Polyamine-Functionalized Carbon Quantum Dots as Fluorescent

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Polyamine-Functionalized Carbon Quantum Dots as Fluorescent Probes for Selective and Sensitive Detection of Copper Ions Yongqiang Dong, Ruixue Wang, Geli Li, Congqiang Chen, Yuwu Chi,* and Guonan Chen Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, and Department of Chemistry, Fuzhou University, Fujian 350108, China S Supporting Information *

ABSTRACT: A novel sensing system has been designed for Cu2+ ion detection based on the quenched fluorescence (FL) signal of branched poly(ethylenimine) (BPEI)-functionalized carbon quantum dots (CQDs). Cu2+ ions can be captured by the amino groups of the BPEI-CQDs to form an absorbent complex at the surface of CQDs, resulting in a strong quenching of the CQDs’ FL via an inner filter effect. Herein, we have demonstrated that this facile methodology can offer a rapid, reliable, and selective detection of Cu2+ with a detection limit as low as 6 nM and a dynamic range from 10 to 1100 nM. Furthermore, the detection results for Cu2+ ions in a river water sample obtained by this sensing system agreed well with that by inductively couple plasma mass spectrometry, suggesting the potential application of this sensing system.

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Most recently, we have prepared a new kind of polyaminefunctionalized CQD by capping with branched poly(ethylenimine) (BPEI).19 The BPEI-functionalized CQDs (i.e., BPEI-CQDs) not only exhibit an excellent fluorescence activity (42.5% FL quantum yield), but also are envisioned to be applicable in chemical sensing due to the polyamine functionalization of the CQDs. It has been reported that BPEI can selectively capture trace-level free copper ions (Cu2+);20 thus, the BPEI-CQDs may serve as a good FL probe for sensitive and selective sensing of Cu2+, which is a well-known heavy metal and plays important roles in biological systems.21−23 Herein, we investigated for the first time the FL interactions between the polyamine-functionalized CQDs and Cu2+ ions in detail and have found the amino groups at the surface of the CQDs can bind the Cu2+ to form cupric amine, leading to a selective and strong quenching of the CQDs’ FL via a so-called inner filter effect (see Figure 1).24−26 On this basis, a rapid, selective, and sensitive sensing method based on the CQD FL probe has been developed for the detection of trace Cu2+ in surface water, such as river water. It is evident from the present work that nanomolar levels of certain metal ions could be detected by using appropriate functionalized CQDs with high FL quantum yield, suggesting promising applications of CQDs in analytical chemistry.

uantum dots (QDs) have received much attention for their unique optical properties and their potential applications in sensing, diagnosis, imaging, and optoelectronic devices.1−4 However, most conventional QDs are based on semiconductors that contain heavy metals, such as Cd, and their applications are thus limited for the well-known toxicity and potential environmental hazard of the heavy metals. Carbon-based QDs (CQDs), mainly including graphite nanoparticles less than 10 nm in size and graphene nanosheets less than 100 nm in width,5,6 are proposed to be promising substitutes of the heavy-metal-containing semiconductor-based QDs. Compared with those heavy-metal-based QDs, CQDs exhibit many advantages, such as low cytoxicity, good stability, easy preparation, and environmental friendliness.5−16 Therefore, more and more attention has been paid to these emerging carbon nanomaterials, including their syntheses, property studies, and applications. In the past several years, most research has been focused on the syntheses and properties of CQDs,5 while not much attention has been paid to the applications of CQDs, except for their applications in bioimaging,11−14 photoreduction of metals,15 and optoelectronic devices.16 Recently, some effort has been made for analytical applications of CQDs based on their fluorescence (FL) properties.17,18 However, the CQDs, as a new type of FL probe, did not show obvious advantages over other organic or other nanomaterial-based FL probes in terms of sensitivity and selectivity, mainly due to the fact that the obtained CQDs did not show satisfied FL activities, leading to low detection sensitivities, or the CQDs were not well functionalized, resulting in bad selectivities. Apparently, the development of specially functionalized CQD FL probes with high quantum yield has become very important for the analytical applications of the fluorescent CQDs. © 2012 American Chemical Society

