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Biophotonics

Biophotonics is an interdisciplinary field that integrates photonics—the science of light generation, detection, and manipulation—with biology and medicine to study, image, and control biological processes at scales ranging from molecules to entire organisms.^[1]^[2] It encompasses the use of light-based technologies to enable non-invasive diagnostics, high-resolution imaging, and targeted therapies, drawing on principles from physics, chemistry, engineering, and life sciences.^[3]^[4] The roots of biophotonics trace back to early optical innovations, such as the invention of the compound microscope in 1595, which laid the foundation for visualizing microscopic biological structures, and progressed significantly in the 20th century with the development of fluorescence microscopy in 1911 and the introduction of lasers in the 1960s that enabled precise light-matter interactions.^[1] Key milestones include the advent of confocal microscopy in the 1950s, two-photon excitation in the 1990s for deeper tissue imaging, and super-resolution techniques, recognized by the 2014 Nobel Prize in Chemistry, which broke the diffraction limit to achieve nanoscale resolution.^[1]^[2]^[5] Central techniques in biophotonics include optical microscopy variants like fluorescence lifetime imaging (FLIM) and multiphoton microscopy for dynamic cellular processes, spectroscopy methods such as Raman and infrared for molecular identification, and sensing tools like optical coherence tomography (OCT) for real-time, high-resolution cross-sectional imaging in clinical settings.^[1]^[2] Manipulation techniques, including optical tweezers for single-cell handling and optogenetics for light-controlled neural activity, further expand its utility in research.^[1] These methods support diverse applications, from neuroscience (e.g., mapping neural connectivity via calcium imaging) to oncology (e.g., tumor detection through hypoxia imaging) and infectious disease diagnostics.^[1]^[3] In medicine, biophotonics drives therapeutics like photodynamic therapy for cancer treatment and laser-based interventions in dermatology and ophthalmology, while also enabling wearable and implantable devices for continuous monitoring.^[2] Beyond healthcare, it extends to environmental monitoring, food safety, and agriculture through light-based sensing of pathogens and pollutants.^[3] As of 2025, integration with artificial intelligence enhances data analysis for personalized medicine, and emerging quantum biophotonics promises ultrasensitive detection, addressing challenges like limited tissue penetration and signaling the field's role in precision health and the "One Health" approach.^[3]^[2]

Fundamentals

Definition and Scope

Biophotonics is defined as the science and technology of generating, detecting, manipulating, and analyzing light (photons) to interact with biological systems, encompassing the application of photonic principles to biology and medicine.^[6] This field centers on the interactions between photons and biological matter, such as cells, tissues, and biomolecules, enabling advancements in understanding and controlling life processes at microscopic scales.^[3] The scope of biophotonics integrates optics, biology, chemistry, and engineering to address key areas including high-resolution imaging, molecular sensing, therapeutic interventions, and precise manipulation of biological entities.^[3] For instance, imaging techniques like optical coherence tomography allow non-invasive visualization of tissues, while sensing methods detect biomarkers with single-molecule sensitivity, and therapeutic approaches such as photodynamic therapy target diseased cells selectively.^[3] Manipulation at cellular and molecular levels, exemplified by optogenetics, uses light to control neural activity, highlighting the field's emphasis on practical, light-driven innovations in healthcare and biotechnology.^[3] Biophotonics distinguishes itself from general photonics, which involves the broader generation and manipulation of light without a biological focus, by specifically targeting photon-biomolecule interactions in living systems.^[3] In contrast to biophysics, which applies a wide range of physical laws to biological phenomena, biophotonics prioritizes light-based technologies and optical methods over other physical principles.^[3] The field gained formal recognition in the 1990s, propelled by advances in laser technology and microscopy that enabled novel applications like optical coherence tomography for retinal imaging.^[7] This period marked the convergence of interdisciplinary efforts and funding, establishing biophotonics as a distinct domain for exploring light's role in biological contexts.^[7]

