chemical sensors & biosensors · deﬁnition of a biosensor “a device that uses speciﬁc...

Stewart SmithBiosensors and InstrumentationBeijing University of Posts and

Telecommunications 2019

Chemical Sensors & Biosensors



Definition of a Biosensor

“A device that uses specific biochemical reactions mediated by isolated enzymes, immunosystems, tissues, organelles or whole cells to detect chemical compounds usually by electrical, thermal or optical signals.”

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.

http://goldbook.iupac.org

https://doi.org/10.1351/goldbook



A Typical BiosensorBiosensors produce an output (often electrical) which is proportional to the concentration of biological analytes.

Signal Conditioning

Analyte

Biological Recognition Element

Transducer

Output Signal



Required Characteristics• Sensitivity

• Low detection limits

• Cost

• Simplicity

• Reliability

• Speed

• Accuracy

• Precision

• Utility

• Field portability

• Ruggedness

• Reproducibility

• Ease of calibration

• Stability

• Room for improvement



Applications of Biosensors

• Medical Care (both clinical and laboratory use)

• Determination of Food Quality

• Detection of Environmental Pollutants

• Industrial Process Control

• Law Enforcement (e.g., Alcohol, Drugs)

• Military (Biodefence)



Biosensing PrinciplesType Examples

Electrochemical: Potentiometric Amperometric FET based Conductometric

Glucose Sensor Neurochemical Sensor for Dopamine, Nitric Oxide, etc.

Optical: Spectroscopy, Fluorescence, Polarimetry

Environmental monitoring

MEMS/Piezoelectric: Quartz crystal microbalance

DNA sequence detection

Thermal: Thermistor, Thermopile

Sensing by Enzymatic Catalysis



Electrochemical Sensors• Potentiometric: Measurement of the emf of an

electrochemical cell at zero current. The emf is proportional to the log. concentration of the analyte.

• Amperometric: An increasing (decreasing) potential is applied to the cell until oxidation (reduction) of the substance to be analyzed occurs. The height of the peak current is directly proportional to the concentration of the electroactive material.

• Conductometric: Most reactions involve a change in the composition of the solution, which can often be measured as a change in its electrical conductivity.



Optical Biosensors• Principle: Changes in light intensity are linked to changes in mass

or concentration. Hence, fluorescent or colorimetric molecules must be present.

• Methods: Optical fibres, surface plasmon resonance, absorbance, luminescence.

LED

Photodetector

Finger

IR light



Glucose Sensor (Electrochemical)

• The analyte, glucose, diffuses through a membrane to the GOD enzyme layer - where glucose is converted to hydrogen peroxide and gluconic acid.

• Both oxygen (being consumed) and H2O2 (being produced) can be measured electrochemically (in an amperometric technique), or the local pH change can be monitored (potentiometrically).

Pt Anode (+)

Ag Cathode (-)

Immobilized Glucose Oxidase (GOD)

Polyurethane Membrane

Glucose → H2O2 + gluconic acid

Glucose oxidase (in presence of oxygen)



Glucose Sensor (Affinity Method)Based on the competitive binding of glucose (plus attached fluorescent label) with receptor sites on an immobilised protein (Concanavalin A). The reduction of fluorescence in the bulk fluid is detected.

3 mm

0.3

mm

Hollow Dialysis Fibre Immobilised Con A

Excitation

Emission

Optical Fiber

Glucose

(Note: Con A has receptor sites that bind specifically to various sugars, glycoproteins

and glycolipids.)



Glucose Sensor (Zhu et al.)

J. Zhu et al, Planar amperometric glucose sensor based on glucose oxidase immobilised by chitosan film on Prussian blue layer, Sensors 2: 127-136, 2002. doi:10.3390/s20400127

Selection of Operational pH

http://dx.doi.org/10.3390/s20400127



Glucose Sensor (Zhu et al.)Effect of Temperature on Response



Glucose Sensor (Zhu et al.)Response Time



Glucose Sensor (Zhu et al.)Testing Interference with ascorbic and uric acid



Glucose Sensor (Zhu et al.)Calibration curve



Glucose Sensor (Zhu et al.)Long Term Stability



Glucose Sensor (Zhu et al.)

No. 1 2 3 4 5 6 7 8 9 10 11

I/nA 291 294 298 292 297 295 290 289 299 295 294

AV 294

RSD 3.0%

PrecisionA measurement can be precise but may not be accurate!

Standard deviation σ is a statistical measure of the precision. (N - number of data; xi - each individual measurement; - mean of all measurements.)x̄

σ =∑N

i=1 (xi − x̄)2

N − 1



Glucose Sensor (Zhu et al.)Accuracy

Correlation and regression line for measurements of 100 human serum samples between spectrophotometry and the glucose sensor.

Correlation Coefficient:

0.996.

chemical sensors & biosensors · deﬁnition of a biosensor “a device that uses speciﬁc...

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