Nanomaterials have become a trending area in science and technology. Numerous metal nanomaterials have been extensively studied for a wide range of applications. Among them, gold nanoparticles have been found to be highly remarkable.
They have several unique properties, which make them a valuable material in a range of fields, including chemistry, physics, biology, medicine, and sensing.
Most of their properties and behavior depend on the size and shape of the nanoparticle. Rodlike gold nanoparticles, for example, have both a longitudinal and transverse absorption peak, and their self-assembly depends on the anisotropy of the shape.
Below, we have explained how gold nanoparticles are different than other nanoparticles, how they are formed, and what are their major benefits and applications. Let’s start with a basic question.
What Are Gold Nanoparticles?
The term “nanoparticles” refers to a material in which at least one dimension (in a 3D space) ranges between 1 and 100 nanometers in size. This is equivalent to the size of 10-1000 atoms closely packed together.
Gold nanoparticles are a cluster of tiny particles with a diameter of 1 to 100 nanometers. This cluster contains a gold core and a surface coating. The core determines the fundamental properties of the gold nanoparticle, while the protective coating (that surrounds the core) can be altered to adjust the properties as per the requirements.
When these nanoparticles are dispersed in water, it is called colloidal gold. The solution is either blue/purple (for nanorods or spherical particles more than 100 nanometers) or intense red (for smaller particles).
Gold Nanoparticle Profile
Size range: 1-100 nm
Molecular Weight: 196.97
Young’s Modulus: 79 GPa
Melting and Boiling Point: 1063 and 2966 °C
How Are They Different From Other Nanoparticles?
Compared to other metal nanoparticles, such as copper, silver, mercury, and platinum, gold nanoparticles are more stable against oxidation and degradation. Their electronic, optical, and molecular-recognition characteristics play a significant role in the advancement of nanomaterials.
Over the past decade, gold nanoparticles have attracted a lot of researchers’ attention. They have been synthesized in various structures and shapes, including nanowires, nanocubes, nanorods, nanospeheres, nanoshells, nanobipyramids, and nanobranches.
How Are They Made?
Although gold nanoparticles have a long history, synthesizing their small and stable structures still remains a challenge in nanotechnology. It requires a lot of bulk material, small gold seeds, gold(III) chloride trihydrate, and various biological extracts to make gold nanoparticles. Plus, these nanoparticles can bind multiple active molecules under specific a temperature and pressure, and have several potential applications in diverse fields.
So far, numerous methods have been developed to control the shape, size, and surface functionality of gold nanoparticles. These methods can be categorized into three key approaches:
Most physical techniques involve adjusting experimental variables (in the presence of a reducing agent) to alter the structure and characteristics of gold nanoparticles without contamination. The two most common physical methods for preparing these nanoparticles are ion implantation and laser ablation.
Ion implantation yields nanoparticles with precise physical, chemical, and biological characteristics. Laser ablation, on the other hand, is used for effectively altering the geometric shape, surface area, fragmentation, and assembly of gold nanoparticles in an aqueous solution.
The chemical reduction of metal ions in solutions is the simplest way of synthesizing gold nanoparticles. Typically, it depends on the reduction of Au(III) (from HAuCl4, hydrogen tetrachloroaurate hydrate) to Au(0) atoms, which are formed as clusters and gathered into polycrystalline particles through aggregation in the presence of stabilizing or reducing agent.
Although this aqueous technique is widely used, the shape of the nanoparticles is not regular and the size distribution obtained is quite poor.
Various studies have shown that the solvent, stabilizing/reducing agent, pH, and temperature of the reaction system play an important role in determining the shape and size of the nanoparticles. The techniques developed in recent years, especially seed-mediated synthesis, have been proven effective in precisely controlling the structure and properties of gold nanoparticles.
While both chemical and physical techniques give a high yield and are inexpensive, they have a few downsides, such as the contamination of precursors, the use of carcinogenic solvents, and high toxicity.
To eliminate these downsides, scientists have studied the potential of microorganisms. It has been found that biological agents and organic solvents can be used to reduce metal ions under specific temperatures and pressure.
