Optical Turn-On Sensor Based on Graphene Oxide for Selective

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Optical Turn-On Sensor Based on Graphene Oxide for Selective Detection of D-Glucosamine Rumei Cheng,† Yong Liu,† Shengju Ou,‡ Yaqiong Pan,† Shu Zhang,† Hao Chen,*,† Liming Dai,*,† and Jia Qu*,† †

Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, and Eye Hospital, Wenzhou Medical College, China ‡ S. C. Energy Technology Co. Ltd., China S Supporting Information *

ABSTRACT: By incorporating the well-known fluorophore 8-aminoquinoline into graphene oxide, we have successfully prepared a turn-on fluorescent sensor capable of specific detection of D-glucosamine with a high selectivity and sensitivity. This methodology provides a new concept for the design and development of highly selective and sensitive turnon optical sensors for selective detection of aminosaccharides and many other biomolecules.

Owing to its unique characteristics, such as good electrical and mechanical properties,7 high water dispersibility,6 large surface area and facile surface modification,8,9 and strong photoluminescence,10 graphene oxide (GO) has been used to construct fluorescent biosensors with improved performance.11,12 Comparing with other sensing techniques, the fluorescent method has many advantages, including greater simplicity, broader applicability, and higher sensitivity. Furthermore, the “turn-on”-type fluorescent sensors, with which the analyte binding leads to an increase in fluorescence intensity from an initially low level, could be extremely sensitive with a low interference from background fluorescence. Here, we report a fluorescent turn-on sensor based on 8aminoquinoline functionalized graphene oxide (GAQ, Scheme 1a) for detection of D-glucosamine with a high selectivity and sensitivity through a photoinduced electron transfer (PET) signaling mechanism. As schematically shown in Scheme 1b, the chemically bonded quinoline fluorophore in GAQ acts as an electron acceptor. Upon optical excitation, an electron in the highest occupied molecular orbital (HOMO) of the quinoline fluorophore is excited into its lowest unoccupied molecular orbital (LUMO). The electron in the HOMO of the donor (GO) then transfers to the HOMO of the fluorophore (the quinoline) through the amide linkage to cause fluorescent quenching. Once binding with the target molecules (i.e., Dglucosamine), the electron transfer from the GO to the quinoline, and the associated fluorescent quenching process, is

The mediation of a large number of biological and pathological events by D-glucosamine is known.1 It is of both biological and biomedical significance to construct sensing devices for probing D-glucosamine over its biological relevant concentration (about 7 mg/L in a man’s serum).2 The development of D-glucosamine sensors with a high sensitivity and selectivity will provide novel approaches for early clinic diagnosis of most D-glucosamine related diseases, such as hepatitis, gastric ulcer, and rheumatic arthritis.2,3 So far, a few of the D-glucosamine sensors have been devised using either boronic acid or transition metals as receptors.4 For instance, Cooper and James5 designed a Dglucosamine fluorescent sensor containing monoaza-18-crown6 ether and boronic acid as the receptor units, in which the boronic acid chelated the diol parts of D-glucosamine and the monoaza-18-crown-6 bound the amine group to enhance the sensing signal. Although boronic acid is well-known to bind diol readily to act as a carbohydrate receptor, many boronic acid based sensors failed to recognize D-glucosamine due to their low selectivities. On the other hand, D-glucosamine chemosenors based on metal complexes in acetonitrile solution have also been reported.6 However, the poor solubility of the metal complexes in water made it difficult, if not impossible, to detect D-glucosamine in aqueous solutions. To date, most of the Dglucosamine sensors suffer multiple shortcomings, including the delayed response, lack of selectivity, and/or precipitation in aqueous solutions. As a result, it is still a big challenge to create synthetic receptors that bind D-glucosamine with a high sensitivity and selectivity. Therefore, it is highly desirable to develop new and improved sensors for detecting D-glucosamine with a high sensitivity and selectivity. © 2012 American Chemical Society

Received: March 20, 2012 Accepted: June 1, 2012 Published: June 1, 2012 5641

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ultrasonication for 1 h. Subsequently, 20 μL of Nhydroxysuccinimide (NHS, 1.0 mM) and 20 μL of N-(3dimethylaminopropyl)-N′-ethylcarbodiimde (EDC, 1.0 mM) solutions were sequentially added into the GO dispersion. After 10 min, 5 mL of an ethanol solution of 8-aminoquinoline (2 mg/mL) was added. The resulting mixture solution was heated up to 50 °C for 2 h, followed by magnetic stirring for 12 h at room temperature. Upon filtration, the GAQ was separated and washed by ethanol for subsequent characterization.

