•Smooth poly(2-hydroxyethyl methacrylate) (PHEMA) films were successfully synthesized by a simple one-step UV curing.•The resulting films generate various swelling-induced wrinkling patterns.•Large area, highly ordered, honeycomb-like wrinkling patterns were obtained from the resulting films.•The regular wrinkling patterns were successfully used for the fabrication of uniform multicellular spheroids.Poly(2-hydroxyethyl methacrylate) (PHEMA) films with gradient crosslinking density are the only hydrogel films capable of generating regular wrinkling patterns upon swelling, however, they were synthesized via a complicated two-stage photopolymerization. In an effort to develop a simple synthesis method, we found that one-step UV-curing of HEMA liquid always results in films with corrugated surface, which is a common issue when curing liquid prepolymers. We further found that the introduction of a physical network in the prepolymer solution could prevent the surface from corrugating during UV curing. For this purpose, linear PHEMA was added into the prepolymer solutions, and PHEMA films with a smooth surface were successfully synthesized by a simple one-step UV curing. The resulting films generate various swelling-induced wrinkling patterns, including random worms, peanuts and long-range ordered hexagons, depending on the contents of crosslinker and linear PHEMA in the prepolymer solution. Particularly, large area, highly ordered, honeycomb-like wrinkling patterns were obtained from films with proper content of crosslinker and linear PHEMA. These regular patterns are expected to find applications such as the fabrication of uniform multicellular spheroids.Download high-res image (184KB)Download full-size image

A strategy was developed for the synthesis of highly ordered 2D arrays of Ag-PNIPAM hybrid microgel. The highly ordered 2D arrays of PNIPAM microgel were prepared by dispersing PNIPAM microgel on a charge-reversible substrate. The microgel spheres self-assembled into a 3D colloidal crystal, and the first 111 plane was fixed in situ onto the substrate as a result of spontaneous charge reversal of the substrate, leaving a high-quality 2D array of PNIPAM microgel. Ag nanoparticles were then synthesized in situ inside the microgel spheres by introduction of Ag+ ions into the microgel spheres and reduction with sodium borohydride. The resulting 2D arrays are highly ordered. The inter-particle distance in the array can be tuned. In addition, the method allows the synthesis of large size arrays and the use of nonplanar substrate.

2D colloidal crystals (CCs) have important applications; however, the fabrication of large-area, high-quality 2D CCs is still far from being trivial, and the fabrication of 2D microgel CCs is even harder. Here, we have demonstrated that they can be facilely fabricated using charge-reversible substrates. The charge-reversible substrates were prepared by modification with amino groups. The amino groups were then protected by amidation with 2,2-dimethylsuccinic anhydride. At acidic pH, the surface charge of the modified substrate will change from negative to positive as a result of the hydrolysis of the amide bonds and the regeneration of the amino groups. 2D microgel CCs can be simply fabricated by applying a concentrated microgel dispersion on the modified substrate. The negatively charged surface of the substrate allows the negatively charged microgel spheres, especially those close to the substrate, to self-assemble into 3D CCs. With the gradual hydrolysis of the amide bonds and the charge reversal of the substrate, the first 111 plane of the 3D assembly is fixed in situ on the substrate. The resulting 2D CC has a high degree of ordering because of the high quality of the parent 3D microgel CC. Because large-area 3D microgel CCs can be facilely fabricated, this method allows for the fabrication of 2D CCs of any size. Nonplanar substrates can also be used. In addition, the interparticle distance of the 2D array can be tuned by the concentration of the microgel dispersion. Besides rigid substrates (such as glass slides, quartz slides, and silicon wafers), flexible polymer films, including polyethylene terephthalate and poly(vinyl chloride) films, were also successfully used as substrates for the fabrication of 2D microgel CCs.

•Silver dendrites were synthesized by first loading Ag+ ions in LBL films and then reducing them using p-hydroquinine.•They are two orders of magnitude larger than those synthesized in homogenous solutions.•They are 2D structures grown on the film surface.Ag dendrites were previously synthesized, with limited success, via the chemical reduction of Ag+ ions with a soluble reducing agent in homogeneous solutions. Here Ag+ ions were first loaded in hydrogen-bonded layer-by-layer (LBL) films fabricated from poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA), then reduced chemically using p-hydroquinone (HQ) as a reducing agent to afford Ag dendritic structures. The dendrites were characterized by SEM, TEM, XRD and optical microscope. Compared with the Ag dendrites synthesized in homogenous solutions, the dendrites obtained here are much larger. The fully developed ones can reach a size over 600 μm, ca. 2 orders of magnitude larger than those synthesized in homogenous solutions (∼1–5 μm). In addition, the dendrites obtained here are 2 dimensional which grow along the surface of the LBL film, instead of 3 dimensional as those obtained in homogenous solutions. A possible 3-step growth mechanism, which involves the rapid reduction of the Ag+ ions in the film, formation of Ag seed particles on the film surface, and fractal dendritic structure formation as described by the diffusion-limited aggregation (DLA) model, was proposed.

