Nanoparticle and Nanotube Size Analysis Using the Scanning Mobility Particle Sizer Spectrometer from TSI Incorporated to Assess Health Risks

Topic List

Introduction
   Understanding the Health Risks of Nanoparticles
   Scanning Mobility Particle Sizer Spectrometer
   Condensation Particle Counters
Typical Applications in Nanotechnology
   Sizing of Nanoparticles in Dry-Synthesis Reactors
   Use of Scanning Mobility Particle Size Spectrometers in Nanotechnology
   Carbon Nanotubes
   Sizing of Nanoparticles Suspended in Colloids
   SMPS Measurements of Zirconia Nanoparticles
Nanoparticle Exposure Analysis

Introduction

The benefits of sizing aerosolized submicrometer particles using an electrical mobility sizing technique have been well documented. The technique is highly accurate and has been shown to size 60 nm and 100 nm Standard Reference Material (SRM) with an uncertainty of only 1%. The National Institute of Standards and Technology (NIST) have been using electrical mobility to measure its 0.1ìm Standard Reference Material (SRM) Particles for well over a decade. Lately, the electrical mobility technique is finding increased use in the in-situ near real-time sizing of engineered nanoparticles synthesized by a variety of aerosol-based processes like diffusion flame synthesis, spray pyrolysis, thermal plasma etc. When combined with electrospray and other dispersion methods, the electrical mobility technique has been shown to accurately size nanoparticles suspended in colloids as well.

Understanding the Health Risks of Nanoparticles

With the commercialization of nanotechnology, occupational health risks associated with manufacturing and handling of nanoparticles is a growing concern. Workers may be exposed to nanoparticles through means of inhalation, at levels that greatly exceed ambient concentrations; and no workplace standards currently exist to limit exposure to nanoparticles. Nanoparticle size governs their deposition pattern in various parts of the lung a nd their ultimate fate within the human body. Thus, ambient measurements of nanoparticle size distributions provided by the electrical mobility technique is a powerful tool in understanding adverse health effects associated with nanoparticle related exposure.

This application note provides a brief overview of the electrical mobility technology as integrated in TSI Scanning Mobility Particle Sizer (SMPS) spectrometer followed by a discussion on applications in nanoparticle synthesis and exposure research.

Scanning Mobility Particle Sizer Spectrometer

The Scanning Mobility Particle Sizer (SMPS) spectrometer consists of a sample preconditioner, a bipolar charger, a nanoparticle size classifier and a nanoparticle detector. Figure 1 depicts a schematic of the entire system. The pre-conditioner (typically an impactor or a cyclone) eliminates large micrometer sized particles. The bipolar charger (also referred to as neutralizer) establishes bipolar charge equilibrium on the particles. This defined charge condition is necessary for the size classification using electrical mobility. Particles are size classified in a Differential Mobility Analyzer (DMA). The charged aerosol passes from the neutralizer into the main portion of the DMA. The DMA shown in the figure is a nano- DMA which is specifically designed for sizing nanoaerosols in the size range of 2 nm to 150 nm. The nano-DMA contains an outer, grounded cylinder and an inner cylindrical electrode that is connected to a negative power supply (0 to 10 kVDC). The electric field between the two concentric cylinders separates the particles according to their electrical mobility which is inversely related to the particle size. Particles with negative charge(s) are repelled towards and deposited on the outer wall. Particles with neutral charge exit with the excess air. Particles with positive charge(s) move rapidly towards the negatively-charged center electrode. Only particles within a narrow range of electrical mobility have the correct trajectory to pass through an open slit near the DMA exit. The electrical mobility of these selected particles is a function of flow rates, geometric parameters and the voltage of the center electrode.

 

Figure 1. Schematic of an SMPS spectrometer

Condensation Particle Counters

The monodisperse particle stream exiting the DMA is counted by a Condensation Particle Counter (CPC). In the CPC, single particles larger than 2 nm are grown to micrometer size by means of condensation of a working fluid (alcohol or water) on the particles. The CPC then optically counts these particles. Particle size distributions are measured by changing the applied high voltage in the DMA, which changes the electrical field, thus scanning the whole size distribution.

Typical Applications in Nanotechnology

Sizing of Nanoparticles in Dry-Synthesis Reactors

The electrical mobility technique is finding increasing use in the in-situ near real-time sizing of engineered nanoparticles synthesized by a variety of aerosol-based processes. The near real-time measurement offered by electrical mobility technique accelerates the research and development process of nanoparticle synthesis since it enhances the understanding of the mechanisms of particle formation and growth. An in-situ measurement eliminates the need for sample collection for off-line methods thus minimizing operator error and providing more consistent repeatable results. Figure 2 gives an overview of important steps in synthesis of nanomaterials in an aerosol based reactor. Real-time sizing of nanoaerosols in these reactors permits the user to follow dynamics of particle formation and growth in highly reacting flows. A precise control of particle size is key; real-time measurement of particle size distributions in the reactor provides the necessary feedback to control reactor conditions to achieve high quality control.

