Fluorescence resonance energy transfer (FRET) based nanoparticles composed of AIE luminogens and NIR dyes with enhanced three-photon near-infrared emission for in vivo brain angiography

Coupling Nanoscale Precision with Multiscale Imaging: A Multifunctional Near-Infrared Dye for the Brain

Live imaging of primary neural cells is crucial for monitoring neuronal activity, especially multiscale and multifunctional imaging that offers excellent biocompatibility. Multiscale imaging can provide insights into cellular structure and function from the nanoscale to the millimeter scale. Multifunctional imaging can monitor different activities in the brain. However, this remains a challenge because of the lack of dyes with a high signal-to-background ratio, water solubility, and multiscale and multifunctional imaging capabilities. In this study, we present a neural dye with near-infrared (NIR) emissions (>700 nm) that enables ultrafast staining (in less than 1 min) for the imaging of primary neurons. This dye not only enables multiscale neural live-cell imaging from vesicles in neurites, neural membranes, and single neurons to the whole brain but also facilitates multifunctional imaging, such as the monitoring and quantifying of synaptic vesicles and the changes in membrane potential. We also explore the potential of this NIR neural dye for staining brain slices and live brains. The NIR neural dye exhibits superior binding with neural membranes compared to commercial dyes, thereby achieving multiscale and multifunctional brain neuroimaging. In conclusion, our findings introduce a significant breakthrough in neuroimaging dyes by developing a category of small molecular dyes.

To see or not to see: In vivo nanocarrier detection methods in the brain and their challenges

Nanoparticles have a great potential to significantly improve the delivery of therapeutics to the brain and may also be equipped with properties to investigate brain function. The brain, being a highly complex organ shielded by selective barriers, requires its own specialized detection system. However, a significant hurdle to achieve these goals is still the identification of individual nanoparticles within the brain with sufficient cellular, subcellular, and temporal resolution. This review aims to provide a comprehensive summary of the current knowledge on detection systems for tracking nanoparticles across the blood-brain barrier and within the brain. We discuss commonly employed in vivo and ex vivo nanoparticle identification and quantification methods, as well as various imaging modalities able to detect nanoparticles in the brain. Advantages and weaknesses of these modalities as well as the biological factors that must be considered when interpreting results obtained through nanotechnologies are summarized. Finally, we critically evaluate the prevailing limitations of existing technologies and explore potential solutions.

Large-depth three-photon fluorescence microscopy imaging of cortical microvasculature on nonhuman primates with bright AIE probe In vivo

Multiphoton microscopy has been a powerful tool in brain research, three-photon fluorescence microscopy is increasingly becoming an emerging technique for neurological research of the cortex in depth. Nonhuman primates play important roles in the study of brain science because of their neural and vascular similarity to humans. However, there are few research results of three-photon fluorescence microscopy on the brain of nonhuman primates due to the lack of optimized imaging systems and excellent fluorescent probes. Here we introduced a bright aggregation-induced emission (AIE) probe with excellent three-photon fluorescence efficiency as well as facile synthesis process and we validated its biocompatibility in the macaque monkey. We achieved a large-depth vascular imaging of approximately 1 mm in the cerebral cortex of macaque monkey with our lab-modified three-photon fluorescence microscopy system and the AIE probe. Functional measurement of blood velocity in deep cortex capillaries was also performed. Furthermore, the comparison of cortical deep vascular structure parameters across species was presented on the monkey and mouse cortex. This work is the first in vivo three-photon fluorescence microscopic imaging research on the macaque monkey cortex reaching the imaging depth of ∼1 mm with the bright AIE probe. The results demonstrate the potential of three-photon microscopy as primate-compatible method for imaging fine vascular networks and will advance our understanding of vascular function in normal and disease in humans.

Coordination-Regulated Terpyridine-Mn(II) Complexes for Photodynamic Therapy Guided by Multiphoton Fluorescence/Magnetic Resonance Imaging

