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Home > Press > Rice expands repertoire with MRI contrast agent: Metal-free fluorinated graphene shows no signs of toxicity in cell culture tests

Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene
This image from a high-resolution transmission electron microscope shows one of Rice University’s graphene-based MRI contrast agents, nanoparticles measuring about 10-nanometers in diameter that are so thin that they are difficult to distinguish.

Image courtesy of C.S. Tiwari/Rice University

Graphene, the atomically thin sheets of carbon that materials scientists are hoping to use for everything from nanoelectronics and aircraft de-icers to and bone implants, may also find use as contrast agents for magnetic resonance imaging (MRI), according to new research from Rice University.

Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows no signs of toxicity in cell culture tests

Houston, TX | Posted on November 10th, 2016

“They have a lot of advantages compared with conventionally available contrast agents,” Rice researcher Sruthi Radhakrishnan said of the graphene-based quantum dots she has studied for the past two years. “Virtually all of the widely used contrast agents contain toxic metals, but our material has no metal. It’s just carbon, hydrogen, oxygen and fluorine, and in all of our tests so far it has shown no signs of toxicity.”

The initial findings for Rice’s nanoparticles — disks of graphene that are decorated with fluorine atoms and simply organic molecules that make them magnetic — are described in a new paper in the journal Particle and Particle Systems Characterization.

Pulickel Ajayan, the Rice materials scientist who is directing the work, said the fluorinated graphene oxide quantum dots could be particularly useful as MRI contrast agents because they could be targeted to specific kinds of tissues.

“There are tried-and-true methods for attaching biomarkers to carbon nanoparticles, so one could easily envision using these quantum dots to develop tissue-specific contrast agents,” Ajayan said. “For example, this method could be used to selectively target specific types of cancer or brain lesions caused by Alzheimer’s disease. That kind of specificity isn’t available with today’s contrast agents.”

MRI scanners make images of the body’s internal structures using strong magnetic fields and radio waves. As diagnostic tests, MRIs often provide greater detail than X-rays without the harmful radiation, and as a result, MRI usage has risen sharply over the past decade. More than 30 million MRIs are performed annually in the U.S.

Radhakrishnan said her work began in 2014 after Ajayan’s research team found that adding fluorine to either graphite or graphene caused the materials to show up well on MRI scans.

All materials are influenced by magnetic fields, including animal tissues. In MRI scanners, a powerful magnetic field causes individual atoms throughout the body to become magnetically aligned. A pulse of radio is used to disrupt this alignment, and the machine measures how long it takes for the atoms in different parts of the body to become realigned. Based on these measures, the scanner can build up a detailed image of the body’s internal structures.

MRI contrast agents shorten the amount of time it takes for tissues to realign and significantly improve the resolution of MRI scans. Almost all commercially available contrast agents are made from toxic metals like gadolinium, iron or manganese.

“We worked with a team from MD Anderson Cancer Center to assess the cytocompatibility of fluorinated graphene oxide quantum dots,” Radhakrishnan said. “We used a test that measures the metabolic activity of cell cultures and detects toxicity as a drop in metabolic activity. We incubated quantum dots in kidney cell cultures for up to three days and found no significant cell death in the cultures, even at the highest concentrations.”

The fluorinated graphene oxide quantum dots Radhakrishnan studies can be made in less than a day, but she spent two years perfecting the recipe for them. She begins with micron-sized sheets of graphene that have been fluorinated and oxidized. When these are added to a solvent and stirred for several hours, they break into smaller pieces. Making the material smaller is not difficult, but the process for making small particles with the appropriate magnetic properties is exacting. Radhakrishnan said there was no “eureka moment” in which she suddenly achieved the right results by stumbling on the best formula. Rather, the project was marked by incremental improvements through dozens of minor alterations.

“It required a lot of optimization,” she said. “The recipe matters a lot.”

Radhakrishnan said she plans to continue studying the material and hopes to eventually have a hand in proving that it is safe and effective for clinical MRI tests.

“I would like to see it applied commercially in clinical ways because it has a lot of advantages compared with conventionally available agents,” she said.

Additional co-authors include Parambath Sudeep and Chandra Tiwary, both of Rice; Atanu Samanta and Abhishek Singh, both of the Indian Institute of Science at Bangalore; Kiersten Maldonado and Sendurai Mani, both of MD Anderson; and Ghanashyam Acharya of Baylor College of Medicine.

The research was supported by the Hamill Foundation through a Hamill Innovation Award to Rice’s Institute of Biosciences and Bioengineering, the Air Force Office of Scientific Research, the Indian Institute of Science at Bangalore’s Supercomputing Education Research Centre and India’s Indo-US Science and Technology Forum.


About Rice University
Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation’s top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,910 undergraduates and 2,809 graduate students, Rice’s undergraduate student-to-faculty ratio is 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for happiest students and for lots of race/class interaction by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger’s Personal Finance. To read “What they’re saying about Rice,” go to http://bit.ly/2g24RkJ .

Follow Rice News and Media Relations on Twitter @RiceUNews.


Rice University
Office of Public Affairs / News & Media Relations


Jeff Falk

Jade Boyd

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Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - Graphene

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Rice expands graphene repertoire with MRI contrast agent: Metal-free fluorinated graphene shows ... - GrapheneThe DOI of the Particle and Particle Systems Characterization paper is: 10.1002/ppsc.201600221:

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Soar cells made from an inexpensive and increasingly popular material called perovskite can more efficiently turn sunlight into electricity using a new technique to sandwich two types of perovskite into a single photovoltaic cell.

Perovskite cells are made of a mix of organic molecules and inorganic elements that together capture light and convert it into electricity, just like today’s more common silicon-based solar cells. Perovskite photovoltaic devices, however, can be made more easily and cheaply than silicon and on a flexible rather than rigid substrate. The first perovskite solar cells could go on the market next year, and some have been reported to capture 20 percent of the sun’s .

In a paper appearing online today in advance of publication in the journal Nature Materials, University of California, Berkeley, and Lawrence Berkeley National Laboratory scientists report a new design that already achieves an average steady-state efficiency of 18.4 percent, with a high of 21.7 percent and a peak efficiency of 26 percent.

Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries

Cross section of the new solar cell, showing the two perovskite layers (beige and red) separated by a single-atom layer of boron nitride and the thicker aerogel (dark gray), which prevents moisture from destroying the perovskite. Gallium nitride (blue) and gold (yellow) electrodes channel the electrons generated when light hits the solar cell.

Nature Materials – Graded bandgap perovskite solar cells

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Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries

Major advance in solar cells made from cheap, easy-to-use perovskite - Coatings Energy Forecast General Research Graphene Latest Innovations Optoelectronics Other Discoveries

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Xiaomi Launches Piston 3 Pro Earphones - Graphene

We have all been witnessing Xiaomi launching new products every now and then, though some products never reach other countries. And, one more such release is a new in-ear headphones, dubbed as ‘Piston 3 Pro’.

The Chinese company has announced that the new earphones will go on sale in China, starting this Friday, and no further information regarding its availability in other countries has been provided yet.

However, it is being said that the new earphone will be priced at CNY 149 that roughly translates to around INR 1,500. The earphones also come with 4 different sizes of ear plugs.

Xiaomi Launches Piston 3 Pro Earphones - Graphene

Technically speaking, the new earphones come with CNC diamond-cut aluminium sound chamber with anodised surface. The Xiaomi Piston 3 Pro also comes with a ‘Graphene diaphragm’, reports Gadgets 360. Also, the company explains on its official product page that this diaphragm helps in producing more natural sounds.

