Introduction to Electrochemical Biosensors. Lecture 3

Transcription

1 1 Introduction to Electrochemical Biosensors Lecture 3

2 2 Summary Introduction to Electrochemical Biosensors Potentiometric sensors Amperometric sensors Oxygen sensing (not a biosensor) Microelectrodes Glucose sensing

3 3 Electrochemical Biosensor An Integrated, Biological Receptor-Transducer, Device Immobilised Biological Receptor Sample A/D Converter Signal Analyte Transducer

4 4 Electrochemical Sensors Three main types of sensor Potentiometric: - Use Ion Selective Electrodes to determine the concentration of chosen ions. Amperometric: - Measure current resulting from redox reactions. Conductometric - Measure changes in ionic composition resulting from an enzyme reaction.

5 5 Electrochemical Sensors Potentiometric: Measure equilibrium E (I = 0) Amperometric: Control E, measure I I E Control or Measurement Working (Indicator, Detector) Electrode Reference (Counter) Electrode

6 6 Electrochemical Sensors Potentiometric: Measure equilibrium E (I = 0) Amperometric: Control E, measure I I E Control or Measurement Working (Indicator, Detector) Electrode Reference (Counter) Electrode

7 7 Potentiometric Sensor ph - Common potentiometric measurement Glass ph Electrode measures concentration of Hydronium (H + ) ions Sensing Electrode (Ag-AgCl) + E Reference Electrode (Ag-AgCl) This, and other potentiometric sensors require a stable reference electrode Porous Frit Glass H + Membrane

8 8 Potentiometric Biosensor Potentiometric urea sensor Two ph sensors: Sensing electrode coated with urease enzyme Bare reference sensor Local ph change at urease coated electrode ph (potential) difference between electrodes proportional to urea conc. Reference ph sensor E Urease

9 9 Potentiometric Biosensors Strengths: A wide concentration range for detection of ions (typically 1 µm to 0.1 M) Can perform continuous measurements (ideal for clinical/environmental use) Inexpensive and portable Weaknesses: ph buffers are often required to maintain optimum enzyme activity

10 10 Electrochemical Sensors Potentiometric: Measure equilibrium E (I = 0) Amperometric: Control E, measure I I E Control or Measurement Working (Indicator, Detector) Electrode Reference (Counter) Electrode

11 11 Amperometric Oxygen Sensor Clark Oxygen Electrode Oxygen reduced at Pt cathode ring Anode coil reference electrode: Ag-AgCl Teflon membrane allows O2 diffusion Current will depend on reaction rate and po2 Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

12 12 Clark Oxygen Sensor Dissolved Oxygen O 2 O 2 O 2 At the Pt cathode: O 2 + 4e + 2H 2 O 4OH At the Ag-AgCl anode: Ag + Cl AgCl + e i d - measured current F - Faraday's constant analyte solution O2 permeable membrane(must stir solution to avoid diffusion barrier ) electrolyte Pt electrode 0.6 to 0.7V P m - permeability of O 2 A - electrode area [O 2 ] - oxygen concentration b - thickness of the membrane

13 13 Electrode Surface Reactions Flux Balance at the Electrode Surface Flux of electrons Flux of Ox Flux of R Planar Diffusion Stirring the solution reduces Diffusion Barrier in Mass Transfer.

14 13 Electrode Surface Reactions Flux Balance at the Electrode Surface Hemispherical Diffusion Flux of electrons Flux of Ox Flux of R Microelectrodes in miniaturised sensors do not require stirring

15 14 Clark Oxygen Sensor Wide range of applications: Environmental studies (e.g. O2-levels in natural waters) Sewage treatment (monitoring bacterial treatment). Alcohol production (O2-levels in fermenters need to be continuously monitored and controlled) Similar uses in other industrial bio-reactors Medicine (invasive and non-invasive monitoring) Biosensor Applications - for example, when combined with an immobilised enzyme?

16 15 Amperometric Glucose Sensor Glucose-oxidaze (GOx/ GOD) enzyme reaction How to measure this? +i Anodic Measure production of hydrogen peroxide? Electrochemical detection: H 2 O 2 O 2 + 2H + + 2e Plateau current depends on H2O2 conc. which depends on glucose conc V -i Cathodic E

17 16 First Generation Sensor Issues H2O2 detection may be confounded with interferant compounds such as uric acid. Use of oxidases means that oxygen is required, so the reaction is dependent on this concentration. How have these issues been addressed?

