Paper-based ELISA to rapidly detect Escherichia coli

A Photonic Immunosensor Detection Method for Viable and Non-Viable E. coli in Water Samples

Detection and enumeration of coliform bacteria using traditional methods and current molecular techniques against E. coli usually involve long processes with less sensitivity and specificity to distinguish between viable and non-viable bacteria for microbiological water analysis. This approach involves developing and validating an immunosensor comprising ring resonators functionalized with specific antibodies surrounded by a network of microchannels as an alternative method for detecting and indirectly enumerating Escherichia coli in samples of water for consumption. Different ELISA assays were conducted to characterize monoclonal and polyclonal antibodies selected as detection probes for specific B-galactosidase enzymes and membrane LPS antigens of E. coli. An immobilization control study was performed on silicon nitride surfaces used in the immunosensor, immobilized with the selected antibodies from the ELISA assays. The specificity of this method was confirmed by detecting as few as 10 CFU/mL of E. coli from viable and non-viable target bacteria after applying various disinfection methods to water samples intended for human consumption. The 100% detection rate and a 100 CFU/mL Limit of Quantification of the proposed method were validated through a comprehensive assessment of the immunosensor-coupled microfluidic system, involving at least 50 replicates with a concentration range of 10 to 106 CFU/mL of the target bacteria and 50 real samples contaminated with and without disinfection treatment. The correlation coefficient of around one calculated for each calibration curve obtained from the results demonstrated sensitive and rapid detection capabilities suitable for application in water resources intended for human consumption within the food industry. The biosensor was shown to provide results in less than 4 h, allowing for rapid identification of microbial contamination crucial for ensuring water monitoring related to food safety or environmental diagnosis and allowing for timely interventions to mitigate contamination risks. Indeed, the achieved setup facilitates the in situ execution of laboratory processes, allowing for the detection of both viable and non-viable bacteria, and it implies future developments of simultaneous detection of pathogens in the same contaminated sample.

Rapid and Ultrasensitive Colorimetric Biosensors for Onsite Detection of Escherichia coli O157:H7 in Fluids

This study presents a breakthrough in the field of onsite bacterial detection, offering an innovative, rapid, and ultrasensitive colorimetric biosensor for the detection of Escherichia coli (E. coli) O157:H7, using chemically modified melamine foam (MF). Different from conventional platforms, such as 96-well plates and fiber-based membranes, the modified MF features a macroporous reticulated three-dimensional (3D) framework structure, allowing fast and free movement of large biomolecules and bacteria cells through the MF structure in every direction and ensuring good accessibility of entire active binding sites of the framework structure with the target bacteria, which significantly increased sensitive and volume-responsive detection of whole-cell bacteria. The biosensing platform requires less than 1.5 h to complete the quantitative detection with a sensitivity of 10 cfu/mL, discernible by the naked eye, and an enhanced sensitivity of 5 cfu/mL with the help of a smartphone. Following a short enrichment period of 1 h, the sensitivity was further amplified to 2 cfu/mL. The biosensor material is volume responsive, making the biosensing platform sensitivity increase as the volume of the sample increases, and is highly suitable for testing large-volume fluid samples. This novel material paves the way for the development of volume-flexible biosensing platforms for the record-fast, onsite, selective, and ultrasensitive detection of various pathogenic bacteria in real-world applications.

Microfluidics-Based Blood Typing Devices: An In-Depth Overview

Identification of correct blood types holds paramount importance in understanding the pathophysiological parameters of patients, therapeutic interventions, and blood transfusion. Considering the wide applications of blood typing, the requirement of centralized laboratory facilities is not well suited on many occasions. In this context, there has been a significant development of such blood typing devices on different microfluidic platforms. The advantages of these microfluidic devices offer easy, rapid test protocols, which could potentially be adapted in resource-limited settings and thereby can truly lead to the decentralization of testing facilities. The advantages of pump-free liquid transport (i.e., low power consumption) and biodegradability of paper substrates (e.g., reduction in medical wastes) make it a more preferred platform in comparison to other microfluidic devices. However, these devices are often coupled with some inherent challenges, which limit their potential to be used on a mass commercial scale. In this context, our Review offers a succinct summary of the recent development, especially to understand the importance of underlying facets for long-term sustainability. Our Review also delineates the role of integration with digital technologies to minimize errors in interpreting the readouts.

A Facile Method to Fabricate an Enclosed Paper-Based Analytical Device via Double-Sided Patterning for Ionic Contaminant Detection

Microfluidic paper-based analytical devices (μPADs) have been developed for use in a variety of diagnosis and analysis fields. However, conventional μPADs with an open-channel system have limitations for application as analytical platforms mainly because of the evaporation and contamination of the sample solution. This study demonstrates the design and fabrication of an enclosed three-dimensional(3D)-μPAD and its application as a primary early analysis platform for ionic contaminants. To generate the hydrophobic PDMS barrier, double-sided patterning is carried out using a PDMS blade-coated stamp mold that is fabricated using 3D printing. The selective PDMS patterning can be achieved with controlled PDMS permeation of the cellulose substrate using 3D-designed stamp molds. We find the optimal conditions enabling the formation of enclosed channels, including round shape pattern and inter-pattern distance of 10 mm of stamp design, contact time of 0.5 min, and spacer height of 300 µm of double-sided patterning procedure. As a proof of concept, this enclosed 3D-μPAD is used for the simultaneous colorimetric detection of heavy metal ions in a concentration range of 0.1-2000 ppm, including nickel (Ni2+), copper (Cu2+), mercury (Hg2+), and radioactive isotope cesium-137 ions (Cs+). We confirm that qualitative analysis and image-based quantitative analysis with high reliability are possible through rapid color changes within 3 min. The limits of detection (LOD) for 0.55 ppm of Ni2+, 5.05 ppm of Cu2+, 0.188 ppm of Hg2+, and 0.016 ppm of Cs+ are observed, respectively. In addition, we confirm that the analysis is highly reliable in a wide range of ion concentrations with CV values below 3% for Ni2+ (0.56%), Cu2+ (0.45%), Hg2+ (1.35%), and Cs+ (2.18%). This method could be a promising technique to develop a 3D-μPAD with various applications as a primary early analysis device in the environmental and biological industries.

