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2. DEFINITION A biosensor may be defined as a device incorporating a biologically active component in intimate contact with a physico-chemical transducer and an electronic signal processor.
4. So what is an biosensor?
5. HISTORY 1962, Clark and Lyons
1967, Updike and Hicks
1975, Yellow Springs Instrument
6. BIOCOMPONENTS Enzymes
7. TRANSDUCERS Electrochemical
8. BIOSENSOR TYPES Enzyme/metabolic biosensors
Enzyme and cell electrodes
9. Enzyme/Metabolic Sensors Enzymes are biological catalysts. There are five main classes of enzymes.
10. Oxidoreductases Dehydrogenases
11. Enzyme/Metabolic Sensors Substrate + Enzyme
12. Bioaffinity Sensors These sensors are based on binding interactions between the immobilised biomolecule and the analyte of interest.
These interactions are highly selective.
Examples include antibody-antigen interactions, nucleic acid for complementary sequences and lectin for sugar.
14. Transducers Electrochemical
15. POTENTIOMETRIC BIOSENSORS In potentiometric sensors, the zero-current potential (relative to a reference) developed at a selective membrane or electrode surface in contact with a sample solution is related to analyte concentration.
The main use of potentiometric transducers in biosensors is as a pH electrode.
16. POTENTIOMETRIC BIOSENSORS E = Eo + RT/nF ln[analyte]
Eo is a constant for the system
R is the universal gas constant
T is the absolute temperature
z is the charge number
F is the Faraday number
ln[analyte] is the natural logarithm of the analyte activity.
17. POTENTIOMETRIC BIOSENSORS The best known potentiometric sensor is the Ion Selective Electrode (ISE).
Solvent polymeric membrane electrodes are commercially available and routinely used for the selective detection of several ions such as K+, Na+, Ca2+, NH4+, H+, CO32-) in complex biological matrices.
The antibiotics nonactin and valinomycin serve as neutral carriers for the determination of NH4+ and K+, respectively.
19. POTENTIOMETRIC BIOSENSORS ISEs used in conjunction with immobilised enzymes can serve as the basis of electrodes that are selective for specific enzyme substrates.
The two main ones are for urea and creatinine.
These potentiometric enzyme electrodes are produced by entrapment the enzymes urease and creatinase, on the surface of a cation sensitive (NH4+) ISE.
20. POTENTIOMETRIC BIOSENSORS
21. AMPEROMETRIC BIOSENSORS With amperometric sensors, the electrode potential is maintained at a constant level sufficient for oxidation or reduction of the species of interest (or a substance electrochemically coupled to it).
The current that flows is proportional to the analyte concentration.
Id = nFADsC/d
25. AMPEROMETRIC BIOSENSORS Amperometric enzyme electrodes based on oxidases in combination with hydrogen peroxide indicating electrodes have become most common among biosensors.
With these reactions, the consumption of oxygen or the production of hydrogen peroxide may be monitored.
The first biosensor developed was based on the use of an oxygen electrode.
27. AMPEROMETRIC BIOSENSORS The drawback of oxygen sensors is that they are very prone to interferences from exogenous oxygen.
H2O2 is more commonly monitored. It is oxidised at +650mV vs. a Ag/AgCl reference electrode.
At the applied potential of anodic H2O2 oxidation, however, various organic compounds (e.g. ascorbic acid, uric acid, glutathione, acetaminophen ...) are co-oxidised.
28. AMPEROMETRIC BIOSENSORS Various approaches have been taken to increase the selectivity of the detecting electrode by chemically modifying it by the use of:
29. AMPEROMETRIC BIOSENSORS
30. AMPEROMETRIC BIOSENSORS
32. Examples of mediators commonly used are:
Ferrocene dicarboxylic acid (soluble)
Methylene Blue (MB)
37. Examples of commonly used polymers are:
38. Electrochemical Transducers
40. OPTICAL BIOSENSORS The area of biosensors using optical detection has developed greatly over the last number of years due mainly to the inherent advantages of optical systems.
