[0001] The present invention relates to a chemiresistive sensor with a low limit for detecting NO<2> at ambient temperature, an active layer of said sensor comprising a few-layer graphene material. The response of the sensor is not altered by relative humidity, and the sensor also demonstrates very low interference with ammonia, allowing this interference to be disregarded in the measurements.
1. A resistive chemical sensor for the detection of NO<2>characterized in that it comprises • an insulating substrate; • at least two metallic electrodes on the insulating substrate; • at least one active layer located on the substrate and the electrodes, connecting said metallic electrodes, wherein the active layer has a thickness between 20 nm and 100 µm and is made up of particles of a size between 20 nm and 1000 nm agglomerated and nanostructured in nanodomains, where said nanodomains are of non-oxidized graphene with few atomic layers, wherein said graphene contains structural defects, and wherein said defects are carbon vacancies in the structure of graphene with few atomic layers. 2. Chemical sensor according to claim 1, wherein the insulating substrate is made of a material selected from FR4, silicon, alumina, polymers, plastics, paper, gallium arsenide, aluminium nitride and glass. 3. Chemical sensor according to any one of claims 1 or 2, wherein the electrodes are at least two and the conductive material that forms them is selected from among copper, gold, platinum, aluminium, chromium, titanium, silver and any alloy thereof, or copper coated with a material selected from gold, platinum, chrome, titanium and any combination thereof. 4. Chemical sensor according to claim 3, wherein the electrodes are of copper coated with gold and both electrodes are interdigitated, with a space between the electrodes between 25 µm and 250 µm, and wherein the confined area between the electrodes is between 0.04 mm<2> of 4 mm<2>. 5. Chemical sensor according to any one of claims 1 to 4, wherein the size of the particles of the active layer is between 50 nm and 450 nm. 6. Chemical sensor according to any one of claims 1 to 5, wherein the active layer has an absorbance in the ultraviolet-visible region with an absorption maximum located between 200 and 400 nm. 7. Chemical sensor according to any one of claims 1 to 6, wherein the active layer has a Raman spectrum comprising 4 characteristic bands, wherein said bands are D, G, D' and 2D, wherein the ratio between the amplitudes of the bands D and G has a value between 0.6 and 1.5, and wherein the ratio between the amplitudes of the bands D' and G has a value between 0.1 and 0.4. 8. Process for obtaining the resistive chemical sensor according to any one of claims 1 to 7, characterized in that it comprises the following steps (a) grinding graphite flakes by means of a high-energy oscillatory dry mechanical method, for a time of at least 80 min at a grinding speed between 1300 and 1700 rpm, and at a temperature between 15 °C and 35 °C; (b) dispersing the powder obtained in step (a) in an organic solvent selected from 1-methyl-2-pyrrolidinone, dimethylformamide or dimethylsulfoxide, and wherein the concentration range varies between 0.1 mg/mL and 10 mg/mL; and (c) drip depositing the solution obtained in step (b) on an integrating circuit comprising an insulating substrate with at least two metallic electrodes on said substrate and evaporating the solvent in the air; optionally, during the step (c) of drip depositing, the resistance between the electrodes is measured. 9. Process for obtaining the chemical sensor according to claim 8, wherein the grinding time of step (a) is between 80 min and 120 min. 10. Process for obtaining the chemical sensor according to any one of claims 8 or 9, wherein the organic solvent is 1-methyl-2-pyrrolidinone. 11. Process for obtaining the chemical sensor according to any of claims 8 to 10, wherein the resistance between the electrodes is measured during step (c) of drip depositing, until the resistance measured between each drop deposited is stable, the resistance remaining without a change greater than 10% for a time of 1 min before depositing the next drop. 12. Use of the sensor according to any one of claims 1 to 7 for detecting the presence of NO<2> in a concentration of at least 25 ppb in air at a temperature between 15°C and 35°C. 13. Use according to claim 12, wherein the sensor is exposed to ultraviolet radiation between 200 nm and 400 nm, preferably between 250 nm and 270 nm. 14. Use according to any one of claims 12 or 13, wherein if in the air there is any of the elements selected from the following list NH<3>, relative humidity between 0.5% and 40% and any combination thereof, the presence of NO<2> is detected at a concentration of at least 25 ppb in air at a temperature between 15 °C and 35 °C. 15. Method for detecting the presence of NO<2> in air, characterized in that it comprises the following steps: a) circulating reference air, without NO<2>, with a flow of 25 mL/min to 1000 mL/min, established by means of a flow controller, through a chamber where the sensor described according to any of claims 1 to 7 is placed for a time between 10 min and 30 min so that the resistance of the sensor has a stable initial value; b) exposing the sensor of step (a) to air to detect the presence of NO<2> until at least the value of the response of the sensor is three times its noise, for a maximum time of 30 min; and c) purging the air inside the chamber for a time between 10 min and 30 min; optionally during step (a) and (b) the sensor is exposed to ultraviolet radiation between 200 nm and 400 nm, preferably between 250 nm and 270 nm.