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.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

Expansion Microscopy with Multifunctional Polymer Dots

Expansion microscopy (ExM) provides nanoscale resolution on conventional microscopes via physically enlarging specimens with swellable polyelectrolyte gels. However, challenges involving fluorophore degradation and dilution during sample expansion have yet to be overcome. Herein, sequential cellular targeting, gel anchoring, and high-fidelity fluorescence reported using multifunctional polymer dots (Pdots) designed for ExM applications are demonstrated. The impressive brightness of the Pdots facilitates multicolor ExM, thereby enabling visualization of a variety of subcellular structures and neuron synapses. The average fluorescence intensities of Pdots in ExM range from ≈3 to 6 times higher than those achieved using commercially available Alexa dyes. Moreover, the fluorescence brightness and optical fluctuation are significantly improved by a surfactant-containing expansion buffer, which enables further resolution enhancement via super-resolution optical fluctuation imaging (SOFI). The combination of ExM and SOFI allows subcellular structures of ≈30 nm to be resolved by conventional microscopes. These results highlight the immense potential of multifunctional Pdots for ExM-enhanced super-resolution imaging.

Superresolution mapping of energy landscape for single charge carriers in plastic semiconductors

The performance of conjugated polymer devices is largely dictated by charge transport processes. However, it is difficult to obtain a clear relationship between conjugated polymer structures and charge transport properties, due to the complexity of the structure and the dispersive nature of charge transport in conjugated polymers. Here, we develop a method to map the energy landscape for charge transport in conjugated polymers based on simultaneous, correlated charge carrier tracking and single-particle fluorescence spectroscopy. In nanoparticles of the conjugated polymer poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)], two dominant chain conformations were observed, a blue-emitting phase (λ_max = 550 nm) and a red-emitting phase (λ_max = 595 nm). Hole polarons were trapped within the red phase, only occasionally escaping into the blue phase. Polaron hopping between the red-emitting traps was observed, with transition time ranging from tens of milliseconds to several seconds. These results provide unprecedented nanoscale detail about charge transport at the single carrier level.

To understand the complex nanoscale structure-property relationships in conjugated polymers for device applications, new methods for tracking charge transport are required. Here, the authors employ superresolution mapping to study the charge carrier dynamics in conjugated polymer nanoparticles.

Enhancing the photostability of poly(phenylene ethynylene) for single particle studies

Enhanced photostability of conjugated polyelectrolytes achieved by using anti-fading agents opens the way for advanced single molecule fluorescence imaging studies.

Direct-Write Optical Patterning of P3HT Films Beyond the Diffraction Limit

Doping-induced solubility control is a patterning technique for semiconducting polymers, which utilizes the reduction in polymer solubility upon p-type doping to provide direct, optical control of film topography and doping level. In situ direct-write patterning and imaging are demonstrated, revealing sub-diffraction-limited topographic features. Photoinduced force microscopy shows that doping level can be optically modulated with similar resolution.

Tracking of single charge carriers in a conjugated polymer nanoparticle

The motion of individual charge carriers in organic nanostructures was tracked by fluorescence microscopy. A twinkling effect is observed in fluorescence microscopy of single conjugated polymer nanoparticles, that is, small displacements in the fluorescence spot of single nanoparticles of the conjugated polymer PFBT are observed over time. There is evidence that superquenching by the charge carrier induces a dark spot in the nanoparticle, which moves with the carrier, resulting in the observed displacements in the fluorescence. Zero-field mobilities of individual charge carriers consistent with highly trapped polarons were obtained from tracking experiments.

Amplified energy transfer in conjugated polymer nanoparticle tags and sensors

Nanoparticles primarily consisting of π-conjugated polymers have emerged as extraordinarily bright fluorescent tags with potential applications in biological imaging and sensing. As fluorescent tags, conjugated polymer nanoparticles possess a number of advantageous properties, such as small particle size, extraordinary fluorescence brightness, excellent photostability, and high emission rate. Exciton diffusion occurring in the nanoparticles results in amplified energy transfer, doubling the energy transfer efficiency in some cases. Amplified energy transfer has been exploited to obtain highly red-shifted emission, oxygen-sensing nanoparticles, and fluorescence photoswitching. Additional observed phenomena are attributable to amplified energy transfer in conjugated polymers, including superquenching by metal nanoparticles, and fluorescence modulation by hole polarons. This feature article presents an overview of recent investigations of optical properties and energy transfer phenomena of this relatively novel type of fluorescent nanoparticle with a viewpoint towards demanding fluorescence-based imaging and sensing applications.

Collective fluorescence blinking in linear J-aggregates assisted by long-distance exciton migration

Fluorescence blinking corresponding to collective quenching of up to 100 dye monomers is reported for individual J-aggregates of a perylene bisimide (PBI) dye. This implies an exciton diffusion length up to 70 nm in these one-dimensional assemblies. The number of quenched monomers was directly measured by comparing the fluorescence brightness of the J-aggregates with that of noncoupled PBI molecules. This brightness analysis technique is useful for unraveling photophysical parameters of any individual fluorescent nanosystem.

Multicolor conjugated polymer dots for biological fluorescence imaging

Highly fluorescent conjugated polymer dots were developed for demanding applications such as fluorescence imaging in live cells. These nanoparticles exhibit small particle diameters, extraordinary fluorescence brightness, and excellent photostability. Single particle fluorescence imaging and kinetic studies indicate much higher emission rates (approximately 10(8) s(-1)) and little or no blinking of the nanoparticles as compared to typical results for single dye molecules and quantum dots. Analysis of single particle photobleaching trajectories reveals excellent photostabilityas many as 10(9) or more photons emitted per nanoparticle prior to irreversible photobleaching. The superior figures of merit of these new fluorescent probes, together with the demonstration of cellular imaging, indicate their enormous potential for demanding fluorescence-based imaging and sensing applications such as high speed super-resolution single molecule/particle tracking and highly sensitive assays.

Charging and discharging of single conjugated-polymer nanoparticles

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

Excited State and Charge Photogeneration Dynamics in Conjugated Polymers

Conjugated polymers are becoming interesting materials for a range of optoelectronic applications. However, their often complex electronic and structural properties prevent establishment of straightforward property-function relationships. In this paper, we summarize recent results on the photophysics and excited state dynamics of conjugated polymers, in order to paint a picture of exciton formation, quenching, and generation of charge carriers.

Size-Dependent Spectroscopic Properties of Conjugated Polymer Nanoparticles

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

Single Molecule Nanoparticles of the Conjugated Polymer MEH−PPV, Preparation and Characterization by Near-Field Scanning Optical Microscopy

We have developed a straightforward method for producing a stable, aqueous suspension of hydrophobic, fluorescent pi-conjugated polymer nanoparticles consisting primarily of individual conjugated polymer molecules. Features of the method are the facile preparation, purity, unique optical properties, and small size (approximately 5-10 nm) of the resulting nanoparticles. The results of TEM, scanning force microscopy, and near-field scanning optical microscopy of particles cast from the suspension indicate that the particles are single conjugated polymer molecules. The NSOM results yield estimates of the optical cross-sections of individual conjugated polymer molecules. The UV-vis absorption spectra of the nanoparticle suspensions indicate a reduction in conjugation length attributed to deformations of the polymer backbone. Fluorescence spectra of the aqueous nanoparticle suspensions indicate interactions between segments of the polymer chain and intramolecular energy transfer.