Alkyl-Doping Enables Significant Suppression of Conformational Relaxation and Intermolecular Nonradiative Decay for Improved Near-Infrared Fluorescence Imaging

Despite various efforts to optimize the near-infrared (NIR) performance of perylene diimide (PDI) derivatives for bio-imaging, convenient and efficient strategies to amplify the fluorescence of PDI derivatives in biological environment and the intrinsic mechanism studies are still lacking. Herein, we propose an alkyl-doping strategy to amplify the fluorescence of PDI derivative-based nanoparticles for improved NIR fluorescence imaging. The developed PDI derivative, OPE-PDI, shows much brighter in n-Hexane (HE) compared with that in other organic media, and the excited state dynamics investigation experimentally elucidates the solvent effect-induced suppression of intermolecular energy transfer and intramolecular nonradiative decay as the underlying mechanism for the fluorescence improvement. Theoretical calculations reveal the lowest reorganization energies of OPE-PDI in HE among various solvents, indicating the effectively suppressed conformational relaxation to support the strongest radiative decay. Inspired by this, an alkyl atmosphere mimicking HE is constructed by incorporating the octadecane into OPE-PDI-based nanoparticles, permitting up to 3-fold fluorescence improvement compared with the counterpart nanoparticles. Owing to the merits of high brightness, anti-photobleaching, and low biotoxicity for the optimal nanoparticles, they have been employed for probing and long-term monitoring of tumor. This work highlights a facile strategy for the fluorescence enhancement of PDI derivative-based nanoparticles.

Bright and Stable Nanomaterials for Imaging and Sensing

This review covers strategies to prepare high-performance emissive polymer nanomaterials, combining very high brightness and photostability, to respond to the drive for better imaging quality and lower detection limits in fluorescence imaging and sensing applications. The more common approaches to obtaining high-brightness nanomaterials consist of designing polymer nanomaterials carrying a large number of fluorescent dyes, either by attaching the dyes to individual polymer chains or by encapsulating the dyes in nanoparticles. In both cases, the dyes can be covalently linked to the polymer during polymerization (by using monomers functionalized with fluorescent groups), or they can be incorporated post-synthesis, using polymers with reactive groups, or encapsulating the unmodified dyes. Silica nanoparticles in particular, obtained by the condensation polymerization of silicon alcoxides, provide highly crosslinked environments that protect the dyes from photodegradation and offer excellent chemical modification flexibility. An alternative and less explored strategy is to increase the brightness of each individual dye. This can be achieved by using nanostructures that couple dyes to plasmonic nanoparticles so that the plasmon resonance can act as an electromagnetic field concentrator to increase the dye excitation efficiency and/or interact with the dye to increase its emission quantum yield.

Recent Advances in Applications of Fluorescent Perylenediimide and Perylenemonoimide Dyes in Bioimaging, Photothermal and Photodynamic Therapy

The emblematic perylenediimide (PDI) motif which was initially used as a simple dye has undergone incredible development in recent decades. The increasing power of synthetic organic chemistry has allowed it to decorate PDIs to achieve highly functional dyes. As these PDI derivatives combine thermal, chemical and photostability, with an additional high absorption coefficient and near-unity fluorescence quantum yield, they have been widely studied for applications in materials science, particularly in photovoltaics. Although PDIs have always been in the spotlight, their asymmetric counterparts, perylenemonoimide (PMI) analogues, are now experiencing a resurgence of interest with new efforts to create architectures with equally exciting properties. Namely, their exceptional fluorescence properties have recently been used to develop novel systems for applications in bioimaging, biosensing and photodynamic therapy. This review covers the state of the art in the synthesis, photophysical characterizations and recently reported applications demonstrating the versatility of these two sister PDI and PMI compounds. The objective is to show that after well-known applications in materials science, the emerging trends in the use of PDI- and PMI-based derivatives concern very specific biomedicinal applications including drug delivery, diagnostics and theranostics.