The Piston 3 Pro comes with three in-line control buttons which help in playing, pausing, and changing music. Users also get 1.25 metre length wire that comes with a matte finish. However, it is being said that the cable is not flat and thus, may be prone to tangle.

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Posted: Nov 09, 2016

(Nanowerk News) The Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS) has developed the world’s thinnest photodetector, that is a device that converts light into an electric current. With a thickness of just 1.3 nanometers – 10 times smaller than the current standard silicon diodes – this device could be used in the Internet of Things, smart devices, wearable and photoelectronics. This 2D technology, published on Nature Communications (“Unusually efficient photocurrent extraction in monolayer van der Waals heterostructure by tunneling through discretized barriers”), uses molybdenum disulfide (MoS2) sandwiched in . The thinnest photodetector in the world is just 1.3 nanometers thick - Graphene (top) Devices with one-layer and seven-layer MoS2 were built on top of a silicon base and compared. Dielectric constants responsible for the difference in electrostatic potentials are shown in parenthesis. (bottom) The device with one-layer MoS2 (inside the violet box) showed better performance in converting light to electric current than the seven-layer device (inside the pink box). (click on image to enlarge) Graphene is a fantastic material: It’s conductive, thin (just one-atom thick), transparent and flexible. However, since it does not behave as a semiconductor, its application in the electronics industry is limited. Therefore, in order to increase graphene’s usability, IBS scientists sandwiched a layer of the 2D semiconductor MoS2 between two graphene sheets and put it over a silicon base. They initially thought the resulting device was too thin to generate an electric current but, unexpectedly, it did. “A device with one-layer of MoS2 is too thin to generate a conventional p-n junction, where positive (p) charges and negative (n) charges are separated and can create an internal electric field. However, when we shine light on it, we observed high photocurrent. It was surprising! Since it cannot be a classical p-n junction, we thought to investigate it further,” explains YU Woo Jong, first author of this study. To understand what they found, the researchers compared devices with one and seven layers of MoS2 and tested how well they behave as a photodetector, that is, how they are able to convert light into an electric current. They found that the device with one-layer MoS2 absorbs less light than the device with seven layers, but it has higher photoresponsitivity. “Usually the photocurrent is proportional to the photoabsorbance, that is, if the device absorbs more light, it should generate more electricity, but in this case, even if the one-layer MoS2 device has smaller absorbance than the seven-layer MoS2, it produces seven times more photocurrent,” describes Yu. The thinnest photodetector in the world is just 1.3 nanometers thick - Graphene Mechanism to explain why the device with one-layer MoS2 generates more photocurrent than the seven-layer MoS2 one. (top) In the one-layer device MoS2 (right), the electron (red circle) has a higher probability to tunnel from the MoS2 layer to the GrT because the barrier (white arch) is smaller in that junction. In the seven-layers MoS2 device (left) instead, the energy barrier between MoS2/GrT and MoS2/GrB is the same so electrons do not have a preferred direction flow. More energy is generated in the one-layer MoS2 device because more electrons flow in the same direction. (bottom) Imagine that people want to cross a mountain without too much effort. If the mountains have different height (right), more people choose to climb (or better, to tunnel) the small mountain, while if the mountains have the same height (left), they do not have a preferred route. (Graphics modified from Freepiks) (click on image to enlarge) The monolayer is thinner and therefore more sensitive to the surrounding environment: The bottom SiO2 layer increases the energy barrier, while the air on top reduces it, thus electrons in the monolayer device have a higher probability to tunnel from the MoS2 layer to the top graphene (GrT). The energy barrier at the GrT/MoS2 junction is lower than the one at the GrB/MoS2, so the excited electrons transfer preferentially to the GrT layer and create an electric current. Conversely, in the multi-layer MoS2 device, the energy barriers between GrT/MoS2 and GrB/MoS2 are symmetric, therefore the electrons have the same probability to go either side and thus reduce the generated current. Imagine a group of people in a valley surrounded by two mountains. The group wants to get to the other side of the mountains, but without making too much effort. In one case ( the seven-layers MoS2 device), both mountains have the same height so whichever mountain is crossed, the effort will be the same. Therefore half the group crosses one mountain and the other half the second mountain. In the second case (analogue to the one-layer MoS2 device), one mountain is taller than the other, so the majority of the group decide to cross the smaller mountain. However, because we are considering quantum physics instead of classical electromagnetism, they do not need to climb the mountain until they reach the top (as they would need to do with classical physics), but they can pass through a tunnel. Although electron tunneling and walking a tunnel in a mountain are very different of course, the idea is that electric current is generated by the flow of electrons, and the thinner device can generate more current because more electrons flow towards the same direction. The thinnest photodetector in the world is just 1.3 nanometers thick - Graphene (a) Illustration of the device with the molybdenum disulfide (MoS2) semiconductor layer sandwiched between top (GrT) and bottom (GrB) graphene layers. Light (green ray) is absorbed and converted into an electric current. When light is absorbed by the device, electrons (blue) jump into a higher energy state and holes (red) are generated in the MoS2 layer. The movement of holes and electrons created by the difference in electronic potential between the GrT-MoS2 and the GrB-MoS2 junctions generates the electric current. Actually, when light is absorbed by the device and MoS2 electrons jump into an excited state, they leave the so-called holes behind. Holes behave like positive charges and are essentially positions left empty by electrons that absorbed enough energy to jump to a higher energy status. Another problem of the thicker device is that electrons and holes move too slowly through the junctions between graphene and MoS2, leading to their undesired recombination within the MoS2 layer. For these reasons, up to 65% of photons absorbed by the thinner device are used to generate a current. Instead, the same measurement (quantum efficiency) is only 7% for the seven-layer MoS2 apparatus. “This device is transparent, flexible and requires less power than the current 3D silicon semiconductors. If future research is successful, it will accelerate the development of 2D photoelectric devices,” explains the professor.

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It has been established that the thickness of the insulating membrane significantly affects the signal to noise ratio (SNR) [1] and spatial resolution [2, 3] in nanopore . Attempts to develop high-resolution resistive pulse sensors for DNA sequencing has encouraged the use of single or multilayer in nanopore devices [49]. The thickness of single-layer graphene is commensurate with the base stacking distance along the DNA backbone, making it an ideal membrane for high-resolution DNA analysis using solid-state nanopores [4]. Moreover, low-aspect-ratio graphene pores can be used for high-resolution probing of proteins, where they can enable detection of conformational changes and protein–protein interactions at the single-molecule level [10]. However, since the first use of graphene for nanopore devices in 2010, only a handful of reports have been published and the potential of graphene nanopores for single-molecule DNA and protein sensing has not been fully realized. This is due to the challenges associated with fabrication of functional graphene pores. The ability to produce graphene nanopore devices capable of analyte detection is limited by the strategies for transferring clean and defect-free graphene and fabricating nanopores of the desired shape and dimensions in the suspended graphene membrane and by the inherent hydrophobicity of graphene, which prevents molecular translocation through the pore.

