Dual-Colored Carbon Dot Ratiometric Fluorescent Test Paper Based

May 18, 2017 - Classical pH test papers are widely used to measure the acid–base degree of media in a qualitative or semiquantitative manner. Howeve...
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Dual-Colored Carbon Dot Ratiometric Fluorescent Test Paper Based on a Specific Spectral Energy Transfer for Semiquantitative Assay of Copper Ions Cui Liu,†,‡ Dianhua Ning,†,‡ Cheng Zhang,†,‡ Zhengjie Liu,†,‡ Ruilong Zhang,§ Jun Zhao,† Tingting Zhao,*,† Bianhua Liu,*,† and Zhongping Zhang†,§ †

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China § School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230601, China ‡

S Supporting Information *

ABSTRACT: Classical pH test papers are widely used to measure the acid−base degree of media in a qualitative or semiquantitative manner. However, the extension of portable and inexpensive methods to a wide range of analytes so as to eliminate the tediousness of instrumental assays remains unsuccessful. Here, we report a novel kind of dual-colored carbon dot (CD) ratiometric fluorescent test paper for the semiquantitative assay of copper ions (Cu2+) by a dosesensitive color evolution. The preparation of the test paper is based on the following two interesting findings: on the one hand, residual p-phenylenediamine at the surface of assynthesized red CDs (r-CDs) efficiently binds Cu2+ ions to produce a strong visible absorption that overlaps the emission of blue CDs (b-CDs); on the other hand, the Cu2+ ions render the adsorption of small b-CDs onto the surface of larger r-CDs through their dual-coordinating interactions with the surface ligands of both r-CDs and b-CDs. These two mechanisms lead to a specific spectral energy transfer to quench the fluorescence of b-CDs with a sensitive detection limit of 8.82 nM Cu2+, whereas the red fluorescence of r-CDs is unaffected as a stable internal standard. Ratiometric fluorescent test papers have been prepared using a mixture of r-CDs and b-CDs (1:7) as ink by jetprinting on a piece of paper. With the addition of Cu2+ ions, the blue test paper produces a consecutive wide-colored evolution from blue to orange-red, with a dose-discerning ability as low as 25 nM. KEYWORDS: carbon dots, ratiometric fluorescence, test paper, energy transfer, copper ions

1. INTRODUCTION

reducing time consumption, the qualitative and quantitative assays on a target species still have to rely on the aid of a fine fluorescence spectrometer. Inspired by the success of classic pH test paper, the problem may be solved by developing fluorescent colorimetric strategies10,11 and visual fluorescent test papers.12−15 To achieve the aim of the observation of qualitative and semiquantitative assays by the naked eye, the fluorescent probes must be highly sensitive to the remarkable variations in spectrum, including color and brightness. Moreover, the fluorescent materials should have high quantum yield and excellent antiphotobleaching ability. QD- and CD-based fluorescent probes can better meet the above requirements because of their high fluorescent brightness, photostability, and tunable emissive color. Recently, our group has developed color-multiplexing fluorescent test papers by the combined use

Instrument-based assays remain predominant in most of analysis-related areas, such as environmental detections, medical diagnosis, food/drug control, and social security screening. Despite their high sensitivity and accuracy, they are unsuitable for on-site assays and also need complicated sample pretreatments, expensive equipment, and well-trained technicians. Accordingly, miniature chemical sensors as a useful supplement to analytical instruments have attracted considerable attention because of their low cost, portability, and easy operation. Among various chemosensory strategies, fluorescent sensors are most extensively explored because of their high sensitivity and simplicity, as well as the wide range of available materials, including organic dyes,1 quantum dots (QDs),2,3 metal clusters,4 graphene oxides,5 rare-earth nanocrystals,6 carbon dots (CDs),7−9 and so on. A fluorescent sensor should be able to specifically bind the molecule of interest and produce measurable spectral responses in the emitting wavelength and/ or intensity. Although the use of fluorescent sensors can exempt the sophisticated sample pretreatment process, thereby © 2017 American Chemical Society

Received: April 26, 2017 Accepted: May 18, 2017 Published: May 18, 2017 18897

DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903

Research Article

ACS Applied Materials & Interfaces

completely and dried in an oven at 70 °C for 3 h. Then, the “ink” of mixing b-CDs and r-CDs (fluorescence intensity ratio = 7:1) was injected into the vacant cartridge. A filter paper was stuck onto a piece of A4 paper. The “ink” was printed on the filter paper through an inkjet printer connected to a computer; the printing process was repeated 20 times. Finally, the filter paper displayed a strong blue fluorescence under a 365 nm UV lamp.

