Smartphone-Based Quantitative Measurement of Cu²⁺: Fluorescent Turn-on Chemosensor via Radical Cation Formation

We report a unique radical cation formation-based fluorescent chemosensor (E)-N’-(4-(diphenylamino)benzylidene)thiophene-2-carbohydrazide (DBTC) that quantitatively determines Cu²⁺ based on the RGB model using a smartphone. DBTC exhibited a weak turquoise fluorescence due to fluorescence suppression by amide isomerization. When Cu²⁺ was added into DBTC, it showed strong light blue fluorescence with a high quantum yield (= 0.470). The detection limit of Cu²⁺ was determined to be 0.40 µM at the concentration range of 0-7.5 µM. In addition, the detection mechanism of DBTC for Cu²⁺ was demonstrated to be an oxidative cyclization reaction through ¹H NMR titration, ESI-MS analysis, and DFT calculation. Remarkably, DBTC could be applied to the quantitative measurement of Cu²⁺ using a smartphone and RGB analysis. The detection limit was calculated to be 0.05 µM, which is the lowest detection limit among chemosensors that could detect Cu²⁺ through smartphone-based fluorescence measurements. Additionally, spike and recovery experiments conducted with different concentrations of Cu²⁺ showed good recovery values. DBTC exhibited its potential as a chemosensor for determining Cu²⁺ through the application of a smartphone-based platform capable of real-time monitoring.