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Ionic liquid-capped graphene quantum dots as label-free fluorescent probe for direct detection of ferricyanide
Author’s Accepted Manuscript Ionic liquid-capped graphene quantum dots as label-free fluorescent probe for direct detection of ferricyanide Xue Sun, Yuting Qian, Yajie Jiao, Jiyang Liu, Fengna Xi, Xiaoping Dong www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(16)31036-0 http://dx.doi.org/10.1016/j.talanta.2016.12.085 TAL17175
To appear in: Talanta Received date: 20 August 2016 Revised date: 27 December 2016 Accepted date: 30 December 2016 Cite this article as: Xue Sun, Yuting Qian, Yajie Jiao, Jiyang Liu, Fengna Xi and Xiaoping Dong, Ionic liquid-capped graphene quantum dots as label-free fluorescent probe for direct detection of ferricyanide, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.12.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Ionic liquid-capped graphene quantum dots as label-free
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fluorescent probe for direct detection of ferricyanide
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Xue Sun, Yuting Qian, Yajie Jiao, Jiyang Liu, Fengna Xi*, Xiaoping Dong*
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Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, China
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Submitted to Talanta
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* Corresponding author. E-mail: [email protected], [email protected]
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Abstract
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Despite complex molecular and atomic doping, efficient post-functionalization strategies for
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graphene quantum dots (GQDs) are of key importance to control the physicochemical properties
17
and broaden the practical applications. With ionic liquid as specific modification agents, herein,
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the preparation of ionic liquid-capped GQDs (IL-GQDs) and its application as label-free
19
fluorescent probe for direct detection of anion were reported. Hydroxyl-functionalized GQDs that
20
could be easily gram-scale synthesized and possessed single-crystalline were chosen as the model
21
GQDs. Also, the most commonly used ionic liquids, water-soluble 1-butyl-3-methyl imidazolium
22
tetrafluoroborate (BMIMBF4) was chosen as the model IL. Under the ultrasonic treatment,
23
BMIMBF4 easily composited with GQDs to form IL-GQDs. The synthesized IL-GQDs were
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characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM),
25
X-ray photoelectron spectroscopy (XPS) and fluorescence (FL) spectrum. After successful
26
combination with IL, the excitation-independent photoluminescence behavior of GQDs presented
27
almost no change, whereas, the anion responsiveness of IL-GQDs drastically improved, which
28
afforded the IL-GQDs a sensitive response to Fe(CN)63−. Based on the strong fluorescence quench,
29
a facile and sensitive detection of Fe(CN)63− was achieved. A wide linear range of 1.0×10-7 to
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2.5×10-3 mol l-1 with a low detection limit of 40 nmol l-1 was obtained. As the composition and
31
properties of IL and GQDs could be easily tuned by varying the structure, ionic liquids-capped
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GQDs might present promising potential for their applications in sensing and catalysis.
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Keywords: Ionic liquid-capped GQDs; Composite; Fluorescent; Direct detection of anion;
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Fe(CN)63−
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1. Introduction
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As a new class of luminescent carbon nanomaterials, graphene quantum dots (GQDs) have
39
rapidly attracted great research interests as intriguing candidates for photoluminescent (PL)
40
materials [1-6]. Compared with conventional semiconductor quantum dots (e.g. CdS, CdSe, CdTe
41
and PbS) and rare elements, GQDs are more biocompatible and environmentally friendly. In
42
addition, the stability of photoemission is higher than organic fluorescent dyes and the cost is
43
lower than noble metal (e.g. Au, Ag) clusters. As great efforts have been paid on developing new
44
preparing strategies, the synthetic methods gradually mature at present and a large number of
45
GQDs have been successfully obtained through both “top-down” and “bottom-up” methodologies
46
[7-13]. For example, Wang et al. reported the gram-scale synthesis of single-crystalline GQDs by
47
a facile molecular fusion route under mild and green hydrothermal conditions [14]. Though GQDs
48
have exhibited significant potential in biology, optical devices and sensing applications, their
49
potential for a spectrum of applications are still ongoing. For instance, the sensing platform for
50
cautions has been widely reported, the application for direct detection of anions is nevertheless
51
still rare (only few indirect detection using on-off strategy have been reported [15-17]).
52
Consequently, further extending the arsenal of GQDs materials is still of critical significance. In
53
spite of complex molecular and atomic doping, the efficient post-functionalization is also
54
extremely important because it provides a convenient route to control the physicochemical
55
properties of GQDs and therefore broadens their practical applications.
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Ionic liquids (ILs) are composed of organic cation and inorganic or organic anion. ILs are
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regarded as one of the most exciting scientific discoveries in chemical science because of their
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unique properties such as negligible vapor pressure, thermal stability, recyclability, ionic
59
conductivity, the ability to dissolve a wide range of chemical species, and easy of structural design
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[18,19]. Moreover, ILs possessed excellent anion exchange properties. Due to those unique
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properties, ILs-capped carbon nanomaterials have represented an interesting class of materials
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owing to their exceptional properties arising from the synergistic combination of both components.
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Due to π-π or cation-π interactions, ILs could easily combine with carbon materials to form
64
composites. For example, ILs-capped carbon nanotubes (CNTs), carbon nanofiber and graphene
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have been extensively reported and applied as matrix with excellent solubility and conductivity for
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electroanalysis [19]. In case of PL carbon materials, ILs-capped carbon quantum dots (CQDs) also
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demonstrated unique properties. For instance, Wang et al. used citric acid monohydrate as a
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carbon source and 1-aminopropyl-3-methyl-imida-zolium bromide (APMImBr) ionic liquid as
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both modification agent and reaction medium to synthesize APMImBr-capped CQDs [20]. With
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the modification of ILs, [APMIm][Br]-capped CQDs showed high thermal stability and anion
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responsiveness i.e., CQDs spontaneously transferred from aqueous phase to ethyl acetate phase
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once the anion of N(CF3SO2)2- was introduced. Wang et al. also prepared highly charged CQDs
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through one-pot pyrolysis with citric acid as carbon source and APMImBr as capping agent. Due
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to the IL modification, the as-prepared CQDs depicted a high quantum yield (25.1%) and its
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amphiphilicity can be facilely tuned by anion exchange, which resulted in spontaneous phase
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transfer between water and oil phase [21]. Liu et al. applied the microwave-hydrothermal
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treatment
78
(1-allyl-3-methylimidazolium chloride, AMIMCl). AMIMCl helped to dissolve the cellulose in
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straw and provided nitrogen atoms for the resulting heteroatom doped CQDs. Due to the surface
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passivation or functionalization of IL, the produced CQDs possessed high quantum yield (22.58%),
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which was higher than those of the other prepared from biomass. In addition, the CQDs can serve
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as an effective label-free fluorescent sensor for the detection of Fe(III) ions with a very low
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detection limit (200 nmol l-1) [22]. To the best of our knowledge, the preparation of ionic
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liquid-capped GQDs composite with anion responsiveness and their application for direct sensing
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of anion were few.
for
rice
straw
in
the
mixed
solvent
of
water
and
ionic
liquid
86
With IL as specific modification agents, in this work, we reported the preparation of
87
IL-capped GQDs and its application as label-free fluorescent probe for direct detection of anion.
88
hydroxyl-functionalized GQDs that possessed single-crystalline and could be easily gram-scale
89
prepared were chosen as the model GQDs. Also, the most commonly used and water soluble ionic
90
liquid, BMIMBF4 was chosen as the model IL. With the ultrasonic treatment, BMIMBF4
91
conveniently was combined with GQDs to form IL-GQDs (Scheme1). The role of ILs might lies
92
in two aspects. Firstly, IL could easily combine with GQDs to form composites due to π-π or
93
cation-π interactions. Secondly, IL possessed excellent anion exchange properties. Consequently,
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more interaction or shorter action distance might occur between IL-GQDs and specific anion,
95
resulting in improved fluorescence quenching and analytical performance. In comparison with
96
GQDs, IL-GQDs presented an almost unchanged PL behavior and the evidently promoted anion
97
responsiveness. Its fluorescence could be strongly quenched in the presence of Fe(CN)63−. On this
98
basis, a facile and sensitive detection of Fe(CN)63− was achieved. The preparation strategy and the
99
main characteristics of IL-GQDs were described and discussed in detail.
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2. Experimental section
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Ionic
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tris(hydroxymethyl)aminomethane (tris) were purchased from Aladdin Chemistry Co. Ltd. (China).
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Pyrene (purity > 97%) was obtained from Chengdu Cologne chemical reagent company (China).
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The other chemicals used in this work were obtained from Hangzhou Gaojing Chemistry Co. Ltd.
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(China). All chemicals were of analytical grade and used without further purification. All aqueous
108
solutions were prepared with ultrapure water (18.2 MΩ cm, Milli-Q, Millipore). Solutions
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containing different cation and anion were applied. The solution of Ag+ was prepared from its
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nitrate salts. The solution of Pb2+ was prepared from its acetic acid salt. The solutions of Fe3+, Al3+,
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Mg2+, Cd2+, Ca2+, Cu2+, Cr3+, Hg2+, K+ and Na+ were prepared from their chloride salts. To avoid
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the hydrolysis of metal ions, pH in the preparation and the following experiments was controlled.
