NANOSENSORS *
CHAPTER 4 CHAPTER 4
4.4 NANOSTRUCTURES AND MATERIALS .1 NANOWIRES
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4.4 NANOSTRUCTURES AND MATERIALS
75 4.4 NANOStRuctuRES ANd MAtERiAlS
4.4.2 CARBON NANOTUBES
A carbon nanotube is an ordered molecule of pure carbon shaped as a cylindrical structure. Many nanomaterials and nanostructures on their own are poor in terms of selectivity, as they are not naturally selective for anything. For this reason, nanotube-based sensors require some type of chemical engineer- ing. This can be accomplished by adding a coating to modify the surface of the nanotube or by doping atoms into the nanotubes increasing their selectivity to a specific analyte (Shelley, 2008) (Fig. 4.9).
A major advantage of nanotubes is their very high surface area. A single-walled carbon nanotube has a surface area of 1600 m2/g. In addition to their massive surface area, nanotubes are a promising material due to other remarkable properties. They are extremely strong, with strengths ranging from 20 to 100 times that of high strength alloys and steels. Nanotubes are also very resilient, with the ability to FIGURE 4.7
An array of silicon nanowires.
From Greene, L. L. (2003). Low-temperature production of ZnO nanowire arrays. Chemie International Edition, 3031–3034.
FIGURE 4.8
A general nanowire sensing device.
From Patolsky, F. L. (2005). Nanowire nanosensors. Materials Today.
76 CHAPTER 4 NANOSENSORS
be fully straightened without damage after extreme bending. Other impressive properties include their high tensile strength (about 200 GPa) and their stiffness (roughly five times that of steel). Nanotubes are also ultralight in weight (Shelley, 2008).
Combining this unique set of properties along with the ability to be modified chemically or biologi- cally in order to optimize targeting, nanotubes are the most likely to make their way to widespread commercial applications (Sinha, 2005). Electrical resistance also changes dramatically upon absorp- tion of certain gaseous molecules (e.g., NO2, NH3, H2), and this can be used in chemical sensing.
Fig. 4.10 shows a carbon nanotube set up as an FET, where when the analyte of interest is adsorbed, the electrical characteristics of the FET change and can be measured (Zhao, 2002).
Nanotubes can also be used as sensors through the use of mechanical resonance frequency shift to detect adsorbed molecules (Riu, 2005).
The nanotube is free to vibrate on one end (as seen in Fig. 4.11), while the other is attached to a negatively charged electrode. Electrons are able to flow to a positively charged electrode near the free end of the nanotube, and the current of the electrons indicates the frequency at which the nanotube is vibrating. By measuring the current, the frequency is being measured, and this indicates whether or not the particles of interest have become attached or not.
The addition of an adsorbed atom or molecule changes the mass of a vibrating nanotube, and this changes the frequency at which the nanotube oscillates.
4.4.3 THIN FILMS
Thin film nanosensors consist of a thin nanocrystalline or nanoporous sensing film, which is capable of interacting with the environment around it.
FIGURE 4.9
the structure of carbon nanotubes.
From Wikipedia. (n.d.). Wikipedia. Retrieved 05 12, 2016, from Carbon nanotube: https://en.wikipedia.org/wiki/Carbon_nanotube.
77 4.4 NANOStRuctuRES ANd MAtERiAlS
Electrical conductance is measured as it changes when gases adsorb or react at the surface of the nanofilms. Various materials have been used in nanoscaled thin film sensing, such as gold, platinum, diamond, titanium, iridium oxide, and various polymers.
An example of a thin film nanosensor is shown in Fig. 4.12. This nanoporous thin film sensor uses a thiol monolayer that selectively binds to lead or other heavy metals. The film acts as an electrode in the detection of heavy metals. This technology could be used in water sampling (Shelley, 2008).
FIGURE 4.10
A nanotube used in an fEt device.
From Namomix, S. (2007). A Nanotechnology Test System. EE-Evaluation Engineering, June 2007.
FIGURE 4.11
A vibrating nanotube. As particles attach to the nanotube, the vibration frequency changes.
From Johnston, H. (2008, July 21). Nanotube cantilever weigh up. Retrieved 10.04.11, from Physicsworld: http://physicsworld.com/
cws/article/news/2008/jul/21/nanotube-cantilever-weighs-up.
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4.4.4 NANOPARTICLES
Nanoparticles are small clusters of hundreds or thousands of atoms, measuring only a few nanometers in length. The energy level can be tuned and manipulated by synthesizing nanoparticles of different diameters (Riu, 2005). A location that can contain a single electric charge is called a quantum dot (QD).
The absence or presence of an electron in a QD changes its properties, and this change in properties can be used for purposes such as information storage or as sensors (Likharev, 1999).
Nanoparticles can be used as nanosensors because they have exceptional size-dependent optical properties. Two types of nanoparticle-based sensors examined in this report are (1) noble metal nano- particles and (2) semiconductor QDs.
