Talanta 138 (2015) 203–208
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Silver nanoparticles-enhanced rare earth co-luminescence effect of Tb (III)–Y(III)–dopamine system Huihui Li, Xia Wu n Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2014 Received in revised form 6 February 2015 Accepted 11 February 2015 Available online 20 February 2015
It was found that silver nanoparticles (AgNPs) could enhance co-luminescence effect of rare earths ions Tb3 þ and Y3 þ . Based on this, a sensitive fluorescence detection method for the determination of dopamine (DA) was proposed. Moreover, the detection limit for DA was very low (down to nM). This is because DA can remarkably enhance the luminescence intensity of the Tb3 þ ion by Y3 þ in the colloidal solution of AgNPs, forming a new co-luminescence system. Furthermore, based on the metal enhanced fluorescence (MEF), AgNPs can sensitize the co-luminescence effect of the complex of Tb3 þ –Y3 þ –DA. In a neutral buffer solution (pH 7.50), the luminescence intensity of the system was linearly related to the concentration of DA in the range of 2.0–100 nM, with a limit of detection as low as 0.57 nM. The proposed method was applied for the determination of DA in dopamine hydrochloride injections and human serum samples with good accuracy and satisfactory recovery. & 2015 Elsevier B.V. All rights reserved.
Keywords: Metal enhanced fluorescence Co-luminescence Tb3 þ Y3 þ Silver nanoparticles
1. Introduction Lanthanide ions (Tb3 þ , Eu3 þ , etc.) as a luminescence probe have interested chemists and biochemists for decades because of their intrinsic characteristics such as narrow emission band widths, large Stocks shift, long luminescence lifetimes and strong binding with biological molecules [1–3]. However, other rare earth ions (La3 þ , Lu3 þ , Tb3 þ , Y3 þ or Gd3 þ etc.) co-existed can unexpectedly enhance their luminescence intensity, which is named as rare earth ions co-luminescence effect (RE-columin) [4]. The sensitivity of a co-luminescence-based analytical method can be significantly improved, compared to that of a single rare earth ion luminescence method [4,5], and the vast majority of methods require that surfactants must exist in the system [6–8]. Our previous work showed that silver island film could sensitize Tb3 þ –Gd3 þ –protein co-luminescence and sensitively detect protein [8], which was conducted in the solid matrix and needed sodium dodecyl benzene sulfonate (SDBS) to coexist. Until now, the analytical method based on RE-columin sensitized by metallic nanoparticles alone without surfactant in aqueous solution has not been reported yet. Silver nanoparticles (AgNPs) have unique optoelectronic properties, such as localized surface plasmon resonance (LSPR), surface-enhanced Raman and surface-enhanced fluorescence [9–11]. n
Corresponding author. Tel.: þ 86 531 88365459; fax: þ86 531 88564464. E-mail address: [email protected] (X. Wu).
http://dx.doi.org/10.1016/j.talanta.2015.02.023 0039-9140/& 2015 Elsevier B.V. All rights reserved.
These optoelectronic properties have been widely used in bioanalytical researches [11–13]. Metal enhanced fluorescence (MEF) [11–14] occurs due to the couple interaction between fluorophores and the plasmon resonance of metal nanoparticles, resulting in an increase in both the radiative decay rate and fluorescence emission. Studies showed that MEF property can be affected by nanoparticle size, particle shape, spacing distance of fluorophore-tometal, and wavelength [11,14,15]. Dopamine (DA), 4-(2-aminoethyl)-benzen-1, 2-diol, a natural substance, is one of the crucial catecholamines. As a neurotransmitter in the brain, DA plays a critical role in the feelings of addiction and excitement, memory, cognition, and motor control [16–18]. An abnormal level of DA in the human body can cause health issues, such as attention deficit hyperactivity disorder (ADHD), anorexia, depression, Parkinson’s disease etc. [19,20]. Therefore, the determination of DA concentration at trace levels in biological fluids such as urine, serum and pharmaceutical formulation is of great importance. Current analytical methods for the determination of DA mainly use electrochemistry [21–25], highperformance liquid chromatography (HPLC) [26,27], capillary electrophoresis (CE) [28,29], chemiluminescence (CL) [30], UV–visible absorption and fluorescence spectrophotometries [31–37]. The methods of HPLC and CE usually need derivatization separation process and is time-consuming. Electrochemical methods have always been a powerful tool for determination of DA. To improve the sensitivity determination of DA, the modified electrodes were adopted, which needed experienced operators.
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As is known, the spectrofluorimetric method for the determination of DA is simple and sensitive. Lately, Yan et al. [33] established a new grapheme oxide-based fluorescence method of selectively determining DA. The detection limit was 94 nM. Kim et al. [31] developed a fluorescence method for determining DA by using AgNPs-enhanced fluorescence of Tb3 þ in alkaline solution. In this work, a facile and sensitive method for the determination of DA was proposed based on Tb3 þ –Y3 þ –DA co-luminescence sensitized by AgNPs in neutral aqueous solution. The interaction mechanism of AgNPs enhancing co-luminescence of the Tb3 þ –Y3 þ –DA system was also discussed.
in a 1 cm quartz cell with the excitation and emission wavelengths of 306 nm and 546 nm, respectively. The excitation and emission slits were both 10 nm and the scan speed was 240 nm/min. The high voltage for the photomultiplier tube was set to 700 V. The enhanced extent of fluorescence intensity is defined as ΔIf ¼If I0, where If and I0 were the fluorescence intensity of the system with and without DA, respectively.
