Eco-Friendly Synthesis of Cerium Oxide Nanoparticles from Lycium cooperi

Abstract

Cerium oxide nanoparticles (CeO₂-NPs) offer promising advantages in semiconductors and biomedical applications due to their optical, electrical, antioxidant, and antibacterial properties. However, the widely reported synthetic strategies for CeO₂-NPs demand toxic precursors and intermediary pollutants, representing a major limitation to CeO₂-NPs applications. Therefore, it is necessary to develop greener strategies that implicate ecological precursors to reduce the negative impact on the scalability of CeO₂-NPs. In this regard, we applied Lycium cooperi (L. cooperi) aqueous extracts as an unexplored potential green reducing agent for the eco-friendly synthesis of CeO₂-NPs. The L. cooperi extract showed the presence of alkaloids, flavonoids, cardiac glycosides, and carbohydrate-derived families, which were assessed for spherical monodispersed CeO₂-NPs under a rapid chemical reduction. Moreover, the elemental composition revealed Ce and O, indicating highly pure CeO₂-NPs characterized by an interplanar cubic crystalline structure. Furthermore, we detected the presence of stabilizing functional groups from L. cooperi, which, after a controlled annealing process, resulted in a band gap energy of 3.9 eV, which was optimal for the CeO₂-NPs. Thus, the results indicate that L. cooperi is an environmentally friendly synthesis method that can open a new route for CeO₂-NPs in biomedical and industrial applications.

1. Introduction

Cerium oxide (CeO₂) is an n-type semiconductor material with a band gap of approximately 3.3 eV [1]. Interestingly, CeO₂-NPs have garnered significant attention due to their improved optical, electrical, catalytic, and thermal properties, along with greater chemical stability compared with non-nanostructured CeO₂ precursors [2]. Moreover, CeO₂-NPs are considered a highly promising material for various applications, including optoelectronic devices [3], energy conversion and storage [4], catalysis [5], tissue engineering [6], biotechnology [7], and nanomedicine and antioxidants in biological systems [8]. In this regard, CeO₂-NPs have been synthesized using different chemical strategies, such as sol-gel, colloidal synthesis, hydrothermal, and precipitation methods [9,10]. These synthetic approaches allow precise control of nanoparticle size, morphology, and chemical composition. However, an important disadvantage of the chemical method is that it often involves toxic solvents and significant pollutant substances. Thus, considering these detrimental issues, green precipitation mechanisms have been widely applied using plant extracts for a facile, economic technique, which is commonly used for CeO₂-NPs. On the other hand, plant extracts contain secondary metabolites (e.g., flavonoids, catechins, coumarins, and more) that act as chelating, reducing, and stabilizing agents to obtain controlled metallic nanoparticles [11,12,13]. In addition, many of these secondary metabolites exhibit antioxidant, anti-inflammatory, and antimicrobial activity, with significant potential biomedical applications.

Considering a novel, greener strategy, the genus Lycium, which belongs to the Solanaceae family with 80 species in different regions worldwide [14,15], is a promising choice for the synthesis of CeO₂-NPs. In recent years, several studies have explored the chemical composition and pharmacology of this genus, leading to the identification and isolation of over 200 compounds. These compounds include flavonoids, alkaloids, polysaccharides, steroids, terpenoids, carotenoids, amides, and essential oils [14]. This diversity of phytochemicals suggests that the Lycium genus is highly promising for the green synthesis of nanoparticles, but it has been scarcely explored for this application. It was recently reported that CeO₂-NPs were synthesized using the green precipitation method, employing various plant extracts such as Origanum majorana L. leaf extract [16], Acorus calamus [17], Calotropis procera flower extract [18], Azadirachta indica [19], Artabotrys hexapetalus leaf extract [20], Abelmoschus esculentus [21], Moringa oleifera leaf extract [22], and Rheum turkestanicum extract [23]. However, these methods require extended synthesis periods, the use of ammonium and sodium hydroxides as precipitants, and show large physicochemical variation, highlighting the importance of green precursor agents. Our methodology presents several notable advantages compared to those reported in literature including minimizing synthesis times to contribute to reduced energy consumption, excluding harmful chemical precursors, and reducing toxic waste generation. These attributes serve to underscore the ecological sustainability of this synthesis process.

