Grantee Research Project Results
2003 Progress Report: A Nanocontact Sensor for Heavy Metal Ion Detection
EPA Grant Number: R829623Title: A Nanocontact Sensor for Heavy Metal Ion Detection
Investigators: Tao, Nongjian
Institution: Arizona State University
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 2002 through December 31, 2004 (Extended to March 19, 2006)
Project Period Covered by this Report: January 1, 2003 through December 31, 2004
Project Amount: $375,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The threat of heavy metal pollution is a serious environmental concern because of the toxicity of such metals to a broad range of living organisms including humans, and the fact that these pollutants are non-biodegradable. Because of the difficulty in the remediation of sites contaminated with heavy metals, there is an urgent demand for an in situ sensor that is sensitive enough to monitor heavy metal ions before the concentration reaches a dangerous level. This project exploits the phenomena of conductance quantization and quantum tunneling to fabricate nanoelectrodes for in situ detection of metal ion pollutants. The objective of this research project is to develop a high-performance and low-cost sensor for the initial onsite screening of surface and groundwater that will provide early warning and prevention of heavy metal ion pollution. The highly integrated sensor will consist of arrays of different nanojunctions, including nanocontacts, molecular junctions, and polymer nanojunctions, for simultaneous detection of a range of different chemical species.
Progress Summary:
We have extended the nanocontact sensor developed in the previous year to molecular junction sensors and polymer nanojunctions. Our progress is summarized below.
Progress to Date
Molecular Junction Sensors for Metal Ion Detection. We have developed a method to wire peptides onto two electrodes to form a molecular junction, which allows us to directly measure the conductance of the peptides. When a metal ion binds onto the peptides, a change in the conductance is detected. The metal ion induced conductance change is sensitive to the sequence of the peptides. By selecting appropriate sequences, different metal ions on the peptides can be detected.
Conducting Polymer Nanojunctions. Another sensor platform that we have developed for metal ion detection is gold/polyaniline/gold junctions. The building blocks consist of an array of microelectrodes on a silicon chip, microfabricated metallic bars, and a thin polyaniline layer deposited on the microelectrodes or on the bars. The individual bars suspended in solution are placed, with the help of a magnetic field, across the microelectrodes to form the junctions. The polyaniline layer is approximately 30 nm thick and modified with Gly-Gly-His oligopeptides. Strong binding of Cu2+ to the oligopeptide is converted into a conductance change at the junctions, allowing selective detection of trace amounts of Cu2+ ions.
Other Relevant Research Activities. We have built a high-resolution differential Surface Plasmon Resonance (SPR) sensor for heavy metal ion detection. The sensor surface is divided into a reference area and a sensing area, and the difference in the resonance angle from the two areas is detected with a quadrant cell photodetector. In the presence of an analyte (metal ions), the SPR signal changes because of specific metal ion binding on the sensing area that is modified with a short peptide, which provides an accurate real-time measurement and quantification of the analyte. Selective detection of Cu2+ and Ni2+ in the ppt-ppb range was achieved by coating the sensing surface with NH2-Gly-Gly-His-COOH and NH2-(His)6-COOH. The copper content of drinking water was tested using this approach.
Major Results to Date
Molecular Junction Sensors. We have studied metal ion binding onto short peptides and observed a conductance increase in each case. The conductance increase is highly sensitive to the sequence of the peptide. For example, the binding of a Cu2+ onto cysteamine-Cys causes only a 10 percent conductance increase, but the binding onto cysteamine-Gly-Gly-Cys increases the conductance approximately 300 times (see Figure 1). Therefore, we can tune the peptide sequence to maximize the metal-ion binding-induced conductance change for sensor applications. The conductance change also is dependent on the type of metal ions. For example, the binding of Ni2+ onto cysteamine-Gly-Gly-Cys increases the conductance approximately 100 times, which is several times smaller than the case of Cu2+ binding. This metal ion dependence shows that we can identify different metal ions from the conductance measurement even if they all bind to the peptide.