Fabrication of graphene nanopores is a complex, labor-intensive and time-consuming process with multiple failure modes. The challenges associated with the individual steps result in a high failure rate, and new researchers face significant barriers upon entering the field. Some process improvements for individual steps have been demonstrated in the materials science literature; however, a comprehensive account of the complete fabrication process is missing; through this report we try to bridge this gap. Most graphene transfer recipes start with coating the graphene surface with a polymer layer (such as polymethylmethacrylate, PMMA), which protects the graphene and serves as a mechanical support during the transfer process. After the transfer is complete, the polymer is dissolved using acetone or another solvent; however, these solvents fail to completely remove PMMA, resulting in dirty membranes with large patches of support polymer remaining [1113]. Several different polymers [1416] and cleaning strategies [4, 13, 14] have been employed to minimize polymer residues on the graphene surface after the transfer. In comparison with solvent cleaning, thermal annealing in a gaseous atmosphere [13, 1719] or a vacuum [20] has been particularly useful in removing residual PMMA and has been shown to produce very clean graphene membranes. Even when nearly complete removal of PMMA from the graphene surface can be achieved; drilling nanopores in the extremely thin graphene using the high-energy electron beam of a transmission electron microscope (TEM), often results in the formation of over-sized pores. In the few journal articles published on graphene nanopores, problems such as pinholes [5], secondary holes, irregularly shaped pores [8] and complete membrane damage [9] during TEM drilling of graphene have been routinely reported (see online supplementary figure S3 for examples of such defects). The challenge is further complicated by the hydrophobicity of graphene, which prevents membrane wetting and consequently the translocation of analyte molecules, thereby rendering graphene pores unusable.

In this paper we report an optimized approach to the fabrication of graphene nanopores and address challenges associated with each fabrication step. The general approach presented here can be used to easily produce functional graphene pores capable of analyte detection. We demonstrate transfer of clean single-layer graphene onto pre-drilled SiO2/SixNy pores and fabrication of size-controlled nanopores in a suspended graphene membrane by electron beam assisted drilling and shrinking. We also show that the electron beam can be used to heal pinholes or any unwanted secondary holes formed during the pore drilling step. Our fabrication workflow also results in a naturally hydrophilic graphene surface, which facilitates pore wetting and analyte translocation. We demonstrate the use of our pores for investigation of single protein translocations and protein–protein complex formation with a high SNR. The experimental approach outlined in this paper can improve the fabrication yield of graphene nanopore devices and enable researchers to carry out single-molecule studies using extremely thin nanopores.

Results and discussion

Graphene transfer process

For the preparation of graphene nanopore devices, graphene grown by chemical vapor deposition (CVD) was transferred onto pre-drilled SiO2/SixNy pores using published recipes [6, 7] with modifications (figure 1). We started with drilling 500 nm diameter pores in free-standing silicon nitride membranes (50 nm thick) using a focused ion beam (FIB; FEI Strata DB 235 FIB) as described earlier [21, 22]. The chips with pores were then coated with a 100 nm thick layer of silicon dioxide on either side using pressure-enhanced CVD (PECVD). These SiO2/SixNy pores served as the receiving substrates onto which graphene was transferred (see below). CVD grown graphene (on Cu/SiO2/Si) was spin-coated (3000 rpm for 30 s) with 1% PMMA prepared in chlorobenzene. The silicon wafer was cut into 3 mm × 3 mm pieces and the SiO2 layer was etched using 7:1 buffered oxide etch (J.T. Baker) for 1 h. During this 1 h etching process, etchant eroded the SiO2 layer from all sides, while the graphene remained safely sandwiched between the PMMA and copper layers. Etching of the SiO2 released a PMMA/graphene/Cu multilayer structure floating in buffered oxide etch (BOE). After three quick washes in deionized water, the copper layer was etched in ammonium persulfate (APS Copper Etch 100, Transene Company Inc., Danvers, MA, USA) solution for 10 min, leaving PMMA/graphene layers floating on APS. Some protocols directly etch the copper layer from PMMA/graphene/Cu/Si or PMMA/graphene/Cu/SiO2/Si multilayer constructs by incubating them in APS for 30 min to 1 h. However, in the direct etching method the graphene layer is also exposed to APS after the copper layer has been etched away, causing chemical insult to the graphene membrane. The sequential etching of SiO2 and copper in our protocol minimized chemical damage to the graphene during the copper etching process since the copper was exposed to APS for only 10 min. Following copper etching, PMMA/graphene layers were washed three times in deionized water and were scooped and transferred onto SiO2/SixNy nanopore chips. The nanopore chips with PMMA/graphene were placed on a hot plate at an angle of 45° and dried (figure 2). Finally, PMMA was removed by two-step thermal annealing in ambient air using a Thermolyne (Thermo Fisher Scientific) benchtop muffle furnace. We heated the chips at 180 °C for 30 min followed by 400 °C for 2.5 h (see the following section for detailed reasoning).

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 1. Process flow for graphene transfer and fabrication of graphene nanopores. (a) A 500 nm pore was drilled in a free-standing silicon nitride membrane using a FIB. (b) A 100 nm layer of silicon dioxide was deposited on either side of the chip using PECVD. (c) SiO2 was etched for 1 h from the PMMA/graphene/Cu/SiO2/Si multilayer structure using 7:1 buffered oxide etch. (d) Copper was etched for 10 min from PMMA/graphene/Cu using ammonium persulfate at room temperature. (e) The PMMA/graphene layer was transferred over the nanopore chip prepared in step (b). (f) Chips were dried at an angle of 45° to achieve asymmetric drying in order to get rid of any residues left behind by evaporating water. (g) PMMA was removed by thermal annealing in ambient air at 180 °C for 30 min followed by 400 °C for 2.5 h. (h) The desired nanopore was then fabricated in the suspended graphene membrane using electron beam assisted drilling and shrinking.

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 2. Effect of symmetrical and asymmetrical drying on the quality of the transfer process. (a) After scooping PMMA/graphene from water onto nanopore chips, the chips were dried by placing them on a filter paper at an angle of 45°. The filter paper was placed on a hot plate maintained at 60 °C. (b) Placing the chips flat on the surface results in symmetrical drying of water from the chips. Since the etch pit and nanopore are in the center of the chip, symmetrical drying results in deposition of contaminants on the suspended graphene by evaporating water. (c) Placing the chips at an angle while drying makes the chips dry asymmetrically. The receding water meniscus leaves no contaminants in the central region of the chip, resulting in clean suspended graphene membranes.

Process improvements to produce clean free-standing graphene

In our graphene transfer process, two steps were critical for obtaining a very clean graphene surface: (a) asymmetrical drying of chips after transferring PMMA/graphene onto SiO2/SixNy pores and (b) removal of PMMA from the graphene surface using thermal annealing. Figure 2 summarizes the effect of the drying angle after transferring graphene on SiO2/SixNy pores. The final step (figure 1(e)) of the graphene transfer process is routinely carried out in deionized water. Nominally, this process results in water getting trapped in the etch pits of the nanopore chips. If the chips were dried flat on a surface, symmetrical evaporation of water left residues focused in the free-standing graphene area, resulting in dirty membranes (figure 2(b)). Placing the chips at an angle of 45° on a hot plate lead to asymmetrical drying, in which the water meniscus moved from the top edge of the graphene to the bottom edge, leaving no residues in the central area where the graphene membrane is suspended (figure 2(c)). Figures 2(b) and (c) also show low-resolution TEM images of 500 nm SiO2/SixNy pores with graphene suspended on them. In the case of symmetrical drying, significant residues were seen in the graphene area. Whereas upon drying the chips asymmetrically very clean graphene was obtained. The strategy of drying substrates at an angle of 45° after the graphene transfer process is particularly helpful when transferring graphene to nanopore devices, since water becomes easily trapped inside the etch pit in silicon. The other critical step for obtaining clean graphene is the removal of PMMA by thermal annealing. We performed thermal treatment in a two-step fashion in ambient air. The first annealing step was designed to remove any folds or strain from the transferred membrane and bring it into conformal contact with the flat surface of the receiving substrate. For the first step, an annealing temperature of 180 °C was maintained for 30 min, which melted the PMMA polymer layer (PMMA melting point = 160 °C) and relaxed the graphene membrane. The second annealing step was designed to boil and completely remove PMMA (PMMA boiling point = 200 °C) from the graphene surface. For the second step, surfaces were annealed at 400 °C for 2.5 h. The chips were placed at an angle of 45° inside the furnace during the thermal annealing process so that no residue was left in the center of the suspended graphene membrane when the PMMA was boiled away.

