Fe3O4@CuS-based immunochromatographic test strips and their application to label-free and dual-readout detection of Escherichia coli O157:H7 in food

Electropositive Magnetic Fluorescent Nanoprobe-Mediated Immunochromatographic Assay for the Ultrasensitive and Simultaneous Detection of Bacteria

Immunochromatographic assays (ICAs) provide simple and rapid strategies for bacterial diagnosis but still suffer from the problems of low sensitivity and high dependency on paired antibodies. Herein, the broad-spectrum capture and detection capability of the antibody-free electropositive nanoprobe are clarified for bacteria for the first time and an ultrasensitive fluorescent ICA platform is constructed for the simultaneous diagnosis of multiple pathogens. A magnetic multilayer quantum dot nanocomposite with an amino-embedded SiO2 shell (MagMQD@Si+) is designed to enrich bacteria from solutions effectively, offer high luminescence, and reduce background signals on test strips, thus greatly improving the sensitivity and stability of ICA technique for pathogen. The superior performance of the MagMQD@Si+-based ICA through the multiplex detection of three common pathogens is demonstrated, namely, Pseudomonas aeruginosa, Streptococcus pneumoniae, and Salmonella typhimurium, showing that this ICA possesses high sensitivity (8-40 cells mL-1), good reproducibility (relative standard deviation <5.4%), and high specificity for the three target bacteria. The clinical utility of the proposed method is verified through the detection of 30 real sputum samples from patients with bacterial respiratory infections, revealing that the MagMQD@Si+-based ICA has massive potential as a powerful inspection tool for the rapid, sensitive, and ultrasensitive diagnosis of bacterial infections. [Correction added on 29 January 2025, after first online publication: Streptococcus typhi was corrected to Salmonella typhimurium.].

Determination of the Infection Dynamics of Escherichia coli O157:H7 by Bacteriophage ΦV10

ΦV10 is an Escherichia coli O157:H7-specific bacteriophage that has been used to develop luminescent reporter assays for the detection of this important foodborne pathogen. Previous work demonstrated the specificity of ΦV10 for infection of E.coli O157:H7 through interaction with the O157 antigen. In addition, modification of the lipopolysaccharide (LPS) via O-acetylation prevents ΦV10 infection in an E. coli O157:H7 expressing a phage-encoded O-acetylase gene. Through assays for phage binding, plaque formation, and lysogeny using non-O157:H7 and O157: non-H7 strains, as well as complementation of an O157:H- strain, it is demonstrated in this study that both the somatic O157 antigen and flagellar H7 antigen are required for productive infection of E. coli O157:H7 by ΦV10. Together, the results indicate that the O157 antigen is required for phage binding and that the H7 antigen is necessary to complete the infection process.

A novel SERS method for detecting E. coli based on an aptamer-functionalized Au-Ag@Si triangular pyramid substrate

Escherichia coli contamination in food and pharmaceutical preparations needs to be strictly controlled according to the regulations in many countries. However, rapid and sensitive E. coli detection is still a challenge. In this study, an aptamer-based surface-enhanced Raman spectroscopy (SERS) sandwich method was developed for the rapid and sensitive detection of E. coli using an aptamer-functionalized Au-Ag@Si triangular pyramid (TP) substrate. The Au-Ag@Si TP substrate was functionalized with E. coli aptamer to work as both the capture probe and SERS tag by integrating with Raman reporter (6-carboxy-X-rhodamine). The fabrication of the capture probe, SH-apt@Au-Ag@Si TP, was simple and rapid (20.5 h). This method could selectively and rapidly detect E. coli with a limit of detection of 2.8 CFU/mL within approximately 3 h. It was successfully applied to a traditional Chinese medicine preparation, Xinhuang tablets, with recoveries ranging from 90.19 % to 104.17 %. The results indicated that the developed method was simple and rapid, and it could be a promising alternative for the on-site detection of E. coli in pharmaceutical and food samples.

