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

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Poongodi, P. Dhanasekaran, Ravikumar Nattudurai, R. Deepika This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7901313/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Feb, 2026 Read the published version in Optical and Quantum Electronics → Version 1 posted 13 You are reading this latest preprint version Abstract 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. Organic nonlinear optical crystals Optoelectronic materials Single crystal growth Second-harmonic generation (SHG) X-ray diffraction MTT Assay. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. INTRODUCTION Nonlinear optical (NLO) materials are indispensable for modern optoelectronic and photonic technologies, enabling applications such as frequency conversion, second-harmonic generation (SHG), optical signal processing, and high-density data storage [ 1 – 4 ]. Among these, organic and amino acid-based crystals have gained prominence due to their enhanced optical nonlinearity, high laser damage threshold, and tunable molecular structures, making them ideal candidates for next-generation photonic devices [ 5 , 6 ]. Recently, organometallic NLO materials have emerged as a promising class of compounds, combining the high hyperpolarizability of organic systems with the mechanical and thermal stability of inorganic frameworks [ 7 ]. This synergy arises from metal–ligand coordination bonds, which facilitate efficient intermolecular charge transfer (ICT) from the metal center to conjugated organic ligands, significantly boosting nonlinear optical responses [ 8 , 9 ]. For practical NLO applications, crystals must exhibit high molecular hyperpolarizability, low optical losses at harmonic wavelengths, thermal robustness, and noncentrosymmetric packing—properties often observed in organometallic complexes [ 10 ]. These materials demonstrate asymmetric molecular alignment in crystalline states, along with exceptional thermal stability, attributed to their unique metal–carbon covalent interactions. Notably, alkali, alkaline earth, and transition metal-based organometallics have shown particular promise due to their tailorable electronic structures and strong NLO activity [ 10 ]. Beyond optoelectronics, amino acid-derived materials are gaining traction in biomedical research, particularly as anticancer agents, owing to their biocompatibility and targeted therapeutic potential [ 11 – 13 ]. Conventional chemotherapy drugs often suffer from systemic toxicity and resistance, driving the search for novel, less toxic alternatives [ 11 ]. In this context, metal-doped amino acid crystals present a unique opportunity, merging NLO functionality for optoelectronic devices with cytotoxic activity for cancer therapy. In this study, we report the growth, characterization, and dual-functional applications of potassium-doped L-histidine hydrochloride single crystals. These crystals exhibit exceptional SHG efficiency (2.4× KDP), robust thermal stability, and dose-dependent cytotoxicity against MCF-7 breast cancer cells (IC₅₀ = 35.96 µg/mL). Our findings highlight their potential as multifunctional materials for both optoelectronic devices and anticancer therapeutics, bridging a critical gap between materials science and biomedicine. 2. MATERIALS AND METHODS 2.1. Crystal growth This study reports the successful growth of potassium ion-doped L-histidine hydrochloride (K + :LHHCl) single crystals using the solvent evaporation technique. The synthesis employed high-purity raw materials: L-histidine (Alfa Aesar, 99.999%) and hydrochloric acid (Merck, 99.99%) in a 1:1 equimolar ratio. The chemical reaction for L-histidine hydrochloride formation is shown below: $$\:{C}_{6}{H}_{9}{N}_{3}{O}_{2}\:+\:HCl\to\:{C}_{6}{H}_{10}{N}_{3}{O}_{2}Cl$$ The solution was thoroughly agitated, followed by the addition of 0.1 mol% potassium chloride as a dopant. The purity of the crystals was further increased through a series of repeated recrystallization processes. A homogeneous mixture of saturated K + :LHHCl solution was prepared at 30°C and agitated for approximately 24 hours using a magnetic stirrer set at a constant 150 rpm. The saturated solution was then filtered through Whatman filter paper to eliminate any suspended impurities. The solution was subsequently placed in a crystallizing vessel covered with a perforated polyethylene sheet and kept undisturbed in a crystallizing chamber. After a period of 20 days, a high-quality, transparent, and colourless single crystal of K + :LHHCl was obtained, as depicted in Fig. 1. Figure 1. Grown potassium (I) ion-doped l -histidine hydrochloride (K + : LHHCl) single crystals. 2.2. Second Harmonic Generation The nonlinear optical properties of potassium-doped L-histidine hydrochloride (K + :LHHCl) crystals were characterized using the modified Kurtz-Perry powder technique. A Q-switched Nd:YAG laser system (1064 nm wavelength, 1.2 mJ/pulse energy, 10 ns pulse width, 10 Hz repetition rate) served as the fundamental beam source. The detection system comprised a photomultiplier tube (Hamamatsu R928) with a 532 nm interference filter and a digital oscilloscope (Tektronix TDS3054C) for signal acquisition. Powdered samples (75–150 µm particle size) of both K + :LHHCl and reference KDP crystals were packed in 1 mm thick glass cells and aligned using a precision goniometer to ensure consistent measurement conditions. All experiments were conducted at room temperature (25 ± 1°C) with controlled humidity (45 ± 5% RH). 2.3. Cytotoxicity Assessment Using MCF-7 Cell Line The MCF-7 breast cancer cell line was procured from the National Centre for Cell Science (NCCS), Pune, and cultured in Eagle’s minimum essential medium supplemented with 10% fetal bovine serum (FBS) [ 14 , 15 ]. The cells were maintained under standard conditions of 37°C, 5% CO 2 , 95% air, and 100% relative humidity. The maintenance cultures were passaged on a weekly basis, and the culture medium was refreshed twice per week. The monolayer cells were detached from the culture surface using trypsin-ethylene diamine tetra acetic acid (EDTA) to obtain single-cell suspensions. The viable cells were counted via a hemocytometer and diluted with medium containing 5% fetal bovine serum (FBS) to achieve a final density of 1x10 5 cells/ml. Subsequently, 100 µl of the cell suspension was seeded into each well of a 96-well plate at a plating density of 10,000 cells/well and incubated at 37°C, 5% CO 2 , 95% air, and 100% relative humidity to allow for cell attachment. After 24 hours, the cells were treated with serial dilutions of the test samples, which were initially dissolved in neat dimethyl sulfoxide (DMSO). An aliquot of the sample solution was diluted to twice the desired final maximum test concentration with serum-free medium, and four additional serial dilutions were made to provide a total of five sample concentrations. Aliquots of 100 µl of these different sample dilutions were added to the appropriate wells containing 100 µl of medium, resulting in the required final sample concentrations. Following sample addition, the plates were incubated for an additional 48 hours at 37°C, 5% CO 2 , 95% air, and 100% relative humidity. The medium without samples served as the control, and triplicate samples were maintained for all concentrations. 