Plant Extracts Activated by Cold Atmospheric Pressure Plasmas as Suitable Tools for Synthesis of Gold Nanostructures with Catalytic Uses

Because cold atmospheric pressure plasma (CAPP)-based technologies are very useful tools in nanomaterials synthesis, in this work we have connected two unique in their classes approaches—a CAPP-based protocol and a green synthesis method in order to obtain stable-in-time gold nanoparticles (AuNPs). To do so, we have used an aqueous Gingko biloba leave extract and an aqueous Panax ginseng root extract (untreated or treated by CAPP) to produce AuNPs, suitable for catalytical uses. Firstly, we have adjusted the optical properties of resulted AuNPs, applying UV/Vis absorption spectrophotometry (UV/Vis). To reveal the morphology of Au nanostructures, transmission electron microscopy (TEM) in addition to energy dispersive X-ray scattering (EDX) and selected area X-ray diffraction (SAED) was utilized. Moreover, optical emission spectrometry (OES) in addition to a colorimetric method was used to identify and determine the concentration of selected RONS occurring at the liquid-CAPP interface. Additionally, attenuated total reflectance Fourier transform-infrared spectroscopy (ATR FT-IR) was applied to reveal the active compounds, which might be responsible for the AuNPs surface functionalization and stabilization. Within the performed research it was found that the smallest in size AuNPs were synthesized using the aqueous P. ginseng root extract, which was activated by direct current atmospheric pressure glow discharge (dc-APGD), generated in contact with a flowing liquid cathode (FLC). On the contrary, taking into account the aqueous G. biloba leave extract, the smallest in size AuNPs were synthesized when the untreated by CAPP aqueous G. biloba leave extract was involved in the Au nanostructures synthesis. For catalytical studies we have chosen AuNPs produced using the aqueous P. ginseng root extract activated by FLC-dc-APGD as well as AuNPs synthesized using the aqueous G. biloba leave extract also activated by FLC-dc-APGD. Those NPs were successfully used as homogenous catalysts for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).


Introduction
Currently, a significant increase in the interest of nanomaterials, especially noble-metal nanostructures among others is noted. These metallic nanoparticles (NPs) are widely applied in different technologies due to a higher number of atoms on their surface compared to their macroscopic forms. Because of a high stability as well as an ability to absorb visible light caused by the plasmonic The issue involving the purification process of nanostructures after their production has been resolved in our research group by using the CAPP-based synthesis. In this case, the reduction of the Au (III) ions to their metallic Au (0) form in nanometric scale was caused by the reactive oxygen and nitrogen species (RONS) generated during the non-thermal plasma operation [41][42][43][44]. Because RONS exhibit excellent oxidation-reduction properties, the AuNPs precursor solution may consist only of Au (III) ions [41]. However, having in mind a potential use of AuNPs as homogenous catalysts, capping agents should be introduced into the system anyway to provide their long-term stability and aqueous plant extracts seem to be perfect for this purpose. To connect these two approaches, that is, the green synthesis method and CAPP-based method for the NPs fabrication, we previously examined how the proper CAPP treatment influences on the granulometric properties of AuNPs obtained by using M. officinalis, M. piperita or S. hispanica aqueous plant extracts [24]. Unfortunately, we observed the increase in the size of the nanostructures treated by CAPP [24].
The main aim of this work was to develop an innovative and low-cost method, which connects the green synthesis along with the CAPP-based approach for the production of AuNPs with catalytic properties. To reach this goal, we compared the optical and granulometric properties of AuNPs produced using untreated aqueous plant extracts with these activated with proper CAPP and revealed how the proper non-thermal plasma treatment affected the Au nanostructures properties. To the best of our knowledge this is the first work in which CAPP was used to activate aqueous G. biloba and P. ginseng plant extracts for their further application in the synthesis of the Au nanostructures that are dedicated to the catalytic use in the form of a homogenous catalyst. The resulted Au nanostructures were characterized using UV/Vis absorption spectrophotometry (UV/Vis) as well as transmission electron microscopy (TEM) supported by energy dispersive X-ray scattering (EDX) and selected area electron diffraction (SAED). Additionally, the identification of selected RONS present at the liquid-CAPP interface was performed using optical emission spectrometry (OES). Nevertheless, the concentration of nitrite in the untreated as well as CAPP-treated aqueous plant extracts was determined using a colorimetric method. To identify the reactive compounds, which might be responsible for the AuNPs surface functionalization, the attenuated total reflectance Fourier transform-infrared spectroscopy (ATR FT-IR) was used. Finally, the selected stable-in-time suspensions of AuNPs were used as homogenous nanocatalysts in the model reaction of the catalytic decomposition of 4-NP to 4-AP.

