Light-induced Surface Research Articles

Hierarchical PEG-Based 3D Patterns Grafting from Polymer Substrate by Surface Initiated Visible Light Photolithography.

The precise construction of a hierarchical complex pattern on substrates is required for numerous applications. Here, a strategy to fabricate well-defined hierarchical three dimensional (3D) patterns on polymer substrate is developed. This technique, which combines photolithography and visible light-induced surface initiated living graft crosslinking polymerization (VSLGCP), can effectively graft 3D patterns onto polymer substrate with high fidelity and controllable height. Owing to the living nature of VSLGCP, hierarchical 3D patterns can be prepared when a sequential living graft crosslinking process is performed on the first formed patterns. As a proof-of-concept, a reactive two layer 3D pattern with a morphology of lateral stripe on vertical stripe is prepared and employed to separately immobilize model biomolecules, e.g., biotin and IgG. This two component pattern can specifically interact with corresponding target proteins successfully, indicating that this strategy has potential applications in the fabrication of polymer-based multicomponent biomolecule microarrays.

Sprayed zinc oxide films: Ultra-violet light-induced reversible surface wettability and platinum-sensitization-assisted improved liquefied petroleum gas response

We report the rapid (superhydrophobic to superhydrophilic) transition property and improvement in the liquefied petroleum gas (LPG) sensing response of zinc oxide (ZnO) nanorods (NRs) on UV-irradiation and platinum (Pt) surface sensitization, respectively. The morphological evolution of ZnO NRs is evidenced from the field emission scanning electron microscope and atomic force microscope digital images and for the structural elucidation X-ray diffraction pattern is used. Elemental survey mapping is obtained from energy dispersive X-ray analysis spectrum. The optical properties have been studied by UV–Visible and photoluminescence spectroscopy measurements. The rapid (120sec) conversion of superhydrophobic (154°) ZnO NRs film to superhydrophilic (7°) is obtained under UV light illumination and the superhydrophobicity is regained by storing sample in dark. The mechanism for switching wettability behavior of ZnO NRs has thoroughly been discussed. In second phase, Pt-sensitized ZnO NRs film has demonstrated considerable gas sensitivity at 260ppm concentration of LPG. At 623K operating temperature, the maximum LPG response of 58% and the response time of 49sec for 1040ppm LPG concentration of Pt- sensitized ZnO NRs film are obtained. This higher LPG response of Pt-sensitized ZnO NRs film over pristine is primarily due to electronic effect and catalytic effect (spill-over effect) caused by an additional of Pt on ZnO NRs film surface.

Entrapment of Xylanase within a Polyethylene Glycol Net-Cloth Grafted on Polypropylene Nonwoven Fabrics with Exceptional Operational Stability and Its Application for Hydrolysis of Corncob Hemicelluloses

Enzyme immobilization is a core technique of enzymatic biochemical engineering because it can remarkably reduce the cost of enzymes and improve the enzyme recovery procedure. The most crucial issues for enzyme immobilization include (1) maintaining its activity, both in the immobilization process and in the batchwise catalyst course; (2) separating the immobilized enzyme from the reaction mixture; and (3) the readiness and cost of the immobilization process. Herein, we report a new strategy to immobilize xylanase within a hydrophilic and nonswelling polyethylene glycol (PEG) net-cloth grafted on a polypropylene nonwoven fabric (PPNWF) membrane by a visible light-induced surface graft cross-linking polymerization. The xylanase was in situ entrapped within the PEG net-cloth. The nonswelling PEG net-cloth can effectively maintain the xylanase without leakage in long-term operation. As for the hydrolysis of corncob hemicelluloses, the experimental results showed that the as-formed immobilized xylanase retained 80% of its original activity after being reused for 25 cycles and 60% after 50 cycles, which is far better than that of other immobilization methods by entrapment. Notably, this simple in situ entrapment of enzymes on routine polymeric matrix would lead to an easy industrial production at low cost, while the form of end-products as a sheet can be readily separated from the reaction mixture and reused for batchwise production. After immobilization, the xylanase showed no significant shift in pH or temperature optima as compared with its free form. These results suggest that the immobilization of xylanase within the PEG net-cloth grafted on PPNWF is promising for industrial applications because of its long-term operation stability and convenient recovery for reuse.

