Plasmonic Enzyme-Linked Immunosorbent Assay Using Nanospherical Brushes as a Catalase Container for Colorimetric Detection of Ultralow Concentrations of Listeria monocytogenes

Phage-induced "one-to-many" FRET sensor for highly sensitive detection of Escherichia coli O157:H7

As a foodborne pathogen capable of causing severe illnesses, early detection of Escherichia coli O157:H7 (E. coli O157:H7) is crucial for ensuring food safety. While Förster resonance energy transfer (FRET) is an efficient and precise detection technique, there remains a need for amplification strategies to detect low concentrations of E. coli O157:H7. In this study, we presented a phage (M13)-induced "one to many" FRET platform for sensitively detecting E. coli O157:H7. The aptamers, which specifically recognize E. coli O157:H7 were attached to magnetic beads as capture probes for separating E. coli O157:H7 from food samples. The peptide O157S, which specifically targets E. coli O157:H7, and streptavidin binding peptide (SBP), which binds to streptavidin (SA), were displayed on the P3 and P8 proteins of M13, respectively, to construct the O157S-M13K07-SBP phage as a detection probe for signal output. Due to the precise distance (≈3.2 nm) between two neighboring N-terminus of P8 protein, the SA-labeled FRET donor and acceptor can be fixed at the Förster distance on the surface of O157S-M13K07-SBP via the binding of SA and SBP, inducing FRET. Moreover, the P8 protein, with ≈2700 copies, enabled multiple FRET (≈605) occurrences, amplifying FRET in each E. coli O157:H7 recognition event. The O157S-M13K07-SBP-based FRET sensor can detect E. coli O157:H7 at concentration as low as 6 CFU/mL and demonstrates excellent performance in terms of selectivity, detection time (≈3 h), accuracy, precision, practical application, and storage stability. In summary, we have developed a powerful tool for detecting various targets in food safety, environmental monitoring, and medical diagnosis.

CLICK-FLISA Based on Metal-Organic Frameworks for Simultaneous Detection of Fumonisin B1 (FB1) and Zearalenone (ZEN) in Maize

Mycotoxins are secondary products produced primarily by fungi and are pathogens of animals and cereals, not only affecting agriculture and the food industry but also causing great economic losses. The development of rapid and sensitive methods for the detection of mycotoxins in food is of great significance for livelihood issues. This study employed an amino-functionalized zirconium luminescent metal-organic framework (LOF) (i.e., UiO-66-NH2). Click chemistry was utilized to assemble UiO-66-NH2 in a controlled manner, generating LOF assemblies to serve as probes for fluorescence-linked immunoassays. The proposed fluoroimmunoassay method for Zearalenone (ZEN) and Fumonisin B1 (FB1) detection based on the UiO-66-NH2 assembled probe (CLICK-FLISA) afforded a linear response range of 1-20 μmol/L for ZEN, 20 μmol/L for FB1, and a very low detection limit (0.048-0.065 μmol/L for ZEN; 0.048-0.065 μmol/L for FB1). These satisfying results demonstrate promising applications for on-site quick testing in practical sample analysis. Moreover, the amino functionalization may also serve as a modification strategy to design luminescent sensors for other food contaminants.

Rapid separation and detection of Listeria monocytogenes with the combination of phage tail fiber protein and vancomycin-magnetic nanozyme

In this work, a lateral flow assay for Listeria monocytogenes was developed based on phage tail fiber protein (TFP) and triple-functional nanozyme probes with capture-separation-catalytic activity. Inspired by interaction between phage and bacteria, TFP of L. monocytogenes phage was immobilized on test line as capture molecule, which replaced traditional antibody and aptamer. After Gram-positive bacteria was captured and separated from samples by nanozyme probes modified with vancomycin (Van), TFP specifically recognized L. monocytogenes and overcame non-specific binding of Van. Special color reaction between Coomassie Brilliant Blue and bovine serum albumin which was an amplification carrier on probe was simply utilized as control zone to replace traditional control line. Relying on enzyme-like catalytic activity of nanozyme, this biosensor realized improved sensitivity and colorimetric quantitative detection with a detection limit of 10 CFU mL^-1. Analytic performance results suggested this TFP-based biosensor provided a portable, sensitive and specific strategy to detect pathogen.

Plasmon-Mediated Peroxidase-like Activity on an Asymmetric Nanotube Architecture for Rapid Visual Detection of Bacteria

Rapid and sensitive detection of bacteria from a complex real media remains a challenge. Herein, we report a visual bacterial sensing assay with excellent specificity, anti-interference ability, and sensitivity based on a surface plasmon resonance (SPR)-enhanced peroxidase (POD) mimetic. The POD mimetic based on Pt nanoparticles (NPs) asymmetrically decorated on Au/TiO₂ magnetic nanotubes (Au/Pt/MTNTs) is designed by combining the intrinsic photocatalytic activity of TiO₂ and the limited transport depth of light. It is revealed that the localized surface plasmon resonance (LSPR) effect of the asymmetric nanotubes is more effective in facilitating the generation of hot electrons, which are subsequently transferred to Pt and MTNTs, thus greatly promoting the catalytic performance. Using Staphylococcus aureus (S. aureus) as a model of Gram-positive bacteria, the dependence of the colorimetric reaction on the active sites of the POD mimetic is used for the sensing of target bacteria. Owing to the specific recognition between S. aureus and peptide, the fluorescein isothiocyanate (FITC) labeled peptide probes are captured by S. aureus and removed from the Au/Pt/MTNTs, leading to the recovery of POD-like activity and fluorescence emission of S. aureus. Particularly, benefiting from the Au-SPR effect and the magnetic feature of the Au/Pt/MTNTs, the recovery of catalytic activity induced an improved colorimetric assay with a wider linear response for S. aureus qualification and a detection limit of four cells, as well as satisfactory selectivity and feasibility for application in real samples. The plasmon-enhanced POD activity would provide a simple-yet-effective approach to achieve a colorimetric bioassay with high efficiency and sensitivity. This asymmetric design can also be utilized to engineer nanozymes in colorimetric assays for the specific detection of biotoxins, biomarkers, and cancer cells.

