In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Building a Low-Cost, Six-Electrode Instrument To Measure Electrical Properties of Self-Assembled Monolayers of Gold Nanoparticles
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Ralph W. Gerber and Maria Oliver-Hoyo* Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *
[email protected] The formation of self-assembled monolayers of gold nanoparticles will result in conductivity after the ninth layer as described by Musick and colleagues (1). In our own research, electrical measurements of the gold nanoparticle layered slides showed intermittent conductivity at layer nine and full conductivity at layer fourteen. The full laboratory experiment is described in “From the Research Bench to the Teaching Laboratory: Gold Nanoparticle Layering” (2). The measurements for conductivity were initially performed with a simple, battery-powered LED utilizing alligator clips to attach the tester to the layered slide. Because of the high susceptibility for scratching of these self-assembled monolayers, measurements with this primitive device damaged the conductive surface and rendered the slide useless for further layering and conductivity measurements. The low-cost instrument described here was designed and built to eliminate scratches on the surface of the gold film. This allows continuation of the layering process with subsequent measurements regarding electrical properties as additional layers of gold nanoparticles are applied. The design of the instrument with this six-electrode configuration is such that not only can current and voltage be measured but, based on “A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape” by L. J. van der Pauw (3), allows for direct measurements of and calculations for resistivity and conductivity of the gold surface. These measurements permit comparison of the gold conductive surface to other media, such as an indium tin oxide (ITO) conductive slide or any other conductive surface, including bulk materials, foils, and conductive liquids. Stepwise assembly of gold monolayers will result in conductivity when the gold nanoparticle orientation is such that
electrons can freely move within the layer. The movement of electrons through the monolayer can be achieved either by direct contact or electron hopping from one nanoparticle to another. In order to monitor the point at which conductivity is achieved during layering, the meter pictured in Figure 1 was constructed. While we built this meter for less than $200, its sensitivity and range of measurement capability competes with other expensive instruments in accuracy, repeatability, and reproducibility. Current measurements can be made between 5 µA–1 A; voltage measurements can be made between 1 mV–10 V. Design Parameters The meter was designed to allow for the following applications and measurements. 1. Multi-position sample stage base plates for 9 mm ⫻ 50 mm coverslips and standard 1 in. ⫻ 3 in. slides. 2. Use of LED’s to identify circuits that are currently in operation and active grounds. 3. Adjust voltage and current from the direct current power supply. 4. Verify raw conductivity with an independent internally powered LED circuit. 5. Measure applied voltage and current at all points of the circuit under investigation by using banana jacks and plugs. 6. Measure adjusted voltage and current at all electrodes. 7. Measure voltage drops across any combination of the six-electrode array.
Figure 1. Photograph of the low-cost, six-electrode multimeter constructed as described in this paper. The Supplemental MaterialW contains a complete parts list with part numbers and pricing, detailed, annotated views in color of all exterior components, a complete wiring diagram, and sample stage upper and lower plates design specifications. Color versions of this image and those in Figures 2–4 are available online.
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In the Laboratory
Meter Fabrication
Sample Stage The sample stage was constructed from 4 ⫻ 4 ⫻ 1 in. acrylic blocks. The top plate as shown in Figure 2 shows the six gold tipped electrodes. The electrodes were installed via counter sunk holes with compression springs to insure good contact with the gold layered surface to be measured. The compression of the springs was adjusted to provide good contact with the surface and to prevent breakage of the coverslips from pressure. The electrodes arrangement was designed with 2.54 cm between the top and bottom electrode. The four center electrodes are spaced 0.5 cm apart in the horizontal and vertical directions, forming a square. The base plates as shown in Figures 3 and 4 were machined to hold the standard microscope slide and the coverslips. All of the blocks were fitted with registration pins and wing nut fasteners to insure consistent orientation during the measurement process. Additional specifications and mechanical drawings are provided in the Supplemental Material.W (The machined sections for slide and coverslip insertion have been shaded in blue in those drawings.)
Figure 2. Six-electrode top plate.
Power Supply and Power Input The 5.0-V, 650-mA dc power supply is connected to a four-position rotary switch equipped with increasing resistance in the different positions to provide varied input voltages.
Adjustable Voltage and Current Controls The meter has two potentiometers: 0–50 Ω and 0–2 kΩ. Either of these units can be placed in series or parallel within a chosen circuit to adjust voltage and current independently or together. This is necessary to decrease the applied voltage to a range that can measure a small voltage drop on the gold monolayers.
Figure 3. 9 mm ⫻ 50 mm base plate.
Output Devices The output readings are made by four analog meters and one digital meter. Current measurements can be made in the range of 5 µA–1 A by utilizing a three-meter array of analog amp meters. Voltage measurements can be made in the range of 1 mV–10 V utilizing both digital and analog meters. Voltage and current measurements can be taken at all points of the circuit since banana plugs and jacks are utilized to make all connections. General Meter Operation A gold layered slide is placed onto the base plate of the sample stage. The top plate of the sample stage (electrode housing) is secured to the base plate via registration pins and wing nuts. Power is applied to the system and various electrodes are tested for conductivity using the LED conductivity test circuit. If conductivity is detected, adjustments are
Figure 4. 1 in. ⫻ 3 in. base plate.
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In the Laboratory
made with the potentiometers to bring the voltage and current into a measurable range. A voltage reading is taken at the circuit just before the electrodes being measured and another reading after the voltage has passed through the gold layer. The difference in these two measurements is the voltage drop attributed to the gold layer. With the measurements of voltage and current, the resistance of the monolayers of gold can be measured. To measure the resistivity of the surface, a current is applied in series to the gold layered slide while voltage measurements are taken in parallel to the current. A full description of this procedure can be found in the Supplemental Material.W Electrical conductivity is a measure of how well a material accommodates the transport of electric charge and is the inverse of the calculated resistivity. Its SI-derived unit is the siemens per meter where 1 siemens is the equivalent of 1 second cubed ampere squared per kilogram per meter squared (1 s3 ⫻ A2 ⫻ kg᎑1 ⫻ m᎑2). Siemens is also the equivalent of 1 ampere per volt (A兾V). The electrical resistivity and conductivity can only be approximated because the thickness of the multiple layers can only be approximated. Additional Uses Fabrication of additional base plates of the sample stage will allow for the measurement of any other conductive surface, such as conductive solid or liquid samples. Since the system utilizes gold-plated electrodes, it is also possible to
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measure conductive liquids, except for those that contain aqua regia. W
Supplemental Material
A number of materials are available in this issue of JCE Online. These include: a complete parts list with part numbers and pricing; detailed annotated views of all exterior components; a complete wiring diagram; sample stage upper and lower plates design specifications; a detailed procedure for measuring current and voltage; and calculations for determining the resistivity and conductivity. Acknowledgments This work has been generously funded by the National Science Foundation through CAREER Award No. REC0346906. Literature Cited 1. Musick, Michael D.; Keating, Christine D.; Lyon, Andrew L.; Botsko, Steven L.; Peña, David J.; Holliway, William D.; McEvoy, Todd, M.; Richardson, John N.; Natan, Michael J. Chem. Mater. 2000, 12, 2869–2881. 2. Oliver-Hoyo, Maria; Gerber, Ralph W. J. Chem. Educ. 2007, 84, 1174–1176. 3. van der Pauw, L. J. Philips Research Reports 1958, 13, 1–9.
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