Self-assembly of Colloidal Particles within Microdroplets: Mechanisms and Applications
Jiyuan Yao is a PhD student in the department Biomedical and Environmental Sensorsystems. (Co)Promotors are prof.dr.ir. L.I. Segerink and dr. S. Pud from the faculty of Electrical Engineering, Mathematics and Computer Science from the University of Twente and prof.dr. L. Shui from the South China Normal University.
The aim of this thesis is to explore evaporation-induced colloidal self-assembly within droplet microfluidics, with particular focus on the underlying mechanisms and potential applications, as detailed in Chapter 2, 4 and 5. To address the limitations associated with alkane oils with specific solvent systems, we developed a robust synthesis protocol for fluorosurfactants and assessed their effectiveness in droplet microfluidics (Chapter 3). The chapters of this thesis are summarized as follows.
In Chapter 2, we developed an electrochemical sensor for methyl parathion (MP) detection by integrating uniform honeycomb carbon nanotube (CNT) microparticles (h-CNT-μPs) with a glassy carbon electrode (GCE). This process leveraged physical interactions — namely the balance among capillary force, electrostatic force, and van der Waals force — to facilitate the self-assembly of binary colloids consisting of carboxylated carbon nanotubes (CNTs-COOH) and silica nanoparticles (SiO2 NPs) into homogenous supraparticles. Subsequently, the h-CNT-μPs were obtained through selectively etching SiO2 NPs with a 20.0 wt% hydrofluoric acid (HF) solution, which allowed for adjustable porosity ratios by varying the ratio of SiO2 NPs.The produced h-CNT-μPs were integrated with the GCE, significantly enhancing surface area and electrical conductivity for electrochemical sensing. Under optimized conditions, h-CNT-μPs/Nafion/GCE exhibited linear response to MP over concentration ranges of 0.3-20.0 μM and 20.0-150.0 μM, with a detection limit of 0.092 μM (S/N = 3), and demonstrated excellent sensitivity, selectivity, reproducibility and anti-interference performance.
Alkane oils such as hexadecane, mineral oil, and silicone oil, when combined with conventional surfactants, exhibit certain limitations in colloidal self-assembly and related to biological applications. Therefore, in Chapter 3, we proposed a robust protocol for fluorosurfactants synthesizing via a two-step reaction, combining a mixed anhydride reaction with an amidation reaction. The achieved yield was comparable to the conventional synthetic protocols, while offering notable advantages including rapid synthesis and tolerance to a wide temperature range (0-60 ℃). We evaluated the interfacial behavior of the synthesized fluorosurfactants at the water-fluorinated oil interface by measuring interfacial tension using the pendant drop technique, demonstrating that the interfacial tension decreases as fluorosurfactants concentration increase. Furthermore, water-in-fluorinated oil microdroplets stabilized by our synthesized fluorosurfactants exhibited satisfactory droplet stability, excellent dye encapsulation, and biocompatibility for cell culture.
Following the protocol developed for synthesizing fluorosurfactant (Chapter 3), we expanded our study in Chapter 4 to focus on the intermediate fluorosurfactants PFPE(H)-COOCOOC2H5 and their interfacial interactions within droplet microfluidics. One such interfacial interactions, driven by the highly electronegative segment of the mixed anhydride between the fluorosurfactants and water, forming hydrogen bond, spontaneously facilitated the formation of satellite droplets and selective transfer of fluorophores. In addition, this fluorosurfactant exhibited another interfacial interaction with metal-based colloids (copper-based oxide nanoparticles, CuxO NPs), which allowed to structure the colloidal self-assembly of the CuxO NPs into the shape of collapsed colloidosomes composed of CuxO NPs, distinct from the typical spherical supraparticles.
In Chapter 5, we further investigated the effect of interfacial jamming of metal-based colloids caused by interactions with the fluorosurfactants during the evaporation-induced self-assembly within microdroplets. Building upon the findings in Chapter 4, we demonstrated that coordination interaction leads to the interfacial trapping of metal-based colloids by fluorosurfactants, influencing the morphology of colloidal self-assembly. In addition, we show that this interfacial interaction can be eliminated by coating of the metal-based colloids with a Si2O layer. The pendant drop technique was employed to visualize the interfacial trapping, and XPS analysis was used to examine the coordination interaction. Furthermore, we found that the morphologies of colloidal self-assembly could be tuned by varying the concentration of metal-based colloids; the morphologies transitioned from colloidosomes to supraparticles when the concentration was high enough to geometrically exclude the possibility of the colloidosome formation. We tested the two types of MS NPs (CdS and ZnS) with three fluorosurfactants, including those synthesized in Chapter 3, PFPE(H)-Tris and the commercial one (Pico SurfTM). The combinations of MS NPs and the fluorosurfactants featured different levels of coordination interaction from strong (ZnS NPs-PFPE(H)-Tris) to weak (CdS NPs-PFPE(H)-Tris, ZnS NPs-PFPE(H)2-ED900, and Zns NPs-Pico SurfTM) and negligible (CdS NPs-PFPE(H)2-ED900). Finally, based on these findings, we constructed binary colloidal self-assemblies with either homogeneous or core-shell distributions.
This thesis provides insights on fluorosurfactants synthesis and how to engineer the morphologies and distributions of colloids in evaporation-induced colloidal self-assembly within microdroplets. It also elucidates the interfacial interactions between fluorosurfactants and metal-based colloids, thereby contributing to the fundamental understanding of the processes driving the colloidal self-assembly.
