Red-emitting DPSB-based conjugated polymer nanoparticles with high two-photon brightness for cell membrane imaging

Recent advances in fluorescent probes for ATP imaging

Adenosine-5'-triphosphate (ATP) is a critical biological molecule that functions as the primary energy currency within cells. ATP synthesis occurs in the mitochondria, and variations in its concentration can significantly influence mitochondrial and cellular performance. Prior studies have established a link between ATP levels and a variety of diseases, such as cancer, neurodegenerative conditions, ischemia, and hypoglycemia. Consequently, researchers have developed many fluorescent probes for ATP detection, recognizing the importance of monitoring intracellular ATP levels to understand cellular processes. These probes have been effectively utilized for visualizing ATP in living cells and biological samples. In this comprehensive review, we categorize fluorescent sensors developed in the last five years for ATP detection. We base our classification on fluorophores, structure, multi-response channels, and application. We also evaluate the challenges and potential for advancing new generations of fluorescence imaging probes for monitoring ATP in living cells. We hope this summary motivates researchers to design innovative and effective probes tailored to ATP sensing. We foresee imminent progress in the development of highly sophisticated ATP probes.

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.

The Role of Stabilizing Copolymer in Determining the Physicochemical Properties of Conjugated Polymer Nanoparticles and Their Nanomedical Applications

Conjugated polymer nanoparticles (CPNs) are a promising class of nanomaterials for biomedical applications, such as bioimaging, gene and drug delivery/release, photodynamic therapy (PDT), photothermal therapy (PTT), and environmental sensing. Over the past decade, many reports have been published detailing their synthesis and their various potential applications, including some very comprehensive reviews of these topics. In contrast, there is a distinct lack of overview of the role the stabilizing copolymer shells have on the properties of CPNs. This review attempts to correct this oversight by scrutinizing reports detailing the synthesis and application of CPNs stabilized with some commonly-used copolymers, namely F127 (Pluronic poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) diacrylate), PSMA (poly(styrene-co-maleic anhydride)), PLGA (poly(D, L-lactide-co-glycolide)) and PEG (polyethylene glycol) derivatives. The analysis of the reported physicochemical properties and biological applications of these CPNs provides insights into the advantages of each group of copolymers for specific applications and offers a set of guidance criteria for the selection of an appropriate copolymer when designing CPNs-based probes. Finally, the challenges and outlooks in the field are highlighted.

Red fluorescent benzothiadiazole derivative loaded in different nanoformulations: Optical properties and their use in bio-imaging

Fluorophores with optimized nonlinear optical properties have become prominent as contrast labels in laser scanning microscopy (LSM). The purpose of this work is to report on a novel benzothiadiazole derivative, namely 4,7-bis(5-((9,9-dioctyl-9H-fluoren-2-yl)ethynyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (EFBT) and its optical performance when it is loaded into organic nanostructures intended as labels for LSM. Four different nanostructured labels were prepared: i) EFBT-loaded silica nanoparticles (SiNPs); ii) folate-bioconjugated SiNPs (SiNPs-FA); iii) EFBT-loaded PEGylated nanoparticles (NPs-PEG); and iv) EFBT-loaded folate-terminated PEGylated nanoparticles (NPs-PEG-FA). All these nanostructures are reported through a comparative study of their linear and nonlinear optical properties, including their performance as exogenous label agents in the cervical cancer cell line HeLa. This assessment of the performance of a specific fluorophore loaded into different nanostructured matrices (labels), and fairly compared under the same characterization conditions, including the LSM settings, is less common while previous reports had focused in comparing silica and PEGylated nanoparticles but loaded with different fluorophores. The results show that the internal molecular organization into each type of organic nanostructure impacted differently the properties of EFBT, where the silica matrix tend to preserve the optical performance of the fluorophore by preventing intermolecular interactions; in contrast, PEGylated nanoparticles favored molecular interactions and introduced non-radiative decay channels that degrades drastically the optical performance. Nevertheless, the use of functionalized ends entities produced a better cellular label uptake with PEGylated that with silica nanoparticles. In overall, the NPs-PEG-FA label produced the best HeLa imaging.

