Surface-initiated Photoiniferter-mediated Polymerization Research Articles

Self-Templating Copolymerization to Produce Robust Conductive Nanocoatings Based on Conjugated Polymer Brushes with Implementable Memristive Characteristics.

An effective synthesis of conductive polymer brushes, i.e., self-templating surface-initiated copolymerization (ST-SICP), is developed. It proceeds through copolymerization of pendant thiophene groups in the precursor multimonomer poly(3-methylthienyl methacrylate) (PMTM) brushes with free 3-methylthiophene (3MT) monomers leading to PMTM-co-P3MT brushes. This approach leads to improved conformational freedom of generated conjugated poly(thiophene)-based chains and their higher share in the brushes with respect to conjugation of pendant thiophene groups only. As a result, best performing conjugated PMTM-co-P3MT brushes demonstrate high ohmic conductivity in both out-of-plane and in-plane direction. Furthermore, thanks to the covalent anchoring as well as intra- and intermolecular connections, highly stable and mechanically robust nanocoatings are produced which can survive mechanical cleaning and long-term storage under ambient conditions. Grafting of ionic poly(sodium 4-styrenesulfonate) (PSSNa) in between PMTM-co-P3MT chains brings new properties to such binary mixed brushes that can operate as thin-film memristive coating with switchable conductance. It is worth mentioning that the crucial synthetic steps, i.e., grafting of precursor PMTM brushes by surface-initiated organocatalyzed atom transfer radical polymerization (SI-O-ATRP) and PSSNa chains by surface-initiated photoiniferter-mediated polymerization (SI-PIMP) are conducted under ambient conditions using only microliter volumes of reagents providing methodology that can be considered for use beyond the laboratory scale.

Temperature Responsive Diblock Polymer Brushes as Nanoreactors for Silver Nanoparticles Catalysis.

Metal nanoparticles are widely used in catalysis. Loading metal nanoparticles into polymer brushes has aroused wide attention, but regulation of catalytic performance still needs to be improved. The novel diblock polymer brushes, polystyrene@sodium polystyrene sulfonate-b-poly (N-isopropylacrylamide) (PSV@PSS-b-PNIPA) and PSV@PNIPA-b-PSS with reversed block sequence, were prepared by surface initiated photoiniferter-mediated polymerization (SI-PIMP) and used as nanoreactors to load silver nanoparticles (AgNPs). The block sequence caused the difference of conformation and further affected the catalytic performance. PSV@PNIPA-b-PSS@Ag was found to be able to control the amount of AgNPs exposed to external reactant of 4-nitrophenol at different temperatures to achieve regulation of the reaction rate due to the hydrogen bonds and further physical crosslinking between PNIPA and PSS.

Anion Exchange Affinity-Based Controllable Surface Imprinting Synthesis of Ultrathin Imprinted Films for Protein Recognition.

Anion exchange affinity-based controllable surface imprinting is an effective approach to overcome low imprinting efficiency and high non-specific binding capacity. The template proteins were first immobilized on the anchored tetraalkylammonium groups of the nanoparticles via anion exchange affinity-based interactions, enabling monolayer sorption using a low template concentration. The combined use of surface-initiated photoiniferter-mediated polymerization to precisely control the imprinted film thickness, allowing the formation of homogeneous binding cavities, and the construction of effective binding sites resulted in a low non-specific binding capacity and high imprinting efficiency. The obtained imprinted films benefited from the anion exchange mechanism, exhibiting a higher imprinting factor and faster binding rate than the reference material. Binding tests revealed that the binding strength and selective recognition properties could be tuned to a certain extent by adjusting the NaCl concentration. Additionally, in contrast to the harsh template elution conditions of the covalent immobilization approach, over 80% of the template molecules were readily removed from the imprinted films using supersonic elution with an aqueous mixture of NaCl and HAc. Introducing template immobilization by anion exchange interactions to the synthesis of imprinted materials may provide a new approach for effective biomacromolecular imprinting.