 

Figure 2. Aerosol processes for synthesis of nanomaterials

Use of Scanning Mobility Particle Size Spectrometers in Nanotechnology

SMPS has been increasingly employed in nanotechnology research. In 1991, Akhtar et al. used an SMPS spectrometer to study vapor synthesis of Titania powder, specifically, the effect of process variables (reactor residence time, temperature, and reactant concentration) on powder size and phase characteristics. The SMPS measured particle size distributions were used to validate particle coagulation model. Somer et al. (1994) used SMPS to study agglomeration of Titanium dioxide aerosol in high intensity field. Ahn et al. (2001) [10] studied silica particle growth characteristics in H2/O2/TEOS diffusion flame. They found close agreement of SMPS measured size as compared to the Transmission Electron Microscope (TEM) image processed size data. Ullman et al. (2002) studied properties of nanoparticle aerosols of size 4.9-13 nm, generated by laser ablation. Measurements of eight materials including Silica, Carbon, Titania, Iron oxide, Tungsten oxide, Niobium oxide, Carbon and Gold were successfully achieved. Other SMPS assisted studies of nanoparticle reactors include liquid flame spray (silver-titania deposit nanoparticles), ethylene flame (soot nanoparticles) [13] and thermal plasma reactors (Si, Ti particles) to name a few. More recently, Zhang et al. (2007) studied temperature effects on Tellurium dioxide synthesis by spray pyrolysis. SMPS data from this reactor (figure 3) clearly shows transition of precursor droplets to product droplets as the temperature increases.

 

Figure 3. Particle size distributions as measured by SMPS at different furnace temperatures.

Carbon Nanotubes

The discussion on nanotechnology is incomplete without the mention of carbon nanotubes (CNTs). Several investigations of gas-phase synthesis of carbon nanotubes (CNTs) have utilized the electrical mobility analysis. For example, Moisala et al. (2005) used the on-line detection of single-walled (SW) CNT formation during aerosol synthesis method using a differential mobility analyzer. Despite the different product morphology and concentration, the authors report that the on-line measurement was able to distinguish SWCNT formation in each experimental set-up as an increase in the geometric mean particle diameter and as a decrease in the total particle number concentration. Furthermore, information regarding the relative SWCNT concentration was also obtained from the DMA measurement. The authors have devepoed a theoretical approach to the mobility of nonspherical particles in the electric field in order to convert the electrical mobility size of the high aspect ratio SWCNTs measured with DMA to the physical size of the product. The authors studied size-selected SWCNTs with transmission electron microscopy in order to find the correlation between the on-line DMA measurement data and the SWCNT morphology.

DMA data analysis for carbon nanotubes is discussed in detail in a paper by Kim and Zachariah (2005) and has been used in subsequent works by the same group and other investigations of carbon nanotubes. Using the analysis described in this paper, it is possible to calculate the length distribution of nanotubes on the basis of the mobility size distribution curve obtained from SMPS. Chiang and Sankaran (2007) used this methodology for gas-phase studies of catalyzed carbon nanotube growth. Figure 4 shows the temperature dependent aerosol size distributions of carbon nanostructures grown on nickel catalysts using microplasma induced synthesis.

 

Figure 4. Temperature-dependent aerosol size distributions of carbon nanostructures grown on Ni catalysts.

Sizing of Nanoparticles Suspended in Colloids

Many wet synthesis methods e.g. sol-gel, microemulsion etc. are now available for producing nanoparticles as colloidal dispersions. The stability of dispersion is a desired trait that ensures nanoparticles are not agglomerating; the level of dispersity can be judged from measured particle size distributions. When combined with electrospray dispersion, the electrical mobility technique has been shown to accurately size nanoparticles suspended in colloids. Figure 5 demonstrates the high size resolution of SMPS. The size distributions of a mixture of nine different proteins and of electrosprayed bovine serum albumin (BSA) nanoparticles were measured with an SMPS (TSI Model 3936-N25). The x-axis denotes electrical mobility based diameter and y-axis denotes particle concentrations in units of counts per cm3 of carrier gas.

 

Figure 5. Size distribution of electrosprayed nanoparticles measured with SMPS. Left, mixture of 9 different proteins, right BSA nanoparticles.

SMPS Measurements of Zirconia Nanoparticles

Figure 6 shows the SMPS measured size distribution of zirconium oxide nanoparticle colloids for tests conducted at TSI. The colloidal particles were prepared in a 20mM ammonium acetate solution in water adjusted to pH 3.8. Plot (a) shows size distribution for a suspension concentration of 20 ìg/ml; the plot (b) shows the size distribution of a diluted suspension with a concentration of 10 ìg/ml. Note that the size distribution remains same, however the relative counts of particles of different sizes is scaled down in proportion to the amount of dilution. This test confirms that the particle size distributions are representative of single particles and not multiplets.

 

Figure 6. Zirconia nanoparticle size distributions for suspension concentrations of (a) 20 ìg/ml (b) 10 ìg/ml.

Nanoparticle Exposure Analysis

Besides the near real-time analysis related to nanoparticle processes discussed above, electrical mobility analysis with SMPS can also be used to monitor process related nanoparticle exposure. The high size resolution allows the calculation of particle area distributions. Figure 7 shows an example: the measurements were taken during emptying an ultrafine Titanium dioxide baghouse into a powder collection bucket. Besides the SMPS data the figure 7a shows total number concentration measured with a CPC and total alveolar deposited surface area concentration measured with a Nanoparticle Surface Area Monitor (TSI NSAM Model 3550). Figure 7b depicts alveolar deposited surface area distribution calculated from SMPS measured particle size distribution in the ambient air close to the material handling operation. A peak in concentrations during middle of process coincided with dumping of a drum of Titania powder in a reservoir.

 

Figure 7. Nanoparticle exposure measurement (alveolar deposited particle surface area) in an ultrafine TiO2 facility. Filling process monitored with SMPS, NSAM and CPC.

Source: TSI Incorporated
For more information on this source please visit TSI Incorporated

Date Added: Apr 21, 2008 | Updated: Jun 11, 2013
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