Technical Note: A monoclonal antibody-based immunoassay for determination of ractopamine in swine feeds1

Two binding epitopes modulating specificity of immunoassay for β-agonist detection: Quantitative structure-activity relationship

A growing number of β-agonists are illegally using for reducing animal fat deposition in animals, but the development of analytical methods always lags behind the emergence of new illegal compounds. Therefore, class specificity antibody-based immunoassays that can detect a great many β-agonists are important for timely supervision. In this study, a competitive inhibition enzyme-linked immunosorbent assay (ciELISA) based on a clenbuterol monoclonal antibody was developed to recognize 23 β-agonists and analogues. Holographic and three-dimensional quantitative structure-activity relationship (HQSAR and 3D QSAR) revealed that there are two critical binding epitopes on β-agonist hapten affecting antibody specificity, and these epitopes have been further validated using a ractopamine antibody with narrow specificity. Tert-butyl at C-2' epitope is needed to generate class specific antibodies, and different characteristics of substituents at benzene ring epitope would adjust antibody specificity. This investigation could provide reference for future design of β-agonist haptens.

A highly specific and sensitive indirect competitive monoclonal ELISA for the detection of brombuterol in animal feed, swine urine, pork and liver samples

Brombuterol, a new β-adrenergic agonist to enhance animal growth and increase feeding efficiency, is forbidden as an additive in animal feed for livestock production due to its adverse effects on consumers. In this study, a highly specific and sensitive monoclonal antibody was obtained and an indirect competitive monoclonal ELISA was developed to detect brombuterol, with an IC₅₀ value of 0.1 μg/kg (μg/L) and no cross-reactivity to other structurally related β-adrenergic agonists. The average recovery of brombuterol using the icELISA method ranged from 72.9% to 106.4% with the coefficient of variation lower than 18.9%, which was determined by analysing spiked animal feed, swine urine, pork and liver samples (n = 5). Finally, the icELISA gave results having a good correlation with those obtained by liquid chromatography-tandem mass spectrometry. These results demonstrated that the developed icELISA for the detection of brombuterol is highly specific, sensitive, and reliable, indicating good potential for use in the area of food safety to improve consumer protection.

Visual upconversion nanoparticle-based immunochromatographic assay for the semi-quantitative detection of sibutramine

Immunochromatographic assay (ICA) has been used widely for the onsite monitoring of illegal additives due to its simplicity, speed, and low cost. However, a scanner is commonly required for ICA to achieve quantitative results. In this work, we developed a visual semi-quantitative ICA for sibutramine, a banned additive in diet foods, without the need for a scanner for measurement. Monoclonal antibodies specific for sibutramine were raised and conjugated with upconversion nanoparticles (UCNPs) as the luminescent tracer. ICA was developed by employing multiple test lines to achieve the semi-quantitative detection of sibutramine. Based on the optimal conditions, the cutoff levels (limit of quantitation, LOQ) of T1 line, T2 line, T3 line, and T4 line were 0.02 μg/mL, 0.15 μg/mL, 1.0 μg/mL, and 7.5 μg/mL, respectively, in buffer system. The ICA demonstrated a LOQ at 0.2 mg/kg for sibutramine in diet food samples. The assay (including pretreatment) can be finished within 30 min without the aid of other instruments, except a laser pen. No false positive or false negative results were observed. The results indicated that the proposed method was reliable, simple, and rapid for the screening of sibutramine abuse in diet food samples.

AgNPs/QDs@GQDs nanocomposites developed as an ultrasensitive impedimetric aptasensor for ractopamine detection

