2.1 Chemicals and reagents

JEJU TECHNOPARK Inc. (Jeju, Korea) provided the S. horneri extract. The following items were purchased from Sigma-Aldrich (St. Louis, MO): silver nitrate, AgNO₃, hydrogen tetrachloroaurate (III) trihydrate, HAuCl₄·3H₂O, methylene blue (MB), rhodamine B (RB), methyl orange (MO), and sodium borohydride (NaBH4).

2.2 Preparation of Sargassum horneri extract

The SH extract was obtained from Jeju Island in Korea’s Jeju Province. Using an electric mixer, SH was dehydrated and uniformed into a fine powder (HMF-3100S, Hanil Electric, Seoul, Korea). To prepare SH solution, the powder was dissolved in 80% ethanol at room temperature. The solution was filtered and concentrated using a rotary vacuum evaporator (Buchi Rotavapor R-144, Buchi Labortechnik, Flawil, Switzerland) and 50 mL of SH was freeze-dried. The powder was preserved at −75 °C until it was needed. Algae powder was dissolved in an aqueous solution at a concentration of 4 mg/mL for the synthesis of NPs. The solution was then filtered and sterilized with a 0.2 µm syringe filter.

2.3 Green synthesis procedure of AgNPs and AuNPs

The concentration, temperature, and reaction time of the SH extract and metal precursor were optimized to synthesize AgNPs and AuNPs. First, 1 µL of AgNO₃ (1 M) aqueous solution was added to 1 mL of the filtered SH extract (2 mg/mL) and incubated for 15 min at 80 °C in water. After 15 min, the tubes containing the colloid were placed on ice for 5 min. The color of the suspensions changed to dark orange, indicating that the synthesis of SH-AgNPs was successful. Similarly, 1 mL of filtered SH extract (2 mg/mL) was added to 1 µL of HAuCl₄·3H₂O (1 M) solution for the synthesis of SH-AuNPs. The transformation of the color of the suspension to dark purple confirmed the successful synthesis of SH-AuNPs (Fig. 1).

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Fig. 1

Schematic diagram of green-synthesis of AgNPs and AuNPs.

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2.4 Characterization of SH-AgNPs and SH-AuNPs

The formation of biosynthesized NPs using SH was recorded using an Ultrospec 6300 pro UV–Vis spectrophotometer (Amersham Biosciences, Buckinghamshire, UK) operating in the 300–800 nm wavelength range. A Zetasizer Nano-ZS90 (Malvern Panalytical, Malvern, UK) was used to analyze the size and zeta potential of the NPs. HRTEM (TALOS F200X (Thermo Scientific, Oregon, USA) with EDX) operating at a potential of 200 kV was used to determine the morphology and particle size distribution of the NPs. The AgNPs and AuNPs were immobilized on a Formvar/Carbon 200 mesh (Electron Microscopy Sciences, Oregon, USA) copper grid. The structural information of the NPs was determined using an X-ray diffractometer, X’Pert3 Powder (XRD Empyrean series 2, PANalytical, Almelo, Netherlands). The XRD patterns and crystal structures were investigated by X-ray diffraction using Cu Kα radiation (1.540 Å) at 40 kV and 30 mA over a 2θ range of 20–80°. The FTIR spectra of the NPs were recorded at 4000–400 cm⁻¹ using a Spectrum GX spectrometer (Perkin Elmer Inc., Boston, MA, USA) equipped with potassium bromide pellets. The biosynthesized NPs were lyophilized for FTIR analysis and ground in a mortar and pestle with KBr powder. During the formation of SH-NPs, various functional groups in the SH extract acted as reducing and capping agents.

2.5 Evaluation of catalytic activity of AgNPs and AuNPs

At room temperature, the catalytic potential of NPs for dye reduction was studied using a UV–visible spectrophotometer. NaBH₄ was prepared ice-cold prior to the analysis. Then, 2 mL of aqueous solutions of MB (0.08 mM), RB (0.05 mM), and MO (0.1 mM) were blended with 1 mL NaBH₄ (10 mM), and the NP solution, SH-AuNP or SH-AgNP, was added as a catalyst. SH-AgNPs (100, 50, and 50 µL) were added to MB, RB, and MO solutions, respectively. The volume of the SH-AuNP solution added to the mixture was 20 µL. The amount of green-synthesized NPs (AgNPs and AuNPs) and the concentrations of NaBH₄ and dyes were optimized to determine the catalytic activity. The kinetics of the catalytic reaction were determined by assuming that the reaction process followed the pseudo-first-order law, which is stated as follows:

The rate constant $k$ , was obtained from the absorbance at time $t$ ( $A_t$ ) and the initial absorbance of the dyes ${(A}_0)$ .

