Green synthesis of gold nanoparticles reduced and stabilized by squaric acid and supported on cellulose fibers for the catalytic reduction of 4-nitrophenol in water

Abstract

We report a simple, fast, and green method to produce gold nanoparticles (AuNPs) that are reduced and stabilized by sodium squarate in water and easily attach to cellulose fibers. The AuNPs and its nanocomposites with cellulose fibers exhibited excellent catalytic activity for the reduction of 4-nitrophenol (4-NP) with NaBH₄. A glass column was packed with the nanocomposites and used for the continuous catalytic reduction of 4-NP with NaBH₄ and demonstrated to be used multiple times (20×) without loss of catalytic activity.

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1. Introduction

The ability of gold nanoparticles (AuNPs) to exhibit size- and shape-dependent electronic, magnetic and optical properties makes them one of the most versatile nanomaterials.^1,2 There are AuNPs designed for numerous applications including, but not limited to catalysis,^3,4 photocatalysis,^5,6 plasmonic photovoltaics,^7,8 photothermal solar distillation,^9,10 chemical sensing,^11,12 biological and medical drug delivery systems,^13,14 biomedicine,¹⁵ bioimaging,^16,17 and photothermal therapy.^18,19

Recently, the application of AuNPs in the catalysis of chemical reactions has garnered much attention²⁰ such as the aldehyde–alkyne–amine (A³) coupling,²¹ the reduction of nitro compounds into amines,²² the Suzuki–Miyaura cross-coupling,²³ the production of hydrogen from carbon monoxide and steam,²⁴ and the oxidation of glucose to gluconic acid.²⁵ However, in most cases, when AuNPs are used in homogeneous catalysis, their aggregation in solution leads to the gradual deactivation of the catalytic activity. Moreover, the separation, recovery, and reuse of the AuNPs from the reaction solution is a real challenge.²⁶ For this reason, AuNPs anchored to a solid support with high-surface area and permeability to the solution reaction is an attractive alternative that can increase the recyclability of the catalyst while minimizing the cost and footprint of the nanoparticles. AuNPs have been successfully supported on bacterial cellulose,²⁶ cellulose nanocrystals,²¹ crystalline cellulose nanofibers,²⁷ mesoporous silica,²⁸ Zeolites,²⁹ Al₂O₃,³⁰ TiO₂,³¹ and mesoporous carbon.³² Since cellulose is one of the most abundant natural polymers, and it is easy to functionalize, it has been a major type of matrix for the support of AuNPs.^33,34 However, to achieve sufficient anchoring of AuNPs, the cellulose fibers are often functionalized with a number of groups, such as –HS,³⁵ –COOH,³⁶ or –NH₃⁺.³⁷

Herein we report for the first time a new and green strategy to prepare active AuNPs that are reduced and stabilized by sodium squarate leading to SA-AuNPs. The SA-AuNPs easily attaches to cellulose fibers (CF) to give nanocomposites CF–AuNPs-2.94 and CF–AuNPs-1.96, which vary only in % AuNPs loading, 2.94% and 1.96% wt, respectively, and maintain the catalytic activity of SA-AuNPs. The SA-AuNPs were characterized with UV-vis spectroscopy, surface enhanced resonance Raman (SERS) spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, dynamic light scattering (DLS) and zeta potential measurements. The cellulose-supported nanocomposites were characterized with scanning electron microscopy (SEM), EDX, TEM, X-ray powder diffraction (XRPD), and FTIR spectroscopy. Both the SA-AuNPs and the cellulose fibers-supported SA-AuNPs samples exhibited excellent catalytic activity for the reduction of 4-nitrophenol (4-NP) with NaBH₄. Moreover, a chromatographic column was packed with the nanocomposite and used for continuous catalytic reduction of 4-NP by using NaBH₄ to evaluate the stability of the nanocomposite.