SERS-based Ag NCs@PDMS flexible substrate combined with chemometrics for rapid detection of foodborne pathogens on egg surface

An innovative method is introduced based on the combination of label-free surface-enhanced Raman scattering with advanced multivariate analysis. This technique allows both quantitative and qualitative assessment of Salmonella typhimurium and Escherichia coli on eggshells. Using silver nanocubes embedded in polydimethylsiloxane, we consistently achieved Raman spectra of bacteria. The stability of the Ag NCs@PDMS substrate is confirmed using rhodamine 6G over 30 days under standard conditions. Principal component analysis (PCA) effectively distinguishes between S. typhimurium and E. coli spectra. Partial least squares regression (PLS) models were developed for quantitative determination of bacteria on egg surfaces, yielding accurate results with minimal error. The S. typhimurium model achieves Rc2 = 0.9563 and RMSEC = 0.601 in calibration, and Rv2 = 0.9113 and RMSEV = 0.907 in validation. Similarly, the E. coli model achieves Rc2 = 0.9877 and RMSEC = 0.322 in calibration, and Rv2 = 0.9606 and RMSEV = 0.579 in validation. Recoveries validate PLS predictions by inoculating egg surfaces with varying bacterial amounts. Our study demonstrates the feasibility of SERS-PLS for quantitative determination of S. typhimurium and E. coli on eggshells, promising enhanced food safety protocols.

Multifunctional nanoenzyme lateral flow immunoassay strip for rapid and ultrasensitive detection of carbofuran in vegetables

A lateral flow immunoassay strip (LFIAS) is one of the most frequently rapid test technologies for carbofuran (CAR). Nevertheless, the LFIAS has a poor quantitative capability and low sensitivity. And, it also requires often complex sample handling steps, making testing time longer. In this study, Fe3O4 nanoparticles were successively modified with MIL-100(Fe)-based metal-organic framework (MOF) and chloroplatinic acid hexahydrate to obtain a core-shell complex of Fe3O4-MOF-Pt. The complex had a peroxidase-mimicking activity catalytic function that enabled signal amplification and sensitivity enhancement. Upon coupling with carbofuran monoclonal antibody (CAR-mAb), the magnetic separation properties of the probe enabled target-specific enrichment. The LFIAS based on Fe3O4-MOF-Pt nanocomposites could detect CAR in the range of 0.25-50 ng mL-1 with a limit of detection (LOD) of 0.15 ng mL-1, enabling colorimetric and catalytic analysis. In addition, the method showed high specificity and stability for detecting CAR in various vegetables, and recovery rates of the spiked samples were 91.40%-102.40%. In conclusion, this study provided one-stop detection of "target enrichment-visual inspection". While lowering the LOD, it reduced the detection time and improved the detection efficiency. The multifunctional Fe3O4-MOF-Pt nanocomposite provides an idea for the construction of novel multifunctional probes to improve the detection performance of conventional LFIAS.

Nanobody and CuS Nanoflower-Au-Based Lateral Flow Immunoassay Strip to Enhance the Detection of Aflatoxin B1

In the realm of analysis, the lateral flow immunoassay (LFIA) is frequently utilized due to its capability to be fast and immediate. However, the biggest challenge of the LFIA is its low detection sensitivity and tolerance to matrix interference, making it impossible to enable accurate, qualitative analyses. In this study, we developed a new LFIA with higher affinity and sensitivity, based on a nanobody (G8-DIG) and CuS nanoflowers-Au (CuS NFs-Au), for the detection of aflatoxin B1 (AFB1) in maize. We synthesized the immunoprobe G8-DIG@CuS NFs-Au, stimulated the in situ development of Au nanoparticles (Au NPs) on Cu NFs by electrical displacement, and obtained Cu NFs-Au for fixing the G8-DIG. G8-DIG@CuS NFs-Au probe-based LFIAs may, in ideal circumstances, use a strip chromatography reader to accomplish sensitive quantitative detection and qualitative visualization. AFB1 has a detection range of 2.82-89.56 µg/L and a detection limit of 0.87 µg/L. When compared with an LFIA based on CuS NFs, this sensitivity is increased by 2.76 times. The practical application of this method in corn flour demonstrated a recovery rate of 81.7% to 117%. Therefore, CuS NFs-Au show great potential for detecting analytes.

