Optical sensing of copper (II) ions using a biofunctional bolaamphiphile self-assembly: Selective binding of copper (II) ions to tyrosine moieties

We demonstrate that a photoluminescent self-assembly of tyrosyl bolaamphiphilic molecules functions as a selective chemosensor of Cu ²⁺ ions. Fluorescence of the photoluminescent self-assembly, which was prepared by association of tyrosyl bolaamphiphilic molecules with Tb ³⁺ ion and salicylic acid (SA), was sensitively quenched by Cu ²⁺ ions, facilitating optical detection. Fluorescence spectroscopy analysis of quenching revealed a static quenching mechanism that involved coordination of a Cu²⁺ ion with the phenol group of a tyrosyl moiety, interfering with the π-π* transition of the phenyl ring. Thus, fluorescence quenching was driven by a reduction in the antenna effect of the tyrosyl moiety as well as by the paramagnetic nature of the Cu²⁺ ion. XPS spectroscopy results confirmed coordination of the Cu²⁺ ion with the phenol group. As an optical chemosensor, the photoluminescent tyrosyl bolaamphiphile assembly responded quickly (<1 min) to the addition of Cu ²⁺ ions, and selectively recognized Cu²⁺ ions at the ppm level amongst diverse cations with distinguishable fluorescence quenching. The presence of other cations did not interfere with Cu²⁺ ion recognition, demonstrating the high selectivity of detection. Photoluminescence was recovered after chemical disintegration of Cu²⁺ ions from the assembly; the tyrosyl bolaamphiphile self-assembly platform presented in this study can therefore function as a reusable optical chemosensor.