Highly Sensitive and Selective Detection of Dopamine Using One-Pot

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Highly Sensitive and Selective Detection of Dopamine Using OnePot Synthesized Highly Photoluminiscent Silicon Nanoparticles Xiaodong Zhang, Xiaokai Chen, Siqi Kai, Hong-Yin Wang, Jingjing Yang, Fu-Gen Wu, and Zhan Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac504520g • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Analytical Chemistry

Highly Sensitive and Selective Detection of Dopamine Using One-Pot Synthesized Highly Photoluminiscent Silicon Nanoparticles

Xiaodong Zhang,† Xiaokai Chen,† Siqi Kai,† Hong-Yin Wang,† Jingjing Yang,†,§ Fu-Gen Wu,*,† and Zhan Chen*,‡

†

State Key Laboratory of Bioelectronics, School of Biological Science and Medical

Engineering, Southeast University, Nanjing 210096, China ‡

Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor,

Michigan 48109, United States §

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189,

China

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ABSTRACT A simple and highly efficient method for dopamine (DA) detection using water-soluble silicon nanoparticles (SiNPs) was reported. The SiNPs with a high quantum yield of 23.6% were synthesized by using a one-pot microwave-assisted method. The fluorescence quenching capability of a variety of molecules on the synthesized SiNPs has been tested; only DA molecules were found to be able to quench the fluorescence of these SiNPs effectively. Therefore, such a quenching effect can be used to selectively detect DA. All other molecules tested have little interference with the dopamine detection, including ascorbic acid, which commonly exists in cells and can possibly affect the dopamine detection. The ratio of the fluorescence intensity difference between the quenched and unquenched cases vs. the fluorescence intensity without quenching (∆I / I) was observed to be linearly proportional to the DA analyte concentration in the range from 0.005 to 10.0 µM, with a detection limit of 0.3 nM (S/N = 3). To the best of our knowledge, this is the lowest limit for DA detection reported so far. The mechanism of fluorescence quenching is attributed to the energy transfer from the SiNPs to the oxidized dopamine molecules through Förster resonance energy transfer (FRET). The reported method of SiNP synthesis is very simple and cheap, making the above sensitive and selective DA detection approach using SiNPs practical for many applications.

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1. INTRODUCTION Dopamine (DA), one of the catecholamine neurotransmitters of the human central nervous system in the brain, plays an important role in many brain functions and behavioral responses, such as feeling, information transduction, and addiction.1,2 Excessive secretion of DA, e.g., due to Huntington's disease, is associated with failure in energy metabolism and causes untimely death. Differently, lack of DA makes muscle out of control, leading to aprosexia and even Parkinson’s disease. Therefore it is important to detect DA and quantify DA concentration. A variety of methods have been developed to detect DA including electrogenerated chemiluminescence,3 electrochemistry,4−7 capillary electrophoresis,8 colorimetry,9 UV-Vis spectrography,10,11 high performance liquid chromatography,12 flow-injection analysis,13 and fluorescent spectroscopy.14−17 Among them, fluorescence method possesses a series of advantages, such as low cost, facile operation, and good reproducibility, and is thus considered to be an ideal method for DA detection. Lv and co-workers designed (3-aminopropyl)trimethoxysilane (APTMS)-capped ZnO quantum dots to detect DA in aqueous and serum samples. The relative fluorescence intensity was linearly proportional to the concentration of DA within the range from 0.05 to 10.0 µM, with a detection limit as low as 12 nM.16 Mu et al. reported an effective fluorescent sensor based on adenosine capped CdSe/ZnS quantum dots for highly sensitive detection of DA in human urine samples with the limit of detection (LOD) of 29.3 nM.17 However, as previously reported, the concentration of DA in living system is very low: 26 to 40 nM or even lower.18,19 Therefore, an extremely sensitive, selective, and quantitative analytical method is urgently needed for

