Design and synthesis of functional polymers, tuning macro and nano scale materials properties by employing controlled surface initiated radical polymerisation techniques, fabrication of functional ceramic and polymeric nanoporous hybrid membranes for various applications like fuel cells, bio-sensing, scaffolds for tissue engineering/regenerative medicines and controlled ion channeling, chemistry of bio-conjugation, surface functionalisation of carbon nanotubes (CNTs) for solar cell applications, synthesis and surface functionalisation of magnetic nanoparticles for environment and biomedical applications, nanotechnology based solutions for health, environment and energy related issues.
Title: “Polycyanurate Thermoset Networks with High Thermal, Mechanical, and Hydrolytic Stability Based on Liquid Multifunctional Cyanate Ester Monomers with Bisphenol A and AF Units”.
Two cyanate ester monomers (CEMs) based on oligomeric aryl ether (OAE) derivatives of bisphenol AF and bisphenol A, with multiple reactive cyanate groups, were developed as technologically highly relevant thermosets. These CEMs are liquids processable at room temperature and can be crosslinked by cyclotrimerisation of the cyanate groups to form extended polycyanurate (PC) networks at lower temperatures than many existing CEMs. The cured PCs have high Tgs, with excellent thermal, mechanical, and dielectric properties. PC nanorods with diameters of 65 or 380 nm could be moulded in porous alumina templates from the OAE-CEMs. The high aspect ratio nanorods with a length in the order of 100 µm were hydrolytically stable upon extended exposure to boiling water. Macromolecular Chemistry and Physics, 2008, 209 (16), 1673–1685.
Title: "Facile Large-Scale Fabrication of Proton Conducting Channels"
A new approach to the facile large-scale fabrication of robust silicon membranes with artificial proton conducting channels is presented. Ordered two-dimensional macroporous silicon was rendered proton conducting by growing a thick uniform polyelectrolyte brush using surface-initiated atom transfer radical polymerisation throughout the porous matrix. With proton conductivities in the range of 10−2 S/cm, these proof-of-concept experiments highlight a promising alternative for producing tailorable proton conducting membranes. Journal of the American Chemical Society, 2008, 30 (39), 13140-13144.
Title: "Polyetheretherketone (PEEK) Surface Functionalization via Surface Initiated Atom Transfer Radical Polymerization".
The interfacial properties of commercial poly(ether ether ketone) (PEEK) have been tailored by tethering polymeric brushes to the PEEK surface via surface-initiated atom transfer radical polymerisation (SI-ATRP). The immobilisation of an ATRP initiator on the PEEK surface was achieved by an unprecedented simple two-step wet chemical method. The keto groups at the PEEK surface were first wet chemically reduced to hydroxyl groups, and then 2-bromoisobutyryl groups were covalently anchored at the PEEK surface as ATRP initiator. SI-ATRP was performed at these functionalised PEEK surfaces with the three different monomers: potassium 3-(methacryloyloxy)propane-1-sulfonate (MPS), monomethoxy-terminated oligo(ethylene glycol)methacrylate (MeOEGMA), and N-isopropylacrylamide (NIPAAm). Atomic force microscopy, scanning electron microscopy, attenuated total reflection infrared spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy ascertained the successful grafting of these polymer brushes at the PEEK surface. These brush-modified PEEK surfaces exhibited fully the physiochemical properties of the respective polymer brush: the surface with polyMPS brush showed selective staining by electrostatic interaction, while the polyMeOEGMA-modified surface was biorepellent. The surface modified with polyNIPAAm brush demonstrated a thermally responsive polarity change. Langmuir 2009, 25 (11),6214-6220
Title: “Highly Proton-Conducting Self-Humidifying Microchannels Generated by Copolymer Brushes on a Scaffold”.
Filling in the gaps: Macroporous silicon membranes modified with sulfonated polymer brushes have been synthesised by pore-filling surface polymerisation (see picture) to give proton-conducting channels with tailor-made, finely tuned physicochemical characteristics. These membranes display high conductivity values (ca. 10−2 S cm−1) regardless of the humidity, thus surpassing the performance of nafion. Angewandte Chemie-International Edition, 2009, 48 (17), 3124-3128.
Title: “Ionic Transport Through Single Solid-State Nanopores Controlled with Thermally Nanoactuated Macromolecular Gates”
Single solid-state nanopores modified with poly-N-isopropylacrylamide (NIPAM) brushes display thermally controlled gating properties. Below the lower critical solubility temperature (LCST) NIPAM brushes are swollen and, consequently, dramatically reduce the effective cross section of the nanopores (see image). Conversely, above the LCST the brushes dehydrate and suffer a transition into a collapsed state, which promotes the widening nanopore and enables a substantial flow of ions. Small, 2009, 5 (11), 1287-1291.
Title: “Single Conical Nanopores Displaying pH-Tunable Rectifying Characteristics. Manipulating Ionic Transport With Zwitterionic Polymer Brushes”.
In this work we describe for the first time the integration of “smart” polymer brushes into single conical nanopores to obtain a new highly functional signal-responsive chemical nanodevice. The responsive brushes were constituted of zwitterionic monomers whose charge is regulated via pH changes in the environmental conditions. The pH-dependent chemical equilibrium of the monomer units provides a fine-tuning of the ionic transport though the nanopore by simply presetting the pH of the electrolyte solution. Our results demonstrate that this strategy enables a higher degree of control over the rectification properties when compared to the nanochannels modified with charged monolayer assemblies. We envision that these results will create completely new avenues to build-up “smart” nanodevices using responsive polymer brushes integrated into single conical nanopores. Journal of the American Chemical Society, 2009, 131 (6), 2070-2071
Title: “Synthetic Proton-Gated Ion Channels via Single Solid-State Nanochannels Modified with Responsive Polymer Brushes”.