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Briefly, pH values for Fe3+, Hg2+, Cr3+, and Al3+ were controlled at 2.2, 2.5, 2.5 and 3.0 using
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glycine-HCl buffer, respectively. Cu2+ (pH 5.0), Zn2+(pH 6.5), Na+(pH 7.0), Pb2+(pH 7.0), K+(pH
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7.0), Cd2+(pH 7.0), Mg2+(pH 7.0), Ca2+(pH 7.0) solutions were prepared in tris-HCl buffer solution.
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For Ag+, pH value was set as 6.5 adjusted using Tris or HNO3. For the used anions, solutions of
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NO3-, S2-, NO2-, S2O32-, HCO3-, H2PO4-, HPO42-, CH3COO- and Cl- were prepared from their
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sodium salts. The solutions of Br-, Cr2O72-, Fe(CN)63-, Fe(CN)64-, S2O82- and SCN- were prepared
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from their potassium salts. The pH of anion solution in the preparation and use was controlled by
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adjusting with tris or HCl. Briefly, pH values for NO3-, NO2-, Cl-, Br-, Fe(CN)63- and Fe(CN)64-
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solutions were set as 7.0. The pH for solutions of S2O32-, HPO42-, and CH3COO- was controlled as
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9.0. The pH for solutions of S2O82-, Cr2O72-, H2PO4-, and SCN- was controlled as 3.5, 4.0, and 4.5,
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respectively. The solution of S2- solutions were prepared in NaHCO3-Na2CO3 buffer solution at pH
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11.5.
2.1 Materials and reagents liquid,
1-butyl-3-methylimidazolium
tetrafluoroborate
(BMIMBF4),,
and
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2.2 Instrumentations
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The fluorescence (FL) spectrum and intensity were recorded on an RF-5301PC
127
spectrofluorometer (Shimadzu Corporation, Japan). Transmission electron microscopic (TEM)
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photograph was taken on a JEM-2100 transmission electron microscope at operating voltage of
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200 kV (JEOL Ltd., Japan). The sample was dispersed in water and then drop-casted on ultrathin
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carbon-coated copper grid. For atomic force microscopy (AFM) characterization, the aqueous
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solution of GQDs or IL-GQDs was spotted onto freshly cleaved mica surface and dried in air. The
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samples were measured in air by tapping mode on MFP-3D AFM microscope (Asylum research).
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X-ray photoelectron spectroscopy (XPS) characterizations were conducted by using a PHI5300
134
electron spectrometer using 250 W, 14KV, Mg Kα radiation (PE Ltd., USA). Fourier transform
135
infrared (FT-IR) spectra of the IL, GQDs and IL-GQDs were obtained using a Spectrum 5700
136
(Nicolet Instrument Co. USA)
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2.3 Preparation of the IL-GQDs
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IL-GQDs were prepared by combination of IL with pre-synthesized GQDs (Scheme 1).
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Firstly, GQDs with hydroxyl groups were prepared according to the literature [14] with little
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modification. The synthesis involved the hydrothermal treatment of 1,3,6-trinitropyrene in alkaline
141
NaOH solution for molecular fusion. Briefly, 1,3,6-trinitropyrene (2 mg/mL) was dispersed in
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NaOH solution (150 mL, 5 mg/mL) by ultrasonically treating for 1 h. The obtained mixture was
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transferred to poly(tetrafluoroethylene)-lined autoclave and heated at 200 oC for 4h. After cooling
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down to room temperature, the resulting GQDs solution was filtered through a 0.45 μm
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microporous membrane to remove undissolved carbon product, and dialysed by dialysis bag (3500
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Da) against water for 3 days to remove the molecular precursors. The obtained solution in dialysis
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bag (GQDs > 3500 Da) was further dialysed with retained molecular weight of 6000Da for 3 days
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to remove impurities with large size. Finally, the obtained GQDs (3500-6000Da) were freeze dried
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to a brown powder.
150
Secondly, IL-GQDs were prepared by mixing GQDs solution (3500-6000Da, 0.02 mg/mL)
151
with BMIMBF4 (100:1, V/V) and ultrasonically treating for 30 min. The formed solution was
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dialyzed by dialysis bag (1000 Da) against water for 1 day to remove unbound IL.
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Quantum yield (QY) of the GQDs and IL-GQDs was measured by using Rhodamine B as the
154
standard sample and was computed according to the following equation:
Yμ = Ys
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Fu As Fs Aμ
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where Yμ and Ys are the quantum yield of the GQDs or IL-GQDs and the standard substance, Fu
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and Fs respectively stand for the integral fluorescence intensity of the test GQDs or IL-GQDs and
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the diluted standard solution and Aμ and As are the maximum absorbance value of GQDs or
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IL-GQDs and the diluted standard solution, respectively [22].
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2.4 Fluorescent detection of Fe(CN)63− using IL-GQDs
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A solution of IL-GQDs (0.02 mg/mL) in 50 mM tris-HCl buffer solution (pH = 7.0) was used
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as an FL probe toward Fe(CN)63−. In a typical run, IL-GQDs solution was mixed with different
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amount of Fe(CN)63−. After the resulting solution was shaken well and incubated for 30 min at
164
room temperature, the FL emission spectra (excited at 470 nm) were recorded. The detection
165
measurements were performed in triplicate. The relative FL intensity (F0- F)/F0 versus Fe(CN)63−
166
concentration were used for calibration. Here, F0 and F are the FL intensities of IL-GQDs in the
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absence and presence of different concentration of Fe(CN)63−, respectively.
168
3. Results and discussion
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3.1 Photcture characterization of the IL-GQDs
170
The PL characteristics of both GQDs and IL-GQDs were investigated to evaluate the effect of
171
IL. Results were given in Fig. 1. As illustrated in the inset of the figure, both GQDs (pH=7.0, 0.02
172
mg/mL) and IL-GQDs (pH=7.0, 0.02 mg/mL calculated using GQDs) were pale-yellow,
173
transparent solution under daylight, indicating their hydrophilic characters. Such property might
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be ascribed to the large amount of hydroxyl groups in GQDs and the hydrophilicity of the
175
water-soluble ionic liquid, BMIMBF4. When the aqueous suspensions of GQDs and IL-GQDs
176
were UV irradiated (365 nm), strong green fluorescence without obvious difference was observed.
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To further study the FL property, FL spectra were recorded. As shown in Fig. 1a, the spectra of
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GQDs in the absence and presence of ionic liquids were almost constant when excited at 470 nm.
179
In addition, the emission spectra of GQDs and IL-GQDs with progressively increased excitation
180
wavelengths from 400 to 470 nm with 10 nm increment were recorded. Excitation-independent PL
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behaviors were revealed, suggesting single-emission fluorescence center and high crystallinity
182
[14]. The maximum excitation and emission wavelengths of the GQDs and IL-GQDs were
183
revealed at 470 nm and 512 nm, respectively (Fig. 1b, Fig. S1). The above results confirmed that
184
the incorporation of IL did not change the FL characteristics of GQDs including the maximum
185
emission wavelength and intensity. Also, significant improvement of the FL intensity was not
186
observed in this IL-GQDs composite.
187
The FL intensity of the IL-GQDs at different pH was studied. The pH of the solution was
188
adjusted by NaOH or HCl (Fig. S2). FL intensity was low at such strong acidic conditions,
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whereas, it increased dramatically with the increase of pH value. When pH was above 6,
190
fluorescent intensity was relative stable. The phenomenon might be ascribed to the ionization of
191
hydroxyl groups of GQDs [14]. It is well known that electronic structure and charge density might
192
affect the opitical characteristics of GQDs. The QYs of GQDs and IL-GQDs were 12.1%，11.8%
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at pH 7.0, respectively. Photo bleaching experiment of IL-GQDs was also performed to evaluate
194
the photo stability. IL-GQDs were irradiated continually for 4 hours under UV lamp (365 nm).
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Results were shown in Fig. S3. It was demonstrated that the FL intensity of IL-GQDs can
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maintain 94% of the original signal with four-hour irradiating under UV light, indicating good
197
photo stability.
198
3.2 Structure characterization of the IL-GQDs
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Elemental analysis of the GQDs and IL-GQDs was performed by XPS. As shown in Fig. 2a,
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the full-scan XPS spectrum confirmed that GQDs were composed of carbon and oxygen.
