Laboratory Experiment pubs.acs.org/jchemeduc
Microwave-Assisted Synthesis of Red-Light Emitting Au Nanoclusters with the Use of Egg White Jinghan Tian,† Lei Yan,† Aohua Sang,‡ Hongyan Yuan,§ Baozhan Zheng,† and Dan Xiao*,†,§ †
College of Chemistry, Sichuan University, No. 29 Wangjiang Road, Chengdu, China College of Chemistry, Sured Instrument Facility, University of British Columbia, Vancouver, British Columbia, Canada § College of Chemical Engineering, Sichuan University, No. 29 Wangjiang Road, Chengdu, China ‡
S Supporting Information *
ABSTRACT: We developed a simple, cost-effective, and eco-friendly method to synthesize gold nanoclusters (AuNCs) with red fluorescence. The experiment was performed using HAuCl4, egg white, Na2CO3 (known as soda ash or washing soda), and a microwave oven. In our experiment, fluorescent AuNCs were prepared within a reaction time of 10 min. The asprepared AuNCs (4 to 8 nm diameter) exhibited an emission peak at 628 nm. The undergraduate students in the laboratory course synthesized fluorescent AuNCs via the proposed synthesis route. Furthermore, the students learned certain basic knowledge regarding nanomaterials and acquired useful instrumental skills.
KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Fluorescence Spectroscopy, Green Chemistry, Instrumental Methods, Nanotechnology, Proteins/Peptides
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accurately.19 However, methanol is often used as a solvent, and the thiolate reagents used in this method are typically toxic.17 Xie et al. described a novel and green method to synthesize AuNCs using bovine serum albumin as both reducing and capping agent.20 Moreover, biomolecules such as lysozyme,21 glutathione,22 and histidine23 have also been used to synthesize AuNCs with blue, green, and red fluorescence. These methods provide a simple and eco-friendly synthesis routine, but the reaction time is relatively long (1 h to1 day).19,24 In addition, several of these reactions require elevated temperatures or boiling to decrease the reaction time.25 At present, microwave-assisted techniques present a promising method to prepare nanomaterials.26−30 In teaching undergraduate laboratory, microwave-assisted methods have been adopted to synthesize aspirin31 and prepare biodiesel from vegetable oil.32
old nanoparticles (AuNPs) or nanoclusters (AuNCs) have attracted substantial attention because of their unique electronic, optical, and catalytic properties.1−5 AuNPs are well-known for their distance-dependent surface plasma resonance.6,7 Specifically, the color of AuNPs changes with the aggregation of AuNPs.8 Unlike AuNPs, AuNCs are composed of several to hundreds of Au atoms and exhibit molecular-like properties, including size-dependent fluorescence and discrete electronic states.9−13 Owing to their good stability, excellent fluorescent property, low cytotoxicity, and biocompatibility, AuNCs have been widely used in sensing and labeling.13−15 The method used to synthesize AuNCs can be divided into two categories: (1) bottom-up, or the chemical reduction of gold salt in the presence of stabilizing agents; and (2) top-down, or etching of bulk gold or AuNPs into smaller AuNCs. Etching is rarely used because of its complicated methodology;16 the reduction of Au3+ by NaBH4 in solution with thiolates as stabilizer is often adopted.17,18 Through this method, the number of Au atoms in each cluster can be controlled © XXXX American Chemical Society and Division of Chemical Education, Inc.
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dx.doi.org/10.1021/ed400605y | J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
Figure 1. A: Fluorescence emission spectra of AuNCs synthesized by different MW irradiation programs: (1) intermittent irradiation program, which involved an initial 2 min MW irradiation, 2 min pause, and then 2 min MW irradiation; and (2) 4 min continuous irradiation. B: Fluorescence emission spectra of the reaction mixture prepared (1) by a typical experiment; (2) by displacing HAuCl4 with DI water; and (3) without MW irradiation. C and D: Fluorescence intensity of AuNCs synthesized using different concentrations of HAuCl4 and Na2CO3. The concentration of Na2CO3 was 0.5 M in C, and the concentration of HAuCl4 was 10 mM in D. The fluorescence spectra were obtained at 370 nm excitation wavelength.
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In this study, we developed a microwave-assisted synthesis of Au nanoclusters with the use of egg white. Eggs are a common and inexpensive material that people encounter in their daily lives. Egg white, a common food stuff, is not usually used as a chemical reagent. Recently, Li et al.33 reported a similar route to synthesize luminescent Au and Pt NCs using egg white at room temperature. Their method offers solid support to our experiment. Another case of egg white protected AuNCs is reported by Kong et al.34 However, both methods involve reaction times that exceed 12 h, which is too long for a laboratory class. In our laboratory experiment, the products can be prepared and characterized in 1 h. The characterization includes UV−visible and fluorescent spectrometric methods, which undergraduate students are required to perform skillfully. Students in our class showed great interest and successfully completed the experiment. In addition, several students remarked about requiring a more specific knowledge about conducting a scientific experiment. More details are given in the Supporting Information.
