Quadruplex gold immunochromatogaraphic assay for four families of antibiotic residues in milk

Ultrasensitive Ti3C2Tx@Pt-Based Immunochromatography with Catalytic Amplification and a Dual Signal for the Detection of Chloramphenicol in Animal-Derived Foods

Herein, a catalytic amplification enhanced dual-signal immunochromatographic assay (ICA) based on Pt nanoparticles (Pt NPs) modified with Ti3C2Tx MXene (Ti3C2Tx@Pt) was first developed for chloramphenicol (CAP) in animal-derived foods. Due to the large specific surface area and abundant active sites of Ti3C2Tx@Pt, they can be loaded with hundreds of Pt NPs to enhance their catalytic activity, resulting in a significant increase in the detection sensitivity; the sensitivity was up to 50-fold more sensitive than the reported ICA for CAP. The LODs of the developed method for milk/chicken/fish were 0.01 μg/kg, the LOQs were 0.03 μg/kg and the recovery rates were 80.5-117.0%, 87.2-118.1% and 92.7-117.9%, with corresponding variations ranging from 3.1 to 9.6%, 6.0 to 12.7% and 6.0 to 13.6%, respectively. The linear range was 0.0125-1.0 μg/kg. The results of the LC-MS/MS confirmation test on 30 real samples had a good correlation with that of our established method (R2 > 0.98), indicating the practical reliability of the established method. The above results indicated that an ICA based on the Ti3C2Tx@Pt nanozyme has excellent potential as a food safety detection tool.

Screening of broad-spectrum aptamer and development of electrochemical aptasensor for simultaneous detection of penicillin antibiotics in milk

Penicillin antibiotics (PENs) play an important role in killing pathogenic bacteria. However, the residues of various penicillin antibiotics in milk gradually accumulate in the human body with the increase of milk intake, which causes direct harm to the human body. Aptamers can be used as recognition element of sensors. It is great significance to use broad-spectrum aptamers for simultaneous detection of PENs. In this study, we reported the screening and identification of DNA aptamers for PENs. The aptamers were screened by graphene oxide-systematic evolution of ligands by exponential enrichment (GO-SELEX). The broad-spectrum aptamers with high affinity and specificity were successfully obtained after 13 rounds of screening. The affinity and specificity of candidate aptamers were analyzed by a GO fluorescence competition method. Further sequence analysis revealed that a truncated 47 nt aptamer (P-11-1) had a higher affinity than the original 79 nt aptamer. The truncated aptamer P-11-1 was used as a recognition element, and an electrochemical aptasensor was prepared using gold nanoparticles (AuNPs) combined with ferroferric oxide-multi walled carbon nanotube (Fe3O4-MWCNTs) complex. The results showed that the developed aptasensor achieved the simultaneous detection of PENs in milk samples across a concentration range of 2 nM-10,000 nM, achieving a limit of detection of 0.667 nM. This methodology provided a simple and sensitive new thinking for antibiotic multi-residue detection.

Occurence of antimicrobial residues in milk and labneh consumed in Lebanon

Antimicrobials are administered in livestock for different uses leading to milk contamination and several undesirable effects. Because there is a lack of surveillance of antimicrobial residues (AMRs) in milk and dairy products in Lebanon, this study aims to determine the occurrence of AMRs in 90 Lebanese samples of milk and labneh (concentrated yoghurt). Multi-residue screening methods with suitable sample preparations were applied to detect 71 AMRs in milk and labneh, respectively, using LC-MS/MS. Of the total number of samples, 71% was contaminated with AMRs and (fluoro)quinolones and macrolides were the most detected families. Additional confirmation tests proved that 6.7% of the milk samples were non-compliant for the macrolides tilmicosin, tulathromycin and spiramycin. Moreover, some labneh prepared from contaminated milk samples was analysed to determine the fate of AMRs during the manufacturing process. The results showed that some AMRs could be concentrated, eliminated or degraded, based on their physicochemical characteristics.