Conducting Polymer Nanojunction Sensors. For metal ion sensor applications using polymer nanojunctions, we have attached oligopeptides to the polyaniline nanojunctions via poly(acrylic acid) incorporated into the nanojunctions. One example is to attach Gly-Gly-His to a 30 nm polyaniline junction. The tripeptide was chosen because of its large binding constant for copper ions. Figure 2 shows the current (I) versus voltage (V) curves of the polymer nanojunction before and after exposure to 10 ppb copper ions. The current change is roughly a linear function of the ion concentration (see Figure 3a). When exposing the sensor to other metal ions, little response was detected, which shows the high degree of selectivity (see Figure 3b). The experiment demonstrates the feasibility of detecting metal ions using the polymer nanojunctions. Although the sensitivity of the preliminary experiment is adequate to meet the National Primary Drinking Water Regulations, it can be further improved by optimizing the polymer nanojunctions and attaching the oligopeptides. For instance, these nanojunctions (assembled with a magnetic-field assisted method) have a thickness of approximately 30 nm, but a literal dimension of approximately 1 µm. Using the electron beam and electrochemical fabricated nanoelectrodes proposed in this project, the literal dimension can be reduced to approximately 50 nm, which should improve the sensitivity.
Figure 1. Individual Conductance Curves, Conductance Histograms (inset of a and b), and I-V Characteristic Curves of Cysteamine-Gly-Gly-Cys before (c), and after (d), the binding to Cu2+. Note that conductance near zero bias increases approximately 300 times upon Cu2+ ion binding.
Metal Ion Detection with SPR. Figure 4 (Left) shows the time course of the SPR measurement upon introduction of nickel ions into the sample cell. The SPR sensing area was modified with cysteamine-(His)6. The increase in the differential signal after each Ni2+ injection indicates that the specific binding of Ni2+ on (His)6 takes place. The measurement is sensitive enough to detect the specific binding of Ni2+ in real time from a solution with a concentration as low as 41 picomolar or 2.4 ppt without preconcentration or stirring of the solution. Figure 4 (Right) shows the corresponding differential signal change as a function of Ni2+ concentration. A linear response with a sensitivity of approximately 0.0020 degrees/nM and a regression coefficient of 0.9854 is observed. The response is sensitive to the method used to modify the sensing surface. For example, if the surface is modified with MUA-(His)6, the sensitivity is poorer than with cysteamine-(His)6, because of the different surface coverage of (His)6.
Figure 2. Left. I-V Characteristics of a Polyaniline Nanojunction Functionalized With Gly-Gly-His Peptide Before and After Exposure to Approximately 10 ppb Cu2+ Dissolved in Water. Right. Schematic Illustration of the Gly-Gly-His Modified Polyaniline Nanojunction Before and After Exposure to Cu2+. The binding of Cu2+ changes the number of protons and the conformation of the tripeptide.
Figure 3. (a) Sensitivity Test. Conductance of the functionalized polymer nanojunction as a function of concentration of Cu2+ at -0.4 V applied bias voltage. (b) Selectivity Test. Lack of response of the nanojunction sensor to other metal ions.
Figure 4. Left: Time Course of the SPR Differential Signal (A-B/A+B) - (C-D/C+D) Upon Ni2+ Injections. The sensing area is modified with a monolayer of (His)6-cysteamine and the reference area is covered with a self-assembled monolayer of dichlorodiphenyltrich loroethane. The Ni2+ concentrations added to the solution cell in each injection are indicated. Right: SPR Differential Signal Change as a Function of Ni2+ Concentration Onto a MUA-(His)6 (Method 1, ) and a Cysteamine-(His)6 (Method 2, ) Modified Sensing Area.
Future Activities:
We will continue to investigate the basic principle of these sensors. We also will start to test a prototype device for field applications. Experiments carried out so far have clearly demonstrated the feasibility of the sensors for metal-ion detections. Planned activities for the following year follow.
