- Email: [email protected]
Aptamer-based colorimetric biosensing of dopamine using unmodified gold nanoparticles
Sensors and Actuators B 156 (2011) 95–99
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Aptamer-based colorimetric biosensing of dopamine using unmodified gold nanoparticles Yu Zheng a,b , Yong Wang a,b , Xiurong Yang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 21 March 2011 Accepted 30 March 2011 Available online 8 April 2011 Keywords: Dopamine-binding aptamer Colorimetry Gold nanoparticles Dopamine Neurotransmitter
a b s t r a c t A simple, sensitive and selective colorimetric biosensor for the detection of dopamine (DA) was demonstrated with a 58-mer dopamine-binding aptamer (DBA) as recognition element and unmodified gold nanoparticles (AuNPs) as probes. Upon the addition of DA, the conformation of DBA would change from a random coil structure to a rigid tertiary structure like a pocket and this change has been demonstrated by circular dichroism spectroscopic experiments. Besides, the conformational change of DBA could facilitate salt-induced AuNP aggregation and lead to the color change of AuNPs from red to blue. The calibration modeling showed that the analytical linear range covered from 5.4 × 10−7 M to 5.4 × 10−6 M and the corresponding limit of detection (LOD) was 3.6 × 10−7 M. Some common interferents such as 3,4dihydroxyphenylalanine (DOPA), catechol, epinephrine (EP), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and ascorbic acid (AA) showed no or just a little interference in the determination of DA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the researches on brain science have become more and more attractive, among which neurotransmitters are widely studied and involve lots of neuropathies. Dopamine (DA) is a member of the catecholamine neurotransmitter family with a variety of functions in the central nervous system. It affects the brain’s control of learning, feeding and neurocognition. In addition, many addictive behaviors are related to dopamine [1]. Thus, disorders in DA levels will bring about a series of neural diseases such as Parkinson’s disease and Alzheimer’s disease. In order to detect dopamine precisely and to study the mechanisms of dopaminerelated diseases, a sensitive and selective analytical method is quite essential. Very recently, a variety of analytical methods have been reported for the determination of DA, such as electrochemistry [2–4], high performance liquid chromatography (HPLC) [5,6] and chemiluminescence [7,8]. However, almost all these methods require sophisticated equipment and/or time-consuming procedures. Thus, a simple, cost-effective and fast alternative without using complex instruments is highly required. Colorimetry has commonly been used for routine analysis since it does not need any special instruments and can be directly observed by the naked eyes. There have been some reports about
∗ Corresponding author. Tel.: +86 431 85262056; fax: +86 431 85689278. E-mail address: [email protected] (X. Yang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.03.077
colorimetric detection of dopamine [9–11]. However, they were limited either with low sensitivity and selectivity or with complicated procedures. To conquer these difficulties, we turned to the colorimetric detection based on unmodified gold nanoparticles (AuNPs) as reporting probes and aptamers as recognition elements and numerous reports have described its application for detecting proteins [12,13], metal ions [14,15] and small molecules [16,17]. The popularity of this aptamer-based colorimetry using unmodified AuNPs was due to two aspects. On the one hand, AuNPs were attractive as colorimetric reporters owing to their simplicity, high extinction coefficients (≥3 orders of magnitude times larger than those of organic dyes) and strongly distance-dependent optical properties [18]. The dispersed AuNP solution is red while the aggregated AuNP solution is blue [19]. On the other hand, aptamers were functional single-stranded oligonucleotides (ssDNA or ssRNA), which were selected in vitro by the systematic evolution of the ligand by exponential enrichment (SELEX) process [20]. They exhibited specific binding ability to target molecules which resulted in their own conformation changing from random coil structures to rigid tertiary structures like hairpin or G-quadruplex [21–23]. SsDNA with random coil structures could adsorb onto the AuNPs and protected AuNPs from salt-induced aggregation while ssDNA with rigid tertiary structures could not react with AuNPs and lost the ability to protect AuNPs. The most important thing for the application of this aptamerbased colorimetry using unmodified AuNPs to detect dopamine was to develop an aptamer specific for dopamine. A RNA aptamer that can specifically bind to DA was selected in vitro from a pool of
96
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
3.4 × 1014 different RNA molecules [24]. The dissociation constant of this RNA aptamer with DA in solution was estimated by equilibrium filtration to be 1.6 M. Upon binding to DA, the RNA aptamer formed a stable framework like a DA pocket with two stems and two loops. However, the RNA aptamer could not be replicated by a deoxy version of the same sequence. Up to date, the DNA homolog of the RNA aptamer was synthesized and demonstrated to retain the function and specificity of the RNA aptamer [25], besides, the DNA aptamer had an affinity to DA higher than the RNA aptamer, with a dissociation constant of 0.7 M. Hence, in this work, we gave the first demonstration of the colorimetric detection of DA using dopamine-binding DNA aptamer (DBA) as recognition element and unmodified gold nanoparticles as probes. The random coil DBA could be adsorbed onto the AuNPs and protect the AuNPs from salt-induced aggregation. However, in the presence of DA, DBA would undergo a conformation variation from a random coil structure to a stem–loop structure [24], and this tertiary structure could not protect the AuNPs effectively, thus resulting in salt-induced aggregation. 2. Materials and methods 2.1. Reagents and materials Hydrogen tetrachloroaurate(III) tetrahydrate, dopamine, 3,4dihydroxyphenylacetic acid, 3,4-dihydroxyphenylalanine, and homovanillic acid were purchased from Acros Organics (NJ, USA). The 58-mer DBA oligonucleotide (5 -GTC TCT GTG TGC GCC AGA GAA CAC TGG GGC AGA TAT GGG CCA GCA CAG AAT GAG GCC C3 ) was synthesized by Sangon Biotechnology Co. Ltd. (Shanghai, China). Before use, the DBA was dissolved in 100 mM PBS buffer solution (pH 7.0). The concentration of the DBA was determined by measuring the UV absorbance at 260 nm. All other reagents were of analytical reagent grade and ultra-pure water (Milli-Q plus, Millipore Inc., Bedford, MA) was used throughout. 2.2. The synthesis of citrate-protected AuNP solution Fifty milliliters aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (1 mM) was heated to boiling while being stirred in a round-bottom flask with a reflux condenser. Then 10 mL of trisodium citrate (38.8 mM) was added into the solution rapidly and the solution was boiled for another 10 min with the color of the solution changing from yellow to wine red. After that, heating the solution was stopped but was stirred until it cooled down to room temperature [26], then stored the AuNP solution in refrigerator at 4 ◦ C. The diameter of the AuNPs synthesized was about 13 nm. According to Beer’s law by using the extinction coefficient (2.7 × 108 M−1 cm−1 ) at 520 nm [27], the concentration of the AuNP solution was calculated to be about 11 nM. 2.3. Instrumentation The recordings of UV–vis absorption spectra and kinetic measurement were conducted on a Cary50 Scan UV–vis spectrometer (Varian, USA) with a 10 mm path length fused-silica cuvette. Circular dichroism spectral measurement was performed on a Jasco J-820 Circular Dichroism Spectropolarimeter (Tokyo, Japan). 2.4. General procedure of colorimetric biosensing of dopamine Two hundred and forty three microliters of DA of different concentrations were respectively added into a 1.5 mL plastic vial containing 6 L DBA solution (10 M). After incubating for 10 min, 135 L of the AuNP solution (11 nM) was added. Reacting for 5 min,
Fig. 1. Mechanism of the colorimetric detection of DA utilizing DBA and unmodified AuNPs.
