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An Efficient on-off Ratiometric Fluorescence Probe for Cyanide Ion Based on Perturbation of the Interaction between Gold Nanoclusters and a Copper (II)-Phthalocyanine Complex Zahra Shojaeifard, Bahram Hemmateenejad, and Mojtaba Shamsipur ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01566 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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ACS Applied Materials & Interfaces

An Effi fficient on-off Ratiometric Fluorescence Probe for Cyanide Ion Based on Perturbation of the Interaction between Gold Nanoclusters and a Copper (II)-Phthalocyanine Complex Zahra Shojaeifard a, Bahram Hemmateenejad a*, Mojtaba Shamsipur b a

Department of Chemistry, Shiraz University, Shiraz, Iran Department of Chemistry, Razi University, Kermanshah, Iran *Corresponding author, Email: [email protected] (B. Hemmateenejad) b

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Abstract A new ratiometric fluorescent sensor was developed for the sensitive and selective detection of cyanide ion (CN–) in aqueous media. The ratiometric sensing system is based on CN– modulated recovery of copper(II) phthalocyanine (Cu(PcTs)) fluorescence signal in the expense of diminished fluorescence intensity of gold nanoclusters (AuNCs). Preliminary experiments revealed that the AuNCs and Cu(PcTs) possess a turn off effect on each other, the interaction of which being verified through studying their interactions by principle component analysis (PCA) and multivariate cure resolution-alternating least squares (MCR-ALS) methods. In the presence of CN– anion, the AuNCs and Cu(PcTs) interaction was perturbed, so that the fluorescence of Cu (PcTs), already quenched by AuNCs, was found to efficiently recovered, while the fluorescence intensity of AuNCs was quenched via the formation of a stable [Au(CN)2]− species. The ratiometric variation of AuNCs and Cu(PcTs) fluorescence intensities lead to designing a highly sensitive probe for CN– ion detection. Under the optimal conditions, CN− anion was detected without needing any etching time, over the concentration ranges of 100 nM–220 µM, with a detection limit of 75 nM, which is much lower than the allowable level of CN− in water permitted by the World Health Organization (WHO). Moreover, the detection of CN− was developed based on persuading of the CN− effects on the blue and red florescent colors of Cu(PcTs) and AuNCs, respectively. The designed probe displays a continuous color change from red to blue by addition of CN−, which can be clearly observed by the naked eye in the range of 7 µM - 350 µM, under UV lamp. The prepared AuNCs/Cu(PcTs) probe was successfully utilized for the selective and sensitive determination of CN− anion in two different types of natural waters (Rodbal dam and rainwater) and aslo real in blood serum as biological sample. Keywords: Gold nanoclusters; Cyanide anion; Ratiometric fluorescence sensor; Phthalocyanine, Multivariate curve resoloution.


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1. Introduction Anions play important role in many biological, environmental and catalysis processes.1 Among them, cyanide (CN−) considered as one of the useful anions in different fields such as silver or gold extraction process,2 fiber and resin manufacturing, metallurgy and herbicide production.3 Despite valuable properties, CN− is one of the most considerable anions owning to its lethal properties,4,5 the human toxicity of which being related to its ability to bind cytochrome c oxidase that causes the inhabitation of oxygen transport to mitochondria and resulting in hypoxia.4,6 Thus, because of its toxicity and industrial functions, designing of selective and sensitive methods for the determination of low levels of CN− is of significant importance. Various analytical methods have been utilized for the determination of CN– anion, including chromatographic,7 electrochemical,8 fluorimetric,9 flow injection,10 and spectrophotometric methods.11,12 Among the proposed methods, fluorimetric techniques offer the opportunity for a highly sensitive, simple, cheap and easy detection methodology. Fluorescence based detection methods can be applied to various measurements. The most frequent form is persuading the intensity changes at a single wavelength, which can give sensitive and reliable responses, although a unique signal output can be eclipsed by the background noises of the sample media and environmental effects. In order to overcome these disadvantages, signal ratios can be taken at two wavelengths, which is called as a ratiometric fluorescent method.13 Ratiometric sensing provides higher sensitivities, built-in correction for environmental effects and also eliminating the differences in instrumental efficiency.14–16 There are some reports about chemosensors acting based on the ratiometric responses toward CN– detection.17–20 While, the reaction based chemosensors are one of the ratiometric flurometric types that suffer from some disadvantages such as high temperature, low selectivity and time consuming response toward cyanide ion detection.21 On the other hand, most of other types of ratiometric responses are dependent on the significant change of one of the fluorescence intensities to small change of another ones, which resulted in less sensitivity. Thus, the development of a 3 ACS Paragon Plus Environment


