Sensors, boronic acid

An example of old PET signaling systems being applied to new purposes appears in the recent report on 23 as a boronic acid sensor by Wang and col-... 

A more recent approach to designing direct fluorescent indicators for glucose involves two-component boronic acid sensors, where the fluorophore—usually anionic in nature—is quenched by a physically separate boronic acid-substituted viologen receptor. As the saccharide binds with the receptor, the quenching efficiency of the viologen is reduced, resulting in an increased emission intensity of the fluorophore (Figure 10.8). 

In 2004, Zhao and James et used the same Binol-based boronie acid chemosensors 4 (Figure 6.2) for enantioseleetive reeognition of D- and L-tartarie acids (Figure 6.3). As there are two a-hydroxy earboxylic acid units in tartaric acid, a cyclic adduction with the Binol based bis-boronic acid sensor can be formed. pH titrations showed that the fluores-eence response of the chemosensor is significant in the acidic pH range. 

Figure 6.3 Chiral polyol substrates that can be selectively recognized by chiral bis-boronic acid sensor 4. ...

Interestingly, the chiral bis-boronic acid sensor 4 show enhanced fluorescence emission towards one enantiomer of the tartaric acid, whereas fluorescence decreased in the presence of another enantiomer (Figures 6.4 and 6.5). This enhancement/reduction in fluorescence response for chiral recognition of enantiomers was rarely reported. Both the fluorescence... 

In 2004, Zhao and James et developed the anthracene-based chiral boronic acid fluorescent sensors 5 and 6, with the a-methylbenzylamine as the chirogenic centre (Eigure 6.6). It is supposed that a cyclic adduct can be formed between the bis-boronic acid sensor and tartaric acids. Based on the fluorescence response of the bis-boronic acid sensor, drastically different fluorescence responses toward the two enantiomers were observed in the acid/ neutral pH range (Figure 6.7). For example, the (i )-sensor shows substantially increased fluorescence in the presence of o-tartaric acid at pH 8.3. In the presence of L-tartaric acid, however, there was no fluorescence enhancement (Figure 6.8a). 

In this case, the sensing mechanism with the bis-boronic acid is open to question. With monoboronic acid sensor 6, fluorescence enhancement was observed in the presence of both d- and L-tartaric acid (Figure 6.8b). For the combination of D-bis-boronic acid sensor and L-tartaric acid, however, no fluorescence enhancement was observed. Herein, we proposed that 1 1 cyclic binding complexes form, otherwise fluorescence enhancement should be observed with the 1 2 binding complexes. Interestingly, the putative 1 1 binding complexes is weakly fluorescent, therefore no significant fluorescence enhancement was observed. The reason for the weak fluorescence of the D-bis-boronic acid sensor/D-(or L)-tartaric acid is unclear. 

Figure 6.9 Single-crystal structure of the binding complex of bis-boronic acid sensor (5,5)-5 complex with L-tartaric acid. (Reproduced by permission of the American Chemical Society.)...

Figure 6.10 a-Methylnaphthylamine based chiral fluorescent bis-boronic acid sensors (R,i )-(-)-7 and (S,5 )-(+)-7. 

In 2009 a d-PET boronic acid was devised by Zhao and James et al (Figure 6.15). An electron-rich carbazole was used as the fluorophore of the chiral bis-boronic acid sensor 9 and a-methylbenzylamine was used as the chirogenic centre of the sensor. To achieve chiral recognition a bis-boronic acid system was used in designing the fluorescent sensor. The fluorescence response of the sensor toward protonation is in contrast to the normal a-PET chemosensors. The fluoreseenee of the sensor is weaker in the acid pH region (Figure 6.16). 

Figure 6.19 Molecular structure of the thienyl carbazole based d-PET bis-boronic acid sensors 11. The monoboronic acid sensors (S)-12 and (i )-12 are also presented.

Boronic acid-derived fluorescent chemosensors are unique in that the inter-molecular interaction is a covalent bond, and not hydrogen bonding as is the case for most conventional fluorescent molecular sensors used for the selective reeognition of hydroxyl carboxylic acids. This chapter summarizes the development of the boronic acid-based chiral fluorescent chemosensors over recent years and the enantioselective fluorescent reeognition of chiral a-hydroxyl carboxylic acids analytes in aqueous solutions. The fundamental scaffolds of these chiral sensors include a fluorophore, an arylboronie aeid binding site, and linker between the two units. The systems usually consist of a bis-boronic acid unit, which is required for enantioselective recognition of the chiral a-hydroxyl carboxylic acid analytes. However, mono-boronic acid fluorescent chemosensors have also been developed. All three components of the chiral boronic acid sensors play an important role in determining the... 

Hall and co-workers have taken the modular approach a step further and employed a solid-phase synthetic approach to optimize structural and electronic properties of boronic acid sensors for oligosaccharides [83]. Through this approach they determined that para electron-withdrawing groups on the boronic acids were beneficial. The approach also allows for the easy introduction of different boronic acid units. Di-... 

If practically useful sensors are to be developed from the boronic acid sensors described above, then they will need to be integrated into a device. One way to help achieve this goal is to incorporate the saccharide-selective interface into a polymer support. 

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