Historical Development

The foundations of biophotonics trace back to 19th-century observations of light interactions with biological materials, particularly bioluminescence and fluorescence. In 1852, George Gabriel Stokes described the phenomenon of fluorescence, noting the shift in wavelength of light emitted by fluorspar upon excitation, which laid the groundwork for understanding light emission in biological systems. Complementing this, Raphaël Dubois isolated luciferin in 1885 from the bioluminescent mollusk Pholas dactylus, demonstrating that bioluminescence arises from an oxidizable substrate interacting with an enzyme, thus establishing key chemical principles for light production in living organisms. These early discoveries highlighted the potential of light as a probe for biological processes, though practical applications remained limited without advanced light sources. The 20th century brought transformative technologies that enabled precise manipulation of light for biological studies. Marvin Minsky patented the concept of confocal microscopy in 1957, introducing a pinhole aperture to eliminate out-of-focus light and achieve optical sectioning, which revolutionized high-resolution imaging of biological specimens.^[8] This was followed by the invention of the ruby laser by Theodore Maiman in 1960, producing the first coherent, monochromatic light beam, which facilitated targeted excitation in biological tissues and paved the way for laser-based diagnostics and therapies. These breakthroughs shifted biophotonics toward engineering-focused applications, integrating optics with biology for enhanced spatial and temporal resolution. By the 1990s, biophotonics emerged as a formalized discipline, driven by interdisciplinary conferences and advances in quantum optics. The Optical Society of America (now Optica) launched its first biennial topical meeting on biomedical optics in 1994, marking a key milestone in uniting researchers across fields and standardizing the term "biophotonics" for light-based biological investigations. Concurrently, quantum optics enabled single-molecule detection, as demonstrated in 1989 by W.E. Moerner and L. Kador, who spectrally resolved individual pentacene molecules in a crystal at low temperatures, extending to room-temperature fluorescence detection by 1993 and influencing biophotonic tools for molecular-scale analysis. Recent milestones up to 2025 have integrated biophotonics with emerging technologies, enhancing its impact in neuroscience and diagnostics. The 2005 development of optogenetics by Edward Boyden and colleagues introduced channelrhodopsin-2 for light-activated neural control in mammals, enabling millisecond-precision manipulation of brain circuits and transforming neuroscientific research.^[9] Post-2010s advancements in artificial intelligence have augmented biophotonic image analysis, with deep learning algorithms improving reconstruction and classification in optical coherence tomography and fluorescence microscopy, as reviewed in symbiotic photonics-AI frameworks that boost diagnostic accuracy.^[10] Nanoscale photonics has further advanced, with plasmonic nanostructures enabling super-resolution biosensing and light delivery at cellular scales, highlighted in 2021 roadmaps projecting applications in targeted therapies through 2030.^[11]

Light-Biological Interactions

Absorption and Scattering Mechanisms

Absorption in biological tissues occurs when photons are captured by biomolecules, exciting electrons from ground to higher energy states and converting light energy into heat or chemical energy. This process is governed by the Beer-Lambert law, which quantifies the attenuation of light intensity due to absorption: I = I_0 e^{-\mu_a x}, where I is the transmitted intensity, I_0 is the incident intensity, \mu_a is the absorption coefficient, and x is the path length through the medium.^[12] In biophotonics, key absorbers include chromophores such as hemoglobin, which exhibits strong absorption in the visible range of 400-600 nm due to its heme groups, with peaks at approximately 415 nm (Soret band) and 540-577 nm for oxyhemoglobin.^[13] This wavelength-specific absorption arises from electronic transitions in molecular orbitals, making hemoglobin a primary determinant of light interaction in vascularized tissues.^[14] Scattering mechanisms in biological tissues redirect photons without energy loss in elastic processes or with minimal change in inelastic ones, primarily due to spatial variations in refractive index from cellular structures like organelles and extracellular matrix. Elastic scattering dominates in tissues and includes Rayleigh scattering for particles much smaller than the light wavelength (\lambda), where intensity scales as \lambda^{-4}, and Mie scattering for larger particles (typically > \lambda / 10), which is less wavelength-dependent and forward-directed.^[14] In biological media, Mie scattering prevails because scatterers such as mitochondria (0.5-1 μm) and collagen fibers are comparable to visible and near-infrared wavelengths, leading to multiple scattering events that diffuse light.^[14] Inelastic scattering, like Raman, is weaker but contributes to spectroscopic signatures.^[14] Tissue optics integrates absorption and scattering to determine light propagation, with penetration depth typically 1-2 mm in skin at near-infrared (NIR) wavelengths (600-1100 nm) due to reduced both effects compared to visible light.^[15] Shorter wavelengths scatter more intensely (higher \mu_s', reduced scattering coefficient), limiting penetration to 0.3-0.5 mm below 440 nm, while longer NIR wavelengths enable deeper propagation by minimizing Rayleigh-like contributions from small scatterers.^[14] Biological specificity arises from chromophores' absorption spectra: melanin provides broad, monotonically decreasing absorption from UV to NIR (e.g., \mu_a scales as (\lambda / 500 \text{ nm})^{-3}), peaking in the 400-600 nm range and varying by skin phototype; water shows low absorption in visible but rises sharply at 970 nm in NIR, influencing overall tissue \mu_a.^[13] These spectral features dictate selective light-tissue interactions fundamental to biophotonic applications.^[14]

