Silver nanoparticles are among the most widely pursued materials in modern nanotechnology, valued for their antimicrobial, electrical, optical, and catalytic properties and for applications spanning electronics, healthcare, textiles, coatings, and food-related technologies. Yet the conventional way of making them relies on chemical reducing agents such as sodium borohydride, which are highly reactive and may generate toxic byproducts, raising environmental concerns that limit their suitability for some biomedical applications. A critical review now published in the journal Bioscience Nanotechnology argues that plant-extract-mediated synthesis, often called green synthesis, offers a potentially more sustainable route, but only if three tightly coupled parameters—pH, temperature, and plant-extract concentration—are treated as an interdependent system rather than as isolated dials.
The review, led by Sidney S. Dos Santos and colleagues, is a critical narrative review with a targeted literature search across Scopus, Web of Science, and Google Scholar, prioritizing the 2020–2025 window while including earlier classic references for mechanistic background. Approximately 100 publications were considered in the narrative synthesis, and 50 primary studies on plant-extract-mediated silver nanoparticle synthesis were selected for a comparative compilation of reaction pH, temperature, and silver nitrate-to-extract ratio. Particle size, morphology, and reaction time were deliberately excluded from that comparison because these properties are inconsistently reported across studies and are often determined using non-equivalent characterization methods, which limits direct comparison.
The mechanism the review describes involves three interdependent stages. First, phytochemical compounds in the plant extract—phenolics, flavonoids, terpenoids, reducing sugars, organic acids, proteins, and polysaccharides—act as reducing agents, converting silver ions, commonly supplied by silver nitrate, into metallic silver. Nucleation and growth of silver colloids then occur, with formed nuclei growing mainly through additional deposition of reduced silver atoms and, depending on conditions, controlled coalescence. Finally, biomolecules adsorb onto the nanoparticle surfaces, limiting further growth, preventing uncontrolled aggregation, and imparting colloidal stability through electrostatic and steric interactions. The relative contribution of each phytochemical class depends on the composition of the specific extract used.
pH emerges as a determining factor because it modifies the ionization state and reactivity of plant-derived metabolites, the complexation and availability of silver ions, the reduction rate, and the surface charge of the resulting nanoparticles. In extracts containing ionizable phenolic and catecholic compounds, acidic conditions favor protonated hydroxyl groups, which may reduce their electron-donating ability. Under alkaline conditions, deprotonation can generate phenolate and catecholate species with greater affinity for silver ions and enhanced electron-transfer capacity, and oxidation of catecholate species to quinone-like structures may release electrons that contribute to silver-ion reduction. The review emphasizes that these effects are system-dependent rather than universally applicable, and it warns that excessively high pH should be avoided because changes in silver speciation and the formation of poorly soluble species may compete with nanoparticle formation. In one cited synthesis, the pH decreased from 9.0 to 7.82 during the reaction, illustrating that pH must be monitored throughout rather than reported only at the initial stage.
Temperature acts on reaction kinetics through the Arrhenius relationship, in which the rate constant increases with temperature because a larger fraction of molecules has sufficient energy to overcome the activation barrier. Faster silver-ion reduction can accelerate nucleation and growth, and cited studies report faster color development and greater surface plasmon resonance band absorbance as temperature increases. However, elevated temperatures may also compromise colloidal stability. Excessive heating can promote oxidation or thermal degradation of phenolic compounds, flavonoids, vitamins, proteins, and polysaccharides, weakening their reducing, complexing, and surface-capping capacity, which may lead to particle growth, agglomeration, and colloidal instability. In one cited study using Litchi chinensis leaf extract, a red shift of the plasmon band from 436 to 445 nm as temperature rose from 30 °C to 100 °C indicated thermal instability and aggregation, and the authors suggested 40 °C as optimal for that system. The review notes that temperatures within roughly 25–60 °C may suit several systems, but that the appropriate temperature must be determined experimentally for each plant-extract–silver nitrate combination.
Plant-extract concentration is presented as the central connecting variable, determining the availability of both reducing and capping agents. At low concentrations, the amount of bioactive molecules is insufficient to efficiently reduce the available silver ions, slowing reduction, limiting nucleation, and leaving formed nuclei poorly stabilized. Within an intermediate range, reduction and capping are balanced, favoring efficient nucleation, controlled growth, and colloidal stability. At excessive concentrations, surplus biomolecules may thicken the organic layer around particles and promote bridging interactions between colloids, increasing polydispersity and aggregation. The review cites work on Acacia cyanophylla extract in which increasing extract concentration raised mean nanoparticle size from 86.98 nm to 347.2 nm and the polydispersity index from 0.280 to 0.412, suggesting that excessive concentration can promote agglomeration and continuous growth.
To move beyond literature synthesis, the authors conducted an experimental case study using aqueous Astrocaryum aculeatum pulp extract, adapted from a previously published methodology. Four extract concentrations were evaluated: 10, 15, 20, and 30 g L⁻¹, equivalent to 1.0, 1.5, 2.0, and 3.0 g of pulp per 100 mL of extraction medium. Five milliliters of each extract were mixed with 50 mL of 0.1 mmol L⁻¹ silver nitrate, the reaction pH was adjusted to 9 with dilute sodium hydroxide, and the mixtures were stirred for 24 hours. Three independent synthesis batches were prepared for each concentration, and formation and colloidal behavior were assessed by visual inspection, UV–Vis spectroscopy, dynamic light scattering, polydispersity index, and nanoparticle tracking analysis, with results expressed as mean ± standard deviation.
