Synthesis of Rhodamine based turn-on Fluorescent Sensors for the Detection of Chromium Ions

Abstract

Selective and sensitive rhodamine-based fluorescent Cr³⁺ sensor, A. ESI mass spectrometry, NMR, and elemental analysis were used to analyze Sensor A, which has been synthesized in high yield. Changes in fluorescence and absorption were used to assess its binding with different metal ions (Ca²⁺, Na⁺, Mn²⁺, Mg²⁺, Fe²⁺, Al³⁺, Ni²⁺, Fe³⁺, Co²⁺, Cu²⁺, Pb²⁺, Cr³⁺, Cd²⁺, Zn²⁺, Sn²⁺, and Hg²⁺). Compared to other metal ions, A exhibits good sensitivity and high selectivity for Cr³⁺. The selectivity regarding A is confirmed by its high binding constant value with Cr³⁺. According to 1H NMR peak broadening tests, Cr³⁺ binds to the sensor's hydroxyl groups and the imine group's nitrogen. Cr³⁺ binding to sensor A's carbonyl oxygen is confirmed by the bright pink color that appears following Cr³⁺ addition. The fluorescence enhancement as dosed the new absorption beyond 400 nm is further confirmed by the quantum yield values for the Cr³⁺-bound form and the ring-opened form of sensor A. According to the findings, the formation regarding the ring-opened form of sensor A upon Cr³⁺ binding is the mechanism responsible for the fluorescence enhancement and new absorption. It was found that A's sensitivity limit to Cr³⁺ was 568 nM. With excess EDTA, the pink color goes away and the binding of Cr³⁺ is reversible. As a result, this sensor could be employed as a reversible fluorescent sensor for Cr³⁺ that is visible to the human eye.