Ion-Fluid Transport-Control Feedback along Nanopore Networks

Biological signaling correlates with the interrelation between ion and nanofluidic transportation pathways. However, artificial embodies with reconfigurable ion-fluid transport interaction aspects remain largely elusive. Herein, we unveiled an intimate interplay between nanopore-driven advancing flow and ion carriage for the spontaneous imbibition of aqueous solutions at the nanoporous thin film level. Ionic factors dominate transport phenomena processing and integration (ions influence fluid motion, which in turn governs the self-regulated ion traveling). We show an ion-induced translation effect that finely converts a chemical input, the nature of ions, into a related fluidic output: modulation of the extent of imbibition. We further find complex imbibition dynamics induced by the ion type and population. We peculiarly pinpoint a stop-and-go effective transport process with a programmable delay time triggered by selective guest-host interactions. The ion-fluid transport interplay is captured by a simple model that considers the counterbalance between the capillary infiltration and solution concentration, owing to water loss at the nanoporous film-air interface. Our results demonstrate that nanopore networks present fresh scenarios for understanding and controlling autonomous macroscopic liquid locomotion and offer a distinctive working principle for smart ion operation.

Ordered Mesoporous Electrodes for Sensing Applications

Electrochemical sensors have become increasingly relevant in fields such as medicine, environmental monitoring, and industrial process control. Selectivity, specificity, sensitivity, signal reproducibility, and robustness are among the most important challenges for their development, especially when the target compound is present in low concentrations or in complex analytical matrices. In this context, electrode modification with Mesoporous Thin Films (MTFs) has aroused great interest in the past years. MTFs present high surface area, uniform pore distribution, and tunable pore size. Furthermore, they offer a wide variety of electrochemical signal modulation possibilities through molecular sieving, electrostatic or steric exclusion, and preconcentration effects which are due to mesopore confinement and surface functionalization. In order to fully exploit these advantages, it is central to develop reproducible routes for sensitive, selective, and robust MTF-modified electrodes. In addition, it is necessary to understand the complex mass and charge transport processes that take place through the film (particularly in the mesopores, pore surfaces, and interfaces) and on the electrode in order to design future intelligent and adaptive sensors. We present here an overview of MTFs applied to electrochemical sensing, in which we address their fabrication methods and the transport processes that are critical to the electrode response. We also summarize the current applications in biosensing and electroanalysis, as well as the challenges and opportunities brought by integrating MTF synthesis with electrode microfabrication, which is critical when moving from laboratory work to in situ sensing in the field of interest.

Electroinduced Surfactant Self-Assembly Driven to Vertical Growth of Oriented Mesoporous Films

Supramolecular soft-templating approaches to mesoporous materials have revolutionized the generation of regular nanoarchitectures exhibiting unique features such as uniform pore structure with tunable dimensions, large surface area, and high pore volume, variability of composition, and/or ease of functionalization with a wide range of organo-functional groups or good hosts for the in situ synthesis of nano-objects. One appealing concept in this field is the development of ordered mesoporous thin films as such a configuration has proven to be essential for various applications including separation, sensing, catalysis (electro and photo), energy conversion and storage, photonics, solar cells, photo- and electrochromism, and low-k dielectric coatings for microelectronics, bio and nanobio devices, or biomimetic surfaces. Supported or free-standing mesoporous films are mostly prepared by evaporation induced self-assembly methods, thanks to their good processing capability and flexibility to manufacture mesostructured oxides and organic-inorganic hybrids films with periodically organized porosity.One important challenge is the control of pore orientation, especially in one-dimensional nanostructures, which is not straightforward from the above evaporation induced self-assembly methods. Accessibility of the pores represents another critical issue, which can be basically ensured in the event of effective interconnections between the pores, but the vertical alignment of mesopore channels will definitely offer the best configuration to secure the most efficient transfer processes through the mesoporous membranes. The orthogonal growth of mesochannels is however not thermodynamically favored, requiring the development of methods enabling self-organization through nonequilibrium states. We found that electrochemistry afforded a real boon to tackle this problem via the electrochemically assisted self-assembly (EASA) method, which not only provides a fast and versatile way to generate highly ordered and hexagonally packed mesopore channels but also constitutes a real platform for the development of functionalized oriented films carrying a wide range of organo-functional groups of adjustable composition and properties.This Account introduces the EASA concept and discusses its development along with the significant progress made from its discovery, notably in view of recent advances on the functionalization of oriented mesoporous silica films, which expand their fields of application. EASA is based on the in situ combination of electrochemically triggered pH-induced polycondensation of silica precursors with electrochemical interfacial surfactant templating, leading to the very fast (a few seconds) growth of vertically aligned silica walls through self-assembly around surfactant hemimicelles transiently formed onto the underlying support. This method benefits from the possibility to deposit uniform thin films onto surfaces of different natures and complex morphologies including at the microscale. From this discovery, our research expanded to cover domains beyond the simple production of bare silica films, turning to the challenge of incorporation and exploitation of organo-functional groups or nanofilaments. So far, the great majority of methods developed for the functionalization of mesoporous silica is based on postsynthesis grafting or co-condensation approaches, which suffer from serious limitations with oriented films (pore blocking, lack of ordering). We demonstrated the uniqueness of EASA combined with click chemistry to afford a versatile and universal route to oriented mesoporous films bearing organo-functional groups of multiple composition. This opened perspectives for future developments and applications, some of which (sensing, permselective coatings, energy storage, electrocatalysis, electrochromism) are also considered in this Account.

