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Chemically Modulated Carbon Nitride Nanosheets for Highly Selective Electrochemiluminescent Detection of Multiple Metal-ions Zhixin Zhou, Qiuwei Shang, Yanfei Shen, Linqun Zhang, Yuye Zhang, Yanqin Lv, Ying Li, Songqin Liu, and Yuanjian Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01062 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 20, 2016

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Analytical Chemistry

Chemically Modulated Carbon Nitride Nanosheets for Highly Selective Electrochemiluminescent Detection of Multiple Metal-ions

Zhixin Zhou†, Qiuwei Shang†, Yanfei Shen‡, Linqun Zhang†, Yuye Zhang†, Yanqin Lv†, Ying Li†, Songqin Liu†, and Yuanjian Zhang*† †

Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, Jiangsu

Optoelectronic Functional Materials and Engineering Laboratory, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China ‡

Medical School, Southeast University, Nanjing 210009, China

1

ACS Paragon Plus Environment


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Abstract Chemical structures of two-dimensional (2D) nanosheet can effectively control the properties thus guiding their applications. Herein we demonstrate that carbon nitride nanosheets (CNNS) with tunable chemical structures can be obtained by exfoliating facile accessible bulk carbon nitride (CN) of different polymerization degree. Interestingly, the electrochemiluminescence (ECL) properties of as-prepared CNNS were significantly modulated. As a result, unusual changes for different CNNS in quenching of ECL due to inner filter effect/electron transfer and enhancement of ECL owing to catalytic effect were observed by adding different metal ions. Based on this, by using various CNNS, highly selective ECL sensors for rapid detecting multiple metal-ions such as Cu2+, Ni2+, and Cd2+ were successfully developed without any labeling and masking reagents. Multiple competitive mechanisms were further revealed to account for such enhanced selectivity in the proposed ECL sensors. The strategy of preparing CNNS with tunable chemical structures that facilely modulated the optical properties would open a vista to explore 2D carbon-rich materials for developing a wide range of applications such as sensors with enhanced performances.

2


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Introduction With respect to the bulk counterparts, two-dimensional (2D) nanosheets have attracted great attention for their intriguing optical, electronic and interfacial properties, thus making them promising for sensors, optoelectronic devices, and energy conversion and storage.1-4 Both theoretical and experimental studies have proved that sizes and edge/plane chemical structures including heteroatom doping, can effectively control the properties thus guiding their applications.5-8 A typical example is graphene nanosheets (GNs) that undergoes an explosion of interest.9,10 Recently, the search for graphene-analogues, but with an opened-up band gap, has attracted tremendous attention, because the transformation of 2D nanosheets into semiconductors is prospective to bring much added-value.11 Among them, polymeric carbon nitride (CN) is a representative polymer featuring a band gap of 2.7 eV that enable immigration of free charger-carriers across interfaces for surface redox reactions upon light excitation.12,13 To fulfill the enormous potential of abundant CN for practical applications such as in biosensing and imaging,14-17 (photo)catalysis,11,18 and photoelectric conversion,3 the peeling of bulk CN into 2D nanosheets has emerged very recently by liquid-exfoliation14,19,20 and thermal oxidation etching.21 However, carbon nitride nanosheets with tunable chemical structures and sizes has been rarely explored so far, thus impeding the further boost of performances. On the other hand, detection of metal-ions as known is crucial for health due to both deficiency and overdose of them will cause the imbalance of homestasis and subsequenet severe diseases.22-25 For this, various analytical techniques have been developed to sense metal-ions, including chemical titration, and UV/atomic absorption/mass spectrometry. However, the drawbacks of them such as time consuming, poisonous chromogenic agents, high price or maintenance costs of instrument,26 limit their widespread application. Among various alternatives,23,27-30 electrochemiluminescent (ECL) analysis including that based on CN luminophor very recently, has attracted considerable attention due to its low background, simple instrument, and high sensitivity.12,31-37 For instance, we showed that dual ECL-signals 3



