Innovations in Nanosensors - An Overview


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This is my national level paper on nanosensors

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Innovations in Nanosensors - An Overview

  1. 1. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 1Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.Innovations in nanosensors – An overviewRamesh Hegdea, Chetan Mb, Dr. P K SrimanicaAssociate professor, Physics department, Acharya Institute of Technology.bII Semester BE E&C, Acharya Institute of Technology.cFormerly Professor of Mathematics, Bangalore University.Abstract:The aim of this paper is to summarize recent developments in the field ofNanosensors in the recent years. Nanosensors are gaining increasing attention due to theneed to detect and measure chemical and physical properties in difficult to reach biologicaland industrial systems that are in the nano-scale region. Nanosensors have superiorproperties over the existing techniques such as high-performance liquid chromatography orgas chromatography, because they can provide rapid, sensitive, simple and low-cost on-fielddetection. The measurement protocols based on nanoparticles and nanotubes are alsosuitable for mass fabrication of miniaturized devices. This conceptual review surveys variousNanosensors, which are categorized into three broad types: optical, electromagnetic andmechanical Nanosensors. Research and development in the nanosensor domain is driven byspecific market or application requirements and societal concerns. As the sensor industry ishighly fragmented, it is difficult to identify the nanosensing product that is likely to do wellcommercially in the immediate future. It is equally tricky to spot the sensor market segmentthat is likely to be most influenced by nanotechnology, as new opportunities are spawnedon a daily basis. The sensing concepts and their corresponding advantages were discussedwith reference to their applications. Future prospects toward the development of selective,sensitive biosensing systems were also discussed in this paper.Key words: Nanosensors; chromatography; nanoparticles; optical; electromagnetic;mechanical; biosensing systems;physical;INTRODUCTION:Most reviews on nanosensors are focused on particular type of sensors, such asnanobiosensors, optical nanosensors etc, with many technical details. This reviewsummarizes the development in the field of nanosensors as a whole from the year 1994 –2012.Nanosensors are sensing devices with at least one of their sensing dimensions beingnot greater than 100 nm. In the field of nanotechnology, nanosensors are instrumental formonitoring physical and chemical phenomena in regions difficult to reach, detectingbiochemicals in cellular organelles, and measuring nanoscopic particles in the industry andenvironment. “Nanotechnology” and “nanosensors” have become some of the common dayto day words showing us growing trend of nanosensors research. Needless to say, a fargreater number can be expected to research on these topics when these sensors becomepart and parcel of our day to day life. Hence I would like to quote “The advance in scientificunderstanding is naturally followed by technological development”.
  2. 2. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 2Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.In order to simplify the explanation of the developments in nanosensors I haveclassified them roughly into three broad categories:i. Optical nanosensors.ii. Electromagnetic nanosensors.iii. Mechanical/Vibrational nanosensors.OPTICAL NANOSENSORS:The first reported optical nanosensor was based on fluorescein which is trappedwithin a polyacrylamide nanoparticle, and was used for pH measurement. Here thefluorescent chemosensors are molecules composed of at least one substrate binding unitand photoactive component. The fluorescein absorbs light of a certain wavelength, which isfollowed by emission of a quantum of light with an energy corresponding to the energeticdifference between the ground and excited states this is also known as luminescencephenomenon. Fig (3) shows a conceptual schematic for a typical luminescent sensor, wherethe reflected light changes in color when the receptor binds with the analyte. The change inphoto-vibrational properties underlies the sensing concept.The most basic type of optical nanosensor is that of a molecular dye probe inside a cell,which is essentially a direct cell loading of fluorescent dyes. An advantage of this basicapproach is to minimize the physical perturbation of the cell, unlike that of the optical-fiberprobe. However, a disadvantage of the free dye is the inherent dye-cell chemicalinterference as a result of protein binding, cell sequestration and toxicity. A slightly differentdeviation from the free dye method is that known as the labeled nanoparticles that consistof a reporter molecule attached to the outside of the nanoparticles. The major differencebetween the labeled nanoparticles as compared to the free dye method is the solid stateand fluid nature of the former and latter, respectively. The labeled nanoparticles are freelyflowing and the reporter molecules are in contact with the intracellular components just likethe free dye. These outer-labeled particles have been used for intracellular sensing, butretain similar drawbacks of using the free fluorescent dyes because the signal is derivedfrom receptor molecules not insulated from the cellular environment.
