Transcript Lecture 19_Anal_Tech_Part2

Nanochemistry
NAN 601
Instructor:
Dr. Marinella Sandros
Lecture 19: Analytical Techniques Part 2
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Singlet
Triplet
Quadruplet
2960
1740
1370
1050
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Zeta Potential
X-ray Diffraction (XRD) Spectroscopy
Transmission Electron Microscopy
Scanning Electron Microscopy
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Almost all particulate or macroscopic
materials in contact with a liquid acquire an
electronic charge on their surfaces.
Zeta potential is an important and useful
indicator of this charge which can be used to
predict and control the stability of colloidal
suspensions.
The greater the zeta potential the more likely
the suspension is to be stable because the
charged particles repel one another and thus
overcome the natural tendency to aggregate.
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The measurement of zeta potential is often
the key to understanding dispersion and
aggregation processes in applications as
diverse as water purification, ceramic slip
casting and the formulation of paints, inks
and cosmetics.
Characteristics of Surface Charge: Definitions
Particle surface
Stern Layer: Rigid layer of ions
tightly bound to particle; ions travel
with the particle
Plane of hydrodynamic shear:
Also called Slipping Plane:
Boundary of the Stern layer:
ions beyond the shear plane do
not travel with the particle
Diffuse Layer: Ions are less firmly
firmly associated
Characteristics of Surface Charge: Definitions
Zeta potential:
The electrical
potential that
exists at the
slipping plane
The magnitude of the zeta potential gives an indication of the
potential stability of the colloidal system
* If all the particles have a large zeta potential they will repel each other
and there is dispersion stability
* If the particles have low zeta potential values then there is no force to
prevent the particles coming together and there is dispersion instability
Zeta Potential and Electrophoretic Mobility
In an applied electric field, charged particles travel
toward the electrode of opposite charge.
When attractive force of the electric field is balanced
by the viscous drag on the particle, the particle
travels with constant velocity.
+
+
-
-
This velocity is the partlcle’s electrophoretic mobility, UE
UE = 2  z f(Ka)/3 
z = Zeta potential
 = dielectric constant (of electrolyte)
= dielectric viscosity (of electrolyte)
f(Ka) = Henry’s function
= ~1.5 (Smoluchowski approximation)
for particles >~ 200 nm and electrolyte ~> 1 x 10-3 M
= ~1.0 (Huckel approximation)
for smaller particles and/or dilute/non-aqueous dispersions
Determination of Zeta Potential
Similar to particle sizing by dynamic light scattering
I.e. what is measured is temporal fluctuations in intensity of light
scattered by the particles in the dispersion.
In light scattering, the fluctuations are related to
Brownian motion of particles.
In ZP, the fluctuations are related to the movement of
the particle in the applied field, i.e. to UE;
The ZP is then calculated from the UE that is
determined by the PALS measurement.
(As in light scattering, the instrument’s autocorrelator
and software take care of the data reduction.)
Zeta Potential vs pH
Typical plot of Zeta Potential vs
pH.
pH dependency of ZP
is very important!
Zeta Potential, mV
Remember, dispersion
stability (or
conversely, ability of
particles to approach
each other) is
determined by ZP, with
~ 30 mV being the
approximate cutoff.
pH
At ZP=0, net charge on particle is 0.
This is called the isoelectric point
[In this example, the
dispersion is stable
below pH ~4 and above
pH ~7.5]
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Colloids with high zeta potential (negative or
positive) are electrically stabilized while
colloids with low zeta potentials tend to
coagulate or flocculate.
Zeta potential [mV]
Stability behavior of the
colloid
from 0 to ±5,
Rapid coagulation or
flocculation
from ±10 to ±30
Incipient instability
from ±30 to ±40
Moderate stability
from ±40 to ±60
Good stability
more than ±61
Excellent stability
X-ray Diffraction
Motivation:
• X-ray diffraction is used to obtain structural
information about crystalline solids.
• Useful in biochemistry to solve the 3D structures of
complex biomolecules.
• Bridge the gaps between physics, chemistry, and
biology.
X-ray diffraction is important for:
• Solid-state physics
• Biophysics
• Chemistry and Biochemistry
• Nanochemist
X-ray Diffractometer
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Wave Interacting with a Single Particle
◦ Incident beams scattered uniformly in all
directions
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Wave Interacting with a Solid
◦ Scattered beams interfere constructively in
some directions, producing diffracted beams
◦ Random arrangements cause beams to
randomly interfere and no distinctive pattern
is produced
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Crystalline Material
◦ Regular pattern of crystalline atoms produces
regular diffraction pattern.
◦ Diffraction pattern gives information on
crystal structure
NaCl
X-ray production typically involves
bombarding a metal target in an x-ray
tube with high speed electrons which have
been accelerated by tens to hundreds of
kilovolts of potential.
The bombarding electrons can eject
electrons from the inner shells of the
atoms of the metal target.