The main objective of this work was to synthesize controlled, well-dispersed, homogenous spherical CeO₂-NPs from secondary metabolites of L. cooperi aqueous extract, taking advantage of its chelating and reducing properties [11,12,13]. The morphological, optical, and structural properties were characterized. To the best of our knowledge, no information exists about the use of this plant for the green synthesis of nanoparticles.

2. Experimental Details

2.1. Morphological Characteristic of L. cooperi Plant

Figure 1a displays the L. cooperi plant specimen harvested under controlled conditions in a greenhouse at the Instituto de Ingeniería, Universidad Autónoma de Baja California, in Mexicali, Baja California, Mexico. Figure 1b shows the characteristics of the leaves and flowers. According to the voucher specimen from the databases of the University of California, Berkeley (Figure 1c), the taxonomic identification is in accordance with L. cooperi [24].

Table 1. Characteristics of L. cooperi plant [24].

L. cooperi Plant
Habit:	Glandular-puberulent; branches rigidly ascending to erect, shrub, leafy.
Leaf:	Size: 1–3 cm, shape: oblanceolate to obovate.
Flower:	Calyx size: 8–15 mm, calyx shape: narrowly bell-shaped, lobe number: 4–5 mm, lobe shape: tube, lobe size: 1.5–3 mm, corolla shape: narrowly funnel-shaped, corolla color: white, corolla size: 9–12 mm.
Fruit:	Size: 5–9 mm, color: yellow to orange.
Seed:	Several
Distribution:	California, Arizona, and Utah.

presents the main morphological characteristics of the plant, along with information about its habitat and geographical distribution [24].

2.2. Preparation of L. cooperi Extract

The L. cooperi leaves were harvested to obtain an aqueous extract. Initially, the leaves were washed with abundant ultrapure deionized (DI) water (milli-Q, type 1) and then left to air-dry at room temperature for one day. Then, 10 g of dry leaves was pulverized and mixed with 100 mL of DI. The mixture was then heated to 60 °C for 30 min with constant stirring at 500 rpm. Finally, the L. cooperi aqueous extract was filtered using filter paper (Whatman N°4, 55 mm) and stored at 4 °C for later use. Figure 2 shows the preparation process of the L. cooperi aqueous extract.

2.3. Phytochemical Characterization of L. cooperi Extract

Phytochemical tests were conducted to identify the secondary metabolites in the aqueous extract of L. cooperi, as follows:

• Alkaloids

A few drops of Hager’s reagent (picric acid) were added to 2 mL of extract. The formation of a yellow color crystalline precipitate indicated the presence of alkaloids.

• Flavonoids

To evaluate flavonoids, we prepared two test tubes containing 2 mL of L. cooperi and 2 mL of DI. Then, for comparison of the samples, 2 mL of NaOH (0.1 M) was added. A change in the color solution to yellow indicated the presence of flavonoids.

• Saponins

A quantity of 5 mL of extract was vigorously shaken and then kept static for 10 min. The presence of a stable froth indicated that saponins were in the extract.

• Tannins

A few drops of FeCl₃ were added to 2 mL of L. cooperi extract. A green condensed formation indicated the presence of tannins.

• Terpenoids

A quantity of 5 mL of aqueous extract was mixed with 2 mL of chloroform and 1 mL of concentrated H₂SO₄. If positive, a red-brown color ring appeared in the lower chloroform layer phase.

• Cardiac glycosides

The aqueous extract (5 mL) was mixed in 2 mL of ferric chloride-glacial acetic acid solution and 1 mL of concentrated H₂SO₄. The appearance of a brown ring at the interface indicated the presence of glycosides.

• Carbohydrates

To the aqueous extract (5 mL), 2 mL of acetic acid and 2 mL of chloroform were added. Subsequently, three drops of concentrated H₂SO₄ were added to the mixture. The presence of an orange precipitate indicated the presence of carbohydrates.

