COURTESY OF REGINALD PENNER, UNIVERSITY OF CALIFORNIA–IRVINE
Stretching t h e
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omeday, you may find yourself driving to your annual health examination in your hydrogen gas-powered dream car with its perfectly tuned fuel flow—monitored by minuscule wire sensors. Then, a mere pinprick by a technician releases a few drops of blood to reveal your state of health from a broad range of blood analytes—using a multifunctional array of nanowire (NW) sensors. These are visions of the near future. These scenarios depend on sensors built with NWs (~10- to 100-nm diam), mesowires (~50- to 200-nm diam), or carbon nanotubes (~1- to 4-nm diam). They may be simple wires or tubes, or they may be functionalized to recognize a specific inorganic molecule or biological analyte. Their tiny size makes them useful for miniaturizing analysis—reducing sample size, analysis time, and waste disposal—and possibly for intracellular analysis in biological systems. Because the materials used for these sensors are different, the mechanism for conductivity change is different in each type of sensor. And because the sensors are so small, they no longer follow the principles of classical physics. In the mysterious quantum world, the phenomena follow their own unique principles, and unexpected responses become the norm. Today, some sensors are on the brink of commercialization, while others are still in the exploration stage. This article describes some recent developments in the field and the corresponding premises for how they work.
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Reinventing the palladium sensor The well-known palladium sensor for hydrogen gas exhibits a drop in conductivity when hydrogen binds to the surface. So,
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imagine the surprise of Reginald Penner of the University of California–Irvine when his group’s palladium mesowires yielded an “inverted response”—higher conductivity (lowered resistance)—when hydrogen was introduced (1, 2). “The first thing that you think,” he says, “is that there’s got to be something wrong.” But, as they cycled through exposure to hydrogen followed by reintroduction of air, the resistance alternately decreased and increased, consistent with hydrogen binding and then releasing from the surface. The group made more wires and checked all the parameters and instruments, and “it became clear that there was something really fundamental going on that we had never seen before.” Scanning electron micrographs (see art on opposite page) revealed contiguous palladium crystals in mesowires deposited on a graphite surface; but after the first exposure to hydrogen, breaks (~40- to 80-nm wide) in the wire appeared at regular intervals of ~2–5 µm, says Penner. Atomic force microscopy of the same wire showed reversible occlusion of hydrogen—breaks opened during exposure to air and closed in the presence of hydrogen. Penner says that these break junctions and changes in resistance can be explained by known morphological changes in palladium after multiple hydrogen/air cycles. “The way we view the initial state is that these grains are really loosely coupled to one another, and there is a lot of resistance at the grain [crystal] boundary. When you expose them to hydrogen for the first time, the lattice swells, the grains get scrunched together, and the boundary resistance goes down. But after hydrogen leaves, the lattice contracts again, and you get breaks.” So, the first exposure to hydrogen causes a phase transition and expansion of the
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palladium lattice, but an air environment reverses the transition. tance of the thinnest NWs decreased as the strength of adsorbateWith subsequent exposures to hydrogen, he says that there is binding increased (3, 4). Thus, the weakly binding dopamine little compression, but “enough to push the grains back and forth causes the smallest change in conductance, followed by 22BPY, along the axis of the wire to open and close break junctions.” and then the strongly binding MPA. Small dimensions confer different electron transport properThe sensor is also quantitative because the measured current deties for NWs than meso- or macrowires. pends on the hydrogen concentration In macrowires, says Tao, the conducand corresponds to the sum of the curtance is directly proportional to the rents of all the wires in the array. “…there was cross-sectional area and indirectly Although macroscopic palladium proportional to the length of the wire. sensors are readily available, Penner But conductivity decreases when elecsays that their response time is too slow something… trons collide with intrinsic impurities or to monitor real-time