NANO LETTERS
Polymeric Nanowire Chemical Sensor Haiqing Liu, Jun Kameoka, David A. Czaplewski, and H. G. Craighead*
2004 Vol. 4, No. 4 671-675
School of Applied and Engineering Physics, Cornell UniVersity, Ithaca, New York 14853 Received January 30, 2004
ABSTRACT We have used a nonlithographic deposition process to form single polymeric nanowire chemical sensors. Oriented polyaniline nanowires, with diameters on the order of 100 nm, were deposited on gold electrodes. The devices showed a rapid and reversible resistance change upon exposure to NH3 gas at concentrations as low as 0.5 ppm. The well-defined single-wire geometry allows for the characterization of the wire material and the device response. The response times of nanowire sensors with various diameters correspond to radius-dependent differences in the diffusion time of ammonia gas into the wires. The nanowire deposition process, utilizing a scanned microfabricated electrospinning source, presents a general method for interfacing polymeric nanowire devices with microfabricated structures.
Nanoscale sensors have been attracting considerable attention in recent years. The application of carbon nanotubes and semiconductor nanowires, for example, has been the topic of significant recent research.1,2 Although the use of nanotubes and nanowires offers the prospect of high sensitivity and rapid detection, the ability to incorporate nanowires into sensor device architectures is limited by the difficulty in manipulating and locating the nanostructures with respect to the microelectrodes. A variety of conducting polymers have shown promise as sensor materials because their properties can be tailored to detect a wide range of chemical compounds.3-5 Conducting polymers also have attractive features such as mechanical flexibility, ease of processing, and modifiable electrical conductivity. In this report, we demonstrate an approach for creating polymeric nanowire sensors that has the advantages of sensitivity, spatial resolution, and rapid response associated with individual nanowires along with the material advantages associated with organic conductors. By using a scanned-tip electrospinning method for depositing isolated and oriented polymeric nanowires, we created individual polyaniline/poly(ethylene oxide) (PANI/PEO) nanowire sensors that can detect NH3 gas at concentrations as low as 0.5 ppm with rapid response and recovery time. The controllable geometry and high surface-to-volume ratio associated with the nanowires yield improved response and increased sensitivity compared to the same properties for previously described polyaniline sensors based on films and random fiber networks.6-8 The organized geometry also allows for better measurement of the wire electrical properties and yields a predictable and reproducible response for the sensor devices. This electrical conductivity of polymers results from the existence of charge carriers (through doping) and from the * To whom correspondence
[email protected]. 10.1021/nl049826f CCC: $27.50 Published on Web 02/27/2004
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ability of those charge carriers to move along the π bonds of the polymer chains.9-11 The polymers may show chemical selectivity, which allows them to act as excellent materials for the immobilization of gas or biological molecules, and they also exhibit highly reversible redox behavior with a distinguishable chemical memory. Conducting polymer nanowires (CPNWs) also have the potential for use as molecular electronic architectures and devices.4,11,12 Electrospinning has been widely used to make PANI and other polymer wires, but these wires have not been deposited as oriented single wires.13-15 As a sensor, the high surface-tovolume ratio of the single nanowires and its small dimensions permit the rapid depletion or accumulation of charge carriers in the nanowires. Among conducting polymers, PANI has received widespread attention because of its simple and reversible doping/ dedoping chemistry, stable electrical conduction mechanism, and high environmental stability. It has been used as a sensing material for a variety of toxic gases such as CO, NO2, and NH3.8,16-20 The detection of NH3 in air is of interest for environmental monitoring and process control because of its high toxicity. The exposure limit in the U.S. is 25 ppm over an 8-h period and 35 ppm over a short-term exposure.21 PANI has an affinity for NH3, resulting from the similarity of the coordinative roles of nitrogen atoms in PANI and NH3. PANI exhibits p-type semiconductor characteristics; consequently, electron-supplying gases such as NH3 reduce the charge-carrier concentration and decrease the conductivity. The sensor devices that we have studied utilized individual oriented polymeric nanowires deposited on lithographically defined microelectrodes. The devices were fabricated by using a recently demonstrated scanned-tip electrospinning deposition method employing a microfabricated source to deposit oriented polymeric nanowires.22-24 A benefit of using this deposition approach is that it does not rely on chemical
Figure 1. Scanning electron micrograph of a single PANI nanowire lying across four-terminal gold electrodes used for conductivity measurements. (Inset) Higher-magnification view of the CPNW of diameter 180 nm.
