Citrate polymer optical fiber for measuring refractive index based on LSPR sensor

Abstract

Fiber optic localized surface plasmon resonance (LSPR) sensors have become an effective tool in refractive index (RI) detection for biomedical applications because of their high sensitivity. However, using conventional optical fiber has caused limitations in implanting the sensor in the body. This research presents the design and construction of a new type of polymer-based LSPR sensors to address this issue. Also, finite element method (FEM) is used to design the sensor and test it theoretically. The proposed polymer optical fiber (POF) based on citrate is biocompatible, flexible, and degradable, with a rate of 22% and 27 over 12 days. The step RI structure utilizes two polymers for light transmission: poly (octamethylene maleate citrate) (POMC) as the core and poly (octamethylene citrate) (POC) as the cladding. The POF core and cladding diameters and lengths are 700 µm, 1400 µm, and 7 cm, respectively. The coupling efficiency of light to the POF was enhanced using a microsphere fiber optic tip. The obtained results show that the light coupling efficiency increased to 77.8%. Plasma surface treatment was used to immobilize gold nanoparticles (AuNPs) on the tip of the POF, as a LSPR-POF sensor. Adsorption kinetics was measured based on the pseudo-first-order model to determine the efficiency of immobilizing AuNPs, in which the adsorption rate constant (k) was obtained be 8.6 × 10^-3 min^-1. The RI sensitivity of the sensor in the range from 1.3332 to 1.3604 RIU was obtained as 7778%/RIU, and the sensitivity was enhanced ~ 5 times to the previous RI POF sensors. These results are in good agreement with theory and computer simulation. It promises a highly sensitive and label-free detection biosensor for point-of-care applications such as neurosciences.

Keywords: Local surface plasmon; Poly (octamethylene citrate) (POC); Poly (octamethylene maleate citrate) (POMC); Polymer optical fiber; Refractive index.

Figure 1

Applications of POF sensors in neurophotonics.

Figure 2

(a) The appearance of AuNP solution. (b, c) TEM image of synthesized NPs.

Figure 3

(a) FTIR spectra of POMC and POC. (b) Thermal characterization of POMC and POC films, DSC thermograph with respective Tg at − 9 °C and − 6 °C.

Figure 4

In-vitro degradation of POMC and POC in PBS (pH 7.4, 37◦C).

Figure 5

(a) Schematic of microsphere fabrication process using CO₂ laser. (b) A typical image of fabricated microsphere on tip of fiber optic.

Figure 6

Schematic of the proposed POF which AuNPs were immobilized to the tip.

Figure 7

The loss coefficient curve with the effective RI of core mode and the LSPR mode according to different wavelengths for external medium RI of n = 1.3332 RIU.

Figure 8

(a) Electric field distribution (V/m) of Core mode of POF from the tip cross-section (x,y). (b) Electric field distribution (V/m) of AuNPs immobilized on POF tip at fiber air interface for the Cross-sectional (y,z) with considering scattering and periodic boundary. (c) Longitudinal (y) view. Finite element analysis of POF, showing the EM field variation excited at the wavelength of 625 nm and RI of bioanalyte n = 1.3332 RIU.

Figure 9

(a) Changing reflected LSPR spectrum of the sensor in different Ris. (b) Obtained RI sensitivity of LSPR–POF sensor.

Figure 10

Optical coupling efficiency of light to the POF versus the microsphere diameter at 625 nm.

Figure 11

Typical images of different manufactured microspheres with a diameter (a) 456.3 µm, (b) 459.6 µm, and (c) 457.8 µm.

Figure 12

3D profiles of different manufactured microspheres, as shown in Fig. 11.

Figure 13

Comparing different profiles of micropheres, (a) 2D profiles and, (b) 3D profiles.

Figure 14

Microsphere inside POMC tube as optic fiber cladding.

Figure 15

The schematic of fixing the microsphere inside the POC tube as cladding of POF.

Figure 16

(a) Flexibility and mechanical properties of polymer POMC, and (b) POC. (c) A citrate-based POF twisted around a tube.

Figure 17

A photo of the POF tip (a) Cutting using a surgical blade. (b) After chemical etching.

Figure 18

Guiding light in homemade POF at the wavelengths of (a) 650 nm, (b) 589 nm, and (c) 532 nm.

Figure 19

Fiber optics in a plasma device.

Figure 20

(a)The POF before, and (b) the POF after AuNPs immobilization. (c) FE-SEM image of NPs immobilized on the POFsurface.

Figure 21

Schematic of the experimental setup.

Figure 22

Stability of light source after 60 min warming up.

Figure 23

(a) Reflectance of the LSPR spectrum. (b) Changing the reflectance versus time during AuNPs immobilization.

Figure 24

Pseudo-first-order kinetic model for the adsorption of AuNPs onto POF.

Figure 25

(a) Normalized LSPR spectrum for different ethanol concentrations. (b) Power intensity changes against RI with a 0.16 AU error bar.

Figure 26

RI sensitivity of the POF tip before Au NPs immobilization obtained from (a) FEM simulation, and (b) experimental results based on intensity modulation.