Fabrication and characterization of chemically sensitive needle tips with aluminum oxide nanopores for pH indication

This paper presents aluminum rods and needle surfaces coated with anodic aluminum oxide (AAO) in order to absorb indicators. The submicron tips of the aluminum needles, produced by the electro-polishing method, were coated with AAO film by anodization. Due to the large surface area of the AAO, the film can absorb indicators to make it chemically sensitive to testing materials. When attaching with pH indicators of bromophenol blue and universal indicator, the AAO film allows sensitive pH detection of solutions and vapor changes. The AAO rods and needles can be used for precise measurement in biotechnology, convenient measurement in industrial engineering, and vapor detection in gas sensors. When AAO carrying bromophenol blue was tested in a dilute HCl solution of pH 2.0 and in DIwater, the color alternated between yellow and violet. When the AAO absorbed with universal indicator was tested in ammonia, water, and hydrochloric acid solutions, the color reversibly changed to green, gold, and red.