Direct and sensitive detection of sulfide ions based on one-step synthesis of ionic liquid functionalized fluorescent carbon nanoribbons

Despite widely reported fluorescence sensors for cations, direct detection of anions is nevertheless still rare. In this work, ionic liquid-functionalized fluorescent carbon nanoribbons (IL-CNRs) are one-step synthesized and serve as the fluorescent probes for direct and sensitive detection of sulfide ions (S2−). The IL-CNRs are synthesized based on electrochemical exfoliation of graphite rods in a water-IL biphasic system. The as-prepared IL-CNRs exhibit uniform structure, high crystallinity, strong blue fluorescence (absolute photoluminescence quantum yield of 11.4%), and unique selectivity towards S2−. Based on the fluorescence quenching of IL-CNRs by S2−, a fluorescence sensor is developed for direct, rapid and sensitive detection of S2− in the range of 100 nM to 1 μM and 1–300 μM with a low detection limit (LOD, 85 nM). Moreover, detection of S2− in a real sample (tap water) is also demonstrated.


Introduction
Inorganic anions play a crucial role in industrial, biological and environmental elds. 1 For instance, the sulde (S 2À ) ion is highly toxic and harmful to human health and the ecological environment. [2][3][4] Usually, the S 2À ion is frequently distributed in natural water and wastewater samples because it is widely used in industrial processes (e.g. sulfur and sulfuric acid production, petroleum reneries, paper and pulp manufacturing, and sewage processing industries) and is also produced in biological systems (e.g. via reduction of sulfate ions by microbes or anaerobic processes of S-containing enzymes). [4][5][6] As the S 2À ion can stimulate the mucosa and has an extreme effect on the nervous system, exposure to S 2À ions leads to unconsciousness and respiratory paralysis and is closely related to various diseases including Alzheimer's disease, Down's syndrome and cirrhosis of the liver. 7-10 Therefore, simple, rapid and sensitive detection of S 2À is of great importance.
In comparison with other technologies including gas chromatography, ion chromatography, titrimetry, colorimetry, and electrochemical method, uorescent sensors are particularly attractive because of high sensitivity, simple operation and rapid detection. 3,[11][12][13][14][15][16][17] Recently, uorescent sensors based on different uorescent probes have been widely reported for detection of cations. 18,19 However, only few researches focus on indirect detection of anions using uorescent off-on strategy. For instance, Na et al. reported that Cu 2+ ions caused aggregation of the GQDs and thereby quenched uorescence. 12 As the GQDs-Cu 2+ aggregates can be dissociated by adding S 2À ions and result in uorescence turn-on, uorescent detection of S 2À ions could be realized. Based on the same off-on strategies, graphitic carbon nitride quantum dots-Hg 2+ , 7 nitrogen and sulfur co-doped carbon dots (CDs)-Hg 2+ , 6 CDs-gold nanoparticles, 10 silver nanoparticles capped with CDs, 9 CDs-MnO 2 nanosheets, 13 quinoline-Zn 2+ , 2 calix [4]arene-Cu 2+ , 11 and organic semiconductor polymer nanodots-Cu 2+ systems 8 were also applied for the detection of S 2À ions. However, it still remains great challenge for improving the chemical selectivity of uorescent probe to realize the direct detection of S 2À .
Carbon-based uorescent nanomaterials have attracted considerable interests as probes in uorescent sensing due to good solubility, highly tunable photoluminescence properties and biocompatibility. [20][21][22][23][24] Carbon nanoribbons (CNRs), which is thin elongated strips of sp 2 bonded carbon atoms, have emerged as an exciting one-dimensional nanomaterial because of their large length-to-width ratio, abundant edge sides, high effective surface area, good exibility, high conductivity, and tunable uorescence. [25][26][27][28] Due to the large interfacial area for pp stacking in basal planes, CNRs exhibit potentials for the synthesis of advanced functional materials through simple modication or functionalization.
Ionic liquids (ILs) have been widely used in many elds because of a series of excellent physical and chemical properties. 21,28,29 Particularly, ILs can easily combine with carbon nanomaterials due to p-p or cation-p interactions and the obtained composite materials exhibit unique characteristics. 21 32 Those IL-modied CDs exhibit capability of anion exchange and are able to act as uorescent probe for direct detection of Fe(CN) 6 3À anion.
Our group also prepared IL-GQDs nanomaterial through post-modication of hydroxyl-functionalized GQDs (OH-GQDs) with 1-butyl-3-methyl imidazolium tetrauoroborate (BMIMBF 4 ) under ultrasonic treatment. The as-prepared IL-GQDs composite also displayed sensitive response towards Fe(CN) 6 3À , owing to the anion exchange ability of IL. 33 Inspired by these evidences, IL modied uorescent nanocarbon materials possess great potentials for the direct detection of anions.
In this work, in contrast to anion detection based on uorescence off-on mode, a new type of ionic liquid-functionalized uorescent carbon nanoribbons (IL-CNRs) were prepared using one-step electrochemical method, which is able to achieve direct and sensitive detection of sulde ions. The IL-CNRs were in situ synthesized based on electrochemical exfoliation of graphite rods in water-IL (1-butyl-3-methylimidazolium hexa-uorophosphate, [BMIM]PF 6 ) biphasic system. The obtained IL-CNRs exhibit bright uorescence, high crystallinity and uniform structure. Based on the selective uorescent quenching by S 2À , IL-CNRs can be served as uorescent probes for direct and sensitive detection of S 2À . In comparison with the previous cation-based uorescent sensor, we for the rst time demonstrate the application of IL-CNRs for direct and selective detection of S 2À .