3. Results and discussion 3.1. Characterization of silver nanoparticles and nanocomposites
2. Experimental section 2.1. Chemicals and materials Dopamine hydrochloride was commercially purchased from the National Institutes for Food and Drug Control (Beijing, China) and used without further purification. The rare earth oxides (99.9%) were obtained from Yuelong Chemical Co., (Shanghai, China). Other reagents used in this study were of analytical grade. The stock solution of DA (1.0 10 2 M) was prepared using 0.1 M hydrochloric acid (1 mL) and deionized water. The stock standard solution (1.0 10 2 M) of rare earth ions was prepared by dissolving the corresponding oxides in hydrochloric acid and then diluting with deionized water. The prepared stock solution was stored in a refrigerator at 4 °C. A 0.05 M Tris–HCl buffer solution was prepared by dissolving 3.03 g of Tris in 500 mL deionized water and then was adjusted using 0.1 M HCl. 2.2. Synthesis of silver nanoparticles Silver nanoparticles (1.0 10 3 M calculated by the concentration of the silver ion added) were synthesized by a chemical reduction method as described in the literature with some modifications [38,39]. Briefly, AgNO3 (17 mg) was dissolved in 50 mL of ultrapure water (18.25 MΩ cm) and heated to 86 °C with vigorous stirring. 3 mL of 1% sodium citrate solution was added dropwise to the silver nitrate solution. Heating was continued for an additional 12 min until the color of the colloidal solution turned light yellow. Then the silver colloidal solution was diluted into 100 mL of a volumetric flask with ultrapure water and stored at 4 °C. The prepared AgNPs were stable for more than 2 months. 2.3. Apparatus Fluorescence spectra and intensity were recorded with a spectrofluorometer (F-7000, Hitachi, Japan) in a 1 cm quartz cuvette. All the absorption spectra were measured using a U-4100 spectrophotometer (Hitachi, Japan). Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-1011 transmission electron microscope operated at an accelerating voltage of 100 kV. Zeta potential was monitored using a Malvern Zetasizer Nano instrument (Zetasizer Nano ZS90). All pH measurements were made with a Delta 320-S acidity meter (Mettler Toledo, Shanghai). 2.4. Measurement procedures Solutions were added in the following order, into a 25 mL test tube: 1.0 mL of Tris–HCl buffer 7.50, 0.3 mL of 1.0 10 3 M Tb3 þ , 0.5 mL of 1.0 10 4 M AgNPs solution, 0.1 mL of 1.0 10 5 M Y3 þ , and DA solution with an appropriate volume. The mixture was diluted to 10 mL with water and was allowed to stand for 30 min. The luminescence spectra of this system were measured
The properties of prepared AgNPs were characterized by Zeta potential and TEM images (Fig. 1). The Zeta potential of AgNPs was 32.57 mV which indicated that high stability AgNPs were obtained. The morphologies of AgNPs and the nanocomposites of AgNPs–DA, Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA were identified by TEM. TEM images showed that AgNPs were spherical particles with average particle diameters approximately 15 73 nm (Fig. 1a). After adding DA to AgNPs, the sizes of their aggregations are about 88 72 nm (Fig. 1b). And the morphologies of the assemblages of Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA were both bead-like and the latter is relatively loose (Fig. 1c and d). 3.2. Fluorescence enhancement Fluorescence spectra of the Tb3 þ –AgNPs–Y3 þ –DA system are shown in Fig. 2. It revealed that with the addition of DA, the emission peak of Tb3 þ –DA (Fig. 2b curve 3 vs curve 1) was enhanced at the wavelengths of 490 nm and 546 nm, which were the intrinsic fluorescence peaks of Tb3 þ and corresponded to the transition of electrons from 5D4-7F6 and 5D4-7F5, respectively. Adding Y3 þ to Tb3 þ –DA, the fluorescence intensity of the system decreased (Fig. 2b curve 2). On the contrary, when the colloidal solution of AgNPs was added to the Tb3 þ –DA or Tb3 þ –Y3 þ –DA system, respectively, the fluorescence intensities of the systems were enhanced (Fig. 2b curves 4 and 5). The extent of the fluorescence enhancement of Tb3 þ –AgNPs–Y3 þ –DA was greater than that of Tb3 þ –AgNPs–DA. It indicated that in the presence of AgNPs, the effective RE-columin effects of the Tb3 þ –Y3 þ –DA system occurred. If there was no AgNPs in the solution, Y3 þ could form a complex with DA and thus compete with Tb3 þ , resulting in reduction in the fluorescence intensity of Tb3 þ . The peak positions of excitation and emission spectra were nearly unchanged. The most intense emission peak was located at 546 nm, and this peak was thus chosen for the luminescence intensity measurement throughout this work. UV–vis absorption experiments were conducted to understand the interaction of AgNPs with DA, Tb3 þ –DA and Tb3 þ –Y3 þ –DA (shown in Fig. 3). It showed that the maximum peak of DA was located at around 280 nm, which was attributed to the π-πn transition. The maximum peaks of the complexes (Tb3 þ –DA, Tb3 þ –Y3 þ –DA, Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA) were almost at the same position as that of DA at around 280 nm, but an obvious increase in absorption intensity was observed, which indicated that a more extensive π-πn conjugation system was formed due to the formation of the coordination bond between ligands and the central ion (Tb3 þ ). The LSPR absorption peak of the prepared AgNPs was at 425 nm (Fig. 3 inset). Meanwhile, in the presence of Tris–HCl buffer solution, the maximum absorption peak of the AgNPs (5.0 10 5 M) appeared at around 269 nm, which was the absorbance of silver nanoparticles clusters, and its plasma resonance absorption peak was very weak merging in the scattering. When DA was added to the AgNPs, the LSPR absorption peak of AgNPs–DA (Fig. 3 curve 8) showed a red-shift of about