  3. 3. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 3Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.Fiber optic nanosensors:Conventional methods for intracellular investigation need “fixing” of cell samplesbefore performing the analysis. This fixing process usually destroys cellular viability andmay, to a considerable extent, change the intracellular structure. Fiber optic nanosensorshave the potential to analyze important cellular processes in vivo. Fundamental monitoringof biological processes at the cellular level is important to enhance further understanding ofdynamic cellular functions. The interaction between the target molecule (A) and thereceptor (R) is designed to produce a physicochemical perturbation that can be convertedinto an electrical signal or other measurable signal:R + A → RA + measurable signalThis measurable signal is then picked up by the optical probe and transmitted into thedatabase. The disadvantage associated with the dye-cell chemical interference prevalent inthe free dye method is overcome by using the optical fiber probe due to the physicalseparation between the environment and the sensing tip. Another advantage of the opticalnanosensor is the minimal invasiveness of this technique as compared to conventional wire-probe devices.Fiber optic nanosensors have so far been successful with their capability in the followingapplications:(a) Measurement of BenzoPyreneTetrol (BPT) and Benzo[a]Pyrene (BaP) inside singlecells. The above biochemicals play a very vital role in cancer studies.(b) Monitoring Apoptosis or programmed cell death which is a process where cellsdegenerate during normal development, due to aging, as a result of disease. Thefiber optic nanosensors have been especially used in monitoring of caspase-9, anapoptosis protein, in human mammary carcinoma cell (MCF-7).(c) Measuring cytochrome-c, which is an important protein involved in the productionof cellular energy as well as in apoptosis. The release of cytochrome-c from themitochondria to the cytoplasm of individual MCF-7 cells is monitored by a fiber opticnanosensor inserted into a single cell, followed by an enzyme-linked immunosorbentassay (ELISA) outside the cell.(d) Fiber optic nanosensors have been developed and used for the detection of cellularpH value as well as ions such as K+and Ca2+, NO, NO2-, Cl-, Na+, Ca2+fluctuation.Due to the small sampling volume probed by the optical fiber nanosensor, the amount oftarget analyte in the excitation volume is small, hence making it a necessity to adopt asensitive optical spectroscopic technique (such as fluorescence) for analysis. Anotherdisadvantage is that in spite of the minimal invasiveness when compared with other wire-probe devices, some amount of physical damage on the cell may occur with the use of theoptical fiber nanosensor.