The synergy of multiphoton fluorescence imaging (MP-FI) and magnetic resonance imaging (MRI) provides an imaging platform with high resolution and unlimited penetration depth for early disease detection. Herein, two kinds of terpyridine-Mn(II) complexes (FD-Mn-O₂NO and FD-Mn-FD) possessing seven and six coordination modes, respectively, were designed rationally for photodynamic therapy (PDT) guided by MP-FI/MRI. The complexes obtain different multiphoton fluorescence/magnetic resonance properties by adjusting the number of terpyridine ligands. Among them, FD-Mn-FD exhibits the following superiorities: (1) The optimal three-photon excitation wavelength of FD-Mn-FD falls at 1450 nm (NIR-II), which brings high sensitivity and deep tissue penetration in MP-FI. (2) FD-Mn-FD has effective longitudinal relaxation efficiency (r₁ = 2.6 m M^-1 s^-1), which can be used for T₁-weighted MRI, overcoming the problems of limited tissue penetration depth and low spatial resolution. (3) FD-Mn-FD generates endogenous ¹O₂ under irradiation by 808 nm light, thereby enhancing the PDT effect in vitro and in vivo. To the best of our knowledge, the complex FD-Mn-FD is the first complex to guide PDT through MP-FI/MRI, providing a blueprint for accurate and effective early detection and timely treatment of the complex in the early stages of cancer.

Optical molecular imaging and theranostics in neurological diseases based on aggregation-induced emission luminogens

Optical molecular imaging and image-guided theranostics benefit from special and specific imaging agents, for which aggregation-induced emission luminogens (AIEgens) have been regarded as good candidates in many biomedical applications. They display a large Stokes shift, high quantum yield, good biocompatibility, and resistance to photobleaching. Neurological diseases are becoming a substantial burden on individuals and society that affect over 50 million people worldwide. It is urgently needed to explore in more detail the brain structure and function, learn more about pathological processes of neurological diseases, and develop more efficient approaches for theranostics. Many AIEgens have been successfully designed, synthesized, and further applied for molecular imaging and image-guided theranostics in neurological diseases such as cerebrovascular disease, neurodegenerative disease, and brain tumor, which help us understand more about the pathophysiological state of brain through noninvasive optical imaging approaches. Herein, we focus on representative AIEgens investigated on brain vasculature imaging and theranostics in neurological diseases including cerebrovascular disease, neurodegenerative disease, and brain tumor. Considering different imaging modalities and various therapeutic functions, AIEgens have great potential to broaden neurological research and meet urgent needs in clinical practice. It will be inspiring to develop more practical and versatile AIEgens as molecular imaging agents for preclinical and clinical use on neurological diseases.

Pulmonary delivery nanomedicines towards circumventing physiological barriers: Strategies and characterization approaches

Pulmonary delivery of nanomedicines is very promising in lung local disease treatments whereas several physiological barriers limit its application via the interaction with inhaled nanomedicines, namely bio-nano interactions. These bio-nano interactions may affect the pulmonary fate of nanomedicines and impede the distribution of nanomedicines in its targeted region, and subsequently undermine the therapeutic efficacy. Pulmonary diseases are under worse scenarios as the altered physiological barriers generally induce stronger bio-nano interactions. To mitigate the bio-nano interactions and regulate the pulmonary fate of nanomedicines, a number of manipulating strategies were established based on size control, surface modification, charge tuning and co-delivery of mucolytic agents. Visualized and non-visualized characterizations can be employed to validate the robustness of the proposed strategies. This review provides a guiding overview of the physiological barriers affecting the in vivo fate of inhaled nanomedicines, the manipulating strategies, and the validation methods, which will assist with the rational design and application of pulmonary nanomedicine.

Deep-Brain Three-Photon Imaging Enabled by Aggregation-Induced Emission Luminogens with Near-Infrared-III Excitation

Understanding the morphology and hemodynamics of cerebral vasculature at large penetration depths and microscale resolution is fundamentally important to decipher brain diseases. Among the various imaging technologies, three-photon (3P) microscopy is of significance by virtue of its deep-penetrating capability and submicron resolution, which especially benefits in vivo vascular imaging. Aggregation-induced emission luminogens (AIEgens) have been recognized to be extraordinarily powerful as 3P probes. However, systematic studies on the structure-performance relationship of 3P AIEgens have been seldom reported. Herein, a series of AIEgens has been designed and synthesized. By intentionally introducing benzene rings onto electron donors (D) and acceptors (A), the molecular distortion, conjugation strength, and the D-A relationship can be facilely manipulated. Upon encapsulation with DSPE-PEG₂₀₀₀, the optimized AIEgens are successfully applied for 3P microscopy with emission in the far-red/near-infrared-I (NIR-I, 700-950 nm) region under the near-infrared-III (NIR-III, 1600-1870 nm) excitation. Impressively, using mice with an opened skull, vasculature within 1700 μm and a microvessel with a diameter of 2.2 μm in deep mouse brain were clearly visualized. In addition, the hemodynamics of blood vessels were well-characterized. Thus, this work not only proposes a molecular design strategy of 3P AIEgens but also promotes the performance of 3P imaging in cerebral vasculature.