2.4. Cell Viability Assessment by MTT Assay The cytotoxic effects of K⁺:LHHCl crystals were quantified using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay, which measures mitochondrial reductase activity in viable cells. This method relies on the enzymatic conversion of the yellow, water-soluble MTT tetrazolium salt to insoluble purple formazan crystals by NAD(P)H-dependent oxidoreductases in metabolically active cells. Following the 48-hour treatment period, 15 µL of MTT solution (5 mg/mL in sterile PBS, pH 7.4) was added to each well, achieving a final concentration of 0.5 mg/mL. The plates were then incubated for 4 hours at 37°C in 5% CO₂ to allow complete formazan formation. After careful removal of the supernatant, the intracellular formazan crystals were solubilized with 100 µL of analytical-grade DMSO per well, with gentle agitation for 10 minutes to ensure complete dissolution. The optical density was measured at 570 nm (reference wavelength: 630 nm) using a microplate reader (BioTek Synergy H1), with blank correction using DMSO-only wells.[ 16 ]. Cell viability was calculated as: $$\:\%\:Cell\:Inhibition\:=\:100-\:Abs\:({K}^{+}:\:LHHCl\:-\:sample)/Abs\:\left(control\right)\:\times\:100.\:$$ where control wells contained untreated cells with 0.1% DMSO vehicle. The percentage growth inhibition was derived by subtracting the viability percentage from 100%. All experiments were performed in triplicate with three independent biological replicates to ensure statistical reliability. 3. RESULTS AND DISCUSSION 3.1. Structural Characterization by Single-Crystal X-ray Diffraction Single-crystal X-ray diffraction analysis confirmed that potassium-doped L-histidine hydrochloride (K + :LHHCl) crystallizes in the noncentrosymmetric orthorhombic space group P2₁2₁2₁ - a prerequisite for second-order nonlinear optical activity. The refined unit cell parameters (Table 1 ) reveal a slight but significant 0.8% volume expansion (V = 943.78(8) ų) compared to undoped L-histidine hydrochlorid [ 17 ], with axes a = 6.8902(3) Å, b = 8.9431(4) Å, and c = 15.3162(8) Å (α = β = γ = 90°). This systematic lattice expansion, while maintaining the parent crystal structure, confirms successful potassium incorporation without phase segregation. The preserved noncentrosymmetric packing arrangement explains the observed SHG efficiency (2.4× KDP), as the P2₁2₁2₁ symmetry allows for optimal alignment of molecular dipoles. The precision of the structural determination is evidenced by the exceptionally low estimated standard deviations (< 0.0008 Å in unit cell parameters), attesting to the high crystalline quality essential for nonlinear optical applications. Table 1 Crystallographic parameters of K + :LHHCl Crystal K + : LHHCl Crystal system Orthorhombic Space group P2 1 2 1 2 1 Unit cell dimensions a = 6.8902(3) Å α = 90° b = 8.9431(4) Å β = 90° c = 15.3162(8) Å γ = 90° Volume 943.78(8) Å 3 3.2. Molecular Vibrational Analysis by FTIR Spectroscopy Fourier-transform infrared spectroscopy confirmed the molecular integrity and functional group composition of potassium-doped L-histidine hydrochloride crystals, as shown in Fig. 2 . The spectrum exhibits characteristic vibrational modes of all fundamental molecular constituents: a broad NH₂ asymmetric stretching band at 3404 cm⁻¹ and ring NH stretching at 3074 cm⁻¹ demonstrate preserved amine functionality, while the carboxylate group shows distinct asymmetric (1610 cm⁻¹) and symmetric (1575 cm⁻¹) CO₂ stretching vibrations. The imidazole ring vibrations appear at 1629 cm⁻¹ (C = C) and 1451 cm⁻¹ (C = N), confirming the aromatic system's structural integrity. Additional fingerprint region absorptions include CH stretching (2993 cm⁻¹), CCH deformation (1296 cm⁻¹), C-C stretching (960 cm⁻¹), and NH deformation (648 cm⁻¹). Notably, the 15–20 cm⁻¹ shifts observed in carboxylate vibrations compared to undoped L-histidine HCl [ 18 , 19 ] suggest potassium ion coordination preferentially affects the carboxyl group's electron density distribution. The absence of unexpected peaks and the sharpness of all vibrational bands (FWHM < 25 cm⁻¹ for major peaks) verify the chemical purity and crystalline perfection of the grown material, consistent with our XRD and EDX results. Table 2 FTIR vibrational assignments of K + :LHHCl Wavenumber (cm⁻¹) Assignment Molecular Origin 3404 NH₂ asymmetric stretch Amino group 3074 Ring NH stretch Imidazole moiety 2993 CH stretch Aliphatic chain 1610 CO₂ asymmetric stretch Carboxylate 1575 CO₂ symmetric stretch Carboxylate 1629 C = C stretch Imidazole ring 1451 C = N stretch Imidazole ring 1296 CCH deformation Side chain 960 C-C stretch Backbone 648 NH deformation Amino group 3.3. Optical Characterization by UV-Vis-IR Spectroscopy The optical transmission properties of potassium-doped L-histidine hydrochloride (K + :LHHCl) single crystals were investigated through ultraviolet-visible-near IR spectroscopy, as shown in Fig. 3 (a). The crystal demonstrated excellent transparency across the visible spectrum (400–800 nm) with a sharp UV cutoff at 251 nm, indicating minimal defects and high crystalline perfection. This low absorption edge suggests strong potential for nonlinear optical applications in the visible and near-UV regions. The observed absorption characteristics arise from electronic transitions between the valence and conduction bands, with the distinct cutoff corresponding to the material's fundamental absorption edge [ 20 ]. The optical band gap was determined using Tauc's plot method (Fig. 3 b), which revealed a direct band gap of 4.90 eV - significantly wider than many organic NLO materials. This large band gap correlates with the crystal's high transmittance and explains its exceptional optical clarity. The steep absorption edge indicates minimal sub-bandgap states, confirming the high purity of the grown crystals. These optical characteristics, combined with thermal stability and SHG efficiency ( discussed in section 3.5 and 3.6 ), position K + :LHHCl as an outstanding candidate for frequency conversion devices operating in the visible spectrum. The wide band gap particularly suggests utility in applications requiring high laser damage thresholds and minimal two-photon absorption at common NLO operating wavelengths [ 21 ]. 3.4. Elemental Composition Analysis by Energy Dispersive X-ray Spectroscopy Energy dispersive X-ray spectroscopy (EDX) analysis confirmed the successful incorporation of potassium dopants in the L-histidine hydrochloride crystals while maintaining excellent chemical purity, as shown in Fig. 4 . The EDX spectrum exhibited characteristic emission peaks for all expected elements: carbon (C Kα at 0.28 keV), nitrogen (N Kα at 0.39 keV), oxygen (O Kα at 0.53 keV), chlorine (Cl Kα at 2.62 keV), and potassium (K Kα at 3.31 keV). Quantitative elemental analysis revealed the following composition: Table 3 Elemental composition of K + :LHHCl single crystals Element Weight (%) Atomic (%) C 18.93 24.45 N 6.08 6.73 O 67.84 65.78 Cl 6.90 3.01 K 0.25 0.03 The measured potassium concentration of 0.25 wt% (0.03 at%) demonstrates effective doping while preserving the host matrix stoichiometry. The oxygen dominance (65.78 at%) agrees with the carboxyl groups in the histidine structure, while the carbon-to-nitrogen ratio (24.45:6.73 at%) closely matches the theoretical 3:1 ratio expected for L-histidine. The absence of extraneous peaks in the spectrum confirms the high phase purity of the crystals. These results verify that the doping process successfully introduced potassium ions into the crystal lattice without compromising the material's chemical integrity, supporting the enhanced thermal and optical properties observed in other characterization studies. The controlled incorporation of dopant atoms at this concentration level (0.03 at%) explains the improved performance while maintaining the crystalline quality essential for optoelectronic applications. 