Promising applications of graphene and graphene-based nanostructures

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Promising applications of graphene and graphene-based nanostructures

Bich Ha Nguyen'-^ and Van Hieu Nguyen'-^

'Advanced Centtt of Physics and hoimie of Materials Science, Viemam Academy of Science aid Technology VAST, 18 Hoang CJuoc Viet Cao Giay, Hanoi. Viemam

Univenity of Engineering and Technology, Viemam National Umversity Hanoi VNUH M4 Xuan Thuv Cau Gray, Hanoi, Viemam

E-mail: [email protected] and [email protected] Received 4 March 2016

Accepted for publication 4 April 2016 A f l l N Published 28 April 2016 \ H / AbMrafl <='"*'•"' The present aittcle is a review of researeh works on pramising applications of gmphene and

giaphene-based nanosmicMres. It contams five maul scientific subjecB. The first one is the raearch on graphene-basod transparent and flexible conductive fihns for displays and electnxles- elficienl mediod ensunng unifotm and controllable deposition of reduced graphene oxide thin films over large areas, large-scale pattem growth of graphene fihns for sttstchble transpaitnt electrodes, ntihzanon of graphene-based mnsparen, conductmg films and graphene oxide-based ones m many photomc and optoelectronic devices and equipments such as the window elKBodes of morganic, organic aod dye-sensifized solar cells, organic hght-omittmg diodes Itght-emittmg electrochemical cells, touch screens, flexible smart windows, graphene-based saturated absorbe,^ m laser cavities for nltrafast generadons, graphene-based flexible, nimsparent Jo h T 'T "^ defoggmg/deichig systems, healable smart windows, graphene elecLles for high-performance orgamc field-eCfect transistors, flexible and transparent acoustic actuators and nantigenerators etc. The second sciendfie subject is the research on conductive inks fo, l ^ t TJT"°" ' ° " ' " ' ' • " ; ' ' " ' « *= =''«"'°'= " " ' ' " y by pnniucing cost-effecdve electronic cncmts and sensors m vety large quantities: preparing high mobihty printable semiconductors tow sintermg temperanire conducting mks, gniphene-based ink by Uquid phase exfoliation of graphite in orgamc soluuons, and developmg Inkjet printing technique for mass production of high-quahly graphene panems widi high resoludon and for fabricadng a variety of good- performance electronic devices, including n»isparent conductors, embedded ri»istois, fldn-fihn mnsistor, and micio supercapacitors. The third scientific subject is die res«nch on grapbene- baa:d ^paranon membranes: molecular dynamics simuladon smdy on die mechanisms of die m ^ p o r i of molecules, vapors and gases dirough nanopores in graphene membranes, experimental works mveshgadng selecdve transport of differem molecules dmmgh nanopores in smgle-layer graphene and graphene-based membranes toward die water desdmiion ctordcal

™ , ^ ' T r T f ^^ """^'- ^ " * ™ ' 'PPUcations of graphene m bic^medicine are d,e conleuB of 0,e fomdi sciennfic subject of die review. Tliey include die DNA translocadons through nanopores m graphene membranes toward die fabricadon of devices for genomic screemng, m particular DNA sequencmg, subnanometre dans-elecdode m e m b r a n e s "

R ^ ^ « Onginal conlent from ihis wori; may be used under the wms m^^^^ of Ihe Creaave Commons AUnbulion 3,0 licence Anv fimbcr d-^buDon of this work mus. ma.niain artnbuuon to the authorfs) and the Uifc of the wort, jounul citation and DOI.

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iy ol Soence & Tectmotogy

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Adtf, Nat SCI.. Nanosa Nanoteehnol, 7 12016) 023002

potential appUcations to the fabrication of very high resolution, high throughput nanopore-based single-molecule detectors; antibacterial activity of graphene. gr^hite oxide, graphene oxide and reduced graphene oxide; nanopore sensors for nucleic acid analysis; utilization of graphene multilayers as the gates for sequential release of proteins from surface; utilization of graphene- based eleclroresponsive scaffolds as implants for on-demand drug delivery etc. The fifth scientific subject of die review is tiie research on tiie utilization of gr^hene in eneigy storage devices: ternary self-assembly of ordered metal oxide-graphene nanocomposites for electrochemical energy storage; self-assembled graphene/carbon nanotube hybrid films for supercapacitors; carbon-based supercapacitors fabricated by activation of graphene;

functionalized graphene sheet-suUure nanocomposite for using as catiiode material in rechaigeable litiiium batteries; tunable ttu^-dimensional pillared carbon nanotube-graphene networks for high-performance capacitance; fabrications of electrachemical micro-capacitors usmg fliin films of carbon nanotubes and chemically reduced graphenes; laser scribing of high- performance and flexible graphene-based electrochemical capacitors; emergence of next- generation safe batteries featunng graphene-supported Li metal anode witfi exceptionally high energy or power densities; fabrication of anodes for lithium ion batteries from crumpled graphene-encapsulated Si nanopartides; liquid-mediated dense integration of graphene materials for compact capacitive energy storage; scalable fabrication of high-power graphene micro- supercapacitors for flexible and on-chip energy storage; superior micro-supercapacitors based on graphene quantum dots; all-graphene core-sheat microfibres for all-solid-state, stretchable fibnform supercapacitors and wearable electronic textiles; micro-supercapacitors wifli high electrochemical perfonnance based on diree-dimensional graphene-carbon nanoUibe carpets- macroscopic mtrogen-doped graphene hydiogels for ultrafast capacitors; manufacture of scalable ulM-tiun and high power density graphene electrochemical capacitor electrodes by aqueous exfoliation and spray deposition; scalable syntiiesis of hierarchically structiired carbon nanotube- graphene fibers for capacitive energy storage; phosphorene-graphene hybrid material as a high- capacity anode material for sodium-ion batteries. Beside above-presented promising applications of graphene and graphene-based nanostructures. otiier less widespread, but perhaps not less important, applications of graphene and graphene-based nanomaterials, are also briefly

discussed. •' Keywords: graphene, graphene oxide, mmsparent, flexible, Inkjet, micio-supereapacitor

Class,ficalionnumbeis:4.00,4.10, 5.01,5 15 F y lu, 1. Introduction

The discovery of graphene by Novoselov « oHl] has opened a new and very promising scicendfic area which has emerged like 'a rapidly rising star on tlie Itorizan ofinalerials science and condensed-matter physics', and revealed 'o cornucopia of new pliystcs and potential applications' [2J. Since diat dme .several reviews on die haste researeh as wdl as on die efficient applicanons of graphene and graphene-based nanos- tmcmres were published [3-7]. Recent advances in expenmenul bas,c research on graphene and graphene-based nanomalenals were reported in our previous review [81 The purpose of present work is to review promising apphcadons of graphene and graphene-based nanosnucnires

hi secdon 2 we summarize die resuto of die smdy on graphene-based oansparenl and flexible conducdve fihns for displays and elecBodes The comen, of section 3 includes CO ducnve inks for printed electronics. Secdon 4 is a review rev^l » T T,- « ' " " • = « - ' ' - « ' »=P>"tion membranes. A rev ew on die uniiadon of graphene in bio-medicine is pre-

c T n r o n ' - a n T t S ^ S o T '""''- ^ ^ " ™ ' ^ ^

2, Graphene-based transparent and flexible conductive films for displays and electrodes Development of transfer printing and solution-based medio, allowed to incoiporate graphene into large area electionics, [9] Chhowalla et al proposed an efficient meUiod ensu umforni and controflable deposition of reduced graphen oxide (RGO) dlin fihns widi diickiiess ranging Irom a singi monolayer to several layers over large areas. The oplo-elc ironic properties can dius be mned over several ordeis magmmde, making diem potentially usefiil for flexible _ dansparem semiconductors or semi-metnis. The ddnnest fitej exhibit graphene-like ambipolar ttansistor characterisfitsj!' whereas diicker films behave as graphite-like semi-metals, oi die whole, die proposed deposition mediod represented i j route for mmslating die fimdaraental properties of giapheiJ

,nto technologically viable devices. ' The large-scale pattern growth of dansparent electiwics:

was successfdily petfonned by Hong « al [10]. The audlon' used die chemical vapor deposition (CVD) on dlin nickel layers and apphed two methods for patterning die films and transferring them to arhitrary substrBtes.

