Unlocking the Potential of Metal Ions in the Formation of Diverse Self-assembled Structures, Room Temperature Phosphorescence, and Sensing

Abstract

Metal ions play indispensable roles across diverse processes, both biological and abiological, but their significance in the formation of complex self-assembled structures is only beginning to be understood. Sodium cholate, a facially amphiphilic molecule, exhibits intriguing properties by self-assembling with metal ions to form gels. Studying its interaction with cadmium ions revealed the formation of diverse structures like particles, nanofibers, and nanorods/plates over time due to a pathway complexity involved in the gelation process. During our studies, it was discovered that the transformation from the nanofiber to the nanorod proceeds via fragmentation, followed by a secondary nucleation mechanism. We also enhanced the mechanical properties of silver cholate hydrogel (AgCh) by co-assembling it with a small amount of gadolinium ions, achieving a 4-fold increase in mechanical strength. This improvement resulted from a morphological transformation from nanoparticles to networks of nanofibrous, which led to a robust gel. Additionally, the silver cholate hydrogel was used as a host matrix for induction of room temperature phosphorescence from organic chromophores 1,3,5 tris(4 carboxyphenyl)benzene (TCB, green emission) and 1,4,5,8-naphthalenetetracarboxylate (NTCA, red emission). The phosphorescence was enhanced further in a co-assembled hydrogel matrix (GdAgCh). We also explored energy transfer from the triplet state of the phosphors to the singlet state of various fluorophores. It led to delayed luminescence and enhanced lifetime of the acceptor and can be considered an artificial light-harvesting system. This system shows good color tunability with persistence and stable luminescent hydrogel. Furthermore, we developed a lanthanide-based ‘turn-off’ sensor for the detection of water in organic solvents. We systematically quantify the water content in the organic solvents such as MeOH, EtOH, CH3CN, and THF. Our developed protocol is simple yet superior and can detect the presence of 0.1 to 5% water in these samples.