RESUMO
Conjugated polyelectrolytes (CPEs) have recently emerged as label-free materials for biosensing due to their intrinsic ability to transduce an amplified optical signal in response to interactions with different analytes. Herein, the conformational change of an anionic oligothiophene is exploited to generate a unique fluorescent response upon interaction with myristoylcholine (MyrCh). The variations observed in spectroscopic signals are explained in terms of a synergistic combination of hydrophobic and electrostatic forces involving the oligothiophene chains and MyrCh molecules, inducing the disassembling of oligothiophene chains. The enzyme acetylcholinesterase (AChE) is able to reverse this effect by catalyzing the hydrolysis of MyrCh; hence, its enzymatic activity can be monitored through the variation of fluorescence emission of the system. The oligothiophene sensing probe retains its conformational sensitivity with regard to the AChE-mediated cleavage of MyrCh upon immobilization onto a quartz substrate, which is accomplished by a "grafting onto" approach based on click chemistry. These results are encouraging for the further development of such a label-free system towards the fabrication of sensing devices that would incorporate CPEs and would be potentially useful for the specific detection of a wide range of bioanalytes.
RESUMO
The positive ion matrix-assisted laser desorption/ionization (MALDI) spectrum of an equimolar blend of Nylon 6 (Ny6) and hydroxyl-terminated polybutyleneterephthalate (PBT) shows a surprisingly strong imbalance between the two components. Since the average molar masses and the polydispersions of the two polymers are comparable, it follows that the efficiency of MALDI is quite different for the two components of the blend. This finding prompted us to a more detailed study, and to synthesize Ny6 and PBT samples terminated with different end groups, in order to analyze their blends by MALDI. The negative ion MALDI spectra of the mixtures investigated show that PBT samples do not yield signals, so that only Ny6 peaks appear in these spectra. By comparing positive and negative ion MALDI spectra of mixtures of Ny6 terminated with various end groups, it was found that the peak intensity depends on the nature of the end groups. The results reported in the present study may help to clarify some fundamental aspects of the mechanisms of ion formation, when MALDI mass spectrometry is applied to macromolecules. End group ionization efficiency appears to be the most important parameter in determining the relative intensity of peaks in the MALDI spectra of the polymer blends investigated. End-group-dependent ionization is the key to the rationalization of the relative peak intensities in MALDI spectra of polymer mixtures.
RESUMO
The determination of molar mass (MM) data for polydisperse polymers by SEC/MALDI involves the fractionation of samples through analytical size exclusion chromatography (SEC). Selected SEC fractions are then analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and the mass spectra of these nearly monodisperse samples allow the determination of the average molar masses. The SEC/MALDI procedure has now been applied to two polycarbonate samples, PC1 and PC2. The results show that the MALDI spectra of the SEC fractions allow not only the detection of linear and cyclic oligomers contained in these samples, but also the simultaneous determination of their average molar masses. Two slightly differing SEC calibration plots were obtained, due to the smaller hydrodynamic volume of the polycarbonate cyclic chains with respect to the linear ones. In agreement with theory, the ratio (M(cycle)/M(linear))(Ve) at a fixed elution volume was found to be 1.22, independent of the molar mass values. Copyright 1999 John Wiley & Sons, Ltd.
RESUMO
We report here a case of apparent failure of the size exclusion chromatography/matrix-assisted laser desorption/ionization (SEC/MALDI) method to provide polymer fractions with narrow molar mass distribution, showing that intermolecular chain association is responsible for this phenomenon. Poly(bisphenol A carbonate) (PC) chains terminated with hydroxyl groups undergo self-association by hydrogen bonding, providing macromolecular aggregates with higher hydrodynamic volume. These aggregates are eluted through SEC columns at the same volume as higher molar mass chains, which remain non-associated. Thus, self-association affects negatively the SEC fractionation experiments, and even the sharpest SEC fractions contain a heterogeneous mixture of PC chains of different size. When the off-line SEC/MALDI procedure is applied, the SEC fraction is diluted in the matrix which, being a dissociating medium (carboxylic acid) for hydrogen-bonded aggregates, suppresses the chains' self-association. Therefore, the MALDI spectra of these PC fractions indicate a polydisperse character, with irregular bimodal distributions of peaks. As a consequence, in the presence of chain association, the SEC/MALDI method for the calculation of molar masses of polymers cannot be directly applied. In the present case we have found that, under opportune experimental conditions, self-association in polycarbonates can be avoided, so that nearly monodisperse SEC fractions can be obtained and the SEC/MALDI method can be applied. Our results also show that MALDI is a very sensitive technique for the detection of association of polymers in dilute solutions. Copyright 1999 John Wiley & Sons, Ltd.
