explain the principles of operation of test strips and biosensors for measuring the concentration of glucose in blood and urine, with reference to glucose oxidase and peroxidase enzymes

Homeostasis in Mammals: Measuring Glucose in Blood and Urine

Maintaining blood glucose within a narrow range is essential for mammalian homeostasis.

Clinical monitoring of glucose uses two main types of devices:

• Colour‑changing test strips (primarily for urine)
• Electrochemical biosensors (hand‑held meters for blood)

Key Enzymes Involved

The reactions that underpin both test strips and biosensors rely on the same enzymes:

1. Glucose oxidase (GOx) – catalyses the oxidation of β‑D‑glucose.
2. Peroxidase (often horseradish peroxidase, HRP) – uses the hydrogen peroxide produced by GOx to oxidise a chromogenic or electroactive substrate.

Biochemical Reactions

The overall sequence can be written as:

\$\text{Glucose} + \text{O}2 \xrightarrow{\text{GOx}} \text{Gluconolactone} + \text{H}2\text{O}_2\$

Followed by the peroxidase‑catalysed step:

\$\text{H}2\text{O}2 + \text{Reduced\;substrate} \xrightarrow{\text{HRP}} \text{Oxidised\;substrate} + 2\text{H}_2\text{O}\$

The amount of oxidised substrate formed is proportional to the original glucose concentration.

Urine Glucose Test Strips

These are simple, disposable strips used for semi‑quantitative screening.

• Sample: a few drops of urine are applied to the reagent pad.
• Reagents: immobilised GOx, HRP, and a chromogenic substrate (e.g., o‑dianisidine).
• Mechanism:

  1. GOx oxidises glucose, generating H₂O₂.
  2. HRP uses the H₂O₂ to oxidise the chromogen, producing a coloured product.
  3. The colour intensity is compared with a printed chart to estimate glucose concentration.

• Result: provides a qualitative/approximate quantitative reading (e.g., negative, trace, +, ++, +++).

Blood Glucose Biosensors (Electrochemical)

Modern hand‑held meters employ a second‑generation amperometric biosensor.

• Sample: a drop of capillary blood (≈0.5 µL) placed on a test strip.
• Strip architecture:

  1. Working electrode (usually carbon) coated with GOx.
  2. Mediator layer (e.g., ferrocene derivative) that shuttles electrons.
  3. Reference and counter electrodes for the potentiostatic circuit.

• Mechanism:

  1. GOx oxidises glucose, producing H₂O₂.
  2. In second‑generation sensors the H₂O₂ is not measured directly. Instead, the enzyme transfers electrons to the mediator, which then transfers them to the electrode.
  3. The resulting current (\$I\$) is proportional to the glucose concentration (\$[G]\$): \$I = k\,[G]\$, where \$k\$ is a calibration constant.

• Result: a digital read‑out in mg/dL or mmol/L with high accuracy (±5 %).

Role of Peroxidase in Different Formats

Peroxidase is essential in colour‑changing strips but is often omitted in modern electrochemical biosensors because the mediator replaces the need for H₂O₂ detection. However, some “third‑generation” sensors still use HRP to directly reduce H₂O₂ at the electrode surface, improving specificity.

Comparison of Test Strips and Biosensors

Clinical Relevance to Homeostasis

Regular monitoring of blood glucose helps detect deviations from the normal range (≈4–7 mmol L⁻¹ fasting). Early detection of hyperglycaemia or hypoglycaemia allows corrective actions (diet, insulin, medication), thereby preserving the homeostatic balance of energy metabolism.

Key Points to Remember

• Both test strips and biosensors rely on the oxidation of glucose by glucose oxidase.
• Peroxidase couples the generated H₂O₂ to a visible colour change in urine strips.
• In blood biosensors the electron flow is usually mediated, giving a direct current proportional to glucose concentration.
• Accurate glucose measurement is vital for maintaining metabolic homeostasis in mammals.