Switch-on fluorescence scheme for antibiotics based on a magnetic composite probe with aptamer and hemin/G-quadruplex coimmobilized nano-Pt-luminol as signal tracer

Label-free electrochemical sensing platform for sensitive detection of ampicillin by combining nucleic acid isothermal enzyme-free amplification circuits with CRISPR/Cas12a

The residue of ampicillin (AMP) in food and ecological environment poses a potential harm to human health. Therefore, a reliable system for detecting AMP is in great demand. Herein, a label-free and sensitive electrochemical sensor utilizing NH2-Co-MOF as an electrocatalytic active material for methylene blue (MB) was developed for rapid and facile AMP detection by combining hybridization chain reaction (HCR), catalytic hairpin assembly (CHA) with CRISPR/Cas12a. The surface of glassy carbon electrode modified with NH2-Co-MOF was able to undergo HCR independent of the AMP, forming long dsDNA complexes to load MB, resulting in strong original electrochemical signal. The presence of AMP could trigger upstream CHA circuit to activate the CRISPR/Cas12a system, thereby achieving rapid non-specific cleavage of the trigger ssDNA of HCR on the electrode surface, hindering the occurrence of HCR and reducing the load of MB. Significant signal change triggered by the target was ultimately obtained, thus achieving sensitive detection of the AMP with a detection limit as low as 1.60 pM (S/N = 3). The proposed sensor exhibited good stability, selectivity, and stability, and achieved reliable detection of AMP in milk and livestock wastewater samples, demonstrating its promising application prospects in food safety and environmental monitoring.

Competitive ECL immunosensor for ultrasensitive 17β-estradiol detection based on synergistic promotion strategy using CdSe-ZnSe nanocomposites

A novel competitive ECL immunosensor for detection of 17β-Estradiol (E2) has been fabricated successfully. CdSe-ZnSe nanocomposites (CdSe-ZnSe NCs) with high catalytic properties, large surface area and good conductivity were used synergistically as the ECL nanocarriers of Pt nanoparticles (PtNPs). The ECL intensity of CdSe-ZnSe NCs increased and stabilized with luminol-PtNPs (luminol-PtNPs@CdSe-ZnSe NCs) because of electron transfer. To achieve the successful assembling of competitive ultrasensitive ECL immunosensor with high sensitivity and synergistic effect, Ag@TiO2 core-shell was introduced as label. Ag@TiO2 acted as a signal amplifier and also exhibited the high catalytic activity towards H2O2. This firmly anchored the E2 Antigen with covalent bond and converted the longer wavelength radiations to shorter wavelength. Under optimized conditions, our proposed strategy quantify the selective and reliable analysis of E2 with detection limit of 2.51 fg/mL (S/N = 3) within the linear range of 0.0001-30 ng/mL. The assembled synergistic strategy-based ECL immunosensor manifested the promising sensitivity, selectability along with high level of repeatability. Thus, the fabricated ECL immunosensor has potential valuable application for E2 detection along with many other environmental pollutants.

Immunochromatographic assays for ultrasensitive and high specific determination of enrofloxacin in milk, eggs, honey, and chicken meat

Enrofloxacin, a veterinary antibiotic that persists in food, poses a risk to human health. Here, a monoclonal antibody against enrofloxacin, 1H12, was prepared based on the hapten ENR-1, and showed excellent sensitivity with a 50% inhibitory concentration (IC₅₀) of 0.03 ng/mL. Using this antibody, 2 lateral-flow immunochromatographic assays were developed for determination of enrofloxacin in egg, milk, honey, and chicken meat samples. The detection ranges (IC₂₀-IC₈₀) were 0.16-0.82 ng/g, 0.24-1.8 ng/g, 0.25-3.6 ng/g, and 0.61-3.9 ng/g by colloidal gold-immunochromatographic sensor (CG-ICS) analysis, and 0.022-0.42 ng/g, 0.054-0.42 ng/g, 0.069-1.4 ng/g, and 0.19-2.2 ng/g by Eu-fluorescence-immunochromatographic sensor (EF-ICS) analysis. The intraassay and interassay recovery rates were 88.9 to 108.5% with coefficients of variation of 1.3 to 7.0% by CG-ICS analysis, and 88.6 to 113.6% with coefficients of variation of 1.3 to 8.1% by EF-ICS analysis. Thus, our newly developed ICS are sensitive and reliable, providing an option for rapid quantitative detection of enrofloxacin in food samples.

