Electrochemical tunnelling sensors and their potential applications

Abstract

the quantum-mechanical tunnelling effect allows charge transport across nanometre-scale gaps between conducting electrodes. application of a voltage between these electrodes leads to a measurable tunnelling current, which is highly sensitive to the gap size, the voltage applied and the medium in the gap. applied to liquid environments, this offers interesting prospects of using tunnelling currents as a sensitive tool to study fundamental interfacial processes, to probe chemical reactions at the single-molecule level and to analyse the composition of biopolymers such as Dna, rna or proteins. this offers the possibility of a new class of sensor devices with unique capabilities. E lectrochemical sensors based on measuring the tunnelling current across nanoscale electrode junctions in solution are emerging as a new class of single-molecule sensors. They combine extremely high spatial resolution with sensitivity to the electronic structure of the analyte. This opens up perspectives towards the label-free detection of small molecules, the characterization of catalytic or enzymatic processes at the single-molecule level as well as the analysis of biopolymers with sub-molecular resolution, Fig. 1. To this end, label-free, fast and inexpensive RNA and DNA sequencing may be in reach, potentially revolutionizing the way we perform gene sequencing today.Charge transfer by quantum-mechanical tunnelling is one of the most ubiquitous elementary processes in nature and features in areas as diverse as electron or hydrogen transfer in enzymatic reactions, interfacial charge transfer at metal electrodes in solution, thin-layer devices in electronics, charge transfer between crystal defects and radioactive α-decay 1 . Important applications exploiting tunnelling transport include the Esaki tunnelling diode and the scanning tunnelling microscope (STM)-both recognized with the Nobel Prize in Physics for their inventors, namely Esaki in 1973 (jointly with Josephson), and Binnig and Rohrer in 1986, respectively.'Tunnelling' refers to the transport of electrons (or other light charge carriers) across a nanometre-sized gap, a molecule or even an insulating layer between two electrodes. Upon application of a bias voltage V bias between the two electrodes, negatively charged electrons are more likely to be transported towards the positively biased electrode and a so-called 'tunnelling current' I t is detected. The latter depends exponentially on gap size, which is the reason for the high spatial resolution of STM, the applied voltage V b and the electronic structure of the medium in the gap. To this end, the electrode junction may be represented by a transmission function that characterizes the probability of electrons moving from one electrode across the gap into the second electrode, summarized over all energy levels.Hence, the more accessible the energy levels in the junction, and the better their electronic coupling to the electrodes, the higher current across the junction. This also implies that the tunnelling current is sensitive to ...

Oligomeric ferrocene rings

“…These were formally attributed to the sequential oxidation of Fe 2+ centres to Fe 3+ . Remarkably, this is the first time successive 1e -redox waves have been resolved in a family of cyclic oligomers (it is notable that cyclo [9] can be interrogated in up to seven different oxidation states). Previous electrochemical studies of large redoxactive rings comprising metallocene units linked by AX n (A = Si, X = Me; M = Fe; n = 2-7) 3,4 have instead extracted a 'three wave rule' for odd n to address the patterns of waves in voltammograms.…”

“…Within the available solvent window, we observed three well-defined redox waves for cyclo [5] (E 4 overlapping with the solvent oxidation wave), four for cyclo [6] and six for cyclo [9] (Fig. 3).…”

“…Applying this methodology to 1,1′-diiodoferrocene (fc 1 I 2 ), 1,1′′′-diiodobiferrocene (fc 2 I 2 ) and 1,1′′′′′-diiodoterferrocene (fc 3 I 2 ) at high dilution, cyclo(1,1′-quinqueferrocene) (cyclo [5]; n = 5), cyclo(1,1′-sexiferrocene) (cyclo [6]; n = 6), cyclo(1,1′-septiferrocene) (cyclo [7]; n = 7) and cyclo(1,1′-noviferrocene) (cyclo [9]; n = 9) were fully or partially isolated for the first time (Fig. 1a).…”

“…1a). To date, cyclo [9] is the largest covalently bound oligomeric ferrocene material ever isolated. No convincing evidence ( 1 H NMR, ES+) for even bigger cyclic oligomers (n > 9)-nor for cyclo [3], cyclo [4] or cyclo [8] (n = 3, 4 or 8)-was obtained.…”

“…cyclo [5] cyclo [6] cyclo [9] Figure 3 | Solution electrochemistry for ferrocene rings. Cyclic (top) and differential pulse (bottom) voltammograms recorded in CH 2 Cl 2 /NaBARF (E vs [FeCp 2 ] + /[FeCp 2 ], corrected for R u ).…”