Within the methods to produce AuNPs to functionalize textiles, chemical reduction is one of the most applied. This method is preferred due to its simplicity, but the current trend is to replace it with more environmental-friendly and inexpensive methods. Researchers often use the traditional two-step functionalization (synthesis and posterior deposition) but also in situ methodologies, where the synthesis and deposition of gold nanoparticles on fabrics takes place in single step. The common reducing agents used in the two-step process are NaBH₄ and sodium citrate. When the two-step method is applied, different methodologies were reported for deposition such as drop-wise deposition, exhaustion, padded, impregnation, and printing of the fabrics with AuNPs dispersion. Kam Ling Chan et al. (2016) have synthesized AuNPs using chloroauric acid (HAuCl₄) as precursor salt, NaBH₄ as reducing agent, and sodium citrate as a capping agent. They were coated on fabrics made with cotton, silk, and wool using drop-wise deposition [48]. Coating of keratin protein in combination with gold nanoparticles onto the cotton fabric by padded method was attempted by O Shanmugasundaram et al. (2018) to improve the antibacterial property. Porosity and water absorbency of fabric was decreased with the coating of keratin and AuNPs. AuNPs were synthesized using a chemical reduction method where HAuCl₄ and trisodium citrate were used as precursor salt and reducing agent, respectively. The synthesized nanoparticles were measured and found to be in the size of 8–30 nm with mean size as 14 nm [49]. T. Abou Elmaaty et al. (2018) have used a simple method, printing and paste, to coat the AuNPs on to the polyester and cotton fabrics. The gold nanoparticles were synthesized using gold (III) chloride hydrate and sodium citrate. Thereafter, the solution was made into paste, and it was printed using flat screen technique. TEM observations revealed that the average diameter was observed in the range between 13 and 20 nm [42]. Yidan Zheng et al. (2013) have produced silk and cotton fabrics coated with gold nanorods for coloring. Au nanorods with the size of 19 nm were synthesized using seed-mediated growth method where they have used HAuCl₄, NaBH₄, and CTAB as precursor solution, reducing agent, and stabilizing agent, respectively. The Au nanorods were deposited onto the fabrics by immersing them in the Au nanorod solutions [50].

The in situ approach allowed to remove one step to functionalize the fabrics. Here, chemical reducing agents are added to the gold salt solution or the chemical groups on the fiber surface may act as reducing agent itself. Xia Lin et al. (2017) have produced AuNPs via reduction method using HAuCl₄ as a precursor solution and trisodium citrate as a reducing agent, where pH value was maintained at 5.0–6.5. The nylon fabrics was functionalized with gold nanoparticles by immersion and heating process. It was observed that the color of the fabric became lighter with an increase in the concentration of the reducing agent [51]. Chitosan being nontoxic and cationic in nature, Iris O. Silva et al. (2019) treated knitted fabric to improve the binding effect of nanoparticles. The AuNPs were chemically synthesized, and NPs with spherical shape and approximately 35 nm in size were obtained. Then, the team immobilized the AuNPs onto the soybean knitted fabric treated with chitosan by exhaustion method. In this process, HAuCl₄, sodium citrate dihydrate, and chitosan acted as precursor salt, reducing agent, and stabilizing agents, respectively. The fabrics developed showed improved thermal stability [52]. Liheng Gao et al. (2020) have produced single-layered Ag-Au-Pt nanocrystal ternary coated biomass textiles using polymer-driven self-assembly strategy. In order to improve the water solubility, oxidative resistance, and affinity to biomass, metal nanoparticles, are transformed into homogenous hyperbranched poly(amide-amine) (HBPAA) encapsulated nanoparticles. In this study, the AuNPs were produced by in situ synthesis, where HAuCl₄.4H₂O and NaBH₄ were used as precursor salt and reducing agent, respectively. AuNPs with a diameter of 6.64 nm were mixed with other nanoparticles, and the textiles were impregnated in the solution [53]. Hanan B. Ahmed et al. (2017) produced AuNPs and coated a silk fabric by immersion with constant shaking and temperature to enhance NPs adhesion and, consequently, the antibacterial properties. In the process of synthesizing AuNPs by in situ reaction, AuCl₃ was used as a precursor salt, silk macro molecular chains were used as reducing agent, and silk fabric itself acted as stabilizing agent. Hydrogen peroxide was used to activate the silk macro molecules, and the size of the AuNPs was in the range of 22–66 and 18–49 nm [54]. Ultrasonic treatment was used by Lin Zhou et al. (2019) for the better deposition of gold nanoparticles on to the silk fabric. HAuCl₄ and sericin in silk were used as precursor salt and reducing agent, respectively, to produce AuNPs. The AuNPs were deposited on the fabric by soaking the fabric in the solution and using ultrasonic treatment. The AuNPs found to have spherical and ellipsoidal shape in the size of 11 ± 4 nm [55]. A few works in the literature exist with this last method, which can be a promising approach to reduce the chemicals during the NPs synthesis, the preparation time, and the energy required. More studies are needed to provide toxicological tests and adapt to industrial processes.

