Introduction

The textile sector significantly contributes to environmental pollution by releasing dyes1. Textile wastewater is an intricate blend of many substances and pigments that have the potential to contaminate groundwater, surface waters and soils2,3. Dyes induce allergic reactions, promote tumour proliferation, and disrupt endocrine function. Before the 19th century, natural dyes were the primary sources of colouring agents. However, with the rise of industrialization, the utilization of synthetic dyes experienced a surge in demand4. Noxious substances released into the environment also increase when businesses develop. Introducing these dangerous substances into the water reduces the probability of having access to clean water5,6. Water is a fundamental constituent of natural resources and is considered a key requirement for all forms of living cells. It is well-known that water covers most of the surface of the earth, accounting for around 75% of its total area. In contrast, living organisms occupy only a small portion, approximately 5%, of the earth’s surface7,8.

The fabrication industry extensively utilizes synthetic dyes due to their high durability and chemical stability9. Textile dyes are categorized as azo, diazo, acidic, basic, metal-complexes, reactive, and anthraquinones. Azo dyes cause serious environmental damage by generating toxic aromatic amines10. These manufactured colours are believed to be very toxic, cancer-causing, and capable of causing genetic mutations11. Discharging dyes-containing effluents into the ambient environment results in severe pollution and harm to aquatic life12. Remediation procedures are intricate and time-consuming operations that effectively eliminate numerous pollutants from wastewater13.

Nanotechnology is advancing, enabling the utilization of nano-particles in specific applications14,15,16. Nanotechnology enables the creation of numerous materials with distinct and extraordinary characteristics17,18. Nanotechnology is frequently utilized because of its significant surface area/volume ratios19. Moreover, it functions as a connection linking the macro and microscopic realms20. Nano-particles purify wastewater and create alternative water sources to enhance water accessibility. Silver nano-particles provide significant therapeutic properties21,22. Despite the availability of multiple methods for producing nano-particles, the increasing need for environmentally friendly practices has led to a rising requirement for green synthesis. The environmentally friendly manufacture of metallic nano-particles improves and safeguards the environment by reducing harmful chemicals and removing biological hazards in bio-medical applications8. Microorganisms, including bacteria, yeast, algae, fungi, and extracts from many plant parts, are employed as reduction agents, converting metals to nano-particles throughout the manufacturing process19. The fabrication of metallic nano-particles (MNPs), such as silver (Ag), platinum (Pt) and cadmium sulphide (CdS), can be accomplished through the fabrication of NPs mediated by algae, a recently developing field of biotechnologies. Microalgae are thought to be a possible source of biologically active compounds. Research on microalgal biotechnology suggests that metallic NPs may be produced on a commercial scale and have an extensive range of potential uses. However, the bio-fabrication of nano-particles (NPs) utilizing microalgae, their biosynthetic methods, and variables influencing biofabrication, characterization, and applications have been the subject of little research. In light of this, the recently published articles provide a thorough review of the several approaches for designing and creating NPs utilizing microalgae as well as the variables influencing the biogenesis of microalgal NPs. The main points of interest are methods for creating biogenic microalgal-based NPs, characterizing them with different methods, and discussing future possibilities for their biological applications. The current review presents a novel idea and fresh perspectives on using microalgae in the biosynthesis of environmentally benign MNPs with a broad range of uses23.

Nano-particles can be synthesized based on their sizes, shapes, large surface areas, and volume ratios. Metal nano-particles, such as silver and gold, can be utilized as photocatalysts to degrade the dyes24. The silver nano-particle exhibits both anti-bacterial and antimicrobial characteristics20. Green synthesis is a synonymous term for biological synthesis. Green synthesis has gained prominence in the world of nanotechnology because of its capacity to produce nano-particles with optimal size and shape. The utilization of plant extract is favoured due to its non-toxic nature, user-friendliness, affordability, and environmental friendliness. Alkaloids, terpenoids, and flavonoids are biomolecules that induce the conversion of molecules into nano-particles in plant extracts12. The primary advantage of this procedure is the complete lack of any hazardous substances. Multiple studies have shown that silver nano-particles serve as highly efficient catalysts in colour reduction techniques. Silver has been applied in the medical profession for numerous years because of its antimicrobial properties. The literature proved that silver can impede the attachment of HIV to host cells. Silver is used to filter both water and air removing the microorganisms. The bactericidal impact of smaller Ag NPs would be greater than that of larger Ag NPs due to the increased surface area available for interaction25. In addition, AgNPs have the capability to interact with the inside of bacteria as well as their membrane surfaces.