Figure 3(a) shows a low-resolution TEM image of a 500 nm SiO2/SixNy pore with graphene suspended over it. The box marked in red in figure 3(a) is shown at a higher magnification in figure 3(b). The red circle in figure 3(b) was used to collect the specific area electron diffraction (SAED) pattern for the graphene shown in figure 3(c). The characteristic diffraction pattern for graphene with hexagonal symmetry was obtained from our transferred graphene membranes. The SAED image was inverted (converted into a negative image) using ImageJ for better representation. The inset shows the inverse intensity plot for the diffraction spots marked by the red rectangle in figure 3(c). The high SNR seen in the inverse intensity plot indicates the presence of clean and crystalline graphene without significant PMMA (amorphous) residues. The intensity ratio between the first and the second nearest diffraction spots in the SAED pattern can be used to determine the number of graphene layers [23]. As seen in the plot, the intensities of the first spots (Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene are greater than those of the second spots (Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene indicating that the suspended membrane was single-layer graphene.

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 3. (a) TEM image of freely suspended graphene over a 500 nm diameter pore. (b) Magnified TEM image of the area enclosed in the red square in (a). (c) The SAED pattern shows hexagonal symmetry characteristic of graphene (the SAED pattern was obtained from area marked with red circle in (b)). The SAED image was inverted in ImageJ. The inset shows inverse intensity profiles of diffraction spots enclosed in the red rectangle in the SAED pattern. An intensity ratio greater than one for the first spots compared with the second spots suggests that the graphene is single layer [23]. (d), (e) Contact angle measurement for the graphene surface when PMMA was removed using acetone and by thermal annealing (in ambient air), respectively. Thermal annealing resulted in much cleaner and a hydrophilic graphene surface.

Improving the wettability of graphene nanopore devices to enable analyte detection

The hydrophobicity of graphene has proved to be a major impediment to using graphene nanopores for analyte translocation. This has prevented the optimal use of graphene nanopores for DNA and protein analysis. The problem of hydrophobicity of solid-state surfaces is routinely addressed by treating them with air/oxygen plasma; however, plasma treatment etches away graphene layers [24] and results in leaky membranes [9]. Several strategies have been adopted to improve the wettability of graphene nanopores, such as flushing the pores with ethanol [4], atomic layer deposition of titanium dioxide [5] and chemical passivation using amphiphilic molecules [9, 25]. While coating with TiO2 can provide a stable solution to improve the wettability of graphene, the process of using ethanol to make bare graphene pores hydrophilic is usually reversible. The strategies to chemically passivate graphene surface have also met with only moderate success. Shan et al reported that no protein translocations could be detected when graphene nanopores were passivated using mercaptohexadecanoic acid (or C16) and only ferritin (and no bovine serum albumin) translocations could be detected when pores were functionalized with phospholipid–polyethylene glycol (DPPE-PEG750) [9]. Schneider et al reported a reduction in nanopore fouling and more stable recording of DNA translocations when the graphene surface was passivated by the amphiphilic product of reaction between 1-aminopyrine and a N-hydroxysuccinimide ester derivative of a 4-mer ethylene glycol [25]. However, from the data presented by Schneider et al it appears that their chemical functionalization significantly increases the baseline noise, resulting in a low SNR [6, 25]. For double-stranded DNA translocations, a higher SNR has been obtained by using a 22 nm uncoated pore (figure 4(a) in Schneider et al [6]), compared with a 10 nm chemically coated pore (figure 4(a) in Schneider et al [25]). Based on these reports, we believe that chemical passivation is not the optimal solution for graphene hydrophobicity. Other methods need to be employed to improve graphene surface properties to achieve analyte translocation.

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 4. (a) Graphene drilling and shrinking scheme. A high current density electron beam at spot modes 1 and 2 was used to easily drill and expand the nanopores in graphene. When the drilled pores were exposed to a low current density beam at spot modes 3 or 4, the pores could be shrunk down to the desired size. The same process was also used to heal any unwanted secondary pores. (b)–(f) Evolution of shrinking nanopore under low current density electron beam exposure (spot mode 3, TEM at 120 keV). (g) Low-magnification view of the nanopore shrunk in (b)–(f). All scale bars: 15 nm.

In our process flow, the thermal annealing method used for removing PMMA from the graphene surface also resulted in naturally hydrophilic graphene. As shown in figures 3(d) and (e), our graphene samples exhibited a significantly lower water contact angle after thermal annealing (35.86°) compared with the samples for which PMMA was removed by dissolution in acetone (65.88°). For contact angle measurements, graphene/PMMA was transferred to SiO2/Si substrates and PMMA was either removed using thermal treatment (as discussed in the previous section) or acetone (see supplementary figure S2 for an optical image of transferred graphene on a SiO2/Si substrate). Recently it has been demonstrated that hydrophilicity of graphene can be improved by thermal annealing at 300 °C in ~0.15 l min−1 flow of O2/Ar (1:2) for 2 h, in order to use graphene as an ultrathin TEM sample support [26]. The authors reported that thermal annealing only resulted in minimal structural changes in graphene as revealed by Raman spectra [26]. In our experiments, similar results were obtained by thermal annealing of PMMA at 400 °C for 2.5 h in ambient air.

Thermal annealing significantly lowered the water contact angle for our transferred graphene; however, this alone was not sufficient to produce graphene nanopore devices capable of analyte translocation. Another critical step to produce functional pores was coating of the silicon nitride pore with silicon dioxide before transferring the graphene (figure 1(b)). We had started out with graphene transferred on FIB drilled pores in bare silicon nitride membranes, and our attempts to use such graphene nanopores for analyte detection were unsuccessful. We observed little or no conductance through the pores despite the low water contact angle observed after thermal annealing. The pore behavior was very similar to what we had experienced in the past with hydrophobic silicon nitride pores. For silicon nitride pores, the problem of hydrophobicity was routinely solved by plasma treating the chips for 2–3 min before use. However, we could not plasma treat the chips with a graphene membrane on them, as plasma etches away graphene and would have caused defects in the membrane [9]. We hypothesized that although graphene was hydrophilic (after thermal annealing), the hydrophobic nature of silicon nitride was preventing adequate chip wetting and ionic conduction through the pore. We needed a strategy to make silicon nitride hydrophilic without affecting the graphene membrane sitting on it. We attempted to thoroughly plasma treat the silicon nitride pore before graphene transfer; however, graphene transfer, PMMA removal and nanopore drilling in graphene takes a good amount of time. Within this time, silicon nitride reverts to the hydrophobic form. To address this issue, we coated the silicon nitride pore with 100 nm SiO2 on either side before the graphene transfer process (figure 1(b)). A SiO2 layer on silicon nitride pores made them permanently hydrophilic (see online supplementary figure S1 for contact angle comparison of bare SixNy and SixNy coated with SiO2) and it combined with the hydrophilic graphene membrane, resulted in fully functional graphene nanopores. These pores did not need any further surface treatment and were able to conduct ionic current immediately after assembly into the flow cell.

