Self-Reporting Conjugated Polymer Nanoparticles for Superoxide Generation and Detection

Conjugated polymer nanoparticles (CPNs or Pdots) have become increasingly popular fluorophores for multimodal applications that combine imaging with phototherapeutic effects. Reports of CPNs in photodynamic therapy applications typically focus on their ability to generate singlet oxygen. Alternatively, CPN excited states can interact with oxygen to form superoxide radical anion and a CPN-based hole polaron, both of which can have deleterious effects on fluorescence properties. Here, we demonstrate that CPNs prepared from the common conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)] (PFBT, also known as F8BT) generate superoxide upon irradiation. We use the same CPNs to detect superoxide by doping them with a superoxide-responsive hydrocyanine dye developed by Murthy and co-workers. Superoxide induces off-to-on fluorescence switching by converting quenching hydrocyanine dyes to fluorescent cyanine dyes that act as fluorescence resonance energy transfer (FRET) acceptors for PFBT chromophores. Amplified FRET from the multichromophoric CPNs yields fluorescence signal intensities that are nearly 50 times greater than when the dye is excited directly or over 100 times greater when signal readout is from the CPN channel. The dye loading level governs the maximum amount of superoxide that induces a change in fluorescence properties and also influences the rate of superoxide generation by furnishing competitive excited state deactivation pathways. These results suggest that CPNs can be used to deliver superoxide in applications in which it is desirable and provide a caution for fluorescence-based CPN applications in which superoxide can damage fluorophores.

Acceleration of the thermal back-reaction and the finding of a non-photochromic isomer for a negative photochromic binaphthyl-bridged imidazole dimer

The development of visible or near-infrared (NIR) light-responsive fast photoswitchable molecules for the real-time, non-contact control of physical and chemical properties has received increased attention because of the non-invasive features to materials and biological tissues. We report a new molecular design to accelerate the thermal back-reaction of the negative photochromic binaphthyl-bridged imidazole dimer, BN-ImD. We also found that irradiation of the BN-ImD derivative with methyl groups on the bridging binaphthyl unit with visible light produced an unprecedented photoreaction product with a unique eight-membered ring structure. These observations provide fascinating clues for the future development of fast negative photochromic molecules.

Facile and wide-range size tuning of conjugated polymer nanoparticles for biomedical applications as a fluorescent probe

Conjugated polymer nanoparticles (Pdots) are expected to be novel bioimaging and sensing probes. However, the size tuning required to control biological interactions has not been well established. Herein, we achieved a size-tunable synthesis of Pdots ranging from 30 to 200 nm by controlling the hydrolysis rate of the stabilising agent and evaluated their cellular imaging properties.

Programmable Assembly of π-Conjugated Polymers

π-Conjugated polymers have numerous applications due to their advantageous optoelectronic and mechanical properties. These properties depend intrinsically on polymer ordering, including crystallinity, orientation, morphology, domain size, and π-π interactions. Programming, or deliberately controlling the composition and ordering of π-conjugated polymers by well-defined inputs, is a key facet in the development of organic electronics. Here, π-conjugated programming is described at each stage of material development, stressing the links between each programming mode. Covalent programming is performed during polymer synthesis such that complex architectures can be constructed, which direct polymer assembly by governing polymer orientation, π-π interactions, and morphological length-scales. Solution programming is performed in a solvated state as polymers dissolve, aggregate, crystallize, or react in solution. Solid-state programming occurs in the solid state and is governed by polymer crystallization, domain segregation, or gelation. Recent progress in programming across these stages is examined, highlighting order-dependent features and assembly techniques that are unique to π-conjugated polymers. This should serve as a guide for delineating the many ways of directing π-conjugated polymer assembly to control ordering, structure, and function, enabling the further development of organic electronics.

Red-Emitting, Acene-Doped Conjugated Polymer Nanoparticles that Respond Ratiometrically to Photogenerated 1O2

Fluorophores that respond to external stimuli on demand have numerous applications in imaging and chemical or biological sensing. In this paper, we describe conjugated polymer nanoparticles (CPNs) that comprise a donor polymer matrix and a red-fluorescent, singlet oxygen-reactive heteroacene dopant (DE-TMT) that display a ratiometric response upon photo-oxidation. This ratiometric response can be tuned by the level of doping of DE-TMT, the identity of the conjugated polymer matrix used, and the blending of two conjugated polymers together to access red-shifted emission wavelengths. We followed a rational design process that combined (i) fundamental understanding of the influence of the chemical structure on luminescence spectra and efficiencies, energy transfer efficiencies, and reactivity and (ii) systematically determining how blending multiple chromophores in nanoparticles influences energy transfer efficiencies and the speed of optical responses to irradiation. Our approach of refining the compositions of these nanoparticles has yielded materials that combine many desirable characteristics for analytical applications-utility in aqueous environments, high quantum yield, emission of red light, and ratiometric luminescent responses. We anticipate that the type of approach described herein can be of use to others in designing CPNs for luminescence applications.

