Dynamic Sensor Concept Combining Electrochemical pH Manipulation and Optical Sensing of Buffer Capacity

Abstract

State-of-the-art electrochemical and optical sensors present distinct advantages and disadvantages when used individually. Combining both methodologies offers interesting synergies and makes it possible to exploit strengths and circumvent possible problems of the individual methods. We report a dynamic sensing concept for buffer capacity by applying water electrolysis to modulate the pH microenvironment in front of an optical pH sensor placed in a flow cell. Using this combinatory approach in a nonequilibrium readout mode allowed us to assess the concentration of different buffer species in relatively short time (1 min per measurement). Theoretical simulations of the system were performed to validate the presented method. Additionally, the dynamic measurement approach enabled in situ determination of the apparent pK a of MOPS (3-(N-morpholino)­propanesulfonic acid) buffer at ambient conditions. The dynamic and combinatory approach presented here holds large potential also for other pH-active analytes.

Reagentless Acid–Base Titration for Alkalinity Detection in Seawater

“…Such a well-controlled sample acidification has been claimed to be very beneficial for a wide range of applications involving either the exhaustive tuning of the pH in the sample or reagentless acidification procedure to fix a desired pH value in the sample. 2,24,28,45,46 Protocol for Reagentless Titration Utilizing Discrete Charge Injections. We next explored the pH that was reached in the sample once several proton packages of a certain duration were increasingly released to the sample, i.e., in a discrete mode.…”

“…2.5. Such a well-controlled sample acidification has been claimed to be very beneficial for a wide range of applications involving either the exhaustive tuning of the pH in the sample or reagentless acidification procedure to fix a desired pH value in the sample. ,,,, …”

“…26,27 More recently, Koren and co-workers substituted the pH detection in such a concept from an electrochemical to an optical sensor and demonstrated the determination of the buffer capacity of several samples. 28 Despite this being an elegant approach, yet, in situ measurements have not been reached and the incorporation of a second different readout in the system seems to complicate the decentralization process needed to realize such a challenging scenario.…”

“…Reagentless acid–base titration was also proposed by van der Schoot et al, who implemented the sensor–actuator via water electrochemical reduction in an electrode surface for the generation of protons and thus water acidification. , More recently, Koren and co-workers substituted the pH detection in such a concept from an electrochemical to an optical sensor and demonstrated the determination of the buffer capacity of several samples . Despite this being an elegant approach, yet, in situ measurements have not been reached and the incorporation of a second different readout in the system seems to complicate the decentralization process needed to realize such a challenging scenario.…”

Total Dissolved Inorganic Carbon Sensor Based on Amperometric CO2 Microsensor and Local Acidification

“…Alternatively, several microfluidic approaches using a combination of pH modification and a sensing platform have been investigated. The pH shift can be achieved by adding acid/base or bulk buffers or by electrochemical methods like water electrolysis and membrane-based proton pumps. , Such local pH modifications were successfully used to determine DIC by a combination with CO 3 2– ion selective electrodes or conductivity measurement . However, most microfluidic devices are inherently prone to clogging and require external pumping systems to introduce sample into the channel.…”