A Novel Approach to Manufacturing an Antioxidant Material, GT-Ag@MSN, Using Recycled Silver and Silicon from Scrapped Photovoltaic Panels

Abstract

This study developed a microporous silica nanosilver antioxidant material (GT-Ag@MSN) from waste photovoltaic (PV) cells by incorporating plant polyphenols in the in situ synthesis. The biosynthesized GT-Ag@MSN had an average size of 296.5 nm, a pore size of 1.96 nm, and an Ag loading of 1.45%. The material was further evaluated through antibacterial tests, antioxidant capacity tests, and a reducing power assay. GT-Ag@MSN exhibited a minimum inhibitory concentration (MIC) of 20 mg/mL for Escherichia coli and Staphylococcus aureus, and a minimum bactericidal concentration (MBC) of 50 mg/mL for both of them, which will need further efforts to improve the performance. However, GT-Ag@MSN exhibited a notable 1,1-Diphenyl-2-picrylhydrazyl (DPPH) scavenging ability of 74.7 ± 1.6% at a concentration of 250 μg/mL, and its reducing power in the range of 10–100 mg was greater than that of ascorbic acid at 10–100 μg/mL. This study proposes a new waste-to-wealth strategy that utilizes purified silicon and silver from recycling used PV modules, encouraging the advancement of PV waste recycling and reuse technology.

1. Introduction

Solar energy is an emerging renewable energy source that can be converted into electricity by solar cells (also known as ‘photovoltaic cells’). Photovoltaic (PV) cells have a service life of about 30 years, and if they are not treated and disposed of or reused after decommissioning, they will not only put heavy pressure on the environment but also constitute a serious waste of resources [1,2,3]. Based on an average lifespan of 25 to 30 years, the annual flux of the first retired PV panels (mainly as monocrystalline and polycrystalline silicon cells) is expected to increase significantly between 2025 and 2030. China is now the fastest-growing country for PV installations, followed by the US [4]. China’s PV panel end-of-life volume is expected to be the highest in the world in 2050, at around 33 million tons [5]. Current research trend on waste solar panel utilization indicates that the hot elements recycled from the crystalline silicon cells include silicon (Si), silver (Ag), and aluminum (Al) [6].

Silver nanoparticles (AgNPs), due to their extremely small size and large specific surface area, can slowly release silver ions. This makes them long-lasting, drug-resistant, and highly effective antimicrobial agents. However, AgNPs are prone to aggregation and oxidation because of their high surface energy. Additionally, the small size and surface characteristics of the nanoparticles can directly damage cell membranes due to the rapid release of Ag⁺, leading to apoptosis or necrosis. This poses limitations on their use as antibacterial materials in clinical practice [7]. To overcome these obstacles, researchers have developed various silver nanocarrier materials for effective and multifunctional action in infection control, wound treatment, and cancer therapy, including silica [8], gelatin [9], and TiO₂ [10]. In recent decades, mesoporous and microporous silica nanoparticles (MSNs) have been extensively utilized as Ag nanocarrier materials because of their high surface area, substantial pore volume, tunable pore morphology, ease of surface modification, and excellent biocompatibility [11,12]. They prevent the aggregation of nanosilver and release Ag⁺ in a controlled manner, producing long-lasting properties while minimizing toxicity. The most studied and representative MSNs include MCM-41, SBA-15, and TUD-1 [12]. Among them, MCM-41 features an ordered ‘honeycomb’ porous structure with adjustable pore sizes ranging from 1.6 to 10 nm and an extremely narrow pore size distribution, with the most typical pore size being approximately 4 nm [13]. Based on the description of PV cells, the high-quality silver and silicon obtained from the stepwise extraction process of crystalline silicon solar cells serve as an excellent source for synthesizing meso(micro)porous silica nanosilver.

