Strategic Pairing of Organic and Inorganic Components for Self-Assembled Gel Formation: A Nickel(II) Metallogel for Selective Cysteine Recognition

Abstract

The strategic pairing of organic gelation components with inorganic cations or anions for gel fabrication remains a relatively unexplored area of research. In this regard, four Schiff-base–based gelation test molecules GTM1 ((1-phenyl-N-(2-(piperazin-1-yl)ethyl)methanimine), GTM2 (N-(2-morpholinoethyl)-1-phenylmethanimine), GTM3 (4-(((2-morpholinoethyl)imino)methyl)benzonitrile), and GTM4 ((2-(piperazin-1-yl)ethyl)imino)methyl)benzonitrile), have been designed, synthesized, and characterized using various spectroscopic techniques. Aqueous solutions of various metal nitrate salts were systematically incorporated into DMSO solution containing the gelation-test molecules GTM1, GTM2, GTM3, and GTM4 to investigate their effects on gel formation. Among all, GTM1 and GTM4 readily form gel with nickel nitrate, specifically as GTM1Ni and GTM4Ni metallogels. To study the effect of anions in gel formation, the gelation test molecules GTM1, GTM2, GTM3, and GTM4 were taken in DMSO and treated with different sodium salts. The gelation components containing the nitrile group, GTM3 and GTM4, readily form an organogel specifically with sodium azide (GTM3N3 and GTM4N3). Rheological studies, including amplitude and frequency sweeps, have been performed on GTM1Ni, GTM4Ni, GTM3N3, and GTM4N3 organogels to characterize their viscoelastic properties. Upon addition of various aqueous amino acid solutions onto the gel beds of GTM1Ni and GTM4Ni metallogels, only cysteine induced a distinct color change from green to brownish-black, indicating selective naked-eye detection of cysteine in both systems. For the sensing of cysteine, the LOD for the GTM1Ni metallogel was determined to be 0.108 μM, whereas the GTM4Ni metallogel exhibited an LOD of 0.078 μM. The GTM3N3 and GTM4N3 systems demonstrate remarkable stimuli-responsive gel formation and reversible behavior, transitioning from gel to sol upon heating and reforming the gel upon subsequent sonication. This study presents a rationally designed gelation component systems in which functional group optimization enables selective metallogel formation with Ni2+ and sodium azide, selectively. The nickel metallogels have been further utilized for selective recognition of cysteine, an approach that remains relatively underexplored.