then 72 L of NaCl (0.25 M) solution was transferred quickly into this vial. After another 5 min of incubation, 300 L of the resulting solution was transferred to a 1 cm path length quartz cuvette for spectral recording. All assays were performed at room temperature. 2.5. Circular dichroism (CD) measurement for DBA Four hundred microliters of 100 mM PBS buffer solution (pH 7.0) containing 1.25 M DBA and different concentrations of DA (0–87.5 M) was respectively added into a cell made of quartz suprasil (10 mm path length, 0.7 mL volume). The CD spectra were measured over the wavelength range from 210 to 350 nm and the scanning speed was 200 nm per minute. 3. Results and discussion 3.1. Principle of the colorimetric detection In this work, the synthesized AuNP solution was stabilized by the citrate anions as their repulsion prevented the AuNPs from aggregating. When added with a high concentration of salt such as NaCl solution, NaCl would neutralize the negative charge of citrate and lead to the AuNP aggregation. However, it has been reported [28,29] that ssDNA with a random coil structure could be adsorbed onto the surface of AuNPs through the coordination interaction between the nitrogen atoms of the exposed bases and the AuNPs, thus added negative charges to the AuNPs, increased their repulsion and enhanced the stability of AuNPs against the NaCl-induced aggregation. Whereas, compared with the random coil ssDNA, the bases of ssDNA with rigid tertiary structures or double-stranded DNA (dsDNA) were not exposed outside and could not be adsorbed onto the AuNPs, hence lost the ability of protecting the AuNPs from NaCl-induced aggregation. Based on the above mentioned, the mechanism of the DA biosensor is shown in Fig. 1. As can be noted in Fig. 1, DA could specifically bind to the dopamine-binding aptamer (DBA) and induced the DBA conformation switching from a random flexible structure to a rigid stem–loop structure. The DBA with a random coil structure could wrap the unmodified AuNPs up while the DBA with rigid tertiary structures could not. After adding a high concentration of NaCl, the AuNPs in the presence of DBA would remain dispersed and the AuNPs with both DBA and DA would aggregate. Based on this rationale, an aptamer-based colorimetric biosensing strategy can be well developed for the detection of dopamine. 3.2. DBA structures characterized by CD CD in the UV region was regarded as the most appropriate technique to distinguish different DNA structures [30]. It was reported
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
97
Fig. 2. CD spectra of 1.25 M DBA in 100 mM PBS buffer (pH 7.0) containing different concentrations of DA (0.00–87.50 M). (a) 0.00 M DA, (b) 8.75 M DA, (c) 25.00 M DA, and (d) 87.50 M DA.
[31] that a hairpin structure possessed a CD spectrum characterized by a positive ellipticity maximum around 280 nm and a negative minimum around 248 nm. In this work, the CD spectra of DBA under different experimental conditions are depicted in Fig. 2. In the absence of DA, there was a positive ellipticity maximum around 280 nm and a negative minimum around 248 nm, indicating that stem–loop structures existed in the unbound DBA solution. However, upon the addition of DA, the positive ellipticity maximum around 280 nm increased greatly, suggesting that more stem–loop structures came into being. Thus, we could suppose that DA possessed the ability of inducing the conformation change of DBA from random coil to stem–loop structure. Indeed, it has been reported [22] that there is a mixture of aptamers of both random coil structures and rigid tertiary structures before binding to target molecules and there is an equilibrium between the two conformations. The existing stem–loop structure in the unbound DBA would affect the sensitivity of the determination of dopamine and it was hard to clear this structure out. Thus, proper concentration of DBA should be chosen to eliminate this influence which has been stated in the following part. 3.3. Spectral characteristics To further prove this concept, a series of UV–vis spectra of AuNPs were measured under different experimental conditions. As was shown in Fig. 3A, in the presence of 40 mM NaCl, the surface plasmon resonance (SPR) absorption band of AuNPs red shifted and broadened. The peak at 520 nm (representing the dispersed AuNPs) decreased and a new peak at about 700 nm (representing the aggregated AuNPs) appeared, indicating that the AuNPs aggregated. However, upon the addition of DBA, it could be noted in Fig. 3A that there was only one peak at about 520 nm, implying that the AuNPs were still dispersed. This was because DBA with a random coil structure was able to be adsorbed onto the AuNPs and protected AuNPs from NaCl-induced aggregation. Then, upon the addition of both DBA and DA, it could be clearly noticed in Fig. 3A that the SPR adsorption band of AuNPs red shifted with a new peak appearing at about 660 nm, besides, the absorption at 660 nm increased with the increase of the concentration of DA (Fig. 3A), indicating the aggregation of AuNPs. Fig. 3B displays a visible color change corresponding to Fig. 3A. From Fig. 3B, it could be noticed that the color of the AuNP solution changed from red to blue after being added with NaCl. Whereas, upon the addition of DBA, Fig. 3B shows that the solution was still red, and when added with both DBA and DA, the solution was purple or blue. Furthermore, Fig. 3C
Fig. 3. (A) UV–vis absorption spectra of AuNPs in the presence of 40 mM NaCl under different experimental conditions, cDBA = 133 nM, cAuNPs = 3.3 nM. (B) Visual color changes corresponding to (A). (C) TEM images of 3.3 nM AuNP solution mixed with 133 nM DBA 5 min after the addition of 40 mM NaCl. (D) TEM images of 3.3 nM AuNP solution in the presence of 133 nM DBA and 3.6 M DA 5 min after the addition of 40 mM NaCl.
