Morphology Engineering of the Asymmetric PS-b-P4VP Block Copolymer: From Porous to Nanodot Oxide Structures.

In the present work, we demonstrate the formation of oxide porous and nanodot structures from the same block copolymer (BCP) by the phase inversion of a BCP template. We investigated the effect of solvent annealing time on the ordering of asymmetric, cylinder forming, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCP. Phase separation of PS-b-P4VP was achieved by solvent vapor annealing (SVA) in a solvent atmosphere that is (partially) selective to P4VP to initially generate hexagonally arranged, cylindrical arrays of the expected structure. The morphology of the BCP changed from P4VP hexagonally packed cylinders to an 'inverse' structure with PS cylinders embedded in a P4VP matrix. This suggests that selective swelling occurs over time such that the swollen P4VP phase becomes the majority volume component. Metal ions (Ga3+, In3+) were infiltrated into the BCP templates by a solution-mediated infiltration approach, followed by an ultraviolet-ozone treatment to remove the polymer and oxidize the metallic ions to their oxides. The findings show that a single BCP can be used to create both metal oxide arrays and porous structures of metal oxides by simply varying the duration of the solvent annealing process. The resulting structures were analyzed through several methods including scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and energy-dispersive X-ray spectroscopy. XPS analyses confirmed the complete elimination of the BCP template and the presence of metal oxides. This study provides important insights into the development of functional BCP materials with inverse structures.

Block Copolymer-Templated Au@CdSe Core-Satellite Nanostructures with Solvent-Dependent Optical Properties.

In the present work, we report the fabrication and characterization of well-defined core-satellite nanostructures. These nanostructures comprise block copolymer (BCP) micelles, containing a single gold nanoparticle (AuNP) in the core and multiple photoluminescent cadmium selenide (CdSe) quantum dots (QDs) attached to the micelle's coronal chains. The asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP was employed to develop these core-satellite nanostructures in a series of P4VP-selective alcoholic solvents. The BCP micelles were first prepared in 1-propanol and subsequently mixed with AuNPs, followed by gradual addition of CdSe QDs. This method resulted in the development of spherical micelles that contained a PS/Au core and a P4VP/CdSe shell. These core-satellite nanostructures, developed in different alcoholic solvents, were further employed for the time-resolved photoluminescence analysis. It was found that solvent-selective swelling of the core-satellite nanostructures tunes the distance between the QDs and AuNPs and modulates their Förster resonance energy transfer (FRET) behavior. The average lifetime of the donor emission varied from 12.3 to 10.3 nanoseconds (ns) with the change in the P4VP-selective solvent within the core-satellite nanostructures. Furthermore, the distances between the donor and acceptor were also calculated using efficiency measurements and corresponding Förster distances. The resulting core-satellite nanostructures hold promising potential in various fields, such as photonics, optoelectronics, and sensors that utilize the FRET process.

Fabrication of High-κ Dielectric Metal Oxide Films on Topographically Patterned Substrates: Polymer Brush-Mediated Depositions.

Fabrication of ultrathin films of dielectric (with particular reference to materials with high dielectric constants) materials has significance in many advanced technological applications including hard protective coatings, sensors, and next-generation logic devices. Current state-of-the-art in microelectronics for fabricating these thin films is a combination of atomic layer deposition and photolithography. As feature size decreases and aspect ratios increase, conformality of the films becomes paramount. Here, we show a polymer brush template-assisted deposition of highly conformal, ultrathin (sub 5 nm) high-κ dielectric metal oxide films (hafnium oxide and zirconium oxide) on topographically patterned silicon nitride substrates. This technique, using hydroxyl terminated poly-4-vinyl pyridine (P4VP-OH) as the polymer brush, allows for conformal deposition with uniform thickness along the trenches and sidewalls of the substrate. Metal salts are infiltrated into the grafted monolayer polymer brush films via solution deposition. Tailoring specific polymer interfacial chemistries for ion infiltration combined with subsequent oxygen plasma treatment enabled the fabrication of high-quality sub 5 nm metal oxide films.

Progress and challenges of graphene and its congeners for biomedical applications.

