Highly Sensitive and Selective Detection of Nanomolar Ferric Ions

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Highly Sensitive and Selective Detection of Nanomolar Ferric Ions Using Dopamine Functionalized Graphene Quantum Dots Ankan Dutta Chowdhury, and Ruey-an Doong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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ACS Applied Materials & Interfaces

Highly

Sensitive and

Nanomolar

Ferric

Selective Ions

Detection

Using

of

Dopamine

Functionalized Graphene Quantum Dots Ankan Dutta Chowdhurya, Ruey-an Doonga,b,*

a

Institute of Environmental Engineering, National Chiao Tung University, 1001, University Road, Hsinchu, Taiwan.

b

Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu, 30013, Taiwan.

ABSTRACT.

The good stability, low cytotoxicity and excellent photoluminescence property of graphene quantum dots (GQDs) make them an emerging class of promising materials in various application fields ranging from sensor to drug delivery. In the present work, the dopaminefunctionalized GQDs (DA-GQDs) with stably bright blue fluorescence were successfully synthesized for low level Fe3+ ions detection. The as-synthesized GQDs are uniform in size with narrow-distributed particle size of 4.5 ± 0.6 nm and high quantum yield of 10.2%. The amide 1

ACS Paragon Plus Environment


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linkage of GQDs with dopamine, confirmed by using XPS and FTIR spectra, results in the specific interaction between Fe3+ and catechol moiety of dopamine at the interfaces for highly sensitive and selective detection of Fe3+. A linear range of 20 nM – 2 µM with a detection limit of 7.6 nM is obtained for Fe3+ detection by DA-GQDs. The selectivity of DA-GQDs sensing probe is significantly excellent in the presence of other interfering metal ions. In addition, the reaction mechanism for Fe3+ detection based on the complexation and oxidation of dopamine has been proposed and validated. Results obtained in this study clearly demonstrate the superiority of surface functionalized GQDs to Fe3+ detection, which can pave an avenue for the development of high performance and robust sensing probes for detection of metal ions and other organic metabolites in environmental and biomedical applications.

Keywords: Ferric ions detection; graphene quantum dots (GQDs); dopamine-functionalized GQDs; fluorescence; selectivity.

INTRODUCTION Graphene quantum dots (GQDs), one of the newly developed members of graphene family, have been discovered very recently as a class of zero-dimensional nanomaterials. Due to their extraordinary optical, thermal and electronic properties, GQDs along with their doped or functionalized nanocomposites have emerged as the most promising nanomaterials in several fields such as electronics,1,

2

energy storage,3-5 catalysis,6 materials science4 and biomedical

engineering.2, 7, 8 Compared with heavy metal-based QDs and other fluorophores developed in recent years, GQDs have attracted growing interest in biosensing,8-10 bioimaging11 and metal ion

2


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sensing12-14 due to their excellent photoluminescence property, good stability, easy preparation, and low cytotoxicity. Up to now, several methods including top-down and bottom-up approaches have been developed to fabricate GQDs with tunable size distribution as well as to improve the quantum yield (QY).15, 16 The former technique involves the exfoliation and break-down from various carbon materials such as carbon fibers, graphite rods and graphene oxides using solvothermal, mechanical, chemical and electrochemical processes.10,

17

Although these GQDs have

fortuitously satisfactory QY values in some specific cases, the distribution of size and thickness (layer numbers) of GQDs are usually broad and non-uniform. On the other hand, the conjugation of small carbon molecules including glucose, citric acid and polyphenols by using the bottom-up strategy results in the well-controlled GQDs with tunable size, shape and property.18, 19 However, the QY of GQDs fabricated by using bottom-up strategy is often lower than the required quality for sensing application. Therefore, the improvement of QY with homogeneous distribution of GQDs after proper functionalization is still a big challenge for their applications, especially in metal ion sensing field. The pure and doped GQDs including N-, S- and B-doped GQDs have been used to detect low level of metal ions.20, 21 However, these nanomaterials usually showed less advantage on serving as the fluorescence probe to the other conventional materials in terms of sensitivity and selectivity. This may be attributed to the fact that the GQDs do not have satisfactory QY and fluorescence activity, leading to the inferior detection performance. In addition, the lack of functionality on GQD surfaces results in the poor selectivity during the detection processes. Therefore, the development of specially functionalized GQDs with high QY has become an essential requirement for analytical applications. 3



