Highly Sensitive Detection of Salmonella typhimurium Using a Colorimetric Paper-Based Analytical Device Coupled with Immunomagnetic Separation

Dengue NS1 detection in pediatric serum using microfluidic paper-based analytical devices

The diagnosis of dengue infection is still a critical factor determining success in the clinical management and treatment of patients. Here, the development of microfluidic paper-based analytical devices (μPADs) utilizing a sandwich immunoassay on wax patterned paper functionalized with anti-dengue NS1 monoclonal antibodies for point-of-care detection of dengue NS1 (DEN-NS1-PAD) is reported. Various assay conditions, including the length of the channel and diluent, were optimized, and the response detected by the naked eye and digitized images within 20-30 min. The DEN-NS1-PAD was successfully tested in the field for detecting dengue NS1 in buffer, cell culture media, and human serum. The limit of detection (LoD) of the DEN-NS1-PAD obtained with the naked eye, scanner, and a smartphone camera was 200, 46.7, and 74.8 ng mL^-1, respectively. The repeatability, reproducibility, and stability of the DEN-NS1-PAD were also evaluated. High true specificity and sensitivity in the serum of pediatric patients were observed. These evaluation results confirm that the DEN-NS1-PAD can potentially be used in point-of-care dengue diagnostics, which can significantly impact on the spreading of mosquito-borne diseases, which are likely to become more prevalent with the effects of global warming. Graphical Abstract.

Rapid Flow Cytometric Detection of Single Viable Salmonella Cells in Milk Powder

Salmonella, a highly virulent food-borne pathogen transmitted through food, can cause severe infectious diseases in a large number of people through a single outbreak, due to its low infective doses. In this study, a flow cytometry (FCM)-based method was developed for the rapid detection of single viable Salmonella cells with dual staining of fluorescein isothiocyanate (FITC)-labeled anti-Salmonella antibody and propidium iodide (PI) dyes. The FCM-based method includes 6 h of pre-enrichment, 40 min of target cell isolation, and 20 min of dual staining and FCM analysis. The developed method demonstrated high specificity for the detection of 23 Salmonella strains and 22 food-borne pathogenic non-Salmonella strains. Furthermore, the analyses of 30 samples of milk powder artificially contaminated with single Salmonella cells, 123 samples of retail milk powder, and 6 samples of Salmonella-positive milk powder were performed by the FCM-based as well as traditional plate-based methods for testing the efficiency of the methods. The two methods yielded similar results for the detection of pathogens in all milk powder samples. In conclusion, the developed FCM-based method was found to be efficient in detecting single viable Salmonella cells in milk powder within 7 h. The proposed dual-color FITC assay combined with pre-enrichment offers a great potential for the rapid and sensitive detection of other pathogens in dairy products.

Microfluidics as an Emerging Platform for Tackling Antimicrobial Resistance (AMR): A Review

Background:

Antimicrobial resistance (AMR) occurs when microbes become resistant to antibiotics causing complications and limited treatment options. AMR is more significant where antibiotics use is excessive or abusive and the strains of bacteria become resistant to antibiotic treatments. Current technologies for bacteria and its resistant strains identification and antimicrobial susceptibility testing (AST) are mostly central-lab based in hospitals, which normally take days to weeks to get results. These tools and procedures are expensive, laborious and skills based. There is an ever-increasing demand for developing point-of-care (POC) diagnostics tools for rapid and near patient AMR testing. Microfluidics, an important and fundamental technique to develop POC devices, has been utilized to tackle AMR in healthcare. This review mainly focuses on the current development in the field of microfluidics for rapid AMR testing.

Method:

Due to the limitations of conventional AMR techniques, microfluidic-based platforms have been developed for better understandings of bacterial resistance, smart AST and minimum inhibitory concentration (MIC) testing tools and development of new drugs. This review aims to summarize the recent development of AST and MIC testing tools in different formats of microfluidics technology.

Results:

Various microfluidics devices have been developed to combat AMR. Miniaturization and integration of different tools has been attempted to produce handheld or standalone devices for rapid AMR testing using different formats of microfluidics technology such as active microfluidics, droplet microfluidics, paper microfluidics and capillary-driven microfluidics.

Conclusion:

Current conventional AMR detection technologies provide time-consuming, costly, labor-intensive and central lab-based solutions, limiting their applications. Microfluidics has been developed for decades and the technology has emerged as a powerful tool for POC diagnostics of antimicrobial resistance in healthcare providing, simple, robust, cost-effective and portable diagnostics. The success has been reported in research articles; however, the potential of microfluidics technology in tackling AMR has not been fully achieved in clinical settings.

