电化学课件 纯英文7Ultramicroelectrodes.ppt

1、UltramicroelectrodesElectroanalytical ChemistryLecture#7UMEs Geometries Disk Recessed diskC.S.Henry;I.Fritsch Anal.Chem.1999,71,550-6.Band Cylinder Materials Carbon fiberUMEs-Advantages Reduced capacitance Low iR dropUMEs-Disadvantages Noise Electrochemical Fouling Analyte Method of electrode prepn(

2、epoxy)Stray capacitance Impurities Maintenance Cost Fragile constructionUMEs-Applications Determine analyte Study reactions at low temperatures=frozen glasses in solvents of moderate to high permittivity w/o electrolyte in solvents of high resistance in solid state in gas phaseUMEs-Applications(cont

3、d)Biological systems Single cells organelles Fast kinetics Development of analytical tools for electrode/solution interface SECM(Bard)EQCM SERS(Van Duyne)Scanning Electrochemical Microscopy(SECM)Diffusion is hemispherical iss=4 n F Co*Do awhere a=diam.of UME Acquire I vs.tip position(x,y)Plot of d v

4、s.tip position piezoelectric positioning BipotentiostatSampleCWRRef.:Bard et al.Science 1991,254,68-74.dSECM(contd)W-100 nm(diam.)Determines resolution 2 nm best at present Sample Conducting Insulator Mineral Biological SemiconductorSampleCWRCommon Modes of Operation Feedback Positive or negative Co

5、llection flux of species generated/consumed probedSECM-Positive Feedback If sample conductive,we get positive feedbacki issConductive SampleCWRROSECM-Negative Feedback If sample insulator,we get negative feedbacki iss Diffusion is hinderedInsulator SampleCWRROOO OSECM-Operation Constant height mode

6、tip scanned across substrate,current monitored In situ can be done in moist airSECM-Applications Analysis Imaging Nanolithography high precision high accuracySECM:Reviews R.C.Engstrom,C.M.Pharr Anal.Chem.1989,61,1099A-1104A.A.J.Bard,G.Denuault,C.Lee,D.Mandler,D.O.Wipf Acc.Chem.Res.1990,23,357-363.M.

7、V.Mirkin Anal.Chem.1996,96,177A-182A.Electrochemical Quartz Crystal Microbalances(EQCM)Very sensitive mass sensors submonolayer Design:2 electrodes sandwiching quartz(piezoelectric)oscillating E surface of quartz wafer oscillation at characteristic frequency(2-20 MHz)electrodeelectrodeQuartz crystal

8、stuffEQCM-Principles of Operation Measure change in oscillation frequency Sauerbrey equation:f=-2(m/A)n fo2/(quartz quartz)1/2wheren=overtone number quartz=shear modulus(2.95 x 1011 g/cms)quartz=density(2.65 g/cm3)EQCM-Principles of Operation(contd)Sauerbrey equation:f=-2(m/A)n fo2/(quartz quartz)1/

9、2 Significance:if m positive,then f _ if m negative,then f _ for 300 m thick quartz wafer oscillating at 5 MHz and m=18 ng/cm2 f 1 Hz(measurable)EQCM-Principles of Operation Solution exerts a dampening effect:f=-f03/2 soln soln/(qaurtz quartz)1/2where:soln=solution viscositysoln=solution densityEQCM

10、-Advantages Sensitivity ng/cm2 Hz In situ technique not a vacuum method Inexpensive$10kEQCM-Disadvantages Selectivity mass sensor Cannot distinguish what is on surfaceEQCM-Applications Mechanism of Electrochemical/Surface phenomena Underpotential deposition(UPD)Deposition of first monolayer at E pos

11、itive of Eo Electroprecipitation polymers prepd electrochemically different from that prepd by other methodsSurface-enhanced Raman Scattering(SERS)1974 R.Van Duyne(Northwestern)Enhancement 1012-1014 compd to nonresonant RamanSERS-Advantages In situ Under potential control XPS,SEM,etc.not in situ Ver

12、y sensitive Good for complex matrices Physiological fluids Detection limit pmole-fmole trace analysisSERS-Advantages(contd)Very selective Only molecules at/near surface probed(up to 16 nm)Surface specificity Vibrational structural probe Info on orientation wrt surfaceSERS-Disadvantages Reproducibili

