melamine or adding both melamine and human serum albumin, resulting in a change

of color of AuNPs based on the degree of aggregation. Tan’s group used an organic

material to fabricate a copper-organic framework (Cu-MOF) [40]. Due to the strong

affinity between Cu NP and pyrophosphate (PPi), PPi inhibited the catalytic activity of

Cu-MOF. However, with the addition of alkaline phosphatase (ALP) that hydrolyzes

PPi, the catalytic activity of Cu-MOF was recovered. Then, the 2,2’-azinobis(3-

ethylbenzothiazoline)-6-sulfonic acid (ABTS) was added to Cu-MOF for investigation

of catalytic activity of Cu-MOF. The colorless ABTS turned green when it was oxidized

by the catalytic active Cu-MOF. The intensity corresponded to the degree of activity of

Cu-MOF. Based on this, an IMPLICATION biologic gate was developed by using PPi

and ALP as inputs and the change of the color of ABTS as an output. Taken together,

with the unique properties of nanomaterials, high specificity, and the electron transfer

efficiency of proteins, research on the multifunctional biologic gate presents the huge

potential for further development in the field of bioelectronic devices.

17.4.3 Biotransistor

The transistor plays an important role in operating computing systems through ampli­

fying or switching electronic signals. It is essential to develop a biotransistor for the

construction of a biocomputer [41]. To solve the limitations of biomaterials for developing

biotransistors, research on the combination of nanomaterials and proteins has been

conducted. Das’s group fabricated a back-gate biotransistor with a 300 nm-long channel

using the Azurin-TiO2 hybrid nanostructure [42]. Azurin has been widely used in bioe­

lectronic devices because of its intrinsic redox property. TiO2 is suitable for UV detection

due to its wide bandgap and high photocatalytic rate but has some limitations such as

low reactivity and weak photocurrent. By combining Azurin and TiO2, the developed

biotransistor exhibited a wide bandgap, high photocatalytic efficiency, fast spectral re­

sponse, and high photocurrent.

In another study, Chaturvedi’s group developed a FET using a hybrid film composed of

bacteriorhodopsin and single-walled carbon nanotubes (SWCNTs) (Figure 17.5c) [36]. The

2D structure of the bacteriorhodopsin formed a photo-active center, and SWCNT acted as

the highly conductive electronic scaffolds. The developed FET did not exhibit gate control

function until the SWCNT/bacteriorhodopsin was immobilized. However, an electrode

immobilized with the SWCNT/bacteriorhodopsin had the properties of gate control si­

milar to semiconductors. This phenomenon was manifested by electron transport of

SWCNT and further enhanced by bacteriorhodopsin used as an optically active proton

pump. The developed FET exhibited the n-type semiconducting characteristics in dark

conditions, but in bright conditions, it had p-type semiconducting characteristics because

of a proton charge transfer from the bacteriorhodopsin to the SWCNT. Furthermore, the

fabricated FET showed an “On” state for positive gate voltages under dark conditions

and negative gate voltages under bright conditions.

Protein-based biotransistors have been also applied to biosensing to broaden their

applicability. The immunoFET, which uses an antibody fixed on the surface of the oxide

between the source and drain electrodes for target molecule sensing, has been reported as

achieving label-free target detection. Kim’s group developed a biosensor for detection of

the SARS-CoC-2 spike protein using an antibody against the SARS-CoV-2 spike protein

immobilized on a graphene-based biotransistor [43]. As such, many studies are being

conducted to improve the stability and performance of biotransistors through combining

nanomaterials and proteins.

Nanomaterial-Assisted Devices

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