Analytica Chimica Acta 812 (2014) 191–198
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Ligand displacement-induced fluorescence switch of quantum dots for ultrasensitive detection of cadmium ions Xianyun Hu a,b,1 , Kao Zhu a,1 , Qingsheng Guo a , Yuqian Liu a , Mingfu Ye a , Qingjiang Sun a,∗ a b
State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing 210096, China Qiannan Medical College for Nationalities, Duyun 558000, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A simple CdTe QD–Phen sensor is constructed for detecting Cd2+ .
• The sensor is operated with a liganddisplacement induced PL switch strategy. • The detection limit of 0.01 nM for Cd2+ is achieved. • This sensor features to discriminate Cd2+ versus Zn2+ , and succeeds in real water samples.
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 31 December 2013 Accepted 5 January 2014 Available online 11 January 2014 Keywords: Quantum dot 1,10-Phenanthroline Photoluminescence Sensor Ligand displacement Cadmium ions
a b s t r a c t This paper reports the construction of a simple CdTe quantum dots (QDs)-based sensor with 1,10phenanthroline (Phen) as ligand, and the demonstration of a novel ligand displacement-induced fluorescence switch strategy for sensitive and selective detection of Cd2+ in aqueous phase. The complexation of Phen at the surface quenches the green photoluminescence (PL) of QDs dominated by a photoinduced hole transfer (PHT) mechanism. In the presence of Cd2+ , the Phen ligands are readily detached from the surface of CdTe QDs, forming [Cd(Phen)2 (H2 O)2 ]2+ in solution, and as a consequence the PL of CdTe QDs switches on. The detection limit for Cd2+ is defined as ∼0.01 nM, which is far below the maximum Cd2+ residue limit of drinking water allowed by the U.S. Environmental Protection Agency (EPA). Two consecutive linear ranges allow a wide determination of Cd2+ from 0.02 nM to 0.6 M. Importantly, this CdTe QDs-based sensor features to distinctly discriminate between Cd2+ and Zn2+ , and succeeds in real water samples. This extremely simple strategy reported here represents an attempt for the development of fluorescent sensors for ultrasensitive chemo/biodetection. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Agricultural activities such as the excessive use of fertilizers and pesticides, irrigation with waste water, industrial applications (nickel–cadmium batteries, dyes and pigments, coating of steel, various alloys) and urban life increase the content of cadmium in soils and waters. As a consequence of food chain system, cadmium exposure can cause anemia, abdominal pain, neurological
∗ Corresponding author. Tel.: +86 25 83790920; fax: +86 25 83792349. E-mail address: [email protected] (Q. Sun). 1 The authors contribute equally. 0003-2670/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2014.01.006
and adverse developmental effects, kidney damage, hypertension, changes in vitamin D metabolism, and even increased risk of cancer [1–3]. The International Agency for Research on Cancer has classified cadmium as category 1 carcinogens [4]. In the United States, the acute and chronic exposure criteria for dissolved cadmium (Cd2+ ) in drinking water are 3.7 and 1.0 g L−1 (32.9 and 8.9 nM), respectively. Therefore, in view of its toxic effects on environment as well as human health, the monitoring of chemical safety through Cd2+ analysis of the most widely used resources, especially drinking water, is of paramount importance. Of various analytical methods [5–10], the most common one still remain the use of fluorescence, which indicates the presence of an analyte by fluorescent changes that can be observed by naked eye or by simple measurements.
192
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Scheme 1. QDs based surface complexation ((A)–(C)) and proposed ligand displacement (D)-induced PL switch strategies for Cd2+ detection.
While a variety of synthetic organic-fluorophore receptors for Cd2+ assay have been designed [11], they often suffer from low solubility in water, low sensitivity, and poor photostability, which restricts their practical applications. Quantum dots (QDs) are a recently developed class of inorganic fluorophores with unique spectral properties including high PL quantum yields and exceptional resistance to photobleaching [12–16]. Recent advances in surface modification of QDs with appropriate receptors, able to interact selectively with an analyte, allow for greater analytical sensitivity in a wide variety of sensing schemes [17,18]. In the past few years, researchers have made efforts toward the development of QDs-based fluorescent sensors for detecting heavy metal ions including Cd2+ [19–22]. Of the fluorescent sensors, a PL switch-on mode, with the reduced chance of false positives, is more preferable than a PL switch-off mode [23]. So far, only a few of the QDs-based PL switch-on sensors for detecting Cd2+ have been reported, which can be classified into three strategies by the analyte-receptor complexation reactions, namely, “surface passivation” [24–28], “host–guest reaction” [29], and “chemical etching” [30,31], as shown in Scheme 1A–C. In these sensors, different Cd2+ receptors such as anions (sulfur or sulfite), N-containing crown ethers (1,10-diaza-18-crown-6), and chelating reagents (ethylene diamine tetraacetic acid, EDTA, or ammonium pyrrolidine dithiocarbamate, APDC) are appended onto the QDs surface, quenching the PL of QDs. Upon exposure to Cd2+ , the analyte-receptor complexation reactions occur at the surface of QDs, which block the non-radiative recombination pathway, inducing the restored PL of QDs. These sensors have achieved the lowest detection limit of 6–10 nM for Cd2+ [30]. However, the detection limit needs to be further lowered for quantitative assay of Cd2+ in drinking water, according to the standards of EPA. In addition, the QDs-based Cd2+ sensors all suffer from the interference of Zn2+ . The
formation of a ZnS passivation layer, or complexation of Zn2+ with N-containing crown ether, EDTA or APDC at the QDs surface can also restore the PL of QDs to some extent [24–31]. The ability of a sensor to distinctly discriminate between Cd2+ and Zn2+ is important because these ions are often found together in nature [32,33]. Therefore, there is an urgent demand to develop novel fluorescent sensors based on QDs for sensitive and selective detection of Cd2+ . Herein, we report an extremely simple PL switch strategy for ultrasensitive detection of Cd2+ , with CdTe QDs as the fluorescent reporter and Phen as the Cd2+ receptor as well as the QDs ligand. Phen is chosen to construct the fluorescent sensor primarily because it is a classic chelating bidentate ligand whose nitrogen atoms are beautifully placed to act cooperatively in binding with metals [34,35]. As illustrated in Scheme 1D, the CdTe QD–Phen sensor is proposed to operate with a novel ligand (receptor) displacement-induced PL switch strategy. In the absence of Cd2+ , the quenched PL of CdTe QDs is mainly ascribed to a PHT process from QDs to the Phen ligand [22,29,36]. Only in the presence of Cd2+ , the chelation of Phen with Cd2+ detaches the Phen ligand from the QDs surface, interrupting the PHT process and switching on the PL of QDs. On basis of this strategy, this sensor has demonstrated to feature excellent selectivity for Cd2+ over Zn2+ . 2. Experimental 2.1. Materials and reagents All the starting materials of synthesis of Thiolglycolic acid (TGA)capped CdTe QDs and mercaptopropionic acid (MPA)-capped CdSe/ZnS core/shell QDs were used without further purification. Chloroform (AR) and anhydrous ethanol (AR) were purchased from NCRC and used for purification of QDs. Various metal salts (>99.99%)
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
were purchased from Sigma-Aldrich. 1,10-Phenanthroline (99%) was purchased from J&K. The pure water ( > 18 M cm−1 ) was obtained from a Pall Cascade AN synthesis system. 2.2. Instruments
193
PL spectrum, K is the average (fwhm)PL , (K was defined as 29 nm), and L (cm) is the path length of the quartz cuvette (L was fixed at 1 cm). 2.5. PL spectra measurements
UV/Vis absorption spectra were performed on a Hitachi U4100 spectrophotometer. PL spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. PL lifetime measurements were performed on a Horiba Jobin Yvon Time-Resolved fluorescence Spectrophotometer. Ultracentrifugation experiments were performed on Optima L-100K ultracentrifuge with the speed of 50,000 rpm (rotations per minute). Fourier Transform Infrared (FTIR) spectra were acquired with Nicolet5700 instrument. Xray photoelectron spectroscopy (XPS) was measured with a PHI 5000 VersaProbe spectrometer (ULVAC-PHI, Inc.) equipped with a monochromatic Al K␣ source at room temperature. Cyclic voltammetry (CV) measurements were performed on a CHI-660D electrochemical workstation, with a typical three-electrode system. Atomic absorption spectroscopy (AAS) analysis was conducted in a Hitachi 180-80 atomic absorption spectrometer. The details in FTIR, XPS, AAS and CV measurements were described in the Supporting information.