In Vivo Fluorescence Imaging-Guided Development of Near-Infrared AIEgens

In vivo fluorescence imaging has received extensive attention due to its distinguished advantages of excellent biosafety, high sensitivity, dual temporal-spatial resolution, real-time monitoring ability, and non-invasiveness. Aggregation-induced emission luminogens (AIEgens) with near-infrared (NIR) absorption and emission wavelengths are ideal candidate for in vivo fluorescence imaging for their large Stokes shift, high brightness and superior photostability. NIR emissive AIEgens provide deep tissue penetration depth as well as low interference from tissue autofluorescence. Here in this review, we summarize the molecular engineering strategies for constructing NIR AIEgens with high performances, including extending π-conjugation system and strengthen donor (D)-acceptor (A) interactions. Then the encapsulation strategies for increasing water solubility and biocompatibility of these NIR AIEgens are highlighted. Finally, the challenges and prospect of fabricating NIR AIEgens for in vivo fluorescence imaging are also discussed. We hope this review would provide some guidelines for further exploration of new NIR AIEgens.

Thiophene α-Chain-End-Functionalized Oligo(2-methyl-2-oxazoline) as Precursor Amphiphilic Macromonomer for Grafted Conjugated Oligomers/Polymers and as a Multifunctional Material with Relevant Properties for Biomedical Applications

Because the combination of π-conjugated polymers with biocompatible synthetic counterparts leads to the development of bio-relevant functional materials, this paper reports a new oligo(2-methyl-2-oxazoline) (OMeOx)-containing thiophene macromonomer, denoted Th-OMeOx. It can be used as a reactive precursor for synthesis of a polymerizable 2,2'-3-OMeOx-substituted bithiophene by Suzuki coupling. Also a grafted polythiophene amphiphile with OMeOx side chains was synthesized by its self-acid-assisted polymerization (SAAP) in bulk. The results showed that Th-OMeOx is not only a reactive intermediate but also a versatile functional material in itself. This is due to the presence of 2-bromo-substituted thiophene and ω-hydroxyl functional end-groups, and due to the multiple functionalities encoded in its structure (photosensitivity, water self-dispersibility, self-assembling capacity). Thus, analysis of its behavior in solvents of different selectivities revealed that Th-OMeOx forms self-assembled structures (micelles or vesicles) by "direct dissolution".Unexpectedly, by exciting the Th-OMeOx micelles formed in water with λabs of the OMeOx repeating units, the intensity of fluorescence emission varied in a concentration-dependent manner.These self-assembled structures showed excitation-dependent luminescence as well. Attributed to the clusteroluminescence phenomenon due to the aggregation and through space interactions of electron-rich groups in non-conjugated, non-aromatic OMeOx, this behavior certifies that polypeptides mimic the character of Th-OMeOx as a non-conventional intrinsic luminescent material.

Perylene diimide-based treatment and diagnosis of diseases

Integrated treatment using imaging technology to monitor biological processes for the precise treatment and diagnosis of diseases to improve treatment outcomes is becoming a hot topic. Accordingly, perylene diimide (PDI) has excellent photothermal conversion and photostability, which can be used as a good material for disease treatment and diagnosis. Herein, we review the latest research progress on the real-time diagnosis of related diseases based on perylene diimide probes in the aspects of bioimaging, detection of biomarkers and determination of the pH in living cells. Furthermore, perylene diimide-based multifunctional nano-delivery systems are particularly emphasized, showing great therapeutic potential in the field of image-guided combination therapy in tumor therapy. Finally, the great opportunities and challenges still faced by perylene diimide before entering the clinic are comprehensively analyzed.