2. Materials and methods

2.1. Chemicals

3,4-Dihydroxy-3-cyclobutene-1,2-dione (squaric acid, 99%), sodium hydroxide (NaOH ≥ 98%), hydrogen tetrachloroaurate trihydrate (HAuCl₄·3H₂O, 99.9%), 4-nitrophenol (4-NP) (O₂NC₆H₄OH ≥ 99%), sodium borohydride (NaBH₄, 99%), and cellulose fibers were obtained from Sigma-Aldrich, USA. Milli-Q water (>18.20 MΩ cm resistivity) was obtained from Milli-Q (Advantage A-10) water filter.

2.2. Synthesis of SA-AuNPs in water

A 5 mL sample of a 38.8 mM aqueous solution of sodium squarate was prepared by adding 22.12 mg (0.194 mmol) of squaric acid and 15.52 mg (0.388 mmol) sodium hydroxide to water. It is worthy of mentioning that the squaric acid is not soluble in water; however, sodium squarate is fairly soluble in water. Then, to a 100 mL round-bottomed flask containing 20 mL of 0.5 mM solution of hydrogen tetrachloroaurate trihydrate, 1 mL of 38.8 mM sodium squarate was added, all at once, while stirring vigorously. The reaction mixture turned into a deep burgundy color after a few seconds of stirring (Scheme 1a), indicating the formation of AuNPs. The stirring was continued for 30 min and the AuNPs solution was preserved under ambient conditions for further characterization, application, and further binding with cellulose fibers. The final SA-AuNPs stock solution had a concentration of 0.0933 mg of Au per 1 mL of solution.

Scheme 1 (a) Synthesis of SA-AuNPs and (b) preparation of cellulose fibers supported AuNPs composites.

2.3. Preparation of cellulose fibers supported SA-AuNPs nanocomposites

Depending upon the desired gold loading on cellulose fibers, different quantities of SA-AuNPs solution and cellulose fibers were used. For example, to prepare a 1.96 wt% SA-AuNPs supported on cellulose fibers, referred to as composite CF–AuNPs-1.96, 196.04 mg of cellulose fibers were added to 42 mL of a stock solution of SA-AuNPs at a concentration of 0.0933 mg Au per 1 mL. This mixture was then honk sonicated for 5 min, which caused SA-AuNPs to bind with the cellulose fibers leading to the decolorization of the reaction solution, Scheme 1b. Composite CF–AuNPs-1.96 was then centrifuged at 4000 rpm for 5 min, washed repeatedly, with DI water, dried under vacuum in a desiccator, and finally placed in an oven, at 60 °C, overnight. In order to prepare a 2.94 wt% AuNPs loaded cellulose fibers, referred to as composite CF–AuNPs-2.94, 194.12 mg of cellulose fibers were added to 63 mL of SA-AuNPs stock solution following the aforementioned procedure. UV-vis spectroscopy of the supernatant showed that no plasmonic absorption band centered at 521 nm remained, which signifies that all SA-AuNPs were loaded onto the cellulose fibers (see ESI†).

3. Characterization of SA-AuNPs and the nanocomposites

3.1. UV-visible spectroscopy of the SA-AuNPs

UV-vis spectrum of a 6× diluted SA-AuNPs solution in H₂O is shown in Fig. 1. A strong absorption band centered at 521 nm is observed, which is the characteristic surface-plasmon resonance absorption band that is usually observed for the AuNPs.^38,39 The SA-AuNPs in H₂O is shown in the inset picture. The molar absorption coefficient of the SA-AuNPs stock solution was calculated to be 1.83 × 10⁶ M⁻¹ cm⁻¹ considering the average particle size of the AuNPs is 21.01 nm.⁴⁰

Fig. 1 UV-vis absorption spectrum of SA-AuNPs in H₂O with the corresponding digital photograph of the solution (inset).

3.2. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis of SA-AuNPs

The size, shape, and dispersity of SA-AuNPs were examined by using Transmission Electron Microscopy (TEM), Fig. 2.

