Colorimetric Immunoassay for Rapid Detection of Vibrio parahemolyticus Based on Mn2+ Mediates the Assembly of Gold Nanoparticles

Advancing foodborne pathogen detection: a review of traditional and innovative optical and electrochemical biosensing approaches

Foodborne diseases are a significant cause of morbidity (600 million cases) and mortality (420,000 deaths) worldwide every year and are mainly associated with pathogens. Besides the direct effects on human health, they have relevant concerns related to financial, logistics, and infrastructure for the food and medical industries. The standard pathogen identification techniques usually require a sample enrichment step, plating, isolation, and biochemical tests. This process involves specific facilities, a long-time analysis procedures, and skilled personnel. Conversely, biosensors are an emerging innovative approach to detecting pathogens in real time due to their portability, specificity, sensitivity, and low fabrication costs. These advantages can be achieved from the synergistic work between nanotechnology, materials science, and biotechnology for coupling biomolecules in nano-matrices to enhance biosensing performance. This review highlights recent advancements in electrochemical and optical biosensing techniques for detecting bacteria and viruses. Key properties, such as detection limits, are examined, as they depend on factors like the design of the biorecognition molecule, the type of transducer, the target's characteristics, and matrix interferences. Sensitivity levels reported range from 1 to 1 × 10⁸ CFU/mL, with detection times spanning 10 min to 8 h. Additionally, the review explores innovative approaches, including biosensors capable of distinguishing between live and dead bacteria, multimodal sensing, and the simultaneous detection of multiple foodborne pathogens - emerging trends in biosensor development.

Ultrasensitive detection platform for Staphylococcus aureus based on DNAzyme tandem blocking CRISPR/Cas12a system

Detection methods based on CRISPR/Cas12a have been widely developed in the application of pathogenic microorganisms to guarantee food safety and public health. For sensitive detection, the CRISPR-based strategies are often in tandem with amplification methods. However, that may increase the detection time and the process may introduce nucleic acid contamination resulting in non-specific amplification. Herein, we established a sensitive S. aureus detection strategy based on the CRISPR/Cas12a system combined with DNAzyme. The activity of Cas12a is blocked by extending the spacer of crRNA (bcrRNA) and can be reactivated by Mn2+. NH2-modified S. aureus-specific aptamer was loaded on the surface of Fe3O4 MNPs (apt-Fe3O4 MNPs) and MnO2 NPs (apt-MnO2 NPs) by EDC/NHS chemistry. The S. aureus was captured to form apt-Fe3O4 MNPs/S. aureus/apt-MnO2 NPs complex and then MnO2 NPs were etched to release Mn2+ to activate DNAzyme. The active DNAzyme can cleave the hairpin structure in bcrRNA to recover the activity of the CRISPR/Cas system. By initiating the whole detection process by generating Mn2+ through nanoparticle etching, we established a rapid detection assay without nucleic acid extraction and amplification process. The proposed strategy has been applied in the ultrasensitive quantitative detection of S. aureus and has shown good performance with an LOD of 5 CFU/mL in 29 min. Besides, the proposed method can potentially be applied to other targets by simply changing the recognition element and has the prospect of developing a universal detection strategy.

Sensing the invisible: Ultrasensitive and selective colorimetric detection of E. coli O157:H7 based on masking the peroxidase-mimetic activity of aptamer-modified Au/Fe3O4

Escherichia coli O157:H7 (E. coli O157:H7) emerges as a significantly worrisome pathogen associated with foodborne illnesses, emphasizing the imperative for creating precise detection tools. In this investigation, we developed a sensitive colorimetric biosensor for detecting E. coli O157:H7. It was constructed using a nanozyme comprised of Au@Fe3O4 NPs, which was fabricated and subsequently modified with an aptamer (Apt). The nanozyme harnesses its inherent peroxidase-like activity to facilitate the transformation of reduced TMB into its oxidized form in the presence of H2O2, resulting in a noticeable shift to a blue color. However, the presence of E. coli O157:H7 effectively diminished the absorbance of oxidized TMB. Consequently, the normalized absorbance at 652 nm demonstrates a linear decrease corresponding to concentrations of E. coli O157:H7 within the range of 101 to 108 CFU mL-1 with a low limit of detection (LOD, S/N = 3) of 3 CFU mL-1.

