Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation EFermi/N1/3, predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon—the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.

/pdf/highly-fluorescent-noble-metal-quantum-dots-52tlt17wo4.pdf

Highly fluorescent noble-metal quantum dots.

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 (∼108 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 photostability—as many as 109 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.

/pdf/multicolor-conjugated-polymer-dots-for-biological-171mggvv2d.pdf

Multicolor Conjugated Polymer Dots for Biological Fluorescence Imaging

The water-soluble, near-IR-emitting DNA-encapsulated silver nanocluster presented herein exhibits extremely bright and photostable emission on the single-molecule and bulk levels. The photophysics have been elucidated by intensity-dependent correlation analysis and suggest a heavy atom effect of silver that rapidly depopulates an excited dark level before quenching by oxygen, thereby conferring great photostability, very high single-molecule emission rates, and essentially no blinking on experimentally relevant time scales (0.1 to >1,000 ms). Strong antibunching is observed from these biocompatible species, which emit >109 photons before photobleaching. The significant dark-state quantum yield even enables bunching from the emissive state to be observed as a dip in the autocorrelation curve with only a single detector as the dark state precludes emission from the emissive level. These species represent significant improvements over existing dyes, and the nonpower law blinking kinetics suggest that these very small species may be alternatives to much larger and strongly intermittent semiconductor quantum dots.

https://www.pnas.org/content/pnas/104/31/12616.full.pdf

Strongly emissive individual DNA-encapsulated Ag nanoclusters as single-molecule fluorophores

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.

Amplified energy transfer in conjugated polymer nanoparticle tags and sensors

Single-molecule spectroscopy can provide insight into the fundamental photophysics of large macromolecules containing tens of thousands of carbon atoms by circumventing disorder broadening. We apply this technique to comparatively ordered ladder-type poly(para-phenylene) and highly disordered poly(phenylenevinylene) (PPV), both of which are materials of substantial technological interest. Identical spectroscopic features are observed on the single-chromophore level, independent of the chemical structure or the chain morphology. Both materials exhibit narrow fluorescence lines down to 0.5 nm wide, which we attribute to the single-chromophore zero-phonon line, accompanied by a discrete vibronic progression providing a signature of the chemical structure. The chromophore units display spectral diffusion, giving rise to dynamic disorder on the scale of the linewidth. Although the energetic range of spectral diffusion is small, it can influence intramolecular excitation energy transfer and thus the overall molecular emission. The spectral diffusion dynamics of single chromophores are identical in both material systems and follow a universal Gaussian distribution. In the case of emission from multiple chromophores situated on the molecule, which we observe for PPV, spectral diffusion follows Lorentzian-like statistics. The fundamental difference between the two materials is the possibility of coherent interchromophoric coupling in PPV, resulting in strong spectral broadening caused by aggregation or superradiance. Such behavior is absent in the ladder-type polymers, where the linewidth of the emissive species is identical for all molecules. Our results demonstrate that structure-property correlations in conjugated polymers derive mainly from chain morphology rather than chromophoric properties and should be considered extrinsic in nature.

https://www.pnas.org/content/pnas/101/41/14695.full.pdf

A universal picture of chromophores in π-conjugated polymers derived from single-molecule spectroscopy

We report the observation of uniformly oriented transition moments perpendicular to the support substrate in single molecules of a conjugated polymer (MEH-PPV) isolated by ink-jet printing techniques. Fluorescence imaging combined with atomic force microscopy and polarization modulation studies, supported by molecular mechanics simulation, provides compelling evidence of polymer nanoparticle (single-molecule) structures with an extraordinary degree of intramolecular order. This is a general technique for preparing oriented nanostructures from structurally similar polymers.

Oriented Nanostructures from Single Molecules of a Semiconducting Polymer: Polarization Evidence for Highly Aligned Intramolecular Geometries

High-resolution fluorescence imaging of isolated nanoparticles of a common semiconducting polymer (poly[2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene, MEH-PPV), produced by ink-jet printing techniques, has revealed highly uniform transition moment orientation perpendicular to the glass substrate. In contrast with the broad emission spectra associated with bulk or single molecules of these species in thin films, we observe narrow photoluminescence emission spectra (10−15 nm fwhm) from individual oriented polymer nanostructures with no evidence of spectral diffusion on time scales of several hundred seconds. The distribution of center frequencies (from several hundred individual nanoparticle measurements) shows clearly defined peaks that can be correlated with excitonic traps of integer multiples of monomer conjugation lengths (8, 9, 10, and 11). The observation of discrete emission characteristics in this important class of materials suggests exciting possibilities in photonics and molecular optoe...

Narrow-bandwidth spontaneous luminescence from oriented semiconducting polymer nanostructures

Fluorescence intensity correlation measurements reveal that z-oriented nanostructures from single chains of a cyano-substituted polyphenylene vinylene (CN-PPV) polymer act as single-quantum emitters. Photon antibunching is observed for the first time on individual polymer nanostructures with a modulation depth exceeding 90%, providing definitive proof that radiative recombination of molecular excitons occurs at a single localized site within the folded polymer chain.

Photon antibunching from oriented semiconducting polymer nanostructures.

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Oriented Nanostructures from Single Molecules of a Semiconducting Polymer: Polarization Evidence for Highly Aligned Intramolecular Geometries

Narrow-bandwidth spontaneous luminescence from oriented semiconducting polymer nanostructures

Photon antibunching from oriented semiconducting polymer nanostructures.