A Biosensor Combining Molecularly Imprinted Polymers (M-MIPs) and Surface Enhanced Raman Spectroscopy (SERS) to Detect Antibiotics in Food Samples

Yi Sun^,1*, Jon Ashley¹, Kaiyu Wu¹, and Anja Bosen¹.

¹ Department of Micro- and Nanotechnology, Technical University of Denmark, Ørsteds Plads, DK-2800 Kgs. Lyngby, Denmark

* Email: sun.yi@nanotech.dtu.dk; Tel.: +45 45256319

In this study, temperature-responsive magnetic molecularly imprinted polymers (M-MIP) nanoparticles were synthesized for the first time for the extraction of cloxacillin in pork products. By combining the M-MIPs with surface enhanced Raman spectroscopy (SERS), a sensitive biosensor was demonstrated to detect cloxacillian with pico-mole sensitivity.

MIPs are synthetic ligands which can be tailored to bind any analyte of choice¹.  They are of great interest due to their thermal stability, robustness, low cost and comparable binding affinity. They have been used in sample preparation and biosensing as an attractive alternative to natural antibodies to capture targets ranging from small molecules to big proteins.  

In this work, the magnetic nanoparticles with MIP-based receptors were synthesized for efficient and rapid extraction of antibiotic residues in pork samples.  Fe₃O₄ nanoparticles were obtained using the solvothermal synthesis.  The resultant nanoparticles were treated with Tetraethyl orthosilicate (TEOS) to form a SiO₂ layer.  Finally a thin MIP layer was polymerized round the nanoparticles using azobisisobutyronitrile (AIBN) as the initiator, ethylene glycol dimethacrylate (EDGMA) as the cross-linker, N-isopropylmethacryamide (NIPAm), methacrylic acid (MAA) as the monomers and the antibiotic as the template.  By adding the monomer NIPAm, the MIPs become temperature responsive, and can swell at low temperature to release the target. The corresponding magnetic non-imprinted polymer nanoparticles (M-NIP) was prepared using the same method in the absence of the template. An Overview of the synthesis strategy is shown in Fig. 1. The resultant M-MIP nanoparticles were characterized using IR, XRD scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Fig. 2).  Both binding affinities of the resultant M-MIPs and M-NIPs were tested using UV absorbance (Fig. 3). M-MIPs with 300-400 nm in size and good binding capacities were obtained.

To demonstrate the feasibility of using M-MIPs for sample preparation, the synthesized M-MIPs were mixed with pork blood samples spiked with Chloxacillian. After incubation at room temperature, the M-MIPs were collected using a magnet and washed by acetonitrile. Owing to the thermos-responsive properties of MIPs, Chloxacillian was easily released by cooling the MIPs to 4 degree. The collected Chloxacillian was dropped on a SERS substrate which contained an array of silicon micropillars coated with silver. The corresponding calibration plots showed a detection limit (LOD) of about 50 pmol (Fig. 4). The biosensor combining M-MIPs and SERS would be widely used on site or in the field for rapidly screening food contaminants to ensure food safety.

Fig. 1: Overview of the synthesis of M-MIPs

Fig.2 IR characterization of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@ SiO₂-MPA and, Fe₃O₄@SiO₂-MIP; XRD of Fe₃O_4.

Fig.3 (A) Binding kinetics and (B) Binding capacity of Cloxacillian MIPs and NIPs.

Fig.4 SERS spectra of cloxacillin in MeOH:acetic acid (9:1) and corresponding calibration plots.

REFERENCES:

1. J. Ashley, M-A. Shahbazi, K. Kant, V. A. Chidambara, A.Wolff, D. D. Bang, Y. Sun, “Molecularly Imprinted Polymers for Sample Preparation and Biosensing in Food analysis: Progress and Perspectives, Biosens. Bioelectron. 2017, 91, 606-615.
2.