R. Griffith Freeman

Structure−Activity Relationships in Gold Nanoparticle Dimers and Trimers for Surface-Enhanced Raman Spectroscopy

Understanding the detailed relationship between nanoparticle structure and activity remains a significant challenge for the field of surface-enhanced Raman spectroscopy. To this end, the structural and optical properties of individual plasmonic nanoantennas comprised of Au nanoparticle assemblies that are coated with organic reporter molecules and encapsulated by a SiO(2) shell have been determined using correlated transmission electron microscopy (TEM), dark-field Rayleigh scattering microscopy, surface-enhanced Raman scattering (SERS) microscopy, and finite element method (FEM) calculations. The distribution of SERS enhancement factors (EFs) for a structurally and optically diverse set of nanoantennas is remarkably narrow. For a collection of 30 individual nanoantennas ranging from dimers to heptamers, the EFs vary by less than 2 orders of magnitude. Furthermore, the EFs for the hot-spot-containing nanoparticles are uncorrelated to aggregation state and localized surface plasmon resonance (LSPR) wavelength but are crucially dependent on the size of the interparticle gap. This study demonstrates that the creation of hot spots, where two particles are in subnanometer proximity or have coalesced to form crevices, is paramount to achieving maximum SERS enhancements.

Structure Enhancement Factor Relationships in Single Gold Nanoantennas by Surface-Enhanced Raman Excitation Spectroscopy

Determining the existence of any direct spectral relationship between the far-field scattering properties and the near-field Raman-enhancing properties of surface-enhanced Raman spectroscopy (SERS) substrates has been a challenging task with only a few significant results to date. Here, we prove that hot spot dominated systems show little dependence on the far-field scattering properties because of differences between near- and far-field localized surface plasmon resonance (LSPR) effects as well as excitation of new plasmon modes via a localized emitter. We directly probe the relationship between the near- and far-field light interactions using a correlated LSPR-transmission electron microscopy (TEM) surface-enhanced Raman excitation spectroscopy (SERES) technique. Fourteen individual SERS nanoantennas, Au nanoparticle aggregates ranging from dimers to undecamers, coated in a reporter molecule and encased in a protective silica shell, were excited using eight laser wavelengths. We observed no correlation between the spectral position of the LSPR maxima and the maximum enhancement factor (EF). The single nanoantenna data reveal EFs ranging from (2.5 ± 0.6) × 10(4) to (4.5 ± 0.6) × 10(8) with maximum enhancement for excitation wavelengths of 785 nm and lower energy. The magnitude of maximum EF was not correlated to the number of cores in the nanoantenna or the spectral position of the LSPR, suggesting a separation between near-field SERS enhancement and far-field Rayleigh scattering. Computational electrodynamics confirms the decoupling of maximum SERS enhancement from the peak of the scattering spectrum. It also points to the importance of a localized emitter for radiating Raman photons to the far-field which, in nonsymmetric systems, allows for the excitation of radiative plasmon modes that are difficult to excite with plane waves. Once these effects are considered, we are able to fully explain the hot spot dominated SERS response of the nanoantennas.

Electrochemical synthesis and optical readout of striped metal rods with submicron features

Abstract Fluorescent molecules are widely used as identification tags in both analytical and bioanalytical chemistry. This manuscript describes an electrochemical approach to the manufacture of submicron metal barcodes for application as identification tags. These Nanobarcodes™ identification tags (NBCs), are prepared via the sequential electrodeposition of various metals within an alumina template. The striped particles grow as replicas of the pores of the membrane, and can be released from the membrane through chemical treatment. The striping pattern of NBCs can be read out via optical microscopy, and allows for the synthesis of large numbers of distinguishable tags. Herein, we address the synthesis of these NBCs and the use of software that can be used to identify specific NBCs in optical microscope images containing several distinct species of NBCs.

Size selection of colloidal gold aggregates by filtration: effect on surface‐enhanced Raman scattering intensities

The effect of colloidal Au particle aggregation on surface-enhanced Raman scattering (SERS) spectra was probed by SERS filtration experiments. In this approach, SERS and optical spectra were recorded for trans-1,2-bis(4-pyridyl)ethylene (BPE)-aggregated solutions of colloidal Au filtered through straight-channel membranes with successively smaller diameters. This allowed the overall SERS intensity to be factored into aggregate size-dependent contributions. Experiments were carried out as a function of adsorbate concentration (0.5–2.5 µM BPE) and initial particle size (12–50 nm diameter colloidal Au). The key findings are as follows: (i) under conditions of minimal aggregation, appreciable SERS intensity derives from aggregates with effective diameters less than 200 nm; (ii) the amount of aggregant clearly controls the average aggregate size; and (iii) similarly aggregated solutions based on different diameter colloidal Au particles give different distributions of aggregates. These studies provide an insight into the dynamics of colloidal Au aggregation, suggest a procedure for signal optimization in colloid SERS experiments, and set the stage for controlled surface confinement of SERS-active particle clusters. Copyright

Use of Nanobarcodes® Particles in Bioassays

We have developed striped metal nanoparticles, Nanobarcodes particles, which can act as encoded substrates in multiplexed assays. These particles are metallic, encodeable, machine-readable, durable, submicron-sized tags. The power of this technology is that the particles are intrinsically encoded by virtue of the difference in reflectivity of adjacent metal stripes. This chapter describes protocols for the attachment of biological molecules, and the subsequent use of the Nanobarcodes particles in bioassays.

Exploring single-molecule SERS and single-nanoparticle plasmon microscopy

In this work we perform correlated structural and optical studies of single nanoparticles as well as explore the generality of SMSERS. First, wide-field plasmon resonance microscopy is used to simultaneously determine the LSPR spectra of multiple Ag nanoprisms, whose structure is determined using TEM. Next, the structure-property relationships for well-defined and easily-controlled nanoparticle structures (e.g. monomers, dimers, and trimers) are studied using correlated TEM, LSPR, and SERS measurements of individual SERS nanotags. We present the SER spectrum of reporter molecules on a single nanotag comprised of a Au trimer. It was determined that of 40 individual nanotags, just 19 exhibited SERS. The remaining nanoparticles were established by TEM to be monomers. These results demonstrate that SERS signal is observed from individual nanotags containing a junction or hot spot. Lastly, we explore crystal violet, a triphenyl methane dye that was used in the seminal SMSERS investigations, and re-examine single-molecule sensitivity using the isotopologue approach.

Nanobarcodes™ Identification Tags in Proteomics and Small Molecule Profiling

We describe novel applications of Nanobarcodes™ Identification Tags (NBCs), nanometer-scale metallic bar codes in proteomics and small molecule profiling. Synthesis, detection and surface derivatization of these nanoparticles are presented. Solid-phase nanoextraction (SPNE) of analytes from biological samples and its unique application in MALDI-TOF mass spectrometry are detailed.