. 2019 May 4;19(9):2076. doi: 10.3390/s19092076

Table 1.

Gas sensors based on optical detection. $λ$ : operating wavelength; l: optical path; MEMS: micro-electro-mechanical systems; FF: face-to-face; //: unreported; LED: light emitting diode; DFB: distributed feedback laser; QCL: quantum cascade laser; MCT: mercury cadmium telluride; WG: waveguide; C $_{3}$ H $_{6}$ O: acetone; NH $_{3}$ : ammonia; CO $_{2}$ : carbon dioxide; CH $_{2}$ O: formaldehyde; NO: nitric oxide; CO: carbon monoxide; CH $_{4}$ : methane; C $_{2}$ H $_{4}$ O $_{2}$ : methyl formate; CH $_{3}$ OH: methanol.

Emitter	Detector	$λ$ [ $μ$ m]	l [cm]	Gas Detected	Detection Limit [ppm]	Topology (cell)	Power Consumption [mW]	Ref.
MEMS	Thermopile	8.26	10	C $_{3}$ H $_{6}$ O	50	FF	//
heater		10.6		NH $_{3}$	10			[63]
LED	Photodiode	4.26	//	CO $_{2}$	//	Planar	35
						(dome)		[47]
MEMS	Bolometer	4.26	8	CO $_{2}$	30	Planar	45
heater						(open)		[73]
LED	Photodiode	0.34	19.5	CH $_{2}$ O	4.3	FF	//	[48]
LED	Photodiode	0.226	50	NO	2	FF	//	[53]
DFB	//	1.65	20	CH $_{4}$	11	FF	//	[75]
MEMS	Pyroelectric	4.65	39.3	CO	8.8	Planar	//
heater		4.26		CO $_{2}$	8.7	(open)
		3.31		CH $_{4}$	10.3			[64]
MEMS	Thermopile	4.26	8	CO $_{2}$	50	FF	350
heater								[19]
//	//	4.26	0.5	CO $_{2}$	∼400	FF	//	[58]
HeNe	PbSe	3.4	0.15	CH $_{4}$	//	Cavity	//
laser						(∼25 μm)		[56]
Lamp	Pyroelectric	4.66	2.6	CO	//	Planar	//
		4.26		CO $_{2}$		(dome)
		3.33		CH $_{4}$				[59]
MEMS	Thermopile	4.26	7.5	CO $_{2}$	//	FF	∼80
heater								[43]
AlInSb	InSb	4.2	8	CO $_{2}$	1000	Planar	//
LED						(open)		[40]
MEMS	Thermopile	4.26	7	CO $_{2}$	//	FF	∼50
heater								[54]
LED	Photodiode	1.66	12	CH $_{4}$	100	FF	//
	(InGaAs)							[76]
MEMS	Pyroelectric	8.4	30	C $_{2}$ H $_{4}$ O $_{2}$	165	FF	//
heater		9.6		CH $_{3}$ OH	184			[77]
Laser	PbTe	3.31	0.5	CH $_{4}$	1000	WG	//	[60]
QCL	MCT	4.23	1	CO $_{2}$	5000	WG	//	[61]
DFB	InGaAs	1.65	10	CH $_{4}$	100	WG	//	[62]