RESEARCH Journal of Fluorescence https://doi.org/10.1007/s10895-025-04473-x responses [7–10]. These advantages have facilitated their extensive adoption in various fields, including environ- mental monitoring, medical diagnostics, pollution control, biomedical research, and quality assessment in the indus- trial and food sectors [11–19]. Recently, increasing atten- tion has been devoted to engineering fluorescent probes capable of selectively recognizing biologically and environ- mentally relevant metal ions [20–24]. Notably, copper (II), a redox-active trace element, plays indispensable roles in vital physiological processes such as mitochondrial respira- tion, connective tissue maturation, and neurological func- tion [22, 25–30]. Despite its essentiality, sustained exposure to elevated copper levels can disturb cellular homeostasis and has been linked to hepatic and renal toxicity, as well as the progression of neurodegenerative disorders, includ- ing Wilson’s, Menkes, and Alzheimer’s diseases [31–35]. In light of these concerns, the World Health Organization has established 2 ppm as the maximum permissible level of cop- per in drinking water [36, 37]. Given its dual role as both a Introduction The pervasive influence of transition and heavy metal ions across biological systems and environmental matrices has positioned their accurate detection as a central challenge in modern analytical chemistry [1–6]. Fluorescence-based detection systems have emerged as a preferred analytical tool due to their exceptional sensitivity, straightforward operation, and ability to provide immediate and real-time  Mahmood Tajbakhsh tajbaksh@umz.ac.ir  Yaghoub Sarrafi ysarrafi@umz.ac.ir Ali Zamani a.zamani17@umail.umz.ac.ir; zamaniali225@gmail.com 1 Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar, Iran Abstract A novel fluorescent and colorimetric probe (L3), based on 2,6-pyridinedicarbohydrazide, was rationally designed and synthesized for the highly selective and reversible detection of Cu²⁺ ions. The structural and optical properties of the probe were comprehensively characterized by FT-IR, ¹H NMR, UV-Vis spectroscopy, fluorescence analysis, ESI⁺-MS, and elemental analysis. The probe L3 displayed a high selectivity, high sensitivity and quick response to Cu 2+ ions in the pres- ence of other competing cations including Ba 2+ , K + , Pb 2+ , Hg 2+ , Al 3+ , Zn 2+ , Fe 2+ , Tl + , Mg 2+ , Ni 2+ , Nd 3+ , Fe 3+ , Ag + , Ca 2+ , Cs + , Co 2+ , Cd 2+ , Sr 2+ , and Mn 2+ ions). Upon interaction with Cu²⁺, probe L3 exhibited distinct optical responses, includ- ing significant fluorescence quenching, a notable redshift in the UV-Vis spectrum, and a rapid, visible color change from colorless to greenish-yellow under aqueous conditions and across a broad pH range. The detection limit was calculated to be as low as 2.28 × 10⁻⁹ M, with an association constant (Ka) of 2.2 × 10¹¹ M⁻², indicating strong binding affinity. Job’s plot and ¹H NMR titration confirmed a 1:2 stoichiometric complex formation between L3 and Cu²⁺ ions. The probe also exhibited excellent reversibility, as confirmed by fluorescence recovery upon adding Na₂EDTA, suggesting its suitability for real-time and reusable sensing applications. Furthermore, the practical utility of L3 was demonstrated using test strips for Cu²⁺ detection in aqueous environments, and its effectiveness was further validated in real water samples, including tap water, drinking water, and seawater, achieving recovery rates up to 99.2%. These findings suggest that L3 is promis- ing for the sensitive, rapid, and cost-effective monitoring of copper ions in environmental and drinking water systems. Keywords Copper ions · Colorimetric · Reversible probe · Pyridine-based sensor · Naked-eye detection Received: 24 May 2025 / Accepted: 15 July 2025 © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2025 A Selective Pyridinedicarbohydrazide-based “naked-eye” Fluorescent Probe for Detecting Cu 2+ Ions in Aqueous Solutions Ali Zamani 1  · Mahmood Tajbakhsh 1  · Yaghoub Sarrafi 1 1 3 Content courtesy of Springer Nature, terms of use apply. Rights reserved.