For example, in 2010, a team of researchers successfully carried out a green synthesis of gold nanoparticles using leaf extract of Rosa rugosa. Although such bacterial-mediated syntheses are environmentally friendly and provide better control over the shape and size of the nanoparticles, they have low yield and are quite difficult to handle.
Different Types Of Gold Nanoparticles
Gold nanoparticles can be categorized into groups based on their shape, size, and physical properties. Gold nanospheres, for example, are the earliest researched gold nanoparticles (though they are not perfectly spherical). Subsequently, other gold nanoparticles with enhanced properties have been reported. Let’s discuss them all.
Gold nanospheres (between 2 and 100 nm in diameter) are prepared by the reduction of an aqueous HAuCl4 solution. The reaction is carried out in the presence of a reducing agent under specific conditions.
Typically, citrate is used as a reducing agent to synthesize monodisperse gold nanospheres. Their size can be controlled by altering the ratio of citrate and gold — lesser amounts of citrate gold yield larger nanospheres.
Gold nanorods can be synthesized by using various techniques. The most common technique is the template method, which involves electrochemical deposition of gold within the pores of alumina template membranes or nanoporous polycarbonate.
The length of the gold nanorod is determined by the amount of gold deposited within the membrane’s pores, while the diameter is based on the width of the membrane’s pore.
Gold nanoshells are fabricated by a seed-mediated growth method which involves attaching gold nanoparticles to the dielectric core and then growing them under a certain condition to form a shell. So basically, gold nanoshells are made of a dielectric core enclosed by a thin gold layer.
The SPR (surface plasmon resonance) of these shells is based on the plasmon difference between the outer and inner shells. One can change the ratio of shell thickness and core radius to tune the SPR wavelength from visible to near-infrared.
Gold Nanocages can be prepared through a galvanic replacement reaction between aqueous HAuCl4 and truncated silver nanocubes. To create silver nanostructures with controlled morphologies, a polyol reduction method is used, which involves reducing AgNO3 by ethylene glycol to produce silver atoms and then nanocrystals.
The wall thickness and dimension of gold nanocages can be precisely controlled by altering the molar ratio of HAuCl4 and silver.
Surface-Enhanced Raman Scattering (SERS) is an optical method that provides several benefits over conventional surface-sensitive techniques, such as robustness, high levels of multiplexing, and improved performance in biological matrices.
SERS Nanoparticles have been extensively studied for applications in imaging and sensing. For example, gold nanospheres (about 13 nm wide) treated with Cy3-labeled, alkylthiol-capped oligonucleotide strands have been used as probes to detect the presence of particular DNA strands.
Gold nanoparticles interact with visible light. They scatter and absorb light, resulting in colors ranging from black to blues to vibrant reds (smaller particles) and to clear and colorless (larger particles). The color primarily depends on the size, shape, and local refractive index of the particles.
The color of these nanoparticles differs because of an optical phenomenon called Localized Surface Plasmon Resonance (LSPR). It occurs when light interacts with conductive nanoparticles that are smaller than the incident wavelength.
As the size of the nanoparticle increases, the wavelength of light absorbed by the particle increases proportionally. 30-nanometer spherical gold particles, for example, have a peak LSPR absorption at 520-nanometer.
The optical properties also change when these nanoparticles aggregate. This happens because the overall shape, size, and dielectric environment change, influencing the observed optical features.
Absorption spectra of gold nanoparticles
Spherical gold nanoparticles, in particular, exhibit a wide range of colors (purple, red, orange, and brown) in an aqueous solution. They usually show a size-depended absorption peak from 490 to 560 nm. Plus, they possess several useful qualities such as low toxicity, excellent biocompatibility, and large surface-to-volume ratios. These features make gold nanoparticles a crucial material in bionanotechnology.
Gold nanoparticles have potential applications in various fields, ranging from electronics to biomedicine. Because of their unique properties, they have been extensively studied, particularly in the medical field, for imaging, diagnostics, therapeutics, and other biological activities.
Since they have weak antibacterial activity than other metal nanoparticles, they can be utilized in gene delivery, drug delivery, tumor therapy, antimicrobial, antioxidant, and improving analytical performances. Let’s discuss their key applications in detail.