Graphene nanopore drilling and shrinking

As we mentioned earlier, the fabrication of graphene nanopores is a complex process, and nanopore drilling in suspended graphene membrane is one of the major failure modes. After successful transfer of graphene onto SiO2/SixNy pores, nanoscale pores are typically drilled in the suspended graphene using the electron beam of a TEM. When the high-energy electron beam of a TEM interacts with atomically thin graphene, carbon atoms are knocked out of the plane, resulting in membrane defects. If the electron beam is focused on a specific area of graphene, these atomic defects can grow and combine to form a nanopore. The minimum incident electron for knocking in-lattice carbon atoms out of the graphene plane is estimated to be around 86 keV [27], and 80 keV electrons are routinely used to prevent beam damage to graphene. However, nanopores can be efficiently sculpted in graphene by using electron energies above 140 keV [28]. Unfortunately, this pore drilling process is difficult to control and usually results in over-sized irregularly shaped pores along with other unwanted damage near the pore being drilled. This electron beam-induced insult to graphene can make the aforementioned transfer process useless. In our experiments, it became evident that the electron beam could be used to create pores in graphene as well as to shrink them. These competing effects observed during exposure of graphene to the electron beam were determined by the beam current density, which in turn could be controlled by the spot modes (SM) in the TEM (see below for a discussion on the pore shrinking mechanism). The lower spot modes (SM1 and SM2) output high beam current densities and result mostly in pore formation, whereas higher spot modes (SM3 and SM4) output low beam current densities and result in pore shrinking. We used a LaB6 thermionic emission TEM (JEOL 2100) operated at 120 or 200 keV for drilling, expanding and shrinking nanopores in the suspended graphene membranes. Both accelerating voltages could be used to drill nanopores when the electron beam was converged; however, 120 keV was less damaging to graphene and required a relatively longer time to drill. Figure 4(a) summarizes our graphene nanopore drilling and shrinking scheme. We took advantage of different spot modes to modulate the current density and cross section of the focused beam, to control the nanopore drilling and shrinking process. When operating the TEM at 200 keV, SM1 and SM2 were used for pore drilling and expansion whereas SM4 was used for shrinking. At 120 keV, SM1 and SM2 were used for drilling and expansion and SM3 was used for shrinking. In some instances, exposure at SM4 (at 200 keV) or SM3 (at 120 keV) could not induce pore shrinking, and in those cases, current density was further lowered using the next higher spot mode. Figures 4(b)–(f) show sequential images of a 25 nm pore shrinking to a 6 nm pore under beam exposure at spot mode 3 when the TEM was operated at 120 keV. Figure 4(g) shows a low-magnification image of the shrunk pore. For shrinking, the electron beam was fully converged on the already drilled nanopore but was spread out intermittently to monitor the size of the shrinking pore and capture the image. It took 3 min to shrink the pore (from 25 nm to 6 nm) presented in figure 4, and the process was stopped at 6 nm diameter, although smaller pore sizes and complete closure can be easily obtained following the same shrink–stop–image routine (see figure 6).

The drilling and shrinking kinetics for nanopores drilled at 200 keV are shown in figure 5. We report drilling kinetics for pores with initial diameters in the range of 40–60 nm because pores larger than 60 nm are seldom used in nanopore sensing, although they can be easily fabricated. The diameter values used in figure 5 were calculated as Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene where A is the area of the pore. The area of the pore was estimated by manually drawing the perimeter of the pore and calculating the area using ImageJ. As seen in figure 5(a), there was some lag in pore formation when drilling using spot mode 2, as compared with spot mode 1, due to the relatively lower beam current density. In figure 5(b), each line on the graph represents the shrinking progression of an individual nanopore. We observed that there was always a 2–3 min time lag before the pores started to shrink. We believe this 2–3 min lag was needed for the adatoms to migrate to the vicinity of the pore and trigger pore shrinking. Nanopores with initial diameters of 40–60 nm could be shrunk down to 2–5 nm within 7–9 min. Once the pores had shrunk down to 10 nm, further shrinking to 2–5 nm was very rapid. At this stage, imaging the pores using a partially converged beam could also result in pore shrinking; however, the shrinking rate was much slower compared with using the fully converged beam. Nevertheless, it allowed for simultaneous imaging and shrinking, enabling us to precisely tune the size of the pore. We were able to capture a real-time pore shrinking video while the pore was shrunk down to 2 nm (see online supplementary video 1).

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 5. (a) Graphene nanopore drilling kinetics at spot modes 1 and 2 when the TEM was operated at 200 keV. Pores were drilled and expanded by converging the beam on the same area on the suspended graphene membrane. (b) Pore shrinking kinetics at spot mode 4 when the TEM was operated at 200 keV. Each line on the graph represents shrinking of an individual pore.

Shrinking of nanopores by in situ heating and electron beam irradiation has recently been reported [29, 30]. It was shown that heating graphene pores to 400–1200 °C using a thermal specimen holder inside the TEM column can result in pore shrinking (22 min to shrink a 9 nm pore) [29]. Xu et al also drew a correlation between initial pore diameter and the membrane thickness to ascertain if the pore would expand or shrink [30]. Shrinking of nanopores in multilayer graphene [7] and magnesium alloys has also been demonstrated using electron beam irradiation [31]. Our shrinking results are similar to those obtained earlier; however, we demonstrate pore shrinking in single-layer graphene, our method is faster, does not require in situ heating and provides more control over the shrinking process.

Perhaps one of the biggest advantages of our method for shrinking and fine-tuning the nanopore diameter is the ability to heal pinholes and unwanted defects without affecting the principal pore. As discussed earlier, drilling nanopores in suspended graphene often results in unwanted membrane damage (see online supplementary figure S3 for examples). Similar to pore shrinking, low-density electron beam exposure can also result in complete healing of pinholes in the graphene membrane. Our ability to heal unwanted membrane damage is demonstrated in figure 6; here peripheral pores were surgically healed and completely closed before the principal pore was size-tuned and shrunk to the desired size. For this demonstration two large pores (30–35 nm diameter, irregularly shaped and <10 nm apart) were created. In figure 6(a), the desired pore is marked with a white arrow, the large peripheral pore is marked with a black arrow and small pinholes are marked with red arrows. Although the distance between the principal pore and the peripheral pore was <10 nm, by converging the beam only on the peripheral pore (black arrow) it could be completely closed while almost maintaining the size of the principal pore (figures 6(b)–(d)). This also resulted in complete healing of the small pinholes (red arrows). After the peripheral pore was completely closed, the principal pore was also shrunk down, as seen in figures 6(e)–(h). The ability to fine tune the size of the nanopores and to heal the unwanted damage caused by the electron beam can significantly increase the usability of graphene nanopores.

