Conjugated‐Polymer‐Based Nanoparticles with Efficient NIR‐II Fluorescent, Photoacoustic and Photothermal Performance

Rational Design of NIR-II Emitting Conjugated Polymer Derived Nanoparticles for Image-Guided Cancer Interventions

Due to the reduced absorption, light scattering, and tissue autofluorescence in the NIR-II (1000-1700 nm) region, significant efforts are underway to explore diverse material platforms for in vivo fluorescence imaging, particularly for cancer diagnostics and image-guided interventions. Of the reported imaging agents, nanoparticles derived from conjugated polymers (CPNs) offer unique advantages to alternative materials including biocompatibility, remarkable absorption cross-sections, exceptional photostability, and tunable emission behavior independent of cell labeling functionalities. Herein, the current state of NIR-II emitting CPNs are summarized and structure-function-property relationships are highlighted that can be used to elevate the performance of next-generation CPNs. Methods for particle processing and incorporating cancer targeting modalities are discussed, as well as detailed characterization methods to improve interlaboratory comparisons of novel materials. Contemporary methods to specifically apply CPNs for cancer diagnostics and therapies are then highlighted. This review not only summarizes the current state of the field, but offers future directions and provides clarity to the advantages of CPNs over other classes of imaging agents.

Near-Infrared Absorbing Para-Azaquinodimethane Conjugated Polymers Synthesized via the Transition-Metal-Free Route toward Efficient Photothermal Conversion

Conjugated polymers with strong absorption in the second near-infrared (NIR-II) window have multiple applications. However, the development of new type of NIR-II conjugated polymers via facile and green methods remains challenging. Herein, this work reports a mild and convenient transition-metal-free method to synthesize near-infrared absorbing quinoidal conjugated polymers containing para-azaquinodimethane (AQM) moieties. The AQM quinoidal conjugated polymers with unique molecular structures and tunable optoelectronic properties can be synthesized by combining the Knoevenagel polycondensation of aromatic dialdehyde monomers with commercially available 1,4-diacetyl-2,5-piperazinedione and the following alkylation reaction. The resultant polymer PQ-DPP shows remarkable NIR-II absorption with a narrow band gap of about 1.08 eV. PQ-DPP nanoparticles exhibit high photothermal conversion efficiency of up to 48% under 1064 nm laser irradiation (1 W cm-2) endowing this polymer with potential in bio-related applications.

Exceptional Electron-Rich Heteroaromatic Pentacycle for Ultralow Band Gap Conjugated Polymers and Photothermal Therapy

Stable redox-active conjugated molecules with exceptional electron-donating abilities are key components for the design and synthesis of ultralow band gap conjugated polymers. While hallmark electron-rich examples such as pentacene derivatives have been thoroughly explored, their poor air stability has hampered their broad incorporation into conjugated polymers for practical applications. Herein, we describe the synthesis of the electron-rich, fused pentacyclic pyrazino[2,3-b:5,6-b']diindolizine (PDIz) motif and detail its optical and redox behavior. The PDIz ring system exhibits a lower oxidation potential and a reduced optical band gap than the isoelectronic pentacene while retaining greater air stability in both solution and the solid state. The enhanced stability and electron density, together with readily installed solubilizing groups and polymerization handles, allow for the use of the PDIz motif in the synthesis of a series of conjugated polymers with band gaps as small as 0.71 eV. The tunable absorbance throughout the biologically relevant near-infrared I and II regions enables the use of these PDIz-based polymers as efficient photothermal therapeutic reagents for laser ablation of cancer cells.

D-A-D organic small molecules with AIE effect for fluorescence imaging guided photothermal therapy

Near infrared (NIR) fluorescent organic molecules as fluorescent probes accurately guide photothermal therapy as a potential antitumor method. However, the aggregation and quenching of organic fluorescent molecules and poor tissue permeability greatly limit their therapeutic effect and clinical transformation. In this paper, with a D-A-D structure as the molecular skeleton, cyclopentadithiophene (CPDT) as the donor (D), diketopyrrolopyrrole (DPP) as the acceptor (A), and long-chain isooctane as the shielding unit, organic fluorescent small molecules with a strong absorption band and bright NIR-II emission were synthesized. Then, tetraphenylethylene (TPE) molecules with typical AIE structure characteristics were introduced on both sides of the organic fluorescent small molecules, and an organic small molecular fluorophore (TDA) with AIE characteristics and the photothermal effect was designed. Through a series of experimental characterization techniques, it is proved that TDA NPs have good biocompatibility and tissue permeability, and can accurately locate the tumor location through NIR-II fluorescence imaging to achieve local photothermal treatment of tumors.

