Green Synthesis of a Gold Nanoparticle–Nanocluster Composite Nanostructures Using Trypsin as Linking and Reducing Agents

We report a green synthesis of novel gold nanoparticle–nanocluster composite nanostructures directly using trypsin as linking and reducing agents. Size exclusion chromatography (SEC) and transmission electron microscopy (TEM) reveals that the as-prepared gold nanocomposite (gold nanoparticles-trypsins-nanoclusters, GNPs-Trys-GNCs) is composed of GNPs (gold nanoparticles) with an average diameter of 5.5 nm and the GNCs (gold nanoclusters, about 1 nm)-embedded trypsins (Trys-GNCs), which are attached to the surface of the GNPs. The specific amino acids in the trypsin molecule, like cysteine, methionine, and tyrosine, combined with the unique spatial structures, enable trypsins to bind and reduce the AuCl4– ions, simultaneously forming GNPs and GNCs in one-pot synthesis. Similar to pure GNPs, the GNPs-Trys-GNCs nanocomposite also exhibits an intense surface plasmon resonance (SPR) absorbance at 520 nm. However, it shows an obvious different optical property in the interaction with Pb2+ ions. In the presence of Pb2+ ions, an increased intensity and a slight red-shift of the SPR peak for the GNPs-Trys-GNCs nanocomposite in the UV–vis spectra were observed, while a decreased intensity and large red-shift for pure GNPs were observed in previous studies. Moreover, we found that the SPR intensity linearly increased with Pb2+ concentration from 1.6 to 32.3 μM (R2 = 0.9731). In addition, high-level Pb2+ ions would induce the aggregation of GNPs-Trys-GNCs nanocomposite accompanied by the formation of precipitate. The unique structure and optical property of the GNPs-Trys-GNCs nanocomposite enable it to be used in heavy metal ions sensing and elimination.