Let There Be Heat: Silica-Coated Gold Nanoparticles as Photothermal Reactors for Chemical Synthesis

Conspectus The heating of matter upon interacting with light is a fundamental process ubiquitous in the natural world. With the rise of nanotechnology over the past decades, a variety of nanomaterials capable of converting light into heat have been discovered and their physicochemical properties investigated. Perhaps the most exotic is the photothermal heating of metallic nanocrystals via surface plasmons. Here an incoming electromagnetic wave triggers the oscillation of the nanoparticle’s electron cloud. When in resonance, this generates an enormous increase to the absorption coefficient, enabling more energy to dissipate as heat. The plasmonic phenomenon has an incredibly diverse range of functions, from the vibrant coloration of medieval stained-glass windows to the localization and enhancement of light at the nanoscale level. Plasmonic heating or thermoplasmonics is a relatively new addition that has gained popularity mainly through applications in therapeutics and biotechnology. With this Account, we aim to put a spotlight on the use of thermoplasmonics to drive chemical synthesis, a rapidly expanding area of research with immense potential. Throughout the long tradition of chemical synthesis, chemists have rarely deviated from the typical oven or hot plate to set and maintain a homogeneous temperature within the reaction vessel. In contrast, the use of thermoplasmonic nanomaterials can introduce heterogeneity to the heating profile of a reaction by forming steep temperature gradients near the surface of nanoparticles. Additionally, photothermal conversion enables heat activated processes to benefit from the advantages of light initiation, e.g., contactless activation and spatial control. Thus, thermoplasmonics offers an attractive alternative to the long-standing norm. Several early studies demonstrated the power of this method, taking advantage of the localized heating to carry out reactions with minimal change to the bulk temperature of the surrounding medium. However, tapping into this potential can be very challenging as colloidal solutions tend to aggregate even with small changes to the environment. Different strategies have been utilized to overcome this obstacle, for example embedding particles into glass or other heterogeneous substrates. Our group has experimented with coating gold nanostructures with a silica shell. This ensures the structural and colloidal stability that is critical for thermoplasmonic chemistry. Recently, we applied this methodology to advance olefin metathesis, the synthesis of iron oxide (IO), palladium (Pd) and silver (Ag) nanoparticles, and the formation of various metal-organic frameworks (MOFs). In addition, highly stable hybrid materials could be isolated as composites of plasmonic particles with polymers, MOFs, and other nanostructures. The large variety of reaction conditions and the different precursors, additives, and catalysts that our method proved to be compatible with highlight the versatility that silica encapsulation provides. The unique properties of plasmonic heating coupled with the added stability can open a wide range of opportunities for more efficient reactions and altogether new reactivity along with the formation of novel composite materials.