Pre-treatments of fabrics are vital to enhance the reactivity onto their surface and enhance the AuNPs deposition rate or to act in synergy with AuNPs improving a specific property. Two different studies emerged in the literature, the first one was the fabric surface activation using plasma treatments and the other one was the pre-functionalization with minerals, by silanization with alkoxysilane molecules or by cross-linking agents. It was reported by Nina Radic et al. (2012) that dielectric barrier discharge and diffuse coplanar surface barrier discharge was used to enhance the deposition of AuNPs. The AuNPs were synthesized by the reduction of HAuCl₄ solution by gallic acid without stabilizer and coating onto the fabric by immersion of the fabric in the solution containing AuNPs. The size of the particles deposited onto the fabrics was in the range of 30–60 nm, and it was observed that they were smaller in solution, which indicates some agglomeration on the fabric [56]. Another methodology was reported by Makoto Ikegami et al. (2012), where the PET (polyethylene terephthalate) non-woven fabric was coated with zirconium dioxide (ZrO₂) fine particles prior to the deposition of AuNPs to enhance the catalytic activity. An in situ reaction method was used to produce and deposit AuNPs by the deposition-precipitation method [57]. Deshan Cheng et al. (2019) synthesized AuNPs via in situ synthesis reduction method and deposited them on cotton fabric by dip-coating. In this work, HAuCl₄ was used as the precursor salt and polydopamine (PDA) acted as reducing agent. AgNPs were deposited prior to the deposition of AuNPs, which acted as catalytic hotspots for enhancing the deposition of AuNPs [58]. Hongjun Liu et al. (2013) used HAuCl₄ to synthesize AuNPs on cotton fibers via in situ synthesis using the amine stabilizers and N-vinyl pyrrolidone (PVP) reductant in an aqueous medium. Fibers of cotton surface were first modified with (3-aminopropyl) triethoxy silane (ATS). The pre-treatment of the surface was found to be essential, without which a small amount of nonuniformly adsorbed AuNPs was formed on the fabric. The size of the nanoparticles was in the range of 2–7 nm and attained spherical shape [59]. Bharat Baruah et al. (2019) have focused on improving the catalytic activity of the fabrics combining zinc oxide (ZnO) and AuNPs. Hence, the fabric was coated with ZnO nanorods prior to the deposition of AuNPs. AuNPs was obtained by ex situ synthesis and citrate reduction method. AuNPs of the size 18.5 ± 2.8 nm were then deposited on cotton fabric coated with ZnO nanorods by the dip-coating method [60]. Hongjuan Ma et al. (2013) have functionalized PE/PP non-woven fabric by gold nanostructured microtubes, where AuNPs were in situ prepared after the grafting of 4-hydroxybutyl acrylate glycidyl ether onto the shell PE layer and posterior reaction with diethylamine. The tertiary amine groups covalently bonded onto the fabric are designed as the adsorbent for metal ions and reducing agent. Spherical-shaped nanoparticles with size of 5–20 nm were obtained in this study [61]. Richard P. Padbury et al. (2015) produced 14.6 ± 1.4 nm gold nanoparticles using HAuCl₄ as precursor salt. The sheets made up of nonwoven nylon-6 were soaked in NP solution and AuNPs are settled on the fabric [62]. It was observed that size and shape of nanoparticles depend upon reducing agent and concentration of the solution. These strategies produced functional textiles with improved properties, opening new perspectives for studies using combined approaches, able to provide the maximum AuNPs effect using low concentrations. Thus, improved methods may be obtained with reduced costs and environmental impact.