201
Fine-structure spectrum of C 1s (Fig. 2a) exhibited two main peaks 286.6eV and 284.6eV,
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implying the C=C and C-OH groups due to the graphitic and hydroxyl groups modified carbon
203
atoms (Fig. 2a). The only one peak for O 1s was ascribed to hydroxyl groups formed from the
204
molecular fusion of 1,3,6-trinitropyrene in alkaline NaOH solution under the hydrothermal
205
treatment (Fig. 2a). Those results were consistent with the results reported in the literature [14]. As
206
known, 1,3,6-trinitropyrene possessed the same mother nucleus structure with graphene and could
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occur molecular fusion in hydrothermal reaction to form graphene framework [14]. As the
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powerful electrophilicity of nitro groups，the addition reaction can happen with electron-rich -OH
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groups and resulted in GQDs with a large amount of hydroxyl groups. In the case of IL-GQDs, the
210
full range XPS spectrum demonstrated the presence of C, N, O, B and F (Fig. 2d). The
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corresponding C1s, N1s, O1s, B1s and F1s peaks centered at 284eV, 403eV, 534eV, 194.6eV and
212
684.5eV, respectively. Despite C=C or C-C (284.6eV) and C-OH (286.8eV), C-N centered at
213
285.9eV was observed corresponding to the presence of N element in IL. Similarly with O 1s of
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GQDs, only OH groups were observed. The N1s peaks at 401.8eV and 403.6eV are ascribed to the
215
imidazole ring and C-N, the two kinds of in amide group and imidazole ring of BMIMBF4. The
216
presence of bromine and fluorine further proved the successful attachment of IL onto GQDs. The
217
interaction between IL and GQDs might lie in π-π or cation-π interactions and a carbon core with
218
peripheral cationic-ionic pairs might form. FTIR spectrum was also used to confirm the groups of
219
GQDs and IL-GQDs (Fig. S4). Both the GQDs and IL-GODs showed absorption of hydroxyl
220
group (3400 cm-1) and C-OH (1150 cm-1). Compared with GQDs, the FT-IR spectrum of IL,
221
IL-GQD showed the additional absorption peak from C-N stretching vibration at 1169cm-1. The
222
peak around 1572 and 1465cm-1 in IL and IL-GQDs were related to the typical stretching of
223
imidazole ring [23]. These results were consistent with the results from XPS and further confirmed
224
the successful hybridization of IL with GQDs.
225
The morphology characteristics of IL-GQDs were investigated using TEM and AFM. TEM
226
images revealed that IL-GQDs were well-dispersed (Fig. 3a, b) with relatively uniform size
227
distribution of 1.6-2.4 nm. The average diameter of the IL-GQDs was calculated to be 2.0 nm
228
(inset of Fig. 3c). High resolution TEM (HRTEM) image (Fig. 3d) showed clear lattice structure
229
of the IL-GQDs, indicating high crystallinity. The lattice spacing was measured to be about 0.23
230
nm, which can be attributed to the (100) facets of graphite [14]. AFM was used to evaluate the
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height profiles of GQDs and IL-GQDs. As demonstrated in Fig. 4, GQDs showed a typical
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topographic height of about 2.8 nm, indicating that most of the GQDs consisted of 3-5 layer of
233
graphite. For IL-GQDs, the topographic height increased to about 3.4 nm, confirming the
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modification of IL onto GQDs.
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3.3 Selective FL response of IL-GQDs towards anion
236
The GQDs-based FL sensing system has been widely reported. The selectivity of cations was
237
mainly achieved based on coordination or electrostatic effect because GQDs usually contained
238
carboxyl or hydroxyl groups or doped hetero atoms (e.g. N, S). However, research on direct anion
239
responsiveness of GQDs is nevertheless still rare. Due to the unique ion-exchange property of IL,
240
the IL-GQDs in this work were expected to possess anion responsiveness.
241
To test the selectivity of anion, the FL quenching experiments of both GQDs and IL-GQDs
242
were investigated. The FL quenching efficiency of (F0-F)/F0 was determined. Here, F0 and F were
243
the FL intensities of GQDs or IL-GQDs in the absence and presence of different ion, respectively.
244
The commonly used anions including HCO3-, H2PO4-, S2O82-, SCN-, HPO42-, Ac-, Cl-, NO3- , S2-,
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NO2-, Br-, S2O32-, Cr2O72-, Fe(CN)63- and Fe(CN)64− were tested. As given in Fig. 5a, the FL of
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GQDs could be slightly quenched by Cr2O72-, Fe(CN)63- and Fe(CN)64− with FL quenching
247
efficiency between 0.1-0.2. The other anions only caused faintly FL quenching with quenching
248
efficiency below 0.1. In case of IL-GQDs, the same anion selectivity was revealed, whereas, the
249
fluorescence quenching efficiency greatly improved towards Fe(CN)63- and Fe(CN)64−. For
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Fe(CN)63-, the highest fluorescence quenching efficiency was observed. In order to eliminate
251
possible cation interference, the selectivity towards the commonly used cations was also
252
determined including Fe3+, Cr3+, Na+, Pb2+, K+, Cu2+, Cd3+, Al3+, Zn2+, Mg2+, Ca2+, Ag+, Hg2+.
253
Among the tested ions, only Fe3+ possessed remarkable quenching for the FL of IL-GQDs (Fig.
254
5b). That may interfere with the detection of Fe(CN)63−. This issue can be circumvented by using
255
ascorbic acid (AA) as chelating agents for Fe3+ ions [24]. The above results showed that Fe(CN)63−
256
present the highest quenching for the FL of IL-GQDs over the other anions and cations. In
257
comparison with GQDs, also, the IL-GQDs as a FL probe promised much improved selectivity for
258
Fe(CN)63−. The reasons might lie in three aspects. Firstly, the used GQDs might have selectivity
259
towards iron based ions. Secondly, the corporation of IL on GQDs might regulate the
260
physicochemical property, especially the electrostatic property, of the GQDs. Compared with
261
GQDs, IL-GQDs might present a carbon core with peripheral cationic-ionic pairs. Thus, the
262
electrostatic effect between Fe(CN)63− and IL-GQDs might be promoted. Thirdly, Fe(CN)63− was
263
bulky anion and could anion-exchange with BF4- of IL (Scheme 1). More systematic work on the
264
mechanism of such selectivity was under way in our laboratory. By controlling the redox property
265
of the medium solution, Fe(CN)63− and Fe(CN)64− can be easily distinguished. For example,
266
Fe(CN)63− will transfer to be Fe(CN)64− in presence of hydroxylamine hydrochloride. On the other
267
hand, Fe(CN)64− will be converted into Fe(CN)63− with oxidizing agent.
268
3.4 Direct detection of Fe(CN)63− using IL-GQDs as FL probe
269
FL quenching of IL-GQDs caused by Fe(CN)63− at different pH was further explored. To
270
adjusted the pH of the solution, glycine-HCl buffer solution (pH=3), tris-HCl buffer solution (pH
271
4-9) and carbonate buffer solution (pH 10-11) were used. The concentration of the buffer solution
272
was set as 30 mmol l-1. As the buffer solution introduced anions and cations, the effect of the
273
buffer solution was firstly studied. The fluorescrnce intensities of IL-GQDs in buffer solution
274
were compared with those obtained by adjusting pH only with NaOH or HCl. Consistent results
275
with change no more than 5% at maximum emission wavelength were obtained (Fig. S5),
276
indicating no significant effect caused by the introduced by anions and cations in the buffer
277
solutions. Then the fluoscrescence quenching caused by Fe(CN)63− was studied by measuring the
278
fluorescence intensity in absence (F0) and presence (F) of Fe(CN)63− (Fig. S5) and calculating the
279
relative change as (F0-F)/F0 (Fig. S6). Obviously, the values of (F0-F)/F0 had great changes at pH
280
3.0-6.0 (Fig. S6). As most of the environmental or biological water bodies are neutral or nearly
281
neutral, the neutral solution (pH 7.0) that gave the highest FL quenching ratio was chosen for
282
further investigation.
283
The kinetic behavior of reactions between the IL-GQDs and Fe(CN)63− was investigated by
284
plotting relative FL intensities (F/F0) as a function of time. The reactions reached the quenching
285
equilibrium in about 30 min (Fig. S7). Therefore, 30 min was chosen as reaction equilibrium time
286
in the subsequent detection experiments.As Fe(CN)63− presented the highest FL quenching
287
efficiency for the
288
Fe(CN)63− based on direct FL quenching of IL-GQDs was performed. IL-GQDs solution reacted
289
with different concentrations of Fe(CN)63− and the FL spectra were measured. As shown in Fig. 6a,
290
the FL intensity of IL-GQDs decreased with increase of the concentration of Fe(CN)63−, proving
291
the validity of the FL assay for detection of Fe(CN)63−. The dependence of (F0-F)/F0 on the
292
concentration of Fe(CN)63− ions was concluded in Fig. 6b. Linear correlation existed between the
293
value of (F0-F)/F0 at 512 nm and the concentration of Fe(CN)63− in the range of 1.0×10-7 to
294
2.5×10-3 mol l-1. A well-resolved linear part (R2 = 0.994) was found in the concentration range of
295
1.0×10-7--5.0×10-5 mol l-1 and another linear part (R2 = 0.995) presented in the concentration range
296
of 5.0×10-5-2.5×10-3 mol l-1. The detection limit was calculated to be 40 nmol l-1 [25]. The direct
297
determination strategy was different form the indirect detection of anion that reported in the
298
literature. For example, Zhao et al. fabricated CQDs containing plenty of carboxylate moieties
IL-GQDs over the other anions and cations, a quantitative analysis of
299
[15]. After coordinated with Eu3+ ions, the fluorescence of CQDs turned off. As Eu3+ displayed
300
higher affinity with oxygen-donor atoms from phosphate (pi) than that with the ones from the
301
carboxylate groups of CQDs, turn-on FL can be observed with the addition of pi due to the strong
302
coordination between pi and Eu3+. Thus, the indirect determination of anion was achieved.
303
Obviously, the direct detection present in this work was much more convenient.
