Optical properties of pH-sensitive carbon-dots with different modifications

Journal of Luminescence 148 (2014) 238–242

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Optical properties of pH-sensitive carbon-dots with different modifications Weiguang Kong a, Huizhen Wu a,n, Zhenyu Ye a, Ruifeng Li a, Tianning Xu a,b, Bingpo Zhang a a b

Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang 310027, People's Republic of China Department of Science, Zhijiang College of Zhejiang University of Technology, Hangzhou, Zhejiang 310024, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 26 August 2013 Received in revised form 12 November 2013 Accepted 6 December 2013 Available online 16 December 2013

Carbon dots with unique characters of chemical inertness, low cytotoxicity and good biocompatibility, demonstrate important applications in biology and optoelectronics. In this paper we report the optical properties of pH-sensitive carbon dots with different surface modifications. The as-prepared carbon dots can be well dispersed in water by modifying with acid, alkali or metal ions though they tend to form a suspension when being directly dispersed in water. We find that the carbon dots dispersed in water show a new emission and absorption character which is tunable due to the pH-sensitive nature. It is firstly proved that the addition of bivalent copper ions offers a high color contrast for visual colorimetric assays for pH measurement. The effect of surface defects with different modification on the performances of the carbon dots is also explored with a core–shell model. The hydro-dispersed carbon dots can be potentially utilized for cellular imaging or metal ion probes in biochemistry. & 2013 Elsevier B.V. All rights reserved.

Keywords: Carbon dots Surface modification pH measurement Bio-imaging Colloid

1. Introduction Colloidal semiconductor quantum dots (QDs) have received considerable attention due to their low cost production and facile processing techniques. QDs have been used as light absorbers and sensitizers in photovoltaic cells, photodiodes and other optical and electronic devices. However, the exploration of QDs applications has mainly focused on cadmium and lead-based QDs, which may have a doubtful future because of their high toxicity [1,2]. A challenge to move away from the toxic QD materials and turn to more environmental friendly semiconductor QDs is urgently needed. Luminescent carbon dots (CDs) which constitute a fascinating class of recently discovered graphene as well as amorphous carbon QDs, have drawn increasing attention owing to their attractive applications in optoelectronic devices and biomedical imaging [3,4,5,6]. Compared with conventional semiconductor quantum dots and organic dyes, luminescent CDs are superior in terms of chemical inertness, large two-photon excitation crosssections, lacking of blinking, low cytotoxicity, and excellent biocompatibility [7–9]. Since carbon quantum dots have been firstly synthesized in 2006 [10], new synthetic methods including cumbersome and

n

Corresponding author. Tel.: þ 86 571 87953885; fax: þ 86 571 87953885. E-mail addresses: [email protected] (W. Kong), [email protected] (H. Wu), [email protected] (Z. Ye), [email protected] (R. Li), [email protected] (T. Xu), [email protected] (B. Zhang). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.12.007

time-consuming (Top-Down) and one-step synthesis methods (Top-Up) emerged [11,12,13,14,15]. However, the luminescent mechanism of CDs remains elusive. One big challenge is that the optical, chemical and even electrical properties of the CDs vary from case to case due to different compositions, structures and the surface ligands [3,8,9]. Therefore, deep and systematic study on the performances of the newly developed CDs is indispensable and should be urgently pursued. Herein, the CDs were synthesized according to a facile and green top-up method developed by Dr. Liu's group [16,17]. The untreated CDs tend to form a suspension when being directly dispersed in water. With the addition of acid, alkali or metal ions, the suspension will swiftly become clear. Particularly, CDs dispersed in water exhibits a pH-sensitive emission and absorption characteristic. Besides, the addition of Cu2 þ proves to offer a high color contrast for visual colorimetric assays for pH measurement. 2. Materials and experiment N-(β-aminoethyl)-γ-aminopropyl methyldimethoxy silane (AEAPMS) and citric acid anhydrous was purchased from Aladdin Company (Shanghai, China). Other solvents and reagents were from Sinopharm chemical reagent Co., Ltd. (Beijing, China). CDs reported in this article were produced by following Dr. Liu's group's method. In brief, heating the AEAPMS to 240 1C, then swiftly adding proper amount of citric acid anhydrous to the solution with vigorous stirring. The whole process took only several minutes and the

