Utilization of UiO-66-NH<sub>2</sub>@cellulose hybrid aerogel for solid-phase extraction of sildenafil in health products

Sildenafil is a prescription drug used to treat pulmonary hypertension and erectile dysfunction. However, the illegal addition of sildenafil to health supplements may be hazardous to human health. Therefore, it is imperative to develop a method for the detection of sildenafil in health products. Solid-phase extraction (SPE) is typically used for the separation, purification, and enrichment of samples, with the aim of reducing the matrix interference of the samples and improving the detection sensitivity. SPE is widely employed in the detection of trace compounds in complex samples. Metal-organic frameworks (MOFs) are hybrid crystalline materials composed of metal ions and organic ligands. MOFs offer the advantages of a large specific surface area, permanent nanopores, good stability, and pore controllability. Owing to their advantages, researchers have attempted to use MOFs for SPE; however, it is difficult to collect MOFs in the powder form when they are directly used in SPE, necessitating the use of a substrate material to fix the MOFs. Hence, this study proposes a novel SPE method with high sensitivity that employs a UiO-66-NH2@cellulose hybrid aerogel, for the detection of sildenafil in health products. First, UiO-66-NH2 was synthesized by a hydrothermal method. Cellulose nanocrystal (CNC) was modified with an aldehyde group to obtain CNC-CHO, and carboxymethyl cellulose (CMC) was modified with a hydrazide group to obtain CMC-NHNH2. Subsequently, UiO-66-NH2 was added to the CNC-CHO solution, mixed with the CMC-NHNH2 solution, and then crosslinked to load UiO-66-NH2 and form a hybrid aerogel as a bulk adsorbent. This bulk hybrid aerogel could be collected easily without extra force for use in SPE. The synthesized hybrid aerogel was characterized by X-ray powder diffraction, scanning electron microscopy, Fourier transform-infrared (FT-IR) spectroscopy, and nitrogen gas adsorption/desorption. The results showed that UiO-66-NH2 was successfully loaded on the pore surface of the hybrid aerogel, which made the pore of aerogel become more regular and a larger surface area. Investigation of the loading amount of UiO-66-NH2 in the hybrid aerogel revealed that a higher amount of UiO-66-NH2 could yield better extraction efficiency. The highest amount of UiO-66-NH2 that could be loaded in the hybrid aerogel was 50%. The experimental conditions affecting the enrichment of sildenafil were optimized, and determined to be the following: pH of the sample, 9.0; extraction time, 60 min; eluent, acetonitrile; elution time, 40 min; elution volume, 3×2 mL; salt ion concentration, 0. Separation was performed on an Agilent Zorbax Eclipse Plus C18 column (150 mm×4.6 mm, 5 μm) with a phosphate solution containing 0.1 mol/L triethylamine (pH=6.50)-acetonitrile (30∶70, v/v) as the mobile phase. The detection wavelength was set at 292 nm. Under the optimal conditions, the UiO-66-NH2@cellulose hybrid aerogel was used as an adsorbent to extract sildenafil in different concentrations. The peak area was proportional to the sildenafil concentration in the range of 10-2000 ng/mL, with limit of detection (LOD, S/N=3) of 2.85 ng/mL and enrichment factor of 59.17. The correlation coefficient (R2) was 0.9950. Compared to previous preconcentration methods for sildenafil, this method offered a wider linear range. Five batches of hybrid aerogels were simultaneously prepared under the same conditions and used to extract sildenafil; the relative standard deviation (RSD, n=3) was 1.71%, indicating that the prepared hybrid aerogels offered good reproducibility. The used composite aerogels were freeze-dried again and reused to extract sildenafil; the recovery was still maintained at 85.23% after five extraction cycles, indicating that the UiO-66-NH2@cellulose hybrid aerogel had good regeneration ability. The feasibility of the developed method was verified by analyzing five health products. The results demonstrated the presence of 3.01 μg/g sildenafil in one of the products and no sildenafil in the others. The recoveries of this SPE method ranged from 74.93% to 89.12%, with RSDs in the range of 2.8%-5.3%, proving the feasibility of this analytical method.