All PL spectra measurements were performed under ambient condition, with samples measured generally 5 min later after mixing. For PL quenching measurements of CdTe QDs, the samples were prepared by adding 100 L of aqueous solutions of Phen with concentrations of 0.1–9.0 M into an equal volume of 200 nM green CdTe QDs solutions, respectively. For CdSe/ZnS QDs, 100 L of Phen solutions with concentrations of 3.33–333.0 M were added into an equal volume of CdSe/ZnS QDs solutions, respectively. For PL restoration measurements of CdTe QDs, 100 L of Cd2+ solutions with concentrations of 0.02 nM–4 M were added into an equal volume of 200 nM green CdTe QD–Phen (the molar ratio of QD to Phen was 1:12) solutions. For CdSe/ZnS QDs, 100 L of Cd2+ solutions with concentrations of 1.33–200.0 M were added into an equal volume of CdSe/ZnS QD–Phen (the fixed concentration of QDs mixed with 40 M Phen) solutions, respectively. 2.6. Selectivity and interference measurements
2.3. Synthesis and purification of water soluble TGA-capped CdTe QDs and MPA-capped CdSe/ZnS QDs Water-soluble TGA-capped CdTe QDs were prepared by a 1.5 h reflux process [37,38]. The typical molar ratio of Cd:Te:TGA was 1:0.5:1.3. The synthesis of green OA-capped CdSe/ZnS quantum dots followed the SILAR method [39]. Water soluble CdSe/ZnS QDs were prepared by replacing the original OA ligands with MPA. The as-prepared CdTe QDs or CdSe/ZnS QDs were subject to thorough purifications. With CdTe QDs as example, 5 mL aqueous solution of as-prepared QDs was mixed with anhydrous ethanol (1:3) and subsequently centrifuged at 10,000 rpm for 10 min. The clear solution of supernatant was discarded, and the precipitate was re-dispersed in pure water. This purification process was repeated for four times, and the final CdTe QDs precipitate could be readily re-dispersed in water with good colloidal stability.
The following metal ions were used for the selectivity experiment: Zn2+ , Li+ , K+ , Na+ , Ca2+ , Mg2+ , Cr2+ , Mn2+ , Ba2+ , Fe2+ , Cu2+ , Co2+ , Ni2+ , Pb2+ , Al3+ , Fe3+ , Hg+ , Ag+ , and Hg2+ . Aqueous solutions of various metal ions with identical concentration (1.0 M) were prepared from stock solutions by serial dilution, and subsequently added into an equal volume of 200 nM green CdTe QD–Phen (the molar ratio of QDs to Phen is 1:12) solutions, respectively. For experiments of the Zn2+ interference, 100 L of Zn2+ solutions with concentrations of 0.04–40.0 M were added into an equal volume of 200 nM green CdTe QD–Phen (1:12) solutions, respectively; and 100 L of Zn2+ solutions with concentration ranges of 1.33–267.0 M were added into an equal volume of CdSe/ZnS QD–Phen solutions, respectively.
3. Results and discussion 2.4. Calculation of the size and concentration of CdTe QDs 3.1. PL switch off of CdTe QDs by Phen The size and concentration of CdTe QDs were calculated according to the method proposed by Peng et al. [38]. Firstly, the particle size could be determined from the wavelength of the first excitonic absorption peak with Eq. (1):
D = 9.8127 × 10−7 max 3 − 1.7147 × 10−3 max 2 + (1.0064) max − (194.84)
(1)
where D (nm) is the size of CdTe QDs, max (nm) is the wavelength of the first excitonic absorption peak of CdTe QDs (503 nm for green CdTe QDs). Then, the extinction coefficient per mole of CdTe QDs (ε) could be obtained from Eq. (2): ε = 10043D2.12
(2)
Finally, the concentration (C) of the CdTe QDs could be calculated using Lambert–Beer’s Law: A =
Am (fwhm)PL K
A = εLC
(3) (4)
where A and Am are the calibrated and measured absorbance, respectively, (fwhm)PL is the full width at the half-maximum of
Green CdTe QDs (d = 2.3 nm, = 551 nm) with the PL quantum yield of ∼32% are used in our experiments (Fig. S1 in ESI). A thorough purification is necessary for constructing an ultrasensitive Cd2+ sensor, since it can remove the excessive TGA ligands as well as the residual unsaturated Cd2+ at the QDs surface, which may hamper the complexation of Phen with QDs or analyte (added Cd2+ ). At a concentration of 0.1 M, the CdTe QDs exhibit adequate PL intensity for high signal-to-noise ratio response, as well as low intensity of the first excitonic absorption peak for minimal self-absorption. Phen can switch off the intense PL of CdTe QDs. As shown in Fig. 1A, the PL of CdTe QDs is gradually quenched with the increasing concentration of Phen under excitation at 400 nm. A concentration of 1.2 M Phen presents a quenching effect of 70%. In addition, the PL excited-state lifetime of CdTe QDs is gradually reduced with the addition of Phen, as shown in Fig. 1B. When the molar ratio of Phen to QDs is increased to be 12:1, the PL excited-state lifetime of CdTe QDs is reduced from 27 ns to 13 ns. The Stern–Volmer plots representing the PL quenching effects of QDs by Phen are depicted in the inset of Fig. 1B. The intensity Stern–Volmer plot shows clear upward curvature, whilst the lifetime Stern–Volmer plot is linear and shows less quenching than the intensity data. This observation is indicative of a combined static
194
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Fig. 1. (A) Evolution of PL intensity of 0.1 M CdTe QDs with the Phen concentration. (B) PL decay curves of CdTe QDs (Black), CdTe QD–Phen with the molar ratios of 1:1 (Red), 1:6 (Green) and 1:12 (Blue), and CdTe QD–Phen (1:12) in the presence of 0.6 M Cd2+ (Violet). Inset: Stern–Volmer plots representing the quenching effects of Phen on CdTe QDs. I0 ( 0 ) and I () are the PL intensities (lifetimes) of CdTe QDs without/with Phen, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and dynamic quenching behavior [40,41]. The PL quenching effects of Phen on QDs can be analyzed by using Eqs. (5) and (6):
of the static Cd–Phen complex at the QDs surface, whilst the N 1s signal corresponds to the formation of dynamic Cd–Phen complex.