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Fe3+ is one of the most essential metal ions in biological systems and plays a crucial role in several physiological and pathological processes including cellular metabolism, enzyme catalysis, oxygen transport and RNA synthesis.22-24 The precise determination of low level of Fe3+ concentration becomes a very important diagnosis strategy to monitor these biological pathways. In addition, the measurement of iron concentration in water samples is crucial for environmental safety.25 Currently, several analytical techniques including spectrophotometry,26, 27

atomic absorption spectrometry,28 inductively coupled plasma mass spectrometry29 have

already been established to determine the concentration of Fe3+. However, these methods require sophisticated instrumentations and laborious sample preparation procedures, which have hampered their practical applications to the on-time and in-situ detection. More recently, the fluorescence assays using fluorophores or transition metal chalcogenide based QDs have become significant to achieve high sensitivity and rapid response to metal ions and organics detection.13, 24, 27, 30 The simple instrumentation with high sensitivity makes the fluorescence method as a prime choice for metal ion sensing. However, the transition metal chalcogenide based QDs and other fluorescence dyes are highly toxic to the environment and human beings. For this reason, non-toxic and high sensitive GQDs along with doped GQDs have been emerged as the promising fluorophores.21 It is noted that most fluorescence assays including GQDs and metal chalcogenide QDs show less specific affinity toward metal ions because of the lack of functionality on the surface and, thus, suffer from poor selectivity on metal ion detection. Several efforts have been made on the functionalization of organic molecules at the edge of GQDs for energy and sensing applications.31,

32

Dopamine is a well-known

catecholamine neurotransmitter and the dopamine-iron complex has been demonstrated to be highly related to the Parkinson’s disease.33 It means that dopamine has good affinity toward iron

4


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ions and can serve as an excellent sensing probe for Fe3+ detection. However, the surface functionalization by anchoring dopamine onto the surface of GQDs has received less attention and the sensing ability as well as selectivity of dopamine-functionalized GQDs (DA-GQDs) toward metal ion detection has rarely been reported.

360 nm

460 nm

200 oC, 25 min

EDC/NHS

Quenching

Fe3+

Fe3+

Scheme 1. The preparation of dopamine-functionalized graphene quantum dots (DA-GQDs) sensor and the proposed mechanism for Fe3+ detection. Herein, we have synthesized soluble DA-GQDs with high QY (Φ = 0.102) for highly sensitive and selective detection of Fe3+. Scheme 1 shows the fabrication and sensing mechanism of DA-GQDs for Fe3+ detection. The water soluble GQDs are originally fabricated from the pyrolysis of citric acid followed by the covalent conjugation with dopamine to serve as the fluorescent probe. After the addition of Fe3+, the catechol moiety of dopamine coordinates with Fe3+ ions and then oxidizes to o-semiquinone. This oxidation process results in the quantitative quenching and the decrease in fluorescence intensity is proportional to the Fe3+ concentration. In addition, the sensitivity as well as selectivity of DA-GQDs toward Fe3+ detection is examined in the presence of a wide variety of metal ions. The DA-GQDs show a sensitive response to Fe3+ in 5



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the concentration range of 20 nM – 20 µM, and the detection limit is found to be 7.6 nM, which can open an avenue to tailor the functionalized GQDs with unique molecules for biomedical sensing, analyte detection and cancer diagnosis. EXPERIMENTAL Reagents.

Citric

acid,

dopamine

hydrochloride,

sodium

borohydride

and

N-(3-

dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC) were purchased from SigmaAldrich. N-hydroxysuccinimide (NHS) was obtained from Alfa Aesar. Analytical grade of iron (III) chloride and other metal salts were used as received without further purification. All the solutions were prepared by using bidistilled deionized water (18.2 MΩ cm) unless otherwise mentioned. The water used throughout the experiments was purified through a UV treated Rephile water system. Preparation of GQDs. The GQDs were prepared by using direct pyrolyzation of citric acid according to the previous study with minor modification.19 In a typical procedure of GQDs preparation, 2 g of citric acid were added into a 100-mL round bottom flask and heated to 200ºC by using a heating mantle in oil bath. The citric acid was liquated and turned to yellowish color after 5 min. Then an orange color was appeared after the heating of 20 min, depicting the formation of GQDs. In addition, different reaction times of preparation ranging from 20 to 45 min were examined and a reaction time of 25 min was chosen in this study to fabricate the asprepared GQDs with high QY. Extra heating time was strictly avoided because the overheating may possibly cause the formation of graphene oxides. The obtained orange liquid containing GQDs was then added dropwisely into 100 mL of 10 g/L sodium hydroxide (NaOH) solution

