Gold nanoparticle-based colorimetric ELISA for quantification of ractopamine

Colorimetric-fluorescent dual-mode nanosensor-powered enzyme immunoassay for ochratoxin A via alkaline phosphatase-mediated silver nanoparticle growth and fluorescence inner filter effect

Ochratoxin A (OTA) seriously threatens public health and agricultural ecosystems, necessitating sensitive detection methods. In view of that traditional immunoassays rely on animal-derived antibodies and toxic substrates, we developed a dual-mode nanosensor integrating colorimetric and fluorescent detection via alkaline phosphatase (ALP)-mediated silver nanoparticles (AgNPs) growth and inner filter effect (IFE). The Ag+-triggered aggregation-induced emission (AIE) enhancement of gold nanoclusters (AuNCs) was investigated for constructing a dual-mode nanosensor. The nanosensor employs an OTA-specific nanobody-ALP fusion (Nb-ALP) to dephosphorylate ascorbic acid 2-phosphate into ascorbic acid (AA), which reduces Ag⁺ to abundant AgNPs with NaBH4-reduced AgNPs seeds. AgNPs amplify plasmonic absorbance for colorimetric ALP detection, while quenching Ag⁺@AuNC fluorescence via IFE for fluorescent detection. The nanosensor was further combined with the Nb-ALP-based immunoassay to develop a dual-mode nanosensor-powered enzyme immunoassay (DMN-EIA). The DMN-EIA exhibited detection limits of 0.14 ng/mL (colorimetric) and 0.28 ng/mL (fluorescent) with high selectivity for OTA, with spike recovery rates of 93.5 %-108.7 % and relative standard deviations not exceeding 20 %. Furthermore, the colorimetric (R2 = 0.96) and fluorescent DMN-EIA (R2 = 0.93) correlated well with high-performance liquid chromatography in detection of ten real pepper samples. Therefore, the developed dual-mode nanosensor and DMN-EIA represent reliable and promising tools for detecting OTA.

Sensing the Future-Frontiers in Biosensors: Exploring Classifications, Principles, and Recent Advances

Biosensors are transforming healthcare by delivering swift, precise, and economical diagnostic solutions. These analytical instruments combine biological indicators with physical transducers to identify and quantify biomarkers, thereby improving illness detection, management, and patient surveillance. Biosensors are widely utilized in healthcare for the diagnosis of chronic and infectious diseases, tailored treatment, and real-time health monitoring. This thorough overview examines several categories of biosensors and their uses in the detection of numerous biomarkers, including glucose, proteins, nucleic acids, and infections. Biosensors are commonly classified based on the type of transducer employed or the specific biorecognition element utilized. This review introduces a novel classification based on substrate morphology, offering a comprehensive perspective on biosensor categorization. Considerable emphasis is placed on the advancement of point-of-care biosensors, facilitating decentralized diagnostics and alleviating the strain on centralized healthcare systems. Recent advancements in nanotechnology have significantly improved the sensitivity, selectivity, and downsizing of biosensors, rendering them more efficient and accessible. The study examines problems such as stability, reproducibility, and regulatory approval that must be addressed to enable the widespread implementation of biosensors in clinical environments. The study examines the amalgamation of biosensors with wearable devices and smartphones, emphasizing the prospects for ongoing health surveillance and individualized medical care. This viewpoint clarifies the distinct types of biosensors and their particular roles, together with recent developments in the "smart biosensor" sector, facilitated by artificial intelligence and the Internet of Medical Things (IoMT). This novel approach seeks to deliver a comprehensive evaluation of the present condition of biosensor technology in healthcare, recent developments, and prospective paths, emphasizing their significance in influencing the future of medical diagnostics and patient care.