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DA detection . Recently, silicon-based nanostructures have drawn extensive attention, due to the wide availability of the source materials to make such nanostructures, and the superb electronic, optical, catalytical, and mechanical properties of these silicon nanomaterials.20−32 Meanwhile, fluorescent silicon nanoparticles (SiNPs), which have a zero-dimensional silicon-based nanostructure, have been widely used in biology owing to their good biocompatibility, low cytotoxicity, and anti-photobleaching capability.33−36 As excellent fluorescent probes, SiNPs can be used in ion detection37 and long-term bioimaging/biosensing.34,38−47 However, it is challenging to synthesize highly fluorescent water-soluble SiNPs using a simple method. Recently, He and co-workers48 developed a facile one-pot synthesis method to prepare water-soluble SiNPs with strong fluorescence emission, favorable biocompatibility, and robust photo- and pH-stability. Herein, we discovered for the first time that the fluorescence of the as-synthesized SiNPs can be sensitively, selectively, and linearly quenched by DA in a broad DA solution concentration range from 0.005 to 10.0 µM. The detection limit was calculated to be as low as 0.3 nM (S/N = 3). To the best of our knowledge, this is the lowest detection limit ever reported for dopamine detection. Besides, we found that the prepared SiNPs showed a superb ability to selectively detect DA over other common molecules in the cell, including amino acids, peptides, proteins, ions, and other neurotransmitters. We believe that the quenching effect of DA on SiNPs is caused by Förster resonance energy transfer (FRET). Scheme 1 shows the mechanism for DA detection using SiNPs. More details will be discussed below.

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Scheme 1. Schematic illustrating the working principle for the DA detection using SiNPs prepared with APTMS.

2. EXPERIMENTAL SECTION Materials.

(3-Aminopropyl)trimethoxysilane

(APTMS),

dopamine

(DA),

serotonin

(5-hydroxytryptamine, 5-HT) hydrochloride, adenosine 5′-triphosphate (ATP), adenosine 5′-diphosphate (ADP), L-arginine (Arg), L-histidine (His), L-lysine (Lys), L-serine (Ser), L-glutamine (Gln), L-cysteine (Cys), L-Tyrosine (Tyr), L-Tryptophan (Trp), L-glutamic acid (Glu), glycine (Gly), D,L-homocysteine (Hcy), L-glutathione (GSH), BSA, glucose, ascorbic acid (AA), KCl, NaCl, CaCl2, MgCl2, and quinine sulfate were purchased from Sigma Aldrich. Norepinephrine (NE) was purchased from J&K Scientific. Sodium citrate dihydrate was obtained from Aladdin (China). All the reagents were used without any further purification. All solutions were prepared with deionized water (18.2 MΩ·cm) purified by a Milli-Q system (Millipore). Synthesis of SiNPs. The synthetic procedure of APTMS SiNPs has been previously reported by He et al..48 Typically, in a 35 mL Ace pressure tube, 12.0 mL aqueous solution dissolved with 0.558 g trisodium citrate dihydrate was bubbled with nitrogen gas for about 5 minutes to 5



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remove oxygen. Then 3.0 mL of APTMS was added into the above solution under vigorous stirring. After filling the tube with nitrogen, the stirring was continued for about 15 min to form SiNP precursors. The resultant precursor solution was then transferred into a microwave reactor and processed under 160 oC for 15 min, and cooled to room temperature. The residual reagents were removed by dialysis (1 kDa). Characterization of SiNPs. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) experiments were carried out on a Tecnai G2 20 transmission electron microscope. Ultraviolet-visible (UV-Vis) spectra were collected on a Shimadzu UV-2600 UV-Vis spectrophotometer. Fluorescence spectra were obtained using a Shimadzu RF-5301PC spectrofluorophotometer. The infrared spectra were collected with a Thermo Scientific Nicolet iS50 FT-IR spectrometer. Fluorescence decay time measurement was performed on a Horiba Jobin Yvon Fluorolog-3-TCSPC spectrophotometer. The final concentrations of the SiNPs and DA used in the fluorescence decay time measurement were 1.0 mg/mL and 1.0 µM, respectively. MTT Assay. To test the cytotoxicity of SiNPs, ATII cells (the human normal lung cells) were used in this study and were cultured in cell medium (DMEM), supplemented with 10% fetal bovine serum, 100 U of penicillin, and 100 µg/mL streptomycin in a humidified incubator at 37 oC and 5% CO2. ATII cells were seeded in a 96-well plate in cell medium overnight and then incubated with different concentrations of SiNPs (0, 10, 30, 100, 300, 600, and 1000 µg/mL) in cell medium for 24 h. After that, 10 µL MTT (5 mg/mL) was added to each well. After incubation for 4 h, 150 µL DMSO was added to each well and shaked for 10 min. The absorbance at 492 nm was measured by a Multiskan FC microplate photometer (Thermo).