The tiansferred graphene fihns showed very low sheet resistance and very high optical d^sparency. At low

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Adv. Wat S d ; Nanosd. Nanoteehnol. 7 (2016) (a3002

^"jperatures the graphene monolayer transferred to SiOj substrata showed high electron mobility and exhibited the half-integCT quantum Hall effecL TTius the quality of graphene grown by CVD is as high as mechanically cleaved gr^hene.

Having eaiqiloyed die outstanding mechanical properties of gr^hene. Ifae authors also demonstrated flie macroscopic use of tfiese highly conducting and transparent electrodes in flexible, stretchable and foldable electronics.

In [11] Colombo ei al demonstrated the large-area syntiiesis of high-quality and uniform graphene films of copper foils. The autiiors grew large-area graphene films of tiie order of centimeters on copper substrates by CVD using metiiane. The fihns were predominantly single-layer graphene, witii a small percent^e (less ttian 5%) of flie area having few layers and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appeared to make fliis growth process self-limiting.

The autiiors also developed tiie graphene fihn transfer process to arbitrary substiates. The dual-gated field-effect transistor fabricated on sificon/silicon dioxide showed electron mobilities as high as 4050 cm^ per volt per second at room temperanire.

Witii tiie exceUent optical and electromc properties of graphene such as high mobility and optical transparency as well as fiexibiUty, robustiiess and environmental stability, it is a promising material for tiie apphcation to photonics and optoelectronics, A comprehensive review on graphene photonics and optoelectronics was presented by Ferrari et al [12].

From tiie rich scientific contents of tiiis review it can be clearly seen tiiat graphene-based transparent conducting films as well as graphene oxide (GO}-based transparent conducting fihns were efficiendy used in tiie fabncations of many pho- tonic and optoelectronic devices and equipments such as tiie window electixides of inorganic, organic and dye-sensitized solar ceUs, organic light-emitting diodes, hght-emitting elec- tiwhemical cells; tiie touch screens; ttie flexible smart windows; tiie graphene-based samrated absorbers in laser cavities for ultrafesl generation etc.

The high-performance, flexible. Iransparent heaters based on large-scale graphene fihns were syntiiesized by Hong et al 113], The autiiors applied ttie CVD on Cu foils and fabricated graphene films witti low sheet resistance and very high (-89%) optical transmittance, which are ideal low-voltage transparent heaters. Time-dependent temperature profiles and heat distribution analyzes showed tiiat tiie performance of graphene-based heaters is superior to tiiat of conventional h^sparcnt heaters based on indium tin oxide (TTO). These graphene-based, flexible, transparent heaters are expected to be widely applied, particularly in automobile defogging/

deicmg systems and beatable smart windows.

In [14] De and Coleman used published transmittance and sheet resistance daU to calculate a figure of merit for transparent conducting graphene films, tiie DC to optical conductivity ratio, CTDC/^OP - 0.7. 4.5 and II, The authors showed tiiat tiiese results represented fiindamental limitmg values for networks of graphene flakes, undoped graphene stacks and graphite films, respectively. The limiting value for graphene flake networics was much too low for transparent-

electrode applications. For graphite, a conductivity ratio of 11 gave a resistance too low compared to ttie minimum nsquirement for transparent conductors in cun«nt driven applications. However, the authors suggested tiiat substrate- induced doping can potentially increase tiie two-dimensional (2D) DC conductivity enough to make graphene a viable transparent conductor.

In [15] Choi and Hong demonstrated high-performance, flexible, transparent heaters based on large-scale ^3phene fihns syntiiesized by CVD on Cu foils. After multiple trans- fers and chemical doping processes, tiie graphene films showed sheet resistance as low as ~ 4 3 n / s q witti 89%

transmittance, which are ideal as low-voltage transparent heaters. Time-dependent temperatiire profiles and heat dis- tiibution analyzes showed tiiat tiie performance of graphene- based heaters is superior to tiiat of conventional ti^sparent heaters based on ITO. In addition, tiie autiiors confirmed ttiat mechanical strain as high as - 4 % did not subslantialy affect heater performance. Therefore, graphene-based, flexible, transparent heaters are expected to find uses in a broad range of applications, including automobile defogging/deicing systems and beatable smart windows.

The graphene electrodes for high-performance organic field-effect transistors were fabricated by Kim el al [16]. In order to optimize tiie performance of ttiese devices ttie auttiors controUed ttie work-fimction of graphene electrodes by fiinctionalizing ttie surfaces of S1O2 substrates (SAMs). The electron-donating NHz-terminated SAMs induced strong n-doping in graphene, whereas tiie CHa-terminated SAMs neutrafized p-doping was induced by SiOz substrates. As ttie result, work-flinctions of graphene electrodes considerably changed. The SAMs were patternable and robust. The result of ttiis work can be applied also to ttie fabrication of many otiier graphene-based elecmsmc and optoelectronic devices.

In a subsequent work [17] Lee et al reported tiie fabrication of flexible organic light-emitting diodes by engmeering tiie graphene electrodes to have high work-functions and low sheet resistances for achieving extiemely high luminous efficiencies.

By using poly(vinytidence fiuoride-trifluoroetiiylene) briefly denoted P(VPF-TrFE), as an effective doping layer between two graphene layers for sigmficanfly decreasing ttie sheet resistance of graphene, Ahn ei 0/ [18] fabricated flexible, transparent acoustic acttiator and nanogenerator based on graphene/P(VPF-TrFE)/graphene multilayer fihn. The prepared acoustic acniator showed good preformance and sensitivity over a broad range of frequency. The output voltage and tiie current density of tiie prepared nanogenerator were comparable to ttiose of ZnO- and PZT-based nanogeneraiois.

The auttiors demonstrated also tiie possibility of roIJable devices based on graphene/P(VDF-TrFE)/graphene multilayer under a dynamical mechanical loading condition.

In tiie miportant experimental work [19] of Cho et al tiie autiiore elaborated an efficient metiiod for fast syntiiesis of high-performance graphene films by hydrogene-fi^ rapid tiiermal chemical vapor deposition (RT-CVD) and roll-to-roll etehing towards its indusuial development for tiie mass-production of graphene films with tiie size, uniformity and

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Adv Nal Sa- Narosci Nanoteehnol 7i20i6 ,2^002

reliability satisfying tiie industrial siandards- The graphene fihn transfer mettiods were also elaborated.

The physical properties of RT-CVD graphene have been carefully characterized by transmission electron microscopy, Raman spectroscopy, chemical grain boundary analysis and various electrical device measurements, showing excellent uniformity and stabihty. Moreover, tiie acmal application of ttie RT-CVD fihns to capacinve multi-touch devices installed in tiie most sophisQcated mobile phone was demonstrated

Beside many superior mechanical and optical properties of graphene films compared to otiier transparent dun films frequentiy used in photonics and optoelectronics, flieir conductivity is inferior to ttiat of conventional ITO electrodes of comparable transparency, resulting in tiie lower performance of the devices using graphene transparent ttun films. To overcome ttiis inconvenience Ahn et al [20] applied an efficient method to improve the performance of graphene films by electrostatically doping them via a ferroelectric polymer.

These graphene films witii ferroelectric polarization were used to fabricate ultrathin organic solar cells (OSCs). Such graphene-based OSCs exhibited an efficiency of 2 07% with a supenor stability compared to chemically doped graphene- based OSCs, Furthermore, OSCs constructed on ultrathin ferroelectric film as a substrate of only a few micrometers showed extremely good mechanical flexibility and durability.

Moreover, ttiey can be rolled up mto a cylinder with 7 mm diameter.