400
3.5 Quantitation detection of Fe(CN)63- in real water samples
401
The applicability of IL-GQDs for detecting Fe(CN)63− in real river water was further
402
evaluated. Determination of Fe(CN)63− in the water from Jinsha Lake in Hangzhou (China) was
403
then carried out. Before use, the river samples were filtered through a membrane (0.22 μm) and
404
then centrifuged (15000 r/min) for 10 min. When Fe(CN)63− (0.015-1.5 mM) was added to such
405
river sample, the recoveries of three samples were between 91.3% and 95.3% (Table 1). The
406
detection measurements were performed in triplicate and the RSD was below 2.0 %. The results
407
indicated that this FL probe is promising for direct Fe(CN)63- detection in real samples.
408
4. Conclusion
409
With ionic liquid as specific modification agents, herein, the preparation of ionic
410
liquid-capped GQDs (IL-GQDs) and its application as label-free FL probe for direct detection of
411
anion were reported. The advantages of the proposed method and the prepared IL-GQDs lie in
412
four aspects. (1) After successful combination with IL, the IL-GQDs did not only preserve the
413
excitation-independent PL behavior of GQDs, but also presented the markedly enhanced anion
414
responsiveness, which afforded the IL-GQDs a sensitive response to anion. (2) Based on the
415
improved FL quenching, a facile and direct FL sensor for sensitive detection of Fe(CN)63− was
416
achieved with wide linear range and low detection limit. (3) Compared with complex molecular
417
and atomic doping, the modification of GQDs with functional ILs provided a simple and efficient
418
post-functionalization way to adjust the physicochemical properties of GQDs. (4) As the
419
composition and properties of IL and GQs could easily be tuned by varying the structure, ionic
420
liquids-capped GQDs might present promising potential for broadening the practical applications
421
of GQDs in sensing and catalysis
422
Acknowledgements
423
The authors gratefully acknowledge the financial support from the National Natural Science
424
Foundation of China (No. 21305127), the Zhejiang Provincial Natural Science Foundation of
425
China (Y15B050022, Y17B050008), the Science Foundation of Zhejiang Sci-Tech University
426
(13062173-Y) and 521 talent project of ZSTU.
References [1] R.L. Sun, Y. Wang, Y.N. Ni, S. Kokot, Talanta 125 (2014) 341. [2] J. Peng, W. Gao, B.K. Gupta, Z. Liu , R.R. Aburto, L.H. Ge, L. Song, Nano Lett. 12(2012) 844. [3] Z. Z. Wu, W.Y. Li, J. Chen, C. Yu, Talanta 119 (2014) 538. [4] X. Yan, X. Cui, B.S. Li , L.S. Li, J, Nano Lett. 10(2010) 1869. [5] X.M. Li, M.C. Rui, J.Z. Song, Z.H. Shen, H.B. Zeng, Adv. Funct. Mater. 25 (2015) 4929. [6] X. Yan, X. Cui, L.S. Li, J. Am. Chem. Soc. 132 (2010) 5944. [7] F.E. Lin, C. Gui, W. Wen, X. Zhang, Talanta 158(2016) 292. [8] S.Y. Bian, C. Shen, H. Hua, L. Zhou, F.N. Xi, RSC Adv. 6 (2016) 69977. [9] X. Wang, G. Sun, N. Li, P. Chen, Chem. Soc. Rev. 45 (2016) 2239. [10] N. Li, A. Than, X. Wang, S. Xu, L. Sun, H. Duan, C.J. Xu, P. Chen, ACS Nano. 10(2016) 3622. [11] X.H. Zhao, L. Gong, Y. Wu, X.B. Zhang, J. Xie, Talanta, 149 (2016), 98. [12] R.L. Liu, D.Q. Wu, X.L. Feng , K. Müllen, J. Am.Chem.Soc.133 (2011) 15221. [13] B. Lin, C. Li, Wang, J.X. Luo, Q.W. Guo, K.Y. Liu, K. Liu, W.J. Zhao, Y.Q. Lin, Talanta 144 (2015) 1301. [14] L. Wang, Y.L. Wang, T. Xu, H. B. Liao, C.J. Yao, Y. Liu , Z. Li, Z.W. Chen, D.Y. Pan, L.T. Sun, M.H. Wu, Nature Commun. 5 (2014) 5357. [15] B.F. Shi, L.L. Zhang, C.Q. Lan, J.J. Zhao, Y.B. Su, S.L. Zhao, Talanta 142 (2015) 131. [16] C.F. Zhang, Y.Y. Cui, L. Song, X. F. Liu, Z.B. Hu, Talanta 150 (2016) 54. [17] H.X. Zhao, L.Q. Liu, Z.D. Liu, Y. Wang, X.J. Zhao , C.Z. Huang, Chem. Commun. 47 (2011) 2604. [18] A.A. Hamad, M.A. Alsaadi, M. Hayyan, I. Juneidi, M.A. Hashim, Electrochim. Acta. 193 (2016) 321. [19] S.G. Zhang, Q.H. Zhang, Y. Zhang, Z.J. Chen, M. Watanabe, Y.Q. Deng, Prog. Mater Sci. 77 (2016) 80. [20] B. G. Wang, W.W. Tang, H.S. Lu, Z.Y. Huang, J. Mater Sci. 50 (2015) 5411. [21] A. Ananthanarayanan, X.W. Wang, P. Routh, B. Sana, S. Lim , D.H. Kim, K.H. Lim, J. Li, P.
14
Chen, Adv. Funct. Mater. 24 (2014) 3021. [22] R. L. Liu , M.P. Gao, J. Zhang, Z. L. Li, J.Y. Chen, P. Liu, D.Q. Wu, RSC Adv. 5 (2015) 24205. [23] J. Ju, W. Chen, Biosens. Bioelectron. 58 (2014) 219. [24] F.X. Wang, Z.Y. Gu, W. Lei, W. J. Wang, X.F. Xia, Q.L. Hao, Sens. Actuators B: Chem. 190 (2014) 516. [25] D. Badocco, I. Lavagnini, A. Mondin, G. Favaro, P. Pastore, Spectrochim. Acta B.114(2015) 81.
15
Table 1 Detection of Fe(CN)63− in water samples. [a] Fe(CN)63− added Fe(CN)63− found Samples (mM) (mM)
Recovery (%)
RSD (%)
Samples 1
1.50
1.43
95.3
0.83
Samples 2 Samples 3
0.150 0.015
0.141 0.0137
94.0 91.3
1.36 1.98
[a] Reaction conditions: IL-GQDs (pH=7.0, tris-HCl buffer solution, 0.02 mg/mL), reactions time: 30 min , excitation wavelengths: 470 nm.
16
Figures Captions Scheme 1. Schematic illustration for the preparation of green IL-GQDs and turn-off detection of Fe(CN)63−. Fig. 1.
(a) FL emission spectra of GQDs and IL-GQDs excited at 470nm. (b) FL emission
spectra of IL-GQDs obtained with excitation wavelength progressively being increased from 400 to 470 nm (wavelength interval between adjacent lines was 10 nm). The insets are the photographs of GQDs (left 1 and 3) and IL-GQDs (left 2 and 4) aqueous solution under visible light and UV light of 365 nm, respectively. Fig. 2.
(a) Full-scan XPS, (B) high-resolution C 1s, (C) high-resolution O 1s spectra of the
GQDs. (d) Full-scan XPS, (e) high-resolution C 1s, (f) high-resolution O 1s, (g) high-resolution N 1s, (h) high-resolution B 1s, (i) high-resolution F 1s spectra of the IL-GQDs. Fig. 3.
(a, b) Low-magnification TEM and (c, d) HRTEM images of the as-prepared IL-GQDs.
The inset in figure c was the corresponding size distribution of the IL-GQDs. The labeled number in figure d was the lattice spacing of IL-GQDs. Fig. 4.
AFM images of the GQDs (a) and IL-GQDs (b) on mica substrate. Inset in each figure
was the height profiles along the lines. Fig. 5.
(a) The variation of (F0-F)/F0 of IL-GQDs and GQDs in presence of different anions
(excitation at 470 nm). The concentration of all anions was set as 1.0×10-3 mol l-1. (b) Variation of (F0-F)/F0 of IL-GQDs and GQDs in presence of different cautions (excitation at 470 nm). The concentration of all cations was set as 1.0×10-3 mol l-1. AA was used as the masking agent and its concentration was set as 2.5×10-3 mol l-1. Fig. 6.
(a) Fluorescent spectra of IL-GQDs in the presence of different concentrations of
Fe(CN)63−. Curves of a-m (from top to bottom) represented the concentrations of Fe(CN)63− as 0, 1.0×10-7, 1.0×10-6, 5.0×10-6, 1.0×10-5, 5.0×10-5, 1.0×10-4, 2.5×10-4, 5.0×10-4, 1.0×10-3, 1.5×10-3, 2.0×10-3, 2.5×10-3 mol l-1, respectively. Insets were photos of IL-GQDs under daylight and 365nm UV light (from left to right) containing Fe(CN)63− (1.0×10-3 mol l-1). (b) Variation of (F0-F)/F0 as a function of the concentration of Fe(CN)63−.
17
Scheme 1.
18
Fig. 1.
19
Fig. 2.
20
Fig. 3.
21
Fig. 4.
22
23
Fig. 5.
24
Fig. 6.
Highlights (1) Ionic liquid-capped GQDs (IL-GQDs) were easily prepared. (2) IL-GQDs was used as label-free fluorescent probe for direct detection of Fe(CN)63−. (3) Sensitive detection of Fe(CN)63− with low detection limit was achieved.
