Newstelescope : Nanotechnology Just another Newstelescope.com weblog

Posted: August 4, 2008

Palm-sized biosensor for point-of-care and field applications

(Nanowerk Spotlight) The term biosensing relates to systems that include electronic, photonic, biologic, chemical and mechanical means for producing signals that can be used for the identification, monitoring or control of biological phenomena. The resulting biosensors are devices that employ biological components such as proteins to provide selectivity and/or amplification for the detection of biochemical materials for use in medical diagnostics, environmental analysis or chemical and biological warfare agent detection.

Surface plasmon resonance (SPR) has become the technology of choice for label-free detection of proteins and other biomolecules. SPR is based on the excitation of a surface plasmon at the interface between a dielectric and a thin layer of metal, typically gold. Once the sample is covered with the nanostructured gold layer (or another suitable metal), the surface absorbs light at certain wavelength. The absorption maximum is influenced by the refractive index of the fluid at the gold surface. This effect is called localized surface plasmon resonance (LSPR) and it is used for detection of presence of biomolecules, such as specific proteins.

Typical systems for LSPR use spectrometers – optical instruments used to measure properties of light over a specific portion of the electromagnetic spectrum. With advances in electronics and optics, spectrometers have become compact electronic devices that are easily plugged into a computer to perform the massive data processing that is required for analysis.

Researchers in Singapore have now developed a palm-sized and battery-operated bio-detection system based on LSPR which can operate on a single chip processor.

Photograph of a complete system with its 4-in. LCD touch screen display. The optical head is electrically connected to the system by two 5-wire cables, one for the LEDs biasing and the other for the optical output. (Image: Dr. Neuzil)

“Both SPR and LSPR systems were recently introduced in portable forms and these solutions are very elegant and space efficient” Dr. Pavel Neuzil tells Nanowerk. “However, the spectrum analyzers on which they rely require complex data processing performed by a PC. Further simplification of the LSPR system would require replacement of the spectrum analyzer with another device giving a straightforward electrical output, such as voltage.”

What Neuzil and his colleague Dr. Julien Reboud at the Institute of Microelectronics in Singapore developed is a way to measure the LSPR effect without a spectrum analyzer. Reporting their findings in the July 1, 2008 online edition of Analytical Chemistry (“Palm-Sized Biodetection System Based on Localized Surface Plasmon Resonance”), the two scientists’ work resulted in a miniaturized system that does not require any external power supply or personal computer and it is therefore suitable for point-of-care and field applications.

Neuzil explains that the typical reflection spectra of an in-house-fabricated LSPR chip exhibits absorption peaks at 555 nm for water and 645 nm for ethanol. “Measuring only the intensity of the reflected light at a few selected wavelengths could lead to a calculation of the peak shift” he says. “In fact, a LSPR system with high reproducibility could even be based on the amplitude of the reflected light intensity at a selected wavelength. The consequence of this is significant as this would allow replacement of the spectrum analyzer and subsequent signal processing scheme by a single source of monochromatic light and a photodiode as detector. The resulting photocurrent output could then be converted into a voltage by an operational amplifier with a resistor in a feedback loop (I/V converter) and detected by a voltmeter.”

The two researchers have started this work accidentally. They were conducting an advanced lithography experiment for researchers from a sister institute using a method called lift-off: silicon wafers with 200 mm diameters were covered with light sensitive material (photo resist) and exposed by deep UV light creating a regular pattern of 150 nm diameter holes in the photoresist. The next step was gold deposition over the photoresist and final step was supposed to be photoresist removal by dissolving in a suitable solvent such as acetone together with the gold above the photoresist.

Neuzil and Reboud found that the gold layer was not removed even though they tried different solvents. At that time they noticed that the nanostructured area of gold started to change color depending on the solvent it was soaked in.

“This was interesting indeed and we started to investigate where the color change was originating from” says Neuzil. “At that point we also started to think about how to measure this effect without using fancy instrumentation.”

To be used as a biosensor, the gold surface is first covered by molecules that are antibody-specific to the target (antigen). Once the antigens are presented they bind to the surface and change the emission peak of the absorption.

