CitationDownload as .RIS
Emerald Group Publishing Limited
Copyright © 2012, Emerald Group Publishing Limited
Environmental sensing and recent developments in graphene
Article Type: Nanosensor update From: Sensor Review, Volume 32, Issue 1
The first part of this update reflects the theme of this issue and considers nanosensors for detecting environmental pollutants. The second part reports on further developments in the “wonder material” graphene. Although considered previously, an update is warranted as progress has been exceedingly rapid during the past year.
Nanosensors for environmental pollutants
The introduction of legislation in most of the world’s industrialised nations has led to environmental sensing emerging as a major application, as all manner of gases, liquid chemicals and other compounds that pose a threat to the ecosystem or to human health are routinely monitored. Environmental sensors are based on a range of established optical, electrochemical and other technologies and have previously been reviewed in this journal (Vol. 28, pp. 275-282) but the research community has recently started to investigate the role of nanosensors to detect environmental pollutants and some recent developments are considered here.
Detecting algal toxins
Eutrophication, the enrichment of waters by nutrients such as nitrates and phosphates, poses a chemical threat to water quality but most importantly it can cause naturally occurring cyanobacteria (blue-green algae) to “bloom” (Figure 1). These blooms contain microcystins, cyclic non-ribosomal peptides (Figure 2) which are amongst the most powerful natural poisons known and have caused the death of wild animals, livestock and domestic pets in many countries. Recreational exposure can result in gastro-intestinal symptoms or pruritic skin rashes in humans. There is some evidence that exposure to high levels of some species of cyanobacteria causes Lou Gehrig’s disease, a motor neuron condition and microcystins have also been linked with liver cancer. Further, there is an interest in the military potential of cyanotoxins as biological weapons. Accordingly, there is a need for techniques to detect these compounds in the field and in real-time, as alternatives to traditional laboratory procedures, and there is an extensive and technologically diverse recent literature. A nanosensor that aims to satisfy this need is being developed by workers from China’s Jiangnan University and the University of Michigan. An antibody (Ab) to microcystin was prepared and dispersed in a solution containing single-walled carbon nanotubes (SWCNTs) at a concentration of 50 mg/mL. The dispersion was used to dip-coat analytical filtration paper, rendering it conductive, and the paper was then freeze-dried under vacuum in order to minimise the Ab denaturation. The resulting nanobiosensor exhibited a change in conductivity proportional to the microcystin concentration and was tested using a standard three-electrode electrochemical arrangement, with the sensor as the working electrode and a platinum wire and a saturated Hg2Cl2 solution acting as the counter and reference electrodes, respectively. The sensor has a linear detection range up to 10 nmol/L and a non-linear response up to 40 nmol/L. The limit of detection was 0.6 nmol/L (0.6 ng/mL), which satisfies the World Health Organisation standard for the maximum microcystin content of drinking water (1.0 ng/mL). The sensor is competitive with alternative detection techniques such as enzyme-linked immunosorbent assay (ELISA) and liquid chromatography/mass spectrometry in terms of sensitivity and also has a far shorter response time. Allied work at the Chinese Nanjing University has investigated the use of single-walled carbon “nanohorns” rather than nanotubes which, it is argued, can enhance sensitivity. Using a similar sensing reaction to the above, the sensors exhibited a linear response to microcystin over the range 0.05-20 μg/L and a significantly enhanced limit of detection of 0.03 μg/L (i.e. 0.03 ng/mL). Several other detection techniques are under development by Chinese groups, such as enhanced ELISAs, quartz microbalances and various optical methods and it is clear that microcystin pollution is a major problem in many parts of the country, perhaps reflecting the largely unregulated use of agrochemicals leading to eutrophication. Although much of the nanosensor work involves CNTs, a technique based on quantum dots (QDs) has been reported by a group from the Korean Gwangju Institute of Science and Technology. This again uses an Ab-based approach whereby the immunological recognition of microcystin using the QD/Ab probe was amplified and converted into an electrochemical signal by measuring the cadmium ions released from the QDs with square-wave stripping voltammetry. A range of 0.227-50 μg/L and a limit of detection of 0.099 μg/L were obtained under optimised conditions.
