Ultrasensitive Electrochemical Sensor Based on Polyelectrolyte Composite Film Decorated Glassy Carbon Electrode for Detection of Nitrite in Curing Food at Sub-Micromolar Level

To ensure food quality and safety, developing cost-effective, rapid and precision analytical techniques for quantitative detection of nitrite is highly desirable. Herein, a novel electrochemical sensor based on the sodium cellulose sulfate/poly (dimethyl diallyl ammonium chloride) (NaCS/PDMDAAC) composite film modified glass carbon electrode (NaCS/PDMDAAC/GCE) was proposed toward the detection of nitrite at sub-micromolar level, aiming to make full use of the inherent properties of individual component (biocompatible, low cost, good electrical conductivity for PDMDAAC; non-toxic, abundant raw materials, good film forming ability for NaCS) and synergistic enhancement effect. The NaCS/PDMDAAC/GCE was fabricated by a simple drop-casting method. Electrochemical behaviors of nitrite at NaCS/PDMDAAC/GCE were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Under optimum conditions, the NaCS/PDMDAAC/GCE exhibits a wide linear response region of 4.0 × 10−8 mol·L−1~1.5 × 10−4 mol·L−1 and a low detection 1imit of 43 nmol·L−1. The NaCS/PDMDAAC shows a synergetic enhancement effect toward the oxidation of nitrite, and the sensing performance is much better than the previous reports. Moreover, the NaCS/PDMDAAC also shows good stability and reproducibility. The NaCS/PDMDAAC/GCE was successfully applied to the determination of nitrite in ham sausage with satisfactory results.


Systhesis and characterization of sodium cellulose sulfate (NaCS)
36 mL of n-propanol was slowly added into a beaker containing 46 mL of concentrated sulfuric acid with ice water bath, and stirred to prevent local overheating at the same time. If n-propanol was added too fast, the color of the solution will turn yellow quickly. Then let the mixture solution rest overnight to obtain an acid-alcohol mixture solution. 2.0135 g of dried cellulose was weighed and then added to a three-necked flask containing an acid-alcohol mixture solution, stirred and reacted in an ice water bath for 16 h. After the reaction is completed, the reaction solution is vacuum filtered, repeated washed with isopropyl alcohol three times. Subsequently, the as-obtained precipitate dissolved into 50 mL of cold deionized water, afterwards 10% sodium hydroxide solution is added dropwise to adjust pH to alkaline, and the filtrate were collected by vacuum filtration. Finally, the equal volume of ethanol was added to the as-obtained filtrate, white viscous sodium cellulose sulfate solid was precipitated out, squeezed and vacuum-dried overnight to obtained sodium cellulose sulfate.
The sodium cellulose sulfate solid was dried in a vacuum and then ground with potassium bromide, and the infrared spectrum was measured by an infrared spectrometer as shown in Fig. S1.

Fig.S1 Infrared spectra of cellulose sodium sulfate
In the infrared spectrum of sodium cellulose sulfate, it can be seen that the stretching vibration absorption peak of C-O-H at 2912 cm -1 and the stretching vibration absorption peak of carbon-hydrogen bond at 3435 cm -1 , 1238 cm -1 It is the peak of the stretching vibration absorption of S=O and -COSO3, and there is a characteristic absorption peak of sulfate in the vicinity of 800 cm -1 . The spectra indicate that sodium cellulose sulfate is successfully synthesized.

Preparation of dimethyldiallylammonium chloride (DMDAAC)
Dimethylamine was added into a three-necked flask equipped with a reflux condenser, and stirred in an ice water bath. With thymolphthalein as indicator, the equimolar sodium hydroxide and allyl chloride were alternately added dropwise, keeping the color of indictor not change. Afterwards the mixture was stirred for 1.5 h, and then two phases were separated by a separating funnel. The upper oil phase was taken out to obtain dimethyl allyl tertiary amine (DMAA), and then sodium hydroxide solid was added into DMAA. After 2.5 h rest, an equal volume of acetone and 20% (V/V) distilled water were added separately. The allyl chloride was added dropwise, and the temperature was raised to 45 °C for 4 h. After the reaction is finished, washed repeatedly with acetone 2 -3 times to obtain dimethyl diallyl ammonium chloride (DMDAAC).

Preparation of poly(dimethyldiallylammonium chloride) (PDMDAAC)
DMDAAC (w = 65%) was added into a four-necked flask equipped with a reflux condenser. Then, ammonium persulphate (APS) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were added as initiator and additive (m(APS) : m(DMDAAC) = 0.35%, m(APS) : m(DMDAAC) = 0.0035%), respectively. The temperature remains below 40 ℃ , and then nitrogen gas was bubbled into the flask for 20 min to drive out the air. Afterwards, the reaction reacted at 45 ℃ for 2 h, 55 ℃ for 3 h, and 67 ℃ for 2 h. Finally, appropriate amount of acetone was added to dissolve the unreacted monomer, and then removed with the helping of the rotary evaporation apparatus.

Infrared spectra for DMDAAC and PDMDAAC
After DMDAAC and PDMDAAC were dried in a vacuum, they were ground with potassium bromide, tableted, and their infrared spectra were measured by an infrared spectrometer.
The infrared spectrum is shown in Fig. S3, where the curve a and b are the infrared spectra of PDMDAAC and DMDAAC, respectively. As can be seen from Fig.3, in the curve b, 3022 cm-1 is a stretching vibration peak of a carbon-hydrogen bond from the double bond (=CH2), 2970 cm -1 is a peak of stretching vibration caused by the methyl/methylene group, and 1642 cm -1 is a stretching vibration absorption peak of a carbon-carbon double bond. 1458 cm -1 is the in-plane bending vibration absorption peak at the ortho position of the N atom, while 1000 cm -1 and 883 cm -1 are out-of-plane bending vibration absorption peaks caused by the carbon-hydrogen bond and the double bond, respectively. In the curve a, it can be seen that the typical infrared absorption peaks at 3093 cm -1 , 3011 cm -1 , 1643 cm -1 and 853~876 cm -1 , have been significantly weakened or disappeared, indicating DMDAAC has been polymerized into PDMDAAC.