Gas exposure for 5 min revealed that the presence of nanoparticlesdecorating graphene entail greater sensitivity (13-fold) in comparisonto the bare graphene performance. Noteworthy, excellent repeatability(0.7% of relative error) and a low limit of detection of 491 ppb wereobtained, together with excellent long-term stability. Besides, anextensive material characterization was conducted, and vibration bandsobtained via Raman spectroscopy confirmed the formation of PPy NPs,while X-ray spectroscopy (XPS) revealed the relative abundance ofthe different species, as polarons and bipolarons. Additionally, XPSanalyses were conducted before and after NH3 exposure toassess the PPy aging and the changes induced in their physicochemicaland electronic properties. Specifically, the gas sensor was testedduring a 5-month period, demonstrating significant stability overtime, since just a slight decrease (11%) in the responses was registered.In summary, the present work reports for the first time the use ofPPy NPs decorating graphene for gas-sensing purposes, revealing promisingproperties for the development of unattended gas-sensing networksfor monitoring air quality.Theoutstanding versatility of graphene for surface functionalizationhas been exploited by its decoration with synthesized polypyrrole(PPy) nanoparticles (NPs). A green, facile, and easily scalable formass production nanocomposite development was proposed, and the resultingPPy@Graphene was implemented in chemoresistive gas sensors able todetect trace levels of ammonia (NH In fact, air pollution is linked to severalmillion premature deaths per year.2 Inconsequence, governments and institutions worldwide are trying toovercome this problem that jeopardizes human health and environmentalquality.During the lasts decades,air pollution has become a growing societalconcern since it is associated with global climate change and showsnoxious effects for both our environment and humankind.Natural sources of air pollution such as volcanic eruptionsarenot significant in comparison to those sources derived from humanactivity, such as industrial and combustion processes. However, sincethe replacement of the current energy sources toward renewable onesis progressive, real-time monitoring of the concentration of key pollutantsis needed. With that, it is possible to reveal in space and time whenthe gases reach the TLV, and subsequently, different actions can betaken to reduce both air pollution and human exposure to those health-threateninglevels.3 Inexpensive,yet reliable and accurate enough, sensor devices are key for achievingsuch gas-sensing networks.4 Some techniques,such as gas chromatography coupled to mass spectrometry (GC–MS),show significant drawbacks, as their complicated miniaturization andslow throughput complicate real-time air quality monitoring, despitebeing highly accurate.5 Other approaches,like the use of electrochemical sensors, present lower cost, but miniaturizationand stability issues associated with electrolytes are still a challenge.6 For that reason, chemoresistive sensors haveemerged as a promising option, as they gather many of the requirementsneeded in the next generation of inexpensive gas sensors.7Nevertheless, for achieving a ubiquitous, real-timemonitoringof gases, developing a widespread sensor network possessing some requisitecharacteristics such as inexpensiveness, accuracy, durability, andlow power consumption is needed.8 However,some drawbacks are preventing their effective implementation in commercialdevices. For instance, the selectivity of MOX-based sensors is stillan issue, and usually, high operating temperatures are needed to activatethe catalytic properties of MOX, leading to a high power consumptionand lower durability due to the coalescence of MOX crystals into largerentities, altering their microstructure and, thus, their gas-sensingproperties.9During the last decades, many researchefforts have been focusedon the development of chemoresistive sensors based on metal oxides(MOX). These devices usually show high sensitivity to gases and fastresponse and recovery dynamics, showing the potential for enablingambient monitoring applications.10 Indeed, graphene-basedgas sensors are power lean and require simpler circuitry because thereis no need to use heating elements. Besides, chemical vapor depositionof graphene is becoming a more mature nanotechnology (now can be producedin large quantities at relatively affordable cost) that shows superiorproperties, such as high carrier density and mobility, which is associatedwith low noise