Enzymatic Precipitation Enhanced Surface Plasmon Resonance Immunosensor for the Detection of Salmonella in Powdered Milk

Subtractive Inhibition Assay Based on PagN-Specific Monoclonal Antibody for the Detection of Salmonella Using Surface Plasmon Resonance

Salmonella is a common foodborne zoonotic pathogen that poses a great threat to human health and breeding industry. The rapid detection of Salmonella is necessary for early prevention and control. In this study, a subtractive inhibition assay (SIA) based on surface plasmon resonance (SPR) for the rapid detection of Salmonella was developed. Mouse-specific monoclonal antibody 3B3 against Salmonella membrane protein PagN was first incubated with Salmonella. The unbound free antibody was separated using a sequential process of centrifugation and then detected using an immobilized goat anti-mouse immunoglobulin G polyclonal antibody on the SPR sensor chip. This SIA-SPR method showed excellent sensitivity for Salmonella with a limit of detection of about 300 CFU mL-1. This method is sensitive to different serotypes of Salmonella strains but not for non-Salmonella strains. It was able to detect Salmonella in the contaminated water and milk powder at less than 102 and 103 CFU mL-1, respectively, which was consistent with the bacterial plate count results. In addition, this method could be used to evaluate the lysis effect of phages on bacteria. Since the culturing detection method needs more than 48 h, this method has the potential for the rapid and sensitive clinical detection of Salmonella. For our knowledge, this is the first report for Salmonella detection using SIA-SPR method.

Rapid detection of Salmonella in milk by labeling-free electrochemical immunosensor based on an Fe3O4-ionic liquid-modified electrode

Electrochemical sensors show distinct advantages over other types of sensors in the rapid detection of microorganisms. Here, we attempted to construct a label-free electrochemical immunosensor based on an Fe3O4-ionic liquid (IL)-modified electrode to rapidly detect Salmonella in milk. The excellent ionic conductivity of the IL facilitated sensor construction, and the large surface area of nano-Fe3O4 provided numerous sites for subsequent experiments. An antibody was fixed on the Fe3O4-IL complex with polyglutamic acid modification by a simple infusion method. The microstructure of the Fe3O4-IL composites was investigated by scanning electron microscopy, and the elements and structures of the composites were analyzed by energy dispersive X-ray and Fourier transform infrared spectroscopy. Under optimized experimental conditions, the detection range of the constructed sensor was 3.65 × 102-3.65 × 108 CFU mL-1, and the LOD was 1.12 × 102 CFU mL-1 (S/N = 3). In addition, the prepared electrochemical immunosensor is convenient for detecting foodborne pathogens because of its outstanding stability, good selectivity, and repeatability.

CRISPR-based biosensors for pathogenic biosafety

Pathogenic biosafety is a worldwide concern. Tools for analyzing pathogenic biosafety, that are precise, rapid and field-deployable, are highly demanded. Recently developed biotechnological tools, especially those utilizing CRISPR/Cas systems which can couple with nanotechnologies, have enormous potential to achieve point-of-care (POC) testing for pathogen infection. In this review, we first introduce the working principle of class II CRISPR/Cas system for detecting nucleic acid and non-nucleic acid biomarkers, and highlight the molecular assays that leverage CRISPR technologies for POC detection. We summarize the application of CRISPR tools in detecting pathogens, including pathogenic bacteria, viruses, fungi and parasites and their variants, and highlight the profiling of pathogens' genotypes or phenotypes, such as the viability, and drug-resistance. In addition, we discuss the challenges and opportunities of CRISPR-based biosensors in pathogenic biosafety analysis.

