Reversible dual-mode detection of Cu2+ and tandem capture of cysteine using a salphen-conjugated microporous polymer

Abstract

Conjugated microporous polymers (CMPs) possess extended π-conjugation combined with microporosity, enabling amplified sensing response even with ultra-trace solution or vapor-phase analytes, and their high sensing response output was demonstrated with several CMPs. However, CMPs exhibiting tandem detection properties, i.e., sequential detection of multiple analytes, are rarely reported and represent the next generation of CMP chemical sensors offering enhanced sensitivity and specificity. Herein, we report the design and synthesis of a salphen-conjugated microporous polymer (pTPE-salphen) for reversible dual-mode (fluorometric/colorimetric) nanomolar detection of Cu²⁺ ions and tandem capture of cysteine (Cys). pTPE-salphen synthesized via Schiff-base condensation between 1,1,2,2-tetrakis(4-hydroxy-3-formylphenyl)ethene and o-phenylenediamine, emits yellow photoluminescence (PL) at λ^max_Em = 537 nm with a PL quantum yield of 5.41%. pTPE-salphen exhibited remarkable thermal stability up to 425 °C and a fused spherical nanoparticle morphology. pTPE-salphen showed strong PL quenching up to 92% when exposed to Cu²⁺ (50 μM), selectively among other metal ions, due to the ground-state complex formation of Cu²⁺@pTPE-salphen. pTPE-salphen was highly sensitive to Cu²⁺ with a detection limit of 5.69 nM and exhibited a high Stern–Volmer constant (K_SV) value of 8.12 × 10⁶ M⁻¹. Notably, the pTPE-salphen-based paper strip sensor showed appreciable sensitivity up to 10⁻¹¹ M Cu²⁺. In addition, strong colorimetric changes from yellow (R/B is 1.9) to black (R/B is 0.53) were also observed upon the formation of Cu²⁺@pTPE-salphen, and the binding of Cu²⁺ was confirmed by XPS analysis. Interestingly, Cu²⁺@pTPE-salphen exposed to cysteine (Cys) exhibited reversible colorimetric response from black to orange (R/B is 1.8) both in dispersion and paper strip sensors due to the formation of Cys–Cu²⁺@pTPE-salphen where Cys binds with Cu²⁺ anchored on the pore surface of pTPE-salphen, and the entire colorimetric process (yellow ⇌ black ⇌ red) is reversible. The binding of Cys to Cu²⁺ and its tandem capture were systematically studied using XPS and NMR. Such sequential detection and capture (tandem process) of Cu²⁺ and Cys using a conjugated microporous polymer sensor is unique and of high significance in environmental and biological applications.