Biosensors and Bioelectronics 74 (2015) 284–290

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Multi-positively charged dendrimeric nanoparticles induced fluorescence quenching of graphene quantum dots for heparin and chondroitin sulfate detection Yan Li a, Hongcheng Sun b, Fanping Shi a, Nan Cai a, Lehui Lu c, Xingguang Su a,n a

Department of Analytical Chemistry, College of Chemistry, Jilin University, Changchun 130012, China State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China c State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 5 May 2015 Received in revised form 13 June 2015 Accepted 15 June 2015 Available online 20 June 2015

A label-free fluorescence assay for rapid and sensitive detection of heparin (Hep) or chondroitin sulfate (CS) was developed by guanidine-terminated poly (amidoanime) (PAMAM-Gu þ ) dendrimers induced aggregation of graphene quantum dots (GQDs). The fluorescence of GQDs was obviously quenched after mixing with PAMAM-Gu þ . However, the addition of highly negatively charged Hep or CS into the fluorescence sensing system resulted in the fluorescence recovery. Because the multi-positively charged PAMAM-Gu þ would prefer to bind with highly negatively charged Hep or CS, resulting in the deaggregation of GQDs. Under the optimized experimental conditions, the recovery of fluorescence intensity ratio I/I0 (I0 and I were the fluorescence intensity of the sensing system in the absence or presence of target analytes, respectively) was proportional to the concentration of target analytes in the range of 0.04–1.6 μg mL  1 for Hep and 0.1–2.5 μg mL  1 for CS. In addition, this method afforded high sensitivity with the detection limit as low as 0.02 μg mL  1 and 0.05 μg mL  1 for Hep and CS, respectively. All results suggested that the fluorescence turn-on method could be successfully employed for sensitive and selective detection of heparin analogs. & 2015 Elsevier B.V. All rights reserved.

Keywords: Graphene quantum dots Guanidyl dendrimer Fluorescence Heparin Chondroitin sulfate.

Introduction Graphene-based materials are promising building blocks for future nanoelectronic devices, electron transportation and supercapacitor due to their fascinating thermal, electronic and mechanical properties (Liao et al., 2010; Novoselov et al., 2004). Recently, many experimental works on graphene-based materials have focused on the optical properties of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). GQDs, which are zero-dimensional graphene sheets smaller than one hundred nanometers in single or few layers, have aroused tremendous research interest in recent years because of their extraordinary optical and electronic properties (Li et al., 2011; Pan et al., 2010; Ponomarenko et al., 2008). Besides the fluorescence property contributed by quantum confinement and edge effects (Girit et al., 2009; Ponomarenko et al., 2008), GQDs also exhibit excellent chemical and physical properties including low toxicity, unique photoluminescence, excellent solubility, high biocompatibility and n

Corresponding author. E-mail address: [email protected] (X. Su).

http://dx.doi.org/10.1016/j.bios.2015.06.032 0956-5663/& 2015 Elsevier B.V. All rights reserved.

robust chemical inertness. These emerging properties of GQDs make them more suitable for cellular imaging, drug delivery, electrochemical and fluorescent biosensor (Dong et al., 2012; Ju and Chen, 2015; Wang et al., 2014; Zhu et al., 2011). Therefore, many researchers have devoted their efforts to explore sensing systems for metal ions (Dong et al.,. 2014; Ju and Chen, 2014; Sun et al., 2013; Zhang and Chen, 2014), glucose (He et al., 2014; Qu et al., 2013), melamine (Li et al., 2014), trypsin (Li et al., 2013) and others (Fan et al., 2012; Liu et al., 2013; Qian et al., 2014). The biological macromolecular polysaccharide-based polyanions, including dextran sulfate, carrageenan, heparin (Hep) and chondroitin sulfate (CS) have unique properties in physiology and food technology (Ayadi et al., 2009; Stalcup and Agyei, 1994). For example, CS is the major sulfated glycosaminoglycan component of native cartilage tissue, which is reported for osteoarthritis therapy because of its high negative charge density, capacity to stimulate the production of extracellular matrix by chondrocytes and ability to induce the chondrogenic differentiation of multipotent stromal cells (Fan et al., 2010; Kubo et al., 2009). Hep, which consists predominantly of a trisulfated disaccharide repeating unit, is a naturally occurring negatively charged linear

