Thermoresponsive and Photocrosslinkable Poly(2-alkyl-2-oxazoline) Toolbox - Customizable Ultralow-Fouling Hydrogel Coatings for Blood Plasma Environments.

This study focuses on developing surface coatings with excellent antifouling properties, crucial for applications in the medical, biological, and technical fields, for materials and devices in direct contact with living tissues and bodily fluids such as blood. This approach combines thermoresponsive poly(2-alkyl-2-oxazoline)s, known for their inherent protein-repellent characteristics, with established antifouling motifs based on betaines. The polymer framework is constructed from various monomer types, including a novel benzophenone-modified 2-oxazoline for photocrosslinking and an azide-functionalized 2-oxazoline, allowing subsequent modification with alkyne-substituted antifouling motifs through copper(I)-catalyzed azide-alkyne cycloaddition. From these polymers surface-attached networks are created on benzophenone-modified gold substrates via photocrosslinking, resulting in hydrogel coatings with several micrometers thickness when swollen with aqueous media. Given that poly(2-alkyl-2-oxazoline)s can exhibit a lower critical solution temperature in water, their temperature-dependent solubility is compared to the swelling behavior of the surface-attached hydrogels upon thermal stimulation. The antifouling performance of these hydrogel coatings in contact with human blood plasma is further evaluated by surface plasmon resonance and optical waveguide spectroscopy. All surfaces demonstrate extremely low retention of blood plasma components, even with undiluted plasma. Notably, hydrogel layers with sulfobetaine moieties allow efficient penetration by plasma components, which can then be easily removed by rinsing with buffer.

The "Tethered Solvent" Effect - H-Bonding-Controlled Thermo-Halochromism of a Push-Pull Azo Chromophore via Its Secondary Amidoalkyl Acrylamide Side Chain.

The fascinating field of thermo-halochromism of azo chromophores still astounds with unexplored facets nourished by the intricate relationship between molecular structure variations and their spectroscopic signatures. In this respect, we investigated the thermally dependent absorption behaviour of acrylamide derivatives of o-methyl red, characterised by two secondary amide linkages with hydrogen bonding-active protons in the pendant alkyl substituent. The systems were studied by a combination of UV-vis, derivative, and difference, as well as 2D-NMR (Nuclear Overhauser Effect Spectroscopy, NOESY) spectroscopy. These experiments show that the thermo-halochromism is specifically influenced by hydrogen bonding interaction of the secondary amidoalkyl acrylamide side chain with the azobenzene core in dependence of the spacer length. Apparently, the substituent acts like a solvent, which is directly tethered to the chromophore and where the tether length determines the interaction by conformational freedom. We refer to this novel phenomenon as "H-bonding-controlled thermo-halochromism".

Plasmonic nanomaterials with responsive polymer hydrogels for sensing and actuation.

Plasmonic nanomaterials have become an integral part of numerous technologies, where they provide important functionalities spanning from extraction and harvesting of light in thin film optical devices to probing of molecular species and their interactions on biochip surfaces. More recently, we witness increasing research efforts devoted to a new class of plasmonic nanomaterials that allow for on-demand tuning of their properties by combining metallic nanostructures and responsive hydrogels. This review addresses this recently emerged vibrant field, which holds potential to expand the spectrum of possible applications and deliver functions that cannot be achieved by separate research in each of the respective fields. It aims at providing an overview of key principles, design rules, and current implementations of both responsive hydrogels and metallic nanostructures. We discuss important aspects that capitalize on the combination of responsive polymer networks with plasmonic nanostructures to perform rapid mechanical actuation and actively controlled nanoscale confinement of light associated with resonant amplification of its intensity. The latest advances towards the implementation of such responsive plasmonic nanomaterials are presented, particularly covering the field of plasmonic biosensing that utilizes refractometric measurements as well as plasmon-enhanced optical spectroscopy readout, optically driven miniature soft actuators, and light-fueled micromachines operating in an environment resembling biological systems.

κ-Carrageenan Enhances the Biomineralization and Osteogenic Differentiation of Electrospun Polyhydroxybutyrate and Polyhydroxybutyrate Valerate Fibers.