This study uses the green bio-synthesis method for synthesizing AgNP’s from Spirulina platensis algae extracts, a low-toxic, economical, straightforward, and environmentally beneficial technique. Numerous bio-activities of AgNPs have been demonstrated, including anti-bacterial and degradation of Azo Dye (Methyl Red) in the wastewater.

Materials and methods

Solution and media

Silver nitrate salt (AgNO3), NaBH2, methyl red dye, Mueller-Hinton agar, antibiotics disks, ascorbic acid, and other chemicals were purchased from Merck, Germany.

Preparation of algal biomass

An isolate of Spirulina platensis was cultivated in the BG-11 medium specifically designed for cyanobacteria. The strain was obtained from the culture collection at the Microbiology Lab of the DNA Research Centre, University of Babylon, Iraq26. The algal growth potential was maintained at 28 °C using regular sub-culturing procedures. The growth conditions included a 16/8 h light/dark cycle with cool fluorescent light (20–30 mol photons m2 s− 1) and BG-11 media with a pH of 9.

Bio friendly-synthesis of silver nano-particles by Spirulina platensis algae extracts

The AgNPs were synthesized by desiccating and pulverizing 5 grammes of thoroughly purified biomass of Spirulina platensis in its exponential development phase. A solution of 50 ml of distilled water was combined with 5 grammes of recently harvested alga powder to become a concentration of extract (10 g/100 ml) at pH 7. The mixture was heated at 60 °C for 15 min before being filtered to obtain the extract. The resulting extract was subsequently stored refrigerated at 4 °C. AgNO3 with a concentration of 1 mM and a volume of 100 ml was prepared. Algae extracts were mixed with this solution before adjusting its pH to 7. The mixture was placed in a 250 ml Erlenmeyer flask and left for 24 h with a ratio of 1 part algae extract to 1 part AgNO3 solution. The user’s text is stated by Bhuiyan et al.27. At a temperature of 25 °C, the metal ions underwent full reduction, forming nano-particles. AgNPs were extracted by centrifuging the samples at a speed of 14,000 rpm for half an hour, then filtering the samples at a 0.22-micrometre membrane filter. The AgNPs were centrifuged at 14,000 rpm for half an hour while simultaneously being cooled. Following this, the AgNPs were washed three additional times with ethyl alcohol. The generated precipitate was collected, dried, and subsequently ground into a powder to create a Scheme 1. AgNPs are identified and characterized by differenties: field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and UV-Vis diffuse reflectance studies28. (The effects of various physicochemical factors such as culture age, AgNO3 concentrations, temperature, pH and NaCl concentration may be studied to determine the optimum growth conditions of AgNPs biosynthesis).

Scheme 1
scheme 1

Biosynthesis of AgNPs and their application as anti-bacterial and dye degradation.

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Activity of AgNPs as anti-bacterial against many multi-drug resistant pathogenic bacteria

The anti-bacterial activity of AgNPs produced by Spirulina platensis algae extracts was tested for its capacity to stop the development of the MDR bacteria under investigation (Streptococcus pneumonia, Staphylococcus aureus, E. coli, and Pseudomonas aeroginosa).

Antibiotic susceptibility

The antibiotic profiling of each identified bacterial isolate was studied using five antibiotic discs: Cephalothin (KF-30 µg), Methicillin (ME-5 µg), Novobiocin (NV-5 µg), Doxycycline (DO-30 µg), and Clarithromycin (CLR-15 µg). The isolates were tested against different concentrations of AgNPs: 31.25, 62.5, 125, 250 and 500 µL.mL− 1. The Kirby-Bauer disk-diffusion technique was employed for all experiments using Mueller-Hinton agar plates from Carl Roth, Germany. Before comparing each identified bacterial isolate with a standard turbidity of 0.5 McFarland (equivalent to 1.5*108 colony forming units per millilitre), as per the guidelines set by the Clinical Laboratory and Standards Institute (CLSI), the inhibition zones that appeared around each disc were evaluated using a Computer-Associated Electronic Zone Analyzer in Single-Disc Antimicrobial Susceptibility Testing.

