Fabrication of cone-shaped boron doped diamond and gold nanoelectrodes for AFM-SECM

We demonstrate a reliable microfabrication process for a combined atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) measurement tool. Integrated cone-shaped sensors with boron doped diamond (BDD) or gold (Au) electrodes were fabricated from commercially available AFM probes. The sensor formation process is based on mature semiconductor processing techniques, including focused ion beam (FIB) machining, and highly selective reactive ion etching (RIE). The fabrication approach preserves the geometry of the original AFM tips resulting in well reproducible nanoscaled sensors. The feasibility and functionality of the fully featured tips are demonstrated by cyclic voltammetry, showing good agreement between the measured and calculated currents of the cone-shaped AFM-SECM electrodes.     

Examination of the spatially heterogeneous electroactivity of boron-doped diamond microarray electrodes

Spatial variations in the electrical and electrochemical activity of microarray electrodes, fabricated entirely from diamond, have been investigated. The arrays contain approximately 50-mum-diameter boron-doped diamond (BDD) disks spaced 250 mum apart (center to center) in insulating intrinsic diamond supports, such that the BDD regions are coplanar with the intrinsic diamond. Atomic force microscopy (AFM) imaging of the surface reveals a roughness of no more than +/-10 nm over the array. Each BDD microdisk within the array contains polycrystalline BDD with a variety of different grains exposed. Using conducting-AFM, the conductivity of the different grains was found to vary within a BDD microdisk. Electrochemical imaging of the electroactivity of the microdisk electrodes using scanning electrochemical microscopy operating in substrate generation-tip collection mode revealed that, under apparently diffusion-limited steady-state conditions, there was a small variation in the response between electrodes. However, the majority of electrodes in the array appeared to show predominantly metallic behavior. For the electrodes that showed a lower activity, all grains within the microdisk supported electron transfer, albeit at different rates, as evidenced by studies on the electrodeposition of metallic silver, at potentials far negative of the flat band potential of oxygen-terminated polycrystalline diamond. The possibility of using these array electrodes for steady-state diffusion-limited measurements in electroanalytical applications is far-reaching. However, caution should be exercised in the kinetic analysis of voltammetric measurements, since wide variations in the electroactivity of individual grains are apparent when the potential is below the diffusion-limited value.     

Fabrication of vertically aligned diamond whiskers from highly boron-doped diamond by oxygen plasma etching

Conductive diamond whiskers were fabricated by maskless oxygen plasma etching on highly boron-doped diamond substrates. The effects of the etching conditions and the boron concentration in diamond on the whisker morphology and overall substrate coverage were investigated. High boron-doping levels (greater than 8.4 × 10(20) cm(-3)) are crucial for the formation of the nanosized, densely packed whiskers with diameter of ca. 20 nm, length of ca. 200 nm, and density of ca. 3.8 × 10(10) cm(-2) under optimal oxygen plasma etching conditions (10 min at a chamber pressure of 20 Pa). Confocal Raman mapping and scanning electron microscopy illustrate that the boron distribution in the diamond surface region is consistent with the distribution of whisker sites. The boron dopant atoms in the diamond appear to lead to the initial fine column formation. This simple method could provide a facile, cost-effective means for the preparation of conductive nanostructured diamond materials for electrochemical applications as well as electron emission devices.     

Batch fabrication of atomic force microscopy probes with recessed integrated ring microelectrodes at a wafer level

A batch fabrication process at the wafer-level integrating ring microelectrodes into atomic force microscopy (AFM) tips is presented. The fabrication process results in bifunctional scanning probes combining atomic force microscopy with scanning electrochemical microscopy (AFM-SECM) with a ring microelectrode integrated at a defined distance above the apex of the AFM tip. Silicon carbide is used as AFM tip material, resulting in reduced mechanical tip wear for extended usage. The presented approach for the probe fabrication is based on batch processing using standard microfabrication techniques, which provides bifunctional scanning probes at a wafer scale and at low cost. Additional benefits of batch fabrication include the high processing reproducibility, uniformity, and tuning of the physical properties of the cantilever for optimized AFM dynamic mode operation. The performance of batch-fabricated bifunctional probes was demonstrated by simultaneous imaging micropatterned platinum structures at a silicon dioxide substrate in intermittent (dynamic) and contact mode, respectively, and feedback mode SECM. In both, intermittent and contact mode, the bifunctional probes provided reliable correlated electrochemical and topographical data. In addition, simulations of the diffusion-limited steady-state currents at the integrated electrode using finite element methods were performed for characterizing the developed probes.     