The functional groups of the secondary metabolites and certain chromophore groups within the aqueous extract of L. cooperi were characterized by FT-IR (PerkinElmer Frontier FT-IR; 400–4000 cm⁻¹) and UV–Vis (Shimadzu 260; 200–800 nm) analyses.

2.4. Green Synthesis of CeO₂-NPs

A precursor solution of 0.1 M Ce³⁺ was prepared by dissolving 2.17 g of cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) in 50 mL of L. cooperi aqueous extract. The mixture was heated to 80 °C for 1 h with constant stirring at 500 rpm to produce cerium hydroxide (Ce(OH)₃) precipitates. Ce(OH)₃ precipitate was centrifuged at 4000 rpm, then vacuum filtered, washed at environmental conditions with DI and absolute ethanol, and dried overnight at 60 °C. Finally, Ce(OH)₃ was calcined at 400 °C in a muffle furnace for two hours in the air to obtain CeO₂-NPs. Figure 3 illustrates the synthesis process of CeO₂-NPs using the green precipitation method.

2.5. Characterization of CeO₂-NPs

The morphology, particle size, and chemical composition were determined using SEM (Tescan LYRA 3 XMH, Tescan, Brno–Kohoutovice, Brno, Czech Republic) equipped with an EDS (Quantax Bruker X-Flash 6160, Tescan, Brno, Czech Republic) and HR-TEM (JEOL 2010, Japan Electron Optics Laboratory, Mitaka, Tokyo, Japan). The SEM micrograph was acquired using an accelerating voltage of 10 kV and a magnification of 40 kX. The mounting conditions for CeO₂ nanoparticles consisted of dispersing 1 mg of CeO₂-NPs in 5 mL of absolute ethanol. This process was conducted using an ultrasonic homogenizer (OMNI Sonic Ruptor 400, OMNI International, Kennesaw GA, USA) for 15 min at a power output of 160 watts and a frequency of 20 kHz. Subsequently, a 10 μL aliquot was extracted, and the dispersion was deposited onto a TEM grid or carbon tape (SEM) for further study. The TEM image was captured using an accelerating voltage of 200 kV. The structural and crystallographic information of the CeO₂-NPs was obtained using TEM and XRD (Panalytical Empyrean, Malvern, Worcestershire, UK) with a Cu Kα radiation source (λ = 0.15406 nm) and a real-time multipass detector X’Celerator, with scanning over the 2–theta range 20–90 at room temperature. FT–IR spectra were recorded in the 400–4000 cm⁻¹ range (Frontier Perkin Elmer, Waltham, MA, USA)) in attenuated total reflection (ATR) mode. For accurate FT–IR analysis of the aqueous extract of Lycium cooperi, aliquots were deposited onto the ATR platform and allowed to dry under ambient conditions until sufficiently dry material was obtained.

The optical properties of the CeO₂-NPs were characterized using UV–Vis analysis from 200 to 800 nm (Shimadzu 260, Shimadzu, Kyoto, Japan). For the UV–Vis analysis, a CeO₂-NPs dispersion was prepared in water, following TEM protocols. An aliquot of 500 μL was then diluted in 5 mL of deionized water (DI) for the measurements.

3. Results and Discussion

3.1. Phytochemical Analysis of L. cooperi Aqueous Extract

In

Table 2. Phytochemical test results of L. cooperi aqueous extract.

Metabolite	Test	Result
Alkaloids	Hager and tannic acid	(+)
Flavonoids	Ammonia	(+)
Saponins	Froth’s	(−)
Tannins	FeCl3	(−)
Terpenoids	Terpenoid	(−)
Cardiac glycosides	Keller–Kellani	(+)
Carbohydrates	Benedict	(+)

, we present the results of the phytochemical tests. Initially, the Hager, ammonia, Keller–Kellani, and Benedict tests showed positive results, indicating the presence of alkaloids, flavonoids, cardiac glycosides, and carbohydrates, respectively. On the other hand, the analyses for saponins, tannins, and terpenoids were negative. The findings demonstrate that the aqueous extract of L. cooperi contains the metabolites flavonoids and carbohydrates, which are chemically highlighted due to the numerous hydroxyl groups (-OH).