Molecularly Imprinted Nanomaterials with Stimuli Responsiveness for Applications in Biomedicine

The review aims to summarize recent reports of stimuli-responsive nanomaterials based on molecularly imprinted polymers (MIPs) and discuss their applications in biomedicine. In the past few decades, MIPs have been proven to show widespread applications as new molecular recognition materials. The development of stimuli-responsive nanomaterials has successfully endowed MIPs with not only affinity properties comparable to those of natural antibodies but also the ability to respond to external stimuli (stimuli-responsive MIPs). In this review, we will discuss the synthesis of MIPs, the classification of stimuli-responsive MIP nanomaterials (MIP-NMs), their dynamic mechanisms, and their applications in biomedicine, including bioanalysis and diagnosis, biological imaging, drug delivery, disease intervention, and others. This review mainly focuses on studies of smart MIP-NMs with biomedical perspectives after 2015. We believe that this review will be helpful for the further exploration of stimuli-responsive MIP-NMs and contribute to expanding their practical applications especially in biomedicine in the near future.

Recent Advances in Biomedical Applications of Polymeric Nanoplatform Assisted with Two-Photon Absorption Process

Polymers are well-recognized carriers useful for delivering therapeutic drug and imaging probes to the target specified in the defined pathophysiological site. The functional drug molecules and imaging agents were chemically attached or physically loaded in the carrier polymer matrix via cleavable spacers. Using appropriate targeting moieties, these polymeric carriers (PCs) loaded with functional molecules were designed to realize target-specific delivery at the cellular level. The biodistribution of these carriers can be tracked using imaging agents with suitable imaging techniques. The drug molecules can be released by cleaving the spacers either by endogenous stimuli (e.g., pH, redox species, glucose level and enzymes) at the targeted physiological site or exogenous stimuli (e.g., light, electrical pulses, ultrasound and magnetism). Recently, two-photon absorption (2PA)-mediated drug delivery and imaging has gained significant attention because TPA from near-infrared light (700-950 nm, NIR) renders light energy similar to the one-photon absorption from ultraviolet (UV) light. NIR has been considered biologically safe unlike UV, which is harmful to soft tissues, cells and blood vessels. In addition to the heat and reactive oxygen species generating capability of 2PA molecules, 2PA-functionalized PCs were also found to be useful for treating diseases such as cancer by photothermal and photodynamic therapies. Herein, insights attained towards the design, synthesis and biomedical applications of 2PA-activated PCs are reviewed. In particular, specific focus is provided to the imaging and drug delivery applications with a special emphasis on multi-responsive platforms.

A zwitterionic red-emitting water-soluble conjugated polymer with high resistance to nonspecific binding for two-photon cell imaging and good singlet oxygen production capability

A zwitterionic red-emitting water-soluble conjugated polymer exhibited better two-photon cell imaging and singlet oxygen production capability than its cationic analogue.

Defects and defect engineering in Soft Matter

Soft matter covers a wide range of materials based on linear or branched polymers, gels and rubbers, amphiphilic (macro)molecules, colloids, and self-assembled structures. These materials have applications in various industries, all highly important for our daily life, and they control all biological functions; therefore, controlling and tailoring their properties is crucial. One way to approach this target is defect engineering, which aims to control defects in the material's structure, and/or to purposely add defects into it to trigger specific functions. While this approach has been a striking success story in crystalline inorganic hard matter, both for mechanical and electronic properties, and has also been applied to organic hard materials, defect engineering is rarely used in soft matter design. In this review, we present a survey on investigations on defects and/or defect engineering in nine classes of soft matter composed of liquid crystals, colloids, linear polymers with moderate degree of branching, hyperbranched polymers and dendrimers, conjugated polymers, polymeric networks, self-assembled amphiphiles and proteins, block copolymers and supramolecular polymers. This overview proposes a promising role of this approach for tuning the properties of soft matter.

Cell Membrane-Specific Fluorescent Probe Featuring Dual and Aggregation-Induced Emissions

A cell membrane-specific fluorescent probe was prepared by conjugating a coumarin dye with a tetraphenylethene (TPE) derivative through an α,β-unsaturated ketone connection. The probe has two absorptions: one from the TPE moiety at 300 nm and a second one due to the coumarin moiety at 458.5 nm. The probe fluoresces at 470 nm in tetrahydrofuran (THF) solution. The probe exhibits a useful aggregation-induced emission (AIE) property. A gradual increase in the water content of a THF solution causes a significant decrease and 12 nm red shift in the fluorescence peak at 470 nm, giving rise to a new strong fluorescence peak at 591 nm at a 95% water content. The probe is hydrophobic with an AIE property and binds to cell membranes, resulting in 591 nm fluorescence upon implantation into cells. The probe possesses a long retention time despite the lack of a long, cell membrane-anchored hydrophobic alkyl chain, which is typical for traditional membrane-specific probes. Our probe also displays low cytotoxicity and excellent photostability.