and D utilized the Transmission Electron Microscope (TEM) technology to characterize the morphology change of AuNPs. Fig. 3C clearly showed that the AuNPs was still dispersed after the addition of NaCl in the presence of DBA. Fig. 3D shows that the AuNPs aggregated after the addition of NaCl in the presence of both DA and DBA. All these results were in good agreement with our assumption, implying that DA induced the conformation of DBA changing from a random coil structure to a stem–loop structure. 3.4. Optimization of experimental conditions In this work, the net absorption ratio between 660 nm and 524 nm, (A660 /A524 ) ((A660 /A524 ) = the absorption ratio A660 /A524 in the presence of DA − the absorption ratio A660 /A524 in the absence of DA), was used to optimize the experimental conditions. The (A660 /A524 ) value was mainly influenced by the concentrations of DBA, the concentration of NaCl, the binding time of DBA and DA, the incubation time after adding AuNPs, the reaction time after adding NaCl and the size of AuNPs. Firstly, the effect of the concentration of NaCl was studied in the concentration range of 0–80 mM. The results showed that (A660 /A524 ) arrived at the maximum value when the concentration of NaCl was 40 mM. Thus, 40 mM was chosen for this work on basis of higher sensitivity. Secondly, the effect of the concentration of DBA was studied in the concentration range of 0–400 nM, the (A660 /A524 ) reached the maximum value when the concentration of DBA was 133 nM. Hence, the concentration of DBA was selected to be 133 nM for this experiment. Thirdly, the effect of
98
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
Fig. 4. (A) UV–vis absorption spectra of AuNPs in the presence of 40 mM NaCl under different concentrations of DA. (B) Derived calibration curve of the adsorption ratio (A660 /A524 ) versus log(concentration of DA). Experimental conditions are: cNaCl = 40 mM, cDBA = 133 nM, cAuNPs = 3.3 nM.
Fig. 5. (A) UV–vis absorption spectra of AuNPs in the presence of 133 nM DBA and 3.6 M dopamine analogs and its metabolite after the addition of 40 mM NaCl, cAuNPs = 3.3 nM. (B) The absorption ratio value A660 /A524 of dopamine analogs and its metabolites. Experimental conditions are as in (A).
binding time of dopamine-aptamer over the range of 0–15 min was investigated, and (A660 /A524 ) increased until the binding time was 10 min. Thus, 10 min was chosen for the following experiment. Subsequently, the incubation time after adding AuNPs was studied over the range of 0–15 min and the results showed that the interaction could be completed within 5 min. Thus, we selected 5 min as the incubation time in this work. Then, the reaction time after adding NaCl over the range of 0–30 min was investigated. It was found that the (A660 /A524 ) increased substantially as the reaction time increased up to 5 min. Therefore, 5 min was chosen as an optimum reaction time. Finally, three different sizes of AuNPs (13 nm, 25 nm and 38 nm) on the effect of the detection of dopamine were examined. The results showed that the (A660 /A524 ) for 13 nm, 25 nm and 38 nm AuNP solution was 0.22, 0.06, −0.05, respectively, under the similar experimental condition. Thus, we used 13 nm AuNPs as the following experiment condition on basis of higher sensitivity.
3.6. Selectivity The selectivity of this colorimetric biosensor to DA was evaluated by measuring the absorption ratio value, A660 /A524 to some common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), epinephrine (EP) and ascorbic acid (AA). As can be observed in Fig. 5A, upon the addition of DA, there was a noticeable change in UV–vis adsorption spectra, while no or just a little spectral change in the absence (blank) or presence of the common interferents occurred. The data derived from Fig. 5A showed that the adsorption ratio value, A660 /A524 , in the presence of DA was considerably larger than those of blank or the common interferents (Fig. 5B). All results indicate that our assay approach has a high specificity to DA. 4. Conclusion
3.5. Calibration modeling for DA Under the above-mentioned optimized experimental conditions, a series of different concentrations of DA was respectively added and their UV–vis spectra were recorded (Fig. 4A). Fig. 4B depicts the derived calibration curves corresponding to Fig. 4A. As can be seen in Fig. 4A and B, the absorption ratio, A660 /A524 , increased proportionally with the concentration of DA in the range of 0.54–5.4 M. It has been reported [32] that the dynamic region of the analyte-induced AuNP aggregation was generally quite narrow. The linear equation could be fitted as A660 /A524 = 0.64 + 0.83 log c (DA, M) (R2 = 0.999) with a limit detection (3) of 0.36 M. Compared to other reported colorimetric methods [9,33], this method has a better sensitivity towards DA.
This work investigated the interaction of dopamine-bindingaptamer and dopamine using unmodified citrate-coated AuNPs as colorimetric signal readout for the first time. In the presence of DA, DBA had a conformation change from a random coil structure to a rigid hairpin structure which has been proved by CD spectroscopic data. The structural change made dopamine-binding-aptamer lose the ability of stabilizing the AuNPs, thus facilitated the NaClinduced AuNP aggregation. This method showed a good sensitivity and selectivity towards DA and avoided the interference from common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), epinephrine (EP) and ascorbic acid (AA). Thus, supposing that aptamers specific for neurotransmitters besides dopamine were selected, this colorimetic method would maybe provide a new
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
