic nanoparticles

In working with nanoparticles, researchers still face two fundamental challenges: how to fabricate the nanoparticles with controlled size and shape and how to characterize them. In this Account, we describe recent advances in laser technology both for the synthesis of organic nanoparticles and for their analysis by single nanoparticle spectroscopy. Laser ablation of organic microcrystalline powders in a poor solvent has opened new horizons for the synthesis of nanoparticles because the powder sample is converted directly into a stable colloidal solution without additives and chemicals. By tuning laser wavelength, pulse width, laser fluence, and total shot number, we could control the size and phase of the nanoparticles. For example, we describe nanoparticle formation of quinacridone, a well-known red pigment, in water. By modifying the length of time that the sample is excited by the laser, we could control the particle size (30-120 nm) for nanosecond excitation down to 13 nm for femtosecond irradiation. We prepared beta- and gamma-phase nanoparticles from the microcrystal with beta-phase by changing laser wavelength and fluence. We present further results from nanoparticles produced from several dyes, C(60), and an anticancer drug. All the prepared colloidal solutions were transparent and highly dispersive. Such materials could be used for nanoscale device development and for biomedical and environmental applications. We also demonstrated the utility of single nanoparticle spectroscopic analysis in the characterization of organic nanoparticles. The optical properties of these organic nanoparticles depend on their size within the range from a few tens to a few hundred nanometers. We observed perylene nanoscrystals using single-particle spectroscopy coupled with atomic force microscopy. Based on these experiments, we proposed empirical equations explaining their size-dependent fluorescence spectra. We attribute the size effect to the change in elastic properties of the nanocrystal. Based on the results for nanoparticles of polymers and other molecules with flexible conformations, we assert that size-dependent optical properties are common for organic nanoparticles. While "electronic confinement" explains the size-dependent properties of inorganic nanoparticles, we propose "structural confinement" as an analogous paradigm for organic nanoparticles.