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Morphology-tunable polydopamine nanoparticles and their application in Fe3+ detection
Author’s Accepted Manuscript Morphology-tunable Polydopamine Nanoparticles and Their application in Fe3+ detection Peng Qi, Dun Zhang, Yi Wan
www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(17)30408-3 http://dx.doi.org/10.1016/j.talanta.2017.03.093 TAL17438
To appear in: Talanta Received date: 6 February 2017 Revised date: 27 March 2017 Accepted date: 29 March 2017 Cite this article as: Peng Qi, Dun Zhang and Yi Wan, Morphology-tunable Polydopamine Nanoparticles and Their application in Fe3+ detection, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.03.093 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Morphology-tunable Polydopamine Nanoparticles and Their application in Fe3+ detection Peng Qi*, Dun Zhang*, Yi Wan Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China [email protected] [email protected]
*Corresponding author,
Abstract In this work, we discovered the morphology transformation property of polydopamine (PDA) nanomaterials, the addition of Fe3+ initiated the dramatic morphology transformation of PDA dots from aggregated plate-like to uniform willow-leaf-like morphology. Further study revealed that this fascinating morphology transformation process could be attributed to the oxidative nature and coordination characteristic of Fe3+. This is the first report on the morphology transformation ability of PDA, and a probable self-assembled mechanism was proposed to explain this issue. Besides, we noticed that morphological and fluorescent properties of PDA dots were closely related, thus a fluorescent Fe3+ detection method was presented based on the morphology-tunable PDA dots. With the proposed method, selective
1
Fe3+ detection was achieved with a wide linear dynamic range of 10 μM to 1 mM. Furthermore, since the morphology tuning behavior of PDA dots was easy to operate, we anticipate this ability will find significant utility in sensing, drug delivery, and tissue engineering. Graphical abstract
Keywords: morphology transformation; polydopamine dots; detection; fluorescent; ferric cations
1. Introduction Designing and synthesis nanomaterials with tailored shape and size was a fundamental goal of material science. Typically, nanomaterials with varied shapes and sizes were synthesized via changing synthetic methods and chemical synthesis conditions [1-3]. However, designing materials that could spontaneously change their shape and size in response to specific stimuli was more profitable in biological and physiological research. For example, an effective imaging or therapeutic agent at the tumor site was developed by
2
transforming into a bulky and more slowly diffusing object under the slightly acidic extracellular microenvironment of tumor tissue (pH 6.6– 7.4) [4]. However, to date, the biological application of stimuli-responsive morphology changing materials was rarely reported, since ideal platforms with high morphology transformation efficiency and excellent biocompatibility remained elusive. For most reported stimuli-responsive morphology transformation materials, polymers were used as the key material by designing the polymeric structure and functional end groups, but their morphology shifting was merely the change in size [5-7]. Additionally, although polymers are very suitable for this application due to their flexible ligands and structural diversity, the complicated synthesis process and low biocompatibility still remain challenges. Dopamine
(2-(3,4-dihydroxyphenyl)ethylamine)
is
a
kind
of
catecholamine
neurotransmitter widely present in the central nervous system. The catechol and amine functional groups of dopamine molecular make it susceptible to undergo oxidative polymerization process, via a series of complex redox reactions, under alkaline conditions [8], and the obtained polymerized dopamine provided an excellent platform for immobilization of biological and chemical molecules [9]. In addition, owing to the presence of ortho-dihydroxyphenyl structure, the metal ion chelation ability of polydopamine (PDA) has also attracted great attention [10, 11]. Recently, it was reported that biocompatible fluorescent polydopamine nanoparticles, with superior optical properties such as excitation-independent fluorescence emission, high fluorescence quantum yield and excellent photostability, can be easily obtained and used as effective fluorescent sensing labels. However, although PDA has been widely used, their stimuli-responsive shape transformation ability has not yet been discovered. In this work, we discovered the morphology tunable ability of PDA dots for the first time.
3
PDA dots were synthesized with a facile one-step approach. A remarkable morphological transformation from worm-like plates to uniform willow-leaf-like nanorods was observed instantaneously under Fe3+ stimuli. Subsequent researches revealed that the fascinating shapeshifing phenomenon was highly selective to Fe3+ stimuli, and the oxidative nature and coordination characteristic of Fe3+ were both responsible for the morphology transformation process. A self-assembly based mechanism was proposed to clarify this interesting phenomenon. In addition, we noticed that the morphological transformation process was closely related with their fluorescent emission property, and then PDA dots were applied as an effective fluorescent probe for selective and sensitive Fe3+ detection. Furthermore, we anticipated that the morphology-tunable PDA nanomaterial could also be applied in drug delivery and tissue engineering, since the morphology tuning process was easy to operate.
2. Experimental 2.1 Chemicals Dopamine hydrochloride (2-(3,4-Dihydroxyphenyl)ethylamine hydrochloride), was purchased from Sigma-Aldrich. Other chemicals used in this paper were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Milli-Q water (Millipore, USA) was used throughout. 2.2 Synthesis of fluorescent PDA dots PDA dots were synthesized according to previous literature with a slight modification [12]. Firstly, 0.2 g dopamine hydrochloride was dissolved in 100 mL 20 mM NaOH solution, and the mixture solution was heated at 50 °C for 20 h. After reaction, the resulting solution was aged for 8 weeks in dark. Then, 25 mL of the aged product was added to a mixture solution containing 15 mL 20% H2O2 and 10 mL 2.5 M NaOH, and the mixture solution was
4
heated to reflux for 30 min under stirring. Finally, the reaction solution was cooled to room temperature and then dialyzed in a dialysis membrane (molecular weight cutoff 1kDa) against 10% ethanol for two days. 2.3 Measurements Morphologies of PDA dots were observed with transmission electron microscopy (TEM) study using JEM 1200 instrument. Fourier transform infrared (FTIR) spectroscopy was used to characterize the binding of PDA and ferric cations using a Thermo Nicolet iS10 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer with an excitation wavelength at 360 nm. Fluorescence emission spectra range was recorded from 380 nm to 700 nm. Excitation and emission slits were set as 5 nm and 2.5 nm, respectively. 2.4 Fluorescent detection of ferric ions Stock solutions of FeCl3 was prepared in PBS. Fe3+ solutions (200 μL) of various concentrations were separately incubated with PDA dots (200 μL, 4 mg mL-1). Then the fluorescence spectra of resulting solutions were recorded at an excitation wavelength of 360 nm. Other metal ions, including Na+, K+, Mg2+, Fe2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Al3+, were used to verify the selectivity for ferric detection. The degradation ratio has been calculated as: Inhibition ratio = (F0 F)/F0, where F and F0 are the fluorescence intensity of PDA dots at 452 nm in the presence and absence of Fe3+ addition, respectively.
