Abstract
The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing. To date, nanoplasmonic sensors typically consist of two-dimensional (2D) nanoparticle arrays and can therefore only probe processes that occur within the array plane, leaving unaddressed the potential of sensing in three dimensions (3D). Here, we present a plasmonic metasurface comprising arrays of stacked Ag nanodisks separated by a thick SiO2 dielectric layer, which, through rational design, exhibit two distinct and spectrally separated LSPR sensing peaks and corresponding spatially separated sensing locations in the axial direction. This arrangement thus enables real-time plasmonic sensing in 3D. As a proof-of-principle, we successfully determine in a single experiment the layer-specific glass transition temperatures of a bilayer polymer thin film of poly(methyl methacrylate), PMMA, and poly(methyl methacrylate)/poly(methacrylic acid), P(MMA-MAA). Our work thus demonstrates a strategy for nanoplasmonic sensor design and utilization to simultaneously probe local chemical or physical processes at spatially different locations. In a wider perspective, it stimulates further development of sensors that employ multiple detection elements to generate distinct and spectrally individually addressable LSPR modes.
Highlights
The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing
This independent function requires sufficient spatial separation between the disks to minimize near-field coupling, and that they exhibit LSPR spectrally separated enough that two independent “peaks” can be resolved in their optical spectrum. With respect to the latter requirement, we aimed to have both LSPR peaks within the visible to near-infrared (NIR) spectral range (i.e., 400−1100 nm) to comply with the most commonly used optical components in the field.[6]. With this constraint at hand, the choice of the metal for the sensing antennas is practically restricted to Ag, which features a narrow LSPR peak and a high interband absorption onset (3.8 eV or ∼325 nm, i.e., beyond the designed wavelength range).[30−32] This is superior to Au, which exhibits broader LSPR modes and has a lower interband transition threshold energy (2.3 eV or ∼540 nm).[30,33]
We achieved this function by vertically stacking in a 3D nanoarchitecture two plasmonic Ag nanodisks with dimensions sufficiently different to ensure spectrally well-separated LSPR modes in the visible−NIR range and by spatially putting them apart by a dielectric SiO2 spacer thick enough to decouple their nearfields
Summary
The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing. Ever since the first demonstration of the nanoplasmonic sensing concept by Englebienne[9] two decades ago, it has developed into a major subfield of plasmonics and has found wide applications such as in bio- and chemosensing[6,7,10,11] and in materials science.[12,13] This, in turn, has spurred the development of a large library of plasmonic nanostructures tailored for various sensing purposes.[6,14−16] as a review of the corresponding literature shows, nanoplasmonic sensors to the largest extent only comprise two-dimensional (2D) arrays of a single type of plasmonic particle on a support They can only detect processes that occur within the plane of the array. To demonstrate the functionality of the sensor in a materials science application recently introduced,[24−29] we simultaneously characterize the layer-specific glass transition temperatures of stacked poly(methyl methacrylate), PMMA, and poly(methyl methacrylate)/poly(methacrylic acid), P(MMA-MAA), copolymer thin films in a single experiment
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