Student Sheet part A: Silver Nanoparticle Synthesis
Introduction
Scientists have recently begun studying and preparing materials which have
intrinsic lengths scales on the order of 1-100 nm (10-9 to 10-7 m). The science of
materials with this length scale is known as nanotechnology.
One of the fundamental mandates and lures of nanotechnology is that material
properties change, sometimes drastically, as the size scale of materials is reduced. On the
simplest terms, we know that solid bulk materials do not appear solid at all on the atomic
scale; most of the atom is in fact empty space.
More recently, some sunscreen manufacturers have begun to use zinc oxide
sunscreens within which the oxide particles are nanoscale. These sunscreens are
transparent (colorless) in the visible region whereas the normal zinc oxide sunscreens
tend to be white and opaque.
Zinc Oxide nanoparticles
Conventional Zinc Oxide
suncreen
Commercially available
product incorporating nanosized zinc oxide particles
http://www.consumerreports.org/cro/health-fitness/nanotechnology-7-07/nanoparticles-in-sunscreens/0707_nano_sunscreen_1.htm
http://science.howstuffworks.com/nanotechnology.htm/printable
http://www.advancednanotechnology.com/zinc_oxide.php
In the lab you will complete, silver is being reduced. In chemical terms, a
chemical is reduced when it has one or more electrons added to it. At the same time,
another chemical will be oxidized, meaning it has one or more electrons taken from it.
Since reduction and oxidation always go in tandem, any reaction where electron(s) are
transferred from one chemical to another is called a redox (reduction-oxidation) reaction.
A useful mnemonic device to remember about redox reactions is OIL RIG.
OIL
Oxidation is Losing electrons
RIG
Reduction is Gaining electrons
“Bulk” silver means silver which is large enough to be seen with the naked eye.
http://z.about.com/d/chemistry/1/0/h/6/silver.jpg
Bulk silver is often made by reducing silver nitrate with a more active metal. The
reaction with magnesium is shown below.
Oxidation Number
2AgNO3 (aq) + Mg  Mg(NO3)2 + 2Ag (s)
+1
0
+2
0
In this reaction, silver (Ag) is reduced because it turns from +1 ions into neutral atoms.
This happens when each +1 ion gains an electron, neutralizing it. Magnesium is
oxidized when it changes from a neutral atom to a +2 ion. Each magnesium atom loses
two electrons. Remember, Oxidation is Losing, Reduction is Gaining.
In this lab you will produce silver, but it will not be bulk silver. Instead, you will
make nanoparticles of silver; silver particles which have a size on the order of 12
nanometer (0.000000012 m).
Directions:
Prelab
Complete a prelab with the normal components.
a. Answer the following prelab questions.
- What happens when a compound is reduced?
One or more electrons are added to a compound when it is
reduced.
- How do you reduce a silver ion?
You add an electron to a silver ion to reduce it.
- Describe some of the material properties of bulk silver.
Bulk silver is conductive, malleable, “silver” colored, reflective
when polished, more dense than water, etc.
-What do you think will happen when you add the sodium
borohydride in step 7)? You may not have any idea, but you can
still make an educated guess.
Answers will obviously vary a lot.
b. Summarize the procedure for the lab (given below) in your laboratory
notebook using pictures to represent the steps.
c. Make a list of the materials you will be using, based on the procedure.
d. Construct a data table. What data will you be collecting?
Procedure
1.) Label a 20 mL glass vial with your name. Using a glass graduated cylinder, measure
9 mL of deionized water and pour it into the vial.
2.) Using a plastic syringe, add 1.0 mL of 1.0 mM AgNO3 to the glass vial.
3.) Add 0.6 mL of 0.7 mM PVP using a plastic syringe into the vial.
4) Add 0.6 mL of 3 mM sodium citrate using a plastic syringe into the vial.
4.) Add 0.25 mL of 3% H2O2 using a plastic syringe in the same vial.
5.) Put the cap on the vial and invert several times in order to mix the reagents
completely.
6.) Check with your teacher to determine how much of the 10 mM NaBH4 to add.
Quickly cap the vial and begin to shake vigorously. (Record the amount of NaBH4 added
to the vial.)
7.) The solution should turn a very pale yellow color at first (A good indicator that the
synthesis is proceeding well). After shaking the vial for additional 5-10 minute,
something very interesting should occur. Continue to shake for a full two minutes after
the change occurs. Make sure to record your observations thoroughly.
8) Record the results of at least four other groups in the class in your data table.
9) (Optional), if equipment is available… Use a spectrophotometer to test the sample
which you prepared. Refer to the specific directions provided or verbally given by your
teacher.
OBSERVATIONS:
All of the liquids used in this lab are initially colorless and transparent. None
have any particle odors. After step 5) in the procedure there is no noticeable change in
the mixture. When the sodium borohydride is added and vigorously shaken, there is
some evidence of gas production, likely due to the decomposition of the hydrogen
peroxide. The reaction also produces gases, but in small amounts. There is also a faint
but definite yellow tint initially which disappears in about 10 seconds as the reaction
flask is shaken. After 5-10 minutes, there is a strong yellow which appears very
dramatically. After another 30 seconds or so, the flask switches colors, moving from the
yellow through oranges, pinks, purples, and blues, depending on how much sodium
borohydride was added.
SAMPLE DATA TABLE:
Amount of Sodium Borohydride Added
(mL)
0.3
0.32
0.35
0.4
0.45
0.5
Final Color Attained
Yellow (max ~ 465 nm)
Light pink (max ~ 505 nm)
Darker Pink (max ~ 510 nm)
Violet (max ~ 545)
Blue/Purple (max ~ 615)
Blue (max ~ 650)
Postlab:
When you are finished with the laboratory, and while you are waiting to use the
spectrophotometer, complete the following questions.
1) Check: have you recorded observations, constructed a data table, and added your own
data as well as that from at least four other groups?
2) What was the independent variable in this lab? The dependent variable?
The independent variable is the amount of sodium borohydride which is added. The
dependent variable which is visible is the color of the solutions. The implied dependent
variable is the same and geometry of the nanoparticles.
3) This reaction produces solid silver. Does the silver look like normal bulk silver?
No, the silver is not visible and does not have the bulk properties. It is not metallic, and
is not silver colored.
4) Why do you think that different colors originate? What’s different in the different
solutions? Why do things appear different colors?
These are very difficult questions for students to answer. Remember that there are no
right answers unless you’ve specifically taught color theory before. Read the teacher
background section for more information on the actual answers to these questions.
5) Pick a color which you did not make. Explain how you might go about producing a
solution with that color. Try it.
There are at least two approaches. Students could mix together two other colors, just like
they do in paint theory (additive addition of colors). For example, blue and yellow
solutions could be combined to obtain a green solution. Alternately, students might
realize that the amount of NaBH4 added leads to different colors. They could ask another
group how their color was made, or extrapolate between two colors to estimate the
amount of NaBH4 which must be added to produce a new color between the other colors.
This question taps into any prior knowledge students have about color theory and
ROYGBIV.
6) What might these solutions be used for?
Again, I suspect your students will come up with numerous creative ideas. They could be
used as dyes, obviously. Silver is known to be antibacterial, so maybe germ deterrent
clothing?