Graphene-based nano bio-sensor: Sensitivity improvement

Transcription

1 Scientia Ianica F (2017) 24(6), 3531{3535 Shaif Univesity of Technology Scientia Ianica Tansactions F: Nanotechnology Gaphene-based nano bio-senso: Sensitivity impovement M. Majdi and D. Fathi School of Electical and Compute Engineeing, Tabiat Modaes Univesity (TMU), Tehan, P.O. Box , Ian. Received 8 August 2016; eceived in evised fom 7 Octobe 2016; accepted 4 Mach 2017 KEYWORDS Abstact. In this pape, we attempt to pesent a multilaye-stuctue biosenso. A Biosenso; Sensitivity enhancement; S-paamete; Suface plasmon esonance; Gaphene. gaphene-based Suface Plasmon Resonance (SPR) biosenso is consideed as a stuctue, which o es enhancement of sensitivity in compaison with the conventional biosensos. The sensitivity impovement caused by the pesence of gaphene is discussed though monitoing of the biomolecula inteactions and following that the Refactive Index (RI) changes. Fo this pupose, an RI change, which inceases duing the couse of biomolecula inteaction, is consideed fom to Ou numeical esults show that the poposed SPR biosenso with a gaphene laye can povide a bette sensitivity due to the eld distibution at the binding egion. This pape investigates the Sensitivity Enhancement Facto (SEF) accoding to the S-paametes and the e ects of design paametes such as thicknesses of the gold and gaphene layes and the efactive index change duing the couse of DNA hybidization in this biosenso Shaif Univesity of Technology. All ights eseved. 1. Intoduction biosenso esponse to the biomolecula inteactions suounding it. Duing the sensing opeation, the adsoption of biomolecules to the ecepto molecules will poduce a local change in the efactive index at the metal-dielectic inteface and enables us to detect these biomolecules by measuing the shift of esonance angle [2]. In this pape, the sensitivity enhancement has been investigated by designing a multilaye stuctue in which a gaphene laye has been applied. The poposed multilaye biosenso has been modelled and simulated using the Finite Element Method (FEM) and the Finite Di eence Fequency Domain (FDFD) method using CST MICROWAVE STUDIO [3]. The aim of this simulation is the investigation of sensitivity enhancement with the measuement of scatteing paametes (S-paametes) with espect to the efactive index occuing duing the biomolecula inteactions and the thicknesses of gold and gaphene layes. The incopoation of gaphene laye in the conventional SPR biosenso can enhance the sensitivity. The lack Suface Plasmon Resonance (SPR) is an optical method based on the excitation of collective fee electons oscillations at the metal-dielectic inteface [1]. The SPR biosensos ae suppoted by the Suface Plasmon Polaiton (SPP) waves in ode to investigate and detect the biomolecula inteactions. In this method, a cetain incidence light angle (esonance angle) can be applied to obtain the equied SPR condition [1]. Unde this condition, momentum matching between the incident photon and the suface plasmon is attained and the Attenuated Total Re ection (ATR) occus [1]. The esonance angle as a function of the efactive index of sensing medium can be measued as the *. Coesponding autho. Tel.: ; Fax: addess: d.fathi@modaes.ac.i (D. Fathi) doi: /sci

2 3532 M. Majdi and D. Fathi/Scientia Ianica, Tansactions F: Nanotechnology 24 (2017) 3531{3535 of band gap in gaphene tuns it to a semi-metal dieent fom othe semiconducto mateials. These unique popeties of gaphene can be used in futue electonic and optoelectonic devices [4]. In this pape, dieent popeties of gaphene have been used in ode to impove sensitivity of the poposed biosenso [4]. One of the disadvantages of conventional SPR biosensos is the lowe limit of detection and thei sensitivity is faced with small ligands and low-concentation analytes [1]. To ovecome this limitation, seveal appoaches have been poposed. In this pape, in ode to esolve this issue, a gaphene-based biosenso has been intoduced. The impoved sensitivity due to the intepolation of a gaphene laye into the conventional SPR biosenso comes fom the inceased adsoption of biomolecules [5]. Moeove, applying a gaphene laye modies the popagation constant of SPP, which will be followed by changing the sensitivity to the efactive index vaiation [5]. 