Figure 2(a) and 2(b) depict the experimental and simulated transmission spectra when the concentration of glucose is 0.1 mol/L, and we can observe that the experimental results are well in agreement with simulation results. When the concentration of glucose is constant, the transmittance at the dip gradually decreases as the number of Ag NPs increases. The resonance wavelength is about 424.681 nm. Due to the increasing of the number of Ag NPs, the distance between Ag NPs becomes closer and closer, more and more photon energy absorbed, resulting in the decreasing transmittance.
When the amount of substance silver nanoparticles is 1.4×10−7 mol, the experimental transmission spectra of different concentration of glucose are shown in Fig. 3(a). We can discover that the resonance wavelength has a slight red shift as the concentration of glucose increases from 0.1 mol/L to 0.4 mol/L. Fig. 3(b) depicts the simulated transmission spectra of different background refractive index, when the number of Ag NPs is 70. We can also discover that the resonant wavelength has a slight red shift as the background refractive index increases, which can be used to detect glucose. The experimental results are consistent with the simulated results.
In addition, we plot the electric field distribution at the dip in Fig. 4, when n=1.33102, N=70, and N=400. We can discover that there is a localized field enhancement on the surface of Ag NPs at the resonant wavelength because of the existence of LSPR. It is more obvious when there are more particles in Fig. 4(b).
As is well known, SPR sensing is sensitive to change of the refractive index at the surrounding medium. Here, we define the sensitivity, \(S=\Delta \lambda /\Delta n\)(nm/RIU). Δλ is the lambda difference, which is resonant wavelength of the dip position of two adjacent refractive index. Δn is adjacent refractive index difference. We can obtain that the maximum sensitivity is S=1144.07407 nm/RIU which is higher than references listed [23, 24], when the refractive index varies from 1.33426 to 1.33588 from Table 1.
Table 1
the relationship between sensitivity and wavelength differences
Δλ | Δn | S |
1.56076 | 0.00162 | 963.43210 |
1.75585 | 0.00162 | 1083.85802 |
1.85340 | 0.00162 | 1144.07407 |