Liquid/liquid interface polymerized porphyrin membranes displaying size-selective molecular and ionic permeability

Thin polymeric membranes have been formed by liquid/liquid interfacial copolymerization of a sterically demanding tetraphenylporphyrin derivative having reactive phenol substituents and a second porphyrin having reactive acid chloride groups. The out-of-plane steric demand is created by 3,5-hexoxyphenyl groups positioned at two of the four meso carbons of the porphyrin ring. The bulky substituents were designed to create local pockets and extended pores within the resulting ester-linked copolymer. Quantitative measures of molecular and ionic transport were obtained by placing membranes over microelectrodes and recording voltammetric responses from redox-active probes. The membranes were found to be permeable to small molecules and ions, but blocking toward larger ones, displaying a sharp size cutoff at a probe diameter of ca. 3.5 A. Molecular transport can be modulated by axially ligating pore-blocking moieties to available porphyrin metal centers.     

Microchip electrophoresis of DNA following preconcentration at photopatterned gel membranes

Rapid separation of nucleic acids by microchip electrophoresis could streamline many biological applications, but conventional chip injection strategies offer limited sample stacking, and thus limited sensitivity of detection. We demonstrate the use of photopatterned polyacrylamide membranes in a glass microfluidic device, with or without fixed negative charges, for preconcentration of double-stranded DNA prior to electrophoretic separation to enhance detection limits. We compared performance of the two membrane formulations (neutral or negatively charged) as a function of DNA fragment size, preconcentration time, and preconcentration field strength, with the intent of optimizing preconcentration performance without degrading the subsequent electrophoretic separation. Little size-dependent bias was observed for either membrane formulation when concentrating dsDNA > 100 bp in length, while the negatively charged membrane more effectively blocks passage of single-stranded oligonucleotide DNA (20-mer ssDNA). Baseline resolution of a six-band dye-labeled ladder with fragments 100-2000 bp in size was obtained in <120 s of separation time, with peak efficiencies in the range of 2000-15,000 plates/cm, and detection limits as low as 1 pM per single dye-labeled fragment. The degree of preconcentration is tunable by at least 49-fold, although the efficiency of preconcentration was found to have diminishing returns at high field and/or long times. The neutral membrane was found to be more robust than the negatively charged membrane, with approximately 2.5-fold larger peak area during the subsequent separation, and less decrease in resolution upon increasing the preconcentration field strength.