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

Tuning aggregation of poly(3-hexylthiophene) within nanoparticles

Nanoparticles derived from π-conjugated polymers have gained widespread attention as active layer materials in various organic electronics applications. The optoelectronic, charge transfer, and charge transport properties of π-conjugated polymers are intimately connected to the polymer aggregate structure. Herein we show that the internal aggregate structure of regioregular poly(3-hexylthiophene) (P3HT) within polymer nanoparticles can be tuned by solvent composition during nanoparticle fabrication through the miniemulsion process. Using absorption spectra and single-NP photoluminescence decay properties, we show that a solvent mixture consisting of a low boiling good solvent and a high boiling marginal solvent results in polymer aggregate structure with a higher degree of uniformity and structural order. We find that the impact of solvent on the nature of P3HT aggregation within nanoparticles is different from what has been reported in thin films.

Highly emissive PEG-encapsulated conjugated polymer nanoparticles

A novel bioimaging probe based on a conjugated polymer, poly(9,9-dihexylfluorene-alt-2,1,3-benzoxadiazole) (PFBD), is demonstrated. Transfer of the hydrophobic polymer into water using a short chain poly(ethylene glycol) (PEG) resulted in conjugated polymer nanoparticles (PEG-PFBD) with a fluorescence quantum yield of 46%. The PEG-PFBD nanoparticles possessed several desirable structural and photophysical properties, such as colloidal stability in a broad range of pH values, sub-20 nm particle size, the presence of surface chemical functionality, as well as desirable excitation and emission spectra, for bioimaging applications. PEG-PFBD nanoparticles were conjugated with cyclic RGDfK targeting peptide for labeling of membrane α(V)β(3) integrin receptors on live HT-29 adenocarcinoma cells. Single nanoparticle microscopy revealed that the PEG-capped PFBD nanoparticles exhibit at least ten times higher emitted photon counts than single quantum dots (QD655) of comparable size. In addition, Fluorescence Lifetime Imaging Microscopy (FLIM) of single PEG-PFBD nanoparticles revealed that the nanoparticles display a clearly resolvable single nanoparticle fluorescence lifetime.

Solvent Vapor Annealing of Single Conjugated Polymer Chains: Building Organic Optoelectronic Materials from the Bottom Up

Optoelectronic devices based on organic materials show a strong relationship between the morphological structure of the material and the function of the device. One of the grand challenges in improving the efficiencies of these devices is hence achieving morphological control throughout the entire course of processing. One of the most important postprocessing methods is solvent vapor annealing, which has repeatedly demonstrated its utility in improving the efficiency of organic-material-based devices by changing bulk-film morphology. This Perspective discusses the recent impact of single-molecule spectroscopy techniques in unraveling morphological changes and molecular dynamics and presents solvent vapor annealing as a tool to build organic optoelectronic materials from the bottom up. In particular, we discuss examples of how solvent vapor annealing at the single-chain level can be split into two different regimes, (i) the solvation regime, in which intrachain interactions and molecular dynamics during solvent vapor annealing can be probed, and (ii) the aggregation regime, in which the influence of interchain interactions can be probed. Finally, it will be shown that solvent vapor annealing in the aggregation regime can be used to build highly ordered mesoscopic objects with distinct properties such as long-range energy transfer.

Fluorescent conjugated block copolymer nanoparticles by controlled mixing

Monitoring of the formation of stable fluorescent nanoparticles from controlled mixing of a THF solution of poly(fluorene ethynylene)-block-poly(ethylene glycol) in a microfluidic laminar flow crossjunction by spatially resolved fluorescence spectroscopy reveals the time scale of particle formation as well as incorporation of small molecule guests and the role of solvent mixing.

Unconventional Kondo effect in redox active single organic macrocyclic transistors

Cyclo[6]- and cyclo[8]pyrrole, two aromatic expanded porphyrins, were studied in a single-molecule transistor (SMT) setup. The analyses of these compounds allowed us to observe an uncommon absence of an even-odd effect in the Kondo resonance in discrete, metal-free organic macrocyclic compounds. The findings from the SMT measurements of these cyclopyrroles were in accord with those from cyclic voltammetry (CV) studies and theoretical analyses. These findings provide support for the notion that SMT measurements could be useful as a tool for the characterization of similar types of aromatic macrocyclic compounds.

