20 research outputs found
Excitonic AND Logic Gates on DNA Brick Nanobreadboards
A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems
Evaluating the quality of social work supervision in UK children's services: comparing self-report and independent observations
Understanding how different forms of supervision support good social work practice and improve outcomes for people who use services is nearly impossible without reliable and valid evaluative measures. Yet the question of how best to evaluate the quality of supervision in different contexts is a complicated and as-yet-unsolved challenge. In this study, we observed 12 social work supervisors in a simulated supervision session offering support and guidance to an actor playing the part of an inexperienced social worker facing a casework-related crisis. A team of researchers analyzed these sessions using a customized skills-based coding framework. In addition, 19 social workers completed a questionnaire about their supervision experiences as provided by the same 12 supervisors. According to the coding framework, the supervisors demonstrated relatively modest skill levels, and we found low correlations among different skills. In contrast, according to the questionnaire data, supervisors had relatively high skill levels, and we found high correlations among different skills. The findings imply that although self-report remains the simplest way to evaluate supervision quality, other approaches are possible and may provide a different perspective. However, developing a reliable independent measure of supervision quality remains a noteworthy challenge
Bridging Lanthanide to Quantum Dot Energy Transfer with a Short-Lifetime Organic Dye
International audienceSemiconductor nanocrystals or quantum dots (QDs) should act as excellent Förster resonance energy transfer (FRET) acceptors due to their large absorption cross section, tunable emission, and high quantum yields. Engaging this type of FRET can be complicated due to direct excitation of the QD acceptor along with its longer excited-state lifetime. Many cases of QDs acting as energy transfer acceptors are within time-gated FRET from long-lifetime lanthanides, which allow the QDs to decay before observing FRET. Efficient QD sensitization requires the lanthanide to be in close proximity to the QD. To overcome the lifetime mismatch issues and limited transfer range, we utilized a Cy3 dye to bridge the energy transfer from an extremely long lived terbium emitter to the QD. We demonstrated that short-lifetime dyes can be used as energy transfer relays between extended lifetime components and in this way increased the distance of terbium-QD FRET to âŒ14 nm
Resonance Energy Transfer in DNA Duplexes Labeled with Localized Dyes
The
growing maturity of DNA-based architectures has raised considerable
interest in applying them to create photoactive light harvesting and
sensing devices. Toward optimizing efficiency in such structures,
resonant energy transfer was systematically examined in a series of
dye-labeled DNA duplexes where donorâacceptor separation was
incrementally changed from 0 to 16 base pairs. Cyanine dyes were localized
on the DNA using double phosphoramidite attachment chemistry. Steady
state spectroscopy, single-pair fluorescence, time-resolved fluorescence,
and ultrafast two-color pumpâprobe methods were utilized to
examine the energy transfer processes. Energy transfer rates were
found to be more sensitive to the distance between the Cy3 donor and
Cy5 acceptor dye molecules than efficiency measurements. Picosecond
energy transfer and near-unity efficiencies were observed for the
closest separations. Comparison between our measurements and the predictions
of FoÌrster theory based on structural modeling of the dye-labeled
DNA duplex suggest that the double phosphoramidite linkage leads to
a distribution of intercalated and nonintercalated dye orientations.
Deviations from the predictions of FoÌrster theory point to
a failure of the point dipole approximation for separations of less
than 10 base pairs. Interactions between the dyes that alter their
optical properties and violate the weak-coupling assumption of FoÌrster
theory were observed for separations of less than four base pairs,
suggesting the removal of nucleobases causes DNA deformation and leads
to enhanced dyeâdye interaction
Evaluating Dye-Labeled DNA Dendrimers for Potential Applications in Molecular Biosensing
DNA nanostructures
provide a reliable and predictable scaffold
for precisely positioning fluorescent dyes to form energy transfer
cascades. Furthermore, these structures and their attendant dye networks
can be dynamically manipulated by biochemical inputs, with the changes
reflected in the spectral response. However, the complexity of DNA
structures that have undergone such types of manipulation for direct
biosensing applications is quite limited. Here, we investigate four
different modification strategies to effect such dynamic manipulations
using a DNA dendrimer scaffold as a testbed, and with applications
to biosensing in mind. The dendrimer has a 2:1 branching ratio that
organizes the dyes into a FRET-based antenna in which excitonic energy
generated on multiple initial Cy3 dyes displayed at the periphery
is then transferred inward through Cy3.5 and/or Cy5 relay dyes to
a Cy5.5 final acceptor at the focus. Advantages of this design included
good transfer efficiency, large spectral separation between the initial
donor and final acceptor emissions for signal transduction, and an
inherent tolerance to defects. Of the approaches to structural rearrangement,
the first two mechanisms we consider employed either toehold-mediated
strand displacement or strand replacement and their impact was mainly
via direct transfer efficiency, while the other two were more global
in their effect using either a belting mechanism or an 8-arm star
nanostructure to compress the nanostructure and thereby modulate its
spectral response through an enhancement in parallelism. The performance
of these mechanisms, their ability to reset, and how they might be
utilized in biosensing applications are discussed
FRET from Multiple Pathways in Fluorophore-Labeled DNA
Because of their ease of design and
assembly, DNA scaffolds provide
a valuable means for organizing fluorophores into complex light harvesting
antennae. However, as the size and complexity of the DNAâfluorophore
network grows, it can be difficult to fully understand energy transfer
properties because of the large number of dipolar interactions between
fluorophores. Here, we investigate simple DNAâfluorophore networks
that represent elements of the more complex networks and provide insight
into the FoÌrster Resonance Energy Transfer (FRET) processes
in the presence of multiple pathways. These FRET networks consist
of up to two Cy3 donor fluorophores and two Cy3.5 acceptor fluorophores
that are linked to a rigid dual-rail DNA scaffold with short interfluorophore
separation corresponding to 10 DNA base pairs (âŒ34 Ă
).
This configuration results in five FRET pathways: four hetero-FRET
and one homo-FRET pathway. The FRET properties are characterized using
a combination of steady-state and time-resolved spectroscopy and understood
using FoÌrster theory. We show that the multiple FRET pathways
lead to an increase in FRET efficiency, in part because homo-FRET
between donor fluorophores provides access to parallel pathways to
the acceptor and thereby compensates for low FRET efficiency channels
caused by a static transition dipole distribution. More generally,
the results show that multiple pathways may be used in the design
of artificial light harvesting devices to compensate for inhomogeneities
and nonideal ensemble effects that degrade FRET efficiency