fiber optic chemical sensor for air pollutant measurement

8/3/2019 Fiber Optic Chemical Sensor for Air Pollutant Measurement

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Asian Journal of Physics - Special issue to be published.

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Fiber Optic Chemical Sensor for Air Pollutant Measurement

Design, Development and Applications

Ang Soo Seng, M. S. John and Anand Asundi

School of Mechanical and Production Engineering

Nanyang Technological University, Singapore 639798

Email: [email protected]

ABSTRACT

In this paper, the development of a sol gel encapsulated fiber optic chemical sensor to detect

the presence and concentration of sulphur dioxide (SO2) and nitrogen dioxide (NO2), two

major air pollutants are discussed. The sensing chemical, rhodamine 6G a fluorescence dye

which absorbs light at 530nm and emits radiation at 560nm, is used to detect SO2 while a

combination of sulfanilamide (SFA) and N, N Dimethyl-1-naphthalamine (DMNA), which

absorb light at 450nm, is used to detect NO2. The sensing chemical used is exclusive to

individual gases and responds to the specific gas. The chemical reaction between the gas and

the sensing chemical changes the transmitted light intensity, which is detected by a

spectrometer. The presence and concentration of the gas is determined by the change in light

intensity and the rate of change in light intensity, respectively. Two probes are developed

an intrinsic fiber optic chemical sensor made up of a sol gel encapsulated chemical coated

optical fiber and an extrinsic fiber sensor which uses a sol gel encapsulated chemical coated

glass plate as the sensing element. Experiments show that the intrinsic sensor is better in

fluorescence quenching test while the extrinsic performs better in light absorption

experiments.


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1. INTRODUCTION

Sol-gel is a process that enables materials to mix on a molecular level from a sol into a colloidal gel.

The moisture in the colloidal gel is removed and after drying the gel will become a porous solid. The

porosity of the solid depends on the catalyst used in the mixing process, an acidic catalyst produce

fast hydrolysis leading to weakly branched microporous structures while basic catalyst produce slow

hydrolysis leading to highly branched structures with large pore size. This process is used to coat the

liquid chemical onto a sample surface (optical fiber or glassplate) then after drying the solidified

chemical will be immobilized on the sample surface. [1-3]

Entrapping a suitable chemical in the sol-gel forms a chemical sensor. The chemical reacts with the

specific gas and changes the spectral characteristics of the transmitting light. In this paper intrinsic

and extrinsic fiber optic chemical sensors are developed for detection and sensing. For the detection

of sulphur dioxide gas, the principle of fluorescence quenching of rhodamine 6G entrapped in sol gel

matrix is used. Similarly, the detection of nitrogen dioxide gas uses the principle of light absorption

of sulfanilamide (SFA) and N, N Dimethyl-1-naphthalamine (DMNA), entrapped in sol gel matrix.

Since the chemicals are gas specific multi-channel chemical sensing is also possible.

2. EXPERIMENTAL DETAILS

In light absorption experiments, the Lambert-Beer law is used to measure the intensity of absorption.

If I0 is the intensity of radiation incident normally on an absorbing material layer of thickness b cm

and concentration c then the intensity of the emergent beam I is, [4,5]

cb

II

= 100 (1)


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AcbI

I==

0

10log (2)

where is the molar absorptivity and A is the absorbance of the sample in the beam. There is a linear

relationship between the absorbance A and concentration c of a given solution if the optical path

length and the wavelength of the light signal are kept constant.

Fluorescence quenching is the decrease in fluorescence intensity in the presence of a particular

substance or gas. The dynamic quenching of fluorescence is described by the Stern-Volmer

equation, [6]

[ ] [ ]QKQkF

FDq

+=+= 11 00

(3)

KD = kq0 (4)

where F0 is the fluorescence intensity in the absence of quencher, F is the fluorescence intensity in

the presence of quencher, kq is the bimolecular quenching constant, 0 is the lifetime of the

fluorophore in the absence of quencher, [Q] is the concentration of quencher, KD is the Stern-Volmer

quenching constant. A linear Stern-Volmer plot usually indicates that a single class of fluorophores

is accessible to the quencher. If two classes of fluorophores are present and one is not accessible then

the Stern-Volmer plot deviates from linearity toward the x-axis.

