based on Graphene Quantum Dots as Fluorescent Probes

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Understanding the Selective Detection of Fe based on Graphene Quantum Dots as Fluorescent Probes: the K of a Metal Hydroxide-Assisted Mechanism sp

Xiaowen Zhu, Zhen Zhang, Zhenjie Xue, Chuanhui Huang, Ye Shan, Cong Liu, Xiaoyun Qin, Wensheng Yang, Xu Chen, and Tie Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02499 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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Analytical Chemistry

Understanding the Selective Detection of Fe3+ based on Graphene Quantum Dots as Fluorescent Probes: the Ksp of a Metal HydroxideAssisted Mechanism Xiaowen Zhu,† Zhen Zhang,‡,§ Zhenjie Xue,‡,§ Chuanhui Huang,‡,§ Ye Shan,‡,§ Cong Liu,‡,§ Xiaoyun Qin,‡,§ Wensheng Yang,† Xu Chen,*,† Tie Wang*,‡,§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China §University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: Graphene quantum dots (GQDs) have been widely used as fluorescence probes to detect metal ions with satisfactory selectivity. However, the diverse chemical structures of GQDs lead to selectivity for multiple metal ions, and this can lead to trouble in the interpretations of selectivity due to the lack of an in-depth and systematic analysis. Herein, bare GQDs were synthesized by oxidizing carbon black with nitric acid and were used as fluorescent probes to detect metal ions. We found that the specific ability of GQDs to recognize ferric ions relates to the acidity of the medium. Specifically, we demonstrated that the coordination between GQDs and Fe3+ is regulated by the pH of the aqueous GQDs solution. Dissociative Fe3+ can coordinate with the hydroxyl groups on the surface of the GQDs to form aggregates (such as iron hydroxide), which induces fluorescence quenching. A satisfactory selectivity for Fe3+ ions was achieved under relatively acidic conditions; this is because of the extremely small Ksp of ferric hydroxide compared to other common metal hydroxides. To directly survey the key parameter for Fe3+ ion specificity, we performed the detection experiment in an environment free of interference from the buffer solution, non-inherent groups, and other complex factors. This study will help researchers understand the selectivity mechanisms of GQDs as fluorescence probes for metal ions, which could guide the design of other GQD-based sensor platforms.

Graphene quantum dots (GQDs) as an emerging member of the carbon-based family, have attracted extensive interest since their discovery. Due to their excellent tunable optical properties,1 photoluminescence stability, biocompatibility and chemical inertness,2 GQDs are promising substitutes for conventional semiconductor-based metal QDs, which are limited by their biological toxicity and environmental hazards.3 In addition, many organic dyes suffer from photobleaching.4 GQDs have been used in such fields as environmental monitoring, bioimaging,5 optoelectronic devices,6 and biosensors.7 Many previous studies have also taken advantage of the sensitive optical signal of GQDs to detect ions, small molecules, and biomolecules.8 More important, the synthesis of GQDs is relatively well [*]E-mail: 82362042

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developed, such that the synthetic route can be selected according to the requirements of the specific application. In the past several years, the detection of metal ions based on GQDs as fluorescent probes has become more important9-11, as metal ions can have deleterious impacts on human and environmental health. Fe3+ is a metal ion essential to both biological systems and environmental processes. Fe3+ ions can coordinate with a variety of regulatory proteins in biological systems. The concentration of Fe3+ ions is a key label for Parkinson’s disease, and unnecessary Fe3+ ions result in cytotoxicity. Fe3+ is also an important pollutant in environmental monitoring.10 For these reasons, there is a need for sensitive and selective sensors of Fe3+ concentrations in both biological systems and the environment. Instrumental techniques and electrochemical methods usually require complex preparation and have a barrier in terms of reproducibility and reliability.12,13 The simplicity

