Development of a chemiluminescent aptasensor for ultrasensitive and selective detection of aflatoxin B1 in peanut and milk

More and more attention about food safety leads to a research hotspot to develop new detection methods for food contaminant. To address the problems of serious interference and low sensitivity, a chemiluminescent aptasensor for the detection of aflatoxin B1(AFB1) in food was developed in this paper. It is based on horseradish peroxidase (HRP) catalyze the luminol chemiluminescence reaction. The hybridization chain reaction (HCR) signal amplification strategy has been used to improve the detection sensitivity. Magnetic separation could further reduce background signal obviously at the same time. AFB1 as a model of analyte to test the capability of our developed assay system. Under the optimal experimental conditions, CL intensity showed a good linear correlation with the concentrations of AFB1 ranging from 0.5 to 40 ng mL^-1. The limit of detection was estimated 0.2 ng mL^-1 based on 3 times of the signal-to-noise ratio which is lower than those of the previously reported sensors. It could be used to detect AFB1 content in real samples, such as peanuts and milk which were purchased in local supermarket. The results proved that the sensing system has good anti-interference and selectivity. In all, it has potential for practical application in food safety field.

Simultaneous detection and characterization of toxigenic Clostridium difficile directly from clinical stool specimens

We employed a multiplex polymerase chain reaction (PCR) coupled with capillary electrophoresis (mPCR-CE) targeting six Clostridium difficile genes, including tpi, tcdA, tcdB, cdtA, cdtB, and a deletion in tcdC for simultaneous detection and characterization of toxigenic C. difficile directly from fecal specimens. The mPCR-CE had a limit of detection of 10 colony-forming units per reaction with no cross-reactions with other related bacterial genes. Clinical validation was performed on 354 consecutively collected stool specimens from patients with suspected C. difficile infection and 45 isolates. The results were compared with a reference standard combined with BD MAX Cdiff, real-time cell analysis assay (RTCA), and mPCR-CE. The toxigenic C. difficile species were detected in 36 isolates and 45 stool specimens by the mPCR-CE, which provided a positive rate of 20.3% (81/399). The mPCR-CE had a specificity of 97.2% and a sensitivity of 96.0%, which was higher than RTCA (x² = 5.67, P = 0.017) but lower than BD MAX Cdiff (P = 0.245). Among the 45 strains, 44 (97.8%) were determined as nonribotype 027 by the mPCR-CE, which was fully agreed with PCR ribotyping. Even though ribotypes 017 (n = 8, 17.8%), 001 (n = 6, 13.3%), and 012 (n = 7, 15.6%) were predominant in this region, ribotype 027 was an important genotype monitored routinely. The mPCR-CE provided an alternative diagnosis tool for the simultaneous detection of toxigenic C. difficile in stool and potentially differentiated between RT027 and non-RT027.

Fluorescent "on-off-on" switching sensor based on CdTe quantum dots coupled with multiwalled carbon nanotubes@graphene oxide nanoribbons for simultaneous monitoring of dual foreign DNAs in transgenic soybean

With the increasing concern of potential health and environmental risk, it is essential to develop reliable methods for transgenic soybean detection. Herein, a simple, sensitive and selective assay was constructed based on homogeneous fluorescence resonance energy transfer (FRET) between CdTe quantum dots (QDs) and multiwalled carbon nanotubes@graphene oxide nanoribbons (MWCNTs@GONRs) to form the fluorescent "on-off-on" switching for simultaneous monitoring dual target DNAs of promoter cauliflower mosaic virus 35s (P35s) and terminator nopaline synthase (TNOS) from transgenic soybean. The capture DNAs were immobilized with corresponding QDs to obtain strong fluorescent signals (turning on). The strong π-π stacking interaction between single-stranded DNA (ssDNA) probes and MWCNTs@GONRs led to minimal background fluorescence due to the FRET process (turning off). The targets of P35s and TNOS were recognized by dual fluorescent probes to form double-stranded DNA (dsDNA) through the specific hybridization between target DNAs and ssDNA probes. And the dsDNA were released from the surface of MWCNTs@GONRs, which leaded the dual fluorescent probes to generate the strong fluorescent emissions (turning on). Therefore, this proposed homogeneous assay can be achieved to detect P35s and TNOS simultaneously by monitoring the relevant fluorescent emissions. Moreover, this assay can distinguish complementary and mismatched nucleic acid sequences with high sensitivity. The constructed approach has the potential to be a tool for daily detection of genetically modified organism with the merits of feasibility and reliability.

Photoelectrochemical CaMV35S biosensor for discriminating transgenic from non-transgenic soybean based on SiO2@CdTe quantum dots core-shell nanoparticles as signal indicators

A methodology for detection of the Cauliflower Mosaic Virus 35S(CaMV35S) promoter was developed to distinguish transgenic from non-transgenic soybean samples by using photoelectrochemical (PEC) biosensor. In this PEC biosensing system, the as-prepared gold nanoparticles-reduced graphene oxide acted as a nanocarrier to immobilize the thiol-functional probe (probe1), and the SiO₂@CdTe quantum dots (QDs) core-shell nanoparticles tagged with the amino-functional probe (probe2) acted as signal indicators, respectively. In the presence of target DNA (tDNA) of CaMV35S, the binding of tDNA with probe1 and probe2 through the high specific DNA hybridization led to the fabrication of sandwich structure, and thus the high loading of the signal indicators SiO₂@CdTe QDs at the electrode surface, which increased the PEC signal. The increased PEC signal depended on the concentration of tDNA, and a wide linear range from 0.1pM to 0.5nM with low detection limit of 0.05pM was obtained. In addition, the PEC biosensor has been successfully used for discriminating transgenic soybean from non-transgenic samples, which was consistent with the polymerase chain reaction (PCR) results, suggesting the proposed PEC biosensor is a feasible tool for the further daily genetically modified organism detection.