Developing easy-to-use and miniaturized sensors for in-field monitoring of targets which is related to human health is necessary. Ractopamine (RAC) is a feed additive with serious side effects that is forbidden in many countries. This study reports the fabrication of an impedimetric aptasensor for ultrasensitive and selective detection of the RAC in human biological fluids. Accordingly, an efficient nanocomposites was synthesized by a beneficial combination of graphene quantum dots (GQDs), quantum dots (QDs) and silver nanoparticles (AgNPs) for modifying a glassy carbon electrode (GCE). This nanocomposite is promising to present a synergistic effect in the increase of the active surface area of the modified electrode to more load the biocapture of the target. Next, the RAC-binding aptamer (Apt) was attached to the AgNPs/QDs@GQDs/GCE surface and a sensitive layer for the RAC detection was embedded. A RAC-Apt complex was formed upon adding the RAC and the changes of the electrochemical behavior were studied by some electrochemical techniques such as electrochemical impedance spectroscopy (EIS). Under optimal conditions, the charge transfer resistance (R_ct) value was increased linearly with increasing of the RAC concentrations in the range of 1 fM to 901.4 nM. Limit of detection (LOD) was calculated to be 330 aM which is superior by other reported electrochemical methods in the RAC sensing. The applicability of the aptasensor was tested in human urine and blood serum as the real samples and satisfactory results of specificity were achieved. It seems that the proposed strategy not only provides a new ultrasensitive strategy for RAC detection but also expands the application of the sensing interface to develop other aptasensors by changing the Apt sequence.

Upconversion luminescence nanoparticles-based immunochromatographic assay for quantitative detection of triamcinolone acetonide in cosmetics

Triamcinolone acetonide (TCA) abuse in cosmetics is a common phenomenon. A rapid lateral flow immunochromatographic assay (ICA) was developed for the quantitative detection of TCA using a probe based on upconversion luminescence nanoparticles. Lanthanide-doped upconversion nanoparticles (UCNPs) were synthesized in a system comprising water and ethylene glycol, and a silicon dioxide layer was covered at the carboxyl site. A binding site protection strategy was used to decrease the background signal of UCNPs-ICA. Using dexamethasone derivative as a coating antigen, the optimal UCNPs-ICA exhibits good dynamic linear detection for TCA in the range 1.0-100 ng mL^-1 with a median inhibitory concentration of 9.8 ng mL^-1. The detection limits for TCA in a cosmetic sample are 20 μg kg^-1. The pretreatment of samples only needs dilution with water, suggesting the assay can quantitate TCA on-site using a portable upconversion luminescence reader with a cumulative analysis time of only 10 min.

A competitive luminol chemiluminescence immunosensor based on a microfluidic chip for the determination of ractopamine

Herein, a competitive luminol chemiluminescence immunosensor based on a microfluidic chip was developed to detect ractopamine (RCT) both in phosphate buffer and swine urine samples. The immunosensor can provide a liner range of 0.5-40 ng/mL and a high sensitivity with a limit of detection of 0.97 ng/mL for RCT detection in swine urine. Good rates of recovery in negative swine urine samples were achieved over the RCT concentration ranging from 0.5 to 40 ng/mL. The proposed method offered a promising analytical scheme for the on-site determination of RCT.

High-specificity quantification method for almond-by-products, based on differential proteomic analysis

A highly specific competitive enzyme-linked immunosorbent assay (ELISA) protocol has been developed to identify and classify almond products based on differential proteomic analysis. We applied two-dimensional electrophoresis to compare the differences between almond and apricot kernels to search for almond-specific proteins. The amino acid of apricot Pru-1 was sequenced and aligned to almond Pru-1. One peptide, RQGRQQGRQQQEEGR, which exists in almond but not in apricot, was used as hapten to prepare monoclonal antibody against almond Pru-1. An optimized ELISA method was established using this antibody. The assay did not exhibit cross-reactivity with the tested apricot kernels and other edible plant seeds. The limit of detection (LOD) was 2.5-100μg/g based on different food samples. The recoveries of fortified samples at levels of twofold and eightfold LOD ranged from 82% to 96%. The coefficients of variation were less than 13.0%. Using 7M urea as extracting solution, the heat-treated protein loss ratios were 2%, 5% and 15% under pasteurization (65°C for 30min), baking (150°C for 30min) and autoclaved sterilization (120°C for 15min), respectively.

