James T. Hugall

Demonstrating photoluminescence from Au is electronic inelastic light scattering of a plasmonic metal: the origin of SERS backgrounds.

Temperature-dependent surface-enhanced Raman scattering (SERS) is used to investigate the photoluminescence and background continuum always present in SERS but whose origin remains controversial. Both the Stokes and anti-Stokes background is found to be dominated by inelastic light scattering (ILS) from the electrons in the noble metal nanostructures supporting the plasmon modes. The anti-Stokes background is highly temperature dependent and is shown to be related to the thermal occupation of electronic states within the metal via a simple model. This suggests new routes to enhance SERS sensitivities, as well as providing ubiquitous and calibrated real-time temperature measurements of nanostructures.

Surface-enhanced Raman spectroscopy of CdSe quantum dots on nanostructured plasmonic surfaces

Although quantum dots (QDs) are widely used as fluorophores they have not so far been used as Raman labels. Here we demonstrate (resonant) surface-enhanced Raman scattering (SERS) of CdSe QDs attached to nanostructured plasmonic surfaces. The 208 cm−1 CdSe longitudinal optical phonon mode is observed for laser excitation at 514, 633, and 785 nm. Tuning the SERS signal into resonance with the localized surface plasmon reveals the effects of optical absorption and emission on QD SERS. Equivalent tuning of the localized plasmons on graded nanovoid samples shows strong resonant SERS enhancements. These results pave the way for exploiting QDs as SERS markers.

SERS from molecules bridging the gap of particle-in-cavity structures

We demonstrate that by combining silver nanoparticles and structured gold SSV surfaces the SERS for those molecules that bridge the nanoparticle-cavity gap is preferentially enhanced using 4-mercaptoaniline and 4-mercaptobenzoic acid as examples.

Ultrasensitive Label-Free Nanosensing and High-Speed Tracking of Single Proteins

Label-free detection, analysis, and rapid tracking of nanoparticles is crucial for future ultrasensitive sensing applications, ranging from understanding of biological interactions to the study of size-dependent classical-quantum transitions. Yet optical techniques to distinguish nanoparticles directly among their background remain challenging. Here we present amplified interferometric scattering microscopy (a-iSCAT) as a new all-optical method capable of detecting individual nanoparticles as small as 15 kDa proteins that is equivalent to half a GFP. By balancing scattering and reflection amplitudes the interference contrast of the nanoparticle signal is amplified 1 to 2 orders of magnitude. Beyond high sensitivity, a-iSCAT allows high-speed image acquisition exceeding several hundreds of frames-per-second. We showcase the performance of our approach by detecting single Streptavidin binding events and by tracking single Ferritin proteins at 400 frames-per-second with 12 nm localization precision over seconds. Moreover, due to its extremely simple experimental realization, this advancement finally enables a cheap and routine implementation of label-free all-optical single nanoparticle detection platforms with sensitivity operating at the single protein level.

Monitoring Early-Stage Nanoparticle Assembly in Microdroplets by Optical Spectroscopy and SERS.

Microfluidic microdroplets have increasingly found application in biomolecular sensing as well as nanomaterials growth. More recently the synthesis of plasmonic nanostructures in microdroplets has led to surface-enhanced Raman spectroscopy (SERS)-based sensing applications. However, the study of nanoassembly in microdroplets has previously been hindered by the lack of on-chip characterization tools, particularly at early timescales. Enabled by a refractive index matching microdroplet formulation, dark-field spectroscopy is exploited to directly track the formation of nanometer-spaced gold nanoparticle assemblies in microdroplets. Measurements in flow provide millisecond time resolution through the assembly process, allowing identification of a regime where dimer formation dominates the dark-field scattering and SERS. Furthermore, it is shown that small numbers of nanoparticles can be isolated in microdroplets, paving the way for simple high-yield assembly, isolation, and sorting of few nanoparticle structures.

Fiber-Based Optical Nanoantennas for Single-Molecule Imaging and Sensing

We present the use of fiber-based resonant dipole and nanogap optical nanoantennas for extreme resolution optical microscopy. A typical optical dipole antenna is only ~100-nm long, as the wavelength of light is typically a million times smaller than for radio waves. We show how by focused-ion-beam milling of metal-coated tapered optical fibers we overcome the challenge of fabricating resonant antennas at such small-length scales, with an accuracy of ~10 nm. The optical fiber provides an ideal interface between the macro- and nanoscales, allowing the manipulation of such a tiny nanoantenna with nanometer precision relative to a sample surface. Imaging single fluorescent molecules and nanobeads, we achieve an optical resolution down to 40 nm (FWHM), far below the Abbe diffraction limit. The strongly localized antenna field results in an enhancement of fluorescence up to 100 ×, while the vectorial nature of the local antenna field allows access to molecules of all orientations. Clearly, dedicated nanofabrication of fiber-based scanning optical antennas is a promising route to push the limits of optical nanoscopy.

Surface plasmon enhanced spectroscopies and time and space resolved methods: general discussion.

F. Javier Garcia de Abajo opened a general discussion of the two papers by Jeremy Baumberg and Javier Aizpurua.

Temperature dependence of surface-enhanced Raman scattering on nanostructured plasmonic surfaces

SERS of sub-monolayers of benzenethiol and quantum dots are studied over a wide temperature range on plasmonic nanostructures. Unusual changes are observed in the background shape and intensity as well as the vibrational signals.