Ultrafast synthesis of gold nanoparticles on cellulose nanocrystals via microwave irradiation and their dyes-degradation catalytic activity

1. Introduction

Cellulose nanocrystal (CNC) has been isolated from various plant-derived natural cellulosic resources such as higher plants wood pulp, bamboo, and cotton by applying acid hydrolysis treatment to amorphous region [1]. The CNC emerged as a top-class renewable carbohydrate biopolymer owing to its unprecedented prominences, such as eco-friendliness, biodegradability, biocompatibility, sustainability, low density, non-toxicity, controllable size, unique morphology, and versatile surface chemistry [2]. Apart from this, the surface of CNC is abundant in electrons owing to hydroxyl and sulfate ester functional groups that serve good colloidal stability in water. These advantageous features make CNC be a suitable supporting material for the synthesis of metal nanoparticles (MNPs) [3]. Nevertheless, the usage of CNC for the synthesis of AuNPs has been very limited. Very few investigations have been reported regarding the synthesis of AuNPs using CNC as both a reducing and a stabilizing agent. Many studies used CNC only as a stabilizing agent and exploited other hazardous chemicals as a reducing agents for the reduction of gold ion [[4], [5], [6]]. Wu et al. synthesized AuNPs by the hydrothermal process using CNC as both a reducing and a stabilizing agent, but it took a prolonged time (more than 10 h) for the synthesis [7].

Recently, MNPs deposited on CNC surface have been extensively used as a recyclable catalyst in various industrial processes. Several characteristics of CNC such as nanometric dispersity, high surface area, and good thermal stability make the nanoparticles be a suitable catalyst carrier. Among the different MNPs, AuNPs are prepared not only for their application in photoelectronics, sensors, medical and pharmaceutical research [8,9] but also for their use as a promising catalyst for many reactions owing to their unique size-dependent physicochemical properties [10,11]. However, AuNPs tends to self-aggregate due to their large active surface energy, which causes a huge drop in catalytic activity [12]. Hence, many polymers, mesoporous matrices, and supporting material have been used with AuNPs to prevent the aggregation [13,14]. Considering the increase in awareness of sustainability initiatives in green synthesis of MNPs, here we are proposing CNC as one of excellent candidates for the green synthesis of nanohybrid (AuNPs/CNC) catalyst.

Nowadays, every manufacturing industry such as paper, textile, leather, cosmetics, foodstuff, and pharmaceutical generate industrial effluents which contain a lot of harmful organic dyes [15]. These dye effluents not only cause significant toxicity to aquatic life and human beings but also induces environmental pollution [16]. Among these, the azo dye is considered as a major source for rapid ecological pollution as it has non-biodegradability and high carcinogenicity [17,18]. In addition, the prolonged intake of other harmful dyes like Congo red, Allura red, Rhodamine B, and Amaranth can result in respiratory malfunction, eye and skin irritation, allergy, tumor formation, and congenital disability in human being [19]. Moreover, all these dyes are extensively soluble in water, so its removal process using various conventional techniques (physical and chemical treatment) has become a challenging issue. In recent years, nanohybrid heterogeneous catalysts have been widely used for the degradation of dyes into non-toxic or less toxic small molecules [[20], [21], [22]]. In this context, AuNPs/CNC film is believed to be a very promising nanocatalyst for the degradation of dyes.

The main focus of our study is to synthesize AuNPs using CNC as both a reducing and a supporting agent. Here, microwave (MW) irradiation played a crucial role as a heating source which was anticipated to produce uniform spherical AuNPs in an energy-efficient manner. Although several investigations have been conducted related to CNC before, no report was found demonstrating a nanohybrid (AuNPs/CNC) synthesized by the application of MW irradiation process, which controls the reaction time as well as avoids side reactions [23]. As compared to traditional synthesis methods, MW irradiation facilitates localized rapid dielectric heating, internally to maintain uniform nucleation and growth rates [24]. Synthesized AuNPs were characterized by UV-vis spectroscopy, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray analysis, X-ray diffraction, and Fourier transform infrared spectroscopy. In addition, we have investigated the effect of nanohybrid heterogeneous catalyst film (AuNPs/CNC) on the degradation of four conventional dyes; Congo red, Allura red, Rhodamine B, and Amaranth with NaBH₄.

