Biosensors
Binding is converted into a current, a potential or an impedance change at an electrode. Amperometric formats use an enzyme label and a redox mediator for amplification; potentiometric and field-effect devices read charge; impedance spectroscopy follows the charge-transfer resistance of the interface label-free.
Our ruleSimulations prepare you for the bench but don't replace hands-on practice.
Electrochemical
Binding is converted into a current, a potential or an impedance change at an electrode. Amperometric formats use an enzyme label and a redox mediator for amplification; potentiometric and field-effect devices read charge; impedance spectroscopy follows the charge-transfer resistance of the interface label-free.
How it differs from the standard method
- Screen-printed electrodes are cheap but vary between units, so calibrate within a batch.
- An enzyme label needs its mediator: without one almost no current flows.
- Turbid and coloured samples are tolerated, unlike in optical formats.
1A recognition layer bolted onto a transducer
A biosensor has two halves. The recognition layer is a biological or biomimetic molecule immobilised on a surface: an antibody, an aptamer, an enzyme, a nucleic-acid probe or a receptor, chosen because it binds one analyte and little else.
Current, potential or impedance
Cheap, small and tolerant of turbid samples. Enzyme labels with a redox mediator give amplification; label-free impedance reads the binding directly.
Refractive index, fluorescence or colour
Label-free and real-time, giving kinetics and affinity as well as concentration, but sensitive to temperature and bulk composition.
A strip anyone can read
Capillary flow carries the sample past a gold-labelled antibody and a striped capture line. No instrument, minutes to a result, limited sensitivity.
The figure of merit
Not the largest signal but the smallest trustworthy one: the detection limit, 3σ of the blank over the calibration slope.
2Why the detection limit is a statistic, not a signal
By convention the limit of detection is the concentration whose signal is three standard deviations of the blank above the blank's mean, which works out as 3σ divided by the calibration slope; ten σ over the slope gives the limit of quantification. Both halves matter equally.
3Kinetics set the low end
Binding takes time. The fraction of sites occupied approaches its equilibrium value with a rate constant ka·c + kd, so a dilute sample is the slowest to equilibrate, and a short incubation costs the low end of the curve disproportionately.
4How biosensors mislead
- An unblocked surface: the label or the sample's abundant proteins adsorb directly, so the blank is high and erratic.
- No reference channel on a refractive-index sensor: bulk composition, temperature and drift are read as binding.
- A capture layer that is only adsorbed: it slowly desorbs, so the baseline runs away and serum displaces it.
- The high-dose hook in one-step sandwich assays: very high analyte saturates the label, so the signal falls again.
- A lateral-flow strip with no control line, where a failed run and a negative result look identical.
- An antibody pair that shares an epitope, which cannot form a sandwich at all.
Key terms
Practise these as flashcards →Lowest concentration distinguishable from a blank, conventionally 3σ of the blank divided by the calibration slope.
Lowest concentration measurable with acceptable precision, conventionally 10σ over the slope.
A small, diffusing molecule that shuttles electrons between an enzyme label and the electrode.
An ordered layer of thiols or silanes used to attach recognition molecules and block the surface.
An otherwise identical sensing channel without the recognition layer, subtracted to remove bulk and drift.
Signal from anything that sticks other than the analyte binding its receptor.
Loss of signal at very high analyte concentration in one-step sandwich formats.
A line on a lateral-flow strip that develops whatever the result, proving the strip ran and the reagents were active.
Seconds for the front to travel 4 cm; a higher number is a slower membrane and usually a more sensitive strip.
When to use it
- When a measurement has to be made outside a laboratory, quickly, or many times over, rather than once with maximum accuracy.
- For continuous or repeated monitoring, such as glucose or a fermentation metabolite, where a plate assay cannot follow the time course.
- When label-free kinetics and affinity, not just concentration, are wanted from a small amount of material.
Compared with alternatives
The plate-based reference method: more sensitive and better standardised, but slower and laboratory-bound.
Label-free optical instruments built for kinetics and affinity rather than for deployment as a sensor.
When many analytes must be measured in one small sample.
The general-purpose optical readout behind many bench assays.
Sub-techniques
Choosing one here switches the whole page, including the protocol and the lab, to it.
Binding is converted into a current, a potential or an impedance change at an electrode. Amperometric formats use an enzyme label and a redox mediator for amplification; potentiometric and field-effect devices read charge; impedance spectroscopy follows the charge-transfer resistance of the interface label-free.
- Screen-printed electrodes are cheap but vary between units, so calibrate within a batch.
- An enzyme label needs its mediator: without one almost no current flows.
- Turbid and coloured samples are tolerated, unlike in optical formats.