Surface chemical functionalization of cylindrical nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer via amidation

Diblock and Triblock Copolymers as Nanostructured Precursors to Functional Nanoporous Materials: From Design to Application

Block copolymers have gained tremendous interest from the scientific community in the last two decades. These macromolecular architectures indeed constitute ideal nanostructured precursors for the generation of nanoporous materials meant for various high added value applications. The parallel emergence of controlled polymerization techniques has notably enabled to finely control their molecular features to confer them with unique structural and physicochemical properties, such as low dispersity values (Đ), well-defined volume fractions, and controlled functionality. The nanostructuration and ordering of diblock or triblock copolymers, which can be achieved through various experimental techniques, including channel die processing, solvent vapor or thermal annealing, nonsolvent-induced phase separation or concomitant self-assembly, and nonsolvent-induced phase separation, allows for the preparation of orientated microphase-separated copolymers whose morphology is dictated by three main factors, i.e., Flory-Huggins interaction parameter between constitutive blocks, volume fraction of the blocks, and polymerization degree. This review article provides an overview of the actual state of the art regarding the preparation of functional nanoporous materials from either diblock or triblock copolymers. It will also highlight the major applications of such peculiar materials.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

We report investigations of the deposition of nanometer-thick Ln(OH)₃ films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)₃ films are deposited from aqueous solutions of LaCl₃ onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH^-, which complexes with Ln³⁺, resulting in the precipitation of Ln(OH)₃. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH₃)₆³⁺ reduction, FcMeOH oxidation, and Fe(CN)₆^4-/3- oxidation/reduction) were investigated at Ln(OH)₃-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)₃ films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H₂O and O₂. We further demonstrate that the thickness of La(OH)₃ films can be controlled by anodic dissolution. Additionally, we show that La³⁺ lowers the supersaturation of dissolved H₂ required to nucleate a stable nanobubble.

Pushing the limits of nanopore transport performance by polymer functionalization

Inspired by the design and performance of biological pores, polymer functionalization of nanopores has emerged as an evolving field to advance transport performance within the last few years. This feature article outlines developments in nanopore functionalization and the resulting transport performance including gating based on electrostatic interaction, wettability and ligand binding, gradual transport controlled by polymerization as well as functionalization-based asymmetric nanopore and nanoporous material design going towards the transport direction. Pushing the limits of nanopore transport performance and thus reducing the performance gap between biological and technological pores is strongly related to advances in polymerization chemistry and their translation into nanopore functionalization. Thereby, the effect of the spatial confinement has to be considered for polymer functionalization as well as for transport regulation, and mechanistic understanding is strongly increased by combining experiment and theory. A full mechanistic understanding together with highly precise nanopore structure design and polymer functionalization is not only expected to improve existing application of nanoporous materials but also opens the door to new technologies. The latter might include out of equilibrium devices, ionic circuits, or machine learning based sensors.

DNA as a Next-Generation Biomonitoring Tool of Hospital Effluent Contamination

A DNA biosensor based on a modified gold electrode with a Au/cysteine/DNA matrix was developed for ultratrace determination of genotoxicity antibiotics. The modified Au/cysteine/DNA electrode was characterized by cyclic voltammetry and impedance spectroscopy methods. The interaction between immobilized DNA and genotoxicity antibiotics in hospital wastewater was investigated using differential pulse voltammetry (DPV) technology. Using this technique, ciprofloxacin and ofloxacin were detected in real time in the hospital wastewater (HW) of the Tunisian cities of Gabes, Tozeur, Sfax, and Gbeli. In addition, physicochemical parameters such as the chemical oxygen demand (COD), biological oxygen demand (BOD), and total organic carbon (TOC) of HW samples that may affect the nature of the samples were studied. Comet assay (single-cell gel electrophoresis) was performed to measure the capacity of xenobiotics to induce DNA damage. In our conditions, this test indicated that all tested wastewater was able to alter cell integrity and cause DNA molecular damage, and the most genotoxic effect was found in the wastewater of Gabes hospital. Results show that the concentrations of the two antibiotics reached 33 and 40 ng/mL in the hospital wastewater of Gabes and Tozeur, respectively. The DNA biosensor based on the modified gold electrode exhibited superb performance and offers a probable application for the detection of genotoxicity antibiotics in hospital wastewater. The level of genotoxicity is proportional to the concentration of antibiotics detected in hospital wastewater. We will explore the application of this model for continuous monitoring downstream of hospital discharge and wastewater treatment plants for effective control of the presence of genotoxic products.

Nanoporous Thin Films Formed from Photocleavable Diblock Copolymers on Gold Substrates Modified with Thiolate Self-Assembled Monolayers

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30-100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1'-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)₆^3- that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.

