Nanosensors in Optics

Nanosensors are devices with dimensions on the nanometer scale that are capable of monitoring the presence of a specific chemical or class of chemicals. Nanosensors which employ optical transduction methods are called optical Nanosensors.

Optical nanosensors can generally be classified into one of two different classes: 1) chemical nanosensors, or 2) nanobiosensors, depending on the type of recognition element (chemical or biochemical) used to provide specificity to the sensor. Small sizes of these sensors allow them to be inserted and precisely positioned within individual cells to obtain spatially localized measurements of chemical species in real time.

Fiber optic nanosensors employ fiber optics that have been tapered on one end to diameters typically ranging between 20 and 100 nm. Excitons or evanescent fields continue to travel through the remainder of the tapered fiber’s tip, providing the necessary excitation energy. Excitation using such a sensor is highly localized, allowing only species close to the fiber’s tip to be excited.

The most significant applications of fiber optic nanoprobes to NSOM analyses of biological samples occurred when a single dye-labeled DNA molecule was detected using near-field surface-enhanced resonance Raman spectroscopy (NFSERRS). In that work, dye-labeled DNA strands were spotted onto a surface-enhanced Raman spectroscopy (SERS) substrate that was prepared by evaporating silver on a nanoparticle-coated surface. Following preparation of the sample, a fiber optic nanoprobe was raster-scanned over the sample’s surface, illuminating it point by point, while the resulting Raman signals were measured with a charge-coupled device (CCD). Based on the intensity of the Raman signals measured at every location, a two-dimensional image of the DNA molecules was reconstructed and normalized
for surface topography based on the intensity of the Rayleigh scatter.

FIBER OPTIC CHEMICAL NANOSENSORS

Fiber optic chemical nanosensors have chemical recognition elements (e.g., fluorescent indicator dyes, etc.) bound to the tapered tip of the fiber to provide a degree of specificity. It is important to employ a sensitive detection system, such as the one shown in the following figure.

Fiber Optic Scanner

In such a system, the sample is excited by launching an intense light source (e.g., laser) into the proximal end of the fiber optic nanosensor. The nanosensor is then positioned in the desired location using an x–y–z micromanipulator or piezoelectric positioning system mounted on a microscope. Once in place, the fluorescent indicator dye immobilized on the tip of the fiber is excited, and the resulting fluorescence emission is collected and filtered by the microscope before being detected with either a photomultiplier tube (PMT) or a CCD.

NANOPARTICLE-BASED OPTICAL NANOSENSORS

One advance in the last several years has been the development of nanoparticle-based optochemical sensors, with nanometer-scale sizes in all three dimensions. Because of the small sizes of these sensors, a large number of them can be implanted within an individual cell at one time, allowing for the monitoring of many locations simultaneously. Although many different nanoparticle-based sensors are currently being developed, three main classes have already shown a great deal of promise for intracellular analyses. These three classes are

· Quantum dot-based nanobiosensors

· Polymer-encapsulated nanosensors known as PEBBLEs

· Phospholipid-based nanosensors

Nanosensors in Biomedical Field - V Unit Notes

Nanosensors in Biomedical Field

Biosensors are sensors for detecting biological entities such as proteins, drugs, specific viruses, cancer cells etc. In vivo detection of these happens in variety of ways naturally. For an example, when a body is first exposed to an allergen, the body creates antibodies that will recognize that allergen if it appear again in the body. This triggers allergy response system the body to release histamine. Glucose detection is important in biosensing. Type I diabetics has to monitor their blood sugar levels continuously. Nanoscale structures may advance it in a big way.

DNA sensing is another important area in which nanosensing can play a potential role. Using the ability of DNA to bind to a complementary strand and not to bind to anything else presence of any microorganism with known DNA sequence. For instance, to sense the structure with the sequence CGCTTC a complementary strand GCGCAAG can be used.

A single strand of, say, six bases can contain 4,096 different combinations. Consequently, if a particular biological target such as botulism or strep or scarlet fever has a known DNA sequence, it is possible to target a short section of that DNA sequence that can be uniquely sensed, without any errors, by an appropriate single-strand complementary structure.
This is called DNA finger printing.

