Plasmonic nanostructures promise to provide sensing capabilities with the potential for sensitive and robust assays in a high parallelization. We present here the use of individual nanostructures for the detection and manipulation of biomolecules such as DNA based on optical approaches [1]. The change in localized surface plasmon resonance of individual metal nanoparticles is utilized to monitor the binding of DNA directly or via DNA-DNA interaction. The influence of different size (length) as well as position (distance to the particle surface) is thereby studied [2]. Holes in a Cr layer present another interesting approach for bioanalytics. They are used to detect plasmonic nanoparticles as labels or to sense the binding of DNA on these particles. This hybrid system of hole and particle allows for simple (just using RGB-signals of a CCD [3]) but a highly sensitive (one nanoparticle sensitivity) detection. On the other hand, the binding of molecular layers around the particles can be detected using spectroscopic features of just an individual one of these systems. Besides sensing, individual plasmonic nanostructures can be also used to manipulate single biomolecular structures such as DNA. Attached particles can be used for local destruction [4] or cutting as well as coupling of energy into (and guiding along) the molecular structure [5].
Noble metal nanoparticles interacting with electromagnetic waves exhibit the effect of localized surface plasmon resonance (LSPR) based on the collective oscillation of their conduction electrons. Local refractive index changes by a (bio) molecular layer surrounding the nanoparticle are important for a variety of research areas like optics and life sciences. In this work we demonstrate the potential of two applications in the field of molecular plasmonics, single nanoparticle sensors and nanoantennas, situated between plasmonics effects and the molecular world.
A newly emerging field in bioanalytics based on biomolecular binding detected label-free at single metal
nanoparticles is introduced. Thereby particles which show the effect of localized surface plasmon resonance
(LSPR) are used as plasmonic transducers. They change their spectroscopic properties (a band in the UV-VIS
range) upon binding of molecules. This effect is even observable at the single nanoparticle level using micro
spectroscopy and presents the base for a new field of single particle bioanalytics with the promise of highly
parallel and miniaturized sensor arrays. The paper describes this approach and shows first result from our work regarding the detection of DNA binding at single nanoparticle sensors.
Charge carrier distribution changes in solid substrates induced by the presence of biomolecules have the potential as
sensoric principle. For a high surface-to-bulk ratio as in the case of nanostructures, this effect can be used for highly
sensitive bioanalytics.
Plasmonic nanosensors represent one possible implementation: The resonance wavelength of the conductive electron
oscillation under light irradiation is changed upon molecular binding at the structure surface. This change can be detected
by spectroscopic means, even on a single nanoparticle level using microspectroscopy.
Other examples are nanowires in electrodes gaps, either by metal nanoparticles arranged in a chain-like geometry or by
rod-like semiconductor nanowires directly bridging the gap. Molecules binding at the surface will lead to changes in the
electrical conductivity which can be easily converted into an electrical readout. The various geometries will be discussed
and their sensoric potential for an electrical detection demonstrated.
Microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next generation
plasmonic devices. This paper details the use of a dynamic chemical deposition technique to demonstrate the wet
chemical deposition of gold and silver nanoparticles (NP) within MOFs with longitudinal, homogenously-distributed
particle densities. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core
fiber. The population density of the NP on the surface, which directly influences the usable / necessary sensor length, can
be tailored via the controlled pre-treatment of the fiber. With the proposed procedure we can coat several meters of fiber
and, afterwards, cut the fiber into the desired lengths. Accordingly, this procedure is highly productive and makes the
resulting MOF-based sensors potentially very cheap. Electron microscope micrographs, taken of the inside of the fiber
holes, confirm the even distribution of the NP. A transversal through-light setup was used for the non-destructive layer
characterization. In proof-of-principle experiments with liquids of different refractive indices, the LSPR dependence on
the surroundings was confirmed and compared with Mie-theory based calculations.
Localized surface plasmons (LSPs) are charge density oscillations caused by an interaction of the external
electromagnetic waves with the interface between metallic nanostructures (e.g. noble metal nanoparticles) and a
dielectric medium. Intensity and frequency of the resulting SP absorption bands are characteristic for the type of material
and depends on the size, shape and surrounding environments of the nanostructures. We have designed core/shellnanostructures
with a defined Au-core and increasing Ag-shell thickness as previously described [17]. We have used
AFM measurement and dark-field microscopy to characterize the nanoparticles, which were immobilized via silane
chemistry on glass substrates. The plasmon band of selected particles was investigated by single particle spectroscopy
(SPS) in transmission and reflection mode. Their potential as optical biosensor was demonstrated by immobilization of a
protein and a protein specific antibody leading to a refractive index change in the local environment of metal
nanoparticles, which causes a characteristic shift of the SP absorption band maximum.
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