Hexagonally ordered arrays of metallic nanodots from thin films of functional block copolymers

Amplified EQCM-D detection of extracellular vesicles using 2D gold nanostructured arrays fabricated by block copolymer self-assembly

Extracellular vesicles (EVs) are routinely released from nearly all cell types as transport vehicles and for cell communication. Crucially, they contain biomolecular content for the identification of health and disease states that can be detected from readily accessible physiological fluids, including urine, plasma, or saliva. Despite their clinical utility within noninvasive diagnostic platforms such as liquid biopsies, the currently available portfolio of analytical approaches are challenged by EV heterogeneity in size and composition, as well as the complexity of native biofluids. Quartz crystal microbalance with dissipation monitoring (QCM-D) has recently emerged as a powerful alternative for the phenotypic detection of EVs, offering multiple modes of analyte discrimination by frequency and dissipation. While providing rich data for sensor development, further progress is required to reduce detection limits and fully exploit the technique's potential within biosensing. Herein, we investigate the impact of nanostructuring the sensor electrode surface for enhancing its detection capabilities. We employ self-assembly of the block copolymer polystyrene-block-poly(4-vinylpyridine) to create well defined 2D gold islands via selective impregnation of the pyridine domain with gold precursors and subsequent removal of the template. When matched to the EV length scale, we find a 4-fold improvement in sensitivity despite a 4-fold reduction in area for analyte and ligand anchoring in comparison to a flat sensor surface. Creation of tailored and confined sensing regions interspersed by non-binding silica provides optimal spatial orientation for EV capture with reduced steric effects and negative cooperativity of grafted antibodies, offering a promising route for facilitated binding and enhanced performance of sensor platforms.

Large area Al2O3-Au raspberry-like nanoclusters from iterative block-copolymer self-assembly

In the field of functional nanomaterials, core-satellite nanoclusters have recently elicited great interest due to their unique optoelectronic properties. However, core-satellite synthetic routes to date are hampered by delicate and multistep reaction conditions and no practical method has been reported for the ordering of these structures onto a surface monolayer. Herein we show a reproducible and simplified thin film process to fabricate bimetallic raspberry nanoclusters using block copolymer (BCP) lithography. The fabricated inorganic raspberry nanoclusters consisted of a ∼36 nm alumina core decorated with ∼15 nm Au satellites after infusing multilayer BCP nanopatterns. A series of cylindrical BCPs with different molecular weights allowed us to dial in specific nanodot periodicities (from 30 to 80 nm). Highly ordered BCP nanopatterns were then selectively infiltrated with alumina and Au species to develop multi-level bimetallic raspberry features. Microscopy and X-ray reflectivity analysis were used at each fabrication step to gain further mechanistic insights and understand the infiltration process. Furthermore, grazing-incidence small-angle X-ray scattering studies of infiltrated films confirmed the excellent order and vertical orientation over wafer scale areas of Al2O3/Au raspberry nanoclusters. We believe our work demonstrates a robust strategy towards designing hybrid nanoclusters since BCP blocks can be infiltrated with various low cost salt-based precursors. The highly controlled nanocluster strategy disclosed here could have wide ranging uses, in particular for metasurface and optical based sensor applications.

High refractive index in low metal content nanoplasmonic surfaces from self-assembled block copolymer thin films

Materials with a high and tunable refractive index are attractive for nanophotonic applications. In this contribution, we propose a straightforward fabrication technique of high-refractive index surfaces based on self-assembled nanostructured block copolymer thin films. The selective and customizable metal incorporation within out-of-plane polymer lamellae produces azimuthally isotropic metallic nanostructures of defined geometries, which were analysed using microscopy and small-angle X-ray scattering techniques. Variable-angle spectroscopic ellipsometry was used to relate the geometrical parameters of the metallic features and the resulting refractive index of the patterned surfaces. In particular, nanostructured gold patterns with a high degree of homogeneity and a gold content as low as 16 vol% reach a refractive index value of more than 3 in the visible domain. Our study thus demonstrates a new route for the preparation of high refractive index surfaces with a low metal content for optical applications.

Uniform metal nanostructures with long-range order via three-step hierarchical self-assembly

Large-scale nanopatterning is a major issue in nanoscience and nanotechnology, but conventional top-down approaches are challenging because of instrumentation and process complexity while often lacking the desired spatial resolution. We present a hierarchical bottom-up nanopatterning routine using exclusively self-assembly processes: By combining crystal surface reconstruction, microphase separation of copolymers, and selective metal diffusion, we produce monodisperse metal nanostructures in highly regular arrays covering areas of square centimeters. In situ grazing incidence small-angle x-ray scattering during Fe nanostructure formation evidences an outstanding structural order in the self-assembling system and hints at the possibility of sculpting nanostructures using external process parameters. Thus, we demonstrate that bottom-up nanopatterning is a competitive alternative to top-down routines, achieving comparable pattern regularity, feature size, and patterned areas with considerably reduced effort. Intriguing assets of the proposed fabrication approach include the option for in situ investigations during pattern formation, the possibility of customizing the nanostructure morphology, the capacity to pattern arbitrarily large areas with ultrahigh structure densities unachievable by top-down approaches, and the potential to address the nanostructures individually. Numerous applications of self-assembled nanostructure patterns can be envisioned, for example, in high-density magnetic data storage, in functional nanostructured materials for photonics or catalysis, or in surface plasmon resonance-based sensing.

