Very rapid synthesis of highly efficient and biocompatible Ag₂Se QD phytocatalysts using ultrasonic irradiation for aqueous/sustainable reduction of toxic nitroarenes to anilines with excellent yield/selectivity at room temperature

Highlights

• •Synthesis of stable nanocatalyst and reduction of nitroarenes in green conditions.
• •Very rapid synthesis of biocompatible Ag₂Se QD catalyst (30 sec, H₂O, rt, air).
• •Use of ultrasonic in the synthesis of very fine (∼7 nm) and monodisperse catalysts.
• •Efficient reduction of nitroarenes (∼1h, H₂O, rt, air) in > 99% yield and selectivity.
• •Easy recycling and reuse of catalyst and good activity in a gram‐scale reaction.

Abstract

There are many problems associated with the synthesis of nanocatalysts and catalytic reduction of nitroarenes - e.g., high temperatures, costs, long reaction/synthesis process times, the toxicity of chemicals/solvents, undesirable byproducts, the toxic/harmful wastes, low efficiency/selectivity, etc. This study represents an attempt to overcome these challenges. To this purpose, biocompatible and highly efficient Ag₂Se quantum dots (QDs) catalysts with antibacterial activity were synthesized in a very rapid (30 sec, rt), simple, inexpensive, sustainable/green, and one-pot strategy in water using ultrasonic irradiation. Characterization of the QDs was performed using different techniques. UV–Vis absorption and fluorescence spectroscopic studies showed an absorption peak at 480–550 nm and a maximum emission peak around 675 nm, which confirmed the successful synthesis of Ag₂Se QDs via the applied biosynthetic method. Subsequently, catalytic reduction of nitroarenes by them was carried out under safe conditions (H₂O, rt, air atmosphere) in ∼ 60 min with excellent yield and selectivity (>99%). Their catalytic activity in the reduction of various toxic nitroarenes to aminoarenes under green conditions was investigated. Thus, a rapid and safe ultrasound-based method was employed to prepare stable and green Ag₂Se QDs phyto-catalysts with unique properties, including exquisite monodispersity in shape (orthorhombic) and size (∼7 nm), air-stability, and good purity and crystallinity. Importantly, instead of various toxic chemicals, the plant extract obtained by rapid ultrasonic method (10 min, rt) was used as natural reducing, capping, and stabilizing agents. Moreover, antibacterial assays results showed that Ag₂Se-QDs catalysts at low concentrations (ppm) have high activity against all tested bacteria, especially E. coli (MIC:31.25 ppm, MBC:125 ppm) which were significantly different from those of Fig extract (MIC = MBC:500 ppm). The data reflect the role of these bio-synthesized Ag₂Se-QDs catalysts in the development of versatile and very safe catalysts with biomedical properties.

1. Introduction

Nitroarenes have different hazardous effects on human health and the environment. They are common organic pollutants and harmful wastes found in industrial water [1]. Hydrogenation reactions of nitroarenes are key processes in the removal of toxic/carcinogenic nitro compounds from environmental matrices as well as in the chemical industry as they produce important and useful building blocks (anilines) for the production of fine/bulk chemicals, pharmaceuticals, dyes/pigments, agrochemicals/herbicides, etc. [2], [3].

On an industrial scale, these materials are produced by catalytic hydrogenation with precious metal catalysts [4]. These conventional reduction techniques require the use of high pressures/temperatures, additional amounts of a reducing metal, reagents, and a flammable gas (H₂). In addition, strong reducing agents in these methods produce undesirable by-products [5]. Therefore, it is important to find a cost-effective, robust, and environmentally friendly catalytic method that does not use hazardous/expensive agents that generate by-products [6]. Sodium borohydride (NaBH₄) is a common reducing agent that could be used for such catalytic reductions. However, current methods usually require it to be used in>100-fold excess over the substrate. Therefore, it is important to search for suitable catalysts that can increase the catalytic efficiency with a minimal amount of NaBH₄ [7].

Nanocatalysts have high catalytic activity and reactivity because of their high surface area to volume ratio. However, many of these catalysts exhibit higher toxicity than comparable bulk catalysts [7], [8], [9]. Moreover, the easy uptake of these nanoparticles by living systems has led to concerns about their effects on human health and the environment [9]. Hence, the selection and synthesis of safe/eco-friendly nanocatalysts, especially with biomedical applications, using non-toxic/benign synthetic methods (i.e., sustainable chemistry) is of utmost importance [10], [11].

One way to achieve this goal is to use plant-derived materials (plant extract) as natural reducing, stabilizing, and capping agents simultaneously with ultrasound-based methods for greener synthesis of eco-friendly nanocatalysts[10]. Ultrasound-based methods are green/eco-friendly and have important advantages, including simplicity, cost-effectiveness, increased efficiency, improved dispersion, enhanced safety, and easy assimilability with other methods [12], [13]. In these methods, water is often used as a suitable green solvent, and nanoparticles with high surface area, high phase purity, and narrow size and shape distribution can be prepared [14], [15].

One of the most important challenges in using nanocatalysts is their separation from the reaction mixture, because of their very small particle size [16]. Phyto-nanocatalysts coated with water-soluble plant organic compounds solve the challenge of catalyst recovery and recycling by allowing extraction in an aqueous biphasic system and simple phase separation with water, with the catalyst remaining in the water [17].

