**1. Introduction**

Biosensor technology is a promising field where many outputs are continuously produced thanks to physicochemical techniques and biological materials developed every day. In this technology, a biological molecule interacts with the analyte on the biosensor and this generates a physicochemical signal which is detected by the transducer. Biosensor systems are divided into two classes in terms of biological molecules used, either catalytic-based which transforms the analyte or affinitybased which binds the analyte directly [1]. In terms of physicochemical signal transmitter, it can be designed as electrochemical, optic or piezoelectric. Systems that are combined within also continue to be open to development today. For example, both electrochemistry and optical measurements can be made in spectroelectrochemical techniques. Among these techniques, electrochemical systems [2], which are produced at low-costs without being interfered by the properties of the analyte solution, are used mostly.

The most important element of the biosensor is the biorecognition agent which shows the affinity for the analyte. These agents are of biological origin and are biomolecules with specific substrates such as proteins [3], enzymes [4], nucleic acids [5], antibodies [6], cells [7], cell surface channels. Biosensors can be designed by immobilizing these biomolecules on a suitable signal transmitter based on the

working principle. When selecting the biorecognition agent, the specimen containing the analyte to be measured and the characteristics of the reaction that will take place should be considered. It is desired that the biorecognition agent has a maximum activity and that it has a low degree of denaturation.

The living organism's structural and functional strategy is hidden in its genes. The gene structure is based on the bases that form nucleic acids, which are divided into two main groups according to their functions, are deoxyribonucleic acid (DNA) and ribonucleic acids (RNA) [8]. DNA carries the genetic information in the double helix structure that includes adenine (A), guanine (G), cytosine (C) and thymine (T) bases. RNA has two main differences when compared to DNA, which are the inclusion of uracil (U) instead of T and the single stranded nucleic acid structure different from DNA. DNA's double helix structure is formed with hydrogen bonds between the DNA bases of the complementary chains. In the same chain, ester bases between the 5 'OH group on the pentose sugar of a nucleotide and the 3' OH group of the other nucleotide sugar form the single chain structure (**Figure 1**).

Thereby, U, A, G, C and T can be ordered in countless combinations to form genetic sequences. These sequences are copied and then transcribed in the ribosomes to take part in the synthesis of peptides and proteins by encoding one amino acid corresponding to all three bases.

Although they have different structures in different living organisms, the basic function of base sequences does not change. Erwin Chargaff showed that A-T and G-C base pairs in DNA sequences are mutually paired with hydrogen bond [9]. The important point here is that the hydrogen bonds formed between DNA helixes directly affect the physical properties of DNA, since there are three hydrogen bonds between G-C, while there are two hydrogen bonds between A-T. If the G-C ratio is high, it requires more energetic power to separate the double helix. This is a considerable structural characteristic for the electrochemical techniques.

On the other hand, RNA is a single chain nucleic acid produced by using DNA sequences. Besides the three main types of RNA in protein encoding (mRNA, tRNA and rRNA), there are also other types of RNAs that serve in post-translational modifications in DNA replication or as regulators. For example, small nuclear RNA (snRNA) involved in RNA cleavage, guide RNA (gRNA) involved in CRISPR-Cas9 system, micro RNA (miRNA), small interfering RNA (siRNA) and viral RNA. These different nucleic acids are worth to identify their function and structure [10].

Today, DNA or RNA analysis can be done quite precisely. Therefore, genetic sequences have a central role in diagnosis and treatment processes and they guide the scientists for development of new techniques. RNA and DNA sequences can be easily illuminated with polymerase chain reactions (PCR) and next-generation sequencing analyzers [11]. Like every method, these methods have limitations. For PCR, it is necessary to use consumables for reproduction and analysis of the nucleic acid sequences. This increases analysis costs and affects the analysis time. The disadvantages of the methods naturally make it inevitable to develop new and efficient methods. Although there are different nucleic acid analysis methods, simple and precise methods are needed. Bioelectronics systems are preferable as they are in the front line with their low cost, fast analysis time, minimum consumable requirement and lower margin of errors. Efficient properties of biological molecules with physicochemical transducer sensitivity increase the preferability. These basic mini analyzers are able to provide fast, low cost and precise analysis, based on the immobilizing biological molecules on a physicochemical transducer. The term biosensor is developed by immobilizing a biomolecule of biological origin that provides the biochemical reaction on a transducer. In terms of classification, it can be divided into different areas according to both the working principle of the biomolecule and the working principle of the transducer.