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could be actuated from CN at different driven potentials to improve the selectivty in sensing.24 Nevertheless, in previous studies, the usage of CN was mostly limited in the pristine form of the bulk powder that was condensed at ca. 550 oC (Figure 1a) or nanosheets by a subsequent exfoliation.12,15,33 The tunable structure of CN with unique properties, however, has been seldom explored for ECL analysis, which hinders the potential application of CN in sensing. In this work, we demonstrate that carbon nitride nanosheets (CNNS) with tunable chemical structures and sizes can be obtained by liquid-exfoliation of facile accessible bulk CN with different polymerization degree. Interestingly, the different chemical structure rendered that the as-prepared CNNS exhibited different ECL behaviors and resulted in distinctive ECL response to several metal-ions. Such advantage was further applied to construct sensors for multiple metal-ions without any labeling and masking reagents, which was rarely achieved previously by ECL sensors based on other luminophors. It would open a vista to explore new functions of 2D carbon-rich materials for developing a wide range of applications such as sensing with enhanced performances.

Experiment Section Reagent.

Dicyandiamide

(DCDA,

99%)

were

purchased

from

Sigma-Aldrich. KCl, tris-hydroxymethyl aminomethane, HCl (37%), and K2S2O8 were of analytic grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Triethanolamine (TEA; 98%) was purchased from Aladdin

Co.

Ltd.

(Shanghai,

China).

CuCl22H2O,

Ni(NO3)26H2O,

CdCl22.5H2O, and other regents were of analytical grade and used as received without further purification. Ultrapure water (18.2 MΩ) was obtained through Thermo purification system. Preparation of bulk CN and sulphur-doped CN. Bulk CN was synthesized according to previous procedures.13 Briefly, 10 g of DCDA was put into a capped crucible and heated at the desired temperature (400 oC to 650 oC) in air for 4 hrs with a 4 hrs ramp time. The obtained products were denoted as CN-T, 4


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where T refers to the final condensation temperature. Sulphur-doped CN was synthesized

by

mixing

10

g

of

urea

and

10

mg

of

2-aminothiophene-3-carbonitrile (ATCN) in 10 mL of water and stirred at room temperature for 12 hrs (Scheme S1).11 Then, the mixtures were stirred at 80 oC to remove water. Finally, the mixtures were ground into powder by agate mortar and heated at 550 oC for 2 hrs with a ramp rate of 5.0 oC/min in air. The obtained product was named as S-CN-550. Preparation of CNNS. The nanosheets was obtained by liquid-exfoliating of as-prepared various bulk CN-T (T=400 oC to 650 oC) and S-CN-550 in water and named as CNNS-T (T=400 oC to 650 oC) and S-CNNS-550, respectively. In details, 100 mg of bulk sample was dispersed in 100 mL of water, and then ultrasonicated for about 16 hrs (KQ-400KDE, 400 W, 40k HZ, China). The initial formed suspension was then centrifuged at 5000 rpm (Eppendorf Centrifuge 5415R) to remove the residual unexfoliated nanoparticle and large area nanosheets before used for further studies. Preparation of the CNNS-Modified glassy carbon electrode (GCE). The GCE (d = 3 mm) was polished to a mirror finish with a 0.3 m alumina slurry. 20 L of CNNS solution was pipetted onto polished glassy carbon electrode and then dried in air at room temperature. After that, 15 L of Nafion (0.05 wt %) was further pipetted onto the surface of the GCE and dried in air at room temperature. Characterization. Fourier transform infrared spectra (FT-IR) were collected with a Nicolet 5700 FTIR spectrometer equipped with an attenuated total reflection (ATR) setup (Thermo, USA). X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance diffractometer equipped with high-intensity Cu-Kα radiation (λ = 1.54178 Å). The transmittance electron microscopy (TEM) images were characterized with JEM-2100 field emission electron microscopy at an acceleration voltage of 200 kV. Energy dispersive X-ray (EDX) spectra were carried out on a Phenom ProX scanning electron microscope (The Netherlands). X-ray photoelectron spectra (XPS) experiments were recorded on a Theta probe (Thermo Fisher) with monochromated 5