6


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under vigorous stirring. After neutralization to pH 7.0 with NaOH, the aqueous solution of GQDs was obtained and preserved at 4 ºC for further use. Functionalization of GQDs with dopamine. The dopamine-functionalized GQDs were prepared by coupling the amine group of dopamine with the carboxylic acid group of GQDs using the standard EDC/NHS reaction at room temperature and at pH 6.34 Briefly, 3 mg of EDC and 4 mg of NHS were added into 10 mL of as-prepared GQDs solution under vigorous stirring. After adding 2 mg of dopamine hydrochloride into the solution, the reaction was performed for 2 h under vigorous stirring at room temperature. Then, the final solution was dialyzed in a 2 kD dialysis bag for 24 h (dialysate was replaced every 8 h) to remove the un-reacted chemicals. Characterization. The transmission electron microscopy (TEM) was used to identify the morphology and size of GQDs by using a JEOL JEM-ARM200F transmission electron microscope and a JEOL JEM-2010 high-resolution TEM (HR-TEM) at 200 kV. Atomic force microscopy (AFM) images were obtained by using Agilent 5500 scanning probe microscope in tapping mode to elucidate the topography of GQDs. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 X-ray diffractometer with Ni-filtered Cu-Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was performed with an ESCA Ulvac-PHI 1600 photoelectron spectrometer from Physical Electronics using Al-Kα radiation photon energy at 1486.6 ± 0.2 eV. UV−vis and fluorescence spectra of GQDs were recorded by Hitachi U-4100 and F-7000 fluorescence spectrophotometers, respectively. The Fourier transform infrared (FTIR) spectra of GQDs were determined by using a Horiba FT720 spectrophotometer. In addition, Raman spectra of QGD-based materials were recorded by using Burker Senterra microRaman spectrometer equipped with an Olympus BX 51 microscope and an Andor DU420-OE CCD camera. 7



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Detection of Fe3+ using DA-GQDs. The detection of Fe3+ by using DA-GQDs was performed at room temperature in the 10 mM phosphate buffered saline (PBS) buffer solutions at pH 6. The choice of mild acidic pH is to minimize the self-oxidation of dopamine to its cyclic form. In addition, phosphate ions can serve as the chelating agent to avoid the precipitation of Fe3+ at pH 6. For Fe3+ detection procedures, 2 mL of diluted stock solution of DA-GQDs were mixed with the Fe3+ solution prepared in PBS buffer at pH 6 to get the final concentration of 20 nM – 20 µM. The resulting solutions were incubated at room temperature under well-mixing conditions. After incubation for 5 min, the change in fluorescence intensity was recorded at the wavelength of 460 nm under the excitation wavelength at 360 nm. In addition, the selectivity of sensing system was conducted by adding 15 different types of 20 µM metal ion solutions into DA-GQD solution and then the quenching of fluorescence was recorded after the incubation of 5 min. The measurements for the detection of metal ions were performed in triplicate. The other characterizations of GQDs and DA-GQDs including the photostability of DA-GQDs and the measurement and calculation of QY were given in the Supporting Information. In addition, the linear relationship between fluorescence curve areas and corresponding optical density from absorbance for as-prepared GQDs and DA-GQDs was shown in Figure S1 (see Supporting Information). RESULTS AND DISCUSSION Characterization of GQDs and DA-GQDs. The synthesis of GQDs was performed following a standardized pyrolysis method of citric acid. The as-prepared GQDs and DA-GQDs were characterized via several techniques to confirm and highlight their properties. The HRTEM image (Figure 1a) shows that the as-prepared GQDs are homogeneously distributed with uniform lateral sizes. The particle size of GQDs shown in Figure 1b is narrow (2 – 9 nm) and the average 8


