New Functional Tracer-Two-Dimensional Nanosheet-Based Immunochromatographic Assay for Salmonella enteritidis Detection

Multifunctional magnetic tags with photocatalytic and enzyme-mimicking properties for constructing a sensitive dual-readout ELISA

ELISA has become the gold standard for detecting harmful substances due to its specific antibody recognition and sensitive enzyme-catalyzed reactions. In this study, multifunctional magnetic Prussian blue nanolabels (MPBNs) were synthesized using a simple gentle two-step method to achieve a dual-readout mode. The MPBNs provide a sensitive colorimetric signal by efficiently catalyzing the oxidation of TMB and exhibit prominent photocatalytic degradation activity towards Rhodamine B (RhB). Supplemented by the quenching effect of oxTMB, the fluorescence was enabled to serve as a sensitive second signal. The magnetic property of the labels facilitates the separation and enrichment of the target, thereby improving sensitivity. Utilizing the versatile MPBNs, the visual limit of detection (vLOD) for Staphylococcus aureus is as low as 100 CFU/mL, with a quantitative analysis range of 102-108 CFU/mL. The introduction of photocatalytic reactions into immunoassay has opened up a new signal response system with strong momentum for development and application.

Nanosheet antibody mimics based label-free and dual-readout lateral flow immunoassay for Salmonella enteritidis rapid detection

Portable devices for on-site foodborne pathogens detection are urgently desirable. Lateral flow immunoassay (LFIA) provides an efficient strategy for pathogens detection, however, antibody labeling independence and detection reliability, are still challenging. Here, we report the development of a label-free LFIA with dual-readout using glucan-functionalized two-dimensional (2D) transition metal dichalcogenides (TMDs) tungsten disulfide (WS₂) as detection probes for sensitive detection of Salmonella enteritidis (S. enteritidis). In particular, glucan-functionalized WS₂, synthesized via liquid exfoliation, are reliable detection antibody candidates which served as antibody mimics for bacteria capturing. This LFIA has not only eliminated the intricate antibody labeling process and screening of paired antibodies in conventional LFIAs, but also promised dual-readout (colorimetric/Raman) for flexible detection. Under optimized conditions, this LFIA achieves selective detection of S. enteritidis with a low visual detection limit of 10³ CFU/mL and a broad linear range of 10³-10⁸ CFU/mL. Additionally, the LFIA could be successfully applied in drinking water and milk with recoveries of 85%-109%. This work is desirable to expand the application of 2D TMDs in biosensors and offers a brand-new alternative protocol of detection antibodies in foodborne pathogens detection.

Joint-detection of Salmonella typhimurium and Escherichia coli O157:H7 by an immersible amplification dip-stick immunoassay

To explore the superiority of multifunctional nanocomposites and realize the joint-detection of foodborne pathogens, an immersible amplification dip-stick immunoassay (DSIA) was exploited for the sensitive detection of Salmonella typhimurium (S. typhi) and Escherichia coli O157:H7 (E. coli O157:H7). Saving for the basic colorimetric performance, the reporter molecule of CoFe₂O₄ (CFO) possesses multivalent elements (Co^2+/3+, Fe^2+/3+) as well as multifunction of superior catalase-like activity and magnetic properties. By dint of the catalytic activity of CFO, a directly immersible amplification can be simply achieved to endure the DSIA with an intensive signal and a dual-visible mode for the determination of S. typhi and E. coli O157:H7. In virtue of the magnetic separation and enrichment capability of the CFO, the DSIA can perform a matrix-interference-free detection and obtain a dynamic detection range of 10²-10⁸ CFU/mL and a low assay limit of 10² CFU/mL. Moreover, the DSIA has reasonable recovery rates for contamination monitoring of two target bacteria in milk and beef samples. Our research provides a persuasive supplement for the application of multifunctional nanocomposites in the ongoing dip-stick immunoassay and an alternative strategy for the efficient detection of foodborne pathogens.

