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An enzyme-free fluorometric assay is described that accomplishes dual signal amplification by making use of a two stirring bars. Two Y-shaped DNA probes were designed and placed on the bars. When the target (with chloramphenicol as model analyte) is added, it triggers target recycling and simultaneously catalyzes hairpin assembly (CHA). A large fraction of DNA primers is released by the analyte from the bar to the supernatant and open hairpins with G-quadruplex DNA sequence. The G-quadruplex can specifically bind thioflavin T (ThT) to emit fluorescence (with excitation/emission maxima at 445 and 485 nm) for quantification of chloramphenicol. An enzyme is not needed. ThT is added to the system as a fluorescent DNA probe. All this strongly reduces the cost for sensor construction and usage. The dual signal amplification steps occur simultaneously which reduces the detection time. The assay was successfully employed to the determination of CAP in spiked milk and fish samples within 60 min and with a 16 pM limit of detection (at S/N = 3). Graphical abstract Schematic representation of a new method for the detection of chloramphenicol by using  two stirring bars. It is based on target recycling and catalyzed hairpin assembly amplification. CAP: chloramphenicol, ThT: thioflavin T, CHA: catalyzed hairpin assembly.

An ultrasensitive colorimetric aptasensor was developed for antibiotics detection, with chloramphenicol (CAP) as model target, using DNAzyme labeled Fe-MIL-88-Pt as novel peroxidase mimic signal tags and target-triggered circular strand-displacement polymerization (CSDP) for signal amplification. The system consists of two components which can partially hybridize with each other: one is capture probe which was formed through immobilizing hairpin DNA containing aptamer sequence on magnetic beads (MB-cDNA), another is signal tag which was constructed through labeling single strand DNAzyme (G-quadruplex/Hemin) which can partially hybrid with cDNA on platinum nanoparticles functionalized Fe-MIL-88 (MIL-88-Pt-DNAzyme). All components of MIL-88, Pt and DNAzyme in the tag can act as peroxidase mimic to triply catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H 2 O 2 into a blue-colored oxidized TMB (oxTMB) for the colorimetric readout. Thus distinctive signal can be observed by naked eye even in presence of 0.02 nM tags. In the presence of target and primer, cDNA loop can open to form cDNA/CAP intermediates, enabling primer to hybridize with the exposed sequences of the cDNA, which initiated target assisted CSDP recycles. Then numerous signal tags were released into supernatant to catalyze TMB for color development. There was a liner relationship between the absorbance and the concentration of CAP in the range of 0.1 pM (0.0323 pg/mL) to 1000 pM (323 pg/mL) with the detection limit of 0.03 pM (0.0097 pg/mL). The ultra-high sensitivity was ascribed to the multiplex catalytic activities from the tags and CSDP based signal amplification. Furthermore, this method can produce signals being observed by naked eye to facilitate in-situ detection and be further extended to detect other antibiotics in food just by simply replacing cDNA on the sensing system., (Copyright © 2018 Elsevier B.V. All rights reserved.)

An ultrasensitive electrochemical aptasensor for multiplex antibiotics detection based on endonuclease and exonuclease assisted dual recycling amplification strategy was proposed. Kanamycin and chloramphenicol were selected as candidates. Firstly, aptamers of the antibiotics were immobilized on bar A and then binding with their endonuclease labeled complementary DNA strands to construct enzyme-cleavage probes. Secondly, The nano zirconium-metal organic framework (NMOF) particles with 1,4-benzene-dicarboxylate (BDC) as linker was defined as UiO-66. And its updated version, hierarchically porous UiO-66 (HP-UIO-66) decorated with different electroactive materials as signal tags were synthesized. Then they were immobilized on bar B linked by two duplex DNA strands which can be specifically cleaved by corresponding enzyme-cleavage probes in bar A. Once targets were introduced into system, aptamers can capture them and then release enzyme-cleavage probes. In the presence of exonuclease-I, exonuclease assisted target recycling amplification was triggered and more enzyme-cleavage probes were released into solution. The probes can trigger endonuclease assisted recycles and repeatedly cleave their corresponding duplex DNA strands on bar B then released numerous signal tags into supernatant. Thus two recycling amplification was performed in the system. Finally, MB and Fc in the signal tags were detected by square wave voltammetry after removing bar A/B and the current intensities were correspondent with the concentration of KANA and CAP respectively. Under the optimum condition, the limits of detection for the KANA and CAP were 35fM and 21fM respectively with a wide linear range from 1 × 10 -4 to 50nM. This dual recycling amplification detection system exhibited high sensitivities and specificity. It can be easily extended to detect other targets if changing the corresponding aptamers and has potential application values for screening of multiplex antibiotics residues in food safety., (Copyright © 2017 Elsevier B.V. All rights reserved.)

