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Synergistic effect of fluorescence recovery and enhancement on ultrasensitive visual assay of cyanide anions based on N-Acetyl-L-Cysteine-Capped CdTe quantum dots and carbon dots
Sensors & Actuators: B. Chemical 301 (2019) 126984
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Synergistic effect of fluorescence recovery and enhancement on ultrasensitive visual assay of cyanide anions based on N-Acetyl-L-CysteineCapped CdTe quantum dots and carbon dots
T
Jing Wanga,b, Daquan Lia, Yu Qiua, Xinyue Liua, Xin Zhangb, Liang Huanga, Huimin Wena, ⁎ Jun Hua, a b
College of Chemical Engineering, Zhejiang University of Technology. Hangzhou 310014, PR China Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Cyanide anions Quantum dots Ratiometric detection Nucleophilic addition Turn-on Visual identification
Developing a reliable fluorescent nanosensor for cyanide anions (CN−), with the potential to enable visual discrimination by naked eye, is highly sought after yet is limited by low sensitivity and selectivity. Herein based on the synergistic effect of fluorescence recovery and enhancement, a specific dual-emission nano-system was designed for ultrasensitive detection of CN−, by employing carbon dots (peaking at 443 nm) as a reference and N-acetyl-L-cysteine (NALC)-capped CdTe quantum dots (QDs, peaking at 611 nm) as a reporter. The red fluorescence of CdTe QDs is first quenched by Cu2+ primarily with electron transfer. Upon addition of CN−, CN− coordinates with Cu2+ to form [Cu(CN)n](n−1)− complex, causing the Cu2+ to detach from the QDs, and consequently fluorescence recovery. More importantly, the CN− can undergo a nucleophilic addition reaction with the carbonyl group of NALC ligands, inducing 1.3-fold fluorescence enhancement towards the original QDs while the blue fluorescence of carbon dots remains constant. This results the fluorescence intensity ratios (I611/I443) are proportional to the concentrations of CN− in the ranges of 0.02―10 and 15―80 μM, and an ultra-low detection limit down to 10.35 nM is achieved. By using both Cu2+-promoted complexation and addition reaction as recognition units, the present method also showed excellent selectivity for CN− over other coexisting anions. Especially, we have already demonstrated, by spiked tests, the practicability of monitoring the concentration changes of CN− in both environmental water and cassava samples, and further realized visual monitoring of CN− changes in aqueous solution and test paper.
1. Introduction Cyanide anions (CN−) is recognized as a hazardous substance in industrial, environmental and biological processes [1,2], and is extremely detrimental to mammals [3]. CN− can bind cytochrome c oxidase to inhibit the oxygen transport to mitochondria, resulting in suffocation and death [2,4]. The highest permissible of CN− concentration in drinking water is 1.9 μM according to the World Health Organization [5]. In addition to being found in many foods and over 2000 plants [6], CN− has been extensively used in industry, raising the potential contamination to ground water and even drinking water accordingly [1–3]. Therefore, pursuing a simple and reliable method for CN− detection from contaminant sources is of particular importance. In this vein, fluorescence (FL) detection technique has aroused significant attention owing to its appealing merits such as easy
⁎
operation [7], low cost [8], and high sensitivity [9]. Until now, many fluorescent chemodosimeters for CN− have been reported, and their ingredients can be mainly classified into organic dyes and inorganic nanoparticles. Organic dyes [1,2,10–21] like azine, azo, benzimidazole, benzothiazole, BODIFY, and their derivatives always display superior selectivity due to their specific binding or chemical reactions with CN−, and some of them with detection limit of sub-micromole CN− were fulfilled in organic solvents [16,18,19]. However, the practical applications of these organic fluorophores is limited by their complicated synthetic procedures [20,21], poor photostability [2,22], and low solubility in aqueous solutions [18,19,23]. Nanoparticles [8,9,22–35] including quantum dots (QDs) [22,24–27], carbon dots [28–30] and clusters of precious metals [8,9,31–33] have been greatly investigated for CN− sensing. The fast time-response [9], high stability [27], and excellent water solubility [9], make nanoparticles promising candidate
Corresponding author. E-mail address: [email protected] (J. Hu).
https://doi.org/10.1016/j.snb.2019.126984 Received 25 March 2019; Received in revised form 15 June 2019; Accepted 15 August 2019 Available online 15 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 301 (2019) 126984
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intensity serves as a stable internal reference. With the change of FL intensity ratios, the FL colors of the system changed correspondingly, offering the possibility for visual identification. It is worthwhile to note that the synergistic effect of FL recovery and enhancement, as well as ratiometric technique was firstly incorporated simultaneously to determine CN−. The skillful strategy grants the detection system with high sensitivity, good selectivity, low cost, and strong stability for detection of CN− in three practical water (tap water, Shangtang river water, and the West Lake water) and in cassava as a plant sample. Besides, a fluorescent test paper was successfully constructed for semiquantitative and visual assay of CN−, which proved the practical application value of the detection system.
in real sample analysis. In addition, the nanoparticles can be readily modified by functional ligands, thus endowing them with novel properties for specific aims [6,36]. Nonetheless, in contrast with conventional organic fluorophores, for which molecule interaction patterns and mechanisms are well established [1,2], the development of generalizable and mechanistically understood methodologies for CN− nanoprobes is still in its infancy and the sensitivity and selectivity of the current nanoprobes are much lower than those of the organic probes [20]. Accordingly, this research field requires, before reaching practical trials, in-depth study to reveal the relationship between nanoparticles’ structure- and ligand-related properties and their performances. As mentioned above, there is still a challenge to construct an exquisite CN− fluorescent assay, which not only has the advantages of low-concentration response and selective recognition of organic probes, but also hold the good water solubility and strong stability of nanoprobes. As a result, it is urgent to explore a facile fabrication strategy, in which the above two requirements can be well-concerned simultaneously. As known, there are two strategies for CN− detection based on organic dyes [1,37]. One is to take advantage of the high binding affinity of CN− to metal ions, H+, or boronic acid derivatives, which is commonly used by nanoparticles. The other is based on the addition reaction of CN− to electrophilic C]C, C]O, C]N, or C = N+ double bonds, which is rarely applied to nanoparticles. Noteworthily, the latter strategy exhibits better specificity and higher sensitivity [1], and its sensing mechanism has also been studied clearly. Inspired by this, it may be an avenue to introduce the chemical reaction recognition mechanism of organic dyes into nanoparticle-based testing system. However, the chemical reaction happened between nanoparticle and target often quenches the FL of the nanoparticle, which improves the specificity of detection but extremely reduces the sensitivity compared with “turn-on” protocols. What's exciting is that we found CN− have an obvious FL enhancement effect on N-acetyl-L-cysteine (NALC)-capped CdTe QDs for the first time. We have proved that the reason for FL enhancement was the nucleophilic addition reaction happened between CN− and C]O double bonds on NALC. The phenomenon demonstrated that the further modification of capping ligands from QDs may improve the analytical performance for CN− detection. Excitingly, ratiometric technique has also been applied to improve the sensitivity in FL-based methods [3,9,38]. Because of the self-calibration