Correction to “Monolayer Graphene Platform for the Study of DNA Damage by Low-Energy Electron Irradiation”

T authors reported a novel approach to studying lowenergy electron-induced damage of short single-strand DNA (ssDNA) oligomers on a graphene coated Au substrate. Two errors are reported. (1) The atomic force microscopy (AFM) image presented in Figure 1b of the original Letter is for long-chain lambda digest DNA samples adsorbed on mica (see www.mtpgroup.nl/otnw-project.aspx). An AFM image of the 35 base pair ssDNA sample studied in our lab is now presented. (2) Figure 3d of the original Letter indicated that the 1090 cm−1 band was correlated with d(CHv) vibrations. The 1090 cm−1 band correlates with PO2 − vibrations. As described in more detail below, these corrections do not change the initial interpretations of the paper. Figure 1a of the published Letter illustrated the experimental approach. The graphene was produced via standard chemical vapor deposition on polycrystalline Cu foil with single-layer film processing techniques using poly(methyl methacrylate) and Cu etching using an aqueous solution of iron chloride. The separated single-layer graphene was then deposited on a nanostructured 30 nm thick Au-covered Si/SiO2 substrate. ssDNA samples (35 base pairs long) were then desalted using standard techniques and deposited on the graphene surface as 0.3 mm droplets within 0.1 mm of each other. These droplets were then allowed to dry under high vacuum conditions, yielding an average surface number density of <10 molecules/ cm. The substrate containing multiple ssDNA sample spots was then exposed to a 20 nA broad (>10 mm) beam of lowenergy (<2 eV) electrons under high vacuum conditions for a known and fixed total dose. The damage was then examined ex situ using Raman microspectroscopy. A typical damage yield of ∼1 DNA break/10 incident electrons was reported for singlecollision damage events. This analysis assumed a uniform ssDNA coverage and essentially no folding or overlap. The folding was initially checked using the unified nucleic acid folding and hybridization package (UNAfold) software. The calculations indicated that the dNA sequences studied do not fold. However, the calculations also indicated that some strands (i.e., T10-A15T10 and A10-T15-A10) may polymerize or form partial complements. In order to check the folding and polymerization, AFM was carried out. The AFM image presented in Figure 1b in the original paper was for long-chain lambda digest DNA samples adsorbed on mica. This modified image was not obtained in our laboratory and does not represent the image of the DNA studied in our work. Figure 1 shows the image of a 35 base pair ssDNA sample adsorbed on the graphene-covered Aucoated Si/SiO2 substrate. This image was obtained in our lab after sample drying under ambient conditions using an XEI Park Systems AFM operated in tapping mode only. The survey scan resolution was only adequate to resolve domains as small as 15−20 nm in diameter and height variations of 2−10 nm (Figure 1a). The bright white spots (Figure 1b) are assumed to be contamination of an unknown chemical composition, and the adsorbed DNA spots are visible as faint white dots about 20−100 nm in diameter with a mean diameter of ∼40 nm. This is larger than the expected size of a free 35 base pair ssDNA strand but possibly close to the size of an extended adsorbed molecule that may be coupled to p-doped regions on the graphene. It may also represent polymerized strands, complements, or clusters. The probability of DNA aggregating/ clustering on nonfunctionalized or undoped graphene can be high and dependent on the base identity. Due to the relative strength of the base interaction with graphene versus the interbase coupling strength, aggregation was shown for pure A and C strands but not for Gand T-containing sequences. The sequences that we examined are T15-G5-T15, G5-T25-G5, T10-A15-A10, A10-T15-A10, and G5-A10-G5-A10-G5. Unfortunately, our AFM measurements are neither the appropriate resolution nor quality to discern whether interbase coupling