Novel electrospun materials for bone tissue engineering were obtained by blending biodegradable polyhydroxybutyrate (PHB) or polyhydroxybutyrate valerate (PHBV) with the anionic sulfated polysaccharide κ-carrageenan (κ-CG) in varying ratios. In both systems, the two components phase separated as shown by FTIR, DSC and TGA. According to the contact angle data, κ-CG was localized preferentially at the fiber surface in PHBV/κ-CG blends in contrast to PHB/κ-CG, where the biopolymer was mostly found within the fiber. In contrast to the neat polyester fibers, the blends led to the formation of much smaller apatite crystals (800 nm vs 7 µm). According to the MTT assay, NIH3T3 cells grew in higher density on the blend mats in comparison to neat polyester mats. The osteogenic differentiation potential of the fibers was determined by SaOS-2 cell culture for 2 weeks. Alizarin red-S staining suggested an improved mineralization on the blend fibers. Thus, PHBV/κ-CG fibers resulted in more pronounced bioactive and osteogenic properties, including fast apatite-forming ability and deposition of nanosized apatite crystals.

Reversibly tunable plasmonic bandgap by responsive hydrogel grating.

Reversible actuating of surface plasmon propagation by responsive hydrogel grating is reported. Thermo-responsive poly(N-isopropylacrylamide)-based (pNIPAAm) hydrogel nanostructure was designed and tethered to a gold surface in order to switch on and off Bragg scattering of surface plasmons which is associated with an occurrence of a bandgap in their dispersion relation. pNIPAAm-based grating with a period around 280 nm was prepared by using photo-crosslinkable terpolymer and laser interference lithography and it was brought in contact with water. The temperature induced swelling and collapse of pNIPAAm hydrogel grating strongly modulates its refractive index (Δn~0.1) which leads to the reversible opening and closing of a plasmonic bandgap. The experiments demonstrate partial opening of a bandgap with the width of 12 nm at wavelength around 800 nm where SPR exhibited the spectral width of about 75 nm.

Tuning the Structure and Rheology of Polystyrene Particles at the Air-Water Interface by Varying the pH.

We form films of carboxylated polystyrene particles (C-PS) at the air-water interface and investigate the effect of subphase pH on their structure and rheology by using a suite of complementary experimental techniques. Our results suggest that electrostatic interactions drive the stability and the structural order of the films. In particular, we show that by increasing the pH of the subphase from 9 up to 13, the films exhibit a gradual transition from solid to liquidlike, which is accompanied by a loss of the long-range order (that characterizes them at lower values of pH). Direct optical visualization of the layers, scanning electron microscopy, and surface pressure isotherms indicate that the particles deposited at the interface form three-dimensional structures involving clusters, with the latter being suppressed and a quasi-2D particle configuration eventually reached at the highest pH values. Evidently, the properties of colloidal films can be tailored significantly by altering the pH of the subphase.

Semifluorinated Alkanes at the Air-Water Interface: Tailoring Structure and Rheology at the Molecular Scale.

Semifluorinated alkanes form monolayers with interesting properties at the air-water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression-expansion cycles. Starting from a reference system perfluor(dodecyl)dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air-water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression-expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure.

Photoswitching the mechanical properties in Langmuir layers of semifluorinated alkyl-azobenzenes at the air-water interface.

Semifluorinated alkyl-azobenzene derivatives (SFAB) can form stable Langmuir layers at the air-water interface. These systems combine the amphiphobic character of semifluorinated alkyl units as structure-directing motifs with photochromic behavior based on the well-known reversible cis-trans isomerization upon irradiation with UV and visible light. Herein, we report our investigations of the structural and dynamic tunability of these SFAB layers at the air-water interface in response to an external light stimulus. The monolayer structures and properties of [4-(heptadecafluorooctyl)phenyl](4-octylphenyl)diazene (F8-azo-H8) and bis(4-octylphenyl)diazene (H8-azo-H8) were studied by neutron reflectivity, surface pressure-area isotherms with compression-expansion cycles, and interfacial rheology. We find that UV irradiation reversibly influences the packing behavior of the azobenzene molecules and interpret this as a transition from organized layer structures with the main axis of the molecule vertically oriented in the trans form to random packing of the cis isomer. Interestingly, this trans-cis isomerization leads to an increase in surface pressure, which is accompanied by a decrease in viscoelastic moduli. These results suggest ways of tailoring the properties of responsive fluid interfaces.

Complex tracer diffusion dynamics in polymer solutions.