Vanadium Disulfide Nanosheet Boosts Optical Signal Brightness as a Superior Enzyme Label to Improve the Sensitivity of Lateral Flow Immunoassay

The color-enzyme lateral flow immunoassay (LFIA) has attracted widespread attention to expand the detection range and improve sensitivity via amplifying the color signal after catalyzing the substrate. As a kind of layered transition-metal dichalcogenide (TMD), the vanadium disulfide nanosheet (VS₂NS) possesses superior peroxidase-like catalytic activity. Here, a VS₂NS was applied as an enzyme label in the LFIA to detect 17β-estradiol (E2). Compared to natural horseradish peroxidase, the VS₂NS expresses a more prominent enzyme catalytic performance, stability, and adsorption ability. Under optimal conditions, the calculated limit of detection (cLOD) of the VS₂NS-based LFIA is 0.065 ng mL^-1 for E2, which is sixfold lower than that of the optimized colloidal nanoparticle-based LFIA (cLOD = 0.406 ng mL^-1). Besides, the detection linear range of the VS₂NS-based LFIA can be widened by 1.5 times after the catalytic reaction. Moreover, the VS₂NS-based LFIA exhibits excellent practicability in real sample detection. Simultaneously, this study helps open up the application of the VS₂NS in the trace analysis of LFIAs, which can broaden TMDs' scope of application and better show their properties of color enzymes.

Graphene-labeled synthetic antigen as a novel probe for enhancing sensitivity and simplicity in lateral flow immunoassay

Lateral flow immunoassay (LFIA), which combines immune-specific recognition properties with sensitive nano-signaling features, has emerged as an excellent tool for point-of-care testing (POCT) in food safety and clinical diagnosis. Exploring novel probes with a simple preparation process, improved signal intensity and good stability is conducive to the development and application of LFIA. Herein, a potent non-antibody probe, graphene-labeled synthetic antigen (G-Ag), was created for LFIA, in which graphene endowed a naked-eye visual colorimetric signal with high sensitivity, and the synthetic antigen competed with the target for binding to the antibody on the test line. During the G-Ag probe manufacturing process, only one simple mixing step was needed because graphene nanosheets presented a strong adsorption capacity toward the protein (BSA) on the synthetic antigen, significantly saving time, labor and cost. Especially, the synthetic antigen forms a fabulous probe element without the need for antibody, and thus the proposed LFIA avoids the destruction of antibody activity, and exhibits excellent sensitivity and stability. After optimization, LFIA was successfully applied to analyze clenbuterol; the lowest visually detectable concentration was 0.1 ng mL^-1, and the probe could be well-applied in pork, mutton, sausage and bacon samples, demonstrating favorable specificity and repeatability. Owing to the advantages of simplicity, non-antibody probe, sensitivity and reliability, G-Ag probe-based LFIA has application potential for small-molecule target monitoring and rapid detection.

"Lighting-up" methylene blue-embedded zirconium based organic framework triggered by Al3+ for advancing the sensitivity of E. coli O157:H7 analysis in dual-signal lateral flow immunochromatographic assay

The sensitive detection of foodborne pathogens is of great significance for ensuring food safety and quality. Herein, on the basis of methylene blue-embedded zirconium based organic framework (UIO@MB) as the remarkable capture carrier and signal indicator, with the Al³⁺-assisted the fluorescent signal response, we developed a label-free and dual-signal lateral flow immunochromatographic assay (LDLFIA) for sensitive detection of Escherichia coli (E. coli) O157:H7. The UIO@MB sensing carrier without monoclonal antibodies (mAbs) was manufactured, which adhered to bacteria to form the UIO@MB-E. coli O157:H7 conjugate, resulting in visible blue band. Then the fluorescent response of the OH-rich UIO@MB was excited by introducing Al³⁺, arising from capturing of Al³⁺ by -OH through coordination and electrostatic affinity, thus generating a green fluorescent band. Impressively, a smartphone-based portable reading system was developed that can reflect the test results of UIO@MB-LDLFIA immediately. Under optimum conditions, UIO@MB-LDLFIA can complete colorimetric and fluorescent mode detection within 90 min, with a detection sensitivity of 10³ CFU/mL, which were 100 times lower than traditional gold nanoparticles-based LFIA and polymerase chain reaction (PCR). Moreover, the feasibility of the method was further evaluated by the determination of E. coli O157: H7 in drinking water and cabbage with average recoveries of 85.1-123.0%.