Conjugated polymers as nanoparticle probes for fluorescence and photoacoustic imaging

In this review, the role of conjugated polymer nanoparticles (CPNs) in emerging bioimaging techniques is described.

When the strategies for cellular selectivity fail. Challenges and surprises in the design and application of fluorescent benzothiadiazole derivatives for mitochondrial staining

Design, synthesis, molecular architecture and the unexpected behavior of fluorescent benzothiadiazole for selective mitochondrial and plasma membrane staining are investigated.

Red emitting conjugated polymer based nanophotosensitizers for selectively targeted two-photon excitation imaging guided photodynamic therapy

Two-photon excitation (2PE) photodynamic therapy (PDT) is a non-invasive technique for the treatment of cancer. However, its clinical applications are limited by small two-photon absorption cross section values of conventional photosensitizers. Here we designed multifunctional conjugated polymer based nanoparticles consisting of a conjugated polymer, a photosensitizer and a red-emitting dye, which can realize simultaneous 2PE red emission imaging and 2PE-PDT activities. The working principle is based on a 2PE fluorescence resonance energy transfer strategy from the conjugated polymer to photosensitizing and imaging agents. In these nanoparticles (NPs), the conjugated polymer, PPBF, was chosen as a two-photon light-harvesting material while the photosensitizer (tetraphenylporphyrin, TPP) and the red-emitting dye (TPD) were chosen as energy acceptors. The 2PE emission of TPP and TPD was enhanced by up to ∼161 and ∼23 times, respectively. The 2PE-PDT activity of these NPs was significantly improved compared with those NPs without PPBF by up to ∼149 times. Further surface-functionalization with folic acid (FA) groups allows these nanoparticles to exhibit selective affinity toward KB cancer cells. These NPs could act as novel 2PE conjugated polymer based nanoparticles combined with the advantages of low dark cytotoxicity, selective targeting and imaging-guided 2PE-PDT activities.

Styrylpyridine salts-based red emissive two-photon turn-on probe for imaging the plasma membrane in living cells and tissues

Based on styrylpyridine salts, a small-molecule red emitting membrane probe with large two-photon absorption cross-section has been synthesized. As a molecular rotor, it enables exclusive lighting up of the plasma membrane in live cells and particular tissues. This probe has the potential to be a powerful tool for bioimaging.

A high brightness probe of polymer nanoparticles for biological imaging

Conjugated polymer nanoparticles (CPNs) with high brightness in long wavelength region were prepared by the nano-precipitation method. Based on fluorescence resonance energy transfer (FRET) mechanism, the high brightness property of the CPNs was realized by four different emission polymers. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) displayed that the CPNs possessed a spherical structure and an average diameter of ~75nm. Analysis assays showed that the CPNs had excellent biocompatibility, good photostability and low cytotoxicity. The CPNs were bio-modified with a cell penetrating peptide (Tat, a targeted element) through covalent link. Based on the entire wave fluorescence emission, the functionalized CPNs1-4 can meet multichannel and high throughput assays in cell and organ imaging. The contribution of the work lies in not only providing a new way to obtain a high brightness imaging probe in long wavelength region, but also using targeted cell and organ imaging.

Controlling photophysical properties of ultrasmall conjugated polymer nanoparticles through polymer chain packing

Applications of conjugated polymer nanoparticles (Pdots) for imaging and sensing depend on their size, fluorescence brightness and intraparticle energy transfer. The molecular design of conjugated polymers (CPs) has been the main focus of the development of Pdots. Here we demonstrate that proper control of the physical interactions between the chains is as critical as the molecular design. The unique design of twisted CPs and fine-tuning of the reprecipitation conditions allow us to fabricate ultrasmall (3.0–4.5 nm) Pdots with excellent photostability. Extensive photophysical and structural characterization reveals the essential role played by the packing of the polymer chains in the particles in the intraparticle spatial alignment of the emitting sites, which regulate the fluorescence brightness and the intraparticle energy migration efficiency. Our findings enhance understanding of the relationship between chain interactions and the photophysical properties of CP nanomaterials, providing a framework for designing and fabricating functional Pdots for imaging applications.