*Nucleic Acids for Electrochemical Biosensor Technology DOI: http://dx.doi.org/10.5772/intechopen.93968*

**Figure 1.** *Representation of the hydrogen bonds between DNA chains.*

In electrochemical nucleic acid biosensor systems, the electrical signal occurs because of the interaction between the biorecognition agent and the analyte. As a result of the biochemical reaction, if an electron is formed, amperometric measurement can be performed. If a molecule is formed it can be detected by potentiometric or direct affinity-based binding can be measured as impedimetric/capacitive. When there is no molecule being exposed, only affinity-based biosensor systems can be designed amperometrically. In this design, when the interaction between biomolecules occurs, a secondary molecule (label) can generate an electrochemical signal. Measurements can be performed through the electrochemical activity of the label used here. Electrical conductivity is an extremely important parameter, especially in biosensor systems. Nucleic acids, on the other hand, can be considered as ideal transistors, because of their structure, they show conductive nanocable characteristics and are very efficient in use in electrochemical biosensors in order to be found in many different conformations. The DNA helix has between 3.4-angstrom base pairs, and the aromatic ring structure facilitates electron flow. This structure, that is the closeness between the bases, is similar to the Z-directional space of graphite and provides conductivity. Moreover, the π electrons on DNA also help electrical conductivity [10]. They help electrochemical conductivity due to structural variability.

On the other hand, beside the electroactivity of DNA, the electrochemical measurement method is important in the design of nucleic acid biosensors. Some electroactive secondary molecules can be used for the electrochemical detection. Mediators, which are frequently used in biosensor systems, can also be used to measure molecules that interact with DNA. Mediator is an intermediary molecule that facilitates electron exchange in an electrochemical reaction and lowers the reduction/oxidation potential of the detection system and also has regeneration potential [1]. The most important feature of a mediator is that it can signal in a low and narrow potential ranges. This feature increases the sensitivity of the biosensor since signaling of other electroactive species can be prevented in lower potentials.

Electroactive species can be determined using techniques such as differential pulse voltammetry (DPV) and cyclic voltammetry (CV) in studies that require the measurement of electroactive species, and the measurement of the amount of DNA through the reduction or oxidation of these species. The basic principle of the measurements is the presence of electroactive species on DNA (guanine base) or the formation of signals by binding some mediators or indicators to DNA. Indicators such as ruthenium complexes, ferrocene and methylene blue are often used for nucleic acid based biosensors. Measurement can be performed by chemically marking the DNA or RNA at the end or forming a complex with DNA helix. Measuring current generated by electroactive species in these electrochemical techniques is extremely important, but can also limit the effectiveness of measurement systems. In other electrochemical measurement systems where mediators are not used, different electrochemical techniques that can measure nucleic acid binding or conformational changes may be used. These methods are performed in redox probe solutions to characterize electrode surface. Here, CV is capable of measuring physical changes on the surface with the help of a redox probe. When the sensitivity of CV in affinity based non-electroactive detections is insufficient, electrochemical impedance spectroscopy (EIS) or capacitance (C) measurement can also be used in affinity-based nucleic acid biosensors. These two methods are really sensitive methods to detect biomolecules, charge transfers and mass transfers with reaction kinetics and also are affected by electrode surface charges [12]. Therefore, nucleic acid length, base composition, and conformational changes after target molecule binding had to be considered before study design. Moreover, the negatively charged redox probe usage reduces the interference of the redox probe interaction between nucleic acid in the measurement.

Another advantage of nucleic acids is that there is no denaturation or loss of activity like proteins. The most important point to consider here is to take measures to reduce the effectiveness of DNase and RNase enzymes that break down nucleic

acids. Some examples of methods that can be used when designing a biosensor system with nucleic acids are given in the following pages.