3. Results and discussion Although polymerization of dopamine under alkaline conditions has been studied widely, the fluorescence properties of PDA were largely unexploited. Until very recently, PDA nanoparticles were firstly reported fluorescent and used for cellular imaging [13]. In this work,
5
PDA dots were synthesized via a green and facile method, and the corresponding morphological and fluorescent properties were investigated. Fig. 1A and B were the TEM images of PDA dots prepared according to the experimental section, the nanomaterials were plate-like with average diameters of 30 to 40 nm. It can be clearly seen that PDA dots were stacked up with each other to form a worm-like morphology, which could be attributed to strong π–π interactions between the PDA plates. Fig. 1C showed that the fluorescence emission intensity of synthesized PDA dots varied with the excitation wavelength ranging from 280 nm to 460 nm, stronger fluorescence intensity was observed when they were excited by 280- 360 nm irradiation, and then the emission intensity decreased with the progressively increased excitation wavelength. The excitation wavelength was set as 360 nm in subsequent researches. Furthermore, the working situations (buffer pH, buffer concentration and temperature) of PDA dots were measured and optimized, and the results were exhibited in Fig. 1D, E and F. As we can see, the fluorescence property of PDA dots was highly stable, they maintained a strong fluorescence spectral response in a wide pH, temperature, and buffer concentration range. Since the interaction of catechol groups with Fe3+ is highly efficient, we inferred that the catechol groups on the PDA dots could coordinate with Fe3+, resulting in the fluorescence quenching of PDA dots with Fe3+ acting as an electron acceptor. Experimental studies confirmed our predictions, the fluorescence emission of PDA dots was completely quenched when 2 mM Fe3+ was added (Fig. 2A). For better understanding the coordination between catechol groups and Fe3+, the morphological properties of PDA dots in the presence of 2 mM Fe3+ were investigated. Surprisingly, the PDA dots exhibited a uniform willow-leaf-like morphology with an average length and width of about 80 and 10 nm (Fig. 2B and C), which were dramatically different from their original morphology (Fig. 1A and B).
6
This is the first time the fascinating morphological transformation phenomenon of PDA nanoparticles was found, so we conducted further investigation to reveal this process. Firstly, the morphology transformation ability of PDA dots were studied under Fe3+ stimuli of varied concentrations. As shown in Fig. 3 A and B, after treatment with 1 mM and 1.5 mM Fe3+ solution, the majority of PDA nanomaterials reassembled as willow-leaf-like morphology, however, the plate-like PDA dots was still observed. When 2 mM Fe3+ was added as stimuli, the morphology of PDA dots was completely changed, their original morphology was no longer found under TEM observation (Fig. 3C). Moreover, we found the morphology transformation degree was highly related with the fluorescence intensity of PDA dots. The addition of 1 mM ferric solution could quench 65.3% of fluorescence emission of PDA dots, and the inhibition ratio of 1.5 mM and 2 mM Fe3+ reached 0.899 and 0.947, respectively. These results indicated that the morphology changing ability of PDA dots was highly related with the concentration of Fe3+ solution, and their fluorescent and morphological properties were closely related. Next, PDA dots were mixed with 3 mM HCl solution to investigate the effect of pH on the morphology transformation process. This is because that ferric chloride solution was strong acidic, the pH value of 4.5 mM ferric chloride solution was measured as 2.58. As we can see from Fig. 4A and B, the PDA dots become irregular and obscure after HCl treatment, but no willow-leaf-like PDA dots were observed, indicating the pH change was not the reason for the dramatic morphology change. These results illustrated that the morphology transformation ability should not only be attributed to solution pH change, and it should be the result of metal coordination. Afterwards, 2 mM Cd2+, Mn2+, Co2+ and Ni2+ were incubated with PDA dots to further confirm the role of metal coordination in the morphology transformation process. Upon
7
treating with Cd2+ and Mn2+, PDA nanomaterials just aggregated together and the outer edge became obscure, but no willow-leaf-like morphology was observed (Fig. 4C and D). In addition, obvious morphological changes were elicited followed by the addition of Co2+ and Ni2+, the PDA dots aggregated together and resulted in a kind of nanosphere structure with diameters of hundreds of nanometers, and nanorods could be observed around the nanospheres (Fig. 4E and F). These results indicated that the binding between metal ion and PDA ligand was not the sole reason for the formation of willow-leaf-like nanorods, and Fe3+ held a special role in the morphology transformation process. Investigation of the interactions of dopamine and Fe3+ is of profound significance, since iron-containing melanins were found in the Substantia nigra of the brains of decreased Parkinsonian patients, and the melanins synthesis process was highly related with the autoxidation of dopamine [14]. The interaction between dopamine and Fe3+ was extensively studied to explore the composition and construction of iron (III)-dopamine complex [10, 15]. Compared with the other metal binding sites, Fe3+ possessed considerable oxidative capacity in acidic conditions. The intramolecular reduction of Fe3+ accompanied by dopamine ligand oxidation would occur during complex formation, since the redox potential of Fe3+/Fe2+ and quinone/catechol is 0.749 and 0.792 V, respectively. According to the classical literature on catechol-iron complex formation, the complex structure transformation process could be illustrated in Scheme 1, the initial rapid formation of an iron (III)-catechol complex (1) was followed by its decomposition to iron (II) benzosemiquinone complex (2). At below pH 2.0, the obtained complex (2) was unstable due to protonation, yielding a semiquinone radical (equation 1). The semiquinone radical could undergo a disproportionation process, generating catechol and o-benzoquinone (equation 2). Otherwise, the obtained complex (2) was stable at pH above 2.0. Thus, the 1:1 iron (II)-PDA complex was the main existing structure after Fe3+
