Interaction of 3-animo phenylboronic acid with ZnS:Cu quantum dots and glucose

Transcription

1 Interaction of 3-animo phenylboronic acid with ZnS:Cu quantum dots and glucose Katarzyna KUR-KOWALSKA, Małgorzata PRZYBYT, Ewa MILLER Keywords: 3-amino phenylboronic acid; quantum dots; fluorescence quenching Abstract: This paper shows the preliminary results of study of interaction between 3-amino phenylboronic acid and glucose or ZnS:Cu quantum dots. QDs were obtained by a new procedure, using 49:1 ratio ZnSO 4 to CuSO 4 and mercaptopropionic acid as the capping agent. These QDs have interesting fluorescence properties and high fluorescence intensity. 3-amino phenylboronic acid has a stable fluorescence intensity at ph 5.5 to 7.5 and forms a reversible ester with glucose. The fluorescence intensity of 3-amino phenylboronic acid is quenched by glucose and by QDs. The Stern-Volmer constants were calculated. The obtained value of Stern-Volmer constant at ph 7 was ± 0.94 L/mol for quenching by glucose and ± 0.57 ml/µg for quenching by QDs. 1. Introduction Because of the great need to find simple and rapid methods for measuring glucose, there are being developed methods for measuring glucose using quantum dots. They are good fluorescent probes because of their properties: photostability, broad absorption spectrum and narrow fluorescence spectrum (Wu et al., 2010). To create a glucose sensor, glucose oxidase (GOx) is often used. This enzyme selectively oxidizes glucose at the presence of oxygen (Egawa et al., 2011). However, this reaction is irreversible, and the use of the enzyme increases the cost significantly. Alternative sensors are non-enzymatic (Sun et al., 2004; Wang et al., 2012) ones acting on the principle of reverse reaction, keeping high stability and sensitivity. For this purpose, boronic acid derivatives which show good binding properties by creating an ester form with sugars are used. The elaboration of two-component glucose sensor using phenylboronic acid (PBA) and quantum dots (QDs) is the goal of many research teams (Cordes et al., 2006; Freeman et al., 2009). This work presents a preliminary study using 3-amino phenylboronic (APBA) acid and ZnS QDs doped with Cu to check their potential to obtain such a sensor.

2 66 PhD Interdisciplinary Journal 2. Experimental section 2.1. Materials and methods Sodium sulfide (Na 2 S 9H 2 O p.a.) and copper sulfate (CuSO 4 5H 2 O p.a.) were purchased from POCH country-region S.A. (Poland). Zinc sulfate (ZnSO 4 7H 2 O, 99.0%) and 3-amino phenylboronic acid (H 2 NC 6 H 4 B(OH) 2 H 2 O, 98%)were purchased from SIGMA-ALDRICH (Germany). 3-Mercaptopropionic acid(mpa, 99.0%) was purchased from Fluka (Germany). Glucose (anhydrous pure p.a.) was purchased from Chempur (Poland). Absorbance spectra were collected using spectrophotometer Nicolet Evolution 300 (Thermo Spectronic). Fluorescence spectra were measured using spectrofuorometer FluoroMax4 (JobinYvon). All experiments were performed at room temperature. Distillate water was used throughout Synthesis of ZnS:Cu quantum dots ZnS:Cu quantum dots were prepared according to (Chen et al., 2012) with slight modifications. 4.9 ml of ID0.1 M0.1 M ZnSO 4, 0.1 ml of ID0.1 M0.1 M CuSO 4 and 0.17 ml of MPA were mixed together, added with water to obtain final volume of 50 ml and adjusted to ph, This mixture was heated for 30 minutes at 95 C. Then the mixture was cooled to room temperature and QDs were precipitated with 75 ml of ethanol. QDs were harvested by centrifugation, washed with ethanol and dried overnight at 40 C. Stock solution was made by dissolving 10 mg of QDS in 2 ml of ID0.01 M0.01 M phosphate buffer, ph7. 3. Results and discussion 3.1. Properties of QDs Absorbance and fluorescence spectra of QDs are shown in Fig. 1A. The radius of obtained QDs was evaluated as 1.7 nm from the absorbance spectrum (Khani et al., 2011). The absorbance spectrum shows a shoulder at 297 nm and fluorescence was excited at this wavelength. The emission fluorescence spectrum shows a narrow band with the maximum at 460 nm Characteristics of 3-amino phenylboronic acid 3-amino phenylboronic acid is a strongly fluorescent compound; therefore, its 10 6 M solution was used in further experiments. The absorbance and fluorescence spectra of APBA at ph 7 are shown on Fig. 1B. The emission fluorescence spectrum (excited at 299 nm) shows a narrow band with the maximum at 460 nm. Its properties were checked at ph range from 5 to 10. ph has no influence on the emission maximum but only on intensity. Fig. 2 shows the dependence of fluorescence intensity on ph. Up to ph 7.5 fluorescence intensity is practically independent on ph and decreased rapidly for higher values. APBA is Lewis acid and at neutral ph it forms the neutral trigonal form, with the sp 2 -hybridization boron atom. In an alkaline environment it is negatively charged with a higher sp 3 electron state, which has a much lower fluorescence (Egawa et al., 2011).