I0 I
=
(1 + KS CPhen ) (1 + KD CPhen )
(5)
3.2. PL switch on of CdTe QD–Phen by Cd2+
0
=
1 + KD CPhen
(6)
where I0 (or 0 ) and I (or ) are the PL intensity (or lifetime) of QDs without/with addition of Phen, respectively; KS and KD are static and dynamic quenching constants, respectively; CPhen is the concentration of added Phen. The obtained KS and KD are 4.3 × 105 L mol−1 and 7.7 × 105 L mol−1 , respectively. Such large values suggest that Phen has a strong quenching ability toward the PL of CdTe QDs. With the PL quenching data in Fig. 1, the relative fraction () of Phen that quenches the QDs PL with a dynamic mechanism due to the formation of dynamic QD–Phen complex can be calculated by using Eq. (7): =
I 0 I0
(7)
At the Phen-to-QDs molar ratio of 12:1, the value of is calculated as 65%. The relative fraction (1 − ) of Phen that quenches the QDs PL with a static mechanism due to the formation of groundstate QD–Phen complex is defined as 35%. The PL quenching are consistent with a gradual attachment of Phen onto the QDs surface by replacing the original TGA ligands, due to the affinity of the two nitrogen atoms of Phen with the surface Cd atoms of QDs. This hypothesis is confirmed by FTIR and XPS measurements. As shown in Fig. 2A, the FTIR spectrum of the precipitate of CdTe QD–Phen shows that the characteristic vibration peaks of aromatic rings (729 cm−1 , 1517 cm−1 ) appear compared to that of TGA-capped CdTe QDs. Fig. 2B presents XPS spectra of TGA-capped CdTe QDs and formed CdTe QD–Phen. The Cd 3d5/2 and N 1s signals are fitted using a Shiley-type background. For the TGA-capped CdTe QDs, the Cd 3d5/2 peak was fitted by three peaks located at 404.8, 404.3, and 403.4 eV, which can be attributed to cadmium binding with S, Te, and O, respectively [42]. The Cd–O coordination is probably caused by surface binding OH groups during purifications. These binding situations remain after the formation of CdTe QD–Phen. However, two additional signals appear at 405.3 and 399.0 eV, respectively. The former signal can be assigned to the Cd–N coordination [43], indicative of the formation
Cd2+ by itself does not affect the PL of TGA-capped CdTe QDs in a wide range of concentrations (Fig. S2 in the ESI). However, the PL of CdTe QD–Phen (the molar ratio of QDs to Phen is 1:12) is continuously restored with the increased concentration of added Cd2+ , as shown in Fig. 3A. The response of this fluorescent sensor is immediate, independent of the incubation time (Fig. S3 in the ESI). The solution changes from dark to bright green emission under a UV lamp which can be seen with the naked eye (inset of Fig. 3A). The detection limit of Cd2+ , calculated following the 3 IUPAC criteria is ∼0.01 nM, which is ∼3 orders-of-magnitude lower than the maximum residue limit of Cd2+ in drinking water allowed by EPA. Even at a Cd2+ concentration as low as 0.02 nM, a 21% enhancement of PL intensity can be clearly observed, demonstrating an ultrasensitive detection. A concentration of 0.6 M Cd2+ results in a 2.5-fold enhancement of PL intensity. At such a concentration, the standard deviation for six replicate measurements is less than 2.2%, indicative of a good repeatability (Fig. S4 in the ESI). Notably, the presence of the higher Cd2+ concentration such as 0.8 M cannot restore the PL of CdTe QD–Phen further. Fig. 3B shows the plot of PL restoration percentage versus the Cd2+ concentration. The PL restoration is proportional to the Cd2+ concentration, exhibiting two consecutive linear relationships: one in the range of 0.02 nM–0.1 M, and another steep linear regression in the range of 0.1–0.6 M. Therefore, the CdTe QD–Phen is suitable for quantitative analysis of Cd2+ residue in drinking water within a wide range of concentrations. The PL excited-state lifetime of CdTe QD–Phen can also be restored by the presence of Cd2+ . In the presence of 0.6 M Cd2+ , the PL excited-state lifetime is defined as ∼23 ns (Fig. 1B). The restored PL intensity and excited-state lifetime are consistent with a continuous detachment of Phen ligands from the QDs surface and subsequent coordination with added Cd2+ . This is confirmed by the FTIR and XPS spectra of the precipitates of the Cd2+ -presenting CdTe QD–Phen. The FTIR characteristic peaks of Phen are gradually reduced by the presence of the increased Cd2+ concentration, until they disappear when the PL of CdTe QD–Phen is fully restored (Fig. 2A). Meanwhile, at the condition of the maximum PL
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
195
Fig. 2. (A) FTIR spectra and (B) XPS spectra of CdTe QDs, CdTe QD–Phen, and the precipitates of CdTe QD–Phen by the presence of Cd2+ , respectively.
restoration, the peak of Cd–N coordination of Cd 3d5/2 signal and the N 1s signal also disappear in the XPS spectra (Fig. 2B). To estimate the binding constant of Phen toward Cd2+ , the PL restoration data in Fig. 3 are further analyzed by using Eqs. (8): 1 1 C + C = I Kb Imax Imax
(8)
where C is the Cd2+ concentration, I is the PL intensity of CdTe QD–Phen at a given Cd2+ concentration, Imax is the maximum PL intensity of CdTe QD–Phen restored by the presence of Cd2+ , and Kb is the binding constant. It shall be noted that such Kb is the binding constant of Phen toward added Cd2+ by competing with that of Phen toward with surface Cd atoms of CdTe QDs. The actual binding constant for Phen compounds toward Cd2+ in the absence of QDs is as high as 1.4 × 1011 L mol−1 [44]. As shown in Fig. 4, the dependence of C/I on C exhibits a good linear relationship in the entire measured range. The obtained Kb is 6.5 × 107 L mol−1 . Such a
high value is indicative of the much higher affinity of Phen toward added Cd2+ in solution than the surface Cd atoms of CdTe QDs. 3.3. Selectivity and reversibility of the present sensor for Cd2+ detection The PL changes of the CdTe QD–Phen sensor in the presence of various metal ions are investigated. As shown in Fig. 5, only the presence of Cd2+ causes the distinct PL switch-on of the sensor. The sensors either do not respond to certain metal ions (Li+ , K+ , Na+ , Ca2+ , Mg2+ , Cr2+ , Ni2+ , Pb2+ , Mn2+ , Ba2+ , Fe2+ , Al3+ , Fe3+ , Hg+ ) or suffer a PL quenching in the presence of transition and heavy metal ions (Co2+ , Ag+ , Cu2+ and Hg2+ ). The PL quenching by Co2+ , Ag+ , Cu2+ and Hg2+ is mainly due to the formation of surface electron transfer pathways from QDs to such metal ions [19]. Particularly, the presence of Zn2+ , which has similar electronic configuration and chemical properties as Cd2+ , cannot cause PL switch of the CdTe QD–Phen sensor in a wide range of concentrations (Fig. S5 in the
Fig. 3. (A) Evolution of PL spectra of 0.1 M CdTe QD–Phen (1:12) with the Cd2+ concentration. Inset: images of CdTe QD–Phen solutions in the absence/presence of 0.5 M Cd2+ under a UV lamp. (B) Relationship between the relative PL intensity (I/I0 ) and the Cd2+ concentration. I and I0 are the PL intensity of CdTe QD–Phen in the presence/absence of Cd2+ , respectively. Insets: linear correlations of the data included in the boxes.
196
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Fig. 4. Langmuir binding isotherm description of the data exhibiting a good linear relationship in the entire measured range of Cd2+ concentrations.
ESI). As shown in the inset of Fig. 5, the PL of CdTe QD–Phen in the presence of 0.5 M Cd2+ is hardly interfered by the coexistence of 20-fold Zn2+ (10 M). The good selectivity of the switch-on sensor toward Cd2+ over other metal ions including Zn2+ , is presumably due to either poor affinity of Phen with alkaline metal or alkaline earth metal ions [34,35], or the binding affinity with transition metal ions, e.g. Zn2+ , not adequate to break the complexation of Phen with surface Cd atoms of QDs. The reversibility of the CdTe QD–Phen sensor is also studied. After the maximum PL restoration of CdTe QD–Phen by the presence of Cd2+ , the solution can be isolated by centrifugation. By re-dissolving the precipitate in an aqueous medium, the solution still shows intense PL of CdTe QDs, which can be subsequently switched off by adding Phen. As shown in Fig. S6 in the ESI, Cd2+ response of the re-dispersed CdTe QD–Phen sensor appears to be satisfactorily reversible. 3.4. Mechanism of the fluorescent sensor The mechanism of chemical as well as electronic interactions underlying the fluorescent “Off-On” conversion of the CdTe
QD–Phen sensor is further clarified. Fig. 6 shows the PL evolution of the CdTe QD–Phen sensor under excitation at 290 nm (the maximum excitation wavelength of Phen). As shown in Fig. 6A, the ultraviolet emission peaked at 369 nm, which exhibits a 5 nm red-shift compared to that of free Phen, is ascribed to the PL of formed Cd–Phen complex at the QDs surface. The PL of Cd–Phen complex is gradually increased with the increased concentration of Phen. During the attachment of Phen, however, the shape and peak position of the quenched PL of CdTe QDs remains unaltered. In addition, the absorption of CdTe QD–Phen does not shift compared to TGA-capped CdTe QDs (Fig. S7 in the ESI). These observations suggest that Phen quenches the QDs PL primarily by an electronic interaction [29] rather than a “surface chemistry” interaction [24–28,30,31]. Under excitation at 290 nm, the CdTe QDs exhibit the same PL quenching rate as those excited at 400 nm, at which Phen does not absorb, indicative of a negligible energy transfer process from Phen ligands to CdTe QDs (Fig. S8 in the ESI). Typical CV measurements were conducted for estimating the energy levels of CdTe QDs and Phen (Fig. S9 in the ESI). The onset potentials of oxidation for CdTe QDs and Phen are 0.74 and 0.45 V, respectively, indicating that the valence band of CdTe QDs is ∼0.3 eV lower than the highest occupied molecular orbital of Phen. Therefore, the electronic reaction occurred between CdTe QDs and Phen is a PHT process: upon excitation, the photogenerated holes on the QDs preferentially transfer to the Phen ligands and become trapped, preventing the effective hole/electron recombination (the inset of Fig. 6A). In the presence of Cd2+ , the PL intensities of both Cd–Phen complex and CdTe QDs are continuously increased until the molar ratio of added Cd2+ to the Phen ligands reaches 1:2, as shown in Fig. 6B. Since the amount of Phen in the system is constant, the increased PL is obviously attributed to the cooperative chelation of Phen with added Cd2+ . In the sensing system, generally, the chelation of added Cd2+ occurs either at the QDs surface with Phen ligands, or in the solution with detached Phen. If in former case, the formed Cd2+ –Phen complexes still act as hole traps at the QDs surface, quenching the QDs PL. In contrast, the observed PL restoration of CdTe QDs reveals that Phen ligands are detached by added Cd2+ , and the originally replaced TGA ligands return to the QDs surface. As a consequence of the Phen detachment, the PHT process (dynamic) as well as the ground-state coordination (static) between CdTe QDs and Phen is interrupted (the inset of Fig. 6B). At a molar ratio of added Cd2+ to Phen higher than 1:2, the PL intensity of CdTe QDs as well as Cd2+ –Phen complexes cannot be further increased, indicating that the chelating stoichiometric ratio of Cd2+ to Phen is 1:2, i.e. the formation of [Cd(Phen)2 (H2 O)2 ]2+ . Therefore, the fluorescence switch strategy for Cd2+ detection by the present sensor is based on subsequent ligand-displacement reactions at the QDs surface, which can be briefly described as: Phen
Cd2+
CdTe@TGA −→ CdTe@Phen + TGA−→ CdTe@TGA + [Cd(Phen)2 (H2 O)2 ]2+
Fig. 5. Effects of various metal ions (0.5 M) on the PL intensity of CdTe QD–Phen (1:12) sensor. The inset shows the PL spectra of CdTe QD–Phen in the absence/presence of 10 M Zn2+ , and in the presence of 0.5 M Cd2+ without/with the coexistence of 10 M Zn2+ , respectively. I and I0 are the PL intensity of CdTe QD–Phen in the presence and absence of ions, respectively.
