Receptor-based sensors

Instead of isolating, stabilizing, and immobilizing chemoreceptors onto electrodes, it is possible to use intact biological sensing structures for determining relevant chemical stimulants (88,89). This novel concept was illustrated with antennule structures of the blue crab. Such structures are part of the crab food- 

FIGURE 6-15 Schematic representation of the ion permeability modulation for cation-responsive voltammetric sensors based on negatively charged lipid membranes. Complexation of the guest cation to the phospholipid receptors causes an increase of the permeability for the anionic marker ion. (Reproduced with permission from reference 49.) 

FIGURE 6-16 Neuronal sensing top view of a flow cell with mounted antennule and the various electrode connections. (Reproduced with permission from reference 52.) 

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Radical ions, 33, 44 Raman spectroelectrochemistry, 45 Randles-Sevcik equation, 31 Rate constant, 12, 18 Rate determining step, 4, 14 Reaction mechanism, 33, 36, 113 Reaction pathway, 4, 33 Reaction rate, 12 Receptor-based sensors, 186 Redox recycling, 135... 

Receptor-based sensors are still quite rare and much work is needed to implement ISEs into the general practice of pharmaceutical analysis. 

Roberts, 1983 Wingard, 1987), might greatly facilitate the development of sensitive and selective receptor-based sensors for the respective ligands. 

Further progress in molecular receptor-based sensors will be a function of advances in molecular biology with respect to enhancing the quantity, purity, and stability of receptors that can be used in vitro. 

It is worth noting that TTF-based sensors for species other than metals, particularly organic species, have been well studied. The principle is, of course, the same a host group capable of recognising a guest molecule is tethered to the signaller TTF, which displays an altered physical response when a guest is bound and when the receptor is free. Systems of this type are of particular interest as biosensors, for example, in the detection of saccharides for disease... 

The dominant attribute that has driven interest in fluorescent conjugated polymers (CPs) sensory materials is their ability to produce signal gain in response to interactions with analytes. The increased sensitivity (amplification) is derived from the ability of a conjugated polymer to serve as a highly efficient transport medium of electronic excitation. Analyte specificity in CP-based sensors results from the covalent or physical integration of receptors, imprinting, and/or the CP s overall electrostatic and chemical characteristics. The observed amplification is a... 

Microcantilever sensors can be operated in modes in which receptor-based coatings are not needed for example, deflagration of adsorbed energetic molecules can induce a measurable response [10]. Since cantilevers can be made extremely sensitive to temperature using bimaterial effect, calorimetric methods can be carried out on cantilevers with adsorbed molecules [11, 12], Exposing the temperature-sensitive cantilevers with adsorbed species to different infrared (IR) wavelengths in a sequential fashion creates mechanical signatures that mimic the IR absorption spectra of the analyte [13]. 

FIGURE 1.6 Schematic representation of cell-based sensor (CBB) for pathogen detection. After binding to receptor on mammalian cells, pathogen or toxin will aid in the release of signaling molecules such as fluorescence or enzyme that can be detected using an appropriate sensor. 

Azpiazu, I., and Gautam, N. (2004). A fluorescence resonance energy transfer-based sensor indicates that receptor access to a G protein is unrestricted in a living mammalian cell./. Biol. Chem. 279, 27709-27718. 

Ferrocene-based bisphosphanes, in hydrogenations, 10, 10 Ferrocene-based receptors anion receptors, 12, 476 cation receptors, 12, 466 for neutral guest species, 12, 483 Ferrocene-based sensors, screen-printed, in biosensing,... 

So far in this section the examples discussed have comprised of hetero aromatic antenna receptors, which mainly function as pH sensors. The detection of metal cations in biological, medical and ecological applications is of major interest. However, this requires more elaborate receptors that can selectively bind a specific metal cation. Naturally, the more elaborate the receptor required, the more challenging its synthesis becomes. The synthesis of lanthanide-based sensors of 53 and 54 has been reported with each ligand incorporating an antenna/receptor selective for zinc(ll) [158,159]. In both cases the receptors are structurally similar, with 53 synthesised from benzylamine and 54 from aniline. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

Although macrocyclic receptors containing N and O donor atoms are suitable for detection of alkali metal cations, the detection of alkali earth cations required application of chelating receptors based on amino acids with EDTA-like spatial arrangement, eg bis(o-amino-phenoxy)-ethane-A, N, N, A -tetraacetic acid (BAPTA) [9,12], The most common calcium PET sensors are shown in Figure 16.6. 

The discussion so far has centred on receptors based on the iminodiacetate element . However, even crown ethers can serve as receptors in sensors where the dicationic alkaline earths can elicit a larger fluorescence response than the alkali monocations. Charge density effects of this sort are particularly noticeable in solvents of relatively low polarity. The best examples are to be found among fluoroionophores [137] with integrated fluorophore and receptor moieties such as (53) [120] and (54) [121]. No PET sensors of this type are currently available, though case (32) [85] is an approximate example of a fluorophore-spacer-receptor system. 

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