3 Quenching of 3-a PBA fluorescence by glucose Due to its structure (two hydroxyl groups) PBA binds diols by the covalent bond and therefore it has good properties to work with sugars, such as glucose, galactose and fructose (Cannizzo et al., 2005). There are studies demonstrating the fluorescence quenching of boronic acid derivatives in the presence of glucose (Fang et al., 2004). Fig. 4 shows the dependence of APBA fluorescence intensity on increasing glucose concentration at ph7. As it can be observed, a significant decrease of APBA fluorescence intensity with the increasing concentration of glucose occurred indicating quenching. From these data the Stern-Volmer constant of APBA fluorescence quenching by glucose was calculated. Fluorescence quenching is described by the Stern-Volmer equation: I 0 I = 1 + K SV [Q] (1) Fig. 1. A)A- absorbance and F- fluorescence spectra of QDs at ph 7, B)A- absorbance and F- fluorescence of APBA at ph 7, 10-6M Fig. 2.. Structure of 3-aminophenylboronic acid

4 68 PhD Interdisciplinary Journal 1. I 0 - initial fluorescence intensity in the absence of a quencher 2. I - fluorescence intensity in the presence of a quencher 3. K sv -static quenching constant 4. [Q] -quencher concentration The obtained value of the Stern-Volmer constant was ± 0.94 L/mol at ph 7 (inset on Fig. 4). The obtained value is reasonable as compared with the values of glucose AMPA binding values obtained by Torun and co-workers (Torun et al., 2009). Boronic acid binds compounds containing diols moieties with high affinity through reversible ester formation (Yan et al., 2004), which show weaker fluorescence intensity. Fig. 3. Dependence of fluorescence intensity APBA as a function of ph Fig. 4. Quenching of AMPA fluorescence intensity by glucose at ph7, λ exc =299 nm; concentration of glucose is varying from 0 to mol/l by 0.001

5 Quenching of APBA fluorescence by ZnS:Cu QDs To check if AMPA interacts with ZnS:Cu QDs, 3 ml of a 10 6 M solution of APBA at ph7 was added with a 3 µl of QDs stock solution. After addition of each portion, fluorescence of the solution was checked. An increase of the fluorescence intensity of QDs and a decrease of the fluorescence intensity of APBA has been observed (Fig. 5) indicating quenching of APBA fluorescence by QDs.3-amino phenylboronic acid has an amino group, which can interact with the carboxyl groups present on the surface of QDs covered with MPA. As the result of such interactions, quenching of APBA fluorescence at the presence of QDs can be observed. The Stern-Volmer constant was evaluated as ± 0.57 ml/µg. This phenomenon is probably characterized by a static quenching which occurs as a result of the formation of a nonfluorescent ground-state complex (Lakowicz, 2006) between APBA and QDs. 4. Conclusion A new method of preparation of ZnS:Cu quantum dots is presented in this work. They are less toxic than the widely studied cadmium dots, which have a carcinogenic potential. ZnS:Cu QD shave a high fluorescence intensity and similar λ exc to 3-amino phenylboronic acid. Preliminary studies of APBA and QDs properties showed that APBA fluorescence is quenched by both glucoseand QDs at ph7. Due to this fact, 3-amino phenylboronic acid which forms a reversible ester with glucose is potentially a good component of the glucose sensor. Interactions between APBA, QDS and glucose could be very interesting, therefore research in this direction will be continued. Another objective is the synthesis of other non-toxic quantum dots with good fluorescence properties. Fig. 5. Fluorescence emission spectra of APBA as a function of increasing QDs concentration, λ exc = 298 nm, ph=7