An amphiphilic aggregate-induced emission polyurethane probe for in situ actin observation in living cells

The specific binding of fluorescent probes or biomolecules to the actin cytoskeleton network is increasingly important for monitoring various complex cellular activities such as cell adhesion, proliferation, locomotion, endocytosis, and cell division. However, improving cell uptake and subcellular resolution is still the main obstacle for successful and wide application of cellular fluorescent probes. Here, we designed and synthesized an amphiphilic block polyurethane with peculiar photophysical properties of aggregation induced emission (AIE), which can be used in living cell imaging to promote selective visualization of cell structures. The AIE effect polyurethane (abbreviated as AIE-PU) was prepared by two-step polymerization of diisocyanate terminated polyethylene glycol and polycaprolactone with hydroxyl terminated AIE dye. A series of characterization techniques proved the successful synthesis of AIE-PU. Due to the amphiphilic chain segment of its linear block molecule, AIE-PU block copolymers can self-assemble into spherical nanoparticles in aqueous solution, showing relatively stable photophysical properties and good water dispersion. Cellular experiments demonstrated that AIE-PUs have low toxicity and high actin network affinity. Moreover, the uptake mechanism was studied by low temperature and metabolic inhibition experiments, showing that AIE-PU nanoparticles could be easily internalized into different living cells through energy-dependent endocytosis, and can be transported from the cellular periphery to the actin network via clathrin- and caveolae-dependent transport pathway. Upon binding with the actin network, the inter-chain AIE mechanism of the probe was significantly enhanced, which is pivotal for the long-term stable fluorescence imaging of actin microfilament network in living cells. Finally, compared with commercial actin dyes, this probe showed higher photostability, even after a longer retention time, without significant fluorescence quenching.

Multifunctional Magnetic-Fluorescent Nanoparticle: Fabrication, Bioimaging, and Potential Antibacterial Applications

Magnetic-fluorescent nanoparticles integrating imaging and therapeutic capabilities have unparalleled advantages in the biomedical applications. Apart from the dual ability of unique biomolecular fluorescent recognition and magnetic modes, the nanoparticle also endows combined effective therapies with high physiological stability, long-term imaging, rapid response time, and excellent circulation ability. Herein, we developed a carboxyl-functionalized magnetic nanoparticle that was further functionalized by polydopamine (PDA) and Schiff base ligand (3-aminopyridine-2-carboxaldehyde N(4)-methylthiosemicarbazone, HL) to form multilayered coating single nanoparticles (Fe₃O₄@PDA@HL). Our work showed that the aggregation-induced emission (AIE) effect could be produced by embedding In³⁺ into the Fe₃O₄@PDA@HL nanostructure, which offered a new opportunity for utilization as a fluorescent detection and therapeutic platform. Cellular fluorescent imaging experiments provided bacterial cell biodistribution, demonstrating their excellent luminescent performance, magnetic aggregation, and separation capability. We simultaneously confirmed that the synergistic antibacterial effect was closely related to both Fe₃O₄@PDA@HL and In³⁺, leading to the disruption of membrane integrity and the leakage of intracellular components, thus inducing bacterial death. This approach presented in our work could promote the development of future bioimaging and clinical therapy applications.

Synthesis, vibrational spectroscopy and X-ray structural characterization of novel NIR emitter squaramides

Two new 2-naphthyl squaramides, 3-methoxy -4-(2-naphtalenylamino)-3-cyclobutene-1,2-dione (SQ-NPh1) and bis-3,4-(2-naphtalenylamino)-3-cyclobutene-1,2-dione (SQ-NPh2) were synthesized via condensation reaction between the dimethylsquarate and 2-naphthylamine. The spectrometric characterization by ¹³C NMR confirmed the obtaining of the squaramide derivative and nor the squaraine analog. This hypothesis was corroborated by Raman and Infrared spectroscopy since the characteristic vibrational bands related to the oxocarbon portion of both structures have been assigned, such as the ones for SQ-NPh1 and SQ-NPh2. The single-crystal X-ray crystallography for SQ-NPh1 crystal structures have been solved and the structure of SQ-NPh2 have been refined using Powder Diffraction state-of-art. The SQ-NPh1 crystallizes in monoclinic system in P2/c space group. Both squaramides present absorption in the ultra-visible (220-370 nm) and fluorescent emission in the near-infrared (780-800 nm), besides they also presented high thermal stability (around 570 °C). Generally, only squaraines are reported as NIR emitters, this is the first description of NIR emission for squaramides, and since the synthesis of squaramides is very easy and the rational design of small-molecule NIR fluorophores is of high priority and great value, these results are very promising for the development of novel NIR fluorescent dyes.

