Nanogap electrode

In the following we will focus on three molecular electronics test beds as developed and employed for applications at electrified solid/liquid interfaces (1) STM and STS, (2) assemblies based on horizontal nanogap electrodes, and (3) mechanically-controlled break junction experiments. For a more detailed description of the methods we refer to several excellent reviews published recently [16-22]. We will also address specific aspects of electrolyte gating and of data analysis. 

Tel-Vered, R., Walsh, D. A., Mehrgardi, M. A. and Bard, A. J. (2006), Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments. Anal. Chem., 78(19) 6959-6966. 

The high spahal in situ STM resolution of biological macromolecules offers new theoretical challenges. Even novel charge transfer phenomena have been revealed as noted. We therefore first overview a few conceptual notions of the fundamental electrochemical ET process, with emphasis on ET phenomena in nanogap electrode systems and in situ STM. 

We focus on the electronic conductivity of molecular monolayers and on single molecules enclosed between a pair of metallic electrodes. We address specifically in situ STM of redox (bio)molecules but concepts and formalisms carry over to other metallic nanogap configurations. Importantly, in addition to the substrate and tip, a third electrode serves as reference electrode [42-44] (Figure 2.2). This allows electrochemical potential control of both substrate and tip. The three-electrode configuration is the basis for two kinds of tunneling spectroscopy unique to electrochemical in situ STM. One is the current-bias voltage relation as for STM in air or vacuum, but with the notion that the substrate (over)potential is kept constant. The other is the current-overpotential relation at constant bias... 

Figure 2.3 Schematic view of a redox molecule in a STM gap or enclosed between two nanogap electrodes, (a) Redox molecule in tunneling gap and (b) electronic energy...

Electronic conductivity of molecules including redox metalloproteins with accessible low-lying redox states in nanogap electrode configurations or in situ STM displays quite different patterns. These are dominated by sequential two-step (or multiple-step) hopping through the redox center, induced both by potential variation and environmental configurahonal fluctuations. Both redox molecules and... 

The double-strand structure of an oligonucleotide is shown schematically in Fig. 6-1. Anticipating discussion in later Sections, the molecule is shown in a upright orientation attached to an atomically planar metallic electrode surface (Au(lll), cf below) by chemisorption via a hexamethylenethiol group. Fig. 6-1 shows the four nucleobases presently in focus. We discuss first concepts and formalism of electron and hole transport of DNA-based molecules in homogeneous solution and at electrochemical interfaces. We then focus on DNA-based molecules in electrochemical nanogaps and STM in electrochemical environments in situ STM). Some case examples illustrate accordance and limitations of current theoretical views of DNA-conductivity. This adds to the comprehensive overview of interfacial electrochemical ET of DNA-based molecules by O Kelly and Hill in Chapter 5. 

Figure 6-3. Schematic view of photo-induced electron or hole transport through the DNA-based molecular chain. Reversible random walk of the charge along the chain is interrupted by irreversible chemical degradation ( quenching ) at given sites. Charge i.e. electron or hole) injection at the terminals corresponds to interfacial charge transfer in the in situ STM or nanogap electrode configurations to be discussed in Section 4.

Figure 8-7. A Schematic view of a redox molecule or metalloprotein in the gap between a substrate and a tip electrode in STM or between two nanogap electrodes. B Electronic energy scheme for in situ STM of the redox molecule. C Tunnel junction with a redox molecule and a metallic nanoparticle in a hybrid structure. D Energy scheme corresponding to the hybrid structure with two potential drops.

We shall allude particularly to in situ STM of a redox (bio)molecule, but as noted, the configuration is representative broadly of redox molecules in electrochemically controlled nanogap electrode configurations and of singlemolecule transistor-like configurations reported lately. In contrast to the latter systems, which only apply in ultrahigh vacuum or at cryogenic... 

Fig. 13.19 SEM images of the gap electrodes (a) the initial electrode pairs with the spacing of 5 mm fabricated by conventional photolithography (b) the nanogap with a separation of 56 nm obtained at an ac sources frequency/= 260 Hz and the series resistances Rj=R = 0. kV (c) the nanogap with a separation of 28 nm obtained at/= 260 Hz and Rj = R = kV (d) the nanogap with a separation of 9 nm at/=820 Hz and R = R = kV. Reprinted with permission from ref. [125]. Copyright 2005, American Institute of Physics...

Chen F, Qing Q, Ren L, Wu ZY, Liu ZF (2005) Electrochemical approach for fabricating nanogap electrodes with well controllable separation. Appl Phys Lett 86 123105... 

Chang, T., Tsai, C., Sun, C., Chen, C., Kuo, L., Chen, P. (2007). Ultrasensitive electrical detection of protein using nanogap electrodes and nanoparticle-based DNA amplification. Biosens Bioelectron 22, 3139-3145. 

Fig. 7.10 Reduced graphene length and nonpmodic boundary conditions, a Structure at t = 1.5 ns with two nucleobases absorbed on left graphene electrode, b Structure at t = 2.5 ns with one nucleobase positioned in the nanogap zone, c vdW energy for gtaphene-ssDNA interaction during the initial 2 ns of equilibration at room temperature, d vdW energy for graphene-ssDNA interaction during the last nanosecond of equilibration at room temperature, and (i) Final conformation at 1 K. Water molecules are omitted in a and b for visuahzation ptrrposes [60]...

Prins F, Barreiro A, Ruitenberg JW, Seldenthuis JS, Aliaga-Alcalde N et al (2011) Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes. Nano Lett 11 4607-4611... 

Rassaei L, Mathwig K, Goluch ED, Lemay SG (2012) Hydrodynamic voltammetry with nanogap electrodes. J Phys Chem C 116 10913-10916... 

The exciting application of an extremely narrow SECM-based nanogap is the electrochemical detection of single molecules [71]. Current measurement with presently available instrumentation requires several thousands of electron-transfer events at an electrode, which corresponds to zeptomoles of molecules [76-77]. In contrast, the positive feedback effect of SECM amplifies a current response to a single molecule to enable its electrochemical detection [78]. Specifically, a small volume of a dilute solution of an electroactive species was trapped in the cavity of a 15-nm-diameter Pt-Ir tip surrounded by insulating wax sheath over indium-tin oxide as a conductive substrate (Figure 1.12a). The tip-substrate distance was adjusted to -10 nm by monitoring the positive feedback current response to the... 

FIGURE 1.16 Scheme of nanogap-based SECM measurements of a fast electron-transfer reaction at a macroscopic snbstrate in the (a) steady-state feedback mode and in quasi-steady-state (b) feedback and (c) SG/TC modes, (d) Quasi-steady-state i-j- — Eg voltammograms of TCNQ in acetonitrile (solid cnrves). The tip was held at —0.235 or 0 V versus an Ag quasireference electrode for feedback or SG/TC modes, respectively. Snbstrate potential was cycled at 50 mV/s. Closed circles and dotted lines are theoretical cnrves for quasi-reversible (k = 7 cm/s and a = 0.5) and reversible snbstrate reactions, respectively, with EP = -88 mV. The inset shows a reversible voltammogram with a peak separation of 61 mV simultaneously measured at the substrate. (Reprinted with permission from Nioradze, N. et al.. Quasi-steady-state voltammetry of rapid electron transfer reactions at the macroscopic substrate of the scanning electrochemical microscope. Anal. Chem., Vol. 83, 2011 pp. 828-835. Copyright 2011, American Chemical Society.)... 

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