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Comparative Solubility of Nanoparticles and Bulk Oxides of Magnesium in Water and Lung Simulant Fluids

John Andrew Pickrell1, Mermagya Dhakal1, Sigfrido D. Castro2, Gunjan Gakhar1, Kenneth J. Klabunde3, and Larry E. Erickson2. (1) Diagnostic Medicine/Pathobiology, Kansas State University, Comparitive Toxicology Laboratories; College of Veterinary Medicine, 1800 N Denison, Manhattan, KS 66506, (2) Chemical Engineering, Kansas State University, Manhattan, KS 66506, (3) Chemistry, Kansas State University, Manhattan, KS 66506

Nanoparticles, particles with at least one dimension < 100 nanometers, can clear airborne smoke particles from the air. Small inhaled particles can deposit in the deep lung, evade phagocytosis and enter the interstitial space of the lung. Thus, the solubility of nanoparticles will determine the amount of time particles remain in the interstitial lung space with potential to irritate and injure. Nanoparticles may have smaller size and diffusion layer thickness as well as greater amounts of their mass on the surface and steeper concentration gradients than conventional macrocrystalline (MC) chemicals. We hypothesized that dissolution of particles of nanomaterials would be more rapid than MC particles of the same chemical. Initially, we investigated the solubility and the rate of dissolution of oxides of magnesium (MgO) at room temperature. We compared the relative solubility of magnesium (nanoactiveTM MgO (NATMMgO) and nanoactiveTM MgO plus (NATM MgO plus) (NanoScale Materials Inc, Manhattan, KS) to that of conventional MC MgO (MC MgO). These materials were added to three 1 liter acid washed glass beakers and the contents continuously stirred by magnetic teflon stirring bars. Lung simulant fluids were Hank's Balanced Salt Solution (HBSS; Sigma-Aldrich) and Dulbecco's Modified Eagle's Medium with low glucose (DMEM, Invitrogen). Dissolved magnesium was measured using ICP-AES; Accuris-141, Fisons Instruments, Beverely, MA). Short-term (20 minute) solubility of all 3 oxides of magnesium was correlated to the bicarbonate concentration of simulant fluids (p< 0.001): DMEM >> HBSS. All forms of MgO totally dissolved at room temperature in DMEM at 50 mg/L. When we repeated the dissolution at 37oC, 2 general trends were noted. First, NA TM MgO plus dissolved more rapidly than did NA TM MgO or MC MgO at 25 mg/500 ml (50 mg/L) for both DMEM and HBSS. NA TM MgO Plus dissolved more rapidly than did NA TM MgO or MC MgO at 250 mg/500 ml (500 mg/L) only in DMEM. NA TM MgO plus dissolves more rapidly, reflecting small microcrystalline size and high surface activity, This trend was amplified by a higher bicarbonate concentration in the lung simulant fluid. These results suggested that the dissolution would be rapid and persistence short for the MgO particles deposited in deep lung that avoided macrophage phagocytosis and entered the interstitial portion of the lung. The second trend was less pronounced and most likely was an auxiliary mechanism in effect only at high levels of aerosol exposures. Lower concentrations of MC MgO dissolved more rapidly than did NA TM MgO plus or NA TM MgO at 1-3 hr for DMEM and 24-48 hr for HBSS. The delay in response most likely reflected the time needed for initial fragmentation of the macrocrystals; the higher bicarbonate concentration that hastened dissolution and shortened the delay. During the time of MC MgO's had higher dissolution, there appeared to be either a smaller diffusion layer reflecting higher crystal convexity, a higher concentration gradient reflecting less intra-crystal stagnation, or both. Risks of health effects especially with NA TM MgO plus in the presence of substantial bicarbonate concentrations in lung simulant fluids would be minimal whether exposures were relatively low or somewhat higher. Future work will investigate the effect of diesel oil or carbon combustion smoke particles binding to NA MgO. This research was partially supported through the award of a contract from the United States Marine Corps Systems Command to M2 Technologies, Inc.