Shape dependent sensing potential of gold nanoparticles in etching based multicolorimetric plasmonic-ELISA

In the present study, a systematic investigation has been carried out for the first time to assess the potential of three different shapes of gold nanoparticles (AuNPs), viz. nanorods (AuNRs), nanotriangles (AuNTs), and nanospheres (AuNSs), to develop a horseradish peroxidase (HRP) enzyme-mediated etching-based plasmonic ELISA (p-ELISA) strategy. The etching of the AuNPs in ELISA is achieved by 3'-3-5'-5-tetramethylbenzidine (TMB²⁺), which is produced by the biocatalytic conversion of chromogenic TMB via HRP. All three types of AuNPs were interacted with varying concentrations of TMB²⁺ (7-131 μM) (product of HRP enzyme reaction) and characterized for visible color change and by UV-Vis spectroscopy and transmission electron microscopy (TEM). From the comparative analysis of all three shapes of AuNPs, AuNRs exhibited vivid visible color change and absorbance intensity change compared to spherical and triangle-shaped nanoparticles. The TEM analysis of the etched nanoparticles revealed the gradual etching pattern of AuNRs compared to AuNTs which resulted in multicolor generation as opposed to AuNTs where the etching was relatively very fast and thus shows a faster shape transformation and poor color discrimination. Further, the potential of the AuNR etching-based optimized strategy was successfully demonstrated to develop an indirect competitive p-ELISA for human IgG detection. The developed p-ELISA showed an ultra-low visual limit of detection of 1 fg mL^-1 (∼6.54 aM) without the aid of any sophisticated instruments. In the future, the developed competitive p-ELISA strategy can be easily employed to develop cost-effective, portable, and point-of-care assays for the detection of various disease biomarkers with ultra-high sensitivity.

Designed protein multimerization and polymerization for functionalization of proteins

Abstract

Multimeric and polymeric proteins are large biomacromolecules consisting of multiple protein molecules as their monomeric units, connected through covalent or non-covalent bonds. Genetic modification and post-translational modifications (PTMs) of proteins offer alternative strategies for designing and creating multimeric and polymeric proteins. Multimeric proteins are commonly prepared by genetic modification, whereas polymeric proteins are usually created through PTMs. There are two methods that can be applied to create polymeric proteins: self-assembly and crosslinking. Self-assembly offers a spontaneous reaction without a catalyst, while the crosslinking reaction offers some catalyst options, such as chemicals and enzymes. In addition, enzymes are excellent catalysts because they provide site-specificity, rapid reaction, mild reaction conditions, and activity and functionality maintenance of protein polymers. However, only a few enzymes are applicable for the preparation of protein polymers. Most of the other enzymes are effective only for protein conjugation or labeling. Here, we review novel and applicable strategies for the preparation of multimeric proteins through genetic modification and self-assembly. We then describe the formation of protein polymers through site-selective crosslinking reactions catalyzed by enzymes, crosslinking reactions of non-natural amino acids, and protein-peptide (SpyCatcher/SpyTag) interactions. Finally, we discuss the potential applications of these protein polymers.

Graphical abstract

Urchin-like PtNPs@Bi2S3: synthesis and application in electrochemical biosensor

Novel urchin-like Pt nanoparticles@Bi2S3 composite materials were prepared by a simple route. The composite nanomaterial was used to modify an electrode for the immobilization of enzyme molecules to construct a sensitive electrochemical biosensor.

Colorimetric determination of Listeria monocytogenes using aptamer and urease dual-labeled magnetic nanoparticles and cucurbit[7]uril-mediated supramolecular assembly of gold nanoparticle

A host-guest colorimetric strategy is described for the detection of Listeria monocytogenes (L. monocytogenes). The optical probes were self-assembled based on the supramolecular interactions between the carbonyl groups of cucurbit[7]uril portals and gold nanoparticles (CB[7]-AuNPs). Aptamer and urease modified magnetic nanoparticles were used to specifically recognize and binding to L. monocytogenes, simultaneously hydrolyzing urea to produce ammonium ion (NH₄⁺) that can reverse CB[7] induced AuNPs aggregation. In the presence of L. monocytogenes, the above-mentioned magnetic conjugates preferentially bind to the bacterial surface, which results in blocking the catalytic active sites, thus inhibiting the production of ammonium ions. The normalized absorbance ratio of A_{700 nm}/A_{525 nm} was proportional to the L. monocytogenes concentration ranging from 10 to 10⁶ cfu·mL^-1, and the visual determination can be done down to 10 cfu·mL^-1. For spiked food samples analyzed without pre-enrichment, recoveries of 98.4% to 99.3% were achieved could be verified and RSD were less than 10%. This work may offer a broad prospect for sensitive and specific determination  of pathogens.

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.

