Application of Au@Pt Nanozyme as Enhancing Label for the Sensitive Lateral Flow Immunoassay of Okadaic Acid

Current trends in colorimetric biosensors using nanozymes for detecting biotoxins (bacterial food toxins, mycotoxins, and marine toxins)

Biotoxins, predominantly bacterial food toxins, mycotoxins, and marine toxins, have emerged as major threats in the fields of seafood, other foods, feeds, and medicine. They have potential teratogenic, mutagenic, and carcinogenic effects on humans, occasionally triggering high morbidity and mortality. One of the apparent concerns relates to the increasing consumption of fast food resulting in the demand for processed food without adequate consideration of the toxins they may contain. Therefore, developing improved methods for detecting biotoxins is of paramount significance. Nanozymes, a type of nanomaterials exhibiting enzyme-like activity, are increasingly being recognized as viable alternatives to natural enzymes owing to their benefits, such as customizable design, controlled catalytic performance, excellent biocompatibility, and superior stability. The remarkable catalytic activity of nanozymes has led to their broad utilization in the development of colorimetric biosensors. This has emerged as a potent and efficient approach for rapid detection, enabling the creation of innovative colorimetric sensing methodologies through the integration of nanozymes with colorimetric sensors. In this review, recent development in nanozyme research and their application in colorimetric biosensing of biotoxins are examined with an emphasis on their characteristics and performance. The study particularly focuses on the peroxidase (POD) activity, oxidase (OXD) activity, superoxide dismutase (SOD), and catalase (CAT) activity of nanozymes in colorimetric biosensors. Ultimately, the challenges and future prospects of these assays are explored.

Au@Pt nanoparticles-based signal-enhanced lateral flow immunoassay for ultrasensitive naked-eye detection of SARS-CoV-2

With SARS-CoV-2 N protein as a model target, a signal-enhanced LFIA based on Au@Pt nanoparticles (NPs) as labels is proposed. This Au@Pt NPs combined the distinguished localized surface plasma resonance (LSPR) effect of Au NPs and the ultrahigh peroxidase-like catalytic activity of Pt NPs. Au@Pt NPs could trigger substrate chromogenic reaction, generating a color signal orders of magnitude darker than their intrinsic color. In the detection, after the coloration of the strips, 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2 were added, and a dark blue chelate (OxTMB) was produced soon, enhancing the band color significantly. After the signal amplification, the naked-eye detection limit for N protein reached 40 pg/mL. The detection sensitivity enhanced more than 1000 times than that without signal amplification. Compared with mainstream LFIA requiring complex readout instruments, the Au@Pt-based LFIA achieved a comparable sensitivity using naked eyes detection. This point is crucial, especially for unprofessional users or low-resource areas. Hence, this signal-enhanced LFIA may serve as a sensitive, cost-effective, and user-friendly detection method. It can shorten the testing window period and help identify early infections.

A graphene oxide-assisted protein immobilization paper-tip immunosensor with smartphone and naked eye readout for the detection of okadaic acid

BACKGROUND

Okadaic acid (OA), as a diarrhetic shellfish poisoning, can increase the risk of acute carcinogenic or teratogenic effects for the ingestion of OA contaminated shellfish. At present, much effort has been made to graft immunoassay onto a paper substrate to make paper-based sensors for rapid and simple detection of shellfish toxin. However, the complicated washing steps and low protein fixation efficiency on the paper substrate need to be further addressed.

RESULTS

A novel paper-tip immunosensor for detecting OA was developed combined with smartphone and naked eye readout. The trapezoid paper tip was consisted of quantitative and qualitative detection zones. To improve the OA antigen immobilization efficiency on the paper substrate, graphene oxide (GO)-assisted protein immobilization method was introduced. Meanwhile, Au nanoparticles composite probe combined with the lateral flow washing was developed to simplify the washing step. The OA antigen-immobilized zone, as the detection zone Ⅰ, was used for quantitative assay by smartphone imaging. The paper-tip front, as the detection zone Ⅱ, which could qualitatively differentiate OA pollution level within 45 min using the naked eye. The competitive immunoassay on the paper tip exhibited a wide linear range for detecting OA (0.02-50 ng∙mL-1) with low detection limit of 0.02 ng∙mL-1. The recovery of OA in spiked shellfish samples was in the range of 90.3 %-113.%.

SIGNIFICANCE

These results demonstrated that the proposed paper-tip immunosensor could provide a simple, low-cost and high-sensitivity test for OA detection without the need for additional large-scale equipment or expertise. We anticipate that this paper-tip immunosensor will be a flexible and versatile tool for on-site detecting the pollution of marine products.

