A nanoengineered surface acoustic wave device for analysis of mercury in gas phase

Abstract

We show that the nickel (Ni)-gold (Au) alloy nanostructures can be directly grown on the electrodes of a surface acoustic wave (SAW) device and can be utilized to detect toxic metal (i.e. elemental mercury (Hg0) in this case), without the requirement of the device having delay line sorption layer and thereby removing the necessity for any additional lithography steps to be undertaken. A set of SAW devices with Ni (100 nm thick) electrodes were fabricated using photo-lithography and wet etching processes where each device contained 180 finger pairs in their input and output transducers with 18 μm width and spacing (i.e. resonance frequency of ∼42.7 MHz). Au nanostructures were then deposited on the Ni surface through galvanic replacement (GR) reaction by utilizing different HAuCl4 concentrations (i.e. 0.5 mM/1 mM) and reaction times (i.e. 10/20/30 min). The results indicated that higher Au concentration can be utilized for acquiring smaller size of Ni-Au alloy nanostructures while the number of Au nanostructures can be increased by the control of the reaction time. It was found that the SAW device with 20 min GR reaction time in 1 mM HAuCl4 (1 mM-20 min GR SAW) solution allowed the optimum growth conditions for Ni-Au alloy nanostructures on the electrode surface for Hg0 vapor sensing. Hg0 vapor testing experiments showed that a limit of detection (LoD) of 1.3 ppbv toward Hg0 vapor can be achieved with the developed 1 mM-20 min GR SAW device tested at 35 °C. This optimum reaction conditions allowed for ∼100% and ∼200% higher response magnitudes than the 0.5 mM-20 min and 1 mM-10 min GR SAW counterparts, respectively when exposed toward low Hg0 vapor concentrations (<400 ppbv). Further analysis showed that the effect of interfering gas species such as ammonia, acetaldehyde, ethyl mercaptan and humidity on the sensor's selectivity toward Hg0 vapor can be reduced by choosing an optimum temperature of 85 °C and an optimum Hg0 vapor exposur