Bifunctional magnetic ZnCdSe/ZnS quantum dots nanocomposite-based lateral flow immunoassay for ultrasensitive detection of streptomycin and dihydrostreptomycin in milk, muscle, liver, kidney, and honey

In this study, bifunctional magnetic ZnCdSe/ZnS quantum dots nanocomposite (MQNs) were synthesized, and firstly used to develop a lateral flow immunoassay (LFIA) for streptomycin (STR) and dihydrostreptomycin (DHSTR) detection in milk, muscle, liver, kidney, and honey simultaneously. The fluorescence signal of MQNs was 9-fold stronger than that of the original quantum dots. The detection limits of the established MQNs-LFIA for STR and DHSTR in five samples were 0.08-1.78 μg/kg, the quantitation limits were 0.26-5.87 μg/kg, the recoveries were between 85.0% and 120.0%, and the coefficient of variations were between 0.8% and 19.3%, respectively. The sensitivity was up to 42-fold more sensitive than the reported LFIAs. The single blind test results of 25 samples were consistent with that of the confirmation method (R² ≥ 0.99). Besides, a portable reader was self-developed and used for rapid quantification. Our study demonstrated MQNs as a promising signal-amplifying tag can be used for ultrasensitive detection of chemical contaminants in foods.

Affinity-Based Analysis Methods for the Detection of Aminoglycoside Antibiotic Residues in Animal-Derived Foods: A Review

With the increasingly serious problem of aminoglycoside antibiotic residues, it is imperative to develop rapid, sensitive and efficient detection methods. This article reviews the detection methods of aminoglycoside antibiotics in animal-derived foods, including enzyme-linked immunosorbent assay, fluorescent immunoassay, chemical immunoassay, affinity sensing assay, lateral flow immunochromatography and molecular imprinted immunoassay. After evaluating the performance of these methods, the advantages and disadvantages were analyzed and compared. Furthermore, development prospects and research trends were proposed and summarized. This review can serve as a basis for further research and provide helpful references and new insights for the analysis of aminoglycoside residues. Accordingly, the in-depth investigation and analysis will certainly make great contributions to food safety, public hygiene and human health.

Dual-readout fluorescence quenching immunochromatographic test strips for highly sensitive simultaneous detection of chloramphenicol and amantadine based on gold nanoparticle-triggered photoluminescent nanoswitch control

Herein, a novel fluorescence quenching immunochromatographic test strip (FQICTS) for simultaneous detection of chloramphenicol (CAP) and amantadine (AMD) was developed on the basis of inner filter effect (IFE), with the combination of gold nanoparticles (AuNPs) and highly luminescent green-emitting gold nanoclusters (AuNCs) as the IFE quencher/donor pair. The AuNPs could quench the excitation light and emission light of AuNCs and achieve a high IFE efficiency due to dual spectral overlapping. Under optimal conditions, the "turn-on" mode of the AuNCs-based dual-readout FQICTS showed good linearity for CAP detection in chicken samples from 0.05 ng/g to 10 ng/g, with a limit of detection (LOD) of 0.043 ng/g. The linear range of AMD is 0.5-50 ng/g, with LOD of 0.45 ng/g. The visual LODs of CAP and AMD in "turn-on" mode were 200 and 10 times lower than that in "turn-off" mode, respectively. The "turn-on" mode of FQICTS showed high recovery for detecting CAP (82.5-94.5%) and AMD (81.9-110.7%) spiked into chicken samples. The performance and practicability of the established method were verified with commercial enzyme-immunoassay kits, and good correlations were observed. Overall, the newly developed AuNCs-based dual-readout FQICTS is a promising on-site screening tool for rapid, high-sensitivity detection of multiple food contaminants in practical applications.

Dual Gold Nanoparticle/Chemiluminescent Immunoassay for Sensitive Detection of Multiple Analytes

Multiple antibiotics and mycotoxins usually simultaneously exist in foods, which poses a serious threat to human health. How to detect them in one test with high sensitivity and fidelity is challenging. In this study, we develop a dual readout lateral flow immunodetection platform that can quantitatively detect five kinds of antibiotics and five kinds of mycotoxins within one sample. The platform is composed of a chip and a portable readout instrument where gold nanoparticle (AuNP)-based and chemiluminescence immunoassays could be performed to reach a maximum throughput of 220 analytes in one setting. For a rapid screen, qualitative analysis by detecting the color change of the deposited AuNPs on the chip could be realized. For quantitative results, chemiluminescence imaging and analysis can be completed within 15 min. Apart from the high throughput and high efficiency, this platform has a high detection sensitivity. For instance, the limit of detection (LOD) for thiamphenicol (a representative antibiotic) and fumonisins B1 (a representative mycotoxin) is 8 times and 40 times lower than those of the previously reported methods, respectively. Thus, this dual readout immunodetection platform is promising as a universal device for rapid and quantitative detection of multiple analytes with high throughput, high sensitivity, and high fidelity.

