50 research outputs found
Optical detection of single non-absorbing molecules using the surface plasmon of a gold nanorod
Current optical detection schemes for single molecules require light
absorption, either to produce fluorescence or direct absorption signals. This
severely limits the range of molecules that can be detected, because most
molecules are purely refractive. Metal nanoparticles or dielectric resonators
detect non-absorbing molecules by a resonance shift in response to a local
perturbation of the refractive index, but neither has reached single-protein
sensitivity. The most sensitive plasmon sensors to date detect single molecules
only when the plasmon shift is amplified by a highly polarizable label or by a
localized precipitation reaction on the particle's surface. Without
amplification, the sensitivity only allows for the statistical detection of
single molecules. Here we demonstrate plasmonic detection of single molecules
in realtime, without the need for labeling or amplification. We monitor the
plasmon resonance of a single gold nanorod with a sensitive photothermal assay
and achieve a ~ 700-fold increase in sensitivity compared to state-of-the-art
plasmon sensors. We find that the sensitivity of the sensor is intrinsically
limited due to spectral diffusion of the SPR. We believe this is the first
optical technique that detects single molecules purely by their refractive
index, without any need for photon absorption by the molecule. The small size,
bio-compatibility and straightforward surface chemistry of gold nanorods may
open the way to the selective and local detection of purely refractive proteins
in live cells
Mode-matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation
Boosting nonlinear frequency conversion in extremely confined volumes remains
a key challenge in nano-optics, nanomedicine, photocatalysis, and
background-free biosensing. To this aim, field enhancements in plasmonic
nanostructures are often exploited to effectively compensate for the lack of
phase-matching at the nanoscale. Second harmonic generation (SHG) is, however,
strongly quenched by the high degree of symmetry in plasmonic materials at the
atomic scale and in nanoantenna designs. Here, we devise a plasmonic
nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode
overlap at both the excitation and the SHG wavelengths. The effective
combination of these features in a single device allows obtaining unprecedented
SHG conversion efficiency. Our results shed new light on the optimization of
SHG at the nanoscale, paving the way to new classes of nanoscale coherent light
sources and molecular sensing devices based on nonlinear plasmonic platforms.Comment: 14 pages, 4 figure
Dielectric function of sub-10 nanometer thick gold films
A smooth gold (Au) film with thickness below 10 nanometer (nm) is hard to fabricate as well as to accurately measure its thickness and the corresponding dielectric function. Here, we report 5.4, 6.6 and 7.5 nm thick continuous Au films prepared on Chromium (Cr) seed layer. The thickness and dielectric function of the Au films are obtained using spectroscopic ellipsometry and first principles calculation. From the fitting results of the ellipsometric parameters, the value of the real part of dielectric function (ε1) is negative almost in the whole spectrum region indicating that the Au films are continuous. For the imaginary part of dielectric function (ε2), it decreases with increasing of the Au film thickness because the surface electrons scattering decreases. Moreover, the calculated and measured results of 5.4, 6.6 and 7.5 nm thick Au films present a good agreement in the wavelength range from 400 to 1600 nm. From the results of the first principles calculation, both ε1 and ε2 decrease with increasing of the Au film thickness. These precise measurement and calculation results of dielectric function are beneficial to nano-photoelectronic devices design with sub-10 nm Au films involved
Through-space transfer of chiral information mediated by a plasmonic nanomaterial
The ability to detect chirality gives stereochemically attuned nanosensors the potential to revolutionise the study of biomolecular processes. Such devices may structurally characterise the mechanisms of protein-ligand binding, the intermediates of amyloidogenic diseases and the effects of phosphorylation and glycosylation. We demonstrate that single nanoparticle plasmonic reporters, or nanotags, can enable a stereochemical response to be transmitted from a chiral analyte to an achiral benzotriazole dye molecule in the vicinity of a plasmon resonance from an achiral metallic nanostructure. The transfer of chirality was verified by the measurement of mirror image surface enhanced resonance Raman optical activity spectra for the two enantiomers of each of ribose and tryptophan. Computational modelling confirms these observations and reveals the novel chirality transfer mechanism responsible. This is the first report of colloidal metal nanoparticles in the form of single plasmonic substrates displaying an intrinsic chiral sensitivity once attached to a chiral molecule