Synthesis and characterization of Zn(II) metalated chitosan based nano-hybrid: Antioxidant and antibacterial activity

Abstract

An efficacious nano-hybrid has been developed comprising of a lacunary polyoxometalate (K₈SiW₁₁O₃₉). The lacunary position has been utilized to incorporate alkyl-silane. In alkyl-silane the NH₂ group is covalently attached to 2-hydroxy-1-naphthaldehyde through Schiff-base reaction and then metalated with zinc salt. After successful metalation it has been encapsulated into chitosan by ion gel technique to develop nano-hybrid. Each step of the reaction has been monitored by FT-IR spectroscopy. Insertion of alkyl-silane into lacunary polyoxometalate followed by Schiff-base reaction and metalation of zinc has been confirmed by UV-Vis spectroscopy. Further the characteristic bands of UV–Visible spectrum have confirmed the final encapsulation into chitosan to develop the nano-hybrid. EDX analysis reveals the successful metalation with zinc and confirms the presence of all elements in their expected stoichiometric ratio. The inductively coupled plasma (ICP) technique further supported and confirmed the EDX analysis. The antibacterial activity of initial moiety and nano-hybrid have been carried out against bacterial strains of E. Coli gram (-)ve and B. subtilis gram (+)ve. The nano-hybrid has shown better antibacterial activity as compared to the initial moieties L-POM@Sil-HN and L-POM@Sil-NH-Zn. We have also studied the antioxidant activity of nano-hybrid by DPPH assay method; it has shown excellent activity.

Keywords

biopolymer chitosan polyoxometalate nano-hybrid antioxidant

Introduction

Chitin derived from the shells of crustaceans and shrimps, undergo chemical treatment by N-deacetylation to produce chitosan (CS).^1,2 CS is a bio-compatible polymer; it has remarkable utilization in the area of therapeutic and medical science.³ CS has enormous advantages in biomedical field, which include wound curing capacity, tissue engineering, transportation of drugs, vaccines and genes in human^4,5 as well as comprise of antifungal, antibacterial, anti-inflammatory and antioxidant activity.⁶ Nano-particles comprising of CS are safe and efficient to develop for targeted drug delivery as nano-carrier system.

Polyoxometalates (POMs) are stable oxo-clusters of transition metals (mostly W, Mo, V) with variable oxidation states. Polyoxometalate (POM) based hybrids have broad range for research since they are vital tools for development of antibiotics, anti-cancer and anti-inflammatory drugs.^7–10 In case of lacunary polyoxometalate (L-POM) the lacunary site can be used to design a variety of valuable hybrids by incorporating versatile components.¹¹ Utilizing bio-degradable CS polymer as an encapsulating unit for POM, various drugs (possessing high stability at diverse pH), antioxidants & catalysts can be designed.^12–16 The area for development of nano-hybrids by usage of CS as the encapsulating unit for POMs has been diminutive explored.

In this manuscript, we have designed a bioactive nano-hybrid by encapsulating lacunary polyoxometalate (L-POM) (K₈[SiW₁₁O₃₉].13H₂O) into CS. But before encapsulation the lacunary position has been incorporated with alkyl-silane [3-(2-Aminoethylamino)-propyltrimethoxysilane)] to give L-POM@Sil. Then L-POM@Sil has been reacted with 2-hydroxy-1-naphtaldehyde to form L-POM@Sil-HN and metalated with zinc chloride anhydrous to form L-POM@Sil-HN-Zn. Addition of 2-hydroxy-1-naphtaldehyde to alkyl-silane and metalation by zinc has been executed to enhance the antibacterial activity. Finally, L-POM@Sil-HN-Zn is encapsulated into CS to give nano-hybrid (L-POM@Sil-HN-Zn-CS). L-POM@Sil-HN-Zn and L-POM@Sil-HN-Zn-CS have been comparatively studied for antibacterial activity against bacterial strains E. Coli gram (-)ve and B. subtilis gram (+)ve respectively. Also, the antioxidant activity of nano-hybrid has been conducted and it has shown good activity.