Review on the Selection of Aptamers and Application in Paper-Based Sensors

An aptamer is a synthetic oligonucleotide, referring to a single-stranded deoxyribonucleic acid or ribonucleic acid ligand produced by synthesis from outside the body using systematic evolution of ligands by exponential enrichment (SELEX) technology. Owing to their special screening process and adjustable tertiary structures, aptamers can bind to multiple targets (small molecules, proteins, and even whole cells) with high specificity and affinity. Moreover, due to their simple preparation and stable modification, they have been widely used to construct biosensors for target detection. The paper-based sensor is a product with a low price, short detection time, simple operation, and other superior characteristics, and is widely used as a rapid detection method. This review mainly focuses on the screening methods of aptamers, paper-based devices, and applicable sensing strategies. Furthermore, the design of the aptamer-based lateral flow assay (LFA), which underlies the most promising devices for commercialization, is emphasized. In addition, the development prospects and potential applications of paper-based biosensors using aptamers as recognition molecules are also discussed.

Highly Accurate Identification of Bacteria's Antibiotic Resistance Based on Raman Spectroscopy and U-Net Deep Learning Algorithms

Bacterial pathogens especially antibiotic-resistant ones are a public health concern worldwide. To oppose the morbidity and mortality associated with them, it is critical to select an appropriate antibiotic by performing a rapid bacterial diagnosis. Using a combination of Raman spectroscopy and deep learning algorithms to identify bacteria is a rapid and reliable method. Nevertheless, due to the loss of information during training a model, some deep learning algorithms suffer from low accuracy. Herein, we modify the U-Net architecture to fit our purpose of classifying the one-dimensional Raman spectra. The proposed U-Net model provides highly accurate identification of the 30 isolates of bacteria and yeast, empiric treatment groups, and antimicrobial resistance, thanks to its capability to concatenate and copy important features from the encoder layers to the decoder layers, thereby decreasing the data loss. The accuracies of the model for the 30-isolate level, empiric treatment level, and antimicrobial resistance level tasks are 86.3, 97.84, and 95%, respectively. The proposed deep learning model has a high potential for not only bacterial identification but also for other diagnostic purposes in the biomedical field.

In Situ Collection and Rapid Detection of Pathogenic Bacteria Using a Flexible SERS Platform Combined with a Portable Raman Spectrometer

This study aims to develop a simple, sensitive, low-cost, environmentally friendly and flexible surface-enhanced Raman scattering (SERS) platform, combined with a portable Raman spectrometer, for the rapid and on-site SERS detection of bacteria. Commercial tobacco packaging paper (TPP) with little background interference was used as a loading medium that effectively adsorbed Au nanoparticles and provided sufficient “hot spots”. This Au-tobacco packaging paper (Au-TPP) substrate used as a flexible SERS platform can maximize sample collection by wiping irregular surfaces, and was successfully applied to the on-site and rapid detection of pathogenic bacteria. Raman fingerprints of pathogenic bacteria can be obtained by SERS detection of spiked pork using wipeable Au-TPP, which verifies its value in practical applications. The results collected by SERS were further verified by polymerase chain reaction (PCR) results. It showed several advantages in on-site SERS detection, including accurate discrimination, simple preparation, easy operation, good sensitivity, accuracy and reproducibility. This study indicates that the established flexible SERS platform has good practical applications in pathogenic bacterial identification and other rapid detections.

A review on urinary tract infections diagnostic methods: Laboratory-based and point-of-care approaches

Urinary tract infections (UTIs) are among the most common infectious diseases worldwide. This type of infections can be healthcare-associated or community-acquired and affects millions of people every year. Different diagnostic procedures are available to detect pathogens in urine and they can be divided into two main categories: laboratory-based and point-of-care (POC) detection techniques. Traditional methodologies are often time-consuming, thus, achieving a rapid and accurate identification of pathogens is a challenging feature that has been pursued by many research groups and companies operating in this area. The purpose of this review is to compare and highlight advantages and disadvantages of the traditional and currently most used detection methods, as well as the emerging POC approaches and the relevant advances in on-site detection of pathogens´ mechanisms, suitable to be adapted to UTI diagnosis. Lately, the commercially available UTI self-testing kits and devices are helping in the diagnosis of urinary infections as patients or care givers are able to perform the test, easily and comfortably at home and, upon the result, decide when to attend an appointment/Urgent Health Care Unit.

