Macromolecular Diffusion in Self-Assembling Biodegradable Thermosensitive Hydrogels.

Hydrogel formation triggered by a change in temperature is an attractive mechanism for in situ gelling biomaterials for pharmaceutical applications such as the delivery of therapeutic proteins. In this study, hydrogels were prepared from ABA triblock polymers having thermosensitive poly(N-(2-hydroxypropyl) methacrylamide lactate) flanking A-blocks and hydrophilic poly(ethylene glycol) B-blocks. Polymers with fixed length A blocks (~22 kDA) but differing PEG-midblock lengths (2, 4 and 10 kDa) were synthesized and dissolved in water with dilute fluorescein isothiocyanate (FITC)-labeled dextrans (70 and 500 kDA). Hydrogels encapsulating the dextrans were formed by raising the temperature. Fluorescence recovery after photobleaching (FRAP) studies showed that diffusion coefficients and mobile fractions of the dextran dyes decreased upon elevating temperatures above 25 °C. Confocal laser scanning microscopy and cryo-SEM demonstrated that hydrogel structure depended on PEG block length. Phase separation into polymer-rich and water-rich domains occurred to a larger extent for polymers with small PEG blocks compared to polymers with a larger PEG block. By changing the PEG block length and thereby the hydrogel structure, mobility of FITC-dextran could be tailored. At physiological pH the hydrogels degraded over time by ester hydrolysis, resulting in increased mobility of the encapsulated dye. Since diffusion can be controlled according to polymer design and concentration, plus temperature, these biocompatible hydrogels are attractive as potential in situ gelling biodegradable materials for macromolecular drug delivery.

Can cyclopropyl-terminated self-assembled monolayers on gold be used to mimic the surface of polyethylene?

This paper describes the formation of a new series of monolayer films generated by the self-assembly of omega-cyclopropylalkanethiols, CyPr(CH(2))(n)SH (n = 9-13), onto the surface of gold. Procedures used to prepare the omega-cyclopropylalkanethiol adsorbates are also reported. Methyl-, vinyl-, and isopropyl-terminated self-assembled monolayers (SAMs) were also prepared and used as reference films to evaluate the structure and properties of the new cyclopropyl-terminated films. Ellipsometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were used to examine the structure of the SAMs. A small but systematically lower thickness of the new films compared to that of analogous methyl-terminated SAMs was observed. Also, the orientation of the ring with respect to the surface normal was observed to vary systematically with the number of methylene groups in the adsorbate backbone (i.e., odd vs even chain lengths). Measurements of wettability by contact angle goniometry also revealed a small but reproducible "odd-even" effect for all contacting liquids used, except hexadecane, which almost completely wet the surfaces (theta(a) = 10-13 degrees ). When compared to the wettability data obtained from methyl- and isopropyl-terminated SAMs, the wettability data obtained from the cyclopropyl-terminated SAMs suggest that these films offer an increased density of atomic contacts per unit area across the surface, and thus enhanced attractive interactions with contacting liquids. Comparison of the wettabilities of vinyl-terminated and cyclopropyl-terminated films is complicated by dipole-induced dipole interactions and/or pi-pi interactions between the surfaces and the probe liquids. Furthermore, the significantly similar wettabilities of the cyclopropyl-terminated SAMs and the surface of polyethylene suggests that these SAMs (and perhaps other SAMs with judiciously designed tailgroups) can be used to mimic the interfacial properties of polymeric materials without complications arising from surface reconstruction.

Responsive hydrogels for label-free signal transduction within biosensors.

Hydrogels have found wide application in biosensors due to their versatile nature. This family of materials is applied in biosensing either to increase the loading capacity compared to two-dimensional surfaces, or to support biospecific hydrogel swelling occurring subsequent to specific recognition of an analyte. This review focuses on various principles underpinning the design of biospecific hydrogels acting through various molecular mechanisms in transducing the recognition event of label-free analytes. Towards this end, we describe several promising hydrogel systems that when combined with the appropriate readout platform and quantitative approach could lead to future real-life applications.

4-Mercaptophenylboronic acid SAMs on gold: comparison with SAMs derived from thiophenol, 4-mercaptophenol, and 4-mercaptobenzoic acid.

We report the formation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of 4-mercaptophenylboronic acid (MPBA) on gold. For comparison, SAMs derived from the adsorption of thiophenol (TP), 4-mercaptophenol (MP), and 4-mercaptobenzoic acid (MBA) were also examined. The structure and properties of the SAMs were evaluated by ellipsometry, contact-angle goniometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Specifically, ellipsometry was used to assess the formation of monolayer films, and contact angle measurements were used to determine the surface hydrophilicity and homogeneity. Separately, PM-IRRAS was used to evaluate the molecular composition and orientation as well as the intermolecular hydrogen bonding within the SAMs. Finally, XPS was used to evaluate the film composition and surface coverage (i.e., packing density), which was observed to increase in the following order: TP < MP < MPBA < MBA. A rationalization for the observed packing differences is presented. The XPS data indicate further that ultrahigh vacuum conditions induce the partial dehydration of MPBA SAMs with the concomitant formation of surface boronic anhydride species. Overall, the analytical data collectively show that the MPBA moieties in the SAMs exist in the acid form rather than the anhydride form under ambient laboratory conditions. Furthermore, stability studies find that MPBA SAMs are surprisingly labile in basic solution, where the terminal B-C bonds are cleaved by the attack of hydroxide ion and strongly basic amine nucleophiles. The unanticipated lability observed here should be considered by those wishing to use MPBA moieties in carbohydrate-sensing applications.

Systematic control of the packing density of self-assembled monolayers using bidentate and tridentate chelating alkanethiols.

The structural and interfacial properties of self-assembled monolayers (SAMs) on gold derived from the adsorption of a series of 1,1,1-tris(mercaptomethyl)alkanes (i.e., CH3(CH2)mC[CH2SH]3, where m = 9, 11, 13, 15) were investigated. The new SAMs, which possess uniformly low densities of alkyl chains, were characterized by ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy. Additional analysis of the SAMs by X-ray photoelectron spectroscopy permitted a direct calculation of the packing densities of the SAMs on gold. The results as a whole, when compared to those obtained on SAMs generated from normal alkanethiols (CH3(CH2)m+2SH), 2-alkylpropane-1,3-dithiols (CH3(CH2)mCH[CH2SH]2), and 2-alkyl-2-methylpropane-1,3-dithiols (CH3(CH2)mC(CH3)[CH2SH]2) having analogous chain lengths, demonstrate that the 1,1,1-tris(mercaptomethyl)alkanes afford SAMs with alkyl chains having the lowest packing density and least conformational order.