capability, ratiometric fluorescent measurement is able to cancel out incident light, probe concentration, as well as environmental fluctuations by calculation of two emission intensity ratio instead of the absolute intensity of one peak [39,40]. Moreover, ratiometric fluorescent measurement enables visual detection by recognizing FL color change [16,36]. Visual detection of CN− based on ratiometric methods have been demonstrated with organic fluorescent dyes. For example, Sun [3] and co-workers developed a ratiometric fluorescent CN− probe bearing a benzo indolium moiety as a fluorophore with a detection limit of 18 nM. Owing to the nucleophilic attack of CN− toward the benzo indolium group of the probe, a markedly FL intensity ratio change as well as a color change appeared. However, to the best of our knowledge, nanoparticle-based ratiometric fluorescent methods for highly sensitive and selective visual detection of the concentration of CN− are still scarce [9]. Here, based on the FL recovery and enhancement synergistic effect of CN− on Cu2+-promoted NALC-capped CdTe QDs, a visual FL ratiometric method was proposed for CN− sensing. As illustrated in Scheme 1a, the FL of NALC-capped CdTe QDs is first quenched by addition of Cu2+, then the introduction of carbon dots constitutes a ratiometric FL detection system. Upon addition of CN−, CN− coordinates with Cu2+ to form [Cu(CN)n](n−1)− complex, which causes the Cu2+ to detach from the QDs [24,27], resulting in FL recovery (Scheme 1b, Route 1). At the same time, the CN− undergoes a nucleophilic addition reaction with the carbonyl group in the NALC, which passivates trap states and enhances the FL of the QDs obviously (Scheme 1b, Route 2). The blue emission of carbon dots, in contrast, was almost unchanged and its
2. Experimental section 2.1. Reagents Cadmium chloride (CdCl2･2.5H2O, 99.95%), sodium borohydride (NaBH4, 98%), sodium tellurite (Na2TeO3, 99.9%), citric acid dihydrate and N-acetyl-L-cysteine (NALC, 99%) were bought from Aladdin Reagent Co., Ltd. Agarose was purchased from Macklin. Glass fiber was obtained from JNBIO. Ethylenediamine, trisodium citrate and other reagents were received from Sinopharm Chemical Reagent Co., Ltd. Phosphate buffered saline (PBS) was prepared by mixing PB solutions with 0.9% NaCl. All solutions were prepared with ultrapure water (≥18.2 MΩ cm−1) by a Millipore water purification system. 2.2. Apparatus and characterization FL emission and excitation spectra of all samples were recorded on a Hitachi F-2700 spectrofluorometer (Tokyo, Japan). Absorption spectra were obtained with a TU-1900 UV–vis spectrometer (Beijing Purkinje General Instrument CO, LTD China). Fourier transform infrared (FT-IR) spectra were measured using a Bruker model VECTOR22 Fouriertransform spectrometer (Bruker Corporation, Germany,). Steady-state FL measurements were carried out on a FLSP920 spectrofluorometer (Edinburgh Analytical Instruments). Transmission electron microscopy (TEM) analysis was performed on FEI-F20 electron microscope operated at 200 kV (FEI Corporation, USA). 2.3. Synthesis and purification of NALC-Capped CdTe QDs and carbon dots The CdTe QDs (4.243 μM) with fluorescent emission wavelength of 611 nm were prepared through our previous reports [41] by controlling reflux time for 11.5 h. After it has been purified by ethanol, the QDs were redispersed in PBS (10 mM) and stored in a fridge at 4 °C for further use. The carbon dots with fluorescent emission wavelength of 443 nm were prepared according the report with some modifications [42]. In brief, citric acid dihydrate (1 g) and ethylenediamine (5 mL) was dissolved in ultrapure water (25 mL). Then the mixture was transferred into a 50 mL teflon-lined stainlesssteel autoclave and heated at 200 °C for 5 h. The obtained carbon dots were dialyzed for further purification through a 500 kDa filter. The resultant carbon dots were stored at 4 °C for later use. 2.4. Preparation of fluorescent test papers Firstly, 10.4 mL PBS (10 mM, pH 8.0), 968.0 μL CdTe QDs, and 193.6 μL of Cu2+ (1 mM) were mixed for 6 min at room temperature. Subsequently, 439.4 μL carbon dots were added to the above mixed solution. Meanwhile, 0.2 g agarose was dissolved in 8 mL water (90 ℃) and stirred for 10 min. Then, the agarose solution was cooled to 60 °C, and the Cu2+-promoted CdTe QDs/carbon dots solution was mixed with agarose solution for another 10 min. After that, the glass fiber was immersed in the above mixed solution for 5 min, then extracted from the solution and freeze-dried. Finally, the obtained fluorescent 2
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Scheme 1. Schematic illustration of the ratiometric detection system for the visual detection CN−.
Fig. 1. FL (solid line) and UV–vis (dot line) spectra of carbon dots (A) and CdTe QDs (B). Insets are digital photos of carbon dots (A) and CdTe QDs (B) under a 365 nm UV lamp. TEM images of carbon dots (C) and CdTe QDs (D) with their size distribution diagram.
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Fig. 2. (A) FL spectra of CdTe QDs in the presence of Cu2+ (0, 0.02, 0.08, 0.2, 0.94 μM, black line), and subsequent addition of CN– (1, 6, 30, 60, 80 μM, red line). (B) FTIR spectra of CdTe QDs (black line), Cu2+-promoted CdTe QDs (red line), Cu2+-promoted CdTe QDs with CN– (blue line), and CdTe QDs with CN– (green line). (C) Normalized Excitation spectra (λem = 611 nm) of the CdTe QDs in the absence (black line) and presence (red line) of 80 μM CN−. (D) Normalized time-resolved FL decay curves (λex = 405 nm, measured at the maximum of the fluorescence) of CdTe QDs and Cu2+-promoted CdTe QDs/carbon dots system in the absence and presence of CN−. The concentrations of QDs and Cu2+ are 20 nM and 0.94 μM respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
by step. One hundred microliter of filtration samples with different concentrations of CN− were put in the testing solution, and their FL spectra measurement procedures were the same as above. One milliliter of samples with different concentrations of CN− were diluted to 10 mL in PBS (pH 8.0, 10 mM) for subsequent detection by fluorescent test paper.
detection paper was stored in refrigerator. 2.5. Standard procedures for CN− detection in aqueous buffer and test paper In a typical fluorescent assay in aqueous buffer, 14.13 μL CdTe QDs, 2.951 mL PBS (10 mM), 28.2 μL of Cu2+ (0.1 mM) was mixed for 6 min at room temperature. Then, 6.4 μL carbon dots were added to the above mixed solution. Subsequently, different concentrations of CN− solution were added and incubated for 15 min. The FL spectra were measured under a single wavelength excitation at 380 nm. The procedure of visually detecting CN− by fluorescent test papers was to soak the test papers in buffer solution containing different amounts of CN− and wait for 30 min, the resultant papers were observed under a 365 nm UV lamp. Then, the photos of these papers were obtained by a digital camera. Subsequently, the hue values of the obtained photographs were analyzed by a Color Analyzer App.
3. Results and discussion 3.1. Characterization of carbon dots and CdTe QDs As shown in Fig. 1A and B, the carbon dots (solid line) had a distinct absorption peak around 346 nm and a strong FL emission with the maximal wavelength at 443 nm, while the characteristic absorption and emission peak of CdTe QDs (dot line) were located at 589 and 611 nm, respectively. The carbon dots and CdTe QDs possessed excellent water dispersibility and their photographs under UV light displayed bright blue (inset in Fig. 1A). and red FL, (inset in Fig. 1B), respectively. The TEM results illustrated that both carbon dots (Fig. 1C) and QDs (Fig. 1D) were nearly spherical and monodispersed, and their average diameters were 4.28 and 3.50, respectively. The stability of as-prepared nanoparticles was investigated (Fig. S1). It was observed that the FL intensity of the QDs and carbon dots remained with no apparent change within 90 days, implying the perfect colloidal stability of the nanoparticles. Furthermore, after 100 min consecutive illuminations under UV light, the FL intensity of the QDs and carbon dots were just slightly diminished, indicating the good photochemical stability of the nanoparticles.