Optical Microscopy Systems for the Detection of Unlabeled Nanoparticles

Label-free detection of nanoparticles is essential for a thorough evaluation of their cellular effects. In particular, nanoparticles intended for medical applications must be carefully analyzed in terms of their interactions with cells, tissues, and organs. Since the labeling causes a strong change in the physicochemical properties and thus also alters the interactions of the particles with the surrounding tissue, the use of fluorescently labeled particles is inadequate to characterize the effects of unlabeled particles. Further, labeling may affect cellular uptake and biocompatibility of nanoparticles. Thus, label-free techniques have been recently developed and implemented to ensure a reliable characterization of nanoparticles.

This review provides an overview of frequently used label-free visualization techniques and highlights recent studies on the development and usage of microscopy systems based on reflectance, darkfield, differential interference contrast, optical coherence, photothermal, holographic, photoacoustic, total internal reflection, surface plasmon resonance, Rayleigh light scattering, hyperspectral and reflectance structured illumination imaging. Using these imaging modalities, there is a strong enhancement in the reliability of experiments concerning cellular uptake and biocompatibility of nanoparticles, which is crucial for preclinical evaluations and future medical applications.

NIR-II Functional Materials for Photoacoustic Theranostics

Photoacoustic imaging (PAI) has attracted great attention in the diagnosis and treatment of diseases due to its noninvasive properties. Especially in the second near-infrared (NIR-II) window, PAI can effectively avoid the interference of tissue spontaneous fluorescence and light scattering, and obtain high resolution images with deeper penetration depth. Because of its ideal spectral absorption and high conversion efficiency, NIR-II PA contrast agents overcome the absorption or emission of NIR-II light by endogenous biomolecules. In recent years, a series of NIR-II PA contrast agents have been developed to improve the performance of PAI in disease diagnosis and treatment. In this paper, the research progress of NIR-II PA contrast agents and their applications in biomedicine are reviewed. PA contrast agents are classified according to their composition, including inorganic contrast agents, organic contrast agents, and hybrid organic-inorganic contrast agents. The applications of NIR-II PA contrast agents in medical imaging are described, such as cancer imaging, inflammation detection, brain disease imaging, blood related disease imaging, and other biomedical application. Finally, the research prospects and breakthrough of NIR-II PA contrast agents are discussed.

Electrospraying as a Technique for the Controlled Synthesis of Biocompatible PLGA@Ag2S and PLGA@Ag2S@SPION Nanocarriers with Drug Release Capability

Ag2S nanoparticles are near-infrared (NIR) probes providing emission in a specific spectral range (~1200 nm), and superparamagnetic iron oxide nanoparticles (SPION) are colloidal systems able to respond to an external magnetic field. A disadvantage of Ag2S NPs is the attenuated luminescent properties are reduced in aqueous media and human fluids. Concerning SPION, the main drawback is the generation of undesirable clusters that reduce particle stability. Here, we fabricate biocompatible hybrid nanosystems combining Ag2S NPs and SPION by the electrospraying technique for drug delivery purposes. These nanostructures are composed of poly(lactic-co-glycolic acid) (PLGA) as the polymeric matrix in connection with both Ag2S NPs and SPIONs. Initially, we fabricate a hybrid colloidal nanosystem composed of Ag2S NPs in connection with PLGA (PLGA@Ag2S) by three different routes, showing good photoluminescent (PL) properties with relatively high average decay times. Then, we incorporate SPIONs, obtaining a PLGA polymeric matrix containing both Ag2S NPs and SPION (PLGA@Ag2S@SPION). Interestingly, in this hybrid system, the location of Ag2S NPs and SPIONs depends on the synthesis route performed during electrospraying. After a detailed characterization, we demonstrate the encapsulation and release capabilities, obtaining the kinetic release using a model chemotherapeutic drug (maslinic acid). Finally, we perform in vitro cytotoxicity assays using drug-loaded hybrid systems against several tumor cell lines.

