Colorimetric and handheld pH meter dual-signal readout platform for E. coli detection based on a cascade reaction.

A dual-signal readout has been designed detecting platform based on a cascade reaction for Escherichia coli (E. coli) detection by using colorimetric approach and a handheld pH meter. The immunoreaction was conducted using polydopamine@copper ferrite-Ag nanoparticles (PDA@CuFe2O4-Ag NP) and a glucose oxidase (GOD)-conjugated graphene oxide-gold nanosheet composite (GOD-GO/Au NS) to synthesize a sandwich complex mode between targets. Together with the formation of immune complexes, the GOD-GO/Au NS can catalyze glucose to produce gluconic acid and hydrogen peroxide (H2O2). The gluconic acid produced altered the pH of the detection solution. Since the PDA@CuFe2O4-Ag NP have good peroxidase-like activity, they can catalyze the oxidation of TMB to the blue product oxTMB once H2O2 is produced in the reaction system, and the absorbance change of oxTMB at 652 nm can be recorded using ultraviolet-visible (UV-Vis) spectroscopy. Interestingly, the PDA@CuFe2O4-Ag NP composites can consume the generated H2O2, and can create a reaction cycle that promotes glucose oxidation. Under optimal conditions, the proposed dual-channel signal platform is proportional to the logarithm of the E. coli concentration within a range of 102-107 cfu mL-1. Additionally, the devised approach was successfully used to detect E. coli at the required levels in real samples. This dual-mode detection method notably enhances the accuracy and diversity of detection, and curbs the false negative and positive rates.

Signal-on photoelectrochemical immunoassay for salivary cortisol based on silver nanoclusters-triggered ion-exchange reaction with CdS quantum dots.

Nowadays, the epidemic, employment, and academic pressures are seriously affecting our physical and mental health. Herein, we designed a magneto-controlled photoelectrochemical immunosensor for noninvasive monitoring of salivary cortisol regarded as a pressure biomarker. A competitive immunoassay model was established by coupling bovine serum albumin-cortisol modified magnetic beads (MB-BSA-cortisol) with silver nanoclusters (Ag NCs)-labelled anti-cortisol antibody, and quantity analysis was operated by photoelectrochemical measurement of the CdS/Au electrode as an ion-exchange platform. Accompanying the formation of immune complexes, the carried Ag NCs were readily dissolved with nitric acid to produce abundant silver ions, which transferred to the electrode for ion-exchange reaction with CdS quantum dots to produce Ag2S, a new electron-hole capture site, leading to a decrease in the photocurrent intensity. The photocurrent signal gradually recovered with the increase of concentration of target cortisol, acquiring the signal-on mode competitive immunosensing system, which is propitious to the detection of small molecules. Within optimal conditions, this sensor had a satisfactory linear relationship in the range of 0.0001-100 ng mL-1 with favorable repeatability, specificity, and acceptable method accuracy. The detection limit was as low as 0.06 pg mL-1. In addition, this strategy provided new thought for the test of other small-molecule analytes and immunosensor applied in the complex biological system.

NIR II light response-based PDA/AuPt@CuS composites: Simultaneous readout of temperature and pressure sensing strategy for portable detection of pathogenic bacteria.

In this study, we developed a simultaneous readout of pressure and temperature dual-signals platform based on the second near-infrared (NIR II) light response-based polydopamine (PDA)-functionalized-AuPt nanoparticles (NPs)@CuS nanosheets (PDA/AuPt@CuS NS) composite. Due to the excellent NIR photothermal performance of PDA/AuPt@CuS NS, it contribute to the decomposition of H2O2 and NH4HCO3 to generate gases (including O2, CO2, and NH3) can be promoted, which can amplify the pressure signals in a sealed container. A sandwich mode is formed between Fe3O4 NPs and PDA/AuPt@CuS NS based on the dual-aptamer when target pathogenic bacteria is present. And, it is possible to convert the molecular recognition signals between the dual-aptamers into amplified pressures and temperatures, which can be read out by a portable pressure meter and smartphones simultaneously. It may offer the possibility for quantitative POCT analysis of Pathogenic Bacteria. Moreover, because of the high photothermal efficiency of this method, the developed dual-mode method can achieve that following the detection of bacteria and killing them immediately. As a result, secondary contamination is eliminated and bacterial transmission is avoided. The developed dual-signal sensing platform is also inexpensive, simple to operate and rapidly, indicating that it can be used for food safety analysis, clinical applications, and environmental monitoring.

Biological redox cycling amplification in a self-powered photoelectrochemical sensor based on TiO2/CdIn2S4/ g-C3N4-WO3 photoanode for sensitive detection of Hg2.