Quantitative Detection of Mastitis Factor IL-6 in Dairy Cow Using the SERS Improved Immunofiltration Assay

Interleukin-6 (IL-6) is generally used as a biomarker for the evaluation of inflammatory infection in humans and animals. However, there is no approach for the on-site and rapid detection of IL-6 for the monitoring of mastitis in dairy farm scenarios. A rapid and highly sensitive surface enhanced Raman scattering (SERS) immunofiltration assay (IFA) for IL-6 detection was developed in the present study. In this assay, a high sensitivity gold core silver shell SERS nanotag with Raman molecule 4-mercaptobenzoic acid (4-MBA) embedded into the gap was fabricated for labelling. Through the immuno-specific combination of the antigen and antibody, antibody conjugated SERS nanotags were captured on the test zone, which facilitated the SERS measurement. The quantitation of IL-6 was performed by the readout Raman signal in the test region. The results showed that the detection limit (LOD) of IL-6 in milk was 0.35 pg mL⁻¹, which was far below the threshold value of 254.32 pg mL⁻¹. The recovery of the spiking experiment was 87.0–102.7%, with coefficients of variation below 9.0% demonstrating high assay accuracy and precision. We believe the immunosensor developed in the current study could be a promising tool for the rapid assessment of mastitis by detecting milk IL-6 in dairy cows. Moreover, this versatile immunosensor could also be applied for the detection of a wide range of analytes in dairy cow healthy monitoring.

Infrared spectroscopy combined with random forest to determine tylosin residues in powdered milk

The contamination of milk by antibiotic residues is a worldwide health and food safety problem. There is a need to develop new methods for the rapid determination of antibiotic residues in milk. A method has been developed for determining tylosin residues directly in powdered milk using Fourier Transformed Infrared spectroscopy (FTIR). Tylosin is a broad-spectrum macrolide antibiotic. The spectra obtained were submitted to chemometric analysis to obtain a prediction model for tylosin concentration in powdered milk. Using the Boruta algorithm, the absorption bands related to the milk contamination by the antibiotic were identified. Random forest was shown to be adequate for the prediction of tylosin residues in milk at low concentrations (≤ 100 μg L^-1) and the prediction model generated showed high correlation and determination coefficients (greater than 0.95). The proposed methodology proved to be efficient for the investigation of antibiotic residues in powdered milk.

Microcystin-LR heterologous genetically engineered antibody recombinant and its binding activity improvement and application in immunoassay

Microcystin-LR (MC-LR) is a high-toxic biohazard that pollutes ecological environment and agroproducts. In this study, a newly recombined genetically engineered antibody (AV_HH-MV_H) with higher thermal stability and binding activity was designed by chain shuffling and based on our previously obtained anti-MC-LR scFv and nanobody. Based on AV_HH-MV_H template, a capacity of 8.99 × 10⁵ CFU/mL of phage display AV_HH-MV_H mutagenesis library was constructed by site-directed mutagenesis in MV_H-CDR3 region, and then used for ultrasensitive mutants screening. Afterwards, a total of five positive AV_HH-MV_H mutants were isolated from the mutagenesis library, and their binding activity was higher than AV_HH-MV_H for MC-LR. The AV_HH-MV_H mutant 3 was cloned into pET-25b vector for soluble expression, and the concentration of target protein expressed in culture system was 43.5 mg/L. An indirect competitive enzyme-linked immunosorbent assay (IC-ELISA) was established based on purified AV_HH-MV_H mutant 3 protein, and it showed ultrasensitive binding activity for MC-LR with the detection limit of 0.0075 μg/L, which was far below the maximum residue limit standard of 1.0 μg/L in drinking water proposed by World Health Organization. The established IC-ELISA shows good accuracy, repeatability, stability and applicability for MC-LR spiked samples, and it is promising for MC-LR ultrasensitive monitoring.

