Comprehensive studies on amino acid based organometallic L-threoninum cobalt (II) sulfate (LTCS) single crystal and its antibacterial and antifungal properties

2.1 Materials and crystal growth

By using familiar slow evaporation solution growth technique with highly purified chemicals purchased from Sigma-Aldrich we have been grown a good quality LTCS nonlinear single crystal. For the growth of LTCS crystal, a sufficient measure of L-Threonine and Cobalt (II) Sulfate heptahydrate was dissolved in highly purified laboratory water with the aid of magnetic stirrer at 40 °C for over almost 3–4 h. Even after the solution as filtered for the removal of contaminants and retained unabated evaporation state for the growth of LTCS crystals, and between 25 and 30 days good quality L-Threonine Cobalt (II) Sulfate heptahydrate crystal were gathered as depicted in Fig. 1 below.

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Fig. 1

Photograph of LTCS single crystal.

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2.2 Characterization techniques

With the help of Bruker Kappa APEXII X-ray technique, the unit cell parameters were determined for the grown LTCS crystals. XPERT-PRO diffractometer was used to determine the PXRD studies with Cu Kα radiation having wavelength λ = 1.5406 Å was scanned over a range from 10˚– 80˚ at the scan rate of 2⁰/min, for analyzing grown crystal’s crystalline nature. The UV–Vis spectrum of grown LTCS sample has been measured over diverse range of 200–800 nm by UV-1700 Series Spectrophotometer (Kalyanaraman et al., 2015). The mechanical behavior of grown crystal was studied with the help of HMV-2 T microhardness tester. Kurtz and Perry technique were applied to determine the SHG efficiency of LTCS crystal (Siva Shankar et al., 2009). With the help of the controls such as Amikacin (for antibacterial) and Nystain (for antifungal) the antimicrobial activities of LTCS was ensured by disk diffusion method.

3.1 Powder X-ray diffraction analysis (PXRD)

LTCS crystal's PXRD assessment was conducted by grinding the defect-free LTCS crystal into small particles and it was analyzed with varying 2θ values’ ranging from 10⁰-80⁰ for the scan rate of 2 degree per minute is illustrated in Fig. 2.

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Fig. 2

Powder XRD pattern of LTCS crystal.

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The sharp and distinct peaks observed at precise 2θ angle confirms high crystalline nature and structural phase purity of the LTCS material (Vivekanandhan et al., 2018 & Araújo et al., 2019).

3.2 Single crystal X-ray diffraction analysis (SXRD)

SXRD method gives the information about unit cell parameters and space group of the LTCS crystal (Abila et al., 2020). The obtained data depicts that the grown crystal belonging towards the class of orthorhombic system by means of non-centrosymmetric space group P2₁2₁2_1, is an elementary condition for SHG application (Suresh and Sagadevan, 2014). The analysis shows that the developed single crystal has a = 5.56 Å, b = 8.19 Å, c = 13.94 Å, α = β = γ = 90⁰ and cell volume obtained was 634 Å³ and this values are concord with the reported literature (Allen Moses et al., 2019).

3.3 UV–Visible spectral analysis

Much knowledge about optical transitions is given by optical constants such as the absorption coefficient, optical extinction coefficient, optical refractive index, and optical (Subhashini et al, 2019; Karuppasamy et al., 2020).

Fig. 3(a) in LTCS crystal shows a wide transparency window from 250 to 800 nm and no absorbance within that range. The titular material is active with 83% transmittance indicating that the LTCS crystal possess greater optical transparency and the prepared crystals were suitable for NLO applications (Hanumantharao and Kalainathan, 2012a, 2012b). By using Tauc’s relationship (Dillip et al., 2012) ‘α’ (optical absorption coefficient) and band gap can be determined using the following expressions (Allen et al., 2019). For LTCS crystal n = 1/2 indicated the direct bandgap nature of the material, and this direct bandgap materials used to make optical devices such as LEDs and semiconductor lasers etc.

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Fig. 3

(a) Transmission spectra of LTCS crystal, (b) Plot of (αhν)² versus (hν) of LTCS crystal, (c) Photon energy versus R, (d) Photon energy versus n, (e) Extinction Coefficient with hν of LTCS crystal, (f) Optical conductivity with hν of LTCS crystal.

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Fig. 3(b) exposed the plot between (αhν)² and hν, it provides the value E_g (band gap) = 5.6 eV for LTCS crystal and this wider E_g reveals LTCS material is suitable for designing and assembly of optoelectronic devices (Chennakrishnan et al., 2017). Reflectance (R) and Refractive indexes (n) of LTCS crystal were carried out by means of the following equation and are plotted versus hν as exposed in Fig. 3 (c & d).

