3.1. Analysis of the synthesized Ag NPs
The electronic bands of UV-vis absorption spectra of Schiff base and AgNPs were recorded in the range of 200–800 nm. The exhibited absorption bands were assigned to intraligand charge transition (ILCT)bands and shown in Fig. 1. For Schiff base, the maximum absorption band at 356 nmwas assigned to π→π* transition of N = N linkage andaromatic rings. Also, the n→π* electronic transition was observed at 422 nm, it is attributed to the azo aromaticchromophore and intra-molecular charge transfer interaction. In the UV-vis absorption spectra of AgNPs, the maximum absorption band was observed at 438 nm (Fig. 1). In this case, the blueshift was found and absorption bands shifted to a lowerwavelength, probably due to the effect of the sizequantization. The decrease in particle size creates a blue shift . The appearance of surface plasma resonance (SPR) band at 438 nm for AgNPs specifies the formation of nanoparticles . Free Schiff base ligand peak at 264 nm shifted to higher wavelength also confirms that the Schiff base strongly interacted with the AgNPs.
The photoluminescence of noble metal NPs could be viewed as an excitation of electrons from occupied d-bands into states above the fermi level. Subsequent electron–phonon and hole-phonon scattering processes led to energy loss and finally photoluminescent recombination of an electron from an occupied sp-band with the hole. Figure 2 shows that the PL spectra of Schiff base and AgNPs recorded at room temperature. It shows that excitation spectrum of AgNPs, which has shown the strong excitation peak at 390 nm with the emission wavelength 484 nm, and Schiff base shown as strong emission peak at 528 nm. PL spectra of Schiff base and AgNPs (Fig. 2) indicate that fluorescence of AgNPs is suppressed compared to Schiff base by a combination of inorganic nanoparticles with the organic ligands . Since photoexcited electrons are transferred from the conduction band of AgNPs to the lowest unoccupied molecular orbital of Schiff base ligand (LUMO), it can modulate the emission behaviour of AgNPs.
FTIR spectra were recorded of Schiff base and Ag NPs between the wavenumber spanning over a range of 4000–400 cm− 1 for the involving functional groups in the preparation of AgNPs as shown in Fig. 3. In the IR spectrum of Schiff base ligand, the azomethine nitrogen (-CH = N-) peak appeared at1621 cm− 1 . This frequency is shifted towards lower frequency at 1571 cm− 1 for AgNPs which shows that azomethine nitrogen was stabilized the metal nanoparticles. A broad peak at3618 cm− 1 in free ligand which does not disappeared in the FTIR spectrum of AgNPs. This result shows that phenolic oxygen atomnot involved in the stabilization of AgNPs. The other prominent IR peaks of Schiff base ligand and AgNPs were 3442.75, 3072.97, 1732.13, 1621.97, 1516.21, 1400.37, 1316.49, 1078.24, 887.14, 800.49 and 611.45 cm− 1 which correspond to –O-H, -C = O, -C-N, -N = O, -C-H, -C-C, and –C = C bands in alcohol or phenol, amines, aromatics, aromatic amines, aliphatic amines, aromatics, nitro compounds, and alkenes presented in the Schiff base. However, the common trend in terms of prominent IR peaks, 3436.24 cm− 1 and 1621.97 cm− 1 were indicates the presence of –O-H and –N = O groups are actively involved in the reduction of Ag+ to Ag0.
