Spectroscopic sensors

Spectroscopic sensing requires that a chromophore must be available and be influenced by the rebinding of the imprinted analyte. This can be accomplished in a variety of ways, the simplest case being when the analyte is itself a chromophore. Sensors based on intrinsic analyte chromophores can benefit from molecular imprinting by both selectivity and sensitivity enhancement. In terms of selectivity, molecules similar to the analyte are likely to have similar spectroscopic parameters that could be the source of interference in a conventional sensing strategy. The inability of an interferent to bind to the imprinted polymer allows discrimination. 

Ionic sensors based on molecularly imprinted polymers 

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Recent advances in instrumentation range from novel (laser) sources and highly compact spectrometers over waveguide technology to sensitive detectors and detector arrays. This, in combination with the progress in electronics, computer technology and chemometrics, makes it possible to realise compact, robust vibrational spectroscopic sensor devices that are capable of reliable real-world operation. A point that also has to be taken into account, at least when aiming at commercialisation, is the price. Vibrational spectroscopic systems are usually more expensive than most other transducers. Hence, it depends very much on the application whether it makes sense to implement IR or Raman sensors or if less powerful but cheaper alternatives could be used. 

As this chapter aims at explaining the basics, operational principles, advantages and pitfalls of vibrational spectroscopic sensors, some topics have been simplified or omitted altogether, especially when involving abstract theoretical or complex mathematical models. The same applies to methods having no direct impact on sensor applications. For a deeper introduction into theory, instrumentation and related experimental methods, comprehensive surveys can be found in any good textbook on vibrational spectroscopy or instrumental analytical chemistry1"4. 

The theory behind molecular vibrations is a science of its own, involving highly complex mathematical models and abstract theories and literally fills books. In practice, almost none of that is needed for building or using vibration spectroscopic sensors. The simple, classical mechanical analogue of mass points connected by springs is more than adequate. 

As vibrational spectroscopic sensors have a high inherent specificity, selective layers are usually not necessary. Still, sensor modifications can strongly enhance the sensitivity. At the same time, in particular for complex multi-component samples with spectrally interfering analytes, also the sensor... 

The continuous determination of compounds, which may adversely affect ecosystems and/or human health, is a major regulative and legislative goal of environmental protection nowadays. Considering the costs and efforts related to this task corroborates a clear demand for portable, real-time, in-situ, field applicable and cost-effective monitoring techniques. Due to their inherent properties, vibrational spectroscopic sensors, in particular fibre-optic sensors show a high potential to contribute to these applications. 

The basis of all determinations under this heading is the Beer-Lambert law [ 14—16]. All reaction spectra obtained by any spectroscopic sensors can be used as long as the Beer-Lambert law is obeyed. For spectra of a reaction containing several components and absorbances measured a number of times at several wavelengths, the matrix form of Equation 8.19 can be used ... 

Cyclodextrins (Sect. 2.2), have the ability to include various organic molecules in their central cavities. Chromophores have been finked to cyclodextrins to build spectroscopic sensors for organic molecules. Many fluorescent cyclodextrins have been prepared to be used as molecular sensors in solution. 

Finally, the robustness of the calibration is a critical parameter that must be established before any noninvasive measurement technology will be useful. Important issues include (1) the ability to collect reliable spectra for each measurement (2) a protocol required to establish a working calibration model (3) time stability of a calibration model and (4) sensitivity of the calibration model to external factors, such as ambient temperature and vibrations. These issues go far beyond demonstrating the feasibility of a noninvasive spectroscopic sensor but pertain to its eventual practical implementation. 

Booksh, K., Henshaw, J.M., Burgess, L.W., and Kowalski, B.R., A second-order standard addition method with application to calibration of a kinetics-spectroscopic sensor for quantitation of trichloroethylene, J. Chemom., 9, 263-282, 1995. 

Rhiel M, Cannizzaro C, Marison I, von Stockar U (2000) All-in-one bioprocess monitoring with a mid-IR spectroscopic sensor. Abstracts of the 219th ACS National Meeting, San Francisco, March 2000... 

Raman spectroscopic sensors for inorganic salts Marc D. Fontana, Kawther Ben Mabrouk and Thomas FI. Kauffmann 40... 

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