# These two authors contributed equally to this paper.
## 1 Introduction  
The prospects of superparamagnetic nanoparticles are more and more extensively thought highly of nowadays. The most unique feature of the magnetic particles is their response to magnetic force, and this feature has been utilized in many fields, such as giant magnetoresistance (GMR) biosensors [1], diagnosis [2], and hyperthermia therapy of cancers [3], drug targeting [4], cell sorting [5], isolation of rare hematopoietic progenitor cells [6], sensitive clinical immunoassays [7], etc. Different synthesis approaches of the applicable magnetic particles or beads were developed for different purposes [8]. However, as a result of anistropic dipolar attraction, nanometer-sized particles intend to aggregate into larger clusters, losing the particular properties associated with their nanometer dimensions. A protection shell of coating materials was prepared to cover the superparamagnetic nanoparticles to prevent the core agglomeration. [PERSON] _et al_[9] introduced the initiator 10-carboxydecanyl 2-bromo-2-methylhiopropanoate onto the surface of maghemite, utilized surface-initiated atom transfer radical polymerization (ATRP) to synthesize perfect polystyrene/maghemite core/shell polymeric nanobeads. [PERSON] _et al_[10] synthesized composite nanostructures from magnetic nanoparticles and cross-linked amphiphilic block copolymers with magnetomicelles. [PERSON] _et al_[11] succeeded in preparing polypyrrole-Fe\({}_{3}\)O\({}_{4}\) magnetic nanocomposite by ultrasonic irradiation, and magnetic nanobeads were endowed with electric conductivity. All these magnetic nanobeads had only one or two single original nanoparticles as their magnetic cores, which would result in weak magnetophoresis ability [6], and in turn the magnetic separation effect would be weakened.  
Recently, we developed a modified suspension polymerization method to prepare superparamagnetic polymers (SPMP) microbeads with nanosized magnetic particles (less than 10 nm in diameter) within their polymeric framework [12]. Not like the normal polymeric beads of several hundred micrometers, these SPMP microbeads were much smaller, around 3 microns in their diameter. Because there were many 8-nm magnetite nanoparticles distributed and immobilized in the inner polymeric frameworks, our SPMP microbeads had much higher magnetite contents and higher magnetophoresis ability than those reported core-shell nanobeads [6, 7(b), 9-11] with only one or two single magnetic original particles as the cores. In this paper, we coupled antibody with the synthesized SPMP microbeads, and set up an enzyme chemiluminescent immuno assay (ECLIA).  
The detection of tumor markers in clinical assays was more and more important in the diagnosis of many cancers. Recently, it was brought forward that free hCG\(\beta\) may replace hCG and hCG\(\beta\) as the tumor marker of procreate neoplasms, renal cell carcinoma (RCC) [13], and Down syndrome [14]. The level of free hCG\(\beta\) is more valuable to diagnose cancers in the reproductive system [15], since there is cross reaction between Human Chorionic Gonodotropin (hCG) and luteinizing hormone (LH), thyroidstimulating hormone (TSH), which were used as markers of tumor assays. In clinical assays, free hCG\(\beta\) was conventionally detected by radioimmuno assay (RIA), electrochemical immunoassay (EIA). Methods that had been reported were in-house ELISA [16], RIA [17], enhanced chemiluminescence immunoassay [18], amperometric immunosensors based on a conducting immunocomposite [19], flowing immunoassay [20]. By far no mature ELICA had been established for the detection for free hCG\(\beta\). We applied these SPMP microbeads to establish an enzyme chemiluminescence immuno assay (ECLIA) of free hCG\(\beta\) in serum, which was an easy-performing, low cost, rapid, and quantitative method.