of QDs and CDs, for the visual detection of arsenic ions12 and blood sugar,15 in which one of two fluorophores was used as the internal color standard to enhance the visualization contrast. The obtained dose-sensitive color evolution was similar to that of the pH test paper, but the poor compatibility between QDs and CDs makes the preparation procedure tedious, as well as the environmental toxicity of QDs limits the use of test papers. Herein, we propose a fluorescent colorimetry strategy for the sensitive detection of copper ions with clear color visualization on test paper using ratiometric fluorescent blue CD (b-CD) and red CD (r-CD) probes. Copper, aside from zinc and iron, is the third essential transition metal in biological systems as well as the pollutant species concerned by environmental protection. The traditional way of determining copper ions in biological and environmental samples involves instrumentbased measurements, including inductively coupled plasma mass spectroscopy,16 potentiometric and voltammetric measurements,17 and atomic absorption spectroscopy/emission spectroscopy.18

3. RESULTS AND DISCUSSION The r-CDs were prepared using p-PDA as starting material by the solvothermal method. Their properties were characterized by transmission electron microscopy (TEM), DLS, UV−vis absorption, and fluorescence spectroscopy. As shown in Figure 1A, the as-prepared r-CDs possessed a good monodispersity

2. EXPERIMENTAL SECTION 2.1. Regents and Instruments. p-Phenylenediamine (p-PDA) was purchased from Aldrich. Sodium citrate, sodium hydroxide, sulfuric acid, ethanol, and all metal salts were supplied by Sinopharm Chemical Reagent Company Co. Ltd (Shanghai, China). All chemicals were used as received, without further purification unless otherwise specified. Ultrapure water (18.2 MΩ cm) was produced with a Millipore water purification system. Structures and morphologies of CDs were examined using a JEOL 2010 transmission electron microscope. Fluorescence spectra were recorded with a Cary Eclipse fluorescence spectrophotometer. UV−vis absorption spectra were obtained with a Shimadzu UV-2550 spectrophotometer. IR spectra were recorded on a Thermo Fisher Nicolet iS10 Fourier transform infrared (FTIR) spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo Scientific ESCALAB 250 high-performance electron spectrometer with an Al Kα (1486.6 eV) radiation. Dynamic light scattering (DLS) was performed by Zetasizer Nano ZS. Fluorescent images were taken under an AGL-9406 portable UV lamp (365 nm) by a Canon EOS 350D digital camera. 2.2. Synthesis of r-CDs. r-CDs were prepared according to the reported solvothermal method, with a slight modification.19,20 First, pPDA (0.5 g) was dissolved in 50 mL of ethanol and the solution was subsequently transferred to a poly(tetrafluoroethylene)-lined autoclave. After heating at 200 °C for 12 h and then cooling to room temperature naturally, the mixture was purified through a silica chromatography column using ethyl acetate as eluent. The final product was obtained by drying in a rotary evaporator, and the purified r-CDs were redispersed in 50 mL of ultrapure water for further use. 2.3. Synthesis of b-CDs. b-CDs were synthesized according to the reported method, with a slight modification.21 Briefly, 0.8 g of sodium citrate and 6.0 g of ammonium hydrogen carbonate were dissolved in 40 mL of ultrapure water and then the solution was sealed in a poly(tetrafluoroethylene)-lined autoclave. After heating at 180 °C for 4 h and then cooling to room temperature, the resultant mixture was purified by dialysis using a membrane with a molecular weight cutoff of 1 kDa for 12 h. Finally, the solution was collected and stored at 4 °C for use. 2.4. Detection of Cu2+ Ions. The ratiometric fluorescent probe was prepared by mixing b-CDs and r-CDs with the fluorescence intensity ratio of 7:1 in 2 mL of HEPES buffer (pH = 7.4). Cu2+ ions with different concentrations were added into the ratiometric fluorescent probe solution and reacted for 3 min, and the resulting fluorescence spectra were recorded with a fluorescence spectrometer. 2.5. Preparation of Test Papers. A commercial ink cartridge was washed with ultrapure water until the ink powder was cleared