Rapid screening and quantitative detection of Salmonella using a quantum dot nanobead-based biosensor

A quantum dot nanobead-based sensor is demonstrated for Salmonella detection with balanced sensitivity, specificity and high accuracy.

Label-Free Detection of Bacteria in Fruit Juice by Nano-Flow Cytometry

Rapid quantification of microbial contamination in fruit juice is highly desired for food safety control. Yet, the complex sample matrix and the diversity of bacterial contaminants present a great challenge. Employing a laboratory-built nano-flow cytometer (nFCM), here we report the development of a label-free approach for the detection of bacteria population in fruit juice. The weak autofluorescence of bacterial cells was used as a hallmark for the identification of bacteria. The sample pretreatment protocol was optimized to reduce fluorescence background, lyse residual plant cells and debris, and attain a good recovery of bacteria from juice samples. It was demonstrated that the nFCM was able to enumerate individual bacteria of very weak autofluorescence, and a clear differentiation from residual juice particulates was achieved. For bacteria spiked in the orange juice, the recovery rate was around 95% and a linear correlation between nFCM analysis and plate counting was acquired in the range of 3 × 10⁴ to 3 × 10⁸ cfu/mL. The assay, including sample pretreatment and instrument analysis, can be accomplished within 1 h, which is far more efficient than plate counting. Using a 40 mL sample volume, the detection limit in apple juice was ∼10² cells/mL. The as-developed method was successfully applied to bacterial measurement of freshly made orange juice and apple juice purchased from grocery stores. We believe it could also have potential practical application in microbial control analysis of other juices and water.

A single cell droplet microfluidic system for quantitative determination of food-borne pathogens

Single-cell detection methods are already of great significance for many bioanalysis applications, and droplet microfluidics technology is understood as particularly a powerful tool. Salmonella infection is a major hygienic problem worldwide that causes major public health and economic damage, and preventing Salmonella outbreaks requires detection food-borne detection methods that are rapid, portable, and reliable, ideally without the need for complicated pre-treatment protocol steps. Herein, we present a single-cell-level analysis method based on droplet microfluidics that can sensitively and rapidly detect Salmonella directly from food samples. Specifically, this method achieves single-cell encapsulation of Salmonella in droplets of a growth medium with resazurin that enables fluorescence-based detection of pathogens within 5 h. The ratio of positive droplets in a Poisson Distribution is used for quantitation, and the detection limit of our system determined to be 50 CFU/mL, a value lower than conventional analytical methods for assessing Salmonella contamination. Our experimental results demonstrate the precise and highly sensitive performance of a single-cell-precision, droplet-based microfluidic chip analytical method for monitoring pathogenic bacteria in food. Beyond our example case of Salmonella detection from milk samples, our work lays the foundation for a new generation of microfluidics-based analytical technologies for both public health and food safety applications which can undoubtedly benefit from increases in the sensitivity and rapidity of food-borne pathogen detection.

Protein-protected metal nanoclusters: An emerging ultra-small nanozyme

Protein-protected metal nanoclusters (MNCs), typically consisting of several to a hundred metal atoms with a protein outer layer used for protecting clusters from aggregation, are excellent fluorescent labels for biomedical applications due to their extraordinary photoluminescence, facile synthesis and good biocompatibility. Interestingly, many protein-protected MNCs have also been reported to exhibit intrinsic enzyme-like activities, namely peroxidase, oxidase and catalase activities, and are consequently used for biological analysis and environmental treatment. These findings have extended the horizon of protein-protected MNCs' properties as well as their application in various fields. Furthermore, in the field of nanozymes, protein-protected MNCs have emerged as an outstanding new addition. Due to their ultra-small size (<2 nm), they usually have higher catalytic activity, more suitable size for in vivo application, better biocompatibility and photoluminescence in comparison with large size nanozymes. In this review, we will systematically introduce the significant advances in this field and critically discuss the challenges that lie ahead. Ultra-small nanozymes based on protein-protected MNCs are on the verge of attracting great interest across various disciplines and will stimulate research in the fields of nanotechnology and biology. This article is characterized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Rapid and sensitive detection of Salmonella with reduced graphene oxide-carbon nanotube based electrochemical aptasensor