vapor deposition or on the selectivity of particular etching processes.25-27 In this method, a droplet of PANI/PEO solution was placed on an arrow-shaped tip that acted as a scanned electrospinning source. The polymer jet, electrostatically extracted from the tip, dried in transit to a substrate on a rotating counterelectrode. This process produced oriented PANI/PEO nanowires with diameters in the range of 100500 nm. Photolithographically patterned four-terminal gold electrodes were positioned on the counterelectrode plate in an orientation perpendicular to the wire deposition direction. A typical scanning electron micrograph showing a single CPNW lying across the gold electrodes is shown in Figure 1. The polymer-blend solution was 2 wt % PANI doped with 10-camphorsulfonic acid in chloroform,28 and 0.25 wt % PEO.29 The scanned tip was fixed at a distance of 1.5 cm from the substrate and held at a potential of 8.5 kV with respect to the grounded counterelectrode. The substrate was rotated, producing a relative velocity of about 200 cm/s for the tip in relation to the substrate. The typical conductivity of our individual PANI/PEO nanowires was about 0.5 S/cm before exposure to NH3. This compares to a conductivity on the order of 0.1 S/cm that is normally observed in PANI films based on the same doping method.30 The higher conductivity achieved in our electrospinning process suggests that there may be alignment of the polymer chains along the wire axis. The bulk conductivity of the polymer is limited by the need for the electrons to hop from one chain to the next. Therefore, interchain interactions, which strongly depend on the local order on nanoscale lengths, play an important role in the conductivity.31 Microstructure studies of PANI indicated that it is an inhomogeneous material. Crystalline regions, where charge delocalization is easier, function as metallic islands connected by lower-conductivity amorphous regions. Better alignment of polymer chains in the disordered regions can facilitate the hopping transport between the crystalline regions.32 Although the microstructure mechanism for increased conductivity of the oriented single wires is not known, the process includes some stretching of the wires as they are 672
deposited on the moving substrate, which is likely to have an impact on the microstructure and molecular orientation. The electrical conductance properties of the CPNW were measured using both two-terminal and four-terminal resistance measurements.33 Resistance measured by the twoterminal method deviated less than 4% from that of the fourterminal method (data not shown), which indicates that the contact resistances between the wires and the electrodes are small. By using a two-terminal or four-terminal measurement, current versus voltage behavior was observed to be linear for all of the devices studied, which indicates that the gold electrodes made ohmic contact to the CPNW. A two-terminal I-V measurement was used for the real-time monitoring of the PANI nanowires’ response to NH3. The current through the wire during the dedoping and doping of the PANI nanowire by the addition and removal of the NH3 gas was recorded at a constant applied potential of 0.5 V. The timedependent conductance response of the CPNW upon exposure to different concentrations of NH3 gas is shown in Figure 2A. The CPNW was exposed to NH3 with a gas tubing source held at a fixed distance of 1 cm above the wire, with a flow rate of 1 L/min.34 As a control, nitrogen flowed over the sensor, and no change in the conductivity of the CPNW was observed. The threshold detection concentration level was ∼0.5 ppm NH3. The time response of the nanowire sensors can be understood by considering the diffusion of ammonia into the wire and the reaction of NH3 with doped PANI. We describe this process with a model of diffusion into a solid cylinder and instantaneous chemical reaction to change the doping.35 The protonation and deprotonation kinetics of PANI have been studied previously.36 Upon exposure to NH3, the gas molecules diffuse into the PANI wire, and the dedoping of the H+-doped PANI by NH3 occurs rapidly. Electroneutrality must be maintained in the polymer, which is achieved by charge transfer between the NH3 molecule and the specific active sites (the H+-doped imine-nitrogen sites of PANI), and this leads to a decrease in the charge-carrier density. We assume that the electrical conductivity of the polymer Nano Lett., Vol. 4, No. 4, 2004
S(t) S0
Figure 2. (A) Measured time-dependent current through an individual CPNW sensor upon exposure to NH3 gas. The nanowire device being tested was about 335 nm in diameter. (B) Timedependent conductance change for two CPNW sensors with wire diameters of 335 and 490 nm, measured upon exposure to 50 ppm NH3. For better visibility, every two out of three data points were removed. The least-squares fits are shown as solid lines.