Molecular Junction Sensors
The observed metal ion binding-induced conductance increase may arise from the following possible mechanisms. First, metal ion binding can introduce new energy levels in the molecular orbitals of the peptide, and thus increase the conductance. If this is a dominant mechanism, the conductance change should be sensitive to the type of metal ion because the energy levels (redox potentials) of different ions are different. Alternatively, the positive charge of a metal ion can lower the tunneling barrier and increase the conductance. To determine if and which of the above two mechanisms is responsible for the observed conductance change, we will study metal ions with different redox potentials and charges. If the first mechanism dominates, the conductance change should correlate strongly with the redox potential of the metal ion. On the other hand, the second mechanism predicts a much greater effect for divalent ions than for monovalent ions. Finally, the strong interaction of a metal ion with the binding sites of a peptide will change the conformation of the peptides, which can shorten the tunneling distance and thus the conductance. For example, when Cu2+ binds onto cysteamine-Gly-Gly-Cys, the peptide will adjust its conformation so that the binding sites wrap around the metal ion to form a peptide-ion complex. Because the metal ion binding-induced conformational change depends on the peptide sequence, we will carry out a systematic study of different sequences.
Conducting Polymer Nanojunctions
This is a highly versatile sensing platform that can be used to detect not only various metal ions but also other chemical and biological species. For metal ion detection, the basic strategy is to use peptides of various sequences, similar to the molecular junction sensors described above. Metal ion detection will continue to be our main focus in the next project period. We will optimize the conducting polymer nanojunctions to achieve the highest sensitivity and response time by tuning the sequences of the peptides. Because both the sensitivity and response also depend on the attachment method of the peptides onto the polymers, we will perform a systematic study of different attachment methods. Finally, we will test metal ion concentrations in drinking water using the polymer nanojunction sensors.
A Prototype Device
Although many scientific questions need further investigations, we believe that our experiments have demonstrated the feasibility of several sensor platforms. We will develop a prototype device using the molecular junctions and conducting polymer nanojunctions or SPR methods. We will collaborate with Paul Westerhoff, Professor of Civil and Environmental Engineering, to develop a practical device for field applications. Our initial focus will be on the detection of metal ions in drinking water.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 54 publications | 13 publications in selected types | All 13 journal articles |
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Boussaad S, Xu BQ, Nagahara LA, Amlani I, Schmickler W, Tsui R, Tao NJ. Discrete tunneling current fluctuations in metal-water-metal tunnel junctions. Journal of Chemical Physics 2003;118(19):8891-8897. |
R829623 (2002) R829623 (2003) R829623 (Final) |
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Boussaad S, Tao NJ. Polymer wire chemical sensor using a microfabricated tuning fork. Nano Letters 2003;3(8):1173-1176. |
R829623 (2003) R829623 (Final) |
Exit |
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He HX, Tao NJ, Nagahara LA, Amlani I, Tsui R. Discrete conductance switching in conducting polymer wires. Physical Review 2003;B68(4):045302. |
R829623 (2003) R829623 (Final) |
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Li XL, He HX, Xu BQ, Xiao XY, Nagahara LA, Amlani I, Tsui R, Tao NJ. Measurement of electron transport properties of molecular junctions fabricated by electrochemical and mechanical methods. Surface Science 2004;573(1):1-10. |
R829623 (2003) R829623 (Final) |
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Xu BQ, Tao NJJ. Measurement of single-molecule resistance by repeated formation of molecular junctions. Science 2003;301(5637):1221-1223. |
R829623 (2003) R829623 (Final) |
not available |
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Zhang HQ, Boussaad S, Ly N, Tao NJJ. Magnetic-field-assisted assembly of metal/polymer/metal junction sensors. Applied Physics Letters 2004;84(1):133-135. |
R829623 (2003) R829623 (Final) |
Exit |
Supplemental Keywords:
nanotechnology, scanning tunneling microscopy, atomic force microscopy, electrochemistry, physics, monitoring and measurement methods, ecosystem protection/environmental exposure and risk, water, analytical chemistry, chemical engineering, chemistry and materials science, electron microscopy, engineering, chemistry, environmental chemistry, environmental engineering, environmental monitoring, monitoring/modeling, groundwater, heavy metal ion detection, heavy metals, measurement, metal-ion pollution, metal ions, nanocontact sensor, sensor, sensor technology., RFA, Scientific Discipline, Water, Ecosystem Protection/Environmental Exposure & Risk, Chemical Engineering, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, Chemistry and Materials Science, Engineering, Chemistry, & Physics, Environmental Engineering, monitoring, metal ion pollution, nanotechnology, metal ions, heavy metal ion detection, surface water, nanocontact sensor, scanning tunneling microscopy, measurement, sensor, heavy metals, sensor technology, groundwaterProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.