Dual Synthetic Receptor-Based Sandwich Electrochemical Sensor for Highly Selective and Ultrasensitive Detection of Pathogenic Bacteria at the Single-Cell Level

"Emerging technologies for detecting foodborne pathogens and spoilage microorganisms in milk: Ensuring safety and quality"

Foodborne and spoilage microorganisms pose significant challenges to both food safety and product integrity, particularly within the dairy industry. These microorganisms can adversely affect milk quality, safety, shelf life, and the overall sustainability of the dairy industry, making their detection a critical concern in food science. This review explores a range of emerging and innovative detection techniques, including molecular, immunological, and sensor-based methods, with a focus on their practical implementation in dairy microbiology. While culture-based methods remain the gold standard, they suffer from several limitations, including being time-consuming, labor-intensive, costly, and less effective at detecting non-culturable microorganisms. Recent advancements in rapid, high-throughput, and high-sensitivity detection technologies have transformed the ability to identify and control both foodborne pathogens and spoilage microorganisms in milk, offering superior accuracy, efficiency, and real-time monitoring capabilities. This review provides a comprehensive overview of cutting-edge microbial detection approaches in milk, highlighting the key characteristics of these emerging methods developed in recent years. Furthermore, global regulatory standards in milk microbiology among eight representative countries were comparatively analyzed to elucidate the current landscape of dairy microbiology regulatory frameworks. Our primary objective is to synthesize current scientific research to offer insights into the effectiveness, practicality, and industry adoption of these emerging detection technologies. We critically examine studies that have contributed to the development of rapid and accurate microbial detection methods, assessing their impact on food safety, quality assurance, and regulatory compliance. By consolidating and analyzing cutting-edge advancements in microbial detection, this review aims to contribute to ongoing efforts to strengthen both food safety and quality assurance in the dairy industry by integrating novel detection technologies into routine monitoring practices. We aim to provide a valuable resource for researchers, industry professionals, and policymakers, facilitating informed decision-making and promoting the adoption of effective microbial detection strategies from dairy farms to consumer products.

Vancomycin-bacterial imprinted polymer hybrid for viable Staphylococcus aureus highly efficient capture, photothermal inactivation, and sensitive detection

To facilitate the capture, inactivation, and detection of viable Staphylococcus aureus (SA) in food, a vancomycin (Van)-bacterial imprinted polymers (BIPs) hybrid receptor was fabricated by introducing Van into SA-imprinted polydopamine (PDA) via oriented surface imprinting. Taking advantage of the dual recognition arising from Van's affinity for peptidoglycan and BIPs' imprinting effect on SA, the Van-BIPs hybrid exhibited better capture performance than Van or BIPs alone. Leveraging the photothermal effect of PDA, the captured SA could be in situ lysed within 5 min under near-infrared irradiation, causing the release of adenosine triphosphate (ATP) from SA. Using ATP as biomarker, as low as 13.7 CFU/mL of viable SA was able to be detected within 38 min by integrating Van-BIPs hybrid with ATP-bioluminescence assay. Spiked food samples were also successfully analyzed with the recoveries of 85.71 %-106.11 %. The Van-BIPs hybrid might offer a promising tool to control SA in food industries.

Rapid point-of-care pathogen sensing in the post-pandemic era

In the post-pandemic era, interest in on-site technologies capable of rapidly and accurately diagnosing viral or bacterial pathogens has significantly increased. Advances in functional nanomaterials and bioengineering have propelled the progress of point-of-care (POC) sensors, enhancing their speed, specificity, sensitivity, affordability, ease of use, and accuracy. Notably, biosensors that utilize surface-enhanced Raman scattering (SERS) technology have revolutionized the rapid and sensitive diagnosis of biomarkers in pathogenic infections. This review of current POC diagnostics highlights the growing emphasis on immunoassays for swift pathogen analysis, augmented by the integration of deep learning for swift interpretation of complex signals through tailored algorithms.