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polysaccharide. It has been clinically used as a major anticoagulant during cardiopulmonary surgery, open heart surgery and in emergency deep venous thrombosis conditions. An overdose of this highly charged polyanion has revealed important adverse effects (Freedman, 1992; Ngo et al., 2008). For example, heparin overdose can induce thrombocytopenia, which is one of the catastrophic complications of heparin treatment. Close monitoring and determination of heparin is much important not only for its regulation in the physiological process but also for its clinical application during surgery and postoperative therapy period. Several methods for monitoring highly negatively charged polyanions have been developed for these polysaccharides in commercial or clinical samples (Fu et al., 2012; Sun et al., 2006; Yu et al., 2009). In recent years, researchers have developed some fluorescence approaches for polyanions detection by electrostatic interaction (Cai et al., 2011; Dai et al., 2011; Kim et al., 2014; Wu et al., 2013). Even though some of these methods showed high sensitivity, most of their positively charged materials used for polyanions detection were insoluble in water and contained few positive charges, which extremely restricted their applications in detection fields. Therefore, it was crucial to develop water-soluble multi-positively charged materials for electrostatic detection of polyanions. As reported in the previous study, Mn-doped ZnSe quantum dots could aggregate in the presence of cationic polyarginine peptide Arg6 via electrostatic interactions that result in the fluorescence quenching (Gao et al., 2012a). However, few works were evolved in dendrimeric nanoparticles induced fluorescence quenching of quantum dots with high efficiency. The success of the previous work promoted us to identify whether dendrimeric nanoparticles could also induced the aggregate of GQDs or not. Poly (amidoanime) (PAMAM) dendrimer, a type of uniform distributed macromolecules with defined molecule size, shape and placement of functional groups, is a near-globular dendrimeric nanoparticles at high generation (Sun et al., 2015). Water-soluble guanidine-terminated dendrimer (PAMAM-Gu þ ) with 128 guanidinium ions (Gu þ ) distributed on the molecular surface might be the most ideal electropositive nanoparticles to interact with polyanions. Therefore, we developed a novel tun-on fluorescence method for sensitive detection of Hep or CS based on the electrostatic aggregation of GQDs with multi-positively charged dendrimers for the first time. PAMAM-Gu þ could bind to the carboxyl group of graphene quantum dots via electrostatic interaction, leading to the degression of the fluorescence intensity. When Hep or CS were introduced to the PAMAM-Gu þ /GQDs system, the PAMAM-Gu þ and highly negatively charged Hep or CS formed a conjugate due to their strong electrostatic attraction, resulting in the fluorescence recovery of GQDs. This turn-on fluorescence method provided a useful strategy for rapid and sensitive detection of Hep or CS in commercial medicinal solutions.

Experimental Reagents and chemicals 1, 2-Ethylenediamine and 1H-pyrazole-1-carboxamidine hydrochloride were purchased from Sinopharm Chemical Reagent Co. Ltd. Methanol was obtained from Beijing Chemical Plant and dried with molecular sieves. Methyl acrylate was purchased from J&K Chemical LTD. Chondroitin sulfate A (CS) from Sangon Biotech (Shanghai) Co., Ltd. Heparin (Hep, 100 U/mL), glucosamine, gluocose, fructose, galactose, lactose and sucrose were purchased from Beijing Dingguo Changsheng Biotechnology Co. Ltd. NaCl, NaAc, Na2HPO4 and Na2CO3 were obtained from Tianjin Guangfu Institute of elaborate chemical industry. Injectable heparin and chondroitin sulfate eye drops were obtained from Bausch & Lomb

