Ratiometric fluorescent chemosensor for mercury(ii) cations in aqueous solution based on the crown-containing bis(chromophoric) 1,8-naphthalimide—styrylpyridine system

The crown-containing bis(chromophoric) chemosensor for mercury(II) cations in an aqueous solution was synthesized using the azide—alkyne 1,3-dipolar cycloaddition click reaction. The synthesized compound contains the 4-methoxy-1,8-naphthalimide fragment acting as the electron excitation energy donor and styrylpyridine being the chromorphore-acceptor. Upon the excitation of the naphthalimide residue with the visible light, the resonance energy transfer (RET) occurs, and its efficiency decreases as a result of complex formation with Hg²⁺ cations. The changes in the fluorescence spectrum observed upon the addition of Hg²⁺ allow one to detect the ratiometric fluorescence response due to an increase in the ratio of emission intensities in the donor and acceptor channels. Based on the spectrophotometric and spectrofluorimetric titration data, the logarithms of the stability constants (logK) for the 1 : 1 metal—ligand complex were calculated to be 5.37±0.05 and 5.81±0.06, respectively. The proposed fluoroionophore is characterized by the detection limit of Hg²⁺ ions in water at pH 4.5 equal to 40 nmol L⁻¹. The ability of the synthesized chemosensor to perform fluorescence imaging of mercury(II) cations in living cells was also analyzed.