Emission Processes

Emission processes in biophotonics describe the mechanisms by which biological molecules release light energy after photoexcitation, primarily through fluorescence and phosphorescence, which are radiative relaxations from excited electronic states. These processes are central to understanding how light interacts with biomolecules, enabling applications in imaging and sensing. The efficiency of emission is quantified by the quantum yield, which represents the ratio of photons emitted to photons absorbed, and is influenced by competing non-radiative pathways.^[16] Fluorescence involves rapid emission on the nanosecond timescale following absorption, where an electron transitions from the ground singlet state (S₀) to the first excited singlet state (S₁) and returns to S₀ by releasing a photon. The Jablonski diagram illustrates this process, highlighting the S₀ to S₁ absorption, followed by vibrational relaxation within S₁, and subsequent emission from S₁ to S₀. This results in a Stokes shift, where the emission wavelength exceeds the absorption wavelength (Δλ = λ_em - λ_abs > 0), due to energy loss via vibrational relaxation in both states.^[17] Quantum yields for fluorescence in biological fluorophores can approach unity in optimal conditions, making it a dominant emission pathway.^[16] Phosphorescence arises from emission on millisecond to second timescales, occurring after intersystem crossing from S₁ to the triplet state (T₁), followed by radiative decay from T₁ to S₀. This transition is spin-forbidden, leading to slower rates and typically lower quantum efficiencies compared to fluorescence, often below 0.1 in oxygenated biological environments due to triplet-state quenching.^[18]^[19] Non-radiative decay competes with these emissions, dissipating excitation energy without photon release through processes like internal conversion (electronic state transitions without spin change) and vibrational relaxation (redistribution of energy within a vibrational manifold). These pathways reduce overall quantum yields, particularly in aqueous biological media where solvent interactions enhance non-radiative rates.^[17]^[20] In biological contexts, endogenous fluorophores such as reduced nicotinamide adenine dinucleotide (NADH) exhibit fluorescence emission peaking around 450 nm upon excitation near 340 nm, providing insights into cellular metabolism. Exogenous dyes, like fluorescein derivatives, are introduced to amplify these signals with tailored emission properties for enhanced biophotonic applications.^[21]^[16]

Biological Light Phenomena

Bioluminescence

Bioluminescence is the emission of light by living organisms through a chemical reaction that does not require external excitation, distinguishing it from fluorescence, which depends on light absorption and re-emission.^[22] The primary mechanism involves the enzymatic oxidation of a substrate called luciferin by the enzyme luciferase in the presence of oxygen, ATP, and magnesium ions, yielding oxyluciferin, carbon dioxide, and light.^[23] In fireflies (Photinus pyralis), this reaction produces yellow-green light peaking at approximately 562 nm with a quantum yield of approximately 0.41, making it one of the most efficient natural light-producing processes.^[24]^[25] Evolutionarily, bioluminescence has arisen independently at least 94 times across taxa, serving roles such as intraspecific communication for mating and interspecific interactions like predation avoidance.^[26] In marine environments, where sunlight is absent below 200 meters, approximately 90% of deep-sea animals exhibit bioluminescence, often using it to counterilluminate and blend with surface light to evade predators, as seen in hatchetfish (Sternoptychidae family) that emit blue light from ventral photophores to match downwelling illumination.^[27]^[28] For predatory species like deep-sea anglerfish (e.g., Ceratias holboelli), symbiotic bacterial luminescence on an esca lures prey, while some variants employ flashes for defensive distraction against larger threats.^[29] In biophotonics, bioluminescence enables non-invasive in vivo imaging through genetic reporters, where luciferase genes are fused to promoters of interest to monitor cellular processes in real time.^[30] Since the 1990s, firefly luciferase has been widely adopted for this purpose, with fusions to green fluorescent protein (GFP) enhancing signal detection in mammalian models by combining bioluminescent and fluorescent outputs for multicolor imaging. Variations include bacterial systems, such as the lux operon (luxCDABE) from Vibrio fischeri, which encodes both luciferase and the fatty acid reductase complex for autonomous light production without exogenous substrates, facilitating synthetic biology applications like whole-cell biosensors. Recent advances include engineered luciferases with improved quantum yields for higher sensitivity in imaging applications.^[31]^[32]

Biofluorescence and Biophosphorescence

Biofluorescence refers to the absorption and prompt re-emission of light by biological molecules, typically occurring on a nanosecond timescale following excitation by external photons. This phenomenon arises from singlet-to-singlet transitions in fluorophores such as proteins and pigments, enabling diverse ecological roles in organisms. In contrast, biophosphorescence involves slower emission from triplet excited states, resulting in delayed light output lasting milliseconds to seconds, which is far less common in natural systems due to efficient quenching of triplets in aqueous environments.^[33]^[34] A prominent example of biofluorescence is the green fluorescent protein (GFP) isolated from the jellyfish Aequorea victoria, where the chromophore formed by residues 65-67 absorbs ultraviolet-blue light at a peak wavelength of 395 nm and emits green light at 509 nm. This protein occurs naturally in the jellyfish's photoproteins, contributing to energy transfer processes. In marine ecosystems, biofluorescence plays a key role in camouflage; for instance, red-fluorescing reef fishes match the fluorescent patterns of coral substrates, enhancing concealment from predators under blue ocean light.^[35]^[33]^[36] Biophosphorescence, characterized by triplet-state emission, is rare in biology and not well-documented in specific organisms beyond certain bacterial or synthetic systems; it is distinct from the more common bioluminescence in fungi. These longer lifetimes, often in the microsecond to millisecond range, facilitate time-resolved spectroscopic studies to distinguish phosphorescent signals from faster fluorescent ones. Quantum mechanically, the fluorescence lifetime \tau for biofluorescent processes is given by