3.1 Green synthesis of SH-AgNPs and SH-AuNPs

SH extract served as both a reducing and capping agent for the reduction and formation of metal salts during the synthesis of AgNPs and AuNPs. Following that, the AgNO₃ solution was added to the SH extract. Within 15 min, as the reaction progressed, the color gradually changed to dark orange (Fig. 2b). Localized surface plasmon resonance (LSPR) was used to observe the color change of the solution. UV–Vis spectroscopy in the range of 300–800 nm also confirmed the formation of SH-AgNPs. The maximum LSPR peak at 420 nm indicated the biosynthesis of SH-AgNPs. Similarly, a HAuCl₄·3H₂O solution was added to the SH extract. Within 15 min of being placed in the water bath, the color of the colloid changed to violet, indicating the formation of SH-AuNPs (Fig. 2c). As indicated in the spectrum, SH-AuNPs exhibited a strong LSPR band at 528 nm. In earlier studies, AgNPs biosynthesized using Sargassum muticum exhibited an LSPR peak at 420 nm, while AuNPs biosynthesized using algae exhibited an LSPR peak at 540 nm (Azizi et al., 2013; Castro et al., 2013). Both studies demonstrated that algae can be successfully used as reducing agents for the synthesis of inorganic nanoparticles.

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Fig. 2

Formation of green-synthesized AgNPs and AuNPs. Ultraviolet–visible (UV–Vis) absorption spectra of (a) Sargassum horneri (SH) extract, (b) SH-silver nanoparticles (SH-AgNPs), and (c) SH-gold nanoparticles (SH-AuNPs).

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3.2 DLS analysis

Table 1 and Fig. 3 show the size and zeta potential of the SH-AgNPs and SH-AuNPs as measured by dynamic light scattering (DLS). SH-AgNPs had an average size of 90.1 ± 8.38 nm when measured by DLS at room temperature, while SH-AuNPs had an average size of 21.0 ± 2.74 nm. SH-AgNPs and SH-AuNPs had polydiversity indices of 0.384 and 0.360, respectively. The determination of zeta potential confirmed the stability of the nanoparticles. The zeta potentials of SH-AgNPs and SH-AuNPs were −43.49 ± 2.74 and −34.57 ± 4.43 mV, respectively, indicating that the synthesized SH-AgNPs and SH-AuNPs (with large negative values of the measured zeta potential) were stable.

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Fig. 3

The size distribution and zeta potential of (a) SH-AgNPs and (b) SH-AuNPs determined by Dynamic light scattering (DLS).

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3.3 HRTEM analysis

High-resolution transmission electron microscopy (HRTEM) was used to observe the morphology, shape, form, and dispersion of the biosynthesized SH-AgNPs and SH-AuNPs. TEM images of the nanoparticles are shown in Figs. 4 and 5. The average sizes of SH-AgNPs and SH-AuNPs shown in the TEM image (Figs. 4a and 5a) are about 22.72 nm and 13.21 nm, respectively. The sizes of the nanoparticles determined using DLS and TEM images did not agree. This can be attributed to the aggregation of the NPs, which reduces the accuracy of the DLS analysis due to the presence of large particles, increasing light scattering and thus shifting the measured particle size to larger values (Souza et al., 2016). The TEM images revealed that, in addition to other shapes, such as pentagons, hexagons, and octagons, the majority of the nanoparticles had a spherical shape. The SAED patterns shown in Figs. 4c and 5c demonstrate that the synthesized nanoparticles have a crystalline structure. The bright rings correspond to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) lattice planes, indicating that the structure is face-centered cubic (FCC). The EDX spectra of the nanoparticles are shown in Figs. 4f and 5f. SH-AgNPs exhibited the highest light absorption peak at 3 keV. Similarly, SH-AuNPs had peaks at 2–2.5 and 9.5–10 keV.

