A basic and affordable experimental apparatus is described that measures the static contact angle of a liquid drop in contact with a solid. The image of the drop is made with a simple digital camera by taking a picture that is magnified by an optical lens. The profile of the drop is then processed with ImageJ free software. The ImageJ contact angle plugin detects the edge of the drop and fits its profile to a circle or an ellipse. The tangent to the triple line contact is calculated and drawn by the ImageJ software, thus, returning the value of the contact angle with acute precision on the measurement.

In molecular electronics, the linker group, which attaches the functional molecular core to the electrode, plays a crucial role in determining the overall conductivity of the molecular junction. While much focus has been placed on optimizing molecular core conductivity, there have been relatively few attempts at designing optimal linker groups to metallic or semiconducting electrodes. The vast majority of molecular electronic studies use thiol linker groups; work probing alternative amine linker systems has only recently been explored. Here, we probe single-molecule conductances in phenylene-ethynylene molecules terminated with thiol and carbodithioate linkers, experimentally using STM break-junction methods and theoretically using a nonequilibrium Green's function approach. Experimental studies demonstrate that the carbodithioate linker augments electronic coupling to the metal electrode and lowers the effective barrier for charge transport relative to the conventional thiol linker, thus enhancing the conductance of the linker-phenylene-ethynylene-linker unit; these data underscore that phenylene-ethynylene-based structures are more highly conductive than originally appreciated in molecular electronics applications. The theoretical analysis shows that the nature of sulfur hybridization in these species is responsible for the order-of-magnitude increased conductance in carbodithioate-terminated systems relative to identical conjugated structures that feature classic thiol linkers, independent of the mechanism of charge transport. Interestingly, in these systems, the tunneling current is not dominated by the frontier molecular orbitals. While barriers >k(B)T to produce the low beta values seen in our experiments. Taken together, these experimental and theoretical studies indicate a promising role for carbodithioate-based connectivity in molecular-scale electronics applications involving metallic and semiconducting electrodes.

Self-assembled monolayers (SAMs) of single-stranded peptide nucleic acids (PNAs) containing 3 to 7 thymine (T) nucleotides, a C-terminus cysteine, and an N-terminus ferrocene group were formed on gold electrodes. The existence of two redox environments for the ferrocene was detected by cyclic voltammetry and was attributed to the presence of "lying-down" and "standing-up" PNA molecules. By exploiting the chemical instability of the ferrocenium ion, electrochemical cycling was used to destroy the ferrocene of "lying-down" molecules while keeping the ferrocene in the "standing-up" molecules intact. Electrochemical measurements were used to determine the electron-transfer rate through the "standing-up" PNA molecules. The tunneling decay constant for these SAMs was determined to be about 0.9 Å -1 . IntroductionThe interest in self-assembled monolayers (SAMs) 1,2 of nucleic acids has increased recently, largely because of their potential applications in molecular electronics, 3 materials science, 4 molecular recognition, 5 biotechnology, and biosensor development. [6][7][8] An understanding of the charge transport (CT) through such SAMs is needed to realize their potential in molecular electronics and biosensing. The past decade has seen progress in understanding charge transfer through deoxyribonucleic acid (DNA), which is believed to occur through either a superexchange mechanism, 9-19 which dominates at short distances, or a hopping mechanism, 20-25 which dominates at large distances.The weak distance dependence of the charge hopping mechanism and its prevalence in duplex DNA systems have motivated the exploration of charge transfer through DNA and its promise for molecular electronics by a large number of different research groups. Nevertheless, only a few research groups have studied CT in DNA monolayers, probably because of the difficulties in creating well-defined DNA assemblies on a metal surface. [26][27][28] For example, Hartwich et al. 29 used cyclic voltammetry to characterize charge transfer in mixed monolayers of DNA having a pyrroloquinoline-quinone redox probe attached to DNA through a spacer and linked to an Au(111) surface through an ethane-thiol linker. These studies determined that the CT rate constant for a 12-base-pair (bp) DNA duplex was 1.5 s -1 , while for the same duplex containing two mismatches it was 0.6 s -1 , and that charge transfer could not be detected for single-stranded (ss) DNA at a scan rate of >10 mV s -1 . Liu et al. 30 argued that CT through a monolayer of a 30-bp double-stranded (ds) DNA takes place through the nucleobase stack and does not involve the DNA backbone. They based their argument on the fact that the rate constant for CT of 30 s -1 was not affected by breaks in the sugar-phosphate backbone and was too small to be measured when a mismatch was introduced in the ds DNA. Interestingly, a similar rate constant for CT was measured for a monolayer of a 15-bp ds DNA. 31 CT rate constants for ss oligonucleotides are also quite high. For example, Kraatz and collaborators r...