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Influence of most reactive inorganic cation in the optical and biological activities of L-Lysine monohydrochloride crystal

Slow evaporation method was used to grow the pure and K+ ion doped L-Lysine monohydrochloride (L-LMHCL) crystals which has optical and antibiotic applications. The space group, structure and slight shifting of peaks are confirmed using single crystal XRD and the powder XRD. The FTIR analysis also shows that the K+ doped L-LMHCL has a slight shifting in the spectrum which indicates the functional group of L-LMHCL and the interaction between the K+ ions. The existence of K+ ion in the doped crystal is assured by the presence of potassium in the EDAX spectrum. The wide optical band gap was found for pure and K+ doped crystal using UV spectra and these are utilized in optoelectronic and nonlinear applications. The Kurtz Perry technique specified the NLO property of grown crystals. The dielectric property crystals was studied by varying the temperature. As a result, the highest dielectric constant is observed in doped crystal. An antibacterial activity against certain bacteria like E-coli, pseudomonas aeruginosa and staphylococcus aureus are provided by mm range for the grown crystals.


Introduction
Aminoacid based organic crystals are potential candidates for optical application as it possesses chiral carbon atom, proton acceptor (NH 2 ) known as zwitterions, non-centrosymmetric space groups and flexibility of molecular design (Shanmuga Sundaram et al., 2019) but it has poor physio-chemical stability and low mechanical strength (Min-hua jiang and Qi Fang., 1999). It can be prone to exhibit wide transparency range, mechanical strength, high polarizability, chemical stability, good second harmonic generation (SHG) and low angular sensitivity (Allen moses et al.,2019; Kandhan et al., 2019) when it combines with inorganic compounds known as semiorganic. Due to the enticing features of hybrid nonlinear optical (NLO) crystals, it is used in the field of data storage, optical communication, laser remote sensing, mode locking, night vision devices etc. (Kui et al., 2017;Kandhan et al., 2018). The novel transparent electro-and elastooptical material is L-Lysine monohydrochloride crystal (Ozga et al., 2008). It owns three reactive groups like a-carboxyl, a and e-amino group also immunostimulatory effect on inoculation site infections (Zhang et al., 2020;Ayala and Krikorian, 1989). Since last decade slow evaporation solution growth was preferred by many researchers to grow defect free crystal due to its simplest and cost-effective method. The various reviews show that the impurities also make changes in the properties of molecules (Helen and Kanchana, 2014). For the first time, in order to enhance the optical, dielectric, nonlinear and antibacterial activities the most reactive potassium ion was introduced as dopant into the lattice of L-LMHCL via slow evaporation method.Obtained crystals are being characterized by using the methods such as Single crystal X-ray Diffraction (SXRD), powder XRD, Fourier Transform Infrared Spectroscopy (FTIR), Energy Dispersive X-ray Analysis (EDX), UV-Visible spectroscopy, Micro hardness, second order nonlinear optical studies, Dielectrics and Antibacterial studies.

Materials
The materials Lysine monohydrochloride and KCl used for this exploration were purchased from Sigma Aldrich Chemicals.
At constant temperature, pure and potassium added L-LMHCL crystals are grown using the conventional slow evaporation technique. The computed amount of L-Lysine monohydrochloride (C 6 H 15 ClN 2 O 2 ) was dissolved in the deionized water. At 40°C temperature, the solution was stirred continuously for about 2 hrs to make sure of homogenous supersaturated solution. To obtain K + ion doped L-LMHCL crystal, 1 mol% of potassium chloride (KCl) was added to the L-LMHCL solution. The solutions were filtered twice in a clean rinsed beaker using whatmann filter paper to remove impurities. A perforated thin polyethylene sheet was used to close the beaker optimally. At room temperature, it was placed undisturbed for slow evaporation. The better-quality crystals which are optically transparent had been harvested within 20 days. The photographs of pure and doped crystals are shown in Fig. 1.

Antibacterial test
The susceptibility action against the pathogenic bacteria were carried out by standard Kirby-Bauer test commonly called disc diffusion test (Shen et al., 2019, Zou et al., 2020. The inhibited range of disease causing bacteria was identified from the zone formed around the compound and the diameter zone was measured in mm range using transparent mm scale.

