AMME5958 Tutorial 2 - BMET5958 PDF

Preview

Summary

AMME5958 Tutorial 2 - BMET5958...

Description

AMME5958 Bonus 2 notes: Electrochemical Processes: •

Occur at electrode-solution interface (not in bulk solution)

•

Electrode interface = junction between ionic and electronic conductors

•

Types of electrochemical cell: 1. Electrolytic: consumes electricity from external source 2. Galvanic: produces electricity (battery)

Main components of biosensor Type of electrical signal used for quantitation: Fixed

Measures

Property

Potentiometry

Current = 0

Potential (Voltage Eeq) for analyte to react

Change in charge distribution across membrane

Potentiostatic/ voltammetry

Voltage

Rate of redox reaction

•

Charge transfer across electrode-solution interface

•

Applied V drive redox reaction

•

Current = rate of electron transfer

2 processes at electrode/solution interface ® •

Direct transfer of electrons (oxidation & reduction) ® both processes produce current Fixed Faradaic Processes (direct electron transfer)

Non-Faradaic Processes

•

Chemical reaction rate at electrode proportional to Faradaic current

• •

Faradaic current caused by electron transfer + obeys Faraday’s law Analyte is electroactive

•

In the potential range that makes electron transfer thermodynamically (spontaneous) or kinetically favoured (fast)

•

Due to change in double layer when E is changed

•

Not good for analysis (charging)

Nernst equation:

Electrode Reactions: •

Total reaction: complex with multiple steps

•

Simple reaction includes:

•

o o o

Mass transport of reactants to electrode surface Electron transfer Mass transport of products back to bulk solution

o

*additional chemical and surface processes

Net reaction = determined by RLS (slowest step) o Affected by: § Electrode material, § Media § Potential § §

Mode of mass transport Time scale

Electrode reactions

Type

Favoured

Mass-transport controlled reactions

Reversible (Nernstian reactions)

Thermodynamically favoured

Chare (electron) transfer controlled reactions

Irreversible

Kinetically favoured

Mass transport of reactants to electrode interface: 1.

Diffusion

Due to concentration gradient (dependent on diffusion coefficient)

2.

Migration

Electrostatic attraction of ion to electrode

3.

Convection

Dependent on stirring or flowing solution (forced/natural density)

Nernst-planck equation for mass flux (mol/cm2s)

Faradic current in mass-limited reactions •

n = no. of electrons

• •

F= faraday’s constant A= surface area of electrode

•

J = mass flux

i = -nFAJ

Non Non--faradic reacti reactions: ons: The elec electric tric double llayer ayer (EDL) •

EDL = Array of charged particles or oriented dipoles that exists in every material interface

•

Ionic zones formed in the solution to compensate for excess charge at the electrode interface

Helmholtz Model of EDL: Compact layer (IHP + OHP)

•

Charges strongly held by electrode

IHP (inner Helmholtz plane)

•

Inner layer, solvent molecules and specifically absorbed ions (not fully solvated)

•

Plane passing through centres of specifically absorbed ions

•

Non-specifically absorbed solvated ions

•

Plane passing through centres of non-specifically absorbed ions

OHP (outer Helmholtz plane)

Gouy-champan model (diffuse layer) •

3D region of scattered ions ® extends from OHP to bulk solution

•

Equilibrium: electric field forces (order) and random thermal motion (disorder)

• •

Concentration of ions decays exponentially with increase distance due to distribution of electrostatic and thermal energy Stern modification: ions have finite size (few nm) ® 1st ions are not at surface but at distance

•

Cdl value » 10-40 uF/cm2

Double layer capacitor: Electrolyte concentration

Properties

Compact layer Capacitance (CH)

Independent

Inversely proportional to distance separation between electrode and counterionic layer

Diffuse layer Capacitance (CG)

dependent

• •

Double layer Capacitance (Cdl)

Dilute soln: Cdl » CG Conc. Soln: Cdl » CH

•

Charging of Cdl = non-faradaic (i.e. electrons not transferred across interface)

•

Residual / transient currents

3-electode electrochemical cell:

Component

Function

Working Electrode (W)

Responds to analyte, cell reaction

Reference Electrode (R) E.g. Ag/AgCl

•

i.e. metal saturated in metal salt soln

• •

Counter/Auxillary electrode (C)

• • •

Conducting solution

Constant potential (due to fast kinetics ® high currents pass without voltage change) Independent of solution properties Other electrode potential measurements made against it Current is applied/measured between working and counter against reference Potential is not measure but adjusted to balance reaction at working electrode May be isolated using glass frit ® prevents byproducts from contaminating test solution

Buffer or electrolyte

Voltametric Techniques Applying potential to measure resulting current from reaction

1.

Linear sweep (cyclic voltammetry)

Voltage

Measures

Properties

Varied (linear scan)

Current

• • • •

Indicates reaction rate Mass transport brings unreacted species to electrode surface Reaction at electrode surface produces concentration gradient with bulk solution NON-STIRRED

2.