Characteristics of biological manganese oxides produced by manganese-oxidizing bacteria H38 and its removal mechanism of oxytetracycline

Oxytetracycline (OTC) is widely used in clinical medicine and animal husbandry. Residual OTC can affect the normal life activities of microorganisms, animals, and plants and affect human health. Microbial remediation has become a research hotspot in the environmental field. Manganese oxidizing bacteria (MnOB) exist in nature, and the biological manganese oxides (BMO) produced by them have the characteristics of high efficiency, low cost, and environmental friendliness. However, the effect and mechanism of BMO in removing OTC are still unclear. In this study, Bacillus thuringiensis strain H38 of MnOB was obtained, and the conditions for its BMO production were optimized. The optimal conditions were determined as follows: optimal temperature = 35 °C, optimal pH = 7.5, optimal Mn(Ⅱ) initial concentration = 10 mmol/L. The results show that BMO are irregular or massive, mainly containing MnCO3, Mn2O3, and MnO2, with rich functional groups and chemical bonds. They have the characteristics of small particle size and large specific surface area. OTC (2.5 mg/L) was removed when the BMO dosage was 75 μmol/L and the solution pH was 5.0. The removal ratio was close to 100 % after 12 h of culture at 35 °C and 150 r/min. BMO can adsorb and catalyze the oxidation of OTC and can produce ·O2-, ·OH, 1O2, and Mn(Ⅲ) intermediate. Fifteen products and degradation pathways were identified, and the toxicity of most intermediates is reduced compared to OTC. The removal mechanism was preliminarily clarified. The results of this study are convenient for the practical application of BMO in OTC pollution in water and for solving the harm caused by antibiotic pollution.

In situ detection of silk fibroin using a dual recognition strategy with a flexible pressure immunosensor

Silk is a symbol of ancient Chinese civilization that has made an indelible contribution to the development of world civilization. However, because ancient artifacts are often contaminated or degraded, it is difficult to detect the presence of silk therein, and the true origin of silk thus remains a mystery. Therefore, this work presents a flexible pressure immunosensor that was designed based on 3D polypyrrole (PPy) foams for the trace detection of silk fibroin at archaeological sites. Initially, silk fibroin (SF) was conjugated with antibody-functionalized copper oxide nanoparticles (CuO NPs) and carboxylated magnetic beads (MBs) to form a sandwich immune complex. Then, the sandwich immune complex was added to hydrogen peroxide (H2O2) by magnetic separation to catalyse the generation of oxygen (O2), which converted the antigen-antibody specific recognition signal to gas pressure. As the pressure within the device increases, the 3D PPy foam, as the sensing layer resistance was 150 Ω, undergoes extrusion and deformation. This deformation leads to alterations in the foam resistance. The flexible pressure immunosensor can sensitively monitor the change in electrical resistance in the system and quantitatively detect silk fibroin. With optimization, the flexible pressure immunosensor demonstrates a dynamic range of operation spanning from 10 ng mL-1 to 100 μg mL-1, exhibiting a remarkable detection limit of 10.58 ng mL-1 specifically for silk fibroin. Notably, this immunosensor surpasses enzyme-linked immunosorbent assay (ELISA) in terms of superior reproducibility, specificity, and accuracy. Therefore, this application provides a new method and technical support for silk detection.

Polychaeta-mediated synthesis of gold nanoparticles: A potential antibacterial agent against Acute Hepatopancreatic Necrosis Disease (AHPND)-causing bacteria, Vibrio parahaemolyticus

Gold nanoparticles (AuNPs) have emerged as a promising application in aquaculture. Their nano-sized dimensions, comparable to pathogens offer potential solutions for combating antibiotic resistance. In this study, AuNPs were synthesized by using polychaetes, Marphysa moribidii as the bio-reducing agent. Modifications were made to reduce agglomeration in green-synthesized AuNPs through ultrasonication. The antibacterial activities of AuNPs against V. parahaemolyticus were evaluated. The physicochemical characteristics of the green synthesized AuNPs were comprehensively investigated. The successful formation of AuNPs was confirmed by the appearance of a red ruby colour and the presence of surface Plasmon resonance (SPR) absorption peaks at 530 nm as observed from UV-vis spectroscopy. Scanning electron microscopy (SEM) revealed spherical-shaped AuNPs with some agglomerations. Transmission electron microscopy (TEM) showed particle size of AuNPs ranging from 10 nm to 60 nm, meanwhile dynamic light scattering (DLS) analysis indicated an average particle size of 24.36 nm. X-ray diffraction (XRD) analysis confirmed the high crystallinity of AuNPs, and no AuNPs were detected in the polychaetes extracts prior to synthesis. A brief ultrasonication significantly reduced the tendencies for AuNPs to coalesce. The green-synthesized AuNPs demonstrated a remarkable antibacterial efficacy against V. parahaemolyticus. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests revealed that a concentration of 0.3 g/ml of AuNPs effectively inhibited V. parahaemolyticus. These findings highlighted the potential of green-synthesized AuNPs as antibacterial agents for the prevention and management of AHPND in aquaculture.

