# Surface Plasmon Resonance-Based Fiber Optic Sensors Utilizing Molecular Imprinting
## 6 Fiber Optic SPR Sensor Based on MITs
### Developments on FO-SPR-MIP-Based Sensors
#### 6.3.2 Tetracycline (TC)  
Tetracycline is a type of antibiotic which is used for the prevention and treatment of several bacterial infections such as acne, pneumonia, etc. [155; 156]. However, large amounts of tetracycline may harm the human body via skin problems, allergic symptoms and skin diseases. Therefore, the monitoring of tetracycline is important to ensure food safety. A TC detection method using MIT and fiber optic SPR technique was reported [157]. The sensing probe was prepared by coating an Ag thin film over the core of the optical fiber followed by a MIP TC layer. MIP TC was synthesized in a polyacrylamide hydrogel medium where acrylamide (AM) and N,N-methylenebisacrylamide (BIS AM) were used as monomer and cross-linking agent while TC was used as template molecule. The sensor operation was checked for the TC concentration range 0-0.96 uM, and for the same concentration change the shift in resonance wavelength was observed to be 35.892 nm. Figure 9a,b show the SPR response and the shift in resonance wavelength for different concentrations of TC sample. Oxytracycline (OTC) is also a member of the TC family. The detection of OTC was also shown in the same study for which OTC was used as template molecule instead of TC.  
Figure 9: (**a**) SPR response; and shift in resonance wavelength for FO-SPR based (**b**) tetracycline (TC) and (**c**) oxytetracycline (OTC) sensor using molecularly imprinted hydrogel for the concentration range of 0–0.96 uM, and (**d**) the comparison of sensitivities evaluated for TC and OTC imprinted sensors. Reprinted with permission from [157]. Copyright 2013 Royal Society of Chemistry.  
The sensing probe was characterized for the OTC concentration range 0-0.96 \(\upmu\)M. In this case the shift in resonance wavelength was found to be 14.668 nm with OTC concentration change as shown in Figure 9c. The sensitivity variation of both the probes (MIP-TC and MIP-OTC) is shown in Figure 9d. The response of both the probes for other analyte is negligible as shown in Figure 9b,c.  
The sensitivity of the TC sensor was further improved by [PERSON] et al. by incorporating the combined phenomenon of SPR and LSPR [65]. In this method, the sensor was fabricated by including the Ag nanoparticle layer between Ag and MIP-TC layer. Silver nanoparticles were synthesized hydrothermally [158] and their various characterizations like scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) were reported as shown in Figure 10.  
The operating range of the sensor was enhanced to 0-10 \(\upmu\)M in this work. The shift in peak absorbance wavelength was found to be around 102 nm for the same TC concentration range and shown in Figure 11. The sensing method showed a detection limit of 2.2 \(\times\) 10\({}^{-9}\) M for TC.  
Figure 11: (**a**) The absorbance spectra and (**b**) calibration curve for the SPR/LSPR based TC sensor utilizing MIP hydrogel in the concentration range of 10\({}^{-8}\)-10\({}^{-5}\) M. Reprinted with permission from [65]. Copyright 2015 Institute of Physics.  
Figure 10: Structural characterization of the silver nanoparticles synthesized in hydrothermal way for the SPR/LSPR based TC sensor with MIP hydrogel. The average size of nanoparticles was found to be in the range of 10–30 nm. Reprinted with permission from [65]. Copyright 2015 Institute of Physics.