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A bisubstrate colorimetric system was developed for rapid and multi-color assay of copper ion (Cu2+). Cu2+-triethylamine (Cu2+-TEA) complex with the peroxidase-like activity can catalyze the 3,3′,5,5′-tetramethylbenzidine (TMB) to produce oxidized TMB (TMB+, color in blue) and dopamine (DA) to oxidized DA (oxDA, color in orange) in the presence of hydrogen peroxide, which can obtain a three-color colorimetric result due to the couple of complementary colors of blue and orange. The determination of Cu2+ is achieved based on the absorbance value at 460 nm of the composite peak of TMB+ and oxDA. After systematically optimizing the reaction conditions, the developed bisubstrate colorimetric method exhibits high sensitivity for the detection of Cu2+ (limit of quantitation (LOQ) is 0.0625 μM) with a wide linear range of 0.0625–8.0 μM (R2 = 0.991). Meanwhile, the reaction solution shows an orange, brown, and green color change with the increase in Cu2+ concentration, which can realize the semi-quantitatively detection by the naked eyes. Furthermore, TMB was also used as a single substrate for Cu2+ detection with a linear range of 0.5–9.0 μM (R2 = 0.994), and the LOQ is determined to be 0.5 μM. The LOQ of bisubstrate system is 8 times lower than that of single substrate system, indicating a higher sensitivity for the detection of Cu2+ using the bisubstrate colorimetric system. In addition, the developed bisubstrate colorimetric method exhibits excellent selectivity for Cu2+ detection, which was applied in the determination of Cu2+ in lake water and soil samples, and satisfactory spiked recoveries were obtained. [Display omitted] • Cu2+-triethylamine (TEA) complex exhibits excellent peroxidase-like activity. • A bisubstrate (TMB + dopamine) colorimetric sensor is developed based on Cu2+-TEA complex. • The developed senor displays three-color change for the detection of Cu2+. • The bisubstrate sensor shows a higher sensitivity than that of single substrate. [ABSTRACT FROM AUTHOR]

The Co 3 (PO 4) 2.8 H 2 O nanozyme was in situ synthesized through direct reaction between cobalt (Ⅱ) and phosphate, and its peroxidase-like activity was studied in the presence of hydrogen peroxide (H 2 O 2) using a chromogenic substrate 3, 3′, 5, 5′ - tetramethylbenzidine (TMB). The effects of pH value, temperature, and incubation time on the conversion of TMB to oxidized TMB (oxTMB) catalyzed by Co 3 (PO 4) 2.8 H 2 O in the presence of H 2 O 2 were optimized. The peroxidase-like activity of Co 3 (PO 4) 2.8 H 2 O conforms to the Michaelis-Menten kinetics, and its Michaelis constant (K m) is 0.073 mM, showing a higher H 2 O 2 affinity than natural horseradish peroxidase (HRP) and some other peroxidase nanozymes. The mechanism exploration experiment confirmed that the hydroxyl radical (•OH) is the main active species in the catalytic reaction of Co 3 (PO 4) 2.8 H 2 O. Finally, based on the inherent peroxidase-like activity of Co 3 (PO 4) 2.8 H 2 O, a simple colorimetric detection method for H 2 O 2 was established, with a linear range of 15.0 − 100.0 μM and a detection limit of 4.385 μM. The method was applied to the determination of H 2 O 2 in milk samples with the spiked recovery of 90.2%− 106.2%. This study reports a novel nanozyme that can be easily produced in situ, which is exempt from the harsh, cumbersome, and time-consuming synthesis and purification procedures. In addition, the H 2 O 2 colorimetric detection method based on the nanozyme has the advantages of low cost, convenient, high sensitivity, and good selectivity. [Display omitted] • A novel nanozyme Co 3 (PO 4) 2.8 H 2 O of excellent catalytic activity is discovered. • This peroxidase-mimic nanozyme is in situ synthesized rapidly. • The hydroxyl radical is the major active specie in the catalytic reaction. • A sensitive colorimetric method for hydrogen peroxide determination is established. [ABSTRACT FROM AUTHOR]

In this study, the cobalt carbonate (CoCO 3) was in situ prepared using cobalt (Ⅱ) and carbonate at room temperature, and the intrinsic peroxidase-like activity of CoCO 3 was demonstrated for the first time through the oxidation of chromogenic substrate 3, 3', 5, 5' - tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H 2 O 2). The effects of pH, reaction temperature, and incubation time on the catalytic activity of CoCO 3 were studied. Under the optimal reaction conditions, the Michaelis constant (K m) of CoCO 3 to H 2 O 2 is 0.443 mM, which is lower than that of natural horseradish peroxidase and the previously reported peroxidase mimics. Furthermore, the mechanism exploration experiments confirm that the catalytic activity of CoCO 3 is mainly attributed to the hydroxyl radical (•OH). Finally, based on the outstanding peroxidase-like activity of CoCO 3 , a colorimetric sensor for the detection of H 2 O 2 , glucose, and ascorbic acid (AA) was developed, which shows satisfactory selectivity and recovery. In addition, the total antioxidant capacity of three types of tea, dried lemon, three kinds of fresh fruit juice, and vitamin C tablet were determined. In short, this study reports a novel in situ generated CoCO 3 , which can be prepared rapidly at room temperature, eliminating the harsh, cumbersome, and time-consuming synthesis and purification process. The developed sensor based on the newly found excellent peroxidase-like activity of in situ prepared CoCO 3 for the determination of H 2 O 2 , glucose, and AA is fast, simple, low cost, and high sensitivity. [Display omitted] • The excellent peroxidase-like activity of CoCO 3 is firstly investigated. • The CoCO 3 is in situ prepared of rapidity and simplicity. • The hydroxyl radical (•OH) is the major active specie in the catalytic reaction. • The nanozyme has preferable kinetics with a low K m value towards substrate H 2 O 2. • A colorimetric detection platform for H 2 O 2 , glucose, and ascorbic acid is developed. [ABSTRACT FROM AUTHOR]