Engineering fluorescent semiconducting polymer nanoparticles for biological applications and beyond

We summarize the recent advances in engineering approaches to obtain functionalized semiconducting polymer nanoparticles (SPNs) for biological applications. The challenges and outlook of fabricating functionalized SPNs are also provided.

Dual-Mode Superresolution Imaging Using Charge Transfer Dynamics in Semiconducting Polymer Dots

In a conjugated polymer-based single-particle heterojunction, stochastic fluctuations of the photogenerated hole population lead to spontaneous fluorescence switching. We found that 405 nm irradiation can induce charge recombination and activate the single-particle emission. Based on these phenomena, we developed a novel class of semiconducting polymer dots that can operate in two superresolution imaging modes. The spontaneous switching mode offers efficient imaging of large areas, with <10 nm localization precision, while the photoactivation/deactivation mode offers slower imaging, with further improved localization precision (ca. 1 nm), showing advantages in resolving small structures that require high spatial resolution. Superresolution imaging of microtubules and clathrin-coated pits was demonstrated, under both modes. The excellent localization precision and versatile imaging options provided by these nanoparticles offer clear advantages for imaging of various biological systems.

Photodynamic Inactivation of ESKAPE Group Bacterial Pathogens in Planktonic and Biofilm Cultures Using Metallated Porphyrin-Doped Conjugated Polymer Nanoparticles

Photodynamic inactivation (PDI) protocols using photoactive metallated porphyrin-doped conjugated polymer nanoparticles (CPNs) and blue light were developed to eliminate multidrug-resistant pathogens. CPNs-PDI protocols using varying particle concentrations and irradiation doses were tested against nine pathogenic bacterial strains including antibiotic-resistant bacteria of the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens group. The bactericidal effect was achieved in methicillin-resistant Staphylococus aureus (S. aureus) strains using low light doses (9.6-14.4 J/cm²), while Gram-negative bacteria required a higher light dose (28.8 J/cm²). The bacteria-CPN interaction was studied through flow cytometry, taking advantage of the intrinsic CPN fluorescence, demonstrating that CPNs efficiently bind to the bacterial envelope. Finally, the performance of CPNs-PDI was explored in biofilms; good antibiofilm ability and almost complete eradication were observed for S. aureus and Escherichia coli biofilms, respectively, using confocal microscopy. Overall, we demonstrated that CPNs-PDI is an efficient tool not only to kill superbugs as sessile cells but also to disrupt and eradicate biofilms of highly relevant pathogenic bacterial species.

Turn-On Mode Fluorescence Switch by Using Negative Photochromic Imidazole Dimer

The development of fluorescence switchable molecules in several polar and apolar environments has been required for fluorescence imaging of nanostructures. Photochromic molecules are an important class for the reversible light-triggered fluorescence switching. Although many studies of fluorescence switching by using photochromic reactions have been reported, the report of photochromic molecules reversibly showing turn-on mode fluorescence switching has been limited in spite of their importance. Herein, we report the photoactivatable fluorescence based on negative photochromism, where the absorption spectrum of the compound after irradiation is blue-shifted relative to that before irradiation. We introduced naphthalimide units as a green fluorophore to the negative photochromic binaphthyl-bridged imidazole dimer. The fluorescence of the naphthalimide unit is efficiently quenched in the initial colored isomer (fluorescence quantum yield: Φ_fluo. = 0.01) by Förster resonance energy transfer. In contrast, the fluorescence quantum yield increases up to 0.75 in the transient isomer formed by the negative photochromic reaction. The fluorescence intensity thermally decreases with the thermal back reaction to form the original stable colored form. These results indicate that the negative photochromic molecules are suitable for turn-on mode fluorescence switches and will give an attractive insight for the development of reversible fluorescence switching molecules.

Conjugated Polymer Nanoparticles with Appended Photo-Responsive Units for Controlled Drug Delivery, Release, and Imaging

Carriers that can afford tunable physical and structural changes are envisioned to address critical issues in controlled drug delivery applications. Herein, photo-responsive conjugated polymer nanoparticles (CPNs) functionalized with donor-acceptor Stenhouse adduct (DASA) and folic acid units for controlled drug delivery and imaging are reported. Upon visible-light (λ=550 nm) irradiation, CPNs simultaneously undergo structure, color, and polarity changes that release encapsulated drugs into the cells. The backbone of CPNs favors FRET to DASA units boosting their fluorescence. Notably, drug-loaded CPNs exhibit excellent biocompatibility in the dark, indicating perfect control of the light trigger over drug release. Delivery of both hydrophilic and hydrophobic drugs with good loading efficiency was demonstrated. This strategy enables remotely controlled drug delivery with visible-light irradiation, which sets an example for designing delivery vehicles for non-invasive therapeutics.

