Large Scale Fabrication of Ordered Gold Nanoparticle-Epoxy Surface Nanocomposites and Their Application as Label-Free Plasmonic DNA Biosensors

Abstract

A robust and scalable technology to fabricate ordered gold nanoparticle arrangements on epoxy substrates is presented. The nanoparticles are synthesized by solid-state dewetting on nanobowled aluminum templates, which are prepared by the selective chemical etching of porous anodic alumina (PAA) grown on an aluminum sheet with controlled anodic oxidation. This flexible fabrication technology provides proper control over the nanoparticle size, shape, and interparticle distance over a large surface area (several cm²), which enables the fine-tuning and optimization of their plasmonic absorption spectra for LSPR and SERS applications between 535 and 625 nm. The nanoparticles are transferred to the surface of epoxy substrates, which are subsequently selectively etched. The resulting nanomushrooms arrangements consist of ordered epoxy nanopillars with flat, disk-shaped nanoparticles on top, and their bulk refractive index sensitivity is between 83 and 108 nm RIU^-1. Label-free DNA detection is successfully demonstrated with the sensors by using a 20 base pair long specific DNA sequence from the parasite Giardia lamblia. A red-shift of 6.6 nm in the LSPR absorbance spectrum was detected after the 2 h hybridization with 1 μM target DNA, and the achievable LOD was around 5 nM. The reported plasmonic sensor is one of the first surface AuNP/polymer nanocomposites ever reported for the successful label-free detection of DNA.

Keywords: DNA biosensor; localized surface plasmon resonance; nanobowled aluminum; nanoparticle lattice; surface nanocomposite.

Figure 1

Comprehensive illustration of the technology to fabricate ordered nanoparticle arrangements on epoxy substrates. The main steps of the process are the following: (1) Preparation (cleaning, mechanical and electrochemical polishing) of the Al sheets. (2) Formation of PAA on aluminum through controlled anodic oxidation. (3) Nanobowled aluminum template formation after PAA removal. (4) Thin film deposition of gold on the template. (5) Nanoparticle arrangement formation through solid-state dewetting. (6) Epoxy casting and curing on top of the gold arrangement. (7) After the removal of the Al sheet the nanoparticles are transferred to the epoxy substrate. The SEM/TEM/EDX/optical images illustrate the various phases of fabrication.

Figure 2

SEM images illustrating the control over the nanoparticle arrangement and sizes on two types of nanobowled Al templates formed by anodization at 25 V in sulfuric acid with cell sizes D = 67 ± 4 nm (A type) and at 40 V in oxalic acid with D = 110 ± 5 nm (B type). The size distributions (d) of particles are the following: 51 ± 5 nm (A1), 60 ± 7 nm (A2), 79 ± 6 nm (B1), 92 ± 6 nm (B2), and 102 ± 9 nm (B3).

Figure 3

Illustration and the effect of selective epoxy etching on B1 type samples. Top row, left: 3D models. Middle: tilted (45°) SEM views. Right: STEM cross-sectional images (bright mode). Etching times from top to bottom: 0, 10, 20, and 40 s. Bottom graphs: detailed XPS spectra of O 1s, C 1s, and Au 4f peaks; collection angle (θ) = 60°. The tables show the estimated atomic concentrations for both standard (θ = 0°) and tilted (θ = 60°) measurements.

Figure 4

Normalized absorbance spectra of (a) A1 type nanocomposites after different times of selective epoxy etching with O₂ plasma, measured in air (data corresponding to Figure 3) with inset of optical microscopy images (transmission) of corresponding samples; (b) A1 and A2 type samples after 30 s selective etching measured in air and in water; (c) B1 and B3 type samples after 30 s selective etching measured in air and in water, respectively.

Figure 5

(a) Position of the LSPR absorbance peak maxima of the etched A1 type nanocomposite samples measured in air and in water, respectively. (b) Calculated bulk refractive index sensitivities of the same sensors. The values on the right side of the graphs represent the condition of the samples after cleaning them with low-power O₂ plasma after 30 days. The samples correspond to the ones presented in Figure 3 and Figure 4a.

Figure 6

(a) Normalized absorbance spectra of a B3 type nanocomposite sample, measured in different media (sucrose solutions). (b) Linear regression of the LSPR peak maxima shown in (a). (c) Normalized absorbance spectra measured in different phases of probe-DNA immobilization and target-DNA hybridization, measured on a B3 type nanocomposite, by using a 0.75 M NaCl–50 mM Na₂HPO₄ buffer.

Figure 7

(a) Absolute LSPR absorbance shift measured after DNA immobilization and subsequent DNA hybridization by using buffers with different ionic strengths on the B3 type nanocomposite. (b) Results of control experiments (performed in a buffer with 0.75 M ionic strength, B3 type composite) aiming to distinguish between the signal contribution of MCH and probe-DNA during immobilization and also negative controls with noncomplementary DNA. (c) Calibration curve of the B3 type nanocomposite. All data are an average of 4–5 measurements.