Some works were found in the literature that described the potential of functional groups in the composition of fibers to act as reducing agents of gold salts under heating. These works synthesized AuNPs in situ of silk fibers due the presence of tyrosine amino acid. Bin Tang et al. (2014) functionalized the pristine silk fabric by in situ synthesis of AuNPs. The silk fabrics with tetracholoroaurate ion (AuCl₄⁻) were heated at 85 °C in solution, and the AuNPs were in situ synthesized on silk fabric. Nanoparticles of size 21.3 ± 3.4 nm and different shapes were observed, and it was noticed that the concentration of the precursor in solution have influenced the shape of the nanoparticles and also improved thermal conductivity [63]. Jun Liu et al. (2016) have produced silk fabrics coated with gold nanoparticles. Gold nanoparticles were in situ synthesized on silk fabrics by heating precursor salt (HAuCl₄.3H₂O). The AuNPs were deposited on the silk fabrics by immersion of fabrics in the solution and heating the solution at 85 ℃ for 60 min. Concentration of the precursor solution had an influence on the shape of the AuNPs and attained spherical, triangular nanoplates, truncated nano prisms, and polygonal shapes with varying concentrations [64]. Zhanyu Zhang et al. (2019) have synthesized AuNPs from HAuCl₄ by in situ process and maintaining pH value at 3. AuNPs coated the silk fabrics by immersion of fabric in solution and applying heat process [65]. These works showed the importance of functional groups in the fiber composition and how they can be used to obtain functional textiles and suggested novel studies using other protein fibers.

