The appeal of ionic liquids begins with their unusual physical properties. Unlike ordinary molecular solvents, they are built entirely from ions, typically a bulky organic cation paired with an organic or inorganic anion. Because the electrostatic attraction between these large, charge-delocalized partners is relatively weak, the salt does not lock into a crystal lattice and instead flows as a liquid. The result is a solvent with negligible vapor pressure, meaning it does not evaporate or release volatile organic compounds, along with nonflammability and high thermal stability. For nanoparticle synthesis, where reactions often require elevated temperatures and where solvent loss can destabilize growing particles, these characteristics are not merely convenient; they are enabling.

Even more important is the structural tunability. The cation and anion of an ionic liquid can be modified independently, which means chemists can, in principle, design a solvent with exactly the polarity, hydrogen-bonding ability, coordinating strength and hydrophobicity a given synthesis demands. This has earned ionic liquids the nickname of designer solvents. Imidazolium-based liquids, built around a five-membered ring containing two nitrogen atoms, have emerged as particularly versatile platforms. By adjusting the length of the alkyl chains attached to the ring, or by swapping the counter-anion, researchers can systematically alter how the liquid organizes itself at the nanoscale, forming polar and nonpolar domains that can template the growth of inorganic structures.

In nanoparticle synthesis, ionic liquids frequently do double or even triple duty. They serve as the reaction medium, but their ions can also act as capping agents, adsorbing onto the surface of nascent particles and preventing them from clumping together. In some cases they even act as reducing agents, supplying the electrons needed to convert metal salt precursors into zero-valent metal atoms. The stabilization they provide arises from a combination of electrostatic interactions, where charged ions form an electrical double layer around each particle, and steric effects, where bulky organic chains physically block particles from approaching one another. This dual mechanism, the review emphasizes, gives ionic liquids an edge over single-mode stabilizers.

The mechanistic picture is grounded in classical colloid science. Nanoparticles in suspension are governed by the balance between attractive van der Waals forces and repulsive electrostatic forces, a framework formalized decades ago in DLVO theory. Capping agents shift this balance toward stability, and the review notes that the choice of capping agent directly affects dispersion quality, which in turn governs the performance of the final material. Studies of silver nanoparticles have shown that different capping agents produce markedly different degrees of aggregation, and that polymer capping layers can even gate electron transfer to and from the particle surface. Ionic liquids, with their dense interfacial ion layers, offer a particularly rich version of this surface chemistry.

Landmark experiments have illuminated how this works at the atomic level. Work on platinum nanoparticles synthesized in imidazolium ionic liquids demonstrated that the imidazolium cation coordinates directly to the metal surface, organizing into ordered layers that both stabilize the particles and dictate their growth. Similar studies on nickel and gold nanoparticles confirmed that the alkyl chains of the cation extend outward from the surface, creating a protective corona whose thickness and fluidity depend on chain length and anion identity. Because these interactions can be tuned independently, researchers can dial in the particle size and morphology they want, something that remains difficult with conventional polymer or small-molecule capping agents.

The range of materials accessible through ionic liquid media is impressive. Transition metal nanoparticles of silver, gold, platinum, nickel and copper have all been prepared with controlled sizes and narrow distributions. Metal oxides present an equally rich target: zinc oxide, magnetite, nickel oxide, ceria, titania and rare earth oxides have been synthesized in ionic liquid systems, often with morphologies, from nanosheets to hexagonal microdisks, that would be hard to achieve otherwise. Electrochemical approaches add another dimension, with nanocrystalline aluminum and other metals electrodeposited directly from ionic liquid electrolytes, where the ionic environment suppresses the coarse grain growth typical of aqueous deposition.

What elevates this work beyond materials science is its biomedical ambition. Ionic-liquid-capped nanoparticles are being explored as antibacterial and antifungal agents, exploiting both the intrinsic antimicrobial activity of certain cations, particularly long-chain imidazolium and choline-derived species, and the well-known ability of silver and zinc oxide nanoparticles to disrupt microbial membranes. Studies have shown that zinc oxide nanoparticles dispersed in ionic liquids exhibit enhanced efficacy against skin-associated bacteria, and that imidazolium-functionalized silver nanoparticles show strong activity against Escherichia coli, Staphylococcus aureus and Enterobacter cloacae. Combination strategies, such as zinc oxide nanoparticles paired with ionic liquids in dissolving microneedle patches, point toward synergistic wound-care applications.

Beyond killing microbes, ionic liquid systems are advancing biosensing and drug delivery. Ionic liquid-modified magnetic nanoparticles have been used to extract drugs from plasma and to adsorb DNA, while platinum nanoparticles dispersed in ionic liquids have formed the basis of enzyme-based biosensors capable of detecting molecules such as adrenaline. Gold nanoparticles stabilized by imidazolium amphiphiles have been loaded with anionic drugs and used as delivery vehicles, and ionic liquids themselves have a track record in transdermal and oral drug delivery, including formulations designed to protect insulin through the digestive tract. The review also highlights protein-nanoparticle interactions as a critical frontier, since the biological identity of a nanoparticle is ultimately determined by the corona of proteins that adsorbs to its surface, a process the surface chemistry of the capping layer directly controls.

The authors are careful to balance enthusiasm with caution. Ionic liquids are not automatically green; their environmental fate and toxicity vary widely with structure, and some imidazolium compounds are persistent and biologically active in undesirable ways. The review calls for the development of low-toxicity, biodegradable ionic liquid systems, alongside a deeper mechanistic understanding of how these liquids interact with nanoparticles and with living tissue. Comparative analyses suggest that, relative to conventional capping agents such as polymers and biological extracts, ionic liquids offer superior control, reproducibility and functional tunability, but realizing their clinical promise will require proving safety as rigorously as performance. If that challenge is met, the marriage of designer salts and nanoscale metals could deliver a versatile platform for bioimaging, diagnostics and therapy, turning a humble molten salt into one of nanomedicine’s most adaptable tools.

Subject of Research: Use of ionic liquids as tunable media and capping agents for metal nanoparticle synthesis and biomedical functionalization

Article Title: Ionic liquids for metal nanoparticle synthesis and bio-functionalization: trends and future perspectives

Article References: Choudhary, G., Dhariwal, J., Behera, K., & Vaya, D. (2026). Ionic liquids for metal nanoparticle synthesis and bio-functionalization: trends and future perspectives. Ionics. https://doi.org/10.1007/s11581-026-07526-8

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07526-8

Keywords: ionic liquids, nanoparticles, metal nanoparticles, metal oxides, imidazolium, capping agents, nanomedicine, antibacterial, biosensing, drug delivery, green chemistry, colloid stability