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lateral size, determined by histogram, is 4.5 ± 0.6 nm (n = 80). It is anticipated that such narrowdistributed particle size with superior homogeneous dispersion can significantly enhance the sensing properties. In addition, the fringes of carbon lattice can be viewed clearly from HRTEM (Figure 1c). The fast Fourier transform (FFT) pattern (Figure 1d) indicates that the GQDs are almost defect-free graphene single crystals with a spacing of 0.21 nm, which corresponds to the (100) plane of graphene (Figure 1e).35 In addition, the AFM image shown in Figure S2 (see Supporting Information) clearly indicates that the QGDs are nanosheets with sizes of 20 – 22 nm. The height found in AFM topography is around 1.0 – 1.4 nm with average of 1.2 nm, which confirms the graphene structure of GQDs.10, 19 Figures 1f and 1g show the change in color after visible and UV lights irradiations, respectively. The solution color of both as-prepared GQDs and buffer solutions is transparent under visible light irradiation, which means that the asprepared GQDs do not have the up-conversion and down-conversion behaviors in visible light region. After irradiation of UV light at 360 nm, however, the as-prepared GQDs emit strongly bright blue fluorescence light, showing the excellent photoluminescence property of GQDs for sensing.

30

Particle Distribution

aa

f

c

b

25 20 15 10 5 0

GQD Buffer 1

2

3

4

5

6

7

8

9

10

Under visible light

Size (nm)

d

g

e

e

Height

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.21nm

GQD Buffer

5 nm 0.0

0.5

1 .0

1.5

D istan ce (n m )

2.0

2 .5

Under 360nm UV light

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. (a) The TEM image, (b) particle size distribution, (c) HRTEM image, (d) fast Fourier transform pattern, (e) lattice spacing of as-prepared GQDs and the change in color of as-prepared GQDs under (f) visible light and (g) 360 nm UV light irradiation. The XPS measurements were performed to determine the change in chemical composition of GQDs before and after the functionalization of dopamine. The XPS survey scan shows that the as-prepared GQDs contain C, O and Na elements (Figure 2a). The sharp C 1s and O 1s peaks appear at 285-288 and 532 eV, respectively. The carbon to oxygen atomic ratio in GQDs is found to be 3.39, which indicates a good contribution from carbonyl and carboxyl groups.35 In addition, the XP spectrum of C 1s shows two adjacent peaks at 284.8 and 288.0 eV (Figure 2b), which is mainly attributed to the graphitic and carboxyl functionalized C1s, respectively.36, 37 After peak deconvolution, C=C group is shown at binding energy of 284.5 eV, which is the representative peak of graphitic carbon of graphene. In addition, several peaks including C-O (285.8 eV), C=O (287.6 eV) and COOH (288.5 eV) groups are observed in deconvoluted C 1s peak. The deconvolution of O 1s also shows C-O and O-H peaks (Figure 2c), confirming the presence of large carboxylic groups on GQDs. After surface modification of dopamine, an additional peak at 400 eV due to N 1s contribution is observed in the survey scan of DA-GQDs spectrum (Figure 2d). It is noteworthy that the peak intensity of C 1s at 284.5 eV decreases dramatically, which means the change in carbon-based species after the addition of dopamine (Figure 2e). The comparison of highresolution C 1s spectra between as-prepared GQDs and DA-GQDs indicates the obvious change in carbon species from graphitic and carboxyl groups (C=C, C–O, C=O, and –COOH) to graphitic and amide groups (C=C, C–N, C=O, and –N–C=O), clearly demonstrating the formation of covalent bonding of carboxylic group of GQDs with amine group of dopamine. The 10


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high resolution N1s spectrum shows N-H linkage (399.1 eV) and –N-C=O bond (401.1 eV) (Figure 2f),36 which is in good agreement with the results shown in C 1s spectrum. It is evident from the XP spectra that amide linkage between dopamine and GQDs has been successful

a

Na1s

Na O1s

formed.

GQD

b

c

C=C

0

200

400

600

800

C=O

529

e

DA-GQD

530

531

200

400

600

800

Binding Energy (eV)

1000

282

283

284

285

286

N-C=O C=O 287

288

289

N-C=O

Intensity

Na

Intensity

Na

N1s Na2s Cl2p

Na2p

0

533

N-H

f

C=C

C-N

532

Binding Energy (eV)

Binding Energy (eV)

C1s

O1s

528

281 282 283 284 285 286 287 288 289 290 291 292

1000

O-H

C-O

Binding Energy (eV)

d

Intensity

COOH

Intensity

Na2p Na2s

C1s

Intensity

C-O

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


290

Binding Energy (eV)