gas flow and that defects in the wire and then scatter difseveral types of molecules poison the going on that fusively. In a copper NW that is ~20sensor by blocking the adsorption site. to 30-nm long and shorter than the Because mesowires have a large surwe had never room temperature electron mean free face/volume ratio, the response time path (the average distance the electron can be as low as 20 ms compared with s e e n b e fo re .” can travel before it collides with an im0.5 s to several minutes for larger senpurity or defect), electron transport is sors; a mesowire sensor also requires “ballistic” with no scattering. But the little power. Penner hypothesizes that —Penner truly quantum effect comes from then the rapid equilibration of hydrogen reducing the diameter of the wire to with palladium in a mesowire explains the scale of an electronic wavelength. the wire’s resistance to poisoning by common contaminants such as methane, oxygen, and carbon Consequently, says Tao, “electrons in a transverse direction form quantum modes that act like standing waves. In an ideal situamonoxide. Penner is currently investigating the commercial possibilities tion, each standing wave gives a transmission of 100% ... and of monitoring the fuel and effluent of hydrogen-powered vehi- the total conductance would be proportional to the number of cles. He believes that the mesowire sensors could also be used standing waves inside the wire ... so that’s why conductance is in biological studies of bacteria that generate or consume hydro- quantized.” The high surface/volume ratio and the number of quantum gen; bacterial production of hydrogen could even be monitored as a possible source of fuel. The sensor could also be a safety fea- modes (proportional to the square of the diameter) account for ture in recharging car batteries to detect dissolved hydrogen be- NW sensitivity and partly explain changes in the conductivity pattern. And, as expected, the larger diameter, higher quantum fore enough gas is released to reach an explosive stage. His group is working with several different noble metal NWs, modes exhibit increasing classical electron scattering. However, detecting other gases, and trying to attach receptor molecules the researchers observe the greatest change in conductivity at to “do sensing in a more general way.” He says that they have the smallest diameter, first quantum level, where almost every taken data for months with some sensors, and he doesn’t “think atom of the wire is on the surface and exposed to adsorbates that can scatter electrons. The greatest sensitivity is at this smallest there’s anything intrinsic that limits their lifetime.” diameter of a few Ångstroms, and, as a result, Tao believes that Metal NWs sense organic molecules NWs could detect even a single molecule. The geometric surface/volume ratio argument doesn’t comNongjian Tao says that his group at Arizona State University “accidentally” moved into investigating sensing properties of pletely explain the NW’s behavior, says Tao, because the scatNWs as a side-path from their initial focus on electron transport tering of electrons depends on the electronic state of the adsorproperties in NWs. In the beginning, Tao says that contaminat- bate. “If a molecule has an electronic state very close [in energy] ing molecules in the air interfered with their study of electron to the energy of the electrons in the NW, there is a greater effect transport properties. “As we made the system cleaner,” he ex- [change in conductance].” That’s also one reason, he says, for a plains, “the data became much more clean. So we thought, since NW’s specificity, because different molecules have different elecit’s so sensitive to the chemical environment, we would put mol- tronic states, so they interact with electrons with a different inecules into the system.” He says that they got excited about tensity—some molecules scatter more than others. chemical sensor applications when they saw “a systematic deTao says that NWs are fairly easy to make, but their lifetimes pendence on molecules in the system.” vary from a few minutes to a few days. He says that introducing Tao says that they chose to test the relative detecting abili- molecules into the system can stabilize it, but introducing other ties of their system with the neurotransmitter dopamine and two chemical species will still cause a responding change in conducother substances that have familiar interactions with gold and tance. Stabilizing molecules could serve a dual function by also copper wires—mercaptopropionic acid (MPA) and 2,2´-bipyri- acting as specific receptors. Tao says that the most important dine (22BPY). This set of analytes demonstrated that conduc- goal is to collect more clues about how it works.