Received: May 6, 2012 Accepted: June 11, 2012 Published: June 11, 2012 6220

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RESULTS AND DISCUSSION FL Response of the BPEI-CQDs to the Cu2+ and FL Quenching Mechanism. The amino-functionalized CQDs (i.e., BPEI-CQDs) synthesized by the previously reported method19 are monodisperse nanoparticles of near spherical morphology distributed in the range of 4−10 nm in size (see Figure S1 in the Supporting Information) and are well capped by BPEI (see Figure S2 in the Supporting Information). Under the excitation of 365 nm, the BPEI-CQD solution exhibits strong blue emission with an FL quantum yield higher than 40%.19 Additionally, there is a good linear relationship between the FL intensity and the CQDs’ concentration in the range of 0−0.168 mg mL−1, but the FL response deviates from linearity in the higher concentration range (Figure S3 in the Supporting Information). The strong blue emission of the BPEI-CQDs can be quenched obviously when Cu2+ ions are added into the solution (see the inset of Figure 2). The corresponding FL

Figure 1. Schematic diagram for the fluorescence of the BPEI-CQDs quenched by copper ions.

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EXPERIMENTAL SECTION Materials. Citric acid (CA) (Alfa Aesar) and branched poly(ethylenimine) (M = 1800) (Aladdin, Shanghai, China) were used to prepare the BPEI-CQDs. All other reagents were of analytical grade and used as received. Doubly distilled water was used throughout the experiments. Phosphate-buffered saline (PBS) solutions of different pH values were prepared by titrating 0.01 mol L−1 phosphoric acid solution with a concentrated sodium hydroxide solution (1 mol L−1) to the required pH values. Synthesis of BPEI-CQDs. The BPEI-capped CQDs were synthesized by pyrolyzing the mixture of CA and BPEI according a method that has been described elsewhere.19 Briefly, 1.0 g of CA and 0.5 g of BPEI were dissolved in a 25 mL beaker with 10 mL of hot water and then heated moderately using a heating mantle (200 °C). Most water was evaporated in about 20 min, leaving a uniform pale-yellow gel. A 1 mL sample of water was added before the gel was scorched and heating was continued. This procedure was repeated about 10 times (in 3 h) until the color of the gel turned to orange, suggesting the formation of CQDs. Finally, the obtained CQDs were adjusted to 10 mL solution using double-distilled water and purified by silica gel column chromatography with 0.01 mol/L HCl solution as the developing solvent. Analysis of a Real Sample. A water sample was collected from Min River (Fujian, China). The sample was filtered through a 0.22 μm membrane (Millipore) prior to the detection. Aliquots (500 μL) of this river water were spiked with standard Cu2+ solutions (10 μL, final concentration 0−9 μM). The spiked samples were then diluted to 1000 μL with PBS (20 mM, pH 4.0) containing BPEI-capped CQDs (final concentration 16.8 μg mL−1) and then analyzed using the developed sensing technique. Instrumentation. Fourier transform infrared (FT-IR) spectra were obtained on an FT-IR spectrophotometer (Thermo Nicolet 360). High-resolution transmission electron microscopy (HRTEM) measurements were performed on an electronic microscope (Tecnai G2 F20S-TWIN 200KV). UV/ vis absorption spectra were recorded by a UV/vis/NIR spectrophotometer (Lambda 750). Fluorescence spectra of the prepared CQDs were obtained by an FL spectrophotometer (Cary Eclipse Varian). An Agilent 7500 ICPMS system (Agilent Technologies, Santa Clara, CA) was used to detect the Cu2+ concentration of the Min River water sample.

