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In this study, a fluorescent biosensor was developed for the sensitive detection of Salmonella typhimurium using a low-gradient magnetic field and deep learning via faster region-based convolutional neural networks (R-CNN) to recognize the fluorescent spots on the bacterial cells. First, magnetic nanobeads (MNBs) coated with capture antibodies were used to separate target bacteria from the sample background, resulting in the formation of magnetic bacteria. Then, fluorescein isothiocyanate fluorescent microspheres (FITC-FMs) modified with detection antibodies were used to label the magnetic bacteria, resulting in the formation of fluorescent bacteria. After the fluorescent bacteria were attracted against the bottom of an ELISA well using a low-gradient magnetic field, resulting in the conversion from a three-dimensional (spatial) distribution of the fluorescent bacteria to a two-dimensional (planar) distribution, the images of the fluorescent bacteria were finally collected using a high-resolution fluorescence microscope and processed using the faster R-CNN algorithm to calculate the number of the fluorescent spots for the determination of target bacteria. Under the optimal conditions, this biosensor was able to quantitatively detect Salmonella typhimurium from 6.9 × 101 to 1.1 × 103 CFU/mL within 2.5 h with the lower detection limit of 55 CFU/mL. The fluorescent biosensor has the potential to simultaneously detect multiple types of foodborne bacteria using MNBs coated with their capture antibodies and different fluorescent microspheres modified with their detection antibodies. [ABSTRACT FROM AUTHOR]

Nucleic acid extraction is crucial for PCR detection of pathogenic bacteria to ensure food safety. In this study, a new magnetic extraction method was developed using 3D printing and magnetic silica beads (MSBs) to extract the target DNA from a large volume of bacterial sample and combined with microfluidic PCR to determine the bacteria. After proteinase K was added into a bacterial sample to lyse the bacteria and release the DNA, it was continuous-flow injected into the serpentine channel of the extraction chip, where magnetic silica bead chains had been formed in advance using a homogeneous magnetic field generated by two concentric semicircle magnets to capture the MSBs. Then, the flowing DNA was captured by the MSB chains, washed with alcohol, dried with gas, and eluted with deionized water to obtain the purified and concentrated DNA. Finally, the extracted DNA templates were injected into a microfluidic PCR chip with lyophilized amplification reagents and determined using a commercial qPCR device. The experimental results showed that the DNA extraction efficiency was more than 90%, and the lower detection limit of Salmonella was 102 CFU/mL. This new Salmonella detection method is promising to provide the rapid, sensitive, and simultaneous detection of multiple foodborne pathogens. [ABSTRACT FROM AUTHOR]

A syringe-driven colorimetric biosensor was constructed for simple detection of Salmonella typhimurium (S. ty) based on nuclear track membrane filtration and nanozyme signal amplification. First, immune gold@platinum nanozymes (IGPNs) and large-volume sample in centrifugal tube were blow-mixed by push-and-pull of syringe. After S. ty -IGPN conjugates and free IGPNs was pulled into syringe, the syringe was connected with a uniform-size nuclear track membrane (NTM) and the mixture was pushed to flow through, resulting in selective trap of larger-size conjugates and effective remove of smaller-size IGPNs. Then, conjugates on the NTM were washed and H 2 O 2 -TMB was pulled to immerse the NTM. The catalysate was finally photographed and measured through smartphone App for detemination of S. ty. This biosensor enabled simple and in-field bacterial detection as low as 12 CFU/mL in 15 min. Compared with common sandwich-structured biosensors using paired antibodies, this biosensor with easier extension, lower cost and shorter time only required one antibody to functionalize nanozymes. • The nuclear track membrane could selectively entrapment and effectively penetration. • The push-and-pull of syringe could realize the blow mixing. • A syringe-driven biosensor enabled detect Salmonella in large-volume sample. • This biosensor was able to detect Salmonella as low as 12 CFU/mL in 15 min. [ABSTRACT FROM AUTHOR]