18 17 Possible Solutions Use of selectively permeable membranes Glucose, Oxygen Outer controls O 2 and glucose flux Inner prevents transport of interferants Other solutions for this Electrode Inner Membrane Immobilised GOx Layer Outer Membrane Interferants involve catalysing H2O2 reactions to lower potential H2O2

19 18 Reactions in GOx Flavin adenine dinucleotide (FAD) at the heart of GOx Redox active site: FAD - Oxidised form FADH 2 - Reduced form Interacting directly with the FAD could replace oxygen Difficult because it is buried away within the molecule GOx(FAD) + glucose GOx(FADH2) + glucono-lactone GOx(FADH2) + O2 GOx(FAD) + H2O2

20 19 Second Generation Sensors Oxygen is replaced by mediator (Med): GOx(FAD) + glucose + H2O GOx(FADH2) + glucono-lactone GOx(FADH2) + 2Med(Ox) GOx(FAD) + 2Med(Red) + 2H + Mediator is oxidised at electrode to transfer electrons: GOx(FADH2) + 2Med(Ox) GOx(FAD) + 2Med(Red) + 2H + Common mediators include ferrocene, ferricyanide, quinones and phenothiazone

21 20 Mediated Electron Transfer i Ferricyanide Reduced Enzyme e Product Substrate (Analyte) Electrode biased at Oxidising Voltage Ferrocyanide Oxidized Enzyme

22 21 Mediated Electron Transfer i Ferricyanide (Ox) GOx(FADH2) (Red) e Gluconolactone Glucose Electrode biased at Oxidising Voltage Ferrocyanide (Red) GOx(FAD) (Ox)

23 22 Mediator Characteristics Mediator allows the biosensor to be independent of oxygen concentration to operate at lower potentials To be effective, the mediator should react rapidly and preferentially with the enzyme and at the electrode be highly soluble and diffuse quickly Be non-toxic and chemically stable

24 23 Direct Electron Transfer Transduced Current i e - Substrate (Analyte) Product Electrode biased at Oxidizing Voltage

25 24 Direct Electron Transfer Direct, non-mediated, electrical contact of two Enzymes Transduced Current i e H 2 O O 2 Substrate (Analyte) Peroxidase Oxidase H 2 O 2 Product Electrode biased at Reducing Voltage

26 25 Further Developments Direct electrical wiring of enzymes to electrodes?

27 26 Implantable Glucose Sensors Holy grail for diabetes control Needs to work for days/ weeks/months Typically they fail within days Subject of huge research effort

28 27 Summary - Glucose Sensors 1st Generation glucose sensor: Measure enyzme reaction products with amperometric sensor 2nd Generation: Use a mediator to transfer electrons to the sensing electrode Future generations Direct wiring of enzymes to electrodes

29 28 Conductometric Biosensors Detect changes in Electrical Conductivity resulting from an Enzyme Reaction. Sources of Conductivity Change Generation of ion groups Separation of different charges Ion migration (proton conduction) Change in association of ion groups Change in size of charged groups Enyzmes Amidases Dehydrogenases & decarboxylases Esterases Kinases Phosphatases & sulphatases

30 29 Conductometric Biosensors Interdigitated electrode design Screen printing of platinum or silver-palladium materials? Vacuum deposition of metals and photolithography Substrate of alumina or glass Immobilised enzyme covalently bound to electroinactive protein (e.g., albumin)

31 30 Conductometric Instrumentation Z 1 Z sensor + Diff. Amp Z 2 Z 3

32 31 Conductometric Instrumentation G sensor R f V in V out + V out = V in R f G sensor

33 32 Impedimetric Biosensors Capacitive sensing Small AC signal applied No DC component Measure changes in dielectric properties Requires sensitive measurement

34 33 Impedimetric Biosensors Interdigitated microelectrode design Various sensor designs measure changes in R or C

35 Electrochemical Impedance 34 Spectroscopy Theory Techniques Equivalent circuits and impedance plane plots Applications

36 Electrochemical Impedance 35 Spectroscopy (EIS) Butler-Vollmer equation defines redox current Small (5-10 mv) AC signal centred on E o Ignore higher order effects for linear output Current (A) Ox + ne Red E1/2 = E o Effects from charge transfer (kinetics) and mass transport Voltage (V) nf I = I o RT (E Eo )

37 36 Impedance Spectroscopy 1 Apply small AC signal: E(ω) = E o + ΔE sin(ωt) Measure response: I I0 I(ω) = I0 + ΔI sin(ωt+φ) Frequency, ω = 2π f E0 E

38 37 Impedance Spectroscopy 2 If ΔE is small then the response ΔE should be linear E0 Impedance is then: Z(!) = E(!) I(!) Complex value: Z(!) = Z(!) e j (!) I0 ΔI

39 38 Contributions to EIS response Three main components of the impedance from a Faradaic reaction: Diffusion of Ox to electrode surface from bulk Reaction kinetics of Ox + ne Red Diffusion of Red to bulk solution Charge transfer resistance Rct and Warburg Impedance ZW ne Ox Red