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Freda Pharmaceutical Co. Ltd. and Xinyi pharmacy, respectively. The water used in all experiments had a resistivity greater than 18 MΩ cm  1. Instrumentation 1 H NMR spectra was measured on a Bruker 510 spectrometer (500 MHz) using CDCl3 or D2O as solvent with tetramethylsilane (TMS) as a reference. Atomic Force Microscopy (AFM) measurements were performed on a NanoScope Multimode AFM (Veeco, USA) using the tapping mode AFM. UV–vis spectrum was obtained with a Shimadzu 3100 UV–vis-NIR Recording Spectrophotometer. Dynamic Light Scattering (DLS) experiments were carried out with Malvern Instrument Zetasizer Nano ZS equipped with a He–Ne laser (633 nm, 4 mW) and an avalanche photodiode detector. Fluorescence measurements were performed on a Shimadzu RF5301 PC spectrafluorophotometer with a 1 cm path-length quartz cuvette.

Synthesis of graphene quantum dots Graphene oxide was prepared by a modified Hummer's method and characterized in our previous work (Gao et al., 2012b). Graphene quantum dots were synthesized according to the previous work (Pan et al., 2010; Peng et al., 2012). Briefly, 0.3 g graphene oxide were added into a mixture of 60 mL H2SO4 (98%) and 20 mL HNO3. The solution was sonicated for 2 h and stirred for 24 h at 80 °C. The mixture was cooled and diluted with 600 mL water and the pH value was adjusted to 7 with NaOH solution. The final product solution was further dialyzed in a dialysis bag (retained molecular weight: 3500 Da) for 7 days. Synthesis of guanidyl dendrimers The synthesis of guanidyl dendrimers (PAMAM-Gu þ ) were shown in Scheme S1. Poly (amino amine) (PAMAM) dendrimers were prepared according to a standard procedure (Krishnan et al., 2009). Briefly, 1, 2-ethylenediamine was employed as core and reacted with methyl acrylate by Michael addition reaction to form quaternary ester. Then, the quaternary ester was aminated with excess 1, 2-ethylenediamine to form quaternary amide, as generation zero (PAMAM G0). We could obtain different generations of PAMAM dendrimers by alternatively Michael additions and amination reactions. Guanidyl dendrimer was synthesized according to the previous report (Tian et al., 2014). Specifically, to a stirred solution of PAMAM G5 in N,N-dimethylformamide, N,Ndiisopropylethylamine (DIEA) and 1H-pyrazole-1- carboxamidine hydrochloride was added. The mixture was stirred at room temperature for 24 h. The reaction mixture was concentrated and dialyzed in a dialysis bag (retained molecular weight: 3500 Da) for 3 days. 1H NMR (Fig. S1-S3) was employed to detect the synthesis procedures of PAMAM-Gu þ . Fluorescence experiments In this study, the detection of Hep or CS was carried out in 50 mM Tris–HCl buffer solution at room temperature. GQDs (25 μg/mL) were firstly mixed with PAMAM-Gu þ , then different concentrations of Hep or CS were added into above solution and allowed to react for 15 min before measurement. The fluorescence spectra were recorded in the 500–740 nm emission wavelength range with the excitation of 480 nm, and the emission and excitation slits were both set at 10 nm.

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Results and discussion Characterization of graphene quantum dots The graphene quantum dots (GQDs) were synthesized from graphene oxide (GO) by chemical oxidation according to the previously described method (Peng et al., 2012). To explore the properties of the GQDs, UV–vis absorption and fluorescence spectra characterizations were carried out (shown in Fig. 1A). The GQDs suspension showed the UV–vis absorption peak around 300 nm, which was consistent with the previous report (Peng

et al., 2012). The fluorescence emissions of GQDs with different excitation wavelengths were shown in Fig. 1A. Like most fluorescent carbon dots, the GQDs also exhibit excitation-dependent fluorescence behavior. With the excitation wavelength changed from 360 to 500 nm, the emission spectra of the GQDs shifts to longer wavelength with the strongest peak at 540 nm when excited at 480 nm. The atomic force microscopy (AFM) image in Fig. 1B–D clearly showed the topographic morphology of GQDs. As shown, the heights were between 1.3 and 3.7 nm, corresponding to 1–3 graphene layers, similar to those previously reported works (Ju and Chen, 2014; Li et al., 2012; Zhu et al., 2011), and the sizes

