Gold nanoparticles (GNPs) are a promising technology with applications in a wide range of fields including catalysis, electronics, materials science, and healthcare. They are of great interest to researchers because of their unusual optical, electronic, and chemical properties.
They can also be easily synthesized in a variety of shapes, including spheres, rods, and stars, with sizes ranging from 1 - 100 nm. They are produced as a suspension - the particles are suspended in a solvent, most often water.
Unique Optical Properties
One of the most useful optical properties of GNPs is that they change color readily, depending on their size, shape, and charge. This makes them, among other things, excellent labels for colorimetric detection of biomolecules.
These optical properties derive from an effect called surface plasmon resonance. Like all metals, gold contains free-moving electrons. When light hits the surface of a nanoparticle, these free electrons interact with the electric fields of the light rays and produce oscillations of charge that resonate with the wavelength of visible light.
The result is that GNPs absorb and reflect light at certain wavelengths, depending on their size, shape and surface chemistry.
For example, small (around 30 nm) GNPs absorb light in the blue-green portion of the spectrum (around 450 nm) and reflect red light (around 700 nm), so they are a rich red color.
As particle size increases, solutions become pale blue or purple as the red light is absorbed and blue light is reflected, until most visible wavelengths are reflected, at which point the solution is translucent.
Another key property of GNPs is that they bind strongly to a range of molecules. This means that they can be coated with all kinds of molecules, such as polymers or biological recognition compounds, and their surfaces are tailored for specific applications.
For example, by coating GNPs with antibodies that bind to specific biomarkers, then measuring how the nanoparticles absorb light, the technology could be used to diagnose cancers and infectious agents.
GNPs are ideally suited for biomedical research because they are biologically inert and generally considered to be non-toxic. There are four main areas of focus: medical imaging, diagnosis, drug delivery and targeted killing of cells.
Gold is a popular choice for diagnostics because it binds strongly to short, single strands of RNA or DNA (oligonucleotides) and changes color easily. By coating GNPs with oligonucleotides, they can be used to capture and identify genetic sequences that can be linked to molecules such as bacteria. Gold nanoparticles are also common in lateral flow immunoassays, a common household example being the home pregnancy test.
Because it is easy to attach molecules to gold, the particles can act as drug delivery vehicles, carrying drugs inside tumors, for example. Nanoparticles get trapped in the porous network of blood vessels that feed a tumor and accumulate there. When light shines on them, they absorb near-infrared wavelengths that pass through tissue without causing harm and start generating heat. This heat can kill cancer cells or release drugs from carriers.
Another approach is to build scaffolds out of GNPs and then arrange DNA or RNA around them (called spherical nucleic acids, or SNAs). They pass easily through the skin’s top layer - offering the potential for treatments for melanoma and other skin conditions – and are also able to cross the blood–brain barrier, so they could be used to target brain tumor cells.
Electronics, Food Science and Other Applications
Apart from life sciences, GNPs can be used in many other areas. These include in electronics as conductors and connectors in products such as printable inks and electronic chips; and in a variety of sensors. For example, a colorimetric sensor based on gold nanoparticles can identify if foods have started to go off.
Other methods, such as surface enhanced Raman spectroscopy, rely on gold nanoparticles as substrates to enable the measurement of vibrational energies of chemical bonds. This technique could be used to detect pollutants and other molecules.
Although gold in bulk is a poor catalyst, it’s a different story for GNPs, which are proving to be important catalysts in fundamental research, in green chemistry where room temperature conversion of biomass and pollutants are crucial, and for fuel cell applications.
For example, GNPs coated onto semiconductor metal oxides catalyze the oxidation of carbon monoxide at relatively low temperatures. In fact, gold catalysts can promote many reactions, at lower temperatures and with higher selectivity than other metal catalysts.
Other examples include the selective oxidation of propylene, alcohols and polyols; selective hydrogenation; and hydrochlorination.
Suppliers for the Life Sciences Industry
There are many companies operating globally to serve the life sciences market. These include Cytodiagnostics, Sigma-Aldrich, BBI Solutions and Nanosphere.
Sigma-Aldrich, in conjunction with Cytodiagnostics, a biotech company based in Ontario, Canada, offers a broad portfolio of gold nanoparticles geared specifically for high-tech applications within life science and materials science. GNPs are available in sizes ranging from 5 - 400 nm in diameter with numerous surface functionalities in a variety of solvent compositions.
While spherical gold nanoparticles are traditionally made using reducing agents such as sodium citrate or sodium borohydride, Cytodiagnostics has a propriety process and formulation to prepare highly spherical gold nanoparticles, without harsh reducing agents. They claim that their proprietary protocols produce particles with uniform shapes and narrow size distributions.
The UK company BBI Solutions offers a range of particle sizes from 2 - 250 nm for a range of applications. BBI says its unique manufacturing technique allows the production of large batches of gold to a high level of reproducibility of size, dispersion, and shape. It claims its gold manufacturing technique guarantees: consistency; high stability; scalability; and quality.
US company Nanosphere, now part of Luminex Corporation, has developed a detector called Verigene based on GNPs coated with oligonucleotides to identify a dozen bacteria known to cause infection. In some cases, it can do this in 2 - 3 hours. Delivery of this time-critical information enables clinicians to provide targeted patient care more quickly than waiting for the results of cultured samples. Verigene is designed to target infections in the bloodstream, respiratory tract, and gastrointestinal tract.
The Future for Life Sciences Applications
Cancer therapy continues to be a major area of interest. An array of approaches are under investigation. These include delivering cancer medication, such as tumour necrosis factor, using GNPs (AstraZeneca/CytImmune); developing SNAs to pass from the bloodstream into the brain to treat brain tumours (AuraSense Therapeutics) and other solid tumours; coating glass shells with GNPs to improve the aim of lasers used to image and zap tumours (Nanospectra Biosciences).
These nanoshells could also serve as a delivery vector for gene silencing (when a gene is switched off). They can carry specific strands of DNA oligonucleotides or RNA molecules that are released when they are exposed to ultraviolet light, and turn off expression of a gene.
Much work continues to focus on diagnostics. Examples include GNP strip tests to detect certain heart attacks by identifying cardiac troponin I (cTn-I), found at several thousand times higher in patients experiencing myocardial infarctions (New York University Polytechnic School of Engineering); flu tests consisting of GNPs coated with antibodies that bind to specific strains of the flu virus (University of Georgia); and a new device that can spot the volatile organic compounds in exhaled breaths of patients with lung cancer (University of Colorado).
References and Further Reading
Gold Nanoparticles: Properties and Applications
Gold Nanoparticle Products
Biomedicine: The New Gold Standard
Targeted Photodynamic Therapy of Breast Cancer Cells Using Antibody-phthalocyanine-gold Nanoparticle Conjugates
Gold Nanoparticles Market Size to Exceed USD 8 Billion by 2022