Reagents and Solutions
G. biloba leaves powder and P. ginseng root powder were purchased from a local Polish company. A tetrachloroauric acid tetrahydrate (HAuCl 4 × 4H 2 O) was bought in Pol-Aura (Olsztyn, Poland). 4-nitrophenol, as well as sodium borohydrite (NaBH 4 ) powders (>99%), were acquired from Sigma-Aldrich Co. (Steinheim, Germany). All chemical reagents were of an analytical grade purity. De-ionized water was used in all experiments.

Plant Extract Preparation
To prepare 2% (m/m) aqueous plant extracts, 2 g of the dry powders of G. biloba leaves or P. ginseng root were mixed with 98 g of de-ionized water and heated to boil. Then, the mixtures were kept boiling for 10 min. Afterwards, they were filtered using 115 qualitative hard filter papers (Chemland, Stargard Szczecinski, Poland). The resulted 2% (m/m) aqueous plant extracts were stored at 4 • C in the dark for further uses.

Plant Extracts Activation by Cold Atmospheric Pressure Plasmas
To activate aqueous plant extracts, the direct current atmospheric pressure glow discharge (dc-APGD) was applied as a CAPP source. dc-APGD was operated in a high-throughput continuous reaction-discharge system, working in two different modes ( Figure 1) [45]. In the first mode, that is, Nanomaterials 2020, 10, 1088 4 of 17 FLA-dc-APGD, dc-APGD was generated in a gap between the surface of a flowing liquid anode (FLA), being an aqueous plant extract solution and a sharpened tungsten cathode. In the second mode, that is, FLC-dc-APGD, dc-APGD was generated in a gap between the surface of a flowing liquid cathode (FLC), being an aqueous plant extract solution and a sharpened tungsten anode. The gap between the FLA (or FLC) and the metallic electrode was ≈ 4.0 mm. In both cases, the aqueous plant extract solutions were delivered to a quartz chamber, applying a four channel peristaltic pump (Masterflex L/S, Cole-Parmer, Vernon Hills, Il, USA), through a graphite/quartz tube (OD = 6 mm). The flow rate of the introduced aqueous plant extracts to the reaction-discharge system was 4.0 mL min −1 . To provide a HV potential to an electrical circuit, a high voltage generator (Dora Electronics Equipment, Wroclaw, Poland) was used. The discharge current of 50 mA was maintained by applying a 10 kΩ ballast resistor (Tyco Electronics, Berwyn, IL, USA). The voltage contact was provided by a Pt wire attached to the graphite/quartz tube. After applying the FLA-dc-APGD or FLC-dc-APGD treatment of the aqueous plant extracts, they were kept for further analyses or immediately mixed with an AuNPs precursor solution in order to produce Au nanostructures. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 16 (FLA), being an aqueous plant extract solution and a sharpened tungsten cathode. In the second mode, that is, FLC-dc-APGD, dc-APGD was generated in a gap between the surface of a flowing liquid cathode (FLC), being an aqueous plant extract solution and a sharpened tungsten anode. The gap between the FLA (or FLC) and the metallic electrode was ≈ 4.0 mm. In both cases, the aqueous plant extract solutions were delivered to a quartz chamber, applying a four channel peristaltic pump (Masterflex L/S, Cole-Parmer, Vernon Hills, Il, USA), through a graphite/quartz tube (OD = 6 mm). The flow rate of the introduced aqueous plant extracts to the reaction-discharge system was 4.0 mL min −1 . To provide a HV potential to an electrical circuit, a high voltage generator (Dora Electronics Equipment, Wroclaw, Poland) was used. The discharge current of 50 mA was maintained by applying a 10 kΩ ballast resistor (Tyco Electronics, Berwyn, IL, USA). The voltage contact was provided by a Pt wire attached to the graphite/quartz tube. After applying the FLA-dc-APGD or FLCdc-APGD treatment of the aqueous plant extracts, they were kept for further analyses or immediately mixed with an AuNPs precursor solution in order to produce Au nanostructures.

Synthesis of Gold Nanostructures Using Untreated as Well as CAPP-Treated Aqueous Plant Extracts
A stock solution (1000 μg mL −1 ) of the Au (III) ions was applied as an Au nanostructures precursor. In order to produce AuNPs using untreated as well as CAPP-activated aqueous plant extracts, a defined volume of the stock solution was immediately mixed with 2% (m/m) aqueous G. biloba or P. ginseng plant extracts. As a result, the respective reaction mixtures were obtained, in which the final concentration of the plant extract was 1% (m/m) and the concentration of the Au (III) ions was 125, 250 or 500 μg mL −1 . The produced AuNPs were kept at 4 °C for further characterization and applications.

