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Hyperbranched polyether hybrid nanospheres with CdSe quantum dots incorporated for selective detection of nitric oxide
Materials Letters 123 (2014) 104–106
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hyperbranched polyether hybrid nanospheres with CdSe quantum dots incorporated for selective detection of nitric oxide Shuiping Liu a,b, Lanming Jin a,b, Ioannis S. Chronakis c,n, Xiaoqiang Li a,b,c,nn, Mingqiao Ge a,b a
Key Laboratory of Eco-Textiles, Ministry of Education (Jiangnan University), Wuxi 214122, China College of Textile & Clothing, Jiangnan University, Wuxi 214122, China c Technical University of Denmark, DTU Food, Søltofts plads, B227, 2800 Kgs. Lyngby, Denmark b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 January 2014 Accepted 21 February 2014 Available online 1 March 2014
In this work, hybrid nanosphere vehicles consisting of cadmium selenide quantum dots (CdSe QDs) were synthesized for nitric oxide (NO) donating and real-time detecting. The nanospheres with QDs being encapsulation have spherical outline with dimension of 127 nm. The fluorescence properties of the mHP conjugated QDs are sensitivity and high selectivity for NO against oxidation products from NO. The QDs-mHP-NO nanospheres provide perspectives for designing a new class of biocompatible NO donating and imaging systems. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles NO detection Quantum dots Functional Polymers
1. Introduction Nitric oxide (NO) is a free radical molecule generated in various tissues from the decomposition of amino acid L-arginine by different forms of nitric oxide synthase [1]. It is always involved in the regulation of the cardiovascular system, the central and peripheral nervous systems, and the immune system [2,3]. Diazeniumdiolates represent a class of compounds containing the anionic [N(O)NO]- functional group, typically synthesized by reactions of a nucleophile with NO at elevated pressure [4,5]. Diazeniumdiolates will decompose spontaneously to generate NO under physiological conditions (37 1C, pH ¼ 7.4). Semiconductor quantum dots (QDs) have unique properties and advantages over conventional organic fluorophores. The fluorescence efficiency of QDs is sensitive to the presence and nature of adsorbates at the QDs surface and any changes in the surface state may significantly influence the photoluminescence characteristics of the QDs [6]. The existence of lone pair electrons makes NO active in interacting with QDs; therefore, the fluorescence changes of the QDs caused by the interaction between them can be used for NO detection. Tan et al. have reported
QDs-carboxymethyl chitosan (CMCS) nanocomposite NO donors capable of releasing NO and simultaneously detecting the NO release [7]. The NO storage of this NO donating system, however, is relatively low due to limited secondary amine groups in the carboxymethyl chitosan. Besides, the selectivity of the polymer encapsulated QDs for NO was not investigated in detail. In another paper, Tan et al. designed chitosan-based diazeniumdiolates with an organic fluorescent molecule as the probe for NO release [8]. Although the organic fluorescent molecule was sensitive to NO with high selectivity, the reaction between the molecule and NO was so rapid that the detection capacity tended to be saturated before the NO release finished, and it is difficult to find a linear relationship between the fluorescence increase and the released amount of NO. In the present work, we synthesized a NO delivery vehicle consisting of modified hyperbranched polyether (mHP) nanospheres with fluorescent cadmium selenide (CdSe) QDs encapsulation. Cd-based QDs were chosen because NO can effectively interact with Cd and give rise to significant fluorescence quenching. The characterization of QDs-mHP nanospheres, and the fluorescence properties of the QDs-mHP-NO were well performed. 2. Experimental
n
Corresponding author. Tel.: þ 45 40206413. Corresponding author at: Technical University of Denmark, DTU FOOD, Lyngby, Denmark. E-mail addresses: [email protected] (I.S. Chronakis), [email protected] (X.Q. Li). nn
http://dx.doi.org/10.1016/j.matlet.2014.02.078 0167-577X & 2014 Elsevier B.V. All rights reserved.
Materials: Boron trifluoride etherate (BF3OEt2,) and 3-ethyl3-oxetanemethanol (EOX) were provided by Sigma-Aldrich. N-(3(Trimethoxysilyl)propyl)ethylenediamine (NTPED) and Ethylenediaminetetraacetic acid (EDTA) were provided by Aladdin Reagent,
S. Liu et al. / Materials Letters 123 (2014) 104–106
105
Scheme 1. NO release and detection mechanism of QDs-mHP-NO nanospheres.
