3.1. Characterizations of T-ZnO-NPs powders and hydrogels
The surface morphology analysis of ZnO-NPs and T-ZnO-NPs depicted in Fig. 1(a and b), reveals distinctive crystal structures produced by the thyme extract. These ZnO-NP crystals exhibit a triangular shape, as evidenced by the SEM images, showcasing a homogeneous particle distribution. Although the SEM images present an overall even dispersion, occasional instances of significant aggregation are observed, a common characteristic in nanoparticles synthesized through green methods. This phenomenon is attributed to the amplified surface area and the persistent affinity of biosynthetic NPs, prompting their tendency to cluster or assemble (Vidya et al. 2013). The particle agglomeration is influenced by the polarity of the nanoparticles and their electrostatic attraction (Aminuzzaman et al. 2018).
The pH level plays a pivotal role in this process, affecting the quantity of H+ and OH− ions, consequently influencing ZnO formation, shape, and structure. Notably, higher pH levels result in the formation of smaller nanoparticles. The presence of additional OH− ions strengthens the Zn-O bond formation in the structure, as these ions are more strongly attracted to the positively charged Zn2+ (Wahab et al. 2009). Wang et al. (2008) contend that modifying the basicity of the solution allows for the tailored design of diverse ZnO morphologies, such as rods and flowers. SEM studies (Fig. 1c and d) reveal that raw HG and T-ZnO-HG materials form cauliflower-like morphological structures, characterized by convoluted composite bodies. In these images, ZnO nanoparticles have formed a core structure, surrounded by a coating of polymeric material (Fig. 1d). The resulting structure is convoluted and compact.
Figure 1
FTIR analysis was carried out on Zn NPs to determine the many unique functional groups associated with the generated nanoparticles. The biomolecules in charge of the production of T-ZnO-NPs, ZnO-NPs, T-ZnO-HG, and raw HG were described and identified using FTIR studies between 4000 and 400 cm− 1 (Fig. 2). Because Zn2+ ions from zinc acetate linked with polyphenols found in the plant extract and reduced to Zn+ during the synthesis of ZnO-NPs, it is known as a complicated production of Zn+-polyphenols in the reaction solution. These intricate events were demonstrated by comparable peaks in the FTIR spectra of raw ZnO-NPs and green T-ZnO-NPs synthesized from plant extracts (Fig. 2a and b). The FTIR spectrum of the raw ZnO-NPs (Fig. 2b) showed absorption bands at 3364, 1499, 1386, 1043, 940, 889, 829, 736, 701, 554, 463 and 416 cm− 1. O-H stretching and deformation, both linked to water adsorption on the metal surface, could be the source of the basic mode of vibration at 3364 cm− 1 in the raw ZnO-NPs FTIR spectrum. It is connected to the asymmetric stretching vibration of C = O at 1386 cm− 1. The binding at 1043 cm− 1 is due to the C-O stretching vibration. The creation of Zn's tetrahedral coordination is the cause of the absorption at 889 cm− 1. The stretching vibrations of ZnO nanoparticles are represented by the peaks that were found, which lie between 736 and 701 cm− 1. The absorption peak at 554 cm− 1 corresponds to the vibration mode linked to metal-oxygen (ZnO stretching vibrations).
Figure 2.
The FTIR spectrum of the T-ZnO-NPs (Fig. 2a) showed absorption bands at 3357, 1572, 1496, 1403, 854, 676, 544, 468 and 415 cm− 1. The broad and very strong band at 3357 cm− 1 may be caused by the hydroxyl functional groups. The peak at 1572 cm− 1 is associated with the C = C stretching vibrations present in the aromatic rings of polyphenolic substances. The COO-stretching mode of symmetrical acids is responsible for the peaks at 1496 and 1403 cm− 1 (Matinise et al. 2017). The FTIR spectra of thymus at 1048 and 1020 cm− 1 can also be linked to the stretching of the C-O, C-C, and C-O-C bonds from saturated esters, alcohols, phenols, cycloalkanes, and acid anhydrides in the plant extract (Arumugam et al. 2021; Gabriela et al. 2017). According to Ishwarya et al. (2018) and Rathnasamy et al. (2019), ZnO's primary absorption band is located between 400 and 600 cm− 1. The broad, strong band at 544 cm− 1 is linked to the Zn-O vibration (Tantiwatcharothai and Prachayawarakorn 2019). The peaks at 854, 676 and 622 cm− 1, alkanes and alkenes (C = C bending) exhibit bending vibrations (Abdullah et al. 2021; Bharathi and Bhuvaneshwari 2019; Larkin 2017).