6 70 PhD Interdisciplinary Journal References Badugu, R., J. R. Lakowicz and Ch. D. Geddes (2004, 522), Fluorescence intensity and lifetime-based cyanide sensitive probes for physiological safegruard, Analityca Chimica Acta pp Cannizzo, C., S. Amigoni-Gerbier and Ch. Larpent (2005), Boronic acidfunctionalized nanoparticles: synthesis by microemulsion polymerization and application as a re-usable optical nanosensor for carbohydrates, Polymer pp. 46, Chen, Y., L. Huang, S. Li and D. Pan (2012), Aqueous synthesis of glutathionecapped cu + and ag + -doped znxcd1-xs quantum dots with full color emission, Journal of Materials Chemistry C. Cordes, D. A., S. Gamsey, Z. Sherrett, A. Miller, P. Thoniyot, R. A. Wessling and B. Singaram (2005), The interaction of boronic-substituted viologens with pyranine: The effects of quencher charge on fluorescence quenching and glucose response, Langmuir pp. 21, Cordes, D. B., S. Gamsey and B. Singaram (2006), Fluorescent quantum dots with boronic acid substituted viologens to sense glucose in aqueous solution, Angewandte Chemie International Edition pp. 45, Egawa, Y., T. Seki, S. Takahashi and J. Anzai (2011), Electrochemical and optical sugar sensors based on phenylboronic acid and its derivatives, Materials Science and Engineering pp. 31, Fang, H., G. Kaur and B. Wang (2004), Progress in boronic acid-based fluorescent glucose sensor, Journal of Fluorescence pp. 14, Freeman, R., B. Bahshi, T. Finder, R. Gill and T. Willner (2009), Competitive analysis of saccharides or dopamine by boronic acid-functionalized cdse-zns quantum dots, Chemical Communications pp Khani, O., H. R. Rajabi, M. H. Yousefi, A. A. Khosravi, M. Jannesari and M. Shamsipur (2011), Synthesis and characterizations of ultra-small zns and zn(1-x)fexs quantum dots in aqueous media and spectroscopic study of their interactions with bovine serum albumin, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy pp. 79, Lakowicz, J. R. (2006), Principles of Fluorescence Spectroscopy, Springer. Springsteen, G. and B. Wang (2002), A detailed examination of boronic acid-diol complexation, Tetrahedron pp. 58, Sun, X.-Y., B. Liu and Y.-B. Jiang (2004), An extremely sensitive monoboronic acid based fluorescent sensor for glucose, Analytica Chimica Acta pp. 515, Torun, O., F. Dudak, D. Bas, U. Tamer and I. Boyaci (2009), Thermodynamic analysis of the interaction between 3-amino phenylboronic acid and monosaccharides for development of biosensor, Sensors and Actuators B: Chemical pp. 140, Wang, Z., H. Lei, Ch. Zhou, F. Liang and L. Feng (2012), Optical probe for d-glucose based on cationic polymer quencher/receptor and onionic dye in aqueous solution, Sensors and Actuators B pp. 163, Wu, W., Z. Zhou, A. Berliner, P. Banerjee and S. Zhou (2010), Glucose-mediated assembly of phenylboronic acid modified cdte/znte/zns quantum dots for intracellular glucose probing, Angewandte Chemie International Edition pp. 49, Yan, J., G. Springsteen, S. Deeter and B. Wang (2004), The relationship among pka, ph, and binding constants in the interactions between boronic acid and diols-it is not as simple as it appears, Tetrahedron pp