possibility for the detection of neurotransmitters and this is being investigated. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20890022), the National Key Basic Research Development Project of China (Nos. 2010CB933602 and 2007CB714500) and the Project of Chinese Academy of Sciences (No. KJCX2-YW-H09). References [1] M. Perry, Q. Li, R.T. Kennedy, Review of recent advances in analytical techniques for the determination of neurotransmitters, Anal. Chim. Acta 653 (2009) 1–22. [2] Y. Wang, Y.M. Li, L.H. Tang, J. Lu, J.H. Li, Application of graphene-modified electrode for selective detection of dopamine, Electrochem. Commun. 11 (2009) 889–892. [3] M.H. Pournaghi-Azar, H. Dastangoo, R. Fadakar bajeh baj, Simultaneous determination of dopamine and its oxidized product (aminochrom), by hydrodynamic amperometry and anodic stripping voltammetry, using the metallic palladium and uranalyl hexacyanoferrate coated aluminum electrodes, Biosens. Bioelectron. 25 (2010) 1481–1486. [4] Y.R. Kim, S. Bong, Y.J. Kang, Y. Yang, R.K. Mahajan, J.S. Kim, H. Kim, Electrochemical detection of dopamine in the presence of ascorbic acid using graphene modified electrodes, Biosens. Bioelectron. 25 (2010) 2366–2369. [5] N. Li, J.Z. Guo, B. Liu, H. Cui, L.Q. Mao, Y.Q. Lin, Determination of monoamine neurotransmitters and their metabolites in a mouse brain microdialysate by coupling high-performance liquid chromatography with gold nanoparticleinitiated chemiluminescence, Anal. Chim. Acta 645 (2009) 48–55. [6] V. Carrera, E. Sabater, E. Vilanova, M.A. Sogorb, A simple and rapid HPLC–MS method for the simultaneous determination of epinephrine, norepinephrine, dopamine and 5-hydroxytryptamine: application to the secretion of bovine chromaffin cell cultures, J. Chromatogr. B 847 (2007) 88–94. [7] S.L. Zhao, Y. Huang, M. Shi, R.J. Liu, Y.M. Liu, Chemiluminescence resonance energy transfer-based detection for microchip electrophoresis, Anal. Chem. 82 (2010) 2036–2041. [8] Y.C. Chen, W.Y. Lin, Enhancement of chemiluminescence of the KIO4 -luminol system by gallic acid, acetaldehyde and Mn2+ : application for the determination of catecholamines, Luminescence 25 (2010) 43–49. [9] R. Baron, M. Zayats, I. Willner, Dopamine-, l-DOPA-, adrenaline-, and noradrenaline-induced growth of Au nanoparticles: assays for the detection of neurotransmitters and of tyrosinase activity, Anal. Chem. 77 (2005) 1566–1571. [10] Z.J. Lin, X.M. Chen, T.T. Jia, X.D. Wang, Z.X. Xie, M. Oyama, X. Chen, Fabrication of a colorimetric electrochemiluminescence sensor, Anal. Chem. 81 (2009) 830–833. [11] Y.F. Zhang, B.X. Li, X.L. Chen, Simple and sensitive detection of dopamine in the presence of high concentration of ascorbic acid using gold nanoparticles as colorimetric probes, Microchim. Acta 168 (2010) 107–113. [12] C. Huang, Y. Huang, Z. Cao, W. Tan, H. Chang, Aptamer-modified gold nanoparticles for colorimetric determination of platelet-derived growth factors and their receptors, Anal. Chem. 77 (2005) 5735–5741. [13] Z.X. Zhang, Z.J. Wang, X.L. Wang, X.R. Yang, Magnetic nanoparticle-linked colorimetric aptasensor for the detection of thrombin, Sens. Actuators B 147 (2010) 428–433. [14] Y. Wang, F. Yang, X.R. Yang, Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer, Biosens. Bioelectron. 25 (2010) 1994–1998. [15] T. Li, S.J. Dong, E.K. Wang, Label-free colorimetric detection of aqueous mercury ion (Hg2+ ) using Hg2+ -modulated G-quadruplex-based DNAzymes, Anal. Chem. 82 (2010) 1515–1520. [16] X.W. Xu, J. Wang, F. Yang, K. Jiao, X.R. Yang, Label-free colorimetric detection of small molecules utilizing DNA oligonucleotides and silver nanoparticles, Small 23 (2009) 2669–2672.
99
[17] F. Li, J. Zhang, X.N. Cao, L.H. Wang, D. Li, S.P. Song, B.C. Ye, C.H. Fan, Adenosine detection by using gold nanoparticles and designed aptamer sequences, Analyst 134 (2009) 1355–1360. [18] N.L. Rosi, C.A. Mirkin, Nanostructures in biodiagnostics, Chem. Rev. 105 (2005) 1547–1562. [19] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, A DNA-based method for rationally assembling nanoparticles into macroscopic materials, Nature 382 (1996) 607–609. [20] S. Tombelli, M. Minunni, M. Mascini, Analytical applications of aptamers, Biosens. Bioelectron. 20 (2005) 2424–2434. [21] Z.S. Wu, H.X. Lu, X.P. Liu, R. Hu, H. Zhou, G.L. Shen, R.Q. Yu, Inhibitory effect of target binding on hairpin aptamer sticky-end pairing-induced gold nanoparticle assembly for light-up colorimetric protein assay, Anal. Chem. 82 (2010) 3890–3898. [22] H. Wei, B.L. Li, J. Li, E.K. Wang, S.J. Dong, Simple and sensitive aptamer-based colorimetric sensing of protein using unmodified gold nanoparticle probes, Chem. Commun. (2007) 3735–3737. [23] H.X. Li, L. Rothberg, Colorimetric detection of DNA sequences based on electrostatic interactions with unmodified gold nanoparticles, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 14036–14039. [24] C. Mannirono, A.D. Nardo, P. Fruscoloni, G.P. Tocchini-Valentini, In vitro selection of dopamine RNA ligands, Biochemistry 36 (1997) 9726–9734. [25] R. Walsh, M.C. DeRosa, Retention of function in the DNA homolog of the RNA dopamine aptamer, Biochem. Biophys. Res. Commun. 388 (2009) 732–735. [26] K.C. Grabar, R.G. Freeman, M.B. Hommer, M.J. Natan, Preparation and characterization of Au colloid monolayers, Anal. Chem. 67 (1995) 735–743. [27] M.M. Maye, L. Han, N.N. Kariuki, N.K. Ly, W.B. Chan, J. Luo, C.J. Zhong, Gold and alloy nanoparticles in solution and thin film assembly: spectrophotometric determination of molar absorptivity, Anal. Chim. Acta 496 (2003) 17–27. [28] H.X. Li, L. Rothberg, Label-free colorimetric detection of specific sequences in genomic DNA amplified by the polymerase chain reaction, J. Am. Chem. Soc. 126 (2004) 10958–10961. [29] H.X. Li, L. Rothberg, DNA sequence detection using selective fluorescence quenching of tagged oligonucleotide probes by gold nanoparticles, Anal. Chem. 76 (2004) 5414–5417. [30] G.D. Fasman, Circular Dichroism and the Conformational Analysis of Biomolecules, Plenum Press, New York, NY, USA, 1996. [31] M. Nakano, E.M. Moody, J. Liang, P.C. Bevilacqua, Selection for thermodynamically stable DNA tetraloops using temperature gradient gel electrophoresis reveals four motifs: d(cGNNAg), d(cGNABg), d(cCNNGg), and d(gCNNGc), Biochemistry 41 (2002) 14281–14292. [32] C. Lu, Y.B. Zu, V. Wing-Wah Yam, Specific postcolumn detection method for HPLC assay of homocysteine based on aggregation of fluorosurfactant-capped gold nanoparticles, Anal. Chem. 79 (2007) 666–672. [33] M. Reza Hormozi Nezhad, J. Tashkhourian, J. Khodaveisic, M. Reza Khoshi, Simultaneous colorimetric determination of dopamine and ascorbic acid based on the surface plasmon resonance band of colloidal silver nanoparticles using artificial neural networks, Anal. Methods 2 (2010) 1263–1269.