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50 nm (from 425 nm to 475 nm) and broadened. It indicated that the AgNPs were aggregated by DA. The LSPR absorption peak intensity of Tb3 þ –AgNPs–DA (Fig. 3 curve 6) was decreased compared to that of AgNPs–DA, which implied the size of Tb3 þ – AgNPs–DA was smaller [40]. Then by adding Y3 þ into the Tb3 þ – AgNPs–DA system, the LSPR absorption peak intensity of Tb3 þ –AgNPs–Y3 þ –DA (Fig. 3 curve 7 vs curve 6) was enhanced. The red-shift and broadening of LSPR absorption peak could cause the increased spectrum overlap between LSPR absorption of AgNPs and the emission of Tb3 þ . In addition, from Fig. 2b, it can be seen that there was no the phenomena of co-luminescence of Tb3 þ –Y3 þ –DA in the absence of AgNPs. However, on adding AgNPs to the system, the fluorescence of the system was significantly enhanced. We think that the complexes of Tb3 þ –DA and Y3 þ –DA bind to the surface of AgNPs and cross-link by DA. Meanwhile, AgNPs can reduce the intermolecular distance between Tb3 þ –DA and Y3 þ –DA and effectively promote the transfer of energy between intramolecular and intermolecular rare
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earth complexes. AgNPs can sensitize the effect of co-luminescence of Tb3 þ –Y3 þ –DA composites. The possible mechanism for the detection of DA is proposed in Scheme 1. As we know, rare earth ion luminescence mainly involves energy transfer from ligand to lanthanide center and reverse energy transition caused by thermal deactivation. Decrease in the non-radiation thermal deactivation is mainly caused by hydroxyl vibration of water molecule around Ln3 þ [41]. In this work, the fluorescence intensity was enhanced markedly by adding AgNPs into the Tb3 þ –Y3 þ –DA system. It is known that dopamine molecules contain one amine group and two hydroxyl groups. Therefore, we think that DA as antenna molecules were binding to Tb3 þ (Y3 þ ) in the form of Tb3 þ (Y3 þ )-catechol complexes (Scheme 1), Y3 þ was a REcolumin ion. AgNPs as fluorescence sensitizers effectively promoted energy transfer of the intramolecular and intermolecular and the complexes of between DA and Tb3 þ , 3þ 3þ Y –DA and Tb –DA. AgNPs also acted as a work platform, which could change the microenvironment around the Tb3 þ –Y3 þ –DA
Fig. 1. TEM images of AgNPs and nanocomposites. (a) AgNPs (1.0 10 3 M), (b) AgNPs–DA, (c) Tb3 þ –AgNPs–DA, and (d) Tb3 þ –AgNPs–Y3 þ –DA. Condition: Tb3 þ : 6.0 10 4 M, AgNPs: 1.0 10 4 M, Y3 þ : 2.0 10 6 M, DA: 5.0 10 5 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH¼ 7.50).
Fig. 2. Fluorescence spectra of the Tb3 þ –AgNPs–Y3 þ –DA system. (a) Excitation spectra (λem ¼546 nm), (b) emission spectra (λex ¼306 nm): Curves: (1) Tb3 þ , (2) Tb3 þ –Y3 þ – DA, (3) Tb3 þ -–DA, (4) Tb3 þ –AgNPs–DA, (5) Tb3 þ –AgNPs–Y3 þ –DA. Conditions: Tb3 þ : 3.0 10 5 M, AgNPs: 5.0 10 6 M, Y3 þ : 1.0 10 7 M, DA: 1.0 10 6 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼ 7.50).
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Table 1 Interference from foreign substances.
Fig. 3. UV–visible spectra of the system. Curves: (1) Tb3 þ , (2) AgNPs, (3) DA; (4) Tb3 þ –DA, (5) Tb3 þ –Y3 þ –DA, (6) Tb3 þ –AgNPs–DA, (7) Tb3 þ –AgNPs–Y3 þ –DA, and (8) AgNPs–DA. Inset: AgNPs. Conditions: Tb3 þ : 3.0 10 4 M, AgNPs: 5.0 10 5 M, Y3 þ : 1.0 10 6 M, DA: 5.0 10 5 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼7.50). Inset: AgNPs: 1.0 10 3 M.
Scheme 1. Schematic illustration of detection strategy by AgNPs MEF-based rare earth ions co-luminescence.
complex and increase the intrinsic radiative decay rate of the fluorophore (Tb3 þ ) [42]. In addition, the gaps existing among the bead-like assemblages of Tb3 þ –AgNPs–Y3 þ –DA (see Fig. 1) could provide an appropriate distance between AgNPs and the fluorophore. This indicates that the complex of Tb3 þ –Y3 þ –DA was closer to the electric field around AgNPs. The coupling efficiency between AgNPs and the complex of Tb3 þ –Y3 þ –DA was higher, which was conducive to enhance the fluorescence of the Tb3 þ –Y3 þ –DA system. 3.3. Optimization of the experimental condition The performance of a method for detecting DA is strongly influenced by the experimental conditions such as pH in the solution, the concentration of compound, and the reaction time. Therefore, the detection parameters were optimized in our study. The effects of pH value (range from 6.8 to 8.0) of the medium on the luminescence intensity of the system were studied (Fig. S1 in the Supporting Information). It showed that a maximum ΔIf was obtained at pH 7.50. The effect of different buffers on the fluorescence intensity of this system was also investigated at pH (7.5070.05). The relative ΔIf (%) for Tris–HCl, HMTA–HCl, HEPES (2-[4-(hydroxyethyl)-1-piperazinyl]
Interferents
Tolerable concentration ( 10 7 M)
Change in luminescence intensity (%)
Mg2 þ , Cl Al3 þ , Cl Ca2 þ , Cl Ba2 þ , Cl K þ , Cl Na þ , Cl Zn2 þ , SO42 Cu2 þ , SO42 Na þ , CO32 Glucose Uric acid Ascorbic acid Citric acid Boric acid Alanine Glycine Tyrosine Cysteine Aspartic acid
10 2.2 5.0 5.0 5.0 5.0 5.0 5.0 30 3.0 8.0 10 8.0 9.0 5.0 2.0 10 5.0 10
0.6 0.3 1.6 4.0 4.2 4.4 4.3 2.5 3.0 4.3 0.7 4.2 3.4 0.8 0.8 5.8 2.3 5.7 4.5
Conditions: Tb3 þ : 3.0 10 5 M, AgNPs: 5.0 10 6 M, Y3 þ : 1.0 10 7 M, DA: 1.0 10 7 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼7.50).