Computational Study of Photochemical Relaxation Pathways of Platinum(II) Complexes

A series of functional platinum(II) complexes (Pt1-Pt3), which present high activity in four-photon absorption, in vivo imaging, and precise cancer therapy, as previously reported by the experimental work of Zhang et al. (Inorg. Chem. 2021, 60, 2362-2371), are computationally investigated in the article. We find that after the complex goes through four-photon absorption to the S₁ state, it undergoes intersystem crossing to the T₂ state and eventually reaches the T₁ state through internal conversion. On the T₁ state, both radiative and nonradiative decay to S₀ exit. The radiative decay forms the basis for the phosphorescence imaging in tissues as reported in the original paper. In addition, the nonradiative decay can simultaneously generate cytotoxic singlet oxygen by the excited energy transfer process, also known as triplet oxygen's quenching of triplet states. We conclude that the phosphorescence property as well as the photosensitizer character jointly bring high activity of in vivo imaging and photodynamic therapy to these complexes.

Practicable Applications of Aggregation-Induced Emission with Biomedical Perspective

Considerable efforts have been made into developing aggregation-induced emission fluorogens (AIEgens)-containing nano-therapeutic systems due to the excellent properties of AIEgens. Compared to other fluorescent molecules, AIEgens have advantages including low background, high signal-to-noise ratio, good sensitivity, and resistance to photobleaching, in addition to being exempt from concentration quenching or aggregation-caused quenching effects. The present review outlines the major developments in the biomedical applications of AIEgens-containing systems. From a literature survey, the recent AIE works are reviewed and the reasons why AIEgens are chosen in various biomedical applications are highlighted. The research activities on AIEgens-containing systems are increasing rapidly, therefore, the present review is timely.

Multi-photon absorption organotin complex for bioimaging and promoting ROS generation

Compared to general fluorescent dyes, multi-photon fluorescent dyes exhibit deeper tissue penetration and lower auto-fluorescence in the bio-imaging field. Therefore, it is necessary to develop an efficient multiphoton imaging agent for deep tissue imaging. In this work, an organotin derivative (HSnBu₃) has been designed and synthesized, which shows multiphoton absorption activity. In constrast to the ignorable three-photon activity of the ligand, the complex (HSnBu₃) exhibits three-photon activity under NIR excitation (1500 nm). Results of chemical and biological tests confirmed that HSnBu₃ was more easily activated by oxygen resulting in a higher level of ¹O₂, which could induce a decrease in mitochondrial membrane potential in HepG2 cells. It suggests that HSnBu₃ has potential in photodynamic therapy.

Modulation of Aggregation-Induced Emission by Excitation Energy Transfer: Design and Application

Excitation energy transfer (EET) as a fundamental photophysical process is well-explored for developing functional materials with tunable photophysical properties. Compared to traditional fluorophores, aggregation-induced emission luminogens (AIEgens) exhibit unique advantages for building EET systems, especially serving as energy donors, due to their outstanding photophysical properties such as bright fluorescence in aggregation state, broad absorption and emission spectra, large Stokes shift, and high photobleaching resistance. In addition, the photophysical properties of AIEgens can be modulated by energy transfer for improved luminescence performance. Therefore, a variety of EET systems based on AIEgens have been constructed and their applications in different areas have been explored. In this review, we summarize recent progress in the design strategy of AIE-based energy transfer systems for light-harvesting, fluorescent probes and theranostic systems, with an emphasis on design strategies to achieve desirable properties. The limitations, challenges and future opportunities of AIE-EET systems are briefly outlined. Design strategies and applications (light-harvesting, fluorescent probe and theranostics) of AIEgen-based excitation energy systems are discussed in this review.

Fluorescent nanorods based on 9,10-distyrylanthracene (DSA) derivatives for efficient and long-term bioimaging