A Universal Bacterial Catcher Au-PMBA-Nanocrab-Based Lateral Flow Immunoassay for Rapid Pathogens Detection

In traditional lateral flow immunoassays (LFIA) for pathogens detection, capture antibody (CA) is necessary and usually conjugated to Au nanoparticles (NPs) in order to label the target analyte. However, the acquisition process of the Au-CA nanoprobe is relatively complicated and costly, which will limit the application of LFIA. Herein, p-mercaptophenylboronic acid-modified Au NPs (namely Au-PMBA nanocrabs), were synthesized and applied for a new CA-independent LFIA method. The stable Au-PMBA nanocrabs showed outstanding capability to capture both Gram-negative bacteria and Gram-positive bacteria through covalent bonding. The acquired Au-PMBA-bacteria complexes were dropped onto the strip, and then captured by the detection antibody on the test line (T-line). Take Escherichia coli O157:H7 as an example, the gray value of T-line was proportional to the bacteria concentration and the linear range was 10³-10⁷ cfu·mL^-1. This CA-independent strategy exhibited higher sensitivity than the traditional CA-dependent double antibody sandwich method, because detection limit of the former one was 10³ cfu·mL^-1 only by visual observation, which was reduced by 3 orders of magnitude. Besides, this platform successfully screened E. coli O157:H7 in four food samples with recoveries ranging from 90.25% to 107.25%. This CA-independent LFIA showed great advantages and satisfactory potential for rapid foodborne pathogens detection in real samples.

'Three-To-One' multi-functional nanocomposite-based lateral flow immunoassay for label-free and dual-readout detection of pathogenic bacteria

Sandwich lateral flow immunoassays (LFIAs) based on paired antibodies are the most frequently used platform for food-borne pathogen detection. Although label-free strategies are used in LFIAs to avoid the utilization of paired antibodies, challenges of probe design and detection reliability still remain. Here, we report a new label-free and dual-readout LFIA (LD-LFIA) mediated by a 'Three-To-One' multi-functional nanocomposite with a unique combination of magnetic-adhesion-color-nanozyme properties. The strengths of the new designed nanocomposite are: (i) the Fe₃O₄ magnetic core simplifies the separation processes; (ii) surface adherent polydopamine (PDA) films exhibit a strong adhesion to pathogenic bacteria and provide colorimetric detection signal; and (iii) the deposited platinum nanoparticles (Pt NPs) can function as nanozymes to generate an extra catalytic signal for constructing a dual-readout mode to improve the detection accuracy. The resulting Fe₃O₄@PDA@Pt nanocomposite-based LD-LFIA can detect highly pathogenic Escherichia coli O157:H7 with limits of detection of 10² and 10 CFU mL^-1 for colorimetric and catalytic quantitative analyses, respectively. Systematic results also reveal that the proposed method exhibited high specificity and applicability for drinking water and chicken samples, serving as a promising tool for real bacterial sample testing. The multi-functional Fe₃O₄@PDA@Pt nanocomposite-based LD-LFIA can provide new ideas for designing new multi-functional probes for improving detection performance of conventional label-free LFIA and constructing more accurate and sensitive detection systems.

Mercury ion-engineering Au plasmonics on MoS2 layers for absorption-shifted optical sensors

Semiconducting MoS₂ layers offer the electrons, reducing conjugated Au(I) to Au atoms, and sebsequently serve as desirable substrates for supporting the interfacial growths of gold nanostructures. Au-covering MoS₂ heterostructures perform morphology-varied optical characteristics, and the surface engineering of MoS₂ involved by Hg²⁺ ions results in the differential growths of nanostructures and morphological diversities. Naked-eye colorimetric responses to mercury ions, with a low limit of detection of 1.27 nM, are achieved based on the in situ grown heterostructures.

Overview of Rapid Detection Methods for Salmonella in Foods: Progress and Challenges

Salmonella contamination in food production and processing is a serious threat to consumer health. More and more rapid detection methods have been proposed to compensate for the inefficiency of traditional bacterial cultures to suppress the high prevalence of Salmonella more efficiently. The contamination of Salmonella in foods can be identified by recognition elements and screened using rapid detection methods with different measurable signals (optical, electrical, etc.). Therefore, the different signal transduction mechanisms and Salmonella recognition elements are the key of the sensitivity, accuracy and specificity for the rapid detection methods. In this review, the bioreceptors for Salmonella were firstly summarized and described, then the current promising Salmonella rapid detection methods in foodstuffs with different signal transduction were objectively summarized and evaluated. Moreover, the challenges faced by these methods in practical monitoring and the development prospect were also emphasized to shed light on a new perspective for the Salmonella rapid detection methods applications.