Characterization of the Optical and Granulometric Properties of Gold Nanostructures
The optical properties of synthesized AuNPs, according to the occurrence of the λmax of the localized surface plasmon resonance (LSPR) absorption band and its absorbance, were determined using UV/Vis absorption spectrophotometry. A Specord 210 Plus (Analytik Jena, Jena AG, Germany) was used to acquire UV/Vis absorption spectra of 25-diluted AuNPs colloidal suspensions. The UV/Vis spectra were recorded in the range from 400 to 700 nm, with a step of 0.2 nm. As a background, de-ionized water was used.

Synthesis of Gold Nanostructures Using Untreated as Well as CAPP-Treated Aqueous Plant Extracts
A stock solution (1000 µg mL −1 ) of the Au (III) ions was applied as an Au nanostructures precursor. In order to produce AuNPs using untreated as well as CAPP-activated aqueous plant extracts, a defined volume of the stock solution was immediately mixed with 2% (m/m) aqueous G. biloba or P. ginseng plant extracts. As a result, the respective reaction mixtures were obtained, in which the final concentration of the plant extract was 1% (m/m) and the concentration of the Au (III) ions was 125, 250 or 500 µg mL −1 . The produced AuNPs were kept at 4 • C for further characterization and applications.

Characterization of the Optical and Granulometric Properties of Gold Nanostructures
The optical properties of synthesized AuNPs, according to the occurrence of the λ max of the localized surface plasmon resonance (LSPR) absorption band and its absorbance, were determined using UV/Vis absorption spectrophotometry. A Specord 210 Plus (Analytik Jena, Jena AG, Germany) was used to acquire UV/Vis absorption spectra of 25-diluted AuNPs colloidal suspensions. The UV/Vis Nanomaterials 2020, 10, 1088 5 of 17 spectra were recorded in the range from 400 to 700 nm, with a step of 0.2 nm. As a background, de-ionized water was used.
The morphology of produced AuNPs in relevance to their size, shape, crystalline structure and elemental composition was assessed using a Tecnai G 2 20 X-TWIN TEM (FEI, Hillsboro, OR, USA) supported by an EDX X-ray microanalyzer and a SAED analyzer (FEI, Hillsboro, OR, USA). To perform TEM, EDX and SAED analyses, a 100 µL of a proper 25-diluted AuNPs colloidal suspension was placed onto a Cu grid (CF400-Cu-UL, GF MICROSYSTEMS, Poznan, Poland) and let to dry. The OES was used to identify the RONS in the untreated as well as FLA-dc-APGD or FLC-dc-APGD-treated aqueous G. biloba leave or P. ginseng root extracts. The OES spectra were acquired in the range of 200 to 900 nm using a Shamrock SR-500i (Andor, UK) spectrometer, equipped with a Newton DU-920P-OE CCD camera. The radiation emitted by CAPP was collimated by an achromatic lens on the entrance slit (10 µm) of the spectrometer. The integration time of the CCD camera was set to 0.10 s.
The concentration of nitrite ions in the analyzed reaction mixtures, containing AuNPs produced using untreated as well as FLA-dc-APGD or FLC-dc-APGD-treated aqueous G. biloba or P. ginseng extracts, was determined applying a commercial colorimetric kit assay (HANNA Instruments, Olsztyn, Poland), employing for that purpose HANNA HI 96708 spectrophotometer (HANNA Instruments, Olsztyn, Poland). All measurements were carried out according to the protocol suggested by the manufacturer (HANNA Instruments, Olsztyn, Poland).

Gold Nanoparticles Stabilization by Active Compounds Originating from Plant Extracts
The presence of active compounds was detected by ATR FT-IR spectroscopy. A Jasco FT/IR-4700 device (Tokyo, Japan), equipped with a diamond ATR attachment (Tokyo, Japan) was used. ATR FT-IR spectra were recorded in the range of 4000-400 cm −1 with 64 scans and resolution of 4 cm −1 . To carry out ATR FT-IR analyses, one drop of analyzed solutions was placed onto a diamond cell and evaporated under vacuum.

Homogenous Catalysis
Selected samples of AuNPs were used as homogenous nanocatalysts in a model reaction of the reduction of 4-NP to 4-aminophenol (4-AP). The reaction was carried out at 20 • C in a quartz cuvette, where 3 mL of a 0.1 mmol L −1 4-NP solution was introduced. Then, 0.3 mL of a 0.1 mol L −1 NaBH 4 solution was added. Afterwards, 0.3 mL of the analyzed AuNPs colloidal suspension was added to the reaction mixture. The catalytic reaction was monitored using UV/Vis absorption spectrophotometry for each stage of the reaction mixture preparation and each 1 min after the nanocatalyst was introduced until the 4-NP was entirely decomposed. The course of the catalytic reaction was monitored using a Jasco V730 UV/Vis absorption spectrophotometer (Tokyo, Japan); suitable spectra were recorded in the range from 200 to 700 nm.
The catalytic activity of selected AuNPs was estimated by defining a maximum absorbance (λ max ) of 4-NP over a time. Because a decrease of λ max was proportional to the concentration of 4-NP, the constant rate (k) was calculated from the slope of the ln (A t /A 0 ) graph versus the time t (in s), where A t is the absorbance and A 0 is the absorbance at the beginning of the process. Then, the k value, expressed in s −1 , was re-calculated to the mass normalized rate constant k m (in s −1 mg −1 ).

