A Simple Amino Acid Reshapes Iron Nanoparticles Without Touching Their Core

Iron is one of the most abundant and inexpensive metals on Earth, and when it is coaxed into particles just tens of nanometers across, it becomes one of the most reactive materials chemists can put into a beaker. Zero-valent iron nanoparticles, or NPs-Fe, carry a metallic iron core with powerful reducing capacity, a vast surface area, and magnetic behavior that lets researchers pull them out of water with an ordinary magnet. Those traits have made them stars of environmental remediation, where they break down or bind contaminants through reduction, adsorption, oxidation, surface complexation, and coprecipitation. Yet the same reactivity that makes them useful also makes them fragile: the metallic surface oxidizes almost immediately in water and air, and magnetic attraction plus van der Waals forces drive the particles into clumps that lose their dispersibility and, with it, much of their reactivity.

A research team at the Institute of Physics of the Benemérita Universidad Autónoma de Puebla in Mexico, led by Azury Nava Guzmán together with Enrique Sánchez Mora, Anabel Romero López, and Mary Carmen Amador Meléndez, has now systematically mapped how a remarkably humble molecule can tame these unruly particles. Their chosen stabilizer is glycine, the simplest of the amino acids that build proteins. Writing in the Journal of Nanoparticle Research, the team shows that the amount of glycine present during synthesis acts as a dial that tunes crystallite size, particle diameter, surface charge, colloidal behavior, and even the thermal response of zero-valent iron nanoparticles, all without disturbing the metallic iron core itself. The work, published as open access on 13 October 2026, was funded by Mexico’s Secretariat of Science, Humanities, Technology and Innovation.

The synthesis itself is a classic of the field. The researchers dissolved ferric chloride in water under a flowing nitrogen atmosphere at 20 degrees Celsius, then injected sodium borohydride, a strong reducing agent, at a controlled rate. The reaction reduces iron ions to metallic iron while releasing hydrogen gas, and it is exothermic enough that the mixture warms to about 42 degrees Celsius at a pH of roughly 9. The resulting black powder was washed an extraordinary 35 times with acetone, using magnetic separation and sonication between washes, to strip away boron byproducts and residual salts. To functionalize the particles, the team simply added glycine to the iron salt solution before reduction, at three doses: 8, 16, and 32 millimoles, producing samples labeled NPs-FeG1, NPs-FeG2, and NPs-FeG3.

X-ray diffraction delivered the first surprise. Every sample, functionalized or not, showed the sharp reflections of body-centered cubic metallic iron, with characteristic peaks near 45.1, 65.3, and 82.8 degrees in two-theta. Glycine left the crystal structure untouched. But the width of the main (110) peak told a subtler story: it broadened steadily as glycine concentration rose, from a full width at half maximum of 0.94 degrees for bare nanoparticles to 1.62 degrees for the highest glycine dose. Applying Scherrer’s equation, the team calculated that crystallite size shrank progressively from 9.55 nanometers in the unfunctionalized particles to 7.81, 6.65, and finally 5.54 nanometers across the three functionalized samples. Glycine, in other words, behaves like a surfactant at the growing crystal surface, capping the domains of metallic iron before they can enlarge.

Scanning electron microscopy then revealed an apparent paradox. While the crystallites got smaller, the particles got bigger. The average diameter of unfunctionalized nanoparticles measured 58.95 plus or minus 12.80 nanometers, based on 1000 particles analyzed per sample, but the glycine-treated particles grew to 75.51, 83.36, and 83.79 nanometers for the three doses, with the two higher doses essentially indistinguishable. The resolution of the paradox lies in scale: each particle seen under the microscope is a polycrystalline aggregate containing many of the smaller crystalline domains measured by diffraction. Glycine-mediated surface interactions appear to promote the assembly of these larger particles during synthesis, even as they suppress growth within each crystalline domain. All samples, meanwhile, assembled themselves into striking chain-like structures, a hallmark of magnetic dipole-dipole interactions pulling quasi-spherical iron particles nose-to-tail.

The chains are not purely magnetic accidents. The researchers propose a layered interaction model in which surface chemistry plays a decisive role. Metallic iron exposed to water and oxygen during synthesis develops a thin skin of hydroxides and oxyhydroxides such as goethite, lepidocrocite, and ferrihydrite. Hydroxyl groups on this skin can hydrogen-bond with one another on bare particles. When glycine is present, its zwitterionic form, carrying both a positively charged ammonium group and a negatively charged carboxylate, can anchor through its amino end to the hydroxylated surface, leaving the carboxylate groups exposed to the surrounding water. Carboxylates on neighboring particles can then pair up, stitching dimers and chains together. Increasing the glycine dose raises the density of these polar groups on the surface, favoring exactly the hydrogen-bonded agglomerates the microscope revealed.

