Confined photo-catalytic oxidation: a fast surface hydrophilic modification method for polymeric materials

Controlled radical polymerization of hydrophilic and zwitterionic brush-like polymers from silk fibroin surfaces

Bombyx mori silk fibroin is a fibrous protein whose tunable properties and biocompatibility have resulted in its utility in a wide-variety of applications, including as drug delivery vehicles, wound dressings, and tissue engineering scaffolds. Control of protein and cell attachment is vital to the performance of biomaterials, but silk fibroin is mostly hydrophobic and interacts nonspecifically with cells and proteins. Silk functionalised with hydrophilic polymers reduces attachment, but the low number of reactive sites makes achieving a uniform conjugation a persistent challenge. This work presents a new approach to grow brush-like polymers from the surface of degradable silk films, where the films were enriched with hydroxyl groups, functionalised with an initiator, and finally reacted with acrylate monomers using atom transfer radical polymerisation. Two different routes to hydroxyl enrichment were investigated, one involving reaction with ethylene oxide (EO) and the other using a two-step photo-catalysed oxidation reaction. Both routes increased surface hydrophilicity, and hydrophilic monomers containing either uncharged (poly(ethylene glycol), PEG) pendant groups or zwitterionic pendant groups were polymerised from the surfaces. The initial processing of the films to induce beta sheet structures was found to impact the success of the polymerizations. Compared to the EO modified or unmodified silk surfaces, the oxidation reaction resulted in more polymer conjugation and the surfaces appear more uniform. Mesenchymal stem cell and protein attachment were the lowest on polymers grown from oxidised surfaces. PEG-containing brush-like polymers displayed lower protein attachment than surfaces conjugated with PEG using a previously reported "grafting to" method, but polymers containing zwitterionic side chains displayed both the lowest contact angles and the lowest cell and protein attachment. This finding may arise from the interactions of the zwitterionic pendant groups through their permanent dipoles and is an important finding because PEG is susceptible to oxidative damage that can reduce efficacy over time. These modified silk materials with lower cell and protein attachments are envisioned to find utility when enhanced diffusion around surfaces is required, such as in drug delivery implants.

Formation of submicron-sized silica patterns on flexible polymer substrates based on vacuum ultraviolet photo-oxidation

Formation of precise and high-resolution silica micropatterns on polymer substrates is of importance in surface structuring for flexible device fabrication of optics, microelectronic, and biotechnology. To achieve that, substrates modified with affinity-patterns serve as a strategy for site-selective deposition. In the present paper, vacuum ultraviolet (VUV) treatment is utilized to achieve spatially-controlled surface functionalization on a cyclo-olefin polymer (COP) substrate. An organosilane, 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS), preferentially deposits on the functionalized regions. Well-defined patterns of TMCTS are formed with a minimum feature of ∼500 nm. The secondary VUV/(O)-treatment converts TMCTS into SiO _x , meanwhile etches the bare COP surface, forming patterned SiO _x /COP microstructures with an average height of ∼150 nm. The resulting SiO _x patterns retain a good copy of TMCTS patterns, which are also consistent with the patterns of photomask used in polymer affinity-patterning. The high quality SiO _x patterns are of interests in microdevice fabrication, and the hydrophilicity contrast and adjustable heights reveal their potential application as a "stamp" for microcontact printing (μCP) techniques.

Mild and Selective C-H Activation of COC Microfluidic Channels Allowing Covalent Multifunctional Coatings

Plastics, such as cyclic olefin copolymer (COC), are becoming an increasingly popular material for microfluidics. COC is used, in part, because of its (bio)-chemical resistance. However, its inertness and hydrophobicity can be a major downside for many bioapplications. In this paper, we show the first example of a surface-bound selective C-H activation of COC into alcohol C-OH moieties under mild aqueous conditions at room temperature. The nucleophilic COC-OH surface allows for subsequent covalent attachments, such as of a H-terminated silane. The resulting hybrid material (COC-Si-H) was then modified via a photolithographic hydrosilylation in the presence of ω-functionalized 1-alkenes to form a new highly stable, solvent-resistant hybrid surface.

