# Kinetically Controlled Morphologies of Magnetic Nanoparticles through Ligand and Precursor Chemistry

## TL;DR

Researchers developed a method to control the shape and magnetic properties of nanoparticles using specific chemical ligands, which could improve biosensing applications.

## Contribution

A capping-ligand-directed approach to tune nanoparticle morphology and magnetic properties through sodium oleate and oleic acid ratios.

## Key findings

- Adding sodium oleate promotes tetrahedral nanoparticle formation with facet-selective passivation.
- Tetrahedral nanoparticles show the highest room temperature saturation magnetization.
- Ligand composition affects ion doping and crystal structure modification.

## Abstract

Kinetically controlled morphologies of colloidal magnetic
nanoparticles
possess unique magnetic properties, making them highly promising for
applications in magnetogenetics as magnetic torque probes. Yet, their
size-controlled chemical synthesis is in its nascent state. Here,
we present a capping-ligand-directed approach to tune the morphology
and magnetic properties of Co
x
Zn
y
Fe3‑(x+y)O4 nanoparticles by adding sodium oleate as a cocapping
ligand to oleic acid during synthesis, resulting in the formation
of monodisperse tetrahedral nanoparticles. Increasing the molar ratio
of sodium oleate to oleic acid promotes facet-selective passivation
along {111} facets, leading to progressive truncation of tetrahedra
and yielding morphologies ranging from truncated tetrahedra to extremely
truncated rod-like shapes. Our electron microscopy studies show that
the synthesis of tetrahedron-shaped nanoparticles does not require
a symmetry-breaking transformation from octahedra, as the initial
crystallite formed is tetrahedra. When sodium oleate is removed from
the synthesis, thermodynamically driven monodisperse octahedral nanoparticles
are formed. We find that ligand composition also influences the doping
of ions into the crystal structure, with higher sodium oleate concentrations
reducing Zn2+ incorporation due to modified metal–ligand
coordination. Tetrahedral nanoparticles synthesized under optimal
conditions exhibit the highest room temperature saturation magnetization
among other morphologies, highlighting their potential for magnetic-nanoparticle-based
biosensing applications. Our study underscores that not only morphology
but also magnetic characteristics of nanoparticles can be tuned by
a ligand-guided chemistry.