Plasma-deposited fluorocarbon films: insulation material for microelectrodes and combined atomic force microscopy-scanning electrochemical microscopy probes

Pinhole-free insulation of micro- and nanoelectrodes is the key to successful microelectrochemical experiments performed in vivo or in combination with scanning probe experiments. A novel insulation technique based on fluorocarbon insulation layers deposited from pentafluoroethane (PFE, CF3CHF2) plasmas is presented as a promising electrical insulation approach for microelectrodes and combined atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) probes. The deposition allows reproducible and uniform coating, which is essential for many analytical applications of micro- and nanoelectrodes such as, e.g., in vivo experiments and SECM experiments. Disk-shaped microelectrodes and frame-shaped AFM tip-integrated electrodes have been fabricated by postinsulation focused ion beam (FIB) milling. The thin insulation layer for combined AFM-SECM probes renders this fabrication technique particularly useful for submicro insulation providing radius ratios of the outer insulation versus the disk electrode (RG values) suitable for SECM experiments. Characterization of PFE-insulated AFM-SECM probes will be presented along with combined AFM-SECM approach curves and imaging.     

Fabrication and characterization of probes for combined scanning electrochemical/optical microscopy experiments

A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was implemented with a new probe tip. The tip for scanning electrochemicaVoptical microscopy (SECM/OM) was constructed by insulating a typical gold-coated near-field scanning optical microscopy tip using electrophoretic anodic paint. Once fabricated, the tip was characterized by steady-state cyclic voltammetry, as well as optical and electrochemical approach experiments. This tip generated a stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. Durable tips whose geometry was a ring with < 1 microm as outer ring radius could be consistently fabricated. Simultaneous electrochemical and optical images of an interdigitated array electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and an electrochemical probe for imaging microstructures. The SECM feedback current measurements were successfully employed to determine tip-substrate distances for imaging.     

Vertically aligned nanowires from boron-doped diamond

Vertically aligned diamond nanowires with controlled geometrical properties like length and distance between wires were fabricated by use of nanodiamond particles as a hard mask and by use of reactive ion etching. The surface structure, electronic properties, and electrochemical functionalization of diamond nanowires were characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) as well as electrochemical techniques. AFM and STM experiments show that diamond nanowire etched for 10 s have wire-typed structures with 3-10 nm in length and with typically 11 nm spacing in between. The electrode active area of diamond nanowires is enhanced by a factor of 2. The functionalization of nanowire tips with nitrophenyl molecules is characterized by STM on clean and on nitrophenyl molecule-modified diamond nanowires. Tip-modified diamond nanowires are promising with respect to biosensor applications where controlled biomolecule bonding is required to improve chemical stability and sensing significantly.     

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.     

Novel ultrananocrystalline diamond probes for high-resolution low-wear nanolithographic techniques

A hard, low-wear probe for contact-mode writing techniques, such as dip-pen nanolithography (DPN), was fabricated using ultrananocrystalline diamond (UNCD). Molding within anisotropically etched and oxidized pyramidal pits in silicon was used to obtain diamond tips with radii down to 30 nm through growth of UNCD films followed by selective etching of the silicon template substrate. The probes were monolithically integrated with diamond cantilevers and subsequently integrated into a chip body obtained by metal electroforming. The probes were characterized in terms of their mechanical properties, wear, and atomic force microscopy imaging capabilities. The developed probes performed exceptionally well in DPN molecular writing/imaging mode. Furthermore, the integration of UNCD films with appropriate substrates and the use of directed microfabrication techniques are particularly suitable for fabrication of one- and two-dimensional arrays of probes that can be used for massive parallel fabrication of nanostructures by the DPN method.     

Surface nanostructuring of boron-doped diamond films and their electrochemical performance

Uniform and vertically aligned nanocone and nanopillar arrays were successfully constructed on heavily boron-doped nanocrysatlline diamond films by carrying out bias-assisted reactive ion etching in hydrogen/argon plasmas. The electrochemical properties of the nanostructured boron-doped diamond films were investigated by cyclic voltammetry using 1 mM [Fe(CN)6](3-/4-) as redox couple. Compared to the planar boron-doped nanocrystalline diamond film electrode, the surface nanostructuring of boron-doped diamond film electrodes demonstrate enhanced sensitivity due to their enlarged electro-active surface areas. The results indicated that boron-doped diamond nanocones and nanopillars are promising electrode materials which benefit to improve the efficiency, sensitivity and reproducibility of biomedical and chemical sensors.     