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Detection of DA in Aqueous Solution. Samples with 1.0 mL of SiNPs dispersion (1.0 mg/mL) each were prepared from SiNPs stock solution by dilution with PBS solution (100 mM, pH = 7.4). Then 1.0 mL of DA solution (diluted by PBS solution) of different concentrations were added into each of the above SiNPs solutions, respectively. After stirring for 3 h, the fluorescence spectra were recorded with an excitation wavelength of 348 nm. Selectivity of DA Detection. Samples with 1.0 mL of SiNP dispersion (1.0 mg/mL) each were prepared from SiNPs stock solution by dilution with PBS solution (100 mM, pH = 7.4). Then 1.0 mL solutions of 20.0 µM DA, NE, 5-HT, ATP, ADP, Arg, His, Lys, Ser, Gln, Cys, Tyr, Trp, Gly, Hcy, Glu, glucose, GSH, BSA, AA, K+, Na+, Mg2+, and Ca2+ in PBS (100 mM, pH = 7.4) were added to each of the above SiNPs solutions respectively to reach an analyte concentration of 10.0 µM in each solution. Besides, a SiNP solution with a lower NE concentration of 1.0 µM was also studied. After stirring for 3 h, fluorescence spectra were recorded with an excitation wavelength of 348 nm. 3. RESULTS AND DISCUSSION Synthesis and characterization of SiNPs. As we stated above, SiNPs were prepared using a simple and “green” (or sustainable) method. The formation mechanism of the SiNPs has been described by He et al..48 In short, APTMS molecules act as silicon sources and sodium citrate is a reduction reagent. Large silicon nanoparticles were firstly formed through the hydrolysis of organosilane after adding silane molecules into the aqueous solution containing the reduction reagents. Under microwave irradiation, the large silicon nanoparticles were rapidly divided into small silicon crystal nuclei through the reduction reaction. When the temperature was higher, the growth of small silicon crystals took place at high super-saturation condition.

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After further hydrolysis and the Ostwald ripening process, the final SiNPs were formed. Figure 1A and 1B show that the synthesized SiNPs have an average size of 2.4 nm. Figure 1C displays the EDS pattern of SiNPs, indicating that SiNPs contain Si, C, O, and N elements. To confirm the presence of various functional groups in SiNPs, FTIR experiments were carried out (Figure 1D). The absorption at around 1112 cm−1 corresponds to the stretching vibrations of Si−O−Si.33 The strong signals at 1410 cm−1 and 1600 cm−1 are assigned to the bending vibrations of the C−H and N−H bonds, respectively. The absorption peaks at 2928 cm 1 and 3368 cm 1 are attributed to the stretching vibrations of the C−H and N−H vibrations, −

−

respectively. We also collected the UV-Vis absorption spectrum from the SiNPs (Figure 2A). There is no absorption detected for the wavelength region > 450 nm. Strong absorption is observed for the wavelength region < 400 nm, with a peak centered near 348 nm. The excitation spectrum also indicates that the maximum excitation peak is centered at 348 nm, and under the 348 nm excitation, the sample generates a fluorescence emission peak centered at about 444 nm (Figure 2B). Using quinine sulfate (QY = 54%) as a reference, the SiNP quantum yield is estimated to be about 23.6%, which is similar to the published result.48

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Figure 1. (A) TEM image, (B) corresponding size distribution histogram, (C) EDS pattern, and (D) FTIR spectrum of the APTMS SiNPs.

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Figure 2. (A) UV-Vis absorption spectrum of the APTMS SiNPs; (B) Fluorescence excitation (black—emission intensity was measured at 444 nm) and emission (red—excitation wavelength was 348 nm) spectra of the SiNPs; Inset: photographs of SiNPs under white (left) and UV (365 nm) light (right). (C) Fluorescence responses of 0.5 mg/mL SiNPs after incubation with different concentrations of DA (0−20.0 µM) for 3 h; (D) Plots of the relationship between ∆I / I0 and [DA] (0−20.0 µM). Inset: expanded linear region of the calibration curve.