2. Mateials and method The schematic of the poposed SPR biosenso is shown in Figue 1. To excite the suface plasmon, the light is coupled though the int glass pism with a specic incidence angle. The base of the pism is coveed by a thin gold lm with the thickness of 50 nm and a gaphene laye on top of the gold laye with the thicknesses of 2 nm. Binding analytes (biotin-steptavidin; DNA hybidization eaction) ae consideed as a laye with an initial efactive index of [6] in an aqueous medium of phosphate bue solution, the efactive index of which is 1.33 [7]. In ode to nd out the best coupling and poduce the necessay esonance condition, a TM-polaized plane wave light with the xed wavelength of nm and dieent incidence angles is coupled into the pism. The optical constants, " = (n; k), of glass pism and gold laye ae set to (1.75, 0) and (0.178, 3.07), espectively, at = 632:8 nm [8]. It has been shown that gaphene has a boad and constant absoption ange in the visible spectal egion [4]. In this method, a model has been developed fo the in-plane optical conductivity of gaphene in the visible ange. In ode to calculate the pemittivity, the tansmittance, and the eectance of gaphene in the visible ange, petubation Hamiltonian is consideed as [4]: H 0 (t) = "(t); "(t) = Re " x e i!t^x ; (1) whee is the electic dipole moment. By applying the suitable appoximation and the simplication of equation, conductivity can be obtained as [4]: (!) = ie2!a X k 2 x ( cc ) E c;k E ;k ~(! + i) ; (2) whee A is the coss-section aea and x is the velocity opeato along x-diection that can be elated to the position opeato x by the equation of motion. As we know, the st Billouin zone of gaphene is a hexagon with a unit cell of two atoms. The conduction and valence bands touch each othe at the cones of hexagon, which ae called the Diac points. Fo an intinsic o lightly doped gaphene, the Femi enegy level is aound the Diac points, whee the chage caies expeience only a linea dispesion [9]. This linea dispesion is called the Diac cone since it is descibed by the elativistic Diac equation [9]. To calculate the optical popeties of gaphene in the visible ange, one can conside only the Diac cone if the photon fequency is low compaed to the esonance fequency and the Femi enegy is nea the Diac points [4,9]. The diect esult of the linea electonic dispesion of Diac cone is the eective massless femions, such that the elated enegy dispesion can be witten as [10]: E = ~ f j ~ kj; (3) Figue 1. 3-D model of the poposed gaphene based biosenso. whee~is the educed Plank's constant, f is the Femi velocity in gaphene, and ~ k is the wave vecto. In ode to obtain an analytical fom of conductivity, we can apply the Diac cone appoximation wheein we assume that conductivity contibutes only though the caies to the Diac cone. Thus, pemittivity, ", and

3 M. Majdi and D. Fathi/Scientia Ianica, Tansactions F: Nanotechnology 24 (2017) 3531{ the optical conductivity of gaphene, 0, in the visible ange ae elated by [4]: "(!) = 5:5" 0 + i 0!d ; (4) whee " 0 is the pemittivity of vacuum and d is the thickness of gaphene. Theefoe, the efactive index of gaphene in the visible ange can obtained [4]: n() = 3 + i 5:446 0 ; (5) 3 whee 0 is the vacuum wavelength. 3. Results and discussion Ou numeical model is based on the biomolecula inteaction of DNA (DNA hybidization). As mentioned in the pevious section, it is dened as a laye with the efactive index of The initial value of coesponds to an ssdna laye with a density of g/cm 3, obtained fom the ellipsomety measuements. Afte the hybidization events, the dsdna laye with a density of g/cm 3 coesponding to the efactive index of 1.52 will be obtained [11]. The optical chaacteistics (e.g., S-paametes) of the biosenso ae aected by the biomolecula inteactions between ssdna and cdna. Measuing the shift of S-paametes is the base of ou investigation in this pape. Fo achieving the coupling between the plasmon esonance and incident photons, the following condition must be maintained to excite the SPs on the metaldielectic inteface [12]: K sp = K ev =! "M " D =! p "p sin es ; (6) c " M + " D c whee K sp and K ev ae the