Some of you may have noticed that it seems increasingly difficult to publish on traditional electroanalytical methods in ACS Sensors. Indeed, techniques that require discrete sampling followed by manual manipulation steps, the adjustment of pH, and the addition of background electrolyte do not describe a sensor, and we would like to encourage our authors to try harder. In environmental sensing, for example, it should be the goal to perform the measurement in the unmodified sample and in situ. In this regard, a recent paper by Steininger, Revsbech, and Koren from Aarhus University, Denmark, published in the July issue of ACS Sensors (DOI: 10.1021/acssensors.1c01140) has impressed me. It describes an electroanalytical sensor for directly measuring dissolved inorganic carbon (DIC) in unmodified aquatic samples. What can be so difficult about that, you ask me? Let me explain. Obviously, measuring dissolved inorganic carbon (the sum of dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate) is of enormous importance as it allows one to understand the rate at which CO₂ from anthropogenic and other sources are taken up by marine systems. Traditionally, this is a laboratory-based technique involving conversion to CO₂ that is then titrated. One may also measure pH and one equilibrium parameter such as CO₂ fugacity or carbonate activity, for example, with potentiometric sensors, in order to deduce the other species by calculation. But it would be desirable to develop a direct probe for DIC. Unfortunately, there are numerous challenges to overcome. The interconversion between bicarbonate and carbon dioxide at natural pH is very slow, taking on the order of minutes, which means that the reduction of one species such as CO₂ will not result in a simultaneous conversion of the coupled carbonate species to give the desired DIC. Even if that problem could be solved, diffusion coefficients for the inorganic carbon species are very different, making such a system difficult to calibrate. Interference from other reducing species, especially oxygen, and the risk of electrode fouling are major additional challenges. The traditional approach would be to develop an electroanalytical technique so that the sample can be diluted, purged from oxygen, and acidified to convert all DIC species to CO₂. But this would not be a sensor. The Aarhus team found a way to solve these problems in an integrated sensing device. An outer, strongly acidified compartment of the microsensor body is placed in contact with the sample through a gelified tip opening. This allows for the rapid and complete local conversion of all DIC species into carbon dioxide. This species is then allowed to diffuse through a silicone gas-permeable membrane into an inner compartment. It is made of an ionic liquid containing a chemical oxygen scavenger and a silver electrode that reduces the carbon dioxide to give the amperometric signal. An additional recessed guard electrode held at the same potential makes sure that no other reducing species are present in the detection compartment. This design alleviates the major challenges mentioned above. The acid rapidly converts the carbon species at the sensor tip, yielding just one diffusing species whose mass transport rate defines the measured current. The mass transport limited process occurs in the interior of the electrode body where the electrolyte composition is constant, eliminating matrix effects. The tip area is gelified to sterically exclude colloids. The silicone membrane protects the oxygen scavenger from the strong acid and avoids other chemical interferences. The conical shape of the electrode body and the narrow opening make sure that the loss of acid is negligible. At the same time, the CO₂ in the conical electrode body is being diluted, which extends the upper measuring range to the desired elevated concentration. Consequently, calibration curves for carbonate and carbon dioxide look identical and are independent of the salt content of the sample. I’d like to congratulate the authors for this elegant concept that may serve as an inspiration to others who aim to transform an electroanalytical principle into a truly useful sensing strategy. This article has not yet been cited by other publications.

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让我们的目标是开发传感器，而不是电分析技术：溶解无机碳的直接检测

你们中的一些人可能已经注意到，在ACS Sensors 中发表关于传统电分析方法的文章似乎越来越困难。事实上，需要离散采样、手动操作步骤、pH 值调整和背景电解质添加的技术并没有描述传感器，我们希望鼓励我们的作者更加努力。例如，在环境传感中，目标应该是在未修改的样品中和原位进行测量。对此，丹麦奥胡斯大学的 Steininger、Revsbech 和 Koren 最近发表在ACS Sensors 7 月刊上的论文（DOI：10.1021/acssensors.1c01140）给我留下了深刻的印象。它描述了一种用于直接测量未改性水生样品中溶解无机碳 (DIC) 的电分析传感器。你问我，这有什么难的？让我解释。显然，测量溶解的无机碳（溶解的二氧化碳、碳酸、碳酸氢盐和碳酸盐的总和）非常重要，因为它可以让人们了解海洋系统吸收人为和其他来源的CO ₂的速度。传统上，这是一种基于实验室的技术，涉及转化为 CO ₂然后进行滴定。还可以测量 pH 值和一种平衡参数，例如 CO ₂逸度或碳酸盐活性，例如，使用电位传感器，以便通过计算推断其他物种。但是开发一种直接探测 DIC 的方法是可取的。不幸的是，有许多挑战需要克服。在自然 pH 值下碳酸氢盐和二氧化碳之间的相互转化非常缓慢，大约需要几分钟，这意味着 CO ₂等一种物质的还原不会导致偶联碳酸盐物质同时转化为所需的 DIC。即使可以解决这个问题，无机碳物种的扩散系数也非常不同，这使得这种系统难以校准。来自其他还原物质（尤其是氧气）的干扰以及电极污染的风险是主要的额外挑战。传统的方法是开发一种电分析技术，以便可以稀释样品、清除氧气并酸化以将所有 DIC 物质转化为 CO ₂. 但这不会是传感器。奥胡斯团队找到了一种在集成传感设备中解决这些问题的方法。微传感器主体的外部强酸化隔室通过凝胶化的尖端开口与样品接触。这允许将所有 DIC 物种快速和完全地本地转化为二氧化碳。然后允许该物质通过硅酮透气膜扩散到内部隔室中。它由含有化学除氧剂和银电极的离子液体制成，可减少二氧化碳以提供电流信号。保持在相同电位的附加凹进保护电极可确保检测室中不存在其他还原物质。这种设计减轻了上述主要挑战。酸在传感器尖端快速转化碳物质，只产生一种扩散物质，其质量传输速率定义了测量的电流。传质受限过程发生在电极体内部，其中电解质成分恒定，消除了基体效应。尖端区域被凝胶化以在空间上排除胶体。硅胶膜可保护除氧剂免受强酸的影响并避免其他化学干扰。电极体的锥形形状和狭窄的开口确保酸的损失可以忽略不计。同时，CO 传质受限过程发生在电极体内部，其中电解质成分恒定，消除了基体效应。尖端区域被凝胶化以在空间上排除胶体。硅胶膜可保护除氧剂免受强酸的影响并避免其他化学干扰。电极体的锥形形状和狭窄的开口确保酸的损失可以忽略不计。同时，CO 传质受限过程发生在电极体内部，其中电解质成分恒定，消除了基体效应。尖端区域被凝胶化以在空间上排除胶体。硅胶膜可保护除氧剂免受强酸的影响并避免其他化学干扰。电极体的锥形形状和狭窄的开口确保酸的损失可以忽略不计。同时，CO锥形电极体中的₂被稀释，这将上限测量范围扩展到所需的升高浓度。因此，碳酸盐和二氧化碳的校准曲线看起来相同，并且与样品的盐含量无关。我要祝贺作者的这个优雅的概念，它可以为其他旨在将电分析原理转变为真正有用的传感策略的人提供灵感。这篇文章还没有被其他出版物引用。