Conjugated Polyelectrolytes as Light‐Up Macromolecular Probes for Heparin Sensing

Two cationic poly(fluorene‐alt‐benzothiadiazole)s with different side chains are designed and synthesized. Both polymers show low fluorescence in aqueous solution due to the charge‐transfer character of the polymer's excited states. Fluorescence turn‐on biosensors for heparin detection and quantification are developed, taking advantage of complexation‐induced aggregation, which increases the polymer fluorescence in aqueous solution. It is found that good polymer water‐solubility is beneficial to the sensitivity and fluorescence contrast of the heparin turn‐on sensor as a result of the low fluorescence background. Moreover, stronger complexation between the polymer/heparin leads to a substantially larger fluorescence increase in the presence of heparin relative to that in the presence of its analog, hyaluronic acid (HA), allowing discrimination of heparin from HA. Heparin quantification with a practical calibration range covering the whole therapeutic dosing levels (0.2–8 U mL⁻¹) is realized based on the polymer with good water‐solubility. This investigation provides a new insight for designing conjugated polymers with a light‐up signature for biomolecular sensing.     

Highly Fluorescent Conjugated Polyelectrolyte Nanostructures: Synthesis,Self‐Assembly,and Al3+ Ion Sensing

A highly fluorescent triazine‐bridged polymer, poly[(diphenylamino‐s‐triazine)‐co‐(2‐methoxy‐5‐propyloxysulfonate‐1,4‐phenylene vinylene)] (DTMSPV), is synthesized from Wittig polycondensation of a triazine monomer with a water‐soluble p‐phenylene vinylene monomer. The fluorescent amphiphilic polymer in aqueous solution self‐assembled into nanoassemblies of micelle‐like nanostructure (MS) and π stacking nanostructure (πS), which have average sizes of 93 to 270 nm, depending on the concentration of DTMSPV. The micelle‐like nanostructure of DTMSPV (MS) shows blue emission at 457 and 488 nm with a high emission quantum yield (Φ_E) of 31% in aqueous solution. On the other hand, the Φ_E of π stacking structures (πS), formed in a highly concentrated solution, is lower than the MS. The MS exhibits fluorescence quenching as well as color change from blue to green/yellow, depending on the kinds of metal ions. The metal ion sensitivity is larger in the order of the main group ions (Na⁺, K⁺) < dicationic transition metal ions (Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Pd²⁺) < trivalent transition metal ions (Fe³⁺, Ru³⁺), with an exception of Al³⁺. In particular, the fluorescence of MS is dramatically quenched with color change to yellow in response to Al³⁺ concentrations. The selectivity and sensitivity of MS to Al³⁺ are unusually high even in the presence of competitive metal ions, which can be attributed to the specific interaction of triazine units with Al³⁺.     

Nanocarrier‐Mediated Codelivery of Small Molecular Drugs and siRNA to Enhance Chondrogenic Differentiation and Suppress Hypertrophy of Human Mesenchymal Stem Cells

Cartilage loss is a leading cause of disability among adults, and effective therapy remains elusive. Human mesenchymal stem cells (hMSCs), which have demonstrated self‐renewal and multipotential differentiation, are a promising cell source for cartilage repair. However, the hypertrophic differentiation of the chondrogenically induced MSCs and resulting tissue calcification hinders the clinical translation of MSCs for cartilage repair. Here, a multifunctional nanocarrier based on quantum dots (QDs) is developed to enhance chondrogenic differentiation and suppress hypertrophy of hMSCs simultaneously. Briefly, the QDs are modified with β‐cyclodextrin (β‐CD) and RGD peptide. The resulting nanocarrier is capable of carrying hydrophobic small molecules such as kartogenin in the hydrophobic pockets of conjugated β‐CD to induce chondrogenic differentiation of hMSCs. Meanwhile, via electrostatic interaction the conjugated RGD peptides bind the cargo siRNA targeting Runx2, which is a key regulator of hMSC hypertrophy. Furthermore, due to the excellent photostability of QDs, hMSCs labeled with the nanocarrier can be tracked for up to 14 d after implantation in nude mice. Overall, this work demonstrates the potential of our nanocarrier for inducing and maintaining the chondrogenic phenotype and tracking hMSCs in vivo.     

