Nanomaterials Monitoring Reactions

Syracuse University Chemists have designed a nanomaterial that changes colour when it interacts with ions and other small molecules during a chemical reaction which enables them to monitor the progress of chemical reactions qualitatively with the naked eye and quantitatively using simple instruments.

Many chemical reactions that occur in aqueous solution involve colourless species. In order to determine how fast the chemical reaction occurs,  Chemists have traditionally tried to "freeze" the reaction at certain points, purify the solution and determine the amounts of unreacted reactants and products produced present at each stage.
Syracuse University Chemists have taken a different route. They are using nanoparticles that react with the byproduct of a reaction. The nanoparticles they used are known as perovskites.
Perovskites are typically composed of metal ions and oxygen. The structure shown below is for a typical perovskite, calcium titanium oxide (CaTiO₃):

Each pale-blue titanium atom is surrounded by 6 red oxygen atoms. The darker-blue calcium atom occupies the space between titanium oxide octahedrons.
The perovskites the researchers used were a bit different to the one shown above. Metal ions were surrounded by halide ions rather than oxygen.
At the nanolevel, perovskites are photo-luminescent, that is, they emit light when "excited" by a laser or a lamp. The colour they emit is largely determined by the concentration of their ions in solution., and it is this property which the researchers used to monitor chemical reactions. It is also this property which is being in exploited in research into light emitting diodes (LEDs), lasers, photodetectors and solar cells.

In this study, perovskites were used to monitor an elimination reaction in which haloalkanes react to form alkenes, eliminating halide ions in the process.
At the start of the reaction, the perovskite fluoresces red.
As the reaction proceeds, halide ions are released which are absorbed by the perovskite nanoparticles, and the fluorescence colour changes from red to yellow to green.
When the fluorescence colour is green, the reaction is over.
The image on the right shows a control colour on the left, and on the right, the changing fluorescence colour of the reaction as it proceeds from 0 minutes at the top to 90 minutes at the bottom.

This technology is patent-pending at the University. In the words of Matthew Maye, Associate Professor of Chemistry, "Who knows, maybe in the future, every chemist will use a Syracuse-based perovskite for monitoring their reactions."

Reference:
Tennyson L. Doane, Kayla L. Ryan, Laxmikant Pathade, Kevin J. Cruz, Huidong Zang, Mircea Cotlet, Mathew M. Maye. Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution. ACS Nano, 2016; DOI: 10.1021/acsnano.6b00806

Further Reading:
Nanotechnology: http://www.ausetute.com.au/nanotech.html
Reaction Rate: http://www.ausetute.com.au/reactrate.html
Ligands and Complex Ions: http://www.ausetute.com.au/ligands.html
Naming Haloalkanes: http://www.ausetute.com.au/namhaloa.html
Naming Alkenes: http://www.ausetute.com.au/namsenes.html
Substitution Reactions of Haloalkanes: http://www.ausetute.com.au/rxreacts.html
Dehydration of Alkanols: http://www.ausetute.com.au/dehydraol.html

Suggested Study Questions:

1. Explain the terms "qualitative" and  "quantitative".
2. Explain the term "reaction rate".
3. Explain the term "nanoparticle".
4. What property of nano-perovskite is being applied by the researchers in this article, and how does this property differ for bulk perovskite?
5. Explain how these perovskites can be used to monitor the reaction qualitatively.
6. Explain how you could use these perovskites to monitor the reaction quantitatively.
7. Discuss the differences between ethane, ethene (ethylene) and bromoethane.
8. Consider ethane and ethene (ethylene), which is likely to be more chemically reactive? Explain your answer.
9. Consider ethane and bromoethane. Which is likely to be more chemically reactive? Explain your answer.
10. Explain what is meant by the term "elimination reaction" as used in the article above.
11. What is the difference between and addition reaction, a substitution reaction and an elimination reaction? Give examples of each type of reaction.
12. Write a chemical reaction to represent the elimination of bromide ions from a bromoethane to produce ethene (ethylene). 
13. Consider the structure of CaTiO₃ given in the article. What is the name of the ligand?
14. Give the formula for the perovskite in which all the oxygen atoms have been replaced with bromine.
15. Could the same perovskite be used to monitor a chemical reaction in which water is eliminated from an alkanol to produce an alkene? Explain your answer.