Self-assembly of highly ordered conjugated polymer aggregates with long-range energy transfer

Applications of conjugated polymers (CP) in organic electronic devices such as light-emitting diodes and solar cells depend critically on the nature of electronic energy transport in these materials. Single-molecule spectroscopy has revealed their fundamental properties with molecular detail, and recent reports suggest that energy transport in single CP chains can extend over extraordinarily long distances of up to 75 nm. An important question arises as to whether these characteristics are sustained when CP chains agglomerate into a neat solid. Here, we demonstrate that the electronic energy transport in aggregates composed of tens of polymer chains takes place on a similar distance scale as that in single chains. A recently developed molecular-level understanding of solvent vapour annealing has allowed us to develop a technique to control the CP agglomeration process. Aggregates with volumes of at least 45,000 nm(3) (molecular weight ≈ 21 MDa) maintain a highly ordered morphology and show pronounced fluorescence blinking behaviour, indicative of substantially long-range energy transport. Our findings provide a new lens through which the ordering of single CP chains and the evolution of their morphological and optoelectronic properties can be observed, which will ultimately enable the rational design of improved CP-based devices.

Silica-shell cross-linked micelles encapsulating fluorescent conjugated polymers for targeted cellular imaging

A bioinspired silification approach was successfully used to encapsulate fluorescent conjugated polymers inside silica-shell cross-linked polymeric micelles (CP-SSCL) in the highly benign synthesis environment of room temperature and near-neutral aqueous environment. Four different conjugated polymers were employed to demonstrate the versatility of the bioinspired silification, resulting in the formation of CP-SSCL with different emission wavelengths across the visible spectrum. The CP-SSCL are characterized by a large absorption coefficient and high quantum yield, indicating that they exhibit the required high fluorescence brightness for cellular imaging application. In addition, the CP-SSCL also exhibit a high colloidal stability and low cytotoxicity. The in vitro studies of using MDA-MB-231 breast cancer cells show that the CP-SSCL are successfully uptaken by the cancer cells and located at the cytoplasm of the cells. Furthermore, by conjugating folic acid on their surfaces, the uptake of CP-SSCL by MDA-MB-231 cells was enhanced significantly, suggesting their great potential for targeted imaging and early detection of cancer cells.

Green fluorescent organic nanoparticles by self-assembly induced enhanced emission of a naphthalene diimide bolaamphiphile

Naphthalene diimide (NDI) bolaamphiphilic molecules (1) self-assemble in water to form organic nanoparticles, which exhibit self-assembly induced preassociated excimer formation and hence an enhanced green fluorescence.

Photosensitizer-doped conjugated polymer nanoparticles with high cross-sections for one- and two-photon excitation

We report a novel nanoparticle that is promising for photodynamic therapy applications, which consists of a π-conjugated polymer doped with a singlet oxygen photosensitizer. The nanoparticles exhibit highly efficient collection of excitation light due to the large excitation cross-section of the polymer. A quantum efficiency of singlet oxygen production of 0.5 was determined. Extraordinarily large two-photon excitation cross-sections were determined, indicating promise for near infrared multiphoton photodynamic therapy. Gel electrophoresis of DNA after near-UV irradiation in the presence of nanoparticles indicated both purine base and backbone DNA damage.

Thermally-Polymerized Rylene Nanoparticles

Rylene dyes functionalized with varying numbers of phenyl trifluorovinylether (TFVE) moieties were subjected to a thermal emulsion polymerization to yield shape-persistent, water-soluble chromophore nanoparticles. Perylene and terrylene diimide derivatives containing either two or four phenyl TFVE functional groups were synthesized and subjected to thermal emulsion polymerization in tetraglyme. Dynamic light scattering measurements indicated that particles with sizes ranging from 70 - 100 nm were obtained in tetraglyme, depending on monomer concentration. The photophysical properties of individual monomers were preserved in the nanoemulsions and emission colors could be tuned between yellow, orange, red, and deep red. The nanoparticles were found to retain their shape upon dissolution into water and the resulting water suspensions displayed moderate to high fluorescence quantum yield.