Comparison of immunoassay and LC-tandem mass spectrometry analyses of ractopamine in hog oral fluid

The accurate detection of ractopamine in food animals is crucial for marketing since some entities require animals or animal carcasses to be free of ractopamine residues. Field-based ractopamine screening tests that are rapid, sensitive, and capable of high-throughput are highly desirable to ensure that inadvertent exposure to ractopamine did not occur in animals marketed as animals that have not been fed ractopamine. An immunochemically based lateral flow assay was used to analyze oral fluids from hogs never exposed to ractopamine and from hogs that were presumed positives and results were confirmed using an enhanced sensitivity LC-MSMS method. We found that an immunochemically based lateral flow system having a working range of 2.5 to 15 ng mL-1 worked well as a screening assay with 1.7% false positive results in freshly collected hog oral fluids. Using ractopamine glucuronide standards and LC-MSMS, we determined that the false positive results were not due to the presence of ractopamine glucuronide metabolites in oral fluids.

Development of a Gold Nanoparticle-Based ELISA for Detection of PCV2

In this new methodology, plasmonic ELISA (pELISA) was used to detect Circovirus porcine2 (PCV2) in serum samples without the need for plate reading equipment. This process occurs by adapting the conventional ELISA test with gold nanoparticles (AuNPs) to promote a color change on the plate and quickly identify this difference with the naked eye, generating a dark purple-gray hue when the samples are positive and red when the samples are negative. The technique demonstrated high efficiency in detecting samples with a viral load ≥ 5 log10 copies/mL. Plasmonic ELISA offers user-friendly, cost-effective, and reliable characteristics, making it a valuable tool for PCV2 diagnosis and potentially adaptable for other pathogen detection applications.

Multifunctional Nanoprobe-Amplified Enzyme-Linked Immunosorbent Assay on Capillary: A Universal Platform for Simple, Rapid, and Ultrasensitive Dual-Mode Pathogen Detection

Although the traditional enzyme-linked immunosorbent assay (ELISA) has been widely applied in pathogen detection and clinical diagnostics, it always suffers from complex procedures, a long incubation time, unsatisfying sensitivity, and a single signal readout. Here, we developed a simple, rapid, and ultrasensitive platform for dual-mode pathogen detection based on a multifunctional nanoprobe integrated with a capillary ELISA (CLISA) platform. The novel capture antibodies-modified capillaries can act as a swab to combine in situ trace sampling and detection procedures, eliminating the dissociation between sampling and detection in traditional ELISA assays. With excellent photothermal and peroxidase-like activity, the Fe₃O₄@MoS₂ nanoprobe with a unique p-n heterojunction was chosen as an enzyme substitute and amplified signal tag to label the detection antibody for further sandwich immune sensing. As the analyte concentration increased, the Fe₃O₄@MoS₂ probe could generate dual-mode signals, including remarkable color changes from the chromogenic substrate oxidation as well as photothermal enhancement. Moreover, to avoid false negative results, the excellent magnetic capability of the Fe₃O₄@MoS₂ probe can be used to pre-enrich the trace analytes, amplifying the detection signal and enhancing the immunoassay's sensitivity. Under optimal conditions, specific and rapid detection of SARS-CoV-2 has been realized successfully based on this integrated nanoprobe-enhanced CLISA platform. The detection limits were 5.41 pg·mL^-1 for the photothermal assay and 150 pg·mL^-1 for the visual colorimetric assay. More importantly, the simple, affordable, and portable platform can also be expanded to rapidly detect other targets such as Staphylococcus aureus and Salmonella typhimurium in practical samples, making it a universal and attractive tool for multiple pathogen analysis and clinical testing in the post COVID-19 era.

Triple Enhancement for Sensitive Immunochromatographic Assay: A Case Study for Human Fatty Acid-Binding Protein Detection

