Ronan J. Cullen

In Situ Studies of the Adsorption Kinetics of 4-Nitrobenzenediazonium Salt on Gold

Self-assembled organic layers are an important tool for modifying surfaces in a range of applications in materials science. Covalent modification of metal surfaces with aryldiazonium cations has attracted much attention primarily because this reaction offers a route for spontaneously grafting a variety of aromatic moieties from solution with high yield. We have investigated the kinetics of this process by performing real-time, in situ nanogravimetric measurements. The spontaneous grafting of 4-nitrobenzene diazonium salts onto gold electrodes was studied via quartz crystal microbalance (QCM) from aqueous solutions of the salt at varying concentrations. The concentration dependence of the grafting rate within the first 10 min is best modeled by assuming a reversible adsorption process with free energy comparable to that reported for arylthiols self-assembled on gold. Multilayer formation was observed after extended grafting times and was found to be favored by increasing bulk concentrations of the diazonium salt. Modified gold surfaces were characterized ex situ with cyclic voltammetry, infrared reflection absorbance spectroscopy, and X-ray photoemission spectroscopy. Based on the experimentally determined free energy of adsorption and on the observed grafting rates, we discuss a proposed mechanism for aryldiazonium chemisorption.

Modulation of Protein Fouling and Interfacial Properties at Carbon Surfaces via Immobilization of Glycans Using Aryldiazonium Chemistry.

Carbon materials and nanomaterials are of great interest for biological applications such as implantable devices and nanoparticle vectors, however, to realize their potential it is critical to control formation and composition of the protein corona in biological media. In this work, protein adsorption studies were carried out at carbon surfaces functionalized with aryldiazonium layers bearing mono- and di-saccharide glycosides. Surface IR reflectance absorption spectroscopy and quartz crystal microbalance were used to study adsorption of albumin, lysozyme and fibrinogen. Protein adsorption was found to decrease by 30–90% with respect to bare carbon surfaces; notably, enhanced rejection was observed in the case of the tested di-saccharide vs. simple mono-saccharides for near-physiological protein concentration values. ζ-potential measurements revealed that aryldiazonium chemistry results in the immobilization of phenylglycosides without a change in surface charge density, which is known to be important for protein adsorption. Multisolvent contact angle measurements were used to calculate surface free energy and acid-base polar components of bare and modified surfaces based on the van Oss-Chaudhury-Good model: results indicate that protein resistance in these phenylglycoside layers correlates positively with wetting behavior and Lewis basicity.

Study of the spontaneous attachment of polycyclic aryldiazonium salts onto amorphous carbon substrates

Diazonium salts of two nitro-substituted polycyclic aromatic compounds were synthesized and their spontaneous covalent attachment onto amorphous carbon surfaces was studied via electrochemical and spectroscopic techniques. In situ spectroscopic monitoring of the grafting of these compounds at amorphous carbon surfaces via attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) highlighted a marked difference in adsorption rates, which was also evident via ex situ electrochemical analysis. We show that adsorption rate differences cannot be explained based on differences in the solvolysis rates of these two molecules. It was found instead that the relative position of the –N2+ groups with respect to the –NO2 groups affected the reduction potential of the diazonium cations and in turn their adsorption rate at amorphous carbon surfaces. We conclude that differences in the electron density at the carbon atom bound to the diazonium group are responsible for the differences observed in the spontaneous attachment at carbon.

Laser-driven rapid functionalization of carbon surfaces and its application to the fabrication of fluorinated adsorbers

The use of laser sources can expand the range of applications of photochemical surface functionalization strategies, increasing reaction rate and sample throughput. However, high irradiances can result in thermal effects and/or changes in the mechanism of photoinduced reactions. In this work we report on the use of a pulsed UV laser source for the modification of carbon surfaces using fluorinated terminal alkenes. A perfluorinated alkene, 1H,1H,2H-perfluoro-dec-1-ene (PFD), was used to modify amorphous carbon surfaces using a pulsed excimer laser (248 nm). The rate and yield of photoinduced PFD chemisorption was measured using Infrared Reflectance Absorption Spectroscopy (IRRAS) and compared to that obtained using a continuous lamp source. Quartz Crystal Microbalance (QCM) measurements were also used to obtain quantitative estimates of surface coverage and quantum yields. We found that, under the experimental conditions investigated, PFD chemisorption rates at bare carbon are proportional to the rate of incident photons. Simulations indicated that thermal effects of laser irradiation are expected to be minor, thus supporting the conclusion that the pulsed source can be used to accelerate the reaction rate without leading to changes in reaction mechanism. However, we observed that the limiting chemisorption yield was ∼30% higher for the laser source. We propose that this difference is due to photochemical formation of multilayers, a reaction that is slower than chemisorption at bare carbon, but that becomes evident when very high total fluence is applied via pulsed sources. Finally, we investigated the influence of reaction conditions on the ability of fluorinated carbon surfaces obtained via laser- and lamp-driven reactions to adsorb and capture fluorinated ligands via non-covalent fluorous–fluorous interactions.

Electroless deposition and characterization of Fe/FeOx nanoparticles on porous carbon microspheres: structure and surface reactivity

There has been great interest in synthetic methods that yield supported iron and iron oxide nanoparticles in order to prevent aggregation and improve their transport properties, handling and surface reactivity. In this work we report on the use of electroless deposition methods for the synthesis of carbon-supported iron/iron-oxide (Fe/FeOx) nanoparticles. We have used carbon porous microspheres synthesized via ultraspray pyrolysis as carbon scaffolds for the nucleation and growth of iron nanoparticles. The reported electroless deposition approach results in composite Fe/FeOx/carbon microspheres of narrowly dispersed size. A combination of X-ray powder diffraction (XRD) and X-ray absorption spectroscopies (EXAFS and XANES) was used in order to determine the structure and composition of the Fe/FeOx/carbon microspheres. Microspheres were found to display (14 ± 1)% iron content (w/w), whereby (12 ± 3)% of iron atoms were present as metallic iron and the remaining as maghemite (Fe2O3). Finally, we show that the removal capacity of Fe/FeOx/carbon microspheres for Cr(VI) is (20 ± 2) mg g−1 and that the maximum surface density for Cr adsorbates is (60 ± 6) μg m−2, thus suggesting that these are promising materials for the removal of water pollutants from aqueous solution.