A versatile CuCo@PDA nanozyme-based aptamer-mediated lateral flow assay for highly sensitive, on-site and dual-readout detection of Aflatoxin B1

Mycotoxin contaminations in food and environment seriously harms human health. Constructing sensitive and point-of-test early-warning tools for mycotoxin determination is in high demand. In this study, a CuCo@PDA nanozyme-based aptamer-mediated lateral flow assay (Apt-LFA) has been elaborately designed for on-site and sensitive determination of mycotoxin Aflatoxin B1 (AFB1). Benefiting from the rich functional groups and excellent peroxidase-like activity, the CuCo@PDA with original dark color can be conjugated with the specific recognition probe (i.e., aptamer), generating colorimetric signal on the test lines of Apt-LFA via a competitive sensing strategy. The signal can further be amplified in-situ by catalytic chromogenic reaction. Therefore, a visual and dual-readout detection of AFB1 has been realized. The developed Apt-LFA provides a flexible detection mode for qualitative and quantitative analysis of AFB1 by naked-eyes observation or smartphone readout. The smartphone-based LFA platform shows a reliable and ultrasensitive determination of AFB1 with the limit of detection (LOD) of 2.2 pg/mL. The recoveries in the real samples are in the range of 95.11-113.77% with coefficients of variations less than 9.84%. This study provides a new approach to realize point-of-test and sensitive detection of mycotoxins in food and environment using nanozyme-based Apt-LFAs.

Advances in electrochemical-optical dual-mode biosensors for detection of environmental pathogens

Electrochemical techniques are commonly used to analyze and screen various environmental pathogens. When used in conjunction with other optical recognition methods, it can extend the sensing range, lower the detection limit, and offer mutual validation. Nowadays, electrochemical-optical dual-mode biosensors have ensured the accuracy of test results by integrating two signals into one, indicating their potential use in primary food safety quantitative assays and screening tests. Particularly, visible optical signals from electrochemical/colorimetric dual-mode biosensors could meet the demand for real-time screening of microbial pathogens. While electrochemical-optical dual-mode probes have been receiving increasing attention, there is limited emphasis on the design approaches for sensors intended for microbial pathogens. Here, we review the recent progress in the merging of optical and electrochemical techniques, including fluorescence, colorimetry, surface plasmon resonance (SPR), and surface enhanced Raman spectroscopy (SERS). This study particularly emphasizes the reporting of various sensing performances, including sensing principles, types, cutting-edge design approaches, and applications. Finally, some concerns and upcoming advancements in dual-mode probes are briefly outlined.

Trifunctional Nanocomposites with Colorimetric Magnetic Catalytic Activities Labels in Sandwich Immunochromatographic Detection of Escherichia coli O157:H7

