Transparent and Robust Silica Coatings with Dual Range Porosity for Enzyme-Based Optical Biosensing

Hierarchically porous transparent silica coatings combine large specific surface area with enhanced pore accessibility for optical biosensing. This paper describes a versatile approach to fabricate optically transparent silica coatings with multiscale porosity. Thin films (around 1 mu m in thickness) of an aqueous suspension of primary silica aggregates form a mesoporous, interconnected matrix, and sacrificial polymer particles template well-defined, discrete macropores with high structural integrity. The total surface area achieved is around 200 m(2) g(-1) with mesopore sizes of 20-40 nm and macropores of 250 nm, with a total porosity of 84%. The macro/meso dual range of porosity allows enhanced biocatalyst loadings of l-lactate dehydrogenase for detection of lactate. The functionalized films showed a linear response within the range of interest of 1-20 x 10(-3) m of lactate. These biosensing coatings therefore strongly enhance sensitivity, speed and reliability of optically based lactate detection as compared to classical thin films with monomodal mesopore structure. Particle-based simulations and experiments reveal that both the location and connectivity of the macropores control the biosensing performance. The coatings and procedure presented here are versatile, scalable, inexpensive, and are therefore compatible with a wide range of deposition techniques suitable for industrial and health care applications.

Controlling Mesopore Size and Processability of Transparent Enzyme-Loaded Silica Films for Biosensing Applications

Heinrich Hofmann, Oswaldo Javier Pérez Anguiano, Raphaël Pugin, Emmanuel Scolan, Bernard Wenger

Silica-based nanoporous thin films including large mesopores are relevant as enzyme supports for applications in biosensing. The diffusion and immobilization of large biomolecules such as enzymes in such porous films require the presence of large mesopores. Creating such morphologies based on a bottom-up synthesis using colloidal templates is a challenge in view of the combination of desired material properties and the robustness of the casting process for the fabrication of thin films. Here a strategy to reproducibly synthesize transparent porous silica thin films with submicrometer thickness and homogeneously distributed porosity is presented. For this purpose, polystyrene-poly-2-vinylpyridine (PS-P2VP) amphiphilic block copolymers are used as porogenic templates. Low-chain alcohols are employed as both selective solvents for the P2VP blocks and reaction media for silica synthesis. Rheology measurements reveal a strong influence of the block copolymer length on the behavior of PS-P2VP micelles in suspension. The pore distribution and accessibility into the film are controlled by adjusting the silica to block copolymer weight ratio. The solvent choice is shown to control not only the micelle size and the generated pore morphology but also the structural homogeneity of the films. Finally, the suitability of the synthesized films as supports for enzymes is tested using a model enzyme, horseradish peroxidase EC 1.11.1.7. Our approach is innovative, robust, and reproducible and provides a convenient alternative to synthesize large mesopores up to small macropores (20100 nm) in nanostructured thin films with applications in biosensing and functional coatings.

Amer Chemical Soc2015

Novel optical fiber components based on thin-film coatings and intra-core fiber gratings

We have designed and developed a new class of optical fiber components based on thin-film coatings and fiber gratings. In our design, the function of the thin-film fiber coating is to provide an electrical means to regulate the optical response of the fiber section underlying the coating. Using this principle, we were able to implement both wavelength tunable filters and optical modulators. We implemented wavelength tunable filters by running an electrical current through a metallic coating. The effect of the current is to directly heat the fiber (which hosts a fiber grating), and the resulting temperature change tunes the filter resonance wavelength(s). The main characteristic of this device are: a) the tuning efficiency is inversely proportional to the length of the heated coating; b) thermal tuning does not induce any spectral profile changes; c) the temperature induced wavelength modulation depends on the packaging and is limited to low frequencies. To produce fiber optic phase modulators, we exploited the change in the refractive index determined by a thin-film piezoelectric coating sandwiched between metal electrodes. The induced strains are proportional to the electric field applied to the piezoelectric layer (zinc oxide or lead titanate zirconate sputtered thin-films). We found that the response is flat at low frequencies and dominated by strong radial mode resonances above a few MHz up to GHz frequencies. We also investigated piezoelectric coated fiber Bragg gratings for intensity and Bragg wavelength modulation. Our results indicate that, in these devices, the refractive index of the fiber grating is changed by the straining action of the piezoelectric film. Finally, we operated an all-fiber Mach-Zehnder interferometer, based on two long period fiber gratings, as an intensity modulator. In this case, a zinc oxide piezoelectric actuator modulates the phase difference between the arms of the interferometer and thus the transmitted intensity. Piezoelectric actuators allowed for wavelength and intensity modulation up to GHz frequencies, with a response similar to that of the phase modulators. In conclusion, the all-fiber modulators and wavelength tunable filters that we have designed and implemented are characterized by low insertion loss, high electrical power efficiency, small size, and high optical power damage threshold. We believe that such novel devices represent a useful addition to the tool kit of the fiber-optic system designer.

EPFL2000

Observation of fracture and plastic deformation during indentation and scratching inside the scanning electron microscope

Jean-Marc Breguet, Franz-Josef Haug

During nanoindentation measurements of thin films the formation of cracks within the film as well as of pile-up or sink-in around the indent are known to affect significantly the precision of hardness and Young's modulus values. The crack pattern in brittle films and the amount of pile-up/sink-in in ductile films allow, however, also to estimate film fracture toughness and to gain knowledge on the film strain hardening behavior, respectively. The shape of the residual imprint and the extension of cracks at the surface after unloading are therefore often analyzed by atomic force microscopy or scanning electron microscopy (SEM). However, the contact area under load, the moment of crack initiation and the extension of the cracks during the loading/unloading cycle cannot be deduced unambiguously by analyzing only the residual imprint and the load-displacement data. We present a miniaturized own nanoindentation and nanoscratch device for use inside a scanning electron microscope. The indentation axis is operated at an inclined angle with respect to the SEM column. This allows looking at the surface around the indent during indentation. Crack and pile-up formation can be directly observed with sub-micrometer resolution and linked to the simultaneously recorded load-displacement data. In a first part of the paper the design concepts of the device will be discussed. In a second part, two case studies - indentation of nanocomposite TiN/SiN x coatings and indentation and scratching of hard diamond-like carbon (DLC) films - will be presented to demonstrate the potential of SEM-indentation and scratching. In the case of indentation of hard coatings, the formation of cracks during loading as well as during unloading was observed. Some cracking events could be correlated to discontinuities in the load-displacement curve. Scratch experiments revealed at which critical load coating failure occurred.

2004

Novel optical fiber components based on thin-film coatings and intra-core fiber gratings

EPFL2000

Observation of fracture and plastic deformation during indentation and scratching inside the scanning electron microscope

Jean-Marc Breguet, Franz-Josef Haug

2004