These metabolites are known for their ability to act as chelating and reducing agents in the formation of nanoparticles in aqueous solutions [25], eliminating the need to add acids or bases. Carbohydrates and flavonoids play multiple roles in the synthesis of nanoparticles acting as chelating, stabilizing, and reducing agents. They can release electrons by breaking the O–H bond, which allows the released electrons to reduce Ce³⁺ ions to Ce⁰, initiating the nucleation process for Ce(OH)₃ formation, which is then calcined to obtain oxidized CeO₂-NPs.

Figure 4a illustrates the FT-IR spectrum of L. cooperi extract. The 3340 cm⁻¹ and 1420 cm⁻¹ peaks correspond to O-H bond stretching and bending; the 2928 cm⁻¹ and 2850 cm⁻¹ peaks correspond to –CH, –CH₂, and –CH₃ stretching vibrations associated with carbohydrates [26]. Moreover, the peak at 1564 cm⁻¹ shows the stretching behavior of the aromatic ring’s C-C bond, the band at 1090 cm⁻¹ is attributed to the C-O stretching, and the shoulder band at 1626 cm⁻¹ is attributed to the stretching vibration of the carbon-oxygen double bond [26]. Finally, a peak is observed at 598 cm⁻¹, suggesting the presence of halide groups in the extract. The families of secondary metabolites identified by the phytochemical reactions in the L. cooperi extract correspond to the functional groups reported in the molecular structure by FT–IR.

The absorption band observed at 225 nm in the UV–Vis spectrum of the L. cooperi extract (Figure 4b) is attributed to the electronic transitions of π → π* orbitals, which occur in the structure of the chromophore groups present in the sample.

The FT–IR and UV–Vis analyses confirm the presence of green-derived metabolites, corresponding to a mixture of flavonoids, cardiac carbohydrates, alkaloids, and polymeric carbohydrates identified through phytochemical tests.

3.2. Physicochemical Analysis of CeO₂-NPs

The SEM micrographs show the agglomeration of CeO₂-NPs, highlighting a homogenous spherical morphology and showing an average size of 86 ± 10 nm (Figure 5a). On the other hand, the chemical composition of the CeO₂-NPs by means of EDS illustrated the presence of Ce and O, suggesting the formation of highly pure CeO₂-NPs (Figure 5b).

Figure 6a displays the TEM micrographs of the CeO₂-NPs, showing a spherical morphology and a size distribution of 6.9 ± 1.5 nm (inset Figure 6a). Compared with the results obtained using SEM (Figure 5a), this size reduction is attributed to the process of dispersion and sample preparation by ultrasound. The nanoparticles size significantly influences their physicochemical properties. For example, the catalytic activity of 7 nm CeO₂ nanoparticles is likely to be enhanced in comparison to that of 86 nm particles, primarily due to the increased surface area associated with the smaller size.

The high-resolution image (Figure 6b) confirms the crystalline structure of CeO₂. We detected an interplanar distance of 0.31 nm on the (111) plane, which is in accordance with previous work [27]. Additionally, the electron diffraction image confirmed the presence of the cubic phase structure of CeO₂, as expected.

In Figure 7, the XRD patterns of the CeO₂-NPs exhibit peaks values (2θ) of 28.35°, 32.85°, 47.39°, 56.09°, 58.79°, 69.09°, 76.37°, 78.96°, and 88.15°, corresponding to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes, respectively. These peaks agree with the typical signals of CeO₂-NPs showing a cubic crystalline structure (ICSD 98-006-1595), as demonstrated in previous studies [17,18,19,20,21,22]. The crystallite size (D) was calculated by Scherrer’s equation:

$$D={\displaystyle \frac{0.9\lambda }{\beta \cos \theta }}$$

(1)

where λ is the wavelength, β is the full width at half maximum (FWHM), and θ is the Bragg’s angle expressed in radians. The calculated crystal size was ~6.5 nm.

Additionally, the interplanar distance ($d$) for plane (111) was determined by Bragg’s equation:

$$d={\displaystyle \frac{n\lambda }{2\sin \theta }}$$

(2)

where n is the order of diffraction. The results indicated that the interplanar distance was 0.31 nm for plane (111), consistent with the HR-TEM findings and supported by previous work on green strategies and thermal CeO₂-NPs [27].