Fluorescent Ag nanoclusters have been easily generated in layer-by-layer (LBL) films of poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA) by photo-reduction of Ag+ ions, which were previously loaded via an ion exchange mechanism. Under UV irradiation, Ag+ ions were reduced to Ag atoms by the photo-generated radicals and then aggregate to form fluorescent nanoclusters. The Ag+ concentration plays a key role in the determination of the growth rate of Ag particles. Careful tuning of the Ag loading in the film prevented the growth of the fluorescent nanoclusters into larger non-fluorescent nanoparticles. Fluorescent Ag nanoclusters can be generated in both wet and dry films. However, the fluorescence intensity achieved in dry films is lower than that in wet films because the tight confinement of the polymer network retards the diffusion and aggregation of the photo-generated Ag atoms. Interestingly the fluorescence intensity can be enhanced significantly by a post-treatment in water, as it allows the Ag atoms to diffuse and aggregate to form more fluorescent nanoclusters. This work shows that the PVPON/PAA LBL films can serve as nanoreactors that allow the in situ generation of fluorescent Ag nanoclusters, yielding fluorescent Ag nanoclusters in a technically important form, in addition to providing a simple way to control the growth of the nanoparticles.

As nanoreactor for the synthesis of metal nanoparticles, hydrogen-bonded layer-by-layer assembled (LBL) films may be a better choice than the electrostatic ones, because more carboxylic acid groups are available for metal ion binding. However, these films should be crosslinked before metal ion loading because of their instability (Chem. Mater., 2005, 17, 1099–1105). Here we report that the uncrosslinked hydrogen-bonded poly(vinyl pyrrolidone)/poly(acrylic acid) (PVPON/PAA) LBL films remain stable during Ag+ loading. FTIR and XPS studies indicate that Ag+ binds with PVPONvia coordination interaction, while it simultaneously binds with PAAvia electrostatic interaction. Therefore the films do not disintegrate upon the disruption of hydrogen bonds between PVPON and PAA. After Ag+ loading, the erosion rate of the film in water decreases, indicating that the long-term stability of the film is actually improved. The loaded Ag+ can be easily unloaded by immersing the film in acidic solutions. The loading and unloading of Ag+ are reversible and can be repeated many times. Silver nanoparticles were synthesized in situ by UV irradiation. The nanoparticles are spherical in shape and present a surface plasmon absorption peak at 434 nm.

A new redox hydrogel using poly(hydroquinone) (PHQ) as polymeric redox couple and chitosan as matrix was synthesized by simple exposing an acidic chitosan/hydroquinone (HQ) solution to the air. PHQ was synthesized in situ by oxidative polymerization of HQ using oxygen as an oxidant. The presence of chitosan increases the reaction rate significantly, suggesting chitosan works as a template for the polymerization of HQ. The reaction rate increases linearly with chitosan concentration when molar ratio of chitosan/HQ is less than 0.96 but keeps constant beyond that point. These results suggest a pick-up mechanism for the template polymerization of HQ. The polymerization of HQ further results in the gelation of the aqueous solution, as physical cross-links forms between PHQ and chitosan. The gelation time decreases with increasing chitosan and HQ concentration and also increasing temperature. The resultant hydrogel are redox-active. Cyclic voltammogram of the hydrogel presents two oxidation peaks at 0.62 and 1.23 V and two reduction peaks at −0.79 and −1.66 V (vs Ag/AgCl). The in situ-formed redox hydrogel may find applications in biomedical areas.

A new principle for PCCA sensing is proposed, in which the analyte-induced swelling of the CCA sphere, rather than that of the hydrogel matrix, is explored for sensing.

