Optical methods offer a versatile and sensitive approach to biosensing, leveraging various optical phenomena to detect chemical and biological species. Here are the key aspects of optical biosensors and their applications:
Components and Working Principle
Schematic of an Optical Biosensor
- An optical biosensor typically consists of a light source (e.g., LED, laser), an optical chip or fiber (optrode), and a detector (e.g., photodiode)[5].
- The optrode contains a chemical indicator with optical properties that change in response to the presence or absence of an analyte. This indicator can exhibit changes in absorption, fluorescence, or other optical properties[5].
Transduction Event
- The presence of the analyte affects the optical properties of the indicator, which is then measured by the detector. This transduction event converts the chemical interaction into an optical signal that can be easily processed[5].
Types of Optical Biosensors
Fluorescence-Based Sensors
- These sensors utilize fluorescent indicators that change their fluorescence properties upon interaction with the analyte. Fluorescence is often preferred over absorbance due to its higher sensitivity[2][5].
- For example, organically modified silica (ormosil) nanoparticles can host probes for oxygen and pH, producing distinct fluorescence peaks that enable simultaneous analysis of these parameters[2].
Raman Spectroscopy Sensing
- Raman spectroscopy, particularly Surface-Enhanced Raman Scattering (SERS), enhances the Raman signal by adsorbing target molecules on rough metal surfaces or nanostructures like silver, gold, or copper nanoparticles. This method can achieve significant signal enhancement, making it highly sensitive[2].
Fiber Optic-Based Sensors
- Fiber optic sensors use optical fibers as both the platform for the biological recognition system and the conduit for the excitation and return signal. These sensors can be configured for various detection modes, including absorbance, reflectance, fluorescence, and chemiluminescence[5].
- Advances in telecommunications have enabled the manufacturing of small, portable fiber optic sensors that can be integrated into arrays, allowing for high-density packing and the monitoring of multiple analytes[5].
Plasmonic Biosensors
- Surface Plasmon Resonance (SPR) and Localized SPR (LSPR):SPR involves propagating surface plasmons at metal-dielectric surfaces, typically using gold or silver films. LSPR sensors use localized surface plasmons on metal nanoparticles[4][5].SPR systems measure biomolecular interactions in real-time by detecting changes in the refractive index near the metal surface, which affects the SPR curve[5].
Interferometric Biosensors
- These sensors use a reference beam and a sample beam that interacts with the bioreceptor, causing phase or intensity changes. The interference pattern with the reference beam is analyzed using a detector. Common types include Michelson interferometer, Mach–Zehnder Interferometer (MZI), and Fabry–Pérot interferometer-based biosensor (FPIB)[5].
Advantages of Optical Biosensors
Sensitivity to Interference
- Optical biosensors are less sensitive to electronic interference because the information is carried as photons rather than electrons, reducing noise and increasing reliability[5].
Chemical and Mechanical Stability
- Optical components, often made from glass chips or fiber-optic cables, exhibit high chemical stability, are resistant to corrosion, and are unaffected by organic solvents. They also display good thermal and mechanical stability[5].
Portability and Miniaturization
- Recent advances have led to the development of portable, handheld equipment, enabling field measurements. The small size of optical fibers allows for miniaturization and integration into wearable or implantable sensors[5].
Applications
Explosive Detection
- Fluorescent polymers like polyarylethynylenes have been used to detect aromatic nitro compounds such as trinitrotoluene (TNT) by quenching the fluorescence of the polymer upon exposure to TNT vapor. This technique is highly sensitive and has led to the development of instruments like FIDO[5].
Food Analysis
- SPR sensors have been used for the direct detection of pathogens like Salmonella in food samples using antibody-modified surfaces[5].
Drug Detection
- SPR and other optical biosensors are applied in various drug detection applications, including the study of protein interactions and the development of immunosensors[5].
Environmental Monitoring
- Optical biosensors are used to monitor environmental pollutants, including the detection of genetically modified organisms (GMOs) and other contaminants in water and soil[1][4].
Sample Preparation and Considerations
Sample Form and Solvent Choice
- Samples are typically in liquid form, often as extracts in water or organic solvents. The choice of solvent is crucial to avoid interfering absorbances or fluorescence characteristics[5].
Particulate Material and Concentration
- Particulate material in the sample can cause light scattering and must be eliminated by filtration. The concentration of the analyte must be optimized to avoid detector saturation or non-compliance with the Beer–Lambert law[5].
Interference Minimization
- For techniques like SPR, sample cleanup methods such as centrifugation or filtration are necessary to remove interferents and prevent non-selective deposition on the sensor surface[5].