The emergence of nanoenzymes has catalyzed the robust advancement of the lateral flow immunoassay (LFIA) in recent years. Among them, multifunctional nanocomposite enzymes with core-shell architectures are considered preferable for promoting the sensing ability due to their good biocompatibility, precise control over size, and surface properties etc. Herein, we developed a dual-channel ensured lateral flow immunoassay (DFLIA) platform utilizing a magnetic, colorimetric, and catalytic multifunctional nanocomposite enzyme (Fe3O4@TCPP@Pd) [TCPP, Tetrakis (4-carboxyphenyl) porphyrin] for the ultrasensitive and highly accurate rapid detection of Escherichia coli O157:H7 (E. coli O157:H7). Fe3O4@TCPP@Pd-mAb exhibits superior performance compared to traditional AuNPs, including enhanced sensitivity and an extended linear detection range, benefiting from its high brightness signal, strong magnetic separation ability, and high peroxidase activity (Vmax = 2.32 μM S1-). Moreover, the Fe3O4@TCPP@Pd-labeled mAb probe exhibited exceptional stability and high affinity toward E. coli O157:H7 (with an affinity constant of approximately 1.723 × 109 M-1), indicating its potential for the efficient capture of the pathogen. Impressively, the developed Fe3O4@TCPP@Pd-DFLIA achieved ultrasensitive detection for E. coli O157:H7 with pre- and postcatalytic naked-eye detection sensitivities of 255 cfu/mL and 77 cfu/mL, respectively, representing an approximately 41-fold improvement over the conventional AuNP-based LFIA and also possessed good specificity and reproducibility [relative standard deviation (RSD) < 10%]. Additionally, the established DFLIA exhibited satisfactory recoveries in detecting pork and milk samples, further validating the reliability of this platform for immunoassays and demonstrating its potential for utilization in bioassays and clinical diagnostics.

Bacterial metabolism-triggered-chemiluminescence-based point-of-care testing platform for sensitive detection and photothermal inactivation of Staphylococcus aureus

Post-operative pathogenic infections in liver transplantation seriously threaten human health. It is essential to develop novel methods for the highly sensitive and rapid detection of Staphylococcus aureus (S. aureus). Interestingly, the combination of the property of bacteria to secrete hydrogen peroxidase, bacterial metabolism-triggered-chemiluminescence (CL)-based bioassays can be as a candidate point-of-care testing (POCT) for the detection of S. aureus against the CL substrate Luminol and hydrogen peroxide without excitation light sources. Here, a CL-based strategy with stable and visualized CL intensity was fabricated according to a hybrid biomimetic enzyme of copper-Hemin metal-organic framework, which enhances the biological enzyme activity while improving the stability and sensitivity of the assay. By further integrating S. aureus-specific capture and one-step separation of the antibody-modified Fe3O4 NPs (Fe3O4 NPs@Ab), the portable device integrated smartphone enables CL-based POCT for specific detection of S. aureus in the range of 101-106 CFU/mL with a limit of detection as low as 1 CFU/mL. Specifically, S. aureus can be eliminated after detection with high antibacterial efficiency due to the excellent photothermal properties of Fe3O4 NPs@Ab. The developed multifunctional platform has the advantages of simplicity of operation and low cost, indicating great potential in clinical applications.

High-throughput, highly sensitive and rapid SERS detection of Escherichia coli O157:H7 using aptamer-modified Au@macroporous silica magnetic photonic microsphere array

The aim of this research was to develop a simple, rapid, sensitive, high-throughput detection method for foodborne Escherichia coli (E. coli) O157:H7 based on the aptamer-modified gold nanoparticles@macroporous magnetic silica photonic microsphere (Au@MMSPM). Such Au@MMSPM array system for E. coli O157:H7 not only integrated sample pretreatment with rapid detection, but also showed highly enhanced effect to develop a highly sensitive SERS assay. The established SERS assay platform gave a wide linear detection range (10-10⁶ CFU/mL) and low limit of detection (2.20 CFU/mL) for E. coli O157:H7. The whole analysis time including sample pretreatment and detection was 110 min. This SERS-based assay platform provided a new high-throughput, highly sensitive and fast detection technology for monitoring E. coli O157:H7 in real samples from the fields of food industry, medicine and environment.

