A Nano-Composite Electrochemical Sensor for Detecting Reactive Oxygen Species (ROS)

Reactive oxygen species (ROS) are highly reactive chemicals formed during the incomplete reduction of molecular oxygen. They encompass various free radicals like superoxide (•O₂⁻), hydroxyl radicals (•OH), alkoxy radicals (RO•), and peroxyl radicals (ROO•), as well as non-radical reactive oxygen intermediates such as lipid hydroperoxide (LOOH), hydrogen peroxide (H₂O₂), and ozone (O₃). These free radicals possess unpaired electrons in their outermost shells, rendering them extremely reactive and constantly seeking to form stable bonds. In physiological processes, ROS serve essential roles such as killing harmful bacteria, mediating intracellular signaling, and activating gene transcription. However, an imbalance between ROS generation and removal can lead to oxidative stress, causing damage to proteins, lipids, and DNA, and ultimately contributing to various diseases, including cancer, diabetes, and neurodegenerative disorders. Recently, the detection of ROS has attracted significant attention across diverse fields. In medicine, early detection of ROS concentration changes is crucial for pathological studies, health screening, and disease diagnosis. Furthermore, industries like fuel cells can benefit from ROS detection due to their known propensity to damage proton exchange membranes, limiting the lifespan of fuel cells.

Despite the growing interest in ROS detection, several challenges remain. ROS possess intrinsic characteristics such as short lifespans, rapid diffusion rates, and diverse sources of production. These features can lead to imprecise and inconsistent measurements. Additionally, their often low and variable concentrations at generating sites, particularly within living cells, can render conventional detection methods inappropriate. Among ROS, hydroxyl radicals (•OH) are particularly reactive and damaging, posing specific challenges for detection. In human cells, mitochondria are primary •OH producers through a multi-step process, involving the reduction of molecular oxygen. Developing precise and reliable detection methods for •OH is essential for understanding and mitigating oxidative stress-related diseases. In addressing the challenges of ROS detection, electrochemical sensing has emerged as a promising approach. Electrochemical sensors offer high sensitivity, selectivity, cost-effectiveness, rapid response times, and the potential for in-situ real-time detection. Various organic and inorganic materials have been integrated into electrochemical sensors for •OH detection, but they often face limitations such as temperature and pH sensitivity, affecting sensor performance.

A recent study published in the Journal of The Electrochemical Society, led by Hamidreza Ghaedamini and Professor Dong-Shik Kim from the University of Toledo in collaboration with Associate Professor Ana Alba-Rubio from Clemson University, presents a significant advancement in the field of electrochemical sensing technology. They developed an innovative method for the detection of reactive oxygen species (ROS), particularly hydroxyl radicals (•OH), using a novel sensor platform comprised of cerium oxide nanoparticles (CeO_x), gold nanoparticles (AuNPs), and a highly conductive carbon. The key innovation in the study involves the use of CeO_x nanoparticles as ROS scavengers. CeO_x exhibits remarkable properties due to its dual oxidation states, switching between Ce³⁺ and Ce⁴⁺ rapidly. Ce³⁺ sites on CeO_x selectively react with hydroxyl radicals (•OH) through an oxidation process, resulting in their conversion  into Ce⁴⁺ sites. This unique feature makes CeO_x an exceptional •OH scavenger. Researchers have focused on reducing CeO_x nanoparticle size to increase the number of Ce³⁺ sites, thereby enhancing •OH scavenging capabilities. This approach has shown promise in improving electrochemical •OH detection.

AuNPs have been widely recognized for their exceptional catalytic activity. They offer the potential to significantly enhance sensor sensitivity by boosting conductivity. The authors deposited AuNPs onto a highly conductive carbon using a deposition-precipitation method, followed by selective deposition of CeO_x nanoislands onto AuNPs through controlled surface reactions (CSR). This innovative approach aimed to create small clusters of CeO_x that could enhance the electrochemical performance of the sensor. Carbon served as a stable platform for AuNPs, providing a conductive matrix for electron transfer between the electrode and the nanocomposite. AuNPs’ high surface area-to-volume ratio allowed for the dispersion of CeO_x domains without agglomeration, further enhancing sensor performance in the Fenton reaction, which is a process that generates •OH through the decomposition of hydrogen peroxide by ferrous ions (Fe²⁺). The researchers’ choice of a screen-printed carbon electrode (SPCE) as a transducer for the sensor base is noteworthy. SPCEs have gained prominence in creating disposable sensors for electroanalysis due to their design versatility, reproducibility in sensor fabrication, and low production cost.

The authors’ experimental details included the synthesis of carbon-supported gold nanoparticles (Au/Carbon) using the deposition-precipitation method. Cerium oxide nanoparticles were selectively deposited onto Au/Carbon using CSR to create the CeO_x-Au/Carbon composite. They used advanced characterization tools such as transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) to examine the distribution of AuNPs on the carbon support and their decoration with CeO_x nanoislands. The actual Au and Ce loadings in the composites were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The CeO_x-Au/Carbon composite was deposited onto the carbon working electrode for electrochemical testing.

The electrochemical characterization of the CeO_x-Au/Carbon-modified electrode demonstrated the successful dispersion of AuNPs on carbon and the selective deposition of CeO_x onto AuNPs. The authors found the electrochemical performance of the CeO_x-Au/Carbon-modified electrode to be significantly improved compared to a CeO_x/Carbon-modified electrode, as evidenced by higher current responses and lower peak potential differences in the presence of hydroxyl radicals (•OH). They determined the effective surface area of the CeO_x-Au/Carbon-modified electrode and reported it to be larger than that of the bare electrode before modification, indicating the presence of more electroactive sites and good electrical conductivity. Moreover, the scan rate effect on the sensor’s performance showed that the redox response was directly proportional to the scan rate, suggesting a classical surface control mechanism. The redox reaction involving Ce³⁺ and Ce⁴⁺ was quasi-reversible. According to the authors, the sensor exhibited excellent selectivity, distinguishing •OH from hydrogen peroxide (H₂O₂), and demonstrated high reproducibility, repeatability, and stability.

In conclusion, the authors’ innovative approach to ROS detection using CeO_x-Au/Carbon-modified electrodes holds great promise in addressing the challenges associated with detecting hydroxyl radicals (•OH) and other ROS. By combining the unique properties of cerium oxide nanoparticles with the catalytic activity of gold nanoparticles on a highly conductive carbon support, the researchers have created a highly sensitive and selective electrochemical sensor. This newly developed sensor offers potential applications in various fields, including biology, medicine, and industry, for monitoring and understanding ROS-related processes and diseases.