Makarychev-Mikhailov Sergey, Shvarev Alexey, Bakker Eric
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
Anal Chem. 2006 Apr 15;78(8):2744-51. doi: 10.1021/ac052211y.
Ion-selective electrodes ideally operate on the basis of the Nernst equation, which predicts less than 60- and 30-mV potential change for a 10-fold activity change of monovalent and divalent ions measured at room temperature, respectively. Typical concentration ranges in extracellular fluids are quite narrow for the electrolytes of key importance. A range of 2.2-2.6 mM for calcium ions, for instance, translates into just a 2.2-mV potential change. The direct potentiometric measurement of physiological electrolytes is certainly possible with direct potentiometry and is done routinely in clinical analyzers and handheld measuring devices. It places, however, strong demands on the precision of the reference electrode and requires careful temperature control and frequent calibration runs. In this paper, a robust 10-20-fold sensitivity enhancement for calcium measurements is attained by departing from the classical response mechanism and operating in a non-Nernstian response mode. Stable and reproducible super-Nernstian responses of these so-called pulstrodes in a narrow calcium activity range can be controlled by instrumental means in good agreement with theory. The potentials may be measured during a galvanostatic excitation pulse (mode I) or immediately after it (mode II), under open-circuit conditions. Subtraction of the potentials, sampled at different times during a single pulse, allows one to obtain a sensitive differential peak-shaped signal at a critical and fully adjustable analyte activity range. Calcium pulstrodes based on the diamide ionophore AU-1 were characterized and applied to the measurement in model physiological liquids. Super-Nernstian responses exceeding 700 mV/decade were observed in a physiological range of calcium concentration. Such remarkable sensitivity of the pulstrodes, complemented with the well-documented high selectivity of these potentiometric sensors, may provide a significant increase in the accuracy and precision of electrolyte measurements in clinical analysis.
离子选择性电极理想情况下是基于能斯特方程工作的，该方程预测在室温下，单价离子和二价离子活性变化10倍时，其电位变化分别小于60 mV和30 mV。对于至关重要的电解质而言，细胞外液中的典型浓度范围相当狭窄。例如，钙离子浓度范围为2.2 - 2.6 mM，这仅转化为2.2 mV的电位变化。生理电解质的直接电位测量当然可以通过直接电位法实现，并且在临床分析仪和手持式测量设备中是常规操作。然而，这对参比电极的精度有很高要求，并且需要精确的温度控制和频繁的校准。在本文中，通过背离经典响应机制并以非能斯特响应模式运行，实现了对钙测量的10 - 20倍灵敏度增强。在狭窄的钙活性范围内，这些所谓的脉冲电极的稳定且可重复的超能斯特响应可以通过仪器手段进行控制，与理论高度吻合。电位可以在恒电流激励脉冲期间（模式I）或之后立即（模式II）在开路条件下进行测量。在单个脉冲期间不同时间采样的电位相减，使得在临界且完全可调节的分析物活性范围内能够获得灵敏的差分峰形信号。基于二酰胺离子载体AU - 1的钙脉冲电极进行了表征，并应用于模型生理液体的测量。在生理钙浓度范围内观察到超过700 mV/十倍的超能斯特响应。脉冲电极如此显著的灵敏度，再加上这些电位传感器已充分证明的高选择性，可能会显著提高临床分析中电解质测量的准确性和精密度。
