Metal nanoclusters (NCs) are nanocrystalline structures composed of tens of atoms that have attracted the attention of the scientific community in the last decades because of their strong photoluminescence and remarkable chemical properties. They represent the bridge between larger metal nanoparticles and organometallic compounds: due to a strong quantum confinement effect, NCs of sizes below 10 nm possess a distinctive molecule-like behavior upon interaction with an electromagnetic field, which is lost in larger nano-objects (Chakraborty and Pradeep, 2017). The first generation of metal NCs was formed by single noble metals such as gold, silver, platinum, copper, etc. (Lu and Chen, 2012) but nowadays the tendency in the field is oriented towards the development of bimetallic NCs or alloys to improve or have synergistic effects on their final optoelectronic and chemical properties. Furthermore, metal NCs exhibit size-tunable photoluminescence ranging from UV to the NIR, high photostability, two-photon absorption, and electroluminescent properties. These characteristics together with chemical ones have enabled the development of a wide panel of applications going from (bio)-sensing and catalysis to bioimaging and therapy (Du et al., 2020; Porret et al., 2020; Tang et al., 2020; Zare et al., 2021). In this research topic, by working with Au-Ag bimetallic NCs, B. Peng et al. demonstrated that the characteristic dual emission observed in this system depends on the pH and the degree of Ag substitution, so the system can be applied for ratiometric pH measurements.
A particularity, from a synthesis point of view, is that NCs need to be synthesized in the presence of a scaffold (ligands, polymers, biomolecules). These scaffolds also play a pivotal role in the optoelectronic properties of the obtained NC as they will modulate the final size and structure of the nanomaterial (Diez and Ras, 2011; Zare et al., 2021). Because of that, the origin of their photoluminescent properties has been unclear since its infancy, fostering thus dedicated research to elucidate these mechanisms. Mainly, two possibilities have been described that do not exclude each other. On one hand, the optoelectronic properties should be related to the intrinsic atomic energy levels of the metal core and thus governed by the quantum confinement. The second component that should be considered is the cluster’s surface where the properties will be governed by metal-ligand interactions (Yang et al., 2020). In this respect, throughout this Research Topic, the reader will find interesting works dedicated to shedding new light on the origin and modulation of the optical properties of bimetallic and metal alloy NCs. Peng et al. were able to establish the origin of the dual emission observed in Au-Ag bimetallic NCs and to attribute it to structural water molecules (SW) found at the interface between the NC surface and the ligand, thereby solving the dilemma between quantum size confinement effect and ligand-related surface mechanisms in this type of metal NCs. Using 1-dodecanethiol as ligand, dual emission at 440 and 630 nm could be observed and assigned to a single emitter, SW, presenting varying binding strengths with surface Au(I)- and Ag(I)- thiolate motifs. Furthermore, by tuning the amount of copper surface doping in [Au9Ag12(SAdm)4 (Dppm)6Cl6](SbF6)3 bimetallic NCs to form a trimetallic one [Au9Ag8Cu4(SAdm)4 (Dppm)6Cl6](SbF6)3, Deng et al. concluded that Cu dopants replace the Ag position in the peripheral DppmAg2Cl2(SR)2 structures and are crucial for explaining the changes in the optical properties of the parent Au9Ag12 NC, thereby highlighting the role of surface properties in the final optoelectronic behavior. However, since Cu oxidizes quickly, this trimetallic NC had time-dependent properties that fade over time under oxidizing environments while its parent bimetallic NC is unaffected.
Particularly, metal NCs are well suited for in vivo biomedical applications owing to their high biocompatibility (non-toxic metal cores and biocompatible surface chemistries), anti-fouling properties stemming from selected ligands allowing good circulation in the body, and ultrasmall sizes enabling a renal clearance pathway. In this special Research Topic, the use of Au NCs as fluorescence imaging contrast agents is exposed, as well as the use of this nanomaterial as a drug delivery system. Regarding the former, Hada et al. prepared bovine serum albumin (BSA)-stabilized gold NCs and carefully investigated their photoluminescence properties through fluorescence lifetime imaging microscopy (FLIM) under one and two-photon excitations, as well as via fluorescence spectroscopy. The nanostructures were characterized by photoluminescence in the first biological window, high photostability under continuous irradiation, and low photobleaching, which, together with their effective detection in agarose phantoms mimicking tissues, confirmed their aptitude to be employed as fluorescent nanotools in tissue bioimaging. Regarding their use as drug delivery vehicles, Wei et al. combined BSA-stabilized Au NCs with curcumin (a natural plant extract with poor water solubility and a regulatory effect on lipid metabolism) to form a new drug, NC-curcumin, with enhanced water solubility and therefore improved cell delivery potential. This nanosystem could effectively be endocytosed by H9c2 cardiomyocyte cells and employed to reduce intracellular lipid accumulation. Furthermore, the system had also an effect at reducing the increase in reactive oxygen species that occurs following lipid imbalances, therefore diminishing the risk of cell death.