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Fig. 4

High resolution transmission electron microscopy (HRTEM) images of SH-AgNPs at various magnifications: the scale bar corresponds to (a) 200 nm, (b) 10 nm, (c) selected area electron diffraction (SAED) patterns, (d)–(e) High-angle annular dark-field (HAADF) images, and (f) energy-dispersive X-ray spectroscopy (EDX) spectra of SH-AgNPs. (a) 58 k, (b) 630 k, and (d)–(e) 115 k.

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Fig. 5

HRTEM images of SH-AuNPs at various magnifications: the scale bar corresponds to (a) 200 nm, (b) 10 nm, (c) SAED patterns, (d)–(e) HAADF images, and (f) EDX spectra of SH-AuNPs. (a) 58 k, (b) 630 k, and (d)–(e) 115 k.

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3.4 XRD analysis

XRD analysis was used to confirm the crystalline phases of the green-synthesized AgNPs and AuNPs. The XRD patterns of the biosynthesized NPs are shown in Fig. 6. Four distinct peaks were observed at 2θ values of 38.2°, 44.2°, 64.6°, and 77.3° for the Bragg reflection peak of SH-AgNPs, which were indexed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of the FCC structure, respectively. Similarly, the peaks of SH-AuNPs at 38.5°, 44.7°, 64.8°, and 77.9° correspond to the same planes as that of SH-AgNPs. We compared these diffraction peaks to the standards (JCPDS card no. 04-0783 and JCPDS card no. 04-0784).

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Fig. 6

X-ray diffraction (XRD) pattern of bio-synthesized nanoparticles. (a) SH-AgNPs and (b) SH-AuNPs.

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3.5 FTIR spectral analysis

The FTIR analysis was used to determine the function of the SH extract as a reducing and capping agent. The FTIR spectrum confirmed the presence of SH chemical compounds on the surface of the biosynthesized NPs. The FTIR spectra of SH extract, SH-AgNPs, and SH-AuNPs are shown in Fig. 7. On analyzing the SH extract, the peaks at 3411 and 1396 cm⁻¹ corresponded to the stretching of —OH bonds in polyphenols or alcohols, which are chemicals found in algae. The peak at 1587 cm⁻¹ corresponds to the N—H stretching vibration of the amine groups. The peak at 1078 cm⁻¹ is attributed to the C—O stretching of alcohols. The peaks at 3412, 1590, 1385, and 1078 cm⁻¹ and peaks at 3423, 1595, 1398, and 1079 cm⁻¹, respectively correspond to the functional components of SH when the SH-AgNPs and SH-AuNPs are analyzed. This suggests that the phytochemicals present in SH act as capping and reducing agents in the synthesis of NPs.

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Fig. 7

Fourier transform infrared (FTIR) spectra of SH extract, SH-AgNPs, and SH-AuNPs.

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3.6 Catalytic activity of SH-AgNPs and SH-AuNPs in reduction of organic dyes

The catalytic activity of the biosynthesized AgNPs and AuNPs was assessed by the degradation of three dyes, methylene blue (MB), rhodamine B (RB), and methyl orange (MO) at room temperature. Double-distilled water was used as a control agent. The degradation reactions of these three dyes were monitored and are shown. A UV–Vis spectrophotometer was used to analyze all reactions. Furthermore, we assumed that the catalytic reaction followed pseudo-first-order kinetics, and calculated the rate constant, k, which is the slope of the graph obtained from the experiment.

MB is a heterocyclic aromatic dye that is used in textiles as well as the medical field and has been the subject of extensive research. It is primarily used to treat methemoglobinemia and malaria (Schirmer et al., 2003). Additionally, it has been reported to have an effect on AD (Oz et al., 2009). Methylene blue, on the other hand, is toxic, and its accumulation in the body, can result in carcinogenicity and biomutation (Payra et al., 2019) as well as serious central nervous system toxicity (Gillman, 2011; Sultan et al., 2018). Fig. 8 shows the UV–VIS spectra of reduced MB in the presence of catalysts (SH-AgNPs and SH-AuNPs) and NaBH₄ or distilled water (control agent) and NaBH₄. Prior to reduction, the UV–Vis spectrum of MB exhibited a maximum absorbance peak at 662 nm and appeared dark blue. After reduction, there was no peak observed at 662 nm, and the solution became colorless. Fig. 8a shows the spectra of MB with NaBH₄ and distilled water without a catalyst. The maximum peak intensity decreased after 40 min. Despite the fact that NaBH₄ is a strong reducing agent, no significant difference was observed. On the other hand, when 100 µL of SH-AgNP (Fig. 8b) or 20 µL of SH-AuNP (Fig. 8c) as a catalyst, the maximum peak intensity of the methylene blue solution was almost completely reduced, and both solutions became transparent after 9 and 3 min, respectively. The natural logarithm of the initial maximum absorbance and the time-varying maximum absorbance are plotted in Fig. 9. The slope of the plot was used to calculate the rate constant, $k$ . Table 2 presents the detailed values.

