Renowned researcher and recipient of the ACS Analytical Division Award in Electrochemistry...
Henry S. White, Ph.D.
Professor (b. 1956) B.S., University of North Carolina
1978 Ph.D., University of Texas
1983 Postdoctoral Associate, Massachusetts Institute of Technology
Adventures in Nanoscale Domains
Dr. Henry White has, as he puts it, “the best job in the world.” He is a Professor of Analytical Chemistry at the University of Utah and head of the White Research Group, a team devoted to studying electrochemistry in nanoscale domains—the driving science underlying several emerging technologies in the fields of energy storage, molecular electronics, and chemical sensors. Among his credits is creation of the nanopore electrode, a nanoscale device with macroscale applications ranging from medical diagnostics to biochemical detection. He has published more than 150 research papers, book chapters, and reviews in the field of electrochemistry, and has earned numerous academic awards—including the 2004 ACS Analytical Division Award in Electrochemistry. We recently sat down with Dr. White to discuss such heady topics as next- generation transdermal patches, futuristic nanobatteries, tomorrow's biowarfare detectors, and whether electron transfer rates obey classical predictions as the electrode is reduced to nanometer dimensions—all projects Dr. White is working on.
Henry S. White, Ph.D.: Before we get started, I would like to thank for its support of the ACS award program.
ColePamer.com: Your receipt of the award seems well deserved. You are performing some very intriguing, cutting-edge research.
White: Everything I've ever done has been in electrochemistry, and in particular molecular transport and shrinking down the size of the electrode. I guess you can say I've sort of been stuck in the mud—although it has given me quite an expertise.
ColePamer.com: Can you give us an example of some current project?
White: We've been developing electrochemical techniques for imaging molecular transport through skin for transdermal drug delivery applications. You've probably seen commercials for the nicotine patch?
ColeParmer.com: Of course.
White: That's transdermal drug delivery. But in the case of the patch it's passive transport—the nicotine contained in the patch diffuses across the skin. The next level of technology—which will be for medical purposes—will include the ability to control the rate at which these molecules are transported across the skin.
ColeParmer.com: And how is that accomplished?
White: One way of doing this is to use an electric current to drive electrically charged drug molecules—either cations or anions—across the skin. The devices that do this are called iontophoresis devices. They are really just miniaturized electrochemical cells, with two electrodes that are in contact with the skin. When you apply a current, the drug molecules under one of these electrodes is then driven across the skin and into the blood system. We are not involved in building these devices but we've constructed a scanning electrochemical microscope to image the mechanism—where the drug molecules go, where the current flows, what are the best pathways into the body, etc.
ColeParmer.com: Are these iontophoresis patches already in the loop or will a whole new generation of patches need to be developed for this?
White: The first of these devices has just been approved by the FDA after about 20 years of development by a company called Alza in California. The device delivers a small organic molecule called Fentanyl that is used for pain management. It's like morphine, basically. It's my understanding that it is used by patients in advanced stages of cancer—the patient simply pushes a button on this little Band-Aid-sized patch that has all of the electrical circuitry built in. That closes the switch in the cell and drives the drug molecule into the body. It's sort of like Star Trek—you know the old devices Doc McCoy use to use?
In our laboratory we've measured how fast the drug molecule actually enters the body—it's within about 50 milliseconds. It takes longer to get a full dosage, but it's quite an interesting device. Completely painless, and it does no perceptible damage to the skin.
ColeParmer.com: So, your team analyzes those channels of entry?
White: Right. When the current flows across the skin it doesn't flow uniformly, and that effects the operation of the device. In fact, there are localized pathways and it turns out, at least based on our measurements, that most of the flux of the molecule goes through hair follicles. That wasn't known before we started our research 15 years ago.
ColeParmer.com: Are there feedback/control implications to this transport information?
White: Yes. The idea behind transdermal drug delivery is to electrochemically pump drug molecules into the body. But if you can push ions into the body this way, you can also pull them out. Imagine you have a dual-function device: it extracts the ions to detect levels of, for example, glucose present in the body, then uses the electrochemical signal it generated to control the release of, in this case, insulin from the patch. In fact, there is a device developed by a research group in San Francisco that actually does monitor the concentration of glucose in the blood stream by exactly the mechanism I described. It's an example of what might be done using one signal in a feedback loop to control how much of a drug molecule you deliver.
