Highly Sensitive Method to Isolate Photocurrent Signals from Large Background Redox Currents on Protein-Modified Electrodes

The high quantum efficiency in converting light energy into a charge-separated state is a major advantage in using photosynthetic proteins in biophotovoltaic applications. Photocurrents are typically measured at open circuit potential (OCP), where the electrochemical redox or faradaic currents are minimized. However, at potentials far from the OCP, the photocurrents produced by the proteins may be impossible to measure against the large background current, owing to electrochemical redox reactions of charge-transfer mediators and/or sacrificial electron donors. Demonstrated here is a highly sensitive method using a sinusoidal-modulated intensity of an LED excitation light source to isolate the protein-based photocurrent component from the total current irrespective of electrode surface coverage. Using a genetically modified photochemical reaction center from Rhodobacter sphaeroides as a proof-of-concept, photocurrents up to 10(4)-10(5) orders of magnitude smaller than the background electrochemical redox current (due to redox reactions directly on the electrode surface) were measured at applied voltages >0.4 V from the OCP. The phase relationship between the optical excitation and photocurrent response was also measured and shown to be analytically useful.

在将光能转化为电荷分离态方面的高量子效率是在生物光伏应用中使用光合蛋白的一个主要优势。光电流通常在开路电位（OCP）下测量，此时电化学氧化还原或法拉第电流最小化。然而，在远离开路电位的电位下，由于电荷转移介质和/或牺牲性电子供体的电化学氧化还原反应，蛋白质产生的光电流可能因大的背景电流而无法测量。这里展示了一种高灵敏度的方法，该方法使用正弦调制强度的LED激发光源，无论电极表面覆盖情况如何，都能从总电流中分离出基于蛋白质的光电流成分。以球形红细菌的一种基因改造的光化学反应中心作为概念验证，在距开路电位>0.4 V的施加电压下，测量到比背景电化学氧化还原电流（由于直接在电极表面的氧化还原反应）小10⁴ - 10⁵个数量级的光电流。还测量了光激发和光电流响应之间的相位关系，并表明其在分析上是有用的。