동적광산란 기준 시료 측정 방법-1

Measuring Latex Standards 

                                by Dynamic Light Scattering  

 

Introduction

 

Dynamic light scattering (DLS) is a non-invasive technique suitable for the size characterization of nanoparticles and low-molecular weight molecules such as proteins and polymers [1-3].  

The technique measures the time-dependent fluctuations in the intensity of scattered light that occur due to the random movement of the particles or molecules undergoing Brownian motion.  

The velocity of this Brownian motion is measured and is called the translational diffusion coefficient (D) which can be converted into a hydrodynamic diameter (DH) using the Stokes-Einstein equation [1-3]. 

 


DLS is an absolute technique using first principles and therefore calibration is not required. However, verification of the instrument should be regularly performed to check correct operation.
 

Polymer latex spheres are very commonly used to verify correct instrument performance. This is because they are available as monosize dispersions of near perfect spheres. The sphere is the only threedimensional shape whose size can be unambiguously described by a single figure and being monodisperse removes any uncertainty regarding the calculation of a mean size.  

 

Polymer latex samples have other benefits. They have a similar density to water, so particles less than 1micron will remain in suspension during measurement. Dispersions can be stored at room temperature and have storage lifetimes of months or years. 

 


A wide range of monodisperse polystyrene lattices are available from a variety of manufacturers. However, not all are supplied with an individual calibration certificate. Thermo Scientific Nanosphere 3000 series size standards [3] are each supplied with a calibration certificate,
measured by transmission electron microscope, (TEM) traceable to NIST [4]. The specification for the standards also includes a hydrodynamic diameter measured by dynamic light scattering (DLS).
 

Thermo Scientific Nanosphere 3000 series size standards are available from 20nm to 900 nm. The easiest sizes to measure are in the range 20nm to 300nm. Particles larger than 60nm are large enough to give very reproducible results at suitable dilutions for DLS. Particles larger than
300nm start to show a marked variation in scattering intensity with angle and measuring standards smaller than this removes the requirement to consider the angle. 

 


The Thermo Scientific Duke Standards 2000 and 4000 Series are NIST traceable standards of sizes which can also be used for DLS verification applications and contain sizes greater than 1 micron.  

 

Certified and Hydrodynamic Sizes  

 

The result quoted on the Thermo Scientific Nanosphere bottle is the certified TEM result. The DLS result (i.e. the hydrodynamic size) is quoted in the specification sheet provided and is not a certified value. For all Thermo Scientific Nanosphere 3000 standards, the size accuracy by DLS should be within the specified hydrodynamic size range ±2% for samples prepared in a 10mM NaCl [2,3]. Sodium chloride is used to suppress the electrical double layer.  

 

Dilution of the standard in deionised water will give an extended double layer and result in an artificially increased size which may be out of specification. 


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