Analytical colorimetric sensors

Colorimetric sensors utilize a change in wavelength, either during absorption, emission, or fluorescence, as the detection method. A chemical interaction between the analyte and the sensor surface produces a change in the optical property. Several methods to detect these changes in optical properties are discussed in this section. 

Electropolymerization of a functionalized precursor represents a straightforward method for the realization of modified electrodes endowed with specific electrochemical or optical properties. Electrode materials based on electrogenerated functional conjugated polymers and their application as electrochemical sensors have been already reviewed [162-165]. On the other hand, the sensitivity of the optical properties of ir-conjugated polymers to conformational changes has led to the realization of colorimetric sensors for the detection of various analytes extending from alkali metal ions to anions and biomolecules [165-168]. In general, the realization of sensors based on functional PTs relies on the fact that complexation at a side chain may lead to perturbation of the polymer conformation, which can be read by either electrochemical or optical methods. 

The analyte-induced aggregation of functionalized poly(thiophene) is reminiscent of colorimetric sensors that are based on the aggregation or dissociation of gold nanopartides. Gold nanoparticles display a red-shift of their plasmon band upon aggregation. This feature has been exploited extensively for (bio)analytical applications. A discussion of this work extends the scope of the present chapter and more details can be found in a number of review artides [25]. 

However, colorimetric sensors have at least one disadvantage with respect to other analytical methods (electrochemical, potentiometric, etc.), namely, they are generally characterized by low sensitivity. However, lower limits of detection can be obtained using fluorescent- or luminescent-based approaches. Therefore, in an effort to improve the sensitivity of an optode, fluorophores are often coupled to the substrate binding subunit. Using this strategy, it is often possible to obtain sensors that produce both colorimetric and luminescent responses that allow analyte detection via instrumental analysis, as well as in certain cases via the naked eye. Currently, the number of colorimetric sensors for lanthanide ions is still limited. Reviewed below, is recent progress in this area. 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

An ideal sensor recognizes analytes in a sensitive, selective, and reversible manner. This recognition is in turn reported as a clear response. In recent years, conducting polymers have emerged as practical and viable transducers for translating analyte-receptor and nonspecific interactions into observable signals. Transduction schemes include electronic sensors using conductometric and potentiometric methods and optical sensors based on colorimetric and fluorescence methods [1]. 

All integrated sensors based on an interaction between the analyte and reagent (neither of which is used in a retained form) and regeneration of the latter rely on chemiluminescent reactions involving electroregeneration of the reagent or a quenching phenomenon. On the other hand, absorptiometric and reflectometric sensors of this type use colorimetric acid-base indicators supported on a suitable material. 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

In the last decade, fiber-optic chemical sensors (FOCS), also known as optrodes, have emerged as alternatives to conventional methods of analysis. FOCS development for a particular analyte depends on the availability of reversible indicating schemes to detect the analyte of interest. Typically, the indicating schemes use commercially available colorimetric or fluorometric indicators (e.g. fluorescein to measure pH (1)). However, the utility of these indicators is limited. Furthermore, indicators may not exist for many analytes. Several reviews discuss the scope of this approach (2,3,4). 

In the same way, the host of a host-guest complex can fimction as, or as part of, a sensor. Sensors are devices that announce the presence of analytes via reversible, real-time signals discernible by one of the human senses. Molecular recognition complexes utilized toward this end fimction as reusable indicators. One school of thought holds that a sensor is a macroscopic device with wires and the like a pH meter is one such embodiment. We prefer the view that molecules themselves can be sensing devices as well. A solution of a colorimetric pH indicator fulfills all the requirements of a sensor the solution announces the presence of protons via a reversible, real-time signal discernible by one of the human senses (sight). 

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