5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin-functionalized urchin-like CuCo2O4 as an excellent artificial nanozyme for determination of dopamine

Integration of Myrica rubra-based N-doped carbon dots with Fe3S4 as excellent peroxidase mimics for colorimetric assay and smartphone-based intelligent sensing of p-aminophenol in waters

To realize the reutilization of waste Myrica rubra in the analytical field, we synthesized Myrica rubra-based N-doped carbon dots (MN-CDs) and further anchored them onto the surface of Fe3S4 to fabricate Fe3S4@MN-CD nanocomposites. The as-fabricated nanocomposites possessed higher peroxidase-mimetic activity than its two precursors, resulting from the synergistic effect between them, and could catalyze colorless 3,3',5,5'-tetramethylbenzidine (TMB) into deep blue oxTMB with a strong 652-nm absorption. Under optimized conditions (initial solution pH, 3.5; incubation temperature, 35 ℃; Fe3S4@MN-CD concentration, 50 µg mL-1, and 652-nm absorption), Fe3S4@MN-CDs were employed for colorimetric assay of p-aminophenol (p-AP) with wide linear range (LR, 2.9-100 µM), low detection limit (LOD, 0.87 µM), and satisfactory recoveries (86.3-105%) in environmental waters. Encouragingly, this colorimetric assay provided the relative accuracy of 97.0-99.4% as compared with  conventional HPLC-UV detection. A portable smartphone-based colorimetric application was developed by combining the Fe3S4@MN-CD-based visually chromogenic reaction with a "Thing Identify" APP software. Besides, we engineered an image-capturing device feasible for field use, in which the internal-compact sealing prevented external light source from entering photography chamber, thereby reducing light interference, and also the bottom light source enhanced the intensity of blue imaging. This colorimetric platform exhibited satisfactory LR (1-500 µM), low LOD (0.3 µM), and fortification recoveries (86.6-99.6%). In the chromogenic reaction catalyzed by Fe3S4@MN-CDs, ·O2- played a key role in concomitant with the participation of •OH and h+. Both the colorimetric assay and smartphone-based intelligent sensing show great promising in on-site monitoring of p-AP under field conditions.

Ultrasensitive Electrochemical Platform for Dopamine Detection Based on CoNi-MOF@ERGO Composite

An electrochemical sensor applied for dopamine (DA) detection was constructed. An easy static way was used to synthesize bimetallic CoNi-MOF. Next, it was mixed with graphene oxide (GO) under ultrasound to get a uniform suspension. Subsequently, the solution was coated on the glassy carbon electrode (GCE) to form CoNi-MOF@ERGO/GCE by the electrochemical reduction method. The interaction between CoNi-MOF and electrochemically reduced graphene oxide (ERGO) enhances the electrocatalytic performance for DA detection. CoNi-MOF@ERGO/GCE has a wider linear range (0.1-400 μM) and a lower detection limit (0.086 μM) under optimum conditions. Furthermore, it has been applied to test DA in human serum samples. The results reveal that the DA sensor shows excellent performance, which will provide a novel idea for more sensitive and quicker DA detection.

A label-free fluorescent sensor for rapid and sensitive detection of ctDNA based on fluorescent PDA nanoparticles

Technological advances in the detection of circulating tumor DNA (ctDNA) have made new options available for diagnosis, classification, biological studies, and treatment selection. However, effective and practical methods for analyzing this emerging class of biomarkers are still lacking. In this work, a fluorescent biosensor was designed for the label-free detection of ctDNA (EGFR 19 del for non-small cell lung cancer, NSCLC). The biosensor was based on the fact that MnO2 nanosheets (MnO2 NSs) have stronger affinity towards single-stranded DNA (ssDNA), as compared with double-stranded DNA (dsDNA). As a high-performance nanoenzyme, MnO2 NSs could oxidize dopamine (DA) into fluorescent polydopamine nanoparticles (FL-PDA NPs), which could be used as a fluorescence signal. The probe ssDNA could be adsorbed on the surface of MnO2 NSs through π-π stacking, and the active site would be masked, causing a lower fluorescence. After the targets were recognized by probe ssDNA to form dsDNA, its affinity for MnO2 NSs decreased and the active site recovered, causing a restored fluorescence. It was verified that Mn ions, •OH radicals and electron transfer were the important factors in the catalytic oxidation of DA. Under the optimal experimental conditions, this biosensor exhibited a detection limit of 380 pM and a linear range of 25-125 nM, providing reliable readout in a short time (45 min). This sensor exhibited outstanding specificity, stability and reproducibility. In addition, this sensor was applied to the detection of ctDNA in serum samples and cell lysates. It is demonstrated that FL-PDA NPs can be used as a fluorescence signal for easy, rapid and label-free detection of ctDNA without any other amplification strategies, and the proposed strategy has great potential for biomarker detection in the field of liquid biopsy.

