Departamento de Química Física y Analítica, Universidad de Oviedo, Asturias, Spain.
Analyst. 2020 May 7;145(9):3431-3439. doi: 10.1039/d0an00477d. Epub 2020 Apr 7.
There is growing interest in the development of simple, fast, sustainable and low-cost analytical methodologies on paper-based platforms. However, sensitive detection strategies that fit properly with these devices are still required. In this work, a calibration-free method is proposed for analytical determinations performed on paper-based electrochemical devices, in this case, for ascorbic acid. Carbon ink is deposited on a hydrophilic working area of the paper delimited with a hydrophobic wax. This maskless procedure is fast and cuts down ink waste. The connection of this working electrode to the potentiostat is provided by reusable gold-plated connector headers that provide also the reference and counter electrodes. The thickness of the paper substrate defines the electrochemical cell and confines a sample volume, ideal for thin-layer coulometry. Controlled-potential coulometry is performed applying a potential of +0.6 V for 50 s. The charge is calculated by measuring the area under the fast chronoamperogram and the concentration is determined following Faraday's law (known number of transferred electrons). This methodology was applied to the determination of ascorbic acid, with a limit of detection of 40 μM. Its concentration in commercial fruit juices can be directly determined in diluted samples. The absence of matrix effects is observed by comparing the results obtained before and after enzymatic reaction of the sample with cucumber ascorbate oxidase. Good accuracy and precision makes this method suitable for quality control of ascorbic acid in commercial juices. Underexploited coulometric readout can be applied as a fast (calibration-free) and low-cost (standards not required) transduction principle for the newly developed paper devices.
人们对开发基于纸张的简单、快速、可持续和低成本分析方法越来越感兴趣。然而，仍然需要与这些设备相匹配的敏感检测策略。在这项工作中，提出了一种用于基于纸张的电化学器件上进行分析测定的无校准方法，在这种情况下，用于测定抗坏血酸。碳墨水沉积在亲水工作区域上，该区域用疏水蜡进行限定。这种无掩模的过程快速且减少了墨水浪费。将工作电极连接到电化学工作站通过可重复使用的镀金连接器头来实现，这些连接器头还提供参考电极和对电极。纸张基底的厚度定义了电化学池并限制了样品体积，非常适合薄层库仑法。通过施加+0.6 V 的电位进行恒电位库仑法 50 s。通过测量快速计时安培图的面积来计算电荷，并根据法拉第定律（已知转移的电子数）来确定浓度。该方法应用于抗坏血酸的测定，检测限为 40 μM。可直接测定稀释样品中商业果汁中的抗坏血酸浓度。通过比较样品与黄瓜抗坏血酸氧化酶进行酶反应前后的结果，观察到不存在基质效应。良好的准确性和精密度使得该方法适用于商业果汁中抗坏血酸的质量控制。未充分利用的库仑读出可以作为新开发的纸张设备的快速（无校准）和低成本（不需要标准）的转换原理。
