# Self-Assembled MoS2/ssDNA Nanostructures for the Capacitive Aptasensing of Acetamiprid Insecticide
## 1 Introduction
[18] have been working intensively on the electrochemical capacitance spectroscopy (ECS) or impedance-derived capacitance approach as a better alternative. They showed that when the system is non-faradaic or faradaic regime with an external redox probe (in solution), the electrochemical capacitance can be represented by the double layer capacitance (\(C_{\text{dl}}\)). In the case of a redox marker attached to the electrode surface, a pseudo-capacitance called redox capacitance (\(C_{\text{r}}\)) that depends on the density-of-states of the confined redox marker was considered instead the \(C_{\text{dl}}\)[19]. This can be explained by the fact that measuring electrochemical impedance spectroscopy (EIS) at the half-wave potential of the confined redox system, the contribution of \(C_{\text{r}}\) is enhanced and the \(C_{\text{dl}}\) remains almost constant and therefore its contribution can be neglected [20]. The \(C_{\text{r}}\) element corresponds to the diameter of the semicircle in Nyquist capacitive plots and its value is obtained by converting the Nyquist EIS plots. This technique has been successfully adapted for various sensing systems [21]. According to [PERSON] et al. [22], the redox capacitance of a faradaic probe confined within a biological film on the surface, is sensitive to the whole system thus can be correlated to the analyte concentration when the biological film is capable of recognizing the target with high specificity. This implies that EIS-derived capacitance signal is based on the charging signal that is generated from the activity of electroactive tethered groups, which is related to its electrostatic environment. The aim of this work was to report a new strategy based on the use of redox-active nanomaterials that will not only replace the confined redox probe but also will enhance the biosensor performances by using their capacitance properties. Particularly, two-dimensional (2D) nanomaterials have shown interesting properties that helped to improve the sensitivity and analytical performances of the developed biosensors [23]. In fact, MoS\({}_{2}\) nanosheets attracted increasing interests for its excellent capacitive properties [24]. Their sheet-like morphology provides large surface area for charge storage that can potentially occur via faradaic charge transfer process on the Mo(+IV) metal cation. The Mo center presenting a range of oxidation states from +2 to +6 can exhibit pseudo-capacitance [25]. Hence, 2D MoS2 is an excellent choice for electrochemical capacitance spectroscopy application. Furthermore, the MoS2 possesses an ultrathin plane structure of atomic thickness making it sensitive to the surrounding environment [26]; thus, an interaction with a target biomolecule can affect its whole thickness. These properties have been explored for design of FET biosensing platform [27] as well as electrochemical biosensors [28]. Additionally, it was demonstrated that MoS2 has a high affinity towards ssDNA oligonucleotides and is able to spontaneously adsorb them via van der Waals interactions between nucleobases and the basal plane of MoS2 nano-sheets [29]. This makes MoS2 nano-sheets suitable for capacitive biosensor. Herein, we report the design of an aptasensor platform in a two-steps process (Scheme 1) by assembling the MoS2 and ssDNA aptamer to sensitively detect acetamiprid. To assure a high stability of DNA on the surface covalent attachment was performed using lipoic acid (LA) self-assembled to MoS2 surface. The sensing platform uses EIS-derived electrochemical capacitance spectroscopy to transduce the recognition event. The novelty in this approach is to perform the electrical measurements without any confined redox probe attached to the surface or in solution as the MoS2 nanosheets exhibit an inherent pseudocapacitance behavior. Upon interaction with acetamiprid, the aptamer will desorb from MoS2 nanosheets to form aptamer/target affinity complex. The aptamer desorption facilitates ionic diffusion of the electrolytes resulting in a signal ON in ECS.