13、ty Theoretical understanding of underlying mechanisms incomplete Limits quantitation Limited number of suitable substrates Cu,Ag,Au-bestRaman-History 1928-C.V.Raman,physicist C.V.Raman and K.S.Krishnan,Nature 1928,121,50.“A New Type of Secondary Radiation”1930 Nobel Prizehttp:/www.rri.res.in/Raman-M

14、echanismRelative IntensityFrequencyincidentincident+incident-StokesAnti-StokesE=0=0=1=1virtualvirtualFewer molecules in v=1 at RT(kT)Less intense because less probable eventhincidenth(incident-)Raman I(ij)2 E Tensoral Isotropic If want to increase I then increase ij(molecular)E Resonance Raman(RR)Mo

15、lecular mechanism:ij=A/(-o+i)+B/(+o+i)Resonance when =o A term blows up Imparts:Selectivity SensitivityRaman-How is it Done?lasermonochromatorCCDCameralensSpinningsamplescramblergratingSERS-3 Mechanisms Electromagnetic(EM)-Moskovits Dominant Preferential utility of Cu,Ag,Au Image Field-Schatz,van Du

16、yne Selection rules Adatom-OttoElectromagnetic For noble M with roughness 99%TNT degradation products:2,4-DNT,1,3-DNB(volatile)J.M.Sylvia,J.A.Janni,J.D.Klein,K.M.Spencer Anal.Chem.2000,72,5834-40.http:/oneworld.org/gallery/landmines/;photos by Nic DunlopEXAMPLE:Contd Proposal:Raman vapor“sniffer”Ins

17、trument:Diode laser(exc=785 nm;100 mW)with fiber optic probe Echelle spectrograph CCD camera(air cooled)SERS:Au foil(-0.3-1.2 V 500 mV/s ORC)Detection of 300 dehydrogenase enzymes Challenges:Electrochemical oxidation Irreversible adsorption of NAD+Electrode deactivation(short term stability)High ove

18、rvoltage(1 V)Abruna,H.see:Anal.Chem.1995,67,3936-44.;Anal.Chem.1996,68,3688-96.;Anal.Chem.1997,69,4065-75.Oxidation of NADH at BDD Device characteristics:Detection limit 10 nM(n=7)Stability-3 mos.Interferents-ascorbic acid(correctable)Bioelectrochemistry:The ExacTech Glucose Electrode Amperometric B

19、iosensor:Mediated electron transferelectrodesurfaceFc+FcFcGlu OxGlu OxGlu Oxglucoseglucolactone+2H+2e-+0.5 V-0.1 Ve-Idea:measure current which is correlated with GlucoseSBP Isolable from soybean seed coat soybean U.S.cash crop seed coat is waste product Single isozyme vs.mixture(HRP)Broad pH stabili

20、ty catalytically active from pH 2-8 Thermostable at elevated temperatures$MillionStatistics:http:/www.amsoy.org/;photo:http:/mars.cropsoil.uga.edu/csi/Applications Using SBP Medical diagnostics Biocatalysis manufacture of phenolic resins(LCDs,microelectronics,epoxy,plastics,etc.)Hair coloring Waste

21、water treatment removal of phenol from waste water and sludgeSBP Sol-Gel H2O2 Sensor Need:H2O2 sensor for low pH media Food,fermentation Approach:Enzyme encapsulated(sol-gel)/GC Methylene blue-dye mediatorWang,B.;Li,B.;Wang,Z.;Xu,G.;Wang,Q.;Dong,S.Anal.Chem.1999,71,1935-9.SBP Sol-Gel H2O2 Sensor Device characteristics:Response time-5 s(fast diffusion)Broad pH response(3-7)High sensitivity(27.5 A/mM)Linearity(0.02-2.6 mM)Detection limit(0.5 M)Reproducibility(1.2%RSD for n=7 at 0.2 mM)Interferents sulfide,ascorbate,fluorideWang,B.;Li,B.;Wang,Z.;Xu,G.;Wang,Q.;Dong,S.Anal.Chem.1999,71,1935-9.