396

397

398

399

400

401

402

403

404

Binding Energy (eV)

Figure 2. The XPS of (a) survey scan, (b) C 1s and (c) O 1s of as-prepared GQDs and (d) survey scan, (e) C 1s and (f) N 1s of dopamine-functionalized GQDs (DA-GQDs). The successful attachment of dopamine is also corroborated with the FTIR spectra. As shown in Figure 3a, the as-prepared GQDs show peaks at 1580 and 1675 cm-1, which indicates the presence of C=C stretching and carbonyl (C=O) groups, respectively.38 A broad peak at 3415 cm-1 is the characteristic peak of hydroxyl group (–OH) from water molecules and carboxylic groups. After surface functionalization with dopamine, a strong peak at 1505 cm-1, which can be assigned as the amide group, appears with the decreased intensity in C-C peak at 1580 cm-1. This result proves again the successful attachment of amine group of dopamine onto GQDs after 11



funactionalization.39 In addition, the DA-GQDs contain two new peaks appearing at 985 and 1101 cm-1, which correspond to the aromatic C-H bending and C-N stretching of dopamine, respectively.40, 41 A broad peak due to N-H stretching is also noted at 3540 cm-1, confirming the presence of secondary amine structure.42 Figure 3b shows the XRD patterns of GQDs and DAGQDs. The as-prepared GQDs exhibit a broad peak centered at 24.2º 2θ, corresponding to the (002) plane of graphene.43 The position of XRD peak at 24.2º 2θ remains unchanged after functionalization with dopamine, indicating that the attachment of dopamine only takes place in –COOH functional groups at the edge of GQDs. In addition, the Raman spectra of GQDs clearly show peaks at 1242 and 1584 cm-1 (Figure 3c), confirming the graphitic nature of the as-prepared GQDs.10

(a)

(b)

1675

Intensity(a.u.)

1580

Transmittance

GQD 3415

DA-GQD

1101

DA-GQD

GQD

985 1505 500

1000

1675 1500

2000

3540 2500

3000

3500

20

25

30

-1

Wavenumber (cm )

35

40

45

50

2θ (degree)

(c) 1584 cm-1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1242 cm-1

900

1200

1500

1800

2100

2400

Raman shift (cm-1)

Figure 3. The (a) FTIR spectra, (b) XRD patterns and (c) Raman spectrum of GQDs-related nanomaterials. 12


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The optical properties of as-prepared GQDs at various pHs were investigated. The UV−vis spectra of as-prepared GQDs at pH 1 and 7 are shown in Figure S3 (see Supporting Information). The as-prepared GQDs show the absorbance at 360 nm due to the n → π* transition, resulting in the emission of bright blue fluorescence. It is important to note that the fluorescence intensity of GQDs at pH 7 is higher than that at pH 1, presumably attributed to the fact that the formation of carboxylic group at pH 1 lowers the fluorescence intensity of GQDs. 600

310 nm 320 nm 330 nm 340 nm 350 nm 360 nm 370 nm 380 nm 390 nm 400 nm 410 nm 420 nm 430 nm

550 500 450 400

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


350 300 250 200 150 100 50 0 400

450

500

550

600

650

Wavelength (nm) Figure 4. The fluorescence emission spectra of DA-GQD solutions at various excitation wavelengths of 310 – 430 nm. Figure 4 shows the emission spectra of DA-GQDs under the excitation wavelength of 310 – 430 nm. The emission wavelength of DA-GDQs can maintain at 460 nm when the solutions are irradiated over a wide range of excitation wavelength from UV to visible light, In addition, the emitted fluorescence intensity at 460 nm increases from 310 to 360 nm and then decreases when the excitation wavelengths are in the range of 370 - 430 nm. These results clearly indicate that the emission wavelength of DA-QGD is completely independent on the excitation wavelength in

