No abstract

Demonstration of a colorimetric approach for nucleic acid detection represents an attractive educational experiment for chemistry undergraduate students in the time of coronavirus pandemic. Herein, a rapid and vivid detection method that visualizes the presence of a specific DNA sequence is described. The plasmon resonance of gold nanoparticles (AuNPs) (∼20 nm in diameter) endows a red color to DNA-functionalized gold nanoparticles (DNA-AuNPs) in a colloidal solution. Simply upon addition of a sample containing the complementary DNA sequence, the DNA-AuNPs dispersed in a strong ionic solution can spontaneously aggregate in minutes and turn to be purple in color, which can be detected using UV–visible spectroscopy as well as the naked eye. In contrast, the DNA-AuNPs remain highly dispersed as a reddish colloid even at high ionic strength when the DNA sequence is terminally mismatched or overhung. According to the teaching experience, this DNA testing experiment can be accomplished by students in pairs or groups under instruction in a three-day lab session. Given the rapid and fail-proof DNA testing, the testing method is also well-suited for hands-on introduction of analytical chemistry and nanotechnology to chemistry freshman and high school students.

Strains in biomolecules greatly restrict their structural flexibility. The effects of DNA's structural flexibility on nanoparticle stability have remained less explored in the field of plasmonic biosensors. In the present study, we discover the opposite effects of a rigid loop and a flexible single-stranded DNA (ssDNA) region in DNAzyme on the colloidal stability of gold nanoparticles (AuNPs), which afford "turnon" plasmonic detection of Pb 2+ . In specific, DNAzyme-functionalized AuNPs undergo spontaneous assembly at high ionic strength upon hybridization to their substrate sequence because of a DNA base stacking interaction. In the presence of Pb 2+ , however, the DNAzyme grafted on the AuNP cleaves the substrate and forms an ssDNA region in the middle of the rigid loop. The induced structural flexibility of the surface-grafted DNAzyme by the ssDNA region in the middle helps elevate interparticle entropic repulsion, thereby bringing AuNP assemblies back to dispersion. We discover that this process can afford a dramatic increase of the AuNPs' plasmon resonance for determination of Pb 2+ concentration. Under optimized conditions, a detection limit of 8.0 nM can be achieved for Pb 2+ by this method with high selectivity. Its applicability to Pb 2+ analysis in tap water samples has also been demonstrated.

Quality control of food products requires the development of powerful analytical tools and regulations to avoid fraud to consumers and ineffective product recalls. DNA barcoding represents a potential strategy for application in food traceability, which involves the use of exogeneous short DNA oligonucleotides (DNA markers) for tracing the origin of food, such as suppliers, producers, attributes, and points of distribution. However, it is generally considered that exogeneous DNA molecules are strongly shielded by complicated interactions with intrinsic ingredients and easily degraded in complex food systems. This consideration leads to the lack of analytical methods to readily identify DNA markers in food, thereby preventing one from exploiting the barcoding capability of DNA markers in food. In this work, we explored the applicability of DNA markers coupled with rapid DNA detection by gold nanoparticles to traceability of various liquid foods, including wines, condiments, and soft drinks. Our results revealed that the gold nanoparticles afford colorimetric identification of DNA markers in liquors, condiments, and milk with a quick (≤5 min) “yes” or “no” readout depending on the solution color. Importantly, the DNA markers stored in these liquid foods remain chemically stable and bioactive in hybridization within months, thereby allowing for barcoding liquid foods with considerably long shelf lives. Given the nutritional value of nucleic acids and the direct DNA identification avoiding complicated isolation and analysis, the present work could provide a practical approach to food traceability.

The currently established DNA nanoprobes for detection of mycotoxin from beverages have been limited by complicated sample pretreatment and uncontrollable nanoparticle flocculation in complex systems. We develop a rapid colorimetric...