Silver selenide (Ag₂Se) or naumannite as a superionic conductor[18] has attracted great attention because of its useful and intriguing properties. These materials are non-toxic semiconductors with a narrow band-gap (0.07–0.15 eV) that can be used in fuel cells, thermochromic materials, batteries, electrocatalysis, solar cells, photosensitizers, solid-state electrochemical sensors, and magnetic field sensors [19], [20], [21], [22], [23]. They are also used for in in-vivo bio-imaging and sensing. Toxicity studies have shown that Ag₂Se QDs have high biocompatibility, high stability, and low toxicity both in vitro and in vivo because they have extremely low solubility product constants [K_sp (Ag₂Se) = 2.0 × 10⁻⁶⁴]. Therefore, only a minimum of Ag⁺ ions are released into biosystems [24], [25] and the ions can thus be rapidly eliminated from the blood and via the kidneys without accumulating in organs [26].

Although anilines have already been synthesized with various nanocatalysts (Table 1), to the best of our knowledge, this is the first study dealing with the synthesis of Ag₂Se QD phytocatalysts, as well as the reduction of aromatic nitro compounds by them under completely green conditions (water, rt, air atmosphere). To this purpose, green/robust/stable/reusable/safe Ag₂Se QD phytocatalysts with plant coating were synthesized in a very fast, safe, and one-pot method (H₂O, rt, air atmosphere) by ultrasonic irradiation for the reduction of nitroarenes using a small amount of NaBH₄ (mild hydride donor) under green conditions (H₂O, rt, air) in a short time (∼60 min) in > 99% yield and selectivity (Scheme 1).

Table 1.

Catalytic reduction of nitrobenzenes with diverse catalysts

catalyst	Fe-phen/C-800	cobalt oxide catalysts	Ni-phen@SiO2-1000	(PNA-BIS-2) microgel-stabilized Ag NPs	Ag-rGO/g-C3N4	This work: Phyto-Ag2Se QDs
	Science, (Jagadeesh et al., 2013)[2]	Nat. Chem, (Westerhaus et al., 2013)[27]	Science, (Ryabchuk et al., 2018)[4]	J. Hazard. Mater. (Begum et al., 2019)[28]	J. Hazard. Mater. (Kumar et al., 2020)[29]
Catalyst synthesis conditions	Need for high temperatures, various toxic and expensive chemicals/organic solvents, N2 or Ar atmosphere, special facilities and equipment, and involve several time-consuming reaction steps					AgNO3, SeO2, Aqueous Fig extract, sonication, rt, 30 sec
Reduction reaction conditions	H2 (50 bar)	H2 (50 bar)	10 bar	NaBH4	N2, methanol, visible	NaBH4
	H2O/THF	H2O/THF	H2O/MeOH	H2O	light	H2O
	120 °C	110 °C	60 °C	r.t	(λ > 400 nm)	rt
	12–24 h	4–12 h	20 h	3–9 h	4–6.5 h	∼1h
yield	86–97%	53–99%	61–99%	75–95	89.2–98.2%	>99%

Scheme 1.

Schematic representation of the method used for phyto-synthesis of Ag₂Se QDs catalysts using Fig extract and their use in the reduction reaction of anilines.

2. Experimental

2.1. Materials and instruments

The ultrasonic irradiation was carried out using digital ultrasonic (WUC-D, Korea) to extract bioactive compounds from Fig fruits as well as phyto-synthesis of Ag₂Se QD phyto-catalysts. The preliminary characterization and light absorption of samples were studied using UV-2501PC (Shimadzu, Japan). Moreover, their functional groups and surface chemistry studies were performed by FT-IR spectrophotometer with a scanning range from 400 to 4000 cm⁻¹ (Bruker Tensor 27, KBr wafer). The conductive materials (Au/Pd) onto the surface of the SEM samples were coated using the SEM sputter coater (Coxem SPT-20, South Korea). The scanning electron microscopy/energy dispersive X-Ray spectroscopy (SEM-EDS) was then utilized to investigate the catalyst’s surface morphology, size, and elemental analysis (FEI ESEM Quanta 200, EDS Silicon Drift 2017, USA). The structural information and phase purity of obtained catalysts were studied using their X-ray diffraction (XRD) spectrum (PW1730, PHILIPS, Netherlands) with Cu Kα source and λ = 1.54056 Å. Finally, biological data were read using the microplate reader BioTek (Epoch 2). The transmission electron microscopy (TEM- EM 208S) with a Tungsten filament (electron gun) and point resolution of 0.45 nm at an accelerating voltage of 100KV was used to obtain the information on the shape and size. Fig fruits (edible fruit of Ficus carica L.) with superior quality were procured from an herbal medicine shop in Ahvaz, Iran, and identified by the plant taxonomist Dr A. Sonboli. All chemicals used in the synthesis of catalyst and nitroarenes reduction were purchased from Sigma-Aldrich & Merck Co. and were used without any purification.

2.2. Ultrasound-assisted synthesis (UAS) of Ag₂Se QDs catalysts: (UAS/extract-capped Ag₂Se QDs)

Dried Figs were washed with distilled water to remove impurities and then powdered by a mechanical grinder. The ultrasound-assisted extraction (UAE) as a fast/green/easy method was used for Fig extraction. Hence, 10 g of dried fruit powder was mixed with 100 mL of distilled H₂O and was then sonicated (10 min, rt, 40 kHz). The content of the flask was filtered using Whatman filter paper to obtain a clear extract that was stored in a sealed container and refrigerated.