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Al-Kα X-rays (hν = 1486.6 eV) as the excitation source. The binding energies obtained in the XPS analysis were corrected for specimen charging by reference C1s to 284.6 eV. UV-vis absorption spectroscopy was taken on Shimadzu UV-2450 spectrophotometer with a diffuse reflectance accessory and BaSO4 was used as the reference sample (100% reflectance). Dynamic light scattering (DLS) was performed on

Malven

Zetasizer

NanoZS.

The

photoluminescence

(PL)

and

electrochemiluminescence (ECL) emission spectra were obtained from fluorescence spectrometer (Fluoromax-4, Horiba Jobin Yvon, Japan). The time-resolved PL spectra were performed with fluorescence spectrometer (PluoroLog 3-TCSPC, Horiba Jobin Yvon, Japan). ECL and cyclic voltammetry measurement were were carried out in a three-electrode cell with an ECL analyzer system (MPI-E, Xi’anruimai Analytical Instruments Co. Ltd., China). A calomel electrode (saturated KCl) and platinum wire were used as reference and counter electrodes, respectively. The supporting electrolyte was 0.10 M KCl with 150 mM K2S2O8 (cathodic ECL coreactant) and 60 mM triethanolamine (anodic ECL coreactant) in 0.01 M Tris-HCl (pH 7.4). Mechanism of the cathodic and anodic ECL for CNNS. Both anodic and cathodic ECL behaviors of bulk CN and CNNS were proposed in previous reports.15,33 In the case of cathodic ECL, the injection of electrons into the conduction band of CNNS from the working electrode produced the negatively charged CNNS (CNNS•-) (eq 1) as the potential was negative enough. Secondly, S2O82- was electro-reduced to SO4•- and SO42- (eq 2). The strong oxidant species (SO4•-) would subsequently capture an electron from the valence band of CNNS•- to produce the excited state CNNS (CNNS*), as shown in eq 3. Finally, an intense blue emission was obtained when CNNS* decays back to the ground state CNNS (eq 4). In the case of anodic ECL, the capture of electrons from the valence band of CNNS by working electrode would produce the positively charged CNNS (i.e., CNNS•+) as the potential was positive enough (eq 5). Similar to other ECL coreactants, coreactant TEA used in this work could be also electro-oxidized to a cation (TEA•+), as shown in eq 6, and then, upon deprotonation, TEA•+ would subsequently decompose to produce a strong reductive radical (TEA•), as shown in eq 7, which can react with the CNNS•+ to produce the 6


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excited state CNNS* via electron transfer (eq 8). Finally, an intense emission was obtained when CNNS* decays back to the ground state CNNS (eq 9).

CNNS  e   CNNS

(1)

S2 O8 2  e   SO 4 2  SO4

(2)

CNNS  SO 4  CNNS* SO 4 2

(3)

CNNS*  CNNS  h

(4)





CNNS  e  CNNS 

TEA  e  TEA 

(5)





TEA  H  TEA

(6) 

(7)

CNNS  TEA   CNNS* TEA oxidant

(8)

CNNS*  CNNS  h

(9)

Result and Discussion Figure 1a shows the general thermal condensation processes for the formation of full condensed CN starting from dicyandiamide (DCDA). Up to ca. 390 oC, melem, an important intermediate for CN, would be essentially formed by polyaddition and polycondensation.38 Further polymerization of this intermediate into melon network and final CN would occur at higher temperature around 520 oC, with the materials becoming slightly decomposed above 600 oC. The elemental analysis (Table S1) quantitatively showed the C/N ratio of the CNs increased, meanwhile Fourier transform infrared spectra (FT-IR, Figure S1b) illustrated the incompletely polymerized

amine

groups

at

terminals

gradually

disappeared

when

the

polymerization degree was improved. Thus, by modulating the synthesis temperature from 400 to 650 oC, the chemical structures of polymeric carbon nitride can be facilely manipulated, and the as-obtained CN was denoted as CN-T where T represents the condensation temperature.

