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In recent years, nanomaterials have displayed potential in effective detection and removal of greenhouse gases, chemical contaminants, organic pollutants, and biological agents. These materials come in various morphologies and have various functions (e.g., adsorbents, catalysts, or membranes). The high reactivity and high surface area of nanomaterials are some of the notable features which provide an advantage in environmental remediation over other conventional alternatives.
Given the abundance of plant resources, plant extracts are the most studied category to date for the synthesis of green nanomaterials. Cellulose is one of the most abundant and pervasive bioipolymers on earth. Cellulose based raw materials have been traditionally used in many fields, because of their unique advantages including renewability, biocompaitiblity, low cost, natural biodegradability and chemical stability. It has been used as an engineering material for thousands of years and continues to be used today in forest products such as paper, textiles, etc. Novel cellulose based functional materials, such as cellulose hydrogel, aerogel and porous materials have been developed in various fields.
Nanocellulose (NC) is one of the most interesting nature-based nanomaterials and is attracting attention in a myriad of fields such as biomaterials, engineering, biomedicine, opto/electronic devices, nanocomposites, textiles, cosmetics and food products. The nanomaterial can be extracted from plant cellulose pulp or synthesized by non-pathogenic bacteria.
Bacterial cellulose nanopaper (BC) is a multifunctional material known for numerous desirable properties: sustainability, biocompatibility, biodegradability, optical transparency, thermal properties, flexibility, high mechanical strength, hydrophilicity, high porosity, broad chemical-modification capabilities and high surface area.
Renewable nanocellulose is being utilized for next generation of ‘green’ electronic devices owing to its low roughness, good thermal stability and excellent optical properties. NC-based platforms could be considered an emerging technology to fabricate efficient, simple, cost-effective and disposable optical/electrical analytical devices for several (bio)sensing applications including health care, diagnostics, environmental monitoring, food quality control, forensic analysis and physical sensing. Various proof-of-concept transparent nanopaper-based electronic devices have been fabricated; these devices exhibit excellent flexibility, bendability and even foldability.
Currently, nanocellulose is under active research for a myriad of applications including filtration, wound dressing, pollution removal approaches and flexible and transparent electronics. Daio Paper Corp., has recently launched a paper toilet cleaner made from cellluose nanofibers (CNF) and Nippon Paper Industries has established Japan’s largest cellulose nanofiber (CNF) production line at its plant in the city of Ishinomaki with a planned annual production of 500 tons.
Many of the (bio)sensors that are currently based on plastic, glass or conventional paper platforms are predicted to transferred to NC and this generation of (bio)sensing platforms could revolutionize the conventional sensing technology.
Nanocellulose and Cellulose Nanopaper
Nanopaper is prepared from the same chemical constituents as regular paper but uses very thin cellulose nanofibrils (<20 nm) instead of thicker fibers. These very thin cellulose nanofibrils can advantageously be obtained from diverse sources (multiple plants or bacteria) including waste sources, instead of thicker fibers that are typically obtained from wood pulp.
Preparing a material from these thinner nanofibrils results in a material that exhibits superior mechanical and barrier properties to regular paper, and significantly, is also optically transparent. The transparent property arises because nanofibrils are much less effective in scattering visible light than larger cellulose fibrils, and can also pack together more efficiently.
Currently, most cellulose comes from wood, in which 40–45% fibers are composed of cellulose. Usually, cellulose fibers have a length of about 1–3 mm and a diameter of 20–50 μm. They consist of bundles with a length greater than 2 μm and a diameter wider than 15 nm. Individual fibrils in those bundles have the same length as the bundle, but a narrower diameter of about 5–10 nm.
Using different methods, two types of nanocellulose (nanocrystalline cellulose and nanofibrillated cellulose) can be isolated from plant cellulose. Cellulose nanocrystal (CNC) or NCC contains rigid rod-like particles resulting from the chemical treatment of cellulose. CNCs display a diameter ranging from 5 to 50 nm and length between 100 and 500 nm, while cellulose nanofibres (CNFs) are classified as aggregates of elementary fibres with a diameter between 20 and 50 nm and length between 500 and 2000 nm. Various terms have been used interchangeably with CNF, such as nanofibrillated cellulose (NFC), nanofibrillar cellulose, nanofibrous cellulose, and bacterial nanocellulose (BC). Due to their high stiffness, pure NCC films are very fragile, which limits their application in flexible electronics.
