Ramjee Balasubramanian

Anion-induced adsorption of ferrocenated nanoparticles.

Au nanoparticles fully coated with omega-ferrocenyl hexanethiolate ligands, with average composition Au225(omega-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing submonolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and there exhibit stable ferrocene voltammetry over long periods of time. The kinetics of forming the robustly adsorbed layer are slow; monolayer and submonolayer deposition can be described by a rate law that is first order in nanoparticle concentration and in available electrode surface. The adsorption mechanism is proposed to involve entropically enhanced (multiple) ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by scanning to positive potentials (through the ferrocene wave) and by high concentrations of Bu4N+ X- electrolyte (X- = ClO4(-), PF6(-)) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is not appreciably diminished by the adsorbed ferrocenated nanoparticles, which are gradually desorbed by scanning to potentials more negative than the electrodes potential of zero charge. At very slow scan rates, voltammetric current peaks are symmetrical and nearly reversible, but exhibit E(fwhm) considerably narrower (typically 35 mV) than ideally expected (90.6 mV, at 298 K) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is qualitatively explicable by a surface-activity effect invoking large, attractive lateral interactions between nanoparticles and, or alternatively, by a model in which ferrocene sites react serially at formal potentials that become successively altered as ion-pair bridges are formed. At faster scan rates, both deltaE(peak) and E(fwhm) increase in a manner consistent with a combination of uncompensated ohmic resistance of the electrolyte solution and of the adsorbed film, as distinct from behavior produced by slow electron transfer.

Self-assembly of Resorcinarene-stabilized Gold Nanoparticles: Influence of the Macrocyclic Headgroup

Gold nanoparticles can be encapsulated by various resorcinarene derivatives and assembled into monolayer films at solvent interfaces. Surface charge plays a critical role in both nanoparticle extraction and self-assembly: the degree of monolayer formation and local two-dimensional (2D) order within the nanoparticle arrays is dependent on the chemical nature of the resorcinarene headgroup as well as the presence of other electrolytes. Cluster size distribution analysis can be used to parameterize local 2D order within the arrays in a quantitative manner, based on mean cluster sizes and fractional hexagonal close-packed (hcp) cluster formation. 2D nanoparticle arrays can also be prepared in some cases using Langmuir–Blodgett techniques. These studies demonstrate that resorcinarenes with chemisorptive headgroups promote the self-assembly of well-ordered 2D arrays.

Extraction and Dispersion of Large Gold Nanoparticles in Nonpolar Solvents

Gold nanoparticles up to 70 nm in diameter could be extracted from aqueous solutions into nonpolar organic solvents by tetrathiolated resorcinarenes 1 and 2. The resorcinarene-coated nanoparticles formed stable dispersions in toluene and chloroform and could be passed through a crosslinked polystyrene column without significant degradation, but exhibited variable resistance to alkanethiol-induced flocculation. Gold nanoparticles encapsulated by resorcinarene 2 were found to be exceptionally stable even in the presence of propanethiol and dodecanethiol, with an approximate dispersion half-life of one month at room temperature.

Encapsulation and Functionalization of Nanoparticles in Crosslinked Resorcinarene Shells

Two resorcinarene-derived tetrathiols with terminal alkene sidechains (tetraarylthiol cavitand 3 and tetrabenzylthiol cavitand 4) were determined to be efficient at extracting colloidal gold nanoparticles from aqueous solutions and stabilizing their dispersion in organic solvents. Treatment of these nanoparticle dispersions with the Grubbs olefin metathesis catalyst resulted in crosslinked resorcinarene shells that were highly resistant to alkanethiol-induced desorption at high temperatures. Nanoparticles in crosslinked shells of tetrabenzylthiol cavitand 4 were particularly robust, and could be precipitated and redispersed many times with minimal attrition. These shells could also withstand oxidative conditions and were amenable to synthetic modifications involving epoxidation and dihydroxylation.