Figure 2. UV absorption spectra (a, b) and FL spectra (excitation spectra, c, d; emission spectra, e, f) of 0.168 mg mL−1 BPEI-CQD solution in the absence (a, c, e) and presence (b, d, f) of 150 μM Cu2+ ions. The inset shows the photos of 0.168 mg mL−1 BPEI-CQD solutions in the absence (left) and presence (right) of 150 μM copper ions illuminated by UV light of 365 nm.

spectra indicate that the addition of Cu2+ ions decreases the FL intensity of the BPEI-CQDs, but has no effect on the FL wavelength (Figure 2). The UV−vis absorption results show that the BPEI-CQDs have two absorption bands centered at 245 and 355 nm. However, the addition of Cu2+ into the BPEICQD solution gives rise to two new UV/vis absorption bands, i.e., one absorption band centered at 275 nm and the other broad band found in the range from 500 to 800 nm. It should be note here that added Cu2+ has nearly no absorption above 240 nm (see Figure S4 in the Supporting Information). The two new absorption bands can also be observed when adding 150 μM Cu2+ into the solution of BPEI (Figure S4). As discussed above, the obtained CQDs are BPEI capped and have abundant amino groups at their surface; thus, the two new absorption bands can be attributed to the cupric amine that formed from the combination of Cu2+ ions with the amine groups at the surface of BPEI-CQDs. Furthermore, the absorption band centered at 275 nm has partial overlap with the excitation spectra of the BPEI-CQDs, and the broad absorption band (500−800 nm) also has some overlap with the emission spectra of the BPEI-CQDs. It has been reported that the absorption of the excitation and/or emission light by absorbers may reduce the FL intensity of the fluorophore; i.e., 6221

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FL can be quenched by the so-called inner filter effect.22−24 Therefore, the quenching of the CQDs’ FL by Cu2+ may result from the inner filter effect related to the specific UV/vis absorption of cupric amine. To further prove that the FL quenching is related to the formation of cupric amine, control experiments were done using two kinds of CQDs without a capping amine, i.e., bare CQDs obtained from activated carbon8 and hydroxyl-functionalized CQDs prepared from the pyrolysis of citric acid.10 It was found that the FL activities of both CQDs without amines were not strongly and specifically quenched by Cu2+ (see Figures S5 and S6 in the Supporting Information), proving that the formation of cupric amine plays an important role in the selective FL response of the BPEI-capped CQDs to the Cu2+. The quenched FL of the CQDs by Cu2+ can be basically recovered by adding a strong metal ion chelator such as ethylenediaminetetraacetate, i.e., EDTA (see Figure S7 in the Supporting Information), indicating that the inner filter effect associated with cupric amine can be eliminated by a competitive complexation reaction and thus the quenching of the CQDs’ FL is reversible. The CQDs’ FL quenching and recovery suggest that it is possible to apply the BPEI-CQDs in sensing Cu2+ ions. Establishment of the FL Sensing Method for Cu2+. As mentioned above, the quenching of CQDs’ FL by Cu2+ ions may be attributed to the formation of cupric amine complexes at the surface of CQDs that can absorb both the excitation and the emission light of BPEI-CQDs; in other words, the addition of Cu2+ changes the CQDs with strong FL (BPEI-CQDs) into CQDs without FL activity (Cu2+-BPEI-CQDs), or the addition of Cu2+ decreases the concentration of the fluorescent BPEICQDs. Apparently, there should be some useful relationship between the Cu2+ concentration and the quenched FL that can be adopted for quantitative analysis of Cu 2+ . Before investigation of the effect of the Cu2+ concentration on the quenched FL intensity, the concentration of BPEI-CQDs should be chosen. From the relationship between the FL intensity and the concentration of the BPEI-CQDs (Figure S3 in the Supporting Information), it can be known that the CQDs’ FL is relatively insensitive to the change of the concentration of CQDs at concentrations higher than 0.168 mg mL−1. This also means that the FL is also insensitive to the addition of Cu2+ (which equals the decrease of the CQDs’ concentration) in this concentration range. Therefore, the concentration of BPEI-CQDs should be chosen in the sensitive and linear response range, i.e., 0−0.168 mg mL−1. Generally, in the presence of quencher of a given concentration, the lower the concentration of fluorophore, the larger the change (F0/F) and thus the higher the sensitivity found.27 However, on the other hand, the signal-to-noise ratio (S/N) would be decreased when using too low a concentration of fluorophore. Finally, 16.8 μg mL−1 BPEI-CQDs was eventually chosen in the detection of Cu2+ for obtaining the widest linear response and the lowest detection limit. The response rate of the FL signal of the BPEI-QDs to Cu2+ was then investigated. As shown in Figure S8 in the Supporting Information, the FL intensity of the BPEI-CQDs is quenched by 75% as soon as 10 μM Cu2+ is added into the BPEI-CQD solution (16.8 μg mL−1) and remains stable in the following 1 h observation. This result indicates that the quenching of the BPEI-CQDs’ FL by Cu2+ is rapid and stable, implying a promising application in a fast sensing of Cu2+ without strict time control.

