UV-Visible spectra of 0.5 mM palladium solution were reordered before and after synthesis of PdNNs compared with plane aspergillus oryzae extract. PdNNs was synthesized within 1 hrs after addition of aspergillus oryzae extract (Fig. 1a). The peak observed at 307 nm and 423 nm indicated the presence of Pd2+ in palladium chloride solution [45]. One hour of stirring at 65˚C with aspergillus oryzae extract, peak observed in palladium chloride solution at 307 nm was completely disappeared and a new absorption peak was observed at about 410 nm instead of 423 nm, which indicated the formation of PdNNs [46].
The X-ray diffraction data was applied to determine the orientation of the crystallographic planes of biosynthesized PdNNs. The reflection pattern of the PdNNs for the untreated sample in the angle of the 2θ range is delineated in Fig. 1b, which shows following patterns: peak1 (110), peak2 (200), and peak3 (220) reflections at 39.61, 46.11, and 67.43 respectively. Since h, k and l are always integers, we can obtain h2 + k2 + l2 values by dividing the sin2θ values for the different XRD peaks with the minimum one in the pattern. The h2 + k2 + l2 value sequence of the peak was 3, 4, 8 respectively which was in agreement with the sequence of the Face-centered cubic pattern. These observations conclude that PdNNs were in a Face-centered cubic lattice form [47]. Whereas the unit-cell parameters for patterns 3.9411 A0 were calculated (using formulae presented in S1).
The morphology and size of the synthesized Nanoneedles were determined by TEM analysis. Typical TEM images obtained for PdNNs showed Nanoneedles of average particle size 3.0 nm width (Fig. 2) (calculated using ImageJ Software and respected value showed in Table S1). The data was supported by EDAX analysis, where the peaks correspond to the binding energy at 2.83 keV confirmed presence of Pd in respected PdNNs sample (Fig. S6).
Interaction of PdNNs with BSA results in complex formation was studied by UV–visible, Fluorescence spectroscopy and Circular dichroism spectroscopy (CD). BSA made up of tryptophan, tyrosine and phenylalanine, which shows absorption peak at 280 nm in UV–visible spectroscopy (Fig. S7) [48]. When BSA forming a complex with PdNNs at different concentrations absorption of complex get increased was depicted in Fig. 3f.
Fluorescence spectroscopy was used to identify the quenching mechanism and the strength of the interaction between BSA with PdNNs. Whereas quenching usually shows two types of mechanisms (dynamic and static) depending on the way of interaction between BSA and quencher [49]. Interaction of BSA and PdNNs was recorded by fluorescence spectroscopy with an increasing amount of PdNNs. It indicates that the fluorescence intensity of BSA was inversely proportional to the increasing concentration of PdNNs. In order to explore the effect of the interaction of BSA, with the various concentrations of PdNNs (9.69 x 10− 7 M to 4.84 x 10− 6 M), (Fig. 4a) the fluorescence quenching data at different temperatures were analyzed by using Stern-Volmer equation (S2) [46]. The value of quenching constant decreases with increasing temperature; this indicated that binding between PdNNs and BSA by complex formation resulted in successful quenching. The value of kq was calculated by considering the average lifetime of protein without the presence of quencher and its value was 10− 8s [50]. The maximum scattering collision quenching constant of various quenchers with the biopolymer is 2 × 1010 L mol− 1 s− 1 which was reported in the literature [51]. In the present work, the quenching constant kq was in the order of 1013 L mol− 1 s− 1 which clearly indicated that the interaction between BSA and PdNNs was accured through a static quenching process [51]. Binding mechanism gives (S3) binding constant (K) between BSA and PdNNs was found in the range of 104 M− 1 and number of binding sites (n) between BSA and PdNNs was approximately equal to 1 delineated in Table S3. Which suggested that PdNNs can easily be complex in protein as well as released in desired target areas and shows moderate affinity [52, 53].
Inorganic molecules interact with BSA by various forces namely hydrogen bonding, electrostatic interaction, van der waals interactions, hydrophilic, and hydrophobic interactions. Which was identifies using Van’t Hoff equation (S4) by evaluating the value of the change in entropy (ΔS°) and enthalpy (ΔH°) for the binding reaction which further gives Gibbs free energy (ΔG°) showed in Table S4. The negative value of free energy (ΔG°) showed that the binding process was spontaneous. In the BSA and PdNNs interaction, the negative value of ∆S° (-274.832 Jmol− 1K− 1) indicates hydrophilic interaction. A negative value of ∆H° (-109.295 KJmol− 1) indicated the hydrogen bonding between BSA and PdNNs [54].