Fluorescent nanoparticles based on 9,10-distyrylanthracene (DSA) derivatives (4,4'-((1E,1'E)-anthracene-9,10-diylbis(ethene-2,1-diyl))bis(N,N-dimethylaniline) (NDSA) and 4,4'-((1E,1'E)-anthracene-9,10-diylbis(ethene-2,1-diyl))dibenzonitrile (CNDSA)) were prepared using an ultrasound aided nanoprecipitation method. The morphologies of the fluorescent nanoparticles could be controlled by adjusting the external ultrasonication time. NDSA or CNDSA could form spherical nanodots (NDSA NDs, CNDSA NDs) in a THF-H2O mixture with an 80% or 70% water fraction when the ultrasonication time was 30 s. When the ultrasonication time was prolonged to 10 min, NDSA and CNDSA could assemble into nanorods (NDSA NRs, CNDSA NRs). Meanwhile, the sizes of NDSA NRs and CNDSA NRs could be controlled by adjusting the water content in the mixture. As the water fraction was increased from 60% to 80%, the sizes of NDSA and CNDSA nanorods or nanodots reduced from 238.4 nm to 140.3 nm, and 482 nm to 198.4 nm, respectively. When the water fraction was up to 90%, irregular morphologies of NDSA and CNDSA could be observed. The nanoparticles exhibited intense fluorescence emission, good anti-photobleaching properties, as well as excellent stability and biocompatibility. In vitro cell imaging experiments indicated that the nanorods prepared by this simple method had the potential to be used for efficient and noninvasive long-term bioimaging.

Functional Platinum(II) Complexes with Four-Photon Absorption Activity, Lysosome Specificity, and Precise Cancer Therapy

Multiphoton materials are in special demand in the field of photodynamic therapy and multiphoton fluorescence imaging. However, rational design methodology for these brands of materials is still nascent. This is despite transition-metal complexes favoring optimized nonlinear-optical (NLO) activity and heavy-atom-effected phosphorescent emission. Here, three four-photon absorption (4PA) platinum(II) complexes (Pt1-Pt3) are achieved by the incorporation of varied functionalized C^N^C ligands with high yields. Pt1-Pt3 exhibit triplet metal-to-ligand charge-transfer transitions at ∼460 nm, which are verified multiple times by transient absorption spectra, time-dependent density functional theory calculations, and low-temperature emission spectra. Further, Pt1-Pt3 undergo 4PA. Notably, one of the complexes, Pt2, has maximum 4PA cross-sectional values of up to 15.2 × 10^-82 cm⁸ s³ photon^-3 under excitation of a 1600 nm femtosecond laser (near-IR II window). The 4PA cross sections vary when Pt2 is binding to lecithin and when it displays its lysosome-specific targeting behavior. On the basis of the excellent 4PA property of Pt2, we believe that those 4PA platinum(II) complexes have great potential applications in cancer theranostics.

Study of laser actions by bird's feathers with photonic crystals

Random lasers had been made by some biomaterials as light scattering materials, but natural photonic crystals have been rarely reported as scattering materials. Here we demonstrate the ability of natural photonic crystals to drive laser actions by sandwiched the feathers of the Turquoise-Fronted Amazon parrot and dye between two plastic films. Parrot feathers comprise abundant photonic crystals, and different color feathers compose of different ratios of the photonic crystal, which directly affect the feather reflectance. In this study, the multi-reflection scattering that occurred at the interface between the photonic crystal and gain media efficiently reduce the threshold; therefore, the more photonic crystal constitutes in the feathers; the lower threshold can be obtained. The random lasers can be easily made by the integration of bird feather photonic crystals and dye with a simple and sustainable manufacturing approach.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

A multi-photon fluorescent probe based on quinoline groups for the highly selective and sensitive detection of lipid droplets

Compared to general fluorescent probes, multi-photon fluorescent probes exhibit deeper tissue penetration, lower auto-fluorescence and lower photo-toxicity in the bio-imaging field. Herein, we synthesized a series multi-photon fluorescent probe (L1-L3) based on quinolone groups. Of notably, the three-photon fluorescence of L3 significantly enhanced when L3 interacted with liposome; moreover, L3 exhibited high selectivity towards lipid droplets in living cells. Due to its large Stokes shift, high selectivity and photon-stability, L3 was successfully used in lipid droplet imaging via multi-photon fluorescence bio-imaging.

Nanomaterials with Supramolecular Assembly Based on AIE Luminogens for Theranostic Applications

One of the major pursuits of biomedical science is to develop advanced strategies for theranostics, which is expected to be an effective approach for achieving the transition from conventional medicine to precision medicine. Supramolecular assembly can serve as a powerful tool in the development of nanotheranostics with accurate imaging of tumors and real-time monitoring of the therapeutic process upon the incorporation of aggregation-induced emission (AIE) ability. AIE luminogens (AIEgens) will not only enable fluorescence imaging but will also aid in improving the efficacy of therapies. Furthermore, the fluorescent signals and therapeutic performance of these nanomaterials can be manipulated precisely owing to the reversible and stimuli-responsive characteristics of the supramolecular systems. Inspired by rapid advances in this field, recent research conducted on nanotheranostics with the AIE effect based on supramolecular assembly is summarized. Here, three representative strategies for supramolecular nanomaterials are presented as follows: a) supramolecular self-assembly of AIEgens, b) the loading of AIEgens within nanocarriers with supramolecular assembly, and c) supramolecular macrocycle-guided assembly via host-guest interactions. Meanwhile, the diverse applications of such nanomaterials in diagnostics and therapeutics have also been discussed in detail. Finally, the challenges of this field are listed in this review.

