Gold nanoparticle induced enhancement of molecular fluorescence for Zn²⁺ detection in aqueous niosome solution

Modified bipyridine-based construct 4-(pyridine-2-yl)-3H-pyrrolo [2,3-c] quinoline (PPQ), and its assemblies with niosomes and gold nanoparticles (GNP) to function as turn-on fluorescent sensors for biologically important Zn²⁺ in aqueous solution are evaluated. Earlier, we have shown fluorescence-based selective ratiometric detection of Zn²⁺ by PPQ in DMF [RSC Adv. 2014, 4, 25605]. This work attempts to overcome the water solubility problem of the hydrophobic PPQ, by using non-ionic surfactant led niosomes. The interest in niosome resides in the fact that they can mimic biological systems. Metals like Ag, Au are known to quench fluorescence, but using metal nanoparticles molecular fluorescence can be enhanced by increasing rate of excitation with concentrating electric field around the fluorophore and also by increasing the rate of radiative decay. This is known as Metal enhanced Fluorescence (MEF). When, PPQ binds with Zn²⁺ and forms the complex in presence of GNP, MEF is evident; whereas no significant fluorescence intensity enhancement is observed for PPQ in absence of either gold nanoparticles or Zn²⁺. The optimum size for the gold nanoparticles used in our study is ~40 nm, which shows the enhancement. The MEF is evident in niosomes, as there is an optimal distance between PPQ-Zn²⁺-PPQ complex (residing in the hydrophobic bilayer) and the nanoparticles (in the water pool). The enhancement in fluorescence intensity is accompanied by decrease in fluorescence lifetimes. The described sensing systems, are capable of detecting Zn²⁺ in HeLa cells at physiological conditions, as shown by bioimaging studies.