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Perform electrical characterisation of biosensors

Posted: 10 Mar 2014  Print Version  Bookmark and Share

Keywords:biosensors  transducer  analyte <b>validation  sensor characterisation  </b>digital multi-meter 

At their most basic, biosensors are devices used to detect an analyte, i.e. a substance or chemical constituent of interest. Essentially, they can allow for understanding bio-composition, structure, and function by converting a biological response into an electrical signal. Proper testing of the electrical portions of these sensors is essential to support their development.

Biosensors are devices that do one or more of the following:
 • Detect, record, convert, process, and transmit information regarding a physiological change or process.
 • Use biological materials to monitor the presence of various chemicals in a substance (analyte).
 • Combine an electrical interface (transducer) with the biologically sensitive and selective element.

More specically, a biosensor contains a bioreceptora biomolecule that recognises the target analyte. The transducer portion of the biosensor converts the recognition event into a measurable signal that correlates with the quantity or presence of the chemical or biological target of interest. Figure 1 illustrates a generalized biosensor model.

Figure 1: A generic biosensor structure.

Performance criteria for a biosensor system include:
 • Speed and ease of use by non-technical personnel.
 • Selectivity to target analyte.
 • Sensitivity/resolution.
 • Linearity.
 • Accuracy/repeatability.
 • Dynamic range. High analyte concentrations will not degrade sensor usability.
 • Robustness (relatively insensitive to temperature, electrical noise, physical shock, vibration, etc.)
 • Usable lifetime/adaptability.
 • Safety/integrity (for personnel, equipment, and analytes.)

Sensor designs
Various biosensor design approaches have been used over the years, including an oligonucleotide sensor and nucleic acid reaction to indicate the presence of a pathogen. Another design employs surface plasmon resonance (SPR) to detect biological molecules such as protein and DNA. An SPR-based sensor can provide label-free studies of molecular interactions in real time using a sensor chip interface that facilitates attachment of specic ligands to the transducer surface and provides a sensitive measurement of surface concentrations.

Tissue-based sensors are also being developed that use living cells on chips that can react functionally to the presence of both biological and chemical threat agents. Because they are designed to mimic the function of multi-cellular human tissue, these sensors should respond to both known and previously uncharacterized agents. The transducer senses small changes in electrical charges on the surface of the living cells.

Electrochemical biosensors are normally based on enzymatic catalysis of a reaction that produces or consumes electrons. The sensor substrate may contain three electrodes: a reference electrode, a working electrode, and a counter electrode. The target analyte is involved in the reaction that takes place on the active electrode surface, and the reaction may cause either electron transfer across the double layer (producing a current) or can contribute to the double layer potential (producing a voltage). One can either measure the current (rate of flow of electrons is now proportional to the analyte concentration) at a fixed potential or the potential can be measured at zero current (which gives a logarithmic response)[1].

An electrochemical potentiometric biosensor (in which potential is produced at zero current) gives a logarithmic response with a high dynamic range. Such biosensors are often made by screen printing the electrode patterns on a plastic substrate that is coated with a conducting polymer, and then some protein (enzyme or antibody) is attached. They have only two electrodes and are extremely sensitive and robust.

All biosensors usually involve minimal sample preparation as the biological sensing component is highly selective for the analyte concerned. The signal is produced by electrochemical and physical changes in the conducting polymer layer due to changes occurring at the surface of the sensor. Field effect transistors (FETs), in which the gate region has been modified with an enzyme or antibody, can also detect very low concentrations of various analytes as the binding of the analyte to the gate region of the FET causes a change in the drain-source current.

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