The fluorescence of BSA was observed due to the presence of Tyrosine residues and Tryptophan residues. Among this tryptophan is the most dominant fluorophore. Most of the metals were bind to one of the active bind sites of protein present in BSA. To find out the binding site synchronous method is used which gives conformational changes in protein molecules. When the difference (Δλ) between excitation wavelength and the emission wavelength was at 15 nm and 60 nm where quenching of the fluorescence intensity indicative of tyrosine and tryptophan residues respectively. The synchronous fluorescence spectra of BSA solutions were recorded in the presence of Nano-sized PdNNs with the increasing concentration by 10min ultra sonication and 20 min incubation treatments (Fig. S6, S7). The fluorescence intensities of tryptophan, as well as tyrosine, reduces consistently by addition of PdNNs but during the interaction, the emission wavelength of the tryptophan residues shows a blue shift with increasing concentration of PdNNs. Whereas tyrosine does not show any conformational change in emission wavelength by addition of PdNNs. It indicates that the interaction of PdNNs with protein affects the conformation of a tryptophan residue. The synchronous measurements confirmed the effective binding of PdNNs with BSA. Hence the strong interaction of these complexes with BSA suggests that the PdNNs may be suitable for anticancerous studies [55].
The CD spectra of BSA with and without PdNNs were measured and delineated in Fig. 3d, There were two negative peaks observed in the CD spectra region at around 208 and 222 nm, which are the characteristics of α- the helical structure of the protein (BSA) [56]. Where MRE208 was the observed MRE value at 208 nm, 4000 was the MRE of the β-form and random coil conformation cross at 208 nm, whereas 33000 was the MRE value of α-helix at 208 nm. By using the equation (S5), the percentage of α -helix of BSA was calculated. The percentage of helicity of BSA was decreased with increasing concentration of PdNNs nanoparticles. The content of α-helix was decreased from 59.79 to 57.47% at 208 nm and 51.10 to 46.68% at 222 nm showed in Table S5. CD analysis confirmed that the binding of BSA with PdNNs alter the secondary structure of BSA.
According to Forster’s resonance energy transfer (FRET) theory, donor fluorophore in its excited state transfers its energy to an acceptor molecule through the non-radiative dipole-dipole coupling, whereas the donor-acceptor distance at which energy transfer is 50% efficient is referred to as the critical distance (R0). The efficiency of this energy transfer can be used to estimate the distance (r) between the PdNNs and fluorophore in the biomolecule [57, 58]. This energy transfer depends on the extent of overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor [59]. The values of R0 and r were calculated from equation (S6) for concentrations from 4.84 x 10− 6 M to 9.69 x 10− 7 M varies to 2.9 to 3.7 nm and 3.2 to 5.4 nm respectively tabulated in Table S6. The shortest donor-acceptor distance arises from FRET between a pair of donor and acceptor. The result showed that PdNNs were strong quencher and they were situated at close proximity to the BSA fluorophore.
The bio-synthesized PdNNs were screened in vitro for anticancer activity against Human Breast Cancer Cell Line MCF-7, COLO-205 and K562. First time we present needle shaped palladium nanoparticles was applied for anticancer study respective literature tabulated in Table S7. The results were expressed in the form of concentration that resulted in a 50% inhibition (IC50 values) whereas the well-known anticancer agents Adriamycin were used for positive controls. GI50 Growth inhibition of 50% was calculated in triplicate for different concentrations of PdNNs such as 10− 7, 10− 6, 10− 5 and 10− 4 M gives average values (Table S8) for cancer cell line MCF-7, COLO-205 and K562 shows in Fig. 5a-c indicated that green synthesized PdNNs showed positive activity against Human Breast Cancer Cell Line MCF-7, COLO-205 and K562.
The ability to trap free radicals is the main property of any antioxidant compound. Therefore, PdNNs has studied for the potential bio-synthesized antioxidant agent. Antioxidant activity was estimated by using the DPPH radical scavenging assay method, were AA consider as a standard antioxidizing agent with which PdNNs property was compared. The DPPH has a half fielded electron configuration so it has the ability to accept an electron or hydrogen free radical. Exactly antioxidant property means odd electron from DPPH forming a pair due to ‘H’ transfer from an antioxidant it results in decrease its absorbance [58]. PdNNs showed antioxidant activity. The higher radical scavenging activity against DPPH (Fig. 6) shows for the concentration of 125µg/ml as 53%.