Bimetallic PdAg nanoparticle arrays from monolayer films of diblock copolymer micelles

The self-assembly technique provides a highly efficient route to generate well-ordered structures on a nanometer scale. In this paper, well-ordered arrays of PdAg alloy nanoparticles on flat substrates with narrow distributions of particle size (6-7 nm) and interparticle spacing (about 60 nm) were synthesized by the block copolymer micelle approach. A home-made PS-b-P4VP diblock copolymer was prepared to obtain a micellar structure in toluene. Pd and Ag salts were then successfully loaded in the micellar core of the PS-b-P4VP copolymer. A self-assembled monolayer of the loaded micelles was obtained by dipping the flat substrate in the solution. At this stage, the core of the micelles was still loaded with the metal precursor rather than with a metal. Physical and chemical reducing methods were used to reduce the metal salts embedded in the P4VP core into PdAg nanoparticles. HRTEM and EDX indicated that Pd-rich PdAg alloy nanoparticles were synthesized by chemical or physical reduction; UV-visible spectroscopy observations confirmed that metallic PdAg nanoparticles were quickly formed after chemical reduction; XPS measurements revealed that the PdAg alloy nanoparticles were in a metallic state after a short time of exposure to O2 plasma and after hydrazine reduction.

High-Resolution Metal Nanopatterning by Means of Switchable Block Copolymer Templates

In this review, recent developments in the fabrication of hexagonal and parallel ordered arrays of metallic nanodomains on a substrate are described. We focus on the nanopatterning approach by means of switchable block copolymer thin films. This approach is highly advantageous, because it can lead to extremely regular patterns with metal subunits of only a few nanometers in diameter and center-to-center distances of tens of nanometers. Hence, the resulting 1D or 2D periodic arrays of metal nanodots and nanowires on silicon substrates can be fabricated with extremely high unit densities and on very large areas. The templated deposition of presynthesized metal nanoparticles on functional block copolymers is described in detail. Current challenges are discussed and an outlook for further developments is given.

Polypropylene/Layered Double Hydroxide (LDH) Nanocomposites: Influence of LDH Particle Size on the Crystallization Behavior of Polypropylene

Highly dispersed isotactic polypropylene (iPP) nanocomposites were prepared by incorporating two different sized Mg-Al LDH nanoparticles with different loadings from 1 to 10 wt % using a modified solvent mixing method. Larger sized LDH nanoparticles (∼3-4 μm) were prepared from the gel form of Mg-Al LDH, and the smaller sized nanoparticles (∼50-200 nm) were prepared by sonication of as-synthesized LDH particles. Such obtained LDH nanoparticles were carefully characterized using wide-angle X-ray diffraction (WAXD), transmission electron microscopy, and scanning electron microscopy. WAXD and atomic force microscopy results indicate that the LDH nanoparticles were highly dispersed in the iPP matrix. The influence of LDH nanoparticles size and concentration on the thermal stability, spherulitic morphology, melting behavior, isothermal crystallization kinetics, and lamellar structure of iPP were investigated. Incorporation of low loadings of sonicated LDH particles (e.g., 1-2.5 wt %) show substantial effect on thermal stability, spherulite size, crystallinity, and crystallization half-time and lamellar morphology of iPP compared to the pure iPP and that of nanocomposites with larger LDH particles with same loadings. The better nucleation ability of iPP in the presence of sonicated LDH can be attributed to the high surface area of LDH nanoparticles along with its better dispersibility within the polymer matrix. The incorporation of LDH nanoparticles does not change the crystallization growth mechanism and crystal structure of iPP.

Double-striped metallic patterns from PS-b-P4VP nanostrand templates

A new nanometallic pattern, characterized by randomly disposed double or twin one-dimensional stripes and that adds to the nanotechnology toolbox, has been obtained from a unique template possessing the nanostrand morphology. This morphology had previously been shown to form in Langmuir-Blodgett films made from a polystyrene-poly(4-vinylpyridine) (PS-P4VP) diblock copolymer blended with 3-n-pentadecylphenol (PDP). The nanostrand backbone is composed of PS, and it is bordered along both sides by a P4VP monolayer, visualized for the first time by high resolution atomic force microscopy. The exposed P4VP alongside the nanostrands serves as sites for depositing compounds attracted selectively to P4VP. Here, both gold ions (HAuCl4·3H2O) and gold nanoparticles (AuNP, 12 nm in diameter, stabilized with sodium citrate) were complexed to the P4VP. Plasma treatment of the gold ions led to double stripes of monolayer metallic gold. To obtain dense deposition of AuNP in double rows, it was necessary to acidify the AuNP aqueous solution (pH 5.2 here). The achievement of the metallic double-stripe patterns also confirms the composition of the nanostrand morphology, which up to now had been deduced indirectly. The double-stripe pattern has possible applications for plasmonic lasers, energy transport, and biosensors.