Afterwards, the aqueous extract (12.5 mL, PH = 8) and AgNO₃ solution (25 mL, 259 × 10^-4 mol. L^-1) were mixed, and the container was placed in the ultrasonic bath (40 kHz, rt). Subsequently, during the ultrasonic process, SeO₂ solution (25 mL, 129 × 10^-4 mol. L^-1) was added to the mixture and subjected to ultrasonic irradiation (30 sec, rt). The observation of turbidity and rapid colour change in the reaction mixture indicates phyto-reducing ions and the production of UAS/phyto-synthesized Ag₂Se QD catalysts (extract-capped). The mixture was centrifuged (6000 rpm, 15 min), washed with distilled H₂O, and dried in vacuo (45 °C, 24 h).

2.3. General procedure for nitrobenzenes reduction using Ag₂Se QD catalysts

The catalytic activity of phyto-Ag₂Se QDs (extract-capped) was evaluated using the reduction of a series of nitroarenes (Table 2). Briefly, the desired nitroarene (0.2 mmol), NaBH₄ (1.0 mmol), water (2.0 mL), and Ag₂Se QD catalyst (10.0 mg, 0.7 mol% Ag) were mixed and stirred at room temperature. The reaction progress was monitored by thin-layer chromatography (TLC). After completion of the reaction, the crude mixture was diluted with water (5 mL), centrifuged, and worked up by EtOAc (2 × 10 mL), and then the organic phase was dried over MgSO₄ and evaporated under a vacuum to give the desired product, which was characterized by ¹H NMR and ¹³C NMR spectroscopy in DMSO‑d₆.

Table 2.

Optimization of conditions of reduction reaction of nitrobenzenes.

Entry	Ag2Se QDs (x mg)	NaBH4 (y mmol)	t (min)	Aniline yield [%]
1	10 mg	1	60	>99
2	7 mg	1	120	55
3	5 mg	1	120	40
4	10 mg	0.5	60	75
5	–	1	120	0
6	10 mg	–	120	0

2.4. In vitro antibacterial activity

The in vitro antibacterial activity of the samples, including UAS/phyto-Ag₂Se QDs catalysts (extract-capped) synthesized by biological method and Fig extract, were evaluated against four bacterial strains including two Gram-negative bacteria species (Pseudomonas aeruginosa and Escherichia coli) and two Gram-positive bacteria species (Bacillus subtilis and Staphylococcus aureus) using disc diffusion method (DDM), minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) tests.

2.4.1. Antibacterial activities by disk diffusion test (DDM)

According to [30], the disk diffusion method (DDM) was utilized to determine the in vitro antibacterial activity of samples, with slight modifications. In this method, different concentrations of the samples (12.5, 25, and 50 mg/mL) as antibacterial compounds were prepared. Filter paper discs with a 6-mm diameter were impregnated with each known concentration of the tested sample solution and then placed on the surface of Mueller-Hinton agar (MHA) loaded with each of the desired organisms. Afterwards, they were incubated (24 h, 37 °C) and then the diameter (mm) of the zone of growth inhibition surrounding the discs was reported as antibacterial activities of the samples [30].

2.4.2. Antibacterial activities by minimum inhibitory concentration (MIC) test

The highest dilution of a sample or minimal inhibitory concentration (MIC) which inhibits the growth of a test microorganism, was evaluated by the broth dilution method [31], [32], with some modifications. The serial dilution procedure as a common standard method gives the best reproducible results in MIC [31], [32]. Briefly, different concentrations of the samples (0.488 to 1000 ppm) were prepared. Muller Hinton Broth and microorganisms were then added to each well containing a sample and then incubated (37 °C, 24 h). The concentration of the last well in the dilution series with no visible growth of bacteria or turbidity was recorded as the MIC value of each sample [33].

2.4.3. Antibacterial activities by minimum bactericidal concentration (MBC) test

According to previously reported [34], the minimum bactericidal concentration (MBC) of samples as complementary to the MIC assay was determined by sub-culturing wells with no visible turbidity/growth in MIC assay, on an area of Muller-Hinton agar (MHA) and incubation (24 h, 37 °C). The lowest sample concentration without visible growth of bacterial colonies on MHA was considered as MBC value in bacterial death (99.9%) [35].

3. Result and discussion

3.1. Synthesis and characterization of the UAS/extract-capped Ag₂Se QD catalysts

The use of biocompatible and safe quantum dots with high antibacterial activity as catalysts in the reduction reaction of toxic nitrorenes is interesting. In some published research, chemically synthesized Ag₂Se QDs have been used as biocompatible and safe QDs in in-vivo imaging[36], [37]. Nevertheless, to increase safety, reduce costs, prevent the use of expensive/unsafe chemicals, and provide an eco-friendly greener synthesis, biocompatible Ag₂Se QD phyto-catalysts were biosynthesized using only two chemicals: i.e., solutions of SeO_2(aq) and AgNO_3(aq), together with aqueous Fig extract (as a natural and cheap reducing/capping/stabilizing agents) and ultrasound irradiation.

In the plant-mediated synthesis of nanoparticles, plant extracts act as reducing, capping and stabilizing agents. Studies have shown that biomolecules and phytochemical content of plant extract (e.g. phenolics, terpenoids, phenolic acid, flavonoids, amino acids, glycosides, organic acids, polysaccharides, vitamins, and proteins) not only play a role in bio-reduction of the ions to nanoparticles but also play an important role in the capping and stabilizing of metal and metal oxide NPs [38], [39].

