Catalysis of Gold Nanoparticles within Lysozyme Single Crystals

Bio–nano hybrid materials have been the focus of numerous studies because they combine the merits of both biomolecules and nanomaterials, with potential for a wide range of applications in catalysis, electronic devices, energy conversion and storage, drug delivery, imaging, and sensing. Because of the promise, many biological molecules, such as DNA, proteins, carbohydrates, bacteria, and viruses, have been used to prepare these bio–nano hybrid materials. Among them, proteins have received special attention due to their nanometer sizes, which are comparable to many nanomaterials, and their diverse structures and functionalities, which allow selective recognition for directed assembly. To date, most studies in this area have focused mainly on synthesis of biomolecules and nanomaterials individually before conjugating them together. Taking advantage of protein single crystals as highly ordered 3D assembled structures, we and others have developed a new approach to synthesis of nanoparticle-in-protein crystals via controllable growth of metal nanoparticles within a protein single crystal. By using the chicken egg white lysozyme as a model system, gold nanoparticles (AuNPs) within lysozyme single crystals (shorten as AuNPs@Lys) have been prepared and carefully characterized (Figure 1a,b). While the work offered insights into AuNP growth in the presence of proteins, the functional properties of the system have not been explored. AuNPs are among the most useful nanomaterials due to their unique optical and catalytic properties, high chemical stability, and wide range of applications. A particularly exciting area of research is the application of AuNPs in catalysis. However, detailed understanding of the mechanism of the AuNP catalysis and fine control of the activity has not been achieved. Herein we demonstrate that this new type of nanoparticle-in-protein crystal can be used for catalytic reduction of p-nitrophenol to p-aminophenol by NaBH4 (Figure 1c). [40] The ability of the protein single crystals to control the size of AuNPs within the crystals made it possible for us to find correlations between the size of the particles and catalytic activity. The catalytic activity could be further modulated by tuning AuNPs@Lys growth with external chemicals. AuNPs@Lys crystals were prepared by in situ growth of crystals from lysozyme and [ClAuS ACHTUNGTRENNUNG(CH2CH2OH)2] solutions, as reported. The crystals were collected at different growth stages and used for catalytic reactions. When NaBH4 was added to the solution of p-nitrophenol and AuNPs@Lys, UV/Vis spectra were recorded to monitor the reaction. Figure 2a shows a typical time-dependent evolution of UV/Vis spectra of the reaction catalyzed by AuNPs@Lys after 10 days of growth. Over time, the intensity of the characteristic absorption peak of p-nitrophenol at 400 nm decreased, thus indicating the successful reduction of p-nitrophenol to p-aminophenol. To further confirm the catalytic activity of the AuNPs@Lys, the same reaction was performed in the absence of the AuNPs@Lys catalysts (Figure S1 in the Supporting Information). The reaction solution remained yellow, and the intensity of the absorption peak did not decrease after overnight incubation, thus indicating that p-nitrophenol was not reduced without the catalyst. In the reaction system, NaBH4 was in approximately 100fold excess, which meant that the reaction rate would follow pseudo-first-order kinetics. The linear regression of the logarithm of absorbance of p-nitrophenol at 400 nm (ln ACHTUNGTRENNUNG(A400)) versus reaction time confirmed the pseudo-first-order kinetics. As shown in Figure 2b, the apparent rate constant (k) of the reaction could be calculated from the slope of ln ACHTUNGTRENNUNG(A400) versus time, which was 0.039 0.002 min . Because the sizes of the AuNPs grown within the single crystals of lysozyme can be precisely controlled, AuNPs@Lys crystals grown at different stages were tested to eluci[a] Dr. H. Wei, Prof. Y. Lu Department of Chemistry University of Illinois at Urbana-Champaign Urbana, Illinois 61801 (USA) Fax: (+1)217-244-3186 E-mail : yi-lu@illinois.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100942. Figure 1. a) Formation of AuNPs@Lys crystals and b) the corresponding optical image after 20 days of growth. c) The catalytic reaction within AuNPs@Lys crystals. Note: Parts (a) and (b) were adapted from previous results.