Ultrastable and Biocompatible NIR‐II Quantum Dots for Functional Bioimaging

Fluorescence bioimaging in the second near‐infrared spectral region (NIR‐II, 1000–1700 nm) can provide advantages of high spatial resolution and large penetration depth, due to low light scattering. However, NIR‐II fluorophores simultaneously possessing high brightness, good stability, and biocompatibility are very rare. Hydrophobic NIR‐II emissive PbS@CdS quantum dots (QDs) are surface‐functionalized, via a silica and amphiphilic polymer (Pluronic F‐127) dual‐layer coating method. The as‐synthesized PbS@CdS@SiO₂@F‐127 nanoparticles (NPs) are aqueously dispersible and possess a quantum yield of ≈5.79%, which is much larger than those of most existing NIR‐II fluorophores. Thanks to the dual‐layer protection, PbS@CdS@SiO₂@F‐127 NPs show excellent chemical stability in a wide range of pH values. The biocompatibility of PbS@CdS@SiO₂@F‐127 NPs is studied, and the results show that the toxicity of the NPs in vivo could be minimal. PbS@CdS@SiO₂@F‐127 NPs are then utilized for in vivo and real‐time NIR‐II fluorescence microscopic imaging of mouse brain. The architecture of blood vessels is visualized and the imaging depth reaches 950 µm. Furthermore, in vivo NIR‐II fluorescence imaging of gastrointestinal tract is achieved, by perfusing PbS@CdS@SiO₂@F‐127 NPs into mice at a rather low dosage. This work illustrates the potential of ultrastable, biocompatible, and bright NIR‐II QDs in biomedical and clinical applications, which require deep tissue imaging.     

A Fluorescent Chemosensor for Long-Term Tracking of Cancer Cell Metastasis and Invasion via Enzyme-Activated Anchoring

Evidence shows that the enzyme human cytochrome P450 1A1 (CYP1A1) is associated with cancer; indeed, it is shown to play a key role in the occurrence of many cancers. Therefore, the molecular imaging of CYP1A1 in cells is of great significance for revealing the process of cancer development. Herein, a chemosensor, DCBEM, is reported that is able to selectively recognize CYP1A1 and achieve long-term labeling of the enzyme through an enzymolysis cascade reaction. The design of DCBEM relies on the reaction between the highly electrophilic intermediate (quinone methide) and the enzyme, which forms a fluorescent label with CYP1A1 via covalent bonding. The chemosensor reveals excellent specificity toward CYP1A1 and achieves high-resolution monitoring of cell migration with a strong retention capacity in vivo (up to 4 days). Further, using DCBEM, the pathways of invasion of colon cancer and breast cancer cells are successfully visualized in living mice. This method of labeling enzymes provides a simple and efficient way to render ordinary fluorescent chemosensors suitable for the long-term tracking of cancer cells, for which such molecular tools are currently lacking.     

Dopamine‐Modified Cationic Conjugated Polymer as a New Platform for pH Sensing and Autophagy Imaging

A dopamine‐modified conjugated polymer PFPDA is synthesized and characterized. At low pH, dopamine exists in its hydroquinone form and lacks the ability to quench fluorescence. At high pH, the proportion of the quinone form of dopamine increases due to its autooxidation, and efficient intramolecular electron transfer from the polymer main chain to quinone occurs, resulting in the quenching of the fluorescence of PFPDA. Thus, PFPDA exhibits a fluorescence “turn‐on” response at low pH. PFPDA possesses excellent photostability and exhibits no cytotoxicity, which makes it a good fluorescent material for pH sensing and cell imaging. A light‐induced hydroxyl anion emitter, MGCB, is also used to change the pH of the solution and thus regulate the fluorescence of PFPDA via remote control under light irradiation. Because the cytoplasm becomes acidic when cell autophagy occurs, PFPDA can also be used for autophagy imaging of HeLa cells with good selectivity.     