Progress toward a Simplified UTI Diagnostic: Pump-Free Magnetophoresis for E. coli Detection

Urinary tract infections (UTIs) are one of the most common infections across the world and can lead to serious complications such as sepsis if not treated in a timely manner. Uropathogenic Escherichia coli account for 75% of all UTIs. Early diagnosis is crucial to help control UTIs, but current culturing methods are expensive and time-consuming and lack sensitivity. The existing point-of-care methods fall short because they rely on indirect detection from elevated nitrates in urine rather than detecting the actual bacteria causing the infection. Magnetophoresis is a powerful method used to separate and/or isolate cells of interest from complex matrices for analysis. However, magnetophoresis typically requires complex and expensive instrumentation to control flow in microfluidic devices. Coupling magnetophoresis with microfluidic paper-based analytical devices (μPADs) enables pump-free flow control and simple and low-cost operation. Early magnetophoresis μPADs showed detection limits competitive with traditional methods but higher than targets for clinical use. Here, we demonstrate magnetophoresis using hybrid μPADs that rely on capillary action in hydrophilic polyethylene terephthalate channels combined with paper pumps. We were able to detect E. coli with a calculated limit of detection of 2.40 × 10² colony-forming units per mL.

Development of a Tetrazolium-Derived Paper-Based Diagnostic Device as an Early, Alternative Bacteria Screening Tool

(1) Background: The complexity, amount of time, and the large amount of resource required to perform gold-standard bacteria culture procedures makes it difficult to perform timely pathogenic analyses, especially in areas where such resources are not readily available. A paper-based biochemical analytical tool can potentially tackle problems economically in terms of time and convenience, potentially finding utility in applications where simple and timely detection of bacteria is necessary; (2) Methods: The utility of paper-based MTT-PMS strips was tested using a simple colorimetric analytical methodology; (3) Results: Sufficient evidence was obtained to suggest that the strips can potentially be used as a rapid and convenient early, alternative bacteria screening tool for a variety of applications; (4) Conclusions: The potential of strips for the rapid detection of bacteria compared to standard bacteria culture is a key advantage in certain clinical, agricultural, and environmental applications.

N, O-codoped hierarchical porous graphitic carbon for electrochemical immunosensing of Lactobacillus rhamnosus GG

An ultrasensitive label-free electrochemical immunosensor was fabricated for quantitative detection of Lactobacillus rhamnosus GG (LGG). The N/O co-doped three-dimensional hierarchical porous graphitic (THPG) carbon was synthesized by a one-step synthesis of polyaniline hydrogel, and followed by simple carbonization and chemical activation procedures. Because of the unique structure design, the obtained THPG carbon networks possess an ultra-large specific surface area of 4859 m² g^-1 along with a class of highly graphitic carbons. The results offer an enormous surface area and excellent electrical conductivity for label-free electrochemical immunosensing of probiotic L. rhamnosus strain. Under optimal conditions, the immunosensor showed a good linear relationship between peak current and concentration of LGG (R² = 0.9976), with a detection limit of 2 CFU mL^-1. Furthermore, this label-free immunosensor also shows good specificity, long-term stability, and reliability, and could be applied to detect probiotic LGG in dairy products and drinks with satisfactory results. The present protocol was shown to be quite promising for practical screening and functional evaluation of probiotic products containing LGG. A ultrasensitive label-free electrochemical immunosensor based on THPG carbon was fabricated for detection of Lactobacillus rhamnosus GG.

Recent advances in magnetic nanoparticle-based microfluidic devices for the pretreatment of pathogenic bacteria

Rapid and sensitive detection of pathogenic bacteria in various samples, including food and drinking water, is important to prevent bacterial diseases. Most bacterial solutions contain only a small number of bacteria in complex matrices with impurities; hence, pretreatment is necessary to separate and concentrate target bacteria before sensing. Among various pretreatment methods, iron oxide magnetic nanoparticle (MNP)-based pretreatment has drawn attention owing to the unique properties of MNP, such as high magnetic susceptibility, superparamagnetism, and biocompatibility. After target bacteria are captured by recognition molecule-functionalized MNPs, bacteria-MNP complexes can be easily separated and enriched by applying an external magnetic field. Various devices, such as optical, electrochemical, and magnetoresistance sensors, can be used to detect target bacteria, and their detection principles have been discussed in numerous review papers. Herein, we focus on recent research advances and challenges in magnetic pretreatment of pathogenic bacteria using microfluidic devices, which offer the advantages of process automation and miniaturization.

Autonomous Internal Reflux of Magnetic Nanoparticle Chains in a Flow Channel for Efficient Detection of Waterborne Bacteria

Herein, we developed a novel method for the efficient capture of waterborne bacteria by creating an autonomous internal reflux of the magnetic nanoparticle chains (MNCs) inside a flow channel. A glass tube containing positively charged polyethyleneimine-coated MNCs (PEI-MNCs) was placed at the center of a Halbach ring, generating a strong and uniform magnetic field inside the ring. When a bacteria-spiked solution was injected into the tube, the target bacteria bound to the PEI-MNCs via an electrostatic interaction remained in the tube, whereas the unbound bacteria left the tube. Some PEI-MNC-bacteria complexes left the glass tube at high flow rates because of the drag force, which reduced the capture efficiency of the device. The loss of the PEI-MNC-bacteria complexes at high flow rates was suppressed by placing a k₀ ring behind the Halbach ring. The k₀ ring was used to apply a magnetic force in the opposite direction of the solution flow and create an autonomous reflux of the PEI-MNCs inside the glass tube, reducing their loss and increasing their capture efficiency. The capture efficiency of Escherichia coli O157 was determined based on the cell count to be greater than 90% at a flow rate of 15 mL/min. E. coli O157 was detected using quantitative polymerase chain reaction, and the limits of detection were 2 and 3 cfu/mL in deionized water and river water, respectively.

