Determination of sialic acid in serum samples by dispersive solid-phase extraction based on boronate-affinity magnetic hollow molecularly imprinted polymer sorbent

Boronate-affinity magnetic hollow molecularly imprinted polymers (B-MhMIPs) were prepared with sialic acid (SA) as the template, 3-aminophenylboronic acid (APBA) as the functional monomer and glycidilmethacrylate (GMA) as the co-monomer to chemisorb Fe3O4 nanoparticles. Furthermore, the hollow structure made the nanoparticles have more binding sites at both internal and external surfaces, which can facilitate the removal of template molecules from polymers and enhance the adsorption abilities towards SA. After optimizing, the adsorption pH was controlled at 4.0, and this was different from most cis-diol-containing compounds. Under the optimal conditions, the limit of detection for SA was 0.025 μg mL−1 (n = 3). This method was applied to analyze serum samples with different spiked levels, and the recoveries of the SA were in the range of 70.9–106.2%. These results confirmed the superiority of the B-MhMIPs for selective and efficient enrichment of trace SA from complex matrices.


Preparation of MCM-48
MCM-48 was synthesized based on the previous report described by Schumacher et al. 1 Firstly, 2.6 g CTAB was dissolved in the 120 mL DDW, followed 4 mL TEOS and 80 mL EtOH were added into the above solution and the mixtures were stirred under room temperature for 30 min.Afterwards, 12 mL NH 3 •H 2 O was added dropwise with continued stirring for another 6 h.
Then, MCM-48/CTAB nanoparticles were collected by filtration and rinsed.The MCM-48 was obtained by calcination to remove CTAB at 823 K (2°C min -1 ) for 6 h.

Optimization of the adsorbents procedure
The performance of the prepared B-MhMIPs adsorbents was evaluated.Various experimental parameters of adsorbents including sample pH, adsorbent amount, sample volume and adsorption time were studied and optimized as follows.

Effect of adsorption pH
The adsorbents were used to adsorb SA under different pH from 2.0 to 10.0.As it is shown in Fig. S2A, the adsorption capacity achieved the maximum at pH 4.0.When the pH is too high, the adsorption capacity decreased gradually.This strategy was found by Otsuka and co-workers that SA exhibit prominent high binding capacity with common boric acid ligand when the pH is lower than the pKa value of the boronic acid, 2 and it can be explained as follows (Fig. S1).The structure of SA is a stable conformation due to the intramolecular hydrogen bonding.When the surrounding turned to lowered pH, the amide NH or CO located at C-5 of SA molecule can form an intermolecular B-N or B-O linkage, which was stable existence. 2In fact, the intramolecular B-N and B-O linkages have been found to be effective strategies for the design and synthesis of boronic acids that can bind to cis-diol compounds at lower pH. 3 However, the boronic acid group will combine with hydroxyl groups on C-8 and C-9 of SA molecules under higher pH.This conformation, with higher energy, Electronic Supplementary Material (ESI) for RSC Advances.This journal is © The Royal Society of Chemistry 2019 the chemical bonds between SA and boronic acid groups can be broken under the alkaline condition. 4Therefore, the pH 4.0 was chosen as optimized adsorption pH.

Effect of the adsorbent amount
To optimize the amount of B-MhMIPs for the exaction of SA, the range of 10-40 mg adsorbents were evaluated.It can be seen from Fig. S2B, 20 mg B-MhMIPs have the highest adsorption capacity.However, the capacity kept nearly constant in the range of 20-40 mg, indicating the remarkable exaction ability of the adsorbents.Thus, 20 mg of B-MhMIPs was used as the suitable adsorbent amount.

Effect of sample volume
The sample volume is one of key factors to affect the adsorption ability for analysis at low concentrations.The values of volume were investigated in the range of 2-10 mL.As shown in Fig. S2C, the curve rises continuously.Therefore, 10 mL sample volume was selected in the present work.

Effect of adsorption time
When 20 mg B-MhMIPs adsorbents were used for the adsorption of SA, effects of the adsorption time were constructed by varying the adsorption time in the range of 10-60 min before phase separation.As shown in Fig. S2D, the adsorption of SA with B-MhMIPs reached equilibrium within 40 min, after which the adsorption amount almost kept constant.Finally, the optimized adsorption time was chosen as 40 min.(Embed Fig. S2)