Uranyl sensor

The quest for improved uranyl sensors has rested with ionophores. lonophores suitable for calcium (5) and lithium (28) are subject to sodium and calcium interferences (Table 3.17) whereas amongst newer ones, (34) particularly is considered to provide a UOj device of unrivalled specificity (65). 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

Figure 22 A calix[6]arene-based uranyl ion sensor [35]...

Since some DNAzymes depend on specific metal ions for activity, they can be employed for the detection of those ions, hi the first example, a previously reported [59] DNAzyme was labeled with a Dabcyl fluorescence quencher at the 3 -end and the corresponding RNA substrate with a TAMRA fluorophore at the 5 -end [60]. Upon addition of Pb " ions, the substrate was cleaved, resulting in dissociation from the DNA enzyme strand. This led to spatial separation of the fluorophore-quencher pair, resulting in fluorescence (Fig. 5). The sensor system was over 80 times more responsive to Pb than to other metal ions, and had a quantifiable detection range of 10 nM to 4 pM. A similar strategy was developed for the detection of Ctf by a DNAzyme that oxidatively cleaves DNA [61]. The system showed a dynamic range of 35 nM to 20 pM and had a metal ion selectivity of a factor of 2000 for Cu over other metal ions. A comparable system was reported for the detection of the uranyl cation (UO/ ), with millionfold selectivity over other metal ions and parts-per-trillion sensitivity. 

As listed in Table 36.1, a good deal of research has been carried out so far on proton conductor-based gas sensors workable at ambient temperature. Various inorganic and organic ion exchangers, such as hydrogen uranyl phosphate (HUP), zirconium phosphate (ZrP), antimonic acid (AA), and NAFION membrane, have been utilized in the form of a disc, thick- or thin-film. The ionic conductivities of these proton conductors, in the range lO " 10 S cm are modest but seem to be still sufficient for chemical sensing devices. 

Organophosphoric acids and a variety of organophosphorus mediators have been examined as liquid ion-exchanger systems for PVC uranyl ISEs (63,64) (Table 3.17). The sensors are readily made by multiple-stage extractions from... 

After removing chloroform, the residual uranyl salt is incorporated with mediator into PVC membranes. This simple technique has proved useful for preparing other sensor material, e.g. zinc and beryllium organophosphates. 

Calcium-phosphate-type sensors are essentially unaffected by large doses of Co-gamma radiation (11) and thus the corresponding uranyl phosphate, and possibly neutral carrier types, should be immune to the very modest doses of internally generated uranium radiation. 

Measurements of nitrate in soils and waste waters using the TOAN-DBP electrode and the brucine method agreed closely. All these nitrate ISEs (66,67) are subject to serious interference from iodide, chlorate and perchlorate. This feature can in turn be exploited, e.g., the Corning nitrate exchanger may be readily converted to a viable chlorate-sensing cocktail using an extraction technique as described for the uranyl phosphate sensor (section 3.2.10). 

Owing to its sensitive response to the uranyl ion (U02 +) in aqueous media, several uranium-sensitive optical sensors based on Arsenazo HI (37) have also been developed for the spectrophotometric determination of this cation at trace... 

Nanda, D., Chauhan, H.P.S. Maiti, B. (2004b) PVC membrane based potentiometric sensor for uranyl ion using thenoyl trifluoro acetone as ionophore. Indian Journal of Chemistry, 43 (8), 1685-1688. 

Wroblewski W, Wojciechowski K, Dybko A et al (2000) Uranyl salophenes as ionophores for phosphate-selective electrodes. Sensors Actuators B 68(1-3) 313-318... 

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