Green synthesis of polysaccharide stabilized gold nanoparticles: chemo catalytic and room temperature operable vapor sensing application

Abstract

A facile, one pot, completely green, and cheap route for the synthesis of gold nanoparticles (AuNPs) has been developed by using locust bean gum (LBG), both as a reducing and a stabilizing agent. Synthesized AuNPs were characterized by UV-vis spectroscopy, TEM, XRD, dynamic light scattering analysis (DLS) and EDAX. A characteristic surface plasmon peak at 537 nm confirmed the formation of AuNPs. Synthesized AuNPs were found to be an efficient catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The reaction follows pseudo-first order kinetics with a rate constant of 14.46 × 10⁻² min⁻¹. Furthermore, the catalytic efficiency of AuNPs for ethanol vapor sensing was investigated by doping AuNPs in a tin oxide (SnO₂) matrix synthesized by a single step thermal decomposition method. The AuNPs doped SnO₂ sensor showed a fast response (∼5 seconds) and excellent ethanol sensing behavior in the range of 10 to 120 ppm at room temperature. A two fold increase in ethanol vapor sensing response was observed with AuNPs doped SnO₂ as compared with the pure SnO₂ sensor.

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Introduction

Increasing awareness about the environment has led researchers to focus on green chemistry strategies by eliminating hazardous ingredients in the design, development and implementation of chemical processes. Use of non-toxic chemicals, environmentally benign solvents and biodegradable materials are some of the important considerations of the green chemistry approach. Nanomaterials are of prime interest to the researchers due to their wide range of applications in the areas of biomedical science, catalysis, optoelectronics, and bio-chemical sensing.^1–5 Among the various metal nanoparticles, gold (Au) is one of the most widespread materials used in wide range of applications because of its stability, less toxicity and biocompatibility.² Green synthesis of Nano Particles (NPs) by using leaf extracts, seed extracts, plant latex, microorganisms and some biopolymers have been reported earlier.^4–6 Various plant polysaccharides, gellan gum, gum arabic have been used as reducing and stabilizing agents for the synthesis of silver and gold NPs.^7–9 Selective synthesis of octahedral Au nanocrystals in high yield by using polyethylene glycol has been reported.¹⁰ In the present work, locust bean gum (LBG) is chosen for green synthesis of AuNPs because it is an easily available, non-toxic, eco-friendly, biodegradable and cost effective biopolymer. It is a polyhydroxylated biopolymer consisting of (1–4)-linked β-D-mannopyranose backbone with branch points from their 6-positions linked to α-D-galactose (i.e., (1–6)-linked α-D-galactopyranose).¹¹ LBG is an effective material used in food and has pharmaceutical applications.¹² Gold nanoparticles have also been widely explored as cancer therapeutics and diagnostic agents in recent years.¹³ The applications of this multifunctional material in new areas like nanoparticle synthesis needs to be explored which can additionally broaden its applications in various areas.

AuNPs are known for their excellent catalytic applications in the selective oxidation of organic compounds including alkenes, alkanes, alcohols etc. from scientific and industrial perspectives.¹⁴ Nitrophenols are widely being used in explosives, dyes and agro-chemicals. They are one of the most common organic pollutants in industrial and agricultural waste water. Therefore, development of efficient methodology for degradation or removal of nitro-phenols is necessary. AuNPs can serve as the electron relay between 4-nitrophenolate ions (oxidant) and BH₄⁻ (reductant) for the catalytic reduction of 4-nitrophenol (4-NP) with sodium borohydride (NaBH₄).^15–17

Besides catalyzing chemical reactions another promising application of AuNPs lies in chemical gas sensors. AuNPs show high catalytic application in alcohol oxidation.¹⁴ Enhanced oxidation of ethanol at the Au based sensor surface improves the sensor performance by several fold.^14,18 The catalytic ability of AuNPs has been widely used to improve the sensing performance and decreasing working temperature of metal oxide based gas sensors.^18–21 Over the past decades, chemical sensors on the basis of semiconductor metal oxide nanomaterials have extensively been developed to detect volatile organic compounds (VOC) and toxic gases.^21–27 Tin oxide (SnO₂), compared to other semiconductor material based sensors exhibited a better response. The low production cost involved and the simple sensor fabrication process are the added advantages in choosing SnO₂ as the sensing material.²⁷ In general, SnO₂ based sensors operate at typical working temperatures between 200 and 450 °C. However, addition of active catalysts is one of the effective strategies used for improving the sensing properties of gas sensors and lowering the working temperature to room temperature.^18–27 For practical applications of chemo-resistive gas sensors with fast response, high sensitivity, low power consumption and mass-produced potency, requires both sensitive materials, and efficient substrate chip for heating and electrical addressing.²⁸ With respect to this SnO₂ nonporous films fabricated on micro-electro-mechanical systems (MEMS) based sensing chip has been developed which showed ultrafast response for ethanol vapors.²⁸ Homogenous thin films are preferable for high-performance gas sensors because of their remarkable reproducibility and long-term stability.²⁹ Low-temperature fabrication route to prepare crack-free and homogenous SnO₂ periodic porous thin films by oxygen plasma irradiation has been reported and found to show response even at 100 ppb of acetone.²⁹

