Mixed protein-templated luminescent metal clusters (Au and Pt) for H2O2 sensing
© Li et al.; licensee Springer. 2013
Received: 3 March 2013
Accepted: 5 April 2013
Published: 19 April 2013
A simple and cost-effective method to synthesize the luminescent noble metal clusters (Au and Pt) in chicken egg white aqueous solution at room temperature is reported. The red-emitting Au cluster is used as fluorescent probe for sensitive detection of H2O2.
Owing to their ultra-small size, good biocompatibility and intriguing physicochemical properties, noble metal clusters show significant promise in biolabeling/bioimaging, sensing, catalysis, and optoelectronic nanodevices [1–7]. In general, there are two pathways to synthesize these fascinating materials: chemical and biological methods. The chemical method mainly includes (1) monolayer-protected method , (2) ligand etching method , (3) protection-deprotection method , and (4) template-assisted method . Although atomically precise clusters with different species have been successfully obtained by these methods, from the ‘12 principles of developing green chemistry,’ there are still many problems to be resolved, such as the elaborate preparation procedure, the heavy use of organic solutes and/or surfactants and/or hazardous regents, and the high reaction temperature and long reaction times . Compared with the chemical method, the biological method particularly refers to the template method, which is inspired by biomineralization behavior of organisms in nature. A variety of biomolecules such as amino acids , peptides , DNA [15, 16], and proteins  have been used as template or scaffold to synthesize noble metal clusters. Among them, protein-protected luminescent noble metal clusters are of particular interest due to their simple preparation and potential applications . Up to now, some proteins (including bovine serum albumin [19–23], lysozyme , transferrin family protein , human serum transferrin , pepsin ) have been widely explored to synthesize noble metal clusters. However, most proteins used are expensive, which hinders their further development in preparing production-level commercial-scale materials. It is worth noting that Shao et al. successfully synthesized Au and Ag clusters by using a kind of cheap materials - egg shell membrane - as template . However, the use of hazardous reducer (NaBH4) and special treatment (UV illumination) is not environmentally friendly. In addition, the resulting products existing only in the form of a solid state greatly hinder their wide-range use. In order to satisfy the trends of developing green nanoscience and industrial production , a simple, green, cost-effective, and flexible strategy of preparing noble metal clusters is urgently required.
Inspired by the work of Shao et al., herein, we report a one-pot green process to synthesize noble metal clusters (Au and Pt) with different luminescences by using chicken egg white as templates at room temperature in aqueous solution. Compared with the existing methods, the egg white-templated synthesis has several prominent advantages: (a) the raw material (chicken egg white) is easily available and rather cheap; (b) the reaction condition is mild, reacts at room temperature, and requires no extra energy consumption (even no stirring); (c) the use of organic solvents, hazardous agents, or surfactants is avoided; (d) the synthesis procedure is very simple, just by mixing two aqueous solutions; (e) the luminescences are strong and tunable by changing the concentrations of metal salt solution; and (f) the resulting products can exist in the form of liquid and solid states, which are flexible to create complex patterns. In addition, these as-prepared clusters were also used to detect H2O2, which shows a high sensitivity with a limited detection of 1.0 × 10−7 M.
Results and discussion
Regarding the formation mechanism, as put forward by Xie et al. , with the egg white as stabilizing host material providing a confined space that limits cluster growth and impedes agglomeration, the formation process consists of the trapping and interacting of metal ions, followed by reduction and growth at highly alkaline pH. During the process, the aromatic amino acids in proteins would donate electrons to reduce metal ions; meanwhile, the broken disulphide bonds would stabilize these nucleated clusters. Considering the complexity of proteins in egg white, it might take us a long time to make the mechanism clear. In spite of this, some questions remain haunting us, such as the following: What happened during the 12 h of evolution of clusters in the mixed proteins ? Is one or more proteins involved in the formation of metal clusters? What is the number of metal atoms in the cluster core? Is it possible to synthesize metal clusters using plant or animal extracts by adopting a similar method [29–31]? What is the luminescent mechanism of metal clusters in mixed proteins? Further work in our group is being actively explored towards these questions.
There are many reports about fabricating luminescent sensors based on metal clusters [32–35]. Herein, the as-prepared Au clusters were also used as a highly sensitive sensor for the identification of H2O2, which is a kind of important small-molecule compounds in the environment and bioanalytical sciences. We found that the luminescence of the Au cluster is quenched in the presence of H2O2.
In conclusion, we have developed ‘a real green way’ to synthesize noble metal clusters (Au and Pt) by using chicken egg white as template. The method is simple; source-, energy-, and cost-effective; and environmentally friendly. The resulting products show high luminescence and stability both in the form of liquid and solid states. The red-emitting Au clusters show high sensitivity to H2O2. By using our method, the commercial production in scale is feasible. We believe that the egg white-templated noble metal clusters (Au and Pt) will find important application potentials in the field of catalysis, bioimaging (contrast agents), biolabeling, sensors, and optoelectronic devices. Following this, some interesting ideas are also suggested: since egg white can also be used for the preparation of vaccines, it seems that our method could throw insight into the development of multi-functional vaccines as well as some multi-functional food additives and antibacterial agents. We will expect a bright future for these clusters.
This work is supported by the China Nano 973 Project (nos. 2010CB933901 and 2011CB933100) and Natural Science Foundation of China (nos. 61008029, 31170961, and 51102049).
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