7



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(a)

(b)

Figure 1. (a) The ideal reaction path for the formation of full condensed CN, (b) the process of exfoliating bulk CN-T (T= 400, 450, 500, 550, 600, 650). Photographs: bulk CN-T and the corresponding CNNS-T solution when irradiated with a UV lamp.

As shown in the XRD patterns (Figure S1a), all CNs polymerized at different temperatures consisted of a layered structure, which made the preparation of various carbon nitride nanosheets (CNNS) with tunable chemical structures feasible by liquid exfoliation (see scheme in Figure 1b and experimental details in SI). The successful exfoliation of various bulk CNs into nanosheets (CNNS-T, T denotes the condensation temperature for the parent bulk CN) was firstly confirmed by transmittance electron microscope (TEM). The nearly transparent feature of the nanosheet (Figure 2a-c) indicated its ultrathin thickness. It was also noted the equivalent size of the CNNS increased firstly and later decreased as the condensation temperature increased from 400 to 650 oC (see size distribution in Figure S2). It could be ascribed to the enlarged 2D network of CNs by connecting more tri-s-triazine units, as the polymerization temperature increased. However, with the further increase of temperature to 650 oC, although the polymerization degree continued to increase, the planar structure of CN became distortion (see more discussion in Figure S1a), and was ready to be broken under the ultrasonic treatment, making the size of as-obtained CNNS decrease instead. 8


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Meanwhile, the XRD pattern (Figure 2d) showed that the intensity of peak at 27.6 o, assignable to the (002) interlayer diffraction of graphitic-like structures, also verifying the successful exfoliation of layered CN.19,20,39,40 (a)

(c)

(b)

(d) CNNS-650 CN-650

Intensity (a.u.)

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


15

30

45

60

2θ (degree)

Figure 2. TEM images of CNNS-400 (a), CNNS-500 (b) and CNNS-650 (c). XRD spectra of bulk CN-650 and CNNS-650 (d).

The chemical structure of the as-obtained CNNS was firstly studied by FT-IR spectroscopy. As shown in Figure 3a, the peaks around 800 cm-1 originated from the tri-s-triazine ring breathing mode were observed in all CNNS. The intensity of peaks at 1545 and 1225 cm-1 attributable to stretching vibration of C-N(-C)-C (full condensation) or C-NH-C (partial condensation), became stronger with the increase of the temperature, suggesting an increased polymerization degree.41 Moreover, characteristic peaks of ν(-NH2) at 3470 and 3325 cm-1 were observed for CNNS-400, but gradually disappeared for CNNS-500 and CNNS-650, indicating abundant -NH2 in CNNS-400.13 The modulated chemical structure of the various CNNS was further confirmed by X-ray photoelectron spectra (XPS). The high-resolution N1s spectra in 9



Figure 3b could be deconvoluted into four Gaussian-Lorenzian peaks with binding energies at 398.6 (N1, sp2-bonded nitrogen in N-containing aromatic rings (C-N=C)), 399.7 (N2, tertiary nitrogen N-(C)3 groups), 400.7 (N3, amino group (C-N-H)) and 404.2 eV (N4, charging effects or positive charge localization in heterocycles).39,42 The peak at 399.7 eV (N2) increased while the peak at 400.7 eV (N3) decreased with the temperature, in agreement with the FT-IR results in Figure 3a. Elemental analysis (EA) clearly revealed that the C/N molar ratio increased with the increase of temperature. Typically, the C/N ratio is 0.60 for CNNS-400, whereas it increases to 0.66 and 0.68 for CNNS-500 and CNNS-650, respectively. Therefore, the polymerization degree, C/N ratio and the amounts of amino groups of the CNNS could be well controlled by exfoliation of bulk CN that were condensed at different temperatures.

