First Evidence of Singlet Oxygen Species Mechanism in Silicate Clay for Antimicrobial Behavior
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First Evidence of Singlet Oxygen Species Mechanism in Silicate Clay for Antimicrobial Behavior Jiun-Chiou Wei1, Yi-Ting Wang1 and Jiang-Jen Lin1* 1 Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan. ABSTRACT Thin silicate nanoplatelets, derived from the exfoliation of natural Sodium montmorillonite (Na+-MMT) clays, show an unexpected antimicrobial property. A physical trapping mechanism has been proposed because the clay nanoplatelets can indiscriminately inhibit the growth of a broad spectrum of bacteria, including drug-resistant species such as methicillin-resistance S. aureus (MRSA) and silver ion-resistant E. coli. The ability to generate singlet oxygen species was first observed for the clay platelets that showed a high-aspect-ratio geometric shape and the presence of surface ionic charges. By comparison, the pristine clay with a multilayered structure failed to generate any singlet oxygen species. The ability to emit singlet oxygen species provides direct evidence for the antimicrobial ability of clay through a non-chemical mechanism, which opens the potential for medical use. INTRODUCTION Recent advances in our understanding of the nanostructures of inorganic materials has often led to the discovery of new functions such as quantum effect [1], conductivity [2], electricity [3], and biomedicinal effects [4]. Various nanosized materials, including C60 derivatives [5] and ZnO [6], are able to generate reactive oxygen species and subsequently inactivate bacteria and viruses. Inorganic materials of particular interest are natural silicate minerals [7] such as MMT, fluorinated micas, hectorite, and saponite, which are conventionally considered to be inert to microorganisms [8,9]. Recent developments have revealed that the hydrated minerals of iron-rich clays could exhibit remarkable antibacterial activity. This bactericidal ability is derived from compositions such as the Fe2+ species that are released from the clay under aqueous buffer pH conditions [10]. Finding new alternatives to inhibit these antibiotic-resistant bacteria is an important research task. Recently, we developed a method to manipulate the natural silicate clay of a multilayered structure into nanoscale silicate platelets (NSP) [11,12]. Examination of these exfoliated nanoplatelets revealed their unexpected antimicrobial property [13]. Previously, a series of tests on cytotoxicity and genotoxicity has demonstrated that these platelets have a low level of acute toxicity [14]. The unusual characteristics of the NSP nanoplatelets prompted us to investigate further their mechanism of antimicrobial activity against the drug-resistant bacterial strains. Here, we used a physicochemical method (electron paramagnetic resonance radical trapping) to detect the generation of reactive oxygen species (ROS) and propose a physical mechanism for their antimicrobial properties. EXPERIMENTAL DETAILS Na+-MMT, a natural smectite aluminosilicate, was obtained from Nanocor Co. (USA). The surfactants, cationic dodecyltrimethyla
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