## 1 Introduction  
Natural products were fruitful sources of bioactive compounds for drug discovery [1]. However, unclear function mechanisms, complex composition, and tiring separation and purification procedures often hinder the research of natural products [2]. Thus, the research on active compounds in natural products would benefit from more efficient and novel screening technologies. Up to now, many screening methods based on solid phase extraction, membrane filtration, and hyphenated instruments were developed [3; 4; 5]. In recent decades, magnetic nanoparticles gained great interests in biological and medicinal fields. Many kinds of magnetic nanoparticles were applied as the catalysts, the carriers, the supports and so on [6; 7]. Unlike the other nanomaterials, well dispersed magnetic nanoparticles in solution could be immediately separated from medium by an external magnet [8]. High magnetic response, high stability and high surface area made magnetic nanoparticles become ideal solid supports for enzyme immobilization [9]. Enzyme modified magnetic nanoparticles could be used as agents in the screening of active and bioactive compounds from natural products. Because of the advantagesin low sample consumption, easy operation and rapid screening, this kind of screening strategy was widely applied [10; 11; 12].  
Xanthine oxidase (XO) is a critical cytosolic enzyme which is widely distributed in mammalian tissues. XO could catalyze the oxidation of hypoxanthine to xanthine and further catalyze the oxidation of xanthine to uric acid [13]. High concentrations of uric acid would result in the hyperuricemia and gout [14]. Accordingly, the inhibition of XO could reduce the production of uric acid and become a of therapeutic method for treating hyperuricemia and gout. Allopurinol is an XO inhibitor and is widely clinical applied in related treatments [15]. However, many adverse reactions to allopurinol, like allergic reactions, nephropathy, and hepatitis, have been observed and have limited the usage of allopurinol [16]. Hence, more potential XO inhibitors from natural products with fewer adverse effects and higher therapeutic activity are needed.  
_Clerodendranthus spicatus_ (Thunb.) is a plant species and a popular folk medicine in China. It was widely distributed in southern China, including in Fujian, Yunnan and Hainan provinces [17]. As a traditional Chinese medicine, _Clerodendranthus spicatus_ has been used in the treatment of urinary lithiasis, rheumat, acute nephritis, chronic nephritis, and cysitis [18]. So far, previous documented reports on the chemical composition of _Clerodendranthus spicatus_ showed the existence of plenty of diterpenes, flavonoids, triterpenes and phenolic acids [19]. Water extract of _Clerodendranthus spicatus_ was often investigated because water soluble phenolic acids were considered to be responsible for those activities [20; 21]. However, more systematical research on _Clerodendranthus spicatus_ was still necessary since many valuable active compounds in _Clerodendranthus spicatus_ were not clear.  
In this study, xanthine oxidase immobilized silica coated Fe\({}_{3}\)O\({}_{4}\) nanoparticles (Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO) were successfully prepared and characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). Previous research in our lab demonstrated that extract of _Clerodendranthus spicatus_ typically inhibited the activity of XO. Consequently, a XO inhibitor screening method using Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO nanoparticles was established and utilized in the extract of _Clerodendranthus spicatus_ based on our previous research. According to our proposed screening method, two compounds were screened out and identified as gardenin B and eupatorin. The inhibitions on XO of gardenin B and eupatorin were further verified and the interactions between these two compounds and XO were investigated by fluorescence spectroscopic methods.