Universal fluorescence nanoprobes to enhance the sensitivity of immunochromatographic assay for detection of 17β-estradiol in milk

The pollution caused by estrogens in the environment and food has received increasing attention. It is still challenging for on-site immunochromatographic assay (ICA) detection of estrogens. The performance of the prepared probes plays a decisive role in the sensitivity and stability of the ICA system. The published probes usually directly couple the detection antibody to the label, ignoring the influence of the label on the activity of the antibody. In this study, 17β-estradiol (E₂) was used as a model analyte for the ICA system. Two universal probes were constructed based on quantum dot nanobeads (QBs), recombinant protein A (SPA, from Staphylococcus aureus), and rabbit anti-mouse immunoglobulin G antibody (anti-IgG). The probes were prepared by coupling QBs with SPA, releasing anti-E₂ monoclonal antibody (mAb), and maintaining its activity. The prepared universal probes can orient recognize the Fc region of mAb and fully expose its Fab region, improving the detection sensitivity of the ICA system. The free anti-E₂ mAb and the universal probe (QBs@SPA or QBs@SPA@anti-IgG) were used as the detection antibodies and signal donors, respectively. The results show that the proposed ICA based on QBs@SPA and QBs@SPA@anti-IgG probes could detect E₂ with IC₅₀ of 8.83 and 0.93 ng/mL, respectively, within 15 min under optimal conditions. The recovery results of ICA based on QBs@SPA and QBs@SPA@anti-IgG probes showed good agreement with the findings of the high-performance liquid chromatography (HPLC) analysis for spiked samples. The developed ICA system based on universal probes was superior in terms of sensitivity, rapidity, and applicability, and held great promise for its implementation in detecting environmental and food small-molecule pollutants.

Nanomaterial application in bio/sensors for the detection of infectious diseases

Infectious diseases are a potential risk for public health and the global economy. Fast and accurate detection of the pathogens that cause these infections is important to avoid the transmission of the diseases. Conventional methods for the detection of these microorganisms are time-consuming, costly, and not applicable for on-site monitoring. Biosensors can provide a fast, reliable, and point of care diagnostic. Nanomaterials, due to their outstanding electrical, chemical, and optical features, have become key players in the area of biosensors. This review will cover different nanomaterials that employed in electrochemical, optical, and instrumental biosensors for infectious disease diagnosis and how these contributed to enhancing the sensitivity and rapidity of the various sensing platforms. Examples of nanomaterial synthesis methods as well as a comprehensive description of their properties are explained. Moreover, when available, comparative data, in the presence and absence of the nanomaterials, have been reported to further highlight how the usage of nanomaterials enhances the performances of the sensor.

(Nano)tag-antibody conjugates in rapid tests

Antibodies (Abs) are naturally derived materials with favorable affinity, selectivity, and fast binding kinetics to the respective antigens, which enables their application as promising recognition elements in the development of various types of biosensors/bioassays, especially in rapid tests. These tests are low-cost and easy-to-use biosensing devices with broad applications including medical or veterinary diagnostics, environmental monitoring and industrial usages such as safety and quality analysis in food, providing on-site quick monitoring of various analytes, making it possible to save analysis costs and time. To reach such features, the conjugation of Abs with various nanomaterials (NMs) as tags is necessary, which range from conventional gold nanoparticles to other nanoparticles recently introduced, where magnetic, plasmonic, photoluminescent, or multi-modal properties play a critical role in the overall performance of the analytical device. In this context, to preserve the Ab affinity and provide a rapid response with long-term storage capability, the use of efficient bio-conjugation techniques is critical. Thanks to their prominent role in rapid tests, many studies have been devoted to the design and development of Abs-NMs conjugates with various chemistries including passive adsorption, covalent coupling, and affinity interactions. In this review, we present the state-of-the-art techniques allowing various Ab-NM conjugates with a special focus on the efficiency of the developed probes to be employed in in vitro rapid tests. Challenges and future perspectives on the development of Ab-conjugated nanotags in rapid diagnostic tests are highlighted along with a survey of the progress in commercially available Ab-NM conjugates.