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and high sensitivity of fluorescent methods make them a potentially powerful tool for detecting harmful metal ions. GQDs can be used as fluorescent probes to detect Fe3+ with a high degree of sensitivity and selectivity. Previously, a sensitive, selective fluorescence probe based on the quenching of fluorescence from GO nanosheets detected Fe3+ ions with a limit of detection as low as 1 ppm. This fluorescence quenching was attributed to the interaction between the alpha-hydroxy quinoid six-membered ring and Fe3+.14 Another study made use of BMIM+functionalized GQDs to detect Fe3+. This study relied on the high binding affinity between the imidazole ring of BMIM+ and Fe3+ where Fe3+ bridged several adjacent GQDs, leading to fluorescence quenching.10 Given this efficient quenching effect, H2O2/Fe2+/CDs systems have been used to investigate the PL center of CDs. Previous studies hypothesized that the carbon cores formed during the hydrothermal reaction of citric acid may result in fluorescence; however, the high quantum yield was attributed to the molecular state.15 Furthermore, Fe3+ is an ideal medium to detect other analytes. Glutathionefunctionalized GQDs (GSH-GQDs) that showed blue fluorescence were used to evaluate the ATP levels in the cell lysate and human blood serum. First the fluorescence of GSH-GQDs was quenched by Fe3+ ions. Then Fe3+ ions were released from GQDs by complexing with ATP molecules, which resulted in fluorescence recovery.16 Similarly, a biosensor for detecting trypsin was designed based on the fact that cytochrome c (Cyt c; which is rich in Fe3+) induced GQDs fluorescence quenching. Trypsin then cleaved Cyt c to the lysine, allowing arginine residues to reduce the GQDs, thus restoring their fluorescence.17 Most reports have suggested that the recognition mechanism results from the special coordination interaction between Fe3+ and the phenolic hydroxyl groups on the surface of the GQDs.10,18,19 The resulting electron-transfer process between the Fe3+ and GQDs induces fluorescence quenching reducing their fluorescence lifetime.18 In addition to inducing dynamic quenching, GQDs can also form complexes with Fe3+ to induce static quenching.20 Although some works have used this sensing mechanism, the idea that Fe3+ can be specifically identified from other metal ions still lacks experimental support and systematic research.8 Most of the research work has assigned the specific recognition of Fe3+ to phenolic hydroxyl groups on the GQDs which then trigger fluorescence quenching. However, the factors that regulate these interactions have not been investigated. In this study, GQDs exfoliated from large carbonaceous materials using an oxidizing acid (without subsequent surface treatment) demonstrated high fluorescence selectivity for Fe3+ over interfering ions. We speculated that a small amount of the residual acid altered the acidity of the solution, which regulated the affinity between metal ions and the hydroxyl groups on the GQDs. Metal ions have different capacities for

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forming complexes with hydroxy groups, where different types of metal ions form complexes at diverse pH values. As the Ksp of ferric hydroxide is small, it can form metal complexes in harsh acidic environments in which other metal ion complexes cannot form. Thus, we studied the influence of pH-regulated Ksp on the fluorescence selectivity for Fe3+ by altering the acidity of the medium. In addition, considering that the residual nitric acid does not just affect the acidity, the nitrate adsorption may also affect the surface state of the GQDs. Thus, we used hydrochloric acid to change the type of anion adsorbed on the surface of GQDs to determine whether the type of adsorption anion affects selectivity. We excluded the interference from the buffer solution, as well as noninherent groups and other complex factors, by operating in a primitive environment to determine key selectivity parameters. Finally, the Ksp of a metal hydroxide-assisted mechanism for the selective detection of Fe3+ based on GQDs as fluorescent probes was proposed.