Conventionally, this peak location is determined by the reflection of white light from the surface and its spectrum detection is performed by a spectrometer. What Neuzil and Reboud have proposed and tested is a simple method to detect the shift of the absorption peak that originated from the sample.

The reflected light originating from the multiple light sources, leading to one photocurrent output of a single detector, can be demultiplexed either by using sequential illumination and synchronized readout or by using a lock-in amplification technique.

“We have upgraded a previously reported single-channel optical system based on a lock-in amplification technique (“An integrated fluorescence detection system for lab-on-a-chip applications”) into a four-channel system capable of simultaneous detection of the intensity of the reflected light from four individual light sources” explains Neuzil. “Each light source was modulated at a different frequency, and the composite signal detected by the photodiode was demodulated to provide four outputs corresponding to four individual signals each related to one light source. The optical system was also redesigned to accommodate four 5-mm-diameter LEDs, mounted around a photodiode detector. As the reflected light intensity from the LSPR chips is much greater than the previously detected fluorescence, the new optical system requires lower efficiency. That was achieved by the longer optical path between LEDs and the photodiode as well as using less sensitive photodiode than earlier.”

After the demodulation and demultiplexing of the composite signal, the four output signals were converted into a digital format by A/D converters and displayed on the 4-in. LCD display. The data processing as well as the display was controlled by a single chip controller. The whole system is shown in the image above.

This novel device is based on a platform of a universal lab-on-a-chip system which is being developed at their Institute of Microelectronics. The institute has been working on molecular diagnostic technologies and devices for some time (for instance see our news item “Bird flu testing on the cheap “) and the scientists have been reporting their findings in several papers recently such as (“Clockwork PCR Including Sample Preparation”) in Angewandte Chemie and (“Disposable real-time microPCR device: lab-on-a-chip at a low cost”) in Molecular BioSystems.

By Michael Berger. Copyright 2008 Nanowerk LLC

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Posted: August 1, 2008

Nanotechnology basics: How does a carbon nanotube grow?

(Nanowerk Spotlight) One of the best ways to gain control over synthesis of nanoparticles is to watch it happen. Take carbon nanotubes (CNTs): Synthesis of CNTs is a field that is growing explosively – but there is a lot that nanotechnology researchers don’t know about how nanotubes form and grow. While there are a number of in situ characterization methods for nanotube synthesis under development worldwide, each with different strengths and weaknesses, much of the information about the nanotube structure is indirect.

Historically, in situ characterization tools have accelerated progress in synthesis for many advanced materials, and there is widespread recognition that in situ tools have the potential to improve CNT synthesis as well. Ideally one would like to detect individual nanotubes and ensembles as they grow and measure their physical properties while imposing minimal constraints on the synthesis method. In other words, with a good understanding of the synthesis process we would be better able to control the product.

Dr. Kazutomo Suenaga, who heads the Nanoscale Characterization Team at the AIST Carbon Center in Japan, explains to Nanowerk that soon after the discovery of single-walled CNTs (SWCNTs), a few possible pathways were proposed for the closed-cap growth of a SWCNT without any catalyst, where carbon clusters could be continuously incorporated into the carbon network of the tubes.

“This growth model has never been experimentally observed because the whole reaction might be highly energetically unfavorable” says Suenaga. “One of the most important approaches is the inner growth of SWCNTs, which has been realized based on so-called peapod structures. By energetic irradiation or heating treatment, the fullerene molecules encapsulated inside the inner hollow cavity of a carbon nanotube could coalesce to form a new SWCNT, which is completed through the initial polymerization and the following series of Stone Wales (SW) transformations.”

He points out that, strictly speaking, this process is ‘transformation’ not real ‘growth’: “The fullerene cages are incorporated in whole, not in isolated carbon atoms and clusters. Besides this, it is rather difficult to investigate the dynamical transformation process since this experiment is hard to monitor, partially due to the short time-scale for SW transformations (in the range of nanoseconds or even faster).”

To solve this drawback, Suenaga and JSPS Fellow Chuanhong Jin introduce an aboratively designed in situ high resolution (HR)-TEM method for the noncatalytic inner growth of SWCNTs.