The FTIR analysis demonstrated that the intermolecular interactions regulated the vibration of functional groups on the superabsorbent hydrogel fragments (Fig. 2c and d). The vibration bands of -OH stretching were responsible for the FTIR peaks detected at 3343cm− 1 (Coates 2006; Siipola et al. 2018; Sánchez-Borrego et al. 2022). The medium-intensity peak at 1426 cm− 1, known as the C = O stretching band, disclosed characteristics of both PVA and SA (Korbag and Saleh 2016). It is evident that when SA is added, the hydroxyl stretches bands widen significantly, providing strong evidence that a hydrogen bond can form between the hydroxyl groups of PVA and this SA group (raw hydrogel) (Fig. 2d). The peak at about 1011 cm− 1 attributed to C-O-C stretching bond from sodium alginate in hydrogel (Isawi 2020). In FTIR spectra, the stretching vibrations of aromatic C-H stretching vibrations and hydrogen-bonding C-OH groups can be seen at 1079 cm− 1 and 821 cm− 1, respectively (Coates 2000; Thayumanavan et al. 2014; Ray et al. 2020). Moreover, at about 484 cm− 1, C = O stretching bonds derived from sodium alginate were seen (Isawi 2020). Additionally, Table 1 displays the elemental compositions of the raw ZnO-NPs, T-ZnO-NPs, Raw HG, and T-ZnO-HG.
Table 1
The functional groups in the raw ZnO, T-ZnO, raw HG and T-ZnO-HG
Raw ZnO | T-ZnO | Raw HG | T-ZnO-HG | Groups/Compound Class | Reference |
3364 | 3357 | 3301 | 3343 | -OH stretching vibration band | Armynah et al. (2019); Coates 2006; Siipola et al. (2018); Sánchez-Borrego et al. (2022) |
| | 2936 | 2918 | C-H stretching vibrations of aliphatic groups | Kang et al. 2012; Shivakumara and Demappa (2019) |
| | 1714 | | C = O stretching | IR spectrum chart |
- | 1572 | | - | C = C stretching in aromatic rings of polyphenolic compounds | Korbag and Saleh (2016); Helmiyati and Aprilliza (2017) |
| | 1594 | 1597 | The asymmetric stretching vibration of COO- groups C = O stretching of PVA | Korbag and Saleh (2016); Helmiyati and Aprilliza, (2017); Ray et al. (2020) |
1499 | 1496 | | | C-H bending of alkanes | IR spectrum chart |
| | 1415 | 1426 | C = O stretching | Korbag and Saleh (2016) |
1386 | 1403 | | | Symmetric stretching vibration of O-C = O | Siipola et al. (2018) |
| | | 1304 | C-H bending | IR spectrum chart |
| | 1251 | | C = O stretching | Armynah et al. (2019) |
| | 1078 | 1079 | C-O stretching (primary alcohol) The peak indicating PVA presence | Thayumanavan et al. (2014); Shivakumara and Demappa, (2019) |
1043 | 1048 | | | various bond stretching (C-O, C-, and C-O-C) | Sethi et al. (2023) |
- | 1020 | 1024 | 1011 | C-O-C stretching bond from sodium alginate | Isawi (2020) |
940 | - | 936 | 939 | C = C bending | IR spectrum chart |
| 854 | | 877 | C-H bending | Coates (2006) |
829 | - | 814 | 821 | aromatic C–H stretching vibration | Ray et al. (2020) |
| 736 | | | C-H bending vibrations | IR spectrum chart |
676 | | | | C = C bending vibrations | Arumugam et al. (2021) |
544 | | | | Zn-O in Zn(OH)2 | Sethi et al. (2023) |
| | | 484 | C = O stretching bonds derived from sodium alginate | Isawi (2020) |
415 | 463 | | 438 | characteristic ZnO absorption band | Kumar et al. (2019); Abdullah et al. (2021) |
Table 1
Figure 3.
