Nanotechnology is gaining popularity due to its wide range of applications. Silver nanoparticles are commonly used nanomaterials that can be synthesized using various methods, including chemical reduction and biological methods. Chemical reduction involves reducing metal ions in a solution containing a reducing agent or organic solvent to create nanoparticles. On the other hand, biological methods use live organisms and biological agents in an eco-friendly manner to reduce metal ions and synthesize nanoparticles in an aqueous solution. By adjusting the quantity of metal ions, biological agent, and reaction time, the size and shape of the nanoparticles can be controlled. The primary difference between the two processes is that chemical reduction uses a reducing agent, while biological methods use live organisms and biological agents to reduce metal ions and create nanoparticles.
In the present study, the extracellularly synthesized Sepioteuthis lessoniana ink AgNPs were confirmed by UV-Visible spectroscopy, which showed the absorbance peak at 407nm. In this study, squid ink acted as the reducing agent. Our findings are consistent with Parthasarathy et al. [16], who observed an absorbance peak at 420 nm and the surface plasmon resonance increase with increasing the reducing agent concentration and times. The S. lessoniana ink was found to contain tyrosinase, melanin, protein, carbohydrate, lipids, L- Dopa, and Dopamine [3, 27] which are responsible for the reduction of metallic silver to silver nanoparticles. The rapid aggregation of AgNPs and their conjugates, known to cause UV shifting, is attributed to the polarity difference between the excited state and ground state, ultimately resulting in a change in the energy gap between the states [28].
FTIR instrument was applied to spot the foremost functional groups on the squid ink silver nanoparticles' surface and their possible involvement in the activities and stabilization of the nanoparticles. In the current scenario, FTIR analysis of the SI-AgNPs revealed the functional groups of amide, carboxylic acid, alkenes, alcohol, ether, and ester. The biological components are known to interact with salt through these functional groups and mediate their reduction to nanoparticles [10].
The XRD spectra of SI-AgNPs in the current investigation revealed a variety of lattice planes at (111), (200), (220), and (311), corresponding to the Bragg's reflection peaks at 2θ values of 38.116°, 44.300°, 64.445°, and 77.399°, respectively. The findings of Parthasarathy et al. [16] provide support for the findings. The XRD patterns undeniably demonstrate that the crystalline squid ink (melanin) was used to bio-reduce copper, silver, and silver composite ions into silver nanoparticles. It is noted that the crystalline size of the generated silver nanoparticles (17nm) is significantly lower than that which has been previously reported [10]. Ibrahim et al. [29] and Roopan et al. [9] used XRD analysis to determine the phase distribution, crystallinity, and purity of synthesized silver nanoparticles.
Scanning Electron Microscopy (SEM) reveals the surface morphology in the form of a picture in the closest view of the synthesized samples. Our present results imply the spherical shape of a particle with an average particle size of 76.5 ± 14.3 nm. The energy dispersive (EDX) analysis presented a clear mapping of the elemental composition of squid ink AgNPs, with distinct peaks identifying silver (Ag) and oxygen (O) and showing the atomic weight % of the analyzed sample elements to be 70.7% and 29.3%, respectively.
The hydrodynamic size distribution of the nanoparticles was measured using DLS. The polydispersity index (PDI), which indicates the breadth of the particle size distribution, is usually used to denote "monodisperse," with a value of 0.1 [30]. The computed PDI for SI-AgNPs is 0.310, and this resoundingly establishes the polydisperse nature of synthesized AgNPs. This DLS result is consistent with the findings of the SEM study. The stability and surface charge of the NPs were confirmed using zeta-potential analysis, which revealed a net charge of -59.56 mV. The negative zeta-potential value demonstrates the increased stability and dispersion of the particles.
AgNPs have demonstrated effective antimicrobial activity compared to other metal nanoparticles, even at low concentrations that are non-toxic to eukaryotic cells [31]. The synthesized squid ink silver nanoparticle-reducing agent has shown significant antimicrobial activity against E. coli and L. monocytogenes, resulting in a greater zone inhibition of food-pathogenic bacteria [10]. The antibacterial effect of AgNPs is size-dependent; that is, nanoparticles less than 10nm in size reduce an electronic effect by interacting with bacterial cell surfaces [32].
In the present study, the percentage of antioxidant activity of SI-AgNPs at different concentrations (20–100 µg/ml) showed excellent inhibition properties. The melanin nanocomposite film with its strong antioxidant activity could be used in food packaging materials to prevent the oxidation of lipid-containing food materials and prevent oxidation, and color changes in packaged foods [33]. Additionally, the squid ink nanoparticles demonstrated good radical scavenging activity.
Because, AgNPs are being used by consumers and health items, public alarm about the threats of utilising those products is growing, as AgNPs may offer possible health risks. Additionally, the widespread use and manufacture of AgNPs would increase their discharge into aquatic environments. AgNPs can build up in aquatic animal tissues and water, then go up the food chain to the human. Hence, understanding how AgNPs interact with mammalian cells is essential for the safe use of these NPs. This enables the development of useful AgNPs that are more biocompatible with mammalian cells and free of unfavorable side effects. This study aims to investigate the cytotoxicity of squid ink AgNPs using a human breast cancer cell line. The results showed a significant reduction in cell growth (74%) at a concentration of 12µg/ml, with an IC50 value of 4.52 µg/ml, indicating cytotoxicity to tumor cells. DNA damage and the release of intracellular Ag were identified as potential mechanisms of cytotoxicity. Previous studies have also reported similar findings, linking AgNPs to increased ROS levels, oxidative DNA damage, and harm to human keratinocytes in a dose-dependent manner. Overall, this study demonstrates the potential of biologically manufactured squid ink AgNPs for various applications, including antibacterial, antibiofilm, and antiproliferative properties, while also highlighting their lack of harm to normal cells.
Artemia is one of the most valuable, interactive, and nonselective filter feeding test organisms available for marine ecotoxicity testing. It is the widespread nutrient-rich live food source for the larvae of a variety of marine organisms which making them the most practical and labor-efficient source for aquaculture [36]. In the present study, Artemia franciscana was treated with squid ink silver nanoparticles for the lethality test at different concentrations (1–5 mg/ml) and showed effective outcomes with LC50 values of 5.090 and 3.303 mg/ml for 24 and 48 hours respectively. Another study by Arulvasu et al. [24] found an LC50 value of 10 nM at concentrations of 2–12 nM. These findings indicate that the toxicity of AgNPs to aquatic animals is dependent on the concentration [37].
The transfer of electrons between the donor and the acceptor drives the catalytic dye degradation. However, significant redox potential differences between the acceptor and the donor may prevent electrons from moving between them. Based on the intermediate redox potential value between the donor and the acceptor, silver, platinum, and gold nanoparticles operate as an efficient catalyst to facilitate the flow of electrons, which may result in an electron relay system [38]. Vidhu et al. [39] focused on the impact of silver nanoparticle size on the rate of methyl orange, methylene blue, and eosin Y degradation by NaBH4. The reported work established the effectiveness of silver nanoparticles as a potential choice for the catalysis of organic dyes by NaBH4 through the electron transfer mechanism, which overlaps with our present finding of dye degradation due to the catalytic reduction potential of SI-AgNPs.