The determination of crystalline nature and structure of the synthesized AgNPs were by using XRD crystallography technique. The XRD pattern of the synthesized Schiff base and its stabilize AgNPs showed the Bragg’s reflection plane in the 2θ range between 20–70°. In the XRD pattern of AgNPs, the diffraction peaks at 2θ = 38.21°, 44.39°, and 64.49° are corresponded to (111), (200), and (220) Miller indices of AgNPs and it was shown in Fig. 4, which interpreted for the face-centred cubic structure of the AgNPs . The broad peaks presented in the XRD pattern indicated that the small crystalline sized nanoparticles were presented in AgNPs. The obtained other peaks illustrate that silver ions had indeed been reduced to Ag by the ligand under reaction conditions.The resulted XRD patter Bragg’s diffraction peaks of AgNPs are well matched and corroborated with database of Joint Committee on Powder Diffraction Standard of AgNPs (JCPDS card No. 04–0783) . The sharp peaks are shown in Fig. 4 might have due to capping agent which stabilize the AgNPs and few unassigned peaks that might be thought due to the crystallization of the Schiff base on the surface of the AgNPs. The lattice constant was calculated from diffraction pattern α = 4.11 Å and d-spacing 2.14 Å of synthesized AgNPs. The synthesized AgNPs crystalline size was calculated using Scherer’s formula:
D = 0.9 λ / β cos θ
Where, D is the crystalline size, β is the full width at half maximum, λ is the wavelength of X-ray used (1.5406 Å) and θ is the Bragg’s angle. The crystallite size was calculated to be approximately 10 ± 2 nm, which is in good agreement with the TEM results.
SEM and TEM
In detail estimation of structure, morphology and size of the synthesized AgNPs were observed from SEM and TEM images and it was shown in Fig. 5. The SEM images of Schiff base shows as needles type structures and Schiff base ligand capped AgNPs were clearly shows that the surface of the AgNPs is uniform and spherical in shape.TEM also clearly shown as synthesized AgNPs have spherical shape with smooth surface and well dispersed. The small dots in the white barebackground were dispersed nanoparticles. It clearly confirms that the synthesized AgNPs are nano in size. The resultant AgNPs completely revealed that the Schiff base can protect Ag nanoparticles from aggregation efficiently. The broad TEM (Fig. 6) was developed by considering AgNPs; it clearly recommends that the mean size dispersion of AgNPs is 10 ± 2 nm. It exhibits the distribution of AgNPs inside ligand matrix that might be attributed to complexionbetween ligand and Ag+ causing the formation of separate AgNPs in ligand matrix. This result, in turn, isinduced by the activity of silver cation with OH and NO2groups inligand and methanol solvent that can reduce the silver ion.
Analysis of Schiff base ligand
The other characterization techniques were analysed for the confirmation of formation of Schiff base such as ESI-Mass, 1H-NMR and elemental composition and it was displayed in Fig. 7. It clearly proved as the M + 1 peak and chemical shifts of protons in Schiff base were well matched.
3.2. Proposed mechanism involved in the development of AgNPs
In this work, we have utilized Schiff base as a capping agent for the development of AgNPs. The structure of Schiff base which is involved in the formation of AgNPs is made up of imine group, which contains the carbonyl, nitro, and hydroxyl groups in abundance. The number of carbonyl and hydroxyl groups helps in the complexation of Ag+ ions. These Ag+ ions oxidize the hydroxyl to carbonyl groups, during which the Ag+ ions are reduced to elemental Ag.
3.3. Catalytic degradation of dyes using the synthesized AgNPs
The synthesized Schiff base ligand capped AgNPs were employed for the catalytic degradation of environmentally persistent dyes such as MB, RhB, and EY dyes in the presence of NaBH4 as a reducing agent (Fig. 8). The degradation of pure dye solution expressed no changes in absorption upon during the catalytic reaction without AgNPs and NaBH4, while the test dye solution having AgNPs in all experiments expressed a gradual decrease in absorbance during the catalytic reaction in the presence of NaBH4. Figure 8 shows that the rate of absorption bands of MB, RhB, and EY from UV–Vis spectrophotometer at 663 nm, 554 nm, and 534 nm due to n → π* transition of C = N, C = O groups. Wavelength for each decreased maxima was recorded and final overlays were formed upon the discolouration. UV–vis overlay spectra show this gradual absorbance reduction corresponds to the increase in degradation efficiency. AgNPs succeeded in degrading 71.58% of MB, 50.16% of RhB, and 80.99% of EY in 80 min with small amount of catalyst (10 µL). The order of degradation of dyes is EY > MB > RhB (Fig. 9). Figure 9 illustrates that the reaction kinetics (ln (A0/At) vs time) for determining the apparent rate constant of catalytic degradation reactions of dyes. For all the dyes, the degradation caused by AgNPs was found to be pseudo first-order kinetics. There was a linear relationship between ln (A0/At) vs time with R2 = 0.98, 0.97, and 0.98 for MB, RhB, and EY, respectively. The apparent rate constants were 0.248 min− 1, 0.186 min− 1, and 0.285 min− 1 were observed for the degradation of MB, RhB, and EY dyes using AgNPs, respectively.