Characterizations
Transmission electron microscopy (TEM) micrographs were obtained using a JEM-2100F transmission electron microscope (JEOL Ltd, Japan) at accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI5300 at operating voltage of 14 kV (PE Ltd., USA).

One-step electrochemical synthesis of IL-CNRs
The IL-CNRs were one-step synthesized by electrochemical exfoliation of graphite rods. A two electrodes system was adopted with graphite rods (d ¼ 3 mm) as both anode and cathode. The electrolyte was water-IL ([BMIm][PF 6 ]) biphasic system (V/V, 4 : 6). Graphite rods were vertically inserted into electrolyte solution at a distance of 3 cm. 21 A static voltage of 8 V was applied on graphite rod for 4 h with stirring the electrolyte. Aerwards, the resultant solution was centrifuged at 12 000 rpm for 10 min to remove large particles. To acquire IL-CNRs, the supernatant solution was then ltered using a 0.22 mm membrane and dialyzed in a dialysis bag (with cut-off molecular weight of 1000 Da), which is followed by freezedrying.

Fluorescent detection of S 2À
The selectivity of IL-CNRs towards different anions and cations were evaluated by measuring the uorescent intensity of IL-CNRs solution in presence of different cations (Na + , K + , Zn 2+ , Mg 2+ , Fe 3+ , Ca 2+ , Al 3+ , Ni 2+ , Cu 2+ , Cd 2+ ) or anions (CH 3 COO À , For S 2À detection, standard stock solutions of S 2À (0.1 M) were prepared and further diluted stepwise to obtain different concentrations. HEPES buffer (0.1 M, pH 7.0) containing AA (25 mM) was applied as the buffer solution. Aer reaction of IL-CNRs with different amount of S 2À for 1 min at room temperature, the uorescent intensity of the solution was recorded with the excitation wavelength xed at 340 nm.
The relative uorescence ratio (F/F 0 ) or relative uorescence quenching ratio (F 0 À F/F 0 ) were used to evaluate the quenching of IL-CNRs caused by ion, where F 0 and F represent the uorescent intensity of IL-CNRs solution in the absence and presence of ion, respectively.

