Advances in Synthesis and Applications of Silver Nanostructures

Background

Metallic nanoparticles have captivated researchers for decades due to their unique optical and electronic attributes. Silver nanoparticles (AgNPs) stand out as the most extensively studied, owing to their superior surface‑plasmon resonance (SPR), catalytic activity, and biocidal performance. The optical response of AgNPs is strongly dependent on morphology—size, shape, and surface chemistry—making precise control over synthesis essential for tailored applications.

Recent work has demonstrated the potential of AgNPs in photo‑electric devices, catalysis, antibacterial coatings, biosensing, and surface‑enhanced Raman scattering (SERS). While traditional chemical, photochemical, and laser‑based methods have produced AgNPs, they often suffer from high energy consumption, harsh conditions, and size heterogeneity. Consequently, there is a growing demand for simpler, greener, and more controllable synthesis routes that yield monodisperse particles with well‑defined shapes and surface functionality.

This review focuses on three core areas: (1) shape‑controlled synthesis of nanocubes, nanowires, nanorods, and nanospheres; (2) size‑specific fabrication of 1–10 nm, 10–100 nm, and larger AgNPs; and (3) emerging biosynthetic approaches that combine environmental friendliness with scalable production. The subsequent sections also highlight key applications—antibacterial activity, fluorescence, catalysis, SPR, and nanosensor development—illustrating how synthesis advances translate into real‑world performance.

Synthetic Methods

Silver nanoparticles are produced via several routes, each with distinct advantages and limitations. Here, we categorize the most impactful methods into six groups, including emerging biosynthetic techniques, and discuss how reaction parameters influence morphology.

Preparation of Different Types of AgNPs

Control over particle shape is pivotal because morphology dictates surface area, plasmonic response, and reactivity. Recent research has produced coral‑like, cage‑like, triangular, and other unconventional geometries, opening new application spaces.

Synthesis of Ag Nanocubes

Xia et al. achieved monodisperse nanocubes (average edge length 175 nm, ±13 nm) by reducing silver nitrate in ethylene glycol with poly(vinyl) pyrrolidone (PVP) as a capping agent. PVP not only stabilizes the cubes but also modulates growth kinetics, yielding smooth surfaces and slightly truncated corners suitable for drug loading or plasmonic devices. A second strategy employs cetyltrimethylammonium bromide (CTAB) in aqueous media; CTAB coordinates with Ag⁺ to form AgBr precipitates that slowly release silver ions, which are then reduced by glucose to produce ~55 nm cubes. CTAB’s surface adsorption suppresses agglomeration and ensures uniform dispersion.

Microwave‑assisted synthesis accelerates cube formation by rapidly heating the reaction mixture. Saraf et al. used gold seeds, a polyelectrolyte, and 60–120 s microwave irradiation to guide faceted growth, highlighting the polyol method’s maturity for scalable production.

Synthesis of Ag Nanowires and Nanorods

Murphy et al. produced high‑aspect‑ratio nanorods and nanowires by seed‑mediated growth in the presence of ascorbic acid, CTAB, and NaOH. The seed size (≈4 nm) and base concentration critically influence aspect ratio. Lee et al. refined this approach, showing that temperature and pH control rod uniformity, achieving monodisperse rods at 30 °C and pH 10.56.

Sun and colleagues developed a silver‑nanowire synthesis that uses ethylene glycol, AgNO₃, seeds, and PVP. Controlled temperature, reaction time, and seeding lead to 30–40 nm diameter wires up to 50 µm long. Subsequent studies identified PVP‑AgO bonds and multiple twinning as key to wire formation, enabling precise tuning of diameter and length via silver nitrate concentration or reduction rate adjustments.

Synthesis of Ag Nanospheres

Quasi‑spherical AgNPs are typically obtained by chemical reduction (NaBH₄, citrate, hydrazine, ascorbic acid, H₂). Xia’s et al. introduced a wet‑etching protocol that transforms nanocubes into uniform spheres, achieving diameters from 25 nm to 142 nm. This method offers a versatile route to monocrystalline spheres suitable for SERS and drug delivery.

Liang et al. reported a high‑temperature (260 °C) PEG/PVP system that yields 54 nm spheres with excellent monodispersity, while Zheng et al. demonstrated solid‑phase thermolysis to produce 2–4 nm luminescent spheres, underscoring the breadth of size control from nanometer to sub‑nanometer scales.

Preparation of Different Sizes of AgNPs

Size dictates many functional properties, from antibacterial potency to catalytic turnover. The following synthesis strategies enable precise size control across three regimes.

Fabrication of 1–10 nm AgNPs

Rapid reduction with NaBH₄ and citrate allows synthesis of 5–10 nm particles (Shekhar et al.), while a single‑source precursor approach (Ag trifluoroacetate + oleic acid) yields 7–10 nm spheres (Lin et al.). Yang et al. introduced an aniline/DBSA system that produces 8.9 ± 1.1 nm particles in 2 min at 90 °C with 94 % yield, maintaining colloidal stability for over a year.

Solid‑phase thermolysis produces 3 nm particles with unique luminescence (Zheng et al.), demonstrating that high‑temperature routes can access sub‑10 nm regimes while preserving structural integrity.

Fabrication of 10–100 nm AgNPs

Electron irradiation (Bogle et al.) and laser ablation (Abid et al.) generate 10–60 nm particles with high productivity and minimal by‑products. Chemical reduction at varied pH levels (Qin et al., Ajitha et al.) tunes diameters from 13 nm to 72 nm. Green syntheses using amino acids (Maddinedi et al.) or plant extracts (Mandal et al.) produce 11–33 nm spheres, highlighting eco‑friendly routes that maintain monodispersity.

Preparation of AgNPs by Biosynthetic Methods

Microbial and plant‑based biosynthesis offers environmentally benign, scalable production of AgNPs. Pseudomonas stutzeri (Klaus et al.) produced 200 nm crystals, while Geotricum sp. yielded 30–50 nm particles via extracellular reduction. Duddingtonia flagrans filtrate enabled 30–50 nm AgNPs without harsh chemicals, showcasing the potential of fungal bioprocesses for high‑yield, low‑toxicity nanoparticle fabrication.