Gold Nanoparticle Paper Immunoassays for Sensing the Presence of Vibrio parahaemolyticus in Oyster Hemolymph

Seafood contamination with Vibrio bacteria is a problem for aquaculture, especially with oysters, which are often consumed raw. Current methods for diagnosing bacterial pathogens in seafood involve lab-based assays such as polymerase chain reaction or culturing, which are time consuming and must occur in a centralized location. Detection of Vibrio in a point-of-care assay would be a significant tool for food safety control measures. We report here a paper immunoassay that can detect the presence of Vibrio parahaemolyticus (Vp) in buffer and oyster hemolymph. The test uses gold nanoparticles conjugated to polyclonal anti-Vibrio antibodies in a paper-based sandwich immunoassay. A sample is added to the strip and wicked through by capillary action. If Vp is present, it results in a visible color at the test area that can be read out by eyes or a standard mobile phone camera. The assay has a limit of detection of 6.05 × 105 cfu/mL and a cost estimate of $5 per test. Receiver operating characteristic curves with validated environmental samples showed a test sensitivity of 0.96 and a specificity of 1.00. Because the assay is inexpensive and can be used on Vp directly without the requirement for culturing, or sophisticated equipment, it has the potential to be used in fieldable settings.

A single thiolated-phage displayed nanobody-based biosensor for label-free detection of foodborne pathogen

Rapid and sensitive detection of bacterial pathogens present in food and environmental samples is of crucial importance to ensure human health and safety. Here, we present a one-step label-free colorimetric strategy based on M13 bacteriophage-displayed nanobody (phage-Nb) derived from camelid heavy-chain antibodies specific to Vibrio parahaemolyticus (V. parahaemolyticus). The thiolation of phage-Nb (Phage-Nb-SH) on pVIII shell proteins induces the aggregation of gold nanoparticles (AuNPs), whereas the specific interaction between nanobody and bacteria prevents the aggregation of AuNPs, resulting in visible color change due to alteration of surface plasmon resonance properties. Based on this phenomenon, a simple and sensitive colorimetric immunosensor for V. parahaemolyticus was developed. The assay can be accomplished within 100 min, and exhibits a visual detection limit of 10⁴ cfu/mL and a quantitative detection limit of 10³ cfu/mL, with no cross-reactivity towards other bacterial species. This strategy takes full advantages of both the high specificity of phage-Nbs and the optical properties of AuNPs, enabling simple and rapid detection of bacterial pathogens.

Gold Nanoparticles-Based Colorimetric Assays for Environmental Monitoring and Food Safety Evaluation

Recent years have witnessed an exponential increase in the research on gold nanoparticles (AuNPs)-based colorimetric sensors to revolutionize point-of-use sensing devices. Hence, this review is compiled focused on current progress in the design and performance parameters of AuNPs-based sensors. The review begins with the characteristics of AuNPs, followed by a brief explanation of synthesis and functionalization methods. Then, the mechanisms of AuNPs-based sensors are comprehensively explained in two broad categories based on the surface plasmon resonance (SPR) characteristics of AuNPs and their peroxidase-like catalytic properties (nanozyme). SPR-based colorimetric sensors further categorize into aggregation, anti-aggregation, etching, growth-mediated, and accumulation-based methods depending on their sensing mechanisms. On the other hand, peroxidase activity-based colorimetric sensors are divided into two methods based on the expression or inhibition of peroxidase-like activity. Next, the analytes in environmental and food samples are classified as inorganic, organic, and biological pollutants, and recent progress in detection of these analytes are reviewed in detail. Finally, conclusions are provided, and future directions are highlighted. Improving the sensitivity, reproducibility, multiplexing capabilities, and cost-effectiveness for colorimetric detection of various analytes in environment and food matrices will have significant impact on fast testing of hazardous substances, hence reducing the pollution load in environment as well as rendering food contamination to ensure food safety.

Recent Progress in Nanotechnology-Based Approaches for Food Monitoring

Throughout the food supply chain, including production, storage, and distribution, food can be contaminated by harmful chemicals and microorganisms, resulting in a severe threat to human health. In recent years, the rapid advancement and development of nanotechnology proposed revolutionary solutions to solve several problems in scientific and industrial areas, including food monitoring. Nanotechnology can be incorporated into chemical and biological sensors to improve analytical performance, such as response time, sensitivity, selectivity, reliability, and accuracy. Based on the characteristics of the contaminants and the detection methods, nanotechnology can be applied in different ways in order to improve conventional techniques. Nanomaterials such as nanoparticles, nanorods, nanosheets, nanocomposites, nanotubes, and nanowires provide various functions for the immobilization and labeling of contaminants in electrochemical and optical detection. This review summarizes the recent advances in nanotechnology for detecting chemical and biological contaminations in the food supply chain.