Functional Up-Conversion Nanoparticle-Based Immunochromatography Assay for Simultaneous and Sensitive Detection of Residues of Four Tetracycline Antibiotics in Milk

An ultrahigh-sensitivity lateral flow immunochromatography (LFIC) assay based on up-converting nanoparticles (UCNPs) was developed to carry out a multi-residue detection of tetracycline in milk. The sensitivity of the immunoassay was greatly improved by the use of a broad-spectrum monoclonal antibody attached to UCNPs to form a signal probe. Under the optimal conditions, the UCNP-LFIC assay enabled sensitive detection of tetracycline (TC) as well as of oxytetracycline (OTC), chlortetracycline (CTC), and doxycycline (DOX) within 10 min, with IC ₅₀ values of 0.32, 0.32, 0.26, 0.22 ng/mL, respectively. There was no cross-reactivity with ten other antibiotics. Similarly, we evaluated the experimental results for matrix effects. Experiments involving spiking showed the four tetracycline antibiotics displaying mean recoveries ranging from 93.95 to 111.90% with relative standard deviations (RSDs) of < 9.95%. The detection results of actual samples using the developed method showed a good correlation (R ² ≥ 0.98) with the results using high-performance liquid chromatography (HPLC). Thus, the assay can achieve an ultrahighly sensitive detection of antibiotics in milk, and can hence promote human health and provides promising applications in the bio-detection field.

Dual near-infrared fluorescence-based lateral flow immunosensor for the detection of zearalenone and deoxynivalenol in maize

A novel dual near-infrared fluorescence-based lateral flow immunosensor was developed to determine zearalenone and deoxynivalenol in maize. Two near-infrared dyes with distinct fluorescence characteristics were utilized to separately label the anti-zearalenone and anti-deoxynivalenol antibodies as detection reagents. The capture antigens zearalenone-BSA and deoxynivalenol-BSA were mixed and immobilized on the same test line of nitrocellulose membrane. This assay format facilitates simultaneous detection of the two mycotoxins on a single test line. After optimizing experimental parameters, the limits of detection for zearalenone and deoxynivalenol were as low as 0.55 μg/kg and 3.8 μg/kg in maize, respectively. The spiking experiment yielded recovery ratios ranging from 81.7% to 107.3% with coefficients of variation less than 14% demonstrating high assay accuracy and precision. Moreover, the actual sample analysis produced consistent results between this method and instrumental method. Therefore, the developed immunosensor can serve as an accurate and efficient approach for monitoring mycotoxins in agricultural products.

High throughput detection of antibiotic residues in milk by time-resolved fluorescence immunochromatography based on QR code

Herein, we have successfully established a novel, rapid, and simple lateral-flow immunoassay based on time-resolved fluorescence and biotin-streptavidin to detect the residues of various antibiotics in milk. The fluorescence signal and sensitivity of immunochromatography were enhanced through biotinylated antibody coupled with streptavidin europium microspheres. Moreover, due to the use of a QR Code and fluorescent reader, quantitative detection and real-time data uploading can be achieved. Under the optimal conditions, the various antibiotic residues were detected in the milk samples. The results showed that the limits of detection of tylosin, lincomycin and doxycycline were 0.10, 0.06, and 0.27 ng/mL, respectively. The recoveries of the spiked milk samples were 88.9%~127%, with coefficients of variation less than 11%, and the test strip can be stored at room temperature for 12 months. This study shows that the proposed time-resolved fluorescence immunoassay is sensitive, rapid and reliable, and has the potential to be used for detection of veterinary antibiotic residues in food safety fields.

Gold nanostars-enhanced Raman fingerprint strip for rapid detection of trace tetracycline in water samples

Rapid and accurate detection of antibiotics at trace levels in food represents a great challenge. Tetracycline (TC), as a sort of broad-spectrum antibiotic, has been extensively used in animal infection therapy and animal husbandry as growth promoters. Large amounts of TC residues in animal-derived foods affect food quality and safety, and cause undesirable side effects such as allergic reactions and bacterial antibiotic resistance. Here, a Raman fingerprint strip sensor was reported based on surface-enhanced Raman scattering technology and demonstrated for ultrasensitive detection of TC. In this approach, 4-aminothiophenol (4-ATP) modified gold nanostars (GNSs) were used as a strong Raman reporter, which was coated with anti-TC monoclonal antibody serving as a biorecognition to acquire both visual and Raman signals on the test line. To demonstrate the performance of this strip, TC standard solutions with concentrations ranging from 0.5 to 50 ng/mL was detected, the limit of the detection (LOD) for the Raman signal was 0.04 ng/mL, which was 100 times more sensitive than those of color intensity quantifications. The other analogues, oxytetracycline, and chlortetracycline were detected using this method, making them suitable for the samples with TC analogues screening.