Anti-bacterial properties of AgNPs

Human pathogens cultured on nutrient agar slants were utilized to evaluate the anti-bacterial properties of AgNPs. The assessment of AgNPs’ anti-bacterial activity adhered to the recommendations of the Clinical and Laboratory Standards Institute [CLSI, 2020]. Triplicates are employed in diluting AgNPs concentrations (500, 250, 125, 62.5, and 31.25 g/ml) in a solvent to assess antibiotic susceptibility and the effectiveness of AgNPs against the microorganisms through disc diffusion experiments. The isolates were first incubated for 15 min at room temperature, followed by overnight incubation at 37 °C when treated with AgNPs against the study isolates. After the incubation period, where the inhibition zones can be seen around the well, a digital Vernier calliper was used to assess its breadth12.

Methyl red dye degradation

A red methyl solution with a 10−3 M concentration was prepared, while sodium borohydride was dissolved in 0.5 Methanol. Then, a solution was prepared by combining 5 ml of a 10−2 M methyl red solution with 1 ml of an ethanolic borohydride solution. A volume of 1.5 millilitres of this solution was gathered and transferred into a UV quartz cuvette. An experiment was carried out using UV-VIS absorption spectroscopy to monitor the decrease in absorbance until the solution became completely colourless.

It is noteworthy to mention that experiments were repeated triple to guarantee the reliability of the results and to avoid human errors.

Results and discussion

UV/vis spectrophotometer

The presence of AgNPs nano-particles in the solution was proven by generating a spectrum in the visible region of 300–800 nm using a UV visible spectrophotometer (Fig. 1). Through this examination, a distinct peak in absorbance was identified at around 400–480 nm, indicating the presence of Ag nano-particles, where AgNPs exhibit an SPR band because of free electron excitation at 450 nm. The SPR of AgNPs is tuned in the visible and near-infrared regions by their shapes and sizes. SPR is the manifestation of a resonance impact because of the interactions of conduction electrons of metal nano-particles with incident photons13. AgNPs exhibit an SPR band because of the excitation of free electrons. The absorption bands in the visible regions are a characteristic of AgNPs. The presence of this peak was observed through the progressive alteration in the colour of the solution, transitioning from transparent to a deep brownish hue after a few minutes of reaction time. The UV-visible spectra further confirmed this observation, as shown in Fig. 2. Various authors have conducted research on the ability of Spirulina platensis algae extract to produce silver nano-particles. The presence of strong peaks at 430 nm in the UV-visible absorption spectra indicates the surface plasmon resonance of these silver nano-particles derived from the aqueous extract of Spirulina platensis algae14.

Fig. 1
figure 1

(a–c) AgNO3 1 mM solution (a), Broth of Spirulina platensis algae extract (b) and AgNPs formation as a positive result: brownish color (c).

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Fig. 2
figure 2

UV-visible spectra of the biosynthesized AgNPs.

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Field Emission-Scanning Electron Microscopy (FE-SEM)

Scanning electron microscopy is usually employed to investigate the surface morphology of synthesized AgNPs. SEM plates were created by adding silver nano-particles smeared with the solution on slides. Conductivity was included in the system by creating a thin platinum film coated on slides. Once the slides were ready, they were scanned at 20 KV accelerating voltage, and high-quality images were captured of silver nano-particles created by the Spirulina platensis algae extract, which have an average size scale of [28.72 nm] with a star-like structure, according to FE-SEM pictures, as in Fig. 3.

Fig. 3
figure 3

FE-SEM of AgNPs synthesis by Spirulina platensis algae extract.