Modifying AgNPs on MSNs in previous studies has required a step-by-step synthesis process, including the synthesis of MSNs, template removal, and the reductive generation of AgNPs, which takes several days. Toxic reducing agents such as sodium borohydride, formaldehyde, and hydrazine are commonly used for the reduction of the final AgNPs, significantly limiting their application in biological fields. In recent years, using extracts from natural materials such as plant leaves, peels, roots, flowers, fruits, and seeds to prepare nanoparticles has become a growing trend [7]. Plant polyphenols, reducing polysaccharides, and amino acids have been shown to function as reducing and stabilizing agents in the synthesis of AgNPs. The plant-based green synthesis method for nanoparticles is quick, cost-effective, easy to reproduce, and yields more stable materials, demonstrating greater development potential than conventional chemical methods. Polyphenols are water-soluble plant phenolic compounds that include phenolic acids (e.g., gallic acid, tannins, ferulic acid, chlorogenic acid), flavonoids (e.g., anthocyanins, catechins), astragals (e.g., resveratrol), and lignans (e.g., schisandrin), found in fruits [14,15], vegetables, and grains [16]. Polyphenols exhibit strong physicochemical properties and biological activities in applications. They can function independently or combine with metal ions, proteins, and other substances to form composites that demonstrate improved performance. This makes them a potentially valuable green ingredient for synthesizing nanomaterials [17]. For example, Li et al. [18] utilized tannic acid to reduce and deposit silver ions on different substrates (including cotton fabrics, copper foils, steel plates, iron sheets, glass sheets, rubber, and plastic sheets). The modified substrates exhibited persistent Ag⁺ release behavior and more than 95% antibacterial effect after five cycles. Chen et al. [11] prepared Ca-TA-MSN@Ag with a Janus structure using tannic acid, silver nanoparticles, and calcium ion-modified MSNs via redox and coordination reactions. Tannic acid and AgNPs synergistically produced excellent antibacterial and coagulation effects. AgNPs prepared by Ebrahim Mohammadzadeh et al. [19] using walnut shell extracts also demonstrated high selectivity and sensitivity to Cd²⁺ and Ni²⁺ for nanosensor detection. Plant polyphenols can enhance the antioxidant, reducing, and stabilizing properties of nanomaterials during the synthesis and application of these AgNPs, demonstrating high removal efficiency and reuse.

Among the natural extracts, tea extracts have become the current hotspot of polyphenol research. Recent studies indicate that the main components of tea extracts include phenol, 1,1′-biphenyl, 2-ethyl, 1,2,3-benzenetriol, 1,3,5-benzenetriol, 6-hydroxy4,4,7a-trimethyl-5,6,7a-tetrahydrobenzofuran, caffeine, zinitrile, and bis(2-ethylhexyl) phthalate [20]. Previous studies have shown that modifying the materials with tea extract as the reducing agent can significantly enhance their reactivity, thus facilitating the reduction and detoxification of target pollutants. For example, a green tea extract impregnation was used to introduce polyphenol functional groups on the surface of attapulgite, and the modified attapulgite achieved efficient reduction and stable remediation of soil Cr(VI) [21]. In addition, a novel adsorbent, ZIF-8-EGCG, was made using ZIF-8 and epigallocatechin gallate (EGCG), a component found in green tea extracts [22]. This composite nanomaterial significantly improved the adsorption (Q_max 136.96 mg/g) and reduction (96%) of Cr(VI) due to the reducing properties of the phenolic hydroxyl groups present in the structure of EGCG itself. Recent studies have also shown that introducing white tea extract as a green reducing and stabilizing agent for trace AgNPs can achieve the stable fixation of Ag⁺ on the surface of Fe₃O₄, and the final product Fe₃O₄@W.tea/Ag NPs demonstrated excellent results in catalysis, antioxidation, and anticancer activity against colon cancer [23]. Hassabo et al. [24] produced AgNPs with the assistance of tea extracts, significantly improving textile durability (increasing physical and mechanical properties and decreasing roughness), antimicrobial properties (against E. coli, S. aureus, and C. albicans), and UV protection (with higher UPF values) in the fabric finishing industry. The above studies show that tea polyphenols play an irreplaceable role in mediating the green synthesis of nanomaterials. However, studies on the preparation of synthetic AgNPs using tea extracts remain quite limited. The Ag precursor solutions utilized are chemical AgNO₃, and the reduction preparations are primarily conducted with media such as magnetite, cotton fabric, and wool. Additionally, there has yet to be an example of polyphenol-induced syntheses of MSNs from the products recovered from scrapped PV cells. Of course, using solid waste to synthesize nanomaterials is not without precedent; fly ash, bottom ash, slag, and waste from the photonics and semiconductor industries have served as sources of nanosilicon preparation [25]. Fu et al. [26] utilized the alkaline molten salt method to extract a silicon source (sodium silicate) from copper-bearing tailings, which subsequently led to the preparation of the mesoporous silica MCM-41.