The work considers a combination of three enhancing approaches for immunochromatographic assay (ICA) and the integration of their impacts into changes of the limit of detection (LOD). Human fatty acid binding protein (FABP), an early biomarker of acute myocardial infarction, was the target analyte. Starting from the common ICA protocol with an LOD equal to 11.2 ng/mL, three approaches were realized: (1) replacement of spherical gold nanoparticles with gold nanoflowers having a branched surface (20-fold lowering the LOD); (2) enhanced labeling of immune complexes via nanoparticle aggregates (15-fold lowering); (3) in-situ growth of bound nanoparticles by reduction of gold salts (3-fold lowering). Single and combined implementations of these approaches have been studied. It has been shown that the LOD decrease for combined approaches is close to the multiplied contribution of each of them. The final LOD for FABP was 0.05 ng/mL, which is 220 times lower than the LOD for the common ICA protocol. The efficiency of the enhanced ICA with three combined approaches was confirmed by testing human serum samples for FABP presence and content. The development presents a new efficient technique for rapid sensitive detection of FABP for medical diagnostics. Moreover, the demonstrated multiple enhancements could be applied for various demanded analytes.

Significantly improved detection performances of immunoassay for ractopamine in urine based on highly urea-tolerant rabbit monoclonal antibody

Highly sensitive and accurate screening of ractopamine (RAC) residue in animal urine is greatly needed to ensure food security. The detection performance of immunoassay for RAC was always seriously harmed by the antibody inactivation derived from urea. Here, we first discovered one rabbit monoclonal antibody (RmAb) to RAC with a high affinity of 0.007 ng mL^-1 and a surprising urea tolerance of 3 M urea, which is beneficial for developing robustly developed immunoassay in urine without sample pretreatment. The limits of detection of developed indirect competitive enzyme-linked immunosorbent assay based on RmAb1 for RAC were 0.0042-0.014 μg L^-1 with the coefficient of variation below 11.7% in swine, sheep, and cow urine, significantly improved 10-100-fold in sensitivity. Moreover, the urea-tolerant mechanism of RmAb1 showed that more non-polar amino acids, more hydrogen bond donors on the surface, and preponderant Pi interaction of antibody-RAC all contributed to the stability of the RmAb1 in a high concentration of urea.

Gold nanoparticle-based optical nanosensors for food and health safety monitoring: recent advances and future perspectives

Modern society has been facing serious health-related problems including food safety, diseases and illness. Hence, it is urgent to develop analysis methods for the detection and control of food contaminants, disease biomarkers and pathogens. As the traditional instrumental methods have several disadvantages, including being time consuming, and having high cost and laborious procedures, optical nanosensors have emerged as promising alternative or complementary approaches to those traditional ones. With the advantages of simple preparation, high surface-to-volume ratio, excellent biocompatibility, and especially, unique optical properties, gold nanoparticles (AuNPs) have been demonstrated as excellent transducers for optical sensing systems. Herein, we provide an overview of the synthesis of AuNPs and their excellent optical properties that are ideal for the development of optical nanosensors based on local surface plasmon resonance (LSPR), colorimetry, fluorescence resonance energy transfer (FRET), and surface-enhanced Raman scattering (SERS) phenomena. We also review the sensing strategies and their mechanisms, as well as summarizing the recent advances in the monitoring of food contaminants, disease biomarkers and pathogens using developed AuNP-based optical nanosensors in the past seven years (2015-now). Furthermore, trends and challenges in the application of these nanosensors in the determination of those analytes are discussed to suggest possible directions for future developments.

Research on Rapid Detection Technology for β2-Agonists: Multi-Residue Fluorescence Immunochromatography Based on Dimeric Artificial Antigen

To detect two types of β2-agonist residues at the same time, we coupled two haptens of clenbuterol (CLE) and ractopamine (RAC) to the same carrier protein through diazotization to prepare dimeric artificial antigen, and a fluorescent lateral flow immunoassay method based on europium nanoparticles (EuNP-FLFIA) was established by combining polyclonal antibodies with europium nanoparticles to form probes. Under optimized conditions, the EuNP-FLFIA could simultaneously detect eight aniline-type and one phenol-type β2-agonists, and the limits of detection (LOD) were 0.11−0.19 ng/mL and 0.12 ng/mL, respectively. The recovery rate of this method was 84.00−114.00%. This method was verified by liquid chromatography−tandem mass spectrometry (LC-MS/MS), and the test results were consistent (R2 > 0.98). Therefore, the method established in this study could be used as a high-throughput screening for the efficient and sensitive detection of β2-agonists in food.

