Versatile Biosensing Toolkit Using an Electronic Particle Counter

Multiple electromagnet synergistic control enabled fast and automatic biosensing of Salmonella in a sealed microfluidic chip

Point-of-care testing of pathogens is vital for prevention of food poisoning. Herein, a colorimetric biosensor was elaborately developed to rapidly and automatically detect Salmonella in a sealed microfluidic chip with one central chamber for housing immunomagnetic nanoparticles (IMNPs), bacterial sample and immune manganese dioxide nanoclusters (IMONCs), four functional chambers for housing absorbent pad, deionized water and H2O2-TMB substrate, and four symmetric peripheral chambers for achieving fluidic control. Four electromagnets were placed under peripheral chambers and synergistically controlled to manipulate their respective iron cylinders at the top of these chambers for deforming these chambers, resulting in precise fluidic control with designated flowrate, volume, direction and time. First, the electromagnets were automatically controlled to mix IMNPs, target bacteria and IMONCs, resulting in the formation of IMNP-bacteria-IMONC conjugates. Then, these conjugates were magnetically separated by a central electromagnet and the supernatant was directionally transferred to the absorbent pad. After these conjugates were washed by deionized water, the H2O2-TMB substrate was directionally transferred to resuspend the conjugates and catalyzed by the IMONCs with peroxidase-mimic activity. Finally, the catalysate was directionally transferred back to its initial chamber, and its color was analyzed by the smartphone APP to determinate bacterial concentration. This biosensor could detect Salmonella quantitatively and automatically in 30 min with a low detection limit of 101 CFU/mL. More importantly, the whole bacterial detection procedure from bacterial separation to result analysis was achieved on a sealed microfluidic chip through multiple electromagnet synergistic control, and this biosensor has great potential for point-of-care testing of pathogens without cross contaminations.

Magnetic relaxation switching immunoassay based on "limited-magnitude" particles for sensitive quantification of aflatoxin B1

Aflatoxin B₁ (AFB₁) is a highly toxic and carcinogenic chemical substance that endangers food safety and human health. Magnetic relaxation switching (MRS) immunosensors are utilized in a variety of applications in food analysis due to its resistance to matrix interferences, but they often suffer from magnetic separation-based multi-washing steps and low sensitivity. Herein, we propose novel MRS strategy for the sensitive detection of AFB₁ using "Limited-Magnitude" size particles: a single millimeter sized polystyrene spheres (PS_mm) and 150 nm superparamagnetic nanoparticles (MNP₁₅₀). Only a single PS_mm is used as the microreactor to enhance all of the magnetic signal on its surface in high concentration by an immune competitive response, successfully preventing signal dilution, which can be transferred by pipette, simplifying the process of separation and washing. The established single polystyrene sphere magnetic relaxation switch biosensor (SMRS) was able to quantify AFB₁ from 0.02 to 200 ng/mL with a detection limit of 14.3 pg/mL. SMRS biosensor has been successfully used for the detection of AFB₁ in wheat and maize samples, and the results in agreement with high performance liquid chromatography-tandem mass spectrometry (HPLC-MS). Benefiting from high sensitivity and convenient operation, the simple and enzyme-free method is promising in trace small molecules applications.

"Lollipop" particle counting immunoassay based on antigen-powered CRISPR-Cas12a dual signal amplification for the sensitive detection of deoxynivalenol in the environment and food samples

Deoxynivalenol is one of the most widely distributed mycotoxins in cereals and poses tremendous threats to the agricultural environment and public health. Therefore, it is particularly important to develop sensitive and interference-resistant deoxynivalenol analysis methods. Here, we establish a "Lollipop" particle counting immunoassay (LPCI) based on antigen-powered CRISPR-Cas12a dual signal amplification. LPCI achieves high sensitivity and accuracy through antigen-powered CRISPR-Cas dual signal amplification combined with particle counting immunoassay. This strategy not only broadens the applicability of the CRISPR-Cas system in the field of non-nucleic acid target detection; it also improves the sensitivity of particle counting immunoassay. The introduction of a polystyrene "lollipop" immunoassay carrier further enables efficiently simultaneous pre-treatment of multiple samples and overcomes complex matrix interference in real samples. The linear detection range of LPCI for deoxynivalenol was 0.1-500 ng/mL with a detection limit of 0.061 ng/mL. The platform greatly broadens the scope of the CRISPR-Cas sensor for the detection of non-nucleic acid hazards in the environment and food samples.