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 6. Surgical shrinking of pores in a graphene membrane. (a) The principal pore is marked with a white arrow, an unwanted peripheral pore is marked with a black arrow and some small pinholes are marked with red arrows. (b)–(d) A low-density electron beam was focused on the peripheral pore (black arrow) and it was fully closed along with the pinholes. (e)–(h) After the peripheral pores were closed, the electron beam was converged on the principal pore and it was shrunk down to a 6 nm pore. All scale bars: 25 nm.

The phenomenon of pore shrinking in graphene has been attributed to the migration of carbon adatoms from adjacent areas to the vicinity of the pore, where they combine with the unsaturated carbon atoms at the pore edge and form stable bonds [29, 30]. The adatoms or self-interstitials are the adsorbed carbon atoms occupying the position between C–C bonds and protruding out of the graphene plane. These adatoms are generally introduced in the graphene structure during growth, chemical treatment or when carbon atoms are knocked out by irradiation (pore drilling) or due to the hydrocarbon adsorbed on the sample. It is worth noting that although we were able to obtain a very clean graphene surface with our transfer recipe, a small amount of PMMA residue and adatoms could still be present and may contribute to pore shrinking. The adatoms can easily migrate along the graphene surface and combine with the unsaturated carbon atoms to facilitate self-healing (or pore shrinking in our case) or combine with other adatoms to form aggregates or hillocks [32, 33]. The activation energy required for migration of adatoms has been documented to be about 0.4 eV, making them highly at elevated temperatures and during electron beam irradiation [34]. In our experiments, electron beam irradiation-induced shrinking was also accompanied by the formation of onion-like graphitic structures around the nanopore and in the shrinking zone when the membrane was irradiated for a long time (see online supplementary figure S4). The graphitic onions became more prominent with longer irradiation times [35] and we believe they are a result of adatom hillocks formed around the nanopore. In comparison with earlier works on graphene nanopore shrinking using a thermal specimen holder to heat up the whole sample [29, 30], our nanopore shrinking process is believed to be driven by the creation of a very localized thermal gradient around the converged electron beam, enabling us to surgically close the unwanted pores in the membrane.

Detection of protein translocation using graphene nanopores

Graphene nanopores have so far been used for DNA analysis, and much less attention has been given to using these devices to detect protein translocations. The motivation for using graphene nanopores for studying protein translocation is to be able to detect different conformational states of proteins, protein–protein interaction and determination of peptide sequences for single protein molecules in real time. Recently, the use of graphene nanopores for protein detection has been explored using molecular dynamics simulations [36, 37]. The authors simulated detection of different protein conformations [36] and multi-step unfolding events [37] when proteins translocate through atomically thin graphene nanopores. So far, only one report on experimental exploration of protein translocation through graphene nanopore is available and, in our opinion, the translocation data presented in the report [9] (current drop ~50 pA, translocation time = 40 ± 20 ms for ferritin translocation at 400 mV) are not characteristic of translocations through a graphene-like thin nanopore. Protein translocation was detected after attempting to make graphene nanopores hydrophilic by coating with DPPE-PEG750 [9] and low SNR translocation data indicate insufficient pore wetting and transient protein–pore interactions. Detection of protein translocation through solid-state nanopores is believed to be more challenging than DNA translocation because of heterogeneous charge distribution, the presence of hydrophobic domains and the relatively low net surface charge density of proteins. In such cases, surface properties of nanopores play a very critical role in minimizing protein–pore interactions and facilitating smooth translocation of protein through the nanopores. The inability to reliably control the surface properties of graphene nanopores has been a major impediment in their use for studying protein translocation.

In order to evaluate the utility of our graphene nanopores, we used them to detect protein translocation and protein–protein interactions. We used antibody–antibody interaction of immunoglobulin-G (IgG) as a model system and studied interaction between Fc-specific rabbit anti-goat IgG (hereafter IgG) and an Fc fragment of goat IgG (hereafter Fc fragment) (Jackson Immuno Research, West Grove, PA, USA) using a 25 nm graphene nanopore. Such protein–protein interactions are routinely used in immunostaining methods where a primary antibody binds to a target antigen and then fluorescently tagged secondary antibodies (specific to the Fc region of the primary antibody) bind to the primary antibody, resulting in signal amplification. Before starting protein experiments, nanopore conductance was measured using 1 M KCl (pH 8, buffered with Tris-EDTA). The graphene nanopore chip was assembled in the flow cell without any pre-treatment and was used within 12 h of pore drilling. Soon after the pore had been assembled in the flow cell and flushed with KCl solution, a stable baseline was observed, characteristic of a sufficiently wet nanopore. Such pore wetting and stable conductance could not be observed when the silicon nitride surface was not coated with silicon dioxide before the graphene transfer process (data not shown). We obtained a conductance of 201.6 nS, which is in agreement with the values previously reported [38]. For translocation experiments, the antibodies were dispersed in 0.2 M KCl (at pH 5.5, buffered using acetate buffer) at a final concentration of 200 nM. IgG is a 150 kDa protein with diameter 12 nm and an isoelectric point ranging from 6.4–9.0 [39], whereas the Fc fragments are 60 kDa proteins with a diameter of around 5 nm. We chose an operating pH of 5.5 for the antibodies to have a dominant positive charge. First, only the IgG molecules were added to the cis chamber of the flow cell and a transmembrane voltage of −400 mV was applied. This resulted in a stream of antibody translocation events, with an event frequency of ~42 events/min. Figure 7(a) (i), shows representative resistive pulses obtained during translocation of IgG molecules. Event statistics for antibody translocation events are captured in figure 7(b), which shows a current drop versus translocation time scatter plot (n = 844) along with the marginal histograms. The histograms of current drop values and translocation time were fitted with Gaussian and log-normal curves, respectively, to obtain the mean values. We observed a mean current drop value of 1420.81 pA (SD = 481.3 pA) and a mean translocation time of 121.76 μs (SD = 27.99 μs).

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

Figure 7. Investigation of protein–protein interaction using a graphene nanopore. (a) Current trace showing resistive pulses corresponding to individual protein translocations for Fc-specific IgG (i), Fc fragment (ii) and Fc–IgG complex. (b) Current drop and translocation time scatter plot and marginal histograms (n = 844)  for IgG translocation data. A mean current drop value of 1420.81 pA (SD = 481.3 pA) and mean translocation time of 121.76 μs (SD = 27.99 μs) were obtained by fitting distribution curves to the population histograms. (c) Current drop histogram for Fc translocation events (n = 223). A mean current drop value of 446.18 pA (SD = 81.05 pA) was obtained by curve fitting. (d) Current drop histogram for Fc–IgG complex translocation events (n = 402). The bimodal distribution results from the presence of free IgG molecules and IgG molecules bound to Fc fragments. Mean current drop values of 1241.58 pA (SD = 267.53 pA) and 1989.10 pA (SD = 267.08 pA) were obtained by curve fitting. (e) Fractional current drop probability density plots for pure analytes (IgG and Fc) and protein complexes (Fc bound to IgG). The curve for Fc (blue trace) is plotted at half the probability density for better representation. All graphs and curves are color coded as free Fc (blue), free IgG (red) and Fc–IgG complex (green).