A time sequential TEM images for the catalyst-free growth of SWCNT inside a DWNT. (a) Originally, two SWCNts are separated with a head-to-head distance of 8.3 nm. There is a local kink across the nanotube walls. No voltage is applied. The distance between two electrodes is about 52 nm. (b, c) Under 1.40 V and 42 µA, the top SWCNT starts to continuously shrink on the cap, and simultaneously, the bottom one grows in its cap. (d) A cone-shaped local protrusion with a height of about 5.5 nm (marked as the filled white triangle) is formed on the cap of the bottom SWCNT, while the cap of the top SWCNT stays round. (e) About 42 s later, a new and solely one SWCNT is formed and the top SWCNT completely disappears. (f) With the voltage further increased to 1.42 V (a current of 44 µA), the newly formed SWCNT further grows, while the kinks across the nanotube walls migrate downward (marked as the unfilled white triangle). Scale bar = 5 nm. (Reprinted with permission from American Chemical Society)

“By well controlling the supply of carbon feedstock through a thermally activated evaporation process, we were able to observe the evolution of a growing SWCNT, with particular attention paid to the following question: How does the cap of a carbon nanotube evolve during the growing process?” Jin explains to Nanowerk.

Jin is first author of a recent paper in ACS Nano where the AIST scientists present their in situ HR-TEM studies on the noncatalytic inner growth of SWCNTs, especially the evolution of the cap (“How Does A Carbon Nanotube Grow? An In Situ Investigation on the Cap Evolution”).

“The cap of a SWCNT is surprisingly found to be kept closed during the growing process, because it was long believed that the cap should be open during the growth of SWCNT” says Jin. “The closed cap should be specific for non-catalytic growth of SWCNT. We also found the cap shape evolves inhomogeneously with a few particular sites growing faster during the growth.”

In this movie, the AIST scientists show the dynamical process of the catalyst-free inner growth of single-wall carbon nanotubes by in situ high resolution transmission electron microscopy.

Suenaga and Jin explain that there has been a long-lasting discussion as to whether SWCNTs could grow with a closed cap through continuous carbon incorporation into the carbon network. While a few groups had proposed theoretical models for the closed growth, until now this has never been experimentally verified.

“In our recent work, in the case of catalyst-free inner growth, our results clearly indicate that SWCNTs prefer to grow with a closed cap, especially for those with small diameters” says Suenaga.

Besides the fundamental importance of understanding the formation mechanism of SWCNTs, which would improve the controllable production of SWCNTs and other related carbon nanostructures, the method presented by Suenaga and Jin should also be of importance for engineering the structure of inner shells of CNTs, such as ‘defects repair’ and ‘cap modification’, and this may be of great interest for electronic and optic applications.

Suenaga and his team believe that it won’t be long before scientists will be able to fully reveal – with atomic resolution – the elementary steps for the catalyst-free growth of SWCNTs through carbon incorporation into the carbon network of a CNT.

In parallel, the group also shows a big interest in the catalyst-assisted growth of SWCNTs by addressing questions which will be of great importance on understanding the formation mechanism like how catalyst and carbon interact; which is the carbon diffusion path; and what is the nucleation process.

By Michael Berger. Copyright 2008 Nanowerk LLC

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Posted: July 29, 2008

Self-healing nanotechnology anticorrosion coatings as alternative to toxic chromium

(Nanowerk Spotlight) Remember the movie blockbuster Erin Brockovich? The film is based on a real world legal case that revolves around hexavalent chromium, also known as chromium (VI), used by the Pacific Gas and Electric Company (PG&E) to control corrosion in cooling towers in its Hinkley, CA compressor station. Chromium (VI), a natural metal, is known to be toxic and is recognized as a human carcinogen via inhalation. It also is widely used by industry in the manufacture of stainless steel, welding, painting and pigment application, electroplating, and other surface coating processes. PG&E for instance would periodically treat the surface of the cooling coils in its Hinkley station with anti-corrosion paint and release the chromium-containing wastewater into the environment.