Those vacancies will be quickly filled by
electrons dropping down from higher
levels, emitting x-rays with sharply
defined frequencies associated with the
difference between the atomic energy
levels of the target atoms.
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X-rays have wavelengths on the order of a
few angstroms (1 Angstrom = 0.1 nm).
This is the typical inter-atomic distance in
crystalline solids, making X-rays the correct
order of magnitude for diffraction of atoms of
crystalline materials.
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When X-rays are scattered from a crystalline solid they can
constructively interfere, producing a diffracted beam.
Constructive vs. Destructive Interference
 Constructive interference occurs when the waves are
moving in phase with each other.
 Destructive interference occurs when the waves are out of
phase.
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Diffraction occurs when each object in a
periodic array scatters radiation coherently,
producing concerted constructive
interference at specific angles.
The electrons in an atom coherently scatter
light.
The electrons interact with the oscillating
electric field of the light wave.
Atoms in a crystal form a periodic array of
coherent scatterers.
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The wavelength of X rays are similar to the
distance between atoms.
Diffraction from different planes of atoms
produces a diffraction pattern, which contains
information about the atomic arrangement
within the crystal
X Rays are also reflected, scattered
incoherently, absorbed, refracted, and
transmitted when they interact with matter.
How Diffraction Works: Schematic
NaCl
http://mrsec.wisc.edu/edetc/modules/xray/X-raystm.pdf
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Data is taken from a full range of angles
For simple crystal structures, diffraction
patterns are easily recognizable
For complicated structures, diffraction
patterns at each angle can be used to
produce a 3-D electron density map
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Rosalind Franklin- physical chemist
and x-ray crystallographer who first
crystallized and photographed BDNA
Maurice Wilkins- collaborator of
Franklin
Watson & Crick- chemists who
combined the information from Photo
51 with molecular modeling to solve
the structure of DNA in 1953
Rosalind Franklin
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Photo 51 Analysis
◦ “X” pattern
characteristic of helix
◦ Diamond shapes
indicate long,
extended molecules
◦ Smear spacing reveals
distance between
repeating structures
◦ Missing smears
indicate interference
from second helix
Photo 51- The x-ray diffraction image
that allowed Watson and Crick to solve
the structure of DNA
www.pbs.org/wgbh/nova/photo51
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Photo 51 Analysis
◦ “X” pattern
characteristic of helix
◦ Diamond shapes
indicate long,
extended molecules
◦ Smear spacing reveals
distance between
repeating structures
◦ Missing smears
indicate interference
from second helix
www.pbs.org/wgbh/nova/photo51
Photo 51- The x-ray diffraction image
that allowed Watson and Crick to solve
the structure of DNA
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Photo 51 Analysis
◦ “X” pattern
characteristic of helix
◦ Diamond shapes
indicate long,
extended molecules
◦ Smear spacing reveals
distance between
repeating structures
◦ Missing smears
indicate interference
from second helix
www.pbs.org/wgbh/nova/photo51
Photo 51- The x-ray diffraction image
that allowed Watson and Crick to solve
the structure of DNA
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Photo 51 Analysis
◦ “X” pattern
characteristic of helix
◦ Diamond shapes
indicate long,
extended molecules
◦ Smear spacing reveals
distance between
repeating structures
◦ Missing smears
indicate interference
from second helix
www.pbs.org/wgbh/nova/photo51
Photo 51- The x-ray diffraction image
that allowed Watson and Crick to solve
the structure of DNA
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Photo 51 Analysis
◦ “X” pattern
characteristic of helix
◦ Diamond shapes
indicate long,
extended molecules
◦ Smear spacing reveals
distance between
repeating structures
◦ Missing smears
indicate interference
from second helix
www.pbs.org/wgbh/nova/photo51
Photo 51- The x-ray diffraction image
that allowed Watson and Crick to solve
the structure of DNA
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Information Gained from Photo 51
◦ Double Helix
◦ Radius: 10 angstroms
◦ Distance between bases: 3.4 angstroms
◦ Distance per turn: 34 angstroms
Combining Data with Other Information
◦ DNA made from:
sugar
phosphates
4 nucleotides (A,C,G,T)
◦ Chargaff’s Rules
 %A=%T
 %G=%C
◦ Molecular Modeling
Watson and Crick’s model
Electron microscopes are scientific instruments that use a beam of
energetic electrons to examine objects on a very
fine scale.
Electron microscopes were developed due to the
limitations of Light Microscopes which are limited by the
physics of light.
In the early 1930's this theoretical limit had been reached
and there was a scientific desire to see the fine details of
the interior structures of organic cells (nucleus,
mitochondria...etc.).
This required 10,000x plus magnification which was not
possible using current optical microscopes.
A "light source" at the top of the microscope emits
the electrons that travel through vacuum in the
column of the microscope.
Instead of glass lenses focusing the light in the light
microscope, the TEM uses electromagnetic lenses
to focus the electrons into a very thin beam.