• A finger-actuated micropump was elaborately developed for accurate liquid injection. • A finger-actuated micromixer with chaotic structure was developed for efficient mixing. • Gold@platinum nanocatalysts were used to amplify biological signals. • The whole bacteria detection procedures were integrated onto a single microfluidic chip. • This microfluidic biosensor could detect Salmonella as low as 350 CFU/mL within 1 h. A microfluidic biosensor was developed for rapid detection of Salmonella using a finger-actuated micropump, a finger-actuated micromixer, gold@platinum nanocatalysts (Au@PtNCs) and a smartphone App. First, immune magnetic nanobeads (MNBs), bacterial sample and immune Au@PtNCs were successively finger-actuated pumped into microfluidic chip. Then, they were fully mixed using finger-actuated micromixer to form MNB- Salmonella -Au@PtNC complexes. After hydrogen peroxide-tetramethylbenzidine was pumped into chip and catalyzed by nanocatalysts on complexes, resulting in color change from colorless to blue, the image of catalysate was collected and finally analyzed by self-developed smartphone App or directly compared with Pantone color card in chip to determine bacterial concentration. Experimental results showed this biosensor could quantitatively detect Salmonella from 3.5 × 102 to 3.5 × 105 CFU/mL in 1 h with lower detection limit of 350 CFU/mL. This biosensor has successfully integrated loading, mixing, incubation, washing, separation and detection onto a chip and might pave a promising way for bacterial detection. [ABSTRACT FROM AUTHOR]

Rapid screening of pathogenic bacteria contaminated foods is crucial to prevent food poisoning. However, available methods for bacterial detection are still not ready for in-field screening because culture is time-consuming; PCR requires complex DNA extraction and ELISA lacks sensitivity. In this study, a microfluidic biosensor was developed for rapid, sensitive and automatic detection of Salmonella using metal-organic framework (MOF) NH 2 -MIL-101(Fe) with mimic peroxidase activity to amplify biological signal and Raspberry Pi with self-developed App to analyze color image. First, the target bacteria were separated and concentrated with the immune magnetic nanobeads (MNBs), and labeled with the immune MOFs to form MNB- Salmonella -MOF complexes. Then, the complexes were used to catalyze colorless o-phenylenediamine and H 2 O 2 to produce yellow 2,3-diaminophenazine (DAP). Finally, the image of the catalysate was collected under the narrow-band blue light and analyzed using the Raspberry Pi App to determine the bacterial concentration. The experimental results showed that this biosensor was able to detect Salmonella Typhimurium from 1.5 × 101 to 1.5 × 107 CFU/mL in 1 h with the lower detection limit of 14 CFU/mL. The mean recovery for Salmonella in spiked chicken meats was ~112%. This biosensor integrating mixing, separation, labelling and detection onto a single microfluidic chip has demonstrated the merits of automatic operation, fast reaction, less reagent and small size, and is promising for in-field detection of foodborne bacteria. • An active vibrating mixer was demonstrated with efficient and continuous-flow mixing. • The improved image processing under blue light was verified with higher sensitivity. • Mixing, separation, catalysis and detection were integrated for automatic operation. • This low-cost biosensor could detect Salmonella as low as 14 CFU/mL within 1 h. [ABSTRACT FROM AUTHOR]

An impedance biosensor using rotary magnetic separation and cascade reaction was developed for rapid and ultrasensitive detection of Salmonella typhimurium. First, magnetic nanoparticles (MNPs) modified with anti- Salmonella monoclonal antibodies were injected into a capillary at the presence of a rotary high gradient magnetic field, which was rotated by a stepper motor. Then, a bacterial sample was injected into the capillary and the target bacteria were continuous-flow captured onto the MNPs. After organic-inorganic hybrid nanoflowers were prepared using manganese dioxide (MnO 2), glucose oxidase (GOx) and anti- Salmonella polyclonal antibodies (pAbs), they were injected to label the bacteria, resulting in the formation of MNP-bacteria-nanoflower sandwich complexes. Finally, glucose (low conductivity) was injected and oxidized by GOx on the complexes to produce H 2 O 2 (low conductivity) and gluconic acid (high conductivity), leading to impedance decrease. Besides, the produced H 2 O 2 triggered a cascade reduction of MnO 2 into Mn2+, leading to further impedance decrease. The impedance changes were measured using an interdigitated microelectrode and used to determine the concentration of target bacteria. This biosensor was able to detect Salmonella ranging from 101 to 106 CFU/mL in 2 h with a low detection limit of 101 CFU/mL and a mean recovery of 100.1% for the spiked chicken samples. • Rotatory magnetic separation was used to separate bacteria for first time. • Two cascade reaction was used to amplify impedance signal. • The method was able to detect Salmonella as low as 101 CFU/mL in 2 h. • The mean recovery for the spiked chicken meat samples was 100.1%. [ABSTRACT FROM AUTHOR]

