All of the chemicals were purchased from Sigma Aldrich Chemicals (USA). Analytical grade solvents were employed during all syntheses. The elemental analyses were recorded on CHNS Analyzer, Perkin Elmer PE2400 Series II at Scientific and Technological Research Equipment Center Chulalongkorn University, Bangkok, Thailand. The morphology of particles was carried out by using Scanning Electron Microscope (Model JCM-6010 LV) and the infrared spectra were recorded on a Perkin Elmer 283-B infrared spectrophotometer.

All ligands were prepared by condensation of syringaldehyde with one of the five hydrazides indicated (Figure 1) in the molar ratio 1:1 in MeOH (5 ml) with constant stirring at 40˚C for 6 hrs and concentrating the reaction mixture before allowing it to cool at room temperature. The products were obtained with high percentages of the yield as shown in Table 1.

Synthesis of nanoparticles by using bottom-up technique between certain metal (II) chloride dihydrate or metal nitrate was added to the methanolic solution of syringaldehyde hydrazones in the molar ratio of 1:1 over a period of 3 hrs with constant stirring in dark. The products were separated by centrifugation and dried in vacuum.

The assay was performed as described Sharma et al. [7] , the DPPH radical scavenging activity of the compounds was measured. Briefly, 0.2 mM, 2-Diphynyl-1-picrylhydrazyl (DPPH) solution was prepared with methanol. 100 µL sample was added to 3.9 cm³ DPPH solution and was incubated in dark for 30 min at a temperature of 37˚C then the absorbance was measured at λ_max = 517 nm, calculate the percentage of anti-free radical activities.

Prepare FRAP (ferric reducing/antioxidant power) by preparing acetate buffer pH 3.6, 10 mM TPTZ (2, 4, 6-tripyridly-s-trizine) in solution 40 mM of HCL, and 20 mM of ferric chloride and then mix at the proportion of 10:1:1 (v/v). This will get a FRAP solution, then prepare a sample solution in purified water with the required concentrations [7] .

All the newly synthesized compounds were screened for antibacterial activity against both gram-negative (Escherichia coli and Salmonella spp.) and gram-positive bacteria (Staphylococcus aureus) by determining minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) using the broth dilution method. Bacterial colonies were taken from fresh cultures and incubated in the tubes containing 5 mL of trypticase soy broth at 37˚C for 6 hours until achieving visible growth. Turbidity was adjusted to 0.5 McFarland standard by adding 0.05 mL of 1% w/v BaCl₂·2H₂O in phosphate buffered saline (PBS) to 9.95 mL of 1% v/v H₂SO₄ in PBS. Finally, 10 mg of each compound was dissolved in 10 mL of dimethyl sulfoxide (DMSO) to obtain concentration of 1 mg/mL [8] .

The research team conducted a preliminary study and selected the best specific compounds against the larveae of the rice dark-head stem borer in the 1.5 × 1.0 × 1.0 m³ plots, each plot containing 40 borer larveae (F1). The experimental plan was completely randomized design (CRD), there were a total of 5 treatments, 3 replications, 20 tillers per plot of rice. The sample was prepared to be concentrated with 10.0 ppm mixed with surfactant and sprayed in the planting rice aged 45 days [9] .

All experiments were carried out in triplicates. Data obtained were analyzed by using one-way analysis of variance (ANOVA) and Pearson’s correlation coefficient was performed. Significant differences between the means at 95% (p < 0.05) level were considered statistically significant. Data was recorded as mean ± standard deviation.

Physical chemistry feature of compound from synthesis, both ligands; Syringaldehyde salicylic hydrazide (L1) Syringaldehyde isonicotinic acid hydrazone (L2), and complexes between ligands and metal ions (Cu²⁺ and Ag⁺) are C1, C2, C3 and C4 as in Figure 1.

The synthesized substances will have colors, molecular formula, melting point, percentage of elemental substances, and percentage of products in different figures as in Table 1 and Table 2, respectively.

All synthesized substances are polar compounds but polarity is high in ligand L1 and L2. These ligands are reacted with Lewis acid (copper and silver) to form C1, C2, C3 and C4 complexes with reduced polarity due to reduced charge. This is caused by the neutralization of negatively charged ligands with positively charged metal ions of copper and silver.

Note: *Cal, ^#Found.

