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Research

IRG2, Molecular and Nanoscale Motors

The theme of IRG 2 is to understand and control of micro- and nanoscale movement. By investigating three classes of motors - catalytic, biological and synthetic - we are endeavoring to bring the study of dynamic nanoscale processes into the realm of engineering science and physics.

Recent highlights:

Catalytic Nanomotors. The broad goals of our work on catalytic nanomotors are: (a) exploring mechanisms for driving catalytically-induced movement of nano/micro-objects and fluids, (b) studying the effect of scaling and geometry, and surface modification (charge, hyrophobicity/hydrophilicity) on movement, (c) discovering alternative catalytic redox reactions that convert chemical energy to motion, and (d) the theoretical modeling of movement. Previously, we showed that the spontaneous decomposition of hydrogen peroxide by Pt/Au nanorods resulted in linear motion at speeds ~10 body-lengths per second (10-20 (micron/s) (J. Am. Chem. Soc. 2004, 126, 13424-13431). In principle, these motors should pump fluid when immobilized on a surface.

Within the past year, we microfabricated silver and gold catalytic surfaces designed to pump fluids. The fluid pumping was confirmed by observing the convective-fluid flow behavior and pattern formation of tracer particles suspended in dilute H₂O₂ solutions (J. Am. Chem. Soc 2005, 127, 17150-1). Remarkably, the pumping/patterning effect occurred only when the silver catalyst and the gold were in electrical contact. Furthermore, tracer particles with differing zeta potentials exhibited different behavior in response to the catalytically generated force. These observations are strongly suggestive of an electrokinetic mechanism of motion described previously by our group (Chem.-Eur. J. 2005, 11, 6462-6470).

This electrokinetic mechanism was further investigated by measuring the short-circuit current between platinum and gold interdigitated microelectrodes in hydrogen peroxide solutions and observing the motion of tracer particles in open and short-circuit conditions. Non-Brownian motion of tracers was found only when the platinum and gold microelectrodes were electrically connected, consistent with an electrokinetic mechanism. We are continuing to study this effect and explore its role in the autonomous movement of platinum/gold nanorods.

Recently, we have observed that hydrazine can also be used as "fuel" for catalytic locomotion thereby extending the repertory of reactions. In the hydrazine system, palladium patterned on gold moves colloids in a manner qualitatively consistent with our electrokinetic predictions.

Hybrid Biological Motors. The overall goal of our biomotor research is to develop new tools to manipulate and control the function of kinesin molecular motors and microtubules for in vitro transport applications and investigations into cytoskeletal organization in cells. When immobilized on surfaces, kinesin motors transport microtubules long distances at ~1micron/s, providing a robust microscale transport system that is a potential alternative to microfluidic pumping. In earlier work, we and others established that microchannels patterned in glass or photoresist can guide the direction of moving microtubules, leaving a need for new fabrication approaches for enclosed microfluidic channels that are biocompatible, transparent, and provide facile sample introduction. We have developed a novel approach for etching channels 5-10 micron wide and ~1 micron deep in glass bonded to a top cover glass with polymethyl methacrylate. Using this approach, a ring structure with an inlet and outlet port performed as a highly efficient microtubule concentrator, accumulating microtubules moving in one direction, and removing within one half revolution any misdirected microtubules. Microtubules moved for over an hour in this ring without using up their ATP fuel, and hundreds of isopolar microtubules accumulated over time, providing a leap in our ability to gather isopolar microtubules for transport application, and investigate in vitro microtubule bundles that mimic cellular structures (Biomed. Microdev. 2007, 9:175-84).

CoFe₂O₄ nanoparticles are another tool we are using to control microtubule assemblies in vitro and control kinesin-driven microtubule movement. When these magnetic nanoparticles are attached to microtubules through biotin-avidin chemistry and placed in a magnetic field, the microtubules were found to align parallel to the magnetic field lines. By then pulling them down to an adsorbent surface, this approach was used to generate millimeter-scale fields of co-aligned microtubules (J. Am. Chem. Soc 2005, 127:15686-7).  Kinesin motors can also transport these particle-functionalized microtubules across surfaces, and we demonstrated that external magnetic fields can be used to control the trajectory of these kinesin-driven microtubules (Small 2007, 3:126-31).

