Graphene oxide nanoparticles (GONPs) are materials composed of single or a few layers of carbon atoms arranged in a hexagonal, honeycomb-like pattern. These ultra-thin sheets are distinguished by oxygen-containing chemical groups attached to their surface and edges, which modifies their characteristics compared to pure graphene. GONPs exist at a nanoparticle scale, typically ranging in lateral size from about 20 to 500 nanometers, with a thickness in the nanometer range. This unique structure and chemical modification provide them with distinct properties, making them promising for diverse applications across various scientific and industrial fields.
Unique Properties and Production
Graphene oxide nanoparticles derive their versatility from specific characteristics and a common production method. A widely used technique for their synthesis is the Hummers’ method, or its modified versions, involving the chemical oxidation of graphite. This process uses strong oxidizing agents, such as potassium permanganate and sodium nitrate, in concentrated sulfuric acid to introduce oxygen-containing groups onto the graphite layers. Following oxidation, the treated graphite is exfoliated and sonicated, separating the layers into individual nanoscale sheets of graphene oxide.
One attribute of GONPs is their high surface area, around 890 square meters per gram, enabling extensive interaction with other molecules. Unlike pure graphene, the oxygen functional groups, including hydroxyl, carboxyl, and epoxy groups, impart a hydrophilic nature to GONPs. This allows them to disperse readily and stably in water, which is beneficial for many applications, particularly in biological and aqueous environments.
The properties of GONPs can be tuned by controlling the degree of oxidation or subsequent reduction processes. This tunability allows modification of their electrical properties, transitioning from an insulating state to a more conductive form when oxygen groups are removed to create reduced graphene oxide. Their optical properties, such as fluorescence and absorption across the visible spectrum, can also be tailored. While oxygen groups can slightly reduce mechanical strength compared to pristine graphene, GONPs retain considerable strength from their carbon lattice, with an effective Young’s modulus around 207.6 gigapascals.
Applications in Biomedicine
Graphene oxide nanoparticles are suitable for various biomedical applications due to their unique properties.
Targeted Drug Delivery
A primary application is targeted drug delivery, where GONPs can be loaded with therapeutic agents like doxorubicin or methotrexate. Their large surface area facilitates efficient binding of these medications. To ensure precise delivery, GONPs can be modified with specific targeting ligands, such as antibodies, folic acid, polyethylene glycol (PEG), or chitosan, which guide them to particular cells, like cancer cells. This approach aims to concentrate medication at disease sites, minimizing adverse effects on healthy tissues throughout the body.
Cancer Therapy
Another application is cancer therapy, particularly through photothermal therapy. These nanoparticles exhibit strong absorption in the near-infrared (NIR) region. When exposed to NIR light, GONPs efficiently convert absorbed light energy into heat. This localized heat can destroy tumor cells, offering a non-invasive method for treating cancers with reduced impact on surrounding healthy tissues.
Bioimaging
GONPs are also used in bioimaging, serving as contrast agents to enhance visibility of biological structures in medical scans. For magnetic resonance imaging (MRI), GONPs can be functionalized with gadolinium ions. This modification significantly improves their contrast-enhancing capabilities. GONPs also show promise in optical imaging techniques such as photoacoustic and thermoacoustic tomography, where they can enhance signal detection for improved diagnostic clarity.
Industrial and Environmental Uses
Graphene oxide nanoparticles have broad utility beyond biomedicine, finding roles in industrial and environmental sectors.
Water Purification
Their large surface area and oxygen-containing functional groups make them effective adsorbents for water purification. GONPs can capture and immobilize pollutants, including heavy metal ions and organic dyes, from contaminated water sources. This adsorption capability helps remove hazardous substances, contributing to cleaner water.
Advanced Sensors
Another application is in advanced sensors, particularly highly sensitive electrochemical biosensors. The inherent defects in graphene oxide promote efficient charge transfer and facilitate the attachment of various sensing elements. This allows for sensors capable of detecting specific chemicals or biological molecules in very small quantities. Examples include the detection of DNA bases, hydrogen peroxide, dopamine, and COVID-19 antibodies from a single drop of blood or saliva, often providing rapid results.
Energy Storage
In energy storage, GONPs improve the performance of both batteries and supercapacitors. For supercapacitors, their high surface area provides extensive sites for ion adsorption and desorption, enabling rapid charging and discharging cycles and promoting a long operational lifespan. In battery technology, incorporating GONPs into electrode materials can lead to higher energy density, allowing devices to store more power and operate for extended periods. GONPs can also facilitate faster charging rates for batteries while enhancing their overall safety by mitigating risks like overheating.
Biocompatibility and Environmental Impact
The safety profile of graphene oxide nanoparticles is actively investigated, as their toxicity depends on multiple factors. These factors include the nanoparticle’s size, shape, concentration, and surface modifications. For instance, some studies suggest that GONPs smaller than 100 nanometers might induce toxicity, while those ranging from 100 to 200 nanometers can function as effective drug carriers. Surface modifications, such as coating with polyethylene glycol (PEG), can significantly reduce potential toxic effects and improve biocompatibility.
A primary concern is their potential to induce oxidative stress, which involves an imbalance leading to increased levels of reactive oxygen species, potentially damaging cellular components like DNA. Physical interactions between GONPs and cell membranes have also been observed, potentially compromising membrane integrity and altering cell morphology. The purity of the GONP material, with impurities and defects, can also influence its cytotoxicity.
From an environmental perspective, researchers study their environmental fate. Some studies indicate that GONPs can undergo biological degradation. The products resulting from this degradation have shown to be non-genotoxic to human lung cells. In aquatic environments, interactions with naturally occurring components like algae have demonstrated the ability to mitigate the acute toxicity of GONPs to organisms such as crustaceans. However, ongoing research continues to explore the long-term environmental fate, including potential accumulation and breakdown processes, of GONPs in various complex natural systems.