Conductive supports for combined AFM-SECM on biological membranes

Four different conductive supports are analysed regarding their suitability for combined atomic force and scanning electrochemical microscopy (AFM-SECM) on biological membranes. Highly oriented pyrolytic graphite (HOPG), MoS(2), template stripped gold, and template stripped platinum are compared as supports for high resolution imaging of reconstituted membrane proteins or native membranes, and as electrodes for transferring electrons from or to a redox molecule. We demonstrate that high resolution topographs of the bacterial outer membrane protein F can be recorded by contact mode AFM on all four supports. Electrochemical feedback experiments with conductive cantilevers that feature nanometre-scale electrodes showed fast re-oxidation of the redox couple Ru(NH(3))(6)(3+/2+) with the two metal supports after prolonged immersion in electrolyte. In contrast, the re-oxidation rates decayed quickly to unpractical levels with HOPG or MoS(2) under physiological conditions. On HOPG we observed heterogeneity in the re-oxidation rate of the redox molecules with higher feedback currents at step edges. The latter results demonstrate the capability of conductive cantilevers with small electrodes to measure minor variations in an SECM signal and to relate them to nanometre-scale features in a simultaneously recorded AFM topography. Rapid decay of re-oxidation rate and surface heterogeneity make HOPG or MoS(2) less attractive for combined AFM-SECM experiments on biological membranes than template stripped gold or platinum?supports.     

Characterization of batch-microfabricated scanning electrochemical-atomic force microscopy probes

A procedure for the batch microfabrication of scanning electrochemical-atomic force microscopy (SECM-AFM) probes is described. The process yields sharp AFM tips, incorporating a triangular-shaped electrode (base width 1 microm, height 0.65 microm) at the apex. Microfabrication was typically carried out on (1)/(4) 3-in. wafers, yielding 60 probes in each run. The measured spring constant of the probes was in the range 1-1.5 N m(-1). To date, processing has been carried out twice successfully, with an estimated success rate for the fabrication process in excess of 80%, based on field emission-scanning electron microscopy imaging of all probes and current-voltage measurements on a random selection of approximately 30 probes. Steady-state voltammetric measurements for the reduction of Ru(NH(3))(6)(3+) in aqueous solution indicate that the electrode response is well-defined, reproducible, and quantitative, based on a comparison of the experimental diffusion-limited current with finite element simulations of the corresponding mass transport (diffusion) problem. Topographical imaging of a sputtered Au film with the SECM-AFM probes demonstrates lateral resolution comparable to that of conventional Si(3)N(4) AFM probes. Combined electrochemical-topographical imaging studies have been carried out on two model substrates: a 10-microm-diameter disk ultramicroelectrode (UME) and an array of 1-microm-diameter UMEs, spaced 12.5 microm apart (center to center). In both cases, an SECM-AFM probe was first employed to image the topography of the substrates. The tip was then moved back a defined distance from the surface and use to detect Ru(NH(3))(6)(2+) produced at the substrate, biased at a potential to reduce Ru(NH(3))(6)(3+), present in bulk solution, at a diffusion-controlled rate (substrate generation-tip collection mode). These studies establish the success of the batch process for the mass microfabrication of SECM-AFM tips.     

Fabrication of boron-doped diamond ultramicroelectrodes for use in scanning electrochemical microscopy experiments

Boron-doped diamond (BDD) ultramicroelectrode (UME) tips were fabricated by the growth of BDD films by chemical vapor deposition onto sharpened tungsten wires. Both nanocrystalline and microcrystalline forms of diamond coatings were examined. The diamond-coated wires were selectively insulated with nail varnish, electrophoretic paint, or fast-setting epoxy to form UME tips of critical dimensions of 1-25 microm. The geometry of the exposed electrode area was disk or hemispherical in most cases. Cyclic voltammetry and chronoamperometry were used to assess exposed electrode area and integrity of the insulation. BDD UMEs were used to obtain SECM approach curves to an insulating and a conducting substrate, which were fitted to the theory appropriate for the observed tip geometry. The tips were used to obtain SECM images of immobilized respiring E. coli, illustrating the suitability of BDD UMEs for electrochemical imaging in biological media.     

Combined scanning electrochemical-atomic force microscopy

A combined scanning electrochemical microscope (SECM)-atomic force microscope (AFM) is described. The instrument permits the first simultaneous topographical and electrochemical measurements at surfaces, under fluid, with high spatial resolution. Simple probe tips suitable for SECM-AFM, have been fabricated by coating flattened and etched Pt microwires with insulating, electrophoretically deposited paint. The flattened portion of the probe provides a flexible cantilever (force sensor), while the coating insulates the probe such that only the tip end (electrode) is exposed to the solution. The SECM-AFM technique is illustrated with simultaneous electrochemical-probe deflection approach curves, simultaneous topographical and electrochemical imaging studies of track-etched polycarbonate ultrafiltration membranes, and etching studies of crystal surfaces.     

Multifunctional Boron‐Doped Diamond Colloidal AFM Probes

Scanning probe microscopy techniques providing information on conductivity, chemical fluxes, and interfacial reactivity synchronized with topographical information have gained importance within the last decades. Herein, a novel colloidal atomic force microscopy (AFM) probe is presented using a spherical boron‐doped diamond (BDD) electrode attached and electrically connected to a modified silicon nitride cantilever. These conductive spherical BDD–AFM probes allow for electrochemical force spectroscopy. The physical robustness of these bifunctional probes, and the excellent electrochemical properties of BDD renders this concept a unique multifunctional tool for a wide variety of scanning probe studies including conductive AFM, hybrid atomic force–scanning electrochemical microscopy, and tip‐integrated chem/bio sensing.     

