Studies On All-solid-state Potentiometric Sensors Based On Nanoporous Carbons

Potentiometric sensors are one of the most importance research areas of electrochemical sensors. Due to their cost-effectiveness, rapid response, and simple operation, they have been widely employed in various fields, such as environmental monitoring, clinical analysis, and process control. Compared to conventional ion-selective electrodes（ISEs）, all-solid-state ISEs are considered to be the new generation of ISEs. Note that all-solid-state ISEs are generally regarded as the future of potentiometric sensors, owing to their excellent properties, including easy miniaturization, convenient storage and maintenance, and durability. However, all-solid-state ISEs are influenced by the large charge-transfer resistance, the low double layer capacitance and the existence of the water layer at the interface between the ion-selective membrane and the electronic conductor, and the potential stabilities of all-solid-state ISEs are relatively poor. In addition, some deficiencies restrict the real applications of potentiometric sensors. For example, the practicability of the all-solid-state ISEs and the sensitivity of the potentiometric aptasensors should be improved. In recent years, nanomaterials have been used as the solid contact between the ion-selective membrane and the electronic conductor in all-solid-state ISEs in order to improve the potential stability due to their large surface area, excellent conductivity and good hydrophobicity. However, challenges still remain in the all-solid-state ISEs with nanomaterials as solid contact. The preparation of nanomaterials-based solid contact is complicated, time-consuming, and may cause aggregation,thus influencing the specific surface area, double layer capacitance, and conductivity at the electrode/membrane interface. In order to improve the stability, practicability and sensitivity of the all-solid-state potentiometric sensors, this dissertation develops several nanoporous carbons and uses them to design all-solid-state potentiometric sensors. The detailed contents are as follows:1. All-solid-state polymeric membrane Pb²⁺-ISE with bimodal pore C60 as solid contact. Compared with pristine C60, bimodal pore C60 synthesized by the liquid–liquid interfacial precipitation method has larger double layer capacitance, faster charge transfers and higher hydrophobicity. Based on its unique properties, we have developed an all-solid-state polymeric membrane Pb²⁺-ISE based on the electrodeposited bimodal pore C60 as solid contact. The proposed Pb²⁺-ISE exhibits a stable Nernstian response with a rapid response time of 10-15 s. The linear range is from 1.0×10^-9 to 1.0×10^-3 M with a detection limit of 5.0×10^-10 M. Moreover, the all-solid-state polymeric membrane Pb²⁺-ISE also displays excellent potential stability, good resistance to O₂, CO₂ and light, and no water layer is existed between the ion-selective membrane and the underlying bimodal pore C60 solid contact. We further investigate the feasibility of the proposed Pb²⁺-ISE. The results show that the bimodal pore C60-based Pb²⁺-ISEs could be used as an effective detection tool for detecting heavy metals in environmental samples.2. An all-solid-state polymeric membrane Cd²⁺-ISE with three-dimensional porous graphene-mesoporous platinum nanoparticle（3D PGR-MPN） composite as solid contact. Three-dimensional porous graphene（3D PGR） with interconnected networks have recently been developed, which not only possess the inherent properties of the twodimensional graphene, but also provide much more “space” for the el ectron/ion, gas and liquid transportation or storage. However, the obtained pore dimensions in the framework of 3D PGR are up to several micrometers, which may restrict the surface area of the 3D PGR. In order to solve this problem, we have synthesized the 3D PGR-MPN composite with adjustable pore dimensions by applying MPNs as cross-linking sites through a one-step hydrothermal co-assembly method. The 3D PGR-MPN composite is used as solid contact for developing an all-solid-state polymeric membrane Cd²⁺-ISE. The proposed Cd²⁺-ISE exhibits a Nernstian response in the range from 1.0×10^-8 to 1.0×10^-4 M. The detection limit calculated as the intersection of the two slope lines is 1.5×10^-9 M. Additionally, the developed electrode also exhibits good long-term stability and is robust to O₂, CO₂ and light interferences. Due to the hydrophobic characteristics of PGR-MPN composite, the presence of undesirable water layer between the polymeric membrane and the underlying PGR-MPN layer is not found.3. An all-solid-state polymeric membrane Cu²⁺-ISE based on the PGR-Pt composite as solid contact. The hydrothermal reduction method used for the fabrication of 3D PGR and its composite always requires high temperatures. Additionally, the 3D PGR-based solid contact on the electrode surface is usually prepared by the drop-casting method, which is tedious and time-consuming. Therefore, it is necessary to develop a new fabrication technique to obtain a solid contact on the electrode. Electrochemical deposition is a rapid, facile, low cost and easily controllable approach to prepare a stable film on the