In the present study, green synthesis of AuNPs has been achieved using LBG as a reducing and stabilizing agent. Catalytic applications of synthesized AuNPs have been studied for the NaBH₄ reduction of 4-nitrophenol to 4-aminophenol. Further, application of AuNPs to improve ethanol vapor sensing property of SnO₂ thin films at room temperature has also been investigated.

Experimental

Materials

HAuCl₄ (chloroauric acid, analytical-reagent-grade) and LBG extracted from the seeds of Ceratonia siliqua were purchased from Sigma Aldrich India and used without any pretreatment. Stannous chloride (SnCl₂) and glacial acetic acid (AR Grade) were purchased form Ranbaxy fine chemicals. All solutions used in the experiments were prepared using deionized water.

Synthesis of gold nanoparticles

Rapid, one pot synthesis of AuNPs was achieved by using LBG as a reducing and stabilizing agent. For the synthesis of AuNPs, 0.2 grams of LBG was dissolved in 100 ml deionized water by heating at 80 °C under constant stirring to achieve 0.2% (w/v) LBG solution. 0.2% LBG and 1 mM HAuCl₄ were mixed in 4 : 1 (v/v) proportion (optimized, see ESI, Fig. S1†) in Erlenmeyer flask and the mixture was autoclaved at 120 °C and 15 psi pressure. In order to study the effect of reaction time, AuNPs synthesis was carried out by autoclaving the reaction mixture containing 0.2% LBG and 1 mM HAuCl₄ in 4 : 1 proportion (v/v) for different time intervals (10, 20 and 30 min). Effect of LBG concentration on the synthesis of AuNPs was studied by carrying out the reactions at various concentrations of LBG (0.1 to 0.5% (w/v)) where the HAuCl₄ concentration was kept constant at 1 mM.

Characterization

UV-vis analysis was performed on Shimadzu 1800 UV spectrophotometer operated at a resolution of 1 nm. Structural and morphological characterization was carried out by using HRTEM (JEM-3010, JEOL, Japan), Particle size distribution was measured by BIC 90 Plus Particle Size Analyser (Brookhaven Instruments Corporation, USA). X-ray diffraction (XRD) was performed using an X-ray diffractometer (Phillips PW1710, Holland) with CuKα radiation λ = 1.5405 Å over the wide range of Bragg angles 10–90°.

Catalytic application of AuNPs for 4-nitrophenol reduction

The reduction of 4-NP was studied as a model reaction to prove the catalytic activity of synthesized AuNPs. The catalytic reaction was performed in a standard quartz cuvette with a 1 cm path length. Double distilled water (1.5 ml) was mixed with 75 μl of 2 mM 4-NP solution followed by the addition of 400 μl of 30 mM freshly prepared NaBH₄ solution. Thereafter 200 μl of AuNPs (1.52 × 10⁻² mg ml⁻¹) were added to the above reaction mixture. Immediately after the addition of AuNPs, the absorption spectra were recorded at 2 min time intervals in the range of 200–600 nm at room temperature.

Ethanol vapor sensing application of AuNPs doped SnO₂

In order to evaluate the application of green synthesized AuNPs to improve ethanol vapor sensing property of SnO₂ thin films, AuNPs were doped in SnO₂ matrix. SnO₂ thin films were deposited essentially as described in the previous report with slight modifications.²⁶ Briefly, 2 grams SnCl₂ was added in 8 ml of distilled water and stirred for half an hour. Then 4 ml of glacial acetic acid was added into SnCl₂ solution and stirred again for 1 h at 90 °C to obtain a transparent solution. Varying volumes (viz. 5, 10, and 15 μl) of AuNPs (0.116 mg ml⁻¹) were added to the above SnCl₂ solution in a final reaction volume of 100 μl to get different concentrations of AuNPs (0, 5, 10 and 15% v/v respectively). AuNPs doped SnO₂ thin films were deposited by spin coating 50 μl of the above solution on 1 × 1 cm² soda glass substrates. The dried films were then annealed in the furnace at 550 °C for 45 min resulting in an adherent film with nano-sized SnO₂. Experimental setup used for VOC sensing is shown in Fig. S3 (see ESI†).