\tau = \frac{1}{k_r + k_{nr}}

where k_r is the radiative decay rate and k_{nr} encompasses non-radiative pathways, providing insight into molecular dynamics in biological media.^[37] Detection of these emissions in vivo faces significant challenges from tissue autofluorescence, which originates from endogenous chromophores like flavins and porphyrins, overlapping spectrally with target signals and reducing signal-to-noise ratios in deep-tissue biophotonics applications. Strategies such as time-gating exploit the shorter lifetimes of autofluorescence compared to phosphorescence to mitigate interference.^[38]

Core Techniques

Spectroscopic Methods

Spectroscopic methods in biophotonics leverage light-matter interactions to probe molecular vibrations and electronic transitions in biological systems, providing label-free, non-destructive analysis of biomolecules such as proteins, lipids, and nucleic acids. These techniques, including Raman and Fourier transform infrared (FT-IR) spectroscopy, generate spectral fingerprints that reveal chemical composition and structural changes at the molecular level, enabling insights into cellular processes and pathological alterations without invasive procedures. By exploiting inelastic scattering or absorption in the visible to infrared range, they offer high specificity for identifying functional groups, such as C-H stretches or amide bonds, which are critical for understanding biological dynamics. Raman spectroscopy relies on the inelastic scattering of monochromatic light, typically from a laser, where incident photons exchange energy with molecular vibrations, producing shifted wavelengths that correspond to vibrational modes. This process yields a spectrum where peaks indicate specific bonds; for instance, the C-H stretching vibration appears around 2900 cm⁻¹, providing a unique fingerprint for hydrocarbons prevalent in biological membranes. The technique's sensitivity to molecular symmetry makes it particularly suited for aqueous environments, as water exhibits weak Raman signals, allowing direct analysis of hydrated tissues or cells.^[39] To enhance detection limits for trace analytes in biology, surface-enhanced Raman spectroscopy (SERS) employs plasmonic nanostructures, such as gold or silver nanoparticles, which amplify signals by factors exceeding 10⁷ through electromagnetic near-field enhancements. This enables single-cell analysis, where SERS probes subcellular components like DNA or proteins with high spatial precision, facilitating studies of cancer cell heterogeneity or bacterial identification without labeling. For example, SERS using colloidal nanoparticles has distinguished lymphoma cells from healthy ones by detecting subtle biomolecular variations.^[40]^[41] FT-IR spectroscopy, in contrast, measures the absorption of infrared light by molecular functional groups, converting interferograms via Fourier transform to yield absorption spectra that highlight vibrational transitions. The amide I band at approximately 1650 cm⁻¹, arising from C=O stretching in peptide bonds, is a hallmark for protein secondary structures, such as α-helices, allowing quantification of conformational changes in tissues. Its advantages in biological imaging stem from broad applicability to solid samples, including fixed or frozen tissues, where it maps chemical distributions with minimal sample preparation. FT-IR excels in delineating lipid-rich regions or protein aggregates, offering complementary insights to Raman by emphasizing polar bonds.^[42]^[43] In biological applications, these methods enable non-invasive disease detection by identifying spectral alterations indicative of pathology, such as shifts in protein or lipid profiles associated with cancer. Raman spectroscopy has achieved up to 100% accuracy in distinguishing breast cancer from benign lesions in vivo through fingerprint region peaks (200–1800 cm⁻¹) linked to nucleic acids and collagen changes. Similarly, FT-IR detects lung cancer signatures in tissue sections via amide and lipid band variations, with combined Raman-FTIR approaches enhancing diagnostic specificity for early-stage tumors. Recent integrations with machine learning have further improved diagnostic accuracy by analyzing complex spectral data, enabling automated classification of pathological states as of 2025.^[44]^[45]^[46]^[47] These spectral fingerprints support rapid, reagent-free screening, reducing the need for biopsies. Instrumentation advancements, such as confocal Raman microscopy, integrate pinhole optics to achieve lateral spatial resolution of approximately 1 μm, approaching the diffraction limit for visible wavelengths. This setup allows three-dimensional mapping of molecular distributions in biological tissues, like skin or brain sections, by scanning focused laser spots while collecting Raman signals from defined focal planes. Confocal configurations minimize out-of-focus interference, enabling high-fidelity imaging of cellular heterogeneity in live or fixed samples.^[48]