8
was added to PDA dots. [FeII (Q·) ]+ + H+ ⇌ Fe2+ + SQ· 2 SQ· ⇌ H2Q + Q
(1) (2)
Hence, we concluded that the morphology transformation ability of PDA dots under Fe3+ activation was the result of comprehensive function of pH, ligand oxidation and metal complexation. In Scheme 2, a probable self-assembly assisted mechanism for the fascinating morphology transformation ability is shown. During PDA dots synthesis, treatment of NaOH and H2O2 would produce additional hydroxyl radicals, which was crucial in decomposition of PDA materials to assemble into PDA dots. The presence of additional hydroxyl groups in PDA dots could effectively prevent the conjugated backbone from π–π stacking [12]. Hence, the strong hydrogen bonds could be considered as the dominated “morphology determining” interaction way among PDA dots, the no orienting binding between hydrogen bonds resulted in the plate-like morphology (Scheme 2A). In comparison, the addition of Fe3+ could oxidize the catechol structure to semiquinone, which destroyed the hydrogen bonding among PDA dots, and the obtained planar iron (II) benzosemiquinone structure favored layered stacking via π-π interactions. In metal-ligand compounds, the π-π stacking was of fundamental important for designing of inorganic superamolecular and tuning of crystal structures [16]. Additionally, when iron was coordinated to the aromatic ligands, the electron-withdrawing effect was enhanced for its positive charge, resulting in a low π-electron density which was more prone to be π-π stacking. During π-πstacking, the iron (II)-ligand complexes assembled longitudinally, and the nanorod-like morphology was originated from the planar sheet structure of the complexes (Scheme 2B). It has to be noted that the π stacking interaction was also correlated with the luminescence properties of the metal complexes, it could induce
9
intramolecular fluorescence quenching [17, 18], which could explain the fluorescence results of Fig. 2A. Since the morphological and fluorescent properties of PDA dots were highly selective to Fe3+ binding, we proposed a novel and rapid method for Fe3+ detection. Fe3+ is one of the most important metal ions in many proteins and enzymes, the high levels of Fe3+ in drinking water could cause serious complications, such as β-thalassemia, hereditary hemochromatosis and Alzheimer's disease [19, 20]. Thus, developing sensitive detection and sensing methods for Fe3+ is important for the environment and human health. The detection method based on morphology-tunable PDA materials was easy to conduct. Fe3+ solutions of various concentrations were separately incubated with 4 mg mL-1 PDA dots simply with volume ratio of 1:1, and then the fluorescence spectra of resulting solutions were recorded under excitation wavelength of 360 nm. As shown in Fig. 5A, with the Fe3+ concentration increased, the fluorescence spectra of PDA dots showed a gradual decrease of fluorescence intensity, and a plot of inhibition ratio versus the Fe3+ concentration is shown in the inset figure. A linear relationship was observed in a broad detection range from 10 to 1000 μM (R2= 0.999), and the detection limit was calculated to be 4.6 μM (based on 3σ/n, where n is the slope of fluorescence intensity against Fe3+ concentration, and σ is the relative standard deviation of blank measurements). Furthermore, the selectivity of Fe3+ detection based on PDA dots was investigated, and the results were shown in Fig .5B. The synthesized fluorescent PDA dots exhibited a significant fluorescence response towards Fe3+ over other cations, and the inhibition effect of Fe3+ ion was not affected in the presence of other cations. These results indicated that the PDA dots showed good selectivity for Fe3+ recognition. The advantages of this approach were that no intricate operation was required and the recognition process was completed almost immediately.
10
The applications of PDA dots for Fe3+ detection in tap water and sea water were further evaluated. Sea water was filtered before use, and tap water was used without pretreatment. Fe3+ stock solutions were added into the environmental water samples containing PDA dots and the final concentration of Fe3+ were 200 μM and 400 μM. The fluorescence intensities of PDA dots were recorded before and after the addition of Fe3+. As shown in Table 1, the quantitative spike recoveries for Fe3+ detection were 102.48% and 99.80% for the tap water samples, and 101.20% and 100.38% for the sea water samples, respectively. These results demonstrated that PDA dots held the potential to be used for Fe3+ detection in real water samples.
4. Conclusions In this work, PDA dots were synthesized via a green and facile method. For the first time, we discovered the morphology tuning ability of polydopamine under a facile stimulation. Introduction of Fe3+ initiated a dramatic change from plate-like to willow-leaf-like morphology. Further studies revealed that the morphology transformation process of PDA dots was selective to Fe3+, and their corresponding morphological and fluorescent properties were highly associated. A probable self-assembled mechanism was proposed to classify these issues. Afterwards, we presented a turn-off fluorescent sensing method for Fe3+ based on PDA dots. The detection process was easy to conduct, and selective Fe3+ detection was achieved with a wide linear dynamic range of 10 μM to 1 mM. On the basis of founding outlined above, we anticipate that the morphology tuning property of PDA dots will find significant utility as a platform in sensing, drug delivery, and tissue engineering application.
11
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 41506094), National Key Basic Research Program of China (2014CB643304), Shandong Provincial Natural Science Foundation (ZR2014DQ009), and Science & Technology Basic Research Program of Qingdao (16-5-1-19-jch).
