Define a biosensor.
A biosensor is a chemical sensing device in which a biologically derived recognition entity is coupled to a transducer, to allow quantitative or qualitative analysis in a complex biochemical matrix.
Biosensors incorporate a specific ____1____ element (that creates a recognition event) and a ____2____ element (that converts the recognition event into a recordable signal).
Basic Principle of a Biosensor
Coupling the biorecognition element to a transducer can be achieved in four ways. State each.
- Membrane Entrapment
- Physical Adsorption
- Porous Entrapment
- Covalent Bonding
A substance or chemical constituent that is of interest in an analytical procedure.
What characteristics must biosensors exhibit? (4)
Repeatability – Intra-assay variability
Reproducibility – Inter-assay variability
Specificity/selectivity (to analyte of interest)
Sensitivity – Linear range, detection limit, response time
Resonance Based Biosensors
In resonant biosensors, a light-wave ____1____ is coupled with an antibody, or bioelement.
When the analyte molecule, or antigen, gets attached to the membrane, the ____2____ of the membrane changes (due to surface flexing).
The resulting change in the mass subsequently changes the resonant frequency of the ____1____ (light hits somewhere else).
This frequency change is then measured.
The degree of flexion = ____3____ of antigen.
Thermal Detection Biosensors
Exploits the absorption or production of heat, that in turn changes the ____1____ of the medium in which the reaction takes place.
Predominantly enzyme based.
When the analyte comes in contact with the enzyme, the energy change of the enzyme reaction is measured and calibrated against the analyte ____2____.
The total heat produced or absorbed is ____3____ to the total number of molecules in the reaction.
Common applications of this type of biosensor include the detection of pesticides and pathogenic bacteria.
Ion-Selective Field Effect Transistors (ISFET)
Ion sensitive biosensors are semiconductor FETs having an ion-sensitive surface.
The surface electrical ____1____ changes when the ions and the semiconductor interact.
This change in the ____1____ can be subsequently measured.
The ISFET can be constructed by covering the sensor electrode with a polymer layer.
This polymer layer is ____2____ permeable to analyte ions.
The ions diffuse through the polymer layer, causing a change in the FET surface ____1____.
This type of biosensor is primarily used for ____3____ detection.
The underlying principle for this class of biosensors is that many chemical reactions produce or consume ions or electrons, causing some change in the electrical properties of the solution that can be used as a measuring parameter.
Electrochemical biosensors can be classified into 3 types, based on measured electrical parameters.
What are they? (name + parameter)
- Conductometric - measures conductance
- Potentiometric - measures changes in potential difference
- Amperometric - measures current
Conductance = inverse of ______
Ohm's Law deals with the relationship between voltage and current. This relationship states what?
The potential difference (voltage) across an ideal conductor is directly proportional to the current through it.
State the equation for Ohm's law.
V = I R
V is the potential difference between two points
I is the current flowing through the resistance.
R is the resistance to current flow
Based on measuring changes in ____1____ of a selective material.
Since the inverse of resistivity is conductivity, these sensors are interchangeably called conductometric sensors or chemiresistors.
There are two types of these sensors:
- A selective material, which can change its conductivity upon interaction with chemical species is clamped between two contact electrodes and the ____1____ of the entire device is measured. Used in____2____ sensing.
- The chemically interactive layer is at the top of an electrode, which is immersed in the solution of ____3____. Used in biosensing
Potentiometric biosensors make use of ion-selective electrodes in order to ____1____ the biological reaction into an electrical signal.
This consists of an immobilised enzyme membrane surrounding the probe from a pH-meter where the catalysed reaction
generates or absorbs ____2____ ions.
The reaction occurring next to the thin sensing glass membrane
causes a ____3____ in pH.
An amperometric biosensor is a high ____1____ biosensor that can detect electroactive species present in biological test samples.
Since the biological test samples may not be intrinsically
electroactive, enzymes are needed to ____2____ the
production of reactive species.
In this case, the measured parameter is current.
Name the 2 major areas of commerical success for biosensors.
- Diabetes monitoring (blood glucose measuring)
- ClearBlue® (pregnancy test)
Blood glucose biosensors account for approximately 85% of the current world market for biosensors (~£2.5billion)
What is the reasons why the glucose market was particularly receptive to the introduction of biosensors?
The high (and increasing) prevalence of diabetes in developed nations.
Most enzyme biosensors (glucose included) rely on ______ enzymes.
The most commonly used enzymes in the design of glucose biosensors contain redox groups that change redox state during the biochemical reaction.
Enzymes of this type are glucose ____1____ (GOx) and glucose ____2____ (GDH).
In nature, ____1____ enzymes such as GOx act by oxidising their substrates, accepting electrons in the process and thereby changing to an inactivated ____3____ state.
State the advantages and disadvantages of glucose oxidase redox enzyme biosensors.
- Requires oxygen as a cosubstrate. Consequently, as oxygen is depleted in the sample, performance decreases.
State the advantages and disadvantages of glucose dehydrogenase redox enzyme biosensors.
- Oxygen independent
- The cofactor (NAD+) is expensive and unstable
State the chemical equation that happens in a glucose oxidase redox biosensor.
Glucose + O2 → gluconolactone + H2O2
State the chemical equation that happens in a glucose dehydrogenase redox biosensor.
Glucose + NAD+ → gluconolactone + NADH
The earliest approaches to the construction of amperometric glucose biosensors were based on GOx immobilised close to an electrode. The depletion of oxygen was monitored, using a ______ oxygen electrode
What are the advantages of using a redox mediator in a biosensor? (3)
- They have a wide range of redox potentials
- The redox potentials are independent of pH
- Easy to manufacture
Name the analyte used in pregnancy testing.
Why is this used?
The hormone HCG.
HCG is present in pregnant women.
Redox enzymes can be divided into 2 classes depending on the mechanism of electron transfer.
What are they? + Examples?
- Intrinsic (e.g. Cytochrome c peroxidase)
- Extrinsic (e.g. Glucose oxidase)
Describe intrinsic redox enzymes. (3)
- Take part naturally in electron-transfer outside the confines of the enzyme.
- Electron transfer between prosthetic group and substrate in vicinity of active centre.
- No requirement for ET pathway from active site to protein surface.
Describe extrinsic redox enymes. (4)