Tuning the Electrostatic Interactions in Nanoionic Precipitates for Heavy Metal Ion Detection

Abstract

The ability to improve and impart new properties in existing nanomaterials is one of the emerging challenges in nanoscience. Our group has recently shown the emergence of selectivity in a traditionally and inherently non-selective carboxylate functionalized gold nanoparticle system ([-] AuNPs). Here, the abilities of different ions were used to break a nanoionic precipitate, where oppositely charged AuNPs are glued together using electrostatic interactions. It was found that the system showed a selective turn on response (appearance of plasmon color) in the presence of Pb2+ over other strongly binding divalent ions (M2+). This system, although conceptually attractive, could only detect ~ mM amounts of Pb2+ ions. The aim of the present work, described in this thesis is to improve the sensitivity of this system by the mere tuning of interparticle forces. These interparticle forces in question are nothing but the electrostatic interactions which hold together the oppositely charged nanoparticles in the nanoionic precipitate. By weakening the electrostatic interactions, we hypothesize that lesser amounts of Pb2+ would be required to break it, thus leading to an improvement in the sensitivity of the detection system. Since the electrostatic interactions are dependent on the ionic strength of the medium and the surface chemistry, they can be used as suitable parameters to control these interactions. In the first part, we study the sensitivity as a function of the ionic strength of the medium. Upon increasing the ionic strength to ~ 300 mM, we observed a ~ 50 fold improvement in the sensitivity of Pb2+ detection. Subsequently, by additionally varying the surface chemistry by the use of mixed charge nanoparticle systems, we observed a further improvement (~ 8 fold) in the sensitivity, which is comparable to/ or better than the best known values with carboxylate functionalized nanoparticles.