1 Introduction

Metal nanostructures have attained considerable attention in the recent times due to their exceptional properties like optical, electrical and physiochemical [1]. Among these, silver nanoparticles (AgNPs) are utilised in numerous applications namely, catalysis [2], biomedical [3], theragnostic [4], photothermal conversion [5], and sensing [6,7,8,9,10,11], etc. These NPs possess a unique characteristic, which is enhanced absorbance and scattering of light of different wavelengths depending on the size, composition and shapes of the particles [12]. This enhanced absorbance is due to localized surface plasmon resonance (LSPR) arising from the collective oscillation of electrons resonating with the external electromagnetic radiation [13]. Size and shape of nanoparticles which determines the LSPR peak position can be altered by varying the synthesis protocols such as pH, temperature, time and solvent media [7, 14].

Various synthesis routes are employed for preparation of AgNPs. Broadly, they are classified into two types, top down and bottom up, where the later one is the highly favoured one. It can be further classified into three types, namely physical, chemical and biological greener routes. But amongst them, greener synthesis protocols are mostly utilised because of low toxicity, cost effectiveness and higher yield [15, 16]. Wide range of plant-based components were used for synthesis [17,18,19,20] such as Aloe vera [21], Camellia sinensis [22], Ocimum sanctum [23,24,25,26,27,28], Mangifera indica [29], Azadirachta indica [30], Magnolia virginiana [31], etc. Here, growth of nanoparticles occurs in three phases: at first ions gets converted into zerovalent atoms by suitable biological reducing agents, nucleates to form seeds through primary nucleation, followed by secondary nucleation which results in formation of NPs. The secondary nucleation highly depends on the suitable solvent used which controls the kinetics and growth of NPs up to a certain desirable size and shape [32, 33].

Recently, nanoparticles synthesized on different solvent media have shown promising potential in diverse array of applications. Various crucial parameters such as dipole moment, dielectric constant, acceptor and donor ability, solubility and cohesive pressure determine the behaviour of the solvent [34]. The polarity of the solvent determines the structural topology and configuration depending on its cohesive nature [35]. The major concern is about the toxicity associated with the use of various organic hydro-carbonated compounds as a solvent. As per a study performed by Sheldon, water is established to be the ideal solvent, due to its cost-effectiveness, higher accessibility, and no toxic effects [36]. Moreover, it favours dissociation of ionic compounds, for their free movement. Nevertheless, the study organic solvents as an active role in reducing and stabilising the nanoparticles still remains as an active field of research. Such as Wu et al., used planar or linear hydrocarbons as solvent for synthesis of gold nanoparticles for obtaining exceptional monodispersed NPs with tuneable size [37, 38]. Though solvent has a crucial role towards fundamental properties, but they are not solely responsible for the creation of nanoparticles. Other parameters such pH also has significant influence.

The reaction rate, size, shape, surface charge and aggregation of nanoparticles relies on pH of the NPs solution [39]. Some studies reported that at lower pH value, especially below 3 aggregation of nanoparticles occurs but if the pH is further lowered down to 2 coagulations of nanoparticles may occur. Meanwhile, higher pH value above 5 acts as a favourable condition for synthesis of nanoparticles, where mono-dispersity in synthesised particles increases with increase in pH value [40].

In the context of this discourse, it is evident that both parameters exert a deterministic influence on the synthesis of nanoparticles. Existing scientific literature reveals a scarcity of investigations exploring the utilization of diverse solvents in nanoparticle synthesis, as well as a limited number of studies investigating alterations in nanoparticle properties resulting from pH variations both pre- and post-synthesis. Here, our work reports synthesis of AgNPs in various solvent media and pH conditions. This study was performed to unravel the effect of solvent on synthesis of nanoparticles via two different reducing agents viz. green and chemical.

