Microplasma electrochemistry controlled rapid preparation of fluorescent polydopamine nanoparticles and their application in uranium detection

Accepted Manuscript Microplasma electrochemistry controlled rapid preparation of fluorescent polydopamine nanoparticles and their application in uranium detection Zhe Wang, Chao Xu, Yuexiang Lu, Guoyu Wei, Gang Ye, Taoxiang Sun, Jing Chen PII: DOI: Reference:

S1385-8947(18)30454-6 https://doi.org/10.1016/j.cej.2018.03.096 CEJ 18703

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

17 January 2018 5 March 2018 19 March 2018

Please cite this article as: Z. Wang, C. Xu, Y. Lu, G. Wei, G. Ye, T. Sun, J. Chen, Microplasma electrochemistry controlled rapid preparation of fluorescent polydopamine nanoparticles and their application in uranium detection, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.03.096

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Microplasma electrochemistry controlled rapid preparation of fluorescent polydopamine nanoparticles and their application in uranium detection Zhe Wang, Chao Xu, Yuexiang Lu*, Guoyu Wei, Gang Ye, Taoxiang Sun, Jing Chen* Institute of Nuclear and New Energy Technology, Collaborative Innovation Centre of Advanced Nuclear Energy Technology, Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing, People’s Republic of China, 100084 Corresponding Author： *E-mail: [email protected] *E-mail: [email protected]

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Abstract: Fluorescent polydopamine nanoparticles (FPD) are a kind of promising fluorescent nanoparticles for biosensing and imaging, while the rapid and controllable synthesis of FPD is still challenging. In this paper, we developed a microplasma electrochemistry strategy to regulate the oxidative polymerization process of dopamine, resulting in a controlled formation of FPD. Treating the dopamine solution with microplasma anode could not only generate oxidative species to trigger the nucleation of polydopamine nanoparticles at the plasma-liquid interface, but also provide an acidic environment to inhibit their further growing up during the diffusion process. Thus, uniform FPD with a diameter of 3.1 nm could be prepared within minutes. And, continuous generation of FPD could be achieved without the formation of aggregates when prolonging the reaction time. The obtained FPD had abundant functional groups on the surfaces, showing tunable fluorescent emission properties. These luminescent nanoparticles were demonstrated for highly selective detection of uranium with a detection limit of 2.1 mg/L. The novel microplasma electrochemistry strategy established in this work provided better opportunity for controllable synthesis of FPD, as well as other luminescent nanoparticles, and broadened their application in chemical sensing area.

Keywords: microplasma, polydopamine, anode, fluorescent, nanoparticle, uranium

1. Introduction

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As a famous neurotransmitter, dopamine (DA) has gained great research interest due to its facile polymerization property under alkaline and oxidant environment [1,2]. Polydopamine (PDA) has been widely used in surface modification [3], drug delivery [4] and adsorption areas [5]. While until 2012, Wei’s group [6] reported the fluorescent property of polydopamine nanoparticles and employed them for cell imaging. Up to now, fluorescent polydopamine nanoparticles (FPD) have been significantly applied in areas such as bioimaging [7] and fluorescent detection [8-12] due to their low toxicity and good biodegradable ability. However, controlling the self-polymerization of dopamine to form fluorescent nanoparticles still remains challenging, because a continuous growth of the PDA nanoparticles usually occurs which leads to the generation of large-sized aggregates with poor fluorescent property [7,8]. Thus, tedious operations [7,12] are often required for carefully controlling the size and oxidation state of the nanoparticles, making it time-consuming and difficult to obtain FPD with uniform size. Therefore, extensive efforts are still required to develop a strategy for the fast and controllable synthesis of uniform FPD. With a nonthermal atmospheric-pressure microplasma as gaseous electrode, microplasma electrochemistry has brought intensive attention in the field of materials synthesis [13]. The charge transfers and plasma neutral reactions at the plasma-liquid interface provide a unique environment for chemical reactions [14,15]. Up to now, most of the researches focus on the application of microplasma cathode for fabricating metal nanoparticles by taking advantage of its reduction property [16-18]. However, research on the controllable synthetic reactions with the assistance of microplasma electrodes, especially microplasma anode, has been rarely reported. Herein, we proposed a facile strategy, by employing a microplasma anode, for controllable preparation of FPD nanoparticles with uniform size and favorable luminescent property. The FPD with a diameter of 3.1 nm were generated in minutes with an initial pH=5.

