Volume 2, Issue 10
Tiny Test Tubes and Nanoscale Membranes
Professor John Arnold also serves as the Associate Editor for the Americas for the scientific journal Dalton Transactions.
It's no surprise that UC Berkeley chemist John Arnold spends most of his time working with test tubes. What's interesting is that some of Arnold's test tubes are thousands of times smaller than the diameter of a human hair. In collaboration with chemistry professor Peidong Yang, Arnold and his colleagues are using Berkeley's latest innovations in nanoscience to synthesize new materials that could someday lead to longer-lasting batteries and ultra sensitive detectors of poisonous arsenic in drinking water.
One of Arnold's essential nanoscale building blocks is nanowires, single crystals of materials like zinc oxide, carbon, silicon, or other semiconductors grown into rods. In recent years, Yang, a fellow faculty scientist at Lawrence Berkeley National Laboratory, has pioneered several methods to fabricate nanowires out of various materials and with unique characteristics. The ability to precisely control the size and composition of the structures affects the nanowires' properties, enabling them to emit light, for example, or act as transistors.
"The nanowires give us a broad canvas to work with," Arnold says.
Recently, Arnold has used nanowires to synthesize membranes that would improve lithium batteries like those in cell phones and laptop computers. In traditional lithium batteries, current is produced when ions travel through an electrolyte between two terminals. The electrolyte allows just the positive ions to move through the battery, thereby freeing electrons to flow to the device requiring power. The problem is that the electrolyte solution itself is often toxic, corrosive, and flammable. In recent years, engineers have developed lithium-ion batteries with a dry polymer film separating the terminals. Unfortunately, the battery's performance is inhibited because the lithium ions don't travel as freely through these "solid electrolytes."
A porous membrane synthesized using an array of nanowires as a template.
"Fortunately, nature has membrane proteins that are quite good at selectively transporting ions through different environments," Arnold says.
The work is being carried out by graduate students Benjamin Rupert and Marty Mulvihill who were recently joined by postdoctoral researcher Seong Huh. The scientists took a cue from biology and fashioned their own nanomembrane. First, they grew an array of nanowires and coated them with a polymer. Next, they dissolved the wires, leaving polymer tubes behind.
"Essentially, we use the nanowires as a mold, or template, to make the polymer nanotubes," Arnold says. "In a battery, if we were able to control the ion-conduction mechanism inside the tubes, the ions could move between the electrodes through the tubes at incredible rates."
The polymer nanotubes have applications beyond better batteries though. The researchers are also able to grow single nanotubes with different functionality on the inner and outer surfaces. For example, the outside might repel water while the inside of the tube is hydrophilic, literally "water-loving." Someday, these kinds of nanotubes might be even be used as a drug delivery system, ferrying medicine through the body until a specific chemical reaction — an encounter with a tumor, for example — spurs its release.
Scanning electron microscope image of silver nanowires aligned like logs on a river and deposited on a silicon wafer. The thin layer of packed nanowires provides a good surface for chemical sensing with Raman spectroscopy. (Peidong Yang/UC Berkeley
In another project, Arnold and Yang hope to explore how silver nanowires could be the basis of a contamination test for water. In Bangladesh alone, arsenic poisoning from tainted well water is expected to cause 10 percent of all future adult deaths. Ideally, remote villages would have access to inexpensive and highly sensitive detectors for ongoing monitoring of their water supplies.
Nanowires may be just the solution. The researchers are functionalizing an array of silver nanowires so that arsenate ions easily bind to them. If a sample contains arsenate, a sensitive analysis technique called surface-enhanced Raman spectroscopy would detect its chemical signature. The researchers hope to eventually shrink the spectroscopy components so that the entire device can fit on a fingernail-sized silicon chip.
"Chemistry is really the only science where you get to make new things from the bottom up," Arnold says.