Sensitive, general and portable detection of RNAs combining duplex-specific nuclease transduction with an off-shelf signalling platform

We report a novel RNA sensing platform by combining the RNA:DNA hybridization, duplex-specific nuclease (DSN) amplification, and personal glucose meter (PGM) readout. The so-called DSN-PGM sensing platform is sensitive, specific and general to both microRNA and long virus RNA. The detection procedure is simple and instrument-free, thus holding particular potential in the development of portable and point-of-care measurements.

Real-time, selective, and low-cost detection of trace level SARS-CoV-2 spike-protein for cold-chain food quarantine

Due to the friendly temperature for virus survival, SARS-CoV-2 is frequently found in cold-chain foods, posing a serious threat to public health. Utilizing an interdigitated microelectrode chip modified with an antibody probe and integrating dielectrophoresis enrichment with interfacial capacitance sensing, a strategy is presented for the detection of trace level spike-protein from SARS-CoV-2. It achieves a limit of detection as low as 2.29 × 10^-6 ng/mL in 20 s, with a wide linear range of 10^-5-10^-1 ng/mL and a selectivity of 234:1. The cost for a single test can be controlled to ~1 dollar. This strategy provides a competitive solution for real-time, sensitive, selective, and large-scale application in cold-chain food quarantine.

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.

Hedgehog-like Bi2S3 nanostructures: a novel composite soft template route to the synthesis and sensitive electrochemical immunoassay of the liver cancer biomarker

A new route was proposed to synthesize novel hedgehog-like Bi₂S₃nanostructure using CTAB-trimellitic acid as a composite soft template and thiourea as the sulfur source, which was used  to construct an effective electrochemical AFP immunosensor.

Controlled copper in situ growth-amplified lateral flow sensors for sensitive, reliable, and field-deployable infectious disease diagnostics

A polyethyleneimine (PEI)-assisted copper in-situ growth (CISG) strategy was proposed as a controlled signal amplification strategy to enhance the sensitivity of gold nanoparticle-based lateral flow sensors (AuNP-LFS). The controlled signal amplification is achieved by introducing PEI as a structure-directing agent to regulate the thermodynamics of anisotropic Cu nanoshell growth on the AuNP surface, thus controlling shape and size of the resultant AuNP@Cu core-shell nanostructures and confining free reduction and self-nucleation of Cu2+ for improved reproducibility and decreased false positives. The PEI-CISG-enhanced AuNP-LFS showed ultrahigh sensitivities with the detection limits of 50 fg mL-1 for HIV-1 capsid p24 antigen and 6 CFU mL-1 for Escherichia coli O157:H7. We further demonstrated its clinical diagnostic efficacy by configuring PEI-CISG into a commercial AuNP-LFS detection kit for SARS-CoV-2 antibody detection. Altogether, this work provides a reliable signal amplification platform to dramatically enhance the sensitivity of AuNP-LFS for rapid and accurate diagnostics of various infectious diseases.

Hydrangea-like mesoporous WO3 nanoflowers with crystalline framework for 3-hydroxy-2-butanone sensing

In this study, a simple and efficient strategy for the construction of hydrangea-like mesoporous WO₃ nanoflowers templated using diblock copolymer PS₁₁₉-PtBA₁₂₉ was developed. The nanoflower shows good gas sensing properties, especially for 3-hydroxy-2-butanone (3H-2B), which is the signature metabolite of Listeria monocytogenes (L. monocytogenes). Therefore, the gas sensing of 3H-2B by hydrangea-like mesoporous WO₃ nanoflowers can be used to detect L. monocytogenes. In the case of 25 ppm 3H-2B as target gas, the response (R_a/R_g) of the hydrangea-like mesoporous WO₃ nanoflowers at 205 °C is 152, where R_a and R_g are the resistances of the sensing device in air and target gas, respectively, and the response and recovery times at 25 ppm are 25 s and 146 s, respectively. Schematic illustration of the formation of hydrangea-like mesoporous WO₃ nanoflowers and its gas sensing implication.

Water-soluble ZnCuInSe quantum dots for bacterial classification, detection, and imaging

Bacteria are everywhere and pose severe threats to human health and safety. The rapid classification and sensitive detection of bacteria are vital steps of bacterial community research and the treatment of infection. Herein, we developed optical property-superior and heavy metal-free ZnCuInSe quantum dots (QDs) for achieving rapid discrimination of Gram-positive/Gram-negative bacteria by the naked eye; driven by the structural differences of bacteria, ZnCuInSe QDs are effective in binding to Gram-positive bacteria, especially Staphylococcus aureus (S. aureus), in comparison with Gram-negative bacteria and give discernable color viewed by the naked eye. Meanwhile, based on its distinctive fluorescence response, the accurate quantification of S. aureus was investigated with a photoluminescence system in the concentration ranges of 1 × 10³ to 1 × 10¹¹ CFU/mL, with a limit of detection of 1 × 10³ CFU/mL. Furthermore, we demonstrated the feasibility of ZnCuInSe QDs as a fluorescence probe for imaging S. aureus. This simple strategy based on ZnCuInSe QDs provides an unprecedented step for rapid and effective bacterial discrimination, detection, and imaging.