Optical Properties of Gold Nanostructures
UV/Vis absorption spectra were acquired 60 min after the AuNPs synthesis. Based on the position of the LSPR absorption band in the UV/Vis spectrum, it was possible to assess the optical properties of AuNPs in addition to a preliminary estimation of their size. Typically, for spherical AuNPs, the LSPR absorption band occurs within 520 to 550 nm [45].
As can be seen from Figure 2 and Table 1, for most of resulted AuNPs the LSPR absorption band shifted towards longer wavelengths with the increasing of concentration of the Au (III) ions in the reaction mixture. It was related to the described in 1908 Mie's light scatter theory [46]. Based on this, it was concluded that the smallest AuNPs were produced using the reaction mixture consisting of the untreated G. biloba aqueous leaves extract and 125 mg L −1 of the Au (III) ions. Furthermore, the biggest AuNPs were obtained when the aqueous G. biloba leaves extract was activated by FLC-dc-APGD and when the final concentration of the Au (III) ions was 500 mg L −1 in the reaction mixture. Taking into account the application of the aqueous P. ginseng root extract in the AuNPs synthesis, the smallest AuNPs were produced using the reaction mixture consisting of the aqueous P. ginseng root extract treated by FLC-dc-APGD and 125 mg L −1 of the Au (III) ions. Additionally, the biggest Au nanostructures were produced when the aqueous P. ginseng root extract was activated by FLA-dc-APGD and when the final concentration of the Au (III) ions was 250 mg L −1 in the reaction mixture. These observations might be related to the characteristics of the applied discharges. When the aqueous plant extract was activated by FLC-dc-APGD, the surface of the FLC was irradiated by the positive ions from the plasma anode [45,47,48]. On the contrary, while the aqueous plant extract was activated by FLA-dc-APGD, the surface of the FLA was irradiated by the electrons from the plasma cathode [45,47,48]. For that reason, the polarity of electrodes used in those systems might have an impact on the type and concentration of the generated RONS. Moreover, these observations might be associated with the influence of the active compounds present in the applied aqueous plant extracts on the position of the LSPR absorption band in the analyzed UV/Vis absorption spectra [32,36]. In addition, for both types of plant extracts activated either by FLC-dc-APGD or FLA-dc-APGD, the increase in the absorbance values was observed. As was suggested by Badi'ah et al. [49], this increase was likely related to the growth of NPs and connected to the enhanced efficiency of the AuNPs synthesis as compared to the untreated aqueous plant extract.
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 16 As can be seen from Figure 2 and Table 1, for most of resulted AuNPs the LSPR absorption band shifted towards longer wavelengths with the increasing of concentration of the Au (III) ions in the reaction mixture. It was related to the described in 1908 Mie's light scatter theory [46]. Based on this, it was concluded that the smallest AuNPs were produced using the reaction mixture consisting of the untreated G. biloba aqueous leaves extract and 125 mg L −1 of the Au (III) ions. Furthermore, the biggest AuNPs were obtained when the aqueous G. biloba leaves extract was activated by FLC-dc-APGD and when the final concentration of the Au (III) ions was 500 mg L −1 in the reaction mixture. Taking into account the application of the aqueous P. ginseng root extract in the AuNPs synthesis, the smallest AuNPs were produced using the reaction mixture consisting of the aqueous P. ginseng root extract treated by FLC-dc-APGD and 125 mg L −1 of the Au (III) ions. Additionally, the biggest Au nanostructures were produced when the aqueous P. ginseng root extract was activated by FLA-dc-APGD and when the final concentration of the Au (III) ions was 250 mg L −1 in the reaction mixture. These observations might be related to the characteristics of the applied discharges. When the aqueous plant extract was activated by FLC-dc-APGD, the surface of the FLC was irradiated by the positive ions from the plasma anode [45,47,48]. On the contrary, while the aqueous plant extract was activated by FLA-dc-APGD, the surface of the FLA was irradiated by the electrons from the plasma cathode [45,47,48]. For that reason, the polarity of electrodes used in those systems might have an impact on the type and concentration of the generated RONS. Moreover, these observations might be associated with the influence of the active compounds present in the applied aqueous plant extracts on the position of the LSPR absorption band in the analyzed UV/Vis absorption spectra [32,36]. In addition, for both types of plant extracts activated either by FLC-dc-APGD or FLA-dc-APGD, the increase in the absorbance values was observed. As was suggested by Badi'ah et al. [49], this increase was likely related to the growth of NPs and connected to the enhanced efficiency of the AuNPs synthesis as compared to the untreated aqueous plant extract.