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final products were purified by precipitation with petroleum ether three times. The CDs without further passivation named “bare dots” were also synthesized in order to understand the forming process of the AEAPMS terminated CDs [16]. The absorption spectra of CDs were recorded at room temperature using a Perkin-Elmer Lambda 9 spectrophotometer; excitation wavelength (λex) dependent fluorescence spectra were measured by an Edinburgh FLS920 spectrophotometer, using a xenon lamp as the excitation source; pH dependent PL spectra were measured on a Zolix Om313005 spectrophotometer with a 405 nm laser as the excitation source. Fourier transform infrared spectra (FTIR) were measured on a Brucker Daltonics APEX III spectrometer. X Ray Diffraction (XRD) patterns were obtained on Panalytical X'Pert PRO diffractometer in the range from 131 to 631. The measurement of energy dispersive X-ray spectroscopy (EDX) was carried out with a Hitachi S4800 operating at 200 kV accelerating voltage.

3.1. Basic characteristic analysis for the as-prepared CDs Fig. 1 is an infrared transmission spectrum which illustrates the differences between the two types of CDs. Two distinct absorption features at 3300 and 1650 cm  1 in AEAPMS-passivated CDs (Fig. 1b) are attributed to amide groups (N–H), which suggests that the amidogen has successfully been merged with the bare CDs and formed a new type CDs. With the same inspiration, it is speculated that Si–O–Si or Si–O–C also terminated the surface of CDs. Fig. 2(a) is an EDX spectrum of CDs passivated with AEAPMS. Atomic percentage content of CDs is consistent with the reference [17]. The XRD pattern in Fig. 2(b) proves that CDs are amorphous, which is similar to most previous reports [6,16,17].

The AEAPMS-passivated CDs exhibit a favorable dispersibility in alcohol, acetone, chloroform and other organic solvents. Although the FTIR spectra shown in Fig. 1 confirm the presence of hydrophilic hydroxyl groups on the CDs, it seems that the asprepared CDs cannot be dispersed in deionized water. If the asprepared CDs are directly added into water, a yellow suspension is formed. It takes several days for the suspension turning into a clear colloid. Fig. 3(a) shows the absorption and PL spectra of the CDs dispersed in different solvents. As shown in Fig. 3(a), there

a

b

O-H 3500

N-H

C-H 3000

2500

2000

1500

20

30

40

50

60

2 theta degree Fig. 2. (a) EDX and (b) powder XRD patterns of the CDs prepared by citric acid as the precursor.

3.2. The effects of hydrolysis on the performance of the CDs

4000

Intensity (a.u.)

3. Results and discussion

Si-O-Si 1000

-1

Wavenumber (cm ) Fig. 1. FTIR transmission spectra of different CDs: (a) bare CDs; and (b) AEAPMSpassivated CDs.

remains only one characteristic peak around 360 nm at low energy side of the absorption band for the CDs dispersed in water, indicating that the hydrolysis has actually happened. The emission peak of the as-prepared CDs initially centers at 520 nm, when being dispersed in the solvents such as water and alcohol the emission peak blue-shifts to 470 nm due to the solvent effects. The absorption features located around 250 and 360 nm are attributed to π–πn transition of C ¼C and n–πn transition of C ¼O, respectively [9,17,18]. The absorption band ranging from 420 to 500 nm originates from the surface states. After hydrolysis, these characteristic absorption features, circled in Fig. 3(a), suffer from an evident weakening compared with the characteristic peak around 360 nm. In contrast, no obvious changes in absorption band edge (as shown in the inset of Fig. 3(a)) and PL are seen before and after the hydrolysis. This fact suggests that the emission is attributed to electron–hole recombination via defect states because the Stokes shift (approximately 314 meV) is too large for direct excitonic recombination [19]. In addition, hydrolysis makes the CDs more stable. This can be demonstrated by the following experiment. When CDs were directly added in hydrogen peroxide (H2O2) solution, white flocculent precipitates emerged along with the generation of a large number of bubbles. When the hydrolysis processed CDs were added into H2O2, there were no precipitates and evident changes in emission properties to be observed. As illustrated in Fig. 3(b), when the hydrolysis processed CDs are dispersed in water they exhibit an obvious λex-dependent emission characteristic. The emission band maximum shifts to longer wavelength almost across the whole visible optical band as

