Functional Fluorescent Organic Nanoparticles

Abstract

This thesis presents an extensive study on fluorescent organic nanoparticles and fluorescent organic binary and ternary nanoassemblies. In particular the attention is focused on the preparation and characterization of organic nanoparticles and new nanocomposites obtained from different types of small organic molecules, their stabilization and the use of these materials for biological and optoelectronics applications. The work deals at the beginning with the description of some methods used to obtain organic nanoparticles, focusing the attention on the reprecipitation process and the classical nucleation theory. On the experimental section, we focus the attention on the preparation and spectroscopic characterization of fluorescent organic nanoparticle (FON) suspensions, their colloidal stability during time and the morphological characterization of the obtained nanostructures. This systematic study allowed to define some basic rules to be followed to generate fluorescent organic nanoparticle suspensions in water: for example, the chromophore fluorescence in the solid state, the non-solubility in water and the final recommended concentration. However, these simple rules cannot reliably predict the stability of the suspensions. In fact, there are plenty of parameters that can affect the stability: molecular structure, polarizability, solubility in water, temperature, concentration, …, and it is very hard to find safe general rules to obtain stable nanoparticle suspensions in water. In the second part we briefly describe the multiphoton microscopy technique and the most popular inorganic and organic (or hybrid organic/inorganic) biphotonic probes, such as quantum dots, dye-loaded silica nanoparticles and nanodots. Then, we focus the attention on the use of fluorescent organic nanoparticles as biphotonic probes and we report a study on two types of FONs (in water) designed for this purpose. These suspensions have been spectroscopically characterized by linear and nonlinear optical techniques, namely absorption, fluorescence and two-photon induced fluorescence. The obtained gigantic two-photon brightness has been exploited for two-photon imaging of living tadpoles. One of the FON suspensions has been found to be extremely good for in-vivo imaging of the circulatory system, in terms of brightness, stability and contrast. All of these characteristics make our fluorescent nanoparticles promising tracers for two-photon in vivo angiography. The third part of the thesis is dedicated to the strategies for stabilizing colloidal nanoparticle suspensions. In the first part of the chapter, the most popular strategies of stabilization are described, such as electrostatic, steric and depletion stabilization processes. In the second part we describe two alternative methods to improve the colloidal stability: the use of different types of additives during the preparation process, and the molecular design. Using the first method, cationic, anionic and non-ionic additives have been tested on our nanoparticle suspensions. The presence of these amphiphilic molecules, probably located on the surface of nanoparticles, plays a key role in changing their surface charge and, consequently, in enhancing the electrostatic repulsions and reducing aggregation phenomena during time. It has also been observed that the nature of the counterion is a very important parameter affecting the colloidal stability of the nanosuspension. The second method, called molecular design, can be briefly described as conceiving organic dyes whose molecular structure is intended to favor stable colloidal suspensions during time. Even small modifications of the molecular structures of the starting compounds can strongly affect the solubility in water, allowing to obtain very high supersaturation levels, fast reprecipitation processes (few seconds) and good colloidal stability of the nanoparticle suspensions (weeks or months). The modification of the molecular structure can also affect the packing of the molecules in the nanoparticle state, hindering for example - stacking and allowing good solid-state and nanoparticle-state fluorescence. The last part of this thesis is focused on novel types of organic nanoassemblies conceived to obtain efficient energy and electron transfer between the chromophores that compose the same nanostructure. In the first part of the chapter we describe energy and electron transfer processes from a theoretical point of view, while in the second part we describe the experimental results. In particular, novel fluorescent binary core@shell nanoassemblies showing high energy transfer rate were prepared using two different chromophores: the energy acceptor compound as core and the energy donor as shell. The synthesis and the spectroscopic and morphological characterization of this new type of nanocomposites have been described in detail. Our novel fully organic and molecular-based core@shell nanostructures show unique photo-luminescence properties. In particular, the luminescence is localized at the shell-to-core nanointerface, thanks to the highly efficient directional energy transfer from the shell to the core, resulting in a striking luminescence enhancement with respect to both single-component nanoparticles. These novel materials can be seen as a promising green and biocompatible alternative to inorganic quantum dots in biological applications, but they may also find interesting applications in energy-harvesting systems. We also report the preparation of ternary core@shell@shell nanoassemblies able to provide energy transfer cascade between the three compounds, and the improvement of their photo-luminescence during time by the use of a commercial polymer as a dopant. The very efficient energy transfer cascade observed for these nanosystems opens a new arena for tuning and enhancing luminescence properties of organic nanostructures by engineering nanointerfaces between polar/polarizable chromophores. In addition, it paves the way for the design of “smart” multi-shell organic nanostructures taking advantage of both directional energy transfer and interfacial phenomena, as demonstrated by the study of core@shell@shell nanoparticles. In the last part of the chapter we describe the preparation and some preliminary experimental results for a core@shell nanostructure designed for electron transfer. The detailed characterization of the properties of this last type of nanosystem is ongoing at present.