3.5. Thermal analysis (TG-DTA) The thermal stability of potassium-doped L-histidine hydrochloride crystals was systematically investigated through simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Measurements were performed on a 4.417 mg single-crystal sample under nitrogen atmosphere (flow rate: 50 mL/min) across a temperature range of 28–600°C with a controlled heating rate of 10°C/min (Fig. 5 ). The TG curve revealed a three-stage decomposition pattern: an initial 8.6% mass loss at 158.7°C (endothermic peak) confirming the anhydrous nature of the crystals, followed by major decomposition at 267.1°C (endothermic) corresponding to the breakdown of the molecular framework. A final exothermic event at 522.7°C with 25.2% mass loss suggested complete oxidative decomposition. The sharp, well-defined thermal transitions observed in both TG and DTA curves attest to the high crystallinity and phase purity of the material, as further supported by our XRD results. Notably, the decomposition onset temperature (158.7°C) represents a significant thermal stability improvement over pure L-histidine hydrochloride (reported decomposition at ~ 145°C [ 22 , 23 ]). This enhanced stability originates from the potassium doping-induced strengthening of intermolecular interactions within the crystal lattice. The absence of solvent-related mass loss below 150°C confirms the successful growth of solvent-free crystals, while the distinct separation between melting and decomposition events (ΔT > 100°C) suggests potential for melt-processing applications. These thermal properties, combined with the previously demonstrated nonlinear optical performance, position K + :LHHCl as a promising candidate for optoelectronic devices requiring operational stability under moderate thermal loads, such as frequency doublers in laser systems or optical modulators. 3.6. Second Harmonic Generation Study of K + : LHHCl Crystals The second harmonic generation (SHG) measurements demonstrated compelling nonlinear optical behavior in the K + :LHHCl crystals. The doped crystals exhibited an SHG efficiency 2.4 times greater than that of standard KDP, though slightly lower than the 3× KDP efficiency observed in undoped L-histidine hydrochloride. This reduction in SHG efficiency compared to the pure crystal suggests that potassium incorporation modifies the molecular dipole alignment while maintaining the noncentrosymmetric structure essential for nonlinear activity. The orthorhombic P2₁2₁2₁ space group, confirmed by single-crystal XRD, provides the necessary asymmetric environment for efficient SHG. Remarkably, the crystals showed excellent stability under laser irradiation, maintaining consistent SHG output over 1000 pulses at 38 MW/cm² intensity with no observable damage. This robust performance, combined with the previously determined thermal stability (up to 158.4°C) and wide optical band gap (4.90 eV), positions K + :LHHCl as a promising candidate for frequency conversion applications in laser systems and integrated photonic devices. The metal-organic charge transfer interactions induced by potassium doping appear to enhance the hyperpolarizability while preserving the crystalline integrity, offering new possibilities for engineering amino acid-based nonlinear optical materials with tailored properties [ 24 ]. 3.7. Dose-Dependent Cytotoxicity in MCF-7 Breast Cancer Cells The anticancer potential of potassium-doped L-histidine hydrochloride (K + :LHHCl) crystals was quantitatively evaluated against MCF-7 breast cancer cells using the MTT assay. Cells were exposed to five concentrations (6.5, 12.5, 25, 50, and 100 µg/mL) for 24 hours at 37°C under 5% CO₂ atmosphere, with viability assessed through formazan quantification. As shown in Fig. 6 , the crystals exhibited a clear dose-response relationship, with cell viability decreasing progressively from 84.97% (6.5 µg/mL) to 33.79% (100 µg/mL), corresponding to growth inhibition values of 15.03%, 30.66%, 44.33%, 55.85%, and 66.21% respectively. Nonlinear regression analysis (Fig. 7 ) yielded an IC₅₀ value of 35.96 µg/mL (95% CI: 32.14–40.22 µg/mL), indicating potent cytotoxic activity comparable to some clinically used chemotherapeutic agents. The observed dose-dependent response suggests that K + :LHHCl crystals interact with specific molecular targets in cancer cells, with efficacy directly proportional to drug availability. Notably, the maximum inhibition (66.21% at 100 µg/mL) was achieved without complete media discoloration, indicating preserved mitochondrial function in remaining viable cells and suggesting selective toxicity rather than generalized cytotoxicity. These results position K + :LHHCl as a promising candidate for further development as an anticancer therapeutic, particularly given its dual functionality as both a cytotoxic agent and nonlinear optical material. 4. CONCLUSION High-quality potassium ion-doped L-histidine hydrochloride (K + :LHHCl) single crystals were successfully grown using the slow evaporation technique at ambient conditions. Single-crystal XRD analysis confirmed the orthorhombic structure (space group P2₁2₁2₁), while FTIR spectroscopy verified the functional group integrity. The material demonstrated excellent optical characteristics, including a wide transmission window and a direct band gap of 4.90 eV, suggesting potential for UV photonic applications. EDX spectroscopy confirmed the successful incorporation of potassium ions into the crystal lattice. Thermal analysis revealed remarkable stability up to 158.4°C, surpassing many organic NLO materials. Most significantly, the crystal exhibited a powder SHG efficiency of 2.4× KDP, making it particularly promising for frequency-doubling applications in laser systems. Beyond its optoelectronic merits, the K + :LHHCl crystal showed pronounced dose-dependent anticancer activity against MCF-7 breast cancer cells, achieving 66.21% growth inhibition at 100 µg/mL with an IC₅₀ of 35.96 µg/mL. This dual functionality – combining exceptional NLO properties with significant cytotoxicity – positions K + :LHHCl as a groundbreaking multifunctional material. The demonstrated properties suggest simultaneous applications in: Nonlinear optical devices (frequency converters, optical modulators), Biomedical platforms (targeted cancer therapy with optical monitoring capability) These findings open new avenues for developing amino acid-based hybrid materials that bridge photonic and biomedical applications, potentially enabling novel optobioelectronic devices. Future work will focus on optimizing crystal properties for specific device integration and investigating the mechanistic pathways of its anticancer activity. Declarations AUTHORS CONTRIBUTIONS N. Poongodi: Investigation, Methodology, Formal analysis, Writing - Original Draft P. Dhanasekaran: Conceptualization, Supervision, Resources, Writing - Review & Editing Ravikumar Nattudurai: Conceptualization, Visualization, Writing - Review & Editing R. Deepika: Data curation, Formal analysis, Validation Conflict of interest The authors declare that they have no conflicts of interest. ACKNOWLEDGMENT The authors gratefully acknowledge the Department of Physics, Bharathiar University, Coimbatore, for providing access to single-crystal X-ray diffraction facilities. We are particularly grateful to Dr. P.K. Das of the Indian Institute of Science (IISc), Bangalore, for performing the second harmonic generation measurements and for his valuable technical insights. N. Poongodi additionally acknowledges Erode Sengunthar College of Engineering for institutional support. DATA AVAILABILITY STATEMENT The manuscript has no associated data. References Medishetty, R., Zaręba, J.K., Mayer, D., Samoć, M., RA Fischer: Chem. Soc. Rev. 46 , 4976 (2017) Ray, P.C.: Chemical reviews 110: 5332. (2010) Suresh, S., Ramanand, A., Jayaraman, D., Mani, P.: Rev. Adv. Mater. Sci. 30 , 175 (2012) Wu, J., Luo, J., Jen, A.K.Y.: J. Mater. Chem. C. 8 , 15009 (2020) Jayaprakash, P., Mohamed, M.P., Krishnan, P., Nageshwari, M., Mani, G., Caroline, M.L.: Phys. B: Condens. 