levels, and high surface area to volume ratio due tobeing a 2D nanomaterial.11 However, animportant drawback of pure graphene-based gas sensors is their lowsensitivity to gases. For that reason, further graphene modificationsare often needed to improve its gas-sensing properties. The most commonmodifications consist of the direct functionalization of graphene or its decorationwith nanoparticles or nanocrystals .13 However, the creation of a wide variety of graphene hybrids or compositesis also feasible.14 With these approachesit is possible to obtain sensitive, inexpensive, and durable devicesleveraging the high versatility of graphene to being surface functionalized.In consequence, duringthe past few years, graphene has been attractinggreat interest as a gas-sensitive material due to its capability towork at room temperature.3. However,their use usually implies working temperatures of about a few hundreddegrees Celsius.15 Conversely, fewer worksare available for detecting NH3 using carbon-based nanomaterialsat room temperature, probably because of their limited charge transferand, in consequence, poor sensing responses.16 Indeed, carbonaceous nanomaterials like graphene and carbon nanotubeshave been mainly focused on the detection of gases from combustionprocesses, such as nitrogen dioxide,17 withhigher charge transfer than NH3, or even significantlyhazardous ones such as benzene or hydrogen sulfide.18To the best of our knowledge, some nanomaterials such as metaloxides have been widely used for detecting NH3 is35 ppm for 15 min.19 Not limited to this,the significant role of NH3 is well-known in additionalenvironmental problems, such as promoting the formation of particulatematter (PM2.5).20 Therefore,there is a need for commercial gas-sensing devices able to selectivelydetect ammonia. In that way, the present paper combines two nanomaterials,graphene and polypyrrole, which have been demonstrated to show promisefor detecting NH3 at room temperature.22Anthropogenic ammonia is related to industrial livestockproduction,crop agriculture, or fertilizer industries. According to the NationalInstitute for Occupational Safety and Health (NIOSH), the permissibletime weighted average (TWA) exposure limit is 25 ppm for 8 h, whereasthe maximum short-term exposure limit (STEL) to NH23 Specifically,polypyrrole has attracted far more research efforts than other polymersdue to its superior properties such as biocompatibility and biodegradability,high conductivity, easy synthesis, and stability.25 In addition, the PPy morphology has a significant effect on thephysicochemical and electronic properties due to the different arrangementsfound in PPy chains and their associated different charge distributions.In consequence, PPy has been synthesized in a wide range of nanostructures,such as nanoparticles,26 nanoribbons,27 and nanofibers,28 to cite some. These PPy forms are usually synthesized through electrochemicalmethods, in which the pyrrole monomers are electropolymerized in anelectrolyte solution. The main advantage of this method is the highcontrol over the geometry and thickness of the PPy synthesized bymodulating the applied voltage and current density.29 However, its main drawbacks are associated with the relativelycomplex instrumentation and the difficulty of large-scale production.30Functional conductive polymers, such as polypyrrole (PPy),polyaniline(PANI), and poly, probably assemble therequirements to be employed in the next generation of sensors. Theirmain advantages lie in their high sensitivity and their capabilityfor room-temperature operation, large-scale production, and easy integrationin wearable and flexible sensors.3 even under room-temperatureconditions, the first nanocomposite of PPy NPs and graphene as a gas-sensitivelayer presents a significant synergistic effect between the two nanomaterials.This novel approach shows outstanding gas-sensing performance, enablinga highly sensitive, reproducible, and stable NH3 detection,differing from the previously reported use of PPy films combined withdifferent graphene configurations such as graphene oxide and reducedgraphene oxide.32 Besides, the synthesis methodproposed has been revealed as an easy, inexpensive, and scalable tolarge production approach. In addition, the nanomaterial depositionover the sensor electrodes through the airbrush method constitutesa straightforward and fast way that implies high throughputs. Butnot limited to this, the present work presents a green-chemistry approachin which low concentrations of NH3 are detected by usinga