Bifunctional KZnF₃:Er³⁺/Mn²⁺ perovskite nanoparticles for achieving pure red upconversion luminescence and magnetic resonance imaging

Exploiting the optical/magnetic bifunctional upconversion nanoparticles (UCNPs) with small size has become a trend in the high-resolution biological imaging. Here, we report the simultaneous size manipulation and color tuning in Er³⁺-sensitized perovskite KZnF₃ UCNPs via Mn²⁺ doping. It was found that the introduction of Mn²⁺ into the KZnF₃ host matrix facilitates the reduction of the nanoparticle size (from 1 μm to 50 nm). Under the excitation of both 980 and 808 nm continuous wave (CW) laser, these KZnF₃:Er³⁺/Mn²⁺ (1/x mol%) UCNPs can efficiently generate tunable upconversion luminescence (UCL) and achieve pure red UCL as the Mn²⁺ concentration reaches 30 mol%. It is suggested that the energy transfer (ET) processes between Er³⁺ and Mn²⁺ ions contribute to the single-band red UC emission, which was well supported by investigating the dependence of UCL intensity on the pump power and the decay lifetimes of ⁴S_3/2 and ⁴F_9/2 states of Er³⁺ ions. In addition, the KZnF₃:Er³⁺/Mn²⁺ UCNPs have been used as the T₁-weighted magnetic resonance imaging (MRI) agents, which exhibits a high optical contrast with the background. These features make the KZnF₃:Er³⁺/Mn²⁺ UCNPs promising for excellent dual-modal agents in UCL imaging and MRI.