Development of Optical Chemical Sensor Based on Pararosaniline in Sol-Gel Matrix for Detection of Formaldehyde in Food Samples

Optical chemical sensor based on immobilesed pararosaniline into sol-gel matrix tetraethyl orthosilicate (TEOS) is a simple tool that can be used to detect the presence of formalin (formaldehide) in food. Pararosaline in sol-gel matrix was developed when contacted with food sample that contains formalin. The optical signal was produced by changing color from purple to yellow, that can be used to detect quantitative formaldehide in sample. The results, chemo sensor optic, have characteristic, maximum wave length 576.42 nm, with linier range 0 100 ppm, linearity coefficient R = 0.999, limit detection (LOD) 0.504 ppm, limit of quantification (LOQ) 1.680 ppm, sensitivity 0.087, disturbed matrix selectivity 1.716 %. The optimum is operational at pH 4, and response time at 150 seconds of 2 ppm. This sensor can be used to detect formalin in food sample in a simple mode and reusable for 4 times application. In addition, the sensor can be regenerated using an acid 0.1 M HCl.


Introduction
Some of food in public marketed discovered contains formaldehyde or usually familiar formaldehyde [1].Formaldehyde is a very dangerous chemical in human health; It gives negative effect to respiration channel, liver and kidney function, and reproducing system [2,3].Based on the moment conditions, detecting process of formaldehyde in food, conducting by laboratory process, used GC, HPLC and spectrometry instrument.The weakness methods of the mentioned impracticably cannot be prepared out of laboratory and need skilled persons who have backgrounds in chemistry specialty.In addition, such methods are not suitable to be employed in the fields [4].Therefore, there is an acute need to develop new and inexpensive methods of assessing the formaldehyde contain in food, particularly those that can be employed in the field.The alternative methods to detecting formaldehyde in food have simple process, low cost, and easy to operate by general society [5,6].
In this respect, the chemical sensor represents tools used for simple, quick and low-cost to detect of formal-dehyde in food [7][8][9][10][11].Developing simple specific optical sensor of formaldehyde is very urgent to give solutions to problems in general public to detect formaldehyde contained in food.The detection of formal-dehyde in food has been proposed using spectrometric [7,11].Pararosaniline is one of the specific reagents to detect formaldehyde, and the reaction between pararosaniline and formaldehyde is presented in Figure 1, next page [5,12].
Sol-gel for instance, there are many advantages, such as its optical clarity, the ability to entrap specific reagent, thermal and chemical stability, simplicity of preparation and flexibility in controlling its pore size and geometry [11,13].
This research uses Tetra Ethyl Ortho Silicate (TEOS) as sol-gel material and shaping as sol-gel granule.The mechanism process, formaldehyde in solution system of diluting food, diffuses sol-gel and reacts with pararosaniline reagent producing change color of sol-gel [12].The specific colors changed sol-gel from yellow to violet indicating that food solution contains formaldehyde, so the food has been solute contains formaldehyde.The solgel optical sensor can be used to detect formaldehyde contains in food as qualitative and quantitative manner.

Reagent
All reagents were used as purchased without further purification.Pararosaniline hydrochloride (SIGMA P3750) was supplied from Sigma (UK).Tetra ethyl orthosilicate (TEOS), hydrochloric acid (HCl 37% pa), Ethanol 96%, Triton X-100, and sodium sulfite (Na 2 SO 3 ) as precursor of sol-gel, were obtained from BDH-Merck (UK).For immobilization a phosphate buffer solution (PBS) with pH 6,5 was prepared by adjusting amounts of NaCl, KCl, Na 2 HPO 4 and KH 2 PO 4 buffer systems; in all cases, the mixture were 0.1 M in each constituent.The standard formaldehyde solutions of (2; 6; 10; 20; 100; 200; 300; 400; 500) ppm (grade of analytical, Merck) were prepared by appropriate dilution with an appropriate buffer solution in order to produce solutions of lower concentration at a desired pH.Salt and sugar use as interference material.All reagents and inorganic salts were of analytical grade and made using double distillate water.

Reagent Immobilizations
Pararosaniline reagent prepared by making solution, take of 0.03 g pararosaniline hydrochloride, diluting by water to total volume 10 mL used volumetric flask, this solution concentration is 3000 ppm.The solution on Na 2 SO 3 , made of 0.1 g Na 2 SO 3 diluting by water to total volume 10 mL used volumetric flask.Pararosaniline sol-gel made by composing of 1.5 mL pararosaniline reagent, 0.5 mL HCl 37%, 250 μL Na 2 SO 3 solution, 2 mL ethanol, 1.75 mL water, and 4.5 mL TEOS composing in beaker glass, stirring a long 3 -5 hours.After that adding 5 drops of triton X-100 and stirring again 30 minutes, after that molding sol-gel as sol-gel.

Optical Fiber Biosensors Construction
The construction of the optical fiber sol-gel chemo sensor has been carried out by carefully placed a single sol-gel of pararosaniline into the specially designed flowcell (Figure 2).This flow-cell (15 × 10 mm and 15 mm depth) has been designed as back pressure free flow cell, so that the effect of pulse from the pump and air bubbles could be removed.Since these problems often faced in flow system, which in turn increasing noise in signal response.This optical chemical sensor design also allows reducing the effect of other incident light levels on the flow-cell and optical system.

Sol-Gel Sensor Product.
The sol-gel sensor product fabrication and it color change before to after interaction with formaldehyde presenting as Figure 3.