It is noteworthy that the aqueous extract of Fig was obtained by ultrasound-assisted extraction (UAE) as a green method, which requires less energy, time, temperature, and solvent than conventional extraction methods[40], [41]. This method increases extraction yield[42] using the acceleration of plant cell wall damage and the mass transfer of bioactive components (such as phenolic compounds) from the solid to the liquid phase[43] by maintaining the quality of the extract[40], [42].

Because of the benefits of ultrasonic irradiation, it was also used for fast bio-reduction of ions and synthesis of Ag₂Se QDs phyto-catalysts. This method was surprisingly useful because it produced Ag₂Se QDs catalysts with extremely small size (∼7 nm), good purity, very uniform shape and size, and without aggregation, in a short time (30 sec). The synthesis of nanoparticles with these properties is important because the size, dimensions, shape, crystallinity, composition, and structure greatly affect their physicochemical properties[44].

These obtained UAS/phyto-Ag₂Se QDs catalysts (extract-capped) were then used to reduce nitrobenzenes under completely green conditions (H₂O, rt) and produced anilines selectively with a very high yield (>99%). Therefore, the comparison of this study with previously published researches (Table 4) shows the use of simple, fast and completely green methods (water, rt), safe materials and biocompatible catalyst, without the use of special equipment at all stages including nanocatalyst synthesis as well as the reduction reaction of toxic nitrorenes. For this purpose, all steps were performed in water and ambient temperature without the use of unsafe chemicals and special equipment with excellent efficiency and without toxic by-products. Characterization of the UAS/phyto-Ag₂Se QDs catalysts was then performed with different methods, including Fourier-transform infrared spectroscopy (FT-IR), UV–Vis spectrophotometry, X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and coupled plasma optical emission spectroscopy (ICP-OES).

Table 4.

Comparison of the catalytic activity of phyto-Ag₂Se QDs with some previously reported catalytic for the reduction of nitroarenes.

Catalyst	Catalyst synthesis conditions	diam (nm)	Reaction reduction conditions	Time	Yield (%)	Ref.
Nanoporous Al75Ag25	1)pure Al, pure Ag, Ar(g), High-frequency induction melting furnace, Single roll melt spinning machine 2)2) NaOH, r.t, 24 h	ligament size: ∼30 nm.	NaBH4, H2O, rt	1–12 h	91–99	(Li et al., 2015)[71]
(PNA-BIS-2) microgel-stabilized Ag NPs	1)SDS, NiPAM, AAm, BIS, water, N2, 30 min, rt, and then 70 °C, 20 min 2)APS, N2, 70 °C, 4.5 h 3)dialysis: 5 days 4)microgel, water, N2, rt, 30 min 5)AgNO3 (aq), NaBH4, N2, rt, 1 h 6)dialysis: 30 min	10–20 nm	NaBH4, H2O, rt	3–9 h	75–95	(Begum et al., 2019)[28]
Ag NPs-MPAT-1	1)SDS, TAA, TEMED, water, conc. HCl (12 N) 2)APS, water, 1 h, then at 348 K, 3 days 3)AgNO3, water, TRIS, 2 min 4)NaBH4, 10 min 5)5) Template-free MPTA-1,TRIS-stabilized Ag-NPs, rt, 1 h	15–20 nm	i-PrOH, NaOH, N2, 80 °C	5–12 h	81–98	(Salam et al., 2014) [72]
Ag@C/ZrPP-500R	1)ZrOCl2·8H2O, reflux, H3PO4, 100 °C, 24 h and drying at 65 °C, 24 h 2)AgNO3, ethanol, phenylacetylene, 1,3-bis(diphenyphosphino) propane, CH2Cl2 3)NaBH4, ethanol, rt, 12 h 4)α-ZrP, DMF, ultrasonic for 30 min 5)Ag cluster ([Ag74(C≡CPh)44](NO3)2), DMF, 100 °C, 5 h and drying (vacuum, 80 °C, 4 h) 6)750 °C (2 °C/min), N2, 3 h 7)500 °C, 1 h 1)8) N2, 10% H2, 300 °C, 2 h	1.0–2.0 nm	H2 (4 MPa), THF, 160 °C	1–2 h	94.5–100	(Gong et al., 2019) [70]
Ag/Fe2O3 nanocatalyst	1)sodium salicylate, NaOH, Iron(III) nitrate, water, pH = 8, 3 h 2)200 °C, 72 h, washed (water and ethanol), extracted (acid–ethanol) 3)S200-BTO (-Fe2O3) nanocrystal, water, sonication 4)AgNO3, ice bath, 30 min 5)NaBH4, ice bath, 3 h and rt, 12 h. washed (water, methanol)	α-Fe2O3nanocrystals (length × width × height: 310 × 220 × 150 nm) and Ag NPs (4–6 nm)	NaBH4 (10 eq), H2O, r.t	30 min	20.1–99.5	(Patra et al, 2017) [73]
HS-Alginate-Ag/Fe3O4	1)halloysite (HS), water, 15 min 2)iron(III) chloride, iron (II), N2, 70 °C 3)ammonia, N2, 70 °C, 1 h 4)halloysite/Fe3O4, water, 20 min, alginate, 5 h 5)HS-Alginate/Fe3O4, water, 25 °C, 90 min, AgNO3 6)hydrazine hydrate, water, 25 °C, 24 h	41–52 nm	NaBH4, H2O, 50 °C	20 min- 2 h	92–100	(Mohammadi et al., 2021) [74]
Ag@MTC	1)glucose, NaOH, water, ice bath, 12 h 2)chloroacetonitrile, isopropanol, 65 °C, 12 h 3)ethanol, dried at 65 °C, 12 h 4)cyanomethyl glucose, zinc chloride, 400 °C, 40 h 5)water, hydrochloric acid solution, 24 h and then washing with water, THF, acetone and drying in a vacuum at 100 °C. 6)glucose mesoporous, water, silver nitrate, NaBH4, rt, 12 h	16–22 nm	NaBH4, H2O, 75 °C	20 min- 4 h	80–100	(Vahedi-Notash et al., 2020) [75]
Ag-rGO/g-C3N4	1)dicyandiamide, thiourea, ball mill (300 rpm min − 1, 50 min), calcination (rt to 550 °C, 4 h) 2)graphene oxide (GO), water, formic acid, sonication, AgNO3, 80 °C, 6 h, NaHCO3 3)g-C3N4, water, 180 °C, 24 h 1)Washing with ethanol and drying, 80 °C, 12 h	10–12 nm	methanol, N2, visible light irradiation (λ > 400 nm)	4–6.5 h	89.2–98.2	(Kumar et al., 2020) [29]
Phyto-Ag2Se QDs	Aqueous Fig extract, AgNO3, SeO2, sonication, rt, 30 Sec	∼ 7 nm	NaBH4, H2O, rt	∼60 min	∼100	This work