Solvent effect in molecular recognition: determining binding constants in different solvents following an extraction based protocol

Semi-rigid molecular tweezers 1, 3 and 4 bind picric acid with more than tenfold increment in tetrachloromethane as compared to chloroform.

Solvent dependent morphologies in thiol-ene photopolymerization: A facile route to the synthesis of resorcinarene nanocapsules

This article describes a direct method for the fabrication of resorcinarene nanocapsules by photopolymerization of resorcinarene tetraalkene tetrathiol, in the absence of any template or preorganization. Further, by varying the polymerization media, a variety of other polymeric architectures like lattices, fibrous networks, and nanoparticles were obtained. The morphology and structure were characterized by transmission electron microscopy, energy dispersive spectroscopy, scanning electron microscopy, dynamic light scattering, infrared and nuclear magnetic resonance spectroscopy. These morphologically distinct resorcinarene polymeric architectures contain residual thiol and ene functional groups offering potential functionalization opportunities.

Multidentate ionic surfactant mediated extraction and dispersion of gold nanoparticles in organic solvents.

Resorcinarenes with three different quaternary ammonium headgroups were synthesized and evaluated for their ability to stabilize gold nanoparticles in organic and aqueous medium. Aqueous dispersions of citrate stabilized gold nanoparticles of dimensions up to 29 nm could be extracted into organic solvents by resorcinarenes functionalized with tetrapyridinium tetrabromide (1), tetratrimethylammonium tetrabromide (2), and tetratributylammonium tetrabromide (3). Such nanoparticles were characterized by TEM, EDS, UV-vis, and IR. Their long-term dispersion stability varied significantly and depended on the nature of the resorcinarene headgroup, and in particular nanoparticles extracted by resorcinarene 1 were stable for several weeks. Nanoparticles passivated by resorcinarenes 1 and 2 were also stable in the presence of thiourea for several hours in both aqueous and organic medium. This is notable as thiourea is known to result in the instantaneous aggregation of citrate stabilized nanoparticles. Remarkably nanoparticles stabilized by resorcinarenes 1 and 2 could be precipitated and redispersed in chloroform without any visible aggregation. The critical parameters controlling the extraction of the nanoparticles into the organic phase have also been evaluated. The resorcinarene surfactant mediated facile phase transfer of gold nanoparticles described here can be readily applied for the stabilization of other citrate stabilized mono- and bimetallic nanoparticles, thus providing opportunities to disperse and stabilize relatively larger nanoparticles in organic solvents using ionic surfactants opening up new applications.

Direct synthesis of hollow polymeric nanocapsules of variable shell thickness and rigidity

The thiol-ene photopolymerization of resorcinarene cavitand thiol with various alkene building blocks led to the formation of hollow nanocapsules of varying thickness and rigidity, depending on the valency of the alkenes.

Tunable gold nanostructures with nanocapsules as template reaction vessels

Hollow, polymeric, thiol-ene resorcinarene nanocapsules can act as template reaction vessels for the synthesis of gold nanoshells or spherical aggregates of gold nanoparticles depending on the reaction conditions. The spherical nanoparticle aggregates were evaluated as catalysts in the biphasic reduction of 4-nitrophenol.

Resorcinarene amine stabilized nanodiamond dispersions in organic solvents: applications in diamond film growth

Nanodiamonds of widely differing sizes (4 to >100 nm), shapes, and origin could be extracted from aqueous dispersions into toluene by a multidentate resorcinarene amine surfactant, in the absence of any elaborate covalent functionalization or extreme conditions. Resorcinarene amine encapsulated nanodiamonds displayed excellent dispersion stability in non-polar organic solvents for several months. The IR spectral analysis of these extracted and further precipitated nanodiamonds suggests that the carboxylic acid groups present on the nanodiamond surfaces are electrostatically stabilized by the resorcinarene amine surfactant. Such nanodiamond dispersions can also act as nucleation sources for the fabrication of continuous diamond films by CVD growth. Notably, this process resulted in diamond films with better uniformity, smaller grain size and reduced surface roughness when compared to the films prepared with unmodified nanodiamonds as nucleating agents.