The structure of citrate adlayers on gold nanoparticles (AuNPs) was investigated. Infrared (IR) and X-ray photoelectron spectroscopy (XPS) analyses indicate citrate anions are adsorbed on AuNPs through central carboxylate groups. A unique structure of adsorbed citrate is determined, and a pH-induced structural transition is presented. IR analysis probes dangling dihydrogen anions (H2Citrate(-)) and hydrogen bonding of carboxylic acid groups between adsorbed and dangling citrate anions. A contribution of steric repulsion between citrate layers to particle stability is characterized. Structure-based modeling, which is consistent with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) images in the literature, suggests organization details relating to the formation of self-assembled layers on (111), (110), and (100) surfaces of AuNPs. Adsorption characteristics of the citrate layer include the interaction between hydrogen-bonded citrate chains, bilayer formation, surface coverage, and chirality. The enthalpic gain from intermolecular interactions and the importance of molecular structure/symmetry on the adsorption are discussed. Combining the enthalpic factor with surface diffusion and adsorption geometry of (1,2)-dicarboxyl fragments on Au(111), H2Citrate(-) anions effectively stabilize the (111) surface of the AuNPs. The detailed understanding of intermolecular interactions in the molecular adlayer provides insight for nanoparticle formation and stabilization. We expect these findings will be relevant for other nanoparticles stabilized by hydroxy carboxylate-based amino acids and have broad implications in NP-based interfacial studies and applications.

The infrared extinction properties of gold nanocrescents fabricated using nanosphere template lithography were studied. The nanocrescents exhibit multiple, structurally tunable localized surface plasmon resonances (LSPRs) across a broad spectral range (560-3600 nm). Plasmon resonances in the infrared have large extinction efficiencies of approximately 20 and peaks as narrow as 0.07 eV. The nanocrescents also have high refractive index sensitivities (370-880 nm/RIU) that are proportional to the LSPR wavelengths. The sensing figure of merit measured for ensembles of nanocrescents is as high as 2.4 for near-infrared plasmon resonances.

Noble-metal nanoparticles exhibit unique plasmon resonances compared to bulk metal that depend on the nanoparticle size, [1] shape, [2,3] and local dielectric environment. [4] At resonance they may be regarded as antennas because they are able to increase the electrical field of an incident plane wave by orders of magnitude in a small volume. This leads, at the same time, to an increase of the emission rate of a radiating molecular dipole that is placed in the volume of enhanced coupling. [5] This antenna property is the basis for applications of subwavelength metal structures in surface-enhanced Raman spectroscopy (SERS), [6] plasmon-enhanced fluorescence spectroscopy, [7] chemical and biological sensing, [8,9] and nearfield microscopy. [10,11] Furthermore, the plasmon resonances of complex metal structures may exhibit far-field responses that cannot be realized by classical molecular resonators. For example, the formation of strong magnetic dipoles in irregularly shaped metal nanoparticles may lead to an especially intriguing application as a material with a negative refractive index [12,13] and highly unusual optical properties. An experimental signature for such an effect has been demonstrated recently at infrared wavelengths (k = 3 lm) for split-ring structures.[14]An essential component for all of these applications is the ability to tailor the particle plasmon resonances according to the desired application. A strong dependence of the strength and wavelength of the plasmon resonances on the geometrical shape of a nanometer-sized metal object is well established for spheroids.[15] More recent calculations [16,17] proved that for increasingly complex structures several distinct and strong resonances may exist and the resonance wavelengths can be tuned by varying the nanoparticle geometry. The same calculations have shown that at resonance the field enhancements may be highly localized and dramatically enhanced at features with small dimensions, such as thin gaps [17] or at the corners [16,18] of a nanoparticle structure. These results imply that for maximized enhancement effects in nano-optical experi-COMMUNICATIONS

A simple method for converting local reflectivity changes measured in surface plasmon resonance (SPR) microscopy to effective adlayer thicknesses and absolute surface coverages of adsorbed species is presented. For a range of high-contrast angles near the SPR resonance where the local metal surface's reflectivity changes linearly with angle, the change in reflectivity at fixed angle is proportional to the change in effective refractive index (eta(eff)) near the surface. This change in eta(eff) can be converted to absolute adsorbate coverage using methods developed for quantitative SPR spectroscopy. A measurement of the change in reflectivity due to changes in refractive index of bulk solutions, i.e., percent reflectivity change per refractive index unit (RIU), is the only calibration required. Application of this method is demonstrated for protein adsorption onto protein/DNA arrays on gold from aqueous solution using an SPR microscope operating at 633 nm. A detection limit of 0.072% change in absolute reflectivity is found for simultaneous measurements of all 200 microm x 200 microm areas within the 24-mm(2) light beam with 1-s time averaging. This corresponds to a change in effective refractive index of 1.8 x 10(-5) and a detection limit for protein adsorption of 1.2 ng/cm(2) (approximately 0.5 pg in a 200-microm spot). The linear dynamic range is Deltaeta(eff) = approximately 0.011 RIU or approximately 720 ng/cm(2) of adsorbed protein. Using a nearby spot as a reference channel, one can correct for instrumental drift and changes in refractive index of the solutions in the flow cell.

Thiols are widely utilized to functionalize metal nanoparticles, including ubiquitous citrate-stabilized gold nanoparticles (AuNPs), for fundamental studies and biomedical applications. For more than two decades, citrate-to-thiol ligand exchange has been used to introduce functionality to AuNPs in the 5-100 nm size regime. Contrary to conventional assumptions about the completion of ligand exchange processes and formation of a uniform self-assembled monolayer (SAM) on the NP surface, coadsorption of thiols with preadsorbed citrates as a mixed layer on AuNPs is demonstrated. Hydrogen bonding between carboxyl moieties primarily is attributed to the strong adsorption of citrate, leading to the formation of a stabilized network that is challenging to displace. In these studies, adsorbed citrates, probed by Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) analyses, remain on the surface following thiol addition to the AuNPs, whereas acetoacetate anions are desorbed. XPS quantitative analysis indicates that the surface density of alkyl and aryl thiolates for AuNPs with an average diameter of ∼40 nm is 50-65% of the value of a close-packed SAM on Au(111). We present a detailed citrate/thiolate coadsorption model that describes this final mixed surface composition. Intermolecular interactions between weakly coordinated oxyanions, such as polyprotic carboxylic acids, can lead to enhanced stability of the metal-ligand interactions, and this needs to be considered in the surface modification of metal nanoparticles by thiols or other anchor groups.