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X-ray diffraction (XRD examination)

The crystal structure of synthesized AgNPs was measured using the XRD technique. The measurement was conducted in sequential phases with a voltage of 40 KV, an electric current of 30 mA, a scanning speed of 2 degrees per minute, and a range of 2 degrees (from 10 to 80 degrees). Figure 4 shows the X-ray diffraction (XRD) pattern of AgNPs formed through biological means and controls the crystalline characteristics of the AgNPs. The pattern displays four clear and weak-medium intensities peaks corresponding to the diffractions from planes 111, 200, 220, and 31129. These planes have a metallic nature and crystal structure, as seen in Fig. 4.

Fig. 4
figure 4

(i) XRD results for Spirulina platensis algae extract without AgNO3 (control). (ii) XRD results for AgNPs synthesized by the reaction of AgNO3 solution with Spirulina platensis algae extract.

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The FTIR analysis was employed to explore the functional groups responsible for reducing and stabilization of the synthesized AgNPs. The noticed intense bands were compared with standard values to recognize the functional groups. Infrared spectroscopy was employed to precisely recognize the chemical aggregates associated with the extracted silver nano-particles. The absorbance was quantified using SHIMADZU equipment within the 4000 –400 cm−¹ wavelength region. The spectra between 1900 and 3700 suggest an increase in the stretching of the O –H bond in alcohols and phenols, specifically due to the presence of hydroxyl. The absorbance peaks at 3398.34 cm-1 correspond to the C –N stretch in amines or amides, whereas the absorbance peaks at 1647.26, 1545.03, 1516.10, 1481.39, and 14 Based on previous observations, the functionalities that are most likely to facilitate the reduction of silver nitrate to AgNPs include hydroxyl groups and primary amines, as shown in Fig. 5. This method is utilized to elucidate the characteristics of the chemical groups that are linked to these particles, which is consistent with (14–16).

Fig. 5
figure 5

FTIR spectra pattern of AgNPs synthesized by the reaction of AgNO3 solution with Spirulina platensis algae extract.

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Antimicrobial activity of AgNPs

Figures 6 and 7 depict the effectiveness of many concentrations of silver nano-particles (AgNPs) in inhibiting the growth of Gram+ ve bacteria, specifically (Staphylococcus aureus and Streptococcus pneumonia), as well as Gram− ve bacteria, namely (Pseudomonas aeruginosa and Escherichia coli). This action is juxtaposed with that of other categories of antibiotics. The bacterial isolates exhibited increased concentration and susceptibility to AgNPs due to the anti-bacterial activity of the AgNPs. E. coli with higher concentrations (500 and 250 g/ml) exhibited increased sensitivity, as indicated by inhibitory zones measuring 20 and 19 mm, respectively. The isolates tested were Staphylococcus aureus, P. aeruginosa, and Streptococcus pneumonia. The inhibitory zones observed for Staphylococcus aureus were 19 mm and 17 mm at AgNPs concentrations of 500 g/ml and 250 g/ml, respectively. For P.aeruginosa, the inhibitory zones were 18 mm and 16 mm at the same AgNPs concentrations. Streptococcus pneumonia showed inhibitory zones of 16 mm and 14 mm at AgNPs concentrations of 500 g/ml and 250 g/ml, respectively. These results are shown in Fig. 8.

Cephalothin (KF-30) exhibited resistance against all tested bacterial isolates. Methicillin (ME-5) demonstrated resistance against Staph. aureus, Strep. pneumonia, and P. aeruginosa. Doxycycline (DO-30) displayed resistance against Staph. aureus, Strep. pneumonia, and E. coli. Novobiocin (NV-5) exhibited resistance against Pseudomonas aeruginosa. On the other hand, Clarithromycin (CLR-15) showed sensitivity towards all bacterial isolates, with identical inhibition zones of 13, 9, 12, and 11 mm for P. aeruginosa, E.coli, Strept. pneumonia, and Staph. aureus, respectively. The findings indicate that AgNPs have the potential to be a powerful antimicrobial agent, maybe surpassing certain traditional antibiotics in effectiveness against the tested bacterial strains. Multiple studies have shown that AgNPs are effective in killing microorganisms, and in certain situations, they have proven to be more efficient than traditional antibiotics in combating specific strains of bacteria30.