EXPERIMENTAL SECTION Materials. Vulcan CX-72 carbon black was purchased from Cabot Corporation (USA). All other reagents were of analytical grade from commercial suppliers and were used as received. Deionized water was used in all the experiments. Synthesis of the GQDs. The GQDs were prepared from Vulcan CX-72 carbon black via chemical oxidation as described elsewhere.21 Briefly, 1 g of carbon black was refluxed with 250 mL of 6 mol⋅L-1 HNO3 at 130 °C for 24 h. After the reaction, the suspension was cooled naturally to room temperature. Then, the suspension was centrifuged for 10 min to separate the dark reddish-brown supernatant from the black sediment. The supernatant was heated at 200 °C to evaporate the water and nitric acid. Approximately 1 h later, a reddish-brown powder was obtained. Finally, the dried sample was dissolved in deionized water at a concentration of 0.1 mg⋅mL-1 for further detection. The pH of the GQD solution was measured to be approximately 3.5, and it could be adjusted to pH 1.4 and 6.0 using concentrated HCl and NaOH, respectively (the volume is negligible). The desorption process for nitrate used an ultrafiltration tube containing a membrane with a molecular weight cut off of 30 kDa. A certain amount of deionized water was added to the GQDs, and then, they were filtered using the ultrafiltration tube. After they were filtered a few times to remove the nitrate, the pH was adjusted by hydrochloric acid to pH 3.5. The obtained solution was used to detect metal ions in the same conditions as before. Detection of Metal Ions. The detection of metal ions was carried out on a spectrophotometer. Different concentration gradients of the metal salts (Pb2+, Co2+, Ni2+, Fe3+, Cu2+, Mg2+, Al3+, Ca2+, K+, Na+ and Zn2+) were dissolved in deionized water. All of the analytes with

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various concentrations were incubated in the GQDs solution for at least 10 min to interact completely. Instrumentation. The Fourier transform infrared (FTIR) spectra were measured on an FT-IR spectrometric spectrophotometer (Bruker VERTOR-22). X-ray photoelectron spectroscopy (XPS) was recorded using an X-ray photoelectron spectrometer (ESCALAB 250XI). UVVis spectra were measured by a spectrophotometer (UV2501PC, Shimadzu, Japan). Photoluminescence (PL) spectra were recorded on a spectrophotometer (RF5301PC, Shimadzu, Japan). The morphology of the obtained nanomaterials was determined using a JEM-1011 electron microscope (TEM). Scanning electron microscopy (SEM) images were recorded on a Zeiss Supra 55 ﬁeld emission scanning electron microscope using an accelerating voltage of 20 kV.

RESULTS AND DISCUSSION Characterization of GQDs. The metal ions sensing platform based on GQDs is shown in Figure 1. The TEM and HRTEM images (Figure 1b) reveal that the synthesized GQDs are monodisperse nanoparticles with a near-spherical morphology distributed in the range of 510 nm in size and GQDs have discernible lattice structures. The XPS results (Figure S1) indicate that the GQDs are composed of carbon, oxygen, and nitrogen. The FT-IR spectrum of the GQDs (Figure S2) confirms the presence of oxygen-containing functional groups, as evidenced by the stretching vibrations of -OH groups and -COOH groups at 3300 and 1380 cm-1. The surface charge of GQDs was characterized in aqueous solutions and showed a potential of -24.2 mV (at a GQDs concentration of 0.1 mg⋅mL-1). These negative potentials derived mainly from carboxyl groups. The UV-Vis absorption spectrum of GQDs in water (Figure S3a) showed typical absorption at 228 nm, which can be attributed to the π→π* transition of aromatic sp2 domains, whereas a broad absorption peak at 350 nm was associated with n→ π* transition (Figure S3a).6 The fluorescence spectra of the GQDs solution across different excitation wavelengths (Figure S3b) showed that the PL of GQDs displayed excitationdependent behavior, which can be explained by the uneven size distribution of the particles or the different emissive states from the functional groups.22 The greatest fluorescence intensity was obtained at an excitation of 460 nm; thus, this wavelength was selected for subsequent experiments. Fluorescence Detection of Fe3+. We hypothesized that Fe3+ coordinates with hydroxyl groups on the surface of GQDs to bridge neighboring GQDs together and that fluorescence quenching may occur as a result of the induced aggregation of GQDs (Figure 1). It was observed that large aggregation areas formed after Fe3+ ions were added to a well-dispersed GQDs solution (Figure 1c). During this time, the strong fluorescence emission of GQDs was obviously quenched (Figure 2a). Changes in the FT-IR spectra of GQDs before and after the addition