2.1 Preparation of Sensor

This process describes the preparation of the intrinsic sensor. A 200 m diameter chemically coated

optical fiber sensor is prepared by first stripping away, from a 4 cm length, the plastic coating from


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the optical fiber. The cladding of the optical fiber is then etched for 1 hour in 50% hydrofluoric acid

to expose the core region. The fiber core is immersed in NHO3 solution for 5 minutes to enhance the

chemical coating process. The rhodamine 6G solution is prepared by dissolving solid rhodamine 6G

with ethanol into a 0.001M solution. The combination of sulfanilamide (SFA) and N, N Dimethyl-1-

naphthalamine (DMNA) solution is prepared by dissolving SFA and DMNA with acetic acid into a

0.01M solution. The sol gel mix is prepared by adding 12 ml of Tetraethyl orthosilicate (TEOS) to

2.8 ml of de-ionised water together with 1 ml of ethanol and stirred. After 20min, 1 ml of rhodamine

6G solution or sulfanilamide (SFA) and N, N Dimethyl-1-naphthalamine (DMNA) solution and 40

l of 37% hydrochloric acid are added and stirred continuously until mixture turns into a sol. The sol

is aged at 24C for 40 hours for rhodamine 6G film or 10 hours for SFA-DMNA film until it

becomes viscous. The prepared fiber core is dip-coated in the viscous solution. The coated fiber core

is left to solidify for 3 days and become glassy. A schematic of the intrinsic Fiber Optic Chemical

Sensor (FOCS) probe is shown in Figure 1(a).

Figure 1. (a) Schematic of the intrinsic FOCS probe and (b) extrinsic FOCS probe

(a)

(b)


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(a) (b)

Figure 2. (a) Schematic of experimental setup for intrinsic fiber optic chemical sensor

(b) Typical spectrum from the NO2 sensor

3. RESULTS

3.1 Results for sensing NO2 using extrinsic fiber optic chemical sensor

The gas pressure in the chamber was varied from 0.06bar to 0.12bar. The following parameters are

kept constant: the wavelength of the light source (455.88nm), the integration time which is the time

interval for the spectrometer to register the data (110ms), the sensing chemical concentration

(0.01Molar) and the data acquisition interval (60s). Figure 3 shows that the rate of decrease of light

intensity increases as the NO2 gas concentration increases.

430 440 450 460 470 480 4901000

1200

1400

1600

1800

2000

2200

NO2

concentration - 232.7ppm

Intensity

Wavelength (nm)

0 min

2 min

4 min6 min

8 min

10 min


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Figure 3. Change in intensity as a function of time for different NO2 concentrations using the

extrinsic FOCS

3.2 Results for sensing NO2 using intrinsic fiber optic chemical sensor

All parameters are the same as for the previous experiment, except that the integration time is set to

20 ms and the data acquisition interval is reduced to 30 s. Figure 4 again shows that the rate of

decrease of light intensity increases as the NO2 gas concentration increases.

0 2 4 6 8 10 12 14 16 18 20

0.5

0.6

0.7

0.8

0.9

1.0wavelength at 455.8nm

NO2concentration - 181.7ppm

Y2=A+B*X

A 0.777

B -7.01x10-3

Y1=A+B*X

A 0.990

B -4.96x10-2

NormalisedInten

sity

Time (min)0 2 4 6 8 10 12 14 16 18 20

0.5

0.6

0.7

0.8

0.9

1.0wavelength at 455.8nm

NO2


Y2=A+B*X

A 0.648

B -2.74x10-3

Y1=A+B*X

A 0.965

B -5.81x10-2

NormalisedIntensity

Time (min)

0 2 4 6 8 10 12 14 16 18 20

0.5

0.6

0.7

0.8

0.9

1.0 wavelength at 455.88nm


Y2=A+B*X

A 0.666

B -9.95x10-3

Y1=A+B*X

A 0.994

B -7.42x10-2

NormalisedIntensity

Time (min)


