A smart photosensitive fluorescent probe for sensing Co²⁺ in extremely alkaline aqueous solution

Area of Science:

• Chemical Sensing
• Fluorescence Spectroscopy
• Coordination Chemistry

Background:

• Development of sensitive and selective probes for metal ion detection is crucial in environmental and biological monitoring.
• Hydrazone-based compounds offer versatile platforms for designing chemosensors due to their tunable electronic properties.

Purpose of the Study:

• To develop a novel polymorphic fluorescence probe (NHP) for the sensitive detection of cobalt ions (Co²⁺).
• To investigate the pH-, viscosity-, and photo-responsive properties of the NHP probe for Co²⁺ sensing.
• To explore the coordination behavior and oxidation states of cobalt complexes formed with NHP.

Main Methods:

• One-step synthesis and purification of the hydrazone-based probe NHP.
• Single crystal X-ray diffraction analysis to resolve different conformations of NHP and its cobalt complexes.
• Spectroscopic studies (UV-Vis absorption and fluorescence) to monitor the sensing mechanism under varying pH and UV irradiation.
• Investigation of the oxidation of Co(II) complexes to Co(III) complexes.

Main Results:

• NHP was synthesized in a single step and characterized by single crystal analysis, revealing polymorphic forms.
• The probe NHP exhibits pH-, viscosity-, and photo-sensitivity, acting as a photo acid generator (PAG).
• NHP selectively chelates Co²⁺, resulting in a yellowish product under UV light and a bright blue-green fluorescent product in alkaline solutions.
• The divalent cobalt complex (NHP₂-Co²⁺) can be oxidized to a trivalent cobalt complex (NHP₃-Co³⁺), confirmed by crystal structure analysis.

Conclusions:

• A novel, easily synthesized, multi-responsive fluorescent probe (NHP) for Co²⁺ detection has been successfully developed.
• The probe's ability to change fluorescence and color upon Co²⁺ binding under different stimuli (pH, UV light) makes it a versatile sensing material.
• The study demonstrates the potential of NHP for selective and sensitive detection of cobalt ions in various conditions, including the formation of different oxidation state complexes.