New sulfonate composite functionalized with multiwalled carbon nanotubes with cryogel solid-phase extraction sorbent for the determination of β-agonists in animal feeds

A new mixed-mode cation-exchange sulfonate composite functionalized with multiwalled carbon nanotubes with polyvinyl alcohol cryogel was fabricated and used for the first time as a solid-phase extraction sorbent for the determination of β-agonists in animal feeds. Feed samples were extracted with 0.20 M phosphoric acid and methanol (1:4, v/v) using ultrasonication, cleaned-up using the developed sorbent to which the β-agonists bound then finally eluted with 5.0% ammonia in methanol and analyzed by high-performance liquid chromatography. Various parameters that affected the extraction efficiency were optimized. Under the optimal conditions, the developed sorbent strongly interacted with β-agonists by cationic exchange and hydrophobic and hydrophilic interactions, that provided a high extraction efficiency in the range of 92.8 ± 3.7-104.4 ± 2.3% over a range of 0.04-2.0 mg/kg for salbutamol and ractopamine, and 0.40-8.0 mg/kg for clenbuterol. The relative standard deviations were less than 6.0%. The developed method was successfully applied for the determination of β-agonists in various types of animal feed and effectively reduced any matrix interference.

Competitive enzyme-linked immunoassay for sialoglycoprotein of edible bird's nest in food and cosmetics

The proliferation of fake and inferior edible bird's nest (EBN) products has recently become an increasingly serious concern. To identify and classify EBN products, a competitive enzyme-linked immunoassay (ELISA) was developed to quantitate sialoglycoprotein in EBN used in food and cosmetic applications. The characteristic sialoglycoprotein in EBN was found, extracted, purified, and analyzed. Sialoglycoprotein, considered the main carrier of sialic acid in EBN, consisted of 106 and 128 kDa proteins. A monoclonal antibody that could recognize both proteins was prepared. The heat-treated process did not change the affinity of sialoglycoprotein with the antibody. An optimized ELISA method was established with a cross-reactivity of less than 0.1% and an IC(50) of 3.3 μg/mL. On the basis of different food and cosmetic samples, the limits of detection (LOD) were 10-18 μg/g. Recoveries of fortified samples at levels of 20 and 80 μg/g ranged from 81.5 to 96.5%, respectively. The coefficients of variation were less than 8.0%.

Direct detection of β-agonists by use of gold nanoparticle-based colorimetric assays

β-Agonists fed to animals for human consumption pose a serious threat to human health. Fast, broad-spectrum detection methods are needed for on-site screening of various types of β-agonists from animal feeds, meats, and animal body fluids. We developed a colorimetric assay that uses gold nanoparticle (AuNP) plasmon absorption to realize quick detection of β-agonists from liquid samples. β-Agonists showed the capability of directly reducing HAuCl(4) into atomic gold, which involved oxidation of the amine or phenol group on the benzene ring of the β-agonists. The resulting atomic gold formed AuNPs spontaneously, which had strong plasmon absorption at 528 nm. The linear relationship between the concentrations of β-agonists and the AuNPs plasmon absorbance granted quantitative determination of β-agonists in solution. The AuNPs colorimetric assay showed different sensitivities toward β-agonists with different substituent groups on the aromatic ring. β-Agonists with phenol groups had a lower limit of quantitation (LOQ) than those with amine groups. High-resolution transmission electron microscopy (TEM) images revealed the sizes of the AuNPs were in the range 15-25 nm, while X-ray energy-dispersive spectroscopic data suggested the smaller particles observed in TEM with lower contrast may be salt particles from the buffer solution. The developed colorimetric assay can potentially be used for the detection of β-agonists and their analogues from serum, urine, and other liquid samples in the presence of interference from common antibiotics and glucose.

