Nanozymes: how can nanotechnology explain this?

Hi all,

Hope you are all good and getting nice results to show in the upcoming RAFA conference in Prague. I’m looking forward to going there and discussing my results with so many experts in the field. It was a very nice experience last time (2017) and I have no doubts that it will be the same this year.

As many of you might remember, my work revolves around the application of nanozymes in biosensors. However, some of you could wonder what is this exactly and how it works. And that is a fair point, I also wondered the same until I started working with them. The first question that arises is: why do we even need them? Biological enzymes have left very little room for improvement in terms of catalytic activities and specificity, following the famous “lock and key” model. Being things this way, one could question the necessity of nanozymes to be developed. However, biological enzymes are thought to be produced by our bodies and to work in physiological conditions, but their use in industrial applications (such as development and commercialisation of biosensors) has some drawbacks. Their production cost and stability in standard conditions are two of them. This is why nanozymes have emerged as potential substitutes for biological enzymes, as they can provide some properties not found elsewhere. First of all, they are cheap to produce and can be obtained in large scales. But also they are very stable in standard conditions, they can be kept for long periods of time and they will still keep their activity and they have a rich surface chemistry, making conjugation easy.

Nevertheless, whereas biological enzymes have been exhaustively studied and their mechanisms understood, there is not much fundamental research to explain action mechanisms in nanoparticles. In any case, for a better understanding of the reasons behind this catalytic properties, there is a clear and accepted scientific evidence that nanozyme activity is a surface phenomenon. Thus, the explanation on this activity lies in the surface gold atoms. It has been proved that different coordination numbers for gold atoms in the surface give as a result diverse catalytic efficiencies (see picture below). As a rule of thumb, the lower the coordination number, the higher the catalytic activity observed, with the exception of CN=7, whose catalytic properties are stronger than for CN=6.

CN

All in all, there are evidences enough that the low-coordinated gold atoms in the surface of gold nanoparticles create an electron-rich interface, in which the catalytic activity observed is based. Empirical research in this line has shown that the catalytic activity is optimum for a nanoparticle diameter of 2 nm and a height of 6 atomic monolayers, thus optimising the number of low-coordinated sites which play a major role in such activity.

As a result, peroxidase-mimicking activity is observed in many metallic nanoparticles, as well as different oxidase activities, catalase, etc. This leads to their main drawback: nanozymes are not specific per se, thus requiring biorecognition elements to be immobilized in their surface if we want to use them in a biosensor development.

All the above explain why when the shape or the size of a nanomaterial is change, their surface properties change. The same happens when any molecule is immobilized on the nanoparticle surface.

Hopefully this has given you an idea of how important it is to choose the shape and size of your nanomaterials properly before using them in your assay, if replacing biological enzymes is to become an advantage for your assay.

See you all in Prague!

Javier

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