13



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the range of 310 – 430 nm and the excitation wavelength at 360 nm can emit maximum fluorescence intensity of DA-GQDs. In addition, both the GQDs and DA-GQDs show good photostability for at least 10 h (Figure S4, Supporting Information). The QY of GQDs is also determined by using fluorescein as the standard solution (Φ = 0.79).44 The QY is calculated to be 0.102, which is much higher than the recently reported data of GQDs (0.06 – 0.09)6, 7, 19 and is satisfactory for fluorophores to serve as the photoluminescence sensing probe for metal ions detection. Ferric ions detection by DA-GQDs. The surface characterization of DA-GDQs clearly show that the functionalized sensing materials (DA-GQDs) have a superior and homogeneous size distribution pattern with small size and excellent stable fluorescence property for further applications. Since the dopamine has a very strong affinity toward Fe3+, the DA-GQDs can be used for the iron detection purpose.45-47 A fluorescence measurement of DA-GQDs with Fe3+ was carried out in a PBS buffer solution at pH 6 and the change in fluorescence intensity was recorded after the incubation of 5 min. Figure 5a shows the change in fluorescence intensity of DA-GQDs solutions after addition of various concentrations of Fe3+ ranging from 20 nM to 20 µM. The quenching of fluorescence intensity of DA-GQDs increases with the increase in Fe3+ concentration. The calibration plot of the change in fluorescence intensity (%∆F) as a function of Fe3+ concentration is shown in Figure 5b. The change in fluorescence intensity increases rapidly in the low concentration range of Fe3+ and then increases slowly to a plateau when Fe3+ concentration increases up to 20 µM. A good linear relationship between the %∆F and Fe3+ concentration with a linear range between 20 nM and 1.5 µM is achieved (Figure 5c). The linear regression equation of %∆F = 6.0 + 629.4 × CFe with a correlation coefficient of 0.9988 (n = 20) is also obtained. The limit of detection (LOD), determined by the 3δ/S (δ is the standard 14


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deviation of the lowest signal and S is the slope of linear calibration plot) is 7.6 nM, which is much lower than the most reported data summarized in Table 1. A previous study has used rhodamine B derivative-functionalized GQD for Fe3+ detection in cancer stem cells and the LOD was found to be 0.02 µM. In this study, the dopamine-functionalized GQDs show superior sensitivity and lower detection limit as compared to the other organic-functionalized GQDs, indicating that the DA-GQDs are excellent sensing probe for detection of trace Fe3+ in aqueous solutions. (a)

(b) 90

600

Change in fluorescence (%∆F)

Fluorescence Intensity

3+

Fe 0

500 400

20 µM

300 200 100 0 400

425

450

475

500

525

550

575

600

625

80 70 60 50 40 30 20 10 0 0

2

4

Wavelength (nm) (c)

70


60

6

8

10

12

14

16

18

20

22

Concentration (µM) (d) Just after addition of Fe3+

600 500 400

50 40

Fl Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 200 100 0 400

30

450

500

550

600

0.1µM

Wavelength (nm)

0.2µM

0.5µM

1 µM

10 µM

0 µM

PBS

20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

After incubation for 10 min

Concentration (µM)

Figure 5. (a) Fluorescence spectra of DA-GQDs at various concentrations of Fe3+ ranging from 0 to 20 µM. The excitation wavelength is fixed at 360 nm, (b) the change in fluorescence intensity as a function of Fe3+ concentration, (c) linear relationship between fluorescence and

15



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Fe3+ concentration at 0 – 1.5 µM (inset is the low level spectra), and (d) the images of the change in fluorescence intensity of DA-GQDs solution in the presence of various concentrations of Fe3+ under the excitation of UV light. Table 1. Comparison of the detection limit of Fe3+ by different sensing systems. Type of probe

Organic-based probe

Sensing probe

Detection limit (µM)

Reference

Pyrazoline derivative

1.4

48

Aminoantipyrine

0.211

49

Rhodamine B Schiff-base

0.11

50

Phosphonic acid- fluorine

0.02

51

Rhodamoine based RPE

5

52

BODIPY

0.13

27

Phosphazene

4.8

14

GO nanosheets

17.9

53

Rhodamine-GQD

0.02

12

GQD-BMIM

7.22

17

N-GQD

0.09

26

S-GQD

0.0042

24

DA-GQD

0.0076

This work

GQD-based probe

Selectivity is one of the most important parameters for sensing application. The selectivity of DA-GQD based fluorescence sensor is presented in Figure 6. Similar to the detection of Fe3+, the quenching of fluorescence intensity (%∆F) of DA-GQDs in the presence of 20 µM metal ions including Fe3+, Fe2+, Cu2+, Al3+, Au3+, Ni2+, Cr3+, Hg2+, Zn2+, Pb2+, Co2+, Cd2+, Ca2+, Mg2+ and