2. Experimental

2.1. Material

Commercial microcrystalline cellulose (MCC), sulfuric acid, tetrachloroaurate trihydrate (HAuCl₄∙3H₂O), Congo red (CR), Rhodamine B (RhB), Amaranth (AM), Uranyl acetate, and NaBH₄ were purchased from Sigma Aldrich Co. (St. Louis MO, USA) and Allura red (AR) AC was obtained from Tokyo Chemical Industry Co., Ltd, Tokyo, Japan.

2.2. Synthesis of CNC

Cellulose nanocrystals were prepared from MCC as per the following method described elsewhere [25]. In brief, 1 g of MCC was acid hydrolyzed (10 mL of 64% H₂SO₄) and stirred at 45 °C for 60 min. The acid hydrolyzed MCC suspension was diluted 10 folds for the termination of the reaction and centrifuged followed by washing the sediment with distilled water. The centrifugation and the washing were repeated until the pH of the suspension was about neutral. The suspension was dialyzed against deionized water to remove residual acid for 2 days at room temperature with the medium being exchanged three times. The suspension was sonicated using a tip type (VC 505, Sonic & Materials, USA) for 35 min at room temperature to obtain colloidal CNC suspension, followed by freeze-drying to get a solid product.

2.3. Synthesis of gold nanoparticles using CNC

Gold nanoparticles were synthesized with aid of CNC using a microwave oven (Midea, MC-E230KW, and 800 W). 2% of CNC suspension was prepared by dispersing 2 g of CNC in 100 mL of distilled water. 6 mL of CNC suspension (2%) was mixed with 2 mL of HAuCl₄ solution (1 mM). The above mixture was contained in a 20 mL vial and kept in the microwave oven for 5-25 s until the reaction mixture turned from colorless to blushing red color. The synthesis conditions were systematically investigated with the concentration of CNC (0.25-2.0), HAuCl₄ (0.1-1 mM), pH (4-12), and MW irradiation time (5-25 s) by varying one parameter with the other parameters being kept constant.

2.4. Catalytic activity of AuNPs/CNC film for dye degradation

In brief, 3.0 mL of nanohybrid (AuNPs/CNC) suspension was put in a Petri dish and kept in an oven thermostated at 50 °C until water was completely evaporated. The dried film of AuNPs/CNC was collected from the Petri dish (Fig. S1) and the AuNPs content was measured (0.21 mg) through atomic absorption spectrometry.

The dye degradation potential of AuNPs/CNC nanocomposite film was determined against four dyes i.e., CR, AR, RhB, and AM with NaBH₄. 1 mL each of RhB, AR, AM, and CR solution (1 mM, in distilled water) was put in a 20 mL vial and mixed with freshly prepared NaBH₄ solution (20 mM) in equi-volumetric ratio. Each reaction mixture was made up to 10 mL by adding distilled water and stirred for 5 min. Later, 3 mL of the reaction mixture was put in a cuvette and treated with various amounts of (2, 4, 6, 8 and 10 mg) AuNPs/CNC film. UV-vis spectrometry was used to observe the progress of the degradation reaction at different time intervals.

2.5. Characterization of AuNPs/CNC

The size, the morphology, and the selected area electron diffraction (SAED) of synthesized AuNPs/CNC were observed on a transmission electron microscope (TEM) (LEO-912AB OMEGA, LEO, Germany, located at Korea Basic Science Institute, Chuncheon, Korea). The pristine CNC and AuNPs/CNC nanocomposite were negatively stained using uranyl acetate aqueous solution (1.2%, w/v). The surface charge and the particle size of AuNPs were measured on a dynamic light scattering equipment (Zeta plus 90, Brookhaven Instrument Co., USA). The mechanism for the formation of AuNPs was confirmed using an FT-IR spectrometer (FT-3000-Excalibur, Varian Inc., USA) with scanning range of 400‒4000 cm^-1. The reaction conditions were optimized using a UV-vis spectrophotometer (UV-vis spectrophotometer, JENWAY, UK) with scanning range of 400-800 nm. The crystal structure of AuNPs/CNC and pristine CNC were investigated by an X-Ray diffractometer (X’pert PRO MPD, PANalytical, Netherland) with the operating voltage of 45 kV and the current of 40 mA at increasing rate of 0.388 cm^-1.