Selective Single-Bacterium Analysis and Motion Tracking Based on Conductive Bulk-Surface Imprinting

Conductive molecular imprinting (MI) shows great potential in enhancing the selectivity of electrochemical bacterial assays, but its efficiency is hindered due to the rigid long-conjugated structure and imprecise specific recognition sites. It is thus urgent to activate the surface MI with clear specific recognition sites toward the bacteria and to develop a single-bacterium monitoring technique for precisely verifying the MI efficiency microscopically. Herein, using lipopolysaccharides and Escherichia coli (E. coli) cells as the surface and bulk templates, respectively, an ideal monomer is successfully predicted by the density functional theory, and the MI with clear and high-precision recognition sites for bacterial matching is prepared. A deep learning-assisted single-bacteria movement trajectory tracking method is developed, and the trained model can effectively recognize and track the movement paths and velocities of both single and group bacteria. Accordingly, the surface MI capture process of the specific recognition sites for E. coli is systematically monitored and analyzed, opening the way for establishing a multidimensional system for characterizing the selective capture process of single and group bacteria by MI polymers. Moreover, the as-prepared electrochemical sensors accomplish the rapid, sensitive sensing of E. coli with a detection limit of 10 CFU/mL and a 433%-increased selectivity, which could promote the development of finer-grained bacterial imprinting techniques and smart bacterial biosensors.

A Dual-Recognition Electrochemical Sensor Using Bacteria-Imprinted Polymer and Concanavalin A for Sensitive and Selective Detection of Escherichia coli O157:H7

The accurate detection and quantification of pathogenic bacteria is crucial for ensuring public health. In this work, we propose a sensitive and selective sandwich electrochemical sensor for detecting Escherichia coli O157:H7 (E. coli O157:H7). The sensor employs a dual-recognition strategy that combines a bacteria-imprinted polymer (BIP) and concanavalin A (ConA). The BIP is formed in situ on the electrode surface as the capture probe, while gold nanoparticles co-functionalized with ConA and the electroactive molecule 6-(ferrocenyl)hexanethiol (Au@Fc-ConA) serve as the signaling probe. When E. coli O157:H7 is present, the bacteria are selectively captured by the BIP. The captured bacteria interact with Au@Fc-ConA through ConA's sugar-binding properties, triggering Fc oxidation and generating a current proportional to the bacterial concentration. The sensor exhibits a linear detection range of 101-105 CFU mL-1 and a low detection limit of 10 CFU mL-1. Additionally, it demonstrates high sensitivity in complex milk samples, detecting E. coli O157:H7 at concentrations as low as 10 CFU mL-1, with recoveries ranging from 94.16% to 110.6%. Even in the presence of a 100-fold higher concentration of E. coli O6, the sensor effectively distinguishes E. coli O157:H7 from it. Moreover, it exhibits high reproducibility with a relative standard deviation of 2%. This study proposes a unique dual recognition strategy that combines simplicity and high performance. This method enables the selective detection of E. coli O157:H7 in real samples, providing a promising tool for food safety monitoring.

Electrochemical/fluorescent dual-mode aptasensor based on 3D porous AuNPs/MXene for detection of ultra-trace mercury (Hg2+)

In this work, the dual-mode aptasensor based on 3D porous AuNPs/MXene using "turn-on" electrochemical method and "turn-off" fluorescent strategy was fabricated. Here, 2D MXene was processed into 3D porous MXene by sacrificial polymethylmethacrylate (PMMA) spherical template. And the meteor hammer-like AuNPs which had good electrochemical properties and quenching effect on fluorescence was synthesized by single electrodeposition. Dual-signal labeled Nile Blue (NB) was in situ grafted to the Hg2+ aptamer ends of 3D porous AuNPs/MXene/GCE, and an efficient and sensitive signal interface was constructed to realize the sensitive detection of Hg2+. 3D porous AuNPs/MXene had the advantages of large specific surface area, excellent electron transmission performance and signal amplification. The experimental results indicated that this sensor exhibited high sensitivity to Hg2+ in both electrochemical and fluorescent sensing, with detection limits of 2.69 fM and 1.60 fM, respectively. Further, the dual-mode aptasensor can ensure the detection accuracy and target quantization. The dual-mode aptasensor has been successfully applied to the ultra-trace detection of Hg2+ in actual water samples, which shows the potential of aptamer sensor in detecting heavy metal ions in environmental monitoring.