The formation of QDs (extract-capped) was initially monitored through the visual color change from clear red to turbid pink under ultrasound irradiation. It was confirmed that the turbidity/color change was due to the phyto-reduction of ions by the biomolecules present in the extract and the formation of Ag₂Se QDs in the reaction mixture. Preliminary characterization and optical properties of QDs were investigated using UV–Vis absorption spectra in the range of 250–700 nm (Fig. 1A). The absorption of light by semiconductor nanoparticles excites the electron and transfers it from the valence band to the conduction band, creating a continuous and wide absorption peak [45]. Because the concentration, pH, distribution, morphology, size, and adsorption of electrophile or nucleophile to the surface of the nanoparticles affect the position of the absorption signal and optical properties of nanoparticles[46], [47], [48], the appearance of a wide peak at 480–550 nm, confirmed the formation of Ag₂Se QD catalysts in the reaction mixture [48], [49].

Fig. 1.

The UV–Vis absorption (A) and PL spectra (B) of the extract-capped Ag₂Se QDs catalysts.

Quantum dots can be excited and emit fluorescence after absorbing (a) certain wavelength(s) of light [50]. Fig. 1B shows the photoluminescence (PL) spectra of extract capped-Ag₂Se QDs catalysts. The emission micrograph shows a maximum PL intensity (a clear and symmetric emission peak) at λ_max = 676 nm. Furthermore, the calculated full width at half-maximum (FWHM) of this emission peak does not exceed 15 nm, which clearly indicates the monodispersed and pristine biosynthesized Ag₂Se quantum dots with a very narrow particle size distribution [51]. Moreover, since the sharp emission intensity is related to the lower density of defect sites, these safe Ag₂Se quantum dots can be used to improve photovoltaic performance as QDs inbuilt by low-defect and PL emission with high quality[52].

It is found that the optical properties of semiconductors depend on their structure, as well as the nature and amount of impurities in the structure [53]. The value of optical band gap energy (E_g) of the biosynthesized semiconductor Ag₂Se QDs can be calculated by Tauc's plot method using following equation: αhν = α₀ (hν - E_g)^1/2.

where α, ν, h, and E_g are the absorption coefficient, frequency, Planck’s constant, and band gap of the nanoparticles, respectively [54], [55]. According to the results, the optical band gap energy of Ag₂Se QDs was obtained (2.4 eV) which was higher than the value of 0.15 eV reported for balk Ag₂Se at room temperature [56]. Therefore, this increase in band gap compared to its bulk value can be due to a decrease in particle size, which is the result of the quantum size confinement effect [54], [57], [58]. The size quantization increases the effective bandgap due to the creation of a series of discrete states in the valence and conduction bands [59]. It should be noted that when materials are extremely small like quantum dots, their optical and electronic properties are significantly distracted from their bulky materials [60].

Fourier Transform Infrared (FT-IR) spectrum brings valuable information about the surface chemistry of bio-synthesized nanoparticles [48] because the stabilizing/capping agents are bound to the surface of Ag₂Se QDs change the FT-IR spectrum [61]. Therefore, this analysis can be used to detect the presence of possible phytochemicals in the aqueous Fig extract that is responsible for the capping and stabilization of Ag₂Se QDs [62]. Fig. 2 shows the FT-IR spectra of Fig extract (black line) and Fig capped Ag₂Se QDs (red line). The FT-IR spectra of Fig extract showed a strong, broadband at 3414 cm⁻¹, which is attributed to the stretching vibration of the OH groups and the formation of intra- and intermolecular hydrogen bonds. The peak at 2925 is due to the C–H symmetric- and asymmetric stretching of the methyl/methylene band. The C = O of ester or carboxylic acid and protein secondary structure (amides I and II) was predominantly observed at 1626 cm⁻¹, while the absorption band at 1406 cm⁻¹ was ascribed to the C–O bonds of ether/ester groups and CH₂ bending. Moreover, the absorption peak at 1075 cm⁻¹ corresponds to the C–O/C–N of aliphatic amines or alcohol/phenol [63], [64], [65]. These absorption bands showed a shift after the reaction with Ag₂Se QDs to 3423, 2923, 1634, 1384, and 1066 cm^{− 1}, respectively. All these peaks show the presence of phytochemicals in extract including polyphenolics, flavonoids, amino acids, etc. which contain different functional groups as phyto-reducing/phyto-capping/phyto-stabilizing agents in the synthesis of Ag₂Se QDs, preventing aggregation of nanoparticles [66]. The results confirmed interactions of Fig extract amino- and hydroxyl groups with Ag₂Se QD catalyst surface. This means that Fig extract has the ability to bind metals and can act as a capping agent for stabilizing the nanocatalysts.

