A Novel Fluorescent Aptasensor Based on Dual-labeled DNA Nanostructure for Simultaneous Detection of Ochratoxin A and Aflatoxin B1

Based on DNA strand replacement reaction and aptamer-specific recognition, a simple dual-labeled DNA nanostructure is designed for the simultaneous detection of Ochratoxin A (OTA) and aflatoxin B1 (AFB₁). C1 is labeled with Cy3 and Cy5, while C2 and C3 are labeled with BHQ2. The fluorescence intensity of DNA nanostructure composed of C1, C2 and C3 is weak because of fluorescence resonance energy transfer. When OTA Aptamer (OTA-Apt) and AFB1 Aptamer (AFB₁-Apt) are added to the homogeneous system at the same time, C1 can be replaced with the help of toehold strand displacement, resulting in fluorescence enhancement. In the presence of both OTA and AFB₁, the toehold strand displacement reaction is inhibited due to preferential binding between the target and their corresponding aptamers. The limit of detection of OTA was 0.007 ng/mL and that of AFB₁ was 0.03 ng/mL. The recoveries of OTA and AFB₁ were 96%-101% and 97%-101% in the corn sample, and 99%-101% and 92%-106% in the wine sample. Compared with other sensors, the preparation of this aptasensor needs simpler experimental steps and a shorter total-preparing time, confirming the convenient, rapid, and time-saving operation process.

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.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

A novel colorimetric immunoassay for sensitive monitoring of ochratoxin A based on an enzyme-controlled citrate-iron(iii) chelating system

Schematic illustration of an enzyme-controlled citrate-iron(iii) chelating system-based colorimetric immunoassay for sensitive determination of ochratoxin A.

Immunoanalytical methods for ochratoxin A monitoring in wine and must based on innovative immunoreagents

Immunochemical methods are highly deployed in analytical laboratories worldwide for monitoring the incidence of mycotoxins in the food chain. Nevertheless, most conventional immunoassays for ochratoxin A (OTA), including commercial kits, show limitations to robustly determine this mycotoxin in grape-derived products below regulated levels (2 ng/mL). Herein, two rapid tests for sensitive OTA determination in wine and must were developed capitalizing on a collection of bioconjugates from innovative synthetic haptens and monoclonal antibodies with subnanomolar affinity. The ELISA (LOD = 8 pg/mL) showed excellent performance in recovery studies, and it was applied to survey commercial wines and musts for OTA contamination. Concerning LFIA, validation according to the Commission Regulation 519/2014 showed that samples exceeding 2 ng/mL were properly scored as uncompliant. More importantly, illegal samples provided a complete inhibition of the test signal, making this test an easy-to-use, rapid, and convenient screening method for in-house control of OTA in wineries.