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Characterizations showed that the fluorescent property and surface functionalities of the FPD were well-preserved. A higher yield of FPD could be obtained with prolonging the treatment time or enhancing the microplasma intensity. The reactive oxygen species (ROS) generated by plasma anode and the dynamic acidic environment in the solution of the anodic side were found to be responsible for rapid synthesis of the well-defined FPD, avoiding the formation of largesized aggregates. Taking advantage of the specific interactions between the functional groups on FPD and U(VI), the FPD were applied for a selective fluorescent detection of uranium with a detection limit of 2.1 mg/L. To the best of our knowledge, this is the first time that the uniform FPD were controllably prepared with microplasma anode and were employed for the fluorescent detection of uranium. 2. Experiment section 2.1 Materials Dopamine hydrochloride (~98%) was received from J&K Scientific Co. Sodium dihydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate (Na2HPO4) for buffer solutions were bought from Alfa Aesar. Other analytical chemicals including sodium hydroxide (NaOH), nitric acid (HNO3), ethanol, hydrochloric acid (HCl) and all the metal salts including AgNO3, KNO3, Mg(NO3)2, Sr(NO3)2, Ba(NO3)2, Cr(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, Cd(NO3)2, Pb(NO3)2 and Ce(NO3)3 were purchased from Beijing Chemical Works. All aqueous solutions were prepared with deionized water and reagents of analytical grade were used as received. 2.2 Characterizations The microplasma device was composed of microplasma anode and Pt cathode as shown in Fig. 1. The plasma was generated with argon passing through a hollow stainless steel tube (diameter ~ 180 mm) under a voltage of 1700-2500 V. The plasma jet was about 2 mm above the solution and the Pt electrode is immersed in solution. A separator was equipped between the anode and

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cathode chambers to separate the solutions of the two sides. Transmission electron microscope (TEM) images of FPD at different reaction times were characterized on HT-7700 microscope. FT-IR spectra of FPD were recorded using the KBr pellets method on a Nicolet Nexus 470 FTIR spectrometer. Fluorescence spectroscopy was examined by FluoroMax-4 spectrophotometer. The UV-Vis absorbance spectra were measured on a Cary 6000i UV-vis-NIR spectrophotometer (Agilent Technologies). Mass spectrometry was carried out using TOF-SIMS 5-100 (ION-TOF GMbH Germany). The spectral data were obtained through a pulsed of 25 keV Bi+ primary ion beam. The X-ray diffraction (XRD) patterns of FPD were obtained on a Rigaku D/max-2400 Xray powder diffractometer with Cu Kα radiation. Equipped with a mono Al Kα X-ray source (1361 eV), the X-ray Photoelectron Spectroscopy (XPS, 250XI) was employed to characterize the elements of FPD. The zeta potential of FPD before and after contact with U(VI) was measured with a Nano ZS90 NanoSizer (Malvern, UK). 2.3 Preparation of fluorescent polydopamine nanoparticles (FPD) Dopamine hydrochloride powder was dissolved into PBS buffer (pH=5.0, 10 mM). In the “H” type reactor, 5 mL solution was added to each side and the reaction began with the microplasma as anode and Pt as cathode [19,20]. The voltage between anode and cathode is about 1700-2500 V under different circumstances. Different reaction conditions were carried out with the reaction current varied from 3 to 9 mA and the reaction time increased from 2 to 30 min. When each reaction was finished, the solutions were taken out and then the UV-Vis spectra and the fluorescent spectra were measured. To obtain more FPD, different concentrations of dopamine changed from 0.1 to 20 mg/mL were also investigated. The obtained products (dopamine concentration=20 mg/mL, reaction current=9 mA, reaction time=30 min) were dialyzed (Mw=1000 Da) in water for 24 h to remove the unreacted reagents for further study. Each experiment was repeated at least three times to reduce the relative error.

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2.4 Fluorescent detection of uranium with FPD The FPD prepared at the following conditions: reaction time=30 min, concentration of dopamine=20 mg/mL, current=9 mA was applied for the detection of uranium experiment. The dialyzed FPD solutions were diluted 300 times by adding uranium solutions with different concentrations at pH=5.0. Then the fluorescence intensity of different solutions was recorded on the fluorescence spectrometer under the excitation wavelength of 360 nm. The experiments were done in triplicates with a relative error smaller than 5%. To test metal selectivity, the prepared FPD diluted 300 times were respectively incubated with various metal ions (100 mg/L each), +

2+

2+

2+

3+

2+

2+

2+

2+

2+

2+

3+

4+

including K , Mg , Sr , Ba , Cr , Co , Ni , Cu , Zn , Cd , Pb , Ce and Th at pH=5.0. The fluorescence emission spectra were also recorded. 3. Results and Discussion 3.1 Characterizations of FPD