Ltd. Ethylene oxide, mercaptoethylamine, ethylene dichloride, cadmium acetate, selenium ( Z99.9%), sodium borohydride, ethyl bromide, and all organic solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. Nitric oxide gas (NO, 99.9%) was purchased from Foshan Kodi Gas Chemical Industry, China. NO solutions of varied concentrations used for investigating the sensitivity of the QDs-mHP-NO nanospheres to NO were prepared by making a series of dilutions of a saturated NO solution (E 1.8 mM). All the chemicals were analytical grade unless otherwise stated. 2.2 Synthesis of modified hyperbranched polyether (mHP) nanospheres. 2.3 Preparation of mHP-NO nanospheres. 2.4 Synthesis of CdSe QDs. 2.5 Preparation of QDs-mHP and QDs-mHP-NO nanospheres. Details of 2.2.–2.5. are presented in Supplementary Materials. Characterization: Transmission electron micrographs (TEM) were recorded on a JEM-2100 transmission electron microscope (JEOL, Japan) at 200 kV. The weight-average molecular weight of HP was measured on a HLC-8320GPC gel permeation chromatography (TOSOH, Japan) using DMF as the eluate. Nuclear magnetic resonance (NMR) tests were carried out on a Varian Mercury Plus400 spectrometer at 400 MHz. Measurement of NO release: Quantitative detection of the NO molecules released from the QDs-mHP-NO nanospheres was carried out by a TBR 4100/1025 free radical analyzer equipped with an ISO-NOP sensor (WPI Ltd., USA). The sensor was calibrated by the addition of 50 μM KNO2 in a mixture of 0.33 g KI and 20 mL of 0.1 M H2SO4. The NO release was measured at 37 1C, where the detection sensitivity was determined to be 2.55 pA/nM. The details are as follows: 0.1 mg of nanospheres was dispersed in 1 mL of 0.2 M PBS buffer (pH ¼7.4), forming a stable suspension. Then the suspension was rapidly injected into 19 mL of the PBS buffer when the ISO-NOP sensor had reached a low, stable current level. The NO probe was immersed about 2 cm into the suspension under magnetic stirring.
3. Results and discussion Scheme 1 depicts the NO release and detection mechanism of the QDs-mHP-NO nanospheres. The hyperbranched polyethers were synthesized by cation ring-opening polymerization. Then the HP was modified by reacting with NTPED in order to obtain secondary amino groups. The modified HP, i.e. mHP was then reacted with NO under pressure in a sodium methoxide methonal solution to generate mHP-NO nanospheres containing diazeniumdiolate functional groups. The CdSe QDs stabilized by mercaptoethylamine were conjugated with the mHP-NO with the aid of EDTA, and the QDsmHP-NO nanospheres with the QDs encapsulated were obtained. In Scheme 1(b), the QDs-mHP-NO nanospheres release NO molecules
Fig. 1. 13C NMR spectra of HP samples polymerized at different temperatures demonstrating the peaks corresponding to the D, L and T units (D, L and T represent the integrations of signals of the dendritic unit, linear unit and terminal unit, respectively).
once they are placed in a PBS buffer where the temperature and pH value are similar to those of physiological environment. Upon release of NO, the fluorescence emitted by the fluorescent QDs-mHP-NO nanospheres greatly decreases, as some of the NO molecules diffuse to coordinate with Cd sites of the encapsulated CdSe QDs near the nanospheres surface, taking advantage of their lone pair electrons. The fluorescence quenching caused by NO can be used to signal the release of NO, based on which detection of NO is achieved. As the precursor and supporting material of the QDs-mHP-NO nanospheres, the HP exerts great influences on the overall properties of the system. It is found that the polymerization temperature is a key factor affecting the feature of the resultant polymer. Fig. 1 shows the region near 43.5 ppm in 13C NMR spectra of HP samples polymerized at different temperatures. The signals are ascribed to quaternary carbon atoms of the dendritic, linear and terminal units. The TEM micrographs of QDs-mHP-NO nanospheres with different magnifications are shown in Fig. 2. The spherical outline suggests the formation of QDs-mHP-NO nanospheres, which had narrow size distribution (Fig. 2a). Hardly any free standing QDs outside the nanospheres are observed, attributed to the large amount of hydroxyl groups that capture the QDs. The in situ synthesized CdSe QDs encapsulated in the mHP have the dimension from 4 to 10 nm (Fig. 2b). It is imperative that the component of fluorescent probes of the QDs-mHP-NO nanosphere system are selective for NO but not for other substances. To examine the selectivity of the QDs-mHP for NO, several relevant substances that may coexist with the NO in
106
S. Liu et al. / Materials Letters 123 (2014) 104–106
Fig. 2. TEM micrographs of QDs-mHP-NO nanospheres.
