Fluorescence Quenching of CdTe Nanocrystals by Bound Gold Nanoparticles in Aqueous Solution

Abstract

Water-soluble gold nanoparticles with an average diameter of 5 nm were prepared with carboxylic acid terminated thiol ligands. These ligands contain zero to eight methylene moieties. CdTe nanocrystals with an average diameter of 5 nm were synthesized with aminoethanethiol capping. These nanocrystals displayed characteristic absorption and emission spectra of quantum dots. The amine terminated CdTe nanocrystals and carboxylic-acid-terminated gold nanoparticles were conjugated in aqueous solution at pH 5.0 by electrostatic interaction, and the conjugation was monitored with fluorescence spectroscopy. The CdTe nanocrystals were significantly quenched upon binding with gold nanoparticles. The quenching efficiency was affected by both the concentration of gold nanoparticles in the complex and the length of spacer between the CdTe nanocrystal and Au nanoparticle. The observed quenching was explained using Förster resonance energy transfer (FRET) mechanism, and the Förster distance was estimated to be 3.8 nm between the donor-acceptor pair.

Fig. 1

TEM images of individual a CdTe nanocrystals, b Au nanoparticles, and c corresponding complexes of CdTe nanocrystals and Au nanoparticles

Fig. 2

Absorbance spectra of tetraoctylammonium-coated gold nanoparticles, tiopronin-coated gold nano nanoparticle, and gold nanoparticles capped with longer linker ligands, and QDot in aqueous solution

Fig. 3

Fluorescence emission spectra of CdTe-tiopronin-protected gold nanoparticles with time (molar ratio of Au particle/QDot=4.5×10^-3). λ_ex=450 nm. The concentration of CdTe nanocrystal was 5.0×10^-7 M in solution

Fig. 4

aChange in CdTe fluorescence intensity at 535 nm during the binding time with different molar ratios of Au nanoparticle/CdTe nanocrystals in solution and b effect of gold nanoparticles concentration on the CdTe fluorescence quenching efficiency. Fluorescence intensity at 535 nm after complete binding [plateau region from figure (a)] was used in this plot. The gold nanoparticles used herein are protected with shortest spacer, tiopronin (methylene number = 0). The concentration of CdTe nanocrystal was 5.0×10^-7 M in solution

Fig. 5

a Change in fluorescence intensity at 535 nm during binding time for the samples with different methylene number as the spacers between the metal particle and QDot in solution. b Effect of spacer length on the fluorescence quenching efficiency. The concentrations of CdTe nanocrystals (5.0×10^-7 M) and metal particles (2.2×10^-7 M) were the same for all the samples in solution

Fig. 6

a Effect of spacer length between CdTe and gold nanoparticles on the rate fluorescence quenching (k_m) and b quenching efficiency. R_o is the Förster distance

Scheme 1

Schematic representation describing the preparation of gold nanoparticles protected with varied ligand-chain length. Tiopronin (shortest ligand used in this study)- protected nanoparticles were obtained as shown in route a, where the tetraoctylammonium ligands were directly displaced with tiopronin. Longer ligand-protected nanoparticles were prepared in a two-step procedure as shown in route b. At first, tetraoctylammonium ligands are displaced with active ester-terminated ligands and subsequent condensation with the γ-aminoalkyl carboxylic acid with varying methylene carbons from 3 to 8

Scheme 2

Schematic representation of electrostatic interactions in the complexes of negatively charged gold nanoparticles and positively charged CdTe nanocrystals in aqueous solution at pH 5.0