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Exploring highly active nanozymes is an important task to realize the real-time detection of some heavy metal ions in water. In this work, yolk–shell Co3S4 microspheres have been verified to possess excellent peroxidase-like activity, which can be further improved by adding Hg2+. Very interestingly, Hg2+ can trigger "ON" the oxidase-like activity of Co3S4 microspheres. The dual peroxidase-/oxidase-like activity of the yolk–shell Co3S4 microspheres is evaluated by using the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB). Furthermore, comprehensive studies verify that the enhanced peroxidase-like activity, together with the "ON" oxidase-like activity of the yolk–shell Co3S4 microspheres, is attributed to the in situ generation of HgS on the surface of Co3S4 microspheres and then the release of more active sites. Importantly, the in situ generated HgS on the surface of Co3S4 microspheres can form a heterojunction, which also accelerates the catalytic process. During the catalytic reaction, some active species (O2− and h+) can be detected by ESR. Thus, a colorimetric sensing platform based on Hg2+-triggered signal amplification has been successfully constructed, which can be validated by the detection of Hg2+ residue in environmental water. [ABSTRACT FROM AUTHOR]

Antioxidants are considered as essential compounds for monitoring human health. In this work, a colorimetric sensor array was developed using oxidase-like (OXD) and peroxidase-like (POD) activities of Co3O4 nanoflowers as sensing elements, together with a substrate, 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB), as a signal reader to effectively identify different antioxidants. In the presence of Co3O4, colorless TMB can be oxidized into blue oxTMB to different degrees in the presence and absence of H2O2. Interestingly, after the addition of antioxidants, the sensor array showed cross-reactions, and different changes in color and absorbance were observed, as TMB and antioxidants competed for binding. Different colorimetric responses were obtained on the sensor array and identified by linear discriminant analysis (LDA). The LDA result indicated that the sensor array can be used to distinguish 4 antioxidants, namely, dopamine (DA), glutathione (GSH), ascorbic acid (AA), and cysteine (Cys) at seven different concentrations, namely, 10, 20, 30, 50, 100, 200, and 250 nM. Different concentrations of antioxidants and proportions of mixed antioxidants were determined. This demonstrates the potential application of sensor arrays in diagnosis and food monitoring. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Some nanoperoxidase-based colorimetric sensors have been used to detect only one molecule at a time. Thus, the simultaneous detection of various molecules coexisting in the same system is a great challenge. In this work, an excellent nanoperoxidase, nickel cobalt Prussian blue analogue-MoS2 nanoboxes (PBA-MoS2), have been successfully prepared by the hydrothermal method and used to construct a colorimetric sensing array to determine a series of reductive substances containing the catechol structure (such as catechol, epinephrine hydrochloride, procyanidin, caffeic acid and dopamine hydrochloride). The excellent peroxidase-like activity of PBA-MoS2 is verified by the chromogenic reaction of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2 in 2 min. The catalytic mechanism of PBA-MoS2 is attributed to generated reactive species including holes (h+) and superoxide radicals (˙O2−) in the process of catalysis. The fast economic H2O2 colorimetric sensing array has been constructed based on the PBA-MoS2 nanoperoxidase. Due to the presence of different reducing substances, the catalytic oxidation of TMB can be restricted to different extents, accompanied by blue colour changes to varying degrees. Therefore, on combining PBA-MoS2 nanoperoxidase with H2O2 and TMB, five reductive substances can be quantitatively distinguished by linear discriminant analysis (LDA) at the 2 mM level. [ABSTRACT FROM AUTHOR]

In this work, a thrombin photoelectrochemical aptasensor was reported based on a photoanode of perylene-3,4,9,10-tetracarboxylic acid (PTCA), Au nanoparticle co-functionalized ZnO nanorods (ZnO NRs) and the "signal-off" amplification effect of Ag@Ag2S. The photocurrent response of the ZnO NRs was improved greatly due to the excellent visible-light photoelectric performance of PTCA and the surface plasmon resonance (SPR) effect of Au nanoparticles. Due to the specific recognition between thrombin and aptamers, the non-conductive complex with a steric hindrance structure blocked the diffusion path of the electron donating ascorbic acid (AA) and then the "signal-off" Ag@Ag2S quencher was captured. The quencher blocked the irradiation light toward the ZnO NRs/PTCA/Au electrode and competitively consumed the electron donor AA that could have been involved in the oxidation reaction with photogenerated holes of PTCA, resulting in the further decrease of the photocurrent. Based on the evident photocurrent response of the photoanode and the superior quenching strategies, the detection limit of thrombin is as low as 33 fM with a wide linear detection range from 0.0001 nM to 50 nM. The prepared biosensor also exhibited good specificity, reproducibility and stability, suggesting potential application in thrombin specific detection. [ABSTRACT FROM AUTHOR]

