Theoretical study on the binding mechanism between N6-methyladenine and natural DNA bases.

N6-methyladenine (m(6)A) is a rare base naturally occurring in DNA. It is different from the base adenine due to its N-CH(3). Therefore, the base not only pairs with thymine, but also with other DNA bases (cytosine, adenine and guanine). In this work, Møller-Plesset second-order (MP2) method has been used to investigate the binding mechanism between m(6)A and natural DNA bases in gas phase and in aqueous solution. The results show that N-CH(3) changed the way of N6-methyladenine binding to natural DNA bases. The binding style significantly influences the stability of base pairs. The trans-m(6)A:G and trans-m(6)A:C conformers are the most stable among all the base pairs. The existence of solvent can remarkably reduce the stability of the base pairs, and the DNA bases prefer pairing with trans-m(6)A to cis-m(6)A. Besides, the properties of these hydrogen bonds have been analyzed by atom in molecules (AIM) theory, natural bond orbital (NBO) analysis and Wiberg bond indexes (WBI). In addition, pairing with m(6)A decreases the binding energies compared to the normal Watson-Crick base pairs, it may explain the instability of the N6 site methylated DNA in theory.

Rapid and sensitive in-situ detection of polar antibiotics in water using a disposable Ag-graphene sensor based on electrophoretic preconcentration and surface-enhanced Raman spectroscopy.

A disposable Ag-graphene sensor was developed for rapid and sensitive in-situ detection of polar antibiotics in water using electrophoretic preconcentration (EP) and surface-enhanced Raman spectroscopy (SERS). The Ag-graphene sensor was fabricated by depositing Ag-graphene nanocomposites synthesized through a facile one-pot method on the disposable screen-printed electrodes (SPEs) and characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). SERS properties and detection applicability of the developed sensor were systematically investigated. It is shown that the polar antibiotics can be selectively adsorbed on the oppositely charged sensors after applying different potentials during the EP procedure, and the SERS signals of antibiotics with an effective amplification can be achieved with proper time of preconcentration. Moreover, the Ag-graphene sensor could facilitate the molecule adsorption through weak π-π interactions between graphene and antibiotics, further improving the sensitivity of SERS detection. Under the optimum EP conditions, the representative SERS spectra of a mixed solution containing four different antibiotics can be obtained within 10 min, and each antibiotic is easily distinguished by its characteristic peaks with a sub-nM detection level. The results demonstrate that the proposed disposable Ag-graphene sensor based on EP-SERS can be used for rapid and sensitive in-situ detection of polar antibiotics in aqueous samples without a pre-separation step.

Fabrication of bimetallic microfluidic surface-enhanced Raman scattering sensors on paper by screen printing.

Au-Ag bimetallic microfluidic, dumbbell-shaped, surface enhanced Raman scattering (SERS) sensors were fabricated on cellulose paper by screen printing. These printed sensors rely on a sample droplet injection zone, and a SERS detection zone at either end of the dumbbell motif, fabricated by printing silver nanoparticles (Ag NPs) and gold nanoparticles (Au NPs) successively with microscale precision. The microfluidic channel was patterned using an insulating ink to connect these two zones and form a hydrophobic circuit. Owing to capillary action of paper in the millimeter-sized channels, the sensor could enable self-filtering of fluids to remove suspended particles within wastewater without pumping. This sensor also allows sensitive SERS detection, due to advantageous combination of the strong surface enhancement of Ag NPs and excellent chemical stability of Au NPs. The SERS performance of the sensors was investigated by employing the probe rhodamine 6G, a limit of detection (LOD) of 1.1×10(-13)M and an enhancement factor of 8.6×10(6) could be achieved. Moreover, the dumbbell-shaped bimetallic sensors exhibited good stability with SERS performance being maintained over 14 weeks in air, and high reproducibility with less than 15% variation in spot-to-spot SERS intensity. Using these dumbbell-shaped bimetallic sensors, substituted aromatic pollutants in wastewater samples could be quantitatively analyzed, which demonstrated their excellent capability for rapid trace pollutant detection in wastewater samples in the field without pre-separation.