Cascade amplification strategy combined with analyte-triggered fluorescence switching of dual-quenching system for highly sensitive detection of isoniazide

A sensitive fluorescence sensing platform consisting of manganese dioxide nanosheets (MnO₂) and gold nanoparticles (AuNPs) as dual nanoquenchers has been constructed to detect isoniazid combined with analyte-triggered cascade reactions. The fluorescence of 2,3-diaminophenazine (DAP) is quenched simultaneously by MnO₂ and AuNPs via inner filter effect. MnO₂ is decomposed by isoniazid to generate Mn²⁺, which makes AuNPs aggregated. The quenching abilities of both the decomposed MnO₂ and aggregated AuNPs are inhibited, causing remarkable fluorescence recovery. The usage of dual nanoquenchers enhances the quenching efficiency and reduces the fluorescence background. Moreover, the isoniazid-triggered cascade reaction further amplifies the readout signal. Thus, this strategy exhibits higher sensitivity towards the detection of isoniazid. Compared with MnO₂-based fluorescence assay, this strategy possesses lower limit of detection. This strategy has been successfully used to detect isoniazid in pharmaceutical preparations, which is of great significance for drug analysis.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

Gold nanoparticle-based optical nanosensors for food and health safety monitoring: recent advances and future perspectives

Modern society has been facing serious health-related problems including food safety, diseases and illness. Hence, it is urgent to develop analysis methods for the detection and control of food contaminants, disease biomarkers and pathogens. As the traditional instrumental methods have several disadvantages, including being time consuming, and having high cost and laborious procedures, optical nanosensors have emerged as promising alternative or complementary approaches to those traditional ones. With the advantages of simple preparation, high surface-to-volume ratio, excellent biocompatibility, and especially, unique optical properties, gold nanoparticles (AuNPs) have been demonstrated as excellent transducers for optical sensing systems. Herein, we provide an overview of the synthesis of AuNPs and their excellent optical properties that are ideal for the development of optical nanosensors based on local surface plasmon resonance (LSPR), colorimetry, fluorescence resonance energy transfer (FRET), and surface-enhanced Raman scattering (SERS) phenomena. We also review the sensing strategies and their mechanisms, as well as summarizing the recent advances in the monitoring of food contaminants, disease biomarkers and pathogens using developed AuNP-based optical nanosensors in the past seven years (2015-now). Furthermore, trends and challenges in the application of these nanosensors in the determination of those analytes are discussed to suggest possible directions for future developments.

Potentiometric aptasensing of Escherichia coli based on electrogenerated chemiluminescence as a highly sensitive readout

We introduce here a versatile approach to read out potentiometric aptasensors by electrogenerated chemiluminescence (ECL), which can amplify the small potential changes induced by the bacterial concentrations via ECL signals. In the present system, the electrode modified with single-walled carbon nanotubes (SWCNTs) and aptamer molecules acts as the reference electrode and is placed in the sample solution for sensing the bacterial concentration changes, while the Ru(bpy)₃²⁺ modified gold electrode serves as the working electrode for generating ECL signals and is placed in the detection solution containing tripropylamine (TPA) spatially separated from the sample solution by a salt bridge. Ru(bpy)₃²⁺ is immobilized on the gold electrode's surface for enhancement of luminous efficiency and reduction of reagent consumption. A moving-part-free fluid flowing system is introduced to promote the mass transport of TPA from the detection solution to the surface of the ECL generating electrode. When a constant potential is imposed between the working and reference electrodes, the potential changes at the SWCNTs-aptamer modified electrode induced by the bacterial concentrations can modulate the potentials at the Ru(bpy)₃²⁺ modified electrode, thus generating the ECL signals. The developed sensing strategy shows a highly sensitive response to E. coli O157: H7 in the linear range of 5-1000 CFU mL^-1 with a low detection limit of 2 CFU mL^-1. We believe that the proposed approach is promising to develop aptasensors for sensitive detection of bacterial cells.

Visual and quantitative detection of E. coli O157:H7 by coupling immunomagnetic separation and quantum dot-based paper strip

Simple and visual quantitative detection of foodborne pathogens can effectively reduce the outbreaks of foodborne diseases. Herein, we developed a simple and sensitive quantum dot (QD)-based paper device for visual and quantitative detection of Escherichia coli (E. coli) O157:H7 based on immunomagnetic separation and nanoparticle dissolution-triggered signal amplification. In this study, E. coli O157:H7 was magnetically separated and labeled with silver nanoparticles (AgNPs), and the AgNP labels can be converted into millions of Ag ions, which subsequently quench the fluorescence of QDs in the paper strip, which along with the readout can be visualized and quantified by the change in length of fluorescent quenched band. Owing to the high capture efficiency and effective signal amplification, as low as 500 cfu mL^-1 of E. coli O157:H7 could be easily detected by naked eyes. Furthermore, this novel platform was successfully applied to detect E. coli O157:H7 in spiked milk samples with good accuracy, indicating its potential in the detection of foodborne pathogens in real samples.

