## 3 The Development of Detection Technology
### Advances in Paper-Based Microfluidic Method  
Paper-based microfluidic technology is a fast detection method with a low cost and real-time detection. The method is mainly used to judge the concentration of the measured substance by colorimetric analysis. Generally, color changes can be judged or read by the naked eye, camera, smart phone, or special chromatic meter. At present, there are four types of paper-based microfluidic technology reported, which are nano gold paper-based, nano silver paper-based, silver nitrate paper-based, and electrode silver paper-based.  
[PERSON] et al. [71] developed a gold nanoparticles test paper for the detection of Cl\({}^{-}\). The film was placed into the gold nanoparticles solution with an ultrasonic load, and the gold nanoparticles were dried to obtain the gold nanoparticle test paper. The working principle of the paper was that Cl\({}^{-}\) could etch gold nanoparticles in nitric acid solution, and the concentration of Cl\({}^{-}\) would be correlated with the etch effect. The quantitative detection of Cl\({}^{-}\) could be realized by optical analysis of the reacted gold nanoparticles through image software, and the minimum detection limit could reach 0.01%. [PERSON] et al. [72] developed an instrument for the determination of sodium chloride concentration in seawater and estuary water based on the color change in gold nanoparticles. The instrument analyzed the color change with Colorgrab application and CIELab color management and could accurately predict the Cl\({}^{-}\) level in water samples.  
[PERSON] et al. [73] prepared a simple, highly sensitive and highly accurate paper-based colorimetric sensor using nano-silver, which could obtain Cl\({}^{-}\) content within 5 min. In the range of 10-1000 mg/L, the paper changed from dark purple to red with the increase in Cl\({}^{-}\) content, and the degree of color change had a good linear correlation with Cl\({}^{-}\) concentration; the detection limit was 1.3 mg/L. The method had a strong anti-interference ability towards common ions in natural water, but weak anti-interference ability towards halogen ions, especially bromine ions. However, the content of these chlorides in natural water was not high.  
[PERSON] et al. [74] used potassium permanganate to oxidize Cl\({}^{-}\) to generate chlorine gas, and then reacted with the nano-silver test paper after extraction. The nano-silver test paper changed color from gray to white. The degree of color change was linearly related to the content of Cl\({}^{-}\). This method could be used for the determination of Cl\({}^{-}\) in pharmaceutical preparations, river water, tomato juice, etc., with high selectivity and no sample pretreatment; the detection range was 0.1-1.5 mg/L.  
[PERSON] et al. [75] transformed the traditional molar precipitation titration method into paper-based microfluidic diagnostic equipment, which only required 5 mL of samples for determination. A trapezoidal channel was used to create a paper reaction surface. Firstly, silver nitrate solution was deposited on the channel at a certain speed. After drying, chromate was deposited on the channel and red precipitate formed. When the paper encountered Cl\({}^{-}\), the silver chromate reacted with Cl\({}^{-}\) to produce silver chloride, and a white band appeared. The length of the band was correlated with the concentration of Cl\({}^{-}\); Cl\({}^{-}\) content could be obtained by measuring the length of the white band.  
[PERSON] et al. [76] reported an electrochemical detection technology using silver electrode filter paper. The paper-based electrochemical detector was used for the detection of Cl\({}^{-}\) in serum and sweat samples, with a detection range of 1-200 mmol/L and a sample dosage of only 10 mL. The recoveries were between 93% and 108%.