Manli Guo

A Highly Selective and Sensitive Fluorescence Detection Method of Glyphosate Based on an Immune Reaction Strategy of Carbon Dot Labeled Antibody and Antigen Magnetic Beads

A sensitive fluorescence detection method for glyphosate (GLY) was established based on immune reaction. First, carbon dot labeled antibodies (lgG-CDs) which were able to specifically identify glyphosate were prepared with the environmentally friendly carbon dots (CDs) and glyphosate antibody (lgG). lgG-CDs could be used to in situ visualize the distribution of glyphosate in plant tissues. In order to eliminate the effects of excess lgG-CDs on the determination of GLY, antigen magnetic beads Fe3O4-GLY based on magnetic nanoparticles Fe3O4 and glyphosate were constructed and utilized to couple with the excess lgG-CDs. After magnetic separation to remove antigen magnetic beads, there was a linear relationship between the fluorescence intensity of lgG-CDs and the logarithmic concentration of glyphosate in the range of 0.01-80 μg/mL with a detection limit of 8 ng/mL. The method was used for the detection of glyphosate in Pearl River water, tea, and soil samples with satisfactory recovery ratio between 87.4% and 103.7%.

Point-of-care testing for streptomycin based on aptamer recognizing and digital image colorimetry by smartphone

The rapid detection of antibiotic residual in everyday life is very important for food safety. In order to realize the on-site and visual detection of antibiotic, a POCT method was established by using digital image colorimetry based on smartphone. Streptomycin was taken as the analyte model of antibiotics, streptomycin aptamer preferentially recognized analyte, and the excess aptamer hybridized with the complementary DNA to form the dsDNA. SYBR Green I combined with the dsDNA and then emitted obvious green fluorescence, thus the fluorescence intensity decreased with the increasing of streptomycin concentration. Then a smartphone-based device was constructed as the fluorescence readout. The smartphone camera acquired the images of the fluorescence derived from the samples, and the Touch Color APP installed in smartphone read out the RGB values of the images. There was a linear relationship between the G values and the streptomycin concentrations in the range of 0.1-100µM. The detection limit was 94nM, which was lower than the maximum residue limit defined by World Health Organization. The POCT method was applied for determining streptomycin in chicken and milk samples with recoveries in 94.1-110%. This method had the advantages of good selectivity, simple operation and on-site visualization.

Fluorescence Determination of Omethoate Based on a Dual Strategy for Improving Sensitivity

Omethoate is a frequently used organophosphorus pesticide, and the establishment of a sensitive, selective, and simple method to determine omethoate is very important for food safety. In this paper, a dual strategy was applied to improve the detection sensitivity of omethoate. In the first strategy, graphene quantum dots (GQDs) were doped with nitrogen to increase the fluorescence quantum yield to 30%. By coupling N-GQDs with omethoate aptamer, an N-GQDs-aptamer probe was synthesized. The fluorescence of the N-GQDs-aptamer probe was turned off by graphene oxide (GO), but recovered by omethoate. Based on this principle, the fluorescence method for detecting omethoate was established with a detection limit of 0.041 nM. To further improve the detection sensitivity, the fluorescence polarization analysis method was applied as another strategy based on the polarization signal of GQDs. The detection limit was decreased to 0.029 pM by using the fluorescence polarization method. The detection limits in this paper were lower than those in other reports. The imaging of omethoate on plant leaves showed that the probe could be used for visual semiquantitative determination of omethoate.

A ratiometric nanoprobe based on silver nanoclusters and carbon dots for the fluorescent detection of biothiols

Ratiometric fluorescent probes could eliminate the influence from experimental factors and improve the detection accuracy. In this article, a ratiometric nanoprobe was constructed based on silver nanoclusters (AgNCs) with nitrogen-doped carbon dots (NCDs) and used for the detection of biothiols. The fluorescence peak of AgNCs was observed at 650nm with excitation wavelength at 370nm. In order to construct the ratiometric fluorescent probe, NCDs with the excitation and emission wavelengths at 370nm and 450nm were selected. After adding AgNCs, the fluorescence of NCDs was quenched. The mechanism of the fluorescence quenching was studied by fluorescence, UV-Vis absorption and the fluorescence lifetime spectra. The results indicated that the quenching could be ascribed to the inner filter effect (IFE). With the addition of biothiols, the fluorescence of AgNCs at 650nm decreased due to the breakdown of AgNCs, and the fluorescence of NCDs at 450nm recovered accordingly. Thus, the relationship between the ratio of the fluorescence intensities (I450/I650) and biothiol concentration was used to establish the determination method for biothiols. Cysteine (Cys) was taken as the model of biothiols, and the working curve for Cys was I450/I650=0.60CCys-1.86 (CCys: μmol/L) with the detection limit of 0.14μmol/L (S/N=3). Then, the method was used for the detection of Cys in human urine and serum samples with satisfactory accuracy and recovery ratios. Furthermore, the probe could be applied for the visual semi-quantitative determination of Cys by naked eyes.