There is growing interest in developing a high-performance sensor array for detection and discrimination of antioxidants owing to their widespread use and essential role in the human body. The present work unveils a novel colorimetric sensor array for colorimetric discrimination of antioxidants based on the red, green, and blue alteration (ΔRGB) pattern recognition. In this sensor array, three concentrations of AgNO3 were used as sensing elements, and gold nanoparticles (AuNPs) were employed as a colorimetric probe. In the presence of antioxidants, the sensor array produces unique colorimetric response patterns for the discrimination of these antioxidants due to different reactivities between three different concentrations of AgNO3 and each antioxidant, leading to deposition of different quantities of Ag nanoshells on the surface of AuNPs, enabling an excellent discrimination of six antioxidants (catechin, epigallocatechin 3-gallate, epicatechin, epigallocatechin, epicatechin 3-gallate, and gallocatechin) at a 20 nM level, when linear discriminant analysis (LDA), hierarchical cluster analysis (HCA), centroid diagram, spidergram, and color contour profiles were smartly combined. Furthermore, different concentrations of antioxidants and binary antioxidant mixtures, even ternary mixtures, could also be discriminated with this sensor array. Finally, the sensor array was successfully used for the discrimination of antioxidants in serum samples, demonstrating its potential applications in the diagnosis of antioxidant-related diseases. [ABSTRACT FROM AUTHOR]

5,10,15,20-Tetrakis(4-carboxyl phenyl)porphyrin (Por) modified Co(OH)2 deposited on the surface of GO nanocomposites (Por/Co(OH)2/GO) were prepared and characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and XRD. For the first time, H2TCPP/Co(OH)2/GO is found to have enhanced peroxidase-like activity and catalyze the oxidation of the substrate 3,3,5,5-tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2). Notably, the colorless TMB rapidly transformed into blue oxTMB in just 60 s, which was easily observed visually. The catalytic kinetics of H2TCPP/Co(OH)2/GO is in accord with the Michaelis–Menten equation. The catalytic mechanism of H2TCPP/Co(OH)2/GO nanocomposites is attributed to hydroxyl radicals (˙OH), due to decomposition of H2O2, which is verified by using terephthalic acid as a fluorescent probe. What's more, H2O2 can be detected in a wide linear detection range from 5 to 35 mM with a detection limit of 0.385 mM. Furthermore, based on the excellent peroxidase-like activity of H2TCPP/Co(OH)2/GO, a colorimetric sensor is established to sensitively detect glutathione (GSH) in a linear range from 10 to 300 μM with a low detection limit of 9.5 μM. [ABSTRACT FROM AUTHOR]

Proteins play a key role in disease diagnosis, and protein discrimination is an important but difficult issue. Here, we report a novel strategy for improving protein discrimination through a facile colorimetric sensor array, which is based on DNA–gold nanoparticle (AuNP) conjugates manipulated by exonuclease I (Exo I). Different proteins exhibit diverse affinities toward the three DNAs, and the DNA–protein binding is resistant to the digestion of Exo I and protects the AuNPs from aggregation in high concentrations of NaCl media, forming distinct response patterns of the array. These response patterns as “fingerprints” can be acquired on the sensor array and then identified by linear discriminant analysis (LDA). The sensor array achieved the correct discrimination of 15 proteins at a 10 nM level in buffer solution and real serum samples. Also, the sensor array had the capability to discriminate individual proteins and the mixtures of them. Remarkably, the practicability of the sensor array was further confirmed by the identification of 35 unknown protein samples with 100% accuracy. [ABSTRACT FROM AUTHOR]

The rapid and sensitive detection of H2O2 is of great importance for a series of industries. In this work, surface-modified FePt nanoparticles were loaded into metal–organic frameworks (MOFs, MIL-101(Fe)) in a facile in situ way and the resulting material (FePt@MOFs NCs) acted as a high efficient colorimetric detection agent. The as-prepared FePt@MOFs NCs possess a well-defined octahedron shape and high stability in water. Via the oxidation reaction of the chromogenic substrate 4,4′-bi-2,6-xylidine, the obtained FePt@MOFs NCs were proved to possess prominent peroxidase-mimic activity. Under the most favorable conditions, the linear range for H2O2 detection was 40 μM to 800 μM and the detection limit was 18.9 μM. Furthermore, FePt@MOFs NCs were successfully applied to the detection of H2O2 in real samples such as fruit juice. The developed colorimetric sensing platform is expected to have potential for the exact detection of H2O2 for the food, mining and bio-pharmaceutical industries, due to its high stability, sensitivity, rapidness and easy preparation. [ABSTRACT FROM AUTHOR]

