A facile method for honey mediated bio-synthesis of nickel nanoparticles and its characterisation

Honey of Phondaghat brand was bought from a local medical shop in Mumbai, India. Nickel chloride hexahydrate (NiCl₂ · 6H₂O Molecular Weight, 237.69 gm), sulfuric acid and sodium hydroxide were purchased from S. D. Fine-Chem Ltd, Mumbai, India.

Using distilled water, 0.01 M Nickel chloride solution of 100 ml was taken in the flask. In this salt solution, honey was added for two hours at 70 rpm under continuous stirring using shaker-bath instrument of Rossari Labtech, Mumbai, India. The synthesis was performed according to the design of experiment 6.0.10 CCD model to investigate the influence of three parameters temperature, the quantity of honey and pH. 0.1 N of sodium hydroxide and 0.1 N of sulphuric acid solutions were used to maintain the pH. During the synthesis process, change in color was observed from sea green to colloidal dull yellow confirming the formation of nanoparticles. The solution was filtered after complete reduction of the nickel chloride and stored for characterisation.

RSM was used as per central composite design (CCD) for optimization of parameters to evaluate the influence of reaction parameters such as reducing agent concentration, temperature, and pH on absorbance of the solution after each experimental run as a response which is shown in table 1.

UV–Visible spectroscopy analysis of NiNPs produced was carried out periodically at fixed time interval on UV–vis spectrophotometer (UV-1800 ENG 240 V, Shimadzu, Japan). Nanoparticle size analyzer (SALD 7500 nano, Shimadzu, Japan) was used to assess the particle size distribution and particle size of nanoparticles. Morphology of the nanoparticle surface was studied with the help of TEM (Phillips TEM-200 Supertwin STEM, accelerating voltage-200 kV, resolution-0.23 nm). Crystallography of the nanoparticles was achieved using x-ray diffractometer (Shimadzu XRD-6100, Japan) with CuKα radiation from 10° to 80°. Chemical group on NiNPs was identified using FTIR (FTIR 8400S Shimadzu, Japan) and analysis of elements was performed in the Na-U channel using EDAX (EDX-720, Shimadzu, Japan).

From table 1, results showed considerable variation in the NiNPs biosynthesis. Treatment run number 3 gave optimized conditions of 100 °C temperature, pH 3 and 7.5 g/100 mL concentration of reducing agent for maximum absorbance (0.6469) among all the runs.

ANOVA results are shown in table 2. The significance and interaction strength of each parameter were checked using P-values. The P-value of NiNPs synthesis was 0.0000176 indicating statistical significance of the obtained model. It can be seen from the probability values of the coefficient that out of the three parameters analyzed, C (reducing agent concentration) showed maximum influence between the three parameters, signifying that 99% of the model is dependent on this parameter. On the other hand, linear and quadratic outcomes of temperature (A) and pH (B) did not show a significant effect on NiNPs biosynthesis. However, the 'lack of fit' value was 0.1402 which proved that 'lack of fit' was not significant.

The small value of R² (coefficient of determination) showed less effect of the dependent variables in the model. The R² value of the model was 0.9527, showing that the regression model was in consonant with the true behavior of the system. The value of the adjusted determination coefficient (Adj. R² = 0.9102) was more than 0.9, justifying the high significance of the model. Thus, the proposed model was considered to be reasonable for the present setup [30].

Polynomial equation of second-order was suggested to compute the optimum values of the input parameters for determining maximum NiNPs biosynthesis as per equation (1).

$$\begin{eqnarray}&&\begin{array}{l}{\rm{Absorbance}}=0.27+0.021A-6.350\times {10}^{-3}B+0.20C\\ \,\,\,\,\,\,+\,0.11\,{A}^{2}-\,0.029{B}^{2}-0.016{C}^{2}\\ \,\,\,\,\,\,-\,0.014AB+0.026AC-0.017BC\end{array}\end{eqnarray} \tag{ 1 }$$

The interaction effects are shown by taking the graph of the response surface curves (figure 1) where the two other parameters were varied while one of the parameters was kept constant at an optimum value. From graphs, we can say that the higher yield of the NiNPs was directly proportional to the reaction temperature and concentration of reducing agent but inversely proportional to the pH (figure 1).