The electron beam then travels through the
specimen you want to study
Depending on the density of the material present,
some of the electrons are scattered and disappear
from the beam.
At the bottom of the microscope the unscattered
electrons hit a fluorescent screen, which gives rise
to a "shadow image" of the specimen with its
different parts displayed in varied darkness
according to their density.
The volume inside the specimen in which interactions occur while
interacting with an electron beam. This volume depends on the
following factors:
• Atomic number of the material being examined; higher atomic
number materials absorb or stop more electrons , smaller interaction
volume.
• Accelerating voltage: higher voltages penetrate farther into the
sample and generate a larger interaction volume
• Angle of incidence for the electron beam; the greater the angle
(further from normal) the smaller the interaction volume.
Specimen must be thin enough to transmit sufficient
electrons to form an image (≤100 nm)
• It should be stable under electron bombardment in a high
vacuum
•Must fit the specimen holder (i.e. < 3 mm in diameter)
• Ideally, specimen preparation should not alter the structure
of the specimen at a level observable with the microscope
• Always research (i.e. literature search) the different
methods appropriate for your sample prep first.
TEM GRIDS
3 mm diameter (Nom. 3.05 mm)
grids used for non self-supporting
specimens
Specialized grids include:
Bar grids
Mixed bar grids
Folding grids (Oyster grids)
Slot grids
Hexagonal grids
Finder grids
Support films (i.e. C or Holey C,
Silicon Monoxide, etc.)
Mesh is designated in divisions per
inch (50 – 2000)
Materials vary from copper and
nickel
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Fixed, dehydrated specimens are embedded in a resin,
hardened, sectioned, stained with heavy metals such as
uranium and lead, and inserted into the electron column in
the microscope.
The electron beam is absorbed or deflected by the heavy
metal stains and shadows are cast onto film or a
phosphorescent plate (image is a shadow) at the bottom of
the column.
2-D image - reveals internal cell structure – high resolution,
high magnification - electron beam is focused by magnetic
field.
faculty.une.edu/com/abell/histo/Histolab4a.htm
(a) TEM image of the
Ag2S(4)/ZnO/TNT
electrode showing the
formation of ZnO on the
TNTs and the Ag2S
nanoparticles inside the
TNTs,
(b) an HR-TEM image of a
deposited Ag2S quantum
dot
(c) the EDX spectrum, and
(d) XRD pattern of the
Ag2S(4)/ZnO/TNTs
Chen et al. Nanoscale Research Letters 2011 6:462
Fixed, dehydrated specimens are mounted on stubs and surfacecoated with gold, palladium or rhodium. The specimen is placed in
a vacuum and an electron beam scans back and forth over it.
Electrons that bounce off the metal-coated specimen surface are
collected, converted to a digital image and displayed on a TV-like
monitor.
SEM:
Gives information about external topography of specimen
Much higher resolution and magnification than possible in LM
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The SEM images the surface structure of bulk samples, from the biological, medical,
materials sciences, and earth sciences up to magnifications of ~100,000x.
The images have a greater depth of field and resolution than optical micrographs
making it ideal for rough specimens such as fracture surfaces and particulate
materials. There is also the option of looking at frozen samples which has applications
in the food technology and pharmaceutical fields.
•A beam of electrons is generated
in the electron gun.
•This beam is attracted through
the anode, condensed by a
condenser lens, and focused as a
very fine point on the sample by
the objective lens.
•The scan coils are energized (by
varying the voltage produced by
the scan generator) and create a
magnetic field which deflects the
beam back and forth in a
controlled pattern, so that the
spot moves across the object.
The electron beam comes from a filament, made of
various types of materials. The most common is the
Tungsten hairpin gun. This filament is a loop of
tungsten which functions as the cathode.
A voltage is applied to the loop, causing it to heat up.
The anode, which is positive with respect to the
filament, forms powerful attractive forces for electrons.
This causes electrons to accelerate toward the anode.
Some accelerate right by the anode and on down the
column, to the sample.
Other examples of filaments are Lanthanum
Hexaboride filaments and field emission guns.
The electron beam hits the sample, producing secondary electrons from the sample.
These electrons are collected by a secondary detector or a backscatter detector,
converted to a voltage, and amplified.
The amplified voltage is applied to the grid of the CRT and causes the intensity of the
spot of light to change.
The image consists of thousands of spots of varying intensity on the face of a CRT that
correspond to the topography of the sample.
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Using the secondary electron detector
produces a clear and focused topographical
image of the sample.
The backscatter electron detector produces
an image that is useful when determining the
make-up of the sample.
Each chemical element in the sample appears
as a different shade, from almost white to
black.
Original Scanning Electron Microscope images of pollen. Panels (a) and (b) are
from the pollen.usda.gov site, while panels (c) and (d) are from the
www.cci.ca.gov/Reference website.
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 http://www.youtube.com/wat
ch?v=lrXMIghANbg&feature=r
elated