Charge-transfer optical absorption mechanism of DNA:Ag-nanocluster complexes

Optical properties of DNA:Ag-nanoclusters complexes have been successfully applied experimentally in Chemistry, Physics, and Biology. Nevertheless, the mechanisms behind their optical activity remain unresolved. In this work, we present a time-dependent density functional study of optical absorption in DNA:${\mathrm{Ag}}_4$. In all 23 different complexes investigated, we obtain new absorption peaks in the visible region that are not found in either the isolated ${\mathrm{Ag}}_4$ or isolated DNA base pairs. Absorption from red to green are predominantly of charge-transfer character, from the ${\mathrm{Ag}}_4$ to the DNA fragment, while absorption in the blue-violet range are mostly associated to electronic transitions of a mixed character, involving either DNA-${\mathrm{Ag}}_4$ hybrid orbitals or intracluster orbitals. We also investigate the role of exchange-correlation functionals in the calculated optical spectra. Significant differences are observed between the calculations using the PBE functional (without exact exchange) and the CAM-B3LYP functional (which partly includes exact exchange). Specifically, we observe a tendency of charge-transfer excitations to involve purines bases, and the PBE spectra error is more pronounced in the complexes where the Ag cluster is bound to the purines. Finally, our results also highlight the importance of adding both the complementary base pair and the sugar-phosphate backbone in order to properly characterize the absorption spectrum of DNA:Ag complexes.

Figure 1

Optical absorption spectra of isolated ${\mathrm{Ag}}_4$ (filled black line), isolated adenine-thymine nucleotide pair (dotted red line), and isolated cytosine-guanine nucleotide pair (dashed green line).

Figure 2

Optimized geometries and TDDFT spectra of CG complexes. Two views of the same geometry are shown for each case.

Figure 3

Optimized geometries and TDDFT spectra of AT complexes. Two views of the same geometry are shown for each case.

Figure 4

Nature of the optical absorption at 3.07 eV in the isolated ${\mathrm{Ag}}_4$. The promotion of an electron from the (a) HOMO to the (b) LUMO+1 accounts for 91% of the spectral weight for this optical absorption. (c) Electronic-density change upon going from the ground state [yellow (light gray)] to the excited state [red (dark gray)] in this optical absorption.

Figure 5

Spectra of all DNA:${\mathrm{Ag}}_4$ complexes with PBE functional for (a) CG complexes and (b) AT complexes and with CAM-B3LYP functional for (c) CG complexes and (d) AT complexes.

Figure 6

Difference density plot of (a) CG-6 complex orbitals involved in the transition that originates the peak at 2.50 eV and (b) AT-9 complex orbitals involved in the transition that originates the peak at 2.52 eV. The negative isodensity values, in yellow (light gray), represent regions from where the electron leaves and the positive isodensity values, in red (dark gray), represent regions to where the electron goes.

Figure 7

Difference of densities of (a) CG-7 complex orbitals involved in the transition that originates the peaks at 2.69 eV and (b) AT-6 complex orbitals involved in the transition that originates the peak at 2.64 eV. The negative isodensity values are in yellow (light gray) and the positive isodensity values are in red (dark gray).

Figure 8

Single-electron-transition picture of CG-9 complex orbitals involved in the transition that originates the peaks at 2.78 eV.

Figure 9

Calculated absorption spectra of CG-6 (filled black line), cytosine:${\mathrm{Ag}}_4$ (dotted red line), and guanine:${\mathrm{Ag}}_4$ (dashed green line).

Figure 10

Calculated absorption spectra of CG-6 (filled black line), and of CG-6 with the sugar-phosphate backbone removed (dotted red line).

Figure 11

Calculated absorption spectra of AT-1 (filled black line) and of AT-1 with the sugar-phosphate backbone removed (dotted red line).