# 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
#### 6.3.6 Ascorbic Acid  
A form of vitamin C, ascorbic acid, is a commonly available nutrient which has anti-oxidizing properties. It is an organic compound that naturally occurs in fruits and vegetables and is available in the form of medicines as well as in some chemically prepared pharmaceutical fluids. An excess intake of it as a food additive results in gastric problems due to its acidic nature. To overcome the limitations such as complexity of the setup, the inability to support online monitoring and remote sensing and needing a pretty long response time for ascorbic acid detection by the existing methods like electrochemical, potentiometry, HPLC, etc., a polyaniline (PANI) MIP-based fiber optic sensor exploiting the principle of SPR was reported [166].  
A synthetic recognition system was fabricated using PANI which involves easy processing to create a mechanically and electrically strong polymer with hydrogen bonds for template binding. The core of the plastic clad silica fiber was used as substrate with 1 cm length of the cladding removed from the central region of the fiber to realize the sensing region. The polymer was prepared using N-methylpyrrolidone as monomer, ammonium persulphate as oxidizing agent and HCl as solvent. Over the 40 nm thick silver film coated on the unclad core, the prepared polyaniline with template molecule was dip coated and then dipped in water for 5 min to create recognition sites. The characterization of the sensor probe was performed for ascorbic acid concentration range \(10^{-8}\) to \(10^{-4}\) M along with the zero concentration. A total shift of 37 nm in resonance wavelength was reported for ascorbic acid solution having pH 7. A comparison plot for the MIP, NIP and simple Ag film coated probes responses are shown in Figure 14a. A maximum sensitivity of \(26.384\times 10^{8}\) nm/M was evaluated near zero concentration with LOD of \(1.28\times 10^{-10}\) M.  
Figure 14: **(a)** Comparison of MIP, NIP and simple Ag film-based SPR sensor in terms of shift in resonance wavelength with varying concentration of ascorbic acid from \(10^{-8}\)-\(10^{-4}\) M. Reprinted with permission from [166]. Copyright 2015 Springer; **(b)** Schematic diagram of the FO-LSPR + SPR-based PANI-Ag MIP probe; **(c)** the absorbance spectra and **(d)** shift in peak absorbance wavelength for the corresponding probe. Reprinted with permission from [66] Copyright 2016 Institute of Physics.  
In 2016, an extension of the work was reported employing both SPR + LSPR techniques along with MIP [66]. A nanocomposite of PANI-Ag was prepared by in-situ polymerization with ascorbic acid as template molecule and the ratio of PANI and Ag was optimized for the best probe performance. The probe with 1 cm sensing region and 40 nm thick silver film over unclad core was coated with PANI-Ag-AA nanocomposite (AA-ascorbic acid). The schematic of the SPR+ LSPR probe is shown in Figure 14b. An optimized dipping time of 30 min in nanocomposite and PANI-Ag ratio of 2:5 was obtained from experimental analysis. The optimum removal time of templates from the nanocomposite to create imprinting sites was found to be 1 min. The characterization of the probe was performed for the concentration range of 0 to 10\({}^{-8}\) M. The absorbance curves for different concentrations are shown in Figure 14c,d with a total shift of 86 nm in peak absorbance wavelength. A linear range was observed for AA concentration range of 0.01-1 uM. A very small response time of 5 s was needed to show the saturation behavior of the curve which surpasses all the existing methods. The sensor possesses 45.1 \(\times\) 10\({}^{8}\) nm/M, 7.383 \(\times\) 10\({}^{-11}\) M and 4.16 \(\times\) 10\({}^{-10}\) M as the sensitivity, LOD and LOQ respectively. The performance parameters of sensors reported using different methods for AA detection have been compared in Table 2.