2 Experimental section

2.1 Materials and instruments

Silver Nitrate (MW-169.87), Propanol (MW-60.10), and Sodium Hydroxide (MW-40) was procured from Thermofisher Scientific. N-Methyl-2-Pyrrolidone (MW-99.13), Ammonia (MW-17.03), and Sodium Borohydride (MW-37.83) was procured from Merck. Ethanol (MW-46.07) was procured from Fisher Chemical and Acetone (MW-58.08) was procured from Fisher Scientific. Hydrochloric Acid (MW-36.46) was purchased from FINAR. A chemical balance (METTLER TOLEDO ME204), an oven (Ecogian series; EQUITRON), a magnetic stirrer (SPINOT-TARSONS), a pH meter (EcoTestr pH1), UV–Visible spectrophotometer (Thermo scientific GENESYS 180), and Advanced X-ray powder diffractometer (Model: D8 FOCUS, Make: BRUKER AXS) were used to conduct optical and structural analysis.

2.2 Preparation of the Ocimum sanctum leaves extract

Ocimum sanctum leaves were procured from nearby village area of Tezpur University. Initially, the leaves were washed properly and then allowed to dry in an oven. Then, to 100 ml of distilled water, 10 g of leaves were added and heated at 100 °C for an hour to obtain the extract. The solution obtained was filtered twice using filter paper no. 1 (Whatman). The extract solution was further refrigerated below 4 °C utilised for synthesis of nanoparticles [41].

2.3 Biosynthesis of silver nanoparticles (AgNPs) in different solvent media

Different solutions of silver nitrate (AgNO3) were prepared in different solvent media by adding 0.00845 g of AgNO3 in 50 ml of various solvents (Distilled water, acetone, ethanol, NMP, ammonia and propanol). To each solution containing 10 ml of AgNO3, 1 ml of Ocimum sanctum leaves extract was added and heated for 5 min at 60 °C. A yellow or brown coloured solution indicated formation of nanoparticles [41] (Figs. 1, 2a, b).

Fig. 1
figure 1

Schematic representing a Synthesis of AgNPs via green and chemical modes prepared in various solvents, and b Mechanism of synthesis process in various pH conditions

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Fig. 2
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a Silver precursor solution in solvents: Water, N-methyl-Pyrrolidone, Ethanol, Acetone, Propanol, and Ammonia, b AgNPs solution prepared by green method using Ocimum sanctum leaves extract in various Solvents Water, N-methyl-Pyrrolidine, Ethanol, Acetone, Propanol, and Ammonia, c AgNPs solution prepared by chemical method in various Solvents Water, N-methyl-Pyrrolidone, Ethanol, Acetone, Propanol, and Ammonia, d Green synthesised AgNPs prepared by using Ocimum sanctum leaves extract by varying pH 1, 3, 5, 6, 8, 9, 12 and 14 (before formation), e Green synthesised AgNPs prepared by Ocimum sanctum leaves extract by varying pH 1, 3, 5, 8, 10,12, and 14 (After formation), f Chemically synthesised AgNPs prepared by varying pH 1, 3, 5. 7, 10, 12, and 14 (Before formation), and g Chemically synthesised AgNPs prepared by varying pH 1, 3, 5. 7, 8, 10, 12, and 14 (After formation)

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2.4 Chemical synthesis of silver nanoparticles (AgNPs) in different solvent media

1 mM of AgNO3 solution was prepared in different solvents. Similarly, sodium borohydride (NaBH4) solution was also prepared in different solvent media by adding 0.00378 g of NaBH4 to various solvents. Then, to 10 ml of NaBH4 solution, 3 ml of AgNO3 was added under constants stirring and heating, where change in colour of the solution indicated that nanoparticles were formed [42] (Figs. 1, 2c).

2.5 Alteration of pH before biosynthesis of nanoparticles

At first, 10 ml of 1 mM AgNO3 was heated under continuous stirring. Then, to it few drops of acidic buffer was added to alter the pH from 1–6. Finally, 200 µl of Ocimum sanctum leaves extract solution was added.

