The small silver nanoparticle-assisted homogeneous sensing of thiocyanate ions with an ultra-wide window based on surface-enhanced Raman-extinction spectroscopy

Abstract

For the first time, we present an original sensing strategy with an ultra-wide detection window from 17 nM to 20 mM to detect SCN⁻ ions. Initially, we investigated the clustering and optical properties of noble metal sol nanoparticles (NPs) due to the competitive interaction of thiocyanate ions (SCN⁻) and cetyltrimethylammonium bromide (CTAB) under weak acidic conditions, and found that different dimensions and scales of nanoclusters containing the alkyne-embedded Au@Ag NPs and relatively small Ag NPs could be achieved by the mediation of CTAB through electrostatic forces and hydrophobic interaction, in which SCN⁻ could be covalently bonded with the silver surface of NPs to form a compact molecular layer (–Ag–S–CN), and CTAB could only occupy remaining sites. In this process, we found that SCN⁻ always runs counter to CTAB and tends to dissolve nanoclusters, so that they occupy the exposed surface of NPs in nanoclusters rather than the binding sites of one another. Remarkably, when the concentration of SCN⁻ initially increased, two highly recognizable SERS emissions, which were assigned to alkyne reporter molecules (2208 cm⁻¹) and CN of SCN⁻ (2110 cm⁻¹), respectively, were rapidly detected, and their ratios (I₂₁₁₀/I₂₂₀₈) increased linearly proportional to the concentration of SCN⁻ over a range of 17 nM to 172 μM, with a limit of detection (LOD) of 10 nM. With the further increase of SCN⁻, small Ag NPs started to desorb from the surface of individual Au@Ag NPs and dissociated in the solution but did not contribute to SERS signals. Instead, the surface plasmon resonance (SPR) peak of pure silver NPs at 385 nm increased gradually in the range from 0.5 to 20 mM with an LOD of 0.2 mM. Of particular significance, this simple sensor in conjunction with surface-enhanced Raman-extinction spectroscopy can be used for the rapid detection of extensive samples with an ultra-wide detection window, such as body fluids (saliva, urine, and serum) and food (milk powder and brassica vegetables), which is far superior to that of ion chromatography (IC).