Rapid detection of foodborne pathogens is crucial as ingestion of contaminated food products may endanger human health. Thus, the objective of this study was to develop a biosensor using reduced graphene oxide-carbon nanotubes (rGO-CNT) nanocomposite via the hydrothermal method for accurate and rapid label-free electrochemical detection of pathogenic bacteria such as Salmonella enterica. The rGO-CNT nanocomposite was characterized using Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction and transmission electron microscopy. The nanocomposite was dropped cast on the glassy carbon electrode and further modified with amino-modified DNA aptamer. The resultant ssDNA/rGO-CNT/GCE aptasensor was then used to detect bacteria by using differential pulse voltammetry (DPV) technique. Synergistic effects of aptasensor was evident through the combination of enhanced electrical properties and facile chemical functionality of both rGO and CNT for the stable interface. Under optimal experimental conditions, the aptasensor could detect S. Typhimurium in a wide linear dynamic range from 10¹ until 10⁸ cfu mL^-1 with a 10¹ cfu mL^-1 of the limit of detection. This aptasensor also showed good sensitivity, selectivity and specificity for the detection of microorganisms. Furthermore, we have successfully applied the aptasensor for S. Typhimurium detection in real food samples.

Rotary manifold for automating a paper-based Salmonella immunoassay

Foodborne pathogens are responsible for hundreds of thousands of deaths around the world each year. Rapid screening of agricultural products for these pathogens is essential to reduce and/or prevent outbreaks and pinpoint contamination sources. Unfortunately, current detection methods are laborious, expensive, time-consuming and require a central laboratory. Therefore, a rapid, sensitive, and field-deployable pathogen-detection assay is needed. We previously developed a colorimetric sandwich immunoassay utilizing immuno-magnetic separation (IMS) and chlorophenol red-β-d-galactopyranoside for Salmonella detection on a paper-based analytical device (μPAD); however, the assay required many sample preparation steps prior to the μPAD as well as laboratory equipment, which decreased user-friendliness for future end-users. As a step towards overcoming these limitations in resource-limited settings, we demonstrate a reusable 3D-printed rotational manifold that couples with disposable μPAD layers for semi-automated reagent delivery, washing, and detection in 65 minutes. After IMS to clean the sample, the manifold performs pipette-free reagent delivery and washing steps in a sequential order with controlled volumes, followed by enzymatic amplification and colorimetric detection using automated image processing to quantify color change. Salmonella was used as the target pathogen in this project and was detected with the manifold in growth media and milk with detection limits of 4.4 × 10² and 6.4 × 10² CFU mL^-1 respectively. The manifold increases user friendliness and simplifies immunoassays resulting in a practical product for in-field use and commercialization.

One-step sensing of foodborne pathogenic bacteria using a 3D paper-based device

Managing food contamination from bacteria has been an ongoing issue in the public health and industrial fields. Enzymatic substrates possessing optical properties, e.g. fluorescence or color manifestation, are widely exploited in pathogenic/non-pathogenic bacteria culture methods. Recently, various chromogenic substrates have been utilized in the development of point-of-care diagnostic tools. Herein, four types of chromogenic substrates were exploited to develop paper-based sensors for major foodborne pathogens. We designed a compact sized three-dimensional paper device with a simple user interface. By inserting functional layers in the middle of multilayers, pre-lysis and pH regulation steps were excluded and the analysis time was subsequently reduced, while only one sample droplet was needed for the whole analysis process. After the enzymatic reactions had proceeded, target-specific colors appeared. When it was combined with enrichment, 101 cfu mL-1 of pathogens were successfully detected in 4-8 hours, while those in milk samples were readily sensed in 12 hours. The proposed bacteria sensor exhibited great advantages of low cost, portability and simple operation, while showing a respectable limit-of-detection as low as 101 cfu mL-1 and below. Significantly, we emphasize that it takes fewer steps than existing methods and provides a reduced analysis time owing to the layer functionalization.