Both ligands and complexes synthesized in this research showed high percentage yields (66.14% - 79.14%) by condensation reaction between syringaldehyde and hydrazide in a 1:1 mass stoichiometric ratio. The complex is green in color, ligands conjugate with metal salts. From the study of chemical composition, it was found that complexes have a general formular as [M(ligand)(H₂O)NO₃/Cl], corresponding to our previous studies [10] [11] and others [12] [13] .

The significant peaks in ir spectra of parent compounds and their probable assignments useful for determining the coordination mode of the coordination mode of the present ligands are summarized in Table 3.

The parent compound exhibits the carbonyl stretching frequency band at 1671 cm⁻¹, when this compound reacts with an amine group, a new Schiff base is formed, this functional group was replaced by the imino group, which showed additional vibration band at 963 - 976 cm⁻¹. There are also stretching frequency bands at 1600 - 1615 cm⁻¹ due to the formation of new hydrazinic N-N and C=O bands. The presence of v (N-H) and v (C=O) bands in the ligands indicate that these compounds are stable in the keto form in nature [14] , but upon interaction of the hydrazinic carbonyl group with metal, they migrate to lower wavenumber (1323 - 1372 cm⁻¹) indicating enolization and metal interaction with the phenolate group, while the presence of a broader band at 1600 - 1615 cm⁻¹ due to the vibration of the azomethine group (>C=N-N=C<) and when complexation with metal the v (N-N) band moves to the higher wavenumber side (as in Table 3). The appearance of new bans around 511 - 570 and 480 - 485 cm⁻¹ were bands between v (Cu-O) and v (Cu-N) [15] . In addition, the vibration attributed to the nitrate group v (NO₃) was observed at 1385 cm⁻¹ and the v (Ag-O) vibration mode was found at 510 - 515 cm⁻¹ in the spectra of complexes C2 and C4, respectively [16] [17] .

The ability of anti-free radical activities by using the DPPH and FRAP methods.

Note: vs = very strong; s = strong; sh = shoulder.

The calculation of inhibitory concentration 50% is illustrated in Table 4, Figure 2 and Figure 3.

It is found that the novel Schiff base ligands anti-free radical DPPH at a high percentage. All compounds (particles) showed anti-free radical DPPH and have a concentration dependence on both the ligands and metal complexes but hydrazonate metal complexes have low proficiency than independent ligands because of the bigger structure resulting in the steric effect that makes it difficult to provide hydrogen free radical. For anti-free radical activity by reducing Fe³⁺ to be Fe²⁺ (FRAP) it is found that ligands and their complexes showed low ability to reduce iron because the molecule has difficulty in providing single electron transfer. All particles showed higher activity in both techniques than the standard ascorbic acid, especially C1 particles showed the best potency because of small particles. Antioxidants have a differing capacity to stop the propagation of free radicals, influencing this both the structure of the antioxidant and the structure of the compounds to be oxidized [18] [19] .

Note: The different of values: a > b > c > d > e > f.

Prepare solution of samples in different concentrations and the screening of the in vitro antibacterial activity against both Gram-negative bacteria (Escherichia coli, Salmonella spp.) and Gram-positive bacteria (Staphylococcus aureus) for 24 hours. Then measure MIC and MBC which find the results as in Table 5, Figure 4 and Figure 5, respectively.

All the green synthesized nanoparticles showed good MIC and MBC antibacterial activity, especially copper nanoparticles (C3) with small particle sizes (56.38 nm) showed the best efficiency to inhibit cell growth. The mechanism of action for antibacterial activity starting from metal ions is released from metal nanoparticles (NPs) during the redox reaction by exchanging electron and ROS free radicals to react with DNA, protein enzymes and mitochondria [20] [21] .

Test results for the ability of nanoparticles against larvae (F1) as in Table 6.

Note: Values are expressed as Mean ± S.D (n = 3).

Note: na = not available. The different of values: a > b > c > d.

It was found that after larvae were exposed to nanoparticles, they move violently at first. Then, the movement became less and less and death began to occur towards the end of the hour. After 24 hours, the mortality rate increased, especially C1 (LT₅₀ 19.00), and by 48 hours, the larvae exposed to C2 particles had a lower survival rate (LT₅₀ 19.33). Nanoparticles have a significant impact on insect antioxidants and detoxifying enzymes leading to oxidative stress and cell death [22] [23] .