On the use of ZX-LiNbO 3 acoustic plate mode devices as detectors for dilute electrolytes

Abstract

Acoustic plate mode (APM) devices on Z-cut, X-propagating LiNbO 3, a high piezoelectric coupling material, are investigated as detectors in dilute electrolyte or metal-ion solutions. The sensing wave is an APM, a slow shear wave coupled to the interdigital transducer, which results in a relatively strong acoustic-ionic interaction. The resulting electrical loading leads to a measurable perturbation in the wave-propagation characteristics, which can then be related to the liquid electrical properties. Experiments conducted at different frequencies with various solutions that compare well with theoretical results show a detector that is at least two orders of magnitude more sensitive than similar APM sensors on quartz. These promising results have also led to the investigation of the detection of aqueous transition-metal ions (Fe 3+, Cu 2+, etc.) using solution conductivity changes rather than the commonly used mass sensitivity. The method involves the bonding of metal ions in soluiton to a ligand-coated silica or LiNbO 3 support, placed adjacent to the APM device and in the path of the liquid flow. The resulting frequency increase is then interpreted in terms of the concentration of the metal ions. X-ray photoelectron spectroscopy analysis of the coated surface is performed to confirm the binding.