These peaks (JCPDS card number: 36-1451) describe the hexagonal wurtzite structure of ZnO nanoparticles (Kaliraj et al. 2019). The XRD diffractograms of the T-ZnO-NPs and raw ZnO-NPs-based manufactured green ZnO photo catalysts are shown in Fig. 3. The XRD patterns in Figs. 3a and b revealed distinct peaks at 2θ = 31.76o, 34.40o, 36.17o, 47.52o, 56.58o, 62.83o, 66.36o, 67.93o, 69.07o, 72.52o, 76.94o and 81.34 o. These peaks match well with the planes Miller indices for T-ZnO-NPs (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202) and (104) (Kaliraj et al. 2019). The observable line widening of the XRD peaks confirms the nanoscale particle size of the material. Based on XRD data, the Debye-Scherrer equation was used to compute the average particle sizes. The results showed that the synthesized T-ZnO-NPs had a size of 19.72 nm. The strong diffraction peaks and lack of impure reflections, respectively, supported the high crystallinity and purity of the produced ZnO-NPs. The ZnO-NPs obtained by green synthesis was completely pure and exceptionally crystalline in its pristine state; no peaks related to impurities were detected. These findings corroborate those of previous studies (Rana et al. 2016; Abdullah et al. 2020). Unlike previous research, which used high temperatures, our experiment was conducted at room temperature (Anbuvannan et al. 2015; Karnan et al. 2016). This implies that a fast, easy, and environmentally safe synthesis method might be used to create highly crystalline nanomaterials. Therefore, in terms of energy efficiency, our study has turned out to be a superior and more feasible choice for synthesizing ZnO-NPs.
3.2. Germination Tests
The results of the germination tests, conducted with two different forms of T-ZnO-NPs powder (suspension in distilled water) and hydrogels, are presented in Fig. 4a-b. In comparison with the control (Fig. 4a), it was observed that the application of T-ZnO-NPs suspension promoted wheat seed germination (p < 0.05). The seed germination percentage exhibited an upward trend with increasing T-ZnO-NPs concentration, reaching a maximum of 100% at higher concentrations of 2000, 3000, and 5000 mg/L. Additionally, a germination percentage of 97% was noted at the concentration of 4000 mg/L T-ZnO-NPs. Limited literature exists on the effects of green-synthesized ZnO-NPs on wheat seeds, and the few available studies suggest that nanoparticles enter through the seed coat pores, enhancing water molecule penetration into the seed during the germination process (Meher et al. 2020; Sharma et al. 2022; Itroutwar et al. 2020). The seed coat, with its selective permeability, plays a crucial role in protecting the embryo, potentially explaining the non-significant differences in seed germination percentage based on T-ZnO-NPs concentration (Asmat-Campos et al. 2022). Consistent with our findings, other studies have also recognized the positive effects of various green-synthesized ZnO-NPs on different seeds, positioning them as a promising agricultural nano-based source (Singh et al. 2019; Rani et al. 2020; Umavathi et al. 2021). Umavathi et al. (2021) synthesized ZnO-NPs using P. hysterophorus leaf extract and demonstrated a significant promotion in the seed germination of S. indicum seeds.
Seed germination is a vital process initiated when a seed absorbs water, leading to the expansion of the embryo and the emergence of the seedling as the root makes contact with water (Jampi et al. 2021). Another aspect of the seed germination test involved the application of T-ZnO-doped hydrogel. This application aimed to assess the efficacy of hydrogels in creating a conducive environment for seed expansion and germination initiation. As depicted in Fig. 4b, the seed germination percentage exhibited an increasing trend with rising T-ZnO-NPs content in hydrogels, reaching a peak at 0.2% T-ZnO-NPs in hydrogels in comparison to raw hydrogels. However, these increases in germination percentages did not surpass those of the control group (p < 0.05). In the hydrogel treatment, the minimum and maximum seed germination percentages were recorded at 0.2% T-ZnO-NPs content in hydrogel (73.3%) and raw hydrogels (40%), respectively (Fig. 4b). Additionally, the germination percentage of wheat seeds in the control group was determined as 93.3%. The results indicate that the presence of T-ZnO-NPs in hydrogel facilitated wheat seed germination, although it did not reach the level observed in the control.
