Annual Review 2024-2025

D I A M O N D L I G H T S O U R C E L I M I T E D 17 A greener route to gold nanoparticles Gold nanoparticles (NPs) are used in a variety of applications including catalysis, drug delivery, biosensing, and electronics. Traditional methods for producing gold NPs often involve harsh conditions and tend to produce larger NPs (10-200 nm). Smaller gold NPs (less than 10 nm) are more desirable for catalysis, because their higher surface area to volume ratio offers a higher number of catalytically active surface sites, and hence greater reactivity. There is a need to develop more sustainable methods of synthesising metal nanoparticles that allow precise control over their size and shape. In addition, there is a lack of sustainable methods for synthesising core-shell NPs, which are composed of two or more materials. Researchers from the University of Oxford developed a more sustainable method for synthesising metal nanoparticles using an isolated enzyme, NAD+ reductase (NRase), to achieve better control over size, shape, and catalytic activity. They used NRase to reduce gold (Au) salts, in a process that involves the oxidation of NADH at the enzyme’s active site, which releases electrons used for the reduction of the metal salts. The new process resulted in the formation of highly uniform, spherical gold nanoparticles. The team was also able to use the process to synthesise core-shell NPs. After forming a gold NP, they found that adding platinum salts and more NADH resulted in the deposition of a platinum (Pt) shell over the gold core. Using HR-STEM at ePSIC allowed them to confirm the core-shell structure of Au@Pt NPs, with the results showing a higher ratio of platinum in the outer layers and gold (Au) in the centre. The ability to simultaneously acquire atomic resolution images - which tells us where the atoms are - with energy dispersive X-ray spectroscopy - which tells us what the atoms are - is a powerful tool. This enables researchers to develop a fundamental understanding of the chemistry that is occurring during a catalytic process, which in turn can help us to develop increasingly efficient catalyst materials. DOI: 10.1002/anie.202404024 I M A G I N G A N D M I C R O S C O P Y G R O U P Synthesis of core-shell Au@Pt NPs using NRase to re- duce Au(III) followed by Pt(IV). A) UV/Vis spectra record- ed after addition of K 2 PtCl 6 and NADH to a solution of NRase and Au NPs (0 h) and after 21 and 48 h. Inset is a HR-STEM (BF) image of a NP after 48 h. B) EDX data of the line scan shown in the inset HR-STEM (HAADF) image, of sample after 48 h, showing a Pt shell and gold core, as depicted in the inset diagram (top left). A B Wavelength (nm) Distance (nm) Absorbance (a.u.) Relative atomic %