Yb^<b>3+</b>-optimized core–shell structured luminescent material for dual-mode encryption and deep learning fluorescence imaging

NaYF₄:Yb³⁺, Nd³⁺Ln³⁺ (Er³⁺, Tm³⁺)@NaYF₄:Yb³⁺ core-shell upconversion luminescent (UCL) materials were prepared by combining the hydrothermal and epitaxial growth methods. Under the excitation of 980 nm laser, the prepared NaYF₄:Yb³⁺, Nd³⁺, Er³⁺@NaYF₄:Yb³⁺ UCL materials can emit green light, and NaYF₄:Yb³⁺, Nd³⁺, Tm³⁺@NaYF₄:Yb³⁺ UCL materials can emit blue light, respectively. Under the optimized Yb³⁺ doping concentration, the luminescence intensity was enhanced by 16.54 and 17.56 times compared to the nucleus, respectively. Anti-counterfeiting ink was made using the resulting UCL materials as its source ingredient. First, through precise screen-printing techniques, we fabricated QR code patterns that demonstrated the unique luminescent characteristics of the core-shell UCL materials. Also, we implemented a Morse code-based dot-line combination pattern to demonstrate the practical application. Multi-level security features integrating structural (Morse code patterns) and information (QR codes) encryption were developed, significantly enhancing information protection reliability. Furthermore, a snowflake-patterned categorization recognition model was constructed through a residual neural network developed on PyCharm platform. This recognition system achieved 96.55% classification accuracy, demonstrating substantial technical advantages for advanced anti-counterfeiting authentication applications.