Farrokhfar Valizadeh Harzand

DLS doesn't actually measure the size distribution of particles in suspension. It measures diffusion coefficients which can be related to size by means of a mathematical engine with assumptions (Stokes-Einstein). An analogous situation exists with sedimentation where Stokes' Law is the mathematical engine used to convert terminal/settling velocity to size.

You've highlighted the conundrum in particle size measurement for irregular particles - more than one number is needed to correctly define an irregular particle, so there are 3 consequences:

• Equivalent diameters are needed if we are to compare on the basis of a single number. See attached
• Visualization is essential. This means electron microscopy
• There is a shape distribution (or, more correctly, many shape distributions) as well as a size distribution

In certain circumstances we can derive fractal information from light scattering but this only provides a single number related to particle geometry. The theoretical history has many examples of work related to the flow and movement of cylindrical and other irregularly shaped particles. See the following webinar (registration needed):

The importance of the measurement of diffusion in 2-phase systems

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150813DiffusionColloidalSuspension.html

Farrokhfar Valizadeh Harzand

DLS doesn't actually measure the size distribution of particles in suspension. It measures diffusion coefficients which can be related to size by means of a mathematical engine with assumptions (Stokes-Einstein). An analogous situation exists with sedimentation where Stokes' Law is the mathematical engine used to convert terminal/settling velocity to size.

You've highlighted the conundrum in particle size measurement for irregular particles - more than one number is needed to correctly define an irregular particle, so there are 3 consequences:

• Equivalent diameters are needed if we are to compare on the basis of a single number. See attached
• Visualization is essential. This means electron microscopy
• There is a shape distribution (or, more correctly, many shape distributions) as well as a size distribution

In certain circumstances we can derive fractal information from light scattering but this only provides a single number related to particle geometry. The theoretical history has many examples of work related to the flow and movement of cylindrical and other irregularly shaped particles. See the following webinar (registration needed):

The importance of the measurement of diffusion in 2-phase systems

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150813DiffusionColloidalSuspension.html

Hi Mr Alan, thank you for your very efficient guidance and correction.

I would also be grateful if you could share more content or article with me.

IHi,

I agree with what Alan said. When I was working with DLS I used an instrument from Brookhaven Instruments.

https://www.brookhaveninstruments.com/what-is-dynamic-light-scattering/

Here is a link for Au nanorod and protein interactions

https://paperity.org/p/4080280/dynamic-light-scattering-for-gold-nanorod-size-characterization-and-study-of-nanorod

As for my experience I was working with enzyme/polymer complexes and using DLS. Not nanorods but might also be at least some additional reference and background. Thesis ENHANCED ENZYMATIC HYDROLYSIS OF CELLULOSIC FIBERS BY CATION...

Good Luck.

Hi Mr John , thank you for your guidance and resources.

Farrokhfar Valizadeh Harzand

Obviously the phrase 'size of nanorods' is an oxymoron. In 2010 I looked at movement of irregular particles when relating rheology and shape in a webinar. Unfortunately since we've sold our rheology business to Netzsch this webinar is no longer available. I attach the slides without commentary. From slides 14 onward you'll see that Perrin and others investigated movement of cylinders in fluids - appropriate literature is referenced. The importance of so-called 'Jeffery orbits' is stressed. Slide 26 provides a summary slide.

Farrokhfar Valizadeh Harzand

I attach a review paper of fractals and light scattering. You can search similarly.

Alan F Rawle

Hi,thank you for your answers and useful content.

They were really helpful.

The equations to determine the hydrodynamic size by DLS, based on the difussion through Brownian motion, assume that the particles to measure should be spherical.

Thus, you can only likely get an approximation of the maximum size of your rods such as the lenght but not the width.

Rebeca Fortes Martín

All irregular particles need more than one number to 'characterize' them. All models to obtain a single 'equivalent' number are based on some shape assumption, usually spherical. The same applies in sedimentation where settling velocity is converted to size by means of the Stokes equation. Also true in image analysis where an irregular particle will be represented by its circular equivalent (same number of pixels) and any calculation for volume/mass assumes something about the z-axis (usually spherical) but discs would give the same image as would the diameter of a cylinder. The best comparisons are where no conversion equation is used - so quote diffusion coefficients in DLS, settling velocity in sedimentation, and numbers of pixels for image analysis. Never make the conversion to particle size... That's why imaging/microscopy is essential but seeing isn't always believing in a quantitative sense.

Rebeca Fortes Martín

Hi

In fact, we have studied some methods and coefficients for measuring the diameter of nanorods by DLS method. We will definitely share our findings with you. Thank you very much.

You may also find these comments related to nanotubes of interest: In certain specific cases the DLS distribution may show two peaks related to the different dimensions (minor and major axis) https://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#nanotubes

Thanks for the question and many thanks to everyone for useful information