The difference between this strategy and those demonstrated previously [24–31] is further validated. Fig. 7 presents the PL evolution of Phen-complexed CdSe/ZnS core/shell QDs as a function of the concentration of Cd2+ as well as Zn2+ . Different with the CdTe QDs, the PL of CdSe/ZnS QDs can be restored in the presence of either Cd2+ or Zn2+ . This restored PL by such metal ions is in contrast to the “surface passivation” and “host-guest reaction” strategies [24–29], where the analyte-receptor complexation occurs at the QDs surface (Scheme 1A and B). It is most conceivable that the complexation of Phen with Cd2+ or Zn2+ at the QDs surface increases the hole traps, leading to PL quenching of QDs instead of PL restoration. As for the “chemical etching” strategy
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
197
Fig. 6. (A) PL spectra representing the quenching of 0.1 M CdTe QDs by Phen with the concentrations of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.2 M, and PL spectra of Phen and [Cd(Phen)2 (H2 O)2 ]2+ . Inset: PHT mechanism based on energy levels of green CdTe QDs versus Phen. (B) PL spectra representing the restoration of the CdTe QD–Phen sensor by Cd2+ with the concentrations of 0.1, 0.5, 1.0, 2.0, 20.0, 100.0, 200.0, 300.0, 400.0, 600.0 and 800.0 nM. Inset: the interrupted PHT process by the detachment of Phen from the QDs surface and the formation of [Cd(Phen)2 (H2 O)2 ]2+ in solution; excitation: 290 nm.
(Scheme 1C), the complexation of a chelating reagent (in our case, Phen) with the shell Zn atoms generates “Zn2+ -imprinted” cavities on the surface of CdSe/ZnS QDs, and only added Zn2+ can repair the surface defects, restoring the QDs PL [30,31]. On contrary, a concentration of 20 M Zn2+ only slightly restores the PL of CdSe/ZnS QDs, while at the identical concentration, the presence of Cd2+ results in a full restoration of the QDs PL (the inset of Fig. 7). This observation is in accordance with the ligand displacement-induced PL switch strategy, where the detachment of ligand is highly dependent on its affinity with metal ions. Although the chelation of Phen with added Cd2+ as well as Zn2+ can break the Zn–Phen complexation at the surface of CdSe/ZnS QDs, Phen has much higher affinity toward added Cd2+ than Zn2+ .
3.5. Analytical performance of the CdTe QD–Phen sensor The analytical performance of QDs-based fluorescent sensors is dependent on the change of the surface states of QDs. In our strategy, the subtle change of surface states of QDs is a result of the ligand (receptor) displacement reaction, which is highly dependent on its relative affinity with metal ions (analyte) against surface metal atoms of QDs. On basis of this strategy, the present CdTe QD–Phen sensor has realized ultrasensitive detection of Cd2+ with wide linear ranges, and for the first time, excellent selectivity for Cd2+ over Zn2+ , as listed in Table 1.
Fig. 7. (A) Evolution of PL spectra of CdSe/ZnS QD–Phen with the Cd2+ concentration. The insets show the structure of CdSe/ZnS QD–Phen sensor and the relationship of relative PL intensity (I/I0 ) of CdSe/ZnS QD–Phen with the concentration of Cd2+ as well as Zn2+ . I and I0 are the PL intensities of CdSe/ZnS QD–Phen in the presence/absence of Cd2+ or Zn2+ , respectively.
Table 1 Comparison of chemical and electronic reactions as well as analytical parameters of the representative QDs-based sensors for Cd2+ detection. References
QDs
Receptor
Chemical reaction
Electronic process
Detection limit
Linear range
Selectivity
This study [24–28] [29] [30,31]
CdTe CdTe CdS:Mn/ZnS CdTe, CdTe/CdS
Phen S2− , SO3 2− 1,10-Diaza-18-Cr-6 EDTA, APDC
Ligand displacement Surface complexation Host–guest reaction Ion exchange
PHT Surface passivation PET Surface passivation
0.01 nM 5–12 nM N/A 6–10 nM
0.02 nM–0.6 M 0.1–15 M N/A 0.05–1 M
Cd2+ Cd2+ , Zn2+ Cd2+ , Zn2+ Cd2+ , Zn2+
Table 2 Real application of the proposed sensor in water samplesa . Samples
Added (nM)
By this sensor (nM)
By the AAS method (nM)
Tap water 1 Tap water 2 Spring water 1 Spring water 2
3.70 114.00 3.70 81.00
3.76 ± 0.08 114.55 ± 1.27 3.66 ± 0.05 82.13 ± 0.70
3.71 ± 0.03 114.32 ± 0.72 3.69 ± 0.02 81.80 ± 0.52
a
All values are reported as average of three determinations ± standard deviation.
198
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
The analytical performance of the present sensor is further evaluated in practical detection of drinking water. Tap water and spring water are used in our experiments. All samples are prepared by spiking with Cd2+ at diverse concentration levels, and analyzed by this fluorescent sensor and the standard AAS method independently. As summarized in Table 2, all results obtained by this sensor are in good agreement with those measured by the AAS method. As is known that many metal ions, as referred in Fig. 5, such as Ca2+ , Mg2+ , Mn2+ , Cu2+ , and Zn2+ , may be present in drinking water. For instance, the maximum allowed concentration levels are 15.7 M for Cu2+ and 15.3 M for Zn2+ , respectively. Our results of the determinations of drinking water samples show that the presence of the relative high concentration levels of such metal ions does not interfere with the detection of trace Cd2+ , owing to the ion-dependent affinity of Phen at the QDs surface.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusions [17]
In summary, this work has demonstrated a surface complexation-originated PHT mechanism of CdTe QDs through the attachment of Phen onto QDs by the formation of Cd–Phen complex at the surface, and a PL switch-on mechanism through the detachment of Phen by the formation of [Cd(Phen)2 (H2 O)2 ]2+ in the solution. This simple CdTe QDs-based fluorescent sensor has achieved ultrasensitive detection of Cd2+ , obtaining the lowest detection limit reported so far. Importantly, by utilizing the iondependent chelating ability of Phen (receptor), this sensor features a distinct discrimination of Cd2+ versus Zn2+ , and succeeds in real water samples. Moreover, this sensor is robust, and exhibits good reversibility and repeatability. Therefore, the QDs-based sensor demonstrated here provides a facile, simple and low cost approach for quantitative determination of Cd2+ residue in drinking water as well as in food.
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Acknowledgments
[34] [35] [36]
This work was funded by NSFC and MOE (Grants: 21005017, 21375015, and 20100092120037).
