nanotechnology enabled sensors

8/7/2019 Nanotechnology Enabled Sensors

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TECHNICAL PAPER

PRESENTATION

TITLE

NANOTECHNOLOGY-ENABLEDSENSORS:

POSSIBILITIES, REALITIES, ANDAPPLICATIONS

Authorised BySANTOSH BHARADWAJ REDDYEmail: [email protected]

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ABSTRACT:

Recent advances in both technology andscience of novel materials have provided

the means to study, understand, control,or even manipulate transitionalcharacteristics between isolated atomsand molecules, and bulk materials.

Various novel nanoscale materials,devices, and systems with remarkableproperties have recently been developed,exhibiting numerous unique applicationsin chemical and biological sensors,nanophotonics, and in-vivo analysis of

cellular processes. Such developments,especially in the last decade, has seededsector specific focus in areas viz.defense, energy, communication,computing, materials production, textile,health care and medicine.

Notwithstanding such advances,perpetual increase in population and thusconsumption of fossil fuels has led toincreased pollution worldwide - a

leading contributor to chronic anddeadly health disorders and diseasesaffecting millions of people each year,hence sustainability. The presentationwill provide an overview of nanotechnology based sensors withapplications in national security andenvironmental pollution sensing andmonitoring.

IMPACT OF

NANOTECHNOLOGY ON

SENSORS

Operating on the scale of atoms and

molecules, emerging nanotechnologies promise dramatic changes in sensordesigns and capabilities.

If you make or use sensors, your business will likely feel the impact ofcurrent and future developments innanotechnology, a very promising newbranch of small-scale technology namedfor the unit of measure at which itoperates: the nanometer, or 0.001

micron. Nanotechnology enables us tocreate functional materials, devices, andsystems by controlling matter at theatomic and molecular scales, and toexploit novel properties and phenomena.Consider that most chemical and biological sensors, as well as manyphysical sensors, depend on interactionsoccurring at these levels and you'll getan idea of the effect nanotechnology willhave on the sensor world.

The trend toward the small began withthe miniaturization of macro techniques,which led to the now well-establishedfield of micro technology. Electronic,optical, and mechanical microtechnologies have all profited from thesmaller, smarter, and less costly sensorsthat resulted from work with ICs, fiberoptics, other micro-optics, and MEMS(micro electro mechanical systems). As

we continue to work with theseminuscule building blocks, there will bea convergence of nanotechnology, biotechnology, and informationtechnology, among others, with benefitsfor each discipline. Substantially smallersize, lower weight, more modest powerrequirements, greater sensitivity, and

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better specificity are just a few of theimprovements we'll see in sensor design.

Nanosensors and nano-enabled sensorshave applications in many industries,

among them transportation,communications, building and facilities,medicine, safety, and national security,including both homeland defense andmilitary operations. Consider nanowiresensors that detect chemicals andbiologics, nanosensors placed in bloodcells to detect early radiation damage inastronauts, and nanoshells that detectand destroy tumors. Many start-upcompanies are already at work

developing these devices in an effort toget in at the beginning. Funding fornanotechnology increased by more thana factor of 5 between 1997 and 2003,and is still on the rise. So this is a goodtime to examine the possibilitiesandthe limitationsof this small new world.

Possibilities

The current global enthusiasm fornanotechnology is an offshoot of severallate 20th century advances. Of particularimportance was the ability to manipulateindividual atoms in a controlled fashion a sort of atomic bricklayingbytechniques such as scanning probemicroscopy. Initial successes inproducing significant amounts of silver

and gold nanoparticles helped to draweven more attention, as did the discoverythat materials and devices on the atomicand molecular scales have new anduseful properties due in part to surfaceand quantum effects.

[Carbon nanotubes can exist in a varietyof forms and can be either metallic orsemi conducting in nature, depending ontheir atomic structure.]