Nanomaterials by virtue of their small size and enhanced surface area, present unique physicochemical properties that enjoy widespread applications in bioengineering, biomedicine, biotechnology, disease diagnosis, and therapy. In recent years, graphene and its derivatives have attracted a great deal of attention in various applications, including photovoltaics, electronics, energy storage, catalysis, sensing, and biotechnology owing to their exceptional structural, optical, thermal, mechanical, and electrical. Graphene is a two-dimensional sheet of sp2 hybridized carbon atoms of atomic thickness, which are arranged in a honeycomb crystal lattice structure. Graphene derivatives are graphene oxide (GO) and reduced graphene oxide (rGO), which are highly oxidized and less oxidized forms of graphene, respectively. Another form of graphene is graphene quantum dots (GQDs), having a size of less than 20 nm. Contemporary graphene research focuses on using graphene nanomaterials for biomedical purposes as they have a large surface area for loading biomolecules and medicine and offer the potential for the conjugation of fluorescent dyes or quantum dots for bioimaging. The present review begins with the synthesis, purification, structure, and properties of graphene nanomaterials. Then, we focussed on the biomedical application of graphene nanomaterials with special emphasis on drug delivery, bioimaging, biosensing, tissue engineering, gene delivery, and chemotherapy. The implications of graphene nanomaterials on human health and the environment have also been summarized due to their exposure to their biomedical applications. This review is anticipated to offer useful existing understanding and inspire new concepts to advance secure and effective graphene nanomaterials-based biomedical devices.

Block Copolymer Template-Directed Catalytic Systems: Recent Progress and Perspectives.

Fabrication of block copolymer (BCP) template-assisted nano-catalysts has been a subject of immense interest in the field of catalysis and polymer chemistry for more than two decades now. Different methods, such as colloidal route, on-substrate methods, bulk self-assembly approaches, combined approaches, and many others have been used to prepare such nano-catalysts. The present review focuses on the advances made in this direction using diblock, triblock, and other types of BCP self-assembled structures. It will be shown how interestingly, researchers have exploited the features of tunable periodicity, domain orientation, and degree of lateral orders of self-assembled BCPs by using fundamental approaches, as well as using different combinations of simple methods to fabricate efficient catalysts. These approaches allow for fabricating catalysts that are used for the growth of single- and multi-walled carbon nanotubes (CNTs) on the substrate, size-dependent electrooxidation of the carbon mono oxide, cracking of 1,3,5-triisopropylbenzene (TIPB), methanol oxidation, formic acid oxidation, and for catalytic degradation of dyes and water pollutants, etc. The focus will also be on how efficient and ease-of-use catalysts can be fabricated using different BCP templates, and how these have contributed to the fabrication of different nano-catalysts, such as nanoparticle array catalysts, strawberry and Janus-like nanoparticles catalysts, mesoporous nanoparticles and film catalysts, gyroid-based bicontinuous catalysts, and hollow fiber membrane catalysts.

Hollow Au@TiO2 porous electrospun nanofibers for catalytic applications.

Catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles were fabricated using a combination of sol-gel chemistry and coaxial electrospinning technique. We report the fabrication of catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles (AuNPs) using a combination of sol-gel chemistry and coaxial electrospinning technique. The coaxial electrospinning involved the use of a mixture of poly(vinyl pyrrolidone) (PVP) and titania sol as the shell forming component, whereas a mixture of poly(4-vinyl pyridine) (P4VP) and pre-synthesized AuNPs constituted the core forming component. The core-shell nanofibers were calcined stepwise up to 600 °C which resulted in decomposition and removal of the organic constituents of the nanofibers. This led to the formation of porous and hollow titania nanofibers, where the catalytic AuNPs were embedded in the inner wall of the titania shell. The catalytic activity of the prepared Au@TiO2 porous nanofibers was investigated using a model reaction of catalytic reduction of 4-nitrophenol and Congo red dye in the presence of NaBH4. The Au@TiO2 porous and hollow nanofibers exhibited excellent catalytic activity and recyclability, and the morphology of the nanofibers remained intact after repeated usage. The presented approach could be a promising route for immobilizing various nanosized catalysts in hollow titania supports for the design of stable catalytic systems where the added photocatalytic activity of titania could further be of significance.

Fluorescence resonance energy transfer in multifunctional nanofibers designed via block copolymer self-assembly.

In the present study, we demonstrate the fabrication of multifunctional nanofibers, loaded with CdSe quantum dots (QDs) and sulforhodamine 101 (S101) dye, via the self-assembly process of a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP). The CdSe QDs and S101 dye were simultaneously incorporated in the cylindrical domains, constituted of P4VP blocks, of the self-assembled BCP structure. The cylindrical domains subsequently were isolated as individual nanofibers via the selective-swelling approach. The confinement imposed due to the nano-dimension geometry of the cylindrical domains enabled the QDs and S101 dye to localize within their Förster radius enabling an efficient fluorescence resonance energy transfer (FRET) between them. The mean lifetime of donor emission varied from 4.56 to 3.38 ns with the change in the ratio of S101 dye and CdSe QDs within the nanofibers. Furthermore, using efficiency measurements and the corresponding Förster distances, donor-acceptor distances were determined. Moreover, the kinetics of energy transfer from CdSe QDs to S101 was studied by the Poisson binding model, to understand the interactions between CdSe QDs and S101 dye molecules. The numbers of dye molecules per CdSe QD were determined, by assuming random distribution of S101 dye molecules around the CdSe QDs in the nanofibers. The results showed that the number of dye molecules per QD increased with increasing concentration of dye molecules in the nanofibers. The resulting multifunctional nanofibers could have potential applications in optoelectronics, photonics and sensors which utilize the FRET process.

