Observation of molecular diffusion in polyelectrolyte-wrapped SERS nanoprobes.

The popularity of nanotechnology-based sensing technologies has rapidly expanded within the past decade. Surface-enhanced Raman spectroscopy (SERS) is one such technique capable of chemically specific and highly sensitive measurements. The careful preparation of SERS-active nanoprobes is immensely vital for biological applications where nanoprobes are exposed to harsh ionic and protein rich microenvironments. Encapsulation of optical reporter molecules via layer-by-layer (LbL) polyelectrolyte wrapping is an emerging technique that also permits facile modification of surface chemistry and charge. LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes. Nonetheless, the stability of its products requires further characterization and analysis. In this study, Raman-active methylene blue molecules were electrostatically encapsulated within alternating layers of cationic and anionic polyelectrolytes surrounding gold nanospheres. We observed molecular diffusion of methylene blue through polyelectrolyte layers by monitoring the change in SERS intensity over a period of more than 5 weeks. To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer. In both cases, molecular diffusion is significantly diminished.

High-index facets in gold nanocrystals elucidated by coherent electron diffraction.

Characterization of high-index facets in noble metal nanocrystals for plasmonics and catalysis has been a challenge due to their small sizes and complex shapes. Here, we present an approach to determine the high-index facets of nanocrystals using streaked Bragg reflections in coherent electron diffraction patterns, and provide a comparison of high-index facets on unusual nanostructures such as trisoctahedra. We report new high-index facets in trisoctahedra and previous unappreciated diversity in facet sharpness.

Plasmonic enhancement of the two photon absorption cross section of an organic chromophore using polyelectrolyte-coated gold nanorods.

The effect of plasmonic enhancement on the two-photon absorption cross section of organic chromophores attached to polyelectrolyte-coated gold nanorods was investigated. The magnitudes of such enhancements were confirmed using single and two photon excitations of the chromophore molecules bound to polyelectrolyte-coated gold nanorods. By synthesizing two-, four-, six-, and eight-polyelectrolyte layer coated nanorods of a particular aspect ratio, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Enhancements of 40-fold were observed for the chromophores nearest to the surface.

A simple millifluidic benchtop reactor system for the high-throughput synthesis and functionalization of gold nanoparticles with different sizes and shapes.

Despite the continuing interest in the applications of functionalized nanomaterials, the controlled and reproducible synthesis of many important functionalized nanoparticles (NPs) above the milligram scale continues to be a significant challenge. The synthesis of functionalized NPs in automated reactors provides a viable approach to circumvent some of the shortcomings of traditional nanomaterial batch syntheses, providing superior control over reagent addition, improved reproducibility, the opportunity to interface real-time product monitoring, and a viable high-throughput synthetic approach. Here, we demonstrate the construction and operation of a simple millifluidic reactor assembled entirely from commercially available components found in almost any chemical laboratory. This reactor facilitates the aqueous gram-scale synthesis of a variety of functionalized gold nanoparticles, including the synthesis of gold nanospheres with tightly controlled core diameters and gold nanorods with controlled aspect ratios between 1.5 and 4.0. The absolute dimensions (i.e., the transverse diameter) of gold nanorods synthesized within the reactor can also be tailored to produce different gold nanorod shapes, including "small" gold nanorods and gold nanocubes. In addition, we show that the reactor can interface with existing purification and monitoring techniques in order to enable the high-throughput functionalization/purification of gold nanorods and real-time monitoring of gold nanoparticle products for quality control. We anticipate that this millifluidic reactor will provide the blueprint for a versatile and portable approach to the gram-scale synthesis of monodisperse, hydrophilically functionalized metal NPs that can be realized in almost any chemistry research laboratory.

Off-resonance surface-enhanced Raman spectroscopy from gold nanorod suspensions as a function of aspect ratio: not what we thought.

Design of nanoparticles for surface-enhanced Raman scattering (SERS) within suspensions is more involved than simply maximizing the local field enhancement. The enhancement at the nanoparticle surface and the extinction of both the incident and scattered light during propagation act in concert to determine the observed signal intensity. Here we explore these critical aspects of signal generation and propagation through experiment and theory. We synthesized gold nanorods of six different aspect ratios in order to obtain longitudinal surface plasmon resonances that incrementally spanned 600-800 nm. The Raman reporter molecule methylene blue was trap-coated near the surface of each nanorod sample, generating SERS spectra, which were used to compare Raman signals. The average number of reporter molecules per nanorod was quantified against known standards using electrospray ionization liquid chromatography mass spectrometry. The magnitude of the observed Raman signal is reported for each aspect ratio along with the attenuation due to extinction in suspension. The highest Raman signal was obtained from the nanorod suspension with a plasmon resonance blue-shifted from the laser excitation wavelength. This finding is in contrast to SERS measurements obtained from molecules dried onto the surface of roughened or patterned metal substrates where the maximum observed signal is near or red-shifted from the laser excitation wavelength. We explain these results as a competition between SERS enhancement and extinction, at the excitation and scattered wavelengths, on propagation through the sample.

Competition Between Extinction and Enhancement in Surface Enhanced Raman Spectroscopy.

Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor and concentrations are indicated. The same processes which give rise to enhancement also lead to increased extinction of both the illumination and the Raman scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise the radiation of the Raman-scattered field (green) is enhanced by the near-by sphere but extinguished by the rest of the spheres in the suspension on propagation.

Surface-Enhanced Raman Spectroscopy of Polyelectrolyte-Wrapped Gold Nanoparticles in Colloidal Suspension.

The rapidly expanding field of surface-enhanced Raman spectroscopy (SERS) has helped fuel an intense interest in noble metal nanoparticle synthesis. An in-suspension approach for quantifying SERS enhancement and relating that enhancement to a spontaneous Raman equivalent signal is described. Gold nanoparticles of various shapes were wrapped with polyelectrolyte multilayers that trapped Raman reporter molecules at defined distances from the metal core. Electrospray ionization liquid chromatography mass spectrometry (ESI-LC-MS) on digested samples was employed to measure the average number of bound Raman reporter molecules per gold nanoparticle, and inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the average number of gold atoms per nanoparticle. Using these data, SERS signal intensity was compared to a spontaneous Raman calibration curve to compute a spontaneous Raman equivalent factor. Three different geometries of gold nanoparticles (cubes, spheres, and trisoctahedra) were synthesized to investigate edge and corner effects using these quantitative techniques. Finite element method (FEM) electromagnetic simulations examined the relationship between the different geometries and the observed SERS signal intensities. The experimental observations and theoretical results indicate that cubic gold nanoparticles have the highest effective signal.

Off-Resonant Two-Photon Absorption Cross-Section Enhancement of an Organic Chromophore on Gold Nanorods.

Surface-plasmon-initiated interference effects of polyelectrolyte-coated gold nanorods on the two-photon absorption of an organic chromophore were investigated. With polyelectrolyte bearing gold nanorods of 2,4,6 and 8 layers, the role of the plasmonic fields as function of distance on such effects was examined. An unusual distance dependence was found: enhancements in the two-photon cross-section were at a minimum at an intermediate distance, then rose again at a further distance. The observed values of enhancement were compared to theoretical predictions using finite element analysis and showed good agreementdue to constructive and destructive interference effects.