Photoswitchable spiropyran-capped hybrid nanoparticles based on UV-emissive and dual-emissive upconverting nanocrystals for bioimaging

Abstract

In this study, two photoswitchable spiropryran-capped hybrid nanoparticles based on two different core/shell upconverting nanoparticles (UCNPs) were reported. Two kinds of LiYF4 core/shell UCNPs doped with Yb3+/Ho3+ and Yb3+/Tm3+ were first prepared and could emit the ultraviolet (UV) and visible light under 980 nm NIR excitation respectively. X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis indicated that both kinds of UCNPs had the tetragonal phase and the single-crystal structure. The photoluminescence (PL) intensities were much enhanced through the active-core/active-shell synthetic strategy. Next, the spiropyran molecules could be coordinated onto the surface of the oleate-capped UCNPs via ligand exchange, which was confirmed by fourier transform infrared (FTIR) spectra. PL spectra for the Ho3+-doped SP-UCNPs hybrid nanoparticles revealed that the green emission band excited by near-infrared (NIR) light could be completely quenched after illuminating SP-UCNPs with UV for 150 s. The cyclical photoswitching tests were further carried out for both kinds of UCNPs by alternating irradiation with UV/visible, UV/NIR, or NIR/visible light. The optical switching of PL intensity of the spiropryran-capped UCPNs did not have apparent decline effect after 10 cycles of on/off test. A reversible photoswitching of the hybrid nanoparticles revealed that both kinds of spiropyran-capped UCNPs had a superior photostability for the biological applications.