Ligand displacement induced morphologies in block copolymer/quantum dot hybrids and formation of core-shell hybrid nanoobjects.

We investigate the self-assembly of a cylinder-forming polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) mixed with trioctylphosphine oxide (TOPO) capped cadmium selenide (CdSe) quantum dots (QDs). The QDs were found to be enthalpically compatible with the P4VP chains via ligand displacement of TOPO from the QD surface. However, the QDs were found to localize preferentially at the PS/P4VP interphase plausibly to gain translational entropy in order to further lower the energetics of the self-assembled structure. Interestingly, the morphological transformation observed with increasing weight fraction of the QDs in the BCP/QD composite was driven by the migration of the displaced TOPO from the QD surface to the PS phase, effectively increasing its total volume fraction. Hence, the PS-b-P4VP BCP with PS as the minority block displayed lamellar morphology in its composite with QDs. Furthermore, the preferred localization of the QDs at the PS/P4VP interface led to the formation of a trilayer lamellar morphology which was deduced from the suppression of the primary scattering peak, relative to higher order peaks in the SAXS data. The morphological transformation was accompanied by a significant increase in the domain spacing due to excessive stretching of the longer P4VP chains of the asymmetric block copolymer. However, in the PS-b-P4VP/CdSe composites with P4VP as the minority block, cylindrical morphology was retained and the domain spacing decreased due to dominance of the co-surfactant effect as well as interfacial localization of CdSe QDs. We also demonstrate that these PS-b-P4VP/CdSe self-assembled hybrid materials could further be used to obtain isolated core-shell nanoobjects, such as nanofibers and nanosheets, containing CdSe QDs. The nanoobjects so obtained exhibited photoluminescence properties typical of CdSe quantum dots. These photoluminescent polymer nanoobjects could have potential applications in biological targeting and fluorescence labeling.

Crystallization behaviour of poly(ethylene oxide) under confinement in the electrospun nanofibers of polystyrene/poly(ethylene oxide) blends.

We have studied the confined crystallization behaviour of poly(ethylene oxide) (PEO) in the electrospun nanofibers of the phase-separated blends of polystyrene (PS) and PEO, where PS was present as the major component. The size and shape of PEO domains in the nanofibers were considerably different from those in the cast films, presumably because of the nano-dimensions of the nanofibers and the extensional forces experienced by the polymer solution during electrospinning. The phase-separated morphology in turn influenced the crystallization behaviour of PEO in the blend nanofibers. At a PEO weight fraction of ≥0.3, crystallization occurred through a heterogeneous nucleation mechanism similar to that in cast blend films. However, as the PEO weight fraction in the blend nanofibers was reduced from 0.3 to 0.2, an abrupt transformation of the nucleation mechanism from the heterogeneous to predominantly homogenous type was observed. The change in the nucleation mechanism implied a drastic reduction of the spatial continuity of PEO domains in the nanofibers, which was not encountered in the cast film. The melting temperature and crystallinity of the PEO crystallites developed in the nanofibers were also significantly lower than those in the corresponding cast films. The phenomena observed were reconciled by the morphological observation, which revealed that the phase separation under the radial constraint of the nanofibers led to the formation of small-sized fibrillar PEO domains with limited spatial connectivity. The thermal treatment of the PS/PEO blend nanofibers above the glass transition temperature of PS induced an even stronger confinement effect on PEO crystallization.

Simultaneous type-I/type-II emission from CdSe/CdS/ZnSe nano-heterostructures.

Core/intermediate/shell (C/I/S) structures with Type-I emission are well-known and are gaining immense importance due to their superior luminescence properties. Here, we report a unique C/I/S structure composed of CdSe/CdS/ZnSe that exhibits both Type-I and Type-II phenomena. The structures have been well characterized using a combination of optical and structural techniques. The photoluminescence (PL) and photoluminescence excitation (PLE) data indicate the formation of a combined Type-I and Type-II structure in one material, results supported by simple theoretical calculations. Single particle fluorescence reveals colocalization of both the emissions. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirm the structure of these particles. The time-resolved fluorescence studies show the possibility of tuning the lifetime of these materials by changing the Type-I/Type-II thickness ratios. It is possible to form these two separate excitons in the same system separated by a CdS intermediate layer that acts both as a barrier and an active member of the Type-II system allowing the generation and recombination of two excitons, in violation of Kasha's rule.