3. Results and discussion

In this study, the AuNPs were synthesized by MW irradiation method using the hydroxyl groups of CNC, which could act as both a reducing and a supporting agent. The synthesized nanohybrid film of AuNPs/CNC served as a heterogeneous catalyst that was used for the catalytic degradation of the harmful dyes (See Scheme 1). When AuNPs were synthesized by a conventional hydrothermal process using CNC as a reducing agent, it took longer time and the resulting particle size was larger [7]. We implemented the MW irradiation method to produce smaller AuNPs in shorter time period (in order of second). The entire one-step green synthesis process was eco-friendly because non-toxic materials (e.g. water and biocompatible CNC) were used in the synthesis. Our synthesis process was ultrafast and energy and cost-effective as compared to conventional methods.

3.1. Chemical composition analysis of AuNPs/CNC nanocomposite

Fig. 1(A and B) represents a TEM image of pristine CNC and AuNPs/CNC nanocomposite. The TEM image demonstrated negatively stained pure CNC, acquired by the acid hydrolysis of MCC (Fig. 1(A)). There were found individual and some aggregated rod-like crystalline cellulose fragments, whose width was 8-15 nm and length was 90-130 nm. The acid hydrolysis using sulfuric acid introduced the sulfate ester groups on the surface of CNC, which provided a good colloidal stability to CNC suspension (pH 6.75). This anionic sulfate groups would induce an electrostatic repulsion among the crystal particles, leading to a stable CNC suspension. Hence, CNC would have a large surface area and be rich in hydroxyl group, which might result in better reducing and stabilizing ability in MNPs synthesis.

Negatively stained AuNPs/CNC shown on TEM image (Fig. 1(B)) were synthesized at the optimal conditions of CNC (2.0%), HAuCl₄ (1 mM), pH 12, and MW irradiation (25 s). Most of the synthesized AuNPs (above 85%) were attached on the CNC surface and exhibited a spherical shape. The AuNPs were evenly distributed on the CNC surface and had an average size of about 8 ± 5.3 nm, as shown in Fig. 1(C). Almost 45% of the particles fell within 6-8 nm, 5% within 10-12 nm, and approximately 3% fell within 12-14 nm, respectively. Fig. 1(D and E) illustrates the HR-TEM image and SAED pattern of the single crystallinity of AuNPs. The space between the lattices fringes was measured to be 0.24 nm (Fig. 1(E)), which matched with the lattice distance of (111) plane of face-centered cubic of gold, indicating that each of the AuNPs/CNC particles was a single crystal [26].

The EDX analysis was evaluated according to the point profile method which confirmed the existence of a gold atom in the AuNPs/CNC. This analysis also exhibited the presence of Au, C, O, and other elements on the surface of AuNPs/CNC where Au peaks were found at 2.18 and 8.45 keV (Fig. 1(F)). The additional peaks including C and O were attributed to the signals of the CNC adhered to AuNPs surface. Cu and U were originated from the copper grid and the staining reagent (i.e. uranyl acetate), respectively. EDX analysis revealed that the production yield of AuNPs was 98%, indicating that almost all the gold ions were reduced to gold nanoparticles.

The evaluation of the crystalline nature of pristine CNC and AuNPs/CNC by XRD were presented in Fig. 2(A). The outcomes of the XRD spectrum of the AuNPs/CNC clearly displayed four characteristic peaks corresponding to the 2θ values of 38.22°, 45.20°, 64.45°, and 74.97°. The resulting peaks were assigned to (111), (200), (220) and (311) planes of face-centred cubic crystals structure of AuNPs. The XRD spectrum of AuNPs/CNC was compared with standard Au crystalline metallic form (JCPDS No 04-07884). The peak produced by (111) reflection was more intensive than the other peaks, which clearly indicated that the crystalline structure of AuNPs was predominantly oriented along (111) plane. The pure CNC and AuNPs/CNC nanocomposite showed four characteristic peaks at 2θ values of 8.94°, 14.33°, 22.40° and 34.38°, which were corresponding to (1-10), (110), (200) and (004) diffraction planes of cellulose, respectively [25]. According to the obtained results, it was noticed that the crystalline structure of CNC was unaltered after the formation of AuNPs.

