1. Introduction
Water quality monitoring is of growing importance all over the
world with increasing demand of the real-time water quality
information. To monitor wastewater quality during different
treatment processes is crucial to ensure the treatment efficiency
and comply with increasingly stringent regulations [1–3].
Chemical oxygen demand (COD) is a common indicator of
organic matter concentration to assess wastewater quality. The
standard COD test involves adding toxic chemicals and is timeconsuming,
requiring 2–4 h until the result is obtained. With the
development of optical techniques, spectroscopic analysis, including
UV–vis spectroscopy [4–7], fluorescence spectroscopy [8–10]