Experimental section

Materials and characterization techniques

Tungstosilicic acid hydrate H₄[SiW₁₂O₄₀].xH₂O, low molecular weight chitosan, zinc chloride anhydrous, alkyl-silane [3-(2-Aminoethylamino)-propyltrimethoxysilane)], 2-hydroxy-1-naphthaldehyde and trimethylorthoformate of A.R. grade have been procured from Merck. Acetic acid, potassium bicarbonate, 1,1-diphenyl-2-picrylhydrazyl (DPPH) and ethanol (A.R. grade) have been procured from CDH and used without further purification. The bacterial strains E. Coli gram (-)ve and B. subtilis gram (+)ve have been procured from IMTECH, Chandigarh, India. Nutrient agar and nutrient broth have been purchased from Titan Biotech Ltd Rajasthan, India. FT-IR spectrum has been recorded in wave number by employing pellets of KBr ranging from 4000 upto 300 cm⁻¹ using spectrophotometer of Perkin Elmer. UV–visible spectra have been obtained with Shimadzu UV-2450. EDX detector coupled with Nova Nano FE-SEM-450 (FEI) is employed for detecting the elemental composition of hybrid. ICP-AES analysis has been performed by SPECTRO Analytical Instruments GmbH- ARCOS, Simultaneous ICP Spectrometer.

Synthesis of (K₈[SiW₁₁O₃₉].13H₂O) L-POM

A similar procedure has been followed as mentioned by A.Tézé and G.Hervé for the synthesis of lacunary polyoxometalate (K₈[SiW₁₁O₃₉].13H₂O) (L-POM) (Figure 1).¹⁷ FT-IR (cm⁻¹, KBr): 710, 742, 797, 852, 870, 909, 950.

Figure 1.

Diagrammatic presentation of L-POM (K₈[SiW₁₁O₃₉].13H₂O).

Synthesis of L-POM@Sil by incorporation of [3-(2-Aminoethylamino)-propyltrimethoxysilane)] alkyl-silane into lacunary position of L-POM

Similar procedure has been followed as reported by Bar-Nahum et al. to incorporate alkyl-silane into the lacunary position of L-POM.¹⁸ Firstly, alkyl-silane [3-(2-Aminoethylamino)-propyltrimethoxysilane)] is activated and then incorporated into L-POM to develop L-POM@Sil (Scheme 1). L-POM@Sil is obtained as tetrahexylammonium (n-hexyl)₄N⁺) salt by adding tetrahexylammonium bromide, i.e., Q₄{SiW₁₁O₃₉[O(Si-CH₂-CH₂-CH₂-NH-CH₂-CH₂·NHHCl)₂]}, (where Q = (n-hexyl)₄N⁺). FT-IR (cm⁻¹, KBr): 710, 742, 797, 853, 870, 910, 950, 1045, 1156, 1230, 1485, 2946, 3241, 3446.

Scheme 1.

Schematic presentation for synthesis procedure of L-POM@Sil.

Synthesis of L-POM@Sil-Hn by reaction between L-POM@Sil & 2-hydroxy-1-naphthaldehyde

For the synthesis of L-POM@Sil-HN (Scheme 2), similar procedure has been followed as reported by S. Arun.¹⁹ Through Schiff-base reaction 2-hydroxy-1-naphthaldehyde is attached to NH₂ site of Q₄{SiW₁₁O₃₉-[O-(Si-CH₂-CH₂-CH₂-NH_2*HCl)₂]} to synthesize Q₄{SiW₁₁O₃₉[O-(Si-CH₂-CH₂-CH₂-N = CH(2-OH-Naph)₂]} (L-POM@Sil-HN) FT-IR (cm⁻¹, KBr): 708, 740, 797, 853, 870, 910, 950, 1040, 1156, 1230, 1485, 1615, 1632, 2944, 3240, 3445.

Scheme 2.

Schematic presentation for synthesis procedure of L-POM@Sil-HN.

Metalation by anhydrous zinc chloride of L-POM@Sil-Hn

L-POM@Sil-HN has been metalated by zinc chloride anhydrous through the similar procedure as stated by P. K. Dutta et al.²⁰ for the synthesis of L-POM@Sil-HN-Zn (Scheme 3).

