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Enhancing the Surface Sensitivity of Metallic Nanostructures Using Oblique-Angle-Induced Fano Resonances

View Article: PubMed Central - PubMed

ABSTRACT

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2°) due to its short decay length. Additionally, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1 ng/ml (0.14 pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73 pg/ml (0.38 fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1–0.2 fg/mm2 under a phase shift of 5 mdeg).

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Surface sensitivity tests for different incident angles with intensity interrogation by measuring the interactions between BSA and anti-BSA.(a) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air, BSA and anti-BSA adsorption conditions for incident angles of 0°, 13° and 19°. (b) The absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. (c) The maximum intensity changes as a function of incident angle for anti-BSA adsorption. (d) The intensity noise and signal-to-noise ratio (SNR) for incident angles of 0°, 13° and 19°. The intensity noise is defined as 3 standard deviations of the response.
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f5: Surface sensitivity tests for different incident angles with intensity interrogation by measuring the interactions between BSA and anti-BSA.(a) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air, BSA and anti-BSA adsorption conditions for incident angles of 0°, 13° and 19°. (b) The absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. (c) The maximum intensity changes as a function of incident angle for anti-BSA adsorption. (d) The intensity noise and signal-to-noise ratio (SNR) for incident angles of 0°, 13° and 19°. The intensity noise is defined as 3 standard deviations of the response.

Mentions: We further compared the surface sensitivities for different incident angles using intensity interrogation. Figure 5a shows spectra for BSA and BSA-Anti-BSA at three different angles: 0°, 13° and 19°. Figure 5b shows the absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. Figure 5c shows the maximum intensity changes as a function of the incident angle. The intensity changes were 56, 103 and 260% for incident angles of 0°, 13° and 19°, respectively. The maximum intensity change was at an angle of 19°, close to the three-mode coupling angle (21°). Compared to the normal incidence, the incident angle of 19° has enhanced the intensity change by a factor of 5. In addition, the measured intensity noises were 1.65%, 0.81% and 0.48% for incident angles of 0°, 13° and 19°, respectively, and the signal-to-noise ratios were 33, 127 and 542, respectively (see Fig. 5d). Obviously, the normal incident has a lower surface sensitivity. The signal-to-noise ratios were improved by factors of 3.8 and 16.4 for incident angles of 13° and 19°, respectively. From equation (5), the surface sensing ability is determined by the bulk sensitivity, the decay length and the resonance slope. For an incident angle of 19°, the backward BW-SPP at the metal/air interface interacted with the cavity mode and was close to the forward BW-SPP at the metal/substrate interface (see Fig. 1c). The split SPR mode (−kSPR) coupled to the cavity mode, which had a decreased decay length. In addition, the coupling between the split SPR (−kSPR) and substrate modes (+kSub) enhanced peak transmission, which resulted in a larger resonance slope and reduced the noise. Therefore, the larger resonance slope, decreased decay length and lower noise enhanced the intensity detection for the incident angle of 19°. It was noted that although the wavelength sensitivity was high for the incident angle of 2°, the intensity sensitivity was low at this angle because of the small resonance slope. The above results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) had a higher wavelength sensitivity for small-angle incidence (2°) due to its shorter decay length. However, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes at large-angle incidence had a higher intensity sensitivity due to the reduced decay length, larger resonance slope and enhanced transmission intensity. It was noted that for the three-mode coupling, the transmission is primarily enhanced by the resonance coupling between the substrate BW-SPP and surface BW-SPP modes. An oblique angle will split the surface BW-SPP (−kSPR) and substrate BW-SPP (+kSub). At a certain angle (~21° in our case), the SP propagation constants on both sides of the nanoslits are matched and result in enhanced transmission. Such transmission enhancement has also been observed in nanohole arrays when the SP resonance energies on both sides of the metal film are matched404142. For three-mode coupling, the resonance coupling of both BW-SPP modes is further coupled with the cavity mode and forms the Fano coupling mode. The surface plasmon field is re-distributed. The cavity mode exhibits localized surface plasmon resonance in the nanoslits with a short decay length, which affects the field distribution of the plasmon mode and results in a shorter decay length for the coupled resonance, as shown in Fig. 4d. Therefore, the transmission intensity and surface sensitivity are greatly enhanced by using three-mode coupling.


