Six-photon upconverted excitation energy lock-in for ultraviolet-C enhancement

Photon upconversion of near-infrared (NIR) irradiation into ultraviolet-C (UVC) emission offers many exciting opportunities for drug release in deep tissues, photodynamic therapy, solid-state lasing, energy storage, and photocatalysis. However, NIR-to-UVC upconversion remains a daunting challenge due to low quantum efficiency. Here, we report an unusual six-photon upconversion process in Gd³⁺/Tm³⁺-codoped nanoparticles following a heterogeneous core-multishell architecture. This design efficiently suppresses energy consumption induced by interior energy traps, maximizes cascade sensitizations of the NIR excitation, and promotes upconverted UVC emission from high-lying excited states. We realized the intense six-photon-upconverted UV emissions at 253 nm under 808 nm excitation. This work provides insight into mechanistic understanding of the upconversion process within the heterogeneous architecture, while offering exciting opportunities for developing nanoscale UVC emitters that can be remotely controlled through deep tissues upon NIR illumination.