Article pubs.acs.org/Langmuir
Self-Polymerization of Dopamine in Acidic Environments without Oxygen Tung-Po Chen,† Tianchi Liu,‡ Tsan-Liang Su,† and Junfeng Liang*,‡ †
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Department of Civil, Environmental and Ocean Engineering, Charles V. Schaefer School of Egnieering and Science, and ‡ Department of Chemistry, Chemical Biology, and Biomedical Engineering, Charles V. Schaefer School of Egnieering and Science, Stevens Institute of Technology, Hoboken, New Jersey 07307, United States ABSTRACT: An alkaline environment and the presence of oxygen are essential requirements for dopamine polymerization. In this study, we are the first to demonstrate the selfpolymerization of dopamine through plasma-activated water (PAW) under acidic environments (pH < 5.5). Resulting poly(dopamine) (PDA) was characterized using Nanosizer, SEM, FTIR, UV−vis, 1H NMR, and fluorescence spectrophotometers and proved to have similar physical and chemical properties to those polymerized under a basic condition, except that the PDA particles formed in PAW were more stable and hardly aggregated at varied pHs. The PAW polymerization method avoids alkaline solutions and the presence of oxygen and thus extends the applications of dopamine polymerization, particularly in biomedical and pharmaceutical sciences.
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INTRODUCTION In recent years, efficient modification of material surfaces has become a crucial strategy in developing original functional materials, bioactive surfaces, and nanocapsules.1−4Among the diverse surface modification methods, poly(dopamine) (also known as PDA or dopamine−melanin) has attracted great attention because of its robust adhesion strength, optical property, and multifunctional biological abilities for wide applications in biomaterials, optics, synthetic membranes, electricity, and nanotechnology.1,2,5−8 More importantly, PDA can be applied to a variety of surfaces,6,8 and those modified surfaces have been developed as an antibacterial layer, fouling-resistant layer, and biocompatibility nanoparticles.1,2,4,8−11 Studies have confirmed that the optimal dopamine polymerization and thus PDA coating conditions are in alkaline solutions (pH > 7.5) and in the presence of oxygen.1−3,12 Some groups claimed that partial removal of oxygen can slow down the kinetics of polymerization, indicating that oxygen plays a crucial role in the polymerization process.1,2 This limits broad applications of PDA coating in many fields where an alkaline condition and oxygen are prohibited.3 In addition, formation of PDA using the conventional method leads to aggregation and precipitation. To solve these problems, other polymerization methods have been developed. It has been reported that singlet oxygen (1O2), superoxide (O2−), peroxide (O22−), and hydroxyl radicals (•OH) are essential active species for the self-polymerization of dopamine.1,13 Du and co-workers demonstrated that using UV irradiation to generate reactive oxygen species (ROS) can accelerate dopamine polymerization under acidic, basic, and neutral conditions.1 However, this method requires exposure to UV light for over an hour with controlled intensity (260 nm, 7.5 mW cm−2), and the kinetics of polymerization decreases under an acidic condition. Zheng et al. investigated how PDA can be polymerized in an acidic condition (pH ∼ 1) © 2017 American Chemical Society
under hydrothermal (high pressure and high temperature) conditions. The self-ionization of water to ROS (i.e., hydronium and hydroxyl ions) occurs under high temperature and high pressure conditions and has a long reaction time (from 16 to 36 h depending on solution pH).12 However, these studies indicated that ROS plays a significant role in initiating the polymerization of dopamine. Recent studies have shown that nonthermal plasmas can be used to generate plasma-activated solutions with reactive oxygen species, reactive nitrogen species, and other species.14−24 In this study, we report dopamine polymerization happens in plasmaactivated water (PAW) under acidic pHs (