Current Advances in Immunoassays for the Detection of β2-Agonists

β2-agonists are a group of synthetic phenylethanolamine compounds which are traditionally used for treating bronchospasm. These compounds can also increase skeletal muscle mass and decrease body fat. The illegal use of β2-agonists in food-producing animals results in residue of β2-agonists in edible tissues and causes adverse health effects in humans. Thus, the detection of β2-agonists at trace level in complex sample matrices is of great importance for monitoring the abuse of β2-agonists. Many methods have been developed to detect β2-agonists. Among them, a variety of antigen–antibody interaction-based techniques have been established to detect β2-agonists in various samples, including animal feed, urine, serum, milk, tissues and hair. In this review, we summarized current achievement in the extraction of β2-agonists from testing samples and detection of β2-agonists using immunological techniques. Future perspectives were briefly discussed.

A high sensitivity electrochemical sensor based on a dual-template molecularly imprinted polymer for simultaneous determination of clenbuterol hydrochloride and ractopamine

A nitrogen-doped Fe-MOF shows a high specific surface area and excellent electrical conductivity after high temperature carbonization. A novel electrochemical sensor based on a N@Fe-MOF@C loaded dual-template molecularly imprinted polymer (DTMIP) modified glassy carbon electrode (GCE) was proposed for the rapid and ultra-sensitive simultaneous detection of clenbuterol hydrochloride (CLB) and ractopamine (RAC). N@Fe-MOF@C combined with a MIP significantly enhanced the electrical signal. Cyclic voltammetry (CV) was used to detect CLB and RAC. The electrochemical polymerization was conducted with O-phenylenediamine as the functional monomer and CLB and RAC as template molecules. The factors affecting the sensor response were optimized. Under the optimal experimental conditions, the CV current response showed a linear range of 10^-12-8 × 10^-9 M for both CLB and RAC, and the detection limit (LOD) for both CLB and RAC was 3.03 × 10^-13 M (S/N = 3). This electrochemical sensing system has high sensitivity, selectivity, excellent reproducibility, repeatability and stability. The recoveries of the actual samples (97.4%-101.2%) and reasonable relative standard deviations (RSDs) (1.06%-3.17%) indicate the practicability of the sensor system. The system has high application value in the rapid detection of CLB and RAC in clenbuterol hydrochloride tablets, human urine and raw pork.

Sensitive recognition of ractopamine using GQDs-DPA as organic fluorescent probe

A novel spectrofluorimetric sensing platform was designed for Ractopamine measurement in aqueous and plasma samples. d-penicillamine functionalized graphene quantum dots (DPA-GQDs) was utilized as a fluorescence probe, which was synthesized through the pyrolysis of citric acid in the presence of DPA. This one-pot down-top strategy causes to high-yield controllable synthesis method. The reaction time and probe concentration were optimized. Then, the fluorescence intensity of aqueous samples containing different Ractopamine concentrations and 500 ppm DPA-GQDs were measured at 25°C with an excitation wavelength of 274 nm. The sensing platform was also applied to detect Ractopamine in untreated plasma samples. The fluorescence spectroscopy technique responses indicated a linear relationship between the peak fluorescence intensity and ractopamine concentration in the range of 0.25-15 ppm with low limit of quantification of 0.25 ppm was for aqueous and plasma samples, respectively.

Rapid Detection of Ractopamine and Salbutamol in Swine Urine by Immunochromatography Based on Selenium Nanoparticles

Purpose

The purpose of this study was to establish a lateral flow immunoassay using selenium nanoparticles (Se-NPs) as a probe to detect ractopamine (RAC) and salbutamol (SAL) in swine urine.

Methods

SDS and PEG were used as templates to prepare Se-NPs; anti-RAC monoclonal antibodies or anti-SAL monoclonal antibodies were labelled with Se-NPs; and rapid detection kits were prepared. The sensitivity, specificity, and stability were measured, and actual samples were analysed.