Highly Emissive,Water‐Repellent,Soft Materials: Hydrophobic Wrapping and Fluorescent Plasticizing of Conjugated Polyelectrolyte via Electrostatic Self‐Assembly

Sulfonated poly(diphenylacetylene) (SPDPA) is used as an anionic conjugated polyelectrolyte to examine stoichiometric electrostatic self‐assembly with homologous cationic surfactants (octadecyl)_X(methyl)_Y ammonium bromides (O_XM_YABs) having different numbers of long hydrophobic tails. The SPDPA–O_XM_YAB complexes formed show significantly increased water contact angle and enhanced fluorescence (FL) emissions compared with the pristine SPDPA. The complexes exist in a gum state at room temperature owing to the plasticizer effect of the hydrophobic tails, hence they are very soft and highly stretchable. The hydrophobicity, softness, and FL quantum efficiency of the SPDPA–O_XM_YAB complexes increase as the number of hydrophobic tails increases. SPDPA adsorbs uniformly onto filter papers to produce fluorescent papers. The SPDPA‐adsorbed papers have many unique applications, including FL image writing, fingerprinting, stamping, and inkjet printing using the surfactant solutions as an ink to reveal high‐resolution FL images. In particular, multideposit inkjet‐printing using SPDPA and O_XM_YAB solutions as inks produces water‐resistant, embedded figures in paper currency.     

Fluorescent Protein Nanovessels: A New Platform to Generate Bio–Abiotic Hybrid Materials for Bioimaging

In this report, a new platform to generate fluorescent protein nanovessels is described. Based on systemic analyses and reconstitution experiments, a combination of protein scaffold and organic dye is identified. Briefly, certain proteins such as bovine serum albumin (BSA) can rapidly form cube‐like scaffold upon heating. This protein scaffolds intrinsically interact with nonfluorescent dyes such as bromophenol blue (BPB), forming BSA‐BPB nanocubes (BBNCs). Moreover, it turns out that the commercially available dye BPB contains aggregation‐induced emission (AIE) properties, allowing the BBNCs emissive upon irradiation. The fluorescent protein nanovessels are highly biocompatible and can be readily internalized by different type of cells. The fluorescent signal of the materials is well‐penetrable from mouse tissues and can be detected at near‐infrared region, making it a useful tool for various biological imaging studies. This platform for making fluorescent protein nanovessels is green, rapid, and cost‐effective and can be extended to other protein scaffolds and possibly other dye/AIE molecules.     

Multi‐Charged Conjugated Polyelectrolytes as a Versatile Work Function Modifier for Organic Electronic Devices

Despite the excellent work function adjustability of conjugated polyelectrolytes (CPEs), which induce a vacuum level shift via the formation of permanent dipoles at the CPE/metal electrode interface, the exact mechanism of electron injection through the CPE electron transport layer (ETL) remains unclear. In particular, understanding the ionic motion within the CPE ETLs when overcoming the sizable injection barrier is a significant challenge. Because the ionic functionality of CPEs is a key component for such functions, a rigorous analysis using highly controlled ion density (ID) in CPEs is crucial for understanding the underlying mechanism. Here, by introducing a new series of CPEs with various numbers of ionic functionalities, energy level tuning at such an interface can be determined directly by adjusting the ID in the CPEs. More importantly, these series CPEs indicate that two different mechanisms must be invoked according to the CPE thickness. The formation of permanent interfacial dipoles is critical with respect to electron injection through CPE ETL (≤ 10 nm, quantum mechanical tunneling limit), whereas electron injection through thick CPE ETL (20–30 nm) is dominated by the reorientation of the ionic side chains under a given electric field.     