2.6. CN− detection in water and cassava samples For the preparation of water and cassava samples, 10 mL of fresh water sample (including tap water, Shangtang river water, the West Lake water) was collected in a sample tube. These samples were filtered five times using a 0.22 μm membrane to remove any solid suspensions and then stored at 4 °C until used. Cassava (2 g) were first crushed and followed by addition of 50 mL water under stirring for one hour. Then, the obtained supernatant was filtered using a 0.22 μm microporous filters and diluted to the detection range with PBS (pH 8.0, 10 mM) step 4
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correspond to the FL enhancement of CdTe QDs by CN–. Interestingly, upon addition of 6 μM CN–, the FL lifetimes of Cu2+-promoted CdTe QDs increased from 31.21 to 37.50 ns, which was higher than that of initial CdTe QDs. However, it can be seen from Fig. 2A, the FL intensity at 611 nm of the Cu2+-promoted CdTe QDs in the presence of 6 μM CN– was lower than that of initial CdTe QDs. This indicated that FL recovery process was accompanied by FL enhancement. Because of the synergistic effect of the FL recovery and enhancement, the FL lifetime (from 31.21 to 38.97 ns) and FL intensity of Cu2+-promoted QDs were greatly improved upon addition of 60 μM CN– (Fig. 2A). In this case, the carbon dot was only acted as a reference. By combining the synergistic effect of FL recovery and enhancement as well as ratiometric technique, an ultrasensitive assay for CN– was proposed.
3.2. Detection mechanism of the Cu2+-promoted CdTe QDs/carbon dots system for CN− Current research shows that there are two main ways for copper ions to quench CdTe QDs. The first approach is cation-exchange or ionbonding [43,44], which is difficult to recover because the ordered structure of CdTe QDs is destroyed, resulting in a large number of defects. The second approach is electron transfer from the conduction band of the QDs to Cu2+ [43], which is reversible owing to the attack of the QD core by Cu2+ could be hindered by high-density ligand on the surface of QDs. In our experiment, the main quenching mechanism is electron transfer. As shown in Fig. 2D, upon addition of Cu2+, the average FL lifetimes of the QDs decreased from 35.00 to 31.21 ns, which indicated the electrons were transferred from QDs to Cu2+ [45]. In addition, the emission wavelength maximum was red-shifted only 1.5 nm (Fig. 2A), and the slight red-shift (1 nm) in UV–vis characterization (Fig. S2) also confirmed this hypothesis [45,46]. We speculated that NALC contained carboxyl and imino groups not only coordinated with Cu2+ but also hindered the ion-bonding between Cu2+ and sulfydryl at the QDs’surface [47,48]. Then the possible mechanism on FL response of Cu2+-promoted QDs by CN− was investigated. Fig. 2A displayed that the FL intensity of Cu2+-promoted QDs was recovered as the increasing amounts of CN−. As known, stable [Cu(CN)n](n–1)– species [22] can be formed through the following reactions [28]. 2Cu2+ + 4 CN− = 2CuCN + (CN)2 −
CuCN + (n-1) CN
3.3. Optimization of the assay conditions To obtain better performance for CN− detection, some related factors such as pH of solution, Cu2+ concentration, and reaction time were optimized. The pH of buffer had a remarkable influence on the FL intensity ratio (I611/I443) of Cu2+-promoted QDs/carbon dots system. It was adverse for detecting CN− under acidic condition due to the formation of hydrogen cyanide (HCN) [23]. As the pH increased from 6.0 to 8.0, the changes of FL intensity ratio (Δ(I611/I443)) in the presence and absence of CN− gradually increased and reached the maximum value at the pH of 8.0 (Fig. 3A). Furthermore, the FL intensity of CdTe QDs before (I0) and after (I) treated with CN− was measured to evaluate the appropriate pH in FL enhancement process. Affected by the pKa of NALC [55], the FL enhancement effect was also best near the pH of 8.0 (Fig. 3A). Therefore, pH 8.0 was chosen as the optimal pH for CN− detection. Then the role of Cu2+ concentration on the detection performance was investigated. As shown in Fig. 3B, the lower concentration of Cu2+ (0.7 μM) limited the response range to CN−. The higher concentration of Cu2+ makes it difficult to recover the FL of Cu2+promoted CdTe QDs, which extremely restrict the sensitivity of the system. In addition, inappropriate concentration of Cu2+ was not conducive to visual discrimination of CN− by naked eye (Inset of Fig. 3B). Comprehensively, the Cu2+ concentration of 0.94 μM was selected. Fig. S6A showed the FL quenching of the CdTe QDs occurred immediately after the addition of Cu2+ and completed in 6 min. It was found that the FL recovery and enhancement occurred after the addition of 80 μM CN− and completed within 15 min (Fig. S6B). Thus, the optimal reaction time was chosen as 6 min and 15 min for the quenching and recovery process, respectively.
(1)
(n–1)–
= [Cu(CN)n]
(2) –
Where n is generally 2 or 4, the stability constants of [Cu(CN)2] and [Cu(CN)4]3– are 1.00 × 1024 and 2.00 × 1030, respectively. Notably, CN− can coordinate with Cu2+, causing the Cu2+ to detach from the QDs, thus leading to the FL recovery. When a high concentration solution of Cu2+-promoted QDs was mixed with enough CN−, some black precipitate was observed, also revealing that the Cu2+ was removed from the surface of QDs. Although similar FL recovery phenomena have been reported [22,27], to our surprise, with the continual increase of CN− concentration, the FL intensity of Cu2+-promoted QDs exceeded the initial FL intensity of QDs and was 2.3 times of the initial intensity at the CN− concentration of 80 μM. We speculated that CN− can interact directly with the QDs to cause FL enhancement of QDs. Fig. S3 presented the obvious FL enhancement effect on the QDs by CN−, which verified our speculation. To gain further insight into the detection mechanism of the system for CN−, the FTIR spectra at different stages were measured. As shown in Fig. 2B, compared with CdTe and Cu2+-promoted CdTe, several peaks disappeared at 1393, 1590, and 1790 cm–1 corresponding to COO– [49], CON–H [50], and C]O [51] respectively in the FTIR spectrum of CdTe and Cu2+-promoted CdTe treated with CN–. Moreover, a new peak appeared at 2080 cm–1 corresponding to C^N [52]. The results indicated that CN– can undergo a nucleophilic addition reaction with the NALC on CdTe QDs’ surface. The variations of UV–vis absorption (Fig. S4) and FTIR spectra (Fig. S5) of NALC molecules before and after addition of CN− also confirmed the carbonyl group on the NALC can undergo nucleophilic addition reaction with the CN−. It is worth mentioning that the changes of ligand components on the surface of QDs will greatly affect the FL of QDs [6,36]. The introduction of C^N functional groups passivated trap states by raising their energy level above that of the CdTe band edge [53,54]. The more tightly CN− bound and the more they raised the energy level of the trap states, the more complete was the passivation and thus the brighter the QDs. Moreover, after addition of CN−, the QDs had a broader excitation spectrum (Fig. 2C), thereby the exited electrons were easily transferred to the LUMO of the QDs, resulting in FL enhancement. FL decay curves were performed for expounding the effect of FL recovery and enhancement. All data were well fitted by a double exponential decay. As shown in Fig. 2D, obvious increase of lifetime from 35.00 to 38.46 ns upon addition of 6 μM CN– was
3.4. Detection of CN− using the Cu2+-promoted CdTe QDs/carbon dots system The dose response of the testing system to CN− has been studied. In the absence of CN−, two well-resolved emission peaks centered at 443 and 611 nm under a single wavelength excitation were observed, which can be ascribed to the FL of carbon dots and QDs, respectively. As presented in Fig. 4A, when the concentration of CN− increased from 0 to 80 μM, the FL intensity at 611 nm increased continuously, while the intensity at 443 nm of the carbon dots hardly changed, Fig. 4B displays that the FL intensity ratios (I611/I443) versus the concentration of CN−. It was clear that the FL intensity ratios (I611/I443) exhibited two linear relationships to the amount of CN− in the ranges from 0.02 to 10 μM and 15 to 80 μM, respectively. Obviously, the linear slope of the former was much higher than that of the latter, suggesting that the first stage is a combination of FL recovery and enhancement, while the second stage is only the result of FL enhancement. The limit of detection (LOD) was determined to be as low as 10.35 nM (LOD = 3Sb/m), where m is the slope of the calibration curve and Sb is the standard deviation of the blank. Compared with those reported CN− assays (Table S1), the sensitivity of present FL assay was superior to most previously reported organic- and nano- probes. The improved sensitivity may be ascribed to 5
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assessed the advantages of ratiometric technique by comparison with the single FL experiment, in which only red Cu2+-promoted CdTe QDs were used for CN− detection. As shown in Fig. S7, the FL intensity ratio (I/I0) expressed a narrower linear relationship in the CN− concentration ranging from 0.06 to 8 μM and the FL color changes of the single CdTe probe are difficult to distinguish. The results distinctly confirmed that the ratiometric detection system provided higher creditability and sensitivity as a visual assay than a single FL signal. More importantly, according to the Color Analyzer App, the hues of test paper for CN− sensing could be obtained. Subsequently, a calibration linearity curve of y = 245.5 + 2.130 [CN−] could be obtained for the CN− determination ranging from 1.0 to 20 μM (Fig. S8). And the detection limit was found to be 0.284 μM. This demonstrated our method can be applied to visually and quantitatively monitor CN− through observing fluorescence color changes of test paper.
