High efficiency reduction of 4-nitrophenol on greenly synthesized gold nanoparticles decorated on chitosan matrix (CS-GLA/AuNPs)

Abstract

An inexpensive bioinspired green approach using Saussurea costus root extract was developed to fabricate CS-GLA/AuNP catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Chitosan beads (CS) supported Au nanoparticles have been modified to improve their mechanical and thermal stability, using Glutaraldehyde (GLA). The modified chitosan beads (CS-GLA) and CS-GLA/AuNPs were investigated using FTIR, SEM, and TEM techniques. The diameter of AuNPs decorated chitosan beads was found to be around 6.0 ± 3 nm. The X-ray diffraction technique confirmed the crystalline nature of (CS-GLA/AuNPs) and AuNPs decorated chitosan beads. The TGA analysis showed that loading of AuNPs on the chitosan matrix increases its thermal stability. The synthesized (CS-GLA/AuNPs) catalysts exhibited excellent activity in reducing 4-nitrophenol to 4-aminophenol, compared to pure AuNPs and chitosan beads (CS-GLA/AuNPs), enabling the conversion of 4-nitrophenol in 30 min with 1 mg of the catalyst. The kinetic study indicated that the reduction of 4-nitrophenol on the CS-GLA/AuNPs catalyst follows the pseudo-first-order model. Moreover, the CS-GLA/AuNPscatalyst could be recycled at least eight times without significant loss of its activity.

Introduction

Water pollution is one of the current problems that raises concern due to its negative impact on the environment and human health¹. The leading reason underlying such an occurrence is the excessive concentration of toxic and recalcitrant pollutants, especially 4-nitrophenol (4-NP), which has wide applications in pharmaceutical and dyeing industries for generating herbicides/insecticides/synthetic dyes/paints/corrosion inhibitors/PAH indicator, etc^2,3. Thus, reducing 4-nitrophenol to 4-aminophenol has become a critical matter, in which 4-aminophenol is considered a less toxic compound⁴. Over the past decades, different techniques have been used to remove the contamination of 4-nitrophenol from water. This includes adsorption⁵, oxidation⁶, biodegradation⁷, ion exchange⁸, photodegradation⁹, and catalytic reduction¹⁰. However, among these methods, reducing 4-nitrophenol to 4-aminophenol could endow an ideal process for organic compounds like 4-nitrophenol. One of the most advantages of reduction technique compared to other techniques is its cost, simplicity and high efficiency¹¹. Reduction reaction using sodium borohydride (NaBH₄) as a source of H₂ (reducing agent)for 4-nitrophenol reduction in combination with metal catalysts, such as Pd^12,13, Au^14,15, Cu¹⁶, Pt^17,18 and their immobilization on supports such as: polymers hybrid, microgels, silica, and graphene oxides¹⁹, has widely applied because itis simple, eco-friendly and the reduction reaction process is straightforward.

Nowadays, much attention has been focused on the design and synthesis of metal nanoparticles/polymer nanocomposites (inorganic-organic hybrid materials), owing to their structurally and compositionally tailorable features and large number of applications in various fields such as biomedical^20,21, sensor²² and catalytic applications^19,23,24.and furthermore, as sustainable catalysts for environmental remediation. Chitosan, a ubiquitous biopolymer, is a derivative gained by deacetylation of chitin from the insect’s exoskeletons and external cell walls²⁵. Although chitin and chitosan are both biocompatible, non-hazardous, and hydrating agents, chitosan shows a high preference compared to chitin, owing to its ease of solubility²⁶. Chitosan composites and derivatives have received wide attention as efficient biosorbents since their cost-effective effectiveness and their amino and hydroxyl functional groups content show significant potential for metal ions, dyes, and proteins remediation from various media^24,25,27. However, chitosan as a support polymer shows some limitations regarding its solubility in most dilute mineral and organic acid solutions. Hence, to overcome this limitation and also recover its mechanical and thermal stability, it can be modified chemically or physically by using cross-linking agents such as glutaraldehyde (GLA), ethylene glycol diglyceryl ether, or triphosphate²⁸. Among them, GLA is commonly used since it does not affect the chitosan adsorption capacity of chitosan.

