1 Introduction

Due to inherent distinct qualities, such as high surface area, reactivity and durability, silver nanoparticles (AgNPs) have become popular as antimicrobial agents. Antibacterial overuses, which concomitantly lead to bacterial resistance have made antibiotics obsolete and/ or less efficient over the past decades; nevertheless, several tactics such as chemical modifications of the obsolete drug and administration of an antibiotic member of a higher generation or combinatorial therapy could be employed to counteract critical clinical problems. Amino acids and DNA interact in the biosynthesis of AgNPs from broken cell proteins, aiding in possible druggable use as antibacterials (Shantkriti et al., 2023). Particularly, compared to conventional physicochemical methods, the synthesis of AgNPs using biological systems offers several advantages, such as the ability to control the size and form of the particles and less negative environmental impacts without any use of external energy. In contrast to expensive and hazardous physicochemical synthesis techniques, biologically generated AgNPs have solubility in water, and yields. Moreover, these techniques produce nanoparticles (NPs) that are bound to health-sensitive substances, which eventually compromise their application in medicine (Zahin et al., 2020). An easy one-step method can be utilized to create biosynthesis of AgNPs using plant extracts, with examples, the dried root of Zingiber officinale (Vijaya et al., (2017), aqueous leaf extracts of Passiflora edulis f. flavicarpa (Thomas et al., 2019), Basella alba leaf (Mani et al., 2021), and using Aspalathus linearis leaf extract by α-Ni(OH)₂ (Diallo et al., 2018) for semiconductor metal oxide nanocrystals via photocatalytic application, which could generate health-sensitive chemicals (Jahangir et al., 2023). Thus, developing host-safe, NP-processing methods with organisms is coveted (Bhatt et al., 2022).

As it is, marine algae synthesis of NPs is done, and those have antibacterial characteristics (Al Nadhari et al., 2021). Currently, the focus is on locating and evaluating more contemporary natural sources as antioxidants in algae. Algal antioxidant qualities are due to phytochemicals that can be crucial and have high redox potentials (Ismail et al., 2021). For example, cyanobacterial phycobiliproteins (PBPs), namely, C-phycocyanin are abundant as the unique pigments employed by cyanobacteria for oxygenic photosynthesis, which explains oxygenation of the nascent earth; oxygenation could be the bold reaction in waiving out reductive oxygen species (ROS) from tissues suffering from ROS; at body metabolism how, NPs might lessen and stabilize the developing cell, as shown (Kannaujiya et al., 2020). Several tests proved the antioxidant activities of AgNPs, the ferric reducing/ antioxidant power (FRAP) assay, metal chelating activity, superoxide radical scavenging activity, and 2,2-diphenylpicrylhydrazyl (DPPH) radical scavenging activity, etc. (Prajapati et al., 2022). Herein, the green synthesis and monitoring of the antibacterial activities of the biosynthesized AgNPs for their plausible health advantages since both cyanobacterial species used have vitamins, minerals, and vital amino acids (Sharma and Phutela, 2023). Indeed, Spirulina sp. and S. subsalsa are photosynthetic and spiral, or coiled filamentous cyanobacteria in morphology belonging to the order Oscillatoriales and with pharmaceutics. Because of the high gamma-linoleic acid (GLA) content, Spirulina is a good dietary supplement (Matufi and Choopani, 2020). Various Spirulina strains have various lipid and fatty acid compositions, which may also depend on the conditions under which they are cultivated (Lupatini et al., 2017). This study evaluates the biosynthesis of AgNPs with the extract of Spirulina sp. and S. subsalsa were characterized and their antibacterial efficacies were examined in vitro.

2 Materials and methods

2.1

2.1 Unialgal biomass production and optimization

Spirulina sp. (strain no. BDU51381) (Ssp) and S. subsalsa (strain no. BDU101022) (Ss) were obtained from Bharathidasan University, Tiruchirappalli, TN, India; and those two taxa were mass-cultured in ASN III medium in a culture room. Individual biomasses were shade-dried for a month at room temperature. The obtained powders were stored in airtight containers. The extracts of Ssp and Ss were individually combined and incubated for one hour at 90 °C with 2 gm of shade-dried cyanobacterial biomass in 100 ml of deionized water for aqueous extracts to get aqueous extracts. Each extract was filtered to produce a clear solution, and each extract was combined individually with ethanol, methanol, butanol, acetone, and water separately to prepare each alga for antibacterial monitoring (Fig. 1).