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ratiometric fluorescence sensor possessing easy operation and fast and selective responses with low detection limit in aqueous media is still a challenge. Phthalocyanine macrocyclic compounds as green/blue pigments (dyes) have much innovative applications in various aspects of chemistry and industry.22 Heteroaromatic π-conjugation units of phthalocyanines containing different central metals located in the core of phthalocyanines, provide easy process ability with good stability and excellent optical and electronic properties.22–24 Their properties have led to significant efforts toward improving their utility for preparation of chemical and electrochemical sensors, 24,25 optical data storage23 and photodynamic therapy. 22 Over the past few decades, gold nanoparticles (AuNPs) have attained considerable attentions in wide areas of science and technology. Different functionalized AuNPs have been used as sensors26 for various analytes such as bioligical compounds,27,28 anions,29 cations,30 and etc. Since the CN– anion can dissolve Au metal in the perescence of oxygen by forming soluble gold cyanide complexes, the AuNPs are good candidates for cyanide determination by diverse methods.29,31 In recent years, nanoclusters (NCs) have been developed as luminescent probes for sensing purposes.32 Gold nanoclusters (AuNCs), with sizes smaller than 3 nm, , have drawn wide attention as a new class of fluorescence receptors, have drawn wide attention due to their long lifetime, large Stokes shift, long term stability, and biocompatibility.33 AuNCs capped with various organic and bioorganicapping agents have been successfully used in the determination of small biomolecules,34 proteins,35,36 various heavy metals ions,37 and also inorganic anions.38,39 In this work, we tried to develop a new method to achieve a more sensitive CN─ anion sensor with lower background effects. Towards reaching this aim, we designed a ratiometric fluorometric system by perturbing the mutual quenching effects of CN─ on the

fluorescence

emission

of

gold

nanoclusters

(AuNCs)

and

copper(II)

phthalocyanine 3,4, 4″,4‴-tetrasulfonic acid, tetrasodium salt (Cu(PcTs)). Among several capping ligands used in the preparation of gold nanoclusters, in this work, glutathione (GSH) was selected as reducing and stabilizing agent for the synthesis of Au nanoclusters. GSH is an environment friendly reducing agent, which also acts as a weak reducer and a strong capping agent allowing a higher amount of thiolate−Au+ 4 ACS Paragon Plus Environment

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monalyaer at the surface of AuNCs core. In fact, the positive charge supplies adequate ability for electrostatic interactions between AuNCs and phthalocyanine.

33,40

The

proposed CN─ ratiometric sensor, revealed improved features such as good sensitivity, high selectivity, and rapid response in comparison with most of the previously reported ratiometric sensors.

14,41,42

Furthermore, this sensor can work directly in

aqueous solution, for monitoring the CN─ ion in water samples and human blood serum.

2. Experimental 2.1 Apparatus Fluorescence spectra were recorded on a Perkin-Elmer LS50B fluorescence spectrophotometer with excitation and emission slits of 10 nm. UV−Vis absorptions were studied by a Hewlett-Packard Model 8452A diode-array spectrophotometer. Transmission electron microscopy (TEM) images were taken on a Zeiss-EM10C microscope operating at 80 KV. X-ray photoelectron spectroscopy (XPS) measurement was performed using a VG Escalab MKII spectrometer. The pH values were adjusted using a Metrohm pH meter model 827. For image analysis, images had been captured by a smart phone camera (Huawei Honor 4X) from different concentration of CN− added to probe under a fluorescent UV lamp (360 nm).

2.2 Materials All chemicals and reagents used in this work were of analytical grade and used without further purification. L-Glutathione reduced (≥ 98.0%), Cu (PcTs), phosphoric acid, acetic acid, and NaOH were obtained from Merck. Gold chloride trihydrate (HAuCl4·3H2O) was purchased from Sigma-Aldrich. Potassium cyanide was purchased from Novopharm. Boric acid was from BDH laboratory reagent. Doubledeionized water was used throughout all parts of this study. All chemicals and reagents used in this work were of analytical grade and used without further purification. Solutions were prepared by dissolving the reagents in deionized water. The standard solutions used for calibration were prepared by gradually diluting the stock solutions (0.02 M) in water freshly.



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2.3 Preparation of GSH-AuNCs The GSH-Au NCs were prepared according to the procedure described previously.40 In brief, 0.50 mL aqueous solution of 20 mM aqueous solutions of HAuCl4 and 0.15 mL aqueous solution of 100 mM GSH were mixed with 4.35 mL of double distillated water at 25 °C. The reaction mixture was heated to 70 °C under gentle stirring at 500 rpm for about 24 h. For more purification, a cut-off dialysis bag was preprocessed for dialyzing GSH-AuNCs against deionized water for 24 h. 2.4 Fluorescent detection of CN− anion by using GHS-AuNCs and Cu(PcTs) For the ratiometric studies, 2.0 mL of aqueous solution of universal buffer of pH 10 was placed in the quartz cell (3.0 cm × 1.0 cm × 1.0 cm). GSH-Au NCs (200 µL with desired concentrations) and Cu(PcTs) 1.6× 10−5 M (30 µL of 1× 10-3 M stock soloution) were added to buffer solution, and then titrated by successive additions of stock solutions of CN− ion. The fluorescence spectra were recorded after incubating for 3 min at excitation wavelength of 345 nm, scan speed of 500 nm min−1 and slit width of 10 nm.

2.5. Image analysis The fluorescence images of the solutions prepared in the previous section, followed by capturing with a smart phone camera under a 366 nm UV lamp. The recorded images, saved in JPEG format, were imported into MATLAB and then were digitized by using the "imread" function. Three output matrices of color values for red (R), green (G), and blue (B) parts of the images have been achieved. Thus, for each picture three data matrices composed of color values of R, G, and B were obtained. However, to avoid edge effect of the pictures, only 50 pixels around the center of the images (from top, down, left and right) were selected. To obtain calibration curve, the changes of color values were plotted against the concentration of CN– anion were plotted.