The FT-IR analysis identified the possible functional groups involved in the reduction of gold ions and the stabilization of AuNPs in the synthesis process. This analysis also provided information about the structural change of CNC after MW irradiations. The FT-IR spectra of the CNC and the AuNPs of AuNPs/CNC are shown in Fig. 2(B). The FT-IR spectrum of pristine CNC denoted some major bands present in the distinctive cellulose characteristics peaks at 3343.75, 2891, 1642, 1172, 1126, 1054, 665 cm^-1, attributed to the signal of OH, CH₂, C=O, C-O-C, asymmetric ring, C-O and C-OH stretching vibrations, respectively [27]. After MW irradiation for 25 s, a new peak appeared at 1723 cm^-1, which was thought to be due to the C=O stretching of carboxyl functional group, indicating that the OH functional groups was involved in the oxidation of CNC. However, the remaining other peaks could not make any considerable change, suggesting that the ring structure of CNC remained intact during the reduction process. The highly reactive primary alcohols were involved in the oxidation process, but not secondary alcohol. Our findings were in agreement with the previous synthesis of AuNPs which were conducted by hydrothermal and NaBH₄ reduction methods [7].

The possible mechanism for the formation of colloidally stabilized AuNPs seemed to be based on two steps. In the first step, the negatively charged CNC would interact with Au³⁺ to form intimidate complex, the hydroxyl groups of CNC would subsequently be oxidized to carbonyl or carboxyl form, thus Au³⁺ would be able to be reduced to AuNPs [28]. In the second step, AuNPs might be stabilized by CNC possibly due to the coordinate bonding of AuNPs with oxygen atoms present in hydroxyl and carbonyl groups of CNC.

The results obtained from TEM, SAED, EDX, XRD, and FT-IR analysis evidently confirmed that the reduction of the gold ion to the gold atom took place successfully with aid of CNC under MW irradiation. AuNPs were attached onto the surface of CNC in a well dispersed manner and the crystallography and the morphology of CNC were unaltered even after the formation of AuNPs.

3.2. Optimization of synthesis conditions

Conditions for the synthesis of AuNPs/CNC were monitored by UV-vis spectroscopy through surface plasmon resonance (SPR). The solution color changed from light yellow to blushing red after MW irradiation for 5-25 s, due to the formation of AuNPs. The reaction conditions were optimized in terms of CNC concentration, HAuCl₄ concentration, MW irradiation time, and pH value.

3.2.1. Effect of the concentration of CNC

The concentration of CNC suspension had a pivotal effect on the formation of AuNPs and its size. Fig. 3(A) represents the UV-vis spectrum of AuNPs synthesized using various concentrations of CNC suspension (0.25%-2.0%) at a fixed concentration of HAuCl₄ (1 mM) and at a fixed MW irradiation time (25 s). As the concentration of CNC suspension increased, the intensity of SPR band increased at 525 nm and reached its maximum at the CNC concentration of 2%. Due to the concentration variation from lower to higher (0.25%-2.0%), the zeta potential value increased, and particle size was decreased, which is shown in Table 1. At a lower CMC concentration (e.g. 0.25%), SPR peak appeared in a longer wavelength range and the intensity was less, possibly due to an insufficient concentration of CNC suspension for the nucleation growth of AuNPs. While the concentration of CNC suspension increased up to 2%, the absorption intensity increased to its maximum. Above the concentration of 2%, no further change in the band intensity was observed, indicating that the precursor had been consumed at the concentration of 2%. These results clearly suggested that the CNC concentration of 2% was sufficient for the formation of AuNPs.

The TEM images (Fig. 4(A-C)) demonstrated the morphology and the size of the AuNPs/CNC nanocomposite prepared using various CNC concentrations (0.25%-2.0%) at a fixed concentration of HAuCl₄ (1 mM) and at a fixed MW irradiation time (25 s). At a lower CMC concentration (e.g. 0.25%), the mean diameter of AuNPs was 35 ± 12.5 nm and many bigger particles were found possibly because the amount of CNC was insufficient to effectively stabilize AuNPs. The diameter of AuNPs decreased from 35 to 20 ± 18.5 nm when CNC concentration increased from 0.25% to 1%. When CNC concentration increased up to 2.0%, the mean diameter decreased to 10 ± 8 nm. A possible reason was the higher CNC concentration would induce more nucleation for the formation of AuNPs. In addition, the agglomeration of AuNPs would be less due to the lower collision of AuNPs at a higher CNC concentration. This finding was consistent with UV-vis spectra analysis showing that the increment of CNC concentration resulted in the formation of more AuNPs without λ max shifting.

