In this study, a combination of carbon dots (CDs) and aptamers is proposed as a novel fluorescence probe for sensitive quantitative detection of Salmonella typhimurium. Carboxyl-modified CDs with better biocompatibility and monodispersity were prepared by a hydrothermal method using citric acid as carbon source. To ensure specificity, amino-modified aptamers were connected to the surface of CDs modified with carboxyl to form the carbon dot–aptamer complexes (CD–apt), which are of extremely high quantum yield (ca. 76%) and specificity. Salmonella typhimurium was taken as a representative sample for fluorescence detection, and incubation time and dosage of CD–apt were investigated. Under the optimized conditions, a linear relationship between concentration of Salmonella typhimurium and fluorescence intensity was obtained in the range of 10³ to 10⁵ cfu mL⁻¹ using the equation I = 82.506 log C − 203.17 with R² = 0.9903, and the detection limit was down to 50 cfu mL⁻¹ without any sample enrichment process. Significantly, the practicability of this method was validated by assaying Salmonella typhimurium in egg samples and tap water. The proposed method is promising for rapid and sensitive detection of other bacteria if suitable aptamers are chosen.

The formation of bacterial biofilms is a key factor in the emergence of chronic infections due to the strong resistance of biofilms to conventional antibiotics. There is an urgent need to develop an effective strategy to control the formation of biofilms. In this study, a nanocomposite of tannic acid and silver (Tannin-AgNPs) was designed and successfully prepared based on the quorum sensing (QS) inhibitory activity of tannic acid and the anti-bacterial activity of silver. The dynamic light scattering and SEM observations indicated that the obtained Tannin-AgNPs were spherical with a mean particle size of 42.37 nm. Tannic acid was successfully modified on the surface of silver nanoparticles and characterized via Fourier transform infrared (FTIR) spectroscopy. The prepared Tannin-AgNPs demonstrated a more effective anti-bacterial and anti-biofilm activity against E. coli than the unmodified AgNPs or tannic acid. In addition, the Tannin-AgNPs can modulate the formation process of E. coli biofilms, shorten the growth period of biofilms and extend the dispersion period of biofilms. Tannin-AgNPs also showed the function of decreasing the production of the QS signal molecule. The proposed strategy of constructing a nanocomposite using AgNPs and natural components with QS inhibitory activity is effective and promising for inhibiting the formation of biofilms.

The thickness of the sensitive layer has an important influence on the sensitivity of a shear horizontal surface acoustic wave (SH-SAW) biosensor with a delay-line structure and lower layer numbers of graphene produce better sensitivity for biological detection. Therefore, a label-free and highly sensitive SH-SAW biosensor with chemical vapor deposition (CVD-)-grown single-layered graphene (SLG) for endotoxin detection was developed in this study. With this methodology, SH-SAW biosensors were fabricated on a 36° Y-90° X quartz substrate with a base frequency of 246.2 MHz, and an effective detection cell was fabricated using acrylic material. To increase the surface hydrophilicity, chitosan was applied to the surface of the SLG film. Additionally, the aptamer was immobilized on the surface of the SLG film by cross-linking with glutaraldehyde. Finally, the sensitivity was verified by endotoxin detection with a linear detection ranging from 0 to 100 ng/mL, and the detection limit (LOD) was as low as 3.53 ng/mL. In addition, the stability of this type of SH-SAW biosensor from the air phase to the liquid phase proved to be excellent and the specificity was tested and verified by detecting the endotoxin obtained from Escherichia coli (E. coli), the endotoxin obtained from Pseudomonas aeruginosa (P. aeruginosa), and aflatoxin. Therefore, this type of SH-SAW biosensor with a CVD-grown SLG film may offer a promising approach to endotoxin detection, and it may have great potential in clinical applications.