the release of NO in real time were fabricated. The nanospheres consist of CdSe QDs as fluorescence probes for NO, diazeniumdiolate moiety as NO donors and mHP as the supporting material. The dose of released NO is quantitatively detected through observable and readily measurable fluorescence changes, with high sensitivity and satisfactory selectivity. This work provides a new NO release and detection system that can be prepared with facile methods and have potentials in biomedical applications.
Acknowledgment
Fig. 3. Effect of increasing concentrations of NO, NO2 and NO3 on the fluorescence signal from the QDs-mHP nanospheres.
physiological environment were introduced to buffered solutions of the QDs-mHP nanospheres, and the fluorescence decrease in each solution over 1 h was recorded (the results presented in Supplementary Materials). Compared with that caused by NO, the fluorescence quenching by its oxidation products NO2 and NO3 as well as the reactive oxygen and nitrogen species (ROS/RNS) including H2O2, ClO and peroxynitrite (ONOO ) was tiny. Although the selectivity for NO was a little inferior to that of the organic probe in Tan et al.'s work [8], these substances would still interfere with the NO detection of the QDs-mHP nanospheres. The quenching of the fluorescence from the QDs-mHP nanospheres by NO at increasing concentration was traced. The influences of two anions (NO2 and NO3 ) on the fluorescence emission from the QDs-mHP-NO nanospheres were also investigated. The results in Fig. 3 show that the extent of quenching increased with increasing NO concentration, gradually reaching a plateau at higher NO concentrations. An approximately linear relationship was observed in the NO concentration range of 35–700 nM, indicating that the quenching effect of NO on the fluorescence emission of the QDsmHP nanospheres can be used for the detection of NO. The two substances did not exert any significant effect on the fluorescence emission of the QDs-mHP nanospheres even at relatively high concentration levels. The limit of detection, calculated following the 3s IUPAC criteria, was 25 nM (0.041 μmol/mg) of NO. It is also noted that the sensitivity of the QDs-mHP nanospheres to NO detection was unaltered after more than 30 days' storage of the nanospheres in the dark, under ambient conditions. 4. Conclusion In this study, hyperbranched polyether-based hybrid nanosphere vehicles with the capability of releasing NO and detecting
We thank Dr. Lianjiang Tan for the Characterization tests of QDsmHP. This study was funded by the Danish Strategic Research Council (DSF -10-93456, FENAMI Project), the Fundamental Research Funds for the Central Universities (JUSRP1041), the Open Project Program of Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University (No. KLET1209), the Jiangsu Provincial Natural Science Foundation of China (NO.BK20130146) and National High-tech R&D Program of China (863 Program, 2012AA030313).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.02.078.
References [1] Butler AR, Williams DLH. The physiological roles of nitric oxide. Chem Soc Rev 1993;22:233–41. [2] Bogdan C. Nitric oxide and the immune response. Nat Immunol 2001;2:907–16. [3] Garthwaite J. Concepts of neural nitric oxide-mediated transmission. Eur J Neurosci 2008;27:2783–802. [4] Saavedra JE, Keefer LK. Nitrogen-based diazeniumdiolates: versatile nitric oxide- releasing compounds in biomedical research and potential clinical applications. J Chem Educ 2002;79:1427–34. [5] Wan A, Gao Q, Li H. Effects of molecular weight and degree of acetylation on the release of nitric oxide from chitosan-nitric oxide adducts. J Appl Polym Sci 2010;117:2183–8. [6] Tan LJ, Wan AJ, Li HL, Lu Q. Biocompatible quantum dots-chitosan nanocomposites for fluorescence detection of nitric oxide. Mater Chem Phys 2012;134:562–6. [7] Tan LJ, Wan AJ, Li HJ, Lu Q. Novel quantum dots-carboxymethyl chitosan nanocomposite nitric oxide donors capable of detecting release of nitric oxide in situ. Acta Biomater 2012;8:3744–53. [8] Tan LJ, Wan AJ, Li HL. Fluorescent chitosan complex nanosphere diazeniumdiolates as donors and sensitive real-time probes of nitric oxide. Analyst 2013;138:879–86.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hyperbranched polyether hybrid nanospheres with CdSe quantum dots incorporated for selective detection of nitric oxide Shuiping Liu a,b, Lanming Jin a,b, Ioannis S. Chronakis c,n, Xiaoqiang Li a,b,c,nn, Mingqiao Ge a,b a