Net-Immobilization of β-glucosidase on Nonwoven Fabrics to Lower the Cost of "Cellulosic Ethanol" and Increase Cellulose Conversions.

The main limitation preventing the use of enzymatic cellulosic ethanol in industrial production is its higher cost which is mainly due to the elevated price of β-glucosidase (BG). Herein, we report on a simple strategy for the in-situ encapsulation of BG for repeated cellulosic ethanol production. In this strategy, BG was net-immobilized into a poly(ethylene glycol) (PEG) net-cloth layer on a PP nonwoven fabric by way of the visible light-induced surface controlled/living graft cross-linking polymerization. The visible light and mild reaction conditions could ensure the activity retention of BG during immobilization, while the non-swelling uniform net-mesh formed by living cross-linking polymerization could prevent the leakage of BG effectively (at the immobilization rate of more than 98.6% and the leakage rate of only 0.4%). When the BG-loaded fabric was used in combination with free cellulase (CEL), the results of the catalytic reaction demonstrated that these BG-loaded fabrics could not only give a 40% increase in cellulose conversions but also be reused for more than fifteen batches without losing the activity. These BG-loaded fabrics with characteristics including easy separation, excellent operation stability, a low cost of the polymeric matrix and a simple fabrication process are particularly interesting for a future bio-fuel production strategy.

Optical Flow Focusing: Light-Induced Destabilization of Stable Liquid Threads

Dispensing droplets in microscale ducts is valuable in several fields, ranging from lab-on-a-chip microanalysis to inkjet printing. The authors locally fragment a liquid thread with a focused laser beam, creating a microscopic version of a controlled dripping faucet. They show how light-induced surface stresses can reversibly force the destabilization of confined threads, providing a tunable, on-demand constriction to control the thread-to-droplet transition for digital microfluidics.

Light-Induced Surface Patterning of Silica.

Manipulating the size and shape of silica precursor patterns using simple far-field light irradiation and transforming such reconfigured structures into inorganic silica patterns by pyrolytic conversion are demonstrated. The key concept of our work is the use of an azobenzene incorporated silica precursor (herein, we refer to this material as azo-silane composite) as ink in a micromolding process. The moving direction of azo-silane composite is parallel to light polarization direction; in addition, the amount of azo-silane composite movement can be precisely determined by controlling light irradiation time. By exploiting this peculiar phenomenon, azo-silane composite patterns produced using the micromolding technique are arbitrarily manipulated to obtain various structural features including high-resolution size or sophisticated shape. The photoreconfigured patterns formed with azo-silane composites are then converted into pure silica patterns through pyrolytic conversion. The pyrolytic converted silica patterns are uniformly formed over a large area, ensuring crack-free formation and providing high structural fidelity. Therefore, this optical manipulation technique, in conjunction with the pyrolytic conversion process, opens a promising route to the design of silica patterns with finely tuned structural features in terms of size and shape. This platform for designing silica structures has significant value in various nanotechnology fields including micro/nanofluidic channel for lab-on-a-chip devices, transparent superhydrophobic surfaces, and optoelectronic devices.

Light-induced metal-like surface of silicon photonic waveguides.

The surface of a material may exhibit physical phenomena that do not occur in the bulk of the material itself. For this reason, the behaviour of nanoscale devices is expected to be conditioned, or even dominated, by the nature of their surface. Here, we show that in silicon photonic nanowaveguides, massive surface carrier generation is induced by light travelling in the waveguide, because of natural surface-state absorption at the core/cladding interface. At the typical light intensity used in linear applications, this effect makes the surface of the waveguide behave as a metal-like frame. A twofold impact is observed on the waveguide performance: the surface electric conductivity dominates over that of bulk silicon and an additional optical absorption mechanism arises, that we named surface free-carrier absorption. These results, applying to generic semiconductor photonic technologies, unveil the real picture of optical nanowaveguides that needs to be considered in the design of any integrated optoelectronic device.