ethanesulfonic acid)–NaOH, formic acid–NaOH and HAc–NaAc was 100, 14.2, 83.1, 35.7 and 42.4, respectively. Experimental results proved that the optimum concentration of Tris–HCl buffer was 5.0 10 3 M. So pH 7.50, 5.0 10 3 M Tris–HCl was chosen for the subsequent experiments. We further investigated the effect of Tb3 þ concentration on the fluorescence intensity of the system (Fig. S2). The enhanced extent of the fluorescence intensity of the system was enhanced on increasing the concentration of Tb3 þ up to 3.0 10 5 M. At this concentration, ΔIf of this system reached its maximum value. On further increasing in the concentration of Tb3 þ , the ΔIf of this system decreased gradually. Therefore, 3.0 10 5 M terbium (III) was used in the following experiments. To further test the effect of AgNPs on the co-luminescence of Tb3 þ – Y3 þ –DA. The effect of the concentration of AgNPs on the fluorescence intensity of the system was examined over the range from 1.0 10 6 M to 7.0 10 6 M (as shown in Fig. S3). The ΔIf reached the maximum value at a concentration of AgNPs at approximately 5.0 10 6 M. Therefore, 5.0 10 6 M AgNPs were used for the following work. Fig. S4 shows the effects of the concentration of Y3 þ on the fluorescence intensity of the system. From Fig. S4, it can be seen that when the Y3 þ concentration was 1.0 10 7 M, the enhancement extent of the fluorescence intensity is the highest. Thus, 1.0 10 7 M Y3 þ ion concentration is used for the further experiment. 3.4. The addition order and stability of the system The effects of reagent addition order on the fluorescence intensity of the system were studied. Using the addition sequence of Tris–HCl buffer solution, Tb3 þ , AgNPs, Y3 þ and DA, the strongest fluorescence enhancement was obtained. Under the optimal conditions, the fluorescence enhancement reached a maximum value after 30 min and remained basically constant for over 3 h. The stabilities of Tb3 þ –Y3 þ –DA and Tb3 þ – AgNPs–DA were also studied under the optimal conditions. After 1.5 h, the fluorescence intensies of Tb3 þ –Y3 þ –DA and Tb3 þ – AgNPs–DA is reduced by 8.7% and 7.2%, respectively. These results showed that the fluorescence intensity of the Tb3 þ –DA system was more stable in the presence of AgNPs and Y3 þ .
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Table 2 Comparison of limits of detection (nM) for DA obtained by the proposed method with other analytical methods. Sensing material
Detection
Linear range
LOD (nM)
Reference
Graphene oxide(GO) Terbium sensitized fluorescence Citrate-capped AgNPs Carboxylated carbonaceous modified electrode Gold nanoparticles dithiobis(succinimidylpropionate)-modified AuNPs KIO4–luminol system by gallic acid, acetaldehyde and Mn2 þ AgNPs–Tb3 þ This method
Fluorescence HPLC Colorimetry and Scattering Electrochemical Colorimetric Colorimetric Chemiluminescence Fluorescence Fluorescence
0–50 μM 1.0–100 μM 0–0.6 μM 0.1–40 μM 10 nM to 1.0 μM 5.0–600 nM 1.0–10 nM 2.4–140 nM 2.0–100 nM
94 70 40 30 5 2 0.63 0.42 0.57
[33] [27] [35] [25] [32] [34] [30] [31] This work
Table 3 Recovery of the DA in serum samples. Samples
Added ( 10 8 M)
Human serum 1.00 2.00 3.00 a
Observed ( 10 8 M) 7 RSDa (%)
Recovery (%)
1.067 2.78 1.99 7 2.34 2.79 7 1.63
106 99.5 93
Relative standard deviation for three replication measurements.
3.5. Interference of coexisting foreign substances To evaluate the present method, the effects of some potentially interfering ions and other bioactive small molecules that may coexist in the pharmaceutical injection and biological fluids were studied. The experimental results are shown in Table 1. The results showed that most of the metal ions and amino acids (except glycine) had little effect on the fluorescence of the system, within 75% relative error.
measurements for DA in the injection samples was 10.4 mg/mL and the relative standard derivation was 3.4% (n ¼5). The results were nearly close to the labeled amount of dopamine hydrochloride injections. Thus, the accuracy and precision of the proposed method were satisfactory and suitable for accurate determination of DA in human serum.
4. Conclusions A sensitive method for the determination of DA was established based on the co-luminescence effect of the system in the presence of AgNPs. The detection limits for the DA were reduced to the 10 10 M level. This method was successfully applied to the determination of the actual sample of dopamine hydrochloride injection. The studies on the interaction mechanism indicated that using AgNPs as sensitizers and work platforms effectively promote the rare earth co-luminescence of the Tb3 þ –Y3 þ –DA system.
3.6. Calibration curve and detection limit of the method
Acknowledgements
According to the above procedures, under the optimal conditions, there was a good linear relationship between the enhanced fluorescence intensity and the concentration of DA over the range from 2.0 10 9 to 1.0 10 7 M with a correlation coefficient of 0.9992 (Fig. S5). The limit of determination was calculated to be 5.7 10 10 M, according to the 3Sb/S criterion, where S is the slope for the range of the linearity used and Sb is the standard deviation of the blank (n ¼11). In comparison with some results reported previously (as shown in Table 2), the LODs (3Sb/S) obtained by this work were lower than those reported in some literatures (Refs. [25,27,32–35]) and comparable with that reported in Ref. [30], but a little higher than that reported in Ref. [31]. However, our method was conducted in a neutral buffer solution, which was close to the pH value of biological fluids. Besides, compared with all these reported methods, the proposed assay method is simple, easy to operate and has high sensitivity.
We gratefully acknowledge the support of the Natural Science Foundation of Shandong Province of China (Grant no. ZR2013BM025). The authors thank Prof. Dr. Yanqin Wang (College of Chemistry and Molecular Engineering, East China University of Science and Technology) for her valuable comments and corrections of the manuscript.
3.7. Sample analysis To evaluate the applicability and the accuracy of this proposed method, DA contents in serum samples and dopamine hydrochloride injections were measured by a standard addition method (Table 3). The spiked serum samples were obtained by adding the standard solution of DA in an appropriate (100-fold) dilution of human serum (Solarbio, Beijing, China). The recoveries were within the range from 93% to 106%. The dopamine hydrochloride injections (Gangzhou Baiyun Shan Ming Xing Pharmaceutical Co., Ltd., China, B. no. 120804, labeled amount: 2 ml: 20 mg) were diluted to a certain concentration with water and were tested by the standard addition method. The mean value of the five
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2015.02.023.
