Reflectometric interference spectroscopy
Encyclopedia
Reflectometric interference spectroscopy (RIfS) is a physical method based on the interference of white light at thin films, which is used to investigate molecular interaction.
.
White light is directed vertically onto a multiple-layer system of a SiO2, a high-refractive Ta2O5 and an additional SiO2 layer (this additional layer can be chemically modified). The partial beams of the white light are reflected at each phase boundary and then refracted (transmitted). These reflected partial beams superimpose which results in an interference spectrum that is detected using a diode array spectrometer.
Through chemical modification the upper SiO2 layer is changed in a way to allow interaction with target molecules. This interaction causes a change in the thickness of the physical layer d and the refractive index
n within this layer. The product of both defines the optical thickness of the layer: n • d.
A change in the optical thickness results in a modulation of the interference spectrum. Monitoring this change over time allows to observe the binding behaviour of the target molecules.
RIfS is used especially as a detection method in chemo-
and biosensor
s.
Chemosensor
s are particularly suitable for measurements under difficult conditions and in the gaseous phase. As sensitive layers, mostly non-selective measuring polymers are used which sort the analytes according to size (the so-called molecular sieve effect when using microporous polymers) or according to polarity (e.g. functionalized polydimethylsiloxane
s). When performing non-selective measurements, a sum signal from several analytes is measured which means that multivariate data analyses such as neural network
s have to be used for quantification. However, it is also possible to use selectively measuring polymers, so-called molecular imprinted polymer
s (MIPs) which provide artificial recognition elements.
When using biosensor
s, polymer
s such as polyethylene glycol
s or dextran
s are applied onto the layer system, and on these recognition elements for biomolecules are immobilized. Basically, any molecule can be used as recognition element (proteins such as antibodies, DNA/RNA such as aptamer
s, small organic molecules such as estrone
, but also lipids such as phospholipid
membranes).
RIfS, like SPR
is a label-free technique, which allows the time-resolved observation of interaction among the binding partners without the use of fluorescence or radioactive labels.
Principle
The underlying measuring principle corresponds to that of the Michelson interferometerMichelson interferometer
The Michelson interferometer is the most common configuration for optical interferometry and was invented by Albert Abraham Michelson. An interference pattern is produced by splitting a beam of light into two paths, bouncing the beams back and recombining them...
.
Realization
White light is directed vertically onto a multiple-layer system of a SiO2, a high-refractive Ta2O5 and an additional SiO2 layer (this additional layer can be chemically modified). The partial beams of the white light are reflected at each phase boundary and then refracted (transmitted). These reflected partial beams superimpose which results in an interference spectrum that is detected using a diode array spectrometer.
Through chemical modification the upper SiO2 layer is changed in a way to allow interaction with target molecules. This interaction causes a change in the thickness of the physical layer d and the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
n within this layer. The product of both defines the optical thickness of the layer: n • d.
A change in the optical thickness results in a modulation of the interference spectrum. Monitoring this change over time allows to observe the binding behaviour of the target molecules.
Application
RIfS is used especially as a detection method in chemo-
Chemosensor
A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. In more general terms, a chemosensor detects certain chemical stimuli in the environment.- Classes :...
and biosensor
Biosensor
A biosensor is an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component.It consists of 3 parts:* the sensitive biological element A biosensor is an analytical device for the detection of an analyte that combines a biological...
s.
Chemosensor
Chemosensor
A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. In more general terms, a chemosensor detects certain chemical stimuli in the environment.- Classes :...
s are particularly suitable for measurements under difficult conditions and in the gaseous phase. As sensitive layers, mostly non-selective measuring polymers are used which sort the analytes according to size (the so-called molecular sieve effect when using microporous polymers) or according to polarity (e.g. functionalized polydimethylsiloxane
Polydimethylsiloxane
Polydimethylsiloxane belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological properties. PDMS is optically clear, and, in general, is...
s). When performing non-selective measurements, a sum signal from several analytes is measured which means that multivariate data analyses such as neural network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
s have to be used for quantification. However, it is also possible to use selectively measuring polymers, so-called molecular imprinted polymer
Molecular imprinted polymer
A Molecularly Imprinted Polymer , or plastic antibody is a polymer that is formed in the presence of a molecule that is extracted afterwards, thus leaving complementary cavities behind. These polymers show a certain chemical affinity for the original molecule and can be used to fabricate sensors,...
s (MIPs) which provide artificial recognition elements.
When using biosensor
Biosensor
A biosensor is an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component.It consists of 3 parts:* the sensitive biological element A biosensor is an analytical device for the detection of an analyte that combines a biological...
s, polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s such as polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
s or dextran
Dextran
Dextran is a complex, branched glucan composed of chains of varying lengths...
s are applied onto the layer system, and on these recognition elements for biomolecules are immobilized. Basically, any molecule can be used as recognition element (proteins such as antibodies, DNA/RNA such as aptamer
Aptamer
Aptamers are oligonucleic acid or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. Aptamers can be used for both basic research and clinical purposes as...
s, small organic molecules such as estrone
Estrone
Estrone is an estrogenic hormone secreted by the ovary as well as adipose tissue.Estrone is one of several natural estrogens, which also include estriol and estradiol...
, but also lipids such as phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
membranes).
RIfS, like SPR
Surface plasmon resonance
The excitation of surface plasmons by light is denoted as a surface plasmon resonance for planar surfaces or localized surface plasmon resonance for nanometer-sized metallic structures....
is a label-free technique, which allows the time-resolved observation of interaction among the binding partners without the use of fluorescence or radioactive labels.
Literature
- G. Gauglitz, A. Brecht, G. Kraus and W. Nahm. Sensor. Actuat. B-Chem. 11, 1993
- A. Jung. Anal. Bioanal. Chem. 372 1, 2002
- F. Gesellchen, B. Zimmermann, F. W. Herberg. Methods in Molecular Biology, 2005
- T. Nagel, E. Ehrentreich-Forster, M. Singh, et al. Sensors and Actuators B-Chemical 129 2, 2008
- P. Fechner, F. Pröll, M. Carlquist and G. Proll. Anal. Bioanal. Chem. Nov 1, 2008