25
Graphic abstract
26
PII: DOI: Reference:
S0039-9140(16)31036-0 http://dx.doi.org/10.1016/j.talanta.2016.12.085 TAL17175
To appear in: Talanta Received date: 20 August 2016 Revised date: 27 December 2016 Accepted date: 30 December 2016 Cite this article as: Xue Sun, Yuting Qian, Yajie Jiao, Jiyang Liu, Fengna Xi and Xiaoping Dong, Ionic liquid-capped graphene quantum dots as label-free fluorescent probe for direct detection of ferricyanide, Talanta, http://dx.doi.org/10.1016/j.talanta.2016.12.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2
Ionic liquid-capped graphene quantum dots as label-free
3
fluorescent probe for direct detection of ferricyanide
4 5
Xue Sun, Yuting Qian, Yajie Jiao, Jiyang Liu, Fengna Xi*, Xiaoping Dong*
6 7
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, China
8 9 10
Submitted to Talanta
11
* Corresponding author. E-mail: [email protected], [email protected]
12 13
14
Abstract
15
Despite complex molecular and atomic doping, efficient post-functionalization strategies for
16
graphene quantum dots (GQDs) are of key importance to control the physicochemical properties
17
and broaden the practical applications. With ionic liquid as specific modification agents, herein,
18
the preparation of ionic liquid-capped GQDs (IL-GQDs) and its application as label-free
19
fluorescent probe for direct detection of anion were reported. Hydroxyl-functionalized GQDs that
20
could be easily gram-scale synthesized and possessed single-crystalline were chosen as the model
21
GQDs. Also, the most commonly used ionic liquids, water-soluble 1-butyl-3-methyl imidazolium
22
tetrafluoroborate (BMIMBF4) was chosen as the model IL. Under the ultrasonic treatment,
23
BMIMBF4 easily composited with GQDs to form IL-GQDs. The synthesized IL-GQDs were
24
characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM),
25
X-ray photoelectron spectroscopy (XPS) and fluorescence (FL) spectrum. After successful
26
combination with IL, the excitation-independent photoluminescence behavior of GQDs presented
27
almost no change, whereas, the anion responsiveness of IL-GQDs drastically improved, which
28
afforded the IL-GQDs a sensitive response to Fe(CN)63−. Based on the strong fluorescence quench,
29
a facile and sensitive detection of Fe(CN)63− was achieved. A wide linear range of 1.0×10-7 to
30
2.5×10-3 mol l-1 with a low detection limit of 40 nmol l-1 was obtained. As the composition and
31
properties of IL and GQDs could be easily tuned by varying the structure, ionic liquids-capped
32
GQDs might present promising potential for their applications in sensing and catalysis.
33
Keywords: Ionic liquid-capped GQDs; Composite; Fluorescent; Direct detection of anion;
34
Fe(CN)63−
35 36
37
1. Introduction
38
As a new class of luminescent carbon nanomaterials, graphene quantum dots (GQDs) have
39
rapidly attracted great research interests as intriguing candidates for photoluminescent (PL)
40
materials [1-6]. Compared with conventional semiconductor quantum dots (e.g. CdS, CdSe, CdTe
41
and PbS) and rare elements, GQDs are more biocompatible and environmentally friendly. In
42
addition, the stability of photoemission is higher than organic fluorescent dyes and the cost is
43
lower than noble metal (e.g. Au, Ag) clusters. As great efforts have been paid on developing new
44
preparing strategies, the synthetic methods gradually mature at present and a large number of
45
GQDs have been successfully obtained through both “top-down” and “bottom-up” methodologies
46
[7-13]. For example, Wang et al. reported the gram-scale synthesis of single-crystalline GQDs by
47
a facile molecular fusion route under mild and green hydrothermal conditions [14]. Though GQDs
48
have exhibited significant potential in biology, optical devices and sensing applications, their
49
potential for a spectrum of applications are still ongoing. For instance, the sensing platform for
50
cautions has been widely reported, the application for direct detection of anions is nevertheless
51
still rare (only few indirect detection using on-off strategy have been reported [15-17]).
52
Consequently, further extending the arsenal of GQDs materials is still of critical significance. In
53
spite of complex molecular and atomic doping, the efficient post-functionalization is also
54
extremely important because it provides a convenient route to control the physicochemical
55
properties of GQDs and therefore broadens their practical applications.
56
Ionic liquids (ILs) are composed of organic cation and inorganic or organic anion. ILs are
57
regarded as one of the most exciting scientific discoveries in chemical science because of their
58
unique properties such as negligible vapor pressure, thermal stability, recyclability, ionic
59
conductivity, the ability to dissolve a wide range of chemical species, and easy of structural design
60
[18,19]. Moreover, ILs possessed excellent anion exchange properties. Due to those unique
61
properties, ILs-capped carbon nanomaterials have represented an interesting class of materials
62
owing to their exceptional properties arising from the synergistic combination of both components.
63
Due to π-π or cation-π interactions, ILs could easily combine with carbon materials to form
64
composites. For example, ILs-capped carbon nanotubes (CNTs), carbon nanofiber and graphene
65
have been extensively reported and applied as matrix with excellent solubility and conductivity for
66
electroanalysis [19]. In case of PL carbon materials, ILs-capped carbon quantum dots (CQDs) also
67
demonstrated unique properties. For instance, Wang et al. used citric acid monohydrate as a
68
carbon source and 1-aminopropyl-3-methyl-imida-zolium bromide (APMImBr) ionic liquid as
69
both modification agent and reaction medium to synthesize APMImBr-capped CQDs [20]. With
70
the modification of ILs, [APMIm][Br]-capped CQDs showed high thermal stability and anion
71
responsiveness i.e., CQDs spontaneously transferred from aqueous phase to ethyl acetate phase
72
once the anion of N(CF3SO2)2- was introduced. Wang et al. also prepared highly charged CQDs
73
through one-pot pyrolysis with citric acid as carbon source and APMImBr as capping agent. Due
74
to the IL modification, the as-prepared CQDs depicted a high quantum yield (25.1%) and its
75
amphiphilicity can be facilely tuned by anion exchange, which resulted in spontaneous phase
76
transfer between water and oil phase [21]. Liu et al. applied the microwave-hydrothermal
77
treatment
78
(1-allyl-3-methylimidazolium chloride, AMIMCl). AMIMCl helped to dissolve the cellulose in
79
straw and provided nitrogen atoms for the resulting heteroatom doped CQDs. Due to the surface
80
passivation or functionalization of IL, the produced CQDs possessed high quantum yield (22.58%),
81
which was higher than those of the other prepared from biomass. In addition, the CQDs can serve
82
as an effective label-free fluorescent sensor for the detection of Fe(III) ions with a very low
83
detection limit (200 nmol l-1) [22]. To the best of our knowledge, the preparation of ionic
84
liquid-capped GQDs composite with anion responsiveness and their application for direct sensing
85
of anion were few.
for
rice
straw
in
the
mixed
solvent
of
water
and
ionic
liquid
86
With IL as specific modification agents, in this work, we reported the preparation of
87
IL-capped GQDs and its application as label-free fluorescent probe for direct detection of anion.
88
hydroxyl-functionalized GQDs that possessed single-crystalline and could be easily gram-scale
89
prepared were chosen as the model GQDs. Also, the most commonly used and water soluble ionic
90
liquid, BMIMBF4 was chosen as the model IL. With the ultrasonic treatment, BMIMBF4
91
conveniently was combined with GQDs to form IL-GQDs (Scheme1). The role of ILs might lies
92
in two aspects. Firstly, IL could easily combine with GQDs to form composites due to π-π or
93
cation-π interactions. Secondly, IL possessed excellent anion exchange properties. Consequently,
94
more interaction or shorter action distance might occur between IL-GQDs and specific anion,
95
resulting in improved fluorescence quenching and analytical performance. In comparison with
96
GQDs, IL-GQDs presented an almost unchanged PL behavior and the evidently promoted anion
97
responsiveness. Its fluorescence could be strongly quenched in the presence of Fe(CN)63−. On this
98
basis, a facile and sensitive detection of Fe(CN)63− was achieved. The preparation strategy and the
99
main characteristics of IL-GQDs were described and discussed in detail.
100 101 102
2. Experimental section
103
Ionic
104
tris(hydroxymethyl)aminomethane (tris) were purchased from Aladdin Chemistry Co. Ltd. (China).
105
Pyrene (purity > 97%) was obtained from Chengdu Cologne chemical reagent company (China).
106
The other chemicals used in this work were obtained from Hangzhou Gaojing Chemistry Co. Ltd.
107
(China). All chemicals were of analytical grade and used without further purification. All aqueous
108
solutions were prepared with ultrapure water (18.2 MΩ cm, Milli-Q, Millipore). Solutions
109
containing different cation and anion were applied. The solution of Ag+ was prepared from its
110
nitrate salts. The solution of Pb2+ was prepared from its acetic acid salt. The solutions of Fe3+, Al3+,
111
Mg2+, Cd2+, Ca2+, Cu2+, Cr3+, Hg2+, K+ and Na+ were prepared from their chloride salts. To avoid
112
the hydrolysis of metal ions, pH in the preparation and the following experiments was controlled.