Energy Transfer Techniques

Förster resonance energy transfer (FRET) is a non-radiative process that enables the study of molecular interactions at the nanoscale through dipole-dipole coupling between an excited donor fluorophore and a ground-state acceptor fluorophore.^[49] This energy transfer occurs without photon emission when the donor's emission spectrum overlaps with the acceptor's absorption spectrum, allowing efficient transfer over distances typically spanning 1 to 10 nm.^[50] The efficiency of FRET, denoted as E, is highly sensitive to the separation distance R between donor and acceptor and follows the relation:

E = \frac{1}{1 + \left( \frac{R}{R_0} \right)^6}

where R_0 is the Förster distance at which transfer efficiency is 50%, commonly ranging from 2 to 6 nm for typical fluorophore pairs in biological systems.^[49]^[50] The value of R_0 is calculated using factors including the quantum yield of the donor, the refractive index of the medium, the overlap integral J of the donor emission and acceptor absorption spectra, and the relative orientation of the transition dipoles, with J quantifying the spectral compatibility essential for efficient coupling.^[49] In biophotonics, FRET serves as a powerful tool for probing dynamic protein-protein interactions in living cells, where changes in proximity trigger measurable shifts in fluorescence signals.^[51] For instance, genetically encoded FRET-based biosensors have been employed to image calcium signaling pathways, revealing real-time conformational changes in proteins like calmodulin upon ion binding, which alters donor-acceptor distances and modulates energy transfer efficiency.^[52] This approach provides insights into intracellular signaling cascades without disrupting cellular function, leveraging the technique's sensitivity to distances on the order of protein domains. A notable variant is bioluminescence resonance energy transfer (BRET), which substitutes the photoexcited donor with a bioluminescent enzyme such as Renilla luciferase, eliminating the need for external excitation light and reducing background autofluorescence.^[53] In BRET, energy from the luciferase-catalyzed oxidation of a substrate transfers to a fluorescent acceptor, enabling similar distance-dependent monitoring of interactions in vivo, often with applications in high-throughput screening of protein associations.^[53] Despite its advantages, FRET is subject to limitations, including dependence on the precise orientation of donor and acceptor dipoles, which can modulate transfer efficiency by up to a factor of 4 if misaligned.^[54] Additionally, photobleaching of the donor or acceptor fluorophores during prolonged imaging can confound efficiency measurements, necessitating careful selection of stable probes and short acquisition times to maintain quantitative accuracy.^[55]

Manipulation and Imaging Methods

Optical Trapping and Surgery

Optical trapping, commonly known as optical tweezers, utilizes a highly focused laser beam to manipulate microscopic particles, including biological cells and organelles, through radiation pressure forces. Pioneered by Arthur Ashkin in 1970, this technique demonstrated the acceleration and stable trapping of micron-sized dielectric particles in optical potential wells generated by a continuous laser beam.^[56] The primary mechanism involves two forces: the scattering force, which propels particles along the beam direction, and the gradient force, which pulls particles toward the region of highest light intensity. For small dielectric particles in the dipole approximation, the gradient force is given by \mathbf{F_g} = \frac{1}{2} \mathrm{Re}(\alpha) \nabla |E|^2, where \alpha is the particle polarizability and E is the electric field; this force is proportional to \nabla I since I \propto |E|^2.^[57] In biophotonics, optical tweezers enable non-contact manipulation of living cells, such as positioning bacteria or yeast for force measurements or sorting, with typical trapping powers in the milliwatt range using infrared lasers to minimize absorption and heating. Applications include studying cell adhesion, motility, and mechanical properties, where forces on the order of piconewtons can be applied and calibrated. Biological impacts are generally benign at low powers, with cell viability exceeding 95% for extended periods, as demonstrated in yeast cells trapped for up to 100 hours without significant damage.^[58] Higher powers or visible wavelengths can induce photodamage, but infrared sources like Nd:YAG lasers at 1064 nm maintain high viability by reducing two-photon absorption.^[59] Laser micro-scalpels employ ultrashort femtosecond pulses to perform precise ablation of biological structures, enabling microsurgery without significant thermal damage due to the rapid deposition of energy in a confined volume. These pulses, typically at 800 nm wavelength and 100 fs duration from Ti:sapphire lasers, facilitate non-thermal ablation by multiphoton ionization and plasma formation, allowing cuts as fine as subcellular scales. A notable example is the targeted cutting of DNA strands or chromatin in chromosomes, where femtosecond lasers disrupt specific genomic regions with minimal collateral damage to surrounding cellular components.^[60]

