(IMCS Second Place Best Paper Award) A High-Efficient Handheld Spectrometer for Ascorbic Acid Detection Based on MnO2 Nanosheet
Authors: Ma Chiyu , Kong Liubing , Sun Xianyou , Zhu Yuxuan , Wan Hao , Wang Ping 
Published: July 2021

Introduction

Ascorbic acid (AA), also known as vitamin C, is a water-soluble vitamin that can promote the body's absorption of iron to prevent iron-deficiency anemia. It also has an antioxidant effect, which can inhibit free radicals from oxidative damage to the human body, thereby preventing tumors. Because of its antioxidant properties, AA can also be used as antioxidants in foods and has a wide range of applications in foods such as cured meat products, canned fruits and vegetables, and beer beverages.
Conventional AA detection methods such as fluorescence [1] and electrochemical methods are complicated and time-consuming, and are not suitable for rapid and portable detection. Based on this, this article proposes a high-efficient handheld spectrometer, which is mainly composed of a miniature spectrometer sensor, a broadband LED light source, an Arduino UNO board and a self-designed circuit board (Figure.1C). This device is equipped with a touch screen and some buttons which are designed to facilitate user operation (Figure.1 D). The entire system is powered by a rechargeable lithium battery. In addition, the device is designed by Solidworks software and fabricated by 3D printing (Figure.1 E). As shown in Figure 1C, the detection system is also equipped with 3D printed guide rails and a light-transmitting bracket, combined with commercial microplates to achieve detection of AA. The detected signal can be transmitted to a smartphone or PC via Bluetooth. The handheld spectrometer has high portability and simplicity and shows excellent sensitivity. Combined with the above high-efficient spectrometer, a colorimetric assay based on MnO2 nanosheet for AA detection was established with great sensitivity and portability.

Method

MnO2 nanosheets were firstly synthesized acting as a biomimetic oxidase with which TMB can be oxidized to oxide TMB with a color change from pale yellow to blue (Figure.1 A). The hydrochloric acid added can turn the color of the solution to yellow with characteristic absorption at 450 nm. When the solution contains AA, MnO2 nanosheets can be reduced to Mn2+ resulting in the oxidation rate decreasing. That induces an absorption change at 450 nm indicating the concentration of AA (Figure.1 B) [2].
All the above reactions were performed in microplates and the microplates were pushed along the track in the middle of the handheld device and pushed out from the other end. The top of the device is equipped with a broadband LED light source. The emitted light passes through the solution in the microplates and is coupled to the micro spectrometer through a collimator below [3]. The optical signal is converted into an electrical signal and processed by the detection circuit board, and then transmitted to the Arduino microcontroller for analyze. Since the detection process is carried out inside the device, in a relatively dark environment, and the first well of the microplates contains the blank solution to eliminate the weak ambient light interference, the handheld spectrometer can be used on almost all occasions.

Results and Conclusions

Under optimized experimental conditions, different concentrations of AA samples were prepared using multiple dilution from 80 μM to 0.1563 μM. The blank solution without AA was also prepared as a control group. All the AA detection experiments were conducted by both the microplate reader and the handheld spectrometer, and the analytical performance was compared. The comparison results demonstrated the two instruments both have good linearity in the AA concentration range from 0.6250 μM to 40 μM. Furthermore, according to the 3σ/slope rule, the LOD of the handheld spectrometer is 0.4946 μM, which is slightly lower than the microplate reader. The good performance of the handheld spectrometer demonstrates it can be a promising tool for POC applications in food security, disease diagnosis, and environmental monitoring.

References

[1] Q. Jin, Y. Li, J.Z. Huo, X.J. Zhao, The “off-on” phosphorescent switch of Mndoped ZnS quantum dots for detection of glutathione in food, wine, and biological samples, Sens. Actuators B Chem. 227 (2016) 108e116.
[2] Y. Gan, N. Hua, C.J. He, S.Q. Zhou, J.W. Tu, T. Liang, Y.X. Pan, D. Kirsanov, A. Legin, H. Wan, P. Wang, MnO2 nanosheets as the biomimetic oxidase for rapid and sensitive oxalate detection combining with bionic E-eye, Biosens. Bioelectron. 130 (2019) 254e261
[3] Bayram, A., Horzum, N., Metin, A. U., Kilic, V., & Solmaz, M. E. (2018). Colorimetric Bisphenol-A Detection With a Portable Smartphone-Based Spectrometer. IEEE Sensors Journal, 18(14), 5948–5955. doi:10.1109/jsen.2018.2843794.

Figure 1