Enhancing red luminescence by doping Yb into Er self-sensitized GdOS 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 Yb ions as mediators in Er self-sensitized GdOS nanoparticles under excitation at 1530 nm. The results show that the GdOS:Yb,Er 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 GdOS:Yb,Er sample is significantly enhanced 3-fold compared with the GdOS:Er sample without Yb 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 Er to YbS (Er) + F (Yb) → I (Er) + F (Yb) and I (Er) + F (Yb) → I (Er) + F (Yb), and further followed by back energy transfer from Yb to Er through I (Er) + F (Yb) → F (Er) + F (Yb). The former cross-relaxation procedure effectively populates the red emission level of F by depopulating the green emission level of S. Our findings provide a feasible way to enhance the red UCL and new insights into red UCL mechanisms in the Er 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.