ColeParmer.com: I understand the White Research Group is also working with magnetic field effects?
White: Yes. The magnetic field effects is a really cool project, too. But it has no practical application as far as we know right now—at least none for the next 10 years or so. But it is good, fundamental science. The work dates back to Michael Faraday, 150 years ago. What he wanted to know was, if you have two electrodes in a cell, and you are running a reaction, and then you apply a magnetic field, can you enhance the rate of that reaction. We ran exactly the same experiments. His experiment failed; ours turned out to be very successful.
ColeParmer.com: How is it you recorded different results?
White: We ran the experiment using really tiny platinum electrodes, about 20 microns in diameter—about 1/4 the diameter of a human hair. By using small electrodes you get very high current densities. And it turns out the magnetic force that you need to influence the chemical reaction is proportional to the current density. The current density in our electrodes is about three to four orders of magnitude higher than what Faraday had, and as a result we recorded tremendous magnetic effects on the electrochemical reaction.
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An example of magnetohydrodynamic transport of electrogenerated molecules. The top image is with the magnetic field off, the bottom with the field on.
The red color is the nitrobenzene radical anion which is generated at the surfaces of the Pt electrodes. The magnetic field acts on this molecule to transport it within a focused molecular beam through the solution. |
ColeParmer.com: As the density increases, the affect is greater?
White: The force that acts on the ions in the solution is actually equal to the product of the current density times the magnetic field. So, you try to get your magnetic field as high as possible and you try to get the current density as high as possible—and you can do that using these really tiny electrodes.
ColeParmer.com: Did anything unusual come out of the work?
White: Well, as a result of these experiments we have learned ways of constructing what we call molecular beams where we can generate molecules at one point in space in a solution and then we can shoot them across the solution a few centimeters in a straight line. People do this in a gas phase all the time. Mass spectrometers are based on using magnetic fields to direct ions in a certain direction. But that is really hard to do in a solution because as soon as I exert a force on that ion, it runs into a neighboring solvent molecule, and changes direction. We have been able to overcome that diffusion process by generating molecules on an electrode surface and making them all move in the same direction through the solution. We can shoot these molecules about two centimeters, which is a big accomplishment. Now it's a matter of using bigger magnets and developing the technology maybe to learn how to shoot these ions a meter in a solution. And if you can do that, you can develop analytical instruments that are analogous to mass spectrometers but which work in a liquid phase.
ColeParmer.com: Are those the “focusing techniques” discussed on your Web site?
White: Yes. And in that same regard we've learned techniques for solution-phase lithography, where we generate a very reactive ion on an electrode and then shoot it toward a silicone surface that's immersed in solution—we've generated some beautiful patterns. I actually have pictures hanging on the wall. It's quite interesting...the physics we are applying to these electrochemical systems on a micron scale is exactly the same physics astronomers apply to describe magnetic fields throughout the universe. And when I look through popular magazines like Science and Nature, and see these beautiful computer-generated images that astrophysicists draw to represent magnetic field lines, well it's exactly the same thing that we actually are taking real pictures of. Only, we are working on a scale that is about 12 orders of magnitude smaller.
ColeParmer.com: Very futuristic work...
White: The applications are still decades away. But I am convinced someone is going to pick up on this; eventually there will be some nice application to come out of it that I just haven't thought of.
ColeParmer.com: And if not?
White: If not, it's okay. It's a lot of fun. The students love it.
ColeParmer.com: Your Web site also lists research into something called “coulomb transport in ultra thin layer electrochemical cells.” What is that study about?
White: “Coulomb transport” is a term we invented for ourselves. This is very recent—all unpublished stuff. It is related to a project we are doing with collaborators at various universities and government labs in which we are trying to develop a new type of battery.
ColeParmer.com: Please explain.