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Figure 4. Change in intensity as a function of time for different NO2 concentrations using the

intrinsic FOCS

3.3 Results for sensing SO2 using extrinsic fiber optic chemical sensor

The fluorescence signal from rhodamine 6G in the plate fiber probe sensor is very weak for the

extrinsic FOCS and thus the data is not reliable. The sensor was not used further although

improvements to the sensitive are being investigated.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0.5

0.6

0.7

0.8

0.9

1.0


Y2=A+B*X

A 0.68292

B -0.00286

Y1=A+B*X

A 0.94009B -0.02333

NormalisedIntensit

y

Time (min)

wavelength at 455.88nm

0 2 4 6 8 10 12 14 16 18 20

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2


Y2=A+B*X

A 0.56993

B -0.00467

Y1=A+B*X

A 1.03059

B -0.04499

NormalisedIntensity

Time (min)


0 5 10 15 20 25 30 35 40 45

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05


Y2=A+B*X

A 0.7908

B -0.00133

Y1=A+B*X

A 0.97596

B -0.01094

NormalisedIntensity

Time (min)



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3.4 Results for sensing SO2 using intrinsic fiber optic chemical sensor

The gas pressure ranges from 0.1bar, 0.2bar, 0.3bar and 0.4bar. The following factors are kept

constant, the wavelength of the light source at 548.59nm, the integration time at 1200ms, the

chemical concentration at 0.001Molar, the time interval at 30s. Figure 5 shows that fluorescence

quenching takes place as the SO2 concentration increases.

Figure 5. Fluorescence quenching as function of time for different SO2 concentrations using the

intrinsic FOCS

0 60 120 180 240 300 360

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

SO2


Y=A+B*X

A 1.00184

B -8.8322x10-5

NormalisedIntensity

Time (sec)

548.59nm

0 60 120 180 240 300 360

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

SO2


Y=A+B*X

A 1.00636

B -1.5986x10-4

NormalisedIntensity

Time (sec)

548.59nm

0 60 120 180 240 300 360

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

SO2


Y=A+B*X

A 0.98831

B -2.0658x10-4

NormalisedIntensity

Time (sec)

548.59nm

0 60 120 180 240 300 360

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

SO2


Y=A+B*X

A 1.00716

B -2.5318x10-4

NormalisedIntensity

Time (sec)

548.59nm


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4. DISCUSSION

4.1 NO2 experiment

Both the intrinsic and extrinsic FOCS showed a decrease in light intensity when exposed to NO2 gas.

When the SFA and DMNA chemical coating reacts with the NO2 gas, the absorbance of the

chemical increases leading to a reduction in light intensity reaching the detector.

Figure 6. Absorption gradient for (a) intrinsic and (b) extrinsic NO2 FOCS

Figure 6 shows the light absorption gradient for the intrinsic and extrinsic NO2 FOCS. It is noticed

that the gradient extrinsic FOCS is greater than that for intrinsic FOCS. Comparing the two gradients

it is noticed that the extrinsic FOCS is 40% more sensitive than the intrinsic FOCS. The chemical

coating process might be the cause for this as the flat surface of the glass plate is easier to coat than

the cylindrical surface of the optical fiber. Further the coating on the glass plate can be made thicker

than that on the cylindrical optical fiber. Since absorbance depends on the thickness of the chemical

coating as well, the glass plate probe sensor is more effective in absorbing the light. There are

several factors to consider when fabricating an effective absorption sensor like the concentration of

the SFA and DMNA chemical coating, the thickness of the chemical coating, the surface roughness

180

190

200

210

220

230

240-5.0

-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0

Rate of change = -6.71x10-2

Absorption gradient for extrinsic FOCSGradient

Gas concentration (ppm)180 190 200 210 220 230 240-7.5-7.0-6.5-6.0-5.5-5.0

Rate of change = -4.85x10-2

Absorption gradient for intrinsic FOCSGradient

NO2concentration (ppm)


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The Stern-Volmer graph is plotted by taking F0 /F (time at 0s / time at 400s) against gas

concentration and is shown in fig. 7(b). The quenching efficiency [6] is

kq = ko. (5)

