DOI: 10.1002/ejic.201800709 Microreview Antibacterial Surfaces Self-Assembled Monolayers of Silver Nanoparticles: From Intrinsic to Switchable Inorganic Antibacterial Surfaces Piersandro Pallavicini,* [a] Giacomo Dacarro, [a] and Angelo Taglietti* [a] Abstract: The layer-by-layer technique allows to graft molec- ular monolayers on bulk surfaces that, in turn, allow to graft monolayers of metal nanoparticles. This microreview focuses on the preparation of such materials featuring a monolayer of sil- ver nanoparticles (AgNP) and their use as antimicrobial surfaces against both planktonic bacteria and biofilms. The role of Ag + release and of direct cell/AgNP contact in the antibacterial ac- tion will be stressed as a function of the adhesive molecular layer, of the AgNP dimension and shape, of their surface den- 1. Introduction The formation of molecular monolayers on bulk surfaces is an important branch of nanochemistry, as the surfaces bearing such monolayers are dimensionally modified on the nanoscale. Implications are enormous, because a simple molecular mono- layer allows the modification of the overall properties of the bulk material interfacing with a medium, e.g. hydrophilicity, hy- drophobicity, solvophobicity, fouling, fogging, conducibility. Last but not least, molecular monolayers deeply influence the behavior of a bulk object in a biologically environment, chang- ing its interactions with cells and tissues, including adhesion, biocompatibility, cytotoxicity, scaffolding and cell growth stimu- lation. [1] An arsenal of materials and of molecules populate the literature in this area. The rationale (Scheme 1A) is to use a molecule featuring a function X capable of specific, stable bind- ing with the atoms of a given surface. The largest number of examples regards gold as a surface and S as the binding group. In the late 80s and early 90s it has been shown by the many papers of scientist like Whitesides, [2,3] Nuzzo [4] and Allara [5] that the gold–sulfur interaction is coordinative (homolytic bond strength = 40 kJ/mol [6,7] ) forming reversible, labile R–S – –Au I bonds. In this case one can properly talk of self-assembled monolayers of R-SH molecules on Au (flat) surfaces, as ordinate, 2D-crystalline structures are obtained in a typical self-assem- bling process, that allows self-correction of architectural mis- takes, as defined by the principles of supramolecular chemis- try. [8] Less properly defined self-assembled monolayers are ob- [a] Department of Chemistry, University of Pavia, viale Taramelli, 12 - 27100 Pavia, Italy E-mail: piersandro.pallavicini@unipv.it angelo.taglietti@unipv.it http://www-5.unipv.it/inlab/ ORCID(s) from the author(s) for this article is/are available on the WWW under https://doi.org/10.1002/ejic.201800709. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 sity, and of the molecular overcoating. While these surfaces dis- play an intrinsic antibacterial action, a further evolution will also be reviewed, in which additional photothermal antibacterial ac- tion can be switched on demand, using near-IR radiation and non-spherical AgNP or a combination of AgNP with non-spheri- cal AuNP. The intrinsic and switchable photothermal action of these surfaces will be unraveled, and their synergistic effect stressed. tained with the silane-SiO 2 surface chemistry, that after Au/S is the most popular grafting function/material couple. [9,10] In this case, R-Si(OR) 3 trialkoxysilanes or R-SiCl 3 trichlorosilanes react with the Si-OH groups on the surface of silica, glass or quartz to form covalent R–Si–O–Si(surface) bonds. Once formed these bonds are not reversible, as a consequence the obtained mono- layer is less ordered (and vertical polymerization is possible), but these materials profit from a much more stable grafting of the R-Si unit, allowing to prepare mechanically and chemically resistant materials. After that the concept of (self)assembled monolayers was established, the idea of the layer-by-layer tech- nique came as an almost obvious consequence. In a seminal paper in 1997, G. Decher [11] introduced the idea of grafting first a (mono)layer of a given molecule M on a bulk surface [S] and then to lay a series of further different (mono)layers one after the other, by successive dipping processes in a solution contain- ing the molecule (or polymer) to be grafted, e.g. N, Scheme 1B. The driving force is the preferential interaction between the moieties of the molecular layer exposed to the solvent and a moiety (or the whole) of the molecule or polymer in solution. The obtained surface can be made in a [S]–M–N–M–N.. replicat- ing fashion (Scheme 1C), or in a [S]–M–N–P–Q.. fashion (Scheme 1D), in principle continuing ad libitum the layering process. Modification of a bulk surface with a monolayer or with mul- tiple molecular or nanoparticles layers can be an answer to the problem of microbial infections spread by surfaces of common use (e.g. touch screens in hospitals) or the problem of the for- mation of bacterial biofilms on the surface of internalized medi- cal devices like prostheses and catheters. [12–15] Biofilms are sessile microbial communities with a strong mechanical and biological resistance due mainly to their self-produced extracel- lular polymeric matrix [16–18] made of polysaccharides and pro- teins. Biofilms form after adhesion of planktonic bacteria to a