Preparation and surface properties of mesoporous silica particles modified with poly(N-vinyl-2-pyrrolidone) as a potential adsorbent for bilirubin removal

Highlights

• PVP functionalized silicas were synthesized via sol–gel method.

• Modification of silica by PVP leads to the formation of mesoporous structure.

• PVP functionalized mesoporous silicas demonstrate good adsorption properties for bilirubin removal.

Abstract

The surface of silica particles was modified with polyvinyl pyrrolidone (PVP) through sol–gel process. The different experimental techniques, i.e., thermogravimetric analysis (TGA and DTG), nitrogen adsorption, scanning electron microscopy (SEM), laser diffraction analysis (LDA), fourier transform spectroscopy (FTIR) are used to characterize the pure non-functionalized and functionalized silicas containing different amount of PVP. It was shown that PVP-modified silica samples have well developed porous structure; the values of specific surface area for PVP-modified silicas are in the range of 140–264 m² g⁻¹. While the non-functionalized silica shows the low surface area (S_BET = 40 m² g⁻¹). The BJH analysis showed that PVP can be used as an effective agent to increase an average pore size and total pore volume. The results indicate that PVP functionalized silicas show a potential as effective adsorbents for bilirubin removal compared to other available adsorbents.

Introduction

Nowadays, the great potentialities of silica gel were demonstrated in numerous areas including composite materials, photonics, thermal insulators, sensors, catalysis, adsorption process, medical diagnostics, and immunoassays [1], [2], [3], [4]. Functionalization of the silica surface using different organosilane precursors or organic polymers play a great role in the adsorption processes [5], [6] mainly because the nature of the functional groups in organosilane or polymers usually have influence on the morphology and structural characteristics such as a specific surface area, pore size, particle size etc [7]. Furthermore, the porous structure is crucial for application in adsorption process or drug delivery systems. The novel adsorbents should possess the following features: ordered pore networks and homogeneous size; high pore volume; high surface area; functional groups grafted onto the surface of the adsorbent.

Several typical sol–gel approaches were described in the literature that has been applied for surface functionalization: (1) silanation of pure silica gel with silane-coupling agents by refluxing in toluene (based on two-step method) [8]; (2) synthesis of hybrid materials using organosilane precursors (based on one-step method) [3], [4], [9]; (3) preparation of organic hybrid materials using natural or synthetic polymers (based on one-step method) [10], [11], [12]. In the synthesis of materials based on polymers incorporation it is very important to keep control of the structure of used polymer because the sol–gel condition may lead to the polymer denaturation and reduce its bioactivity, especially, for natural polymers such as proteins [13]. Therefore, the analog synthetic polymers with different functional groups can be used for the surface functionalization. These polymers are characterized by a good solubility and do not undergo serious conformational changes during sol–gel synthesis [14]. The nature of guest molecule (used polymer) incorporated in a silica matrix can be different that opens a great opportunity for treatment of different toxicological diseases and drug delivery systems because the functional groups of the host polymer can easily attach to toxin molecules via different covalent and non-covalent interactions.

One of the best-known endogenous toxins is bilirubin [15]. Bilirubin is a dicarboxylic acid, circulates in human blood plasma where is bound to serum albumin to form a water-soluble complex [16]. It is transported to the liver as a complex with albumin, where it is normally conjugated with glucuronic acid and excreted into bile [17]. But the dysfunction in bilirubin metabolism leads to high concentration (hyperbilirubinemia) of free bilirubin in the blood plasma. Nowadays it is established, that hyperbilirubinemia can cause irreversible brain damage. So searching the ways to remove bilirubin excess from organism is still actual. In medical treatments, many techniques, such as plasma apheresis, hemodyalysis, photocatalytic or affinity membrance chromatography, have been develop to remove excess bilirubin from the plasma [18]. One of the most promising is hemoperfusion [19] and successful hemoperfusion technique requires the specific modified adsorbents with high selectivity and adsorption capacity relative to bilirubin. The bilirubin molecule contains carboxyl and imine groups (Fig. 1). Therefore, several works were reported where the polymers containing amine and hydroxyl groups were in bilirubin adsorption through electrostatic interactions or hydrogen bonding [20], [21]. Some researchers used the hydrophobic polymers for successful bilirubin utilization [22], [23]. In a recent paper, the novel adsorbent based on carbon nanotubes was synthesized and demonstrate a good adsorption capacity for bilirubin removal due to their larger pore and cavity volume and spherical morphology [24]. In many works the albumin is considered as an effective agent for surface modification which based on the formation of strong water-soluble complex with bilirubin [25]. Sideman et al. suggested to use the albumin-deposited macroreticular resin for bilirubin removal [26]. C. Alvarez et al. also used serum albumin as a ligand for bilirubin utilization [27].

A lot of works concerning application of PVP were published. Graf et al. reported that PVP is considered as a dispersant to improve dispersion stability of colloid particles to perform silica coating in ethanol media [28]. PVP can be immobilized onto a broad range of materials such as metal oxide (TiO₂, iron oxide, alumina) [29], polystyrene [30], silica [31], graphite [32] etc. However, there were still not reported the possible application of PVP in bilirubin utilization. A comparative study has been made of the complexing capacity of serum albumin and polyvinyl pyrrolidone with respect to bilirubin [33]. It was proved the possible application of polyvinyl pyrrolidone (PVP) in bilirubin binding. Therefore, PVP can be used as a synthetic analog for surface modification in synthesis of highly effective hemoadsorbents. This polymer is stable in water and many nonaqueous solutions what makes it suitable for sol–gel process. Thus, the unique properties of PVP such as its high solubility in different solvents, emulsion-stabilizing effects and binding properties enable to consider PVP as one of the most promising agents for pharmaceutical technology and treatment of toxicological diseases such as hyperbilirubinemia. In this current study, we have described the preparation and characterization of novel PVP functionalized silicas with the following application of these materials as adsorbents for bilirubin removal. The BET and BJH analysis are used to investigate the surface properties of the modified silica with the following application of the obtained materials as adsorbents for study of bilirubin removal from water at pH 7.4. We investigated the influence of the incorporated PVP into molecular structure of silica and particle size. Also the thermal stability of incorporated PVP was studied.