Nanosensors for pesticides
Pesticides are a major source of water pollution and as with cyanotoxins, most determinations are made using often time-consuming techniques in the laboratory. There is an extensive body of research into sensors that can determine these compounds in the field and QDs-based nanobiosensors have been demonstrated by several groups. An example of recent work is by a group from the Harbin Institute of Technology, China, where bi-enzymes of acetylcholinesterase (AChE) and choline oxidase are used as the biological receptors and cadmium telluride (CdTe) QDs act as the fluorescent probes for the optical transduction of the enzyme activity. The aim is to detect organophosphorus pesticides (OPs) and increasing amounts of OPs lead to a decrease of the enzymatic activity and thus a decrease in the production of hydrogen peroxide which can quench the fluorescence of the QDs. This sensor has been used to detect three types of commonly used OPs (paraoxon, dichlorvos and parathion) at picomolar concentrations and the linear detection range covers six orders of magnitude, from 10−12 to 10−6 M. Workers from the Chinese Ningbo University have recently reported a disposable, electrochemical nanobiosensor for OPs based on a screen-printed carbon electrode, modified with AChE-coated Fe3O4/gold magnetic composite nanoparticles. This showed a linear response to the pesticide dimethoate from 1.0×10−3 to 10 ng/mL, with a detection limit of 5.6×10−4 ng/mL. Details of the sensor are shown in Figure 3. The use of AChE to detect OPs has been studied extensively and several groups have recently combined this reaction with a range of nanomaterials. In addition to CdTe QDs, examples include multi-walled CNTs, gold and zirconia nanoparticles and cadmium sulphide nanoparticles and QDs. These nanomaterial matrices confer significant enhancements to OP determinations, with the thiocholine oxidation occurring at much lower oxidation potentials. Moreover, nanomaterial-based AChE sensors exhibit rapid responses and increased operational and storage stability and as such are well suited for OP determination over wide concentration ranges. Figure 4 shows cyclic voltammograms of solutions containing different malathion concentrations derived from a sensor based on gold nanoparticles and AChE and Figure 5 shows a calibration plot for a surface plasmon resonance (SPR) sensor with AChE immobilised onto a monolayer of self-assembled gold nanoparticles for sensing the OP paraoxon in the range 3.63×10−9 to 0.36×10−6 M.
Heavy metal sensing
Heavy metals such as cadmium, mercury, copper and lead are major pollutants of watercourses and sensors based on all of the previously mentioned nanomaterials (QDs, CNTs and nanoparticles) have been used for their detection in experimental devices. For example, workers in the USA from Missouri State University and Rowan University have functionalised CNTs with cysteine and incorporated them into the surface of an electrode. Owing to their affinity to cysteine, metals accumulated on the electrode and by using differential pulse anodic stripping voltammetry as the detection technique, the sensors were shown to exhibit detection limits of 1 and 15 ppb to Pb2+ and Cu2+, respectively. Gold nanoparticles have been functionalised with monoazacrown ether ionophores by workers from Iran’s Razi University. In the presence of Pb2+ ions the particles exhibit a colour change caused by the appearance of the surface plasmon band at 520 nm. Importantly, the sensor is virtually immune to interferences from alkali, alkaline earth and transition metal ions. In other instances, gold nanoparticles have been functionalised with fluorescent molecules such as rhodamine B, whose fluorescence is stimulated by the presence of mercury ions and workers from China’s Anqing Normal College reported a gold nanoparticle-based fluorescent sensor, functionalised with rhodamine 6G, which achieved an exceedingly low detection limit of 6.0×10−11 M Hg2+. Several groups have functionalised QDs with metal receptor molecules, as previously considered in this journal (Vol. 30 No. 4). An example of recent work is research by a group from the Russian State University of Informational Technologies, Mechanics and Optics, together with colleagues from Belarus and Ireland. They have combined the azo dye 1-(2-pyridilazo)-2-naphtol (PAN), a well-known metallochromic indicator which can be used for the colourimetric determination of metal ions such as Ni2+ and Co2+, with luminescent CdSe/ZnS core/shell QDs. Thin polymer films with thicknesses of 1-3 μm containing QD/PAN complexes were prepared which exhibited changes in their absorption spectra and an increase in the QD’s luminescent intensity when immersed in solutions containing nickel or cobalt ions. It was found that concentrations of 10−8 M Co2+ and 10−8 M Ni2+ in aqueous solutions can be determined with experimental errors of ~15 per cent, even without optimisation of the sensor parameters. These values are more than an order of magnitude lower than the detection limits of 6.7 ng/mL for Co2+ and 3.2 ng/mL for Ni2+ obtained by colourimetric measurements with PAN as a complexing reagent in the aqueous phase.
