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. 2017 Jul 25;9(8):303. doi: 10.3390/polym9080303

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© 2017 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Wearable gas sensors and its integrated systems. (a) Photograph of transparent and flexible single-layer graphene (SLG) sensor channel-bilayer graphene (BLG) heater on a polyethersulfone (PES) substrate (scale bar 7 mm). (b)Temperature distribution along transverse (x-axis) and longitudinal (y-axis) direction of sensor-heater device structured as laterally intercalated SLG sensor channel (6 mm width) between BLG heaters (7 mm width) with applied 1.7 W of electric power. Here the red dot and blue dot are temperature profiles of thermal image in inset along x-axis and y-axis with origin at center on channel, respectively. Inset: Spatial temperature distribution of graphene heaters (7 mm width) which intercalate 6 mm width graphene sensor with applied 1.7 W. Here three broken squares indicate center channel and side heaters area, respectively (scale bar 7 mm). (c) Recovering time constant τ_r as a function of heater temperature. Inset: the recovering curves of the ΔR/R₀ as a function of time under different temperature range from room temperature to 250 °C. (d) The relative resistance variation ΔR/R₀ of SLG channels as a function of time including recovery step with 100 to 165 °C heating under different NO₂ gas concentration from 40 to 0.5 ppm. Reprinted with permission from Ref. [37] Copyright 2014, John Wiley and Sons. (e) Response curves of the sensor to NO₂ of different concentrations. Inset: The sensor response depends linearly on NO₂ concentration. (f) Response curves of the sensor to 50 ppm of NO₂ when tested under different bending angles. (g) Response curves of the sensor tested before and after bending 1000 and 5000 times (bending angle = 50°). Reprinted with permission from Ref. [114] Copyright 2014, John Wiley and Sons. (h) Photograph of microfabricated flexible room temperature ionic liquid (RTIL) based gas sensor (scale bars 1 cm and 2 mm, respectively). (i) Current versus time curve at various oxygen concentrations when the potential is held at −1.4 V vs. Au. Nitrogen is the background gas. Oxygen concentration steps up from 0% to 21% and steps down from 21% to 0%. Reprinted with permission from Ref. [117] Copyright 2013, IEEE. (j) Schematic illustration of the preparation of the PDA/MoS₂ film and the sensor upon exposure to DMF vapor. (k) UV-vis spectra of polydiacetylene (PDA)/MoS₂ composites with an increased ratio of MoS₂ to PDA in the absence and presence of 0.1% DMF vapor. (l) UV-vis spectra of PDA/MoS₂ films exposed to N,N-dimethylformamide (DMF) vapor with different concentrations. (m) UV-vis spectra of the PDA/MoS₂ film upon exposure to different (5%) vapors, in comparison with 2% DMF vapor. (n) Flexible transparent wrist strap with DMF sensing ability. Reprinted with permission from Ref. [110] Copyright 2017, Royal Society of Chemistry.

Wearable gas sensors and its integrated systems. (a) Photograph of transparent and flexible single-layer graphene (SLG) sensor channel-bilayer graphene (BLG) heater on a polyethersulfone (PES) substrate (scale bar 7 mm). (b)Temperature distribution along transverse (x-axis) and longitudinal (y-axis) direction of sensor-heater device structured as laterally intercalated SLG sensor channel (6 mm width) between BLG heaters (7 mm width) with applied 1.7 W of electric power. Here the red dot and blue dot are temperature profiles of thermal image in inset along x-axis and y-axis with origin at center on channel, respectively. Inset: Spatial temperature distribution of graphene heaters (7 mm width) which intercalate 6 mm width graphene sensor with applied 1.7 W. Here three broken squares indicate center channel and side heaters area, respectively (scale bar 7 mm). (c) Recovering time constant τ_r as a function of heater temperature. Inset: the recovering curves of the ΔR/R₀ as a function of time under different temperature range from room temperature to 250 °C. (d) The relative resistance variation ΔR/R₀ of SLG channels as a function of time including recovery step with 100 to 165 °C heating under different NO₂ gas concentration from 40 to 0.5 ppm. Reprinted with permission from Ref. [37] Copyright 2014, John Wiley and Sons. (e) Response curves of the sensor to NO₂ of different concentrations. Inset: The sensor response depends linearly on NO₂ concentration. (f) Response curves of the sensor to 50 ppm of NO₂ when tested under different bending angles. (g) Response curves of the sensor tested before and after bending 1000 and 5000 times (bending angle = 50°). Reprinted with permission from Ref. [114] Copyright 2014, John Wiley and Sons. (h) Photograph of microfabricated flexible room temperature ionic liquid (RTIL) based gas sensor (scale bars 1 cm and 2 mm, respectively). (i) Current versus time curve at various oxygen concentrations when the potential is held at −1.4 V vs. Au. Nitrogen is the background gas. Oxygen concentration steps up from 0% to 21% and steps down from 21% to 0%. Reprinted with permission from Ref. [117] Copyright 2013, IEEE. (j) Schematic illustration of the preparation of the PDA/MoS₂ film and the sensor upon exposure to DMF vapor. (k) UV-vis spectra of polydiacetylene (PDA)/MoS₂ composites with an increased ratio of MoS₂ to PDA in the absence and presence of 0.1% DMF vapor. (l) UV-vis spectra of PDA/MoS₂ films exposed to N,N-dimethylformamide (DMF) vapor with different concentrations. (m) UV-vis spectra of the PDA/MoS₂ film upon exposure to different (5%) vapors, in comparison with 2% DMF vapor. (n) Flexible transparent wrist strap with DMF sensing ability. Reprinted with permission from Ref. [110] Copyright 2017, Royal Society of Chemistry.