Highly sensitive detection of multiple antibiotics based on DNA tetrahedron nanostructure-functionalized magnetic beads

Functional DNAs-functionalized magnetic beads (MBs) offer great potential in bioanalysis field because of their target recognition and magnetic separation functions. However, the recognition capability and hybridization affinity of DNA probes often suffer from limited available space, poor probe conformation and non-selective adsorption. To overcome these limitations, we herein used aptamer-pendant DNA tetrahedron nanostructure-functionalized MBs (TETapt-tet MBs) to develop a target-response fluorescence method with tetracycline (TET) as a model. In the absence of TET, 6-carboxy-X-rhodamine-labeled complementary DNAs (ROX-cDNAs) were assembled on the surface of MBs. Upon the addition of target TET, the ROX-cDNAs were separated and released from the MBs to generate fluorescence signal. The limit of detection and limit of quantification for TET were found to be 6 pg mL^-1 and 20 pg mL^-1, respectively. Compared with ssDNA-functionalized MBs surface, the designed DNA tetrahedron nanostructure-based surface could decrease the hybridization time and reduce false positives, ensuring the accuracy of TET detection in complex samples. The presented method was successfully employed for TET detection in honey samples. Moreover, this functionalization strategy could be extended to detect multiple antibiotics by simply substituting different aptamer sequences. Therefore, the proposed method has great potential in the field of food safety and public health.

Ovalbumin antibody-based fluorometric immunochromatographic lateral flow assay using CdSe/ZnS quantum dot beads as label for determination of T-2 toxin

This work describes an anti-ovalbumin antibody-based lateral flow immunoassay (LFI) for T-2 toxin. The antibody uses a coating antigen as a bifunctional element for universality and introduces preincubation to improve the detection limits of the method. T-2 toxin and ovalbumin-modified T-2 toxin competitively binds on the anti-T-2 toxin monoclonal antibody modified on CdSe/ZnS quantum dot beads during preincubation. The modified T-2 toxin acts as a bifunctional element that forms immuno complexes during preincubation and combines with anti-ovalbumin antibody coated in the test line through the ovalbumin terminal. Fluorescence is detected at 610 nm on the test zone following photoexcitation at 365 nm. It has a reverse dose-effect relationship with the amount of T-2 toxin. The calibration plot is linear in the 20-110 fg mL^-1 T-2 toxin concentration range, and the limit of detection (LOD) is 10 fg mL^-1, which is lower by 8-fold than that of the traditional LFI system (LOD 80 fg mL^-1) and one order of magnitude than those of LFIs with labels of colloidal gold nanoparticles (LOD 150 fg mL^-1) or fluorophores (LOD 190 ng mL^-1). Universality was verified through aflatoxin B₁ detection using the established ovalbumin antibody-based LFI system (LOD 10 fg mL^-1). The performance of the method was compared with that of established systems and a commercial ELISA kit (LOD 360 fg mL^-1). Graphical abstractSchematic representation of ovalbumin antibody-based immunochromatographic lateral flow assay for T-2 toxin. Preincubation is introduced for high sensitivity. T-2- anti-ovalbumin acts as a bi-functional element for universality. CdSe/ZnS quantum dot beads act as label. Fluorometric signal is detected at 610 nm.

