## 2 Materials, Methods and Experimental Setup
### The Experimental Setup
In an indirect aerosoliser, a submerged piezoelectric plate, oscillating at a constant resonance frequency, produces vibrations in a volume of water acting as an auxiliary fluid, which, in turn, transmits these vibrations to the meniscus of a separated liquid phase where a reagent is dissolved, giving a stream of aerosolised droplets. The auxiliary liquid has the sole function of energy transfer between the piezoelectric transducer and the liquid subject to aerosolisation, which is located in a thin-wall polymeric vessel surrounded by the auxiliary liquid. On the other hand, a direct aerosolisation technique relies on direct contact between an oscillating metal plate and a liquid containing a reagent, without resorting to an auxiliary fluid acting as an energy vector. In particular, the liquid containing a reagent is placed in a container, where a wick is located in the middle, which imbibes by capillarity, bringing the liquid into contact with the vibrating metal plate. The latter, 16 mm in diameter, has a finely perforated central area, 3.5 mm in diameter, which is permeable to the solution. The latter is thus finely atomised, producing a stream of droplets containing a reagent. An enlarged image of the micro-holes on the vibrating lamella is shown in Figure 4.  
Figure 2: Schematic diagram of the Venturi nozzle adopted for NPs captured in the liquid phase. In Figure 5a, a photograph of the ultrasonic atomiser driver circuit, connected with the atomiser plate, is reported. The operating tension is 5 V, with a power input of 2 W and a resonant frequency in the range 103-113 kHz. The frequency transducer, namely the atomising device, acts as a capacitor and consists of two metallic plates separated by a dielectric layer. The upper plate is shaped in the form of a circular crown, while the lower one has a central pitted area, as previously said. In its original commercial version, as reported in Figure 5b, the vibrating plate is suitable for atomising solutions with low or negligible electrical conductivity, typical of weakly polar organic substances dispersed in an aqueous solvent. This makes the device unsuitable for atomising saline solutions, whose electrical conductivity compromises the operation of the capacitive atomiser, resulting in overheating and rapid deterioration of the power circuit, which happened during the first experiments using a commercial product as is. For this reason, the atomiser plate has been customarily modified as depicted in Figure 5c by adopting the following amendments:  
* One of the two capacitor armatures has been coated with insulation paint to minimise electrical conduction between the armatures and the liquid drops. Figure 4: An enlarged photograph of the perforated area, located in the centre of the vibrating plate of the piezoelectric sensor operating via a direct aerosolisation technique. Figure 3: Schematic diagram of two different devices for liquid nebulisation. The left figure refers to an indirect atomization technique, while the right one refers to a direct atomization method. The liquid in the cavity of the indirect aerosoliser has the sole function of transmitting the vibrations produced by a piezoelectric transducer. The electrical connections between the armatures and the external circuit were redesigned to make them as far apart as possible. This is to avoid possible short circuits between electric terminals, induced by contact with the conductive solution. The driver module of Figure 5a has been mounted on suitable supports connected to heat sinks with forced ventilation, adopting a solution similar to cooling systems for computer CPUs.