• Atom Scale Junctions <download slide pdf>
  We have long been interested in the universal phenomenon of adsorbate-induced surface wave-packet scattering as monitored through the in-plane conductance for chemical sensor applications.  Recently we have pushed this experiment to its extreme by forming wires that, at their narrowest point, are only a single metal atom wide.  These atom-scale junctions are formed between two Au thin film electrodes by a combination of lithography, microfluidics, and electrochemistry, specifically to be used as the active nanosensing elements in a conductance-based sensor. The improvements in fabrication strategy have advanced the state-of-the-art in nanowire fabrication to the point where it is essentially possible to “dial-in” a specific number of conductance quanta for the wire to be fabricated. Prototype sensing measurements made by measuring ac impedance while hexadecanethiol (HDT) was chemisorbed onto the atom-scale junction yield a signal with an equivalent molecular fluctuation of 5 molecules.  Although the development of this particular device is at an early stage its relevance to nanosensor technology is clear, because it is extremely sensitive, and because,  once used, the sensing structure can be regenerated for multiple sensor readout cycles.

• Sub-Wavelength Aperature Arrays <download slide pdf>
  A critical problem in a variety of sensor strategies is the need for specific labeling the of the compound of interest.  Strategies which can detect the analyte species without labeling, i.e. label-free detection, are of significant interest. In this regard we are targeting the development of advanced plasmonic nanoelectrode assemblies (PNAs) for simultaneous imaging of complex fluidic environments with high spatial and temporal resolution and mediation of information-rich bioelectrochemical transformations.  When metallized appropriately, sub-wavelength aperture arrays form 2D plasmonic crystals that can be used to monitor the full three-dimensional dielectric response function, e(k,w) of complex analyte-bearing fluids. Monitoring refractive index distributions in the analyte stream at high sensitivity and spatial resolution coupled to bioelectrochemical recognition constitutes a powerful, and heretofore unexploited means of multi-modal characterization.  A key element of this approach is the use of advanced methods of chemical patterning to add functionality to the PNAs such that they can achieve three discrete, but coupled, functions simultaneously: (1) a fluidic structure capable of switched fluid transport with sub-attoliter volume precision; (2) a multi-modal sensor capable of coupled refractive index imaging and high selectivity; and (3) when derivatized with wall/surface-immobilized biomolecular arrays a catalytic reactor of ultrahigh efficiency.

For more information about these efforts, please see the graphics gallery that follows and these original papers:

Drake, P.A.; Bohn, P.W. “Studies of Anomalous Diffusion. Surface Plasmon Resonance Measurements as Probes of Nanometer-Scale Film-Swelling Dynamics for CH₃OH in Poly(methylmethacrylate),” Analyt. Chem. 1995, 67, 1766-1771.

Zhang, Y.; Terrill, R.H.; Bohn, P.W. “In-Plane Resistivity of Ultrathin Gold Films: A High Sensitivity, Molecularly Differentiated Probe of Mercaptan Chemisorption at the Liquid-Metal Interface,” J. Amer. Chem. Soc. 1998, 120, 9969-9970.

Zhang, Y.; Terrill, R.H.; Bohn, P.W. “Chemisorption and Chemical Reaction Effects on the Resistivity of Ultra thin Gold Films at the Liquid-Solid Interface,” Analyt. Chem. 1999, 74, 119-125.

Fried, G.A.; Zhang, Y.; Bohn, P.W. “Effect of molecular adsorption at the liquid-metal interface on electronic conductivity: The role of surface morphology.,” Thin Solid Films 2001, 401, 171-178.

Swint, A.L.; Bohn, P.W. “Effect of Acidic and Basic Surface Dipoles on the Depletion Layer of Indium Tin Oxide as Measured by In-Plane Conductance,” Appl. Phys. Lett. 2004, 84, 61-63.

Kirk, J.S.; Bohn, P.W. “Surface Adsorption and Transfer of Organomercaptans to Colloidal Gold and Direct Identification by Matrix Assisted Laser Desorption/Ionization Mass Spectrometry, J. Am. Chem. Soc. 2004, 126, 5920-5926.

Swint, A.L.; Bohn, P.W. “Effect of the Interfacial Chemical Environment on In-Plane Electronic Conduction of Indium Tin Oxide: Role of Surface Charge, Dipole Magnitude and Carrier Injection,” Langmuir, 2004, 20, 4076-4084.

Castle, P.J.; Bohn, P.W. “Interfacial Scattering at Electrochemically Fabricated Atom-Scale Junctions between Thin Gold Film Electrodes in a Microfluidic Channel,” Analyt. Chem. 2005, 77, 243-249.