References [1] Y.T. Yu, Q.H. Zhang, B.Q. Xu, Shape-controlled syntheses of metal nanoparticles, Prog. Chem. 16(4) (2004) 520-527. [2] F. Basiri, M. Taei, Application of spinel-structured MgFe2O4 nanoparticles for simultaneous electrochemical determination diclofenac and morphine, Microchim. Acta 184(1) (2017) 155-162. [3] X. Liu, G. Liu, L. Wang, Y. Li, Y. Ma, J. Ma, Morphology- and facet-controlled synthesis of CuO micro/nanomaterials and analysis of their lithium ion storage properties, J. Power Sources 312 (2016) 199-206. [4] A. Ghosh, M. Haverick, K. Stump, X. Yang, M.F. Tweedle, J.E. Goldberger, Fine-Tuning the pH Trigger of Self-Assembly, J. Am. Chem. Soc. 134(8) (2012) 3647-3650. [5] J. Kim, Y.M. Lee, H. Kim, D. Park, J. Kim, W.J. Kim, Phenylboronic acid-sugar grafted polymer architecture as a dual stimuli-responsive gene carrier for targeted anti-angiogenic tumor therapy, Biomaterials 75 (2016) 102-111. [6] A.K. Bajpai, J. Bajpai, R. Saini, R. Gupta, Responsive Polymers in Biology and Technology, Polym. Rev. 51(1) (2011) 53-97. [7] F. Liu, M.W. Urban, Recent advances and challenges in designing stimuli-responsive polymers, Prog. Polym. Sci. 35(1-2) (2010) 3-23. [8] Y. Tan, W. Deng, Y. Li, Z. Huang, Y. Meng, Q. Xie, M. Ma, S. Yao, Polymeric Bionanocomposite Cast
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Thin Films with In Situ Laccase-Catalyzed Polymerization of Dopamine for Biosensing and Biofuel Cell Applications, J. Phys. Chem. B 114(15) (2010) 5016-5024. [9] P. Qi, D. Zhang, Y. Wan, D. Lv, A facile approach to construct versatile signal amplification system for bacterial detection, Talanta 118 (2014) 333-338. [10] R.C. Hider, A.R. Mohdnor, J. Silver, I.E.G. Morrison, L.V.C. Rees, Model compounds for microbial iron-transport compounds. Part 1. Solution chemistry and Mössbauer study of iron (II) and iron (III) complexes from phenolic and catecholic systems, Dalton Trans. (2) (1981) 609-622. [11] N. Schweigert, A.J.B. Zehnder, R.I.L. Eggen, Chemical properties of catechols and their molecular modes of toxic action in cells, from microorganisms to mammals, Environ. Microbiol. 3(2) (2001) 81-91. [12] J. Lin, C. Yu, Y. Yang, W. Tseng, Formation of fluorescent polydopamine dots from hydroxyl radical-induced degradation of polydopamine nanoparticles, Phys. Chem. Chem. Phys 17(23) (2015) 15124-15130. [13] X. Zhang, S. Wang, L. Xu, L. Feng, Y. Ji, L. Tao, S. Li, Y. Wei, Biocompatible polydopamine fluorescent organic nanoparticles: facile preparation and cell imaging, Nanoscale 4(18) (2012) 5581-5584. [14] F.A. Zucca, J. Segura-Aguilar, E. Ferrari, P. Muñoz, I. Paris, D. Sulzer, T. Sarna, L. Casella, L. Zecca, Interactions of iron, dopamine and neuromelanin pathways in brain aging and Parkinson's disease, Prog. Neurobiol. [15] Y. Xu, B. Zhang, S. Wu, Y. Xia, The adsorption of dopamine on gold and its interactions with iron(III) ions studied by microcantilevers, Anal. Chim. Acta 649(1) (2009) 117-122. [16] C. Janiak, A critical account on π-π stacking in metal complexes with aromatic nitrogen-containing ligands, Dalton Trans. (21) (2000) 3885-3896. [17] C. Pan, K. Sugiyasu, Y. Wakayama, A. Sato, M. Takeuchi, Thermoplastic Fluorescent Conjugated
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Polymers: Benefits of Preventing π-π Stacking, Angew. Chem. Int. Ed. 52(41) (2013) 10775-10779. [18] S. Bevers, S. Schutte, L.W. McLaughlin, Naphthalene- and perylene-based linkers for the stabilization of hairpin triplexes, J. Am. Chem. Soc. 122(25) (2000) 5905-5915. [19] R.E. Fleming, P. Ponka, Iron Overload in Human Disease, N. Engl. J. Med. 366(4) (2012) 348-359. [20] E. Nemeth, Iron regulation and erythropoiesis, Curr Opin in Hematol 15(3) (2008) 169-175.
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Figure captions
Scheme 1. Structure transformation process of iron-PDA complex under Fe3+ stimulation. Scheme 2. Probable mechanism for the morphology transformation of PDA dots. Figure 1. TEM images of PDA dots (A and B), fluorescence emission spectra of synthesized PDA dots under varied excitation wavelength (C), and fluorescence property of PDA dots under varied pH (C), temperature (D), and buffer concentration (E). Figure 2. Fluorescence spectra of PDA dots in the absence and presence of 2 mM Fe3+ (A), and TEM images of PDA dots in the presence of 2 mM Fe3+ (B and C). Figure 3. TEM images of PDA dots after addition of 1 mM (A), 1.5 mM (B) and 2 mM (C) Fe3+. Figure 4. TEM images of PDA dots after treated with HCl (A and B), and TEM images of PDA dots after treated with 2 mM Cd2+ (C), Mn2+ (D), Co2+ (E)and Ni2+ (F).
Figure 5. Fluorescence spectra of PDA dots in the absence and presence of Fe3+ of 10 μM, 20 μM, 50 μM, 100 μM, 200 μM, 500 μM, 1 mM, 1.5 mM, 2 mM, 2.5 mM, 3 mM, 3.5 mM, 4 mM and 4.5 mM from top from bottom. Inset: a plot of inhibition ratio at 452 nm versus the Fe3+concentration (A). The inhibition effect of other anions (from left to right: Na+, K+, Mg2+, Mn2+, Cd2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Al3+) towards PDA dots in the absence (green bars) and presence (red bars) of Fe3+, the concentration of Fe3+ and other metal ions was 2 mM (B).
Table 1 Spike and recovery test for Fe3+ detection in natural samples Fe3+ added
Fe3+ found
RSD
Recovery
(μM)
(μM)
(%, n=3)
(%)
Tap water sample 1
200.00
207.97
3.48
102.48
Tap water sample 2
400.00
399.22
4.96
99.80
Samples
15
Sea water sample 1
200.00
202.407
6.43
101.20
Sea water sample 2
400.00
401.532
5.18
100.38
Highlights ● ● ● ●
The morphology transformation ability of PDA dots was reported for the first time. Dramatic morphology change of PDA dots was initiated by stimuli of ferric ion. A self-assembled mechanism was proposed to explain the stimuli responsive property. A rapid and simple method based on PDA dots was proposed for selective Fe3+ detection .