Fabrication, Characterization, and Analytical Application of Silica Nanopore Array-Modified Platinum Electrode

In this work, we report a new approach to fabricate the nanopore array electrode (NAE) by transferring silica nanochannel membrane (SNM) to the surface of Pt electrode (0.5 mm in diameter) sealed by glass capillary (designated as Pt-NAE for simplicity). The SNM is supported via the irreversible covalent-bond formation with the surrounding glass capillary treated by plasma, thus providing long-term stability to Pt-NAE. Meanwhile, this fabrication process does not require pregrafting or premodification of Pt electrode surface, providing well-defined active surface domains. Thanks to the small pore diameter (∼2.3 nm) and negatively charged channel walls, the SNM is permselective and thus the electrochemical behavior of Pt-NAE is dependent on both electrolyte concentration and charge state of redox molecules. The permeability of SNM was determined by the scanning electrochemical microscopy (SECM) approach curve measurements coupled with finite-element simulations from a quantitative viewpoint. The permeability of anionic Ru(CN)₆^4- was varied from 150 to 10.3 μm s^-1 as the electrolyte concentration decreased from 1.0 to 0.01 M, while there is no obvious change for cationic Ru(NH₃)₆³⁺. Finally, the as-prepared Pt-NAE is able to continuously monitor dissolved oxygen for up to 2 h in a solution containing biofouling reagents, exhibiting an enhanced antifouling ability and therefore excellent current stability. We believe the NAE with unique mass transport properties can be extended further for other analytical applications.

Electrochemical Manipulation of Aligned Block Copolymer Templates

Block copolymers have a wide range of functions in advanced electrochemistry because of their ability to self-assemble into ordered nanometer-sized structures, resulting in their extensive usage as nanoporous templates that can be electrochemically manipulated. These highly ordered nanoporous templates are used as working electrodes for electrodeposition and electropolymerization to build nanoelectrode arrays and can serve as models to study the diffusion pathway of redox-active units with regard to chemical modification of pores. The block copolymer system allows different morphologies to be utilized, but the most exploited structures are standing cylinders of the minority block that are etched to expose highly aligned porous nanoelectrode array templates. This review starts with introducing alumina and track-etched membranes as pioneer porous templates transitioning to the production of block copolymer films as succeeding templates, with a particular focus on both poly(styrene)-block-poly(methylmethacrylate) (PS-b-PMMA) and poly(styrene)-block-poly(lactide) (PS-b-PLA). The aim is to give fundamental insights of electrochemistry where functionality extends beyond to applications in the nanoscience field of biosensors and plastic electronics.

Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes

Original porous catalytic supports can be engineered via an effective and straightforward synthetic route to polystyrene-block-poly(d,l-lactide) diblock copolymer precursors displaying an acid-cleavable acetal junction between both blocks. To this purpose, we synthesized an acetal-containing heterodifunctional initiator, thus enabling to combine two different polymerization methods, i.e., first atom transfer radical polymerization (ATRP) of styrene, and then ring-opening polymerization (ROP) of d,l-lactide. Thanks to the labile nature of the acetal junction, oriented porous frameworks could be obtained upon trifluoroacetic acid-mediated cleavage of the latter, after orientation of the block copolymer nanodomains by solvent vapor annealing. The resulting porous materials bearing a reactive aldehyde function at the pore surface allowed for further chemical modification via reductive amination with amino-containing compounds, such as tetraethylenepentamine, thus leading to amine-functionalized porous polystyrene. In situ generated gold nanoparticles could then be immobilized within such functionalized porous nanoreactors, and these hybrid materials could find interesting applications in heterogeneous supported catalysis. In this regard, model catalytic reactions, including C-C homocoupling of benzeneboronic acid derivatives, hydride-mediated reduction of nitroaromatic compounds, and especially unprecedented "one-pot" cascade reactions consisting of the latter consecutive reactions from 3-nitrobenzeneboronic acid, were successfully monitored by different chromatographic and spectroscopic techniques.

“Clickable” thiol-functionalized nanoporous polymers: from their synthesis to further adsorption of gold nanoparticles and subsequent use as efficient catalytic supports

Thiol-coated nanoporous materials are engineered from polystyrene-block-poly(d,l-lactide) precursors having a disulfide junction. Further adsorption of GNPs on pore surface provides novel supported catalysts.

Transition from star-like to crew-cut micelles induced by UV radiation

In the present article, the effect of UV on PS-b-PMMA micelles in solution is discussed. Micellar solutions of the amphiphilic poly(styrene-b-methylmethacrylate) block copolymer in selective solvent (methanol for the PMMA block) were exposed to UV radiation, which has simultaneously led to cross linking of the micellar core (PS) and degradation of the micellar corona (PMMA). The kinetics of such process were investigated in situ by means of dynamic light scattering, allowing the measurement of hydrodynamic radius as a function of UV exposure time. Results indicate that the size of micelles has decreased with UV exposure time down to a minimum value. Such reduced size resulted from PMMA degradation, which later promoted aggregation and coagulation because the micellar core was no longer well protected by PMMA. Addition of good solvent for both blocks (toluene) to non-UV exposed micelles has led to core swelling (PS) and, ultimately, system disassembly (free copolymer chain). The effect of adding toluene on the UV-exposed micelles has only caused core swelling as a consequence of the PS cross-linking.