Smartphone surface plasmon resonance imaging for the simultaneous and sensitive detection of acute kidney injury biomarkers with noninvasive urinalysis

A surface plasmon resonance imaging (SPRi) platform integrated with a smartphone was constructed for the simultaneous and sensitive detection of acute kidney injury (AKI) biomarkers. The smartphone SPRi platform was developed without the requirement of additional light and power sources. The LED flash of the smartphone was used as the light source for the excitation of surface plasmon resonance of a gold sensor chip based on the Kretschmann configuration, while the reflected light was collected by the camera of the smartphone. This smartphone SPRi system was conveniently fabricated by 3D printing and showed a sensitivity of 1.78 × 10^-5 refractive index unit (RIU). In addition, based on a magnetic nanoparticle-enhanced sandwich immunoassay, the smartphone SPRi system with a gold array chip was employed for the detection of multiple AKI biomarkers, with a low limit of detection (LOD) of 0.19 ng ml^-1, 0.51 ng ml^-1 and 0.7 ng ml^-1 for the simultaneous detection of neutrophil gelatinase-associated lipocalin (NGAL), interleukin-18 (IL-18) and retinol-binding protein (RBP) in urine, respectively. The biosensors demonstrated high specificity and sensitivity for the simultaneous detection of multiple AKI biomarkers in PBST and urine. The smartphone SPRi system provided a portable and cost-effective platform for point-of-care diagnosis, in-field healthcare and environmental monitoring.

Toward intelligent food packaging of biosensor and film substrate for monitoring foodborne microorganisms: A review of recent advancements

Microorganisms in food do harms to human. They can cause serious adverse reactions and sometimes even death. So it is an urgent matter to find an effective method to control them. The research of intelligent- biosensor packaging is in the ascendant in recent years, which is mainly promoted by reflecting on food safety and reducing resource waste. Intelligent biosensor-packaging is an instant and efficient intelligent packaging technology, which can directly and scientifically manifest the quality of food without complex operation. In this review, the purposes of providing relevant information on intelligent biosensor-packaging are reviewed, such as types of biosensors for monitoring foodborne microorganism, the suitable material for intelligent biosensor-packaging and design and fabrication of intelligent biosensor-packaging. The potential of intelligent biosensor-packaging in the detection of foodborne microorganisms is emphasized. The challenges and directions of the intelligent biosensor-packaging in the detection of foodborne pathogens are discussed. With the development of science and technology in the future, the intelligent biosensor-packaging should be commercialized in a real sense. And it is expected that commercial products can be manufactured in the future, which will provide a far-reaching approach in food safety and food prevention. HighlightsSeveral biosensors are suitable for the detection of food microorganisms.Plastic polymer is an excellent choice for the construction of intelligent biosensor packaging.Design and fabrication can lay the foundation for intelligent-biosensor packaging.Intelligent biosensor-packaging can realize fast and real-time detection of microorganisms in food.

Boosting the detection performance of severe acute respiratory syndrome coronavirus 2 test through a sensitive optical biosensor with new superior antibody

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus emerged in late 2019 leading to the COVID-19 disease pandemic that triggered socioeconomic turmoil worldwide. A precise, prompt, and affordable diagnostic assay is essential for the detection of SARS-CoV-2 as well as its variants. Antibody against SARS-CoV-2 spike (S) protein was reported as a suitable strategy for therapy and diagnosis of COVID-19. We, therefore, developed a quick and precise phase-sensitive surface plasmon resonance (PS-SPR) biosensor integrated with a novel generated anti-S monoclonal antibody (S-mAb). Our results indicated that the newly generated S-mAb could detect the original SARS-CoV-2 strain along with its variants. In addition, a SARS-CoV-2 pseudovirus, which could be processed in BSL-2 facility was generated for evaluation of sensitivity and specificity of the assays including PS-SPR, homemade target-captured ELISA, spike rapid antigen test (SRAT), and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Experimentally, PS-SPR exerted high sensitivity to detect SARS-CoV-2 pseudovirus at 589 copies/ml, with 7-fold and 70-fold increase in sensitivity when compared with the two conventional immunoassays, including homemade target-captured ELISA (4 × 103 copies/ml) and SRAT (4 × 104 copies/ml), using the identical antibody. Moreover, the PS-SPR was applied in the measurement of mimic clinical samples containing the SARS-CoV-2 pseudovirus mixed with nasal mucosa. The detection limit of PS-SPR is calculated to be 1725 copies/ml, which has higher accuracy than homemade target-captured ELISA (4 × 104 copies/ml) and SRAT (4 × 105 copies/ml) and is comparable with qRT-PCR (1250 copies/ml). Finally, the ability of PS-SPR to detect SARS-CoV-2 in real clinical specimens was further demonstrated, and the assay time was less than 10 min. Taken together, our results indicate that this novel S-mAb integrated into PS-SPR biosensor demonstrates high sensitivity and is time-saving in SARS-CoV-2 virus detection. This study suggests that incorporation of a high specific recognizer in SPR biosensor is an alternative strategy that could be applied in developing other emerging or re-emerging pathogenic detection platforms.