  4. 4. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 4Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.PEBBLEs:In order to overcome the shortcomings associated with both the free fluorescentdyes method and the optical fiber method, the Photonic Explorers for Bio-analysis withBiologically Localized Embedding (PEBBLE) was introduced. PEBBLEs are nano-scale sensingdevices which encapsulate an analyte-specific dye and a reference dye inside a biologicallyinert matrix. Due to the absence of a long probe connecting the sensor in the cell to theoutside of the cell, PEBBLEs are less physically disruptive to the cellular environment.Furthermore, the encapsulation of the dyes within an inert matrix ensures that the sensingphase is separated from the cell environment, thereby preventing chemical interference.PEBBLEs can be categorized into four types according to their distinct matrices, twocategories on the basis of their working principles, and four methods of PEBBLE delivery intothe cell. The four types of PEBBLE matrices are:(a) Polyacrylamide: They are made by polymerizing a solution of monomer, sensing dye,reference dye and, to control the size, a surfactant. The dye molecules are simplyincorporated in the matrix by being in the solution during polymerization.(b) Polydecylmethacrylate (PDMA): These are polymerized within a hydrophobicenvironment without the presence of dye molecules or other sensing components.The hydrophobic sensing components, such as dyes, ionophores and ionic additivesare then introduced by swelling the matrix of the nanospheres with a polar solution(tetrahydrofuran/water) in the presence of the relevant components.(c) Sol-gel: These are synthesized using “soft” techniques that allow the inclusion of del-icate biological molecules. The sol-gel nanoparticle preparation is carried out in thepresence of the sensing components. Sol-gel PEBBLEs are coated withpoly (ethyleneglycol) in order to enhance the biocom-patibility.(d) Organicallymodifiedsilicates (Ormosils): They are prepared in two steps. In the firststage, the core formation takes place by hydrolyzing phenyltrimethoxysilane withinacidic environment, followed by silane condensation within alkaline environment.The nanoparticle cores are then coated with the ormosil layer. Finally, the sensingelements are incorporated into the ormosil PEBBLEs just before the second layerforms.The two working mechanisms of PEBBLEs are:(a) Direct measurement PEBBLEs: This mechanism applies for sensing both ions andsmall molecules. These allow the analyte to permeate the matrix and interact withthe indicator dye directly and selectively, thereby causing stimulation or quenchingof fluorescence. They can sense H+, Ca2+, Mg2+, Zn2+, glucose, dissolved molecularoxygen and carbon dioxide.(b) Ion-correlation PEBBLEs: This mechanism consist of silent-ionophore and chromo-ionophore bound together as a pair working in a synergistic manner. The silent-ionophore has high affinity toward the ion of interest and due to the change in thecharge of the pair, the chromo-ionophore emits the fluorescent indication. They cansense H+, pH, K+, Na+and Cl-.
  5. 5. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 5Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.The latest breakthrough in the optical sensor method is SERS (Surface Enhanced RamanScattering), where the hybrid sensor consists of gold or silver nanoparticles with an attachedreporter species. The sensor can be detected and imaged based on the SERS signature of thereporter. This results in several advantages, such as high spectral specificity, multiplexcapabilities, improved contrast, and photostability. SERS sensors not only highlight cellularstructures, based on enhanced Raman spectra of intrinsic cellular molecules measured inthe local optical fields of the gold nanoparticles, they also provide molecular structuralinformation on their cellular environment. Moreover, the SERS signature of the reporter candeliver information on the local pH value inside a cell at subendosomal resolution. The SERSdevices are now capable of even identifying the bacteria present on tooth enamel.ELECTROMAGNETIC NANOSENSORS:Under the category of electromagnetic nanosensors, we have two types of sensorsbased on their physical mechanisms:(a) Detection by electrical current measurement.