3D Capillary-Driven Paper-Based Sequential Microfluidic Device for Electrochemical Sensing Applications

This article describes the device design and fabrication of two different configurations (flow-through and stopped-flow) of a sequential fluid delivery platform on a microfluidic paper-based device. The developed device is capable of storing and transporting reagents sequentially to the detection channel without the need for external power. The device comprises two components: an origami folding paper (oPAD) and a movable reagent-stored pad (rPAD). This 3D capillary-driven device eliminates the undesirable procedure of multiple-step reagent manipulation in a complex assay. To demonstrate the scope of this approach, the device is used for electrochemical detection of biological species. Using a flow-through configuration, a self-calibration plot plus real sample analysis using a single buffer introduction are established for ascorbic acid detection. We further broaden the effectiveness of the device to a complex assay using a stopped-flow configuration. Unlike other electrochemical paper-based sensors in which the user is required to cut off the device inlet or rest for the whole channel saturation before measurement, herein a stopped-flow device is carefully designed to exclude the disturbance from the convective mass transport. As a proof of concept, multiple procedures for electrode modification and voltammetric determination of serotonin are illustrated. In addition, the research includes an impedimetric label-free immunosensor for α-fetoprotein using the modified stopped-flow device. The beneficial advantages of simplicity, low sample volume (1 μL), and ability to perform a complex assay qualify this innovative device for use with diverse applications.

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.

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.

Multifunctional Paper-Based Analytical Device for In Situ Cultivation and Screening of Escherichia coli Infections

Point-of-care testing (POCT) for uropathogen detection and chemical screening has great benefits for the diagnosis of urinary tract infections (UTIs). The goal of this study was to develop a portable and inexpensive paper-based analytical device (PAD) for cultivating bacteria in situ and rapidly testing for nitrite on the same device. The PAD was fabricated using a wax printing technique to create a pattern on Whatman No. 1 filter paper, which was then combined with a cotton sheet to support bacterial growth. Nitrite detection was based on the principle of the Griess reaction, and a linear detection range of 0-1.6 mg/dL (R² = 0.989) was obtained. Scanning electron microscopy (SEM) analysis demonstrated that the bacteria were able to grow and formed a cluster on the cellulose fibres within 2 hours. The enzyme β-glucuronidase, which is specifically produced by Escherichia coli, was able to convert the pre-immobilized 5-bromo-4-chloro-3-indolyl-β-D-glucuronide sodium salt (X-GlcA), a colourless substrate, generating a blue colour. Under optimum conditions, the proposed device allowed bacterial concentrations in the range of 10⁴-10⁷ colony forming units (CFU)/mL to be quantified within 6 hours. Moreover, the use of this device enables the identification of E. coli pathogens with selectivity in real urine samples. In conclusion, the PAD developed in this study for UTI screening provides a rapid, cost-effective diagnostic method for use in remote areas.

Colorimetric-Fluorescent-Magnetic Nanosphere-Based Multimodal Assay Platform for Salmonella Detection

Rapid and sensitive foodborne pathogen detection assay, which can be applied in multiple fields, is essential to timely diagnosis. Herein, we proposed a multisignal readout lateral flow immunoassay for Salmonella typhimurium ( S. typhi) detection. The assay employs colorimetric-fluorescent-magnetic nanospheres (CFMNs) as labels, which possess multifunctional target separation and enrichment, multisignal readout, and two formats of quantitation. The assay for S. typhi detection involves magnetic separation and chromatography. First, the S. typhi were separated and enriched from matrix by antibody labeled CFMNs, and then the S. typhi-containing suspension is added onto the sample pad to flow up the test strip. The introduction of magnetic separation enhances anti-interference ability and 10-fold sensitivity, making the assay possible for practical application. The assay has realized naked eye detection of 1.88 × 10⁴ CFU/mL S. typhi, and 3.75 × 10³ CFU/mL S. typhi can be detected with a magnetic assay reader, which is 2-4 orders of magnitude lower than other label-based LFIAs, with a quantitation range of 1.88 × 10⁴ to 1.88 × 10⁷ CFU/mL by measuring the fluorescence intensity and magnetic signal. Moreover, the successful detection of S. typhi in complex matrix (tap water, milk, fetal bovine serum, and whole blood) indicated its potential application in real samples.

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.

A multicolorimetric assay for rapid detection of Listeria monocytogenes based on the etching of gold nanorods

Listeria monocytogenes (L. monocytogenes) is one of the most common food-borne pathogens. The authors describe a sensitive and reliable multicolorimetric assay for L. monocytogenes using a sensing system based on TMB²⁺ etching of gold nanorods. Apt-MNP was used as the capture probe, and IgY-BSA-MnO₂ NPs was chosen as an oxidase-like nano-artificial enzyme to oxidize TMB to generate TMB²⁺. Under the optimized conditions, the longitudinal shift of localized surface plasmon resonances had a linear correlation with the L. monocytogenes concentration in the range between 10 to 10⁶ cfu mL^-1. Meanwhile, the sensing system can generate vivid color responses as colorful as a rainbow, and the limit of detection is as low as 10 cfu mL^-1 at a glance. Recoveries ranging from 97.4 to 101.3% are found when analyzing spiked food samples without pre-enrichment. In our perception, it shows promise in rapid instrumental and on-site visual detection of L. monocytogenes.

