@Parisa Eram: best turquoise and saffron from Mashad.....

To add to Ulf's excellent response, intensity distributions from DLS are weighted to volume squared or diameter to the 6th power whereas number distributions (from microscopy) are not weighted in such a fashion.  Both are correct - see the attached Basic Principles - but the number distribution from microscopy will always be smaller than the intensity distribution from DLS for a polydisperse sample.  So your results are in line with expectation.

Also note that electron microscopy responds to the electron-dense part of the particle - thus to the Pt part of your platinum nanoparticles. DLS in contrast (bad pun!) is relating how the particle moves in suspension and this diffusion coefficient is related to hydrodynamic size via the Stokes-Einstein equation. Thus the hydrodynamic size is a reflection of the transport properties of the particle and takes into account any protecting (surfactant, steric layer) or stabilizing layer that may surround the particle. 

You get complementary, not competitive, information on your system with both microscopy and DLS techniques.

Two main reasons are that DLS is an intensity based measurement and that the nanoparticles in DLS are measured in solution and thus with solvent layer (and potentially a tendency to aggregate).

http://www.materials-talks.com/blog/2017/01/23/intensity-volume-number-which-size-is-correct/

@Parisa Eram: best turquoise and saffron from Mashad.....

To add to Ulf's excellent response, intensity distributions from DLS are weighted to volume squared or diameter to the 6th power whereas number distributions (from microscopy) are not weighted in such a fashion.  Both are correct - see the attached Basic Principles - but the number distribution from microscopy will always be smaller than the intensity distribution from DLS for a polydisperse sample.  So your results are in line with expectation.

Also note that electron microscopy responds to the electron-dense part of the particle - thus to the Pt part of your platinum nanoparticles. DLS in contrast (bad pun!) is relating how the particle moves in suspension and this diffusion coefficient is related to hydrodynamic size via the Stokes-Einstein equation. Thus the hydrodynamic size is a reflection of the transport properties of the particle and takes into account any protecting (surfactant, steric layer) or stabilizing layer that may surround the particle. 

You get complementary, not competitive, information on your system with both microscopy and DLS techniques.

In addition to the above, note that an electron beam used to probe in TEM has a de Broglie wavelength ~ 1 nm (smaller than your particles) compared to the photon wavelength ~ 600 nm or so (way larger than your particles) used in DLS. One might expect the size distribution resolution for the latter from scattering theory should be much harder to determine with accuracy than the former given positional accuracy of the encounter has greater uncertainty. Still, after calibration only getting 20 nm difference is not bad, and is often attributed to the role of solvent binding in diffusing the scattering edge of the particle itself! 

Hi Parisa

In recommendation to above answers i would like to add, DLS  measures the relaxation time for the decay of the autocorrelation function of the scattered light, from which a diffusion coefficient inversely proportional to particle size can be investigated. If you have a monodisperse size distribution,  then a size measurement by TEM should give a size similar to that measured by DLS. If you have a distribution of particle sizes then things become interesting. Your size distribution measured by counting particles and distributing them into a range of size bins, give a number average size. DLS weights the distribution differently, by size to the power 6, essentially giving a z-average distribution. Larger particles are therefore given more weight, making the "average" size look larger. The other point is that part of the weighting arises as from the intensity of the light scattered by your particles and are independent of the angle at which scattering is measured. Hence,  DLS gives the hydrodynamic radius while TEM gives the actual size. So the size you got from TEM will be the actual size of your NPs

TEM measured the exact size of nanoparticles while DLS is volume / intensity based crude analysis, therefore, always you get higher nanoparticle size in DLS analysis as you are considering the outer ionic layer also . When light heats the particle the ionic layer also reflects it which is then scattered by dynamic light scattering principle and is detected to give an overall larger particle size as compared to TEM

@ Ulf Nobbmann @Alan F Rawle  @John Canning @Anupma Thakur @ J. C. Tarafdar

Thank you very much for all of you that have answered my question.Thanks for your great directions.

Dear Anupma Thakur. Allow me to express my opinion on your phrase. " If you have a monodisperse size distribution,  then a size measurement by TEM should give a size similar to that measured by DLS." If monodisperse particles, then there is no distribution of these particles. Monodisperse particles have a hydrated sphere and they will be larger than those of the TEM. 