Figure 4.
Existing literature suggests that synthesized hydrogels can influence the seed germination process, although studies have often focused on different hydrogels and plant species. For instance, Jampi et al. (2021) synthesized NaOH/urea and cellulose fibers-based epichlorohydrin hydrogels and assessed their effects on maize seed germination. Their findings revealed that the presence of hydrogel in the germination media did not significantly affect germination percentage compared to the control. The maximum germination percentage was observed in the treatment with 5 g water and 0.055 g dried hydrogel, while the treatment with only 5 g swelled hydrogel exhibited a lower germination percentage than the control group, similar to the observations in this study. In another study, Mat Nayan et al. (2018) demonstrated that PVA/Chitosan/fertilizer (N/P/K) hydrogel had a positive impact on the germination energy of okra seeds, attributing it to the hydrogel's ability to absorb large amounts of water and retain moisture effectively.
While germination percentages serve as the initial indicators of plant response to biotic or abiotic stress factors, significant differences in this parameter may not always be apparent. Consequently, the first assessment after germination percentages involves observing root and shoot elongation. As roots are the initial plant structures encountering external factors, they typically exhibit more pronounced reactions than shoots. Evaluating vegetative growth, such as root-shoot elongation, aids in determining the beneficial threshold of nanoparticles (Azim et al. 2022). Several studies have demonstrated that, despite the toxic effects of higher concentrations of nanoparticles (NPs) on wheat growth and development, lower concentrations can have positive effects (Prakash and Chung 2016; Baddar and Unrine 2018; Srivastav et al. 2021).
The impact of synthesized T-ZnO-NPs and T-ZnO-doped hydrogels (T-ZnO-HG) on the root and shoot elongation of wheat seedlings is illustrated in Fig. 4c and d. The results indicate that the T-ZnO-NPs suspension enhances root elongation at low concentrations (250–2000 mg/L) compared to the control group (p < 0.05), with inhibitory effects observed at higher concentrations (3000–5000 mg/L) (Fig. 4c) (p < 0.05). Besides that, significant differences in shoot elongation were observed between the control and T-ZnO-NPs treatment (p < 0.05), and the maximum decrease was determined at the 3000 mg/L concentration (p < 0.05). The initial growth and development period of all plants, encompassing seed germination and seedling formation, is crucial. In regions with insufficient soil moisture, such as arid and semi-arid areas, seed activities often slow down or are constrained (Abobatta 2018). Efficient agricultural management necessitates adequate and consistent water availability in the environment (Sasmal and Patra 2022).
In the hydrogel treatment, an opposing effect was observed compared to T-ZnO-NP suspensions. In this case, shoot lengths exceeded root lengths, except in the control and raw hydrogel treatment (Fig. 4d). The maximum root and shoot length were determined at 0.1% T-ZnO-NPs content in the hydrogel. Additionally, in this treatment, root elongation did not reach the level observed in the control, while shoot length exceeded that of the control. The presence of green-synthesized ZnO NPs using thyme in the hydrogel provided favorable germination conditions for wheat seeds, as evidenced by higher shoot elongation than root length. The application of hydrogels to create these conditions has garnered attention from researchers in the agricultural sector (Abobatta 2018). Research on this subject aligns with our findings. Sasmal and Patra (2022) investigated the effects of wheat straw-based cellulose and N,N-methylenebisacrylamide hydrogels on the seedling stage of corn, cucumber, okra, radish, and wheat seeds. The authors demonstrated that the hydrogel treatment promoted both shoot and root growth.
