Development of a surface acoustic wave micromixer for enhanced chemiluminescence microanalysis: application to pesticides

This study introduces a new concept in active microfluidics as it employs surface acoustic waves (SAW) to induce active microfluidic mixing in a microanalytical flow system. A SAW driven microfluidic platform integrated with a flow injection analysis (FIA) system is presented. The platform is equipped with a micromixer and a sensitive photodetector for the rapid on-line quantification of chemiluminescent species. The microfabricated mixer consists of a 100 µL chamber cast in polydimethylsiloxane (PDMS) with an embedded single interdigitated SAW transducer. The reagent and sample are injected into the chamber and actively mixed using unidirectional SAW streaming. Several experimental configurations were optimised by varying the power and duration of the SAW input, fluid flow rate and the orientation of the micromixer inlets relative to the SAW aperture. The mixing efficiency of the SAW Micromixer was quantified as a function of the spatial shifting of a fluorescent tracer in the SAW Micromixer and later as a function of the chemiluminescent output of a chemiluminescent reaction under different conditions using image analysis. The effect of SAW on the microanalytical sensitivity was assessed using the chemiluminescent species tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate. The microanalytical sensitivity was increased by two orders of magnitude with the use of SAW active mixing in comparison with conventional FIA systems, resulting in a theoretical detection limit of approximately 0.02 µgL-1 for the chemiluminogenic test substance L-proline. Application to selected chemiluminogenic pesticides resulted in double the sensitivity of conventional FIA systems, with a theoretical detection limit of approximately 0.96 µgL-1 for the pesticide glyphosate using tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate as the chemiluminescent reagent.