Plasmonic Stimulation of Gold Nanorods for the Photothermal Control of Engineered Living Materials

Engineered living materials (ELMs) use encapsulated microorganisms within polymeric matrices for biosensing, drug delivery, capturing viruses, and bioremediation. It is often desirable to control their function remotely and in real time. Suitable, genetically engineered microorganisms respond to changes of their environment. Here, we combine this local sensitivity with a nanostructured encapsulation material to sensitize the ELM for infrared light. Previously, blue light has been used to stimulate microorganisms that contain optogenetic modules responsive to those wavelengths without the need for exogenous cofactors. Here, we use plasmonic gold nanorods (AuNR) that have a strong absorption maximum at 808 nm, a wavelength where human tissue is relatively transparent. Biocompatible composites of a Pluronic-based hydrogel and AuNR are prepared without agglomeration; they react to illumination by local heating. We measure a photothermal conversion efficiency of 47 % in transient temperature measurements. Steady-state temperature profiles from local photothermal heating are quantified using infrared photothermal imaging, correlated with measurements inside the gel, and applied to stimulate thermoresponsive bacteria. Using a bilayer ELM construct with the thermoresponsive bacteria and the thermoplasmonic composite gel in two separate but connected hydrogel layers, it is shown that the bacteria can be stimulated to produce a fluorescent protein using infrared light in a spatially controlled manner.