Luo, YijunLuo, YuxiaLiu, YuxiHuang, YinYu, PingMa, HaoLi, XinpingZhang, ZhaoZhang, CuilingChen, ChaohaoGale, Philip A.Bao, Guochen2025-05-232025-05-230956-5663PubMed:39657555WOS:001431099400001http://www.scopus.com/inward/record.url?scp=85211086244&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733750928Upconversion nanoparticles (UCNPs) are characterized by high photostability, narrow spectral bands, excellent tuneability, and low biotoxicity, facilitating a broad range of biomedical applications. However, the small size required in many biological applications implies a lower luminescent brightness, as large surface-to-volume ratio is always accompanied with severe surface quenching. Herein, we introduce a strategy to overcome the surface quenching by incorporating an acceptor dye, sulforhodamine B (SRB) to surpass energy relaxation on long-lived lanthanide excited states. The surface modification of SRB led to up to 98.8% energy transfer efficiency, accompanied with the emergence of an intense SRB emission, with four orders of magnitude of change in the SRB/UCNPs emission ratio. The further structural optimisation led to an 8-fold upconversion emission enhancement. Moreover, the system exhibits excellent photostability, with only a 25% reduction over 2 h under intense irradiation. By incorporating a pH responsive 5-carboxytetramethylrhodamine (5-TAMRA) to the UCNPs, we achieved a self-referencing protochromic sensor that are specific to protons and resistant to interference from various metal ions. This work provides a facile method for enhancing small-sized nanocrystals for potential biomedical sensing and imaging applications.The authors would like to acknowledge the UTS Chancellor's Research Fellowship Program (G.B., PRO23-17800), the Dust Diseases Board (G.B., PRO23-16473), the National Health and Medical Research Council of Australia (G.B., 2034651) and the National Natural Science Foundation of China (Y.L., Grant 12004232, 22171170).8en© 2024 The AuthorsEmission enhancementOrganic-inorganic hybridspH sensingPhoton upconversionSmall-sized nanoparticles2024-11-2910.1016/j.bios.2024.11700385211086244