Aggregation-Induced Emission Luminogen with Near-Infrared-II Excitation and Near-Infrared-I Emission for Ultradeep Intravital Two-Photon Microscopy

Currently, a serious problem obstructing the large-scale clinical applications of fluorescence technique is the shallow penetration depth. Two-photon fluorescence microscopic imaging with excitation in the longer-wavelength near-infrared (NIR) region (>1100 nm) and emission in the NIR-I region (650-950 nm) is a good choice to realize deep-tissue and high-resolution imaging. Here, we report ultradeep two-photon fluorescence bioimaging with 1300 nm NIR-II excitation and NIR-I emission (peak ∼810 nm) based on a NIR aggregation-induced emission luminogen (AIEgen). The crab-shaped AIEgen possesses a planar core structure and several twisting phenyl/naphthyl rotators, affording both high fluorescence quantum yield and efficient two-photon activity. The organic AIE dots show high stability, good biocompatibility, and a large two-photon absorption cross section of 1.22 × 10³ GM. Under 1300 nm NIR-II excitation, in vivo two-photon fluorescence microscopic imaging helps to reconstruct the 3D vasculature with a high spatial resolution of sub-3.5 μm beyond the white matter (>840 μm) and even to the hippocampus (>960 μm) and visualize small vessels of ∼5 μm as deep as 1065 μm in mouse brain, which is among the largest penetration depths and best spatial resolution of in vivo two-photon imaging. Rational comparison with the AIE dots manifests that two-photon imaging outperforms the one-photon mode for high-resolution deep imaging. This work will inspire more sight and insight into the development of efficient NIR fluorophores for deep-tissue biomedical imaging.

Aggregation-Induced Emission Luminogen with Deep-Red Emission for Through-Skull Three-Photon Fluorescence Imaging of Mouse

Imaging the brain with high integrity is of great importance to neuroscience and related applications. X-ray computed tomography (CT) and magnetic resonance imaging (MRI) are two clinically used modalities for deep-penetration brain imaging. However, their spatial resolution is quite limited. Two-photon fluorescence microscopic (2PFM) imaging with its femtosecond (fs) excitation wavelength in the traditional near-infrared (NIR) region (700-1000 nm) is able to realize deep-tissue and high-resolution brain imaging. However, it requires craniotomy and cranial window or skull-thinning techniques due to photon scattering of the excitation light. Herein, based on a type of aggregation-induced emission luminogen (AIEgen) DCDPP-2TPA with a large three-photon absorption (3PA) cross section at 1550 nm and deep-red emission, we realized through-skull three-photon fluorescence microscopic (3PFM) imaging of mouse cerebral vasculature without craniotomy and skull-thinning. Reduced photon scattering of a 1550 nm fs excitation laser allowed it to effectively penetrate the skull and tightly focus onto DCDPP-2TPA nanoparticles (NPs) in the cerebral vasculature, generating bright three-photon fluorescence (3PF) signals. In vivo 3PF images of the cerebral vasculature at various vertical depths were obtained, and a vivid 3D reconstruction of the vascular architecture beneath the skull was built. As deep as 300 μm beneath the skull, small blood vessels of 2.4 μm could still be recognized.

Red emissive AIE nanodots with high two-photon absorption efficiency at 1040 nm for deep-tissue in vivo imaging

Deep-tissue penetration is highly required in in vivo optical bioimaging. We synthesized a type of red emissive fluorophore BT with aggregation-induced emission (AIE) property. BT molecules were then encapsulated with amphiphilic polymers to form nanodots, and a large two-photon absorption (2PA) cross-section of 2.9 × 10(6) GM at 1040 nm was observed from each BT nanodot, which was much larger than those at the wavelengths of 770 to 860 nm. In addition, 1040 nm light was found to have better penetration and focusing capability than 800 nm light in biological tissue, according to the Monte Carlo simulation. The toxicity and tissue distribution of BT nanodots were studied, and they were found to have good biocompatibility. BT nanodots were then utilized for in vivo imaging of mouse ear and brain, and an imaging depth of 700 μm was obtained with the femtosecond (fs) excitation of 1040 nm. The red emissive AIE nanodots with high 2PA efficiency at 1040 nm would be useful for deep-tissue functional bioimaging in the future.

Modeling the tight focusing of beams in absorbing media with Monte Carlo simulations

A severe drawback to the scalar Monte Carlo (MC) method is the difficulty of introducing diffraction when simulating light propagation. This hinders, for instance, the accurate modeling of beams focused through microscope objectives, where the diffraction patterns in the focal plane are of great importance in various applications. Here, we propose to overcome this issue by means of a direct extinction method. In the MC simulations, the photon paths' initial positions are sampled from probability distributions which are calculated with a modified angular spectrum of the plane waves technique. We restricted our study to the two-dimensional case, and investigated the feasibility of our approach for absorbing yet nonscattering materials. We simulated the focusing of collimated beams with uniform profiles through microscope objectives. Our results were compared with those yielded by independent simulations using the finite-difference time-domain method. Very good agreement was achieved between the results of both methods, not only for the power distributions around the focal region including diffraction patterns, but also for the distribution of the energy flow (Poynting vector).