Supramolecular hierarchy among halogen and hydrogen bond donors in light-induced surface patterning

Halogen bonding is an interaction of choice in supramolecular polymers for efficient light-induced surface patterning.

Visible light-induced surface initiated atom transfer radical polymerization of methyl methacrylate on titania/reduced graphene oxide nanocomposite

Visible light assisted copper catalyzed surface initiated atom radical transfer polymerization system for polymer growth from titania/reduced graphene oxide (TiO2/rGO) has been described.

Synthesis and Light-Induced Surface Potential Observation of Retinal Prosthesis Using Polyethylene Thin Films Immobilized with Photoelectric Dyes

Synthesis and Light-Induced Surface Potential Observation of Retinal Prosthesis Using Polyethylene Thin Films Immobilized with Photoelectric Dyes

A mild strategy to encapsulate enzyme into hydrogel layer grafted on polymeric substrate.

Although the hydrogel network has been widely investigated as a carrier for enzyme immobilization, to in situ encapsulate enzymes into a hydrogel network in an efficient, practical, and active way is still one of the great challenges in the field of biochemical engineering. Here, we report a new protocol to address this issue by encapsulating enzyme into poly(ethylene glycol) (PEG) hydrogel network grafted on polymeric substrates. In our strategy, isopropyl thioxanthone semipinacol (ITXSP) dormant groups were first planted onto the surface of a plastic matrix with low density polyethylene (LDPE) film as a model by a UV-induced abstracting hydrogen-coupling reaction. As a proof of concept, lipase, which could catalyze esterification of glucose with palmitic acid, then was in situ net-immobilized into a PEG-based hydrogel network layer through a visible light-induced surface controlled/living graft cross-linking polymerization. This strategy demonstrates the following novel significant merits: (1) in comparison with the UV irradiation or high temperature, the visible light and room temperature used provide a friendly condition to maintain activity of enzyme during immobilization; (2) the uniqueness of controlled/living cross-linking polymerization not only makes it easy to form a uniform PEG hydrogel network, which is a benefit to avoid the leakage of net-immobilizing enzyme, but also to tune the net-thickness or capacity to accommodate enzyme; and (3) as compared to systems of nanoparticles and porous matrixes, the flexible/robust end-products of the surface net-immobilizing enzyme with polymer film are more suitable to be applied in a bioreactor due to their features of easier separation and reuse. We confirmed that this catalytic film could retain almost all of its initial activity after seven batches of 24 h esterifications. The proposed strategy provides an extremely simple, effective, and flexible method for enzyme immobilization.

The effectiveness of indoor photocatalytic paints on NOx and HONO levels

There is an increasing concern about the indoor air environment, where we spend most of our time. Common methods of improving indoor air quality include controlling pollution sources, increasing ventilation rates or using air purifiers. Photocatalytic remediation technology was suggested as a new possibility to eliminate indoor air pollutants instead of just diluting or disposing them. Titanium dioxide (TiO2) is a widely used photocatalyst, which is aimed to eliminate organic and inorganic pollutants.Here, we demonstrate that indoor photocatalytic paints which contain TiO2 can substantially reduce the concentrations of nitrogen dioxide (NO2). We show that the efficiency of nitrogen dioxide (NO2) removal increases with the quantity of TiO2 in the range 0–7%. The geometric uptake coefficients increase from 5×10−6 to 1.6×10−5 under light irradiation of the paints. On the other hand, during the reactions of NO2 with this paint (7% of TiO2) nitric oxide (NO) and nitrous acid (HONO) are formed. Nitrous acid (HONO) is an important harmful indoor pollutant and its photolysis leads to the formation of highly reactive OH radicals. Maximum conversion efficiencies of NO2 to HONO and NO of 15% and 33% were observed at 30% RH, respectively.A dynamic mass balance model applied to typical indoor environment predicts a steady state mixing ratio of 5.6ppb of HONO released upon light-induced surface reaction of NO2 with a photocatalytic paint (7% of TiO2) and considering the photolysis process as the most important loss of HONO.The quantity of TiO2 embedded in the paint is of crucial importance with respect to nitrogen oxides NO2 remediation, but may also influence the formation of harmful intermediates like nitrous acid (HONO), which should be considered for future optimization of photocatalytic paints aimed for indoor applications.