Different methods have been successfully reported on the Au preparation in a chitosan matrix. However, while these approaches are varied, there is a thoughtful concern allied with them due to the employment of harsh, expensive, high-temperature, irradiation, instrumentally complicated processes, or using toxic chemical reducing agents or surfactants, and the metal leaching problem. Consequently, there is a need to develop environmentally safe and green strategies without using toxic chemicals. Interestingly, several biological systems such as bacteria, viruses, fungi, and plants have been employed for AuNPs preparation, among which, plant extract-mediated synthetic approaches^29–33 have several advantages as compared to the other mentioned routes, as they are more scalable, economic, simple, available, nontoxic, and eradicate the complication of cell culture techniques.

Saussurea costus is widely distributed in different regions of the world. It is also well known in many Arab countries as a folklore medicinal plant. The medicinal properties of Saussurea costus are related to numerous bioactive constituents such as polyphenols, alkaloids, triterpenes, flavonoids, proteins, carbohydrates and tannins³⁴. It also exhibits efficient anti-inflammatory, analgesic, anticancer, antiviral, and antioxidative properties. Consequently, because of the presence of these Phyto -molecules in root extract, they might be acting as reducing and capping agents during the synthesis of AuNPs. During synthesis, Phyto-molecules present in Saussurea costus root extract mainly polyphenols acan bind with Au⁺³ ions to form metal complexes and then reduced into Au⁰ by proving electrons through a redox reaction. Afterwards, other Phyto-molecules such as flavonoids, alkaloids, proteins, carbohydrates, etc., instantaneously capped Au zero-valent species to stabilize them. A similar mechanism of metal nanoparticles synthesis using plant extract was also reported by^35,36.

In this work, AuNPs-based catalyst decorated on chitosan cross-linked matrix (CS-GLA/AuNPs) was synthesized by a clean and green strategy using Saussurea costus root extract as reducing agent for the bio-reduction of Au⁺³ ions to Au⁰. The as-prepared nanocomposite was used to catalyze the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Furthermore, structural features as well as catalytic activity of the prepared nanocomposite have been investigated in more detail.

Materials and methods

Materials

HAuCl₄.3H₂O, chitosan flakes, glutaraldehyde 50% and 4-nitrophenol were purchased from Sigma Aldrich. All chemicals were used as analytical grade. All aqueous solutions were made using double-distilled water.

Preparation of saussurea costus root extract

Saussurea costus root plant used in these experiments was obtained from Alahsa local market. A combination of 1 g of Saussurea costus root powder and 10 ml of deionized water was used to prepare the extract. The mixture was boiled and stirred magnetically for 10 min. After the settlement of the extract Whatman filter paper was used for the isolation of the extract. The filtrate was kept in the refrigerator at 4 °C for extra experiments³⁶.

Preparation of chitosan cross-linked beads (CS-GLA)

To prepare Chitosan cross-linked beads, 2 g of chitosan flakes was dissolved in 50 ml of acetic acid solution (5%). The solution was vortexed and reserved overnight, and then this viscous solution was drop-wise into NaOH solution (0.5 M), with stirring to form spherical gel uniform beads. The formed gel beads were extensively washed with distilled water until neutrality (pH = 7) and air-dried before use. Finally, the resultant beads were suspended in 2.5% GLA solution with continuous stirring for 24 h at room temperature. Afterward, the freshly prepared chitosan-GLA beads were filtered, rinsed with water, air dried, and ground to size (< 200 μm) before use.