3.5. Selectivity and anti-interference ability study In CN− detection system, decreasing the interference from other anions is a challenge, thus the selectivity of the assay was tested by various coexisting anions (80 μM) including NO3−, NO2−, Br−, BrO3−, CO32−, HCO3− SO42−, HSO4−, HSO3−, C2O42−, B4O72−, I−, IO4−, IO3−, F− and SCN−. As demonstrated in Fig. 5A, the FL intensity ratio of I611/I443 increased obviously after addition of 80 μM CN−, resulting in a remarkable FL color change (blue to red) under a UV lamp. However, the addition of NO3−, NO2−, Br−, BrO3−, CO32−, HCO3− SO42−, HSO4−, HSO3−, C2O42−, B4O72−, I−, F−, and SCN− did not cause any obvious changes in both FL intensity ratio and color, demonstrating the excellent selectivity of the assay. We infer the reason was due to the combination of Cu2+-promoted complexation and nucleophilic addition reaction as recognition units. Although IO3− and IO4− may interfere the detection of CN− based on their oxidation capacity, research revealed that IO3− and IO4− can be masked by addition of I− during the pretreatment of samples [56]. After that, we evaluated the anti-interference ability of our method. In contrast to adding individual CN−, nearly the same FL intensity ratio and color were observed when adding equivalent CN− to the interference existed solution. The results demonstrate our assay can realize nearly ideal identify CN− from other coexisting anions even in the presence same amount of interferences. Moreover, As shown in Figs. S9 and S10, the anti-interference ability of our sensing system in the presence of various environmentally relevant metal ions was also investigated. Our results showed excellent anti-interference ability for CN− detection in the presence of Na+, K+, Ca2+, Mg2+, Ba2+, Al3+, Fe3+, Zn2+, Ni2+, Pb2+, Co2+, Ag+. Also, it was noted that the presence of Hg2+ or Cu2+, had an interference for CN− detection. However, the Cu2+ and Hg2+
Fig. 3. (A) Influence of pH (10 mM PBS) on the FL intensity ratios of Cu2+promoted QDs/carbon dots (I611/I443) before and after treatment with CN− (10 μM, left coordinate). Influence of pH on the FL intensity ratio (I/I0) of CdTe QDs after treatment with CN− (10 μM, right coordinate). (B) FL response (I611/I443) of the Cu2+-promoted QDs/carbon dots system versus CN− concentration under different concentrations of Cu2+. The fluorescent photos of the system under different concentrations of Cu2+ (top to down: 0.7, 0.94, and 1.2 μM) versus CN− concentrations (left to right: 0, 0.1, 0.6, 1, 2, 5, 10, 20, 40, 80 and 100 μM) were taken under a 365 nm UV lamp.
the synergistic effect of FL recovery and enhancement as well as ratiometric technique. Owing to the changes in the FL intensity ratio of the two emission wavelengths, a distinguishable color change from blue to pink through purple, ranging from 0 to 100 μM CN− could be observed under a 365 nm UV lamp (inset of Fig. 3B and Fig. 4C), which was favorable for visual detection by the naked eye. Besides, we
Fig. 4. (A) FL response of Cu2+-promoted CdTe QDs/carbon dots system (CdTe QDs: 20 nM, Cu2+: 0.94 μM) upon addition of various concentrations of CN− in a PBS (pH 8.0) solution. The concentrations of CN− from down to top are 0, 0.02, 0.06, 0.08, 0.2, 0.4, 0.6, 1, 2, 4, 6, 8, 10, 15, 20, 30, 40, 50, 60, 70, 80 and 100 μM, respectively. (B) FL response (I611/I443) of the Cu2+-promoted CdTe QDs/carbon dots system versus CN− concentration, the insets are the linear calibration plots of I611/I443 as a function of CN− and the photos of test paper with various concentrations of CN− taken under a 365 nm UV lamp. (C) The CIE 1931 chromaticity diagram of Cu2+-promoted CdTe QDs/carbon dots system in the presence of CN− with different concentration (0 to 100 μM). 6
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Fig. 5. (A) Selectivity and anti-interference ability of Cu2+-promoted CdTe QDs/carbon dots system for CN− over other coexisting anions in PBS (pH 8.0), insets are FL photos under a 365 nm UV lamp. The concentrations of all anions were 80 μM. (B) Photographs of testing reagents (top) and papers (down) impregnated with CN− in various samples under a 365 nm UV lamp. The concentrations of CN− are 0, 0.2, 2, and 8 μM, respectively.
can form CuI precipitation and HgI42− ions with I− respectively [57,58], thus the interference could be significantly decreased after simple pretreatment with I–. It’s worth pointing out that the concentrations of these metal ions in real water samples were generally much lower than the values used in our experiment [8].
paper experiment. We believe our research will provide a novel pathway for future design of CN− FL sensors with high sensitivity, good selectivity, and robust practicality. Declaration of Competing Interest The authors declare no competing financial interest.
3.6. Determination of CN− in tap water, Shangtang river water, the West Lake water, and cassava samples
Acknowledgments The feasibility of utilizing our method for detecting CN− in tap water, Shangtang river, the West Lake and cassava samples has been explored. We applied a spiked method to estimate the concentrations of CN− in different samples. Studies were carried out with four concentrations (0, 0.2, 2 and 8 μM) spiked in each real sample. Each measurement was done in quintuplicate and the analytical results are shown in Table S2. It was found that the recoveries of all the measurements are statistically close to those values added and the obtained RSD were satisfactory for quantitative assays. The excellent analytical performance of the assay towards CN− encouraged us to check its practical applicability as on-site detection kit. The preparation of the testing reagents and papers was described in the experiment section. As shown in Fig. 5B, we can see the solution and paper impregnated with different amounts of CN− emitted blue-purple-pink luminescence under 365 nm UV lamp illumination, and the emission color changes can be identified by naked eyes. The results confirmed our detection system can meet with the requirements for the visual detection CN− in real samples.