Self-aligned, full solution process polymer field-effect transistor on flexible substrates

Conventional techniques to form selective surface energy regions on rigid inorganic substrates are not suitable for polymer interfaces due to sensitive and soft limitation of intrinsic polymer properties. Therefore, there is a strong demand for finding a novel and compatible method for polymeric surface energy modification. Here, by employing the confined photo-catalytic oxidation method, we successfully demonstrate full polymer filed-effect transistors fabricated through four-step spin-coating process on a flexible polymer substrate. The approach shows negligible etching effect on polymeric film. Even more, the insulating property of polymeric dielectric is not affected by the method, which is vital for polymer electronics. Finally, the self-aligned full polymer field-effect transistors on the flexible polymeric substrate are fabricated, showing good electrical properties and mechanical flexibility under bending tests.

Chemoselective phototransformation of C-H bonds on a polymer surface through a photoinduced cerium recycling redox reaction

It is generally accepted that Ce(4+) is unable to directly oxidize unreactive alkyl C-H bonds without the assistance of adjacent polar groups. Herein, we demonstrate in our newly developed confined photochemical reaction system that this recognized issue may be challenged. As we found, when a thin layer of a CeCl(3)/HCl aqueous solution was applied to a polymeric substrate and the substrate subjected to UV irradiation, Ce(3+) was first photooxidized to form Ce(4+) in the presence of H(+), and the in situ formed Ce(4+) then performs an oxidation reaction on the C-H bonds of the polymer surface to form surface-carbon radicals for radical graft polymerization reactions and functional-group transformations, while reducing to Ce(3+) and releasing H(+) in the process. This photoinduced cerium recycling redox (PCRR) reaction behaved as a biomimetic system in an artificial recycling reaction, leading to a sustainable chemical modification strategy for directly transforming alkyl C-H bonds on polymer surfaces into small-molecule groups and polymer brushes. This method is expected to provide a green and economical tool for industrial applications of polymer-surface modification.

Modification of indium tin oxide with persulfate-based photochemistry toward facile, rapid, and low-temperature interface-mediated multicomponent assembling

The well-controlled material assembly and patterning on indium tin oxide (ITO) coating layer is of great importance for the practical fabrication of a functional device. Nonetheless, the conventional way to achieve this aim is still mainly based on the combination of photolithography with pattern transfer techniques (e.g., wet/dry etching, μCP) due to the lack of one method that is able to directly afford site-selective ITO surface tailoring and subsequent templating for material assembly. Herein, we reported a novel, fast, and efficient photochemical reaction to accurately tailor the surface property of ITO with light-controlled site-selectivity, thus resulting in direct photoresist-free and etching/contact-free lithographic patterning of building blocks, e.g., ZnO, BaTiO3, CdS, lipid membrane, conductive polymers, colloids, and liquid crystals. The entire process reveals new interfacial chemistry suitable for inorganic metal oxide and its important versatile implications for low-cost fabrication of large-area flat and flexible optical/electronic/biorelated devices.