White: Batteries available today are what we call two-dimensional batteries: basically, there are two electrodes that are parallel, and the ions flow from one electrode, through a solution, to the other electrode. That particular design has a key limitation in that the ions have to move across the separator between the two electrodes, and that separator has to be typically 50 to 100 microns in thickness. In terms of ion transport that's a huge gap. It limits the power that you can get out of the battery, and it limits how fast you can discharge the battery—which places limitations on battery applications. And there are some military applications, for instance, where you want to get maximum current and maximum voltage simultaneously. So, we have been developing designs for three dimensional batteries, where, instead of two parallel plates, you have interdigitated electrodes, and instead of two electrodes, we might have a billion anodes and a billion cathodes maybe a few microns in diameter. Our contribution to this project has been looking at the theory of what happens when you put two electrodes really close together.
We have discovered something totally unexpected—mainly, that when two electrodes have less than one-micron separation, the electrical fields from the two electrodes overlap, and instead of the ions diffusing across the separator between the two electrodes at a limited rate, the ions are driven across by the electrical fields. So, at least theoretically, you get this enormous enhancement in transport rates.
ColeParmer.com: Does that generate a burst...?
White: Not necessarily. I mean, it may be that you discharge this thing so fast that you only get a burst of current and it may work against you. Right now, these results are just coming out of numerical calculations. No one has done the experiment. And it will not be easy to experimentally place two electrodes that close together. But it has interesting applications in terms of power sources for either batteries or electrochemical capacitors—if you can change the transport rate by orders of magnitude, you can build many different types of devices.
ColeParmer.com: When you are talking about one-micron gaps, it is a little mind-boggling for the layman. How do you physically work in nanoscale domains?
White: That really is the result of 20 years of effort. For example, we've recently built what we call a nanopore electrode. This is a platinum wire sealed in glass, with a recessed tip that sits in a nanoscale pore.
ColeParmer.com: How is that designed and constructed?
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| Figure A |
White: Before we seal it up, we electrochemically etch the end of the platinum wire so that we create a very sharp tip—an atomically sharp tip. We can use this in a scanning tunneling microscope to image atoms, so we know this tip is really sharp. Then we gently seal the wire in glass that has a low softening point so as not to damage the wire, and we polish the end of it until we expose just the very end of this sharpened tip—so what we're left with is a little platinum disk at the end. What we do now is chemically etch the platinum wire and as the tip dissolves back into the glass it creates a little pore in the tip of the glass and at the bottom of that pore, you still have a platinum disk. [See Figure A]
ColeParmer.com: So, the “nanopore” is what's left after the platinum it etched away?
White: Yes. The pore is a cone-shaped pore with the platinum at the base.
ColeParmer.com: How small is it?
White: Over the years we have learned how to polish this glass to expose the disk so that you are only exposing five or 10 nanometers of radius of this platinum.
ColeParmer.com: How can you possibly measure the current?
White: It's actually very easy. The current is typically picoamps—1012 amps—which you can measure in the lab just using a commercial potentiostat that can be purchased for about $1,000.
ColeParmer.com: How does the electrode work?
White: This is a really neat device. Let's say you want to measure the current for the oxidation-reduction of some molecule. That molecule, in order to get to the electrode surface, has to go through the orifice at the top of that pore and diffuse down. But you are also letting other species diffuse down with it. So, what you can do is chemically modify the surface of the pore—attach molecules to the surface—so that only certain molecules are allowed to enter. In other words, the pore is acting as a chemical separator. On a nanoscale, we are basically doing chromatography inside the pore and at the bottom of the pore, you've got your electrochemical detector. So you've got your separation chemistry built right into the device with which you are doing the electrical measurement.
ColeParmer.com: What type of experiments have you run with it?
White: We've done a proof of concept experiment here where we've modified the surface with an amino silene—this is basically a hydrocarbon chain with an amine group and a neutral pH attached. And the amine is proteinated so it's a positively charged ammonium group. We used that molecule to coat the walls of the pore and inserted it into a solution that contains anions and cations. The electrode will preferentially sense the anions in the solution because the anions are electrostatically attracted to the positive charge on the wall of the pore and will enter inside. But the cation solution is electrostatically repelled from the pore. So, we have demonstrated that we can selectively detect negatively charged species in a solution when you have a mixture of both positively and negatively charged species. It's not a useful thing yet, it's just demonstrating that we can do this.