In developing approaches for patterning motors and microtubules, we have pioneered a novel method using deep UV ablation to produce high contrast protein patterns on glass and quartz surfaces.  This approach is inexpensive and generalizable to any functional protein in principle.

In ongoing work, we are investigating electric field control of microtubule motility by embedding electrodes into microchannels.  We can control the accumulation of microtubule with high spacial resolution using dielectrophoresis and AC electroosmotic flow.  Besides the contribution of these techniques to microscale transport applications, this suite of tools is being applied to investigating in vitro models of microtubules and motors such as the mitotic spindle. This level of manipulation using defined components provides a bridge between in vitro single-molecule investigations and studies with intact cells.

Synthetic Motors. The goal of our synthetic motors research is to explore the fundamental limits of driven motion. We have expanded the synthesis of molecular motors to include motion driven not only by electric fields but also by photons, chemical binding, and mechanical manipulation. We continue to work closely between molecular design, theory and measurement. Three systems are under study - caltrops, double decker phthalocyanine/porphyrins, and hybrid systems of oligophenylene ethynylenes with porphyrins.

We have measured the thermal motion of "nanocars" as well as STM-driven motion. By varying the molecular design between two different three-wheeled nanocars, we were able to show that the fullerene "wheels" rotate, rather than skid across the surface. This work is being extended to include a "chassis" that can hold cargo molecules while moving across the surface.

With double decker phthalocyanine/porhpyrin molecules, we discovered that the spacings and orientations can be selectively controlled by functionality on or off the surface (Ye et al., submitted). Further, through proper functionalization, we can make the molecular conformations chemically selective, and have observed these changes in single molecules upon capture of ions between the sandwiching ligands.

Likewise, reversible conformational changes were induced in single molecules using selective photoexcitation. These are azobenzene-containing OPEs, and measurements were once again performed using two custom-built STMs. The quenching problem plaguing others' work in this area has been overcome and is being measured as a function of separation between functionality and the metal surface.

Theory efforts focus on controlling the rotational barriers in double-decker molecules: first-principles density functional calculations reveal that ligands added for solvation are also a source of steric hindrance. Model calculations proceed to identify distinct sources of disspation as critical degrees of freedom in the motors interpose themselves between the fundamental motor degree of freedom (e.g. rotational angle of an axle) and external reservoirs. A novel mechanisms of power law dissipation, without the need for multiple simultaneous timescales, has been identified for progressive elastic collisions with a damped element.

For molecular and catalytic nanomotors, the most important challenges are to understand motion at the fundamental level and to design new systems to extend the concepts. Catalytic motors that are more efficient and can therefore scale to smaller dimensions are needed. This will require investigating different kinds of reactions and new motor geometries. If we are successful in scaling catalyzed motion down to 10's of nanometers and below, we may achieve the ultimate goal of unifying the "bottom up" synthetic motors approach with catalytic engines.

With double decker phthalocyanine/porhpyrin molecules, we discovered that the spacings and orientations can be selectively controlled by functionality on or off the surface (Ye et al., submitted). Further, through proper functionalization, we can make the molecular conformations chemically selective, and have observed these changes in single molecules upon capture of ions between the sandwiching ligands.

Likewise, reversible conformational changes were induced in single molecules using selective photoexcitation. These are azobenzene-containing OPEs, and measurements were once again performed using two custom-built STMs. The quenching problem plaguing others' work in this area has been overcome and is being measured as a function of separation between functionality and the metal surface.

Theory efforts focus on controlling the rotational barriers in double-decker molecules: first-principles density functional calculations reveal that ligands added for solvation are also a source of steric hindrance. Model calculations proceed to identify distinct sources of disspation as critical degrees of freedom in the motors interpose themselves between the fundamental motor degree of freedom (e.g. rotational angle of an axle) and external reservoirs. A novel mechanisms of power law dissipation, without the need for multiple simultaneous timescales, has been identified for progressive elastic collisions with a damped element.

For molecular and catalytic nanomotors, the most important challenges are to understand motion at the fundamental level and to design new systems to extend the concepts. Catalytic motors that are more efficient and can therefore scale to smaller dimensions are needed. This will require investigating different kinds of reactions and new motor geometries. If we are successful in scaling catalyzed motion down to 10's of nanometers and below, we may achieve the ultimate goal of unifying the "bottom up" synthetic motors approach with catalytic engines.