Fig. 2.

The FT-IR spectra of UAS/phyto-Ag₂Se QDs (red) and Fig extract (black) by UAE method. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The size details, morphology, size distribution, and elemental composition of the Ag₂Se QDs were determined using SEM-EDS (scanning electron microscopy/energy dispersive spectroscopy) and TEM analysis, as depicted in Fig. 3 (A-D). The resulting images showed the presence of a large number of very fine spherical nanoparticles with a very narrow size distribution, without any aggregation, which had an average diameter of ∼ 7 nm. To our knowledge, there are no phyto-synthesized Ag₂Se QDs using ultrasonic with a very uniform, extremely fine, and very good monodispersity in shape and size in the literature.

Fig. 3.

SEM images (A&B), TEM images (C&D), and EDX pattern (E) of phyto-synthesized Ag₂Se QDs by UAS method.

The EDX spectrum (Fig. 3E) displays peaks corresponding to Ag and Se, which confirmed the successful synthesis of UAS/phyto-Ag₂Se QDs. It is noteworthy that the extract phytochemicals, which capped the surface of the QDs, generate other signals that can be seen in Fig. 3E [66]. Furthermore, the results of inductively coupled plasma optical emission spectroscopy (ICP-OES) exhibited that Ag content in Ag₂Se QDs was 1.5 wt%.

The phase purity and crystal structure of the Ag₂Se was investigated by the x-ray diffraction (XRD) method. The X-ray diffractogram is shown in Fig. 4 in the 2θ range from 15^° to 80^°, which was assigned with Miller indices (hkl) values to each reflection peak. The X-ray diffraction peaks were identical and matched with those reported in the appropriate literature database (reference code: 96–223-0973). The successful synthesis of Ag₂Se QDs phyto-catalysts could be made evident from the appearance of the characteristic peaks of Ag₂Se in the XRD pattern [67]. As shown (Fig. 4), some peaks corresponding to orthorhombic Ag₂Se were observed at the position (2θ) of 23.05°, 26.90°, 31.07°, 32.88°, 33.63°, 34.90°, 37.09°,40.15, 42.76°, 45.14°, 46.86°, 48.71°, 50.07°, 53.38°, 55.20°, 57.41°, 59.29°, 62.52°, 65.92°, 69.61°, 71.33, 75.28, and 79.21 which were attributed to (0 0 2), (1 1 1), (1 0 2), (1 2 0), (1 1 2), (1 2 1), (0 1 3), (0 3 1), (1 1 3), (0 3 2), (0 0 4), (0 1 4), (1 3 2), (1 1 4), (2 2 2), (0 4 2), (2 3 1), (2 0 4), (3 0 1), (1 2 5), (1 5 1), (3 2 2), and (2 4 3) crystallographic planes, respectively. The results revealed the pure phase of the Ag₂Se QDs had an orthorhombic crystal structure.

Fig. 4.

XRD patterns of phyto-Ag₂Se QDs using ultrasound assisted synthesis (UAS).

3.2. Catalytic selectivity and activity

Different industries, including those associated with pharmaceuticals and fine chemicals, use many toxic, hazardous chemicals and chemical processes and produce relatively much more waste, which have adverse effects on nature, the environment, and human health [68]. Therefore, the use of green chemistry and safe materials and processes such as the use of water, recyclable/biocompatible catalysts, and the prevention and minimal use of chemicals, etc. are very important.

Water as green, inexpensive, non-toxic, abundant, non-flammable, and the available solvent is the ideal solvent. In addition, because of its high polarity, new reactivity and selectivity can be expected for organometallic catalysis in water[17].

Importantly, the use of the biocompatible/green catalyst (phyto-synthesized Ag₂Se QDs) coated with water-soluble plant organic compounds solved the problem of catalyst recovery and recycling by extraction in an aqueous biphasic system and by simple phase separation, using water with the catalyst remaining in the water.

The use of plant extracts to prevent the use of chemicals in nano-catalysts synthesis is also significant. The complexity and synergistic effect of all the components of plant extracts have increased their therapeutic effects and have caused them to act simultaneously as reducing, stabilizing, and capping agents with a natural origin in the synthesis of eco-friendly nanoparticles[69].

To demonstrate the applicability of UAS/phyto-synthesized Ag₂Se QDs as biocompatible/green catalysts, the catalytic hydrogenation of industrially important substrates (nitrobenzenes) under completely green conditions was tested.

