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Abstract A kind of optical fiber based on fluorescence quenching oxygen sensors. Cellulose acetate membranes were prepared and studied the sensors performance, the results show that the sensing film uniformity , stability, fast response.Using phase-locked amplification technology to achieve the detection of weak fluorescence signal. The sensors has high precision, good repeatability and stability.

Key words optical fiber oxygen sensors sensing membrane phase lag lock enlarge

Detection of oxygen and other gases in the industrial and agricultural production , environmental monitoring, medical and many other fields are very important. Conventional oxygen sensors used to detect most of the probe through the change of resistance or capacitance to measure the oxygen concentration, vulnerable to external electric field, magnetic field interference, but also in flammable and explosive environment, which will create security risks. Since fiber optic sensors has high precision, free from external electromagnetic interference, the use of the advantages of safety, therefore, developed a new type of high performance fiber optic gas sensors become increasingly been paid attention [1 _ 4] .

Optical fiber gas sensors, the key is to solve low-light sensing film preparation and signal detection are two problems. This paper developed a fiber-optic based on fluorescence quenching oxygen sensors of the sensing membrane and cellulose acetate method and performance. The fluorescent indicator fluorescence emitted by the excited weak, so we use the phase-locked amplifier technology, the sensors by measuring the fluorescent probe to determine the lag phase shift the concentration of oxygen to obtain good results.

In the experimental part of the

1, reagents and equipment

Fluorescent indicator for the ruthenium ( I ) bipyridine Ru (bpy) 3 Cl 2 , prepared by the method reported in the literature and purification [5] . It it were of analytical grade, commercial products, direct use. Lock-in amplifier from the United States Stanford Research Systems purchase, model SR830 .Binocular microscope purchased from Chongqing Optical Instrument Factory, model XSZ - 3G , maximum magnification of 1600 times.

2, the sensing membrane and

Certain amount of cellulose acetate ( CA), added the right amount of acetone and a certain concentration of fluorescent indicator solution , in the room under the conditions of mild agitation sealed 6h , CA completely dissolved to form solution. Apply a small amount of this solution drop in fiber optic sensors probe end, and make it spread evenly in the air for some time, that formed sensors film in the end , the thickness may drop in the probe end of the volume control. Binocular microscope with the status of film.

3 , fluorescent test principle and devices

Oxygen for some fluorescent substances (such as Ru (bpy) 3 Cl 2 ) of the fluorescence quenching effect, resulting in the decrease of its fluorescence intensity and fluorescence lifetime shortened. The fluorescence intensity of fluorescent material or the relationship between life and oxygen concentration can be Stern - Volmer equation to describe:

2 = 1 + K [Q]

(1)

Where I . , I , r ., R respectively, oxygen and oxygen-free conditions, the fluorescence intensity and lifetime, [Q] is the concentration of oxygen, K for the Stern-Volmer constants of quenching agent for a particular value is fixed. By measuring 1 . , 1 , or &, r , to determine the concentration of oxygen.

Phase shift method we use to achieve the determination of fluorescence lifetime . The laser used in light source through the sinusoidal modulation , so the indicator 's fluorescence intensity also showed sinusoidal changes, but relative to the excitation light has a phase shift lagging 0,0 of the tangent function and the fluorescence lifetime T has

If T Relationship :

tan0 = w r (2) where w is angular velocity of sinusoidal modulation , o) = 27rf , f is the frequency of sinusoidal modulation signal.

To rule out the excitation light on the impact of test results, we use technology to lock the required amplification of fluorescence signal to lock and zoom. Modulating signal source to provide sinusoidal modulation, sinusoidal modulation signal of light modulated to produce light intensity by the sine law of change excitation signals, resulting in fluorescence output also by sine law changes, but the fluorescence output relative to the excitation signal, there is a lag phase shift 0 . 0 and the fluorescence lifetime r has the following relationship:

tan0 = 27rfT (3)

Where f is frequency of sinusoidal modulation. Thus, by measuring the 0 to determine r .

Where 0 . , 0 respectively, without oxygen and oxygen lag phase shift, Co 2 for the oxygen concentration. Therefore, the determination of the different

Oxygen concentration of 0 , the value of oxygen concentration can be derived. Since detection of the signal being measured and the reference object is the phase difference between signals, rather than changes in light intensity, which can eliminate stray light effects on the fluorescence signal, has strong anti-interference ability and higher accuracy.

Experimental setup is shown in Figure 1 below. Were controlled by gas flow meter O 2 and N 2 flow , the mixing chamber after mixing, then get different O 2 concentrations of the gas mixture.

First, open only to control the nitrogen gas flow , after a period of ventilation, so sensors chamber in the gas environment and gas mixing chamber in the same environment, and observe the output of the phase lock-in amplifier is a constant value of the 'record the phase value 'This is the oxygen concentration in the lag under zero phase shift 0 . .Regulating gas flow meter, so mixing chamber for containing a certain concentration of oxygen gas mixture, obtained by the same method the oxygen concentration in a certain environment, the lag phase shift 0 °

Third, the results and discussion

1, the sensing film properties

We fixed aqueous acetone and the volume of fluorescent indicator, respectively Sml and 0. 5ml , change the CA amount to respectively 0. 15,0. 20,0. 25,0. 30,0. 35,0. 40 0. 45g , found that when the CA in the amount of W ca = O . 30g , the best film uniformity, pore size distribution. When W ca adding amount of time for other, less uniformity of membrane pore size distribution is uneven. Therefore, the preparation of CA sensors membrane ratio of the best ingredients: W ca : W nm V water = 0. 30g : 8. OML : 0.50ml .Click here to prepare the ingredients than the sensing film placed at room temperature 4 months, and its morphology is essentially the same, indicating that the sensing film has good stability.

In order to examine the response time of the sensing films, we adjust the oxygen concentration of Co 2 to the 100% change to 0 and then change to 100% , found that the lag phase shift 0 and steady changes in the time required when no more than 5 seconds , indicating that the response time of the sensing films t <5 seconds. Chemical sensors for this response is relatively fast, indicating that the sensing film has a better sensors performance.

2, oxygen concentration and the lag phase shift 0 between

We set the excitation frequency f = 40KHz, at room temperature measured at different oxygen concentrations C 02 sensors head under the lagging phase shift, the variation of Figure 2 shows.

From Figure 2, the known , " 2 and ta n 0 . / tan0 larger concentration range between a good linear relationship within the 'high sensitivity. We were under the same conditions many times repeated experiments, the error less than 2% , indicating that this sensors has good repeatability. We're still f = 60KHz , f = 100KHz determined under the conditions of the oxygen concentration and the lag between the phase shift and found that C 02 and ta n 0 . / t an 0 as in the larger concentration range between a good linear relationship within, and as f increases , C 02 to 100% of the time tan0 . / tan0 and C 02 to 0 when the tan0 . / ta n 0 increased the difference, which indicates that f increases help improve the sensitivity of the sensors. However, if f is too high, the detection circuit is more demanding.

3, the stability of the sensors of a

Will be coated with fluorescent indicator probe placed 4 months , making the determination, the lag phase shift changes its rate of decline