Type of Plant
Au(III)

Granulometric Properties of Gold Nanostructures
To reveal the structural properties of AuNPs obtained using untreated as well as activated by FLA-dc-APGD or FLC-dc-APGD aqueous plant extracts, several experimental techniques including TEM, EDX and SAED were used.  Table 2. The average size of AuNPs was calculated based on the 300-700 NPs, depending of the well-observed grain boundaries of analyzed nanostructures.
As can be seen from Figures 3 and 4, the application of untreated aqueous plant extracts resulted in the fabrication of mainly spherical AuNPs. A similar effect was observed in the case of the application of plant extracts activated using FLC-dc-APGD. The utilization of FLA-dc-APGD seemed to result in decreasing the content of spherical AuNPs (see Table 2). Accordingly, a greater amount of triangular and rod-like Au nanostructures was observed. These differences were likely linked with the nature of the CAPP source, as explained by the LSPR bands shifts described above (see Section 3.1 for more details). In the case of the aqueous P. ginseng extract the smallest in size AuNPs were produced after the plant extract was activated using FLC-dc-APGD (14.2 ± 3.8 nm), while considerably bigger AuNPs (21.3 ± 12.3 nm) were observed in the case of the plant extract activation by FLA-dc-APGD (see Table 2). The effect was opposite in the case of the aqueous G. biloba extract because its FLA-dc-APGD activation led to the formation of smaller AuNPs as compared to the FLC-dc-APGD (24.4 ± 4.4 vs. 29.0 ± 3.5 nm). However, considering the standard deviation values (STD) (see Table 2), these sizes could be considered as very close to each other, hence, it was concluded that the average diameter of AuNPs was depended on either the CAPP source or/and the characteristics of the plant extract. This observation was supported by the fact that the plant extract activation by non-thermal plasma slightly increases the average size of resultant AuNPs (Table 2), as compared to the nanostructures fabricated using the untreated aqueous plant extracts. Hence, it was expected that the plant extract activation either by FLC-dc-APGD or by FLA-dc-APGD led to increase the concentration of RONS in the plant extracts, hence, was responsible for their increased reducing potential. are present in the leaves of G. biloba in high amounts. Based on the EDX patterns, it might be concluded that the green synthesis of biogenic AuNPs was successful in all the described cases. Taking into account the crystalline structure of the produced AuNPs based on the SAED diffractograms, the following d-spacing was noted for all fabricated biogenic AuNPs: 1.26, 1.52, 2.15 and 2.44 Å. This was consistent with the literature data [51,52], proving that the detected nanostructures were indeed AuNPs of the face-centered cubic (fcc) structure.  concluded that the green synthesis of biogenic AuNPs was successful in all the described cases. Taking into account the crystalline structure of the produced AuNPs based on the SAED diffractograms, the following d-spacing was noted for all fabricated biogenic AuNPs: 1.26, 1.52, 2.15 and 2.44 Å. This was consistent with the literature data [51,52], proving that the detected nanostructures were indeed AuNPs of the face-centered cubic (fcc) structure.   Table 2. The summary of the shape and size distribution determined for AuNPs synthesized using untreated, FLA-dc-APGD-activated or FLC-dc-APGD-activated aqueous plant extracts. The concentration of the Au (III) ions used in the reaction mixture was 500 mg L −1 .

Shape Distribution (%)
Diameter ( In order to reveal the elemental composition as well as the crystalline structure of the so-obtained Au nanostructures, the EDX and SAED analyses were carried out. In all the acquired EDX spectra (Figures 3 and 4), the presence of Au (from AuNPs), Cu (from the Cu grid), O and C (from organic compounds present in the plant material extracts) and Cl (from the nanostructures precursor) were detected. Additionally, in the case of the AuNPs produced using the aqueous G. biloba leaves extract, the presence of K and Mg was also identified. As was suggested by Koczka et al. [50], these elements are present in the leaves of G. biloba in high amounts. Based on the EDX patterns, it might be concluded that the green synthesis of biogenic AuNPs was successful in all the described cases. Taking into account the crystalline structure of the produced AuNPs based on the SAED diffractograms, the following d-spacing was noted for all fabricated biogenic AuNPs: 1.26, 1.52, 2.15 and 2.44 Å. This was consistent with the literature data [51,52], proving that the detected nanostructures were indeed AuNPs of the face-centered cubic (fcc) structure.

