Tom Vosch

Oligonucleotide-stabilized Ag nanocluster fluorophores.

Single-stranded oligonucleotides stabilize highly fluorescent Ag nanoclusters, with emission colors tunable via DNA sequence. We utilized DNA microarrays to optimize these scaffold sequences for creating nearly spectrally pure Ag nanocluster fluorophores that are highly photostable and exhibit great buffer stability. Five different nanocluster emitters have been created with tunable emission from the blue to the near-IR and excellent photophysical properties. Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.

The Rylene Colorant Family—Tailored Nanoemitters for Photonics Research and Applications

This Review summarizes the latest advances in the field of rylene dyes and rylene nanoemitters for applications in photonics, and describes the influence of the dye design on the optical properties, the self-assembly, the molecular interactions, as well as the labeling specificity of the compounds. The interplay between tailored (macro)molecular design and bulk/single-molecule spectroscopy enables complex processes to be explained, for example, the kinetics of energy-transfer processes or (bio)catalysis. Such investigations are essential for the ultimate design of optimized nanoemitters, and require a close cooperation between spectroscopists and preparative organic chemists.

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

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.

Rapid Detection of MicroRNA by a Silver Nanocluster DNA Probe

MicroRNAs (miRNAs) are regulatory small RNAs that have important roles in numerous developmental, metabolic, and disease processes of plants and animals. The individual levels of miRNAs can be useful biomarkers for cellular events or disease diagnosis. Thus, innovative new tools for rapid, specific, and sensitive detection of miRNAs are an important field of research. Using the fluorescence properties of DNA-nanosilver clusters (DNA/AgNC), we have designed a DNA/AgNC probe that can detect the presence of target miRNA. Here, we show that the red fluorescence of the DNA/AgNC probe is diminished upon the presence of target miRNA without pre- or postmodification, addition of extra enhancer molecules, and labeling. The DNA/AgNC probe emission was lowest when the complementary miRNA target was present and was significantly higher for four other control miRNA sequences. Also, when adding whole plant endogenous RNA to the DNA/AgNC probe, the emission was significantly higher for the mutant where miRNA was deficient. On the basis of these findings, we suggest that these DNA/AgNC probes could be developed into a new, simple, inexpensive, and instant technique for miRNAs detection.

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

Design aspects of bright red emissive silver nanoclusters/DNA probes for microRNA detection.

The influence of the nucleic acid secondary structure on the fast (1 h) formation of bright red emissive silver nanoclusters (AgNCs) in a DNA sequence (DNA-12nt-RED-160), designed for the detection of a microRNA sequence (RNA-miR160), was investigated. The findings show that especially the propensity for mismatch self-dimer formation of the DNA probes can be a good indicator for the creation and stabilization of red emissive AgNCs. Also, the role of the thermal stability of the secondary DNA structures (mismatch self-dimer and hairpin monomers) and the observed AgNC red emission intensity were investigated. These findings can form the basis for a rationale to design new red emissive AgNC-based probes. As an example, a bright red emissive AgNC-based DNA probe was designed for RNA-miR172 detection. The latter opens the possibility to create a variety of AgNC-based DNA probes for the specific detection of plant and animal miRNAs.

Optically modulated fluorophores for selective fluorescence signal recovery.

Fluorescence imaging in biological sciences is hindered by significant depth-dependent signal attenuation and highly fluorescent backgrounds. We have developed optically modulated near-IR-emitting few-atom Ag nanodots that are selectively and dynamically photobrightened upon simultaneous excitation with a secondary laser, enabling high-sensitivity image extraction to reveal only the demodulated fluorophores. Image demodulation is demonstrated in high-background environments to extract weak signals from completely obscuring background emission.

Characterization of fluorescence in heat-treated silver-exchanged zeolites.

Thermal treatment of Ag(+)-exchanged zeolites yields discrete highly photostable luminescent clusters without formation of metallic nanoparticles. Different types of emitters with characteristic luminescence colors are observed, depending on the nature of the cocation, the amount of exchanged silver, and the host topology. The dominant emission bands in LTA samples are situated around 550 and 690 nm for the samples with, respectively, low and high silver content, while in FAU-type materials only a broad band around 550 nm is observed, regardless of the degree of exchange. Analysis of the fluorescent properties in combination with ESR spectroscopy suggests that a Ag(6)(+) cluster with doublet electronic ground state is associated with the appearance of the 690-nm emitter, having a decay of a few hundred microseconds. Tentatively, the nanosecond-decaying 550-nm emitter is assigned to the Ag(3)(+) cluster. This new class of photostable luminescent particles with tunable emission colors offers interesting perspectives for various applications such as biocompatible labels for intracellular imaging.

Conformational rearrangements in and twisting of a single molecule

Single molecule spectroscopy is used to obtain detailed information on the photophysical properties of immobilized perylenediimide-based molecules, substituted in the bay positions. The fluorescence spectra recorded for numerous single molecules show a clear bimodal distribution of the peak position. Within the low energy component of the distribution, two different vibronic shapes of the emission spectrum can be seen, which can be correlated to different decay times. We show that former observation can be explained by conformational changes of the bay substituents while the latter are related to twisting of the single molecule around the central perylenediimide long axis.

Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric molecules

We demonstrate measurements of the efficiency of competing Förster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energy-transfer pathways have to be considered in the analysis of single-molecule trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.