Scheme 3.

Schematic metalation presentation of L-POM@Sil-HN with zinc chloride anhydrous to synthesize L-POM@Sil-HN-Zn.

Scheme 4.

Schematic presentation for synthesis of nano-hybrid (L-POM@Sil-HN-Zn-CS) by encapsulation of L-POM@Sil-HN-Zn into Chitosan (CS).

Encapsulation of L-POM@Sil-Hn-Zn into Cs matrix for synthesis of nano-hybrid (L-POM@Sil-Hn-Zn-Cs)

Ionotropic gelation technique has been employed for the synthesis of nano-hybrid (L-POM@Sil-HN-Zn-CS), by the reported method of P.K. Dutta et al.²¹ Successful encapsulation has been achieved due to cationic and anionic interaction in which POM, CS act as cation and anion respectively (Scheme 4).

Antioxidant activity of nano-hybrid (L-POM@Sil-Hn-Zn-Cs) using DPPH radical-scavenging assay

DPPH is hydrophobic in nature which has been used to study antioxidant activity. DPPH is a radical which becomes stable by accepting electron or hydrogen radical and switch into a diamagnetic molecule.²² DPPH possessing antioxidant activity is extensively exploited worldwide due to its viable availability.

Antibacterial study

Synthesized nano-hybrid as well as its initial moieties, L-POM@Sil-NH-Zn and L-POM@Sil-HN have been studied for antibacterial activity against two bacterial strains namely E. coli gram (-)ve and B. subtilis gram (+)ve. For the antibacterial assessment, agar well-diffusion method has been applied as described by P.K. Dutta et al.²¹ The inoculum of bacterial strains has been prepared from the fresh overnight culture in sterilized Nutrient Broth (13 g/L) by incubation at 37°C. According to well-diffusion technique the sterilized nutrient agar has been poured into petri-dishes then left undisturbed to dry. Consequently, active broth culture of each bacterial strain (1% of the bacterial culture in order to obtain 105 CFU ml⁻¹). Inoculated agar plates have been treated with nano-hybrid L-POM@Sil-HN-Zn-CS, L-POM@Sil-NH-Zn and L-POM@Sil-HN in 100μl of aqueous solution and put in wells (8 mm). Then incubation has been continued overnight at 37°C, and finally the zone of inhibition (diameter) was measured in (mm) to determine the antibacterial efficacy.

Results and discussion

FT-IR spectra

The recorded FT-IR spectrum of L-POM [536 (ʋ_sym WOW), 710 (ʋ_asym WOW), 742 (ʋ_asym WOW), 797 (ʋ_asym WOW), 852 (ʋ_asym WOW), 870 (ʋ_asym WOW), 909 (ʋ_asym W = O), 950 (ʋ_asym W = O)] (Figure 2(a)) is similar as reported in reference¹⁷ and it has confirmed the synthesis of L-POM. FT-IR spectrum of L-POM@Sil (Figure 2(b)) shows the presence of 1045 (ʋ_asym Si-O-Si), 1156 (C-N), 1230(Si-C), 1485 (C-H), 2946 (C-H), 3241 (N-H), 3446 (N-H) bands along with L-POM bands which give evidence for successful incorporation of alkyl-silane into L-POM.¹⁹ In Figure 2(c) the presence of two new bands 1615 and 1632 for C = N confirmed that 2-hydroxy-1-naphthaldehyde is covalently linked through the C atom of carbonyl group to N atom of alkyl-silane. Presence of these two new bands along with above mentioned IR bands with minute shifts confirms the formation of L-POM@Sil-NH. To detect for metalation of POM@Sil-NH with Zn for synthesis of POM@Sil-NH-Zn, the FT-IR spectrum is recorded up to far infra-red region i.e., 450 cm⁻¹. In FT-IR spectra stretching vibration band of Zn-N has been recorded at 520 cm⁻¹ which means that zinc is coordinated to N of C = N. Similarly, the stretching vibration band of Zn-O has been recorded at 592 cm⁻¹ which means that zinc is coordinated to oxygen, which is due to deprotonation of O-H group of 2-hydroxy-1-naphthaldehyde.²³ The characteristic FT-IR peaks of CS are shown in Figure 3(a). In Figure 3(a) observed peaks at 1636 and 1072 cm⁻¹ for N-H bending vibration of amines and bridge oxygen stretching vibration in CS respectively.²⁴ The encapsulation of POM@Sil-NH-Zn into CS to produce nano-hybrid POM@Sil-NH-Zn-CS is confirmed by presence of characteristic FT-IR peaks of CS along with FT-IR bands of POM@Sil-NH-Zn.