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21875218, 21501191, 51672248 and 51872261) and Zhejiang Provincial Natural Science Foundation (LR19E020002). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126984. The supporting data includes UV–vis, FL, and FTIR characterizations of the presented system, the single FL experiment for CN− detection, hue response of test paper versus CN− concentrations, the anti-interference ability of Cu2+-promoted CdTe QDs/carbon dots system to various cations for CN− detection, testing results of four real samples and the comparison of current method with existing FL detection methods. References
4. Conclusions
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−
In summary, we presented a ratiometric FL method for CN detection through simple combination of carbon dots and Cu2+-promoted CdTe QDs. The addition of CN− induced a significantly enhanced emission intensity of QDs while the FL intensity of carbon dots undergoes slight change, leading to FL intensity ratio response toward CN−. Taking advantages of the synergistic effect of FL recovery and enhancement, our method exhibited remarkable sensitivity to CN− (detection limit 10.35 nM) in a wide concentration range compared to other reported methods. Through Cu2+-promoted complexation and addition reaction as recognition units, the present method showed excellent selectivity and successfully identified CN− from other coexisting anions. Furthermore, the tunable FL colors from blue to pink allowed our method to be used as a sensitive colorimetric luminescent sensor to directly map the CN− concentration distribution (0 to 100 μM). The practical utility of the method as a potential on-site detection of CN− in four actual samples has also been demonstrated by testing reagent and 7
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Synergistic effect of fluorescence recovery and enhancement on ultrasensitive visual assay of cyanide anions based on N-Acetyl-L-CysteineCapped CdTe quantum dots and carbon dots
T
Jing Wanga,b, Daquan Lia, Yu Qiua, Xinyue Liua, Xin Zhangb, Liang Huanga, Huimin Wena, ⁎ Jun Hua, a b
College of Chemical Engineering, Zhejiang University of Technology. Hangzhou 310014, PR China Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Cyanide anions Quantum dots Ratiometric detection Nucleophilic addition Turn-on Visual identification
Developing a reliable fluorescent nanosensor for cyanide anions (CN−), with the potential to enable visual discrimination by naked eye, is highly sought after yet is limited by low sensitivity and selectivity. Herein based on the synergistic effect of fluorescence recovery and enhancement, a specific dual-emission nano-system was designed for ultrasensitive detection of CN−, by employing carbon dots (peaking at 443 nm) as a reference and N-acetyl-L-cysteine (NALC)-capped CdTe quantum dots (QDs, peaking at 611 nm) as a reporter. The red fluorescence of CdTe QDs is first quenched by Cu2+ primarily with electron transfer. Upon addition of CN−, CN− coordinates with Cu2+ to form [Cu(CN)n](n−1)− complex, causing the Cu2+ to detach from the QDs, and consequently fluorescence recovery. More importantly, the CN− can undergo a nucleophilic addition reaction with the carbonyl group of NALC ligands, inducing 1.3-fold fluorescence enhancement towards the original QDs while the blue fluorescence of carbon dots remains constant. This results the fluorescence intensity ratios (I611/I443) are proportional to the concentrations of CN− in the ranges of 0.02―10 and 15―80 μM, and an ultra-low detection limit down to 10.35 nM is achieved. By using both Cu2+-promoted complexation and addition reaction as recognition units, the present method also showed excellent selectivity for CN− over other coexisting anions. Especially, we have already demonstrated, by spiked tests, the practicability of monitoring the concentration changes of CN− in both environmental water and cassava samples, and further realized visual monitoring of CN− changes in aqueous solution and test paper.
1. Introduction Cyanide anions (CN−) is recognized as a hazardous substance in industrial, environmental and biological processes [1,2], and is extremely detrimental to mammals [3]. CN− can bind cytochrome c oxidase to inhibit the oxygen transport to mitochondria, resulting in suffocation and death [2,4]. The highest permissible of CN− concentration in drinking water is 1.9 μM according to the World Health Organization [5]. In addition to being found in many foods and over 2000 plants [6], CN− has been extensively used in industry, raising the potential contamination to ground water and even drinking water accordingly [1–3]. Therefore, pursuing a simple and reliable method for CN− detection from contaminant sources is of particular importance. In this vein, fluorescence (FL) detection technique has aroused significant attention owing to its appealing merits such as easy
⁎
operation [7], low cost [8], and high sensitivity [9]. Until now, many fluorescent chemodosimeters for CN− have been reported, and their ingredients can be mainly classified into organic dyes and inorganic nanoparticles. Organic dyes [1,2,10–21] like azine, azo, benzimidazole, benzothiazole, BODIFY, and their derivatives always display superior selectivity due to their specific binding or chemical reactions with CN−, and some of them with detection limit of sub-micromole CN− were fulfilled in organic solvents [16,18,19]. However, the practical applications of these organic fluorophores is limited by their complicated synthetic procedures [20,21], poor photostability [2,22], and low solubility in aqueous solutions [18,19,23]. Nanoparticles [8,9,22–35] including quantum dots (QDs) [22,24–27], carbon dots [28–30] and clusters of precious metals [8,9,31–33] have been greatly investigated for CN− sensing. The fast time-response [9], high stability [27], and excellent water solubility [9], make nanoparticles promising candidate
Corresponding author. E-mail address: [email protected] (J. Hu).
https://doi.org/10.1016/j.snb.2019.126984 Received 25 March 2019; Received in revised form 15 June 2019; Accepted 15 August 2019 Available online 15 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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intensity serves as a stable internal reference. With the change of FL intensity ratios, the FL colors of the system changed correspondingly, offering the possibility for visual identification. It is worthwhile to note that the synergistic effect of FL recovery and enhancement, as well as ratiometric technique was firstly incorporated simultaneously to determine CN−. The skillful strategy grants the detection system with high sensitivity, good selectivity, low cost, and strong stability for detection of CN− in three practical water (tap water, Shangtang river water, and the West Lake water) and in cassava as a plant sample. Besides, a fluorescent test paper was successfully constructed for semiquantitative and visual assay of CN−, which proved the practical application value of the detection system.
in real sample analysis. In addition, the nanoparticles can be readily modified by functional ligands, thus endowing them with novel properties for specific aims [6,36]. Nonetheless, in contrast with conventional organic fluorophores, for which molecule interaction patterns and mechanisms are well established [1,2], the development of generalizable and mechanistically understood methodologies for CN− nanoprobes is still in its infancy and the sensitivity and selectivity of the current nanoprobes are much lower than those of the organic probes [20]. Accordingly, this research field requires, before reaching practical trials, in-depth study to reveal the relationship between nanoparticles’ structure- and ligand-related properties and their performances. As mentioned above, there is still a challenge to construct an exquisite CN− fluorescent assay, which not only has the advantages of low-concentration response and selective recognition of organic probes, but also hold the good water solubility and strong stability of nanoprobes. As a result, it is urgent to explore a facile fabrication strategy, in which the above two requirements can be well-concerned simultaneously. As known, there are two strategies for CN− detection based on organic dyes [1,37]. One is to take advantage of the high binding affinity of CN− to metal ions, H+, or boronic acid derivatives, which is commonly used by nanoparticles. The other is based on the addition reaction of CN− to electrophilic C]C, C]O, C]N, or C = N+ double bonds, which is rarely applied to nanoparticles. Noteworthily, the latter strategy exhibits better specificity and higher sensitivity [1], and its sensing mechanism has also been studied clearly. Inspired by this, it may be an avenue to introduce the chemical reaction recognition mechanism of organic dyes into nanoparticle-based testing system. However, the chemical reaction happened between nanoparticle and target often quenches the FL of the nanoparticle, which improves the specificity of detection but extremely reduces the sensitivity compared with “turn-on” protocols. What's exciting is that we found CN− have an obvious FL enhancement effect on N-acetyl-L-cysteine (NALC)-capped CdTe QDs for the first time. We have proved that the reason for FL enhancement was the nucleophilic addition reaction happened between CN− and C]O double bonds on NALC. The phenomenon demonstrated that the further modification of capping ligands from QDs may improve the analytical performance for CN− detection. Excitingly, ratiometric technique has also been applied to improve the sensitivity in FL-based methods [3,9,38]. Because of the self-calibration capability, ratiometric