Graphene: a review of recent progress
Graphene, the two-dimensional form of carbon (Figure 6) earned its discoverers at the University of Manchester a Nobel Prize for Physics in 2010. It exhibits a number of unique electrical, mechanical, optical and other properties (see Sensor Review, Vol. 31, pp. 178-80) and a selection of significant, recent developments are considered here.
Researchers at the Lawrence Berkeley National Laboratory and the University of California at Berkeley have shown that they can tune graphene to responds to light with THz frequencies. This could play a critical role in future generations of THz sensing and imaging systems in the healthcare, security and other industries. They have developed a device which consists of an array of graphene nanoribbons whose response to THz radiation can be tuned by varying the width of the ribbons and the number of charge carriers (electrons and holes) in the structures. Varying these two parameters allows the control of the collective oscillations of electrons (plasmons) in the ribbons and it is these plasmons that strongly couple to the THz light. The tuning mechanisms are shown in Figure 7. As THz radiation lies in the wavelength range between ~1 and 0.03 mm and the width of the graphene ribbons is just 1-4 μm, the ribbon is a so-called “metamaterial”; one that consists of structures with dimensions much smaller than the associated wavelength. Metamaterials exhibits optical properties which are distinctly different from those of the bulk material and the research group notes that “We have not only made the first studies of light and plasmon coupling in graphene, we’ve also created a prototype for future graphene-based metamaterials in the terahertz range”. Gas sensing with nanomaterials has attracted huge interest and some research has previously been considered in the journal but a potentially significant development has recently been reported by researchers at Wayne State University, together with colleagues from the Korean Ajou University. They have made progress in developing what is often regarded as the “holy grail” of gas sensing: a non-optical sensor for carbon dioxide (CO2) that operates unheated. The sensor was fabricated from graphene flakes produced by mechanical cleavage, deposited on a silicon substrate and unlike most other solid-state gas sensors it can be operated at room temperature. The sensor’s conductance varies linearly on exposure to CO2 over the concentration range 10-100 ppm and most significantly, it exhibits a fast response and recovery time, low power consumption and high sensitivity.
