Nanoparticles of Organic Electronic Materials for Biomedical Applications

Photoresponsive Hydrogels for Tissue Engineering

Hydrophilic and biocompatible hydrogels are widely applied as ideal scaffolds in tissue engineering. The "smart" gelation material can alter its structural, physiochemical, and functional features in answer to various endo/exogenous stimuli to better biomimic the endogenous extracellular matrix for the engineering of cells and tissues. Light irradiation owns a high spatial-temporal resolution, complete biorthogonal reactivity, and fine-tunability and can thus induce physiochemical reactions within the matrix of photoresponsive hydrogels with good precision, efficiency, and safety. Both gel structure (e.g., geometry, porosity, and dimension) and performance (like conductivity and thermogenic or mechanical properties) can hence be programmed on-demand to yield the biochemical and biophysical signals regulating the morphology, growth, motility, and phenotype of engineered cells and tissues. Here we summarize the strategies and mechanisms for encoding light-reactivity into a hydrogel and demonstrate how fantastically such responsive gels change their structure and properties with light irradiation as desired and thus improve their applications in tissue engineering including cargo delivery, dynamic three-dimensional cell culture, and tissue repair and regeneration, aiming to provide a basis for more and better translation of photoresponsive hydrogels in the clinic.

Preparation of Ultrasmall AIE Nanoparticles with Tunable Molecular Packing via Freeze Assembly

Advances in the development of aggregation-induced emission luminogens (AIEgens) depend on understanding how the molecular packing affects their luminescent properties and on making nanoparticles (NPs) with desired sizes. Although reported strategies have advanced the field, rational control of molecular packing and efficient fabrication of AIEgen NPs sub-5.5 nm in diameter remain pressing issues. Here we report a "freeze assembly" strategy, in which the diameter of AIEgen NPs can be precisely tuned from ∼3 nm to hundreds of nanometers, and a molecular packing in kinetically trapped states that are not easily captured by conventional assembly methods can be obtained, leading to tunable fluorescence emissions. Therefore, this study provides a significant tool to fabricate organic luminescent nanomaterials with diameters smaller than 5 nm, which is of critical importance for biomedical applications; meanwhile, tuning molecular packing in nanoparticles displaying different fluorescence may help to shed new light on the mechanism of AIEgens.

Alcian Blue Staining to Visualize Intracellular Hyaluronic Acid-Based Nanoparticles

Investigating at transmission electron microscopy the intracellular trafficking of hyaluronic acid-based nanoparticles remains a challenge due to their intrinsic weak electron density. Here we describe a simple protocol to stain hyaluronic acid that allows visualization of hyaluronic acid-based nanoparticles inside cells at both light and electron microscopy. By applying the critical-electrolyte-concentration Alcian blue method, these nanoparticles were observed as blue dots at bright-field microscopy or filled with fine electron dense precipitates at transmission electron microscopy.

NIR-II AIEgens with Photodynamic Effect for Advanced Theranostics

Phototheranostics that concurrently integrates accurate diagnosis (e.g., fluorescence and photoacoustic (PA) imaging) and in situ therapy (e.g., photodynamic therapy (PDT) and photothermal therapy (PTT)) into one platform represents an attractive approach for accelerating personalized and precision medicine. The second near-infrared window (NIR-II, 1000-1700 nm) has attracted considerable attention from both the scientific community and clinical doctors for improved penetration depth and excellent spatial resolution. NIR-II agents with a PDT property as well as other functions are recently emerging as a powerful tool for boosting the phototheranostic outcome. In this minireview, we summarize the recent advances of photodynamic NIR-II aggregation-induced emission luminogens (AIEgens) for biomedical applications. The molecular design strategies for tuning the electronic bandgaps and photophysical energy transformation processes are discussed. We also highlight the biomedical applications, such as image-guided therapy of both subcutaneous and orthotopic tumors, and multifunctional theranostics in combination with other treatment methods, including chemotherapy and immunotherapy; and the precise treatment of both tumor and bacterial infection. This review aims to provide guidance for PDT agents with long-wavelength emissions to improve the imaging precision and treatment efficacy. We hope it will provide a comprehensive understanding about the chemical structure-photophysical property-biomedical application relationship of NIR-II luminogens.