To find the optimal conditions for the greenest, easiest, and most environmentally friendly hydrogenation of nitroarenes with the highest possible efficiency as well as to evaluate the selectivity and activity of Ag₂Se-Qs phyto-catalysts, 4-nitroaniline was selected as a model substrate. Thus, optimization of catalytic reduction reactions was performed under completely green conditions (H₂O, rt), and other common parameters such as the amount of Ag₂Se QD catalysts and NaBH₄ were tested. Initially, this reaction was tested by decreasing the amount of nano-catalyst. The Ag₂Se QD catalyst (rt, 60 min) in the presence of NaBH₄ (1 mmol) as the mild hydrogen donor was able to reach exclusively dianiline as the product in a > 99% yield, selectively (Table 2, entry 1). Moreover, reaction efficiency in the presence of 7 and 5 mg catalyst was significantly dropped to 55% and 40%, respectively (Table 2, entries 2 and 3). It was also found that by reducing the amount of NaBH₄ (0.5 mmol), the reaction efficiency decreased to 75% (Table 2, entry 4). Nevertheless, blank runs showed that in the absence of the Ag₂Se QD catalyst or NaBH₄, no reaction occurred, and the starting 4-nitroaniline was recovered (Table 2, entries 5 and 6). This suggested that both Ag₂Se QD phyto-catalysts and NaBH₄ have important roles in the reduction of 4-nitroaniline.

As a result, the best catalytic activity was obtained using the Ag₂Se QD phyto-catalyst (10 mg) and NaBH₄ (1 mmol) under green conditions (H₂O, rt) which provided the product with excellent yield and selectivity (>99%). It should be noted that under these conditions, the reduction reaction was carried out more rapidly by the Ag₂Se QD catalyst (∼1h) than previously reported similar reactions [28], [29], [70], [71], [72]. Thus, this phyto-nanocatalyst plays an important role in the green/environmentally friendly catalytic reduction with the excellent yield and selectivity.

Next, to reach a deeper understanding of the generality and limitations of the catalytic system as well as the ability of UAS/phyto-Ag₂Se QD catalysts in eco-friendly hydrogenation of nitrobenzene under green optimal reaction conditions, hydrogenations of diverse nitrobenzenes were investigated (Table 3). Corresponding anilines were characterized by ¹H NMR and ¹³C NMR. As shown in Table 3, nitroarenes substituted with electron-withdrawing and electron-donating groups including NH₂, OH, CH₃, Cl, Br, −COCl, −CHO, −OCOCH₃, −CH₂Cl, −CH₂OH, −CH₂Br, −CHCHCHO, etc. were readily hydrogenated to the targeted anilines in excellent selectivity and yield (>99%) without any byproduct. It was also found that both carbonyl and nitro groups in the aqueous medium were reduced (Table 3, 7 –10).

Table 3.

Substrate scope for the hydrogenation of nitroarenes.

Entry	Sub.	Product	t (min)	Conv. [%]	Sel. [%]
1			60	>99	>99
2			60	>99	>99
3			75	>99	>99
4			70	>99	>99
5			60	>99	>99
6			55	>99	>99
7			60	>99	>99
8			55	>99	>99
9			55	>99	>99
10			70	>99	>99
11			60	>99	>99
12			65	>99	>99
13			65	>99	>99
14			60	>99	>99
15			75	>99	>99

The synthesis conditions of Ag₂Se QD phyto-catalysts as well as their performance (effectiveness) in the reduction of nitroarenes were compared to those obtained for other catalytic systems containing Ag reported in the literature (Table 4). The results showed that not only was the synthesis of the Ag₂Se QD catalysts performed much faster, easier, with minimal use of toxic chemicals, and in completely green conditions than other catalysts in the previous literature; also, they are highly active in the catalytic reduction of various nitroarenes. It was surprisingly found that Ag₂Se QD-catalyzed hydrogenation of nitrobenzenes under green conditions (rt, H₂O) could be performed within ∼ 60 min in excellent yield and selectivity (>99%).

3.3. Gram scale experiment

Because of the excellent reduction results obtained for the reduction of nitrobenzenes into corresponding arylamines on small scale, the usability of this procedure in the gram scale experiment was investigated. To this purpose, a gram-scale catalytic reduction of 4-nitroaniline (10 mmol) to 1,4-diaminobenzeneas as the model reaction was chosen. This reaction was carried out using the phyto-Ag₂Se QD catalyst and NaBH₄ in water (green solvent) with a good yield. 1,4-Diaminobenzene as the reaction product was then isolated and purified. This method demonstrates the technical feasibility of replacing precious metal catalysts with inexpensive biocompatible catalysts, and it is expected that this protocol will expand the use of these catalysts in the hydrogenation of nitroarenes.

3.4. Catalyst recycling experiments

The recyclability and stability of the phyto-Ag₂Se QD catalyst is one of the important properties of the catalyst, and performance metrics for its use in cost-effective industrial processes, for use in consecutive runs, was also assessed. To this purpose, after diluting each reduction reaction mixture with water, it was centrifuged (6000 rpm, 35 min), and worked up with ethyl acetate for use in the next run. The Ag₂Se QD catalyst remained in the aqueous phase. The aqueous mixture containing nanocatalyst was reused again under identical reaction conditions. The results of recycling tests are shown in Fig. 5. The catalyst recycling results showed that they were surprisingly usable in five consecutive 4-nitroaniline reduction cycles, without loss of selectivity and need for reactivation (Fig. 5). As shown in Fig. 5, the targeted aniline was formed with excellent efficiency in the first five runs, but from the sixth cycle onwards, the reaction was performed with reduced efficiency. Recyclability of the catalyst is due to its good stability, obtained from covering its surface with plant phytochemicals. These phyto-Ag₂Se QD catalysts are completely safe for humans and the environment. Unlike Raney Ni, it is an eco-friendly catalyst and can be easily used under an air atmosphere without any danger or deactivation.

