Introduction
AuNPs (or gold nanoparticles) are gold nanoparticles, ranging in size from 1 to 100 nanometres, that exhibit different physical and chemical compositions from the bulk version. For their unique properties (surface plasmon resonance (SPR), biocompatibility and easy functionalisation), gold nanoparticles have become important targets in medicine, environmental science and nanotechnology.
Properties of Gold Nanoparticles
Gold nanoparticles possess a number of properties that are not found in bigger particles or whole gold.
Physical Properties
AuNPs have small size, surface-to-volume ratio and a distinctive geometry that could be spherical, rod-shaped, triangular or more complicated. Perhaps their most noticeable physical property is their optics, in particular their surface plasmon resonance (SPR). When illuminated by light, AuNPs undergo collective oscillations of their electrons, producing a distinct absorption peak, often in the visible region. This property is widely exploited in applications such as sensors and imaging.
| Particle Size | SPR Absorption Peak Position | Optical Properties | Explanation |
| 1-2 nm | About 520-540 nm | Weak absorption, broad SPR peak | Ultra-small gold nanoparticles show minimal optical response, weak absorption, and broad SPR peaks, mainly influenced by surface effects. |
| 2-5 nm | About 530-550 nm | Strong absorption, broad SPR peak | Gold nanoparticles in this size range start to exhibit stronger absorption, with SPR peaks still broad, influenced by surface effects. |
| 5-10 nm | About 540-570 nm | Strong absorption and scattering, broad SPR peak | Gold nanoparticles in this range show significant absorption and scattering with broad SPR peaks, surface effects remain dominant. |
| 10-20 nm | About 550-580 nm | Strong absorption and scattering, narrow SPR peak | Nanoparticles in this size range show noticeable absorption and scattering, with narrower SPR peaks and more stable optical properties. |
| 20-30 nm | About 570-600 nm | Strong absorption and scattering, narrow SPR peak | Gold nanoparticles in this size range exhibit enhanced scattering and stable SPR absorption peaks. |
| 30-50 nm | About 580-620 nm | Significant absorption and scattering, narrow SPR peak | As the particle size increases, scattering becomes more prominent, and the SPR peak shifts to a longer wavelength. |
| 50-75 nm | About 600-650 nm | Strong scattering, weak absorption | Larger particles primarily scatter light, with weaker absorption, and the SPR peak shifts to longer wavelengths. |
| 75-100 nm | About 620-670 nm | Weak absorption, significant light scattering | Larger particles show reduced absorption, with scattering dominating and SPR peaks moving further into the longer wavelength range. |
| > 100 nm | About 650-700 nm | Significant light scattering, almost no absorption | With even larger particles, scattering becomes the dominant optical effect, and absorption becomes minimal, with the SPR peak moving to the infrared region. |
Chemical Properties
The surface of gold nanoparticles is highly reactive which makes functionalization quite straightforward. An attachable functional group is able to attach to the surface, so AuNPs can associate with many biological and chemical species. This reactivity is also part of their stability because AuNPs can be tuned to be stable in any environment and thus versatile across many applications.
Biocompatibility
In contrast to most nanoparticles, gold nanoparticles are very biocompatible and toxicity free, which is one of the attractive features for biomedical applications. They can be sized and surface charged so as to cause as few negative effects as possible when put into living organisms.
Preparation Methods of Gold Nanoparticles
Gold nanoparticles can be synthesised by several different methods with varying advantages for different properties.
Sodium Citrate Reduction Method
Combine a solution of chloroauric acid (HAuCl4) with a solution of sodium citrate and boil. The solution goes from yellow to wine red. Continue stirring and then cool the solution to obtain gold nanoparticles.
For example, heating 100 mL of a 0.3 mmol/L chloroauric acid solution to 100 °C and adding 2 mL of a 38.7 mmol/L sodium citrate solution, then continuing to heat until the color changes and allowing it to cool, will yield gold nanoparticles.
