Getting Molecules To Hold Still and Blink
New approach lets scientists trap single molecules and measure electron transfers
Results: Like little kids during family pictures, molecules like to move around. Scientists at Pacific Northwest National Laboratory devised a method to hold certain types of molecules still and measure if they blink; that is, gain or lose electrons.
Why It Matters: Mother Nature is very good at making catalysts: proteins that quickly add and remove electrons to create the desired products. Scientists would like to design molecules that mimic Mother Nature's catalysts, but do not yet know how. This research provides an important tool to learn if, when and why single molecules transfer electrons. By understanding electron transfers, scientists can gain the insights needed to design, atom by atom, catalysts that effectively turn water into hydrogen fuel or other reactions.
Methods: The scientists combined two existing methods to study immobilized individual molecules of the dye cresyl violet. The first technique is electrochemistry, which allows scientists to add and remove electrons to molecules at specific intervals. The second method is to use a single-molecule fluorescence microscope to record the light emission of the molecules. It uses a dye that gives off light under specific conditions. For example, when cresyl violet loses an electron, it gives off a red glow. When it gains an electron, it goes dark.
So, the researchers built an electrochemical cell that can measure electron movement on a type of conductive glass surface. Then, they placed a few drops of a clay solution on the glass. The liquid dried onto the glass, forming a transparent clay film. Then, they added cresyl violet. The dye attached to the clay film, holding the dye in place. This left the scientists with a piece of glass with a few dye molecules securely attached to it.
Next, they added to and removed electrons from the molecules at regular intervals by changing the electrochemical voltages. Simultaneously, they recorded the light flashes using a single-molecule fluorescence microscope at the Department of Energy's EMSL, a national scientific user facility at PNNL.
The method worked well, providing information about single-molecule effectiveness; that is, single molecules regularly gained and lost electrons while the molecules underwent cycles of blinking, dark and then bright.
What's Next: The researchers plan to use this method to interrogate other molecules and gain the knowledge necessary to design catalysts that mimic Mother Nature's most effective molecules.
Acknowledgments: DOE's Office of Basic Energy Sciences funded this research. The work, including use of microscopy instruments, was performed at DOE's EMSL, a scientific user facility. This research was done by Chenghong Lei, Dehong Hu, and Eric Ackerman at PNNL. Dr. Ackerman is now with Sandia National Laboratories.
Reference: Lei C, D Hu, and EJ Ackerman. 2009. "Clay nanoparticles-supported single-molecule fluorescence spectroelectrochemistry." Nano Letters 9(2):655-658.