Advanced Imaging Modalities

Advanced imaging modalities in biophotonics leverage hybrid optical and acoustic or interferometric techniques to achieve high-resolution visualization of biological structures, surpassing the limitations of purely optical methods in scattering tissues. These approaches enable non-invasive, label-free imaging with resolutions down to sub-micrometer scales and penetration depths extending beyond 1 mm, facilitating detailed studies of cellular and vascular dynamics in vivo.^[61] Photoacoustic microscopy (PAM) operates on the photoacoustic effect, where pulsed laser light is absorbed by endogenous chromophores, generating thermoelastic expansion and ultrasound waves that are detected to form images. This hybrid opto-acoustic method provides optical absorption contrast without the diffusion limits of light scattering, achieving lateral resolutions below 1 μm in optical-resolution PAM (OR-PAM) through tight laser focusing. Imaging depths exceed 1 mm, up to several millimeters in acoustic-resolution PAM (AR-PAM), making it suitable for volumetric microvascular mapping. A key example is the high-contrast imaging of hemoglobin-rich structures, such as single red blood cells and tumor vasculature, due to the strong absorption of hemoglobin at near-infrared wavelengths. Practical implementations, including coaxial beam alignment and synthetic aperture focusing, further enhance signal-to-noise ratio and speed, with scanning rates up to 900 Hz for B-scans.^[61]^[62] Optical coherence tomography (OCT) employs low-coherence interferometry to measure backscattered light from tissue, reconstructing high-resolution cross-sectional images based on optical path length differences between a reference arm and the sample. The axial resolution, typically 1-10 μm, is determined by the coherence length of the broadband light source, enabling precise structural profiling of layered tissues like the retina. Lateral resolution is governed by the focused beam size, often around 10-20 μm. Swept-source OCT (SS-OCT) variants improve imaging speed by using a rapidly tunable laser source with sweep rates over 100 kHz, allowing real-time volumetric acquisition with reduced motion artifacts and enhanced sensitivity up to 94 dB. These advancements support biophotonic applications in non-invasive depth-resolved imaging of biological microstructures.^[63] Multiphoton microscopy utilizes nonlinear excitation processes, particularly two-photon absorption, where simultaneous absorption of two near-infrared photons excites fluorophores at the focal plane, minimizing out-of-focus photodamage and scattering. Excitation at wavelengths around 800 nm reduces tissue scattering compared to single-photon visible light, enabling deeper penetration depths of several hundred micrometers to over 1 mm in scattering media like brain tissue. This technique confines emission to the focal volume, providing intrinsic optical sectioning for three-dimensional imaging with resolutions approaching the diffraction limit, around 0.3-0.5 μm laterally. It excels in visualizing endogenous fluorophores and second-harmonic signals from collagen, offering reduced photobleaching and improved signal-to-noise for live-cell studies. Recent advances as of 2025 have integrated artificial intelligence (AI) with these modalities to enhance image quality and automate analysis in label-free biophotonic setups for pathology. These AI-driven methods streamline workflows in digital pathology and enable faster, nondestructive visualization of biological samples, with ongoing developments addressing generalizability across tissue types.^[64]

Medical Applications

Diagnostics

Biophotonics plays a pivotal role in diagnostics by leveraging light-matter interactions to enable non-invasive, high-resolution detection of diseases in clinical environments. Techniques such as Raman spectroscopy, fluorescence endoscopy, optical coherence tomography (OCT), and photoacoustic microscopy (PAM) allow for real-time imaging and spectroscopic analysis of biological tissues, facilitating early identification of pathological changes without the need for ionizing radiation. These methods draw from core spectroscopic and imaging principles to provide label-free or minimally invasive assessments, enhancing diagnostic accuracy in oncology, ophthalmology, and vascular disorders. Raman spectroscopy and Fourier-transform infrared (FT-IR) spectroscopy are key label-free diagnostic tools for cancer detection, as they analyze molecular vibrations to identify biochemical alterations in tissues. In breast cancer diagnostics, Raman spectroscopy combined with principal component analysis has achieved approximately 85% accuracy in distinguishing malignant from benign tissues by detecting shifts in lipid and protein signatures. Similarly, FT-IR spectroscopy has been applied to cervical cancer screening, where spectral differences in amide bands enable differentiation of dysplastic cells with sensitivities exceeding 90% in clinical trials. These vibrational techniques offer molecular specificity without exogenous agents, making them suitable for intraoperative guidance during biopsies. Fluorescence endoscopy utilizes endogenous fluorophores like NADH or exogenous probes to highlight early lesions in hollow organs and the brain. For instance, 5-aminolevulinic acid (5-ALA), a prodrug that induces protoporphyrin IX accumulation in tumor cells, enhances visualization of brain tumors during neurosurgery, improving resection margins and progression-free survival, with studies showing an absolute increase of approximately 20% in 6-month progression-free survival rates compared to white-light surgery.^[65] This approach provides real-time, high-contrast imaging of precancerous or malignant tissues, with studies demonstrating improved detection rates for esophageal dysplasia compared to white-light endoscopy alone. Optical coherence tomography (OCT) and photoacoustic microscopy (PAM) extend biophotonics diagnostics to structural and functional imaging in ophthalmology and dermatology. OCT, which employs low-coherence interferometry, is widely used for retinal imaging to detect glaucoma through measurement of retinal nerve fiber layer thickness, with resolutions down to 5-10 micrometers enabling early-stage identification before visual loss occurs. In parallel, PAM combines optical excitation with ultrasonic detection for vascular mapping, such as in melanoma assessment, where it visualizes tumor angiogenesis non-invasively with depths up to 3 mm and contrasts based on hemoglobin absorption, aiding in staging and biopsy guidance. The primary advantages of biophotonics diagnostics include their non-ionizing nature, which minimizes patient risk, and their capacity for real-time, in vivo analysis, allowing immediate clinical decision-making. As of 2024, the FDA has approved several new biophotonic imaging devices for cancer screening, enhancing early detection capabilities.^[66] However, challenges such as tissue heterogeneity, which can introduce spectral noise and reduce reproducibility, and the need for advanced signal processing to mitigate autofluorescence interference, continue to drive research toward improved algorithms and hybrid systems.