Results and discussion

Synthesis of gold nanoparticles

LBG, a non-toxic, eco-friendly, biodegradable and cost effective biopolymer is found to be a good reducing and stabilizing agent for the synthesis of AuNPs. Structure of LBG polymer and mechanism of synthesis of AuNPs at the reducing end of the LBG is shown in Fig. 1. LBG is a polyhydroxylated biopolymer consisting of (1 → 4)-linked β-D-mannopyranose backbone with branch points from their 6-positions linked to α-D-galactose.¹¹ The extensive number of hydroxyl groups and the hemiacetal reducing ends on LBG polysaccharide act as active reaction centers to facilitate the reduction of Au³⁺ to Au⁰ as depicted in Fig. 1. AuNPs, thus formed, get embedded and stabilized within the polymer matrix. UV-vis absorption spectra of AuNPs synthesized by autoclaving the reaction mixture containing 0.2% LBG and 1 mM HAuCl₄ for 20 min is shown in Fig. 2. Appearance of characteristic surface plasmon peak at 537 nm indicates the formation of AuNPs. Inset of Fig. 2 shows the color of LBG solution and of LBG stabilized AuNPs. Appearance of characteristic pink red color indicates the formation of AuNPs.

Fig. 1 Schematic of LBG structure and reaction mechanism of AuNPs synthesis at the reducing end of LBG biopolymer.

Fig. 2 UV-vis spectra of LBG without AuNPs and LBG stabilized AuNPs. Inset figure shows color of LBG solution and LBG stabilized colloidal solution of AuNPs.

Effect of reaction time on synthesis of AuNPs is shown in Fig. 3a. UV-vis absorption peak intensity at 537 nm was found to increase with increase in reaction time from 10 to 20 min, which can be attributed to the increased reduction of Au³⁺ to Au⁰. However, red shift and decrease in absorption peak intensity was observed with further increase in reaction time to 30 min. Hence the autoclaving reaction mixture for 20 min is the optimized reaction time for AuNPs synthesis. As proposed earlier, autoclaving at 120 °C, under the influence of temperature and pressure, biopolymers expand and become more accessible for the metal ions to interact with the available functional groups on the gum.^5,30,31 The large number of hydroxyl groups and the hemiacetal reducing ends on LBG accelerates the reduction of Au(III) to AuNPs. FTIR spectra shows peak at 1728 cm⁻¹ which arises from the carbonyl stretching vibrations at the reducing end of LBG similar to that reported for gum kondagogu (see ESI Fig. S5†).³¹ Also increase in band intensities of carboxylate groups were found after autoclaving the LBG which suggests more extensive oxidation of the gum during AuNPs synthesis.³¹ Similar observations have been reported for gum kondagogu.³¹ In solution, part of the end groups of polysaccharides existed in open chain form. Due to addition of Au³⁺ ions in the reaction mixture, –CHO groups at the end of the polysaccharide chain get oxidized to –COOH group. Disappearance of peak at 1728 cm⁻¹ and increase in peaks intensity at 1642 and 1554 cm⁻¹ (for –COO stretching vibrations) of LBG stabilized AuNPs indicates that the reduction of Au³⁺ is coupled to the oxidation of hemiacetal/aldehyde groups in the formation of AuNPs (see ESI Fig. S5†).

Fig. 3 (a) UV-vis spectra of AuNPs synthesized by autoclaving reaction mixture at 120 °C and 15 psi pressure for different time interval. (b) UV-vis absorption spectra of the AuNPs solutions synthesized at various concentrations of LBG (0.1, 0.2, 0.3, 0.4 and 0.5%) and 1 mM HAuCl₄.