Conformation Variation and Tunable Protein Adsorption through Combination of Poly(acrylic acid) and Antifouling Poly(N-(2-hydroxyethyl) acrylamide) Diblock on a Particle Surface.

Adsorption and desorption of proteins on biomaterial surfaces play a critical role in numerous biomedical applications. Spherical diblock polymer brushes (polystyrene with photoiniferter (PSV) as the core) with different block sequence, poly(acrylic acid)-b-poly(N-(2-hydroxyethyl) acrylamide) (PSV@PAA-b-PHEAA) and poly(N-(2-hydroxyethyl) acrylamide)-b-poly(acrylic acid) (PSV@PHEAA-b-PAA) were prepared via surface-initiated photoiniferter-mediated polymerization (SI-PIMP) and confirmed by a series of characterizations including TEM, Fourier transform infrared (FTIR) and elemental analysis. Both diblock polymer brushes show typical pH-dependent properties measured by dynamic light scattering (DLS) and Zeta potential. It is interesting to find out that conformation of PSV@PAA-b-PHEAA uniquely change with pH values, which is due to cooperation of electrostatic repulsion and steric hindrance. High-resolution turbidimetric titration was applied to explore the behavior of bovine serum albumin (BSA) binding to diblock polymer brushes, and the protein adsorption could be tuned by the existence of PHEAA as well as apparent PAA density. These studies laid a theoretical foundation for design of diblock polymer brushes and a possible application in biomedical fields.

Conformation Study of Dual Stimuli-Responsive Core-Shell Diblock Polymer Brushes.

Stimuli-responsive nanoparticles are among the most popular research topics. In this study, two types of core-shell (polystyrene with a photoiniferter (PSV) as the core and diblock as the shell) polymer brushes (PSV@PNIPA-b-PAA and PSV@PAA-b-PNIPA) were designed and prepared using surface-initiated photoiniferter-mediated polymerization (SI-PIMP). Moreover, their pH- and temperature-stimuli responses were explored by dynamic light scattering (DLS) and turbidimeter under various conditions. The results showed that the conformational change was determined on the basis of the competition among electrostatic repulsion, hydrophobic interaction, hydrogen bonding, and steric hindrance, which was also confirmed by protein adsorption experiments. These results are not only helpful for the design and synthesis of stimuli-responsive polymer brushes but also shed light on controlled protein immobilization under mild conditions.

Salt- and thermo-responsive polyzwitterionic brush prepared via surface-initiated photoiniferter-mediated polymerization

Zwitterionic polymer brushes with strong anti-polyeletrolyte effect have shown great potential in applications of tunable friction, protein absorption/desorption, and “killing and release” antibacterial surfaces. However, such polymer brushes are presently prepared by surface-initiated atom transfer polymerization (SI-ATRP) which is a sophisticated and time-consuming process. In this paper, a simple method, i.e. surface-initiated photoiniferter-mediated polymerization (SI-PIMP) was used to prepare poly (3-(dimethyl (4-vinylbenzyl) ammonio) propyl sulfonate) (polyDVBAPS) brush on silicon wafer. It was shown that the reaction solution had a significant effect on the growth rate and surface roughness of the resulting brushes. In suitable solution (e.g. 0.53 M NaCl solution) brushes with controllable thickness and smooth surface could be obtained. Contact angle and surface friction measurement indicated that the brush prepared by SI-PIMP showed identical salt-responsive properties compared to the brushes from SI-ATRP. In addition, the thermo-responsive properties of polyDVBAPS brush was also investigated, which was further used for tuning surface friction of the brush. Due to the thermo-responsive properties, polyDVBAPS brush showed super lubrication (u ∼ 10−3) in water and NaCl solutions with low concentrations (0.1 M and 0.53 M) when the temperature was increased to 70 °C, 65 °C and 55 °C respectively. Although ultralow friction could be achieved under the conditions of different temperature and salt concentration, the stability against long-term friction highly depended on the environmental condition. At 40 °C and in 1.0 M NaCl solution, ultralow friction was well retained up to 1500 s, while under other conditions (saturated NaCl solution of 25 °C and 0.53 M NaCl solution of 55 °C), aqueous lubrication was rapidly deteriorated. This work not only demonstrated the practicability of the SI-PIMP for the preparation of polyDVBAPS brush, but also realized the tunable surface friction of such polyzwitterionic brush under different environmental conditions, extending its applications in both academic and industrial communities.