To alter the pH from 8–14, 0.01 M of NaOH solution was added dropwise followed by addition of 200 µL of Ocimum sanctum leaves extract under constant heating and stirring (Fig. 2d).

2.6 Alteration of pH after biosynthesis of nanoparticles

Firstly, 10 ml of 1 mM AgNO3 was heated under continuous stirring, and then 200 µl of Ocimum sanctum leaves extract was added to the solution. After formation of nanoparticles, few drops of acidic buffer were added to alter the pH from 1–6.

Similarly, 0.01 M of NaOH solution was added to alter the pH from 8 to 14 (Fig. 2e).

2.7 Alteration of pH before chemical synthesis of nanoparticles

At first 10 ml of NaBH4 solution was heated under constant stirring and to it few drops of 0.001 M of HCl was added to alter the pH from 1–6 followed by dropwise addition of 3 ml of AgNO3 solution.

To vary the pH from 8–14, 0.01 M of NaOH was added before addition of AgNO3 solution (Fig. 2f).

2.8 Alteration of pH after chemical synthesis of nanoparticles

Firstly, 10 ml of NaBH4 solution was heated under constant stirring and to it 3 ml of AgNO3 solution was added dropwise. To alter the pH from 1–6, few drops of 0.001 M of HCl was added to the colloidal solution. Likewise, to vary the pH from 8–14, 0.01 M of NaOH was added to the solution (Fig. 2g).

3 Results and discussion

3.1 Optical study

3.1.1 Silver nanoparticles prepared in different solvent media

3.1.1.1 Biosynthesis mode

In this study, we employed UV–Visible spectroscopy to elucidate the optical characteristics of silver nanoparticles synthesised in various solvent media. Notably, the process unravelled the distinct behaviours of nanoparticles depending on the choice of solvents. Immediate formation and agglomeration of NPs was observed when employing organic solvents such as ethanol and acetone. In contrast, the use of NMP, resulted in formation of NPs after a brief incubation period of 3 min, while water required 5 min for nanoparticle synthesis. UV–Visible analysis of the resultant AgNPs displayed characteristic absorbance peak for water, NMP, ethanol and acetone. However, as acetone and ethanol are less polar solvents, exhibited red shifted, broad absorbance peak indicating formation of highly polydisperse NPs with larger size due to pronounced agglomeration. Conversely, NMP yielded less broadening of absorbance peak, signifying synthesis of less polydisperse nanoparticles. Additionally, a slight red shift relative to AgNPs synthesised in water, suggested increase in particle size. Notably, no absorbance peak was observed in ammonia and propanol, indicating that these solvents are not suitable for nanoparticles synthesis [43, 44].

The observed variations in nanoparticle optical characteristics can be attributed to the polarity of the solvent and its ability to dissociate ionic compounds. Polar solvents, such as NMP and water, enables rapid dissociation of ionic compounds because of the existence of polar functional groups and hydrogen bonding capabilities, respectively. In case of NMP, its highly polar nature enables it to perform as a weak reducing agent, facilitating nanoparticles formation without addition of any external reducing agent. Conversely, higher concentrations of less polar organic solvents such as acetone and propanol disrupted nanoparticle stability, leading to increased zeta potential values and subsequent agglomeration, driven by elevated surface charge. The literature has also noted that ammonia, despite its polarity, inhibited nanoparticle formation by creating an oxidizing complex compound (Ag [NH3]2+), resulting in reduced reaction rates when ammonia concentrations were low. At higher ammonia concentrations, complete inhibition of nanoparticle formation occurred. Furthermore, propanol, being a less polar solvent than ethanol and containing a longer carbon chain, exhibited reduced overall polarity, preventing the dissociation of precursor ionic compounds and, consequently, nanoparticle formation [45,46,47,48] (Fig. 3a, Table 1a).

