Electrochemical Study and Application on Shikonin at Poly（diallyldimethylammonium chloride） Functionalized Graphene Sheets Modified Glass Carbon Electrode

The electrochemical behaviors of shikonin at a poly（diallyldimethylammonium chloride） functionalized graphene sheets modified glass carbon electrode（PDDA-GS/GCE） have been investigated. Shikonin could exhibit a pair of well-defined redox peaks at the PDDA-GS/GCE located at 0.681 V（Epa） and 0.662 V（Epc）[vs. saturated calo- mel electrode（SCE）] in 0.1 mol/L phosphate buffer solution（pH=2.0） with a peak-to-peak separation of about 20 mV, revealing a fast electron-transfer process. Moreover, the current response was remarkably increased at PDDA- GS/GCE compared with that at the bare GCE. The electrochemical behaviors of shikonin at the modified electrode were investigated. And the results indicate that the reaction involves the transfer of two electrons, accompanied by two protons and the electrochemical process is a diffusional-controlled electrode process. The electrochemical para- meters of shikonin at the modified electrode, the electron-transfer coefficient（a）, the electron-transfer number（n） and the electrode reaction rate constant（ks） were calculated to be as 0.53, 2.18 and 3.6 s^-1, respectively. Under the optimal conditions, the peak current of differential pulse voltammetry（DPV） increased linearly with the shikonin concentra- tion in a range from 9A72×10^-8 mol/L to 3,789×10^-6 mol/L with a detection limit of 3,157× 10^-8 mol/L. The linear regression equation was Ip=O.7366c＋0.7855（R=0.9978; lp： 10-7 A, c： 10-8 mol/L）. In addition, the modified glass carbon electrode also exhibited good stability, selectivity and acceptable reproducibility that could be used for the sensitive, simple and rapid determination of shikonin in real samples. Therefore, the present work offers a new way to broaden the analytical application of graphene in pharmaceutical analysis.

Detection of dopamine at graphene-ZIF-8 nanocomposite modified electrode

A hybrid graphene-ZIF-8（G-ZIF-8） nanocomposite modified electrode was prepared in our work. SEM characterization shows that nanocrystals of zeolitic imidazolate frameworks（ZIF-8） were homogeneously well-intergrown on the surface of graphene and thus the graphene sheets were refrained from restacking, which implies the high accessible surface area. The BET results further testifies that G-ZIF-8composites had a larger surface area than 3D graphene. G-ZIF-8 modified electrode exhibited excellent electroanalytical performance for dopamine. The linear concentration range was from 3.0 mmol/L to1.0 mmol/L with the detection sensitivity of 0.34 m A/mmol/L and the detection limit of 1.0 mmol/L was obtained. The interference study, electrode stability and reproducibility were carried out. In addition, the prepared sensor was applied to the detection of DA in serum sample with recoveries from 96.8% to100.7%. It is believable that the structure characteristic of G-ZIF-8 nanocomposite is favorable for using MOFs to fabricate highly sensitive electrochemical sensor

A nanosilica/exfoliated graphene composite film-modified electrode for sensitive detection of methyl parathion

Graphene nanosheets （GS） were easily prepared through liquid-phase exfoliation of graphite powder in N,N-dimethylformamide （DMF） with the assistance of sodium citrate. Then, GS was coated onto a glassy carbon electrode （GCE） surface by drop to fabricate a GS]GCE nanointerface. Subsequently, by using tetraethylorthosilicate sol as precursor, nanosilica was electrochemically deposited onto the GS]GCE surface to produce a nanocomposite film electrode （nanosilicaJGSJGCE）. Electrochemical behaviors of methyl parathion （MP） on the nanosilica/GS/GCE surface were investigated thoroughly. It was found that the nanosilicaJGS nanocomposites can improve the redox peak currents of MP significantly due to the synergetic effect. The oxidation peak current was linearly related to MP concentration in the range from 0.0005 μmol/L to 5.6 μmol/L. The detection limit was calculated to be 0.07 nmol/L （SJN = 3）. The developed method was used to determine MP in real samples. The recoveries were in the range from 95.4% to 104.2%, demonstrating satisfactory results.