Aperiodic interferometer for six degrees of freedom position measurement.

We present a new class of interferometer system that is capable of simultaneous measurement of absolute position and rotation in all six degrees of freedom (DOF) with nanometer precision. This novel capability is due to the employment of a system of interference fringes that is not periodic. One of the key strengths offered by this new approach is that the absolute position of the system can be determined with a single measurement, rather than by counting fringes during displacement from a known location. The availability of a simultaneous measurement of all six DOF eliminates many problems associated with conventional interferometry.

Polyaniline Langmuir-Blodgett films: formation and properties.

The deposition and characterisation of Langmuir-Blodgett (LB) layers of polyaniline (PAN) on solid supports is described. Langmuir films were spread as a mixture of PAN and dodecylbenzenesulfonic acid (DBSA) at the water/air interface and deposited on either glass or indium tin oxide (ITO). Mono- and multi-layer films of PAN/DBSA were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), absorption spectroscopy and cyclic voltammetry (CV). The ultrathin films produced were found to be highly uniform and very stable. Further characterisation of the films was accomplished by scanning electrochemical microscopy (SECM) in the feedback mode. It was found that the conductivity depended strongly on the pH of the solution and the number of layers deposited. Values for the pH-dependent lateral conductivity of PAN LB films are reported.

In-situ atomic force microscopy (AFM) imaging: influence of AFM probe geometry on diffusion to microscopic surfaces.

The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH 3) 6 (3+)) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 microm and 1.6 microm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.

Synthesis of azide/alkyne-terminal polymers and application for surface functionalisation through a [2 + 3] Huisgen cycloaddition process, "click chemistry".

Living radical polymerization of methyl methacrylate (MMA) and a fluorescent comonomer with 2-bromo-2-methylpropionic acid 3-azidopropyl ester and 2-bromo-2-methylhept-6-yn-3-one as initiators has been successfully employed for the synthesis of fluorescently tagged azide and alkyne terminated PMMA with close to that predicted, PDI < 1.20, and good first order kinetics as expected for a living polymerisation. Cotton and organic resin surfaces have been functionalised with alkyne groups using a condensation with 4-chlorocarbonylbutyric acid prop-2-ynyl ester. The surfaces have been further modified using a Huisgen [2 + 3] cycloaddition ("click") reaction of polymeric and small molecule azides. Different functional azides, mono azido-PEG and a new fluorescent hostasol derivative have been prepared and tested as model substrates for cotton surface modification. FTIR, tensiometry, FE-SEM and confocal spectroscopy have been used to characterize the modified surfaces. Tensiometry shows an increase in the hydrophobicity of the surface; verified by FE-SEM which shows a change in surface morphology. The use of the fluorescence label allows fluorescent and confocal microscopy to demonstrate the surface reactions. This approach is shown to be very general allowing soft and hard surfaces with different geometries to be modified. In particular it is an excellent method to alter the nature of organic resins allowing the incorporation of many different functionalities.

Electron beam lithographically-defined scanning electrochemical-atomic force microscopy probes: fabrication method and application to high resolution imaging on heterogeneously active surfaces.

This paper describes in detail the use of electron beam lithography (EBL) to successfully batch microfabricate combined scanning electrochemical-atomic force microscopy (SECM-AFM) probes. At present, the process produces sixty probes at a time, on a 1/4 of a three-inch wafer. Using EBL, gold triangular-shaped electrodes can be defined at the tip apex, with plasma enhanced chemical vapor deposited silicon nitride serving as an effective insulating layer, at a thickness of 75 nm. The key features of the fabrication technique and the critical steps are discussed. The capability of these probes for SECM-AFM imaging in both tapping and constant distance mode is illustrated with dual topographical-electrochemical scans over an array of closely-spaced 1 microm diameter Pt disc electrodes, held at a suitable potential to generate an electroactive species at a transport-limited rate. As highlighted herein, understanding diffusion to heterogeneous electrode surfaces, including array electrodes, is currently topical and we present preliminary data highlighting the use of SECM-AFM as a valuable tool for the investigation of diffusion and reactivity at high spatial resolution.

Scanning electrochemical microscopy as a probe of Ag+ binding kinetics at Langmuir phospholipid monolayers.

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag+ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 microm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag+ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag+. Since the Ag+/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag+ away from the electrode, which, in turn, is governed by the interaction of Ag+ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag+ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.

Nanowire probes for high resolution combined scanning electrochemical microscopy - atomic force microscopy.

We describe a method for the production of nanoelectrodes at the apex of atomic force microscopy (AFM) probes. The nanoelectrodes are formed from single-walled carbon nanotube AFM tips which act as the template for the formation of nanowire tips through sputter coating with metal. Subsequent deposition of a conformal insulating coating, and cutting of the probe end, yields a disk-shaped nanoelectrode at the AFM tip apex whose diameter is defined by the amount of metal deposited. We demonstrate that these probes are capable of high-resolution combined electrochemical and topographical imaging. The flexibility of this approach will allow the fabrication of nanoelectrodes of controllable size and composition, enabling the study of electrochemical activity at the nanoscale.