From Figure 7, the gradient of the lines gives the constant, KD= 3.4x10-4

M-1

. With 0 3.7ns [7], the

bimolecular quenching constant, kq from the Stern-Volmer equation is

KD = kq0

Hence,

4

9

4

0

1012.9107.3

104.3x

x

xKk Dq ===

M

-1s

-1

The diffusioncontrolled bimolecular rateconstant (k0)can be found by solving the Smoluchowski

equation [6] in order to calculate the quenching efficiency,

( )( )qfqf DDRR

Nk ++=

1000

40

(6)

where Rf is the molecular radius of rhodamine 6G, Rq is the molecular radius of SO2, Df is the

diffusion coefficient of rhodamine 6G, Dq is the diffusion coefficient of SO2 and N is the Avogadros

number. The molecular radius is given as [8]

3

1

Vr (7)


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where r is the molecular radius and V is the volume of the molecule. The molecular radius is

approximately equal to the cube root of the volume of molecule. The diffusion coefficient of SO2

can be obtained from Stokes-Einstein equation [6],

RkTDq 6/= (8)

where k is the Boltzmann constant, is the solvent viscosity, R is the molecular radius and T is the

temperature. Solving the diffusioncontrolled bimolecular rate constant gives k0 =3.45x105M

-1s

-1.

Thus the quenching efficiency is as follows:

26.01045.3

1012.95

4

0

===x

x

k

kq

Thus only 26% of the molecular collisions between the rhodamine 6G and SO2 are effective in

fluorescence quenching. The reasons that might affect the quenching efficiency are the concentration

of the rhodamine 6G chemical coating, the thickness of the chemical coating, the surface roughness

of the chemical coating. The chemical coating process and drying process are also very important in

improving the efficiency of the fluorescence quenching. This could be one of the reasons that the

extrinsic FOCS is not sensitive enough. Furthermore for the intrinsic FOCS the fluorescence is

integrated over the length of the fiber, while for the extrinsic FOCS, the fluorescence is confined to a

small area and thus the signal is weaker.

5. CONCLUSION

Fiber Optic Chemical Sensors (FOCS) for detecting NO2 and SO2 have been designed, developed

and tested. Two sensing configurations have also been investigated for each of these sensors:


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(1)Chemical coated fiber sensor (intrinsic FOCS)

(2)Glass plate probe sensor (extrinsic FOCS).

The extrinsic FOCS is found to be better suited for absorption experiments as the thickness of the

coating can be large as well more uniform. However for fluorescence measurements, since the

intensity is weak, the intrinsic FOCS, with a larger sensing area, gives a stronger signal than the

extrinsic FOCS. The preparation of the chemical coating is also crucial in the design of an effective

gas sensor. Miscalculation in the chemical proportion or incorrect mixing of chemicals will inhibit

the sol from setting. Besides this, the concentration of the sensing chemicals also affects the

sensitivity of the gas sensor.

6. REFERENCES

1. C. McDonagh, B. D. MacCraith, A. K. McEvoy, Tailoring of Sol Gel Films for Optical Sensing

of Oxygen in Gas and Aqueous Phase, Anal. Chem. 1998, 70, pp45-50

2. R.W. Jones, Fundamental Principles of Sol-Gel Technology, Institute of Metals (1988)

3. Lisa C. Klein, Sol-Gel optics: Processing and Applications, Kluwer Academic Publishers

(1994)

4. C. Burgess and A. Knowles, Standards in Absorption Spectroscopy, Chapman and Hall

5. D. A. Krohn, Fiber optic sensors - Fundamentals and Application, Instrument Society of

American (1988)

6. Lakowicz Joseph R., Principles of Fluorescence Spectroscopy, Plenum Press (1983)

7. Lifetime decay of Rhodamine 6G with two-photon excitation,

http://www.mi.infm.it/~biolab/tpe/fh6g.htm

8. Robert H. Perry and Don W. Green, Perrys Engineers Handbook (7 th edition), Mcgraw Hill,

pp2-230, pp5-50
http://www.mi.infm.it/~biolab/tpe/fh6g.htmhttp://www.mi.infm.it/~biolab/tpe/fh6g.htm

fiber optic chemical sensor for air pollutant measurement

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