Optical Transducers and Biosensors: Principles and Applications

TRANSDUCER

A transducer is a device that converts an observed change (physical, chemical, or biological) into a measurable signal.

Optical Transducers

Absorption spectroscopy, fluorescence spectroscopy, luminescence, internal reflection, light scattering, surface plasmon resonance, ellipsometry, and fiber optics.

TOTAL INTERNAL REFLECTION and EVANESCENT FIELD

The refractive index n1 is larger for the incident medium than the refractive index n2 of the reflecting medium. Sin(THETA)=n2/n1. However, the electric field of the light penetrates into the second medium and is called the EVANESCENT FIELD. The evanescent field has an extension of several hundred nanometers and can be used as an optical probe if one or more additional layers are present at the interface.

ELLIPSOMETRY

Ellipsometry is based on measurements of the change of the polarization state of a light beam reflected from a surface. It has a high surface sensitivity, which makes it powerful for studies of thin films. Laser--Polarizer--Substrate--Analyzer--CCD camera.

HOW does ELLIPSOMETRY WORK?

1. Light from a light source.
2. The light is polarized by passing through a linear polarizer.
3. The light is then elliptically polarized by passing through a compensator.
4. The light hits the sample, is reflected, and is linearly polarized.
5. The analyzer detects the change of polarization.
6. The detector catches the light and sends it to the computer to process the data.
7. The measured data combined with computerized optical modeling gives information of the film thickness and refractive index values of a sample.

SURFACE PLASMON RESONANCE

Measuring the interaction between a wave vector (induced by a light beam) and a wave vector in a metal film (caused by so-called “surface plasmon”). Surface Plasmons are charged density waves propagating along the interface of a metal and dielectric media. The refractive index near the sensor surface changes because of binding of macromolecules to the surface. As a result, the SPR angle will change according to the amount of bound macromolecules.

ADVANTAGES OF SPR

• Allows to study interactions between molecules in real time
• No need for labeling
• Time-saving
• No change of the native condition of the biomolecules
• Molecules are not destroyed during monitoring

Disadvantages

• Commercial products are expensive
• Sensitivity depends on optical thickness of adsorbed layer, small molecules cannot be measured in low concentration
• Only interaction predictions are possible, no information of the 3D structure.

FIBER OPTIC BIOSENSORS

Fiber-optic biosensors use optical fibers as the transduction element and rely on optical transduction mechanisms for detecting target biomolecules.