Following the antibody translocation experiment, the flow cell was thoroughly washed and Fc fragments were introduced and detected in the similar manner. The current drop signals obtained for Fc fragment translocation events are shows in figure 7(a) (ii). Because of the small size of Fc protein (compared with the nanopore diameter), the SNR of translocation events was lower than the SNR observed for antibody translocation. As a rule of thumb, a high SNR can be obtained in nanopore experiments when 0.4dpore < danalyte < 0.8dpore; however, in the case of the Fc fragment, the diameter of the protein was only 0.24dpore, resulting in a low SNR. The current drop statistics for Fc translocation are presented in figure 7(c). Similar to the antibody data, a histogram was created for the current drop values and log-normal curve fitting was performed to obtain an average current drop value of 446.18 pA (SD = 81.05 pA). In addition to the 446.18 pA peak captured by curve fitting, there is an additional bump in the histogram around 850 pA (figure 7(c)), which can be attributed to existence of Fc dimers in the solution.

After studying the translocation behavior of individual protein components, their interaction with each other was investigated. For this experiment, Fc fragments were mixed with anti-Fc IgG in a limiting concentration (Fc:IgG 1:2) and incubated at room temperature for 2 h. After incubation, the protein complex was introduced into one of the chambers of the flow cell and translocation events were captured under similar experimental conditions. Figure 7(a) (iii) shows the characteristic signals obtained for translocation of protein–protein complexes. When the current drop data were plotted as a histogram, we observed a bimodal distribution, with one peak close to the current drop values originally observed for the pure antibody sample and a new peak shifted towards higher values (figure 7(d)). The first peak indicated the existence of free IgG molecules in the solution and the second shifted peak suggested formation of Fc–IgG complexes and an increase in the size of protein molecules. The protein–protein interaction behavior observed here is similar to interaction between HIV protein gp120 (5 nm) and anti-gp120 IgG reported earlier by our group [40]. We fitted the histogram with Gaussian mixed model to obtain the characteristic values for the two peaks. The average values obtained after curve fitting for the first and the second peaks were 1241.58 pA (SD = 267.53) and 1989.10 pA (SD = 267.08) respectively.

For low-aspect-ratio pores like the one used in this study the diameter of the molecule is much larger than the pore length and the ionic current blockades can be directly related to the cross-sectional area of the pore occupied by the translocating molecule. The magnitude of the current blockades can be estimated by the following simple relationship [41]:

Hydrophilic and size-controlled graphene nanopores for protein detection - Graphene

where ΔI is the magnitude of the current blockades, I0 is the baseline current and dmolecule and dpore are diameters of the molecule and the nanopore, respectively. We plotted the ΔI/I0 values for the three analytes in a probability density plot, shown in figure 7(e). Considering a 25 nm pore, the right-hand side of above equation results in 0.04, 0.23 and 0.46 for a 5 nm Fc fragment, 12 nm IgG and 17 nm Fc–IgG complex, respectively. The peak values we obtained in the probability density plot agree very nicely with the theoretical values calculated based on size, except for the Fc fragment. The overestimation of ΔI/I0 values for Fc can be a result of bias towards larger current drop events because of the low SNR signal for the small protein. The clear peak separation observed in the current drop histogram and probability density curve for Fc–IgG complexes (figures 7(d) and (e) green trace), demonstrate the advantage of ultrathin graphene nanopores for distinguishing the sub-populations existing in a protein sample.


In summary, we demonstrate a robust way to transfer clean, defect-free and hydrophilic graphene onto nanopore chips and fabricate size-controlled nanopores in a suspended graphene membrane using electron beam-induced drilling and shrinking. Electron beam-induced shrinking was also used to heal any secondary unwanted pores, resulting in functional graphene nanopore devices. Our protein experiments demonstrate the utility of graphene nanopores for investigating single protein molecules and protein–protein interactions, which as far as we know has not been reported before. The chip preparation routine presented here improves wettability of the graphene membrane and allows for detection of complex analytes with a low charge to mass ratio.


Graphene fabrication

The graphene layer was synthesized on the copper surface by a CVD process using RF plasma. A 300 nm thick copper film deposited on top of a silicon wafer with a 300 nm thick silicon dioxide (SiO2) layer, was inserted in an inductively coupled plasma (ICP) CVD system. After ramping up the temperature to 725 °C under Ar ambient at 50 mTorr, the sample was treated by H2 plasma with a gas flow rate of 40 sccm and RF plasma power of 50 W for 2 min. A gas mixture of Ar and C2H2 (Ar:C2H2 = 40:1 sccm), was then flowed into the chamber with 150 W RF plasma for graphene synthesis.

Experimental set-up for detection of protein translocation

The graphene nanopore chip was sandwiched between two PDMS gaskets and was assembled in a custom-built flow cell. The electrolyte filled cavities served as the cis and trans chambers. Ag/AgCl electrodes were inserted into the two electrolyte chambers and were connected to a Molecular Devices Axopatch 200B patch clamp amplifier. The current data were sampled at 200 kHz, digitized using a MD Digidata 1440 A digitizer and analyzed using pClamp 10.3 software. Recorded data were pre-conditioned for analysis by electronic low-pass Bessel filtering (10 kHz) and manual baseline correction.


This work was financially supported by the National Science Foundation Nanomanufacturing Program (CMMI 1345000) and the Korea National Research Foundation grants NRF-2015K1A4A3047100, Global Frontier (CASE-2011-0032147) and NRF-2014M3A7B6034494

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Nano-scale electronics score laboratory victory - Coatings Electronics Energy General Research Graphene Latest Innovations Medical Optoelectronics Other Discoveries Water Purification

At just one atom thick, tungsten disulfide allows to switch off and on — important for nano-scale electronic transistors — but it also absorbs and emits light, which could find applications in , sensing, and flexible . The NYU logo shows the monolayer material emitting light. Researchers at NYU Tandon reported success in growing the promising monolayer material. Credit: NYU Tandon

Researchers at the NYU Tandon School of Engineering have pioneered a method for growing an atomic scale electronic material at the highest quality ever reported. In a paper published in Applied Physics Letters, Assistant Professor of Electrical and Computer Engineering Davood Shahrjerdi and doctoral student Abdullah Alharbi detail a technique for synthesizing large sheets of high-performing monolayer tungsten disulfide, a synthetic material with a wide range of electronic and optoelectronic applications.

“We developed a custom reactor for growing this material using a routine technique called . We made some subtle and yet critical changes to improve the design of the reactor and the growth process itself, and we were thrilled to discover that we could produce the highest quality monolayer tungsten disulfide reported in the literature,” said Shahrjerdi. “It’s a critical step toward enabling the kind of research necessary for developing next-generation transistors, wearable electronics, and even flexible biomedical devices.”

The promise of two-dimensional electronic has tantalized researchers for more than a decade, since the first such material—graphene—was experimentally discovered. Also called “monolayer” materials, and similar two-dimensional materials are a mere one atom in thickness, several hundred thousand times thinner than a sheet of paper. These materials boast major advantages over silicon—namely unmatched flexibility, strength, and conductivity—but developing practical applications for their use has been challenging.

Graphene (a single layer of carbon) has been explored for electronic switches (transistors), but its lack of an energy poses difficulties for semiconductor applications. “You can’t turn off the graphene transistors,” explained Shahrjerdi. Unlike graphene, tungsten disulfide has a sizeable . It also displays exciting new properties: When the number of atomic layers increases, the band gap becomes tunable, and at monolayer thickness it can strongly absorb and emit light, making it ideal for applications in optoelectronics, sensing, and flexible electronics.

Efforts to develop applications for monolayer materials are often plagued by imperfections in the material itself—impurities and structural disorders that can compromise the movement of charge carriers in the semiconductor (carrier mobility). Shahrjerdi and his student succeeded in reducing the structural disorders by omitting the growth promoters and using as a carrier gas rather than a more common choice, argon.