To facilitate the use of gold nanoparticles in the health sector and minimize the environmental impact, it is essential not to use any toxic or harmful chemical during the synthesis. Hence, researchers have been shifted towards green synthesis methods, where the reducing and stabilizing agents are obtained from plants or a few at times bacteria. All the works that evolve biological methods for AuNPs are quite recent, being this approach a high research tendency. R.M. Ganesan et al. (2015) have synthesized AuNPs using HAuCl₄ as a precursor and extract of Acorus calamus rhizome as a reducing agent. Then, the cotton fabrics were coated by the pad-dry-cure method. The synthesized AuNPs were small and big spherical shape and had different sizes depending upon concentration of the solution [66]. Nabil A. Ibrahim et al. (2016) have biosynthesized AuNPs using HAuCl₄3H₂O as a precursor salt and bacterial isolates (Streptomyces sp.) as a reducing agent. The surfaces of cotton and viscose knitted fabrics were modified using plasma treatment before the functionalization with gold nanoparticles in combination with TiO₂NPs or ZnONPs. The AuNPs were observed to have spherical shape with size in the range of 4–13 nm. The nanoparticles were deposited on to the knitted fabrics by sonication [67]. Bin Tang et al. (2017) have used in situ synthesis method to prepare AuNPs onto a cotton fabric using HAuCl₄ solutions at the concentrations of 0.025, 0.05, 0.075, 0.10, and 0.125 mM. Cellulose acted as reducing and stabilizing agents. The obtained AuNPs have exhibited different shapes depending on the content of gold such as spherical and triangular nanoplates with different sizes. The coated fabric displayed catalytic activity and improved UV protection [68]. To obtain a green approach, Jinlon Tao et al. (2018) functionalized cotton and polyester fabric by hybrid colloids of AuNPs and NRP (natural rubber particles), which was obtained through in situ synthesis of AuNPs in NRL (natural rubber latex) matrix. In this process, NRL act as both reducing and capping agent. HAuCl₄ solution and NRL was employed for obtaining AuNP@ NRP hybrid latex. The team induced the hierarchical nature into the material by the addition of NRL, which lead to the phenomena of hydrophobicity of the treated fabrics. The fabric surface was coated with AuNPs of size 31 nm using the dip and dry method [69]. P. Boomi et al. (2019) have functionalized cotton fabric using gold nanoparticles produced by green synthesis reduction method by maintaining pH value equal to 7. In this study, AuNPs were synthesized by reducing HAuCl₄ with Coleus aromaticus leaf extract. The AuNPs were coated on the cotton fabric by immersion of the fabric in the colloidal solution. The obtained nanoparticles were of spherical and triangular shape and different sizes were measured [70]. Pandi Boomi et al. (2020) have synthesized AuNPs and deposited them on the cotton fabric to improve their antibacterial and anticancer properties. The gold nanoparticles were synthesized by green synthesis, where Croton sparsiflorus leaves extract acted as both reducing and stabilizing agent and HAuCl₄ as precursor salt. The cotton fabric was coated with pristine leaf extract through the pad-dry-cure method. Different sizes between 16.6 and 17 nm were obtained using high concentration and low concentration solution, respectively [71]. Simone Haslinger et al. (2019) reported a novel strategy that was attempted for the first time. Noble metal nanoparticles were added into the cellulose pulp by a hydrothermal approach and subsequently subjected to dry-wet spin process. They have functionalized cellulose-based textiles with Au and AgNPs. In this study, bleached birch prehydrolyzed kraft pulp acted as a reducing agent and HAuCl₄ as precursor salt to synthesize AuNPs. These nanoparticles were incorporated into the textile by dry-jet wet spinning process, which improved the UV protection and helped to achieve bright colors [72]. Some attempts to use the combination of both chemical and green methodologies to study the synergetic effect of both methodologies have been performed. Velmurugan et al. (2016) synthesized AuNPs using in situ synthesis method onto leather, silk, and cotton fabrics by three different methods that include green, chemical, and a combination of green and chemical synthesis. Ginkgo biloba Linn leaf powder extract, HAuCl₄, and potassium borohydride (KBH₄) were used in green and chemical synthesis. For the combination of green and chemical synthesis, Ginkgo biloba Linn leaf powder extract and KBH₄ were used, and the obtained nanoparticles were deposited by immersion of the fabrics in the solution. TEM observations had revealed nanoparticles in the range of 10–75 nm with either rectangular, spherical, hexagonal with smooth edges, or roughly circular in shape [73]. The use of biological methods showed several advantages, but more studies are needed to solve reproducibility issues, understand the influence of AuNPs attached groups in the assigned properties, and implement them at the commercial level.

Electrochemical synthesis is a simple and inexpensive method to obtain NPs but a few studies were found in the literature in the textile field. Mauro Pasta et al. (2012) have produced a conductive textile by coating single-walled carbon nanotubes on a polyester textile substrate. Subsequently, AuNPs were prepared using HAuCl₃ in the presence of HCl electrolyte. Later, AuNPs were electrodeposited onto the conductive textiles and the size of the nanoparticles was found with a mean diameter of 50 nm [76].