2.6 Data analysis for studying the interaction between GSH-AuNCs and Cu(PCTs) All data analysis and image analysis calculations were performed in the MATLAB environment. Fluorescence spectroscopy was applied to monitoring the interaction 6 ACS Paragon Plus Environment

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between GSH-AuNCs with Cu(PCTs). Two different experiments were carried out to get the expanding data matrix for MCR−ALS in BRB solution of pH 10 at room temperature. Experiment 1: the volume of GSH-AuNCs was fixed at 200 µL and different amounts of Cu(PCTs) (0–8.0 × 10−5 mol L−1, total of 35 solutions) were added to the solution. Experiment 2: the concentration of Cu(PCTs) was kept at 1.6× 10−5 mol L−1, and various amounts of AuNCs (0−600 µL at an interval of 20 µL, total of 31solutions) were added. After each addition of the solutions, the fluorescence spectra were collected over the range of 376−630 nm, at an excitation wavelength 345 nm. Therefore, two data matrices including D1 for Au titrated with Cu(PcTs) (31 × 573) and D2 for Cu(PcTs) titrated with GSH-AuNCs (31×573) were obtained from these measurements, and a row-wise expanded data matrix was constructed.

2.7 Multivariate Curve Resolution−Alternating Least Squares (MCR−ALS) Method

Multivariate curve resolution−alternating least squares (MCR−ALS) have been applied to resolve multiple component responses in unknown mixtures.43 The main goal of MCR-ALS as a self-modeling method is to decompose mixture-measured profiles such as concentration or spectra profiles into different pure profiles for each species of mixture and also to investigate the interaction and molecular complex formation process. MCR-ALS analysis facilitates the bilinear decomposition of the data matrix D (r × c) into a matrix of pure concentration profiles, C (r × n), and a transpose matrix of pure spectral profiles, ST (n × c). D= CST + E

(1)

T

Product of CS reconstructs the original data matrix D with the optimal fit, i.e., gives the minimum residual error (E). Performing the MCR-ALS procedure is based on the following steps: (i) The data of each experiment are collected in a D (r × c) matrix, the rows contain the recorded spectra, and the columns are the wavelengths. (ii) The number of components or chemical species is estimated with the use of singular value decomposition (SVD), principal component analysis (PCA), or some related technique based on factor analysis, such as evolving factor analysis (EFA). 44 7 ACS Paragon Plus Environment


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(iii) Initial estimates of the concentration profiles and spectra profile are generally generated by applying EFA to data matrix D. (iv) In each iterative cycle of the optimization, the C and S matrices are calculated in turn by a constrained least squares method, and the iterative process is repeated until convergence.

3. Result and discussion 3.1. Characterization of GSH-AuNCs and Cu(PcTs) The preparation of GSH-protected AuNCs were performed as reported earlier.40 The TEM and XPS were employed for the characterizations of the prepared AuNCs. As shown in Figure 1A, GSH-AuNCs have an ultrasmall size with an average diameter of 1.85 nm, close to the previous reports.40,45 The oxidation state of Au in the luminescent GSH-AuNCs was investigated by the XPS spectra (Figure 1B). The Au 4f spectrum could be deconvoluted into two distinct spectra with the binding energies of 84.3 and 83.7 eV, which are assigned to Au(I), and Au(0) respectively. The fraction of Au(I) reached to about 75% in the luminescent GSH-AuNCs. The XPS result verified the synthesis of GSH-AuNCs core, in which each NC is stabilized with a monolayer of thiolated−Au+ complexes. The high Au(I) content of GSH functionalized AuNCs can provide the ability of more electrostatic interaction in comparison with other common type of NCs such as bovine serum albumin (BSA) capped NCs with only about 17% of Au(I) content in its structure.46 The investigations of optical properties are shown in Figures 1C and 1D. AuNCs appeared as yellowish in solution and is transparent under daylight, while emitted a red emission under the UV light (inset of Figure 1C). The UV−Vis spectra of GSH-AuNCs show a shoulder peak at 400 nm. The excitation wavelength of AuNCs varied between 300 to 400 nm so that at an excitation wavelength of 345 nm showed an emission peak at 580 nm. The Cu(PcTs) complex (Figure S1) is a negatively charged ion and hence it is water-soluble. The color of Cu(PcTs) solution is blue under visible and UV light (inset of Figure 1D). The electronic absorption spectral features of Cu(PcTs), shown in Figure 1D, consists of B and Q-bands located in the UV-Vis region at about 340 nm 8 ACS Paragon Plus Environment

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and 610 nm, respectively.47,48 As shown in inset of Figure 1D, Cu(PcTs) possesses an emission peak at 420 nm when excited at 345 nm. In addition, the quantum yields (QY) of both luminescent GSH-AuNCs and Cu(PcTs) complex were determined by comparision with the known QY of Quinine Sulfate (0.58) at λex=347 nm. The quantum yields were determined to be 0.13 % and 0.33% for GSH-AuNCs and Cu(PcTs), respectively.