In Fig. 3(c and d) it is understood that these optical parameters suchlike R and n depend on hν shows the LTCS materials are suitable for manufacturing optoelectronic devices. The extinction coefficient (K) indicates the absorptivity of the grown material and it can be calculated by the following equation:

Where all symbols have their own significance.

Fig. 3(e) shows the material is highly translucent in the lower energy region because the small value of extinction coefficient, while a larger value indicates higher degrees of opacity. For use as optical material, a low extinction coefficient is essential.

Using the above equation, we can easily find out the optical conductivity of the prepared LTCS crystal.

The variation of Optical conductivity with respect to hν for LTCS crystal is exposed in Fig. 3(f). It displays that, how when optical conductivity drops significantly, hv is increased and is essential for optoelectronic applications. (Subhashini et al., 2019).

3.4 Mechanical studies

The mechanical property of the prepared material was measured by Vickers micro-hardness test (Surekha et al., 2014). Herein the indentation mark of the LTCS crystal was measured by varying the load (25 g, 50 g, and 100 g) for indentation time 10 s. H_ν (hardness number) can be calculated using the following relation

Where all the symbols have their own significance.

It is observed from the Fig. 4(a) that H_ν of the LTCS crystal increases up to a load of 100 g enunciate the RISE (reverse indentation size effect) displayed by the crystal and it is useful for any application (Sangwal 2000).

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Fig. 4

(a) Hardness versus load P of LTCS crystal, (b) Log P versus Log d of LTCS crystal.

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The type of the crystal was determined by means of following Meyer’s law.

The Meyer index ‘n’ can be determined from the slop of a graph drawn for log d versus log P.

The work hardening value 1.94 is estimated from Fig. 4(b) As Onitsch explained the value of 'n' for harder materials is between 1and 1.6, over 1.6 the substance shall be known as soft material (Onitsch 1956). Anyhow, the LTCS material is beneficial for NLO applications because this material is a softer category (Hanumantharao and Kalainathan, 2012).

3.5 Nonlinear optical studies

In order to test the nonlinear SHG efficiency, the titular compound was illuminated with Q-switched Nd: YAG laser (1064 nm) and the green light of wavelength 532 nm was emitted by LTCS crystal was measured and the result obtained is compared with KDP (Bhuvaneswari and Nalini, 2018; Kurtz and Perry, 1968). This confirms that the SHG output of grown LTCS crystal was 1.32 times that of standard KDP crystal (MartinBritto et al., 2008; UmaDevi et al., 2008). It is clear from the observation that the presence of Co⁺ ions which enhances the effect of titular crystal. Here the greater SHG performance of the grown sample ensures it can be beneficial for different NLO applications (Chandrasekaran et al., 2012).

3.6 Antimicrobial activity

The microbial activities of LTCS material were demonstrated using the disc diffusion process. To understand the bacterial activity, antibacterial susceptibility testing is performed for LTCS material using microorganisms namely Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus and the results are presented in Table 1. The result obtained shows that LTCS material resists the growth of specified above most susceptible microorganisms, nevertheless, on a comparative the result indicated that LTCS material showed a substantial resistivity against Klebsiella pneumonia than the other two microorganisms as shown in Fig. 5 (a, b, & c).The inhibitory activity on Escherichia coli of LTCS material is greater than that of macrocyclic dinuclear Zn(II) complex (Kou et al., 2020). Because of the result obtained, the developed material may be suitable for the treatment of various diseases caused by the above mentioned microorganisms (Aneeba et al., 2020; Priya et al., 2020).

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Fig. 5

Antibacterial activity of LTCS crystal for three bacterial pathogens (a) Escherichia coli (b) Klebsiella pneumoniae (c) Staphylococcus aureus, Antifungal activity of LTCS crystal for three fungal pathogens (d) Candida albicans (e) Candida parapsilosis (f) Aspergillus flavus.

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To access the antifungal activity, LTCS material were screened against three different test organisms namely Candida albicans, Candida parapsilosis and Aspergillus flavus as shown in Fig. 5 (d, e & f) and the obtained results are presented in Table 1 .The result suggest that among three fungi, LTCS highly inhibited the activity of Candida albicans (15 mm) than other two fungi. However, according to the results obtained, LTCS sample makes resistance to the most prevalent opportunistic fungal pathogens. In the context of above concern the grown material can be used to treat various diseases caused by the above-mentioned three pathogenic fungi (Aneeba et al., 2020 & Abid et al., 2019).