Ratiometric luminescence nanothermometry is rapidly evolving as a promising alternative to traditional thermometry for remote and sensitive temperature sensing. This is crucial for biomedical applications which require robustness, minimal invasiveness, and biocompatibility. In this project, we synthesize and characterize nanomaterials as ratiometric luminescent nanothermometers using the bottom-up approach and we test their toxicity to healthy human cells for theranostic applications.