Effect of block sequence on responsive behavior of core-shell diblock polymer brushes

In this work, two kinds of core-shell diblock polymer brushes, i.e., poly(acrylic acid)-b-poly(N-isopropylacrylamide) and poly(N-isopropylacrylamide)-b-poly(acrylic acid) diblock polymer brushes with polystyrene cores (PSV@PAA-b-PNIPA and PSV@PNIPA-b-PAA), were prepared by surface-initiated photoiniferter-mediated polymerization (SI-PIMP). Both of them are demonstrated to be dually responsive to pH and temperature. Compared to brushes with random polymer chains of PAA and PNIPA, the control of PAA and PNIPA block sequence not only affects the responsive behavior, but also changes the competition between intermolecular and intramolecular forces which results in remarkable conformation variation. The “inverse” temperature response of PSV@PAA-b-PNIPA could shine light on structure design of polymer brushes for applications such as protein separation.

Development of hierarchical Fe3O4 magnetic microspheres as solid substrates for high sensitive immunoassays.

Improving the detection sensitivity of enzyme linked immunosorbent assay (ELISA) is of the utmost importance for meeting the demand of early disease diagnosis. In this work, a sensitive solid substrate for ELISA, i.e., hierarchical iron oxide magnetic microspheres, Fe3O4@mSiO2@poly[poly(ethylene glycol) methacrylate-co-glycidyl methacrylate], was developed via a novel surface-initiated photoiniferter-mediated polymerization (SI-PIMP) strategy. The magnetic microspheres consist of a magnetic Fe3O4 core that gives a high magnetic response, a 3D backbone, a mesoporous SiO2 middle layer, that facilitates microsphere stability and provides anchoring sites, and polymer brushes, that serve as an antifouling and oriented antibody immobilization layer. As a result, the as-prepared microspheres possess a high antibody loading capacity, an enhanced detection signal and a dramatically improved sensitivity, resulting in a 25-fold improvement over conventional ELISA solid substrates.

Temperature-Responsive Hierarchical Polymer Brushes Switching from Bactericidal to Cell Repellency.

Unlike conventional poly(N-isopropylacrylamide) (PNIPAM)-based surfaces switching from bactericidal activity to bacterial repellency upon decreasing temperature, we developed a hierarchical polymer architecture, which could maintain bactericidal activities at room temperature while presenting bacterial repellency at physiological temperature. In this architecture, a thermoresponsive bactericidal upper layer consisting of PNIPAM-based copolymer and vancomycin (Van) moieties was built on an antifouling poly(sulfobetaine methacrylate) (PSBMA) bottom layer via sequential surface-initiated photoiniferter-mediated polymerization. At room temperature below the lower critical solution temperature (LCST), the PNIPAM-based upper layer was stretchable, facilitating contact killing of bacteria by Van. At physiological temperature (above the LCST), the PNIPAM-based layer collapsed, thus leading to the burial of Van and exposure of bottom PSBMA brushes, finally displaying notable performances in bacterial inhibition, dead bacteria detachment, and biocompatibility, simultaneously. Our strategy provides a novel pathway in the rational design of temperature-sensitive switchable surfaces, which shows great advantages in the real-world infection-resistant applications.

