Fast Potentiometric CO₂ Sensor for High-Resolution in Situ Measurements in Fresh Water Systems

Abstract

We present a new potentiometric sensor principle and a calibration protocol for in situ profiling of dissolved CO with high temporal and spatial resolution in fresh water lakes. The sensor system is based on the measurement of EMF between two solid-contact ion selective electrodes (SC-ISEs), a hydrogen ion selective and a carbonate selective sensor. Since it relies on SC-ISEs, it is insensitive to changes in pressure, thus suitable for in situ studies. Also, as it offers a response time ( t) of <10 s, it allows for profiling applications at high spatial resolution. The proposed optimum in situ protocol accounts for the continuous drift and change in offset that remains a challenge during profiling in natural waters. The fast response resolves features that are usually missed by standard methods like the classical Severinghaus CO probe. In addition, the insensitivity of the presented setup to dissolved sulfide allows also for measurements in anoxic zones of eutrophic systems. Highly resolved CO concentration profiles obtained by the novel and robust SC-ISE setup along with the developed optimum in situ protocol allow investigating hotspots of biogeochemical processes, such as mineralization and primary production in the water column and help improving estimates for CO turnover in freshwater systems.

Ion-to-electron capacitance of single-walled carbon nanotube layers before and after ion-selective membrane deposition

“…Single-walled and multi-walled carbon nanotubes were introduced as a transducing material for SC-ISEs about a decade ago [6,28]. It is today commercially available and widely used for potentiometric sensor preparation [21][22][23][24][25]. After deposition on the electrode surface, the carbon nanotubes create a network of well-interconnected fiber-like wires as shown previously by means of environmental [6] and traditional scanning electron microscopy [29].…”

“…Typically, the capacitance of the transducer layer is tested after it is covered with the ion-selective membrane. However, since the transducer materials (i.e., carbon-based [6,8,9,[21][22][23] or metal-based nanomaterials [16][17][18][19]) are often deposited manually from a suspension phase, the process is prone to undesirable variability. It would be preferable to test the transducer layer capacitance before the layer is covered with the ion-selective membrane so that the amount of transducer material may be adjusted as needed.…”

“…In this work, an optimized protocol for the chronopotentiometric estimation of the double-layer capacitance of ion-to-electron single-walled carbon nanotube transducing layers is proposed as a quality control test that can be easily performed at the stage of solid-contact ion-selective electrode (SC-ISE) preparation. Single-walled carbon nanotubes (SWCNTs) functionalized with octadecylamine were chosen for this study, owing to their recurrent use in potentiometric sensors exploited in environmental analysis [21][22][23] and wearable sensing platforms [24,25].…”

A Droplet Microfluidic-Based Sensor for Simultaneous in Situ Monitoring of Nitrate and Nitrite in Natural Waters

“…It also indicates that the sensor can be used on profiling platforms or moving vehicles. 40 Despite the high measurement frequency, the low total flow rate within the sensor meant the fluid consumption of the sensor was markedly lower than previously reported systems. Low fluid consumption is important as this, along with power considerations, determines the length and/or measurement frequency of a sensor deployment.…”

Comparison of Different Commercial Conducting Materials as Ion-to-Electron Transducer Layers in Low-Cost Selective Solid-Contact Electrodes

“…Specifically, nearly 200 papers have been published during the last five years [1]. The advantages of SCISEs over the conventional ion-selective electrodes (ISEs) have been demonstrated in a variety of applications, including medical analysis [2][3][4][5][6], agriculture [7,8], environmental [9][10][11] and water quality monitoring [12][13][14] and process control [15]. Improvements in signal stability, detection limits, reduction of water accumulation at the SC-membrane interphase, transmembrane ion fluxes, reproducibility, and research efforts in the development of new SC materials have been reported [14,[16][17][18][19][20].…”