(b) CNNS-400

CNNS-400 N3 N4

CNNS-500

4000

CNNS-650

1546

3000

N2

CNNS-500

CNNS-650 -NHx

(c)

N1

2000 -1

Wavenumber (cm )

1225 1000

406

404

402

400

398

CNNS-400 CNNS-650

CNNS-500 S-CNNS-550

ECL intensity (a.u.)

(a)

Intensity (a.u.)

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

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396

Binding Energy (eV)

400

500

600

Wavelength (nm)

Figure 3. FT-IR (a) and XPS N1s (b) spectra of CNNS-400, CNNS-500 and CNNS-650. ECL emission spectra of CNNS-400, CNNS-500 and CNNS-650, and S-CNNS-550 (c).

The as-prepared CNNS could be well dispersed in water (see Tyndall effect in Figure S3) and exhibited high stability of optical properties under continuous UV exposure, high ionic concentration and a wide range of pH value (Figure S4). All these features would greatly facilitate their applications, e.g. as luminophor-probes for sensing and bioimaging. It was observed that the ECL spectra (Figure 3c) matched well with the PL spectra of CNNS (Figure S5) and the ECL emission peaks of the 10


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CNNS also gradually red-shifted from 398 to 450 nm with the increase of the polymerization degree. It indicated that the excited-state CNNS generated from electrochemical reactions was similar to that by photoexcitation, thus the ECL emission could be ascribed to a band-gap luminescence.33 More interestingly, the optical properties of CNNS can be further engineered by heteroatom doping. For instance, when 2-aminothiophene-3-carbonitrile (ATCN) were used as the dopants in copolymerization with bulk CN (see experimental details in SI), the PL and ECL emission peak of the corresponding S-CNNS-550 can further red-shift ca. 80 nm, reaching 531 nm (Figure 3c and S5). In principle, this strategy allowed the insertion of any desired molecule/element to the final framework of CNNS, with the possibility to further tune the optical properties according to the required applications. Reminiscent of those methods previously reported for preparation of 2D nanosheet with tunable structure that requires the harsh reaction conditions such as hydrothermal treatment, hazardous chemicals or complicated synthetic routes,43-45 our presented approach is safe, robust and highly suitable for large-scale preparation.

Table 1. Elemental analysis of as-prepared CNNS-400, CNNS-500 and CNNS-650. Sample

Ca

Na

Ha

C/Nb

CNNS-400

30.10

58.05

3.45

0.60

CNNS-500

31.72

56.30

2.79

0.66

CNNS-650

34.91

59.96

2.12

0.68

a

wt.%; b molar ratio

11



(I0-I)/I0

Quenching Coefficient ((I0-I)/I0)

1.0

0.5

0.5

0.0 0

60 120 180 240 [Ni2+]/ nM


(b) 1.0

(a)1.0

0.5

0.0

Cd2+

blank Na+

Co2+ Fe3+ Fe2+

Ni2+

Cu2+ Zn2+

-0.5

0.0 blank Na+ Cd2+ Fe3+ Fe2+ Co2+ Cu2+ Zn2+ Ni2+

CNNS-500

CNNS-500

(I0-I)/I0


0.9

0.5

0.6 0.3 0.0

0

5 10 [Cd2+]/ M

0.0 blank Na+ Zn2+ Ni2+ Co2+ Fe2+ Fe3+ Cu2+ Cd2+


(d)

(c) 1.0

0.6

0.3 blank

0.0

Cd2+

Ni2+ Co2+

Na+

Fe2+ Fe3+ Cu2+

Zn2+

-0.3

-0.6 CNNS-650

CNNS-650

0.5

(f)

0.9

CNNS-500+Ni

0.6 0.3 0.0 0

2

4 6 8 [Cu2+]/M

10

2+

ECL Intensity (a.u.)