Gold Immunochromatographic Assay for Rapid On-Site Detection of Lincosamide Residues in Milk, Egg, Beef, and Honey Samples

Lincosamides (LMs), include clindamycin (CLIN), lincomycin (LIN), and pirlimycin (PIR), that are widely used as veterinary drugs. LM residues in edible animal origin foods endanger human health and are in urgent need of establishing fast, simple, and highly sensitive detection methods. A gold immunochromatographic strip is prepared to detect CLIN, LIN, and PIR residues simultaneously with a single monoclonal antibody. This antibody is obtained with the design of a novel Hapten and can simultaneously recognize CLIN, LIN, and PIR. Under optimized conditions, the strip results can be semi-quantitatively evaluated with the naked eye within 15 min, with cut-off values in phosphate-buffered saline of 1 ng mL^-1 for CLIN, 10 ng mL^-1 for LIN, and 25 ng mL^-1 for PIR, respectively. Besides, the strip can also be quantified using a hand-held strip scanner, and the spiked samples are used for establishing matrix curves. The limits of detection for CLIN, LIN, and PIR in spiked milk, egg, beef, and honey samples can satisfy the detection requirement. The utility of this strip is also confirmed by positive honey sample. In short, this strip should be expected to be a useful tool for the rapid on-site screening of lincosamide residues in milk, egg, beef, and honey samples.

A label-free colorimetric aptasensor based on controllable aggregation of AuNPs for the detection of multiplex antibiotics

We devise a novel colorimetric aptasensor for multiplex antibiotics based on an ss-DNA fragment coordinately controlling gold nanoparticles (AuNPs) aggregation. The multifunctional aptamer (Apt) was elaborately designed to be adsorbed on AuNPs surfaces acting as a binding element for antibiotics and a molecular switch. Chloramphenicol (CAP) and tetracycline (TET) were selected as the model antibiotics. When one kind of antibiotics was added, the specifically recognized fragment of Apt can bind to it and dissociated, and the non-specific one coordinately controls AuNPs aggregation under high-salt conditions. Hence, different color changes of AuNPs solution can be used as the signal readout. The aptasensor exhibited remarkable selectivity and sensitivity for separate detection of TET and CAP, and the detection limits are estimated to be 32.9 and 7.0 nM, respectively. The analysis with the absorption spectroscopy and the smartphone are applied to detect antibiotics in real samples with consistent results and desirable recoveries.

An electrochemical sensing platform based on ladder-shaped DNA structure and label-free aptamer for ultrasensitive detection of ampicillin

Herein, an electrochemical aptasensor is described for detection of ampicillin (Ampi). The sensing strategy is based on the application of a ladder-shaped DNA structure as a multi-layer physical block on the surface of gold electrode. Attributing to the electrostatic repulsion and physical prevention of the ladder-shaped DNA structure, ultrasensitive detection of Ampi was achieved with a detection limit as low as 1 pM. In the presence of Ampi, the ladder-shaped DNA structure is disassembled and detached from the electrode surface. This leads to the high access of [Fe(CN)₆]^3-/4- as a redox indicator to the electrode surface and a strong redox peak. The aptasensor response for Ampi detection was in the linear range from 7 pM to 100 nM with the detection limit of 1 pM. The presented analytical strategy showed its application in detecting Ampi in the spiked milk samples with satisfactory performance. This work can be easily expanded for different targets by alternating the corresponding aptamers.

Quantitative and rapid detection of amantadine and chloramphenicol based on various quantum dots with the same excitations

Herein, we developed a sensitive and quantitative flow assay for simultaneous detection of amantadine (AMD) and chloramphenicol (CAP) in chicken samples based on different CdSe/ZnS quantum dots (QDs). In contrast to other reports, the QDs could be excited by the same excitations that lowered the requirements for the matching instruments. Under the optimal conditions, the strategy permitted sensitive detection of AMD and CAP in a linear range of 0.23 to 1.02 ng/g and 0.02 to 0.66 ng/g. The limits of detection were 0.18 ng/g and 0.016 ng/g, respectively. Moreover, the whole detection process could be completed within 20 min with no additional sophisticated instruments and complicated operations. Spiked samples were analyzed using both QD-based lateral flow immunoassay (QD-LFIA) and commercial ELISA kits with good correlation (R² = 0.96). Moreover, this study laid the foundation and simplified the development of the requisite instrument. Graphical abstract ᅟ.