In this study, waste PV solar panels were used as the research subject. High-quality sources of Si and Ag were sourced by dismantling components and applying chemical leaching. Based on this, we intended to prepare microporous silica nanosilver antioxidant and antibacterial materials in an environmentally friendly manner by combining the reducing properties of tea extract. This study innovatively employed scrapped PV panels for resource recycling to develop a program for producing AgNP-based antioxidant materials, which could improve the recycling technology and standard formulation of the PV waste industry.

2. Methods and Materials

2.1. Chemicals and Reagents

Sodium hydroxide (NaOH, AR), nitric acid (HNO₃, AR), hydrochloric acid (HCl, AR), potassium ferrocyanide (K₃[Fe(CN)₆], AR), Triton X-100 (CR), hexadecyl trimethyl ammonium Bromide (CTAB), ferric chloride (FeCl₃, AR), ascorbic acid (AA, AR), sodium chloride (NaCl, AR), phosphate buffering solution (PBS, 0.2 M, pH 6.6), 2,2-diphenyl-1-picrylhydrazyl (DPPH, 99.25%, AR), trichloroacetic acid (TCA, AR), analytical pure hydrazine hydrate (N₂H₄⋅H₂O, ≥85%), anhydrous ethanol (99.7%), and agar (10000561, BR) were supplied by Shanghai Guoyao Group Chemical Reagent Co., Ltd. Two bacterial strains, Escherichia coli (E. coli, CCTCC AB 93154) and Staphylococcus aureus (S. aureus, CCTCC AB91093), were obtained from the China Center for Type Culture Collection. McFarland turbidity tube 0.5 was sourced from Bkmam^® Co., Ltd. (Changsha, China). Yeast extract powder and tryptone were sourced from Shanghai Shengsi Biochemical Technology, Co., Ltd. (Shanghai, China). The green tea was purchased from Hunan Xiangliang Dongting Tea Co., Ltd. (Changde, China).

2.2. Solar Panel Separation and Preparation of MSN

A scrapped solar panel was provided by a local user with the trademark of TNG solar (Grade A PNG335, P/72), which is a widely distributed model in rural areas of China. The scrapped solar panel was first physically separated to obtain the crystalline Si cell, which was further etched and purified using the combined NaOH and HNO₃ etching procedure, as set out in a previous study [27]. In brief, 225 min 3 M HNO₃ etching at room temperature, followed by 40 min 3 M NaOH etching at 70^◦C completely removed the deep blue anti-reflective coating SiN_x and successfully removed metallic impurities such as Ag and Al. The above procedure could produce a clean Si wafer with a Si recovery rate of 99% and a purity of 93.2%, better than the recovery rate of 92.74% in the study by Wang and Ma [28]. A certain amount (1.1 g) of the cleaned Si wafer was then dissolved in 20 mL of 3 M NaOH to make a sodium silicate (Na₂SiO₃) solution (equal to 20% Si). The preparation of MSN was conducted using surfactants Triton X-100 and CTAB as templates following the previous procedure described in Yue et al. [27]. The dissolved Ag in the HNO₃ etching solution was then collected through a precipitation-reduction procedure using HCl, NaOH, and hydrazine hydrate. The extracted Ag⁺ was completely converted to elemental Ag with a high recovery rate of 92%. The above procedure is illustrated in Figure 1.