Engineering of a Dual-Recognition Ratiometric Fluorescent Nanosensor with a Remarkably Large Stokes Shift for Accurate Tracking of Pathogenic Bacteria at the Single-Cell Level

Rapid, accurate, reliable, and risk-free tracking of pathogenic microorganisms at the single-cell level is critical to achieve efficient source control and prevent outbreaks of microbial infectious diseases. For the first time, we report a promising approach for integrating the concepts of a remarkably large Stokes shift and dual-recognition into a single matrix to develop a pathogenic microorganism stimuli-responsive ratiometric fluorescent nanoprobe with speed, cost efficiency, stability, ultrahigh specificity, and sensitivity. As a proof-of-concept, we selected the Gram-positive bacterium Staphylococcus aureus (S. aureus) as the target analyte model, which easily bound to its recognition aptamer and the broad-spectrum glycopeptide antibiotic vancomycin (Van). To improve the specificity and short sample-to-answer time, we employed classic noncovalent π-π stacking interactions as a driving force to trigger the binding of Van and aptamer dual-functionalized near-infrared (NIR) fluorescent Apt-Van-QDs to the surface of an unreported blue fluorescent π-rich electronic carbon nanoparticles (CNPs), achieving S. aureus stimuli-responsive ratiometric nanoprobe Apt-Van-QDs@CNPs. In the assembly of Apt-Van-QDs@CNPs, the blue CNPs (energy donor) and NIR Apt-Van-QDs (energy acceptor) became close to allow the fluorescence resonance energy transfer (FRET) process, leading to a remarkable blue fluorescence quenching for the CNPs at ∼465 nm and a clear NIR fluorescence enhancement for Apt-Van-QDs at ∼725 nm. In the presence of S. aureus, the FRET process from CNPs to Apt-Van-QDs was disrupted, causing the nanoprobe Apt-Van-QDs@CNPs to display a ratiometric fluorescent response to S. aureus, which exhibited a large Stokes shift of ∼260 nm and rapid sample-to-answer detection time (∼30.0 min). As expected, the nanoprobe Apt-Van-QDs@CNPs showed an ultrahigh specificity for ratiometric fluorescence detection of S. aureus with a good detection limit of 1.0 CFU/mL, allowing the assay at single-cell level. Moreover, we also carried out the precise analysis of S. aureus in actual samples with acceptable results. We believe that this work offers new insight into the rational design of efficient ratiometric nanoprobes for rapid on-site accurate screening of pathogenic microorganisms at the single-cell level in the early stages, especially during the worldwide spread of COVID-19 today.

Chemiluminescence assay for Listeria monocytogenes based on Cu/Co/Ni ternary nanocatalyst coupled with penicillin as generic capturing agent

Bacterial pathogen control is important in seafood production. In this study, a Cu/Co/Ni ternary nanoalloy (Cu/Co/Ni TNA) was synthesized using the oleylamine reducing method. It was found that Cu/Co/Ni TNA greatly enhanced the chemiluminescence (CL) signal of the hydroxylamine-O-sulfonic acid (HOSA)-luminol system. The CL properties of Cu/Co/Ni TNA were investigated systemically. The possible CL mechanism also was intensively investigated. Based on the enhanced CL phenomenon of Cu/Co/Ni TNA, a Cu/Co/Ni TNA, penicillin, and anti-L. monocytogenes (Listeria monocytogenes) antibody-based sandwich complex assay for detection of L. monocytogenes was established. In this sandwich CL assay, penicillin was employed to capture and enrich pathogenic bacteria with penicillin-binding proteins (PBPs) while anti-L. monocytogenes antibody was adopted as the specific recognition molecule to recognize L. monocytogenes. L. monocytogenes was detected sensitively based on this new Cu/Co/Ni TNA-HOSA-luminol CL system. The CL intensity was proportional to the L. monocytogenes concentration ranging from 2.0 × 10² CFU ml^-1 to 3.0 × 10⁷ CFU ml^-1 and the limit of detection wa 70 CFU ml^-1 . The reliability and potential applications of our method was verified by comparison with official methods and recovery tests in environment and food samples.

Stochastic DNA Dual-Walkers for Ultrafast Colorimetric Bacteria Detection

Rapid, sensitive, and reliable pathogen detection is growing in importance in human health and safety. In this work, we report a stochastic DNA dual-walker-based colorimetric biosensor for bacterial detection. In the presence of target bacteria, two kinds of released multiple walking strands are allowed for continuous walking on the Au nanoparticle (AuNP)-based 3D track, resulting in destabilized aggregation of AuNP-based probes. The induced color change from red to blue can serve as an analytical signal for colorimetric detection of target bacteria. We demonstrated that this mothed enables sensitive and specific bacterial detection within 15 min due to its ultrafast reaction kinetics and sensitive color change, showing a linear response ranging from 10⁰ to 10⁵ CFU/mL with a limit of detection of 1 CFU/mL. Moreover, we also realized analysis of practical samples using this colorimetric biosensor. Given its features of rapid, sensitive, specific, and reliable analysis, our stochastic dual-walker-based colorimetric biosensor shows much promise in point-of-care testing for bacteria detection.

