Facile rhodamine-based colorimetric sensors for sequential detections of Cu(ii) ions and pyrophosphate (P₂O₇ ⁴⁻) anions

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 Rh2via 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.