Length-Independent Charge Transport in Chimeric Molecular Wires.

Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA-like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high-performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.

Bulk protonic conductivity in a cephalopod structural protein.

Proton-conducting materials play a central role in many renewable energy and bioelectronics technologies, including fuel cells, batteries and sensors. Thus, much research effort has been expended to develop improved proton-conducting materials, such as ceramic oxides, solid acids, polymers and metal-organic frameworks. Within this context, bulk proton conductors from naturally occurring proteins have received somewhat less attention than other materials, which is surprising given the potential modularity, tunability and processability of protein-based materials. Here, we report proton conductivity for thin films composed of reflectin, a cephalopod structural protein. Bulk reflectin has a proton conductivity of ~2.6 × 10(-3) S cm(-1) at 65 °C, a proton transport activation energy of ~0.2 eV and a proton mobility of ~7 × 10(-3) cm(2) V(-1) s(-1). These figures of merit are similar to those reported for state-of-the-art artificial proton conductors and make it possible to use reflectin in protein-based protonic transistors. Our findings may hold implications for the next generation of biocompatible proton-conducting materials and protonic devices.

Electrokinetic assembly of one-dimensional nanoparticle chains with cucurbit[7]uril controlled subnanometer junctions.

One-dimensional (1D) nanoparticle chains with defined nanojunctions are of strong interest due to their plasmonic and electronic properties. A strategy is presented for the assembly of 1D gold-nanoparticle chains with fixed and rigid cucurbit[n]uril-nanojunctions of 9 Å. The process is electrokinetically accomplished using a nanoporous polycarbonate membrane and controlled by the applied voltage, the nanoparticle/CB[n] concentration ratio, time and temperature. The spatial structure and time-resolved analysis of chain plasmonics confirm a growth mechanism at the membrane nanopores.

A ruthenocene-PNA bioconjugate--synthesis, characterization, cytotoxicity, and AAS-detected cellular uptake.

Labeling of peptide nucleic acids (PNA) with metallocene complexes is explored herein for the modulation of the analytical characteristics, as well as biological properties of PNA. The synthesis of the first ruthenocene-PNA conjugate with a dodecamer, mixed-sequence PNA is described, and its properties are compared to a ferrocene-labeled analogue as well as an acetylated, metal-free derivative. The synthetic characteristics, chemical stability, analytical and thermodynamic properties, and the interaction with cDNA were investigated. Furthermore, the cytotoxicity of the PNA conjugates is determined on HeLa, HepG2, and PT45 cell lines. Finally, the cellular uptake of the metal-containing PNAs was quantified by high-resolution continuum source atomic absorption spectrometry (HR-CS AAS). An unexpectedly high cellular uptake to final concentrations of 4.2 mM was observed upon incubation with 50 µM solutions of the ruthenocene-PNA conjugate. The ruthenocene label was shown to be an excellent label in all respects, which is also more stable than its ferrocene analogue. Because of its high stability, low toxicity, and the lack of a natural background of ruthenium, it is an ideal choice for bioanalytical purposes and possible medicinal and biological applications like, e.g., the development of gene-targeted drugs.

Impact of single basepair mismatches on electron-transfer processes at Fc-PNA⋠DNA modified gold surfaces.

Gold-surface grafted peptide nucleic acid (PNA) strands, which carry a redox-active ferrocene tag, present unique tools to electrochemically investigate their mechanical bending elasticity based on the kinetics of electron-transfer (ET) processes. A comparative study of the mechanical bending properties and the thermodynamic stability of a series of 12-mer Fc-PNA⋅DNA duplexes was carried out. A single basepair mismatch was integrated at all possible strand positions to provide nanoscopic insights into the physicochemical changes provoked by the presence of a single basepair mismatch with regard to its position within the strand. The ET processes at single mismatch Fc-PNA⋅DNA modified surfaces were found to proceed with increasing diffusion limitation and decreasing standard ET rate constants k(0) when the single basepair mismatch was dislocated along the strand towards its free-dangling Fc-modified end. The observed ET characteristics are considered to be due to a punctual increase in the strand elasticity at the mismatch position. The kinetic mismatch discrimination with respect to the fully-complementary duplex presents a basis for an electrochemical DNA sensing strategy based on the Fc-PNA⋅DNA bending dynamics for loosely packed monolayers. In a general sense, the strand elasticity presents a further physicochemical property which is affected by a single basepair mismatch which may possibly be used as a basis for future DNA sensing concepts for the specific detection of single basepair mismatches.