2.3. Production of GT-Ag@MSN

The raw Ag collected from the above procedure was further dissolved in 0.1 M HNO₃ to make an Ag stock solution (calculated to be 0.04 mol/L). Tea extract was used as a reducing agent for AgNP synthesis. Ten grams of dried green tea were added to 100 mL of ultrapure water in a 250 mL conical flask. The mixture was heated to 85 °C for 30 min, then cooled and filtered, and the filtrate was stored as a stock solution at 4 °C and used within one week. The total organic carbon (TOC) content of the tea extract was approximately 12.4 g/L. The stock solution of tea extract was diluted to 10%, 25%, 50%, and 100% (v/v) for use as reducing and capping solutions.

Then, 0.3 g of MSN was dissolved in 20 mL of anhydrous ethanol and ultrasonicated to mix. Subsequently, the MSN was transferred to 20 mL of 0.04 mol/L AgNO₃ and stirred at room temperature for 24 h. The product was then centrifuged and washed with deionized water several times to collect the MSN-Ag⁺. The concentration of tea leaf extract was evaluated for its reduction effect on forming AgNPs. The MSN-Ag⁺ product was redistributed in 50 mL of green tea extract solutions at the following concentrations: 10%, 25%, 50%, and 100%. After continuous stirring at 40 °C and 200 rpm for 24 h, GT-Ag@MSN was obtained through centrifugation at 4000 rpm. Finally, the product was vacuum-dried at 60 °C for 4 h and stored at 4 °C for further characterization.

The dosage of Ag was set up at various concentrations to evaluate the loading effect. In a similar procedure, 0.3 g of MSN was dissolved in 20 mL of anhydrous ethanol and ultrasonicated to mix. Subsequently, the MSN was dispersed separately in 20 mL of 0.0005, 0.01, and 0.02 mol/L AgNO₃ solutions. The MSN-Ag⁺ product was collected as above and then redistributed in 50 mL of 100% green tea extract. The mixture was stirred continuously at 40 °C and 200 rpm for 24 h. The final products were then vacuum-dried at 60 °C for 4 h and stored at room temperature for further characterization. The whole procedure of synthesizing the GT-Ag@MSN is shown in Figure 2.

2.4. Antibacterial Experiment

First, 2.0 g of tryptone, 1 g of yeast extract, and 2.0 g of NaCl were added to a 200 mL beaker with 200 mL of distilled water and stirred until fully dissolved to make the LB culture medium. The pH of the medium was adjusted to be approximately 7–7.4 by adding 40 μL of 5 M NaOH. Using a graduated cylinder, 50 mL of the prepared liquid medium was measured and poured it into a 250 mL conical flask. The mouth of the flask was covered with a pierced rubber cap to prevent contamination while ensuring good ventilation. Test tubes, with 5 mL of liquid medium in each, were covered with tube caps and secured with rubber bands. The remaining 130 mL of liquid medium was added to 1.95 g of agar powder (1.5% of the liquid medium), mixed well, gently poured into Petri dishes, and covered. The flask, tubes, and Petri dishes were all wrapped with double-layered newspaper and subjected to high-pressure steam sterilization. After cooling to room temperature, 100 µL of active bacterial stock was then transferred to the sterilized LB liquid medium flask for culture for 15 h at 200 rpm, 37 °C. The concentration of the bacterial stock was compared to the 0.5 McFarland turbidity tube (1 × 10⁸ CFU/mL).

