Combination of Multivariate Standard Addition Technique and Deep Kernel Learning Model for Determining Multi-Ion in Hydroponic Nutrient Solution

• Main Authors: Vu Ngoc Tuan, Abdul Mateen Khattak, Hui Zhu, Wanlin Gao, Minjuan Wang
• Format: Article
• Language: English
• Published: MDPI AG 2020-09-01
• Series: Sensors
• Subjects: ion-selective electrode; multi-ion sensor array; artificial neural network; gaussian process; deep kernel learning; hydroponics
• Online Access: https://www.mdpi.com/1424-8220/20/18/5314

Ion-selective electrodes (ISEs) have recently become the most attractive tools for the development of efficient hydroponic systems. Nevertheless, some inherent shortcomings such as signal drifts, secondary ion interferences, and effected high ionic strength make them difficult to apply in a hydroponic system. To minimize these deficiencies, we combined the multivariate standard addition (MSAM) sampling technique with the deep kernel learning (DKL) model for a six ISEs array to increase the prediction accuracy and precision of eight ions, including <inline-formula><math display="inline"><semantics><mrow><mi>N</mi><msubsup><mi>O</mi><mn>3</mn><mo>−</mo></msubsup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>N</mi><msubsup><mi>H</mi><mn>4</mn><mo>+</mo></msubsup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><msup><mi>K</mi><mo>+</mo></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>C</mi><msup><mi>a</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>N</mi><msup><mi>a</mi><mo>+</mo></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mi>C</mi><msup><mi>l</mi><mo>−</mo></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><msub><mi>H</mi><mn>2</mn></msub><mi>P</mi><msubsup><mi>O</mi><mn>4</mn><mo>−</mo></msubsup></mrow></semantics></math></inline-formula>, and <inline-formula><math display="inline"><semantics><mrow><mi>M</mi><msup><mi>g</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></semantics></math></inline-formula>. The enhanced data feature based on feature enrichment (FE) of the MSAM technique provided more useful information to DKL for improving the prediction reliability of the available ISE ions and enhanced the detection of unavailable ISE ions (phosphate and magnesium). The results showed that the combined MSAM–feature enrichment (FE)–DKL sensing structure for validating ten real hydroponic samples achieved low root mean square errors (RMSE) of 63.8, 8.3, 29.2, 18.5, 11.8, and 8.8 <inline-formula><math display="inline"><semantics><mrow><mi>mg</mi><mo>·</mo><msup><mi mathvariant="normal">L</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> with below 8% coefficients of variation (CVs) for predicting nitrate, ammonium, potassium, calcium, sodium, and chloride, respectively. Moreover, the prediction of phosphate and magnesium in the ranges of 5–275 mg·L⁻¹ and 10–80 <inline-formula><math display="inline"><semantics><mrow><mi>mg</mi><mo>·</mo><msup><mi mathvariant="normal">L</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> had RMSEs of 29.6 and 8.7 <inline-formula><math display="inline"><semantics><mrow><mi>mg</mi><mo>·</mo><msup><mi mathvariant="normal">L</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></semantics></math></inline-formula> respectively. The results prove that the proposed approach can be applied successfully to improve the accuracy and feasibility of ISEs in a closed hydroponic system.