Brian D. MacCraith

Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication

Investigations into the structuring by two-photon polymerization of a nonshrinking, photosensitive, zirconium sol-gel material are presented. This hybrid material can be photostructured even when it contains up to 30 mol % of zirconium propoxide (ZPO); by varying the materials inorganic content, it is possible to modify and tune its refractive index. The introduction of ZPO significantly increases the photosensitivity of the resulting photopolymer. The fabricated three-dimensional photonic crystal structures demonstrate high resolution and a clear band-stop in the near-IR region. In contrast to common practice, no additional effort is required to precompensate for shrinkage or to improve the structural stability of the fabricated photonic crystals; this, combined with the possibility of tuning this materials optical, mechanical, and chemical properties, makes it suitable for a variety of applications by two-photon polymerization manufacturing.

Tailoring of sol-gel films for optical sensing of oxygen in gas and aqueous phase.

Sol-gel-based optical sensors for both gas-phase and dissolved oxygen have been developed. Both sensors operate on the principle of fluorescence quenching of a ruthenium complex which has been entrapped in a porous sol-gel silica film. A comprehensive investigation was carried out in order to establish optimal film-processing parameters for the two sensing environments. Both tetraethoxysilane and organically modified sol-gel precursors such as methyltriethoxysilane and ethyltriethoxysilane were used. Film hydrophobicity increases as a function of modified precursor content, and this was correlated with enhanced dissolved oxygen (DO) sensor performance. Extending the aliphatic group of the modified precursor further improved DO sensitivity. The influence of water/precursor molar ratio, R, on the sol-gel film microstructure was investigated. R value tailoring of the microstructure and film surface hydrophobicity tailoring were correlated with oxygen diffusion behavior in the films via the Stern-Volmer constants for both gas phase and DO sensing. Excellent performance characteristics were measured for both gas-phase and DO oxygen sensors. The long-term quenching stability of DO sensing films was established over a period of 6 months.

Fibre optic oxygen sensor based on fluorescence quenching of evanescent-wave excited ruthenium complexes in sol–gel derived porous coatings

A simple, low-cost technique for the fabrication of optical sensors for oxygen is described and preliminary results obtained using these sensors are reported. The technique is based on coating a declad portion of an optical fibre with a microporous glass film prepared by the sol–gel process. A ruthenium complex [RuII–tris-(2,2′-bipyridine) or RuII–tris(4,7-diphenyl-1,10-phenanthroline)] is trapped in the nanometre-scale cage-like structure of the porous film. In this sensor configuration the complex is excited by the evanescent field of the 488 nm radiation guided by the optical fibre. The luminescence from such complexes is known to be quenched by oxygen and the sensors exhibit repeatable quenching behaviour when exposed to various concentrations of oxygen. The ratio R=I0/I100 where I0 and I100 represent the detected signals from a sensor exposed to 100% nitrogen and 100% oxygen, respectively, is used as a measure of the sensitivity of the sensor. Sensors based on the diphenylphenanthroline complex exhibit greater sensitivity than those based on the bipyridine complex, in accordance with theoretical predictions. More importantly, however, the design potential of the sol–gel process for sensor fabrication is demonstrated by the achievement of a substantial increase in R when the process parameters are adjusted to increase the pore volume.

Sol-gel coatings for optical chemical sensors and biosensors☆

In this paper an overview is presented of the state-of-the-art of optical sensors employing sol-gel-derived coatings. The technique is particularly suited to the side-coating of optical fibres or waveguides in evanescent-wave sensors because precise control of sensitivity-determining parameters, such as the coating thickness and length, is achievable. Sensors based on entrapped organic and inorganic dyes, enzymes and other biomolecules have been reported. The main features of the process are illustrated by examples of chemical sensors and biosensors from the literature. In particular, the development of an oxygen sensor based on the quenching of fluorescence from a sol-gel-entrapped ruthenium complex and an absorption-based pH sensor are described. Issues that require further investigation before this technology can proceed to the stage of industrial development are also highlighted.

Sol–gel based optical carbon dioxide sensor employing dual luminophore referencing for application in food packaging technology

An optical sensor for the measurement of carbon dioxide in Modified Atmosphere Packaging (MAP) applications has been developed. It is based on the fluorescent pH indicator 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilised in a hydrophobic organically modified silica (ormosil) matrix. Cetyltrimethylammonium hydroxide was used as an internal buffer system. Fluorescence is measured in the phase domain by means of the Dual Luminophore Referencing (DLR) sensing scheme which provides many of the advantages of lifetime-based fluorometric sensors and makes it compatible with established optical oxygen sensor technology. The long-term stability of the sensor membranes has been investigated. The sensor displays 13.5 degrees phase shift between 0 and 100% CO2 with a resolution of better than 1% and a limit of detection of 0.08%. Oxygen cross-sensitivity is minimised (0.6% quenching in air) by immobilising the reference luminophore in polymer nano-beads. Cross-sensitivity towards chloride and pH was found to be negligible. Temperature effects were studied, and a linear Arrhenius correlation between ln k and 1/T was found. The sensor is stable over a period of at least seven months and its output is in excellent agreement with a standard reference method for carbon dioxide analysis.