fluorescent measurement is able to cancel out incident light, probe concentration, as well as environmental fluctuations by calculation of two emission intensity ratio instead of the absolute intensity of one peak [39,40]. Moreover, ratiometric fluorescent measurement enables visual detection by recognizing FL color change [16,36]. Visual detection of CN− based on ratiometric methods have been demonstrated with organic fluorescent dyes. For example, Sun [3] and co-workers developed a ratiometric fluorescent CN− probe bearing a benzo indolium moiety as a fluorophore with a detection limit of 18 nM. Owing to the nucleophilic attack of CN− toward the benzo indolium group of the probe, a markedly FL intensity ratio change as well as a color change appeared. However, to the best of our knowledge, nanoparticle-based ratiometric fluorescent methods for highly sensitive and selective visual detection of the concentration of CN− are still scarce [9]. Here, based on the FL recovery and enhancement synergistic effect of CN− on Cu2+-promoted NALC-capped CdTe QDs, a visual FL ratiometric method was proposed for CN− sensing. As illustrated in Scheme 1a, the FL of NALC-capped CdTe QDs is first quenched by addition of Cu2+, then the introduction of carbon dots constitutes a ratiometric FL detection system. Upon addition of CN−, CN− coordinates with Cu2+ to form [Cu(CN)n](n−1)− complex, which causes the Cu2+ to detach from the QDs [24,27], resulting in FL recovery (Scheme 1b, Route 1). At the same time, the CN− undergoes a nucleophilic addition reaction with the carbonyl group in the NALC, which passivates trap states and enhances the FL of the QDs obviously (Scheme 1b, Route 2). The blue emission of carbon dots, in contrast, was almost unchanged and its
2. Experimental section 2.1. Reagents Cadmium chloride (CdCl2･2.5H2O, 99.95%), sodium borohydride (NaBH4, 98%), sodium tellurite (Na2TeO3, 99.9%), citric acid dihydrate and N-acetyl-L-cysteine (NALC, 99%) were bought from Aladdin Reagent Co., Ltd. Agarose was purchased from Macklin. Glass fiber was obtained from JNBIO. Ethylenediamine, trisodium citrate and other reagents were received from Sinopharm Chemical Reagent Co., Ltd. Phosphate buffered saline (PBS) was prepared by mixing PB solutions with 0.9% NaCl. All solutions were prepared with ultrapure water (≥18.2 MΩ cm−1) by a Millipore water purification system. 2.2. Apparatus and characterization FL emission and excitation spectra of all samples were recorded on a Hitachi F-2700 spectrofluorometer (Tokyo, Japan). Absorption spectra were obtained with a TU-1900 UV–vis spectrometer (Beijing Purkinje General Instrument CO, LTD China). Fourier transform infrared (FT-IR) spectra were measured using a Bruker model VECTOR22 Fouriertransform spectrometer (Bruker Corporation, Germany,). Steady-state FL measurements were carried out on a FLSP920 spectrofluorometer (Edinburgh Analytical Instruments). Transmission electron microscopy (TEM) analysis was performed on FEI-F20 electron microscope operated at 200 kV (FEI Corporation, USA). 2.3. Synthesis and purification of NALC-Capped CdTe QDs and carbon dots The CdTe QDs (4.243 μM) with fluorescent emission wavelength of 611 nm were prepared through our previous reports [41] by controlling reflux time for 11.5 h. After it has been purified by ethanol, the QDs were redispersed in PBS (10 mM) and stored in a fridge at 4 °C for further use. The carbon dots with fluorescent emission wavelength of 443 nm were prepared according the report with some modifications [42]. In brief, citric acid dihydrate (1 g) and ethylenediamine (5 mL) was dissolved in ultrapure water (25 mL). Then the mixture was transferred into a 50 mL teflon-lined stainlesssteel autoclave and heated at 200 °C for 5 h. The obtained carbon dots were dialyzed for further purification through a 500 kDa filter. The resultant carbon dots were stored at 4 °C for later use. 2.4. Preparation of fluorescent test papers Firstly, 10.4 mL PBS (10 mM, pH 8.0), 968.0 μL CdTe QDs, and 193.6 μL of Cu2+ (1 mM) were mixed for 6 min at room temperature. Subsequently, 439.4 μL carbon dots were added to the above mixed solution. Meanwhile, 0.2 g agarose was dissolved in 8 mL water (90 ℃) and stirred for 10 min. Then, the agarose solution was cooled to 60 °C, and the Cu2+-promoted CdTe QDs/carbon dots solution was mixed with agarose solution for another 10 min. After that, the glass fiber was immersed in the above mixed solution for 5 min, then extracted from the solution and freeze-dried. Finally, the obtained fluorescent 2
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Scheme 1. Schematic illustration of the ratiometric detection system for the visual detection CN−.
Fig. 1. FL (solid line) and UV–vis (dot line) spectra of carbon dots (A) and CdTe QDs (B). Insets are digital photos of carbon dots (A) and CdTe QDs (B) under a 365 nm UV lamp. TEM images of carbon dots (C) and CdTe QDs (D) with their size distribution diagram.
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Fig. 2. (A) FL spectra of CdTe QDs in the presence of Cu2+ (0, 0.02, 0.08, 0.2, 0.94 μM, black line), and subsequent addition of CN– (1, 6, 30, 60, 80 μM, red line). (B) FTIR spectra of CdTe QDs (black line), Cu2+-promoted CdTe QDs (red line), Cu2+-promoted CdTe QDs with CN– (blue line), and CdTe QDs with CN– (green line). (C) Normalized Excitation spectra (λem = 611 nm) of the CdTe QDs in the absence (black line) and presence (red line) of 80 μM CN−. (D) Normalized time-resolved FL decay curves (λex = 405 nm, measured at the maximum of the fluorescence) of CdTe QDs and Cu2+-promoted CdTe QDs/carbon dots system in the absence and presence of CN−. The concentrations of QDs and Cu2+ are 20 nM and 0.94 μM respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
by step. One hundred microliter of filtration samples with different concentrations of CN− were put in the testing solution, and their FL spectra measurement procedures were the same as above. One milliliter of samples with different concentrations of CN− were diluted to 10 mL in PBS (pH 8.0, 10 mM) for subsequent detection by fluorescent test paper.
detection paper was stored in refrigerator. 2.5. Standard procedures for CN− detection in aqueous buffer and test paper In a typical fluorescent assay in aqueous buffer, 14.13 μL CdTe QDs, 2.951 mL PBS (10 mM), 28.2 μL of Cu2+ (0.1 mM) was mixed for 6 min at room temperature. Then, 6.4 μL carbon dots were added to the above mixed solution. Subsequently, different concentrations of CN− solution were added and incubated for 15 min. The FL spectra were measured under a single wavelength excitation at 380 nm. The procedure of visually detecting CN− by fluorescent test papers was to soak the test papers in buffer solution containing different amounts of CN− and wait for 30 min, the resultant papers were observed under a 365 nm UV lamp. Then, the photos of these papers were obtained by a digital camera. Subsequently, the hue values of the obtained photographs were analyzed by a Color Analyzer App.
3. Results and discussion 3.1. Characterization of carbon dots and CdTe QDs As shown in Fig. 1A and B, the carbon dots (solid line) had a distinct absorption peak around 346 nm and a strong FL emission with the maximal wavelength at 443 nm, while the characteristic absorption and emission peak of CdTe QDs (dot line) were located at 589 and 611 nm, respectively. The carbon dots and CdTe QDs possessed excellent water dispersibility and their photographs under UV light displayed bright blue (inset in Fig. 1A). and red FL, (inset in Fig. 1B), respectively. The TEM results illustrated that both carbon dots (Fig. 1C) and QDs (Fig. 1D) were nearly spherical and monodispersed, and their average diameters were 4.28 and 3.50, respectively. The stability of as-prepared nanoparticles was investigated (Fig. S1). It was observed that the FL intensity of the QDs and carbon dots remained with no apparent change within 90 days, implying the perfect colloidal stability of the nanoparticles. Furthermore, after 100 min consecutive illuminations under UV light, the FL intensity of the QDs and carbon dots were just slightly diminished, indicating the good photochemical stability of the nanoparticles.
2.6. CN− detection in water and cassava samples For the preparation of water and cassava samples, 10 mL of fresh water sample (including tap water, Shangtang river water, the West Lake water) was collected in a sample tube. These samples were filtered five times using a 0.22 μm membrane to remove any solid suspensions and then stored at 4 °C until used. Cassava (2 g) were first crushed and followed by addition of 50 mL water under stirring for one hour. Then, the obtained supernatant was filtered using a 0.22 μm microporous filters and diluted to the detection range with PBS (pH 8.0, 10 mM) step 4
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correspond to the FL enhancement of CdTe QDs by CN–. Interestingly, upon addition of 6 μM CN–, the FL lifetimes of Cu2+-promoted CdTe QDs increased from 31.21 to 37.50 ns, which was higher than that of initial CdTe QDs. However, it can be seen from Fig. 2A, the FL intensity at 611 nm of the Cu2+-promoted CdTe QDs in the presence of 6 μM CN– was lower than that of initial CdTe QDs. This indicated that FL recovery process was accompanied by FL enhancement. Because of the synergistic effect of the FL recovery and enhancement, the FL lifetime (from 31.21 to 38.97 ns) and FL intensity of Cu2+-promoted QDs were greatly improved upon addition of 60 μM CN– (Fig. 2A). In this case, the carbon dot was only acted as a reference. By combining the synergistic effect of FL recovery and enhancement as well as ratiometric technique, an ultrasensitive assay for CN– was proposed.