We employ fluorescence correlation spectroscopy (FCS) and coarse-grained molecular dynamics simulations to study the mobility of tracers in polymer solutions. Excluded volume interactions result in crowding-induced slowdown, depending only on the polymer concentration. With specific tracer-polymer attractions, the tracer is slowed down at much lower concentrations, and a second diffusion component appears that is sensitive to the polymer chain length. The two components can be resolved by FCS, only if the distance traveled by the tracer in the polymer-bound state is greater than the FCS focal spot size. The tracer dynamics can be used as a sensitive probe of the nature and strength of interactions, which-despite their local character-emphasize the role of chain connectivity.

Correction: Jaik et al. Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties-Thermoresponsive and Non-Thermoresponsive Gels. Gels 2022, 8, 541.

In the original publication [...].

Simultaneous measurement of mechanical and surface properties in thermoresponsive, anchored hydrogel films.

Hydrogel films have been used extensively in the preparation of biosensors and biomedical devices. The characteristics of the aqueous interface of the polymer layer are significant for the biosensor or device function; likewise, the changing mechanical properties of thermoresponsive polymers are an important feature that affects the polymer behavior. Atomic force microscopy was used here to characterize both the surface and the mechanical properties of polymeric hydrogel films prepared from a thermoresponsive terpolymer of N-isopropylacrylamide and acrylic acid with benzophenonemethacrylate as a photoreactive cross-linker comonomer. The force-distance curves thus obtained were analyzed to assess both the surface forces and the mechanical response that were associated with the hydrogel. These properties were investigated as a function of temperature, in water and in Tris buffer, for different degrees of polymer cross-linking. For samples in water, the distance over which the surface forces were effective was found to remain constant as the temperature was increased from 26 to 42 °C, even though the mechanical response indicated that the samples had been heated past the lower critical solution temperature, or LCST. The bulk of the polymer becomes less soluble above the LCST, although this does not seem to affect the surface properties. This may be due to the segregation of the acrylic acid-rich polymer segments near the gel surface, which is in agreement with reports for related systems.

Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics.

The swelling and collapsing of thermo-responsive poly(N-isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties-Thermoresponsive and Non-Thermoresponsive Gels.

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

Ruthenium(II) Polypyridyl Complexes for Antimicrobial Photodynamic Therapy: Prospects for Application in Cystic Fibrosis Lung Airways.

Antimicrobial photodynamic therapy (aPDT) depends on a variety of parameters notably related to the photosensitizers used, the pathogens to target and the environment to operate. In a previous study using a series of Ruthenium(II) polypyridyl ([Ru(II)]) complexes, we reported the importance of the chemical structure on both their photo-physical/physico-chemical properties and their efficacy for aPDT. By employing standard in vitro conditions, effective [Ru(II)]-mediated aPDT was demonstrated against planktonic cultures of Pseudomonas aeruginosa and Staphylococcus aureus strains notably isolated from the airways of Cystic Fibrosis (CF) patients. CF lung disease is characterized with many pathophysiological disorders that can compromise the effectiveness of antimicrobials. Taking this into account, the present study is an extension of our previous work, with the aim of further investigating [Ru(II)]-mediated aPDT under in vitro experimental settings approaching the conditions of infected airways in CF patients. Thus, we herein studied the isolated influence of a series of parameters (including increased osmotic strength, acidic pH, lower oxygen availability, artificial sputum medium and biofilm formation) on the properties of two selected [Ru(II)] complexes. Furthermore, these compounds were used to evaluate the possibility to photoinactivate P. aeruginosa while preserving an underlying epithelium of human bronchial epithelial cells. Altogether, our results provide substantial evidence for the relevance of [Ru(II)]-based aPDT in CF lung airways. Besides optimized nano-complexes, this study also highlights the various needs for translating such a challenging perspective into clinical practice.

Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface.

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.

Surface initiated polymerization on pulsed plasma deposited polyallylamine: a polymer substrate-independent strategy to soft surfaces with polymer brushes.

The deposition of polyallylamine (PAA) adlayers by pulsed plasma polymerization on various types of polymeric substrates has been explored as a general route to amino functionalized polymeric surfaces. These amino groups are highly suitable for anchoring an atom transfer radical polymerization (ATRP) initiator via a robust amide linkage. Subsequent surface initiated ATRP (SI-ATRP) of monomethoxy oligo(ethylene glycol) methacrylate (MeOEGMA) resulted in polyMeOEGMA brush grafted polymer surfaces. This combined strategy of pulsed plasma polymerization with SI-ATRP was demonstrated for five different polymeric substrates namely polyether ether ketone (PEEK), polyethylene terephthalate (PET), polyimide (PI), polypropylene (PP), and polytetrafluoroethylene (PTFE). Analysis of brush layers by attenuated total reflection infrared (ATR-IR) spectroscopy as well as X-ray photoelectron spectroscopy (XPS) fully corroborated the success of the proposed strategy for all substrate types.

