Attenuated Total Reflectance Crystal of Silicon for Rapid Nitrate Sensing Combining Mid-Infrared Spectroscopy

Rapid detection of NO₃ ^--N is critical to address the challenges of food security, environmental degradation, and climate change. Conventional methods for sensing NO₃ ^--N in water demand pretreatments and chemical reagents, which are time- and cost-consuming. Consequently, Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy has been well applied for the determination of NO₃ ^--N. However, the conventional ATR crystals, i.e., zinc selenide (ZnSe) and diamond, showed a weakness in duration or cost since the ZnSe material was relatively soft and diamond was relatively expensive. In this study, comparing with ZnSe-ATR and diamond-ATR, a silicon-based ATR (Si-ATR) accessory was developed and used to explore the applicability and stability for sensing NO₃ ^--N combining mathematic algorithms. It was found that partial least-squares regression (PLSR) showed a good performance comparing with the algorithms of principal component analysis (PCA) and linear regression (LR), and it was recommended for quantifying NO₃ ^--N. For ZnSe-ATR, the residual prediction deviation (<i>RPD</i>) was more than 1.80, the determination coefficient (<i>R</i> ²) was more than 0.7725, and the root-mean-square error (<i>RMSE</i>) was less than 2.73 mg L^-1. For diamond-ATR, the <i>RPD</i> was more than 1.76, the <i>R</i> ² was more than 0.7309, and the <i>RMSE</i> was less than 8.22 mg L^-1. For Si-ATR, the <i>RPD</i> was more than 1.42, the <i>R</i> ² was 0.5198, and the <i>RMSE</i> was less than 11.02 mg L^-1. It was confirmed that all three types of ATR could be applied in the quantification of NO₃ ^--N in water for high nitrate concentrations. However, for the quantification of low nitrate concentrations (0-1 mg L^-1 NO₃ ^--N), ZnSe-ATR and diamond-ATR acquired the same accuracy, while Si-ATR had a lower accuracy. The pretreatment of Si-ATR-based spectra using the deconvolution algorithm could improve the prediction accuracy compared to water deduction for predicting low NO₃ ^--N. Furthermore, a Si-ATR accessory was developed using Si-ATR, which was reliable for NO₃ ^--N concentration quantification in water with the advantage of its low cost and long durability. Totally, samples with high nitrate concentrations implied a more reliable prediction for all crystals, and comprehensively, ZnSe-ATR was recommended for sensing low nitrate concentrations; diamond-ATR was recommended for samples with strong acid or base corrosion; and for sensing relatively high nitrate concentrations, such as in natural water bodies, Si-ATR was more economical because of its low cost and relatively long use life.