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Fig. 1

Experimental setup and biomass production of Ssp and Ss extracts.

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3 Results

3.1

3.1 Biosynthesis of AgNPs

Additionally, a color reaction was observed, whereby the transparent AgNO₃ solution took on a reddish-brown color, suggesting that silver nanoparticles may eventually surface, as illustrated (Fig. 2). The appearance of a brown color upon visual inspection indicated that Ssp and Ss were utilized in the synthesis of AgNPs.

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Fig. 2

Formation of color changed after the biosynthesis of Ss-AgNPs and Ssp-AgNPs.

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3.2

3.2 Characterizations

3.2.1

3.2.1 Uv–vis spectroscopy of biosynthesized Ssp-AgNPs and Ss-AgNPs

Using UV–visible spectroscopy, Spirulina sp., and S. subsalsa produced AgNPs with a distinct reddish-brown color (Fig. 2), surface plasmon variation (SPR) caused the color shift, and a prominent peak was seen at 432 and 458 nm in the spectral spectrum of Ssp. and Ss respectively (Fig. 3). SPR in the 450–500 nm spectral band is indicated for optical filtering applications in Ag–TiO₂ nano cermet thin films formed by two different sputtering techniques, as the smaller silver grains spread out from the first heterogeneous layer produced the massive, flat Ag grains that make up the overlayer. The weak SPR absorption at about 700 nm is caused by these anisotropic granules (Dakka et al., 2000). It was in line with previous research, which indicated that the usual SPR peak of AgNPs was located between 440 and 450 nm (Dar et al., 2013). ZnO nanoparticles that have been biosynthesized have been effectively produced utilizing Moringa oleifera extract and have been characterized by several approaches. After annealing, pure, single-phase absorbance profiles with a fall beginning at 350 nm and an inflection point at 380 nm, which is equivalent to 3.2 eV and in good agreement with the bandgap of ZnO, are obtained (Matinise et al., 2017). ZnSnO₃-NPs were shown to have UV–visible diffused reflectance, an absorption edge at 354 nm, a band gap of 3.50 eV, and possible photocatalytic activity using Aspalathus linearis natural extracts (Mayedwa et al., 2018a), and another study revealed a peak of SPR of AgNPs was noted the range of 420 nm (Bishoyi et al., 2021).

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Fig. 3

Observation of SPR absorbance peaks of (A) Ss- AgNPs and (B) Ssp.-AgNPs by UV–vis spectroscopy analysis.

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3.2.2

3.2.2 FE-SEM of biosynthesized Ssp-AgNPs, and Ss-AgNPs

The morphology of the nanoparticles was assessed by FESEM analysis (Fig. 4). The smaller particles in the FESEM investigation had aggregated to form a larger particle-like appearance. As revealed by SEM investigation, the dispersion of silver nanoparticles on the surface grid amply illustrates the morphological regularity. SEM tests indicated that the O. princeps extract generated spherical AgNP units with diameters ranging from 100–200 nm during the manufacture of O. princeps-AgNPs (Bishoyi et al., 2021). The synthetic Spirulina-AgNPs used in this investigation had a quasi-spherical form and ranged in size from 40-50 µm.

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Fig. 4

Observation of size of the particles of Ss-AgNPs (A, B), and Ssp-AgNPs, (C, D) FESEM analysis.

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3.2.3

3.2.3 FT-IR spectrometric profile of biosynthesized Ssp-AgNPs and Ss-AgNPs

Spirulina sp. and S. subsalsa exhibit distinct peaks at 3332, 1636, and 536 cm⁻¹ and 1634, 1634, and 536 cm⁻¹, respectively, FTIR spectra (Fig. 5, Table 1). The Spirulina sp. and S. subsalsa peaks, respectively, correspond to the N–H group of amino acids at 3350 and 3332 cm⁻¹. One can observe a diketone group as a peak at 1634 and 1636 cm⁻¹. The halogen compound of C caused the peaks at 536 and 524 cm⁻¹. (Fig. 6).

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Fig. 5

Observation of peaks of functional groups by FT-IR analysis (A) Ss-AgNPs and (B) Ssp-AgNPs.