In this work, a novel aptamer functionalized magnetic adsorbent was developed and combined with magnetic dispersive solid phase extraction (MDSPE) for selective enrichment of several amphenicol antibiotics residues (chloramphenicol(CAP), thiamphenicol(TAP) and florphenicol(FF)) in foodstuff then determined by High Performance Liquid Chromatography (HPLC)-Diode array detector(DAD). Firstly, a magnetic silica-coated Fe 3 O 4 microsphere(Fe 3 O 4 @SiO 2 ) was synthetized by sol-gel method, then it was functionalized by amino groups through 3-Aminopropyltriethoxysilane (APTES) reagent to form Fe 3 O 4 @SiO 2 -NH 2 ; Thirdly, the amino group on Fe 3 O 4 @SiO 2 -NH 2 was transferred to carboxylic group via the succinic anhydride to form Fe 3 O 4 @SiO 2 -COOH. Finally a kind of DNA aptamer with amino group which can simultaneously recognize CAP, TAP, FF, was functionalized on Fe 3 O 4 @SiO 2 -COOH through adding the EDC/sulfo-NHS coupling reagent (Fe 3 O 4 @SiO 2 @Apt). Therefore, a very stable and useful adsorbent was synthesized for the detection of chloramphenicol series antibiotics. This adsorbent can specifically and simultaneously recognize and enrich CAP, TAP, and FF with high adsorption amount from some complicated food matrix, e.g. milk based on the high affinity of aptamer towards the analytes. The saturated extraction capacities for CAP, TAP and FF by the adsorbent were 2.82, 2.56, 2.72μg/g (mass of target/adsorbent)respectively and the enrichment folds were more than 100 times. Afterwards, the target analytes were washed away by pH 8.5 0.1M Tris-HCl buffer solution and detected by HPLC-DAD. The parameters including extraction temperature, extraction capacity, extraction & desorption pH, extraction & desorption time were investigated. With the optimized conditions, the limits of detection (LOD) and limits of quantitation (LOQ) were 0.12-0.17ng/ml and 0.40-0.55ng/ml for the amphenicols in milk. The adsorbent also has good reproducibility for extraction which can be reused at least for 60 cycles with the recovery over 80% (Fig. S2). The MDSPE combined with HPLC-DAD detection possessed the advantages of high selectivity, extraction capacity and very convenient for magnetic separation. In addition, this method is environment friendly and employed no organic solution in the period of pretreatment and extracting. It is a universal platform which can be extended to selective enrichment other organic pollutants residues if changing the modified aptamers., (Copyright © 2017. Published by Elsevier B.V.)

