Synthesis of nanoparticles composed of silver and silver chloride for a plasmonic photocatalyst using an extract from a weed Solidago altissima (goldenrod)

Fresh plant leaves of S. altissima were collected on the Kawagoe Campus, Toyo University, Japan and were cleaned with millipore water to remove the surface contaminants. Some 10 g of leaf pieces were heated in 100 ml of millipore water at 90 °C for 90 min, followed by filtration (Whatman Cat No 1002-185) to obtain an aqueous plant leaf extract. The leaf extract was refrigerated at 4 °C for further use. Some 20 ml of the aqueous leaf extract was mixed with 60 ml of 1 mM aqueous solution of Ag nitrate (AgNO₃) (Sigma-Aldrich, Japan) at 4 °C, and the mixture was stirred for 30 min in the dark and then was exposed to light (power density: 1 sun = 100 mW cm⁻², exposed area: 3 × 3 cm²) (HAL 320, Asahi spectra Co., Ltd.) for 30 min. The mixture was centrifuged at 15 000 rpm for 40 min to pelletize the product. The pellet was purified by dispersing it in millipore water, followed by centrifugation at 15 000 rpm for 40 min.

The optical properties of as-synthesized NPs were characterized by an UV–vis–NIR spectrophotometer (DU 730, Beckman Coulter) in the range from 200 to 1000 nm at a resolution of 1 nm and a UV diffuse reflectance spectroscope (DH2000-DUV, USB2000, Ocean Optics, Inc.) in the range from 250 to 600 nm (figure 1). The morphology of the particles was analyzed using a transmission electron microscope (TEM) (JEM2200FS, JEOL) at an acceleration voltage of 200 kV with a scan snap, orius camera, and software (Gatan). Samples for TEM observation were prepared on a carbon (C) coated copper (Cu) grid (Cu 200 mesh, JEOL), dropping and drying NP suspension on the grid. The elemental composition of the particles was analyzed by energy dispersive x-ray spectroscopy (EDS) (SU6600, Hitachi equipped with OXFORD X-Max^N) (figure 2). The crystal nature of the particles was determined by an x-ray diffractometer (XRD) (Smart Laboratory, Rigaku), using a Cu Kα x-ray of a 0.154 nm wavelength supplied by a 9 kW rotating Cu anode x-ray generator and employing a grazing-incidence x-ray diffraction geometry with a scanning diffraction angle (2θ) from 25° to 90° at 1.5° min⁻¹ at an incident angle of 0.1°. The elemental and chemical bonding states of the NPs were studied by an x-ray photoelectron spectroscope (XPS) (Axis-HIs, Kratos Analytical) equipped with a nonmonochromatic Al Kα x-ray of an energy of 1486.61 eV, pass energy of 80 eV, and step energy of 0.1 eV at lower than 1.0 × 10⁻⁸ torr (figure 3). Samples for XPS were prepared on a hydrofluoric acid treated silicon (Si) wafer, dropping and drying NP suspension on the wafer. The photoluminescence (PL) spectra of the particles were determined using a spectrofluorometer (FP 6500, Jasco).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The chemical functional groups of the organic compounds responsible for the reduction reaction were analyzed by Fourier transform infrared spectroscopy (FTIR) (Nicolet iS50, Thermo Fischer Scientific), scanning the IR from 400 to 4000 cm⁻¹ at a resolution of 4 cm⁻¹. The leaf extract was centrifuged at 15 000 rpm for 45 min, the supernatant was removed, and the pellet was dried in vacuum. Samples for the FTIR measurement were prepared by the KBr pellet method [32]. The elemental composition of the leaf extract was analyzed by EDS and XPS. Samples for EDS and XPS were prepared on a hydrofluoric acid treated Si wafer, dropping and drying an aqueous leaf extract on the wafer.