Another metiiod to enhance the performance of die flexible graphene-based OSCs was elaborated by Gradei5ak et al [21]. These authors showed that the high efficiency can be achieved via tiiermal u^atmeut of M0O3 electron blocking layer and direct deposition of ZnO electron transporting layer on graphene. The authors also denionstraicd ciuphene-based flexible OSCs on polyelhykne naphlh.ilaiL' subsuate. The fabricated flexible OSCs wuh ^ra|>lK'in: :uKide and catiiode achieved record-high power coim-r^mR elliuencies of 6 1%

and 7.1%, respectively. Thus this work piived a way to fuHv graphene elecu'odes based flexible OSCs IIMIIJ; a simple ;i reproducible process.

In a short review of above-mentioned t:\pcnmenia;

works Ahn and Hong [22] concluded thai graph^jne has emerged as a promising material for transparcni and il-'iiblc electrodes. The use of graphene-based transpareni CLMUIJCS has been already demonstrated in various photoiiK aul optoelecnonic devices. Ahn and Hong anticipated il ,,•:

applications of graphene to flat and simple structures sucli,.

touch screens, smart windows, electromagnetic interference shields, lighting and transparent heater will be the first to be realized, whereas applications to flexible displays and microeleclronic devices will follow some years later 3. Conductive inks for printed electronics Having noted that for at least past ten years printed electronics has promised to revolutionize the electronic industry by producing cost-effective electronic circuits and sensors in very laige quantities, Noh ei ai [23] indicated also the need to

find suitable fimctional inks, mainly higfr i,niobilit35 semiconductors and low sintenng " ' f ^apabli inks as weU as to develop printing '"^ convey resolution and uniformity compared l" ' ^ ,revioi In fact diis need was responded m ^°^^' , ™,nL.^

In [24] Jang et al fabncated paitem^ ^ grapnem an inkjet printing technique. High Im-' resoiuHoJ tained electrical conductivity were aclneved, sheet resistance was dependent on the concentra|

ink and flie number of print layers. The pattemf based tiiin film was also applied as a praetiQS dipole antenna

SubsequL'oliy Ferrari el al [25] demons!

printing as a viable method for large-area fabric;

phene devices. The authors produced the graphen^

by liquid phase exfoliation of graphite in N-iai done. The prepared ink was used to print thin- with mobilities up to ~95 cm^ V~^ s~^ as weU and conductive patterns with '"80% transiniO

~30 kf! cm~^ sheet resistance. These resitit pav^i aU-printed, flexible and transparent graphene' arbitrary substrates.

In [26] Ostling et al demonstrated an efficit inkjet printing technology for mass productinn of!

graphene patterns witii a high resolution. Typi passes of printing and a simple baking aUowed variety of good-performance electronic devices transparent conductors, embedded resistors, sistors and micro-supercapacitors.

Recendy Torrisi and Coleman [27] describf phene can be produced and tiien used in conduoj Inkjet printing. First, a large quantity of prisi nanosheets, typically hundreds of nanometers '"I nm thick, can be produced quickly and easi|

phase exfoliation in readily printable liquids sui and organic solvents. The resulting ink is stable, in ambient conditions, and has high batch-to-bal cibility as well as good rheological properties for;

I,-Gating.

Some heterostrucmres were fabricated by us;

1 the experimental work of Cas "lmi eet-b;

28] llic ,i,jt.hors noted tiiat the possibility of '2'^- oi d u i a . m 2D materi;

unprecedented ,.uu'i,i

^^ie^ of Ihe re^ulun.^

in one stack over the electronic and 0[

'raphene, he:

'il;-'hidc ( M o S j ) . '^agonal hoi 01 ^•lerial. These 2D m a t e r i d ^ a i ! nitride (hBN) a

used II

can be"""^ ^ " ^ ° ' " ' ^^'"onstrated diat

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t' I ,1 'pin!; the larEe-arp ' et al [291 d o . . nMraej ^ ^ " ' a i b l e e l e c t l W ">1 rapidly produt,- ,.„„d„„i,j "'= p,.„ting ofg ""l The amhoi, pre,,.,,,-,, su.iable -^C' °" O ^ W s i •*»

meters enabling ,hu l,,hntauo„ „" ""1 ihose pri, "'»

down to 30/,m. A inilu .lnnealing^'^n,^ w ' "

lines with high rchabiiny and „ "P yiuded ^ " l "

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-..^ W**- Na. Scu NanoscL NanotechnoL 7 (2016) 0230OZ

^ efficient metiiod for the integration of ^aphene into large-

"^ area printed and fiexible electtonics.

In [30] Coleman et al demonstrated Inkjet printing of nanoshe« of both graphene and M0S2 prepaied by Uquid exfoliation. The authors described a procedure for preparing inks &Dm nanosheets with well-defined size distribution and concentration up to 6 mg m l ^ \ Graphene traces were printed at low tenqierature (<70''C) wittiout subsequent ttiermal or diemical treatment Thin tiaces displayed percolation effects while traces wilh ttiickness above 160 nm displayed thickness-independent conductivity 3000 S m " ^ The auttiors also demonstrated fee printing of semiconducting traces using , solvent-exfohated, size-selected MoS2 nanosheets. Such tta- r, ces can be combined with inkjet-printer graphene inter- , digitated array electiodes to produce all-printed , photodetectors.

^ A review on recent developments of the smdy on conductive nanom^eiials and their applications in printed electronics was presented by Magdasi and Kamyshny [31]. The authors particularly emphasized on inkjet printing of ink formulations based on metal nanoparticles, carbon nanotubes and graphene rfieets. The review described ttie basic properties of conductive nanomaterials suitable for printed electronics, their st^ilization in dispersions, formulations of conductive inks and various sintering mettiods to obtain conductive pattems. AppUcations of conductive nanomaterials for electronic devices (transparent electrodes, metalliza- tion of solar cells, RFED antennas, hght emitting devices etc) were also briefly reviewed.

In a recent work [32] Hersam et al demonstrated ttie mtense pulsed light annealing of graphene inks for rapid post- processing of inkjet-prinled pattems on various subsUates. A conductivity of -25COOSm"' was achieved foUowing a simple printing pass using a concentrated ink containing 20 mg ml" graphene. estabhshing tiiis strategy as a practical and effective approach for ttie versatile and high-performance integration of graphene in printed and flexible electronics.

In anottier recent work [33] Park ei al performed ttie direct printing of RGO on planar or highly curved surface with a high resolution using electrodynamic technology. The authors demonstrated the electrodynamic inkjet pnnting of RGO to form complex geometric devices widi a high resolution. Both planar and highly curved surfaces (witii ttie radius of curvature ~60mm) can be used as substrates.

Demonstration of counterfeit com recognition using RGO pattems and aU-printed RGO transistors suggested substantial promise for application in security and electronics.

4. Graph»ifr-based separation membranes The idea to use graphene sheets containing nanopores as tiie separation membranes was emerged since long time from the theoretical simulation smdies. Kr^ et al [34] designed fimctionalized nanopores in graphene monolayers and showed by molecular dynamics simulations tiial ttiey provide highly selective passage of hydraled ions. Only ions ttiat can be partly stripped of ttieir hydration sheUs can pass ttirough ttiese

ultrasmall pores with diameter ~5 A. For example, a fluorine- nilrogen-tenninated pore allows the passage of Li*, Na* and K* cations witii fee ratio 9:14:33. but it blocks fee passage of aiuon. Tbe hydrogen-terminated pore allows the passage of F~, c r , Br~ anions witii fee ratio 0:17:33 but it blocks fee passage of cations. These nanopores could have potential applications in molecular separation, desalination, and energy storage systems.

Subsequently Jiang et al [35] investigated fee perme- abifity and selectivity of graphene sheets wttti designed subnanometer pores using first principles density functional calculations. The aufeors found high selectivity on fee order of 10^ for H3/CH4 wife a high performance of H2 for a nitrogen-functionaUzed pore. Moreover, fee aufeors found extremely high selectivity on fee order of 10^^ for Hj/CH, at an aU-hydrogen passivated pore whose small widfe (at 2,5 A) presents a formidable barrier (1.6eV) for CH4 but easily surmountable for H2 (0.22 eV). These results suggested feat feese pores are far superior to traditional polymer and sihca membranes, where buUc solubiUty and diffusivity dominate fee transport of gas molecules through fee material. The aufeors proposed to use porous graphene sheets as one-atom- thin, highly efficient, and highly selective membranes for gas separation. Such fee pores could have widespread impact on numerous energy and technological applications.