A third type of nanocellulose, BNC, is produced by bacteria. Different from the rod-like NCCs and fiber-shape NFCs isolated from plant cellulose, BNC is produced by Acetobacter xylinum and some other species, which consists of ribbon-shape nanofibers in a web-like network. BNC shares the same structural unit as NFC and NCC, which possess some specific properties, such as high crystallinity, high chemical purity (about 100%), high degree of polymerization and ultrafine web-like structure;
Cellulose Nanopaper (CNP) is an architecture composed of cellulose nanofibres (CNFs) or nanocrystals (CNCs). When the nanocelluloses form a self-standing nanopaper film, they interconnect with each other by establishing interfibre hydrogen bonds, yielding a “nanodimensioned network” similar to the one formed by standard paper fibres.
However, due to the strong hydrogen bonding between the nanofibers, the mechanical fibrillation requires significant time and energy, which is a big barrier for commercial applications. To commercialize transparent nanopaper made of NFC or NFC-based composites, scientists have attempted to develop efficient manufacturing techniques.
Nanocellulose used to develop viable, environmentally-friendly alternative to Styrofoam
Washington State University researchers have developed an environmentally-friendly, plant-based material that for the first time works better than Styrofoam for insulation. The foam is mostly made from nanocrystals of cellulose, the most abundant plant material on earth. The researchers also developed an environmentally friendly and simple manufacturing process to make the foam, using water as a solvent instead of other harmful solvents.
Researchers have been working to develop an environmentally friendly replacement for polystyrene foam, or Styrofoam. The popular material, made from petroleum, is used in everything from coffee cups to materials for building and construction, transportation, and packaging industries. But, it is made from toxic ingredients, depends on petroleum, doesn’t degrade naturally, and creates pollution when it burns.
The work, led by Amir Ameli, assistant professor in the School of Mechanical and Materials Engineering, and Xiao Zhang, associate professor in the Gene and Linda School of Chemical Engineering and Bioengineering, is published in the journal Carbohydrate Polymers (“Strong ultralight foams based on nanocrystalline cellulose for high-performance insulation”).
To make cellulose nanocrystals, researchers use acid hydrolysis, in which acid is used to cleave chemical bonds.
In their work, the WSU team created a material that is made of about 75 percent cellulose nanocrystals from wood pulp. They added polyvinyl alcohol, another polymer that bonds with the nanocellulose crystals and makes the resultant foams more elastic. The material that they created contains a uniform cellular structure that means it is a good insulator. For the first time, the researchers report, the plant-based material surpassed the insulation capabilities of Styrofoam. It is also very lightweight and can support up to 200 times its weight without changing shape. It degrades well, and burning it doesn’t produce polluting ash.
“We have used an easy method to make high-performance, composite foams based on nanocrystalline cellulose with an excellent combination of thermal insulation capability and mechanical properties,” Ameli said. “Our results demonstrate the potential of renewable materials, such as nanocellulose, for high-performance thermal insulation materials that can contribute to energy savings, less usage of petroleum-based materials, and reduction of adverse environmental impacts.”
Nanopaper as Plastic Substitute
The problem of plastic waste is one of the greatest challenges faced by the current generation. Every year, millions of tons of plastic pollutes our oceans resulting in potentially major damage to marine life, biodiversity, food security, and human health. The development of renewable, sustainable, and biodegradable alternatives for plastic materials is therefore essential. Nanopaper has attracted attention as a low-cost, environmentally friendly, high-performance material with strong potential to replace plastic substrates in many electronic and material applications.
