Competitive ELISA of Chloramphenicol: Influence of Immunoreagent Structure and Application of the Method for the Inspection of Food of Animal Origin

Novel hapten design, highly sensitive monoclonal antibody production, and immunoassay development for rapid screening of illegally added chloramphenicol in cosmetics

Hapten design and synthesis have been regarded as the key factor to generate high-quality antibodies. In the present study, a novel hapten of chloramphenicol was synthesized, characterized and compared with two conventional haptens. The new hapten generated mAb 4B5 showed higher sensitivity and titer than the other two haptens-based mAbs. The haptens synthesized with the structure of chloramphenicol base generated more sensitive antibodies than the hapten with chloramphenicol succinate, and the spacer arm linked to the phenyl group hapten elicited the strongest antibody response. After optimization, a direct competitive enzyme-linked immunosorbent assay (dcELISA) and a lateral flow immunoassay (LFIA), both based on the mAb 4B5, were developed. The dcELISA had a half maximum inhibition concentration of 0.23 ng/mL and the LFIA showed a cutoff value of 5-10 ng/mL. The LFIA was applied to detect illegally-added chloramphenicol samples in anti-acne cosmetics, five out of 19 samples were tested chloramphenicol containing within 10 min, which result was confirmed with the dcELISA and HPLC. The LFIA has an adequate sensitivity and can be used as a point of care diagnostic device for rapidly screening chloramphenicol in cosmetics.

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.

Mathematical Modeling of Bioassays

The high affinity and specificity of biological receptors determine the demand for and the intensive development of analytical systems based on use of these receptors. Therefore, theoretical concepts of the mechanisms of these systems, quantitative parameters of their reactions, and relationships between their characteristics and ligand-receptor interactions have become extremely important. Many mathematical models describing different bioassay formats have been proposed. However, there is almost no information on the comparative characteristics of these models, their assumptions, and predictive insights. In this review we suggested a set of criteria to classify various bioassays and reviewed classical and contemporary publications on these bioassays with special emphasis on immunochemical analysis systems as the most common and in-demand techniques. The possibilities of analytical and numerical modeling are discussed, as well as estimations of the minimum concentrations that may be detected in bioassays and recommendations for the choice of assay conditions.

A homogenous “signal-on” aptasensor for antibiotics based on a single stranded DNA binding protein-quantum dot aptamer probe coupling exonuclease-assisted target recycling for signal amplification

The method is based on replacement reaction, just by mixing the probes with targets for detection, which is simple and easy for manipulation.

An aptamer-based effective method for highly sensitive detection of chloramphenicol residues in animal-sourced food using real-time fluorescent quantitative PCR

Chloramphenicol (CAP) residues can not only harm human health through entering food chain, but also cause the spreading of drug-resistant bacteria, thereby leading to secondary environmental pollution. Therefore, it is in urgent need of establishing an efficient technology to detect CAP residues in animal-sourced food. In this study, a novel sensitive approach for detection of CAP was designed based on a CAP specific aptamer and real-time fluorescent quantitative PCR (qRT-PCR). The CAP specific aptamer was firstly hybridized with a biotin modified complementary probe, and then was immobilized on streptavidin conjugated magnetic beads through biotin. When CAP was added, the aptamer would specifically bind with CAP by forming a hairpin structure and be released from the magnetic beads for CAP detection by qRT-PCR. Factors (i.e., probe strand length, aptamer concentration, NaCl concentration and incubation time) that would influence the determination accuracy of this aptamer-based detection system were optimized. Under the optimized conditions, the present detection system exhibited a high sensitivity toward CAP with a limit of detection of 0.1ng/mL (linear range from 0.1 to 20ng/mL). Moreover, this detection system also showed high selectivity against thiamphenicol (TAP) and florfenicol (FF), which are CAP's structure analogs. Eventually, this detection system was applied for detecting CAP in real spiked milk. The recovery rate of CAP from spiked milk samples ranged from 94.0-102.0%. These results indicated this developed detection system a promising high sensitive and specific method of CAP residues detection in animal-sourced food.