Biographies Yu Zheng obtained her bachelor’s degree in 2009, University of Science and Technology of China, and she has been a graduate student of the State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science. Her research interests focus mainly on the field of biosensor. Yong Wang has been a PhD candidate since 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Science. His current research is biosensor. Xiurong Yang is a professor of the State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science. Her current research includes study of biomolecules interaction, fabrication of biosensors, and synthesis of functional nanomaterials.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Aptamer-based colorimetric biosensing of dopamine using unmodified gold nanoparticles Yu Zheng a,b , Yong Wang a,b , Xiurong Yang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 21 March 2011 Accepted 30 March 2011 Available online 8 April 2011 Keywords: Dopamine-binding aptamer Colorimetry Gold nanoparticles Dopamine Neurotransmitter
a b s t r a c t A simple, sensitive and selective colorimetric biosensor for the detection of dopamine (DA) was demonstrated with a 58-mer dopamine-binding aptamer (DBA) as recognition element and unmodified gold nanoparticles (AuNPs) as probes. Upon the addition of DA, the conformation of DBA would change from a random coil structure to a rigid tertiary structure like a pocket and this change has been demonstrated by circular dichroism spectroscopic experiments. Besides, the conformational change of DBA could facilitate salt-induced AuNP aggregation and lead to the color change of AuNPs from red to blue. The calibration modeling showed that the analytical linear range covered from 5.4 × 10−7 M to 5.4 × 10−6 M and the corresponding limit of detection (LOD) was 3.6 × 10−7 M. Some common interferents such as 3,4dihydroxyphenylalanine (DOPA), catechol, epinephrine (EP), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and ascorbic acid (AA) showed no or just a little interference in the determination of DA. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the researches on brain science have become more and more attractive, among which neurotransmitters are widely studied and involve lots of neuropathies. Dopamine (DA) is a member of the catecholamine neurotransmitter family with a variety of functions in the central nervous system. It affects the brain’s control of learning, feeding and neurocognition. In addition, many addictive behaviors are related to dopamine [1]. Thus, disorders in DA levels will bring about a series of neural diseases such as Parkinson’s disease and Alzheimer’s disease. In order to detect dopamine precisely and to study the mechanisms of dopaminerelated diseases, a sensitive and selective analytical method is quite essential. Very recently, a variety of analytical methods have been reported for the determination of DA, such as electrochemistry [2–4], high performance liquid chromatography (HPLC) [5,6] and chemiluminescence [7,8]. However, almost all these methods require sophisticated equipment and/or time-consuming procedures. Thus, a simple, cost-effective and fast alternative without using complex instruments is highly required. Colorimetry has commonly been used for routine analysis since it does not need any special instruments and can be directly observed by the naked eyes. There have been some reports about
∗ Corresponding author. Tel.: +86 431 85262056; fax: +86 431 85689278. E-mail address: [email protected] (X. Yang). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.03.077
colorimetric detection of dopamine [9–11]. However, they were limited either with low sensitivity and selectivity or with complicated procedures. To conquer these difficulties, we turned to the colorimetric detection based on unmodified gold nanoparticles (AuNPs) as reporting probes and aptamers as recognition elements and numerous reports have described its application for detecting proteins [12,13], metal ions [14,15] and small molecules [16,17]. The popularity of this aptamer-based colorimetry using unmodified AuNPs was due to two aspects. On the one hand, AuNPs were attractive as colorimetric reporters owing to their simplicity, high extinction coefficients (≥3 orders of magnitude times larger than those of organic dyes) and strongly distance-dependent optical properties [18]. The dispersed AuNP solution is red while the aggregated AuNP solution is blue [19]. On the other hand, aptamers were functional single-stranded oligonucleotides (ssDNA or ssRNA), which were selected in vitro by the systematic evolution of the ligand by exponential enrichment (SELEX) process [20]. They exhibited specific binding ability to target molecules which resulted in their own conformation changing from random coil structures to rigid tertiary structures like hairpin or G-quadruplex [21–23]. SsDNA with random coil structures could adsorb onto the AuNPs and protected AuNPs from salt-induced aggregation while ssDNA with rigid tertiary structures could not react with AuNPs and lost the ability to protect AuNPs. The most important thing for the application of this aptamerbased colorimetry using unmodified AuNPs to detect dopamine was to develop an aptamer specific for dopamine. A RNA aptamer that can specifically bind to DA was selected in vitro from a pool of
96
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
3.4 × 1014 different RNA molecules [24]. The dissociation constant of this RNA aptamer with DA in solution was estimated by equilibrium filtration to be 1.6 M. Upon binding to DA, the RNA aptamer formed a stable framework like a DA pocket with two stems and two loops. However, the RNA aptamer could not be replicated by a deoxy version of the same sequence. Up to date, the DNA homolog of the RNA aptamer was synthesized and demonstrated to retain the function and specificity of the RNA aptamer [25], besides, the DNA aptamer had an affinity to DA higher than the RNA aptamer, with a dissociation constant of 0.7 M. Hence, in this work, we gave the first demonstration of the colorimetric detection of DA using dopamine-binding DNA aptamer (DBA) as recognition element and unmodified gold nanoparticles as probes. The random coil DBA could be adsorbed onto the AuNPs and protect the AuNPs from salt-induced aggregation. However, in the presence of DA, DBA would undergo a conformation variation from a random coil structure to a stem–loop structure [24], and this tertiary structure could not protect the AuNPs effectively, thus resulting in salt-induced aggregation. 2. Materials and methods 2.1. Reagents and materials Hydrogen tetrachloroaurate(III) tetrahydrate, dopamine, 3,4dihydroxyphenylacetic acid, 3,4-dihydroxyphenylalanine, and homovanillic acid were purchased from Acros Organics (NJ, USA). The 58-mer DBA oligonucleotide (5 -GTC TCT GTG TGC GCC AGA GAA CAC TGG GGC AGA TAT GGG CCA GCA CAG AAT GAG GCC C3 ) was synthesized by Sangon Biotechnology Co. Ltd. (Shanghai, China). Before use, the DBA was dissolved in 100 mM PBS buffer solution (pH 7.0). The concentration of the DBA was determined by measuring the UV absorbance at 260 nm. All other reagents were of analytical reagent grade and ultra-pure water (Milli-Q plus, Millipore Inc., Bedford, MA) was used throughout. 2.2. The synthesis of citrate-protected AuNP solution Fifty milliliters aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (1 mM) was heated to boiling while being stirred in a round-bottom flask with a reflux condenser. Then 10 mL of trisodium citrate (38.8 mM) was added into the solution rapidly and the solution was boiled for another 10 min with the color of the solution changing from yellow to wine red. After that, heating the solution was stopped but was stirred until it cooled down to room temperature [26], then stored the AuNP solution in refrigerator at 4 ◦ C. The diameter of the AuNPs synthesized was about 13 nm. According to Beer’s law by using the extinction coefficient (2.7 × 108 M−1 cm−1 ) at 520 nm [27], the concentration of the AuNP solution was calculated to be about 11 nM. 2.3. Instrumentation The recordings of UV–vis absorption spectra and kinetic measurement were conducted on a Cary50 Scan UV–vis spectrometer (Varian, USA) with a 10 mm path length fused-silica cuvette. Circular dichroism spectral measurement was performed on a Jasco J-820 Circular Dichroism Spectropolarimeter (Tokyo, Japan). 2.4. General procedure of colorimetric biosensing of dopamine Two hundred and forty three microliters of DA of different concentrations were respectively added into a 1.5 mL plastic vial containing 6 L DBA solution (10 M). After incubating for 10 min, 135 L of the AuNP solution (11 nM) was added. Reacting for 5 min,
Fig. 1. Mechanism of the colorimetric detection of DA utilizing DBA and unmodified AuNPs.