The huge economic impact of the corrosion of metallic structures is a very important issue for all modern societies. Estimates for the cost of corrosion degradation run to about €200 billion a year in Europe and over $270 billion a year in the U.S. The annual cost of corrosion consists of both direct costs and indirect costs. The direct costs are related to the costs of design, manufacturing, and construction in order to provide corrosion protection, and the indirect costs are concerned with corrosion-related inspection, maintenance and repairs.

In spite of its toxicity, chromium (VI) has remained an essential ingredient in the metal finishing industry for corrosion control. But combine the economic impact of corrosion damage, the environmental and health problems cause by chromium (VI), and the increasing regulatory restrictions, scientists have a huge incentive to develop a new generation of protective coating systems.

The advanced materials that are being developed and used in modern industries require increasingly sophisticated coatings for improved performance and durability. With a degrading environment due to industrial factors, environmental compatibility is an aspect that gains in importance during the design phase of novel materials – and chromium (VI) compounds certainly wouldn’t make the list. Furthermore, while conventional anticorrosion coatings are just passive barriers that prevent the interaction of corrosive species with a metal, future nanotechnology based protective coatings will be ’smart’, i.e. they will provide several functionalities that will in effect result in self-healing capabilities.

The whole concept of ’smart’ materials that react on external impact (pH, humidity changes, or distortion of the coating integrity) and repair themselves has experienced a tremendous boost with the advent of nanotechnology. The nanoscale multilayer structure of a coating, in which the components are integrated and mutually reactive, is a main point in sophisticated and strong corrosion protection.

Researchers in Germany now have developed a novel method of multilayer anticorrosion protection including the surface pre-treatment by sonication and deposition of polyelectrolytes and inhibitors. This method results in the formation of a smart polymer nanonetwork for environmentally friendly organic inhibitors.

“Our novel coating exhibits very high resistance to corrosion attack, long term stability in aggressive media and an environmentally friendly, easy and economical preparation procedure,” Dr. Daria Andreeva tells Nanowerk. “We have demonstrated the general procedure for a surface important for the aircraft industry but it is similarly applicable for many types of surfaces, thus enabling many applications in advanced technologies.”

Andreeva is a researcher at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany. Together with her colleagues Dmitri Fix, Dr. Dmitry Shchukin and Dr. Helmuth Möhwald, she published a paper on the design of the group’s novel anticorrosion system in Advanced Materials (“Self-Healing Anticorrosion Coatings Based on pH-Sensitive Polyelectrolyte/Inhibitor Sandwichlike Nanostructures”).

21-day-corrosion test in 0.1M NaCl solution: scanning electron microscopy image and photograph of uncovered aluminum plate with corrosion degradation (above) and corrosion resistant aluminum plate covered by polyelectrolyte coating (below). (Image: Dr. Andreeva, Max Planck Institute of Colloids and Interfaces)

The main novelty of the proposed system is the multi-level protection approach, where the protective systems – the ’smart’ multilayers – will not only be a barrier to external impacts, but also respond to changes in their internal structure, and combine in the same system different damage prevention and reparation mechanisms.

The Max-Planck scientists started with the assumption that the layer-by-layer (LbL) deposition procedure would be a very effective solution for the preparation of self-healing anticorrosion coatings. The LbL process involves the stepwise electrostatic assembly of oppositely charged species (e.g., polyelectrolytes and inhibitors or nanoparticles) on a substrate surface with nanometer-scale precision, and allows the formation of a coating with multiple functionality.

A novel step in this anticorrosion system is the surface pretreatment of aluminum alloy by intensive sonication in water with an ultrasonic horn. Although the typical aluminum surface is covered by a 3-7 nm thick natural oxide film, this thin layer is not sufficient to protect against corrosion agents and does not yield good adhesion to subsequent layers of the coating.

“The ultrasonic pretreatment is crucial for formation of a uniform film” says Andreeva. “The surface of ultrasonically pretreated samples exhibits better wettability, adhesion, and chemical bonding with the polymer layers of the subsequent LbL coating. It results in a homogeneous distribution of the polymer film on the aluminum surface.”

