Introduction of Optical Sensors
Optical sensors rely on the optical transducer of the signal and comprise ultraviolet, visible and infrared spectrophotometry in transmission or reflectance modes. The relationship between the incident light intensity and the transmitted radiation is given by the Lambert-Beer law:
ε = ελ . l . c
where Aλ = log10 Io/I is the absorbance at a given wavelength λ (being Io and I the incident and transmitted light intensities); ε is the molar absorptivity; l is the optical path through the absorbing sample and c is the molar concentration of absorbing analyte. The transmitted light is then detected by a photodiode or phototube which transduces light into an electrical signal.
More recently “optrodes” were introduced (the term optrode or optode was coined in analogy with “electrode” in electrochemical sensors) with fiber optics covered at one end with reagent immobilized in a gel or membrane which produces a change in color in the presence of the analyte. The light is then transmitted to the solution and partly reflected and detected in a bifurcated fiberoptics. The advantages of “chemosensor optodes” are that they do not require an extra reference like the reference electrode in potentiometric sensors and that are insensitive to electrical noise, but double transduction chemical-optical-electrical is needed at the end signal.
In addition to absorption-transmission optical sensors, luminescent sensors detect light emission that results from the absorption of a photon by a molecule and then the excited state decays with either fluorescent or phosphorecent emission. These techniques are highly sensitive and the emitted light of higher wavelength against zero background enhances the sensitivity and signal-to-noise ratio allows extraction of chemical information from complex samples.
Aplication Of Optical Sensors
Optical sensors can be used to monitor pH, metal ions, oxygen, glucose and other enzyme substrates, etc., by appropriate choice of the chemical sensing layer. They are limited, however, to transparent and colorless samples.
Biosensors based on evanescent waves have had a high impact in recent years, for instance surface plasmon resonance (SPR) biosensors. Surface plasmon resonance arises from the interaction of light with suitable metal or semiconductor surface which generates a quantum optical-electrical phenomenon. Under certain conditions, the photon energy can be transferred to the surface of the metal as packets of electrons called plasmons. This energy transfer occurs at a particular wavelength of light, when the quantum energy carried by the photon exactly matches the quantum energy level of the plasmons.
The incident light is almost completely absorbed at the wavelength that excites plasmons, that is the resonance wavelength. Using monochromatic light at the resonance wavelength and changing the incidence angle, the maximum absorption occurs at a particular angle of incidence. The interaction between the plasmon’s electric field and analyte molecules within this field determines the resonant condition (wavelength at constant angle or conversely plasmon resonant angle at constant wavelength). Therefore, any change in composition adjacent to the surface plasmon alters the plasmon resonance angle and the SPR angle shift is directly and linearly proportional to the change in surface composition. This has been used to detect biomolecules such as antibody-antigen or single stranded DNA hybridization with success. Optical sensors waveguide and fiber optics work in a similar way differing only in geometry.