Surface-Plasmon-Coupled Fluorescence Enhancement Based on Ordered Gold Nanorod Array Biochip for Ultrasensitive DNA Analysis

Abstract

An innovative gold nanorod (GNR) array biochip was developed to systematically investigate the localized surface plasmon resonance (LSPR)-coupled fluorescence enhancement for signal amplification in molecular beacon detection. An ordered GNR assembly in vertical standing array on a glass surface was fabricated as plasmonic substrates, resulting in dramatically intensified LSPR between adjacent nanoparticles as compared to that from an ensemble of random nanorods. We have shown that the plasmonic response of the nanoarray can be tuned by the proper choice of GNR size to overlap the fluorophore excitation and emission wavelengths greater than 600 nm. Plasmon-induced fluorescence enhancement was found to be distance-dependent with the competition between quenching and enhancement by the metal nanostructures. The augmented fluorescence enhancement by the GNR array can efficiently overcome the quenching effect of the gold nanoparticle even at close proximity. The enhancement correlates with the spectral overlap between the fluorophore excitation/emission and the plasmonic resonance of the GNR array, indicating a surface-plasmon-enhanced excitation and radiative mechanism for the amplification. From these results, the applicability of the ordered GNR array chip was extended to molecular fluorescence enhancement for practical use as a highly functional and ultrasensitive plasmonic DNA biochip in molecular beacon fashion.

Figure 1.

(A) Schematic of the fabrication of ordered arrays of standing gold nanorods on glass surface by controlled evaporation, followed by the fluorophore conjugate. The spacer thickness is tunable to adjust the distance between the fluorophore and nanorod surface. (B) Electron microscopic images (top view) of the GNR nanoarray of varying sizes with the respective LSPR peak wavelength as noted.

Figure 2.

Simulation results showing the electric field distribution of the GNR nanoarray. (A) Side view and (B) top view. The incident light wavelength was 664 nm, and the excitation polarization direction was along the long axis of GNR (z direction) with the electric intensity of 1 × 10⁸ V/m. The amplied surface plasmon intensity “hot spot” is located at the tips of the nanorod cluster. Inset: SEM image of the GNR array revealing the close packed hexgonal structure of adjacent nanorods.

Figure 3.

Comparison of the ordered GNR array with the random assembly on substrate for the distance-dependent fluorescence enhancement. Inset: schematic of the fluorophore nearby the GNR surface with different spacer thickness.

Figure 4.

Effect of spectral overlap of the gold nanoparticles with the excitation/emission wavelengths of Quasar670 on the fluorescence enhancement.

Figure 5.

Practical use of the ordered GNR array chip for DNA detection based on surface plasmon enhanced fluorescence upon hybridization.

Figure 6.

(A) Quasar670 fluorescence intensity change during hybridization from the GNR664 nanoarray biochip (⧫) and comparison to that from control sample (■). (B) Calibration curve of the GNR nanoarray based DNA chip as a function of the increasing target ssDNA concentration (r² = 0.99).