Another major contributor was thecreation of carbon nanotubes (CNTs),extremely narrow, hollow cylindersmade of carbon atoms. Both single- andmulti-walled CNTs could, for example,be functionalized at their ends to act as biosensors for DNA or proteins. Thesingle-walled versions can have differentgeometries. Depending on the exactorientation of the carbon atoms, a CNTcan exhibit either conducting (metallic)or semiconducting properties. Thischaracteristic and the ability to growCNTs at specific locations andmanipulated afterward, make it likelythat the tubes will be important forelectronics and sensors. For instance,they can be used in the fabrication ofnano field-effect transistors forelectronics or as biological probes forsensors, either singly or as an array.

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Increasingly Integrated

Technologies.

The technologies associated withmaterials, devices, and systems were

once relatively separate, but integrationhas become the ideal. First, transistorswere made into ICs. Next came theintegration of micro-optics andmicromechanics into devices that were packaged individually and mounted onPCBs. The use of flip chips (where thechip is the package), and placement of passive components within PCBs, areblurring the distinction between devicesand systems. The high levels of

integration made possible bynanotechnology has made the (verysmart) material essentially the deviceand possibly also the system. LarryBock, chief executive for Nanosys,recently noted that "nanotech takes thecomplexity out of the system and puts itin the material".

We can now seriously contemplatesensing the interaction of a small number

of molecules, processing andtransmitting the data with a smallnumber of electrons, and storing theinformation in nanometer-scalestructures. Fluorescence and other meansof single-molecule detection are beingdeveloped. IBM and others are workingon data storage systems that use proximal probes to make and readnanometer-scale indentations in polymers. These systems promise

read/write densities near 1 1012

bits/sq.in., far in excess of current magneticstorage capabilities. Although presentinga significant challenge, integration ofnano-scale technologies could lead totiny, low-power, smart sensors thatcould be manufactured cheaply in largenumbers. Their service areas could

include in situ sensing of structuralmaterials, sensor redundancy in systems,and size- and weight-constrainedstructures such as satellites and spaceplatforms.

Nanomaterials and nanostructures areother promising application areas. Twofunctions often separated in manysensors, especially those for chemicalsand biological substances, arerecognition of the molecule or otherobject of interest and transduction of thatrecognition event into a useful signal.Nanotechnology will enable us to designsensors that are much smaller, less

power hungry, and more sensitive thancurrent micro- or macro sensors. Sensingapplications will thus enjoy benefits far beyond those offered by MEMS andother micro sensors.

Manufacturing Advances

Recent advances in top-down

manufacturing processes have spurred both micro- and nanotechnologies.Makers of leading-edge ICs uselithography, etching, and deposition tosculpt a substrate such as silicon and build structures on it. Conventionalmicroelectronics has approached thenanometer scaleline widths in chipsare near the 100 nm level and arecontinuing to shrink. MEMS devices areconstructed in a similar top-down

process. As these processes beginworking on smaller and smallerdimensions, they can be used to make avariety of nanotechnology components,much as a large lathe can be used tomake small parts in a machine shop.

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In the nano arena, various bottom-upmethods use individual atoms andmolecules to build useful structures.Under the right conditions, the atoms,molecules, and larger units can self-

assemble. Alternatively, directedassembly can be used.

In either case, the combination of nano-scale top-down and bottom-up processesgives materials and device designers awide variety of old and new tools.Designers can also combine micro- andnanotechnologies to develop new sensorsystems.

Computational Design

Recently developed experimental tools,notably synchrotron X-radiation andnuclear magnetic resonance, haverevealed the atomic structures of manycomplex molecules. But this knowledgeis not enough; we need to understand theinteractions of atoms and molecules inthe recognition and sometimes thetransduction stages of sensing. The

availability of powerful computers andalgorithms for simulating nano-scaleinteractions means that we can designnanosensors computationally, and not just experimentally, by using themolecular dynamics codes andcalculations that are already fundamentaltools in nanotechnology.