Key Laboratory of Eco-Textiles, Ministry of Education (Jiangnan University), Wuxi 214122, China College of Textile & Clothing, Jiangnan University, Wuxi 214122, China c Technical University of Denmark, DTU Food, Søltofts plads, B227, 2800 Kgs. Lyngby, Denmark b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 January 2014 Accepted 21 February 2014 Available online 1 March 2014
In this work, hybrid nanosphere vehicles consisting of cadmium selenide quantum dots (CdSe QDs) were synthesized for nitric oxide (NO) donating and real-time detecting. The nanospheres with QDs being encapsulation have spherical outline with dimension of 127 nm. The fluorescence properties of the mHP conjugated QDs are sensitivity and high selectivity for NO against oxidation products from NO. The QDs-mHP-NO nanospheres provide perspectives for designing a new class of biocompatible NO donating and imaging systems. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles NO detection Quantum dots Functional Polymers
1. Introduction Nitric oxide (NO) is a free radical molecule generated in various tissues from the decomposition of amino acid L-arginine by different forms of nitric oxide synthase [1]. It is always involved in the regulation of the cardiovascular system, the central and peripheral nervous systems, and the immune system [2,3]. Diazeniumdiolates represent a class of compounds containing the anionic [N(O)NO]- functional group, typically synthesized by reactions of a nucleophile with NO at elevated pressure [4,5]. Diazeniumdiolates will decompose spontaneously to generate NO under physiological conditions (37 1C, pH ¼ 7.4). Semiconductor quantum dots (QDs) have unique properties and advantages over conventional organic fluorophores. The fluorescence efficiency of QDs is sensitive to the presence and nature of adsorbates at the QDs surface and any changes in the surface state may significantly influence the photoluminescence characteristics of the QDs [6]. The existence of lone pair electrons makes NO active in interacting with QDs; therefore, the fluorescence changes of the QDs caused by the interaction between them can be used for NO detection. Tan et al. have reported
QDs-carboxymethyl chitosan (CMCS) nanocomposite NO donors capable of releasing NO and simultaneously detecting the NO release [7]. The NO storage of this NO donating system, however, is relatively low due to limited secondary amine groups in the carboxymethyl chitosan. Besides, the selectivity of the polymer encapsulated QDs for NO was not investigated in detail. In another paper, Tan et al. designed chitosan-based diazeniumdiolates with an organic fluorescent molecule as the probe for NO release [8]. Although the organic fluorescent molecule was sensitive to NO with high selectivity, the reaction between the molecule and NO was so rapid that the detection capacity tended to be saturated before the NO release finished, and it is difficult to find a linear relationship between the fluorescence increase and the released amount of NO. In the present work, we synthesized a NO delivery vehicle consisting of modified hyperbranched polyether (mHP) nanospheres with fluorescent cadmium selenide (CdSe) QDs encapsulation. Cd-based QDs were chosen because NO can effectively interact with Cd and give rise to significant fluorescence quenching. The characterization of QDs-mHP nanospheres, and the fluorescence properties of the QDs-mHP-NO were well performed. 2. Experimental
n
Corresponding author. Tel.: þ 45 40206413. Corresponding author at: Technical University of Denmark, DTU FOOD, Lyngby, Denmark. E-mail addresses: [email protected] (I.S. Chronakis), [email protected] (X.Q. Li). nn
http://dx.doi.org/10.1016/j.matlet.2014.02.078 0167-577X & 2014 Elsevier B.V. All rights reserved.
Materials: Boron trifluoride etherate (BF3OEt2,) and 3-ethyl3-oxetanemethanol (EOX) were provided by Sigma-Aldrich. N-(3(Trimethoxysilyl)propyl)ethylenediamine (NTPED) and Ethylenediaminetetraacetic acid (EDTA) were provided by Aladdin Reagent,
S. Liu et al. / Materials Letters 123 (2014) 104–106
105
Scheme 1. NO release and detection mechanism of QDs-mHP-NO nanospheres.