In the antibacterial test, 0.2 g of GT-Ag@MSN was first dispersed in 5 mL of sterilized LB liquid medium, which was further diluted using serial dilution to obtain final concentrations of 50, 40, 30, 20, 10, 5, 2.5, 1.25, 0.625, 0.313, 0.156, 0.078, 0.039, and 0.02 mg/mL. Then, 200 μL of E. coli (1 × 10⁵ CFU/mL, using the bacterial stock and diluted to 1/1000) was added to each solution. For each concentration, two test tubes were prepared: one with 1 mL of sterile saline and 1 mL of bacterial suspension as the positive control, the other with 1 mL of sterile saline and 1 mL of the antimicrobial solution (e.g., 10 mg/mL) as the negative control. All test tubes were well shaken and incubated at 37 °C for 20 h. After incubation, the tubes’ turbidity was compared with the positive control to determine the lowest concentration of the antimicrobial agent that results in clear liquid, which is the minimum inhibitory concentration (MIC). Subsequently, an inoculating loop was used to transfer the contents from the clear test tubes onto solid agar plates, which were incubated for another 20 h. The lowest concentration of the antibacterial material that completely inhibits bacterial growth is the minimum bactericidal concentration (MBC). The experiment was repeated three times, and the average values were calculated. The procedure for testing the antibacterial activity against S. aureus was identical.

2.5. DPPH Free Radical Scavenging Experiment

To evaluate the antioxidant potential of GT-MSN@Ag, the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging method was used following a method in Negi et al. [29]. GT-MSN@Ag was sonicated in ethanol to obtain suspensions with different concentrations (100–250 μg/mL). These were then mixed with 400 μL of DPPH solution (0.1 mM in ethanol). After shaking, the reaction mixture was placed in the dark for 30 min, and the absorbance at 517 nm was measured. Ascorbic acid was used as a reference, and the blank control consisted of the reaction mixture without GT-MSN@Ag. Lower absorbance values indicated high scavenging activity. The percentage of inhibited or scavenged radicals was calculated using the following equation.

$$\mathrm{I}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{\%})={\displaystyle \frac{{\mathrm{A}}_1-{\mathrm{A}}_2}{{\mathrm{A}}_1}}\times 1$$

where A₁ represents the absorbance of the blank control, while A₂ represents the absorbance of the test sample solution.

2.6. GT-Ag@MSN Reduction Ability

Different masses of GT-MSN@Ag (10, 50, 100 mg) were mixed with phosphate buffer PBS (0.2 M, pH 6.6) and K₃[Fe(CN)₆] (1%). The reaction mixture was then incubated at 50 °C for 20 min. This mixture was then added to 2.5 mL trichloroacetic acid (10%) and centrifuged at 3000 rpm for 10 min to obtain the supernatant. The absorbance of Prussian blue (Fe₄[Fe(CN)₆]₃) was determined at 700 nm by mixing 2.5 mL superserum mixture with 2.5 mL distilled water and 0.5 mL FeCl₃ (to 0.01%). Ascorbic acid (AA) was used as the standard, and PBS as the control. The increase in absorbance of the reaction solution indicates that the reduction ability of the system is enhanced.

2.7. Characterization

Phase and structure identification of metal Ag and MSN was conducted by X-Ray Diffraction (XRD, Rigaku SmartLab SE, Tokyo, Japan). The pore diameter, pore volume, and specific surface area of MSN were measured using the Automatic Specific Surface Area and Pore Analyzer (BET, American Micromeritics ASAP 2460, Norcross, GA, USA). The morphology and elemental composition of PT-cells and MSN were observed using a scanning electron microscopy-energy dispersive spectrometer (SEM-EDS ZEISS Sigma 300, Oberkochen, Baden-Württemberg, Germany). The particle size distribution of the GT-Ag@MSN was measured on a Zetasizer Nano ZS90 (Malvern, Worcestershire, UK). The TOC of the green tea extract was measured using a Carbon Analyzer– (TOC-VCPH), Shimadzu, Kyoto, Japan.