Facile Preparation of a Thienoisoindigo-Based Nanoscale Covalent Organic Framework with Robust Photothermal Activity for Cancer Therapy

The covalent organic frameworks (COFs) so far are usually built with small aromatic subunits, which makes their absorption spectra mainly located in the high-energy part of the visible region. In this work, we have developed a COF with a low band gap by integrating electron-deficient thienoisoindigo and electron-rich triphenylamine. The intramolecular charge-transfer effect combining the extended length of the π-conjugated backbone of COF endow it with broad absorption even to the second near-infrared region. After optimizing the solvent, a uniform size and colloidal stable COF is obtained. Benefiting from the coplanar structure of the monomer, this COF achieves a considerable photothermal conversion efficiency (PCE) of 48.2%. With these advantages, it displays convincing cancer cell killing effect upon laser irradiation in vitro or in vivo. This work provides a simple and practical method to acquire promising a COF-based phototherapy reagent that is applied in biomedicine field.

Graphene quantum dots alleviate ROS-mediated gastric damage

The gastrointestinal (GI) tract is one of the major sites for reactive oxygen species generation (ROS). Physiological ROS, lower than the threshold concentration, is beneficial for human physiology to preserve gut functional integrity. However, ROS generated in large quantities in presence of external stimuli overwhelms the cellular antioxidant defense mechanism and results in oxidative damage and associated physiological disorder. Graphene quantum dots (GQDs) are a class of carbon-based nanomaterials that have attracted tremendous attention not only for their tunable optical properties but also for their broad-spectrum antioxidant properties. In this report we have shown that GQDs are highly efficient in scavenging ROS and suppressing stress-induced gastric ulcers by targeting the MMP-9 pathway and reducing the inflammatory burden by suppressing excessive oxidative stress by inducing high caspase activity, overproduction of Bax, and downregulation of BCL2.

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The gastrointestinal tract is one of the major sites for ROS generation

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Graphene quantum dots (GQDs) have broad-spectrum antioxidant properties

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GQDs scavenge the ROS and suppress gastric ulcers by targeting the MMP-9 pathway

EGFR-Targeted Photodynamic Therapy

The epidermal growth factor receptor (EGFR) plays a pivotal role in the proliferation and metastatization of cancer cells. Aberrancies in the expression and activation of EGFR are hallmarks of many human malignancies. As such, EGFR-targeted therapies hold significant potential for the cure of cancers. In recent years, photodynamic therapy (PDT) has gained increased interest as a non-invasive cancer treatment. In PDT, a photosensitizer is excited by light to produce reactive oxygen species, resulting in local cytotoxicity. One of the critical aspects of PDT is to selectively transport enough photosensitizers to the tumors environment. Accordingly, an increasing number of strategies have been devised to foster EGFR-targeted PDT. Herein, we review the recent nanobiotechnological advancements that combine the promise of PDT with EGFR-targeted molecular cancer therapy. We recapitulate the chemistry of the sensitizers and their modes of action in PDT, and summarize the advantages and pitfalls of different targeting moieties, highlighting future perspectives for EGFR-targeted photodynamic treatment of cancer.

Synthesis of Uniform Polymer Encapsulated Organic Nanocrystals through Ouzo Nanocrystallization

Nanocrystals (NCs) are widely used in optoelectronics, photocatalysis, and bioimaging. As the surface area to volume ratio increases with a decrease in the size of NCs, strategies to control the size of NCs are highly valuable for many applications. Given the importance of photoluminescent dyes, especially those with aggregation-induced emission, the transformation from an amorphous to a crystalline state can yield a drastic enhancement in their optical properties, which is of significance for biomedical applications. Till now, there is no general method available for the synthesis of small NCs with accurate control over the size and uniformity. Herein, a simple and general approach of ouzo nanocrystallization is presented for the synthesis of small (<100 nm) and highly uniform (polydispersity index~0.1) NCs with good control over the size. The process of nanoprecipitation is used to synthesize uniform nanoparticles (NPs) with different size, which is followed by solvent addition to form swollen NPs. Further, the amorphous core of swollen NPs is converted into NCs within polymer shell under Ouzo zone, which restricts NCs to grow above certain size. To demonstrate the general applicability of ouzo nanocrystallization, two different classes of luminescent materials are used as examples to fabricate small and highly uniform NCs.