A novel label-free, universal, and high throughput aptasensor was developed based on a microchip electrophoresis (MCE) platform for automatic detection of antibiotic residues in food. Firstly, chloramphenicol (CAP) was employed as a model to be captured by its aptamer probe (Apt). Then, the partial complementary oligonucleotide of CAP's aptamer (C-DNA) was introduced into the reaction system. Because the Apt-CAP complex can't further hybrid with free C-DNA, the amount of hybrid Apt-C-DNA double strand DNA (dsDNA) was less than that without adding the target. Finally, the above mixture was introduced into the microchip electrophoresis (MCE) platform for detection, both dsDNA and Apt-CAP can be separated and produce different fluorescence signals in the MCE. In a certain concentration range, the ratio of signal between dsDNA and Apt-CAP (I dsDNA /I Apt-CAP ) was proportional to the concentration of targets. Under the optimum conditions, the ratio showed a satisfactory linearity range from 0.008 to 1ng/mL of CAP with a detection limit of 0.003ng/mL. Thus, a universal MCE-based assay was developed for quantifying CAP automatically. The method was also successfully applied in the different food samples for CAP detection, which showed a good recovery (Milk: 91.1-108%, Fish: 86.1-114%) and the results were consistent with that of ELISA. This method owned many merits as follows: firstly, MCE was a high throughput screening platform and the detection time is limited to 3min for each sample. Secondly, the aptamer probes can be directly used for detection without labeling any signal tag which can facilitate the preparation procedures of probes. Thirdly, the operation was easy just by the following steps: firstly, the mixture of aptamer probes were incubated followed adding C-DNA; then measurement was performed. Moreover, the assay with MCE platform can be used to detect other targets just by changing the corresponding aptamer probe; it can even realize simultaneous detection when the targets have aptamers with different number of base pairs. Above all, it's a high- throughput and prospective method which can be applied in high throughput screening of antibiotics in food safety., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Herein, a smart single-stranded DNA binding protein (SSB)-assisted fluorescence aptamer switch based on fluorescence resonance energy transfer (FRET) was designed. The FRET switch was synthesized by connecting SSB labeled quantum dots (QDs@SSB) as donor with aptamer (apt) labeled gold nanoparticles (AuNPs@apt) as acceptor, and it was employed for detecting chloramphenicol (CAP) in a homogenous solution. In the assay, the interaction between core-shell QDs@SSB and AuNPs@apt leads to a dramatic quenching (turning off). After adding CAP in the detection system, AuNPs@apt can bind the target specifically then separate QDs@SSB with AuNPs@apt-target, resulting in restoring the fluorescence intensity of QDs (turning on). Consequently, the fluorescence intensity recovers and the recovery extent can be used for detection of CAP in homogenous phase via optical responses. Under optimal conditions, the fluorescence intensity increased linearly with increasing concentrations of CAP from 0.005 to 100ngmL -1 . The limit of this fluorescence aptamer switch was around 3pgmL -1 for CAP detection. When the analyte is changed, the assay can be applied to detect other targets only by changing relative aptamer in AuNPs@apt probe. Furthermore, it has potential to be served as a simple, sensitive and portable platform for antibiotic contaminants detection in biological and environmental samples., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The present study aimed to explore a novel triple-amplification electrochemiluminescence (ECL) assay for detecting of chloramphenicol (CAP). This strategy was based on single-stranded DNA-binding protein (SSB) and horseradish peroxidase (HRP) enzyme-linked polymer (EnVision reagent, EV) labeled on Au nanoparticles (EV-Au-SSB) as nanotracer and exonuclease-assisted target recycling. The composite probes were prepared via immunoreactions between the CdS nanocrystal (CdS NC)-functionalized partial complementary DNA and aptamer (CdSNCs/Apt-ssDNA1) as capture probes, and EV-Au-SSB as nanotracer. When the composite probe solution co-existed with CAP and Exo I, the aptamer on the capture probes preferentially combined with CAP, and then CAP-Apt and nanotracer complex were released into the solution. Subsequently, Exo I in the solution could further digest the CAP-Apt from the 3'-end of the aptamer and release CAP, which could participate in further reaction with the probes. It was worth mentioning that EV contained a large number of HRPs on its dendritic chain. In the EV-Au-SSB, Au could enhance ECL intensity of CdS NCs by surface plasmon resonance. What's more, HRPs on EV could catalyze the reaction of H2O2, which could obviously enhance ECL intensity of CdS NCs. This study demonstrated excellent performance of the triple-amplification ECL assay, which makes this aptasensor system suitable and promising for the practical application of CAP residues in fish samples. Moreover, the assay might provide a promising avenue to develop efficient aptasensors to determine small-molecule harmful substances in environmental monitoring and food safety., (Copyright © 2016 Elsevier B.V. All rights reserved.)

