776 Phys. Chem. Chem. Phys., 2013, 15, 776--786 This journal is c the Owner Societies 2013 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 776 Electrochemistry and in situ scanning tunnelling microscopy of pure and redox-marked DNA- and UNA- based oligonucleotides on Au(111)-electrode surfaces† Allan G. Hansen, a Princia Salvatore, a Kasper K. Karlsen, b Richard J. Nichols, c Jesper Wengel b and Jens Ulstrup* a We have studied adsorption and electrochemical electron transfer of several 13- and 15-base DNA and UNA (unlocked nucleic acids) oligonucleotides (ONs) linked to Au(111)-electrode surfaces via a5 0 -C6-SH group using cyclic voltammetry (CV) and scanning tunnelling microscopy in aqueous buffer under electrochemical potential control (in situ STM). 2,2 0 ,6 0 ,2 00 -Terpyridine (terpy) onto which the transition metal ions Fe 2+/3+ , Os 2+/3+ and Ru 2+/3+ could be coordinated after UNA monolayer formation was attached to UNA via a flexible linker. The metal centres offer CV probes and in situ STM contrast markers, and the flexible UNA/linker a potential binder for intercalation. CV of pure and mercaptohexanol diluted ON monolayers displayed reductive desorption signals but also, presumably capacitive, signals at higher potentials. Distinct voltammetric signals arise on metal binding. Those from Ru-binding are by far the strongest and in accord with multiple site Ru-attachment. In situ STM disclosed molecular scale features in varying coverage on addition of the metal ions. The Ru-derivatives showed a bias voltage dependent broad maximum in the tunnelling current–overpotential correlation which could be correlated with theoretical frames for condensed matter conductivity of redox molecules. Together the data suggest that Ru-units are bound to both terpy and the UNA–DNA backbone. 1. Introduction DNA-based molecules in their double-strand (ds) form in homo- geneous solution can transfer electronic charge (electrons or holes) over long distances through their molecular structure. 1–13 Charge transfer mapping has rested on photo-induced, i.e. high-energy charge injection, establishing long-range DNA-based photo-induced charge transfer as a combination of hopping, superexchange, and gating. ‘‘Hopping’’ involves electron transfer (ET) or hole transfer between physically populated electron or hole states, 14 usually cationic states of guanine and guanine doublets or triplets. 15–21 The individual ‘‘hops’’ involve superexchange via intermediate higher-energy ionized states of the other bases (A, C, and T) and are restricted to short distances less than a few base pairs. These higher ionized states are not populated, but represent tunnelling barriers. The combination of ‘‘hopping’’ by superexchange of the individual hops operates over long distances but guanine cationic states still involve high-energy states of the photo-generated holes. DNA conductivity has also been addressed as interfacial electrochemical ET between an electrode and a variety of organic 22–30 and transition metal 28,31 redox molecular entities intercalated or covalently bound in ds oligonucleotides (ONs). Facile long-range interfacial ET even up to 100 base pairs 24 has been reported. The interfacial ET processes are highly sensitive to local structural ds features, base pair mismatches, etc. DNA conductivity has also been brought to the single-molecule level in controlled nanogap electrodes or electrochemical scanning tunnelling micro- scopy (in situ STM). 32–36 Long-range charge transfer with variable but broadly weak distance attenuation was also observed in these cases. Long-range charge transfer through ds-DNA under the ‘‘mild’’ conditions in the electrochemical approach offers con- ceptual challenges. The high oxidation and low reduction potentials of the bases are prohibitive for common views of facile interfacial electrochemical ET or hole transfer without inter- ference from Au-electrode oxidation, dioxygen evolution, etc. a Department of Chemistry, Building 207, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. E-mail: ju@kemi.dtu.dk b Department of Physics, Chemistry and Pharmacy, Nucleic Acid Center, University of Southern Denmark, Campusvej 55, DK-5230, Odense M, Denmark c The Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK † Electronic supplementary information (ESI) available: Details of the synthesis and purification of the ONs; MALDI-MS spectra of the oligonucleotides; tunnelling STS of the Ru-derivative of terpy 15-base ss DNA; formalism of STS bandwidth dependence of the ionic strength. See DOI: 10.1039/c2cp42351k Received 14th May 2012, Accepted 28th September 2012 DOI: 10.1039/c2cp42351k www.rsc.org/pccp PCCP PAPER Downloaded by DTU Library on 10 January 2013 Published on 28 September 2012 on http://pubs.rsc.org | doi:10.1039/C2CP42351K View Article Online View Journal | View Issue