Study of Highly Efficient Au/Pt Nanoparticles for Rapid Screening of Clostridium difficile

This study synthesized core/shell gold-platinum nanoparticles and characterized their colorimetric properties; ultraviolet-visible spectroscopy revealed that the synthesized nanoparticles exhibited distinct colors from conventional gold nanoparticles. Furthermore, the nanoparticles were subjected to lateral flow assays using Protein A, and the results revealed that they outperformed conventional spherical gold nanoparticles in terms of color development. This improvement can be attributed to the distinct core/shell structures of our nanoparticles. Further evaluation revealed that these nanoparticles could facilitate the detection of Clostridium difficile Toxin B visually at an extremely low concentration (1 ng/mL) without the requirement for advanced instrumentation. This substantial improvement in sensitivity can be attributed to the meticulous design and nanoscale engineering of the structure of the nanoparticles.

An AgPd NP-based lateral flow immunoassay for simultaneous detection of glycocholic acid and alpha-fetoprotein

Hepatocellular carcinoma (HCC) remains a leading cause of cancer-related mortality globally, ranking third in cancer deaths. Early diagnosis of HCC markers is imperative for effective prognosis and treatment. This study explores the utility of glycocholic acid (GCA) and alpha-fetoprotein (AFP) as biomarkers for liver diseases, with a specific focus on their simultaneous detection for enhanced diagnostic and prognostic capabilities. Harnessing the benefits of lateral flow immunoassay (LFIA), such as operational simplicity, speed, and accuracy, we engineered AgPd nanocomposites with antibodies targeting GCA and AFP. Under the optimized conditions, the visual detection limit for GCA was established at 50 ng mL-1 and the cut-off value at 104 ng mL-1. And for AFP, the visual detection limit was 0.1 ng mL-1 and the cut-off value was 500 ng mL-1. The accuracy and feasibility of the strips were validated through the detection of 39 actual serum samples. The results highlight the potential of LFIA as a rapid and effective tool for clinical diagnosis. The developed LFIA method not only demonstrates accuracy and feasibility but also presents a promising avenue for the early diagnosis of hepatocellular carcinoma.

Highly Sensitive Immunochromatographic Detection of Porcine Myoglobin as Biomarker for Meat Authentication Using Prussian Blue Nanozyme

This study was aimed at the sensitive immunodetection of porcine myoglobin (MG) as a species-specific biomarker in meat products. The enhanced lateral flow immunoassay (LFIA) was created in the sandwich format using monoclonal antibodies (Mab) with specificity to porcine MG and labeled by Prussian blue nanoparticles (PBNPs) as peroxidase-mimicking nanozymes. Signal amplification was provided by the colored product of oxidation catalyzed by the PBNPs. Several Mab-PBNP conjugates with different antibody loads were synthesized; the one that provided the best analytical characteristics of the LFIA was selected. Advanced optimization of the test system was carried out. As a result, the visual limit of detection (LOD) of MG was 1.5 ng/mL. Involvement of the catalytic nanozyme properties allowed the LOD to be decreased by ~9 times in comparison to the LFIA based on gold nanomarkers, and by ~27 times compared to the LFIA based on PBNP coloration. The assay time was 30 min, including catalytic enhancement. A simple technique of meat sample pre-treatment aimed at effective MG extraction and matrix disposal was proposed. The specificity of the LFIA towards the pork meat was demonstrated. The applicability of the created test system was shown by testing extracts obtained from finished meat products.

Post-Assay Chemical Enhancement for Highly Sensitive Lateral Flow Immunoassays: A Critical Review

Lateral flow immunoassay (LFIA) has found a broad application for testing in point-of-care (POC) settings. LFIA is performed using test strips-fully integrated multimembrane assemblies containing all reagents for assay performance. Migration of liquid sample along the test strip initiates the formation of labeled immunocomplexes, which are detected visually or instrumentally. The tradeoff of LFIA's rapidity and user-friendliness is its relatively low sensitivity (high limit of detection), which restricts its applicability for detecting low-abundant targets. An increase in LFIA's sensitivity has attracted many efforts and is often considered one of the primary directions in developing immunochemical POC assays. Post-assay enhancements based on chemical reactions facilitate high sensitivity. In this critical review, we explain the performance of post-assay chemical enhancements, discuss their advantages, limitations, compared limit of detection (LOD) improvements, and required time for the enhancement procedures. We raise concerns about the performance of enhanced LFIA and discuss the bottlenecks in the existing experiments. Finally, we suggest the experimental workflow for step-by-step development and validation of enhanced LFIA. This review summarizes the state-of-art of LFIA with chemical enhancement, offers ways to overcome existing limitations, and discusses future outlooks for highly sensitive testing in POC conditions.