Preparing Au NPs decorated chitosan-GLA beads (CS-GLA/AuNPs)

A certain amount of synthesized chitosan-GLA beads (0.5 g) was mixed with 30 mL of an aqueous solution of HAuCl₄ (1mM) and kept under stirring for 1 h at room temperature. 20 ml of Saussurea costus root extract was added to chitosan-GLA beads (appearing dark red), and the mixture was left stirring overnight to ensure the completion of the bioreduction process. At the same time, the pH of the solution was kept at normal pH = 3.98. After the complete reduction, CS-GLA/AuNPs beads were filtered, rinsed with deionized water, dried, and then stored in a dry cabinet for further use.

Characterization methods

FT-IR analysis for all samples was done using (Cary 630) FT-IR spectrophotometer model at a spectral range of 400 to 4000 cm⁻¹.The crystallinity nature of AuNPs and chitosan materials was analyzed by X-ray diffraction on an XRD BrukerD8 Advance diffractometer at (λ = 1.5418 Å, kV = 40, mA = 40) in the range 10–90º 2θ scale. Emission scanning electron microscopy (FESEM) JSM-6701 F and transmission electron microscope (TEM, JEOL JEM-2100 instrument) were used to analyze the morphology of the synthesized (CS-GLA/AuNPs). Thermal analysis (TGA) was carried out on A Mettler Toledo TGA/SDTA851E instrument in a nitrogen atmosphere at a temperature range of 50–1000 °C with a heating rate of 10 °C min⁻¹. It was used to determine the thermal stability of the supported catalysts.

Catalytic reduction of 4-nitrophenol to 4-aminophenol

3 mL of 4-nitrophenol solution (20 mgL⁻¹) was used to examine the catalytic activity of (CS-GLA/AuNPs). The solution was mixed with 1 mg NaBH₄ and 10 mg of the prepared catalyst. The concentration of 4-nitrophenol was observed as a function of time using a UV-Vis spectrophotometer at 400 nm. 4-nitrophenol removal percentage was calculated with Eq. (1):

1

A_t and A₀ are the absorbance of 4—nitrophenolate ion at any time t and at time zero, respectively. Pseudo-first-order is used to study the kinetics of the reduction reaction. The linear form of this kinetic model is given in Eq. (2). The rate constants (k) were evaluated by plotting Ln (At) versus time. The slope value of the straight line represents the rate constant k (min − 1).

2

Results and discussion

Synthesis and structural characterization of (CS-GLA/AuNPs)

First, to improve the mechanical strength, chemical stability, and biocompatibility of the raw chitosan flakes, they were chemically modified using glutaraldehyde (GLA), as one of the most commonly used cross-linking agents³⁷. Due to the gel formation, the cross-linking process can expand the porous network of chitosan beads and increase their surface area, increasing the adsorption property of chitosan-GLA beads(CS-GLA). It can be considered a popular adsorbent that can coordinate with Au⁺³ ions since it has many hydroxyl functional groups and a Schiff-base structure³⁸. This was clearly observed when the chitosan-GLA (CS-GLA) beads were added to an aqueous solution of HAuCl₄; the color of the beads was gradually changed from orange to dark red, confirming the adsorption (loading) of Au⁺³ ions in the biopolymer matrix. Additionally, it works with dual rule as a sorbent to coordinate with Au (III) ions and as a dispersant to trap the initially-formed gold seeds and prevent them from further growth and aggregation in the synthesis process of AuNPs. Finally, after mixing these beads with an aqueous solution of Saussurea costus root extract, under stirring conditions at room temperature, the color of (CS-GLA) beads changed from dark red to black. This is due to the complete reduction of coordinated Au⁺³ ions to elemental Au and the assembly of AuNPs in the biopolymer matrix (Fig. 1). Saussurea costus root extract has different phytochemical constituents, such as polyphenols, alkaloids, triterpenes, flavonoids, and tannins, some of these constituents, such as polyphenols, could be responsible for the reduction and stabilization of AuNPs.Thus, no other protective or reducing agent was added to the synthesis system; these results demonstrated that Saussurea costus root extract acted as a reducing agent. Amina M et al.. and Najlaa S. Al-Radadi reported the same finding using Saussurea costus root extract to prepare MnO₂ and Pd nanoparticles^38–40.

Fig. 1.

