Development of negatively charged particulate surfaces through a dry plasma-assisted approach

A dry two-step plasma process is introduced for the fabrication of particulate surfaces showing negative charges over a wide range of pH. Plasma polymerized thiophene (PPT) was initially deposited onto silica particles using an inductively coupled plasma polymerization reactor fitted with a rotating barrel. Sulfur-functionalized particles were further chemically modified through an oxidative air or water plasma treatment. Wide ranges of plasma specific energies (0.06-2.4 kJ cm(-3)) and treatment times (5-60 minutes) were employed to manipulate the surface chemistry, hydrophobicity and surface charge of the silica particles. Surface chemistry of the modified silica particles was studied using X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). Changes in hydrophobicity and surface charge of the modified particles were quantified via Washburn capillary rise measurements and electrokinetic analysis, respectively. Plasma treatment of PPT coated particles resulted in homogenous formation of -SOx(H) functionalities such as sulfonate (SO3-), sulfonic acid (SO3H), and sulfate (SO42-) on surfaces. Such changes in surface chemistry significantly decreased the zeta potential and isoelectric point of the particles as well as their degree of hydrophobicity. In comparison to air plasma, water plasma was found to be a better candidate for the treatment of PPT coated particles as it produced surfaces with lower zeta potentials and isoelectric points. Our introduced solvent-free approach is applicable for the modification of almost any other particles regardless of their shape and surface chemistry. Such surface engineered particles could be utilized as protein detectors/adsorbents, solid-state catalysts, and heavy metal removal agents.