W. Kong et al. / Journal of Luminescence 148 (2014) 238–242

Relative Intensity (a.u.)

PL of CDs dispersed in water PL of CDs dispersed in alcohol PL of As-prepared CDs

electron transition is greatly restricted

Relative Intensity (a.u.)

240

314 meV

Band edge Wavelength (nm)

Absorption of CDs in water Absorption of CDs in alcohol

300

400

500

600

700

Wavelength (nm)

λex=330 nm

Relative PL Intensity (a.u.)

λex= 360 nm λex=390 nm λex=405 nm λex=420 nm λex=450 nm λex=480nm λex=530 nm λex=540 nm

350

400

450

500

550

600

650

700

750

800

Wavlength (nm) Fig. 3. (a) Absorption and PL spectra of CDs dispersed in different solvents. Inset: The absorption spectra: the absorption band edge locates at 420 nm (2.95 eV). (b) The λex-dependent PL spectra of the CDs.

the excitation wavelength increases. When excitation light wavelength increases from 330 to 450 nm with an interval of 30 nm, the emission peak almost keeps unchanged at around 470 nm while showing a slight change in intensity. When the λex increases from 450 to 480 nm, the emission peak appears around 500 nm. As the λex becomes further longer, the emission peak ultimately red-shifts even to around 620 nm with the presence only at specific wavelengths. This phenomenon is quite different from previous reports that the emission peak changes continuously with the λex. The fact that the emission peak being not continuous with variation of excitation wavelength implies that there exist discrete energy levels in the CDs. 3.3. pH-dependent emissions of the CDs after hydrolysis It is proved that adding certain amount of alkali or acid can effectively accelerate the hydrolysis making the suspension instantly clear. Fig. 4(a) shows the pH-dependent PL spectra for the CDs excited at 405 nm. It is illustrated in Fig. 4(b) and (c) that with pH increasing from 1 to 12 the PL intensity steadily increases to a level almost 5 times that of the specimen with pH at 1, along with the peak position blue-shifting from 520 to 463 nm under excitation at 405 nm. The full width at half maximum (FWHM) of the PL remains smaller than 80 nm under the condition of pH 47. In acidic condition, FWHM increases from around 80 to 4110 nm.