Selection and Identification of Chloramphenicol-Specific DNA Aptamers by Mag-SELEX

Chloramphenicol (CAP) has been widely used to treat bacterial infections in livestock and aquatic animals. To reduce the risk of CAP residues, an efficient technology to rapidly detect CAP residues in animal-sourced food is expressly needed. In this study, magnetic bead-based systematic evolution of ligands by exponential enrichment (Mag-SELEX) strategy was performed to select and identify CAP-specific single-stranded DNA (ssDNA) aptamers from a random oligonucleotide library. After nine rounds of selection, five potential ssDNA aptamers were selected. Low homology indicated that they might belong to different families. To identify an aptamer with the highest affinity for CAP, the dissociation constant (K _d) values of these selected aptamers were determined. The lowest K _d values of two potential aptamers (i.e., No. 4 and No. 5) were, respectively, 0.10162 ± 0.0111 and 0.03224 ± 0.00819 μM, which were much lower than previously reported lowest K _d value (i.e., 0.766 μM) of CAP aptamer. Moreover, compared with No. 4, aptamer No. 5 had higher binding rate, which is quite different among those with CAP and with CAP's structural analogs (i.e., thiamphenicol (TAP) and florfenicol (FF)). These results indicated that the potential aptamer No. 5 with highest specificity and affinity for CAP would be an ideal aptamer for future detection of residual CAP in animal-sourced food.

Development of an Immunoassay for Chloramphenicol Based on the Preparation of a Specific Single-Chain Variable Fragment Antibody

Specific antibodies are essential for the immune detection of small molecule contaminants. In the present study, the heavy and light variable regions (V(H )and V(L)) of the immunoglobulin genes from a hybridoma secreting a chloramphenicol (CAP)-specific monoclonal antibody (mAb) were cloned and sequenced. In addition, the light and heavy chains obtained from the monoclonal antibody were separated using SDS-PAGE and analyzed using Orbitrap mass spectrometry. The results of DNA sequencing and mass spectrometry analysis were compared, and the V(H) and V(L) chains specific for CAP were determined and used to construct a single-chain variable fragment (scFv). This fragment was recombinantly expressed as a soluble scFv-alkaline phosphatase fusion protein and used to develop a direct competitive ELISA. Compared with the parent mAb, scFv exhibits lower sensitivity but better food matrix resistance. This work highlights the application of engineered antibodies for CAP detection.

A sensitive electrochemical aptasensor for multiplex antibiotics detection based on high-capacity magnetic hollow porous nanotracers coupling exonuclease-assisted cascade target recycling

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.

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

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-H2O2 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 100ng mL(-1) with detection limit of 0.0005ng 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.

Chemiluminescent aptasensor for chloramphenicol based on N-(4-aminobutyl)-N-ethylisoluminol-functionalized flower-like gold nanostructures and magnetic nanoparticles

A novel chemiluminescent aptasensor for the highly sensitive detection of chloramphenicol (CAP) in milk was successfully developed using biotinylated CAP aptamer-functionalized magnetic nanoparticles (MNPs) as capture probes and thiolated hybridized complementary strand-modified N-(4-aminobutyl)-N-ethylisoluminol (ABEI)-functionalized flower-like gold nanostructures (AuNFs) as signal probes. P-iodophenol (PIP) was also added to form an ABEI-H2O2-PIP steady-state chemiluminescence (CL) system. Based on a competitive format, the CL intensity was negatively correlated with the concentration of CAP in the range of 0.01-0.20 ng/mL and the detection limit was 0.01 ng/mL in buffer and 1 ng/mL in milk. The proposed method was successfully applied to measure CAP in milk samples and compared to a commercial ELISA method. The high sensitivity of AuNFs, excellent selectivity and stability of aptamers, and good overall stability of the chemiluminescent bioassay with magnetic separation make them a promising approach for the detection of small molecular illegal additives. Additionally, the high sensitivity, easy operation, and good reproducibility exhibited by the stable chemiluminescent bioassay demonstrate its applicability for the trace detection of CAP in applications, such as animal husbandry.