Incorporating functionalized polyethylene glycol lipids into reprecipitated conjugated polymer nanoparticles for bioconjugation and targeted labeling of cells

We report a simple and rapid method to prepare extremely bright, functionalized, stable, and biocompatible conjugated polymer nanoparticles incorporating functionalized polyethylene glycol (PEG) lipids by reprecipitation. These nanoparticles retain the fundamental spectroscopic properties of conjugated polymer nanoparticles prepared without PEG lipid, but demonstrate greater hydrophilicity and quantum yield compared to unmodified conjugated polymer nanoparticles. The sizes of these nanoparticles, as determined by TEM, were 21-26 nm. Notably, these nanoparticles were prepared with several PEG lipid functional end groups, including biotin and carboxy moieties that can be easily conjugated to biomolecules. We have demonstrated the availability of these end groups for functionalization using the interaction of biotin PEG lipid conjugated polymer nanoparticles with streptavidin. Biotinylated PEG lipid conjugated polymer nanoparticles bound streptavidin-linked magnetic beads, while carboxy and methoxy PEG lipid modified nanoparticles did not. Similarly, biotinylated PEG lipid conjugated polymer nanoparticles bound streptavidin-coated glass slides and could be visualized as diffraction-limited spots, while nanoparticles without PEG lipid or with non-biotin PEG lipid end groups were not bound. To demonstrate that nanoparticle functionalization could be used for targeted labelling of specific cellular proteins, biotinylated PEG lipid conjugated polymer nanoparticles were bound to biotinylated anti-CD16/32 antibodies on J774A.1 cell surface receptors, using streptavidin as a linker. This work represents the first demonstration of targeted delivery of conjugated polymer nanoparticles and demonstrates the utility of these new nanoparticles for fluorescence based imaging and sensing.

Preparation of high-quality organic semiconductor nanoparticle films by solvent-evaporation-induced self-assembly

Organic semiconductor nanoparticles are expected to be used in organic optical and electronic devices due to their unique optical and electrical properties. However, no method has been reported for the preparation of high-quality organic nanoparticle films without remaining additives and being capable of dealing with binary nanoparticle blends. We developed a simple approach to fabricate high-quality organic semiconductor nanoparticle films from their aqueous solutions by solvent-evaporation-induced self-assembly. Only volatile solvents are employed in the nanoparticle solutions, so the self-assembled nanoparticle films are free of additives. Moreover, this method is also suitable for fabricating thin films containing binary nanoparticles. Therefore, it paves the way for potential applications of organic semiconductor nanoparticles in nanoscale optical and electronic devices.

Bioconjugation of ultrabright semiconducting polymer dots for specific cellular targeting

Semiconducting polymer dots (Pdots) represent a new class of ultrabright fluorescent probes for biological imaging. They exhibit several important characteristics for experimentally demanding in vitro and in vivo fluorescence studies, such as their high brightness, fast emission rate, excellent photostability, nonblinking, and nontoxic feature. However, controlling the surface chemistry and bioconjugation of Pdots has been a challenging problem that prevented their widespread applications in biological studies. Here, we report a facile yet powerful conjugation method that overcomes this challenge. Our strategy for Pdot functionalization is based on entrapping heterogeneous polymer chains into a single dot, driven by hydrophobic interactions during nanoparticle formation. A small amount of amphiphilic polymer bearing functional groups is co-condensed with the majority of semiconducting polymers to modify and functionalize the nanoparticle surface for subsequent covalent conjugation to biomolecules, such as streptavidin and immunoglobulin G (IgG). The Pdot bioconjugates can effectively and specifically label cellular targets, such as cell surface marker in human breast cancer cells, without any detectable nonspecific binding. Single-particle imaging, cellular imaging, and flow cytometry experiments indicate a much higher fluorescence brightness of Pdots compared to those of Alexa dye and quantum dot probes. The successful bioconjugation of these ultrabright nanoparticles presents a novel opportunity to apply versatile semiconducting polymers to various fluorescence measurements in modern biology and biomedicine.

Mechanism of cellular uptake of highly fluorescent conjugated polymer nanoparticles

Conjugated polymer nanoparticles are formed by precipitation of highly fluorescent conjugated polymers to form small nanoparticles with extremely bright fluorescence. We characterized cellular uptake and cytotoxicity of 18 ± 5 nm PFBT conjugated polymer nanoparticles in J774A.1 cells. Significant nanoparticle uptake was observed, indicating efficient nanoparticle entry into cells, even for short (1 h) incubations. The high fluorescence of these nanoparticles allows extremely low loading concentrations; PFBT nanoparticle fluorescence in cells could be detected with loading concentrations of 155 pM (270 ppb). Cellular uptake slows at low temperature, consistent with endocytic entry. Nanoparticles colocalize with Texas Red dextran and are trafficked to lysosomes, as demonstrated by the location of nanoparticle fluorescence in perinuclear organelles that also stain with an anti-LAMP-1 antibody. Inhibition of uptake by phosphoinositide 3-kinase inhibitors implicates macropinocytosis as the operative endocytic mechanism. No significant cytotoxic or inflammatory effects could be observed, making PFBT nanoparticles attractive probes for live cell imaging.