Generalization of this method leads to lab-on-a-chip concept. Microlaboratories capable of sensing viral and bacterial diseases are possible with this technology. Finally, biochips could be used to sense either particular DNA signatures or particular protein signatures known to be defects that can result in disease.

One of the great challenges of DNA sensing is to amplify the effects of hybridization so that they can be easily measured. One way to provide this amplification is to change the optical properties of gold or silver nanodots that are attached to the DNA. Chad Mirkin, Robert Letsinger, and their groups at Northwestern pioneered the combination of quantum optical effects and molecular recognition (complementary DNA binding). Their scheme and some actual results are shown in the following figure.

DNA Viral Detection

The upper schematic shows how the nanodots in a colorimetric sensor are brought together upon binding to the DNA target (in this case anthrax). The clustered dots have a different color than the unclustered ones as is shown in the photograph below them.

By exposing the single strands of DNA that are attached to the gold nanodots, the sensor recognizes the target strands of DNA, which causes the gold nanospheres to come closer together and, as in those recurring stained glass windows, change color.

In a protein biosensor a molecular nanostructure containing a biological binding site is attached to the gold nanoparticles. The binding site is designed to recognize a particular protein analyte. When that analyte appears in solution, it binds to the recognition site, which changes the chemical and physical environment of the gold dot, whose color is then slightly changed. This change can be measured.

In the electronic nose, a random polymer, or mix of polymers, is spread between electrodes. When the molecules to be smelled land on the polymer(s), the conductivity properties in particular regions will change in a particular way that is specific to any given analyte.

Nanomaterials Applications in Electronics - V Unit Notes

· Transistors have gotten smaller through nanotechnology. Around 2001, a typical transistor was 130 to 250 nanometers in size. In 2014, Intel created a 14 nanometer transistor, then IBM created the first seven nanometer transistor in 2015, and then Lawrence Berkeley National Lab demonstrated a one nanometer transistor in 2016. Research is on going to use single molecules as transistors.

·  Using magnetic random access memory (MRAM), computers will be able to “boot” almost instantly. MRAM is enabled by nanometer‐scale magnetic tunnel junctions and can quickly and effectively save data during a system shutdown or enable resume‐play features.

·  Ultra-high definition displays and televisions are now being sold that use quantum dots to produce more vibrant colors, while being energy efficient.

· Flexible, bendable, foldable, rollable and stretchable electronics are reaching into various sectors and are being integrated into a variety of products, including  wearables, medical applications, aerospace applications, and the Internet of Things. Flexible electronics have been developed using, for example, semiconductor nanomembranes for applications in smartphone and e-reader displays. Nanomaterials like graphene and cellulosic nanomaterials are being used for various types of flexible electronics to enable wearable and “tattoo” sensors, photovoltaics that can be sewn onto clothing, and electronic paper that can be rolled up. Making flat, flexible, lightweight, non-brittle, highly efficient electronics opens the door to countless smart products.

· Computing and electronic products include Flash memory chips for smart phones and thumb drives; ultra-responsive hearing aids; antimicrobial/antibacterial coatings on keyboards and cell phone casings; conductive inks for printed electronics for RFID/smart cards/smart packaging; and flexible displays for e-book readers.

·   Nanoparticle copper suspensions have been developed as a safer, cheaper, and more reliable alternative to lead-based solder and other hazardous materials commonly used to fuse electronics in the assembly process.

·    DNA can be used as scaffolding for assembling molecules into electronic circuitry. This is used to integrate novel devices at densities far beyond those possible lithographic techniques. Studies on DNA showed a wide range of electron transport behavior. DNA can act as an insulator, a semiconductor, a conductor and a super conductor offering future bio-compatible devices and circuits.

·   Nanomaterials research, especially molecular electronics has demonstrated the new electrical element ‘Memristor’. Memristor has the capability of remembering the current it experienced in the past. So Memristor research offers a possibility to implement 50Gbytes of low-power memory in mobile devices.

·   Nanomagnetics provide an opportunity to realize zero-energy switching logic gates and memory. Phase change memory (PCM) is considered one of the most promising candidates for next-generation nonvolatile memory, based on its excellent characteristics of high speed, large sense margin, good endurance, and high scalability.

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