Efficient removal of nitrate, manganese, and tetracycline by a polyvinyl alcohol/sodium alginate with sponge cube immobilized bioreactor

The co-existence of nitrate, manganese (Mn), and antibiotics are of a wide concern. In this study, a denitrifying and manganese-oxidizing Zoogloea Q7 bacterium was immobilized using polyvinyl alcohol/sodium alginate with sponge cube (PVA/SA@sponge cube) in the reactor. The optimal operation parameters of the bioreactor were explored. Maximum nitrate, Mn(II), and tetracycline (TC) removal efficiencies of 93.00, 72.34, and 57.32% were achieved with HRT of 10 h, pH of 6.5, Mn(II) concentration of 20 mg L^-1, and TC of 1 mg L^-1, respectively. Fluorescence excitation-emission matrix (EEM) proved that the microorganism in the bioreactor was greatly active. Scanning electron microscope (SEM) images demonstrated that Zoogloea Q7 was commendably immobilized on the novel material. X-ray diffraction (XRD) analysis suggested that the bioprecipitate was mainly composed of MnO₂ and MnCO_3. Through high-throughput analysis, Zoogloea sp. Q7 was considered to be the dominant bacteria present in the bioreactor.

One-step colorimetric detection of Staphylococcus aureus based on target-induced shielding against the peroxidase mimicking activity of aptamer-functionalized gold-coated iron oxide nanocomposites

Staphylococcus aureus (S. aureus) is one of the most threatened food-borne pathogens. Thus, it is necessary to establish fast, portable and reliable tools to realize the identification of S. aureus. Herein, the authors describe an effective colorimetric-based biosensor for the detection of S. aureus in multiple types of samples. Initially, a nanozyme composed of gold and iron oxide nanoparticles was synthesized and further modified with S. aureus-specific aptamer via Au-S bond. By utilizing the intrinsic peroxidase-like activity of the above magnetic conjugates, 3,3',5,5'-tetramethylbenzidine (TMB) can be transferred to oxTMB by oxidation of hydrogen peroxide (H₂O₂), resulting in a visible blue color. However, the introduction of S. aureus can turn off the UV-vis absorbance signals of TMB-H₂O₂ system, due to the identification property of the nanozyme probe. Consequently, the optical density of the mixed solution measured at 652 nm decreased linearly as the concentration of S. aureus increased from 10 to 10⁶ CFU mL^-1, with the visible limit of detection as low as 10 CFU mL^-1. The as-prepared sensor can detect S. aureus in spiked water, milk and urine samples quantitatively during 12 min without any pre-enrichment, separation or washing steps. In our perception, the one-step colorimetric assay show promise in practical on-site detection of S. aureus.

Advances in Portable Visual Detection of Pathogenic Bacteria

Food safety and regulation of consumer welfare are of great concern, so it is necessary to be able to detect pathogenic bacteria quickly and effectively. Although traditional methods of pathogen detection are reliable and widely used, the detection and analysis processes are cumbersome and time-consuming, which is not conducive to fast assays in the field. New detection strategies have emerged in recent years, especially point-of-care testing (POCT) methods, which do not rely on the laboratory and have become an important development direction for pathogen detection. Many visual detection schemes have been developed that integrate portable glucose meters (PGMs), test strips, smartphones, and other portable devices. Importantly, portable and ultrasensitive biosensors have vast promise in detecting pathogens, as they can be suitable tools for clinical diagnosis and the regulation of food safety. This Review focuses on the latest advances in portable device-based methods for visual detection of pathogens, evaluating their advantages and disadvantages.

A nanozyme-linked immunosorbent assay based on metal-organic frameworks (MOFs) for sensitive detection of aflatoxin B1

In order to avoid the occurrence of false positives and false negatives caused by conventional enzyme-linked immunosorbent assay (ELISA), we established a novel indirect competitive MOF-linked immunosorbent assay (MOFLISA) method for the high throughput and high sensitive detection of aflatoxin B₁. This method replaces the natural enzyme with functional MOFs to catalyze a chromogenic system. As a result, the limit of detection (LOD) of the MOFLISA method was 0.009 ng·mL^-1 with a linear working range from 0.01 to 20 ng·mL^-1. The developed MOFLISA method for AFB₁ has a 20-fold improved LOD value compared with the conventional ELISA. The recoveries and relative standard deviations (RSD) ranged from 86.41 to 99.74% and 2.38-9.04%, respectively. The results demonstrate that the recovery rate and accuracy of this detection method is better than that of conventional ELISA, reducing risks offalsepositive andfalsenegativeresults.

