Nanoscale material has attracted the interest of many researchers because of its special physicochemical and surface properties. In this study, a nanocomposite was prepared based on cellulose nanocrystals (CNCs) in situ forming ultra-fine size gold nanoparticles. To form the gold nanoparticles, chloroauric acid was reduced with an aqueous mixture of ethanol and CNC. The gold nanoparticles were less than 8 nm in size, with 80% of them less than 5 nm. Nanocellulose in the preparation system acted as a stabilizer. The size and dispersity of the ultra-fine gold nanoparticles (UAuNPs) were controlled by adjusting the pH. The resulting UAuNPs/CNCs composite exhibited excellent catalytic performance to eliminate solubilized NaNO2 mixed with NH4Cl at room temperature. The apparent rate constant of the reaction was 4.12 × 10-5 s-1. The catalytic effect of the generated UAuNPs on NO2- reduction has potential use in many fields, particularly in the food and environmental sectors.
Preparation of Gold Nanoparticles on Nanocellulose and their Catalytic Activity for Reduction of NO2–
Zihe Guo and Shiyu Fu *
Nanoscale material has attracted the interest of many researchers because of its special physicochemical and surface properties. In this study, a nanocomposite was prepared based on cellulose nanocrystals (CNCs) in situ forming ultra-fine size gold nanoparticles. To form the gold nanoparticles, chloroauric acid was reduced with an aqueous mixture of ethanol and CNC. The gold nanoparticles were less than 8 nm in size, with 80% of them less than 5 nm. Nanocellulose in the preparation system acted as a stabilizer. The size and dispersity of the ultra-fine gold nanoparticles (UAuNPs) were controlled by adjusting the pH. The resulting UAuNPs/CNCs composite exhibited excellent catalytic performance to eliminate solubilized NaNO2 mixed with NH4Cl at room temperature. The apparent rate constant of the reaction was 4.12 × 10-5 s-1. The catalytic effect of the generated UAuNPs on NO2– reduction has potential use in many fields, particularly in the food and environmental sectors.
Keywords: Ultra-gold nanoparticles; Cellulose nanocrystals; Catalytic
Contact information: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China; *Corresponding author: firstname.lastname@example.org
Gold is a useful element for chemists because it can be made into special functional materials of various shapes. Nanoparticles and self-assembled monolayers (SAMs) comprised of gold are some of the emerging applications in the field of nanoscience and nanotechnology (Bumbudsanpharoke and Ko 2015). Gold nanoparticles (AuNPs) refer to nanosized gold particles with a large specific surface area, high surface energy, and high activity for catalytic reactions. These AuNPs exhibit unique physical and chemical properties that bulk gold does not possess. The surface effect, surface plasmon resonance effect, and quantum size effect are some of the unique effects from AuNPs (Halperin 1986; Daniel and Astruc 2004; Ghosh and Pal 2007). Due to their unique properties, AuNPs are widely applied in many areas such as industrial catalysis, biological medicine, biological analytical chemistry, and rapid detection (Panigrahi et al. 2007; Ai et al. 2009; Marques et al. 2016; Paukkonen et al. 2017). Both physical and chemical methods can be used to fabricate AuNPs. Physical methods such as vacuum evaporation, soft landing, and laser ablation can produce high quality gold particles with few impurities (Veith et al. 2005). However, physical methods are costly and require special equipment. Additionally, physical fabrication methods do not provide much control over the size and shape of the AuNPs. Some of the main chemical methods include sodium citrate reduction, crystal seed growth, two-phase method, and self-assembly. The resulting AuNPs from chemical methods are too large for many applications, have uneven size distribution of particles, and tend to exhibit agglomeration of particles.
The most widely used method for AuNPs production is the self-assembly of the gold atom by reducing chloroauric acid with ethanol, which may affect the size of the AuNPs. The most established and widely used reduction method is the sodium citrate reduction method proposed by Turkevich many years ago (Turkevich et al. 1951). When sodium citrate reduction is used to prepare AuNPs, the sodium citrate acts as both a reducing agent to obtain spherical AuNPs and as a protective agent to stabilize the AuNPs. Adjusting the ratio of sodium citrate and chloroauric acid has been shown to produce AuNPs 16 to 147 nm in size (Frens 1973). Polyethylene glycol (PEG) was used as a reductant to prepare 15 to 25 nm AuNPs, which were applied to reduce p-nitro-toluene (Yan et al. 2016).