Preparation and structure characterizations of IL-CNRs
Electrochemical synthesis has particularly attracted attention owning to the inexpensive apparatus and easy operation under This journal is © The Royal Society of Chemistry 2019 ambient conditions. [34][35][36] As illustrated in Fig. 1, ionic liquidfunctionalized uorescent carbon nanoribbons (IL-CNRs) were easily synthesized through one-step electrochemical exfoliation of graphite rods in water-IL (1-butyl-3-methylimidazolium hex-auorophosphate, [BMIM]PF 6 ) biphasic system. Owing to high ionic conductivity and wide electrochemical potential window, ILs shows potential as electrolyte in electrochemical synthesis. 17,24 In the process of electrochemical exfoliation, the colour of electrolyte turned from colourless to pale yellow and aerward turned dark brown due to rapid aking-off of abundant IL-CNRs. In comparison with the same procedure in pure water, this phenomenon cannot be observed, indicating the importance of ILs for improved conductivity and facilitated exfoliation. The formation of IL-CNRs can be observed by transmission electron microscopy (TEM) (Fig. 2). The length and width of IL-CNRs were about 40 nm and 5 nm (Fig. 2a-c), respectively. Clear lattice lines can be seen from the highresolution TEM (HRTEM) image (Fig. 2d), indicating good crystallinity. The lattice spaces are 0.26 nm and 0.38 nm, that are consistent with the (100) and (002) lattice of graphite, respectively. 21 The chemical composition of the as-prepared IL-CNRs were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3a, the XPS survey spectrum of IL-CNRs contains ve distinct peaks of C 1s, N 1s, O 1s, P 2p, F 1s. The appearance of N, P, F reveals the formation of IL-CNRs nanocomposites. The C-C/C]C (sp 2 C) peak is prominent in high resolution spectrum of C 1s, suggesting the graphitic structure in CNRs (Fig. 3b). This result is consistent with the good crystallinity demonstrated in HRTEM image (Fig. 2d). The appearance of C-N in high resolution spectrum of C 1s (Fig. 3b), N-C]C and quaternary N in high resolution spectrum of N 1s (Fig. 3c) conrm the existence of BMIM + in IL-CNRs. The appearance of O 1s reveals the abundant oxygen-containing groups (Fig. 3d), suggesting good solubility of the as-prepared IL-CNRs. The F 1s peak (Fig. 3e) and P 2p peak (Fig. 3f) also prove the presence of In comparison with IL, UV-Vis spectrum of IL-CNRs exhibits the same large peak of p-p* transition ($240 nm) and new peak at about 326 nm that corresponds to n-p* transition (Fig. S2 in ESI †). Taken together, [BMIM]PF 6 are proven to be composited with CNRs. According to the above results, we propose that the mechanism for the preparation of IL-CNRs includes oxidative cleavage, ionic liquid intercalation and hybridization (Fig. 1). 21,[28][29][30][31] Firstly, the oxidation of water is expected to produce radicals (e.g. hydroxyl and oxygen radicals) when a high oxidation voltage is applied. Secondly, the oxidation of the edge planes opens up the edge sheets and leads to the expansion of the graphite anode, facilitating intercalation by PF 6 À between the graphene layers. Thirdly, the oxidative cleavage of the expanded graphene sheets generates CNRs. Finally, interaction between CNRs and BMIM + via p-p or cation-p interactions leads to stable IL-CNRs nanocomposites. In comparison with the preparation of CNRs using electron or ion beam etching, chemical ultrasound, lithographic patterning, oxidative cleavage, and chemical vapor deposition (CVD), 37-40 this  electrochemical method is simple, green and possesses potential for scalable production owing to inexpensive apparatus and easy operation.

Fluorescence properties of IL-CNRs
The uorescence properties of IL-CNRs were investigated and results were demonstrated in Fig. 4. The IL-CNRs show good solubility in water and appears light-yellow under daylight, while exhibit strong blue luminescence under UV light (365 nm) (inset in Fig. 4b). The maximum emission is located at 430 nm and the maximum excitation wavelength is 340 nm (Fig. 4a). Like most luminescent carbon nanomaterials, the IL-CNRs also exhibit excitation-dependent uorescence behavior (Fig. 4b). IL-CNRs prepared under the same conditions independently exhibit the same maximus emission wavelength and a relative standard deviation (RSD) of 2.9% in uorescence intensity (at 430 nm), indicating high stability and repeatability of the preparation. As the excitation wavelength increases, the uorescence emission slightly red-shis, which could be ascribed to size heterogeneity and distribution of different emissive sites on the carbon ribbons. Absolute photoluminescence quantum yield of IL-CNRs is measured to be 11.4%, which may be ascribed to the high crystallinity of IL-CNRs and the presence of BMIM + on IL-CNRs surface which has electron-withdrawing nitrogen groups. The lifetime of B-GQDs is 5.9 ns (Fig. S3 in ESI †). The IL-CNRs also exhibits good photostability. As shown in Fig. S4 (ESI †), even aer being illuminated for 4 h under UV light (365 nm), the uorescent intensity remains 91%, indicating good anti-photobleaching property.