Ar gas flow

Pt cathode

Current source

Plasma anode

Separator Fig. 1 Illustration of the microplasma electrochemical device. The fluorescent polydopamine nanoparticles (FPD) were prepared with the treatment of microplasma anode. Briefly, 20 mg/mL dopamine solution was added to an “H” type reactor (Fig. 1) and then treated with microplasma anode and Pt cathode. The initial pH of dopamine

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solution was adjusted to 5 with PBS buffer solution to prevent the auto-polymerization. The TEM image (Fig. 2a) demonstrated that well-defined fluorescent polydopamine nanoparticles were obtained. Through statistical analyses of 50 FPD nanoparticles, the average diameter was calculated to be about 3.1 nm (Fig. 2b). XRD analysis of the FPD nanoparticles (Fig. 2c) suggested that there was no obviously lattice structure in their structure. And, the FPD was mainly composed of amorphous

a)

B

b) 30

Average=3.1 nm

Percentage (%)

25

20 nm

20 15 10 5 0

B

c)

1.5

Intensity (a.u.)

Intensity (a.u.)

15 10 5 0 20

30

40

50

60 o

2 theta/( )

70

80

2.5

3.0

3.5

4.0

4.5

Size (nm)

d)

25 20

2.0

Ex 320 nm Ex 340 nm Ex 360 nm Ex 380 nm Ex 400 nm Ex 420 nm Ex 440 nm

350 400 450 500 550 600 650

Wavelength (nm)

Fig. 2 (a) TEM image, (b) particle size distribution, (c) XRD spectrum and (d) fluorescence excitation spectra (dilute for 100 times) of FPD. Concentration of dopamine solution =20 mg/mL, current=9 mA, reaction time=30 min, pHinitial=5. carbon instead of graphic carbon. The fluorescent properties of FPD were examined as shown in Fig. 2d. When the excitation wavelengths progressively increased from 320 nm to 440 nm, the

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peaks of the emission spectra shifted to higher wavelength positions accordingly. This excitation-dependent photoluminescence (PL) property was similar to the FPD reported previously, which could be attributed to the different sizes of the FPD as well as the heterogeneous structures of each particle [6,8-10]. The maximum emission was recorded at the 360 nm excitation wavelength and the peak intensity was around 440 nm. Thus, the 360 nm excitation wavelength and the emission at 440 nm were selected for further applications. The quantum yield of FPD was calculated to be about 0.58% with quinone sulphate as standard. Fig. 3a displayed the UV-Vis absorption spectra and the UV-Vis absorption peak occurred at about 360 nm, which indicated the quinone formation [21] and data at 360 nm was selected for analysis. The FTIR spectrum in Fig. 3b displayed the ν(O-H/N-H) at 3200-3400 cm-1, ν(=C-H) on the benzene ring at 3050 cm-1, ν(C=O) at 1705 cm-1 and ν(C=C) on the benzene ring at 1610 -1

cm . Besides, the XPS analysis of FPD was also measured by drying the FPD solution in a vacuum oven to obtain the powder FPD. As shown in Fig. 3c, it was clear that the FPD was composed of C, N and O elements and the content percent of these three elements was 61.9%, 9.0% and 29.1%, respectively (Table 1). Also, the N/C ratio of FPD was computed to be 1:8 and that was quite similar with the structure of dopamine. While, the O/C ration of FPD was about 1:2.8 which was lower than dopamine molecular of 1:2. It was possibly caused by the dehydration of FPD during the drying procedure. Moreover, the C1s peak of FPD was further analyzed in Fig. 3d and the peak binding energy and the corresponding percent were summarized in Table 2. The C element was connected with three kinds of chemical bonds which were CC/C=C, nitrous carbon and oxygenated carbon. These functional groups identified by XPS confirmed the formation of FPD nanoparticles which were of great potential to be used as fluorescent probes.

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Abs

a)

Transmittance (%)

4

Absorbance

Smoothed Y 2

b) 3 2 1

100

0 300

400

500

600

80 70

B

C=O

=C-H 60 O-H/N-H

C=C

3500 3000 2500 2000 1500 1000

700

Wavelength (nm)

c)

90

-1

Wavenumber (cm )

d)

O

800

Intensity

Intensity

C

N

600

400

Binding energy (eV)

200

291

288

285

282

Binding energy (eV)

Fig. 3 (a) UV-Vis spectra, (b) FT-IR analysis, (c) XPS analysis and (d) the C1s peak analysis of FPD at concentration of dopamine solution=20 mg/mL, current=9 mA, reaction time=30 min, pHinitial=5. Table 1 XPS analysis of FPD Elements

C1s

N1s

O1s

Atomic (%)

61.9

9.0

29.1

Table 2 XPS analysis of FPD (C1s analysis) C-C/C=C

Nitrous Carbon

Oxygenated Carbon

Peak binding energy (eV)

283.8

285.4

287.4

FPD (%)

31.99

35.52

32.49

9

a)

B

B

b)

6

3x10

0.8

6

2x10

0.6

I/I0

Intensity (a.u.)