3.2. Interaction between GSH-AuNCs and Cu(PcTs) In preliminary experiments, the mutual effects of Cu(PcTs) and GSH-AuNCs on their fluorescence emission spectra was investigated. In this regard, two different experiments were performed in BRB solution of pH 10, at room temperature, for studying the effects of GSH-AuNCs and Cu (PcTs) on their fluorescences. In the first experiment, a constant amount of GSH-AuNCs (200 µL) was used to investigate the variation in GSH-AuNCs fluorescent in the presence of Cu (PcTs). As shown in Figure 2A, GSH-AuNCs show a fluorescence emission in 580 nm (at λem= 345) with no fluorescence peak at 420 nm. By addition of different concentration of Cu(PcTs) (0– 8.0 × 10−5 mol L−1, total of 35 solutions) to the GSH-AuNCs, an enhancement in fluorescence intensity of Cu(PcTs) and a decrease in the GSH-AuNCs intensity along with a blue shift to lower wavelengths was observed. In the second experiment, a constant concentration of Cu(PcTs) (1.6× 10−8 M) was titrated with different amounts of GSH-AuNCs. As is obvious from Figure 2B, by addition of the different volumes of GSH-AuNCs to Cu(PcTs), a simultaneous enhancement in GSH-AuNCs emission at 560 nm and quenching of Cu(PcTs) emission at 420 nm were occurred. These two reversed experiments clearly demonstrated the turn off fluorescence effects of GSH-AuNCs and Cu(PcTs) on each other. This quenching effect can be related to electrostatic interactions between the positivly charged GSH-AuNCs and the negativly charged Cu(PcTs). Furthermore, as reported before,49 metals can interact with aromatic part of phthalocyanine by π−electrostatic interactions which lead to the affinity of Au atoms to aromatic part of Cu(PcTs).



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In order to investigate the turn off effects in more details, the recorded fluorescence data were subjected to chemometrics analyses. The fluorescent emission data of each experiment were collected in two different data matrices D1 and D2 belonging to titration of GSH-AuNCs with Cu(PcTs) and Cu(PcTs) with AuNCs, respectively. In the first step, for finding the number of chemical species coexisted in the reaction system, the data were subjected to principal component analysis (PCA) using singular value decomposition (SVD) and evolving factor analysis (EFA) (Figure S2).50 The eigen-value (EV), reduced eigenvalue (REV) and root mean square (RMS) as mathematical criteria were used to confirm the number of factors (see Table S1).51 In order to obtain more reliable results, PCA was run on the extended data matrix obtained by stacking the data matrices of both experiments in column-wise augmentation. In summary, principal component analysis (PCA)44 on mixture of GSHAuNCs and Cu(PcTs) individually and also in augmentations forms revealed the presence of three significant chemical species (see ESI section, FigureS2 and Table S1 for details). The three species observed can be attributed to the pure GSH-AuNCs, pure Cu(PcTs), and a new species, which resulted from interaction of these two species (namely, GSH-AuNCs-Cu(PcTs) complex). In the second step, two D1 and D2 matrices were augmented coloumn wise for estimating the trends of concentration and spectra of three compounents. By applying MCR-ALS analysis, the estimated pure spectra and concentration profile of columnwise augmented matrix of the two titration experiments were resolved, as shown in Figure S3. As expected, in both cases, i.e., GSH-AuNCs titrated by Cu(PcTs) and vice versa, the concentration of one of species decreased (fluorescence quenching) while that of another one increased (fluorescence enhancement). Moreover, in both experiments, the evolution of the GSH-AuNCs-Cu(PcTs) complex as a third component, was clearly observed. 3.3. Effect of CN− anion on the interaction of GSH-AuNCs and Cu(PcTs) It has been previously reported that, in the presence of CN−, the fluorescence of AuNCs capped with lysozyme and BSA is quenched in the presence of CN− anion. 10 ACS Paragon Plus Environment

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39,52

Thus, in this work, we firstly examined the effect of cyanide ion on the

fluorescence signal of each reagent, separately. As it is shown in Figure 3A, similar to previous observations, the addition of cyanide ion represented a quenching effect on the fluorescence of the GSH-capped AuNCs, while its addition to Cu(PcTs) solution resulted in no measurable quenching on the copper complex fluorescence. As it was explained in the previous section, addition of Cu(PcTs) to the solution of GSH-AuNCs causes a decreasing in the fluorescence intensity of the nanoclusters around 580 nm, and the addition of GSH-AuNCs can also cause fluorescence quenching of Cu(PcTs) signal at around 420 nm. However, as shown in Figure 3B, a mixture of GSH-AuNCs and Cu(PcTs) presents a twin-peak emission spectrum with wavelengths of maximum intensity at around 420 nm and 580 nm. While, the addition of CN− ion to this mixture, shows a competition with Cu(PcTs) ion for interaction with positively charged GSH-AuNCs. As shown in Figure 3B, by addition of CN− ion, a decrease in the fluorescence intensity of GSH-AuNCs is accompanied with an increase in the emission peak of Cu(PcTs). This spectral changes can be attributed to the higher affinity of the Au component of the GSH-AuNCs towards the CN− ion to form a stable [Au(CN)2]– complex by oxidation of the GSH-AuNCs in the presence of O2 (Equation. 2). Attachment of CN− ion that quench the fluorescence of GSH-AuNCs at 580 nm, removes the Cu(PcTs) complex from the surface of AuNCs and turns on the quenched fluorescence of Cu(PcTS) at around 420 nm. These processes are described schematically in Scheme 1. 4Au + 8CN− + 2H2O + O2 → 4[Au (CN)2]− + 4OH−