Fractionation of block copolymers for pore size control and reduced dispersity in mesoporous inorganic thin films

Mesoporous inorganic thin films are promising materials architectures for a variety of applications, including sensing, catalysis, protective coatings, energy generation and storage. In many cases, precise control over a bicontinuous porous network on the 10 nm length scale is crucial for their operation. A particularly promising route for structure formation utilizes block copolymer (BCP) micelles in solution as sacrificial structure-directing agents for the co-assembly of inorganic precursors. This method offers pore size control via the molecular weight of the pore forming block and is compatible with a broad materials library. On the other hand, the molecular weight dependence impedes continuous pore tuning and the intrinsic polymer dispersity presents challenges to the pore size homogeneity. To this end, we demonstrate how chromatographic fractionation of BCPs provides a powerful method to control the pore size and dispersity of the resulting mesoporous thin films. We apply a semi-preparative size exclusion chromatographic fractionation to a polydisperse poly(isobutylene)-block-poly(ethylene oxide) (PIB-b-PEO) BCP obtained from scaled-up synthesis. The isolation of BCP fractions with distinct molecular weight and narrowed dispersity allowed us to not only tune the characteristic pore size from 9.1 ± 1.5 to 14.1 ± 2.1 nm with the identical BCP source material, but also significantly reduce the pore size dispersity compared to the non-fractionated BCP. Our findings offer a route to obtain a library of monodisperse BCPs from a polydisperse feedstock and provide important insights on the direct relationship between macromolecular characteristics and the resulting structure-directed mesopores, in particular related to dispersity.

Unraveling the molecular interactions involved in phase separation of glucocorticoid receptor

BACKGROUND

Functional compartmentalization has emerged as an important factor modulating the kinetics and specificity of biochemical reactions in the nucleus, including those involved in transcriptional regulation. The glucocorticoid receptor (GR) is a ligand-activated transcription factor that translocates to the nucleus upon hormone stimulation and distributes between the nucleoplasm and membraneless compartments named nuclear foci. While a liquid-liquid phase separation process has been recently proposed to drive the formation of many nuclear compartments, the mechanisms governing the heterogeneous organization of GR in the nucleus and the functional relevance of foci formation remain elusive.

RESULTS

We dissected some of the molecular interactions involved in the formation of GR condensates and analyzed the GR structural determinants relevant to this process. We show that GR foci present properties consistent with those expected for biomolecular condensates formed by a liquid-liquid phase separation process in living human cells. Their formation requires an initial interaction of GR with certain chromatin regions at specific locations within the nucleus. Surprisingly, the intrinsically disordered region of GR is not essential for condensate formation, in contrast to many nuclear proteins that require disordered regions to phase separate, while the ligand-binding domain seems essential for that process. We finally show that GR condensates include Mediator, a protein complex involved in transcription regulation.