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Cite Share Download PDF Status: Published Journal Publication published 19 Feb, 2026 Read the published version in Optical and Quantum Electronics → Version 1 posted Editorial decision: Revision requested 30 Dec, 2025 Reviews received at journal 30 Dec, 2025 Reviews received at journal 28 Dec, 2025 Reviewers agreed at journal 17 Dec, 2025 Reviewers agreed at journal 08 Dec, 2025 Reviews received at journal 18 Nov, 2025 Reviews received at journal 15 Nov, 2025 Reviewers agreed at journal 27 Oct, 2025 Reviewers agreed at journal 25 Oct, 2025 Reviewers invited by journal 22 Oct, 2025 Editor assigned by journal 21 Oct, 2025 Submission checks completed at journal 21 Oct, 2025 First submitted to journal 19 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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crystal.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/d35277615a86c8799d3f137b.png"},{"id":95030590,"identity":"9fa74f4e-afbf-4597-b0a8-5a84b34b3746","added_by":"auto","created_at":"2025-11-03 14:14:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":347953,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) UV‒Vis optical absorption, (b) (αhν)\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e vs hν (Tauc plot) of K\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e-doped LHHCl single crystals.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/0a938841c1308b6331ee3f25.png"},{"id":95221440,"identity":"b5b2d8fd-64d2-4139-a4b5-71b36afb1eda","added_by":"auto","created_at":"2025-11-05 16:19:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":51778,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eEDX spectrum of a grown K\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e:LHHCl crystal.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/673620776567f2422e11d97f.png"},{"id":95221744,"identity":"b48660ac-4fa2-43f1-abf8-88f30c1d003a","added_by":"auto","created_at":"2025-11-05 16:19:39","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":12781,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eTG-DTA analysis of grown K\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e:LHHCl crystals.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/e36ee7865147ceca33ed8953.png"},{"id":95030593,"identity":"13d487e9-afa4-4a7b-80ba-47e236b62577","added_by":"auto","created_at":"2025-11-03 14:14:36","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":716979,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMorphological changes in cultured breast cancer cells treated with different concentrations of K\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e:LHHCl crystals.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/f69332839280150a671fb462.png"},{"id":95030594,"identity":"7b8b13d9-74b2-4849-b6d4-e302b9f51867","added_by":"auto","created_at":"2025-11-03 14:14:36","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":19980,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e% cell inhibition vs. the concentration of potassium ion-doped LHHCl crystals.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/978536513c894b2a70c4463b.png"},{"id":103251185,"identity":"ae13277a-3fc9-459b-8b00-ab4c4bc617bb","added_by":"auto","created_at":"2026-02-23 16:05:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2171675,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7901313/v1/33ae7e5c-d696-41ce-aa41-77aed2176bb4.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Multifunctional Potassium-Doped L-Histidine Hydrochloride Crystals: Optoelectronic Properties and Cytocompatibility Studies","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eNonlinear optical (NLO) materials are indispensable for modern optoelectronic and photonic technologies, enabling applications such as frequency conversion, second-harmonic generation (SHG), optical signal processing, and high-density data storage [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Among these, organic and amino acid-based crystals have gained prominence due to their enhanced optical nonlinearity, high laser damage threshold, and tunable molecular structures, making them ideal candidates for next-generation photonic devices [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Recently, organometallic NLO materials have emerged as a promising class of compounds, combining the high hyperpolarizability of organic systems with the mechanical and thermal stability of inorganic frameworks [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. This synergy arises from metal\u0026ndash;ligand coordination bonds, which facilitate efficient intermolecular charge transfer (ICT) from the metal center to conjugated organic ligands, significantly boosting nonlinear optical responses [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFor practical NLO applications, crystals must exhibit high molecular hyperpolarizability, low optical losses at harmonic wavelengths, thermal robustness, and noncentrosymmetric packing\u0026mdash;properties often observed in organometallic complexes [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. These materials demonstrate asymmetric molecular alignment in crystalline states, along with exceptional thermal stability, attributed to their unique metal\u0026ndash;carbon covalent interactions. Notably, alkali, alkaline earth, and transition metal-based organometallics have shown particular promise due to their tailorable electronic structures and strong NLO activity [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eBeyond optoelectronics, amino acid-derived materials are gaining traction in biomedical research, particularly as anticancer agents, owing to their biocompatibility and targeted therapeutic potential [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Conventional chemotherapy drugs often suffer from systemic toxicity and resistance, driving the search for novel, less toxic alternatives [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In this context, metal-doped amino acid crystals present a unique opportunity, merging NLO functionality for optoelectronic devices with cytotoxic activity for cancer therapy.\u003c/p\u003e\u003cp\u003eIn this study, we report the growth, characterization, and dual-functional applications of potassium-doped L-histidine hydrochloride single crystals. These crystals exhibit exceptional SHG efficiency (2.4\u0026times; KDP), robust thermal stability, and dose-dependent cytotoxicity against MCF-7 breast cancer cells (IC₅₀ = 35.96 \u0026micro;g/mL). Our findings highlight their potential as multifunctional materials for both optoelectronic devices and anticancer therapeutics, bridging a critical gap between materials science and biomedicine.\u003c/p\u003e"},{"header":"2. MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Crystal growth\u003c/h2\u003e\u003cp\u003eThis study reports the successful growth of potassium ion-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) single crystals using the solvent evaporation technique. The synthesis employed high-purity raw materials: L-histidine (Alfa Aesar, 99.999%) and hydrochloric acid (Merck, 99.99%) in a 1:1 equimolar ratio. The chemical reaction for L-histidine hydrochloride formation is shown below:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{C}_{6}{H}_{9}{N}_{3}{O}_{2}\\:+\\:HCl\\to\\:{C}_{6}{H}_{10}{N}_{3}{O}_{2}Cl$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe solution was thoroughly agitated, followed by the addition of 0.1 mol% potassium chloride as a dopant. The purity of the crystals was further increased through a series of repeated recrystallization processes. A homogeneous mixture of saturated K\u003csup\u003e+\u003c/sup\u003e:LHHCl solution was prepared at 30\u0026deg;C and agitated for approximately 24 hours using a magnetic stirrer set at a constant 150 rpm. The saturated solution was then filtered through Whatman filter paper to eliminate any suspended impurities. The solution was subsequently placed in a crystallizing vessel covered with a perforated polyethylene sheet and kept undisturbed in a crystallizing chamber. After a period of 20 days, a high-quality, transparent, and colourless single crystal of K\u003csup\u003e+\u003c/sup\u003e:LHHCl was obtained, as depicted in Fig.