Nature of Plant Extracts Activated by Proper CAPP Source
To identify the RONS responsible for the aqueous plant extracts activation the OES was used. A typical emission spectra of FLA-dc-APGD and FLC-dc-APGD systems, recorded during the activation of both aqueous plant extracts, are presented in Figure 5. As can been seen, the following molecular species NO, OH, N 2 , N 2 + and NH were identified in the spectra. In the 200-260 nm spectral region, several bands of the γ-system (A 2 Σ + -X 2 Π) of the NO molecule, that is, (2-0) at 204.7 nm, (1-0) at 214.9 nm, (2-2) at 221.6 nm, (0-0) at 226.3 nm, (0-1) at 236.3 nm, (0-2) at 247.1 nm and (0-3) at 258.7 nm, were identified. This molecule was produced as a result of the chemical reaction between N 2 coming from the air atmosphere and O radicals, which takes place as follows: N 2 + O = NO + N. In addition, two strong bands of the OH molecule with the band heads (1-0) at 282.9 and (0-0) at 308.9 nm, belonging to the A 2 Σ-X 2 Π system, were observed for both FLA-dc-APGD and FLC-dc-APGD sources. In opposite to NO, the highly oxidative OH radicals were generated as a result of the water (or its ion) dissociation and/or recombination processes, which occur as follows At high concentrations of the Au (III) ions, their fast reduction on the surface of AuNPs was likely established, according to the Finke-Watzky two-step mechanism [54]. This would make the synthesized AuNPs larger in size. The produced ions as a result of plasma liquid interaction may also electrostatically stabilize the AuNPs due to the occurrence of the defined charge onto their surface, as was previously suggested by Patel and co-workers [55].

Gold Nanoparticles Stabilization by Active Compounds Originating from Plant Extracts
In order to evaluate the interactions between the surface of the synthesized Au nanostructures and active compounds, originating from the applied aqueous plant extracts, ATR FT-IR spectroscopy was used. As can be seen from Figure 6 and Table 3, Amide A and Amide I bands were identified in the ATR FT-IR spectra of the untreated aqueous plant extracts. These bands occurred at 3243, 3254 and 1635 cm −1 for the Amide A, Amide I and Amide A bands, respectively. Additionally, these bands were also associated with the presence of N-H and O-H (Amide A) and C=O (Amide I) functional groups. Based on this, it was suspected that these functional groups originated from active compounds present in the prepared aqueous plant extracts, including ginkgolic acids, terpenes trilactiones, flavonoids and proanthocyanidins [32,36]. Interesting, these bands did not disappeared when the aqueous plant extracts were activated by FLC-dc-APGD or FLA-dc-APGD, suggesting that the process did not entirely decomposed the above-mentioned capping agents. However, all the bands in quest slightly shifted as a result of the AuNPs synthesis. This was primarily visible in the In order to determine the concentration of the NO molecule in the form of the NO 2 − ions, the colorimetric analyses were performed. It was found that after the impact of FLA-dc-APGD and FLC-dc-APGD on the aqueous plant extracts, the concentration of NO 2 changed as compared to the untreated aqueous plant extracts (data not shown). In both cases, the concentration of NO 2 − in the FLA-dc-APGD and FLC-dc-APGD treated plant extracts slightly increased as compared to this in the untreated plant extracts. It was noticed that a variety of RONS as well as H radicals were generated in the aqueous plant extracts activated by FLA-dc-APGD or FLC-dc-APGD, as a result of the plasma-liquid interactions [53]. As was suggested by others, the presence of OH, NO and H radicals, hydrogen peroxide (H 2 O 2 ), nitrate (NO 3 − ) and nitrite (NO 2 − ) ions might be responsible for a partial decomposition of the active organic species occurs in mentioned aqueous plant extracts [41,53]. These species along with long-lived active species could participate in the AuNPs nucleation as well as in their growth in the prepared mixtures. Namely, at low concentrations of the Au (III) ions, H 2 O 2 molecules likely enhanced their reduction to Au 0 as a result of the following reaction: 2AuCl 4 − + 3H 2 O 2 = Au 0 + 3O 2 + 6H + + 8Cl − , leading to the formation of a relatively high number of the nucleation seeds. At high concentrations of the Au (III) ions, their fast reduction on the surface of AuNPs was likely established, according to the Finke-Watzky two-step mechanism [54]. This would make the synthesized AuNPs larger in size. The produced ions as a result of plasma liquid interaction may also electrostatically stabilize the AuNPs due to the occurrence of the defined charge onto their surface, as was previously suggested by Patel and co-workers [55].

