Materials Included In Kit

Additional Materials Required

Safety Precautions

Dilute hydrogen tetrachloroaurate solution may be irritating to the eyes, skin and gastrointestinal tract. Use a “hot hands” heat protector or gloves to handle hot glassware. The potential health effects of nanoparticles have not been fully identified. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The colloidal gold solution is very stable and may be stored indefinitely. Keep the solution in a dark bottle to avoid exposure to light. Because of the unknown potential health hazards of colloidal gold, we do not recommend disposing of colloidal gold down the drain. The colloid may be broken by adding 6 M hydrochloric acid, which precipitates the gold. Solid gold may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Excess hydrogen tetrachloroaurate solution should be stored for future use.

Lab Hints

• Using a more concentrated solution of HAuCl₄ in the colloidal gold preparation produces a cloudy, dark blue dispersion of gold nanoparticles. The blue mixture consists of larger particles (ca. 100 nm), and the visible absorbance shifts to longer wavelength. The product gives considerable light scattering.
• The wavelength of maximum absorbance at 520 nm correlates with the formation of gold nanoparticles having an average diameter of 20–40 nm. The “peak width at half-maximum” for this colloidal gold preparation is quite broad (130 nm) and indicative of a fairly wide distribution of particle sizes around the mean. In general, the wavelength of maximum absorbance shifts to higher wavelengths (> 520 nm) when the mean particle size increases above 40 nm, and the peak width increases when there is a larger variation in particle sizes. The color change of the gold colloid from red to blue when sodium chloride is added illustrates this effect. Adding NaCl, a strong electrolyte, shields the negative charges of the colloidal gold nanoparticles and causes them to clump together to form larger particles.
• Scanning tunneling microscopy (STM) is the most important tool for studying the size and shape of nanoparticles. Using a non-optical, scanning probe microscope, scientists are able to “see” individual atoms and molecules at a resolution of 0.2 nm (2 x 10^–10 m). A tiny electrical probe or stylus is moved across a surface, producing a weak electrical current between the tip and the surface. The locations of atoms on the surface are visualized as regions of high electron density due to changes in the magnitude of the current or the position of the stylus. Heinrich Rohrer and Gerd Binnig of the IBM Research Laboratory in Zurich, Switzerland, received the Nobel Prize in Physics in 1986 for their invention of the STM. Visit the official website of the Nobel Foundation at http://nobelprize.org (accessed November 2014) to view an impressive gallery of “atomic” photos obtained with the STM.

Answers to Prelab Questions

1. The line drawn on the side of this page is 220 mm long. Assume that this line represents the average diameter of a red blood cell, which is 1 micrometer or micron (1 μm = 1 x 10^–6 m).
   1. What is the diameter of a red blood cell in nanometers?
      {14036_PreLabAnswers_Equation_3}
   2. Calculate the length of the line segment in millimeters that would be needed to represent an object that is one nanometer in diameter on this line.

      If 1000 nm corresponds to 220 mm on the line, then 220/1000 = 0.22 mm would be needed to represent an object that is 1 nm in diameter.

2. Mark off line segments of the appropriate length on the line to represent (a) the average size of the influenza virus (100 nm), (b) a colloidal gold nanoparticle (40 nm) and (c) the width of the DNA double helix (2 nm).

   The length of the line segments should be (a) 22 mm for the influenza virus, (b) 9 mm for the gold nanoparticle and (c) 0.4 mm for the DNA helix.

3. Complete the following table to compare and contrast the properties of solutions, colloids and suspensions.
   {14036_PreLabAnswers_Table_2}

Sample Data

Ruby-Red Colloidal Gold

Observation Table

Spectrum of Colloidal Gold

Answers to Questions

1. Compare the Tyndall effect observed in colloidal gold (step 9) with what happens when light is shone through water. What is responsible for the Tyndall effect?

   Water is transparent—light passes through it, but the path of light is not visible. The Tyndall effect arises due to light scattering by larger particles in a colloid or suspension.

2. Plot the absorbance of colloidal gold versus wavelength on the following graph.
   {14036_Answers_Figure_1}
3. Relate the wavelength(s) of maximum light absorption to the observed (transmitted) color of the liquid.

   The wavelength of maximum light absorption occurs at 520 nm, in the green region of the spectrum. This is the complementary wavelength or color to the observed red (transmitted) color of colloidal gold.

4. Adding sodium chloride causes the gold nanoparticles in colloidal gold to clump together into larger particles. Based on the observed color change when NaCl was added to colloidal gold, would you expect its wavelength of maximum absorption to shift toward higher or lower wavelengths?

   The color changed from red to purple when sodium chloride was added. The complementary color or wavelength of light is yellow, 560–580 nm. The wavelength of maximum absorption would shift to higher wavelengths.

