Synthesis, stabilization, and functionalization of silver nanoplates for biosensor applications

Silver nanoplates (NPTs) were prepared by a seed-mediated growth method with different diameters (d = 25, 32, 53, and 100 nm) and with thicknesses of approximately 10 nm in all cases. As the concentration of silver seeds increased, the diameter of the nanoplates increased, resulting in an overall shift in the localized surface plasmon resonance (LSPR) band maximum from 570 to 900 nm, thus providing a novel method to tune the plasmon resonance. The LSPR was calculated from theory for both triangular and circular nanoplate geometries. In agreement with transmission electron micrographs, the model results confirmed the shape of nanoplates as being truncated prisms, intermediate between that of a prism and a disk. Because of the toxicity of the surfactant hexadecyltrimethylammonium bromide (CTAB), the stabilizing CTAB bilayer surrounding the NPT was replaced by a nontoxic alkanethiol with surfactant properties. This enabled the extraction of metal nanoparticles into deionized water or buffer for bioconjugation without aggregation. Silver nanoplates were also coated with polyelectrolyte layers using the standard layer-by-layer (LbL) method. The LSPR was found to be very sensitive to the addition of polyelectrolyte layers, with a plasmon band shift from 728 to 740 nm after adding only one monolayer (thickness ~1.5 nm). Bioconjugation of these nanoplates was achieved with the addition of a mercaptolinker containing a carboxyl group. The carboxyl groups were activated with 1-ethyl-3-(3- dimethylaminopropyl) hydrochloride (EDC)/N-hydroxysuccinimide (NHS) and conjugated to green fluorescent protein (GFP) in order to validate the potential of the NPTs for enhancement of bioassays. The fluorescence of the conjugated NPTs was 5.6-fold brighter than that of NPTs added to GFP without activation. © 2009 American Chemical Society

Plasmonically Enhanced Electron Escape from Gold Nanoparticles and Their Polarization-Dependent Excitation Transfer along DNA Nanowires

Here we show plasmon mediated excitation transfer along DNA nanowires over up to one micrometer. Apparently, an electron excitation is initiated by a femtosecond laser pulse that illuminates gold nanoparticles (AuNP) on double stranded DNA (dsDNA). The dependency of this excitation on laser wavelength and polarization are investigated. Excitation of the plasmon resonance of the AuNPs via one- and two-photon absorption at 520 and 1030 nm, respectively, was explored. We demonstrate an excitation transfer along dsDNA molecules at plasmon supported four-photon excitation of AuNP cluster or at laser field driven nanoparticle electron tunneling for an alignment of the attached dsDNA to the polarization of the electric field of the laser light. These results extend the previously observed plasmonically induced three-photon excitation transfer along DNA nanowires to another nanoparticle material (gold) and the adapted irradiation wavelengths

Tuning of Spectral and Angular Distribution of Scattering from Single Gold Nanoparticles by Subwavelength Interference Layers

Localized surface plasmon resonance (LSPR) as the resonant oscillation of conduction electrons in metal nanostructures upon light irradiation is widely used for sensing as well as nanoscale manipulation. The spectral resonance band position can be controlled mainly by nanoparticle composition, size, and geometry and is slightly influenced by the local refractive index of the near-field environment. Here we introduce another approach for tuning, based on interference modulation of the light scattered by the nanostructure. Thereby, the incoming electric field is wavelength-dependent modulated in strength and direction by interference due to a subwavelength spacer layer between nanoparticle and a gold film. Hence, the wavelength of the scattering maximum is tuned with respect to the original nanoparticle LSPR. The scattering wavelength can be adjusted by a metallic mirror layer located 100–200 nm away from the nanoparticle, in contrast to near-field gap mode techniques that work at distances up to 50 nm in the nanoparticle environment. Thereby we demonstrate, for the first time at the single nanoparticle level, that dependent on the interference spacer layer thickness, different distributions of the scattered signal can be observed, such as bell-shaped or doughnut-shaped point spread functions (PSF). The tuning effect by interference is furthermore applied to anisotropic particles (dimers), which exhibit more than one resonance peak, and to particles which are moved from air into the polymeric spacer layer to study the influence of the distance to the gold film in combination with a change of the surrounding refractive index

Novel disposable biochip platform employing supercritical angle fluorescence for enhanced fluorescence collection

This paper presents an overview of development of a novel disposable plastic biochip for multiplexed clinical diagnostic applications. The disposable biochip is manufactured using a low-cost, rapid turn- around injection moulding process and consists of nine parabolic elements on a planar substrate. The optical elements are based on supercritical angle fluorescence (SAF) which provides substantial enhancement of the fluorescence collection efficiency but also confines the fluorescence detection volume strictly to the immediate proximity of the biochip surface, thereby having the potential to discriminate against background fluorescence from the analyte solution. An optical reader is also described that enables interrogation and fluorescence collection from the nine optical elements on the chip. The sensitivity of the system was determined with a biotin-avidin assay while its clinical utility was demonstrated in an assay for C-reactive protein (CRP), an inflammation marker