"Lock-and-key" recognizer-encoded lateral flow assays toward foodborne pathogen detection: An overview of their fundamentals and recent advances

In light of severe health risks of foodborne pathogenic bacterial diseases, the potential utility of point-of-care (POC) sensors is recognized for pathogens detection. In this regard, lateral flow assay (LFA) is a promising and user-friendly option for such application among various technological approaches. This article presents a comprehensive review of "lock-and-key" recognizer-encoded LFAs with respect to their working principles and detection performance against foodborne pathogenic bacteria. For this purpose, we describe various strategies for bacteria recognition including the antibody-based antigen-antibody interactions, nucleic acid aptamer-based recognition, and phage-mediated targeting of bacterial cells. In addition, we also outline the technological challenges along with the prospects for the future development of LFA in food analysis. The LFA devices built based upon many recognition strategies are found to have great potential for rapid, convenient, and effective POC detection of pathogens in complex food matrixes. Future developments in this field should emphasize the development of high-quality bio-probes, multiplex sensors, and intelligent portable readers.

NIR II light response-based PDA/AuPt@CuS composites: Simultaneous readout of temperature and pressure sensing strategy for portable detection of pathogenic bacteria

In this study, we developed a simultaneous readout of pressure and temperature dual-signals platform based on the second near-infrared (NIR II) light response-based polydopamine (PDA)-functionalized-AuPt nanoparticles (NPs)@CuS nanosheets (PDA/AuPt@CuS NS) composite. Due to the excellent NIR photothermal performance of PDA/AuPt@CuS NS, it contribute to the decomposition of H₂O₂ and NH₄HCO₃ to generate gases (including O₂, CO₂, and NH₃₎ can be promoted, which can amplify the pressure signals in a sealed container. A sandwich mode is formed between Fe₃O₄ NPs and PDA/AuPt@CuS NS based on the dual-aptamer when target pathogenic bacteria is present. And, it is possible to convert the molecular recognition signals between the dual-aptamers into amplified pressures and temperatures, which can be read out by a portable pressure meter and smartphones simultaneously. It may offer the possibility for quantitative POCT analysis of Pathogenic Bacteria. Moreover, because of the high photothermal efficiency of this method, the developed dual-mode method can achieve that following the detection of bacteria and killing them immediately. As a result, secondary contamination is eliminated and bacterial transmission is avoided. The developed dual-signal sensing platform is also inexpensive, simple to operate and rapidly, indicating that it can be used for food safety analysis, clinical applications, and environmental monitoring.

Ultrasensitive Hybridization Chain Reaction-Assisted Multisite Exonuclease III Amplification Strategy Combined with a Direct Quantitative Fluorescence Lateral Flow Technique for Multiple Bacterial 16S rRNA Detection

Accurate and in-time detection of bacteria conduces to preventing their rapid spread around the environment, while a nucleic acid test (NAT) is a powerful tool for early diagnosis of pathogens. Herein, we propose a hybridization chain reaction (HCR)-mediated multisite exonuclease III (Exo-III) amplification strategy (HCR/Exo-III amplifier) to achieve the one-pot and ultrasensitive isothermal amplification of bacterial 16S rRNA and a portable fluorescence detection device (PFD) to directly read signals in a lateral flow assay (LFA). In detail, the target-initiated HCR products present multiple binding sites for triggering the Exo-III amplifier that produces numerous target amplicons. Following that, the target amplicons travel up on the strip and bridge between the DNA-CdTe/CdS probes and the capture DNA to form a positive fluorescence line. After that, the strip is inserted into the PFD to accomplish the fluorescence signal reading. The constructed HCR/Exo-III amplifier-based PFD-LFA implemented the simultaneous and specific detection of three bacteria with a detection limit of a few tenths of fM for synthetic 16S rRNA fragments and dozens of CFU/mL for Staphylococcus aureus, Listeria monocytogenes, and Salmonella typhimurium in pure cultures. The sensing platform features isothermal amplification, convenient operation, and good economy, displaying great potential for on-site testing toward multiple nucleic acid analytes.