Best regards.

Above reasons are correct. Yet, here I have included another reason. In case of TEM study, a drop of dilute dispersion has placed on a carbon-coated copper grid and air-dried. The samples were then analyzed to obtain transmission electron microscopy. But in case of size measurement by DLS study, only dilute solution have used, there are no needed to dry the smaples. At the time of air-dried process, size were compressed in case of TEM study.

I am always amused by comments that TEM is the 'right' answer and DLS is 'wrong' - usually based on larger numbers by DLS or 'seeing is believing'. Both techniques measure different properties of the particle and, as Yuri hints above, the hydration sphere and surfactant shell coverage of the particle is just as important as the core (usually) electron-dense phase.  The surface shell is just as much a part of the particle as the core.  The extent of sample preparation, hinted by Biplab above, is crucial, and always underestimated. Take a monodisperse set of spherical standard material and prepare it in a routine way for TEM - disperse in epoxy resin and microtome.  This generates a set of discs of which only those of maximum diameter reflect the 'true' size of the particle.  All the rest are sections from sphere down to the limit of the technique -  a size distribution results where there was no size distribution to begin with!   Microscopy is essential but one needs a combination of techniques including DLS to get a handle on the critical quality attributes or product performance indicators.

Remember too, as stated by Heywood many years ago, shape is a 3-dimensional property and all we have to look at with micrographs is a 2-dimensional image.  We need some form of confocal microscopy to get an idea of the z-axis.

Because DLS determines the hydrodynamic diameter of the tested particles which is relatively higher than the real diameter of the same particles determined by TEM

@ John Canning You make an interesting point of the relation of wavelength of the incident radiation with that of the size of the particle.  In this regard one needs to think of ensemble-based techniques or scattering (e.g. the scattering on sunlight from oxygen and nitrogen molecules in the sky) in contrast to to the situation where we're considering a train of light impinging and interacting with a single small particle.  The latter was dealt with very simply over a hundred years ago by John William Strutt a.k.a. Lord Rayleigh.  Small particles or molecules (essentially they're the same from a light scattering perspective - a boundary or heterogeneity exists causing scattering) way below the size of the incident radiation scatter weakly and behave as point scatterers.  But scatter light they do proportional to their size (radius to the 6th power or volume squared in the Rayleigh region) and this (isotropic for unpolarized radiation) scattered light can be easily detected either with a photon counter (such as an APD) or with traditional silicon photodiode detectors if there is enough total scattering.

@ Eman Bayoumi  Real diameter?  Is the picture you're showing a disc/coin or a 2-D representation of a 3-D sphere?

dear, In DLS we measure the size of the naoparticles which are dispersed in nano fluid. Thus the average of the size is measure and above that the size what we get as results is about the hydrodynamic size which are measured from scatterd light. 

The sem results are mainly taken as the best particle and smallest one from the sample thus the size will be always smaller than the DLS

Hope this helped you.  I strongly recommend @Alan F Rawle's answers too. 

@Chingakham_Chinglenthoiba

Thank you for your help

 Dear Parisa,

First of all, it depends on the sample preparation. If you have drop casted the same sol on the copper grid, which you have analysed in DLS, and get this result, then the probable answer is because of "Hydrodynamic boundary layer". In DLS, it is calculating the hydrodynamic dia via diffusivity due to Brownian motion, which can be different from the actual particle size due to absorbed surfactants and polymer etc. Even if the particles are forming any type of floc, then in DLS it will be treated as a single particle. But in case of well dispersed naked particles, the results from both the analysis must be nearly same.

@ Nitai: 'actual particle size'?  The hydrodynamic size is as much a property of the particle as the inner electron-dense core. Both are correct and features of the particle's behavior (critical quality attributes or product performance indicators).  The data are also processed in different manners in DLS and microscopy - intensity  and number distributions respectively.  For a polydisperse material, the intensity mean must be larger than the number mean and thus Parisa's results are in line with expectation.  There is no right or wrong, correct or incorrect, here.