Analytical techniques for characterisation Flashcards
of nanoparticles
What does chemical characterisation include?
Chemical composition
Surface composition and density
Structural properties
Mass concentration
What are some examples of physical properties?
Mean size, size distribution
Shape and morphology
Surface charge (zeta potential is a proxy) - if there is no surface charge they are likely to collapse into other nanoparticles (aggregate)
Physical stability – constant properties? Aggregation? Change in morphology?
What are some examples of chemical characterisation techniques?
XDR (x-ray diffraction) - crystalline structure of nanoparticles
XPS (x-ray photoelectron spectroscopy) - element and surface composition
SEM-EDX – elemental and surface composition
NMR – chemical and surface composition
FTIR – surface composition
Raman spec – surface composition
ICP-OES (inductively coupled plasma – optical emission spectroscopy) or ICP-MS - elemental composition, mass concentration
HPLC – chemical composition
UV-vis spectroscopy – optical properties, nanoparticle concentration
What is TEM?
(transmission electron microscopy)
< 0.5nm resolution, 4 million times better than naked eye
Electron source in vacuum for imaging, focused using electromagnetic lenses into sample, CCD camera, can digitally convert image
Sample is made of materials, some of which can absorb electrons, some cannot
Sample prep – sample solution pipetted onto carbon coated grid, dried and loaded under vacuum conditions. carbon membrane on grid can absorb sample, and doesn’t absorb electrons.
Negative staining – sample embedded by a dried amorphous layer of heavy metal-containing cationic or anionic salt. Provides contrast for example when the nanoparticles do not absorb electrons enough
Cryo-EM – sample cooled to cryogenic temperatures (do not add any contrast, so requires very powerful microscopes compared to negative staining) and embedded into an environment of vitreous water. No drying preserves the structure. Mostly used for structural biology (direct imaging of macromolecules)
Pros – near atomic resolution, direct and accurate measurement nanoparticle size distribution, provides information about shape/morphology of nanoparticles
Cons – sample processing may alter nanoparticles or generate artifacts, dried samples are far from native conditions, time consuming, expensive to run/buy and may have a massive footprint
What is ATM?
(atomic force microscopy)
Form of surface probing microscopy that uses interatomic forces to image topography on the nanometer scale. Physically “feels” the sample rather than “looking” at it like in SEM, and has the ability to measure intermolecular forces between atoms
Can image almost any type of surface (polymers, glass, composites, biological samples etc.) The surface topology often does not correlate to the material properties (strength, malleability, etc.)
Rough surfaces (with ridges etc.) are more likely to grow unwanted bacteria but also better at cellular integration
Advanced imaging modes (quantitative data) - friction, electrical forces, capacitance, magnetic forces, conductivity, viscoelasticity, surface potential and resistance
Made up of:
Scanner – needs fine control, most AFMs use piezoelectric materials
Tip – originally diamond, now silicon/silicon nitrate, generally pyramidal or tetrahedral, around 5-15nm radius around the apex
Cantilever – contact mode needs cantilevers to deflect easily without damaging the sample or tip, and high resonant frequency to avoid vibrational instability, 0.3-2um thickness, 100-200um length. A laser is focused onto the back reflective surface of the cantilever and reflects in a photodetector to determine deflection of the cantilever
3 modes – contact (causes damage to sample but high detail), non-contact (no damage to sample but low detail) and tapping (oscillating between the two other modes)
What is DLS?
(dynamic light scattering) laser scattering technique
Determining size distribution of nanoparticles using Brownian motion
Diffusion coefficient is directly proportional to the temperature
If a particle is non-spherical it still moves with the same Brownian motion
Pros – fast routine method, benchtop instrument, accessible cost, almost no training needed, nanoparticles are in solution in their native environment, direct and accurate measurement of nanoparticle size distribution
Cons – no information about shape/morphology of nanoparticles, the hydrodynamic diameter is larger than the solid diameter (?), the scattering intensity is proportional to the 6th power of the size:aggregates (large debris affect the accuracy), unsuitable for polydisperse particles
What is NTA?
(nanoparticle tracking analysis) a laser scattering technique used for analysing particles in liquid that relates the rate of brownian motion to particle size.
Pros – opposite to DLS (can deal with polydisperse particles), benchtop instrument, accessible cost, direct and accurate measurement of nanoparticle size distribution
Cons – more time consuming than DLS, particle concentration must be within a precise window (sample dilution may alter the properties), no information about shape/morphology, the hydrodynamic diameter is larger than the solid diameter (?*), large debris may prevent analysis
What is ELS?
(electrophoretic light scattering) a laser scattering technique based on DLS, the electrophoretic mobility of the particle is measured which can be converted to zeta potential and effective charge. This allows comparison of materials under different experimental conditions. Particles are in solution or suspension in ELS.
In ELS an electric field induced particle motion, leading to a shift in the frequency of scattered light. The light scattered is detected by interference with a reference beam.
The doppler effect - if the observed wavelength of electromagnetic radiation is longer than the emitted source (moving away) this is red shift. If the wavelength is shorter than the emitted source (moving toward) this is blue shift.
