Localized electrografting of vinylic monomers on a conducting substrate by means of an integrated electrochemical AFM probe.

Combinations of scanning electrochemical microscopy (SECM) with other scanning probe microscopy techniques, such as atomic force microscopy (AFM), show great promise for directing localized modification, which is of great interest for chemical, biochemical and technical applications. Herein, an atomic force scanning electrochemical microscope is used as a new electrochemical lithographic tool (L-AFM-SECM) to locally electrograft, with submicrometer resolution, a non-conducting organic coating on a conducting substrate.

Exploring the motional dynamics of end-grafted DNA oligonucleotides by in situ electrochemical atomic force microscopy.

We introduce herein the use of atomic-force electrochemical microscopy (AFM-SECM) to simultaneously probe locally the conformation and motional dynamics of nanometer-sized single-stranded (ss) and double-stranded (ds) DNA oligonucleotides end-tethered to electrode surfaces. The ss-DNA system studied here consists of a low-density monolayer of (dT)20 oligonucleotides, 5'-thiol end-tethered onto a flat gold surface via a C6 alkyl linker and bearing at their free 3'-end a redox ferrocene label. It is shown that, as a result of the flexibility of the relatively long C6 linker, hinge motion, rather than elastic deformation of the DNA chain, is the major component of the dynamics of both the (dT)20 strand and its post-hybridized (dT-dA)20 duplex. DNA chain elasticity is nevertheless sufficiently contributing to the overall dynamics to result in approximately 4 times slower dynamics for (dT-dA)20 than for (dT)20. Taking advantage of this dissimilar dynamical behavior of ss- and ds-DNA, it is demonstrated that hybridization can be easily locally detected at the scale of approximately 200 molecules by AFM-SECM.

Nano-Electrochemistry and Nano-Electrografting with an Original Combined AFM-SECM.

This study demonstrates the advantages of the combination between atomic force microscopy and scanning electrochemical microscopy. The combined technique can perform nano-electrochemical measurements onto agarose surface and nano-electrografting of non-conducting polymers onto conducting surfaces. This work was achieved by manufacturing an original Atomic Force Microscopy-Scanning ElectroChemical Microscopy (AFM-SECM) electrode. The capabilities of the AFM-SECM-electrode were tested with the nano-electrografting of vinylic monomers initiated by aryl diazonium salts. Nano-electrochemical and technical processes were thoroughly described, so as to allow experiments reproducing. A plausible explanation of chemical and electrochemical mechanisms, leading to the nano-grafting process, was reported. This combined technique represents the first step towards improved nano-processes for the nano-electrografting.

Characterization of a modified gold platform for the development of a label-free anti-thrombin aptasensor.

This work reports the characterization of a modified gold surface as a platform for the development of a label free aptasensor for thrombin detection. The biorecognition platform was obtained by the self-assembly of 4-mercaptobenzoic acid onto a gold surface, covalent attachment of streptavidin and further immobilization of the biotinylated anti-thrombin aptamer. The biosensing platform was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring. The biorecognition event aptamer-thrombin was detected from changes in the SPR angle produced as a consequence of the molecular interaction between the aptasensor and the target protein. The biosensing platform demonstrated to be highly selective for human thrombin even in the presence of large excess of bovine thrombin, bovine serum albumin, cytochrome C, lysozyme and myoglobin. The relationship between the changes in the SPR angle and thrombin concentration was linear up to 0.19 µmol L(-1) (R(2)=0.992) while the detection limit was of 12.0 nmol L(-1) (240 fmol in the sample). This new sensing approach represents an interesting and promising alternative for the SPR-based quantification of thrombin.

Multilayer assemblies of polyelectrolyte-gold nanoparticles for the electrocatalytic oxidation and detection of arsenic(III).

Multilayers of poly(diallyldimethylammonium chloride) (PDDA) and citrate capped Au nanoparticles (AuNPs) anchored on sodium 3-mercapto-1-propanesulfonate modified gold electrode by electrostatic layer-by-layer assembly (LbL) technique are shown to be an excellent architecture for the direct electrochemical oxidation of As(III) species. The growth of successive layers in the proposed LbL architecture is followed by atomic force microscopy, UV-vis spectroscopy, quartz crystal microbalance with energy dissipation, and electrochemistry. The first bilayer is found to show rather different physico-chemical characteristics as compared to the subsequent bilayers, and this is attributed to the difference in the adsorption environments. The analytical utility of the architecture with five bilayers is exploited for arsenic sensing via the direct electrocatalytic oxidation of As(III), and the detection limit is found to be well below the WHO guidelines of 10ppb. When the non-redox active PDDA is replaced by the redox-active Os(2,2'-bipyridine)(2)Cl-poly(4-vinylpyridine) polyelectrolyte (PVPOs) in the LbL assembly, the performance is found to be inferior, demonstrating that the redox activity of the polyelectrolyte is futile as far as the direct electro-oxidation of As(III) is concerned.

Electrochemical atomic-force microscopy using a tip-attached redox mediator. Proof-of-concept and perspectives for functional probing of nanosystems.

This paper presents the first steps toward the development of a new type of high-resolution AFM-SECM microscopy which relies on the use of tip-attached redox-labeled polymer chains as mediators to probe the local electrochemical reactivity of a planar substrate at the nanoscale. Submicrometer-sized combined gold AFM-SECM probes were functionalized by linear, nanometer-sized, flexible, PEG3400 chains bearing a ferrocene (Fc) redox label at their free end. Analysis of the force and current approach curves recorded when such Fc-PEGylated probes (tips) were approached to a bare gold substrate allowed the presence of the Fc-PEG chains at the very tip end of the combined probes to be specifically demonstrated. It also allowed the chain coverage, configuration, and dynamics to be determined. When the Fc-PEGylated probe is positioned some approximately 5 nm above the substrate, only a few hundred chains are actually electrochemically contacting the surface, thus reducing the size of the tip-substrate interaction area to 20-40 nm. Most importantly, we have shown that the tip-borne PEG chains are flexible enough to allow their Fc heads to efficiently "sense" locally the electrochemical reactivity of the substrate, thus validating the working principle of the new AFM-SECM microscopy we propose. This innovative microscopy, we label Tarm (for tip-attached redox mediator)/AFM-SECM, should be particularly suitable for probing the activity of slowly functioning nanometer-sized active sites on surfaces, such as individual enzyme molecules, because it is, by design, free of the diffusional constraints which hamper the characterization of such nanosystems by classical SECM.

Electrochemical atomic force microscopy using a tip-attached redox mediator for topographic and functional imaging of nanosystems.

We describe the development of a new type of high-resolution atomic force electrochemical microscopy (AFM-SECM), labeled Tarm (for tip-attached redox mediator)/AFM-SECM, where the redox mediator, a ferrocene (Fc), is tethered to the AFM-SECM probe via nanometer long, flexible polyethylene glycol (PEG) chains. It is demonstrated that the tip-attached ferrocene-labeled PEG chains effectively shuttle electrons between the tip and substrate, thus acting as molecular sensors probing the local electrochemical reactivity of a planar substrate. Moreover the Fc-PEGylated AFM-SECM probes can be used for tapping mode imaging, allowing simultaneous recording of electrochemical feedback current and of topography, with a vertical and a lateral resolution in the nanometer range. By imaging the naturally nanostructured surface of HOPG, we demonstrate that Tarm/AFM-SECM microscopy can be used to probe the reactivity of nanometer-sized active sites on surfaces. This new type of SECM microscopy, being, by design, free of the diffusional constraints of classical SECM, is expected to, in principle, enable functional imaging of redox nanosystems such as individual redox enzyme molecules.