Therapies

Biophotonics encompasses several light-based therapeutic approaches that leverage photochemical, photothermal, and photobiomodulatory effects to treat diseases, particularly cancers and inflammatory conditions. These therapies exploit the interaction of light with biological tissues or exogenous agents to induce targeted cellular damage or stimulate repair mechanisms, offering minimally invasive alternatives to traditional treatments. Key modalities include photodynamic therapy, photothermal therapy, and low-level laser therapy, each with distinct mechanisms and clinical applications.^[67] Photodynamic therapy (PDT) involves the administration of a photosensitizer, such as porfimer sodium (Photofrin), which accumulates preferentially in diseased tissues. Upon activation by specific wavelengths of light, typically in the red to near-infrared range, the photosensitizer transfers energy to molecular oxygen, generating cytotoxic singlet oxygen and other reactive oxygen species that cause oxidative damage to cellular components, leading to apoptosis or necrosis of target cells. This process is highly selective due to the localized light delivery and photosensitizer retention in abnormal tissues. In skin cancer treatment, PDT has demonstrated high efficacy, with response rates approaching 90% for superficial basal cell carcinomas when using topical or systemic photosensitizers.^[67]^[68]^[69] Photothermal therapy utilizes near-infrared light-absorbing nanoparticles, such as gold nanorods, to convert absorbed photon energy into localized heat. Gold nanorods, with their plasmon resonance tuned to around 808 nm, efficiently absorb laser light and elevate temperatures above 50°C within targeted tumors, inducing thermal ablation by denaturing proteins and disrupting cell membranes. This approach minimizes damage to surrounding healthy tissue due to the precise spatial control of the laser beam and the nanoparticles' tumor accumulation via the enhanced permeability and retention effect. Preclinical studies have shown complete tumor regression in murine models following such treatments.^[70]^[71]^[72] Low-level laser therapy (LLLT), also known as photobiomodulation, employs low-intensity light in the 600-1000 nm range at power densities of 1-100 mW/cm² to stimulate cellular processes without generating significant heat. The primary mechanism involves photodissociation of nitric oxide from cytochrome c oxidase in the mitochondrial electron transport chain, enhancing ATP production, reducing oxidative stress, and modulating inflammatory pathways such as cytokine release. This biostimulatory effect promotes tissue repair and reduces inflammation in conditions like wound healing and musculoskeletal disorders. Clinical applications have reported significant reductions in inflammatory markers, such as TNF-α, following treatment.^[73]^[74] As of 2025, biophotonic therapies have achieved regulatory milestones, including FDA approval of porfimer sodium-based PDT for various cancers since the 1990s and verteporfin (Visudyne) for age-related macular degeneration in 2000, enabling non-thermal vascular occlusion in choroidal neovascularization. Emerging combinations with immunotherapy, such as PDT or photothermal therapy paired with immune checkpoint inhibitors, enhance antitumor immune responses by releasing tumor antigens and promoting T-cell infiltration, with phase II trials showing improved progression-free survival in melanoma and lung cancer patients. These integrations address limitations like immune evasion, positioning biophotonics as a synergistic pillar in multimodal oncology.^[75]^[76]^[77]^[78]