The results reveal a clear concentration threshold rather than a linear benefit. Samples prepared with 10, 15, and 20 g L⁻¹ extract exhibited surface plasmon resonance maxima at 406.3 ± 3.2, 410.0 ± 2.0, and 407.0 ± 1.0 nm, respectively, whereas no clearly resolved plasmon maximum was observed at 30 g L⁻¹ in any replicate, a broad and unresolved optical profile the authors associate with increased light scattering, polydispersity, and aggregation. Intensity-weighted DLS measurements yielded mean hydrodynamic diameters of 185.8 ± 18.2, 220.0 ± 19.0, 589.2 ± 29.7, and 786.3 ± 108.8 nm across the four concentrations, showing a pronounced increase in apparent diameter at the two highest levels. Mean PDI values rose from 0.267 ± 0.038 and 0.338 ± 0.015 at the two lowest concentrations to 0.849 ± 0.068 and 0.939 ± 0.050 at the two highest. One-way ANOVA confirmed a significant effect of extract concentration on PDI [F(3,8) = 160.50, p < 0.001, η² = 0.984], and Tukey’s test showed no significant difference between the two lowest concentrations or between the two highest, but significantly higher PDI for the two highest than the two lowest, indicating a marked increase in polydispersity between 15 and 20 g L⁻¹.
Nanoparticle tracking analysis added a number-based perspective that DLS alone cannot provide. Mean particle sizes were 64.7 ± 11.3, 69.2 ± 2.4, 82.7 ± 3.3, and 93.0 ± 7.5 nm for the four concentrations, and particle-number concentrations were (7.34 ± 0.23) × 10⁹, (5.41 ± 0.18) × 10¹⁰, (4.92 ± 0.08) × 10¹⁰, and (9.45 ± 0.28) × 10⁹ particles mL⁻¹. The 15 g L⁻¹ sample showed the highest particle-number concentration, followed by the 20 g L⁻¹ sample, while the 30 g L⁻¹ sample exhibited a larger mean size and a lower particle-number concentration, consistent with reduced particle number density and increased aggregation at the highest extract concentration. The divergence between DLS and NTA sizes reflects the intensity-weighted nature of DLS, in which even a small fraction of large aggregates can strongly inflate the apparent mean diameter, versus the number-based tracking approach of NTA. Taken together, the authors identify 15 g L⁻¹ as the most favorable concentration among the tested conditions.
The review concludes that future optimization studies should evaluate pH, temperature, and extract concentration as a coupled reaction system, using factorial designs, response surface methodology, and multivariate modeling to identify synthesis windows in which reduction rate, nucleation density, growth control, and colloidal stabilization are appropriately balanced—approaches not applied in the present case study and proposed as directions for future research. The authors also acknowledge a key limitation: the A. aculeatum extract was not phytochemically characterized, so the specific metabolites responsible for silver-ion reduction, surface capping, and stabilization could not be identified or quantified, limiting the generalizability of the concentration-dependent trends. Because phytochemical composition varies with plant species, plant part, cultivar, geographic origin, season, cultivation and harvest conditions, and extraction methods, reproducibility cannot be achieved solely by controlling reaction parameters. The authors argue that scalable production will require botanical authentication, standardized sourcing and storage, controlled extraction procedures, batch-specific quality control, extract dosage normalized to dry mass or phytochemical markers rather than raw plant material, and early evaluation of residual silver speciation, colloidal stability, cytotoxicity, and ecotoxicity as process development proceeds.
Subject of Research: Plant-extract-mediated green synthesis of silver nanoparticles and the interdependent effects of pH, temperature, and extract concentration
Article Title: Green synthesis of silver nanoparticles mediated by plant extracts: a critical review and experimental insights on the integrated effects of pH, temperature, and concentration
Article References: Dos Santos, S. S., De Moraes, G. E. R., Cerqueira, M. Â., Aouada, F. A., Tanaka, F. C., Grillo, R., Perotti, G. F., & de Moura, M. R. (2026). Green synthesis of silver nanoparticles mediated by plant extracts: a critical review and experimental insights on the integrated effects of pH, temperature, and concentration. Bioscience Nanotechnology, 2(1), Article 24. https://doi.org/10.1186/s44331-026-00033-7
Image Credits: AI Generated
DOI: 10.1186/s44331-026-00033-7
Keywords: silver nanoparticles, green synthesis, plant extracts, pH, temperature, extract concentration, colloidal stability, polydispersity, UV-Vis spectroscopy, dynamic light scattering, nanoparticle tracking analysis, Astrocaryum aculeatum
News Source: Gregory Coleman. (October 28, 2026). Plant Extracts Can Make Silver Nanoparticles, But Only Within a Narrow Concentration Window. Scienmag.