Involving Synergy of Green Light and Acidic Responses in Control of Unimolecular Multicolor Luminescence

Conversion of multicolor luminescence is one of desirable goals in study and development of next-generation molecular emitters, whereas involving visible light into the control of the above-mentioned ability has been poorly addressed due to the need of a relatively complicate molecular design. In this work, we present a novel dyad with a linkage of 4-piperazinyl-1,8-naphthalimide and cyanostyryl-modified azulene moiety, upon which the luminescence signal can be orthogonally controlled by protonation and green light irradiation. The superior features of the protonation induced excited state energy alteration, followed by green light driven photoisomerization led to a progressive luminescent color conversion among blue, yellow and green at the single molecular level. This strategy may bring in novel insights for preparing advanced function-integrated optoelectronic materials.

Rational Design of a Green-Light-Mediated Unimolecular Platform for Fast Switchable Acidic Sensing

A controllable sensing ability strongly connects to complex and precise events in diagnosis and treatment. However, imposing visible light into the molecular-scale mediation of sensing processes is restricted by the lack of structural relevance. To address this critical challenge, we present the rational design, synthesis, and in vitro studies of a novel cyanostyryl-modified azulene system for green-light-mediated fast switchable acidic sensing. The advantageous features of the design include a highly efficient green-light-driven Z/E-isomerization (a quantum yield up to 61.3%) for fast erasing chromatic and luminescent expressions and a superior compatibility with control of ratiometric protonation. Significantly, these merits of the design enable the development of a microfluidic system to perform a green-light-mediated reusable sensing function toward a gastric acid analyte in a miniaturized platform. The results may provide new insights for building future integrated green materials.

Dual Colorimetric and Fluorescent Authentication Based on Semiconducting Polymer Dots for Anticounterfeiting Applications

Semiconducting polymer dots (Pdots) have recently emerged as a novel type of ultrabright fluorescent probes that can be widely used in analytical sensing and material science. Here, we developed a dual visual reagent based on Pdots for anticounterfeiting applications. We first designed and synthesized two types of photoswitchable Pdots by incorporating photochromic dyes with multicolor semiconducting polymers to modulate their emission intensities and wavelengths. The resulting full-color Pdot assays showed that the colorimetric and fluorescent dual-readout abilities enabled the Pdots to serve as an anticounterfeiting reagent with low background interference. We also doped these Pdots into flexible substrates and prepared these Pdots as inks for pen handwriting as well as inkjet printing. We further applied this reagent in printing paper and checks for high-security anticounterfeiting purposes. We believe that this dual-readout method based on Pdots will create a new avenue for developing new generations of anticounterfeiting technologies.

Hollow Au-Cu Nanocomposite for Real-Time Tracing Photothermal/Antiangiogenic Therapy

High absorption in the near-infrared (NIR) region is essential for a photoabsorbing agents to realize efficient photothermal therapy (PTT) for cancer. Here, a novel hollow Au-Cu nanocomposite (HGCNs) is developed, which displays a significantly enhanced NIR surface plasmon resonance absorption and photothermal transduction efficiency. Besides, fluorescent polymer dots poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (PFBT) and chemotherapeutic mammalian target of rapamycin (mTOR) inhibitor agent rapamycin (RAPA) are attached onto the HGCNs (RAPA/PFBT-HGCNs) for real-time NIR fluorescence tracing and combined PTT/antiangiogenesis therapy. In particular, due to the fluorescence resonance energy transfer effect, RAPA/PFBT-HGCNs can act as NIR-activatable on/off probe system for real-time tracing of tumor tissues. A standard in vitro cellular uptake study, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, dual-staining study, and flow cytometry assay reveal that the RAPA/PFBT-HGCNs combined with NIR laser exhibit higher drug accumulation and cytotoxicity in both tumor cells and epithelial cells. Moreover, the margins of tumor and normal tissue can be accurately indicated by NIR-stimulated dequenched PFBT after 24 h intravenous administration. Further, tumor growth can be considerably hampered by the optimal formulation plus laser treatment with relatively lower side effects. Consequently, the work highlights the real-time tracing and enhanced PTT/antiangiogenesis therapy prospects of the established HGCNs with tremendous potential for treatment of cancer.

Photoswitchable Emission Color Change in Nanodots Containing Conjugated Polymer and Photochrome

A simple approach for the preparation of conjugated polymer (CP)-based fluorescent nanodots containing photochrome (dithienylethene, DTE) is reported. The CP in the nanodots was designed to exhibit dual emissions of blue and green. The photochromic, fluorescent, composite nanodots (PNDs) were able to tune the emission color from green to blue using selective energy transfer from the CP to DTE under ultraviolet (UV) irradiation. The UV-irradiation-induced ring closure of the DTE within the PNDs provided a spectral overlap between the green emission of the CP and the absorption of DTE, leading to quenching of the green emission and, concomitantly, maintaining of the blue emission. The photoswitchable fluorescent PNDs with high on-off green fluorescence contrast were successfully applied in a living zebrafish imaging. Our design strategy provided a versatile tool for constructing a special photomodulated color-changeable nanostructure in bioimaging.