Electronic Supplementary Material Electrochemical simultaneous assay of chloramphenicol and PCB72 using magnetic and aptamer-modified quantum dot-encoded dendritic nanotracers for signal amplification Meng Chen,a Ning Gan*a, Huairong Zhanga, Zhongdan Yana, Tianhua Lia, Yinji Chenb, Qing Xu**a and Qianli Jiangc a State Key Laboratory Base of Novel Functional Materials and Preparation Science, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China. b Faculty of Food Science and Engineering, Nanjing University of Finance and Economics, Nanjing 210000, China. c Department of Hematology, Nanfang Hospital, Southern Medical University, Guangzhou 510515, China. * Corresponding author, Email: [email protected], Tel: +86-574-87609987; Fax: +86-574-87609987 ** Corresponding author, Email: [email protected] Experimental Section Preparation of Fe3O4 nanoparticles Fe3O4 nanoparticles were synthesized through a solvothermal reaction. Typically, FeCl3·6H2O (8.1 g) and sodium acetate anhydrous (NaAc, 21.6 g) were dissolved in 300 mL of ethylene glycol under magnetic stirring for 30 min. Subsequently, the homogeneous yellow solution was transferred to Teflon-lined stainless-steel autoclaves and sealed to heat at 200 °C. After reaction for 8 h, the autoclaves were cooled to room temperature. The obtained black Fe3O4 nanoparticles were separated by a magnet and washed three times with deionized water and ethanol respectively. Finally, they were dried under vacuum at 60 °C. Preparation of the Fe3O4/Au nanoparticles Magnetic gold nanoparticles (Fe3O4/Au) were synthesized with our previously reported method . In brief, 0.02 g Fe3O4 NPs was dispersed in 5 mL 3% PDDA aqueous solution and then stirred for 30 min. After magnetic separation and removal of the unbounded PDDA, the resulting solid was spreaded in 130 mL Au NPs solution and stirred for 8 h at room temperature. The dark purple Fe3O4/Au NPs were obtained until the supernatant was colorless through magnetic separation. After washed several times with deionized water and ethanol respectively, the Fe3O4/Au NPs were dispersed into 100 mL ultrapure water. Prior to use, the solution was stored at 4 °C. Fig. S1 TEMs and their size distributions of CdS/EV (a) and PbS / EV (b). Fig. S2 EIS of bare GCE (a) and Bismuth film modified GCE (b) at frequency range 0.1–1.0×105 Hz. Fig. S3. The effect of the different volume ratio of CdS (a), PbS (b) QDs to EV. Fig.S4. Effect of (a) Incubation time and (b) Incubation temperature and (c) PH on the stripping peak current using 10 ng mL-1 CAP and PCB72. Fig. S5. SWVs responses (a) and calibration plots of CAP (b) and PCB72 (c) at different concentrations (from 0.001 ng mL-1 to 100 ng mL-1). Table S1 Comparison of the detection limit and linear range of different determination assays References Method Solid  phase chromatography Detection limit Linear range microextraction—Liquid (SPME-LC) determination of 0.1 ng mL-1 0.1 -10 ng mL-1 0.82 ng mL-1 2.0 -200 ng mL-1 0.03 ng mL-1 0.1-300 ng mL-1 0.39 ng mL-1 0.3-100 ng mL-1. 0.5 ng mL-1 0.1 -100 ng mL-1 chloramphenicol Amperometric immunosensor for chloramphenicol  dcetection Electrochemical sensor for the determination of  chloramphenicol Direct electrochemical immunosensor for  polychlorinated biphenyls aptamers against polychlorinated biphenyls as  potential biorecognition elements for PCB77 [email protected] in the multi-residue analysis of 0.02 ng g-1 (PCB28)  1-500 ng g-1 pesticides and PCBs by GC–MS/MS 0.33 pg mL-1 (CAP) 0.0001-100 ng mL-1 The proposed method 0.35 pg mL-1 (PCB72) Table S2. The spiked recovery of CAP and PCB72 for fish sample. Found Added Detection Samples Recovery (%) (ng mL-1) ( ng mL-1) ( ng mL-1) number CAP PCB72 0.014±0.00 0.053±0.00 3 8 0.026±0.00 0.061±0.00 5 7 0.089±0.00 0.021±0.01 6 0 1 2 3 CAP PCB72 CAP PCB72 CAP PCB72 0.050 0.050 0.062 0.099 96.0±3.7 92.0±4.6 0.100 0.100 0.128 0.155 102.0±6.2 94.0±7.6 1.000 1.000 1.053 0.998 96.4±4.3 97.7±4.8 2.500 2.500 2.492 2.553 99.7±7.4 101.1±5.2 5.000 5.000 5.106 4.890 102.1±5.4 97.8±3.9 0.025±0.00 4 ND 6 5 ND ND ND: not found References  Aresta A, Bianchi D, Calvano C D, Zambonin C G (2010) Solid phase microextraction—Liquid chromatography (SPME-LC) determination of chloramphenicol in urine and environmental water samples. J Pharm Biomed Anal 53 (3): 440-444.  He G, Yang X, Hu Y, Hu Y, Zhang F (2014) A Sensitive and Selective Amperometric immunosensor for chloramphenicol detection based on magnetic nanocomposites modify screen-printed carbon electrode as a disposable platform. Int J Electrochem Sci 9: 6962-6974.  Liu G, Chai C (2015) Towards the development of a sensitive electrochemical sensor for the determination of chloramphenicol residues in milk. Anal Methods-UK 7(4): 1572-1577.  Bender S, Sadik O A (1998) Direct electrochemical immunosensor for polychlorinated biphenyls. Environ Sci Technol 32(6): 788-797.  Xu S, Yuan H, Chen S, Xu A, Wang J, Wu L (2012) Selection of DNA aptamers against polychlorinated biphenyls as potential biorecognition elements for environmental analysis. Anal Biochem 423(2): 195-201.  Peng X, Jiang L, Gong Y, Hu X Z, Peng L J, Feng Y Q (2015) Preparation of mesoporous ZrO 2-coated magnetic microsphere and its application in the multi-residue analysis of pesticides and PCBs in fish by GC–MS/MS. Talanta 132: 118-125.