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Fig. 8

Methylene blue (MB) dye reduction. UV–Vis absorption spectra of MB Reduction by NaBH₄ (a) in the absence of catalyst and in presence of (b) SH-AgNPs or (c) SH-AuNPs.

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Fig. 9

Kinetics of catalytic reaction. Plot of $\ln \frac{A_t}{A_0}$ versus time during MB degradation by (a) SH-AgNPs and (b) SH-AuNPs. $A_t$ is the maximum absorbance of MB at time $t$ and $A_0$ is the maximum absorbance at $t=0$ .

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RB is commonly used in water as a tracer to measure the velocity and direction of water movement, and is widely used as a colorant in the textile and food industries (Sharma et al., 2019; Le and Tran, 2020). However, RB is a potent carcinogen and also exhibits other forms of toxicity, including reproductive and neurotoxic effects (Ahmad et al., 2021). RB had a maximum absorbance of 553 nm and lends a pink color to the solution. Fig. 10a displays the UV–Vis absorption spectra of RB in the presence of NaBH₄ and distilled water without catalysts. For 20 min, the maximum absorbance of RB decreased slightly. The absorbance of RB was measured at regular time intervals using SH-AgNPs (50 µL) and SH-AuNPs (20 µL) as catalysts as shown in Fig. 10b and c. The maximum absorbance peak at 553 nm was completely lost within 15 and 3 min respectively, as the solution became colorless. A plot of $\ln \frac{A_t}{A_0}$ against time is shown in Fig. 11. The alternative detailed values, including the RB degradation rate constant, are listed in Table 2.

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Fig. 10

Rhodamine B (RB) dye reduction. (a) UV–Vis absorption spectra of RB Reduction by only NaBH₄ without catalyst, reduction by NaBH₄ in presence of (b) SH-AgNPs or (c) SH-AuNPs.

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Fig. 11

Kinetics of catalytic reaction. Plot of $\ln \frac{A_t}{A_0}$ versus time during RB degradation by (a) SH-AgNPs and (b) SH-AuNPs. $A_t$ is the maximum absorbance of RB at time $t$ and $A_0$ is the maximum absorbance at $t=0$ .

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MO is an azo dye characterized by chromogenic azo bonds (-N = N-) that is used in textiles, laboratories, and commercial products (Martínez-Huitle and Brillas, 2009; Chaukura et al., 2016). However, methyl orange is a known carcinogen (Azami et al., 2012; Gong et al., 2013; Hildenbrand et al., 1999). Additionally, it degrades the photosynthetic efficiency of aquatic organisms and is toxic (Huang et al., 2008). MO is orange in color. At 464 nm, the maximum absorbance was determined. The UV–Vis absorption spectra of MO, which was stable for 50 min in the absence of the catalyst is shown in Fig. 12a. However, the maximum absorbance of MO at 464 nm when SH-AgNPs (50 µL) and SH-AuNPs (20 µL) were used as catalysts decreased significantly within 12 and 15 min, respectively, and the MO became colorless (Fig. 12b, c). Graphs of $\ln \frac{A_t}{A_0}$ versus time for the catalytic reduction of MO are shown in Fig. 13, and additional details, including the rate constant, are listed in Table 3.

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Fig. 12

Methyl orange (MO) dye reduction. UV–Vis absorption spectra of MO Reduction by NaBH₄ (a) in the absence of catalyst and in the presence of (b) SH-AgNPs or (c) SH-AuNPs.

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Fig. 13

Kinetics of catalytic reaction. Plot of $\ln \frac{A_t}{A_0}$ versus time during MO degradation by (a) SH-AgNPs and (b) SH-AuNPs. $A_t$ is the maximum absorbance of MO at time $t$ and $A_0$ is the maximum absorbance at $t=0$ .

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