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Mn2+ were analyzed after incubation of 5 min. Of all the ions tested, only Fe2+ and Cu2+ have shown the small quenching effect on fluorescence intensity, presumably attributed to the adsorption of metal ion onto GQDs and slight oxidation ability toward dopamine. It is noteworthy that catechol group in dopamine has high affinity toward trivalent cations because of the high charge density. In this study, the fluorescence intensity of DA-GQDs only decreases 7 % for Al3+ detection, showing that charge density of metal ions is not the main sensing mechanism of DA-GQDs for metal ions detection. In addition, the selectivity and efficiency of DA-GQDs were examined in a mixture which contained Fe3+ and other metal ions. It is clear that addition of various types of metal ions does not influence the quenching effect of Fe3+ on the fluorescence intensity as well as the maximum emission wavelength, showing that the DA-GQDbased sensing probes possess high selectivity which can effectively detect Fe3+ specifically from the mixture that contains all possibly competitive metal ions.

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40

20

0

k I) I) I) I) I) I) I) I) X I) I) I) I) I) I) I) (II Fe(I Cu(I Al(II u(II Ni(I r(II Hg(I Zn(I Pb(I Co(I Cd(I Ca(I g(I n(I MI Blan Fe M M C A

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Figure 6. The competitive effects of 20 µM various types of metal ions on the detection of Fe3+ ions. Mechanism for Fe3+ sensing. The sensing mechanism of Fe3+ by using DA-GQDs is mainly based on the specific interaction between Fe3+ and catechol group of dopamine which has been already established in human body and Parkinson's disease.45-47 Alternatively, several studies have used the fluorescence property of graphene as the main detectable signal for detection of metal ions and organic molecules.7, 17, 20, 54 However, most of the optical sensing pathways that used GQDs and doped GQDs as the probes were mainly focused on the adsorption of metal ions onto the graphitic plane of GQDs for the metal ions detection when the fluorescence of bound GQDs was quenched.7, 17, 31 In this study, a different reaction mechanism based on the complexation and oxidation of catechol moiety in Fe-dopamine complexes has been proposed. The successful attachment of dopamine at the edge of GQDs can build a highly sensitive and selective sensor to detect Fe3+ in a very specific manner. Similar to the as-prepared GQDs, the DA-GQDs also exhibits the strong and stable fluorescence intensity at 460 nm after the excitation at 360 nm. The dopamine attached at the edge of GQDs remains in its original form without any obvious quenching effect at pH 6. When Fe3+ ions are added into the solution, Fe3+ ions will complex with dopamine quickly and then the catechol moiety of dopamine in Fe-dopamine complex is oxidized to osemiquinone,46, 47 resulting in the decrease in fluorescence intensity of DA-GQDs. It is well-known that the oxidation of dopamine is highly pH-dependent and the control of pH plays the crucial role in Fe3+ detection by DA-GQDs.54 Figure S5 (see Supporting Information) shows the fluorescence intensity of DA-GQDs as a function of pH values before and after the addition of Fe3+. The fluorescence intensity of DA-GQDs increases with the increase in pH

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ranging from 1 to 6 and then decreases at 7 – 8. It is noteworthy the self-oxidation of dopamine may occur under alkaline conditions,54 and the decrease in fluorescence intensity of DA-GQD at pH 7 – 8 is presumably due to the oxidation of dopamine. After addition of Fe3+, the fluorescence intensity decreases significantly at pH 6. This means that the self-oxidation of dopamine can be prevented under acidic conditions and pH 6 is optimal for DA-GQD to detect Fe3+ ions. 900

Fe3+

Fe3+

Fl Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


750

GQD

600

DA-GQD

450

Cyst-GQD

300 150

Fe3+ 0 400 425 450 475 500 525 550 575 600 625 650

Wavelength (nm) Figure 7. The sensing mechanism for Fe3+ detection by using three GQD-based sensing probes. The DA-GQDs in the right figure shows the superior quenching effect to the as-prepared GQDs and cysteamine-functionalized GQDs. To further elucidate the reaction and sensing mechanism of DA-GQDs at pH 6, the detection behaviors of Fe3+ were studied and compared by using different GQD-based sensing probes including as-prepared GQDs, DA-GQDs and cysteamine-GQDs. All the GQD-based sensing probes were synthesized following the same fabrication route and then the Fe3+ sensing was performed under identical conditions. As shown in Figure 7, the original fluorescence intensity of GQD-based probes follows the order as-prepared GQDs > DA-GQDs > cysteamine-GQDs,