1.0

0.4

6

1x10

0.2

0

1

2

3

4

pH

5

6

7

0.0

0.0

0.5

1.0

1.5

2.0

Salt concentration (mol/L)

Fig. 4 Fluorescent intensity of FPD (a) at different pH values and (b) in NaCl solutions with different concentrations, I0 and I represented the fluorescent intensity of FPD in de-ionized water and in different concentration of NaCl solutions, respectively. Excitation wavelength was 360 nm, and the peak emission at 440 nm was recorded for analysis.

The fluorescent stability of FPD at different pH values and in NaCl solutions with various concentrations was also investigated. As shown in Fig. 4a, the fluorescent intensity slightly increased when pH rising from 1 to 5 and then appeared no obviously quenching. Moreover, the intensity remained almost the same at different concentrations of NaCl in Fig. 4b and it verified that the FPD possessed a good salt stability. A series of condition experiments were designed to make the formation mechanism more clear. As shown in Fig. 5, the color of the products, UV-Vis absorbance intensity and the fluorescent intensity increased accordingly with the enhancement of the reaction conditions including the treatment time, the reaction current and the initial concentration of dopamine. These rising amounts of FPD were due to the increased oxygen radical produced by the plasma. These results reconfirmed that the plasma anode could controllably synthesize uniform FPD

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nanoparticles without the addition of other reagents. And the as-prepared FPD are more convenient for further application without aggregation. a)

2 min 5 min 10 min 20 min 30 min

M

6

6

6x10

4 3 2 1 0

b)

3 mA 4.5 mA 6 mA 7.5 mA 9 mA

30 min 20 min 10 min 5 min 2 min

6

Intensity (a.u.)

Absorbance

5

5x10

6

4x10

6

3x10

6

2x10

6

1x10

0

0

5

10

15

20

25

30

Time (min)

4

400

I

450

500

600

6

3.5x10

9.0 mA 7.5 mA 6.0 mA 4.5 mA 3.0 mA

6

Intensity (a.u.)

3.0x10

Absorbance

550

Wavelength (nm)

3

2

6

2.5x10

6

2.0x10

6

1.5x10

6

1.0x10

5

5.0x10

1

c)

10 5 2 20 0.1 mg/mL mg/mL mg/mL mg/mL mg/mL

0.0

3

4

5

6

7

Current (mA)

8

9

400

450

500

6

Intensity (a.u.)

Absorbance

3 2 1

600

20 mg/mL 10 mg/mL 5.0 mg/mL 2.0 mg/mL 1.0 mg/mL 0.1 mg/mL

5 4

550

Wavelength (nm)

O1

1.5x10

6

1.0x10

5

5.0x10

0.0

0 0

5

10

15

20

Concentration (mg/mL)

400

450

500

550

600

Wavelength (nm)

Fig. 5 Photographs, UV-Vis absorbance (data at 360 nm) and fluorescence spectra changes of FPD in different reaction conditions (excited at 360 nm). (a) different reactions time with 5 mg/mL dopamine and current=6 mA; (b) different current with 5 mg/mL dopamine and time=10 min; (c) different concentration of dopamine solution with reaction time=10 min and current=6 mA, for the fluorescent spectra the reaction time=30 min and current=9 mA. 3.2 Mechanism of the synthesis of FPD The formation mechanism of FPD was further studied. In common dopamine polymerization process, the pH value of the solution and the reactive oxygen species dissolved in the solution were the two key parameters that affect the polymerization behavior [22-25]. With an initial pH