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In addition, verification of the optimized study was done practically to compare with the predicted data. The experimental absorbance value was 0.6469 at λ_max = 333 nm which was in agreement with predicted value (0.624976) as per the model. Therefore, the optimized values of the process parameters for NiNPs biosynthesis using honey were 100 °C temperature, pH 3.7 and 7.5 g of reducing agent.

The absorption spectra of NiNPs for different time scales are shown in figure 2. It is observed that absorbance increased as the reaction time progressed leading to a higher concentration of NiNPs. The peak around 320–350 nm indicated the presence of NiNPs [31–33].

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From figure 3, change in color was observed from sea green to colloidal dull yellow for the biosynthesis of NiNPs at optimum reaction conditions. Due to a phenomenon known as plasmon absorbance, the distinctive colors of colloidal nickel were observed [34]. However, such color changes might also occur due to the variation in the nature, size, and shape of the metal nanoparticles [35].

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The TEM image (figure 4(A)) shows that the particles were predominantly ellipsoidal in shape. Agglomeration of nanoparticles was observed which could be due to possible sedimentation after a certain period of time. It can also be noted from the images that the particles were somewhat coated with a thin layer, which could be the capping components from honey [36].

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In the selected area electron diffraction pattern (SAED) ring-like diffraction pattern was observed which indicated the crystalline nature of the synthesised nanoparticles as shown in figure 4(B). The diffraction rings correspond to the different crystallographic planes of elemental nickel [37].

TEM analysis gave the size of nanoparticles in the range of 12 to 45 nm (average diameter 28 nm) (figure 4(A)). Same particle size range was obtained using nanoparticle size analyzer with 31 nm being median particle size as shown in figure 5.

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The XRD measurement of NiNPs (figure 6) showed four distinct diffraction peaks at 37.6378°, 43.8638°, 64.2226° and 77.3990° representing (010), (111), (200) and (220) Miller indices respectively which are catalogued in Joint Committee on Powder Diffraction Standards (JCPDS), file No. 04–0850 [31, 38].

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The observed peaks revealed that the obtained particles are face-centered cubic [23] and showed crystallinity of 20.23% which is supported by the sharpness of the major peak (111). The peaks get shifted towards lower diffraction angle (2θ) by a value of 0.6362 as against a bulk value (2θ = 44.5; JCPDS 04-0850) indicating the increase in lattice constant [39]. The 32 nm particle size of NiNPs was estimated using Debye–Scherrer's formula [40, 41]. The crystal size calculated by different methods is tabulated in table 3.

EDX spectroscopy analysis gave a sharp signal in the nickel region thus the formation of NiNPs was confirmed (figure 7). Due to surface plasmon resonance, the optical absorption peak at 7.5 keV was shown by metallic nickel nanocrystals [42].

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Figure 8 shows, the FTIR spectrum of the colloidal NiNPs and honey in order to establish possible reducing as well as capping agents.

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From figure 8, the broad band observed around 3300 cm⁻¹ was attributable to the O-H stretching of the honey components. The peaks such as small band located at 2932 cm⁻¹ (C–H stretch of carboxylic acids and amine stretching band present in free amino acids), 1043 cm⁻¹ (C–O stretch occurring in the C–OH group) and 1254 cm⁻¹ (C–C stretch of carbohydrates). Similarly, the absorption band noted at 1027–1045 cm⁻¹ can be related to the C-O stretching of the glucose or sucrose molecules. Thus, the FTIR analysis indicates the presence of glucose, fructose and other honey components in the samples of NiNPs. It is well known that proteins are capable of binding to nanoparticles either through carboxylate ions or free amine groups of the amino acid residues. But it is possible that other bioorganic compounds that exist in honey can cause the reduction of nickel ions and its stabilization through surface capping [36].