Light-Driven Surface Patterning of Supramolecular Polymers with Extremely Low Concentration of Photoactive Molecules.

Light-induced surface patterning in azobenzene-containing polymers and other materials is a widely studied phenomenon with possible applications in fields ranging from photonics to biology. Yet, the fundamental understanding of this purely photodriven mass transport remains inadequate, and existing literature fails to define a threshold chromophore content for mass transport to occur, if such a limit exists. This letter presents a systematic study of the relationship between chromophore concentration and mass transport using hydrogen-bonded polymer-azobenzene complexes, in which the chromophore concentration can be freely adjusted while keeping the polymer backbone unchanged. Essentially, we demonstrate that effective surface patterning can be induced even at an extremely low chromophore content of 1 mol %, when only every tenth polymer chain carries a single azobenzene molecular motor. Importantly, the results highlight the extraordinary photomechanical power of azobenzene and contribute to the fundamental understanding of the light-induced motions.

Anomalous photoelectric effect of a polycrystalline topological insulator film.

A topological insulator represents a new state of quantum matter that possesses an insulating bulk band gap as well as a spin-momentum-locked Dirac cone on the surface that is protected by time-reversal symmetry. Photon-dressed surface states and light-induced surface photocurrents have been observed in topological insulators. Here, we report experimental observations of an anomalous photoelectric effect in thin films of Bi2Te3, a polycrystalline topological insulator. Under illumination with non-polarised light, transport measurements reveal that the resistance of the topological surface states suddenly increases when the polycrystalline film is illuminated. The resistance variation is positively dependent on the light intensity but has no relation to the applied electric field; this finding can be attributed to the gap opening of the surface Dirac cone. This observation of an anomalous photoelectric effect in polycrystalline topological insulators offers exciting opportunities for the creation of photodetectors with an unusually broad spectral range. Moreover, polycrystalline topological insulator films provide an attractive material platform for exploring the nature and practical application of topological insulators.

Spatial charge separation in asymmetric structure of Au nanoparticle on TiO2 nanotube by light-induced surface potential imaging.

Both enhancing the excitons' lifetime and ingeniously controlling the spatial charge transfer are the key to the realization of efficiently photocatalytic and artificially photosynthetic devices. Nanostructured metal/metal-oxide interfaces often exhibit improved energy conversion efficiency. Understanding the surface potential changes of nano-objects under light illumination is crucial in photoelectrochemical cells. Under ultraviolet (UV) illumination, here, we directly observed the charge separation phenomena at the Au-nanoparticle/TiO2-nanotube interfaces by using Kelvin probe force microscopy. The surface potential maps of TiO2 nanotubes with and without Au nanoparticles were compared on the effect of different substrates. We observed that in a steady state, approximately 0.3 electron per Au particle of about 4 nm in diameter is effectively charged and consequently screens the surface potential of the underlying TiO2 nanotubes. Our observations should help design improved photoelectrochemical devices for energy conversion applications.

Light-induced surface graft polymerizations initiated by an anthraquinone dye on cotton fibers

Anthraquinone and its derivatives could serve as photo-sensitizers and generate radicals and reactive oxygen species in polymers under exposure of UVA or day light. Such a property was utilized in development of novel light-induced surface radical graft polymerizations on cotton fibers that were dyed with an anthraquinone derivative, 2-ethylanthraquinone. Several functional monomers were directly grafted onto the dyed cotton fibers upon UVA exposure. The chemical and morphological structures and thermal properties of the grafted fibers were confirmed and characterized by Fourier transform infrared spectrometer (FTIR), scanning electron microscope (SEM) and thermal gravimetric analysis (TGA). Reaction conditions including concentrations of the photosensitizer, the amount of monomers, as well as UVA irradiation time could influence grafting efficiencies. More interestingly, the surface graft polymerization did not significantly change the light active functions of the agent, evidenced by the light-active antimicrobial functions of the grafted fibers.