Simultaneously Enhanced Singlet Oxygen and Fluorescence Production of Nanoplatform by Surface Plasmon Resonance Coupling for Biomedical Applications

Photodynamic therapy (PDT) and fluorescence imaging offer the possibility of precise and personalized treatment of cancer, but low singlet oxygen production of a commercial photosensitizer and the quenching effect of fluorescent dyes limit the further application of PDT treatment and fluorescence imaging. In addition, the single nanoplatform that simultaneously achieved singlet oxygen and fluorescence enhancement is rare. In this paper, a novel simultaneously enhanced singlet oxygen and fluorescence production nanoplatform of AuNR@mSiO₂-Ce6-Cy5.5 has been successfully designed and synthesized by surface plasmon resonance coupling. The as-synthesized nanoplatform achieved a 1.8-fold enhancement of the singlet oxygen production of Ce6 and a 5.0-fold enhancement of the fluorescence production of Cy5.5 by surface plasmon resonance coupling. The as-synthesized nanoplatform simultaneously enhances the photodynamic therapy and fluorescence imaging of cancer, which will have great potential in biomedical applications.

Enhanced three-photon activity triggered by the AIE behaviour of a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge

Multiphoton bioimaging benefits from good penetration of tissue, low phototoxicity and high resolution. Hence, development of efficient multiphoton imaging agents is highly desirable but remains challenging. Herein, a novel terpyridine-based Zn(ii) complex bearing a thiophene bridge was designed rationally and fabricated. Thanks to its aggregation-induced emission (AIE), DZ1 emitted bright yellow-green fluorescence (λ em = 575 nm) under physiological conditions. The three-photon spectral changes of DZ1 when binding with RNA unambiguously reflected its RNA-specific targeting behaviour, resulting in twofold enhancement in three-photon action cross-sections located at the second near-infrared window (1700 nm).

High-efficiency fluorescent and magnetic multimodal probe for long-term monitoring and deep penetration imaging of tumors

High-quality multimodal imaging requires exogenous contrast agents with high sensitivity, spatial–temporal resolution, and high penetration depth for the accurate diagnosis and surveillance of cancer.

Crown-substituted naphthalocyanines: synthesis and supramolecular control over aggregation and photophysical properties

Tetra-15-crown-5-naphthalocyanines as first representatives of crown-substituted π-extended phthalocyanines were synthesized and characterized. The possibility to control their aggregation and photophysical properties by reversible formation of supramolecular assemblies in the presence of KOAc was demonstrated.

Bright Near-Infrared Aggregation-Induced Emission Luminogens with Strong Two-Photon Absorption, Excellent Organelle Specificity, and Efficient Photodynamic Therapy Potential

Far-red and near-infrared (NIR) fluorescent materials possessing the characteristics of strong two-photon absorption and aggregation-induced emission (AIE) as well as specific targeting capability are much-sought-after for bioimaging and therapeutic applications due to their deep penetration depth and high resolution. Herein, a series of dipolar far-red and NIR AIE luminogens with a strong push-pull effect are designed and synthesized. The obtained fluorophores display bright far-red and NIR solid-state fluorescence with a high quantum yield of up to 30%, large Stokes shifts of up to 244 nm, and large two-photon absorption cross-sections of up to 887 GM. A total of three neutral AIEgens show specific lipid droplet (LD)-targeting capability, while the one with cationic and lipophilic characteristics tends to target the mitochondria specifically. All of the molecules demonstrate good biocompatibility, high brightness, and superior photostability. They also serve as efficient two-photon fluorescence-imaging agents for the clear visualization of LDs or mitochondria in living cells and tissues with deep tissue penetration (up to 150 μm) and high contrast. These AIEgens can efficiently generate singlet oxygen upon light irradiation for the photodynamic ablation of cancer cells. All of these intriguing results prove that these far-red and NIR AIEgens are excellent candidates for the two-photon fluorescence imaging of LDs or mitochondria and organelle-targeting photodynamic cancer therapy.