Paul S. Russo

Synthesis and Rapid Characterization of Amine-Functionalized Silica

Amine-functionalized colloidal silica finds use in a variety of applications and fundamental investigations. To explore convenient methods of synthesis and characterization of research-grade materials in relatively large quantities, nearly monodisperse colloidal silica particles were prepared by base-catalyzed hydrolysis of reagent-grade tetraethyl orthosilicate (TEOS) without the traditional time- and energy-consuming distillation step. Radius was varied reliably from 30 to 125 nm by changing the water/TEOS ratio. Asymmetric flow field flow fractionation (AF4) methods with online light scattering detection proved effective in assessing the uniformity of the various preparations. Even highly uniform commercial standards were resolved by AF4. The surface of the colloidal silica was decorated with amino groups using (3-aminopropyl) trimethoxysilane and spacer methyl groups from methyl-trimethoxysilane. The surface density of amino groups was quantified spectrophotometrically after reaction with ninhydrin; the nature of this analysis avoids interference from sample turbidity. As an alternative to the ninhydrin test, an empirical relationship between surface density of amino groups and zeta potential at low pH was found. The size of the colloidal silica was predictably decreased by etching with HF; this method will be effective for some preparations, despite a modest reduction in size uniformity.

Characterization of the Self-Assembly of meso-Tetra(4-sulfonatophenyl)porphyrin (H2TPPS4–) in Aqueous Solutions

The aggregation of meso-tetra(4-sulfonatophenyl)porphyrin (H(2)TPPS(4-)) in phosphate solutions was investigated as a function of pH, concentration, time, ionic strength, and solution preparation (either from dilution of a freshly prepared 2 mM stock or by direct preparation of μM solution concentrations) using a combination of complementary analytical techniques. UV-vis and fluorescence spectroscopy indicated the formation of staggered, side-by-side (J-type) assemblies. Their size and self-associative behavior were determined using analytical ultracentrifugation and small-angle X-ray scattering. Our results indicate that in neutral and basic solutions of H(2)TPPS(4-), porphyrin dimers and trimers are formed at micromolar concentrations and in the absence of NaCl to screen any ionic interactions. At these low concentrations and pH 4, the protonated H(4)TPPS(2-) species self-assembles, leading to the formation of particularly stable aggregates bearing 25 ± 3 macrocycles. At higher concentrations, these structures further organize or reorganize into tubular, rod-like shapes of various lengths, which were imaged by cryogenic and freeze-fracture transmission electron microscopy. Micron-scale fibrillar aggregates were obtained even at micromolar concentrations at pH 4 when prepared from dilution of a 2 mM stock solution, upon addition of NaCl, or both.

Two-directional cascade molecules: synthesis and characterization of [9]-n-[9] arborols

A series of two-directional cascade molecules (arborols) has been synthesized by a two-step nucleophilic substitution–amidation procedure; the [9]-10-[9] arborol forms a thermally reversible aqueous gel, which has been investigated via microscopy and light scattering techniques.

Silica−Polypeptide Composite Particles: Controlling Shell Growth

A method is presented for preparing core-shell silica-polypeptide composite particles with variable and controllable shell growth. The procedure is demonstrated using poly(carbobenzoxy-L-lysine) and poly(benzyl-L-glutamate); after deprotection, these can lead to the most common basic and acidic homopolypeptides, poly(L-lysine) and poly(L-glutamic acid). Control over shell thickness is made possible by sequential addition of N-carboxyanhydride peptide monomer to surfaces that have been functionalized with an amino initiator combined with a surface passivation agent. This results in a series of particles having different shell thicknesses. Variation of shell thickness was evident both in light scattering and in thermogravimetric assays. The shells were visible by transmission electron microscopy; these images along with light scattering measurements suggest the polymers in the shells are highly solvated.

The solubility and surface activity of the Ceratocystis ulmi toxin cerato-ulmin

The glycoprotein cerato-ulmin is one of several toxic metabolites isolated from Ceratocystis ulmi , the causal agent of Dutch elm disease. We present a number of physico-chemical measurements on cerato-ulmin in water and in ethanol/water. Surface tension measurements in water reveal cerato-ulmin to be surface active, even at concentrations as low as 3×10 −8 g cm −3 . This property relates also to the change in drop si2e observed as cerato-ulmin elutes during chromatography. Light scattering shows that cerato-ulmin is only sparingly soluble in 70% ethanol/water (3·7×10 −4 g cm −3 ) and much less soluble in water. A limited X-ray analysis provides an estimate of the molecular weight and some information about the aggregation of cerato-ulmin. Few resonances appear in the 13 C nuclear magnetic resonance (n.m.r.) spectrum for cerato-ulmin in water. However, more resonances appear in 75% ethanol/water, and many more in. 6 m urea. From these results we gain insight as to the intra- and intermolecular interactions of the macromolecule in these solvents. A possible mechanism for the toxic effect of cerato-ulmin at ultra-low concentrations is put forth.

Sculpting the internal architecture of fluorescent silica particles via a template-free approach.

