The previously reported relay network conductivity model has shed some light on the structure and mechanisms behind long-distance extracellular electron transfer (EET) in Geobacter biofilms. The structuration of c-type cytochromes (c-Cyt) in supramolecular complexes and their interactions with pili, as a requirement for achieving external intermolecular ET, were put forward. Such an arrangement supports a redox gradient-driven process limited by potential loss along the biofilm, which ultimately limits technological developments on bioanodes. Geobacter cells display wide respiratory versatility, including uranium, palladium, silver and gold salt reduction, which often yields nanoparticles (NPs). Here, we took advantage of the ability of G. sulfurreducens to produce monodisperse AuNPs (G.Au) to interpret and improve the EET mechanism. Both metabolic stratification and co-localization of c-Cyt and AuNPs were analyzed by TEM microscopy and Raman spectroscopy to evaluate the relation between these elements and reveal the spatial organization of redox proteins, giving support to the 2-fold increase in the current density production that was measured as a consequence of improving cell connectivity with gold nucleation. The final corroboration of specific interactions between AuNPs and c-Cyt came from the electrophoretic analysis of the nanostructure isolated fractions. We observed that electrons accumulated in the absence of polarization reduced Au(III) throughout the biofilm and can also be drained through a poised microelectrode located at 100 μm from the basal electrode used for biofilm growth, thus probing no predetermined directionality in the EET network, other than that dictated by the potential. While presenting gold nucleation as an alternative to overcome limitations in current production, these results corroborate main concepts of the relay network model, pushing towards more efficient applications for bio-hybrid nanostructured materials in the field of bioelectronics.

中文翻译：

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呼吸金成核和微电极技术揭示了细菌导电基质的关键特征

先前报道的中继网络电导率模型揭示了Geobacter生物膜中远距离细胞外电子转移（EET）背后的结构和机制。提出了超分子复合物中c型细胞色素（c-Cyt）的结构及其与菌毛的相互作用，这是实现外部分子间ET的要求。这种布置支持氧化还原梯度驱动的过程，该过程受沿生物膜的潜在损失的限制，这最终限制了生物阳极的技术发展。地细菌细胞显示出广泛的呼吸功能，包括减少铀，钯，银和金盐，这通常会产生纳米颗粒（NPs）。在这里，我们利用了G.硫还原菌的能力产生单分散的AuNPs（G.Au）来解释和改善EET机制。通过TEM显微镜和拉曼光谱分析了c-Cyt和AuNPs的代谢分层和共定位，以评估这些元素之间的关系并揭示氧化还原蛋白的空间组织，为电流密度增加2倍提供了支持这是由于通过金成核作用改善了细胞连通性的结果。AuNP和c-Cyt之间特定相互作用的最终确证来自对纳米结构分离级分的电泳分析。我们观察到在没有极化的情况下积累的电子还原了Au（III）贯穿整个生物膜，并且也可以通过位于距用于生物膜生长的基底电极100μm处的平衡微电极排放，因此除了可能的电势之外，在EET网络中没有探测到任何预定的方向性。尽管提出了金成核法来克服当前生产中的局限性，但这些结果证实了中继网络模型的主要概念，从而推动了在生物电子领域中生物混合纳米结构材料的更有效应用。