3. Results and Discussion

3.1. Characterization of GT-Ag@MSN

The produced GT-Ag@MSN had an irregular spherical shape with an average diameter of 296.5 nm (Figure 3a,c). The N₂ adsorption–desorption isotherm of GT-Ag@MSN shown in Figure 3b resembles a Type I pattern in the IUPAC classification, which is a typical microporous adsorption pattern resulting from volume filling. The material had a specific surface area of 580.61 m²/g, an average pore size of 1.94 nm, and a pore volume of 0.2816 cm³/g, suggesting the microporous nature and high surface area of GT-Ag@MSN. As compared to the MSN from our previous study (specific surface area 855.30 m²/g, average pore size 1.85 nm, pore volume 0.3963 cm³/g) [27], the in situ formation of AgNPs on the material caused a reduced surface area but a slightly increased pore size and pore volume. According to the EDS analysis, the elemental compositions of the material followed an order of O (42.97%) > Si (28.18%) > C (27.22%) > Ag (1.45%) > N (0.19%). The C on the GT-Ag@MSN might come from the residue of surfactant templates due to incomplete combustion, while the negligible amount of N may particularly originate from the CTAB surfactant.

The changes in GT extract concentration and Ag⁺ concentration both had an obvious effect on forming a final product with sufficient antibacterial ability. Ag had the typical and strongest diffraction peaks at the (111), (200), (220), and (311) facets, similar to other reported studies [8,10,24], at the undiluted tea leaf extract (100%), and the initial Ag⁺ concentration of 0.04 mol/L (Figure 4). Therefore, the following experiments all used the GT-Ag@MSN produced under these conditions.

3.2. Antibacterial Performance of GT-Ag@MSN

In this study, two microbial strains, namely E. coli (gram-negative) and S. aureus (gram-positive), were used to study the antibacterial performance of GT-Ag@MSN. The GT-Ag@MSN materials were prepared in a series of solutions using the dilution method (50, 40, 30, 20, 10, 5, 2.5, 1.25, 0.625, 0.313, 0.156, 0.078, 0.039, 0.02 mg/mL) against the two strains of bacteria, and the MIC values were determined by turbidimetry using the broth microdilution method based on the clarity of the solution [30]. In Figure 5, the ‘+’ sign indicates turbidity, and the ‘−’ sign suggests clarity. The inhibition of the growth of both bacteria with the increasing concentrations of GT-Ag@MSN explains the concentration-dependent behavior of AgNPs [31]. The GT-Ag@MSN antibacterial powder had an indistinguishable MIC of 20 mg/mL for both E. coli and S. aureus, where the solutions were clarified at the high concentration, suggesting that the material concentrations at and above this level were able to inhibit the growth of the two strains. Regarding the MBC, no bacterial growth occurred at a material concentration of 50 mg/mL, suggesting that the MBC value of GT-Ag@MSN was 50 mg/mL. It is noteworthy that although the GT-Ag@MSN exhibited an obvious antibacterial function, the ability was compromised and not comparable to some reported AgNPs. The first reason may be due to the low loading of Ag on the MSN (100 mg MSN with only 1.45 mg AgNPs). The loadings of Ag directly determine the antibacterial efficiency. For example, the surface-coated SiO₂ with 0.1, 0.3, 0.5 g of Ag had a more significant antibacterial activity with an inhibition zone of 10–12 mm [8], whereas an acrylic polymer-coated Ag@SiO₂ with an Ag loading of 12.58% (higher than our study) only had an E. coli and S. aureus inhibition zone of 2–3 mm [32]. Secondly, the supporting MSN material showed a certain aggregation behavior (not easily dispersed after addition of PVP during synthesis, Figure S1) [33,34], which limited the interaction of AgNPs with bacteria. Therefore, the antibacterial performance could be further improved by optimizing the synthesis parameters of the GT-Ag@MSN.