Uranium Glass

Uranium compounds have been used to colour glass since ancient times. A mosaic found in a Roman villa dated to about 79 AD used yellow glass containing 1% uranium oxide. Between 1880 and the 1920s, uranium glass was quite popular, not only because of its interesting yellow-green colour, but because it fluoresces in ultra-violet light as shown in the image of a vase below.
The image on the left hand side shows the vase under normal daylight, while the image on the right shows the same vase in the dark exposed to UV light.

Why does uranium glass glow green in the dark like this?
Well, it has nothing to do with the radioactivity of uranium isotopes. The green glow does not occur because of changes within the nucleus of uranium atoms, it occurs because of changes in the energy of electrons surrounding the nucleus.
A ground state electron in a uranium atom absorbs the energy of a photon  of ultra-violet light causing the electron to jump to a higher energy level known as the excited state. This new excited state electron configuration is not stable, so, the electron falls back down to to the ground state energy level, which is a lower energy state, by losing energy which it does by emitting a photon of light. Some of the energy of the original photon used to excite an electron is dispersed as molecular vibration and heat, so the energy of the photon emitted when the excited electron falls back to the ground state is less than the energy originally absorbed.
That is: E = hν/λ
where E = energy, h = Planck's constant, ν = speed of light , λ = wavelength of light
If the speed of light is a constant, then E ∝ 1/λ or Eλ = a constant
in other words, the more energy the photon of light has the shorter its wavelength is.
In the case of uranium glass, the absorbed photon of ultra-violet light has a wavelength of about 300 nm, while the emitted photon of green light has a wavelength of about 550 nm.
The wavelength of the emitted photon of light corresponds to the green part of the visible spectrum.
The emission of visible light in this way is known as fluorescence.

Further Reading:
Isotopes
Radioactivity: Properties and Uses
Emission Spectra
Flame Tests

Suggested Study Questions

1. Uranium is found in nature as one of three isotopes, uranium-234, uranium-235 and uranium-238. Explain how atoms of each isotope are:
• the same
• different
2. Atoms of neptunium-234, neptunium-235 and neptunium-238 have been synthesized. Explain the similarities and differences between each of the following pairs of atoms:
   • neptunium-234 and uranium-234
   • neptunium-235 and uranium-235
   • neptunium-238 and uranium-238
3. The value for Planck's constant is 6.626 070 040 x 10^-34 J s and the speed of light is given as 300 000 000 m / s. Calculate the energy of each of the following photons of light:
• ultra-violet light, λ = 300 nm
• green light, λ = 550 nm
4. Using the results of question 3, compare the energy of the photons used to excite uranium atoms and the energy of the photons emitted by uranium atoms and explain the difference.
5. Explain why the same wavelength of light is always emitted when these uranium atoms are exposed to ultra-violet light.
6. Would you expect the uranium glass to fluoresce if it is exposed to infra-red light rather than to ultra-violet light? Explain your answer.
7. Optical brighteners are often used in laundry detergents to make your old, yellowy-looking white clothes look whiter. Typically these compounds absorb ultra-violet light with a wavelength of around 300 nm and emit light with a wavelength of around 450 nm.
   • What colour is the light emitted?
   • What is the energy of the emitted light?
   • Why is the energy of the light emitted not the same as the energy absorbed when the incoming photon hits the "optical brightening compound"?
   • Would you expect a laundry detergent containing "optical brightening compounds" to glow in the dark when exposed to UV light? Explain your answer.

Glow in the Dark Ice

No (chemistry) party is complete without edible "glow in the dark" ice cubes.
To make your ice cubes:

•  open up some tonic water (or a bottle of "bitter lemon")
• pour it into an ice cube mold
• place the mold in the freezer until the tonic water solidifies (freezes)

To make your ice cubes glow in the dark:

• place some ice cubes in a glass of water (or cordial or carbonated beverage)
• place the glass under a UV ("black") light (even strong fluorescent light will work but the effect is not as dramatic!) and turn off the room's lights

Results : your glass of water and ice should glow a nice blue colour.


