illustrates the transmission spectra of the tapered and un-tapered fibers. It is seen that the peak value of the transmitted intensity of the un-tapered fiber is more than 15,000 a.u., which is higher than that of the tapered fiber. This result indicates that the structure of the tapered fiber results in more light-leakage, and meanwhile, the evanescent field is enhanced through fiber tapering.
transmittance of tapered fiber and un-tapered fiber.
illustrates the transmitted power spectra of a single-mode tapered fiber tested in air and water, modified with APTMS as a function of wavelength. The obvious decline in water λ
= 1,240 nm and λ
= 1,380 nm are pronounced, and are shown as the curve (b). The absorption peaks of water are approximately 1,240 and 1,380 nm [20
]. Comparing the curve (b) with the curve (a), approximately 14.6 dB and 10.1 dB fall at λ
= 1,240 nm and λ
= 1,380 nm. The obvious decline in water illustrates that the APTMS on the fiber surface can absorb the water molecules. Depending on the successful APTMS surface modification, the gold nanoparticles can be absorbed by the silane coupling agent on the fiber surface.
Transmitted power spectra of the silane coupling agent modified fiber tested in air and water.
The spectra of the fiber modified using APTMS after gold-nanoparticle conjugation are shown in . Curve (a) denotes the spectrum of the modified fiber in air. Curve (b) represents the result when the modified fiber was placed in 98% ethanol; a clear decrease compared to curve (a) is evident. Curve (c) shows the result when the modified fiber was placed in 0.1 mM gentian violet. Curve (d) shows the result when the concentration was raised to 1 mM. Different concentrations resulted in different peak values in the transmission spectra. Furthermore, the transmission spectra of the fibers modified by the gold nanoparticles in APTMS varied when the solutions were changed.
A comparison between and shows that the transmission spectra of the modified tapered fiber using MPTMS decrease more obviously than when using APTMS. In order to better analyze the data, the y-axis is presented as −log (I/I0). I denotes the intensity of light exiting the fiber, where the gold nanoparticle-modified fiber is immersed in a medium. I0
denotes the intensity of light exiting a bare fiber, where there is no gold nanoparticle on the core surface and the fiber is immersed in a blank. The absorption peak of gentain violet is approximately 590 nm [21
Transmittance of the modified fiber by APTMS.
Transmittance of the modified fiber by MPTMS.
With the same reagent but with different concentrations, the MPTMS-modified fiber decreased by 0.048 a.u. at 590 nm, whereas the APTMS-modified fiber decreased by merely 0.015 a.u. at 590 nm. Based on the measurement principle of the transmission spectra that the distribution of the spectra represents the characteristics of the surface modification, these results indicate that surface modification of the gold nanoparticles using MPTMS is better than that using APTMS.
illustrates the transmission spectra of the modified fiber treated with t-dodecylmercaptan. Gold nanoparticles modified using APTMS were further tested for comparison; results are shown in . The gold nanoparticles were wrapped with t-dodecylmercaptan to remove electrostatic force. shows that the spectra decrease slightly under the same conditions of because LSPR of the gold nanoparticles continued to be absent from the fiber surface. This result shows that an electrostatic force exists between the nanoparticles and amino groups.
Transmittance of the modified fiber by APTMS after treatment with t-dodecylmercaptan.
After the MPTMS-modified nanoparticle fiber was treated with t-dodecylmercaptan, the spectra of the treated fiber markedly varied in different solutions, as shown in .
Transmittance of the modified fiber by MPTMS treated with t-dodecyl mercaptan.
Notably, the force between the gold-nanoparticles and mercapto group was not electrostatic. Based on a study of MPTMS-modified nanoparticles in Au-S on wafer [16
], MPTMS is able to modify nanoparticles in Au-S on the fiber surface.
To verify reproducibility, we further performed tests on another MPTMS-modified fiber. It is seen from that the two tests displayed similar results. The peak value of the transmission spectra changed greatly when the MPTMS-modified fibers were immersed in different media in both tests. Note that different conditions of the surface modification lead to different curve shapes. The peak values of the transmission spectra in are different because the tapering conditions of the two fibers are not the same. With the same reagent but with different concentrations, the MPTMS-modified fibers decreased by 900 a.u. at 590 nm wavelength for in both the tests.
Transmittance of the modified fiber by MPTMS in two tests. (a) 1st test (b) 2nd test.
illustrates the transmission spectra of the MPTMS-modified fiber tested in different concentrations of ethanol. The peak value occurs at different wavelengths for difference cases, i.e., at 600 nm for 10% ethanol, 590 nm for 20% and 30% ethanol, and 580 nm for 40% ethanol. The refractive index is also different with different concentrations ethanol. illustrates the relationship between wavelengths and refractive index, based on which the refractive index sensitivity of MPTMS-modified fiber could be obtained as 1,190.5 nm/RIU.
Refractive index sensitivity test. (a) Transmittance of the modified fiber by MPTMS; (b) Relationship between wavelength and refractive index.