3.3. Pot experiments
In this investigation, three distinct pot experiments, NPs suspension, foliar, and hydrogel treatments, were conducted to assess their impact on plant growth. The T-ZnO-NPs suspension treatment in soil demonstrated an increase in plant growth with rising T-ZnO-NPs concentration (Fig. 5a) (p < 0.05). The maximum and minimum plant heights for wheat seedlings were recorded at 4000 mg/L and 250 mg/L T-ZnO-NPs concentrations, measuring 38.9 cm and 29.3 cm, respectively, while the average plant height for control groups was 33.9 cm. For the foliar treatment, all applications of T-ZnO-NPs, even at low concentrations, significantly promoted wheat plant height compared to control plants (Fig. 5b) (p < 0.05). Similar results were observed in T-ZnO-doped hydrogel treatments, except for the 0.5% T-ZnO-HG concentration (Fig. 5c), where increasing T-ZnO-NPs content in hydrogel led to decreases in plant heights (p < 0.05). To the best our knowledge there were not any similar studies in the literature that about compare these treatment types and effects of synthesized ZnO nanoparticles. However, the studies in the literature about the hydrogel applications in agricultural showed the uses of hydrogel is suitable advance material due to their water uptake capacity, fertilizer reservoir, and lightweight properties (Calcagnile et al. 2019; Palanivelu et al. 2022). The quantity of additives (chitosan, biochar, cellulose, green-synthesized materials, etc.) in hydrogels emerges as a crucial factor for healthy plant growth (Doğaroğlu et al. 2023; Wang et al. 2023).
Figure 5.
Chlorophyll content is a vital parameter for plant health, and our results demonstrate that soil and foliar treatment of T-ZnO-NPs suspension promoted wheat chlorophyll content at low concentrations (250 and 500 mg/L, p < 0.05), with no significant changes at higher concentrations compared to control (Fig. 5d and e). This aligns with previous studies by Dimpka et al. (2019) and Das et al. (2023), indicating increased photosynthetic pigment content in sorghum and wheat plants with foliar application of ZnO-NPs and zinc-chitosan-salicylic acid nanoparticles. T-ZnO-doped hydrogel treatment in soil also exhibited significantly increases in chlorophyll content with rising T-ZnO-NP content in hydrogels (Fig. 5f) (p < 0.05), consistent with findings by Liu et al. (2016), who reported enhanced chlorophyll content in young coffee trees with superabsorbent polymer treatment. The foliar application method, particularly at low concentrations, emerged as the most effective in promoting chlorophyll content, as indicated by the highest SPAD value. The maximum chlorophyll content was determined at the concentration of 250 mg/L T-ZnO-NPs suspension treatment in soil as 31.9 SPAD value, while it was 35.9 SPAD value (500 mg/L) for foliar treatment and 31.96 SPAD value (0.4%) for T-ZnO-doped hydrogel treatment. When the findings were evaluated, it was determined that the foliar application method had the best chlorophyll value at low concentrations. Hu and Xianyu (2021) highlighted that the majority of biochemical agents commonly employed in studies to foster healthy plant growth exhibit limited penetration to the intended target area. The utilization of nanotechnology to address this limitation is gaining prevalence. Recent research has demonstrated the crucial role of nanoparticles in facilitating the delivery of biochemicals to plants. For instance, CeO2, SiO2, and carbon dots, when applied foliar, exhibit distribution percentages of 100% to guard cells, 90.3% to the extracellular space, and 55.8% to chloroplasts in corn and cotton leaves (Hu et al. 2020; Hu and Xianyu 2021). Furthermore, previous studies, such as Adhikari et al. (2016), have indicated that the application of ZnO-NPs can enhance both plant growth performance and chlorophyll content.
