of the computer that conducts logical operations including AND, NOR, and OR logic

functions through the conversion of input signals to the one binary output signals. These

logical operations can be demonstrated by using enzymatic reactions for implementing

Boolean functions on biochips to develop a biologic gate. For instance, by combining

various enzymes (e.g., lactate dehydrogenase, LDH) and their reactions, output signals

from logic operations were determined by added input substances (e.g., lactate) to de­

monstrate certain logic gates such as AND or NAND gates (Figure 17.2c) [9]. However, as

discussed previously, bioelectronic devices implemented using only proteins have lim­

itations such as weak signals or difficulties in implementing into more complex electronic

devices and the inherent instability of biomaterials. Recently, a lot of research on the

fusion of protein-based electronic devices and nanomaterials has been conducted, which

will be the focus of later sections.

17.2.2 Nucleic Acid–Based Bioelectronic Devices

A transistor is one of the most essential electronic devices that can amplify or switch

current to operate a computing system. The increase of transistor density is an important

issue because Si-based electronic devices typically face physical limitations at sizes below

100 nm. Therefore, the demand for materials that can replace Si is increasing, and nucleic

acids are one of the most attractive candidates for replacing Si due to their unique

properties such as charge transfer and molecular rectification [10].

Gottarelli’s group developed the field-effect transistors (FETs) using guanosine

(Figure 17.3a) [11]. Guanosine has the lowest oxidation potential among the nucleobases

and a unique sequence of hydrogen bonding sites. The low oxidation potential of gua­

nosine is suitable for charge carrier transport and forms long ribbon-shaped supramo­

lecular assemblies via hydrogen bonding. In addition, a strong dipole moment can be

formed along the ribbon axis, which induces a commutation of the current for enhancing

the transistor density. Stefanović’s group developed an array of three ribozyme-based

biologic gates to realize an artificial decision-making network in a biocompatible and

autonomous manner (Figure 17.3b) [12]. A Boolean algebra was performed with a total of

three fluorescence dye–modified ribozymes by applying two ribozymes for the XOR gate

construction and another ribozyme for the AND gate construction to generate different

outputs. Two oligonucleotides were used as random two inputs, which allosterically

activate the ribozyme. The output was also an oligonucleotide, and two different col­

orimetric fluorescence detection systems had applied to distinguish these two output

values. In addition, Lee’s group fabricated a global positioning system (GPS) using a

DNA bioprocessor that designated two physical locations, the current location of the

processor and the final destination, and specified the optimal or shortest route based on

the six routes stored in the database (Figure 17.3c) [13]. For simplicity, the map was

simplified into six main pieces of information. An indication that connected the physical

location with the connection route was made by utilizing the length of single-stranded

DNA, which represents the length of the linkage pathway.

In addition, Suyama’s group has developed a biomemory using DNA hybridization [15].

The memory strand was composed of DNA with a hairpin structure. The data strand

consisted of “address” and “content,” where the address was a linear DNA sequence that

can complementarily be hybridized with the memory strand, and the content was a DNA or

RNA sequence (state 1). “Writing” and “erasing” of the DNA-based biomemory were

performed by temperature control. The data strand was hybridized with the memory

strand as the hairpin structure of the memory strand changed by writing temperature (TW)

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