Figure 2.

FT-IR spectrum of (a) L-POM (b) L-POM@Sil (c) L-POM@Sil-NH.

Figure 3.

FT-IR spectrum of (a) CS (b) L-POM@Sil-Zn (c) L-POM@Sil-NH-Zn-CS.

EDX and ICP analysis

Analysis of nano-hybrid L-POM@Sil-NH-Zn-CS by EDX shows the presence of W atoms and Zn atoms in the expected stoichiometric ratio i.e., per 11 atoms of W, 1 atom of Zn is present (Figure 4). EDX analysis also confirmed the presence of all expected elements i.e., C, N, O in the predictable ratio.²⁵ Additionally, ICP study also supported the EDX analysis by exhibiting the presence of Si (Conc.: 0.004 gm/L, Wavelength: 255.60 nm), W (Conc.: 0.128 gm/L, Wavelength: 242.70 nm), and Zn (Conc.: 0.006 gm/L, Wavelength: 337.40 nm) in the same amount as expected in POM@Sil-NH-Zn-CS. Calculated % Si, 2.55; W, 61.95; Zn,1.80; found % Si, 2.58; W, 61.98; Zn,1.84.

Figure 4.

EDX analysis of POM@Sil-NH-Zn-CS.

UV-Vis-spectroscopy

UV-Vis spectrum has been recorded in the region between 200 to 700 nm for L-POM in DMSO medium at 298 K. In UV-Vis spectrum (Figure 5(a)) the peaks at 209 and 265 nm has been assigned to the charge transfer from W⁶⁺ to O²⁻ in the lacunary structure of α-K₈SiW₁₁O₃₉ at W-O-W and W = O. UV-Vis spectra of L-POM@Sil-NH-Zn (Figure 5(b)) show the absorbance peak of the Zn (II) ion at 437 and 512 nm which indicates that the coordination geometry of the Zn(II) ion may be tetrahedral. The persistence of two absorbance peaks of L-POM with the minute bathochromic shift in the UV-Vis spectrum of L-POM@Sil-NH-Zn illustrates the successful incorporation of metalated alkyl-silane into L-POM.

Figure 5.

(a) UV-Vis spectrum of L-POM (b) L-POM@Sil-NH-Zn.

Figure 6.

UV Vis Spectra of (a) First Reading was taken just after sonication of nano-hybrid (L-POM@Sil-NH-Zn-CS) was stopped (b) Second Reading after two hours of continuous stirring of colloidal suspension of nano-hybrid (L-POM@Sil-NH-Zn-CS) (c) Third Reading after ten hours of continuous stirring of colloidal suspension of nano-hybrid (L-POM@Sil-NH-Zn-CS) (d) Fourth Reading was taken after centrifugation of nano-hybrid (L-POM@Sil-NH-Zn-CS).

Further the UV-Vis spectroscopy technique has been used to study the encapsulation of L-POM@Sil-NH-Zn into CS. For this study four readings have been taken during the synthesis procedure of nano-hybrid keeping the concentration of L-POM@Sil-NH-Zn constant. The first reading in UV-Vis spectrum of supernatant has been obtained just after sonication was stopped (Figure 6(a)). In first reading all the absorption peaks of L-POM@Sil-NH-Zn were present. Second reading of supernatant has been taken after two hours of continuous stirring of colloidal suspension of L-POM@Sil-NH-Zn and CS (Figure 6(b)). In second reading all the absorption peaks that were present in Figure 6(a) got diminished which indicate slight entrapment of L-POM@Sil-NH-Zn into CS. Third reading has been taken after ten hours of continuous stirring (Figure 6(c)) and the fourth reading of supernatant was taken after centrifugation (Figure 6(d)). Finally, characteristic absorption peaks of L-POM@Sil-NH-Zn vanished from the third and fourth readings of UV-Vis spectra. This indicates that LPOM-Sil-NH-Zn has been successfully encapsulated into CS after centrifugation.²⁰

Figure 7.