Fig. 3
figure 3

UV–Vis plot of AgNPs solution prepared by a green method using Ocimum sanctum leaves extract and b chemical method in various solvents Water, N-methyl-Pyrrolidine, Ethanol, Acetone, Propanol, and Ammonia

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Table 1 Comparative study of peak position and absorbance obtained for different solvents
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3.1.1.2 Chemical synthesis mode

The optical characteristics of the synthesized nanoparticles (NPs) across different solvent environments was also carried out through UV–Visible spectroscopy. Notably, a rapid emergence of NPs was observed when utilizing water, ethanol, and NMP as the solvents. In the case of ethanol and NMP, broad absorption spectra were noted, accompanied by a noticeable shift toward longer wavelengths. This spectral shift indicated an augmentation in particle size and a decline in the stability of the nanoparticles. These changes were attributed to the amplification of positive surface charge on the nanoparticles when exposed to these solvents. Conversely, no discernible absorption peak was observed in ammonia, acetone, and propanol. Additionally, in the case of NMP, an intriguing new peak emerged at 373 nm. This spectral feature may be attributed to out-of-phase quadrupole and dipole resonances within the silver nanoparticles due to interactions between the silver ions from the reducing agent and the NMP solvent [49, 50].

In line with the findings from our previous biosynthesis of AgNPs, both propanol and acetone in this chemical synthesis setting exhibited hindrance in the dissociation of ionic compounds, primarily due to their lower polarity. As a result, despite the addition of the reducing agent, nanoparticles formed in these solvents due to the inability of the silver ions to dissolve in acetone and propanol. This absence of silver ions in solution was a contributing factor [46, 48].

Moreover, similar to our earlier observations, ammonia also demonstrated a hindrance to nanoparticle formation. The precursor compound, when dissolved in ammonia, formed a complex compound denoted as Ag [NH3]2+, which exhibited oxidizing properties surpassing those of Ag+. Consequently, this complex formation led to a reduction in the reaction rate, particularly when lower concentrations of ammonia were present. In instances of higher ammonia concentrations, a complete inhibition of nanoparticle formation ensued [47] (Fig. 3b, Table 1b).

3.1.2 Silver Nitrate in different solvent media

Metallic precursor solution in NMP displayed two significant absorbance peak one at 430 nm, corresponding to characteristic LSPR peak of AgNPs, and another small peak was obtained at 320 nm which is due to out of plane quadrupole plasmon resonance occurring in nanoparticles. It is also evident that due to high polarity due presence of the pyrrolidone functional group the solvent NMP can also function as a good green reducing agent for synthesis of plasmonic nanoparticles acting as an electron donor [51] (Fig. 4).

Fig. 4
figure 4

UV–Vis plot of AgNO3 in various Solvents Water, N-methyl-Pyrrolidine, Ethanol, Acetone, Propanol, and Ammonia

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3.1.3 AgNPs synthesized under different pH conditions

pH plays a significant role in determining the shape and size of the AgNPs, owing to its varied LSPR characteristics. On increasing the pH, rate of the reaction increased along with decrease in reaction time especially when pH > 7 for both green assisted and chemically synthesized AgNPs. Initially, a slight shift in colour from colourless solution (pH < 6) towards darker shade of brown (pH > 6) was observed suggesting that no significant formation of AgNPs occurred in pH < 6. This was indicative from the UV–Vis spectra which displayed no characteristic LSPR peaks in that acidic pH range. The absorbance spectrum also revealed that on increasing the pH from neutral to basic, significant absorbance peaks with increase in absorbance were observed suggesting basic and neutral medium to be favourable for synthesis of nanoparticles. This behaviour of pH on the growth of nanoparticles can be attributed to its capability to modify the charge of bio-compounds present in the leaf extract [52]. Additionally, optimal value of pH for synthesis of AgNPs attributed to high absorbance peak value, low λmax (wavelength value for maximum absorbance/LSPR peak position) and high population of smaller sized NPs were found to be pH 12 and pH 10 for biosynthesized and chemically synthesized AgNPs respectively [53,54,55] (Fig. 5, Table 2). This phenomenon can be attributed to the enhanced reactivity of the biological components within the extract at a pH of 12. This heightened activity can lead to the formation of a significant quantity of smaller-sized nanoparticles, particularly at this pH value. Conversely, a pH of 10 was identified as optimal for the chemical reduction of silver ions into nanoparticles. At this pH, the reducing agent, sodium borohydride, efficiently generated smaller-sized particles.

