PubMed Health⌕ Search

Biomedical subjects

Niek F van Hulst

Publications and source records attributed to Niek F van Hulst.

12 recordsLinked to original sources
PubMed2007

Lambda/4 resonance of an optical monopole antenna probed by single molecule fluorescence.

We present a resonant optical nanoantenna positioned at the end of a metal-coated glass fiber near-field probe. Antenna resonances, excitation conditions, and field localization are directly probed in the near field by single fluorescent molecules and compared to finite integration technique simulations. It is shown that the antenna is equivalent to its radio frequency analogue, the monopole antenna. For the right antenna length and local excitation conditions, antenna resonances occur that lead to an enhanced localized field near the antenna apex. Direct mapping of this field with single fluorescent molecules reveals a spatial localization of 25 nm, demonstrating the importance of such antennas for nanometer resolution optical microscopy.

Fluorescence↗
PubMed2006

DNA-based molecular wires: multiple emission pathways of individual constructs.

The extent of photon energy transfer through individual DNA-based molecular wires composed of five dyes is investigated at the single molecular level. Combining single-molecule spectroscopy and pulse interleaved excitation imaging, we have directly resolved the time evolution spectral response of individual constructs, while simultaneously probing DNA integrity. Our data clearly show that intact wires exhibit photon-transfer efficiencies close to 100% across five dyes. Dynamical and multiple pathways for the photon emission resulting from conformational freedom of the wire are readily uncovered. These results provide the basis for guiding the synthesis of DNA-based supramolecular arrays with improved photon transport at the nanometer scale.

DNA, Single-Stranded↗
PubMed2006

Effect of disorder on ultrafast exciton dynamics probed by single molecule spectroscopy.

We present a single-molecule study unraveling the effect of static disorder on the vibrational-assisted ultrafast exciton dynamics in multichromophoric systems. For every single complex, we probe the initial exciton relaxation process by an ultrafast pump-probe approach and the coupling to vibrational modes by emission spectra, while fluorescence lifetime analysis measures the amount of static disorder. Exploiting the wide range of disorder found from complex to complex, we demonstrate that static disorder accelerates the dephasing and energy relaxation rate of the exciton.

Journal Article↗
PubMed2006

Synthesis and characterization of long perylenediimide polymer fibers: from bulk to the single-molecule level.

The synthesis and characterization of perylenediimide polyisocyanides is reported. In addition to short oligomers, our synthetic approach results in the formation of extremely long, well-defined, and rigid perylenediimide polymers. Ordering and close-packing of the chromophores in these long polymers is guaranteed by attachment to a polyisocyanide backbone with amino acid side chains. Hydrogen bonding interactions between those groups stabilize and rigidify the helical polymer structure. The rodlike nature of the synthesized long perylenediimide pendant polyisocyanides as well as the helical arrangement of the chromophores is demonstrated by means of atomic force microscopy. Remarkably, polymer fibers up to 1 mum in length have been visualized, containing several thousands of perylenediimide molecules. Circular dichroism spectroscopy reveals the chiral organization of the chromophore units in the polymer, whereas absorption and emission measurements prove the occurrence of excited-state interactions between those moieties due to the close packing of the chromophore groups. However, an intricate optical behavior is encountered in bulk as a result of the coexistence of short oligomers and long polymers of perylenediimide, a situation subsequently uncovered by means of single-molecule experiments. Individual long helical perylenediimide polyisocyanides exhibit a typical red-shifted fluorescence spectrum, which, together with depolarized emission continuously decreasing in time, demonstrate that fluorescence arises from multiple excimer-like species in the polymer. Upon continuous irradiation of these long polymers, a fast decay in fluorescence lifetime is observed, a situation explained by photoinduced creation of quenching sites. Radical/ion formation by intramolecular electron transfer between close-by perylenediimide moieties is the most probable mechanism for this process. Appropriate control of the electron-transfer process might open the possibility of applying these polymers as perylenediimide-based supramolecular nanowires.

Anisotropy↗
PubMed2005

Power-law-distributed dark states are the main pathway for photobleaching of single organic molecules.

We exploit the strong excitonic coupling in a superradiant trimer molecule to distinguish between long-lived collective dark states and photobleaching events. The population and depopulation kinetics of the dark states in a single molecule follow power-law statistics over 5 orders of magnitude in time. This result is consistent with the formation of a radical unit via electron tunneling to a time-varying distribution of trapping sites in the surrounding polymer matrix. We furthermore demonstrate that this radicalization process forms the dominant pathway for molecular photobleaching.

