Nanotechnology can be termed as the design, production and application of structures, devices and systems by controlled management of size and shape at the nanoscale that produces structures, devices and systems with at least one new property (1).
1 nanometer is one billionth of a meter. The physical and chemical properties of material get changed significantly at nanoscale. For e.g. Metals with average size of 10 nm are significantly harder and tougher when compared to hardness and toughness at the size of 1000 nm. This difference in properties is ascribed to two main reasons (2):
1. Nanomaterial has large surface area when compared to same mass in larger form. This makes it highly reactive and has an effect on the material's strength and electrical properties (2).
2. Quantum effects begin to increase with decrease in size which has an impact on optical, electrical and magnetic properties of material (2).
Nanotechnology has many applications such as nanomedicine, nanofluidics, molecule self assembly, intelligent drug delivery systems, nanomachines. One of these is nanobiosensors (2).
Components of nanobiosensors:
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They are made up of 3 parts (3):
sensitive biological element:
These can be either living biological materials (for ex. Microorganisms, cell receptors etc.) or biological molecules (for ex. Organelles, enzymes, sensing organs, antibodies, synthetic receptors, nucleic acids etc.)
The transducer (4):
It measures the physical change due to the reaction of sensitive biological element with bioreceptor. They work directly or indirectly depending upon which biosensors are classified as:
Direct detection biosensors: the biological reaction can be directly measured. E.g. biosensors using antibodies or cell receptors (i.e. non catalytic ligands).
Indirect detection biosensors: the biological reaction is detected by secondary detector. E.g. labelled antibodies, catalytic enzymes.
Signals from the transducer are amplified, analyzed, converted to concentration units and then stored in storage device.
Techniques of producing nanomaterials:
There are two techniques (5) of producing nanomaterials. They are:
Top down techniques
Bottom up techniques
Top down technique produces very small structures from larger ones while 'bottom up technique' constructs nanomaterials by joining atom by atom or molecule by molecule. In bottom up technique, nanomaterials can be produced either by self assembly, in which atoms or molecules get arranged due to their natural properties or by positional assembly, in which tools are used to move each atom or molecule individually. However, the latter method is tedious and not suitable for industrial applications.
Nanomaterials can be produced in one, two or three dimensions (5).
Nanomaterial in one dimension:
These include thin films and engineered surfaces which have been developed and used for decades. For e.g. in silicon integrated circuit industry many devices depend on thin films for their operation. Advances are being made to control the composition and smoothness of surfaces (5).
Nanomaterials in two dimensions:
These include nanotubes and nanowires. These have generated interest among the researchers in past few years (5).
carbon nanotubes (CNTs):
These were first observed by Sumjo lijima (7) in 1991. CNTs (5, 6) may be single walled (one tube) or multiwalled (several concentric tubes). CNTs are few nanometers in diameter and several nanometers to centimeters long. These have gained a lot of importance in nanoworld because of their high mechanical strength (Young's modulus over 1 i.e. as hard as diamond), flexibility and good electrical conductance (2, 5).
These are based on layered compounds such as molybdenum disulfide, silicon (5, 8). They have excellent lubricant properties, resistance to shockware impact, catalytic reactivity and high capacity for hydrogen and lithium storage (5).
These are ultrafine wires or linear arrays of dots formed by self assembly. These semiconductor nanowires are made from a wide range of materials like silicon, gallium nitride, indium phosphide etc. they have remarkable optical, electrical and magnetic properties. These are prepared by self assembly techniques. Nanowires have wide applications in high density date storage, either as magnetic read heads or as patterned storage media and electronic and opto electronic nanodevices for metallic interconnects of quantum devices and nanodevices (2, 9).
Biopolymers such as DNA molecules can be coated in metal. The self assembly of organic backbone nanostructures is controlled by weak interactions like hydrogen bonds, hydrophobic or van der waal's interactions (generally in aqueous environment) and hence requires quite different synthesis strategies to CNTs (5).
Nanomaterials in three dimensions:
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nanoparticles have diameter less than 100 nm. These exhibit new or enhanced size dependant properties compared with large particles of the same material. Nanoparticles serve as the raw materials, ingredients or additives in various products (5, 10).
In the mid of 1980, Kroto and Richard Smalley discovered a new class of carbon compounds carbon 60 and named it "buckminsterfullerene" (11). C60 are spherical molecules about 1nm in diameter. They are applied as miniature ball bearings to lubricate surfaces, drug delivery vehicles and electronic circuits (5).
Dendrimers are spherical polymer molecules formed through self assembly process. These are used in coatings and inks. They can also be used as nanoscale carrier molecules in drug delivery (5, 12).
If semiconductor particles are made small enough, quantum effects begin to dominate. There is change in chemical, optical and electrical properties. Such nanoparticles are called quantum dots (5).
