Colorimetric Probe-Infused Polymeric Solid-State Sensors for Co²⁺-Specific Visual Detection and Quantification

This work focuses on developing simple and fast probe-decorated macro-/mesoporous polymer monolithic solid-state visual colorimetric sensors for quantifying ultratrace amounts of Co2+. The polymer monolith, namely, poly(N-allylthiourea-co-trimethyl propane trimethacrylate) (poly(NAT-co-TM)), is constructed through a precise stoichiometric combination of monomers, crosslinkers, and porogenic solvents to architect the structures with the combinational matrix consisting of a macro-/mesoporous framework that imposes enhanced structural and pore properties for the effective incorporation of probe molecules. The optimized amounts of two indigenously prepared chromoionophoric probes, namely, 5-(diethylamino)-2-(quinolin-8-yldiazenyl)phenol (DEQY) and 2,4-dimethyl-6-(quinolin-8-yldiazenyl)phenol (DMQY), are physically impregnated onto the fabricated polymer framework to evolve into two Co2+ sensors to interplay an efficient role in Co2+-ion-sensing studies. The structurally instilled probe molecules within the porous framework form stable ligand-to-metal charge-transfer complexes, thus delivering a concentration-proportionate color transition from light orange to dark purple for the poly(NAT-co-TM)DEQY sensor and yellow ocher to dark brown for the poly(NAT-co-TM)DMQY sensor. Moreover, the characteristic structural information of the polymer template poly(NAT-co-TM) and the probe-infused poly(NAT-co-TM)DEQY/DMQY sensor materials are studied through FE-SEM, EDAX, HR-TEM, SAED, p-XRD, BET isotherm analyses, etc. These structural features enhance ion selectivity and sensitivity and produce intense visual color transformation upon Co2+-ion complexation. The lower detection limit values are 0.14 and 0.18 ppb for poly(NAT-co-TM)DEQY and poly(NAT-co-TM)DMQY sensors, respectively. Both sensors offered a quick signal response to Co2+ by providing a maximum signal response within 45 s. The proposed sensor materials are cheap, portable, environmentally friendly, and more flexible in the sensing process without sample pretreatment steps. The sensors offered good data reliability and reproducibility when applied to various environmental water samples with RSDs ≤1.6% and ≤1.8% for poly(NAT-co-TM)DEQY and poly(NAT-co-TM)DMQY sensors, respectively.