Optical nanosensors for biofilm detection in the food industry: principles, applications and challenges

Biofilms are the universal lifestyle of bacteria enclosed in extracellular polymeric substances (EPS) on the contact surfaces of food processing facilities. The EPS-encapsulated foodborne bacterial pathogens are the main food contaminant sources, posing a serious threat to human health. The microcrystalline, sophisticated and dynamic biofilms necessitate the development of conventional microscopic imaging and spectral technology. Nanosensors, which can transfer the biochemical information into optical signals, have recently emerged for biofilm optical detection with high sensitivity and high spatial resolution at nanoscale scopes. Therefore, the aim of this review is to clarify the main detection scope in biofilms and the detection principles of optical nanosensors arousing Raman enhancement, fluoresce conversion and color change. The difficulties and challenges of biofilm characterization including the secretion and variation of main biochemical components are first discussed, the details about the principles and application examples of bioassays targeting foodborne pathogens based on optical nanosensors are then summarized. Finally, the challenges and future trends in developing optical nanosensors are also highlighted. The current review indicates that optical nanosensors have taken the challenges of detecting biofilm in complex food samples, including the characterization of biofilm formation mechanism, identification of microbial metabolic activities, diagnosis of potential food pathogens and sanitation monitoring of food processing equipment. Numerous in-depth explorations and various trials have proven that the bioassays based on multifunctional optical nanosensors are promising to ensure and promote food safety and quality. However, there still remains a daunting challenge to structure reproducible, biocompatible and applicable nano-sensors for biofilm characterization, identification, and imaging.

A universal signal-on electrochemical assay for rapid on-site quantitation of vibrio parahaemolyticus using aptamer modified magnetic metal-organic framework and phenylboronic acid-ferrocene co-immobilized nanolabel

Sensitive and rapid detection of pathogenic bacteria remains important and challenging for food safety and preventing outbreaks of foodborne disease. The major limitations of standard analytical methods for detecting vibrio parahaemolyticus (V.P) lie in their bulky equipment and tedious and long-time operation. This study presents an electrochemical aptasensor for the rapid on-site quantification of V.P in seafood. Magnetic nanoscale metal-organic frameworks (Fe₃O₄@NMOF) labeled with an aptamer against V.P served as capture probes, while gold nanoparticles combined with phenylboronic acid and ferrocene acted as the nanolabels. When detecting V.P, the sandwich-type complex of capture probe-V.P-nanolabel was formed and magnetically attached to a screen-printed electrode (SPE) for signal measurement. Under optimal conditions, the increase in the ferrocene electrochemical signals could assess the V.P amount; the quantified concentration range was 10-10⁹ cfu/mL. Then, the developed signal-on sensor successfully detected V.P in real seafood samples, exhibiting many advantages. It could not only specifically enrich and rapidly separate the V.P in complex samples but also largely amplify the signal. Moreover, using compact SPE with a detection time of maximum 20 min as the measurement platform allows rapid on-site assays. Thus, the proposed method is a feasible strategy for screening V.P in seafood.

A manganese-oxidizing bacterial consortium and its biogenic Mn oxides for dye decolorization and heavy metal adsorption

Manganese (Mn) contamination is a common environmental problem in the world and manganese oxidizing bacteria (MOB) play important roles in bioremediation of heavy metal and organic pollution. In this study, a novel MOB consortium AS containing core microbes of Sphingobacterium and Bacillus was acclimated from Mn-contaminated rivulet sediments. The MOB consortium AS presented good Mn(II) removal performance under 500-10,000 mg/L Mn(II), with Mn(II) removal capacities ranging from 481 to 3478 mg/L. In coexistence systems of Mn(II) and Fe(II), Ni(II), Cu(II), and Zn(II), the MOB consortium AS removed 98%, 91%, 99%, and 76% of Mn(II), respectively. Additionally, the MOB consortium AS could utilize multiple carbon sources (e.g., Chitosan, β-Cyclodextrin, and Phenanthrene) to remove Mn(II), with Mn(II) removal efficiencies ranging from 11% to 97%. Meanwhile, XRD, XPS, FTIR, SEM, and EDS analyses reflected that biogenic Mn oxides (bio-MnOx-C) contained C, O, Mn (Mn(II) and Mn(IV)) and embodied in rhodochrosite and birnessite. The bio-MnOx-C exhibited second-order kinetic reaction for removal of dye, with corresponding decolorization capacities of 22.0 mg/g for methylene blue and 23.8 mg/g for crystal violet. In addition, bio-MnOx-C showed adsorption capacities of 159.0 mg/g for Cu(II), 130.7 mg/g for Zn(II), and 123.3 mg/g for Pb(II). Overall, this study illustrates consortium AS and bio-MnOx-C have great potentials in remediation of pollution caused by heavy metals and organic pollutants.