Layer-by-layer assembly of magnetic-core dual quantum dot-shell nanocomposites for fluorescence lateral flow detection of bacteria

Lateral flow immunoassay (LFA) strips are extensively used for rapid tests of various biochemical molecules, but these strips still have some limitations in bacterial detection due to their low sensitivity and poor stability in complex samples. In this study, we reported a highly sensitive and quantitative fluorescent LFA strip for bacterial detection by using novel magnetic-core@dual quantum dot (QD)-shell nanoparticles (Fe3O4@DQDs) as multifunctional fluorescent labels. The Fe3O4@DQDs were prepared through a polyethyleneimine (PEI)-mediated layer-by-layer (LBL) assembly method, and they possess monodispersity, high magnetic responsiveness, good stability, and superior fluorescence properties. Based on these merits, the Fe3O4@DQDs were used to capture and enrich bacteria from complex samples and then used as advanced fluorescent labels of LFA strips for the quantitative detection of bacteria. Under optimal conditions, the assay ultra-sensitively detected Streptococcus pneumoniae with a low limit of detection of 8 cells per mL and a wide dynamic linear range of 10 cells per mL to 107 cells per mL. Systematic comparison revealed that the fluorescence detection limit of the Fe3O4@DQD-based strip was 55 and 1000 times higher than those of Fe3O4-core@QD-shell nanocomposite (Fe3O4-QD)-based and conventional QD microsphere-based strips, respectively. The proposed method also exhibited high specificity and selectivity for biological samples (human whole blood and sputum) and is thus a promising tool for real bacterial sample testing.

A novel fluorescence method for the rapid and effective detection of Listeria monocytogenes using aptamer-conjugated magnetic nanoparticles and aggregation-induced emission dots

The sensitive and specific detection of L. monocytogenes through immunomagnetic separation and fluorescence response produced by recognition of IgG-coated TPE-OH@BSA nanoparticles.

Three-dimensional ring-oven washing technique for a paper-based immunodevice

Washing is a standard step for enzyme-linked immunosorbent assays (ELISA) performed on a paper-based chip, in which nonspecific-binding antibodies and antigens should be removed completely from the paper surface. In this study, a novel three-dimensional (3D) washing strategy using a heating ring-oven was carried out on a paper-based chip. Compared with a plane washing mode by a ring-oven, this 3D washing strategy obtained a lower background, as gravity played an important role in the washing step. The paper-based chip was placed on a 3D plastic holder and the waste area was connected to a heating ring. Use of a heating waste area meant that the nonspecific-binding protein was continuously carried to the waste area through gravity and capillary action. The angle between the plastic holder and the ring plane was carefully selected. The effect of washing on different parts of the detection area was investigated by upconversion fluorescence and chemiluminescence (CL). This novel 3D washing strategy was performed for carcinoembryonic antigen detection through CL and a lower detection limit of 2 pg ml^-1 was obtained. This approach provides an effective washing strategy to remove nonspecific-binding antibody from a paper-based immunodevice.

Plasmonic rK28 ELISA improves the diagnosis of canine Leishmania infection

In this study, we evaluated the performance of a new enzyme-linked immunosorbent assay (ELISA) variant known as indirect "plasmonic ELISA" (pELISA) for the detection of Leishmania spp. infection. Serum samples from 170 dogs from an area where canine leishmaniosis (CanL) is endemic and from 26 healthy dogs from a nonendemic area were tested by indirect pELISA, and the results were compared to those of an indirect ELISA (both with recombinant antigen rK28) and those of an immunochromatographic test (dual-path platform, TR-DPP®) using real-time PCR on blood samples or conjunctival swabs as the gold standard. The pELISA, indirect rK28 ELISA and the TR-DPP® immunochromatographic test presented sensitivities of 94.7%, 89.5% and 79.0% and specificities of 100%, 92.7% and 91.5%, respectively. The analysis of the results revealed that the specificity of the indirect pELISA was greater than that of the method recommended by the Ministry of Health in Brazil and may increase the feasibility of diagnosis in resource-constrained countries because it does not require sophisticated instruments to read. Thus, this method can be used as an additional tool for the detection of Leishmania spp. infection in these areas.

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.

Fluorescence immunoassay through histone-ds-poly(AT)-templated copper nanoparticles as signal transductors for the sensitive detection of Salmonella choleraesuis in milk

The rapid and sensitive detection of foodborne pathogens is one of the most important issues in food safety control. In this work, we developed a novel fluorescence immunoassay method for the sensitive detection of Salmonella choleraesuis. The method uses the fluorescent signals of histone-ds-poly(AT)-templated copper nanoparticles (His-pAT CuNP) as signal transducers and glucose oxidase as an alternative for horseradish peroxidase for the generation of hydrogen peroxide (H₂O₂) through the catalysis of glucose. The H₂O₂ is then further converted into hydroxyl radical (·OH) by Fenton reagents. Owing to the ultrahigh sensitivity of His-pAT CuNP synthesis toward ·OH, the proposed fluorescence immunoassay method exhibited excellent sensitivity for S. choleraesuis, with a limit of detection of 8.04 × 10¹ cfu/mL, which is 3 orders of magnitude lower than that of the tetramethylbenzidine-based traditional immunoassay. The reliability of the proposed method was evaluated by using spiked milk samples with S. choleraesuis concentration ranging from 8.8 × 10¹ to 8.8 × 10⁴ cfu/mL. The average recoveries for the intra- and inter-assay ranged from 73.52 to 96.59% and from 66.99 to 98.24% with a coefficient of variation from 6.85 to 31.26% and 5.46 to 17.99%, respectively. These results indicated that the proposed fluorescence immunoassay possesses a great potential for ultra-sensitive detection of foodborne pathogens in food safety control.

