Ultra-sensitive gas phase detection of 2,4,6-Trinitrotoluene by non-covalently functionalized Graphene Field Effect Transistor
The high energy density (4.2 MJ/kg) and low vapor pressure (7.2 x 10-9 atm) of chemical explosives such as TNT (2,4,6-Trinitrotoluene) poses grave security risk demanding immediate attention. Detection of such hazardous and highly challenging chemical demands specific, ultra-sensitive and rapid detection platforms that can concomitantly transduce the signal as electrical readout. Although chemo-sensitive strategies have been investigated, a majority of them are restricted to detecting TNT from solutions and is therefore not implementable in real-time, on-field situations. Addressing this demand, we report a ultra-sensitive (parts-per-billion) and rapid (~ 40 s) detection platform for TNT based on non-covalently functionalized graphene field effect transistors (GFETs). This multi-parametric GFET detector exhibits a reliable and specific modulation in its Dirac point upon exposure to TNT in vapor phase. The chemical specificity provided by 5-(4-hydroxyphenyl)-10,15,20-tri(p-tolyl) zinc porphyrin (ZnTTPOH) is synergestically combined with the high surface sensitivity of graphene through non-covalent functionalization approach to realise p-doped GFETs (Zn-GFETs). Such a FET platform exhibits extremely sensitive shifts in Dirac point (ΔDp) that correlate with the number of nitro-groups present in the analyte. Analytes with mono-, di-, tri-nitro substituted aromatic molecules exhibits distinctly different ΔDp, leading to unprecedented specificity towards TNT. Additionally, the Dirac point of Zn-GFETs is invariant for common and potential interferons such as acetone and 2-propanol (perfume emulsifiers) thereby validating its practical applicability. Further, the ΔDp is also manifested as changes in the contact potential of GFETs, indicating that sub-monolayer coverage of ZnTTPOH is sufficient to modulate the transfer characteristics of GFETs over an area 1000 times larger than the dopant dimensions. Specifically, ZnTTPOH-functionalized GFETs exhibits p-doped behaviour with positive ΔDp with respect to pristine GFETs. Such p-doped Zn-GFETs undergoes selective charge-transfer mediated interactions with TNT that results in enhanced electron withdrawal from Zn-GFETs. Thus the ΔDp shifts to higher positive gate voltage leading to the dichotomous combination of highest signal generation (1.2 x 1012 V/mole) with ppb level molecular sensitivity. Significantly, the signal generated due to TNT is 105 times higher in magnitude compared to other potential interferons. The signal reliability is established in cross-sensitivity measurements carried out with TNT-mDNB (1:10 molar ratio) mixture pointing to high specificity for immediate applications in atmospherically relevant conditions pertaining to homeland security and global safety.