Luminol-Eu-based ratiometric fluorescence probe for highly selective and visual determination of tetracycline

In this work, a ratiometric fluorometric method based on luminol-Europium complex (luminol-Eu) was constructed for the detection of tetracycline (TC). Luminol-Eu, synthesized by self-assembly reaction, displayed a strongly emission peak at 453 nm under excitation at 360 nm which was derived from the aggregation-induced emission (AIE) of the luminol-Eu. In the present of TC, the fluorescence of luminol-Eu at 453 nm was quenched based on inner filter effect (IFE). Meantime, the characteristic emission peak of Eu³⁺ at 626 nm can be observed thank to antenna effect (AE). Therefore, we proposed a ratiometric fluorometric method for detection of TC, which allowed detection of TC from 0.5 to 80 μM with the detection limit of 39 nM. In addition, the luminol-Eu-based test paper was prepared for visual semi-quantitative detection of TC in real samples based on the color of luminol-Eu change from blue to red under 365 nm ultraviolet light. All of those results indicated that the ratiometric fluorometric strategy was fast, sensitive, and visual for detection of TC.

Aptamer-based fluorometric determination of chloramphenicol by controlling the activity of hemin as a peroxidase mimetic

A method for the aptamer-based determination of chloramphenicol (CAP) was developed by exploiting the peroxidase mimicking activity of hemin. The method includes two hemin-modified DNA probes termed P1 and P2. P1, which was modified at its 5' end with one hemin monomer, contains the CAP-binding sequence. The hybridization between P1 and P2 brings the two hemin monomers in close proximity, resulting in the formation of a hemin dimer with low peroxidase mimicking activity. The duplex structure was dehybridized in the presence of CAP. The formed hemin monomer featured a strong peroxidase mimicking activity and catalyzed the conversion of non-fluorescent tyramine into fluorescent dityramine by hydrogen peroxide. Fluorescence (with an excitation/emission maxima at 320 and 410 nm, respectively) increased linearly in the 0.1 ng mL-1 to 10 ng mL-1 CAP concentration range. The detection limit based on the 3σ/k criterion reached 0.07 ng mL-1. The proposed assay was successfully employed for CAP detection in (spiked) honey samples with recoveries of 94.3-117.2%. Given its high sensitivity and good stability, this method shows potential in providing a platform for antibiotic detection.

Oligonucleotides and pesticide regulated peroxidase catalytic activity of hemin for colorimetric detection of isocarbophos in vegetables by naked eyes

A novel colorimetric sensing platform based on the peroxidase activity of hemin regulated by oligonucleotide and pesticide was reported for the ultrasensitive and selective detection of isocarbophos. Oligonucleotides can accumulate on the surface of hemin in acid condition and temporarily inhibit its catalytic activity, which results in the loss of one electron of TMB molecule and produce the blue products. With the addition of isocarbophos, the pesticide molecules can interact with oligonucleotides to form some complexes, which relieve the inhibition of ssDNA to hemin and further enhance its catalytic activity. Thus, the TMB molecules are further oxidized to lose another electron and produce the yellow product in a few minutes, which has the characteristic absorption peak at 450 nm. The color change of the sensing system is related to the amount of isocarbophos, so this method can quickly discriminate whether the target pesticide exceeds the maximal residue limit just by naked eyes. To improve the performance of sensing platform, some important parameters like buffer condition and ssDNA have been investigated, and the peroxidase activity of hemin was further studied to verify the catalytic mechanism. The proposed sensing platform has a detection limit as low as 0.6 μg/L and displays good selectivity against other competitive pesticides. Moreover, the developed sensing platform also exhibits favorable accuracy and stability, indicating that it has potential applications in the detection of pesticide residues in agricultural products. Graphical abstract A novel colorimetric sensing platform based on oligonucleotides and pesticide regulation; the peroxidase catalytic activity of hemin was firstly reported for the ultrasensitive and selective detection of isocarbophos pesticide.