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of 5, the auto-polymerization of dopamine was inhibited. When discharge began, at the -

microplasma anodic side, there are amounts of reactive oxygen species (ROS), including O2• and HO2•, as well as OH• in the plasma system [14,15]. These strong oxidants could turn dopamine to quinone and then polymerized. At the same time, the charge transferred at the plasma-liquid interface caused the electrolysis of water resulting in a more acidic environment, in which the polymerization progress self-stopped rapidly. Thus, the dopamine could only polymerize to nanoparticles without further aggregation. As shown in Fig. 6a, with the increasing reaction time, the nanoparticle size remained almost the same and only the amount of the nanoparticles accumulated. For comparison, at the microplasma cathode, although polydopamine nanoparticles were also generated at the first few minutes, they grew bigger and bigger with the reaction time (Fig. 6b). That is because, the charge transfer at the microplasma cathodic side would result in a more alkalescent environment, which was suitable for the auto- polymerization of dopamine to fast and controllably form polydopamine membrane on various substrates [25].

a)

b)

2 min

10 min

30 min

20 nm

20 nm

20 nm

2 min

10 min

30 min

20 nm

200 nm

0.2 μm

Fig. 6 TEM images of polydopamine treated at (a) plasma anode and (b) plasma cathode for different reaction time.

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Besides, the possible chemical structures of FPD were also analyzed through TOF-SIMS spectra as it can detect some fragments [22,23]. As shown in Fig. 7, the signal corresponding to the dopamine dimer and trimer fragments could be observed at m/z 297/302 and 438/441/445, respectively. The possible structures of the dimer and trimer fragments were presented in Fig. 5 and they could transform to one another dynamically [26]. The spacing was consistent with the proposed repeat units of 5,6-dihydroxyindole in polydopamine which should be the basic unit of the polymerization of dopamine [22]. In addition, there were lots of hydroxyl groups on the FPD which could be attributed to the ROS produced by plasma anode. That was quite familiar with the polydopamine nanoparticles prepared by Jia-Hui Lin [27] with hydroxyl radical-induced method and they also verified the formation mechanism discussed above. Therefore, the plasma anode provided a unique condition for the synthesis of FPD, without external adding of acids or H2O2. 441

438

445 H

302 297

302

α: OH β: O

319 323 297 α

485

α

α 340

314

α 336

345 300

330

424 401

384 α 362 α 379 α 360

445

α 441 421 α438 β 457 469 α

390

420

450

486

480

m/z G

Fig. 7 TOF-SIMS spectra of FPD (dopamine concentration 20 mg/mL, plasma anode at 9 mA for 30 min) and the possible chemical structures of the fragments of FPD. A series of peaks referring to different fragments of the FPD were also presented in the spectra.

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3.3 Fluorescent detection of U(VI) with FPD Uranium is a valuable raw material for nuclear power plants as well as a toxic radioactive element [28], it is crucial to develop a simple method for the rapid and on-site detection of uranium in the waste water produced during the mining and processing progress [29,30]. The FPD prepared at the condition of 9 mA, 30 min and 20 mg/mL dopamine were employed for the sensitive and selective detection of uranium in aqueous solution. From Fig. 8a, the fluorescent intensity of FPD decreased gradually with the increasing concentration of U(VI) after adding FPD to the solution. And the fitting curve between the fluorescent intensity of FPD and the concentration of uranium changed from 0 to 100 mg/L displayed a good linear relationship (Fig. 8b) with a limit of detection of about 2.1 mg/L. The limit of detection was calculated through the equation LOD=3σ/s, where σ and s represent the standard deviation of the blank solution and sensitivity of the calibration plot respectively. The mechanism of the fluorescent quenching of FPD was proposed and investigated by the time-correlated single-photon counting (TCSPC) and dynamic light scattering method of FPD in the absence and presence of U(VI). As shown in Fig. 8c, the fluorescent lifetime of the FPD contained two components of 3.39 ns (ca. 83%) and 9.82 ns (ca. 17%). After coordination with U(VI), the lifetime were 2.97 ns (ca. 82.9%) and 9.28 ns (ca. 17.1%) and no obvious change in lifetime was observed after the introduction of U(VI). The results implied the quenching mechanism tend to be a static quench process and suggested that a possible chemical reaction occurred between the FPD and U(VI). [12,31,32] Then the dynamic light scattering was carried out to measure the particle size of the solutions. The size distribution results in Fig. 8d and Table 3 demonstrated that the pure FPD solution owned a very small particle size below 100 nm and it grew over to about1000 nm after the adding of uranium. That might be caused by the special

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interaction of FPD and U(VI) and then the U(VI) induced the aggregation of FPD thus to quench the fluorescence of FPD [33,34].