wave vectos of the suface plasmon and the evanescent wave, espectively, c is the speed of light, " M and " D ae the dielectic constants of metal and dielectic laye, espectively, and es is the esonance angle [12]. The S-paametes ae complexvalued wavelength-dependent matices expessing the device chaacteistics (e.g., the tansmission and e- ection of electomagnetic enegy) using the amount of absoption o tansmission. Fo a two-pot device, the S-paametes ae dened as [13]: S11 S S = 12 ; (7) S 22 S 21 whee the matix elements S 11, S 12, S 21, and S 22 ae known as the scatteing paametes o the S- paametes. In the meantime, S 11 and S 22 epesent e- ection coecients, wheeas S 12 and S 21 ae dened as the tansmission coecients. In addition, we have [13]: Powe eected fom 1 S 11 = Powe incident on 1 ; Powe deliveed to 2 S 21 = Powe incident on 1 : (8) The magnitude of S 11 (js 11 j) gives the nomalized eectance. The nomalized eectance fo this multilaye SPR is calculated using [1]: S 11 = M 2 11 M 12 exp(i' 11 ); (9) whee ' 11 contains the phase infomation of S 11. The total M-matix of the whole stuctue can be expessed as the poduct of inteface and laye matices that descibe the eects of the individual intefaces and layes of the entie statied stuctue, taken in pope ode, as: M = with: M11 M 12 M 21 M 22 = I 01 L 1 I 12 L 2 I 23 L 3 I 34 L 4 :::I j L ; (10) j J j = 1 1 t jk j 1 Kzj ; jk = K zk " j " k K zj " j + K ; zk " k (11) whee t jk and jk ae the Fesnel complex tansmission and eection coecients, espectively. Also, the laye matix (phase matix) descibing the popagation though laye j is dened as: L j = e e id zkzj 0 0 e idzkzj ; (12) with:! K zj = c 2 "j " 0 sin 2 ; (13) whee " j and d j ae the dielectic constant and the thickness of the jth laye, espectively. Fo the detection of biomolecules, vaiations of S-paametes along with thei phase chaacteistics esulting fom biomolecula inteactions with the sensing suface ae studied in this pape. Thus, in ode to investigate the senso pefomance, we have studied the change of S-paametes fo detection and sensitivity enhancement. In ou poposed design, the esonances befoe and afte binding occu at the incidence angles of and degees, espectively. As shown in Figue 2, befoe the esonance with the angle of 45 degees, the intensity plasmonic wave is eected back wheeas at the esonance condition with the angle of degees, the oscillation of plasmonic wave is maximum.

4 3534 M. Majdi and D. Fathi/Scientia Ianica, Tansactions F: Nanotechnology 24 (2017) 3531{3535 Figue 2. Absolute electic eld: (a) Befoe and (b) at esonance condition. Figue 3. The peak values of SPR sensitivity as a function of the gold thickness changing fom 10 to 90 nm. Figue 5. Changes of dieent S-paametes vesus the incidence angle. Figue 4. Vetical eld along the z axis, E z, when the SPR stuctue pocesses the gold laye with the thicknesses of 45, 60, 75, and 90 nm. Figue 3 shows the peak values of sensitivity as a function of gold thickness in the ange of 10 to 90 nm. It is clealy obseved in this gue that the maximum sensitivity has been obtained as high as 3.82 at an optimal thickness of 60 nm. This is because the thicke gold laye leads to a esonance excitation of suface plasmons in a highe momentum. The vetical eld along the z axis (E z ) fo the gold laye thicknesses of 45, 60, 75, and 90 nm is shown in Figue 4. Accoding to this gue, as the gold laye thickness inceases fom 10 to 90 nm, the sensitivity inceases at the initial stage with incease in gating thickness. Howeve, when it exceeds 60 nm, the sensitivity begins to decease (see [14] fo moe infomation). Figue 6. Phase vaiation of S 11 (ag S 11) vesus the incidence angle befoe and afte the biomolecula inteaction. Figue 5 shows the changes of S-paametes vesus the incidence angle fo the poposed SPR biosenso. It is outlined that the maximum values of changes in S- paametes, js 11 j, js 12 j, js 22 j, and js 21 j, appea at thei esonance angles and the geatest change is elated to the S 11 paamete. In Figue 6, the phase plot of S 11 vesus the incidence angle befoe and afte the biomolecula inteaction is shown. It is obseved that thee is a apid tansition in the phase of S 11 at the esonance angle of degees befoe binding.