solvent-free synthesis procedure and biocompatible nanomaterials.Thereby, it is worth noting that sincepolymers usually lead tosensitive gas detection of NH34 Poly (PVA) (Merck KGaA) was chosen as a water-solublepolymer to act as a dispersion aid. Since previous works already reportedthat greater molecular weights of PVA lead to smaller PPy nanoparticles,35 a PVA with Mw = 186 000 was chosen.First, a 7.5 wt % solution of PVA dissolved in distilled water wasprepared under magnetic stirring at room temperature for 10 min. Afterward,3.5 wt % iron chloride (FeCl3) (Merck KGaA) was added tothe aqueous PVA solution, waiting 1 h to achieve the equilibration.In that time, the solution appearance changed from limpid to orangebecause FeCl3 acts as an oxidizing agent, forming the PVA/ironcation complex. Subsequently, the pyrrole monomer (C4H5N) (Merck KGaA) was added into the aqueous PVA/FeCl3 solution dropwise, with a molar ratio FeCl3/pyrrole of2.5. Then, the solution was kept overnight under vigorous stirringto complete the polymerization. Meanwhile, the appearance changedfrom orange to black, indicating that polymerization took place derivedfrom the contact of the pyrrole monomer with the iron cation. Nevertheless,the resulting nanoparticles should be extracted and cleaned via thecentrifugation method from the solution. Specifically, three consecutivecentrifugation processes (10 000 rpm and 10 min) were conducted,removing the dispersion solution and washing with abundant distilledwater after each centrifugation. The resulting PPy NPs presented theform of a black powder that was dried in an oven at moderate temperature(80 °C).The synthesis of polypyrrolenanoparticles was conducted via chemicalpolymerization, modifying a protocol described in the literature.Afterward, a graphene solution in distilled water(0.5 mg/mL) was prepared by using graphene nanoflakes . Subsequently, to achieve a suitable graphene exfoliation, thesolution was placed in an ultrasonic bath at high frequency (35 kHz)for 1 h. Then, 5% wt of PPy NPs was added to the graphene solutionand immediately mixed under vigorous stirring for 2 h. This methodis based on PPy NPs supported on liquid-phase exfoliated (LPE) grapheneand constitutes an inexpensive, simple, and fast procedure to obtaingraphene nanocomposites with suitable decoration.An additionalgraphene-based solution was prepared to act as areference sample. In this case, a bare graphene solution was preparedand deposited as mentioned above. In consequence, the effect of thesensing performance induced by the presence of PPy NPs can be elucidatedby comparing the sensing results obtained using the reference sample(bare graphene) and the PPy-decorated one (PPy@Graphene).Asit was mentioned before, the PPy NP synthesis and graphene decorationwere conducted using distilled water and room-temperature workingconditions. Furthermore, the products obtained (PPy and graphene)are biocompatible, constituting a green experimental procedure.Figure S1, Supporting Information (SI)]. Subsequently, the gas sensors wereplaced in an airtight Teflon chamber with a volume of 35 cm3. The sensing chamber was connected to a gas mixing and deliverysystem that used calibrated gas cylinders and pure dry air (Air Premierpurity: 99.999%) as carrier. Then, the resistance of the differentsensors was monitored using a multimeter , andresistance changes were recorded while different concentrations ofgases were applied. In order to reduce the power consumption of thesystem and to work under more realistic experimental conditions, thetotal flow was adjusted at a low rate (100 mL/min) using a set ofmass-flow controllers (Bronkhorst High-Tech B.V.) and electrovalves.The sensors were stabilized under dry air for 15 min before beingexposed to a given concentration of a gaseous species for 5 min. Theresponses to several concentrations were recorded by applying successivedilutions of gases and defining sensor response as ΔR/R0 expressed in percentage,where ΔR corresponds to the resistance changesrecorded over 5 min of gas exposure, while R0 is given by the resistance of the sensor in air (or baseline).The resulting graphene nanoflakes decorated with polypyrrole NPswere deposited via the airbrushing method, using nitrogen as a carriergas while the hot plate  was kept at 115 °C.The solution was deposited onto alumina substrates that comprisedplatinum screen-printed electrodes [In order to characterize the gas-sensing performance under a humidatmosphere, a controller evaporator mixer (Bronkhorst High-Tech B.V.)was used to humidify the