Intensity (a.u.)

blank 2 mg/L 5 mg/L 10 mg/L 20 mg/L 40 mg/L 50 mg/L 75 mg/L 100 mg/L

400

500

550

Wavelength (nm)

1.0

1.0 0.9

0.7 0.6 0.5

600

d)

0

20

0.6 0.4 0.2

40

60

80

100

Concentration (mg/L)

12

FPD FPD with U(VI)

0.8

R2=0.995

0.8

Intensity

Normalized intensity

c)

450

B Linear Fit of B

b)

I/I0

a)

FPD FPD with U(VI)

8

4

0.0 80

90

100

Time (ns)

110

0 0.1

1

10

100

Size (nm)

1000 10000

Fig. 8 (a) The fluorescence spectra of FPD in different concentration of uranium solutions at pH=5 (excited at 360 nm); (b) the linear fitting line between the fluorescence intensity and the concentration of uranium, peak data at 440 nm was selected for analysis; (c) the time-correlated single-photon counting (TCSPC) of FPD with and without U(VI) at excitation wavelength of 372 nm; (d) the size distribution of FPD with the dynamic light scattering method in the presence and absence of U(VI).

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Table 3 The size distribution of FPD with the dynamic light scattering method in the presence and absence of U(VI) FPD

FPD with U(VI)

Peak

Peak 1

Peak 2

Peak3

Peak 1

Peak 2

Size (nm)

0.27

5.69

73.85

425.5

2291

Percent (%)

32.4

40.4

19.7

23.9

76.1

According to other researches on the adsorption of uranium with the polydopamine composite material, the functional groups especially hydroxyl groups on polydopamine have a special interaction with uranium. [35-37] Therefore, the aggregation occurred after the FPD interacted with U(VI) to cause a fluorescent quenching of the FPD. Otherwise, the zeta potential of FPD before and after the addition of U(VI) at pH=5 was also measured. The initial zeta potential of bared FPD was 17.7±0.7 mV which indicated the surface of FPD was positively charged. After interacted with U(VI), the aggregation took place and the zeta potential decreased to about 12.5±0.4 mV. The decreased zeta potential could be attributed to the aggregation of FPD after interacting with U(VI). The selectivity of FPD was also investigated and various metal ions including K+, Mg2+, Sr2+, Ba2+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Ce3+ and Th4+ were challenged with the FPD probe. As shown in Fig. 9, U(VI) caused a dramatically fluorescent quenching. Thus the FPD can be considered as a fluorescent probe candidate for the detection of U(VI).

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B

0.5 0.4

(I0-I)/I0

0.3 0.2 0.1 0.0 -0.1

K

Mg

Sr

Ba

Cr

Co

Ni

Cu

Zn

Cd

Pb

Ce

Th

U

Fig. 9 The selectivity of FPD to uranium with a Aserious of competing metal ions. Excitation wavelength=360 nm, the data at 440 nm was selected for analysis, pH=5, [M]=100 mg/L. 4. Conclusion In conclusion, we present in this work a novel method for facile and controllable synthesis of fluorescent polydopamine nanoparticles (FPD) with the assistance of microplasma anode without introducing extra reagents. TEM and FT-IR analysis indicated that well-defined FPD were obtained with a large number of functional groups preserved on the surfaces. Mechanistic study suggested that, at anodic side, a high concentration of ROS generated by the microplasma triggered the polymerization reaction of dopamine at a localized area. Whereas the concurrently generated acidic environment inhabited the further growth of the FPD nanoparticles to form large-sized aggregates. The nanoparticles were applied as nanoprobe for the detection of uranium with good sensitivity and selectivity. Overall, this work brings forward a new strategy for the controllable polymerization of dopamine, and is expected to open a new direction for the application of microplasma electrochemistry. Acknowledgment

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We acknowledge the National Natural Science Foundation of China (Grant No. 51425403, 21390413, 21775087 and 91426302), Program for Changjiang Scholars and Innovative Research Team in University (IRT13026) and Tsinghua University Initiative Scientific Research Program (2014z22063). References [1] H. Lee, B.P. Lee, P.B. Messersmith, A reversible wet/dry adhesive inspired by mussels and geckos, Nature. 448 (2007) 338-341. [2] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057-5115. [3] Q. Ye, F. Zhou, W. Liu, Bioinspired catecholic chemistry for surface modification, Chem. Soc. Rev. 40 (2011) 4244-4258. [4] A.I. Neto, A.C. Cibrão, C.R. Correia, R.R. Carvalho, G.M. Luz, G.G. Ferrer, G. Botelho, C. Picart, N.M. Alves, J.F. Mano, Nanostructured Polymeric Coatings Based on Chitosan and Dopamine-Modified Hyaluronic Acid for Biomedical Applications, Small. 10 (2014) 2459-2469. [5] Z. Dong, D. Wang, X. Liu, X. Pei, L. Chen, J. Jin, Bio-inspired surface-functionalization of graphene oxide for the adsorption of organic dyes and heavy metal ions with a super high capacity, J. Mater. Chem. A. 2 (2014) 5034-5040. [6] 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 (2012) 5581-5584.

