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, therefore, a need to develop more sustainable methods of synthesising metal nanoparticles that allow precise control over their size and shape. However, bio-based synthesis methods using plant extracts or microorganisms often result in poor uniformity. In addition, there is a lack of sustainable methods for synthesising core-shell NPs, which are composed of two or more materials. In work recently published in Angewandte Chemie, researchers from the University of Oxford demonstrated a mild synthesis method that produced NPs with high uniformity of size and shape. Using high-resolution scanning transmission electron microscopy (HR-STEM) at the electron Physical Science Imaging Centre (ePSIC), they showed that the synthesis could also form core-shell Au@Pt nanoparticles. Their results suggest that this approach could be used to develop a new type of self-synthesised chemo-biocatalyst with wide-ranging applications in biotechnology.
Metal nanoparticles have a wide variety of uses, from drug delivery to catalysis. Smaller NPs are more desirable for catalysis due to their greater reactivity, and gold is often combined with platinum group metals in core-shell NPs to improve reactivity and stability. As traditional synthesis methods rely on harmful chemicals or high temperatures, there is a need to develop more sustainable processes. However, bio-based strategies using plant extracts or micro-organisms struggle to produce NPs with the high uniformity required.
In this work, a research team 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. By varying the concentration of NRase, the researchers were able to precisely control the size of the resulting nanoparticles; higher concentrations of NRase led to smaller nanoparticles, indicating that the enzyme acts as a template for nanoparticle formation.
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.
The team used several imaging techniques to characterise the synthesised nanoparticles, including UV and visible light spectroscopy to monitor the formation of nanoparticles and to estimate their average diameter and transmission electron microscopy (TEM) to directly observe the size, shape, and structure of the nanoparticles. 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.
Christopher Allen, Principal Electron Microscopist at ePSIC commented:
At ePSIC, 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 an incredibly powerful tool. This enables us 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. The work by Professor Vincent's group at ePSIC is a great example of how information about atomic structure can enable us to understand the macroscopic properties of materials.
The researchers also demonstrated that the catalytic activity of the biohybrid Au@Pt NPs created via this process is different to that of either standard metal NPs or NRase. Further experiments showed that NRase-Au@Pt NPs can be used for continuous, H2-driven, atom-efficient recycling of NADH.
This work shows that using the enzyme NRase allows for the synthesis of gold nanoparticles in water under mild conditions, providing a more sustainable alternative to traditional methods that often require high temperatures or harsh chemicals. The resulting NPs are highly uniform in size and shape and can be coated with platinum to form Au@Pt core-shell structures; the final enzyme-metal NP hybrids retain the reactivity of the NRase enzyme, giving the composite catalysts properties of both the enzyme and metal.
This research has significant potential for the development of new technologies and applications in various fields, particularly in biocatalysis, chemical synthesis, and sustainable chemistry, including the development of advanced biocatalytic systems that leverage the unique properties of enzyme-metal nanoparticle hybrids.
To find out more about the electron Physical Science Imaging Centre (ePSIC) or discuss potential applications, please contact Principal Electron Microscopist Chris Allen: [email protected].
Browne LBF et al. Controlled Biocatalytic Synthesis of a Metal Nanoparticle‐Enzyme Hybrid: Demonstration for Catalytic H2‐driven NADH Recycling. Angewandte Chemie International Edition (2024): e202404024. DOI: 10.1002/anie.202404024
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