Nanozyme-based cascade SPR signal amplification for immunosensing of nitrated alpha-synuclein

A self-assembled nanozyme of iron porphyrin mediated supramolecular modified gold nanoparticles (FpA) was fabricated to determine nitrated alpha-synuclein as the Tyr 39 residue (nT39 α-Syn) of a potential biomarker for early diagnosis of Parkinson's disease (PD). Mechanically, localized surface plasmon resonance (LSPR) and the mass effect caused by catalytic deposition of the nanozyme contributed to a cascade signal amplification strategy. The sensor allowed a signal amplification and selective nT39 α-Syn bioanalysis with a 1.34-fold enhancement by cascade amplified SPR signal and double specific recognition. The detection limit was 1.78 ng/mL in the detection range of 7-240 ng/mL. Benefiting from the excellent immunosensor, this method can distinguish healthy people and PD patients using actual samples. Overall, this strategy provides a nanozyme-based biosensing platform for the early diagnosis of PD and can be applied to detect other protein biomarkers, such as PD-L1.

Colorimetric detection of Salmonella typhimurium based on hexadecyl trimethyl ammonium bromide-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles

We developed an effective and specific colorimetric strategy to detect Salmonella typhimurium (S. typhimurium) based on hexadecyl trimethyl ammonium bromide (CTAB)-induced supramolecular assembly of β-cyclodextrin-capped gold nanoparticles (β-CD-AuNPs). In this study, ssDNA aptamer of S. typhimurium could combine with CTAB to form the supramolecular ssDNA-CTAB composite, so the ssDNA aptamer was applied to control the concentration of CTAB. In the presence of S. typhimurium, ssDNA aptamers selectively bound to S. typhimurium but not to CTAB, leading to the host-guest chemistry reaction of CTAB and β-CD resulting in β-CD-AuNP supramolecular assembly aggregation with an obvious color change. The ratio of absorption at 650 and 520 nm (A_650nm/A_520nm) has a linear correlation to the log scale of the concentration of the bacteria (1 × 10²-1 × 10⁷ CFU/mL) with a low limit of detection (LOD) of 13 CFU/mL. In addition, this optical sensor has good selectivity and practicability. In milk samples, the recovery was 93.55-111.32%, which suggested its potential application in real samples.

Isothermal RNA Amplification for the Detection of Viable Pathogenic Bacteria to Estimate the Salmonella Virulence for Causing Enteritis

Viable foodborne pathogens can cause intestinal infection and food poisoning. Herein, we reported an RNA assay allowing for sensitive (close to 1 CFU and 1% viable bacteria detectable) and rapid (within 2.5 h) detection of viable pathogenic bacteria by coupling isothermal RNA amplification (nucleic acid sequence-based amplification, NASBA) with a CRISPR/Cas13a system. NASBA allowed direct amplification of 16S rRNA extracted from viable S. enterica (RNAs degrade rapidly in dead bacteria), and the specificity of amplification was ensured using Cas13a/crRNA to recognize the amplicons. We used the CRISPR/Cas13-based NASBA assay (termed cNASBA assay) to investigate the in vivo colonization and intestinal infection of S. enterica in mice. We found that S. enterica was mainly colonized at the cecum, colon, and rectum, and the severity of enteritis caused by S. enterica was determined by the number of viable S. enterica rather than the total count of S. enterica. The cNASBA assay can quantify viable S. enterica and thus can improve the accuracy of virulence estimation compared to qPCR. It shows promise as a reliable tool for monitoring pathogen contamination and biosafety control.