Photoactive Surface‐Grafted Polymer Brushes with Phthalocyanine Bridging Groups as an Advanced Architecture for Light‐Harvesting

Surface-grafted polymer brushes of novel ladder-like architecture were proposed for inducing ordering of chromophores embedded therein. The brushes with acetylene side groups were obtained by surface-initiated photoiniferter-mediated polymerization. The acetylene moieties reacted then through a "click" process with an axially azide-bifunctionalized silicon phthalocyanine bridging the neighboring chains that inherently adopt extended conformations in dense brushes. FTIR, quartz crystal microbalance, and atomic force microscopy were used to study formation and structure of the photoactive brushes varying in grafting densities. Importantly, photophysical properties of the chromophores were virtually unaffected upon embedding them into the brushes, as evidenced by UV/Vis absorption and emission spectroscopy. Owing to the unique ordering of the chromophores, the proposed method may open new opportunities for the fabrication of light-harvesting systems suitable for photovoltaic or sensing applications.

Amino Acid-Based Zwitterionic Polymer Surfaces Highly Resist Long-Term Bacterial Adhesion.

The surfaces or coatings that can effectively suppress bacterial adhesion in the long term are of critical importance for biomedical applications. Herein, a group of amino acid-based zwitterionic polymers (pAAZ) were investigated for their long-term resistance to bacterial adhesion. The polymers were derived from natural amino acids including serine, ornithine, lysine, aspartic acid, and glutamic acid. The pAAZ brushes were grafted on gold via the surface-initiated photoiniferter-mediated polymerization (SI-PIMP). Results show that the pAAZ coatings highly suppressed adsorption from the undiluted human serum and plasma. Long-term bacterial adhesion on these surfaces was investigated, using two kinds of representative bacteria [Gram-positive Staphylococcus epidermidis and Gram-negative Pseudomonas aeruginosa] as the model species. Results demonstrate that the pAAZ surfaces were highly resistant to bacterial adhesion after culturing for 1, 5, 9, or even 14 days, representing at least 95% reduction at all time points compared to the control unmodified surfaces. The bacterial accumulation on the pAAZ surfaces after 9 or 14 days was even lower than on the surfaces grafted with poly[poly(ethyl glycol) methyl ether methacrylate] (pPEGMA), one of the most common antifouling materials known to date. The pAAZ brushes also exhibited excellent structural stability in phosphate-buffered saline after incubation for 4 weeks. The bacterial resistance and stability of pAAZ polymers suggest they have good potential to be used for those applications where long-term suppression to bacterial attachment is desired.

Hierarchical Polymer Brushes with Dominant Antibacterial Mechanisms Switching from Bactericidal to Bacteria Repellent.

Although polycationic surfaces have high antimicrobial efficacies, they suffer from high toxicity to mammalian cells and severe surface accumulation of dead bacteria. For the first time, we propose a surface-initiated photoiniferter-mediated polymerization (SI-PIMP) strategy of constructing a "cleaning" zwitterionic outer layer on a polycationic bactericidal background layer to physically hinder the availability of polycationic moieties for mammalian cells in aqueous service. In dry conditions, the polycationic layer exerts the contact-active bactericidal property toward the adherent bacteria, as the zwitterionic layer collapses. In aqueous environment, the zwitterionic layer forms a hydration layer to significantly inhibit the attachment of planktonic bacteria and the accumulation of dead bacteria, while the polycationic layer kills bacteria occasionally deposited on the surface, thus preserving the antibacterial capability for a long period. More importantly, the zwitterionic hydrated layer protects the mammalian cells from toxicity induced by the bactericidal background layer, and therefore hierarchical antibacterial surfaces present much better biocompatibility than that of the naked cationic references. The dominant antibacterial mechanism of the hierarchical surfaces can switch from the bactericidal efficacy in dry storage to the bacteria repellent capability in aqueous service, showing great advantages in the infection-resistant applications.

Fast clean-up and selective enrichment of florfenicol in milk by restricted access media molecularly imprinted magnetic microspheres based on surface-initiated photoiniferter-mediated polymerization

A novel RAM-MIMM with water-compatible, exclusion biomacromolecules and selective enrichment analytes was prepared using a surface-initiated iniferter technique for magnetic dispersion extraction of trace florfenicol from milk samples.