A colorimetric analytical method based on a TCPP-CuCo2O4-like peroxidase for the detection of trichlorfon

In this work, a highly sensitive colorimetric sensing platform was designed for the detection of trichlorfon based on inhibiting thiocholine (TCh)-induced redox reaction. 5,10,15,20-Tetracarboxyphenylporphyrin (TCPP) functionalized CuCo2O4 (TCPP-CuCo2O4) was synthesized to construct a colorimetric sensing platform for trichlorfon. In the presence of H2O2, TCPP-CuCo2O4 can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue ox-TMB, accompanied by a strong absorption peak at 652 nm, while acetylcholinesterase (AChE) can specifically hydrolyze acetylthiocholine (ATCh) into TCh, which can reduce ox-TMB back into colorless TMB, resulting in a lower absorbance at 652 nm. Trichlorfon can irreversibly inhibit the activity of AChE and thus recover the absorption peak. Under the optimized conditions, detection of trichlorfon has a wide linear range of 40-4000 ng mL-1 with a linear correlation coefficient of 0.9904. The proposed method can be applied to the detection of trichlorfon in vegetables and has good application prospects.

Balancing "on" and "off" response of hydroxyl groups to a nanozyme-catalyzing system for the construction of an ultra-sensitive and selective "signal-on" detection platform for dopamine

Targeting the functional groups present in analytes by nanozyme-catalyzed systems is a promising strategy to construct sensitive and selective platforms for the sensing of specific analytes. Herein, various groups (-COOH, -CHO, -OH, and -NH₂) on benzene were introduced in an Fe-based nanozyme system with MoS₂-MIL-101(Fe) as the model peroxidase nanozyme, H₂O₂ as the oxidizing agent, and TMB as the chromogenic substrate, and the effects of these groups at both a low concentration and high concentration were further investigated. It was found that the hydroxyl group-based substance catechol showed an "on" effect at a low concentration to increase the catalytic rate and enhance the absorbance signal, whereas an "off" effect at a high concentration with a decreased absorbance signal. Based on these results, the "on" mode and "off" mode for the biological molecule dopamine, a type of catechol derivative, were proposed. In the control system, MoS₂-MIL-101(Fe) catalyzed the decomposition of H₂O₂ to produce ROS, which further oxidized TMB. In the "on" mode, the hydroxyl groups of dopamine could combine with the Fe(iii) site of the nanozyme to lower its oxidation state, resulting in higher catalytic activity. In the "off" mode, the excess dopamine could consume ROS, which inhibited the catalytic process. Under the optimal conditions, by balancing the "on" and "off" modes, the "on" mode for the detection of dopamine was found to have better sensitivity and selectivity. The LOD was as low as 0.5 nM. This detection platform was successfully applied for the detection of dopamine in human serum with satisfactory recovery. Our results can pave the way for the design of nanozyme sensing systems with sensitivity and selectivity.

