4-Nitrophenol (4NP) is a vital intermediate in organic industries, and its exploitation creates serious environmental issues. We propose a fluorescence quenching-based strategy with nitrogen and sulfur co-doped carbon dots (NS-CDs) for highly sensitive 4NP detection with excellent selectivity. The NS-CDs are produced through the hydrothermal process, in which citric acid serves as a carbon source and cysteamine hydrochloride as a source of N and S. The effect of doping was also studied by synthesizing undoped CDs and examining their properties. As-developed NS-CDs exhibit a bright cyan blue color with maximum emission centered at 465 nm. The fluorescence of NS-CDs is significantly quenched in an approximately linear fashion with increasing 4NP concentration (7.5–97.5 μM). The inner filter effect (IFE) and static quenching (SQ) between NS-CDs and 4NP are responsible for such fluorescence reduction. The fluorimetry technique enables the quantification of 4NP with a limit of detection (LOD) of about 0.028 μM. Moreover, the fluorescence quenching is tested for several other chemical compounds but they generate false quenching signals; only 4NP leads to fluorescence quenching of NS-CDs, demonstrating excellent selectivity. The “turn-off” fluorescence properties and visually apparent color change of the fluorescent probe reveal the excellent performance for 4NP sensing. The NS-CDs’ capability of quantifying 4NP in real water samples (tap water and drinking water) produces an excellent recovery rate ranging between 96.24 and 98.36%.

Inner filter effect-based highly sensitive quantification of 4-nitrophenol by strong fluorescent N, S co-doped carbon dots
    Abstract
    1 Introduction
    2 Experimental
        2.1 Chemicals
        2.2 Synthesis of NS-CDs
        2.3 Instrumentation
        2.4 Quantum yield (QY) measurement
        2.5 Analytical procedure for the 4NP detection
        2.6 Analysis in real water samples
    3 Results and discussion
        3.1 Analysis of morphology and surface functional groups
        3.2 Optical properties
        3.3 Photostability of NS-CDs
        3.4 Sensitive detection of 4NP
        3.5 Possible quenching mechanism
        3.6 Selectivity of 4NP detection
        3.7 Detection in real water samples
    4 Conclusions
    Acknowledgements 
    References

• Kandasamy Sasikumar(Department of Physics, Gachon University, Seongnam‑si, Gyeonggi‑do 13120, Republic of Korea) Corresponding author
• Ramar Rajamanikandan(Department of Physics, Gachon University, Seongnam‑si, Gyeonggi‑do 13120, Republic of Korea)
• Heongkyu Ju(Department of Physics, Gachon University, Seongnam‑si, Gyeonggi‑do 13120, Republic of Korea)