Plastic tip arrays for force spectroscopy

The mechanical stability and viability of molecules investigated with the atomic force microscope (AFM) continue to be limiting factors in the duration of force spectroscopy measurements. In an effort to circumvent this problem, we have fabricated an all-plastic array of over 30 000 tips with dimensions similar to common AFM probes using silicon micromolding techniques. This approach enables rapid fabrication of tip arrays with improved properties, as compared to tip arrays made entirely of silicon.     

Role of current profiles and atomic force microscope tips on local electric crystallization of amorphous silicon

Various types of conductive tips in atomic force microscope (AFM) are used to localize field-enhanced metal-induced solid phase crystallization (FE-MISPC) of amorphous silicon at room temperature down to nanoscale dimensions. The process is driven by electrical currents ranging from 0.1 nA to 3 nA between the tip and the bottom nickel electrode. The amplitude of the current is controlled by a metal-oxide-semiconductor field-effect transistor-based regulation circuit using proportional and derivative feedback loops. We analyze the results of the FE-MISPC process as a function of exposition current profiles, topographic changes, local conductivity changes (using current-sensing AFM) and regulation parameters. We found out that the FE-MISPC crystallization requires fluctuations of the exposition current rather than its stability. This is independent of the actual current set-point level. We also show the influence of the process on the AFM probes employed and vice versa. Bulk diamond probes exhibit superior endurance compared to bare or coated silicon probes, nevertheless all tips produce similar FE-MISPC results.     

Fabrication and characterization of a silicon cantilever probe with an integrated quartz-glass (fused-silica) tip for scanning near-field optical microscopy

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.     

High-resolution mapping of redox-immunomarked proteins using electrochemical-atomic force microscopy in molecule touching mode

We explore the possibility of using molecule touching atomic force electrochemical microcopy (Mt/AFM-SECM) for high-resolution mapping of proteins on conducting surfaces. The proposed imaging strategy relies on making surface-immobilized proteins electrochemically "visible" via redox-immunomarking by specific antibodies conjugated to poly(ethylene glycol) (PEG) chains terminated by redox ferrocene (Fc) heads. The flexibility and length of the PEG chains are such that, upon approaching a combined AFM-SECM microelectrode tip toward the surface, the Fc moieties can efficiently shuttle electrons from the surface to the tip. The so-generated SECM positive feedback tip current allows the specific localized detection of the sought protein molecules on the surface. This new electrochemical imaging scheme is validated experimentally on the basis of a model system consisting of mouse IgGs adsorbed onto electrode surfaces and recognized by Fc-PEG-labeled antimouse antibodies. In order to estimate the resolution of Mt/AFM-SECM for protein imaging, regular arrays of submicrometer-sized spots of mouse IgGs are fabricated onto gold electrode surfaces using particle lithography. The Fc-PEG-immunomarked mouse IgG spots are imaged by Mt/AFM-SECM operated in tapping mode. Both an electrochemical image, reflecting the surface distribution of the redox-labeled IgGs, and a topography image are then simultaneously and independently acquired, with a demonstrated resolution in the ~100 nm range. The strength of Mt/AFM-SECM imaging is to combine the nanometric resolution of AFM with the selectivity of the electrochemical detection, potentially allowing individual target proteins to be identified amidst similarly sized "nano objects" present on a conducting surface.     

Electrochemical detection of arsenic(III) using iridium-implanted boron-doped diamond electrodes

Iridium-modified, boron-doped diamond electrodes fabricated by an ion implantation method have been developed for electrochemical detection of arsenite (As(III)). Ir+ ions were implanted with an energy of 800 keV and a dose of 10(15) ion cm(-2). An annealing treatment at 850 degrees C for 45 min in H2 plasma (80 Torr) was required to rearrange metastable diamond produced by an implantation process. Characterization was investigated by SEM, AFM, Raman, and X-ray photoelectron spectroscopy. Cyclic voltammetry and flow injection analysis with amperometric detection were used to study the electrochemical reaction. The electrodes exhibited high catalytic activity toward As(III) oxidation with the detection limit (S/N = 3), sensitivity, and linearity of 20 nM (1.5 ppb), 93 nA microM(-1) cm(-2), and 0.999, respectively. The precision for 10 replicate determinations of 50 microM As(III) was 4.56% relative standard deviation. The advantageous properties of the electrodes were its inherent stability with a very low background current. The electrode was applicable for analysis of spiked arsenic in tap water containing a significant amount of various ion elements. The results indicate that the metal-implanted method could be promising for controlling the electrochemical properties of diamond electrodes.