Gold Nanoparticles Stabilization by Active Compounds Originating from Plant Extracts
In order to evaluate the interactions between the surface of the synthesized Au nanostructures and active compounds, originating from the applied aqueous plant extracts, ATR FT-IR spectroscopy was used. As can be seen from Figure 6 and Table 3, Amide A and Amide I bands were identified in the ATR FT-IR spectra of the untreated aqueous plant extracts. These bands occurred at 3243, 3254 and 1635 cm −1 for the Amide A, Amide I and Amide A bands, respectively. Additionally, these bands were also associated with the presence of N-H and O-H (Amide A) and C=O (Amide I) functional groups. Based on this, it was suspected that these functional groups originated from active compounds present in the prepared aqueous plant extracts, including ginkgolic acids, terpenes trilactiones, flavonoids and proanthocyanidins [32,36]. Interesting, these bands did not disappeared when the aqueous plant extracts were activated by FLC-dc-APGD or FLA-dc-APGD, suggesting that the process did not entirely decomposed the above-mentioned capping agents. However, all the bands in quest slightly shifted as a result of the AuNPs synthesis. This was primarily visible in the case of the Amide A bands, observed for the aqueous G. biloba leaves extract activated by both CAPPs. In this case, the above-mentioned bands were shifted to 3245, 3246 and 3245 cm −1 after the AuNPs synthesis (Table 3). This suggested that the prepared aqueous plant extracts participated in the formation of AuNPs as well as their further capping and stabilization [17].
Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 16 case of the Amide A bands, observed for the aqueous G. biloba leaves extract activated by both CAPPs. In this case, the above-mentioned bands were shifted to 3245, 3246 and 3245 cm −1 after the AuNPs synthesis (Table 3). This suggested that the prepared aqueous plant extracts participated in the formation of AuNPs as well as their further capping and stabilization [17].

Catalytic Activity
The presented green approach was aimed to produce stable-in-time AuNPs that granulometric properties would be controlled by activating the aqueous plant extract with a proper CAPP source, that is, FLA-dc-APGD or FLC-dc-APGD. Because the stability of the resultant Au nanostructures is very important in the processes of the homogenous catalysis, all the samples containing obtained AuNPs were kept at 4 °C for 14 days prior to the tests on their catalytic activity. After this time, it was

Catalytic Activity
The presented green approach was aimed to produce stable-in-time AuNPs that granulometric properties would be controlled by activating the aqueous plant extract with a proper CAPP source, that is, FLA-dc-APGD or FLC-dc-APGD. Because the stability of the resultant Au nanostructures is very important in the processes of the homogenous catalysis, all the samples containing obtained AuNPs were kept at 4 • C for 14 days prior to the tests on their catalytic activity. After this time, it was determined that neither of the samples tends to aggregate. Additionally, because the application of the FLC-dc-APGD-activated plant extracts tended to decrease the average diameter of the AuNPs in the case of the aqueous P. ginseng root extract, only the samples containing AuNPs produced with the aid of the FLC-dc-APGD-activated aqueous of G. biloba and P. ginseng extracts were used for the tests on their catalytic activity.
The catalytic reactions were carried out under the supervision of UV/Vis absorption spectrophotometry (Figure 7). The bands at 318 nm was attributed to the presence of 4-NP. After the addition of a NaBH 4 solution, this band shifted to 400 nm, which was attributed to the reduction of 4-NP and the presence of the 4-nitrophenolate anion. It is well known this reaction requires a catalyst, otherwise, the system remains unchanged [56]. After the addition of AuNPs fabricated using a 500 mg L −1 Au(III) solution and both aqueous plant extracts activated by FLC-dc-APGD, the band at 400 nm immediately started to fade (following spectra, Figure 7A,B). Due to the lack of any diffusion limitations (homogenous catalysis), the reaction mixtures responded so rapidly that in case of the application of the aqueous P. ginseng root extract with AuNPs it was impossible to record UV/Vis absorption spectrum fast enough before almost half of 4-nitrophenolate anion decomposed. At the same time, another band near 295 nm appeared. This was linked with the formation of 4-aminophenol, indicating the successfulness of the process. Based on the decrease of the absorbance of the band at 400 nm, it was possible to calculate the constant rate (k). I was 0.31 and 0.52 s −1 for the CAPP-activated aqueous G. biloba leave extract and the CAPP-activated aqueous P. ginseng root extract, respectively. As a result, the complete reduction of 4-NP was achieved within 8 and 6 min, respectively.  Based on the initial concentration of the Au (III) in the samples (500 mg L −1 ) and assuming their entire reduction to AuNPs, it was possible to recalculate the k values into the mass-normalized constant rates km, being 2.04 and 3.50 s −1 mg −1 in case of AuNPs obtained using the CAPP-activated aqueous G. biloba leaves and P. ginseng root extracts, respectively. This enabled to compare the present results for homogenous nanocatalysts with others, being reported in the literature. The suitable summary, setting the proposed approach towards synthesis of homogenous nanocatalysts with comparison of works of other researchers are displayed in Table 4.  Based on the initial concentration of the Au (III) in the samples (500 mg L −1 ) and assuming their entire reduction to AuNPs, it was possible to recalculate the k values into the mass-normalized constant rates k m, being 2.04 and 3.50 s −1 mg −1 in case of AuNPs obtained using the CAPP-activated aqueous G. biloba leaves and P. ginseng root extracts, respectively. This enabled to compare the present results for homogenous nanocatalysts with others, being reported in the literature. The suitable summary, setting the proposed approach towards synthesis of homogenous nanocatalysts with comparison of works of other researchers are displayed in Table 4. Considering the catalytic decomposition of 4-NP using Au-based homogenous catalysts reported in literature, it was concluded that the approach proposed in the present work presented an increased feasibility of the process. FLC-dc-APGD-based activation of the prepared plant extracts allowed not only to efficiently reduce the Au (III) ions to AuNPs and stabilize them but also was responsible for their favourable physiochemical and granulometric characteristics. This enabled the achievement of high k m values, 10-fold exceeding the km values reported for other Au-based nanocatalysts (see Table 4). This was linked with a high reduction efficiency of the Au (III) ions provided by the CAPP-activated plant extracts. The proposed CAPP-based approach, due to its flow-through nature, could be easily adopted for continuous systems, transferring the achieved efficiencies into real-life systems.

