Biosensors

Biosensors

what are bio sensors
1. Biosensors are analytical devices which are capable of providing either qualitative or quantitative results.
Clinical relevance
1. Point of Care (PoC) analysis becoming increasingly important
  1. primary and secondary care
2. Biosensor devices - where speed makes difference in early diagnosis and Infection
  1. trying to reduce the time between diagnostic information and when treatment is provided. particulary in an area where time is of essence e.g in some disease cases. (allows information to be available in minutes rather than days)
3. Enable personalised medicines - allows for stratified clinical interventions and improved patient outcome
  1. reduces waste
How does the natural world sense things
1. Drug/ explosive sniffer dogs better than any artificial devices
2. Moth Pheromones Able to detect ONE MOLECULE!
3. Human sense of smell thousands of smells at ppm
sensors
1. Integrated device detects its environment and gives the user a response Sensitivity can be as high as ppt detection Specificity often high sometimes broader
  1. physical sensors
    1. heat, light, temperature, radiation (smoke alarm)
  2. pH, metal ions, oxygen, toxic gases
    1. these are all examples of sensors but they are not biosenseors
  3. ppt= parts per trillion
compoents of a bio sensor
1. recognition component
  1. All are, or rely on, proteins
    1. electronics
    2. stability
    3. environment
    4. limitations of molecular recognition
      1. Antibodies – size of target Antibodies – toxicity in host Enzyme - finding one suitable
  2. Recognition - Catalytic binding events
    1. Enzymes
      1. selective binding of analyte, enzyme mediated chemical reaction, reaction products detected in transduction system eg. Peroxides from oxidases, NADH from dehydrogenases, only limited number of suitable enzymes available, Km of enzyme dictates analytical range, can be fragile / costly, can use chain of enzymes to achieve results, immobilization can affect performance
        Problem is that there is a limited number of enzymes.
      2. sensitivity and affinity
        km is a function of the sensitivity of the biosensor it is where you see half the reaction velocity (1/2 vmax)
    2. glucose sensors
      1. e.g glucose oxidases
        enzyme assay: glusose sensors all use the same sensor. glucose oxidase. it oxidises glucose down to gluconic acid. this reaction also produces hyrogen peroxide which can be detected very easily at the surface of an electrode, the hdrogen peroxide is reduced and produces electrons which go into the sensor and that is what you see.
      2. sense blood glucose levels and glucose levels in sweat.
  3. Recognition - Affinity binding events
    1. Antibodies
      1. Binding only – no chemical reaction, suitable for large molecules, less useful for small molecules, raised via animals , wide applicability, high affinity, can be fragile and costly
      2. sensitivity and affinity
    2. Molecularly Imprinted Polymers
      1. suitable for large and small molecules Moderate affinity Robust Cheap
      2. possible improvements are increasing affinity, Imprinting on surfaces, Novel transduction methods, Computational Molecular Modelling for design of Polymer
      3. how its done
        (i) pre-polymerisable complex (ii) polymerisation (iii) template removal
  4. Recognition - Whole Cell
    1. Detecting physiological changes
      1. Enzyme activity / products Extracellular molecular release Morphological changes mortality
2. Transduction component
  1. Purpose - to monitor recognition component
  2. Must only respond to analyte binding NOT another interfering species
  3. Application dictates type of transduction eg. Is sample highly opaque? …can’t use many optical methods
  4. Electrochemical transduction
    1. Amperometry - Current
      1. uses a change in the concentration of a redox species produced as a consequence of analyte presence
    2. Potentiometry - Voltage
      1. uses a change in the concentration of an ionic species produced as a consequence of analyte presence eg. NH4+ produced on enzyme Urease acting on Urea
    3. Impediometric - Impedance
      1. uses changes in electrical impedance brought about by interaction of the analyte
  5. Transduction component = an electrode
  6. Optical transduction
    1. spectroscopic properties of the analyte / analyte reaction products / labelled species
    2. optical fibres and optical waveguides carry information about the reactions/ interactions at the recognition site
    3. properties include: absorbance / fluorescence/ chemiluminescence /
  7. Mass Based methods
    1. In principle, the simplest form of transduction weigh the analyte after binding onto immobilized affinity receptor sites
    2. in practice, v. tricky and not a very convenient sensor Can be very sensitive to non-specific changes
    3. What would we need? Property of a solid that changes due to minute changes in the mass of that solid Property that can be monitored very accurately
      1. Piezoelectric materials
        a single crystal of quartz can be made to oscillate at its resonant frequency Most crystals of current interest resonate between 5 to 30 MHz
        The resonant frequency can be measured very accurately
        When mass changes, resonant frequency of system changes This change can also be measured very accurately The change is proportional to the increase in mass Can be used to monitor binding to an affinity surface
        These devices are called: Quartz Crystal Microbalances (QCMs)
    4. Mass transduction: surface acoustic waves
      1. The speed a wave travels across a surface changes when mass increase
      2. The increase in mass due to an ‘affinity binding event’ can therefore be monitoring the time taken for wave to move a set distance
      3. Various modes of wave propogation are available
        Transverse (90deg to plane of crystal and direction of travel)
        Longitudinal (In plane of crystal and direction of travel
        Shear (In plane of crystal and 90deg to direction of travel
Benefits of Biosensors
1. Fast
2. Suitable for PoC applications
3. Needs of an ageing population
4. Cheap
  1. Instrument cheap, sensor can be v. cheap often disposable if “one-shot”
5. Portable
  1. Diagnose wherever it is needed “hostile” environments OK
6. Non-skilled operation
  1. No pr little sample prep. Electronics interpret the signal
7. Can be robust
"a device that recognizes an analyte in an appropriate sample and interprets its concentration as an electrical signal via a suitable combination of a biological recognition system and an electrochemical transducer.”
1. biorecognition is the part of the sensor that interacts with the environment. it reacts/ bind/ targets or reacts with one specific molecule within a mixture of a lot of molecules.

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