Nanoplasmonic sensing (NPS)
NPS is a versatile and extremely surface sensitive optical technique for studies of properties in molecular scale films, using gold nanoparticles as local sensing elements. The nanofabricated plasmonic gold discs of the Insplorion sensors are embedded in a custom-made dielectric material offering optimal protection and tailored surface chemistry of the sensor.
Insplorion has taken the general concept of LSPR-based sensing and made it applicable to a wide range of areas, where the gold Nano discs act as optical antennas, which respond to processes at the sensor/sample interface. The technique constitutes a very versatile sensing platform that enables detection and monitoring of a large variety of material and interface processes both in research, under in situ conditions, and in applied functions like battery sensors and air quality sensors.
NPS based on the LSPR phenomenon
Insplorion’s technology Nanoplasmonic Sensing (NPS) exploits a physical phenomenon called Localized Surface Plasmon Resonance (LSPR).
First of all, what is a plasmon? “In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantization of plasma oscillations, just like phonons are quantizations of mechanical vibrations. Thus, plasmons are collective (a discrete number) oscillations of the free electron gas density. For example, at optical frequencies, plasmons can couple with a photon to create another quasiparticle called a plasmon polariton.” Wikipedia Plasmon
Localized Surface Plasmon
A localized surface plasmon (LSP) is a coherent, collective spatial oscillation of the free electrons in a metallic nanoparticle. LSPs can be excited by the electromagnetic field of near visible light. When white light passes through a plasmonic sensor, due to absorption and scattering of light by the particles, a peak in the extinction spectrum emerges. The resonance peak position is determined by the size, shape and material of the nanoparticle, and more importantly, it also depends on the refractive index of the medium in close proximity to the nanoparticle. Thus, by monitoring changes in the resonance peak, it is possible to detect and monitor processes influencing the dielectric environment of the nanoparticles on the sensor surface.
Localized Surface Plasmon Resonance
The LSPR is a coherent, collective spatial oscillation of the conduction electrons in a metallic nanoparticle, which can be directly excited by near-visible light. The LSP resonance condition (i.e. the wavelength/color of light which can excite the LSPR) is defined by a combination of:
• the electronic properties of the nanoparticles
• the nanoparticle size and shape
• the nanoparticle temperature
• the dielectric environment in close proximity of the nanoparticles
The dielectric environment of the nanoparticles is a consequence of the locally enhanced plasmonic near field (with respect to the incoming field). The field is exponentially decaying from the nanoparticle surface. Within this “nanovolume” of locally enhanced field, small changes in the local dielectric environment (e.g. caused by molecular adsorption or thermic processes) affect the resonance. The changes in resonance, in turn, changes the amount of scattered and/or absorbed light at different wavelengths. These changes can be measured with high spectral resolution, in a simple optical transmission or reflection experiment, making LSPRs excellent nano-sensors.
NPS-chip Nanoarchitecture and Functionality
Nanoplasmonic Sensing in general, exploits metallic nanoparticles, usually gold (Au) or silver (Ag), as local sensing elements, which offer a combination of unique properties; including ultrahigh sensitivity, small sample amount/volume (due to the tininess of the “sensor”, i.e. a nanoparticle typically in the 50 – 100 nm size range) and capability for fast, real-time (millisecond time resolution) remote readout.
In Insplorion’s patent, "Applied Nanoplasmonic Sensing (NPS) technology", the sensing is realized through nanofabricated arrays of non-interacting, identical gold nanodisks on a glass surface. This gold nanodisk array (the “sensor”) is then covered with a thin film of a dielectric spacer layer. The sensor nanoparticles are thereby embedded in the sensor surface and not physically interacting with the studied nanomaterial, except via the LSPR dipole field.
The field penetrates though the spacer layer and has considerable strength also on and in proximity to its surface and can, therefore, sense dielectric changes.
The glass surface with the deposited Au sensor particles and the covering spacer layer can be regarded as a general sensor chip. In the unique nano-architecture of the Insplorion NPS sensor chip the spacer layer exerts the following functions:The glass surface with the deposited Au sensor particles and the covering spacer layer can be regarded as a general sensor chip. In the unique nano-architecture of the Insplorion NPS sensor chip the spacer layer exerts the following functions:
• protection of the Au nanosensors from structural re-shaping;
• protection of the Au nanosensors from chemical interaction with the sample material;
• protection of the Au nanosensors from harsh/reactive environments;
• providing a tailored surface chemistry of the sensor chip, and thereby
• be either an inert substrate for the sample material, or
• participate actively in the studied process (e.g. spillover effects).
A major advantage of the NPS approach is that any material shape and size and any material type can be studied on a nano level on a wide range of substrate materials.