iMAT News: A Simple Ion Swap Improves Gold Nanoparticle Synthesis

17 November 2025 by Laura Rammelt

Gold nanoparticles are widely used in sensing, medicine, catalysis, and optics because of their unique physical and chemical properties. Producing them reliably, however, can be challenging. Even well-established synthesis methods sometimes fail unexpectedly or yield inconsistent results. A new study shows that part of this unpredictability may come from an overlooked source: the choice of metal counterions in the reaction.

Researchers at Aarhus University and collaborators found that simply changing the accompanying metal ion in standard gold nanoparticle recipes can have a major effect on nanoparticle quality. In particular, swapping the common sodium or potassium salts for lithium-based salts made the particles more stable, more uniform in size, and more resistant to clumping. These findings provide a new design principle for colloidal synthesis and could help make nanoparticle production more robust, reproducible, and scalable.

Key Findings

The study provides clear design rules for choosing reagents in gold nanoparticle chemistry
Gold nanoparticles synthesized with lithium ions are more stable and well-defined than those made with sodium or potassium ions
The stabilizing effect of lithium was most visible under challenging synthesis conditions (e.g., high precursor concentration).
The type of cation used affects the growth and stability of nanoparticles across multiple reaction types, including both citrate- and borohydride-based methods.

A Subtle but Powerful Change

In colloidal gold nanoparticle synthesis, the gold salt (usually HAuCl₄) is reduced in the presence of stabilizers like citrate or surfactants. These recipes almost always use sodium salts, such as sodium citrate, because they are inexpensive and widely available. Until now, the accompanying sodium ion was assumed to play no significant role in particle formation.

The research team decided to test that assumption by systematically replacing sodium with lithium or potassium in both the citrate-based and borohydride-based synthesis routes. The results were striking. Lithium-based syntheses consistently yielded smaller (around 8 nm) and more uniformly sized spherical nanoparticles that remained stable over time. In contrast potassium-based syntheses were more prone to aggregation and shape changes. Sodium salts gave intermediate results. In other words, lithium allows for more variations while giving the same results, whereas potassium required a very narrow window to avoid failure.

Figure 1: Electron microscopy images show the gold nanoparticles synthesized with different counterions X (K⁺, Na⁺, and Li⁺) at two citrate-to-gold ratios (XCit/Au = 20 and 30). The images show how the choice of countercation affects particle size and shape. [1]

Better Control, Better Science

The lithium-based method did more than improve stability. It also allowed the researchers to push the reaction conditions further. For example, using lithium citrate, they successfully synthesized gold nanoparticles from solutions with five times the usual gold concentration, something that failed when using sodium or potassium citrate. This makes the method more efficient and potentially more scalable, which is especially important for applications requiring large quantities of nanoparticles.

Because the Li⁺-stabilized particles were more uniform and could be made in higher concentrations, they were also better suited for detailed structural analysis. The researchers used small-angle X-ray scattering and total X-ray scattering (PDF analysis) to confirm the internal structure of the nanoparticles. This kind of analysis would not have been possible with less stable particles.

How Did They Do It?

The team combined experiments with a range of analytical techniques:

UV–Vis spectroscopy tracked how quickly and reliably the characteristic gold nanoparticle color developed in each reaction.
STEM microscopy was used to measure particle size and shape directly.
Centrifugation and shelf-life tests revealed which formulations remained stable over time.
X-ray total scattering and PDF analysis reveal the atomic arrangement and size of particles, using synchrotron radiation.

Why Does It Matter?

This work offers a simple way to improve the reproducibility and scalability of gold nanoparticle production without changing the core chemistry. It also fills a gap in our understanding of colloidal synthesis by showing that counterions, often considered “spectators,” play an active role in stabilizing growing nanoparticles.

By choosing lithium salts instead of sodium or potassium, researchers can produce higher quality nanoparticles and avoid frustrating failures due to hidden recipe sensitivities. This could benefit a wide range of applications that rely on precisely engineered nanomaterials, from drug delivery and diagnostics to plasmonic sensors and catalytic devices.

AU Materials Aarhus University Materials Research Cluster

Small Ions, Big Impact on Gold Nanoparticle Formation