Ltd. Ethylene oxide, mercaptoethylamine, ethylene dichloride, cadmium acetate, selenium ( Z99.9%), sodium borohydride, ethyl bromide, and all organic solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. Nitric oxide gas (NO, 99.9%) was purchased from Foshan Kodi Gas Chemical Industry, China. NO solutions of varied concentrations used for investigating the sensitivity of the QDs-mHP-NO nanospheres to NO were prepared by making a series of dilutions of a saturated NO solution (E 1.8 mM). All the chemicals were analytical grade unless otherwise stated. 2.2 Synthesis of modified hyperbranched polyether (mHP) nanospheres. 2.3 Preparation of mHP-NO nanospheres. 2.4 Synthesis of CdSe QDs. 2.5 Preparation of QDs-mHP and QDs-mHP-NO nanospheres. Details of 2.2.–2.5. are presented in Supplementary Materials. Characterization: Transmission electron micrographs (TEM) were recorded on a JEM-2100 transmission electron microscope (JEOL, Japan) at 200 kV. The weight-average molecular weight of HP was measured on a HLC-8320GPC gel permeation chromatography (TOSOH, Japan) using DMF as the eluate. Nuclear magnetic resonance (NMR) tests were carried out on a Varian Mercury Plus400 spectrometer at 400 MHz. Measurement of NO release: Quantitative detection of the NO molecules released from the QDs-mHP-NO nanospheres was carried out by a TBR 4100/1025 free radical analyzer equipped with an ISO-NOP sensor (WPI Ltd., USA). The sensor was calibrated by the addition of 50 μM KNO2 in a mixture of 0.33 g KI and 20 mL of 0.1 M H2SO4. The NO release was measured at 37 1C, where the detection sensitivity was determined to be 2.55 pA/nM. The details are as follows: 0.1 mg of nanospheres was dispersed in 1 mL of 0.2 M PBS buffer (pH ¼7.4), forming a stable suspension. Then the suspension was rapidly injected into 19 mL of the PBS buffer when the ISO-NOP sensor had reached a low, stable current level. The NO probe was immersed about 2 cm into the suspension under magnetic stirring.
3. Results and discussion Scheme 1 depicts the NO release and detection mechanism of the QDs-mHP-NO nanospheres. The hyperbranched polyethers were synthesized by cation ring-opening polymerization. Then the HP was modified by reacting with NTPED in order to obtain secondary amino groups. The modified HP, i.e. mHP was then reacted with NO under pressure in a sodium methoxide methonal solution to generate mHP-NO nanospheres containing diazeniumdiolate functional groups. The CdSe QDs stabilized by mercaptoethylamine were conjugated with the mHP-NO with the aid of EDTA, and the QDsmHP-NO nanospheres with the QDs encapsulated were obtained. In Scheme 1(b), the QDs-mHP-NO nanospheres release NO molecules
Fig. 1. 13C NMR spectra of HP samples polymerized at different temperatures demonstrating the peaks corresponding to the D, L and T units (D, L and T represent the integrations of signals of the dendritic unit, linear unit and terminal unit, respectively).
once they are placed in a PBS buffer where the temperature and pH value are similar to those of physiological environment. Upon release of NO, the fluorescence emitted by the fluorescent QDs-mHP-NO nanospheres greatly decreases, as some of the NO molecules diffuse to coordinate with Cd sites of the encapsulated CdSe QDs near the nanospheres surface, taking advantage of their lone pair electrons. The fluorescence quenching caused by NO can be used to signal the release of NO, based on which detection of NO is achieved. As the precursor and supporting material of the QDs-mHP-NO nanospheres, the HP exerts great influences on the overall properties of the system. It is found that the polymerization temperature is a key factor affecting the feature of the resultant polymer. Fig. 1 shows the region near 43.5 ppm in 13C NMR spectra of HP samples polymerized at different temperatures. The signals are ascribed to quaternary carbon atoms of the dendritic, linear and terminal units. The TEM micrographs of QDs-mHP-NO nanospheres with different magnifications are shown in Fig. 2. The spherical outline suggests the formation of QDs-mHP-NO nanospheres, which had narrow size distribution (Fig. 2a). Hardly any free standing QDs outside the nanospheres are observed, attributed to the large amount of hydroxyl groups that capture the QDs. The in situ synthesized CdSe QDs encapsulated in the mHP have the dimension from 4 to 10 nm (Fig. 2b). It is imperative that the component of fluorescent probes of the QDs-mHP-NO nanosphere system are selective for NO but not for other substances. To examine the selectivity of the QDs-mHP for NO, several relevant substances that may coexist with the NO in
106
S. Liu et al. / Materials Letters 123 (2014) 104–106
Fig. 2. TEM micrographs of QDs-mHP-NO nanospheres.