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples-A Review

In recent years, microfluidic lab-on-paper devices have emerged as a rapid and low-cost alternative to traditional laboratory tests. Additionally, they were widely considered as a promising solution for point-of-care testing (POCT) at home or regions that lack medical infrastructure and resources. This review describes important advances in microfluidic lab-on-paper diagnostics for human health monitoring and disease diagnosis over the past five years. The review commenced by explaining the choice of paper, fabrication methods, and detection techniques to realize microfluidic lab-on-paper devices. Then, the sample pretreatment procedure used to improve the detection performance of lab-on-paper devices was introduced. Furthermore, an in-depth review of lab-on-paper devices for disease measurement based on an analysis of urine samples was presented. The review concludes with the potential challenges that the future development of commercial microfluidic lab-on-paper platforms for human disease detection would face.

Development of a fluorescent distance-based paper device using loop-mediated isothermal amplification to detect Escherichia coli in urine

The highly sensitive and selective determination of Escherichia coli (E. coli) in urine was achieved using a SYBR™ safe loop-mediated isothermal amplification (LAMP) method with a distance-based paper device. New primers set specific to multi-copy the 16s rRNA gene of E. coli were designed and used in this study. The detection sensitivity of these primers was higher than in related work and they could be incorporated with a low-cost paper-based device to quantify E. coli in urine at a concentration lower than 101 CFU mL-1. Regarding standard artificial urine, a linear range of a 10-fold dilution of E. coli concentration (105-100 CFU mL-1) with an R-square value (R2) = 0.9823 was observed directly using a fluorescent migratory distance of the 4 μL reaction mixture in the detection zone under blue light without the need for postreaction staining process. Based on the device, E. coli infection could be significantly categorized into 3 groups; none, light, and heavy levels, which is beneficial for UTI diagnosis. Hence, this paper-based device is suitable for use with the SYBR™ Safe-LAMP assay to semi-quantify E. coli, especially in resource-limited settings due to advantages of low cost, simple fabrication and operation, and no requirement for sophisticated instruments, as well as its disposability and portability.

A Bioinspired Peptide in TIR Protein as Recognition Molecule on Electrochemical Biosensors for the Detection of E. coli O157:H7 in an Aqueous Matrix

Currently, the detection of pathogens such as Escherichia coli through instrumental alternatives with fast response and excellent sensitivity and selectivity are being studied. Biosensors are systems consisting of nanomaterials and biomolecules that exhibit remarkable properties such as simplicity, portable, affordable, user‑friendly, and deliverable to end‑users. For this, in this work we report for the first time, to our knowledge, the bioinformatic design of a new peptide based on TIR protein, a receptor of Intimin membrane protein which is characteristic of E. coli. This peptide (named PEPTIR‑1.0) was used as recognition element in a biosensor based on AuNPs‑modified screen‑printed electrodes for the detection of E. coli. The morphological and electrochemical characteristics of the biosensor obtained were studied. Results show that the biosensor can detect the bacteria with limits of detection and quantification of 2 and 6 CFU/mL, respectively. Moreover, the selectivity of the system is statistically significant towards the detection of the pathogen in the presence of other microorganisms such as P. aeruginosa and S. aureus. This makes this new PEPTIR‑1.0 based biosensor can be used in the rapid, sensitive, and selective detection of E. coli in aqueous matrices.

Turntable Paper-Based Device to Detect Escherichia coli

Escherichia coli has been known to cause a variety of infectious diseases. The conventional enzyme-linked immunosorbent assay (ELISA) is a well-known method widely used to diagnose a variety of infectious diseases. This method is expensive and requires considerable time and effort to conduct and complete multiple integral steps. We previously proposed the use of paper-based ELISA to rapidly detect the presence of E. coli. This approach has demonstrated utility for point-of-care (POC) urinary tract infection diagnoses. Paper-based ELISA, while advantageous, still requires the execution of several procedural steps. Here, we discuss the design and experimental implementation of a turntable paper-based device to simplify the paper-based ELISA protocols for the detection of E. coli. In this process, antibodies or reagents are preloaded onto zones of a paper-based device and allowed to dry before use. We successfully used this device to detect E. coli with a detection limit of 10⁵ colony-forming units (colony-forming unit [CFU])/mL.

Portable Device for Quick Detection of Viable Bacteria in Water

(1) Background: Access to clean water is a very important factor for human life. However, pathogenic microorganisms in drinking water often cause diseases, and convenient/inexpensive testing methods are urgently needed. (2) Methods: The reagent contains 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and phenazine methosulfate (PMS) and can react with succinate dehydrogenase within bacterial cell membranes to produce visible purple crystals. The colorimetric change of the reagent after reaction can be measured by a sensor (AS7262). (3) Results: Compared with traditional methods, our device is simple to operate and can provide rapid (i.e., 5 min) semi-quantitative results regarding the concentration of bacteria within a test sample. (4) Conclusions: This easy-to-use device, which employs MTT-PMS reagents, can be regarded as a potential and portable tool for rapid water quality determination.

Symptoms, risk factors, diagnosis and treatment of urinary tract infections

Urinary tract infection (UTI) is a common microbial infection found in all ages and sexes which involves inflammation of the urinary tract. These infections can range from simple bladder inflammation, that is, cystitis, to severe cases of uroseptic shock. UTI ranks as the number 1 infection that leads to a prescription of antibiotics after a doctor's visit. These infections are sometimes distressing and even life threatening, and both males (12%) and females (40%) have at least one symptomatic UTI throughout their lives. Diagnostic failures in case of bacterial infections are the main contributing factor in improper use of antibiotics, delay in treatment and low survival rate in septic conditions. So, early diagnosis and appropriate therapy with antibiotics are the most significant requirements for preventing complicated UTI conditions such as urosepsis. This review article summarises the symptoms of the UTIs and the associated risk factors to it. The various conventional and recent diagnostic methods were also discussed in this review, along with treatment therapies with or without antibiotics.

