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In 1994, a new type of atom probe instrument, named the scanning atom probe (SAP), was proposed. The unique feature of the SAP is the introduction of a small extraction electrode, which scans over a specimen surface and confines the high field, required for field evaporation of surface atoms in a small space, between the specimen and the electrode. Thus, the SAP does not require a sharp specimen tip. This indicates that the SAP can mass analyze the specimens which are difficult to form in a sharp tip, such as organic materials and biomolecules. Clean single wall carbon nanotubes (CNT), made by high-pressure carbon monoxide process are found to be the best substrates for biomolecules. Various amino acids and dipeptide biomolecules were successfully mass analyzed, revealing characteristic clusters formed by strongly bound atoms in the specimens. The mass analysis indicates that SAP analysis of biomolecules is not only qualitative, but also quantitative.
The merits of atom-probe tomography (APT) of inorganic materials are well established, as described in this volume. However, one of the long-held aspirations of atom-probe scientists, structural and chemical characterization of organic and biological materials at near-atomic resolution, has yet to be fully realized. A few proof-of-concept type investigations have shown that APT of organic materials is feasible, but a number of challenges still exist with regard to specimen preparation and conversion of raw time-of-flight mass spectrometry data into a three-dimensional map of ions containing structural and chemical information at an acceptable resolution. Recent research aided by hardware improvements and specimen preparation advances has made some progress toward this goal. This article reviews the historical developments in this field, presents some recent results, and considers what life science researchers might expect from this technology.
Atom probe (AP) is known to be a unique instrument that makes possible to mass analyze a specimen at atomic level. However, its application is mostly limited to metals and semiconductors because the AP analysis proceeds by field evaporating surface atoms applying the high field, 20-40 V/nm, on the specimen surface. In order to generate such a high field the analyzed area is an apex of a sharp tip. Metals and semiconductors can be formed in such a sharp tip easily. However, the formation of a sharp organic and bio molecule tip is not easy. Thus, we introduced a funnel shaped micro extraction electrode that scans over a specimen surface and confines the high field in a narrow space between the micro open hole at the apex of the electrode and a micro protrusion on a specimen surface. Thus, this type of the AP is named as scanning atom probe (SAP). Then, organic and bio molecules can be deposited on the micro protrusion on the specimen surface. The AP analysis of metals indicates that the field evaporation of metal atoms proceeds one atom by one atom implying that the binding between metal atoms are uniform and non-directional. On the other hand most atoms of non-metallic specimens are field evaporated as clustering atoms. For example, doubly charged thiophene monomers are detected when polythiophene is analyzed. This indicates that one sulfur and four carbon atoms are strongly bound. Similarly, the mass spectra of highly pure single walled carbon nano tubes (SWCNT) exhibit sharp mass peaks of C2+ and C+ indicating that carbon atoms are bound by non-directional strong bonds. This implies that the unique feature of the AP is not only in the identification of individual ion but also in the investigation of binding states of the atoms forming the materials. For the present analysis amino acids are deposited on a small ball of the SWCNT fibers in order to avoid the catalytic reaction of metals. The SWCNT ball is dipped in a solution of sample molecules. The glycine solution is made by dissolving 1 gram of glycine in 15 ml pure water. Cystine, leucine and methionine solutions are made by dissolving 50 mg of the molecules in 1 ml of 0.1 N HCl. Discrimination of the carbon ions of the SWCNT from the fragment ions of the molecules is relatively easy because nearly all of the SWCNT carbon ions are detected as C2+ and C+. Glycine is the smallest amino acid formed by a carboxy group, an amino group and a CH2 group. Thus, it is assumed that the analysis will provide a guideline for the analysis of larger molecule. However, the identification of fragments ions is not easy because many different fragments have the same mass such as CH3 and NH. This indicates that mass analyzer for the bio-molecules requires a mass spectrometer with the mass resolution m/Δm higher than 10,000. The characteristic mass spectra of the amino acids and the structure of a new SAP with a position sensitive ion detector with a spiral delay line will be presented.
Calcium phosphate films were deposited on titanium electrodes cathodically from CaCl2·2H2O and Ca(H2PO4)2 · H2O aqueous solutions. In this study, H2O2 addition into electrolytes was applied to enhance the electrochemical process at the solution/electrode at a smaller cathodic potential than no H2O2 addition. Deposited films were analyzed by scanning electron microscopy (SEM) observation and X-ray diffraction (XRD). Cathodic current of the Ti electrode decreased once and increased in the solution with H2O2. It shows a cathodic current peak at c.a. 25 min in the case of potentiostatic condition at - 0.756 V (vs. Ag - AgCl, sat. KCl). The calcium phosphate film grows mainly with the decrease in current after the cathodic peak. The characteristics for the electrodeposited film such as crystal morphology depends on cathodic potential, solution pH, deposition temperature and amount of H2O2 addition. Dense calcium phosphate film composed of relatively good crystalline was obtained at pH 5.5 and – 0.756 V. Film adhesion on Ti appeared to be strong by peeling test. At larger cathodic potential of – 1.156 V, the film coverage on titanium plates was smaller and film adhesion worsened. Larger cathodic polarization of more than – 1.556 V was necessary to reduce water in case without H2O2 addition.
The analyzing area of a conventional atom probe is an apex of a long, sharp tip. However, the fabrication of such a filamentary tip is extraordinary difficult for most materials, such as organic materials, ceramics and semiconductors with multi-layer structures. Accordingly, the application of the atom probe are severely restricted In order to overcome this difficulty, Nishikawa proposed to develop a scanning atom probe (SAP) introducing a funnel-shaped microextraction electrode to the conventional AP in order to confine the high field into a small space between a tip apex and an open end of the electrode. Thus, the SAP allows to analyze not only an apex of a sharp slender tip but also an apex of a few micron high minute cusp which normally exists on an unsmoothened specimen surface.
The microextraction electrode scans over a rugged specimen surface at a bias voltage and stands still right above an apex of a micro cusp, FIG. 1.
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