Topic 5: Biosensors

Question 1

• What is a biosensor?

Answer 1

• A device which uses a living organism or biological molecules, especially enzymes or antibodies, to detect the presence of chemicals.

Question 2

• What does a glucose biosensor do?

Answer 2

• Measures the blood glucose levels, generally for diabetes management

Question 3

• What are some of the initial requirements and hurdles posed with developing a glucose biosensor?

Answer 3

• Daily tests need to be cost effective
• Must be able to give reading in the range of normal levels (4.4-6.6 mM)
• Difficult to directly oxidise/reduce glucose as has a high E_o value

Question 4

• What is the general method of making blood glucose biosensors?

Answer 4

• Indirect electrochemical detection via a redox active enzyme though a lock and key mechanism

Question 5

• Describe the 1^st generation glucose sensor

Answer 5

• Oxygen used as oxidising agent in the glucose oxidase (GO_x- enzyme that does rxn on glucose) cycle
• Glucose docks with enzyme, electron transfer occurs between FAD and glucose
• FAD (oxidised state)/FADH – species switching oxidation state when oxidising glucose
• Enzyme converted back in feedback cycle via oxygen oxidising enzyme back

Question 6

• Give the general equation for this 1^st generation biosensor

Answer 6

Question 7

• How does the 1^st generation glucose biosensor detect glucose levels.

Answer 7

• Electrochemically detects output signalof H₂O₂
• If not glucose, no reaction, no signal due to H₂O₂
• i ∝ H₂O₂ ∝ glucose

Question 8

• Describe the Clark cell for oxygen sensing

Answer 8

• Sensor that aided that invention of the glucose sensor
• Membrane acts as excluder of interferences meaning only O₂ turnover at electrode

Question 9

• What is the problem with using O₂ in biosensors

Answer 9

• Very little O₂ reacts with glucose to turn it back to original state
• Normal O₂ concentration around 10x smaller than physiological levels of glucose, not enough to to convert enzyme back.

Question 10

• Describe 2^nd generation glucose biosensors

Answer 10

• Replace O₂ with a synthetic oxidising agent
• To oxidise FADH₂ –> FAD, synthetic reagent needs E^O > 0.031V or will not be sufficient to oxidise back

Question 11

• Give an example of a synthetic oxidising agent that is used in 2^nd gen glucose biosensors. Give equations for the reactions occurring between this agent and glucose oxidase, as well as the reaction occurring at the electrode

Answer 11

• Fe²⁺/ Fe³⁺ based mediators
• E.g. ferrocene/ferrocyanide/ferrycyanide

Question 12

• Draw a sequence of events of the 2^nd generation glucose sensor and use it to describe the resulting CSV.

Answer 12

• No limiting current when no glucose present

Question 13

• Describe a third-generation glucose sensor

Answer 13

• Direct communication with FAD|FADH₂ of the enzyme
• In theory can apply potential to switch enzyme back to oxidised state

Question 14

• What is a problem currently facing 3^rd gen glucose sensors?

Answer 14

• Electrode/wire directly communicates with enzyme so no need for mediator
• However, it is very challenging to place electrode at centre of enzyme as must electron tunnel out of centre through insulating bulk (blocking communication) of enzyme to electrode.

Question 15

• Describe a commercial home test glucose strip kit

Answer 15

• Electrode detects mediator after blood drop soaks in to enzyme

Question 16

• Describe a quantum dot-based biosensor

Answer 16

• Changing size of dot gives different colour light out.
• Reduction gives higher energy
• Can be used as a link to antigen associated with cancerous tumours
• In vivo protection prevents excretion of quantum dot.
• Au nanoparticles can be used to prevent luminescence
• Target cancerous tumours to cleave nanoparticles, resulting in luminescence of quantum dot at tumour site