## 1 Introduction  
In the past few decades, chemical gas sensors have been widely used in several applications, including the detection of combustible, flammable, and toxic gases [1; 2; 3; 4; 5]. Various semiconductive materials are used as transducers for chemical sensors, and extensive investigations have been conducted to improve their performance [6; 7; 8; 9]. The critical indicators for sensor performance include high sensitivity to the target analyte, short response and recovery times, and cycle stability [10; 11]. Among these, the sensitivity and cycle stability are optimized via the control of the morphology of electrode materials. However, the response and recovery times can be adjusted by modifying the semiconductive properties of the electrode material, unlike other factors.  
Conducting polymers, which are semiconductive materials, have been investigated for sensor applications, owing to their versatility originating from the conjugated backbone structure consisting of alternating single and double bonds [12; 13]. Conducting polymers have been used as sensor transducers with inherent electronic, optical, and mechanical transducing properties because of their unique properties [14; 15; 16]. Considerable efforts have been directed toward the fabrication of nanometer-scale conducting polymers, owing to their beneficial characteristics, such as small size, high surface-to-volume ratio, and amplified signals, to enhance the sensitivity [17; 18; 19; 20]. In recent years, several studies have focused on increasing the surface area of conducting polymer nanomaterialsto maximize the interactions between the transducers and analytes [21; 22; 23; 24]. [PERSON] et al. fabricated multidimensional conductive nanofilms that consisted of graphene-coated carboxyl polypyrrole nanowires for application as a Hepatitis B surface antigen aptamer sensor electrode [21]. [PERSON] et al. synthesized three-dimensional polyaniline hydrogels with high surface areas and conductivities for application in glucose oxidase sensors [24]. However, research on controlling the electrical signal transmission speed of the electrode materials is limited.  
Among the most popular conducting polymers, polypyrrole (PPy) exhibits several beneficial characteristics, including high electrical conductivity, redox property, and environmental stability [25; 26; 27]. PPy has been extensively investigated in sensor applications, owing to its unique chemical and electrical properties [28; 29; 30]. In particular, PPy has different electronic band structures, such as neutral, polaron, and bipolaron states, which can be facilely modified by controlling the doping ratio [31; 32]. However, its structure rather than the doping ratio is more often modified for application in sensor transducer materials.  
Herein, the sensing performance of polypyrrole nanoparticles (PPyNPs) as a sensor transducer in different chemical states is investigated. First, PPyNPs with a uniform diameter of 60 nm were synthesized via a modified monodisperse method, and then subjected to chemical treatment while using aqueous solutions of different pH levels (pH 1-13). The charge carrier (hole) density and diameter of the particles decreased with an increase in pH, owing to the deprotonation of the polymer chain under basic conditions. The nanoparticles with controlled protonation were subsequently introduced on an interdigitated array electrode surface via spin-coating for application as an NH\({}_{3}\) gas-sensing transducer. The sensor electrode at pH 1 exhibited high sensitivity to the target analyte (down to 1 ppb of NH\({}_{3}\)), with short response and recovery times of less than 20 s and 50 s, respectively, at 25 \({}^{\circ}\)C.