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Silver nanoparticles-enhanced rare earth co-luminescence effect of Tb (III)–Y(III)–dopamine system Huihui Li, Xia Wu n Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2014 Received in revised form 6 February 2015 Accepted 11 February 2015 Available online 20 February 2015
It was found that silver nanoparticles (AgNPs) could enhance co-luminescence effect of rare earths ions Tb3 þ and Y3 þ . Based on this, a sensitive fluorescence detection method for the determination of dopamine (DA) was proposed. Moreover, the detection limit for DA was very low (down to nM). This is because DA can remarkably enhance the luminescence intensity of the Tb3 þ ion by Y3 þ in the colloidal solution of AgNPs, forming a new co-luminescence system. Furthermore, based on the metal enhanced fluorescence (MEF), AgNPs can sensitize the co-luminescence effect of the complex of Tb3 þ –Y3 þ –DA. In a neutral buffer solution (pH 7.50), the luminescence intensity of the system was linearly related to the concentration of DA in the range of 2.0–100 nM, with a limit of detection as low as 0.57 nM. The proposed method was applied for the determination of DA in dopamine hydrochloride injections and human serum samples with good accuracy and satisfactory recovery. & 2015 Elsevier B.V. All rights reserved.
Keywords: Metal enhanced fluorescence Co-luminescence Tb3 þ Y3 þ Silver nanoparticles
1. Introduction Lanthanide ions (Tb3 þ , Eu3 þ , etc.) as a luminescence probe have interested chemists and biochemists for decades because of their intrinsic characteristics such as narrow emission band widths, large Stocks shift, long luminescence lifetimes and strong binding with biological molecules [1–3]. However, other rare earth ions (La3 þ , Lu3 þ , Tb3 þ , Y3 þ or Gd3 þ etc.) co-existed can unexpectedly enhance their luminescence intensity, which is named as rare earth ions co-luminescence effect (RE-columin) [4]. The sensitivity of a co-luminescence-based analytical method can be significantly improved, compared to that of a single rare earth ion luminescence method [4,5], and the vast majority of methods require that surfactants must exist in the system [6–8]. Our previous work showed that silver island film could sensitize Tb3 þ –Gd3 þ –protein co-luminescence and sensitively detect protein [8], which was conducted in the solid matrix and needed sodium dodecyl benzene sulfonate (SDBS) to coexist. Until now, the analytical method based on RE-columin sensitized by metallic nanoparticles alone without surfactant in aqueous solution has not been reported yet. Silver nanoparticles (AgNPs) have unique optoelectronic properties, such as localized surface plasmon resonance (LSPR), surface-enhanced Raman and surface-enhanced fluorescence [9–11]. n
Corresponding author. Tel.: þ 86 531 88365459; fax: þ86 531 88564464. E-mail address: [email protected] (X. Wu).
http://dx.doi.org/10.1016/j.talanta.2015.02.023 0039-9140/& 2015 Elsevier B.V. All rights reserved.
These optoelectronic properties have been widely used in bioanalytical researches [11–13]. Metal enhanced fluorescence (MEF) [11–14] occurs due to the couple interaction between fluorophores and the plasmon resonance of metal nanoparticles, resulting in an increase in both the radiative decay rate and fluorescence emission. Studies showed that MEF property can be affected by nanoparticle size, particle shape, spacing distance of fluorophore-tometal, and wavelength [11,14,15]. Dopamine (DA), 4-(2-aminoethyl)-benzen-1, 2-diol, a natural substance, is one of the crucial catecholamines. As a neurotransmitter in the brain, DA plays a critical role in the feelings of addiction and excitement, memory, cognition, and motor control [16–18]. An abnormal level of DA in the human body can cause health issues, such as attention deficit hyperactivity disorder (ADHD), anorexia, depression, Parkinson’s disease etc. [19,20]. Therefore, the determination of DA concentration at trace levels in biological fluids such as urine, serum and pharmaceutical formulation is of great importance. Current analytical methods for the determination of DA mainly use electrochemistry [21–25], highperformance liquid chromatography (HPLC) [26,27], capillary electrophoresis (CE) [28,29], chemiluminescence (CL) [30], UV–visible absorption and fluorescence spectrophotometries [31–37]. The methods of HPLC and CE usually need derivatization separation process and is time-consuming. Electrochemical methods have always been a powerful tool for determination of DA. To improve the sensitivity determination of DA, the modified electrodes were adopted, which needed experienced operators.
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As is known, the spectrofluorimetric method for the determination of DA is simple and sensitive. Lately, Yan et al. [33] established a new grapheme oxide-based fluorescence method of selectively determining DA. The detection limit was 94 nM. Kim et al. [31] developed a fluorescence method for determining DA by using AgNPs-enhanced fluorescence of Tb3 þ in alkaline solution. In this work, a facile and sensitive method for the determination of DA was proposed based on Tb3 þ –Y3 þ –DA co-luminescence sensitized by AgNPs in neutral aqueous solution. The interaction mechanism of AgNPs enhancing co-luminescence of the Tb3 þ –Y3 þ –DA system was also discussed.
in a 1 cm quartz cell with the excitation and emission wavelengths of 306 nm and 546 nm, respectively. The excitation and emission slits were both 10 nm and the scan speed was 240 nm/min. The high voltage for the photomultiplier tube was set to 700 V. The enhanced extent of fluorescence intensity is defined as ΔIf ¼If I0, where If and I0 were the fluorescence intensity of the system with and without DA, respectively.