Enhancing the Surface Sensitivity of Metallic Nanostructures Using Oblique-Angle-Induced Fano Resonances
Surface sensitivity tests for different incident angles with intensity interrogation by measuring the interactions between BSA and anti-BSA.(a) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air, BSA and anti-BSA adsorption conditions for incident angles of 0°, 13° and 19°. (b) The absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. (c) The maximum intensity changes as a function of incident angle for anti-BSA adsorption. (d) The intensity noise and signal-to-noise ratio (SNR) for incident angles of 0°, 13° and 19°. The intensity noise is defined as 3 standard deviations of the response.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5016831&req=5

f5: Surface sensitivity tests for different incident angles with intensity interrogation by measuring the interactions between BSA and anti-BSA.(a) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air, BSA and anti-BSA adsorption conditions for incident angles of 0°, 13° and 19°. (b) The absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. (c) The maximum intensity changes as a function of incident angle for anti-BSA adsorption. (d) The intensity noise and signal-to-noise ratio (SNR) for incident angles of 0°, 13° and 19°. The intensity noise is defined as 3 standard deviations of the response.
Mentions: We further compared the surface sensitivities for different incident angles using intensity interrogation. Figure 5a shows spectra for BSA and BSA-Anti-BSA at three different angles: 0°, 13° and 19°. Figure 5b shows the absolute spectral intensity changes caused by anti-BSA adsorption for different incident angles from 0° to 50°. Figure 5c shows the maximum intensity changes as a function of the incident angle. The intensity changes were 56, 103 and 260% for incident angles of 0°, 13° and 19°, respectively. The maximum intensity change was at an angle of 19°, close to the three-mode coupling angle (21°). Compared to the normal incidence, the incident angle of 19° has enhanced the intensity change by a factor of 5. In addition, the measured intensity noises were 1.65%, 0.81% and 0.48% for incident angles of 0°, 13° and 19°, respectively, and the signal-to-noise ratios were 33, 127 and 542, respectively (see Fig. 5d). Obviously, the normal incident has a lower surface sensitivity. The signal-to-noise ratios were improved by factors of 3.8 and 16.4 for incident angles of 13° and 19°, respectively. From equation (5), the surface sensing ability is determined by the bulk sensitivity, the decay length and the resonance slope. For an incident angle of 19°, the backward BW-SPP at the metal/air interface interacted with the cavity mode and was close to the forward BW-SPP at the metal/substrate interface (see Fig. 1c). The split SPR mode (−kSPR) coupled to the cavity mode, which had a decreased decay length. In addition, the coupling between the split SPR (−kSPR) and substrate modes (+kSub) enhanced peak transmission, which resulted in a larger resonance slope and reduced the noise. Therefore, the larger resonance slope, decreased decay length and lower noise enhanced the intensity detection for the incident angle of 19°. It was noted that although the wavelength sensitivity was high for the incident angle of 2°, the intensity sensitivity was low at this angle because of the small resonance slope. The above results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) had a higher wavelength sensitivity for small-angle incidence (2°) due to its shorter decay length. However, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes at large-angle incidence had a higher intensity sensitivity due to the reduced decay length, larger resonance slope and enhanced transmission intensity. It was noted that for the three-mode coupling, the transmission is primarily enhanced by the resonance coupling between the substrate BW-SPP and surface BW-SPP modes. An oblique angle will split the surface BW-SPP (−kSPR) and substrate BW-SPP (+kSub). At a certain angle (~21° in our case), the SP propagation constants on both sides of the nanoslits are matched and result in enhanced transmission. Such transmission enhancement has also been observed in nanohole arrays when the SP resonance energies on both sides of the metal film are matched404142. For three-mode coupling, the resonance coupling of both BW-SPP modes is further coupled with the cavity mode and forms the Fano coupling mode. The surface plasmon field is re-distributed. The cavity mode exhibits localized surface plasmon resonance in the nanoslits with a short decay length, which affects the field distribution of the plasmon mode and results in a shorter decay length for the coupled resonance, as shown in Fig. 4d. Therefore, the transmission intensity and surface sensitivity are greatly enhanced by using three-mode coupling.

View Article: PubMed Central - PubMed

ABSTRACT

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2°) due to its short decay length. Additionally, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1 ng/ml (0.14 pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73 pg/ml (0.38 fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1–0.2 fg/mm2 under a phase shift of 5 mdeg).

No MeSH data available.


Related in: MedlinePlus