Electrostatically driven self-assembly of CdTe nanoparticles with organic chromophores probed via Ham effect

Abstract

We demonstrate that the polarity influence on the pyrene fluorescence band ratios (I₃/I₁) termed as the Ham effect can be effectively utilized to study the electrostatic interaction of CdTe nanoparticles (ζ = −46 mV) with the oppositely charged organic molecule, pyrenemethylamine hydrochloride (PMAH). As compared to water, the less polar nature of the QD's surface resulted in an increase in I₃/I₁ peak ratio of PMAH (from 0.34 to 0.54) when brought into close proximity. The lowering of the asymmetric stretching frequency of the carboxylate group of thioglycolic acid (1570 cm⁻¹ to ∼1550 cm⁻¹) and the drop in zeta potential of the nanoparticle from −46 mV to −8 mV indicate the interaction of PMAH with the nanoparticles. Control experiments with capping ligands, QDs with different surface charges (−46, −39, −35, −31, −20 and +21 mV) and negatively charged pyrene molecules invariably support the role of nanoparticles and their surface charge. The combined effects of charge neutralization and hydrophobicity increase lead to the organization of QDs into 2D sheet like superstructures, preserving their initial individuality.