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Nanowire pH Sensors


CA State Teaching Standards 


3.  Heat and Thermodynamics

d. Students know that most processes tend to decrease the order of a system over
time and that energy levels are eventually distributed uniformly.

Polyaniline can be doped with extrinsic dopants such as acids. The new levels in conductivity are due to the changes in the chemical potential of the material. The addition of a donor level within the polymer. Over time these levels are distributed uniformly as the doping levels revert to an undoped state due to interactions with the environment.

e. Students know that entropy is a quantity that measures the order or disorder of a
system and that this quantity is larger for a more disordered system.

Polyaniline nanofibers form intrinsically and the entropy of the system increases as molecular chains become entangled within each other. Polymer formation and its morphology is governed by an entropic formalism. Nanofibers look ordered but the molecular chains that compose them are disordered.

4.  Waves

e. Students know radio waves, light, and X-rays are different wavelength bands in
the spectrum of electromagnetic waves whose speed in a vacuum is approximately
3Ê _Ê 108 m/s (186,000 miles/second).

Color in polyaniline can be explained via band-theory. The electromagnetic spectrum of visible light possesses the required amount of energy to excite electrons and cause color change. This happens as charge carriers are promoted from the valence band to the conduction band.

5.  Electric and Magnetic Phenomena

d. Students know the properties of transistors and the role of transistors in electric

Polyaniline is an organic semiconductor which can be used in p-n junctions, and therefore can serve as a transistor. I-V curves would show rectifying behavior.


2.  Chemical Bonds

a. Students know atoms combine to form molecules by sharing electrons to form
covalent or metallic bonds or by exchanging electrons to form ionic bonds.

Covalent linkages connect atoms in polyaniline; conductivity is one dimensional as charge carriers travel via conjugated bonds. This is different from a three dimensional conductor that possesses metallic bonds but it allows for an interesting comparison.

c. Students know salt crystals, such as NaCl, are repeating patterns of positive and
negative ions held together by electrostatic attraction.

Dopants protonate the backbone of the polymer chain but counter anions remain electrostatically coupled to the positive charge on the polymer.

3.  Conservation of Matter and Stoichiometry

d. Students know how to determine the molar mass of a molecule from its chemical
formula and a table of atomic masses and how to convert the mass of a molecular
substance to moles, number of particles, or volume of gas at standard temperature
and pressure.

Polymers offer the concept of average molecular weight, and the concept of polydispersity due to a distribution of molecular weights present in the final product.

e. Students know how to calculate the masses of reactants and products in a chemical
reaction from the mass of one of the reactants or products and the relevant
atomic masses.

Aniline is the monomer and ammonium peroxydisulfate is used in the synthesis of polyaniline, a ratio of 1:4 (oxidant: monomer) is used. Atomic masses are available in the MSDS of this materials (free of charge in the internet). Assuming a final molecular weight of 30,000 g/mol and an overall yield of 35% vs. monomer, student can therefore carry out a simple calculation.

f.* Students know how to calculate percent yield in a chemical reaction.

A 35% yield vs. monomer takes place in polyaniline synthesis.

g.* Students know how to identify reactions that involve oxidation and reduction and
how to balance oxidation-reduction reactions.

Polyaniline is synthesized via a radical oxidative polymerization.

5.  Acids and Bases

a. Students know the observable properties of acids, bases, and salt solutions.

Polyaniline is doped by an acid and dedoped by a base, protonation, and deprotonation respectively. Since this behavior is reversible the polymer is stable under acidic and basic conditions.

e.* Students know the Arrhenius, Brønsted-Lowry, and Lewis acid–base definitions.

Proton donor and acceptors (Brønsted-Lowry) apply for polyaniline chemistry.

6.  Solutions

d. Students know how to calculate the concentration of a solute in terms of grams per
liter, molarity, parts per million, and percent composition.

Polyaniline nanofibers are dispersed in a medium and concentration in dispersions are calculated in terms of grams / liter of material.

f.* Students know how molecules in a solution are separated or purified by the methods
of chromatography and distillation.

Our polymer can be purified by centrifugation or dialysis, however the monomer is purified by distillation which removes color impurities and can affect percent yield. Morphology isn’t affected.

8.  Reaction Rates

b. Students know how reaction rates depend on such factors as concentration, temperature, and pressure.

Polyaniline can be synthesized with or without initiators which accelerate the reaction rate. Also if synthesized at higher than room temperature this synthesis yields higher percentages of final product.

10.  Organic Chemistry and Biochemistry

a. Students know large molecules (polymers), such as proteins, nucleic acids, and
starch, are formed by repetitive combinations of simple subunits.

Macromolecules like polyaniline are made up of monomers.

b. Students know the bonding characteristics of carbon that result in the formation of
a large variety of structures ranging from simple hydrocarbons to complex polymers
and biological molecules.

The backbone of polyaniline is a hydrocarbon chain containing a heteroatom, nitrogen.

e.* Students know how to identify the functional groups that form the basis of
alcohols, ketones, ethers, amines, esters, aldehydes, and organic acids.

When other functionalized monomers are used like 2-chloroaniline the final product will be a halogenated polymer. Other functionalities are possible.