A fully integrated wireless microfluidic immunosensing system for portable monitoring of Staphylococcus aureus

The advanced devices that function fully without the need for external accessories are regarded as a pinnacle goal in the design and construction of modern ones. Staphylococcus aureus (S. aureus), a prominent human pathogen, is responsible for causing a wide variety of infections and chronic diseases. Herein, we present the first instance of a fully integrated wireless microfluidic immunosensing system (FIWMIS) capable of conducting point-of-care S. aureus monitoring in real samples of S. aureus-spiked commercial purified drinking water and S. aureus-spiked watermelon juice. The development of the proposed FIWMIS became a reality by conquering significant engineering hurdles in seamlessly integrating a microfluidic unit for liquid sample transport without the need of an external pump, an immunosensing unit for S. aureus monitoring, and an electronic control unit for signal conversion and wireless transmission. Such full integration culminated in a FIWMIS that upholds its pump-free, wireless, and low-cost characteristics for portable monitoring of S. aureus.

Au NPs modified Ni-B nanosheets/graphene oxide three-dimensional network as label-free electrochemical immunosensor for the detection of diethylstilbestrol

Three-dimensional (3D) network provide a promising platform for construction of high sensitive electrochemical immunosensor due to the benefits of high specific surface area and electron mobility. Herein, a sensitive label-free electrochemical immunosensor based on Au nanoparticles modified Ni-B nanosheets/graphene matrix was constructed to detect diethylstilbestrol (DES). The 3D network not only could increase the electron transport rate and surface area, but also could provide confinement area, which is conducive to increases the collision frequency with the active site. Moreover, Au NPs also have good biocompatibility, which is beneficial for ligating antibodies. Benefiting from the 3D network structure and Au collective effect, the electrochemical immunosensor possess sterling detection ability with wide linear response range (0.00038-150 ng/mL) and low detection limit (31.62 fg/mL). Moreover, the constructed immunosensor can also be extend to detect DES in Tap-water and river water. This work may provide a novel material model for the construction of high sensitive immunosensor.

Hazardous Materials from Threats to Safety: Molecularly Imprinted Polymers as Versatile Safeguarding Platforms

Hazards associated with highly dangerous pollutants/contaminants in water, air, and land resources, as well as food, are serious threats to public health and the environment. Thus, it is imperative to detect or decontaminate, as risk-control strategies, the possible harmful substances sensitively and efficiently. In this context, due to their capacity to be specifically designed for various types of hazardous compounds, the synthesis and use of molecularly imprinted polymers (MIPs) have become widespread. By molecular imprinting, affinity sites with complementary shape, size, and functionality can be created for any template molecule. MIPs' unique functions in response to external factors have attracted researchers to develop a broad range of MIP-based sensors with increased sensitivity, specificity, and selectivity of the recognition element toward target hazardous compounds. Therefore, this paper comprehensively reviews the very recent progress of MIPs and smart polymer applications for sensing or decontamination of hazardous compounds (e.g., drugs, explosives, and biological or chemical agents) in various fields from 2020 to 2024, providing researchers with a rapid tool for investigating the latest research status.

An antifouling electrochemical biosensor using self-signal for Salmonella typhimurium direct detection in food sample

Eating food contaminated by foodborne pathogens can lead to illness. The development of electrochemical sensors for pathogen detection has received widespread attention. However, the analytical performance of electrochemical sensors is inevitably affected by the non-specific adsorption of molecules in the sample. Moreover, the external signal probes might be affected by the complex components in the sample accompanied with signal suppression. This work presents an electrochemical aptasensor for Salmonella typhimurium detection based on the self-signal of poly-xanthurenic acid and the antifouling ability of chondroitin sulfate. The detection time was 60 min. The linear range was from 101 to 107 CFU/mL, and the detection limit was 3 CFU/mL. The biosensors presented good repeatability and storage stability. And the biosensors has been successfully applied in milk and orange juice. This strategy is expected to be applied in the design of other antifouling biosensors, to achieve rapid detection of pathogens and ensure food safety.