While composed of a reduced number of contributions, this Research Topic addresses the key points in the NCs field and illustrates well the forefront and the trends of this promising group of nanomaterials. Therefore, we encourage the readers to explore it.
Statements
Author contributions
LTA and MT wrote the editorial, which was revised, proofed, and accepted by all the authors.
Acknowledgments
The Topic Editors deeply acknowledge the authors of all contributions composing this Research Topic. They also acknowledge the constructive comments and engagement of all the reviewers, and the editorial support from Frontiers throughout the publication process. MT wishes to thank CONICET and the Agencia Nacional de Promoción de la Investigación, el Desarrollo Tecnológico y la Innovación for funding.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
References
1
ChakrabortyI.PradeepT. (2017). Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles. Chem. Rev.117 (12), 8208–8271. 10.1021/acs.chemrev.6b00769
2
DíezI.RasR. H. A. (2011). Fluorescent Silver Nanoclusters. Nanoscale3, 1963–1970. 10.1039/c1nr00006c
3
DuY.ShengH.AstrucD.ZhuM. (2020). Atomically Precise Noble Metal Nanoclusters as Efficient Catalysts: A Bridge between Structure and Properties. Chem. Rev.120 (2), 526–622. 10.1021/acs.chemrev.8b00726
4
LuY.ChenW. (2012). Sub-nanometre Sized Metal Clusters: from Synthetic Challenges to the Unique Property Discoveries. Chem. Soc. Rev.41, 3594–3623. 10.1039/c2cs15325d
5
PorretE.Le GuévelX.CollJ.-L. (2020). Gold Nanoclusters for Biomedical Applications: toward In Vivo Studies. J. Mater. Chem. B8, 2216–2232. 10.1039/c9tb02767j
6
TangJ.ShiH.MaG.LuoL.TangZ. (2020). Ultrasmall Au and Ag Nanoclusters for Biomedical Applications: A Review. Front. Bioeng. Biotechnol.8, 1019. 10.3389/fbioe.2020.01019
7
YangT.-Q.PengB.ShanB.-Q.ZongY.-X.JiangJ.-G.WuP.et al (2020). Origin of the Photoluminescence of Metal Nanoclusters: From Metal-Centered Emission to Ligand-Centered Emission. Nanomaterials10, 261. 10.3390/nano10020261
8
ZareI.ChevrierD. M.Cienfuentes-RiusA.MoradiN.XianyuY.GhoshS.et al (2021). Protein-protected Metal Nanoclusters as Diagnostic and Therapeutic Platforms for Biomedical Applications. Mater. Today. 10.1016/j.mattod.2020.10.027
Summary
Keywords
synthesis, metal nanoclusters (NCs), bioapplications, optical properties, characterization
Citation
Trapiella-Alfonso L, Tasso M, Ramírez García G, Martín-Yerga D and Montoro Bustos AR (2022) Editorial: Design, Synthesis, Characterization and Applications of Nanoclusters. Front. Chem. 10:898480. doi: 10.3389/fchem.2022.898480
Received
17 March 2022
Accepted
21 March 2022
Published
26 April 2022
Volume
10 - 2022
Edited and reviewed by
Chen Zhou, University of Central Missouri, United States
Updates
Copyright
© 2022 Trapiella-Alfonso, Tasso, Ramírez García, Martín-Yerga and Montoro Bustos.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Antonio R. Montoro Bustos, antonio.montorobustos@nist.gov
‡These authors have contributed equally to this work and share first authorship
ORCID ID:
Laura Trapiella-Alfonso
Marian Tass
Gonzalo Ramirez Garcia
Daniel Martin-Yerga
Antonio R. Montoro Bustos
This article was submitted to Nanoscience, a section of the journal Frontiers in Chemistry
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.