SEM image of L-POM@Sil-NH-Zn-CS.

SEM image of L-POM@Sil-Nh-Zn-Cs

Scanning Electron Microscope (SEM) image of L-POM@Sil-NH-Zn-CS represents size distribution, morphology and particle size of the nano-hybrid in Figure 7. Homogeneous size, morphology distribution is illustrated in Figure 7 and the particle size was found to be in the physiologically relevant range from 80–130 nm with an average around 110 nm as processed by Image software.

Antioxidant activity by DPPH assay method

DPPH is hydrophobic in nature which is used to study antioxidant activity. DPPH is a radical which become stable by accepting electron or hydrogen radical and converts into a diamagnetic molecule.²⁶ Here antioxidant activity was executed by DPPH scavenging method, in which different concentrations of the sample (L-POM@Sil-HN-Zn-CS) were prepared from 0.2 mg/mL to 2 mg/mL and these different concentrations of the sample have been analyzed at 517 nm absorbance by UV-vis spectrophotometer.²⁷ The results showed that on increasing sample concentration from 0.2–2 mg/mL, the absorbance value decreases, and therefore the percentage of scavenging activity of DPPH increases from 10% to 58.5% (Figures 8). The results interpreted for antioxidant property of L-POM@Sil-HN-Zn-CS is as follows at 0.2 mg/mL (10%), 0.4 mg/mL (14.8%), 0.6 mg/mL (17.2%) so on and finally at 2 mg/mL it shows 58.5% antioxidant property. The result shows that L-POM@Sil-HN-Zn-CS possesses excellent antioxidant properties.

Figure 8.

Antioxidant activity of L-POM@Sil-HN-Zn-CS performed by DPPH scavenging method.

Figure 9.

Antibacterial activity of L-POM@Sil-HN (a), L-POM@Sil-NH-Zn (b), L-POM@Sil-HN-Zn-CS(c) against gram positive (B. subtilis) [I] and gram negative (E. coli) [II] bacteria.

The DPPH scavenging activity was calculated by the equation given below:

DPPH Scavenging activity (%) = \frac{A_{B l a n k} - A_{0.2 - 2 m g / m L}}{A_{B l a n k}} \times 100

Where A_Blank is the absorbance value obtained by blank solution and A_0.2–2mg/mL is the absorbance value obtained by the sample solutions at different concentrations.

Biological application of synthesized material

Antibacterial screening

For the antibacterial assessment, Agar well-diffusion method has been used. The synthesized L-POM@Sil-HN-Zn-CS and its initial components L-POM@Sil-NH, L-POM@Sil-NH-Zn have been tested against gram negative (E. coli) and gram positive (B. subtilis) bacteria (Figure 9). The results have been investigated in terms of Zone of Inhibition (ZOI) which is reported in mm and the concentration of samples taken was 0.5 mg/mL. Agar solidified plates have been prepared and then broth which is a bacterial medium was spread on it appropriately. After this holes have been created by sterilized two sided open test tube in agar plates. Each created holes were filled with 80μL of sample solutions and then incubated for 12 h at 37°C. Analysis result shows that L-POM@Sil-HN-Zn-CS has larger ZOI as compared to its initial components L-POM@Sil-HN, L-POM@Sil-NH-Zn (Table 1). The results are shown in Table 1.

Table 1.

Zone of Inhibition (mm) of L-POM@Sil-HN, L-POM@Sil-NH-Zn, L-POM@Sil-HN-Zn-CS.