DIFFERENT SENSORS

Physical Sensors

Researchers at the Georgia Institute of

Technology led by Walter de Heerdevised the world's smallest "balance" by taking advantage of the uniqueelectrical and mechanical properties ofcarbon nanotubes. They mounted asingle particle on the end of a CNT andapplied an electrical charge to it. Actingmuch like a strong, flexible spring, theCNT oscillated, without breaking, andthe mass of the particle was calculatedfrom changes in the resonance

vibrational frequency with and withoutthe particle. This approach may allowthe mass of individual biomolecules tobe measured.

[The mass of a carbon sphere shifts theresonance frequency of the carbonnanotube to which it is attached.]

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Electrometers

Cleland and Roukes at the CaliforniaInstitute of Technology reported thefabrication and characterization of a

working, submicron mechanicalelectrometer. This device hasdemonstrated charge sensitivity below asingle electron charge per unit bandwidth (~0.1 electrons/ Hz at 2.61MHz), better than that of state-of-the-artsemiconductor devices.

[A nanometer-scale mechanical

electrometer consists of a torsionalmechanical resonator, a detectionelectrode, and a gate electrode used tocouple charge to the mechanicalelement. A schematic and micrographsof a single element and an array ofelements are shown.]

Chemical Sensors

Various nanotube-based gas sensors

have been described in the past fewyears. Modi et al. have developed aminiaturized gas ionization detector based on CNTs. The sensor could beused for gas chromatography.

Titania nanotube hydrogen sensors havebeen incorporated in a wireless sensor

network to detect hydrogenconcentrations in the atmosphere. AndKong et al. have developed a chemicalsensor for gaseous molecules such asNO2 and NH3 that is based on nanotube

molecular wires.

Datskos and Thundat used a focused ion beam technique to fabricatenanocantilevers and have developed anelectron transfer transduction approachto measure cantilever motion. Theresults might be sensitive enough todetect single chemical and biologicalmolecules. Structurally modifiedsemiconducting nanobelts of ZnO have

also been demonstrated applicable tonanocantilever sensors.

[This nano-array incorporates capacitivereadout cantilevers and electronics forsignal analysis.]

Biosensors

Nanotechnology will also enable thevery selective, sensitive detection of abroad range of biomolecules. By usingthe sequential electrochemical reductionof the metal ions onto an aluminatemplate, we can now create cylindricalrods made up of metal sections 50 nm to5 microns long. These particles,

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trademarked Nanobarcodes, can becoated with analyte-specific entities suchas antibodies for selective detection ofcomplex molecules. DNA detection withthese nano-scale coded particles has also

been demonstrated.

[DNA and other biomaterials can besensed using encoded antibodies onNanobarcodes particles.]

Researchers at NASA Ames ResearchCenter have taken a different route. Theycover the surface of a chip with millionsof vertically mounted CNTs 3050 nmin dia. When the DNA moleculesattached to the ends of the nanotubes are placed in a liquid containing DNA

molecules of interest, the DNA on thechip attaches to the target and increasesits electrical conductivity. Thistechnique, expected to reach thesensitivity of fluorescence-baseddetection systems, may find applicationin the development of a portable sensor.

[Vertical carbon nanotubes are grown ona silicon chip. DNA molecules attachedat the ends of the tubes detect specifictypes of DNA in an analyte.]

Deployable Nanosensors

The SnifferSTAR, a lightweight,portable chemical detection system , is agood example of nanotechnology's potential for field applications. Thisunique system combines a nanomaterialfor sample collection and concentrationwith a MEM-based chemical lab-on-a-chip detector. SnifferSTAR will likelyfind work in defense and homelandsecurity and is ideal for deployment onunmanned systems such as micro

unmanned aerial vehicles.

[The Sniffer STAR is a nano-enabledchemical sensor integrated into a microunmanned aerial vehicle.]

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And More

Other areas we expect to benefit fromnanotechnology-based sensors includetransportation (land, sea, air, and space);

communications (wired and wireless,optical, and RF); buildings and facilities(homes, offices, factories); humans(especially for health and medicalmonitoring); and robotics of all types.

We'll also see nano-enabled sensorsincreasingly integrated into commercialand military products. Many newcompanies will make nano materials andsome will make sensors based on them.