Fig. 2 (a) and (b) Typical TEM images of SA-AuNPs in H₂O with different magnifications; (c) size distribution of SA-AuNPs based on TEM; (d) EDX spectrum of SA-AuNPs. Scale bars 100 nm for both images.

TEM of SA-AuNPs in H₂O shows that the AuNPs are relatively spherical and well dispersed in solution with little to no aggregation and have a range of core diameter from 8 to 35 nm, with an average size of 21.01 nm.

Energy-dispersive X-ray analysis was conducted on SA-AuNPs to determine the elemental analyses of the nanoparticles, Fig. 2d. EDX spectrum shows a high abundance of gold in the nanoparticles samples, as well as traces of C and O.

3.3. Surface enhanced resonance Raman (SERS) of SA-AuNPs

Fig. 3 shows the Raman scattering spectrum of SA-AuNPs and the sodium squarate solution (1.8 mM) in water as the control. It is observed that the intensity of the characteristic peaks that originated from the sodium squarate solution (trace a) is slightly enhanced in SA-AuNPs (trace b). Similar enhancement in the intensity of Raman signal of 4-aminothiophenol and 4-nitrothiophenol was observed in other studies where they were adsorbed on the AuNPs surfaces.⁴¹ The binding of squarate with AuNPs also reveals a small shift in the carbonyl peaks from 1573 to shoulder peak at 1556 cm⁻¹. This effect indicates that squarate ion is structurally bonded with the surface of the AuNPs.

Fig. 3 Raman spectrum of (a) aqueous sodium squarate solution (1.8 mM) and (b) SA-AuNPs solution. * – instrumental artifact at baseline.

The Raman spectra of SA-AuNPs reveals two new peaks at 2129 and 2144 cm⁻¹. These are assigned to be the characteristic peaks of carbon monoxide (CO) perpendicularly bound to the AuNPs surface, which appears to be originated from the degradation of sodium squarate. Similar peaks were observed in other studies where sodium squarate was bound to the gold and platinum electrode surfaces.^42,43

3.4. Dynamic light scattering of SA-AuNPs

Dynamic light scattering (DLS) was carried out on a 6× diluted solution of SA-AuNPs in water (see ESI†) to determine the size distribution. The average hydrodynamic diameter was observed to be 44.21 nm. However, the average size in DLS analysis is almost double than the TEM analysis. This is because in solution the DLS gives the hydrodynamic size, whereas, TEM gives the size of the gold nanoparticle core only.

3.5. Zeta potential measurements of SA-AuNPs

Zeta potential measurements on 6× diluted of SA-AuNPs in water (see ESI†) showed negative values of −32.6 mV. This negative zeta potential signifies that the AuNPs surface has a net negative charge, which may be due to the binding of squarate molecules on the nanoparticles, and explain the high dispersion stability in solution due to the interparticle electrostatic repulsive forces.

3.6. Scanning electron microscopy (SEM) and EDS of the nanocomposites

Fig. 4 shows the typical SEM images of cellulose fibers before and after treatment with SA-AuNPs. The cellulose fibers have diameter of about 25 micrometer with varying lengths. After treatment with SA-AuNPs, the cellulose fibers maintain their morphology.

Fig. 4 (a) and (b) Typical SEM images of cotton fibers and nanocomposite CF–AuNPs-2.94 respectively; (c) and (d) respective EDX spectrum of cotton fibers and composite CF–AuNPs-2.94 respectively.

EDS analysis of CF–AuNPs-2.94 shows the presence of high abundance of carbon, oxygen and gold in the spectrum.

3.7. Transmission electron microscopy (TEM) of the nanocomposite

Typical TEM image of a respective nanocomposite is shown in Fig. 5. The image was obtained by staining the sample with uranylacetate before imaging. The TEM image shows the presence of SA-AuNPs bound to the cellulose fibers.

Fig. 5 TEM image of CF–AuNPs-2.94 showing the SA-AuNPs bound on the surface of cellulose fibers.

