How Does A Biosensor
Introduction to the Biotin Avidin System
Characterization of c-Si/Biotin-Avidin
The analysis of molecular interactions is a key area of research in the
healthcare, pharmaceutical and biotechnology fields. Molecular recognition at
solid surfaces forms the basis of a large number of bio and immunosensor
A biosensor is an analytical device that uses biological molecules to detect
other biological molecules or chemical substances. Typically the detector
molecule must be connected to a sensor that can be monitored by a computer,
which converts the biological systems with the computing power of the
How Does A Biosensor Work?
A biosensor consists of 3 parts:
- The sensitive biological element (biological material e.g. tissue,
microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids
etc.), a biologically derived material or biomimic). The sensitive elements can
be created by biological engineering.
- The transducer in between (associates both components).
- The detector element (works in a physicochemical way; optical,
piezoelectric, electrochemical, thermometric, or magnetic).
Schematic diagram showing the main components of a biosensor. This device
consists of: (a) a biocatalyst that converts the substrate to product; (b) the
transducer that determines the reaction and converts it to an electrical; and
the signal output is (c) amplified, (d) processed, and (e) displayed.
Introduction to the Biotin Avidin System
There are many potential application of biosensors of various types. The main
requirements for a biosensor approach to be valuable in terms of research and
commercial applications are the identification of a target molecule,
availability of a suitable biological recognition element, and the potential for
disposable portable detection systems to be preferred to sensitive
laboratory-based techniques in some situations. Some examples are given below:
glucose monitoring in diabetes patients <-- historical market driver, other
medical health related targets, environmental applications e.g. the detection of
pesticides and river water contaminants, remote sensing of airborne bacteria
e.g. in counter-bioterrorist activities, detection of pathogens...
The key goal in the development of such devices is the immobilization of
proteins onto the transducer element in such a way as to keep maximum
biochemical activity and minimum non specific interactions.
Avidin is a glycoprotein found in egg white. Biotin is a member of the
B-vitamin family, also known as vitamin-H.
The biotin-avidin system plays a major role in these applications as it
exhibits highly specific and strong binding affinity. Another distinctive
advantage of this system are the four identical binding sites of avidin for
biotin ensuring that the binding is directed only to the target of interest.
Adsorbing biotin on solid surfaces and assembling avidin makes it possible to
attach active ligands to the biosensor surface, thereby identifying target
species in the sample and increasing the selectivity of the biosensor by
reducing interferences from non-specific interactions.
Schematic of the formation of the biotin-avidin complex
When specific species come into contact with the avidin/ biotin surface and
bind to it there is an increase in the thickness of the deposited film. By
measuring an increase in the film thickness one can tell if a biomolecule has
bound to the sensor surface. The thickness increase is small, typically of the
order of 20 Å, requiring a sensitive and reliable technique for the
Such a technique is Spectroscopic Ellipsometry (SE), a non
destructive and very accurate technique to detect ultra-thin layers as well as
investigate the interactions that can happen at the solid-liquid interface. A
major advantage of the technique is that investigations can be made in ambient
air, liquid or vacuum environment.
Ellipsometric data were collected at an angle of incidence of 70° using the
Spectroscopic Phase Modulated Ellipsometer from Horiba Scientific across the spectral range 260-830 nm. The combination of the phase
modulation technology of the UVISEL and
its modular design makes it a powerful tool for biofilm measurement.
Characterization of c-Si/Biotin
A biotin film was deposited on the silicon substrate and is represented by a
single layer. The measurement was performed in ambient air environment, the
sample is simply placed on the sample stage to perform the non-destructive
Biotin Optical Constants
Characterization of c-Si/Biotin-Avidin Complex
The measurement was performed in a liquid environment; the sample is placed
in a liquid cell filled with demineralised water. A single layer was used to
model the sample. We made the hypothesis that the refractive index of the biotin
film and the biotin-avidin complex was the same. The increase of the thickness
found shows the strong binding affinity of the biotin-avidin system.
The figure below shows the excellent agreement between the experimental data
and the model.
Experimental and generated data
This application note illustrates the suitability of the UVISEL
Spectroscopic Phase Modulated Ellipsometer as a very accurate instrument for
studying solid/solid or solid/liquid interface reactions, and measurement of
Source: Horiba Scientific – Thin Films Division
For more information on this source please visit Horiba
Scientific – Thin Films Division