Thermally Activated Delayed Fluorescence Material: An Emerging Class of Metal-Free Luminophores for Biomedical Applications

The development of simple, efficient, and biocompatible organic luminescent molecules is of great significance to the clinical transformation of biomaterials. In recent years, purely organic thermally activated delayed fluorescence (TADF) materials with an extremely small single-triplet energy gap (ΔEST ) have been considered as the most promising new-generation electroluminescence emitters, which is an enormous breakthrough in organic optoelectronics. By merits of the unique photophysical properties, high structure flexibility, and reduced health risks, such metal-free TADF luminophores have attracted tremendous attention in biomedical fields, including conventional fluorescence imaging, time-resolved imaging and sensing, and photodynamic therapy. However, there is currently no systematic summary of the TADF materials for biomedical applications, which is presented in this review. Besides a brief introduction of the major developments of TADF material, the typical TADF mechanisms and fundamental principles on design strategies of TADF molecules and nanomaterials are subsequently described. Importantly, a specific emphasis is placed on the discussion of TADF materials for various biomedical applications. Finally, the authors make a forecast of the remaining challenges and future developments. This review provides insightful perspectives and clear prospects towards the rapid development of TADF materials in biomedicine, which will be highly valuable to exploit new luminescent materials.

Organic Persistent Luminescent Materials: Ultralong Room-Temperature Phosphorescence and Multicolor-Tunable Afterglow

Organic persistent luminescent materials have attracted special attention due to their significant applications in optoelectronics, sensors, and security technology areas. In this work, a series of organic compounds (1-4) with twisted electron donor-acceptor structures are successfully designed and synthesized, and then the resultant compounds are dissolved in methyl methacrylate (MMA), and afterward, in situ polymerization realizes single-molecular organic room-temperature phosphorescent (RTP) materials (P1-P4). All RTP materials show long lifetime, especially P2 exhibits ultralong lifetime of 1.51 s. When the compounds are grown into single crystals, multicolor-tunable afterglow is obtained at different delay times due to the dual emission of phosphorescence and delayed fluorescence, which is promising to be applied in high-level anticounterfeiting.

Anti-Sandwich Structured Photo-Electronic Wound Dressing for Highly Efficient Bacterial Infection Therapy

Photo-electronic devices based on reactive oxygen species (ROS) generation suffer a crucial limitation in wound treatment due to their sandwich structure, which prevents the contact of ROS with wound tissue. In this work, the first anti-sandwich structured visible-light/electricity dual-responsive wound dressing is constructed for treatment of methicillin-resistant Staphylococcus aureus (MRSA), based on selenoviologen-appendant polythiophene (SeV²⁺ -PT)-containing polyacrylamide hydrogels. The new wound dressing is named an anti-sandwich structured photo-electronic wound dressing (PEWD). The unique structure of PEWD enables its use in synergistic electrodynamic and photodynamic therapy (EDT and PDT), providing rapid, on-demand, and sustained generation of ROS in situ via short-time light irradiation and/or wireless-controlled electrification. The PEWD possesses good flexibility, excellent biocompatibility, and fast response, as well as sustained ROS generation in a physiological environment. Animal experiments demonstrate effective ROS generation in 6 s under irradiation and electrification, inhibiting infection at an early stage, and substantially shortening the healing time of bacterial infection (to within 7 days). This proof-of-concept research holds great promise in developing new flexible PEWD, and novel strategies to improve wound treatment.

Human Serum Albumin-Oligothiophene Bioconjugate: A Phototheranostic Platform for Localized Killing of Cancer Cells by Precise Light Activation

The electronic, optical, and redox properties of thiophene-based materials have made them pivotal in nanoscience and nanotechnology. However, the exploitation of oligothiophenes in photodynamic therapy is hindered by their intrinsic hydrophobicity that lowers their biocompatibility and availability in water environments. Here, we developed human serum albumin (HSA)-oligothiophene bioconjugates that afford the use of insoluble oligothiophenes in physiological environments. UV-vis and electrophoresis proved the conjugation of the oligothiophene sensitizers to the protein. The bioconjugate is water-soluble and biocompatible, does not have any "dark toxicity", and preserves HSA in the physiological monomeric form, as confirmed by dynamic light scattering and circular dichroism measurements. In contrast, upon irradiation with ultralow light doses, the bioconjugate efficiently produces reactive oxygen species (ROS) and leads to the complete eradication of cancer cells. Real-time monitoring of the photokilling activity of the HSA-oligothiophene bioconjugate shows that living cells "explode" upon irradiation. Photodependent and dose-dependent apoptosis is one of the primary mechanisms of cell death activated by bioconjugate irradiation. The bioconjugate is a novel theranostic platform able to generate ROS intracellularly and provide imaging through the fluorescence of the oligothiophene. It is also a real-time self-reporting system able to monitor the apoptotic process. The induced phototoxicity is strongly confined to the irradiated region, showing localized killing of cancer cells by precise light activation of the bioconjugate.