113
Briefly, pH values for Fe3+, Hg2+, Cr3+, and Al3+ were controlled at 2.2, 2.5, 2.5 and 3.0 using
114
glycine-HCl buffer, respectively. Cu2+ (pH 5.0), Zn2+(pH 6.5), Na+(pH 7.0), Pb2+(pH 7.0), K+(pH
115
7.0), Cd2+(pH 7.0), Mg2+(pH 7.0), Ca2+(pH 7.0) solutions were prepared in tris-HCl buffer solution.
116
For Ag+, pH value was set as 6.5 adjusted using Tris or HNO3. For the used anions, solutions of
117
NO3-, S2-, NO2-, S2O32-, HCO3-, H2PO4-, HPO42-, CH3COO- and Cl- were prepared from their
118
sodium salts. The solutions of Br-, Cr2O72-, Fe(CN)63-, Fe(CN)64-, S2O82- and SCN- were prepared
119
from their potassium salts. The pH of anion solution in the preparation and use was controlled by
120
adjusting with tris or HCl. Briefly, pH values for NO3-, NO2-, Cl-, Br-, Fe(CN)63- and Fe(CN)64-
121
solutions were set as 7.0. The pH for solutions of S2O32-, HPO42-, and CH3COO- was controlled as
122
9.0. The pH for solutions of S2O82-, Cr2O72-, H2PO4-, and SCN- was controlled as 3.5, 4.0, and 4.5,
123
respectively. The solution of S2- solutions were prepared in NaHCO3-Na2CO3 buffer solution at pH
124
11.5.
2.1 Materials and reagents liquid,
1-butyl-3-methylimidazolium
tetrafluoroborate
(BMIMBF4),,
and
125
2.2 Instrumentations
126
The fluorescence (FL) spectrum and intensity were recorded on an RF-5301PC
127
spectrofluorometer (Shimadzu Corporation, Japan). Transmission electron microscopic (TEM)
128
photograph was taken on a JEM-2100 transmission electron microscope at operating voltage of
129
200 kV (JEOL Ltd., Japan). The sample was dispersed in water and then drop-casted on ultrathin
130
carbon-coated copper grid. For atomic force microscopy (AFM) characterization, the aqueous
131
solution of GQDs or IL-GQDs was spotted onto freshly cleaved mica surface and dried in air. The
132
samples were measured in air by tapping mode on MFP-3D AFM microscope (Asylum research).
133
X-ray photoelectron spectroscopy (XPS) characterizations were conducted by using a PHI5300
134
electron spectrometer using 250 W, 14KV, Mg Kα radiation (PE Ltd., USA). Fourier transform
135
infrared (FT-IR) spectra of the IL, GQDs and IL-GQDs were obtained using a Spectrum 5700
136
(Nicolet Instrument Co. USA)
137
2.3 Preparation of the IL-GQDs
138
IL-GQDs were prepared by combination of IL with pre-synthesized GQDs (Scheme 1).
139
Firstly, GQDs with hydroxyl groups were prepared according to the literature [14] with little
140
modification. The synthesis involved the hydrothermal treatment of 1,3,6-trinitropyrene in alkaline
141
NaOH solution for molecular fusion. Briefly, 1,3,6-trinitropyrene (2 mg/mL) was dispersed in
142
NaOH solution (150 mL, 5 mg/mL) by ultrasonically treating for 1 h. The obtained mixture was
143
transferred to poly(tetrafluoroethylene)-lined autoclave and heated at 200 oC for 4h. After cooling
144
down to room temperature, the resulting GQDs solution was filtered through a 0.45 μm
145
microporous membrane to remove undissolved carbon product, and dialysed by dialysis bag (3500
146
Da) against water for 3 days to remove the molecular precursors. The obtained solution in dialysis
147
bag (GQDs > 3500 Da) was further dialysed with retained molecular weight of 6000Da for 3 days
148
to remove impurities with large size. Finally, the obtained GQDs (3500-6000Da) were freeze dried
149
to a brown powder.
150
Secondly, IL-GQDs were prepared by mixing GQDs solution (3500-6000Da, 0.02 mg/mL)
151
with BMIMBF4 (100:1, V/V) and ultrasonically treating for 30 min. The formed solution was
152
dialyzed by dialysis bag (1000 Da) against water for 1 day to remove unbound IL.
153
Quantum yield (QY) of the GQDs and IL-GQDs was measured by using Rhodamine B as the
154
standard sample and was computed according to the following equation:
Yμ = Ys
155
Fu As Fs Aμ
156
where Yμ and Ys are the quantum yield of the GQDs or IL-GQDs and the standard substance, Fu
157
and Fs respectively stand for the integral fluorescence intensity of the test GQDs or IL-GQDs and
158
the diluted standard solution and Aμ and As are the maximum absorbance value of GQDs or
159
IL-GQDs and the diluted standard solution, respectively [22].
160
2.4 Fluorescent detection of Fe(CN)63− using IL-GQDs
161
A solution of IL-GQDs (0.02 mg/mL) in 50 mM tris-HCl buffer solution (pH = 7.0) was used
162
as an FL probe toward Fe(CN)63−. In a typical run, IL-GQDs solution was mixed with different
163
amount of Fe(CN)63−. After the resulting solution was shaken well and incubated for 30 min at
164
room temperature, the FL emission spectra (excited at 470 nm) were recorded. The detection
165
measurements were performed in triplicate. The relative FL intensity (F0- F)/F0 versus Fe(CN)63−
166
concentration were used for calibration. Here, F0 and F are the FL intensities of IL-GQDs in the
167
absence and presence of different concentration of Fe(CN)63−, respectively.
168
3. Results and discussion
169
3.1 Photcture characterization of the IL-GQDs
170
The PL characteristics of both GQDs and IL-GQDs were investigated to evaluate the effect of
171
IL. Results were given in Fig. 1. As illustrated in the inset of the figure, both GQDs (pH=7.0, 0.02
172
mg/mL) and IL-GQDs (pH=7.0, 0.02 mg/mL calculated using GQDs) were pale-yellow,
173
transparent solution under daylight, indicating their hydrophilic characters. Such property might
174
be ascribed to the large amount of hydroxyl groups in GQDs and the hydrophilicity of the
175
water-soluble ionic liquid, BMIMBF4. When the aqueous suspensions of GQDs and IL-GQDs
176
were UV irradiated (365 nm), strong green fluorescence without obvious difference was observed.
177
To further study the FL property, FL spectra were recorded. As shown in Fig. 1a, the spectra of
178
GQDs in the absence and presence of ionic liquids were almost constant when excited at 470 nm.
179
In addition, the emission spectra of GQDs and IL-GQDs with progressively increased excitation
180
wavelengths from 400 to 470 nm with 10 nm increment were recorded. Excitation-independent PL
181
behaviors were revealed, suggesting single-emission fluorescence center and high crystallinity
182
[14]. The maximum excitation and emission wavelengths of the GQDs and IL-GQDs were
183
revealed at 470 nm and 512 nm, respectively (Fig. 1b, Fig. S1). The above results confirmed that
184
the incorporation of IL did not change the FL characteristics of GQDs including the maximum
185
emission wavelength and intensity. Also, significant improvement of the FL intensity was not
186
observed in this IL-GQDs composite.
187
The FL intensity of the IL-GQDs at different pH was studied. The pH of the solution was
188
adjusted by NaOH or HCl (Fig. S2). FL intensity was low at such strong acidic conditions,
189
whereas, it increased dramatically with the increase of pH value. When pH was above 6,
190
fluorescent intensity was relative stable. The phenomenon might be ascribed to the ionization of
191
hydroxyl groups of GQDs [14]. It is well known that electronic structure and charge density might
192
affect the opitical characteristics of GQDs. The QYs of GQDs and IL-GQDs were 12.1%，11.8%
193
at pH 7.0, respectively. Photo bleaching experiment of IL-GQDs was also performed to evaluate
194
the photo stability. IL-GQDs were irradiated continually for 4 hours under UV lamp (365 nm).
195
Results were shown in Fig. S3. It was demonstrated that the FL intensity of IL-GQDs can
196
maintain 94% of the original signal with four-hour irradiating under UV light, indicating good
197
photo stability.
198
3.2 Structure characterization of the IL-GQDs
199
Elemental analysis of the GQDs and IL-GQDs was performed by XPS. As shown in Fig. 2a,
200
the full-scan XPS spectrum confirmed that GQDs were composed of carbon and oxygen.
