This investigation, following our recent report on the one-pot hemi-micellar interfacial synthesis of Janus gold nanoclusters and the inter-cluster electron coupling establishing insulator-metal transition in the oriented Janus monolayers [Langmuir, 2010, 26(17), 14047], was to fabricate modified electrodes for sensing dopamine, the neurotransmitter. With a detection limit in the sub-nanomolar range, the apparent electron transfer rate constants for dopamine detection signified an intricate Janus cluster 2D phase dependency. Surface pressure as a thermodynamic variable controlled the electronic communication between the clusters as a result of varied inter-cluster distance and size, ultimately reflecting on the sensitivity and detection limit for dopamine sensing. The non-covalent nature of the ligands on the core metal clusters facilitated the overall electro-catalytic oxidation of dopamine. The notable feature of this precise work was that it established a more effective phase- and orientation-specific Janus cluster sensing than those reported through patterned gold nanowire based sensors.

A one-pot hemimicellar synthesis of oriented, amphiphilic, and fluorescent Janus gold clusters, establishing the Janus character in terms of ligand asymmetry and distribution, has been demonstrated. The method was based on the efficient Langmuir strategy, where the in situ two-dimensional (2D) reduction of Au(3+) in the sprayed micellar electrostatic complex, TOA(+)-AuCl(4)(-), was accomplished by subphase tryptophan that acted as the hydrophilic protecting ligand on one hemisphere of the spherical gold cluster. In contrast to the reported micelle-assisted Janus cluster formation, here the cluster growth occurred inside the surface pressure driven hemimicelles, which rapidly formed 2D cluster arrays without any interfacial reorientation. The Janus structure was validated using angle dependent polarized Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS), where orientation dependent vibrational changes in the adsorbed ligand functionalities were detected. Electrochemical impedance measurements of the transferred Janus layers onto hydrophobized ITO revealed the heterogeneous electron transfer rate constant k(ET) to show a clear orientational odd-even parity effect with the odd layers showing much higher rates. Isobaric area relaxation investigations further evidenced toward a hemispherical instantaneous nucleation with edge growth mechanism of the nanoclusters formed at the tryptophan subphase. Surface pressure as a thermodynamic variable effectively controlled the interparticle separation; intercluster electron coupling exhibited insulator-metal transition in the Janus cluster monolayers through scanning electrochemical microscopy investigations.

On the basis of the formation of a noncovalent inclusion complex through crown ether-ammonium ion interaction, a new ligand, 18-crown-6-aminoethanethiol (18C6-AET), was predesigned and synthesized with a formation constant of 4.26 x 10(2) M(-1)s(-1) for the 1:1 noncovalent complex. Consequently, stable crown ether-protected gold nanoclusters were synthesized with 18C6-AET in an aqueous medium with a rate of formation of 3.688 x 10(-4) min(-1). The newly formed 18C6-AET-capped gold nanoclusters showed quantized double-layer charging, revealing their concentric capacitor structure, and effective cluster capacitance, C(CLU) was calculated to be 0.93 aF from the single-electron events. A larger C(CLU) in comparison to that of monolayer-protected gold clusters with longer chain lengths was attributed to a smaller ligand shell thickness as well as the cationic nature of the nanoclusters formed in an aqueous electrolyte medium.

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