Bacteria	Zone of Inhibition (ZOI) in mm
Bacteria	L-POM@Sil-HN(A)	L-POM@Sil-NH-Zn(B)	L-POM@Sil-HN-Zn-CS(C)
E. coli	22 ± 0.3 (3)	29 ± 0.1 (3)	35 ± 0.5 (3)
B. subtilis	18 ± 0.2 (3)	25 ± 0.4 (3)	30 ± 0.0 (3)

From the results we can conclude that the nano-hybrid and its initial components showed greater antibacterial activity in gram negative than gram positive bacteria. Also from this study we can conclude that initial components (i.e., L-POM@Sil-HN and L-POM@Sil-NH-Zn) showed lesser antibacterial activity than L-POM@Sil-NH-Zn-CS nano-hybrid. This confirms that encapsulation of L-POM@Sil-NH-Zn into chitosan enhanced antibacterial activity than its initial moieties.

Conclusions

A new class of nano-hybrid has been fabricated by using lacunary polyoxometalate through multistep reactions. After metalation with anhydrous zinc salt, it was encapsulated into chitosan by ionotropic gelation technique. Characteristic bands of UV–Visible spectrum have confirmed the final encapsulation into chitosan to develop the nano-hybrid with excellent yield. Further, EDX and ICP techniques have been employed to authenticate the successful synthesis of nano-hybrid. Antioxidant activity has been performed by DPPH scavenging method, in which different concentrations of L-POM@Sil-HN-Zn-CS have been employed. The result shows that the L-POM@Sil-HN-Zn-CS possesses excellent antioxidant properties. For the antibacterial assessment with gram negative (E. coli) and gram positive (B. subtilis) bacteria have been demonstrated. A comparative antibacterial study between L-POM@Sil-HN, L-POM@Sil-NH-Zn and L-POM@Sil-HN-Zn-CS have been reported. Metalation with zinc salt and its encapsulation into chitosan have enhanced the antibacterial activity of L-POM@Sil-HN-Zn-CS as compared to other components.

Footnotes

Acknowledgment

PS is heartily thankful to the Science and Engineering Research Board, New Delhi, India for Teachers Associateship for Research Excellence Grant (Project No. TAR/2021/000075). SA is grateful to Hon’ble Vice-chancellor Prof. Sanjay Singh, Dr Shakuntala Misra National Rehabilitation University Lucknow for University-Post Doctoral Fellowship (2209/1466 DSMNRU Research Cell 2020-21, dated 18 January 2022). We are also thankful to Punjab University and Motilal Nehru National Institute of Technology Allahabad for recording for analytical data.

ORCID iDs

Puspendra Singh

Pradip Kumar Dutta

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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Synthesis and characterization of Zn(II) metalated chitosan based nano-hybrid: Antioxidant and antibacterial activity

Abstract

Keywords

Introduction

Experimental section

Materials and characterization techniques

Synthesis of (K8[SiW11O39].13H2O) L-POM

Synthesis of L-POM@Sil by incorporation of [3-(2-Aminoethylamino)-propyltrimethoxysilane)] alkyl-silane into lacunary position of L-POM

Synthesis of L-POM@Sil-Hn by reaction between L-POM@Sil & 2-hydroxy-1-naphthaldehyde

Metalation by anhydrous zinc chloride of L-POM@Sil-Hn

Encapsulation of L-POM@Sil-Hn-Zn into Cs matrix for synthesis of nano-hybrid (L-POM@Sil-Hn-Zn-Cs)

Antioxidant activity of nano-hybrid (L-POM@Sil-Hn-Zn-Cs) using DPPH radical-scavenging assay

Antibacterial study

Results and discussion

FT-IR spectra

EDX and ICP analysis

UV-Vis-spectroscopy

SEM image of L-POM@Sil-Nh-Zn-Cs

Antioxidant activity by DPPH assay method

Biological application of synthesized material

Antibacterial screening

Conclusions

Footnotes

Acknowledgment

ORCID iDs

Funding

Declaration of conflicting interests

References

Synthesis of (K₈[SiW₁₁O₃₉].13H₂O) L-POM