Schematic diagram summarizes the synthesis of cross-link chitosan beads decorated by greenly synthesized AuNP prepared via using Saussurea costus root extract.

FTIR spectrophotometric

FT-IR spectroscopy can be used to examine the successful cross-linking modification of chitosan partially, the coordination of Au (III) ions into the biopolymer matrix, and phytochemical reduction. As shown in Fig. In (Fig. 2a), a strong broad vibration at around 3423 cm⁻¹ for chitosan flakes, which can be assigned to the O-H and N-H groups vibrations²⁶. In addition, the NH₂ group of primary amines shows a bending vibration peak at 1652 cm⁻¹. After the cross-linking process of chitosan with glutaraldehyde (Fig. 2b), it’s clear that the disappearance of the -NH₂ band is owing to its contribution to the cross-linking process. Furthermore, the absorption band intensity for beaks in the range of 2800–3000 cm^−1, which corresponded to the -CH₂ and -CH₃ groups of chitosan and GLA, was enlarged. The appearance of a new peak at 1663 cm⁻¹ for Schiff base (C = N) confirmed the effective cross-linking process. Also, the nonappearance of any peak at range 1500–1700 cm⁻¹ proves the reaction of the aldehyde group and the completion of the cross-linking process⁴¹.

Fig. 2.

FTIR of (a) Chitosan flakes, (b) CS-GLA, and (c) (CS-GLA/AuNPs).

The FTIR spectrum of (CS-GLA/AuNPs) beads is provided in (Fig. 2c). It was clear from the figure that the C = N was shifted from 1662 to 1637 cm^−1, which is revealing of the metal-ligand interaction. Moreover, the existence of a new peak at 603 cm⁻¹ indicates the bonding between gold nanoparticles and chitosan-GLA matrix (CS-GLA)⁴². Additionally, the intensity of the O-H stretching band from the hydroxyl groups of chitosan rings at 3445 cm⁻¹ decreased when Au was loaded to the chitosan matrix, again, signifying the chelation between Au with imine and hydroxyl groups of modified chitosan (CS-GLA).

Elemental analysis

The modification of chitosan flakes was studied using elemental analysis, as shown in Table 1. The nitrogen content was obviously decreased from 7.80 to 6.95% (Table 1, entries 1 and 2), whereas the C and H contents were increased. This is owing to the successful cross-linking process using (GLA). The amount of Au in chitosan-GLA/Au (III) and CS-GLA/AuNPs beads was measured using atomic absorption spectroscopy (Table, entries 3 and 4). It displayed that most of the Au (III) ions in the chitosan matrix have been reduced to Au (0) with Saussurea costus root extract.

Table 1.

Chitosan flakes and modified Chitosan with GLA (CS-GLA) and CS-GLA/AuNPs elemental analysis.

Entry	Samples	Elements
		C%	%H	%N	Au
					% Atom	mmol g−1
1	Chitosan flakes	40.91	7.54	7.80	-	-
2	Chitosan-GLA beads	43.58	7.81	6.95	-	-
3	Chitosan-GLA/Au3+				2.033	0.103
4	Chitosan-GLA/AuNPs	51.63	8.83	5.50	2.024	0.102

XRD characterization

The XRD analysis was used to confirm the crystalline nature of AuNPs in the CS-GLA/AuNPs matrix under the phytochemical reduction of Au (III) ions using Saussurea costus root extract. Figure 3a displays the XRD patterns of the chitosan flakes, which show the presence of two broad peaks observed in the 2θ range of 12◦ to 26◦, which are assigned to the crystallinity of original chitosan flakes⁴³. These peaks are still well-maintained even after the cross-linking modification, as shown in Fig. 3b. AuNPs showed distinguishing peaks at 38.2◦, 44.6◦, and 64.6◦ representing face-centered cubic lattice planes of Au NPs in chitosan matrix (JCPDS card No. 4-784)⁴⁴ (Fig. 3c), indicating that the crystalline nature of Au nanoparticles does not change even after embedment of AuNPs in biopolymer matrix. The AuNPs’ average size was assessed using the Scherrer equation to be ~ 8.8 nm.