Many nanobiosensors have been developed. Some of them are as follows (table 1):
Colorimetric: colour changes
Photometric: light intensity changes
Diagnosis of cancer, direct glucose level detection in blood
(13, 14, 15)
Based on changes in electric changes
Label free DNA detection, diagnosis of different diseases
Sensitive biomolecules immobilized
Electrochemical detection of blood glucose levels, determination of DNA and its effectors
Hybridization of nucleic acids from different sources
Detection of genes and mutant genes related to inherited human diseases
(18, 19, 20)
Used in place of ELISA- based methods, improvement over PCR based detection, cheaper, faster method
Sensitive biomolecules immobilized
Used for immunoassay, cancer diagnosis and therapy, diagnosis of Alzheimer's disease
Sensitive biomolecules immobilized
Diagnosis of cancer, drug discovery, treatment of diseases, kinetic studies of fundamental biochemical reactions
Nanotube based nanobiosensors
Carbon nanotubes used as electrodes as well as immobilization phase
Used in immunoassays, nucleic acid probe assays, clinical chemistry assays
(25, 26, 27)
Some of the nanobiosensors and principle behind their working are described below:
Fluorescent nanobiosensors for enzymatic activity detection based on liposomes:
Liposomes are nanoscale spherical shells which consist of lipid bilayers with aqueous phase inside. They can be easily formed and they are stable for prolonged period without significant differences in size and structure (28). They have biocompatible microenvironment and their physicochemical properties can be controlled. They can be effectively used as carriers for many functional substances and drugs. Release of these substances from liposomes can be controlled based on their physicochemical properties. Enzymes can be considerably protected from unfolding and from the attack of outer agents like proteases inside the microenvironment of liposomes. Enzymes embedded in lipopsomes can maintain their activity at low concentration (29). Liposomes are optically transluscent and thus can be employed for the formation of nanosized optical biosensors under specific experimental conditions. Initially liposome based electrochemical biosensors were developed with glucose oxidase (30, 31) on screen printed electrodes (32) and on chitosan gel beads (33). Porins can be entrapped within liposomes which facilitate incorporation of enzyme molecules in the internal aqueous phase of liposomes. Sensitive transduction can be achieved with the help of pH sensitive fluorescence probe (34) which is inserted within liposome. This provides a simple self contained stable nanobiosensor.
The porins incorporated on liposome membrane allow substance's entry in the internal microenvironment of the liposome where they can react with enzyme. Change in the pH leads to change in the colour of fluorescent indicator. This change is directly proportional to substrate concentration. Due to fluorescence it is easy to obtain higher sensitivity, lower detection limits and wider concentration range (34). Incorporation of fluorescent indicator within nanobiosensor avoids two step detection of enzyme activity (34). A sharp fluorescent signal is obtained when substrate is added to the enzyme containing liposome. Increase in the substrate concentration is inversely proportional to intensity of fluorescence. Liposomes are very small and not easy to handle. Therefore they are immobilized in a sol gel matrix. This matrix does not introduce errors in the fluorescent signals by forming an additional diffusion barrier or does not affect enzme kinetics. These nanobiosensors can be employed in the detection of organophosphorus pesticides and other toxic AchE inhibitors (34, 35).
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Organophosphates are the most widely used pesticides due to their ability of rapid insect irradication, short environmental half life. They are highly toxic and exert many ill effects on CNS, respiratory, myocardial and neuromuscular malfunction which may lead to death. Their residues are found in the soil, atmosphere, and agricultural products as well as in ground water (35).
They are structural analogs of Ach and irreversibly inhibit Ach (35). They block active site on AchE; prevent the binding of Ach and its break down to choline and acetic acid. This decreases acetic acid production (35, 36). The fluorescence signal of fluorescent indicator decreases. The reduction in response is correlated with concentration of pesticide in solution.
Glucose biosensors using protein immobilized within gold/ polycarbonate nanopores:
The glucose galactose receptor (GGR) protein belongs to bacterial periplasmic protein (bPBP) superfamily. It is used as biorecognition element in glucose biosensors. It is preferred as the active component of glucose biosensors (38, 39) because it has high solubility and stability upto 600C. These proteins undergo large physiological change i.e. wide amplitude, hinge twist motion upon binding to the glucose. This change can be detected using various analytical techniques such as fluorescence, quartz- crystal microbalance, electrochemical impedance and surface plasmon resonance detection. One of these methods is immobilizing this receptor protein within Au/ polycarbonate nanopores (38, 39). The gold nanoisland is prepared with the help of polycarbonate Au template.
The original GGR protein does not contain cysteine residues. These cysteine residues are incorporated by genetic engineering. Immobilization of these proteins occurs due to Au-S bond formation (40).
The cells consist of two Teflon blocks with cylindrical bore in between. Fluid inlet and outlet are present on both the sides to rapidly drain the electrolyte. The Au coated electrolyte is mounted between two Teflon blocks. One electrode acts as working electrode while the other serves as both reference as well as conductor electrode. The impedance obtained is directly proportional to the resistance or inversely proportional to the conductivity between two electrodes. (R=1/C). Impedance decreases with increased glucose concentration. The greatest accuracy in the measurement of impedance is obtained when the diameter of the nanopores is smallest. This method provides an alternative reagentless biosensor for detection and measurement of glucose biosensors. Glucose binding to protein is detected as an increase in electrolyte conductivity as an increase in the electrolyte conductivity within the nanopores caused by reduction in the effective protein film thickness. Hence its concentration can be determined (41).