3.2.2. Effect of the concentration of HAuCl₄

Fig. 3(B) represents the UV-vis spectrum of gold nanoparticles synthesized using various HuACl₄ concentrations (0.1-1 mM) at a fixed CNC concentration (2.0%) and at a fixed MW irradiation time (25 s). It is known that AuNPs displays a strong SPR absorption peak around 525 nm. When the concentration was 0.1 mM, the absorption band appeared in slightly longer wavelength range with very low peak intensity at 535 nm. As HAuCl₄ concentration increased from 0.25 to 1 mM, the absorbance curve became more bulging but the SPR position remained unchanged (525 nm). No considerable changes were identified in zeta potential values, but the size decreased with increasing HAuCl₄ concentration (Table 1). Hence, the obtained results clearly indicated the successful formation of AuNPs while increase in concentration of HAuCl₄, but due to constant stabilizing effect of CNC, the zeta potential value remained constant.

3.2.3. Effect of the pH value

Fig. 3(C) shows the UV-vis spectrum of gold nanoparticles synthesized at various pH values with HuACl₄ concentration (1 mM), CNC concentration (2%), and MW irradiation time (25 s) being kept constant. When the pH value was 3.52-7, no substantial SPR bands were found, indicating that AuNPs were not formed. This was possibly because strong hydrogen bonds among OH functional groups would hardly allow CNC to act as a reducing agent. As the pH value increased from 7 to 12, the SPR peak at 525 nm was growing, suggesting that AuNPs were formed more as the medium was more alkali. Under strong alkali conditions, the hydrogen bonds are weak, and thus OH groups would be actively involved in the reduction process to produce AuNPs.

3.2.4. Effect of MW irradiation time

Fig. 3(D) shows the UV-vis spectrum of gold nanoparticles synthesized by varying MW irradiation time (5-25 s) with HuACl₄ concentration (1 mM), CNC concentration (2%), and pH value (12) being kept constant. The color of reaction solution began to change within 5 s of time, demonstrating that the reduction reaction was ultrafast. The SPR at 525 nm increased without any shift of wavelength while increasing the MW irradiation time. These results clearly indicated that a larger quantity of gold ions were reduced to AuNPs within 25 s. Due to the variation in MW irradiation time from 5 to 25 s, the particle size was decreased and zeta potential value increase, which is shown in Table 1. MW irradiation for more than 25 s did not make any considerable change in the peak intensity, indicating that the precursors had been exhausted during the MW irradiation time of 25 s.

3.3. Catalytic degradation of dyes

Over the last few decades, the treatment of industrial effluents through a chemical degradation technique has been one of the major concerns, owing to their toxicity and harmfulness to the environment. The reduction of stable dyes with an excess amount of NaBH₄ is thermodynamically favourable because the reduction rate is very slow. However, when a small amount of MNPs is induced, the reduction rate is accelerated and it converted to be kinetically favourable. A reason is possibly due to the large difference in reduction potential that hinders the electron transfer between NaBH₄ (a donor) and dyes (an acceptor). An alternative way to reduce the activation energy of reaction is to use metal nanoparticles, which enhance the electron transfer and shorten the degradation process time. AuNPs were attached onto CNC, which can help the reactants to come closer to AuNPs surface. Besides, the nanoparticles play a pivotal role as an electron transmitter, which facilitates the transfer of electrons from donor to acceptor molecule. This transformation of electron makes the rate of reaction more favourable both thermodynamically and kinetically. The assumed mechanism of the catalytic action of AuNPs/CNC is schematically represented in Fig. 5.

3.3.1. Catalytic degradation of Congo red

CR dye is utilized as an anionic dye in textile, plastic, paper, and rubber industries [29]. The presence of CR dye in industrial effluents is harmful to environments and human. The catalytic degradation of CR with NaBH₄ in the presence of eco-friendly prepared AuNPs/CNC as a catalyst was examined. The absorptions band of CR at 495 nm was for π- π* and 345 nm was for n- π* transition which was related to two azo bonds (-N = N-) [30]. The absorption band of CR was well separated from the SPR of AuNPs/CNC, and hence the catalytic reaction could be easily monitored spectrophotometrically.

In the process of the catalytic degradation of CR with NaBH₄ when AuNPs/CNC catalyst was absent, only 10%-15% degradation was completed after 240 min (Fig. S2). The progress of the degradation of CR dye in the presence of AuNPs/CNC could be easily monitored by observing a time-dependent decrease in absorbance at 495 nm and a time-dependent increase in absorbance at 285 nm (Fig. 6(A)). Almost 100% degradation of CR dye took place within 180 s in the presence of AuNPs/CNC and the color of the dye solution changed from red to colorless. After the completion of the degradation of CR, a very weak peak was observed at 522 nm, which might be the SPR of AuNPs. This results implied that the nanocatalyst was involved in an electron relay process during reduction reaction.