Emerging Graphitic Carbon Nitride-based Nanobiomaterials for Biological Applications

Graphitic carbon nitride (g-CN) based nanostructures are distinctive materials with unique compositional, structural, optical, and electronic properties with exceptional band structure, moderate surface area, and exceptional thermal and chemical stability. Because of these properties, g-CN based nanomaterials have shown promising applications and higher performance in the biological avenue. This review covers the state-of-the-art synthetic strategies used for the preparation of the materials, the basic structure, and a panorama of different optimization strategies leading to improved physicochemical properties responsible for the biological application. The following sections include the recent progress in the use of g-CN based nanobiomaterials for biosensors, bioimaging, photodynamic therapy, drug delivery, chemotherapy, and the antimicrobial segment. Furthermore, we have summarized the role and evaluation of biosafety and biocompatibility of the material. Finally, the unresolved issues, plausible challenges, current status, and future perspectives for the development and design of g-CN have been summarized and are expected to promote a clinical path for the medical sector and human well-being.

Artificial Intelligence-Assisted Digital Immunoassay Based on a Programmable-Particle-Decoding Technique for Multitarget Ultrasensitive Detection

The development of a multitarget ultrasensitive immunoassay is significant to fields such as medical research, clinical diagnosis, and food safety inspection. In this study, an artificial intelligence (AI)-assisted programmable-particle-decoding technique (APT)-based digital immunoassay system was developed to perform multitarget ultrasensitive detection. Multitarget was encoded by programmable polystyrene (PS) microspheres with different characteristics (particle size and number), and subsequent visible signals were recorded under an optical microscope after the immune reaction. The resultant images were further analyzed using a customized, AI-based computer vision technique to decode the intrinsic properties of polystyrene microspheres and to reveal the types and concentrations of targets. Our strategy has successfully detected multiple inflammatory markers in clinical serum and antibiotics with a broad detection range from pg/mL to μg/mL without extra signal amplification and conversion. An AI-based digital immunoassay system exhibits great potential to be used for the next generation of multitarget detection in disease screening for candidate patients.

Integrating magnetic metal-organic frameworks-based sample preparation with microchannel resistance biosensor for rapid and quantitative detection of aflatoxin B1

Aflatoxin B₁, a secondary metabolite produced by fungi, is one of the most toxic mycotoxins that poses a major food security and public health threat worldwide. Effective sample pretreatment and high sensitivity detection techniques are urgently needed due to its trace amount in complex samples. Herein, an integrated detection strategy was developed by combining Mg/Zn-metal organic framework-74 modified Fe₃O₄ magnetic nanoparticles (Mg/Zn-MOF-74 @Fe₃O₄ MNPs)-based sample preparation and microchannel resistance biosensor for rapid and highly sensitive detection of aflatoxin B₁ in food samples. The synthesis and characterization of Mg/Zn-MOF-74 @Fe₃O₄ MNPs was reported, which exhibited efficient separation and enrichment capacity when exposed to complex grain samples. The competitive immunoassay-based microchannel resistance biosensor enabled specific and high-sensitive analysis of aflatoxin B₁ by using current as a readout, which caused by the blocking effect between the functionalized-polystyrene microspheres and microchannel. Under optimized conditions, this biosensor was capable to quantitatively analysis aflatoxin B₁ from 10 pg/mL to 20 ng/mL, and with a limit of detection of 4.75 pg/mL. This integrated detection strategy has been tested for the quantitative detection of aflatoxin B₁ in grain samples that is a potential protocol for food safety control and environmental monitoring.

One-step homogeneous micro-orifice resistance immunoassay for detection of chlorpyrifos in orange samples

In this work, a one-step homogeneous micro-orifice resistance immunoassay has been proposed for chlorpyrifos detection by integrating functionalized polystyrene (PS) microsphere probes with particle counting technology. The particle counter is highly sensitive and accurate for detecting the state of PS microspheres, where the particles of different states exhibit significant differences in resistance. The state of the functionalized PS microspheres is altered from dispersed to aggregated during the antigen-antibody recognition. Based on the degree of aggregation of the functionalized PS microsphere probes, chlorpyrifos can be quantitatively detected through the competitive immune response between PS antibodies and PS complete antigens. This one-step homogeneous micro-orifice resistance immunoassay simplified the procedures and greatly increased the sensitivity of detection, which has been successfully applied to detect chlorpyrifos in orange samples within 0.5 h, with the detection limit of 0.058 ng/mL.

Simple and programmed three-dimensional DNA tweezer for simultaneous one-step detection of ochratoxin A and zearalenone

Three-dimensional (TD) deoxyribonucleic acid (DNA) tweezers were programmed for one-step identification and detection of ochratoxin A (OTA) and zearalenone (ZEN). The unfolding of the TD-DNA tweezers by aptamers specific to these two mycotoxins "turned" the fluorescent signals "on." The bonding of the aptamers to their corresponding targets in OTA and ZEN "turned" the fluorescent signals and the DNA tweezers "off." The detection limit of the TD-DNA tweezers for OTA and ZEN was 0.032 and 0.037 ng mL^-1, respectively. The feasibility of this method was tested using two samples. Detection via this method increased the recovery of OTA and ZEN from 95.8% to 110.2%. Spike recovery and certified food products were used to detect applicability in actual situations. Analyte detection in complex samples using TD-DNA tweezers is rapid, as the process involves a single operational step. This proposed design has considerable potential for application in mycotoxin detection.

