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PROGRESS REPORT FOR AINGRA07173
Date electronic copy received at AINSE: 22 September 2008
Ion selective atomic force microscopy: a powerful new imaging technique for materials science
Institution and Department
Professor Roland De Marco
Applied Chemistry, Curtin University of Technology
Prof. Erno Pretsch, ETH Zürich Prof. Eric Bakker, Purdue University
Mr Adam Martin, Honours Student, Department of Applied Chemistry, Curtin University of Technology
Dr Kathryn Prince
The overarching aim of this project is to develop an innovative approach for chemically modified atomic force microscopy (AFM) using tips comprising miniaturized ion-selective sensor nanoparticles. These ion-selective sensor modified AFM tips or nanosensors will permit atomic resolution force imaging of unique molecular structures, and the Project Team will employ these AFM tips in the imaging of ion efflux events in model or ion channel mimetic systems, as well as real ion channels in living cells.
In this project, we have characterized silver sulfide nanoparticles that were synthesized in a spinning disc reactor using secondary ion mass spectrometry, and the project has specifically targeted a determination of whether the particles had the required stoichiometry (i.e., Ag2S), or a non-stoichiometric composition ascribable to the presence of occluded salts and/or partially oxidized particles.
In this project, we spin coated the nanoparticles of silver sulfide onto a mica substrate from an aqueous suspension of nanoparticles to produce an even distribution of nanoparticles on the substrate. Since the mica susbstrate is electrically insulating, the samples were coated with 20 nm of gold using the sputter coater at ANSTO. This strategy was insufficient to combat the charging of the samples, so the Project Team also employed the SIMS instrument’s electron flood gun to yield reliable depth profiles on 500 μm2 square areas of the specimens. This strategy was successful, as evidenced by a depth profile of one of the specimens (see Figure 1 including mica and silver sulfide elemental profiles, and Figure 2 only showing the silver, sulfur, gold and cesium ion profiles). It is evident that, as the silver sulfide particles are sputtered away, there are peaks in the silver and sulfur ion counts at around 75 to 120 seconds, and a diminution in the gold counts after approximately 120 seconds. We integrated the intensities under the silver and sulfur ion curves, determined the ratios of silver-to-sulfur ion intensities, and compared these ratios with the one obtained on a standard sample of silver sulfide prepared using a conventional precipitation technique.
On each specimen, SIMS analyses were conducted in duplicate, and fourteen samples corresponding to different nanoparticle sizes obtained by controlling the experimental conditions in the spinning disc reactor were examined. The SIMS analyses confirmed that the nanoparticles had bulk compositions commensurate with those of silver sulfide, as evidenced by silver-to-sulfur intensity ratios of 1.9 ± 0.6 for the 28 assays on the bulk nanoparticles and a corresponding silver-to-sulfur intensity ratio of 1.7 ± 0.5 on the standard silver sulfide material.
Most significantly, these silver sulfide nanoparticles have been used successfully in ion-selective atomic force microscopy, and two papers are pending publication.
PROGRESS REPORT and RESEARCH OUTCOMES
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