The pH value of the solution is another key factor that affects the sensing system, because the initial FL intensity (in the absence of Cu2+) and the quenched FL intensity (in the presence of Cu2+) of the BPEI-CQDs are both pH-dependent (see Figure 3). The BPEI-CQDs (blue columns in Figure 3)

Figure 3. FL responses of 16.8 μg mL−1 BPEI-CQDs in the absence (blue) and presence (red) of 10 μM copper ions at different pH values.

have strong FL activities in the range of pH 4−6, but have weak FL activities at other pH values, especially at pH > 10. This indicates that the weakly acidic media (pH 4−6) are suitable for sensing. The addition of Cu2+ leads to the quenching of BPEICQDs’ FL over the wide pH range from 2 to 12 (see pink columns in Figure 3); however, the quenching efficiencies (F0/ F) at these pH values are quite different. In strongly acidic media (pH ≤ 3.0), the addition of Cu2+ has nearly no effect on the BPEI-CQDs’ FL, which may be attributed to the fact that the amino groups at the surfaces of the BPEI-CQDs are well protonated and are thus unable to complex with Cu2+ to form FL-quenching cupric amine moieties. In alkaline solutions (pH > 7.0), the quenching efficiencies are not satisfied either, which may result from the fact that partial hydrolysis of Cu2+ ion in the alkaline media inhibits the complex reaction between Cu2+ and the amines of BPEI-CQDs. In contrast, in the weakly acidic media (pH 4−6), the quenching efficiencies have similar high values, suggesting that these weakly acid media can be chosen for the sensitive detection of Cu2+. The selectivity of this BPEI-CQD FL sensing system was estimated. As shown in Figure 4, besides Cu2+, the effects of 17 other kinds of cations, including Hg2+, Co2+, Ni2+, Fe2+, Fe3+, Ag+, Li+, Mn2+, Na+, K+, Mg2+, Ca2+, Zn2+, Pb2+, Ba2+, NH4+, and Cd 2+ , on the FL response of BPEI-CQDs were investigated. Most cations have no effects on the FL emission, except that Hg2+ may quench slightly the FL emission of the BPEI-CQDs in pH 4.0 aqueous solutions. However, at pH higher than 4.0, the sensing system shows poor selectivity. For example, at pH 5.0, not only does Hg2+ have weak quenching to the FL emission of the BPEI-CQDs, but also Ni2+ and Fe2+ may interfere seriously with the detection of Cu2+ (Figure S9 in the Supporting Information). Apparently, the lower pH (pH < 5) may prevent Ni2+ and Fe2+ from combining with amino groups of the BPEI-CQDs. Finally, pH 4.0 is chosen for the detection of Cu2+ in a real water sample to decrease the possible interferences. It should be noted here that, in the complexation reactions with amines, Ag+ and Cu2+ may have similar reactivities. However, Ag+ does not inhibit the FL response 6222

