Mechanisms sensing chemicals

Few chemical sensing mechanisms (other than bulk quenching) have been described for neutral molecules nevertheless, many small analytes of interest are uncharged glucose, for example. Of course, most carbohydrates are neither fluorescent nor are they fluorescence quenchers novel signal transduction mechanisms are required. 

In the near-IR, sensors almost exclusively rely on silica fibres (standard or low-OH) as they are accepted as industrially fully applicable32, 33 Silica-based glass fibres are chemically and mechanically robust, easy to handle, inexpensive, available with various core and outer diameters, a core-clad transfer fibres or bare sensing fibres, and have successfully been optimised to their theoretical attenuation limit.34. The spectral window allows application up to 2,5 pm. 

Fig. 36 Sensing mechanism of the TBA probe for the detection of Pb2+ and Hg2+ through enhanced RET-type quenching. (Reprinted with permission from [212]. Copyright 2009 American Chemical Society)...

Volume 4 is intended to summarize the principles required for these biomedical applications of time-resolved fluorescence spectroscopy. For this reason, many of the chapters describe the development of red/NIR probes and the mechanisms by which analytes interact with the probes and produce spectral changes. Other chapters describe the unique opportunities of red/NIR fluorescence and the types of instruments suitable for such measurements. Also included is a description of the principles of chemical sensing based on lifetimes, and an overview of the ever-important topic of immunoassays. 

If enzymes are described under tbe aspect of reaction mechanisms, the maximal rate of turnover Vmax. the Michaelis and Menten constant Km, the half maximal inhibitory concentration ICso, and tbe specific enzyme activity are keys of characterization of the biocatalyst. Even though enzymes are not catalysts in a strong chemical sense, because they often undergo an alteration of structure or chemical composition during a reaction cycle, theory of enzyme kinetics follows the theory of chemical catalysis. 

SAMFETs have also been used in chemical sensing. The a-substituted quincpiethiophene SAMFETs were covered with a 10-nm pinhole-riddled iron tetraphenylporphyrin chloride layer, that acts as a receptor to nitric oxide (NO), an important biomarker [74]. The threshold voltage, measured by the FET transfer characteristics with the porphyrin receptor shifts upon increased exposure to NO. Annealing the monolayer FET in vacuum restores the initial FET behavior. Also, in the single monolayer HBC assembled FETs between metallic SWCNT source and drain electrodes increased current levels were measured in /d-Fds and Aj-Fg characteristics (Fig. 9) upon exposure to solutions of the electron acceptor TCNQ [68]. While the mechanism of response is not known, TCNQ has an affinity for coronene, and likely gives rise to charge transfer between electron-deficient TCNQ... 

It is important to have a clear picture of the detection mechanism before we introduce the different types of field-effect transistor (FET) devices and their gas sensing properties. The sensing mechanism is largely independent of the device type, since the chemical reactions responsible for the gas response are defined by the type of catalytic material processed onto the device and the operation temperature [1,2, 20, 21]. Even at a temperature of 600°C, chemical reactions occur on the catalytic metal surface at a rate of a few milliseconds, which is slower than the response time of the devices. 

The sweet taste and olfactory responses to a variety of stimuli are examples of chemical senses that utilize protein receptors for initial detection of the stimulus. Most bitter compounds have a hydrophobic component which enables their direct interaction with the cell membrane however, some evidence suggests a protein receptor mechanism. The cooling sensation is treated as a chemesthetic sense, where stimulation takes place at the basal membrane. However, compounds that evoke this response have very specific structural limitations, and most are related to menthol. For purposes of discussion, bitter and cooling sensations will be discussed under generalized receptor mechanisms. 

Koratkar, N. (2003) Nanoscale devices for thermal, mechanical and chemical sensing. Proc. Inti. Conf. on Nano Science and Technology, Kolkata, India, Dec. 17-20, 2003, p. 55. 

The application of semiconductor luminescence to chemical sensing can rely on the chemical, electrical, and optical properties of II-VI and III-V semiconductors [1]. These properties provide the binding capability, transducing mechanism, and signal required for chemical sensing. The diverse chemical compositions of semiconductor materials provide a range of surfaces for molecular binding. 

This also characterizes the very style of the experimental studies. Even until recently the hydrodynamic theory of the detonation velocity, which was excellently confirmed in experiments, created a sense of contentedness and did not inspire the search for the chemical reaction mechanism or investigation of the conditions at the detonation wave front. If our paper brings about new experimental studies which penetrate deeper into the essence of the phenomenon, then our task will have been accomplished. 

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