Thermo-activatable PNIPAM-functionalized lanthanide-doped upconversion luminescence nanocomposites used for in vitro imaging

Abstract

Stimuli-responsive nanocomposite has a great potential application for cell imaging, drug delivery, and improving therapeutic effect. Rare-earth upconversion nanoparticles (UCNPs) have exhibited an excellent properties as a promising biological luminescent labels for fluorescence imaging. In this work, we prepared β-NaLuF₄:Yb³⁺,Er³⁺ UCNPs coated with thermo-responsive copolymer poly(N-isopropyl acrylamide-co-acrylic acid) (P(NIPAM-co-AA)) by using ligand exchange strategy to obtain novel nanocomposite UCNP@P(NIPAM-co-AA). The acrylic acid moieties of P(NIPAM-co-AA) coordinate with the surfaces of UCNPs, and the PNIPAM moieties can induce the UCNPs dispersion or aggregation in the aqueous solution below and above the lower critical solution temperature (LCST) of PNIPAM. The UCNP@P(NIPAM-co-AA) exhibited weak upconversion luminescence(UCL) fluorescence intensity below the LCST but an 80-fold enhanced UCL intensity at 31 °C above their LCST with only 1 °C temperature transition range. The novel nanocomposite also shows a remarkable reversible property in aqueous solution and low cytotoxicity. For in vitro imaging, a 13-fold signal-to-noise ratio can be obtained below and above the LCST for the novel thermo-activatable UCL imaging, which implies potential application for intelligent biomedical agent.