基于石墨相氮化碳量子点直接荧光猝灭法检测碘离子的研究

石墨相氮化碳(g-C₃N₄)荧光纳米材料具有原料便宜、制备容易、荧光量子产率高、光学稳定性好、毒性低等优点，并且避免有机荧光染料复杂的合成步骤或者金属半导体量子点对环境潜在的危害，这些优点使得g-C₃N₄纳米材料成为新兴的荧光探针用于检测金属离子。最近，已有文献报道重金属汞离子能够高灵敏高选择性地猝灭g-C₃N₄量子点的荧光，加入碘离子能够提取被键合的汞离子形成碘化汞(HgI₂)进而恢复g-C₃N₄量子点的荧光，从而建立一种高灵敏检测碘离子的荧光传感器。然而，该方法依然需要重金属汞离子的参与，限制了该方法的推广应用。通过硝酸氧化块体g-C₃N₄并结合水热法处理制备了一种水溶性好、荧光强度高的g-C₃N₄量子点。该量子点的荧光发射波长位于368 nm，且其荧光发射波长不随激发波长的改变而改变，表明该量子点的尺寸比较均一。笔者发现碘离子在220 nm处有一个较强的吸收峰，与该量子点的激发光谱(中心波长245 nm)具有较大的重叠，从而产生内滤效应引起该量子点的荧光发生猝灭。利用这一性质，构建了一种选择性检测碘离子的新型荧光传感器。在最优检测条件下，g-C₃N₄量子点的荧光猝灭强度(ΔF)与碘离子浓度(X，μmol·L-1)在10～400 μmol·L-1之间具有良好的线性关系，线性方程为ΔF=0.325 79X+6.039 05(R2=0.999 5)，检出限为5.0 μmol·L-1。通过“混合即检测”并且不需要借助与重金属离子的配位作用就能够检测碘离子，因此该方法具有快速、环保以及操作简便等优点。

Study of Direct Fluorescence Quenching of Graphitic Carbon Nitride for the Detection of Iodine Ions

Iodine is one of essential trace elements. Deficiency of iodine and excess intake of iodine both can lead to thyroid diseases. Therefore, it is of great significance to develop a highly sensitive and selective method for the detection of iodine ions. Traditional analytical methods for iodine ions are usually involved in complex sample pretreatment and precision instruments, which are unfavorable for in-situ rapid detection. Fluorescent methods have been attracted great attentions due to their high sensitivity, high selectivity and easy operation. However, the present probes for iodine ions usually need complex organic syntheses and iodine ions are detected by means of coordination with heavy metal ions, which are unfavorable to promote the use of these methods. Fluorescent graphitic carbon nitride (g-C₃N₄) nanomaterial has attracted more attention due to the advatages of low-cost, easy preparation, high quantum yield, excellent photostability, and low toxicity. Furthermore, these nanomaterials can avoid complex synthesis for organic fluorophores or potential damage to environment for metal semiconductor quantum dots. These features make g-C₃N₄ nanomaterial an emerging fluorescent probe for the detection of metal ions. Recently, it was reported that Hg2+ ions could selectively and sensitively quench the fluorescence of g-C₃N₄ quantum dots (QDs). The addition of iodine ions could Abstract the bound Hg2+ ions to form HgI₂ complexes and restored the fluorescence of g-C₃N₄ QDs. Therefore, the fluorescent sensor for iodine ions could be developed. However, heavy metal ions (Hg2+) are also involved in this method, which limits its application. In this work, water-soluble g-C3N4 QDs with high fluorescence emission were prepared by using chemical oxidation of bulk g-C₃N₄ in nitric acid and hydro-thermal treatment. The maximal emission wavelength of g-C₃N₄ QDs located at 368 nm and did not change with the excitation wavelength, which indicates the size of g-C₃N₄ QDs is relatively uniform. There was a strong absorption peak at around 220 nm for iodine ions, which was overlapped with the fluorescent excitation spectrum of g-C₃N₄ QDs. On addition of iodine ions, the fluorescence of g-C₃N₄ QDs was quenched due to the inner filter effect. Therefore, a sensitive and selective fluorescent sensor for iodine ions was developed. Under the optimal conditions, there was a linear relationship between the fluorescence quenching (ΔF) of g-C₃N₄ QDs and the concentration (X, μmol·L-1) of iodine ions over the range of 10～400 μmol·L-1. The linear equation is ΔF=0.325 79X+6.039 05 (R2=0.999 5). The limit of detection is 5.0 μmol·L-1. The detection of iodine ions can be completed by “mixing and testing” without the need of coordination with heavy metal ions. Thus, this sensor is rapid, environment-friendly, simple and convenient.