A carbon-dot fluorescence capillary sensor for the determination of chemical oxygen demand

Abstract

The development of a highly sensitive, reusable, and microportable sensor for determining the chemical oxygen demand (COD) is very important for performing in situ measurements. A reusable carbon-dot fluorescence capillary sensor is reported here. Citric acid/urea-carbon dots were synthesized by hydrothermal treatment and immobilized on the inner surface of a capillary using sol–gel technology. The sensor were used in conjunction with a recirculating-flow system to establish a set of methods for the indirect determination of COD in water by reaction with KMnO4. The carbon dots reduce KMnO4 to produce MnO2, thereby quenching the fluorescence of the sensor through the adsorption/inner filter effect. We harnessed this mechanism to successfully determine the CODMn (permanganate index) in the range of 0.2–40 mg/L (ΔF = -48.32c + 678.25, r = 0.9996, 0.2–12 mg/L; ΔF = -16.31c + 792.61, r = 0.9998, 8–40 mg/L). The sensor was subsequently regenerated by using ascorbic acid to reduce the carbon dots and remove the MnO2 adsorbed on the inner surface. The sensor could be reused at least 20 times with a relative standard deviation of less than 0.59 %. The proposed method was applied to quantify the COD in potable, waste and natural water. Combining the sensor with the automated flow system is a potential method for in situ real-time monitoring of COD in water.