New Core-Shell Nanostructures for FRET Studies: Synthesis, Characterization, and Quantitative Analysis

Abstract

This work describes the synthesis and characterization of new core-shell material designed for Förster resonance energy transfer (FRET) studies. Synthesis, structural and optical properties of core-shell nanostructures with a large number of two kinds of fluorophores bound to the shell are presented. As fluorophores, strongly fluorescent rhodamine 101 and rhodamine 110 chloride were selected. The dyes exhibit significant spectral overlap between acceptor absorption and donor emission spectra, which enables effective FRET. Core-shell nanoparticles strongly differing in the ratio of donors to acceptor numbers were prepared. This leads to two different interesting cases: typical single-step FRET or multistep energy migration preceding FRET. The single-step FRET model that was designed and presented by some of us recently for core-shell nanoparticles is herein experimentally verified. Very good agreement between the analytical expression for donor fluorescence intensity decay and experimental data was obtained, which confirmed the correctness of the model. Multistep energy migration between donors preceding the final transfer to the acceptor can also be successfully described. In this case, however, experimental data are compared with the results of Monte Carlo simulations, as there is no respective analytical expression. Excellent agreement in this more general case evidences the usefulness of this numerical method in the design and prediction of the properties of the synthesized core-shell nanoparticles labelled with multiple and chemically different fluorophores.

Keywords: FRET; TiO2@SiO2; core-shell nanostructures; luminescent materials.

Scheme 1

Synthesis of TiO₂@SiO₂-(CH₂)₃-NH-D/A.

Figure 1

TEM image and diagram of the particle diameter of TiO₂@SiO₂-(CH₂)₃-NH-D/A nanoparticle. The solid green line corresponds to the Gaussian distribution with the mean value $\langle R\rangle$ = 74.58 nm and variance $\sigma$ = 5.82 nm.

Figure 2

Fourier transform infrared spectra of TiO₂@SiO₂-(CH₂)₃-NH-R101/110 with a different ratio of the donor (D—rhodamine 110 chloride) to acceptor (A—rhodamine 101).

Figure 3

Diagrams of zeta potential for TiO₂ and TiO₂@SiO₂.

Figure 4

Normalized absorption (solid lines) and emission (dashed lines) spectra of donor (R110) and acceptor (R101). ${\lambda }_{exc}=440 \mathrm{nm}$.

Figure 5

Donor fluorescence intensity decays for (a) D_MIGA and (b) D_TRA systems, bonded covalently to the core-shell surface.

Figure 6

Mean number of excitation energy jumps in the presence of energy migration for nanoparticles of different mean radius.

Figure 7

Relative mean squared displacement of excitation energy versus the number of donors on the nanoparticle.

Scheme 2

Illustrative picture of TiO₂@SiO₂-(CH₂)₃-NH-D/A.