CC BY
60
Section 5. Chemistry
https://doi.org/10.29013/AJT-19-11.12-25-28
Buronov Akrom Oydinkulovich, Tashpulatov Khurshid Shukurovich, Nasimov Abdullo Murodovich, Samarkand State University, Samarkand, Uzbekistan
E-mail: [email protected]
DEVELOPMENT OF OPTICAL SENSOR FOR DISSOLVED AMMONIA WITH SOL-GEL ROUTE
Abstract. Optical sensor for the detection of microconcentration of dissolved ammonia was developed using the sol-gel technology. Parameters affecting the sensor performance were evaluated and optimal conditions for the reaction was proposed. Sensor show linear relationship to the concentration of ammonia in a wide range.
Keywords: indicator, sol-gel, absorption, immobilization, ammonia sensor.
1. Introduction. ing of water quality in polluted places, especially in
Ammonia is produced in vast quantities world- regions close to farms is utmost important. Because wide to produce further fertilizers and as a primary ammonia is notorious to be toxic at concentrations
above 25 Hg/L (1-3).
Numerous methods of ammonia detection of dissolved ammonia have been proposed. Because of availability of LEDs, photodiodes and cheap fluorescent and non-fluorescent dyes, optical sensor for detection of dissolved ammonia have been developing rapidly [3-6]. Many optical sensors for dissolved ammonia detection use polymer membrane as PVC, PVA or ethylcellulose.
In this paper we use tetraethoxysilane matrix and incorporated a pH-sensitive indicator bromocresol purple using the sol-gel route. Using i-butylalcohol leads to improved sensor property because of optimal match with the indicator dye. Developed optical ammonia sensors were investigated in different media and there performance was evaluated.
source of nitrogen to prepare a series of chemicals. Main reaction there is known as Haber process and 450 °C and high pressure (100 atm) along with Fe catalyst are used to promote the reaction between hydrogen and nitrogen gas. Ammonia is water soluble weak base and in aqueous solution the following equilibrium is established:
NH3( ) + H2O(l) ^ NH4+( ) + OH-( )
3(aq) 2 (l) 4 (aq) (aq)
pKb = 4.75
And main emission sources of ammonia to aqueous media in environment are decomposition of biological waste, use of fertilizers, agricultural run-off, and fowl-farm where excretion of waste from domestic birds. As gaseous ammonia is decomposed easily by photolytic reaction, dissolved ammonia in environment causes many problems. Thus monitor-
2. Experimental.
2.1. Materials and methods
Tetraethoxysilane (TEOS) was purchased from Haihang Industry Co., Ltd (PRC); ethanol (EtOH), bromocresol purple (BCP), hydrochloric
acid (HCl) and nitric acid (HNO3) were analytical grade and used without any purification. All buffers and solutions prepared using chemical pure grade reactants and doubly distilled water used as solvent.
//
C2H5 O
H5C2—O—Si—O—C2H5 O
C2H5
left right
Scheme 1. Structure of BCP (left) and TEOS (right)
2.2. Standard solutions
Ammonia test solutions were prepared using 25% (mass) aqueous concentration of ammonia solution and consequently diluted to reach the desired concentration. Doubly distilled water was used to prepare all solutions throughout the experiment. In order to keep solutions concentration constant, fresh test solutions were used in each experiment.
2.3. Preparation of sol-gel membranes Sol-gel solutions were prepared mixing 2 ml of
TEOS, 3.31 ml of i-CHOH for 30 minutes. 1.3 ml
' 4 9
Table 1. - Content of solutions used in experiment
0,01M HCl aqueous solution was added in order to start hydrolysis and condensation reactions. pH of the final solution was adjusted to 2 since this solution was found to be optimal. Resulting solution was mixed for 4 hours at room temperature. Then 40 ^l of 0.1M BCP in C2H5OH solution was added and another 30 minutes was mixed. In order to study water to alkoxide ratio R several sol cocktails were prepared and the content is given in (Tab. 1).
