Simple synthesis of carboxyl-functionalized upconversion nanoparticles for biosensing and bioimaging applications

We report a simple one-step hydrothermal method for the synthesis of hydrophilic luminescent upconversion nanoparticles (UCNPs) using malonic acid as the stabilizer and functional agent. Using this method, two UCNPs with different colors of upconversion luminescence were synthesized. The surface of the as-prepared UCNPs was capped with carboxyl groups, which not only resulted in the UCNPs having good dispersity in water, but also allowed further conjugation with other functional molecules, thus indicating the potential applications in biosensing and bioimaging. To demonstrate this, amino-labeled single-stranded DNA (ssDNA) was conjugated on the surface of the UCNPs. Based on the different absorption and luminescence quenching abilities of graphene oxide (GO) to ssDNA-modified UCNPs before and after exonuclease I (Exo I)-triggered hydrolysis of ssDNA, a detection platform was developed for the detection of Exo I activity with a detection limit of 0.02UmL−1. The prepared hydrophilic UCNPs were also used successfully for in vivo upconversion luminescence imaging of nude mice. A simple one-step hydrothermal method was reported for the synthesis of hydrophilic luminescent upconversion nanoparticles (UCNPs) by using malonic acid as the stabilizer and functional agent, and the feasibility of the prepared UCNPs in biosensing and bioimaging applications was demonstrated. [Display omitted] •A simple one-step method for upconversion nanoparticles (UCNPs) is reported.•The prepared UCNPs are well-hydrophilic and can be easily further functionalized.•A sensitive exonuclease I-sensing platform is developed using the prepared UCNPs.•The prepared UCNPs are also used successfully for in vivo bioimaging of nude mice.