1.3 Fundamentals of light-matter interactions in biological systems

Light-matter interactions in biological systems are the foundation of biophotonics. These interactions, including absorption, scattering, and fluorescence, determine how light behaves in tissues and cells, enabling various imaging and sensing techniques.

Understanding these interactions is crucial for developing optical biosensors and imaging methods. By manipulating light's properties like wavelength, polarization, and coherence, researchers can probe biological systems with unprecedented precision and sensitivity.

Light Interaction with Tissues

Absorption, Scattering, and Fluorescence in Biological Tissues

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• Light absorption in biological tissues converts light energy into other forms (heat or chemical energy) when photons interact with molecules
• Absorption of light in biological tissues depends on wavelength, with different molecules absorbing light at specific wavelengths based on their molecular structure and electronic transitions
• Light scattering in biological tissues results from variations in the refractive index within the tissue due to the presence of various structures (cells, organelles, and extracellular matrix components)
• Elastic scattering events (Rayleigh and Mie scattering) do not involve energy loss, while inelastic scattering events involve energy loss
• Fluorescence in biological tissues occurs when fluorophores absorb light at a specific wavelength and emit light at a longer wavelength due to the relaxation of excited electrons
• Quantum yield, the ratio of the number of photons emitted to the number of photons absorbed, characterizes the efficiency of fluorescence
• Fluorescence lifetime, the average time a molecule spends in the excited state before emitting a photon, is another important parameter in characterizing fluorescence in biological systems

Measuring and Applying Optical Properties in Biological Systems

• Refractive index describes how light propagates through a material and varies among different biological tissues and components
• Differences in refractive index between biological structures lead to scattering and refraction of light, generating contrast in imaging techniques (phase contrast microscopy and differential interference contrast (DIC) microscopy)
• Absorption coefficient quantifies the rate at which light is absorbed by a material as a function of wavelength and depends on the concentration and molar extinction coefficient of the absorbing molecules in biological tissues
• Beer-Lambert law relates the attenuation of light to the absorption coefficient and the path length of light through the material, enabling quantitative analysis of light absorption in biological samples
• Scattering coefficient describes the rate at which light is scattered by a material as a function of wavelength and depends on the size, shape, and refractive index of the scattering particles in biological tissues
• Reduced scattering coefficient characterizes scattering in biological tissues by accounting for both the scattering coefficient and the anisotropy factor, which describes the directionality of scattering
• Optical properties of biological tissues can be measured using techniques such as spectrophotometry, integrating sphere measurements, and diffuse reflectance spectroscopy, providing valuable information for diagnostic and therapeutic applications

Optical Properties of Biomolecules

Intrinsic Optical Properties of Biological Molecules

• Biological molecules (proteins, nucleic acids, and metabolites) have unique optical properties determined by their chemical structure, including the presence of chromophores (light-absorbing groups) and fluorophores (light-emitting groups)
• Proteins containing aromatic amino acids (tryptophan, tyrosine, and phenylalanine) exhibit intrinsic fluorescence, which can be used to study protein structure, dynamics, and interactions
• Nucleic acids (DNA and RNA) have characteristic absorption spectra in the UV region due to the presence of nucleobases, which can be used for quantification and purity assessment
• Intrinsic optical properties of biological molecules can be exploited for label-free biosensing applications, such as monitoring protein folding or detecting nucleic acid hybridization

Extrinsic Labels and Biosensing Techniques

• Extrinsic fluorescent labels (fluorescent dyes and quantum dots) can be attached to biological molecules to enhance their optical properties and enable sensitive detection
• Förster Resonance Energy Transfer (FRET) is a widely used biosensing technique that relies on the distance-dependent energy transfer between two fluorophores, allowing for the study of molecular interactions and conformational changes
• Surface plasmon resonance (SPR) is an optical biosensing technique that detects changes in refractive index near a metal surface, enabling label-free detection of biomolecular interactions
• Extrinsic labels and advanced biosensing techniques expand the range of applications for studying biological molecules and their interactions, providing high sensitivity and specificity

Wavelength, Polarization, and Coherence Effects

Wavelength-Dependent Light-Tissue Interactions

• Wavelength of light significantly influences its interaction with biological tissues, as different wavelengths are absorbed, scattered, and penetrate tissues to varying degrees
• Shorter wavelengths (UV and blue light) are strongly absorbed by biological molecules and have limited tissue penetration, while longer wavelengths (near-infrared) have lower absorption and can penetrate deeper into tissues
• Therapeutic window (600-1200 nm) is a range of wavelengths where light has minimal absorption by water, hemoglobin, and melanin, allowing for deeper tissue penetration and reduced photodamage
• Wavelength selection is crucial for optimizing light-based diagnostic and therapeutic applications, such as photodynamic therapy and optical imaging

Polarization and Coherence in Biomedical Applications

• Polarization of light affects its interaction with anisotropic biological structures (collagen fibers and muscle tissues), leading to birefringence and polarization-dependent scattering
• Polarization-sensitive imaging techniques (polarization-sensitive optical coherence tomography (PS-OCT)) provide additional information about tissue structure and organization
• Coherence of light, the degree of phase correlation between light waves, influences its ability to interfere and form high-contrast images in techniques such as optical coherence tomography (OCT) and holography
• Coherent light sources (lasers) are often used in biomedical imaging and sensing applications due to their high spatial and temporal coherence, enabling high-resolution and depth-resolved imaging
• Polarization and coherence properties of light can be exploited to enhance contrast, resolution, and depth penetration in various biomedical imaging and sensing techniques

Refractive Index, Absorption, and Scattering in Biology

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