Figure 1. Characterizations and optical properties of CDs. (A) TEM image of r-CDs (the inset is the size distribution of r-CDs). (B) UV− vis absorbance and fluorescence emission of r-CDs (insets a and b show the corresponding colors under a 365 nm UV lamp and daylight). (C) TEM image of b-CDs (the inset is the size distribution of b-CDs). (D) UV−vis absorbance and fluorescence emission of bCDs (insets a and b show the corresponding colors under a 365 nm UV lamp and daylight).

with a diameter of ∼20 nm. The DLS result showed that the hydrate particle size of r-CDs was ∼45 nm (Figure S1), much larger than the size examined by TEM, which reveals the excellent hydrophilicity of r-CDs. The fluorescence spectrum of r-CDs exhibited an emission peak at 615 nm with an excitation of 360 nm (the red curve in Figure 1B), and a bright orange-red fluorescence was observed under a 365 nm UV lamp (inset a in Figure 1B). Moreover, r-CDs have excellent antiphotobleaching ability compared to that of common organic dyes (Figure S3A), and the ratiometric fluorescence obtained by mixing r-CDs and b-CDs also kept the same photostability (Figure S3B). The UV−vis spectrum showed two characteristic absorption peaks centered at 239 and 281 nm, with a shoulder at 304 nm (the black curve in Figure 1B). The two peaks are assigned to the π−π* transitions of CC and CN bonds in the aromatic rings, respectively, which are identical to those of p-PDA.19,22,23 The weak band at 304 nm is attributed to the transition from π−p conjugation by the unshared electrons of −NH2 and the 18898

DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903

Research Article

ACS Applied Materials & Interfaces benzenoid ring.23 A light orange appearance was observed under daylight (inset b in Figure 1B). Moreover, FTIR spectroscopy, mass spectrometry, elemental analysis, and XPS were carried out to investigate the possible chemical bonds and functional groups in r-CDs. The FTIR spectrum in Figure S4 shows a band at 3406 cm−1, which could be assigned to the N−H stretching vibration, three sharp peaks at 1639, 1515, and 1320 cm−1 ascribed to the CN, CC, and C−N stretching vibrations, respectively, and a peak at 818 cm−1, which corresponded to the out-plane bending of the benzene ring.19,20,22 Additionally, the mass spectrum of r-CDs displayed that no free p-PDA (m/z = 108.14) was detected in the r-CD solution (Figure S5) and the elemental analysis indicates 66.04% C, 17.36% N, and 6.83% H in r-CDs. These results show that the p-PDA material was not completely carbonized in the high-temperature thermal degradation process.19,24 On the other hand, in the XPS spectra (Figure S6), the deconvolutions of the N 1s band into three peaks at 398.6, 399.5, and 400.3 eV are associated with pyridinic N, amino N, and pyrrolic N, respectively.19,20 Together with the UV−vis spectrum, these measurements confirm that the incomplete carbonization resulted in a large amount of residue of p-PDA ligands at the surface of r-CDs. The b-CDs were prepared by the hydrothermal method using sodium citrate and ammonium hydrogen carbonate as the starting materials. The b-CDs (2 nm) were highly monodispersive, as examined by TEM (Figure 1C). Moreover, the ultrasmall b-CDs exhibited a crystalline structure, with the clear lattice space discernable by high-resolution transmission electron microscopy (Figure S7). The fluorescence spectrum of b-CDs exhibited a blue emission centered at 440 nm, with the excitation of 360 nm (Figure 1D), and a bright blue fluorescence was observed under a 365 nm UV lamp (inset a in Figure 1D). The UV−vis spectrum showed two characteristic absorption peaks centered at 234 and 340 nm (black line in Figure 1D), and the aqueous b-CDs are almost transparent under daylight (inset b in Figure 1D). Meanwhile, the FTIR spectrum result showed a band at 3200−3600 cm−1 and peaks at 1597 and 1376 cm−1, resulting from the stretching vibrations of hydroxyl and carboxylate groups,19,21 respectively (Figure S8). Together with the analysis of XPS (Figure S9), these indicate the presence of −COOH at the surface of b-CDs. Furthermore, the DLS of b-CDs shows a hydrate particle of size ∼10 nm (Figure S10), which was much bigger than the particle size examined by TEM, also suggesting the highly hydrophilic −COOH ligands at the surface of b-CDs. Because of the presence of −COOH and p-PDA ligands at the surfaces of b-CDs and r-CDs, respectively, the metallic ions have a strong interaction with b-CDs and r-CDs, by the surfacecomplexing reaction. Interestingly, after the addition of Cu2+ ions to the mixture of b-CDs and r-CDs, TEM clearly revealed that small b-CDs are completely adsorbed onto the surface of larger r-CDs, as shown in Figure 2A. Because Cu2+ ions could simultaneously coordinate with −COOH and p-PDA ligands,8,25,26 the interaction resulted in the conjugation of ultrasmall b-CDs onto the surface of larger r-CDs, as shown in Figure 2B. To better understand the interaction mechanism, Figure 3A illustrates the coordinating action of Cu2+ ions at the surface of r-CDs alone. The large π-conjugation system of the p-PDA ligand will lead to the transition of π electrons to the unoccupied d orbitals of the Cu2+ ion.27−31 The ligand-to-metal charge transfer (LMCT) is demonstrated by the measurements

