Surface-enhanced Raman scattering (SERS) is a fascinating process by which normally weak Raman signals can be amplified by 8–10 orders in magnitude.[1] These large enhancements are mainly caused by the strong, light-induced electromagnetic fields (E-fields) at the surface of a metallic nanostructure.[2] The superb sensitivity of SERS has shaped the mainstream view of this method as one primarily to be implemented for trace detection. Yet, SERS does not need single-molecule sensitivity to be useful. This is because SERS can reveal the structural information of molecules adsorbed on the surface of a Au or Ag nanoparticle. These surfaces continue to gain importance as nanoparticle engineering and surface functionalization become evermore sophisticated to meet the demands of new applications. One application where this surface plays a pivotal role is the photothermal (PT) effect. The PT effect occurs when a metal nanoparticle absorbs light and releases it as heat.[3] This heat can affect the molecules on the nanoparticle s surface and heat up the local environment, both of which have been actively exploited for drug delivery,[4] cancer therapy,[5] and lithography applications.[6] In the PT effect, a nanoparticle s surface plays a key role in its utilization as molecules are often released from this surface or change as a result of the released heat. Quantifying the heat released and the temperature gradients generated by the PT effect is therefore essential for engineering the nanoparticle and its surface for the aforementioned applications. While an array of techniques have been developed to quantify the PT effect over varying timescales, these approaches rely on indirectly inferring the heat generated by the PT effect through bubble formation,[7] ice melting,[8] computations,[9] and ultrafast absorption techniques.[10] Herein we show, for the first time, that SERS can be employed as a sensitive tool to probe the PT effect, leading to direct examination of the heat generated at the nanoparticle s surface.

Both SERS and the PT effect share the same fundamental mechanism of plasmon excitation that generates strong local E-fields for SERS and heat for the PTeffect, respectively. This same origin makes SERS a very simple and attractive technique for probing the PT effect without the involvement of sophisticated equipment or analysis. This simplicity is also a big advantage for nanoparticles having complex shapes, morphologies, and compositions, where modeling is rather complicated and assumptions about the nanoparticle s parameters may become untenable. Figure1 shows SEM and TEM images of the Ag nanocubes and Au–Ag nanocages used in the present work and their corresponding localized