An improved recombinase polymerase amplification assay for visual detection of Vibrio parahaemolyticus with lateral flow strips

Vibrio parahaemolyticus is an important pathogenic bacterium in both food safety management and mariculture. Rapid and accurate detection technologies are critical for effective control of its outbreak and spreading. Conventional technologies and polymerase chain reaction (PCR)-based approaches have limited usage because of the requirement of laboratory instruments and trained personnel. Using the isothermal recombinase polymerase amplification (RPA) technology, several detection assays have been developed with added convenience. Combining the lateral flow strip (LFS) test with RPA can further simplify the detection. In this study, an improved RPA assay using LFS for visual detection of V. parahaemolyticus was developed. Primers were designed targeting the virulence genes and screened for amplification efficiency, nonspecific amplification, and primer-dimer formation. Probes were designed for the best primer pairs, and the weakness of LFS tests, being easily affected by primer-dependent artifacts, was overcome by sequence modifications on primers and probe. The RPA-LFS assay took 25 min at 35 to 45 °C, and showed excellent specificity. It detected as low as one colony forming unit (CFU) of V. parahaemolyticus per reaction without DNA purification, or 10 CFU/10 g spiked food samples with 2 hr of enrichment. The detection limit was better than the currently available RPA-based detection methods. Application of the RPA-LFS assay for simulated samples or real clinical samples showed accurate and consistent detection results compared to bioassay and quantitative PCR. The RPA-LFS assay provided a rapid, accurate, and convenient V. parahaemolyticus detection method suitable for on-site detection in resource-limited conditions. PRACTICAL APPLICATION: This research developed a rapid and visual detection technology for Vibrio parahaemolyticus that is not dependent on complicated equipment. The detection process takes 25 min and the result is read with the naked eye. A detection kit can be developed based on this technology for on-site detection of V. parahaemolyticus in resource-limited regions for food safety management and mariculture.

Colorimetric aptasensor for ochratoxin A detection based on enzyme-induced gold nanoparticle aggregation

An innovative colorimetric method based on enzyme-induced gold nanoparticle aggregation was developed to detect the activity of alkaline phosphatase (ALP), and it was further applied to construct an aptasensor to monitor ochratoxin A (OTA) concentrations. In the presence of ALP, the substrate ascorbic acid 2-phosphate was hydrolyzed to generate ascorbic acid (AA). Subsequently, reduction of MnO₂ nanosheets by AA produced manganese ions, which mediated gold nanoparticle aggregation. The color of the detection solution changed from brown-red to purple to blue as the ALP concentration increased, and a detection limit of 0.05 U·L^-1 was achieved. Furthermore, this strategy was successfully utilized to devise a target-responsive aptasensor for colorimetric detection of an important mycotoxin, OTA, which causes food poisoning and has various toxic effects on humans. The proposed method offers high sensitivity with a detection limit as low as 5.0 nM together with high specificity. When applied to analyze red wine and grape juice samples, no complex sample pretreatment or bulky instruments were required. Overall, a colorimetric platform based on enzyme-induced gold nanoparticle aggregation was successfully established to improve the simplicity and sensitivity of ALP and OTA detection. This platform appears highly promising for mycotoxin-related food safety monitoring.

Multicolor colorimetric detection of ochratoxin A via structure-switching aptamer and enzyme-induced metallization of gold nanorods

Colorimetric aptasensors have been intensively studied for the ochratoxin A (OTA) detection, but they mostly exhibit just one-color change, resulting in poor visual resolution and limited use for semi-quantitative analysis. Thus, we designed a high-resolution colorimetric assay on the basis of aptamer structural switching and enzyme-induced metallization of gold nanorods (AuNRs). DNA-alkaline phosphatase (ALP)-immobilized magnetic beads were prepared. The aptamer bounded to OTA to form G-quadruplexes, releasing ALP-labelled complementary DNA (cDNA-ALP). After magnetic separation, cDNA-ALP catalyzed the decomposition of ascorbic acid 2-phosphate to ascorbic acid that reduced Ag⁺, forming an Ag shell on the surface of AuNRs. This caused a blue-shift of the longitudinal local surface plasmon resonance peak of the AuNRs and a naked eye visible multicolor change. Under optimal conditions, the assay exhibited a 9.0 nM detection limit for OTA, with high specificity. This method is promising for the on-site visual semi-quantitative detection of mycotoxins in foods.