Sensitive sandwich-type electrochemical immunosensing of p53 protein based on Ti3C2Tx MXene nanoribbons and ferrocene/gold

Since the p53 protein is an important promising biomarker of lung tumor and colorectal tumor, it is very essential to design a highly effective mean to monitor the degree of p53 for the early clinical analysis/therapy of the related tumors. In this work, a sandwich-type electrochemical immunosensing (SES) platform is proposed for the first time to detect p53 via synthesizing Ti3C2Tx MXene nanoribbons (Ti3C2Tx Nb) and ferrocene/gold nanoparticles (Fc/Au) respectively as the sensing substrate and signal-amplifier. The superior electrical property and large surface area of Ti3C2Tx Nb are beneficial to assemble the initial p53-antibodies (Ab1), while the synthesized Fc/Au is devoted to assemble the secondary p53 antibodies (Ab2) and gives a magnified signal. By adopting the Fc molecules as the probes, the experiments reveal the response current of Fc resulted from the SES structure increases along with the p53 increase from 1.0 to 200.0 pg mL-1. A considerable low detection limit (1.0 pg mL-1) is achieved after optimizing several key conditions, it is thus confirmed the as-proposed SES mean exhibits significant application in the detection of p53 protein and other targets.

Portable Saliva Sensor Based on Dual Recognition Elements for Detection of Caries Pathogenic Bacteria

Dental caries is one of the most common diseases affecting more than 2 billion people's health worldwide. In a clinical setting, it is challenging to predict and proactively guard against dental cavities prior to receiving a confirmed diagnosis. Streptococcus mutans (S. mutans) in saliva has been recognized as the main causative bacterial agent that causes dental caries. High sensitivity, good selectivity, and a wide detection range are incredibly important factors to affect S. mutans detection in practical applications. In this study, we present a portable saliva biosensor designed for the early detection of S. mutans with the potential to predict the occurrence of dental cavities. The biosensor was fabricated using a S. mutans-specific DNA aptamer and S. mutans-imprinted polymers. Methylene blue was utilized as a redox probe in the sensor to generate current signals for analysis. When S. mutans enters complementarily S. mutans cavities, it blocks electron transfer between methylene blue and the electrode, resulting in decreases in the reduction current signal. The signal variations are associated with S. mutans concentrations that are useful for quantitative analysis. The linear detection range of S. mutans is 102-109 cfu mL-1, which covers the critical concentration of high caries risk. The biosensor exhibited excellent selectivity toward S. mutans in the presence of other common oral bacteria. The biosensor's wide detection range, excellent selectivity, and low limit of detection (2.6 cfu mL-1) are attributed to the synergistic effect of aptamer and S. mutans-imprinted polymers. The sensor demonstrates the potential to prevent dental caries.

Ultrasensitive and Highly Selective Detection of Staphylococcus aureus at the Single-Cell Level Using Bacteria-Imprinted Polymer and Vancomycin-Conjugated MnO2 Nanozyme

