Particle Size Analysis of Nanoscale Biopolymers by Laser Diffraction

By AZoNano

Table of Contents

Overview
Introduction
Proving Ability to Detect Larger Population
Experimental Procedure
Results and Discussion
Test for Repeatability
Laser Diffraction for Process Control
Conclusions
About Horiba

Overview

A large amount of research has been conducted exploring the use of nanoparticles composed of biopolymers as a drug delivery vehicle. Since the design goal for nanoparticles is in the range of 100 nm, much of the particle size analysis in this field has been done using dynamic light scattering (DLS). However, certain materials will feature larger particles that are outside the upper size range of DLS, but within the capability of laser diffraction. Here lies the unique value of a single analyzer, which can precisely measure particles both less than 100 nm and greater than several microns. This application note describes two experiments where laser diffraction was capable of determining both the base biopolymer nanoparticles and also larger particles outside of the range of DLS.

Introduction

Biodegradable polymers are most commonly studied as potential carriers for controlled release formulations of active pharmaceutical ingredients (APIs). One common biopolymer used for drug delivery is polylactide. Poly(lactic acid) or polylactide (PLA) is a thermoplastic aliphatic polyester as shown in Figure 1 obtained from renewable resources, for e.g. corn starch. PLA used for drug delivery has been studied for several decades. PLA or PLA surface modified with polyethylene glycose (PEG) can be produced as nanoparticles in the range of 50 – 500 nm using a range of techniques.

Figure 1. Polylactic acid (PLA)

Proving Ability to Detect Larger Population

The first experiment was developed to confirm the ability of laser diffraction to measure both PLAPEG nanoparticles in the 100 nm range and spiked 1 µm latex spheres meant to model the presence of agglomerates. The particles that were used for this study were supplied by a potential customer who made the test particles using a double emulsion (W/O/W) method in a sonicator. Sample 1 comprises only the PLA-PEG nanoparticles. Sample 2 comprises the nanoparticles and a second population of several percent 1 µm (nominal) polystyrene latex particles. The particle size distribution analysis was done using a competitive DLS system and on the HORIBA LA-950 laser diffraction analyzer as shown in Figure 2.

Figure 2. HORIBA LA-950

Experimental Procedure

Two LPA nanoparticle samples (1 and 2) were studied using both DLS and laser diffraction. The DLS measurements were done on a competitive system, so specific analysis procedures were not available to report. The samples were analyzed by laser diffraction using the HORIBA LA-950 system using the Fraction Cell accessory in order to minimize the amount of sample required for the measurement. The Fraction Cell is a distinct LA-950 accessory that minimizes the needed sample volume to less than 1 mg.

The samples were studied using the rapid and easy procedure detailed below:

  1. The Fraction Cell is filled with DI water
  2. The magnetic stirrer is activated
  3. The automatic system is aligned
  4. A background reading is taken
  5. The sample is pipetted directly into the fraction cell
  6. Desired concentration (%T) is measured
  7. The measurement is done three times and COV is calculated

Results and Discussion

The results from studying the samples 1 and 2 using DLS reported as intensity distributions are shown in the Figures 3 and 4.

Figure 3. DLS results for sample 1

Figure 4. DLS results for sample 2

The peak of the nanoparticles alone as shown in Figure 3 is centered at 143 nm. When the 1 µm PSL particles are added the reported bimodal distribution seen in Figure 4 shows two peaks centered at 160 nm and 465 nm. Both of these peaks are not in the right position. The vendor and the customer tried in vain to optimize the algorithm to properly split the distributions. The results for the two similar samples studied by laser diffraction are shown in the Figures 5 and 6.

Figure 5. LA-950 Results for sample 1

Figure 6. LA-950 Results for sample 2

The results for sample 1 in Figure 5 report the main population as being centered at 92 nm based on the volume distribution, lesser than the reported size based on intensity distribution by DLS as anticipated. The results for sample 2 shows the first peak shifted slightly to the larger size, but very accurately reports the 1 µm particles at 1.02 µm. Additionally, a third peak of larger agglomerates at 46 µm is sensed well beyond the range of any DLS system.

Test for Repeatability

Sample 2 was measured three times to test for repeatability, which provided excellent results as shown in the Figure 7 where the coefficient of variation (CV) is 0.44 % for the D (v,0.5).

Figure 7. Repeatability of sample 2

Laser Diffraction for Process Control

Another PLA-based engineered nanoparticle used for drug delivery was studied by the LA-950 regularly at a customer site as a QA and process control tool. This customer encapsulates API’s into a matrix of biocompatible and biodegradable polymers engineered to provide the desired drug release profile. Optimal batches of the product comprised only a single population centered near 80 nm as shown in Figure 8.

Figure 8. LA-950 results of PLA nanoparticles, good batch

On very few occasions, the same formulation generated the same 80 nm particles as well as a tiny population of agglomerates in the range of anywhere between 10 and 50 µm. It was essential for customers to detect batches having these agglomerates so all batches were routinely tested on the LA-950 to find out if they were present. The results from a bad batch of the PLA nanoparticles are shown in Figure 9.

Figure 9. LA-950 results of PLA nanoparticles, bad batch

Data interpretation is crucial in order to preset specifications to identify the bad batch since the D(v,0.1), D(v,0.5), and D(v,0.9) for both batches are mostly identical. But the volume mean D(4,3) reports a nine fold increase from 0.082 to 0.731 µm. Therefore, the volume mean is the optimum result value to be used to identify the presence of the agglomerates.

Conclusions

The HORIBA LA-950 can detect both nano-scaled particles smaller than 100 nm along with larger model or agglomerated particles. This highlights several innovative features including the low end dynamic range to measure down to 30 nm via laser diffraction, the ability to accurately split multiple populations, and the sensitivity to sense a small percentage of agglomerates in the presence of a main peak in the nanoparticle range. The LA-950 is both sensitive enough for the most challenging R&D requirements and easy enough to use that it can be a day to day process control monitor.

About Horiba

HORIBA Scientific is the new global team created to better meet customers’ present and future needs by integrating the scientific market expertise and resources of HORIBA. HORIBA Scientific offerings encompass elemental analysis, fluorescence, forensics, GDS, ICP, particle characterization, Raman, spectral ellipsometry, sulfur-in-oil, water quality, and XRF. Prominent absorbed brands include Jobin Yvon, Glen Spectra, IBH, SPEX, Instruments S.A, ISA, Dilor, Sofie, SLM, and Beta Scientific. By combining the strengths of the research, development, applications, sales, service and support organizations of all, HORIBA Scientific offers researchers the best products and solutions while expanding our superior service and support with a truly global network.

This information has been sourced, reviewed and adapted from materials provided by Horiba.

For more information on this source, please visit Horiba.

Date Added: Oct 27, 2011 | Updated: Jan 16, 2014
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