Conclusions
The present work proposes a novel unique approach for AuNPs production suitable for application in homogenous catalysis. The synthetic route involved the CAPP-activation of the aqueous plant extracts of G. biloba and P. ginseng that were further applied as the reducing and capping agents for the AuNPs fabrication. It was found that the CAPP-treatment might alter the organic species present in the plant extracts that are responsible for the reduction of the Au (III) ions and the stabilization of AuNPs. This, linked with the introduction of the excessive RONS as well as H radicals derived from both FLA-dc-APGD and FLC-dc-APGD, enhanced the reduction of the AuNPs precursor and caused an increased number of the nucleation seeds. As a result, the CAPP-treatment of the aqueous plant extracts resulted in the formation of the colloidal suspensions of AuNPs. These colloidal suspensions were characterized by the increased content of the rod-like and triangular shaped nanostructures, as compared to those produced with the aid of the untreated aqueous plant extracts. The latter ones led to the formation of mainly spherical Au nanostructures. Besides, the FLC-dc-APGD activation of the aqueous P. ginseng root extract allowed to obtain the biogenic AuNPs of the smallest average size (14.2 ± 4.4 nm). These AuNPs were selected to test their catalytic activity in the reduction of 4-NP to 4-AP. The so-prepared homogenous nanocatalyst revealed the constant rate of 0.52 s −1 , which was indeed better as compared to the constant rate of 0.31 s −1 that was provided with the AuNPs (29.0 ± 3.5 nm), produced using the untreated aqueous G. biloba leave extract. The performance of the presented catalytic systems was particularly tempting as compared to this reported in the literature. The determined mass normalized constant rates were approx. 10-times greater as compared to those given in similar literature reports.
Based on the obtained results, it was concluded that the application of the aqueous P. ginseng extract resulted in the formation of smaller in size AuNPs that were much favoured in the catalytic decomposition of 4-NP as compared to the G. biloba extract. We believe that the proposed approach might be a very tempting and efficient alternative over other protocols used in the bio-inspired synthesis of the nanocatalysts containing AuNPs.
Author Contributions: A.D. conceptualized all the experiments. A.D. and P.J. adjusted the reaction-discharge system for the effective plant extracts activation by CAPP. A.D., P.J. and W.M. carried out the synthesis of AuNPs. A.D. and W.M. characterized the optical and granulometric properties of AuNPs. P.J. carried out the OES measurements. P.C. and D.J.-B. performed the catalytic characteristic of the analyzed nanomaterials. A.D. and P.C. wrote the presented paper. P.C. provided graphical support. D.J.-B., P.P. and P.J. took a part in the discussion and re-vised the following manuscript. All authors have read and agreed to the published version of the manuscript.

Funding:
The scientific activity of Anna Dzimitrowicz is supported by the Polish Ministry of Science and Higher Education; program Outstanding Young Scientist (UMO-532/STYP/13/2018). The presented work was co-financed by the statutory activity, subsisted to the Faculty of Chemistry of Wroclaw University of Science and Technology.