Optical nanobiosensors for intracellular or subcellular analyte detection:
PEBBLEs (Probe Encapsulated By Biologically Localized Embedding) are mono or multicomponent spherical devices of nano size made up of sensors entrapped in inert chemical matrix. They occupy as small as 1 ppm of the cell's volume (42). Small size of sensors has many advantages (43) such as accurate detection of intracellular analyte, greater resolution of chemical images, lower detection limits and rapid response. These are used for real time sensing of ions and small molecules. Inert matrix prevents protein binding of sensors (for e.g. non specific protein binding of dyes) and also protects cellular components from harmful effects of chemical sensors (42). Depending upon the type of sensor material, three different matrices are used: hydrophilic, hydrophobic, amphilic. There is a very little or no photobleaching and leaching of dyes. These PEBBLEs can be used singly or in group of single analyte PEBBLEs or sets of multi analyte PEBBLEs or "ensembles" of sets of groups. "Esembles" enable either multi analyte detection or simultaneous chemical images of complex cellular biochemical processes. These can be inserted into the cell using a gene gun, microprojector, biolistics, as liposomes or by natural uptake (phagocytosis) (41).
Advantages of these PEBBLEs are:
They allow detection of intracellular analytes for which no intracellular fluorescent indicators are available. For e.g. sodium, nitrite, chloride etc.
It is possible to differentiate between the fluorescence of the indicator and fluorescence of intracellular organelles of the cell. These PEBBLEs can be employed for the detection of magnesium, potassium, calcium, oxygen and pH changes due to cellular changes (41, 44).
Development of nanobiosensors for monitoring individual cells:*
Heat and pull technique is used in this method. In this technique, a silica optical fibre is fitted into a fibre pulling device at both the ends. It is heated at the midpoint using a CO2 laser or a heat filament. The heated filament is pulled using a pulling device. The shape of the tip is dependent on temperature and timing of the procedure. One end of silica fibre is highly polished which leads to excitation of electrons to attach with biosensing element. The heated surface when pulled leads to the formation of nanosized tips. These nanotips are coated within a thin layer of gold, aluminium or silver so that the tip does not lose the excited light. The distal end is left uncoated to immobilize biosensing elements like antibodies, synthetic peptide substrates etc. The sensing element coupled with fluorephore is covalently bonded to uncoated end. The biosensor is inserted into cells using various techniques previously mentioned. The fluorescence emitted is collected using microscope objective and passed through dichroic mirror. It is detected with photomultiplier tube (PMT). Thus biochemical processes can be monitored (45).
These have capacity of detecting and distinguishing complex odorant mixtures using broad spectrum network of sensors. A characteristic response pattern is generated that enables identification and discrimination of odorant present in the mixture. This sensing depends on various techniques such as electrical, optical, colorimetric, gravimetric techniques etc. to estimate parameters responsible for identifying odorants in the mixture. Electrical noses have specificity, stability, objectivity, rapidity and are not hampered by the presence of water (46).
Nanobiosensors and cancer:
Nanobiosensors are useful for early diagnosis of cancer and detection of cancer agents such as environmental pollutants, dangerous gases and pathogens. The use of nanobiosensors in cancer clinical testing have been increased due to high speed and reduced cost for diagnosis, automation and multi target analysis (4, 47).
Antitumor drugs can be associated with nanoparticles. This avoids cellular and non cellular mechanisms of resistance, improves selectivity of drugs towards tumor cells and decreases their toxicity towards healthy tissues.
Advantages (48) of nanobiosensors in cancer:
Due to extreme small size, they are easily absorbed into human body.
They can easily penetrate cell membrane and non hazardous since they are made up of biodegradable polymers.
Nanoparticles are stable and can form colloidal dispersions.
Drugs can be adsorbed on particle surface or dissolved in particle matrix.
They can target drugs effectively to tumor cells, identify physiological changes in tumor cells and detect residual tumor cells.
They have large surface area to volume ratio which allows rapid diffusion into cells as well as impart high thermal and chemical resistance.
In cancer treatment, sensors conjugated with biological sensing element such as cancer specific antibody or other biological ligands are used to target proteins or to seize tumor cells. This yields electrical, optical or mechanical signals which are detected by transducer (49). Microelectrical mechanical system sensors and quantum dots have been used for DNA detection. Recently, a method of finding specific DNA sequence by using a semiconductor crystal, biological probes and laser have been developed. In this method, DNA is made to light up beneath the microscope and then the sequence is determined.
Point of care testing:
It is one of the hopeful methods for faster and cheaper diagnosis of cancer. Cancer biomarkers are identified from genomic and proteomic analysis and validated. Probes are developed for these markers and conjugated with detectors to form biosensors. This technique is helpful for cancer care (4, 47).
There has been an exponential growth in the applications of nanobiosensors since last few years including medicine and health care. Many nanobiosensors are available in the market based on latest technology. In future, nanobiosensors will detect the onset of dreadful diseases like cancer or heart disease at the earliest stage possible. They are being looked as an innovative and cheaper alternative to the current trends of diagnosis and treatment. Nanotechnology will soon become the next frontier of medical research.