An immunofiltration strip method based on the photothermal effect of gold nanoparticles for the detection of Escherichia coli O157:H7

An immunofiltration strip is a promising rapid and powerful tool for the routine monitoring of foodborne pathogenic bacteria. However, the strip assay is generally less sensitive than other immunological techniques. A novel immunofiltration strip method based on the photothermal effect of gold nanoparticles (GNPs) was developed for the sensitive, rapid, simple handheld and low-cost detection of Escherichia coli O157:H7. The photothermal effect of GNPs was used to achieve the amplification of signals to improve the sensitivity of the strip method. The thermal contrasts caused by the photothermal effect were proportional to the bacteria concentrations and yield an almost logarithmic relation. Under optimal conditions, the detection limit was 1.95 × 104 CFU mL-1, and the sensitivity was improved about ten times compared to that of the conventional visual strip method. In brief, the photothermal based immunofiltration strip could be used for the rapid and sensitive detection of other pathogens as a great potential food quality control technique.

Nanomaterials-Based Colorimetric Immunoassays

Colorimetric immunoassays for tumor marker detection have attracted considerable attention due to their simplicity and high efficiency. With the achievements of nanotechnology and nanoscience, nanomaterials-based colorimetric immunoassays have been demonstrated to be promising alternatives to conventional colorimetric enzyme-linked immunoassays. This review is focused on the progress in colorimetric immunoassays with the signal amplification of nanomaterials, including nanomaterials-based artificial enzymes to catalyze the chromogenic reactions, analyte-induced aggregation or size/morphology change of nanomaterials, nanomaterials as the carriers for loading enzyme labels, and chromogenic reactions induced by the constituent elements released from nanomaterials.

Dual-mode fluorescent and colorimetric immunoassay for the ultrasensitive detection of alpha-fetoprotein in serum samples

We present a novel dual-mode fluorescent and colorimetric immunosensor based on conventional immunoassay platforms by utilizing a gold nanoflower (AuNF)-loaded fluorescein molecule (AuNF@Fluorescein) as signal output. The AuNFs were modified with thiolated carboxyl ligand, which consisted of a hydrophobic alkane chain as hydrophobic wallet for fluorescein encapsulation, a tetra (ethylene glycol) unit for biocompatibility and solubility, and a functional carboxyl group for the conjugation of biorecognition molecules for biosensing. The resultant AuNFs showed a high loading capacity of 3.74 × 10⁶ fluorescein molecules per AuNF because of its flower-like shape with many complex branches. By adjusting the solution pH to 8.0, the fluorescein molecules can almost entirely be released from the hydrophobic wallet of AuNF@Fluorescein, which led to strong fluorescent-signal amplification. Under the optimal detection conditions, the proposed immunoassay based on fluorescent signal exhibited ultrahigh sensitivity for alpha-fetoprotein (AFP) detection, with a limit of detection (LOD) of 29 fg/mL. This value is approximately 9.3 × 10³-fold lower than that of corresponding horseradish peroxidase (HRP)-based immunoassay (LOD = 270 pg/mL). The fluorescein molecule also had intrinsic peroxidase-like activity to catalyze 3,3',5,5'-tetramethylbenzidine oxidation with hydrogen peroxide for colorimetric signal. The proposed method with colorimetric mode further exhibited a sensitivity with a LOD of 17.7 pg/mL, which is about 15-fold lower than that of conventional HRP-based immunoassay. The recoveries of the proposed dual-mode immunoassay for AFP spiked serum samples ranged within 89.85%-100.0%, with the coefficient of variations ranging from 0.5% to 2.4%, indicating acceptable accuracy and precision for AFP quantitative detection. The reliability of the developed dual-mode immunoassay was further compared with a commercial chemiluminescence immunoassay kit by analyzing 20 clinical serum samples, showing that the two methods well agreed with each other, with high correlation coefficients of 0.997 and 0.986 based on recorded fluorescence and colorimetric signals, respectively. In summary, the proposed method was highly suitable for the ultrasensitive analysis of biomarkers or infectious diseases by fluorescence mode and can be used for routine clinical diagnosis by colorimetric mode.

Combined use of vancomycin-modified Ag-coated magnetic nanoparticles and secondary enhanced nanoparticles for rapid surface-enhanced Raman scattering detection of bacteria

Background

Pathogenic bacteria have always been a significant threat to human health. The detection of pathogens needs to be rapid, accurate, and convenient.

Methods

We present a sensitive surface-enhanced Raman scattering (SERS) biosensor based on the combination of vancomycin-modified Ag-coated magnetic nanoparticles (Fe₃O₄@Ag-Van MNPs) and Au@Ag nanoparticles (NPs) that can effectively capture and discriminate bacterial pathogens from solution. The high-performance Fe₃O₄@Ag MNPs were modified with vancomycin and used as bacteria capturer for magnetic separation and enrichment. The modified MNPS were found to exhibit strong affinity with a broad range of Gram-positive and Gram-negative bacteria. After separating and rinsing bacteria, Fe₃O₄@Ag-Van MNPs and Au@Ag NPs were synergistically used to construct a very large number of hot spots on bacteria cells, leading to ultrasensitive SERS detection.

Results

The dominant merits of our dual enhanced strategy included high bacterial-capture efficiency (>65%) within a wide pH range (pH 3.0–11.0), a short assay time (<30 min), and a low detection limit (5×10² cells/mL). Moreover, the spiked tests show that this method is still valid in milk and blood samples. Owing to these capabilities, the combined system enabled the sensitive and specific discrimination of different pathogens in complex solution, as verified by its detection of Gram-positive bacterium Escherichia coli, Gram-positive bacterium Staphylococcus aureus, and methicillin-resistant S. aureus.

Conclusion

This method has great potential for field applications in food safety, environmental monitoring, and infectious disease diagnosis.