Polydopamine nanoparticle-mediated, click chemistry triggered, microparticle-counting immunosensor for the sensitive detection of ochratoxin A

A rapid and accurate detection method is needed for the quantitation of ochratoxin A in agricultural products due to its high toxicity. A microparticle-counting immunosensor based on polydopamine nanoparticle-mediated click chemistry was established for the highly-sensitive detection of ochratoxin A. Polydopamine nanoparticles with good biocompatibility and a strong metal-chelating ability were synthesized and conjugated with the antibody. The Coupled compounds were then used as an immune carrier to change the Cu²⁺ concentration via an immuno-reaction. Some of the remaining Cu²⁺ ions were reduced to Cu⁺ ions, which caused azide-polystyrene microspheres and alkyne-polystyrene microspheres to aggregate via a Cu⁺ ion-mediated click reaction. Particle counting was used to distinguish changes in the sizes of the polystyrene microspheres from dispersed to aggregated to detect ochratoxin A. It showed a wide linear detection range of 0.5-800 ng/mL, and a detection limit of 0.2 ng/mL. This assay provides an attractive analytical tool for the accurate detection of trace targets in complex samples.

Click Chemistry-Mediated Particle Counting Sensing via Cu(II)-Polyglutamic Acid Coordination Chemistry and Enzymatic Reaction

An electrical resistance-based particle counter (ERPC) with simple operation and high resolution has proved to be a promising biosensing toolkit, whereas amplification-free ERPC biosensors are incapable of analyzing trace small molecules due to their relatively low sensitivity. In this work, click chemistry-mediated particle counting sensing of small-molecule hazards in food samples with high sensitivity was developed. In this strategy, unbound alkyne-functionalized polystyrene microspheres were collected by magnetic separation from the copper-ion-mediated click reaction between alkyne-functionalized polystyrene microspheres and azido-functionalized magnetic beads, which could be used as signal probes for the readout. This click chemistry-mediated ERPC biosensor converts the detection of targets to the quantification of copper ions or ascorbic acid by performing competitive immunoassay-based coordination chemistry and enzymatic reaction, respectively. The sensitivity of the ERPC biosensor has been improved by an order of magnitude due to the signal amplification effects of click chemistry, coordination adsorption, and enzyme catalysis. Furthermore, because of the efficient separation and enrichment of immunomagnetic beads and the robustness of click chemistry, the interference from food matrixes and immunoassay is effectively reduced, and thus, our strategy is exceedingly suitable for detecting trace targets in complex samples.

Retroreflection-based sandwich type affinity sensing of isothermal gene amplification products for foodborne pathogen detection

Loop-mediated isothermal amplification (LAMP) is an outstanding method for molecular diagnostics, as the rapid, specific, and sensitive amplification of target genes is possible. However, it is necessary to measure fluorescence in the quantitative analysis of LAMP products, so a sophisticated optical setup is required. This study tried to develop a novel sensing method that can quantify target analytes with simple equipment, such as nonspectroscopic white light and a CMOS camera. To achieve this, a retroreflective Janus particle (RJP) as a probe and specially designed loop primers, fluorescein isothiocyanate (FITC)- and biotin-modified loop primers, were introduced into the LAMP system. By performing LAMP in the presence of designed primers, double-stranded amplicons possessing FITC and biotin labels at each end are generated in proportion to the quantity of the target pathogen. Using the anti-FITC antibody-modified sensing surface and streptavidin-conjugated RJP probes, the amplicons can be captured in sandwich-configuration and detected under nonspectroscopic conditions composed of white light and a camera. To confirm the feasibility of the sensing system, the invA gene of Salmonella was selected as the target. It was possible to quantitatively analyze the Salmonella concentration from 0 to 10⁶ colony-forming units, sufficiently covering the required detection range. In addition, quantitative analyses of pathogens in contaminated food sources, including milk and chicken meat, were successfully conducted with a limit of detection of 10 CFU.

Low-Cost and Convenient Microchannel Resistance Biosensing Platform by Directly Translating Biorecognition into a Current Signal

We report a low-cost and convenient microchannel resistance (MCR) biosensing platform that uses current signal to report biorecognition. The biorecognition behavior between targets and biometric molecules (antigens, antibodies, or oligonucleotides) immobilized on magnetic beads and polystyrene (PS) microspheres induces a quantitative change in the unreacted PS microspheres. After magnetic separation, the unreacted PS microsphere solution is passed through the microchannel, leading to an obvious blocking effect, resulting in an increase in resistance, which can in turn be measured by monitoring the electric current. Thus, the biorecognition is directly converted into a detectable current signal without any bulky instruments or additional chemical reactions. The MCR biosensing platform is cost-effective and user-friendly with high accuracy. It can be an appropriate analysis technique for point-of-care testing in resource-poor settings.