Thione–gold nanoparticles interactions: Vroman-like effect, self-assembly and sensing

The chemistry and interaction of thiones towards passivation, and self-assembly of metal nanoparticles is nascent and poorly understood. Here, we describe a sensitive (15 ppb) and rapid (<1 min) thiourea (TU) based self-assembly of citrate stabilized gold nanoparticles (AuNPs) into 1D arrays (or nanochains) through a non-crosslinking mechanism. The underlying principle for the nanochain formation is as follows: the displacement of negatively charged citrate with neutral TU in a Vroman-like effect introduces a surface polarity, which then drives the self-assembly of AuNPs into nanochains via the dipole–dipole interactions. Moreover, we find that the size of the nanochains can be controlled by regulating the strength of the resultant dipole, which depends on the physico-chemical parameters such as the zeta potential and hydrodynamic size. We further substantiate the dipole-based mechanistic pathway by demonstrating that addition of TU leads to a selective aggregation of electrostatically stabilized AuNP systems while sterically stabilized ones remain unaffected. The mechanism of ligand displacement (akin to Vroman effect) is supported by an increased sensitivity of the assembly process in the presence of NaHSO4. The soft–soft interactions of the gold–thione pair and the kinetics of ligand exchange were investigated and found to be influenced by the manner in which the substituents attached to the TU moiety modify the electron density around the thione sulphur. As demonstrated for TU detection (9.35 ppb) in sweet lime juice, the method serves as a simple, sensitive, selective, and rapid colorimetric assay for TU.