[37]
Appendix A. Supplementary data
[38] [39]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.01.006.
[40] [41] [42]
References [1] R. Hernández-Martínez, I. Navarro-Blasco, Food Control 26 (2012) 6–14. [2] T. Schwerdtle, F. Ebert, C. Thuy, C. Richter, L.H.F. Mullenders, A. Hartwig, Chem. Res. Toxicol. 23 (2010) 432–442.
[43] [44]
G. Kazantzis, Am. J. Ind. Med. 20 (1991) 701–704. I.A. Darwish, D.A. Blake, Anal. Chem. 74 (2002) 52–58. Y. Wang, Y.H. Wang, Z.L. Fang, Anal. Chem. 77 (2005) 5396–5401. M. Suzuki, K. Ohta, Anal. Chem. 54 (1982) 1682–1689. R. Laocharoensuk, A. Bulbarello, S.B. Hocevar, S. Mannino, B. Ogorevc, J. Wang, J. Am. Chem. Soc. 129 (2007) 7774–7775. T. Shtoyko, S. Conklin, A.T. Maghasi, A. Piruska, J.N. Richardson, C.J. Seliskar, W.R. Heineman, Anal. Chem. 76 (2004) 1466–1473. S.P. Dolan, D.A. Nortrup, P.M. Bolger, S.G.J. Capar, Agric. Food Chem. 51 (2003) 1307–1312. K. Tsuji, K. Nakano, Anal. Chem. 84 (2012) 636–668. J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, Trends Anal. Chem. 25 (2006) 207–218. M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013–2015. W.C.W. Chan, S. Nie, Science 281 (1998) 2016–2018. I.L. Medintz, A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nat. Mater. 2 (2003) 630–638. Q.J. Sun, Y.A. Wang, L.S. Li, D.Y. Wang, T. Zhu, J. Xu, C.H. Yang, Y.F. Li, Nat. Photonics 1 (2007) 717–722. Q.J. Sun, S. Guru, L.M. Dai, C. Michael, C. Angela, N. Rajesh, G. James, Y.Q. Wang, ACS Nano 3 (2009) 737–743. X. Michalet, F. Pinaud, T.D. Lacoste, M. Dahan, M.P. Bruchez, A.P. Alivisatos, S. Weiss, Single Mol. 2 (2001) 261–276. D. Knopp, D.P. Tang, R. Niessner, Anal. Chim. Acta 647 (2009) 14–30. R. Freeman, T. Finder, I. Willner, Angew. Chem. Int. Ed. 48 (2009) 7818–7821. Y. Chen, Z. Rosenzweig, Anal. Chem. 74 (2002) 5132–5138. M.J. Ruedas-Rama, E.A.H. Hall, Anal. Chem. 80 (2008) 8260–8268. J. Völker, X. Zhou, X. Ma, S. Flessau, H. Lin, M. Schmittel, A. Mews, Angew. Chem. Int. Ed. 49 (2010) 6865–6868. E.M. Nolan, S.J. Lippard, Chem. Rev. 108 (2008) 3443–3480. M. Algarra, B.B. Campos, B. Alonso, M.S. Miranda, A.M. Martinez, C.M. Casado, J.C.G. Esteves da Silva, Talanta 88 (2012) 403–407. H.B. Li, Y. Zhang, X.Q. Wang, Sens. Actuators, B 127 (2007) 593–597. Y.L. Li, J. Zhou, C.L. Liu, H.B. Li, J. Mater. Chem. 22 (2012) 2507–2511. R. Gui, X. An, W. Huang, Anal. Chim. Acta 767 (2013) 134–140. H. Xu, R. Miao, Z. Fang, X. Zhong, Anal. Chim. Acta 687 (2011) 82–88. S. Banerjee, S. Kar, S. Santra, Chem. Commun. (2008) 3037–3039. P. Wu, X.P. Yan, Chem. Commun. 46 (2010) 7046–7048. R. Gui, X. An, H. Su, W. Shen, Z. Chen, X. Wang, Talanta 94 (2012) 257–262. S. Dobson, Cadmium-Environmental Aspects, World Health Organization, Geneva, 1992. L. Friberg, C.G. Elinger, T. Kjellstrom, Cadmium, World Health Organization, Geneva, 1992. P.G. Sammes, G. Yahioglu, Chem. Soc. Rev. 23 (1994) 327–334. A. Bencini, V. Lippolis, Coord. Chem. Rev. 254 (2010) 2096–2180. L. Zhang, K. Zhu, T. Ding, X.Y. Hu, Q.J. Sun, C.X. Xu, Analyst 138 (2013) 887–893. A.L. Rogach, T. Franzl, T.A. Klar, J. Feldmann, N. Gaponik, V. Lesnyak, A. Shavel, A. Eychmu1ller, Y.P. Rakovich, J.F. Donegan, J. Phys. Chem. C 111 (2007) 14628–14637. W.W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854–2860. J.J. Li, Y.A. Wang, W. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, J. Am. Chem. Soc. 125 (2003) 12567–12575. J.R. Lakwicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Press, New York, NY, 1999. I. Yildiz, M. Tomasulo, F.M. Raymo, PNAS 103 (2006) 11457–11460. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenburg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. L. Jing, C. Yang, R. Qiao, M. Niu, M. Du, D. Wang, M. Gao, Chem. Mater. 22 (2010) 420–427. Z.S. Yang, J.S. Yu, H.Y. Chen, Chin. J. Inorg. Chem. 4 (2002) 373–377.
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Ligand displacement-induced fluorescence switch of quantum dots for ultrasensitive detection of cadmium ions Xianyun Hu a,b,1 , Kao Zhu a,1 , Qingsheng Guo a , Yuqian Liu a , Mingfu Ye a , Qingjiang Sun a,∗ a b
State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing 210096, China Qiannan Medical College for Nationalities, Duyun 558000, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A simple CdTe QD–Phen sensor is constructed for detecting Cd2+ .
• The sensor is operated with a liganddisplacement induced PL switch strategy. • The detection limit of 0.01 nM for Cd2+ is achieved. • This sensor features to discriminate Cd2+ versus Zn2+ , and succeeds in real water samples.
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 31 December 2013 Accepted 5 January 2014 Available online 11 January 2014 Keywords: Quantum dot 1,10-Phenanthroline Photoluminescence Sensor Ligand displacement Cadmium ions
a b s t r a c t This paper reports the construction of a simple CdTe quantum dots (QDs)-based sensor with 1,10phenanthroline (Phen) as ligand, and the demonstration of a novel ligand displacement-induced fluorescence switch strategy for sensitive and selective detection of Cd2+ in aqueous phase. The complexation of Phen at the surface quenches the green photoluminescence (PL) of QDs dominated by a photoinduced hole transfer (PHT) mechanism. In the presence of Cd2+ , the Phen ligands are readily detached from the surface of CdTe QDs, forming [Cd(Phen)2 (H2 O)2 ]2+ in solution, and as a consequence the PL of CdTe QDs switches on. The detection limit for Cd2+ is defined as ∼0.01 nM, which is far below the maximum Cd2+ residue limit of drinking water allowed by the U.S. Environmental Protection Agency (EPA). Two consecutive linear ranges allow a wide determination of Cd2+ from 0.02 nM to 0.6 M. Importantly, this CdTe QDs-based sensor features to distinctly discriminate between Cd2+ and Zn2+ , and succeeds in real water samples. This extremely simple strategy reported here represents an attempt for the development of fluorescent sensors for ultrasensitive chemo/biodetection. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Agricultural activities such as the excessive use of fertilizers and pesticides, irrigation with waste water, industrial applications (nickel–cadmium batteries, dyes and pigments, coating of steel, various alloys) and urban life increase the content of cadmium in soils and waters. As a consequence of food chain system, cadmium exposure can cause anemia, abdominal pain, neurological
∗ Corresponding author. Tel.: +86 25 83790920; fax: +86 25 83792349. E-mail address: [email protected] (Q. Sun). 1 The authors contribute equally. 0003-2670/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2014.01.006
and adverse developmental effects, kidney damage, hypertension, changes in vitamin D metabolism, and even increased risk of cancer [1–3]. The International Agency for Research on Cancer has classified cadmium as category 1 carcinogens [4]. In the United States, the acute and chronic exposure criteria for dissolved cadmium (Cd2+ ) in drinking water are 3.7 and 1.0 g L−1 (32.9 and 8.9 nM), respectively. Therefore, in view of its toxic effects on environment as well as human health, the monitoring of chemical safety through Cd2+ analysis of the most widely used resources, especially drinking water, is of paramount importance. Of various analytical methods [5–10], the most common one still remain the use of fluorescence, which indicates the presence of an analyte by fluorescent changes that can be observed by naked eye or by simple measurements.