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Table 1 Peaks of functional groups of Ssp-AgNPs and Ss-AgNPs by FT-IR spectroscopy.

AgNPs	Peak (cm−1)	Functional groups
Ssp- AgNPs	3332	Amino acid (N–H)
	1636	Diketones
	524	Halogen compound (C-I)
Ss- AgNPs	3350	Amino acid (N–H)
	1634	Diketones
	536	Halogen compound (C-I)

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Fig. 6

Observation of size of NPs (A) Ss- AgNPs and (B) Ssp- AgNPs by DLS analysis.

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3.2.4

3.2.4 DLS of biosynthesized Ssp-AgNPs, and Ss-AgNPs

The DLS distribution of Spirulina AgNPs size was found to range between 30 and 60 nm, however, the previous results were with an average computed size of 26.57 nm and a maximum mean intensity of 3.02 % found to be consistent with the dispersed light intensity value of 976.11 or the mean intensity value of 16.93 (Bishoyi et al., 2021). Pedalium murex produced biogenic AgNP with an average DLS of 73.14 nm (Bishoyi et al., 2021).

3.2.5

3.2.5 XRD of biosynthesized Ss, and Ssp-AgNPs

Moreover, crystalline structures were generated in 0–70 theta (Θ) ranges individually in both Spirulina species, the analysis of this study was presented (Fig. 7), and the peaks list of biosynthesized AgNPs (Ss-AgNPs, and Ssp-AgNPs) indicated the crystal size of NPs showed by XRD analysis (Tables S1, and S2). The previous study of XRD analysis with values of 10.3°, 38.90°, and 63.9°, and verified the production of AgNPs. The synthesized AgNPs were generated at two different temperatures, 39.01°, and 64.72°, according to the crystalline pattern (Bishoyi et al., 2021).

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Fig. 7

Observation of crystalline structure of (A) Ss- AgNPs and (B) Ssp- AgNPs by XRD analysis.

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3.2.6

3.2.6 Antibacterial properties of biosynthesized Ssp-AgNPs, and Ss-AgNPs

The produced Ssp-AgNPs and Ss-AgNPs exhibited remarkable antibacterial efficacy against bacterial strains, E. coli, A. baumannii, and S. pyogenes. The MIC values for the bacterial species were 50–100 µg/ml, the MBC value ranged from 80-140 µg/ml where the ZOI of −ve control was 0 mm, + ve control (Gentamycin) ranged from 17 mm to 19 mm. The ZOI of Ssp-AgNPs showed higher potencies of killing MDR bacteria than Ss-AgNPs (Table 2, Fig. 8). However, the early study ZOI of S platensis extracts ranged from 5 to 45 mm at 200 µl/well. The mean ZOI (30 mm) in the S. platensis nanomaterial extract inhibited Gram-positive bacteria with ZOI as given, Pseudomonas aeruginosa (12 mm), Staphylococcus aureus, Enterococcus faecalis (6 mm), and Klebsiella pneumoniae (11 mm) (Awadalla et al., 2021).

Table 2 Antibacterial activities (ZOI, MICs, and MBCs) of biosynthesized Ssp-AgNPs and Ss-AgNPs against bacteria, A. baumannii, E. coli, and S. pyogenes.

AgNPs	Bacterium	ZOI of −ve control (mm)	ZOI of + ve control (mm)	ZOI of Ssp-AgNPs and Ss-AgNPs (mm)	MIC (µg/ml)	MBC (µg/ml)
Ssp-AgNPs	A. baumannii	0	19	15	80	100
Ssp −AgNPs	E. coli	0	18	14	50	80
Ssp −AgNPs	S. pyogenes	0	17	14	100	140
Ss-AgNPs	A. baumannii	0	18	14	80	120
Ss-AgNPs	E. coli	0	18	13	75	130
Ss-AgNPs	S. pyogenes	0	17	12	60	90

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Fig. 8

Observations of antibacterial activity, (A) Ss-AgNPs against A. baumannii, (B) Ssp-AgNPs against S. pyogenes.

