Facile rhodamine-based colorimetric sensors for sequential detections of Cu(ii) ions and pyrophosphate (P2O74−) anions

Abstract

Two rhodamine hydrazine derivatives Rh1 and Rh2 with catechol and ether functionalities have been synthesized and utilized towards sequential colorimetric detections of Cu(II) and pyrophosphate (PPi) ions in CH₃CN–H₂O (v/v = 9 : 1, 5 mM Tris–HCl, pH 7.4) semi-aqueous medium. Notably, Rh1 and Rh2 are the first example of colorimetric rhodamine-based probes for the sequential detections of Cu²⁺ ion and PPi anion. Based on the significant colorimetric responses (from colorless to pink) of Rh1 and Rh2 to Cu²⁺ ions, the detection limits were estimated as low as 1.22 × 10⁻⁸ M and 8.0 × 10⁻⁷ M, respectively, which signified the utilities of designed probes towards facile detections of Cu²⁺ ions. Furthermore, upon the successive addition of PPi ion to Rh1–Cu²⁺ and Rh2–Cu²⁺ complexes, it has been altered and restored to its origin of Rh1 and Rh2 via re-coordination of PPi to Cu²⁺ ion, which was confirmed by color changes from pink to colorless. Moreover, computational DFT calculations provided more insights into HOMO–LUMO structures of rhodamine derivatives and their copper complexes. Additionally, the solid state strip-based colorimetric detections of Cu²⁺ ion were supplemented as a real time application.