Macromolecular shape and interactions in layer-by-layer assemblies within cylindrical nanopores

Protein-based layer-by-layer films for biomedical applications

The surface engineering of biomaterials is crucial for their successful (bio)integration by the body, i.e. the colonization by the tissue-specific cell, and the prevention of fibrosis and/or bacterial colonization. Performed at room temperature in an aqueous medium, the layer-by-layer (LbL) coating method is based on the alternating deposition of macromolecules. Versatile and simple, this method allows the functionalization of surfaces with proteins, which play a crucial role in several biological mechanisms. Possessing intrinsic properties (cell adhesion, antibacterial, degradable, etc.), protein-based LbL films represent a powerful tool to control bacterial and mammalian cell fate. In this article, after a general introduction to the LbL technique, we will focus on protein-based LbL films addressing different biomedical issues/domains, such as bacterial infection, blood contacting surfaces, mammalian cell adhesion, drug and gene delivery, and bone and neural tissue engineering. We do not consider biosensing applications or electrochemical aspects using specific proteins such as enzymes.

In Situ Study of Layer-by-Layer Polyelectrolyte Deposition in Nanopores of Anodic Aluminum Oxide by Reflectometric Interference Spectroscopy

The modification of cylindrical anodic aluminum oxide (AAO) nanopores by alternating layer-by-layer (LBL) deposition of poly(sodium-4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) was studied in situ by reflectometric interference spectroscopy (RIfS). In particular, the kinetics of polyelectrolyte deposition inside the pores with a diameter of 37 ± 3 nm and a length of 3.7 ± 0.3 μm were unraveled, and potential differences in the LBL multilayer growth compared to flat silicon substrates as well as the effect of different ionic strengths and different types of ions were investigated. RIfS measures the effective optical thicknesses, which is-for a constant pore length-proportional to the effective refractive index of the AAO sample, from which, in turn, the deposited mass of the polymer or the corresponding layer thickness can be estimated. Compared to the multilayer growth by the LBL deposition on the flat aminosilane-primed silicon wafers, which was assessed by spectroscopic ellipsometry, the thickness increment per deposited bilayer, as well as the dependence of this increment on the ionic strength (0.01-0.15) and the counterion type (Na⁺ vs Ca²⁺) inside the aminosilane-primed nanopores, was for the first bilayers to within the experimental error identical. For thicker multilayers, the pore diameter became smaller, which led to reduced thickness increments and eventually virtually completely filled the pores. The observed kinetics is consistent with the mass-transport-limited adsorption of the polyelectrolyte to the charged surface according to a Langmuir isotherm with a negligible desorption rate. In addition to fundamental insights into the buildup of polyelectrolyte multilayers inside the AAO nanopores, our results highlight the sensitivity of RIfS and its use as an analytical tool for probing processes inside the nanopores and for the development of biosensors.

Layered self-assemblies for controlled drug delivery: A translational overview

Self-assembly is a prominent phenomenon observed in nature. Inspired by this thermodynamically favorable approach, several natural and synthetic materials have been investigated to develop functional systems for various biomedical applications, including drug delivery. Furthermore, layered self-assembled systems provide added advantages of tunability and multifunctionality which are crucial for controlled and targeted drug release. Layer-by-layer (LbL) deposition has emerged as one of the most popular, well-established techniques for tailoring such layered self-assemblies. This review aims to provide a brief overview of drug delivery applications using LbL deposition, along with a discussion of associated scalability challenges, technological innovations to overcome them, and prospects for commercial translation of this versatile technique. Additionally, alternative self-assembly techniques such as metal-phenolic networks (MPNs) and Liesegang rings are also reviewed in the context of their recent utilization for controlled drug delivery. Blending the sophistication of these self-assembly phenomena with material science and technological advances can provide a powerful tool to develop smart drug carriers in a scalable manner.

Nanoporous thin films in optical waveguide spectroscopy for chemical analytics

Spectroscopy with planar optical waveguides is still an active field of research for the quantitative analysis of various supramolecular surface architectures and processes, and for applications in integrated optical chip communication, direct chemical sensing, etc. In this contribution, we summarize some recent development in optical waveguide spectroscopy using nanoporous thin films as the planar substrates that can guide the light just as well as bulk thin films. This is because the nanoporosity is at a spacial length-scale that is far below the wavelength of the guided light; hence, it does not lead to an enhanced scattering or additional losses of the optical guided modes. The pores have mainly two effects: they generate an enormous inner surface (up to a factor of 100 higher than the mere geometric dimensions of the planar substrate) and they allow for the exchange of material and charges between the two sides of the solid thin film. We demonstrate this for several different scenarios including anodized aluminum oxide layers for the ultrasensitive determination of the refractive index of fluids, or the label-free detection of small analytes binding from the pore inner volume to receptors immobilized on the pore surface. Using a thin film of Ti metal for the anodization results in a nanotube array offering an even further enhanced inner surface and the possibility to apply electrical potentials via the resulting TiO₂ semiconducting waveguide structure. Nanoporous substrates fabricated from SiN_x thin films by colloid lithography, or made from SiO₂ by e-beam lithography, will be presented as examples where the porosity is used to allow for the passage of ions in the case of tethered lipid bilayer membranes fused on top of the light-guiding layer, or the transport of protons through membranes used in fuel cell applications. The final example that we present concerns the replication of the nanopore structure by polymers in a process that leads to a nanorod array that is equally well suited to guide the light as the mold; however, it opens a totally new field for integrated optics formats for direct chemical and biomedical sensing with an extension to even molecularly imprinted structures. Graphical abstract.

Real time monitoring of layer-by-layer polyelectrolyte deposition and bacterial enzyme detection in nanoporous anodized aluminum oxide

Porous anodized aluminum oxide (pAAO) is a nanostructured material, which due to its optical properties lends itself to the design of optical biosensors where interactions in the pores of this material are transduced into interferometric reflectance shifts. In this study, a pAAO-based biosensor was developed as a biosensing platform to detect proteinase K, an enzyme which is a readily available model system for the proteinase produced by Pseudomonas aeruginosa. The pAAO pore walls are decorated by means of the layer-by-layer (LbL) deposition technique using poly(sodium-4-styrenesulfonate) and poly-l-lysine as negatively and positively charged polyelectrolytes, respectively. Interferometric reflectance spectroscopy utilized to observe the optical properties of pAAO during LbL deposition shows that the deposition of the polyelectrolyte onto the pore walls increases the net refractive index, thus red-shifting the effective optical thickness (EOT). Upon incubation with proteinase K, a conspicuous blue shift of the EOT is observed, which is attributed to the destabilization of the LbL film upon enzymatic degradation of the poly-l-lysine components. This result is confirmed by scanning electron microscopy results. Finally, as a proof-of-principle, we demonstrate the ability of the label-free pAAO-based biosensing platform to detect the presence of the proteinase K in human wound fluid, highlighting the potential for detection of bacterial infections in chronic wounds.

Sorption of hydrophilic dyes on anodic aluminium oxide films and application to pH sensing

The sorption of selected hydrophilic pH-sensitive dyes (bromophenol blue, bromothymol blue, bromocresol purple, alizarin red, methyl orange, congo red, rhodamine 6G) on films of anodized aluminium oxide (AAO) was investigated in this study.