16
Fig. 1
17
Fig. 2
18
Fig. 3
19
Fig. 4
20
Fig. 5
21
Scheme 1
22
Scheme 2
23
www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(17)30408-3 http://dx.doi.org/10.1016/j.talanta.2017.03.093 TAL17438
To appear in: Talanta Received date: 6 February 2017 Revised date: 27 March 2017 Accepted date: 29 March 2017 Cite this article as: Peng Qi, Dun Zhang and Yi Wan, Morphology-tunable Polydopamine Nanoparticles and Their application in Fe3+ detection, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.03.093 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Morphology-tunable Polydopamine Nanoparticles and Their application in Fe3+ detection Peng Qi*, Dun Zhang*, Yi Wan Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China [email protected] [email protected]
*Corresponding author,
Abstract In this work, we discovered the morphology transformation property of polydopamine (PDA) nanomaterials, the addition of Fe3+ initiated the dramatic morphology transformation of PDA dots from aggregated plate-like to uniform willow-leaf-like morphology. Further study revealed that this fascinating morphology transformation process could be attributed to the oxidative nature and coordination characteristic of Fe3+. This is the first report on the morphology transformation ability of PDA, and a probable self-assembled mechanism was proposed to explain this issue. Besides, we noticed that morphological and fluorescent properties of PDA dots were closely related, thus a fluorescent Fe3+ detection method was presented based on the morphology-tunable PDA dots. With the proposed method, selective
1
Fe3+ detection was achieved with a wide linear dynamic range of 10 μM to 1 mM. Furthermore, since the morphology tuning behavior of PDA dots was easy to operate, we anticipate this ability will find significant utility in sensing, drug delivery, and tissue engineering. Graphical abstract
Keywords: morphology transformation; polydopamine dots; detection; fluorescent; ferric cations
1. Introduction Designing and synthesis nanomaterials with tailored shape and size was a fundamental goal of material science. Typically, nanomaterials with varied shapes and sizes were synthesized via changing synthetic methods and chemical synthesis conditions [1-3]. However, designing materials that could spontaneously change their shape and size in response to specific stimuli was more profitable in biological and physiological research. For example, an effective imaging or therapeutic agent at the tumor site was developed by
2
transforming into a bulky and more slowly diffusing object under the slightly acidic extracellular microenvironment of tumor tissue (pH 6.6– 7.4) [4]. However, to date, the biological application of stimuli-responsive morphology changing materials was rarely reported, since ideal platforms with high morphology transformation efficiency and excellent biocompatibility remained elusive. For most reported stimuli-responsive morphology transformation materials, polymers were used as the key material by designing the polymeric structure and functional end groups, but their morphology shifting was merely the change in size [5-7]. Additionally, although polymers are very suitable for this application due to their flexible ligands and structural diversity, the complicated synthesis process and low biocompatibility still remain challenges. Dopamine
(2-(3,4-dihydroxyphenyl)ethylamine)
is
a
kind
of
catecholamine
neurotransmitter widely present in the central nervous system. The catechol and amine functional groups of dopamine molecular make it susceptible to undergo oxidative polymerization process, via a series of complex redox reactions, under alkaline conditions [8], and the obtained polymerized dopamine provided an excellent platform for immobilization of biological and chemical molecules [9]. In addition, owing to the presence of ortho-dihydroxyphenyl structure, the metal ion chelation ability of polydopamine (PDA) has also attracted great attention [10, 11]. Recently, it was reported that biocompatible fluorescent polydopamine nanoparticles, with superior optical properties such as excitation-independent fluorescence emission, high fluorescence quantum yield and excellent photostability, can be easily obtained and used as effective fluorescent sensing labels. However, although PDA has been widely used, their stimuli-responsive shape transformation ability has not yet been discovered. In this work, we discovered the morphology tunable ability of PDA dots for the first time.
3
PDA dots were synthesized with a facile one-step approach. A remarkable morphological transformation from worm-like plates to uniform willow-leaf-like nanorods was observed instantaneously under Fe3+ stimuli. Subsequent researches revealed that the fascinating shapeshifing phenomenon was highly selective to Fe3+ stimuli, and the oxidative nature and coordination characteristic of Fe3+ were both responsible for the morphology transformation process. A self-assembly based mechanism was proposed to clarify this interesting phenomenon. In addition, we noticed that the morphological transformation process was closely related with their fluorescent emission property, and then PDA dots were applied as an effective fluorescent probe for selective and sensitive Fe3+ detection. Furthermore, we anticipated that the morphology-tunable PDA nanomaterial could also be applied in drug delivery and tissue engineering, since the morphology tuning process was easy to operate.
2. Experimental 2.1 Chemicals Dopamine hydrochloride (2-(3,4-Dihydroxyphenyl)ethylamine hydrochloride), was purchased from Sigma-Aldrich. Other chemicals used in this paper were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Milli-Q water (Millipore, USA) was used throughout. 2.2 Synthesis of fluorescent PDA dots PDA dots were synthesized according to previous literature with a slight modification [12]. Firstly, 0.2 g dopamine hydrochloride was dissolved in 100 mL 20 mM NaOH solution, and the mixture solution was heated at 50 °C for 20 h. After reaction, the resulting solution was aged for 8 weeks in dark. Then, 25 mL of the aged product was added to a mixture solution containing 15 mL 20% H2O2 and 10 mL 2.5 M NaOH, and the mixture solution was
4
heated to reflux for 30 min under stirring. Finally, the reaction solution was cooled to room temperature and then dialyzed in a dialysis membrane (molecular weight cutoff 1kDa) against 10% ethanol for two days. 2.3 Measurements Morphologies of PDA dots were observed with transmission electron microscopy (TEM) study using JEM 1200 instrument. Fourier transform infrared (FTIR) spectroscopy was used to characterize the binding of PDA and ferric cations using a Thermo Nicolet iS10 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer with an excitation wavelength at 360 nm. Fluorescence emission spectra range was recorded from 380 nm to 700 nm. Excitation and emission slits were set as 5 nm and 2.5 nm, respectively. 2.4 Fluorescent detection of ferric ions Stock solutions of FeCl3 was prepared in PBS. Fe3+ solutions (200 μL) of various concentrations were separately incubated with PDA dots (200 μL, 4 mg mL-1). Then the fluorescence spectra of resulting solutions were recorded at an excitation wavelength of 360 nm. Other metal ions, including Na+, K+, Mg2+, Fe2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Al3+, were used to verify the selectivity for ferric detection. The degradation ratio has been calculated as: Inhibition ratio = (F0 F)/F0, where F and F0 are the fluorescence intensity of PDA dots at 452 nm in the presence and absence of Fe3+ addition, respectively.