Fig. 3.

XRD patterns of (a) chitosan flakes, (b) (CS-GLA), and (c) CS-GLA/AuNPs beads.

FESEM imaging

Figure 4a shows the (FESEM) images of the prepared CS-GLA/AuNPs sample. As is clear, only a limited number of the spherical particles of Au were spotted on the chitosan surface (highlighted by yellow arrows in Fig. 4b and in contrast, the AuNPs are believed to be mostly captured into the chitosan matrix owing to the modification by the cross-linking agent. An elemental analysis of the samples was implemented by energy-dispersive X-ray.

Fig. 4.

FESEM image and EDX of (a, c) CS-GLA (b, d) CS-GLA/AuNPs beads.

spectroscopy (EDS) as shown in Fig. 4c and d, no appearance of Au nanoparticles in chitosan-GLA beads, while in the (CS-GLA/AuNPs), the content of Au is about 31.49 wt%, 48.65 wt% carbon, and 19.86 wt% oxygen. This clearly demonstrated that Saussurea costus root extract acted as a reducing agent and chitosan-GLA beads with dual roles to coordinate with Au(III) ions and stabilization agent to prevent further growth and aggregation of AuNPs.

The elemental mappings of CS-GLA/AuNPs beads (Fig. 5) show the sites where AuNPs were distributed homogeneously on or within the CS-GLA surface. It provides qualitative information on the presence of Au along with carbon and oxygen atoms in the chitosan matrix. Since no other reducing or stabilization agents were added during the synthesis process, the results thus clearly demonstrated that Saussurea costus root extract acted as a reducing and stabilization agent.

Fig. 5.

Elemental mapping of chitosan-GLA/AuNPs bead.

TEM imaging

To confirm the successful assembly of AuNPs in the chitosan matrix, the morphology of (CS-GLA/AuNPs) was analyzed using transmission electron microscopy imaging (TEM) (Fig. 6a). The image in (Fig. 6b) displays well-separated spherical AuNPs with a mean size of 6.0 ± 3 nm, as shown in the histogram box, while the lattice spacings of AuNPs are 0.236 nm, which match well the d-spacings of the Au (111). These data were in agreement with XRD and FESEM results. A control test was conducted to evaluate the possible formation of AuNPs using Saussurea costus root extract without chitosan cross-linked beads (Fig. 6c). The images showed AuNPs with uncontrolled structure and size. The effect of controlling of chitosan on AuNPs morphology and aggregation is probably due to the steric stabilization factor, in which AuNPs are protected from aggregation by surrounding them in a protective layer (Fig. 6d). This layer prevents the direct contact between particles, steric hinderance effectively counteracts the attractive force, and promotes^44,45.

Fig. 6.

TEM images of (a) chitosan-GLA/AuNPs nanocomposite, and (b) the lattice fringes of the particles with measured lattice d-spacing. (c) AuNPs image with Saussurea costus root extract without using cross-linked chitosan. (d) TEM image of AuNPs trapped in the polymeric matrix of chitosan.

