Dye-Doped Silica Nanoparticles

Fluorescent nanoparticles (NPs) hold considerable promise for technological applications in biochemical, bio-analytical, and medical areas. Current medical and biological fluorescent imaging methods are mainly based on dye markers, which have moderate light emission per molecule, as well as limited photostability In the last decades, semiconductor quantum dots (Qdots) have  emerged as possible bright and photostable substitutes but due to their high price, toxicity and difficult disposal, their use is somewhat impractical.

Recently, it has been shown that an enhancement of fluorescence properties such as quantum efficiency and particle brightness can be obtained by encapsulating covalently single or multiple dyes in silica-based materials, and then making small changes to the internal architecture of the particles  thus formed. More specifically, an effective building process leading to the formation of highly fluorescent monodisperse silica NPs consists essentially of two distinct phases, namely the formation of a fluorophore-rich core inside a silica precursor, followed by the addition of a siliceous shell to the system; the latter step serves the purpose of protecting the fluorescent core material from external perturbations, such as solvent interactions, which can negatively affect the photostability of the complex.

Examination of the photophysical characteristics of these complex systems at each stage of the synthesis, performed by means of fluorescence correlation spectroscopy (FCS), revealed that the cores had a lower brightness in comparison with the core-shell particles and the free dyes. Conversely, the addition of a silica shell to the core significantly enhanced the brightness of the particles and at the same time caused an in-crease in the radiative and a decrease in the non-radiative decay rates of the dye.

Part of the effect can be attributed to a suppression of intermolecular quenching mechanisms connected with the effective removal of interactions between the dye and the surrounding solvent; however, another cause of the enhanced brightness could be represented by silica caging effects reducing the mobility and flexibility of the encapsulated dye.

Despite the wide use of inorganic matrices as shields between the solvent and the dyes, their radiative properties are not yet com-pletely understood and are the subject of continued investigations.

In this context, theoretical studies can provide valuable information on the photophysical and spectroscopic properties of fluorophores embedded into different chemical environments. The possibility to simulate absorption and emission spectra of dye-doped NPs is in the first place a tool for interpretation of experimental results; moreover, it can be exploited for the design of new nano-architectures with improved performances.

In the our group, accurate quantum mechanical (QM) calculations, based on Density Functional Theory (DFT) and its extensions to excited states (Time Dependent DFT, TD-DFT) have been extensively used for the simulation of spectral properties of isolated dyes (in gas phase or in solution), [1-4] dimeric species, [5] and coumarin dyes adsorbed on a realistic model of MCM-41.[6]

An integrated strategy based on the combined use of different computa-tional methods, like e.g. classical molecular dynamics simulations based on purposely tailored force-fields and TDDFT quantum mechanical calculations, has allowed for the first time to simulate realistic models of silica nanoparticles incorporating the TRITC molecule (see Fig. 1) [7,8], and to elucidate the mechanisms behind the observed brightness enhancement while the combination of TDDFT calculations with stochastic approaches has allowed us to simulate the timeresolved spectra of coumarin molecules embedded in silica nanoparticles. [4]

References.

• 1. Pedone, A.; Bloino, J.; Monti, S.; Prampolini, G.; Barone, V. Absorption and Emission UV-Vis Spectra of the TRITC Fluorophore molecule in solution: a quantum mechanical study. Phys. Chem. Chem. Phys. (2010) 12, 1000-1006, DOI:10.1039/b920255b

  2. Pedone, A.; Barone, V. Unraveling Solvent Effects on the Electronic Absorption Spectra of TRITC Fluorophore in Solution: a Theoretical TD-DFT/PCM study. Phys. Chem. Chem. Phys. 2010, 12, 2722-2729. DOI:10.1039/b923419e 

  3. Barone, V.; Bloino, J.; Monti, S.; Pedone, A.; Prampolini, G. A Theoretical Multi-level Approach for the Study of Optical Properties of Organic Dyes in Solution. Phys. Chem. Chem. Phys.  2010, 12, 10550–10561, DOI: 10.1039/c002722g

1. Barone, V.; Bloino, J.; Monti, S.; Pedone, A.; Prampolini, G. Fluorescence Spectra of Organic Dyes in Solution: A Time Dependent Multilevel Approach. Phys. Chem. Chem. Phys., 2011, 13, 2160-2166. DOI: 10.1039/c0cp01320j

2. Pedone, A.; Gambuzzi, E.; Barone, V.; Bonacchi, S.; Genovese, D.; Rampazzo, E.; Prodi, L.; Montalti, M. Undestanding the photophysical properties of coumarin-based Pluronic-Silica (PLUS) silica nanoparticles by means of time-resolved emission spectroscopy and accurate TDDFT/stochastic calculations. Phys. Chem. Chem. Phys., 2013, DOI: 10.1039/C3CP51943K

3. Pedone, A.; Bloino, J.; Barone, V. Role of Host-Guest Interactions in Tuning the Optical Properties of Coumarin Derivatives Incorporated in MCM-41: A TD-DFT Investigation. J. Phys. Chem. C, 2012, 116, 17807−17818

4. Pedone, A.; Prampolini, G.; Monti, S.; Barone, V. Absorption and Emission Spectra of Fluorescent Silica Nanoparticles from TD-DFT/MM/PCM calculations. Phys. Chem. Chem. Phys. 2011, 13, 16689–16697 DOI:10.1039/C1CP21475h

5. Pedone, A.; Prampolini, G.; Monti, S.; Barone, V. Realistic Modelling of fluorescent Dye-Doped silica nanoparticles: A Step Toward the Understanding of their Enhanced Photophysical Properties. Chemistry of Materials 2011, 23, 5013-5023