3.2. Detection mechanism of the Cu2+-promoted CdTe QDs/carbon dots system for CN− Current research shows that there are two main ways for copper ions to quench CdTe QDs. The first approach is cation-exchange or ionbonding [43,44], which is difficult to recover because the ordered structure of CdTe QDs is destroyed, resulting in a large number of defects. The second approach is electron transfer from the conduction band of the QDs to Cu2+ [43], which is reversible owing to the attack of the QD core by Cu2+ could be hindered by high-density ligand on the surface of QDs. In our experiment, the main quenching mechanism is electron transfer. As shown in Fig. 2D, upon addition of Cu2+, the average FL lifetimes of the QDs decreased from 35.00 to 31.21 ns, which indicated the electrons were transferred from QDs to Cu2+ [45]. In addition, the emission wavelength maximum was red-shifted only 1.5 nm (Fig. 2A), and the slight red-shift (1 nm) in UV–vis characterization (Fig. S2) also confirmed this hypothesis [45,46]. We speculated that NALC contained carboxyl and imino groups not only coordinated with Cu2+ but also hindered the ion-bonding between Cu2+ and sulfydryl at the QDs’surface [47,48]. Then the possible mechanism on FL response of Cu2+-promoted QDs by CN− was investigated. Fig. 2A displayed that the FL intensity of Cu2+-promoted QDs was recovered as the increasing amounts of CN−. As known, stable [Cu(CN)n](n–1)– species [22] can be formed through the following reactions [28]. 2Cu2+ + 4 CN− = 2CuCN + (CN)2 −
CuCN + (n-1) CN
3.3. Optimization of the assay conditions To obtain better performance for CN− detection, some related factors such as pH of solution, Cu2+ concentration, and reaction time were optimized. The pH of buffer had a remarkable influence on the FL intensity ratio (I611/I443) of Cu2+-promoted QDs/carbon dots system. It was adverse for detecting CN− under acidic condition due to the formation of hydrogen cyanide (HCN) [23]. As the pH increased from 6.0 to 8.0, the changes of FL intensity ratio (Δ(I611/I443)) in the presence and absence of CN− gradually increased and reached the maximum value at the pH of 8.0 (Fig. 3A). Furthermore, the FL intensity of CdTe QDs before (I0) and after (I) treated with CN− was measured to evaluate the appropriate pH in FL enhancement process. Affected by the pKa of NALC [55], the FL enhancement effect was also best near the pH of 8.0 (Fig. 3A). Therefore, pH 8.0 was chosen as the optimal pH for CN− detection. Then the role of Cu2+ concentration on the detection performance was investigated. As shown in Fig. 3B, the lower concentration of Cu2+ (0.7 μM) limited the response range to CN−. The higher concentration of Cu2+ makes it difficult to recover the FL of Cu2+promoted CdTe QDs, which extremely restrict the sensitivity of the system. In addition, inappropriate concentration of Cu2+ was not conducive to visual discrimination of CN− by naked eye (Inset of Fig. 3B). Comprehensively, the Cu2+ concentration of 0.94 μM was selected. Fig. S6A showed the FL quenching of the CdTe QDs occurred immediately after the addition of Cu2+ and completed in 6 min. It was found that the FL recovery and enhancement occurred after the addition of 80 μM CN− and completed within 15 min (Fig. S6B). Thus, the optimal reaction time was chosen as 6 min and 15 min for the quenching and recovery process, respectively.
(1)
(n–1)–
= [Cu(CN)n]
(2) –
Where n is generally 2 or 4, the stability constants of [Cu(CN)2] and [Cu(CN)4]3– are 1.00 × 1024 and 2.00 × 1030, respectively. Notably, CN− can coordinate with Cu2+, causing the Cu2+ to detach from the QDs, thus leading to the FL recovery. When a high concentration solution of Cu2+-promoted QDs was mixed with enough CN−, some black precipitate was observed, also revealing that the Cu2+ was removed from the surface of QDs. Although similar FL recovery phenomena have been reported [22,27], to our surprise, with the continual increase of CN− concentration, the FL intensity of Cu2+-promoted QDs exceeded the initial FL intensity of QDs and was 2.3 times of the initial intensity at the CN− concentration of 80 μM. We speculated that CN− can interact directly with the QDs to cause FL enhancement of QDs. Fig. S3 presented the obvious FL enhancement effect on the QDs by CN−, which verified our speculation. To gain further insight into the detection mechanism of the system for CN−, the FTIR spectra at different stages were measured. As shown in Fig. 2B, compared with CdTe and Cu2+-promoted CdTe, several peaks disappeared at 1393, 1590, and 1790 cm–1 corresponding to COO– [49], CON–H [50], and C]O [51] respectively in the FTIR spectrum of CdTe and Cu2+-promoted CdTe treated with CN–. Moreover, a new peak appeared at 2080 cm–1 corresponding to C^N [52]. The results indicated that CN– can undergo a nucleophilic addition reaction with the NALC on CdTe QDs’ surface. The variations of UV–vis absorption (Fig. S4) and FTIR spectra (Fig. S5) of NALC molecules before and after addition of CN− also confirmed the carbonyl group on the NALC can undergo nucleophilic addition reaction with the CN−. It is worth mentioning that the changes of ligand components on the surface of QDs will greatly affect the FL of QDs [6,36]. The introduction of C^N functional groups passivated trap states by raising their energy level above that of the CdTe band edge [53,54]. The more tightly CN− bound and the more they raised the energy level of the trap states, the more complete was the passivation and thus the brighter the QDs. Moreover, after addition of CN−, the QDs had a broader excitation spectrum (Fig. 2C), thereby the exited electrons were easily transferred to the LUMO of the QDs, resulting in FL enhancement. FL decay curves were performed for expounding the effect of FL recovery and enhancement. All data were well fitted by a double exponential decay. As shown in Fig. 2D, obvious increase of lifetime from 35.00 to 38.46 ns upon addition of 6 μM CN– was
3.4. Detection of CN− using the Cu2+-promoted CdTe QDs/carbon dots system The dose response of the testing system to CN− has been studied. In the absence of CN−, two well-resolved emission peaks centered at 443 and 611 nm under a single wavelength excitation were observed, which can be ascribed to the FL of carbon dots and QDs, respectively. As presented in Fig. 4A, when the concentration of CN− increased from 0 to 80 μM, the FL intensity at 611 nm increased continuously, while the intensity at 443 nm of the carbon dots hardly changed, Fig. 4B displays that the FL intensity ratios (I611/I443) versus the concentration of CN−. It was clear that the FL intensity ratios (I611/I443) exhibited two linear relationships to the amount of CN− in the ranges from 0.02 to 10 μM and 15 to 80 μM, respectively. Obviously, the linear slope of the former was much higher than that of the latter, suggesting that the first stage is a combination of FL recovery and enhancement, while the second stage is only the result of FL enhancement. The limit of detection (LOD) was determined to be as low as 10.35 nM (LOD = 3Sb/m), where m is the slope of the calibration curve and Sb is the standard deviation of the blank. Compared with those reported CN− assays (Table S1), the sensitivity of present FL assay was superior to most previously reported organic- and nano- probes. The improved sensitivity may be ascribed to 5
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assessed the advantages of ratiometric technique by comparison with the single FL experiment, in which only red Cu2+-promoted CdTe QDs were used for CN− detection. As shown in Fig. S7, the FL intensity ratio (I/I0) expressed a narrower linear relationship in the CN− concentration ranging from 0.06 to 8 μM and the FL color changes of the single CdTe probe are difficult to distinguish. The results distinctly confirmed that the ratiometric detection system provided higher creditability and sensitivity as a visual assay than a single FL signal. More importantly, according to the Color Analyzer App, the hues of test paper for CN− sensing could be obtained. Subsequently, a calibration linearity curve of y = 245.5 + 2.130 [CN−] could be obtained for the CN− determination ranging from 1.0 to 20 μM (Fig. S8). And the detection limit was found to be 0.284 μM. This demonstrated our method can be applied to visually and quantitatively monitor CN− through observing fluorescence color changes of test paper.