\u0026nbsp;1.\u003c/p\u003e\u003cp\u003e\u003cem\u003eFigure 1. Grown potassium (I) ion-doped\u003c/em\u003e \u003cspan type=\"ItalicSmallCaps\" class=\"ItalicSmallCaps\" name=\"Emphasis\"\u003el\u003c/span\u003e\u003cem\u003e-histidine hydrochloride (K\u003c/em\u003e\u003csup\u003e\u003cem\u003e+\u003c/em\u003e\u003c/sup\u003e:\u003cem\u003eLHHCl) single crystals.\u003c/em\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Second Harmonic Generation\u003c/h2\u003e\u003cp\u003eThe nonlinear optical properties of potassium-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) crystals were characterized using the modified Kurtz-Perry powder technique. A Q-switched Nd:YAG laser system (1064 nm wavelength, 1.2 mJ/pulse energy, 10 ns pulse width, 10 Hz repetition rate) served as the fundamental beam source. The detection system comprised a photomultiplier tube (Hamamatsu R928) with a 532 nm interference filter and a digital oscilloscope (Tektronix TDS3054C) for signal acquisition. Powdered samples (75\u0026ndash;150 \u0026micro;m particle size) of both K\u003csup\u003e+\u003c/sup\u003e:LHHCl and reference KDP crystals were packed in 1 mm thick glass cells and aligned using a precision goniometer to ensure consistent measurement conditions. All experiments were conducted at room temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C) with controlled humidity (45\u0026thinsp;\u0026plusmn;\u0026thinsp;5% RH).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Cytotoxicity Assessment Using MCF-7 Cell Line\u003c/h2\u003e\u003cp\u003eThe MCF-7 breast cancer cell line was procured from the National Centre for Cell Science (NCCS), Pune, and cultured in Eagle\u0026rsquo;s minimum essential medium supplemented with 10% fetal bovine serum (FBS) [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The cells were maintained under standard conditions of 37\u0026deg;C, 5% CO\u003csub\u003e2\u003c/sub\u003e, 95% air, and 100% relative humidity. The maintenance cultures were passaged on a weekly basis, and the culture medium was refreshed twice per week.\u003c/p\u003e\u003cp\u003eThe monolayer cells were detached from the culture surface using trypsin-ethylene diamine tetra acetic acid (EDTA) to obtain single-cell suspensions. The viable cells were counted via a hemocytometer and diluted with medium containing 5% fetal bovine serum (FBS) to achieve a final density of 1x10\u003csup\u003e5\u003c/sup\u003e cells/ml. Subsequently, 100 \u0026micro;l of the cell suspension was seeded into each well of a 96-well plate at a plating density of 10,000 cells/well and incubated at 37\u0026deg;C, 5% CO\u003csub\u003e2\u003c/sub\u003e, 95% air, and 100% relative humidity to allow for cell attachment. After 24 hours, the cells were treated with serial dilutions of the test samples, which were initially dissolved in neat dimethyl sulfoxide (DMSO). An aliquot of the sample solution was diluted to twice the desired final maximum test concentration with serum-free medium, and four additional serial dilutions were made to provide a total of five sample concentrations. Aliquots of 100 \u0026micro;l of these different sample dilutions were added to the appropriate wells containing 100 \u0026micro;l of medium, resulting in the required final sample concentrations. Following sample addition, the plates were incubated for an additional 48 hours at 37\u0026deg;C, 5% CO\u003csub\u003e2\u003c/sub\u003e, 95% air, and 100% relative humidity. The medium without samples served as the control, and triplicate samples were maintained for all concentrations.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Cell Viability Assessment by MTT Assay\u003c/h2\u003e\u003cp\u003eThe cytotoxic effects of K⁺:LHHCl crystals were quantified using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay, which measures mitochondrial reductase activity in viable cells. This method relies on the enzymatic conversion of the yellow, water-soluble MTT tetrazolium salt to insoluble purple formazan crystals by NAD(P)H-dependent oxidoreductases in metabolically active cells. Following the 48-hour treatment period, 15 \u0026micro;L of MTT solution (5 mg/mL in sterile PBS, pH 7.4) was added to each well, achieving a final concentration of 0.5 mg/mL. The plates were then incubated for 4 hours at 37\u0026deg;C in 5% CO₂ to allow complete formazan formation. After careful removal of the supernatant, the intracellular formazan crystals were solubilized with 100 \u0026micro;L of analytical-grade DMSO per well, with gentle agitation for 10 minutes to ensure complete dissolution. The optical density was measured at 570 nm (reference wavelength: 630 nm) using a microplate reader (BioTek Synergy H1), with blank correction using DMSO-only wells.[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Cell viability was calculated as:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\%\\:Cell\\:Inhibition\\:=\\:100-\\:Abs\\:({K}^{+}:\\:LHHCl\\:-\\:sample)/Abs\\:\\left(control\\right)\\:\\times\\:100.\\:$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere control wells contained untreated cells with 0.1% DMSO vehicle. The percentage growth inhibition was derived by subtracting the viability percentage from 100%. All experiments were performed in triplicate with three independent biological replicates to ensure statistical reliability.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Structural Characterization by Single-Crystal X-ray Diffraction\u003c/h2\u003e\u003cp\u003eSingle-crystal X-ray diffraction analysis confirmed that potassium-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) crystallizes in the noncentrosymmetric orthorhombic space group P2₁2₁2₁ - a prerequisite for second-order nonlinear optical activity. The refined unit cell parameters (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) reveal a slight but significant 0.8% volume expansion (V\u0026thinsp;=\u0026thinsp;943.78(8) \u0026Aring;\u0026sup3;) compared to undoped L-histidine hydrochlorid [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], with axes a\u0026thinsp;=\u0026thinsp;6.8902(3) \u0026Aring;, b\u0026thinsp;=\u0026thinsp;8.9431(4) \u0026Aring;, and c\u0026thinsp;=\u0026thinsp;15.3162(8) \u0026Aring; (α\u0026thinsp;=\u0026thinsp;β\u0026thinsp;=\u0026thinsp;γ\u0026thinsp;=\u0026thinsp;90\u0026deg;). This systematic lattice expansion, while maintaining the parent crystal structure, confirms successful potassium incorporation without phase segregation. The preserved noncentrosymmetric packing arrangement explains the observed SHG efficiency (2.4\u0026times; KDP), as the P2₁2₁2₁ symmetry allows for optimal alignment of molecular dipoles. The precision of the structural determination is evidenced by the exceptionally low estimated standard deviations (\u0026lt;\u0026thinsp;0.0008 \u0026Aring; in unit cell parameters), attesting to the high crystalline quality essential for nonlinear optical applications.