Optimization of Experimental Parameters 1) Optimum Wave Length Operational and Linearity Range Concentration
The first step in parameter optimization is to finding the optimal wave length, base on the scanning the spectra of blank solution and some standard formaldehyde solution, 2 ppm, 10 ppm, 20 ppm, 100 ppm, 200 ppm, 800 ppm and 1000 ppm.Result of scanning as presenting by Figure 4, from this spectra, have been result the optimum wave length base on the correlation between standard formaldehyde concentrations with intensity of reflectance produced by sol-gel sensor after reacted with formaldehyde in solution.
Based on Figure 4, it is able to resume the reflectance intensity as Table 1 follows.
2) Test the Confidence Level Linearity Linearity test includes a margin of error sensitivity (slope error) and the margin of error intercept (intercept error).The results of calculations with a 95% of confidence level obtained error bounds of sensitivity 0.087 ± 0.0003 and a margin error of intercept 10.31 ± 0.0109 [10,16].
3) Limit of Detection (LOD) and Limit of Quantification (LOQ) Referring to Figure 4 and Table 1 further tested the linearity of the calibration curve in detail for the concentration range of 0 ppm to 100 ppm, with measurements repeated 7 times for each standard solution.The concentration of the standard solution used is a concentration of 0 ppm, 2 ppm, 4 ppm, 6 ppm, 8 ppm, 10 ppm, 12 ppm, 14 ppm, 16 ppm, 18 ppm, 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm.The next linearity test based on the average concentration measurements every standard.The test results give the following data as Table 2 and Figure 5.
Based on data from the linearity of the curve in Table 2 and Figure 5, it obtained the limit of detection (LOD) worth 0.5041 ppm and limit of quantitation (LOQ) of 1.6804 ppm [16].

4) Sensor Reproducibility
The data resumes to detecting Sensor Repro-ductivebility Measurement from 7 repeatbles as presenting Table 3 followed.Table 3 data give reality, the variance coefficient measurements base of reflectance signal, minimum 0.025% and optimum 0.557%.The variance coefficient measurements base of formalin contains, minimum 0.8768% and optimum 4.7875%.This condition are lowest of 5%, so the chemical sensor is usable as formalin detector [10,16].The responses time of sensor has been affected by concentration of formaldehyde in solution system.Type of formaldehyde concentration affected to sensor response time presenting like Figure 6, follow.Base on Figure 6, the response time of sensor between 75.88 seconds for 500 ppm to 150 seconds for 2 ppm formaldehyde concentrations.

6) Operational Conditions of pH Sensor
The reaction between pararosaniline with formalin affected by the pH, the influence of pH conditions the sample system to the intensity of reflectance sensor interaction with formalin results provide Figure 7, presented above.Based on the Figure 7, obtained information that the system for the detection of formaldehyde in the solution pH conditions are optimal system operating at pH 4.   The selectivity of the sensor for the identification of formaldehyde in the system through the test solution with sugar and salt.Selectivity trials conducted with formalin concentration ratio, salt and sugar 1:10, 1:100 and 1:1000.Sample spectra pattern measurements sugar disorders and formalin shown as Figure 8 and the data as Table 4(a) the following.

7) Sensor Selectivity
According to the test result tampering sugar and salt at Table 4(a), obtained information that the existence of sugar and salt in aqueous system can provide a distrac- tion to the measurement of the levels of formaldehyde in solution.The higher the concentration of sugar and salt, or in solution, the greater the percentage of its disorders.Selectivity of the sensor towards the sugar and salt as the interference was 1.716% [10,16].

8)Sensors Acurration
The accuracy of the sensors are tested through sensor application to a sample simulation known concentrations of formaline content.Testing using two sample object i.e. meat fish and noodles soggy, each with 4 kinds of concentration, that is 10 ppm, 20 ppm, 60 ppm and 100 ppm.Testing is done through standard methods and standard addisi.The test results are shown in 9) Reuse Sensor Test reuse (regeneration) is done by using a solution of formalin solution and blank with concentration of (10 ppm, 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm).The test results give a picture of a decrease in the performance of the sensor as shown in Table 5.
Based on the data in Table 5, obtained the fact that optical chemical sensors can be used repeatedly for four times, because its still 92.611%compared to the initial state, means meets the minimum limit analysis method rule capabilities sensor 90 % for reuse.

Applications Sensor for Real Samples
Application sensors for real samples performed using standard additions methods tested Sea-Fish meat and noodles soggy use of 5 sample objects.The results of measurements of formaldehyde content in the real sample by using optical chemical sensors and UV-Vis as a comparison method, as shown in the Table 6 below.
Based on the data in Table 6 brings about reality, that the determination of formaldehyde in food samples between using the chemical sensors and methods UV-Vis provides a different quantity.But quantitatively the difference is relatively low still meet the criteria analysis.Furthermore if the review of the results of the analysis of the paired t-tests (Paired t-Test), found no difference between the two methods of determination of the results of the two system analysis [10,16].

Conclusion
Based on the test results of the operational characteristics of the optic chemical sensor fabrication yield, it can be concluded that the sensor has the feasibility to use in the process of identifying and determinating the presence of formalin in food.In addition, the sensor can be regenerated using an acid solution.

Figure 4 .
Figure 4. Spectra profile of formaldehyde by pararosaline TEOS optical chemical sensor.

Figure 5 .
Figure 5. Test product of linearity on concentration range of 0 ppm -100 ppm, on wave length 576.42 nm, by 7 time repetition for each standard solution.

Figure 6 .
Figure 6.Response time of sensor base formaldehyde concentration.

Figure 7 .
Figure 7. Effect of pH on the intensity of reflectance operational.

Figure 8 .
Figure 8. Sensor selectivity curve formalin to interference ratio of sugar-salt at a concentration of 1:100.