Besides, in the acidic solution, CDs appear orange-yellow, and gradually turn to light green in neutral and almost colorless as the pH increases. This vivid color change can be clearly observed by naked eyes as shown in Fig. 4(d). It is well-known that the increased FWHM could result from exciton–phonon scattering and ionized impurity scattering in crystalline semiconductor QDs. These two scattering mechanisms which are sensitive to temperature may not be suitable for the scenario, since all the tests were performed under steady temperature. It was speculated that the pH reversible response of the emission is attributed to the concentration of the H þ and OH  causing electronic transition changes of π–πn and n–πn in CDs by refilling or depleting their valence bands. Thus the protonation and deprotonation of carboxylate CDs due to changes in pH may cause electrostatic charging to the CDs, thereby shifting the Fermi level similar to carboxylate SWNTs [18,20,21]. However, this speculation which is based on the theory of organic electronics is mainly for bulk materials comprised of organic molecules, not for low dimensional systems such as QDs. Further, it cannot explain the intensity variation of the emission. Herein, we propose a different mechanism for the pH reversible response of the fluorescence of the CDs which will be discussed later in Section 3.5. The as-prepared CDs can be swiftly dispersed in NaOH and HCl aqueous solutions. However, it is still arguable that the dispersion is due to other cations or anions such as Na þ and Cl  . Table 1 implies the effects of different cations and anions on the emission peak positions of the CDs. The figures in parentheses denote the pH value of the solution. “Concentration” particularly refers to the molar density of Na þ in aqueous solution. The fluorescence properties of CDs are only sensitive to the concentration of H þ or OH  , e.g., in the case of NaCl, as the concentration of NaCl aqueous solution increases the emission peak remains unchanged. 3.4. Effects of metal ions on the optical properties of the CDs after hydrolysis To explore the effects of metal ions on CDs, few drops of aqueous solution of copper acetate was added into the CD colloid before and after hydrolysis, respectively. With the addition of Cu2 þ , the hydrolysis processed CDs instantly became purple. As shown in Fig. 5(a), there arises an obvious absorption peak around 550 nm demonstrating that Cu2 þ has successfully combined with the CDs. NaOH was used as OH  provider. In order to avoid disturbance of Cl  , dilute nitric acid was used as H þ provider. The colloid appears yellow under acidic condition, green in weak acid, purple when pH ¼8 and dark blue in alkaline solution without a precipitate. It is noted that the yellowish CDs colloid at low concentration becomes stained with the proper addition of copper acetate aqueous solution. Surprisingly, with the pH variation the post-treated CDs colloid shows more color changes as vividly shown in Fig. 5(b). The visual color contrast and diversity are effectively promoted. PL characteristics of CDs capped with Cu2 þ are still pHsensitive as shown in Fig. 5(c). As pH value increases from 1 to 10, the emission peak blue-shifts from 490 to 470 nm. When pH 410, the emission peak keeps unchanged. The FWHM also shrinks from 110 to around 70 nm. It is necessary to mention that when added with Cu2 þ , the suspension of the CDs without hydrolysis gradually became clear along with an obvious color change from yellow to dark green. However, a blue precipitate appeared under alkaline condition which indicates that the Cu2 þ was only physically attached to the CDs surface. Besides, other metal ions such as Ag þ , Cd2 þ , Zn2 þ , Mg2 þ , Mn2 þ , as well as Al3 þ only coexist with CDs under weak alkaline or acidic condition. In a strong alkaline condition, the presence of the precipitate makes it hard to identify whether these metal ions have attached to the CDs surface. It

pH=1 pH=2 pH=3 pH=4 pH=5 pH=6 pH=7 pH=8 pH=9 pH=10 pH=11 pH=12

0.8 0.6 0.4 0.2

241

115

520

0.0

510

PL Peak Position

110

FWHM

105

500

100

490

95 90

480

85 470

80

460 400

450

500

550

600

650

700

750

75 0

800

2

4

6

8

10

Full Width at Half Maximum (nm)

1.0

Peak position (nm)

Normalized PL Intensity (a.u.)

W. Kong et al. / Journal of Luminescence 148 (2014) 238–242

12

pH

Intensity Ratio (a.u.)

Wavelength (nm)

0

2

4

6

8

10

12

Hydrogen Ion Concentration Fig. 4. (a) pH-dependent PL spectra for CDs excited at 405 nm. (b) Relative intensity variation of PL spectra with the increase of pH value from 1 to 12. The intensity represents the integral area of the PL peak. (c) Variations of FWHM and peak positions with the increase of pH value from 1 to 12. (d) Photographs of CDs aqueous colloid with different pH values under room light (up) and UV lamp illumination (down). Table 1 PL peak positions of CDs tuned by different reagents in relevant aqueous solution. Reagent

NaOH KOH Na2SO4 NaCl Na2CO3 C6H5Na3O7  2H2O

Concentration (pH)

Sodium hydroxide Potassium hydroxide Sodium sulfite anhydrous Sodium chloride Sodium carbonate anhydrous Sodium citrate tribasic di-hydrate

1  10  4 M

1  10  3 M

1  10  2 M

1  10  1 M

469 nm (10) 469 nm (10) 473 nm (8) 473 nm (8) 471 nm (9) 473 nm (8)