Differential pulse voltammetry

Varied (scan) but increase in pulses

Current

Quiet or stirred solution

3.

Fixed potential (amperometry)

Fixed

Current

Stirred solution ® prevent mass transport from being RLS

Cyclic voltammetry: Epa = Peak anodic potential Epc = Peak cathodic potential Ipa = Peak anodic current Ipa = Peak cathodic current

•

Fast electron transfer compared with other processes

•

Reversible reaction: o 2 peaks with similar magnitude o o o

•

Peaks are independent of scan rate Displays symmetry Controlled by only mass-transfer

Shape controlled by that system: o o o

Temperature Concentration of analyte Medium composition

Distance between anodic and cathodic peak potential at room temp. is:

•

n: number of electrons in the reaction

•

For reversible reaction, 25 0C and transfer of 1 e-: ΔEp=59 mV

•

CV can be used to estimate number of electrons in the reaction

Redox potential is:

*Note: multistep electron transfer possible (e.g. reduction of fullerenes)

Cyclic voltamm voltammetry: etry: Reversible reaction Peak current: • • • • •

N: no. of electrons A: electrode surface area (cm2) C: analyte concentration (mol/cm3) D: diffusion coefficient (cm2/s) V: voltage scan rate (V/s)

Key points: 1. Linear correlation between Ip and v1/2 indicates diffusion controlled 2. Increase in scan rate ® thinner diffusion layer ® higher current\

A: Irreversible reactions •

Controlled by charge transfer

•

Peaks smaller and separated

•

Peak potential shifts with scan rate (dependent)

•

Peak current correlated with analyte conc.

B: Quasi-reversible reactions •

Controlled by charge transfer (more) + mass transport

•

Either reversible or irreversible (depending on conditions)

•

Large peak separation compared with reversible

Cyclic voltamm voltammetry: etry: IInterpretation nterpretation •

Observation with CNT: ü higher peak current magnitude, a ü Anodic peak potential shifts to lower magnitude ü Closer peaks (faster reaction)

•

In the presence of CNT: ü Enhanced response: higher currents ü Efficient charge transfer in the presence of CNT – electrons can move faster (reaction became more reversible), require less applied potential

Interpretation of experimental results ü Linear correlation between peak current magnitude (Ip) and the square root of the scan rate (𝒗1/2)→diffusion controlled reaction • Lower scan rate ® large diffusion layer ® lower current

Differential Pulse Voltammetry (DPV)

Lower detection limit of voltametric reactions

Aim:

Increasing the difference between faradaic and non-faradaic currents

Method

Fixed magnitude pulses superimposed on linear potential map

Benefits

Current sampled before and after each pulse (increased sensitivity)

Limitations Only one way

Amperometry (biosensors)

Properties: • Fixed potential (step) •

Current measured over time (current changes as concentration of material changes)

•

Stirred solution to ensure fast mass transport (i.e. slow electron transfer ® RLS) o

V1 = no faradaic reaction

o

V2 = maximum reaction rate (= peak potential from cyclic voltammetry) ® since peak potential is where most reactions occur and most specific

•

Competitive enzymatic reaction for the substrate glucose (oxidase and kinase)

•

(1) 𝑔𝑙𝑢𝑐𝑜𝑠𝑒 +𝑂2→𝑔𝑙𝑢𝑐𝑜𝑛𝑖𝑐𝑎𝑐𝑖𝑑+𝐻 𝑂 ( 𝑔𝑙𝑢𝑐𝑜𝑠𝑒𝑜𝑥𝑖𝑑𝑎𝑠𝑒 = 2 2 𝐺𝑂𝑋)

•

(2) 𝑔𝑙𝑢𝑐𝑜𝑠𝑒 + 𝐴𝑇𝑃 → 𝑔𝑙𝑢𝑐𝑜𝑠𝑒 − 6 − 𝑃 + 𝐴𝐷𝑃 (𝐻𝑒𝑥𝑜𝑘𝑖𝑛𝑎𝑠𝑒 = 𝐻𝐸𝑋)

•

The electrochemical signal is generated by the oxidation of H O at the electrode (in the reaction 2 2 catalyzed by GOX).

•

In presence of ATP→HEX catalyzes consumption glucose in a different mecanism →current signal decreases (less H O is produced) → proportional to 2 2 ATP concentration

Calculation of DC capacitance from CV measurements • The DC capacitance is derived from the oxidation current versus the scan rate data according to:

0.15 V/s

0.05 V/s 0.015 V/s

𝑑𝑉

• 𝑖 = 𝐶 𝑑𝑡

• 𝑖 - charging current • 𝐶 - DC capacitance is the slope of graph current vs scan rate •

𝑑𝑉 𝑑𝑡

- scan rate

The Charging current is half of the total amplitude (half is charging and half is discharging)....