The URLs of some of these companiesare given after the Reference listings.

Realities

Although the excitement overnanotechnology and its prospective usesis generally well founded, thedevelopment and integration of

nanosensors must take into account therealities imposed by physics, chemistry, biology, engineering, and commerce.For example, as nanotechnologies areintegrated into macro-sized systems,we'll have to provide for and control theflow of matter, energy, and informationbetween the nano and macro scales.

The Nokia Morph is a mobile phonewonder in a true sense and it can deceive

anyone with its amazing design andlooks. Morph uses nanotechnology andliquid batteries to enable the flexibilityand transparency of the materials used.Millions of tiny micro-fibres help to holdthe handset in a new position, whilemicroscopic sensorswithin the materialsmake the whole handset work like a very

thin, very flexible touch-screen. Morphis a joint Nano Technology of NokiaResearch Center (NRC) and theUniversity of Cambridge (UK) and itwas launched at the Design and the

Elastic Mind exhibition.

[NOKIA MORPH is a piece of

translucent green plastic complete with a

circular earpiece clipped to it. Blink, andit has been folded into three, making it

look like a regular mobile phone. Also

the phone can be used in wrist with

Morph Wrist Mode. What else the

camera is excellent capable of capturing

marvelous images and pictures. Also

drop any liquid, however sticky, onto it

and it will simply roll off.]

The Usual Design Problems

Intensified.

Many of the design considerations fornanosensors are similar to those for

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micro sensors, notably interfacerequirements, heat dissipation, and theneed to deal with interference and noise, both electrical and mechanical. Eachinterface in a micro system is subject to

unwanted transmission of electrical,mechanical, thermal, and, possibly,chemical, acoustical, and optical fluxes.Dealing with unwanted molecules andsignals in very small systems oftenrequires ancillary equipment and low-temperature operation to reduce noise.Flow control is especially critical inchemical and biological sensors intowhich gaseous or liquid analytes are brought and from which they are

expelled. Furthermore, the verysensitive, tailored surfaces of thesesensors are prone to degradation fromthe effects of foreign substances, heat,and cold. But the ability to installhundreds of sensors in a small spaceallows malfunctioning devices to beignored in favor of good ones, thusprolonging a system's useful lifetime.

Risk and Economics

The path from research to engineering to products to revenues to profits tosustained commercial operations,difficult for technologies of any scale, is particularly challenging for nanotechnologies. One major impediment to their adoption is thecommon reluctance to specify newtechnologies for high-value systems.Another is that at present most nano-

scale materials are hard to produce inlarge volumes, so unit prices are highand markets are limited. Costs willdecrease over time, but smallcompanies may have a strugglemaking their profit goals quicklyenough to survive.

Applications

Few sensors today are based on purenanoscience, and the development ofnano-enabled sensors is in the earlystages; yet we can already foresee someof the possible devices and applications.Sensors for physical properties were thefocus of some early development efforts,but nanotechnology will contribute mostheavily to realizing the potential ofchemical and biosensors for safety,medical, and other purposes. Vo-Dinh,

Cullum, and Stokes recently provided anoverview of nanosensors and biochipsfor the detection of bimolecules.

CONCLUSION:

Nanotechnology is certain to improveexisting sensors and be a strong force in

developing new ones. The field isprogressing, but considerable work mustbe done before we see its full impact.Among the obvious challenges arereducing the cost of materials anddevices, improving reliability, and packaging the devices into usefulproducts. Nevertheless, we are beginningto see nano-scale materials and devicesbeing integrated into real-world systems,and the future looks very bright indeed

for technology on a tiny scale.

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R eferences

1. "Small Wonders, Endless Frontiers: AReview of the National NanotechnologyInitiative," National Academy Press,2002.

2. "Space Mission for Nanosensors,"The Futurist, Nov./Dec. 2002, p. 13.

3. IBM Research News.

Authorised BySANTOSH BHARADWAJREDDYEmail: [email protected]

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