3. Results and discussion Although polymerization of dopamine under alkaline conditions has been studied widely, the fluorescence properties of PDA were largely unexploited. Until very recently, PDA nanoparticles were firstly reported fluorescent and used for cellular imaging [13]. In this work,
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PDA dots were synthesized via a green and facile method, and the corresponding morphological and fluorescent properties were investigated. Fig. 1A and B were the TEM images of PDA dots prepared according to the experimental section, the nanomaterials were plate-like with average diameters of 30 to 40 nm. It can be clearly seen that PDA dots were stacked up with each other to form a worm-like morphology, which could be attributed to strong π–π interactions between the PDA plates. Fig. 1C showed that the fluorescence emission intensity of synthesized PDA dots varied with the excitation wavelength ranging from 280 nm to 460 nm, stronger fluorescence intensity was observed when they were excited by 280- 360 nm irradiation, and then the emission intensity decreased with the progressively increased excitation wavelength. The excitation wavelength was set as 360 nm in subsequent researches. Furthermore, the working situations (buffer pH, buffer concentration and temperature) of PDA dots were measured and optimized, and the results were exhibited in Fig. 1D, E and F. As we can see, the fluorescence property of PDA dots was highly stable, they maintained a strong fluorescence spectral response in a wide pH, temperature, and buffer concentration range. Since the interaction of catechol groups with Fe3+ is highly efficient, we inferred that the catechol groups on the PDA dots could coordinate with Fe3+, resulting in the fluorescence quenching of PDA dots with Fe3+ acting as an electron acceptor. Experimental studies confirmed our predictions, the fluorescence emission of PDA dots was completely quenched when 2 mM Fe3+ was added (Fig. 2A). For better understanding the coordination between catechol groups and Fe3+, the morphological properties of PDA dots in the presence of 2 mM Fe3+ were investigated. Surprisingly, the PDA dots exhibited a uniform willow-leaf-like morphology with an average length and width of about 80 and 10 nm (Fig. 2B and C), which were dramatically different from their original morphology (Fig. 1A and B).
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This is the first time the fascinating morphological transformation phenomenon of PDA nanoparticles was found, so we conducted further investigation to reveal this process. Firstly, the morphology transformation ability of PDA dots were studied under Fe3+ stimuli of varied concentrations. As shown in Fig. 3 A and B, after treatment with 1 mM and 1.5 mM Fe3+ solution, the majority of PDA nanomaterials reassembled as willow-leaf-like morphology, however, the plate-like PDA dots was still observed. When 2 mM Fe3+ was added as stimuli, the morphology of PDA dots was completely changed, their original morphology was no longer found under TEM observation (Fig. 3C). Moreover, we found the morphology transformation degree was highly related with the fluorescence intensity of PDA dots. The addition of 1 mM ferric solution could quench 65.3% of fluorescence emission of PDA dots, and the inhibition ratio of 1.5 mM and 2 mM Fe3+ reached 0.899 and 0.947, respectively. These results indicated that the morphology changing ability of PDA dots was highly related with the concentration of Fe3+ solution, and their fluorescent and morphological properties were closely related. Next, PDA dots were mixed with 3 mM HCl solution to investigate the effect of pH on the morphology transformation process. This is because that ferric chloride solution was strong acidic, the pH value of 4.5 mM ferric chloride solution was measured as 2.58. As we can see from Fig. 4A and B, the PDA dots become irregular and obscure after HCl treatment, but no willow-leaf-like PDA dots were observed, indicating the pH change was not the reason for the dramatic morphology change. These results illustrated that the morphology transformation ability should not only be attributed to solution pH change, and it should be the result of metal coordination. Afterwards, 2 mM Cd2+, Mn2+, Co2+ and Ni2+ were incubated with PDA dots to further confirm the role of metal coordination in the morphology transformation process. Upon
7
treating with Cd2+ and Mn2+, PDA nanomaterials just aggregated together and the outer edge became obscure, but no willow-leaf-like morphology was observed (Fig. 4C and D). In addition, obvious morphological changes were elicited followed by the addition of Co2+ and Ni2+, the PDA dots aggregated together and resulted in a kind of nanosphere structure with diameters of hundreds of nanometers, and nanorods could be observed around the nanospheres (Fig. 4E and F). These results indicated that the binding between metal ion and PDA ligand was not the sole reason for the formation of willow-leaf-like nanorods, and Fe3+ held a special role in the morphology transformation process. Investigation of the interactions of dopamine and Fe3+ is of profound significance, since iron-containing melanins were found in the Substantia nigra of the brains of decreased Parkinsonian patients, and the melanins synthesis process was highly related with the autoxidation of dopamine [14]. The interaction between dopamine and Fe3+ was extensively studied to explore the composition and construction of iron (III)-dopamine complex [10, 15]. Compared with the other metal binding sites, Fe3+ possessed considerable oxidative capacity in acidic conditions. The intramolecular reduction of Fe3+ accompanied by dopamine ligand oxidation would occur during complex formation, since the redox potential of Fe3+/Fe2+ and quinone/catechol is 0.749 and 0.792 V, respectively. According to the classical literature on catechol-iron complex formation, the complex structure transformation process could be illustrated in Scheme 1, the initial rapid formation of an iron (III)-catechol complex (1) was followed by its decomposition to iron (II) benzosemiquinone complex (2). At below pH 2.0, the obtained complex (2) was unstable due to protonation, yielding a semiquinone radical (equation 1). The semiquinone radical could undergo a disproportionation process, generating catechol and o-benzoquinone (equation 2). Otherwise, the obtained complex (2) was stable at pH above 2.0. Thus, the 1:1 iron (II)-PDA complex was the main existing structure after Fe3+
