Roles of zwitterionic charges in polymers on synthesis of Ag seeds with anisotropic growth properties

Abstract

Polymeric materials have been widely used for the shape-controlled synthesis of metallic nanoparticles. In addition to coordination between the polymers and the particle surfaces during seed growth, a few studies have reported the anisotropic growth as the result of polymer intervention solely during seed formation. However, this mechanism has not been thoroughly elucidated. In this work, we synthesize silver nanoplates (AgNPLs) and investigate the influence of two representative zwitterionic polymers, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and poly(sulfobetaine methacrylate) (PSBMA), on Ag seed formation. PMPC and PSBMA carry net zero global charges and non-zero local charges, which are ideal for investigating polymer–Ag⁺ interactions during the early stage of nanoparticle synthesis. The non-zwitterionic polymers carry different global charges, and they are examined in parallel with the zwitterionic polymers. X-ray photoelectron spectroscopy reveals that coordinate bonding between Ag⁺ and oxygen atoms in polymer functional groups strongly promotes the anisotropic growth of AgNPLs, even without the addition of polymers during growth. In contrast, nitrogen atoms in the polymers inhibit anisotropic growth. Our findings are further generalized in the context of seed-mediated growth by growing the seeds synthesized with the zwitterionic polymers at various concentrations, with various number average molecular weights, and at different temperatures.

Introduction

Anisotropic silver (Ag) nanomaterials exhibit distinct chemical and physical properties, such as surface plasmon resonance and surface-enhanced Raman scattering, that depend on their structures and sizes [1], [2], [3]. In the seed-mediated growth of an anisotropic Ag nanomaterial, shape is directed during the growth of Ag seed particles using various shape-directing agents (SDAs) [4]. These include cetyltrimethylammonium bromide (CTAB) [5], [6], citrate ions [7], halide ions [8], [9], [10], DNA [11], [12], and polymers [13], [14], [15], [16], [17], [18], [19], [20]. Although the fundamental synthetic mechanisms are not fully understood, SDAs are believed to selectively bind to specific facets of the seed particles. Growth in the corresponding crystallographic directions is inhibited, and the Ag nanomaterials grow anisotropically. Efforts have also been made to manipulate the properties of the seeds during their formation to achieve anisotropic growth without the additional SDAs during the growth. A powerful strategy involves removing unstable, non-twinned seeds using an oxidative etchant, such as H₂O₂. Only twinned seeds remain, which can then grow into anisotropic silver nanoplates (AgNPLs) without an SDA [7]. Another approach is to employ a polymeric material, such as poly(sodium 4-styrenesulfonate) (PSSS), during seed formation to direct nanoparticle growth [18]. While the specific mechanism is somewhat unclear, Ag seeds synthesized with PSSS grow into highly uniform AgNPLs, the size of which can be easily controlled. Subsequent investigation suggests that the charge of the polymer governs anisotropic seed growth [21]. The addition of a neutral or anionic polymer reproducibly affords seeds that can grow into AgNPLs with a narrow size distribution. In contrast, the use of a cationic polymer results in the formation of randomly shaped nanoparticles with non-uniform sizes. These findings strongly suggest that polymers and silver ions (Ag⁺) interact during seed formation. Two cooperative binding mechanisms have been proposed, one being electrostatic attraction between the negatively charged polymers and Ag⁺. Alternatively, coordinate bonds may form between Ag⁺ and oxygen (O) or nitrogen (N) atoms in the polymers via their lone pairs. Although electrostatic interactions provide a reasonable and persuasive explanation, they do not explain the results obtained with uncharged polymers. Specific coordination chemistry could provide an alternative explanation, but it is rarely reported as part of the synthetic mechanism. Importantly, electrostatic attraction alone does not fully explain interactions between Ag⁺ and a diffuse layer of polymer molecules. This is because the charge of a polymer utilized for seed formation is difficult to control.

In contrast to negatively charged and uncharged polymers, zwitterionic polymers can reveal information about the roles of polymers in AgNPL synthesis. This is because they carry a net charge of zero at neutral pH while carrying local anionic and cationic charges simultaneously. To date, zwitterionic polymers have mainly been used for antifouling surface modification, hydrogel synthesis, drug delivery, and as anti-freezing agents [22], [23], [24], [25], [26], [27]. These conventional applications exploit the electrostatic properties of zwitterionic polymers, which has immensely expanded the respective fields of research [28], [29], [30], [31], [32]. The dispersion and stabilization of colloidal and two-dimensional (2D) nanomaterials using zwitterionic polymers has recently attracted interest [33]. For example, various poly(sulfobetaine)s were used for surface modification of Au, Ag, and Pd nanoparticles [34]. Surface modification significantly increased particle stability, even at high salt concentrations. This result was attributed to the “salting-in” behavior of zwitterionic polymers, wherein they become more soluble as the ionic strength increases. Gold nanoparticles coated with a different zwitterionic sulfobetaine polymer dispersed uniformly, regardless of temperature [35]. The excellent thermostability of the particles was ascribed to the formation of a dense polymer layer on their surfaces and gelation after swelling. The unique molecular structures of zwitterionic polymers and their correspondingly unique properties have been exploited to control the chemical and physical properties of nanoparticles. To the best of our knowledge, however, zwitterionic polymers are rarely utilized as SDAs for the synthesis of anisotropic metal nanoparticles.

Herein, we demonstrate how the charges and molecular structures of polymeric materials influence the chemical properties of Ag seed particles, which govern their growth into AgNPLs. Although these phenomena have been touched upon in previous reports, no detailed mechanistic study has been conducted owing to the limited range of the examined polymers. In this work, zwitterionic polymers were used as functional chemical tools to further our understanding of the roles of polymers during Ag seed particle formation and their subsequent anisotropic growth. This enabled us to identify different types of polymer–Ag⁺ interactions and elemental trends at the molecular level. The main text of the article should appear here with headings as appropriate.