Journal Article↗
PubMed2005

Single-molecule pump-probe detection resolves ultrafast pathways in individual and coupled quantum systems.

We report the first experimental study of individual molecules with femtosecond time resolution using a novel ultrafast single-molecule pump-probe method. A wide range of relaxation times from below 100 up to 400 fs is found, revealing energy redistribution over different vibrational modes and phonon coupling to the nanoenvironment. Addressing quantum-coupled molecules we find longer decay times, pointing towards inhibited intramolecular decay due to delocalized excitation. Interestingly, each individual system shows discrete jumps in femtosecond response, reflecting sudden breakup of the coupled superradiant state.

Journal Article↗
PubMed2005

Energy transfer in single-molecule photonic wires.

Molecular photonics is a new emerging field of research around the premise that it is possible to develop optical devices using single molecules as building blocks. Truly technological impact in the field requires focussed efforts on designing functional molecular devices as well as having access to their photonic properties on an individual basis. In this Minireview we discuss our approach towards the design and single-molecule investigation of one-dimensional multimolecular arrays intended to work as molecular photonic wires. Three different schemes have been explored: a) perylene-based dimer and trimer arrays displaying coherent exciton delocalisation at room temperature; b) DNA-based unidirectional molecular wires containing up to five different chromophores and exhibiting weak excitonic interactions between neighbouring dyes; and c) one-dimensional multichromophoric polymers based on perylene polyisocyanides showing excimerlike emission. As a whole, our single-molecule data show the importance of well-defined close packing of chromophores for obtaining optimal excitonic behaviour at room temperature. Further improvement on (bio)chemical synthesis, together with the use of single-molecule techniques, should lead in the near future to efficient and reliable photonic wires with true device functionality.

Journal Article↗
PubMed2005

Molecular printboards on silicon oxide: lithographic patterning of cyclodextrin monolayers with multivalent, fluorescent guest molecules.

Three compounds bearing multiple adamantyl guest moieties and a fluorescent dye have been synthesized for the supramolecular patterning of beta-cyclodextrin (CD) host monolayers on silicon oxide using microcontact printing and dip-pen nanolithography. Patterns created on monolayers on glass were viewed by laser scanning confocal microscopy. Semi-quantitative analysis of the patterns showed that with microcontact printing approximately a single monolayer of guest molecules is transferred. Exposure to different rinsing procedures showed the stability of the patterns to be governed by specific supramolecular multivalent interactions. Patterns of the guest molecules created at CD monolayers were stable towards thorough rinsing with water, whereas similar patterns created on poly(ethylene glycol) (PEG) reference monolayers were instantly removed. The patterns on CD monolayers displayed long-term stability when stored under N(2), whereas patterns at PEG monolayers faded within a few weeks due to the diffusion of fluorescent molecules across the surface. Assemblies at CD monolayers could be mostly removed by rinsing with a concentrated CD solution, demonstrating the reversibility of the methodology. Patterns consisting of different guest molecules were produced by microcontact printing of one guest molecule and specific adsorption of a second guest molecule from solution to non-contacted areas, giving well-defined alternating assemblies. Fluorescent features of sub-micrometer dimensions were written using supramolecular dip-pen nanolithography.

Acrylonitrile↗
PubMed2004

A simple approach to sensor discovery and fabrication on self-assembled monolayers on glass.

Self-assembled monolayers (SAMs) on glass were used as a platform to sequentially deposit fluorophores and small molecules for ion sensing. The preorganization provided by the surface avoids the need for complex receptor design, allowing for a combinatorial approach to sensing systems based on small molecules. The resulting libraries are easily measured and show varied responses to a series of both cations and anions. This technology is transferable from the macro- to the microscale both via microcontact printing (microCP), where the fluorophore is printed onto a glass surface, and via direct attachment of the fluorophore to microchannel walls. The ease of miniaturization of this technology may make the generation of a wide variety of simple yet efficient microarrays possible.

Journal Article↗
PubMed2004

Segment dynamics in thin polystyrene films probed by single-molecule optics.

Single fluorescent molecules (represented by spheres with a volume equal to the actual van der Waals volume of the molecule) has been embedded in a polystyrene matrix (left). Such molecules act as probes for the study of polymer nanoscale (segmental scale) dynamics in thin films deposited on a glass cover slide (right).

Journal Article↗