Single crystal X-Ray diffraction (SXRD)
ENRAF NONIUS CAD4 X-ray diffractometer was used to collect the single crystal XRD of pure and doped L-LMHCI crystals. The collected lattice parameter values show that the grown crystals belong to monoclinic structure with space group P2 1 because lattice parameters a, b, c are different and two interfacial angles are equal to 90 0 and other is not. Non-centro symmetric nature also confirmed from space group which is a rudimentary condition for SHG applications. The obtained lattice parameters of crystals are well compliance with the reported values (Ramesh Babu et al., 2006). Slight difference in unit cell parameters are generally caused by the change of periodicity in lattice vibrations at high temperature, concentration of dopant, electronic and size effect (Deepa and Philominathan, 2017). Here the reduction in the lattice parameters of K + doped L-LMHCL crystal was due to the effect of dopant K + ion in the lattices of L-LMHCL crystals.

Powder X-Ray diffraction (XRD) analysis
X'Pert Pro -PAnalytic powder diffractometer was used for powder X-ray diffraction studies. The studies were carried out with 0.2°/sec scan speed and CuK a radiation of wavelength 1.5418 Å. The powder X-ray diffraction pattern of undoped and doped L-LMHCl is given in Fig. 2a.
Scanty shift in the intensity of peak and FWHM is obtained in the Fig. 2b as a result of strain in the crystalline lattice. There is no additional phase recommended since K + ion does not affect the entire lattice of L-LMHCL, but there is minute lattice distortion.

Fourier Transform Infrared (FTIR) spectroscopic study
FTIR spectra is a unique fingerprint of molecule that elucidate the various functional groups exist in the growncrystals. The FTIR analysis of the grown crystals were recorded in the middle IR region (400-4000 cm À1 ) by Thermo Nicolet Avatar 370 spectrometer (Fig. 3). This range is the most remarkable range in the study of organic, inorganic and semi organic compound (Pasupathi and Philominathan, 2008;Balasubramanian et al., 2010;Ushasree et al., 1999). The FTIR vibrational spectra and its assignments of gown crystals agree well with the literature values (Vasudevan et al., 2010) (see Table 1).
The band observed at 3443 cm À1 is assigned as NH 3 + asymmetric stretching vibrations. The absorption band at 1624 cm À1 and 1506 cm À1 are attributed to weak asymmetric NH 3 + bending and strong symmetric NH 3 + bending.CH 2 twisting acknowledged at 1348 cm À1 . The band observed at 1140 cm À1 is attributed to NH 2 and NH 3 rocking. The peak observed at 996 cm À1 is due to C-C stretching. O-H-O out -of-plane bending type of vibrations of hydrogen bonds observed at 861 cm À1 . The band imputed at 553 cm À1 is the result of COO À wagging. Table 2 shows the functional group assignments of obtained crystals. The spectrum of K + doped L-LMHCL provide the similar features of pure L-LMHCL, although there is shifting exist for most of the band insinuate that there is wide range of interactions between the K + ions with the function group of L-LMHCL.

Energy Dispersive X-ray analysis (EDX)
EDX is the most positive way to recognize the presence of accompanied elements in the obtained crystal. Fig. 4displays the accounted EDAX spectrum, which was taken to confirm the presence of potassium in the doped crystal. It distinctly indicates the presence of various elements such as oxygen, carbon, potassium and chloride in the K + ion doped crystal. The existence of k + ion in the doped crystal was confirmed by the trace of potassium in the EDAX spectrum.

Optical transmission spectral study
One of the important properties of crystal is its optical behaviour which mainly embraces the ultraviolet radiation which can either be absorbed or transmitted by the sample. The wide transparency nature of the crystal plays the main role in UV-tunable lasers, photonics and NLO device applications. In order to find the optical property, Perkin Elmer (Lambda 35) in the wavelength range of 198-800 nm was used. The polished crystal samples were subjected to be measured. The recorded transmittance spectrum of impure L-LMHCL crystal collated with undoped L-LMHCL crystal is shown in Fig. 5(a). It indicates the presence of transmission in the whole UV-visible range. It is particularly seen in the second harmonic wavelength (532 nm) region, and also the transparency of K + ion doped L-LMHCL is slightly increased and cut-off wavelength shifted towards lower wavelength side because of the lack of grain and defect boundaries (Rathika and Ganapathi Raman, 2014). As the lower cut-off wavelength and optical transparency of good NLO material lies between 200 and 400 nm, the grown crystal can be considered as an NLO material (Kalaivani et al., 2015).

