Plant’s Nanomachinery for Photosynthesis and Nanotechnology for
Solar Energy Conversion
I. Introduction
This education module is developed to help students and teachers understand the miscroscopic
photosynthetic chemical reaction by observing gas production by fresh spinach that convert CO2 to
and water into sugar, giving off oxygen under a simulated light source. Oxygen released from the
stomata of fresh spinach diffuses into water and then air which can be detected using a volumeric
sensor built by the module developers. Actual amount of oxygen production and its relationship to
light irradation time is measured quantitatively, and compared with two other control samples to
compare the gas releasing process and relative efficiency. One control sample is a artificial
photocatalyst made of TiO2 nanoparticles suspended in baking soda solution to convert CO2 and
water into oxygen and other products(e.g., methane and hydrogen). Understanding the energy
conversion capability of photosynthesis and the artificial nanostructured photocatalysts would not
only benefit public awareness of renewble energy conversion methods and nanotechnology, but
also the training and education of the next generation for advancing research nanosciences that
would help solve the challenging energy issues in the future.
II.
Engage
As a team of scientists working on the moon, the generation of oxygen is critical to survival. You
and your team members are working toward inexpensive, sustainable but efficient ways to generate
oxygen gas quickly. You are currently considering plants, like spinach for the production of
oxygen or other substances such as particular nanoparticles. Why would your team consider
using plants to generate oxygen? What would be needed for the plants to generate oxygen?
Have students (in partners) write their response to the scenario above.
Or you may choose to do the following:
Ask students what they know about photosynthesis, in particular what materials are needed for
photosynthesis and what products of the important energy conversion system. Allow them to
discuss their ideas to ascertain their prior knowledge of the photosynthetic process.
III. Materials check list
1: plastic knife X1
2: plastic spoon X1
3: baking soda X1
4: transparent plastic cup X2
5: 50 ml test tube X3
6: 4 L regular water
7: fresh spinach bag
8: TiO2 titanium dioxide nanoparticle solution X1
9: 1/16’’ ID rubber tube connected to a black rubber stopper for test tube (5) X 3
10: 1/16 ‘’ glass capillary tube X3
11: color indicator solution with Rhodamin B dye solution
12: 100 Watt desk lamp
13: clock for timing
14: waste bottle
15: gloves
16: plastic transfer pipette
17: graduated cylinder
Figure 1. Materials and tools provided for this activity
IV: Explore: Which design is most time efficient?
The purpose of the following activity is to detect gas product of photosynthesis using a volumetric
sensor, which indicates the amount of gas by measuring its volume, and then compare the gas
production efficiency with two control samples with soda solution in the presence and absence of
TiO2 nanoparticles. The motivation is to introduce artificial nanomaterials that can utilize solar
energy to convert CO2 to useful carbon hydrides and oxygen and strengthen our understanding of
photosynthesis.
A. Procedures (note: tools and items for the activity are indicated by the numerical label shown in
Figure 1.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
Measure 2 grams (~1plastic spoon) (2) of baking soda (3), and transfer it into one of the
clean plastic cups (4).
Add water into the above cup (4) to dissolve the baking soda powder by stirring the
solution with plastic knife(1), continue adding water to fill the plastic cup to obtain clear
aqueous solution of baking soda. Fill 80% of the cup with water to avoid spilling the
solution on table.
Label the three test tubes (5) with A, B and C, respectively.
Take 3-4 pieces of clean spinach leaves from fresh spinach bag (7), and cut them to
quarter inch in size and place all of pieces into test tube B.
Fully load the plastic transfer pipette (16) with 1% TiO2 nanoparticle solution (8), and add
the TiO2 nanoparticles into test tube C. Repeat the transfer step two more times.
Fill each of the test tubes A, B and C with <40 mL baking soda solution (prepared in step
b) using a graduated cylinder (17). Ask your team member to help hold the test tube in
vertical to avoid spilling of solution.
Plug the three back rubber stoppers (9) in all three individual test tubes A, B and C,
respectively to seal the necks of all tubes to prevent gas leakage.
Place all three test tubes in the second plastic cup (4), and fill the cup with ~160 ml water
(called a water bath). This would help place all test tube stand alone on a desk, and avoid
heating the test tube under light because gas can expand its volume by increasing its
temperature at constant pressure (ideal gas law!) At this point, you should keep all three
test tubes stable to avoid any volume changes to happen.