Mechanistic studies of Fc-PNA(⋠DNA) surface dynamics based on the kinetics of electron-transfer processes.

N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(⋅DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(⋅DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA⋅DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k(0). An in-depth understanding of the dynamics of surface-bound PNA and PNA⋅DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers.

A single-electrode, dual-potential ferrocene-PNA biosensor for the detection of DNA.

A Fc-PNA biosensor (Fc: ferrocenyl, C(10)H(9)Fe) was designed by using two electrochemically distinguishable recognition elements with different molecular information at a single electrode. Two Fc-PNA capture probes were therefore synthesized by N-terminal labeling different dodecamer PNA sequences with different ferrocene derivatives by click chemistry. Each of the two strands was thereby tethered with one specific ferrocene derivative. The two capture probes revealed quasi-reversible redox processes of the Fc(0/+) redox couple with a significant difference in their electrochemical half-wave potentials of Delta E(1/2)=160 mV. A carefully designed biosensor interface, consisting of a ternary self-assembled monolayer (SAM) of the two C-terminal cysteine-tethered Fc-PNA capture probes and 6-mercaptohexanol, was electrochemically investigated by square wave (SWV) and cyclic voltammetry (CV). The biosensor properties of this interface were analyzed by studying the interaction with DNA sequences that were complementary to either of the two capture probes by SWV. Based on distinct changes in both peak current and potential, a parallel identification of these two DNA sequences was successful with one interface design. Moreover, the primary electrochemical response could be converted by a simple mathematical analysis into a clear-cut electrochemical signal about the hybridization event. The discrimination of single-nucleotide polymorphism (SNP) was proven with a chosen single-mismatch DNA sequence. Furthermore, experiments with crude bacterial RNA confirm the principal suitability of this dual-potential sensor under real-life conditions.

"Four-potential" ferrocene labeling of PNA oligomers via click chemistry.

The scope of the Cu(I)-catalyzed [2 + 3] azide/alkyne cycloaddition (CuAAC, click chemistry) as a key reaction for the conjugation of ferrocene derivatives to N-terminal functionalized PNA oligomers is explored herein (PNA: peptide nucleic acid). The facile solid-phase synthesis of N-terminal azide or alkyne-functionalized PNA oligomer precursors and their cycloaddition with azidoferrocene, ethynylferrocene, and N-(3-ethylpent-1-yn-3-yl)ferrocene-carboxamide (DEPA-ferrocene) on the solid phase are presented. While the click reaction with azidomethylferrocene worked equally well, the ferrocenylmethyl group is lost from the conjugate upon acid cleavage. However, the desired product was obtained via a post-SPPS conversion of the alkyne-PNA oligomer with azidomethylferrocene in solution. The synthesis of all ferrocene-PNA conjugates (trimer t(3)-PNA, 3, 4, 5, 6; 12mer PNA, 10 - t c t a c a a g a c t c, 11 - t c t a c c g t a c t c) succeeded with excellent yields and purities, as determined by mass spectrometry and HPLC. Electrochemical studies of the trimer Fc-PNA conjugates 3, 4, 5, and 6 with four different ferrocene moieties revealed quasi-reversible redox processes of the ferrocenyl redox couple Fc(0/+) and electrochemical half-wave potentials in a range of E(1/2) = -20 mV to +270 mV vs FcH(0/+) (Fc: ferrocenyl, C(10)H(9)Fe). The observed potential differences ΔE(1/2)(min) are always greater than 60 mV for any given pair of Fc-PNA conjugates, thus allowing a reliable differentiation with sensitive electrochemical methods like e.g. square wave voltammetry (SWV). This is the electrochemical equivalent of "four-color" detection and is hence denoted "four-potential" labeling. Preparation and electrochemical investigation of the set of four structurally different and electrochemically distinguishable ferrocenyl groups conjugated to PNA oligomers, as exemplified by the conjugates 3, 4, 5, and 6, demonstrates the scope of the azide/alkyne cycloaddition for the labeling of PNA with electrochemically active ferrocenyl groups. Furthermore, it provides a PNA-based system for the electrochemical detection of single-nucleotide polymorphism (SNP) in DNA/RNA.

Synthesis of organometallic PNA oligomers by click chemistry.

The facile side-specific insertion, on the solid phase, of one or two ferrocene moieties into peptide nucleic acid (PNA) oligomers by click chemistry is presented.