Rapid determination of ractopamine residues in edible animal products by enzyme-linked immunosorbent assay: development and investigation of matrix effects

To determine ractopamine residues in animal food products (chicken muscle, pettitoes, pig muscle, and pig liver), we established a rapid direct competitive enzyme-linked immunosorbent assay (ELISA) using a polyclonal antibody generated from ractopamine-linker-BSA. The antibody showed high sensitivity and specificity in phosphate buffer, with an IC(50) of 0.6 ng/mL, and the limit of detection was 0.04 ng/mL. The matrix effect of the samples was easily eliminated by one-step extraction with PBS, without any organic solution or clean-up procedure such as SPE or liquid-liquid extraction, making it a much more simple and rapid method than previously reported ones. The detection limit in blank samples was 0.2 mug/kg. To validate this new RAC (ractopamine hydrochloride) ELISA, a RAC-free pig liver sample spiked at three different concentrations was prepared and analyzed by HPLC and ELISA. The results showed a good correlation between the data of ELISA and HPLC (R(2) > 0.95), which proves that the established ELISA is accurate enough to quantify the residue of RAC in the animal derived foods.

A Chromatographic Strip Test for Rapid Detection of One Lineage of the H5 Subtype of Highly Pathogenic Avian Influenza

A rapid immune colloidal gold test strip for detecting 1 lineage of the H5 subtype of highly pathogenic avian influenza was developed based on membrane chromatography with monoclonal antibodies (mAbs) against hemagglutinin (HA) of the H5 subtype of Avian influenza virus (AIV). Bagg albino C mice that were immunized with AIV A/Goose/Guangdong/3/96(H5N1) yielded 4 hybridomas secreting mAbs (1A6, 1B8, 3C4, and 8F6) specific for HA. Monoclonal antibody 3C4 was conjugated with colloidal gold as the detector antibody. On the test strip, 1A6, in combination with rabbit anti-H5N1 polyclonal antibody, was used as the capture complex at the “test line,” and goat antimouse immunoglobulin G antibody was used as the capture antibody at the “control line.” The strip test correctly detected the H5 subtype of AIV in known positive samples and did not react with the standard antigens of AIV H1-H4 or H6-H14, or the antigens of Newcastle disease virus, Infectious bursal disease virus, Infectious bronchitis virus, or Avian infectious laryngotracheitis virus. When 483 clinical specimens suspected of infection with H5N1 were tested, results from the strip test were nearly identical with those from the HA/hemagglutination inhibition test, reverse transcription polymerase chain reaction, and a commercial enzyme-linked immunosorbent assay kit. Because the newly developed strip was specific, rapid, and sensitive, it should be useful for diagnosing the H5 subtype of AIV and for investigating its epidemiology. The test strip is only reliable for the lineage of H5N1. As for the other strains of AIV, further experiments will be needed.

Development and Identification of Monoclonal Antibodies Against Ractopamine

Ractopamine was reacted with two carrier proteins, human serum albumin and bovine thyroglobulin, as immunogen and coating antigen, respectively. Using a conventional immunization protocol, we generated a stable murine monoclonal antibody toward ractopamine, which had high affinities. The clone was found to be of IgG(2a) subclass with kappa light chain. An indirect competitive enzyme-linked immunosorbent assay for the determination of ractopamine has been optimized and characterized. The sensitivity, estimated as the IC(50) value, was 21.25 ng/mL, with a practical working range between 2.9 and 450 ng/mL. The limit of detection was 1.5 ng/mL. Moreover, other phenethanolamine beta-agonists showed low cross-reactivity with the monoclonal antibody. In addition, the indirect competitive enzyme-linked immunosorbent assay for the detection of ractopamine in animal feed was also developed using this antibody.

Time-resolved fluoroimmunoassay for ractopamine in swine tissue

This study describes the development and validation of a time-resolved fluoroimmunoassay (TR-FIA) for screening ractopamine (RAC) in swine tissue. The method is based on the direct competitive-type immunoassay using europium-labeled anti-RAC monoclonal antibody as a tracer and RAC-ovalbumin as a solid-phase antigen. When RAC was spiked at levels of 1-10 microg kg(-1), recoveries ranged from 88.2 to 118.5% for swine liver and muscle with coefficients of variation from 7.1 to 20.5%. The detection limit was 0.1 microg kg(-1). The proposed TR-FIA method was applied to the determination of RAC in an actual residue study and the applicability was confirmed by liquid chromatography-tandem mass spectrometry.