Take one of the three glass capillary tube (10) and check if it is clean and dry. Ask the
teacher to replace one for you if it already contains water droplet or contaminated. Dip
about 1 cm of one side of the tube into the color indicator (pink) solution (11). The pink
solution will be taken into the dipped portion of the tube due to capillary force of the
tube.
Make sure you turn off the room light. We will use a light source to trigger the
photosynthesis reaction in the following steps.
Insert the colored side of the capillary tube into the open rubber tube side of the test tube
while holding the capillary tube lying in horizontal position. Please not to apply too much
force when the capillary tube is inserted to the plastic tube to avoid breaking the glass
tube.
Repeat step i and j for test tube B and C using the other two available capillary tubes after
loading them with color indicator. Always hold the capillary tubes lying in horizontal
position and the solution in your test tube stable!
Place three capillary tubes loaded with pink color indicator on the printed paper ruler,
and align the end of the pink column along the zero cm as shown in Figure 2. Tube A
contains baking soda, water; B contains baking soda, water and spinach; and C contains
water, baking soda, and TiO2, respectively. Make sure to make a note to identify which
capillary tube belongs to which test tube. It is important not to apply any force to the
rubber stoppers or the plastic tube in order to maintain the zero location of the pink color
indicators.
Caution: Check the color indicators to make sure they are stable in the capillary tubes and
do not move; otherwise you will disconnect the capillary tube from the rubber tube and
reset the indicator to the end of the capillary tube, then make connection again.
m. Use a clock (13) to time the reaction of the three test tubes. Begin by turning on the light
and place the lamp about 15 cm away from the test tubes as shown in Figure 3. Record
the location of the pink color column along the paper ruler. Take one data point per 30
sec and record your data in table 1.
Figure 2. Alignment of the pink color indicator in capillary tube with a paper ruler.
Figure 3. Three test tubes containing soda water (A), spinach baking soda water (B), and TiO2
nanoparticles baking soda (C) under light illumination from a lamp.
30
60
90
120
150
180
210
240
270
300 330
360
390
Sec
A
B
C
Table 1. Record the location (by cm) of the pink color indicator column in each of the capillary
tube from A to C under light.
Graph the data for Test tubes A, B, and C (Distance vs Time)
IV. Explain
(1) Do you see any location change for any of the three pink color indicators? Explain how
you think the colored liquid in an empty capillary tube moves? Rank the speed of the three
pink colored liquids from fastest to slowest when they move inside the capillary tubes
under light.
(2) Which colored liquid did not move? How can you explain this?
(3) Which test tube (A, B and C) did the indicator move fastest? What are the contents in the
test tube? What do you think is causing the pink liquid to move?
(4) Explain why the test tube A and B are used for the movement of their corresponding pink
colored liquids?
(5) Compare test tube A and B, which one has chemical reaction occurring under light? What
are the products of the reaction? Why do you need spinach leaf?
(6) Examining test tube B, what is the purpose of the light? What is the purpose of baking
soda?
(7) Compare B and C, which indicator moves fast? Do both of them move? Explain what
might be produced in C if the pink colored liquid moves?
Explain what you think causes the difference in the movement of pink colored liquids in
test tube B and C?
Teacher Background and Discussion for the Explain:
Photosynthesis is one of the energy conversion processes taking place in plants that utilizes sun
light to power all kinds of activities of an organism. Typically, sun light, CO2 and water are
converted in the presence of chlorophyll into glucose, water while oxygen is released to the
atmosphere to be used by other living organism including ourselves. At the microscopic level,
photosynthesis takes place by absorbing sun light using the plant’s photosynthetic reaction centers,
the chloroplasts. The photosynthetic center contains chlorophylls that capture solar energy and
starts the process by splitting water with oxygen being given off through stomata. Hydrogen is then
chemically combined with carbon dioxide to form a simple sugar (C6H12O6) and water. During cell
respiration glucose in the cells is oxidized to release energy which is used to convert adenosine
diphosphate to adenosine triphosphate (ATP), the energy currency of the cells. ATP releases
energy for cellular activity when a phosphate is split off and it become ADP.
