Nanosphere Monolayer on a Transducer for Enhanced Detection of Gaseous Heavy Metal

This study reports for the first time that polystyrene monodispersed nanosphere monolayer (PS-MNM) based Au (Au-MNM) and Ag (Ag-MNM) nanostructures deposited on quartz crystal microbalance (QCM) transducers can be used for nonoptical based chemical sensing with extremely high sensitivity and selectivity. This was demonstrated by exposing the Au-MNM and Ag-MNM based QCMs to low concentrations of Hg⁰ vapor in the presence interferent gas species (i.e., H₂O, NH₃, volatile organics, etc.) at operating temperatures of 30 and 75 °C. At 30 °C, the Au-MNM and Ag-MNM based QCMs showed ∼16 and ∼20 times higher response magnitude toward Hg⁰ vapor concentration of 3.26 mg/m³ (364 parts per billion by volume (ppb_v)) relative to their unmodified control counterparts, respectively. The results indicated that the extremely high sensitivity was not due to the increased surface area (only 4.62 times increase) but due to their long-range interspatial order and high number of surface defect formation which are selectively active toward Hg⁰ vapor sorption. The Au-MNM and Ag-MNM also had more than an order of magnitude lower detection limits (<3 ppb_v) toward Hg⁰ vapor compared to their unmodified control counterparts (>30 ppb_v). When the operating temperature was increased from 30 to 75 °C, it was found that the sensors exhibited lower drift, better accuracy, and better selectivity toward Hg⁰ vapor but at the compromise of higher detection limits. The high repeatability (84%), accuracy (97%), and stability of Au-MNM in particular make it practical to potentially be used as nonspectroscopic based Hg⁰ vapor sensor in many industries either as mercury emission monitoring or as part of a mercury control feedback system.