Biotin-exposure-based immunomagnetic separation coupled with nucleic acid lateral flow biosensor for visibly detecting viable Listeria monocytogenes

Infectious diseases caused by Listeria monocytogenes pose a great threat to public health worldwide. Therefore, a rapid and efficient method for L. monocytogenes detection is needed. In this study, a biotin-exposure-based immunomagnetic separation (IMS) method was developed. That is, biotinylated antibody was first targeted to L. monocytogenes. Then, streptavidin-functionalized magnetic nanoparticles were added and anchored onto L. monocytogenes cells indirectly through the strong noncovalent interaction between streptavidin and biotin. Biotin-exposure-based IMS exhibited an excellent capability to enrich L. monocytogenes. Specifically, more than 90% of L. monocytogenes was captured when the bacterial concentration was lower than 10⁴ colony-forming units (CFU)/mL. Importantly, the antibody dosage was reduced by 10 times of that in our previous study, which used antibody direct-conjugated magnetic nanoparticles. Propidium monoazide (PMA) treatment prior to PCR amplification could eliminate the false-positive results from dead bacteria and detected viable L. monocytogenes sensitively and specifically. For viable L.monocytogenes detection, enriched L. monocytogenes was treated with PMA prior to asymmetric PCR amplification. The detection limits of the combined IMS with nucleic acid lateral flow (NALF) biosensor for viable L. monocytogenes detection were 3.5 × 10³ CFU/mL in phosphate buffer solution and 3.5 × 10⁴ CFU/g in lettuce samples. The whole assay process of recognizing viable L. monocytogenes was completed within 6 h. The proposed biotin-exposure-mediated IMS combined with a disposable NALF biosensor platform posed no health risk to the end user, and possessed potential applications in the rapid screening and identification of foodborne pathogens.

Circular exponential amplification of photoinduced electron transfer using hairpin probes, G-quadruplex DNAzyme and silver nanocluster-labeled DNA for ultrasensitive fluorometric determination of pathogenic bacteria

The authors describe a fluorometric strategy for the detection of pathogenic bacteria with ultrasensitivity and high specificity. This strategy relies on the combination of target-modulated photoinduced electron transfer (PET) between G-quadruplex DNAzyme and DNA (labeled with silver nanoclusters) along with hairpin probe-based circular exponential amplification. The reaction system involves three hairpin probes (H1, H2 and H3). Probe H1 contains an aptamer against S. Typhimurium and the recognition sequence for nicking endonuclease. It is used to recognize S. Typhimurium and participates in polymerase-catalyzed target recycle amplification and secondary-target recycle amplification. Probe H2 contains an aptamer against hemin and is used to form the G-quadruplex DNAzyme in the presence of hemin and potassium ion. It acts as the electron acceptor and quenches the fluorescence of the labeled DNA. Fluorescence is best measured at excitation/emission wavelengths of 567/650 nm. Probe H3 contains the template sequence for the synthesis of AgNCs and the H2-annealing sequence. Both H2 and H3 are utilized to perform a strand displacement reaction and to achieve PET between G-quadruplex DNAzyme and DNA/AgNCs. To the best of our knowledge, this is the first example of a PET between G-quadruplex DNAzyme and DNA/AgNCs coupled with circular exponential amplification. The assay has an ultra-low detection limit 8 cfu·mL^-1 of S. Typhimurium. The assay is rapid, accurate, inexpensive and simple. Hence, the strategy may represent a useful platform for ultrasensitive and highly specific detection of pathogenic bacteria as encountered in food analysis and clinical diagnosis. Graphical abstract The reaction system includes three hairpin probes (H1, H2 and H3), primer probe (P), Phi 29 DNA ploymerase (Phi 29) and nicking endonuclease Nt.AlwI (Nt.AlwI). Phi 29 and Nt.AlwI -assisted signal amplification leads to the recycling of target and produces numerous single stranded-DNAs (S). Strand displacement amplification leads to photoinduced electron transfer (PET) between G-quadruplex DNAzyme and DNA/AgNCs. HAP-based circular exponential amplification of PET results in an ultrasensitive fluorometric assay.

Simple and Sensitive Quantification of MicroRNAs via PS@Au Microspheres-Based DNA Probes and DSN-Assisted Signal Amplification Platform

Identifying the microRNA (miRNA) expression level can provide critical information for early diagnosis of cancers or monitoring the cancer therapeutic efficacy. This paper focused on a kind of gold-nanoparticle-coated polystyrene microbeads (PS@Au microspheres)-based DNA probe as miRNA capture and duplex-specific nuclease (DSN) signal amplification platform based on an RGB value readout for detection of miRNAs. In virtue of the outstanding selectivity and simple experimental operation, 5'-fluorochrome-labeled molecular beacons (MBs) were immobilized on PS@Au microspheres via their 3'-thiol, in the wake of the fluorescence quenching by nanoparticle surface energy transfer (NSET). Target miRNAs were captured by the PS@Au microspheres-based DNA probe through DNA/RNA hybridization. DSN enzyme subsequently selectively cleaved the DNA to recycle the target miRNA and release of fluorophores, thereby triggering the signal amplification with more free fluorophores. The RGB value measurement enabled a detection limit of 50 fM, almost 4 orders of magnitude lower than PS@Au microspheres-based DNA probe detection without DSN. Meanwhile, by different encoding of dyes, miRNA-21 and miRNA-10b were simultaneously detected in the same sample. Considering the ability for quantitation, high sensitivity, and convenient merits, the PS@Au microspheres-based DNA probe and DSN signal amplification platform supplied valuable information for early diagnosis of cancers.