Digital dipstick: miniaturized bacteria detection and digital quantification for the point-of-care

Established digital bioassay formats, digital PCR and digital ELISA, show extreme limits of detection, absolute quantification and high multiplexing capabilities. However, they often require complex instrumentation, and extensive off-chip sample preparation. In this study, we present a dipstick-format digital biosensor (digital dipstick) that detects bacteria directly from the sample liquid with a minimal number of steps: dip, culture, and count. We demonstrate the quantitative detection of Escherichia coli (E. coli) in urine in the clinically relevant range of 102-105 CFU ml-1 for urinary tract infections. Our format shows 89% sensitivity to detect E. coli in clinical urine samples (n = 28) when it is compared to plate culturing (gold standard). The significance and uniqueness of this diagnostic test format is that it allows a non-trained operator to detect urinary tract infections in the clinically relevant range in the home setting.

Paper-based pump-free magnetophoresis

Microfluidic magnetophoresis is a powerful technique that is used to separate and/or isolate cells of interest from complex matrices for analysis. However, mechanical pumps are required to drive flow, limiting portability and making translation to point-of-care (POC) settings difficult. Microfluidic paper-based analytical devices (μPADs) offer an alternative to traditional microfluidic devices that do not require external pumps to generate flow. However, μPADs are not typically used for particle analysis because most particles become trapped in the porous fiber network. Here we report the ability of newly developed fast-flow microfluidic paper-based analytical devices (ffPADs) to perform magnetophoresis. ffPADs use capillary action in a gap between stacked layers of paper and transparency sheets to drive flow at higher velocities than traditional μPADs. The multi-layer ffPADs allow particles and cells to move through the gap without being trapped in the paper layers. We first demonstrate that ffPADs enable magnetic particle separations in a μPAD with a neodymium permanent magnet and study key factors that affect performance. To demonstrate utility, E. coli was used as a model analyte and was isolated from human urine before detection with a fluorescently labeled antibody. A capture efficiency of 61.5% was then obtained of E. coli labeled magnetic beads in human urine. Future studies will look at the improvement of the capture efficiency and to make this assay completely off-chip without the need of a fluorescent label. The assay and device described here demonstrate the first example of magnetophoresis in a paper based, pump free microfluidic device.

Nano-enabled sensing approaches for pathogenic bacterial detection

Infectious diseases caused by pathogenic bacteria, especially antibiotic-resistant bacteria, are one of the biggest threats to global health. To date, bacterial contamination is detected using conventional culturing techniques, which are highly dependent on expert users, limited by the processing time and on-site availability. Hence, real-time and continuous monitoring of pathogen levels is required to obtain valuable information that could assist health agencies in guiding prevention and containment of pathogen-related outbreaks. Nanotechnology-based smart sensors are opening new avenues for early and rapid detection of such pathogens at the patient's point-of-care. Nanomaterials can play an essential role in bacterial sensing owing to their unique optical, magnetic, and electrical properties. Carbon nanoparticles, metallic nanoparticles, metal oxide nanoparticles, and various types of nanocomposites are examples of smart nanomaterials that have drawn intense attention in the field of microbial detection. These approaches, together with the advent of modern technologies and coupled with machine learning and wireless communication, represent the future trend in the diagnosis of infectious diseases. This review provides an overview of the recent advancements in the successful harnessing of different nanoparticles for bacterial detection. In the beginning, we have introduced the fundamental concepts and mechanisms behind the design and strategies of the nanoparticles-based diagnostic platform. Representative research efforts are highlighted for in vitro and in vivo detection of bacteria. A comprehensive discussion is then presented to cover the most commonly adopted techniques for bacterial identification, including some seminal studies to detect bacteria at the single-cell level. Finally, we discuss the current challenges and a prospective outlook on the field, together with the recommended solutions.

Paper chip-based colorimetric assay for detection of Salmonella typhimurium by combining aptamer-modified Fe3O4@Ag nanoprobes and urease activity inhibition

A rapid and sensitive colorimetric assay is described for Salmonella typhimurium (S. typhimurium) detection using urea/phenol red impregnated test paper. Aptamer-modified Fe₃O₄@Ag multifunctional hybrid nanoprobes (apt-Fe₃O₄@Ag NPs) were used to specifically captured S. typhimurium; the nanoprobes were quickly etched by H₂O₂ to form Ag⁺. The generated Ag⁺ can inhibit the urease-catalyzed hydrolysis reaction of urea to produce NH₄⁺. Consequently, the as-prepared test paper displayed a yellow color. In the presence of S. typhimurium, the target bacteria can cause aggregation of apt-Fe₃O₄@Ag NPs, and the deposited Ag on the nanoprobe's surface is shielded against H₂O₂-induced oxidative decomposition leading to reduced Ag⁺ production. The catalytic activity of urease cannot be inhibited completely by inadequate amount of Ag⁺. An obvious color change from yellow to pink can be monitored directly using our test paper as a result of increased NH₄⁺. The entire assay procedure could be completed within 1 h. A limit of detection of 48 cfu/mL is achieved with a linear range of 1 × 10² to 1 × 10⁶ cfu/mL. The recoveries of S. typhimurium spiked in pure milk samples were 92.48-94.05%. Graphical abstract Schematic diagram of the proposed colorimetric assay for S. typhimurium detection based on etching of bifunctional apt-Fe₃O₄@Ag NPs and inhibiting catalytic activity of urease by Ag⁺. A color change from yellow to pink can be observed and correlated to the concentration of S. typhimurium.

