Novel dual-responsive phenothiazine-based probes for moisture detection and Cu2+ sensing

TL;DR: Researchers developed two phenothiazine-based probes that can detect moisture in organic solvents and Cu2+ in ethanol with high sensitivity, exhibiting potential for on-site monitoring and Cu2+ sensing applications.

Abstract: Development of a fluorescent probe with an ability to detect two different analytes is challenging. Implanting two distinct properties into a single molecule is necessary to attain this dual-responsiveness. In the current study, we prepared two phenothiazine-based probes (probes 1 and 2) with the abilities to detect both moisture in organic solvents and Cu2+ in ethanol (EtOH). The presence of moisture in organic solvents is often detrimental for various applications including preparations of pharmaceuticals and fine chemicals, while high levels of Cu2+ are harmful to human health. Probe 1 exhibited luminescence with different colors in various solvents depending on the solvent polarity. Luminescence of this probe was responsive to a trace amount of moisture present in commonly used organic solvents such as DMSO, DMF, and EtOH. Due to a dramatic change in luminescence intensity and/or color, the probe showed the ability to differentiate water percentages in the organic solvents. The minimum limit of detection (LOD) of this probe for moisture sensing was 0.002 %. Intriguingly, the probe displayed selective sensing of Cu2+ in EtOH among various metal ions. Upon Cu2+ binding, the luminescence color of the probe was changed from orange-red to greenish-blue, allowing us to use a ratio-metric approach to obtain a fluorescence-based calculated LOD of 1.0 nM and observed LOD of 0.2 µM. This dual-sensing ability achieved by a single probe (i.e., moisture sensing and Cu2+ detection) was well reproduced even in solid-state probe 1 using cotton and filter paper. Thus, the phenothiazine-cored probe holds significant potential in on-site monitoring of moisture in organic solvents and Cu2+ sensing.