Au Nanoparticle-Based Amplified DNA Detection on Poly-l-lysine Monolayer-Functionalized Electrodes

Affinity sensing of nucleic acids is among the most investigated areas in biosensing due to the growing importance of DNA diagnostics in healthcare research and clinical applications. Here, we report a simple electrochemical DNA detection layer, based on poly-l-lysine (PLL), in combination with gold nanoparticles (AuNPs) as a signal amplifier. The layer shows excellent reduction of non-specific binding and thereby high contrast between amplified and non-amplified signals with functionalized AuNPs; the relative change in current was 10-fold compared to the non-amplified signal. The present work may provide a general method for the detection of tumor markers based on electrochemical DNA sensing.

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.

Recent advances in Surface Plasmon Resonance (SPR) biosensors for food analysis: a review

Food safety is the prime area of concern that builds trust. With the prevailing advancements, it has become facile to ensure safety in almost all aspects. Technology has grown from tedious lab techniques to modern chromatographic techniques and immunoassays, progressed with more precise and rapid sensing through the advent of Biosensors. Biosensors provide an automated technology by presenting superfast, nondestructive and cost-effective detection in food analysis. SPR biosensor is an optical biosensor known for its versatility and has wider applications in food testing and analysis. It has an optical system for excitation and interrogation of surface plasmons, and a biomolecular recognition element to detect and seize the target analyte present in a sample. The optical signal detects the binding analyte, on the recognition element, which results in a change in refractive index at the surface and modifies the surface plasmons' propagation constant. SPR aids in label-free detection of various components such as adulterants, antibiotics, biomolecules, genetically modified foods, pesticides, insecticides, herbicides, microorganisms and microbial toxins in food and assures safety. The distinct advancements of SPR in food analysis have been found and discussed. The review also provides knowledge on the advantages and the key challenges encountered by SPR.

Improving the detection limit of Salmonella colorimetry using long ssDNA of asymmetric-PCR and non-functionalized AuNPs

A colorimetric sensor based on gold nanoparticles (AuNPs) and single-stranded DNA (ssDNA) is a simple and rapid method for detecting foodborne pathogens. However, the colorimetric method employed in previous studies involved short ssDNA (<100 nucleotides), including the aptamer and PCR products, resulting in the high detection limit of this technique. In this study, a colorimetric sensor was developed based on long ssDNA of asymmetric PCR (aPCR) and non-functionalized AuNPs for detecting Salmonella Typhimurium (S. Typhimurium). In the presence of S. Typhimurium, the long ssDNA (547 nt) amplified by aPCR-protected AuNPs from NaCl-induced aggregation, while the solution retained a red color. After optimizing parameters, the limit of detection (LOD) of the colorimetric sensor was 2.56 CFU/mL with high specificity. Recovery studies showed its feasibility for detecting S. Typhimurium (10² CFU/mL, 10⁴ CFU/mL, and 10⁶ CFU/mL) in spiked lettuce samples. This colorimetric sensor provides new opportunities for the highly sensitive detection of bacteria in real food samples.

