4.1. Back-gated GFETs: Back gated GFETs are less frequently employed as DNA sensors, likely due to the difficulties associated with producing devices requiring small gate voltages (VG) rather than the more commonly and readily prepared higher gate voltage devices, which require VG on the order of tens to hundreds of volts [51] to achieve significant gain. However, back-gated GFETs have clear sensing advantages over liquid-gated assemblies in conditions in which the analyte solution composition may vary, and in situations where the analyte is not in a solution matrix, e.g., vapor detection [52]. Kybert et al. have recently reported the use of DNA- decorated graphene for arrays of chemical vapor sensors which demonstrated a significant shift in the VG required to observe the minimum in the device conductance (VG, min) after DNA deposition [48]. ssDNA was adsorbed onto the surface of graphene, which allowed for chemical vapor sensing down to parts-per-billion as analyte binding further shifted the VG, min. Fig. 7 shows the device setup, current voltage curves demonstrating the value of VG, min, which is also termed the Dirac voltage, for the back-gated GFETs before and after the addition of DNA, a mobility histogram and the dispersion of VG, min values observed for an array of devices. These authors explain the positive shift in VG, min as counter- acting the negative field produced by the phosphate backbone of the adsorbed ssDNA. The gate voltage shifts positive to overcome the negative field induced by DNA in order to maintain a similar charge state for graphene after adsorption. Shifts of VG, min in the direction of a more positive gate voltage were previously reported by the same group (Fig. 8) [48]. Generally, back-gated GFETs show relatively large VG, min shifts compared to liquid-gate schemes discussed in the next section. Similar electronic principles apply to liquid-gated GFETs but generally require lower potentials compared to back-gated devices. [48] N.J. Kybert, G.H. Han, M.B. Lerner, E.N. Dattoli, A. Esfandiar, A.T.C. Johnson, Scalable arrays of chemical vapor sensors based on DNA-decorated graphene, Nano Res. 7 (2014) 95–103. Figure is as seen in review article by N. Green and M. Norton – review article text below Histogram of 56 devices measured ambient/dry

Detection of DNA and poly-l-lysine using CVD graphene-channel FET biosensors Aniket Kakatkar1, T S Abhilash2, R De Alba2, J M Parpia2 and H G Craighead1 Nanotechnology 26 (2015) (5pp) All measurements are performed with a source–drain voltage of VSD = 50 mV. The Dirac point is observed at ∼ 2 V, indicating that the transfer scheme is relatively clean. The deviation from the ideal Dirac point at 0 V (unmodified gfet - dn) could result from a combination of trapped charges in the oxide and the substrate [24] as well as the graphene quality. ~50umx50um gfets Upon rinsing under running water and drying, the DNA entirely desorbs from the graphene (see supplementary section 2). The biosensors have a detection limit of 11 pM for poly-l-lysine and 8 pM for λ DNA. This is calculated from three times the standard deviation3 of the Dirac peak voltage shift at zero concentration, which are 1.4 V and 1.2 V for poly-l-lysine and DNA respectively. + charged amino acids - charged nucleic acids measured ambient/dry

Summary of recordings done at WSU by david neff Weidong zhang and elloitt brown on October using Phi’s chip number I do not have the recordings of this device as measured by Phi before sending to Weidong. I believe that Weidong has these plots and that they match closely with our pre-modification measurements. This chip was fabricated by Phi on Si (doped? resistivity? – WZ says high resistivity) with a 90nm oxide layer. -OTS monolayer on the SiO2 before applying graphene – YES To prevent ‘doping’ of graphene by SiO2. -Benzimidazole NOT used on graphene before applying to SiO2/OTS (can see in the relatively low Dirac point of ~30V, not 100V as Phis says is case in doped samples at WSU – device

2.0nM back gate drain source Previous expts. (in adsorption kinetics) done by M.Rahman – concentration here is.3nM dna origami ON HOPG 0.3nM These origami are somewhat different design (arm anchors present) than those used in GFET expts. at WSU THz/GFET transmission setup at WSU at WSU – device

2.0nM dna origami added to GFET - settling time 17 minutes Water added to GFET - settling time 3 minutes Rinsed GFET – settling time 25 minutes THz transmission AU All measurements taken after GFET is blown dry. Multiple plots at each stage represent multiple scans through Vgs 15-35V. Scans were repeated until the GFET response stopped trending with time. Buffer control (not shown) prior to dna addition showed much quicker (3 minutes) settling time than seen with dna addition. Ids (amps) at Vds = 0.05V THz transmission AU Ids (amps) at Vds = 0.05V All plots show THz transmission and DC measurements of device prior to any analyte treatment. Blue vertical line shows Dirac point of device prior to any solution exposure.

at WSU – device DNA ON SURFACE BETWEEN SOURCE AND DRAIN AFTER RINSING 0.4um x 0.4um

After return to Marshall U, we measured the same device for DC response. We varied the relative humidity of the air over the GFET from 2% - 30% with no apparent effect on DC current measurements at Vsd = 0.05 and Vgs = 5V. About 1 hour was given for the GFET to equilibrate with atmosphere at each RH. AFM images reveal that the rinsed GFET is still covered with much dna origami. This extensive coverage does not seem to affect electronic properties of graphene as profoundly as is seen in some literature: N.J. Kybert, G.H. Han, M.B. Lerner, E.N. Dattoli, A. Esfandiar, A.T.C. Johnson, Scalable arrays of chemical vapor sensors based on DNA-decorated graphene, Nano Res. 7 (2014) 95–103 and Detection of DNA and poly-l-lysine using CVD graphene-channel FET biosensors Aniket Kakatkar1, T S Abhilash2, R De Alba2, J M Parpia2 and H G Craighead1 Nanotechnology 26 (2015) (5pp) The devices in these studies were <100um in any dimension. David Neff will visit Brown lab at WSU this week ( ) while Phi is there to repeat measurements with new devices. Norton lab (Abhijit R.) is performing experiments to determine if Mg++ is protective against the dissolution seen when dna origami adsorbs to HOPG/graphene. Also we are eploring ways to reverse dna binding to HOPG/graphene. These studies are in part to answer reviewers at WSU – device