Fig. 1. The UV–vis absorption and fluorescence spectra of GQDs at different excitation wavelengths (A) and the atomic force microscopy image of GQDs (B–D).

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Fig. 2. The dynamic light scattering analysis of the GQDs (A) and GQDs in the presence of 1.2 (B) and 2.4 μg/mL (C) PAMAM-Gu þ . The fluorescence spectra of GQDs with different concentrations of PAMAM-Gu þ (D) and The calibration curve between the fluorescence intensity ratio I/I0 and concentrations of PAMAM-Gu þ ranging from 0 to 1.2 μg/mL (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2 μg/mL) (E). I0 and I were the fluorescence intensity of GQDs in the absence and presence of PAMAM-Gu þ , respectively.

were distributed in the range of 20–40 nm. As shown in Fig. 2A, dynamic light scattering (DLS) also showed the diameters of GQDs was about 38 nm. Compared with the blue-fluorescent GQDs, the as-prepared GQDs could be applied to complex biological samples due to the longer wavelength excitation of these GQDs. Design of the florescence sensing system In this work, a novel optical sensing method for turn-on detection of heparin (Hep) and chondroitin sulfate (CS) was developed via the fluorescent signal change of GQDs. Scheme 1 illustrated the principle of our fluorescence sensing system for the Hep or CS determination. Guanidyl dendrimer (PAMAM-Gu þ ), with 128 terminal guanidinium ions around the surface, is a nearglobular organic molecule with a diameter of 5–6 nm. GQDs would aggregate in the presence of PAMAM-Gu þ via electrostatic interactions, resulting in fluorescence quenching. Upon the addition of Hep and CS, the multi-positively charged PAMAM-Gu þ would prefer to bind with highly negatively charged Hep or CS instead of

GQDs, and the fluorescence intensity of the sensing system could also be recovered. The fluorescence spectra of GQDs sensing system in the presence or absence of target analytes were shown in Fig. 3A. It could be seen that the fluorescence of GQDs was effectively quenched in the presence of PAMAM-Gu þ , while the fluorescence recovered after the addition of Hep and CS. The extent of fluorescence recovery induced by Hep was higher than that by CS at the same concentration, resulting from the different amount sulfate ions of the polyanions (Wu et al., 2013). DLS was used to investigate the electrostatic induced GQDs/PAMAM-Gu þ aggregates at the final PAMAM-Gu þ concentration of 1.2 and 2.4 μg/mL. From Fig. 2A–C, it can be seen that the diameter of GQDs in the presence of 1.2 and 2.4 μg/mL PAMAM-Gu þ changed from 38 nm to 342 and 615 nm, respectively. The results demonstrated that the fluorescence quenching was resulted from the GQDs aggregates induced by this dendrimeric nanoparticle.

Scheme 1. Schematic illustration of the fluorescence turn-on system for Hep or CS determination.

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Fig. 3. The fluorescence spectra (A) and the effect of the incubating time (B) on the fluorescence intensity of GQDs sensing system. (a) GQDs; (b) GQDs þ PAMAM G4.5; (c) GQDs þ PAMAM-Gu þ þHep; (d) GQDs þPAMAM-Gu þ þ CS; (e) GQDs þ PAMAM-Gu þ .

