Plasmonic fluorescent quantum dots

Abstract

Combining multiple discrete components into a single multifunctional nanoparticle could be useful in a variety of applications. Retaining the unique optical and electrical properties of each component after nanoscale integration is, however, a long-standing problem. It is particularly difficult when trying to combine fluorophores such as semiconductor quantum dots with plasmonic materials such as gold, because gold and other metals can quench the fluorescence. So far, the combination of quantum dot fluorescence with plasmonically active gold has only been demonstrated on flat surfaces. Here, we combine fluorescent and plasmonic activities in a single nanoparticle by controlling the spacing between a quantum dot core and an ultrathin gold shell with nanometre precision through layer-by-layer assembly. Our wet-chemistry approach provides a general route for the deposition of ultrathin gold layers onto virtually any discrete nanostructure or continuous surface, and should prove useful for multimodal bioimaging, interfacing with biological systems, reducing nanotoxicity, modulating electromagnetic fields and contacting nanostructures.

Figure 1. Schematic of gold-shell-encapsulated quantum dots (QDs)

a, Hydrophobic QDs coated with trioctyphosphine oxide (molecules on QD surface, blue) were solubilized with a lipid-PEG-COOH conjugate (purple). The water-soluble QDs were then coated with PLH (blue curves) for immobilization of Au³⁺ ions at high density. Addition of a mild reducing agent, hydroxylamine, led to gold nucleation on the PLH template and formation of a thin gold shell. b, The distance between the QD core and the gold shell can be precisely tuned by coating QDs with extra polyelectrolyte bilayers (cationic PAH and anionic PSS, red and green curves) by means of layer-by-layer (LBL) assembly before PLH coating. This figure is not drawn to scale.

Figure 2. TEM imaging of QD-gold hybrid nanoparticles

a-d, TEM images of QDs in chloroform (a), lipid-PEG-COOH conjugate-coated water-soluble QDs (b), gold-shell-encapsulated QDs shown at a magnification of 245,000 (c) and 340,000 (d). These TEM studies clearly show that virtually every QD is encapsulated by a thin gold shell (2-3 nm) with a gap between.

Figure 3. Optical properties of the QD-gold core-shell nanoparticles

a, UV-vis absorption of the QDs before and after gold encapsulation. The black (original) and green (absorption value ×10 for better visualization) curves show the absorption profile of the water-soluble PLH-coated QD. After gold shell formation, the QD absorption was buried by a strong surface plasmon resonance (SPR) band centred at 583 nm. b, Fluorescence spectra of the original organic-soluble QDs (green), water-soluble QDs in the presence of Au³⁺ ions (yellow), and gold-shell-encapsulated QDs (red). Although the peak position did not change, the fluorescence intensity decreased 51.9% upon addition of Au³⁺ salt, and decreased another 23.7% after gold shell formation. c, To improve the QD fluorescence efficiency, polyelectrolyte bilayers composed of cationic PAH and anionic PSS were deposited onto the QD surface before PLH coating to increase the spacing between the QD core and the gold shell. Quantitative spectroscopic measurements indicated that the QD fluorescence was significantly improved with one to two layers of polyelectrolyte coating. The final QD quantum yield increased to 32.8% with one layer of PAH/PSS and 39.0% with two layers of PAH/PSS. d,e, Representative TEM images of the QD-gold core-shell nanoparticles with one and two layers of polyelectrolyte spacers. f, Histograms of the core-shell separation distribution of particles with zero bilayers (top), one bilayer (middle) and two bilayers (bottom) of polyelectrolytes. Each histogram was plotted based on >100 nanoparticles. Insets show the TEM measurements of representative particles.

Figure 4. Photostability and dual imaging modalities of the QD–gold nanonanoparticles

a, Normalized fluorescence intensities of organic soluble QDs, water-soluble QDs and gold-coated QDs. Under identical illumination conditions (480/40 nm), the QD–gold nanoparticles are significantly more photostable than the lipid-PEG-coated water-soluble QDs, and even slightly outperformed the original organic-soluble QDs. b-g, Fluorescence and dark-field imaging of single nanoparticles spread between two glass coverslips. For QDs without gold shell coating (b-d), only fluorescence signals were observed, but no scattering signal. In contrast, because of the metallic gold shell, the encapsulated QDs (e-g) not only exhibited strong fluorescence, but also became excellent probes for surface plasmon scattering.