In [36] Strano ei al stufeed fee mechanisms of gas permeation through single layer graphene membranes. The auttiors derived analytical expressions for gas permeation through atomicaUy fein membranes in various Umit of gas diffusion, surface adsorption, or pore translocation. Gas permeation can proceed via direct gas-phase interaction wife fee pore, or mteracuon via fee adsorbed phase on fee membrane exterior surface. A series of van der Waals force fields aUowed for fee estimation of fee energy barriers in various types of graphene nanopiores.

Using molecular dynamics simulations Xue el al [37]

investigated fee separation of COz from a mixtiire of CO2 and N2 by means of porous graphene membranes. The effects of chemical fimctionalization of fee graphene sheet and pore nm on fee gas separation performance of porous graphene membranes were examined. The aufeora found feat chemical fimctionalization of fee graphene sheet can mcrease fee absorption ability of CO2, while chemical fimcUonahzation of fee pore nm can sigmficantiy improve fee selectivity of COz over N2. Obtained results demonstrated fee potential use of fiinctionahzed porous graphene as single-atom-tiiick membrane for CO2 and Nj separation. Thus fee aufeoK proposed an effective way to improve fee gas separ^ion performance of porous graphene membranes.

The utihzation of nanoporous graphene (NPG) for water desaUnauon was proposed by Grossman and Cohen-Tanugi [38]. Using classical molecular dynamics, fee aufeors showed feat nanometer-scale pores in single-layer freestanding graphene can effectively filter NaCI from water. Moreover, fee aufeors smdied fee desalinaUon performance of such membranes as a fiinction of pore size, chemical ftincuonalization and applied pressure Obtained results indicated feat fee membrane's abiUty to prevent fee salt passage depends

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Adv. NaL Sg-. Wanosa. WarKMachno,. 7 (20,6) 023002

critically on pore diameter widi adequately sized pores excluding Cl" ions even at moderate solution ionic sEiengA,, allowing for water flow while blocking ions. Furdier, an TTiese results are useftd for the design of water desaljnalioti mvestigation on the role of chemical functional groups bon- membranes.

ded to die edge of graphene pores suggested that commonly In a recent work [43] Grossman and CobeB-Tanugi \ occurring hydroxyl groups can roughly double die water suggested to reconsider the water permeability of NPG aff!

diardts to their hydrophylic character. However, die mcrease realistic pressute for RO desaUnation. Tile ptnbiem is as m water flux takes place at die expense of less consistent salt follows: NPG shows tiemendous promise as an ultia-penne- ? rejection performance, which can be attributed to the ability able membrane for water desalination thanks to its atomic : of hydroxyl fimcdonal group to substimte for water molecules diickness and precise sieving properties- However, a sig. ' m die hydration shell of die ions. OveraU, obtamed results nificant gap exists between die ideal conditions assumed for' mdicated diat die water pemieability of diis material is several NPG desahnation and die physical environment inherent to, i]

orders of magnitiide higher dian conventional reverse osmosis RO system. In particular, die water permeability of NPG has ' (RO) membranes, and diat NPG may have a valuable role to been calculated previously based on very high ptessoiBS play for water purification (1000-2000 bats). Does NPG maintain its uloahigh warn At die same time Karmk el nf [39] smdied die selective penneabifity under realworld RO pressures (< 100 bars)? "ITie nansport of molecules tinough innuisic defects m smgle audiors answered diis question by drawmg results fiom layers of CVI> graphene widi nomuial areas more dian molecular dynamics sunulations and indicating tiiat NPG 25 mm which were dien ttansferred onto porous poly- mahitains its ultiahigh petmeabifity even at low pressures, carbonate substiates. A combmation of pressure-dnven and allowing a permeated waBr flux of 6.01h"'bar per pore, ot diiiiisivc transport measurements provided evidence of size- equivalendy 1041 ± 201m-^hbar assuming a nanopore selective nansport of molecules duough die membranes, density of 1.7 x 10'^ cm^

which was attributed to die low-frequency occunence of Lophcadons of penneation duough mlnnsic defiicts in 1-13 nm diameter pores m CVD graphene. Thus die audlon, graphene on die design of defect-tolerant membranes for .as demonstrated die fim step toward die realization of graphene- separation were investigated by Kamik el al [44) The audlon, based selecnon membrane. demonsttated diat independent stacking of graphene layers on By applying molecular dynamics simulations Jiang « al a porous support exponentially decreases die flow dirough [40] demonstrated diat porous graphene of a certam pore size defects. On die basis of experimental data die audioB c „ eflicendy separate CO, fiom N, widi a high perfor- developed a gas ttansport model fliat elucidated separa^

H n ^ * ' = s ™ ™ widi die recent experimental finding conttibudons of tears and inttimiic defects on gas iLm.

u L ? o f r n X » ' ^ «"=«e<i "• the much higher duough diese membranes. Tlie model showed that d i e ^ number of CO^passing-dmiugh events dian diat of Njfl^ra size of die porous support and its pemieance c r i S

mnd was hnlher corrobolated by dte ftee energy bamer of presence of nonselective defects, even for sui.le-layer mem r o T o Z h ^ h ' . CO,/N selectivity was around branes. Obtinned results provided a f S L e w o k l u X n n : / ° ' . ! ! i * , ? J T . ? - ' " . r „ r f ' " ^ ^ ^ . ° = " " V " " " ' * s - a n l i n g g a s t t a n s p o r t i n g i p h e n e m e m b r a n e s a r d S d ^ I

design of practical, selectively penneable graphene mem- COj/Nj selectivity makes NPG a promising membrane for

post-combustion separation.

Subsequently to die suggestion of Grossman [38] die sunnladon smdy of graphene-based water desalinaoon membranes was carried out again by Sttiolo et al [42] The andion, apphed d,e molecular dynamics simulations to investigate die ttansport of water and ions duough die pores created on die basal plane of die graphene sheet. Graphene pore diametenj ranged ftom 7.5 to 145 A. Different pore fimcnonalities obtained by tediering various fimcnonal groups to die tenninal cartion atom were considered. The ease of ion and water tt^nslocatii

branes for gas separation.

In anodier work of Kamik « ai [45] die selective ttansport of ions duough tiinable subnanometer poles in single- layer graphene membranes was investigati^. Isolated, reactive defects were mttoduced into die graphene lattice duough ion bombardment and subsequendy enlarged by oxidative etching into permeable pores widi diameters of 0.40 ± 0.24 nm and densities exceeding 1 0 " cm"', while retaimng sttuctiiral integrity of graphene. Transport measurements revealed diat die created pores were canon-selective at short

to die graphene sheet. The r : l u ^ 1 a T d t a t X t i t : i™ I T ' t l i t " " ' ? " " " ' " " ^ " ' ^ " ^ ^ " " ^ ^ "^ f""

exclusion can be achieved onlv whm n„„fi . ,• j = * " • ^1 longer oxidative times, tile pores allowed ttansport (pristine, pores have Z l t ^ J ^ SAZI^^ZZ^T' " V * . " " ' P ™ " - " " - P " " "^ ' « S - "^anic molecnte easily penettate pristine pores of diameKralTo s l d M 5 A ' " f T ™ /'""' "" " " " = " " • ' ^ """•>• "" ° ™ " ^

**^sirr^f=s^-.9;-'-- ^"^-« ^'"™"" ~ ''""^'**

ohrd™r:rsrthr„tr;Xores- ---'-r-"''-

tionalized wife hvdmyvl arv,„„. ,„irTK?^^ The mechanisms of moli tionalized with hydroxyl

pores promised fee development of advanced pronounced as NPG membranes for nanofiltration. desaUnation, gas sepaia- grouDs n-main^H »ff ^ —« »i molecular pcimcation tiiTOUgh NPG g ups remained effective at membranes were studied by Hadjiconstanfeiou « a/[46J. By

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t^-Ht-ScuNtmatbtmitM-,,:

* applying molecular dynamics simulatiom, die aufliora mves-

^ S r '°'1'^°^' S a ^ - ' " ' ' ™ . Hydrogen, ninogen and

• " " " • ' ' ^ T h e y showed diat m addition to die direct (gas-

^ tonetic) flux of molecules crossmg fiom Uie bulk phase on , one side of flie graphene R, tile bulk phase on die otiier side

•" , , ? / T ^ * " ' ^ * ' " " ° " « 8 ^ 1 ' e n e , s i 8 n i f i c a m c o n t t i b - '

^ unon tt) flie flux across die membnme was obtaiicd ftom a 1, ' ™ '""'Mnism by which molecules cross after being

^ adsorbed onM die graphene suifiice.