The degradation reaction followed a pseudo-first order, possibly due to the relativeley high concentration of NaBH₄ in comparison to CR and catalyst. A plot of ln(A₀/A_t) vs time was drawn to calculate the reaction rate constant of CR degradation (Fig. 6(B)). Here, A_t was the dye concentration at a given time and and A₀ was the initial concentration. AuNPs/CNC could degrade CR dye much faster than previously reported nanocatalysts, which was summarized in Table 2.

The effect of AuNPs/CNC (catalyst 2-10 mg) various amounts on the degradation of CR was examined, and the rate related constant is shown in Fig. S3(A). The effect of temperature (15-35 °C) on the degradation of CR was investigated, while the other parameters were kept constant. A plot of lnk vs 1/T was drawn and it was found to be linear. The rate related constant was proportional to temperature (Fig. S3(B)). Arrhenius equation (lnk=lnA-E_a/(RT)) was used for the calculation of the activation energy (E_a) of the reaction, where k denotes the reaction rate constant at temperature T (in K), A is the frequency factor and R is the universal gas constant.

3.3.2. Degradation of Allura red

The dye degradation potential of eco-friendly synthesized AuNPs/CNC was studied using AR dye with NaBH₄ as a model of the reduction reaction. AR is a red dye having staining properties and used in food, cosmetics industries, and biological field [31]. The typical absorption band of AR was found at 504 nm for the n- π* transition of the azo bond (-N = N-) [32]. In the absence of AuNPs, AR with NaBH₄ exhibited 5% reduction in the absorbance at 504 nm in 240 min (Fig. S4), indicating that the degradation reaction was quite slow. Upon the addition of AuNPs/CNC as a catalyst, the absorbance of AR dye quickly decreased and a new peak appeared at 332 nm (Fig. 7(A)). The degradation time reduced to below 220 s and the rate related constant is shown in Fig. 7(B). This clearly demonstrated the degradation reaction was accelerated by the nanocatalyst (AuNPs/CNC). AuNPs/CNC could degrade AR dye much faster than previously reported nanocatalysts, which is summarized in Table 2.

Fig. S5(A) shows the rate related constant values obtained by varying the amounts of nanocatalyst (AuNPs/CNC) ranging between (2-10 mg). The rate related constant increased with the increase in the concentration of AuNPs/CNC. The temperature-dependent catalytic activity was studied in 15-35 °C and the plots of lnk vs 1/T along with the rate related constants are shown in Fig. S5(B). The results clearly indicated the rate related constant increased with the increase in the temperature.

3.3.3. Degradation of Rhodamine B

The reduction of RhB dye by NaBH₄ is highly favourable thermodynamically but not kinetically, due to a huge difference in redox potential between RhB (-0.48 V) and NaBH₄ (-1.33 V), and this reduction reaction took longer time in the absence of AuNPs/CNC. However, when a small amount of AuNPs/CNC was added, the reduction reaction was completed ultrafast possibly because AuNPs redox potential can facilitate electron relay process between donor (NaBH₄) and acceptor (RhB). RhB is one of the most common xanthene dyes extensively used in many industries as fluorescent probes [33]. It has become one of the organic pollutants in industrial wastewater [34].

In our study, RhB exhibited a strong absorption band at 554 nm. Without AuNPs/CNC, the catalytic reduction reaction of RhB along with NaBH₄ was very slow and only 5% of the dye was degraded within 240 min (Fig. S6). Upon the addition of AuNPs/CNC, the reduction started immediately and almost 100% of the dye was degraded within 220 s (Fig. 8(A)). The rate related constant is shown in Fig. 8(B). A weak peak appeared at 525 nm at the end of the reaction, confirming the SPR of nano gold catalyst (AuNPs/CNC). This result implied that the nanocatalyst was involved in an electron relay process during the reduction reaction. AuNPs/CNC exhibited an improved degradation activity for RhB dye compared to previously reported nanocatalysts, which is summarized in Table 2.

We also examined the effect of different amounts (2-10 mg) of AuNPs/CNC and temperatures (15-35 °C) on the degradation of RhB dye and reported the rate related constant in Fig. S7(A and B). When AuNPs/CNC concentration and temperature increased, the related rate constant increased. The obtained results are plotted using lnk vs 1/T along with temperature related rate constant graph.