Fig. 5.

Catalyst recycling experiments.

3.5. Evaluation of antibacterial properties using DDM, MIC, and MBC assays

The antibacterial activities of phyto-synthesized Ag₂Se QDs catalysts under UAS (UAS/phyto-Ag₂Se QDs), as well as UA-based Fig extract, were also investigated against the following important microorganisms: P. aeruginosa, E. coli, B. subtilis, S. aureus bacteria by disk diffusion (DDM), MIC, and MBC assays.

The antibacterial results of samples at different concentrations (12.5, 25, and 50 mg/ml) were recorded as the diameter of the inhibition zone (mm) of bacterial growth using the DDM method (Fig. 6). A comparison between the in vitro antibacterial activity data by the DDM method showed that the tested samples have different effects on preventing the growth of bacteria and UAS/phyto-Ag₂Se QDs catalyst had more antibacterial activity (inhibition zone) than Fig extract against all selected bacteria. Also, there is a direct relationship between the concentration of samples and their antibacterial activity. Hence, the UAS/phyto-Ag₂Se QDs catalyst at 50 mg/mL concentration had the highest effect against both tested gram-positive and gram-negative bacterial strains (Fig. 6). As found, the samples showed the most antibacterial activity against E. Coli bacteria, which is attributed to the structure of their cell wall structure and the permeability of the samples to their cell wall [76].

Fig. 6.

Antibacterial studies result of Fig extract (A) and phyto-Ag₂Se QDs catalysts samples (B) using disc diffusion method (DDM).

Moreover, to further investigate the antibacterial properties of these phyto-nanocatalysts and corresponding Fig extract, the minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays were performed at lower samples concentrations (1000 to 0.488 ppm) on four different bacteria strains (Table 5).

Table 5.

MIC and MBC of UAE-based Fig extract (ppm) and Ag₂Se QDs (ppm) against Gram-positive/negative bacterial strains.

Samples	S. aureus		B. subtilis		E. coli		P. aeruginosa
	MIC	MBC	MIC	MBC	MIC	MBC	MIC	MBC
UAS/phyto-Ag2Se QDs catalysts	500	>1000	62.5	125	31.25	125	125	500
Fig extract	>1000	>1000	250	500	500	500	1000	>1000

The MIC and MBC values showed that there is a considerable difference in antibacterial activities of samples against tested bacteria as well as the UAS/phyto-Ag₂Se QDs catalyst coated with Fig phytochemicals (plant-coated) is the most active sample with maximum antibacterial effect against all tested bacteria, especially E. coli bacteria (MIC:31.25 ppm, MBC:125 ppm).

Therefore, biological results showed that the UAS/phyto-Ag₂Se QDs catalysts in very low concentrations have antibacterial activities. This could be due to their high available surface area, extremely small size as well as the Fig extract coating on their surface, which enhances the friction and contact between the phytochemicals in extract coating and microorganisms’ cells and thus facilitates their penetration into the cell and causes cell death [77], [78].

4. Conclusion

Ag₂Se QD was previously synthesized and used in various industries, e.g., as a non-toxic/safe agent in in-vivo imaging. For the first time, the rapid/one-pot synthesis of stable/biocompatible Ag₂Se QD phyto-catalysts (30 sec) with high antibacterial activity (ppm), as well as the hydrogenation of nitroarenes by these catalysts (∼60 min) were carried out in very high yields under completely green conditions (H₂O, rt, air atmosphere) by simpler/faster/safer/cheaper methods than other reported works, without the need for high pressure equipment or N₂/Ar atmosphere as well as without the use or production of hazardous/toxic substances and by-products. For a greener/faster synthesis of Ag₂Se QD phyto-catalyst, a combination of facile plant-mediated and sonochemical methods was used. Importantly, the present method did not need organic solvents nor unsafe and expensive chemicals; instead, aqueous Fig extract was used as capping, reducing, stabilizing agents, and water was used as a green and low-cost solvent. The use of green ultrasound irradiation greatly accelerated cell wall destruction and the extraction (10 min) as well as the synthesis of Ag₂Se QDs phyto-catalyst (30 sec) and simultaneously improved catalyst properties. This means that the obtained nanocatalysts were highly monodisperse, fine (∼7 nm), with good crystallinity and purity, and were generated without aggregation. A safe, silver selenide-catalyzed reduction of nitroarenes under absolutely green conditions (H₂O, rt, air atmosphere) in ∼ 60 min with excellent selectivity and yield (>99%) was performed. The catalytic activity results showed that these stable/biocompatible/eco-friendly Ag₂Se QD catalysts exhibit high activity in nitroarene reduction. Moreover, the product and this reusable catalyst can be easily separated from the reaction mixture by simple extraction. Hence, this safe phyto-catalyst offers direct access to a variety of anilines (important building in chemicals, pharmaceutical, and agrochemical industries) from toxic nitroarenes with excellent selectivity and yield as well as they can be used as reusable and efficient catalysts with good antibacterial activity to remove toxic nitroarenes from environmental effluents, intending to create a cleaner environment for sustainable development. With their operational simplicity and absolutely green conditions, they could enable wide applications in research and industries.

CRediT authorship contribution statement

Maryam Shahzad Shirazi: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Alireza Foroumadi: Validation, Resources. Iraj Saberikia: Software, Formal analysis. Mahdi Moridi Farimani: Conceptualization, Validation, Writing – review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.