Synthesis of Fe-Doped Peroxidase Mimetic Nanozymes from Natural Hemoglobin for Colorimetric Biosensing and In Vitro Anticancer Effects

Despite their efficiency and specificity, the instability of natural enzymes in harsh conditions has inspired researchers to replace them with nanomaterials. In the present study, extracted hemoglobin from blood biowastes was hydrothermally converted to catalytically active carbon nanoparticles (BDNPs). Their application as nanozymes for the colorimetric biosensing of H₂O₂ and glucose and selective cancer cell-killing ability was demonstrated. Particles that were prepared at 100 °C (BDNP-100) showed the highest peroxidase mimetic activity, with Michaelis-Menten constants (K_m) of 11.8 mM and 0.121 mM and maximum reaction rates (V_max) of 8.56 × 10^-8 mol L^-1 s^-1 and 0.538 × 10^-8 mol L^-1 s^-1, for H₂O₂ and TMB, respectively. The cascade catalytic reactions, catalyzed by glucose oxidase and BDNP-100, served as the basis for the sensitive and selective colorimetric glucose determination. A linear range of 50-700 µM, a response time of 4 min, a limit of detection (3σ/N) of 40 µM, and a limit of quantification (10σ/N) of 134 µM was achieved. In addition, the reactive oxygen species (ROS)-generating ability of BDNP-100 was employed for evaluating its potential in cancer therapy. Human breast cancer cells (MCF-7), in the forms of monolayer cell cultures and 3D spheroids, were studied by MTT, apoptosis, and ROS assays. The in vitro cellular experiments showed dose-dependent cytotoxicity of BDNP-100 toward MCF-7 cells in the presence of 50 µM of exogenous H₂O₂. However, no obvious damage was induced to normal cells in the same experimental conditions, verifying the selective cancer cell-killing ability of BDNP-100.

Mismatched duplexed aptamer-isothermal amplification-based nucleic acid-nanoflower for fluorescent detection of okadaic acid

We developed a highly sensitive fluorescent assay to detect okadaic acid (OA), a prevalent aquatic toxin posing serious health risks. Our approach uses a mismatched duplexed aptamer (DA) immobilized on streptavidin-conjugated magnetic beads (SMBs) to create a DA@SMB complex. In the presence of OA, the cDNA unwinds, hybridizes with a G-rich segment pre-encoding circular template (CT), and undergoes rolling circle amplification (RCA) to produce G-quadruplexes, which are detected using the fluorescent dye thioflavine T (ThT). The method has a LOD of 3.1 × 10^-3 ng/mL, a linear range of 0.1 ∼ 1.0 × 10³ ng/mL, and was successfully applied to shellfish samples with spiked recoveries of 85.9% ∼ 102.2% and RSD less than 13%. Furthermore, instrumental analysis confirmed the accuracy and reliability of this rapid detection method. Overall, this work represents a significant advancement in the field of rapid aquatic toxin detection and has important implications for public health and safety.

Single-Atom Fe Nanozyme with Enhanced Oxidase-like Activity for the Colorimetric Detection of Ascorbic Acid and Glutathione

Single-atom nanozymes (SAzymes) have drawn ever-increasing attention due to their maximum atom utilization efficiency and enhanced enzyme-like activity. Herein, a facile pyrolysis strategy is reported for the synthesis of the iron-nitrogen-carbon (Fe-N-C) SAzyme using ferrocene trapped within porous zeolitic imidazolate framework-8 (ZIF-8@Fc) as a precursor. The as-prepared Fe-N-C SAzyme exhibited exceptional oxidase-mimicking activity, catalytically oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) with high affinity (K_m) and fast reaction rate (V_max). Taking advantage of this property, we designed two colorimetric sensing assays based on different interaction modes between small molecules and Fe active sites. Firstly, utilizing the reduction activity of ascorbic acid (AA) toward oxidized TMB (TMBox), a colorimetric bioassay for AA detection was established, which exhibited a good linear range of detection from 0.1 to 2 μM and a detection limit as low as 0.1 μM. Additionally, based on the inhibition of nanozyme activity by the thiols of glutathione (GSH), a colorimetric biosensor for GSH detection was constructed, showing a linear response over a concentration range of 1-10 μM, with a detection limit of 1.3 μM. This work provides a promising strategy for rationally designing oxidase-like SAzymes and broadening their application in biosensing.

A Sensitive Fluorescence Polarization Immunoassay for the Rapid Detection of Okadaic Acid in Environmental Waters

In this study, a homogeneous fluorescence polarization immunoassay (FPIA) for the detection of hazardous aquatic toxin okadaic acid (OA) contaminating environmental waters was for the first time developed. A conjugate of the analyte with a fluorophore based on a fluorescein derivative (tracer) was synthesized, and its interaction with specific anti-OA monoclonal antibodies (MAbs) was tested. A MAbs-tracer pair demonstrated highly affine immune binding (KD = 0.8 nM). Under optimal conditions, the limit of OA detection in the FPIA was 0.08 ng/mL (0.1 nM), and the working range of detectable concentrations was 0.4-72.5 ng/mL (0.5-90 nM). The developed FPIA was approbated for the determination of OA in real matrices: river water and seawater samples. No matrix effect of water was observed; therefore, no sample preparation was required before analysis. Due to this factor, the entire analytical procedure took less than 10 min. Using a compact portable fluorescence polarization analyzer enables the on-site testing of water samples. The developed analysis is very fast, easy to operate, and sensitive and can be extended to the determination of other aquatic toxins or low-molecular-weight water or food contaminants.