Biosensors for rapid detection of Salmonella in food: A review

Salmonella is one of the main causes of foodborne infectious diseases, posing a serious threat to public health. It can enter the food supply chain at various stages of production, processing, distribution, and marketing. High prevalence of Salmonella necessitates efficient and effective approaches for its identification, detection, and monitoring at an early stage. Because conventional methods based on plate counting and real-time polymerase chain reaction are time-consuming and laborious, novel rapid detection methods are urgently needed for in-field and on-line applications. Biosensors provide many advantages over conventional laboratory assays in terms of sensitivity, specificity, and accuracy, and show superiority in rapid response and potential portability. They are now recognized as promising alternative tools and one of the most on-site applicable and end user-accessible methods for rapid detection. In recent years, we have witnessed a flourishing of studies in the development of robust and elaborate biosensors for detection of Salmonella in food. This review aims to provide a comprehensive overview on Salmonella biosensors by highlighting different signal-transducing mechanisms (optical, electrochemical, piezoelectric, etc.) and critically analyzing its recent trends, particularly in combination with nanomaterials, microfluidics, portable instruments, and smartphones. Furthermore, current challenges are emphasized and future perspectives are discussed.

Microbial biosensors for recreational and source waters

Biosensors are finding new places in science, and the growth of this technology will lead to dramatic improvements in the ability to detect microorganisms in recreational and source waters for the protection of public health. Much of the improvement in biosensors has followed developments in molecular biology processes and coupling these with advances in engineering. Progress in the fields of nano-engineering and materials science have opened many new avenues for biosensors. The adaptation of these diverse technological fields into sensors has been driven by the need to develop more rapid sensors that are highly accurate, sensitive and specific, and have other desirable properties, such as robust deployment capability, unattended operations, and remote data transfer. The primary challenges to the adoption of biosensors in recreational and source water applications are cost of ownership, particularly operations and maintenance costs, problems caused by false positive rates, and to a lesser extent false negative rates, legacy technologies, policies and practices which will change as biosensors improve to the point of replacing more traditional methods for detecting organisms in environmental samples.

Application of Biosensors for Detection of Pathogenic Food Bacteria: A Review

The use of biosensors is considered a novel approach for the rapid detection of foodborne pathogens in food products. Biosensors, which can convert biological, chemical, or biochemical signals into measurable electrical signals, are systems containing a biological detection material combined with a chemical or physical transducer. The objective of this review was to present the effectiveness of various forms of sensing technologies for the detection of foodborne pathogens in food products, as well as the criteria for industrial use of this technology. In this article, the principle components and requirements for an ideal biosensor, types, and their applications in the food industry are summarized. This review also focuses in detail on the application of the most widely used biosensor types in food safety.

Versatile Bioconjugation Strategies of PEG-Modified Upconversion Nanoparticles for Bioanalytical Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) display highly beneficial photophysical features for background-free bioimaging and bioanalysis; however, they are instable in high ionic strength buffers, have no functional groups, and are nonspecifically interacting. Here, we have prepared NIR-excitable UCNPs that are long-term colloidally stable in buffered media and possess functional groups. Heterobifunctional poly(ethylene glycol) (PEG) linkers bearing neridronate and alkyne or maleimide were attached to UCNPs via a ligand exchange. Streptavidin (SA)-conjugates were prepared by click reaction of UCNP@PEG-alkyne and SA-azide. Antihuman serum albumin pAbF antibody was modified with azide groups and conjugated to UCNP@PEG-alkyne via click reaction; alternatively, the antibody, after mild reduction of its disulfide bonds, was conjugated to UCNP@PEG-maleimide. We employed these nanoconjugates as labels for an upconversion-linked immunosorbent assay. SA-based labels achieved the lowest LOD of 0.17 ng/mL for the target albumin, which was superior compared to a fluorescence immunoassay (LOD 0.59 ng/mL) or an enzyme-linked immunoassay (LOD 0.56 ng/mL).