Rapid detection of biofilm with modified alcian blue staining: In-vitro protocol improvement and validation with clinical cases

For chronic wounds, biofilm infection is a critical issue because it can tip the scales toward an unhealing state. Biofilm-based wound therapy has been extensively advocated. However, point-of-care biofilm diagnosis still largely relies on clinical judgment. In this study, we aimed to develop a rapid tool for diagnosing wound biofilm presence by alcian blue staining. First, we sought to optimize alcian blue staining using a colorimetric-based approach to detect the biofilm, specifically targeting polysaccharides in the extracellular polymeric substances. Among examined transfer membranes and cationic detergents at various concentrations, we selected a positively charged nylon transfer membrane for sample loading, and 1% cetyl trimethyl ammonium chloride (CTAC) as the blocking solution. After sample loading and blocking, the membrane was immersed in alcian blue solution for staining, followed by immersion in 1% CTAC to decrease background noise. Each step required only 30 seconds, and the whole procedure was completed within a few minutes. In the second part of this study, we enrolled 31 patients with chronic wounds to investigate the predictive validity of biofilm detection for unhealed wounds at a 1-month follow-up visit. Among the 18 cases with positive wound biofilm staining, 15 wounds (83.3%) were not healed at the 1-month follow-up visit. Only three unhealed wounds (30%) produced in negative staining cases. This finding indicates that biofilm infection is associated with poor healing outcome for chronic wounds. Moreover, our staining results correlated well with the clinical microbiological culture assessment (83.9% consistency; 95.2% sensitivity, and 60% specificity). In conclusion, the modified alcian blue staining protocol used here represents a rapid and sensitive procedure for detecting biofilm in chronic wounds. This technique provides a practical point-of-care approach for detection of wound biofilm, the implementation of which may improve clinical outcomes for chronic wound patients. Additional studies are required to validate this method.

A novel and facile immunosensor based on a barometer: Application for rapid analysis of Escherichia coli in waters

A facile immunosensor was constructed based on a barometer indicator using a double-antibody- sandwich method for rapid and sensitive detection of Escherichia coli (E. coli) from water samples. At the present study, Anti- E. coli DH-5 polyclonal antibody was modified through enriching carboxylated magnetic beads and catalase functionalized gold nanoparticles (AuNPs)- loaded nanospheres. The functionalized AuNPs-loaded nanospheres exhibited an excellent catalysis towards decomposition of hydrogen peroxide (H₂O₂), generating a large volume of oxygen (O₂) into waters, increasing the pressure inside the glass vial, which in turn raised the water level in barometer. Under the optimized experimental conditions, the proposed method showed wide linear ranges (10²- 10⁷ cfu mL^-1), good accuracy and precision (recoveries, 86.7- 107%; CV, 3.2- 8.1%) with a limit of detection (LOD, S/N = 3) and the limit of quantification (LOQ, S/N = 10) were 80 cfu mL^-1and 267 cfu mL^-1, respectively. Furthermore, the fabricated portable immunosensor device showed some distinct features in low cost and visibility, suggesting great potential for rapid and on-site analysis of this bacteria from waters in less developed areas of developing countries.

A Sensitive and Simple Enzyme-Linked Immunosorbent Assay Using Polymer as Carrier

In this study, a new and sensitive enzyme-linked immunosorbent assay (ELISA) was developed by introducing a polymer as a reaction carrier. The results suggest that the newly developed ELISA method is more convenient than the existing paper-based ELISA method and applicable to a wider range of environments. In addition, the sensitivity of the new method is much higher than that of the existing paper-based ELISA method and even higher than that of the traditional ELISA method.

Recent Advances of Fluid Manipulation Technologies in Microfluidic Paper-Based Analytical Devices (μPADs) toward Multi-Step Assays

Microfluidic paper-based analytical devices (μPADs) have been suggested as alternatives for developing countries with suboptimal medical conditions because of their low diagnostic cost, high portability, and disposable characteristics. Recently, paper-based diagnostic devices enabling multi-step assays have been drawing attention, as they allow complicated tests, such as enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR), which were previously only conducted in the laboratory, to be performed on-site. In addition, user convenience and price of paper-based diagnostic devices are other competitive points over other point-of-care testing (POCT) devices, which are more critical in developing countries. Fluid manipulation technologies in paper play a key role in realizing multi-step assays via μPADs, and the expansion of biochemical applications will provide developing countries with more medical benefits. Therefore, we herein aimed to investigate recent fluid manipulation technologies utilized in paper-based devices and to introduce various approaches adopting several principles to control fluids on papers. Fluid manipulation technologies are classified into passive and active methods. While passive valves are structurally simple and easy to fabricate, they are difficult to control in terms of flow at a specific spatiotemporal condition. On the contrary, active valves are more complicated and mostly require external systems, but they provide much freedom of fluid manipulation and programmable operation. Both technologies have been revolutionized in the way to compensate for their limitations, and their advances will lead to improved performance of μPADs, increasing the level of healthcare around the world.