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Semiconducting NWs provide variety
Carbon nanotube counterparts
Boron-doped silicon NWs with different biological adducts re- Some researchers are exploring single-walled carbon nanotubes spond to a different type of electronic disruption than metal (SWNTs) as sensors. Like NWs, these hollow tubes have a high surNWs and mesowires (5). Yi Cui, working with Charles Lieber’s face/volume ratio, and adsorbates affect their conductivity (6–8). group at Harvard University, explains that these p-type semi- Hongjie Dai of Stanford University believes SWNTs can be useconductors act as field-effect transistors ful for analyzing automobile exhaust, and exhibit increased or decreased conindustrial work environments and effluductance with negatively or positively ents, and even for monitoring breath“One goal for charged adsorbates, respectively. able air in homes. These properties make an effective SWNTs can be semiconductors or pH monitor out of a silicon NW that “metallic” conductors depending on us now is…[to] has been modified with 3-aminoprotheir chirality. The molecular structure is pyltriethoxysilane (APTES). APTES, similar to graphite, explains Dai, with make a multisp2-hybridized carbon atoms held towith both an NH2 moiety that can be gether in a hexagonal pattern. Spiralling protonated and a SiOH moiety that functional hexagons impart semiconducting propcan be deprotonated, conveniently acts erties, while hexagons lined up along a as either a positive or a negative chemN W s e n s o r. ” tube’s axis give metallic properties. A ical gate. Protonation at low pH depaper by Alex Zettl of the University of creases the hole density and conducCalifornia–Berkeley describes SWNTs tance in the semiconductor. On the —Cui as behaving like one-dimensional quanother hand, conductance increases with tum wires (9). Zettl also reports that deprotonation at high pH. Experiments exposing semiconductors to oxygen at showed a linear, reversible change in room temperature converts them into metallic conductors. conductance over a 2–9 pH range. Several groups have found that SWNTs behave like p-type The sensitivity of these sensors is illustrated by the irreversible reaction of a biotin-functionalized NW with streptavidin. Bind- doped semiconductors with hole conduction. Unlike boroning negatively charged (under experimental conditions) strep- doped silicon nanowires, Dai describes SWNTs as charge-transtavidin increased conductivity, corresponding with concentra- fer devices. “When amines adsorb, they donate a tiny amount tions as low as 10 pM—lower than found by previous methods. of charge. That can cause a dramatic change in the electronic The researchers believe that their NWs could be used for conductance of nanotubes, and that’s the basis for a nanotube real-time measurements of protein concentrations to aid med- sensor,” he says. Adsorbed electron-donating molecules reduce ical diagnostics or monitor protein expression. Reversible bind- hole carriers and conductance, while electron-withdrawing ing between monoclonal antibiotin antibodies and the biotin molecules (such as NO2 and O2) increase hole carriers and conligand illustrates one example. The positively charged antibody ductance. Dai says that these sensors regenerate in about 10 s. decreased conductance, but addition of a buffer caused disso- “UV light works very efficiently,” he says. “You shine a light on ciation of the antibody–antigen complex and higher conduc- the sensor and the molecules go away.” Dai says that there are problems to address but that he’s contance within seconds. Concentrations between ~4–10 nM were fident that any remaining problems can be solved and that some directly proportional to the change in conductivity. Also important in biological systems is Ca2+, which is involved sensors are “ready to be tested in realistic situations.” He says that in cellular processes such as growth and death. Calmodulin im- functionalizing sensors for particular analytes is one way to elimimobilized on a NW acts as a sensor for Ca2+. Ions were detect- nate interfering signals from contaminants. Dai adds that nanotubes ed by decreased conductance, as expected for positive gating are “extremely robust and they live for a very long time. You resubstances. ally have to do very nasty things to [them] to destroy them.” Only charged analytes have been reported in published work, but Cui says that they have also detected polar molecules such as References water. “One goal for us now is ... to make a NW array and func- (1) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002,74, 1546. tionalize different NWs with different receptors, so basically we (2) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, can make a multifunctional NW sensor,”—hence, a “smart” sen293, 2227. sor for blood and other analyses. (3) Bogozi, A.; et al. J. Am. Chem. Soc. 2001,123, 4585. Cui says that reproducible, 5- to 30-nm-diam silicon NWs are (4) Li, C. Z.; He, H. X.; Bogozi, A.; Bunch, J. S.; Tao, N. J. Appl. Phys. Lett. very easy to make. He says that it’s also very easy to link func2000,76, 1333. tional groups to the silicon NW through the surface hydroxyl (5) Cui, Y.; Qingqiao, W.; Park, H.; Lieber, C. M. Science 2001,293, 1289. group, thus fine-tuning the electronic properties of the NW. (6) Kong, J.; Dai, H. J. Phys. Chem. B 2001,105, 2890. Silicon semiconductor sensors have lifetimes of at least a few (7) Kong, J.; et al. Science 2000,287, 622. months, but they have not yet been tested for longer periods of (8) Zhou, C.; Kong, J.; Dai, H. Appl. Phys. Lett. 2000,76, 1597. time. Nevertheless, he says, they have commercial promise. (9) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000,287, 1801. A P R I L 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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