Study of Insulin Electrochemical Reaction Mechanism on Modified Ni-Chit/SPCE Electrode
Authors: Orinakova Renata , Chovancová Frederika , Šišoláková Ivana , Shepa Jana 
Published: December 2024

Insulin is the main hormone that regulates the glucose level in blood. Dysfunction in insulin production or action causes a serious disease called diabetes mellitus. Currently the glucose sensors are using for monitoring the course of the disease, but during treating diabetes, it is also necessary to know the concentration of insulin in body fluids. It offers information about the function of the pancreas and how much insulin was produced. A change in the concentration of glucose in the blood is the body's reaction to low insulin concentration or low sensitivity of cells to the presence of insulin [1].
The insulin detection could be provided by immunoassay, chromatographic methods, electrochemical sensors etc. Among them electrochemical sensors display various advantages such as fast response, simple preparation, inexpensive instruments, and easy-to-use device. Modification of sensors working electrode improves electroanalytical properties of studied sensors. Various metal nanoparticles, metal oxide nanoparticles, and carbon nanomaterials are used for sensors modification. These nanomaterials are frequently fixed on the surface of electrode by using polymer film. Among polymer materials Nafion, chitosan, polypyrrole, and polyaniline are the most often used materials [2].
Herein, we are focused on development of electrochemical sensor based on the nickel nanoparticles immobilised by chitosan film on the surface of screen printed carbon electrode (Ni-chit/SPCE). The mechanism of electrochemical reaction plays key role in the insulin electrochemical detection. There is the possibility to improve electrode material taking into consideration reaction mechanism and rate determining step. For studying of reaction mechanism was used cyclic voltammetry.
The cyclic voltammograms for 2 μM insulin in 0.1 M NaOH and PBS at Ni-chit/SPCE are depicted in Figure 1. The cyclic voltammograms for the insulin solution display one anodic peak at approx. 0.7 V and a cathodic peak at approx. 0.4 V. According to the its shape and the distance between the oxidation and reduction peak, the electrochemical oxidation of insulin is irreversible process. Moreover, the oxidation peaks display a higher peak area compared to the reduction peak; it could indicate the EC mechanism of the electrochemical reaction. Upon analysis, Figure 1B presents a linear relationship (R2=0.92) between the current response and scan rate. Furthermore, Figure 1D reveals the relationship between the logarithmic peak current (I) and logarithmic scan rate (v). The derived equation log Ip = 0.606 log v – 1,787 (R2 = 0.92) with the slope value of 0.606 suggests that the insulin oxidation reaction is controlled by diffusion. The sluggish diffusion of the insulin molecules could be notch up to the size of insulin molecule. Therefore, the alkali solution displays a catalytic influence on the electrochemical detection of insulin. In alkali solution Ni nanoparticles will oxidise to Ni(OH)2 and following to NiOOH, as shown in Equations (1) and (2).
Ni + 2 OH- ↔ Ni(OH)2 + 2e− (1)
Ni(OH)2 + OH− ↔ NiOOH + H2O + e− (2)
NiOOH + insulin ↔ Ni(OH)2 + product (3)
The next step of the oxidation mechanism occurs after the oxidation of adsorbed insulin on the electrode surface. It could be expected, that NiOOH will reduce to Ni(OH)2, as shown in Equation (2). Simultaneously, the adsorbed insulin will be oxidized during the electrochemical reaction. Based on proposed mechanism of electrochemical reaction, the main role in the reaction mechanism plays presence of NiOOH and enhancement of insulin diffusion. Both processes are influenced by presence of NaOH in the studied solution because alkali solution promotes the NiOOH creation during the electrochemical reaction. These findings make it possible to significantly improve the properties of the tested sensor only by adding NaOH to the solution and thus improve sensor electroanalytical properties.

Figure 1 Cyclic voltammograms of 2 µM insulin on Ni-chit/SPCE at different scan rates (A). The dependences of the peak current on scan rate (B), peak current on the square root of scan rate (C), and of logI on logv (D).

Acknowledgement

This research was sponsored by the NATO Science for Peace and Security Programme under grant id G6106

References

[1] Šišoláková, I., Gorejová, R., Chovancová, F. et al.; Electrocatalysis 2023, 14, 697–707. https://doi.org/10.1007/s12678-023-00827-w
[2] Shepa J, Šišoláková I, Vojtko M, et al.; Sensors (Basel). 2021 Jul 26;21(15):5063. doi: 10.3390/s21155063.

Figure 1