Controlled synthesis of SPION@SiO₂ nanoparticles using design of experiments

The synthesis of single-core superparamagnetic iron oxide nanoparticles (SPIONs) coated with a silica shell of controlled thickness remains a challenge, due to the dependence on a multitude of experimental variables. Herein, we utilise design of experiment (DoE) to study the formation of SPION@SiO2 nanoparticles (NPs) via reverse microemulsion. Using a 33 full factorial design, the influence of reactant concentration of tetraethyl orthosilicate (TEOS) and ammonium hydroxide (NH4OH), as well as the number of fractionated additions of TEOS on the silica shell was investigated with the aim of minimising polydispersity and increasing the population of SPION@SiO2 NPs formed. This investigation facilitated a reproducible and controlled approach for the high yield synthesis of SPION@SiO2 NPs with uniform silica shell thickness. Application of a multiple linear regression analysis established a relationship between the applied experimental variables and the resulting silica shell thickness. These experimental variables were similarly found to dictate the monodispersity of the SPION@SiO2 NPs formed. The overall population of single-core@shell particles was dependent on the interaction between the number of moles of TEOS and NH4OH, with no influence from the number of fractionated additions of TEOS. This work demonstrates the complexity of the preparative method and produces an accessible and flexible synthetic model to achieve monodisperse SPION@SiO2 NPs with controllable shell thickness

Reversible Microscale Assembly of Nanoparticles Driven by the Phase Transition of a Thermotropic Liquid Crystal

The arrangement of nanoscale building blocks into patterns with microscale periodicity is challenging to achieve via self-assembly processes. Here, we report on the phase-transition-driven collective assembly of gold nanoparticles in a thermotropic liquid crystal. A temperature-induced transition from the isotropic to the nematic phase under anchoring-driven planar alignment leads to the assembly of individual nanometer-sized particles into arrays of micrometer-sized agglomerates, whose size and characteristic spacing can be tuned by varying the cooling rate. Phase field simulations coupling the conserved and nonconserved order parameters exhibit a similar evolution of the morphology as the experimental observations. This fully reversible process offers control over structural order on the microscopic level and is an interesting model system for the programmable and reconfigurable patterning of nanocomposites with access to micrometer-sized periodicities

Reversible Microscale Assembly of Nanoparticles Driven by the Phase Transition of a Thermotropic Liquid Crystal

The arrangement of nanoscale building blocks into patterns with microscale periodicity is challenging to achieve via self-assembly processes. Here, we report on the phase-transition-driven collective assembly of gold nanoparticles in a thermotropic liquid crystal. A temperature-induced transition from the isotropic to the nematic phase under anchoring-driven planar alignment leads to the assembly of individual nanometer-sized particles into arrays of micrometer-sized agglomerates, whose size and characteristic spacing can be tuned by varying the cooling rate. Phase field simulations coupling the conserved and nonconserved order parameters exhibit a similar evolution of the morphology as the experimental observations. This fully reversible process offers control over structural order on the microscopic level and is an interesting model system for the programmable and reconfigurable patterning of nanocomposites with access to micrometer-sized periodicities.</p

Reversible microscale assembly of nanoparticles driven by the phase transition of a thermotropic liquid crystal

The arrangement of nanoscale building blocks into patterns with microscale periodicity is challenging to achieve via self-assembly processes. Here, we report on the phase-transition-driven collective assembly of gold nanoparticles in a thermotropic liquid crystal. A temperature-induced transition from the isotropic to the nematic phase under anchoring-driven planar alignment leads to the assembly of individual nanometer-sized particles into arrays of micrometer-sized agglomerates, whose size and characteristic spacing can be tuned by varying the cooling rate. Phase field simulations coupling the conserved and nonconserved order parameters exhibit a similar evolution of the morphology as the experimental observations. This fully reversible process offers control over structural order on the microscopic level and is an interesting model system for the programmable and reconfigurable patterning of nanocomposites with access to micrometer-sized periodicities

Stimuli Responsive Liquid Crystal-Nanoparticle Composites

Composites of liquid crystals and nanoparticles offer unique properties for applications including displays, sensors, plasmonics and metamaterials. Their potential is two-fold: the anisotropic and ordered nature of liquid crystals provides a promising medium in which to control characteristics and direct self-assembly of nanoparticles; likewise, inclusions of nanoparticles tune attributes of liquid crystals, such as the ordering and optical properties. In this work, the thermotropic liquid crystal, 4-cyano-4’-pentylbiphenyl, acted as a complex medium for the manipulation of functionalised gold nanoparticles. The research presented herein follows three streams. First, to develop a deep understanding of the materials used, novel methodologies were developed. The nanoparticle entropy provides an assumption-free measurement of dispersity for nanoparticle populations. Furthermore, a spatial distribution function was constructed for the 2D evaluation of colloidal ordering that highlights structure, periodicity, and anisotropic ordering. Software was produced for both techniques and made freely available. Second, the synthesis and functionalisation of gold nanoparticles was studied and optimised to promote miscibility in the liquid crystal. A design of experiments-based approach was implemented to develop an understanding of key experimental conditions and interactions during synthesis and functionalisation. Third, the effects of various stimuli on the liquid crystal mediated self-assembly of gold nanoparticles were investigated. The isotropic to nematic phase transition was leveraged to form reversible arrays of nanoparticle aggregates with tuneable size and interaggregate spacing. Furthermore, procedures for the induction of concentration gradients of nanoparticles in the composite material were established through exposure to light and heat. Overall, this work provides a framework for the reliable preparation of highly soluble gold nanoparticles in complex media as well as novel insights on stimuli-responsive liquid crystal-nanoparticle composites with respect to temperature and light. Finally, both accompanying methodologies, i.e. the nanoparticle entropy and spatial distribution function, can be applied to a wider range of scientific problems