Rapid surface enhanced Raman scattering detection method for chloramphenicol residues

Chloramphenicol (CAP) is a widely used amide alcohol antibiotics, which has been banned from using in food producing animals in many countries. In this study, surface enhanced Raman scattering (SERS) coupled with gold colloidal nanoparticles was used for the rapid analysis of CAP. Density functional theory (DFT) calculations were conducted with Gaussian 03 at the B3LYP level using the 3-21G(d) and 6-31G(d) basis sets to analyze the assignment of vibrations. Affirmatively, the theoretical Raman spectrum of CAP was in complete agreement with the experimental spectrum. They both exhibited three strong peaks characteristic of CAP at 1104 cm(-1), 1344 cm(-1), 1596 cm(-1), which were used for rapid qualitative analysis of CAP residues in food samples. The use of SERS as a method for the measurements of CAP was explored by comparing use of different solvents, gold colloidal nanoparticles concentration and absorption time. The method of the detection limit was determined as 0.1 μg/mL using optimum conditions. The Raman peak at 1344 cm(-1) was used as the index for quantitative analysis of CAP in food samples, with a linear correlation of R(2)=0.9802. Quantitative analysis of CAP residues in foods revealed that the SERS technique with gold colloidal nanoparticles was sensitive and of a good stability and linear correlation, and suited for rapid analysis of CAP residue in a variety of food samples.

An antibody-graphene oxide nanoribbon conjugate as a surface enhanced laser desorption/ionization probe with high sensitivity and selectivity

Antibody-functionalized graphene oxide nanoribbons were synthesized and applied as highly sensitive and selective SELDI probes for mass spectrometry detection.

Development of a new fluorescence immunochromatography strip for detection of chloramphenicol residues in chicken muscles

BACKGROUND

Chloramphenicol (CAP), an antimicrobial drug that is widely used in animal feed, would have a negative effect on human health due to its low elimination rate and relatively high residue in animal food. It is important to develop a rapid and economic method to determine CAP in animal food to ensure that human health is not affected.

RESULTS

A new fluorescence immunochromatography strip was developed and established for the detection of CAP residue in chicken muscles for the first time. A CAP-bovine serum albumin conjugate, monoclonal antibody and polyclonal antibody against CAP were applied to constitute a fluorescence immunochromatography strip. The fluorescence intensity was detected by a charge-coupled device scanner and transformed to a digital value. The CAP linearity working range was from 0.1 ng mL(-1) to 20 ng mL(-1) with a limit of detection of 0.1 ng mL(-1) within 10 min. The performance of the strip assay was compared with a commercial ELISA kit and the correlation coefficient was 0.99, which indicated that the new strip assay had a good quantification ability for CAP.

CONCLUSION

The fluorescence immunochromatography strip was successfully applied to the detection of CAP residues in chicken samples. To our knowledge, it is the first report regarding the development of a fluorescence immunochromatography method for screening CAP in animal samples.

Development of a homogeneous immunoassay based on the AlphaLISA method for the detection of chloramphenicol in milk, honey and eggs

BACKGROUND

A homogenous light-induced chemiluminescence immunoassay was developed using AlphaLISA technology for the detection of chloramphenicol (CAP). This technology is based on two different kinds of bead, namely light-sensitive donor beads and beads containing chemiluminescers, also called acceptor beads. A competitive CAP AlphaLISA method was established using artificial antigen-coated acceptor beads, polyclonal antibodies, biotinylated goat anti-rabbit IgG and streptavidin-coated donor beads.