. „ , , . ' * ! " * ° " l"'^'^ O'O « " v e coutiibutions of die

" ' " ' a n d surfiice mechanisms and showed fliat die direct flux . ™ ! r f ^ reasonably and accmBtely by kinetic dieory provided die later is appropriately modified assmmng steric molecule-pore inttiractions, wid, gas molecules behavmg as J f ^ «Pl]=«« of known kmetic diametera. The surface flil is

**, Z ^ ; , . °. f T " "'^ <= t^H* "•• N^)"'"**

^entified die nanopore geometty Uiat is penneable tt, H , and He, s i g n ^ U y less penneable to N „ and essentially m^emieaWe to C t t , dins validated previous suggestions Uiat NPG membrane can he used for gas separation. T i e audiora also showed diat molecular pemieation is stiongly affected by pore fimctionahzation. Tbis observation may be sufficient to explain Uie large discrepancy between simulated and experi- menttdly measured ttansport ndes dirough NPG membranes In die search of graphene-based materials for filttation and separation techniques Geun ct al [47] found Uiat sub- nucromettir-Uiick membranes made ftom GO can be com- pletely mipermeable to hquids, vapors and gases, hicludmg hehum, but diese membranes allowed nmmpeded penneation , r S o ' " ^ ' ^ <X">''e>' graphene-based membranes at least 10 nmes fasBr dian He). The audiors anributed Uiese seemmgly incompatible observations to a low-fiiction flow of a monolayer of water Uuough 2D capillaries fomied by closely spaced graphene sheeK. Diffiision of odicr molecules was blocked by reveraible nanowmg of die capUlancs m low humidity and/or by diett cloggings witii water

Ahnost at die same nme Bunch el al [41] investigated selective molecular sieving duough porous graphene The andiora nodal diat membranes act as selective banier and play ail important role m processes such as cellular compartnen- M^ization and industtial-scale chemical and gas pmification The membranes should be as dun as possible to maxhnize flux, mechanically robust to prevent fiscttue, and have weU- deflned pore sizes to increase selectivity. Graphene is an excefiem stinting pomt for developmg size-selective membranes because of its atomic duckness, high mechanical snengUi, relative inertness and impemieabUity to aU standanl gases. However, pores dat can exclude larger molecules but allow smaller molecules tt, pass duough would have to be intiDduced mto die matisrial. The auUion, showed diat ulna violet-induced oxidative euihmg can create pores in micro meter-sized graphene membranes, and die resulting membranes can he used as molecular sieves. A pressurized bhsKr test and mechamcal resonance were used to measure die ttansport of a range of gases (Hj, COj, Ar, Nj CH. and SF.) duough die pores. The experimentiOly measured leak

rate, separation factors and Raman spectium agree wefl wiUi models based on effusion duough a small number of ang- strom -sized pores.

SnbsequenUy Choi el al [48] mvestigated die selective gas ttansport fluough few-layered graphene and GO membranes. The audiots demonsoattxl fliat few-and several- kyered graphene and GO sheeti, can be engineered to exhibit ttie desued gas separation characteristics. Selective gas diffiision was achieved by conttolling gas flow chamiels and JHJres via different sacking mediods. For layered (3-10 nm) OO membranes nmable gas ttansport behavior was stton.ly dependent on die degree of mterlockhig widun tile G<3 SBckmg sttucttue. High carbon dioxide/nimjgen selectivity was achieved by well-interlocked GO membranes in relative high hmmdity, which was most suimble for post combustion carbon droxide capnue processes.

At Ore same time die ultaUnn molecular-sieving GO membranes for selective hydrogen separation were mvestigated also by Yu c, al [49]. The audiois noted diat ulttafliin molecular-sievnig have great potential to reaUze high-flux' high-selectivity mixmre separation at low energy cost' However, cmrem microporous membranes wifli die pore size

<1 mn are usually relatively diick. Therefore, widi die use of cunem membnme materials and techniques, it is difficult to prepare microporous membranes dunner dian 20 nm widiout mttoducing extta defects. The audiors have succeeded ui prepanng ulttadun GO membranes widi die diickness approachmg 1.8 mn by a facile filttation process. These S,^,!^ T n ' ^ ' " " " " ^ " ^ ' ™ '"'Pa^'ioh selectivities as high as 3400 and 900 for H,/CO, and H , / N , mixttires, respective^

Uuough selective snucnual defects on GO.

Recendy ptecise and ulttafas, molecular sievuig duough GO membranes was perfonned by Geim e, al [50]. Having noted fliat graphene-based materials can have wefl-defined mmopores as weU as can exhibit low ftictional water flow mside Uiem, making dieir properties of intiirest for filttation tmd separation, die audiora mvestigated die pemieation duough micromena-diick laminates prepared by means of vacnmn filttatton of GO suspensions. The laminates are vacnmn-ttght in die dry state but, if immersed in water, diey K:1 as molecular sieves, blockmg all solutes widi hydrated radu larger dian 4.5 A. Smaller ions perineal duough Z membranes at rates of diousands times faster Uian SL. is expected for srniple diffiision. Tie auUiors beheved t t a l u ^ t T r T"1'"' " °="""'= " ' »="«apiliaries diat open 1 1 ^ i " ^ ™ " " " " " P " "-'y " P " ' " ' * a t fit in. m e anomalously fast pemieation was amibnted to capillaiy-fike pressure acting on ions inside graphene capillarieT

Soon after Park „ al [51] observed Uie ulumatt per- rmi°n°d ™ b i ™ ' ' " ' * ' " P " " " ^ ^ " P ' ' ^ ^ - Having ^a^

m e J r l e f " ' ° P " " " ' " ' ^ " " ">'*= *= ideal membrane for separation of chemical mixmres because its

; t S T l f ^ ' P " " ™ " ' ' " • ' ™ penneation tie aufliors beheved diat graphene, widi great mechi^cal smingUi, chemical smbihty and inherenf i m p e ™ a W i „ offeis a unique 2D system wifli which to r e a l i i r s mem^

brane, and smdied die mass ttansport The audiors d ™ Stinted the highly efficient mass 1 ^ 2 ^ ! ° ^ , ^ ^ ^

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Mil. NaL Sa.: Nanosa Nanoteehnol. 7 (2016) 023002

perforated double-layer gr^hene, having up to a few million pores wife narrowly fesnibuted diameters between less fean 10 nm and I iim. The measured transport rates were in agreement wife predictions of 2D transport feeory. Attiibuted to their atomic thickness, feese porous graphene membranes showed permeance of gas, hquid and water vapor far in excess of feose shown by finite-tiiickness menferanes, having highlighted fee ultimate permeation feese 2D membranes can provide.