David R. Ballerini et al. (2014) have synthesized AuNPs by the Turkevich method using HAuCl₄ as precursor salt and sodium citrate tribasic dihydrate as a reducing agent. AuNPs of size of 20 and 60 nm were obtained and deposited on the cotton thread by soaking them individually in the solution of AuNPs. The cotton threads were treated with cationic polyacrylamide (CPAM) before the deposition of AuNPs [77]. Cristina Battesini Adamo et al. (2020) have synthesized spherical-shaped gold nanoparticles of size 20–40 nm using the Turkevich method. In this study, HAuCl₄ solution (30 wt%) and sodium citrate were used. The AuNPs were wicked on to the cotton thread by capillary action when threads were immersed in the solution [78]. Md. Tariqul Islam et al. (2016) have produced AuNPs with average size of 21.01 nm where HAuCl₄ was used as a precursor salt and sodium squarate in water was used as a reducing and stabilizing agent. The gold nanoparticles were synthesized in water and attached to the cellulose fibers by sonication [79]. Hyung Ju Park et al. (2016) have fabricated cotton yarns decorated with Au core-shell nanoparticle by a solution-based approach. The AuNPs were synthesized by a chemical reduction method where HAuCl₄ was used as a precursor salt solution and trisodium citrate dihydrate was used as a reducing agent and stabilizing agent. The solution was maintained at pH between 3 and 4, and the obtained nanoparticles were of the size 13 nm [80].

Some reports conducted with in situ methods have been reported, but in all studies, additional chemical reducing agents were used. Youvi Xia et al. (2011) have produced sulfonated polyaniline-modified silk fibroin fibers coated with AuNPs. AuNPs of size in the range of 50–100 nm were synthesized using HAuCl₄ via in situ reduction method where sulfonated polyaniline acted as reducing agent. They were deposited by soaking fibers in the solution that contains AuNPs [81]. Bin Tang et al. (2015) have synthesized AuNPs through in situ synthesis using HAuCl₄ with different concentrations and NaBH₄. The pH value of the solution was maintained such that it had acidic condition. They were deposited on to the ramie fibers by immersion of fibers in the solution [82]. Tariqul Islam et al. (2017) have used in situ synthesis where HAuCl₄ was used as precursor salt and sodium rhodizonate was used as a reducing and stabilizing agent to produce AuNPs. The AuNPs were found to have spherical shape and the size of 7 and 11 nm. The stabilized AuNPs were readily adsorbed on cellulose fibers [83]. Hossam E. Emam et al. (2017) have used one-pot fabrication of AgNPs onto a cellulosic solid support. AuNPs of 26.1 nm were synthesized through green synthesis using gold chloride (AuCl₃), NaBH₄ as reducing agent, and cellulosic macromolecules as stabilizing agent. They were deposited onto cellulose solid support by one-step method [84]. Yong Ju Yun et al. (2017) have fabricated gold/graphene yarns using a solution-based process to potentially use them for flexible and wearable electronics. HAuCl₄ was used as precursor salt and hydroxylamine was used as a reduced agent for the synthesis of AuNPs. They were deposited uniformly on the surface by electroless deposition and found to have from spherical shape to plate and grow vertically with time, the size of the AuNPs was 40 nm in diameter [85]. Qian Yu et al. (2018) have produced multifunctional cellulose fiber Au composites via decorating regenerated cellulose fiber with AuNPs. HAuCl₄ was used as a precursor salt and trisodium citrate was used as a reducing and stabilizing agent to produce gold nanoparticles following Natan’s method. The obtained spherical AuNPs with size of 40–50 nm were decorated on to the surface of cellulose fiber by the immersion of fibers in the solution of Au colloids. The fibers were grafted with positive charge according to the method proposed by Tabba and co-workers with some modifications [86].