Invited review: Advancements in lateral flow immunoassays for screening hazardous substances in milk and milk powder

Dairy-related food safety outbreaks, such as food-borne pathogen contamination, mycotoxin contamination, and veterinary drug contamination, sometimes happen and have been reported all over the world, affecting human health and, in some cases, leading to death. Thus, rapid yet robust detection methods are needed to monitor milk and milk powder for the presence of hazardous substances. The lateral flow immunoassay (LFI) is widely used in onsite testing because of its rapidity, simplicity, and convenience. In this review, we describe some traditional LFI used to detect hazardous substances in milk and milk powder. Furthermore, we discuss recent advances in LFI that aim to improve sensitivity or detection efficiency. These advances include the use of novel label materials, development of signal amplification systems, design of multiplex detection systems, and the use of nucleic acid-based LFI.

Multiplex immunoassay based on biochip technology for the screening of antibiotic residues in milk: validation according to the European guideline

The Infiniplex for milk® (IPM) kit is a quick method for the simultaneous and qualitative detection of more than 100 molecules including antibiotic residues, mycotoxins, anti-inflammatories and antiparasitic drugs into a single test that does not require milk treatment. The IPM® kit was validated according to the European decision EC/2002/657 and according to the European guideline for the validation of screening methods (2010). Our validation was focused only on antibiotic residues. The washing step was identified as the most critical step of the assay. Insufficient washes could cause a significant background noise that prevents imaging. Positive controls have to be freshly prepared each day (insufficient stability). The method was specific with a low false-positive rate of 1.7% on 5 discrete test regions (DTR) ((beta-lactams, lincomycin, virginiamycin, quinolones and sulphonamides)) and a false-positive rate of 0% on the 26 other DTR. During our validation, the 42 determined detection capabilities CCβ for 12 antibiotic families (aminoglycosides, cephalosporins, lincosamides, macrolides, miscellaneous antibiotics, penicillins, phenolated polymixins, polypeptide antibiotics, quinolones, sulphonamides, tetracyclines) were at between once and twice the decision levels stated by the manufacturer. Forty CCβ determined were lower than the respective regulatory limits (i.e. MRL, RC, MRPL) in milk, except for tilmicosin (1.5 times the MRL) and neospiramycin (>1.25 times the MRL). The estimated CCβ of thiamphenicol, cloxacillin, danofloxacin, sulphathiazol, ceftiofur and sulphamonomethoxine were lower than or at the MRL. However, it was difficult to approach an accurate CCβ with only qualitative results. It is impossible to know whether or not we were close to the cut-off value. The software could be improved by differentiating between low-positive and high-positive results. The results of our participation in three qualitative proficiency tests in 2016 and 2017 for the detection of quinolones, tetracyclines and sulphonamides in cows' milk were very satisfactory.

Visual and fluorometric lateral flow immunoassay combined with a dual-functional test mode for rapid determination of tetracycline antibiotics

A fluorometric immunochromatographic assay (FICA) is described where ZnCdSe/ZnS quantum dots (QDs) act as fluorescent label and gold nanoparticles (AuNPs) act as quencher. The assay works in the "turn-on" mode, i.e. the fluorescent signal (best measured at excitation/emission wavelengths of 302/525 nm) increases with the increase of analyte concentration. This assay can detect tetracycline antibiotics including tetracycline, oxytetracycline, chlortetracycline, and doxycycline. It is not interfered by other veterinary drugs. The visual limits of detection (LODs) for the tetracycline antibiotics are 2 μg·L^-1 in buffer, 20 μg·L^-1 in milk, and 40 μg·kg^-1 in animal muscle tissue. The assay (including sample treatment) can be performed within 30 min. The FICA based on "turn on" mode is more sensitive than the colloidal gold-based immunochromatographic assay (CGICA) and quantum dot-based immunochromatographic assay (QDICA) based on "turn off" mode using either AuNPs or QDs as signal labels. One strip can simultaneously provide the fluorescent test results in the "turn on" mode on the basis of QD luminescence quenching under UV light. The colorimetric test is of the "turn off" mode based on the formation of a red coloration due to the use of AuNPs under natural light. The use of such a dual-functional test mode allows for rapid semi-quantitative determination of tetracycline antibiotics in milk and tissue samples. Graphical abstract Schematoc of a fluorometric immunochromatographic assay (FICA) based on fluorescence quenching of quantum dot (QD) by gold nanoparticle (AuNP) combined with a dual-functional test mode under UV light (turn on mode) and natural light (turn off mode) to visually detect tetracycline antibiotics.