atmosphere during some of the measurements.With that, the effect of the ambient moisture on sensor response wasevaluated as well.2O3 substrate and ultrasonically dispersed for 20min, at 37 kHz and 100% of power, in a mixture of methanol and water.One drop of the dispersion was then deposited on a lacy carbon filmsupported by a copper grid (300 mesh). Figure S2a, SI) with a size of about 270 × 400 nm2. An imageobtained at higher magnification  . For TEM investigationof bare graphene, the layout deposited was scratched from an AlFigure S2b (SI) shows a narrow nanoparticle size distribution, situatingthe highest frequency in the range from 100 to 120 nm. An AG-Ultra 55 (Zeiss) field emissionscanning electron microscope(FESEM) was used to analyze both nanomaterials separately and theresulting sensitive film. The synthesized PPy NPs suspended in waterwere deposited on a piece of a silicon wafer, showing monodispersednanoparticles 1b. Besid36 This broadening is associated with the scattering of X-raysfrom the PPy chains at the interplanar spacing.37An X-ray diffraction (XRD) analysiswas conducted using a Bruker-AXSD8-Discover diffractometer (Bruker Co.) with a parallel incident beam(Incoated GmbH). The X-ray diffractometer was equipped with a HI-STARgeneral area diffraction system (GAADS) detector from Bruker Co. TheXRD measurement was conducted at 40 kV and 40 mA to generate the CuKα radiation, and the data were collected stepwise over therange 2θ = 5°–70°. The synthesized PPy NPs were also characterized by employing Ramanspectroscopy. The PPy powder was placed onto a piece of a siliconwafer and the analysis was conducted using a Raman spectrometer (Renishawplc) with a coupled confocal Leica DM2500 microscope (Leica MicrosystemsGmbH). The laser employed had a wavelength of 514 nm and the acquisitiontime was set to 30 s, while three cycles or repetitions were acquiredover the spectra to reduce the noise level. 38 A polaron is formed when a π-electron is moved out from theneutral PPy, while a bipolaron can be formed when a second electronis removed. Both forms act as charge carriers along the conductivepolymer.39 Thus, these electron relocationsas polarons or bipolarons induce crystal lattice distortions in thePPy rings.40 Raman spectroscopy for barePPy NPs . These oxidizedstates are associated with intermediate energy levels within the bandgap, resulting in polarons and bipolarons.PPy NPs 2breveal3 configuration (Cα) at 285.3 eV.This type of carbon bonding is associated with sp3 latticedefects and amorphous carbon, being consistent with the low crystallinityregistered for the XRD analysis. The component centered at 284.4 eVreflects photoelectrons emitted from the sp2 carbon configuration(Cβ), which is related to aromatic carbon bonds,whereas the components shifted to higher binding energies are attributedto the C–N (or C–O), C=N (or C=O), andN–C=O (or O–C=O), appearing at 286.5,287.4, and 288.7 eV, respectively. The components related to oxygen-containingfunctional groups , such as carbonyl and carboxylterminations, probably originated during PPy polymerization, in whichthe terminations of the chains can interact with water to form theoxygen groups.43X-ray spectroscopy(XPS) has been conducted to analyze the chemicalcomposition of synthesized PPy NPs using a VERSAPROBE PHI 5000  equipped with a monochromatic Al Kα X-raysource. High-resolution XPS spectra were recorded, accounting foran overall energy resolution of about 0.6 eV. +) and bipolarons (C=N+) at 401.2 and 402.5 eV, respectively. As a result, polaronsare more predominant than bipolarons in the synthesized PPy NPs.The deconvolution of the N 1score level from PPy NPs is depictedin 2 adsorption/desorption results are summarizedin Figure S4 and Table S1 (SI). Specifically,the BET area obtained for bare graphene (730 m2/g) wasslightly higher than PPy@Graphene (644 m2/g). This resultcan be expected since PPy NPs probably present a significantly lowerBET area. Nevertheless, negligible changes in the average pore diametersand pore volumes were observed between both samples, revealing thatthese parameters are mainly ruled by graphene nanoplatelets.Nitrogen adsorption–desorption analysis of bare and PPy-decoratedgraphene was conducted using a Quadrasorb SI surface analyzer (QuantachromeInstruments) for determining the BET (Brunauer–Emmett–Teller)area, average pore size, and pore volume. The samples were degassedovernight at 120 °C before this measurement. The specific surfacearea was calculated through the BET method and the pore size distributionwas determined