Information Entropy as a Reliable Measure of Nanoparticle Dispersity

Nanoparticle size impacts properties vital to applications ranging from drug delivery to diagnostics and catalysis. As such, evaluating nanoparticle size dispersity is of fundamental importance. Conventional approaches, such as standard deviation, usually require the nanoparticle population to follow a known distribution and are illequipped to deal with highly poly- or heterodisperse populations. Herein, we propose the use of information entropy as an alternative and assumption-free method for describing nanoparticle size distributions. This approach works equally well for mono-, poly- and heterodisperse populations and provides an unbiased route to evaluation and optimisation of nanoparticle synthesis. We provide an intuitive tool for analysis with a user-friendly macro and provide guidelines for interpretation with respect to known standards

Application of the Spatial Distribution Function to Colloidal Ordering

2D colloidal assembly is a vital process in the fabrication of nanostructured devices and remains of widespread interest in fundamental research. Characterising the ordering is crucial to develop an understanding of the driving forces behind the assembly and to optimise processing conditions. Image analysis offers a direct evaluation pathway, typically via the radial distribution function or the 2D-fast Fourier transform. Both methods have inherent limitations; the former provides no angular dependence while the latter is challenged when confronted with imperfection on the mean size, spacing and coverage of the building blocks. Here, we introduce the 2D spatial distribution function (SDF) as an alternative pathway to evaluate colloidal ordering. We benchmark the method in case studies of prominent examples and provide a tool-kit for implementation, either as imageJ plugin or standalone software. Application and interpretation is straightforward and particularly powerful to analyse and compare colloidal assemblies with limited order

DoE-It-Yourself: A Case Study for Implementing Design of Experiments into Nanoparticle Synthesis

Predictable and repeatable outcome is a major issue in nanoparticle synthesis. Traditionally, a one-factor-at-a-time (OFAT) method is relied upon to investigate and optimise synthetic processes; however, this method is inefficient and often misleading. Design of experiments (DoE), in contrast, can provide a greater amount of information in fewer experiments and lends itself to more reproducible results. Nevertheless, DoE techniques are only used by a relatively low number of practitioners in nanoparticle research. Here, we provide a step-by-step tutorial for the synthesis of oleylamine-capped gold nanoparticles (AuNPs). Through the use of DoE, we are able to achieve a marked reduction in dispersity and develop a model for detailed control over the mean diameter of the nanoparticle populations. Principles of the case study presented herein are applicable and should serve for facile implementation of DoE to other synthetic routes

A Toolkit to Quantify Target Compounds in Thin Layer Chromatography Experiments

Thin layer chromatography (TLC) is one of the basic experimental procedures in chemistry and allows the demonstration of various chemical principles in an educational setting. An often-overlooked aspect of TLC is the capability to quantify isolated target compounds in an unknown sample. Here, we present a suitable route to implement quantitative analysis in a lesson plan. We provide a free, stand-alone software that allows students to obtain quantitative information and present two suitable experiments, namely the absorbance-based quantification of the colorant Sudan IV and the fluorescence-based quantification of Rhodamine 6G, a fluorophore widely used in biotechnology. Students conduct TLC experiments following established routes, then take pictures of their TLC plates with their mobile phones and finally quantify the amount of different compounds in the separate bands they observe

Comparative characterisation of non-monodisperse gold nanoparticle populations by X-ray scattering and electron microscopy

Accurate nanoparticle size determination is essential across various research domains, with many functionalities in nanoscience and biomedical research being size-dependent. Although electron microscopy is capable of resolving a single particle down to the sub-nm scale, the reliable representation of entire populations is plagued by challenges in providing statistical significance, suboptimal preparation procedures and operator bias. While alternative techniques exist that provide ensemble information in solution, their implementation is generally challenging for non-monodisperse populations. Herein, we explore the use of small-angle X-ray scattering in combination with form-free Monte Carlo fitting of scattering profiles as an alternative to conventional electron microscopy imaging in providing access to any type of core size distribution. We report on a cross-method comparison for quasi-monodisperse, polydisperse and bimodal gold nanoparticles of 2-7 nm in diameter and discuss advantages and limitations of both techniques