Figure 2. (A) TEM image of small b-CDs adsorbed onto lager r-CDs after the addition of Cu2+ ions. (B) Schematic illustration of the conjugation of b-CDs and r-CDs through a Cu2+-ion bridge.

of UV−vis spectra in Figure 3B,C. Upon the addition of Cu2+ ions to the r-CD solution, a new absorption band ranging from 400 to 600 nm appeared and became stronger with increasing amount of Cu2+ ions and the aqueous solution changed from light orange to brownish red. Similar evolutions were also observed on adding Cu2+ ions to pure p-PDA solution. However, the addition of Cu2+ ions to other aqueous amines, such as ethylenediamine, phenylenediamine, m-phenylenediamine, o-phenylenediamine, pyridine, and pyrrole, did not generate a new absorption peak (Figure S11). On the other hand, the UV−vis spectrum of b-CDs also kept constant after the addition of Cu2+ ions (Figure S12). These above results confirm the charge transfer from the p-PDA ligand to Cu2+ ions at the surface of r-CDs, as shown in Figure 3A. Interestingly, the new absorption band corresponding to the coordination of Cu2+ with r-CDs has a large spectrum overlapping with the emission of b-CDs, as shown in Figure 3D. Thus, when Cu2+ ions are added to the mixture of b-CDs and r-CDs to form ensembles by “Cu2+ bridge” (Figure 2A),32 fluorescence resonance energy transfer (FRET) will occur from b-CDs to the Cu2+−p-PDA complex at the surface of r-CDs, leading to the quenching of the fluorescence of b-CDs. The mechanism of energy transfer is shown in Figure 3E. The investigation of the fluorescence spectra further confirms the above mechanism. With the addition of Cu2+ ions to b-CDs and to r-CDs, their respective fluorescence emissions remained almost unchanged (Figure 4A,B, respectively), which was also confirmed by the observation of their fluorescence under a UV lamp (insets in Figure 4A,B). However, after the addition of Cu2+ ions to the mixture of b-CDs and r-CDs, the blue fluorescence of b-CDs was drastically quenched but the red fluorescence of r-CDs kept highly stable (Figure 4C), similar to 18899

DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903

Research Article

ACS Applied Materials & Interfaces

Figure 4. Fluorescence emission spectra (λex = 360 nm) of (A) b-CDs and (B) r-CDs with the addition of Cu2+ ions. (C) The evolution of fluorescence spectra of the mixture of b-CDs and r-CDs with the addition of Cu2+ ions (The initial fluorescence intensities of b-CDs and r-CDs were adjusted to a ratio of 7:1 by their concentrations). The inset shows the calibration curve of fluorescence intensity ratio (I440/ I615) of the ratiometric probe vs Cu2+-ion concentration, and the fitted equation is y = −0.027x + 6.82, with R2 = 0.9986. The inset photos show the corresponding color evolutions under a 365 nm UV lamp.

Figure 3. (A) Interaction between r-CDs and Cu2+ ions by LMCT. (B, C) Evolutions of UV−vis spectra of r-CDs after the addition of Cu2+ ions and pure p-PDA (0.1 mg/mL in 2 mL of ethanol) after the addition of Cu2+ ions under nitrogen atmosphere. Insets show the corresponding colors under daylight. (D) b-CD and r-CD fluorescence spectra together with the absorption spectrum of r-CDs with the addition of Cu2+ ions. (E) Schematic illustration of the Cu2+-initiated energy transfer from b-CDs to r-CDs.