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indicating the slight decrease in fluorescence intensity after functionalization of organic molecules on GQDs. After the addition of 20 µM Fe3+, around 50% of fluorescence intensity is quenched in the presence of as-prepared GQDs. However, the fluorescence intensity of DAGQDs at 460 nm is nearly complete diminished in the presence of 20 µM Fe3+. Several studies have indicated that the quenching of fluorescence intensity of as-prepared GQDs by Fe3+ is mainly due to the adsorption phenomenon.26,

53

This result also indicates that the quenching

efficiency attributed from the adsorption of Fe3+ ions onto the surface of as-prepared GQDs is inferior to that of DA-GQDs. When cysteamine-GQDs are used as the sensing probe, the extent of quenching is even lower than that of as-prepared GQDs, clearly demonstrating that the incorporation of dopamine at the edge of GQDs plays the crucial role in promoting the interaction between Fe3+ and catechol structure on the DA-GQDs. Therefore, we can conclude that when Fe3+ ions are added to the DA-GQDs solutions, the Fe3+ would complex and oxidize the catechol moiety of dopamine to o-semiquinone, and subsequently lead to the quench of fluorescence of DA-GQDs, as shown in Scheme 1. To further confirm the dopamine-Fe3+ interaction, the DA-GQD-Fe3+ complex along with asprepared GQD-Fe3+ and cysteamine-GQD-Fe3+ complexes were treated with 0.1 mM EDTA solution (Figure S6, see Supporting Information). The fluorescence of as-prepared GQDs and cysteamine-GQDs is re-generated after the addition of EDTA, which confirms the adsorption phenomena between GQD layer and Fe3+ ions. Interestingly, the fluorescence intensity of DAGQDs is not well recovered even after the treatment of EDTA, proving the presence of strong complexation interaction between dopamine and Fe3+. The complexation and oxidation between Fe3+ and dopamine result in effective quenching of DA-GQDs, which have the advantages of

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acting as the main driving force to achieve the highly sensitive and selective detection of Fe3+ in aqueous solutions.

CONCLUSIONS In this study, we have successfully synthesized a new class of dopamine-functionalized GQDs by using one-step EDC/NHS coupling reaction for Fe3+ detection. The functionalization between -COOH group of GQD molecule and amine group of dopamine results in the strong complexation interaction between catechol moiety of DA-GQD and Fe3+. The further oxidation of dopamine by Fe3+ enhances the sensitivity and selectivity of sensing probes. The DA-GQDbased sensor shows excellent blue fluorescent behavior at 460 nm with high photostability and can selectively bind with Fe3+ in nanomolar range with the detection limit of 7.6 nM. To the best of our knowledge, this is the first time that GQDs with high quantum yield of 0.102 are functionalized with dopamine to detect Fe3+ based on the complexation and oxidation principles. The excellent linearity of calibration curve and the selectivity of Fe3+ among the other competitive metal ions make DA-GQDs an excellent sensing probe for on-time and in-situ iron detection. Results obtained in this study have opened an avenue to surface functionalization of GQDs or doped GQDs with small organic molecules, which can serve as a superior platform in a wide variety of application including sensing, biomedical diagnosis and environmental monitoring. ASSOCIATED CONTENT Supporting Information.

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Quantum yield measurement and calculation of GQD; AFM image of GQDs; UV-Vis spectra of GQDs at various pHs; photostability of GQDs and DA-QGDs; fluorescence intensity as a function of pH; Effect of EDTA on fluorescence intensity. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author Ruey-an Doong, E-mail address: [email protected]; [email protected]; Phone number: +886-3-5726785. Fax number: +886-3-5718649. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank the Ministry of Science and Technology (MOST), Taiwan for financial support under contact Nos. 102-2113-M-007-002-MY3 and 105-2113-M-009-023-MY3. ADC is acknowledged to the MOST, Taiwan for financial support. REFERENCES 1. Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I., Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574-578. 2. Lalwani, G.; Sundararaj, J. L.; Schaefer, K.; Button, T.; Sitharaman, B., Synthesis, Characterization, in vitro Phantom Imaging, and Cytotoxicity of a Novel Graphene-Based

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1 ACS Paragon Plus Environment

Highly Sensitive and Selective Detection of Nanomolar Ferric Ions

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