Subsequentiy to fee previous work [50] Geim et al [52]

continued to smdy fee proton transport through one-atom- feick crystals. The aufeors performed transport and mass spectroscopy measurements which demonstrated feat monolayer of graphene and hexagonal boron nitride (hBN) are highly penneable to feermal protons under ambient conditions, whereas no proton Uanspon was detected for thicker crystal such as monolayer molybdenum disulphide, bilayer graphene or multilayer hBN. Protons present an intermediate case between electrons (which can tunnel easily tiuough atomically fein bamers) and atoms, yet fee measured trans- pon rates were unexpectedly high and raised fiindamental question about fee details of fee transport process. The aufeors observed fee highest room-temperature proton conductivity wife monolayer hBN, for which fee aufeois measured a resistivity to proton flow of about lOIicm^ and a low activation energy of about 0,3 eV. At higher temperamre hBN was outperformed by graphene. fee resistivity of which was estimated to fall below lOIJcm" above 250 °C. Moreover, proton transport was furfeer enhanced by decorating fee graphene and hBN membranes wife catalytic metal nanoparticles. "Die high, selective proton conductivity and stability m ^ e one-atom-feick crystals promising candidates for use in many hydrogen-based technologies.

Very recentiy. after a short review of fee research on water desalination. Koh and Lively [53] concluded feat water desalination membranes can be created by etching nanometer- sized pores in a single layer of graphene. In particular, fee aufeors highly evaluated fee promising results of fee research of Mahurin et al related to fee water permeability of single- layer graphene membranes toward fee application to water desalination. Indeed, a comprehensive experunental research work on water desalination using nanoporous single-layer graphene has been performed by Mahurin et al [54]. The aufeors have experimentally examined fee Uanspori of ions and water across a suspended, single-layer graphene membrane wife stable nanometer-sized pores generated by oxygen plasma etching in order to validate fee effectiveness of graphene-based desalination of water. These membranes exhibited bofe high salt rejection and exceptionally rapid water transport properties. Using aberration-corrected scanning transmission electron microscopy imaging, fee aufeors cor- related fee porosity of graphene membrane wife transport properties and determined fee optimum pore size for effective desalination. The mechanism of water ttansport was explored suggesting feat graphene may be suitable bofe for membrane disullation and RO. The content of fee research consisted of tiiree parts: preparation and characterizaUon of graphene membranes, water transport and salt rejection measurements

and analysis of transport mechanisnis. FoUowing concli^ons !

were attained: ft The potential utility of NPG as a selective membrane feat I

can be used for water desalination was demonstrated. It was \ shown feat oxygen plasma can be used as a veiy convenient "!

mefeod for fabricating tailored nanopon^ of desired dimen- ^ sion (and probably altered chemical properties) in suspended ^ single-layer graphene. wife high precision. The resulting ' nanopores showed Demendous water molecule selectivi^' , over dissolved ions (K^. Na"*", Li^, Cl~). The selectivi^;' exceeded five orders of magnitude for low porosities, but.' precipitously decreased at higher porosities, most probifely due to enlargement of fee nanopores. Based on fee estimated nanopore density (0.01 nm~^). fee estimated water flux tiirough a single nanopore can reach fee tiemendously higji u value of 3 molecules per picosecond. At fee same time, fe?, ^ water flux in a conventional geometry wife an osmotic pressure gradient and liquid water on bofe sides of fee porous graphene membrane showed smaller water flux values of 2G0 molecules per microsecond. Alfeough scaling up feese membranes for use in industiial and commercial processes remains a significant challenge, fee present work represented a proof-of-concept of fee effectiveness and potential of NPG for desalination applications.

As fee continuation of fee previous wori: [41] on selective molecular sieving tiuough porous graphene Bunch et al [55] fabricated molecular valves for controlling gas phase transport by using graphene wife discrete Sngsti^im-sized pores. The aufeors demonstrated feat gas flux tiirough disctete Sngstrdm-sized pores in monolayer graphene can be delected and feen conti'olled using nanometer-sized gold clusters, which are formed on fee surface of fee graphene and can migrate and partially block a pore. In samples wifeout gold clusters fee aufeors observed stochastic switching of fee magnimde of fee gas permeance attributed to molecular rearrangement of fee pore. The fabricated molecular valvts could be used, for example, to develop unique approaches tQ I molecular synfeesis feat are based on fee controllable ^ switching of molecular gas flux.

5. Graphene applications in bio-mediclne

The efficient applications of graphene in bio-medicine were simultaneously developed by tiiree independent research groups wife fee leaderships of Dradfc, Dekker and Golov- chenko. In [56] Dmdfc et al demonstiated fee DNA translo- cations tiirough nanopores created in graphene membranes.

The devices consisted of 1-5 nm tiiick graphene membranes wife election-beam sculpted nanopores from 5 to lOnm in diameter. Due to fee thin namre of fee graphene membranes, fee aufeors observed larger blocked currents fean for tiadi- tional solid-state nanopores. However, ionic cuneni noise levels were several order of magnitude larger fean feose for silicon nitride nanopores. TTiese flucfeations were reduced wife fee atomic-layer deposition of 5 nm of TiOj over the device. Unlike traditional solid-state nanopore materials tiiat are insulating, graphene is an excellent electrical conducWr.

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,- ttenowJ. NanotectMwi 7 fi

The use of graphene as a membrane material opened die door J til a new class of nanopore devices m which electionic sen-

^ smg and conttol are perfoimed direcfly at die pore.

^ taanoUlerworic on DNA ttanslocationdnough graphene

^ nanopores [57J Dekker e, al also noted Uiat nanopoies- , nanosized holes Uiat can nansport ions and molecules-are

^ veiy promising devices for genomic screening, m particular DNA sequencing. SoUd-stiite nanopores cunentiy suffer ftom die drawback, however, Uiat die channel constiniting die pore is long, ~I(X) times die distimce between two bases of DNA molecules (0.5 om for smgle-sttanded DNA). The anUiots provided die proof-of^xincept diat was possible tt> reaUze and use nltiadiin nanopores fabricated m graphene monolayers for single-molecule DNA nanslocation. The pores were obained by pkcmg a graphene flake over a microsize bote m a sihcon mttide membrane and drilhng a nanosize hole m die graphene usmg an electton beam. As mdividual DNA molecules ttanslocaUKl Unough die pore, characteristic temporary con- ducttmce changes were observed ui die ionic cmrent dnough die nanopore, sedhig die sage for fiimre smgle-raolecule genomic screening devices.

The Utihzation of graphene as a subnanomette ttans- electtodc membrane was realized by Golovchenko el ai [58].

The audioni nooal Uiat a graphene membrane separating two tonic solutions in electtical contact is sttongly ionically msulating despite bemg atomically dun, and has m-plane electionic properties dependem on die interfacial environment. The atomic Unnness, stitbility and electiical sensitivity of graphene motivated die auUiois to investigate Uie potential use of graphene membranes and graphene nanopores tt>

charactenze single molecules of DNA in ionic solution. The audlora showed diat when immeraed in an ionic solution, a hiyer of graphene becomes a new electtnchemical sttucnire caUed a tians-electtode. The nans-elecOTKlc's unique properties are die consequence of die atomic-scale proximity of its two oppostiig Uquid-sohd mtetfaces togedier wiUi graphene's weU-known m-plane conductivity. The audiors showed diat several tians-electitKle properties were revealed by ionic conductance raeasuremeno, on a graphene membrane diat separated two aqueous ionic solutions, Aldiough die used membranes were only one to two atomic layers dnck, die audiors found diat diey were remarkable ionic insulators widi a very small sttdile conductimce diat depended on die ion species in solution. Electtical measurements on graphene membranes, in which a smgle nanopore has been drilled, showed Uiat die membrane's effective insulating diickness was less dian one nanometer. This small effective duckness makes graphene an ideal snbstiate for very high resolution, high Ulrough put nanopore-based smgle-molecule detectore The sensitivity of graphene's m-plane electinnic conductivity to its immediate surface environment and tians-membrane solution potentials offered new insighB mto atomic surface processes and sensor development opportumties

The antibacterial activity of graphite, graphite oxide, GO and RGO was investigated by Chen et al |59I. The audiors noted diat graphene has stiong cytotoxicity toward bacteria To bener nnderetand its antimicrobial mechamsm U,e audiors compared Uie antibacterial activity of four types of graphene-

based materials: graphite (Gt), graphite oxide (GtO), rGO and RGO towanl a bacteria model—Esckericliia coli. Under sunilar concentration and incubation conditions, GO showed die highest antibacterial activity, sequentially foUowed by iGO, Gt, and GtO. Scanning electton microscopy (SEM) and dynamic hght scattering analyzes showed diat GO aggregates have tile smallest average size among four types of materials.