Fig. 5
figure 5

UV–Vis plot of a Green assisted, and b Chemically synthesised AgNPs prepared by varying pH

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Table 2 AgNPs synthesised under various pH conditions
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3.1.4 Stability of AgNPs under different pH conditions

The pH of the synthesized AgNPs solutions were altered from acidic to basic which resulted in change in colour of the solution. The yellow coloured AgNPs solution turned colourless in very acidic condition. However, on further increment of pH, colour of the solution tended towards darker shade till pH 14 in case of biosynthesized AgNPs but in case of chemically synthesized AgNPs solution, colour changed to lighter for pH > 8. From UV–Vis absorbance spectrum it was confirmed that the synthesised NPs displayed quite stable behaviour in neutral and basic medium. This was observed from enhanced absorption bands for pH > 5 with sharp LSPR peaks and narrow width. The stability in such alkaline pH is mainly contributed by the antioxidants of the leaf extract [56]. However, in acidic medium (pH < 5) absorbance decreased significantly attributing to protonation which weakened the Ag–O bonds dissociating more Ag+ into the solution indicating conversion of NPs to ions [57]. The as-synthesised NPs were found to be optimal for storage and preservation at pH 14 and pH 10 for green and chemically synthesized respectively as it displayed comparatively sharp peak with less broadening at lower λmax value indicating monodisperse distribution of particles with smaller sizes [39] (Fig. 6, Table 3).

Fig. 6
figure 6

UV–Vis plot of to determine the stability of a Green assisted, and b Chemically synthesised AgNPs

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Table 3 Stability of AgNPs at various pH conditions
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3.2 Structural analysis

3.2.1 Biosynthesis mode

Biosynthesised nanoparticles prepared in water, NMP and acetone displayed distinct diffraction peaks at four positions corresponding to Bragg diffraction planes (111), (200), (220) and (311), thereby affirming Face Centred Cubic (FCC) crystalline structure of the NPs. This finding aligns with the X-ray diffraction (XRD) patterns found in the JCPDS database, specifically under file 04–0783. These database patterns exhibit four well-defined diffraction peaks at positions similar to those seen in our experimental results. Additionally, in acetone, emergence of two new peaks between 50–60° were observed, corresponding to bio-organic phase which is present on the outer shell of the particles, caused by the interaction of the solvent media and the reducing agent [58] (Fig. 7a–c).

Fig. 7
figure 7

XRD plots of biosynthesised AgNPs using various solvents a Water, b NMP and c Acetone

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3.2.2 Chemical synthesis mode

The crystalline nature of the synthesized nanoparticles was validated by comparing them with the X-ray diffraction (XRD) spectra found in the JCPDS database, specifically under file 04–0783. The presence of analogous diffraction peaks corresponding to the Bragg diffraction planes (111), (200), (220), and (311) in both our experimental results and the database confirms the Face Centred Cubic (FCC) crystalline structure of nanoparticles [59] (Fig. 8a, b).

Fig. 8
figure 8

XRD plots of chemically synthesised AgNPs using various solvents a Water, and b NMP

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4 Conclusion

The study reports synthesis of nanoparticles in various solvent media, where it was observed that propanol and ammonia were not suitable for silver nanoparticles (AgNPs) formation. A substantial absorbance peak indicated that NMP was an effective reducing agent, facilitating nanoparticle formation without the need for heating or stirring. X-ray diffraction (XRD) analysis also established a face-centred cubic (FCC) crystal structure for all the synthesized nanoparticles. Ultimately, it was determined that a basic or neutral medium was optimal for both the formation and storage of these nanoparticles.