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[36] F. Wu, N. Pu, G. Ye, T. Sun, Z. Wang, Y. Song, W. Wang, X. Huo, Y. Lu, J. Chen, Performance and Mechanism of Uranium Adsorption from Seawater to Poly (dopamine)Inspired Sorbents, Environ. Sci. Technol. 51 (2017) 4606-4614. [37] Y. Yang, J. Wang, F. Wu, G. Ye, R. Yi, Y. Lu, J. Chen, Surface-initiated SET-LRP mediated by mussel-inspired polydopamine chemistry for controlled building of novel core–shell magnetic nanoparticles for highly-efficient uranium enrichment, Polym. Chem. 7 (2016) 24272435. List of Figures and table Fig. 1 Illustration of the microplasma electrochemical device. Fig. 2 (a) TEM image, (b) particle size distribution, (c) XRD spectrum and (d) fluorescence excitation spectra (dilute for 100 times) of FPD. Concentration of dopamine solution =20 mg/mL, current=9 mA, reaction time=30 min, pHinitial=5. Fig. 3 (a) UV-Vis spectra, (b) FT-IR analysis, (c) XPS analysis and (d) the C1s peak analysis of FPD at concentration of dopamine solution=20 mg/mL, current=9 mA, reaction time=30 min, pHinitial=5. Fig. 4 Fluorescent intensity of FPD (a) at different pH values and (b) in NaCl solutions with different concentrations, I0 and I represented the fluorescent intensity of FPD in de-ionized water and in different concentration of NaCl solutions, respectively. Excitation wavelength was 360 nm, and the peak emission at 440 nm was recorded for analysis. Fig. 5 Photographs, UV-Vis absorbance (data at 360 nm) and fluorescence spectra changes of FPD in different reaction conditions (excited at 360 nm). (a) different reactions time with 5 mg/mL dopamine and current=6 mA; (b) different current with 5 mg/mL dopamine and time=10

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min; (c) different concentration of dopamine solution with reaction time=10 min and current=6 mA, for the fluorescent spectra the reaction time=30 min and current=9 mA. Fig. 6 TEM images of polydopamine treated at (a) plasma anode and (b) plasma cathode for different reaction time. Fig. 7 TOF-SIMS spectra of FPD (dopamine concentration 20 mg/mL, plasma anode at 9 mA for 30 min) and the possible chemical structures of the fragments of FPD. A series of peaks referring to different fragments of the FPD were also presented in the spectra. Fig. 8 (a) The fluorescence spectra of FPD in different concentration of uranium solutions at pH=5 (excited at 360 nm); (b) the linear fitting line between the fluorescence intensity and the concentration of uranium, peak data at 440 nm was selected for analysis; (c) the time-correlated single-photon counting (TCSPC) of FPD with and without U(VI) at excitation wavelength of 372 nm; (d) the size distribution of FPD with the dynamic light scattering method in the presence and absence of U(VI). Fig. 9 The selectivity of FPD to uranium with a serious of competing metal ions. Excitation wavelength=360 nm, the data at 440 nm was selected for analysis, pH=5, [M]=100 mg/L. Table 1. XPS analysis of FPD Table 2. XPS analysis of FPD (C1s analysis) Table 3. The size distribution of FPD with the dynamic light scattering method in the presence and absence of U(VI)

24

Fig. 1

Ar gas flow

Pt cathode

Current source

Plasma anode

Separator Fig. 2

25

a)

B

b) 30

Average=3.1 nm

Percentage (%)

25

20 nm

20 15 10 5 0

B

c)

Intensity (a.u.)

Intensity (a.u.)

15 10 5 0 20

30

40

50

60 o

2 theta/( )

70

80

2.0

2.5

3.0

3.5

4.0

4.5

Size (nm)

d)

25 20

1.5

Ex 320 nm Ex 340 nm Ex 360 nm Ex 380 nm Ex 400 nm Ex 420 nm Ex 440 nm

350 400 450 500 550 600 650

Wavelength (nm)

Fig. 3

26

Abs

a)

Smoothed Y 2

b) Transmittance (%)

4

Absorbance

3 2 1

100

0 300

c)

400

500

600

90 80 70

B

C=C

3500 3000 2500 2000 1500 1000

700

Wavelength (nm)

C=O

=C-H 60 O-H/N-H

-1

Wavenumber (cm )

d)

O

Intensity

Intensity

C

N

800

600

400

Binding energy (eV)

291

200

288

285

282

Binding energy (eV)

Fig. 4

a)

B

B

b)

6

3x10

0.8

6

2x10

0.6

I/I0

Intensity (a.u.)