Seed Growth Method
First, make smaller gold nanoparticle seeds and grow them further in a growth solution using the correct chemical reagents (CTAB, NaBH4, etc.) to form larger gold nanoparticles.
With 2.8 nm gold nanoparticles as seeds, larger gold particles can be produced by diluting chloroauric acid with hydrazine.
In Situ Synthesis Method
At room temperature, synthesize gold nanoparticles of different shapes and sizes directly by adding phosphates (such as Na₂HPO₄ or NaH₂PO₄) along with a reducing agent (such as HEPES). This process can be done once and it only takes 15 minutes.
For instance, Na2HPO4:HEPES and NaH2PO4:HEPES can be tuned by altering the molar ratio of Na2HPO4:HEPES and NaH2PO4:HEPES to adjust gold nanoflowers and gold nanosheets.
Electrochemical Synthesis Method
Stabilize the nanoparticles with tetraalkylammonium salts (via electrochemistry). This is an easy process with low equipment, cost, and low temperature. This method offers advantages such as simple equipment, low cost, and low processing temperatures.
Laser Synthesis Method
Form gold nanoparticles in a liquid environment by laser-evaporating metal films or mixtures. This method allows for the efficient and high-quality preparation of nanoparticles.
Microfluidic Reactor Method
Create quality, uniformly sized gold nanoparticles with microfluidic reactors for two-phase flow. This is a reproducible and robust method.
Self-Assembly Technique
Create patterns of gold nanoparticles on a PDMS substrate through self-assembly, then transfer the particles to the target substrate using transfer techniques.
Polymer-Mediated Method
Protect them with polyvinylpyrrolidone (PVP) to produce gold nanoparticles smaller than 10 nm in diameter by low-temperature modified sol-gel synthesis.
Each of these approaches is also a mixed bag, and selecting a suitable synthesis approach must be tailored to the application and experimental situation.
Surface Modification and Functionalization of Gold Nanoparticles
The surface of gold nanoparticles can be changed to give it a better or more functional effect. Functionalisation usually consists of gluing organic molecules, polymers or biological components to the nanoparticle surface. It is this change that enables AuNPs to bind controlled targets and system.
Polyethylene Glycol (PEG) Modification
Among the most popular surface functionalization processes for gold nanoparticles is PEG modification. The dispersibility and biocompatibility of thiolated polyethylene glycol (PEG-SH) can be made much more easily achievable by linking it to gold nanoparticles. The use of mPEG-SH, for instance, is an excellent hydrophobic coating; Cy5-PEG-thiol is an ideal fluorescent labelling material for optical detection and tracking. Additionally, PEG modification can control the coating thickness and the number of reactive terminal groups by adjusting the polymer ratio, thereby enabling flexible functionalization.
Carboxyl Modification
The carboxyl functional groups on the gold surface of carboxylated gold nanoparticles can be functionalized so that they react with amino, thiol and other functional groups in molecules. The modified process is stable and easy to use, making it very common for drug delivery, biosensing and imaging applications.
Thiol Modification
Surface functionalization of gold nanoparticles is also done through thiol modification. By bonding thiolated ligands (Thiolated polyethylene glycol or Thiolated peptides) onto the gold nanoparticles, biocompatibility and stability can be increased. Thiolated polyethylene glycol (PEG-SH), for example, has very strong coordination with the gold surface and so the nanoparticles become stable.
Multifunctional Polymer Modification
PEG, thiol and other polymers with many multifunctional functions can also be modified gold nanoparticles. Modifications with polymers such as polyvinylpyrrolidone (PVP) or polyamidoamine (PAMAM) can also add stability and functionality to gold nanoparticles, for instance.
Biomolecule Modification
Add biomolecules to gold nanoparticles (eg, proteins, antibodies, amino acids) to create targeted delivery and biosensing functions. Changes in the surface chemistry of gold nanoparticles, for instance, can be made using lysine and lysine peptides, which facilitate their assembly.