ColeParmer.com: But it holds promise?
White: Absolutely! This is a very mechanically robust device—which is the key for a sensor that you can take out into the field.
ColeParmer.com: What do you mean by “robust?”
White: What I mean is that I can wave this thing around in the air, since it is all self contained, and it's so small that you can do anything to it and you won't damage it. You won't alter the chemistry that is going on inside. There are no moving parts.
ColeParmer.com: What are the potential applications?
White: We are not developing it specifically for any one application—in part because this is really new. But it has enormous potential...wastewater, medical uses, also the military—and in fact this is a defense-supported project because it is so portable and so robust it can be used by soldiers out in the field.
ColeParmer.com: For detection?
White: I can't exactly say. But if there was one organic molecule, for instance, say a neurotoxin, that the army was really interested in, then you could tailor the chemistry of the pore to be selective for that molecule. Then it would function as a sensor.
ColeParmer.com: What are the dimensions of the device as it exists now?
White: The active part of the device is a glass tube about a millimeter in diameter with a wire running through it. It's long because we haven't miniaturized it. It's still in the lab. But I can imagine that when you build it as a sensor you can easily get it down to a few cubic millimeters.
ColeParmer.com: How large is the opening?
White: Right now, our smallest pore opening is about 15 nanometers.
ColeParmer.com: How does that compare to the size of a molecule?
White: A molecule would be about one nanometer or less.
ColeParmer.com: Why is the size of the opening so critical?
White: The reason this electrode is successful in selectivly detecting certain species over others is the fact that the hole is so small. You want to control the chemistry of that opening to make it harder for some molecules to go down the pore and easier for others. And you don't get that chemical interaction until the molecules get really close, within molecular dimensions. If you have a large opening—so big that molecules can go right down the middle without interacting with the chemistry on the surface of the wall—then you lose all of your selectivity.
ColeParmer.com: Would it be to your advantage to make it even smaller?
White: We would like to make it three nanometers, reproducible, day in and day out. We don't know how to do that yet. We need to learn how to make the tip of the platinum wire sharper and we need to be able to seal it in glass without the tip structure changing. What we think is happening is as we seal these wires in the glass, at the very end of this wire—the last few atoms—the shape is changing. The temperatures used in the sealing process are not high enough to melt bulk platinum, but they might be high enough to change the shape of the tip. That's what is limiting us from making truly molecular-sized openings.
I don't know, maybe the device won't work if we make it that small. Right now we are down to around 10 to 15 nanometers and we'd like to go down to at least five nanometers; at five nano, we think we can do everything we want to do.
ColeParmer.com: What problems do you run into operating in nanoscale domains?
White: Convincing people that the geometry of the device you are describing is actually the true geometry. The problem is that we make a platinum disk electrode that we claim is 10 to 15 nanometers in size and we measure that size by electrochemical measurements—and what people want, in order to believe it, is a visual picture of the device. And for that we have to use electron microscopy. But if you have a 10-nanometer chunk of platinum stuck in a huge piece of glass, it is sort of like finding a needle in a haystack. Electron microscopy only sees the surface. So, imagine you have this gigantic cornfield and somewhere in that corn field is the head of a nail sticking up and you've got to find that and take a picture of it to convince your colleagues that the geometry you claim to have is the true geometry. Making the device and running the experiments is fun. But then you've got to spend months trying to characterize it and trying to take pictures of it—that's the difficult part.
ColeParmer.com: Still, it all sounds very exciting.
White: I've got the best job in the world. I can come in and do whatever I want to any day—just pick the most interesting projects to work on without worrying too much about whether or not a product comes out at the end. Plus, we are educating a lot of students—sending a lot of Ph.D. students out into the world who develop their own new technologies. There are so many enjoyable aspects to this job. I just love it.
ColeParmer.com: Thank you.