Laser-patterned paper-based sensors for rapid point-of-care detection and antibiotic-resistance testing of bacterial infections

Antimicrobial resistance (AMR) has been identified by the World Health Organisation as a global threat that currently claims at least 25,000 deaths each year in Europe and 700,000 globally; the number is projected to reach 10 million per year between 2015 and 2050. Therefore, there is an urgent need for low-cost but reliable point-of-care diagnostics for early screening of infections especially in developing countries lacking in basic infrastructure and trained personnel. This work is aimed at developing such a device, a paper-based microfluidic device for infection testing by an unskilled user in a low resource setting. Here, we present our work relating to the use of our laser-patterned paper-based devices for detection and susceptibility testing of Escherichia coli, via a simple visually observable colour change. The results indicate the suitability of our integrated paper devices for timely identification of bacterial infections at the point-of-care and their usefulness in providing a hugely beneficial pathway for accurate antibiotic prescribing and thus a novel route to tackling the global challenge of AMR.

Developing a toll-like receptor biosensor for Gram-positive bacterial detection and its storage strategies

Conditions to store toll-like receptor2/6 sensors and use them to detect bacterial analytes, including pathogen-associated molecular patterns and bacterial cultures.

Rapid and accurate detection of Escherichia coli O157:H7 in beef using microfluidic wax-printed paper-based ELISA

Escherichia coli O157:H7 is a severe foodborne pathogen. Paper-based ELISA can rapidly and accurately detect E.coli O157:H7 in beef. The method has good sensitivity, specificity and repeatability. It is suitable for point-of-care testing and offers new ideas for the detection of other foodborne pathogens.

Novel paper-based colorimetric immunoassay (PCI) for sensitive and specific detection of salbutamol residues in flesh of swine and urine using Ag3 PO4 /Ag nanocomposite as label

Salbutamol (SAL) can cause potential hazards to human health and its use as a growth promoter in meat-producing animals is illegal. This work reports a novel approach for competitive paper-based colorimetric immunoassay (PCI) using the Ag₃ PO₄ /Ag nanocomposite as label for sensitive and specific determination of SAL in flesh of swine and urine. The Ag₃ PO₄ /Ag nanocomposite was synthesized by a one-step chemical bath method, which could instantly oxidize a chromogenic substrate for the color development under acidic conditions without the participation of H₂ O₂ . This approach provides high affinity between the Ag₃ PO₄ /Ag nanocomposite and the substrate (with the Michaelis-Menten constant of 0.44 mM). In addition, the fabrication process of the PCI was simple and cost-effective. Particularly, the novel PCI also exhibits simplicity and cost-effectiveness of the fabrication process through a simple wax screen-printing, which requires inexpensive equipment and material including a screen, wax, a squeegee, and a hair dryer. Under optimal conditions, the competitive PCI exhibited a linearity range of 0.025 to 1.00 µg/L. The developed approach offers advantages over the conventional ELISA for the purpose of routine use because it requires a shorter incubation time (<1 hr), significantly small volumes of reagents and samples (<100 µL each), and an inexpensive consumer-grade digital camera coupled with a simple gray-scale transformation of the RGB (Red Green Blue) color image for the purpose of quantification of the detection. PRACTICAL APPLICATION: Salbutamol (SAL) can cause potential hazards to human health and the use of which as growth promoter in meat-producing animals is illegal. This work introduces a novel approach for competitive immunoassay on paper-based colorimetric immunoassay using the Ag₃ PO₄ /Ag nanocomposite as the label (instead of using natural enzyme) for low-cost, sensitive, and specific determination of SAL residues at low level in flesh of swine and urine samples. The proposed approach offers advantages over the conventional ELISA for the purpose of routine use.

Two Potential Clinical Applications of Origami-Based Paper Devices

Detecting small amounts of analyte in clinical practice is challenging because of deficiencies in specimen sample availability and unsuitable sampling environments that prevent reliable sampling. Paper-based analytical devices (PADs) have successfully been used to detect ultralow amounts of analyte, and origami-based PADs (O-PADs) offer advantages that may boost the overall potential of PADs in general. In this study, we investigated two potential clinical applications for O-PADs. The first O-PAD we investigated was an origami-based enzyme-linked immunosorbent assay (ELISA) system designed to detect different concentrations of rabbit IgG. This device was designed with four wing structures, each of which acted as a reagent loading zone for pre-loading ELISA reagents, and a central test sample loading zone. Because this device has a low limit of detection (LOD), it may be suitable for detecting IgG levels in tears from patients with a suspected viral infection (such as herpes simplex virus (HSV)). The second O-PAD we investigated was designed to detect paraquat levels to determine potential poisoning. To use this device, we sequentially folded each of two separate reagent zones, one preloaded with NaOH and one preloaded with ascorbic acid (AA), over the central test zone, and added 8 µL of sample that then flowed through each reagent zone and onto the central test zone. The device was then unfolded to read the results on the test zone. The three folded layers of paper provided a moist environment not achievable with conventional paper-based ELISA. Both O-PADs were convenient to use because reagents were preloaded, and results could be observed and analyzed with image analysis software. O-PADs expand the testing capacity of simpler PADs while leveraging their characteristic advantages of convenience, cost, and ease of use, particularly for point-of-care diagnosis.

Atom transfer radical polymer-modified paper for improvement in protein fixation in paper-based ELISA

A newly modified paper-based enzyme-linked immunosorbent assay (P-ELISA) was established by immobilizing more proteins on the paper surface through an atom transfer radical polymerization (ATRP) reaction. In addition, introducing graphene oxide (GO) sheets, Au nanoparticles (AuNps) and two primary antibodies (Ab1s) led to signal amplification and cost reduction.