Special Functionalization
In addition to the common methods mentioned above, gold nanoparticles can be endowed with greater reactivity and application potential through special functionalization techniques (such as "Click" chemistry and alkyne modification). For instance, alkyne modification can enhance the performance of gold nanoparticles in click chemistry reactions.
The surface modification and functionalization techniques for gold nanoparticles are diverse, with each method offering unique advantages and application scenarios. By selecting appropriate modification strategies, the performance of gold nanoparticles can be significantly improved to meet the research and application needs in various fields.
Applications of Gold Nanoparticles
Gold nanoparticles can be used in a multitude of fields due to their properties.
Medical Applications
- Drug Delivery and Targeted Therapy
Gold nanoparticles make for the perfect platform for delivery of drugs because they are small, biocompatible and easily functionalised. With the surface grafted on, or loaded in the nanoparticle, AuNPs allow therapeutic agents to be released. And they can be designed with an extra-cellular marker on their surface, which could be used for personalised medicine and cancer treatment.
- Imaging and Diagnostics
For their optical character, AuNPs are a common imaging material in computed tomography (CT) and magnetic resonance imaging (MRI). They can brighten the contrast in scans to detect disease in time.
- Photothermal Therapy
AuNPs, in photothermal therapy, eject light into the atmosphere, where they heat up to burn cancer cells. This uses the SPR function of gold nanoparticles for local heat-sequestration and treatment.
Environmental Applications
There is also the use of gold nanoparticles for environmental cleanup. The large surface area and reactivity of these substances allow them to effectively trap heavy metals and contaminants from water and be used in water purification.
Electronics and Energy
Because they are conductive, AuNPs are used to build nanoscale electronic devices. They are also used in solar panels and energy storages, where they are stable and electrically conductor.
Catalysis and Sensors
Because of the gold nanoparticles' catalytic activity, they work well to induce chemical reactions, especially in organic synthesis and in environmental environments. AuNPs are also deployed in gas, biological molecules and pollutants sensors because of their reactivity and sensitivity on the surface.
Challenges and Future Prospects
Even with all of these promising applications, there are still many issues in the gold nanoparticles space.
Stability and Toxicity
AuNPs are biocompatible, at least for the short term, but their long-term stability and toxicity in vivo are unknown. It is crucial that gold nanoparticles remain stable and harmless in medical applications if they are to be commercialized.
Commercialization Barriers
It is hard to increase production of gold nanoparticles at the same scale and maintain uniform size, shape and quality. There is also the production cost and regulatory complexity to bring AuNP-based technologies to the market.
Future Developments
The future research is likely to address how to synthesise better for faster production, how to functionalize better, and what new applications there might be in nanomedicine, renewable energy, and environmental monitoring.
Conclusion
Gold nanoparticles are becoming an all-purpose material for applications with different uses because of their unique nature and functional flexibility. From medical treatments and diagnostics to environmental and energy management, AuNPs can be used in everything from the environment to energy. But the issues of stability, toxicity and scale-up will have to be overcome before these nanoparticles become commercially viable. More research on gold nanoparticles will result in revolutionary solutions for several industries and make them an integral part of the future of nanotechnology.
References
- Reznickova, A., et al. "PEGylated gold nanoparticles: Stability, cytotoxicity and antibacterial activity." Colloids and Surfaces A: Physicochemical and Engineering Aspects 560 (2019): 26-34.
- Du, Y. Q., et al. "A cancer-targeted drug delivery system developed with gold nanoparticle mediated DNA–doxorubicin conjugates." RSC advances 4.66 (2014): 34830-34835.
- Khademi, S., et al. "Targeted gold nanoparticles enable molecular CT imaging of head and neck cancer: an in vivo study." The international journal of biochemistry & cell biology 114 (2019): 105554.
- Neshastehriz, A., et al. "Photothermal therapy using folate conjugated gold nanoparticles enhances the effects of 6 MV X-ray on mouth epidermal carcinoma cells." Journal of Photochemistry and Photobiology B: Biology 172 (2017): 52-60.