The basis of these systems is that enzymatic reactions alter the optical properties of some substances allowing them to emit light upon illumination.
Means of optical detection include fluorescence, phosphorescence, chemi/bioluminescence...
41. OPTICAL BIOSENSORS Advantages of optical biosensors include
due to fibre optics, miniaturisation is possible
in situ measurements are possible
in vivo measurements are possible
diode arrays allow for multi-analyte detection
signal is not prone to electromagnetic interference
42. Disadvantages include:
ambient light is a strong interferent
fibres are very expensive
indicator phases may be washed out with time
43. Fibre optics are a subclass of optical waveguides which operate using the principle of total internal reflection.
Light incident on the interface between two dielectric media will be either reflected or refracted according to Snellís Law.
45. If light is entered into a fibre (surrounded by a medium of lower refractive index) at a shallow enough angle, the light will be confined within.
Thus, the optical fibres consist of a core of high refractive index surrounded by a cladding of slightly lower refractive index, with the whole fibre protected by a non-optical jacket.
47. Light input, and hence output, is dependent on the diameter of the fibre.
As a very small diameter is required for flexible fibres, this size is a limiting factor in the fabrication of the fibres.
For this reason, fibres are made from bundles which have the advantage of efficient light collection and flexibility.
Fibre bundles of 8, 16 and more fibre strands are available.
48. Generally, fibre-optic based biosensors employ fluorescence or chemiluminescence as the light medium.
This is due to the fact that fluorescence is intrinsically more sensitive than absorbance.
It is also more flexible due to the fact that a great variety of analytes and influences are known to change the emission of particular fluorophores.
49. There are basically two different configurations used at the tip of the fibre-optic probe
the distal cuvette configuration
waveguide binding configuration
50. The distal cuvette configuration involves immobilisation of detection molecules in a porous, transparent medium at the fibre tip.
The fluorescence changes when the analyte diffuses and is bound.
Excitation comes from out of the fibre and emission is coupled back into the fibre.
52. The waveguide binding tip configuration involves the binding of fluorescent-labelled detector molecules (e.g. antibodies) to covalently attached analyte molecules on the fibre surface.
As the label is close to the surface it is excited by the evanescent wave emanating from the fibre and the resulting fluorescence is coupled back into the fibre.
Free analyte competes for the binding sites on the recognition molecules, permitting them to diffuse away from the surface with a resultant decrease in fluorescence.
54. Piezoelectric Transducers The principle of this sensor type is based on the discovery of there being a linear relationship between the change in the oscillating frequency of a piezoelectric (PZ) crystal and the mass variation on its surface.
Sauerbrey discovered in 1959 that the change in mass is inversely proportional to the change in frequency of the resonating crystal (usually at MHz frequencies).
55. Piezoelectric Transducers DF = -2.3x106F2DM/A (Sauerbrey equation)
The change in mass occurs when the analyte interacts specifically with a biospecific agent immobilised on the crystal surface.
The crystal may be coated with antibodies, enzymes or organic materials.
Frequency changes smaller than 1MHz may be measured providing nanogram sensitivity.
57. SAW Transducers Surface acoustic wave (SAW) devices operate by the propogation of acoustoelectric waves, either along the surface of the crystal or through a combination of bulk and surface.
The oscillation of the crystal in SAW devices is greater, by at least a factor of ten, than the oscillation of the crystal used in PZ devices.
58. Calorimetric Transducers Enzyme-catalysed reactions exhibit the same enthalpy changes as spontaneous chemical reactions.
Considerable heat evolution is noted (5-100kJ/mol).
Thus, calorimetric transducers are universally applicable in enzyme sensors.
59. Calorimetric Transducers The thermal biosensors constructed have been based on:
direct attachment of the immobilised enzyme or cell to a thermistor
Immobilisation of the enzyme in a column in which the thermistor has been embedded.
DT = nDH/cp