Transcript electrochemical biosensors

ELECTROCHEMISTRY
CHEM 4700
CHAPTER 6
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 6
ELECTROCHEMICAL SENSORS
SENSOR
- A small device used for direct measurement of a physical
quantity of an analyte in a sample matrix
- Response is continuous and reversible
- Sample is not perturbed
- Does not require sample collection and preparation
SENSOR
- Consists of a transduction element covered by a recognition layer
- Recognition layer may be chemical or biological
- Recognition layer interacts with target analyte
- Transduction element translates the chemical changes
into electrical signals
ELECTROCHEMICAL BIOSENSORS
- Produces an electrical signal that is related to the concentration
of an analyte
- Biological recognition processes are converted into quantitative
amperometric or potentiometric response
ELECTROCHEMICAL BIOSENSORS
- Two categories depending on the nature of the biological
recognition process
A. Biocatalytic Devices
- Utilizes enzymes, cells, tissues as immobilized biocomponents
B. Affinity Sensors
- Utilizes antibodies, membrane receptors, nucleic acids
ELECTROCHEMICAL BIOSENSORS
A. Enzyme-Based Electrodes
- Enzymes are proteins that catalyze chemical reactions in
living things
- Based on coupling a layer of an enzyme with an electrode
(enzyme is immobilized on an electrode)
- Electrode serves as a transducer
- Very efficient and extremely selective
ELECTROCHEMICAL BIOSENSORS
A. Enzyme-Based Electrodes
Enzymes (biocatalytic) layer immobilized on an electrode
Electrode
Biocatalytic Layer
ELECTROCHEMICAL BIOSENSORS
A. Enzyme-Based Electrodes
- Polymeric films are used to entrap enzyme (Nafion, polypyrrole)
Enzyme may be trapped
- between electrode and a dialysis membrane
- by mixing with carbon paste
- by surface adsorption
- by covalent binding
Applications
- Useful for monitoring clinical, environmental, food samples
ELECTROCHEMICAL BIOSENSORS
AI. Glucose Sensors
- For determination of glucose in blood
- For diagnosis and therapy of diabetes
- Glucose oxidase is trapped between polyurathene and
permselective membrane on a Pt WE
Glucose + O2
Glucose
oxidase
→
Gluconic acid + H2O2
ELECTROCHEMICAL BIOSENSORS
AI. Glucose Sensors
- The libration of H2O2 in the enzymatic reaction is
monitored at the electrode surface
H2O2
electrode
→
O2 + 2H+ + 2e-
- Enzymatic reaction can also be followed by consumption of O2
ELECTROCHEMICAL BIOSENSORS
AI. Glucose Sensors
Hand-held glucose monitoring devices
- Makes use of mediators
- Disposable strips are PVC and screen-printed carbon electrodes
- Contains a mixture of glucose oxidaze and a mediator
- Applies potential-step (chronoamperometric) operation
ELECTROCHEMICAL BIOSENSORS
AII. Ethanol Sensors (Ethanol Electrodes)
- For amperometric sensing of ethanol
- Employs immobilization of ADH and NAD+ to C or Pt anode
- Based on enzymatic reaction of ethanol with NAD+
in the presence of ADH
C2H5OH +
NAD+
ADH
→
C2H5O + NADH
ELECTROCHEMICAL BIOSENSORS
AII. Ethanol Sensors (Ethanol Electrodes)
- NADH is produced upon reduction of NAD+ by alcohol
- NAD+ is regenerated by electrochemical oxidation of NADH
NADH → NAD+ + 2e- + H+
- The resulting anodic current is measured
NAD+: nicotinamide adenine dinucleotide
ADH: alcohol dehydrogenase
ELECTROCHEMICAL BIOSENSORS
AIII. Urea Electrodes
- For sensing urea in the presence of urease enzyme
- Makes use of ammonium ion-selective electrode
- Electrode is modified with a gel containing the urease enzyme
NH2CONH2 + 2H2O +
H+
urease
→ 2NH4+ + HCO3-
ELECTROCHEMICAL BIOSENSORS
AIV. Other Enzyme Electrodes
- Cholesterol electrodes
- Lactate electrodes
- Penicillin electrodes
- Uric acid electrodes
ELECTROCHEMICAL BIOSENSORS
AV. Tissue and Bacteria Electrodes
- Use of plant tissues and bacterial cells for enzymatic activity
- Very important since some enzymes are very expensive
and not available in the pure state
Examples
- Banana tissue mixed with carbon paste for dopamine sensor
(enzyme is polyphenol oxidase)
- Use of microorganisms
(employs changes in the respiration activity of microorganisms)
ELECTROCHEMICAL BIOSENSORS
B. Affinity Biosensors
- Based on selective binding of certain biomolecules towards
specific species that triggers electrical signals
- Measures electrochemical signals resulting from
the binding process