Semiconducting Perylene Diimide Nanostructure: Multifunctional Phototheranostic Nanoplatform

Precision medicine requires noninvasive and accurate early diagnosis and individually appropriate treatments. Phototheranostics has been considered a frontier precision medical technology to provide rapid and safe disease localization and efficient cure. Harnessing the power of advanced nanomedicine with photonics, phototheranostics is rapidly developing and progressively becoming irreplaceable in modern medicine. Nanoscale semiconducting materials, such as inorganic semiconductors, organic conjugated polymers, and small molecules with photonic properties, have been extensively explored in medical imaging (fluorescence imaging, optical coherence tomography, and photoacoustic [PA] imaging) and phototherapy (photothermal, photodynamic, and photocontrolled combination therapies). In practical clinical applications, organic semiconducting materials, because of their biocompatibility and natural metabolism, are preferred over inorganic materials for phototheranostics. Supramolecular self-assembly is considered a significant method for preparing organic detachable and multifunctional phototheranostics, as supramolecular interactions, such as π-π interactions, hydrogen bonding, hydrophobic effects, and electrostatic interactions, are non-covalent and dynamic. Developing new and effective organic supramolecular phototheranostics requires exploration of well-designed basic building blocks with optical properties, understanding of the assembly at the nanoscale, and optimization of the phototheranostics with unique and distinctive multifunctional efficacy. In this Account, we summarize our recent work on the development of small molecular semiconducting perylene diimide (SPDI) for advanced phototheranostics. SPDI is modified to have strong near-infrared absorption beyond 700 nm by the push-pull electronic effect and owns the merits of remarkable photostability, large extinction coefficient, and high photothermal conversion efficiency. By hydrophilic modification, the amphiphile can self-assemble into a nanomicellar structure that allows PA imaging and can serve as a photothermal conversion agent. After theranostics delivery is achieved, this SPDI can be further functionalized for multimodality imaging and photothermally triggered multimodal synergistic therapy. Several well-designed asymmetric structures of SPDI can be obtained by stepwise modification of imides. It is noteworthy that the self-assembly of SPDI is controllable, allowing the preparation of different-sized spherical nanoparticles and rodlike nanoparticles and nanodroplets. For biomedical applications of SPDI phototheranostics (SPDIPTs), the size effect of SPDIPTs has been highlighted in lymph node mapping and cancer imaging. The PA properties and targeting peptide modification of SPDIPTs have brought about the ultrasensitive imaging of early thrombus. The supramolecular nanoconstructs of SPDIPTs further permit multimodality-imaging-guided cancer therapy. In brief, the design of SPDIPTs considers synthetic chemistry, supramolecular self-assembly, nanotechnology, and photonics. Furthermore, SPDIPTs have diverse biomedical applications and offer many opportunities for advancing nanomedicine.

Fluorescent Supramolecular Polymeric Materials

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly-disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π-π stacking interactions, metal-coordination, van der Waals interactions and host-guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Fluorescent Polymer Nanoparticles Based on Dyes: Seeking Brighter Tools for Bioimaging

Speed, resolution and sensitivity of today's fluorescence bioimaging can be drastically improved by fluorescent nanoparticles (NPs) that are many-fold brighter than organic dyes and fluorescent proteins. While the field is currently dominated by inorganic NPs, notably quantum dots (QDs), fluorescent polymer NPs encapsulating large quantities of dyes (dye-loaded NPs) have emerged recently as an attractive alternative. These new nanomaterials, inspired from the fields of polymeric drug delivery vehicles and advanced fluorophores, can combine superior brightness with biodegradability and low toxicity. Here, we describe the strategies for synthesis of dye-loaded polymer NPs by emulsion polymerization and assembly of pre-formed polymers. Superior brightness requires strong dye loading without aggregation-caused quenching (ACQ). Only recently several strategies of dye design were proposed to overcome ACQ in polymer NPs: aggregation induced emission (AIE), dye modification with bulky side groups and use of bulky hydrophobic counterions. The resulting NPs now surpass the brightness of QDs by ≈10-fold for a comparable size, and have started reaching the level of the brightest conjugated polymer NPs. Other properties, notably photostability, color, blinking, as well as particle size and surface chemistry are also systematically analyzed. Finally, major and emerging applications of dye-loaded NPs for in vitro and in vivo imaging are reviewed.