5. The colloidal gold in this experiment was prepared starting with 20 mL of 1 x 10^–3 M HAuCl₄.
   1. How many grams of gold (Au = 197 g/mole) are contained in the flask of colloidal gold?

      The mole ratio is one mole of gold produced per mole of HAuCl₄ added to the flask.
      1 x 10^–3 moles/L x 0.020 L x 197 g/mole = 0.0039 g of gold

       

   2. At a current price of $1225 per Troy ounce (1 Troy ounce = 31.1 g) for gold, how much is the gold in the flask worth?

      0.0039 g x (1 Troy ounce/31.1 g) × ($1225/Troy ounce) = $0.15 (The gold is worth fifteen cents!)

6. Estimate the number of gold atoms in a single gold nanoparticle that is 40 nm in diameter. Use the following assumptions: (a) The radius of a gold atom is 0.15 nm. (b) Both particles are the shape of a sphere. The volume of a sphere is 4/3πr³, where r is the radius. (c) Only 74% of the total volume of the nanoparticle is physically occupied by gold atoms. (The rest of the volume is “empty space” between atoms.)

   The total volume of the gold nanoparticle is equal to 4/3(3.14)(20 nm)³ or 3.3 x 10⁴ nm³
   The volume of a gold atom is equal to 4/3(3.14)(0.15 nm)³ or 0.014 nm³.
   The effective volume of the nanoparticle that is occupied by gold atoms is 74% of the total volume, or (0.74)( 3.3 x 10⁴ nm³) = 2.4 x 10⁴ nm³.
   The approximate number of gold atoms can be obtained by dividing the effective volume of the nanoparticle by the volume of one gold atom:

   {14036_Answers_Equation_4}
   There are almost two million gold atoms in a single nanoparticle!

References

Liz-Marzán, Luis M. “Nanometals: Formation and Color” Materials Today 2004, 7, 26–31.

Turkevich, J. “Colloidal Gold. Part I. Historical and Preparative Aspects, Morphology and Structure” Gold Bulletin 1985, 18, 86–91.

Introduction

From nanotech fibers and nanosensors to nanobots, nanotechnology has created so much “buzz” that it is hard to tell where the science ends and the science fiction begins. Wherever it may lead in the future, the science of nanotechnology begins with solid particles called nanoparticles that are 1–100 nm in size. Shrinking the size of solid-phase particles to the nanometer scale—one billionth of a meter—changes their physical and chemical properties. The surprising properties of “colloidal gold” are a good example of this phenomenon. Whereas normal or “bulk” gold is a bright, shiny, metallic yellow, colloidal gold nanoparticles are red or blue and not at all shiny. Let’s investigate the preparation, properties and uses of colloidal gold.

Concepts

• Nanotechnology
• Colloids vs. solutions
• Redox reaction
• Metric measurements

Background

Nanoscience or nanotechnology involves the preparation, characterization, and uses of nano-sized particles having dimensions in the 1–100 nm range. Nanoparticles have unique physical and chemical properties that are significantly different from the macroscopic properties of traditional or bulk solids. Many of these properties have taken on special importance in recent years as applications of nanotechnology have been developed. In particular, the electronic, magnetic and optical properties of nanoparticles have proven to be very useful in the creation of new products using nanotechnology. Quantum dots, for example, are nanocrystalline fluorescent semiconductors that are used in high definition DVD players and video game consoles.

Gold nanoparticles are one of the most widely used materials in nanotechnology, and they are certainly the oldest. Colloidal gold consists of gold(0) nanoparticles that range in size from approximately 5–50 nm and are uniformly dispersed in water. Michael Faraday (1791–1867) published a scientific report of the preparation and properties of colloidal gold in 1857, but the art of using colloidal gold as a decorative pigment in glassmaking dates back more than 1000 years. The Lycurgus Cup at the British Museum in London, England, a Roman artifact from the fourth century, is the most famous example of ancient “gold ruby glass.” The presence of tiny, 50-nm crystals of silver and gold “dissolved” in the glass gives the cup a lustrous red appearance when light shines through it. This demonstrates the most striking and beautiful feature of gold nanoparticles—their color. Depending on the size and shape of the particles, the color of gold nanoparticles varies from red to purple. The optical properties of gold nanoparticles are not only unique, they are also useful, providing the basis for commercial products such as medical diagnostic kits for HIV detection, biosensors for DNA analysis, lasers, and optical filters. From a chemical standpoint, gold nanoparticles have a very active surface chemistry and are thus valuable catalysts for pollution control, fuel cells, and the synthesis of specialty chemicals. The ability of various electron-donating groups to bind to the surface of gold nanoparticles is being investigated as a way to direct the self-assembly of complex structures for futuristic nanocomputers and other nanoelectronic devices.