then 72 L of NaCl (0.25 M) solution was transferred quickly into this vial. After another 5 min of incubation, 300 L of the resulting solution was transferred to a 1 cm path length quartz cuvette for spectral recording. All assays were performed at room temperature. 2.5. Circular dichroism (CD) measurement for DBA Four hundred microliters of 100 mM PBS buffer solution (pH 7.0) containing 1.25 M DBA and different concentrations of DA (0–87.5 M) was respectively added into a cell made of quartz suprasil (10 mm path length, 0.7 mL volume). The CD spectra were measured over the wavelength range from 210 to 350 nm and the scanning speed was 200 nm per minute. 3. Results and discussion 3.1. Principle of the colorimetric detection In this work, the synthesized AuNP solution was stabilized by the citrate anions as their repulsion prevented the AuNPs from aggregating. When added with a high concentration of salt such as NaCl solution, NaCl would neutralize the negative charge of citrate and lead to the AuNP aggregation. However, it has been reported [28,29] that ssDNA with a random coil structure could be adsorbed onto the surface of AuNPs through the coordination interaction between the nitrogen atoms of the exposed bases and the AuNPs, thus added negative charges to the AuNPs, increased their repulsion and enhanced the stability of AuNPs against the NaCl-induced aggregation. Whereas, compared with the random coil ssDNA, the bases of ssDNA with rigid tertiary structures or double-stranded DNA (dsDNA) were not exposed outside and could not be adsorbed onto the AuNPs, hence lost the ability of protecting the AuNPs from NaCl-induced aggregation. Based on the above mentioned, the mechanism of the DA biosensor is shown in Fig. 1. As can be noted in Fig. 1, DA could specifically bind to the dopamine-binding aptamer (DBA) and induced the DBA conformation switching from a random flexible structure to a rigid stem–loop structure. The DBA with a random coil structure could wrap the unmodified AuNPs up while the DBA with rigid tertiary structures could not. After adding a high concentration of NaCl, the AuNPs in the presence of DBA would remain dispersed and the AuNPs with both DBA and DA would aggregate. Based on this rationale, an aptamer-based colorimetric biosensing strategy can be well developed for the detection of dopamine. 3.2. DBA structures characterized by CD CD in the UV region was regarded as the most appropriate technique to distinguish different DNA structures [30]. It was reported
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
97
Fig. 2. CD spectra of 1.25 M DBA in 100 mM PBS buffer (pH 7.0) containing different concentrations of DA (0.00–87.50 M). (a) 0.00 M DA, (b) 8.75 M DA, (c) 25.00 M DA, and (d) 87.50 M DA.
[31] that a hairpin structure possessed a CD spectrum characterized by a positive ellipticity maximum around 280 nm and a negative minimum around 248 nm. In this work, the CD spectra of DBA under different experimental conditions are depicted in Fig. 2. In the absence of DA, there was a positive ellipticity maximum around 280 nm and a negative minimum around 248 nm, indicating that stem–loop structures existed in the unbound DBA solution. However, upon the addition of DA, the positive ellipticity maximum around 280 nm increased greatly, suggesting that more stem–loop structures came into being. Thus, we could suppose that DA possessed the ability of inducing the conformation change of DBA from random coil to stem–loop structure. Indeed, it has been reported [22] that there is a mixture of aptamers of both random coil structures and rigid tertiary structures before binding to target molecules and there is an equilibrium between the two conformations. The existing stem–loop structure in the unbound DBA would affect the sensitivity of the determination of dopamine and it was hard to clear this structure out. Thus, proper concentration of DBA should be chosen to eliminate this influence which has been stated in the following part. 3.3. Spectral characteristics To further prove this concept, a series of UV–vis spectra of AuNPs were measured under different experimental conditions. As was shown in Fig. 3A, in the presence of 40 mM NaCl, the surface plasmon resonance (SPR) absorption band of AuNPs red shifted and broadened. The peak at 520 nm (representing the dispersed AuNPs) decreased and a new peak at about 700 nm (representing the aggregated AuNPs) appeared, indicating that the AuNPs aggregated. However, upon the addition of DBA, it could be noted in Fig. 3A that there was only one peak at about 520 nm, implying that the AuNPs were still dispersed. This was because DBA with a random coil structure was able to be adsorbed onto the AuNPs and protected AuNPs from NaCl-induced aggregation. Then, upon the addition of both DBA and DA, it could be clearly noticed in Fig. 3A that the SPR adsorption band of AuNPs red shifted with a new peak appearing at about 660 nm, besides, the absorption at 660 nm increased with the increase of the concentration of DA (Fig. 3A), indicating the aggregation of AuNPs. Fig. 3B displays a visible color change corresponding to Fig. 3A. From Fig. 3B, it could be noticed that the color of the AuNP solution changed from red to blue after being added with NaCl. Whereas, upon the addition of DBA, Fig. 3B shows that the solution was still red, and when added with both DBA and DA, the solution was purple or blue. Furthermore, Fig. 3C
Fig. 3. (A) UV–vis absorption spectra of AuNPs in the presence of 40 mM NaCl under different experimental conditions, cDBA = 133 nM, cAuNPs = 3.3 nM. (B) Visual color changes corresponding to (A). (C) TEM images of 3.3 nM AuNP solution mixed with 133 nM DBA 5 min after the addition of 40 mM NaCl. (D) TEM images of 3.3 nM AuNP solution in the presence of 133 nM DBA and 3.6 M DA 5 min after the addition of 40 mM NaCl.