Noble Metal-Assisted Surface Plasmon Resonance Immunosensors

For the early diagnosis of several diseases, various biomarkers have been discovered and utilized through the measurement of concentrations in body fluids such as blood, urine, and saliva. The most representative analytical method for biomarker detection is an immunosensor, which exploits the specific antigen-antibody immunoreaction. Among diverse analytical methods, surface plasmon resonance (SPR)-based immunosensors are emerging as a potential detection platform due to high sensitivity, selectivity, and intuitive features. Particularly, SPR-based immunosensors could detect biomarkers without labeling of a specific detection probe, as typical immunosensors such as enzyme-linked immunosorbent assay (ELISA) use enzymes like horseradish peroxidase (HRP). In this review, SPR-based immunosensors utilizing noble metals such as Au and Ag as SPR-inducing factors for the measurement of different types of protein biomarkers, including viruses, microbes, and extracellular vesicles (EV), are briefly introduced.

Detecting Fingerprints of Waterborne Bacteria on a Sensor

Human fecal contamination is a crucial threat that results in difficulties in access to clean water. Enterococcus faecalis is a bacteria which is utilized as an indicator in polluted water. Nevertheless, existing strategies face several challenges, including low affinity and the need for labelling, which limit their access to large scale applications. Herein, a label-free fingerprint of the surface proteins of waterborne bacteria on a sensor was demonstrated for real-time bacteria detection from aqueous and water samples. The kinetic performance of the sensor was evaluated and shown to have a range of detection that spanned five orders of magnitude, having a low detection limit (3.4 × 104 cfu/mL) and a high correlation coefficient (R2 = 0.9957). The sensor also designated a high selectivity while other competitor bacteria were employed. The capability for multiple usage and long shelf-life are superior to other modalities. This is an impressive surface modification method that uses the target itself as a recognition element, ensuring a broad range of variability to replicate others with different structure, size and physical and chemical properties.

Quartz Crystal Microbalance Detection of Poly(ADP-ribose) Polymerase-1 Based on Gold Nanorods Signal Amplification

Recent findings have thrust poly(ADP-ribose) polymerase-1 (PARP-1) into the limelight as a potential biomarker and chemotherapeutic target for cancer. Thus, a sensitive method for detection of PARP-1 is necessary for early diagnosis of cancer and drug development. However, the poor electrochemical and optical activity of PARP-1 and its product poly(ADP-ribose) (PAR) prompted researchers to develop more methods. Here, we developed an efficient method for the determination of PARP-1 by using quartz crystal microbalance (QCM) because it is mass-sensitive. Once activated by the specific DNA, PARP-1 cleaves nicotinamideadenine dinucleotide (NAD⁺) into nicotinamide and ADP-ribose to synthesize a hyperbranched poly(ADP-ribose) polymer. Although QCM is mass-sensitive, it is not sensitive enough to discern PAR effectively. So, positively charged cetyltrimethylammonium bromide (CTAB)-coated gold nanorods (GNRs) were introduced to increase the frequency change significantly because of the strong electrostatic interaction between them with negatively charged PAR. PARP-1 ranging from 0.06 to 3 nM can be facilely detected with a low detection limit of 0.04 nM. The strategy has been used to evaluate PARP-1 inhibitors and to detect PARP-1 activity in real cancer cells lysate with satisfactory results, indicating that it was a promising candidate for clinical diagnosis and drug screening in the future.

A non-spectroscopic optical biosensor for the detection of pathogenic Salmonella Typhimurium based on a stem-loop DNA probe and retro-reflective signaling

