external stimulation. Using this switch of the metal ion, metalloproteins can exist with

two different states, which can be distinguished and defined as “0” and “1.” This de­

monstrates the binary-based memory functions in conventional electronic devices [6].

Moreover, since some metalloproteins have different types of metal ions, a higher-order

multibit biomemory can be developed through the simultaneous introduction of different

metalloproteins. In one study, Choi’s group developed the multilevel biomemory device

by using two metalloproteins composed of a recombinant azurin and cytochrome c [7].

Using the cysteine residue of a recombinant azurin and electrostatic interaction between

azurin and cytochrome c, the heterolayer of metalloproteins was prepared on the gold

(Au) substrate, and the redox states of two metalloproteins were regulated by electro­

chemical stimulation to implement the multilevel memory functions (Figure 17.2a). In

addition, metalloproteins have been used to develop other types of bioelectronic devices

such as a biotransistor using their inherent redox properties with excellent reproducibility

(Figure 17.2b) [8].

Another important protein that can be used for bioelectronic devices is an enzyme. An

enzyme acts as a biological catalyst in the metabolic processes of living organisms.

Various enzymes conduct lots of enzymatic reactions to maintain living things. These

numerous enzymatic reactions between enzymes and substances can be used to imple­

ment the logic gate functions on the biochip. The logic gate is one of the core components

FIGURE 17.2

(a) A multilevel biomemory device. Adapted with permission [ 7]. Copyright (2010) John Wiley and Sons. (b) An

azurin-based field-effect biotransistor and transistor properties. Adapted with permission [ 8]. Copyright (2017)

Elsevier. (c) A biologic gate based on enzymatic reactions. Adapted with permission [ 9]. Copyright (2010)

American Chemical Society.

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