Results

The Se-NPs were spherical with a diameter of 40.63 ± 5.91 nm, and were conjugated successfully with an anti-RAC antibody to give a total diameter of 82.33 ± 17.91 nm. The detection limit of a RAC kit in swine urine was 1 ng/mL, and that of a SAL kit was 3 ng/mL. Both procedures could be completed within 5 minutes. No cross-reaction occurred with clenbuterol, bambuterol and phenylethanolamine A. Samples were tested consistently across different batches of kits for swine urine. The results of the kits were identical to those of actual clinical samples analysed by ELISA, and the coincidence rate was 100%.

Conclusion

The assay kit does not require any special device for reading the results, and the readout is a simple colour change that can be evaluated with the naked eye. It is easy to operate, sensitive, specific, and stable This kit is suitable for the rapid and real-time detection of RAC and SAL residues in swine urine samples.

Clinical Trial Registration

Swine urines samples were used under approval from the Experimental Animal Ethics committee of the Joint National Laboratory for Antibody Drug Engineering, Henan University.

Nanozyme and aptamer- based immunosorbent assay for aflatoxin B1

Traditional enzyme-linked immunosorbent assay (ELISA) suffers from the limitations of relatively low sensitivity and stability, and enzyme-labelled antibodies are hard to be prepared and purified. Based on a nanozyme, an aptamer and Fe₃O₄ magnetic nanoparticles (MNP), a nanozyme and aptamer-based immunosorbent assay (NAISA) was developed for aflatoxin B1 (AFB1) detection with simpler operation and separation. In this work, mesoporous SiO₂/Au-Pt (m-SAP) were prepared to act as signal labels, which showed high catalase-like activity and was denoted as nanozyme. Aptamer was adopted to specifically recognize with AFB1, and MNP facilitated to realize magnetic separation. To verify the performance of NAISA, traditional ELISA (t-ELISA) and enhanced ELISA (e-ELISA) using MNP and m-SAP nanozyme were applied in AFB1 detection. The NAISA method showed the lowest limit of detection (LOD) with 5 pg mL^-1 (n = 3, ±4.2 %), 600 and 12-fold lower than that of t-ELISA (3 ng mL^-1) and e-ELISA (0.06 ng mL^-1), respectively. In the interference tests, AFB1 can be identified among six different interfering substances. The NAISA method, thus, can be of great importance as it allows selective and sensitive AFB1 detection, while providing the simplicity of use and need for screening hazardous materials.

Ultrasensitive determination of ractopamine based on dual catalytic signal amplification by Pd nanocubes and HRP using a flow injection chemiluminescence immunoassay

A rapid and highly sensitive flow injection chemiluminescence immunoassay was developed to detect ractopamine residues in pork products. Palladium nanocubes with excellent catalytic performance for a traditional luminol-PIP-H2O2 chemiluminescence system were used as effective carriers to connect a ractopamine antibody and horseradish peroxidase (HRP). Double amplification of a chemiluminescence signal was realized because of palladium nanocubes and HRP. Carboxyl-modified resin beads were used as suitable materials to load ractopamine-coating antigens due to their good biocompatibility and large specific surface area. Based on the principle of competitive immunity, ractopamine standard solution would compete with antigen loaded on carboxyl resin beads for limited binding sites on a ractopamine antibody. Thus, the chemiluminescence intensity of an immunosensor has a linear negative correlation with the logarithm value of ractopamine concentration. Under optimal experimental conditions, the detection range of ractopamine was 0.005-1000 ng mL-1, and the limit of detection (LOD) was 1.7 pg mL-1 (S/N = 3). The proposed immunoassay possessed acceptable accuracy, high specificity and reproducibility and RAC was examined in pork and pig feed with satisfactory results, which would provide a better prospect for the detection of small molecules in food and environment analysis.