Measurement of optical band gap
The type of electronic transition and energy gap mainly depend on optical absorption coefficient and photon energy. The optical absorbance coefficient (a) is find out from the Eq. (1): where 'T' and 'd' are transmittance and thickness of the crystal. Direct band gap was recommends from the energy dependence of absorption coefficient and hence for high photon energy it obeys Tauc's relation is given in Eq. (2): (Tauc, 1974). Fig. 5(b) illustrates the extrapolated graph between (ahm) 2 and hm owing to the direct allowed transition of crystals and the graph imputed the value of E g . By the extrapolation of straight-line portion toðahmÞ 2 ¼ 0, 5.52 eV for undoped and 5.89 eV for K + doped L-LMHCL crystals were founded, and this wideband gap values utilized in optoelectronic and nonlinear optical application. It also shows the ability of dielectric (crystal) medium to be polarized when external field is applied.

Nonlinear optical (NLO) study
Kurtz-perry powder technique was the important tool used to assess obtained crystal's second harmonic generation (SHG) (Kurtz and Perry, 1968). The green light (532 nm) emitted by the crystals when it was exposed to radiation from Nd: YAG laser of wavelength 1064 nm confirms the NLO nature of the crystals. When Compared to standard KDP crystal's SHG efficiency, the pure and doped crystal's SHG efficiency are 1.2 and 1.6 times greater. From the observation it is clear that the presence of K + ion enhance the SHG efficiency of the L-LMHCL. This result strongly recommends that the harvested crystals are propitious for SHG implementation.

Dielectric analysis
The study of dielectric behaviour at different temperature with various frequencies of applied field gives important electrical properties of the materials like polarizability, phase change, nature of atoms and charge transport mechanism of the crystal (Karuppasamy et al., 2017). With the use of two probe set up, dielectric behaviour was measured. In the middle of the copper electrode which behaves as parallel plate capacitor double side graphite coated crystal was placed this double side graphite coated crystal act as dielectric material. By changing the temperature from 40°C to 140°C and frequency from 100 HZ to 1 MHZ corresponding capacitance values were noted. Dielectric constant ðe r Þ was measured by the Eq.  Table 1 Unit cell parameters of pure and K + doped L-LMHCL crystals.  when the frequency increases suggest that these polarization diminishes gradually and electronic polarization only exist and indicate less detect (Sivavishnu et al., 2018). These attributes of the crystals are used in the photonics and NLO applications because polarizability depend on linear susceptibility, and linear susceptibility proportional to SHG (Raval et al., 2019). Further, correlating the magnitude of e r of K + mixed and unmixed L-LMHCL crystals reveals that potassium added crystals possess high e r and tand. High dielectric constant material exposes high polarizability due to mobility of dipoles, which type of materials are contribute high SHG efficiency. The change in conductivity at various temperature and angular frequency is shown in Fig. 6(e) and (f). The conductance of doped crystal is more than pure crystal. There is motion of charge carrier increases with temperature which lead to high value of conductivity.

Antibacterial activity
Antibacterial activities of pure and K + ion doped L-LMHCL crystals assessed quantitatively by most widely used antibiotic susceptibility test Kirby-Bauer method. The organism was susceptible by the sample and can be identified from the diameter of zone formation around the grown powdered crystal sample. Observed inhibition zone in the millimeter (mm) range, compared to the standard drug Amikacin is shown in Table 3, indicating that doped crystal showed good inhibitory action against pseudomonas aeruginosa and staphylococcus aureus bacteria than pure crystal and the antibacterial activity of pure L-LMHCl crystal towards Escherichia coli is more than doped L-LMHCL crystal. Zone resisting action of crystals against pseudomonas aeruginosa is leading than HTNN crystal (Ravathi and Karthik, 2019) also it have more inhibitory action on Staphylococcus aureus than Ag nanoparticles. (Prakasha et al., 2013).

Conclusion
Single crystals of pure and k + doped L-LMHCL were grown by slow evaporation method. Monoclinic structure and the P2 1 space group nature of the obtained crystal was revealed when it undergoes X-ray diffraction analysis. The K + ion does not affect the entire lattice of L-LMHCL, identified from the single-phase nature of powder XRD. The existence of functional groups and elements were corroborate by FTIR and EDX analysis. The optical absorbance study reveals that the percentage of transmission was increased by adding K + ions. The obtained crystal's SHG efficiency were 1.2 and 1.6 times higher than that of KDP. Dielectric measurements of crystals were carried out and the results were discussed. The pure and doped L-LMHCL crystals show good inhibitory antibacterial property when exposed to bacteria. The above said studies confirmed that K + doped L-LMHCL crystals have enhanced properties than pure L-LMHCL crystals and could be treated as a promising material for fabricating the optoelectronic devices and antibiotics.