Electromagnetic and optical characteristics of Nb-doped double-crossover and salmon DNA thin films.

We report the fabrication and physical characteristics of niobium ion (Nb)-doped double-crossover DNA (DX-DNA) and salmon DNA (SDNA) thin films. Different concentrations of Nb ([Nb]) are coordinated into the DNA molecules, and the thin films are fabricated via substrate-assisted growth (DX-DNA) and drop-casting (SDNA) on oxygen plasma treated substrates. We conducted atomic force microscopy to estimate the optimum concentration of Nb ([Nb] = 0.08 mM) in Nb-doped DX-DNA thin films, up to which the DX-DNA lattices maintain their structures without deformation. X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical nature of the intercalated Nb in the SDNA thin films. The change in peak intensities and the shift in binding energy were witnessed in XPS spectra to explicate the binding and charge transfer mechanisms between Nb and SDNA molecules. UV-visible, Raman, and photoluminescence (PL) spectra were measured to determine the optical properties and thus investigate the binding modes, Nb coordination sites in Nb-doped SDNA thin films, and energy transfer mechanisms, respectively. As [Nb] increases, the absorbance peak intensities monotonically increase until ∼[Nb] and then decrease. However, from the Raman measurements, the peak intensities gradually decrease with an increase in [Nb] to reveal the binding mechanism and binding sites of metal ions in the SDNA molecules. From the PL, we observe the emission intensities to reduce them at up to ∼[Nb] and then increase after that, expecting the energy transfer between the Nb and SDNA molecules. The current-voltage measurement shows a significant increase in the current observed as [Nb] increases in the SDNA thin films when compared to that of pristine SDNA thin films. Finally, we investigate the temperature dependent magnetization in which the Nb-doped SDNA thin films reveal weak ferromagnetism due to the existence of tiny magnetic dipoles in the Nb-doped SDNA complex.