Fabrication of optical sensors

Caputo, G. Delia, C. L. General method for the covalent immobihsation of indicators in, or onto a support useful for the fabrication of optical sensors. Eur. Pat. Appl. EP 1213583, 2002 Chem. Abstr. 2002,137,14966. 

Optical sensors are relatively new and not much work has been carried out in this field. From first principles, the sensitivity of an optical sensor would depend on the amount of fluorescence emitted which in turn is proportional to the available area of the interface. Thus by change in morphology, if pores or pockets could be introduced the sensitivity would also increase proportionately. In the case of nanofibers this is inadvertent as the available area by itself is quite large and when porous fibers are used, the further increase is manifold. Thus, nanofibers would be the best-suited materials for fabrication of optical sensors. 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

BronkKS, Walt DR. Fabrication of patterned sensor arrays with aryl azides on a polymer-coated imaging optical fiber bundle. Analytical Chemistry 1994, 66, 3519-3520. 

The importance of these kinetic phase-separation effects will depend strongly on the relative concentrations of the components, of course, since the kinetics of every elementary step leading to gel formation are concentration dependent. From the point of view of creating functional materials that require site accessibility, the base-catalyzed situation is preferable. Unfortunately for those apphcations that rely on optical properties as well as site accessibihty (such as fabrication of optically based sensors), the use of base catalysis typically affords opaque materials. For catalysis and separations applications, however, the optical properties are of little importance (except for characterization of the... 

This study also explored the scope of the utility of direct dyes as Immobilized Indicators at hydrolyzed cellulose acetate films for the construction of optical sensors which exhibit a rapid response and are easily fabricated. In addition, and perhaps more importantly, the "proof of concept" design of a sensor and preliminary evaluation of characterization with a response over a broad pH range was demonstrated. Illustrating opportunities provided by this approach for the fabrication of sensors with new and/or Improved performance characteristics. Studies to explore the fundamental Interactions that govern the formation, structure, reactivity, and stability of such sensors are currently underway. 

NCF is the candidate textile architecture for the incorporation of optical sensor fibres because of the absence of crimp in the fibres, which reduces the risk of micro-bending of the optical fibre. This would create losses in the fibres and would endanger their function. Another advantage with respect to conventional woven stmc-tures is that several layers can be stitched together, including the layer oriented at 0°, where the optical sensor fibres can be placed, and the fabric can be made as thick as desired for the application, so that one single fabric is sufficient. Also, the stitch can be designed to obtain a stable fabric, even with open stmctures. 

Arregui, F.J., et al.. An experimental study about hydrogels for the fabrication of optical fiber humidity sensors. Sensors and Actuators B Chemical, 2003. 96(1-2) p. 165-172. 

Through the examples presented above, MIPs appear to be very pronusing novel bioelements. They are more stable and more reproducible than natural antibodies. They can be used in combination with various transducers (electrochemical, optical, itnmunosensors, etc.). The synthesis can be set up so that the MIPs are directly synthesized on the transducer. A large diversity of functional monomers and cross-linking polymers as well as synthesis methods is available. An easy fabrication and characterization of the MIP could give them the potential to be widely used in an industrial fabrication of the sensors. 

The advantages of miniaturization are now being exploited in areas beyond microelectronics. Adaptation of materials and processes originally devised for semiconductor manufacture has allowed fabrication of sensors (for example, pressure meters and accelerometers used in the automotive industry) (6,7), complex optical (8) and micromechanical (6,7,9) assembHes, and devices for medical diagnostics (6,7,10) using Hthographic resists. 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Clark H.A., Kopelman R., Tjalkens R., Philbert M.A., Optical Nanosensors for Chemical Analysis inside Single Living Cells. 1. Fabrication, Characterization, and Methods for Intracellular Delivery of PEBBLE Sensors, Anal. Chem. 1999 71 4831— 4836. 

Sensitivity impacts upon the limit of detection and resolution of the device, making it a key performance parameter. Recently, several strategies have been developed in order to provide sensitivity enhancements for optical sensor platforms based on both optical absorption and fluorescence phenomena. These strategies are the result of rigorous theoretical analyses of the relevant systems and, combined with polymer processing technology and planar fabrication protocols, provide a viable route for the development of low-cost, efficient optical sensor platforms. 

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