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was added to PDA dots. [FeII (Q·) ]+ + H+ ⇌ Fe2+ + SQ· 2 SQ· ⇌ H2Q + Q
(1) (2)
Hence, we concluded that the morphology transformation ability of PDA dots under Fe3+ activation was the result of comprehensive function of pH, ligand oxidation and metal complexation. In Scheme 2, a probable self-assembly assisted mechanism for the fascinating morphology transformation ability is shown. During PDA dots synthesis, treatment of NaOH and H2O2 would produce additional hydroxyl radicals, which was crucial in decomposition of PDA materials to assemble into PDA dots. The presence of additional hydroxyl groups in PDA dots could effectively prevent the conjugated backbone from π–π stacking [12]. Hence, the strong hydrogen bonds could be considered as the dominated “morphology determining” interaction way among PDA dots, the no orienting binding between hydrogen bonds resulted in the plate-like morphology (Scheme 2A). In comparison, the addition of Fe3+ could oxidize the catechol structure to semiquinone, which destroyed the hydrogen bonding among PDA dots, and the obtained planar iron (II) benzosemiquinone structure favored layered stacking via π-π interactions. In metal-ligand compounds, the π-π stacking was of fundamental important for designing of inorganic superamolecular and tuning of crystal structures [16]. Additionally, when iron was coordinated to the aromatic ligands, the electron-withdrawing effect was enhanced for its positive charge, resulting in a low π-electron density which was more prone to be π-π stacking. During π-πstacking, the iron (II)-ligand complexes assembled longitudinally, and the nanorod-like morphology was originated from the planar sheet structure of the complexes (Scheme 2B). It has to be noted that the π stacking interaction was also correlated with the luminescence properties of the metal complexes, it could induce
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intramolecular fluorescence quenching [17, 18], which could explain the fluorescence results of Fig. 2A. Since the morphological and fluorescent properties of PDA dots were highly selective to Fe3+ binding, we proposed a novel and rapid method for Fe3+ detection. Fe3+ is one of the most important metal ions in many proteins and enzymes, the high levels of Fe3+ in drinking water could cause serious complications, such as β-thalassemia, hereditary hemochromatosis and Alzheimer's disease [19, 20]. Thus, developing sensitive detection and sensing methods for Fe3+ is important for the environment and human health. The detection method based on morphology-tunable PDA materials was easy to conduct. Fe3+ solutions of various concentrations were separately incubated with 4 mg mL-1 PDA dots simply with volume ratio of 1:1, and then the fluorescence spectra of resulting solutions were recorded under excitation wavelength of 360 nm. As shown in Fig. 5A, with the Fe3+ concentration increased, the fluorescence spectra of PDA dots showed a gradual decrease of fluorescence intensity, and a plot of inhibition ratio versus the Fe3+ concentration is shown in the inset figure. A linear relationship was observed in a broad detection range from 10 to 1000 μM (R2= 0.999), and the detection limit was calculated to be 4.6 μM (based on 3σ/n, where n is the slope of fluorescence intensity against Fe3+ concentration, and σ is the relative standard deviation of blank measurements). Furthermore, the selectivity of Fe3+ detection based on PDA dots was investigated, and the results were shown in Fig .5B. The synthesized fluorescent PDA dots exhibited a significant fluorescence response towards Fe3+ over other cations, and the inhibition effect of Fe3+ ion was not affected in the presence of other cations. These results indicated that the PDA dots showed good selectivity for Fe3+ recognition. The advantages of this approach were that no intricate operation was required and the recognition process was completed almost immediately.
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The applications of PDA dots for Fe3+ detection in tap water and sea water were further evaluated. Sea water was filtered before use, and tap water was used without pretreatment. Fe3+ stock solutions were added into the environmental water samples containing PDA dots and the final concentration of Fe3+ were 200 μM and 400 μM. The fluorescence intensities of PDA dots were recorded before and after the addition of Fe3+. As shown in Table 1, the quantitative spike recoveries for Fe3+ detection were 102.48% and 99.80% for the tap water samples, and 101.20% and 100.38% for the sea water samples, respectively. These results demonstrated that PDA dots held the potential to be used for Fe3+ detection in real water samples.
4. Conclusions In this work, PDA dots were synthesized via a green and facile method. For the first time, we discovered the morphology tuning ability of polydopamine under a facile stimulation. Introduction of Fe3+ initiated a dramatic change from plate-like to willow-leaf-like morphology. Further studies revealed that the morphology transformation process of PDA dots was selective to Fe3+, and their corresponding morphological and fluorescent properties were highly associated. A probable self-assembled mechanism was proposed to classify these issues. Afterwards, we presented a turn-off fluorescent sensing method for Fe3+ based on PDA dots. The detection process was easy to conduct, and selective Fe3+ detection was achieved with a wide linear dynamic range of 10 μM to 1 mM. On the basis of founding outlined above, we anticipate that the morphology tuning property of PDA dots will find significant utility as a platform in sensing, drug delivery, and tissue engineering application.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 41506094), National Key Basic Research Program of China (2014CB643304), Shandong Provincial Natural Science Foundation (ZR2014DQ009), and Science & Technology Basic Research Program of Qingdao (16-5-1-19-jch).