Gold nanoparticles are conjugated with recognition moieties like oligonucleotides and antibodies to detect target biomolecules (or diseases like cancer). They play an important role in the ultrasensitive technique called bio-barcode assay for detecting various protein and nucleic acid targets.
The versatile electronic and optical characteristics of gold nanoparticles have been utilized for cell imaging. Researchers have used various methods to achieve better results, such as Raman spectroscopy, optical coherence tomography, dark-field light scattering, and computed tomography (CT). For instance, these nanoparticles act as contrast agents for CT imaging based on the electron density of gold.
Functionalized gold nanoparticles have been studied to analyze the interaction with cell membranes to improve delivery efficiency. It has been found that nanoparticles functionalized with an ordered arrangement of amphiphilic molecules can easily penetrate the cell membrane.
These studies have helped researchers develop effective targeting and delivery mechanisms using gold nanoparticles for therapeutic applications such as genetic regulation, drug treatment, and photothermal therapy.
Overall, gold nanoparticles are a shockingly versatile material for advanced nanotechnology and biomedical applications.
Recent advances in chemical synthesis and nanofabrication have extended the scope of gold nanoparticles from conventional homogeneous nanospheres to various hybrid nanostructures with programmable size, shape, and composition.
In 2018, a group of researchers at Rutgers University developed star-shaped gold nanoparticles coated with a semiconductor. They found that these nanoparticles can generate hydrogen from water four times more efficiently than other techniques.
In 2019, scientists at Mount Sinai School of Medicine designed biocompatible gold nanoparticles to safely transform near-infrared light into heat. They then used this technique in the clinical trial to target and treat prostate cancer cells. They were able to achieve selective hyperthermic cell death, without destroying adjacent healthy tissue.
In 2021, researchers at North Carolina State University obtained desirable optical properties by stretching shape-memory polymers embedded with gold nanoparticles. The material can be used to make sensors that use optical characteristics to track an object or surrounding’s thermal history.
Another milestone was achieved by researchers at the Tokyo Institute of Technology. In 2021, they developed a green method for preparing gold nanoparticles with optimized morphology for near-infrared light absorption. They used a biomolecule named B3 peptide to synthesize gold nanoparticles with controllable structure.
The method is quite effective in killing cancer cells. (Gold nanoparticles are infused with cancer cells, heated up, and destroyed via near-infrared light. The light is then absorbed by gold nanoparticles). It also provides detailed insights into the development of next-generation, non-invasive cancer therapy.
Future Market Size Of Gold Nanoparticles
The global gold nanoparticle market size is expected to grow at a CAGR of 12.7% from 2021 to 2026. It will surpass the value of $7.7 billion by 2026. The ever-increasing demand for nanotech-induced diagnostics and treatments, high demand from various end-use industries, and technological advancements are the major key factors behind this growth.
By end-user industry, the market can be segmented into healthcare, electronics, and chemicals. By application, the markets can be segmented into sensors, imaging, catalysis, diagnostics, probes, and targeted drug delivery.
North America is most likely to lead this market in the next decade, owing to the high demand for advanced devices in medical diagnostics, nanoparticles in detecting tumor cells, and growing R&D projects.
Frequently Asked Questions
Who invented gold nanoparticles?
Although gold nanoparticles have a long history, the first report on this topic was published by Michael Faraday in 1857. He described the experimental relations of gold (and other metals) to light.
In 1951, John Turkevich developed a technique for synthesizing gold nanoparticles by treating hydrogen tetrachloroaurate with citric acid in boiling water. Gert Frens further refined this technique by altering the ratio of gold and citrate in the reaction. This allowed him to control the sol stability and particle size.
Are gold nanoparticles toxic to humans?
Gold nanoparticles are usually nontoxic at low doses, but an acute burst of exposure is dangerous to human cells. The extent of toxicity is determined by the properties and preparation method of these nanoparticles.
While some studies have shown that gold nanoparticles are not toxic, many studies contradict this statement. More research is required to get a holistic inference.
How much do they cost?
One milligram of gold nanoparticles costs between $30 and $90 (depending on the particle size). For example, gold particles that are 10-nm in diameter cost about $35 per milligram or $35,000 per gram. In contrast, 24-carat gold goes for about $65.