- Highly sensitive and selective
ELECTROCHEMICAL BIOSENSORS
BI. Immunosensors
- Based on immunological reactions
- Useful for identifying and quantifying proteins
ELECTROCHEMICAL BIOSENSORS
BII. DNA Hybridization Biosensors
- Nucleic acid recognition layers are combined with
electrochemical transducers
- Used to obtain DNA sequence information
- Electrochemical response of DNA is strongly dependent
on DNA structure
ELECTROCHEMICAL BIOSENSORS
BII. DNA Hybridization Biosensors
Other Applications
- For chemical diagnosis of infectious diseases
- For environmental monitoring
- For detecting drugs, carcinogens, food containing organisms
- For criminal investigations
ELECTROCHEMICAL BIOSENSORS
BIII. Receptor-Based Sensors
- Make use of chemoreceptors as biological recognition elements
- Class-specific device (binds specific classes of substances that
possess similar chemical properties)
Receptors
- Protein molecules embedded in the cellular membrane to which
target analytes specifically bind
Chemoreceptor (chemosensor)
- Converts chemical signal into potential
ELECTROCHEMICAL BIOSENSORS
BIV. Molecularly Imprinted Polymer Sensors
- Based on polymerization process
- Target analyte binds functional monomers by covalent or
noncovalent bonding in a polymerization process
- Results in a polymer that contains recognition sites that have
high affinity for print molecules
- Has long term stability and resistant to harsh environments
GAS SENSORS
- For monitoring gases such as CO2, O2, NH3, H2S
- Device is known as compound electrode
- Highly sensitive and selective for measuring dissolved gases
- For environmental monitoring
- For clinical and industrial applications
GAS SENSORS
- Gas permeable membrane (teflon, polyethylene) is immobilized
on a pH electrode or ion-selective electrode
- Thin film of electrolyte solution is placed between
electrode and membrane (fixed amount, ~0.1 M)
- Inbuilt reference electrode
- The target analyte diffuses through the membrane and comes
to equilibrium with the internal electrolyte solution
GAS SENSORS
- The target gas then undergoes chemical reaction and the
resulting ion is detected by the ion-selective electrode
- Electrode response is directly related to the concentration
of gas in the sample
- Two types of polymeric materials are used
Microporous and Homogeneous
- Membrane thickness is ~ 0.01 – 0.10 mm
- Membrane is impermeable to water and ions
GAS SENSORS
CO2 Sensors
- Consists of pH electrode covered by a CO2 selective
membrane (silicone)
- Electrolyte between electrode and membrane is
NaHCO3-NaCl solution
- pH of inner solution lowers when CO2 diffuses through membrane
- Inner glass electrode senses changes in pH
- Overall potential is determined by CO2 concentration in sample
GAS SENSORS
CO2 Sensors
RT
EK
ln[CO 2]
F
HCO3- solution
CO2 + H2O ↔ H+ + HCO3H+ lowers pH
pH glass electrode
Membrane
(silicone)
GAS SENSORS
NH3 Sensors
- Consists of pH electrode covered by NH3 selective
membrane (teflon or polyethylene)
- Electrolyte between electrode and membrane is
NH4+-KCl solution
- NH3 goes through membrane and raises pH
- Inner glass electrode senses changes in pH
- Increase in pH is proportional to amount of NH3 in sample
GAS SENSORS
Other Gas Sensing Devices
NO2 and SO2
- Makes use of modified pH electrode
H2S
- Makes use of S2- ISE or modified pH electrode
HF
- Makes use of F- ISE or modified pH electrode
GAS SENSORS
Oxygen Sensors
- Based on amperometric measurements
(gas sensors discussed earlier are potentiometric)
- Consists of a pair of electrodes (Ag anode and Pt cathode) in
an electrolyte solution
- Electrodes are separated by a gas-permeable
hydrophorbic membrane
- Membrane may be teflon, silicon rubber, polyethylene
GAS SENSORS
Oxygen Sensors
- Electrolyte is a solution of KCl and buffer
- O2 diffuses through the membrane and is reduced
- Electrolytic current is then measured
SOLID-STATE DEVICES
- Known as ion-selective field effect transistors (ISFET)
Consists of
- a conductive base layer (supports a sensitive membrane)
and
- a guard ring layer (located below the base layer)
- Provides the ability to sense several ions
(Na+, Ca2+, K+, pH in blood samples, etc)
- For detection of hydrocarbons and NOx in exhaust
SOLID-STATE DEVICES
- Known as ion-selective field effect transistors (ISFET)
Examples
Na+ ISFET
NH3 ISFET
Cl- ISFET