Yan Liu et al. (2017) produced AuNPs with size of 15 nm using HAuCl₄ solution and trisodium citrate by chemical reduction method. AuNPs were coated on the CNTs, which were priorly functionalized with poly(diallyldimethylammonium chloride) (PDDA) using sonication method, and these were deposited on to the cotton thread by soaking cotton threads in the solution that contains AuNPs/CNTs. To improve the wicking function of the cotton thread, chemical treatments are necessary to eliminate the surface contaminants [87]. Xiaobo Jia et al. (2017) have synthesized AuNPs with size of 15 ± 3 nm using HAuCl₄ solution and trisodium citrate by chemical reduction method. These nanoparticles are mixed with CNT and subsequently, the cotton thread device was constructed for carcinoembryonic antigen (CEA) detection. It was constructed by soaking the cotton thread in the solution containing AuNPs-coated CNTs that were previously functionalized with PDAA [88].

Bin Tang et al. (2011) have synthesized AuNPs using HAuCl₄ as precursor salt and citrate, malate, and tartrate as reducing agents by heating and photochemistry processes. The solution was used to color silk and nylon fibers, and AuNPs were deposited using the exhaustion process. Nanoparticles were observed to have egg, spherical shape and various sizes depending on the type of reducing agent [75].

Naseeb Ullah et al. (2019) have used HAuCl₄ as a precursor salt and Osmanthus fragrans leaves as reducing and capping agent irradiated by natural sunlight to produce AuNPs. The cotton fibers were then immersed in the AuNPs solution, and they were deposited on the fabrics through in situ reduction in the presence of direct sunlight. Nanoparticles of size 40 and 60 nm in diameter were observed in TEM and appeared to have spherical and hexagonal shapes [89]. Victor Nolasco-Arizmendi et al. (2012) have synthesized AuNPs using HAuCl₄ as the base solution and citrus paradise extract as the reducing agent, and the AuNPs were deposited on the silk fabric by impregnating the fabric in the solution. Quasi-spherical-shaped nanoparticles were observed [90].

The two-step chemical reduction method, the AuNPs synthesis and posterior textiles functionalization, was widely applied. Wei Wang et al. (2010) have produced AuNPs–Bacterial cellulose (BC) nanocomposites. In this study, AuNPs were synthesized by the reduction of HAuCl₄ in the presence of NaBH₄ that acted as reducing agent, mixed with BC nanofibers. Produced AuNPs have measured approximately 9 nm. The AuNPs were deposited on fibers by immersing them in the solution of BC nanofibers and applying ultrasonic cell disruption system at room temperature. [93]. Hirotaka Koga et al. (2010) have synthesized AuNPs on crystalline cellulose single nanofibers (CSNFs). Preparation of AuNPs includes the addition of CSNFs in the solution of HAuCl₄ followed by reduction with NaBH₄. The nanoparticles of size less than 5 nm were deposited because of immersion of fibers in nanoparticles in solution [91]. Taiji Zhang et al. (2010) have obtained spherical-shaped AuNPs of approximately 9 nm by the reduction of HAuCl₄ in the presence of polyethyleneimine (PEI) that played the role of both reducing and stabilizing agent, and it also acted as a linking agent to uniformly coat the AuNPs on to the BC nanofibers. AuNPs were coated on to the BC nanofibers by suspending them in the solution [92]. Bin Zhou et al. (2014) have produced antibacterial multilayer films coated with gold nanoparticles on nanofibers. Gold nanoparticles were synthesized using chemical reduction method. HAuCl₄ solution and trisodium citrate were used as precursor salt, reducing and stabilizing agent for the synthesis of AuNPs, and they were coated on to the electrospun cellulose acetate fiber mats by immersion of mats into colloidal gold nanoparticles solution [94].

