## 2 Materials and Methods  
Figure 1 shows experiment scheme on the formation and thermal sintering of printed lines from nanoparticles on various substrates for fabricating current-carrying and gas-sensitive microstructures. The formation of printed lines with a width of 100-150 \(\upmu\)m and a length of 1.5-2.0 mm was carried out due to the focused deposition of aerosol nanoparticles on moving substrates. Common materials in printed electronics, such as alumina Al\({}_{2}\)O\({}_{3}\) (VK-100 ceramic, C-Component Ltd., Moscow, Russia), borosilicate glass BG (GETECH, Shenzhen, China), and silicon Si (Si-CZ, Microchemicals GmbH, Ulm, Germany), were used as substrates. Alumina and borosilicate glass are dielectrics and silicon has a resistivity equal to 1-10 \(\Omega\)-cm, which is several orders higher than the resistance of copper lines. Silicon also contained a natural dielectric layer of SiO\({}_{2}\) on the surface with a thickness of several nanometers. In the experiments, the electrical and gas-sensitive characteristics of printed lines made of nanoparticles were investigated. Thermal sintering of lines to obtain conductive microstructures was carried out using a tube furnace in a hydrogen atmosphere. In turn, gas-sensitive microstructures, which are the original unsintered lines, were tested in the experimental setup \"Mikrogaz-F\" [24] for the detection of ammonia (NH\({}_{3}\)) and carbon monoxide (CO). In the process of \"dry\" aerosol jet printing, nanoparticles were synthesized directly in a spark discharge due to electrical erosion of copper electrodes (Cu \(\geq\) 99.90%, KUZOCM, JSC, Kamensk-Uralsky, Russia) in an argon-hydrogen flow (Ar 95% + H\({}_{2}\) 5%). This gas mixture was used as a reducing agent to limit the oxidation of nanoparticles during their synthesis. An optimized process for the synthesis of nanoparticles using a spark discharge was previously reported in works [25; 26]. The size, morphology, and crystal structure of the synthesized nanoparticles were investigated using a transmission electron microscope (JEM-2100, JEOL Ltd., Tokyo, Japan). Real-time particle size distribution was measured using an aerosol spectrometer (SMPS 3936, TSI Inc., Shoreview, MN, USA). Further, aerosol nanoparticles were focused and deposited on substrates through a coaxial micro nozzle with an outlet diameter of 100 \(\upmu\)m. Aerosol nanoparticles collided with the substrate at high speed (50-150 m/s) and attached to its surface due to the van der Waals force. The width of the nanoparticle beam was controlled by the flow rates of the carrier gas \(Q_{\text{a}}\) and the sheath flow \(Q_{\text{sh}}\), respectively. At the same time, the sheath flow \(Q_{\text{sh}}\) limited the expansion of the nanoparticle beam and protected the nozzle clogging. The focusing process of aerosol nanoparticles was described in more detail in the works [15; 27; 28]. In the presented experiments, \(Q_{\text{a}}\) and \(Q_{\text{sh}}\) were equal to 50 and 20 sccm, respectively. The distance from the nozzle to the substrate and the speed of substrate were 0.3 mm and 0.42 mm/min, respectively. The number of print layers varied from 2 to 5. Thermal sintering of lines was used to reduce oxidized copper nanoparticles and obtain conductive copper microstructures. The printed lines were sintered at various temperatures ranging from 350 to 650 \({}^{\circ}\)C for 60 min using a tube furnace using H\({}_{2}\) as a reducing agent at a flow rate of about 50 sccm. The length and internal diameter of the tube furnace chamber were 560 and 30 mm, respectively. The sample-heating rate was 5 \({}^{\circ}\)C/min with natural cooling in a controlled atmosphere. The electrical resistivity \(\rho\) of current-carrying microstructures was investigated depending on the sintering temperature \(T_{\text{sint}}\) and the type of substrate material. There were 5 lines on each type of substrate formed for each temperature sintering to define the accuracy.