The most common method for the preparation of colloidal gold involves the reduction of gold(III) ions by citrate ions in dilute (0.1 mM) aqueous solution. The gold(III) ions are usually added to water in the form of hydrogen tetrachloroaurate (HAuCl₄). Citrate ions (C₆H₅O₇^3–), which are most likely oxidized to acetone dicarboxylate ions (C₅H₄O₅^2–) in the process, act as a two-electron reducing agent. The half-reactions for the synthesis of colloidal gold are given in Equations 1 and 2.

The average diameter of gold nanoparticles produced by this method depends on temperature and the concentration ratio of gold(III) ions and citrate ions in solution. The gold nanoparticles are stabilized by the presence of citrate ions adsorbed on the surface of the particles. Adsorption of citrate ions gives the gold particles an overall negative charge and is the principal factor responsible for the formation of a stable colloid. Mutual repulsion of the small, negatively charged particles prevents them from coagulating to form larger particles that might eventually settle out of solution.

The absorbance of visible light by gold and other metal nanoparticles has been attributed to a unique phenomenon called surface plasmon resonance (SPR). This phenomenon is very different from the “normal” visible spectrum of colored dye molecules, which is due to the promotion of electrons from the ground state to an excited state when light of a specific wavelength is absorbed. SPR is defined as the “collective oscillation of conduction band electrons resulting from the interaction with electromagnetic radiation.” In laymen’s terms, the incoming electromagnetic radiation induces the formation of a dipole on the surface of a nanoparticle, which then oscillates in phase or in resonance with the electric field of the incoming light. This occurs at a specific frequency (and wavelength or color) of light, depending on the size, shape and form of the nanoparticles.

Solutions and colloids, which differ in the size of the particles that are dispersed throughout a continuous phase, may be distinguished based on their properties. Colloids are defined as mixtures in which the dispersed particles are small enough to pass through a filter but too large to pass through a semipermeable membrane. The particles in a colloid are large enough that they will reflect or scatter light in all directions. The scattering of light by particles in a mixture is called the Tyndall effect and makes it possible to view a beam of light as it passes through a colloid or a suspension. In a true solution, the dispersed particles are too small to scatter.

Experiment Overview

The purpose of this activity is to prepare colloidal gold and investigate its properties.

Materials

Prelab Questions

1. The line drawn on the side of this page is 220 mm long. Assume that this line represents the average diameter of a red blood cell, which is 1 micrometer or micron (1 μm = 1 x 10^–6 m).
   1. What is the diameter of a red blood cell in nanometers?
   2. Calculate the length of the line segment in millimeters that would be needed to represent an object that is one nanometer in diameter on this line.
2. Mark off line segments of the appropriate length on the line to represent (a) the average size of the influenza virus (100 nm), (b) a colloidal gold nanoparticle (40 nm) and (c) the width of the DNA double helix (2 nm).
3. Complete the following table to compare and contrast the properties of solutions, colloids and suspensions.
   {14036_PreLab_Table_1}

Safety Precautions

Dilute hydrogen tetrachloroaurate solution may be irritating to the eyes, skin and the gastrointestinal tract. Use a “hot hands” heat protector or gloves to handle hot glassware. The potential health effects of nanoparticles have not been fully identified. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron.

Procedure

1. Measure 20 mL of 1 mM hydrogen tetrachloroaurate solution in a graduated cylinder, and pour the solution into a 250-mL Erlenmeyer flask.
2. Add distilled water to the 200-mL mark on the Erlenmeyer flask, and place a glass stirring rod into the solution.
3. Place the Erlenmeyer flask on a hot plate at a medium-high setting and heat the diluted gold chloride solution to a gentle boil.
4. Using a graduated, Beral-type pipet, add 2 mL of 1% trisodium citrate solution to the boiling solution in the Erlenmeyer flask.
5. Observe and record the color changes in the solution as gold(III) ions react with citrate.
6. Continue heating the solution at a gentle boil for approximately 10 minutes until the solution is ruby- or wine-red and the color no longer changes.
7. Carefully remove the Erlenmeyer flask from the hot plate using a heat protector. Place the flask on a heat-resistant surface or ceramic fiber square and allow the solution to cool. Add distilled water to bring the total volume of liquid back up to 200 mL, if necessary.
8. When the solution has cooled to room temperature, pour some of the “colloidal gold” into a small beaker. Observe and record the properties of the product.
9. Shine a laser pointer or flashlight through the colloidal gold solution and observe the “path” of the light through the solution. Record observations.
10. Pour the colloidal gold into two medium-size test tubes, filling each test tube about one-third full.
11. Add an equal volume of distilled water to the first test tube and an equal volume of 1 M sodium chloride solution to the second test tube. Carefully swirl each test tube to mix the contents. Observe and record any color changes.
12. Fill a cuvet approximately two-thirds full with colloidal gold solution and measure the absorbance every 20 nm from 420 to 700 nm using a spectrophotometer.

Student Worksheet PDF

14036_Student1.pdf