Electrochemical aptasensor for ampicillin detection based on the protective effect of aptamer-antibiotic conjugate towards DpnII and Exo III digestion

A simple and sensitive electrochemical method was developed for ampicillin detection based on the protective effect of aptamer-antibiotic conjugate towards endonuclease DpnII activity. Without ampicillin, DNA aptamer firstly hybridizes with the capture probe to form double strand DNA (dsDNA) structure. Then, dsDNA is cleaved by DpnII restriction endonuclease to form two dsDNA fragments. In which, one fragment is released from electrode surface and the other fragment is kept on electrode surface. Then, the dsDNA fragment kept on electrode surface is further digested by Exo III, which leads to the release of the dsDNA fragment from electrode surface. Thus, the electrochemical signal increases due to the decrease of the interface electron transfer resistance causing by the release of dsDNA from electrode surface. However, the formation of dsDNA is blocked when forming aptamer-ampicillin conjugate, which makes the obstruction of the digestion of DpnII and Exo III towards capture probe. Thus, a weak electrochemical signal is achieved due to the increase of the interface electron transfer resistance causing by the dsDNA on the electrode surface. Based on the relationship between ampicillin concentration and the decrease of the electrochemical signal, antibiotic is detected with low detection limit of 32 pM under optimal conditions, which is lower than the mandated maximum residue limit of European Union (9.93 nM). The developed method also presents good detection selectivity. Moreover, the applicability is confirmed by detecting antibiotic in milk and water samples with satisfactory results.

Label-Free Aptamer-Based Biosensor for Specific Detection of Chloramphenicol Using AIE Probe and Graphene Oxide

A facile, sensitive, and label-free aptamer-based fluorescent biosensor (aptasensor) for chloramphenicol (CAP) detection was successfully developed based on an aggregation-induced emission (AIE) probe and graphene oxide (GO). In this aptasensor, the specific aptamer of CAP (C-Apt) is used as the recognition part, an AIE molecule, 9,10-distyrylanthracene (DSA) derivative with short alkyl chains (9,10-bis{4-[2-(N,N,N-trimethylammonium)-ethoxy]styrene}anthracene dibromide, DSAC2N), as the fluorescent probe, and GO with a low oxidation degree as the fluorescent quencher. Initially, the AIE probe DSAC2N and C-Apt could be adsorbed on GO through π-stacking interactions, and the fluorescence of DSAC2N could be efficiently quenched due to the energy transfer between DSAC2N and GO. When CAP is added, C-Apt can preferentially bind with CAP and the newly formed complex (C-Apt-CAP) can be released from GO, resulting in the recovery of the fluorescence signal of DSAC2N. Thus, with the aid of GO, turn-on detection of CAP can be readily realized by monitoring the fluorescence signal of DSAC2N from "off" to "on". Under the optimized conditions, the aptasensor exhibits a high sensitivity toward CAP with a limit of detection of 1.26 pg/mL. Besides, we have successfully applied this aptasensor to the detection of CAP in spiked milk.

Aptasensors as the future of antibiotics test kits-a case study of the aptamer application in the chloramphenicol detection

Antibiotics are a type of antimicrobial drug with the ubiquitous presence in foodstuff that effectively applied to treat the diseases and promote the animal growth worldwide. Chloramphenicol as one of the antibiotics with the broad action spectrum against Gram-positive and Gram-negative bacteria is widely applied for the effective treatment of infectious diseases in humans and animals. Unfortunately, the serious side effects of chloramphenicol, such as aplastic anemia, kidney damage, nausea, and diarrhea restrict its application in foodstuff and biomedical fields. Development of the sufficiently sensitive methods to detect chloramphenicol residues in food and clinical diagnosis seems to be an essential demand. Biosensors have been introduced as the promising tools to overcome the requirement. As one of the newest types of the biosensors, aptamer-based biosensors (aptasensors) are the efficient sensing platforms for the chloramphenicol monitoring. In the present review, we summarize the recent achievements of the accessible aptasensors for qualitative detection and quantitative determination of chloramphenicol as a candidate of the antibiotics. The present chloramphenicol aptasensors can be classified in two main optical and electrochemical categories. Also, the other formats of the aptasensing assays like the high performance liquid chromatography (HPLC) and microchip electrophoresis (MCE) have been reviewed. The enormous interest in utilizing the diverse nanomaterials is also highlighted in the fabrication of the chloramphenicol aptasensors. Finally, some results are presented based on the advantages and disadvantages of the studied aptasensors to achieve a promising perspective for designing the novel antibiotics test kits.