In this work, a novel homogeneous and signal "off-on" aptamer based fluorescence assay was successfully developed to detect chloramphenicol (CAP) residues in food based on the fluorescence resonance energy transfer (FRET). The vesicle nanotracer was prepared through labeling single stranded DNA binding protein (SSB) on limposome-CdSe/ZnS quantum dot (SSB/L-QD) complexes. It was worth mentioning that the signal tracer (SSB/L-QD) with vesicle shape, which was fabricated being encapsulated with a number of quantum dots and SSB. The nanotracer has excellent signal amplification effects. The vesicle composite probe was formed by combining aptamer labeled nano-gold (Au-Apt) and SSB/L-QD. Which based on SSB's specific affinity towards aptamer. This probe can't emit fluoresce which is in "off" state because the signal from SSB/L-QD as donor can be quenched by the Au-aptas acceptor. When CAP was added in the composite probe solution, the aptamer on the Au-Apt can be preferentially bounded with CAP then release from the composite probe, which can turn the "off" signal of SSB/L-QD tracer into "on" state. The assay indicates excellent linear response to CAP from 0.001 nM to 10 nM and detection limit down to 0.3 pM. The vesicle probes with size of 88 nm have strong signal amplification. Because a larger number of QDs can be labeled inside the double phosphorus lipid membrane. Besides, it was employed to detect CAP residues in the milk samples with results being agreed well with those from ELISA, verifying its accuracy and reliability., (Copyright © 2016 Elsevier B.V. All rights reserved.)

A novel fluorescence aptasensor was successfully developed to respond to chloramphenicol (CAP) in food based on magnetic aptamer-liposome vesicle probe. In order to fabricate it, aptamer labeled on functionalized magnetic beads (MB) was firstly employed as capture adsorbent (MB-Apt), then SSB (single-stranded DNA binding protein) and DIL (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanineperchlorate) coimmobilized liposomes (SSB/DIL-Lip) was employed as vesicle signal tracer. The composite vesicle probe is formed between SSB/DIL-Lip and MB-Apt based on SSB's specific recognition towards aptamer on vesicle signal tracer. Upon the vesicle probe solution reacted with CAP, the aptamer on the magnetic beads preferentially bounded with CAP, and then released SSB/DIL-Lip vesicle signal tracer in the supernatant after magnetic separation. The released tracer can emit fluorescence which was correspondence with the concentration of the analyte. At the optimum conditions, the aptasensor exhibited a good linear response for CAP detection in the range of 0.003-10nM with a detection limit of 1pM. Importantly, the methodology was further validated for analyzing CAP in fish samples with consistent results obtained by ELISA kit, thus providing a promising approach for quantitative monitoring of CAP and significant anti-interference ability in food safety., (Copyright © 2016 Elsevier B.V. All rights reserved.)

