Mono- (Au, Ag and Cu) and bi-metallic (Au–Ag and Au–Cu) nanoparticles were deposited on TiO₂ and tested for the photocatalytic degradation of sulfamethoxazole using either UV-C or simulated sunlight. The optimal loading of metallic nanoparticles was determined as 1.5 wt% for Au and Ag, and 1.0 wt% for Cu. In the case of bi-metallic nanoparticles, only the ratio 1:0.5 wt% for both Au–Ag and Au–Cu was tested. In experiments using UV-C light, the highest degradation performance was found for Ag/TiO₂, while bi-metallic nanoparticles supported on TiO₂ also showed increased photocatalytic activity compared with unmodified TiO₂. In simulated sunlight irradiation tests, Au/TiO₂ showed to be the most efficient material. Complete mineralization of sulfamethoxazole was achieved when surface-modified materials were tested in both UV-C and simulated sunlight experiments. Photolysis was efficient to fully degrade sulfamethoxazole, although mineralization was lower than 10% for both luminic sources. The main by-products of sulfamethoxazole were determined in photolysis and photocatalysis tests using UV-C light, and degradation paths were proposed. By-products showed non-toxicity and low antibiotic activity. Reuse of the catalysts upon three reaction cycles did not result in the loss of activity.

Nowadays, heterogeneous photocatalysis for water treatment and hydrogen production are topics gaining interest for scientists and developers from different areas, such as environmental technology and material science. Most of the efforts and resources are devoted to the development of new photocatalyst materials, while the modeling and development of reaction systems allowing for upscaling the process to pilot or industrial scale are scarce. In this work, we present what is known on the upscaling of heterogeneous photocatalysis to purify water and to produce green H₂. The types of reactors successfully used in water treatment plants are presented as study cases. The challenges of upscaling the photocatalysis process to produce green H₂ are explored from the perspectives of (a) the adaptation of photoreactors, (b) the competitiveness of the process, and (c) safety. Throughout the text, Green Chemistry and Engineering Principles are described and discussed on how they are currently being applied to the heterogeneous photocatalysis process along with the challenges that are ahead. Lastly, the role of automation and high-throughput methods in the upscaling following the Green Principles is discussed.

Heterogeneous photocatalysis is a promising advanced oxidation process for water purification, given its potential to fully oxidize organic pollutants and to inactivate microorganisms. Due to its versatility and high performance in a broad range of conditions, titanium dioxide (TiO2)-based photocatalysis has been systematically used at laboratory scale to treat water of different quality. Even though TiO2 is an exceptional photocatalyst, its broad band gap value (3.2 eV) makes necessary the use of UV light to achieve the photoactivation. This results in the underutilization of the material in sunlight-driven photocatalysis schemes. In order to overcome this handicap, the synthesis of heterostructures using low band gap semiconductors coupled with TiO2 has brought exceptional materials for visible light-driven photocatalysis. In this chapter, the fundamentals of the synthesis and photoactivation of TiO2-low band gap semiconductor heterostructures are explored. The mechanisms leading to the increase of the photocatalytic activity of such heterostructures are described. A summary of the available data on the photocatalytic performance of TiO2-based heterostructures is presented, in terms of degradation of organic pollutants in water using visible light and sunlight. A comparison of the depuration performance of powdered and thin film heterostructures is given at the end of the chapter.

ZnO nanostructures have attracted attention as transducer materials in optical biosensing platforms due to their wide bandgap, defect-mediated photoluminescence, high surface-to-volume ratio, and tunable morphology. This review examines how the dimensionality of ZnO nanostructures affects biosensor performance, particularly in terms of charge transport, signal transduction, and biomolecule immobilization. The synthesis approaches are discussed, highlighting how they influence crystallinity, defect density, and surface functionalization potential. The impact of immobilization strategies on sensor stability and sensitivity is also assessed. The role of ZnO in various optical detection schemes, including photoluminescence, surface plasmon resonance (SPR), localized (LSPR), fluorescence, and surface-enhanced Raman scattering (SERS), is reviewed, with emphasis on label-free and real-time detection. Representative case studies demonstrate the detection of clinically and environmentally relevant targets, such as glucose, dopamine, cancer biomarkers, and SARS-CoV-2 antigens, with limits of detection in the pico- to femtomolar range. Recent developments in ZnO-based hybrid systems and their integration into fiber-optic and microfluidic platforms are explored as scalable solutions for portable, multiplexed diagnostics. The review concludes by outlining current challenges related to reproducibility, long-term operational stability, and surface modification standardization. This work provides a framework for understanding structure–function relationships in ZnO-based biosensors and highlights future directions for their development in biomedical and environmental monitoring applications.