1.0 (I0-I)/I0

(e) Quenching Coefficient ((I0-I)/I0)

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

Page 12 of 21

0.0 blank Na+ Fe2+ Fe3+ Ni2+ Zn2+Co2+ Cd2+ Cu2+

0

CNNS-400

100

200

300

Time (s)

Figure 4. ECL response of CNNS-500 (a), CNNS-650 (c) and CNNS-400 (e) after the addition of various metal-ions in the cathodic potential range. Inset: calibration curve for detecting Cd2+ (a), Ni2+ (c) and Cu2+ (e). I0 and I are the ECL intensity before and after addition of metal-ions, respectively. ECL response of CNNS-500 (b) and CNNS-650 (d) after the addition of various metal-ions in the anodic potential range. The ECL intensity change of CNNS-500 after adding Ni2+ with time (f).

12


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As shown in Figure S6a, CNNS-500 was ECL-active at both cathode and anode potential ranges (see mechanism in eq 1-9 and more discussion in supporting information). Moreover, the ECL intensity was stable by ten continuous cycles of cyclic voltammetry scan at cathode (0 to -1.6 V) and anode (0 to 1.4 V) potential ranges, respectively, implying its good reliability for ECL reactions. In this regrad, the ECL responses of CNNS in the presence of representative metal-ions, including Cd2+, Cu2+, Mg2+, Na+, Ni2+, Fe2+, Zn2+ and Fe3+ were studied and depicted in Figure 4a-e. In cathode potential range, it was found that CNNS-500 exhibited excellent selectivity for Ni2+ (quenching co-efficiency: 98%) over other metal-ions, while CNNS-650 and CNNS-400 exhibit superior selectivity for Cd2+ (quenching co-efficiency: 96%) and Cu2+ (quenching co-efficiency: 85%), respectively. These results demonstrated that the as-prepared CNNS with tunable chemical structures, could be developed as an excellent platform for a new class of sensory devices. For sensitivity study, as shown in Figure 4, the ECL intensity decrement was proportional to Cu2+, Ni2+, and Cd2+ concentration ranging from 0.4 to 6 μM, 0 to 120 nM, and 0.1 to 4 μM, respectively. The detection limit of Cu2+, Ni2+, and Cd2+, estimated at a signal-to-noise of 3 were 250 nM, 1 nM and 20 nM, respectively, which was 2~340 times lower than the maximum level in drinking water permitted by World Health Organization (WHO). The degree of quenching could be quantitatively described by the Stern–Volmer equation: I0/I = 1 + KSV[Q], where I0 and I represent the ECL intensities in the absence and presence of metal ions, respectively; KSV is the quenching rate constant; [Q] is the concentration of metal ions. The KSV of 8.12×106 M-1 for Ni2+, 2.19×105 M-1 for Cd2+, and 1.31×105 for Cu2+ can be calculated, which are comparable with those of other metal ions-sensitive systems.46,47 More interestingly, the anodic ECL intensity of both CNNS-500 and CNNS-650 in the presence of some specific metal-ions was improved instead of the conventional quenching (NB: due to the low signal-to-noise ratio the anodic ECL of CNNS-400 was not investigated). For instance, Ni2+ exhibited remarkable enhancement for ECL of CNNS-500 (Figure 4b), while Cd2+ showed evident improvement for ECL of CNNS-650 (Figure 4d). Indeed, in cathode potential range, there are some 13



