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stirred δ : const. ( əC A / əx) = conc. gradient = slope of conc. profile α diffusion rate. i = nFAD A ( ə C A / ə x) = nFAD A [(C A - C A º ) / δ ]. as C A º → 0 i l = nFAD A C A / δ = k A C A. unstirred δ: ↑ w time. For A + ne - = P - PowerPoint PPT Presentation

### Transcript of electrode surface

• electrode surfacestirred: constunstirred: w timeunstirred: w timei = nFADA (CA / x) = nFADA [(CA - CA ) /](CA / x) = conc. gradient= slope of conc. profile diffusion rate as CA 0il = nFADACA / = kACAFor A + ne- = PEapp = E1/2 (0.0592 / n) log (CP / CA) CP : surface conc. of product PCA : surface conc. of analyte A

• 1 ms5 msi (CA / x) conc. gradient diffusion rate10 msEapplSolution: stirred vs. unstirredEappl: linear scan vs. steppeddiffusion layer w time

• FIGURE 25-9 Current response to a stepped potential for a planar electrode in an unstirred solution. (a) Excitation potential. (b) Current response.

• FIGURE 25-10 Concentration distance profiles during the diffusion-controlled reduction of A to give P at a planar electrode. (a) Eappl = 0 V. (b) Eappl = point Z in Figure 25-6; elapsed time: 1, 5, and 10 ms.

• FIGURE 25-16 Voltammogram for the reduction of oxygen in an air-saturated 0.1-M KCl solution. The lower curve is for a 0.1-M KCl solution in which the oxygen is removed by bubbling nitrogen through the solution.deaerated (by N2)air-saturatedDME0.1 M KCl

• FIGURE 25-17(a) A schematic of a voltammtric system for detecting electroactive species as they elute from a column. The cell volume is determined by the thickness of the gasket.Amperometric Detector in Liquid ChromatographyAmperometry: to measure i at the fixed applied potentialiEappliedEthin-layer cellamperometric

• FIGURE 25-17(b) Detail of a commercial flow cell assembly.

• FIGURE 25-17(c) Configurations of working electrode blocks. Arrows shown the direction of flow in the cell.

• Professor Kissinger is a part time faculty member and entrepreneur who founded the drug development company Bioanalytical Systems, Inc. He regularly counsels students on career opportunities in the pharmaceutical industry and is active in the Indiana Instrumentation Institute (III) and development of the Purdue Research Park. Professor Analytical Chemistry Department of Chemistry, Purdue UniversityPeter T. KissingerB.S., 1966, Union College; Ph.D., 1970, University of North Carolina; Postdoctoral Associate, 1970-1972.

• Parkinsons Disease ()Dopamine () a neurotransmitter () , , mouse ()rat () LC UV LC Fluorescence LC EC

• pumpLC-ECmicro-dialysis

• From columnTo wasteUV/Vis LC Detector

• FIGURE 25-18 The Clark voltammetric oxygen sensor. Cathodic reaction: O2 + 4H+ + 4e- 2H2O. Anodic reaction: Ag + Cl- AgCl(s) + e-1956Clark oxygen sensorTo determine the dissolved oxygen in a variety of aqueous environment, e.g., sea water, blood, sewage, etc.PtAgTeflon orPolyethylene (PE)

• (NH2)2CO + 2H2O + H+ 2NH4+ + HCO3- (23-23)

ureasepH 7.5urea2NH3 + 2H+Clinical test () BUN (Blood Urine Nitrogen)FIGURE 23-13 Enzyme electrodes for measuring urea.

• ISEImmunosensors Antigen = target analyte

• FIGURE 21-14 Surface plasmon resonance. Laser radiation is coupled into the glass substrate coated with a thin metal film by a half-cylindrical prism. If total internal reflection occurs, an evanescent wave is generated in the medium of lower refractive index. This wave can excite surface plasmon waves. When the angle is suitable for surface plasmon resonance, a sharp decrease in the reflected intensity is observed at the detector.Au

• antibodyAu filmantigen= target analyteSurface plasmon resonanace: a label-free immunosensor

• FIGURE 25-19(a) A: electrode containing immobilized antibody (Y); B: binding of target analyte () to electrode-bound antibody; C: binding of alkaline phosphatase-labeled antibody to electrode-bound analyte; D: application of 320 mV to the electrode and addition of hydroquinone diphosphate (HQDP). Electrochemical oxidation of AP-generated hydroquinone (HQ) generates a current at the electrode that is proportional to the amount of analyte bound to the electrode. Label Fluorescent moleculeenzyme (alkaline phosphatase)Sandwich assayantibodyantigen= target analyteW.E.hydroquinone diphosphatehydroquinonequinonei [analyte]

• FIGURE 25-19(b) Photograph of the biosensor showing the arrangement of lrOx 1-mm-diameter working electrodes, 4-mm-diameter counter electrode, 7-mm-outside-diameter Ag-AgCl reference electrode, and electrical contacts on the substrate (28 35 1 mm). For clarity, the sample well is not shown.

• at a fixed applied potentialend pointARA + RA + R PAnalyte ReagentiEEappliedtitration reaction:StirredP: electro-inactiveX20

• FIGURE 25-21 (a) Side view of an RDE showing solution flow pattern. (b) Bottom view of a disk electrode. rotate the electroderotating disk electrode, RDE

• FIGURE 25-21 (c) Photo of a commercial RDE. (d) Bottom view of a ring-disk electrode.X

• Rotating Disk Electrode, RDE The most common method for obtaining a rigorous description of the hydrodynamic flow of stirred solution is based on a rotating disk electrode. Numerous studies of the kinetics and the mechanisms of electrochemical reaction have been performed with rotating disk electrode.

il = 0.620 n F A D2/3 CA -1/6 1/2 (25-15) : kinematic viscosity (cm2/s) : angular velocity = 2 f f : rotation speed ( r p m )limiting iLevich equation:

• FIGURE 25-22 Disk (a) and ring (b) current for reduction of oxygen at the rotating-ring-disk electrode. hydrodynamicX

• FIGURE 25-16 Voltammogram for the reduction of oxygen in an air-saturated 0.1-M KCl solution. The lower curve is for a 0.1-M KCl solution in which the oxygen is removed by bubbling nitrogen through the solution.deaerated (by N2)air-saturatedDME0.1 M KCl

• Cyclic Voltammetry (CV)potential waveform Ei(initial potential)forward scanreverse scanswitching potentialin unstirred solnscan rate: 50 mV/s23

• FIGURE 25-24 (a) Potential versus time waveform (b) cyclic voltammogram for a solution that is 6.0 mM in K3Fe(CN)6 and 1.0 M in KNO3. Stationary solnPt-disk electrodeno reaxat EiFe(CN)63- + e- Fe(CN)64-Fe(CN)64- Fe(CN)63- + e- for reversible reaction : Epa Epc = 0.0592 / n ipc ipa E = (Epc + Epa) / 2

for diffusion-controlled reax : ip 1/2a: anodic c: cathodic