Applications of gold nanoparticles in ELISA, PCR, and immuno-PCR assays: A review

Development of state-of-the-art assays for sensitive and specific detection of disease biomarkers has received significant interest for early detection and prevention of various diseases. Enzyme Linked Immunosorbent assays (ELISA) and Polymerase Chain Reaction (PCR) are two examples of proteins and nucleic acid detection assays respectively, which have been widely used for the sensitive detection of target analytes in biological fluids. Recently, immuno-PCR has emerged as a sensitive detection method, where high specificity of sandwich ELISA assays is combined with high sensitivity of PCR for trace detection of biomarkers. However, inherent disadvantages of immuno-PCR assays limit their application as rapid and sensitive detection method in clinical settings. With advances in nanomaterials, nanoparticles-based immunoassays have been widely used to improve the sensitivity and simplicity of traditional immunoassays. Owing to facile synthesis, surface functionalization, and superior optical and electronic properties, gold nanoparticles have been at the forefront of sensing and detection technologies and have been extensively studied to improve the efficacies of immunoassays. This review provides a brief history of immuno-PCR assays and specifically focuses on the role of gold nanoparticles to improve the sensitivity and specificity of ELISA, PCR and immuno-PCR assays.

Ultramarine blue nanoparticles as a label for immunochromatographic on-site determination of ractopamine

A competitive immunochromatographic assay (ICA) is presented and used for on-site determination of ractopamine (RAC). Ultramarine blue nanoparticles were directly separated from ultramarine blue industrial products by centrifugation (< 10,000 rpm and > 4000 rpm) and used as visible labels in ICAs. The ultramarine blue nanoparticles were coated by polyacrylic acid (PAA), which provides carboxyl groups on the surface of ultramarine blue nanoparticles. An anti-RAC monoclonal antibody (mAb) was covalently immobilized on the carboxyl-modified ultramarine blue nanoparticle surface via diimide-activated conjugation between the carboxyl groups on the ultramarine blue nanoparticle surface and the amino groups of the antibodies. RAC and BSA-modified RAC competitively bind to the anti-RAC mAb on the ultramarine blue nanoparticles. The blue band in the test line is generated by the accumulation of ultramarine blue nanoparticles and is negatively associated with the RAC content. Under optimal conditions, the visual limit of detection (vLOD) of this ICA for RAC is 2.0 ng mL^-1, 2.0 ng mL^-1, and 1.0 ng mL^-1 in phosphate-buffered saline (PBS), feed samples, and pork samples, respectively. The ultramarine blue nanoparticle-based ICA also shows no cross activity with salbutamol, clorprenaline, clenbuterol, or terbutaline. Graphical abstract Schematic representation of the ultramarine blue nanoparticles immunochromatographic assay for detection of ractopamine (RAC) based on competitive method. The ultramarine blue nanoparticles were screened from commercial ultramarine pigments for the first time and used to detect ractopamine.

Development of a Nanoparticle-based Lateral Flow Strip Biosensor for Visual Detection of Whole Nervous Necrosis Virus Particles

Effective analysis of pathogens causing human and veterinary diseases demands rapid, specific and sensitive detection methods which can be applied in research laboratory setups and in field for routine diagnosis. Paper lateral flow biosensors (LFBs) have been established as attractive tools for such analytical applications. In the present study a prototype LFB was designed for whole particles (virions) detection of nodavirus or fish nervous necrosis virus. Nodavirus is an important threat in the aquaculture industry, causing severe economic losses and environmental problems. The LFB was based on polyclonal antibodies conjugated on gold nanoparticles for signal visualization. Brain and retinas from fish samples were homogenized, centrifuged and the supernatant was directly applied on the LFB. Formation of a red test line was indicative of nodavirus virions presence. Nodavirus visual detection was completed in short time (30 min). Key factors of the LFB development influencing the assays’ detection limit were characterized and the optimum parameters were determined, enabling increased efficiency, excluding non-specific interactions. Therefore, the proposed LFB assay consists a robust, simple, low cost and accurate method for detection of nodavirus virions in fish samples. The proposed biosensor is ideal for development of a commercial kit to be used on aquaculture facilities by fish farmers. It is anticipated that disease monitoring and environmental safety will benefit from the simplification of time consuming and costly procedures.

