Sensing films

In Ref. 11, IT voltammograms of NH4 and N03 were obtained when a 0-pipette was exposed to vapors of ammonia and nitric acid, and linear dependence of the voltam-metric response on concentration of vapor-generating solution has been demonstrated. The surface liquid layer in all pipettes used in that work was aqueous, and only the detection of water-soluble gases was discussed. However, the detection of organic compounds in the gas phase may also be possible using a 0-pipette with a nonaqueous sensing film. 

C. Hu, X. Chen, and S. Hu, Water-soluble single-walled carbon nanotubes films preparation, characterization and applications as electrochemical sensing films. J. Electroanal. Chem. 586, 77-85 (2006). 

Mensitieri, G. Venditto, V. Guerra, G., Polymeric sensing films absorbing organic guests into a nanoporous host crystalline phase, Sens. Actuators B 2003, 92, 255 261... 

To overcome the limitation of detecting only a color change in the sensing colloidal crystal films, we apply a differential spectroscopy measurement approach coupled with the multivariate analysis of differential reflectance spectra. In differential spectroscopy, the differential spectrum accentuates the subtle differences between two spectra. Thus, in optical sensing, when the spectral shifts are relatively small, it is well accepted to perform measurements of the differential spectral response of sensing films before and after analyte exposure6 19. Therefore, the common features in two spectra of a sensing film before and after analyte exposure cancel and the differential spectrum accentuates the subtle differences due to analyte response. 

The spectral reflectivity of the sensing film before and after the exposure to different vapors (all a.tP/P0 = 0.1) is illustrated in Figs. 4.7 and 4.8. Similar to other photonic nanostructured sensors8 19 34 35, the spectral shifts upon response to low vapor concentrations are relatively small. Thus, to accentuate the subtle differences due to vapor response, we measured the differential reflectance spectra AR as described by equation (4.1). 

Changes in the differential reflectance spectra AR of the sensing film upon exposure to different vapors at various concentrations are presented in Fig. 4.9. These spectra illustrate several important findings. For polar vapors such as water and ACN (see Fig. 4.9a, b respectively), the differential reflectance spectra have a stable baseline and consistent well-behaved changes in the reflectivity as a function of analyte concentration. The response of the colloidal crystal film to nonpolar vapors such as DCM and toluene (see Fig. 4.9c, d respectively) is quite different compared with the response to polar vapors. There are pronounced analyte concentration-dependent baseline offsets that are likely due to... 

Fig. 4.7 The spectral reflectivity of the sensing film measured over the range 400 1,000 nm before (solid line) and after (dotted line) the exposure to different vapors (all at P/P0 0.1) (a) water,...

The reversibility of interactions of the composite colloidal crystal film with different vapors was also evaluated. Figure 4.10 illustrates the dynamic response of the sensing film at several wavelengths (770, 835, and 870 nm) upon triplicate exposures to water vapor at four concentrations (0.02, 0.04, 0.07, and 0.1 PIPo). The response and recovery kinetics upon exposure to water vapor were fully reversible and rapid (under 5 s). A comparison of dynamic response of the sensing film at five different wavelengths upon exposures to water and toluene vapors at four concentrations is presented in Fig. 4.11. These data illustrate that the direction, magnitude, and kinetics of the responses to these vapors were quite different. 

Fig. 20a.3. Schematic of optical sensing film mounted in a flow-through cell for absorbance measurements.

Zinc-5,10,15,20-tetraphenylporphyrin (ZnTPP) has been used as a coating material in ammonia sensors by immobilizing it on the surface of silicone rubber. Absorbance and fluorescence emission were the modes of detection. A spectral change is caused by the coordination of NH3 molecules to the Zn11 ion in the immobilized metalloporphyrins. Sensing films made from the ZnTPP immobilized in silicone rubber were found to be the most sensitive for NH3 sensing (20). 

Chronoamperometric transduction can be applied to electroinactive analytes as well as electroactive, which are sorbed by the MIP film and then undergo an electrochemical reaction [25]. In the latter case, the analyte should be able to diffuse freely both towards and away from the electrode surface for the current to flow. The primary requirement of chronoamperometric sensing is a linear relationship between the current measured at the constant potential and the concentration of the analyte. Moreover, the electrochemically generated species should readily diffuse away from the electrode surface coated by the sensing film. By way of example, a few representative chronoamperometric sensors based on MIPs are presented below. 

The NH4+ sensing film optode was fabricated from TD19C6, KD-M13, K-TCPB, and NPOE under optimum conditions, which correspond to the NH4+ ionophore, the color-changeable dye of pfCa 7.9, the lipophilic anionic additive, and a membrane solvent, respectively, included in a PVC membrane [23,24], These chemical structures are shown in Fig. 13. KD-M13 becomes yellow in its protonated form and turns blue in the deprotonated form. When the quantity of the protonated form of the dye equals that of the deprotonated form, the mixture becomes green. 

The ion-sensing film optode responds to NH4+ on the basis of the ion-pair extraction/exchange mechanism as shown in the following reaction [25-27] ... 

Fig. 17 The reproducibility test (a) and the ion selectivity (b) of the NH4+ sensing film optode...

In this section, a visual and colorimetric NH4+ sensing film was described. The observed color gradation was suitable for human visual perception over a wide dynamic range. This NH4+ sensing film is useful for routine semiquantification of NH4+. 

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