A multiplex electrochemical aptasensor was developed for simultaneous detection of two antibiotics such as chloramphenicol (CAP) and oxytetracycline (OTC), and high-capacity magnetic hollow porous nanotracers coupling exonuclease-assisted target recycling was used to improve sensitivity. The cascade amplification process consists of the exonuclease-assisted target recycling amplification and metal ions encoded magnetic hollow porous nanoparticles (MHPs) to produce voltammetry signals. Upon the specific recognition of aptamers to targets (CAP and OTC), exonuclease I (Exo I) selectively digested the aptamers which were bound with CAP and OTC, then the released CAP and OTC participated new cycling to produce more single DNA, which can act as trigger strands to hybrid with nanotracers to generate further signal amplification. MHPs were used as carriers to load more amounts of metal ions and coupling with Exo I assisted cascade target recycling can amplify the signal for about 12 folds compared with silica based nanotracers. Owing to the dual signal amplification, the linear range between signals and the concentrations of CAP and OTC were obtained in the range of 0.0005-50 ng mL(-1). The detection limits of CAP and OTC were 0.15 and 0.10 ng mL(-1) (S/N=3) which is more than 2 orders lower than commercial enzyme-linked immunosorbent immunoassay (ELISA) method, respectively. The proposed method was successfully applied to simultaneously detection of CAP and OTC in milk samples. Besides, this aptasensor can be applied to other antibiotics detection by changing the corresponding aptamer. The whole scheme is facile, selective and sensitive enough for antibiotics screening in food safety., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A "signal-on'' aptasensor was developed for simultaneous detection of chloramphenicols (CAP) and polychlorinated biphenyl-72 (PCB72) with a novel multi-metal ions encoded nanospherical brushes as nanotracers. To construct the assay, the respective aptamer of CAP and PCB72 labeled magnetic gold nanoparticles as capture probes (aptamer-MGPs), and their complementary single strand DNA (s-DNA) encoded metal ions (Cd(2+) and Pb(2+)) on nanospherical branched polyethylene imine brushes as tracers (s-DNA-MSPEIs), were simultaneously synthesized. After that, the capture probe and tracers were connected through a hybridization reaction between s-DNA and aptamers. In the presence of CAP and PCB72, the analytes could react with the aptamers on capture probes and release the tracers into supernatant after magnetic separation. The released tracers with metal ions (Cd(2+) and Pb(2+)) could be simultaneously detected through the square wave voltammetry (SWV) without acid dissolution, which can switch the signals of the biosensor to "on'' state. Under optimal conditions, the assay could detect CAP and PCB72 as low as 0.3 pg mL(-1) with the dynamitic range from 0.001 to 100 ng mL(-1) and exhibited excellent selectivity. More importantly, the strategy can be extended easily to other targets after changing the corresponding aptamers and other metal ions tracers, which provides a promising and facile approach in multiplex detection of ultra-trace level of pollutants in food safety without more complex separation and washing steps., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A novel type of "dual-potential" electrochemiluminescence (ECL) aptasensor array was fabricated on a homemade screen-printed carbon electrode (SPCE) for simultaneous detection of malachite green (MG) and chloramphenicol (CAP) in one single assay. The SPCE substrate consisted of a common Ag/AgCl reference electrode, carbon counter electrode and two carbon working electrodes (WE1 and WE2). In the system, CdS quantum dots (QDs) were modified on WE1 as cathode ECL emitters and luminol-gold nanoparticles (L-Au NPs) were modified on WE2 as anode ECL emitters. Then the MG aptamer complementary strand (MG cDNA) and CAP aptamer complementary strand (CAP cDNA) were attached on CdS QDs and L-Au NPs, respectively. The cDNA would hybridize with corresponding aptamer that was respectively tagged with cyanine dye (Cy5) (as quenchers of CdS QDs) and chlorogenic