Hydroxylation of organic polymer surface: method and application

It may be hardly believable that inert C-H bonds on a polymeric material surface could be quickly and efficiently transformed into C-OH by a simple and mild way. Thanks to the approaches developed recently, it is now possible to transform surface H atoms of a polymeric substrate into monolayer OH groups by a simple/mild photochemical reaction. Herein the method and application of this small-molecular interfacial chemistry is highlighted. The existence of hydroxyl groups on material surfaces not only determines the physical and chemical properties of materials but also provides effective reaction sites for postsynthetic sequential modification to fulfill the requirements of various applications. However, organic synthetic materials based on petroleum, especially polyolefins comprise mainly C and H atoms and thus present serious surface problems due to low surface energy and inertness in reactivity. These limitations make it challenging to perform postsynthetic surface sequential chemical derivatization toward enhanced functionalities and properties and also cause serious interfacial problems when bonding or integrating polymer substrates with natural or inorganic materials. Polymer surface hydroxylation based on direct conversion of C-H bonds on polymer surfaces is thus of significant importance for academic and practical industrial applications. Although highly active research results have reported on small-molecular C-H bond activation in solution (thus homogeneous), most of them, featuring the use of a variety of transition metals as catalysts, present a slow reaction rate, a low atom economy and an obvious environmental pollution. In sharp contrast to these conventional C-H activation strategies, the present Spotlight describes a universal confined photocatalytic oxidation (CPO) system that is able to directly convert polymer surface C-H bonds to C-OSO3(-) and, subsequently, to C-OH through a simple hydrolysis. Generally speaking, these newly implanted hydroxyl groups preserve their own reactivity toward other complementary compounds, thus creating a novel base with distinct surface properties. Thanks to this functionalized platform, a wide range of organic, inorganic and metal materials have been attached to conventional organic polymer substrates through the rational engineering of surface molecular templates from small functional groups to macromolecules. It is expected that the proposed novel CPO method and its versatile usages in advanced material applications will offer new opportunities for a variety of scientific communities, especially for those working on surface/interface modulation.

Construction of DNA microarrays on cyclic olefin copolymer surfaces using confined photocatalytic oxidation

A novel strategy for DNA immobilization on cyclic olefin copolymer surfaces.

Pyrolyzed carbon film diodes

We have previously reported pyrolyzed parylene C (PPC) as a conductive carbon electrode material for use with micropipets, atomic force microscopy probes, and planar electrodes. Advantages of carbon electrode fabrication from PPC include conformal coating of high-aspect ratio micro/nanoscale features and the benefits afforded by chemical vapor deposition of carbon polymers. In this work, we demonstrate chemical surface doping of PPC through the use of previously reported methods. Chemically treated PPC films are characterized by multiple spectroscopic and electronic measurements. Pyrolyzed parylene C and doped PPC are used to construct diodes that are examined as both p-n heterojunction and Schottky barrier diodes. Half-wave rectification is achieved with PPC diodes and demonstrates the applicability of PPC as a conductive and semiconductive material in device fabrication.

An extremely simple and effective strategy to tailor the surface performance of inorganic substrates by two new photochemical reactions

This article reports on a new sequential strategy to fabricate monolayer functional organosilane films on inorganic substrate surfaces, and subsequently, to pattern them by two new photochemical reactions. (1) By using UV light (254 nm) plus dimethylformamide (DMF), a functional silane monolayer film could be fabricated quickly (within minutes) under ambient temperature. (2) The organic groups of the formed films became decomposed in a few minutes with UV irradiation coupled with a water solution of ammonium persulfate (APS). (3) When two photochemical reactions were sequentially combined, a high-quality patterned functional surface could be obtained thanks to the photomask.

Photoactivation of alkyl C-H and silanization: a simple and general route to prepare high-density primary amines on inert polymer surfaces for protein immobilization

Surface modification through implanting functional groups has been demonstrated to be extremely important to biomedical applications. The usage of organic polymer phase is often required to achieve satisfactory results. However, organic surfaces usually have poor chemical reactivity toward other reactants and target biomolecules because these surfaces usually only consist of simple alkyl (C-H) and/or alkyl ether (ROR') structures. For the first time, we here report the potential to perform silanization techniques on alkyl polymer surface, which provide a simple, fast, inexpensive, and general method to decorate versatile functional groups at the molecular level. As an example, high-density primary amines could be obtained on a model polymer, polypropylene substrate, through the reaction between amine-capped silane, 3-aminopropyltriethoxysilane (APTES) and hydroxylated polypropylene surface. A model protein, immunoglobulin (IgG), could be effectively immobilized on the surface after transforming amines to aldehydes by the aldehyde-amine condensation reaction between glutaraldehyde (GA) and amines. The routes we report here could directly make use of the benefits from well-developed silane chemistry, and hereby are capable of grafting any functionalities on inert alkyl surfaces via changing the terminal groups in silanes, which should instantly stimulate the development of many realms such as microarrays, immunoassays, biosensors, filtrations, and microseparation.