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4 Discussion

Cyanobacteria as microalgae, are highly sought after as naturally occurring sources of bioactive compounds with antibacterial, antiviral, anticancer, antioxidant, and anti-inflammatory qualities. They also yield several interesting compounds with a wide spectrum of biological functions (Hassan et al., 2022). Also, plant extracts, Aspalathus linearis with green-synthesized palladium and palladium oxide nanoparticles showed two distinct peaks: a broad peak including flavonoids that was centred around 348.5 nm, and a sharp peak containing the aspalathin component that was centred around 284.5 nm (Ismail et al., 2017). Pant-mediated green production of NPs of nickel oxide, palladium, and palladium oxide was particularly promising, because Aspalathus linearis natural extracts of NPs function as both a capping and reducing agent (Mayedwa et al., 2018b).

Along with the first production and viability of consistently replicating stoichiometric pure textured VO2 nano-structures, a potential vacuum sputtering technique utilizing an inverted cylindrical magnetron to produce nanostructured VO2 thin films was disclosed (Kana Kana et al., 2008). Generally nano system consists of a single dimension that is 100 nm or less, anisotropic, isotropic, and single-dimensional nanoparticles can be produced, in addition to two-dimensional systems like sheets or thin films and multilayered systems like nanocomposites (Henini et al., 2022).

The nanomaterial had the largest ZOI against Yersinia pestis, 45 mm, when compared to an organic solvent extract (Na-citrate) against Staphylococcus aureus (ZOI, 20 mm) (Awadalla et al., 2021). Instead of using prescription antimicrobials, S. platensis is an affordable antibacterial agent that can be used to combat harmful bacteria. It has been demonstrated that phycobiliproteins isolated from these species have several beneficial health effects, including boosting immune response. These pigments are widely used in the food, pharmaceutical, and cosmetics industries as natural alternatives to artificial colorants. Spirulina sp. is filamentous, multicellular, photoautotrophic biomass cyanobacteria resistant to the extreme environments seen in wetlands. It is rich in essential proteins, minerals, vitamins, and fatty acids used in many industries.

The cyanobacteria that produce oxygen share cytological similarities with Gram-negative bacteria, which are recognized for possessing phycobiliproteins in red and blue colors that are soluble in both water and chlorophyll a. Because of their chromatic adaptation mechanisms, cyanobacteria can survive in harsh environments with high salinity, low water content, heat, cold, nitrogen deficiency, photo-oxidation, osmotic stress, and ultraviolet light (Yarkent et al., 2020). Several compounds have undergone preclinical and/or clinical testing as possible anticancer medications, including the anti-microtubule medicines dolastatin 10 and curacin A (Barreca et al., 2020).

5 Conclusion

This research describes the characterization of products such as UV–vis spectroscopy results that confirmed the formation of AgNPs, and the FTIR analysis that identified the biomolecules responsible for the stability and reduction of the nanoparticles. XRD analyses were utilized to ascertain the dimensions, shape, and crystalline nature of the generated AgNPs. The significant antibacterial efficacy of AgNPs against bacteria was suggested by the potential application demonstrated by their ZOI, and MICs/MBC indicating the potential utility of medications of innovative antimicrobials. Future research efforts must focus on the in vitro assessment of these biosynthesized AgNPs and the development of practical applications were successfully synthesized by biological methods and combined with S. subsalsa and Spirulina sp. extracts. This study has shown the antibacterial activities of synthesized cyanobacterial extracts with AgNO₃ against bacteria. A synergistic impact against E. coli, A. baumannii, and S. pyogenes was seen when conjugating AgNPs claimed that all conjugates with effects are responsible for increasing the efficacy of various extracts and conventional antibacterial drugs.

Funding information

MSK acknowledges the generous support from the Research supporting project (RSP2024R352) by King Saud University, Riyadh, Kingdoms of Saudi Arabia. The extramural fund was unavailable; S. Bej, as the Ph.D. research fellow (Regd. no. 218https://doi.org/1002054/2021) and the work was supported by SOA University, Bhubaneswar.

CRediT authorship contribution statement

Shuvasree Bej: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Data curation, Conceptualization. Surendra Swain: Writing – review & editing, Visualization, Formal analysis. Ajit Kumar Bishoyi: Writing – review & editing, Visualization, Conceptualization. Chita Ranjan Sahoo: Writing – review & editing, Visualization, Conceptualization. Bigyan Ranjan Jali: Writing – review & editing, Methodology. Mohd Shahnawaz Khan: Visualization, Funding acquisition. Rabindra Nath Padhy: Writing – review & editing, Visualization, Supervision, Funding acquisition.