acid (CA) (as quenchers of l-Au NPs) using poly(ethylenimine) (PEI) as a bridging agent. PEI could lead to a large number of quenchers on the aptamer, which increased the quenching efficiency. Upon MG and CAP adding, the targets could induce strand release due to the highly affinity of analytes toward aptamers. Meanwhile, it could release the Cy5 and CA, which recovered cathode ECL of CdS QDs and anode ECL of L-Au NPs simultaneously. This "dual-potential" ECL strategy could be used to detect MG and CAP with the linear ranges of 0.1-100 nM and 0.2-150 nM, with detection limits of 0.03 nM and 0.07 nM (at 3sB), respectively. More importantly, this designed method was successfully applied to determine MG and CAP in real fish samples and held great potential in the food analysis., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Herein, an ultrasensitive and selective colorimetric assay for antibiotics, using chloramphenicol (CAP) as the model analyte, was developed based on magnetic aptamer-HRP-platinum composite probes and exonuclease-assisted target recycling. The composite probes were prepared through immunoreactions between the double stranded DNA antibody (anti-DNA) labeled on core-shell Fe3O4@Au nanoparticles (AuMNP-anti-DNA) as the capture probe, and the double stranded aptamer (aptamer hybrid with its complementary oligonucleotides) labeled on Pt@HRP nanoparticles as the nanotracer (ds-Apt-HRP-PtNPs). When the CAP samples were incubated with the probes for 30 min at room temperature, they could be captured by the aptamer to form a nanotracer-CAP complex, which was then released into the supernatant after magnetic separation. This is because the anti-DNA on the capture probes cannot recognize the single strand aptamer-CAP complex. The exonuclease I (Exo I) added into the supernatant can further digest the aptamer-CAP from the 3'-end of the aptamer and the CAP in the aptamer-CAP complex can be released again, which can further participate in a new cycling process to react with the probes. Pt and HRP in the nanotracer could both catalyze and dual amplify the absorbance at 650 nm ascribed to the 3,3',5,5'-tetramethylbenzidine (TMB)-H2O2 system. Moreover, Exo I can assist the target recycling, which can further amplify the signal. Thus, the triple amplified signal can be quantified by ultraviolet-visible spectroscopy. The experimental results showed that the CAP detection possessed a linear range of 0.001-10 ng mL(-1) and a detection limit of 0.0003 ng mL(-1) (S/N = 3). The assay was successfully employed to detect CAP in milk, which is much more facile, time saving, and sensitive than the commercial ELISA kits.

A portable and antibody-free sandwich assay was fabricated for determination of chloramphenicol (CAP) in animal-derived food by a personal glucose meter (PGM). The sandwich-type strategy was developed on the basis of magnetic molecularly imprinted probe (m-MIP) nanoparticles and a β-cyclodextrin/invertase-functionalized signal tag. Firstly, the m-MIPs were fabricated using polydopamine molecularly imprinted film modified Fe3O4 nanoparticles with 2,2-dichloroacetamide as a template that could capture the 2,2-dichloroacetamide segment of CAP. Secondly, β-cyclodextrin/invertase polymer bioconjungate was synthesized as a signal tag that could recognize the nitrobenzene segment of CAP through host-guest interaction. The dual-specificity recognition model relies on the formation of a sandwich between m-MIPs, different segments of CAP, and the β-cyclodextrin-functionalized signal tag. The sandwich-type complex formed was then subjected to detection with a PGM. The complexes can hydrolyze sucrose to glucose, which can be used for detection with a PGM through invertase. According to our experiment, the concentration of CAP was proportional to the amount of glucose formed, which could quantitatively assess the CAP with a dynamic range of 0.5-50 ng mL(-1) and a detection limit of 0.16 ng mL(-1) (signal-to-noise ratio of 3). The detection time of the assay was about 1 h, which was obviously shorter than that of competitive ELISA. More importantly, we have successfully applied this on-site assay for CAP screening in animal-derived food.

