Carbon Nanomaterials (CNMs) and Enzymes: From Nanozymes to CNM-Enzyme Conjugates and Biodegradation

Abstract

Carbon nanomaterials (CNMs) and enzymes differ significantly in terms of their physico-chemical properties-their handling and characterization require very different specialized skills. Therefore, their combination is not trivial. Numerous studies exist at the interface between these two components-especially in the area of sensing-but also involving biofuel cells, biocatalysis, and even biomedical applications including innovative therapeutic approaches and theranostics. Finally, enzymes that are capable of biodegrading CNMs have been identified, and they may play an important role in controlling the environmental fate of these structures after their use. CNMs' widespread use has created more and more opportunities for their entry into the environment, and thus it becomes increasingly important to understand how to biodegrade them. In this concise review, we will cover the progress made in the last five years on this exciting topic, focusing on the applications, and concluding with future perspectives on research combining carbon nanomaterials and enzymes.

Keywords: carbon nano-onions; carbon nanodots; carbon nanomaterials; carbon nanotubes; enzymes; fuel cells; graphene; medicine; nanozymes; sensing.

Figure 1

Seven enzyme classes constituting the first component of the Enzyme Commission (EC) number.

Figure 2

Main types of carbon nanostructures (not to scale). Reproduced from [26] under a Creative Commons license. The CNO schematic structure is adapted with permission from [27], copyright ©1996, Elsevier.

Figure 3

A literature search on carbon nanomaterials and enzymes focused on the last decade (Source: Scopus 14 November 2021).

Figure 4

CNMs typically used for peroxidase mimicry (left) and a possible reaction mechanism that ultimately generates hydroxyl radicals for the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to a colored product (oxTMB). Reprinted with permission from [300], Copyright © 2022, American Chemical Society.

Figure 5

(a) Optimized binding modes between hydrogen peroxide and CNTs either in their pristine form (p-CNTs) or with different oxygen-bearing functional groups. (b) Binding energies between hydrogen peroxide and the various CNT types shown in (a). (c) Schematic illustration of peroxidase mimicry by oxidized CNTs. Reprinted with permission from [320], Copyright © 2022 American Chemical Society.

Figure 6

Front-view (A) and vertical section (B) of the surface of the active pocket of acetylcholinesterase, with the peripheral anionic site (PAS) giving access, through a narrow gorge, to the catalytic active site (CAS). (C) Fullerene (brown sphere) can interact with the PAS through hydrophobic interactions with the enzyme surface, whose lipophilic potential (LP) is color-coded from brown (highest hydrophobicity) to blue (highest hydrophilicity). Reproduced with permission from [349], Copyright © 2022, American Chemical Society.

Figure 7

Graphene is one of the most popular CNMs, employed in a variety of biosensing devices thanks to its exceptional electronic and mechanical properties. Reproduced with permission from [368], Copyright © 2022 American Chemical Society.

Figure 8

Screen-printed electrode preparation using a conductive ink based on graphite and CNOs. Reproduced from [369], under a Creative Commons license.

Figure 9

Schematic representation of a biofuel cells with enzymes at the bioanode, where the fuel is oxidized, and the biocathode, where oxidants are reduced. Reproduced with permission from [374], Copyright © 2022, American Chemical Society.

Figure 10

Enzymatic biodegradation of GO as a green production method of graphene quantum dots. Reproduced with permission from [389], Copyright © 2022, American Chemical Society.