The creation of switchable and tunable nanodevices displaying transport properties similar to those observed in biological pores poses a major challenge in molecular nanotechnology. Here, we describe the construction of a fully “abiotic” nanodevice whose transport properties can be accurately controlled by manipulating the proton concentration in the surrounding environment. The ionic current switching characteristics displayed by the nanochannels resemble the typical behavior observed in many biological channels that fulfill key pH-dependent transport functions in living organisms, that is, the nanochannel can be switched from an “off” state to an “on” state in response to a pH drop. The construction of such a chemical nanoarchitecture required the integration of stable and ductile macromolecular building blocks constituted of pH-responsive poly(4-vinyl pyridine) brushes into solid state nanopores that could act as gate-keepers managing and constraining the flow of ionic species through the confined environment. In this context, we envision that the integration of environmental stimuli-responsive brushes into solid-state nanochannels would provide a plethora of new chemical alternatives for molecularly design robust signal-responsive “abiotic” devices mimicking the function of proton-gated ion channels commonly encountered in biological membranes. Nano Letters, 2009, 9 (7), 2788-2793.
Title: “Construction of Hybrid Polymer-Silicon Proton Conducting Membranes via a Pore-Filling Surface-Initiated Polymerization Approach”.
An alternative approach for the creation of proton conducting platforms is presented. The methodology is based on the so-called “pore-filling concept”, which relies on the filling of porous matrices with polyelectrolytes to obtain proton conducting platforms with high dimensional stability. Polymer−silicon composite membranes, with well-defined polyelectrolyte microdomains oriented normal to the plane of the membrane, were prepared using photoelectrochemically etched silicon as a microstructured scaffold. Ordered two- dimensional macroporous silicon structures were rendered proton conducting by filling the micropores via a surface-initiated atom transfer radical polymerisation process. The morphological aspects, chemical stability, and performance of the hybrid assemblies were characterised by a set of techniques including scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, nuclear magnetic resonance and impedance spectroscopy, among others. The fabricated silicon-poly(sodium 2-acrylamide-2-methylpropane sulfonate) hybrid membranes displayed proton conductivities in the range of 1 × 10−2 S/cm. This work illustrates the potential of hybrid polymer−silicon composite membranes synthesised by pore-filling surface-initiated polymerisation to create proton conducting platforms in a simple and straightforward manner. Versatility and relative ease of preparation are two key aspects that make this approach an attractive alternative for the molecular design and preparation of proton conducting systems. ACS Applied Materials & Interfaces, 2010, 2 (1), 279-287.
Title: “A Facile Route for the Preparation of Azide-Terminated Polymers."Clicking" Polyelectrolyte Brushes on Planar Surfaces and Nanochannels”.
In this work we describe the facile preparation of azide-terminated polymers by conventional radical polymerisation (cRP) using azo initiators bearing azidegroups. We show that cRP provides a convenient avenue for the preparation of azide end-functional polymers in a one-step process. The versatility of this chemical methodology was demonstrated by the synthesis of unprecedented azide end group-functionalised sodium polystyrene sulfonate (PSSNa) and poly(2-methacryloyloxyethyl-trimethylammonium chloride) (PMETAC) which were then “clicked” onto alkyne-terminated silicon surfaces and polyethylene terephthalate nanochannels to form polyelectrolyte brush layers. The facile synthesis of the end-functionalised macromolecular building blocks will enable the creation of a wide variety of “clickable” architectures using very simple synthetic tools. We are confident that these results will constitute a key element in the “click” chemistry toolbox and, as such, will have strong implications for the molecular design of interfaces using macromolecular architectures. Polymer Chemistry, 2010, 1, 183-192.
Title: “Proton-regulated rectified ionic transport through solid-state conical nanopores modified with phosphate-bearing polymer brushes.
We describe the use of polyprotic polymer brushes to construct robust signal-responsive chemical devices mimicking the transport properties of proton regulated biological channels. Chem. Commun.”, 2010, 46, 1908-1910.
Title: “Polycyanurate Nanorod Arrays for Optical-Waveguide-based Biosensing”.
We demonstrate high-sensitivity biosensing by optical waveguide spectroscopy (OWS) at visible wavelengths using aligned polycyanurate thermoset nanorods (PCNs) arranged in extended arrays as waveguides. The PCNs formed by thermal polymerisation of a cyanate ester monomer in self-ordered nanoporous alumina templates were 60 nm in diameter and 650 nm in length. Subtle refractive index changes of the medium surrounding the nanorods could be detected by monitoring the angular shifts of waveguiding modes. The sensing figure of merit thus achieved amounted to 196 reciprocal refractive index units and is, therefore, higher than that of other sensors based on angular modulation, while the configuration used here is eligible for further surface functionalisation. Kinetics of the binding of taurine to the surface cyanate groups of the PCNs was monitored by OWS. Thus, modified PCNs bearing sulfonic acid groups at their surfaces were obtained. PCN arrays may represent a versatile platform for the design of biosensors. Nano Letters, 2010, 10 (6), 2173-2177.
Title: “Surface Initiated Polymerization on Pulsed Plasma Deposited Polyallylamine: A Polymer Substrate-Independent Strategy to Soft Surfaces with Polymer Brushes”.
Pulsed plasma deposited polyallylamine leads to amine functionalised soft surfaces, which is demonstrated as a general route to surface tethering an initiator for highly resourceful atom transfer radical polymerisation (ATRP) through a robust amide linkage. The successful fabrication of polymer brushes is ascertained by ATR-IR spectroscopy and XPS. The general scope of presented strategy is demonstrated by employing five different polymer types. Macromolecular Rapid Communications, 2011, DOI: 10.1002/marc.20110036