Plasmon-Based Colorimetric Nanosensors for Ultrasensitive Molecular Diagnostics

Colorimetric detection of target analytes with high specificity and sensitivity is of fundamental importance to clinical and personalized point-of-care diagnostics. Because of their extraordinary optical properties, plasmonic nanomaterials have been introduced into colorimetric sensing systems, which provide significantly improved sensitivity in various biosensing applications. Here we review the recent progress on these plasmonic nanoparticles-based colorimetric nanosensors for ultrasensitive molecular diagnostics. According to their different colorimetric signal generation mechanisms, these plasmonic nanosensors are classified into two categories: (1) interparticle distance-dependent colorimetric assay based on target-induced forming cross-linking assembly/aggregate of plasmonic nanoparticles; and (2) size/morphology-dependent colorimetric assay by target-controlled growth/etching of the plasmonic nanoparticles. The sensing fundamentals and cutting-edge applications will be provided for each of them, particularly focusing on signal generation and/or amplification mechanisms that realize ultrasensitive molecular detection. Finally, we also discuss the challenge and give our future perspective in this emerging field.

Enhanced Fluorescence ELISA Based on HAT Triggering Fluorescence "Turn-on" with Enzyme-Antibody Dual Labeled AuNP Probes for Ultrasensitive Detection of AFP and HBsAg

At present, enzyme-linked immunosorbent assay (ELISA) is considered to be the most appropriate approach in clinical biomarker detection, with good specificity, low cost, and straightforward readout. However, unsatisfactory sensitivity severely hampers its wide application in clinical diagnosis. Herein, we designed a new kind of enhanced fluorescence enzyme-linked immunosorbent assay (FELISA) based on the human alpha-thrombin (HAT) triggering fluorescence "turn-on" signals. In this system, detection antibodies (Ab₂) and HAT were labeled on the gold nanoparticles (AuNPs) to form the detection probes, and a bisamide derivative of Rhodamine₁₁₀ with fluorescence quenched served as the substrate of HAT. After the sandwich immunoreaction, HAT on the sandwich structure could catalyze the cleavage of the fluorescence-quenched substrate, leading to a strong fluorescence signal for sensing ultralow levels of alpha fetoprotein (AFP) and hepatitis B virus surface antigen (HBsAg). Under the optimized reaction conditions, AFP and HBsAg were detected at the ultralow concentrations of 10^-8 ng mL^-1 and 5 × 10^-4 IU mL^-1, respectively, which were at least 10⁴ times lower than those of the conventional fluorescence assay and 10⁶ times lower than those of the conventional ELISA. In addition, we further discussed the efficiency of the sensitive FELISA in clinical serum samples, showing great potential in practical applications.

Sensitive detection of Listeria monocytogenes based on highly efficient enrichment with vancomycin-conjugated brush-like magnetic nano-platforms

Pathogens pose a significant threat to public health worldwide. Despite many technological advances in the rapid diagnosis of pathogens, sensitive pathogen detection remains challenging because target pathogenic bacteria usually exist in complex samples at very low concentrations. Here, the construction of multivalent brush-like magnetic nanoprobes and their application for the efficient enriching of pathogens are demonstrated. Brush-like magnetic nanoprobes were constructed by modification with poly-L-lysine (PLL) onto amino-modified magnetic beads, followed by coupling of PEG (amine-PEG₅₀₀₀-COOH) to the amine sites of PLL. Subsequently, vancomycin (Van), a small-molecule antibiotic with affinity to the terminal peptide (D-alanyl-D-alanine) on the cell wall of Gram-positive bacteria, was conjugated to the carboxyl of the PEG. The use of multivalent brush-like magnetic nanoprobes (Van-PEG-PLL-MNPs) results in a high enrichment efficiency (>94%) and satisfactory purity for Listeria monocytogenes (employed as a model) within 20min, even at bacterial concentrations of only 10²cfumL^-1. Integrated with the enrichment of the Van-PEG-PLL-MNP nano-platform and electrochemiluminescence (ECL) detection, Listeria monocytogenes can be rapidly and accurately detected at levels as low as 10cfumL^-1. The approach described herein holds great potential for realizing rapid and sensitive pathogen detection in clinical samples.

Enzyme-free colorimetric determination of EV71 virus using a 3D-MnO2-PEG nanoflower and 4-MBA-MA-AgNPs

We present a simple colorimetric assay for EV71 virus detection based on the aggregation of 4-mercaptobenzoic acid (4-MBA) and melamine (MA) modified silver nanoparticles (4-MBA-MA-AgNPs) in the presence of Mn²⁺. The EV71-Ab₁ was incubated on a 96-well plate and the EV71-Ab₂ was labeled on the surface of three-dimensional nanoflower-like MnO₂-PEG (3D-MnO₂-PEG). After layer-by-layer immunoreactions, the EV71 virus and the corresponding 3D-MnO₂-PEG-Ab₂ were captured on the plate. With the addition of Vitamin C (Vc), Mn²⁺ was released from the 3D-MnO₂-PEG and then the aggregation of the 4-MBA-MA-AgNPs was induced, allowing a naked-eye detection limit of EV71 virus to be as low as 5 × 10⁴ particles per mL, which is about three orders of magnitude lower than the conventional enzyme-linked immunosorbent assay (ELISA). This enzyme-free immunoassay based on a hybrid 3D-MnO₂ features signal amplification strategies via a simple reduction reaction.