Modification-free carbon dots as turn-on fluorescence probe for detection of organophosphorus pesticides

It is important to detect pesticides residues due to the concern over food safety. In this work, the burning ash of waste paper was used as carbon source to synthesize carbon dots (C-dots). The fluorescence of obtained C-dots could been turn off by Fe3+ which was from Fe2+ oxidized by H2O2, organophosphorus pesticides could effectively inhibit the production of H2O2 by destroying the acetylcholinesterase activity, so the fluorescence of C-dots hold turning on in the presence of organophosphorus pesticides. Based on above principle that the fluorescence intensity of C-dots was proportional to the pesticides concentration, take chlorpyrifos for example, a universal method for pesticides detection was established. The linear range was 0.01-1.0 μg/mL with detection limit of 3 ng/mL. The method was reliable and sensitive to actual samples. The imaging of chlorpyrifos on cabbages leaves indicated this method could be used for visualization detection of organophosphorus pesticides in vegetables.

Ultrasensitive electrochemiluminescence detection of Staphylococcus aureus via enzyme-free branched DNA signal amplification probe

As a main cause of foodborne diseases, pathogenic bacteria have threatened the health and well-being of human communities. There is a need of fastness, accuracy and sensitivity in the method of detecting pathogenic bacteria. Classical signal amplification assays usually employ enzymes as biocatalysts to generate amplified signals, but the strict experimental conditions and complicated instruments restrict their application. In this work, we demonstrated an enzyme-free branched DNA (bDNA)-based signal amplification electrochemiluminescence (ECL) assay for ultrasensitive detection of pathogenic bacteria. Firstly, the capture probes and the amplification probes group were carefully designed by our research group. The detecting ECL signal of Staphylococcus aureus (S. aureus) was amplified by bDNA technique through the layer-by-layer signal amplification. The sensitivity was greatly improved by the use of multiple Ru(bpy)32+ (TBR)-labeled ECL probes. Secondly, the whole process of the detection was carried out in the absence of enzyme, without the need to control the reaction conditions strictly. Thirdly, the designed amplification probes group could be used for the analysis of other pathogenic bacteria, virus, tumor markers, biomarkers, etc. For the detection of S. aureus, the limit of detection (LOD) of the method was 2 pM for standard DNA, with the linear range from 20 pM to 100 nM. Last but not least, the LOD of the S. aureus asymmetric PCR products was 5 pM, with the linear range from 10 pM to 50 nM. The sensitivity was 1-2 orders in magnitude higher than that of the common detection assays.

Novel Fluorescence Sensor Based on All-Inorganic Perovskite Quantum Dots Coated with Molecularly Imprinted Polymers for Highly Selective and Sensitive Detection of Omethoate

All-inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I) have attracted considerable attention with superior electrical and photophysical properties. In this study, luminescent perovskite (CsPbBr3) quantum dots (QDs) as sensing elements combined with molecularly imprinted polymers (MIPs) are used for the detection of omethoate (OMT). The new MIPs@CsPbBr3 QDs were synthesized successfully through the imprinting technology with a sol-gel reaction. The fluorescence (FL) of the MIPs@CsPbBr3 QDs was quenched obviously on loading the MIPs with OMT, the linear range of OMT was from 50 to 400 ng/mL, and the detection limit was 18.8 ng/mL. The imprinting factor was 3.2, which indicated excellent specificity of the MIPs for the inorganic metal halide (IMH) perovskites. The novel composite possesses the outstanding FL capability of CsPbBr3 QDs and the high selectivity of molecular imprinting technology, which can convert the specific interactions between template and the imprinted cavities to apparent changes in the FL intensity. Hence, a selective and simple FL sensor for direct and fast detection of organophosphorus pesticide in vegetable and soil samples was developed here. The present work also illustrates the potential of IMH perovskites for sensor applications in biological and environmental detection.

Tunable and Nontoxic Fluorescent Probes Based on Carbon Dots for Imaging of Indole Propionic Acid Receptor in Plant Tissues in Situ

Indole propionic acid (IPA) is one of the important plant growth hormones for promoting rooting and fruiting. Labeling IPA receptor in plant tissues is able to further track the signal transduction processes of IPA and uncover the function mechanism of IPA on crop productions. In this paper, a tunable and nontoxic fluorescent probe for IPA receptors was designed and synthetized base on carbon dots (C dots). Firstly carboxyl-modified carbon dots were prepared by high temperature cracking of citric acid. The fluorescence emission wavelengths of C dots varied with the excitation wavelengths change. Then IPA-modified carbon dots (IPA-C dots) were prepared by coupling the amino of tryptophan with the carboxyl of as-prepared carbon dots. Compared with C dots, the fluorescence intensity of IPA-C dots was double and the fluorescence stability was satisfactory under various conditions. This probe retained the biological activity of IPA and acted as target recognition of IPA receptors in plant tissues. The probe could avoid green fluorescence background of plants. The imaging results showed that the IPA receptors mainly existed on the membrane of stele. The toxicity test indicated the probe was less toxic than traditional inorganic semiconductor quantum dots.