Optimization of the fluorescence sensing system The performance of the established turn-on sensing system is closely related to some important experimental conditions. In this study, the effect of the concentration of PAMAM-Gu þ on the fluorescence intensity of GQDs was investigated firstly. Fig. 2D showed the fluorescence spectra of GQDs with different concentrations of PAMAM-Gu þ , from which it could be seen that the fluorescence intensity of GQDs gradually decreased with the increasing of PAMAM-Gu þ concentration due to the aggregates between GQDs and PAMAM-Gu þ . Fig. 2E also showed a good linear relationship between the fluorescence intensity ratio I/I0 (I0 and I were the fluorescence intensity of GQDs in the absence and presence of PAMAM-Gu þ , respectively) and concentrations of PAMAM-Gu þ ranging from 0 to 1.2 μg/mL. And we chose 1.2 μg/mL PAMAM-Gu þ for Hep or CS detection in the further experiments. We also studied the effect of the incubating time on the fluorescence intensity of the GQDs sensing system, and the results were shown in Fig. 3B. It showed that the fluorescence intensity of GQDs sharply decreased after the addition of PAMAM-Gu þ at room temperature and remained nearly constant after 1 min. When Hep or CS was added into the GQDs/PAMAM-Gu þ system, the fluorescence intensity of the sensing system increased quickly with the increasing of the incubation time and reaching a plateau after 15 min. Thus the incubation time of 15 min for the target analytes detection was adopted in the following experiments. The results also revealed that the proposed method for Hep and CS determination was more rapid compared with other previous report (Hung and Tseng, 2014; Jin et al., 2009). To further confirm the fluorescence quenching of GQDs was due to the electrostatic interaction, a comparative experiment was performed in which the PAMAM-4.5 without any amino groups around the surface was

mixed with GQDs. From Fig. 3B, it could be seen that the fluorescence quenching extent induced by PAMAM-Gu þ was obviously greater than that by PAMAM-4.5, which indicated that PAMAMGu þ could efficiently quench the fluorescence of GQDs through electrostatic interactions. Hep or CS detection To further study the performance of the turn-on sensing system for Hep or CS detection, we investigated the fluorescence emission spectra of this system with different concentrations of Hep or CS under the optimized conditions. As shown in Fig. 4A, the fluorescence intensity of sensing system increased upon the increasing concentration of Hep from 0 to 10.0 μg/mL. The inset of Fig. 4A showed a good linear relationship between the fluorescence intensity ratio I/I0 (I0 and I were the fluorescence intensity of the sensing system in the absence and presence of Hep, respectively) and the concentrations of Hep in the range of 0.04– 1.60 μg/mL with a correlation coefficient R2 ¼0.998. The linear regression equation was I/I0 ¼1.092 þ0.698CHep (μg/mL) and the detection limit for Hep was 0.02 μg/mL based on 3s rule, the relative standard deviation (RSD) for six replicate measurements of 0.1 μg/mL Hep was 4.7%. The fluorescence spectra of the sensing system with and without CS were shown in Fig. 4B, and the inset of Fig. 4B revealed a good linear correlation between the fluorescence intensity ratio I/I0 (I0 and I were the fluorescence intensity of the sensing system in the absence and presence of CS, respectively) and the concentrations of CS in the range of 0.1–2.5 μg/mL. The linear regression equation for CS was as follows: I/ I0 ¼1.020 þ0.341CCS (μg/mL) (R2 ¼ 0.998), and CS can be detected as low as 0.05 μg/mL. Compared with the previous reports for Hep and CS determination in linear range and detection limit (Tables S1 and S2), it could be see that our method obtained the similar or

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Fig. 4. (A) Fluorescence emission spectra of GQDs sensing system with different concentrations of Hep (0, 0.04, 0.06, 0.4, 0.8, 1.2, 1.6, 2.0, 3.0, 4.0, 5.0, 10 μg/mL), The inset of (A) showed the calibration curve between the fluorescence intensity ratio I/I0 and the concentration of Hep in the range of 0.04 to 1.6 μg/mL. (B) Fluorescence emission spectra of GQDs sensing system with different concentrations of CS (0, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 10 μg/mL), The inset of (B) showed the calibration curve between the fluorescence intensity ratio I/I0 and the concentration of CS in the range of 0.1–2.5 μg/mL. I0 and I were the fluorescence intensity of the sensing system in the absence and presence of target analytes, respectively.