Responsive Hydrogel Binding Matrix for Dual Signal Amplification in Fluorescence Affinity Biosensors and Peptide Microarrays.

A combined approach to signal enhancement in fluorescence affinity biosensors and assays is reported. It is based on the compaction of specifically captured target molecules at the sensor surface followed by optical probing with a tightly confined surface plasmon (SP) field. This concept is utilized by using a thermoresponsive hydrogel (HG) binding matrix that is prepared from a terpolymer derived from poly(N-isopropylacrylamide) (pNIPAAm) and attached to a metallic sensor surface. Epi-illumination fluorescence and SP-enhanced total internal reflection fluorescence readouts of affinity binding events are performed to spatially interrogate the fluorescent signal in the direction parallel and perpendicular to the sensor surface. The pNIPAAm-based HG binding matrix is arranged in arrays of sensing spots and employed for the specific detection of human IgG antibodies against the Epstein-Barr virus (EBV). The detection is performed in diluted human plasma or with isolated human IgG by using a set of peptide ligands mapping the epitope of the EBV nuclear antigen. Alkyne-terminated peptides were covalently coupled to the pNIPAAm-based HG carrying azide moieties. Importantly, using such low-molecular-weight ligands allowed preserving the thermoresponsive properties of the pNIPAAm-based architecture, which was not possible for amine coupling of regular antibodies that have a higher molecular weight.

A new ultralow fouling surface for the analysis of human plasma samples with surface plasmon resonance.

Surface plasmon resonance (SPR) has been widely used to detect a variety of biomolecular systems, but only a small fraction of applications report on the analysis of patients' samples. A critical barrier to the full implementation of SPR technology in molecular diagnostics currently exists for its potential application to analyze blood plasma or serum samples. Such capability is mostly hindered by the non-specific adsorption of interfering species present in the biological sample at the functional interface of the biosensor, often referred to as fouling. Suitable polymeric layers having a thickness ranging from 15 and about 70 nm are usually deposited on the active surface of biosensors to introduce antifouling properties. A similar approach is not fully adequate for SPR detection where the exponential decay of the evanescent plasmonic field limits the thickness of the layer beyond the SPR metallic sensor surface for which a sensitive detection can be obtained. Here, a triethylene glycol (PEG(3))-pentrimer carboxybetaine system is proposed to fabricate a new surface coating bearing excellent antifouling properties with a thickness of less than 2 nm, thus compatible with sensitive SPR detection. The high variability of experimental conditions described in the literature for the quantitative assessment of the antifouling performances of surface layers moved us to compare the superior antifouling capacity of the new pentrimeric system with that of 4-aminophenylphosphorylcholine, PEG-carboxybetaine and sulfobetaine-modified surface layers, respectively, using undiluted and diluted pooled human plasma samples. The use of the new coating for the immunologic SPRI biosensing of human arginase 1 in plasma is also presented.

Self assembling cluster crystals from DNA based dendritic nanostructures.

Cluster crystals are periodic structures with lattice sites occupied by several, overlapping building blocks, featuring fluctuating site occupancy, whose expectation value depends on thermodynamic conditions. Their assembly from atomic or mesoscopic units is long-sought-after, but its experimental realization still remains elusive. Here, we show the existence of well-controlled soft matter cluster crystals. We fabricate dendritic-linear-dendritic triblock composed of a thermosensitive water-soluble polymer and nanometer-scale all-DNA dendrons of the first and second generation. Conclusive small-angle X-ray scattering (SAXS) evidence reveals that solutions of these triblock at sufficiently high concentrations undergo a reversible phase transition from a cluster fluid to a body-centered cubic (BCC) cluster crystal with density-independent lattice spacing, through alteration of temperature. Moreover, a rich concentration-temperature phase diagram demonstrates the emergence of various ordered nanostructures, including BCC cluster crystals, birefringent cluster crystals, as well as hexagonal phases and cluster glass-like kinetically arrested states at high densities.