Studies On Potentiometric Sensing Platforms Based On Novel Molecular Recognition Mechanisms

Molecular recognition is of great importance for chemical and biological sensing of specific targets. Highly selective measurements can be achieved using traditional ion-selective electrodes by doping the membranes with specific ionphores. Since 1960 s, with the development of biochemistry and supramolecular chemistry, many macromolecular and small molecular receptors have been discovered/synthesized and used as efficient ionphores for potentiometric sensing. Although the discovery of low-detection-limit ion-selective electrodes in 1990 s has further promoted the development of this technique, very limited new ionphores(especially for anion ionphores) have been reported in the past two decades. The main reasons may be that the findings in supramolecular chemistry have not been used to ion-selective electrodes and the intrinsic difficulties of constructing new ionphores/receptors for anions/neutral species. Based on novel molecular recognition mechanisms originating from covalent, coordination and hydrogen bonding interactions and free radical polymerization reactions, potentiometric sensing platforms for several kinds of targets have been developed.1. Potentiometric detection of polyols based on oligomerization with a diboronic acid. The instabilities originating from the intrinsic variable features of the enzymes may prevent the enzyme-based sensors from being widely utilized. Moreover, polymeric membrane electrodes for highly specific and enzyme free glucose detection have rarely been reported, probably due to the inability of glucose to induce potential response directly and unavailability of potentiometric reporters to specially recognize glucose and transduce the target-binding events. Based on the specific covalent reactions, the diboronic acid condenses with glucose via its two cis-diol units to form cyclic or linear oligomeric polyanions, which can interact electrostatically with protamine, thus efficiently decreasing the potentiometric response of protamine on a polycation-sensitive membrane electrode. Although fructose, galactose and mannose show even higher binding affinities to the diboronic acid as compared to glucose, these monosaccharides with only one cis-diol unit cannot oligomerize with the receptor, which efficiently excludes the interferences from the glucose’s stereoisomers. In this study, high selective glucose detection can be achieved when a specific receptor is unavailable. This sensing strategy can also be used for the detection of polyphenols from plants.2. Potentiometric sensing of aqueous phosphate by competition assays. Although covalent interactions are promising for highly affinitive recognition, specific functional groups are always required, thus restricting their wide applications. Considering that coordination interactions are very powerful and exist between most aninons and metal ions, we propose a potentiometric sensing platform for phosphate by competition assays. Using Zn2+-BPMP or Cu2+-BPMP as receptor and o-mercaptophenol as indicator to initiate the signal by the oxidation of the indicator, potentiometric sensing of aqueous phosphate can be achieved. To give more insights into the detection mechanism, the binding constants between the phosphate/indicator and the receptors have been determinated by potentiometric titration experiments and competition assays. Interferences from other anions are excluded in this study because of their inabilities to displace the indicator from the receptors. This method has been successfully used for detection of phosphate levels in mineral water and human urine and saliva samples, and the results are in good accordance with those obtained by the standard method.3. Cationic and netrual antimony based ionphores for potentiometric sensing of fluoride. Fluoride is one of the most hydrated anions which is in the front of the Hoffmeister series, and highly affinitive ionophores are needed to extract the aqueous fluoride into the polymeric membrane. Using cationic and netrual antimony compands as ionophores, highly selective fluoride sensors have been developed by optimizing the compotents(such as the types and amounts of the ion-exchangers) of the polymeric membranes. Anti-Hoffmeister series behaviors can be observed for most of the polymeric membranes doped with cationic and netrual antimony compands, which are significantly different from those of the TDMACl doped membranes. The complex formation constants between the anions and the ionphores in the solvent polymeric membranes have been determinated with segmented sandwich membranes. It is shown that the selectivity coefficients of the polymeric membrane electrodes reflect the complex formation constants. The proposed sensors can be used for aqueous fluoride detection with a high sensitivity, and are promising in real word applications.4. Potential responses to neutral thiophenols of polymeric membrane electrodes and their applications in potentiometric biosensing. It is difficult to develop receptors for electrically neutral species by using covalent and coordination bonds mentioned above. According to the traditional theory of phase boundary potential, only charged species can induce potential responses on the polymeric membranes. It has been reported recently that electrically neutral species such as surfactants, phenols and boronic acids can show significant potential responses on the ion exchanger-doped polymeric membranes by disturbing the charge separation layer at the oil/water interface. However, electrically neutral species that can induce potential responses are rather limited. In this work, the unexpected potential responses to electrically neutral thiophenols of polymeric membranes with the assistant of hydrogen bonds are shown. The complexations between thiophenols and quaternary ammonium salts in homogeneous and heterogeneous organic solutions have been studied by titration and extraction experiments, respectively. The potential response mechanism has been investigated. The potential responses to thiophenols of the polymeric membranes provide a simple way to differentiate thiophenols from biologically important bioactive aliphatic thiols. By using thiophenols as reducing substrates, a potentiometric sensing platform for oxidases and their related reactions has been developed.5. Label-free polymerization amplified potentiometric sensing platform for radical reactions. The methods developed in the above chapters are suitable for targets with stable physical and chemical properties. It should be noted that it is difficult to design receptors for chemically active species such as radicals. In this work, selective and sensitive sensors for these species are proposed using specific reactions. Taking advantage of the “polymerization for amplification” strategy by using enzymes or other radical initiators to produce polymers via free radical polymerization of cationic/anionic and aromatic monomers and using the polyion sensitive membrane electrodes as transducers, a highly sensitive and label-free potentiometric sensing platform for free radical reactions can be developed. By carefully examing the response characteristics of the ion-exchanger doped polymeric membrane electrodes to polycation/polyanion and their corresponding monomers, it has been found that the membrane electrodes are at least two orders of magnitude more sensitive to the polymers generated from reversible addition-fragmentation chain transfer polymerization(RAFT) and free radical polymerization(FRP) reactions than to the monomers. Thus, highly sensitive polyion detection can be achieved even in the presence of large concentrations of background monomers. We demonstrate here the use of this platform to detect low concentrations of radical initiators(such as horseradish peroxidase, G-quadruplex, as well as trace concentrations of Fe2+ and Cu2+) and quencher(such as catalase). It is believed that DNA/aptamer based biosensing can also be achieved using this platform with similar sensitivities.