Infrared spectroscopy backed this mechanism with molecular-level detail. The functionalized particles showed weak O-H stretching bands between 3727 and 3696 inverse centimeters, indicating that glycine modifies the environment of surface hydroxyls. Bands between 2670 and 2678 inverse centimeters, tied to the ammonium group of zwitterionic glycine interacting with surface hydroxyls, appeared only in the functionalized samples. The carboxylate-asymmetric-stretch and ammonium-deformation bands near 1615 to 1565 inverse centimeters shifted progressively to lower frequencies with increasing glycine dose, and a mixed CH2 bending and NH2 torsion band near 810 inverse centimeters moved as well, both signatures of a changing vibrational environment for adsorbed glycine. Bands between 664 and 602 inverse centimeters confirmed Fe-O vibrations from the surface oxide and oxyhydroxide species, which XRD suggested must be present only in small or poorly crystalline amounts, since no bulk oxide phases were detected.

Perhaps the most practically consequential result came from zeta potential measurements, a proxy for colloidal stability. Bare nanoparticles carried a zeta potential of minus 22.95 millivolts, indicating only limited stability in water. The lowest glycine dose pushed the value to minus 32.83 millivolts, the most negative of all samples, reflecting exposed carboxylate groups and stronger electrostatic repulsion between particles. But the story reversed at higher doses: the 16 and 32 millimole samples dropped to just minus 10.07 and minus 9.68 millivolts. The team attributes this to charge shielding by dense glycine coverage, which lets hydrogen bonding and dipole-dipole forces between amino and carboxyl groups outweigh electrostatic repulsion, promoting the agglomeration seen by electron microscopy. The lesson is that more stabilizer is not always better; for zero-valent iron, the sweet spot for colloidal stability sits at the lowest glycine concentration tested.

Thermal analysis added a final dimension. Thermogravimetric curves showed multistage behavior for all samples: water desorption below 150 degrees Celsius, dehydroxylation of oxyhydroxides and gradual oxidation of metallic iron between 200 and 600 degrees, and a pronounced mass gain above 600 degrees as iron takes up oxygen. The functionalized samples added a distinct mass loss between 150 and 350 degrees Celsius corresponding to glycine decomposition, and the oxidation-related mass changes were less pronounced than in bare particles, suggesting glycine partially delays oxidation. The team also tracked the Curie transition near 770 degrees Celsius, where the particles lose their ferromagnetism and the apparent mass recorded by the balance jumps, an instrumental artifact the authors carefully disentangled from real oxidation. Raman spectra of the heat-treated residues revealed hematite and beta-FeOOH in all samples, with the functionalized residues additionally showing lepidocrocite signatures, confirming that the organic layer reshapes the thermal degradation pathway of the particles.

What makes this study notable is its systematic dose-response design applied to metallic iron rather than the iron oxides that dominate the glycine literature. Most prior work on glycine and iron nanomaterials focused on magnetite and related oxides, whose chemistry cannot simply be extrapolated to a reactive metallic core. By demonstrating that glycine concentration independently tunes crystallite growth, particle aggregation, surface charge, and thermal stability while preserving the body-centered cubic iron phase, the Mexican team has provided a practical recipe book for engineers designing zero-valent iron materials. The implications reach into groundwater remediation, where better-dispersed particles could intercept contaminants more efficiently; into catalysis, where surface chemistry governs electron transfer; and toward emerging biomedical and biosensing applications, where amino-acid coatings offer a biocompatible interface. The next step, the authors note, will be surface-sensitive techniques such as X-ray photoelectron or Mössbauer spectroscopy to pin down the exact oxidation states at the glycine-iron interface.

Subject of Research: Effect of glycine concentration on the physicochemical properties of glycine-functionalized zero-valent iron nanoparticles

Article Title: Glycine-functionalized zero-valent iron nanoparticles: effect of glycine concentration on physicochemical properties

Article References: Glycine-functionalized zero-valent iron nanoparticles: effect of glycine concentration on physicochemical properties. (n.d.). https://doi.org/10.1007/s11051-026-06787-8

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06787-8

Keywords: zero-valent iron, nanoparticles, glycine, surface functionalization, X-ray diffraction, zeta potential, colloidal stability, FTIR spectroscopy, thermogravimetric analysis, Raman spectroscopy, environmental remediation, nanoparticle synthesis