# Self-Assembled MoS2/ssDNA Nanostructures for the Capacitive Aptasensing of Acetamiprid Insecticide
## 3 Results and Discussions
### Analytical Performances  
#### 3.4.1 Acetamiprid Detection  
The aptasensing platform was incubated in various concentrations of acetamiprid ranging from 50 M to 450 fM and the ECS was used as transduction method. The capacitance signal, derived from EIS measurements, increased proportionally with the increase acetamiprid concentrations (Figure 4a). The observed behavior can be explained by strong affinity of target analyte to the aptamer, leading to desorption of the immobilized aptamer from MoS2 surface upon formation of the aptamer-target complex. This phenomenon was also observed in the case of DNA hybridization with MoS2 where hybridization reaction led to the desorption of dsDNA from the surface [29]. The loop formation of complex is distant from the surface which facilitates ion diffusion into MoS2 interlayers, where there are more sites available for ion exchange. A linear calibration curve of the average variation of normalized redox capacitance ((C\({}_{\text{O}^{\prime}}\)-C\({}^{\prime}\)/C\({}_{\text{O}^{\prime}}\))*100) with acetamiprid concentration ([ACE]) was plotted (Figure 4b). The latter shows a linear equation regression:  
\[\Delta\text{C}^{\prime}/\text{C}_{0}^{\prime}=6733+0.03[\text{ACE}]/(\text{fM} )\;(\text{R}^{2}=0.999)\]  
High coefficient of regression in the concentration ranging from 50 to 450 fM is obtained that confirms the linearity of the measurement. Furthermore, the detection limit was calculated to be 14 fM by considering the criteria of signal-to-noise ratio equals three. The reproducibility of the sensor was determined by measuring five different electrodes. The relative standard deviation (RSD) was calculated at 5.4%, which indicates the sensor has good electrode-to-electrode reproducibility thanks to the electrodeposition method  
Figure 4: (**a**) Capacitance curves before and after incubation at various acetamiprid concentrations in Phosphate-buffered saline (PBS); (**b**) Calibration curve of the biosensor displaying the relative variation of redox capacitance %.\(\Delta\text{C}^{\prime}/\text{C}_{\text{O}^{\prime}}\) vs. [ACE].  
Figure 3: Electrochemical characterization in solution of 5 mM [Fe(CN)6]\({}^{3/4-}\) and 0.1 MKCl: (**a**) CVs recorded at scan rate 100 mV/s; (**b**) EIS obtained with frequency range: 100 KHz to 0.1 Hz with DC of 10 mV with (curve a) SPCE/LA–MoS2 and (curve b) SPCE/LA–MoS2/APT.  
which allows high reproducibility of MoS\({}_{2}\) layers in addition of DNA covalent attachment. This biosensor presents a good performance with comparable analytical performance with other systems described in introduction without amplification strategy where signal readout was measured directly after detection. It takes also advantage of the chemical stability of MoS\({}_{2}\) where storage of the MoS\({}_{2}\) and LA-MoS\({}_{2}\) was checked within long time without any variation of properties. In addition aptamer are known to have high stability compared to others biological system [41]. The biosensor is proposed for single use without regeneration.  
This detection domain obtained does not include the concentration values of the maximum levels of acetamiprid set by US EPA and EFSA. It is worthy to note that reel samples have to be diluted further before acetamiprid detection, which helps to further decrease the matrix effect. So, taking into consideration this dilution step, it is of high interest to develop aptasensor for acetamiprid detection in diluted extracted samples.