Scheme 1. (a) Synthetic Route to GAQ and (b) Schematic Representation of the Mechanism for the Detection of DGlucosamine

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RESULTS AND DISCUSSION To follow the chemical reaction shown in Scheme 1a, we performed Fourier transform-infrared (FT-IR) spectroscopic measurements. As expected, the FT-IR spectrum of GAQ given in Figure 1A shows a very strong O−H vibration at 3524 cm−1,

disrupted due to the “blocking” effect induced by binding of the target molecule to the GAQ at the amide side (Scheme 1b), leading to a significantly enhanced fluorescence emission. Although the possibility for D-glucosamine to bind onto the GO basal plane in GAQ cannot be ruled out, our measurements on a controlled experiment involving the unfunctionalized GO and D-glucosamine mixture did not show any noticeable change in the fluorescent emission intensity.

Figure 1. (A) FT-IR spectra of GO and GAQ and (B) Raman spectra of GAQ and GO.

along with the CO stretching at 1730 cm−1. Upon the chemical bonding with 8-aminoquinoline, the amide band at 1631 and 1518 cm−1, and aromatic C−C stretch (in ring) at 1455 and 1340 cm−1 appeared.14 Bands over 8501250 cm−1 can be attributed to the =C−H bend, C−O, and C−H stretching. Comparing the Raman spectrum of GAQ to that of GO in Figure 1B revealed a significant decreased in the ID/IG intensity ratio, suggesting an increase in the average size of the sp2 domains for GAQ via conjugation of GO with aminoquinoline.15 It was also found that the G band of GAQ was shifted by 10 cm−1 compared to that of GO, indicating the electron transfer from the GO sheet to quinoline16 to quench its fluorescent emission, as mentioned earlier. The weak bands over 615, 1488, and 2050 cm−1 shown in the Raman spectrum of GAQ can be attributable to typical quinoline vibrations, which are absent in GO (Figure 1B). Further evidence for the chemical structure changes comes from X-ray photoelectron spectroscopic measurements. Figure 2a shows the XPS survey spectra of GO and GAQ. The former shows the C 1s (∼285 eV) and O 1s (∼530 eV) peaks only while the latter shows an additional N 1s peak at about 400 eV attributable to the aminoquinoline moieties grafted onto GO in the GAQ with a C/N ratio17 of 26:1 (Figure 2a). The C1s peak of GO in Figure 2b can be fitted to C−C, C−O, CO, and C(O)O components at 285.0, 286.4, 287.5, and 289.1, respectively. In addition to the above-mentioned carbon components, the curve-fitted C1s peak of GAQ in Figure 2c contains the C−N at 285.9 eV and amide carbon at 288.2 eV.18 The corresponding N1s peak from GAQ shown in Figure 2d can be fitted to quinoline N at 400.5 eV and −NH−CO at 402.3 eV. These XPS results clearly show that aminoquinoline molecules have been successfully attached onto GO, as shown in Scheme 1.

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EXPERIMENTAL SECTION Materials and Methods. All reagents used were analytical grade. D-glucosamine was purchased from Sigma. All other reagents were obtained from Shanghai Reagents Company. The atomic force microscopy (AFM) image (Figure S1a in the Supporting Information) of the GAQ was observed using a SPI3800N microscope operating in the tapping mode. The transmission electron microscope (TEM) image (Figure S1b in the Supporting Information) was taken by using a PHILIPS EM400ST microscope at an accelerating voltage of 150 kV. Xray photoelectron spectroscopy (XPS) was performed on ESCALB MK-II spectrometer. Fourier transform-infrared spectra (FT-IR) were recorded on a PE Spectrum One spectrometer with KBr pellets. UV−vis spectra were measured on a Perkin-Elmer Lambda 35 spectrometer. Raman spectra were recorded with a Renishaw 2000 equipped by an Ar+ ion laser having the excitation line of 514.5 nm and an air-cooling charge-coupled device as the detector. Fluorescence emission spectra were obtained using an AB-series2 luminescence spectrometer. Fluorescence determination was done in a 1:1 ethanol−water (volume ratio) solution. Diluted HCl and NaOH were used to adjust the pH values of solutions. All solutions were freshly prepared immediately before use. The titration experiments were carried out at a pH = 6.5. Synthesis of Nanosensor GAQ. GO was prepared from natural graphite (Shanghai Reagents Company) using the Hummer’s method,13 through the acid-oxidation exfoliation, followed by filtration and vacuum drying. Scheme 1a shows the reaction of 8-aminoquinoline with graphene oxide in aqueous solution to produce GAQ. In a typical experiment, the resultant GO (16 mg) was dissolved in 20 mL of distilled water under 5642

