Heterogeneous catalysis in 4-nitrophenol degradation and antioxidant activities of silver nanoparticles embedded in Tapioca starch

2.1 Materials

Tapioca starch was purchased from Sigma-Aldrich. (AgNO₃) and NaOH were supplied from Sigma–Aldrich Co., USA (analytical grade). Deionized water was used for all dilutions and characterizations.

2.2 Methods

At the first, (1.0 g) of Tapioca starch was mixed with 0.35 g of NaOH platelets together using pestle, then added to 100 mL of distilled water and heated at 40 °C for 20 min. Then, certain amount of (AgNO₃) is dissolved in 5 ml of water along with 3 ml isopropyl alcohol to volatile easily during the synthesis of Ag-NPs. AgNO₃ solution was added dropwise to alkaline treated starch under continuous stirring. The reaction temperature was set to 80 °C and allowed to react for several hours. At the end of reaction, the system colour turns to dark brown which confirms the synthesis of Ag-NPs.

2.3 Characterization of silver nanoparticles (Ag-NPs)

UV–vis absorption measurements were done using a (Shimadzu UV 2401pc) spectrophotometer in the 300–700 nm wavelength range as the first indication for Ag-NPs. Fourier infrared (FT-IR) spectroscopy of starch and Ag-NPs is recorded with Nicolet-Nexus 670. The XRD measurements were conducted using an X-ray diffractometer (Empyrean, PANalytical, Netherlands). The morphological features of Ag-NPs were measured by TEM (JEOL JEM-3010 electron microscope).

2.4 Catalytic activity examination

In starch-Ag-NPs (S-Ag-NPs)-catalysed (4-NP) hydrogenation reaction, 10.0 wt.% of (S-Ag-NPs) was added to 5 mL of a solution containing 60 mg of (4-NP), 6 mg of catalyst and 180 mg of NaBH₄. The sonication was used to mixture and in every 3 min, monitored by UV spectrophotometer in the wavelength range of 250–600 nm. The characteristic maximum absorbance of (4-NP) at 401 nm was used as indicator the (4-NP) remaining, and the absorbance at 300 nm was utilized to identify the (4-AP).

2.5 DPPH radical scavenging assay

The DPPH assay was used to show the free radical scavenging activity of the (S-Ag-NPs) and monitored according to the method explained by Sharifi et al. (2013) and Sharifi et al. (2013).

At the first, 0.002% of DPPH solution was prepared in methanol, and 3 ml of this solution was added to (S-Ag-NPs) samples at different concentrations (50, 100, 150, 200 and 250 μg/ml) and vortexed thoroughly. The mixtures were kept in dark condition at room temperature for 30 min. As a positive control (standard) and blank, ascorbic acid and methanol were used respectively. The experiments were performed in triplicate.

The Inhibition ability of free radical DPPH in one per cent was evaluated based on the following:

The radical scavenging ability of antioxidants is indicated by the decrease in absorbance of DPPH mixed reaction mixture on addition of the antioxidant (Rajan et al., 2015).

3.1 UV-visible spectra and TEM studies

Surface Plasmon Resonance (SPR) is a characteristic absorption band of metal nanoparticles in the UV–visible region, which is related to conduction electrons oscillation induced by the interacting electromagnetic field (Raghunandan et al., 2010). Fig. 2 shows the UV–visible spectra of Ag-NPs. According to this spectrum, intense absorption peak at 411 nm indicated the production of Ag-NPs. Results show that, there is no absorption peak for alkaline starch in the range from 410 to 420.

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Figure 2 UV–vis spectra of the (S-Ag-NPs).

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TEM has been applied for morphological study of nanoparticles surface. The TEM images Fig. 3(A) and histogram in Fig. 3(B) are used for clear investigation of the size distribution and shape of the synthesized (S-Ag-NPs). According to Fig. 3, Ag-NPs are successfully synthesized in spherical shape with good distribution and size of 11 ± 4.75 nm. Alkaline treated starch has two main actions towards Ag⁺, first reducing the silver cations to Ag⁰ and then stabilization of Ag-NPs via capping with hydroxyl groups of starch molecules during the fabrication process (Cano et al., 2016; Hebeish et al., 2016).

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Figure 3 TEM micrographs of the Ag-NPs (A), and histogram illustrating the particle (B).

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3.2 XRD patterns and FT-IR spectra of Ag-NPs

According to Fig. 4(A), the broad peaks at 1351 cm⁻¹ and 1523 cm⁻¹ correspond to the symmetric and asymmetric C-O stretching vibrations of carboxylate group respectively (Kizil et al., 2002). The absorption bands at 833 and 563 cm⁻¹ are attributed to the whole glucose ring stretching vibrations.

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Figure 4 (A) FTIR spectrum of starch and (S-Ag-NPs), and (B) X-ray diffraction patterns of synthesized (S-Ag-NPs).

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Another characteristic bands at around 1050 cm⁻¹ in the fingerprint region are due to the C–O–C ether–stretching vibration in glucose bonds (Hebeish et al., 2016; Rao et al., 2010). Moreover, two bands observed at 3100–3200 cm⁻¹ for C–H stretching and 1640–1660 cm⁻¹ for O–H stretching are attributed to the tightly bound water presented in the starch molecule. Starch spectrum shows also a characteristic vibration bands at 3200 cm⁻¹ (O–H stretching) (Hebeish et al., 2016). The FTIR spectrum of (S-Ag-NPs) exhibits a similar pattern to that of starch. However, a shift in frequencies is detected for the signals associated with the –OH functional group, noticeably at 3290 cm⁻¹ (OH stretching) and 1050 cm⁻¹ to higher 1076 cm⁻¹ evidently indicating the interaction of –OH groups with silver nanoparticles (Raghavendra et al., 2016; Mandal et al., 2015).