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.

Fluorescent nanoparticles based on self-assembled pi-conjugated systems

pi-Conjugated molecules are interesting components to prepare fluorescent nanoparticles. From the use of polymer chains that form small aggregates in water to the self-assembly of small chromophoric segments into highly ordered structures, the preparation of these materials allows to develop systems with applications as sensors or biolabels. The potential functionalization of the nanoparticles can lead to specific probing. This progress report describes the recent advances in the preparation of such emittive organic nanoparticles.

Colloidal and optical stability of PEG-capped and phospholipid-encapsulated semiconducting polymer nanospheres in different aqueous media

Aqueous dispersions of fluorescent semiconducting polymer nanospheres (SPNs) have been synthesised by two methods; miniemulsion and micellar encapsulation. The colloidal and optical stability of SPNs synthesised by these two methods has been compared in order to assess the potential of these fluorescent nanoparticles for use in biological applications. The SPNs were dispersed in water, phosphate buffer solution (PBS) and bovine serum albumin (BSA). The optical stability was studied by absorption and emission spectroscopy, and the colloidal stability was studied by dynamic light scattering (DLS) over a one month period. The results indicate that the micelle-encapsulated SPNs exhibit favourable optical and colloidal stability, and seem promising for use in biological sciences.

Conjugated polymer nanoparticles

Conjugated polymer nanoparticles are highly versatile nano-structured materials that can potentially find applications in various areas such as optoelectronics, photonics, bio-imaging, bio-sensing and nanomedicine. Their straightforward synthesis in desired sizes and properties, biocompatibility and non-toxicity make these materials highly attractive for the aforementioned applications. This feature article reviews the recent developments in the synthesis, characterization, properties and application of these exciting nanostructured materials.

Nanoscale 3D tracking with conjugated polymer nanoparticles

Small ( approximately 15 nm diameter), highly fluorescent conjugated polymer nanoparticles were evaluated for nanoscale 2D and 3D tracking applications. Nanoparticles composed of conjugated polymers possess high absorption cross sections, high radiative rates, and low or moderate aggregation quenching, resulting in extraordinarily high fluorescent brightness. The bright fluorescence ( approximately 200 000 photons detected per particle per 20 ms exposure) yields a theoretical particle tracking uncertainty of less than 1 nm. A lateral tracking uncertainty of 1-2 nm was determined from analysis of trajectories of fixed and freely diffusing particles. Axial (Z) position information for 3D particle tracking was obtained by defocused imaging. Nanoscale tracking of single particles in fixed cells was demonstrated, and a range of complex behaviors, possibly due to binding/unbinding dynamics, were observed.

Non-radiative resonance energy transfer in bi-polymer nanoparticles of fluorescent conjugated polymers

This work demonstrates the comparative studies of non-radiative resonance energy transfer in bi-polymer nanoparticles based on fluorescent conjugated polymers. For this purpose, poly[(9,9-dihexylfluorene) (PF) as a donor (D) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an acceptor (A) have been utilized, from which four different bi-polymer nanoparticle systems are designed and synthesized. Both, steady-state fluorescence spectra and time-resolved fluorescence measurements indicate varying energy transfer efficiencies from the host polymer PF to the acceptor polymer MEH-PPV depending on the D-A distances and structural properties of the nanoparticles. The first approach involves the preparation of PF and MEH-PPV nanoparticles separately and mixing them at a certain ratio. In the second approach, first PF and MEH-PPV solutions are mixed prior to nanoparticle formation and then nanoparticles are prepared from the mixture. Third and fourth approaches involve the sequential nanoparticle preparation. In the former, nanoparticles are prepared to have PF as a core and MEH-PPV as a shell. The latter is the reverse of the third in which the core is MEH-PPV and the shell is PF. The highest energy transfer efficiency recorded to be 35% is obtained from the last system, in which a PF layer is sequentially formed on MEH-PPV NPs.

