Free radical sensors based on inner-cutting graphene field-effect transistors

Due to ultra-high reactivity, direct determination of free radicals, especially hydroxyl radical (•OH) with ultra-short lifetime, by field-effect transistor (FET) sensors remains a challenge, which hampers evaluating the role that free radical plays in physiological and pathological processes. Here, we develop a •OH FET sensor with a graphene channel functionalized by metal ion indicators. At the electrolyte/graphene interface, highly reactive •OH cuts the cysteamine to release the metal ions, resulting in surface charge de-doping and a current response. By this inner-cutting strategy, the •OH is selectively detected with a concentration down to 10−9 M. Quantitative metal ion doping enables modulation of the device sensitivity and a quasi-quantitative detection of •OH generated in aqueous solution or from living cells. Owing to its high sensitivity, selectivity, real-time label-free response, capability for quasi-quantitative detection and user-friendly portable feature, it is valuable in biological research, human health, environmental monitoring, etc.


Supplementary Note 1. Experimental details of preparation of graphene
Monolayer graphene was produced by chemical vapor deposition (CVD) as reported previously 1 . The graphene was grown on 25 μm thick Cu foils placed in the center of a tube furnace (GSL 1200X). The tube furnace was first heated to 1000 °C under a 4 sccm flow of H 2 (99.999%) within 30 min and kept at 1000 °C for 30 min.
And then, 16 sccm CH 4 (99.999%) was introduced as the carbon source in the atmosphere of 4 sccm H 2 . The growth process lasted for 15 min. Finally, the tube furnace was rapidly cooled to room temperature under the protection of H 2 gas, and the graphene film was obtained on the surface of Cu foil.
The graphene film was transferred to a clean SiO 2 /Si wafer by the electrochemical bubbling method. The surface of the graphene-on-Cu was first coated with poly-methyl methacrylate (PMMA) by spin-coating. And then, the graphene film was removed from the Cu foils by electrochemical bubbling at a potential of 2.5 V in 0.5 M NaOH aqueous solution. The PMMA/graphene was released from Cu substrate and washed by distilled water for three times, and then was lifted from the solution and transferred on the surface of clean SiO 2 /Si. The graphene film on SiO 2 /Si was obtained by further annealing in Ar (99.999%) at 400 °C for 30 min to remove the PMMA.

Supplementary Note 2. Calculation of the surface charge density
According to the literature 2 , the surface charge density on the graphene surface can be calculated by the equation where Δn is the change of surface charge density. C lq is the total interfacial capacitance between electrolyte and graphene, which consists of the double layer capacitance and the quantum capacitance 3 . The C lq is around 3 μF cm −2 , as previously reported 4

Supplementary Note 3. Debye length
The Debye Length (λ d ) is one of the key parameters in liquid gated FET sensor.
Within λ d , the charged target molecules will introduce a current response in the conducting channel. If the distance between the charged target molecules and the channel is larger than λ d , a significant deterioration of the sensor performance will be expected 5 . Here, the λ d in 0.01×PBS is around 7.3 nm. In the FET sensor, the distance between the indicators (Cd 2+ , Zn 2+ or Mg 2+ ) and the graphene surface is ~6 nm, according to the AFM result in Supplementary Fig. 2. The value is smaller than the λ d , allowing the sensitive response of the FET sensor upon the change of the amount of metal ion indicators on the graphene surface.

Supplementary Note 4. Cell culture and fluorescent imaging.
Hela cells were cultured in Dulbecco's modified Eagle's medium (DMEM) including high glucose with 10% (v/v) fetal bovine serum, penicillin (100 units mL −1 ), and streptomycin (100 μg mL −1 ). The cells were seeded in a 25 cm 2 culture bottle and incubated in the incubator for 12 h in an atmosphere of 5% CO 2 and 95% air at 37ºC. 4 To culture the Hela cell on the FET sensor, the device was put into a culture bottle, and then the Hela cells were seeded and incubated on the device in the same condition.
To capture a fluorescent image, a device on glass substrate with Hela cells was incubated in 2 μM DCFH-DA solution for 20 min. After washing the extra DCFH-DA by PBS, the device was incubated in 10 μg mL −1 LPS for 30 min. The Hela cells generated the •OH, which reacted with the DCFH-DA fluorescent probe. By using a