and D utilized the Transmission Electron Microscope (TEM) technology to characterize the morphology change of AuNPs. Fig. 3C clearly showed that the AuNPs was still dispersed after the addition of NaCl in the presence of DBA. Fig. 3D shows that the AuNPs aggregated after the addition of NaCl in the presence of both DA and DBA. All these results were in good agreement with our assumption, implying that DA induced the conformation of DBA changing from a random coil structure to a stem–loop structure. 3.4. Optimization of experimental conditions In this work, the net absorption ratio between 660 nm and 524 nm, (A660 /A524 ) ((A660 /A524 ) = the absorption ratio A660 /A524 in the presence of DA − the absorption ratio A660 /A524 in the absence of DA), was used to optimize the experimental conditions. The (A660 /A524 ) value was mainly influenced by the concentrations of DBA, the concentration of NaCl, the binding time of DBA and DA, the incubation time after adding AuNPs, the reaction time after adding NaCl and the size of AuNPs. Firstly, the effect of the concentration of NaCl was studied in the concentration range of 0–80 mM. The results showed that (A660 /A524 ) arrived at the maximum value when the concentration of NaCl was 40 mM. Thus, 40 mM was chosen for this work on basis of higher sensitivity. Secondly, the effect of the concentration of DBA was studied in the concentration range of 0–400 nM, the (A660 /A524 ) reached the maximum value when the concentration of DBA was 133 nM. Hence, the concentration of DBA was selected to be 133 nM for this experiment. Thirdly, the effect of
98
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
Fig. 4. (A) UV–vis absorption spectra of AuNPs in the presence of 40 mM NaCl under different concentrations of DA. (B) Derived calibration curve of the adsorption ratio (A660 /A524 ) versus log(concentration of DA). Experimental conditions are: cNaCl = 40 mM, cDBA = 133 nM, cAuNPs = 3.3 nM.
Fig. 5. (A) UV–vis absorption spectra of AuNPs in the presence of 133 nM DBA and 3.6 M dopamine analogs and its metabolite after the addition of 40 mM NaCl, cAuNPs = 3.3 nM. (B) The absorption ratio value A660 /A524 of dopamine analogs and its metabolites. Experimental conditions are as in (A).
binding time of dopamine-aptamer over the range of 0–15 min was investigated, and (A660 /A524 ) increased until the binding time was 10 min. Thus, 10 min was chosen for the following experiment. Subsequently, the incubation time after adding AuNPs was studied over the range of 0–15 min and the results showed that the interaction could be completed within 5 min. Thus, we selected 5 min as the incubation time in this work. Then, the reaction time after adding NaCl over the range of 0–30 min was investigated. It was found that the (A660 /A524 ) increased substantially as the reaction time increased up to 5 min. Therefore, 5 min was chosen as an optimum reaction time. Finally, three different sizes of AuNPs (13 nm, 25 nm and 38 nm) on the effect of the detection of dopamine were examined. The results showed that the (A660 /A524 ) for 13 nm, 25 nm and 38 nm AuNP solution was 0.22, 0.06, −0.05, respectively, under the similar experimental condition. Thus, we used 13 nm AuNPs as the following experiment condition on basis of higher sensitivity.
3.6. Selectivity The selectivity of this colorimetric biosensor to DA was evaluated by measuring the absorption ratio value, A660 /A524 to some common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), epinephrine (EP) and ascorbic acid (AA). As can be observed in Fig. 5A, upon the addition of DA, there was a noticeable change in UV–vis adsorption spectra, while no or just a little spectral change in the absence (blank) or presence of the common interferents occurred. The data derived from Fig. 5A showed that the adsorption ratio value, A660 /A524 , in the presence of DA was considerably larger than those of blank or the common interferents (Fig. 5B). All results indicate that our assay approach has a high specificity to DA. 4. Conclusion
3.5. Calibration modeling for DA Under the above-mentioned optimized experimental conditions, a series of different concentrations of DA was respectively added and their UV–vis spectra were recorded (Fig. 4A). Fig. 4B depicts the derived calibration curves corresponding to Fig. 4A. As can be seen in Fig. 4A and B, the absorption ratio, A660 /A524 , increased proportionally with the concentration of DA in the range of 0.54–5.4 M. It has been reported [32] that the dynamic region of the analyte-induced AuNP aggregation was generally quite narrow. The linear equation could be fitted as A660 /A524 = 0.64 + 0.83 log c (DA, M) (R2 = 0.999) with a limit detection (3) of 0.36 M. Compared to other reported colorimetric methods [9,33], this method has a better sensitivity towards DA.
This work investigated the interaction of dopamine-bindingaptamer and dopamine using unmodified citrate-coated AuNPs as colorimetric signal readout for the first time. In the presence of DA, DBA had a conformation change from a random coil structure to a rigid hairpin structure which has been proved by CD spectroscopic data. The structural change made dopamine-binding-aptamer lose the ability of stabilizing the AuNPs, thus facilitated the NaClinduced AuNP aggregation. This method showed a good sensitivity and selectivity towards DA and avoided the interference from common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), epinephrine (EP) and ascorbic acid (AA). Thus, supposing that aptamers specific for neurotransmitters besides dopamine were selected, this colorimetic method would maybe provide a new
Y. Zheng et al. / Sensors and Actuators B 156 (2011) 95–99
