NIR-SERS studies of DNA and DNA bases attached on polyvinyl alcohol (PVA) protected silver grass-like nanostructures

• The fabrication process of the PVA–Ag–GNS is very facile and inexpensive.
• The PVA–Ag–GNS is biocompatible and highly NIR-SERS active, with the EF of 108.
• DNA adsorbed on silver surface via the intra-annular nitrogen of the DNA bases.
• DNA bases in DNA adsorbed with the silver surface is in the order T < C < A < G.

In this work, polyvinyl alcohol (PVA) protected silver grass-like nanostructure (PVA–Ag–GNS) with near infrared surface-enhanced Raman scattering (NIR-SERS) activity was prepared and employed to detect DNA and DNA bases. The PVA–Ag–GNS demonstrated high NIR-SERS activity and good optical reproducibility in the detection of adsorbates such as the case of crystal violet, DNA and DNA bases. By using of the tested molecule of thymine, the PVA–Ag–GNS shows a high enhancement factor (EF) of ∼108. For NIR-SERS detection of DNA molecules, Raman signals from the DNA bases of guanine (630 cm−1) and adenine (720 cm−1) are greatly enhanced. For DNA molecules NIR-SERS detection, Raman signals from the DNA bases of guanine (630 cm−1), adenine (720 cm−1) and cytosine (1010 cm−1) are greatly enhanced. The experimental results show that the NIR-SERS spectrum of DNA is dominated by guanine mode, which is followed by adenine and cytosine modes, respectively. Meanwhile, the NIR-SERS signal intensities of the DNA bases increase in the order of thymine (T) < cytosine (C) < adenine (A) < guanine (G). One can conclude that the adsorption strength of the DNA bases in DNA molecule with the silver surface is in the order T < C < A < G, which is different from that of the four DNA bases in individual molecule adsorbed on silver surface (T < A < G < C). On the other hand, the geometry optimization and calculated wavenumber of the complexes of adenine–Ag, guanine–Ag, cytosine–Ag and thymine–Ag for the ground states are performed with DFT, B3LYP functional and the LanL2DZ basis set. The calculated wavenumbers match well with the experimental results. According to our experiment and calculations, DNA base molecules adsorbed on silver surface via the intra-annular nitrogen atom which is adsorbed on the silver nanoparticle and formed metal–molecule complexes by the available lone pair.