Determination Of Phenolic Compounds Based On Multi-walled Carbon Nanotubes Composite Modified Electrodes

Up to now,phenolics compounds are mainly used in environment and food,such as butylated hydroxyanisole and tert-butyl hydroquinone for synthetic phenolic antioxidants in food,luteolin which are commonly used in food and drugs,and dihydroxybenzene isomer,considered as environmental pollutant with high toxicity and low degradability in the ecological environment.However,the use level of phenolic compounds increased year after year with the rapid development of economy.Meanwhile,control of the content for luteolin is very important in food and drugs.Therefore,the design and development of effective analytical methods for the determination of phenolics compounds are thus necessary.Chemical modified electrodes for quantitative analysis are the accumulation of separation,enrichment and detection,greatly improve the sensitivity and selectivity of the electrode.In this paper,we developed a series of electrochemical sensors based on multi-walled carbon nanotubes,ionic liquids,manganese dioxide semiconductor materials,dye organic molecules and various composite materials for the determination of phenolics compounds.These sensors own the synergies effects of composite materials and showed good selectivity,high sensitivity,low detection limit and long-term stability.Meanwhile,the electrochemical redox mechanisms of phenolics compounds were studied.The primary contents are summarized as follows?(1)The electroanalytical determination of luteolin based on poly(crystal violet)-multi-walled,carbon nanotubes modified electrode was studied.A novel poly(crystal violet)/multi-walled carbon nanotubes modified electrode(PCV/MWCNTs/GCE)was fabricated by casting multi-walled carbon nanotubes(MWCNTs)onto the surface of glassy carbon electrode(GCE),then electo-polymerizing crystal violet(CV)on the MWCNTs/GCE matrix.The prepared electrodes were characterized by cyclic voltammetry,electrochemical impedance spectroscopy(EIS)and scanning electron microscopy(SEM).Compared with the bare GCE,PCV/GCE and MWCNTs/GCE,PCV/MWCNTs/GCE exhibited the higher electrocatalytic ability toward the oxidation of luteolin.It leads to a considerable improvement of the redox peak current for luteolin and allows the development of a sensitive voltammetric sensor for the determination of luteolin.The increased stability in the presence of MWCNTs can be attributed to strong interactions that occur between MWCNTs and PCV that can be electrostatic,between negatively charged carboxylate groups on the MWCNTs and positively charged nitrogen in PCV and/or?-? interaction.PCV acts as an electron transfer mediator to accelerate electron transfer rate for oxidation of luteolin.A series of experimental parameters were optimised.The oxidative peak currents increased linearly for the concentration of luteolin in the range of 2.0×10-8 to 7.0×10-5 M with a detection limit of 5.0×10-9 M(S/N = 3).The novel modified electrode demonstrated high sensitivity,good reproducibility and long-term stability.(2)Electrochemical determination of luteolin using multi-walled carbon nanotubes-ionic liquid composite electrode was studied.A simple and sensitive multi-walled carbon nanotubes(MWCNTs)-ionic liquids(1-butyl-3-methylimidazolium hexafluorophosphate,BMIMPF6,IL)composite modified glassy carbon electrode(GCE)was successfully developed for the electrochemical determination of trace-level luteolin.The prepared composites werecharacterized by scanning electron microscopy(SEM).Characterization of the novel modified electrode was carried out by cyclic voltammetry.The electrochemical behaviors of luteolin indicated that the MWCNTs-BMIMPF6/GCE can greatly enhance the electrocatalytic activity in the redox process of luteolin due to the synergic effects of good electrical properties and large surface-to-volume ratio ofMWCNTs together with the good conductivity and wide potential windows of IL.Under the optimized conditions,the electrochemical sensor can be applied to the quantification of luteolin with a linear range covering 5×10-9-1×10-6 mol·L-1(with a correlation coefficient of 0.9969)and the limit detection is 5×10-10mol·L-1(S/N=3).Moreover,the method was successfully applied for the determination of luteolin in Chrysanthemum as a real sample with satisfying results.(3)A voltammetric sensor based on multi-walled carbon nanotubes-MnO2 nanowires composite film for simultaneous determination of hydroquinone and catechol was studied.An electrochemical method for the simultaneous detection of hydroquinone(HQ)and catechol(CC)was developed,based on a multi-walled carbon nanotubes and MnO2 nanowires composite(MWCNTs-MnO2)modified glassy carbon electrode(GCE).Electrochemical impedance spectra studies showed a small charge transfer resistance(Rct)of the MWCNTs-MnO2 composite film due to the incorporation of MWCNTs.Compared with the bare GCE,both HQ and CC exhibited well-defined redox peaks and much larger peak currents at MWCNTs-MnO2/GCE,which was due to the higher specific surface area and catalysis of MWCNTs-MnO2 film.Mn02 film acted as an electron transfer mediator to accelerate electron transfer rate for oxidation of HQ and CC.Due to the large separation of oxidation peak potentials(102 mV),the concentrations of HQ and CC can be easily determined simultaneously.The oxidation peak currents were linear to HQ/CC in the range from 5×10-7 to 8×10-5M with the detection limits of 5×10-8 M(S/N=3)for HQ and 8 ×10-8 M(S/N=3)for CC,respectively.Simultaneous determination of HQ and CC with such electrode was conducted in tap water samples with reliable recovery.(4)Simultaneous determination of TBH2Q and BHA antioxidants in food samples using eosin Y film modified electrode and the electrochemical behaviors of two antioxidants were studied.A glass carbon electrode(GCE)was modified with eosin Y that was electrodeposited on GCE via continuous cycling between-1.6 and 1.5 V(vs.Ag/AgCl).This electrode was characterized by scanning electron microscopy and electrochemical impedance spectra.The resulting electrode exhibited excellent electrocatalytic activity towards the oxidation of butylated hydroxyanisole(BHA)and tert-butyl hydroquinone(TBH2Q),in addition,the oxidation products of BHA and TBH2Q were found to be the same.Under the optimized condition,the oxidation peak currents were linear to BHA/TBH2Q in the range from 0.10 ?g·mL-1 to 7.00 ?g·mL-1 with the detection limits of 0.01 ?g·mL-1(S/N=3)for BHA and 0.015 ?g·mL-1(S/N=3)for TBH2Q,respectively.Moreover,the reproducibility and repeatability of the electrode were determined.The proposed method was successfully applied in the simultaneous determination of BHA and TBH2Q in several edible oil samples,and satisfactory results when compared with those obtained using high-performance liquid chromatography(HPLC).Then the redox mechanisms of butylated hydroxyanisole(BHA)and tert-butyl hydroquinone(TBH2Q)were studied by the techniques of cyclic voltammetry(CV),in situ FT-IR spectroelectrochemistry(FT-IR),cyclic voltabsorptometry(CVA)and derivative cyclic voltabsorptometry(DCVA).We can track simultaneously the concentration changes of relevant redox species(reactant,final product)during electrochemical process.The reasonable electrochemical oxidation mechanism of TBH2Q in acetonitrile was proposed.Meanwhile,the final(?)product of TBH2Q and BHA was tert-butyl benzoquinone(TBQ)in acetonitrile.