Pathogenic bacterial infections, even at extremely low concentrations, pose significant threats to human health. However, the challenge persists in achieving high-sensitivity bacterial detection, particularly in complex samples. Herein, we present a novel sandwich-type electrochemical sensor utilizing bacteria-imprinted polymer (BIP) coupled with vancomycin-conjugated MnO2 nanozyme (Van@BSA-MnO2) for the ultrasensitive detection of pathogenic bacteria, exemplified by Staphylococcus aureus (S. aureus). The BIP, in situ prepared on the electrode surface, acts as a highly specific capture probe by replicating the surface features of S. aureus. Vancomycin (Van), known for its affinity to bacterial cell walls, is conjugated with a Bovine serum albumin (BSA)-templated MnO2 nanozyme through EDC/NHS chemistry. The resulting Van@BSA-MnO2 complex, serving as a detection probe, provides an efficient catalytic platform for signal amplification. Upon binding with the captured S. aureus, the Van@BSA-MnO2 complex catalyzes a substrate reaction, generating a current signal proportional to the target bacterial concentration. The sensor displays remarkable sensitivity, capable of detecting a single bacterial cell in a phosphate buffer solution. Even in complex milk matrices, it maintains outstanding performance, identifying S. aureus at concentrations as low as 10 CFU mL-1 without requiring intricate sample pretreatment. Moreover, the sensor demonstrates excellent selectivity, particularly in distinguishing target S. aureus from interfering bacteria of the same genus at concentrations 100-fold higher. This innovative method, employing entirely synthetic materials, provides a versatile and low-cost detection platform for Gram-positive bacteria. In comparison to existing nanozyme-based bacterial sensors with biological recognition materials, our assay offers distinct advantages, including enhanced sensitivity, ease of preparation, and cost-effectiveness, thereby holding significant promise for applications in food safety and environmental monitoring.

Recent Advances in Aptamer-Based Biosensors for Bacterial Detection

The rapid and sensitive detection of pathogenic bacteria is becoming increasingly important for the timely prevention of contamination and the treatment of infections. Biosensors based on nucleic acid aptamers, integrated with optical, electrochemical, and mass-sensitive analytical techniques, have garnered intense interest because of their versatility, cost-efficiency, and ability to exhibit high affinity and specificity in binding bacterial biomarkers, toxins, and whole cells. This review highlights the development of aptamers, their structural characterization, and the chemical modifications enabling optimized recognition properties and enhanced stability in complex biological matrices. Furthermore, recent examples of aptasensors for the detection of bacterial cells, biomarkers, and toxins are discussed. Finally, we explore the barriers to and discuss perspectives on the application of aptamer-based bacterial detection.

Simultaneous Electrochemical Detection of Multiple Bacterial Species Using Metal-Organic Nanohybrids

Organic metallic nanohybrids (NHs), in which many small metal nanoparticles are encapsulated within a conductive polymer matrix, are useful as sensitive electrochemical labels because the constituents produce characteristic oxidation current responses. Gold NHs, consisting of gold nanoparticles and poly(m-toluidine), and copper NHs, consisting of copper nanoparticles and polyaniline, did not interfere with each other in terms of the electrochemical signals obtained on the same electrode. Antibodies were introduced into these NHs to function as electrochemical labels for targeting specific bacteria. Electrochemical measurements using screen-printed electrodes dry-fixed with NH-labeled bacterial cells enabled the estimation of bacterial species and number within minutes, based on the distinct current response of the labels. Our proposed method achieved simultaneous detection of enterohemorrhagic Escherichia coli and Staphylococcus aureus in a real sample. These NHs will be powerful tools as electrochemical labels and are expected to be useful for rapid testing in food and drug-related manufacturing sites.

The Compact Integration of Multiple Exonuclease III-Assisted Cyclic Amplification Units for High-Efficiency Ratiometric Electrochemiluminescence Detection of MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) exhibits multiresistance to a plethora of antibiotics, therefore, accurate detection methods must be employed for timely identification to facilitate effective infection control measures. Herein, we construct a high-efficiency ratiometric electrochemiluminescent (ECL) biosensor that integrates multiple exonuclease (Exo) III-assisted cyclic amplification units for rapid detection of trace amounts of MRSA. The target bacteria selectively bind to the aptamer, triggering the release of two single-stranded DNAs. One released DNA strand initiates the opening of a hairpin probe, inducing exonuclease cleavage to generate a single strand that can form a T-shaped structure with the double strand connecting the oxidation-reduction (O-R) emitter of N-(4-aminobutyl)-N-ethylisoluminol gold (ABEI-Au). Consequently, ABEI-Au is released upon Exo III cleavage. The other strand unwinds the hairpin DNA structure on the surface of the reduction-oxidation (R-O) emitter ZIF-8@CdS, facilitating the subsequent release of a specific single strand through Exo III cleavage. This process effectively anchors the cathode-emitting material to the electrode. The Fe(III) metal-organogel (Fe-MOG) is selected as a substrate, in which the catalytic reduction of hydrogen peroxide by Fe(III) active centers accelerates the generation of reactive oxygen species and enhances signals from both ABEI-Au and ZIF-8@CdS. In this way, the two emitters cooperate to achieve bacterial detection at the single-cell level, and a good linear range is obtained in the range of 100-107 CFU/mL. Moreover, the sensor exhibited excellent performance in detecting MRSA across various authentic samples and accurately quantifying MRSA levels in serum samples, demonstrating its immense potential in addressing clinical bacterial detection challenges.

