One Polymer, One Step: Gold Nanoparticles Tailored by the Solvent They Grow In

Gold nanoparticles are prized for the way they scatter and absorb light, a property that underpins applications from catalysis to sensing. Yet most reliable recipes for making them demand extra reducing agents, capping molecules, or multi-step phase-transfer chemistry, especially when the goal is a dispersion that survives in organic solvents rather than water. A new study in Gold Bulletin reports a strikingly simpler route: a single polymer, poly(N,N-dimethylaminoethyl methacrylate), or PDMAEMA, does the work of both reducing the gold precursor to metallic gold and stabilizing the resulting particles, in water and in a range of protic and aprotic organic solvents, all at room temperature.

The research team, led by Maryam Raeesi and Ali Reza Mahdavian with colleagues at their institution, built on their earlier demonstration that PDMAEMA could produce water-dispersible gold nanoparticles with triangular, star-like, and dendritic morphologies. The tertiary amine groups along the polymer backbone are the key chemical actors: they reduce Au3+ to Au(0) and then anchor to the nanoparticle surface through metal-amine complexation. Because PDMAEMA also dissolves in solvents spanning a wide range of dielectric constants and dipole moments, the researchers asked whether the same one-pot chemistry could yield stable, well-defined nanoparticles directly in organic media, with no additional reagents.

The experimental design was deliberately systematic. PDMAEMA was first synthesized by solution radical polymerization of DMAEMA monomer, initiated by AIBN in 1,4-dioxane under nitrogen at 80 degrees Celsius, yielding polymer with a molecular weight of about 29,000 grams per mole. Gold nanoparticles were then grown by injecting a dilute stock solution of hydrogen tetrachloroaurate into polymer solutions held at a fixed nitrogen-to-gold molar ratio of 100. Nine continuous phases were tested: deionized water, ethanol, isopropyl alcohol, dichloromethane, dichloroethane, tetrahydrofuran, chloroform, dimethyl formamide, and dimethyl sulfoxide. Each synthesis was run in triplicate, and reactions were allowed to proceed until the localized surface plasmon resonance peak, the optical signature of nanoparticle formation, stopped growing in intensity.

Optical monitoring told a story of dramatically different reaction speeds. In water, nucleation and growth began almost immediately, and the solution turned red. Ethanol and isopropyl alcohol lagged slightly behind. In dichloroethane, dichloromethane, tetrahydrofuran, and chloroform, no sign of nanoparticle formation appeared until 8, 16, 18, and 21 hours respectively, pointing to a progressive weakening of PDMAEMA’s reducing ability as the dielectric constant of the medium dropped. In dimethyl formamide and dimethyl sulfoxide, solvents with moderate dielectric constants but high dipole moments, almost no color change occurred even after three days, and ultraviolet-visible spectra confirmed only poor reduction of the gold precursor.

To quantify these differences, the team extracted kinetic data from the time-resolved spectra and fitted the curves to the Finke-Watzky two-step model, which describes slow continuous nucleation followed by fast autocatalytic surface growth. The fitted rate constants revealed a clear trend in PDMAEMA’s reducing power across the solvents: tetrahydrofuran, then dichloromethane, then dichloroethane, then isopropyl alcohol, then ethanol, then water, from slowest to fastest. Synthesis in protic solvents, water, ethanol, and isopropyl alcohol, was much faster than in the aprotic pair of dichloromethane and dichloroethane and tetrahydrofuran, and both nucleation and growth rate constants fell as the dielectric constant decreased. Notably, no matching trend emerged with dipole moment, which pointed the researchers toward a second, structural mechanism.

That mechanism involves the physical shape of the polymer chains in solution. Using Ubbelohde viscometry, the team measured the intrinsic viscosity of PDMAEMA in each solvent and found it declined with the solvent’s dipole moment. In high-dipole-moment media, hydrogen bonding and dipole-dipole interactions between solvent molecules and the polymer’s amine and carbonyl groups keep the chains extended, leaving tertiary amines exposed and accessible to the aurate ions. In low-dipole-moment solvents, the polymer collapses into compact coils that bury those reactive groups inside, slowing access of the gold precursor and retarding the redox chemistry. The researchers also note that in the highly polar dimethyl formamide and dimethyl sulfoxide, solvent molecules effectively cage the amine groups and stabilize the aurate ions, further weakening the interaction needed for reduction.