40 Other Contributions to 39 Electrochemical Impedance Counter electrode is large so that it does not limit the current (Rct(CE) Rct(WE)) Significant capacitance from the electrical double layer (which we ll get to) C dl appears in parallel with Rct, ZW Final component is related to the solution conductivity: RΩ

41 Electrical Double Layer 40 Helmholtz Layer Electrode Surface Diffuse Layer Capacitance C dl made up of two parts, compact Stern layer and the diffuse layer 1 = C dl C H C d Diffuse layer width decreases with solution concentration, increasing Cd C H C d

42 41 Randles Equivalent Circuit R Ω C dl R ct W Zw At low frequencies the limiting values of Z: Re(Z) =R + R ct +! 1/2 Im(Z) =! 1/ C dl Eliminate! 1/2 to leave: Im(Z) =Re(Z) (R + R ct 2 2 C dl )

43 42 Randles Equivalent Circuit R Ω C dl R ct W Zw At high frequencies the limiting values of Z: Re(Z) =R + R ct 1+! 2 C 2 dl R2 ct Eliminate ω to leave: Im(Z) =!C dlr 2 ct 1+! 2 C 2 dl R2 ct Re(Z) R R ct 2 2 +( Im(Z)) 2 = Rct 2 2

44 43 Complex Impedance Polar co-ordinates: Z(ω) = Z(ω) e jϕ(ω) Z magnitude, ϕ phase shift Cartesian: Z(ω) = Zr(ω) + jzj(ω) Z r real part, Zj imaginary part Z = (Zr 2 + Zj 2 ), ϕ = tan 1 (Zr / Zj) Zr = Z cos(ϕ), Zj = Z sin(ϕ)

45 44 Impedance Plane Plots Plot of the complex impedance plane Related to Argand, Nyquist and Cole-Cole plots Real part of Z on X-axis Imaginary part of Z on Y-axis Each point on the curve is a particular frequency ω

46 45 Impedance Plane Plots Im Z Z = R j/ωc C Increasing ω R R Re Z

47 46 Impedance Plane Plots Double layer capacitance Cdl Im Z ω = 1/RctCdl C dl R ct μf cm 2 Charge transfer resistance Faradaic reaction 0 ϕ Z ω Rct ω = 0 Re Z ω =

48 47 Impedance Plane Plots R ct Includes solution resistance RΩ Im Z R Ω C dl Series resistance shifts response ω = ω ω = 0 along Re Z axis 0 RΩ RΩ + Rct Re Z

49 48 Impedance Plane Plots R Ω C dl Im(Z) Kinetic Control ω = 1/R ct C dl ω Mass Transport Control 45 o R ct W Z w At low frequencies: R Ω ω = R ct +R Ω (R ct + R Ω 2σ 2 C dl ) Re(Z) Im(Z) =Re(Z) (R + R ct 2 2 C dl ) Straight line, unit gradient, intersects axis at: Re(Z) =R + R ct 2 2 C dl

50 49 Impedance Plane Plots R Ω C dl Im(Z) Kinetic Control ω = 1/R ct C dl ω Mass Transport Control 45 o At high frequencies: (Semi)circle with radius Rct /2, centre: R ct W Z w Re(Z) R R ct 2 R Ω 2 +( Im(Z)) 2 = Im(Z) =0, Re(Z) =R + R ct ω = R ct +R Ω (R ct + R Ω 2σ 2 C dl ) 2 Rct 2 2 Re(Z)

51 50 Real Data At maximum Im Z, f = 1.1 khz C dl R ct Rct Organic LED Charge transport in organic semiconductors

52 51 Bode Plot

53 52 Biosensor Applications Review paper: E. Katz and I. Willner, Electroanalysis, vol. 15, 2003 EIS is ideal for the measurement of changes to a surface caused by attachment of biomolecules.

54 53 Biosensor Applications Functionalised microelectrodes Attachment changes EIS response Measurement of changes in Rct or Cdl (a) Bare electrode (b) Functionalised electrode (c) With antibody attachment

55 54 EIS - Microelectrodes C dl Not diffusion limited at low f Modified Randles equivalent circuit Im Z ω = R Ω R ct ω R nl W Z w ω = 0 ZW 0 as f 0 Rnl - non-linear 0 RΩ Re Z RΩ + Rct RΩ + Rct + Rnl

56 55 EIS - Microelectrodes 50µm Pt microelectrode EIS performed in solution of: 5mM ferricyanide/ 5mM ferrocyanide DC = 0.19 V f:1000 Hz to 0.1 Hz

57 56 EIS Summary Electrochemical Impedance Spectroscopy Measurement basics, small AC signal at E o Contributions to impedance and equivalent circuits Impedance plane plots and physical relevance Applications in Biosensing