3.4. Sensing activity for the detection of Fe2+
To test their analytical application as a fluorescent sensor, we studied their SPR properties and tendency to agglomerate in the presence of the analyte. The employability of AgNPs as an optical probe for harsh metal ions is significantly centered on the examination of variations in position and/or intensity of plasmon band and color of the nanoparticle solution. If the interaction of nanoparticles is selective towards only one metal ion, this may become the basis of metal detection [47–48]. Based on this observation, initially we inspected visually, the interaction of different metal ions (Pb2+, Cd2+, Co2+, Fe2+, K+, Mg2+, Ni2+, Hg2+, Sr2+, Ca2+, Ba2+, Al3+ and Cr3+) individually with AgNPs and later obtained the emission spectra of the assay solutions (Fig. 10). Out of these metal ions, only Fe2+ ion exhibited significant change the PL intensity in the emission spectra and the color of the solution, which can be easily observed with the naked eye. This indicates that AgNPs can be used as a fluorescent sensor to detect Fe2+ ion in an aqueous medium either without any modification or sample pre-treatment other than metal ions. Therefore, the selective interaction of AgNPs with Fe2+ ions over the other environmentally relevant heavy metal ions could be due to the presence of nitro (‒NO2) and hydroxyl (‒OH) groups on the surface of AgNPs. It is noticed that only Fe2+ ion exhibited a substantial change in the spectrum with a red-shift of the λmax from 472 to 485 nm with intensity decreasing and a sharp color change from yellow to orange, detectable by naked-eye. The shift in λmax of the plasmon band and the color change of the solution is due to aggregation of AgNPs induced by Fe2+ ion through the coordination covalent bonding between the surface functional groups of Ag NPs and Fe2+ ion. However, there is a drastic change in the absorption spectra and color of AgNPs upon the addition of Fe2+ ion, confirming that the Schiff base consisting electron donors on the surfaces of AgNPs play key role to interact with Fe2+ via coordinate covalent bond, resulted in a red shift and a color change.
For practical applications, it is essential to establish the sensing response and minimum detection limit of the system. Therefore, to quantitatively establish the dynamic detection concentration of Fe2+ ions by AgNPs, several repeated experiments were carried with varying dilutions of Fe2+. The concentration of the solution is increasing from 1 µM to 100 µM, the alteration in its intensity from higher to lower (Fig. 10). Furthermore, a large decline in intensity with distinct color changes was observed with increasing at the concentration of 100 µM. This interaction of Fe2+ ions leads to the disintegration of AgNPs that result in a decrease in the concentration of free scattered colloidal AgNPs. It was observed that this diluted colloidal AgNPs solution showed a lower limit of detection (LOD) = 0.284 µM (284 nM), which might be due to the reason that dilution results into excellent dispersal of AgNPs and addition of even very little amount of Fe2+ ions are enough for etching of available particles. Based on these investigations, it can be clinched that more dilution of AgNPs solution may result in detection of mercury ions to further lower limit.
The Stern-Volmer constant (KSV) of the Schiff base ligand and its capped AgNPs were determined by Stern-Volmer equation:
I / I0 = 1 + Ksv [Q]
KSV is the Stern-Volmer quenching constant, [Q] is the concentration of quencher (ligand, Ag NPS), I and I0 are the presence and absence of quencher fluorescence intensities. Ksv of values of ligand and AgNPs are 3.62 × 104 and 2.87 × 105. The obtained Ksv of values of prepared compounds indicate that AgNPs has better sensing activity than ligand towards the detection of Fe2+ in nano-molar range of concentration.