We herein present a simple, low-cost, and ultrasensitive colorimetric sensing strategy for the detection of mercury ions (Hg2+) that takes advantage of the natural pore structure in rose petals to encapsulate gold nanoparticles (AuNPs). AuNPs easily aggregate in high salt concentrations, which restrict their application, for example, as a catalyst in real sample analysis. Rose petals offer a three-dimensional (3D) space for more efficient dispersability of AuNPs in their extensive network of channels, which will increase the specific surface area, thereby enhancing the catalytic capability of AuNPs. The presence of Hg2+ triggers the formation of gold amalgam, which results in remarkable enhancement of the catalytic ability of citrate-capped AuNPs. On the basis of this effect, a colorimetric signal amplification is achieved via the use of gold amalgam to catalyze the reaction of methylene blue (MB) and sodium borohydride. A limit of detection (LOD) as low as 10 fM can be obtained by naked eye observation. Our sensor has promising potential for detection and quantitation of Hg2+ in environmental samples. [ABSTRACT FROM AUTHOR]

Breaking the restrictions of a lock-and-key sensing strategy which relies only on the most dominant interactions between the sensing element and target, here, we develop a colorimetric sensor array with three kinds of cationic polymers (polydiallyl dimethylammonium chloride (PDDA), chitosan (CTS), and cetyltrimethylammonium bromide (CTAB)) as nonspecific receptors. Diverse interactions between cationic polymers and proteins make gold nanoparticles (Au NPs) exhibit different aggregation behaviors, resulting in changes in color and absorbance of Au NPs. Based on the diverse colorimetric response patterns, seven proteins were successfully discriminated visually at the 20 nM level by linear discrimination analysis (LDA). Furthermore, the practicability of the sensor array was validated by the successful identification of proteins without any overlap in human serum samples. [ABSTRACT FROM AUTHOR]

We present an ultrasensitive colorimetric nucleic acid assay based on the intensity of gold nanoparticles (Au NPs) using dark field microscopy. In the absence of target DNA, two hairpin-like DNA strands with protruding single-stranded DNA (ssDNA) can be absorbed onto the Au NP surface via non-covalent interactions between the exposed nitrogen bases of ssDNA and Au NPs, which inhibits Au NPs from aggregating in a high concentration of salt media, while in the presence of target DNA, two hairpin DNA strands hybridize with target DNA to form double-stranded DNA (dsDNA). After hybridization chain reaction (HCR) amplification, rigid dsDNA polymers are formed, which results in serious Au NP aggregation in the salt environment. By measuring the intensity change of yellow and red dots on a dark-field image, the concentration of target DNA can be accurately quantified with a limit of detection (LOD) of 66 fM. [ABSTRACT FROM AUTHOR]

The rapid and sensitive detection of H2O2 is of great importance for a series of industries. In this work, surface-modified FePt nanoparticles were loaded into metal–organic frameworks (MOFs, MIL-101(Fe)) in a facile in situ way and the resulting material (FePt@MOFs NCs) acted as a high efficient colorimetric detection agent. The as-prepared FePt@MOFs NCs possess a well-defined octahedron shape and high stability in water. Via the oxidation reaction of the chromogenic substrate 4,4′-bi-2,6-xylidine, the obtained FePt@MOFs NCs were proved to possess prominent peroxidase-mimic activity. Under the most favorable conditions, the linear range for H2O2 detection was 40 μM to 800 μM and the detection limit was 18.9 μM. Furthermore, FePt@MOFs NCs were successfully applied to the detection of H2O2 in real samples such as fruit juice. The developed colorimetric sensing platform is expected to have potential for the exact detection of H2O2 for the food, mining and bio-pharmaceutical industries, due to its high stability, sensitivity, rapidness and easy preparation. [ABSTRACT FROM AUTHOR]

Exploring highly active nanozymes is an important task to realize the real-time detection of some heavy metal ions in water. In this work, yolk–shell Co3S4 microspheres have been verified to possess excellent peroxidase-like activity, which can be further improved by adding Hg2+. Very interestingly, Hg2+ can trigger "ON" the oxidase-like activity of Co3S4 microspheres. The dual peroxidase-/oxidase-like activity of the yolk–shell Co3S4 microspheres is evaluated by using the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB). Furthermore, comprehensive studies verify that the enhanced peroxidase-like activity, together with the "ON" oxidase-like activity of the yolk–shell Co3S4 microspheres, is attributed to the in situ generation of HgS on the surface of Co3S4 microspheres and then the release of more active sites. Importantly, the in situ generated HgS on the surface of Co3S4 microspheres can form a heterojunction, which also accelerates the catalytic process. During the catalytic reaction, some active species (O2− and h+) can be detected by ESR. Thus, a colorimetric sensing platform based on Hg2+-triggered signal amplification has been successfully constructed, which can be validated by the detection of Hg2+ residue in environmental water. [ABSTRACT FROM AUTHOR]