Up conversion luminescence in Er and Tm activated NaYF4 microparticles

A luminescence material with pure red emission under an excitation of 980 nm is suitable for bio-applications due to the high penetration depth in biological tissue and no cell damage. In this work, β-NaYF4:15Er,xTm (x = 0, 0.5, 1, 2) Up-conversion Micro-Particles were synthesized at 180 °C in 24 h using a hydrothermal method. The structural and morphological studies reveal pure hexagonal crystals and a branch structure with a size of several hundred nanometers. Most interestingly, depending on Tm doping concentrations UCMP exhibits an intense red emission with R/G ratio from 30 to 46. The emission mechanism of NaYF4:Er,Tm UCMP under 980 nm excitation was also suggested. The co-doping of Tm3+ ions into β-NaYF4:Er3+ leads to a decrease of green emission (Er3+:2H11/2/4S3/2 → 4I15/2) and enhances the red emission (Er3+:4F9/2 → 4I15/2). Besides, the UCMP modified with polyacrylic acid (PAA) shows a zeta potential as high as −56.6 mV. These findings demonstrate that the β-NaYF4:Er,Tm@PAA UCMP with high signal-to-background ratio are promising candidates for bio-applications.