3.5. Antimicrobial activity
Antimicrobial action of Schiff base capped AgNPs was tested against Gram-negative (P. aeruginosa and E. coli) and Gram-positive (B. subtilis and S. aureus) bacterial strains and results were shown in Fig. 11. The outcomes demonstrated that the synthesized AgNPs have discrete antibacterial action against pathogenic microorganisms at 5 µg/mL concentration. AgNPs were contrasted positively with silver nitrate, Schiff base and standard antibiotic, ampicillin at equal concentration . The AgNPs revealed more bacterial growth inhibition action than silver nitrate and Schiff base. AgNPs were genuinely lethal to S. aureus, B. subtilis, and E. coli with inhibition zone of 17.4, 16.6, and 18.6 mm. The AgNPs indicated less antibacterial action against E. coli, S. aureus and B. subtilis and furthermore, high activity against P. Aeruginosa (20.8 mm) can be inferred that the blended AgNPs demonstrated noteworthy antibacterial activity on both gram classes of microorganisms . The cell membraneof bacteria consists of proteins containing sulfur, and the AgNPs interact with these proteins as well as thephosphorus-containing compounds like DNA. Therefore, these AgNPs can cause structural changes in the bacterialcell wall and nuclear membrane ultimately leading to celldistortion and death . For Schiff base capped AgNPs, the higher activity compared to the free ligand may berelated to chelation of the metal ion with donor atoms of the ligand  that reduces polarity of the metal ion. As aresult, an increase occurs in the lipophilic character, favouring the permeation through lipid layers of the bacterial membrane that damages the outer cell membrane and consequently inhibits the growth of bacteria .
In addition, the synthesized AgNPs were exhibited high antifungal activity, and this property can be very useful, especially against microorganisms resistant to conventional antimicrobials such as A. niger and C. albicans showed high sensitivity to AgNPs (Fig. 11). The antifungal activity results of 20 µg of AgNPs can be compared with the activity of ketoconazole as a standard antifungal. Among the tested fungi, A. niger had the most (12.1 mm) and C. ablicans (10.5 mm) had the least sensitive fungus detected by using the synthesized Schiff base and AgNPs. The percentage of growth inhibition due to the effect of AgNPs was analyzed by statistical software SAS. Results confirmed a significant effect of AgNPs on fungal growth inhibition at 1% confidence interval.
3.6. Mechanism of antimicrobial activity of Ag NPs
The mechanism of antimicrobial activity of metal nanoparticles such as AgNPs has been testified by numerous researchers, the cell death and leakage of cell membrane is because of the discharge of Ag+ ion and generation of reactive oxygen species (singlet oxygen, superoxide anion radical, hydroxyl radical and hydrogen peroxide) the smaller size particles gives a more reactive surface zone to interact with the bacteria enhancing a superior anti-bacterial ability. The production ofthe reactive oxygen specieson the surface of the AgNPs, when the light causes oxidative stress in the bacterial cell wall, eventually it leads to the death of the cells . Ag with a positive charge and cell membrane with negative charge mutually attract. Further, Ag+ enters into the cell membrane and reacts with the thiol groups present on the cell membrane and destruction it leading to the death of the cells.
3.7. Antioxidant activity
In the present study, the antioxidant activity of the synthesized Schiff bas capped AgNPs was studied by using DPPH method. The antioxidant activity of synthesized AgNPs was assessed in terms of percentage inhibition of DPPH radicals in the presence of ascorbic acid. The DPPH is considered more stable nitrogen-cantered free radical due to exhibiting a higher degree of accepting hydrogen atoms or electrons from antioxidant materials . The DPPH solution color change was observed on the addition of AgNPs, which is due to the scavenging action of DPPH by addition of hydrogen to form the yellow-colored DPPH. The scavenging ability was quantified using spectrophotometer by taking optical density at 517 nm. The percentage of antioxidant activity of Schiff base and AgNPs were calculated and compared with Schiff base. The resultsare clearly indicated as AgNPs exhibited high DPPH activity (68.24%) than the Schiff base (44.86%), moreover the standard compound (ascorbic acid) shown maximum DPPH activity (86.38%) and the results were illustrated in Fig. 12. However, the AgNPs showed very good or almost equivalent free radical scavenging activity. The free radical scavenging activity of synthesized AgNPs might be due the active components present on the surface of the nanoparticles and they were ready to give up hydrogen atom from their hydroxyl groups to free radicals and form stable phenoxy radicals.