Unexpected Dual Action of Cetyltrimethylammonium Bromide (CTAB) in the Self‐Assembly of Colloidal Nanoparticles at Liquid–Liquid Interfaces

Abstract

elegant and effective way to obtain 2D NP arrays is through bottom-up self-assembly of colloidal particles at the interface between two immiscible liquids. [5][6][7][8] We are interested in preparing interfacial NP arrays using methods which cause minimum perturbation of the NP surface chemistry so that the chemical properties of the constituent particles are retained in the array. In particular, it is important to avoid modifying the surfaces of particles through adsorption of strongly bound molecular passivating layers because these will interfere with the functionality of the array, e.g., by preventing intimate contact between the surface and reactants in catalysis or hindering the close approach of target analytes to the surface which is essential for surface-enhanced Raman spectroscopy (SERS). [9,10] The ideal method for interfacial assembly would be one in which the initial colloidal suspension is simply shaken with pure nonaqueous solvent and the particles spontaneously migrate to the interface. Unfortunately, while adsorption of solid NPs to liquidliquid interfaces (LLIs) is highly favorable, since it significantly lowers the energy of the interface, this is counterbalanced by electrostatic repulsion between NPs. This means that NPs seldom spontaneously migrate to LLIs in significant numbers when aqueous colloids are simply mixed with highly immiscible oils. [11] Conventionally, films may be formed by functionalizing the surfaces of charged colloidal NPs with charge-neutral organic "modifiers" to reduce repulsion but this means that the surface of the NPs is covered in a layer of strongly adsorbed organic capping agents, which is undesirable for the reasons outlined above. [12,13] Alternatively, less polar co-solvents such as acetone and ethanol may be added, which reduce the ability of the aqueous phase to solvate ionic compounds and again significantly weaken the electrostatic repulsion between NPs. [14,15] Although this method can be readily applied to assemble various types of NPs, the loss of surface charge also leads to unwanted aggregation of the particles and the formation of structural defects in the product NP array. [16] We have previously shown a third approach to induce selfassembly of colloidal NPs into defect-free NP arrays, which is based on adding amphiphilic salts to the biphasic system. [10,17,18] Self-assembly of colloidal nanoparticles at water-oil interfaces offers an efficient way to construct multi-dimensional arrays. Self-assembly is generally induced by using adsorbing molecular "modifiers" or co-solvents to remove the nanoparticles' surface charge. Here, it is shown that cetyltrimethylammonium bromide (CTAB), which is commonly used in bulk quantities in colloidal synthesis, can induce self-assembly of negatively charged colloidal nanoparticles at water-oil interfaces, even at sub-micromolar levels but it does this by providing charge screening rather than removing the surface charge. Since this is a physical effect, CTAB can promote assembly of nanoparticles regardless of their ...

Self-assembly of colloidal nanoparticles into 2D arrays at water–oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response

“…In addition to the work using combinations of agents for nanoparticle self-assembly, we have recently shown that it is also possible for a single compound to assume dual roles in nanoparticle self-assembly. In this work we showed that hexadecyltrimethylammonium bromide (CTAB), which is an amphiphilic surfactant molecule commonly used as a growth directing agent and stabilizing agent in the synthesis of plasmonic nanoparticles can also induce self-assembly of Au nanoparticles at final concentrations between 3.5×10 -7 -5.6×10 -6 M. [61] Moreover, we showed that depending on its concentration CTAB acted as a modifier and/or promoter in the self-assembly system. This is possible because a proportion of the CTAB added to the oil-water system will dissolve into the oil phase as CTA + anion and act as a promoter while the remainder of the CTAB will adsorb onto the surface of Au particles and act as charge-neutral CTAB modifiers.…”

Mimicking the Microbial Oxidation of Elemental Sulfur with a Biphasic Electrochemical Cell

“…The preparation of homogeneous and stable AuNP films at immiscible L|L interfaces is a major challenge. [21,23,24] Usually the surface is not homogeneously covered, significant aggregation of particles takes place at the interface and potential cycling can induce movement of the particles generating an interfacial convection (interfacial stirring). [25] Citrate ions are typically used to stabilize AuNP suspensions in bulk aqueous solutions.…”

“…[30][31][32] The latter act as "promoters" if the salts contain organic soluble ions of opposite charge to the NPs that screen inter-particle electrostatic repulsion. [23] Here, we replace the citrate buffer solution with a phosphate buffered saline (PBS) solution, pH 7.4, see electrochemical cell 1 in Scheme 1. The PBS-capped AuNP suspensions remain stable in the bulk aqueous phase.…”

From Dilute to Multiple Layers: Bottom‐Up Self‐Assembly of Rough Gold Nanorods as SERS Platform for Quantitative Detection of Thiram in Soil

“…[ 24–26 ] Among these techniques, bottom‐up self‐assembly is a simple, scalable, and low‐cost approach to achieve uniform and highly effective SERS substrates, such as droplet evaporation, [ 27–29 ] electrostatic interaction, [ 30,31 ] and interface self‐assembly. [ 32–37 ]…”