Conjugated Polymer Nanodots as Ultrastable Long‐Term Trackers to Understand Mesenchymal Stem Cell Therapy in Skin Regeneration

Stem cell–based therapies hold great promise in providing desirable solutions for diseases that cannot be effectively cured by conventional therapies. To maximize the therapeutic potentials, advanced cell tracking probes are essential to understand the fate of transplanted stem cells without impairing their properties. Herein, conjugated polymer (CP) nanodots are introduced as noninvasive fluorescent trackers with high brightness and low cytotoxicity for tracking of mesenchymal stem cells (MSCs) to reveal their in vivo behaviors. As compared to the most widely used commercial quantum dot tracker, CP nanodots show significantly better long‐term tracking ability without compromising the features of MSCs in terms of proliferation, migration, differentiation, and secretome. Fluorescence imaging of tissue sections from full‐thickness skin wound–bearing mice transplanted with CP nanodot‐labeled MSCs suggests that paracrine signaling of the MSCs residing in the regenerated dermis is the predominant contribution to promote skin regeneration, accompanied with a small fraction of endothelial differentiation. The promising results indicate that CP nanodots could be used as next generation of fluorescent trackers to reveal the currently ambiguous mechanisms in stem cell therapies through a facile and effective approach.     

Gadolinium‐Doped Persistent Nanophosphors as Versatile Tool for Multimodal In Vivo Imaging

Recent breakthroughs in the rational development of multifunctional nanocarriers have highlightened the advantage of combining the complementary forces of several imaging modalities into one single nanotool fully dedicated to the biomedical field and diagnosis applications. A novel multimodal optical‐magnetic resonance imaging nanoprobe is introduced. Designed on the basis of a spinel zinc gallate structure doped with trivalent chromium and gadolinium, this nanocrystal bears the ability to serve as both a highly sensitive persistent luminescence nanoprobe for optical imaging, and a negative contrast agent for highly resolved magnetic resonance imaging (MRI). Additional proof is given that surface coverage can be modified in order to obtain stealth nanoparticles highly suitable for real‐time in vivo application in mice, showing delayed reticulo‐endothelial uptake and longer circulation time after systemic injection.     

Ultrasensitive,Biocompatible, Quantum‐Dot‐Embedded Silica Nanoparticles for Bioimaging

The successful development of highly sensitive, water‐compatible, nontoxic nanoprobes has allowed nanomaterials to be widely employed in various applications. The applicability of highly bright quantum dot (QD)‐based probes consisting of QDs on 120 nm silica nanoparticles (NPs) with silica shells is investigated. Their substantial merits, such as their brightness and biocompatibility, for effective bioimaging are demonstrated. Silica‐coated, QD‐embedded silica NPs (Si@QDs@Si NPs) containing QDs composed of CdSe@ZnS (core‐shell) are prepared to compare their structure‐based advantages over single QDs that have a similar quantum yield (QY). These Si@QDs@Si NPs exhibit approximately 200‐times stronger photoluminescence (PL) than single QDs. Cytotoxicity studies reveal that the Si@QDs@Si NPs are less toxic than equivalent numbers of silica‐free single quantum dots. The excellence of the Si@QDs@Si NPs with regard to in vivo applications is illustrated by significantly enhanced fluorescence signals from Si@QDs@Si‐NP‐tagged cells implanted in mice. Notably, a more advanced version of QD‐based silica NPs (Si@mQDs@Si NPs), containing multishell quantum dots (mQDs) composed of CdSe@CdS@ZnS, are prepared without significant loss of QY during surface modification. In addition, the Si@mQDs@Si NPs display a fivefold higher fluorescence activity than the Si@QDs@Si NPs. As few as 400 units of Si@mQDs@Si‐ NP‐internalized cells can be detected in the cell‐implanted mouse model.     

