The electrochemical behaviour of cysteine (Cys) at a graphite electrode modified with gold nanoparticles (GAu nano electrode) was studied by cyclic voltammetry. It was found that the graphite electrode-Au nanoparticles show an electrocatalytic activity towards the oxidation of Cys in 0.1 M NaOH. At 0.05 V, there is an "inverse" maximum in the cathodic voltammogram of Cys. Using a G-Au nano electrode, the dependence of the peak current of the "inverse" maximum on Cys concentration was linear in the range from 1 to 14 pM, and the detection limit was 0.6 pM. The proposed analytical method is simple, rapid and sensitive.

It was established that the silver nanoparticles (AgNPs) in an equimolar ratio have the highest electrochemical activity on a graphite electrode (GE) surface in an alkaline medium. The electrocatalytic oxidation mechanism of hydrogen peroxide on the GE surface was proposed. We detected an additional maximum at E = 0.3 V on the cathodic branch of the cyclic curve in the potential range from +2.0 to -1.0 V. The appearance of this maximum indicates the reduction of hydrogen peroxide generated during AgNPs electrooxidation in the potential range from -1.0 to +2.0 V.

The “inverse” cathodic peak of gold nanoparticles is observed in the reaction mixture used to obtain gold nanoparticles HAuCl4:Na3C6H5O7:NaBH4=125:8:1 and accumulation time is 90 s. The conditions in which methionine has the greatest electrochemical activity were determined. They are as follows: the molar ratio of reagents HAuCl4:Na3C6H5O7:NaBH4=125:8:1 and accumulation time is 90 s, 0.1 M NaOH. The mechanism of methionine oxidation is proposed to be on the surface of the graphite electrode modified with gold nanoparticles in 0.1 M NaOH. The determination limit of methionine is 0.7 •10-14 M. The proposed method is simple, sensitive, and does not need toxic substances.

The shape, size, optical properties and electrochemical activity of the silver nanoparticles (AgNPs) were studied, as well as their dependence on the amount of the reducing agent, the method of initiation of the formation of nanoparticles and the introduction order of reagents in the reaction mixture. It was determined that in the sols obtained without polyethylene glycol (PEG) AgNPs are predominant with the average size of 40 nm. With PEG the triangular AgNps predominate with the average size of 20 nm due to the silver ions reduction on the surface of the nuclei. It was determined that AgNPs are most active when obtained with PEG and the molar ratio [Ag]:[C 6 H 5 O73-] = 1:5. On the anodic branch of the cyclic AgNPs curve the splitting of anodic maximum into two due to AgNPs oxidation was observed. AgNPs obtained in the presence of PEG by light have the most electrochemical activity. The processes of AgNPs oxidation become easier when PEG is present which is caused by the formation of the most stable silver oxides.

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