The detection of foodborne pathogenic microorganisms is an essential issue in molecular diagnostics. Fluorescence-based assays have been widely utilized in molecular diagnostics because of their ability to detect and measure low analyte concentrations. However, conventional fluorescence-based assays require sophisticated optics systems, such as a specific light source and light filter. To overcome these limitations, we developed an optical sensing system using a retroreflective Janus microparticle (RJP) as a signaling probe. Compared to fluorescent dyes, RJPs have the advantage of not requiring complicated optic systems because they can be observed using visible light without a filter. To confirm that RJPs can be used as a probe for molecular diagnostics, Salmonella was detected using a biotinylated stem-loop DNA probe to capture the target gene DNA and a streptavidin-conjugated RJP (SA-RJP) as the detection molecule. When the target gene DNA was present at the sensing surface where the stem-loop DNA probe was immobilized, the biotinylated stem-loop DNA probe was stretched, exposing biotin, which can react with SA-RJP. Since the amount of exposed biotin increased according to the concentration of the applied target gene DNA, the number of observed RJPs on the sensing surface increased with the concentration of the target gene DNA. Consequently, the concentration of Salmonella could be quantitated by counting the number of observed RJPs. Using this system, Salmonella at concentrations ranging from 0 to 100 nM could be analyzed, with high sensitivity and selectivity, with a limit of detection of 2.48 pM.

Salmonella Typhimurium Sensing Strategy Based on the Loop-Mediated Isothermal Amplification Using Retroreflective Janus Particle as a Nonspectroscopic Signaling Probe

Loop-mediated isothermal amplification (LAMP) is a powerful gene amplification method, which has many advantages, including high specificity, sensitivity, and simple operation. However, quantitative analysis of the amplified target gene with the LAMP assay is very difficult. To overcome this limitation, we developed a novel biosensing platform for molecular diagnosis by integrating the LAMP method and retroreflective Janus particle (RJP) together. The final amplified products of the LAMP assay are dumbbell-shaped DNA structures, containing a single-stranded loop with two different sequences. Therefore, the concentration of the amplified products can be measured in a manner similar to the sandwich-type immunoassay. To carry out the sandwich-type molecular diagnostics using the LAMP product, two DNA probes, with complementary sequences to the loop-regions, were prepared and immobilized on both the sensing surface and the surface of the RJPs. When the amplified LAMP product was applied to the sensing surface, the surface-immobilized DNA probe hybridized to the loop-region of the LAMP product to form a double-stranded structure. When the DNA probe-conjugated RJPs were injected, the RJPs bound to the unreacted loop-region of the LAMP product. The number of RJPs bound to the loop-region of the LAMP product was proportional to the concentration of the amplified LAMP product, indicating that the concentration of the target gene can be quantitatively analyzed by counting the number of observed RJPs. Using the developed system, a highly sensitive and selective quantification of Salmonella was successfully performed with a detection limit of 10² CFU.

Ultra-sensitive detection by metal nanoparticles-mediated enhanced SPR biosensors

Surface plasmon resonance (SPR), as an optical technique, has widely been used for the detection of biomarkers. Various investigations have been conducted to address the impacts of SPR on the kinetics of biological interactions between the ligand and its cognate bio-element. Up until now, different biofunctionalized metal nanoparticles (NPs) have been used for the ultrasensitive detection of biomarkers in the enhanced SPR. The enhancement of plasmonic properties and refractive index by means of metal NPs in SPR-based biosensors have significantly improved the diagnosis and monitoring of molecular markers in different disesaes including malignancies. In all the enhanced SPR systems utilized for the direct/sandwich assay, each NP is covalently modified with the analyte molecules like antibody (Ab) or a nucleic acid such as DNA/RNA aptamer (Ap) capable of interaction with the related biomarker(s). The increasing of density near the gold surface and plasmonic coupling of gold film and NPs can provide a large shift in the refractive index enhancing the plasmonic resonance because the SPR response unit is sensitive to alteration of the refractive index and the mass shifting onto the chip surface. In this study, we review the potential applications of two major NPs for enhancing the SPR signals for the detection of molecular biomarkers, including gold and magnetic NPs.