Fig. 4(B) shows the XRD patterns of synthesized (S-Ag-NPs). The XRD peaks in the wide angle range of 2θ (30° < 2θ < 85°) ascertained that the peaks in 38.63°, 44.26°, 64.75°, 77.44° and 82.20° can be attributed to the 111, 200, 220, 311 and 222 crystalline structures of the face cantered cubic (fcc) synthesized Ag nano-crystal, respectively (Ag XRD Ref. No. 00-087-0719) (Shameli et al., 2012). The average particle size of Ag-NPs can be calculated via Debye–Scherrer equation: $$n=\frac{K\text{ƛ}}{\beta \mathit{Cos}\theta }$$ where K (shape factor) is the Scherrer constant with value from 0.9 to 1, and λ is the X-ray wavelength (1.5418 Å), β_1/2 is the width of the XRD peak at half height and θ is the Bragg angle. From this equation the average crystallite size of silver nanoparticles is found to be around 11 nm, which are also in line with the observation of the TEM results discussed later. The peaks revealed that the main crystalline phase was silver and there were no other impurity phases found in the XRD patterns. Therefore, this gives good evidence for the presence of silver nanoparticles in the (S-Ag-NPs).

3.3 Antioxidant activity

An antioxidant scavenging activity widely is assessed by DPPH as a stable free radical. In 1995, Williams et al., described a method for measurement of DPPH scavenging activity, with slight modifications. In this method, with the presence of a hydrogen donating antioxidant and methanol solution, DPPH is reduced due to the formation of the non-radical form of DPPH (Kharat and Mendhulkar, 2016). The antioxidant activity of S-Ag-NPs was evaluated using DPPH scavenging assay. The results obtained are summarized in Table 1.

The finding shows significant shift in the DPPH radical scavenging ability for studied samples. The recorded scavenging ability for the lowest concentration of the synthesized S-Ag-NP (50 μg/ml) was 17.29 ± 0.06 and this scavenging ability was increased to 78.60 ± 0.05 (Table 1), when concentration was increased to 250 μg/ml.

3.4 Catalytic activity of S-Ag-NPs

Metal nanoparticles have high surface area to volume ratio and make them good catalysts compared to bulk materials (Rajan et al., 2015).

The catalytic activities of the obtained S-Ag-NPs were evaluated for the aqueous reduction of (4-NP) to (4-AP) with Sodium borohydride. This method has been used as an effective technique for the removal of (4-NP).

The UV–visible spectra for the catalytic reduction of (4-NP) by NaBH₄ in the presence of S-Ag-NPs obtained from 5 ml of extract are shown in Fig. 5. According to our findings, maximum adsorption peak is observed by addition of the NaBH₄ into an aqueous solution of (4-NP). Moreover the UV–visible spectrum shows a shifting to the higher wavelength (red shift) from 321 to 401 nm because of the (4-NP) formation under alkaline conditions (Fig. 5A). This peak at 401 nm remained unaltered in the absence of S-Ag-NPs. The maximum absorption at 401 nm did not change over time even after the addition of superfluous NaBH₄ solution confirming that the reduction did not proceed by aqueous NaBH₄ solution (Dong et al., 2014).

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Figure 5 (A) UV–vis spectra of (4-NP) and alkaline (4-NP) by NaBH₄, and (B) UV–vis spectra for the reduction analysis of (4-NP).

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S-Ag-NPs act as the hydrogen donor, and a sample was withdrawn every 3 min to monitor (Fig. 5B). With the addition of prepared S-Ag-NPs to the reaction mixture, the peak at 401 nm disappeared with the appearance of new peak at 321 nm and the light yellow colour of solution is changed into a deep yellow showing the presence of (4-NP) ion indicating the formation of (4-AP) (Bindhu and Umadevi, 2015; Chen et al., 2006). As expected, no target product could be found in the absence of catalyst after 60 min.

This results confirmed the catalytic activity of S-Ag-NPs in the short induction period (3 min in water); the completion reaction continued within 15 min. Ag NPs supported can catalyse the reaction by facilitating electron relay from BH₄- to (4-NP) (Rostami-Vartooni et al., 2016). Some factors have influence on hydrogenation of nitro compounds such as electron transfer into nitro compound and proton accessibility (Khandanlou et al., 2016).

Briefly, catalytic reduction included essential steps: on the surface of catalyst, BH₄⁻ reacts with S-Ag-NPs to form the metal hydride, then discharging of electrons from borohydride ions via the metal to the acceptor. Hydrogen ions are required to complete the reduction reaction which was provided by water as a polar protic solvent (Gopal and Vydehi, 2013). Obviously, in catalysis phenomena, adsorption has crucial role. In this work, the S-Ag-NPs catalyst had adsorption sites for borohydride ions and nitro aromatic compounds. Moreover, the electrons transfer from the borohydride ion donor to the nitro compound acceptor facilitated by catalyst.

Several parameters such as reaction conditions, size of nanoparticles and the number of surface silver atoms are crucial for silver nanoparticles-mediated catalysis (Narayanan and Sakthivel, 2011).