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. 2017 May 1;8(5):3812-3820.
doi: 10.1039/c6sc05584b. Epub 2017 Mar 2.

Catalyst Displacement Assay: A Supramolecular Approach for the Design of Smart Latent Catalysts for Pollutant Monitoring and Removal

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Free PMC article

Catalyst Displacement Assay: A Supramolecular Approach for the Design of Smart Latent Catalysts for Pollutant Monitoring and Removal

Cheuk-Fai Chow et al. Chem Sci. .
Free PMC article

Abstract

Latent catalysts can be tuned to function smartly by assigning a sensing threshold using the displacement approach for targeted analytes. Three cyano-bridged bimetallic complexes were synthesized as "smart" latent catalysts through the supramolecular assembly of different metallic donors [FeII(CN)6]4-, [FeII(tBubpy)(CN)4]2-, and FeII(tBubpy)2(CN)2 with a metallic acceptor [CuII(dien)]2+. The investigation of both their thermodynamic and kinetic properties on binding with toxic pollutants provided insight into their smart off-on catalytic capabilities, enabling us to establish a threshold-controlled catalytic system for the degradation of pollutants such as cyanide and oxalate. With these smart latent catalysts, a new catalyst displacement assay (CDA) was demonstrated and applied in a real wastewater treatment process to degrade cyanide pollutants in both domestic (level I, untreated) and industrial wastewater samples collected in Hong Kong, China. The smart system was adjusted to be able to initiate the catalytic oxidation of cyanide at a threshold concentration of 20 μM (the World Health Organization's suggested maximum allowable level for cyanide in wastewater) to the less harmful cyanate under ambient conditions.

Figures

Scheme 1
Scheme 1. Bimetallic complexes 1–3 used as the CDA systems.
Fig. 1
Fig. 1. ESI-MS spectra of the isotopic distribution of (a) complex 1, (b) complex 2, and (c) complex 3: and (inset) simulations based on their molecular formulas. All of the experiments were conducted in DI water/methanol.
Scheme 2
Scheme 2. Proposed mechanism for the overall CDA process. The release of the active catalyst is triggered by cyanide/oxalate in solution, which then catalyzes the oxidation of free cyanide/oxalate to the products cyanate/carbon dioxide, respectively. The development of the colored solution from the Fe(CN)2(L)4 complex indicates the reaction has taken place.
Fig. 2
Fig. 2. UV-vis spectroscopic titration curves of (a) 1, (b) 2, and (c) 3 with CN. Addition of cyanide anions to 1–3 restored the characteristic spectroscopic properties of [FeII(CN)6]4–, [FeII(tBubpy)(CN)4]2–, and FeII(tBubpy)2(CN)2, respectively. All of the titrations were examined in aqueous DMF (1 : 1 v/v, 1.50 mL aqueous HEPES buffer at pH 7.4 + 1.50 mL DMF) at 298 K with complex concentrations of 1 × 10–4 M and CN concentrations from 0 to 2.0 × 10–4 M.
Fig. 3
Fig. 3. Kinetic plots of the apparent association rate constant k obs (s–1) versus cyanide concentration. The rate constant values were calculated from the slopes of the curves (y = mx; 1 = 18.8 M–1 s–1; 2 = 32.0 M–1 s–1; and 3 = 58.3 M–1 s–1).
Fig. 4
Fig. 4. (a) Formation of cyanate from the oxidation of cyanide (1 mM) in the presence of 1 (), 2 (), and 3 () against time. The formation of cyanate in the absence of H2O2 () or catalyst () concentration. All of the experiments were performed with a 10 : 65 : 1 molar ratio of cyanide, H2O2, and the “CuII(dien)2+” of 1, 2, or 3 at room temperature in an ambient atmosphere. Formation of cyanate in the presence of (b) 1, (c) 2, and (d) 3 with respect to different initial concentrations of cyanide versus time. All of the experiments were performed with a 65 : 2 molar ratio of H2O2 (6.53 mM) and the “CuII(dien)2+” of 1, 2, or 3 at room temperature in an ambient atmosphere. The error bars are the mean value of three independent experiments.

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