Simple Strategy for Rapid and Sensitive Detection of Avian Influenza A H7N9 Virus Based on Intensity-Modulated SPR Biosensor and New Generated Antibody

In 2013 a new reassortant avian influenza A H7N9 virus emerged in China, causing human infection with high mortality. An accurate and timely diagnosis is crucial for controlling the outbreaks of the disease. We therefore propose a simple strategy for rapidly and sensitively detecting the H7N9 virus using an intensity-modulated surface plasmon resonance (IM-SPR) biosensor integrated with a new generated monoclonal antibody. The novel antibody exhibits significant specificity to recognize H7N9 virus compared with other clinical human influenza isolates (p < 0.01). Experimentally, the detection limit of the proposed approach for H7N9 virus detection is estimated to be 144 copies/mL, which is a 20-fold increase in sensitivity compared with homemade target-captured ELISA using the identical antibody. For the measurement of mimic clinical specimens containing the H7N9 virus mixed with nasal mucosa from flu-like syndrome patients, the detection limit is calculated to be 402 copies/mL, which is better than conventional influenza detection assays; quantitative reverse transcription polymerase chain reaction (qRT-PCR) and rapid influenza diagnostic test (RIDT). Most importantly, the assay time took less than 10 min. Combined, the results of this study indicate that the proposed simple strategy demonstrates high sensitivity and time-saving in H7N9 virus detection. By incorporating a high specific recognizer, the proposed technique has the potential to be used in applications and development of other emerging or re-emerging microbe detection platforms.

Prussian Blue Nanoparticles as a Catalytic Label in a Sandwich Nanozyme-Linked Immunosorbent Assay

Enzyme immunoassays are widely used for detection of analytes within various samples. However, enzymes as labels suffer several disadvantages such as high production cost and limited stability. Catalytic nanoparticles (nanozymes) can be used as an alternative label in immunoassays overcoming the inherent disadvantages of enzymes. Prussian blue nanoparticles (PBNPs) are nanozymes composed of the Fe₄[Fe(CN)₆]₃-based coordination polymer. They reveal peroxidase-like activity and are capable of catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂ to form intensely blue product. Here, we introduce the method for conjugation of PBNPs with antibodies and their application in nanozyme-linked immunosorbent assay (NLISA). Sandwich NLISA for detection of human serum albumin in urine was developed with limit of detection (LOD) of 1.2 ng·mL^-1 and working range up to 1 μg·mL^-1. Furthermore, the microbial contamination of Salmonella Typhimurium in powdered milk was detected with LOD of 6 × 10³ colony-forming units (cfu)·mL^-1 and working range up to 10⁶ cfu·mL^-1. In both cases, a critical comparison with the same immunoassay but using native peroxidase as label was realized. The achieved results confirmed the suitability of PBNPs for universal and robust replacement of enzyme labels.

Carboxymethylated Dextran-Modified N-Heterocyclic Carbene Self-Assembled Monolayers on Gold for Use in Surface Plasmon Resonance Biosensing

Surface chemistry is a key enabler for various biosensing applications. Biosensors based on surface plasmon resonance routinely employ thiol-based chemistry for the linker layer between gold-coated support surfaces and functional biosensor surfaces. However, there is a growing awareness that such sensor surfaces are prone to oxidation/degradation problems in the presence of oxygen, and previous efforts to improve the stability have shown limited advancements. As an alternative, recent studies employing N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) deposited on gold have shown significant promise in this area. Here, we describe a sensor surface employing an NHC SAM to couple a modified carboxymethylated dextran onto a gold surface. Such a dextran matrix is also used for affinity chromatography, and it is the most commonly employed matrix for commercial biosensor surfaces today. The performance reliability of the dextran-modified NHC chip to act as an alternative biosensing platform is compared with that of a thiol-based commercial chip in the proof-of-concept tests. The resultant NHC sensor surface shows a higher thermal stability compared to thiol analogues. Moreover, the plasma protein/drug and antibody/antigen interactions were validated on the NHC-based dextran chip and showed similar performance as compared to the thiol-based commercial chip. Ultimately, this study shows the strong potential applicability of chemical modifications to gold surfaces using NHC ligands for biosensing applications.