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log(F0/F ) = 48800C + 0.0108

(R2 = 0.998)

(1)

where F0 and F are the FL intensities of CQDs in the absence and presence of Cu2+ and C represents the concentration of Cu2+. The detection limit of Cu2+ (at a S/N of 3) was calculated to be 6 nM. These results show that the aminefunctionalized CQDs with strong FL activity have very promising application in the detection of Cu2+. Application. The applicability of this sensing system for detecting Cu2+ in a real sample was further evaluated. We applied a standard addition method to detect the concentration of Cu2+ in the sample of Min River (the largest river in Fujian Province, China). The present approach provides a linear response to Cu2+ ions in spiked samples at concentrations over the range from 0 to 9 μM (log(F0/F) = 46500C + 0.0466, R2 = 0.999) (see Figure S11 in the Supporting Information). The concentration of Cu2+ ions in the Min River sample (n = 10) detected using this new approach is 1.924 ± 0.075 μM (or 122.17 ± 4.76 ppb); this value is consistent with the result obtained by the ICPMS method, namely, 1.953 ± 0.059 μM (or 125.7 ± 3.75 ppb). These results confirm the validity of this CQD-based FL sensing method for the detection of Cu2+ in real samples.

Figure 4. Selectivity of the BPEI-CQD-based sensor for copper ions over other ions in pH 4 PBS solution. The concentrations of BPEICQDs and metal ions were 16.8 μg mL−1 and 10 μM, respectively.

of BPEI-CQDs (Figure 4), and the presence of Ag+ does not affect the quenching activity of Cu2+ either (data not shown). This might result from the fact that the formed silver amine complexes have no absorption in the wavelength range of BPEI-CQDs’ FL spectra (Figure S10 in the Supporting Information). This means that the silver amine has no inner filter effect on the FL of the BPEI-CQDs and thus Ag+ has nearly no interference in the detection of Cu2+. The sensitivity, the linear response range, and the detection limit of the CQD-based sensing system are measured under the optimum experimental conditions. As shown in Figure 5, the FL intensity of the BPEI-capped CQDs is sensitive to Cu2+ ions and decreases with increasing concentration of Cu2+. There is a good semilogarithmic correlation between the quenching efficiency (F0/F) and the concentration of Cu2+ in the range from 10 to 1100 nM (see the inset of Figure 5) via the following equation:

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CONCLUSION The polyamine-functionalized CQDs were found to be excellent FL probes for Cu2+ detection. Cu2+ ions can react with the amino groups at the surface of the BPEI-CQDs to form an absorbent complex, resulting in the sensitive quenching of CQDs’ FL via an inner filter effect. After optimization of the experimental conditions, an excellent FL sensing system has been developed for the detection of Cu2+ ions in aqueous solutions. The CQDs’ FL-based sensing system shows many advantages, including rapid detection, high sensitivity, good selectivity, wide linear response range, and low cost, and has been demonstrated to have promising applications in the detection of Cu2+ in environmental water samples.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax/phone: +86-591-22866137. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (Grant 21075018), Program for New Century Excellent Talents in Chinese University (Grant NCET-10-0019), Natural Science Funds for Distinguished Young Scholar of Fujian Province, China (Grant 2009J06003), National Basic Research Program of China (Grant 2010CB732400), and Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT1116).

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Figure 5. FL response of 16.8 μg mL−1 BPEI-CQDs upon addition of various concentrations of copper ions (from top to bottom, 0, 0.01, 0.05, 0.25, 0.5, 0.75, 1, 3, 5, 7, 9, and 11 μM) in a pH 4 PBS solution. Inset: semilogarithmic plot of F0/F of 16.8 μg mL−1 BPEI-CQDs vs the concentration of Cu2+.

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