Particles with an open, porous structure can be used to deliver payloads. It is often of interest to detect such particles in tissue or materials, which is facilitated by addition of dye. A straightforward approach leading to fluorescent, porous silica particles is described. The particles are etched with 3mM aqueous sodium hydroxide, taking advantage of the etching rate difference between normal silica and an interior band of silica that contains covalently attached dye. No additional steps, such as dye labeling or thermal annealing, are required. Etching modeled the internal structure of the fluorescent silica particles by creating meso/macropores and voids, as reflected by nitrogen absorption measurements. In order to investigate whether a polymer shell influences etching, certain composite particles are top-coated with poly(l-lysine) representing neutral or positive charged surfaces under typical pH conditions in living systems. The polypeptide-coated fluorescent silica cores exhibit the same porous morphology as uncoated homologs. The polypeptide topcoat does little to alter the permeation by the etching agent. Preservation of size during etching, confirmed by dynamic light scattering, transmission electron microscopy and small-angle X-ray scattering, simplifies the use of these template-free porous fluorescent particles as platforms for drug encapsulation, drug carriers and in vivo imaging.

Surface Properties of a Series of Amphiphilic Dendrimers with Short Hydrophobic Chains

A series of tree-shaped, amphiphilic dendrimers was synthesized. The products belong to the family of one-directional arborols of the form ([9]-n), where the notation signifies that each molecule has nine hydroxyl groups ([9]-) as the hydrophilic head and an alkyl chain as the hydrophobic moiety (n = 6, 8, or 10 carbon atoms). The surfactant character changes dramatically as the number of methylene groups increases. The critical micelle concentration of [9]-6 was determined, and pressure-area isotherms of the less soluble [9]-8 and [9]-10 were obtained. Large structures existed atop the spread layers. Large structures were also found in solutions of [9]-6.

Polypeptide-Coated Silica Particles Dispersed in Lyotropic Liquid Crystals of the Same Polypeptide

When a particle is introduced into a liquid crystal (LC), it distorts the LC director field, leading to new arrangements of the particles. This phenomenon is ordinarily studied using >100 nm particles and ∼2 nm mesogens. Usually the particle surface and mesogens are chemically distinct, which adds an enthalpic effect, even though the more interesting interactions are entropic. To raise the structures to the visible regime, while minimizing chemical differences between the particle surface and mesogen, silica particles coated with an α-helical polypeptide have been prepared and dispersed in lyotropic polypeptide LCs. The polypeptide is poly(γ-stearyl-α,l-glutamate) or PSLG. To make the particles easy to manipulate and easy to find, the silica core included superparamagnetic magnetite (Fe3O4) and covalently attached dye. Two methods were used to place polypeptides on these magnetic, fluorescent particles: a multistep grafting-to approach in which whole polypeptides were attached and a one-pot grafting-from approach in which the polymerization of the monomers was initiated from the particle surface. These approaches resulted in sparse and dense surface coverages, respectively. The influence of surface curvature and polypeptide molecular weight on the design of sparsely covered particles was investigated using the grafting-to approach. The aggregated grafting-from particles when freshly dispersed in a PSLG/solvent matrix disrupted the orientation of the characteristic cholesteric LC (ChLC) phase directors. In time, the hybrid particles were expelled from some domains, enabling the return of the familiar helical twist of the cholesteric mesophase. The sparsely coated grafting-to hybrid particles when inserted in the PSLG/solvent matrix assembled into stable islet-like formations that could not be disrupted even by an external magnetic field. The bulk particles aligned in chains that were easily manipulated by a magnetic field. These results indicate that polypeptide ChLCs can control and facilitate colloidal assembly of particles with matching surfaces.

Separation and Characterization of Poly(tetrafluoroethylene) Latex Particles by Asymmetric Flow Field Flow Fractionation with Light-Scattering Detection

Poly(tetrafluoroethylene) (PTFE) latex particles have been analyzed and sorted according to size using asymmetric flow field flow fractionation (AF4) coupled with multiple-angle light scattering (MALS). Characterization of fractions by regular and depolarized dynamic light scattering confirmed that smaller particles elute prior to larger ones, as expected for field flow fractionation. The measured radii of the optically and geometrically anisotropic particles are consistent with those determined from transmission electron microscopy (TEM). A certain amount of heterogeneity remains in the fractions, but their uniformity for use as diffusion probes is improved. Full characterization of PTFE colloids will require a difficult assessment of the distribution, even within fractions, of the optical anisotropy. A general method to obtain number versus size distributions is presented. This approach is valid even when an online concentration detector is not available or ineffective. The procedure is adaptable to particles of almost any regular shape.

Effect of length on the diffusion of a rodlike polymer at concentrations spanning the isotropic-lyotropic transition.

The optical tracer self-diffusion of rodlike poly(benzyl-alpha,l-glutamate) (PBLG) is measured in pyridine solutions spanning the isotropic-liquid crystalline phase boundary for two different molecular weights, M = 134 500 (contour length L = 921 A, axial ratio x = 58) and M = 24 600 (L = 168 A, x = 10.5). The results are compared to previously published data for PBLG with M = 232 000 (L = 1590 A, x = 99). Under the conditions of the measurements, the two longer rodlike polymers align in the liquid crystalline phase with their cholesteric screw axis in the vertical direction. The diffusion is measured in a particular direction normal to this over distances that are long compared to the polymer lengths. Except for the shortest polymer, whose cholesteric screw axis does not assume the vertical alignment, the diffusion suddenly increases as the liquid crystalline phase is entered, then declines, signaling the disappearance of topological constraints in the isotropic phase having a vertical component. The solutions also contain a small, fluorescent component whose mobility was not at all affected by the isotropic-liquid crystalline phase transition.