Enhancement of photoelectrocatalytic performance of copper cobaltate nanoflowers modified with 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin for methanol oxidation under light

With the continuously increasing global energy demand, there is an urgent requirement to find efficient methanol oxidation reaction (MOR) catalysts that can replace precious metals. In this work, we have elaborately integrated 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (H₂TCPP) with copper cobaltate (CuCo₂O₄), which possesses efficient separation of photogenerated charges and increased active sites. The mass activity of H₂TCPP/CuCo₂O₄ (534.75 mA mg^-1) toward MOR is higher than that of pure CuCo₂O₄ (291.75 mA mg^-1) under light. In addition, H₂TCPP/CuCo₂O₄ can catalyze the oxidation of other alcohols, such as ethanol, ethanediol, isopropanol, and glycerol. This study demonstrates that it is feasible to enhance the MOR activity by the modification of bimetallic transition metal oxides with porphyrins.

Zinc pyrovanadate nanorods with excellent peroxidase-like activity at physiological pH for the colorimetric assay of H2O2 and epinephrine

Exploring highly active peroxidase mimics at physiological pH is important for the construction of efficient and convenient colorimetric sensing platforms for detecting small biomolecules. In this work, prepared zinc pyrovanadate (Zn₃V₂O₇(OH)₂·2H₂O) nanorods exhibit excellent peroxidase-like activity, which is verified by the fast oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into a blue product (oxTMB) by H₂O₂ at physiological pH (pH = 7) in 2 min. In addition, the catalytic behaviors of Zn₃V₂O₇(OH)₂·2H₂O as a peroxidase-like nanozyme conform to the Michaelis-Menten equation. Scavenger experiments prove that the catalytic activity of Zn₃V₂O₇(OH)₂·2H₂O is ascribed to ˙O₂^- radicals generated in the process of catalysis. Based on the peroxidase-like activity of the Zn₃V₂O₇(OH)₂·2H₂O nanozyme, a fast and convenient colorimetric sensor has been constructed to detect H₂O₂ and epinephrine (EP) under physiological pH. The detection limit of EP is as low as 0.26 μM. In addition, the feasibility of the proposed sensor has been validated to detect H₂O₂ in milk and EP in serum.

Heparin-enhanced peroxidase-like activity of iron-cobalt oxide nanosheets for sensitive colorimetric detection of trypsin

Iron-cobalt oxide nanosheets (FeCo-ONSs) were proved to have intrinsic peroxidase-like activity. Additionally, the peroxidase-like activity of FeCo-ONSs toward the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) was dramatically enhanced after heparin addition due to the stronger affinity toward TMB. Protamine combines with heparin, so the promotion of peroxidase-like activity of FeCo-ONSs with heparin was suppressed. With the addition of trypsin, protamine was hydrolyzed and the enhancement effect of catalytic activity of FeCo-ONSs was recovered. Based on above process, a sensitive colorimetric platform for trypsin activity determination was constructed through measuring the absorbance of produced oxTMB at 652 nm, providing a linear detection range of 5 to 500 ng/mL and a low detection limit of 2.8 ng/mL. The method was applied to trypsin determination in real samples (human urine sample and multienzyme tablet sample) with satisfactory results, illustrating the potential application of this biosensor.

Self-assembled recombinant camel serum albumin nanoparticles-encapsulated hemin with peroxidase-like activity for colorimetric detection of hydrogen peroxide and glucose

The construction of enzyme mimics using protein protection layers possesses advantages of high biocompatibility and superior catalytic activity, which is desirable for biomedical applications including diseases diagnosis. Here, from E. coli expression system, recombinant protein of camel serum albumin (rCSA) from Camelus bactrianus was successfully obtained to encapsulate hemin via the self-assemble method without additional toxic organic reagents. As compared with that of horseradish peroxidase, the produced rCSA-hemin nanoparticles exhibited enhanced enzyme-mimicking activity and stability under harsh experimental conditions. Additionally, the steady-state kinetic analysis of rCSA-hemin in the solution revealed its higher affinity to the substrates. Therefore, a colorimetric detection method of H₂O₂ and glucose was constructed with a linear range of 2.5-500 μM with an LOD of 2.39 and 2.42 μM, respectively, which was also applied for the determination of glucose in the serum samples with satisfying recovery ratio ranging from 101.1% to 112.1%. The constructed camel protein-derived nanozyme system of remarkable stability holds promising potentials for the versatile biomedical uses.