Incoherent-Light-Excitable Lanthanide Upconversion Enabled by Highly Hydrophilic and Photostable Dye Sensitization

Achieving efficient lanthanide upconversion excited by low-power incoherent light remains a daunting challenge due to the low extinction coefficients of lanthanide ions. Although dye sensitization is a promising route for enabling such excitation, dye-sensitized UCNPs (dsUCNPs) typically suffer from hydrophobicity and photolability, thus hindering their practical applications. In order to overcome these limitations, we present a "one-stone-two-birds" strategy by coating dsUCNPs with unsaturated fatty acid salts (UFAS). By leveraging the amphiphilic and antioxidant properties of UFAS, the coating imparts both excellent water dispersibility and photostability to dsUCNPs. We demonstrate that the photostability of UFAS-coated dsUCNPs increases with the increasing degree of unsaturation (i.e., the content of carbon-carbon double bond) in the UFAS molecules, with sodium linolenate (SLn) offering the most significant improvement. SLn-coated dsUCNPs exhibit a photobleaching half-life of 260 min, which is 87 times longer than those coated with conventional materials such as F-127, DSPE-PEG2000, and mesoporous silica. Importantly, SLn-coated dsUCNPs are excitable under low-power incoherent-light excitation (100 mW/cm²) and enable high-contrast cell imaging using light-emitting diodes (LED), making it possible to obviate the need for a high-intensity laser as an excitation source and significantly minimizing phototoxicity. This work offers a simple and effective strategy for producing highly hydrophilic and photostable dsUCNPs of practical significance, paving the way for their practical use in a broad range of applications involving incoherent low-power light excitation.