Surface properties of pulsed laser deposited Ir, Rh, and Ir0.9Rh0.1 thin films for use as microelectrode arrays in electroanalytical heavy metal trace sensors

Pulsed laser deposition (PLD) of iridium, rhodium, and Ir0.9Rh0.1 thin films onto a-SiC:H-coated silicon substrates for use in heavy metal sensors has been achieved by ablating iridium and/or rhodium targets with a KrF excimer laser. The deposited films are polycrystalline and exhibited very smooth surfaces with an average roughness value (R-a) of similar to 1 nm. The wettability by mercury of their surface was investigated by means of the contact angle measurement technique, and compared to that of Ag and Pt films. The PLD films (whether Ir, Ph, or Ir0.9Rh0.1) were found to present an almost identical Hg-wetting behavior, which is characterized by a high static contact angle of 132 +/- 2 degrees. Moreover, in contrast to the cases of Pt or Ag films, where amalgamation with Hg rapidly occurs during the contact angle measurements, no evident interaction of Hg with the surface of the PLD films was observed. Microelectrode arrays of each of the three films (Ir, Ph and Ir0.9Rh0.1) were fabricated and used as a conducting base onto which Hg microdrops are electroplated. Reproducible and uniform Hg deposits on the microelectrode arrays were obtained. The electroanalytical performance of these Hg-electroplated microelectrode arrays based sensors was then evaluated by means of Square Wave Anodic Stripping Voltammetry (SWASV), in synthetic solutions containing Zn, Cd and Pb ion traces. Over a concentration range as wide as 0.2-20 ppb, the detected signals are found to exhibit a strong linear correlation with ion concentration. For a preconcentration time of only 5 min, detection limits as low as 0.2 ppb for both Cd and Pb, and 0.5 ppb for Zn were achieved. (C) 1999 Elsevier Science B.V. All rights reserved.