Light-fuelled transport of large dendrimers and proteins.

This work presents a facile water-based supramolecular approach for light-induced surface patterning. The method is based upon azobenzene-functionalized high-molecular weight triazine dendrimers up to generation 9, demonstrating that even very large globular supramolecular complexes can be made to move in response to light. We also demonstrate light-fuelled macroscopic movements in native biomolecules, showing that complexes of apoferritin protein and azobenzene can effectively form light-induced surface patterns. Fundamentally, the results establish that thin films comprising both flexible and rigid globular particles of large diameter can be moved with light, whereas the presented material concepts offer new possibilities for the yet marginally explored biological applications of azobenzene surface patterning.

Quantitative Light-Induced Fluorescence to Measure Enamel Remineralization in vitro

The aim of this study was to compare the ability of quantitative light-induced fluorescence (QLF) and surface microhardness (SMH) to measure the remineralization of enamel subsurface lesions, using a pH-cycling model including treatment with 0-ppm, 550-ppm or 1,100-ppm sodium fluoride (NaF) dentifrices. Methods: Subsurface lesions were created in human enamel specimens (n = 36) and exposed to a remineralization pH-cycling model for 14 days. The pH-cycling model was performed in an automated system where specimens were subjected to a demineralizing solution for 20 min and treatment for 1 min and were then remineralized for 7 h 39 min, 3 times daily. The treatments consisted of 3 NaF, silica-containing dentifrices (0 ppm F; 550 ppm F; 1,100 ppm F). The outcome variables were: change from baseline in surface hardness and percentage change from baseline in fluorescence. An ANCOVA explored differences between different treatment groups (at the p < 0.05 level). Associations between QLF and SMH were evaluated using Spearman's correlation coefficient. Results: The percentage SMH changes were 14.9 ± 2.1%, 56.6 ± 9.6% and 103.9 ± 14.6% for the 0-, 550- and 1,100-ppm F dentifrices, respectively. The percentage fluorescence changes were 15.6 ± 7.1%, 59.8 ± 11.9% and 85 ± 13.2%, respectively. The differences between all pairwise comparisons were statistically significant for both methods (p = 0.001). QLF correlated with SMH (r = 0.67). Conclusions: Both the SMH and QLF methods demonstrated a significant F dose response for toothpaste in this in vitro remineralization model, and both methods were able to distinguish treatments with different F levels.

Enhancement of visible light-induced surface photo-activity of nanostructured N–TiO2 thin films modified by ion implantation

This work reports the morphological and chemical modifications induced in TiO2 thin films by bombardment with high energy N+ ions at different temperatures and their different photo-activity responses after implantation under visible and UV light illumination. When implanted samples are illuminated with visible light, no dye photo-decolouration takes place despite that light transformed the surfaces from hydrophobic to hydrophilic. In agreement with the Wenzel model of wetting, correlation is found between visible light photo-activity and film morphology. We conclude that the photo-activity response can be separated into shallow and Schottky barrier photo-activity, this latter involving a thicker layer of material.

Nanoindentation study of light-induced softening of supramolecular and covalently functionalized azo polymers

Nanoindentation studies on thin films of the widely used azo polymer pDR1A and a supramolecular polymer–azobenzene complex p4VP(DY7)0.5 demonstrate significant light-induced softening upon visible-light irradiation due to trans–cis–trans photoisomerization of the azobenzene units. More specifically, the strain-rate sensitivities of pDR1A and p4VP(DY7)0.5 upon 532 nm irradiation increase by 80% and 120%, respectively. These results imply a photosoftening contribution to the mechanisms of light-induced surface patterning of azo polymers and the photomechanical effect. The finding that under the experimental conditions used photosoftening is more significant in the supramolecular complex than in the covalently functionalized polymer highlights the potential of noncovalent functionalization strategies in designing materials with efficient photomechanical response, and nanoindentation provides a powerful technique to quantify the connection between the photoinduced changes in mechanical properties and photoinduced macroscopic movement of azo polymer films.