Toluene chemical sensors

The sPS has been exploited as sensitive material for fiber optic chemical sensors based on reflectance measurements and aimed to detection of chloroform and toluene in water and air environments48 50. The refractive index of sPS thin films is estimated to be about 1.578. The effect of the analyte sorption in the crystalline domain was modeled as an increase in the material density, which in turn leads to an increase in the refractive index according to the Lorentz-Lorenz law ... 

As expected, solutions containing mixtures of nanocrystal sizes exhibited spectral profiles corresponding to the sum of the spectra of the individual sizes (Fig. 5.3). The initial PL peak positions of 2.8 and 5.6-nm CdSe nanocrystals in toluene solution were at 541 and 636 nm, respectively. We incorporated the CdSe nanocrystal mixtures into polymer matrices rationally selected based on polymer-sensing properties (Table 5.1). CdSe nanocrystals exhibited spectral shifts upon their immobilization in polymer films and photoactivation with the 407-nm diode laser. Photoactivation is an important aspect of the performance of the CdSe and other semiconductor crystals as chemical sensor materials.16 Different spectral shifts of steady-state PL emission and different emission intensity were observed upon incorporation of the 2.8 and 5.6-nm CdSe nanocrystals in polymer films as illustrated in Fig. 5.4. Polymer 2 (PMMA) was selected for the detailed evaluation because it has been used previously as a matrix for incorporation of CdSe nanocrystals.1636... 

CdSe/PMMA nanocomposites have also been used as chemical sensor for detection of aromatic hydrocarbons [196]. In that study, a series of surface-modified QDs, which utilized both TOPO and carboxylic acids attached to the surface of CdSe SC QDs, were developed. Unmodified QDs and surface-modified nanocrystals were incorporated into PMMA matrices, and the PL of nanocomposites was studied as a function of a series of exposures to toluene and xylene vapors ranging in concentration from ppm to percent levels. It was reported that reversible enhancement and quenching of the PL on film exposure to toluene and xylene vapors were both observed for the QDs modified by carboxylic acid systems. PL enhancement was observed at low target gas concentrations, with the onset of the quenching process for each of the films being dependent on the target gas type and its concentration [196]. 

Figure 8. Seventy-six sensor beads (Jupiter C4/Nile Red) monitored to show that the average responses for three consecutive 0.38-s exposures of 50% saturated vapor levels result in reproducible and high-speed response profiles. The sensors are positioned on the distal tip of an optical imaging fiber and relative analyte concentrations are 0.5 and 18700 ppm for 1,3-DNB and toluene, respectively. Reprinted with permission from ref 12a. Copyright 2000 American Chemical Society.

In the following, the concept of micro modular process engineering is introduced together with the backbone interface developed in order to realize this modular approach. The integration of sensors and an electronic bus system is also described, and the physical characterization of the backbone is discussed within a case study of the enantioselective synthesis of organoboranes. Within the second case study, the sulfonation of toluene with gaseous sulfur tri oxide, the backbone system together with the micro structured devices used is finally assessed based on its application to chemical synthesis. 

Fast response is possible with swelling-based sensing mechanisms. For example, the mapping of a chemical plume caused by a release into the air could employ such a sensor positioned on an unmanned aerial vehicle (UAV). With this vehicle traveling at 40 miles/h, subsecond response times would be required to locate and map out the released analyte. An example of this is shown in Fig. 16, which describes a thin film of poly(2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole-co-tetrafluoroethylene), Teflon AF placed on an interferometer which in turn was placed in the nose cone of an UAV, which was positioned in a wind tunnel with the wind moving at 40 mph. Small amounts of toluene vapor were introduced into the air stream to get an idea of whether the sensor would work in such an application. The sensor responded quickly, established equilibrium in seconds, and reversibly returned to baseline after the material passed. 

PL peak intensity of the CdSe-FBA film as a function of time for chemical vapor exposures (a) toluene and (b) xylenes in nitrogen. Reprinted from Sensors and Actuators B, 123, 522 (2007). Copyright (2007), with permission from Elsevier. 

Besides the utilization of PANI/Sn02 as a humidity sensor device, many scientific works emphasized that nanocomposites developed from polyaniline (PANI) and metal oxides—especially Sn02—are good candidates as gas sensor deviees. Taking into account all these observations, the literature supports fabrication of PANI/Sn02 hybrid materials with different amounts of Sn02 content prepared by in situ chemical oxidative polymerization method, with the sensitivity of materials toward toluene... 

We had initially limited our studies to polypyrrole based sensors with several different chemically inert VOCs and to polythiophene sensors with toluene analyte. However, because of the relatively low sensitivity of the polypyrrole sensors and because of photosensitivity of the polythiophene sensors we have recently directed our attention to a new type of oligomer of aniline which we have recently synthesized and characterized (67, 62). 

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