Moreover, SPR phenomena are also found in noble metal nanoparticles, namely the localized surface plasmon resonance (LSPR).Researchers have combined the advantages of optical fiber sensors and the performance of LSPR to improve the sensitivity [21,22]. Noble metal nanoparticles are decorated on the surface on the sensing fiber, and a localized surface plasmon resonance occurs which is excited by the evanescent wave around the bare core. A novel class of fiber-optic evanescent-wave sensor is constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled gold colloids and the colloidal gold surface is functionalized with biotin, with a detection limit for streptavidin of 9.8 �� 10?11 M [23].

To achieve stable test results, most commercial LSPR sensors use gold for the metal nanoparticles, but for the LSPR effect, silver nanoparticles are better [24]. In this paper, we provide a complete explanation of the sensor fabrication and propose a MEMS microchip in addition to describing the sensing fiber decoration in detail. Meanwhile, previous researchers always talk about the local surface plasma resonance peak wavelength variation due to the concentration of the analyte, but here we mainly focus on the intensity changes caused by the analyte absorbance, which is found to be more sensitive. There are still some unexplained details such as what is the specific difference effect of the modification and how much is the sensitivity really improved for testing the same analyte.

In this work, the evanescent field optical fiber sensor with silver nanoparticle modification has been successfully fabricated and experiments performed with different concentrations of analytes. Comparison of the same sensing fiber without any decoration and modified with silver nanoparticles for the detection of methylene blue solutions has also been presented.2.?TheoryOptical fibers transmit light on the basis of the principle of total internal reflection (TIR) as shown in Figure 1.Figure 1.Schematic representation of the light path in optical fibers.When light propagates from the core to a cladding with low refractive index n2 and the incident angle is larger than the critical angle (��i �� ��c), total reflection occurs. For the evanescent field optical sensor, the cladding is always peeled off and substituted by the absorptive analyte.

Then the refractive index of GSK-3 the cladding region should be described as n2 = n2r + jn2j, where the real part shows the transmission characteristics of the refractive index, and the imaginary part represents the absorption properties. The propagation constant can also be written as �� = ��2r + j��2j. In order to simplify the calculation process, here we consider the propagating wave near the refractive point to be the plane wave.