Some 2 mg of NPs were mixed with 2 ml of a 10 mg l⁻¹ aqueous solution of RhB (Tokyo Chemical Industry Co., Ltd.) for 30 min in the dark for the establishment of adsorption/desorption equilibrium of RhB on the particles. Then, the mixture was transferred to a quartz photoreactor for the measurement of the photocatalytic activity of the nanoparticles. The photoreactor was irradiated with light from a solar simulator without any cutoff filters (power density: 1 sun = 100 mW cm⁻² and an exposed area of 3 × 3 cm²) (HAL 320, Asahi Spectra Co., Ltd.) ranging from 350 to 1100 nm to evaluate the degradation of RhB. Aliquots of the reaction mixture were collected and were centrifuged at 15 000 rpm for 10 min to separate the photocatalyst from the mixture, and UV–vis–NIR absorption by the supernatant was recorded by the UV–vis–NIR spectrophotometer for 60 min at regular intervals of time to measure the degradation of RhB (figure 4). Degraded RhB was further confirmed by a mass spectroscope (amaZon speed, Brüker Daltonics) by the direct probe method using an atmospheric pressure chemical ionization source at the mass-to-charge ratio (m/z) ranging from 15 to 600 amu. For examining the enhancement of the photocatalytic efficiency of the present NPs, another experiment was carried out using a commercialized photocatalyst Ag₃PO₄, the size of which varied from 25 to 600 nm (Sigma-Aldrich, Japan) under the same experimental conditions.

Zoom In Zoom Out Reset image size

The SERS characteristics were investigated using an aqueous solution of RhB. A 0.1 mM RhB solution was dropped onto dried Ag and AgCl NPs placed on a glass substrate, and the Raman spectra were obtained using a micro-Raman spectroscopic system (Laboratory Ram HR800UV, Horiba Jobin Yvon S.A.S.) with an excitation wavelength of 633 nm (figure 5). The spot diameter and power of the laser beam were 2 μm and 43.6 μW, respectively, and the spectra were obtained with a 5 s integration.

Zoom In Zoom Out Reset image size

To examine the antibacterial activity of the Ag and AgCl NPs, the time variation of the bacterial growth was measured for 15 h, culturing two bacterial species; Escherichia coli (gram-negative) and Bacillus subtilis (gram-positive) in nutrient broth No. 2 (Oxoid) media supplemented with an aqueous solution of Ag and AgCl NPs, the concentration of which was changed, 5, 10, and 20 μg ml⁻¹ at 37 °C under a constant shaking at 150 rpm. The optical density (O.D.) values at 600 nm were recorded every 30 min using an O.D. monitor (C&T, Taitec Corp.).

We successfully synthesized Ag and AgCl NPs by the following two-step procedure; (1) the reduction of Ag⁺ (AgNO₃) using an aqueous leaf extract of S. altissima to synthesize AgCl NPs and (2) the photoreduction of AgCl NPs to synthesize Ag and AgCl NPs. The chemical compounds present in the aqueous leaf extract, such as terpenoids, glycosides, acetylenes, phenols, and components containing Cl⁻ [29, 33–38] were supposed to be responsible for the reduction reaction to synthesize AgCl NPs in the first procedure [39, 40] noting that no external Cl components or any additional chemicals were supplied during the synthesis (see figure A1 in the appendix for FTIR, EDS, and XPS analyses of the leaf extract). The second procedure was performed by irradiation of light into the reaction mixture. Ag and AgCl NPs were synthesized owing to the high photosensitivity of AgCl [6, 41]. A pair of electrons and holes is generated in AgCl via photon activation, and, therefore, it is supposed that the photogenerated electron promoted the conversion of the Ag⁺ ion into the Ag⁰ atom [7, 41, 42], resulting in the synthesis of Ag and AgCl NPs. The progress of the synthesis of Ag and AgCl NPs was confirmed by the time variation of the UV–vis–NIR spectrum of the reaction mixture during the synthetic procedure. As shown in figure 1, the time variation of the peak intensity observed in the vis region depicts the gradual synthesis of Ag and AgCl NPs. Figure A2 in the appendix shows the UV–vis–NIR spectrum of the purified Ag–AgCl NPs dispersed in millipore water where the plasmonic resonance peak at 462 nm is attributed to Ag, whereas, the absorption peak at 246 nm is attributed to AgCl [43]. The UV–vis diffuse reflectance spectrum of dried Ag and AgCl NPs shows some absorbance in the vis light region due to SPR in metallic Ag clusters in the powders [44], while the absorbance in the UV region is caused by AgCl NPs coupled with AgNPs (see figure A3 in the appendix) [45], which suggests that the present Ag and AgCl NPs can be used as an effective plasmonic photocatalyst in vis light [46]. The PL spectrum of the Ag and AgCl NPs at room temperature (24 ± 2 °C) showed emission bands at 318 and 470 nm when excited with photons of a 200 nm wavelength, which also clarifies the semiconductor features of the present Ag and AgCl NPs (see figure A4 in the appendix).