192
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Scheme 1. QDs based surface complexation ((A)–(C)) and proposed ligand displacement (D)-induced PL switch strategies for Cd2+ detection.
While a variety of synthetic organic-fluorophore receptors for Cd2+ assay have been designed [11], they often suffer from low solubility in water, low sensitivity, and poor photostability, which restricts their practical applications. Quantum dots (QDs) are a recently developed class of inorganic fluorophores with unique spectral properties including high PL quantum yields and exceptional resistance to photobleaching [12–16]. Recent advances in surface modification of QDs with appropriate receptors, able to interact selectively with an analyte, allow for greater analytical sensitivity in a wide variety of sensing schemes [17,18]. In the past few years, researchers have made efforts toward the development of QDs-based fluorescent sensors for detecting heavy metal ions including Cd2+ [19–22]. Of the fluorescent sensors, a PL switch-on mode, with the reduced chance of false positives, is more preferable than a PL switch-off mode [23]. So far, only a few of the QDs-based PL switch-on sensors for detecting Cd2+ have been reported, which can be classified into three strategies by the analyte-receptor complexation reactions, namely, “surface passivation” [24–28], “host–guest reaction” [29], and “chemical etching” [30,31], as shown in Scheme 1A–C. In these sensors, different Cd2+ receptors such as anions (sulfur or sulfite), N-containing crown ethers (1,10-diaza-18-crown-6), and chelating reagents (ethylene diamine tetraacetic acid, EDTA, or ammonium pyrrolidine dithiocarbamate, APDC) are appended onto the QDs surface, quenching the PL of QDs. Upon exposure to Cd2+ , the analyte-receptor complexation reactions occur at the surface of QDs, which block the non-radiative recombination pathway, inducing the restored PL of QDs. These sensors have achieved the lowest detection limit of 6–10 nM for Cd2+ [30]. However, the detection limit needs to be further lowered for quantitative assay of Cd2+ in drinking water, according to the standards of EPA. In addition, the QDs-based Cd2+ sensors all suffer from the interference of Zn2+ . The
formation of a ZnS passivation layer, or complexation of Zn2+ with N-containing crown ether, EDTA or APDC at the QDs surface can also restore the PL of QDs to some extent [24–31]. The ability of a sensor to distinctly discriminate between Cd2+ and Zn2+ is important because these ions are often found together in nature [32,33]. Therefore, there is an urgent demand to develop novel fluorescent sensors based on QDs for sensitive and selective detection of Cd2+ . Herein, we report an extremely simple PL switch strategy for ultrasensitive detection of Cd2+ , with CdTe QDs as the fluorescent reporter and Phen as the Cd2+ receptor as well as the QDs ligand. Phen is chosen to construct the fluorescent sensor primarily because it is a classic chelating bidentate ligand whose nitrogen atoms are beautifully placed to act cooperatively in binding with metals [34,35]. As illustrated in Scheme 1D, the CdTe QD–Phen sensor is proposed to operate with a novel ligand (receptor) displacement-induced PL switch strategy. In the absence of Cd2+ , the quenched PL of CdTe QDs is mainly ascribed to a PHT process from QDs to the Phen ligand [22,29,36]. Only in the presence of Cd2+ , the chelation of Phen with Cd2+ detaches the Phen ligand from the QDs surface, interrupting the PHT process and switching on the PL of QDs. On basis of this strategy, this sensor has demonstrated to feature excellent selectivity for Cd2+ over Zn2+ . 2. Experimental 2.1. Materials and reagents All the starting materials of synthesis of Thiolglycolic acid (TGA)capped CdTe QDs and mercaptopropionic acid (MPA)-capped CdSe/ZnS core/shell QDs were used without further purification. Chloroform (AR) and anhydrous ethanol (AR) were purchased from NCRC and used for purification of QDs. Various metal salts (>99.99%)
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
were purchased from Sigma-Aldrich. 1,10-Phenanthroline (99%) was purchased from J&K. The pure water ( > 18 M cm−1 ) was obtained from a Pall Cascade AN synthesis system. 2.2. Instruments
193
PL spectrum, K is the average (fwhm)PL , (K was defined as 29 nm), and L (cm) is the path length of the quartz cuvette (L was fixed at 1 cm). 2.5. PL spectra measurements
UV/Vis absorption spectra were performed on a Hitachi U4100 spectrophotometer. PL spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. PL lifetime measurements were performed on a Horiba Jobin Yvon Time-Resolved fluorescence Spectrophotometer. Ultracentrifugation experiments were performed on Optima L-100K ultracentrifuge with the speed of 50,000 rpm (rotations per minute). Fourier Transform Infrared (FTIR) spectra were acquired with Nicolet5700 instrument. Xray photoelectron spectroscopy (XPS) was measured with a PHI 5000 VersaProbe spectrometer (ULVAC-PHI, Inc.) equipped with a monochromatic Al K␣ source at room temperature. Cyclic voltammetry (CV) measurements were performed on a CHI-660D electrochemical workstation, with a typical three-electrode system. Atomic absorption spectroscopy (AAS) analysis was conducted in a Hitachi 180-80 atomic absorption spectrometer. The details in FTIR, XPS, AAS and CV measurements were described in the Supporting information.
All PL spectra measurements were performed under ambient condition, with samples measured generally 5 min later after mixing. For PL quenching measurements of CdTe QDs, the samples were prepared by adding 100 L of aqueous solutions of Phen with concentrations of 0.1–9.0 M into an equal volume of 200 nM green CdTe QDs solutions, respectively. For CdSe/ZnS QDs, 100 L of Phen solutions with concentrations of 3.33–333.0 M were added into an equal volume of CdSe/ZnS QDs solutions, respectively. For PL restoration measurements of CdTe QDs, 100 L of Cd2+ solutions with concentrations of 0.02 nM–4 M were added into an equal volume of 200 nM green CdTe QD–Phen (the molar ratio of QD to Phen was 1:12) solutions. For CdSe/ZnS QDs, 100 L of Cd2+ solutions with concentrations of 1.33–200.0 M were added into an equal volume of CdSe/ZnS QD–Phen (the fixed concentration of QDs mixed with 40 M Phen) solutions, respectively. 2.6. Selectivity and interference measurements
2.3. Synthesis and purification of water soluble TGA-capped CdTe QDs and MPA-capped CdSe/ZnS QDs Water-soluble TGA-capped CdTe QDs were prepared by a 1.5 h reflux process [37,38]. The typical molar ratio of Cd:Te:TGA was 1:0.5:1.3. The synthesis of green OA-capped CdSe/ZnS quantum dots followed the SILAR method [39]. Water soluble CdSe/ZnS QDs were prepared by replacing the original OA ligands with MPA. The as-prepared CdTe QDs or CdSe/ZnS QDs were subject to thorough purifications. With CdTe QDs as example, 5 mL aqueous solution of as-prepared QDs was mixed with anhydrous ethanol (1:3) and subsequently centrifuged at 10,000 rpm for 10 min. The clear solution of supernatant was discarded, and the precipitate was re-dispersed in pure water. This purification process was repeated for four times, and the final CdTe QDs precipitate could be readily re-dispersed in water with good colloidal stability.
The following metal ions were used for the selectivity experiment: Zn2+ , Li+ , K+ , Na+ , Ca2+ , Mg2+ , Cr2+ , Mn2+ , Ba2+ , Fe2+ , Cu2+ , Co2+ , Ni2+ , Pb2+ , Al3+ , Fe3+ , Hg+ , Ag+ , and Hg2+ . Aqueous solutions of various metal ions with identical concentration (1.0 M) were prepared from stock solutions by serial dilution, and subsequently added into an equal volume of 200 nM green CdTe QD–Phen (the molar ratio of QDs to Phen is 1:12) solutions, respectively. For experiments of the Zn2+ interference, 100 L of Zn2+ solutions with concentrations of 0.04–40.0 M were added into an equal volume of 200 nM green CdTe QD–Phen (1:12) solutions, respectively; and 100 L of Zn2+ solutions with concentration ranges of 1.33–267.0 M were added into an equal volume of CdSe/ZnS QD–Phen solutions, respectively.
