Ionophore-Based\nSERS Sensing for Electrolyte Cations

We present here a surface-enhanced\nRaman spectroscopy (SERS)-based\nplatform for selectively detecting electrolyte cations. This platform\nfacilitates the reorientation of the chromoionophore I (CHI) molecule\npositioned on the surface of silver nanoparticles, regulated by the\ntargeted electrolyte cation within the ionophore-based ion-selective\nsensing framework. When exposed to the target ions, leading to deprotonation,\nthe aromatic plane of the CHI molecule shifts from an endwise to an\nedgewise configuration on the SERS substrate surface. This reorientation\nimproves the coupling of the induced dipole within the molecule and\nthe induced electromagnetic field normal to the surface, leading to\na heightened and more discernible SERS signal. Additionally, NMR data\nrevealed a protonation-dependent conformational change in the CHI\nmolecules. Upon protonation, the side chain of the CHI molecule extends\naway from its aromatic ring, whereas upon deprotonation, the carbon\nchain folds back and closely approaches the edge of the aromatic plane,\nfurther confirming this conclusion. Using Ca<sup>2+</sup> and Na<sup>+</sup> as examples, this method shows a detection limit of 0.1 μM\nfor Ca<sup>2+</sup> and 1 μM for Na<sup>+</sup> with high selectivity.\nThe successful application of the versatile and rapidly advancing\nSERS technique for electrolyte cation detection shows a promising\npathway for enhancing current ion detection capabilities. As a preliminary\ndemonstration, the total Ca<sup>2+</sup> concentration in undiluted\nhuman serum was successfully determined, with common matrix interference\neffectively circumvented.