An All-in-One Nanoheater and Optical Thermometer Fabricated from Fractal Nanoparticle Assemblies

We designed and optimized a dual-functional photothermal agent that performs as a nanoheater and real-time optical thermometer by leveraging gold nanoparticle (AuNP) self-assembly and anti-Stokes thermometry. We engineered colloidally stable fractal AuNP clusters with well-defined nanogaps to absorb strongly in the near-infrared and enhance anti-Stokes vibrational modes via surface-enhanced Raman scattering (SERS) for electromagnetic (EM) hotspot-localized thermometry during plasmonic heating. Photothermal characterization and simulations of a range of AuNP building block sizes demonstrated that 40 nm AuNPs are optimum for combined plasmonic heating and SERS due to the high probability of in resonance chains within assemblies. We explored the relationship between the far-field of our AuNP clusters and the near-field enhancement of anti-Stokes modes in the context of SERS thermometry, setting out design considerations for applying SERS thermometry. Finally, using a single near-infrared (NIR) laser source, we demonstrated plasmonic heating of a colloidal system with simultaneous accurate temperature measurement from EM hotspots via the thermal information encoded in the anti-Stokes mode of surface-bound Raman reporter molecules. Ultimately, our approach could enable real-time noninvasive temperature feedback from plasmonic nanoparticles within tumor tissue environments to guide safe and effective temperature increases during cancer photothermal therapy.