3.8. X-ray powder diffraction (XRPD) of the nanocomposites

Fig. 6 shows the X-ray diffraction patterns of crystalline cellulose as well as the nanocomposites. Three main diffraction peaks at 2θ = 14.5°, 16.4° and 22.4° corresponds to the crystalline faces of 100, 010 and 110 of cellulose Iα, or the 11̄ 0, 110, and 200 crystalline faces of cellulose Iβ allomorph.^44,45 The XRD peaks that originate from these two allomorphs of cellulose usually locate very close to each other, which is difficult to distinguish from XRD spectrum only.

Fig. 6 XRD patterns of (a) cellulose fibers; (b) and (c) composites of CF–AuNPs-2.94 and CF–AuNPs-1.96 respectively.

The additional peaks at 37.7°, 43.8°, 64.1° and 77.2° correspond to the Au planes 111, 200, 220 and 311, respectively, and indicate the presence of face-center cube (fcc) AuNPs.³⁷ Moreover, it can be observed that the intensity of the peaks that originated from the AuNPs increases with the % loading of AuNPs on the cellulose fibers.

3.9. FTIR of the nanocomposites

FTIR was conducted on the nanocomposites of SA-AuNPs, however, only the characteristic peaks of cellulose corresponding to the O–H stretching at 3260 cm⁻¹, the C–H stretching at 2860 cm⁻¹, the C–O stretching at 1024 cm⁻¹, and C–H bending at 1307 cm⁻¹ was observed (see ESI†).

4. Results and discussion

4.1. Synthesis of SA-AuNPs and its nanocomposites

The SA-AuNPs were prepared by adding an aqueous solution of 3.88 equivalents of sodium squarate into a round-bottomed flask containing aqueous solution of hydrogen tetrachloroaurate (HAuCl₄·3H₂O) with vigorous stirring at room temperature. The reaction mixture turned into a deep burgundy color after a few seconds of stirring, indicating the formation of AuNPs as a result of direct reduction by squarate ions and further stabilization by excess squarate.

The nanocomposites were prepared by the addition of cellulose fibers into the SA-AuNPs solution, followed by honk sonication for five minutes, Scheme 1. The nanoparticle solution of SA-AuNPs turned clear without any trace of plasmonic absorption, indicating that all the SA-AuNPs were taken up by the cellulose fiber.

4.2. Catalytic reduction of 4-NP by SA-AuNPs

All experiments were carried out under ambient conditions. The catalytic activity of SA-AuNPs was examined by the reduction of 4-NP into 4-aminophenol (4-AP) in the presence of NaBH₄ as the reducing agent (Scheme 2). The reduction of 4-NP to 4-AP was monitored by UV-vis spectroscopy. The amount of SA-AuNPs used in the catalysis experiments was 5.8 mol% with respect to 4-NP. For the reduction of 4-NP to 4-AP, 0.2 mL of 4-NP aqueous solution (0.001 M) was mixed with 0.8 mL of Milli-Q water followed by 0.5 mL of freshly prepared NaBH₄ aqueous solution (0.1 M) in a standard quartz cuvette. Afterwards, 50 μL of AuNPs stock solution was added into the quartz cuvette and mixed quickly. The reaction course was monitored, every 30 seconds, by the gradual decrease in the absorbance of 4-nitrophenolate at the absorption maximum wavelength at 400 nm. However, the formation of a new absorption band centered at 310 nm was also observed, which is assigned to the formation of 4-AP.⁴⁶

Scheme 2 The SA-AuNPs catalyzed reduction of 4-NP to 4-AP.

Fig. 7 shows the time-dependent UV-vis spectrum and the kinetics of the reduction reaction of 4-NP into 4-AP catalysed by SA-AuNPs. Fig. 7a shows the reduction of 4-NP, where a continuous decrease in the absorbance centered at 400 nm is observed with the lapse in reaction time. This decrease in absorbance is indicative of the reduction of 4-NP into 4-AP. Whereas, the time dependent UV-vis of an un-catalyzed reaction where no AuNPs were used, shows negligible decrease in absorbance at 400 nm even after 16 min of reaction time (see ESI†). This signifies the robust nature of 4-NP to undergo reduction with only NaBH₄.