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Page 14 of 21

interference in the detection of metal ions. For CNNS-500, Zn2+ and Cu2+ may interfere with detection of Ni2+; for CNNS-650, Cu2+ and Fe3+ may interfere with the detection of Cd2+; and for CNNS-400, Co2+ and Cd2+ may interfere with the detection of Cu2+. However, in the anodic range, for CNNS-500, Ni2+ could enhance the ECL intensity, but Zn2+ and Cu2+ could quench the ECL intensity; and for CNNS-650, Cd2+ could enhance the ECL intensity, but Cu2+ could Fe3+ quenched the ECL intensity. In this sense, according to different case of quenching and enhancement, the interference with other ions could be eliminated and the false-positive result could be largely avoided. Compared with previous sensors based on carbon nitride which could not eliminate interference or need masking reagent to improve selectivity,23,47 the selectivity of our sensors based on dual ECL-signal toward metal ions were largely improved. Not only limited to Ni2+, Cu2+, and Cd2+, thanks to the diversity of chemical structures of CNNS, it was possible in principle to develop a set of multiple distinctive ECL signals for each metal-ions.48 By fitting the multiple ECL intensity changes with the calibration curves of each metal-ion, the concentration of each metal ion in a mixture could be obtained, if each metal-ion changed the ECL intensity of CNNS independently. In addition, the response time of ECL intensity of CNNS to metal-ions was investigated. The time-dependent ECL spectra of CNNS/metal-ions solution (Figure 4f and Figure S6) showed that after the addition of metal ions into the electrolyte, the ECL intensity decreased remarkably, and remained unchanged after 2 min, indicating that the proposed sensor for detection of metal ions was rapid. All these results indicated that the proposed CNNS-based ECL sensors were highly selective, sensitive, rapid and stable, implying promising applications in a fast screening of metal-ions without time control.

14


CNNS-500-ECL CNNS-500 CNNS-500+Ni2+

Absorbance

2

1

0

400

600

(b)

CNNS-500 CNNS-500+Ni2+

Intensity (a.u.)

(a)

0

800

20

2

1

0 200

400

600

60

(d)

800

CNNS-400 CNNS-400+Cu2+

Intensity (a.u.)

CNNS-400-ECL CNNS-400 CNNS-400+Cu2+


(c) 3

40 Time (ns)

Wavelength (nm)

Absorbance

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0

Wavelength (nm)

30

60

Time (ns)

Figure 5. UV-vis absorption spectra in the absence and presence of Ni2+ and ECL emission spectra of CNNS-500 (a). Time-resolved PL spectra of CNNS-500 in the absence and presence of Ni2+ (b). UV-vis absorption spectra in the absence and presence of Cu2+ and ECL emission spectra of CNNS-400 (c). Time-resolved PL spectra of CNNS-400 in the absence and presence of Cu2+ (d).

To reveal the mechanism of ECL intensity changes upon different metal-ions is important to avoid uncertainness in practical sensing application. The possible explanations are listed as follows: (1) the absorption of the emission light by absorbers would reduce the ECL intensity of CNNS, i.e., inner filter effect (IFE) between CNNS and metal-ions occurred;22,40 (2) an efficient electron transfer between CNNS and metal-ions happened.33 In this sense, the influences of optical properties of various CNNS upon metal-ions were investigated by the ultraviolet-visible (UV-vis) absorption spectra and time-resolved decay spectra. As displayed in Figure 5a, CNNS-500 presented a wide band from 300 to 600 nm, with a strong absorption peak 15