possibility for the detection of neurotransmitters and this is being investigated. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20890022), the National Key Basic Research Development Project of China (Nos. 2010CB933602 and 2007CB714500) and the Project of Chinese Academy of Sciences (No. KJCX2-YW-H09). References [1] M. Perry, Q. Li, R.T. Kennedy, Review of recent advances in analytical techniques for the determination of neurotransmitters, Anal. Chim. Acta 653 (2009) 1–22. [2] Y. Wang, Y.M. Li, L.H. Tang, J. Lu, J.H. Li, Application of graphene-modified electrode for selective detection of dopamine, Electrochem. Commun. 11 (2009) 889–892. [3] M.H. Pournaghi-Azar, H. Dastangoo, R. Fadakar bajeh baj, Simultaneous determination of dopamine and its oxidized product (aminochrom), by hydrodynamic amperometry and anodic stripping voltammetry, using the metallic palladium and uranalyl hexacyanoferrate coated aluminum electrodes, Biosens. Bioelectron. 25 (2010) 1481–1486. [4] Y.R. Kim, S. Bong, Y.J. Kang, Y. Yang, R.K. Mahajan, J.S. Kim, H. Kim, Electrochemical detection of dopamine in the presence of ascorbic acid using graphene modified electrodes, Biosens. Bioelectron. 25 (2010) 2366–2369. [5] N. Li, J.Z. Guo, B. Liu, H. Cui, L.Q. Mao, Y.Q. Lin, Determination of monoamine neurotransmitters and their metabolites in a mouse brain microdialysate by coupling high-performance liquid chromatography with gold nanoparticleinitiated chemiluminescence, Anal. Chim. Acta 645 (2009) 48–55. [6] V. Carrera, E. Sabater, E. Vilanova, M.A. Sogorb, A simple and rapid HPLC–MS method for the simultaneous determination of epinephrine, norepinephrine, dopamine and 5-hydroxytryptamine: application to the secretion of bovine chromaffin cell cultures, J. Chromatogr. B 847 (2007) 88–94. [7] S.L. Zhao, Y. Huang, M. Shi, R.J. Liu, Y.M. Liu, Chemiluminescence resonance energy transfer-based detection for microchip electrophoresis, Anal. Chem. 82 (2010) 2036–2041. [8] Y.C. Chen, W.Y. Lin, Enhancement of chemiluminescence of the KIO4 -luminol system by gallic acid, acetaldehyde and Mn2+ : application for the determination of catecholamines, Luminescence 25 (2010) 43–49. [9] R. Baron, M. Zayats, I. Willner, Dopamine-, l-DOPA-, adrenaline-, and noradrenaline-induced growth of Au nanoparticles: assays for the detection of neurotransmitters and of tyrosinase activity, Anal. Chem. 77 (2005) 1566–1571. [10] Z.J. Lin, X.M. Chen, T.T. Jia, X.D. Wang, Z.X. Xie, M. Oyama, X. Chen, Fabrication of a colorimetric electrochemiluminescence sensor, Anal. Chem. 81 (2009) 830–833. [11] Y.F. Zhang, B.X. Li, X.L. Chen, Simple and sensitive detection of dopamine in the presence of high concentration of ascorbic acid using gold nanoparticles as colorimetric probes, Microchim. Acta 168 (2010) 107–113. [12] C. Huang, Y. Huang, Z. Cao, W. Tan, H. Chang, Aptamer-modified gold nanoparticles for colorimetric determination of platelet-derived growth factors and their receptors, Anal. Chem. 77 (2005) 5735–5741. [13] Z.X. Zhang, Z.J. Wang, X.L. Wang, X.R. Yang, Magnetic nanoparticle-linked colorimetric aptasensor for the detection of thrombin, Sens. Actuators B 147 (2010) 428–433. [14] Y. Wang, F. Yang, X.R. Yang, Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer, Biosens. Bioelectron. 25 (2010) 1994–1998. [15] T. Li, S.J. Dong, E.K. Wang, Label-free colorimetric detection of aqueous mercury ion (Hg2+ ) using Hg2+ -modulated G-quadruplex-based DNAzymes, Anal. Chem. 82 (2010) 1515–1520. [16] X.W. Xu, J. Wang, F. Yang, K. Jiao, X.R. Yang, Label-free colorimetric detection of small molecules utilizing DNA oligonucleotides and silver nanoparticles, Small 23 (2009) 2669–2672.
99
[17] F. Li, J. Zhang, X.N. Cao, L.H. Wang, D. Li, S.P. Song, B.C. Ye, C.H. Fan, Adenosine detection by using gold nanoparticles and designed aptamer sequences, Analyst 134 (2009) 1355–1360. [18] N.L. Rosi, C.A. Mirkin, Nanostructures in biodiagnostics, Chem. Rev. 105 (2005) 1547–1562. [19] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, A DNA-based method for rationally assembling nanoparticles into macroscopic materials, Nature 382 (1996) 607–609. [20] S. Tombelli, M. Minunni, M. Mascini, Analytical applications of aptamers, Biosens. Bioelectron. 20 (2005) 2424–2434. [21] Z.S. Wu, H.X. Lu, X.P. Liu, R. Hu, H. Zhou, G.L. Shen, R.Q. Yu, Inhibitory effect of target binding on hairpin aptamer sticky-end pairing-induced gold nanoparticle assembly for light-up colorimetric protein assay, Anal. Chem. 82 (2010) 3890–3898. [22] H. Wei, B.L. Li, J. Li, E.K. Wang, S.J. Dong, Simple and sensitive aptamer-based colorimetric sensing of protein using unmodified gold nanoparticle probes, Chem. Commun. (2007) 3735–3737. [23] H.X. Li, L. Rothberg, Colorimetric detection of DNA sequences based on electrostatic interactions with unmodified gold nanoparticles, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 14036–14039. [24] C. Mannirono, A.D. Nardo, P. Fruscoloni, G.P. Tocchini-Valentini, In vitro selection of dopamine RNA ligands, Biochemistry 36 (1997) 9726–9734. [25] R. Walsh, M.C. DeRosa, Retention of function in the DNA homolog of the RNA dopamine aptamer, Biochem. Biophys. Res. Commun. 388 (2009) 732–735. [26] K.C. Grabar, R.G. Freeman, M.B. Hommer, M.J. Natan, Preparation and characterization of Au colloid monolayers, Anal. Chem. 67 (1995) 735–743. [27] M.M. Maye, L. Han, N.N. Kariuki, N.K. Ly, W.B. Chan, J. Luo, C.J. Zhong, Gold and alloy nanoparticles in solution and thin film assembly: spectrophotometric determination of molar absorptivity, Anal. Chim. Acta 496 (2003) 17–27. [28] H.X. Li, L. Rothberg, Label-free colorimetric detection of specific sequences in genomic DNA amplified by the polymerase chain reaction, J. Am. Chem. Soc. 126 (2004) 10958–10961. [29] H.X. Li, L. Rothberg, DNA sequence detection using selective fluorescence quenching of tagged oligonucleotide probes by gold nanoparticles, Anal. Chem. 76 (2004) 5414–5417. [30] G.D. Fasman, Circular Dichroism and the Conformational Analysis of Biomolecules, Plenum Press, New York, NY, USA, 1996. [31] M. Nakano, E.M. Moody, J. Liang, P.C. Bevilacqua, Selection for thermodynamically stable DNA tetraloops using temperature gradient gel electrophoresis reveals four motifs: d(cGNNAg), d(cGNABg), d(cCNNGg), and d(gCNNGc), Biochemistry 41 (2002) 14281–14292. [32] C. Lu, Y.B. Zu, V. Wing-Wah Yam, Specific postcolumn detection method for HPLC assay of homocysteine based on aggregation of fluorosurfactant-capped gold nanoparticles, Anal. Chem. 79 (2007) 666–672. [33] M. Reza Hormozi Nezhad, J. Tashkhourian, J. Khodaveisic, M. Reza Khoshi, Simultaneous colorimetric determination of dopamine and ascorbic acid based on the surface plasmon resonance band of colloidal silver nanoparticles using artificial neural networks, Anal. Methods 2 (2010) 1263–1269.
Biographies Yu Zheng obtained her bachelor’s degree in 2009, University of Science and Technology of China, and she has been a graduate student of the State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science. Her research interests focus mainly on the field of biosensor. Yong Wang has been a PhD candidate since 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Science. His current research is biosensor. Xiurong Yang is a professor of the State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science. Her current research includes study of biomolecules interaction, fabrication of biosensors, and synthesis of functional nanomaterials.