Dithieno[3,2-b:2',3'-d]silole-based conjugated polymers for bioimaging in the short-wave infrared region

The short-wave infrared window (SWIR, 900–1700 nm) fluorescence imaging has been demonstrated to have excellent imaging performance in signal/noise ratio and tissue penetration compared to the conventional NIR biological window (NIR-I, 700–900 nm). Conventional organic SWIR fluorescent materials still suffer from low fluorescence quantum efficiency. In this work, a donor unit with sp³ hybrid configuration and an acceptor unit with small hindered alkyl side chains are employed to construct donor–acceptor (D–A) type conjugated polymers P1 and P2, which were substituted with one or two fluorine atoms. These structural features can alleviate the aggregation-caused quenching (ACQ) and contribute to charge transfer, resulting in a significantly improved fluorescence quantum efficiency. The SWIR fluorescent quantum efficiencies of P1 and P2 nanoparticles are 3.4% and 4.4%, respectively, which are some of the highest for organic SWIR fluorophores reported so far. Excellent imaging quality has been demonstrated with P2 nanoparticles for SWIR imaging of the vascular system of nude mice. The results indicate that our design strategy of introducing sp³ hybrid configuration and small hindered alkyl side chains to fabricate conjugated polymers is efficient in improving the fluorescent quantum efficiency as SWIR fluorescent imaging agents for potential clinical practice.

A D–A type polymer with a SWIR fluorescence quantum efficiency of 4.4% was obtained after structural optimization.

Charge Transfer Metal-Organic Framework Containing Redox-Active TTF/NDI Units for Highly Efficient Near-Infrared Photothermal Conversion

Metal-organic frameworks (MOFs), as a class of new inorganic-organic hybrid crystal materials, could have important applications in near-infrared (NIR) photothermal conversion. Herein, a new charge-transfer MOF (Co-MOF) with mixed ligands of H₄ TTFTB and bpmNDI incorporating redox-active tetrathiafulvalene/naphthalene diimide (TTF/NDI) units into one system is reported. Due to the presence of TTF/NDI oxidative and reductive couples, stable radicals can be observed in the MOF. In addition, charge transfer from the electron donor (TTF) to the acceptor (NDI) results in a broad absorption in the NIR region. The Co-MOF exhibited an efficient photothermal effect induced by irradiation with a NIR laser. Under the 808 nm laser (0.7 W cm^-2 ) illumination, the temperature of the Co-MOF increased from room temperature to 201 °C in only 10 s. Furthermore, a series of polydimethylsiloxane (PDMS) films doped with trace amounts of Co-MOF showed efficient NIR photothermal conversion. When a Co-MOF@PDMS (0.6 wt %) film is irradiated by 808 nm laser with power of 0.5 W cm^-2 , it's temperature can reach a plateau at 62 °C from 20 °C within 100 s. Our experimental results from the Co-MOF@PDMS film demonstrate that the effectiveness and feasibility of the material is promising for photothermal applications.

Thienothiadiazole-Based NIR-II Dyes with D-A-D Structure for NIR-II Fluorescence Imaging Systems

Fluorescence imaging (FI) in the second near-infrared optical window (NIR-II, 1000-1700 nm) has received increasing focus due to its capacity of high spatiotemporal resolution, rapid real-time imaging, and deep penetration depth. In addition, D-A-D-based organic small molecules have also attracted wide attention due to their designed chemical structure and rapid renal metabolism. However, most of the fluorescent cores were based on benzobisthiadiazole (BBTD) and 6,7-diphenyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TTQ). The design and development of fluorescent core still remain challenging. Therefore, two NIR-II dyes based on the acceptor 4,6-di(2-thienyl)thieno[3,4-c][1,2,5]thiadiazole (TTDT) were designed and developed with donors tributyl(5-(9,9-dioctyl-9H-fluoren-2-yl)thiophen-2-yl)stannane (TF) and (5-(9,9'-spirobi[fluoren]-2-yl)thiophen-2-yl)tributylstannane (TSF) by the Stille coupling reaction, respectively. Subsequently, the corresponding nanoparticles were prepared, and then TTDT-TF-based nanoparticles with superior photostability and strong NIR-II fluorescence signals were chosen for NIR-II FI. More importantly, the in vivo experiments suggested that TTDT-TF NPs exhibited significant accumulation at tumor sites and high signal-to-background ratio (SBR). The above results indicated that the two D-A-D-type fluorophores based on TTDT have potential for NIR-II FI with superior imaging quality and imaging-guided surgery or therapy.