is proportional to the time-dependent H+ concentration [H+] and that this is equal to the concentration prior to NH3 exposure minus the concentration of NH4+, [H+] ) [H+]0 - [NH4+] We assume that the chemical reaction by which NH4+ is formed proceeds much more rapidly that the diffusion process and that local equilibrium exists between bound and free NH3 such that [NH4+] ) K[NH3]. The local time-dependent conductivity is therefore proportional to [H+] ) [H+]0 - K[NH3], where [NH3] is the local free-NH3 concentration due to diffusion into the wire. The time-dependent conductance of the wire S(t) is proportional to the total number of NH3 molecules entering the wire by diffusion. This can be obtained from the solution of the diffusion equation for a cylinder:35 Nano Lett., Vol. 4, No. 4, 2004
)1-
(
K[NH3]0 [H+]0
1-
∞
∑ n)1
4
Rn2
e-Rn Dt/a 2
)
2
(1)
where S0 is the initial conductance of the wire, [NH3]0 is the supplied constant NH3 concentration, a is the radius of the wire, and D is the apparent diffusion coefficient. Rn are roots of J0(Rn) ) 0, where J0 is the Bessel function of the first kind of order zero. We measured the response times of CPNW sensors of different wire diameters. The conductance changes as a function of time for 335- and 490-nm-diameter wires are shown in Figure 2B. In both cases, sensors were exposed to NH3 at a concentration of 50 ppm. The response times (defined as the time duration for the conductance to decrease to 1/e of the whole conductance change) for the two wires are 74.8 s for the nominal 335-nm-diameter wire and 162.3 s for the nominal 490-nm-diameter wire. A least-squares fit was applied to the experimental data using eq 1, and the apparent diffusion coefficients for the two wires were determined to be 5.47 × 10-13 cm2/s for the nominal 335nm-diameter wire and 5.25 × 10-13 cm2/s for the nominal 490-nm-diameter wire. The fact that these apparent diffusioncoefficient values agree essentially to within our experimental error indicates that the differences in wire response are due to radius-dependent diffusion, not material differences. Basically, the response time of the wire scales as the wire radius squared, a2, indicating the response time value of narrow wires. If the factor K is 100 (99% of the NH3 reacted), then the diffusion coefficient, which is 1/(K + 1) of the apparent diffusion coefficient, would be 5.52 × 10-11 cm2/s for the nominal 335-nm-diameter wire and 5.30 × 10-11 cm2/s for the nominal 490-nm-diameter wire. These values are comparable to diffusion coefficients of a similar gas in polymer substances,36-38 although direct measurements of NH3 gas diffusivity in PANI/PEO are not available from previous study. The electrical properties of the wires are systematic with respect to their response to gas concentration and to diameter, as seen in Figure 2A and B, which indicates reproducible materials properties for the spun wires. For the measured current versus time curves in Figure 2A, which reflect the gas-concentration dependence of the temporal conductance behavior, the lower-gas-concentration behaviors do not fit well with the simple theory given above for the diffusion coefficient. For the diffusion of gas in high-polymer materials, the gas diffusivity can be concentration-dependent.35,39 There are also more complex processes, in addition to diffusion, involved in the dedoping such as the electrontransport properties of the H+-doped PANI.36 Our simple single-wire geometry should enable systematic studies of these phenomena. The well-defined device geometry allows us to observe the diameter-dependent behavior of the CPNW sensor. The results indicate that the wire diameter affects the response time of the sensor, with the smaller-diameter wires having a faster response associated with the more rapid diffusion of gas molecules through the wire. Previously reported PANI 673
Figure 3. Steady-state CPNW sensitivity to 0.5-120 ppm concentrations of NH3 gas. Each data point with an error bar is based on three independent measurements of the same sensor, with a wire diameter of 300 nm.
