5 research outputs found
Strong Resistance of Citrate Anions on Metal Nanoparticles to Desorption under Thiol Functionalization
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
Structural Study of Citrate Layers on Gold Nanoparticles: Role of Intermolecular Interactions in Stabilizing Nanoparticles
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 (H<sub>2</sub>Citrate<sup>–</sup>) 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), H<sub>2</sub>Citrate<sup>–</sup> 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
Mid-Infrared Localized Plasmons through Structural Control of Gold and Silver Nanocrescents
Metal nanoarchitectures producing
optical responses in the visible
and near-infrared form the foundation for most plasmonic studies.
In contrast, a relative lack of infrared-active substrates has limited
the exploration of plasmonic behavior beyond the near-infrared. In
this study, we investigate the polarization-dependent, multimodal
localized plasmon resonances of asymmetric nanocrescents for large
diameter structures composed of gold and silver. The extended size
(0.5–3.0 μm) shifts the plasmon resonances into the mid-infrared
(mid-IR) spectral range. Polarization-dependent localized surface
plasmon resonance (LSPR) behavior is maintained for nanocrescent diameters
up to several microns due to the preservation of nanoscale structural
features that result in high aspect ratios. Simulations of the extinction
spectra and near-field distributions support experimentally observed
plasmonic behavior. Manipulation of nanocrescent plasmon resonances
in the mid-IR spectral range through structural-based tuning and polarization
control of incident light will find application in IR-related detection,
light guiding, and surface-enhanced IR-based spectroscopies
Robust Polymer-Coated Diamond Supports for Noble-Metal Nanoparticle Catalysts
Much
research has been done using polymer and silica particles
as support materials for catalytically active noble metal nanoparticles,
but these materials have limited stability in organic solvents or
under extreme reaction conditions such as high pH. Here we present
a robust and versatile composite polymer-diamond support for ultrasmall
noble metal nanoparticles combining chemical and mechanical stability
of diamond with the chemical versatility of a polymer. By exploiting
the rich surface chemistry of nanodiamond and incorporating a reactive
thiol–ene polymer, a thinly coated polymer-diamond composite
was formed. Fourier transform infrared spectroscopy (FTIR), X-ray
photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA)
confirmed the presence of the polymer. High resolution scanning transmission
electron microscopy (S/TEM) analysis showed that <i>in situ</i> growth of gold, platinum and palladium nanoparticles produced high
density coverage at the polymer-diamond support surface. Energy dispersive
spectroscopy mapping and S/TEM imaging indicated spatial alignment
of nanoparticles with chemical groups present in the polymer used
for particle tethering. The polymer-diamond supported nanoparticles
catalyze the NaBH<sub>4</sub> reduction of para-nitrophenol to para-aminophenol
and possess better stability than silica supports which dissolve at
high pH resulting in nanoparticle aggregation. With the high robustness
of the diamond and the ability to tailor the monomer combinations,
this polymer-diamond support system may be expanded to a wide range
of nanoparticle compositions suitable for various reaction conditions
Gold Core Nanoparticle Mimics for Asphaltene Behaviors in Solution and at Interfaces
Asphaltenes are a
poorly defined class of self-assembling and surface
active molecules present in crude oils. The nature and structure of
the nanoaggregates they form remain subjects of debate and speculation.
In this exploratory work, the surface properties of asphaltene nanoaggregates
are probed using electrically neutral 5 nm diameter gold-core nanoparticles
with alkyl, aromatic, and alkanol functionalities on their surfaces.
These custom synthesized nanoparticles are characterized, and their
enthalpies of solution at near infinite dilution and the interfacial
tensions of solutions containing these nanoparticles are compared
with the corresponding values for Athabasca pentane asphaltenes. The
enthalpies of solution of these asphaltenes in toluene, heptane, pyridine,
ethanol, and water are consistent with the behavior of gold-alkyl
nanoparticles. The interfacial tension values of these asphaltenes
at toluene–water and (toluene + heptane)–water interfaces
are consistent with the behavior of gold-biphenyl nanoparticles as
are the tendencies for these asphaltenes and gold-biphenyl nanoparticles
to “precipitate” in toluene + heptane mixtures. Gold-alkyl
nanoparticles are minimally surface active at toluene–water
and (toluene + heptane)–water interfaces and remain dispersed
in all toluene + heptane mixtures. The behavior of these asphaltenes
in solution and at interfaces is inconsistent with the behavior of
gold-<i>n</i>-alkanol nanoparticles. The outcomes of this
formative work indicate potential roles for aromatic submolecular
motifs on aggregate surfaces as a basis for interpreting asphaltene
nanoparticle flocculation and interfacial properties, while alkyl
submolecular motifs on aggregate surfaces appear to provide a basis
for interpreting other aspects of asphaltene solution behavior. A
number of lines of inquiry for future work are suggested