3.3. DPPH Free Radical Scavenging Assay

Free radicals, such as hydroxyl, superoxide, and peroxide, are known to be highly responsible for cell damage and DNA damage and related to many diseases upon aging, including Parkinson’s disease, neural disorders, mild cognitive impairment, etc. [35]. AgNPs have been intensively studied for their efficient antioxidant capabilities [9,29,36,37]. In this study, the antioxidant activity of GT-Ag@MSN was also assessed against a stable free radical DPPH (Figure 6a), and the results showed that the synthesized GT-Ag@MSN possessed significant antioxidant ability. The free radical scavenging activity of the material increased with increasing concentrations (100–250 μg/mL), which is in line with previous studies [31,36]. The GT-Ag@MSN exhibited the highest DPPH inhibition activity (74.7 ± 1.6%) at the concentration of 250 μg/mL, with the scavenging activity enhanced by almost seven times. The DPPH scavenging capability of GT-Ag@MSN was also compared with that in some existing studies (Figure 6b). The highest percentage of scavenging at 250 μg/mL was better than many existing antioxidants, such as the AgNPs biosynthesized by Spirulina platensis extract [38], the AgNPs synthesized by Ceropegia debilis leaf extract [39], the AgNPs@gelatin bionanocomposite [9], the Aerva lanata flower extract-mediated AgNPs [37], and the white tea extract-modified magnetite-supported AgNPs [23].

3.4. GT-Ag@MSN Reducing Power

The strong reducing power given by natural plant extracts has been reported frequently in the biosynthesis of AgNPs. The solution’s increased absorbance is often positively correlated with enhanced reducing power [36]. In this study, the absorbance of AA at low concentration (10 μg/mL) was very low (0.04), suggesting almost no reducing power (Figure 6c). With the increase in GT-MSN@Ag mass (10–100 mg), the absorbance increases from 0.278 to 0.342, indicating that the reducing capacity of GT-MSN@Ag increases with the increase in concentration. The absorbance of 10 mg GT-MSN@Ag was close to 50 μg/mL AA (0.278 vs. 0.268), indicating that the reducing capacity of 10 mg GT-MSN@Ag was close to 50 μg/mL AA. GT-MSN@Ag at 50 mg began to have better reducing power than AA at 100–200 μg/mL (0.302 vs. 0.273–0.288). The reducing ability of GT-MSN@Ag at 100 mg was significantly higher than that of 200 μg/mL AA, indicating that it has strong reducing ability. The exceptionally high reducing ability of GT-Ag@MSN may come not only from AgNPs but also from the remaining reducing power of green tea extract left on the material as a capping and stabilizing agent during formation. The key function of polyphenols in the GT-Ag@MSN may be summarized as follows [17]: firstly, polyphenols act as stabilizers or capping agents that prevent the aggregation of nanoparticles and enhance the surface stability material; secondly, polyphenols function as reducing agents due to their strong reducing properties, which can reduce or eliminate the ROS activity; thirdly, polyphenols serve as chelating agents, with functional groups such as -OH and -COOH chelating the materials with Ag⁺ ions, which ensures the sustainable release of Ag⁺.

4. Conclusions

Rapid development of solar energy and the pending retirement of the first batch of commissioned solar panels has aroused widespread public concern pertaining to the reuse and recycling strategies for them. Any conversion of waste solar panels should conform to the principle of green chemistry. In this study, green tea extract was used as a reducing agent in the insitu biosynthesis of AgNPs on MSN using AgNO₃ solution and Na₂SiO₃ solution readily derived from the extraction and purification of scrapped crystalline Si cells. The as-formed GT-Ag@MSN, with an average size of 296.5 nm and pore size of 1.96 nm, demonstrated clear antioxidant, antibacterial, and reducing abilities. The waste-to-wealth conversion proposed by the study is a feasible, straightforward, cost-effective, and environmentally friendly strategy, providing a feasible pathway to a circular economy and sustainable environment. However, this study only provides a possible solution for transforming the waste solar cell elements into a medical antibacterial material. Future research is still needed to improve the antibacterial ability, considering the low mass loading of AgNPs and the aggregation behavior of MSN. Studies on the full life cycle assessment of this resource recovery route are also an indispensable aspect required for real application on an industrial scale.