CONCLUSIONS

We show that GR foci have properties of liquid condensates and propose that active GR molecules interact with chromatin and recruit multivalent cofactors whose interactions with additional molecules lead to the formation of a focus. The biological relevance of the interactions occurring in GR condensates supports their involvement in transcription regulation.

Tuning Pore Dimensions of Mesoporous Inorganic Films by Homopolymer Swelling

The functionality and applications of mesoporous inorganic films are closely linked to their mesopore dimensions. For material architectures derived from a block copolymer (BCP) micelle coassembly, the pore size is typically manipulated by changing the molecular weight corresponding to the pore-forming block. However, bespoke BCP synthesis is often a costly and time-consuming process. An alternative method for pore size tuning involves the use of swelling agents, such as homopolymers (HPs), which selectively interact with the core-forming block to increase the micelle size in solution. In this work, poly(isobutylene)-block-poly(ethylene oxide) micelles were swollen with poly(isobutylene) HP in solution and coassembled with aluminosilicate sol with the aim of increasing the resulting pore dimensions. An analytical approach implementing spectroscopic ellipsometry (SE) and ellipsometric porosimetry (EP) alongside atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) in transmission and grazing-incidence (GISAXS) modes enabled us to study the material evolution from solution processing through the manifestation of the mesoporous inorganic film after BCP removal. The in-depth SE/EP analysis evidenced an increase of more than 45% in mesopore diameter with HP swelling and a consistent scaling of the overall void volume and number of pores. Importantly, our analytical toolbox enabled us to study the effect of swelling on the connecting necks between adjacent pores, with observed increases as high as ≈35%, offering novel pathways to sensing, electrochemical, and other mass-transfer-dependent applications.

TiO2 mesoporous thin film architecture as a tool to control Au nanoparticles growth and sensing capabilities

In this paper, a systematic study regarding the effect of the mesoporous structure over Au nanoparticles (NPs) growth inside and through the pores of mesoporous TiO2 thin films (MTTFs) is presented, and the effect of such characteristics over the composites' sensing capabilities is evaluated. Highly stable MTTFs with different pore diameters (range: 4-8 nm) and pore arrangements (body- and face-centered cubic) were synthesized and characterized. Au NPs were grown inside the pores, and it was demonstrated-through a careful physicochemical characterization-that the amount of incorporated Au and NP size depends on the pore array; being higher for bigger pore diameters and face-centered cubic structures. The same structure allows the growth of more and longer tips over Au NPs deposited at the thin film-substrate interface. Finally, to confirm the effect of the structural characteristics of the composites over their possible applications, the materials were tested as surface-enhanced Raman scattering (SERS)-based substrates. The composites with a higher amount of Au and more ramified NPs were the ones that presented better sensitivity in the detection of a probe molecule (4-nitrothiophenol). Overall, this work demonstrates that the pore size and ordering in MTTFs determine the materials' accessibility and connectivity, and therefore, have a clear impact on their potential applications.

Chemical Stability of Mesoporous Oxide Thin Film Electrodes under Electrochemical Cycling: from Dissolution to Stabilization

Mesoporous oxide thin films (MOTF) present very high surface areas and highly controlled monodisperse pores in the nanometer range. These features spurred their possible applications in separation membranes and permselective electrodes. However, their performance in real applications is limited by their reactivity. Here, we perform a basic study of the stability of MOTF toward dissolution in aqueous media using a variety of characterization techniques. In particular, we focus in their stability behavior under the influence of ionic strength, adsorption of electrochemical probes, and applied electrode potential. Mesoporous silica thin films present a limited chemical stability after electrochemical cycling, particularly under high ionic strength, due to their high specific surface area and the interactions between the electrochemical probes and the surface. In contrast, TiO₂ or Si_0.9Zr_0.1O₂ matrices present higher stability; thus, they are an adequate alternative to produce accessible, sensitive, and robust permselective electrodes or membranes that perform under a wide variety of conditions.