Graphene-based biosensors in milk analysis: A review of recent developments

Cow's milk, an excellent source of fat, protein, amino acids, vitamins and minerals, is currently one of the most consumed products worldwide. Contaminations originating from diverse sources, such as biological, chemical, and physical, cause dairy product quality problems and thus dairy-related disorders, raising public health issues. For this reason, legal authorities have deemed it necessary to classify certain contaminations in commercial milk and keep them within particular limitations; therefore, it is urgent to develop next-generation detection systems that can accurately identify just the contaminants of concern to human health. This review presents a detailed investigation of biosensors based on graphene and its derivatives, which offer superior sensitivity and selectivity, by classifying the contaminants under the headings biological, chemical, and physical, in cow's milk according to their sources. We reviewed the current status of graphene-based biosensor (GBs) technology for milk or dairy analysis, highlighting its strengths and weaknesses with the help of comparative studies, tables, and charts, and we put forward a novel perspective to handle future challenges.

A highly sensitive fluorescence biosensor for detection of Staphylococcus aureus based on HCR-mediated three-way DNA junction nicking enzyme assisted signal amplification

Sensitive and efficient monitoring of food-borne bacteria is of great importance for food safety control. Herein, a novel biosensor for highly sensitive detection of Staphylococcus aureus (S. aureus) was constructed by combining hybridization chain reaction (HCR) and nicking enzyme. Different from the upstream-downstream based circuit, the proposed biosensor integrated HCR circuit and three-way DNA junction nicking enzyme assisted signal amplification (3WJ-NEASA) into a virtuous circle of promotion. In the HCR-mediated 3WJ-NEASA sensing strategy, target DNA of S. aureus initiated the self-assembly between HCR hairpins (H1 and H2), which exposed the gap to capture molecular beacon (MB) and construct the 3WJ structure. Meanwhile, MB increased the stability of HCR nanowires and enhanced the efficiency of the HCR circuit, and thus more 3WJ-NEASA circuits were generated in HCR nanowires. Benefiting from the synergistic amplification coupling HCR and 3WJ-NEASA, this isothermal biosensor can detect as low as 6.7 pM of target DNA in one step within only 30 min. Furthermore, the HCR-mediated 3WJ-NEASA assay has been applied in the detection of S. aureus with a limit of detection (LOD) as low as 1.2 × 10¹ cfu mL^-1, and has exhibited reliable practicability in spiked milk. It is the first time that a DNA biosensor combining HCR and 3WJ-NEASA for dual signal amplification was developed and has been adopted to the sensitive analysis of food-borne bacteria. Additionally, this strategy can serve as a universal platform for monitoring other analytes, and therefore possesses broad application prospects in food safety and environmental monitoring.

Applications of Catalytic Hairpin Assembly Reaction in Biosensing

Nucleic acids are considered as perfect programmable materials for cascade signal amplification and not merely as genetic information carriers. Among them, catalytic hairpin assembly (CHA), an enzyme-free, high-efficiency, and isothermal amplification method, is a typical example. A typical CHA reaction is initiated by single-stranded analytes, and substrate hairpins are successively opened, resulting in thermodynamically stable duplexes. CHA circuits, which were first proposed in 2008, present dozens of systems today. Through in-depth research on mechanisms, the CHA circuits have been continuously enriched with diverse reaction systems and improved analytical performance. After a short time, the CHA reaction can realize exponential amplification under isothermal conditions. Under certain conditions, the CHA reaction can even achieve 600 000-fold signal amplification. Owing to its promising versatility, CHA is able to be applied for analysis of various markers in vitro and in living cells. Also, CHA is integrated with nanomaterials and other molecular biotechnologies to produce diverse readouts. Herein, the varied CHA mechanisms, hairpin designs, and reaction conditions are introduced in detail. Additionally, biosensors based on CHA are presented. Finally, challenges and the outlook of CHA development are considered.