Fluorimetric detection of distinct lyotropic anion interactions on nanoscopic surfaces

Abstract The molecular structure, stability and function of proteins, nucleic acids, and many other colloidal entities including nanomaterials depend greatly on their highly specific response towards surrounding solvent molecules and inorganic ions. In order to earn an unambiguous mechanistic insight into the origin of these effects, water-soluble fluorescent carbon nanomaterials having amide groups in a hydrophobic microenvironment were successfully employed as the model colloidal system in this research study. Quantum dots with acidic functionalities were first of all prepared using a simple carbon precursor. Subsequently, chemical modification of carbon quantum dots (CQDs) was carried out with isopropylamine to impart protein-like amide richness to the nanoparticle surface. Moreover, the prepared CQDs were found to be highly fluorescent. Fluorescence intensity, interestingly, showed unique modulation patterns on incubation with different types of anions. Lipophilic chaotropic anions such as I−, SCN−, and Br− showed concentration-dependent fluorescence quenching whereas hydrophilic kosmotropic anions like SO42− had almost no impact on the nanomaterial fluorescence. This striking observation in the case of chaotropic anions can be correlated to their strong partitioning affinity for the amide groups on the nanomaterial surface. High charge density anions like sulphate, on the other hand, are mostly excluded from the colloidal interfaces. The current study emphasizes the fact that the functional groups available on the surface of macromolecules as well as anion's inherent thermodynamic parameters in a solvent play a pivotal role in the observed anion specificity.