Synthesis of small conjugated polymer nanoparticles (Pdots) with bright and stable fluorescence is an active challenge. Here, the authors introduce a strategy to fabricate ultrasmall Pdots with high fluorescence intensity by using twisted, rather than planar, conjugated polymers, lending new insight into the molecular design of Pdots.

Preparation of Sialic Acid-Imprinted Fluorescent Conjugated Nanoparticles and Their Application for Targeted Cancer Cell Imaging

Fluorescent conjugated polymer nanoparticles have attracted great interest for applications in biological imaging owing to their excellent optical properties and low cytotoxicity; however, a lack of effective targeting limits their use. In this work, we design and synthesize a fluorescent conjugated polymer modified with a phenylboronic acid group, which can covalently bind with cis-diol-containing compounds, such as sialic acid (SA), by forming a cyclic ester. However, the obtained conjugated polymer nanoparticles failed to discriminate between cancer cells, with or without SA overexpressed surfaces (such as DU 145 and HeLa cells, respectively). To address this problem, we introduced SA template molecules into the polymer nanoparticles during the reprecipitation process and then removed the template by adjusting the solution pH. The SA-imprinted nanoparticles showed a uniform size around 30 nm and enhanced fluorescence intensity compared with unmodified polymer nanoparticles. The SA-imprinted nanoparticles exhibited selective staining for DU 145 cancer cells and did not enter HeLa cells even after long incubation times. Thus, we present a facile method to prepare fluorescent nanoparticles for applications in targeted cancer cell imaging.

Organic Dots Based on AIEgens for Two-Photon Fluorescence Bioimaging

Two-photon fluorescence imaging technique is a powerful bioanalytical approach in terms of high photostability, low photodamage, high spatiotemporal resolution. Recently, fluorescent organic dots comprised of organic emissive cores and a polymeric matrix are emerging as promising contrast reagents for two-photon fluorescence imaging, owing to their numerous merits of high and tunable fluorescence, good biocompatibility, strong photobleaching resistance, and multiple surface functionality. The emissive core is crucial for organic dots to get high brightness but many conventional chromophores often encounter a severe problem of fluorescence quenching when they form aggregates. To solve this problem, fluorogens featuring aggregation-induced emission (AIE) can fluoresce strongly in aggregates, and thus become ideal candidates for fluorescent organic dots. In addition, two-photon absorption property of the dots can be readily improved by just increase loading contents of AIE fluorogen (AIEgen). Hence, organic dots based on AIEgens have exhibited excellent performances in two-photon fluorescence in vitro cellular imaging, and in vivo vascular architecture visualization of mouse skin, muscle, brain and skull bone. In view of the rapid advances in this important research field, here, we highlight representative fluorescent organic dots with an emissive core of AIEgen aggregate, and discuss their great potential in bioimaging applications.

Nanomaterial-based activatable imaging probes: from design to biological applications

Activatable imaging probes as alternatives to "always on" imaging probes have attracted more and more attention due to their improved sensitivity and specificity. They are commonly designed to amplify or boost imaging signals only in response to specific biomolecular recognition or interaction. Thus, the design strategies play a vital role in the fabrication of activatable imaging probes. In this review, we focus on the design mechanisms and biological applications of those nanomaterial-based activatable imaging probes reported in the past five years, benefitting greatly from the good development of nanotechnology. These probes not only include the most studied activatable fluorescence imaging probes, but also cover more activatable MR imaging probes based on nanoparticle contrast agents and activatable photoacoustic imaging probes, providing more bases for clinical translation.

Sensitive Conjugated-Polymer-Based Fluorescent ATP Probes and Their Application in Cell Imaging

Three cationic conjugated polyelectrolytes (CPEs) with a common poly(p-phenylene ethynylene terthiophene) backbone and side chains of different lengths, named as PPET3-N1, PPET3-N2, and PPET3-N3, were designed and synthesized. The UV-vis absorption and fluorescence spectra of the polymers vary strongly with solvent composition, suggesting that the polymers are strongly aggregated in H2O. In addition, the spectroscopic properties of the polymers are affected by small-molecule ATP, characterized by significant fluorescence intensity decreases and red shifts of their absorption bands. Further application of these polymers in cell imaging was studied by confocal fluorescence microscopy, which demonstrated that all of the polymers were localized on the cell membrane and partially inside of cells and that the staining effect gradually increased with the length of the polymer side chains. On the basis of the low cytotoxicity and efficient quenching of PPET3-N2 by ATP, the dose and time effects of ATP on PPET3-N2 imaging were studied, and the results indicated that this polymer might have potential in cell imaging for ATP semiquantification in vivo.