Photosynthesis is one energy conversion method that stores solar energy in chemical bonds that
can be used for many applications. Similar to several artificial energy conversion systems (e.g., solar
cell, and photoelectrochemical water splitting catalysts), photosynthesis produces useful energy that
can be used to keep organisms alive and to power cell activities. In comparison to photosynthesis,
the above-mentioned artificial energy conversion systems use more stable inorganic or organic
materials with much simpler charge storage/conversion configuration upon light absorption.
Energy conversion process of 25 nanometer (in diameter, 1 nanometer=10-9 meter) TiO2
nanoparticles is
Light (UV and blue) + TiO2 + CO2+H2O →H2 + CH4 + O2
TiO2 nanoparticles only absorb ultraviolet (UV) light with wavelengths less than 400 nm. When
light in this short wavelength is absorbed by TiO2, the electrons in TiO2 are excited to higher
energy levels. Since the excited electrons have very negative energy level they can reduce CO2
and proton to produce hydrogen gas and methane, while the positive charges left would oxidize
hydroxide to produce oxygen. Hydrogen production is important as it can be used as energy
storage to be supplied to fuel cells with high energy intensity, and the product of burning
hydrogen in oxygen is water so it serves as clean energy source without producing CO2.
Our module shows gas production of TiO2 is not as efficient as photosynthesis of spinach. This is
because: 1) TiO2 absorbs only light the portion of the light near ultraviolet light from the lamp
which is only a very small portion of the radiation of the solar energy (or our lamp), while
chlorophyll in spinach captures a range of wavelengths in the visible light region; 2) spinach uses
more sophisticated chemical reactions upon light absorption to convert to oxygen and CO2 to
food effectively; and 3) the structure of TiO2 nanoparticles are not optimized for effective charge
storage into chemical bonds. In addition, TiO2 sample contains small nanoparticles which total
surface area much large than spinach, and the light scattering by the nanoparticle may decrease the
actual light absorption by the nanoparticle.
Not like plants that needs to be regenerated to provide a nanomachinery of photosynthesis to
convert CO2 to other energy sources under light, photocatalysts like TiO2 has low cost and can
be reused over an extended period of time. Therefore this kind of artificial photocatalytic
nanomaterials can be used a pigment or as a UV absorber in sunscreens, and for helping clean
contaminates on its surface under sun light. TiO2 can also be used to destruct organic pollutants
in waste water. In this module, plant produces oxygen more efficient because of strong light
absorption in visible region than TiO2.
Other methods for detecting gas production of photosynthesis include:
1. Colormetric assay of iodine to show the presence of starch produced by photosynthesis
(http://www.hometrainingtools.com/starch-test/a/1497/)
2. Colormetric method of using dye
(http://www.dijitalimaj.com/alamyDetail.aspx?img=%7BB8A089A5-8ED5-4237-8131C87D5C91C33B%7D
3. Floating of spinach leaves (http://www.youtube.com/watch?v=i3D2L9DIUrQ)
4. CO2 sensor and gas sensors (http://www.hansatech-instruments.com/ciras2.htm)
V. Elaborate:
Now that you have carried out you experiments for examining the efficiency of oxygen production
with spinach vs titanium dioxide. As a scientific team member, you have to write up a short report
about which of the two designs (photosynthetic vs artificial systems)would be most effective for
oxygen production on the moon station. What are aspects that you will have to consider to make
this a long term, sustainable and inexpensive solution for oxygen production.
VI. Evaluation questions:
Based on the activities, discussion and above information on artificial photocatalyst (TiO2)
following questions can be discussed further to strengthen students’ understanding of these two
similar systems:
 How does surface area difference and actual amount of materials (reactants) affect the the
total amount of gas production efficiency?




How might one improve the TiO2 or Photosythetic systems to be more efficient?
What other methods might one can use to measure gas production?
Can CO2 reduction with TiO2 and photosynthesis help stop global warming?
Why do we still need other energy conversion systems like fuel cells, solar cells other than
photosynthesis?
Reference:
１． High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon
Fuels Oomman K. Varghese, Maggie Paulose, Thomas J. LaTempa, and Craig A.
Grimes，Nano Lett., 2009, 9 (2), 731-737
２． Titania based catalysts for photoreduction of carbon dioxide: Role of modifiers，
eyalakshmi, V，Mahalakshmy, R，Krishnamurthy, K R，Viswanathan, B，Indian
Journal of Chemistry -Section A (IJC-A), 2012, 51A, 1263-1283.
３． http://en.wikipedia.org/wiki/Photosynthesis