According to Abdulazeem et al.31, AgNPs had rapid bactericidal activity against all tested Gram-ve bacteria, resulting in a 99.9% decrease in bacterial count within 1–2 h. This demonstrates that AgNPs have the potential to be developed as a new type of antimicrobial therapy, namely for the treatment of microbial infections that are unresponsive to several medications. As stated by Pandiyan et al.32, AgNPs demonstrated significantly higher anti-bacterial effectiveness against a wide range of Gram-ve and Gram+ve strains of bacteria, compared to the plant extracts used in their production. Bhuiyan et al.27 demonstrated that biologically synthesized AgNPs exhibited a stronger anti-bacterial effect compared to chemically generated AgNPs, while also causing minimal harm to cells. Rippka et al.30 observed that even when administered at non-lethal levels, combinations of antibiotics and AgNPs had anti-bacterial properties, such as the concentration of the AgNPs and other factors, such as the strain of bacteria used, whether it is resistant to antibiotics or sensitive.

Fig. 6
figure 6

Anti-bacterial effect of many concentrations of AgNPs biosynthesis by Spirulina platensis algae extract different traditional antibiotics against G⁻ve bacteria E. coli and P. aeruginosa.

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Fig. 7
figure 7

Anti-bacterial effect of different concentrations of AgNPs biosynthesis by Spirulina platensis algae extract different traditional antibiotics against G⁺⁺ve bacteria S. pneumonia and S. aeures.

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Fig. 8
figure 8

The sensitivity test of AgNPs showed different concentrations of Gram-positive, Streptococcus pneumoniae (A), Gram-negative, and Pseudomonas aeroginosa. (B) Cultured on Muller-Hinton agar.

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The cell membrane is mostly composed of proteins and DNA bases, which contain phosphorus and sulphur. These biomolecules carry a negative charge. Due to its positive charge, silver has a tendency to interact with these biomolecules. Silver nano-particles induce changes in the structure of bacterial cells by causing damage to their cell wall and membrane33. Mukundan et al.34 have demonstrated the efficacy of AgNPs against both Gram+ ve and Gram− ve bacteria. The basic characteristics to describe nano-particles include their size, which must be between 1 and 100 nm, as well as their favourable surface-to-volume ratio and shape. Every single one of these aspects holds significance35. The size of nano-particles has a major role in influencing their activity against bacyeria. Multiple studies demonstrate that the ability of a nano-particle to penetrate bacteria is enhanced when the size of the nano-particle decreases36,37,38,39.

The exact mechanism by which AgNPs effect as anti-bacterial on microorganisms remains uncertain. We present a hypothetical mechanism of action of AgNP in Scheme 2, which could potentially result in anti-bacterial activity. Silver nano-particles exhibit a continuous release of silver ions, which could serve as a means of eradicating germs. Due to the strong electrostatic attraction between silver ions and sulphur proteins, the silver ions can easily adhere to the cell wall and cytoplasmic membrane40,41,42,43,44,45. The attachment of silver ions with the bacterial cell wall or with bacterial cytoplasmic membrane increase the cell’s permeability, leading to cell disintegration and simultaneous sabotage of the bacterial cell envelope. Silver ions, when they enter cells, inhibit respiratory enzymes, leading to the generation of reactive oxygen species that halt the production of adenosine triphosphate. ROS is the main species responsible for causing DNA modification and cell membrane rupture. Phosphorus and sulphur are essential constituents of DNA. However, the interactions between AgNPs and the sulphur and phosphorus in DNA might hinder DNA replication, impede cell growth, and perhaps lead to bacterial death. Silver ions have the potential to induce denaturation of the ribosome in the cytoplasm, leading to the inhibition of protein synthesis46,47,48,49,50.

Scheme 2
scheme 2

Explains the mechanical strategy used by NPs to combat bacteria. The bacterial cell wall is penetrated by the nano-particles, which then change the various metabolic pathways. It interferes with protein synthesis and DNA replication because oxidative stress causes the formation of ROS. Additionally, it finally results in cell death38.