Hui-Hui Cheng et al. (2016) have fabricated thermoplastic polyurethane electrospun fiber mat and deposited the gold nanoparticles after the functionalization of the mat with chitosan by dip-coating method. The AuNPs of size 16 nm were produced using HAuCl₄, trisodium citrate as reducing agent, and chitosan as stabilizing agent [95]. Xinglong Yang et al. (2017) have produced antibiotic intermediate capped AuNPs by chemical reduction of HAuCl₄ by NaBH₄. The process was done in the presence of antibiotic intermediates. These nanoparticles of size approximately 3 nm have been embedded on to the polycaprolactone (PCL)/gelatin nanofibers in the form of scaffolds during electrospinning [96]. Ying Li et al. (2017) have produced a bacterial cellulose (BC) membrane decorated by AuNPs and modified with 4,6-diamino-2-pyrimidinethiol (DAPT) for treating bacterially infected wounds. In this research work, AuNPs of size 3 nm were produced via in situ synthesis method where HAuCl₄ and DAPT solution was used. The AuNPs were deposited on to the BC membranes by soaking them in the solution [97]. Xu Fang et al. (2010) have produced electrospun poly(ethyleneimine)/polyvinyl alcohol nanofibers and immobilized them with round-shaped gold nanoparticles of size 11.8 nm. In this study, the AuNPs were synthesized and deposited on to the electrospun fibers via in situ reduction of AuCl₄⁻ ions using NaBH₄ as reducing agent [98]. Zhen Liu et al. (2011) have produced electrospun porous polyacrylonitrile nanofibers and functionalized them with AuNPs. AuNPs were synthesized by in situ reduction of HAuCl₄ to Au (0) by 4-(dimethylamino) benzaldehyde. The electrospun membrane was immersed into the buffered hydrogen tetrachloroaurate (III) trihydrate solution and then, AuNPs of size 6 nm, were adsorbed onto the nanofibers [104]. Anitha Senthamizhan et al. (2014) have produced DTT capped AuNPs by in situ synthesis using the gold solution and dithiothreitol. AuNPs of the size 2.5 ± 0.5 nm were deposited on the cellulose acetate fibrous membranes by soaking them in the solution [105].

Xiaodong Chen et al. (2014) have functionalized zein ultrafine fibers by AuNPs. The zein ultrafine fibers were produced by electrospinning and, subsequently, AuNPs were added on to the fibers by one step reduction method (in situ polymerization) using HAuCl₄ as precursor salt and poly(ethyleneimine) (PEI) as reducing agent and cross-linking agent. The pH was maintained between 3 and 7, and the size of the AuNPs were observed to be 90.9 nm in diameter [106]. Han Zhu et al. (2014) have synthesized AuNPs embedded polyacrylonitrile nanofibers by the combination of in situ reaction and electrospinning. AuNPs were produced using the HAuCl₄ solution via in situ reduction method. The nanofibrous mats with AuNPs embedded in it were achieved by the electrospinning technique and spherical-shaped AuNPs with size of 2.3 ± 0.5 nm [107]. Diana Serbezeanu et al. (2015) have produced gold-containing polyimide fibers. AuNPs were synthesized using an in situ electrospinning approach where HAuCl₄.3H₂O was used as precursor salt and polyimide was used as stabilizing agent. The reduction takes place due to the thermal reactions and the SEM observations displayed nanoparticles of the size 9–22 nm [108]. Simón Yobanny Reyes-López et al. (2015) have proposed a facile method to produce gold-coated polycaprolactone nanofibers. Reduction method was employed to synthesize AuNPs, round-shaped 5–6 nm nanoparticles, using HAuCl₄ and NaBH₄ as a reducing agent. The gold nanoparticles were separated in a rotavapor, were added into the viscous solution of PCL, and subjected to electrospinning [99].