3. Results and discussion 3.1. Characterization of silver nanoparticles and nanocomposites
2. Experimental section 2.1. Chemicals and materials Dopamine hydrochloride was commercially purchased from the National Institutes for Food and Drug Control (Beijing, China) and used without further purification. The rare earth oxides (99.9%) were obtained from Yuelong Chemical Co., (Shanghai, China). Other reagents used in this study were of analytical grade. The stock solution of DA (1.0 10 2 M) was prepared using 0.1 M hydrochloric acid (1 mL) and deionized water. The stock standard solution (1.0 10 2 M) of rare earth ions was prepared by dissolving the corresponding oxides in hydrochloric acid and then diluting with deionized water. The prepared stock solution was stored in a refrigerator at 4 °C. A 0.05 M Tris–HCl buffer solution was prepared by dissolving 3.03 g of Tris in 500 mL deionized water and then was adjusted using 0.1 M HCl. 2.2. Synthesis of silver nanoparticles Silver nanoparticles (1.0 10 3 M calculated by the concentration of the silver ion added) were synthesized by a chemical reduction method as described in the literature with some modifications [38,39]. Briefly, AgNO3 (17 mg) was dissolved in 50 mL of ultrapure water (18.25 MΩ cm) and heated to 86 °C with vigorous stirring. 3 mL of 1% sodium citrate solution was added dropwise to the silver nitrate solution. Heating was continued for an additional 12 min until the color of the colloidal solution turned light yellow. Then the silver colloidal solution was diluted into 100 mL of a volumetric flask with ultrapure water and stored at 4 °C. The prepared AgNPs were stable for more than 2 months. 2.3. Apparatus Fluorescence spectra and intensity were recorded with a spectrofluorometer (F-7000, Hitachi, Japan) in a 1 cm quartz cuvette. All the absorption spectra were measured using a U-4100 spectrophotometer (Hitachi, Japan). Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-1011 transmission electron microscope operated at an accelerating voltage of 100 kV. Zeta potential was monitored using a Malvern Zetasizer Nano instrument (Zetasizer Nano ZS90). All pH measurements were made with a Delta 320-S acidity meter (Mettler Toledo, Shanghai). 2.4. Measurement procedures Solutions were added in the following order, into a 25 mL test tube: 1.0 mL of Tris–HCl buffer 7.50, 0.3 mL of 1.0 10 3 M Tb3 þ , 0.5 mL of 1.0 10 4 M AgNPs solution, 0.1 mL of 1.0 10 5 M Y3 þ , and DA solution with an appropriate volume. The mixture was diluted to 10 mL with water and was allowed to stand for 30 min. The luminescence spectra of this system were measured
The properties of prepared AgNPs were characterized by Zeta potential and TEM images (Fig. 1). The Zeta potential of AgNPs was 32.57 mV which indicated that high stability AgNPs were obtained. The morphologies of AgNPs and the nanocomposites of AgNPs–DA, Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA were identified by TEM. TEM images showed that AgNPs were spherical particles with average particle diameters approximately 15 73 nm (Fig. 1a). After adding DA to AgNPs, the sizes of their aggregations are about 88 72 nm (Fig. 1b). And the morphologies of the assemblages of Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA were both bead-like and the latter is relatively loose (Fig. 1c and d). 3.2. Fluorescence enhancement Fluorescence spectra of the Tb3 þ –AgNPs–Y3 þ –DA system are shown in Fig. 2. It revealed that with the addition of DA, the emission peak of Tb3 þ –DA (Fig. 2b curve 3 vs curve 1) was enhanced at the wavelengths of 490 nm and 546 nm, which were the intrinsic fluorescence peaks of Tb3 þ and corresponded to the transition of electrons from 5D4-7F6 and 5D4-7F5, respectively. Adding Y3 þ to Tb3 þ –DA, the fluorescence intensity of the system decreased (Fig. 2b curve 2). On the contrary, when the colloidal solution of AgNPs was added to the Tb3 þ –DA or Tb3 þ –Y3 þ –DA system, respectively, the fluorescence intensities of the systems were enhanced (Fig. 2b curves 4 and 5). The extent of the fluorescence enhancement of Tb3 þ –AgNPs–Y3 þ –DA was greater than that of Tb3 þ –AgNPs–DA. It indicated that in the presence of AgNPs, the effective RE-columin effects of the Tb3 þ –Y3 þ –DA system occurred. If there was no AgNPs in the solution, Y3 þ could form a complex with DA and thus compete with Tb3 þ , resulting in reduction in the fluorescence intensity of Tb3 þ . The peak positions of excitation and emission spectra were nearly unchanged. The most intense emission peak was located at 546 nm, and this peak was thus chosen for the luminescence intensity measurement throughout this work. UV–vis absorption experiments were conducted to understand the interaction of AgNPs with DA, Tb3 þ –DA and Tb3 þ –Y3 þ –DA (shown in Fig. 3). It showed that the maximum peak of DA was located at around 280 nm, which was attributed to the π-πn transition. The maximum peaks of the complexes (Tb3 þ –DA, Tb3 þ –Y3 þ –DA, Tb3 þ –AgNPs–DA and Tb3 þ –AgNPs–Y3 þ –DA) were almost at the same position as that of DA at around 280 nm, but an obvious increase in absorption intensity was observed, which indicated that a more extensive π-πn conjugation system was formed due to the formation of the coordination bond between ligands and the central ion (Tb3 þ ). The LSPR absorption peak of the prepared AgNPs was at 425 nm (Fig. 3 inset). Meanwhile, in the presence of Tris–HCl buffer solution, the maximum absorption peak of the AgNPs (5.0 10 5 M) appeared at around 269 nm, which was the absorbance of silver nanoparticles clusters, and its plasma resonance absorption peak was very weak merging in the scattering. When DA was added to the AgNPs, the LSPR absorption peak of AgNPs–DA (Fig. 3 curve 8) showed a red-shift of about