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at 310 nm. After the addition of Ni2+, the intensity of absorption band of CNNS-500 between 300 and 600 nm which had a remarkable overlap with ECL emission, increased evidently (Figure 5a), while that of CNNS-400 and CNNS-650 (Figure S8a, b) almost kept unchanged. It could be attributed to the increase of the combination of Ni2+ with CNNS-500,22,49 which resulted in IFE and quenching of the ECL emission. It should be noted that added Ni2+ has negligible absorption bands (Figure S7). In order to further ascertain the main reason for the quenching, the PL lifetime of CNNS in the presence and absence of metal-ions was investigated. As shown in Figure 5b, the lifetime of CNNS-500 in the absence and presence of Ni2+ remained constant (2.3 ns) under an excitation of 370 nm. These results excluded the possibility of the electron transfer process between CNNS and metal-ions. Therefore, the IFE could be considered as one major process in the ECL quenching of CNNS-500 by Ni2+. Similarly, the intensity of absorption band between 400 and 600 nm of CNNS-650 increased evidently after the addition of Cd2+, compared with that without Cd2+ (Figure S8c). Meanwhile, the lifetime of CNNS-650 with and without Cd2+ did not change (1.3 ns, Figure S8d), indicating that IFE could also be considered as the major quenching process. However, the UV spectra of CNNS-400 in the presence and absence of Cu2+ are almost the same (Figure 5c) and the lifetime of CNNS-400 (17 ns) was significantly reduced after addition of the Cu2+ (15 ns, Figure 5d). Therefore, for CNNS-400, the ECL quenching by Cu2+ was attributed to nonradioactive electron-transfer from the conduction band of CNNS to Cu2+. In the anodic range, the enhancement of ECL intensity for CNNS in the presence of Ni2+, Co2+, and Cd2+ was more complicated, compared with that of the cathodic ECL. It was observed that the electrochemical current in the anodic potential range at the CNNS-modified electrode increased after addition of Ni2+, Co2+, and Cd2+, while remained the same or decreased after addition of other metal ions.24 Together considering the fact that the reactions with eq 5 and 6 by these metal ions were likely prohibited in thermodynamics, as catalysts, Ni2+, Co2+, and Cd2+ ions may accelerate the reaction between CN nanosheets•+ and TEA• (eq 8), resulting in enhancement of the ECL intensity.24 16


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Another key question for the future development of CNNS-based sensor is how chemical structure of CNNS influence the selectivity of the proposed ECL sensor. Previous report has reported chemical modifications to the materials structure could be used to alter the sensing response towards a particular analytes.2,48 Similarly, different chemical structure of CNNS may result in specific interactions between CNNS and metal ions. The complex formation of CNNS with metal-ions was confirmed by energy dispersive X-ray (EDX) spectra. As shown in the EDX spectra (Figure S9), the characteristic peaks for each metal ions were selectively observed in CNNS/metal ions. However, the interaction between CNNS and metal-ion alone was not sufficient to account for all ECL observations. Our data showed that the electronic band structure of CNNS which was largely depended on the chemical structure (see UV-vis absorption and VB XPS spectra in Figure S10), was also likely a key factor to the observed ECL response upon adding metal ions. In general, the redox potentials of these metal ions all lay between the conduction band (CB) and valence band (VB) of CNNS, but the efficiency of electron transfer from the CB of CNNS to metal-ions was closely depended on the matching degree of their energy levels.33 Taking the preferred nonradioactive electron-transfer from the CB of CNNS to Cu2+ (Figure 5c, 5d) as an example, it was presumably because CNNS-400 had a favorable CB position relative to the redox potential of Cu2+ in comparison with other CNNS. In these regards, multiple competitive mechanisms were involved simultaneously. Overall, this behavior of the CNNS used here is unique as compared with other known categories of ECL luminophor.

Conclusion In summary, we have descried that CNNS with tunable chemical structures can be obtained by liquid exfoliation of bulk CN with different polymerization degree. Taking advantage of tunable chemical structures, ECL of CNNS was modulated, and thus could be used to reliably distinguish different metal-ions by distinctive changes in ECL quenching due to inner filter effect/electron transfer and enhancement owing to catalytic effect. As a result, the highly selective, sensitive, and rapid responsive 17



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ECL sensors based on CNNS family for detecting multiple metal ions, e.g., Cu2+, Ni2+, and Cd2+, were successfully developed with detection limits as low as 250 nM, 1 nM and 20 nM, respectively. Rational manipulation of the chemical structure of CNNS allows for future development of improved sensor materials that can exhibit unique selective detection. CN family would thus provide an exciting and powerful platform for the development of sensors with excellent performances

ASSOCIATED CONTENT Supporting Information. Figure S1-S10, Scheme S1, Table S1, and more discussions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported in part by the National Natural Science Foundation of China (91333110 and 21305065), Natural Science Foundation of Jiangsu Province (BK20130788), and the Fundamental Research Funds for the Central Universities. We thank Prof. Fugen Wu (SEU, China) for help in dynamic light scattering measurement.

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