Supplementary Information Experimental section Materials: Graphene ...

Electronic Supplementary Material (ESI) for Chemical Communications This journal is © The Royal Society of Chemistry 2011

Supplementary Information Experimental section Materials: Graphene oxide was synthesized by a modified Hummers method1 from graphite powder (Alfa Aesar, 20 + 80 mesh). Ethylenediamine and 18-crown-6 ether were both brought from Sigma-Aldrich.

1-Ethyl-3(3-dimethylaminopropyl)

carbodiimide

hydrochloride

(EDC),

and

N-Hydroxysuccinimide (NHS) were purchased from Alfa Aesar (Ward Hill, MA). Dialysis membrane was purchased from Pierce (3000 MWCO). All other regents were of analytical reagent grade. Ultra-pure water (>18.2 MΩ, Milli-Q, Millipore) was used for the preparation of all the solutions. Instrumentation: Fluorescence measurements were carried out using a JASCO FP-6500 spectrofluorometer with the slit width for the excitation and emission of 5 nm. Fourier transform infrared spectroscopy (FTIR) measurements were carried out with a BRUKER Vertex 70 FTIR spectrometer. The sample was prepared as pellets using spectroscopic grade KBr. The thermogravimetric analysis (TGA) was recorded with a RIGAKU Standard type with a heating rate of 10 ºC min-1 from room temperature to 800 ºC. Atomic-force microscopy (AFM) measurements were performed using a Nanoscope V multimode atomic force microscope (Veeco Instruments, USA) under ambient conditions. AFM samples were prepared by dropping the solution on mica. After 30 min, samples were washed once by ultra-pure water. The in-situ energy dispersive X-Ray spectroscopy (EDS) analysis were performed using a using a HITACHI S-4500 instrument. To determine the elemental composition at each point on the sample, the substrate was accurately positioned using an encoded x–y translation stage, and the locations were recorded. The EDS samples were prepared by casting 18C6E-rGO (washed with ultrapure water for three times after

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equilibrated with cations) on silicon sheet. Sensing of potassium (K+): In a typical measurement, a 20 μg mL-1 Am-CD and 14 μg mL-1 18C6E-rGO solution was prepared in 20-fold diluted (with 5 mM MES buffer, pH 6.8, 80 vol% MeOH) mimic extracellular matrix. Thereafter, different concentrations of KCl were added to the above solution and the mixtures incubated for 10 min before measurement. Preparation of aminated carbon dots (Am-CDs): Carbon dots (CDs) were prepared from candle soot as the reported method.2 Candle soot was refluxed in 5 M nitric acid for 12 h, and purified by centrifugation at 13000 rpm for 30 min to remove unreacted soot and non-fluorescent large sized particles. The obtained supernatant CDs were neutralized by Na2CO3 and dialyzed against pure water. The CDs were then aminated with ethylenediamine through an EDC-NHS coupling protocol. The reaction product was subsequently dialyzed for obtaining Am-CDs. The fluorescence intensity of Am-CDs remains unchanged under continuous 356 nm illumination for up to 60 min, which suggests good photostability. Preparation of 18C6E-rGO hybrids: For the preparation of the18C6E-rGO hybrids, the homogeneous graphene oxide dispersion was mixed with 18C6E aqueous solution and sodium hydroxide, and then the mixture was reduced with hydrazine at 80 ºC for 24 h.3 After reduction and non-covalent fictionalization, a black dispersion was obtained, and the excess of 18C6E was removed with five successive cycles that involved centrifugation, decantation, and resolubilization to ultimately yield 18C6E-rGO hybrid. After that, the 18C6E-rGO hybrid was dialyzed through semipermeable menbrames to remove impurities and excess 18C6E. References 1.

(a) W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339; (b) Y. Xu, H. Bai, G. Lu, C. Li and G. Shi, J. Am. Chem. Soc., 2008, 130, 5856. S2


2.

X. Wang, K. Qu, B. Xu, J. Ren, X. Qu, Nano Research, 2011, DOI: 10.1007/s12274-011-0147-4.

3.

(a) W. Wei, K. Qu, J. Ren, X. Qu, Chem. Sci., 2011, 2, 2050-2056; (b) L. Feng, C. Chen, J. Ren and X. Qu, Biomaterials, 2011, 32, 2930.

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Fig. S1 AFM topographical images and height profiles of rGO and Am-CD mixture.

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Fig. S2 AFM topographical images and height profiles of the 18C6E-rGO-Am-CD complex.

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Am-CDs Am-CDs+bare rGO

250

Fluorescence (a.u.)