Synthesis and self-assembly of polymer and polymer-coated ag nanoparticles by the reprecipitation of binary mixtures of polymers

Binary polymer nanoparticles were synthesized by the reprecipitation of poly(4-vinylpyridine) in the presence of poly(diallyldimethylammonium chloride) and further used to make polymer-coated Ag nanoparticles. Polymer shells around Ag nanoparticles were formed by two methods: the reduction of Ag(2)O in the presence of the polymer nanoparticles and by mixing the polymer nanoparticles with already-made Ag nanoparticles. The resulting nanoparticles were coated with layers of the two polymers with the hydrophilic polymer on the outside providing their stability in water. The exposure of the polymer-coated Ag nanoparticles to unmodified Ag nanoparticles resulted in spontaneous self-assembly due to the electrostatic attraction. The polymer-coated nanoparticles and the nanoparticle assemblies were characterized by UV-vis, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy.

Polarization portraits of single multichromophoric systems: visualizing conformation and energy transfer

A novel technique, two-dimensional (2D) polarization single-molecule imaging, is presented. It is based on measurements and analysis of fluorescence intensity as a function of excitation and emission polarization angles. The technique allows recording of full information on the steady-state polarization properties of fluorescent objects. It is particularly suitable for application to single multichromophoric systems (molecules or nanoparticles) with energy transfer (ET) between different chromophores (e.g., single fluorescent pi-conjugated polymer chains). The 2D polarization data simultaneously provide information on the conformation of the system and the efficiency of its internal excitation ET. The technique is used to characterize single chains and different kinds of chain aggregates of different conjugated polymers at different temperatures. The 2D polarization measurements reveal a dramatic difference in ET taking place in these systems. Clear temperature dependence of ET is observed for individual aggregates as well as for their statistical ensembles. Also, a dependence on solvent and aggregate size is shown. Additionally, extensive "traditional one-dimensional" polarization results on the polarization anisotropy of fluorescence excitation and emission are presented. These results and findings are discussed in relation to internal organization of the nano-objects under study.

Coherent intrachain energy migration in a conjugated polymer at room temperature

The intermediate coupling regime for electronic energy transfer is of particular interest because excitation moves in space, as in a classical hopping mechanism, but quantum phase information is conserved. We conducted an ultrafast polarization experiment specifically designed to observe quantum coherent dynamics in this regime. Conjugated polymer samples with different chain conformations were examined as model multichromophoric systems. The data, recorded at room temperature, reveal coherent intrachain (but not interchain) electronic energy transfer. Our results suggest that quantum transport effects occur at room temperature when chemical donor-acceptor bonds help to correlate dephasing perturbations.

What can be learned from single molecule spectroscopy? Applications to sol-gel-derived silica materials

Single molecule spectroscopic methods are now being widely employed to probe the nanometer scale properties of sol-gel-derived silica materials. This article reviews a subset of the recent literature in this area and provides salient examples of the new information that can be obtained. The materials covered include inorganic and organically-modified silica, along with surfactant-templated mesoporous materials. Studies of molecule-matrix interactions based on ionic, hydrogen bonding and hydrophobic interactions are reviewed, highlighting the impacts of these interactions on mass transport phenomena. Quantitative investigations of molecular diffusion by single molecule tracking and fluorescence correlation spectroscopy are also covered, focusing on the characterization of anisotropic and hindered diffusion in mesoporous systems. Single molecule polarity studies are described and the new information that can be obtained from the resulting inhomogeneous distributions is discussed. Likewise, single molecule studies of silica acidity properties are reviewed, including observation of nanoscale buffering phenomena due to the chemistry of surface silanols. Finally, related single nanoparticle studies of macroporous silicas are also discussed.

Quantum efficiency enhancement in film by making nanoparticles of polyfluorene

We report on conjugated polymer nanoparticles of polyfluorene that were formed to exhibit higher fluorescence quantum efficiency in film (68%) and reduce undesired emission peak wavelength shifts in film (by 20 nm), compared to the solid-state polymer thin film made directly out of the same polymer solution without forming nanoparticles. Using the facile reprecipitation method, solutions of poly[9,9-dihexyl-9H-fluorene] in THF were added at different volume ratios to obtain different size distributions of nanoparticle dispersions in water. This allowed us to control the sizedependent optical emission of our polyfluorene nanoparticles. Such organic nanoparticles hold great promise for use as efficient emitters in optoelectronic device applications.