Polymeric AIE-based nanoprobes for biomedical applications: recent advances and perspectives

The development of polymeric luminescent nanomaterials for biomedical applications has recently attracted a large amount of attention due to the remarkable advantages of these materials compared with small organic dyes and fluorescent inorganic nanomaterials. Among these polymeric luminescent nanomaterials, polymeric luminescent nanomaterials based on dyes with aggregation-induced emission (AIE) properties should be of great research interest due to their unique AIE properties, the designability of polymers and their multifunctional potential. In this review, the recent advances in the design and biomedical applications of polymeric luminescent nanomaterials based on AIE dyes is summarized. Various design strategies for incorporation of these AIE dyes into polymeric systems are included. The potential biomedical applications such as biological imaging, and use in biological sensors and theranostic systems of these polymeric AIE-based nanomaterials have also been highlighted. We trust this review will attract significant interest from scientists from different research fields in chemistry, materials, biology and interdisciplinary areas.

Facile preparation of biocompatible and robust fluorescent polymeric nanoparticles via PEGylation and cross-linking

Novel cross-linked copolymers of PEG-IM-PhNH2 are successfully synthesized through PEGylation via radical polymerization of 2-isocyanatoethyl methacrylate and poly(ethylene glycol) monomethyl ether methacylate and subsequent cross-linking with an amino-terminated aggregation-induced emission fluorogen. Such obtained amphiphilic copolymers can self-assemble to form uniform fluorescent polymeric nanoparticles (FPNs) and be utilized for cell imaging. These cross-linked FPNs are demonstrated good water dispersibility with ultralow critical micelle concentration (∼ 0.002 mg mL(-1)), uniform morphology (98 ± 2 nm), high red fluorescence quantum yield, and excellent biocompatibility. More importantly, this novel strategy of fabricating cross-linked FPNs paves the way to the future development of more robust and biocompatible fluorescent bioprobes.

Preparation of emissive glucose-containing polymer nanoparticles and their cell imaging applications

Water dispersible, bright and biocompatible fluorescent glycopolymer nanoparticles were facilely fabricated, and their cellular imaging application was successfully demonstrated.

Fabrication of photostable PEGylated polymer nanoparticles from AIE monomer and trimethylolpropane triacrylate

Fabrication of biocompatible and photostable PEGylated nanoparticles from AIE monomer and trimethylolpropane triacrylate for cellular imaging.

Fluorescent polymeric nanoparticles with ultra-low CMC for cell imaging

Fluorescent polymeric nanoparticles (FPNs) with ultra-low critical micelle concentration were facilely fabricated through radical polymerization and ring-opening crosslinking, and utilized for cell imaging.

Preparation of biocompatible and photostable PEGylated red fluorescent nanoparticles for cellular imaging

Biocompatible and photostable PEGylated red fluorescent nanoparticles were preparedviasurface-initiated ATRP and their cellular imaging application was successfully demonstrated.

Supramolecular electron donor–acceptor complexes formed by perylene diimide derivative and conjugated phenazines

The nanostructure and binding mode of the perylene diimide–phenazine complex can be modulated by the phenazine derivative substituents.

Fluorescent conjugated polymers based on thiocarbonyl quinacridone for sensing mercury ion and bioimaging

Highly sensitive FRET-based thiocarbonyl quinacridone fluorescent conjugated polymers for the sensing and bioimaging of mercury ion have been developed.