Toward Ideal Sphere Packing: Solvent-Free Single-Layer Assembly for Chromatography

Ordered packing structures are considered the holy grail in liquid chromatography (LC) to overcome the 50 – 60% separation loss due to the infamous eddy dispersion in randomly structured beds. However, with the current state-of-the-art column packing technology involving spherical silica particle slurries pushed into stainless steel columns or fused silicabased capillaries under high pressure, we are far from accomplishing the aspired ordered structures. To solve this hurdle, our group is focused on developing alternative approaches to assemble the optimized LC particles into ordered structures for miniaturized LC devices, driving technological advances in column separation efficiency. Miniaturization is also needed to reduce solvent waste for a greener and more sustainable chemical industry. In particular, we recently introduced a vacuum-assisted assembly approach of microspheres onto perforated silicon membranes and a wet-rubbing assembly technique to capture particles inside an interconnected micro-groove structure, leading to a novel sealed microgroove column.
Recently, droplet microfluidics have been reported to produce the required monodisperse silica microspheres with high precision and well-defined porosity in a high-throughput fashion with little solvent consumption. This unlocks the opportunity to explore the assembly of these high-performance chromatographic microspheres for planar chromatography utilizing a solvent-free rubbing assembly approach.
Here, we utilize our recently developed solvent-free rubbing method to assemble ordered monolayers comprising monodisperse silica or polymer (e.g., PS and PMMA) beads with diameters ranging between 500 nm and 10 μm in < 20 s on fluorocarbon-coated substrates. Our results elucidate that triboelectric charging and elastic deformations promote the formation of hexagonal-closely packed (HCP) single-layer crystals. Tribocharging embodies the charge-exchanging process when bodies are brought into frictional contact, e.g., when rubbing a balloon across our hair. The order of our monolayers is assessed by means of the Voronoi tessalation approach.
Given these advancements in droplet microfluidics and solvent-free assembly, we pursue the development of an innovative thin-layer chromatography (TLC) plate comprising a hexagonal closely packed single layer of monodisperse 9 μm porous particles (normal phase or reverse phase (RP)) on polymer carrier (e.g., PDMS) covering a glass substrate. These ordered monodisperse particles into a single layer may contribute to ideal separation, enhancing chromatographic performance. Furthermore, these novel TLC plates add to sustainability aspects, as less material is needed to produce plates, potentially reducing sample and solvent consumption. Our study involves attempting to separate either lipophilic or water-soluble dye mixtures to optimize our TLC plate in terms of solvent use, stationary phase functionality, and carrier substrate. In addition, we explore the integration of our novel TLC plates with the recent open-source 2LabsToGo system.
Next to TLC plates, our study on ordered particles opens the avenues for developing novel Labon- a-chip analytical devices, e.g., immunoassays or SERS. Furthermore, the perfect assembly of such ordered single particle layers may be a crucial first step towards developing the highly anticipated ordered stationary phases in HPLC to decrease the A-term in the van Deemter equation significantly.