The morphology and crystalline nature of the particles were investigated by SEM, EDS, TEM, and XRD. SEM, EDS, TEM, and FFT images of the particles are shown in figure 2. A large number of clusters of particles, which were composed of Ag and Cl, were produced (see figures 2(a) and (b)). It is clearly shown that each cluster was composed of Ag and AgCl (figures 2(c)–(f)), noting that the lattice fringe distance of 0.23 nm corresponds to Ag (111), whereas, 0.27 nm corresponds to AgCl (200) (figures 2(d)–(f)). The peaks in the diffractogram also confirm the cubic AgCl and cubic Ag structures, respectively, with the lattice constants of 5.553 050 Å (ICDD 01-071-5209) and 4.097 667 Å (ICDD 00-041-1402) (see figure 3).

The elemental composition of the as-synthesized NPs was analyzed by XPS (see figure A5 in the appendix). The wide scan spectrum indicates the presence of Ag, Cl, C, and O (see figure A5(a)). Ag3d_5/2 and Ag3d_3/2 centred at 367.75 and 373.76 eV represent AgCl (figure A5(b)) [47, 48], whereas, Cl2p_3/2 and Cl2p_1/2 centred at 197.90 and 199.57 eV represent Cl present in Ag and AgCl NPs (figure A5(c)) [49, 50].

The photocatalytic efficiency of the as-synthesized Ag and AgCl NPs was evaluated via the degradation of an organic dye RhB (10 mg l⁻¹) under the irradiation of light generated from the solar simulator. RhB molecules in an aqueous solution show three major peaks at 258, 354, and 554 nm in the UV–vis absorption spectrum. Figures 4(a) and (b) show the time variations of the absorption spectra in the presence of Ag and AgClNPs, and Ag₃PO₄NPs. The decrease in the peak intensity and the shift of the peak position toward shorter wavelength regions elucidate the degradation of RhB molecules and N-deethylation [51]. The photocatalytic activity was evaluated by the UV–vis absorption at 554 nm. The time variation of −ln (C/C₀), where C is the concentration of the dye at regular intervals of time t and C₀ is the concentration at t = 0, is shown in figure 4(c). The rate constant k for the degradation reaction of RhB based on the first-order reaction kinetics (ln (C/C₀) = −kt) is calculated to be 0.0387 min⁻¹ in the case of Ag and AgCl NPs, whereas, k = 0.0237 min⁻¹ in the case of Ag₃PO₄. The high photocatalytic performance of Ag and AgCl NPs owes to the SPR of AgNPs and the semiconductor features of AgCl NPs [47, 52]. A further confirmation of the degradation of RhB by Ag and AgCl NPs was carried out by mass spectrometric analysis. Figure A6 in the appendix shows the peaks at 442, 413, 386, 357, and 329 amu, which correspond to RhB and its N-deethylated intermediates [53]. It is clearly shown that the degradation of RhB is encouraged by the present Ag and AgCl NPs.

The change in the ratio of Ag and AgCl in the Ag and AgCl NPs before and after the photocatalytic experiment was evaluated by XRD analysis (see figure A7 in the appendix). The peak intensities in the XRD spectrum and weight percentages of Ag and AgCl changed quite significantly after the experiment due to the photosensitivity of AgCl, noting that no cutoff filter was used during light irradiation in the present study. The previous studies showed that Ag NPs on the AgCl NPs were structurally stable and catalytically reusable thanks to the cutoff filter of UV light [7, 54], and therefore, we believe that the catalytic performance of the present Ag and AgCl will be improved by introducing the cutoff filter of UV light.

Raman spectra of an aqueous solution of RhB, which was dropped onto Ag and AgCl NPs placed on a glass substrate, was obtained (see figure 5). It is clearly shown that Raman signals were greatly enhanced in the presence of Ag and AgCl NPs, noting that each peak coincides with that of RhB [55] and, therefore, the present Ag and AgCl NPs are highly SERS active thanks to Ag NPs [56, 57], whereas, there were no recognizable signals of RhB when the solution was dropped onto the surface of a glass substrate.

The antibacterial activity of the Ag and AgCl NPs was investigated culturing Escherichia coli (gram-negative) and Bacillus subtilis (gram-positive) in the presence of Ag and AgCl NPs in the liquid medium. The time variation of the bacterial growth for 15 h is shown in figure A8 in the appendix. The growth of Escherichia coli and Bacillus subtilis in the presence of Ag and AgCl NPs was reduced even when the concentration of Ag and AgCl NPs was as low as 5 μg ml⁻¹ [58, 59].