In recent years, many researchers have begun to shift their focus onto the synthesis of nanomaterials as this field possesses an immense potential that may provide incredible technological advances in the near future. The downside of conventional synthesis techniques, such as co-precipitation, sol-gel and hydrothermal methods, is that they necessitate toxic chemicals, produce harmful by-products and require a considerable amount of energy; therefore, more sustainable fabrication routes are sought-after. Biological molecules have been previously utilized as precursors for nanoparticle synthesis, thus eliminating the negative factors involved in traditional methods. In addition, transition-metal nanoparticles possess a broad scope of applications due to their multiple oxidation states and large surface areas, thereby allowing for a higher reactivity when compared to their bulk counterpart and rendering them an interesting research topic. However, this field is still relatively unknown and unpredictable as the biosynthesis of these nanostructures from fungi, bacteria and plants yield undesired diameters and morphologies, rendering them redundant compared to their chemically synthesized counterparts. Therefore, this review aims to obtain a better understanding on the plant-mediated synthesis process of the major transition-metal and transition-metal oxide nanoparticles, and how process parameters—concentration, temperature, contact time, pH level, and calcination temperature affect their unique properties such as particle size, morphologies, and crystallinity.

Human breath investigation offers a non-invasive and quick strategy for recognizing different volatile organic compounds (VOCs) that are markers for various diseases. Scientists have shown that breath acetone is a successful biomarker of Type 2 diabetes which is the most common type of diabetes. The generation of acetone is a consequence of the body processing fats as an alternative of glucose to produce energy. Thus, detection of breath acetone can be a rapid, viable, and patient compliant alternative to the conventional methods of blood glucose determination. To achieve this goal, metal oxide nanostructures with various shapes through different synthesis routes in the nanometer scale, can be used. Owing to its properties such as high surface-to-volume ratios and subsequently large number of surface sites exposed to acetone gas, metal oxide nanostructures facilitate a well-built gas-sensing layer interaction and consequently compared to conventional materials, present a higher sensitivity. This work, presents the progress in metal oxides nanostructures (semiconductor nanomaterials) as gas sensing materials for the exhaled acetone detection, which offers the possibility to help people living with diabetes to screen their disease. The different types of metal oxides materials used in Breath acetone sensors, their limitations and future perspectives have been highlighted.