3.5. Selectivity and anti-interference ability study In CN− detection system, decreasing the interference from other anions is a challenge, thus the selectivity of the assay was tested by various coexisting anions (80 μM) including NO3−, NO2−, Br−, BrO3−, CO32−, HCO3− SO42−, HSO4−, HSO3−, C2O42−, B4O72−, I−, IO4−, IO3−, F− and SCN−. As demonstrated in Fig. 5A, the FL intensity ratio of I611/I443 increased obviously after addition of 80 μM CN−, resulting in a remarkable FL color change (blue to red) under a UV lamp. However, the addition of NO3−, NO2−, Br−, BrO3−, CO32−, HCO3− SO42−, HSO4−, HSO3−, C2O42−, B4O72−, I−, F−, and SCN− did not cause any obvious changes in both FL intensity ratio and color, demonstrating the excellent selectivity of the assay. We infer the reason was due to the combination of Cu2+-promoted complexation and nucleophilic addition reaction as recognition units. Although IO3− and IO4− may interfere the detection of CN− based on their oxidation capacity, research revealed that IO3− and IO4− can be masked by addition of I− during the pretreatment of samples [56]. After that, we evaluated the anti-interference ability of our method. In contrast to adding individual CN−, nearly the same FL intensity ratio and color were observed when adding equivalent CN− to the interference existed solution. The results demonstrate our assay can realize nearly ideal identify CN− from other coexisting anions even in the presence same amount of interferences. Moreover, As shown in Figs. S9 and S10, the anti-interference ability of our sensing system in the presence of various environmentally relevant metal ions was also investigated. Our results showed excellent anti-interference ability for CN− detection in the presence of Na+, K+, Ca2+, Mg2+, Ba2+, Al3+, Fe3+, Zn2+, Ni2+, Pb2+, Co2+, Ag+. Also, it was noted that the presence of Hg2+ or Cu2+, had an interference for CN− detection. However, the Cu2+ and Hg2+
Fig. 3. (A) Influence of pH (10 mM PBS) on the FL intensity ratios of Cu2+promoted QDs/carbon dots (I611/I443) before and after treatment with CN− (10 μM, left coordinate). Influence of pH on the FL intensity ratio (I/I0) of CdTe QDs after treatment with CN− (10 μM, right coordinate). (B) FL response (I611/I443) of the Cu2+-promoted QDs/carbon dots system versus CN− concentration under different concentrations of Cu2+. The fluorescent photos of the system under different concentrations of Cu2+ (top to down: 0.7, 0.94, and 1.2 μM) versus CN− concentrations (left to right: 0, 0.1, 0.6, 1, 2, 5, 10, 20, 40, 80 and 100 μM) were taken under a 365 nm UV lamp.
the synergistic effect of FL recovery and enhancement as well as ratiometric technique. Owing to the changes in the FL intensity ratio of the two emission wavelengths, a distinguishable color change from blue to pink through purple, ranging from 0 to 100 μM CN− could be observed under a 365 nm UV lamp (inset of Fig. 3B and Fig. 4C), which was favorable for visual detection by the naked eye. Besides, we
Fig. 4. (A) FL response of Cu2+-promoted CdTe QDs/carbon dots system (CdTe QDs: 20 nM, Cu2+: 0.94 μM) upon addition of various concentrations of CN− in a PBS (pH 8.0) solution. The concentrations of CN− from down to top are 0, 0.02, 0.06, 0.08, 0.2, 0.4, 0.6, 1, 2, 4, 6, 8, 10, 15, 20, 30, 40, 50, 60, 70, 80 and 100 μM, respectively. (B) FL response (I611/I443) of the Cu2+-promoted CdTe QDs/carbon dots system versus CN− concentration, the insets are the linear calibration plots of I611/I443 as a function of CN− and the photos of test paper with various concentrations of CN− taken under a 365 nm UV lamp. (C) The CIE 1931 chromaticity diagram of Cu2+-promoted CdTe QDs/carbon dots system in the presence of CN− with different concentration (0 to 100 μM). 6
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Fig. 5. (A) Selectivity and anti-interference ability of Cu2+-promoted CdTe QDs/carbon dots system for CN− over other coexisting anions in PBS (pH 8.0), insets are FL photos under a 365 nm UV lamp. The concentrations of all anions were 80 μM. (B) Photographs of testing reagents (top) and papers (down) impregnated with CN− in various samples under a 365 nm UV lamp. The concentrations of CN− are 0, 0.2, 2, and 8 μM, respectively.
can form CuI precipitation and HgI42− ions with I− respectively [57,58], thus the interference could be significantly decreased after simple pretreatment with I–. It’s worth pointing out that the concentrations of these metal ions in real water samples were generally much lower than the values used in our experiment [8].
paper experiment. We believe our research will provide a novel pathway for future design of CN− FL sensors with high sensitivity, good selectivity, and robust practicality. Declaration of Competing Interest The authors declare no competing financial interest.
3.6. Determination of CN− in tap water, Shangtang river water, the West Lake water, and cassava samples
Acknowledgments The feasibility of utilizing our method for detecting CN− in tap water, Shangtang river, the West Lake and cassava samples has been explored. We applied a spiked method to estimate the concentrations of CN− in different samples. Studies were carried out with four concentrations (0, 0.2, 2 and 8 μM) spiked in each real sample. Each measurement was done in quintuplicate and the analytical results are shown in Table S2. It was found that the recoveries of all the measurements are statistically close to those values added and the obtained RSD were satisfactory for quantitative assays. The excellent analytical performance of the assay towards CN− encouraged us to check its practical applicability as on-site detection kit. The preparation of the testing reagents and papers was described in the experiment section. As shown in Fig. 5B, we can see the solution and paper impregnated with different amounts of CN− emitted blue-purple-pink luminescence under 365 nm UV lamp illumination, and the emission color changes can be identified by naked eyes. The results confirmed our detection system can meet with the requirements for the visual detection CN− in real samples.
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21875218, 21501191, 51672248 and 51872261) and Zhejiang Provincial Natural Science Foundation (LR19E020002). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126984. The supporting data includes UV–vis, FL, and FTIR characterizations of the presented system, the single FL experiment for CN− detection, hue response of test paper versus CN− concentrations, the anti-interference ability of Cu2+-promoted CdTe QDs/carbon dots system to various cations for CN− detection, testing results of four real samples and the comparison of current method with existing FL detection methods. References
4. Conclusions
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−
In summary, we presented a ratiometric FL method for CN detection through simple combination of carbon dots and Cu2+-promoted CdTe QDs. The addition of CN− induced a significantly enhanced emission intensity of QDs while the FL intensity of carbon dots undergoes slight change, leading to FL intensity ratio response toward CN−. Taking advantages of the synergistic effect of FL recovery and enhancement, our method exhibited remarkable sensitivity to CN− (detection limit 10.35 nM) in a wide concentration range compared to other reported methods. Through Cu2+-promoted complexation and addition reaction as recognition units, the present method showed excellent selectivity and successfully identified CN− from other coexisting anions. Furthermore, the tunable FL colors from blue to pink allowed our method to be used as a sensitive colorimetric luminescent sensor to directly map the CN− concentration distribution (0 to 100 μM). The practical utility of the method as a potential on-site detection of CN− in four actual samples has also been demonstrated by testing reagent and 7
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