Paper-based materials like nanopaper are excellent substrates for functionalization by nanoparticles (NPs) because the porous structure allows for high NP loadings. Additionally, paper-based materials can be effectively functionalized by a wide variety of NPs, resulting in materials suitable for a wide range of applications. For example, the use of inorganic NPs (e.g., TiO2, Au, Ag) can produce paper-based materials with excellent catalytic, antibacterial, sensing, and anticounterfeit properties.
Netherlands Researchers have developed transparent UV-blocking nanopaper by embedding tunable UV-absorbing NPs from ethyl cellulose into nanopaper. These functional nanopaper films are highly transparent, selectively block UV light, and show excellent photostability, therefore with great potential as high-performance, renewable, sustainable, and biodegradable materials for photoprotection applications.
Green Electronics
Cellulose nanopaper is used to replace traditional glass and plastic substrates in energy devices, and a new scientific term “Green: electronics has been created. The aim of green electronics is used to utilize compounds of natural materials (such as cellulose and gelatin) and produce ecofriendly electronics through new green and efficient routes. Electronic devices should also be manufactured by low energy consumption processes to realize a sustainable and clean society. The biodegradability of cellulose nanopaper is settled. Microorganisms (moulds, fungi, and bacteria) can degrade cellulose by the intervention of extracellular enzymes known as cellulases.
Paper is one of the most and most popular “green substrate” materials. Paper based energy devices exhibit many particular advantages including sustainability, lighter, cheaper and more flexibility. Flexible electronics or printable electronics on paper promise excellent flexibility, low cost, light weight, inertness, recyclability and high mechanical strength when compared to the silicone-based or plastic-based electronics .
Nevertheless, the porous structure, high surface roughness, optical opaqueness or energy intensive manufacturing of the typical paper produced could not satisfy all the requirements for the next generation of ‘green’ electronics. In order to overcome these problems and to expand the opportunity of using cellulose as a host substrate for electronic devices, a thin layer of passivation is usually coated on the surface of regular papers by calendaring to reduce the surface roughness.
Alternatively, the devices were first fabricated on other flat and smooth substrates like glass, and then transferred to the paper surfaces, which is more complex and incurred additional cost. However, most passivation coating alters the surface properties of the cellulose substrate and may reduce the printability of the paper due to the change of surface energy.
Paper electronics could be manufactured using high speed and high volume printing technology. Thus various kinds of biodegradable and biocompatible electronics e.g. super-capacitors, antenna, photoanode, and inductors are fabricated on cellulose based substrates.
Transistors
Thin film transistor (TFT) is one important component for microelectronic devices to amplify or switch electronic signals. Different from regular paper, nanocellulose paper exhibits low surface roughness, strong mechanical strength and excellent optical properties, which make it very suitable as a flexible and transparent substrate to fabricate TFT. Huang et al. reported a highly transparent and flexible organic transistor device by using a NFC paper as substrate. The nanopaper transistor exhibits good electrical characteristics; the carrier mobility is around 4.3 × 10−3 cm2/(Vs) and Ion/Ioff ratio can reach up to 200.
A Biodegradable Computer Chip
Researchers at the University of Wisconsin, led byZhenqiang (Jack) Ma, a professor of electrical and computer engineering, have developed Electronic components of gallium arsenide on silicon wafer, and then through a rubber stamp lifted them from the silicon wafer and transferred them to a new surface made of nanocellulose. This reduced the amount of semiconducting material used by a factor of up to 5,000, without sacrificing performance.
The results also show that a transparent, wood-derived material called nanocellulose paper is an attractive alternative to plastic as a surface for flexible electronics. The inventors argue that the new chips could help address the global problem of rapidly accumulating electronic waste, some of which contains potentially toxic materials.
In two recent demonstrations, Ma and his colleagues showed they can use nanocellulose as the support layer for radio frequency circuits that perform comparably to those commonly used in smartphones and tablets. They also showed that these chips can be broken down by a common fungus.
Memory
Non-volatile memory is an essential component in portable and free-standing electronic components. Nagashima et al. proposed an ultra-flexible resistive non-volatile memory device using Ag-decorated CNF nanopaper. The device exhibits stable non-volatile memory effects and excellent mechanical flexibility without degradation after being bent down to a radius of 350 μm. This study paves the way for the development of inexpensive, environmentally friendly and mechanically flexible memory devices for portable flexible electronics.
