Controlling Silver Nanoparticle Morphology via CTAB‑Capped Seeds and Aging Time

Background

Silver nanoparticles (AgNPs) are renowned for their surface plasmon resonance, quantum confinement, and macroscopic quantum tunneling, enabling applications ranging from antimicrobial coatings to cancer therapeutics, catalysis, DNA sensing, and drug delivery.1–12 The physical and chemical properties of anisotropic AgNPs—nanorods, nanowires, and nanoplates—are highly size‑ and morphology‑dependent, underscoring the need for precise synthesis control.13–15

Wet‑chemical reduction remains the most scalable method for uniform AgNP production, with key advances from polyvinyl pyrrolidone‑mediated nanowires to light‑induced triangular nanoparticles.16–20

The seed‑mediated strategy, pioneered by Murphy et al. in 2001, offers tunable size and shape but suffers from mixed spherical by‑products, low yield, and a limited seed window.21–24 CTAB’s role in promoting anisotropic growth, particularly in gold nanorods, highlights its importance.25

CTAB’s high affinity for the (110) facet and its ability to form micelles above its critical micelle concentration (CMC) make it a powerful surfactant that can suppress aggregation and direct crystal orientation.26–28

In this study, we introduce a refined seed‑mediated method that incorporates CTAB during seed synthesis. By controlling seed aging, we can produce nanospheres, nanorods, or triangular nanoplates in a single reaction system, with seed stability extending beyond 52 h. This innovation simplifies the production of shape‑specific AgNPs and enhances process efficiency.

Methods

To investigate CTAB‑capped seed aging effects, we prepared silver seed crystals with a defined CTAB concentration and aged them for varying durations before using them to grow AgNPs.

Materials

Analytical‑grade silver nitrate (AgNO₃), potassium borohydride (KBH₄), sodium hydroxide (NaOH), trisodium citrate (TSC), and ascorbic acid (V_c) were used without further purification. Cetyltrimethylammonium bromide (CTAB) was purchased from AMRESCO LLC. All solutions were prepared with double‑distilled water.

Instruments

Seed size distribution was measured by Malvern Zetasizer Nano ZS90 in the dynamic light scattering (DLS) mode, with signal detection via an avalanche photodiode. UV‑vis spectra were recorded on a U‑3900 spectrophotometer. TEM images were captured on a JEM‑1400 microscope.

Preparation of Silver Seeds

In 19.0 mL distilled water, we added 0.2 mL of 0.1 M CTAB, 0.5 mL of 0.01 M AgNO₃, and 0.5 mL of 0.01 M TSC. Rapidly, 0.6 mL of freshly prepared ice‑cold 0.01 M KBH₄ was introduced, and the mixture was gently stirred at 28 °C. The solution brightened to yellow, then yellow‑green after ~10 min, indicating silver nanocrystal formation. Seeds aged from 0 to 52 h were harvested and used directly. For comparison, seeds prepared without TSC but with CTAB followed the same protocol.

Preparation of Silver Nanoparticles

In a 50‑mL flask, 15.0 mL of 0.1 M CTAB and 0.5 mL of 0.01 M AgNO₃ were mixed. Then, 0.25 mL of seed colloid aged for the desired period was added. Finally, 1.0 mL of 0.1 M V_c and 3.0 mL of 0.1 M NaOH were introduced, and the mixture was stirred vigorously for 3 min. The solution color—deep yellow, brownish‑red, or blue‑black—reflected the seed aging time. Seeds aged without TSC produced a constant yellow solution, confirming the critical role of citrate.

Results and Discussion

Influence of Seed Aging on AgNP Morphology

Silver nanorods exhibit two plasmon bands (transverse ~400 nm and longitudinal), while triangular nanoparticles show characteristic peaks at ~350, 420, and 500 nm.29–31

Figure 5a displays UV‑vis spectra of AgNPs grown from seeds aged 0–30 min. Fresh seeds produced a single peak at ~412 nm, indicating nanospheres. Seeds aged 10 min introduced a minor 480 nm shoulder, suggesting nascent rods. At 15 min, a pronounced 345 nm shoulder emerged, while 30 min aging produced the full triangular signature, with a red‑shifted λ_max and enhanced 500 nm intensity.

TEM images (Fig. 1b, c, e) confirm these morphologies: nanospheres (~41 ± 14 nm), nanorods (~53.9% of the population), and triangular nanoplates (~56.3%). Shape‑distribution histograms (Fig. 1d, f) illustrate the progressive shift from spheres to rods to plates as seed age increases.

Contrary to earlier reports that seeds must be used within 2 h, our CTAB‑capped seeds remain stable up to 52 h, eliminating the time constraint and enabling efficient, scalable synthesis.

Role of CTAB in Seed Preparation

CTAB’s inclusion alters the selective adsorption landscape. With both CTAB and TSC, aging time tunes the competition between citrate and CTAB, directing growth along specific crystal facets. Without TSC, seeds yield only spherical nanoparticles regardless of aging, confirming citrate’s necessity for anisotropy.32–34

CTAB concentrations between the first (0.72 mM) and second (9.6 mM) CMCs form spherical micelles, providing a template for Ag atom deposition and preventing aggregation.35–37

Computational and experimental evidence indicates that Ag⁺ preferentially reacts with Br⁻ to form AgBr, which moderates reduction rates and promotes controlled nucleation.38–40

Seed Aging Dynamics

UV‑vis spectra of seeds aged 0–360 min (Fig. 5a, b) show a λ_max shift from 411 nm (fresh) to 408 nm after 20 min, with a narrowing full width at half maximum (FWHM) over time, indicating particle growth and reduced polydispersity. The emergence of absorption >600 nm after 30 min reflects increasing surface‑state charge density due to CTAB‑citrate competition.

DLS measurements (Fig. 6) reveal hydrodynamic diameters expanding from 3.8 nm (5 min) to 17.5 nm (120 min), corroborated by TEM images. Seed aggregation becomes evident after 120 min, but remains manageable within the 6‑hour window.

Collectively, these observations demonstrate that seed aging modulates facet‑specific adsorption, enabling precise morphological control of AgNPs.

Conclusions

By incorporating CTAB into seed synthesis and manipulating seed aging, we achieved shape‑selective AgNPs—nanospheres, nanorods, and triangular nanoplates—within a single, reproducible reaction system. The CTAB‑capped seeds remain functional for over 52 h, removing the short seed lifespan limitation of conventional methods. Seed aging governs the competitive adsorption between CTAB and citrate, dictating facet‑specific growth and, consequently, particle morphology. This strategy offers a scalable, versatile platform for tailoring silver and other metal nanoparticle shapes with high yield and reproducibility.

Abbreviations

AgNPs:

Silver nanoparticles

CMC:

Critical micelle concentration

CTAB:

Cetyltrimethylammonium bromide

DLS:

Dynamic light scattering

FWHM:

Full width at half maximum

PVP:

Polyvinyl pyrrolidone

TEM:

Transmission electron microscope

TSC:

Trisodium citrate

V_c

Ascorbic acid

λ_max

Maximum absorption wavelength