Gold nanocrystals are particles of gold so small they are measured in nanometers. At this minuscule level, the familiar properties of the metal transform entirely. While the gold in jewelry is stable and chemically non-reactive, as a nanocrystal it becomes a different material with new behaviors. The size and shape of these particles dictate their function, opening doors to uses that would be impossible with bulk gold.
Unique Properties of Gold at the Nanoscale
A striking characteristic of gold nanocrystals is their interaction with light, which gives them vibrant colors. This phenomenon is known as localized surface plasmon resonance (LSPR). When light hits a gold nanoparticle, its energy causes the free electrons on the metal’s surface to oscillate together in a collective wave. This resonance results in the strong absorption and scattering of specific wavelengths of light, producing colors like deep red or blue depending on the particle’s dimensions.
The optical response of the nanocrystals is highly tunable by carefully controlling the size and shape of the particles. For example, small, spherical gold nanocrystals around 20 nanometers in diameter suspended in water absorb green light, making the solution appear ruby red. This sensitivity of the LSPR to the particle’s immediate surroundings also makes them effective sensors. A change in the refractive index near the particle surface, such as when a protein binds to it, causes a detectable shift in the absorbed light.
Beyond their optical qualities, gold nanocrystals possess a high surface-area-to-volume ratio. As a particle shrinks, a greater proportion of its atoms are located on the surface, providing an abundance of active sites where chemical reactions can occur. This increased surface area makes them effective catalysts.
While bulk gold is catalytically inert, gold nanocrystals smaller than 10 nanometers are highly reactive and can speed up reactions like oxidation and reduction. This property allows industrial processes to run more efficiently and under milder conditions, such as lower temperatures and pressures. The catalytic activity increases as the particle size decreases, linking their nanoscale dimensions to their chemical functionality.
Methods of Creation
Scientists employ several techniques to produce gold nanocrystals, often using “bottom-up” methods where particles are built from individual atoms. One of the most common approaches is the Turkevich method, a form of chemical reduction in a liquid solution. This process synthesizes spherical gold nanoparticles with a relatively uniform size distribution, in the range of 10 to 30 nanometers.
The Turkevich method begins with a heated solution of a gold salt, such as chloroauric acid (HAuCl₄). To this solution, a reducing agent like sodium citrate is added. The citrate serves two functions: it reduces the positively charged gold ions (Au³⁺) into neutral gold atoms (Au⁰) and then acts as a stabilizing agent, coating the new particles to prevent them from clumping together.
The process unfolds through seed-mediated growth. Immediately after the citrate is added, tiny “seed” particles form. Subsequent gold atoms deposit onto the surface of these existing seeds rather than forming new ones. This controlled growth ensures the final particles are very close in size, which is important for consistent properties. The final size can be tuned by adjusting the ratio of citrate to gold salt.
Applications in Biomedicine
The properties of gold nanocrystals make them useful tools in medicine, particularly in diagnostics. They are a component in many lateral flow assays, the technology behind rapid diagnostic tools like the home pregnancy test. In these tests, gold nanoparticles are attached to antibodies that are specific to a target molecule, such as the hormone human chorionic gonadotropin (hCG) in pregnancy.
When a sample containing the target molecule is applied, it flows along the strip and binds to the antibody-coated gold nanocrystals. This complex then continues to migrate until it is captured by a line of immobilized antibodies. This concentrates the colored particles, and the intense color from the nanoparticles’ LSPR allows for a clear, visible signal on the test strip.
Gold nanocrystals also serve as vehicles for targeted drug delivery. Therapeutic molecules can be attached to the surface of a nanoparticle, which acts as a carrier to transport the drug through the bloodstream. The surface of the nanocrystal can be functionalized with specific targeting agents, such as antibodies, that recognize and bind only to receptors on diseased cells, like cancer cells. This approach delivers medication directly where it is needed, increasing its efficacy while minimizing side effects.
Another medical application is in photothermal therapy (PTT), a non-invasive treatment for cancer. In this technique, gold nanocrystals, often engineered into shapes like rods or shells, are introduced into the body and accumulate in tumors. A laser emitting near-infrared light, which can penetrate biological tissues, is then aimed at the tumor. The nanocrystals absorb this light energy intensely and convert it into localized heat, destroying cancerous cells without significant damage to surrounding tissue.
Uses in Technology and Industry
Outside of medicine, the catalytic abilities of gold nanocrystals are leveraged to improve a range of industrial chemical processes. For example, they can facilitate the conversion of alcohols into more valuable chemical compounds under much milder conditions than traditional methods. This makes manufacturing processes more energy-efficient and environmentally friendly.
These nanocatalysts are often stabilized on a support material, such as silica or various metal oxides, which helps maintain their small size and prevents them from clumping. Researchers are also developing bimetallic nanoparticles, combining gold with other metals like platinum or palladium, to create catalysts with enhanced performance and stability for specific reactions.
In electronics, gold nanocrystals are enabling the creation of new types of highly sensitive sensors. Thin films composed of gold nanoparticles can function as chemiresistors, devices that detect the presence of specific chemical vapors. When molecules of a target substance adsorb onto the film, they change the distance between the nanoparticles and alter the film’s overall electrical resistance.
The conductive properties of gold at the nanoscale are also being explored for use in printable electronics and microchip fabrication. Suspensions of gold nanocrystals can be formulated into conductive inks, allowing circuits to be printed onto flexible or unconventional surfaces. These particles provide a pathway for creating the microscopic connections needed to link elements on a chip.
Safety and Environmental Profile
While bulk gold is chemically inert and safe, the increased reactivity of gold at the nanoscale necessitates a careful evaluation of its safety. The biocompatibility and potential toxicity of gold nanocrystals are complex issues, with effects depending on the particles’ physical characteristics. Factors such as size, shape, surface coating, and electrical charge all influence how these particles interact with biological systems.
Studies have shown that these characteristics determine how the particles are distributed and cleared from the body. For instance, smaller nanocrystals may be more readily excreted through the kidneys, whereas larger ones might accumulate in organs like the liver and spleen. The molecules used to coat the nanoparticles are important, as coatings like polyethylene glycol (PEG) can render the particles more biocompatible.
The environmental impact of gold nanocrystals arises not from the particles themselves but from their synthesis, which often requires significant energy and chemical inputs. Life cycle assessments have identified the manufacturing stage as the primary source of their environmental burden. To address this, scientists are developing greener synthesis methods and exploring ways to recycle nano-waste.