1.0

0.4

6

1x10

0.2

0

1

2

3

4

pH

5

6

7

0.0

0.0

0.5

1.0

1.5

2.0

Salt concentration (mol/L)

27

Fig. 5

a)

2 min 5 min 10 min 20 min 30 min

M

6

6

6x10 4 3 2 1 0

b)

3 mA 4.5 mA 6 mA 7.5 mA 9 mA

30 min 20 min 10 min 5 min 2 min

6

Intensity (a.u.)

Absorbance

5

5x10

6

4x10

6

3x10

6

2x10

6

1x10

0 0

5

10

15

20

25

30

Time (min)

4

400

I

450

500

600

6

3.5x10

9.0 mA 7.5 mA 6.0 mA 4.5 mA 3.0 mA

6

Intensity (a.u.)

3.0x10

Absorbance

550

Wavelength (nm)

3

2

6

2.5x10

6

2.0x10

6

1.5x10

6

1.0x10

5

5.0x10

1

c)

10 5 2 20 0.1 mg/mL mg/mL mg/mL mg/mL mg/mL

0.0

3

4

5

6

7

Current (mA)

8

9

400

450

500

O1

6

Intensity (a.u.)

Absorbance

3 2 1

600

20 mg/mL 10 mg/mL 5.0 mg/mL 2.0 mg/mL 1.0 mg/mL 0.1 mg/mL

5 4

550

Wavelength (nm)

1.5x10

6

1.0x10

5

5.0x10

0.0

0 0

5

10

15

20

Concentration (mg/mL)

400

450

500

550

600

Wavelength (nm)

Fig. 6

28

a)

2 min

10 min

30 min

20 nm

20 nm

20 nm

2 min

10 min

30 min

20 nm

200 nm

0.2 μm

b)

Fig. 7 441

438

445 H

302 297

302

α: OH β: O

319 323 297 α

485

α

α 340

314

α 336

345 300

330

424 401

384 α 362 α 379 α 360

445

α 441 421 α438 β 457 469 α

390

420

450

486

480

m/z G

Fig. 8

29

Intensity (a.u.)

blank 2 mg/L 5 mg/L 10 mg/L 20 mg/L 40 mg/L 50 mg/L 75 mg/L 100 mg/L

400

500

550

Wavelength (nm)

1.0

1.0 0.9

0.7 0.6 0.5

600

0

d)

20

0.6 0.4 0.2

40

60

80

100

Concentration (mg/L)

12

FPD FPD with U(VI)

0.8

R2=0.995

0.8

Intensity

Normalized intensity

c)

450

B Linear Fit of B

b)

I/I0

a)

FPD FPD with U(VI)

8

4

0.0 80

90

100

110

Time (ns)

0 0.1

1

10

100

Size (nm)

1000 10000

Fig. 9

B

0.5 0.4

(I0-I)/I0

0.3 0.2 0.1 0.0 -0.1

K

Mg

Sr

Ba

Cr

Co

Ni

Cu

Zn

Cd

Pb

Ce

Th

U

A

30

Table 1 Elements

C1s

N1s

O1s

Atomic (%)

61.85

9

29.15

Table 2 C-C/C=C

Nitrous Carbon

Oxygenated Carbon

Peak binding energy (eV)

283.8

285.4

287.4

FPD (%)

31.99

35.52

32.49

Table 3 FPD

FPD with U(VI)

Peak

Peak 1

Peak 2

Peak3

Peak 1

Peak 2

Size (nm)

0.27

5.69

73.85

425.5

2291

Percent (%)

32.4

40.4

19.7

23.9

76.1

31

Table of Contents Graphic

ROS

H+

Microplasma anode

U(VI)

Fluorescent polydopamine nanoparticles

With the assistant of microplasma anode, the uniform fluorescent polydopamine nanoparticles (FPD) could be prepared continuously and the FPD can be applied for fluorescent chemical sensing of uranium.

32

Highlights 1. Developed a novel microplasma method for facile and controllable synthesis of FPD. 2. The FPD could be prepared continuously without additional reagents. 3. The possible formation mechanism of FPD was studied. 4. The FPD were first applied for the detection of U(VI). 5. The fluorescent quench detection mechanism was also investigated.

33