Hydrogen-bonded layer-by-layer assembled films from poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA) were fabricated. Fabry–Perot fringes were used as a simple but efficient method for studying the structure changes in the films during salt treatment. Results indicate that these films are very sensitive to salt when incubated in water. A brief treatment in salt solution of low concentration results in microphase separation in the films, the extent of which increases with increasing pH and salt concentration. The microphase separation in PVPON–PAA films is explained by the enhanced dissociation of PAA, which partially breaks the hydrogen bonds in the films. More importantly, the erosion of the PVPON–PAA films accelerates greatly in salt solution. Morphology changes during the erosion suggest a dewetting process of the films, which facilitates their dissolution. Finally, salt-triggered, rapid release of ibuprofen was achieved by the quick, salt-induced erosion of the hydrogen-bonded films.

•An optical glucose sensor was designed using GOD-containing hydrogel films as both sensing material and Fabry-Perot cavity.•The new sensor works well under physiological conditions.•Potential interferents for PBA-based sensors and GOD-based electrochemical sensors, do not influence the new sensor.•The new sensor responds linearly within the clinically relevant glucose range.•The response of the new sensor to glucose is quick.Hydrogel biosensors usually suffer from a slow response, which severely hinders their practical applications. Here a new optical glucose biosensor was designed, using glucose-sensitive hydrogel films as both glucose-sensing material and Fabry-Perot cavity. The film was fabricated by layer-by-layer assembly from partially oxidized dextran (PO-Dex), chitosan, and glucose oxidase (GOD). The film responds to glucose because the incorporated GOD converts glucose to gluconic acid, and thus lowers the local pH in the film, and, in turn, triggers the pH-sensitive film to swell. The glucose-induced swelling causes a shift of Fabry−Perot fringes on the reflection spectra of the film, from which the glucose concentration can be reported. The new sensor works well under physiological conditions. Potential interferents, such as diols for phenylboronic acid-based sensors and electroactive compounds for electrochemical sensors, do not influence the new sensor. The sensor can respond reversibly over a wide range of glucose concentration. Particularly, it responds linearly within the clinically relevant glucose range (0–20 mM). More importantly, because the film is very thin, the new sensor can respond quickly, making it potential for real-time, continuous glucose monitoring.Download high-res image (360KB)Download full-size image

A new principle for PCCA sensing is proposed, in which the analyte-induced swelling of the CCA sphere, rather than that of the hydrogel matrix, is explored for sensing.

Fluorescent Ag nanoclusters have been easily generated in layer-by-layer (LBL) films of poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA) by photo-reduction of Ag+ ions, which were previously loaded via an ion exchange mechanism. Under UV irradiation, Ag+ ions were reduced to Ag atoms by the photo-generated radicals and then aggregate to form fluorescent nanoclusters. The Ag+ concentration plays a key role in the determination of the growth rate of Ag particles. Careful tuning of the Ag loading in the film prevented the growth of the fluorescent nanoclusters into larger non-fluorescent nanoparticles. Fluorescent Ag nanoclusters can be generated in both wet and dry films. However, the fluorescence intensity achieved in dry films is lower than that in wet films because the tight confinement of the polymer network retards the diffusion and aggregation of the photo-generated Ag atoms. Interestingly the fluorescence intensity can be enhanced significantly by a post-treatment in water, as it allows the Ag atoms to diffuse and aggregate to form more fluorescent nanoclusters. This work shows that the PVPON/PAA LBL films can serve as nanoreactors that allow the in situ generation of fluorescent Ag nanoclusters, yielding fluorescent Ag nanoclusters in a technically important form, in addition to providing a simple way to control the growth of the nanoparticles.

As nanoreactor for the synthesis of metal nanoparticles, hydrogen-bonded layer-by-layer assembled (LBL) films may be a better choice than the electrostatic ones, because more carboxylic acid groups are available for metal ion binding. However, these films should be crosslinked before metal ion loading because of their instability (Chem. Mater., 2005, 17, 1099–1105). Here we report that the uncrosslinked hydrogen-bonded poly(vinyl pyrrolidone)/poly(acrylic acid) (PVPON/PAA) LBL films remain stable during Ag+ loading. FTIR and XPS studies indicate that Ag+ binds with PVPONvia coordination interaction, while it simultaneously binds with PAAvia electrostatic interaction. Therefore the films do not disintegrate upon the disruption of hydrogen bonds between PVPON and PAA. After Ag+ loading, the erosion rate of the film in water decreases, indicating that the long-term stability of the film is actually improved. The loaded Ag+ can be easily unloaded by immersing the film in acidic solutions. The loading and unloading of Ag+ are reversible and can be repeated many times. Silver nanoparticles were synthesized in situ by UV irradiation. The nanoparticles are spherical in shape and present a surface plasmon absorption peak at 434 nm.