# Electrochemical Biosensors Based on Conducting Polymers: A Review
## 4 Conducting Polymer-Based Electrochemical Biosensors
### Conducting Polymer-Based DNA Biosensors  
There is a great interest in the development of easy-to-use DNA biosensors, since detection of specific DNA sequences is of great importance in medical research and clinical diagnosis, in particular for DNA diagnostics and gene analysis. DNA biosensors generally rely on the immobilization of a single  
Figure 5: Schematic representation of the protocol for surface modification and immunosensing. SPCE: Screen-printed carbon electrodes, PANI: polyaniline, AuNCs: gold nanocrystals, HSA: human serum albumin, Ab-HSA: anti-HAS, EIS: electrochemical impedance spectroscopy. (**a**) screen-printed working electrode ; (**b**) polyaniline ; (**c**) Au nanocrystals (**d**) oxidized HSA antibody (**e**) BSA (**g**) the evolution of impedance (**g**) HSA Reproduced with permission of [122], Copyright 2019, MDPI. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Active Layer** & **Target** & **Detection Mode** & **Linear Range** & **Detection Limit** & **Ref.** \\ \hline Polyaniline-poly(vinylsulfonic acid) & \multirow{2}{*}{atrazine} & \multirow{2}{*}{amperometry} & 0.12-5 uM & 0.1 \(\mu\)g/L & [118] \\ Polythiophene derivative & & & & & \\ (with—COOH groups) & & & & & \\ Polypyrrole-polythionine & neuron-specific conlase & voltammetry & 0.001–100 pg/mL & 0.65 pg/mL & [120] \\ Polyyrrole-polythionine & carcinoma antigen-125 & 1–20 mM & 0.000–1000 U/mL & 0.00125 U/mL & [121] \\ Polyaniline/Au nanocrystals & human serumalbumin & voltammetry + impedometry & 3–300 ug/mL & 3 \(\mu\)g/mL & [122] \\ Polyaniline-poly(sodium styrene sulfonate) & voltammetry & 0.01–1000 pg/L & 3.7 fg/mL & [123] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of conducting polymer-based immunosensor performances. stranded DNA (ssDNA) probe onto a surface to recognize its complementary DNA target sequence by hybridization. Conducting polymers are frequently used for fabrication of DNA biosensors since the immobilization of oligonucleotide (ODN) probes onto conducting polymers generally provides an electrochemical response which allows a direct way to detect hybridization events. adsorption method for probe DNA immobilization into conducting polymers offers the simplest methodology but suffers from poor stability and response if not properly optimized. One of the most convincing studies using the adsorption method has been carried out by [PERSON] et al. who used few layered MoS2 nanosheets blended with conducting polyaniline to perform DNA sensing via differential pulse voltammetry technique [124]. This biosensor worked well even at concentrations as low as \(10^{-15}\) M of target DNA and showed highly satisfactory results in case of serum samples. It is also possible to use ODN entrapment in conducting polymer films even if this method is not the most widely used in the area of DNA sensors since it can be difficult for the DNA target to access the entrapped ODN. However, some works exist such as the one of [PERSON] et al. who entrapped an ODN probe in a polypyrrole film during its electropolymerization, leading to a low detection limit of _Alicyclobacillus acidoterrestris_ DNA [125]. Another strategy was used by [PERSON] et al. who synthesized a polypyrrole derivative to facilitate the covalent attachment of an ODN. Indeed, the copolymer poly [3-acetic acid pyrrole, 3-\(N\)-hydroxophthalimide pyrrole]) was electropolymerized, then a direct chemical substitution of the leaving \(N\)-hydroxophthalimide group by the oligonucleotide was realized leading to the formation of amide bonds between the ODN probe bearing a terminal amino group on its 5' phosphorylated position and the copolymer film [126]. The hybridization reactions with the DNA complementary target and non-complementary target were then investigated by both amperometric and impedimetric analyses which demonstrated a good sensitivity and low detection limit (1 pmol).