Over the last few years, various organic supports have been utilised to stabilise metallic nanoparticles [7-9]. For instance, Dokoutchaev et al. reported the fabrication of polystyrene (PS) microspheres coated with Au, Pt and Pd nanoparticles [10]. Whereas, Jeon et al. synthesised hierarchically structured microspheres composed of a PS-b-PEO diblock copolymer and gold nanoparticles [11]. These hybrid systems possess weak mechanical properties and low catalytic activities and are often complex to produce [12].

Among all the polymers used as a support, a little attention has been devoted to polyamide 66 (PA66). This polymer, classified in the engineering semi-crystalline thermoplastic family, possesses excellent thermal, mechanical and chemical resistance properties [13]. Additionally, this type of polymer can easily form spherical nanostructures called spherulites via a crystallisation process from solution (Figure ​(Figure1)1) [14]. Compared to PS microspheres which need to be functionalised by an amine or carboxylic acid group to coordinate the noble metal nanoparticles, PA66 has its polymer chain functional amide group to host a metal complex [8,12]. These well-defined PA66 functionalized spherulites could support noble metal nanoparticles onto their surfaces in order to collect their intriguing physical properties.

Controlling the nanoparticle diameter is found to be the key factor for harnessing their catalytic properties. Indeed, gold and palladium in bulk state are chemically inert but become chemically active for many reactions at a nanoscale level. It has been found that decreasing the particle size to nanoscale decreases their redox potential to a negative value [15-19]. Due to their high specific surface area and their low redox potential, stabilised noble metal nanoparticles are usually used in the chemical field as an effective catalyst [5,12].

Herein, we propose a simple concept to coat PA66 spherulites with gold nanoparticles (Au NPs) prepared in situ via a wet chemical approach. This cost-effective method offers the advantages of controlling the particle size, the size distribution and also the organic-inorganic interactions [20].

As observed on the TEM micrograph, Au NPs synthesised in situ are located at the surface of the PA66 spherulites (Figure ​(Figure3).3). The size and size distribution of Au NPs, which are the key parameters in the production of an effective catalytic nanoparticle, have been determined from TEM micrographs (insert in Figure ​Figure3).3). The average particle diameter of Au NPs was estimated at around 13 nm (insert in Figure ​Figure3).3). The SEM and TEM results demonstrate that the PA66 spherulite can be used as an effective support to stabilise Au NPs.

The crystalline structure of gold was accessed via XRD spectrometry. The XRD pattern of the PA66/Au NP hybrid material is displayed in Figure ​Figure4.4. The peaks, distinguished at 2θ = 38.09°, 44.3° and 64.7°, are assigned to the (111), (222) and (220) lattice planes of gold in cubic structure [21]. The result demonstrates that the gold precursor is reduced to form crystallised Au NPs after adding NaBH₄ into the solution.

Based on the TEM and SEM results, a possible mechanism for the metallisation of PA66 spherulites with Au NPs is proposed in Figure ​Figure5.5. PA66 microspheres obtained by precipitation (Figure ​(Figure1)1) were re-dispersed in water medium. Adding the gold precursor into the PA66 solution leads to the acidification of the medium since the gold precursor dissociates in water to form hydrogen ions (H⁺) and gold complex ([AuCl₄]^-). Thus, the amide group, protonated at the spherulite surface by the hydrogen ions into the solution, can interact with the gold complex charged negatively [22] (Figure ​(Figure5).5). The metal coordination is the key factor to anchor Au NPs at the PA66 surfaces after the reduction process.

To validate the hypothesis that PA66 interacts with Au NPs, ATR-FTIR spectrometry measurements were conducted on PA66 and PA66/Au NP hybrid powders (Figure ​(Figure6).6). The characteristic peaks of PA66 and PA66 coated with Au NPs have been identified and listed in Table ​Table1.1. Characteristic vibration frequency peaks of PA66 are found at 3,298 (N-H stretching), 2,933 (CH₂ stretching), 1,631 (C = O stretching, amide I), 1,536 (N-H bending vibration) and 686 cm^-1 (N-H bending vibration) [23]. The presence of Au NPs at the surface of the PA66 spherulite did not change the vibration frequency of the carbonyl group of PA66 but slightly shifts the vibration frequency of the amine group (Table ​(Table1).1). This variation could indicate that Au NPs are interacting with the amine group of PA66 via physical bonds.

PA66 spherulites coated with Au NPs as a catalyst were demonstrated by investigating the reduction of MB to leuco MB (LMB) (Figure ​(Figure7)7) as a function of time by UV-Vis spectrometry in the wavelength range between 400 and 800 nm (Figure ​(Figure8)8) at room temperature. To understand the effect of the hybrid material on the reduction rate of MB, further investigations need to be conducted regarding the amount of the metallised PA66 microspheres and the temperature.

In chemistry, this substance is recognised as a redox indicator since it can easily change its colour in a specific environment [4,24]. Indeed, MB, initially blue in an oxidizing environment, undergoes a definite colour change by becoming colourless in the presence of a reducing agent such as sodium borohydride [25]. The MB reduction reaction, leading to the formation of LMB, is described in Figure ​Figure77[25].

In aqueous medium, MB exhibits a main absorption peak at 664 nm with a shoulder at 614 nm as shown in Figure ​Figure8.8. It has been reported that the main absorption peak at 664 nm corresponds to the n-π* transition of MB [26,27]. The reduction of MB as a function of time has been investigated in the presence of sodium borohydride. Relative absorbance of the peak at 664 nm is plotted as a function of time to evaluate the MB reduction reaction rate (Figure ​(Figure8b).8b). Incorporation of the reducing agent into the MB solution decreases slightly the absorbance intensity of the peak at 664 nm with the time (Figure ​(Figure8b,8b, see Additional file 2). This decreasing trend indicates that MB starts to reduce in the presence of NaBH₄; however, the reaction is slow. After 20 min, the PA66/Au NP hybrid system was incorporated into the MB/NaBH₄ solution. Interestingly, a strong decrease of the UV-Vis absorbance intensity of MB is observed in the presence of the hybrid material (Figure 8a, b). Additionally, the plot of the relative absorbance of the peak at 664 nm reveals that the complete reduction of MB to LMB is accomplished in less than 20 min in the presence of the hybrid materials since the curve tends to stabilise at the end (Figure ​(Figure8b).8b). This result confirms that PA66/Au NP hybrid microspheres act as an effective catalyst in the reduction of MB.

The catalytic ability of the coated PA66 spherulites depends on the size of Au NPs produced. Indeed, gold in bulk state is chemically inert since the redox potential of this noble metal is positive [15]. It has been reported by Haruta et al. that gold is becoming catalytically active for many chemicals at a nanoscale level (diameter below 10 nm) due to the reduction of its redox potential to a negative value [19,28,29]. Thus, to act as an effective catalyst, the redox potential of Au NPs needs to be found between the redox potential of the donor and the acceptor system [17,30]. In this case, noble metal nanoparticles are considered as an electron relay in the redox reaction to transfer the electron from the donor (B₂H₄/BH₄^-) to the acceptor system (LMB/MB) since Au NPs act as both donor and acceptor of electrons [17] (Figure ​(Figure9).9). Experimental results demonstrate that PA66 metallised with Au NPs accelerates the reduction of MB because Au NPs act as an electron relay in the MB reduction reaction. Based on this observation, it is possible to deduce that the redox potential of the Au NPs produced in this investigation is located between the redox potential of MB (E°(MB/LMB) = -1.33 V) and that of sodium borohydride (E°(B₂H₄/BH₄^-) = -0.21 V) [30] (Figure ​(Figure99).