465 nm (11) 465 nm (11) 473 nm (8) 473 nm (8) 469 nm (10) 473 nm (8)

463 nm (12) 463 nm (12) 473 nm (8) 473 nm (8) 469 nm (10) 473 nm (8)

463 nm (13) 463 nm (13) 473 nm (8) 473 nm (8) 469 nm (10) 473 nm (8)

seems that only bivalent copper ions can terminate the surface of CDs. 3.5. Model of the luminescent CDs with different surface modifications Similar to the well-known effect of photo-activation (P-A) for CdSe QDs [23], absorbed OH þ passivates the defects on the surface of the CDs and creates a surface modified organic shell layer, i.e., a protective shell on the CD surface makes the CDs more or less isolated and lowers down non-radiative recombination rate [22]. Thus the quantum yield of the CDs is greatly enhanced. The protective shell also makes the CD diameter effectively smaller, leading to the emission of the CDs blue-shift. Also, bivalent copper ions can be easily absorbed by the hydrophilic shell making the electronic distribution on the surface states of CDs change which can be seen from the UV–vis absorption spectra of Cu2 þ passivated CDs under different pH values in Fig. 5(a). H þ can recreate the defects which were passivated by OH þ and introduce new surface defects on the CDs. Further, H þ may enlarge the diameter of CDs by prolonging the conjugated chain length. The pH-dependent behaviors of both the emission and absorption can be

well explained by this hypothesis. The formation of shell layer on the surface of CDs effectively reduces the size of QDs and the quantum size effect makes the emission blue-shift. The core–shell model is illustrated in Fig. 6. On the CDs surface there exist diverse defects which are formed during the synthesis process. The inherent surface defects including various dangling bonds, reactive groups and carrier capturing centers are critical for CDs being emissive and responding to the ambient change. The passivation/modification of the surface defects can greatly enhance the quantum yield (QY), dispersibility in different solvents and stability of the CDs [24]. The key point is the fluorescence and absorption characteristics of the CDs become tunable according to the design by modifying the surface defects, e.g., the increased pH makes the absorption and PL spectra of CDs blue shifts.

4. Conclusions In conclusion, by adding acid, alkali or metal ions, CDs become easier to be dispersed in water. Furthermore, CDs dispersed in

W. Kong et al. / Journal of Luminescence 148 (2014) 238–242

Absorption Intensity (a.u.)

242

Copper acetate pH>7 pH<7 pH=7

300

400

500

600

700

Wavelength (nm)

Fig. 6. The effect of different surface modifications on the performance of the CDs.

pH measurement easier and more practical. Different surface functional groups and defects of CDs render different optical and chemical properties. By modifying the surface of the CDs, their fluorescence, absorption characteristics and dispersibility in different solvents will be greatly changed and/or enhanced. As the pathological cells shows different pH from the surrounding body fluid or tissues, these hydro-soluble CDs may be utilized as the bio-probe because they have no bio-toxicity which will be explored in the future work.

Acknowledgments This work was supported by National Key Basic Research Program of China (Grant No. 2011CB925603), Natural Science Foundation of China (Grant Nos. 61290305, 11374259).

PL Intensity (a.u.)

pH=1 pH=2 pH=3 pH=7 pH=8 pH=10

400

450

500

550

600

650

References

700

Wavelength (nm) Fig. 5. (a) The UV–vis absorption of Cu2 þ passivated CDs under different pH values. In inset are photographs of Cu2 þ passivated CDs with different pH under indoor lighting. (b) Photographs of as-prepared (up) and Cu2 þ passivated (down) CDs aqueous colloid with the same low concentration at different pH under indoor lighting. (c) The pH-dependent behavior for the PL of CDs with Cu2 þ passivation excited by a 405 nm laser.

water become more stable in redox and colorful at different pH. When CDs are terminated with Cu2 þ , the absorption characteristics become more pH sensitive. This behavior can be exploited for fluorescent and visual colorimetric assays for pH measurement. As metal ions and/or pH probes usually operate through characterizations of PL or absorption, the visual color change makes the

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