of Fe3+ further confirmed this hypothesis. Specifically, multiple peaks at approximately 1434 cm-1 and 1339 cm-1 were converted into a strong narrow peak at approximately 1380 cm-1 after the addition of Fe3+ (Figure S2); we attributed this to Fe3+ ions binding to surface hydroxyl groups. The corresponding UV-Vis spectrum showed an apparent increase in absorption that was different from spectral changes prompted by the addition of other metal ions (Figure S4a and Figure S5). Timeresolved FL decay spectra are shown in Figure S4b, and the average GQDs fluorescent lifetime was 3.68 ns. No obvious change in lifetime was observed after the introduction of Fe3+. When combined with the UV-Vis absorption spectrum results, this suggests that Fe3+ quenching of PL could be attributed to static quenching from Fe3+-induced aggregation of GQDs. Both the O 1s and Fe 2p XPS spectra further suggest the complexation between Fe3+ and -OH groups of the GQDs at pH 3.5 (Figure S6). Based on the affinity between Fe3+ and hydroxyl groups, GQDs could be an excellent sensing platform for Fe3+ ions. As shown in the inset of Figure 2a, this could be represented by a semilogarithmic linear relationship when the concentration of Fe3+ ranged from 0 to 60 μM (as shown by Equation 1). The detection limit was determined to be 0.45 μM (at a signal-to-noise ratio of 3). These results demonstrate that the synthesized GQDs are capable of detecting Fe3+ ions based on aggregation-induced fluorescence quenching.

Figure 1. (a) Schematic for GQDs fluorescence quenched by Fe3+ ions and selectivity of GQDs for Fe3+. (b) TEM image of the GQDs. Inset: Lattice fringes of GQDs. (c) SEM image of Fe3+-induced GQDs aggregation. Understanding the Selectivity of the GQDs. Bare GQDs are sensitive to many metal ions, and certain heavy or transition metals can selectively quench fluorescence. Zong et al. designed an “off–on” fluorescent sensor for the sensitive detection of Cu2+ and L-Cys using carbon dots (CDs).23 In this scheme, Cu2+ absorbed on the surface of CDs significantly quenched their fluorescence. L-Cys has the ability to remove Cu2+ from the surface of CDs and restore the fluorescence. CDs showed an obvious

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selectivity for Cu2+ over other metal ions (including Ag+, Co2+, Cu2+, Hg2+, Mn2+ and Ni2+) under the same conditions. Previously, Liu et al. synthesized ultraviolet and blue emitting GQDs.20 The structure and surface states of GQD-B and GQD-UV are distinctive, where each has different energy levels and metal binding affinities. Herein, the two GQDs exhibited different optical properties and recognition abilities toward diverse metal ions. GQD-B showed a sensitivity to Cu2+, whereas GQDUV was sensitive to Fe3+. In addition to surface functionalized GQDs (which induce a special combination of Fe3+), bare GQDs are also selective for Fe3+. Studies on the selective detection of Fe3+ have focused mostly on how Fe3+ quenches GQDs fluorescence. The special coordination interaction between Fe3+ ions and the phenolic hydroxy groups of the GQDs or the electron/energy transfer process has been widely used to explain the origin of the fluorescence quenching.18,24 However, detailed explanations have not been proposed for why metal ions could be selectively detected using bare GQDs. Such a specific mechanism requires further research.

Figure 2. (a) PL intensity attenuation of GQDs in the presence of 0 μM, 3 μM, 15 μM, 20 μM, 30 μM, 35 μM, 40 μM, 50 μM, and 60 μM Fe3+. Inset: A linear calibration plot for Fe3+ detection. (b) Selectivity of the GQDs for Fe3+ over other common metal ions. The concentrations of all the metal ions were 5 μM. lg(F0/F) = 0.0099 C + 0.0187

(R2=0.994)

(1)

where F0 and F represent the PL intensities of GQDs in the absence and presence of Fe3+, and C represents the concentration of Fe3+ (μM). In our study, bare GQDs showed superior selectivity for Fe3+ at pH 3.5 when other metal ions (including Ni2+, Co2+, Mg2+, Zn2+, Cu2+, Pb2+, Ca2+, K+, Al3+, and Na+) were present as a background matrix (Figure 2b). It is worth noting that we did not treat GQDs to induce superior selectivity to Fe3+; our raw materials were very simple and resulted from a one-pot synthetic process. Considering that the residual nitric acid was difficult to completely remove, and likely impacted the GQDs acidity of the solution, we hypothesize that observed superior selectivity is related to pH.