(b) Detection by magnetic measurement.Detection by electrical Measurement:A salient advantage of this approach is the label-free methodology over the use ofdyes. We review the category of electrical current measurement for two cases:I. Detection by current inhibition –Geng, studied the interaction between hydrogen sulfide and gold nanoparticles, and foundthat the adsorption of hydrogen sulfide molecules onto the nanoparticles change thehopping behavior of the electrons through the particles, hence the suppressed hoppingphenomenon. The hopping of electrons was measured by recording the current and voltageacross chromium and gold electrodes in the presence of an applied electrical field Fig (2).Without exposure to hydrogen sulfide, the current increases with the applied voltage, butloss of current was observed with the exposure to hydrogen sulfide. It is known that thecurrent loss is due to a change in surface properties of the gold nanoparticles following theadsorption of hydrogen sulfide molecules as a result of the strong chemical affinity betweengold and sulfur atoms. Chemical adsorption of the hydrogen sulfide molecules onto thenanoparticles brings about partial substitution of the citrate layer, producing possible Au-Sor Au-SH type species on the gold nanoparticle surface. Consequently, a sulfide shell isproduced, thereby inhibiting the transfer of charge from one nanoparticle to the next. Thebyproduct is released into the gas phase as hydrogen molecules:Au + H2S → AuS + H2 ↑2Au + 2H2S → 2AuSH + H2 ↑
  6. 6. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 6Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.II. Detection by current enhancement –The critical components of these types of nanosensors are carbon nanotubes (CNTs). Theincorporation of CNTs can be done either as vertically aligned arrays to form coating forelectrode transducers or by embedment to form nanocomposite electrodes. Such CNTelectrodes have been used for monitoring of oxidase such as glucose. Specifically, Davis andBesteman employed single-walled CNTs (SWCNT) whilst Ye adopted multi-walled CNTs(MWCNT) for the detection of glucose. Apart from glucose, another oxidase detection usingCNTs is lactate oxidase. In addition to oxidase, CNT-based nanosensors have been used forthe detection of enzymes including de-hydrogenase, peroxidase (such as horseradish-peroxidase), hydrogen peroxide and catalase. Other enzymes detectable using this categoryof nanosensor include organo-phosphorous pesticides and hydrolase. In view of thesignificance of testing genetic and infectitious diseases, CNT-based nanosensors have beenused for detection of DNA and the latest breakthroughs have been pressure sensors andlarge scale gas detection and water solvents detection.The attachment of Abl on silicon nanowire as a detector ATP and the use of Gleevec as thecompetitor binder were performed and it was found that the conductance of the siliconnanowire increases linearly with the concentration of ATP without Gleevec. However, nosignificant change was observed in the presence of Gleevec. This result shows the ability ofthe nanowire for the detection of small-molecule-mediated inhibition of protein-proteininteractions with the potential impact in drug discovery and chemical genetics.
  7. 7. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 7Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.Detection by magnetic measurement:In nuclear magnetic resonance (NMR), the spin-spin relaxation time is defined as thetime to reduce the transverse magnetism by a factor of e, i.e. 2.718281828. The spin-spinrelaxation time is a bio-logical parameter that is used in magnetic resonance imaging (MRIs)to distinguish between tissue types and is called T2. Some examples of T2 readings are 40ms, 90 ms, 180 ms and 2500 ms for muscle, fat, blood and water respectively. It has beenpostulated that magnetic nanosensors composed of magnetic nanoparticles can be used fordetecting molecular interactions by magnetic resonance techniques. When these magneticnanoparticles bind to their intended molecular target, they form stable nanoassemblies,thereby leading to a corresponding decrease in T2 of the surrounding molecules, this isbecause when the superparamagnetic nanoparticles assemble into clusters the effectivecross-sectional area becomes larger, the nanoassembly becomes more efficient at de-phasing the spins of surrounding water protons, leading to an enhancement of therelaxation rates. This technique of measuringT2 has been performed in a number ofexperiments to detect enzymatic activity and viral particles in serum.The unique sensing technique of magnetic nanoparticle sensor technology enablesquick detection of targets without extensive purification of the sample or signalamplification. Since light is not used it bears no influence on the outcome of the assay, andexperimentation can be performed in turbid, light-impermeable media such as cellsuspension, lipid emulsion, blood, culture media and even entire tissue. Since the iron oxidenanoparticles used are non-toxic, this technology can be applied for in vivo sensing ofmolecular targets by MRI. Apart from bioscience application, magnetic nanosensors – on thebasis of magnetoresistance (MR) – have potential application for the electronics industry.MR is the phenomenon whereby the electrical resistance of a metal or semiconductorchanges as a result of the application of a magnetic field. Extraordinary magnetoresistance(EMR) is also found and depends on the detailed shape of a device made fromsemiconductor