Pathways of cylindrical orientations in PS-b-P4VP diblock copolymer thin films upon solvent vapor annealing

The orientation changes of perpendicular cylindrical microdomains in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin films upon annealing in different solvent vapors were investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS) and ex situ scanning force microscopy (SFM). The swelling of P4VP perpendicular cylinders (C⊥) in chloroform, a non-selective solvent vapor, leads to the reorientation to in-plane cylinders through a disordered state in a particular kinetic pathway in the phase diagram upon drying. On the other hand, the swelling of the P4VP perpendicular cylinders in a selective solvent vapor (i.e., 1,4-dioxane) induces a morphological transition from cylindrical to ellipsoidal as a transient structure to spherical microdomains; subsequent solvent evaporation resulted in shrinkage of the matrix in the vertical direction, merging the ellipsoidal domains into the perpendicularly aligned cylinders. In this paper, we have discussed the mechanism based on the selectivity of the solvent to the constituting blocks that is mainly responsible for the orientation changes.

2D and 3D nanopatterning of titanium for enhancing osteoinduction of stem cells at implant surfaces

The potential for the use of well-defined nanopatterns to control stem cell behaviour on surfaces has been well documented on polymeric substrates. In terms of translation to orthopaedic applications, there is a need to develop nanopatterning techniques for clinically relevant surfaces, such as the load-bearing material titanium (Ti). In this work, a novel nanopatterning method for Ti surfaces is demonstrated, using anodisation in combination with PS-b-P4VP block copolymer templates. The block copolymer templates allows for fabrication of titania nanodot patterns with precisely controlled dimensions and positioning which means that this technique can be used as a lithography-like patterning method of bulk Ti surfaces on both flat 2D and complex shaped 3D surfaces. In vitro studies demonstrate that precise tuning of the height of titania nanodot patterns can modulate the osteogenic differentiation of mesenchymal stem cells. Cells on both the 8 nm and 15 nm patterned surfaces showed a trend towards a greater number of the large, super-mature osteogenic focal adhesions than on the control polished Ti surface, but the osteogenic effect was more pronounced on the 15 nm substrate. Cells on this surface had the longest adhesions of all and produced larger osteocalcin deposits. The results suggest that nanopatterning of Ti using the technique of anodisation through a block copolymer template could provide a novel way to enhance osteoinductivity on Ti surfaces.

Quantum-dot-based photoelectrochemical sensors for chemical and biological detection

Quantum-dot-based photoelectrochemical sensors are powerful alternatives for the detection of chemicals and biochemical molecules compared to other sensor types, which is the primary reason as to why they have become a hot topic in nanotechnology-related analytical methods. These sensors basically consist of QDs immobilized by a linking molecule (linker) to an electrode, so that upon their illumination, a photocurrent is generated which depends on the type and concentration of the respective analyte in the immediate environment of the electrode. The present review provides an overview of recent developments in the fabrication methods and sensing concepts concerning direct and indirect interactions of the analyte with quantum dot modified electrodes. Furthermore, it describes in detail the broad range of different sensing applications of such quantum-dot-based photoelectrochemical sensors for inorganic and organic (small and macro-) molecules that have arisen in recent years. Finally, a number of aspects concerning current challenges on the way to achieving real-life applications of QD-based photochemical sensing are addressed.

Novel anodization technique using a block copolymer template for nanopatterning of titanium implant surfaces

Precise surface nanopatterning is a promising route for predictable control of cellular behavior on biomedical materials. There is currently a gap in taking such precision engineered surfaces from the laboratory to clinically relevant implant materials such as titanium (Ti). In this work, anodization of Ti surfaces was performed in combination with block copolymer templates to create highly ordered and tunable oxide nanopatterns. Secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) analyses showed that the composition of the anodized structures was mainly titania with small amounts of nitrogen left from the block copolymer. It was further demonstrated that these nanopatterns can be superimposed on more complex shaped Ti surfaces such as microbeads, using the same technique. Human mesenchymal stem cells were cultured on Ti microbead surfaces, with and without nanopatterns, in vitro to study the effect of nanotopography on Ti surfaces. The results presented in this work demonstrate a promising method of producing highly defined and well-arranged surface nanopatterns on Ti implant surfaces.

Periodic titania nanostructures using block copolymer templates

The deposition of periodic titania nanostructures, templated by a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer, is reported. When cast as a thin film (30-50 nm thick), the PS-b-P4VP forms a morphology that consists of P4VP cylinders that are orientated perpendicular to the substrate. The P4VP phase was lightly cross-linked by exposing the film to diiodobutane. When the block copolymer film was exposed to the sol-gel titania precursor, titanium(IV) bis(ammonium lactate) dihydroxide (TALH), titania was formed in the P4VP phase. The resulting titania structures were identical in size to the P4VP cylinders and only formed (under the deposition conditions used in this study) when the block copolymer film was present on the substrate, thus providing evidence that the block copolymer indeed acts as a template. The process works for both silicon and indium tin oxide substrates.