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weaker and remained unchanged even after the addition of Dglucosamine under the same conditions (Figure S3 in the Supporting Information). Therefore, we have demonstrated a fluorescent turn-on sensor based on 8-aminoquinoline functionalized graphene oxide (GAQ, Scheme 1a) for detection of D-glucosamine with a high sensitivity. As shown in Scheme 1b and described in the associate text above, a PET signaling mechanism is responsible for the sensing process.20 The fluorescence response of GAQ to various biological molecules/ions and its selectivity for D-glucosamine are illustrated in Figure 4. As can be seen, the presence of a 2-

Figure 2. XPS spectra of GO and GAQ: (a) survey spectra of GAQ and GO, (b) C1s peak of GO, (c) C1s peak of GAQ, and (d) N1s peak of GAQ. Figure 4. Fluorescence response of GAQ (25.0 mg/L) in 1:1 ethanol− water solution with addition of a 2-fold excess concentration of various ions/molecules including: (1) GAQ, (2) glucosamine, (3) glucose, (4) glycine, (5) ethanolamine, (6) Na+, (7) K+, (8) Mg2+, (9) Ca2+, (10) Zn2+, (11) DNA oligomer, and (12) vasoactive intestinal peptide, respectively.

Having successfully functionalized GO with quinoline fluorephore, we demonstrated the potential application of GAQ as fluorescent sensors by carrying out UV−vis and photoluminescence measurements. Figure 3a shows UV−vis

fold excess concentration of K+ increased the luminescent intensity by about 5%. The presence of other physiologically important metal ions which exist in living cells, such as Ca2+, Mg2+, Na+, Zn2+, and Fe3+, did not change the luminescent intensity much under the same conditions nor the presence of glycine, D-glucose, ethanolamine, DNA oligomer, and vasoactive intestinal peptide. The simple mixture of GO and 8aminoquinoline did not show any response to the above species (Figure S4 in the Supporting Information). These results clearly indicate that the GAQ fluorescent sensor is highly selective.21 More interesting, the GAQ fluorescent sensor was demonstrated to be reusable in the pH range from 5.5 to 7.5 (Figures S5 and S6 in the Supporting Information).

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CONCLUSIONS In summary, we have for the first time demonstrated that 8aminoquinoline could be covalently grafted onto graphene oxide for detection of D-glucosamine with a high sensitivity and selectivity through a photoinduced electron transfer (PET) signaling mechanism. Owing to the generic nature of the PET signaling, the concept demonstrated in this study should have important implications for the development of other fluorescent turn-on sensing for detection of various species of biological significance.

Figure 3. (a) UV−vis spectra of 8-aminoquinoline, GO, and GAQ. (b) Fluorescence response of GAQ (25.00 mg/L) upon addition of different amounts of D-glucosamine in 1:1 ethanol−water solution (excitation at 330 nm). (c) Emission intensity at 560 nm vs concentrations of D-glucosamine.

spectrum of GAQ in 1:1 water/ethanol solution, which exhibited two broad bands at 243 and 310 nm attributable to the n → π* transition of amide groups, and the π → π* transition of aromatic carbon bonds, respectively.19 Upon excitation at 330 nm, the GAQ solution showed a strong emission band at 560 nm (Figure 3b). The addition of Dglucosamine significantly enhanced the luminescent intensity of GAQ. A linear response between the amount of D-glucosamine and the luminescent intensity was obtained over the concentration range from 1.8 to 40 mg/L with a detection limit of 1.0 mg/L (3c). Although 8-aminoquinoline also exhibited an emission peak at 560 nm, its intensity was much

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.C.); liming.dai@hotmail. com (L.D.); [email protected] (J.Q.). 5643

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Notes

Chem. 2010, 20, 4328−4332. Zhou, X.; Yu, B.; Guo, Y.; Tang, X.; Zhang, H.; Liu, W. Inorg. Chem. 2010, 49, 4002−4007. (20) De Silva, A. P.; Fox, D. B.; Huxley, A. J. M.; Moody, T. S. Coord. Chem. Rev. 2000, 205, 41−57. Ou, S.; Lin, Z.; Duan, C.; Zhang, H.; Bai, Z. Chem. Commun. 2006, 4392−4394. (21) Liu, Y.; Liu, C. Y.; Liu, Y. Appl. Surf. Sci. 2011, 257, 5513−5515.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education of China (Grants 20103321120003, IRT1077, and 211069), the Chinese National Nature Science Foundation (Grant 81000663), the National “Thousand Talents Program”, the Ministry of Science and Technology of China (Grant 2009DFB30380), and the Zhejiang Department of Education (Grant T200917) are acknowledged. The Science Foundation of Wenzhou Medical College (Grant KYQD110104) is also acknowledged.

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