201
Fine-structure spectrum of C 1s (Fig. 2a) exhibited two main peaks 286.6eV and 284.6eV,
202
implying the C=C and C-OH groups due to the graphitic and hydroxyl groups modified carbon
203
atoms (Fig. 2a). The only one peak for O 1s was ascribed to hydroxyl groups formed from the
204
molecular fusion of 1,3,6-trinitropyrene in alkaline NaOH solution under the hydrothermal
205
treatment (Fig. 2a). Those results were consistent with the results reported in the literature [14]. As
206
known, 1,3,6-trinitropyrene possessed the same mother nucleus structure with graphene and could
207
occur molecular fusion in hydrothermal reaction to form graphene framework [14]. As the
208
powerful electrophilicity of nitro groups，the addition reaction can happen with electron-rich -OH
209
groups and resulted in GQDs with a large amount of hydroxyl groups. In the case of IL-GQDs, the
210
full range XPS spectrum demonstrated the presence of C, N, O, B and F (Fig. 2d). The
211
corresponding C1s, N1s, O1s, B1s and F1s peaks centered at 284eV, 403eV, 534eV, 194.6eV and
212
684.5eV, respectively. Despite C=C or C-C (284.6eV) and C-OH (286.8eV), C-N centered at
213
285.9eV was observed corresponding to the presence of N element in IL. Similarly with O 1s of
214
GQDs, only OH groups were observed. The N1s peaks at 401.8eV and 403.6eV are ascribed to the
215
imidazole ring and C-N, the two kinds of in amide group and imidazole ring of BMIMBF4. The
216
presence of bromine and fluorine further proved the successful attachment of IL onto GQDs. The
217
interaction between IL and GQDs might lie in π-π or cation-π interactions and a carbon core with
218
peripheral cationic-ionic pairs might form. FTIR spectrum was also used to confirm the groups of
219
GQDs and IL-GQDs (Fig. S4). Both the GQDs and IL-GODs showed absorption of hydroxyl
220
group (3400 cm-1) and C-OH (1150 cm-1). Compared with GQDs, the FT-IR spectrum of IL,
221
IL-GQD showed the additional absorption peak from C-N stretching vibration at 1169cm-1. The
222
peak around 1572 and 1465cm-1 in IL and IL-GQDs were related to the typical stretching of
223
imidazole ring [23]. These results were consistent with the results from XPS and further confirmed
224
the successful hybridization of IL with GQDs.
225
The morphology characteristics of IL-GQDs were investigated using TEM and AFM. TEM
226
images revealed that IL-GQDs were well-dispersed (Fig. 3a, b) with relatively uniform size
227
distribution of 1.6-2.4 nm. The average diameter of the IL-GQDs was calculated to be 2.0 nm
228
(inset of Fig. 3c). High resolution TEM (HRTEM) image (Fig. 3d) showed clear lattice structure
229
of the IL-GQDs, indicating high crystallinity. The lattice spacing was measured to be about 0.23
230
nm, which can be attributed to the (100) facets of graphite [14]. AFM was used to evaluate the
231
height profiles of GQDs and IL-GQDs. As demonstrated in Fig. 4, GQDs showed a typical
232
topographic height of about 2.8 nm, indicating that most of the GQDs consisted of 3-5 layer of
233
graphite. For IL-GQDs, the topographic height increased to about 3.4 nm, confirming the
234
modification of IL onto GQDs.
235
3.3 Selective FL response of IL-GQDs towards anion
236
The GQDs-based FL sensing system has been widely reported. The selectivity of cations was
237
mainly achieved based on coordination or electrostatic effect because GQDs usually contained
238
carboxyl or hydroxyl groups or doped hetero atoms (e.g. N, S). However, research on direct anion
239
responsiveness of GQDs is nevertheless still rare. Due to the unique ion-exchange property of IL,
240
the IL-GQDs in this work were expected to possess anion responsiveness.
241
To test the selectivity of anion, the FL quenching experiments of both GQDs and IL-GQDs
242
were investigated. The FL quenching efficiency of (F0-F)/F0 was determined. Here, F0 and F were
243
the FL intensities of GQDs or IL-GQDs in the absence and presence of different ion, respectively.
244
The commonly used anions including HCO3-, H2PO4-, S2O82-, SCN-, HPO42-, Ac-, Cl-, NO3- , S2-,
245
NO2-, Br-, S2O32-, Cr2O72-, Fe(CN)63- and Fe(CN)64− were tested. As given in Fig. 5a, the FL of
246
GQDs could be slightly quenched by Cr2O72-, Fe(CN)63- and Fe(CN)64− with FL quenching
247
efficiency between 0.1-0.2. The other anions only caused faintly FL quenching with quenching
248
efficiency below 0.1. In case of IL-GQDs, the same anion selectivity was revealed, whereas, the
249
fluorescence quenching efficiency greatly improved towards Fe(CN)63- and Fe(CN)64−. For
250
Fe(CN)63-, the highest fluorescence quenching efficiency was observed. In order to eliminate
251
possible cation interference, the selectivity towards the commonly used cations was also
252
determined including Fe3+, Cr3+, Na+, Pb2+, K+, Cu2+, Cd3+, Al3+, Zn2+, Mg2+, Ca2+, Ag+, Hg2+.
253
Among the tested ions, only Fe3+ possessed remarkable quenching for the FL of IL-GQDs (Fig.
254
5b). That may interfere with the detection of Fe(CN)63−. This issue can be circumvented by using
255
ascorbic acid (AA) as chelating agents for Fe3+ ions [24]. The above results showed that Fe(CN)63−
256
present the highest quenching for the FL of IL-GQDs over the other anions and cations. In
257
comparison with GQDs, also, the IL-GQDs as a FL probe promised much improved selectivity for
258
Fe(CN)63−. The reasons might lie in three aspects. Firstly, the used GQDs might have selectivity
259
towards iron based ions. Secondly, the corporation of IL on GQDs might regulate the
260
physicochemical property, especially the electrostatic property, of the GQDs. Compared with
261
GQDs, IL-GQDs might present a carbon core with peripheral cationic-ionic pairs. Thus, the
262
electrostatic effect between Fe(CN)63− and IL-GQDs might be promoted. Thirdly, Fe(CN)63− was
263
bulky anion and could anion-exchange with BF4- of IL (Scheme 1). More systematic work on the
264
mechanism of such selectivity was under way in our laboratory. By controlling the redox property
265
of the medium solution, Fe(CN)63− and Fe(CN)64− can be easily distinguished. For example,
266
Fe(CN)63− will transfer to be Fe(CN)64− in presence of hydroxylamine hydrochloride. On the other
267
hand, Fe(CN)64− will be converted into Fe(CN)63− with oxidizing agent.
268
3.4 Direct detection of Fe(CN)63− using IL-GQDs as FL probe
269
FL quenching of IL-GQDs caused by Fe(CN)63− at different pH was further explored. To
270
adjusted the pH of the solution, glycine-HCl buffer solution (pH=3), tris-HCl buffer solution (pH
271
4-9) and carbonate buffer solution (pH 10-11) were used. The concentration of the buffer solution
272
was set as 30 mmol l-1. As the buffer solution introduced anions and cations, the effect of the
273
buffer solution was firstly studied. The fluorescrnce intensities of IL-GQDs in buffer solution
274
were compared with those obtained by adjusting pH only with NaOH or HCl. Consistent results
275
with change no more than 5% at maximum emission wavelength were obtained (Fig. S5),
276
indicating no significant effect caused by the introduced by anions and cations in the buffer
277
solutions. Then the fluoscrescence quenching caused by Fe(CN)63− was studied by measuring the
278
fluorescence intensity in absence (F0) and presence (F) of Fe(CN)63− (Fig. S5) and calculating the
279
relative change as (F0-F)/F0 (Fig. S6). Obviously, the values of (F0-F)/F0 had great changes at pH
280
3.0-6.0 (Fig. S6). As most of the environmental or biological water bodies are neutral or nearly
281
neutral, the neutral solution (pH 7.0) that gave the highest FL quenching ratio was chosen for
282
further investigation.
283
The kinetic behavior of reactions between the IL-GQDs and Fe(CN)63− was investigated by
284
plotting relative FL intensities (F/F0) as a function of time. The reactions reached the quenching
285
equilibrium in about 30 min (Fig. S7). Therefore, 30 min was chosen as reaction equilibrium time
286
in the subsequent detection experiments.As Fe(CN)63− presented the highest FL quenching
287
efficiency for the
288
Fe(CN)63− based on direct FL quenching of IL-GQDs was performed. IL-GQDs solution reacted
289
with different concentrations of Fe(CN)63− and the FL spectra were measured. As shown in Fig. 6a,
290
the FL intensity of IL-GQDs decreased with increase of the concentration of Fe(CN)63−, proving
291
the validity of the FL assay for detection of Fe(CN)63−. The dependence of (F0-F)/F0 on the
292
concentration of Fe(CN)63− ions was concluded in Fig. 6b. Linear correlation existed between the
293
value of (F0-F)/F0 at 512 nm and the concentration of Fe(CN)63− in the range of 1.0×10-7 to
294
2.5×10-3 mol l-1. A well-resolved linear part (R2 = 0.994) was found in the concentration range of
295
1.0×10-7--5.0×10-5 mol l-1 and another linear part (R2 = 0.995) presented in the concentration range
296
of 5.0×10-5-2.5×10-3 mol l-1. The detection limit was calculated to be 40 nmol l-1 [25]. The direct
297
determination strategy was different form the indirect detection of anion that reported in the
298
literature. For example, Zhao et al. fabricated CQDs containing plenty of carboxylate moieties
IL-GQDs over the other anions and cations, a quantitative analysis of
299
[15]. After coordinated with Eu3+ ions, the fluorescence of CQDs turned off. As Eu3+ displayed
300
higher affinity with oxygen-donor atoms from phosphate (pi) than that with the ones from the
301
carboxylate groups of CQDs, turn-on FL can be observed with the addition of pi due to the strong
302
coordination between pi and Eu3+. Thus, the indirect determination of anion was achieved.
303
Obviously, the direct detection present in this work was much more convenient.