and conductive metal, and could be applied for producing computer disc-drive read-heads that are faster and capable of storing higher densities of information thancurrent read-heads, which rely on giant magnetoresistance (GMR). Since EMR read-headsdo not have magnetic materials, they emit lesser noise than GMR read-heads, hencepointing towards an enhanced working performance. A number of possible applicationshave been suggested, such as position-sensing robot as well as speed and position sensorsin industry.MECHANICAL/VIBRATIONAL NANOSENSORS:The earliest mechanical nanosensor was proposed for measuring the vibrational andelastic characteristics of a nanosphere attached to a tapered cantilever. This work isimportant for application in nanodevices components and nano-scale sub-assemblies inmicroelectronic devices. Instead of measuring the vibrational and elastic properties of thesub-assemblies attached to a surface.Binh introduced the concept of producing replicas of these objects from heating offine wires terminated with sharp tips. Experimental studies verify the possibility of a solid
  8. 8. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 8Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.drop formation that is connected to the rest of the wire by a narrow neck. It was shown thatthis technique is capable of producing a sphere of 102nm diameter connected to the shankby a slender neck of 101nm diameter and 102nm length, which results in a resonancefrequency of 102MHz and spring constants between 10-2to 101Nm.Hierold explored the possibility of down-scaling the mechanical inertia sensors fromthe micro-scale to nano-scale. The sensing force is measured as a result of pressure,acceleration and yaw rate that displaces the sensing electrode against the spring force. Thechange of distance with respect to the counter electrode is then measured by a change ofthe capacitance. Such micro-scale mechanical inertial sensors could be scaled down intonanosensors provided that self-assembly of nanostructures becomes a well controlledfabrication technology.Although development in mechano-vibrational nanosensors is not as remarkable asthat of photo-chemical or electro-chemical nanosensors, one may expect an increasingprogress in the former due to advances made in the nano-scale enabling technologiesA summary on various types of nanosensors and their applications:NanosensorType:Subcategory: Applications:Optical Fiber opticPEBBLESERSBenzopyrene tetrol, benzo[a]-pyrene, caspase-9 (anapoptosisprotein), cytochrome c (a pro-tein involved inproducing cellu-lar energy), pH, K+,Ca2+, NO, NO2−, Cl−,Na+H+, Ca2+, Mg2+, Zn2+, glucose, dissolved O2, K+, Na+, Cl−All the above along with pH levels in terms of graph,bacteria in tooth enamel, dissolved impurities in largescale etc. (Highly under research)Electromagnetic CurrentmeasurementMagnetismmeasurementH2S, GOx, lactase oxidase,dehydrogenase, peroxidase,hydrogen peroxide, catalase,organophosphorus pesticides, organophosphorussubstrates of organophosphorus hydrolase,DNA, ATP, pressure sensing and gas sensing,temperature nanosensors are also being developed inthis particular type.Molecular interactions, oligonu-cleotide sequences,enzymatic activity, viral particles, magneticfield, speed, position sensingMechanical - Resonance frequency, springConstant, Pressure, acceleration, yaw rate etc.
  9. 9. INNOVATIONS IN NANOSENSORS – AN OVERVIEWAcharya Institute of Technology, BangaloreP a g e | 9Acharya Institute of Technology,Acharya Dr. Sarvapalli Radhakrishnan Road,Soldevanahalli, Hesarghatta Road, Bangalore – 560090.CONCLUSION:A wide range of nanosensors has been surveyed, categorized and discussedaccording to their working mechanism. In general, nanosensors are useful for detection ofchemicals and their properties from single cell organisms to most complex organisms likehumans, they can also detect the electro-magnetic properties and physio-mechanicalproperties, thus making nanosensors a huge field for research and development. As futureISRO missions attempt to identify the life in other planets nanosensors are expected to playa key role in analyzing the nature of organism present there. In spite of the relatively shorthistory of nanosensors, the advances made in this area have been remarkable. With thecontinuing progress in nanotechnology tools and increasing insight on the nano-scalephenomena, one may expect further advancement in the area of nanosensors throughenhance performance of existing nanosensors and newer nanosensors based on novelmechanisms.REFERENCES:[1] Nanosensors:physical chemical and biological [2012] – Vinod Kumar Khanna.[2] - Wikipedia Nanosensors article.[3] A Conceptual Review of Nanosensors - Teik-Cheng Lim and Seeram Ramakrishna.[4] Graphene-based wireless bacteria detection on tooth enamel – Nature Communications.[6] - Nanosensors portal.[7] Tata McGrawHill – Nanotechnology an Introduction.[8] - Wikipedia Graphene article