A novel kind of electrochemical biocodes based on nanoscale metal organic framework (NMOF) was developed for simultaneous detection of multiple antibiotics such as kanamycin (KANA) and chloramphenicol (CAP). Herein, amine-functionalized NMOF (UiO-66-NH 2 ) was firstly employed as substrate to carry different metal ions (Pb 2+ or Cd 2+ ) respectively (M-NMOF). The adsorption capacities were 293.52 mg g −1 for Pb 2+ and 268.37 mg g −1 for Cd 2+ . Secondly, the M-NMOFs were respectively labeled with the complementary DNA strands of aptamers toward KANA and CAP to obtain two signal tags. Finally, both of the tags were conjugated with aptamers functionalized magnetic beads (MB@Apt1/Apt2) to obtain the biocodes. Upon aptamers bound to targets (KANA and CAP) specifically, the corresponding M-NMOFs were released into supernatant after magnetic separation, which can be detected by square wave voltammetry (SWV). Results showed the high sensitivity with detection limits as low as 0.16 pM and 0.19 pM (S/N = 3) toward KANA and CAP respectively, which is 3 or 4 orders of magnitude lower than that of ELISA. Moreover, it exhibited excellent selectivity and more resistant to interference in food analysis. Furthermore, this NMOF based biocodes can be extended to other simultaneous detection system just by changing the aptamer and increasing the types of metal ions. [ABSTRACT FROM AUTHOR]

Herein, a promising colorimetric strategy for small molecule detection was developed based on one novel magnetic single-stranded DNA binding protein (SSB)-linked composite probe. Firstly, a magnetic probe was synthesized by labeling SSB on gold magnetic particles (Au MNPs-SSB) and detection probe by co-immobilizing aptamer (Apt) and horseradish peroxidase (HRP) on the core-shell SiO 2 @Au nanoparticles (Apt-SiO 2 @Au-HRP); furthermore, Au MNPs-SSB was bound with Apt-SiO 2 @Au-HRP to construct a composite probe relied on the specific affinity between SSB and free aptamer. When the target, with chloramphenicol (CAP) as model, was introduced into the reaction system, the formation of the target-Apt-SiO 2 @Au-HRP complex can release from its parent probe to supernatant after magnetic separation because SSB can’t bind target-aptamer complex. The target-Apt-SiO 2 @Au-HRP complex in supernatant could efficiently catalyze 3,3′,5,5′-tetramethylbenzidine(TMB)-H 2 O 2 system for the color development. Under the optimized reaction conditions, the linear range was from 0.05 to100 ng mL −1 with a low detection limit of 0.02 ng mL −1 for CAP. Moreover, the aptamer assay was employed to detect CAP in milk and the results were consistence with those from ELISA. In addition, the assay can be easily extended to other small molecules detection when changing the corresponding aptamer on the detection probe. [ABSTRACT FROM AUTHOR]

A novel ratiometric electrochemiluminescent (R-ECL) aptasensor array was fabricated on a homemade screen-printed carbon electrode (SPCE) for antibiotic chloramphenicol (CAP) based on the ratio of working signal to internal reference signal. The SPCE substrate consists of a common Ag/AgCl reference electrode, carbon counter electrode and two spatial-resolved carbon working electrodes (WE1 and WE2). In the system, luminol-gold nanoparticles (L-Au NPs) as working signal tags were modified on WE1, and CdS quantum dots (QDs) were modified on WE2 as internal reference to provide a correction for avoiding the environmental effects. The L-Au NPs at WE1 were functionalized with a CAP aptamer′s DNA complementary (DNA1) and then CAP aptamer (DNA2) that was tagged with chlorogenic acid (CA) as quenchers of L-Au NPs. Upon CAP adding to WE1, the DNA2-CAP complexes were formed due to the highly affinity of CAP towards DNA2. Meanwhile, it could release the CA, which recovered the ECL of L-Au NPs. This ratio of ECL signals for quantification between target probes (at WE1) to internal standard probes (at WE2) could reduce the matrix environmental interference to a certain degree. Moreover, the two probes immobilized on two working electrodes while not one electrode, could also reduce cross-talk interference between each other. Such R-ECL approach offers a selective and sensitive platform and shows a great utility for detection of CAP in the food analysis. [ABSTRACT FROM AUTHOR]

