Determination of refractive index, size, and concentration of nonabsorbing colloidal nanoparticles from measurements of the complex effective refractive index

Abstract

We describe a method for obtaining the refractive index (RI), size, and concentration of nonabsorbing nanoparticles in suspension from relatively simple optical measurements. The method requires measuring the complex effective RI of two dilute suspensions of the particles in liquids of different refractive indices. We describe the theoretical basis of the proposed method and provide experimental results validating the procedure.

Measuring the refractive index and sub-nanometre surface functionalisation of nanoparticles in suspension

“…As the illuminated volume of the capillary is only ∼2 μL, the amount of sample required for this technique is a fraction of that used in other techniques such as SAXS 23,24 or turbidimetry. 38,39 For example, turbidimetry requires volume fractions on the order of ∼1%, whereas here the volume fractions required to distinguish the CDs are ∼0.01%. This limit of detection will scale both with the difference in volume of the functionalisation layer and the particle radius; it is much easier to determine a difference in gradient for a particle with a large corona to core volume ratio.…”

“… 31 Yet, there are downsides to these methods. For example, single particle techniques 37 are statistically limited whereas absorption-based techniques 38 suffer from the need for very high volume fractions (∼1%). In addition, many methods, including NTA and holography, have a minimum size below which characterisation is impossible.…”

“…31 The RI of the nanoparticles themselves is utilised extensively in fields such as tailored or tunable optical materials [32][33][34] created through doping, as well as optical cloaking. 35,36 Measurements of nanoparticle RIs have been performed using techniques such as inline holography, 37 turbidimetry, 38,39 nanoparticle tracking analysis, 40 and SPR itself. 31 Yet, there are downsides to these methods.…”

Determination of the Refractive Index of Particles Through the Immersion Solid Matching Method

“…Both methods are accurate (0.0001-0.00001 RI unit), but they are expensive and require samples with very smooth surfaces [2]. Nanoparticle tracking analysis (NTA) is a technique for determining the RI of suspended nanoparticles based on the rate of their Brownian motion [3]. However, that approach is possible only for the nanoparticles composed of high RI materials, such as metals.…”

Determining Number Concentrations and Diameters of Polystyrene Particles by Measuring the Effective Refractive Index of Colloids Using Surface Plasmon Resonance

“…However, its applicability is restricted because the number concentration ( c p ) must already be known, and the requirement to measure both the real and imaginary parts of the difference between the n eff and the n of the dispersion media ( n m ). Marquez-Islas et al , developed another method for determining the n part , c p , and d part based on the van der Hulst model. It requires the measurement of the n eff of two samples that are diluted to the same concentrations with media having different n m .…”