EXPERIMENTAL SECTION
Chemicals
Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O) was obtained from Aldrich (Milwaukee, WI, USA). Na2CO3was obtained from Chengdu Kelong Chemicals (Chengdu, China). Fresh chicken eggs (stored for less than 10 days since they were laid) were purchased from Wangjiang campus supermarket. Doubly distilled deionized water was used throughout all the experiments. A Panasonic NN-5208 microwave oven (Kyoto, Japan) was used to provide microwave irradiation. Synthesis Procedure
All glassware was washed with deionized (DI) water. In a typical experiment, 100 μL of 0.5 M Na2CO3 solution was added to 500 μL of egg white in a 30 mL glass vial, and then 500 μL of 10 mM HAuCl4 solution was added. Then the mixture was thoroughly mixed by shaking and then heated by low microwave (MW) irradiation (100 to 200 W) twice: 2 min MW irradiation, 2 min pause, and then another 2 min MW irradiation. The reaction mixture changed from turbid light yellow to clear dark yellow. The mixture also displayed red fluorescence under UV light, which indicates the formation of AuNCs. The as-prepared AuNCs were then characterized by B
dx.doi.org/10.1021/ed400605y | J. Chem. Educ. XXXX, XXX, XXX−XXX
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fluorescence and UV−visible spectroscopy, as well as transmission electron microscopy (TEM). Instrumentation
Fluorescence spectra were obtained on a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan). Absorption spectra were obtained on a Techcomp UV1100 UV−visible absorption spectrophotometer (Beijing, China). TEM was performed by an FEI Tecnai F-20 field-emission HRTEM (Hillsboro, OR, USA) operating at 200 kV.
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HAZARDS Hydrogen tetrachloroaurate(III) (HAuCl4) is corrosive and could erode exposed skin. HAuCl 4 solutions at low concentration are relatively safe, but should be handled with special caution. The temperature of the glass vial is very high right after MW irradiation; thus, students should touch the vial only after cooling.
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RESULTS AND DISCUSSION A microwave oven generates an alternating electric field, which induces the vibration of the molecules of the material in the chamber. Heat is then released as a result of molecular friction. These steps explain how a microwave oven works as a heating device.26 Water is an ideal solvent to absorb MW irradiation because of its high dielectric loss constant.30 Egg white is 70% water, and thus it can absorb MW irradiation to create fast heating. In addition, microwave radiation has superheating and nonthermal effects on proteins. Thus, egg white-protected AuNCs with strong fluorescence can be obtained rapidly. Furthermore, an intermittent irradiation program was designed to prevent overheating. AuNCs that are prepared by continuous irradiation for 4 min exhibit considerably lower emissions than those prepared using the intermittent program of an initial 2 min MW irradiation, 2 min pause, and then another 2 min MW irradiation (Figure 1A). A wavelength of 370 nm was selected as the optimum excitation wavelength, and a broad band (570 to 680 nm) with an emission peak at 628 nm is distinguished in Figure 1A, which is consistent with the descriptions of Li et al.33 As shown in Figure 1B, egg white is composed of different proteins and displays background fluorescence emission in a broad wavelength range. Curve 2 of Figure 1B shows the fluorescence of egg white mixed with Na2CO3 after MW irradiation. The background fluorescence emission of the egg white is significantly weaker than that of the AuNCs (curve 1). Curve 3 in Figure 1B shows that the reaction mixture does not exhibit fluorescence without MW irradiation. The concentrations of HAuCl4 and Na2CO3 are crucial to the formation of AuNCs. Figure 1C shows that the highest fluorescence of the products in our experiments is achieved with 10 mM HAuCl4. When the HAuCl4 concentration exceeds 10 mM, fluorescence intensity decreases with the increase in HAuCl4 concentration. Previous reports stated that tyrosine or tryptophan can reduce Au3+ in an alkaline pH.35 According to the literature, ∼6.5% of egg white (weight of dry protein) consists of tyrosine and tryptophan.36 As a weak base, Na2CO3 performs an important function of maintaining a balanced alkaline condition. The detailed mechanism needs further exploration, and our group is still working on discovering more chemical functions of egg white. Figure 2A shows the UV−visible absorption spectra of the reaction mixture before (1) and after (2) MW irradiation. The difference in absorbance indicates the formation of a new
Figure 2. A: UV−visible absorption spectra of the reaction mixture before (1) and after (2) MW irradiation. The inserted images show the reaction mixture before (1) and after (2) MW irradiation under daylight (left) and 365 nm UV light (right). B: TEM image of the egg white-AuNCs prepared in a typical experiment.
substance and the change in the microscopic structure of the egg white. In our experiment, the thick and viscous egg white is ideal for trapping and isolating AuNCs from each other. During the reaction, Au atoms form by the in situ reduction of Au3+, and then neighboring Au atoms aggregate to form AuNCs. Simultaneously, egg white surrounds the clusters to impede the agglomeration of too many atoms and prevent the further growth of the clusters. Thus, we can obtain protein-protected AuNCs in appropriate sizes. The TEM image (Figure 2B) shows that the particle sizes of the AuNCs range from 4 to 8 nm. If the reaction mixture was heated too violently (e.g., in the continuous 4 min MW irradiation program), the reaction would be so rapid that the egg white could not coat the clusters. This effect explains the need for an intermittent heating program. Moreover, owing to the protection of egg white, our red fluorescent AuNCs are stable in mildly acidic (>pH 3) and mildly alkaline (