Simple Way To Fabricate Novel Paper-Based Valves Using Plastic Comb Binding Spines

A novel strategy for fabricating the paper-based valves on microfluidic paper-based analytical devices (μPADs) was described to control fluid in a user-friendly way. Initial prototypes of 3D μPADs manipulate the spatial distribution of fluid within the device. The movable paper channel in a different layer could be achieved using the channel's connection or disconnection to realize the valve function using plastic comb binding spines (PCBS). The entire valve manipulation process was similar to a desk calendar that can be flipped over and turned back. It is notable that this kind of PCBS valve can control a fluid in a simple and easy way without the timing setting or any trigger, and this advantage makes it user-friendly for untrained users to carry out the complex and high throughput operations. The reusable plastic comb binding spines greatly reduce the cost of fabricating paper-based valves. To evaluate the performance, the actual samples of Fe (II) and nitrite were successfully analyzed. We hope this method will introduce a new approach to fabrication of paper-based valves on μPADs in the future.

High-throughput deposition of chemical reagents via pen-plotting technique for microfluidic paper-based analytical devices

The deposition of chemical reagent inks on paper is a crucial step in the development and fabrication of microfluidic paper-based analytical devices (μPADs). A pen-plotting approach, delivering chemical ink deposition using technical pens filled with reagents and inserted into a desktop electronic plotter, is shown herein to be a versatile, low-cost, simple, rapid, reproducible, and high-throughput solution. The volume of the deposited ink was quantified gravimetrically, confirming that nanoliter volumes of reagents can be deposited reproducibly (e.g. 7.55 ± 0.37 nL/mm for a plotting speed of 10 cm/s) in detection zones of μPADs, typically spatially defined using wax printing. This approach was further investigated with regard to deposition of reagents in different geometrical forms (circular and linear), so demonstrating its applicability for preparation of μPADs with flexible design and application. By adjusting the plotting speed for linear deposition, lines with a relatively large range of widths (≈628-1192 μm) were created. Circular deposition was also demonstrated via delivery of reagents within wax printed circular fluidic barriers of a range of diameters (inner diameter = 1.5-7 mm). These capabilities were practically demonstrated via the fabrication of μPADs, based upon differing detection principles for determination of aluminum in natural waters using Morin as the fluorescent reagent. Traditional μPADs based on digital image colorimetry (DIC) were produced using circular deposition, whilst distance-based μPADs exploited linear deposition. Both types of μPADs developed using this method showed excellent precision for determination of trace concentrations of aluminium (average RSDs = 3.38% and 6.45%, and LODs = 0.5 ng (0.25 ppm) and 2 ng (0.5 ppm), for traditional and distance-based detection, respectively).

Colorimetric immunoassay for Listeria monocytogenes by using core gold nanoparticles, silver nanoclusters as oxidase mimetics, and aptamer-conjugated magnetic nanoparticles

The authors describe a rapid colorimetric assay for Listeria monocytogenes (L. monocytogenes) based on the o-phenylenediamine-mediated deaggregation of gold nanoparticles. Silver nanoclusters are used as an artificial enzyme that can oxidize o-phenylenediamine to form o-benzoquinone diamine. Aptamer and IgY antibodies were chosen to conjugate with magnetic beads and silver nanoclusters, respectively, which can recognize and bind L. monocytogenes at different specific binding sites. This results in the disassembly of colloidal gold nanoparticles which is accompanied by a color change from blue to red, with peaks at 730 and 525 nm, respectively. The method allows L. monocytogenes to be colorimetrically determined in the 10 to 10⁶ cfu·mL^-1 concentration range without pre-enrichment, and the limit of detection is as low as 10 cfu·mL^-1. Recoveries ranging from 97.4 to 101.3% are found when analyzing spiked food samples. The assay is rapid, sensitive and specific. Graphical abstract Schematic illustration of a colorimetric method for detection of L. monocytogenes based on silver nanoclusters-catalyzed oxidation of OPD and de-aggregation of GNPs. A color change from blue to red can be observed and correlated to the concentration of L. monocytogenes.