Dissolved oxygen sensor based on fluorescence quenching of oxygen-sensitive ruthenium complexes immobilized in sol–gel-derived porous silica coatings

A dissolved oxygen sensor based on the quenching of fluorescence from a ruthenium dye complex entrapped in a porous sol–gel film is reported. Sol–gel-derived silica films were fabricated by dip-coating on to planar and optical fibre substrates. The films were pre-doped with the oxygen-sensitive ruthenium complex [RII–tris(4,7-diphenyl-1,10-phenanthroline)], the fluorescence of which is quenched in the presence of oxygen. The structure and behaviour of sol–gel films are dependent on the fabrication parameters. In particular, enhancement of the surface hydrophobicity increases the quenching response in water. This is achieved by using suitable proportions of modified precursors of silica of the form R(OEt)3Si, where R is an alkyl group, in the standard fabrication procedure. It is shown that by increasing the ratio of modified precursor, the quenching response in the aqueous phase increases. A very low limit of detection, 6 ppb, was determined for the modified films. Using a high-brightness blue LED, combined with a miniature photodiode detection system, these results indicate the potential for a low-cost, high-performance, portable dissolved oxygen sensor for use in many varied situations from aeration control to on-line river pollution monitoring.

Characterisation of porosity and sensor response times of sol-gel-derived thin films for oxygen sensor applications

Abstract In recent years, sol–gel-derived films have played a significant role in optical sensor development. This paper focuses on the characterisation of oxygen-sensitive tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS)-based films. Film porosity and sensor response times are reported for a range of films fabricated under different conditions. Porosity data are correlated with predicted film behaviour and also with previous characterisation studies. Oxygen diffusion coefficients are estimated from the response times. The enhanced diffusion coefficient of MTEOS films compared to TEOS films is discussed in terms of the relative oxygen solubility of the films. Comparisons are made with data for oxygen-sensitive polymer films, indicating an enhanced solubility for sol–gel films compared with typical polymer films.

Leaching in sol-gel-derived silica films for optical pH sensing

Abstract The effect of different processing parameters on the efficiency of entrapment of pH indicators in a sol–gel-derived silica film is reported. An evanescent wave absorption approach was used to monitor dopant-leaching from these films. The issues investigated include the effect on the rate of leaching of the water:alkoxide ratio, the pH of the leachant solution and the choice of silicon alkoxide. Results indicate that the most suitable sol–gel films in terms of leaching and response time are those prepared using tetraethoxysilane as precursor, acid catalysis and a water:alkoxide ratio of at least 4. Leaching is more pronounced for films used directly after preparation and varies for different pH environments. Implications of leaching for the long-term use of sol–gel based pH sensors are discussed.

Silane–dextran chemistry on lateral flow polymer chips for immunoassays

The prognosis for patients suffering from cardiovascular and many other diseases can be substantially improved if diagnosed at an early stage. High performance diagnostic testing using disposable microfluidic chips can provide a platform for realizing this vision. Amic AB (Uppsala, Sweden) has developed a new microfluidic test chip for sandwich immunoassays fabricated by injection molding of the cycloolefin-copolymer Zeonor. A highly ordered array of micropillars within the fluidic chip distributes the sample solution by capillary action. Since wetting of the pillar array surface is the only driving force for liquid distribution precise control of the surface chemistry is crucial. In this work we demonstrate a novel protocol for surface hydrophilization and antibody immobilization on cycloolefin-copolymer test chips, based on direct silanisation of the thermoplastic substrate. Dextran is subsequently covalently coupled to amino groups, thus providing a coating with a low contact angle suitable for antibody immobilization. The contact angle of dextran coated chips is stable for at least two months, which enables production of large batches that can be stored for extended periods of time. We demonstrate the utility of the presented platform and surface chemistry in a C-reactive protein assay with a detection limit of 2.6 ng ml(-1), a dynamic range of 10(2) and a coefficient of variance of 15%.

Characterisation of sol-gel-derived silica films

Abstract Sol-gel-derived silica films were fabricated by dip-coating onto silicon and glass substrates. Film properties such as thickness and temporal stability were monitored as a function of dip speed, water:precursor ratio, sol aging time and time after dipping. Film behaviour was interpreted in terms of the dependence of hydrolysis and condensation rates on the interplay between sol pH and water:precursor ratio. Film thickness was found to increase by approximately a factor of two as water:precursor ratio increased from two to six. Film thickness also increased with sol pre-polymerisation time. The time to achieve film stability was decreased at higher water:precursor ratios. Surface quality was correlated with processing conditions.