Positive and negative ZnO micropatterning on functionalized polymer surfaces

Patterned ZnO deposition on substrates has received increasing attention because of its great potential in photocatalysis, energy conversion, and electro-optical techniques. Chemical solution growth is especially promising for organic substrates due to its very mild reaction conditions. Here this method is used on functionality-patterned polymer surfaces in order to fabricate positive and negative ZnO micropatterns. A ZnO film made of arrayed rods, typically 500-750 nm in diameter and 2.5 microm in length, is selectively obtained on sulfated and hydroxylated regions of biaxially oriented poly(propylene), giving rise to positive patterns. For reactive polyesters such as poly(ethylene terephthalate), the ZnO rods selectively remain on the unmodified original regions, creating negative patterns. Unlike complex photolithography procedures, the irradiation and patterning processes do not require the use of positive or negative photoresists, and possible damage from acidic solutions on the underlying substrate during the chemical etching process is avoided. The process thus proves to be a simple, creditable, and low-cost method, which could be easily applied on a variety of inert and reactive polymer surfaces.

Positive and Negative TiO2 Micropatterns on Organic Polymer Substrates

Ordered titanium dioxide (TiO2) films have received increasing attention because of their great potential in photocatalysis, energy conversion, and electrooptical techniques. Such films are often fabricated as coatings on various substrates such as silicon or a variety of polymers. Liquid-phase deposition (LPD) of TiO2 films is especially promising for organic substrates due to its very mild reaction conditions. In the present paper, LPD is conducted on a wettability-patterned polypropylene surface to fabricate positive and negative TiO2 micropatterns. A thin layer of ammonium persulfate in an aqueous solution was sandwiched between two biaxially oriented polypropylene (BOPP) films, and a photomask was employed to control the irradiation region. Within a short time interval, a high hydrophilicity could be obtained on the irradiation region, and an effective wettability contrast between the irradiated and unirradiated regions could be created to further induce the formation of two types of TiO2 micropatterns. Up until now, most approaches for micropatterning have been based on self-assembled monolayers on surfaces of gold (or other noble metals), silicon, and various polyesters. With the present method, however, there is no longer any limitation in the type of substrate used. Our work demonstrates that an anatase TiO2 film could be selectively deposited on a hydrophilic region, giving rise to a positive pattern with significant bonding strength and good line edge acuity, providing an effective solution toward the microfabrication on various inert polymer substrates. More surprisingly, we find, for the first time, that TiO2 could also be selectively retained on a hydrophobic region to form a negative pattern by simply adjusting the reaction conditions. Further analysis of the mechanism shows that, independent of the deposition conditions, the TiO2 deposition pattern changes gradually, from being initially negative to becoming positive as the deposition time increases. The surface functionality changes (from sulfate to hydroxyl groups) during the deposition, and the resulting difference in the affinity for TiO2 is used to interpret this negative-to-positive pattern change. Such negative patterns refute the conventional opinion that only hydrophilic regions favor the formation of TiO2 films and could be used to fabricate large areas (mm2) of interconnected TiO2 micronetworks. Such networks are difficult to obtain by conventional metallic masks, and the present method is expected to provide new strategies in the fabrication of flexible photomasks and macro/mesoporous TiO2 films. An example is given wherein a patterned photografting of poly(acrylic acid) on the surface of BOPP is achieved by using such a polymer-based photomask. The innovativeness of this method arises from its ability to provide negative patterning, whereas present related approaches have been found only to give positive patterns from an equivalent photomask. Unlike complex photolithography procedures, our irradiation and patterning process does not require the use of positive or negative photoresists, and should thus prove to be a simple, fast, and low-cost method.