Integration of a Rhodamine-Bromophenol Blue into Portable Paper-Based and Arduino-Assisted Platforms for Selective Optical Sensing.

A novel dual-mode chemosensor, BPRH, was designed by conjugating a Rhodamine derivative with Bromophenol Blue, thereby integrating colorimetric/fluorometric sensing and pH-indicator functions into a single molecular framework. BPRH exhibited pronounced pH-dependent optical behavior, undergoing a visually distinct color transition from blue to pink under acidic conditions (pH < 3), while remaining stable at neutral and basic pH, confirming its robust acid-base sensitivity. The sensing material demonstrated exceptional selectivity toward Al³⁺, Hg²⁺, and Cu²⁺ among a wide panel of metal ions, each eliciting unique and easily discernible color changes: Al³⁺ and Cu²⁺ induced a pink hue, whereas Hg²⁺ produced an apricot color, enabling straightforward visual discrimination. Fluorescence studies revealed that both Al³⁺ and Hg²⁺ significantly enhanced emission at 578 nm, with Al³⁺ showing the highest fluorescence intensity, indicative of superior binding affinity and activation efficiency. In contrast, Cu²⁺ produced only a weak fluorescence response, further supporting the sensing material's higher selectivity for Al³⁺ and Hg²⁺. The optical responses were most prominent under acidic (pH 2) and neutral (pH 7) conditions, with Al³⁺ exhibiting maximal fluorescence enhancement at physiological pH, suggesting optimal complexation in biologically relevant environments. The practical utility of BPRH was extended to paper-based analytical devices and an Arduino-assisted RGB color detection platform, enabling rapid, instrument-free, and quantitative detection of target ions. These features-high selectivity, strong fluorescence response, dual-environment functionality, and compatibility with low-cost, portable platforms-position BPRH as a highly promising chemosensor.