Green Synthesis of Gold and Silver Nanoparticles with Platycodon Saponins from Platycodi Radix

Background

With growing environmental concerns, green chemistry has become pivotal across research domains. Natural products, particularly plant extracts, are increasingly employed to replace hazardous reductants in metallic nanoparticle (MNP) synthesis. Such approaches reduce chemical waste, enhance biocompatibility, and are scalable for industrial applications. Gold and silver nanoparticles have garnered attention for catalysis, drug delivery, sensing, and antimicrobial use due to their unique optical and surface properties. Traditional reduction methods often rely on toxic agents like sodium borohydride; in contrast, plant‑derived saponins provide a sustainable alternative.

Platycodi Radix, a traditional Chinese medicine, contains triterpenoidal saponins with diverse pharmacological activities (anti‑inflammatory, anti‑oxidant, anti‑tumor, etc.). Among these, platycodin D (PD) is a major marker compound. Through enzymatic conversion of platycoside E and platycodin D₃, a PD‑enriched fraction was isolated, yielding a natural reducing agent for AuNP and AgNP synthesis.

Materials and Instruments

PD‑enriched fraction prepared as previously reported. Metal salts (HAuCl₄·3H₂O, AgNO₃) and reagents (NaBH₄, 4‑NP) sourced from Sigma‑Aldrich. UV‑vis spectra recorded on Shimadzu UV‑2600. HR‑TEM (JEM‑3010, 300 kV) on carbon‑coated copper grids; AFM (Bruker Dimension Icon, tapping mode) on mica; XRD (Bruker D8 Discover, Cu Kα, 20°–90° 2θ); FT‑IR (Nicolet 6700, KBr pellet). Samples were freeze‑dried (FD5505) before analysis.

Green Synthesis of PD‑AuNPs and PD‑AgNPs

For AuNPs: 0.05 % PD‑enriched fraction + 0.2 mM HAuCl₄·3H₂O, 1 mL, 5 min at ambient temperature. For AgNPs: 0.01 % PD‑enriched fraction + 0.8 mM AgNO₃, 1 mL, 3 h at 80 °C followed by 21 h at room temperature. UV–vis spectra (300–700 nm) confirmed complete reduction.

AFM Image Enhancement via Curvature‑Dependent Evolution

We applied a principal curvature flow (κ₂) to raw AFM height data, improving boundary delineation and enabling precise size measurements. Enhanced images were generated using 500 evolution steps with Δt = 10⁻⁷.

Catalytic Activity of PD‑AuNPs

PD‑AuNPs (0.04 mM) catalyzed the reduction of 4‑NP (0.113 mM) to 4‑AP in the presence of excess NaBH₄ (4.13 mM). The reaction proceeded to completion within 720 s, with a pseudo‑first‑order rate constant of 3.4 × 10⁻³ s⁻¹, as evidenced by UV–vis monitoring of the 400 nm absorbance decay and emergence of a 300 nm peak.

Results and Discussion

Colorimetric Confirmation of Nanoparticle Formation

PD‑AuNPs exhibited a wine‑purple hue with SPR at 536 nm; PD‑AgNPs appeared yellow with SPR at 427 nm, confirming successful reduction.

High‑Resolution TEM

AuNPs: spherical, 14.94 ± 2.14 nm; minor triangular/polygonal forms observed. AgNPs: spherical, 18.40 ± 3.20 nm. Gaussian size distributions centered at ~15 nm (Au) and ~18 nm (Ag). PD‑enriched fraction acted as a capping agent, preventing aggregation.

AFM Analysis

Both AuNPs and AgNPs appeared spherical in 2D height and 3D amplitude error images. Section analyses measured heights of 10.44–10.47 nm (Au) and 7.46–10.35 nm (Ag). Curvature‑dependent enhancement yielded diameters of 19.14 nm (Au) and 29.93 nm (Ag), slightly larger due to substrate effects.

Crystallinity (HR‑XRD)

Face‑centered cubic structures confirmed. AuNPs: (111) at 38.2°, (200) at 44.4°. AgNPs: (111) 38.2°, (200) 44.4°, (220) 65.2°, (311) 78.0°. Scherrer analysis estimated sizes of 11.05 nm (Au) and 12.54 nm (Ag).

Functional Groups (FT‑IR)

Shifts in –OH (3421→3426 cm⁻¹), aromatic C=C (1654→1633 cm⁻¹), C–O/C–H (1035→1043/1058 cm⁻¹) indicate active participation of these groups in reduction.

Catalytic Performance

PD‑AuNPs achieved a rate constant of 3.4 × 10⁻³ s⁻¹ for 4‑NP reduction, comparable to or exceeding many plant‑derived AuNP catalysts. Future work will explore purification effects and in‑vitro/in‑vivo synergistic bioactivities.

Conclusions

PD‑enriched Platycodi Radix provides an efficient, eco‑friendly route to spherical AuNPs and AgNPs with face‑centered cubic structures. The –OH, aromatic, C–O, and C–H groups act as natural reductants and stabilizers. PD‑AuNPs exhibit promising catalytic activity for 4‑NP reduction, illustrating the potential of plant saponins in multifunctional nanomaterials. Further studies will assess combined therapeutic effects of these nanoparticles in biomedical contexts.