Table 1 Detection of Hep or CS in medicinal solutions using this proposed fluorescence method. Medicinal solutions

Determined (μg/mL)

Hep

0.195

CS

0.498

Added (μg/mL) 0.200 0.600 0.500 1.000

Found (μg/mL) 0.390 0.840 1.000 1.558

Recovery (%)

RSD (n ¼3, %)

97.5 107.5 100.0 106.0

1.5 2.9 1.0 3.6

superior detection limit and linear range.

by standard addition method were shown in Table 1. The obtained values of target analytes were calculated to be 6094 U/mL for injectable heparin and 0.996 mg/mL for chondroitin sulfate eye drops by the proposed method. These results were close to 6250 U/mL and 1.0 mg/mL labeled in the instruction of commercial medicinal solutions. The recoveries of the added Hep or CS were found to be in the range of 97–108% and 100–106%, respectively. And the relative standard deviations (RSD) were less than 5.0%. The results summarized in Table 1 indicated the potential applicability of this method for the detection of Hep or CS in complicated real samples.

Interference study For further evaluating the selectivity of the present fluorescence method, we investigated the fluorescence response of the sensing system to sugars and other interfering substances included bovine serum albumin (BSA), human serum albumin (HSA), trypsin, pepsin, lysozyme (Lys), papain, adenosine triphosphate (ATP), adenosine diphosphate (ADP), NaCl, NaAc, Na2HPO4, Na2CO3, glucosamine (Gl), glucose (Glu), fructose (Fru), galactose (Gal), lactose (Lac) and sucrose (Sur). The concentration of coexisting substances was 10-folds higher than Hep or CS (2.0 μg/mL). The results were shown in Fig. S4. It could be seen that Hep or CS caused the significant fluorescence recovery, while less fluorescence intensity changes were observed in the presence of other interfering substances. These results also suggested that the proposed fluorescence turn-on method exhibited excellent selectivity for highly negatively polyanions. However, the application in more complicated samples (various biological fluids) could limit the usefulness of the proposed method, because the components of these samples could potentially prevent this electrostatic binding and absorb exciting or emitted light. Therefore, it will cause the necessity in some pre-treatment of the samples or in a special selection of the wavelengths used. Detection of hep or CS in commercial medicinal solutions In order to demonstrate the practical application of this fluorescence turn-on assay for heparin and CS detection in complex samples, the developed method had been applied to determine Hep or CS in commercial medicinal solutions. The results obtained

Conclusion In summary, we have developed a fluorescence turn-on method for highly negatively charged Hep or CS detection based on GQDs and multi-positively charged dendrimers. Under the optimized conditions, good linear relationships and detection limits for Hep or CS were obtained, respectively. The proposed method was applied to the determination of Hep or CS in commercial medicinal solutions with satisfactory results. In comparison with the previous methods for Hep or CS detection, the present method has some advantages: (1) the GQDs with excitation of 480 nm can be applied to complex biological samples which have a highly variable fluorescence background under shortwave excitation; (2) the tight binding between the multi-positively charged PAMAM-Gu þ and negatively charged polysaccharide imparts the turn-on method high sensitivity and selectivity; (3) this method offers a rapid and convenient approach for the detection of these polyanions.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21075050 and No. 21275063), the Science and Technology Development project of Jilin province, China (No. 20150204010GX) and the Open Founds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201401).

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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.bios.2015.06.032.

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Multi-positively charged dendrimeric nanoparticles induced fluorescence quenching of graphene quantum dots for heparin and chondroitin sulfate detection.

A label-free fluorescence assay for rapid and sensitive detection of heparin (Hep) or chondroitin sulfate (CS) was developed by guanidine-terminated p...
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