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eCrystallographic parameters of K\u003csup\u003e+\u003c/sup\u003e:LHHCl\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCrystal\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e: LHHCl\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCrystal system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eOrthorhombic\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpace group\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eP2\u003csub\u003e1\u003c/sub\u003e2\u003csub\u003e1\u003c/sub\u003e2\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eUnit cell dimensions\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ea\u0026thinsp;=\u0026thinsp;6.8902(3) \u0026Aring;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eα\u0026thinsp;=\u0026thinsp;90\u0026deg;\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eb\u0026thinsp;=\u0026thinsp;8.9431(4) \u0026Aring;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eβ\u0026thinsp;=\u0026thinsp;90\u0026deg;\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ec\u0026thinsp;=\u0026thinsp;15.3162(8) \u0026Aring;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eγ\u0026thinsp;=\u0026thinsp;90\u0026deg;\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eVolume\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003e943.78(8) \u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Molecular Vibrational Analysis by FTIR Spectroscopy\u003c/h2\u003e\u003cp\u003eFourier-transform infrared spectroscopy confirmed the molecular integrity and functional group composition of potassium-doped L-histidine hydrochloride crystals, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The spectrum exhibits characteristic vibrational modes of all fundamental molecular constituents: a broad NH₂ asymmetric stretching band at 3404 cm⁻\u0026sup1; and ring NH stretching at 3074 cm⁻\u0026sup1; demonstrate preserved amine functionality, while the carboxylate group shows distinct asymmetric (1610 cm⁻\u0026sup1;) and symmetric (1575 cm⁻\u0026sup1;) CO₂ stretching vibrations. The imidazole ring vibrations appear at 1629 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;C) and 1451 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;N), confirming the aromatic system's structural integrity. Additional fingerprint region absorptions include CH stretching (2993 cm⁻\u0026sup1;), CCH deformation (1296 cm⁻\u0026sup1;), C-C stretching (960 cm⁻\u0026sup1;), and NH deformation (648 cm⁻\u0026sup1;). Notably, the 15\u0026ndash;20 cm⁻\u0026sup1; shifts observed in carboxylate vibrations compared to undoped L-histidine HCl [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] suggest potassium ion coordination preferentially affects the carboxyl group's electron density distribution. The absence of unexpected peaks and the sharpness of all vibrational bands (FWHM\u0026thinsp;\u0026lt;\u0026thinsp;25 cm⁻\u0026sup1; for major peaks) verify the chemical purity and crystalline perfection of the grown material, consistent with our XRD and EDX results.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eFTIR vibrational assignments of K\u003csup\u003e+\u003c/sup\u003e:LHHCl\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWavenumber (cm⁻\u0026sup1;)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAssignment\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMolecular Origin\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3404\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNH₂ asymmetric stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAmino group\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3074\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRing NH stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eImidazole moiety\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2993\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCH stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAliphatic chain\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1610\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCO₂ asymmetric stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCarboxylate\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1575\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCO₂ symmetric stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCarboxylate\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1629\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC\u0026thinsp;=\u0026thinsp;C stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eImidazole ring\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1451\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC\u0026thinsp;=\u0026thinsp;N stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eImidazole ring\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1296\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCCH deformation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSide chain\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e960\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC-C stretch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBackbone\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e648\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNH deformation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAmino group\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Optical Characterization by UV-Vis-IR Spectroscopy\u003c/h2\u003e\u003cp\u003eThe optical transmission properties of potassium-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) single crystals were investigated through ultraviolet-visible-near IR spectroscopy, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a). The crystal demonstrated excellent transparency across the visible spectrum (400\u0026ndash;800 nm) with a sharp UV cutoff at 251 nm, indicating minimal defects and high crystalline perfection. This low absorption edge suggests strong potential for nonlinear optical applications in the visible and near-UV regions. The observed absorption characteristics arise from electronic transitions between the valence and conduction bands, with the distinct cutoff corresponding to the material's fundamental absorption edge [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe optical band gap was determined using Tauc's plot method (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), which revealed a direct band gap of 4.90 eV - significantly wider than many organic NLO materials. This large band gap correlates with the crystal's high transmittance and explains its exceptional optical clarity. The steep absorption edge indicates minimal sub-bandgap states, confirming the high purity of the grown crystals. These optical characteristics, combined with thermal stability and SHG efficiency (\u003cem\u003ediscussed in\u003c/em\u003e section \u003cspan refid=\"Sec12\" class=\"InternalRef\"\u003e3.5\u003c/span\u003e and \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003e3.6\u003c/span\u003e), position K\u003csup\u003e+\u003c/sup\u003e:LHHCl as an outstanding candidate for frequency conversion devices operating in the visible spectrum. The wide band gap particularly suggests utility in applications requiring high laser damage thresholds and minimal two-photon absorption at common NLO operating wavelengths [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Elemental Composition Analysis by Energy Dispersive X-ray Spectroscopy\u003c/h2\u003e\u003cp\u003eEnergy dispersive X-ray spectroscopy (EDX) analysis confirmed the successful incorporation of potassium dopants in the L-histidine hydrochloride crystals while maintaining excellent chemical purity, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The EDX spectrum exhibited characteristic emission peaks for all expected elements: carbon (C Kα at 0.28 keV), nitrogen (N Kα at 0.39 keV), oxygen (O Kα at 0.53 keV), chlorine (Cl Kα at 2.62 keV), and potassium (K Kα at 3.31 keV). Quantitative elemental analysis revealed the following composition:\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eElemental composition of K\u003csup\u003e+\u003c/sup\u003e:LHHCl single crystals\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElement\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eWeight (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAtomic (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e18.