Manganese Oxide Nanomaterials: Synthesis, Properties, and Theranostic Applications

Despite the comprehensive applications in bioimaging, biosensing, drug/gene delivery, and tumor therapy of manganese oxide nanomaterials (MONs including MnO₂ , MnO, Mn₂ O₃ , Mn₃ O₄ , and MnO_x ) and their derivatives, a review article focusing on MON-based nanoplatforms has not been reported yet. Herein, the representative progresses of MONs on synthesis, heterogene, properties, surface modification, toxicity, imaging, biodetection, and therapy are mainly introduced. First, five kinds of primary synthetic methods of MONs are presented, including thermal decomposition method, exfoliation strategy, permanganates reduction method, adsorption-oxidation method, and hydro/solvothermal. Second, the preparations of hollow MONs and MON-based composite materials are summarized specially. Then, the chemical properties, surface modification, and toxicity of MONs are discussed. Next, the diagnostic applications including imaging and sensing are outlined. Finally, some representative rational designs of MONs in photodynamic therapy, photothermal therapy, chemodynamic therapy, sonodynamic therapy, radiotherapy, magnetic hyperthermia, chemotherapy, gene therapy, starvation therapy, ferroptosis, immunotherapy, and various combination therapy are highlighted.

Rapid visualized isothermal nucleic acid testing of Vibrio parahaemolyticus by polymerase spiral reaction

The aim of this study was to develop an effective and specific visual method to rapidly detect and identify Vibrio parahaemolyticus (V. parahaemolyticus) based on the polymerase spiral reaction (PSR). The method utilized only two pairs of primers designed specifically to target the conserved tlh gene sequence of V. parahaemolyticus. Nucleic acid amplification can be achieved under isothermal conditions using DNA polymerase. The reaction could be accomplished in < 40 min with high specificity and sensitivity. The limits of detection of V. parahaemolyticus in purified genomic DNA and pure culture were 300 fg/μL and 2.4 CFU/mL per reaction, respectively, which were 100-fold more sensitive than with conventional PCR. The model food samples showed consistent specificity and sensitivity to the pure bacterial culture. With these encouraging results, it is expected that the novel, effortless and reliable isothermal nucleic acid testing assay developed in this study has potential to be applied to screening for V. parahaemolyticus in seafood samples.

Sensitive and Matrix-Tolerant Lateral Flow Immunoassay Based on Fluorescent Magnetic Nanobeads for the Detection of Clenbuterol in Swine Urine

The lack of sensitivity and poor matrix tolerance are the main bottlenecks of the lateral flow immunoassay (LFIA). Here, a sensitive and matrix-tolerant method that integrated immunomagnetic separation and fluorescent lateral flow immunoassay (IMS-FLFIA) based on fluorescent magnetic nanobeads was developed to detect the clenbuterol (CLE) residue in swine urine. The limit of detection (LOD) of IMS-FLFIA is 4 times lower than that of traditional colloidal gold LFIA. This method, which exhibits similar LOD and linearity range in both phosphate-buffered saline and urine swine, is highly correlated with liquid chromatography-tandem mass spectrometry for the detection of real swine urine samples. The result indicated that IMS-FLFIA has a universal resistance to the swine urine matrix. The merits of this assay, high sensitivity, matrix tolerance, accuracy, and specificity, ensure a promising future in detection of veterinary drug residues.

Visualized Detection of Vibrio parahaemolyticus in Food Samples Using Dual-Functional Aptamers and Cut-Assisted Rolling Circle Amplification

A biosensor using two aptamers (Dual-Apt) and cut-assisted rolling circle amplification (CA-RCA) for rapid and visualized detection of Vibrio parahaemolyticus was established. The anchoring aptamer (A-Apt) that specifically binds to the surface of V. parahaemolyticus was applied to separate and enrich the bacterium from the food matrix with the help of streptavidin magnetic beads. While the detecting aptamer (D-Apt), binding on the different sites of the cell surface, was used as a signal reporter. CA-RCA with an enhanced amplification rate was fabricated here to amplify the D-Apt to produce the monomeric G4 sequence that catalyzes the oxidation of ABTS^2-, resulting in the coloration visible to the naked eye. Under optimal conditions, as low as 10 colony-forming units (CFU)/mL (g) of V. parahaemolyticus can be visibly detected in real food samples. Free from DNA extraction, visualized signal output and no need for expensive instruments enable Dual-Apt and CA-RCA to be a promising strategy for on-spot rapid detection.