film sensors have longer response times than we have observed, often accompanied by hysteresis.7,40 We expect this to be the result of the diffusion time associated with these geometries. Other results of this single CPNW structure, compared to random mats of fibers, may be its mechanical stability and well-defined current path so that variations in the “interconnect” resistance through the many contacts in a fiber network are eliminated. This could be valuable for the reproducibility and signal/noise ratios of practical sensors.8 A four-terminal I-V measurement was used to measure the steady-state conductance by measuring the voltage across the wire while supplying a constant current of 50 nA at different NH3 concentrations. The steady-state gas-sensing experiment was carried out by placing the sensor in a sealed 90-mL container with electrical feed-through and a gas inlet/ outlet. NH3 gas diluted with nitrogen (0.5 to 120 ppm) was supplied at a flow rate at 1 L/min. Resistance change was monitored and recorded. The response of the CPNW to different concentrations of NH3 (Figure 3) indicates that the change in resistance ∆R (where ∆R ) R - R0, R0 is the initial resistance in the absence of NH3, and R is the resistance measured after a 15-min exposure to gas) appears to saturate at high concentrations (>80 ppm), which suggests that limited imine-nitrogen sites can be doped by protons in the wire. The sensitivity [(R/R0) - 1)] was approximately 30 times higher than that of the traditional film-based sensors.6,7 This is likely due to the small diameter of the nanowires that gives rise to the high surface-to-volume ratio. A typical response of the single CPNW to cyclic NH3/air exposure (10 s on/2-4 min off) demonstrates the speed of the CPNW sensing performance (Figure 4). After the NH3 flow was replaced by pure nitrogen, the conductance of the CPNW sensor recovered quickly, on the order of 10 s to a few minutes, with a recovery time depending on the NH3 gas concentration. Devices were cycled up to 40-50 times while retaining resistance reversibility. Long-term (several 674
Figure 4. Real-time CPNW response to different concentrations of NH3 gas. Cyclic NH3/air exposure (10 s on/2-4 min off).
months to years) stability tests were not performed because of time considerations. Although not investigated in this study, another advantage of using single CPNW chemical sensors is its spatial resolution. As with carbon nanotubes and other nanowire sensors, the detection area is small, hence this can be used to localize the detection of chemicals spatially. Arrays of detectors could provide both spatial and temporal information about chemical concentration. This could be used, for example, with an appropriate sensing material, to study the chemical release from cellular systems.41 This work demonstrates a new approach to forming organized self-assembled polymeric electronic devices. Advantages of this approach include integration with wellestablished microfabrication technologies. Although we have demonstrated the method for creating individual nanowire gas sensors, the same approach can be used to interface deposited nanowires to microfabricated electrodes, or electronic devices can clearly be used for other systems such as switches, transistors, displays, or memory elements. This represents a possible manufacturing approach, utilizing the best aspects and diversity of polymers and self-assembled materials combined with the best aspects of microelectronics technology. Acknowledgment. This work was supported by the Nanobiotechnology Center (NBTC) and the STC Program of the National Science Foundation under agreement no. ECS-9876771. The work was performed in part at the Cornell Nano-Scale Science & Technology Facility (a member of the National Nanofabrication User Network). References (1) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622-625. (2) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 12891292. (3) Sadik, O. A. Electroanalysis 1999, 11, 839-844. (4) Janata, J.; Josowicz, M. Nat. Mater. 2003, 2, 19-24. (5) Nguyen, T. A.; Barisci, J. N.; Partridge, A.; Wallace, G. G. Synth. Met. 2003, 137, 1445-1446. Nano Lett., Vol. 4, No. 4, 2004
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