(2)

As shown in the inset of Figure 3B, such fluorescence changes in the presence of cyanide ion can be distinguished by the naked eye. The shining of red and blue emission of GSH-AuNCs and Cu(PcTs), respectively, is evident in the inset of Figure 3B. By mixing of these two components the red emission is faded, due to the quenching effect of Cu(PcTs) on the fluorescence of GSH-AuNCs the red emission is faded (Figure 3B, b). Following the CN− addition, the red emission of gold nanoclusters is decayed in the expense of the increased blue emission of the second 11 ACS Paragon Plus Environment


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component, so that it reaches the blue color of pure Cu(PcTs) (inset of Figure 3B (cg)). The suggested mechanism for effect of cyanide ion on interaction of GSHAuNCs and Cu(PcTs) can also be described by the UV/Vis absorbance spectroscopy. As shown in supplementary section Figure S4, the spectra of mixture of reagents (GSH-AuNCs and Cu(PcTs)) are close to the summation of the absorbance spectrum of their single ones. By addition of cyanide ion to this mixture, a significant change is observed in the absorbance peak of the gold nanoclusters is observed, while no obvious changes is observed in the absorbance peak of the Cu complex. The observed spectral changes can also be monitored by visual inspection. As shown in the inset of Figure S3, by addition of the blue Cu(PcTs) solution to the yellowish solution of GSHAuNCs soloution, a green color solution is formed. Meanwhile, by addition of colorless cyanide ion to this solution, the blue color changes to dark blue (the color of Cu complex in the absence of nanoclusters). The observed changes in absorbance spectra and color of solutions confirm that cyanide ion interacts with GSH-AuNCs but not with the Cu(PcTs) complex. It should be noted that, according to previous reports,53–57CN− interacts with Cu2+ ion to form [(Cu(CN)x)n−] complex. However, the obtained data in this essay suggest higher stability of Cu(PcTs) compared to [(Cu(CN)x)n−]. In overall, the enhancement in fluorescence emission intensity at 420 nm in the presence of increasing amount of cyanide ion, which is accompanied with the quenching of emission of GSH-AuNCs at 580 nm (Figure 3B), provides the basis for ratiometric fluorescence detection of CN− ion.

3.4. Ratiometric determination of cyanide ion According to the explanations given in the previous section, the ratio of fluorescent intensity at 420 nm to 560 nm (F420/F560) was selected as the analytical signal for measurement of cyanide ion. To gain higher sensitivity, reaction conditions including amounts of the reagents, pH and reaction time were optimized. To optimize the amounts of reagents, 12 mixtures of the two reagents (i.e., GSH-AuNCs and Cu (PcTs)) in different concentrations and ratios were prepared. Then, cyanide ion 12 ACS Paragon Plus Environment

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solutions of variable concentrations were added to each solution and calibration curves were obtained for each mixture of reagents. Then, the binary mixture for which the calibration curve of highest slope (sensitivity) and wider linear range obtained was selected as the optimum reagent mixture. It was found that the mixture involves 200 µL of GSH-AuNCs and 1.6 × 10-5 M of Cu(PcTs) resulted in the best performances. Cyanide ion as a weak acid can be protonated by capturing the available protons in solutions of low pH and forms hydrocyanic acid (HCN). Among these two acidic and basic forms of cyanide, only CN− can etch the gold atoms. On the other hand, the spectral response of Cu(PcTs) is independent on pH whereas GSH-AuNCs do not show good stability at high pH values (i.e., pH >11). Therefore, we studied the influence of solution pH on the fluorescence quenching effect of cyanide ion in the limited pH range of 7.0–10.0. To achieve this pH range, four different buffering systems including Britton–Robinson buffer (BRB), phosphate, Tris-HCl, and borate buffers (all buffers are 0.1 M) were used. As shown in Figure 4A, the change of fluorescence intensity reached its maximum in BRB of pH 10. Figure 4B shows the optimized buffer concentration for BRB buffer as 0.1 M. The rate of reaction between CN− anion and gold atoms is an important factor that has significant effect on the fluorescence intensity.44,49 Thus, the optimization of reaction time using fluorescence intensity ratio F420/F560 was investigated in the BRB buffer of pH 10 at 20×10−6 M concentration of CN− anion. As it is obvious from Figure 5, after addition of cyanide ion the signal increases immediately and then remains constant. In order to have a precise determination, the fluorescence changes were examined 3 min after addition of the analyte. This reaction time is lower than that reported in previous works.52,58 The calibration curve was then obtained under the optimal conditions. Figure 6A shows the changes in the fluorescence spectrum of the mixture of reagents in the presence of increasing amounts of the cyanide ion. Obviously, as the concentration of CN− is rising, the fluorescence at 560 nm is decreased while that at 420 nm is increased. As shown in Figure 6B, the logarithm of the ratiometric fluorescence intensity exhibited an excellent linear relationship with CN− concentration, over a wide range of from 100 nM to 220 µM. By using the proposed ratiometric senbsor, the 13 ACS Paragon Plus Environment