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50 nm (from 425 nm to 475 nm) and broadened. It indicated that the AgNPs were aggregated by DA. The LSPR absorption peak intensity of Tb3 þ –AgNPs–DA (Fig. 3 curve 6) was decreased compared to that of AgNPs–DA, which implied the size of Tb3 þ – AgNPs–DA was smaller [40]. Then by adding Y3 þ into the Tb3 þ – AgNPs–DA system, the LSPR absorption peak intensity of Tb3 þ –AgNPs–Y3 þ –DA (Fig. 3 curve 7 vs curve 6) was enhanced. The red-shift and broadening of LSPR absorption peak could cause the increased spectrum overlap between LSPR absorption of AgNPs and the emission of Tb3 þ . In addition, from Fig. 2b, it can be seen that there was no the phenomena of co-luminescence of Tb3 þ –Y3 þ –DA in the absence of AgNPs. However, on adding AgNPs to the system, the fluorescence of the system was significantly enhanced. We think that the complexes of Tb3 þ –DA and Y3 þ –DA bind to the surface of AgNPs and cross-link by DA. Meanwhile, AgNPs can reduce the intermolecular distance between Tb3 þ –DA and Y3 þ –DA and effectively promote the transfer of energy between intramolecular and intermolecular rare
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earth complexes. AgNPs can sensitize the effect of co-luminescence of Tb3 þ –Y3 þ –DA composites. The possible mechanism for the detection of DA is proposed in Scheme 1. As we know, rare earth ion luminescence mainly involves energy transfer from ligand to lanthanide center and reverse energy transition caused by thermal deactivation. Decrease in the non-radiation thermal deactivation is mainly caused by hydroxyl vibration of water molecule around Ln3 þ [41]. In this work, the fluorescence intensity was enhanced markedly by adding AgNPs into the Tb3 þ –Y3 þ –DA system. It is known that dopamine molecules contain one amine group and two hydroxyl groups. Therefore, we think that DA as antenna molecules were binding to Tb3 þ (Y3 þ ) in the form of Tb3 þ (Y3 þ )-catechol complexes (Scheme 1), Y3 þ was a REcolumin ion. AgNPs as fluorescence sensitizers effectively promoted energy transfer of the intramolecular and intermolecular and the complexes of between DA and Tb3 þ , 3þ 3þ Y –DA and Tb –DA. AgNPs also acted as a work platform, which could change the microenvironment around the Tb3 þ –Y3 þ –DA
Fig. 1. TEM images of AgNPs and nanocomposites. (a) AgNPs (1.0 10 3 M), (b) AgNPs–DA, (c) Tb3 þ –AgNPs–DA, and (d) Tb3 þ –AgNPs–Y3 þ –DA. Condition: Tb3 þ : 6.0 10 4 M, AgNPs: 1.0 10 4 M, Y3 þ : 2.0 10 6 M, DA: 5.0 10 5 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH¼ 7.50).
Fig. 2. Fluorescence spectra of the Tb3 þ –AgNPs–Y3 þ –DA system. (a) Excitation spectra (λem ¼546 nm), (b) emission spectra (λex ¼306 nm): Curves: (1) Tb3 þ , (2) Tb3 þ –Y3 þ – DA, (3) Tb3 þ -–DA, (4) Tb3 þ –AgNPs–DA, (5) Tb3 þ –AgNPs–Y3 þ –DA. Conditions: Tb3 þ : 3.0 10 5 M, AgNPs: 5.0 10 6 M, Y3 þ : 1.0 10 7 M, DA: 1.0 10 6 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼ 7.50).
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Table 1 Interference from foreign substances.
Fig. 3. UV–visible spectra of the system. Curves: (1) Tb3 þ , (2) AgNPs, (3) DA; (4) Tb3 þ –DA, (5) Tb3 þ –Y3 þ –DA, (6) Tb3 þ –AgNPs–DA, (7) Tb3 þ –AgNPs–Y3 þ –DA, and (8) AgNPs–DA. Inset: AgNPs. Conditions: Tb3 þ : 3.0 10 4 M, AgNPs: 5.0 10 5 M, Y3 þ : 1.0 10 6 M, DA: 5.0 10 5 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼7.50). Inset: AgNPs: 1.0 10 3 M.
Scheme 1. Schematic illustration of detection strategy by AgNPs MEF-based rare earth ions co-luminescence.
complex and increase the intrinsic radiative decay rate of the fluorophore (Tb3 þ ) [42]. In addition, the gaps existing among the bead-like assemblages of Tb3 þ –AgNPs–Y3 þ –DA (see Fig. 1) could provide an appropriate distance between AgNPs and the fluorophore. This indicates that the complex of Tb3 þ –Y3 þ –DA was closer to the electric field around AgNPs. The coupling efficiency between AgNPs and the complex of Tb3 þ –Y3 þ –DA was higher, which was conducive to enhance the fluorescence of the Tb3 þ –Y3 þ –DA system. 3.3. Optimization of the experimental condition The performance of a method for detecting DA is strongly influenced by the experimental conditions such as pH in the solution, the concentration of compound, and the reaction time. Therefore, the detection parameters were optimized in our study. The effects of pH value (range from 6.8 to 8.0) of the medium on the luminescence intensity of the system were studied (Fig. S1 in the Supporting Information). It showed that a maximum ΔIf was obtained at pH 7.50. The effect of different buffers on the fluorescence intensity of this system was also investigated at pH (7.5070.05). The relative ΔIf (%) for Tris–HCl, HMTA–HCl, HEPES (2-[4-(hydroxyethyl)-1-piperazinyl]
Interferents
Tolerable concentration ( 10 7 M)
Change in luminescence intensity (%)
Mg2 þ , Cl Al3 þ , Cl Ca2 þ , Cl Ba2 þ , Cl K þ , Cl Na þ , Cl Zn2 þ , SO42 Cu2 þ , SO42 Na þ , CO32 Glucose Uric acid Ascorbic acid Citric acid Boric acid Alanine Glycine Tyrosine Cysteine Aspartic acid
10 2.2 5.0 5.0 5.0 5.0 5.0 5.0 30 3.0 8.0 10 8.0 9.0 5.0 2.0 10 5.0 10
0.6 0.3 1.6 4.0 4.2 4.4 4.3 2.5 3.0 4.3 0.7 4.2 3.4 0.8 0.8 5.8 2.3 5.7 4.5
Conditions: Tb3 þ : 3.0 10 5 M, AgNPs: 5.0 10 6 M, Y3 þ : 1.0 10 7 M, DA: 1.0 10 7 M, and Tris–HCl buffer solution: 5.0 10 3 M (pH ¼7.50).