Fig. 3b shows the UV-vis absorption spectra of the resultant AuNPs solutions obtained by autoclaving the reaction mixture containing varying concentrations of LBG (0.1, 0.2, 0.3, 0.4 and 0.5%) with a 1 mM HAuCl₄ for 20 min at 120 °C and 15 psi pressure. UV-vis absorption peak intensity was found to be increased with increase in LBG concentration from 0.1–0.5%. At 0.1% LBG concentration, purple-blue colloidal solution of AuNPs was obtained with a weak and broad absorption peak at 579 nm (Fig. 3b) because this concentration was not sufficient to reduce all Au(III) ions and stabilize the AuNPs effectively as observed in earlier reports.⁸ When the LBG concentration was in the range of 0.2–0.5% (w/v), a slight blue shift was observed in the characteristic absorption peak of the resultant Au colloid solutions, with the increase in LBG concentration. At higher gum concentrations, rate of capping of nanoparticles was excellent due to adequate availability of biopolymer resulting in reduced particle size and increased narrow size distribution.³¹ Dynamic light scattering (DLS) analysis of AuNPs synthesized by autoclaving reaction mixture containing different ratio (v/v) of 1 mM AuHCl₄ and 0.2% LBG at 120 °C and 15 psi pressure shows that the size of the AuNPs decreases with increased volume of LBG (see ESI, Fig. S2†).

Morphologies of the synthesized AuNPs were investigated by TEM analysis. HR-TEM image (Fig. 4a) showed that NPs are of spherical shape. SAED pattern showing bright circular rings assigns to (111), (200), (220 and) (311) planes exhibited the face centered cubic (fcc) crystalline structure of AuNPs (right inset of Fig. 4a). The crystalline nature of AuNPs was further confirmed by X-ray diffraction (XRD) analysis as shown in Fig. 4b. Characteristic peaks for AuNPs were observed at 38.32, 44.33, 64.73, 77.80 and 81.79 corresponding to (111), (200), (220), (311) and (222) planes which further confirms the fcc crystalline structure of AuNPs (JCPDS file no. 4-0784) based on the above XRD data. SEM-EDX analysis confirms the presence of gold as shown in Fig. S4 (see ESI†).

Fig. 4 (a) TEM analysis of AuNPs. Inset left and right of (a) shows HRTEM image with clear lattice fringe and SAED pattern of AuNPs respectively. (b) XRD pattern of AuNPs.

Catalytic application of AuNPs for 4-nitrophenol reduction

Catalytic ability of AuNPs was investigated by studying the reduction of 4-NP with NaBH₄. Absorption at 400 nm remains unaltered for several hours in the absence of AuNPs. However in the presence of AuNPs, the yellow color of the solution gradually disappears which indicates the conversion of 4-NP to 4-amino phenol (4-AP).³² As shown in Fig. 5a, absorption peak intensity of 4-NP at 400 nm decreases gradually with respect to time with subsequent appearance of new peaks at 230 and 300 nm corresponding to the 4-AP.^15,17,33 In order to confirm the catalytic ability of LBG stabilized AuNPs for the reduction of 4-NP to 4-AP, absorbance of 4-NP with NaBH₄ at 400 nm was monitored after addition of LBG and LBG stabilized AuNPs separately. As seen in inset of Fig. 5a, absorbance of 4-NP remained unchanged as a function of reaction time in presence of LBG whereas a gradual decrease was observed in presence of LBG stabilized AuNPs. As initial concentration of NaBH₄ was largely greater than the initial concentration of 4-NP, the rate of reduction is independent of the concentration of NaBH₄, and the reaction could be considered pseudo-first order with respect to the concentration of 4-NP. Fig. 5b shows plot of ln(A_t/A₀) versus time at different concentrations of AuNPs. The linear correlation suggests that the reaction catalyzed by AuNPs followed the pseudo-first order kinetics. The kinetic reaction rate constants (k) were determined from the slopes of the linear relationship and were found to be 7.0 × 10⁻², 12.5 × 10⁻² and 14.46 × 10⁻² min⁻¹ for 0.59 × 10⁻², 1.06 × 10⁻² and 1.52 × 10⁻² mg ml⁻¹ AuNPs respectively. An increase in the rate constants was observed with an increase in the concentration of the AuNPs. The extent of AuNPs capping with LBG was found to have a significant effect on its catalytic performance. Catalytic performance was found to decrease with increase in concentration of capping agent. In preliminary investigation, AuNPs capped with 0.2% LBG was found to give highest catalytic activity as compared with AuNPs capped with 0.3–0.5% LBG and hence all catalytic experiments were carried out using 0.2% LBG capped AuNPs.

Fig. 5 (a) Time-dependent UV-vis absorption spectra for NaBH₄ reduction of 4-NP catalyzed by AuNPs. Inset fig shows absorbance variation with time at 400 nm for NaBH₄-reduction of 4-NP with LBG and LBG stabilized AuNPs. (b) Plots of ln(A_t/A₀) versus time for NaBH₄-reduction of 4-NP at different AuNPs concentration.

