The formation of mixed micelles composed of dodecyltrimethylammonium bromide (C(12)TAB) and a hexamethylated p-sulfonatocalix[6]arene (SC6HM) was studied by several techniques. It was found that above the critical aggregation concentration the concentrations of free and micellized surfactant are strongly related to that of SC6HM. When there is free SC6HM in solution, the addition of C(12)TAB mainly results in an increase in the concentration of micellized surfactant, but when all SC6HM has been aggregated, the addition of C(12)TAB results in a substantial increase in the concentration of free surfactant in solution. When the concentration of free surfactant is equal to the critical micelle concentration of the pure system, a second independent aggregation process is observed. This aggregation behavior has many features that are similar to those of more complex systems that involve surfactants in the presence of oppositely charged polyelectrolytes. In this way, calixarenes can serve as simple models to mimic polyelectrolytes and to gain insight into the complex behavior displayed by these macromolecules.
We present a simple procedure for the synthesis of quasi-spherical Au nanoparticles in a wide size range mediated by macrocyclic host molecules, ammonium pillar[5]arene (AP[5]A). The strategy is based on a seeded growth process in which the water-soluble pillar[5]arene undergoes complexation of the Au salt through the ammonium groups, thereby avoiding Au nucleation, while acting as a stabilizer. The presence of the pillar[5]arene onto the Au nanoparticle particle surface is demonstrated by surface-enhanced Raman scattering (SERS) spectroscopy, and the most probable conformation of the molecule when adsorbed on the Au nanoparticles surface is suggested on the basis of theoretical calculations. In addition, we analyze the host-guest interactions of the AP[5]A with 2-naphthoic acid (2NA) by using (1)H NMR spectroscopy and the results are compared with theoretical calculations. Finally, the promising synergetic effects of combining supramolecular chemistry and metal nanoparticles are demonstrated through SERS detection in water of 2NA and a polycyclic aromatic hydrocarbon, pyrene (PYR).
The complexation of an anionic guest by a cationic water-soluble pillararene is reported. Isothermal titration calorimetry (ITC), (1)H NMR, (1)H and (19)F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and (1)H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.
In this work, the formation of supramolecular mixed micelles from a hexamethylated p-sulfonatocalix[6]arene (SC6HM) derivative and a conventional cationic surfactant (dodecyltrimethylammonium bromide, CTAB) was investigated by surface tension and using pyrene as a micropolarity fluorescent probe to gain insights into the role of the calixarene concentration on the aggregation behavior. The formation of micelles at a concentration well below the critical micelle concentration of pure surfactant was observed in the presence of very low concentrations of SC6HM (below the micromolar range). Interestingly, the critical micelle concentration of the mixed system was shown to be rather insensitive to the concentration of SC6HM. On the other hand, the concentration of mixed micellar aggregates was demonstrated to be highly dependent on the macrocycle concentration and less dependent on the CTAB concentration in the range between the critical micelle concentrations of the mixed systems and pure surfactant.
We present here a simple procedure for the surface modification of plasmonic nanoparticles (NPs) with a cationic water-soluble ammonium pillar[5]arene (AP[5]A) in order to create selective surface-enhanced Raman scattering (SERS) spectroscopy based sensors. The strategy is based on a ligand exchange reaction between the AP[5]A and the stabilizing agent of the as-prepared plasmonic NPs. The approach could be applied to plasmonic nanoparticles either negatively charged, stabilized by citrate ions (Au spheres) or positively charged, stabilized by cetyltrimethylammonium bromide (Au and Au@Ag nanorods). The SERS performance of all systems was studied as a function of NP size and excitation laser line by using an analyte with no affinity towards the metal surface such as pyrene. The analytical enhancement factor (AEF) for the different systems was estimated between 0.55 3 10 4 and 1.49 3 10 5 . Finally the synergistic effect of combining supramolecular chemistry and plasmonic NPs is demonstrated through SERS-based detection, in aqueous media, of molecules with no affinity towards a bare plasmonic substrate such as the contaminant pyrene or the biomolecule pyocyanin with nanomolar limit of detection.
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