TGA analysis

The as-prepared samples’ thermal stabilities were studied. The TGA curves of chitosan flakes, chitosan-GLA(CS-GLA), and (CS-GLA/AuNPs) beads are shown in Fig. 7. In chitosan flakes, Fig. 7a shows the weight loss at two steps, one for the loss of moisture (8.3%) at ~ 100° C, while the second weight loss (71.3%) occurred at 310° C, due to decomposition of polymer chains. Fi. 7b also shows two weight loss steps for chitosan-GLA beads; first at 200 °C (33.4%), which may be due to the degradation of glutaraldehyde chains, and the second step at 380 °C (31.8%), which is presumably attributed to the thermal degradation path of the chitosan chains. For CS-GLA/AuNPs Fig. 7c, there are two peaks of weight loss: the first one at 230° C, with weight loss of 35.1%, which could be attributed to the biomolecules of Saussurea costus root extract capped on AuNPs, and the second peak at 430° C, with weight loss of 31.8%. As seen in the TGA curves, the decomposed rate of unmodified chitosan flakes was significantly higher than that of cross-linked and AuNPs-loaded beads, and among these three, the mass loss of (CS-GLA/AuNPs) beads was the lowest. In addition to a strengthening effect of the cross-linking process on the thermal stability of (CS-GLA/AuNPs) beads, the presence of AuNPs in the chitosan matrix could increase its thermal stability⁴⁶. One can assume that the higher surface energy of AuNPs, combined with oxygen in water molecules, can increase the opportunity for the formation of hydrogen bonding. The thermal stability of the obtained (CS-GLA/AuNPs) beads can also be related to the gold nanoparticles’ capacity to resist thermal decomposition. This result is in line with Mohamed Hosny et al.⁴⁷. The results verified that the modification of chitosan with GLA increase the thermal stability when it was doped with AuNPs.

Fig. 7.

TGA thermograms of (a) chitosan flakes, (b) chitosan-GLA beads, and (c) (CS-GLA/AuNPs).

Activity of catalysts

The catalytic activity of CS-GLA/AuNPs beads was also studied for the reduction of 4-nitrophenol to 4-aminophenol. The yellow color of 4-nitrophenol showed characteristic absorption peak at 318 nm as shown in Fig. 8a. After the addition of NaBH4 to 4-nitrophenol in absence of a catalyst, the color of solution immediately Changed to dark yellow with shifting in the peak from 318 to 400 nm, presumably due to the 4-nitrophenolate anion formation, that responsible from the bright yellow color of the mixture. When the 10 mg of CS-GLA/AuNPs beads catalyst was added into the quartz cell with 4-nitrophenol solution (20 ppm) and NaBH4(1% W/V), a gradual decrease in the adsorption intensity of 4-nitrophenol at 400 nm (Fig. 8b) was observed with the presence of a new peak at 280 nm. The completion of the hydrogenation reaction was reached almost within 30 min at room temperature, and the solution color of changed from bright yellow to colorless.

Fig. 8.

Time-dependent UV-vis spectral Changes of the 4-nitrophenol solution(a) After adding NaBH₄ without catalyst, (b) catalyzed by :) CS-GLA/AuNPs, (c) AuNPs, and (d) CS-GLA.

Figure 8 (c &d) shows the time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol using Au nanoparticles and cross-linked chitosan beads (CS-GLA). The difference between CS-GLA/AuNPs beads, Au nanoparticles, and (CS-GLA) is the catalytic efficiency and the time-dependent total reduction of 4-nitrophenol to 4-aminophenol. CS-GLA/AuNPs beads showed the fastest rate of reduction, while the cross-linked chitosan beads sample showed very slow reduction of the peak. The high efficiency of CS-GLA/AuNPs beads can be explained by the good loading and distribution of Au nanoparticles on the cross-linked chitosan beads surface, leading to effective accessibility of the reactants to the active sites, resulting in high catalytic performance.

The reduction percentage of 4-nitrophenol as a function of time for three samples is shown in Fig. 9a. As a function of time, a 96% reduction percentage of 4-nitrophenol using CS-GLA/AuNPs catalyst is achieved in a very short time of 9 min, compared to 85% and 5% achieved by AuNPs and CS-GLA, respectively. The whole reduction was observed in 30 min for both samples, CS-GLA/AuNPs and AuNPs, while the CS-GLA sample achieved only a 45% reduction after 30 min. The performance of the nanocomposites prepared by loading Au nanoparticles on the chitosan matrix (CS-GLA/AuNPs) showed higher activity than Au nanoparticles and CS-GLA. This is mostly related to improving the distribution of Au nanoparticles, which leads to an increase the surface area and the number of active sites available for the reduction process. CS-GLA/AuNPs catalyst exhibited higher catalytic activity compared to other Au-based catalysts used for the reduction of 4-nitrophenol, such as Au-SiO₂(OPL)⁴⁸, Au-RS-SR-NH-SiO₂-Fe₃O₄⁴⁹, and Fe₃O₄-Au.(Table 2), since the CS-GLA/AuNPs catalyst has the advantages of a simple, green, and economically synthetic process under mild conditions.

