A hybrid plasmonic-photonic nanodevice for label-free detection of a few molecules
(Nanowerk Spotlight) Much is being written about nanotechnology's role in vastly improving the detection and treatment of cancer (read our Spotlight: On beating cancer with nanotechnology). Detection of cancer at the earliest stage provides the greatest chance of survival. Unfortunately, cancer has a logarithmic growth rate. A one cubic centimeter size tumor may have 40-50 cell divisions and typically doctors don't see 80% of the life of a tumor.
The detection of a protein pattern in blood serum can be helpful in evidencing a possible presence of cancer at an early stage. The problem is that 'early' means the capability of detecting very few molecules in dilute conditions. Now, in another step to improve the design and fabrication of devices for single molecule detection, new research has demonstrated an experimental capability of detecting down to as few as 10 organic molecules deposited on a quantum dot.
"Our findings are original because we combined photonic and plasmonic working principles on the same device in order to reach a sensitivity close to single molecule level," Dr. Enzo Di Fabrizio tells Nanowerk. "Moreover, our optical detection configuration is in the far field, that means that we use a standard microscope in order to detect a Raman spectroscopical signature of the molecule."
Di Fabrizio is a professor in the Department of Medicine at the Universití della Magna Graecia in Catanzaro, Italy, the Chief Scientist of the university's BIONEM laboratory, and also a senior scientist and group leader of LILIT Beam Line - INFM (National Institute For Matter Physics) at the TASC-INFM National Laboratory in Trieste.
In a recent paper in the July 17, 2008 online edition of Nano Letters ("A Hybrid Plasmonic-Photonic Nanodevice for Label-Free Detection of a Few Molecules"), Di Fabrizio and a team of colleagues from BIONEM and TASC report on the design and fabrication of a novel photonic-plasmonic device which demonstrates label-free detection capabilities on single inorganic nanoparticles and on monolayers of organic compounds.
Fabrication setup and SENSe (surface-enhanced nanosensor) device. (a) Scheme of the dual beam fabrication system based on focused ion milling (step 1) and electron beam induced deposition (step 2) with a gas injection system. (b) SEM image of the whole device including the photonic cavity and, at its center, the plasmonic nanoantenna. The latter is 2.5 µm high, and its size gradually decreases from 90 nm in diameter at bottom down to 10 nm radius of curvature at the tip. (c, d) SEM details of the nanoantenna tip and its radius of curvature. (Reprinted with permission from American Chemical Society)
Specifically, this novel technique combines atomic force spectroscopy, with topographic resolution below 10 nm, with chemical information also with a spatial resolution below 10 nm. This means that in the near future it will be possible to obtain topographic and label-free spectroscopic information directly from a living cell.
"Our results, demonstrating label-free detection of a few molecules in subwavelength regime and in far field configuration, open up new perspectives toward single-molecule detection" says Di Fabrizio.
"In our experiments, the number of molecules involved covers a range between 10 and 200; the detection is accomplished by far field Raman scattering spectroscopy operating in the subdiffraction regime." says Di Fabrizio. "The device can be straightforwardly integrated in a scanning probe apparatus with the possibility to match topographic and label-free spectroscopic information in a wide range of geometries."
The design of the nano-optical device developed by the team in Italy allows its fabrication on an atomic force microscope (AFM) cantilever. They actually have already fabricated such a prototype and demonstrated its detection capabilities on silicon oxide nanoparticles, on a single cadmium/selenium quantum dot, and on a monolayer of organic compounds.
The working principle of the Italian team's technique combines the light harvesting capabilities of a dielectric photonic crystal cavity with the extraordinary confining properties of a metallic nanowaveguide, leading to efficient optical excitation of target samples through surface plasmon polariton modes localized at the nanoscale.
Overview of an AFM cantilever with the photonic cavity (PC) and plasmonic nanoantenna fabricated on its surface (the details of the circled area are shown in (b) in the figure above. The silicon nitride cantilever is locally thinned on one side in order to obtain a membrane thickness of 100 nm necessary for the fabrication of the desired PC cavity. The local thinning of the membrane doesn't affect the mechanical properties of the cantilever. The aim of this architecture is to combine AFM and Raman Scattering technique. (Reprinted with permission from American Chemical Society)
The nanolithography techniques of choice in this work rely on two powerful fabrication methods: focused ion beam (FIB) milling and chemical vapor deposition (CVD) induced by a focused electron beam.
"Our device consists of a plasmonic nanoantenna with an ogival-shaped tip made of a nobel metal, such as gold or silver, combined with a photonic crystal (PC) cavity" explains Di Fabrizio. " The PC cavity produces an efficient coupling between the external optical source and the nanoantenna – direct coupling of the far field to the nanoantenna in the absence of a PC is inefficient, as we observed both theoretically and experimentally."
The scientists note that the high sensitivity and the high signal-to-noise ratio found in the experimental Raman spectra of the their work open bright perspectives for application to single molecule detection.
With a view towards their continued work on scanning probes and protein nanoarrays, Di Fabrizio says that one of the interesting questions he and his colleagues will try to answer is if – with such plasmonic and SERS (Surface Enhanced Raman Scattering) devices – it will be possible to collect information on the secondary, ternary and quaternary structure of proteins while they are sitting on the cell membrane without the use of protein crystallization methods.
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