Capillary methacrylate-based monoliths by grafting from/to γ-ray polymerization on a tentacle-type reactive surface for the liquid chromatographic separations of small molecules and intact proteins

Highlights

• Capillary methacrylate-based monoliths were synthesized with tentacle-type inner wall activation.

• Both thermal and radiolytic polymerization were employed.

• Monoliths were characterized by FT-IR, solid state NMR, NMR cryoporosimetry, and SEM.

• High efficiencies for separations of small molecules in isocratic mode and proteins in gradient elution were achievable.

• Thermal and radiolytic monoliths were compared in terms of morphology and chromatographic behaviours.

Abstract

Capillary methacrylate-based monoliths were prepared for the high performance liquid chromatography (HPLC) separation of both small molecules and large biomolecules. An efficient grafting from/to synthetic approach was adopted introducing a network of activated sites in the inner wall surface using the new silanization agent (N-trimethoxysilylpropyl)-polyethylenimine. Copolymerization of lauryl methacrylate monomer and 1,6-hexanediol dimethacrylate cross-linker in the presence of porogenic solvents was obtained under continuous γ-ray exposure with high conversion yield.

The morphology and porous structure of the resulting monoliths have been investigated by Scanning Electron Microscopy (SEM) and ¹H NMR cryoporosimetry. By chromatographic investigation, the new capillary columns attested high kinetic performance (with efficiency larger than 100,000 theoretical plate/m for small molecules at optimum mobile phase linear velocity of about 0.5 mm/s) and also excellent mechanical stability and repeatability.

The new methacrylate-based monolithic capillary columns have been successfully employed for efficient reversed-phase separation of intact proteins and peptides.

Introduction

Monolithic materials are versatile adsorbents widely employed in separation science, sample preparation and as supports for flow-through applications (e.g. heterogeneous catalysis, ion-exchange, solid-phase extraction, etc.) [1], [2], [3], [4]. Interest around their preparation and applications has been rapidly growing in recent years. Some reviews about the use of monoliths as separation media for analytical chromatography have been published [5], [6], [7], [8], [9], [10]. Two of the most important groups of monoliths are based on inorganic silica chemistry, and those derived from organic monomers. Silica-based monoliths consist of a bi-continuous mesoporous skeleton as result of the sol-gel preparation method designed by Tanaka in the 90 s [11]. On the other hand, polymeric organic ones have a normally globule-like backbone. They are most commonly obtained by a single-step polymerization process starting from a bulk mixture of monomers, cross-linkers (difunctional monomers) and porogens. In both cases, monoliths are characterized by a single-body mesoporous structure with interconnected channels (flow-through pores).

Thanks to the possibility of modulating the skeleton thickness with respect to the width of the flow-through pores, monoliths combining high efficiency and high permeability can be prepared. They have been proven to be particularly suitable for high efficiency separations of large biomolecules, which are excluded by the mesoporous network and do not experience the usually slow mass transfer therein [12], [13], [14]. A further considerable advantage also includes the simplicity of in-situ preparation and, consequently, monolithic chromatographic columns of virtually any geometry and shape can be easily prepared. This flexibility allows to overcome the constraints related to both packing and miniaturization of the conventional particle-based chromatographic columns. Nowadays, polymer monolithic column technology has been successfully applied for the HPLC separation of large molecules such as intact proteins, synthetic polymers, peptides [15], [16], [17], [18], and used to improve the sensitivity when directly interfaced to UV absorbance detection or MS [19], [20]. The majority of recent developments have been focused on the optimization of their morphology to achieve a better efficiency and enhanced mass transport of solutes, as reported by Nischang [21] and Shen et al. [22]. Different monomers (acrylamide, acrylate, methacrylate, vinylbenzene), cross-linkers and monomer/cross-linker ratio have been evaluated to induce a control of pore dimensions and to improve efficiency and biocompatibility [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. New solvent systems as ionic liquids have also been introduced in a microwave-assisted polymerization process [33]. Polymerization of active precursors involves a free radical process mainly thermal or photo-induced, but several less common free radical polymerization techniques (γ-ray, electron beam, living processes, polycondensation methods) have also emerged as reviewed by Svec [34]. Among them, γ-ray induction does not require an initiator and the polymerization can be carried out at room temperature in almost any confinement, including the stainless steel columns usually employed for the preparation of HPLC columns [35], [36]. On the other hand, the use of this powerful technology requires access to γ-ray sources, which are expensive and need dedicated laboratory. In the preparation of monolithic columns, a crucial step is the strong adhesion of the polymer backbone to the inner walls of the holder where the polymerization is performed (e.g., the capillary column), to reduce the possibility of bed heterogeneity and void spaces close to the wall. Without an efficient adhesion, the quality of separation in terms of efficiency and reproducibility is compromised. To obtain a strongly tethered organic monolith, usually, the capillary surface is subjected to a two-step treatment: the so-called etching step firstly makes available the silanols groups present on the inner wall; then, the superficial grafting procedure (silanization step) introduces on the silanols the reactive units that will be covalently embedded into the monolithic skeleton during the polymerization process. Usually, acidic and alkaline solutions are alternately used in the etching procedure, while the surface modification is carried out by anchoring 3-((trimethoxysilyl)propyl) methacrylate [37], [38], [39], [40], [41], [42].

In this work, we present an innovative grafting synthetic approach on a multisite tentacle-type inner-wall activated surface obtained by using (N-trimethoxysilylpropyl)-polyethylenimine as silanization agent and methacrylic anhydride. The “octopus-like” surface modification permits the generation of a so-called grafting from/to polymerization process since the covalently anchored active units (vinyl groups) take part to the free radical polymerization happening in the bulk phase, possessing the active moieties of precursors, both monomers and cross-linkers. The new methacrylate-based monoliths are extensively characterized from a morphological viewpoint by employing a series of advanced techniques including FT-IR (Fourier transform infrared spectroscopy), solid state ¹³C CPMAS NMR (Cross-Polarization Magic Angle Spinning Nuclear Magnetic Resonance), SEM (Scanning Electron Microscopy) and ¹H NMR cryoporosimetry. The chromatographic performance of these columns for the reversed-phase separation of small molecules and intact proteins is discussed.