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Degradation of methyl red dye by AgNPs

The fragile balance of the environment is at risk of being disrupted. In this investigation, a concentration of 4.0 g/ml of produced AgNPs was introduced to the dye in order to expedite its breakdown. The result is presented in a Scheme 2. Based on the UV plot, the absorptions peak of the dye molecules gradually decreases over time until it completely disappears, resulting in a change in the solution’s colour from red to colourless. The Shimudzu-1800 UV-VIS spectrophotometer was utilized to monitor the progression of methyl red degradation. Methyl red dissolved in water displays an SPR band at a wavelength of 390 nm, which corresponds to a transition from the π orbital to the π* orbital. Additionally, it exhibits an electron transfer related to azo at a wavelength of 380 nm, corresponding to a transition from the n orbital to the π* orbital. This information is depicted in Fig. 9.

Fig. 9
figure 9

Study of kinetics of the methyl red dye degradation by the bio-synthesis of AgNPs from Spirulina platensis algae extract at a 2 min time interval.

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Silver nano-particles (AgNPs) facilitate the electron transfer from BH-₄ molecules (donor) to the azo bond in methyl red (acceptor) when sodium borohydride present. However, AgNPs do not significantly impact the rate of degradation of methyl red solution. Figure 10 clearly demonstrates that the powerful reducing agent NaBH4 causes a very slow reduction of the methyl red solution. However, when methyl red dye solution is combined with a mixture of NaBH4 in anhydrous ethanol and AgNPs (0.4 g/ml), the dye is completely degraded within minutes. This result is shown in Fig. 10. Therefore, AgNPs derived from Spirulina platensis algae extract can be effectively utilized to eliminate methyl red colouring from wastewater11,51.

Fig. 10
figure 10

Study of kinetics of the methyl red dye degradation by NaBH2 at a 2 min time interval.

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The dye and NaBH4 molecule initially binds to the surfaces of the AgNPs. The dye acts as an electrophilic agent upon absorption, whereas NaBH4 acts as a powerful nucleophilic agent. AgNPs serve as a relay mechanism in the solution, facilitating the transfer of the electron needed for dye degradations from NaBH4 to the dye molecules52,53,54. During the process of dye degradations, the dye molecules underwent decomposition into small, colourless chemicals such as SO42, CO2, H2O, and others55. Based on the latest research, silver nano-particles have the ability to function as a catalyst in efficiently breaking down both individual and combined colour combinations seen in artificial wastewater. The catalytic activity of silver nano-particles can be further evaluated to eliminate toxic colours from wastewater. The current work provides a concise and non-hazardous description of the dye degradation process. The user’s text is stated by Gola et al.11.

Conclusion

Nanotechnology utilizes the fabrication of minuscule particles that can fulfil many functions. Research has demonstrated the potential of nanotechnology to address various challenges in the domains of medicine, food, agriculture, textiles, and the environment, among others. Recently, wastewater treatment with nanotechnology has shown successful outcomes. Waterways may be cleansed of various contaminants utilizing different metallic and non-metallic nano-particles. Spirulina platensis algae extract was used to generate silver nano-particles from silver nitrate solution, adopting an environmentally acceptable and effective green approach. Production of AgNPs with a 28.72 nm average particle size and a star-like shape. The current approach for generating silver nano-particles shows promising results in action against both Gram+ ve and Gram− ve bacteria. The produced AgNPs also displayed excellent catalytic activity toward methyl red dye degradation, suggesting they have promise for industrial application. However, to achieve long-term sustainability in real-world settings, it is also necessary to research the right fusion of the catalyst with traditional technologies, as well as its stability and recyclability.

By using adsorption, oxidation, and reduction, among other processes, these nano-particles may effectively remove contaminants from wastewater, including heavy metals, organic compounds and dyes. Being less costly, less damaging to the environment, and easily scaled up for large-scale manufacturing, this technology has several benefits over chemical and physical processes. Because of the wide range of possible applications for silver nano-particles, more studies should focus on creating novel applications in industries including power, health care, and environmental remediation. It is imperative to carefully examine how nano-particles affect the environment to prevent unexpected repercussions, just like with any newly developed technology. Therefore, the main focus of future studies should be on examining the effects of nano-particles on the environment, including their persistence in the environment, capacity to assemble in living things, and effects on ecosystems. To fully understand the processes underlying AgNP cytotoxicity, more research is necessary.