SEM unages displayed diat die duect contiicts wiUi graphene nanosheets dismpt ceU membrane. No superoxide anion (Oj) mduced reactive oxygen species production was detected.

However, four types of materials can oxidize glutiiUuone, which serves as redox sttwe mediator m bacteria. Conductive iGO and Gt have higher oxidation capacities dian insulating GO and GtO. The audiois envisioned Uiat physicochemical properties of graphene-based materials, such as density of fiinctional groups, size and conductivity can be precisely tiulored to eiUier reducing dieu- healdi and envnonmentiil nsks or mcreasmg Uien appUcation potentials.

hi [60] Ruiz el al perfonned die characterization of antimicrobial properties of GO and its biocompatibility wiUi maramahan ceUs. Audiore showed Uiat when GO was added to a bacterial cultiue at 25 ^ ml"', bacteria grew faster. SEM images mdicated diat bacteria fonned dense biofilms in die presence of GO. On Biters coated widi 25 and 75 (jg of GO bacteria grew two and diree times better dian on filKra wiUiout GO. Closer analysis showed diat bacteria were able to attach and proUferate preferentially m areas conttunmg GO at highest levels. Furthennore, GO acts as a general enhancer of cellular growdi by increasmg cell attachemem and prohferadon.

The applicanons of nanopore technology in DNA sequencing, genetics and medical diagnostics were presented m die review on nanopore sensors for nucleic acid analysis of Venkatesan and Bashn [61]. Tbe audlon, ,ndicatt»l diat nanopore analysis is an emergmg technique diat mvolves usmg a volage to drive molecules duough a nanoscale pore m a membrane between tivo electiolytes, and raomtoring how die ionic cunent dnough die nanopore changes as single molecules pass dnough it. This approach allowed charged polymers (uicludiig single-sttanded DNA, double-sBainded DNA and RNA) to be analyzed widi subnanomette resolution and wiUlout Uie need for labels or amplification. Recent advances suggested diat nanopore-based sensors could be competitive widi oUier diini-generation DNA sequencmg technologies, and might be able to rapidly and retiably sequence die human genome.

The Utilization of graphene multilayers as die gates for sequential release of proteins from surfaces was perfonned by Hammond et al [62]. Protein-loaded polyelecttolyte multilayer fihns were fabricated using layer-by-layer assembly mcoqKiranng a hydrolytically degradable cationic polwfl- ammo ester) (Poly I) wiU, a model protein antigen, ovabnmin (ova), m a bilayer arehitecnire along widi positively and negatively fiinctionahzed GO capping layers for die degrad- abte protem fihns. Ova release widiout die GO layers akes place m less dian Ih but can be mned lo release from 30 to 90 days by varyuig die number of bilayers of fiinctionahzed GO m die multilayer arehitecnire. TTie audiora demonsttated dial

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Adw. Mat Sd Nanosa. ManolBchnol- 7 (2016) 023002

proteins can be released in sequence wife mutd-day gaps detection, fee DNA passed t h r o n g tbe nanopore too q between fee release of each species by incorporating GO for individual bases to be resolved, and achieving single- layers between protein-loaded layers. This qqiroach provided resolution wife graphene nanopores remains an u n m o l ^

1 route for storage of feer^ieutics in a solid-state thin film ftir subsequent delivery in a time-controlled and sequraiial fashion.

Singular phase nano-optics in plasmoiuc metamaterials for label-&ee single-molecule detection was investigated by K^asbin, Grigorcnko el al [63]. Tlie aufeors showed feat properly designed plasmonic metamaterials exhibit topologi- cally protected zero refla:tion yielding to sharp phase changes

challenge. For any solid-state oanopcnes-be it graphene, s con lutride or something else-there is still a variety of fi damental issues feat need to be resolved befcne ] sequencing wife ionic currents could become a r e ^ ^ , \ example, should single-or double-stranded DNA be used? Ig specific range of nanopore diameters required, and bow p cisely will feey have to be manufectured? ImprovemHits ^ fee nanopore devices will also be required. In paiticE nearly, which can be employed to radically improve fee current noise needs to be lowered and signal-to-noJsc i^«

sensitivity of deteaors based on plasmon resonance. By using increased at high bandwidfes, or fee translocation of fee DI reversible hydrogenation of graphene and binding of slrep- needs to be slowed down wifeout more noise being tavidin-biotin, fee aufeors demonstrated an areal mass sensi- duced into fee signal. There are also more prosaic butciutsSi tivity at a level of fg mm"^ and detection of individual details feat need to be dealt wife: enough devices have to^ihi biomolecules. The proof-of-concept results of fee aufeors tested and reproduced by independent labs to draw M offered a router towards simple and scalable single-molecule conclusion, a task feat requires sldU. time and effort . ^ label-free biosensing technologies. Simultaneously wife above-mentioned carefiil attiftid^

Wife fee intention to explore fee interface between fee Dmfec', Kostarelo and Novoselov [67] presented an m f l | research on graphene and fee bioscience, Kostarelos and mistic point of view on fee potential of graphene in WomB, Novoselov [64] ouflined tiiree issues of complexity feat are dical applications. Graphene materials (GMs- a femUy 4' interconnected and need to be considered carefuUy m fee materials mcluding pristine graphene sheets, few-layer gta- development of graphene for use in biomedical applications: phene flakes, GO and many ofeers) offer a range of nniqufl, matenal charactenstics; interactions of graphene wife biolo- versatile and tmiable proprieties feat can be creativelTMed gical components (tissues, cells, proteins etc) and biological for biomedical purposes. Graphene applications in biomedi- aclivity outcomes. Conceming fee fir.t issue fee aufeors noted cine, even feough still in feeir infancy, can be divided into S ! ^ ^ ^ Z ^ ^ * ' ^ ^ ' ^ " ^ ^ ^ ^ S "^ °*«^ -J'^- - - ^"gineering and biological agents (for example Liti- S n n P y ™ * ^ - T?«^. P™If!^^ «>"'d ^--^le fee microbials). TTie unique properties of graphene to be used^in development of multifimctional biomedical devices. Con- biomedicine are: 2D flat s ^ , largfavailable a r e ^ T . ^ nents the aufeors emphasized feat fee determmation of fee gap, aqueous solubility (in fee case of GO) v e r ^ t i l i w ^ T i S r ^ e Z ^ H ' ' " " H r ^ ^ ' ' ^ ' ' ' ' ^ " ™ ' " " ""'"^"'^ fimctionlization. Tbe feasible b ^ m « l ^ c 7 5 ^

types of materials wife definite exposure.

"Hie utilization of graphene-based eleclroresponsive scaffolds as implants for on-demand drug deUvery was proposed by Kostarelov ei al [65]. The aufeors fabncated graphene hydrogel hybrid electro-active scaffolds capable of controlled smaU molecule release. Pristine ball-milled graphene sheets were incorporated into a tiuee-dimensional {3D)

implants. The opportunities are: responsive to a wide rangeflf' parameters, high sensitivity, multiple read-out routes. mR and speed of degradation. However, feere exist also fl»

challenges: unknown cytotoxic limitations, controltoble dimensions, determination of j

kinetics. biodegradabi%

macroporous hydrogel matiix to obtam hybrid eels with fi n » i n h » » » i -

enhanced mechamcal, electtical and diemial p r o ^ s C s t ^ " ' " ^ ^ ^ * " ' " = « elecooactive scaffolds demonsttati«l conttolled dmg release n „ r i „ . i . , «

in a pdsatile fashion upon die ON/OIT appUcato of low b e n e ^ l , T "^"^ ° ° " ™ " ' " ' " " ' ^ l " "

electtical volmges, at low concenttations of ™ h e n " ^ ™ ' » ' , , 7 > S ' ' ' ™ " » / * e v e d significant progress. I t e ( 0 . 2 m g m r ' ) and by maintainmg dieir s t t n c m r a l S r ^ ^ o , ^ T f ,"' " " ^ " ^ ™ ' ^ o«de-graphen.