Fig. 9.

(a) reduction of p-nitrophenol catalytic efficiency using CS-GLA/AuNPs, Au NPs and CS-GLA (b) pseudo-first-order kinetics reduction of p-nitrophenol plot using CS-GLA/AuNPs, Au NPs and CS-GLA. The reduction conditions were: 3mL of 20 ppm 4-NP solution, 1 mg NaBH₄, and 10 mg of the catalyst.

Table 2.

Comparison of the catalytic activity of previously reported Au based nanocatalysts for the reduction of 4-nitrphenol.

Catalyst	4-Nitrophenol concentration mg/L	Catalyst dose (mg)	Time (min)	Ref
CS-GLA/AuNPs	20	1	30	This work
Au-SiO2(OPL)	5	2	45	48
Au-RS-SR-NH-SiO2-Fe3O4	5	2	60	49
Au–Ce nanorods	14	0.7	30	50
Fe3O4-Au	5	1	40	51
Au-ECCG-CF	1	1	25	52
g-C3N4/Au	2.5	10	120	53

Pseudo-first-order kinetic model applied to fit the data as shown in the Fig.9b. The results show that all the samples fit well with the pseudo-first-order kinetic model with R² value as follows: 0.9194 for CS-GLA/AuNPs, 0.9447for AuNPs, and 0.9000 for CS-GLA.

The reduction conditions were: 3mL of 20 ppm 4-NP solution, 1 mg NaBH4, and 10 mg of the catalyst.

Catalyst recyclability

From the viewpoints of environmental friendliness and cost-effectiveness, the reusability of CS-GLA/AuNPs was studied in the reduction of 4-nitrophenol (Fig. 10). The as-prepared catalyst shows catalytic activity, which can be easily recycled. After completion of the first run, the catalyst was removed by filtration, washed with water, and dried in a vacuum oven at 100 ◦C. The results show that the catalyst can be recycled and reused eight times with stable catalytic activity. It can be suggested that the CS-GLA/AuNPs were not deactivated or poisoned during the catalytic process. These results indicate that CS-GLA/AuNPs are stable and could be used as an excellent catalyst for 4-nitrophenol reduction by NaBH₄.

Fig. 10.

The reusability of CS-GLA/AuNPs catalyst.

Conclusion

A green and cost-effective method has been utilized to assemble stable AuNPs into a chitosan matrix with the aid of Saussurea costus root extract. Modification of chitosan to cross-linking chitosan using glutaraldehyde increases its capability for better loading of Au (III) ions to be reduced to Au (0) by Saussurea costus root extract. The biogenic AuNPs are well dispersed with spherical morphology and particle size (< 10 nm). The as-prepared CS-GLA/AuNPs catalyst exhibits high performance of catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4 compared to pure AuNPs and CS-GLA, since it needs only 9 min to reduce 96% of 4-nitrophenol, while 85% and 5% achieved by AuNPs and CS-GLA, respectively. Moreover, the CS-GLA/AuNPs catalyst showed high reusability up to four cycles of catalytic reaction without significant loss of its catalytic activity. The results demonstrate that CS-GLA/AuNPs can be a promising green catalyst with enormous potential for the reduction of 4-nitrophenol or the remediation of other organic pollutants after more intensive study to optimize both the synthesis and the operation conditions.

Author contributions

Conceptualization, A.T.; methodology, A.T.; data curation, A.T. and N.S.A.; writing—original draftpreparation, A.T.; editing, A.T and N.S.A.; visualization, A.T.; supervision, A.T. Project administration, A.T and N.S.A. Fundingacquisition: N.S.A. All the authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Grant No. KFU253243].

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.