No. Alkoxide Solvent R ratio Catalyst
1. TEOS i-CHOH 4 9 1:1 HCl
2. TEOS 1-CHOH 4 9 1:2 HCl
3. TEOS i-CHOH 4 9 1:3 HCl
4. TEOS i-CHOH 4 9 1:4 HCl
5. TEOS i-CHOH 4 9 1:5 HCl
6. TEOS i-CHOH 4 9 1:6 HCl
Solution of sol then remained for 24 hours for aging. Microscope slides were taken and cut into 0.6x4 cm pieces. All glasses were activated in the aqueous solution of nitric acid for 1 hour and rinsed
with ethanol and copious amount of water before dip coating process. Preparation steps of samples are given in (Fig. 1).
Figure 1. Schematic diagram
2.4. Spectroscopic studies of prepared membranes
UV-vis spectrophotometer EMC-30PC-UV (EMC Labs Germany) was used to record the absorption spectra of the sensors. The surface of the sensor films was investigated using the Optika (Germany). Sensor membranes was rinsed in distilled water and dried before each experiment.
3. Results and discussion.
Measurements were made after one week prior the gelation process completed. All sensor samples were kept in a sealed polymer bag. Sensor layers show no deterioration or any significant change during preser-
of sample preparation steps. vation. In aqueous ammonia solution sensor films converted to violet color immediately. This shows pores of sol-gel film perfectly fulfill to serve as a membrane for the selected indicator dye. Moreover spectrochemical properties of BCP show no change during the immobilization process. This is very crucial, since non-covalent attachment ofdye molecule is driving force to maintain photochemical properties of the dopant.
Figure 2 show the absorption spectra of BCP in different aqueous ammonia solutions. BCP shows an intense peak around 590 nm in basic media. Increase in ammonia concentration also increases the intensity of the peak.
<
500
Wavelength (nm)
600
Figure 2. Absorption spectrum of BCP@TEOS sensor in different concentrations of aqueous ammonia (red - 1M, black - 0.2M). Intensity at 588 nm decreases as the concentration of ammonia decreases
0,32
0,30
Ä
0 o c ro
-Q s—
O w
-Q
<
0,28
0,26
0,24
0,22
R2=0.98967
0,2
0,4 0,6 0,8
Concentration (M)
—i— 1,0
Figure 3. Calibration curve of BCP@TEOS
The calibration curve of the sensor is shown in Figure 3. As can be seen from the figure, the absor-bance and the concentration shows a good linear relationship in the range of 1M to 0.1M concentration. We also performed experiments with higher ammonia concentrations, but deviation observed
in the linear relationship. Possible reason may be related of saturation of the sensor film with NH,, , and regeneration of sensor films have little effect on the stable signal reproduction.
Using other solvents as CH3OH, n-C3H7OH, i-C3H7OH show similar behavior, but the surface shown more cracks and we decided to use i-C4H9OH as a solvent. Moreover sensors prepared using i-C4H9OH show significant improvement in the sensor quality and no practical leaching was observed.
sensors for aqueous ammonia detection
The study also aimed to investigate different parameters to the sensor. For this purpose sensor layers are subjected to hot solutions and other acidic solutions. The sensors show stable response at different temperatures. As the sensing scheme is based on the pH-change, effect ofpH was not studied. Acidic media does not interfere to determine dissolved ammonia.
Sensors stored in a sealed polymer bag remained stable for 6 months. However the initial absorbance shown little decrease in intensity.
Conclusions
Optical sensor for the determination of dissolved ammonia is developed. The sensor shows a linear relationship to the concentration of ammonia in a wide range. Effects of different parameters also studied during the experiment. Sensors shown stable response to dissolved ammonia in different conditions.
References:
1. Timmer B., Olthuis W., van der Berg A., Sens. Actuators B107, 2005.- P. 666-677.
2. Porello S., Ferrari G., Lenzi M., Persia E. Aquaculture 219, 2003.- P. 485-494.
3. Waich K., Mayr T., Klimant I. Talanta 77, 2008.- P. 66-72.
4. Lieberzeit P. A., Dickert F. L. Anal. Bioanal. Chem. 387, 2007.- P. 237-247.
5. Malins C., Butler T. M., MacCraith B. D. Thin Solid Films 368, 2000.- P. 105-110.
