Fluorescent and colorimetric dual-mode chemosensor for easy detection of Cu²⁺: Practical and smartphone-based application to test strip and water sample

A fluorescent and colorimetric dual-mode chemosensor HNHQ, based on the julolidine and naphthol moieties, was synthesized. Compound HNHQ exhibited unique photophysical properties of an intense fluorescence with a high quantum yield (Φ = 0.363) and a light yellow color through the ICT mechanism. HNHQ could detect Cu²⁺ through both a fluorescence turn-off and a colorimetry variation of light yellow to yellowish brown. HNHQ reacted with Cu²⁺ with a binding ratio of 1:1. The detection limit for Cu²⁺ was analyzed to be 1.60 μM for fluorescence and 1.32 μM for UV–vis, considerably below the safe value of 31.5 μM established by the World Health Organization (WHO). HNHQ analyzed Cu²⁺ in the pH range of 6–8 and reversibly identified Cu²⁺ by using ethylenediaminetetraacetic acid (EDTA). Moreover, HNHQ could effectively detect Cu²⁺ in mineral and tap water samples, in addition to test strips through fluorescent and colorimetric changes. As a portable probe, HNHQ was successfully applied to a smartphone-based RGB analysis for the quantitative examination of Cu²⁺. The detection mechanism of HNHQ for Cu²⁺ was illustrated by ¹H NMR titration, ESI-MS analysis, and density functional theory (DFT) calculations.