Abstract
We report on a quasi-three-dimensional (3D) plasmonic nanowell array with high structural uniformity for molecular detection. The quasi-3D plasmonic nanowell array was composed of periodic hexagonal Au nanowells whose surface is densely covered with gold nanoparticles (Au NPs), separated by an ultrathin dielectric interlayer. The uniform array of the Au nanowells was fabricated by nanoimprint lithography and deposition of Au thin film. A self-assembled monolayer (SAM) of perfluorodecanethiol (PFDT) was coated on the Au surface, on which Au was further deposited. Interestingly, the PFDT-coated Au nanowells were fully covered with Au NPs with an ultra-high density of 375 μm−2 rather than a smooth film due to the anti-wetting property of the low-energy surface. The plasmonic nanogaps formed among the high-density Au NPs led to a strong near-field enhancement via coupled localized surface plasmon resonance and produced a uniform surface-enhanced Raman spectroscopy (SERS) response with a small relative standard deviation of 5.3%. Importantly, the highly uniform nanostructure, featured by the nanoimprint lithography and 3D growth of densely-packed Au NPs, minimizes the spatial variation of Raman intensity, potentially providing quantitative analysis. Moreover, analyte molecules were highly concentrated and selectively deposited in nanowells when a water droplet containing the analyte was evaporated on the plasmonic substrate. The analyte formed a relatively thick overcoat in the nanowells near the triple line due to the coffee-ring effects. Combining 3D plasmonic nanowell substrates with molecular enrichments, highly sensitive detection of lactic acid was demonstrated. Given its combination of high sensitivity and signal uniformity, the quasi-3D plasmonic nanowell substrate is expected to provide a superior molecular detection platform for biosensing applications.
Highlights
Plasmonic nanostructures provide strong light localization, through which the electromagnetic field is drastically enhanced in the nanogaps known as “hotspots”
The Au nanowell substrate was incubated for 2 h in a square dish to form a PFDT self-assembled monolayer (SAM)
The nanostructure was sketched based on scanning electron microscope (SEM) observations using Autodesk Fusion 360, and the data were subsequently exported into finite-difference time-domain (FDTD) software
Summary
Plasmonic nanostructures provide strong light localization, through which the electromagnetic field is drastically enhanced in the nanogaps known as “hotspots” This phenomenon has led to the development of an ultrasensitive molecular sensing method referred to as surface-enhanced Raman spectroscopy (SERS) [1–9]. The conventional top-down nanofabrication techniques to produce accurate structural arrangement and geometries for SERS applications include electron-beam lithography [12,13] and laser-interference lithography [14–20] These techniques enable the elaborate construction of plasmonic nanostructures, the low-throughput and high-cost of production restrict practical uses. To further enhance Raman intensity, on the other hand, it has been suggested to concentrate molecules in the nanogaps It is highly demanded, for the development of pragmatic sensing systems, to fabricate plasmonic nanostructures with a high density of nanogaps, through the NIL, that can concentrate and position probe molecules at the hotspots. The limit of detection (LOD) was measured to be 0.05 mM, which was 6 times lower than the LOD in the previous study [27]
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