Enhancing red luminescence by doping Yb3+ into Er3+ self-sensitized Gd2O2S upconverting nanoparticles under excitation at 1530 nm.

Red upconversion luminescence (UCL) nanoparticles are of significant importance for applications in the fields of deep tissue imaging, photothermal therapy and security ink. In this work, a highly efficient red emission was achieved by introducing Yb3+ ions as mediators in Er3+ self-sensitized Gd2O2S nanoparticles under excitation at 1530 nm. The results show that the Gd2O2S:Yb3+,Er3+ nanoparticles synthesized by a homogeneous precipitation method exhibit a uniform spherical shape and narrow size distribution with a mean particle diameter of ≈65 nm. Moreover, the integral emission intensity ratio of red to green of the Gd2O2S:Yb3+,Er3+ sample is significantly enhanced 3-fold compared with the Gd2O2S:Er3+ sample without Yb3+ doping. The enhancement mechanisms are discussed in detail on the basis of steady-state luminescence spectra and decay dynamics measurements under various excitations at 380, 808, 980 and 1530 nm, respectively. It has been demonstrated that the enhanced red luminescence is induced by cross-relaxation energy transfer from Er3+ to Yb3via4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+) and 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+), and further followed by back energy transfer from Yb3+ to Er3+ through 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2 (Er3+) + 2F7/2 (Yb3+). The former cross-relaxation procedure effectively populates the red emission level of 4F9/2 by depopulating the green emission level of 3S3/2. Our findings provide a feasible way to enhance the red UCL and new insights into red UCL mechanisms in the Er3+ self-sensitized system under ≈1500 nm excitation, by combining with the nontoxic oxysulfide host, indicating their potential application as safe fluorescent nanoprobes in the bio-field.