Tunable Plasmonic Microcapsules with Embedded Noble Metal Nanoparticles for Optical Microsensing

We report a comprehensive investigation of the synthetic conditions leading to the formation of tunable plasmonic microcapsules (MCs) made of a hydrophobic liquid core encapsulated into a hard silica shell embedding plasmonic metallic nanoparticles (NPs). The distinctive and remarkable features of the prepared MCs are the inert nanometer-thin silica shell and the small plasmonic NPs embedded in it, which confer interesting optical absorbance properties. We tie the mechanical robustness of the MCs to the thickness of their silica shell. We show that several oils can be used for the synthesis of MCs and we evidence how the relative solubility of the silica precursor and the polarity of the oil phase influence the final MC characteristics. We also evidence the synthesis of “monoflavor” or “multiflavor” MCs with, respectively, a single type of NPs or a mixture of metallic NPs, respectively, embedded in the silica shell. Using experiments and simulations, we demonstrate that the optical response of the MCs can be finely tuned by choosing the right ratio between Ag and Au NPs initially suspended in the solution. Our heterogeneous hybrid MCs exhibit optical properties directly resulting from the choice of NP composition and shell thickness, making them of great interest not only for mechanical and chemical microsensing but also for applications in photothermal therapy, surface-enhanced Raman spectroscopy studies, microreactor vesicles for interfacial electrocatalysis, antimicrobial activity, and drug delivery. Our simple and versatile emulsion template method holds great promise for the tailored design of a generation of multifunctional MCs consisting of modular nanoscale building blocks.