## 1 Introduction  
Organic dyes are colored substances that are extensively used in industries such as pharmaceuticals, textiles, cosmetics, and food coloring [1, 2, 3, 4]. However, their accumulation in wastewater poses significant concerns for both human health and the environment. The catalytic reduction process of these dyes into non-toxic by biomimetic nanomaterials has emerged as a promising approach [5, 6, 7, 8]. Biomimetic materials are synthetic substances inspired by nature to simulate desirable properties. Among them, a polymer so-called PDA, which is inspired by the adhesive proteins in blue mussel byssus, has gained attention as an excellent support material for nanocatalyst stabilization and decoration [9]. Owing to its high biocompatibility and robust adhesive properties, PDA serves as an environmentally friendly platform for various applications such as catalysis, biomedicine, or sensing [10]. It is commonly synthesized as a coating layer, enabling the facile functionalization of surfaces and the attachment of diverse molecules and nanoparticles [11]. Due to the presence of catechol and amine groups, PDA can act both as a reducing agent and a stabilizing matrix for metal nanoparticles. Additionally, PDAcan be synthesized in the form of nanospheres with tunable sizes, which serve as effective templating materials and are well dispersed in aqueous solutions [12; 13; 14]. PDA NSs can be used as suitable substrates for the in situ decoration of nanoparticles such as Au, Pt, Pd, Rh, Ru, and CdS to obtain high-efficient hybrid nanocatalysts [15; 16; 17; 18; 19]. In this context, AuNPs are of particular interest in catalysis because of their high specific surface area that enables higher catalytic activity [20]. Moreover, the applications of PDA-Au have been extended not only for catalysis but also for a variety range of applications. For instance, [PERSON] et al. decorated AuNPs on mussel-inspired PDA as a sensitive SERS substrate for the selective detection of phthalate plasticizers [21]. [PERSON] et al. develop Au nanostars based on PDA-Au for highly sensitive cancer biomarker detection [22]. [PERSON] et al. used PDA-Au as a substrate for the construction of Co\({}^{2+}\)-based copeptin immunostaining [23]. In dye degradation reactions, in the presence of NaBH\({}_{4}\) as a reducing agent, AuNPs act as catalysts to accelerate the electron transfer from NaBH\({}_{4}\) to dye molecules, leading to their decomposition and decolorization of the polluted solution [24; 25; 26]. It is well known that smaller AuNPs exhibit higher catalytic efficiency due to their increased surface-to-volume ratio and greater number of surface-active sites [27; 28; 29]. However, small-size nanoparticles are prone to aggregation, thus limiting their catalytic activity and reusability. Immobilizing AuNPs onto a substrate such as PDA is a suitable approach for preventing nanoparticle aggregation and facilitating catalyst recollection. Additionally, substrate interactions can enhance the electronic properties of AuNPs, thereby improving their catalytic performance [30]. For example, [PERSON] et al. reported that gold nanoparticles supported by hollow PDA nanoreactors is a highly efficient catalyst for dye degradation [31]. Several studies also have investigated the in situ formation of gold ions into metallic AuNPs on PDA surfaces [32; 33]. However, due to the mild reducing capability of PDA, the in situ synthesis of AuNPs often requires a long reaction time ranging from a few hours to a day. For instance, [PERSON] et al. demonstrated that AuNP decoration on PDA NSs required up to 24 h to complete [16]. Therefore, improving the decoration process of AuNPs on PDA NSs is necessary to fully realize the potential of PDA-Au hybrids. To overcome the limitation of the slow reduction process, we propose a new strategy using cold atmospheric pressure plasma as an alternative approach to induce Au ion reduction. Plasma generates reactive species and high-energy electrons, which can dissolve in a liquid medium in contact with the plasma.