by the BJH  method. Theobtained N3 was evaluated in similar conditions to those neededin real-time monitoring. First measurements consisted of repeatedresponse and recovery cycles to 25, 50, 75, and 100 ppm of NH3. Figure S5 (SI), showing that graphene decorated with PPy offers up to 7-foldhigher responses  than bare graphene. Besides, since sensitivityis given by the slope of the calibration curves, PPy@Graphene presents14 times higher sensitivity than bare graphene.The ability of the hybrid nanomaterial developedfor detectingNH44 In consequence, both nanomaterials employed, oxygen-defectivegraphene and Cl–-doped PPy, show a mild p-type behavior.When the sensitive layer interacts with an electron-donating moleculesuch as NH3, the hole density is lowered, thus resultingin an increased electrical resistance. According to the experimentalfindings, it can be assumed that bare graphene can interact with NH3 molecules; however, the presence of PPy NPs decorating thiscarbon nanomaterial involves a more intense interaction. In otherwords, the presence of both nanomaterials creates a synergistic sensingeffect, in which graphene and PPy are interacting with the targetgas, thus resulting in superior sensing performance. Besides, bothnanomaterials show a noteworthy interaction through strong hydrogenbonds and π–π-stacking.45 As a result, long-term sensor stability can be attributed to theirefficient interaction.46 In addition, thelarge graphene surface area and its superior electron mobility inducea fast and efficient charge transfer with PPy NPs, leading to an effectivetransduction.47Conjugated polymers are well-known semiconductor materials,whereasPPy stands out due to the high positions of its HOMO and LUMO levels,being easily p-doped and conferring high stability.3 is well-known, especiallyunder room-temperature conditions.48 Inparticular, oxygenated defects and functionalgroups grafted at graphene act as adsorption sites for ammonia molecules,leading a charge transfer toward the sensitive film as follows:The somewhat limitedinteraction between graphene and NH3, thus resulting in better sensing performance. Overall, whengraphene nanoflakes are decorated with PPy, these NPs contribute tothe NH3 sensing through the donation of a lone pair ofelectrons of nitrogen from the target gas to the initially oxidizedPPy, which results in a neutralized PPy through the reduction of theirdoping level, thus decreasing conductivity. Indeed, the ammonia–polymerinteraction is based on a compensation effect, in which the dopinglevel of the polymer chains varies through two reversible pathways.49 The first one is related to an electron transfer,in which the positively charged PPy NPs (PPy+) are reducedto their neutral form (PPy0) when interacting with NH3 molecules, whereas the dopant anion (Cl–) that was compensating the charged PPy+ is reversiblydisplaced as follows:50Nevertheless, PPy NPs tend to show higher interaction with NH50The second feasible sensing mechanism involves a proton transferbetween ammonia, which is a strong base, with the PPy NPs. Specifically,the PPy deprotonation toward ammonia results in the formation of theammonium ion that induces a compensation effect through the dopantanion. Therefore, the doping level is reduced as follows:51 However, graphene decoratedwith PPy NPs showed better sensing performance than bare graphenedespite its lower BET area. This fact is revealing the essential roleof PPy in the NH3 gas detection, enabling a significantelectronic sensitization that compensates this lower specific areaand confirming the importance of the sensing mechanisms proposed.Nevertheless, the mere presence of the nanoparticles cannot explainthe outstanding enhancement in the sensing responses to NH3. Therefore, a synergistic effect between graphene and PPy is probablyheld. For that reason, their interface has been extensively studied.Roughly, both nanomaterials are composed of sp2 conjugatedcarbons resulting in pristine graphene and neutral polypyrrole, showingsemimetallic behavior. Theoretically, pristine graphene is a zeroband gap semiconductor with a tiny overlap between the valence andconduction bands.52 Nonetheless, XPS analysisrevealed that graphene employed in this work shows a significant oxygencontent , which results in p-type doping. This chemical functionalizationand the presence of the sp3 carbon configuration inducea band gap opening in graphene  using a Cary 5000 UV–vis–NIR spectrophotometer(Agilent). Subsequently, the band gap was determined through a Taucplot, in which the gap energy