Shahrjerdi noted that comprehensive testing of their material revealed the highest values recorded thus far for carrier mobility in monolayer tungsten disulfide. “It’s a very exciting development for those of us doing research in this field,” he said.

Nano-scale electronics score laboratory victory - Coatings Electronics Energy General Research Graphene Latest Innovations Medical Optoelectronics Other Discoveries Water Purification Explore further: Method for creating high-quality two-dimensional materials could enable industrial-scale production

More information: Abdullah Alharbi et al. Electronic properties of monolayer tungsten disulfide grown by chemical vapor deposition, Applied Physics Letters (2016). DOI: 10.1063/1.4967188

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An electrical noise is one of the key parameters determining the performance of modern electronic devices. However, it has been extremely difficult, if not impossible, to image localized noise sources or their activities in such devices. We report a “noise spectral imaging” strategy to map the activities of localized noise sources in domains. Using this method, we could quantitatively estimate sheet resistances and noise source densities inside graphene domains, on domain boundaries and on the edge of graphene. The results show high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. Additionally, we showed the upper layer in double-layer graphene had lower noises than single-layer graphene. This work provides valuable insights about the electrical noises of graphene. Furthermore, the capability to directly map noise sources in electronic channels can be a major breakthrough in electrical noise research in general.

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Graphene as a high-mobility semiconductor has been considered as the most prospective sensitive material in novel sensor development. Its sensor applications for the detection of electrically charged analytes in aqueous-media has become an attractive hotspot over the recent decade. nanosensors designed under the field-effect transistor (GFET) configuration can transduce analyte-induced electric excitation into device conductivity response that enable simple sensing operations via the measurement of electrical current. In this case, graphene with outstanding carrier mobility confers GFET nanosensors significantly improved sensitivity, compare with other sensitive materials.A newly structured high-κ solid-gate graphene nanosensor - Graphene

While the GFET nanosensors have made great progress, the devices were mostly configured based on a classical liquid-gate transistor structure, which supply gating electrical field through aqueous-solutions. This configuration undesirably requires an external metal wire as the gate electrode limits the device integration degree and hinders the device practicability in the on-site applications. For a compact structure, the back-gate GFET configuration has also been proposed, however, it usually requests insecure high gate voltage and exhibits low sensitivity. Electrical analyses attribute these shortcomings to the low gating capacitance provided by the SiO2 dielectric layer, compared with solutions. Although the back-gate GFET configuration was rarely used, it lightens an inspiration that a planar gating configuration with high permittivity may enable high device practicability and ideal sensitivity, concurrently.

A newly structured high-κ solid-gate GFET nanosensor is proposed by Prof He and his team. By employing a planar gate electrode buried by high-κ HfO2 dielectric layer, the GFET nanosensor simultaneously achieves a fully integrated structure for the practical applications, and significantly enhances the sensitivity performance over the conventional liquid-gate GFET nanosensors under the safe gate voltage. Theoretical discussion suggests that the sensitivity improvement is attributed to the enhancement of gating capacitance, the carrier mobility of graphene is well-preserves. These results also suggest that more advanced solid-gating structures suppling either thinner dielectric layer or higher permittivity may further improve the device’s performance.

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Smart composite materials have become essential in 21st century lifestyle. We can find them in phone , diabetes , thermo-jackets or aircraft blades. Composite materials combine structural and functional capabilities and the recent integration of into ceramic and polymeric matrices could open new opportunities on the design novel composites. However, to materialize this goal we need to be able to design and build structures to take full advantage of graphene’s (monoatomic layer of carbon) unique properties, such as it being the material of highest electrical conductivity or electron mobility. In this project we shall use a bottom up strategy to design new graphene/ceramic composite materials. Taking inspiration from biomaterials hierarchical structures will be developed. We will explore graphene oxide (chemically exfoliated graphite) suspensions processing in order to build 3D graphene networks which in turn will serve as scaffolds to host ceramic and ceramic based materials. And we will take advantage of novel technical ceramic manufacturing routes such as freeze casting or to fabricate multi-materials and build devices The challenge is to design, build up, and evaluate a new composite with damage sensing capabilities. The goal “a novel composite with damage self-monitoring capabilities” would for instance save unexpected break-up stop costs in a broad range of applications from energy to transportation.

In this project a wide variety of skills and techniques will be developed and used. The first stage of the project will deal with characterization and processing of materials and different techniques such as particle size distribution, zeta potential, XRD, SEM, TEM among others will be used. Skills in wet processing of graphene and ceramics together with their shaping by 3D printing and/or freeze casting will also be acquired and thus fundamental learning in rheology will be developed. Finally the achievement of the new composite will also demand training in mechanical and electrical characterization. The project will establish external collaborations with the Centre of Advanced Structural Ceramics at Imperial College London, Queen Mary University London and collaborations with graphene and carbon based materials suppliers such as “GRAPHENEA” and “HAYDALE” may also arise. Moreover, across college collaborations may be also possible due to the multidisciplinary character of the research proposed. For instance Dr Wayne Nishio Ayre at Cardiff School of Dentistry will bring his experience in the design, development and testing of devices and biomaterials. His experience will help us to find new applications for the novel composite. Research on is carrying out At the School of Physics and Astronomy they are looking at Nanodiamond and nanocarbons and we will find support to understand and measure graphene properties at atomic scale.
The ground breaking materials and methods for material fabrication that will be developed within the scope and throughout the duration of this project will definitely contribute to the continuation of UK’s industrial competitiveness.

Candidates should hold or expect to gain a first class degree or a good 2.1 and/or an appropriate Master’s level qualification (or their equivalent).

Applicants whose first language is not English will be required to demonstrate proficiency in the English language (IELTS 6.5 or equivalent)

Funding Notes

The studentship is funding through the EPSRC Doctoral Training Partnership and Cardiff School of Engineering. It consists of full UK/EU tuition fees, as well as a Doctoral Stipend matching UK Research Council National Minimum (£14,296p.a. for 2016/17, updated each year). Additional funding is available over the course of the programme and will covers costs such as research consumables, training, conferences and travel.

Eligibility: We welcome applications from both UK and EU applicants.


In the first instance interested applicants are invited to send a CV and covering email/letter to:

[email protected]


Shortlisted candidates will be invited to attend an interview after the closing date.

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Benefit from Graphene in Your Composite Development - Graphene

Meet and interact with experts at the Graphene Connect workshop taking place at Messe Düsseldorf, conference room 814 A & B, during Composites Europe in Düsseldorf, Germany on 29 November to 1 December 2016.

Graphene is an innovative carbon-based material made of robust, flexible, electrically conductive sheets, having a thickness of one atom and a lateral size of several microns. “It can be produced on large scale from the exfoliation of conventional graphite. Due to its unique combination of superior properties graphene can improve the functionality of composite materials. It is lightweight but at the same time incredibly strong, transparent, conducts heat and electricity and has a large surface area,” states Vincenzo Palermo, Graphene Flagship leader of Polymer Composites and keynote speaker at the workshop.

The Graphene Flagship states that it carries out advanced research, creating unique and innovative materials by incorporating graphene into composites. There are many potential applications including and industries, applications, structural foams, films and .

Join the Graphene Flagship workshop and learn more and interact with Europe’s leading graphene experts.





Photo provided by Graphene Connect

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