Transmission electron microscopy then connected the kinetics to particle architecture. Nanoparticles grown in water were nearly spherical and smallest, averaging 13 nanometers, consistent with the highest nucleation rate generating many growth sites. Ethanol and isopropyl alcohol dispersions yielded larger spheres of 14 and 18 nanometers, and dichloroethane and tetrahydrofuran gave spherical particles of 23 and 24 nanometers. The striking exception was dichloromethane, whose low dielectric constant and lowest dipole moment produced well-defined projected hexagonal particles with no spherical population at all, averaging 61 nanometers. Field-emission scanning electron microscopy revealed these particles to be highly faceted icosahedra with sharp, symmetrical vertices, and energy-dispersive X-ray analysis confirmed the expected elemental composition of polymer-coated gold.

X-ray diffraction supported the morphological assignment. The diffractogram of the dichloromethane-grown particles showed the characteristic face-centered cubic reflections of gold, with the intensity ratio of the (111) to (200) peaks reaching 4.1, well above the 2.53 measured for the spherical water-grown particles. That elevated ratio indicates preferential growth along the (200) direction, a hallmark of icosahedral nanocrystals. The authors attribute this shape deviation to kinetically controlled growth: as the solvent’s polarity drops, PDMAEMA can no longer uniformly cover all crystallographic facets of the growing particle, so site-specific adsorption directs regioselective deposition of gold atoms onto particular faces. A weaker reducing agent, they add, similarly pushes the system away from thermodynamically favored shapes.

The optical consequences were equally systematic. The wavelength of maximum absorbance shifted to longer values as both dielectric constant and dipole moment decreased, from 522 nanometers for the water-grown spheres to 550 nanometers for the dichloromethane icosahedra, reflecting the variable sizes and shapes of the particles. Chloroform behaved anomalously: its spectra showed broadened plasmon peaks with a new shoulder near 620 nanometers, and electron microscopy revealed that the roughly 35-nanometer polyhedral particles had self-assembled into large aggregates, a behavior the authors link to the low dielectric constant and compact polymer conformation in that solvent. They note that such electromagnetic coupling between adjacent particles in aggregates is precisely what enhances surface-enhanced Raman scattering, suggesting a potential route to SERS-active substrates.

Colloidal stability over storage emerged as another solvent-dependent property. Water, ethanol, and isopropyl alcohol dispersions retained their red color and unchanged plasmon peaks after three months, while dichloroethane and tetrahydrofuran samples remained stable for a month, with a slight intensity increase attributed to solvent evaporation. Dichloromethane dispersions precipitated after a month, though gentle bath sonication restored them to their original state. Chloroform fared worst, with complete irreversible precipitation and loss of the plasmon peak within 15 days. The authors attribute these failures to the same factors that shaped the particles: limited polymer solubility and stabilizing capacity in low-dipole-moment solvents, combined with the greater aggregation tendency of larger, denser particles. Overall, the study demonstrates that simply choosing the synthesis medium, at room temperature, in a single step, and without any extra reducing agent, stabilizer, or phase-transfer agent, offers a tunable handle on the size, shape, dispersibility, and plasmonic response of gold nanoparticles, with potential extensions to other metals such as silver.

Subject of Research: Single-step synthesis of water- and organo-dispersible gold nanoparticles using PDMAEMA, with solvent properties controlling kinetics, morphology and plasmonic features

Article Title: Kinetic and morphological studies on water and organo-dispersible gold nanoparticle formation and the influence of synthesis media on dispersibility and plasmonic features

Article References: Raeesi, M., Hamrang, V., Alinejad, Z., Salehi-Mobarakeh, H., & Mahdavian, A. R. (2025). Kinetic and morphological studies on water and organo-dispersible gold nanoparticle formation and the influence of synthesis media on dispersibility and plasmonic features. Gold Bulletin, 58(1), Article 3. https://doi.org/10.1007/s13404-025-00356-y

Image Credits: AI Generated

DOI: 10.1007/s13404-025-00356-y

Keywords: gold nanoparticles, PDMAEMA, nanoparticle synthesis, solvent effects, plasmon resonance, Finke-Watzky kinetics, colloidal stability, icosahedral nanoparticles, TEM analysis, UV-Vis spectroscopy, organo-dispersible nanomaterials, Kinetic