Multifunctional Potassium-Doped L-Histidine Hydrochloride Crystals: Optoelectronic Properties and Cytocompatibility Studies

High-quality potassium-doped L-histidine hydrochloride (K ⁺ :LHHCl) single crystals were successfully grown via slow evaporation technique for dual optoelectronic and biomedical applications. Single-crystal XRD confirmed an orthorhombic system (space group P2₁2₁2₁) with 0.8% lattice expansion compared to undoped crystals, while EDX spectroscopy verified potassium incorporation (0.03 at%). FTIR analysis demonstrated preserved molecular functionality with carboxylate vibrational shifts indicating K⁺ coordination. The crystals exhibited exceptional optical properties with wide transparency (251–1200 nm) and a direct band gap of 4.90 eV. Remarkably, the material showed 2.4× higher second harmonic generation efficiency than KDP, attributed to its noncentrosymmetric structure. Thermal analysis revealed stability up to 158.7°C, while MTT assays demonstrated dose-dependent cytotoxicity against MCF-7 breast cancer cells (IC₅₀ = 35.96 µg/mL, 66.21% inhibition at 100 µg/mL). These results position K+:LHHCl as a novel multifunctional material combining nonlinear optical performance for frequency conversion devices with potential anticancer therapeutic applications.