Generic Method of Preparing Multifunctional Fluorescent Nanoparticles Using Flash NanoPrecipitation

There is increased demand for nanoparticles with a high fluorescence yield that have the desired excitation wavelength, surface functionalization, and particle size to act as biological probes. Here, a simple, rapid, and robust method, Flash NanoPrecipitation (FNP), to produce such fluorescent nanoparticles is described. This process involves encapsulation of a hydrophobic fluorophore with an amphiphilic biocompatible diblock copolymer in a kinetically frozen state. FNP is used to produce nanoparticles ranging from 30 to 800 nm with fluorescence emission peaks ranging from, but not limited to, 370 nm to 720 nm. Such fluorescent nanoparticles remain stable in aqueous solutions, and, in contrast to soluble dyes, show no photobleaching. Fluorophores and drugs are incorporated into a single nanoparticle, allowing for simultaneous drug delivery and biological imaging. In addition, functionalization of nanoparticle surfaces with disease‐specific ligands permits precise cell targeting. These features make FNP‐produced fluorescent nanoparticles highly desirable for various biological applications.     

Cationic Conjugated Polyelectrolytes with Molecular Spacers for Efficient Fluorescence Energy Transfer to Dye‐Labeled DNA

Two water‐soluble conjugated polyelectrolytes, poly(9,9′‐bis(6‐N,N,N‐trimethylammoniumhexyl)fluorene‐alt‐1,4‐(2,5‐bis(6‐N,N,N‐trimethylammoniumhexyloxy))phenylene) tetrabromide ( P1i ) and poly((10,10′‐bis(6‐N,N,N‐trimethylammoniumhexyl)‐10H‐spiro(anthracene‐9,9′‐fluorene))‐alt‐1,4‐(2,5‐bis(6‐N,N,N‐trimethylammoniumhexyloxy))phenylene) tetrabromide ( P2i ) are synthesized, characterized, and used in fluorescence resonance energy transfer (FRET) experiments with fluorescein‐labeled single‐stranded DNA (ssDNA‐Fl). P1i and P2i have nearly identical π‐conjugated backbones, as determined by cyclic voltammetry and UV‐vis spectroscopy. The main structural difference is the presence of an anthracenyl substituent, orthogonal to the main chain in each of the P2i repeat units, which increases the average interchain separation in aggregated phases. It is possible to observe emission from ssDNA‐Fl via FRET upon excitation of P2i . Fluorescein is not emissive within the ssDNA‐Fl/ P1i electrostatic complex, suggesting Fl emission quenching through photoinduced charge transfer (PCT). We propose that the presence of the anthracenyl “molecular bumper” in P2i increases the distance between optical partners, which decreases PCT more acutely relative to FRET.     

Electrostatic Self‐Assembly Conjugated Polyelectrolyte‐Surfactant Complex as an Interlayer for High Performance Polymer Solar Cells

A simple method is demonstrated to improve the film‐forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely‐charged materials provides the driving force to form a stable CPE‐surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk‐heterojunction (BHJ) films, e.g. poly(3‐hexylthiophene):[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer‐based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, ～20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer‐based BHJ systems.     

Conjugated Polymer Nanoparticles for Imaging,Cell Activity Regulation,and Therapy

Conjugated polymer nanoparticles, considered as a class of promising nanomaterials for biomedical application, are extensively employed in bioimaging, anti‐microorganism and antitumor, gene and drug delivery/release in the past decade. By virtue of unique photoelectric properties, such as strong light absorbing quality, high brightness, good photostability, tunable spectra property, and favorable compatibility, conjugated polymer nanoparticles attract increasing attention and are used in more emerging aspects in biological and biomedical fields. This review summarizes the recent (2014–2018) development of conjugated polymer nanoparticles, including design, synthesis, and biomedical applications. Especially, their abilities of bioimaging, cell activity regulation, anti‐microorganism and antitumor therapy are discussed in detail. Finally, the challenges and outlooks in the field are highlighted.