When CNCs were used as a reducing agent and stabilizer, AuNPs of approximately 25 nm were produced with a good photothermal effect (Hu et al. 2017a,b). In the above reaction system, the prepared AuNPs need stabilizing agents such as PEG, CNC, carbon, silicon, or metal oxides because AuNPs alone are unstable and easily aggregate in the solution due to the van der Waals forces. Compared to other agents, CNCs are natural and biodegradable (Xu et al. 2017), while exhibiting excellent performance properties such as high crystallinity, high purity, high Young’s modulus, high hydrophilicity, ultra-fine structure, and high transparency (Abdul Khalil et al. 2012; Mittal et al. 2017). Nevertheless, monofunctional nanofillers only improve upon a specific property for host matrices, whereas multifunctional nanofillers can provide multiple properties for the resulting nanocomposites.
Cellulose nanocrystals can function as stabilizers, dispersants, and templates for the carrying of gold nanoparticles (Hajian et al. 2017; Xiong et al. 2018). For example, Zhang et al. (2018) succeeded in synthesizing palladium and gold nanoparticles by using dialdehyde nanocellulose as a template and reducing agent. Similarly, Koga et al. (2010) synthesized in situ highly dispersed AuNPs on the surface of 2,2,6,6-tetramethylpiperidinyl-1-oxyl free radical (TEMPO)-oxygenized cellulose nanofibers in the presence of NaBH4. The prepared AuNPs on carriers were applied in catalysis for oxidation or reduction because of the surface effect of AuNPs with high surface energy and surface binding energy, which refers to the nanoparticles distinguished from the body of gold (Lopez et al. 2004). Specific surface area refers to the total area of a unit mass of material. The prepared AuNPs are spheroidal. Therefore, the smaller the particle size of the AuNPs, the larger the specific surface area and the larger the surface energy and surface binding energy.
Gold nanoparticles with a diameter of less than 5 nm are defined here as ultra-fine gold nanoparticles (UAuNPs). When the diameter of the gold particles is 1 nm, UAuNPs are formed and the specific surface area is the largest. At this point, the surface energy and surface binding energy of the UAuNPs are greatly increased, leading to a significant improvement in the catalytic performance of UAuNPs.
Due to their high surface energy and surface binding ability, UAuNPs have high activity for catalysis. These properties allow UAuNPs/CNCs to catalyze the reduction of nitrite with NH4Cl, which takes place in the laboratory to produce nitrogen or to remove nitrite. Usually, this reaction should be carried out at 60 °C or above. When UAuNPs are used as a catalyst, the reaction can happen at room temperature with a second-order. This catalytic reaction can potentially be applied to remove nitrite substances in food.
Cotton linters with a moisture content of 8% were supplied by Fumin Chemical Fiber Co. Ltd. (Shandong, China). Analytical grades of sulfuric acid (98 wt. %), sodium hydroxide, gold(III) chloride trihydrate, anhydrous ethanol, ammonium chloride, and sodium nitrite were purchased from the Guangzhou Chemical Reagent Factory (Guangzhou, China). All chemicals were used as received.
Preparation of CNCs
Cellulose nanocrystals were prepared from cotton linters by acid hydrolysis according to a previously reported method (Tian et al. 2014). A quantity of 64 wt% H2SO4 and dry cotton linters were added in a ratio of 13.5 to 1 (w/w). The mixture was stirred at 45 °C for 90 min, then diluted 10 times to terminate the reaction with water. After centrifugation washing two to three times, the reaction solution was dialyzed for two to three days until the pH was neutral. Finally, a uniform CNC was obtained by ultra-sonication at 40 °C for 3 min and stored at 4 °C.
Preparation of CNCs-Supported UAuNPs Suspensions (UAuNPs/CNCs)
A modified method was used to prepare UAuNPs in the presence of CNCs (Hu et al. 2017b). A 5 mL quantity of anhydrous ethanol and 542 μL of HAuCl4 (9.71 mM) were added to a 50 mL CNC (0.5 wt%) suspension at 70 °C under constant magnetic stirring for 45 min. The pH was set at 12 and was adjusted by using 0.5 M NaOH. The UAuNPs/CNCs suspension contained 20.74 ppm of UAuNPs and was stored at 4 °C for further use.
Characterization of UAuNPs/CNCs Nanocomposites
X-ray diffraction (XRD) patterns of freeze-dried AuNP/CNCs were collected on a D8 Advance X-ray Diffractometer (Bruker Corporation, Karlsruhe, Germany) using Ni-filtered Cu KR radiation (λ = 0.154 18 nm) at 40 kV and 20 mA. Data were collected over the 2θ range of 5 to 90° with a scan step size of 0.04° at a rate of 1° per min. Ultraviolet-visible (UV-Vis) absorption spectra from 400 to 1000 nm were obtained using an Agilent Technologies HP-8453 spectrophotometer (Santa Clara, CA, USA) with deionized water as the blank.