ethanesulfonic acid)–NaOH, formic acid–NaOH and HAc–NaAc was 100, 14.2, 83.1, 35.7 and 42.4, respectively. Experimental results proved that the optimum concentration of Tris–HCl buffer was 5.0 10 3 M. So pH 7.50, 5.0 10 3 M Tris–HCl was chosen for the subsequent experiments. We further investigated the effect of Tb3 þ concentration on the fluorescence intensity of the system (Fig. S2). The enhanced extent of the fluorescence intensity of the system was enhanced on increasing the concentration of Tb3 þ up to 3.0 10 5 M. At this concentration, ΔIf of this system reached its maximum value. On further increasing in the concentration of Tb3 þ , the ΔIf of this system decreased gradually. Therefore, 3.0 10 5 M terbium (III) was used in the following experiments. To further test the effect of AgNPs on the co-luminescence of Tb3 þ – Y3 þ –DA. The effect of the concentration of AgNPs on the fluorescence intensity of the system was examined over the range from 1.0 10 6 M to 7.0 10 6 M (as shown in Fig. S3). The ΔIf reached the maximum value at a concentration of AgNPs at approximately 5.0 10 6 M. Therefore, 5.0 10 6 M AgNPs were used for the following work. Fig. S4 shows the effects of the concentration of Y3 þ on the fluorescence intensity of the system. From Fig. S4, it can be seen that when the Y3 þ concentration was 1.0 10 7 M, the enhancement extent of the fluorescence intensity is the highest. Thus, 1.0 10 7 M Y3 þ ion concentration is used for the further experiment. 3.4. The addition order and stability of the system The effects of reagent addition order on the fluorescence intensity of the system were studied. Using the addition sequence of Tris–HCl buffer solution, Tb3 þ , AgNPs, Y3 þ and DA, the strongest fluorescence enhancement was obtained. Under the optimal conditions, the fluorescence enhancement reached a maximum value after 30 min and remained basically constant for over 3 h. The stabilities of Tb3 þ –Y3 þ –DA and Tb3 þ – AgNPs–DA were also studied under the optimal conditions. After 1.5 h, the fluorescence intensies of Tb3 þ –Y3 þ –DA and Tb3 þ – AgNPs–DA is reduced by 8.7% and 7.2%, respectively. These results showed that the fluorescence intensity of the Tb3 þ –DA system was more stable in the presence of AgNPs and Y3 þ .
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Table 2 Comparison of limits of detection (nM) for DA obtained by the proposed method with other analytical methods. Sensing material
Detection
Linear range
LOD (nM)
Reference
Graphene oxide(GO) Terbium sensitized fluorescence Citrate-capped AgNPs Carboxylated carbonaceous modified electrode Gold nanoparticles dithiobis(succinimidylpropionate)-modified AuNPs KIO4–luminol system by gallic acid, acetaldehyde and Mn2 þ AgNPs–Tb3 þ This method
Fluorescence HPLC Colorimetry and Scattering Electrochemical Colorimetric Colorimetric Chemiluminescence Fluorescence Fluorescence
0–50 μM 1.0–100 μM 0–0.6 μM 0.1–40 μM 10 nM to 1.0 μM 5.0–600 nM 1.0–10 nM 2.4–140 nM 2.0–100 nM
94 70 40 30 5 2 0.63 0.42 0.57
[33] [27] [35] [25] [32] [34] [30] [31] This work
Table 3 Recovery of the DA in serum samples. Samples
Added ( 10 8 M)
Human serum 1.00 2.00 3.00 a
Observed ( 10 8 M) 7 RSDa (%)
Recovery (%)
1.067 2.78 1.99 7 2.34 2.79 7 1.63
106 99.5 93
Relative standard deviation for three replication measurements.
3.5. Interference of coexisting foreign substances To evaluate the present method, the effects of some potentially interfering ions and other bioactive small molecules that may coexist in the pharmaceutical injection and biological fluids were studied. The experimental results are shown in Table 1. The results showed that most of the metal ions and amino acids (except glycine) had little effect on the fluorescence of the system, within 75% relative error.
measurements for DA in the injection samples was 10.4 mg/mL and the relative standard derivation was 3.4% (n ¼5). The results were nearly close to the labeled amount of dopamine hydrochloride injections. Thus, the accuracy and precision of the proposed method were satisfactory and suitable for accurate determination of DA in human serum.
4. Conclusions A sensitive method for the determination of DA was established based on the co-luminescence effect of the system in the presence of AgNPs. The detection limits for the DA were reduced to the 10 10 M level. This method was successfully applied to the determination of the actual sample of dopamine hydrochloride injection. The studies on the interaction mechanism indicated that using AgNPs as sensitizers and work platforms effectively promote the rare earth co-luminescence of the Tb3 þ –Y3 þ –DA system.
3.6. Calibration curve and detection limit of the method
Acknowledgements
According to the above procedures, under the optimal conditions, there was a good linear relationship between the enhanced fluorescence intensity and the concentration of DA over the range from 2.0 10 9 to 1.0 10 7 M with a correlation coefficient of 0.9992 (Fig. S5). The limit of determination was calculated to be 5.7 10 10 M, according to the 3Sb/S criterion, where S is the slope for the range of the linearity used and Sb is the standard deviation of the blank (n ¼11). In comparison with some results reported previously (as shown in Table 2), the LODs (3Sb/S) obtained by this work were lower than those reported in some literatures (Refs. [25,27,32–35]) and comparable with that reported in Ref. [30], but a little higher than that reported in Ref. [31]. However, our method was conducted in a neutral buffer solution, which was close to the pH value of biological fluids. Besides, compared with all these reported methods, the proposed assay method is simple, easy to operate and has high sensitivity.
We gratefully acknowledge the support of the Natural Science Foundation of Shandong Province of China (Grant no. ZR2013BM025). The authors thank Prof. Dr. Yanqin Wang (College of Chemistry and Molecular Engineering, East China University of Science and Technology) for her valuable comments and corrections of the manuscript.
3.7. Sample analysis To evaluate the applicability and the accuracy of this proposed method, DA contents in serum samples and dopamine hydrochloride injections were measured by a standard addition method (Table 3). The spiked serum samples were obtained by adding the standard solution of DA in an appropriate (100-fold) dilution of human serum (Solarbio, Beijing, China). The recoveries were within the range from 93% to 106%. The dopamine hydrochloride injections (Gangzhou Baiyun Shan Ming Xing Pharmaceutical Co., Ltd., China, B. no. 120804, labeled amount: 2 ml: 20 mg) were diluted to a certain concentration with water and were tested by the standard addition method. The mean value of the five
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2015.02.023.
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