Ultrasensitive label-free immunochromatographic strip sensor for Salmonella determination based on salt-induced aggregated gold nanoparticles

Here we present an innovative label-free immunochromatographic strip (ICTS) sensor, in which salt-induced aggregated gold nanoparticles (SIA-AuNPs) act as the signal probe, allowing in 14 min the identification and sensitive quantification of Salmonella as model targets. It has been evidenced that SIA-AuNPs could be absorbed on the surface of bacteria based on van der Waals forces. The SIA-AuNPs@Salmonella complex was captured by anti-Salmonella polyclonal antibody deposited on the test zone. With the label-free ICTS sensor, we successfully detected Salmonella in a concentration range of 10³-10⁸ CFU/mL and a visual detection limit of 1 × 10³ CFU/mL. The band of test zone could be distinguished at a concentration of 10³ CFU/mL by naked eye, which is 100-fold lower than the cationic AuNPs based method. The strip sensor was further validated with real samples including cabbage and drinking water with excellent precision and showed to provide excellent recovery.

Graphene biosensors for bacterial and viral pathogens

The infection and spread of pathogens (e.g., COVID-19) pose an enormous threat to the safety of human beings and animals all over the world. The rapid and accurate monitoring and determination of pathogens are of great significance to clinical diagnosis, food safety and environmental evaluation. In recent years, with the evolution of nanotechnology, nano-sized graphene and graphene derivatives have been frequently introduced into the construction of biosensors due to their unique physicochemical properties and biocompatibility. The combination of biomolecules with specific recognition capabilities and graphene materials provides a promising strategy to construct more stable and sensitive biosensors for the detection of pathogens. This review tracks the development of graphene biosensors for the detection of bacterial and viral pathogens, mainly including the preparation of graphene biosensors and their working mechanism. The challenges involved in this field have been discussed, and the perspective for further development has been put forward, aiming to promote the development of pathogens sensing and the contribution to epidemic prevention.

Gold nanoparticles-functionalized three-dimensional flower-like manganese dioxide: A high-sensitivity thermal analysis immunochromatographic sensor

A sensitive photothermal immunochromatographic test strip (PITS) for the detection of deoxynivalenol (DON) was developed using flower-like gold nanoparticle-deposited manganese dioxide nanocarrier (FMD-G NC) labeled antibodies (Abs) as the photothermal-sensing probe. FMD was used as a template to deposit small gold nanoparticles (GNPs) to synthesize FMD-G NC with large specific surface area and significant photothermal conversion property. The FMD-G-Ab probe was competitively captured by DON target and antigen coated on test line (T-line), forming colorimetric signals under naked eyes and photothermal signals under an 808 nm laser. Under optimal conditions, the PITS exhibited sensitive and specific detection of DON from 0.19 ng mL^-1 to 12 ng mL^-1 with detection limits of 0.013 ng mL^-1, which were over 15-fold and 58-fold more sensitive than visual FMD-G-ITS and traditional GNPs-ITS. In addition, the novel FMD-G-PITS possessed a universal applicability, which could be well applied in green bean, corn, and millet.

Metal-polydopamine framework based lateral flow assay for high sensitive detection of tetracycline in food samples

Gold nanoparticles (AuNPs)-based lateral flow assay (LFA) enables a rapid detection of tetracycline (TET) in food samples but suffers from low sensitivity. Herein, metal-polydopamine framework (MPF), as a label, was employed to load monoclonal antibodies (mAbs) directly as the probe in LFA for highly sensitive sensing of TET. Combining zeolitic imidazolate framework (ZIF-67) and polydopamine (PDA), a stable MPF with large size, well water-dispersible, excellent affinity and optical properties was prepared through a versatile layer-by-layer assembly (LLA) strategy. Under optimized conditions, the biosensor (MPF-LFA) exhibited a great linear relationship in the range of 0.09-6 ng/mL and a detection limit of 0.045 ng/mL for TET detection, which was over 66-fold more sensitive than traditional AuNPs based LFA. Furthermore, the MPF-LFA was successfully applied to the screening of TET in fish, chicken, milk and shrimp samples with satisfied recoveries from 91% to 114%.