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experimental detection limit of CN− anion was evaluated as 75.0 nM, which is ~28 times lower than the maximum level (2 µM) of cyanide in drinking water permitted by the World Health Organization (WHO). In comparison to the previously reported ratiometric sensors of CN− based on GSH-functionalized AuNCs alone, , reported for CN−, 39,52 the mixed reagent sensor proposed in this essay has the advantages of wider linear range, lower detection limit and lesser response time. It should be noted that an even wider linear range can be achieved by supplying the fluorescence response GSHAuNCs to lower concentrations of cyanide ion and that of Cu(PcTs) to higher concentrations of the analyte (see the inset of Figure 6A). In Table 1, the analytical characteristics of the proposed sensor are compared with those the previously reported single and ratiometric fluorescence sensors for the determination of CN−. Obviously, the mixed reagent sensor of GSH-AuNCs/Cu(PcTs) has lower detection limit and wider linear range for the determination of CN− in aqueous solution, in comparison with most of the previously reported single flurometric sensors and ratiometric chemical sensors. As shown in Table 1, just one of the reported sensors have lower detection limit, in which the enhancing fluorescence sensor suffered from its nonaqueous reaction environment (DMSO). 59

3.4.1 Colorimetric determination Of various chemosensory protocols, the color change observed by the naked eye was considered as a feasible and simple way to indicate the presence of an analyte. Owing to the changes in the intensities in the red and blue emission wavelengths, the fluorescence color of the AuNCs/Cu(PcTs) solution was changed by adding CN− anion. As demonstrated in Figure 7A, increscent of increasing in the concentration of CN− changes the color from red to blue when it is irradiated by a UV lamp (366 nm). By this distinguishable color changes, the visual detection of CN− anion by the naked eye under UV lamp can be accessible. In this regard, the images recorded by a smart mobile phone were analyzed (see experimental section) and then the color value was plotted against the concentration of CN− to obtain an image analysis-based calibration curve. As is interestingly observed from Figure 7, there is a linear relationship between


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CN− concentration and RGB color of the images over the concentration range of 7.0 × 10-6 M to 3.5 ×10-4 M.

3.5. Selectivity and specificity of GSH-AuNCs/Cu(PcTs) sensor The selectivity of the fluorescence-based GSH-AuNCs/Cu(PcTs) sensor was explored by studying the effects of foreign anionic species on the ratiometric analytical signal of cyanide ion. As shown in Figure 8a, among the studied anions including CN−; N3−, SCN−,Br−,CO32−, Cl−, NO3−, SO42−, CN− ,Ac−, PO43−, NO2−, IO3−, citrate, C2O4−, F−, and I−, the anion CN− is the only species that could induce a drastic change in the interaction of GSH-AuNCs with Cu(PCTs). Also, Figure 8B shows the tolerance concentrations of studied anions in detecting CN−. At concentration levels of at least 500 times of the CN− concentration, these anions do not exert any significant effect on the ratiometric signal of cyanide ion. Moreover, the response of GSHAuNCs/Cu(PcTs) sensor to the presence of various environmentally relevantmetal ions was investigated. The observed excellent selectivity of the designed sensor against two common CN− interference anions (i.e., F– and Ac–) can be related to the well affinity of cyanide toward Au atoms (Elsner reaction),60 and also the selectivity of Cu(PcTs) toward other anions. Experimental results showed that only S2− had some interfering effect on the florescence intensity of the sensing system. To improve the selectivity of the proposed probe for CN−, we used Pb2+ as a masking agent61 to take advantage of the stronger formation constant of PbS (log Kf∼ 28), whereas the log Kf values of Pb(CN)42− was 2.2 × 108.

62,63

As indicated in Figure 8 in BRB soloution (PH 10)

containing the masking agents (100 µM Pb(NO3)2), proposed sensor demonstrated good selectivity for CN− ions with respect to the S2− ion at the same concentration. Furthermore, the response of the sensing system to some environmentally relevant metal cations was also investigated. As demonstrated in Figure S5, our sensing system showed good selectivity for CN− over the studied cations up to 50 times of CN− concentration. Higher concentrations of the cations increas the probability of precipitation in basic solutions. Among cations, it was noted that the presence of Ag+ and Hg2+ had an effect on the fluorescence intensity of GSH-AuNCs. However, in the presence of masking agent, 2,6-pyridinedicarboxylic acid (PDCA), 15 ACS Paragon Plus Environment


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BSA and GSH,52 the interference of such cations could significantly decrease (Figure S5B). As a matter of fact, the concentrations of these metal ions are much lower of determined values in real samples. In addition, the possible interfering effects of some biologically important species such as urea, histidine, cystine and glocouse were studied. As shown in Figure S6 A and B, thee compounds did not exhibit effect on the designed sensing system up to concentrations of 150-fold higher than cyanide ion. However, cysteine can effect on the flurescnce intensity of GSH-AuNCs/Cu(PcTs) sensor at the concentrations larger than 8. µM. Since the concentration of cysteine in human blood is in micro molar levels, the interference of cysteine can be decrease by de-proteination and dillution of blood serum.

