1.3 Fundamentals of light-matter interactions in biological systems
4 min read•august 14, 2024
Light-matter interactions in biological systems are the foundation of biophotonics. These interactions, including , , and , determine how light behaves in tissues and cells, enabling various 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
- 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 , providing valuable information for diagnostic and therapeutic applications
Optical Properties of Biomolecules
Intrinsic Optical Properties of Biological Molecules
- Biological molecules (, , 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
- (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
- (SPR) is an optical biosensing technique that detects changes in refractive index near a metal surface, enabling 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 ( (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 (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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Key Terms to Review (20)
Absorption: Absorption is the process by which matter takes in light energy, converting it into other forms of energy, usually heat. This phenomenon is critical in understanding how biological tissues interact with light, influencing both imaging techniques and treatment methods. Absorption properties of tissues vary based on factors like wavelength and tissue composition, making it essential for the design of optical biosensors and light source selection in biophotonics.
Beer-Lambert Law: The Beer-Lambert Law is a fundamental principle that describes the relationship between the absorption of light and the properties of the material through which the light is traveling. It states that the absorbance of a substance is directly proportional to its concentration and the path length of light passing through it. This law is essential for understanding how light interacts with biological tissues and is crucial for techniques like absorption spectroscopy, which are used to analyze materials based on their light absorption characteristics.
Biosensor sensitivity: Biosensor sensitivity refers to the ability of a biosensor to detect and quantify small concentrations of specific biological analytes. It plays a crucial role in determining the performance of biosensors, as higher sensitivity allows for more accurate measurements and earlier detection of target substances in complex biological systems. This concept is closely linked to light-matter interactions, as the effectiveness of biosensors often relies on optical methods that utilize light to interact with biological materials, enhancing their detection capabilities.
Fluorescence: Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. This process occurs when the absorbed energy excites electrons to a higher energy state, and as these electrons return to their ground state, they emit light of a longer wavelength. Fluorescence is fundamental in understanding light-matter interactions, particularly in biological systems, where it plays a critical role in imaging and detecting various biomolecules, enhancing the capabilities of optical biosensing techniques.
Fluorometer: A fluorometer is an analytical instrument used to measure the fluorescence emitted by a sample when it is exposed to light of a specific wavelength. This device plays a vital role in understanding light-matter interactions, particularly in biological systems, as it allows researchers to study various biomolecules that exhibit fluorescence, such as proteins and nucleic acids. By analyzing the intensity and wavelength of the emitted light, scientists can gain insights into the properties and behaviors of these molecules within complex biological environments.
Förster resonance energy transfer: Förster resonance energy transfer (FRET) is a distance-dependent process where energy is transferred from a donor chromophore to an acceptor chromophore through non-radiative dipole-dipole coupling. This phenomenon is crucial in understanding how light interacts with biological systems, enabling the study of molecular interactions and dynamics within cells using optical biosensors and imaging techniques.
Imaging: Imaging refers to the process of creating visual representations of biological structures or processes, using various techniques that utilize light and other forms of radiation. It plays a crucial role in both healthcare and research, enabling the visualization of tissues, cells, and molecular interactions. The ability to capture images at different scales and resolutions allows scientists and clinicians to gain insights into biological mechanisms and diagnose diseases effectively.
Label-free detection: Label-free detection is a method that allows for the identification and quantification of biomolecules without the need for fluorescent or radioactive tags. This technique relies on the inherent properties of light-matter interactions, enabling sensitive analysis of biological systems. It provides significant advantages such as preserving the native state of biomolecules, reducing background noise, and allowing for real-time monitoring of interactions.
Laser: A laser is a device that emits light through a process called stimulated emission, producing a coherent and monochromatic beam. Lasers are vital in biophotonics, as they enable precise light delivery and manipulation in biological systems, enhancing imaging, diagnostics, and therapeutic applications.
LED: A Light Emitting Diode (LED) is a semiconductor device that emits light when an electric current passes through it. LEDs are highly efficient light sources, making them essential in biophotonics applications, where light interaction with biological tissues is crucial. They provide a wide range of wavelengths, which can be tailored for specific absorption and scattering characteristics in various biological systems.
Maxwell's Equations: Maxwell's equations are a set of four fundamental equations in classical electromagnetism that describe how electric and magnetic fields interact with each other and with matter. These equations are essential for understanding phenomena such as light propagation, electromagnetic radiation, and light-matter interactions, especially within biological systems where light is used for imaging and sensing applications.
Nucleic Acids: Nucleic acids are large biomolecules essential for all known forms of life, primarily responsible for storing and transferring genetic information. They come in two main forms: DNA (deoxyribonucleic acid), which encodes genetic instructions, and RNA (ribonucleic acid), which plays a key role in translating those instructions into proteins. Their interactions with light are fundamental to understanding many biological processes, and they can be analyzed using advanced techniques in biosensing and drug discovery.
Optical Coherence Tomography: Optical coherence tomography (OCT) is a non-invasive imaging technique that uses light to capture high-resolution, cross-sectional images of biological tissues. It provides detailed structural information, making it invaluable in fields like ophthalmology and dermatology, while also being applicable in various medical and research settings.
Polarization-sensitive optical coherence tomography: Polarization-sensitive optical coherence tomography (PS-OCT) is a non-invasive imaging technique that utilizes the polarization state of light to enhance contrast and provide additional information about tissue microstructure. This method helps in understanding the organization of collagen fibers and other anisotropic structures in biological tissues, revealing details that standard OCT might miss. By analyzing how light is polarized as it interacts with biological samples, PS-OCT can provide valuable insights into various biological processes and conditions.
Proteins: Proteins are large, complex molecules made up of long chains of amino acids, which perform a vast array of functions within living organisms. They are essential for the structure, function, and regulation of the body's tissues and organs, playing critical roles in processes like enzyme activity, immune response, and cell signaling. Understanding proteins is vital for exploring various interactions with light, as well as applications in biosensors and drug discovery.
Quantum Mechanics: Quantum mechanics is a fundamental theory in physics that describes the behavior of matter and energy at atomic and subatomic scales. It introduces concepts like wave-particle duality, superposition, and entanglement, which are crucial for understanding how light interacts with biological systems and materials at the nanoscale, such as semiconductor quantum dots and plasmonic nanoparticles.
Scattering: Scattering is the process by which light is forced to deviate from a straight trajectory due to interactions with particles or irregularities within a medium. This phenomenon plays a crucial role in how light interacts with biological tissues, influencing the interpretation of images and signals in medical diagnostics. Scattering can provide insights into the structure and composition of tissues, helping to differentiate healthy from diseased states based on how light behaves in those environments.
Spectrophotometer: A spectrophotometer is an analytical instrument used to measure the intensity of light at different wavelengths as it passes through a sample. This tool helps researchers understand how light interacts with matter, making it essential in studies involving biological systems and their components, particularly when assessing the absorption of light by various molecules in a sample.
Spectroscopy: Spectroscopy is a scientific technique used to analyze the interaction of light with matter, allowing researchers to determine the composition and properties of various substances. This method is vital for understanding how biological tissues absorb and scatter light, providing insights into their structure and function, while also being integral to the development of advanced imaging technologies and biosensors.
Surface Plasmon Resonance: Surface plasmon resonance (SPR) is a sensitive optical technique that detects changes in refractive index near a metal surface, particularly involving biomolecular interactions. It exploits the resonant oscillation of conduction electrons at the interface between a metal and a dielectric material, which can be influenced by the binding of molecules to the metal surface, making it invaluable for studying interactions in biological systems.