Linker-based control of electron propagation through ferrocene moieties covalently anchored onto insulator-based nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer

This paper reports the effects of linker length on electron propagation through ferrocene moieties covalently anchored onto insulator-based cylindrical nanopores derived from a cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymer. These nanopores (24 nm in diameter, 30 nm long) aligned perpendicular to an underlying gold electrode were modified via esterification of their surface COOH groups with OH-terminated ferrocene derivatives having different alkyl linkers (FcCO(CH(2))(n)OH; n = 2, 5, 15). Cyclic voltammograms were measured in 0.1 M NaBF(4) at different scan rates to assess the efficiency of electron propagation through the ferrocene moieties. The redox peaks of the anchored ferrocenes were observed at nanoporous films decorated with FcCO(CH(2))(15)OH and FcCO(CH(2))(5)OH, but not at those with FcCO(CH(2))(2)OH. Importantly, the higher electron propagation efficiency was observed in the use of the longer linker, as shown by the apparent diffusion coefficients (ca. 10(-12) cm(2)/s for n = 15; ca. 10(-13) cm(2)/s for n = 5; no electron propagation for n = 2). The observed electron propagation resulted from electron hopping across relatively large spacing that was controlled by the motion of anchored redox sites (bounded diffusion). The longer linker led to the larger physical displacement range of anchored ferrocene moieties, facilitating the approach of the adjacent ferrocene moieties within a distance required for electron self-exchange reaction. The linker-based control of redox-involved electron propagation on nanostructured, insulating surfaces will provide a means for designing novel molecular electronics and electrochemical sensors.

Electrochemical impedance spectroscopy studies of organic-solvent-induced permeability changes in nanoporous films derived from a cylinder-forming diblock copolymer

In this paper we report electrochemical investigations of the influence of organic solvents dissolved in aqueous solution on the permeability of nanoporous films derived from a cylinder-forming polystyrene-poly(methyl methacrylate) diblock copolymer (CF-PS-b-PMMA). The nanoporous films (ca. 30 nm in pore diameter) were prepared on planar gold electrodes via UV-based degradation of the cylindrical PMMA domains of annealed CF-PS-b-PMMA films (30-45 nm thick). The permeability of the electrode-supported nanoporous films was assessed using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The faradic current of Fe(CN)(6)(3-/4-) decreased upon immersion in aqueous solutions saturated with toluene or methylene chloride (5.8 mM and 0.20 M, respectively). EIS data indicated that the decrease in faradic current mainly reflected an increase in the pore resistance (R(pore)). In contrast, R(pore) did not change in a saturated n-heptane solution, 0.17 M ethanol, or 5.8 mM aqueous solutions of methylene chloride, diethyl ether, methyl ethyl ketone, or ethanol. Atomic force microscopy images of a nanoporous film in aqueous solution with and without 5.8 mM toluene showed a reversible change in the surface morphology, which was consistent with a toluene-induced change in R(pore). The solvent-induced increase in R(pore) was attributed to the swelling of the nanoporous films by the organic solvents, which decreased the effective pore diameter. The reversible permeability changes suggest that the surface of CF-PS-b-PMMA-derived nanoporous films can be functionalized in organic environments without destroying the nanoporous structure. In addition, the solvent-induced swelling may provide a simple means for controlling the permeability of such nanoporous films.

Surface chemical properties of nanoscale domains on UV-treated polystyrene-poly(methyl methacrylate) diblock copolymer films studied using scanning force microscopy

This paper reports the surface chemical properties of ca. 20 nm wide domains on a UV-treated thin film of a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction). UV irradiation and subsequent acetic acid (AcOH) treatment were used for selectively etching horizontally aligned PMMA domains on a thin PS-b-PMMA film to obtain nanoscale trenches and ridges. The surface charge and hydrophilicity of the trenches (etched PMMA domains) and ridges (PS domains) were investigated using three approaches based on scanning force microscopy. Chemical force titration data with a COOH-terminated tip showed a prominent decrease in adhesion force from pH 3 to 4.5 due to electrostatic repulsion between negatively charged functional groups on the tip and film surface but could not clarify the difference in chemical properties between the two nanoscale domains. Friction force images in n-dodecane showed higher friction over etched PMMA and PS domains with an OH-terminated tip and a CH(3)-terminated tip, respectively, exhibiting higher hydrophilicity of the etched PMMA domains. In an atomic force microscopy image of a UV/AcOH-treated PS-b-PMMA film upon immersion in a ferritin solution, approximately 80% of the ferritin deposited on the film was found on the PS domains. The preferential deposition of ferritin on the PS domains was probably due to the electrostatic repulsion between negatively charged ferritin and negatively charged etched PMMA surface in addition to the hydrophobic interaction between ferritin and the PS surface. These results indicated that the etched PMMA domains were more hydrophilic than the PS domains due to the presence of acidic functional groups (e.g., -COOH groups) at a higher density.