Functional nanozyme mediated multi-readout and label-free lateral flow immunoassay for rapid detection of Escherichia coli O157:H7

To overcome the drawbacks of antibody labeling dependence and single-readout system in the conventional lateral flow immunoassays (LFIAs) as well as the non-targeted combination of new capture agents reported recently for pathogen detection, in this work, a multi-readout and label-free LFIA was proposed for rapid detection of Escherichia coli O157:H7 (E. coli O157:H7) based on a nanozyme-bacteria-antibody sandwich pattern. A type of functional nanozyme-mannose modified Prussian blue (man-PB), was introduced as the recognition agent as well as signal indicator. Apart from original signal intensity on the T-line, the peroxidase-like catalytic activity-driven generation of colorimetric signal could be used as another format of quantitation. Importantly, such LFIA could exhibit excellent performance for target pathogens detection separately with a quantitative range of 10²-10⁸ cfu·mL^-1 and a low detection limit of 10² cfu·mL^-1 based on different readout formats, indicating the application potential of the proposed LFIA in real samples.

Development of Nanozymes for Food Quality and Safety Detection: Principles and Recent Applications

The public concerns about agrifood safety call for innovative and reformative analytical techniques to meet the inspection requirements of high sensitivity, specificity, and reproducibility. Enzyme-mimetic nanomaterials or nanozymes, which combine enzyme-like properties with nanoscale features, emerge as an excellent tool for quality and safety detection in the agrifood sector, due to not only their robust capacity in detection but also their attraction in future-oriented exploitations. However, in-depth understanding about the fundamental principles of nanozymes for food quality and safety detection remains limited, which makes their applications largely empirical. This review provides a comprehensive overview of the principles, designs, and applications of nanozyme-based detection technique in the agrifood industry. The discussion mainly involves three mimicking types, that is, peroxidase, oxidase, and catalase-like nanozymes, capable of detecting major agrifood analytes. The current principles and strategies are classified and then discussed in details through discriminating the roles of nanozymes in diverse detection platforms. Thereafter, recent applications of nanozymes in detecting various endogenous ingredients and exogenous contaminants in foods are reviewed, and the outlook of profound developments are explained. Evidenced by the increasing publications, nanozyme-based detection techniques are narrowing the gap to practical-oriented food analytical methods, while some challenges in optimization of nanozymes, diversification of recognition-to-signal manners, and sustainability of methodology need to conquer in the future.

Dual recognition strategy and magnetic enrichment based lateral flow assay toward Salmonella enteritidis detection

As a rapid and facile means for foodborne bacteria detection in situ, lateral flow immunoassay (LFA) still has intrinsic limitations in the construction of the existing sandwich LFA format, e.g. screening difficulties of paired antibodies (Abs), poor stability of Ab probe, etc. Here, combined the strong affinity of antibiotic with the superior specificity of antibody molecules, a novel and robust LFA based on a dual recognition strategy and magnetic separation was designed to achieve specific and sensitive determination of Salmonella enteritidis (S. enteritidis). In this work, ampicillin (Amp), a broad-spectrum antibiotic against bacteria, was employed as an ideal Ab replacer to anchor cells of target bacteria. By coating Amp on magnetite nanoparticles (MNPs), the Amp-MNPs showed remarkable binding, separation and enrichment capacities toward bacteria even under complex sample matrices. To ensure the selectivity of this protocol, anti-S. enteritidis monoclonal antibody was then adopted as the second anchoring agent to form a sandwich complex with Amp-MNPs. Based on these facts, S. enteritidis, as low as 10²-10³ CFU/mL, could be detected by naked eyes in food samples. Therefore, this creative antibiotic-bacteria-antibody LFA sandwich pattern shows great application potential in the monitoring of food contamination and infectious diseases caused by pathogenic bacteria. Compared to the common paired Abs based sandwich method, the proposed approach was cost-effective, non-labor intensive, stable, sensitive and efficient.