Functional biocompatible luminescent quantum dots (QDs) and plasmonic metal nanoparticles (NPs) have been successfully developed as separate nanoparticles in the past. Size and shape dependent optical properties of these nanomaterials have attracted tremendous attention in modern nanomedicine . Notably, advancements in nanoparticle synthesis in past decades, pushing the limits of size and shape control, have paved the way for the development of more sensitive probes with high imaging contrast, being capable to selectively target subcellular structures. However, ensuring efficient delivery of NPs into the interior of the cell is still the bottle-neck preventing NPs to reach their full potential as cellular imaging probes. As their cellular uptake proceeds through endocytosis, they end up in the endosomes which eventually fuse into lysosomes, in which degradation takes place. Hence, NPs lose their functionalities when they remain trapped inside endosomes. Such issues can be overcome by the combination of both functionalities into a single nano-object. This can make for a multifunctional nano-object that simultaneously allows for cytosolic translocation, molecular detection, specific targeting and optical imaging of intracellular structures.

Here, we report on the synthesis and optical characterization of CdSe/CdS@SiO2 nanoparticles functionalized with plasmonic gold nanocrystals. This is achieved by encapsulating flash CdSe/CdS quantum dots in a silica matrix through a water-in-oil microemulsion approach that leads to water soluble QDs@SiO2 architectures. These QDs@SiO2 are functionalized with plasmonic gold nanocrystals through specific molecular linkers. The process allows for an easy adjustment of the SiO2 shell thickness and the gold surface loading, parameters we find have a major impact on the photoluminescence brightness of the entire nano-object. While thicker SiO2 shells and high gold loadings invariably quench photoluminescence, we demonstrate an enhanced brightness for QD@SiO2/Au nano-objects with ≈10 nm thick SiO2 shells and sub-monolayer gold coating.

After a proper surface derivation to prevent aggregation in cellular media, the combination of plasmonic and luminescent functionalities in a single nano-object as demonstrated here may enable intracellular imaging through externally triggered endosomal escape.