400
3.5 Quantitation detection of Fe(CN)63- in real water samples
401
The applicability of IL-GQDs for detecting Fe(CN)63− in real river water was further
402
evaluated. Determination of Fe(CN)63− in the water from Jinsha Lake in Hangzhou (China) was
403
then carried out. Before use, the river samples were filtered through a membrane (0.22 μm) and
404
then centrifuged (15000 r/min) for 10 min. When Fe(CN)63− (0.015-1.5 mM) was added to such
405
river sample, the recoveries of three samples were between 91.3% and 95.3% (Table 1). The
406
detection measurements were performed in triplicate and the RSD was below 2.0 %. The results
407
indicated that this FL probe is promising for direct Fe(CN)63- detection in real samples.
408
4. Conclusion
409
With ionic liquid as specific modification agents, herein, the preparation of ionic
410
liquid-capped GQDs (IL-GQDs) and its application as label-free FL probe for direct detection of
411
anion were reported. The advantages of the proposed method and the prepared IL-GQDs lie in
412
four aspects. (1) After successful combination with IL, the IL-GQDs did not only preserve the
413
excitation-independent PL behavior of GQDs, but also presented the markedly enhanced anion
414
responsiveness, which afforded the IL-GQDs a sensitive response to anion. (2) Based on the
415
improved FL quenching, a facile and direct FL sensor for sensitive detection of Fe(CN)63− was
416
achieved with wide linear range and low detection limit. (3) Compared with complex molecular
417
and atomic doping, the modification of GQDs with functional ILs provided a simple and efficient
418
post-functionalization way to adjust the physicochemical properties of GQDs. (4) As the
419
composition and properties of IL and GQs could easily be tuned by varying the structure, ionic
420
liquids-capped GQDs might present promising potential for broadening the practical applications
421
of GQDs in sensing and catalysis
422
Acknowledgements
423
The authors gratefully acknowledge the financial support from the National Natural Science
424
Foundation of China (No. 21305127), the Zhejiang Provincial Natural Science Foundation of
425
China (Y15B050022, Y17B050008), the Science Foundation of Zhejiang Sci-Tech University
426
(13062173-Y) and 521 talent project of ZSTU.
References [1] R.L. Sun, Y. Wang, Y.N. Ni, S. Kokot, Talanta 125 (2014) 341. [2] J. Peng, W. Gao, B.K. Gupta, Z. Liu , R.R. Aburto, L.H. Ge, L. Song, Nano Lett. 12(2012) 844. [3] Z. Z. Wu, W.Y. Li, J. Chen, C. Yu, Talanta 119 (2014) 538. [4] X. Yan, X. Cui, B.S. Li , L.S. Li, J, Nano Lett. 10(2010) 1869. [5] X.M. Li, M.C. Rui, J.Z. Song, Z.H. Shen, H.B. Zeng, Adv. Funct. Mater. 25 (2015) 4929. [6] X. Yan, X. Cui, L.S. Li, J. Am. Chem. Soc. 132 (2010) 5944. [7] F.E. Lin, C. Gui, W. Wen, X. Zhang, Talanta 158(2016) 292. [8] S.Y. Bian, C. Shen, H. Hua, L. Zhou, F.N. Xi, RSC Adv. 6 (2016) 69977. [9] X. Wang, G. Sun, N. Li, P. Chen, Chem. Soc. Rev. 45 (2016) 2239. [10] N. Li, A. Than, X. Wang, S. Xu, L. Sun, H. Duan, C.J. Xu, P. Chen, ACS Nano. 10(2016) 3622. [11] X.H. Zhao, L. Gong, Y. Wu, X.B. Zhang, J. Xie, Talanta, 149 (2016), 98. [12] R.L. Liu, D.Q. Wu, X.L. Feng , K. Müllen, J. Am.Chem.Soc.133 (2011) 15221. [13] B. Lin, C. Li, Wang, J.X. Luo, Q.W. Guo, K.Y. Liu, K. Liu, W.J. Zhao, Y.Q. Lin, Talanta 144 (2015) 1301. [14] L. Wang, Y.L. Wang, T. Xu, H. B. Liao, C.J. Yao, Y. Liu , Z. Li, Z.W. Chen, D.Y. Pan, L.T. Sun, M.H. Wu, Nature Commun. 5 (2014) 5357. [15] B.F. Shi, L.L. Zhang, C.Q. Lan, J.J. Zhao, Y.B. Su, S.L. Zhao, Talanta 142 (2015) 131. [16] C.F. Zhang, Y.Y. Cui, L. Song, X. F. Liu, Z.B. Hu, Talanta 150 (2016) 54. [17] H.X. Zhao, L.Q. Liu, Z.D. Liu, Y. Wang, X.J. Zhao , C.Z. Huang, Chem. Commun. 47 (2011) 2604. [18] A.A. Hamad, M.A. Alsaadi, M. Hayyan, I. Juneidi, M.A. Hashim, Electrochim. Acta. 193 (2016) 321. [19] S.G. Zhang, Q.H. Zhang, Y. Zhang, Z.J. Chen, M. Watanabe, Y.Q. Deng, Prog. Mater Sci. 77 (2016) 80. [20] B. G. Wang, W.W. Tang, H.S. Lu, Z.Y. Huang, J. Mater Sci. 50 (2015) 5411. [21] A. Ananthanarayanan, X.W. Wang, P. Routh, B. Sana, S. Lim , D.H. Kim, K.H. Lim, J. Li, P.
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Chen, Adv. Funct. Mater. 24 (2014) 3021. [22] R. L. Liu , M.P. Gao, J. Zhang, Z. L. Li, J.Y. Chen, P. Liu, D.Q. Wu, RSC Adv. 5 (2015) 24205. [23] J. Ju, W. Chen, Biosens. Bioelectron. 58 (2014) 219. [24] F.X. Wang, Z.Y. Gu, W. Lei, W. J. Wang, X.F. Xia, Q.L. Hao, Sens. Actuators B: Chem. 190 (2014) 516. [25] D. Badocco, I. Lavagnini, A. Mondin, G. Favaro, P. Pastore, Spectrochim. Acta B.114(2015) 81.
15
Table 1 Detection of Fe(CN)63− in water samples. [a] Fe(CN)63− added Fe(CN)63− found Samples (mM) (mM)
Recovery (%)
RSD (%)
Samples 1
1.50
1.43
95.3
0.83
Samples 2 Samples 3
0.150 0.015
0.141 0.0137
94.0 91.3
1.36 1.98
[a] Reaction conditions: IL-GQDs (pH=7.0, tris-HCl buffer solution, 0.02 mg/mL), reactions time: 30 min , excitation wavelengths: 470 nm.
16
Figures Captions Scheme 1. Schematic illustration for the preparation of green IL-GQDs and turn-off detection of Fe(CN)63−. Fig. 1.
(a) FL emission spectra of GQDs and IL-GQDs excited at 470nm. (b) FL emission
spectra of IL-GQDs obtained with excitation wavelength progressively being increased from 400 to 470 nm (wavelength interval between adjacent lines was 10 nm). The insets are the photographs of GQDs (left 1 and 3) and IL-GQDs (left 2 and 4) aqueous solution under visible light and UV light of 365 nm, respectively. Fig. 2.
(a) Full-scan XPS, (B) high-resolution C 1s, (C) high-resolution O 1s spectra of the
GQDs. (d) Full-scan XPS, (e) high-resolution C 1s, (f) high-resolution O 1s, (g) high-resolution N 1s, (h) high-resolution B 1s, (i) high-resolution F 1s spectra of the IL-GQDs. Fig. 3.
(a, b) Low-magnification TEM and (c, d) HRTEM images of the as-prepared IL-GQDs.
The inset in figure c was the corresponding size distribution of the IL-GQDs. The labeled number in figure d was the lattice spacing of IL-GQDs. Fig. 4.
AFM images of the GQDs (a) and IL-GQDs (b) on mica substrate. Inset in each figure
was the height profiles along the lines. Fig. 5.
(a) The variation of (F0-F)/F0 of IL-GQDs and GQDs in presence of different anions
(excitation at 470 nm). The concentration of all anions was set as 1.0×10-3 mol l-1. (b) Variation of (F0-F)/F0 of IL-GQDs and GQDs in presence of different cautions (excitation at 470 nm). The concentration of all cations was set as 1.0×10-3 mol l-1. AA was used as the masking agent and its concentration was set as 2.5×10-3 mol l-1. Fig. 6.
(a) Fluorescent spectra of IL-GQDs in the presence of different concentrations of
Fe(CN)63−. Curves of a-m (from top to bottom) represented the concentrations of Fe(CN)63− as 0, 1.0×10-7, 1.0×10-6, 5.0×10-6, 1.0×10-5, 5.0×10-5, 1.0×10-4, 2.5×10-4, 5.0×10-4, 1.0×10-3, 1.5×10-3, 2.0×10-3, 2.5×10-3 mol l-1, respectively. Insets were photos of IL-GQDs under daylight and 365nm UV light (from left to right) containing Fe(CN)63− (1.0×10-3 mol l-1). (b) Variation of (F0-F)/F0 as a function of the concentration of Fe(CN)63−.
17
Scheme 1.
18
Fig. 1.
19
Fig. 2.
20
Fig. 3.
21
Fig. 4.
22
23
Fig. 5.
24
Fig. 6.
Highlights (1) Ionic liquid-capped GQDs (IL-GQDs) were easily prepared. (2) IL-GQDs was used as label-free fluorescent probe for direct detection of Fe(CN)63−. (3) Sensitive detection of Fe(CN)63− with low detection limit was achieved.
25
Graphic abstract
26