3 - Structures

An optrode (or optode) consists of two (maybe one or more than two) fibers for the input and output light. The operation is based on spectral change of the output light caused by different physical and chemical effects at the tip of the sensor:

• Colorimetric detection is based on the color changes of indicators entrapped in the material at the sensor tip. Practically the reflected light spectra are to be measured. A special case of that method is the simple reflectance/absorbance measurement using a monochromatic light source or a special wavelength region. Sometimes the spectral change is caused directly by the color change of the analyte of interest.
• Fluorimetric detection is based on the phenomenon of fluorescence quenching. The absorbed incident light excites a secondary light wave, the spectrum of which differs from that of the incident light spectrum. The intensity of the emitted light may vary according to environmental changes.
• Phosphorescence behavior is also a secondary light emission as an answer for the excitation, however, the emission decays slowly and is present for a given period of time even when the excitation has already been switched out. Not only spectral changes, but decay time measurements can be used for analytical purposes.
• Catalyzed light emission caused by chemiluminescence or bioluminescence reaction can also be used for sensor purposes. There is no need for incident input light in this case.

Optrodescan be used for the detection of gases, ions, enzyme substrates, as well as macromolecules. The active material and the membrane at the tip may contain immobilized ionophores, indicators, fluorescent dyes, chemi- or bioluminescence enzymes, organic adsorbents, etc., according to the application purposes and operation principles. Immobilizing selective reagents and/or applying permselective membranes can increase chemical sensitivity and selectivity.

In core-based sensors, the core-analyte interaction is directly utilized as the basis of operation. Some core-based sensors use porous fibers with chemically sensitive reagents immobilized physically or chemically on the surfaces of the pores to sensitize them to the analyte of interest. Porous fibers may exhibit very high gas permeability and liquid impermeability so they can be used for the detection of gases in liquids. Vapors permeating into the porous zone can produce a spectral change in transmission. For better sensitivity and selectivity, colorimetric reagents can be trapped into the pores. The observed light intensity is a function of the concentration of the analyte to be measured, c, and can be expressed: I = I0 exp[- h l f (c )] where I0 is the intensity of the incident light; h is the extinction coefficient, both of light absorption and of light scattering; l is the light passing length of the sensing segment; and f (c ) is the concentration of the absorption and scattering centers.

Cladding-based chemical fiber-optic sensors can be made using microporous or other type sensitive claddings. Evanescent-wave refractometric cladding-based sensors detect the absorption of the species in the polymeric cladding, which leads to a variation of its refractive index, thus, also to a variation of the overall transmission efficiency of the fiber. Porous polymer cladding-type sensors can be used, for instance, for humidity measurements. The quantity of moisture absorbed by the microporous cladding varies with humidity. The optical power level of the transmitted light in the core varies according to the moisture absorption quantity because of the refractive index change of the cladding. Another example is the application of organopolysiloxanes as cladding materials since they can change their refractive index when they adsorb and/or absorb molecules from the surrounding medium. Also evanescent wave excited fluorescence quenching can be used for detection. In special cases, the surrounding medium itself plays the role of the cladding; the de-coupling light from a fiber-core having no special cladding on its surface follows the refractive index changes of the environment.

Both conventional fiber-optic and integrated optical interferometers can also be used as sensors. Their operation is based on the optical interference between reference and sensitive lightwaves. According to the well-known rules of light interference, the resultant intensity is a function of the difference in optical path length of the branches, which can be modulated by a change of either the refractive index or the geometrical length difference between the paths. The former one is the Mach-Zehnder interferometer consisting of two Y-branches. The incoming light is split into two parts, which are guided in the two branches of the interferometer. For sensor applications, one branch is usually affected by the quantity to be measured, while the other provides the reference phase. If one of the branches of the integrated structure is covered with a sensitive polymer film, the refractive index of which is a function of its surrounding medium, the effective refractive index of this waveguide branch is a function of the quantity to be measured. Thus, the interferometer acts as a sensor based on the refractive index modulation caused by the evanescent field effect in the branch covered with the sensitive polymer.

In reflection-type interferometer sensors the superposition of the two partial beams reflected from the waveguide/polymer and polymer/air interfaces, respectively, can be influenced either by swelling of the film caused by permeation of gases and liquids or by adsorption of particles on the top of the film, which will introduce an additional reflection. Moreover, the introduced analyte can interact with the polymer film, thus influencing the value of the refractive index. Spectral interferometry allows these effects to be discriminated to a certain extent.

Surface plasmon resonance (SPR) is one of the surface oriented biosensing techniques that can be used to monitor biomolecular interactions. Surface plasmons are the quanta of plasma waves propagating along a metal interface. Surface plasmons can be excited by lightwaves, when an intensity decrease occurs in the propagating beam; this phenomenon is called SPR. For sensor purposes, the mostly used excitation arrangement is the attenuated total reflection (ATR) or Kretschmann configuration. A recently developed version is shown in the animation. It consists of a glass chip coated by a thin gold film closed into a flow-through cell. The surface of the gold is activated by receptor molecules. Plasmons are excited optically on the metal-glass interface, at a characteristic angle of the incident light (Qsp). In practical arrangements, a convergent light beam falls through a cylindrical prism onto the sensitive area. The reflected light is detected by a photodiode array, where the critical angle can be determined directly. The resonance-angle shifts are measured when molecules or clusters are bound to the receptors. Further chemical reactions of the immobilized molecules may also be followed.

The basic principle of the OWLS method is the following: linearly polarized light (laser) is coupled by a diffraction grating into the waveguide layer, provided that the incoupling condition is fulfilled. The incoupling is a resonance phenomenon that occurs at a precise angle of incidence, which depends on the refractive indices of the medium covering the surface of the waveguide. The light is guided by total internal reflection to the ends of the waveguide layer where it is detected by photodiodes. By varying the angle of incidence of the light the mode spectrum can be obtained from which the effective refractive indices and hence the adsorbed mass per unit area can be calculated.