Figure 3. (a) Diagram for insoluble metal hydroxide solubility-pH.25 (b) Schematic diagram combining the relationship between the GQDs and the metal ions at pH 1.4, pH 3.5 and pH 6.0. Corresponding selectivity of the GQDs at pH 1.4, pH 3.5 and pH 6.0. The concentrations of all the metal ions were 5 μM at different pH values. Nitrogen on the surface of the GQDs exists in three forms: pyrrolic N, pyridinic N and nitrate N (Figure S1b). Adsorbed nitrate may affect both the acidity and composition of GQDs. Thus, we adjusted the acidity of the GQDs solution and tested for selectivity of metal ions known to quench GQDs fluorescence, including Fe3+, Co2+, Ni2+, and Cu2+. A desirable selectivity for Fe3+ can be achieved when the pH of the GQDs solution is 3.5 (Figure 2b). To study the effect of the pH on selectivity, we controlled for other variables and observed the degree of quenching under different pH values. GQDs fluorescence did not change significantly with pH (Figure S7). Thus, we adjusted the pH of the GQDs solution to 1.4 and 6.0 using concentrated HCl and NaOH, respectively. At pH of 6.0, contrast metal ions (Co2+, Ni2+, and Cu2+) showed nonnegligible interference with Fe3+ (Figure 3b); In addition, GQDs showed poor selectivity for Fe3+ at pH of 1.4 (Figure 3b). Different metal hydroxides form under diverse conditions. Specifically, the pH under which metal hydroxides are formed changes according to metal species. For example, ferric hydroxide forms when both

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concentration and pH are on or to the right of the curve (Figure 3a). Based on this simple consideration, the relationship between the pH and the selectivity for a different metal has been discussed in depth. At pH 1.4, only the forming conditions of ferric hydroxide are satisfied. However, the formation of insoluble hydroxides relates not only to pH but also to the concentration of metal ions (detailed theory is discussed in the Supporting Information). However, a precipitate cannot form if the metal ion concentration is extremely low. Fe3+ at micromolar concentrations (such as under sensitive detection conditions) cannot effectively complex to the GQDs hydroxyl group. All metal ions had negligible quenching effects at a pH of 1.4. In contrast, a very desirable selectivity for Fe3+ was observed at a pH of 3.5. This is because of the lower Ksp of the metal hydroxides, and at a lower pH, the precipitate appears when the metal ion concentration is at the same value. The Ksp of Fe(OH)3 is much smaller than that of the other three metal hydroxides (2.8×10-39 for Fe(OH)3, 2.2×10-20 for Cu(OH)2, 5×10-16 for Ni(OH)2, 2.3×10-16 for Co(OH)2). Thus, it is easier to form Fe(OH)3 at a low pH and therefore induce GQDs aggregation. With increasing pH, the tendency to form metal hydroxides with a larger Ksp than Fe(OH)3 increases. This means that copper hydroxide, nickel hydroxide and cobalt hydroxide are more likely to produce. Most experiments that use GQDs fluorescence quenching to detect metal ions are conducted in a buffer solution, such as phosphate-buffered saline (PBS). When the pH remains stable, it is difficult to determine the relationship between pH and GQDs selectivity. To further verify our theory, we used carbon dots (CDs) and carbon nitride (g-C3N4) nanosheets as contrast fluorescent probes. Results for detecting metal ions using CDs at different pHs were similar to those for GQDs (Figure S8). However, the element composition and electronic properties of gC3N4 are significantly different from those of GQDs, which affects how they complex with metal ions (Figure S9). Surface states have a great influence on GQDs optical properties. For example, GQDs absorption bands can be regulated by surface passivation or other modifications.6 Furthermore, excitation-dependent properties of GQDs relate heterogeneity in size and surface state.26 Herein, residual nitric acid absorbs to the surface of GQDs, which likely impacts how GQDs complex to different metal ions. To study this, we washed adsorbed nitrate off of the surface of GQDs and adjusted the solution pH back to 3.5 using concentrated hydrochloric acid. This process replaces the nitrate anion with chloride anion, enabling us to determine whether the adsorption of the nitrate anion on the GQDs impacts Fe3+ selectivity (the scheme in Figure 4). The excellent selectivity for Fe3+ was unchanged for chloride ion-absorbed GQDs at a pH of 3.5 (Figure 4b). We thus hypothesize that nitrate plays two roles. Although adsorbed on the surface of the GQDs, it does not affect its surface state and has no effect on how

