Surface-engineered cationic silver nanoparticles as a selective colorimetric sensor for mercury(II) detection in...

Synthesis of silver (Ag) nanoparticles was done using chemical reduction of AgNO₃ in ice cold NaBH₄, and functionalization of the surface was done using cysteamine via Ag-S thiolate bonding at pH 9.0. Quaternization of the surface was achieved via exhaustive N-methylation of the terminal amine group at the surface using iodomethane (CH₃I). This converts the surface -NH₂ groups stepwise into permanently positively charged quaternary ammonium moieties (-N⁺(CH₃)₃). The resulting surface-modified nanoparticles (AgNPs-QCy) were comprehensively characterized using UV-visible spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy coupled with Energy-Dispersive X-ray spectroscopy (SEM-EDX), Dynamic Light Scattering (DLS), and zeta potential measurements. Surface quaternization was confirmed by the red shift in the Localized Surface Plasmon Resonance (LSPR) band from ∼385 nm to 393 nm, attributed to changes in the local dielectric environment at the nanoparticle surface following quaternization, and accompanied by a decrease in the hydrodynamic diameter from 314.2 nm to 149.9 nm, consistent with improved colloidal uniformity upon surface charge stabilization. Zeta potential measurements confirmed the pH-independent positive surface charge, thus confirming the stability of the permanent quaternary ammonium group over a broad range of pH. AgNPs-QCy exhibited high selectivity toward Hg²⁺ ions in the presence of several competing metal ions, including Fe²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Pb²⁺, Al³⁺, Co²⁺, Ni²⁺, Cu²⁺, As⁵⁺, Cr³⁺, and La³⁺. The selective response toward Hg²⁺ is attributed to its strong interaction with the functionalized AgNP surface, leading to pronounced changes in nanoparticle interfacial interactions and plasmonic properties. These changes result in aggregation and suppression of the LSPR signal, producing a distinct colorimetric response that can be visually observed. AgNPs-QCy-based sensors exhibit a limit of detection (LOD) of 50.38 nM and limit of quantification (LOQ) of 167.9 nM. The sensing platform was successfully validated in real water matrices including lake, tap, and underground water collected from the Riyadh region (Kingdom of Saudi Arabia), confirming its practical applicability for environmental Hg²⁺ monitoring, and supporting its potential as a rapid, low-cost, single-use screening tool for Hg²⁺ detection.