Aptamer-Based Biosensors for Antibiotic Detection: A Review

Antibiotic resistance and, accordingly, their pollution because of uncontrolled usage has emerged as a serious problem in recent years. Hence, there is an increased demand to develop robust, easy, and sensitive methods for rapid evaluation of antibiotics and their residues. Among different analytical methods, the aptamer-based biosensors (aptasensors) have attracted considerable attention because of good selectivity, specificity, and sensitivity. This review gives an overview about recently-developed aptasensors for antibiotic detection. The use of various aptamer assays to determine different groups of antibiotics, like β-lactams, aminoglycosides, anthracyclines, chloramphenicol, (fluoro)quinolones, lincosamide, tetracyclines, and sulfonamides are presented in this paper.

Development and characterization of DNA aptamers against florfenicol: Fabrication of a sensitive fluorescent aptasensor for specific detection of florfenicol in milk

Specific ssDNA aptamers for the antibiotic florfenicol (FF) were developed from an enriched nucleotide library using magnetic beads-based SELEX (Systematic Evolution of Ligands by EXponential enrichment) technique with high-binding affinity. After 12 rounds of selection, thirty-six sequences were obtained that were then divided into five major families, according to the primary sequence similarity. Binding affinity analyses of three fluorescently tagged aptamers belonging to different families demonstrated that the dissociation constants (K_d) were in the low nanomolar range (K_d = 52.78-211.4 nmol L^-1). Furthermore, to verify the potential application of the aptamers, a fluorescent aptasensor was fabricated for detecting the FF residue in raw milk samples based on the energy transfer between graphene oxide as the acceptor and fluorescently tagged FF-specific aptamer as the donor. Under optimal conditions, the aptasensor displayed a wide linear range from 5 to 1200 nmol L^-1 and a detection limit of 5.75 nmol L^-1 with excellent selectivity in milk. The recovery rate in the milk was between 101% ± 0.14% and 110% ± 2.8%, indicating high accuracy. This fluorescent aptasensor possessed considerable potential for rapid analysis of FF in raw milk because of its simplicity of detection. Moreover, the interaction between the aptamer and FF was studied using molecular modeling.

Magnetic Nanoparticles for Antibiotics Detection

Widespread use of antibiotics has led to pollution of waterways, potentially creating resistance among freshwater bacterial communities. Microorganisms resistant to commonly prescribed antibiotics (superbug) have dramatically increased over the last decades. The presence of antibiotics in waters, in food and beverages in both their un-metabolized and metabolized forms are of interest for humans. This is due to daily exposure in small quantities, that, when accumulated, could lead to development of drug resistance to antibiotics, or multiply the risk of allergic reaction. Conventional analytical methods used to quantify antibiotics are relatively expensive and generally require long analysis time associated with the difficulties to perform field analyses. In this context, electrochemical and optical based sensing devices are of interest, offering great potentials for a broad range of analytical applications. This review will focus on the application of magnetic nanoparticles in the design of different analytical methods, mainly sensors, used for the detection of antibiotics in different matrices (human fluids, the environmental, food and beverages samples).

Quantitative determination of VEGF165 in cell culture medium by aptamer sandwich based chemiluminescence assay

In this work, we have developed a sensitive and selective chemiluminescence (CL) assay for vascular endothelial growth factor (VEGF165) quantitative detection based on two specific VEGF165 binding aptamers (Apt). VEGF is a predominant biomarker in cancer angiogenesis, and sensitive detection method of VEGF are highly demanded for both academic study and clinical diagnosis of multiple cancers. In our experiment, VEGF165 was captured in a sandwich structure assembled by two binding aptamers, one capture aptamer was immobilized on streptavidin-coated magnetic beads (MBs) and another VEGF-binding aptamer was labeled by biotin for further phosphatase conjunction. After Apt-VEGF-Apt sandwich was formed on MBs surface, alkaline phosphatase (ALP) was modified to the second aptamer to catalyze CL reaction. By applying 4-methoxy-4-(3-phosphatephenyl)-spiro-(1,2-dioxetane-3,2-adamantane) (AMPPD) as CL substrate, strong signal intensity was achieved. VEGF165 content as low as 1ng/mL was detected in standard spiked samples by our assay, and linear range of working curve was confirmed from 1 to 20ng/mL. Then our method was successfully applied for cell culture medium analysis and on-chip hypoxic HepG2-HUVEC co-culture model study with excellent accuracy equal to ELISA Kit. Our developed assay demonstrated an outstanding performance in VEGF165 quantification and may be further extended to clinical testing of important biomarkers as well as probing microchip cell culture model.

Novel single-stranded DNA binding protein-assisted fluorescence aptamer switch based on FRET for homogeneous detection of antibiotics

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.