Magnetic and optical properties of NaGdF₄:Nd³⁺, Yb³⁺, Tm³⁺ nanocrystals with upconversion/downconversion luminescence from visible to the near-infrared second window

We have designed and synthesized NaGdF₄:Nd³⁺, Yb³⁺, Tm³⁺ magnetic nanophosphors with combined dual-mode downconversion (DC) and upconversion (UC) photoluminescence upon 800 nm excitation. Hexagonal-phase NaGdF₄:Nd³⁺, Yb³⁺, Tm³⁺ nanocrystals (NCs) with an average size of 21 nm were synthesized using a solvothermal approach. Nd³⁺, Yb³⁺, Tm³⁺ triple-doped NaGdF₄ NCs exhibit a broad range of photoluminescence peaks covering a near infrared first/second window (860–900, 1,000, and 1,060 nm), and visible emission including blue (475 nm), green (520 and 542 nm) and yellow (587 nm) after excitation at 800 nm. A mechanism involving circulation of energy over Gd³⁺ sublattices as bridge ions and final trapping by the initial activator ions (Nd³⁺) has been proposed. Penetration depth studies indicate that NIR emission is easily detected even at a large tissue thickness of 10 mm. These paramagnetic nanophosphors demonstrate a large magnetization value of 1.88 emu/g at 20 kOe and longitudinal relaxivity value of 1.2537 mM⁻¹·S⁻¹ as a T₁-weighted magnetic resonance imaging contrast agent. These NaGdF₄:Nd³⁺, Yb³⁺, Tm³⁺ NCs are promising for applications in biological and magnetic resonance imaging.