A selective and facile fluorescence “switch-on” scheme is developed to detect antibiotics residues in food, using chloramphenicol (CAP) as model, based on a novel magnetic aptamer probe (aptamer–Pt–luminol nanocomposite labeled with hemin/G-quadruplex). Firstly, the composite probe is prepared through the immuno-reactions between the capture beads (anti-dsDNA antibody labeled on magnetic Dynabeads) and the nanotracer (nano-Pt–luminol labeled with double-strand aptamer, as ds-Apt, and hemin/G-quadruplex). When the composite probe is mixed with CAP, the aptamer preferentially reacted with CAP to decompose the double-strand aptamer to ssDNA, which cannot be recognized by the anti-dsDNA antibody on the capture probes. Thus, after magnetic separation, the nanotracer can be released into the supernatant. Because the hemin/G-quadruplex and PtNPs in nanotracer can catalyze luminol–H 2 O 2 system to emit fluorescence. Thus a dual-amplified “switch-on” signal appeared, of which intensity is proportional to the concentration of CAP between 0.001 and 100 ng mL −1 with detection limit of 0.0005 ng mL −1 ( S / N =3). Besides, our method has good selectivity and was employed for CAP detection in real milk samples. The results agree well with those from conventional gas chromatograph–mass spectrometer (GC–MS). The switch-on signal is produced by one-step substitution reaction between aptamer in nanotracer and target. When the analyte is changed, the probe can be refabricated only by changing the corresponding aptamer. Thus, all features above prove our strategy to be a facile, feasible and selective method in antibiotics screening for food safety. [ABSTRACT FROM AUTHOR]

In this work, a fluorescence aptasensor has been developed to detect trace chloramphenicol based on FRET and exonuclease-assisted target recycling. Firstly, the composite probe for CAP was prepared through the immunoreactions between the capture probe based on dsDNA antibody labeled on Fe 3 O 4 @Au nanoparticles and the nanotracer using double strand aptamer (aptamer hybrid with its complementary DNA) labeled on core-shell SiO 2 @Au. When the composite probe solution was mixed with CAP, the aptamer on the nanotracer preferentially bounded with CAP, and then released cDNA-SiO 2 @Au and CAP-Apt complex to the supernatant. This is because anti-DNA on capture probes can’t recognize the ssDNA. The exonuclease I (Exo I) added into the supernatant can further digest the aptamer-CAP from the 3′-end of aptamer and release CAP again, which can further participate in new cycle to react with the probes. The supernatant containing numerous SiO 2 @Au could efficiently quench the fluorescence response of CdSe QDs by FRET. Experimental results showed the CAP detection owning a linearity range of 0.001–10 ng mL −1 and detection of limit (LOD) of 0.0002 ng mL −1 . Besides, the results of our method for CAP detection in the fish samples agreed well with those from ELISA, verifying its accuracy and reliability. [ABSTRACT FROM AUTHOR]

A sensitive and facile colorimetric aptasensor for chloramphenicol (CAP) detection based on enzyme-linked polymer nanotracers labeled by double-stranded DNA (ds-DNA) antibody. In this work, the ds-DNA containing thiolated aptamer for CAP (Apt) and its complementary strand (cDNA) was immobilized on Fe 3 O 4 @Au magnetic nanoparticles (AuMNPs) as capture probe. An enzyme-linked polymer nanotracer was fabricated by co-immobilizing the ds-DNA antibody and horseradish peroxidase (HRP) labeled nanogold on Envision reagents (ds-DNA Ab/EV-AuNPs-HRP) as signal tag. EnVision reagent (EV) contains about 100 HRP and some anti-IgG antibody molecules. In the presence of CAP, the aptamer preferentially bound with CAP to release ss-DNA that cannot be recognized by ds-DNA Ab in the nanotracers and then the signal tag was substituted into supernatant after magnetic separation. The supernatant containing numerous HRP could efficiently catalyze 3,3′,5,5′-tetramethylbenzidine (TMB)-H 2 O 2 solution for the color development quantified by ultraviolet–visible spectroscopy. Experimental results showed the CAP detection owning a linear range of 0.05–100 ng mL −1 and sensitively respond down to 0.015 ng mL −1 . Besides, the results of this method for CAP detection in the fish samples agreed well with the ELISA results, verified its accuracy and reliability. The developed method can be used for in situ CAP detection in food safety. [ABSTRACT FROM AUTHOR]