Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.
Polymer modification of mesoporous materials is a relevant topic for applications from sensing and separation to drug delivery. Especially the combination of structure and responsive, charged polymer functionalization opens new possibilities, such as gating of drug release. Thereby, zwitterionic polymers are interesting, because of their antifouling characteristics and their influence on ionic permselectivity. The control on polymerization in confinement of a mesopore including spatial polymer location is crucial for the resulting mesopore function such as ionic permselectivity. The amount of generated polymer and polymerization kinetics should be influenced by the confinement of mesopores: Diffusion is limited by the pore size, pore connectivity, and wall charge. These parameters influence termination, which depends on radical concentration and proximity. Concerning the total surface area, the size of the internal surface area of mesoporous materials largely dominates over the size of the outer surface area. This might tempt one to relate observed functionalization effects to the internal surface. Here, we investigate iniferter-initiated polymerization in thin mesoporous silica films concerning the type of iniferter, the generated amount of polymer, the polymerization inside the mesopores versus the external surface, and the used monomer. Our results clearly indicate potential bottlenecks of iniferter-initiated polymerizations in mesopores. The charge of the monomer can be crucial for the generated amount of polymer inside the mesopores and the exterior surface can dominate the polymerization. These results emphasize that polymer distribution in mesoporous materials must be analyzed carefully before data interpretation and that polymerization inside nanometer-sized pores can be controlled by confinement effects. We expect these results to have great impact, e.g., on the design of miniaturized separation and sensing devices, and to open new confinement-controlled functionalization strategies.
Creating switchable and gradually tunable pores or channels that display transport control similar to biological pores remains a major challenge in nanotechnology. It requires the generation and manipulation of complex charge situations at the nanoscale and the understanding of how confinement influences chemistry and transport. Here, two different pore sizes, ∼100 nm and less than 10 nm, functionalized with varying amounts of responsive zwitterionic polycarboxybetaine methyl acrylate (PCBMA) give fascinating insight into the confinement controlled ionic transport of pores functionalized with pH-dependent zwitterionic polymers. Under basic conditions, the zwitterionic state offers complex, strongly pore-size-dependent ionic permselectivity characteristics. For mesoporous films with pore sizes smaller than 10 nm, complete ion exclusion is observed after reaching a critical zwitterionic polymer amount, clearly indicating an electrostatic behavior of "bipolar charged" pores. This ion exclusion is not observed for pore diameters of ∼100 nm. In addition, the solution pH of equal pore accessibility for oppositely charged ions and pore sizes smaller than 10 nm shifts with increasing polymer amount from a pH of 2.5 to 8.2, and the quantity of ions accessing the pores depends on the pore size. These observations clearly show the potential of controlling pore accessibility based on controlled functional composition at the nanoscale without changing the components themselves as well as the influence of spatial confinement on pore accessibility in the presence of complex (zwitterionic) charged states.
Nanopores play a decisive role in different technologies from oil production, separation, and sensing to drug delivery or catalysis and energy conversion. In recent years, abilities to functionalize nanopores have advanced significantly. Thereby, nanopores functionalized with polyelectrolytes or responsive polymers show fascinating transport properties, such as gated or gradually controlled ionic permselectivity. Nonetheless, understanding the influence of external parameters such as ion type or concentration on nanopore performance, and thus on the mentioned applications, remains a challenge but is crucial for applications. In this work, the effect of different counterions on the wetting and ionic transport in poly(2-(methacryloyloxy)ethyltrimethylammonium chloride)-functionalized silica mesopores (pore diameter <10 nm) was experimentally and theoretically investigated. Static contact angles covered a range from 45 to almost 90° by exclusively changing the counterion. Ionic pore accessibility was also strongly dependent on the counterion present and was found to gradually change from accessible pores up to complete, pH-independent ion exclusion. On the basis of molecular theory calculations, these experimental observations were rationalized on the basis of ion binding between the [2-(methacryloyloxy)ethyl]trimethylammonium chloride monomers and the counterions. In addition, the theoretical framework provided a nanoscopic view into the molecular organization inside the pores, showing a strong dependence of ion concentration and ion distribution profiles along the pore radius in dependence of the present ions. The obtained insights on the role of counterion type and ion binding in nanopores are expected to have direct impact on the above-mentioned applications.
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