The FT-IR spectrum (Figure 8) exhibited a peak at 405 cm⁻¹, which corresponds to the Ce–O stretching vibrational mode [19]. Low-intensity peaks were displayed at 1315, 1502, and 1630 cm⁻¹ and are attributed to traces of organic matter from the phytocompounds of the extract. The peak at 3350 cm⁻¹ indicates the presence of O–H groups; nonetheless, it was a low-intensity peak [17,22].

The optical bandgap of the CeO₂-NPs was determined using the Tauc plot method (Figure 9) from the UV–Vis absorption spectrum (Figure 9 inset). The Tauc expression relates the optical band gap, E_g, and absorption coefficient, α, in the following expression:

αhν = B [(hν − E_g)]ⁿ

(3)

where hν is the photon energy, B is the proportionality constant, and n is a number that depends on the nature of the electronic transitions responsible for the absorption. E_g was determined by fitting the linear zone of (αhν)² vs. hν dependence and extrapolating to the energy. The optical band gap obtained for CeO₂-NPs was ~3.29 eV. This value corresponds to the band gap energy reported in the literature for CeO₂-NPs [1,18]. The bandgap energy of CeO₂-NPs indicates their potential for photocatalytic applications. Under the influence of UVA light, these nanoparticles can generate reactive oxygen species, which facilitate the degradation of contaminants in water. Moreover, CeO₂-NPs could be used in sensors to detect UVA light.

Table 3. A comparison of the green precipitation method for the synthesis of CeO₂-NPs reported in the literature with the results obtained in this work.

Plant Extract	Size of NPs	Band Gap	Synthesis Time and Temperature	Calcination Temperature and Time	Ref.
Origanum majorana L.	~20 nm	-	48 h at 100 °C	450 °C for 4 h	[16]
Acorus calamus	~22 nm	-	4 h at RT	400 °C for 2 h	[17]
Calotropis procera	~21 nm	~3.3 eV	3 h at 85 °C	400 °C for 2 h	[18]
Azadirachta indica	~28 nm	-	-	-	[19]
Artabotrys hexapetalus	-	~3.2 eV	6 h at 80 °C	-	[20]
Abelmoschus esculentus	~36 nm	-	6 h at 120 °C	600 °C for 4 h	[21]
Moringa oleifera	~17 nm	~2.6 eV	2 h at 80 °C	600 °C for 2 h	[22]
Rheum turkestanicum	~30 nm	~3.3 eV	24 h at 80 °C	400–600 °C for 2 h	[23]
Lycium cooperi	~7 nm	~3.3 eV	1 h at 80 °C	400 °C for 2 h	This work

compares the results reported in the literature with our findings regarding size and optical properties, as well as the reaction time of the CeO₂-NPs obtained through green synthesis using various plant extracts. Our method results in shorter synthesis times compared with those reported in the literature [16,17,18,19,20,21,22,23].

4. Conclusions

CeO₂ nanoparticles were synthesized by the green precipitation method from an aqueous extract of Lycium cooperi. The SEM and EDS results indicate the formation of spherical CeO₂-NPs, with an average size of ~85 nm and an elemental composition of Ce and O atoms. However, the TEM results indicate an average size of ~7 nm; this reduction in the CeO₂-NPs’ size is attributed to the ultrasonic dispersion process. Thus far, this extract has shown great potential as a green precursor for controlling the synthesis of spherical CeO₂-NPs.

The XRD and HR-TEM analyses revealed the formation of cubic-structured CeO₂-NP crystals, and the FT-IR spectrum confirmed the presence of the Ce-O stretching bond. Moreover, the calculated CeO₂-NPs bandgap was ~3.29 eV, determined from the UV–Vis absorbance spectra and Tauc plot method.

This study demonstrates an affordable and eco-friendly method for obtaining CeO₂-NPs. Notably, it reports for the first time the application of L. cooperi extract as a green-derived strategy for controlling the size, morphology, and distribution of cerium oxide nanoparticles. The environmentally friendly synthesis method and the characteristics of the resulting CeO₂-NPs can open a new route for extensive biomedical and industrial applications.