Fabrication of laser printed microfluidic paper-based analytical devices (LP-µPADs) for point-of-care applications

Microfluidic paper-based analytical devices (µPADs) have provided a breakthrough in portable and low-cost point-of-care diagnostics. Despite their significant scope, the complexity of fabrication and reliance on expensive and sophisticated tools, have limited their outreach and possibility of commercialization. Herein, we report for the first time, a facile method to fabricate µPADs using a commonly available laser printer which drastically reduces the cost and complexity of fabrication. Toner ink is used to pattern the µPADs by printing, without modifying any factory configuration of the laser printer. Hydrophobic barriers are created by heating the patterned paper which melts the toner ink, facilitating its wicking into the cross-section of the substrate. Further, we demonstrate the utilization of the fabricated device by performing two assays. The proposed technique provides a versatile platform for rapid prototyping of µPADs with significant prospect in both developed and resource constrained region.

Wax-Printed Fluidic Time Delays for Automating Multi-Step Assays in Paper-Based Microfluidic Devices (MicroPADs)

Microfluidic paper-based analytical devices (microPADs) have emerged as a promising platform for point-of-care diagnostic devices. While the inherent wicking properties of microPADs allow for fluid flow without supporting equipment, this also presents a major challenge in achieving robust fluid control, which becomes especially important when performing complex multi-step assays. Herein, we describe an ideal method of fluid control mediated by wax-printed fluidic time delays. This method relies on a simple fabrication technique, does not utilize chemicals/reagents that could affect downstream assays, is readily scalable, and has a wide temporal range of tunable fluid control. The delays are wax printed on both the top and bottom of pre-fabricated microPAD channels, without subsequent heating, to create hemi-/fully-enclosed channels. With these wax printed delays, we were able to tune the time it took aqueous solutions to wick across a 25 mm-long channel between 3.6 min and 13.4 min. We then employed these fluid delays in the sequential delivery of four dyes to a test zone. Additionally, we demonstrated the automation of two simple enzymatic assays with this fluid control modality. This method of fluid control may allow future researchers to automate more complex assays, thereby further advancing microPADs toward real-world applications.

Emerging Point-of-care Technologies for Food Safety Analysis

Food safety issues have recently attracted public concern. The deleterious effects of compromised food safety on health have rendered food safety analysis an approach of paramount importance. While conventional techniques such as high-performance liquid chromatography and mass spectrometry have traditionally been utilized for the detection of food contaminants, they are relatively expensive, time-consuming and labor intensive, impeding their use for point-of-care (POC) applications. In addition, accessibility of these tests is limited in developing countries where food-related illnesses are prevalent. There is, therefore, an urgent need to develop simple and robust diagnostic POC devices. POC devices, including paper- and chip-based devices, are typically rapid, cost-effective and user-friendly, offering a tremendous potential for rapid food safety analysis at POC settings. Herein, we discuss the most recent advances in the development of emerging POC devices for food safety analysis. We first provide an overview of common food safety issues and the existing techniques for detecting food contaminants such as foodborne pathogens, chemicals, allergens, and toxins. The importance of rapid food safety analysis along with the beneficial use of miniaturized POC devices are subsequently reviewed. Finally, the existing challenges and future perspectives of developing the miniaturized POC devices for food safety monitoring are briefly discussed.

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.

Gold nanoparticles-based multifunctional nanoconjugates for highly sensitive and enzyme-free detection of E.coli K12

Immobilization of proteins on a biocompatible conductive interface is highly desirable for the fabrication of biosensors. In this study, a nanocomposite has been prepared by assembling well-distributed gold nanoparticles (AuNPs) on the surface of a polypyrrole-reduced graphene oxide (PPy-rGO) composite through electrostatic adsorption. This serves as a platform for immobilization of a capture antibody, which was conjugated onto the ferrocene doped polypyrrole-gold nanoparticles (PPy@Fc/AuNPs) composite. The design and performance of the biosensor was tested against detection of a whole-cell bacteria E. coli K12. This nanocomposite has a high surface area, good conductivity and biocompatibility, which is shown to be very suitable for enzyme-free detection of this bacteria. Results show excellent analytical performance with a linear range from 1.0 × 10¹ to 1.0 × 10⁷ CFU mL^-1 and a low detection limit of 10 CFU mL^-1. The sensor has high selectivity, excellent reproducibility, and good stability.

Electrically-receptive and thermally-responsive paper-based sensor chip for rapid detection of bacterial cells

Although significant technological advancements have been made in the development of analytical biosensor chips for detecting bacterial strains (E. coli, S. Mutans and B. Subtilis), critical requirements i.e. limit of detection (LOD), fast time of response, ultra-sensitivity with high reproducibility and good shelf-life with robust sensing capability have yet to be met within a single sensor chip. In order to achieve these criteria, we present an electrically-receptive thermally-responsive (ER-TR) sensor chip comprised of simple filter paper used as substrate coated with composite of poly(N-isopropylacrylamide) polymer (PNIPAm) - graphene nanoplatelet (GR) followed by evaporation of Au electrodes for capturing both Gram-positive (S. mutans and B. subtilis) and Gram-negative (E. coli) bacterial cells in real-time. Autoclave water, tap water, lake water and milk samples were tested with ER-TR chip with and without bacterial strains at varying concentration range 10¹-10⁵ cells/mL. The sensor was integrated with in-house built printed circuit board (PCB) to transmit/receive electrical signals. The interaction of E. coli, S. mutans and B. subtilis cells with fibers of PNIPAm-GR resulted in a change of electrical resistance and the readout was monitored wirelessly in real-time using MATLAB algorithm. Finally, prepared ER-TR chip exhibited the reproducibility of 85-97% with shelf-life of up to four weeks after testing with lake water sample.