*4.6. Sample Preparation*

A total of 5 g of rice was extracted and vortex mixed with 25 mL 50% methanol for 30 min. Then, the supernatant fluid was first filtered with a double filter paper before it was further purified with a 0.22 μm filter membrane. Then, 250 μL of the filtrate was watered down with 750 μL deionized water, samples of which were analyzed with the FPIA method [43].

#### *4.7. Comparison with HPLC Analysis*

Samples were extracted with ochratoxin A immunoaffinity columns similar to those previously described. Briefly, rice samples were finely ground and homogenized and then mixed with 80% acetonitrile and half volume hexane by gently mixing. The extracts were filtered with a filter paper, and then the filtrate was collected and centrifuged. The bottom layer was evaporated to dry below a flow of nitrogen. After dilution with acetonitrile and PBS, the extracted samples were loaded into the IAC columns. OTA was eluted with 2% methanol/acetic acid solution, and then dry-evaporated. A 50 μL reconstituted sample (1 mL acetonitrile) was injected into the chromatograph [44,45]. The HPLC analysis was run applying a C18 column on a Waters Alliance 2695 chromatographic system in isocratic conditions at ambient temperature with the moving phase of CH3CN:NH3/NH4Cl (20 mM, pH = 9.8) (*v*/*<sup>v</sup>* <sup>=</sup> 15:85); the column was acquired using Waters XTerra® (3 <sup>μ</sup>m, 2.1 <sup>×</sup> 250 mm), the injection volume was 20 μL, and the flow velocity was 0.2 mL per minute; the FLD determination was acquired using a Waters 474 Scanning Fluorescence Detector (λex 380 nm, λem 440 nm; attenuation 32; gain 7 × 100; bandwidth 40 nm); the analyte holding time was 20 times the retention time, corresponding to the column void volume; no chemical compound could be used as an internal reference for the OTA extraction [46].

**Author Contributions:** Conceptualization, X.T., X.H. and Q.Z.; methodology, X.T. and X.H.; software, X.H.; validation, X.T., X.H. and Q.Z.; formal analysis, X.T. and X.H.; investigation, X.T. and X.H.; resources, Q.Z., P.L., X.Q., W.Z., J.J. and H.L.; data curation, X.T. and X.H.; writing—original draft preparation, X.H.; writing—review and editing, X.H., Q.Z. and A.J.; visualization, X.H.; supervision, X.T. and Q.Z.; project administration, Q.Z., P.L., W.Z., J.J. and H.L.; funding acquisition, Q.Z. and P.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China (2018YFC1602505), Agricultural Science and Technology Innovation Program of CAAS (Chinese Academy of Agricultural Sciences) (CAAS-ZDRW202011), the Natural Science Foundation of China (31801665)

**Acknowledgments:** The Fluorescein thiocarbamyl ethylenediamine (EDF), Fluorescein thiocarbamyl butane diamine (BDF), fluorescein thiocarbamyl hexame (HDF) and amino-methyl fluorescein (AMF) were provided by Sergei A. Eremin, a professor of Department of Chemistry, Lomonosov Moscow State University.

**Conflicts of Interest:** The authors declare no conflict of interest. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
