Inspired by the fact that thionin (TN) is an effective mediator of extracellular electron-transfer reactions, we assessed the potential of TN as a solder paste for carbon nanotube (CNT) circuits. When TN (10 mM in water) was delivered by conductive-mode atomic force microscopic (AFM) tips to conductive indium-doped tin oxide (ITO) glass slides via dip-pen nanolithography (DPN), the ink formed cylinder-shaped nanodots on the sites after a bias voltage (Vtip) ranging from −3 to −8 V at a relative humidity (RH) level > 50% was applied to the tips. If the tip bias polarity was reversed or the RH lowered to < 50% or |Vtip| to < 3 V, no dots appeared. An AFM tip field-induced local oxidation (ALO) and a water meniscus located between the tip and the deposited ink droplets featured prominently in the formation of these nanostructures. The ink could also be hardened on CNTs under similar conditions. As applied to CNTs that had been positioned on ITO electrodes, the solder could bind the tubes to the electrodes and substantially lower the CNT|ITO junction resistance. The TN-based DPN was shown to be a potential room-temperature nanospot welding technique for the assembly of CNT circuits.
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