Plasmonic nanoparticles, particularly gold (Au) and silver (Ag), have emerged as highly efficient materials for the degradation of pharmaceutical contaminants, including antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs). Their unique optical properties, driven by localized surface plasmon resonance (LSPR), enable light absorption and scattering at specific wavelengths, which can be harnessed for photocatalytic applications. When these nanoparticles are integrated with semiconductor supports, they form hybrid systems that enhance degradation efficiency through synergistic mechanisms such as hot-electron injection and near-field enhancement.
The LSPR phenomenon occurs when incident light interacts with the conductive electrons of plasmonic nanoparticles, causing collective oscillations. For Au and Ag nanoparticles, LSPR peaks typically lie in the visible to near-infrared spectrum, making them suitable for solar-driven photocatalysis. Upon excitation, these nanoparticles generate highly energetic hot electrons that can be injected into the conduction band of an adjacent semiconductor, such as TiO2 or ZnO. This electron transfer facilitates the generation of reactive oxygen species (ROS), including hydroxyl radicals (•OH) and superoxide anions (•O2−), which are critical for breaking down pharmaceutical molecules.
Hot-electron injection is a key mechanism in plasmonic photocatalysis. When Au or Ag nanoparticles absorb photons, electrons in the metal are excited to higher energy states. A fraction of these electrons, termed hot electrons, possess sufficient energy to overcome the Schottky barrier at the metal-semiconductor interface. Once injected into the semiconductor, these electrons participate in redox reactions. For instance, they can reduce oxygen to form •O2−, while the remaining holes in the metal oxidize water or organic molecules directly. The efficiency of hot-electron transfer depends on factors such as nanoparticle size, shape, and the electronic coupling between the metal and semiconductor.
Near-field enhancement is another critical aspect of plasmonic photocatalysis. The intense electromagnetic fields generated around Au and Ag nanoparticles under LSPR excitation can enhance the absorption of light by nearby semiconductor materials. This effect increases the generation of electron-hole pairs in the semiconductor, further boosting photocatalytic activity. The spatial distribution of the enhanced field is highly localized, typically within a few nanometers of the nanoparticle surface, making the design of plasmonic-semiconductor heterostructures crucial for maximizing this effect.
Hybrid systems combining plasmonic nanoparticles with semiconductors exhibit superior performance compared to individual components. For example, Au-TiO2 composites have demonstrated enhanced degradation rates for antibiotics like ciprofloxacin and tetracycline under visible light. The plasmonic component extends the photocatalytic activity of TiO2 into the visible spectrum, while the semiconductor provides a stable platform for charge separation and ROS generation. Similarly, Ag-ZnO systems have shown high efficiency in degrading NSAIDs such as ibuprofen and diclofenac due to the combined effects of LSPR and ZnO’s intrinsic photocatalytic properties.
Analytical techniques play a vital role in tracking the degradation pathways and intermediate byproducts of pharmaceuticals. High-performance liquid chromatography (HPLC) coupled with mass spectrometry (LC-MS) is widely used to identify transformation products and quantify degradation efficiency. For instance, the breakdown of sulfamethoxazole, a common antibiotic, has been monitored using LC-MS, revealing intermediates such as 3-amino-5-methylisoxazole and sulfanilic acid. Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy provide insights into structural changes during degradation, while electron paramagnetic resonance (EPR) spectroscopy confirms the presence of ROS.
Ecotoxicity assessments are essential to ensure that the degradation process does not produce harmful byproducts. Plasmonic photocatalytic systems have been evaluated using bioassays with organisms like Daphnia magna and Vibrio fischeri to measure acute and chronic toxicity. Studies indicate that while complete mineralization of pharmaceuticals is ideal, some intermediates may retain biological activity. For example, the degradation of amoxicillin generates intermediates that exhibit lower toxicity than the parent compound but still require further treatment. Advanced oxidation processes (AOPs) combining plasmonic photocatalysis with other techniques, such as ozonation or Fenton reactions, can improve mineralization efficiency and reduce ecotoxicological risks.
The stability and reusability of plasmonic nanoparticles are critical for practical applications. Ag nanoparticles are prone to oxidation and dissolution, which can limit their long-term performance. Strategies such as coating with inert shells (e.g., silica) or embedding in protective matrices have been explored to enhance stability. Au nanoparticles, being more chemically inert, are often preferred for repeated use. However, both materials may suffer from aggregation under operational conditions, necessitating careful design of support structures and surface functionalization.
In summary, plasmonic nanoparticles and their hybrid systems offer a promising solution for the degradation of pharmaceutical pollutants. The interplay of LSPR-driven hot-electron injection, near-field enhancement, and synergistic effects with semiconductors underpins their high photocatalytic activity. Advanced analytical techniques enable precise tracking of degradation pathways, while ecotoxicity assessments ensure environmental safety. Future research should focus on optimizing nanoparticle-semiconductor interfaces, scaling up synthesis methods, and integrating these systems into real-world water treatment technologies.