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e24.45\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eN\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6.08\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6.73\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e67.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e65.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCl\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.01\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eK\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe measured potassium concentration of 0.25 wt% (0.03 at%) demonstrates effective doping while preserving the host matrix stoichiometry. The oxygen dominance (65.78 at%) agrees with the carboxyl groups in the histidine structure, while the carbon-to-nitrogen ratio (24.45:6.73 at%) closely matches the theoretical 3:1 ratio expected for L-histidine. The absence of extraneous peaks in the spectrum confirms the high phase purity of the crystals. These results verify that the doping process successfully introduced potassium ions into the crystal lattice without compromising the material's chemical integrity, supporting the enhanced thermal and optical properties observed in other characterization studies. The controlled incorporation of dopant atoms at this concentration level (0.03 at%) explains the improved performance while maintaining the crystalline quality essential for optoelectronic applications.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.5. Thermal analysis (TG-DTA)\u003c/h2\u003e\u003cp\u003eThe thermal stability of potassium-doped L-histidine hydrochloride crystals was systematically investigated through simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Measurements were performed on a 4.417 mg single-crystal sample under nitrogen atmosphere (flow rate: 50 mL/min) across a temperature range of 28\u0026ndash;600\u0026deg;C with a controlled heating rate of 10\u0026deg;C/min (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The TG curve revealed a three-stage decomposition pattern: an initial 8.6% mass loss at 158.7\u0026deg;C (endothermic peak) confirming the anhydrous nature of the crystals, followed by major decomposition at 267.1\u0026deg;C (endothermic) corresponding to the breakdown of the molecular framework. A final exothermic event at 522.7\u0026deg;C with 25.2% mass loss suggested complete oxidative decomposition. The sharp, well-defined thermal transitions observed in both TG and DTA curves attest to the high crystallinity and phase purity of the material, as further supported by our XRD results.\u003c/p\u003e\u003cp\u003eNotably, the decomposition onset temperature (158.7\u0026deg;C) represents a significant thermal stability improvement over pure L-histidine hydrochloride (reported decomposition at ~\u0026thinsp;145\u0026deg;C [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]). This enhanced stability originates from the potassium doping-induced strengthening of intermolecular interactions within the crystal lattice. The absence of solvent-related mass loss below 150\u0026deg;C confirms the successful growth of solvent-free crystals, while the distinct separation between melting and decomposition events (ΔT\u0026thinsp;\u0026gt;\u0026thinsp;100\u0026deg;C) suggests potential for melt-processing applications. These thermal properties, combined with the previously demonstrated nonlinear optical performance, position K\u003csup\u003e+\u003c/sup\u003e:LHHCl as a promising candidate for optoelectronic devices requiring operational stability under moderate thermal loads, such as frequency doublers in laser systems or optical modulators.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.6. Second Harmonic Generation Study of K\u003csup\u003e+\u003c/sup\u003e: LHHCl Crystals\u003c/h2\u003e\u003cp\u003eThe second harmonic generation (SHG) measurements demonstrated compelling nonlinear optical behavior in the K\u003csup\u003e+\u003c/sup\u003e:LHHCl crystals. The doped crystals exhibited an SHG efficiency 2.4 times greater than that of standard KDP, though slightly lower than the 3\u0026times; KDP efficiency observed in undoped L-histidine hydrochloride. This reduction in SHG efficiency compared to the pure crystal suggests that potassium incorporation modifies the molecular dipole alignment while maintaining the noncentrosymmetric structure essential for nonlinear activity. The orthorhombic P2₁2₁2₁ space group, confirmed by single-crystal XRD, provides the necessary asymmetric environment for efficient SHG. Remarkably, the crystals showed excellent stability under laser irradiation, maintaining consistent SHG output over 1000 pulses at 38 MW/cm\u0026sup2; intensity with no observable damage. This robust performance, combined with the previously determined thermal stability (up to 158.4\u0026deg;C) and wide optical band gap (4.90 eV), positions K\u003csup\u003e+\u003c/sup\u003e:LHHCl as a promising candidate for frequency conversion applications in laser systems and integrated photonic devices. The metal-organic charge transfer interactions induced by potassium doping appear to enhance the hyperpolarizability while preserving the crystalline integrity, offering new possibilities for engineering amino acid-based nonlinear optical materials with tailored properties [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.7. Dose-Dependent Cytotoxicity in MCF-7 Breast Cancer Cells\u003c/h2\u003e\u003cp\u003eThe anticancer potential of potassium-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) crystals was quantitatively evaluated against MCF-7 breast cancer cells using the MTT assay. Cells were exposed to five concentrations (6.5, 12.5, 25, 50, and 100 \u0026micro;g/mL) for 24 hours at 37\u0026deg;C under 5% CO₂ atmosphere, with viability assessed through formazan quantification. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the crystals exhibited a clear dose-response relationship, with cell viability decreasing progressively from 84.97% (6.5 \u0026micro;g/mL) to 33.79% (100 \u0026micro;g/mL), corresponding to growth inhibition values of 15.03%, 30.66%, 44.33%, 55.85%, and 66.21% respectively. Nonlinear regression analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e) yielded an IC₅₀ value of 35.96 \u0026micro;g/mL (95% CI: 32.14\u0026ndash;40.22 \u0026micro;g/mL), indicating potent cytotoxic activity comparable to some clinically used chemotherapeutic agents. The observed dose-dependent response suggests that K\u003csup\u003e+\u003c/sup\u003e:LHHCl crystals interact with specific molecular targets in cancer cells, with efficacy directly proportional to drug availability. Notably, the maximum inhibition (66.21% at 100 \u0026micro;g/mL) was achieved without complete media discoloration, indicating preserved mitochondrial function in remaining viable cells and suggesting selective toxicity rather than generalized cytotoxicity. These results position K\u003csup\u003e+\u003c/sup\u003e:LHHCl as a promising candidate for further development as an anticancer therapeutic, particularly given its dual functionality as both a cytotoxic agent and nonlinear optical material.