was calculated by extrapolating thelinear portion of the graph between the function (αhν)2 versus the photon energy (eV), as shown in Figure S6b (SI). A relatively wide band gap (3.1eV) for graphene nanoflakes resulted.It is worth notingthat, roughly, greater specific BET areas usuallyled to improved sensing performance owing to a larger area for adsorbinggas species.graphene 5. Indeed53 resulting in poor electronicconductivity. Nevertheless, it is possible to increase the electronictransport by adding new energy levels within this band gap  and the lowest unoccupied molecular orbital (LUMO).Specifically, previous works report a 3.2 eV band gap for PPy in itsneutral state,band gap 5. IndeedEC) relative to the valence bands (ΔEV), owing to the closer levels of valence bandsbetween the two nanomaterials according to XPS analysis.56 Therefore, excitations of the sensitive layertranslate into positive charges (holes) being emitted from the grapheneover the barrier into the PPy NPs. In consequence, near the boundaryof the interface a depletion region can be formed,57 specifically located on the graphene surrounding PPy. Conversely,PPy NPs probably increase carrier concentration, thus being consideredas accumulation regions. As a result, the PPy@Graphene sensing performancewas enhanced because the NPs would have an excess of positive chargesthat can be, to some extent, recombined with the negative chargeswhen interacting with electron donor gas molecules such as NH3. Meanwhile, graphene is lowering its concentration of majoritycharge carriers (holes), further decreasing its conductivity and enhancinggas-sensing response. This fact is worth mentioning considering thatgraphene is mainly acting as a transducer (since limited interactionwith gases cannot be ruled out) while the main gas interaction isprobably located at the PPy NPs, acting as gas recognition elements.Indeed, the potential applied to the transducer element (graphene)is increasing the charge flow and in consequence favoring the holemovement from graphene to PPy NPs over the barrier.3 is a donor of electronic chargewhile the sensitive film is the receptor, the HOMO level of the gasmolecule interacts with the LUMO level of the sensor.583. This electron-donating gas specie is attached to the sensitivefilm via coupling π-bonds.59 As aresult, once the gas donates electrons to the PPy@Graphene layer,the Fermi level in dry air conditions (E0F) undergoes an upward shift, reaching EgF, which results in the lowering of the majoritycarrier (holes) concentration.Considering that NH3. In consequence, once ammonia adsorbs at the hybrid nanomaterial,the energy gap at the interface between the valence band of grapheneand the HOMO of PPy NPs widens , which favors holes from the valence band of graphene totransfer toward the HOMO of PPy NPs. In other words, upon exposureto ammonia, the graphene layer loaded with PPy NPs sees the concentrationof its majority charge carriers (holes) reduced, which translatesinto a significant change in the overall sensor resistance.This shift in the Fermi levelfor graphene and PPy NPs is associatedwith a band-bending readjustment when the sensor is exposed to NH3 in the range of 25–100 ppm and the improvedsensing performance when graphene is loaded with PPy NPs, furthermeasurements at decreased target gas concentrations were performed.The intermediate range of ammonia concentrations ranging from 5 to25 ppm was studied, and results can be found in the Supporting Information. Figure S7 (SI) shows the sensing responses and calibration curves obtained.It is worth noting that this range of concentration has significantinterest for monitoring ammonia in the ambient because it is closeto the TWA and STEL values defined by the agency NIOSH. 3 concentration levels (from 1 to 5 ppm). Even at this range of concentrations,a significant response repeatability and baseline stability was achievedunder room-temperature operation conditions. Conversely, under theseexperimental conditions, the bare graphene sample shows a significantbaseline drift  and poorsensitivity. Indeed, Considering the very low noise levels achieved in the detectionof NH60 In addition,adsorbed water molecules might enhance the proton exchange with theNH3 molecule, inducing a further decrease in the hole concentration,thus leading to higher resistance levels.Nevertheless, the presence of ambient moisture is a key influencingfactor on the gas-sensing performance due to its interfering effect.In consequence, the previous experiments carried out under dry conditionswere reproduced