Ultrasensitive multichannel immunochromatographic assay for rapid detection of foodborne bacteria based on two-dimensional film-like SERS labels

Rapid and sensitive detection of multiple foodborne bacteria without DNA amplification is still challenging. Here, we proposed an immunochromatographic assay (ICA) with multiplex analysis ability and high sensitivity for direct detection of bacteria in real food samples, based on an improved surface-enhanced Raman scattering (SERS) sensing strategy. Multifunctional Au shell-coated graphene oxide nanosheets (GO@Au) were fabricated and for the first time introduced into the ICA system as a two-dimensional film-like SERS label, which possessed huge surface area, excellent stability, and superior SERS activity. Different from the conventional spherical nanotags, the antibody-conjugated GO@Au nanosheet effectively and rapidly adhered to bacterial cells, improved the dispersibility of bacteria-nanolabel complexes on the ICA strips, and provided numerous stable hotspots for SERS signal enhancement. The combination of GO@Au labels and the ICA system achieved the multiplex and ultrasensitive determination of three major foodborne pathogens, namely, Staphylococcus aureus, Escherichia coli O157:H7, and Salmonella typhimurium, in a single test, with low detection limits (8, 10, and 10 cells/mL) and short detection time (20 min). The proposed biosensor demonstrated high stability and good accuracy in various food samples and is thus a promising tool for the rapid identification of bacteria.

A review on recent advances in the applications of composite Fe3O4 magnetic nanoparticles in the food industry

Fe3O4 magnetic nanoparticles (MNPs) have attracted tremendous attention due to their superparamagnetic properties, large specific surface area, high biocompatibility, non-toxicity, large-scale production, and recyclability. More importantly, numerous hydroxyl groups (-OH) on the surface of Fe3O4 MNPs can provide coupling sites for various modifiers, forming versatile nanocomposites for applications in the energy, biomedicine, and environmental fields. With the development of science and technology, the potential of nanotechnology in the food industry has also gradually become prominent. However, the application of composite Fe3O4 MNPs in the food industry has not been systematically summarized. Herein, this article reviews composite Fe3O4 MNPs, including their properties, modifications, and physical functions, as well as their applications in the entire food industry from production to processing, storage, and detection. This review lays a solid foundation for promoting food innovation and improving food quality and safety.

Multifunctional Au@Pt@Ag NPs with color-photothermal-Raman properties for multimodal lateral flow immunoassay

Multimodal lateral flow immunoassay (LFIA) has displayed its potential to improve practicability and elasticity of point-of-care testing. Herein, multifunctional core-shell-shell Au@Pt@Ag NPs loaded with dual-layer Raman reporter molecules of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) with a characteristic combination of color-photothermal-Raman performance were constructed for colorimetric LFIA (CM-LFIA), photothermal LFIA (PT-LFIA) and surface-enhanced Raman scattering-based LFIA (SERS-LFIA), respectively. The highly specific nanoprobes, being obtained through the combination of the resulted dual-layer DTNB modified Au@Pt@Ag NPs with the antibody, were triumphantly utilized in exploring multimodal LFIA with one visual qualitative and two optional quantitative modes with excellent sensing sensitivity. Under optimal conditions, the limit of detection (LOD) for the model hazardous analyte dehydroepiandrosterone (DHEA) were 1.0 ng mL^-1 for CM-LFIA, 0.42 ng mL^-1 for PT-LFIA, and 0.013 ng mL^-1 for SERS-LFIA, three of which were over 100-fold, 200-fold and 7 000-fold more sensitive than conventional visual AuNPs-based LFIA, respectively. In addition, the quantitative PT-LFIA and SERS-LFIA sensors worked well in spiked real samples with acceptable recoveries of 96.2 - 106.7% and 98.2 - 105.2%, respectively. This assay demonstrated that the developed multimodal LFIA had a great potential to be a powerful tool for accurate tracing hazardous analytes in complex samples.

