Solving Analytical Challenges with Thin Layer Electrochemistry

Abstract: The decentralization of chemical sensing to attain environmental-related information is today highly desirable to increase the knowledge on biological or geological events as well as effluents. The current state of the field moves toward submersible probes; chemical sensors implemented into submersible devices for quantifying analytes over extended times. However, many sensors are still not robust enough for such applications. Additionally, the detections of most analytes require reagent addition and other steps before analysis (i.e., pre-treatments). For such analyses to be implemented in decentralized measurements, it would be beneficial to find reagentless approaches to modify samples and avoid waste associated with the reagent addition. This thesis aimed to develop such strategies using different solid materials capable of imposing ion-transfer events (actuators) under electrochemical control, to achieve measuring the analytes in the same sample using chemical sensors. Both actuators and sensors were jointly employed in thin layer (or near thin layer) samples, inside newly designed 3D-printed cells. This allowed for small sample volumes (ca 100 µm thicknesses) down to 0.5 µL to be analyzed, and resulted in fast, non-diffusion limited measurements that facilitated the sensor-actuator concepts. First, acidification of thin layer samples using polyaniline (PANI) was investigated. By electrochemical oxidation of PANI, its molecular structure changed resulting in hydrogen ions (acid) being delivered to the thin layer sample within two minutes or less, shifting its pH from ca 8 down to 2–3. By combining PANI and pH-sensors, reliable detection of alkalinity in real and artificial water samples could be achieved for a period of two weeks and possibly more. Also, by combining the PANI-based acidification with planar optodes capable of measuring pH or CO2 with high spatial resolution, buffer capacity or dissolved inorganic carbon (DIC) gradients could be resolved in a 2D domain with sub-mm resolution. PANI-based acidification was tested for sensing several environmental samples, including freshwater plants, brackish water, seawater, and soil, presenting great versatility in analytical performance. Second, a concept of selective deionization of thin layer samples was developed. The importance of such a concept is related to the selectivity of ion-based measurements, where ions such as Li+ or NH4+ are difficult to detect in real samples because of interfering ions increasing their limit of detection (LOD). Non-faradic processes were explored to remove such interferents by using carbon nanotubes (CNTs) for modulating the ion transfer with the sample. To facilitate selective deionization to only remove one ion species, the CNTs were covered with ultra-thin ion-selective membranes (ISMs; ca 200 nm thick). The tandem of CNTs-ISM was found to be capable of selectively removing multiple different cations, proven with the monitoring from both potentiometric sensors and optodes additionally implemented into the thin layer sample. Overall, the CNTs-ISM tandem shows great promise for lowering the LOD of chemical sensors in complex matrixes such as biological or environmental samples, which could aid to decentralized measurements in the future.