Fig. 7 (a) UV-vis time-dependant of the reduction of 4-NP catalyzed by SA-AuNPs and (b) pseudo-first order kinetics of catalyzed and uncatalyzed reaction.

Fig. 7b shows the kinetics of the catalytic reactions. A linear relationship between −ln(C_t/C₀) and reaction time (t) at ambient temperature is indicative that the kinetics of the catalysis reaction follows the Langmuir–Hinshelwood (LH) model, where C₀ represents the initial concentration of 4-NP and C_t represents the concentration of 4-NP at time (t). Moreover, the catalytic experiment, where −ln(C_t/C₀) increases linearly with respect to time indicates the course of the reaction follows pseudo-first-order kinetics, which eventually follows the equation kt = −ln(C_t/C₀). As shown in Fig. 7b, the apparent rate constant (k_app) of the catalytic reduction of 4-NP in the presence of SA-AuNPs was calculated to be 5.365 × 10⁻³ s⁻¹. Whereas, the apparent rate constant of the un-catalyzed reaction is calculated to be 2.138 × 10⁻⁴ s⁻¹. It is noteworthy to observe that the induction time for the SA-AuNPs catalyzed reaction is very low or zero, which indicates the fast catalytic activity of the nanoparticles.

To rule out any possibility that there might be any ionic (un-reduced) gold present in the nanoparticles solution, which could account for the catalysis reaction, two complementary control experiments were carried out. First, a stock solution of SA-AuNPs was centrifuged at a speed of 15 000 rpm for 1 hour. The supernatant obtained by this was added to NaBH₄ and UV-vis was taken in situ. There was no color change observed, both by the naked eyes and UV-vis (see ESI†), which implies that there was no ionic gold present in the AuNPs solution and that squarate completely reduced the gold ions to their elemental form.

In the second control experiment, the centrifuged supernatant was used for the catalytic reaction in the same way as the AuNPs catalysed reaction (see ESI†). There was no or a negligible decrease in absorbance at 400 nm observed, which implies that there was no ionic (un-reduced) gold present in the AuNPs stock solution which might have catalysed the reaction mentioned in Scheme 2. This is because sodium squarate was used in large excess compared to HAuCl₄. This also implies that the squarate ions do not catalyze the reactions as well. Therefore, this shows that the SA-AuNPs are the species, which catalysed the reduction of 4-NP to 4-AP in the presence of excess NaBH₄.

4.3. Catalytic reduction of 4-NP by nanocomposites

All experiments were carried out under ambient conditions. In a standard quartz cuvette, 3 mg of the nanocomposite of each type, was added to 2.5 mL of a 1 mM aqueous solution 4-NP, followed by a bath sonication for 5 min. Afterwards, 1 mL of freshly prepared 0.159 M solution NaBH₄ in water was added and mixed quickly. The reaction course was monitored by UV-vis spectroscopy using kinetics software that monitored the lowering of the absorbance at 400 nm with respect to time with a time interval of 10 s per measurement. Fig. 8 shows the kinetics of the reduction of the 4-NP into 4-AP by CF–AuNPs-2.94 and CE–AuNPs-1.96, and cellulose fibers as a control. The results show that −ln(C_t/C₀) increases with respect to the time, which indicates that the reaction course follows pseudo-first-order kinetics. The apparent rate constants of the catalytic reactions are calculated to be 4.280 × 10⁻³ s⁻¹, 1.713 × 10⁻³ s⁻¹ and 5.33 × 10⁻⁵ s⁻¹, for CF–AuNPs-2.94, CE–AuNPs-1.96 and pristine cellulose fibers, respectively. However, it was observed that the cellulose fibers supported SA-AuNPs catalysed reactions had induction periods of about 2 min and 7 min for CF–AuNPs-2.94 and CE–AuNPs-1.96, respectively. This induction period is usually observed mostly in heterogeneous catalysis to obtain the adsorption and desorption equilibrium of the reactants and the products.

