Instructor: Instructor:

Dr. Marinella SandrosDr. Marinella Sandros

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NanochemistrNanochemistryy

NAN 601NAN 601

Lecture 19: Analytical Techniques Part 2

TripletSinglet

Quadruplet

2960

1050 1370

1740

Zeta Potential X-ray Diffraction (XRD) Spectroscopy Transmission Electron Microscopy Scanning Electron Microscopy

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.

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.

Diffuse Layer: Ions are less firmly firmly associated

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

Particle surface

Characteristics of Surface Charge: Definitions

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.

UE = 2 z f(Ka)/3

=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

z = Zeta potential

+ -+

-

This velocity is the partlcle’s electrophoretic mobility, UENote relationship of

zeta potential and electrophoretic mobility; therefore…

Zeta potential can be determined by measuring UE

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

pH dependency of ZP is very important!

Remember, dispersion stability (or conversely, ability of particles to approach each other) is determined by ZP, with ~ 30 mV being the approximate cutoff.

[In this example, the dispersion is stable below pH ~4 and above pH ~7.5]

At ZP=0, net charge on particle is 0. This is called the isoelectric point

Typical plot of Zeta Potential vs pH.

Zeta

Pote

nti

al,

m

V

pH

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 DiffractionMotivation:

• 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

Wave Interacting with a Single Particle◦ Incident beams scattered uniformly in all

directions 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

Crystalline Material◦ Regular pattern of crystalline atoms produces

regular diffraction pattern.◦ Diffraction pattern gives information on crystal

structureNaCl

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.

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.

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.

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.

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

http://mrsec.wisc.edu/edetc/modules/xray/X-raystm.pdf

NaCl

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

Rosalind Franklin- physical chemist and x-ray crystallographer who first crystallized and photographed B DNA

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

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

Photo 51- The x-ray diffraction image that allowed Watson and Crick to solve the structure of DNA

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

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

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

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

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:

sugarphosphates

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 veryfine scale.

Electron microscopes were developed due to thelimitations of Light Microscopes which are limited by thephysics of light.

In the early 1930's this theoretical limit had been reachedand there was a scientific desire to see the fine details ofthe interior structures of organic cells (nucleus,mitochondria...etc.).

This required 10,000x plus magnification which was notpossible using current optical microscopes.

A "light source" at the top of the microscope emitsthe electrons that travel through vacuum in thecolumn of the microscope.

Instead of glass lenses focusing the light in the lightmicroscope, the TEM uses electromagnetic lensesto focus the electrons into a very thin beam.

The electron beam then travels through thespecimen you want to study

Depending on the density of the material present,some of the electrons are scattered and disappearfrom the beam.

At the bottom of the microscope the unscatteredelectrons hit a fluorescent screen, which gives riseto a "shadow image" of the specimen with itsdifferent parts displayed in varied darknessaccording 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.

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3 mm diameter (Nom. 3.05 mm) grids used for non self-supporting

specimensSpecialized 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

TEM GRIDS

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  

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Fixed, dehydrated specimens are mounted on stubs and surface-coated 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 specimenMuch higher resolution and magnification than possible in LM

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 amagnetic 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 thatcorrespond to the topography of the sample.

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.

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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.

http://www.youtube.com/watch?v=lrXMIghANbg&feature=related