surface of electrode without any further treatment. In this work, the 3D PGR-Pt composites can be prepared directly on the surface of gold electrodes by one-step electrochemical deposition co-reduction. The electrochemical properties of the 3D PGR-Pt composite have been characterized by electrochemical impedance cyclic voltammetry and spectroscopy. The results indicate that the 3D PGR-Pt composite has large double layer capacitance and fast charge-transfer. We have developed an all-solid-state polymeric membrane Cu²⁺-ISE using 3D PGR-Pt composite as the solid contact. The proposed Cu²⁺-ISE shows a stable Nernstian response in the range from 1.0×10^-8 to1.0×10^-4 M. The detection limit calculated as the intersection of the two slope lines is 1.7×10^-8 M. The results of chronopotentiometry indicate that the Cu²⁺-ISE has good potential stability.4. An all-solid-state polymeric membrane screen-printed Cu²⁺-ISE based on the 3D PGR as solid contact. The commonly used all-solid-state ISEs require external reference electrodes, which may cause inconvenience for in-situ measurements. Screen-printed electrodes（SPEs） possess several advantages, such as flexible design, simple construction, good consistence, low cost, and ease to mass production. Additionally, SPEs can be screen-printed to the multiple-electrode systems for integration. The multiple-electrode systems resolve the operation difficulties of the single electrode system for the online monitoring processes, which can enhance the potential stability and reproducibility and further expand the application of the SPEs in the field of environmental analysis. We have developed a novel screen-printed Cu²⁺-ISE with a double-electrode system using 3D PGR as the solid contact prepared by constant potential reduction of the GO onto the surface of the SPE. The obtained Cu²⁺-ISE shows a stable Nernstian response in the range of 1.0×10^-6-1.0×10^-3 M. The detection limit calculated as the intersection of the two slope lines is 3.9×10^-7 M. Due to the hydrophobic characteristics of PGR, no water layer between the sensing membrane and the underlying PGR solid contact is formed. Moreover, the screen-printed system is also composed of the solid-state reference electrode, which can satisfy the demand for in-situ analysis.5. An all-solid-state polymeric membrane Cu²⁺-ISE with graphene foam as a freestanding electrode. Generally, the fabrication of an all-solid-state polymeric membrane ISE requires two steps:（1） solid contact layer formed on the substrate electrode surface is prepared by drop-casting or electrodeposition method, and（2） an ion-selective membrane is drop-casted on the top of the solid contact layer. Such steps are complicated. Graphene foam（GF） is composed of thin layers of stacked graphene sheets and possesses interconnected framework, which is essential for excellent electron transfer in the GF. The porous structures in the framework of the GF offer a large surface area, which has the potential to make the GF used as a freestanding electrode. We have developed an all-solid-state polymeric membrane Cu²⁺-ISE with graphene foam as a freestanding electrode. The GF is used as both electrode material and solid contact in the Cu²⁺-ISE, which simplifies the fabrication procedure compared with the conventional all-solid-state ISEs. The proposed Cu²⁺-ISE exhibits a Nernstian response in the range of 1.0×10^-7-1.0×10^-3 M. The detection limit calculated as the intersection of the two slope lines is 2.5×10^-9 M. The developed electrode is also robust to light, O₂, and CO₂ interferences. Additionally, due to the hydrophobic characteristics of PGR, the undesirable water layer between the sensing membrane and the underlying solid contact is not found.6. Porous graphene-based potentiometric aptasensors for determination of acetamiprid. Unlike the ISEs that respond to the target ions following the Nernst equation, the potentiometric aptasensors based on a different response model have also attracted much attention. In the potentiometric aptasensors, the receptors are directly linked to the nanomaterial instead of being embedded into the polymeric membrane. Aptamers can be covalently or non-covalently attached to the carbon-based nanomaterials due to their large specific surface area, strong affinity, and good biocompatibility. In this work, we have developed a potentiometric aptasensor for detection of a nicotinamide insecticide acetamiprid. The porous graphene and aptamer are used as transducer layer and sensing layer of the aptasensor, respectively. The specific recognition event between the aptamer and the target analyte induces a conformational change in the aptamer, which combines the phosphodiester negative charges of the aptamer to the porous graphene, thus resulting in a potential change. Results show that the proposed aptasensor exhibits a good linear relationship to acetamiprid in the concentration range of 5×10^-10 M-1.0×10^-6 M and the detection limit is 3×10^-10 M. The developed aptasensor has a good sensitivity to acetamiprid, providing a possibility for the determination of acetamiprid in real samples. This aptasening strategy can also be used to detect other organic pesticides using the different aptamers.