Selective extraction and detection of β-agonists in swine urine for monitoring illegal use in livestock breeding

The illegal use of β-agonists often endangers animal-derived food safety. In this study, a selective detection method for β-agonists in swine urine was established via the combination of polymeric ionic liquid-molecularly imprinted graphene oxide-miniaturized pipette tip solid-phase extraction and high-performance liquid chromatography. It is worth noting that this method relied mainly on the designed adsorbent, which presented a rich adsorption mechanism, fast mass transfer rate, and high selectivity, and was successfully utilized in the selective extraction of β-agonists from swine urine samples. The proposed method has low LOD (0.20-0.56 ng/mL), high recovery (94.9-107.9%), and high reusability (4 times, 91.9-108.8%), which indicates its high potential as a selective, sensitive, accurate, and nonfatal method for monitoring the illegal use of β-agonists in the livestock breeding stage.

Development of plasmonic ELISA for the detection of anti-Leishmania sp. IgG antibodies

Recently, a novel Enzyme-Linked Immunosorbent Assay (ELISA) strategy has emerged, known as "plasmonic ELISA" (pELISA), which enables the detection of disease biomarkers at low concentrations with the naked eye. For the first time, this research has developed a signal-generation mechanism for the detection of anti-Leishmania sp. IgG antibodies with the naked eye using pELISA. The immunoassay incorporates an indirect ELISA with successive growth of gold nanoparticles to obtain blue or red-colored solutions in the presence or absence of anti-Leishmania sp. IgG antibodies in canine serum, respectively. The technique we developed was successfully tested in canine serum positive and negative for canine leishmaniasis (CanL), and was shown to be an effective method that could be used as an additional tool for CanL diagnosis. It will be particularly useful in resource-constrained countries, because it does not require sophisticated instruments to read the results, increasing the practicality of CanL detection in these areas.

Photoelectrochemical determination of ractopamine based on inner filter effect between gold nanoparticles and graphitic carbon nitride-copper(II) polyphthalocyanine coupled with 3D DNA stabilizer

Copper(II) polyphthalocyanine (CuPPc) was combined with graphitic carbon nitride (g-C₃N₄) to form a heterojunction with enhanced photoelectrochemical (PEC) signal. A sensitive PEC method was developed for determination of ractopamine based on a PEC inner filter effect between gold nanoparticles (AuNPs) and the g-C₃N₄/CuPPc. A gold electrode was modified with g-C₃N₄/CuPPc and the DNA was linked to the AuNPs. Initially, the PEC signal is weak due to the inner filter effect between the AuNPs and g-C₃N₄/CuPPc. In the presence of ractopamine, it interacts with the aptamer and the complementary chain (C chain) is released. This triggers the entropy-driven cyclic amplification and results in the release of the substrate B chain (SB chain) from three-dimensional DNA stabilizer. The probe is released from the electrode due to the interaction of probe DNA and the SB chain. As a result, the PEC signal increases linearly in the 0.1 pmol·L^-1 to 1000 pmol·L^-1 ractopamine concentration range. The detection limit is 0.03 pM, and the relative standard deviation is 3.4% (at a 10 pmol·L^-1 level; for n = 11). The method has been successfully applied to the determination of ractopamine in pork samples. Graphical abstract Schematic presentation of detection method based on PEC inner filter effect between AuNPs and the g-C₃N₄/CuPPc being fabricated for ractopamine. 3D DNA was used as stabilizer to decrease the PEC blank signal.