3.6. Application of the designed sensor To assess applicability of the designed probe in real samples, the ratiometric GSHAuNCs/Cu(PcTs) sensor was used to detect CN− in two different matrices. Two types of water (i.e., rain water and dam water) were selected as the first real matrix. Rainwater samples were collected manually on a time in pre-cleaned glass bottles. Samples were collected for different rainfall hours. The BRB solution of pH 10 was prepared in the collected rainwater. Samples of dam water were collected from Rodbal dam in Darab (Iran). The pH of dam water was adjusted to pH 10 by the addition of BRB components and NaOH. In the case of both water samples, the provided BRB solutions were filtered by a Whatman filter paper (40 m) before analysis. In addition, a blood serum was also used after de-proteination by the previously reported method.64 For analysis of the serum samples, each real sample was diluted 4 times with BRB (pH 10). By de-proteination and dillution of blood serum the concentration of cysteine become neglible. Standard addition method was then applied for CN− determination in water and serum samples. As shown in Tables 2, the results demonstrate satisfactory recoveries for determination of CN− in the studied 3 real samples. These results showed that this new ratiometric sensing system has great potential for quantitative analysis of cyanide levels in various matrixes such as environmental and biological samples. 16 ACS Paragon Plus Environment

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4. Conclusion In summary, this work has demonstrated a new concept and utility for selective and sensitive detection of CN− anion in aqueous and biological environments by a ratiometric fluorescence sensor. The concept takes advantage of turn off effect of GSH-AuNCs and Cu(PCTs) on each other. The presence of CN− in solutions can be detected by the fluorescence changes of the ratiometric probe. In this way, the Cu(PcTs) intensity enhanced while GSH-AuNCs fluorescence quenched by the CN− anion. The probe can also be used to quantify CN− in solutions based on fluorescence color changes of the ratiometric probe from red to blue. This method exhibits significant sensitivity and selectivity compared to other common anions, including F−, Ac− and etc. Moreover, the ratiometric fluorescence probe was applied to real water samples and serum blood successfully, which validate the efficiency of the proposed probe for the determination of CN− in complex environments Supporting Information

The results of EFA, MCR-ALS, absorbance spectra of AuNCs, Cu(PcTs) and their mixtures in the presence of CN−, selectivity and specificity of GSH-AuNCs/ Cu (PcTs) probe over cations and biocoumpounds are provided in a separate supporting information document.



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Figure 1 (A) TEM image of the luminescent GSH-AuNCs. (B) Au XPS spectra of synthesized luminescent AuNCs. (C) UV−vis absorption (black line (a)) and fluorescence emission (red line (b)) spectra of AuNCs. (D) UV−vis absorption of Cu (PcTs). (Inset (D) is fluorescence emission of Cu (PcTs)) (Digital photos of AuNCs and Cu (PcTs) under visible and UV light are shown in Insets (C) and (D), respectively.)



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Figure 2 (A) Changes in fluorescence emission spectra of 200 µL GSH-AuNCs in the presence different amounts of Cu (PCTs) (0– 8.0 × 10

−8

molL−1). Inset (A) shows the

enhancement and quenching of Cu (PcTs) and GSH-AuNCs fluorescence, respectively (B) Variation in fluorescence emission spectra of 1.6× 10

−8

M Cu (PcTs) with various amounts

of GSH-AuNCs (0−600 µL at an interval of 20 µL) at pH 10 (BRB). Inset (B) shows the descending and ascending variations of Cu (PcTs) and GSH-AuNCs fluorescence, respectively. λex is 345 nm in both measurements.


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Figure 3 (A) Fluorescence emission spectra of (a) Au NCs, (b) Au NCs+ CN−, (c) Cu (PcTs) (d) Cu (PcTs) + CN−. The volume and concentrations of Au NCs, Cu (PcTs), and CN− were 200 µL, 1.6× 10

−8

M, and 10 µM, respectively. (B) Fluorescence spectra of

mixture of 200 µL Au NCs and mixture of GSH-AuNCs and 1.6× 10

−8

M Cu(PcTs) (a

and b). Fluorescence spectra of Au NCs/Cu(PcTs) probe toward different concentrations of CN−. The concentrations of CN−

(c-g) were 0.01, 0.05, 0.1, 0.22, 0.33 mM,

respectively. Fluorescence emission spectra of 1.6× 10

−8

M Cu(PcTs) (h). The

fluorescence spectra were recorded upon a 345 nm excitation in BRB solution pH 10.



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Figure 4 (A) The fluorescence response of the GSH-AuNCs/ Cu (PcTs) probe to 10×10−6 M CN− at different pH values in diverse buffer solutions. (A– B Phosphate buffer, pH 7– 8. C– E, Tris–HCl buffer, pH 7– 9. F– H, Borate buffer, pH 8– 10. I–

L, BRB, pH 7– 10.) (B) The relationship between ratiometric intensity and the concentration of universal buffer at pH 10. R420/ R560 is ((I420)/ (I420)0) / ((I560)/ (I560)0).