Sensitivity of SiNPs for DA detection. The fluorescence quenching of SiNPs by different concentrations of DA can be seen in the fluorescence spectra shown in Figure 2C. As the concentration of DA increased, the fluorescence intensity of SiNPs was monotonically

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decreased. Figure 2D shows that the relative change of the fluorescence intensity vs. the initial intensity (∆I / I) at 444 nm as a function of DA concentration ([DA]). The ∆I / I values were measured over the DA concentration range of 0.005 to 20.0 µM and it is well described by the Stern−Volmer equation: I0 / I = 1 + Ksv [DA], ∆I / I = Ksv [DA] where I0 and I are the fluorescence intensities in the absence and presence of the quencher (DA), respectively. ∆I is the difference between I0 and I. Ksv is the Stern−Volmer constant representing the affinity between fluorophore and quencher. The plot of ∆I / I at 444 nm versus [DA] showed a linear relationship from 0.005 to 10.0 µM with a Ksv value of 1.76 × 106 M–1 and the square of correlation coefficient (R2) of 0.998 (inset in Figure 2D). Moreover, the detection limit (LOD) of the method is as low as 0.3 nM (calculated at a signal to noise ratio of 349), which makes the SiNP a very sensitive probe for detecting DA.

Selectivity of SiNPs for DA detection. Since the SiNPs are intended to be used for detecting DA, we also studied the possible fluorescence quenching effect of SiNPs caused by other common molecules in the cell. As shown in Figure 3, the fluorescence intensity of the SiNPs exhibited a significant decrease by adding DA to the solution, whereas other molecules such as amino acids (including Arg, His, Lys, Ser, Gln, Cys, Tyr, Trp, and Hcy), peptide (GSH), protein (BSA), ascorbic acid (AA), neurotransmitters (5-HT, ATP, ADP, Gly, and Glu), and ions (including K+, Na+, Mg2+, and Ca2+) did not cause obvious fluorescence changes. Besides, as shown in Figure 3, NE at 10.0 µM can reduce the fluorescence intensity of SiNPs by ~25% by quenching, while 1.0 µM NE only slightly influences the fluorescence intensity

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of SiNPs (less than 3%). The concentration of NE in a living system is less than 300 pg/mL (~1.8 nM),51 which is less than one-tenth of that of DA (26 to 40 nM).18,19 Thus we believe that NE would have little influence on the DA detection in a living system.

Figure 3. Fluorescence responses of APTMS SiNPs with different guest molecules. The concentrations of all analytes were 10.0 µM except for NE (10.0 µM and 1.0 µM).

We have tested the fluorescence intensities of SiNPs in solutions containing DA and other neurotransmitters (NE, 5-HT, ATP, ADP, Gly, and Glu) to further confirm the selectivity. Solutions with 10.0 mM 5-HT, 10.0 mM ATP, 10.0 mM ADP, 10.0 mM Gly, and 10.0 mM Glu at pH = 7.4 along with DA (0.10, 1.0, and 10.0 µM) and NE (0.010, 0.10, and 1.0 µM) were investigated. The determined DA concentrations were determined from the above presented calibration curve (Figure 2D, insert) and the results are summarized in Table 1. Table 1 shows that the obtained results are in good agreement with the DA concentrations in the solutions. Furthermore, we demonstrated that the fluorescence emission of SiNPs does not vary substantially as a function of temperature (Figure S1) and SiNPs have low

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cytotoxicity (Figure S2). All these results illustrated the feasibility of using SiNPs to detect DA in complex sample environments (e.g., in vivo ).

Table 1. Determination of DA in the mixed solutions containing 10.0 mM 5-HT, 10.0 mM ATP, 10.0 mM ADP, 10.0 mM Gly, 10.0 mM Glu and NE (0.010 µM in Sample 1, 0.10 µM in Sample 2, and 1.0 µM in Sample 3) using APTMS SiNPs. Sample

Standard (µM)

Measured (µM)