5 M. Majdi and D. Fathi/Scientia Ianica, Tansactions F: Nanotechnology 24 (2017) 3531{ Figue 7. Vaious gaphs demonstating the eects due to the thickness of taget biomolecules on the js 11j paamete. Figue 7 shows the investigation of js 11 j paamete changes vesus the incident angle fo dieent thicknesses of gaphene laye. As can be obseved in this gue, with incease in the thickness of gaphene laye, the magnitude of S 11 begins to decease. 4. Conclusions In this pape, we pesented the design and analysis of a multilaye SPR biosenso using the FDFD method. The ssdna was used as a ecepto molecule, which was mobilized on the senso suface to specically adsob its complementay countepat (cdna) immesed in a phosphate bue solution. With employing the S- paametes-based detection, it was demonstated that the poposed stuctue would impove sensitivity and speed of detection. Refeences 1. Choi, S.H., Kim, Y.L. and Byun, K.M. \Gapheneon-silve substates fo sensitive suface plasmon esonance imaging biosensos", Optics Expess, 19(2), pp (2011). 2. Jung, W.K., Kim, N.H. and Byun, K.M. \Numeical study on an application of subwavelength dielectic gatings fo high-sensitivity plasmonic detection", Applied Optics, 51(20), pp (2012). 3. Available at: 4. Lin, I.T. \Optical popeties of gaphene fom the THz to the visible spectal egion", Thesis of Maste of Science, Univesity of Califonia, Los Angeles, UCLA (2012). 5. Wu, L., Chu, H.S., Koh, W.S. and Li, E.P. \Highly sensitive gaphene biosensos based on suface plasmon esonance", Optics Expess, 18(14), pp (2010). 6. Mandenius, C.F., Chollet, A., Mecklenbug, M., Lundstvm, I. and Mosbach, K. \Optical suface methods fo detection of nucleic acid binding", Analytical Lettes, 22(15), pp (1989). 7. Islam, M.S., Kouzani, A.Z., Dai, X.J., Schietz, H.F., Lim, Y.C. and Michalski, W.P. \Sensitivity analysis of a sub-wavelength localized suface plasmon esonance gaphene biosenso", Complex Medical Engineeing (CME), 2012, ICME Intenational Confeence on. IEEE (2012). 8. Palik, E.D., Handbook of Optical Constants of Solids, 3, Academic pess (1998). 9. Taft, E.A. and Philipp, H.R. \Optical popeties of gaphite", Physical Review, 138(1A), pp. A197-A202 (1965). 10. Neto, A.C., Guinea, F., Pees, N.M., Novoselov, K.S. and Geim, A.K. \The electonic popeties of gaphene", Reviews of Moden Physics, 81(1), pp (2009). 11. Kim, K., Yoon, S.J. and Kim, D. \Nanowie-based enhancement of localized suface plasmon esonance fo highly sensitive detection: A theoetical study", Opt. Exp., 14, pp (2006). 12. Homola, J. \Electomagnetic theoy of suface plasmons", Suface Plasmon Resonance Based Sensos, 4, pp (2006). 13. Islam, M.S. and Kouzani, A.Z. \A S-paametes-based detection method fo a multilaye SPR biosenso", Annual Intenational Confeence of the IEEE Engineeing in Medicine and Biology Society, IEEE (2012). 14. Majdi, M. and Fathi, D. \Impovement of the sensitivity of nano biosensos by multilayeed stuctues", 24th Ianian Confeence on Electical Engineeing (ICEE) (2016). Biogaphies Mahdieh Majdi eceived the MSc degee in Optoelectonics fom Tabiat Modaes Univesity (TMU), Tehan, Ian, in He eseach inteests include gaphene plasmonics, plasmonic waveguides, plasmonic gates, and switches. Davood Fathi eceived the BSc degee in Electonic Engineeing fom Amikabi Univesity of Technology, Tehan, Ian, in 1990, and the MSc degee in Biomedical Engineeing fom Shaif Univesity of Technology, Tehan, Ian, in Afte some yeas of woking in industy, he woked towad the PhD degee between in Nanoelectonics with the Nanoelectonic Cente of Excellence, Thin Film and Photonics Reseach Laboatoy, School of Electical and Compute Engineeing, Univesity of Tehan, Tehan, Ian. D. Fathi joined the School of Electical and Compute Engineeing at Tabiat Modaes Univesity (TMU), Tehan, Ian, in 2010, as a faculty membe. His cuent eseach inteests include nanoelectonics, nanophotonics and optoelectonics, sola cells, and quantum tanspot. He is also autho o coautho of moe than 50 jounal and confeence papes in vaious elds of nanoelectonics and nanophotonics.