Ethanol vapor sensor

AuNPs doped SnO₂ thin films were found to show two fold increase in ethanol vapor sensing property as compared to pure SnO₂. For AuNPs doped SnO₂ nanostructure, the sensor response was found to be highly dependent on ethanol concentration, while no remarkable sensor response was observed for SnO₂ nanostructure without AuNPs. This enhanced vapor sensing may be due to the increased surface to volume ratio of the AuNPs doped SnO₂ which significantly improves the gas diffusion and mass transport in the sensing layers. When ethanol vapor is exposed to Au-based sensors, the high activity of Au in alcohol oxidation may also contribute to the sensor performance.²¹

Fig. 6 shows the response of sensors with various percent of AuNPs (0, 5, 10 and 15% v/v) doped in SnO₂ for different concentrations of ethanol vapor. Here, the sensor response was defined as R_air/R_gas (R_air is the resistance of sensor in atmospheric air; R_gas is the resistance of sensor in gas). The response of SnO₂ without doped AuNPs was found to be negligible up to 80 ppm of ethanol vapor, above which a slight increase in sensing of ethanol vapor was observed up to 120 ppm and saturation thereafter. Sensor response for 10 to 100 ppm ethanol vapors was found to be increased with increase in concentration of AuNPs (0, 5, 10 and 15%) in AuNPs doped SnO₂ thin films. SnO₂ doped with 15% AuNPs showed increased response in the lower concentration range of ethanol vapors (from 10 to 80 ppm) whereas response was found to be decreased with further increase in ethanol vapor concentration. When an n-type oxide semiconductor is exposed to a reducing gas, the resistance is significantly decreased by the release of electrons during the oxidation reaction between the reducing gas and the negatively charged surface oxygen (O²⁻ or O⁻). If the sensor temperature is too low, it is difficult to oxidize the reducing gas by the negatively charged surface oxygen. Therefore, most of the semiconducting metal oxide based gas sensors require high temperature operation. Doping of metal NPs remarkably increases the sensitivity of metal oxides based gas sensors by several folds even at room temperature.^18–27

Fig. 6 Sensor response with different concentration of AuNPs doped in SnO₂ as a function of ethanol concentration at room temperature.

Comparison of some previously reported sensors for detection of ethanol vapors based on metal NPs doped SnO₂ with the present sensor is summarized in Table 1. It can be noted that most of them require high operating temperature. In the present study room temperature operable, green synthesized AuNPs doped SnO₂ based sensor is reported. The present AuNPs doped SnO₂ sensor shows faster response time (∼5 s) than the previously reported Pd doped SnO₂ (50 to 850 s) and Pt doped SnO₂ (∼15 s) sensors.^22,23 Jun Zhang et al. reported catalytic application of AuNPs doped in SnO₂ matrix for enhanced sensing performance for ethanol vapors.²¹ Beside the good sensing response and fast response time (3 s), sensor requires high operating temperature in the range of 180 to 380 °C with maximum sensing response at 300 °C.²¹ Although the present sensor exhibited low sensing range (10 to 120 ppm) as compared to the reported sensors it has the advantage of room temperature operation over the other sensors which are operated at temperature ≥250 °C. Sensing range as well as response of the sensor can be improved with increasing the number of layers, to have full advantage of Debye length with increase in thickness and firing temperature, which has a dominant role in sensing mechanism.³⁴

Table 1 Comparison of some previously reported sensors for detection of ethanol vapors based on metal NPs doped SnO₂ with the present sensor

Material	Operating temperature (° C)	Range of detection (ppm)	Response time	Reference
SnO2 (gold NPs)	300	10–600	3 s	21
SnO2 (Pd)	250	50–800	50 to 850 s	22
SnO2 (Pt)	300	1–1000	∼15 s	23
SnO2 (Cd)	225	0–500	15 to 25 s	24
SnO2 (gold NPs)	Room temperature	10–120	∼5 s	Present work

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Conclusion

Herein, a simple green approach for the synthesis of AuNPs using locust bean gum as a reducing and stabilizing agent has been reported. Synthesized AuNPs were found to be an efficient catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The reaction follows pseudo-first order kinetics with rate constant 14.46 × 10⁻² min⁻¹. AuNPs were doped in SnO₂ matrix and were tested for room temperature operable vapor sensing applications. AuNPs doped SnO₂ showed fast response (∼5 seconds) and good ethanol sensing behaviour at room temperature.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of optimization of LBG and HAuCl₄ (v/v) ratio for AuNPs synthesis, DLS and EDX analysis of AuNPs, Experimental setup for VOC sensing, FTIR analysis. See DOI: 10.1039/c4ra02972k

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