Moreover, die /„ v/rro perfomi^ce of Ihes, .JS^T^... Z l Z T ^ I" ' ^ " • ^ ' ^ » " 8 y ^ " - ^ e »"«

demonsttated by Uu a al [68], Previously surfactant or , virro perfonnance of dlese electtoactive

scaffolds to release drag molecules widiout any 'resistive heating' was demonsttated.

Aldiough diere was die snidy of sequencmg wiUi graphene nanopores [61], at die presem time Uiere still exists die question: wheUler nanopores created in graphene could fiilfiU all of Uie requirements needed for sequencmg, as tins was asked by Dmdic' 166], hideed. in all expedments widi DNA

polymer directed self-assembly was widely used to pnipan ' nanosttuctiired metid oxides, semiconducton, and polymea, but dus approach was mosdy Umited to two-phase materiali, orgamc/morganic hybrids, and nanopailide or polymH', based nanocomposites. In Uie present work die auUiots ' demonsttated a ttmary self-assembly approach using gia-i phene as flindamenttU buildmg blocks to consttuct onieiKl^

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^^ '^tfa>.Scl:NanMd.NanoiBcnnoL 7 (20,6) 023002

^ metal oxide-graphene nanocomposites. A new class of

* 3 " ^ nanocompositts was formed contiuning stidjle, J ordered alumating layos of nanocrystallhie metid oxides

^, will" sraphcne or graphene stacks. Alternatively, Ute graphene I or graphene stacks can be uicoiporated into Bquid-ciysal- I templated nanoporous stincttnes tt) form high surfiice area, J conductive networks. 110 self-assembly mediod can also be

^ t«»»i to fiibncatefiee-sttmdmg,aexible metid oxide-graphene

^ nanocompositt films and electiodes. The audiors investigatirf

^ die U-lon insertion propenies ot Uie self-assembled electin-

^ des for energy stinage and showed diat SnOj-graphene

^ nanocomposiB fihns can achieve near Uieoretical specific

• energy density widiout significant charge/discharge degradation.

T i e self-assembled graphene/caiton nanotiibe hybrid films for snpereapacittmi were fabricated by Dai cr of [69J. In Ulis worit stiible aqueous dispeisions of polymer-modified graphene sheeK were prepared via in situ reduction of exfo- liated graphite oxides m die presence of cationic poly(eUiy- lenenmne) (PEI). The resultant water-soluble PEI-modified graphene sheeti, were Uien used for sequential self-assembly widl acid-oxidized moltiwalled carbon nanottibes, fonnmg hybnd carijon fihns. These hybrid fihns were demonsttated to pos'iess an mtereonnected network ot carbon sttticttnes widi weD-defined nanopores to be promising for snpercapacitor electtodes. exhibiting a nearly recangular cyclic voltammo- gram even at an exceedingly high scan rate of 1 V s"' widi an average specific capacittmce of 120 F g " ' .

In [70] Ruoff et al demonstiatiMl die caibon-based supereapacitors fabricated by activation of graphene The supercapacitors store electtical charge on high-surface-aiea conducting materials. Then widespread use is hmited by Uteh- low energy storage density and relatively high effective series resistimce. Usmg chemical activation of exfohated graphite oxide, die audiora syntiiesized a porous carijon widl a Bni- nauer-Emmen-Telier surface area up o 3 1 0 0 m ' g " ' a high electiical conductivity, and a low oxygen and hydrogen content This sp'-bonded carbon has a continuous dnee- dunentional netivorit ot highly curved, atom-duck walls tiiat fonn primarily 0.6- to 5-nanomeKr-widdi pores. Two-electiode supereapacitor cells constiucted widi Uns cariiou yielded high values of gnvimettic capacittmce and energy density wiUl oiganic and tonic hquid electtolytes. The processes used to make Uiis cartion are readily scalable to mdusttial levels.

The fimctionaUzed graphene sheet-sulfure (FGSS) iianocomposite tor usmg as cadiode material m rechargeable liduum batteries was syntiiesized by Liu et al [71]. The sttucmre has a layer of fimctionahzed graphene sheeK/sBcks and a layer ot sulfure nanoparticles creating a 3D sandwich- type arehiti»,tine. This unique FGSS nanoscale layered composite has a high loading (70 wt%) ot active matimal (S) a high tap density ot - 9 2 g cm"', and a reveraible capacity of

~505niAh_g (-464 mAg-^) at a cunem density of l680niAg . When coated wiUi a Uiin layer of cation exchange Nafion fihn, die migration of dissolved polysulfide anions ftom die FGSS nanocomposit was effectively reduced leading to a good cycUng stitbiUty of 75% capacity retention over 100 cycles. This sandwich-stinctined composite

conceptiially provided a new sttategy to design electtodes for energy sKirage applications.

In [72] Cui et al syntiiesized graphene-sulfiue composite material by wrappmg polyfediylene glycol) (PEG)-coated submicrometii sulfiue particles widl mildly oxidized GO sheets decorated by carbon black nanoparticles. The PEG and graphene coating kyera are important to accommodating volume expansion of die coatixl sulfiir particles during discharge, ttappmg soluMe polysulfide muimediales, and ren- dering die suHur particle electtically conducting. The resulting graphene-sulfiire composite showed high and stidile specific capacities up to - S O O m A h g - ' over more Ulan 100 cycles, representing a promismg cadiode matenal for rechargeable UUtimn batteries widl high eneigy density.

Hierarehically porous graphene was used as hduum-an battery electiode by Zhang et al [73]. The audiors demon- stiattd diat a novel an electiode consisting of an unsual hlerarehical anangement of fimctionalized graphene sheets (witiiout catiilyst) dehveied an exceptionally high capacity of 15000mAhg ' m liduum-02 batteries which was die highest value ever reported. This excellent perfonnance was attributiid to die unique biomodal porous stiucttne of die elecnode consisting of microporous channels faciUtitting rapid O2 diffiision while die highly density ot teactive sites tor l i - O j reactions. Further, die anUiors showed Uiat Uie defects and fimctional groups on graphene favored die for- mation of isolattal nanosized LiaOj particles and helped to prevent an blocking m tile air elecnode. The hierarehically onieied porous sttucnire m bulk graphene enabled its practical appUcations by promoting accessibihty ra most graphene sheets in this stracnire.

In [74] Liu et al perfonned die revereible sodium ion msertion m smgle crystidlhte manganese oxide nanowhes wiUi long cycle hte. The auUiora prepared singte crysttdhne Na4Mn90,a nanowires by a polymer-pyrolysis raeUiod This material showed a high, reveraibk sodium ion insertion/

exttaction capacity, excellent cycUng abihty, and promismg rate capability for sodium-ion batteiy apphcadons.

The nmable 3D pillared carbon nanombe-graphene net- woriis for high-performance capacittmce were fabricated and mvestigati»i by Dai a al [75]. The auUioni have developed a rationalsttategy for creating die 3Dpillaredvenicallyahgned cariKin nanombe (VACNT)-graphene arehitecnne by mter- calated growUi of VACNT into diennally expanded highly onlered pyrolytic graphite. By conttolhng die fabrication process, die lengUi of VACNT pillara can be timed hi con- junction witii die electtodeposition of nickel hydroxide to mttnduce die pseudocapacittmce, diese 3D pillared VACNT- graphene arehitecttues wid, a conttoUable nanombe lengUi were demonsttated to show a high specific capacittmce and remarirable rate capabihty, and diey significanUy out- perfomied many elecnode materials cunendy used in die stiitt of die art supercapacitois.

The dlin fihns of cartion nanottibes and chemically reduced graphenes were used for fabncating electtochemica]

micro-capaciton, by Hammond, Shao-Hom et al [76] The authors prepared Uie stiucmre comiisting of chemically reduced graphene (CRG) sheets separated by layer-by-layer-

Promising applications of graphene and graphene-based nanostructures