Moreover, the fluorescence intensity ratio decreased proportionately with a good linearity ranging from 0 to 225 nM Cu2+ ions by the correlation coefficient (R2) of 0.9986 (inset plot in Figure 4C). The detection limit was defined as 3 times of the standard deviation of background (3σ) and calculated to be 8.82 nM. The dynamic experiment demonstrated that the fluorescence response to Cu2+ ions was completed in ∼3 min (Figure S14). The ratiometric fluorescent probe could also be used efficiently for the detection of Cu2+ ion with spiked concentration in tap water and lake water, with recovery in the range of 98.6−104.3% (Table S1). Meanwhile, the spectral response to Cu2+ ions rendered the color evolution gradually from blue to purple to pink to orangered under a 365 nm UV lamp (inset photo in Figure 4C). Detailed experiments demonstrated that the widest and consecutive color evolution could be obtained at the 7:1 fluorescence intensity ratio of b-CDs to r-CDs (Figure S15). The sensitive spectral response and color evolution suggest a novel strategy for the visual detection of Cu2+ ions using the ratiometric fluorescent probe through mixing b-CDs and rCDs. To examine the selectivity of the ratiometric fluorescence for Cu2+ ions, the fluorescence intensity ratio (I440/I615) was measured by adding various metal ions to the ratiometric fluorescent probe solution under the same conditions. As

the case of pure r-CDs. Significantly, the fluorescence intensity ratio (I440/I615) remarkably reduced from the initial value of 7:1 to 0.6:1 at 225 nM Cu2+-ion concentration, suggesting its ultrasensitivity to Cu2+. The changes in the fluorescence intensity ratio can further confirm that the Cu2+ bridge between b-CDs and r-CDs induces the transfer of spectral energy from b-CDs to the Cu2+−p-PDA complex (Figure S13). After the −COOH ligands at the surface of b-CDs were first coordinated with Hg2+ ions, the resultant Hg2+−b-CDs were mixed with r-CDs and the fluorescence intensity ratio was also adjusted to 7:1. With the subsequent addition of 225 nM Cu2+ ions, however, only a slight decrease in the fluorescence intensity ratio was measured in the mixing system, in which the change was much less than that in Figure 4C. This is to say, the inner filter effect by absorbing the fluorescence emission of b-CDs by the Cu2+−pPDA complex is very slight. The predominant mechanism should be the formation of the Cu2+ bridge between b-CDs and r-CDs, resulting in a significant quenching of the fluorescence of b-CDs by the energy transfer, as shown in Figure 2B. 18900

DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903

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ACS Applied Materials & Interfaces shown in Figure 5, when the I440/I615 ratio is quenched about 85.5% by Cu2+ ions at 0.2 μM, no obvious change in the ratio

Figure 5. Ratiometric fluorescent responses to various metal ions (black bars) and their mixture with Cu2+ ions (red bars). The selectivity tests were done in HEPES buffer (pH = 7.4) with the addition of various metal ions (2 μM) into the mixture of b-CDs and rCDs (fluorescence intensity ratio, 7:1). The anti-interferences were tested using the mixture of an excessive interfering ion (2 μM) with a Cu2+ ion (0.2 μM). The inset shows the corresponding fluorescence observed under a 365 nm UV lamp.

Figure 6. (A) Visualization of Cu2+ ions using the fluorescent test papers prepared by printing dual-colored CD ink onto a piece of filter paper. Visual detection of Cu2+ ions in (B) tap water and (C) lake water. The photos were taken under a 365 nm UV lamp.

clearly discern the color change from sky blue to blue purple even at Cu2+-ion concentration as low as 25 nM. Every dose interval of 25 nM corresponds to a discernable color by the naked eye. That is to say, the fluorescent test papers possess the semiquantitative analytical capability for the detection of aqueous Cu2+ ions. We further examined the applicability of the fluorescent test papers for the detection of Cu2+ ions in real water samples. Tap water and lake water were first filtered to remove undissolved substances and then spiked with Cu2+ ions at concentrations of 75, 150, and 225 nM. The fluorescent test papers showed the obvious color responses to the different Cu2+ concentrations, and the corresponding colors are identical in both cases (Figure 6B,C). Moreover, the tendency of color evolutions for the detection of real samples is very similar to that in Figure 6A. The excellent visual effect and accuracy suggest that our fluorescent test papers can meet the requirements of the visual detection of Cu2+ ions in water samples.