Ion selectivity of IL-CNRs
Ionic liquids modied carbon materials have proven to exhibit anion exchange ability and have potential in direct detection of anion. Thus, the selectivity of the as-prepared IL-CNRs towards different anions were investigated. As revealed in Fig. 5a, the uorescence of IL-CNRs is strongly quenched by S 2À , but not the other physiologically or environmentally relevant anions (Fig. 5a). The lifetime of IL-CNRs in presence S 2À is also revealed to be 5.9 ns (Fig. S5 in ESI †). The unchanged uorescence lifetime of IL-CNRs in the presence of S 2À indicates no charge transfer and exciton recombination process. 18 The quenching might be ascribed to the ability of anion exchange originated from IL. 32,33,41 As uorescent carbon materials are usually easy to interact with metal cations and cause uorescence quenching, the selectivity of IL-CNRs towards cations were also investigated. As demonstrated in Fig. 5b, Fe 3+ cause uorescent quenching, while other common cations did not signicantly affect the uorescence intensity of the IL-CNRs. The interactions between IL-CNRs and Fe 3+ could be efficiently masked by ascorbic acid (AA). Thus, IL-CNRs have potential for direct and selective detection of S 2À ions.

Detection of S 2À ions using IL-CNRs as uorescent probe
The detection of S 2À ions using IL-CNRs as uorescent probe was investigated. To obtain the highest sensitivity, the detection conditions including pH and incubation time were optimized. As illustrated in Fig. S6a (ESI †), S 2À ions could quench the uorescence of IL-CNRs very quickly. As the incubation time goes beyond 60 s, the uorescence quenching reaches a plateau. Obviously, IL-CNRs possesses great potential for fast detection of S 2À ions. Accordingly, the reaction time with IL-CNRs was set as 60 s in the following experiments. In order to avoid protonation of S 2À ions, uorescence quenching by S 2À ions was investigated in near neutral solutions. The highest uorescent quenching is observed at pH 7, which is chosen for further investigated (Fig. S6b in ESI †).
Owing to the fast and sensitive uorescence quenching of IL-CNRs by S 2À ions (insets in Fig. 6a), the uorescent detection   was performed under the optimum conditions. As demonstrated in Fig. 6a, S 2À ions caused uorescence quenching in dose dependent manner. Good linear correlation was found between (F 0 À F)/F 0 and the concentration of S 2À ions from 100 nM to 1 mM and 1 to 300 mM (Fig. 6b). The limit of detection (LOD) for S 2À ions is as low as 85 nM at a signal-to-noise ratio of 3. Thus, IL-CNRs based method is sensitive enough to detect the maximum allowable level of S 2À ions in drinking water recommended by World Health Organization. 42,43 As shown in Table  S1 (ESI †), the detection limit obtained with the present method was lower than those obtained by carbon dots (CDs)-Ag + , 44 covalent linking uorescein isothiocyanate with branchedpolyethylenimine (PEI-FITC), 43 triarylimidazole chromophore (TPI-H)-Cu 2+ , 45 Au nanoclusters, 46 lysozyme-stabilized silver nanoclusters (Lys-Ag NCs), 47 graphene quantum dot (GQDs)-Cu 2+ , 48 but higher than that obtained using Cu nanoclusters, 49,50 Au nanoclusters-Ce(III), 51 and silver nanoparticles capped with carbon dots (AgNPs-CDs). 52

Real sample analysis
In order to demonstrate the practical application of the developed uorescent sensor in detection of S 2À in real samples, tap water samples were analyzed by standard addition method. The recoveries ranges from 93.7-103.8% and the relative standard deviations (RSD) is less than 3.0% (Table S2 †), indicating the potential for the detection of S 2À in complicated real samples.

Conclusions
Ionic liquid-functionalized carbon nanoribbons (IL-CNRs) were readily synthesized using one-step electrochemical exfoliation of graphite rod in water-IL biphasic system. The hybridization of IL with CNRs not only results in bright uorescence of CNRs but also endows IL-CNRs with specic interaction with S 2À ions. Taking the advantages of bright photoluminescence and selectivity towards S 2À , IL-CNRs are applied for fast, selective and sensitive detection of S 2À ion. In contrast with uorescent detection of S 2À ion using turn-off-on mode, this IL-CNRs demonstrate its potential for practical use in direct detection of anions.

Conflicts of interest
There are no conicts to declare. 14 X. Chen, S. Yu, L. Yang, J. Wang and C. Jiang, Fluorescence and visual detection of uoride ions using a photoluminescent graphene oxide paper sensor, Nanoscale, 2016, 8, 13669-13677.