under humid conditions (50% relative humidity at 23°C). However, since thesehigher sensing responses can be beneficialto lower the LOD or improve other sensing parameters, variable relativehumidity levels registered under real conditions of operation canconstitute a significant drawback. To solve that, an alternative mightbe the creation of a sensor array, in which a specific humidity sensoris integrated. Therefore, with the calibration of the PPy@Graphenesensor at multiple ambient moisture levels, this issue could be overcome.y is thestandard deviation of y-residuals and b corresponds to the sensitivity (slope) from the calibration curve.As a result, the PPy@Graphene shows a LOD of 491 ppb for NH3, which is comparable to other types of organic-based nanomaterials.For instance, the use of a conductive sublayer of perfluoro-copperphthalocyanine (Cu(F16Pc) covered by a lutetium bisphthalocyanine(LuPc2) sensitive layer reveals a LOD of 140 and 2000 ppb dependingon if DMBz or TFBz was respectively grafted to the ITO substrate.61 Conversely, Figure S8b (SI) shows a significant baseline drift and poor sensing responsesfor bare graphene samples, denoting the enhancement effect of PPyNPs decorating graphene nanoflakes. Furthermore, cross-sensitivitywas assessed (6H6) and toluene (C7H8). Asa result, negligible resistance changes were obtained toward theseanalytes in comparison to NH3, making the PPy@Graphenegas sensor reliable and promising to detect ammonia in the environment.Considering the gas-sensing results obtained under a humid atmosphere,the limit of detection (LOD) was estimated through the following equation3 pulses for 5 min and recovery steps of 15 min between gas exposures. 3 were also evaluated by reducing the exposure timefrom 5 to 1 min, while the duration of recovery steps was loweredfrom 15 to 5 min . Only a small decrease in response was registered afterthis 5-month period. Nevertheless, to better our understanding, thesensing results were correlated with an XPS analysis performed onsamples freshly synthesized and after 5 months of continuous use,in order to assess aging.Despite these superior sensing properties, a demonstrationof highstability and durability is also required before the potential implementationof this sensor in commercial devices. For that reason, a stabilitytest was performed by measuring and comparing the sensing responsestoward 50 ppm of ammonia for 5 months (3 gassensing (Table S2 (SI)summarizes the relative areas of the different components, showingthe absence of significant differences between both C 1s spectra.Indeed, just a slight decrease in the sp2 configurationcan be observed, while a limited increase in both the sp3 carbon configuration and oxygen content through a higher intensitypeak of C–O/C–N and C=O/C=N is obtained.These results indicate a slight aging of the sensitive film after5 months due to the slight oxidation of the hybrid nanomaterial, demonstratingsuperior reusability and stability. N 1s core levels studied on thePPy@Graphene sample before  and an increase in the C=N defectsof the PPy due to the loss of conjugation during aging . Interestingly, at higher bindingenergy a new component appears when the sensor is exposed to NH3  for a PPy@Graphenesample. The analysis of the O 1s spectrum shows an oxygen contentrelated to the partial oxidation of PPy and the oxygen-containinggroups grafted to graphene. Interestingly, a decrease in the abundanceof the C=O bond was observed while C–O experiences asignificant increase. This fact can be derived from the conjugationof carbonyl groups due to the oxidation of PPy and the interactionwith the gaseous environment.The C 1s core levels were analyzedbefore the NHsensing 10a and asensing 10b. Table before 10c and ae before 10d NH3 sd to NH3 10d. In f3 as an oxidant elementleads to higher stability of electrical conductivity than other oxidantssuch as Fe2(SO4)3 or (NH4)2S2O8.62 The reason is that some anions like SO4–2 tend to experience higher migration to pyrrole nitrogen than Cl– anions, resulting in the quaternization of the nitrogen,thus increasing the PPy degradation and instability. Besides, sincesunlight and operating conditions well above room temperature arewell-known parameters that induce the aging and degradation of PPy,64 the gas-sensing measurements have been conducted under dark conditionsand ambient temperature. Both parameters can be easily implementedin airtight testing chambers, favoring the preservation and stabilityof PPy NPs. Even more noticeably, graphene probably plays an essentialrole