Fig. 8 Plot of −ln(C_t/C₀) vs. reaction time during the reduction of 4-NP catalyzed by (a) CF–AuNPs-2.94, (b) CE–AuNPs-1.96 and (c) pristine cellulose fibers (blank catalysis).

In order to evaluate the practical applicability of the cellulose supported SA-AuNPs, a glass chromatographic column having dimensions I.D. × O.D × L = 20 × 26 × 457 mm was packed about 19 cm with the cellulose supported SA-AuNPs mixed with sand (Fig. 9). In detail, 11 mL of SA-AuNPs stock solution (0.0933 mg mL⁻¹) was mixed with 500 mg of cotton fibers followed by sonication for 5 min. The SA-AuNPs bound to the cellulose fibers was thoroughly mixed with 90 g of sea sand with particle size range of 125 to 500 micrometers. Afterwards, the column was filled, by wet method, with cotton and coarse sea sand supports at the bottom of the column. The top of the column was also filled with coarse sea sand for about 4 cm.

Fig. 9 UV-vis spectrum of the inlet 4-NP (4× diluted) and the eluent 4-AP solutions. Inset: an operating column reactor packed with cellulose fibers supported SA-AuNPs mixed with sand.

The column was first washed with 500 mL of DI water. For the continuous catalysis reaction, 60 mg of NaBH₄ (1.586 mmol) was dissolved in 60 mL of 0.5 mM 4-NP solution with constant stirring. The solution was then continuously fed into the column with an initial flow rate of 2.25 mL min⁻¹. However, a gradual decrease in flow was observed because of the back-pressure created by the hydrogen gas that evolved from the NaBH₄. When the whole column was filled with the reaction solution, an average flow of 1.25 mL min⁻¹ was observed. The eluted reaction solution looked clear with generation of micro bubbles, which indicated the excess NaBH₄ was coming out along with 4-AP. The UV-vis spectrum of the eluted solution did not have any absorption band centered at 400 nm. However, a new absorption band was originated at 310 nm, which is the characteristic band observed for 4-AP. This confirms the complete reduction of the 4-NP into 4-AP. The column was used for 20 such successive reactions over a period of 6 months and every time, all 4-NP was reduced to 4-AP, without the loss of any catalytic activity, showing the robustness of the nanocomposites. The % conversion of 4-NP to 4-AP in such 10 cycles (10 to 20) of reduction reaction is shown in Fig. 10. It shows that the 4-NP was reduced to 4-AP more than 99% in every cycle.

Fig. 10 % conversion of 4-NP to 4-AP in 10 successive cycles (10 to 20) of reaction in the column reactor.

5. Conclusions

In conclusion, we report a simple, fast, and green method to prepare gold nanoparticles (AuNPs) using squaric acid as a reducing agent and stabilizer. These nanoparticles can easily attach to cellulose fibers to make active nanocomposites that exhibit excellent catalytic activity for the reduction of 4-NP with NaBH₄. Moreover, a chromatography column was packed with the nanocomposites and demonstrated for the continuous catalytic reduction of 4-NP with NaBH₄ under flow conditions.

Acknowledgements

Financial support from NSF grants CHE-0748913, DMR PREM-1025302, the NEWT-1449500, and USDA 2014-38422-22078 are gratefully acknowledged. We thank Dr Jorge Gardea-Torresdey for kind assistant with the DLS and Zeta-potential measurements.

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Footnote

† Electronic supplementary information (ESI) available: Cheracterization technique, TEM image, dynamic light scattering, zeta potential measurements, Uv-vis and FTIR spectroscopy. See DOI: 10.1039/c6ra17480a

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