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. CONCLUSION","content":"\u003cp\u003eHigh-quality potassium ion-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e:LHHCl) single crystals were successfully grown using the slow evaporation technique at ambient conditions. Single-crystal XRD analysis confirmed the orthorhombic structure (space group P2₁2₁2₁), while FTIR spectroscopy verified the functional group integrity. The material demonstrated excellent optical characteristics, including a wide transmission window and a direct band gap of 4.90 eV, suggesting potential for UV photonic applications. EDX spectroscopy confirmed the successful incorporation of potassium ions into the crystal lattice. Thermal analysis revealed remarkable stability up to 158.4\u0026deg;C, surpassing many organic NLO materials. Most significantly, the crystal exhibited a powder SHG efficiency of 2.4\u0026times; KDP, making it particularly promising for frequency-doubling applications in laser systems.\u003c/p\u003e\u003cp\u003eBeyond its optoelectronic merits, the K\u003csup\u003e+\u003c/sup\u003e:LHHCl crystal showed pronounced dose-dependent anticancer activity against MCF-7 breast cancer cells, achieving 66.21% growth inhibition at 100 \u0026micro;g/mL with an IC₅₀ of 35.96 \u0026micro;g/mL. This dual functionality \u0026ndash; combining exceptional NLO properties with significant cytotoxicity \u0026ndash; positions K\u003csup\u003e+\u003c/sup\u003e:LHHCl as a groundbreaking multifunctional material. The demonstrated properties suggest simultaneous applications in: Nonlinear optical devices (frequency converters, optical modulators), Biomedical platforms (targeted cancer therapy with optical monitoring capability)\u003c/p\u003e\u003cp\u003eThese findings open new avenues for developing amino acid-based hybrid materials that bridge photonic and biomedical applications, potentially enabling novel optobioelectronic devices. Future work will focus on optimizing crystal properties for specific device integration and investigating the mechanistic pathways of its anticancer activity.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAUTHORS CONTRIBUTIONS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eN. Poongodi: Investigation, Methodology, Formal analysis, Writing - Original Draft\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eP. Dhanasekaran: Conceptualization, Supervision, Resources, Writing - Review \u0026amp; Editing\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRavikumar Nattudurai: Conceptualization, Visualization, Writing - Review \u0026amp; Editing\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eR. Deepika: Data curation, Formal analysis, Validation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest.\u003c/p\u003e\n\u003ch2\u003e\u0026nbsp;\u003c/h2\u003e\n\u003ch2\u003eACKNOWLEDGMENT\u003c/h2\u003e\n\u003cp\u003eThe authors gratefully acknowledge the Department of Physics, Bharathiar University, Coimbatore, for providing access to single-crystal X-ray diffraction facilities. We are particularly grateful to Dr. P.K. Das of the Indian Institute of Science (IISc), Bangalore, for performing the second harmonic generation measurements and for his valuable technical insights. N. Poongodi additionally acknowledges Erode Sengunthar College of Engineering for institutional support.\u003c/p\u003e\n\u003ch2\u003eDATA AVAILABILITY STATEMENT\u003c/h2\u003e\n\u003cp\u003eThe manuscript has no associated data.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMedishetty, R., Zaręba, J.K., Mayer, D., Samoć, M., RA Fischer: Chem. Soc. Rev. \u003cb\u003e46\u003c/b\u003e, 4976 (2017)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRay, P.C.: Chemical reviews 110: 5332. (2010)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSuresh, S., Ramanand, A., Jayaraman, D., Mani, P.: Rev. Adv. Mater. Sci. \u003cb\u003e30\u003c/b\u003e, 175 (2012)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, J., Luo, J., Jen, A.K.Y.: J. Mater. Chem. 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Electron. \u003cb\u003e34\u003c/b\u003e, 522 (2023)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSurya, S., Gunasekaran, B.: J. Mater. Sci.: Mater. Electron. \u003cb\u003e33\u003c/b\u003e, 26383 (2022)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"optical-and-quantum-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"oqel","sideBox":"Learn more about [Optical and Quantum Electronics](https://www.springer.com/journal/11082)","snPcode":"11082","submissionUrl":"https://submission.nature.com/new-submission/11082/3","title":"Optical and Quantum Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Organic nonlinear optical crystals, Optoelectronic materials, Single crystal growth, Second-harmonic generation (SHG), X-ray diffraction, MTT Assay.","lastPublishedDoi":"10.21203/rs.3.rs-7901313/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7901313/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHigh-quality potassium-doped L-histidine hydrochloride (K\u003csup\u003e+\u003c/sup\u003e: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\u0026ndash;1200 nm) and a direct band gap of 4.90 eV. Remarkably, the material showed 2.4\u0026times; higher second harmonic generation efficiency than KDP, attributed to its noncentrosymmetric structure. Thermal analysis revealed stability up to 158.7\u0026deg;C, while MTT assays demonstrated dose-dependent cytotoxicity against MCF-7 breast cancer cells (IC₅₀ = 35.96 \u0026micro;g/mL, 66.21% inhibition at 100 \u0026micro;g/mL). These results position K+:LHHCl as a novel multifunctional material combining nonlinear optical performance for frequency conversion devices with potential anticancer therapeutic applications.\u003c/p\u003e","manuscriptTitle":"Multifunctional Potassium-Doped L-Histidine Hydrochloride Crystals: Optoelectronic Properties and Cytocompatibility Studies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-03 14:14:31","doi":"10.21203/rs.3.rs-7901313/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-30T08:20:15+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-30T07:23:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-28T12:42:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"338671883912445622740002002306377558691","date":"2025-12-18T00:29:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"76076254689295784646438859444528282821","date":"2025-12-09T01:54:58+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-18T09:07:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-15T07:05:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"109584149500282472268413619002335766734","date":"2025-10-28T01:42:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"160915413536611289085541045483274029126","date":"2025-10-25T06:51:46+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-22T17:47:31+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-22T00:29:38+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-21T11:01:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"Optical and Quantum Electronics","date":"2025-10-20T01:51:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"optical-and-quantum-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"oqel","sideBox":"Learn more about [Optical and Quantum Electronics](https://www.springer.com/journal/11082)","snPcode":"11082","submissionUrl":"https://submission.nature.com/new-submission/11082/3","title":"Optical and Quantum Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9dcb7131-5eb8-49da-b811-d4ae19002a29","owner":[],"postedDate":"November 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T16:02:30+00:00","versionOfRecord":{"articleIdentity":"rs-7901313","link":"https://doi.org/10.1007/s11082-026-08718-2","journal":{"identity":"optical-and-quantum-electronics","isVorOnly":false,"title":"Optical and Quantum Electronics"},"publishedOn":"2026-02-19 15:58:01","publishedOnDateReadable":"February 19th, 2026"},"versionCreatedAt":"2025-11-03 14:14:31","video":"","vorDoi":"10.1007/s11082-026-08718-2","vorDoiUrl":"https://doi.org/10.1007/s11082-026-08718-2","workflowStages":[]},"version":"v1","identity":"rs-7901313","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7901313","identity":"rs-7901313","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}