A photoelectrochemical (PEC) sensor is proposed with a TiO2/CdIn2S4 co-sensitive structure and a g-C3N4-WO3 heterojunction as the photoanode to form a self-powered system. The photogenerated hole-induced biological redox cycle of TiO2/CdIn2S4/g-C3N4-WO3 composites is used as a signal amplification strategy for Hg2+ detection. In the test solution, ascorbic acid is first oxidized by the photogenerated hole of the TiO2/CdIn2S4/g-C3N4-WO3 photoanode, which triggers the ascorbic acid－glutathione cycle to achieve signal amplification and increase the photocurrent. However, in the presence of Hg2+, glutathione forms a complex with Hg2+, which destroys the biological cycle and leads to a decreased of photocurrent, thus achieving detection of Hg2+. Under optimal conditions, the proposed PEC sensor has a wider range (from 0.1 pM to 100 nM), and lower limit of Hg2+ detection (0.44 fM) than most other Hg2+ detection methods. In addition, the developed PEC sensor can be used to detect of real samples.

A ratiometric electrochemical strategy based on Fe (III) and Pt (IV) for immobilization-free detection of Escherichia coli.

A new ratiometric electrochemical strategy for immobilization-free detection of Escherichia coli (E. coli) was constructed by using a capture DNA-polyaniline/copper ferrite nanoparticles/graphene oxide (cDNA-PANI/CuFe2O4/GO) composite as capture probes, which has a high specific surface area and good magnetic properties. Then trigger DNA/Au nanoparticles (tDNA/Au NPs) were used as signal amplification labels, and Pt (IV) and Fe (III) were chosen as the signal probes. In the presence of targets, the sandwich format among cDNA-PANI/CuFe2O4/GO, E. coli and auxiliary DNA (aDNA) was realized by using the aptamer recognition system. Then, the tDNA/Au binding could be anchored on the sandwich format due to the principle of base complementation between unpaired aDNA and tDNA. And the unbounded tDNA of tDNA/Au NPs could bind an amount of Pt (IV). After separation using a magnet, a handful of unbound Pt (IV) which remained in the supernatant reacted with a large number of Fe (III) ions, leading to a markedly increased IFe(III)/IPt(IV) value. Oppositely, the sandwich format could not appear in the absence of targets, and even the tDNA/Au could not be immobilized on it. So, the redox reaction between a large amount of Pt (IV) residue in the supernatant and Fe (III) was significantly successful, causing a low IFe(III)/IPt(IV) value. Under optimal conditions, we found that IFe(III)/IPt(IV) was linearly related to the logarithmic E. coli concentration with a low limit of detection (1.862 × 103 cfu mL-1). This devised ratiometric electrochemical method may develop into a powerful and effective means for the detection of E. coli in real samples, which may also be developed as a universal tool for another microorganism.

A simple colorimetric method based on "on-off-on" mode for detection of H2S and Hg2+ in water.

It is of great significance to develop efficient platforms for the detection of hypertoxic Hg2+ and H2S. Colorimetric have received much attention for the detection of H2S and Hg2+ in the last decades. In this work, an "on-off-on" mode colorimetric method based on MnO2/multi-wall carbon nanotubes (MnO2/MWCNTs) composite was constructed. MnO2/MWCNTs composite can oxidize TMB directly to form blue product (ox TMB) with a good simulated oxidase activity. In the presence of H2S, it can decompose the MnO2/MWCNTs composite causing the absorbance of the chromogenic system to decrease. When Hg2+ is introduced, the formation of Hg-S bond between Hg2+ and H2S inhibited the decomposition ability of H2S toward MnO2 composite, thus resulting in a color change from colorless to blue. Based on this phenomenon, the proposed "on-off-on" colorimetric sensor can be used for detection of H2S (off) and Hg2+ (on). Under optimized experimental conditions, this sensor showed a satisfactory linear relationship of H2S and Hg2+ with pleasant repeatability, acceptable method accuracy and stability. More importantly, the proposed colorimetric sensor has been successfully applied to the detection of H2S and Hg2+ in real samples, which not only provides a simple and cost-effective method to detect H2S and Hg2+ but also hopefully makes a certain contribution to environmental protection.

Kinetic trapping of a cobalt(ii) metallocage using a carbazole-containing expanded carbaporphyrinoid ligand.

The meso-unsubstituted expanded porphyrinoid 3, incorporating two carbazole moieties, acts as an effective ligand for Co(ii) and permits the isolation and X-ray diffraction-based characterization of a 6 : 3 metal-to-ligand metallocage complex that converts spontaneously to the constituent 2 : 1 metal-to-ligand metalloring species in chloroform solution. The discrete metalloring is formed directly when the Co(ii) complex is crystallized from supersaturated solutions, whereas crystallization from more dilute solutions favors the metallocage. Studies with two other test cations, Pd(ii) and Zn(ii), revealed exclusive formation of the monomeric metalloring complexes with no evidence of higher order species being formed. Structural, electrochemical and UV-vis-NIR absorption spectral studies provide support for the conclusion that the Pd(ii) complex is less distorted and more effectively conjugated than its Co(ii) and Zn(ii) congeners, an inference further supported by TD-DFT calculations. The findings reported here underscore how expanded porphyrins can support coordination modes, including bimetallic complexes and self-assembled cage structures, that are not necessarily easy to access using more traditional ligand systems.