(257f) Electrophoretic and Dielectrophoretic Assembly of Nanocolloids in Conic Nanopipettes

We report and analyze a new electrokinetic technique for assembling fewer than one hundred 20-nm metal and dielectric nanoparticles at the tip of a nanopipette that is fabricated by laser-assisted drawing techniques. The nanocolloid assembly mechanism is driven by two fundamental phenomena at nanoscales: the high ionic strength at the tip due to sub-Debye length Boltzmann equilibrium and singular field focusing towards the tip. The enhanced Debye screening due to the former and the enhanced (reactive) induced particle dipole due to the latter conspire to produce a submicron slug of nanoparticle crystal at the tip under certain conditions. Using an extended DLVO theory, we are able to collapse the measured  phase boundaries for such confined-domain nanoparticle assembly at different electric field intensity/frequency, particle size/property and bulk ionic strength.  Interestingly, by monitoring the slow ion current dynamics, as well as short current jump events due to single nanoparticle or single aggregate translocation, we are able to decipher the packing dynamics and crystal geometry using a statistical scaling theory.  Such sub-micron nanoparticle crystals are used for generating nm-sized plasmonic hot spots that allow single-molecule detection and even single-molecule spectroscopy. We report a single-stranded microRNA detection experiment with functionalized hairpin oligos and FRET (Forster Resonance Energy Transfer) fluorescent reporters on gold nanoparticles. Sub-femtomolar limit of detection is achieved with fewer than 100 target molecules. We also show single-mismatch selectivity because of the high shear rate during the nanoparticle assembly process.