Antifouling gold surfaces grafted with aspartic acid and glutamic acid based zwitterionic polymer brushes.

We report two new amino acid based antifouling zwitterionic polymers, poly(N(4)-(2-methacrylamidoethyl)asparagine) (pAspAA) and poly(N(5)-(2-methacrylamidoethyl)glutamine) (pGluAA). The vinyl monomers were developed from aspartic acid and glutamic acid. Surface-initiated photoiniferter-mediated polymerization was employed to graft polymer brushes from gold surfaces. Different thickness of polymer brushes was controlled by varying UV irradiation time. The nonspecific adsorption from undiluted human blood serum and plasma was studied by surface plasmon resonance (SPR). With the polymer film as thin as 11-12 nm, the adsorption on pAspAA from serum and plasma was as low as 0.75 and 5.18 ng/cm(2), respectively, and 1.88 and 10.15 ng/cm(2), respectively, for pGluAA. The adsorption amount is comparable to or even better than other amino acid based zwitterionic polymers such as poly(serine methacrylate), poly(lysine methacrylamide), and poly(ornithine methacrylamide) and other widely used antifouling polymers such as poly(sulfobetaine methacrylate), even under thinner polymer film thickness. The pAspAA and pGluAA grafted surfaces also showed strong resistance to endothelial cell attachment. The possession of both zwitterionic structure and hydrophilic amide groups, biomimetic property, and multifunctionality make pAspAA and pGluAA promising candidates for biocompatible antifouling functionalizable materials.

Amino acid-based zwitterionic polymers: antifouling properties and low cytotoxicity

A group of five amino acid containing zwitterionic vinyl monomers, based on serine, lysine, ornithine, glutamic acid, and aspartic acid, respectively, were proposed and developed for potential antifouling applications. Their polymer brushes were grafted on gold chips by surface-initiated photoiniferter-mediated polymerization. We then compared their performance in resisting protein adsorption from full human serum and plasma. All five polymers can reduce protein adsorption by more than 90% compared to the unmodified gold. The ornithine-based and aspartic acid-based poly(methacrylamide) can most strongly resist protein adsorption from serum and plasma, compared to the other three. The ability of surfaces to suppress bacterial adhesion is another criterion in evaluating antifouling properties of materials. Our results show that the five polymer-grafted surfaces can significantly suppress Escherichia coli K12 adhesion to 99% compared to the bare gold surface. The zwitterionic structure of amino acids, with homogenously distributed and balanced positive and negative charges, is responsible for the outstanding antifouling properties. Considering multiple potential applications (e.g. medical devices and drug delivery) of the antifouling materials, we further systematically evaluated the cytotoxicity of both monomers and polymer nanogels for all five materials at various concentrations. Very low cytotoxicity was observed for all tested amino acid-based monomers and nanogels, which is comparable or even lower than the traditional and some newly developed antifouling materials, which might be related to the biomimetic nature of amino acids.

Two amino acid-based superlow fouling polymers: Poly(lysine methacrylamide) and poly(ornithine methacrylamide)

We developed and investigated two new antifouling zwitterionic polymers, poly(lysine methacrylamide) (pLysAA) and poly(ornithine methacrylamide) (pOrnAA), both derived from natural amino acids – lysine and ornithine, respectively. The pLysAA and pOrnAA brushes were grafted on gold via the surface-initiated photoiniferter-mediated polymerization, with the polymer film thickness controlled by the UV-irradiation time. Nonspecific adsorption from human blood serum and plasma was investigated by surface plasmon resonance. Results show that the adsorption level decreased with the increasing film thickness. With the thin films of ∼14.5nm, the minimal adsorption on pLysAA was 3.9ngcm−2 from serum and 5.4ngcm−2 from plasma, whereas the lowest adsorption on pOrnAA was 1.8 and 3.2ngcm−2, from serum and plasma, respectively. Such protein resistance is comparable to other widely reported antifouling surfaces such as poly(sulfobetaine methacrylate) and polyacrylamide, with a much thinner polymer film thickness. Both pLysAA and pOrnAA showed better protein resistance than the previously reported serine-based poly(serine methacrylate), whereas the pOrnAA is the best among three. The pLysAA- and pOrnAA-grafted surfaces also highly resisted the endothelial cell attachment and Escherichia coli K12 bacterial adhesion. Nanogels made of pLysAA and pOrnAA were found to be ultrastable in undiluted serum, with no aggregation observed after culturing for 24h. Dextran labeled with fluorescein isothiocyanate (FITC–dextran) was encapsulated in nanogels as a model drug. The encapsulated FITC–dextran exhibited controlled release from the pOrnAA nanogels. The superlow fouling, biomimetic and multifunctional properties of pLysAA and pOrnAA make them promising materials for a wide range of applications, such as implant coating, drug delivery and biosensing.