Screening of pathogenic bacteria in foods is an effective way to prevent foodborne diseases. In this study, an impedance biosensor was developed for rapid and sensitive detection of Salmonella typhimurium using multiple magnetic nanobead (MNB) nets in a ring channel for continuous-flow separation of target bacteria from 10 mL of sample, manganese dioxide nanoflowers (MnO 2 NFs) for efficient amplification of biological signal, and an interdigitated microelectrode for sensitive measurement of impedance change. First, the MNBs modified with capture antibodies were vortically injected from outer periphery of this ring channel to form multiple ring MNB nets at specific locations with high gradient magnetic fields. Then, the bacterial sample was continuous-flow injected, resulting in specific capture of target bacteria onto the nets, and the MnO 2 NFs modified with detection antibodies were injected to form MNB-bacteria-MnO 2 NF complexes. After the complexes were washed with deionized water to remove excessive nanoflowers and residual ions, H 2 O 2 with poor conductivity was injected to reduce MnO 2 NFs to conductive Mn2+ at neutral medium, leading to impedance decrease. Finally, impedance change was measured using the microelectrode for quantitative determination of Salmonella. This biosensor was able to separate ~60% of Salmonella from 10 mL of bacterial sample and detect Salmonella with a linear range of 3.0 × 101 to 3.0 × 106 CFU/mL in 1.5 h with lower detection limit of 19 CFU/mL. This biosensor might be further improved with higher sensitivity using a larger volume (100 mL or more) for routine screening of foodborne bacteria without bacterial pre-culture. • Magnetic nanobead nets were formed to separate Salmonella from large-volume sample. • MnO 2 nanoflowers were first used to amplify impedance signal for bacteria detection. • This impedance biosensor could detect Salmonella as low as 19 CFU/mL in 1.5 h. • The mean recovery of Salmonella in spiked chicken samples was 101.8%. • This biosensor might be promising for bacterial routine screening without pre-enrichment. [ABSTRACT FROM AUTHOR]

• Pt@MnO 2 nanoflowers were synthesized for dual mimic enzymatic catalysis of H 2 O 2. • Piezoresistor was combined with smartphone to real-time monitor pressure change. • This pressure biosensor was able to detect Salmonella as low as 13 CFU/mL in 1.5 h. • The mean recovery of Salmonella in the spiked chicken samples was 96.8%96.8 %. Salmonella is the leading factor for microbial food poisoning. In this study, a facile pressure biosensor was developed for rapid and sensitive detection of Salmonella using magnetic nanobeads (MNBs) to separate target bacteria, platinum nanoparticle loaded manganese dioxide nanoflowers (Pt@MnO 2 NFs) to amplify detection signal, and a thin-film piezoresistor based pressure detector to monitor pressure change. First, the capture antibodies (CAbs) modified MNBs were used to specifically separate Salmonella from sample to form MNB-CAb- Salmonella complexes (magnetic bacteria). Then, the detection antibodies (DAbs) modified Pt@MnO 2 NFs were used for labelling magnetic bacteria to form MNB-CAb- Salmonella -DAb-Pt@MnO 2 NF complexes (nanoflower bacteria). After nanoflower bacteria were resuspended with H 2 O 2 in a sealed centrifuge tube, H 2 O 2 was catalyzed by Pt@MnO 2 NFs to produce O 2 , resulting in the increase on pressure. Finally, the pressure increase was real-timely monitored by piezoresistor based pressure detector and transferred to smartphone App via Bluetooth for analysis and determination of Salmonella. This biosensor could quantitatively detect Salmonella from 1.5 × 101 to 1.5 × 105 CFU/mL in 1.5 h with low detection limit of 13 CFU/mL. The Pt@MnO 2 NFs with high loading capacity of platinum nanoparticles were demonstrated as dual mimic enzymatic catalyst of H 2 O 2 to greatly enhance the sensitivity. More importantly, it is the first time to combine a thin-film piezoresistor with a smartphone App for realtime monitoring of the pressure change resulting from the mimic enzymatic catalysis of H 2 O 2 into O 2 by the Pt@MnO 2 NFs on the target bacteria to determine pathogenic bacteria in food samples. [ABSTRACT FROM AUTHOR]