Highly sensitive analysis of four hemeproteins by dynamically-coated capillary electrophoresis with chemiluminescence detector using an off-column coaxial flow interface

Dynamic coating of the surface in capillary electrophoresis with chemiluminescence detection (CE-CL) using an off-column coaxial flow interface for the determination of four hemeproteins was developed. This method is based on the luminol-hydrogen peroxide reaction catalyzed by metalloproteins in alkaline medium. The experimental setup of the CE-CL system with the proposed off-column coaxial interface was evaluated by separation and detection of dopamine and catechol based on inhibition of the luminol-potassium ferricyanide reaction. Highly efficient separation of the two model compounds with symmetrical peak shape and satisfactory reproducibility was achieved by using this interface. In addition, in order to obtain a good resolution for hemeproteins, polyvinylpyrrolidone (PVP) combined with sodium dodecyl sulfate (SDS) were introduced as dynamic modifiers to reduce the unwanted adsorption of non-specific protein. Several parameters affecting the CE separation and CL detection were investigated in detail. Under the optimized conditions, a mixture of the four hemeproteins (horseradish peroxidase (HRP), catalase (Cat), myoglobin (Mb) and cytochrome C (Cyt C)) could be well separated within 20 min. The linear ranges of the four proteins were 5.7 × 10(-8) to 1.1 × 10(-6) mol L(-1) for HRP, 4.0 × 10(-8) to 2.0 × 10(-6) mol L(-1) for Cat, 1.1 × 10(-10) to 5.6 × 10(-8) mol L(-1) for Mb, and 3.8 × 10(-7) to 7.7 × 10(-6) mol L(-1) for Cyt C. The limits of detection (LODs) (S/N = 3) for HRP, Cat, Mb and Cyt C were 2.2 × 10(-8) mol L(-1) (104.5 amol), 1.6 × 10(-8) mol L(-1) (74 amol), 5.6 × 10(-11) mol L(-1) (0.26 amol), and 1.95 × 10(-7) mol L(-1) (0.89 fmol), respectively. The proposed method has been successfully applied to the analysis of low-level Mb in a spiked human urine sample and the recoveries were above 97%. Our primary result demonstrated that the proposed CE-CL method has great potential for Mb determination in clinical diagnosis.

MACRO: a combined microchip-PCR and microarray system for high-throughput monitoring of genetically modified organisms

The monitoring of genetically modified organisms (GMOs) is a primary step of GMO regulation. However, there is presently a lack of effective and high-throughput methodologies for specifically and sensitively monitoring most of the commercialized GMOs. Herein, we developed a multiplex amplification on a chip with readout on an oligo microarray (MACRO) system specifically for convenient GMO monitoring. This system is composed of a microchip for multiplex amplification and an oligo microarray for the readout of multiple amplicons, containing a total of 91 targets (18 universal elements, 20 exogenous genes, 45 events, and 8 endogenous reference genes) that covers 97.1% of all GM events that have been commercialized up to 2012. We demonstrate that the specificity of MACRO is ~100%, with a limit of detection (LOD) that is suitable for real-world applications. Moreover, the results obtained of simulated complex samples and blind samples with MACRO were 100% consistent with expectations and the results of independently performed real-time PCRs, respectively. Thus, we believe MACRO is the first system that can be applied for effectively monitoring the majority of the commercialized GMOs in a single test.

Capillary electrophoresis with electrochemiluminescent detection for highly sensitive assay of genetically modified organisms

A capillary electrophoresis coupled with electrochemiluminescent detection system (CE-ECL) was developed for the detection of polymerase chain reaction (PCR) amplicons. The ECL luminophore, tris(1,10-phenanthroline) ruthenium(II) (Ru(phen)(3)(2+)), was labeled to the PCR primers before amplification. Ru(phen)(3)(2+) was then introduced to PCR amplicons by PCR amplification. Eventually, the PCR amplicons were separated and detected by the homemade CE-ECL system. The detection of a typical genetically modified organism (GMO), Roundup Ready Soy (RRS), was shown as an example to demonstrate the reliability of the proposed approach. Four pairs of primers were amplified by multiple PCR (MPCR) simultaneously, three of which were targeted on the specific sequence of exogenous genes of RRS, and another was targeted on the endogenous reference gene of soybean. Both the conditions for PCR amplification and CE-ECL separation and detection were investigated in detail. Results showed that, under the optimal conditions, the proposed method can accurately identifying RRS. The corresponding limit of detection (LOD) was below 0.01% with 35 PCR cycles.