Enhanced red upconversion luminescence in Fe3+-activated NaYF4: Yb, Er nanocrystals for rapid latent fingerprint recognition

Traditional methods for latent fingerprint (LFP) detection often suffer from insufficient contrast and background interference, limiting their effectiveness in forensic applications, particularly in complex environments. This study introduces an innovative approach that combines Fe³⁺-doped NaYF₄:Yb, Er upconversion nanoparticles (UCNPs) with digital image processing technology. By optimizing the Fe³⁺ doping concentration (15 % mmol) and synthesis temperature (220 °C), the red emission intensity at 655 nm was enhanced by 13.96 times compared to undoped samples. The amplified red emission originates from energy transfer of |²F_7/2, ⁴T_1g⟩ (Yb³⁺-Fe³⁺ dimer) to ⁴F_9/2 (Er³⁺) states, coupled with crystalline field symmetry distortion induced by Fe³⁺ codoping. Dynamic studies of ⁴S_3/2 and ⁴F_9/2 states under 980 nm pulsed laser excitation, alongside diffuse reflectance analysis, corroborate the proposed upconversion mechanism. Additionally, The optimized UCNPs demonstrated high biocompatibility and stability, achieving LFP visualization with contrast ratios up to 85.03 % and structural similarity scores exceeding 91.58 % via MATLAB-based digital image processing, a significant improvement over conventional methods. This work establishes a robust, non-toxic LFP detection system with potential applications in forensic science.