In this investigation, we explored the crucial growth parameter of the dry-to-fresh weight ratio in wheat plants subjected to various treatments. Surprisingly, the hydrogel treatment outperformed both T-ZnO-NPs suspension in soil and foliar treatment, yielding the maximum dry-to-fresh weight ratio for wheat (Fig. 6). Besides that, there were significant changes between different concentrations of T-ZnO-NPs suspension in soil or foliar treatment (p < 0.05). However, these changes were not obvious as like hydrogel treatment (Fig. 6a and b). The trend depicted in Fig. 6c indicated a notable increase in the dry-to-fresh weight ratio with escalating T-ZnO-NPs content in hydrogels until the 0.1% T-ZnO-HG treatment (p > 0.05), beyond which the ratio stabilized (p < 0.05). While conventional studies often report fresh and dry weights separately or as the ratio of leaf dry weight to fresh weight, our focus on the ratio of the entire plant provides a more comprehensive understanding of growth dynamics (Solanki and Laura 2018; Huang et al. 2019; Rai-Kalal and Jajoo 2021; Farooq et al. 2023). The proportional increase observed in the T-ZnO-doped hydrogel treatment suggests improved plant nutrition or enhanced uptake of essential elements such as zinc and phytochemicals from thyme. Zinc, a known essential element, promotes enzymatic and hormonal activity in plants (Asmat-Campos et al. 2022). Corroborating this, Shoo et al. (2021) emphasized the relevance of dry matter in cereal plant yield, while Rai-Kalal and Jajoo (2021) reported increased fresh and dry weights in wheat seedlings treated with ZnO-NPs. Essentially, the ratios of leaf dry weight to fresh weight vary across different individual leaves, challenging the assumption of a constant value. Despite this variability, many researchers adopt leaf dry weight as a representative measure of leaf biomass when investigating the scaling relationship between leaf biomass and area. This scaling has direct implications for the light-capturing surface area, influencing the investment of dry biomass. It is important to note that, for most plant species, leaf dry weight does not exhibit a proportional relationship with leaf fresh weight (Huang et al. 2019). In another study, the combined application of ZnO + CaO at a concentration of 50 ppm demonstrated positive effects on shoot length, number of shoots, number of roots, yield per plant, fruit weight, and leaf area. Conversely, root length, plant weight, and fruit diameter exhibited higher values at a concentration of 200 ppm of ZnO + CaO. Additionally, a zinc nano-spray treatment on tomato plants resulted in a 7.7% increase in fresh plant weight compared to conventional zinc salt-treated tomato plants (Farooq et al. 2023). In addition, growing plants in coco peat, perlite and vermiculite medium fertilized with Pusa HG + Vermicomposting gave very positive results. This approach resulted in the highest values for the number, length, and breadth of leaves, plant dry weight, number of roots and flowers, stalk length, flower head diameter, days taken for flower senescence, and vase life (Verma et al. 2019).
Figure 6
Zinc is an essential mineral for plants that affects many different aspects of their lives, including protein synthesis, gene expression, metabolic functions, water uptake and transport, and enzyme activity (Hafeez et al. 2013). As a result, an increase in Zn level benefited the plants and enhanced growth. It was determined that T-ZnO-NPs suspension administered to the soil significantly increased the Zn content in plants up to a concentration of 4000 mg/L (Fig. 6d) (p < 0.05). When the T-ZnO-NPs concentration in the environment increased to 5000 mg/L, the Zn2+ content taken into the plant decreased to the level of control plants again (Fig. 6d). The maximum Zn2+ uptake was determined at the 2000 mg/L as 20.32 mg/g dw (p < 0.05), while the control plants had 13.26 mg Zn2+/g dw. In other words, plants did not uptake the Zn2+ in the environment when T-ZnO-NPs suspension were present at concentrations as high as 5000 mg/L (13.72 mg Zn/g DW) in the environment. Plants treated with foliar spray showed a considerable rise in Zn2+ content between 250 and 3000 mg/L; at 4000 mg/L ZnO-NPs concentration, Zn2+ uptake declined once more; and at 5000 mg/L concentration, plant Zn2+ content dropped to that of control plants (Fig. 6e) (p < 0.05). In other words, at a concentration of 5000 mg/L, plants did not uptake Zn2+. In the foliar treatment, the maximum Zn uptake was determined at the 3000 mg/L T-ZnO-NPs concentration as 25.59 mg Zn2+/g dw, while the control plants had 12.41 mg Zn2+/g dw. Depending on the concentration increase, HG in the environment enhanced Zn2+ uptake in ZnO-NPs given to the environment as HG (Fig. 6f). The highest increase was measured at 13.51 mg Zn2+/g dw (p < 0.05) at 0.3% T-ZnO-HG treatment. Furthermore, the control plants had a Zn2+/g dw of 2.97 mg (Fig. 6f). The hydrogel treatment in soil demonstrated a similar tendency between the fresh weight to dry weight ratio and the Zn2+ content in plants, but the other treatment types (foliar treatment and suspension application in soil) did not demonstrate the same trend as clearly as the hydrogel treatment (Fig. 6). Additionally, while the hydrogel application had the highest values in plant biomass, it was also shown to have the lowest Zn uptake. Furthermore, the optimal Zn content and biomass for spray application were identified.