The morphologies and sizes of the gold nanoparticles were observed by transmission electron microscopy (TEM) (H-7650, Hitachi, Japan) at 80 kV accelerating voltage. Atomic force microscopy (ATM) images of the AuNP/CNC nanocomposite films were recorded on a Bruker Multimode 8 atomic force microscope.
Catalytic Reaction of NH4Cl and NaNO2
The reaction of NH4Cl and NaNO2 can produce N2, H2O, and NaCl under suitable conditions. In the reaction, 20 mL of UAuNPs/CNCs suspension (total amount of UAuNPs: 0.002 mM) were added into the mixture of 0.05 M NH4Cl and 0.05 M NaNO2 at 25 °C with constant stirring. Nitrogen gas produced over time was collected to determine the reaction rate.
RESULTS AND DISCUSSION
Preparation of UAuNPs/CNCs Nanocomposites
The prepared AuNPs samples have potential use in catalysis and photothermal effect materials (Panigrahi et al. 2007; Zhang et al. 2017). When CNCs were used as the scaffold and reducer of AuNPs, the AuNPs/CNCs composite was formed through the photothermal effect, as observed in previous research (Hu et al. 2017b). When ethanol was used as the reducing agent, an ultra-fine gold nanoparticle (UAuNP) suspension was obtained in the presence of CNC at pH 11.47. The UAuNP sample was 10 nm in size. The XRD spectra of the CNCs and UAuNP/CNC nanocomposites can be seen in Fig. 1. The results from Fig. 1 indicate that the CNC had a typical cellulose I structure with characteristic peaks of 2θ angles at 15.1°, 22.5°, and 34.5°. Compared with CNC, the UAuNPs/CNCs had five extra diffraction peaks at 2θ angles of approximately 38.1°, 44.1°, 64.6°, 77.4°, and 81.7°. These extra peaks were indexed as the 111, 200, 220, 311, and 222 lattice planes of the standard face-centered cubic phase of metallic gold, further indicating the formation of crystalline gold (Arockiya Aarthi Rajathi et al. 2014; Shi et al. 2015).
Fig. 1. X-ray diffraction patterns of CNCs and UAuNP/CNC nanocomposites
The particle size and crystalline gold structure were analyzed through AFM and TEM imagery. The AFM image of the UAuNP/CNC nanocomposite and the TEM images of the UAuNP/CNC nanocomposites are shown in Fig. 2. In Fig. 2a, the rod-like shape substances were CNCs 100 to 300 nm in length and 10 to 30 nm in width. The UAuNPs were uniformly dispersed on the well-dispersed CNCs. The crystal diffraction and shape of CNCs in Fig. 1 were in agreement with the previous results from many reports, which indicates that CNCs act as carriers for UAuNPs and do not change physically (Arockiya Aarthi Rajathi et al. 2014; Shi et al. 2015). It is probable that the reductive end of the cellulose molecule on the surface of the cellulose crystal was oxidized to the carboxyl group by gold(III) during the reaction. The formed carboxyl group may provide negative charges that are beneficial for the dispersal of CNCs, as seen by the well dispersed CNCs in Fig. 2a. Figures 2b and 2d show the size and crystal diffraction pattern of the UAuNPs. Figure 2c illustrates that the visible particles originated from gold crystal. The distribution of the small particles was very even, as can be seen in Fig. 2d. When the viewing area in Fig. 2b was magnified, it was apparent that all of the gold particles in the darker area were less than 5 nm in size. To determine which particles in the darker areas were gold, the crystal diffraction pattern of the particles was measured, as shown in Fig. 2c. The diffraction pattern of the measured particles matched that of typical gold crystal, confirming that the particles were UAuNPs. While many previous studies have prepared gold nanoparticles, this was the first time that ultra-fine gold nanoparticles have been achieved. Yan et al. prepared 15 to 25 nm AuNPs using PEG as a reductant (2016). Siddiqi and Husen (2017) produced gold nanoparticles of about 25 nm in size and studied their application in biological systems. Chen et al. (2015) made AuNP/AOBC composites larger than 10 nm and applied them to the catalysis. Zhang and Zhao (2013) prepared AuNPs of about 10 nm in size and used them to make shape memory materials. There was no published research on the use of CNCs as a substrate and ethanol as a reducing agent to obtain UAuNPs through pH adjustment.