3. Results and discussion 2.4. Calculation of the size and concentration of CdTe QDs 3.1. PL switch off of CdTe QDs by Phen The size and concentration of CdTe QDs were calculated according to the method proposed by Peng et al. [38]. Firstly, the particle size could be determined from the wavelength of the first excitonic absorption peak with Eq. (1):
D = 9.8127 × 10−7 max 3 − 1.7147 × 10−3 max 2 + (1.0064) max − (194.84)
(1)
where D (nm) is the size of CdTe QDs, max (nm) is the wavelength of the first excitonic absorption peak of CdTe QDs (503 nm for green CdTe QDs). Then, the extinction coefficient per mole of CdTe QDs (ε) could be obtained from Eq. (2): ε = 10043D2.12
(2)
Finally, the concentration (C) of the CdTe QDs could be calculated using Lambert–Beer’s Law: A =
Am (fwhm)PL K
A = εLC
(3) (4)
where A and Am are the calibrated and measured absorbance, respectively, (fwhm)PL is the full width at the half-maximum of
Green CdTe QDs (d = 2.3 nm, = 551 nm) with the PL quantum yield of ∼32% are used in our experiments (Fig. S1 in ESI). A thorough purification is necessary for constructing an ultrasensitive Cd2+ sensor, since it can remove the excessive TGA ligands as well as the residual unsaturated Cd2+ at the QDs surface, which may hamper the complexation of Phen with QDs or analyte (added Cd2+ ). At a concentration of 0.1 M, the CdTe QDs exhibit adequate PL intensity for high signal-to-noise ratio response, as well as low intensity of the first excitonic absorption peak for minimal self-absorption. Phen can switch off the intense PL of CdTe QDs. As shown in Fig. 1A, the PL of CdTe QDs is gradually quenched with the increasing concentration of Phen under excitation at 400 nm. A concentration of 1.2 M Phen presents a quenching effect of 70%. In addition, the PL excited-state lifetime of CdTe QDs is gradually reduced with the addition of Phen, as shown in Fig. 1B. When the molar ratio of Phen to QDs is increased to be 12:1, the PL excited-state lifetime of CdTe QDs is reduced from 27 ns to 13 ns. The Stern–Volmer plots representing the PL quenching effects of QDs by Phen are depicted in the inset of Fig. 1B. The intensity Stern–Volmer plot shows clear upward curvature, whilst the lifetime Stern–Volmer plot is linear and shows less quenching than the intensity data. This observation is indicative of a combined static
194
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Fig. 1. (A) Evolution of PL intensity of 0.1 M CdTe QDs with the Phen concentration. (B) PL decay curves of CdTe QDs (Black), CdTe QD–Phen with the molar ratios of 1:1 (Red), 1:6 (Green) and 1:12 (Blue), and CdTe QD–Phen (1:12) in the presence of 0.6 M Cd2+ (Violet). Inset: Stern–Volmer plots representing the quenching effects of Phen on CdTe QDs. I0 ( 0 ) and I () are the PL intensities (lifetimes) of CdTe QDs without/with Phen, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and dynamic quenching behavior [40,41]. The PL quenching effects of Phen on QDs can be analyzed by using Eqs. (5) and (6):
of the static Cd–Phen complex at the QDs surface, whilst the N 1s signal corresponds to the formation of dynamic Cd–Phen complex.
I0 I
=
(1 + KS CPhen ) (1 + KD CPhen )
(5)
3.2. PL switch on of CdTe QD–Phen by Cd2+
0
=
1 + KD CPhen
(6)
where I0 (or 0 ) and I (or ) are the PL intensity (or lifetime) of QDs without/with addition of Phen, respectively; KS and KD are static and dynamic quenching constants, respectively; CPhen is the concentration of added Phen. The obtained KS and KD are 4.3 × 105 L mol−1 and 7.7 × 105 L mol−1 , respectively. Such large values suggest that Phen has a strong quenching ability toward the PL of CdTe QDs. With the PL quenching data in Fig. 1, the relative fraction () of Phen that quenches the QDs PL with a dynamic mechanism due to the formation of dynamic QD–Phen complex can be calculated by using Eq. (7): =
I 0 I0
(7)
At the Phen-to-QDs molar ratio of 12:1, the value of is calculated as 65%. The relative fraction (1 − ) of Phen that quenches the QDs PL with a static mechanism due to the formation of groundstate QD–Phen complex is defined as 35%. The PL quenching are consistent with a gradual attachment of Phen onto the QDs surface by replacing the original TGA ligands, due to the affinity of the two nitrogen atoms of Phen with the surface Cd atoms of QDs. This hypothesis is confirmed by FTIR and XPS measurements. As shown in Fig. 2A, the FTIR spectrum of the precipitate of CdTe QD–Phen shows that the characteristic vibration peaks of aromatic rings (729 cm−1 , 1517 cm−1 ) appear compared to that of TGA-capped CdTe QDs. Fig. 2B presents XPS spectra of TGA-capped CdTe QDs and formed CdTe QD–Phen. The Cd 3d5/2 and N 1s signals are fitted using a Shiley-type background. For the TGA-capped CdTe QDs, the Cd 3d5/2 peak was fitted by three peaks located at 404.8, 404.3, and 403.4 eV, which can be attributed to cadmium binding with S, Te, and O, respectively [42]. The Cd–O coordination is probably caused by surface binding OH groups during purifications. These binding situations remain after the formation of CdTe QD–Phen. However, two additional signals appear at 405.3 and 399.0 eV, respectively. The former signal can be assigned to the Cd–N coordination [43], indicative of the formation
Cd2+ by itself does not affect the PL of TGA-capped CdTe QDs in a wide range of concentrations (Fig. S2 in the ESI). However, the PL of CdTe QD–Phen (the molar ratio of QDs to Phen is 1:12) is continuously restored with the increased concentration of added Cd2+ , as shown in Fig. 3A. The response of this fluorescent sensor is immediate, independent of the incubation time (Fig. S3 in the ESI). The solution changes from dark to bright green emission under a UV lamp which can be seen with the naked eye (inset of Fig. 3A). The detection limit of Cd2+ , calculated following the 3 IUPAC criteria is ∼0.01 nM, which is ∼3 orders-of-magnitude lower than the maximum residue limit of Cd2+ in drinking water allowed by EPA. Even at a Cd2+ concentration as low as 0.02 nM, a 21% enhancement of PL intensity can be clearly observed, demonstrating an ultrasensitive detection. A concentration of 0.6 M Cd2+ results in a 2.5-fold enhancement of PL intensity. At such a concentration, the standard deviation for six replicate measurements is less than 2.2%, indicative of a good repeatability (Fig. S4 in the ESI). Notably, the presence of the higher Cd2+ concentration such as 0.8 M cannot restore the PL of CdTe QD–Phen further. Fig. 3B shows the plot of PL restoration percentage versus the Cd2+ concentration. The PL restoration is proportional to the Cd2+ concentration, exhibiting two consecutive linear relationships: one in the range of 0.02 nM–0.1 M, and another steep linear regression in the range of 0.1–0.6 M. Therefore, the CdTe QD–Phen is suitable for quantitative analysis of Cd2+ residue in drinking water within a wide range of concentrations. The PL excited-state lifetime of CdTe QD–Phen can also be restored by the presence of Cd2+ . In the presence of 0.6 M Cd2+ , the PL excited-state lifetime is defined as ∼23 ns (Fig. 1B). The restored PL intensity and excited-state lifetime are consistent with a continuous detachment of Phen ligands from the QDs surface and subsequent coordination with added Cd2+ . This is confirmed by the FTIR and XPS spectra of the precipitates of the Cd2+ -presenting CdTe QD–Phen. The FTIR characteristic peaks of Phen are gradually reduced by the presence of the increased Cd2+ concentration, until they disappear when the PL of CdTe QD–Phen is fully restored (Fig. 2A). Meanwhile, at the condition of the maximum PL
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
195
Fig. 2. (A) FTIR spectra and (B) XPS spectra of CdTe QDs, CdTe QD–Phen, and the precipitates of CdTe QD–Phen by the presence of Cd2+ , respectively.