4.4.4.1 Noble metal nanoparticles
Nanostructures of noble metals that are smaller than the de Broglie wavelength for electrons are capa- ble of intense absorption in visible and UV region that is absent in the spectrum of the same bulk mate- rial (Riu, 2005).
The de Broglie Wavelength is given by
λ h p,
where h is Plank’s constant and p is the momentum (Richards, 2009).
FIGURE 4.12
A thin film nanoscale lead sensor.
From Shelley, S. (2008). Nanosensors: evolution, not revolution...yet. CEP, 1–5.
79 4.4 NANOStRuctuRES ANd MAtERiAlS
The excitation of the surface by light is called localized surface plasmon resonance (LSPR). Surface plasmons are electron oscillations at the interface between two materials.
LSPR sensing is based on the sensitivity of plasmon absorbance of metal nanostructures as a response to the changes in dielectric properties of the contacting medium. Typically, a recognition interface is constructed on a metal nanostructure, and specific binding is transduced into an optical signal (Vaskecich, 2008).
The size, material, and shape of the nanoparticle as well as the external environment control the LSPR spectrum. This makes noble metals extremely valuable as possible materials for nanosensors.
When target molecules attach to the metal nanoparticles, the local refractive index changes. LSPR spectra are extremely sensitive to these changes in refractive index, and the shift produced in the LSPR spectra can be used to detect molecules. The sensing principle of LSPR sensors is shown in Fig. 4.13.
In this example, a silver nanoparticle is used. When the antibody is present, it binds to the antigen, and a change in local refractive index occurs. This results in a shift in the LSPR spectra that can be interpreted as an “on” (presence) or “off” (absence) position.
4.4.4.2 Quantum dots
QDs are small semiconductor crystals that range from a few to a few hundred nanometers. They are commonly made of cadmium serenade, cadmium sulfide, or cadmium telluride. Because of the toxicity of cadmium, QDs usually have an inert polymer coating to protect human cells. The polymer coating is also advantageous in that it facilitates the attachment of various other molecules that can be used to optimize the selectivity of the target species. The conductivity of QDs as well as the wavelength of emitted light can also be manipulated by doping the dots with certain atoms.
FIGURE 4.13
the biosensing mechanism of a silver nanoparticle using lSpR.
From Riboh, J. H. (2003). A nanoscale optical biosensor: real-time immunoassay in physiological buffer enabled by improved nanoparticle adhesion. The Journal of Physical Chemistry B, 107, 1772–1780.
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QDs are able to demonstrate different optical properties as a result of shape, size, and composition, and this can be exploited to custom design optical responses. By controlling the size, shape, or com- position of the QD, one can custom design it to emit a certain light under certain conditions, creating a color-coded response.
Semiconductor QDs can be used to develop optical sensors based on fluorescence measurements. A smaller nanocrystal has a larger difference between energy levels and a shorter wavelength of fluores- cence. By adjusting the size of the semiconductor QD, any fluorescence colors in the visible spectrum can be obtained. For example, CdSe nanocrystals of about 2.5 nm diameters have green fluorescence, while those with diameters of about 7 nm have red fluorescence (Smith, 2010).
QDs mainly work as sensors because of the Forster resonance energy transfer (FRET) effect, which changes the fluorescence in nanoparticles into either an “on” or “off” state.
FRET is a nonradiative energy transfer from a donor fluorophore to an acceptor fluorophore in close proximity. When a donor fluorophore is in an excited state due to an outside energy source, energy from a donor can be transferred to an acceptor fluorophore located within a range of about 2–8 nm. This transfer of energy will only occur if there is sufficient spectral overlap between the donor emission and the acceptor absorption. Upon energy transfer, the donor fluorescence is reduced, and acceptor fluores- cence is increased (Barroso, 2011).
QDs have several characteristics, such as having broad excitation spectra making them ideal candi- dates for donor fluorophores in an FRET reaction (Fig. 4.14).
4.4.5 POROUS SILICON
Porous silicon is identical to the silicon used in many technological applications today with the differ- ence that the surface contains tiny pores, ranging from less than 2 nm to microns, capable of absorbing and emitting light. The material is a complicated network of silicon threads of thicknesses that range from 2 to 5 nm. This semiconductor material has a very large internal surface area to volume ratio, which can be as high as 500 m2/cm3. Light emission takes place in the visible region of the electromag- netic spectrum and is due to quantum confinement effects. The unique property of this material is that FIGURE 4.14
Qd in an fREt reaction.
From Zhang, C.-Y., Yeh, H.-C., Kuroki, M.T., Wang, T.-H. (2005). Single-quantum-dot-based DNA nanosensor.
Nature Materials 4, 826–831.
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the porosity of the material controls the wavelength of the emitted light. With a lower material porosity, longer wavelengths will be emitted, and with a higher material porosity, shorter wavelengths will be emitted. For example, materials with about 40% porosity will emit red light, while materials with a porosity of greater than 70% emit a blue/green light (Riu, 2005).
The luminescence porous silicon can also be altered when molecules are incorporated into the porous layer. This unique property has allowed for the design of gas sensors in which the response can be monitored by visually observing a change in colors.