Guaiazulene-Based 2‑Hydrazinylpyridine Probe (GAHP550) for Ultralow, Reversible Cu²⁺ Detection and Molecular Logic...

A guaiazulene-derived 2-hydrazinylpyridine probe (GAHP550) was developed for the highly sensitive, selective, and reversible detection of Cu²⁺ ions, with seamless integration into a smartphone-based analytical platform. The probe was synthesized via a simple two-step procedure from commercially available guaiazulene in 88.7% overall yield, without the need for chromatographic purification. The structure was confirmed using NMR, HRMS, FTIR, and single-crystal X-ray diffraction. GAHP550 exhibits a rapid, visually perceptible green-to-brown color change upon Cu²⁺ binding, with a linear response between 5.0 × 10^-7 and 1.2 × 10^-3 M and an ultralow detection limit of 9.45 × 10^-7 M (55.4 ppb), far below WHO (2.0 ppm) and EPA (1.3 ppm) recommended limits for drinking water. High selectivity was observed against a broad panel of competing cations and anions, and reversible sensing was demonstrated via multiple Cu²⁺/Na₄EDTA cycles. The binding stoichiometry was determined as 1:2 (Cu²⁺: GAHP550) with a strong binding constant of 1.21 × 10⁶ M^-2. Interestingly, the probe functions as an INHIBIT-type molecular logic gate using Cu²⁺ and Na₄EDTA as inputs. A smartphone application was developed to quantify Cu²⁺ concentrations in real-time using RGB analysis of the colorimetric transition. GAHP550 was then used to successfully quantify Cu²⁺ in tap and bottled water, achieving excellent recovery values of 83.5-104.2%, confirming the robustness and applicability in practical aqueous solutions. The integration of ultrahigh sensitivity, fast reversible response, logic gate functionality, and mobile-device compatibility establishes GAHP550 as a powerful, portable tool for environmental and field-based Cu²⁺ detection.