A Universal Bacterial Catcher Au-PMBA-Nanocrab-Based Lateral Flow Immunoassay for Rapid Pathogens Detection

In traditional lateral flow immunoassays (LFIA) for pathogens detection, capture antibody (CA) is necessary and usually conjugated to Au nanoparticles (NPs) in order to label the target analyte. However, the acquisition process of the Au-CA nanoprobe is relatively complicated and costly, which will limit the application of LFIA. Herein, p-mercaptophenylboronic acid-modified Au NPs (namely Au-PMBA nanocrabs), were synthesized and applied for a new CA-independent LFIA method. The stable Au-PMBA nanocrabs showed outstanding capability to capture both Gram-negative bacteria and Gram-positive bacteria through covalent bonding. The acquired Au-PMBA-bacteria complexes were dropped onto the strip, and then captured by the detection antibody on the test line (T-line). Take Escherichia coli O157:H7 as an example, the gray value of T-line was proportional to the bacteria concentration and the linear range was 10³-10⁷ cfu·mL^-1. This CA-independent strategy exhibited higher sensitivity than the traditional CA-dependent double antibody sandwich method, because detection limit of the former one was 10³ cfu·mL^-1 only by visual observation, which was reduced by 3 orders of magnitude. Besides, this platform successfully screened E. coli O157:H7 in four food samples with recoveries ranging from 90.25% to 107.25%. This CA-independent LFIA showed great advantages and satisfactory potential for rapid foodborne pathogens detection in real samples.

Modification of a nitrocellulose membrane with nanofibers for sensitivity enhancement in lateral flow test strips

Lateral-flow analysis (LFA) is a convenient, low-cost, and rapid detection method, which has been widely used for screening of diseases. However, sensitivity enhancement in LFA is still a focus in this field and remains challenging. Herein, we propose an electrospinning coating method to modify the conventional nitrocellulose (NC) membrane and optimize the liquid flow rate for enhancing the sensitivity of the NC based LFA strips in the detection of human chorionic gonadotropin (HCG) and luteinizing hormone (LH). It can be seen that coating the NC membrane with nitrocellulose fibers could obtain a NC based strip with HCG and LH detection limits of 0.22 and 0.36 mIU mL-1 respectively, and a quantitative linear range of 0.5-500 mIU mL-1. The results show that electrospinning is effective in modifying conventional NC membranes for LFA applications.

Progress and prospects of magnetic iron oxide nanoparticles in biomedical applications: A review

Nanoscience has been considered as one of the most substantial research in modern science. The utilization of nanoparticle (NP) materials provides numerous advantages in biomedical applications due to their unique properties. Among various types of nanoparticles, the magnetic nanoparticles (MNPs) of iron oxide possess intrinsic features, which have been efficiently exploited for biomedical purposes including drug delivery, magnetic resonance imaging, Magnetic-activated cell sorting, nanobiosensors, hyperthermia, and tissue engineering and regenerative medicine. The size and shape of nanostructures are the main factors affecting the physicochemical features of superparamagnetic iron oxide nanoparticles, which play an important role in the improvement of MNP properties, and can be controlled by appropriate synthesis strategies. On the other hand, the proper modification and functionalization of the surface of iron oxide nanoparticles have significant effects on the improvement of physicochemical and mechanical features, biocompatibility, stability, and surface activity of MNPs. This review focuses on popular methods of fabrication, beneficial surface coatings with regard to the main required features for their biomedical use, as well as new applications.

Sensitive detection of Escherichia coli O157:H7 and Salmonella typhimurium in food samples using two-channel fluorescence lateral flow assay with liquid Si@quantum dot

Here, we proposed a silica-quantum dot (QD)-based fluorescent lateral flow immunoassay (LFA) method with high sensitivity for the simultaneous qualification of Salmonella typhimurium and Escherichia coli O157:H7 in food samples. The silica-QD nanobead (Si@DQD) with dual-QD shell was introduced into the two-channel LFA strip as the advanced fluorescent tag, thus providing superior fluorescence signal, monodispersity, and excellent stability for actual sample detection. The liquid Si@DQD tags were mixed with sample solution and directly loaded onto the LFA strip for the quantitative analysis of target bacteria within 15 min. The detection limit of the proposed assay reached 50 cells/mL for both S. typhi/E. coli and was approximately 200 times more sensitive than the colloidal gold (AuNP)-based LFA strips. The Si@DQD-LFA also exhibited the advantages of good stability, specificity, and easy operation, suggesting its great potential for real bacterial sample detection.

