• Integrated Microfluidics <download slide pdf>
  One important goal of our program is to establish intelligent control of molecular transport in space and time at small length scales.Intelligent control implies materials and structures that can sense molecular characteristics, e.g. size, charge, molecular shape, etc., and then generate control signals that control transport based on those molecular characteristics.  Specifically we seek to manipulate, (separate, isolate, react, detect) low-mass samples with the same precision and level of control currently possible with bench-scale samples by combining microfluidic and nanofluidic structures to achieve integrated microfluidics capable of addressing the challenging problems posed by multi-dimensional separations and analysis with low mass samples. 
  
  Molecular transport in structures of nanometer (1 nm <d < 100 nm) characteristic dimensions is a critical component of a large number of separation technologies and sensor paradigms. Independent of the type of force field used to drive transport, the unique characteristics of nanoscale structures ensure that transport is fundamentally different than in larger mm and mm-scale structures.  The similarity of the Debye length, and the channel diameter, accesses a new electrokinetic flow regime ka ~ 1), that is not available at longer lengthscales, even in mm-scale capillaries. By changing k^-1 it is possible to move from a regime, ka >> 1, where flow is dominated by electrophoresis to a regime, ka ~ 1, where electroosmotic flow is the dominant transport mechanism. This is possible, because in the nanochannels there is a preponderance of counterions over co-ions; in fact it is easy to achieve conditions where every mobile counterion in the pore is of one polarity, i.e. there are no co-ions.

• Multidimensional Chemical Analysis of Mass Limited Samples <download slide pdf>
  Two sample types naturally demand that samples be handled in low masses: (a) availability-limited samples – where the mass is limited by the inherent amount of sample available for characterization, and (b) attribute-limited samples - where the mass is limited by some characteristic, e.g. toxicity or cost, of the analyte. Key to the concept of manipulating small-mass analyte “packets” for multidimensional analysis is the ability to move exceptionally small fluidic volume elements (voxels) to desired physical locations at desired times with high precision and with flexible control elements.Skoog et al. define the unit operations of chemical analysis at the bench scale (weighing, volume measurement, filtration, etc. and an analogous set of operations could be defined for low-mass analyses including, for example: (a) separation, (b) isolation, (c) reaction, and (d) detection.
  

• Nanofluidics <download slide pdf>
  The vast preponderance of studies of fluid transport in nanocapillary array membranes (NCAMs) rely on composite measurements over an ensemble of pores.  We take up the quantitative characterization of nanopore flow using single nanopores as a uniquely powerful experimental model system. NCAMs, containing 10⁸-10⁹ pores cm^-2, are naturally characterized by dispersion in the pore characteristics, e.g. different pores have slightly different angles relative to the surface plane, and therefore slightly different lengths; the pores do not have uniform diameters along their length, i.e. they are not strictly cylindrical; and over sufficiently large areas, defects, such as branched pores, always exist. To circumvent these difficulties we have developed a 4p microscope capable of individually detecting single molecules at the entrance and the exit of a membrane containing a single cylindrical nanopore.  The dual confocal single-molecule-counting 4p microscope is comprised of two identical, axially opposed single-molecule sensitivity laser-induced fluorescence microscopes.  This arrangement allows photon bursts from the entrance and exit channels to be cross-correlated in time to tease out velocities of individual probe molecules.

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

Kemery, P.J.; Steehler, J.K.; Bohn, P.W. “Electric Field Mediated Transport in Nanometer Diameter Channels,” Langmuir 1998, 14, 2884-2889.

Kuo, T.C.; Sloan, L.A.; Sweedler, J.V.; Bohn, P.W. “Manipulating Molecular Transport Through Nanoporous Membranes by Control of Electrokinetic Flow:  Effect of Surface Charge Density and Debye Length,” Langmuir, 2001, 17, 6298-6303.

Kuo, T.C.; Cannon, D.M. Jr.; Feng, W.; Shannon, M.A.; Sweedler, J.V; Bohn, P.W. “Gateable Nanofluidic Interconnects in Multilevel Microanalytical Systems,” Analyt. Chem. 2003, 75, 1861-1867.

Kuo, T.C.; Cannon, D.M. Jr.; Feng, W.; Shannon, M.A.; Sweedler, J.V; Bohn, P.W. “Hybrid Three-Dimensional Nanofluidic/Microfluidic Devices Using Molecular Gates,” Sens. Actuat. A 2003, 102/3, 223-233.

Cannon, D.M. Jr.; Kuo, T.C.; Sweedler, J.V.; Shannon, M.A.; Bohn, P.W. “Nanocapillary Array Interconnects for Gated Analyte Injections and Electrophoretic Separations in Multilayer Microfluidic Architectures,” Analyt. Chem.2003, 75, 2224-2230.

Angew. Chem. Intl. Ed. Engl. 2004, 43, 1862-1865.

Tulock, J.J.; Shannon, M.A.; Bohn, P.W.; Sweedler, J.V. “Microfluidic Separation and Gateable Fraction Collection for Mass-Limited Samples,” Analyt. Chem. 2004, 76, 6419-6425.

Swearingen, C.B.; Wernette, D.P.; Cropek, D.M.; Lu, Y.; Sweedler, J.V.; Bohn, P.W. “Immobilization of a Catalytic DNA Molecular Beacon on Au for Pb(II) Detection,” Analyt. Chem. 2005, 77, 442-448.

Chang, I.H.; Tulock,J.J.; Liu, J.; Kim, W.S.; Cannon, D.M. Jr.; Lu, Y.; Bohn, P.W.; Sweedler, J.V.; Cropek, D.M. “Miniaturized Lead Sensor Based on a Lead Specific Catalytic DNAzyme in a Nanocapillary Interconnected Microfluidic Device,” Environm. Sci. Technol., 2005, 39, 3756-3761.

Chatterjee, A.N.; Cannon, D.M Jr.; Gatimu, E.N; Sweedler, J.V.; Aluru, N.R.; Bohn, P.W. “Modeling and Simulation of Ionic Currents in Three-Dimensional Microfluidic Devices with Nanofluidic Interconnects,” J. Nanopart. Res. 2005, 7, 507-16.

Fa, K.; Tulock, J.J.; Sweedler, J.V.; Bohn, P.W. “Profiling pH Gradients across Nanocapillary Array Membranes Connecting Microfluidic Channels,” J. Am. Chem. Soc.2005, 127, 13928-13933.

Kirk, J.S.; Sweedler, J.V.; Bohn, P.W. “Nanofluidic Injection and Heterogeneous Kinetics of Organomercaptan Adsorption to Colloidal Gold in a Microfluidic Stream,” Analyt. Chem., 2006, 78, 2335-2341.

Gatimu, E.N.; King, T.L.; Sweedler, J.V.; Bohn, P.W. “Three-Dimensional Integrated Microfluidic Architectures Enabled through Electrically Switchable Nanocapillary Array Membranes,” Biomicrofluidics 2007, 1, 021502(1-11).