Ca-Doped BaCeO₃ Nanoperovskites for Enhanced Photocatalytic Degradation and Electrochemical Detection of Trimethoprim

The world is increasingly cluttered with pharmaceuticals that nature struggles to break down. Among these persistent contaminants is trimethoprim, a common antibiotic used to treat infections in humans and animals. Because it is chemically stable and resists natural decay, it slips through wastewater treatment plants and accumulates in rivers and groundwater. While present in tiny amounts, its continuous release poses a threat to ecosystems and public health, potentially fostering antibiotic-resistant bacteria and causing harm to aquatic life. To tackle this, scientists look toward two distinct but complementary strategies: destroying the pollutant and detecting its presence. One approach uses light to trigger chemical reactions that break complex molecules into harmless substances, a process known as photocatalysis. The other relies on sensitive electrical sensors to measure exactly how much of the drug is present in a solution. Ideally, a single material could perform both tasks, offering a streamlined solution for environmental monitoring and cleanup.

In a recent study, researchers from the University of Mysore and affiliated institutions in India explored whether a specific type of engineered crystal could serve as this dual-purpose tool. They focused on a material called barium cerate, a crystalline structure known for its stability and catalytic properties. To make it more effective, they introduced a small amount of calcium into the crystal lattice, a process called doping. By swapping some of the larger barium atoms with smaller calcium atoms, the team created a new material with a modified internal structure. They synthesized these particles using a sol-gel method, a technique that mixes chemicals in a liquid solution to form a gel, which is then dried and heated to create a fine, porous powder. The goal was to see if this calcium-doped version could outperform the original material in both breaking down trimethoprim under sunlight and sensing the drug through electrical signals.

The team began by confirming the structure of their new material. Using X-ray diffraction, a technique that reveals the arrangement of atoms inside a solid, they verified that the calcium had successfully entered the crystal structure without creating unwanted impurities. The analysis showed that the calcium atoms had squeezed into the spaces normally occupied by barium, causing a slight shrinkage in the crystal lattice. This structural change was crucial because it created tiny defects and missing oxygen atoms within the material. These defects acted as traps for electrical charges, preventing them from canceling each other out too quickly. The researchers also examined the material under electron microscopes, revealing that the particles were nanoscale in size, roughly twenty to thirty nanometers across, and formed porous, flower-like clusters. This porous shape provided a vast surface area, allowing more space for pollutants to attach and react.

When the researchers tested the material's ability to absorb light, they found that the calcium doping had shifted its capabilities. Both the original barium cerate and the new calcium-doped version were able to absorb light in the visible region, with the original material showing an absorption edge around 560 nm. However, the calcium-doped version displayed a pronounced absorption feature near 525 nm, indicating a shift in its behavior. This shift occurred because the structural defects lowered the energy barrier required to excite electrons within the material. With an optical band gap of 2.1 electron volts, the material could utilize a broader spectrum of solar energy. This change was the key to unlocking its potential as a photocatalyst. When exposed to sunlight, the material generated pairs of electrons and "holes" (positive charges) that could drive chemical reactions.

In the degradation experiments, the team placed the calcium-doped powder into water containing trimethoprim and exposed the mixture to natural sunlight. They systematically adjusted variables such as the amount of catalyst, the concentration of the drug, the temperature, and the acidity of the water to find the ideal conditions. They discovered that at a neutral pH, a temperature of 40 degrees Celsius, and a specific dosage of the catalyst, the material achieved remarkable results. Within 45 minutes, it broke down approximately 96 percent of the trimethoprim in the solution. To understand how this happened, the researchers added specific chemicals that neutralize different types of reactive species. They found that superoxide radicals, which are highly reactive oxygen molecules, played the most significant role in destroying the antibiotic, followed by hydroxyl radicals and positive charges. The calcium doping enhanced the generation of these radicals by keeping the electrical charges separated for longer, allowing them to attack the pollutant more effectively. The material also proved durable, maintaining its high performance even after being reused for five consecutive cycles.

Beyond cleaning water, the team investigated whether the same material could detect the presence of trimethoprim. They coated a carbon electrode with the calcium-doped powder and placed it in a solution containing the drug. When they applied an electrical voltage, the material facilitated the oxidation of the trimethoprim molecules, generating a measurable electrical current. The calcium-doped electrode responded much more strongly than the undoped version, producing a clear signal even at very low concentrations. The sensor worked best in a slightly alkaline environment, where the chemical reaction proceeded most efficiently. The device showed a linear response, meaning the electrical signal increased steadily as the amount of drug increased, allowing for precise quantification. The researchers determined that the sensor could detect trimethoprim at concentrations as low as 0.03 micromolar, a level sensitive enough to monitor trace amounts in environmental samples.

The study concludes that the strategic addition of calcium to barium cerate creates a versatile nanomaterial capable of addressing pharmaceutical pollution in two ways. By engineering the crystal structure to create specific defects, the researchers improved the material's ability to harvest sunlight and transfer electrical charges. This enhancement allowed the material to act as both a powerful agent for degrading antibiotics under natural light and a highly sensitive tool for detecting them. The work demonstrates that modifying the internal architecture of a simple oxide can yield a dual-functional material, offering a promising avenue for developing cost-effective solutions to monitor and clean up water contaminated with persistent drugs.