After pretreatment, 5-10 nm thick layers of polyelectrolytes and inhibitor were formed by LbL deposition on the freshly sonicated aluminum alloys.

The scientists were amazed that even the nanometer-thick polyelectrolyte/ inhibitor coating provides effective corrosion protection for the aluminum alloy. They explain that the nature and properties of their novel anticorrosion coating simultaneously provide three mechanisms of corrosion protection: 1) passivation of the metal degradation by controlled release of inhibitor; 2) buffering of pH changes at the corrosive area by polyelectrolyte layers; and 3) self-curing of the film defects owing to the mobility of the polyelectrolyte constituents in the layer-by-layer assembly.

Schematic mechanism of corrosion protection. (Reprinted with permission from Wiley)

Since the release of the inhibitor is stimulated by corrosive species and corrosion products, this ’smart’ coating enables prolongated self-healing activity.

This anticorrosion protection method has a very broad range of potential applications due to its versatility. All components (polyelectrolytes and inhibitors) could be adjusted for a particular surface. The novel coating could be applied in aerospace, automotive and maritime industry and all other areas that suffer from corrosion damage, such as for instance oil and gas pipelines.

“Although we concentrated on corrosion, our method could also be more generally applied for self-repairing coatings like antifungal or antifriction applications” Andreeva points out.

One of the practical problems the Max-Planck team is still working on is the automation of the layer formation procedure in order to allow the scaling up of their technique for industrial applications. Beyond that, they are already looking to introducing other components with other mechanisms of corrosion protection into the system such as the combination of several inhibitors or self-polymerized compounds.

By Michael Berger. Copyright 2008 Nanowerk LLC

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While everybody talks about oil prices, water scarcity and water pollution are two increasingly pressing problems that could easily and quickly surpass the oil issue. Renewable energy sources can substitute for fossil fuels - but freshwater can’t be replaced. This makes the ability to remove toxic contaminants from aquatic environments rapidly, efficiently, and within reasonable costs an important technological challenge. Nanotechnology could play an important role in this regard. An active emerging area of research is the development of novel nanomaterials with increased affinity, capacity, and selectivity for heavy metals and other contaminants. The benefits from use of nanomaterials may derive from their enhanced reactivity, surface area and sequestration characteristics. Numerous nanomaterials are in various stages of research and development, each possessing unique functionalities that are potentially applicable to the remediation of industrial wastewater, groundwater, surface water and drinking water. The main goal for most of this research is to develop low-cost and environmentally friendly materials for removal of heavy metals from water. A recent example is a novel low-cost magnetic sorbent material for the removal of heavy metal ions from water, developed by scientists in China.

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The term ‘mechanical engineering’ generally describes the branch of engineering that deals with the design and construction and operation of machines and other mechanical systems. Students training to become engineering professionals have to delve into subjects such as instrumentation and measurement, thermodynamics, statics and dynamics, heat transfer, strengths of materials and solid mechanics with instruction in CAD and CAM, energy conversion, fluid dynamics and mechanics, kinematics, hydraulics and pneumatics, engineering design and so on. If you are currently doing coursework in mechanical engineering, better add nanotechnology courses to your core curriculum.

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Photonic crystals are similar to semiconductors, only that the electrons are replaced by photons (i.e. light). By creating periodic structures out of materials with contrast in their dielectric constants, it becomes possible to guide the flow of light through the photonic crystals in a way similar to how electrons are directed through doped regions of semiconductors. The photonic band gap (that forbids propagation of a certain frequency range of light) gives rise to distinct optical phenomena and enables one to control light with amazing facility and produce effects that are impossible with conventional optics. A prominent example of a photonic crystal is the naturally occurring gemstone opal. The problem with artificial opals, which limits their applications, is that they lack in pattern variety and their fabrication requires very expensive equipment and sophisticated processes. In contrast, natural photonic crystals have various patterns that are quite promising structural matrices for creating novel optical devices. One example are peacock feathers, whose iridescent colors are derived from the 2D photonic crystals structure inside the cortex.