RESULTS

The sensitivity of detection was 0.0086 ng mL⁻¹ and the working range was from 0.0096 to 25 ng mL⁻¹. The intra- and inter-assay coefficients of variation were both below 10%. The average recovery rates at spiked levels of 0.05-10 ng mL⁻¹ were 103.2, 108.4 and 91.6% for milk, honey and eggs respectively. The data obtained from the samples showed good correlation with ELISA results.

CONCLUSION

The CAP AlphaLISA method is highly sensitive, specific and rapid and is suitable for screening large quantities of samples.

Development of a chemiluminescent immunosensor for chloramphenicol

A direct competitive chemiluminescent immunosensor system that exploits the competition between chloramphenicol (CAP) as an analyte and CAP-horseradish peroxidase conjugate as a tracer for binding to an anti-CAP antibody on a solid support was devised by installing a flow-through cell which was connected to an injector and a peristaltic pump inside a dark box, followed by positioning a photomultiplier tube as light detector in front of it. The anti-CAP antibody was immobilized onto positively charged Biodyne B membrane pieces by a dipping procedure. The operating conditions for the immunosensor were selected with respect to substrate composition (0.25, 13.3 and 0.66 mM for luminol, H2O2 and p-iodophenol, respectively), injection volume of the substrate solution (200 microL) and the concentrations of antibody for immobilization (0.10 mg mL(-1)) and tracer (0.030 mg mL(-1)). At these conditions, sensor response according to analyte concentration was well fitted to a linear equation when plotted in semi-logarithmic scale, with the limit of detection for CAP of 10(-8) M. By using the immunosensor, CAP measurement in the model samples prepared from five food materials was conducted.

Development of a chemiluminescent ELISA for determining chloramphenicol in chicken muscle

An indirect competitive enzyme-linked immunosorbent assay (ELISA) with chemiluminescent (CL) detection for chloramphenicol (CAP) in chicken muscle was developed. CAP-specific polyclonal antibody was raised in rabbit with a CAP-succinate derivative conjugated with bovine serum albumin. Luminol solution was used as the substrate of horseradish peroxidase. The detection limit was 6 ng/L. The CL-ELISA was 10 times more sensitive compared to the colorimetric-ELISA. When CAP was spiked in chicken muscle at levels of 0.05-5 microg/kg, recoveries ranged from 97 to 118% with coefficients of variation of 6-22%. In an actual residue study, the results obtained by CL-ELISA correlated well with those obtained by gas chromatography with microcell electron capture detector. The residue levels of CAP in treated chicken decreased with time and dropped rapidly after the first 6 h from around 50 to 10 microg/kg. After 3 days, CAP was not detected in chicken muscle. The developed method is therefore suitable for screening of CAP in chicken muscle samples.

A monoclonal antibody-based time-resolved fluoroimmunoassay for chloramphenicol in shrimp and chicken muscle

A time-resolved fluoroimmunoassay (TR-FIA) for determination of chloramphenicol (CAP) in shrimp and chicken muscle was developed. The method was based on a direct competitive immunoassay using europium-labeled anti-CAP monoclonal antibody (MAb) and CAP-ovalbumin as coated antigen. The limit of detection was 0.05 ng g(-1) and limit of quantification was 0.1 ng g(-1). Recoveries ranged from 101.2 to 112.5% for shrimp and 104.9 to 115.3% for chicken muscle at spiked levels of 0.1-5 ng g(-1), with intra-assay and inter-assay variations 8.7-14.6 and 9.6-17.8%, respectively. The results obtained by the TR-FIA and ELISA correlated well. The established TR-FIA was validated for the determination of incurred shrimp samples and confirmed by gas chromatography with microcell electron capture detector (GC-microECD).

