How gold nanoparticles defy conventional physics by exhibiting magnetic properties at room temperature
Imagine holding a piece of gold jewelry to a magnet and watching it stubbornly refuse to interact—this is our everyday experience with the precious metal. Gold, in its bulk form, is decidedly diamagnetic, meaning it weakly repels magnetic fields rather than being attracted to them. This fundamental property has been documented in physics textbooks for centuries.
In its natural form, gold repels magnetic fields with a magnetic susceptibility (χd) of -1.42 × 10⁻⁷ emu g⁻¹ Oe⁻¹ 1 .
At the nanoscale, gold exhibits ferromagnetic-like behavior that persists at room temperature.
This surprising phenomenon represents more than just a laboratory curiosity; it challenges our fundamental understanding of materials and demonstrates how quantum effects can dominate at the nanoscale.
The discovery of magnetic gold nanoparticles opens up exciting possibilities across medicine, electronics, and information technology, potentially leading to more effective cancer treatments, novel computing architectures, and advanced sensors. As we explore this mysterious quantum behavior, we find ourselves at the frontier of materials science, where the familiar rules of physics give way to strange new possibilities.
To appreciate the strangeness of magnetic gold nanoparticles, we must first understand why bulk gold is non-magnetic in the first place. The magnetic properties of any element stem from the arrangement and behavior of its electrons. In bulk gold, the 5d electron bands are fully occupied, with a balanced number of spin-up and spin-down electrons. This balanced configuration results in no net magnetic moment, making gold diamagnetic.
The diamagnetism of gold is so well-established that it serves as a reference standard in magnetic measurements. When placed in a magnetic field, bulk gold will develop a weak magnetization that opposes the applied field—the hallmark of diamagnetic behavior. For centuries, this was considered an immutable property of the element, until scientists began exploring the nanoscale world where different rules apply.
At dimensions typically below 2 nanometers (for comparison, a DNA helix is about 2 nanometers wide), gold undergoes a dramatic transformation. The once continuous electronic bands break down into discrete, atomic-like energy levels, creating what scientists call a "HOMO-LUMO gap" rather than the continuous conduction band of larger nanoparticles 4 .
| Property | Bulk Gold | Gold Nanoparticles (<2 nm) |
|---|---|---|
| Electronic Structure | Continuous bands | Discrete energy levels |
| Primary Magnetic Behavior | Diamagnetic | Ferromagnetic-like |
| Surface Plasmon Resonance | Strong at ~520 nm | Absent or weak |
| Influence of Capping Molecules | Minimal | Significant |
To understand how researchers confirm and study this unexpected magnetism, let's examine a typical experiment that demonstrates the effect.
Using a modification of the Brust-Schiffrin method 1 , researchers start with tetrachloroauric acid (HAuCl₄) as the gold precursor.
This compound is transferred to an organic solvent using tetraoctylammonium bromide (TOAB) as a phase-transfer catalyst 3 .
The critical step: adding a thiol-based ligand, typically dodecanethiol or 2-phenylethanethiol, which serves as a capping agent.
Sodium borohydride (NaBH₄) is added in large excess, rapidly reducing the gold to neutral gold atoms that form stable clusters 3 .
This synthesis is often performed in an oxygen-free argon environment inside a glove box to prevent oxidation 3 .
The most direct measurements come from SQUID (Superconducting Quantum Interference Device) magnetometry, which can detect extremely weak magnetic signals 2 .
Results show ferromagnetic-like behavior at room temperature, complete with hysteresis loops featuring both remnant magnetization (Mᵣ) and coercive field (H꜀) 2 .
| Technique | What It Reveals |
|---|---|
| UV-visible Spectroscopy | Confirms quantum nature with multiple absorption peaks |
| XPS | Reveals modified electronic structure via Au 4f₇/₂ peak shifts |
| MALDI-TOF MS | Provides precise size/composition (e.g., Au₂₅(SR)₁₈) |
The study of magnetic gold nanoparticles requires specialized materials and reagents, each playing a critical role in the synthesis and stabilization of these quantum-sized structures.
Gold precursor - Source of gold atoms for nanoparticle formation
Capping ligands - Bind to gold surface, modifying electronic properties and preventing aggregation
Phase-transfer catalyst - Moves gold precursors from aqueous to organic phases
Reducing agent - Converts ionic gold to neutral atoms, forming nanoparticles
Reaction medium - Prevents oxidation during synthesis, ensuring well-defined nanoparticles
Oxygen-free environment - Essential for preventing oxidation during synthesis
The experimental evidence for magnetism in gold nanoparticles is compelling, but what explains this counterintuitive phenomenon? Several theoretical models have emerged to account for these observations.
When thiol molecules bind to gold surfaces, they form a strong Auδ⁺–Sδ⁻ bond that involves significant charge transfer from gold to sulfur 1 .
This bonding modifies the electronic structure of surface gold atoms, creating unoccupied states in the 5d bands—a phenomenon confirmed by XANES (X-ray absorption near-edge structure) spectroscopy 1 .
As nanoparticle size decreases, the proportion of surface atoms increases dramatically, making this surface-induced modification increasingly significant.
One intriguing hypothesis suggests that the magnetism might originate from self-sustained persistent currents within the nanoparticles 2 .
According to this model, the magnetic behavior could be a room-temperature quantum effect where electrons circulate continuously within the nanoscale structures, generating magnetic moments without resistance.
This explanation is particularly appealing because it would account for several puzzling observations, including the weak temperature dependence of the magnetism and the wide variability in magnetic strength between different samples 2 .
Below a critical size (around 2-3 nm), electronic structure transitions from band-like to molecular-like 4
Gold's strong spin-orbit interaction enhances these effects compared to other metals 1
Non-fcc atomic packing structures often found in nanoclusters versus the fcc structure of bulk gold 4
The discovery of magnetism in gold nanoparticles represents a fascinating example of how nanoscale materials can defy our macroscopic expectations.
What was once considered a definitively non-magnetic element has been transformed into a potentially magnetic material through the power of nanotechnology and quantum effects.
Their biocompatibility makes them promising for targeted drug delivery and hyperthermia cancer treatment, where they could be guided by magnetic fields and activated to destroy tumor cells.
In information technology, they could contribute to high-density data storage or spintronic devices that use electron spin rather than charge to process information 2 .
Perhaps most importantly, the study of magnetic gold nanoparticles reminds us that even the most familiar materials can surprise us when viewed from a new perspective—in this case, the quantum perspective of the nanoscale.
As research continues to unravel the mysteries of this phenomenon, we're likely to discover even more remarkable properties hidden within the elements we thought we knew so well.