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Carbon nanotubes (CNTs) have been hyped as the wunderkind material of the 21st century. And while researchers have developed numerous CNT applications, ranging from nanoelectronics to nanomedicine and military armor, the actual properties of CNTs fell way short of what the theory predicted. For instance, quantum mechanics calculations predict that defect-free single-walled carbon nanotubes possess a tensile strength of well over 100 gigapascals - which translates into the ability to endure weight of over 10,000 kg on a cable with a cross-section of 1 square millimeter. In practice, CNT tensile strength of only up to 28 GPa have been measured. The problem lies not so much with the actual CNTs but rather with the mechanical tests that have been employed so far. It is very difficult to produce testable samples without damaging the tubes (which in turn adversely affects their properties), and to image the test with high enough resolution to determine the exact nature of the fracture. First experimental measurements of the mechanical properties of carbon nanotubes have now been made that directly correspond to the theoretical predictions.

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The success of the semiconductor industry has been due in large part to its ability to continuously increase the complexity, and therefore the processing power, of integrated circuits at a given manufacturing cost. Moore?s Law observes that the number of transistors in a computer chip doubles every two years, whilst the cost of making the chip remains the same, due to miniaturization of the components. In order to produce the next generation of computer chips it is necessary to continue to shrink the size of the components on the chip. The miniaturization upon which Moore?s Law rests has been achieved through advances in the photolithographic process used to pattern the components onto to the silicon wafer. A beam of light is projected through a shadow-casting reticule and the light pattern is then directed onto a silicon wafer coated with a photochemically sensitive material, known as a resist. The solubility of the resist is modified by exposure to the light, allowing specific areas of the resist film to be removed, whilst other areas remain as a mask, so that the silicon wafer can be selectively etched, metallized or doped. For many years it has been predicted that the end of photolithography is approaching, and that further miniaturization will require next generation lithography techniques, such as EUV lithography. However, photolithography has proved remarkably resilient, and continues to improve. Unfortunately, whilst the ability of photolithography to pattern small features continues to improve, the industry is beginning to challenge the capabilities of the photosensitive resist.

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More than half a century ago, Erwin Schroedinger, nobel laureate in physics, claimed that it is ‘impossible to carry out experiments on single molecules or atoms’. Today, the detection, tracking and study of single molecules and atoms has become an omnipresent tool in biology, chemistry and physics alike. For example, sequencing DNA one base pair (or letter) at a time currently provides the most likely solution to fulfill the quest for a $1,000 human genome. Nevertheless, observation of a single molecule, especially with standard light microscopes requires a good deal of laboratory skills. This is mostly due to the fact that a single molecule only gives a miniscule amount of detectable signal. In fact, people using light as a probe have relied exclusively on the use of fluorescence, the emission of lower energy light following absorption of radiation at a certain energy. In this scheme, the signal from the molecule of interest can be easily separated from residual excitation light or background fluorescence simply by filtering the detected light spectrally and only detecting the color that is emitted by the molecule. In this way, it is possible to suppress unwanted signals from the billions of other molecules that are in the vicinity of the molecule of interest. As powerful as this approach has been, it also has one major limitation: it is only possible to study molecules that are highly fluorescent, i.e. emit lower energy light with high efficiency. Scientists from the ETH Zurich have recently demonstrated a major step towards the detection and study of single molecules in absorption.

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Our title today refers to the 1960 article by Yuri Artsutanov in Pravda: ‘To the Cosmos by Electric Train’. This article is the granddaddy of all ’space elevator’ concepts and first to propose the idea that a cable-based transport system could become an alternative to rockets for launching people and payload into space. The single most difficult task in building the Space Elevator is achieving the required tether strength-to-weight ratio - in other words, developing a material that is both strong enough and light enough to support the up to 100,000 km long tether. Thanks to nanotechnology, this material has become available in the form of carbon nanotubes (CNTs). The challenge ahead is to weave these raw CNTs into a useful form - a space worthy climbable ribbon. Assembling carbon nanotubes into commercially usable fibers is still one of the many challenges that nanotechnology researchers are faced with when trying to exploit the amazing properties of many nanomaterials.

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