Tao Niu et al. (2014) have produced cellulose-based membranes and coated the membranes with AuNPs. In this research study, AuNPs were synthesized with KAuCl₄ and NaBH₄ and these nanoparticles were deposited on membranes previously coated with Titania gel [109]. Riyas Subair et al. (2016) synthesized gold nanoparticles on the PET track-etched micro-porous membranes. Before synthesis on nanoparticles, these membranes were coated with dopamine and poly(ethyleneimine). The authors followed two types of methodologies for the synthesis of AuNPs. AuNPs were synthesized by self-reduction of AuCl₄⁻ immobilized on the surface of dopamine membrane, and AuNPs were synthesized via in situ reduction with NaBH₄ on PEI membranes. In both the cases, AuNPs were deposited by immersion of the membranes into nanoparticles solution and maintaining continuous shaking condition [100]. Mohammed Awad Abedalwafa et al. (2019) have functionalized polyamide nanofiber membranes using AuNPs. The AuNPs were synthesized via citrate reduction technique where HAuCl₄ and trisodium citrate dihydrate were used as precursor salt and reducing agent, respectively. AuNPs were functionalized with melamine (MA) and were immobilized on polyamide 6 nanofiber electrospun membranes by dropping MA@AuNPs on the membranes. AuNPs and MA@AuNPs were found to have the size of 20.1 ± 1.7 and 28.4 ± 2.3 nm, respectively [101]. Yan Chen et al. (2016) have deposited AuNPs on bacterial cellulose nanofibers treated with 2,2,3,3-tetramethylpiperidine-l-oxyl (TEMPO)-oxidized (TOBCNS). AuNPs were produced by chemical reduction method using HAuCl₄ and NaBH₄, and simultaneously, they were deposited on to the TOBCNS by immersing nanofibers in the solution. It was observed from TEM that nanoparticles were spherical having size of 4.30 ± 0.97 nm [102].

Fadwa H. Anka et al. (2012) have produced polyacrylonitrile nanofibers using electrospinning, and they have been functionalized with AuNPs. The AuNPs were synthesized via in situ photoreduction. In this study, HAuCl₄ was the precursor salt and sodium alginate acted as reducing agent. UV light has been used during electrospinning. Spherical shape was observed with a size of 21.4 and 5.8 nm [111]. Koichi Sawada et al. (2013) have produced PAN (Polyacrylonitrile) nanofibrous fabrics and deposited AuNPs onto the fabrics. PAN fabrics were produced by electrospinning of PAN solution and HAuCl₄. AuNPs were formed on the fabrics by in situ gold formation using UV irradiation. The average size of the gold nanoparticles obtained was 4.7–5.4 nm [112].

Xiaodong Wu et al. (2014) have used one-pot and green synthesis to synthesize and deposit the gold nanoparticles on to cellulose nanocrystals. By means of hydrothermal reaction AuNPs were deposited on to the cellulose nanocrystals. In this work, they have used HAuCl₄ solution and cellulose nanocrystals as a both reducing and stabilizing agent to synthesize AuNPs with size of 30.5 ± 13.4 nm. These cellulose nanocrystals are derived from cotton fabrics. In the same study, unsupported AuNPs were synthesized, using hydrazine hydrate as reducing agent, with size of 18.3 ± 3.5 nm [103]. Xu Zhou et al. (2018) have synthesized AuNPs on BC using the photoinduction method. In this study, HAuCl₄ was used as precursor salt and xenon lamp was used as reducing agent. AuNPs were deposited by immersing the BC hydrogels in the HAuCl₄ solution [113].

Moeng G. Motitswe et al. (2020) have used the green technique for the synthesis of AuNPs where banana (Musa paradisiaca) was used as a reducing agent and HAuCl₄ was used as the precursor salt. The spherical-shaped 9 nm AuNPs were deposited on to the polyacrylonitrile nanofibers using electrospinning technology [2].

Maggalena Aflori et al. (2015) have incorporated gold nanoparticles into electrospun polyimide fibers. In this work, HAuCl₄ was used as gold precursor and treated thermally at 200 °C for 6 h to reduce Au³⁺ to Au⁰. The electrospinning technique was used to incorporate the AuNPs into polyimide fibers [18].

Amir Shahin Shamsabadi et al. (2019) have produced AuNPs by laser ablation in the solution of PAN in dimethyl formamide. After that, spherical-shaped AuNPs were mixed with the PAN solution to subject to electrospinning and produce PAN nanofibers incorporated with AuNPs [114].