NIR-II bioimaging of small organic molecule

In recent years, people have been actively exploring new imaging methods with high biological imaging performance because the clinical image definition and depth in vivo cannot meet the requirements of early diagnosis and prognosis. Based on the traditional near-infrared region I (NIR-I), the molecular probe of the near-infrared region II (NIR-II) is further explored and developed. In the NIR-II region due to the wavelength is longer than the NIR-I region can effectively reduce the molecular scattering, optical absorption of the organization, the organization of spontaneous fluorescence negligible, thus the NIR-II Fluorescence imaging (FI) can get deeper penetration depth, higher signal-to-background ratio (SBR) and better spatiotemporal resolution, FI in NIR-II region are an important and rapidly developing research region for future imaging. In the NIR-II fluorophore, small organic molecule fluorophore has attracted much attention because of its good biocompatibility and good pharmacokinetic properties. In this review, we briefly introduced the existing NIR-II organic small molecule fluorophores, and introduced the existing relatively mature methods for improving quantum yield and water solubility, and the small molecule dyes on FI of various improvement methods, also briefly introduces the small molecules of photoacoustic imaging (PAI), and a brief introduction of imaging-guided surgery (IGS) for some small organic molecules, finally, a reasonable prospect is made for the development of small organic molecules.

Double-acceptor conjugated polymers for NIR-II fluorescence imaging and NIR-II photothermal therapy applications

Nanoparticles based double-acceptor conjugated polymers were developed by conventional methods. And subsequently NPs with bright NIR-II fluorescence signals and superior NIR-II PTT efficiency were successfully applied for NIR-II FI guided NIR-II PTT.

A design strategy for D–A conjugated polymers for NIR-II fluorescence imaging

The development of bioimaging technology in recent years has shown that second near-infrared (NIR-II) fluorescence imaging (FI) is gradually replacing the traditional visible light and first near-infrared (NIR-I) FI.

Excretable IR-820 for in vivo NIR-II fluorescence cerebrovascular imaging and photothermal therapy of subcutaneous tumor

Rationale: Cerebrovascular diseases, together with malignancies, still pose a huge threat to human health nowadays. With the advantages of its high spatial resolution and large penetration depth, fluorescence bioimaging in the second near-infrared spectral region (NIR-II, 900-1700 nm) and its related imaging-guided therapy based on biocompatible fluorescence dyes have become a promising theranostics method. Methods: The biocompatibility of IR-820 we used in NIR-II fluorescence bioimaging was verified by long-term observation. The model of the mouse with a cranial window, the mouse model of middle cerebral artery occlusion (MCAO) and a subcutaneous xenograft mouse model of bladder tumor were established. NIR-II fluorescence cerebrovascular functional imaging was carried out by IR-820 assisted NIR-II fluorescence microscopy. Bladder tumor was treated by NIR-II fluorescence imaging-guided photothermal therapy. Results: We have found that IR-820 has considerable NIR-II fluorescence intensity, and shows increased brightness in serum than in water. Herein, we achieved real time and in vivo cerebrovascular functional imaging of mice with high spatial resolution and large penetration depth, based on IR-820 assisted NIR-II fluorescence microscopy. In addition, IR-820 was successfully employed for NIR-II fluorescence imaging and photothermal therapy of tumor in vivo, and the subcutaneous tumors were inhibited obviously or eradicated completely. Conclusion: Due to the considerable fluorescence intensity in NIR-II spectral region and the good photothermal effect, biocompatible and excretable IR-820 holds great potentials for functional angiography and cancer theranostics in clinical practice.