ZnO@Ag-Functionalized Paper-Based Microarray Chip for SERS Detection of Bacteria and Antibacterial and Photocatalytic Inactivation

Bacterial infections caused by pathogenic microorganisms have become a serious, widespread health concern. Thus, it is essential and required to develop a multifunctional platform that can rapidly and accurately determine bacteria and effectively inhibit or inactivate pathogens. Herein, a microarray SERS chip was successfully synthesized using novel metal/semiconductor composites (ZnO@Ag)-ZnO nanoflowers (ZnO NFs) decorated with Ag nanoparticles (Ag NPs) arrayed on a paper-based chip as a supporting substrate for in situ monitoring and photocatalytic inactivation of pathogenic bacteria. Typical Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli and Vibrio parahemolyticus were selected as models. Partial least-squares discriminant analysis (PLS-DA) was performed to minimize the dimensionality of SERS spectra data sets and to develop a cost-effective identification model. The classification accuracy was 100, 97.2, and 100% for S. aureus, E. coli, and V. parahemolyticus, respectively. The antimicrobial activity of ZnO@Ag was proved by the microbroth dilution method, and the minimum inhibitory concentrations (MICs) of S. aureus, E. coli, and V. parahemolyticus were 40, 50, and 55 μg/mL, respectively. Meanwhile, it demonstrated remarkable photocatalytic performance under natural sunlight for the inactivation of pathogenic bacteria, and the inactivation rates for S. aureus, E. coli, and V. parahemolyticus were 100, 97.03 and 97.56%, respectively. As a result, the microarray chip not only detected the bacteria with high sensitivity but also confirmed the antibacterial and photocatalytic sterilization properties. Consequently, it offers highly prospective strategies for foodborne diseases caused by pathogenic bacteria.

A Novel Dual Bacteria-Imprinted Polymer Sensor for Highly Selective and Rapid Detection of Pathogenic Bacteria

The rapid, sensitive, and selective detection of pathogenic bacteria is of utmost importance in ensuring food safety and preventing the spread of infectious diseases. Here, we present a novel, reusable, and cost-effective impedimetric sensor based on a dual bacteria-imprinted polymer (DBIP) for the specific detection of Escherichia coli O157:H7 and Staphylococcus aureus. The DBIP sensor stands out with its remarkably short fabrication time of just 20 min, achieved through the efficient electro-polymerization of o-phenylenediamine monomer in the presence of dual bacterial templates, followed by in-situ template removal. The key structural feature of the DBIP sensor lies in the cavity-free imprinting sites, indicative of a thin layer of bacterial surface imprinting. This facilitates rapid rebinding of the target bacteria within a mere 15 min, while the sensing interface regenerates in just 10 min, enhancing the sensor's overall efficiency. A notable advantage of the DBIP sensor is its exceptional selectivity, capable of distinguishing the target bacteria from closely related bacterial strains, including different serotypes. Moreover, the sensor exhibits high sensitivity, showcasing a low detection limit of approximately 9 CFU mL-1. The sensor's reusability further enhances its cost-effectiveness, reducing the need for frequent sensor replacements. The practicality of the DBIP sensor was demonstrated in the analysis of real apple juice samples, yielding good recoveries. The integration of quick fabrication, high selectivity, rapid response, sensitivity, and reusability makes the DBIP sensor a promising solution for monitoring pathogenic bacteria, playing a crucial role in ensuring food safety and safeguarding public health.