Two most important structural parameter of a molecule is size and shape. While there is many spectroscopic (like DLS, neutron scattering, fcs) and microscopic (SEM,tem, AFM, etc); for shape measurement as I know, we depend on microscopic techniques.
I want to know other than such imaging techniques are there any such techniques that can measure the shape of nanoparticle or protein?
@ Nilimesh, beside the technique you have mentioned the other techniques are: Surface Plasmon Resonance, Ionic Spectroscopy, Electron Spectroscopy, Tunable Resistive Pulse Sensing etc.
You can also use diffraction techniques. The most precise answer on the shape of a protein molecule is obtained from the single crystal diffraction technique. It results in all atomic positions and thus in the shape of a protein molecule.
Small angle scattering measures the intensity in a diffraction pattern at small angles (first few degrees), and this gives size and shape information. for a protein the contrast between the molecule and the solvent is usually not very good, but it can be done.
The size, shape and internal structure of nanoparticles can be determined by analyzing the full diffraction pattern as well, as the profile of the Bragg reflections depends on these parameters. A rough measure is the Scherrer equation
D = k*lambda/(beta*cos(Theta), where k is a parameter approximately 0.9, lambda the wave length beta the FWHM in radians and Theta is half the diffraction angle. A full simulation of the diffraction pattern using the Debye-Scattering-Equation gives you full info on the nanoparticle.
If the diffraction pattern is measured with short wavelengths one can run a Fourier transform of the powder diffraction pattern to obtain the Pair Distribution Function. This is currently a very popular way to analyze nanoparticles.
Nilimesh Das
Fractal analysis is possible for light scattering data. This does give an average number only and no visual or distribution data. Useful for comparison purposes though.
Agnishwar Girigoswami
Yes, microscopy is essential but may not be quantitative and we often have a low contrast image for the materials that are described. Usually though we only have a 2-D image of a 3-D particle and 'sample preparation' (especially TEM) is a trap for the unwary. Cryo-TEM has had excellent success though.
We also have to be aware that there's usually a shape distribution as well as a size distribution and many shape descriptors are ratios of 2-D parameters (e.g. aspect ratio). There's also a statistical element in that (for example) to calculate the mean to 1% standard error would require data on 10000 particles.
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