Design of gold-dot pattern with high sensitivity
Figure 1 displays the schematic illustration of the finite-difference time-domain (FDTD) simulation model for the Au-dot pattern chip. To improve the sensitivity of the LSPR sensor, it is important to understand, for each Au-dot pattern, how the LSPR peak shifts under the presence of the influenza virus (model protein). The FDTD simulation was therefore used to determine the diameter (D) and pitch (P) of Au-dot pattern structure suitable for the influenza virus detection from the peak shift of the LSPR spectra when a single model protein was placed on the Au-dot surface. The result of the FDTD simulation in Supplementary Fig. S1 suggests that the Au-dot pattern structure with the peak position in the near-infrared region (780–2500 nm) is useful for detecting protein capture. The LSPR spectrum is heavily affected by water vapour in the atmosphere because water absorption bands occur at 1400 and 1800 nm. In this study, the pattern of D400P800 (Supplementary Fig. S1) with a peak wavelength of about 1200 nm was used to prevent deformation of the LSPR peak profile caused by moisture adsorbed on the sensor surface.
Fabrication of gold-dot pattern chip by the nano-imprint method
Several techniques have been developed for fabricating nano-scale structures, including the Au-dot pattern. Self-organisation techniques, such as colloidal lithography37 and nano-sphere lithography38,39, are well known for their low cost; however, these methods are unable to provide high precision nano-structures over large areas. On the other hand, the direct writing techniques such as electron beam lithography40 requires high manufacturing cost due to the long processing time. To put it simply, these methods are not suitable for the mass production of nano-scale structures. Nano-imprint lithography41,42 has thus been a promising approach for fabricating uniform nano-structures over a large area. Au-dot pattern chips in this study were thus prepared by the thermal nano-imprinting method (Fig. 2).
The main issue in manufacturing the Au-dot pattern is weak adhesion between Au and quartz substrate. Generally, a metal adhesion underlayer such as silicon (Si), titanium and chromium is interposed between Au and quartz substrate to enhance their adhesion. However, the metal underlayer considerably reduces the plasmon dephasing time, causing degradation of LSPR sensitivity43. Jo et. al.27 reported that LSPR sensitivity could be improved by partially removing the metal underlayer through piranha treatment despite the decrease of the mechanical and chemical stabilities of the Au-dot pattern chip. In this study, we simultaneously improved LSPR sensitivity and mechanical strength by oxidising the Si underlayer via heating at 450 ℃; Si forms a eutectic with Au at more than 370 ℃44. The improvement of the mechanical strength is likely attributed to the occurrence of Au–Si eutectic reaction.
Detection experiments of influenza virus using the gold-dot pattern chip
A sialic sugar chain with a small molecular weight was used as a recognition molecule for detecting HA with high sensitivity herein. Polysaccharides play an important role as ligands specifically recognisable by various HA variants in influenza virus infections into host cells. The receptor specificity of influenza viruses has been primarily stratified by recognition of sialic acid species (N-acetyl- or N-glycolylneuraminic acid) and the type of glycosidic linkage between sialic acid and penultimate galactose (α2,3 or α2,6)45, 46; α2,3 sialic acid is known to strongly bind H5 HA and α2,6 sialic acid to H1 HA. We observed that H5 and H1 HAs can be distinguished with high sensitivity by immobilising α2,3 and α2,6 sialic acids on the Au-dot pattern chip, respectively.
Figure 3a represents the LSPR spectral change of the α 2,3 sialic acid-coated chip that was employed to detect 1 fg mL−1 of avian influenza virus (H5N1). To investigate the ability to distinguish virus type and the possibility of false-positive occurrence, 1 pg mL−1 of human influenza virus (H1N1) with a 1000-fold higher concentration than H5N1 virus was added dropwise on the chip. The peak shift of the LSPR spectrum barely occurred when 1 pg mL–1 of H1N1 was dripped on the chip. Subsequently, 1 fg mL–1 of H5N1 was dripped on the chip, resulting in the red-shift of the peak by about 10 nm.
Figure 3b represents the LSPR spectral change of the α 2,6 sialic acid-coated chip that was employed to detect 1 fg mL–1 of H1N1. H5N1 (1 pg mL–1) was dripped on the chip, followed by 1 fg mL−1 of H1N1. Although the peak shift of the LSPR spectrum barely occurred after the dripping of 1 pg mL–1 H5N1, the dripping of 1 fg mL–1 H1N1 generated the red-shift of the peak by about 10 nm. As the evaluation of other protein contaminants, the change in the LSPR spectrum was observed when 1 μg mL−1 bovine serum albumin was dripped on the chip. The LSPR spectrum was barely shifted. It is evident that the influence of contaminants was rather low. These results show that the Au-dot pattern chip can discriminate between H1 and H5 HAs with high sensitivity.