the release of NO in real time were fabricated. The nanospheres consist of CdSe QDs as fluorescence probes for NO, diazeniumdiolate moiety as NO donors and mHP as the supporting material. The dose of released NO is quantitatively detected through observable and readily measurable fluorescence changes, with high sensitivity and satisfactory selectivity. This work provides a new NO release and detection system that can be prepared with facile methods and have potentials in biomedical applications.
Acknowledgment
Fig. 3. Effect of increasing concentrations of NO, NO2 and NO3 on the fluorescence signal from the QDs-mHP nanospheres.
physiological environment were introduced to buffered solutions of the QDs-mHP nanospheres, and the fluorescence decrease in each solution over 1 h was recorded (the results presented in Supplementary Materials). Compared with that caused by NO, the fluorescence quenching by its oxidation products NO2 and NO3 as well as the reactive oxygen and nitrogen species (ROS/RNS) including H2O2, ClO and peroxynitrite (ONOO ) was tiny. Although the selectivity for NO was a little inferior to that of the organic probe in Tan et al.'s work [8], these substances would still interfere with the NO detection of the QDs-mHP nanospheres. The quenching of the fluorescence from the QDs-mHP nanospheres by NO at increasing concentration was traced. The influences of two anions (NO2 and NO3 ) on the fluorescence emission from the QDs-mHP-NO nanospheres were also investigated. The results in Fig. 3 show that the extent of quenching increased with increasing NO concentration, gradually reaching a plateau at higher NO concentrations. An approximately linear relationship was observed in the NO concentration range of 35–700 nM, indicating that the quenching effect of NO on the fluorescence emission of the QDsmHP nanospheres can be used for the detection of NO. The two substances did not exert any significant effect on the fluorescence emission of the QDs-mHP nanospheres even at relatively high concentration levels. The limit of detection, calculated following the 3s IUPAC criteria, was 25 nM (0.041 μmol/mg) of NO. It is also noted that the sensitivity of the QDs-mHP nanospheres to NO detection was unaltered after more than 30 days' storage of the nanospheres in the dark, under ambient conditions. 4. Conclusion In this study, hyperbranched polyether-based hybrid nanosphere vehicles with the capability of releasing NO and detecting
We thank Dr. Lianjiang Tan for the Characterization tests of QDsmHP. This study was funded by the Danish Strategic Research Council (DSF -10-93456, FENAMI Project), the Fundamental Research Funds for the Central Universities (JUSRP1041), the Open Project Program of Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University (No. KLET1209), the Jiangsu Provincial Natural Science Foundation of China (NO.BK20130146) and National High-tech R&D Program of China (863 Program, 2012AA030313).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.02.078.
References [1] Butler AR, Williams DLH. The physiological roles of nitric oxide. Chem Soc Rev 1993;22:233–41. [2] Bogdan C. Nitric oxide and the immune response. Nat Immunol 2001;2:907–16. [3] Garthwaite J. Concepts of neural nitric oxide-mediated transmission. Eur J Neurosci 2008;27:2783–802. [4] Saavedra JE, Keefer LK. Nitrogen-based diazeniumdiolates: versatile nitric oxide- releasing compounds in biomedical research and potential clinical applications. J Chem Educ 2002;79:1427–34. [5] Wan A, Gao Q, Li H. Effects of molecular weight and degree of acetylation on the release of nitric oxide from chitosan-nitric oxide adducts. J Appl Polym Sci 2010;117:2183–8. [6] Tan LJ, Wan AJ, Li HL, Lu Q. Biocompatible quantum dots-chitosan nanocomposites for fluorescence detection of nitric oxide. Mater Chem Phys 2012;134:562–6. [7] Tan LJ, Wan AJ, Li HJ, Lu Q. Novel quantum dots-carboxymethyl chitosan nanocomposite nitric oxide donors capable of detecting release of nitric oxide in situ. Acta Biomater 2012;8:3744–53. [8] Tan LJ, Wan AJ, Li HL. Fluorescent chitosan complex nanosphere diazeniumdiolates as donors and sensitive real-time probes of nitric oxide. Analyst 2013;138:879–86.