200

150

100

50

0 500

520

540

560

580

600

620

640

wavelength (nm)

Fig. S3 Fluorescence emission spectra of Am-CDs (20 μg mL-1) in the presence and absence of 14 μg mL-1 bare rGO in 5 mM MES buffer (pH 6.8, 80 vol% MeOH). No obvious signal change was observed, and this implied that no nonspecific adsorption of Am-CDs on the rGO surface occurs. One thing should be mentioned that there is hardly any oxygen-related group on the surface of rGO (data not shown). This is totally different to graphene oxide.

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Fig S4 Fluorescent photograph of the sensor in absence (left) and presence (right) of potassium ion (10 mM).

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5

Equation

y = a + b*x

Weight

No Weighting 0.2598

Residual Sum of Squares

4

Pearson's r

0.99126

Adj. R-Square

0.97679

(F-F0)/F0

Value

3

Standard Erro

C

Intercept

-0.1122

0.23965

C

Slope

4.86006

0.37349

2

1

200 μL

0 1

10

Sample volume (mL)

Fig S5 Linear relationship of fluorescence of 18C6E-rGO-Am-CDs system against sample volume by keeping the potassium concentration at 10-5 M. About 2.8 μg 18C6E-rGO in rGO was dispersed in 10-5 M KCl with sample volume ranging from 1 to 10 mL. After equilibrium, each sample was concentrated to 0.2 mL through centrifugation. Subsequently, 4 μg Am-CD was added to each sample and the fluorescence intensity at 520 nm was monitored.

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250

A

+

K

100 mM 50 mM 20 mM 10 mM 7.5 mM 5.0 mM 2.0 mM 1.0 mM 0.5 mM extracellular matrix

Fluorescence (a.u.)

200

150

100

50

0 500

520

540

560

580

600

620

640

Wavelength (nm) 4

B

(F-F0)/F0

3

2

1

Equation

y = a + b*x

Weight

Instrumental

Residual Sum of Squares

6.20272

Pearson's r

0.99376

Adj. R-Square

0.98444 Value

Standard Error

L

Intercept

0.26223

0.07548

L

Slope

0.31211

0.01752

0 0

2

4

6

8

10

+

K (mM)

Fig. S6 Fluorescence titration spectra of 18C6E-rGO-Am-CD complex in the presence of different concentrations of KCl (0.5-100 mM) in diluted mimic extracellular matrix. (b) Linear relationship between donor fluorescence intensity and potassium concentrations. The F0 and F are fluorescence intensities at 520 nm in the absence and the presence of K+, respectively. Excitation: 472 nm.

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Percentage (%)

B 45 36 27 18 9 0

0

1

2

3

4

5

6

Height (nm)

Fig. S7. AFM characterization of Am-CDs. The result shows that the average particle size of the Am-CDs is 3.2±0.9 nm.

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Transmittance (%)

a 1760 1597

b 1667 1640

4000

3000

1596

2000

1000

500

-1

Wavenumber (cm )

Fig. S8 FTIR spectra of (a) as prepared CDs, and (b) Am-CDs. The FTIR spectrum of Am-CDs shows the characteristic amide I band (C=O) at 1667 cm-1 and amid II band at 1640 cm-1, which provides the qualitative evidence that amino groups have been covalently linked on the surface of CDs. Besides, after amination, the band at 1760 cm-1 (-COOH) disappeared. This also indicated that the carboxyl groups on the surface of as prepared CDs were reacted with ethylenediamine. The results of zeta potential measurements provide further evidence for supporting this analysis. The zeta potential of the pristine CDs was -12.6 mV, suggesting the surface of CDs was negatively charged due to carboxyl groups. While the zeta potential of Am-CDs was 13.8 mV after amination, indicating the surface was positively charged, due to the conversion of carboxyl groups to amino-groups.

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a b

Transmittance (a.u.)

c

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Fig. S9 FTIR spectra of (a) rGO, (b) 18C6E-rGO, and (c) 18C6E. The appearance of characteristic infrared bands of 18C6E indicate clearly that 18C6E cannot be removed from 18C6E-rGO hybrids by dialysis; thus, 18C6E molecules have covered the surface of rGO.

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nm

2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

µm

Fig. S10 AFM characterization of 18C6E-rGO hybrid. The AFM height image shows that the thickness of the 18C6E-rGO is about 2 nm

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100

Weight (%)

80

a

60

b

40

20

c 0 0

100

200

300

400

500

600

700

800

O

Temperature ( C)

Fig. S11 TGA analysis of (a) rGO, (b) 18C6E-rGO, and (c) 18C6E. According to the result, the amount of 18C6E molecules in 18C6E-rGO hybrids was determined to be 20 wt%.

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