The effect of functionalization on solubility and plasmonic features of gold nanoparticles

Highlights

• Functionalization of gold nanoparticles change their plasmonic features.

• Functionalization make gold NPs safer for using in hyperthermia.

• Adding ligands on the gold NPs make them stable and prevent them from aggregating

• Functionalized gold nanoparticles are more soluble than naked ones.

Abstract

Effect of functionalization on stability, solubility, and plasmonic features of gold nanoparticle with the general formula of Au₁₈(SR)₁₄ in water solvent has been studied in this work. Thiol functional groups including 1,1-mercapto-ethyl alcohol, s-cysteamine, thioglycolic acid, and beta-mercaptoethanol have been used. Electronic band-gap, excitation energies, dipole moment, and hardness for all gold nanoparticles in water solvent were investigated using the quantum mechanical approach. Intermolecular forces, radial distribution function (RDF), mean square displacement (MSD), and solvation free energy were calculated by using simulation methods. Electronic band-gap, and excitation energy analysis show that surface modification of gold nanoparticles can change their electronic and plasmonic properties. The analysis of dipole moments indicates that ligands affect the nanoparticle’s solubility. An increase of hardness and therefore chemical stability can be observed for functionalized nanoparticles compared to the bare structure. Intermolecular energies analyses suggest that structure with 1,1-mercapto ethyl alcohol ligand has the strongest interaction with the solvent. The analysis of RDF diagrams also indicates that the molecule with 1,1-mercapto ethyl alcohol ligand has the sharpest pick. The slope of the linear part of MSD diagrams that is the criterion of solute’s lateral diffusion is the highest value for nanoparticle with 1,1-mercapto ethyl alcohol ligand. Furthermore, functionalization also affects solvation free energy contributions. According to obtained data of quantum mechanical calculations and molecular dynamics simulations, it may be concluded that particle with 1,1-mercapto ethyl alcohol is the best ligand for increasing solubility, stability, and plasmonic functions of Au₁₈(SR)₁₄ structures among the examined ones.

Introduction

In recent years, nanoparticles have attracted extensive attention. By reducing a material size to the nanometre length scale, the role of the surface increase and consequently some of their properties will change [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Bulk metals are chemically inert but some metals such as gold, platinum, and silver in nano scale size are active and they can be easily oxidized [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. To improve the application of nanoparticles, three factors should be considered, stability, reactivity, and function [22]. Due to high reactivity, they can easily interact with each other and with functional groups; thus, they are thermodynamically and kinetically unstable, especially in solvents [[23], [24], [25], [26]].

To prevent nanoparticles from aggregation, a stabilizing environment is needed [27,28]. Some ligands may be used on the surface of these nanoparticles to make them stable and they are called surface chemical microenvironment (SCME) [[29], [30], [31]]. Moreover, SCME can affect the application of NPs ranging from biotechnology to microelectronics [32]. Some atomic groups in the surface of nanoparticle’s structure such as thiolates, amines, carboxylates, and phosphines enable them to modulate by suitable water-soluble ligands and have good dispersion efficiency in solvents such as water [[33], [34], [35], [36]].

Gold nanoparticles are some of the nanomaterial having been studied very much. They have some good properties such as surface plasmon resonance (SPR) that make them useful in many applications from simple optical sensing to solar energy conversion, and so on [[37], [38], [39], [40]]. Furthermore, they are capable to be used in other fields such as electronics, catalysis, biotechnology, and cancer therapy (drug delivery agents and hyperthermia). To protect gold NPs from aggregation, they are capped by organic layer (organic ligands) [41]. Thiols are commonly used in the synthesis of AuNPs as surfactants [[42], [43], [44], [45]]. Without using proper stabilizers on the gold NPs, they cannot maintain their structures and will aggregate, causing them to lose their plasmonic functionality and their solubility [[46], [47], [48], [49], [50]].

Brian J. Heinz et al. by using molecular dynamics simulations examined density, binding energy, and solubility of functionalized gold nanoparticles. They used some functional groups such as CH₃, OH, and NH₂ at the tail of alkanethiol ligands (S–(CH₂)₈–X) and observed that with increasing electronegativity of terminal groups, particle-particle binding energy (solubility parameter) is minimized [51]. Jia-Qi Lin et al., using coarse-grained molecular dynamics simulations, studied the dynamics of 2.2 nm monolayer protected gold nanoparticles in solvents. They examined the effects of ligand length, ligand terminal chemistry, solvents, and temperature. It had been found that AuNPs with unmodified alkanethiol and with short ligand tails formed stable and amorphous aggregations in water respectively whereas long-tailed AuNPs aggregated into a spherical cluster. In addition, it was investigated that increasing the polarity of ligand terminals weakened the tendency of aggregation of AuNPs in water [52]. Elena Heikkila et al. studied charged monolayer-protected gold nanoparticles in aqueous solution by using atomistic molecular dynamics simulations. They studied Au₁₄₄ nanoparticles that have a spherical Au core and functionalized alkanethiol chains. Amin and carboxyl-terminal groups and Cl⁻/Na⁺ counter ions were used to make cationic and anionic AuNPs. Results revealed that side chains and terminal groups have significant flexibility. Moreover, it was found that long-range electrostatic interactions play a significant role in determining nanoparticle properties in aqueous solutions [53]. Silvia Barbosa et al. studied gold nanoparticles as core particles functionalized with the anticancer drug, doxorubicin, with a cleavable heterobifunctional cross-linker to make its release easy under the action of reducing enzymes. They also used folic acid as a cancer cell identifier on the surface of gold nanoparticles. It was observed that cellular uptake enhance when the AuNPs decorated with targeting ligands [54]. Ryo Lida et al. studied assembly behaviours of gold nanoparticles with 3, 5, and 10 nm diameter coated with a self-assembled monolayer of ethyl, iso-propyl, and propyl headed oligo ethylene glycol ligands at different temperatures. They found that slight changes in the hydrophobicity of the alkyl head and diameter can affect their assembly temperature. The ligands with shorter tail length provide lower assembly temperature [55]. Christopher J et al. have reported on the results of Brust et al. [56] for organothiol monolayer-protected gold clusters. They tested the capacity to stabilize Au₅₅ cores by exchanging thiolate-for-phosphine. They found that straight chains as long as Pentanthiol on the NPs can make remarkable stability, but for structures with C₃; C₄ alkanethiols have lower stability [57].Many other works have studied gold nanoparticles with different water soluble functions on them such as homocysteine, cysteine, glutathione, thioglycolic acid,Tetraethylen glycol, lysin, mercaptobenzoic acid, mercapto propionic acid, methionine, triphenylphosphine, mercapto-poly carboxylic acids, tiopronin, and so on [[58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73]]. In addition, many studies have been conducted to investigate the effect of different ligands such as polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB), polyvinyl pyrrolidone (PVP), some anticancer drugs such as doxorubicin, etc. on their plasmonic excitation and colloidal stability [[74], [75], [76], [77], [78], [79]].

Our work is aimed at studying the effect of different ligands (anchoring groups) on the dispersion stability and electronic properties of Au₁₈S₁₄ nanoparticle as the smallest crystallography thiolate gold structure found until now [80]. To investigate the effect of ligands on the solubility degree of NPs, quantum mechanical DFT calculations and molecular dynamics simulations have been performed and some properties such as electronic band-gap, excitation energies, excited state life-time, dipole moment, chemical potential, hardness, intramolecular energies, radial distribution function, mean square displacement, and solvation free energies have been examined.