Gold
Nanoisland Films as Reproducible SERS Substrates
for Highly Sensitive Detection of Fungicides
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Abstract
A wet-chemical approach is used to
fabricate centimeter-scale gold
nanoisland films (NIFs) with tunable morphology of islands and with
strong electromagnetic coupling between them. The approach consists
in a uniform seeding of small gold nanoparticles on a glass or silicon
substrate, followed by controllable growth of the seeds into small
nanoislands. A special technique for TEM sampling was developed to
follow the gradual formation of larger-sized isolated nanoparticles,
nanoislands of sintered overgrown seeds, and a complete gold layer
with nanoscale cracks. The electromagnetic field distribution inside
the fabricated NIFs was calculated by FDTD simulations applied to
actual TEM images of the fabricated samples rather than to artificial
models commonly used. SERS measurements with 1,4-aminothiophenol (ATP)
molecules demonstrated the analytical enhancement factor about of
10<sup>7</sup> and the fundamental enhancement factor about of 10<sup>8</sup> for optimized substrates. These values were at least 1 order
of magnitude higher than that for self-assembled arrays of gold nanostars
and silver nanocubes. SERS spectra of independent samples demonstrated
good sample-to-sample reproducibility in terms of the relative standard
deviation (RSD) of the main peaks less than 20%. Additionally, Raman
maps with 1 μm increment in <i>X</i>–<i>Y</i> directions of NIFs (800 spectral spots) demonstrated good
point-to-point repeatability in the intensity of the main Raman vibration
modes (RSD varied from 5% to 15% for 50 randomly selected points).
A real-life application of the fabricated SERS substrates is exemplified
by the detection of the thiram fungicide in apple peels within the
5–250 ppb linear detection range. Specifically, the NIF-based
SERS technology detected thiram on apple peel down to level of 5 ng/cm<sup>2</sup>