SUNSET-1 utilizes a dopant set of Er 3+ , Nd 3+ , Yb 3+ because this combination of dopants has been used to sensitize upconversion and optogenetic activity with 800 nm excitation, a wavelength that lies in the NIR-I biological imaging window. Segregation of these dopants into different shells of UCNP heterostructures has been shown to dramatically enhance emission. In these systems, Nd is typically included to sensitize the absorption of the 800 nm excitation, Er to upconvert absorbed energy and emit UV/visible light, and Yb to act as a conduit to transfer energy between Nd and Er dopants that would otherwise quench each other via cross-relaxation energy transfer. The nanoparticle heterostructures sampled in SUNSET-1 are variable, with core radii ranging from 1-4 nm and up to 3 shells, each measuring between 1-2.5 nm in thickness, as depicted in Figure . To probe the extrapolatory power of developed models, we simulate explicit 4-shell nanoparticles and hold them out of training data to use as an out-of-distribution (OOD) test set. Thus, SUNSET-1 as a whole has nanoparticle radii spanning from 1-13.6 nm, and the brightest particle has an intensity of ∼ 20,000 cps. I vis-U V most closely follows an exponential distribution, so we use the log(I vis-U V ) as the target label for model training.