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Figure 5 Time independent fluorescence response of the GSH-AuNCs/ Cu (PcTs) probe to 20 ×10−6 M CN−. The inset shows R420/ R560 plotted against time in the presence of 20 ×10−6 M CN− in 0.1M BRB solution. (R420/ R560 ) can be explained as (I420/I560) /(I420/ I560)0.



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Figure 6 (A) The fluorescence spectra (λex = 365 nm) of the ratiometric GSHAuNCs/ Cu (PcTs) probe upon the exposure to different concentrations of CN−. Inset shows the fluorescence response of GSH-AuNCs and Cu (PcTs) upon addition of CN− anion individually. (I420)/ (I420)0 and (I560)/ (I560)0 are the relative intensity of GSH-AuNCs and Cu (PcTs) respectively. (B) Plot of the ratiometric response of GSH-AuNCs/ Cu (PcTs) probe as a function of the CN− concentrations in BRB pH 10. (R420/ R560 ) is (I420/I560) /(I420/ I560)0.


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Figure 7 Plot of changing in the color of the ratiometric probe as a function of CN− concentrations. RGB/RGB0 variations show the changes in the ratiometric probe colors in the absence and presence of different concentrations of CN− anion, respectively. (Inset upset: Changes in fluorescence colors of the ratiometric probe upon the exposure to different concentrations of CN− in BRB (pH10).) The fluorescence photos were taken under a UV lamp (excitation wavelength was 366 nm).



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Figure 8 (A) Selectivity of the ratiometric GSH-AuNCs/ Cu (PcTs) probe to various anions in the BRB (pH = 10, 0.1 M). Concentration of each of the anions was 5×10-5 M. Selectivity of the sensor for S2− is in the peresence of masking agent (Pb(NO3)2 (100 µM)) (B) ((R420/ R560 ) plotted against 1×10-5 M CN─ and 500 ×10-5 M of other anions. Relative phosphorescence intensity changes for 1×10-5 M CN─ and 5 ×10-5 M S2− is in the peresence of masking agent (Pb(NO3)2 (100 µM)).


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Scheme 1 Scheme of a ratiometric chemosensor for cyanide anion using AuNCs/ Cu(PcTs) “On-Off “” fluorescence response.



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Table 1 Comparison of analytical data of fluorescence method for CN− sensors Reagents

Linear range (M)

Detection limit (nM)

Solvent

Mode of assay

Phosphorescent molecular gold(I) cluster in a macroporous polymer film

─

2000

Solid state

Quenching

58

Fluorescein isothiocyanate capped BSA Au nanoparticles

0 – 1.0 × 10−5

1000

Water

Enhancement

61

Organic turn-on fluorescence probe

5.0 × 10−7 – 8.0 × 10−6

45

DMSO

Enhancement

59

BSA-stabilized AuNCs

2 × 10–7 M – 9.5 × 10–6

200

Water

Quenching

52

Lysozyme-stabilized AuNCs

5.0 × 10−6 – 1.2 × 10−4

190

Water

Quenching

39

3.2 × 10−6 – 3.4 × 10−5 and 3.81 × 10−5 – 1.04 × 10−4

180

Water

Quenching

65

1.0 × 10−5– 4.5 × 10−5

─

Tris.HCl/ ethanol (8 : 2)

Enhancement

66

Salicylideneaniline-based fluorescent sensor

0 – 2 × 10−4

240

THF/H2O (8:2, v/v)

Ratiometric

14

Hybrid coumarin–hemicyanine dye

0 – 4 × 10−6

600

MeOH/ Tris.HCl (1 : 1, v/v)

Ratiometric

41

Hybrid coumarin-hemicyanine

9.8 × 10−6 – 20 × 10−4

9800

MeOH/ H2O (1 : 1, v/v)

Ratiometric

42

Fluorescent chemosensor, N-2anthracenyl trifluoroacetamide

0 – 2.3 × 10−5

─

Acetonitrile/ water 95:5 (v/v)

Ratiometric

67

1.0 × 10−7 – 2.2 × 10−4

75

Water

Ratiometric

L-Amino acid oxidase-protected AuNCs Fluorescent chemodosimeter based on anthracene– hemicyanine conjugation

Cu (PcTs) and GSH-AuNcs


Ref.

This work

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Table 2 The Detection of CN− in CN−-Spiked real samples by the Ratiometric Probe Serum sample

Found (µM )

Recovery (%)

RSD a

−

Not detected

−

−

10

9.78± 0.0099

97.8

0.95

30

29.44± 0.020

98.1

1.6

50

49.57± 0.057

99.1

4.0

70

73.53± 0.056

105.0

3.3

90

90.41 ± 0.11

100.4

5.5

Rodbal dam

−

Not detected

water

30

28.58± 0.011

− 95.2

− 1.0

50

50.35± 0.048

100.7

4.1

70

71.18± 0.063

101.6

4.7

80

78.14± 0.057

97.6

3.9

− 10

Not detected 9.74± 0.0037

− 97.4

− 0.34

30

30.37± 0.013

101.2

1.1

50

49.55± 0.011

99.1

0.81

70

69.26± 0.015

98.9

0.96

100

100.96± 0.015

100.9

0.72

Rain Water

Human blood serum

a

Added (µM)

RSD determined for n=3



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Table of Contents Graphic


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