1 2 3

0.10 1.0 10.0

0.090 0.92 9.87

Possible mechanism of the quenching. As previously reported, DA and NE were easily oxidized to quinone by ambient O2 in basic solutions.16,17,50 Here DA quinone and NE quinone were prepared by the addition of a basic solution to the DA and NE solution. The UV-Vis absorption spectra have been measured (Figure 4). DA and NE only have a strong peak around 280 nm. However, DA quinone has another peak centered around 450 nm, which is very close to the emission peak of SiNPs (444 nm). In contrast, the absorbance intensity of NE quinone around 450 nm is much weaker than that of DA quinone at the same concentration. So the quenching efficiency of DA quinone is much higher than that of NE quinone, due to the Förster resonance energy transfer (FRET) effect from the donor (SiNPs) to the acceptor (DA quinone). When a DA solution sample was tested, the DA molecules would oxidize in air-containing solution and be adsorbed onto SiNP surfaces due to the strong noncovalent interactions (such as electrostatic interaction and hydrogen bonding formation). The short distance between the

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donor (SiNP) and the acceptor (DA quinone), together with the overlap of the emission spectrum of SiNPs and the absorption spectrum of DA quinone, fulfill the requirements of the FRET process. During the FRET process, the energy transferred from the photo-excited SiNPs to the oxidized dopamine quinone molecules, resulting in the fluorescence quenching of SiNPs.

Figure 4. UV-Vis absorption spectra of DA, DA quinone, NE, and NE quinone at the same concentration of 125 µM.

As previously reported, since SiNPs themselves have superior photostability,48 the time course of the fluorescence intensity change after the addition of DA can be measured to study the effect of DA. Here, as shown in Figure 5, the fluorescence intensity of SiNPs after adding 10.0 µM DA to the solution decreased to 5.4% after 3 h and became stable thereafter. Thus, all the fluorescence spectra exhibited in this work were collected after incubation of SiNPs and DA for at least 3 h. Besides, the fluorescence intensity of SiNPs decreased gradually, which implied the gradual occurrence of the oxidation of DA.

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Figure 5. Time course of the fluorescence intensity change of APTMS SiNPs after adding DA to the SiNP solution to reach a final DA concentration of 10.0 µM.

To better understand the quenching mechanism, the time-dependent SiNP fluorescence decay induced by DA was investigated by time-resolved fluorescence spectroscopy (Figure 6). In general, the fluorescence decay curve can be fitted by an exponential function: I(t) = ΣAiexp(−t/τi) where τi represents the decay time of the fluorescence emission; Ai represents the relative weights of the decay components at t = 0. The fluorescence decay curve of the SiNPs alone can be fitted by a double exponential function, with τ1 of 4.8 ns and τ2 of 10.3 ns, respectively. Such a double exponential fitting result is similar to that reported in a previous work.36 The decreased PL lifetimes (τ1 from 4.8 to 2.7 ns, τ2 from 10.3 to 7.2 ns) after the introduction of DA implied that dopamine quinone adsorbed onto the surface of SiNPs probably acted as an energy acceptor to quench the fluorescence of SiNPs.

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Figure 6. Time-resolved fluorescence decay curves of (A) APTMS SiNPs and (B) APTMS SiNPs + DA.

4. CONCLUSION Highly fluorescent water-soluble SiNPs were synthesized in a simple, one-pot method and used as a fluorescent probe for DA detection. It was found that the fluorescence of SiNPs can be linearly quenched by DA in a broad DA concentration range from 0.005 to 10.0 µM. The detection of DA by SiNPs shows a very low LOD of 0.3 nM, which makes it the most sensitive DA detection method ever reported. In addition, it was found that other common molecules in the living system did not interfere with the DA detection, making it possible to use SiNPs for DA detection in a living system. To summarize, the present work shows a simple, low-cost, highly sensitive, and selective detection method for DA.

ASSOCIATED CONTENT Supporting Information Effect of temperature on fluorescence intensity of SiNPs and MTT assay results. This material is available free of charge via the Internet at http://pubs.acs.org. 16


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AUTHOR INFORMATION Corresponding Authors *Fu-Gen Wu, E-mail: [email protected]; *Zhan Chen, E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (21303017), the Natural Science Foundation of Jiangsu Province (KB20130601), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities. ZC thanks the University of Michigan to support his sabbatical. We thank Prof. Zheng-Jian Qi (School of Chemistry and Chemical Engineering, Southeast University) for her help on the time-resolved fluorescence experiments.

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Highly Sensitive and Selective Detection of Dopamine Using One-Pot

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