Comparing aerosol surface-area measurements of monodisperse ultrafine silver agglomerates by mobility analysis, transmission electron microscopy and diffusion charging

Abstract

Three methods—scanning mobility particle sizer (SMPS), transmission electron microscopy (TEM), and diffusion charging (DC)—for estimating aerosol surface area were evaluated and compared. The aerosol used was monodisperse silver particles, having morphologies that range from spherical to agglomerated particles, with corresponding fractal dimensions from 1.58 to 1.94. For monodisperse silver particle agglomerates smaller than 100 nm, the DC response was proportional to the mobility diameter squared, regardless of morphology. For particle sizes from 80 to 200 nm, the DC response varied as the mobility diameter to the power 1.5. The projected surface area of agglomerates analyzed by TEM agreed well with that estimated from particle mobility diameters for particles smaller than 100 nm. The surface area of monodisperse particles, measured by DC and SMPS, was comparable to the geometric surface area below 100 nm, but in the size range of 100–200 nm, the methods used underestimated the geometric surface area. SMPS, TEM, and DC-based measurements of surface area were in good agreement with one another for monodisperse aerosol particles smaller than 100 nm.

Introduction

The toxicity of most inhaled aerosol particles is generally accepted to be associated with particulate mass. However, several studies in recent years have indicated that the toxicity of low-solubility inhaled particles may be more appropriately associated with particulate surface area (Oberdörster et al., 1995, Oberdörster, 2000, Brown et al., 2001). Ultrafine particles (particles below 100 nm) have a higher specific surface area (area unit per mass) than coarser particles, and it is plausible that evaluating exposures to such particles by their mass concentration may lead to the underestimation of health risks. While this hypothesis probably will not gain wide approval until extensive data relate occupational health effects to aerosol surface area, intuitively, an association between the surface area of insoluble lung deposits and health effects is anticipated.

Although workplace exposures to ultrafine particles are widespread and predicted to increase due to the advent of mainstream nanotechnology, the means of measuring aerosol exposures in terms of surface area are not readily available. The standard method for measuring surface area (the Brunauer–Emmett–Teller (BET) method (Brunauer et al., 1938)) is appropriate for relatively large quantities of powder only, but not suited to a rapid evaluation of aerosol surface area, particularly at low concentrations.

Research to find alternative methods for analyzing nanotechnology exposures from ultrafine particles in the workplace is ongoing. Woo et al. (2001) and Maynard (2003) have estimated surface area from measurements of number and mass concentration and, in the case of Woo et al. (2001) charge measurement. However, while these two methods may be suitable for rough estimates of surface area concentration, they are not designed for accurate exposure measurements in this emerging technology.

Other possible methods and instruments for measuring aerosol surface area include the diffusion charging (DC), transmission electron microscopy (TEM), and scanning mobility particle sizer (SMPS). Recently, Bukowiecki et al. (2002) used real-time instruments, including DC and particle counter, for characterizing combustion aerosols in the air. These experiments showed the possibilities offered by DC for the real-time monitoring of ambient aerosols.

In the present paper, three techniques—DC, TEM, and SMPS—for estimating aerosol surface area are evaluated and compared. Monodisperse silver particle agglomerates were used as the test aerosol. In addition, the responses from two DC models were investigated systematically for various particle morphologies. Unlike BET, these techniques are only sensitive to the outer surface of particles being assessed.