‘Wet-chemical’ bottom-up chemical synthetic routes can lead to nanomaterials with well-defined features, such as size, shape and crystal structure. As a result of this, further physical properties can be tuned, which enable these nanomaterials to be promising for a range of applications. Typical parameters which can be adjusted in order to achieve the production of high-quality nanomaterials are the amount and type of precursors, surfactants, reducing agents and solvents, as well as the reaction temperature. Apart from the reactivity of the above reagents, which affects the growth mechanism and the resulting characteristics of the produced nanoparticles, other factors such as the preference for using non-toxic, eco-friendly, energy-efficient and cost-effective reactants is also taken into account. In addition, researchers bear in mind that apart from the academic insights that can be gained upon pursuing a novel synthetic protocol for a given material, successful experiments may also sometimes open the possibility for ‘real-world’ applications. In that case, issues that might arise from the scale-up of a laboratory–scale reaction to the industrial sector with the objective of commercialization need to also be considered.

Though great progress has been achieved in the colloidal synthesis of a range of different nanomaterials (metals, alloys, oxides, metal chalcogenides, etc.) during the last decades, I felt that the role of reducing agents was somewhat ‘unsung’ compared to that of solvents, precursors and capping ligands. So I decided to invite a team of experts from around the world in order to compose this book, which contains a collection of chapters on different types of reducing agents. Efforts have been made to not only present a summary of the state of the art, but also to give the main insights for each kind of reductant, discussing their roles, together with advantages and limitations. Of course, some reducing agents have dual roles, acting also as surfactants, but this is also discussed in the relevant parts of the book.

In the beginning, the Introduction of the book, after providing a historical point of view, gives some of the main points worth knowing about different types of materials which have been employed so far as reducing agents. Redox processes and redox potentials are also described, when possible.

In Chapter 2, the authors present how alcohols can function in dual roles as solvents and reductants in colloidal nanoparticle synthesis. They show how differences of alcohol chain length or alcohol concentration can lead to the preparation of nanoparticles in a range of morphologies. Polyols are discussed in Chapter 3, where their redox mechanisms are demonstrated, together with the main findings and concepts for the fabrication of metals, oxides and bimetallic nanostructures. Following this in Chapter 4 is the presentation of phenol and its derivatives, which act as effective capping and reducing agents for the acquisition of metal nanoparticles with desired features in a cost-efficient, green and controlled way. Gases (H₂ and CO) are highlighted in Chapter 5. The authors explain that apart from the fact that the use of gases leaves no or few residues at the nanoparticles surface after reaction, gases can also affect the shape evolution mode of the growing nanostructures. In addition, the particles prepared in the presence of excess gases are particularly interesting for a variety of catalytic applications.

Amines and amine-boranes are analyzed in Chapter 6. Examples of these compounds are alkylamines, aryl amines, hydrazine and distinct amine–borane complexes. Differences in the reducing power, toxicity, shape control and stabilization capabilities of those molecules are provided. Chapter 7 addresses the use of various kinds of acids in the synthesis of nanoparticles such as iron, copper, gold and silver. The authors give insights on the use of carboxylic, phenolic, but also amino acids, though the latter ones are the main focus of Chapter 8. Still, Chapter 7 provides interesting pieces of knowledge for those acids, too. In fact, Chapter 8 overviews the role of amino acids and small peptides not only as reductants but also as a matrix to stabilize colloidal nanoparticles.

The role of hydrides as widely known reducing agents in the synthesis of nanoparticles composed of metals, alloys and ceramics is summarized in Chapter 9. Small nanoparticles produced through hydride-mediated routes display remarkable catalytic applications. A fruitful degree of overlap with parts of the content of Chapter 6 will help the reader to access the relevant insights in a complementary manner. For the utilization of polysaccharides as reductants, Chapter 10 provides an overview of the progress achieved on the use of starch, chitosan, dextran and cellulose, among other materials. Stabilizing and structure-directing functions for the resulting nanoparticles are also discussed. The dual capping/reducing operation of other polymeric compounds such as (but not limited to) polyethyleneimines is given in detail in Chapter 11. ‘Green’ materials such as plant derivatives (extracts of leaves, fruits and roots) as well as microorganisms (fungi, bacteria, yeast) are the topic of Chapter 12. How proteins and peptides function during colloidal nanoparticle synthesis is elaborated in Chapter 13. A small degree of overlaps with some of the previous chapters will help the readers to see the discussed topics from different angles. Chapter 14 presents the reducing agents used for the synthesis of silicon nanoparticles and carbon dots. In addition, it shows that those materials (Si particles and C dots) can be used as reductants themselves for noble metal nanoparticle synthesis. The authors describe that the structure of produced nanomaterials depends on the size and surface functional groups of Si particles and C dots. Finally, Chapter 15 analyzes the use of miscellaneous reagents for nanoparticle synthesis. Those materials include dimethylformamide (DMF), H₂O₂, organosilanes, polyoxometalates and CTAB, among others. The authors choose to also describe the use of natural materials such as plant-leaf extracts, and this complements very nicely the content of Chapter 12.

I believe that the publication of this book comes in a timely and worthy manner. Since a critical presentation mode was pursued (when possible), the readers will be able to understand why the so-called ‘green’ methods, though so much desired, have still to go through certain challenges before replacing the more traditional, and sometimes ‘harsh’ chemical reagents. The interest for the book is expected to be multidisciplinary, coming from chemists, materials scientists, biologists and engineers. A better understanding of how reducing agents act in colloidal nanoparticle synthesis will inspire researchers to improve the literature protocols and design their own ones, aiming to better tune nanoparticles' structural features and properties. In this way, the produced nanomaterials will become more competitive for application in a variety of fields.

I would like to thank the main chapter authors (R. Contreras-Caceres, K. Pitchumani, C. Dendrinou-Samara, S. Kinayyigit, K. Soulantica, D. Ciuculescu-Pradines, S. M. Haske, D. Haldar, M. Comesana-Hermo, W. van Zyl, F. A. Yagci, L. Pereira da Costa, G. Zheng, Y. Liu and T. Yonezawa) and their colleagues who have made the book to be of value and rich in content. I also thank Prof. Thanh Nguyen from University College London who planted the idea of my editing a book relevant to my expertise in nanoparticle synthesis. The editorial staff and the production team of the Royal Society of Chemistry are also acknowledged for their help; they always answered my questions in a swift manner.

Stefanos Mourdikoudis

University College London, UK

present address: University of Chemistry and Technology, Prague, Czech Republic