GQDs complex with metal ions. However, the residual acid alters the GQDs solution pH, which directly influences interactions between GQDs and metal ions, as discussed previously.

Figure 4. (a) Schematic for nitrate desorption and replacement of NO3− by Cl− on the GQDs surface. (b) Selectivity of GQDs for Fe3+ using different adsorbed anions. The concentration of all the metal ions was 5 μM.

CONCLUSIONS To give an in-depth and systematic interpretation of the selective detection of metal ions based on GQDs as fluorescence probes, we prepared bare GQDs via the exfoliation of carbon black with nitric acid to test the Fe3+ ions. The distinct recognition abilities of the bare GQDs for Fe3+ were revealed at various pH values. Unlike other studies that attribute selectivity to surface recognition groups, we demonstrated that complexation between GQDs and Fe3+ is seriously altered by the acidity of the GQDs solution. Fe3+ can coordinate with hydroxyl groups on the GQDs surface to form aggregates (like iron hydroxide), quenching GQDs fluorescence. On account of the lower Ksp value of the metal hydroxide, the corresponding precipitate will appear at a lower pH. Under relatively acidic conditions, we achieved satisfactory selectivity for Fe3+ ions, owing to the extremely low Ksp of ferric hydroxide. We determined key selectivity parameters by operating in a primitive environment, thus excluding interference from the buffer solution, non-inherent groups, and other factors. We believe that our results are critical for future studies, including the development of detection systems that can selectively determine metal ions. Our work also contributes critical instructive for understanding the intrinsic properties of the pure GQDs.

ASSOCIATED CONTENT Supporting Information XPS, FT-IR, UV-vis absorption, PL spectra of GQDs and contrast experiments for other carbon-based fluorescent probes and the theoretical derivation for the relationship

between the solubility and solution acidity are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author E-mail: [email protected]. Phone: 010-64435271. E-mail: [email protected]. Phone: 010-82362042.

ORICD

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(10) Ananthanarayanan, A.; Wang, X. W.; Routh, P.; Sana, B.; Lim, S.; Kim, D. H.; Lim, K. H.; Li, J.; Chen, P. Adv. Funct. Mater. 2014, 24, 3021－3026. (11) Goncalves, H. M. R.; Duarte, A. J.; da, Silva, J. C. G. E. Biosens. Bioelectron. 2010, 26, 1302－1306.

Xu Chen: 0000-0001-6187-3890 Tie Wang: 0000-0001-5965-6520 Notes The authors declare no competing financial interest.

(12) Du, Y.; Lim, B. J.; Li, B. L.; Jiang, Y. S.; Sessler, J. L.; Ellington, A. D. Anal. Chem. 2014, 86, 8010-8016.

ACKNOWLEDGMENT

(13) Zhang, J. F.; Zhou, Y.; Yoon, J. Y.; Kim, S. Chem. Soc. Rev., 2011, 40, 3416–3429.

This research was financially supported by the 1,000 Young Talents program, the National Natural Science Foundation of China (Grant Nos. 21656001, 21521005, 21422507, 21321003) and the Chinese Academy of Sciences.

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