This is an example of fluorescence, the emission of light by a substance that has absorbed electromagnetic radiation. In the case of the tonic water, there is a compound in the tonic water that absorbs light in the ultraviolet region of the electromagnetic spectrum (wavelength ~ 350 nm), and emits light in the visible region of the electromagnetic spectrum (wavelength ~ 450 nm corresponding to bright blue or cyan).

The compound in the tonic water that fluoresces is known as quinine, with the molecular formula C₂₀H₂₄N₂O₂ and the structural formula shown below:

Quinine occurs naturally in the bark of the cinchona tree which is found in the tropical Andes forests of western South America. Quinine was the first effective treatment for malaria. The first medicinal uses involved drying the bark of the cinchona tree then grinding it into a powder which was then mixed into a drink (often wine), which was then drunk. The effective medicinal ingredient of the bark, quinine, was finally isolated in 1820 by French researchers Pierre  Joseph Pelletier and Joseph Bienaime Caventou.
During World War II, the Axis Powers had control over most of the commercial quinine production centres, so the Allied Powers were cut off from their supply of quinine, a necessary war-time commodity for fighting in the tropics. Then in 1944, the American chemists R.B. Woodward and W.E. Doering succeeded in producing quinine in the laboratory.




Quinine is no longer recommended as a first-line treatment of malaria, instead another plant-derived organic compound is used, artemisinin, but that's a different story.

Further Reading:
Emission Spectroscopy
Empirical Formula
Relative Molecular Mass (molecular weight, formula mass, formula weight)
Percent Composition
Parts per Million Concentration
Functional Groups


Suggested Study Questions:

1. Draw a diagram to describe what happens when quinine absorbs UV light and emits bright blue/cyan light.
2. Imagine you were to view the light emitted by the tonic water through a spectroscope. Draw a sketch of the emission spectrum you would expect to see.
3. Use the molecular formula for quinine to find its empirical formula.
4. Calculate the relative molecular mass (molecular weight, formula mass, formula weight) of quinine.
5. Calculate the percentage of each element present in quinine.
6. Assume a 1L bottle of tonic water contains 15ppm quinine. Calculate the
   • mass of quinine contained in the bottle
   • moles of quinine in the bottle
   • quinine concentration in mol L^-1
7. Using the structural formula for quinine, identify an aliphatic double bond (that is, a double bond that does not occur in an aromatic ring), an aromatic (benzene) ring, and an hydroxyl group

Fluorescence and Counterfieting

Fluorescence is an emission of electromagnetic radiation. Certain organic dyes are fluorescent, when hit within a certain wavelength range they emit light at a greater wavelength. The wavelength and intensity of this light depends on the physical and chemical properties of the materials to which the dye was applied.

To make a product counterfeit-proof, multiple dyes can be added to the material in very minute quantities, such as just a few parts per billion, to create a marker. At such low concentrations, the marker is virtually impossible to decode, and the counterfeit protection cannot be removed since it permeates the whole material. Fluorescent dye markers produce precise characteristics so it is easy to recognize if the material is original or if it is a copy.

Reference:
Fraunhofer-Gesellschaft (2010, June 15). Brilliant counterfeit protection. ScienceDaily. Retrieved June 18, 2010, from http://www.sciencedaily.com­ /releases/2010/06/100614093627.htm

Study Questions

1. What is meant by the term electromagnetic radiation?
2. Fluorescein is an example of a fluorescent dye that has been used as a marker for airplanes downed at sea. Find the structural formula for fluorescein.
3. Some fluorescent compounds are used as fabric whiteners in washing powders and are referred to as optical bleaches. Find an example of an optical bleach and describe how it works.
   
4. A number of naturally occurring minerals are fluorescent. Can you list examples of these?
5. Phosphorescence is similar to fluorescence. What is the difference between phosphorescence and fluorescence?
6. If an aqueous solution of fluorescein has a concentration of 175ppb, what is its equivalent concentration in,

• ppm
• parts per hundred
• g/mL
• μg/mL
• mol/L