DOI: 10.1002/cssc.201402126 Unveiling the Chemistry behind the Green Synthesis of Metal Nanoparticles Sónia A. O. Santos,* [a] Ricardo J. B. Pinto,* [a, b] Sílvia M. Rocha, [c] Paula A. A. P. Marques, [b] Carlos Pascoal Neto, [a] Armando J. D. Silvestre, [a] and Carmen S. R. Freire [a] Introduction Nanometer-scale metallic particles are probably one of the main classes of nanomaterials investigated and used currently because of the wide spectrum of potential applications in sev- eral fields that arises from their unique and distinct properties in relation to the bulk counterparts. [1] The recent advances in the fields of nanoscience and nanotechnology lead to a variety of physical and chemical strategies for the selective prepara- tion of inorganic nanoparticles (NPs) with precise control over their shape and dimensionality. [2] Among the chemical method- ologies, those used most commonly are based on soluble metal salt precursors and different reducing agents, which can act also as stabilizers to avoid the coalescence of the NPs. However, most of the reducing agents reported, for example, hydrazine, sodium borohydride, or N,N-dimethylformamide, are reactive chemicals that are commonly associated with huge environmental risks and toxicity. [3] The green synthesis of metal NPs has been studied as a relia- ble and promising alternative to minimize or eliminate the use of these harmful substances. [4] Green synthetic processes com- prise either micro-organisms, such as bacteria [5] or fungi, [6] as well as plant biomass [7] and several plant extracts. [8] The use of plant extracts from different morphological parts, which in- clude leaves, fruits, seeds, and bark, presents some advantages over the use of micro-organisms because of the high diversity and abundance of plant extracts and the simplicity, easy scale- up, and cost-effectiveness of the method. [4, 9, 10] Despite the remarkable number of reports in the literature on the synthesis of metal NPs using distinct plant extracts, as highlighted recently by Iravani, [10] a precise understanding of their formation mechanism and the clear-cut role of the natu- ral compounds involved in this process is still imprecise. Some reports have put forward hypothetical mechanisms that pro- pose that the reduction of the metal ions to metal NPs may be caused by the distinct compounds present in the extracts, such as reducing sugars, phenolic compounds (e.g., flavo- noids), and proteins. [9, 11] However, these generic suggestions have been based essentially on qualitative routine spectropho- tometric analysis, such as FTIR spectroscopy, colorimetric assays, or UV spectroscopy, [12] which cannot unambiguously differentiate the presence or absence of the different families of compounds in the extracts and resulting NPs. In this vein, and in consideration of the relevance of such fundamental knowledge, particularly in the optimization and control of the final properties of the NPs and on the up-scaling of these green processes, in the present study we intended to clarify the role of the different components of a plant extract in a NP formation mechanism, kinetics, and stabilization. Here, an aqueous extract of Eucalyptus globulus Labill. bark, a typical residue from the pulp and paper industry that is rec- ognized for its high content of phenolic compounds, [13] was Nanobiotechnology has emerged as a fundamental domain in modern science, and metallic nanoparticles (NPs) are one of the largest classes of NPs studied because of their wide spec- trum of possible applications in several fields. The use of plant extracts as reducing and stabilizing agents in their synthesis is an interesting and reliable alternative to conventional method- ologies. However, the role of the different components of such extracts in the reduction/stabilization of metal ions has not yet been understood clearly. Here we studied the behavior of the main components of a Eucalyptus globulus Labill. bark aqueous extract during metal-ion reduction followed by advanced chro- matographic techniques, which allowed us to establish their specific role in the process. The obtained results showed that phenolic compounds, particularly galloyl derivatives, are mainly responsible for the metal-ion reduction, whereas sugars are essentially involved in the stabilization of the NPs. [a] Dr. S. A. O. Santos, + Dr. R. J. B. Pinto, + Prof. Dr. C. P. Neto, Prof. Dr. A. J. D. Silvestre, Dr. C. S. R. Freire Department of Chemistry-CICECO University of Aveiro 3810-193 Aveiro (Portugal) Fax: (+ 351) 234-401-470 E-mail : santos.sonia@ua.pt r.pinto@ua.pt [b] Dr. R. J. B. Pinto, + Dr. P. A. A. P. Marques TEMA-NRD, Mechanical Engineering Department and Aveiro Institute of Nanotechnology (AIN) University of Aveiro 3810-193 Aveiro (Portugal) [c] Prof. Dr. S. M. Rocha Department of Chemistry-QOPNA University of Aveiro 3810-193 Aveiro (Portugal) [ + ] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402126.  2014 Wiley-VCH Verlag GmbH & Co. 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