As well as forming the basis of novel sensors, graphene may ultimately provide a means of powering them. A group from the Rensselaer Polytechnic Institute have shown that graphene can be used to harvest energy from flowing fluids (Figure 8). The power generated by the flow of 0.6 M HCl solution at 0.01 m/s was found to be about 85 nW for a 30×16 μm graphene film, which equates to a power per unit area of approximately 175 W/m2. While a similar effect has been observed with CNTs, this is the first such study with graphene. The energy harvesting capability of graphene is at least an order of magnitude superior to nanotubes and an advantage of the flexible graphene sheets is that they can be wrapped around almost any shape. Conventional thinking is that free electrons on the surface of CNTs and graphene can interact with ions in the flowing fluid; the ions drag the electrons in the flow direction, creating an electric current. It is curious, however, that the flowing water stream creates a voltage, as it does not contain ions – a mystery that a new study plans to address. Additionally, the team will investigate how water flowing inside CNTs and within layered graphene can be harnessed to generate additional power. Applications are anticipated in hydrocarbon exploration which involves deep drilling to detect the presence of oil or natural gas. Exploration companies would like to augment this process by deploying large numbers of microscale or nanoscale sensors into new and existing wells. These would travel laterally through the earth, carried by pressurised water pumped into the wells, and into the network of cracks beneath the surface. Companies would no longer be limited to vertical exploration and data collected from the sensors would provide more information regarding the best locations to drill. By coating the sensors with graphene they would be truly autonomous and could be deployed in this manner. This work is part of a three-year study, funded by the Advanced Energy Consortium, entitled “Nanofluidic power generation using one-dimensional (carbon nanotube) and two-dimensional (graphene) nanomaterials.”
Being a “new” material, there is understandably an ever-growing body of research into graphene processing technologies. Workers at the Seoul National University and the Samsung Advanced Institute of Technology have developed a simple but efficient low-temperature route to prepare metallic nanowire-graphene (NW-G) hybrid nanostructures for use as flexible field emission devices. Combining vertically aligned metallic NWs with graphene is expected to allow the fabrication of various flexible electronic components. Recently, nitrogen-doped CNTs and ZnO NWs have been grown on reduced graphene but most methods are based on high-temperature processing and the growth of vertically aligned metallic NWs on graphene has not been demonstrated before. In this new method, a graphene layer is transferred onto an anodic aluminium oxide template and vertically aligned gold NWs are grown on the graphene surface via electrode position. This process allows NW-G hybrid nanostructures to be prepared with a controlled length and diameter. The technique avoids any high temperature steps or unconventional lithographic procedures, which suggests that it could be applied to a variety of substrates, including soft materials. To fabricate completely flexible field emission devices, the NW-G hybrid nanostructures and another graphene layer on a polydimethylsiloxane substrate are configured as a cathode and an anode, respectively. These devices exhibit a high emission current density and stable field emission currents even when bent to a radius of 25 mm, indicating that the device forms a stable NW-graphene junction under the bent conditions and that the contact resistance between the graphene and the NWs is very low. This technique for fabricating flexible hybrid nanostructures suggests a number of applications such as biosensors and chemical sensors, pressure sensors and field emission devices, as well as novel battery electrodes. Details of the technology are shown in Figure 9.
Hexagonal patterns with 4.8 and 3.2 nm line widths produced by the SPL/controlled gas technique
A group at Sejong University in South Korea has recently reported a graphite patterning method based on scanning probe lithography (SPL) that benefits from a controlled gas environment and which could offer a route to preparing graphene-based electronic devices. Patterning of graphite or graphene by SPL has typically been performed in air, as water molecules form a meniscus between the tip and sample which mediates the etching reaction. This water meniscus may prohibit uniform patterning due to its strong surface tension or large contact angle on the sample surfaces. To combat this effect, the group patterned samples in a chamber where the environment could be controlled with methyl alcohol, oxygen or isopropanol gases and vapours. It was found that methyl alcohol facilitates graphite etching and creates a line width as narrow as 3 nm. The line drawing speed was about 100 nm/s, which is 20 times faster than other STM-based etching techniques. This improvement in speed is attributed to the lower viscosity of methyl alcohol and the reduction in enthalpy change compared with reactions occurring in water. By applying a top layer transfer technique to the graphite sample, controlled gas environment SPL could be used to generate graphene-based nanodevices such as sensors and electronic components. Figure 10 shows hexagonal patterns with 4.8 and 3.2 nm line widths produced by this technique.
Robert BogueAssociate Editor, Sensor Review