3.3.4. Degradation of Amaranth

The catalytic efficiency of bio-synthesized AuNPs/CNC was examined using AM dye with NaBH₄. AM is a water-soluble red dye, contains single azo group (-N = N-), and comprehensively utilized as a coloring agent for textile, paper, leather, foodstuff, wood, and cosmetics, etc [35]. This dye has some carcinogenic and toxic effects in the case of a prolonged intake in human beings. Hence, the degradation of AM dye is one of the major challenges in present days.

The catalytic degradation of AM dye was monitored by UV-vis spectroscopy. With time lapse, the intensity of an absorbance peak at 520 nm decreased and a peak at 378 nm appeared and grew up. Only 2% of the dye with NaBH₄ was degraded in 240 min (Fig. S8), indicating that the degradation of AM dye in the absence of AuNPs was insubstantial. In contrast, upon the addition of AuNPs/CNC, the degradation was completed within 170 s. Fig. 9(A) and (B) shows the continuously decreasing absorption peak at 520 nm with increasing time and the rate related constant, respectively. The degradation of AM with NaBH₄ is favoured only thermodynamically, however the addition of AuNPs/CNC reduced the activation energy of reaction and would enable the degradation to be favoured thermodynamically as well as kinetically. AuNPs/CNC showed an improved degradation activity for AM dye compared to previously reported nanocatalysts, which is summarized in Table 2.

The effect of AuNPs/CNC various amounts (2-10 mg) on the reduction of AM was studied and the related rate constant was shown in Fig. S9(A). The obtained results indicated that the degradation of AM increased with increasing AuNPs/CNC concentration. The effect of the temperature (15-35 °C) on the reduction of AM was examined with the other parameters being kept constant. A plot of lnk vs 1/T was drawn along with the rate related constants, which is shown in Fig. S9(B). The plot was linear and suggested that the rate of the catalytic reduction increased with increasing temperature.

At present, a powder form of metal nanoparticles has been used for catalytic dye degradation. To develop novel catalysts, engineering facile and efficient recovery processes is getting much attention. However, the recovery processes are very expensive, time- and energy- consuming due to the involvement of complex steps such as filtration, centrifugation, magnetic separation, and precipitation [47,48]. In this regard, our study presented an efficient process of separating catalyst from the reaction medium, which can minimize possible health and ecological concerns caused by the release of nanoparticles. Moreover, the AuNPs/CNC film has been investigated for its reusability. For instance, the repeated use of AuNPs/CNC film in 5 cycles for the reduction of dyes demonstrated almost similar catalytic activity in first two cycles, but a small decrease from third cycles was observed possibly due to the deactivation of the catalytic surface with the repeated absorption of reactants (Fig. S10). Therefore, this study proposes free-standing film of the nanocatalysts which facilitate the separation of reaction medium, without using additional processes.

The stability of the catalyst and the recycled catalyst were examined using FT-IR and XRD analysis (Fig. 10(A) and (B)). The obtained results clearly indicated that there were no considerable changes in FT-IR and XRD before and after the reduction of dyes. Therefore, the roles of the functional groups present on the nanocatalyst surface had no major contribution in catalysis and the crystal structure was retained after participating in the catalytic reduction reactions.

4. Conclusion

In this study, we have presented a novel ultrafast and facile MW irradiation synthesis of AuNPs by using cellulose nanocrystals as a reducing and a supporting agent. This whole green process was a very simple, rapid and energy-efficient route and also suitable for the large-scale production of AuNPs. Besides, most of the prepared AuNPs were successfully attached onto the surface of the cellulose nanocrystals. The synthesized AuNPs had a uniform spherical shape, the mean diameter was 8 ± 5.3 nm, and they were single-crystalline in nature. The diameter of AuNPs/CNC nanocomposite was strongly influenced by CNC concentration. The biosynthesized AuNPs/CNC nanocomposite exhibited excellent degradation properties of organic dyes incluing AR, CR, RhB, and AM. The degradation reactions were ultrafast and well-fitted by pseudo-first order kinetics. The facile synthetic strategy allowed for the bio-reduction of gold ions to spherical AuNPs in an ultrafast period of time. The synthesized AuNPs/CNC nanocomposite would be applicable to the ultrafast degradation of various industrial organic effluents.

Acknowledgments

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A1A03025582).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.11.003.

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