Antennae
Antennae can convert electric power to radio waves and vice versa, which have been broadly used in various communication systems, such as portable phones, computers and radios. In order to meet the development of next-generation flexible and wearable electronics, the size and weight of antennae should be reduced.
Due to its foldability, light weight and low roughness, nanopaper is an excellent substrate to print small and flexible antennae. Nogi et al. demonstrated a V-shaped antenna by screen-printing silver nanowire ink on the smooth surface of nanopaper. The antenna exhibits good sensitivity with a loss less than −26 dB.
Solar cells
The high potentialities shown by nanopaper for electrode deposition have turned on a relevant interest in the demonstration of optoelectronic devices deposited on CNP. A benefit of the high optical haze of transparent nanopaper is that it is very attractive for high-efficiency organic solar cells with light management. Significant advances have been made in the past several years. Zhou et al. demonstrated an efficient organic solar cell by using optical transparent NCC film as a substrate. A power conversion efficiency (PCE) of 2.7% can be obtained.
Hu et al. reported a highly transparent with large light scattering nanopaper, which can be successfully deposited with conductive materials, like ITO, carbon nanotubes and silver nanowires. This transparent conductive paper can be used in many applications such as displays, touch screens and solar cells. They demonstrated an organic bulk of heterojunction solar cells, and a PCE of 0.4%. The cell efficiency can be further improved by controlling surface smoothness during fabrication.
Nanopaper as an optical sensing platform
An international team led by the ICREA Prof Arben Merkoçi seeks to design, fabricate, and test simple, disposable and versatile sensing platforms based on this material. They designed different bacterial cellulose nanopaper-based optical sensing platforms.
In the article, the authors describe how the material can be tuned to exhibit plasmonic or photoluminescent properties that can be exploited for sensing applications. Specifically, they have prepared two types of plasmonic nanopaper and two types of photoluminescent nanopaper using different optically active nanomaterials.
The researchers took advantage of the optical transparency, porosity, hydrophilicity, and amenability to chemical modification of the material. The bacterial cellulose employed throughout this research was obtained using a bottom-up approach and it is shown that it can be easily turned into useful devices for sensing applications using wax printing or simple punch tools.
The scientific team also demonstrates how these novel sensing platforms can be modulated to detect biologically relevant analytes such as cyanide and pathogens among others. According to the authors, this class of platforms may prove valuable for displaying analytical information in diverse fields such as diagnostics, environmental monitoring and food safety.
Moreover, since bacterial Nanopaper as optical sensing platform cellulose is flexible, lightweight, biocompatible and biodegradable, the proposed composites could be used as wearable optical sensors and could even be integrated into novel theranostic devices. Theranostic nanodevices are a combination of therapy and diagnostics in the same device, to treat and image efficacy with one single tool.
In general, paperbased sensors are known to be simple, portable, disposable, low power-consuming and inexpensive devices that might be exploited in medicine, detection of explosives or hazardous compounds and environmental studies.
Organic light-emitting diodes
OLEDs are promising for commercialization in displays and lighting with their light-weight, thin and energy efficient characteristics. Usually, OLEDs are fabricated on glass or plastic film substrates like PET and polyethylene naphthalate. However, the glass substrate cannot be used in the roll-to-roll fabricating process, and the large coefficient of thermal expansion of plastic films results in thermal instability, which is unfavorable for OLEDs.
As a recyclable and sustainable material, nanopaper is an attractive substrate for OLEDs due to its excellent thermal stability, light weight, flexibility, high optical transmittance and compatibility with roll-to-roll manufacturing. More recently, flexible OLEDs based on 100% pure nanocellulose have been reported. Zhu et al. demonstrated highly flexible OLEDs on 100% CNF nanopaper.
Simultaneous strength and toughness properties of cellulose nanopaper
The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. So far there exists only limited success of attaining both strength and toughness, which often needs material-specific, complicated, or expensive synthesis processes and thus can hardly, is applicable to other materials.