and fluorescence was detected with the additions of 2 μM Na+, K+, Mg2+, Zn2+, Al3+, Co2+, Cr3+, Cd2+, Fe3+, Hg2+, Mn2+, Ca2+, Fe2+, Ni2+, and Pb2+ into the probe solution (black bars in Figure 5). The fluorescence response was not influenced even if 10-fold excesses of the interfering ions coexisted (red bars in Figure 5). Furthermore, the simultaneous addition of 2 μM Na+, K+, Mg2+, Zn2+, Al3+, Co2+, Cr3+, Cd2+, Fe3+, Hg2+, Mn2+, Ca2+, Fe2+, Ni2+, and Pb2+ into the probe solution quenched the I440/I615 ratio by only about 20% (Figure S16). These data suggest that the ratiometric fluorescent probe has excellent selectivity and anti-interference ability. Following the above strategy, we have prepared a fluorescent test paper for the visual semiquantification detection of Cu2+ ions. The b-CDs and r-CDs were mixed at a ratiometric fluorescence intensity ratio of 7:1, and the aqueous mixture was used as fluorescent ink for jetprinting on a piece of filter paper to prepare the fluorescent test paper using a computer. The fluorescence brightness of the test paper could be enhanced by repeated printing. The test papers with the fluorescent ink were pale pink under room light (Figure S17) and displayed a highly uniform blue fluorescence on the whole piece of paper under the irradiation of a UV lamp at 365 nm. Here, we tested the capability of high-quality test papers to detect Cu2+ ions in water by the direct observation of the colors of the test papers under UV lamp. On dropping an aqueous solution of Cu 2+ ions, the test papers gradually and consecutively showed blue to purple to pink to orange-red fluorescence with the increment of Cu2+-ion concentration from 0 to 225 nM (Figure 6A). The temporal color evolution with the addition of 225 nM Cu2+ ions displayed a series of intermediate colors from blue to red in 2 min (Figure S18), further suggesting that the color evolution on the test paper is due to the gradual quenching of the blue fluorescence of b-CDs by the Cu2+ ions. In addition, Figure 6A shows that we could

4. CONCLUSIONS In summary, we have demonstrated a strategy to produce dualcolored CD ratiometric fluorescent test paper for the semiquantitative assay of Cu2+ ions, with high selectivity and sensitivity, by a dose-sensitive color evolution. The ratiometric fluorescent test paper to realize visual assay of Cu2+ ions has been successfully designed in three significant points. (1) The residual p-PDA on the surface of r-CDs efficiently binds Cu2+ ions to produce a strong visible absorption that overlaps the emission of b-CDs. (2) Cu2+ ion as a bridge renders the small b-CDs to be adsorbed onto the surface of larger r-CDs through its dual-coordinating interactions with the surface ligands of both r-CDs and b-CDs. (3) A specific spectral energy transfer from b-CDs to the r-CD−Cu2+ complex occurs so as to quench the blue fluorescence of b-CDs, inducing a consecutive widecolored evolution from blue to orange-red. These discoveries in the surface interactions and the Cu2+-sensitive mechanism have 18901

DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903

Research Article

ACS Applied Materials & Interfaces been exploited to fabricate ratiometric fluorescent test papers. The inkjet-printed test papers produced by mixing the CD ink exhibited a dose-sensitive color response, with a discernable scale as low as 25 nM Cu2+, which can be observed by the naked eye. The results reported here imply the advantages of the unique optical properties of nanomaterials to develop visual test papers for promising applications in environmental and medical assays.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05827. DLS, photostability, FTIR spectra, XPS, mass spectra, UV−vis spectra, fluorescence spectra, recovery tests (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.Z.). *E-mail: [email protected] (B.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Basic Research Program of China (2015CB932002), China-Singapore Joint Project (2015DFG92510), Science and Technology Service Network Initiative of Chinese Academy of China (KFJ-SWSTS-172), the National Natural Science Foundation of China (Nos. 21275075, 21335006, 21475135, 21375131, 21277145, and 21275145), and the Natural Science Foundation of Anhui Province (1608085QB32).

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DOI: 10.1021/acsami.7b05827 ACS Appl. Mater. Interfaces 2017, 9, 18897−18903