- Gold nanoparticles (AuNPs) are nanoscale particles of gold, typically ranging from 1 to 100 nanometers in diameter, that exhibit exceptional physical, chemical, and optical properties distinct from those of bulk gold. Their stability, biocompatibility, and tunable surface chemistry make them one of the most extensively studied nanomaterials in modern science. The most remarkable feature of gold nanoparticles is their surface plasmon resonance (SPR)—a phenomenon in which conduction electrons on the nanoparticle surface oscillate collectively in response to light. This gives AuNPs unique optical absorption and scattering characteristics, resulting in vivid colors (ranging from red to purple depending on particle size and shape) and making them invaluable for sensing, imaging, and biomedical applications.
- Structurally, gold nanoparticles can be synthesized in various shapes—spherical, rod-like, cubic, star-shaped, or shell-coated—each conferring distinct optical and functional properties. Their surfaces can be readily modified with thiol (-SH), amine (-NH₂), or carboxyl (-COOH) groups, allowing the attachment of biomolecules such as DNA, peptides, antibodies, or drugs. This surface functionalization capability underlies their versatility across fields from nanomedicine and diagnostics to catalysis and electronics. Gold nanoparticles are typically synthesized through chemical reduction methods, such as the Turkevich method, where chloroauric acid (HAuCl₄) is reduced by sodium citrate to form spherical nanoparticles. Alternative techniques, including photochemical, electrochemical, and biological (green) synthesis, enable better control over size, shape, and biocompatibility, often using plant extracts or microorganisms as reducing agents.
- In biomedical applications, gold nanoparticles play a central role due to their biocompatibility, inertness, and ability to interact with light and biomolecules. In drug delivery, AuNPs can be functionalized to target specific tissues or cells, releasing therapeutic agents in response to external stimuli such as light, heat, or pH. Their tunable size and surface chemistry allow them to penetrate biological barriers effectively, enhancing drug efficacy while minimizing side effects. In cancer therapy, gold nanoparticles are used in photothermal therapy (PTT), where they absorb near-infrared light and convert it into heat, selectively destroying cancer cells without harming surrounding healthy tissues. They also serve as contrast agents in imaging techniques such as computed tomography (CT), photoacoustic imaging, and surface-enhanced Raman spectroscopy (SERS), improving the sensitivity and precision of disease diagnosis.
- In biosensing and diagnostics, gold nanoparticles are employed in assays due to their strong optical properties and ability to produce visible color changes upon aggregation. The classic example is the lateral flow assay used in home pregnancy tests and rapid disease diagnostics, where AuNPs are conjugated with antibodies to detect specific biomolecules. Similarly, in molecular diagnostics, functionalized gold nanoparticles can hybridize with target DNA sequences, allowing for colorimetric detection of genetic markers or pathogens with high sensitivity. These properties make AuNP-based biosensors highly valuable for point-of-care testing and personalized medicine.
- Beyond medicine, gold nanoparticles have numerous applications in catalysis, electronics, and environmental science. Their large surface area and catalytic activity at the nanoscale make them effective catalysts in oxidation and reduction reactions, such as carbon monoxide oxidation and green chemical synthesis. In electronics, AuNPs are used in conductive inks, sensors, and nanoelectronic devices due to their excellent electrical conductivity and chemical stability. Their ability to absorb and scatter light efficiently also makes them promising materials for plasmonic and photonic devices, solar cells, and optical filters.
- Despite their many advantages, the use of gold nanoparticles raises important considerations regarding toxicity, biodistribution, and environmental impact. Although elemental gold is chemically inert and generally considered biocompatible, nanoparticle size, shape, coating, and dosage can influence cellular uptake and biological responses. Smaller particles, for instance, may enter cells more readily and interact with subcellular structures, potentially leading to oxidative stress or inflammation. Thus, understanding their toxicological profile and long-term biostability remains an active area of research, especially as their use expands in medicine and consumer products.