References [1] Y.T. Yu, Q.H. Zhang, B.Q. Xu, Shape-controlled syntheses of metal nanoparticles, Prog. Chem. 16(4) (2004) 520-527. [2] F. Basiri, M. Taei, Application of spinel-structured MgFe2O4 nanoparticles for simultaneous electrochemical determination diclofenac and morphine, Microchim. Acta 184(1) (2017) 155-162. [3] X. Liu, G. Liu, L. Wang, Y. Li, Y. Ma, J. Ma, Morphology- and facet-controlled synthesis of CuO micro/nanomaterials and analysis of their lithium ion storage properties, J. Power Sources 312 (2016) 199-206. [4] A. Ghosh, M. Haverick, K. Stump, X. Yang, M.F. Tweedle, J.E. Goldberger, Fine-Tuning the pH Trigger of Self-Assembly, J. Am. Chem. Soc. 134(8) (2012) 3647-3650. [5] J. Kim, Y.M. Lee, H. Kim, D. Park, J. Kim, W.J. Kim, Phenylboronic acid-sugar grafted polymer architecture as a dual stimuli-responsive gene carrier for targeted anti-angiogenic tumor therapy, Biomaterials 75 (2016) 102-111. [6] A.K. Bajpai, J. Bajpai, R. Saini, R. Gupta, Responsive Polymers in Biology and Technology, Polym. Rev. 51(1) (2011) 53-97. [7] F. Liu, M.W. Urban, Recent advances and challenges in designing stimuli-responsive polymers, Prog. Polym. Sci. 35(1-2) (2010) 3-23. [8] Y. Tan, W. Deng, Y. Li, Z. Huang, Y. Meng, Q. Xie, M. Ma, S. Yao, Polymeric Bionanocomposite Cast
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Thin Films with In Situ Laccase-Catalyzed Polymerization of Dopamine for Biosensing and Biofuel Cell Applications, J. Phys. Chem. B 114(15) (2010) 5016-5024. [9] P. Qi, D. Zhang, Y. Wan, D. Lv, A facile approach to construct versatile signal amplification system for bacterial detection, Talanta 118 (2014) 333-338. [10] R.C. Hider, A.R. Mohdnor, J. Silver, I.E.G. Morrison, L.V.C. Rees, Model compounds for microbial iron-transport compounds. Part 1. Solution chemistry and Mössbauer study of iron (II) and iron (III) complexes from phenolic and catecholic systems, Dalton Trans. (2) (1981) 609-622. [11] N. Schweigert, A.J.B. Zehnder, R.I.L. Eggen, Chemical properties of catechols and their molecular modes of toxic action in cells, from microorganisms to mammals, Environ. Microbiol. 3(2) (2001) 81-91. [12] J. Lin, C. Yu, Y. Yang, W. Tseng, Formation of fluorescent polydopamine dots from hydroxyl radical-induced degradation of polydopamine nanoparticles, Phys. Chem. Chem. Phys 17(23) (2015) 15124-15130. [13] X. Zhang, S. Wang, L. Xu, L. Feng, Y. Ji, L. Tao, S. Li, Y. Wei, Biocompatible polydopamine fluorescent organic nanoparticles: facile preparation and cell imaging, Nanoscale 4(18) (2012) 5581-5584. [14] F.A. Zucca, J. Segura-Aguilar, E. Ferrari, P. Muñoz, I. Paris, D. Sulzer, T. Sarna, L. Casella, L. Zecca, Interactions of iron, dopamine and neuromelanin pathways in brain aging and Parkinson's disease, Prog. Neurobiol. [15] Y. Xu, B. Zhang, S. Wu, Y. Xia, The adsorption of dopamine on gold and its interactions with iron(III) ions studied by microcantilevers, Anal. Chim. Acta 649(1) (2009) 117-122. [16] C. Janiak, A critical account on π-π stacking in metal complexes with aromatic nitrogen-containing ligands, Dalton Trans. (21) (2000) 3885-3896. [17] C. Pan, K. Sugiyasu, Y. Wakayama, A. Sato, M. Takeuchi, Thermoplastic Fluorescent Conjugated
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Polymers: Benefits of Preventing π-π Stacking, Angew. Chem. Int. Ed. 52(41) (2013) 10775-10779. [18] S. Bevers, S. Schutte, L.W. McLaughlin, Naphthalene- and perylene-based linkers for the stabilization of hairpin triplexes, J. Am. Chem. Soc. 122(25) (2000) 5905-5915. [19] R.E. Fleming, P. Ponka, Iron Overload in Human Disease, N. Engl. J. Med. 366(4) (2012) 348-359. [20] E. Nemeth, Iron regulation and erythropoiesis, Curr Opin in Hematol 15(3) (2008) 169-175.
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Figure captions
Scheme 1. Structure transformation process of iron-PDA complex under Fe3+ stimulation. Scheme 2. Probable mechanism for the morphology transformation of PDA dots. Figure 1. TEM images of PDA dots (A and B), fluorescence emission spectra of synthesized PDA dots under varied excitation wavelength (C), and fluorescence property of PDA dots under varied pH (C), temperature (D), and buffer concentration (E). Figure 2. Fluorescence spectra of PDA dots in the absence and presence of 2 mM Fe3+ (A), and TEM images of PDA dots in the presence of 2 mM Fe3+ (B and C). Figure 3. TEM images of PDA dots after addition of 1 mM (A), 1.5 mM (B) and 2 mM (C) Fe3+. Figure 4. TEM images of PDA dots after treated with HCl (A and B), and TEM images of PDA dots after treated with 2 mM Cd2+ (C), Mn2+ (D), Co2+ (E)and Ni2+ (F).
Figure 5. Fluorescence spectra of PDA dots in the absence and presence of Fe3+ of 10 μM, 20 μM, 50 μM, 100 μM, 200 μM, 500 μM, 1 mM, 1.5 mM, 2 mM, 2.5 mM, 3 mM, 3.5 mM, 4 mM and 4.5 mM from top from bottom. Inset: a plot of inhibition ratio at 452 nm versus the Fe3+concentration (A). The inhibition effect of other anions (from left to right: Na+, K+, Mg2+, Mn2+, Cd2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Al3+) towards PDA dots in the absence (green bars) and presence (red bars) of Fe3+, the concentration of Fe3+ and other metal ions was 2 mM (B).
Table 1 Spike and recovery test for Fe3+ detection in natural samples Fe3+ added
Fe3+ found
RSD
Recovery
(μM)
(μM)
(%, n=3)
(%)
Tap water sample 1
200.00
207.97
3.48
102.48
Tap water sample 2
400.00
399.22
4.96
99.80
Samples
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Sea water sample 1
200.00
202.407
6.43
101.20
Sea water sample 2
400.00
401.532
5.18
100.38
Highlights ● ● ● ●
The morphology transformation ability of PDA dots was reported for the first time. Dramatic morphology change of PDA dots was initiated by stimuli of ferric ion. A self-assembled mechanism was proposed to explain the stimuli responsive property. A rapid and simple method based on PDA dots was proposed for selective Fe3+ detection .
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Scheme 1
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Scheme 2
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