Figure 3c shows the shift amount of the LSPR spectrum at H5N1 concentrations ranging from 1 ag mL−1 to 1 ng mL−1. The shift amount of the LSPR spectrum was about 8 nm at the concentration above 1 fg mL−1, and about 0.96 nm at the concentration of 100 ag mL−1. The shift of the LSPR spectrum could not be observed at the concentration of 1 and 10 ag mL−1. These results indicate that the detection limit of the Au-dot pattern chip is 1 fg mL−1 of HA. The HA included in the dropped solution (10 μL of 1 fg mL−1 HA) was estimated to have 100 HA molecules. Such HA amount is smaller than that in a single influenza virus containing approximately 500 HA. This means that the Au-dot pattern chip can detect a single influenza virus in about 10 min. Since the detection sensitivity of our chip is 109 times higher than currently used immunochromatography (detection sensitivity: viral protein concentration 10 μg mL−1), our Au-dot pattern chip is extremely useful as an inspection chip.
The presence of HA on the chip surface was validated by using a nano-search microscope. Figure 4a shows the atomic force microscopy (AFM) image of the surface after dropping 1 fg mL–1 of H5. The AFM image demonstrates that a sphere of about 50 nm was present on the surface. The oxygen plasma treatment was implemented to prove that this sphere was HA. The plasma treatment got rid of all traces of organic matter and left no residues unless excessive inorganic contaminants were present on the sample. If this sphere disappeared after the plasma treatment, the sphere was likely an organic matter. Figure 4b shows the LSPR spectrum before and after the oxygen plasma treatment. The LSPR spectrum shifted by about 10 nm after dropping 1 fg mL–1 of H5, but it returned to its original state upon the oxygen plasma treatment. In addition, comparing AFM images, spherical bodies disappeared due to oxygen plasma treatment. These results suggest that a wavelength shift in the LSPR spectrum occurred due to the capture of HA on the surface of the Au dot.
What led to the detection of the influenza virus with ultra-high sensitivity?
This study suggests that the Au-dot pattern chip can detect influenza viruses with ultra-high sensitivity. The FDTD simulations reveal the reasons for the high sensitivity. Figure 5b shows the FDTD simulations of the electromagnetic field intensity distribution in the simulation area presented in Fig. 5a. The electric field was remarkably enhanced in the rim of the Au dot along the electric field polarisation direction. The electric field intensity enhancement (|E|2) can achieve a maximum of 24.8, 27.49, 29.78 and 26.97 at the wavelengths of 1204.05, 1329.85, 1442.91 and 1576.99 nm, respectively. These simulations suggest that electric field intensity enhancement depends on the wavelength. Figure 5c shows the spectrum of the normalised electric field intensity in the near-infrared wavelength range from 0.4 to 1.8 µm. Sharp peaks were found at the wavelength between 0.5 and 0.65 µm, which are attributed to the size of the single Au dot. A broad peak was observed at the wavelength between 0.65 and 1.8 µm, which can reach up to 34.4 times compared with the incident electric field intensity.
The FDTD simulation results also indicate that the Au-dot pattern structure clearly affected the electric field intensity around the Au dot (Fig. S6). The D and P/D are important for controlling the peak position of the electric field intensity at the top edge of the Au dot. As the D of the Au dot increased, the peak position shifted to the higher wavelength, and the peak area increased. For the same D of the Au dot, the Au-dot P was affected by the peak area. To understand the correlation between the electric field intensity and the detection sensitivity of model proteins, we investigated the peak area change of the electric field intensity relative to the peak shift in the LSPR spectrum when a single model protein was placed on the Au-dot pattern surface (Fig. 6). The results show that the peak shift in the LSPR spectrum is negligible (~2 nm) when the peak area was below 15 (D ≤ 300). When the peak area is more than 15 (D ≥ 350), a positive correlation between the peak area and the amount of shift was displayed. At the peak shift of about 10 nm, which was comparable to that observed in the detection experiments of the influenza virus, the peak area was about 19. These results imply that the enhancement and localisation of the electric field intensity in the Au-dot pattern are significant for the sensitive detection of the dielectric constant change associated with model protein adsorption.