New fluorescent compounds produced by femtosecond laser surgery in biological tissues: the mechanisms

Gentle label-free nonlinear optical imaging relaxes linear-absorption-mediated triplet

Sample health is critical for live-cell fluorescence microscopy and has promoted light-sheet microscopy that restricts its ultraviolet-visible excitation to one plane inside a three-dimensional sample. It is thus intriguing that laser-scanning nonlinear optical microscopy, which similarly restricts its near-infrared excitation, has not broadly enabled gentle label-free molecular imaging. We hypothesize that intense near-infrared excitation induces phototoxicity via linear absorption of intrinsic biomolecules with subsequent triplet buildup, rather than the commonly assumed mechanism of nonlinear absorption. Using a reproducible phototoxicity assay based on the time-lapse elevation of auto-fluorescence (hyper-fluorescence) from a homogeneous tissue model (chicken breast), we provide strong evidence supporting this hypothesis. Our study justifies a simple imaging technique, e.g., rapidly scanned sub-80-fs excitation with full triplet-relaxation, to mitigate this ubiquitous linear-absorption-mediated phototoxicity independent of sample types. The corresponding label-free imaging can track freely moving C. elegans in real-time at an irradiance up to one-half of water optical breakdown.

Long-term in vivo imaging of mouse spinal cord through an optically cleared intervertebral window

The spinal cord accounts for the main communication pathway between the brain and the peripheral nervous system. Spinal cord injury is a devastating and largely irreversible neurological trauma, and can result in lifelong disability and paralysis with no available cure. In vivo spinal cord imaging in mouse models without introducing immunological artifacts is critical to understand spinal cord pathology and discover effective treatments. We developed a minimally invasive intervertebral window by retaining the ligamentum flavum to protect the underlying spinal cord. By introducing an optical clearing method, we achieve repeated two-photon fluorescence and stimulated Raman scattering imaging at subcellular resolution with up to 15 imaging sessions over 6-167 days and observe no inflammatory response. Using this optically cleared intervertebral window, we study neuron-glia dynamics following laser axotomy and observe strengthened contact of microglia with the nodes of Ranvier during axonal degeneration. By enabling long-term, repetitive, stable, high-resolution and inflammation-free imaging of mouse spinal cord, our method provides a reliable platform in the research aiming at interpretation of spinal cord physiology and pathology.

Noninvasive two-photon optical biopsy of retinal fluorophores

High-resolution imaging techniques capable of detecting identifiable endogenous fluorophores in the eye along with genetic testing will dramatically improve diagnostic capabilities in the ophthalmology clinic and accelerate the development of new treatments for blinding diseases. Two-photon excitation (TPE)-based imaging overcomes the filtering of ultraviolet light by the lens of the human eye and thus can be utilized to discover defects in vitamin A metabolism during the regeneration of the visual pigments required for the detection of light. Combining TPE with fluorescence lifetime imaging (FLIM) and spectral analyses offers the potential of detecting diseases of the retina at earlier stages before irreversible structural damage has occurred. The main barriers to realizing the benefits of TPE for imaging the human retina arise from concerns about the high light exposure typically needed for informative TPE imaging and the requirement to correlate the ensuing data with different states of health and disease. To overcome these hurdles, we improved TPE efficiency by controlling temporal properties of the excitation light and employed phasor analyses to FLIM and spectral data in mouse models of retinal diseases. Modeling of retinal photodamage revealed that plasma-mediated effects do not play a role and that melanin-related thermal effects are mitigated by reducing pulse repetition frequency. By using noninvasive TPE imaging we identified molecular components of individual granules in the retinal pigment epithelium and present their analytical characteristics.

Probing Intracellular Dynamics Using Fluorescent Carbon Dots Produced by Femtosecond Laser In Situ

Fluorescent particle tracking is a powerful technique for studying intracellular transport and microrheological properties within living cells, which in most cases employs exogenous fluorescent tracer particles delivered into cells or fluorescent staining of cell organelles. Herein, we propose an alternative strategy, which is based on the generation of fluorescent species in situ with ultrashort laser pulses. Using mouse germinal vesicle oocytes as a model object, we demonstrate that femtosecond laser irradiation produces compact dense areas in the intracellular material containing fluorescent carbon dots synthesized from biological molecules. These dots have tunable persistent and excitation-dependent emission, which is highly advantageous for fluorescent imaging. We further show that tight focusing and tuning of irradiation parameters allow precise control of the location and size of fluorescently labeled areas and minimization of damage inflicted to cells. Pieces of the intracellular material down to the submicrometer size can be labeled with laser-produced fluorescent dots in real time and then employed as probes for detecting intracellular motion activity via fluorescent tracking. Analyzing their diffusion in the oocyte cytoplasm, we arrive to realistic characteristics of active forces generated within the cell and frequency-dependent shear modulus of the cytoplasm. We also quantitatively characterize the level of metabolic activity and density of the cytoskeleton meshwork. Our findings establish a new technique for probing intracellular mechanical properties and also promise applications in tracking individual cells in population or studies of spatiotemporal cell organization.

Adaptive optics two-photon microscopy enables near-diffraction-limited and functional retinal imaging in vivo

In vivo fundus imaging offers non-invasive access to neuron structures and biochemical processes in the retina. However, optical aberrations of the eye degrade the imaging resolution and prevent visualization of subcellular retinal structures. We developed an adaptive optics two-photon excitation fluorescence microscopy (AO-TPEFM) system to correct ocular aberrations based on a nonlinear fluorescent guide star and achieved subcellular resolution for in vivo fluorescence imaging of the mouse retina. With accurate wavefront sensing and rapid aberration correction, AO-TPEFM permits structural and functional imaging of the mouse retina with submicron resolution. Specifically, simultaneous functional calcium imaging of neuronal somas and dendrites was demonstrated. Moreover, the time-lapse morphological alteration and dynamics of microglia were characterized in a mouse model of retinal disorder. In addition, precise laser axotomy was achieved, and degeneration of retinal nerve fibres was studied. This high-resolution AO-TPEFM is a promising tool for non-invasive retinal imaging and can facilitate the understanding of a variety of eye diseases as well as neurodegenerative disorders in the central nervous system.

Wavelength-Selective Nonlinear Imaging and Photo-Induced Cell Damage by Dielectric Harmonic Nanoparticles

We introduce a nonlinear all-optical theranostics protocol based on the excitation wavelength decoupling between imaging and photoinduced damage of human cancer cells labeled by bismuth ferrite (BFO) harmonic nanoparticles (HNPs). To characterize the damage process, we rely on a scheme for in situ temperature monitoring based on upconversion nanoparticles: by spectrally resolving the emission of silica coated NaGdF4:Yb³⁺/Er³⁺ nanoparticles in close vicinity of a BFO HNP, we show that the photointeraction upon NIR-I excitation at high irradiance is associated with a temperature increase >100 °C. The observed laser-cell interaction implies a permanent change of the BFO nonlinear optical properties, which can be used as a proxy to read out the outcome of a theranostics procedure combining imaging at 980 nm and selective cell damage at 830 nm. The approach has potential applications to monitor and treat lesions within NIR light penetration depth in tissues.