in preventing PPy aging thanks to its high hydrophobicity. Whenwater molecules directly interact with PPy chains, the electron densityof nitrogen atoms is changed,65 weakeningthe interaction between the PPy and the dopant anion, in this case,Cl–. As a result, ambient moisture reduces the electricalconductivity of PPy NPs, accelerating their aging. However, the hydrophobiccharacter of graphene may protect the PPy NPs toward their aging derivedfrom the interaction with water molecules. This fact was already demonstratedfor other types of sensitive nanomaterials, such as halide perovskites,in which graphene increases their lifetime and stability toward thepresence of relative humidity in gas sensors66 and solar cells.67Therefore, the low-rate decay of response intensitywith time andits consequent low aging rate could be attributed to several reasons.The PPy NPs synthesis using FeClTable S4 (SI) is summarizingseveral works that report the use of different PPy/graphene hybridconfigurations. Overall, an increase in the sensing performance wasachieved when the PPy is present in the sensitive film. However, essentialinformation is frequently missing, such as the flow rate applied,LOD, and different tests to assess the repeatability, stability, andeffect of the ambient moisture. Table S4 (SI) depicts that graphene nanoflakes loaded with PPy NPs show anoutstanding sensitivity coefficient within the works that report anin situ chemical polymerization. It is worth noting that electrochemicalpolymerization can lead to better sensing responses, since high controlover the thickness and PPy geometry can be achieved. However, theiruse involves more expensive synthesis procedures and fewer possibilitiesfor industrial-scale production. Besides, Table S4 (SI) shows that the present work uses a low flow rate, 10-foldlower than those reported in other works, as well as the use of airinstead of nitrogen as carrier gas. With these experimental parameters,the sensing performance is probably underestimated to some extentbecause higher flow rates or sensing in a nitrogen atmosphere canboost up the resistance changes. But experimental conditions appliedhere are closer to those needed in real-time monitoring of ambientpollutants.Finally, 3 under ambient temperature operating conditions. Thechemical polymerization of PPy NPs offers a low-cost and mass-scalablesynthesis method, resulting in biocompatible nanomaterials with outstandingelectrical properties. Indeed, the development of a chemical resistivedevice able to operate at room temperature paves the way for achievingwidespread air quality monitoring systems due to their simple readoutinterface circuitry, inexpensiveness, low-power consumption, miniaturization,and high portability. Besides, high gas sensitivity and remarkablerepeatability and LOD were obtained for detecting NH3.The sensing mechanisms are probably dominated by electron transfer,in which the presence of PPy decorating the graphene nanoflakes inducesa larger and more effective charge transfer interaction, leading toenhanced sensing properties.A solvent-free, green method was proposedto obtain a graphenenanocomposite loaded with polypyrrole nanoparticles for detectingNHNevertheless, despite the reducedfootprint of this technology,high stability over time is mandatory for gas sensors. In consequence,the sensing properties were assessed for 5 months, and XPS analysiswas conducted to study the aging of the hybrid nanomaterial. Afteran intensive use of the developed sensors, a suitable stability wasobserved. The slight oxidation of PPy observed in the XPS analysisis in agreement with the slight decrease registered in response intensityafter several months of use in humid atmospheres. These results indicatethat these sensors meet many of the specifications for being employedsuccessfully in ambient monitoring applications.Noteworthy,the present work reports the first use of a nanocompositecomprising PPy NPs morphology and graphene, revealing an outstandingsensing performance owing to the significant synergistic effect derivedfrom the integration of both nanomaterials. With all of these, thecombination of both graphene and PPy NPs is paving the way towardlow power consumption and highly durable devices thanks to their capabilityto be operated at ambient temperature. Thereby, the as developed grapheneloaded with PPy NPs shows unprecedented sensing properties, increasingits potential to be employed in commercial devices. However, furtheroptimizing this hybrid gas-sensitive material  will be the subject of future research.