Ratiometric fluorescent immunoassay for the cardiac troponin-I using carbon dots and palladium-iridium nanocubes with peroxidase-mimicking activity

A nanozyme-linked immunosorbent assay is described for cardiac troponin I which is a biomarker for myocardial infarction. The method is based on the use of Pd-Ir nanocubes with excellent peroxidase-like activity. The nanocubes catalyze the oxidization of nonfluorescent o-phenylenediamine (OPD) by H₂O₂ to form a yellow fluorescent product (oxOPD) with excitation/emission maxima at 400/570 nm. Carbon dots are added as a reference fluorophore. Under the same excitation wavelength, they display blue fluorescence (450 nm). The ELISA uses the Pd-Ir nanocubes as a label for the secondary antibody and OPD as substrate. The ratio of fluorescence intensities at 570 and 450 nm increases in the 1 pg·mL^-1 to 1 ng·mL^-1 cardiac troponin I concentration range, and the detection limit is 0.31 pg·mL^-1. The method was applied to analyze spiked serum samples, and the results compared well with those obtained by a commercial chemiluminescence assay. Graphical abstract Schematic presentation of the ratiometric fluorescence immunoassay for cardiac troponin-I. Pd-Ir nanocubes were employed to fabricate nanozyme-based signal labels for its excellent peroxidase-mimicking activity.

Post-assay growth of gold nanoparticles as a tool for highly sensitive lateral flow immunoassay. Application to the detection of potato virus X

This article demonstrates a new kind of a highly sensitive lateral flow immunoassay (LFIA). It is based on the enlargement of the size of gold nanoparticles (GNPs) directly on the test strip after a conventional LFIA. Particle size enlargement is accomplished through the catalytic reduction of HAuCl₄ in the presence of H₂O₂ and through the accumulation of additional gold on the surface of the GNPs. To attain maximal enhancement of the coloration of the zone in the test strip and to achieve a minimal background, the concentration of precursors, the pH value, and the incubation time were optimized. GNPs on the test strip are enlarged from 20 to 350 nm after a 1-min treatment at room temperature. The economically important and widespread phytopathogen potato virus X (PVX) was used as the target analyte. The use of the GNP enlargement method results in a 240-fold reduction in the limit of the detection of PVX, which can be as low as 17 pg·mL^-1. The total duration of the assay, including virus extraction from the potato leaves, lateral flow, and the enhancement process, is only 12 min. The diagnostic efficiency of the technique was confirmed by its application to the analysis of potato leave samples. No false positives or false negatives were found. The technique does not depend on specific features of the target analyte, and it is conceivably applicable to numerous GNP-based LFIAs for important analytes. Graphical abstract An enlargement solution (containing HAuCl₄ and H₂O₂) was dripped on the strip after common lateral flow immunoassay. Gold nanoparticles on the strip (20 nm) catalyze gold reduction and the formation of larger particles (up to 350 nm), resulting in a 240-fold lower detection limit within 1 min.

Colorimetric and visual detection of mercury(II) based on the suppression of the interaction of dithiothreitol with agar-stabilized silver-coated gold nanoparticles

A colorimetric and visual method is described for the determination of mercury(II) ion. A gel consisting of agar-stabilized silver-coated gold nanoparticles (Au@Ag NPs) was prepared. The reaction with dithiothreitol (DTT) via thiol-Ag chemistry results in an orange to purple color change of the gel. However, in the presence of Hg(II), the reaction of DTT with the silver shells is suppressed due to the strong thiophilicity of Hg(II). The color of the gel changes from purple to red to orange in the presence of increasing concentrations of Hg(II). The Au@Ag NPs therefore are a viable optical probe for Hg(II) which can be detected in concentration as low as 78 nM via dual-wavelength ratiometric absorbance (A₃₉₀/A₅₂₀), and at 1 μM levels with bare eyes. The use of agar as a support is mandatory to prevent the aggregation of the NPs and also improves selectivity. The method was applied to the analysis of spiked samples, and recoveries ranged between 96.3 and 104%. The assay is easy, inexpensive, and in our perception represents an attractive tool for on-site visual detection of Hg(II). Graphical abstract Schematic of the assay. With increasing concentrations of Hg(II), the oxidative etching of silver shells caused by dithiothreitol (DTT) is gradually inhibited, and the color of agar-stabilized Au@Ag NP gel varies from purple to red, and finally to orange. This can be used for visual detection of Hg(II).