A novel smartphone-based colorimetric aptasensor for on-site detection of Escherichia coli O157:H7 in milk

Effective testing tools for Escherichia coli O157:H7 can prevent outbreaks of foodborne illness. In this paper, a smartphone-based colorimetric aptasensor was developed using functionalized gold nanoparticles (GNP) and multi-walled carbon nanotubes (MWCNT) for monitoring E. coli O157:H7 in milk. The maximum absorption peak of GNP bonded with aptamer (Apt) generated evident transformation from 518 to 524 nm. The excess GNP-Apt was removed by functionalized MWCNT magnetized with carbonyl iron powder (CIP) and hybridized with a DNA probe, whereas the GNP-Apt immobilized on E. coli O157:H7 remained in the system. In the presence of a high-salt solution, the GNP-Apt that captured E. coli O157:H7 remained red, but the free GNP-Apt aggregated and appeared blue. The chromogenic results were analyzed by a smartphone-based colorimetric device that was fabricated using acrylic plates, a light-emitting diode, and a mobile power pack. To our knowledge, this was the first attempt to use a smartphone-based colorimetric aptasensor employing the capture of GNP-Apt coupled with separation of MWCNT@CIP probe to detect E. coli O157:H7. The aptasensor exhibited good reproducibility and no cross-reaction for other bacteria. A concentration of 8.43 × 10³ cfu/mL of E. coli O157:H7 could be tested in pure culture, and 5.24 × 10² cfu/mL of E. coli O157:H7 could be detected in artificially contaminated milk after 1 h of incubation. Therefore, the smartphone-based colorimetric aptasensor was an efficient tool for the detection of E. coli O157:H7 in milk.

Dual-signal lateral flow assay using vancomycin-modified nanotags for rapid and sensitive detection of Staphylococcus aureus

This paper reports a colorimetric-fluorescent dual-signal lateral flow assay (LFA) based on vancomycin (Van)-modified SiO2-Au-QD tags for sensitive and quantitative detection of Staphylococcus aureus (S. aureus). The combination of high-performance Van-tags and detection antibodies integrated into the LFA system produced assays with high sensitivity and specificity. The visualization limit of the colorimetric signal and the detection limit of the fluorescence signal of the proposed method for S. aureus can reach 104 and 100 cells mL-1, respectively.

Biosensing platform on ferrite magnetic nanoparticles: Synthesis, functionalization, mechanism and applications

Ferrite magnetic nanoparticles (FMNPs) are gaining popularity to design biosensors for high-performance clinical diagnosis. The fusion of information shows that FMNPs based biosensors require well-tuned FMNPs as detection probes to produce large and specific biological signals with minimal non-specific binding. Nevertheless, there is a noticeable lacuna of information to solve the issues related to suitable synthesis route, particle size reduction, functionalization, sensitivity towards targeted intercellular biological tiny particles, and lower signal-to-noise ratio. Therefore it allows exploring unique characteristics of FMNPs to design a suitable sensing device for intracellular measurements and diseases detection. This review focuses on the extensively used synthesis routes, their advantages and limitations, crystalline structure, functionalization, along with recent applications of FMNPs in biosensors, taking into consideration their analytical figures of merit and range of linearity. This work also addresses the current progress, key factors for sensitivity, selectivity and productivity improvement along with the challenges, future trends and perspectives of FMNPs based biosensors.