Methods for depositing thin films are important in generating functional materials for diverse applications in a wide variety of fields. Over the last half-century, the layer-by-layer assembly of nanoscale films has received intense and growing interest. This has been fueled by innovation in the available materials and assembly technologies, as well as the film-characterization techniques. In this Review, we explore, discuss, and detail innovation in layer-by-layer assembly in terms of past and present developments, and we highlight how these might guide future advances. A particular focus is on conventional and early developments that have only recently regained interest in the layer-by-layer assembly field. We then review unconventional assemblies and approaches that have been gaining popularity, which include inorganic/organic hybrid materials, cells and tissues, and the use of stereocomplexation, patterning, and dip-pen lithography, to name a few. A relatively recent development is the use of layer-by-layer assembly materials and techniques to assemble films in a single continuous step. We name this "quasi"-layer-by-layer assembly and discuss the impacts and innovations surrounding this approach. Finally, the application of characterization methods to monitor and evaluate layer-by-layer assembly is discussed, as innovation in this area is often overlooked but is essential for development of the field. While we intend for this Review to be easily accessible and act as a guide to researchers new to layer-by-layer assembly, we also believe it will provide insight to current researchers in the field and help guide future developments and innovation.

Coordination chemistry of natural polyphenols and transition metals allows rapid self-assembly of conformal coatings on diverse substrates. Herein, we report that this coordination-driven self-assembly process applies to simple phenolic molecules with monotopic or ditopic chelating sites (as opposed to macromolecular, multitopic polyphenols), leading to surface-confined amorphous films upon metal coordination. Films fabricated from gallic acid, pyrogallol, and pyrocatechol, which are the major monomeric building blocks of polyphenols, have been studied in detail. Pyrocatechol, with one vicinal diol group (i.e., bidentate), has been observed to be the limiting case for such assembly. This study expands the toolbox of available phenolic ligands for the formation of surface-confined amorphous films, which may find application in catalysis, energy, optoelectronics, and the biomedical sciences. ■ INTRODUCTIONModular control over the rational design of supramolecular architectures has been achieved in the last two decades by smart engineering of coordination-driven self-assembly processes. 1 Early prediction of the inherent preferences for directionality and binding affinity within the complementary building blocks of coordination complexes has paved the way for fabricating structures with extended networks of metal clusters bridged by compatible organic ligands. 2,3 Porous coordination polymers or metal−organic frameworks (MOFs) with distinct spatial and geometrical arrangements of the interconnecting motifs are examples of such organic−inorganic hybrid materials. 4−7 These crystalline materials with structurally encoded nano-and microporosities have potential application for gas storage, separations, and sensing. 8−13 On the other hand, surface-bound or freestanding amorphous thin films/coatings are another class of network materials of importance in several branches of science, 14−16 where polymeric compounds are commonly used structural components. Research has also focused on exploring novel strategies to incorporate inorganic moieties in polymeric films to obtain functional hybrid materials that exploit the synergistic effects of the organic and inorganic constituents. 17,18 In this context, processes utilizing self-assembly of coordination complexes are a promising strategy toward facile engineering of thin films with defined properties.Recently, we reported a facile assembly approach that exploits metal−polyphenol interactions, specifically between tannic acid (TA) and iron(III) (Fe III ) ions, to form thin films. 19 Our interest in these metal−polyphenol systems arises from the facile and versatile nature of the assembly process, which produces tunable, dynamic materials. Using TA as a ligand, we demonstrated the formation of capsules with engineered pHresponsive degradation, luminescence, and positron emission, by judicious choice of the incorporated metal, 20 as well as pHresponsive drug delivery vectors 21 and cytoprotective coatings. 22 Furthermore, we reported the assembly of Fe IIIpolyphenol capsules fro...

Over the past few decades, nanoengineered particles have gained increasing interest for applications in the biomedical realm, including diagnosis, imaging, and therapy. When functionalized with targeting ligands, these particles have the potential to interact with specific cells and tissues, and accumulate at desired target sites, reducing side effects and improve overall efficacy in applications such as vaccination and drug delivery. However, when targeted particles enter a complex biological environment, the adsorption of biomolecules and the formation of a surface coating (e.g., a protein corona) changes the properties of the carriers and can render their behavior unpredictable. For this reason, it is of importance to consider the potential challenges imposed by the biological environment at the early stages of particle design. This review describes parameters that affect the targeting ability of particulate drug carriers, with an emphasis on the effect of the protein corona. We highlight strategies for exploiting the protein corona to improve the targeting ability of particles. Finally, we provide suggestions for complementing current in vitro assays used for the evaluation of targeting and carrier efficacy with new and emerging techniques (e.g., 3D models and flow‐based technologies) to advance fundamental understanding in bio‐nano science and to accelerate the development of targeted particles for biomedical applications.

Ezrin is a membrane-cytoskeleton linker protein that can bind F-actin in its active conformation. Several means of regulation of ezrin's activity have been described including phosphorylation of Thr-567 and binding of L-α-phosphatidylinositol-4,5-bisphosphate (PIP(2)). However, the relative contributions of these events toward activation of the protein and their potential interdependence are not known. We developed an assay based on solid-supported membranes, to which different ezrin mutants (ezrin T567A (inactive mutant), wild-type, and T567D (active pseudophosphorylated mutant)) were bound, that enabled us to analyze the influence of phosphorylation and PIP(2) binding on ezrin's activation state in vitro. The lipid bilayers employed contained either DOGS-NTA-Ni to bind the proteins via an N-terminal His-tag, or PIP(2), to which ezrin binds via specific binding sites located in the N-terminal region of the protein. Quantitative analysis of the binding behavior of all three proteins to the two different receptor lipids revealed that all three bind with high affinity and specificity to the two receptor lipids. Fluorescence microscopy on ezrin-decorated solid-supported membranes showed that, dependent on the mode of binding and the phosphorylation state, ezrin is capable of binding actin filaments. A clear synergism between phosphorylation and the receptor lipid PIP(2) was observed, suggesting a conformational switch from the dormant to the active, F-actin binding state by recognition of PIP(2), which is enhanced by the phosphorylation.

Phosphoinositides and in particular L-α-phosphatidylinositol-4,5-bisphosphate (PIP2) are key lipids controlling many cellular events and serve as receptors for a large number of intracellular proteins. To quantitatively analyze protein-PIP2 interactions in vitro in a time-resolved manner, planar membranes on solid substrates are highly desirable. Here, we describe an optimized protocol to form PIP2 containing planar solid supported membranes on silicon surfaces by vesicle spreading. Supported lipid bilayers (SLBs) were obtained by spreading POPC/PIP2 (92:8) small unilamellar vesicles onto hydrophilic silicon substrates at a low pH of 4.8. These membranes were capable of binding ezrin, resulting in large protein coverage as concluded from reflectometric interference spectroscopy and fluorescence microscopy. As deduced from fluorescence microscopy, only under low pH conditions, a homogeneously appearing distribution of fluorescently labeled PIP2 molecules in the membrane was achieved. Fluorescence recovery after photobleaching experiments revealed that PIP2 is not mobile in the bottom layer of the SLBs, while PIP2 is fully mobile in the top layer with diffusion coefficients of about 3 μm(2)/s. This diffusion coefficient was considerably reduced by a factor of about 3 if ezrin has been bound to PIP2 in the membrane.