Chemiluminescence enzyme immunoassay (CLEIA) for the determination of chloramphenicol residues in aquatic tissues

A competitive indirect chemiluminescence enzyme immunoassay (ic-CLEIA) for chloramphenicol (CAP) residues in shrimp has been developed. After optimization (incubation time, concentration of Tween-20, concentration of PBS and pH), the method gave a limit of detection of 0.01 ng/mL and a detection range of 0.03-23.7 ng/mL, with an ED(50) of 0.47 ng/mL. The method has been validated on spiked shrimp samples in terms of precision (intra- and interassay coefficient variations of less than 10% and 15%, respectively) and accuracy (mean recovery 95-123%). The assay performance is better than the ELISA method which is widely used to detect chloramphenicol and indicates that the CLEIA method can be used to test aquatic samples instead of ELISA.

In-house validation of a liquid chromatography/electrospray tandem mass spectrometry method for confirmation of chloramphenicol residues in muscle according to Decision 2002/657/EC

In this work we present an in-house validation study for the confirmatory analysis of chloramphenicol (CAP) in muscle according to the Commission Decision 2002/657/EC requirements. CAP is extracted in acetonitrile and after liquid-liquid partitioning with n-hexane is identified and quantitatively determined by ion trap liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/ESI-MS/MS) analysis in the negative ion mode. CAP was identified using the precursor ion and at least two product ions, meeting the qualitative and quantitative criteria set by the European Commission in the Decision 2002/657/EC for confirmation of prohibited veterinary drug residues. We calculated mean drug recoveries, CCalpha and CCbeta of the method, and reported data on specificity, ruggedness and within-laboratory reproducibility. Finally, we point out and discuss some problems and questions arising from controversy about the application of Decision 2002/657/EC.

Development of a liquid chromatography/electrospray tandem mass spectrometry method for confirmation of chloramphenicol residues in milk after alfa-1-acid glycoprotein affinity chromatography

In this work we present a method for confirmatory analysis of chloramphenicol (CAP) in bovine and buffalo raw milk. CAP is extracted in acetonitrile and purified by affinity chromatography on an alpha-1-acid glycoprotein (AAG) column, then is identified and determined by ion-trap liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/ESI-MS/MS) analysis in the negative ion mode. CAP was identified at the minimum required performance limit (MRPL) of 0.30 ppb, by monitoring the [M-H]- ion and at least two product ions, meeting the qualitative and quantitative criteria set by the European Commission in Decision 2002/657/EC for confirmation of prohibited veterinary drugs. The trueness and within-day and between-day repeatability data are also reported. Moreover, the loading capacity of affinity columns towards CAP was tested. This method, based on the molecular recognition between drug and AAG during the purification step to improve sample cleanup, represents a quantitative and repeatable procedure for confirmatory analysis, and fits the requirements for the routine official control of CAP residues in raw milk.

Development of a direct-binding chloramphenicol sensor based on thiol or sulfide mediated self-assembled antibody monolayers

A batch-type antibody-immobilized quartz crystal microbalance (QCM) system for detecting chloramphenicol (CAP) was developed. To bind an anti-CAP antibody onto the gold electrode surface of piezoelectric crystals, self-assembled monolayers (SAMs) of different thiols or sulfides were formed by a chemisorption procedure. Then, the anti-CAP antibody was covalently linked to the pre-formed monolayers by an activation procedure using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysulfosuccinimide. The antibody-immobilized QCM chip thus prepared was installed in a well holder and was measured for sensor response. Compared with the bare QCM chip and the QCM chip only coated with 3-mercaptopropionic acid (MPA), the antibody-immobilized sensor showed greatly enhanced frequency shifts by 10-50-fold after CAP injection. In this case, CAP detection which was indicated by steady-state resonant frequency shift was accomplished within 10 min. When CAP solution was injected into the reaction cell in 50mM concentration, the frequency shifts obtained were, respectively, 530 and 505 Hz in case of thiosalicylic acid and MPA immobilization. Repeated use of the sensor chips up to eight times was possible after 1 min regeneration with 0.1M NaOH. This system demonstrated a potential application of thiol or sulfide mediated SAMs as the pre-coatings of a real-time detection on CAP in solution.