Fabricating a Cycloolefin Polymer Immunoassay Platform with a Dual-Function Polymer Brush via a Surface-Initiated Photoiniferter-Mediated Polymerization Strategy

The development of technologies for a biomedical detection platform is critical to meet the global challenges of various disease diagnoses. In this study, an inert cycloolefin polymer (COP) support was modified with two-layer polymer brushes possessing dual functions, i.e., a low fouling poly[poly(ethylene glycol) methacrylate] [p(PEGMA)] bottom layer and a poly(acrylic acid) (PAA) upper layer for antibody loading, via a surface-initiated photoiniferter-mediated polymerization strategy for fluorescence-based immunoassay. It was demonstrated through a confocal laser scanner that, for the as-prepared COP-g-PEG-b-PAA-IgG supports, nonspecific protein adsorption was suppressed, and the resistance to nonspecific protein interference on antigen recognition was significantly improved, relative to the COP-g-PAA-IgG references. This strategy for surface modification of a polymeric platform is also applicable to the fabrication of other biosensors.

Surface modification of cycloolefin polymer via surface-initiated photoiniferter-mediated polymerization for suppressing bioadhesion

Cycloolefin polymer was modified via surface-initiated photoiniferter-mediated polymerization for suppressing bioadhesion.

Amino Acid-Based Zwitterionic Poly(serine methacrylate) as an Antifouling Material

A serine-based zwitterionic poly(serine methacrylate) (pSerMA) was developed in this work to be used as a potential antifouling material. A surface-initiated photoiniferter-mediated polymerization (SI-PIMP) method was used to graft polymer brushes on gold surfaces. The pSerMA-grafted samples with different polymer film thicknesses were readily prepared by varying the UV-irradiation time. With the optimal film thickness, the adsorptions from bovine serum albumin, human serum, and human plasma onto the pSerMA-grafted surfaces, as evaluated by a surface plasmon resonance (SPR) biosensor, were 1.8, 9.2, and 12.9 ng/cm(2), respectively, comparable to the traditional antifouling material such as poly(ethylene glycol). The pSerMA-grafted surfaces also strongly resisted adhesion from bovine aortic endothelial cells. This is the first work to develop an amino acid-based zwitterionic polymer as an antifouling material, demonstrating that pSerMA is a promising alternative to the traditional ethylene glycol-based antifouling materials.

Internal Architecture of Zwitterionic Polymer Brushes Regulates Nonfouling Properties

In this work, we study how film thickness and chain packing density affect the protein-resistant properties of polymer brushes in complex media. Polymer brushes based on dual-functional poly(carboxybetaine acrylamide) (pCB) were prepared via surface-initiated photoiniferter-mediated polymerization. By adjusting UV radiation time and solvent polarity, pCB films with different thicknesses can be achieved and characterized using an ellipsometer. The packing density of pCB polymer chains is directly related to the swelling ratio of swollen to collapsed film thicknesses. Results showed that the dry film thickness alone, used often in the literature, is not sufficient to correlate with nonfouling properties and the chain packing density must be considered for the design of nonfouling surface coatings.