Photostable far-red emitting pluronic silicate nanoparticles: perfect blood pool fluorophores for biphotonic in vivo imaging of the leaky tumour vasculature

A new non-diffusible fluorescent probe for two photon microscopy, comprising a hydrophobic push-pull dye in the apolar core of Pluronic F127–silica nanoparticles, shows intense red emission (Φ_f 39% at 650 nm) and two-photon absorption properties in the NIR.

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.

Conjugated Polymer-Based Hybrid Nanoparticles with Two-Photon Excitation and Near-Infrared Emission Features for Fluorescence Bioimaging within the Biological Window

Hybrid fluorescent nanoparticles (NPs) capable of fluorescing near-infrared (NIR) light (centered ∼730 nm) upon excitation of 800 nm laser light were constructed. A new type of conjugated polymer with two-photon excited fluorescence (TPEF) feature, P-F8-DPSB, was used as the NIR-light harvesting component and the energy donor while a NIR fluorescent dye, DPA-PR-PDI, was used as the energy acceptor and the NIR-light emitting component for the construction of the fluorescent NPs. The hybrid NPs possess δ value up to 2.3 × 10(6) GM per particle upon excitation of 800 nm pulse laser. The excellent two-photon absorption (TPA) property of the conjugated polymer component, together with its high fluorescence quantum yield (ϕ) up to 45% and the efficient energy transfer from the conjugated polymer to NIR-emitting fluorophore with efficiency up to 90%, imparted the hybrid NPs with TPEF-based NIR-input-NIR-output fluorescence imaging ability with penetration depth up to 1200 μm. The practicability of the hybrid NPs for fluorescence imaging in Hela cells was validated.

Biocompatible Green and Red Fluorescent Organic Dots with Remarkably Large Two-Photon Action Cross Sections for Targeted Cellular Imaging and Real-Time Intravital Blood Vascular Visualization

Fluorescent organic dots are emerging as promising bioimaging reagents because of their high brightness, good photostability, excellent biocompatibility, and facile surface functionalization. Organic dots with large two-photon absorption (TPA) cross sections are highly desired for two-photon fluorescence microscopy. In this work, we report two biocompatible and photostable organic dots fabricated by encapsulating tetraphenylethene derivatives within DSPE-PEG matrix. The two organic dots show absorption maxima at 425 and 483 nm and emit green and red fluorescence at 560 and 645 nm, with high fluorescence quantum yields of 64% and 22%, respectively. Both organic dots exhibit excellent TPA property in the range of 800-960 nm, affording upon excitation at 820 nm remarkably large TPA cross sections of 1.2×10(6) and 2.5×10(6) GM on the basis of dot concentration. The bare fluorophores and their organic dots are biocompatible and have been used to stain living cells for one- and two-photon fluorescence bioimagings. The cRGD-modified organic dots can selectively target integrin αvβ3 overexpressing breast cancer cells for targeted imaging. The organic dots are also applied for real-time two-photon fluorescence in vivo visualization of the blood vasculature of mouse ear, providing the spatiotemporal information about the whole blood vascular network. These results demonstrate that the present fluorescent organic dots are promising candidates for living cell and tissue imaging.

Unraveling the effect of polymer dots doping in inverted low bandgap organic solar cells

In this study, molecular doping with polymer dots was designed to unravel its effect on the photoconductivity in organic solar cells. The photocurrent in organic solar cells exhibited a considerable increase under optimal doping concentration, leading to an ultimate enhancement of power conversion efficiency from 2.30% to 3.64%. This can be attributed primarily to the improvement of the initial boost in charge carriers due to the background carriers induced by the polymer dots and increased tail absorption by the active layer. Based on single carrier device and impedance measurements, polymer dopant can efficiently decrease charge recombination and improve charge carriers mobilities. The obtained achievements pave an approach of molecular doping in affecting the operation of organic solar cells.