The UMD team pursued the development of a strong and tough material by exploring the mechanical properties of cellulose, the most abundant renewable bio-resource on Earth. Researchers found that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) compared to regular notebook paper, as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher.
Further fundamental mechanistic studies reveal that reduced intrinsic defect size and facile (re)formation of strong hydrogen bonding among cellulose molecular chains is the underlying key to this new scaling law of mechanical properties. The smaller the cellulose fibers more are the hydrogen bonds per square area. Once broken, the hydrogen bonds can reform on their own—giving the material a ‘self-healing’ quality.
These high performance yet lightweight cellulose-based materials might one day replace conventional structural materials (i.e. metals) in applications where weight is important. It could lead, to more energy efficient and “green” vehicles. In addition, team members say, transparent cellulose nanopaper may become feasible as a functional substrate in flexible electronics, resulting in paper electronics, printable solar cells and flexible displays.
Touch sensors
Paper-based touch sensors have attracted tremendous attention since they are lightweight, portable and flexible. Fang et al. designed a bilayer transparent nanopaper by using regular wood fibers as the backbone and NFC as fillers. Such a hybrid nanopaper exhibits excellent optical transmittance and superior surface roughness. A thin layer of CNT was then deposited on the surface by rod coating to make it conductive. A four-wire resistive touch screen was fabricated using this transparent conductive paper. This sensor can sense physical touch, with the signal being transferred to the computer with an external controller.
Nanopaper-based energy harvester
Liangbing Hu of the University of Maryland; Jun Zhou of Huazhong University of Science & Technology, in China; and their colleagues have designed a nanopaper-based generator that can generate electrical power from a user’s touch.
The researchers deposit carbon nanotubes on the nanopaper to make a pair of electrodes, and then sandwich a polyethylene film in between. The generator works via electrostatic induction. Pressing one side of the device causes a change in the charge balance between the nanotube electrodes, resulting in a flow of current through the device.
Releasing the pressure causes electrons to flow back, so repeated pressing and releasing creates continuous current. The researchers demonstrated that the generator could produce enough power when pressed to light up a small liquid-crystal display. The paper energy-harvester could be used to make disposable, self-powered touch screens that fold; interactive light-up books; touch-sensitive skin for prosthetics; and security systems for art and documents, according to the researchers.
Your paper notebook could become your next tablet
Purdue engineers developed a simple printing process that renders any paper or cardboard packaging into a keyboard, keypad or other easy-to-use human-machine interfaces. This technology is published in the Aug. 23 edition of Nano Energy. “This is the first time a self-powered paper-based electronic device is demonstrated,” said Ramses Martinez, an assistant professor in Purdue’s School of Industrial Engineering and in the Weldon School of Biomedical Engineering in Purdue’s College of Engineering. “We developed a method to render paper repellent to water, oil and dust by coating it with highly fluorinated molecules. This omniphobic coating allows us to print multiple layers of circuits onto paper without getting the ink to smear from one layer to the next one.”
Purdue University engineers developed a simple printing process that renders any paper or cardboard packaging into a keyboard, keypad or other easy-to-use human-machine interfaces. (Image provided)
Martinez said this innovation facilitates the fabrication of vertical pressure sensors that do not require any external battery, since they harvest the energy from their contact with the user. This technology is compatible with conventional large-scale printing processes and could easily be implemented to rapidly convert conventional cardboard packaging or paper into smart packaging or a smart human-machine interface.
“I envision this technology to facilitate the user interaction with food packaging, to verify if the food is safe to be consumed, or enabling users to sign the package that arrives at home by dragging their finger over the box to properly identify themselves as the owner of the package,” Martinez said. “Additionally, our group demonstrated that simple paper sheets from a notebook can be transformed into music player interfaces for users to choose songs, play them and change their volume.”
Martinez and his team have worked with the Purdue Research Foundation Office of Technology Commercialization to patent some of his technologies related to robots and other design innovations. For more information on licensing a Purdue innovation, contact the Office of Technology Commercialization at otcip@prf.org.
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