Technological Components

Laser Sources

Gas lasers represent one of the earliest and most reliable coherent light sources in biophotonics, valued for their stable output and specific wavelengths that interact predictably with biological materials. The helium-neon (He-Ne) laser emits continuous-wave light at 632 nm, making it suitable for low-power applications such as alignment in optical setups and preliminary studies of cellular responses due to its minimal thermal effects on tissues.^[79] In contrast, the carbon dioxide (CO2) laser operates at 10.6 μm in the mid-infrared, where its emission is strongly absorbed by water molecules, enabling precise tissue ablation with controlled vaporization depths of several millimeters, ideal for surgical interventions.^[80] Diode lasers have become ubiquitous in biophotonics owing to their compact design, high electrical-to-optical efficiency often exceeding 50%, and tunability across key wavelengths such as 650-980 nm, which align with absorption peaks of photosensitizers used in photodynamic therapy (PDT).^[81] Their semiconductor-based operation allows for integration into portable devices, facilitating targeted light delivery in clinical settings with power outputs from milliwatts to watts while maintaining beam quality for fiber-optic coupling.^[82] Solid-state lasers offer versatility through host materials doped with rare-earth ions, providing robust performance in demanding biophotonic environments. The neodymium-doped yttrium aluminum garnet (Nd:YAG) laser fundamentally emits at 1064 nm in the near-infrared but is frequently frequency-doubled via nonlinear crystals to produce 532 nm green light, which is preferentially absorbed by hemoglobin and used in ophthalmology for procedures like posterior capsulotomy with minimal collateral damage.^[83] Similarly, the titanium-sapphire (Ti:sapphire) laser, pumped by green sources, achieves broad tunability from 700 to 1000 nm and generates femtosecond pulses, positioning it as the standard excitation source for multiphoton microscopy where nonlinear excitation enables deep-tissue imaging without photodamage to superficial layers.^[84] Ultrashort pulse lasers extend the capabilities of biophotonics by delivering energy in femtosecond durations, promoting nonlinear processes like photodisruption for precise microsurgery, as seen in corneal flap creation with pulse energies below 10 μJ to confine effects to sub-micrometer scales.^[85] Terahertz (THz) sources, operating in the 0.1-10 THz range, probe low-energy vibrational modes in proteins, revealing dynamics such as collective motions in hydrated biomolecules that are inaccessible to higher-frequency optical lasers.^[86] Biological suitability of these lasers hinges on wavelength-dependent interactions with tissues, where near-infrared light in the 700-900 nm "optical window" optimizes penetration depths up to several millimeters by minimizing hemoglobin absorption and Rayleigh scattering, thus enabling effective delivery for imaging and therapy while reducing phototoxicity.^[87]

Specialized Light Sources

Specialized light sources in biophotonics encompass non-laser technologies optimized for low-intensity, quantum-sensitive, or bio-integrated applications, enabling precise probing of biological systems at the molecular and cellular levels. These sources prioritize broadband emission, single-photon emission statistics, or integration with biological media to minimize damage while enhancing sensitivity in imaging and spectroscopy. Single-photon sources, such as quantum dots and nitrogen-vacancy (NV) centers in diamond, are critical for quantum-limited biophotonic techniques like super-resolution imaging. Quantum dots, particularly all-inorganic lead halide perovskites like CsPbI₃, serve as room-temperature single-photon emitters with high purity, achieving second-order correlation functions g^{(2)}(0) as low as 0.02, confirming near-ideal single-photon antibunching without resonant excitation or cavities.^[88] These sources support quantum imaging by providing photons with enhanced spatial resolution and sensitivity for biological samples. Similarly, NV centers in nanodiamonds enable sub-20 nm super-resolution localization microscopy through photoluminescence blinking, where single NV emitters exhibit ~50% on/off duty cycles over hundreds of cycles, allowing precise tracking of nanoscale biological structures without bleaching.^[89] Biolasers leverage biological materials as gain media within optical cavities to produce coherent emission tailored for cellular-scale biophotonics. These devices use cavity-enhanced stimulated emission from biomolecules like green fluorescent protein (GFP) in microcavities, where recombinant GFP solutions or live cells expressing GFP form the active medium, yielding directional, narrowband laser output with clear threshold behavior upon nanosecond pulsed pumping.^[90] Threshold pump fluences are on the order of ~μJ/cm², enabling lasing without damaging the biological components and preserving cell viability post-emission.^[90] This bio-integration facilitates applications in single-cell analysis, where the laser modes directly reflect intracellular properties. LED and supercontinuum sources provide broadband illumination essential for spectroscopic investigations in biophotonics. White-light LEDs emit across 400–700 nm, offering a compact, incoherent alternative for absorption and fluorescence spectroscopy of biomolecules, with balanced spectral output suitable for portable biomedical devices like smartphone-based analyzers.^[91] Supercontinuum sources, generated via nonlinear effects in photonic crystal fibers pumped by femtosecond lasers, deliver coherent broadband spectra (e.g., 730–1350 nm at ~1 mW/nm) for multispectral biophotonic spectroscopy, enabling high signal-to-noise ratios in tissue imaging and optical coherence tomography without multiple discrete sources.^[92] Terahertz (THz) sources support time-domain spectroscopy to probe low-frequency biomolecular vibrations in the 0.1–10 THz range, where intermolecular modes and hydrogen-bond dynamics produce distinct spectral fingerprints. Photoconductive antennas and electro-optic crystals generate broadband THz pulses for THz time-domain spectroscopy (THz-TDS), revealing conformational changes in proteins, nucleobases, and hydration shells with sub-picosecond temporal resolution and sensitivity to weak vibrational couplings.^[86] These sources enable non-invasive analysis of biomolecular interactions, such as protein folding kinetics, by detecting THz absorption variations tied to structural dynamics.^[86]

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