restoration, the peak of Cd–N coordination of Cd 3d5/2 signal and the N 1s signal also disappear in the XPS spectra (Fig. 2B). To estimate the binding constant of Phen toward Cd2+ , the PL restoration data in Fig. 3 are further analyzed by using Eqs. (8): 1 1 C + C = I Kb Imax Imax
(8)
where C is the Cd2+ concentration, I is the PL intensity of CdTe QD–Phen at a given Cd2+ concentration, Imax is the maximum PL intensity of CdTe QD–Phen restored by the presence of Cd2+ , and Kb is the binding constant. It shall be noted that such Kb is the binding constant of Phen toward added Cd2+ by competing with that of Phen toward with surface Cd atoms of CdTe QDs. The actual binding constant for Phen compounds toward Cd2+ in the absence of QDs is as high as 1.4 × 1011 L mol−1 [44]. As shown in Fig. 4, the dependence of C/I on C exhibits a good linear relationship in the entire measured range. The obtained Kb is 6.5 × 107 L mol−1 . Such a
high value is indicative of the much higher affinity of Phen toward added Cd2+ in solution than the surface Cd atoms of CdTe QDs. 3.3. Selectivity and reversibility of the present sensor for Cd2+ detection The PL changes of the CdTe QD–Phen sensor in the presence of various metal ions are investigated. As shown in Fig. 5, only the presence of Cd2+ causes the distinct PL switch-on of the sensor. The sensors either do not respond to certain metal ions (Li+ , K+ , Na+ , Ca2+ , Mg2+ , Cr2+ , Ni2+ , Pb2+ , Mn2+ , Ba2+ , Fe2+ , Al3+ , Fe3+ , Hg+ ) or suffer a PL quenching in the presence of transition and heavy metal ions (Co2+ , Ag+ , Cu2+ and Hg2+ ). The PL quenching by Co2+ , Ag+ , Cu2+ and Hg2+ is mainly due to the formation of surface electron transfer pathways from QDs to such metal ions [19]. Particularly, the presence of Zn2+ , which has similar electronic configuration and chemical properties as Cd2+ , cannot cause PL switch of the CdTe QD–Phen sensor in a wide range of concentrations (Fig. S5 in the
Fig. 3. (A) Evolution of PL spectra of 0.1 M CdTe QD–Phen (1:12) with the Cd2+ concentration. Inset: images of CdTe QD–Phen solutions in the absence/presence of 0.5 M Cd2+ under a UV lamp. (B) Relationship between the relative PL intensity (I/I0 ) and the Cd2+ concentration. I and I0 are the PL intensity of CdTe QD–Phen in the presence/absence of Cd2+ , respectively. Insets: linear correlations of the data included in the boxes.
196
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
Fig. 4. Langmuir binding isotherm description of the data exhibiting a good linear relationship in the entire measured range of Cd2+ concentrations.
ESI). As shown in the inset of Fig. 5, the PL of CdTe QD–Phen in the presence of 0.5 M Cd2+ is hardly interfered by the coexistence of 20-fold Zn2+ (10 M). The good selectivity of the switch-on sensor toward Cd2+ over other metal ions including Zn2+ , is presumably due to either poor affinity of Phen with alkaline metal or alkaline earth metal ions [34,35], or the binding affinity with transition metal ions, e.g. Zn2+ , not adequate to break the complexation of Phen with surface Cd atoms of QDs. The reversibility of the CdTe QD–Phen sensor is also studied. After the maximum PL restoration of CdTe QD–Phen by the presence of Cd2+ , the solution can be isolated by centrifugation. By re-dissolving the precipitate in an aqueous medium, the solution still shows intense PL of CdTe QDs, which can be subsequently switched off by adding Phen. As shown in Fig. S6 in the ESI, Cd2+ response of the re-dispersed CdTe QD–Phen sensor appears to be satisfactorily reversible. 3.4. Mechanism of the fluorescent sensor The mechanism of chemical as well as electronic interactions underlying the fluorescent “Off-On” conversion of the CdTe
QD–Phen sensor is further clarified. Fig. 6 shows the PL evolution of the CdTe QD–Phen sensor under excitation at 290 nm (the maximum excitation wavelength of Phen). As shown in Fig. 6A, the ultraviolet emission peaked at 369 nm, which exhibits a 5 nm red-shift compared to that of free Phen, is ascribed to the PL of formed Cd–Phen complex at the QDs surface. The PL of Cd–Phen complex is gradually increased with the increased concentration of Phen. During the attachment of Phen, however, the shape and peak position of the quenched PL of CdTe QDs remains unaltered. In addition, the absorption of CdTe QD–Phen does not shift compared to TGA-capped CdTe QDs (Fig. S7 in the ESI). These observations suggest that Phen quenches the QDs PL primarily by an electronic interaction [29] rather than a “surface chemistry” interaction [24–28,30,31]. Under excitation at 290 nm, the CdTe QDs exhibit the same PL quenching rate as those excited at 400 nm, at which Phen does not absorb, indicative of a negligible energy transfer process from Phen ligands to CdTe QDs (Fig. S8 in the ESI). Typical CV measurements were conducted for estimating the energy levels of CdTe QDs and Phen (Fig. S9 in the ESI). The onset potentials of oxidation for CdTe QDs and Phen are 0.74 and 0.45 V, respectively, indicating that the valence band of CdTe QDs is ∼0.3 eV lower than the highest occupied molecular orbital of Phen. Therefore, the electronic reaction occurred between CdTe QDs and Phen is a PHT process: upon excitation, the photogenerated holes on the QDs preferentially transfer to the Phen ligands and become trapped, preventing the effective hole/electron recombination (the inset of Fig. 6A). In the presence of Cd2+ , the PL intensities of both Cd–Phen complex and CdTe QDs are continuously increased until the molar ratio of added Cd2+ to the Phen ligands reaches 1:2, as shown in Fig. 6B. Since the amount of Phen in the system is constant, the increased PL is obviously attributed to the cooperative chelation of Phen with added Cd2+ . In the sensing system, generally, the chelation of added Cd2+ occurs either at the QDs surface with Phen ligands, or in the solution with detached Phen. If in former case, the formed Cd2+ –Phen complexes still act as hole traps at the QDs surface, quenching the QDs PL. In contrast, the observed PL restoration of CdTe QDs reveals that Phen ligands are detached by added Cd2+ , and the originally replaced TGA ligands return to the QDs surface. As a consequence of the Phen detachment, the PHT process (dynamic) as well as the ground-state coordination (static) between CdTe QDs and Phen is interrupted (the inset of Fig. 6B). At a molar ratio of added Cd2+ to Phen higher than 1:2, the PL intensity of CdTe QDs as well as Cd2+ –Phen complexes cannot be further increased, indicating that the chelating stoichiometric ratio of Cd2+ to Phen is 1:2, i.e. the formation of [Cd(Phen)2 (H2 O)2 ]2+ . Therefore, the fluorescence switch strategy for Cd2+ detection by the present sensor is based on subsequent ligand-displacement reactions at the QDs surface, which can be briefly described as: Phen
Cd2+
CdTe@TGA −→ CdTe@Phen + TGA−→ CdTe@TGA + [Cd(Phen)2 (H2 O)2 ]2+
Fig. 5. Effects of various metal ions (0.5 M) on the PL intensity of CdTe QD–Phen (1:12) sensor. The inset shows the PL spectra of CdTe QD–Phen in the absence/presence of 10 M Zn2+ , and in the presence of 0.5 M Cd2+ without/with the coexistence of 10 M Zn2+ , respectively. I and I0 are the PL intensity of CdTe QD–Phen in the presence and absence of ions, respectively.
The difference between this strategy and those demonstrated previously [24–31] is further validated. Fig. 7 presents the PL evolution of Phen-complexed CdSe/ZnS core/shell QDs as a function of the concentration of Cd2+ as well as Zn2+ . Different with the CdTe QDs, the PL of CdSe/ZnS QDs can be restored in the presence of either Cd2+ or Zn2+ . This restored PL by such metal ions is in contrast to the “surface passivation” and “host-guest reaction” strategies [24–29], where the analyte-receptor complexation occurs at the QDs surface (Scheme 1A and B). It is most conceivable that the complexation of Phen with Cd2+ or Zn2+ at the QDs surface increases the hole traps, leading to PL quenching of QDs instead of PL restoration. As for the “chemical etching” strategy
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
197
Fig. 6. (A) PL spectra representing the quenching of 0.1 M CdTe QDs by Phen with the concentrations of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.2 M, and PL spectra of Phen and [Cd(Phen)2 (H2 O)2 ]2+ . Inset: PHT mechanism based on energy levels of green CdTe QDs versus Phen. (B) PL spectra representing the restoration of the CdTe QD–Phen sensor by Cd2+ with the concentrations of 0.1, 0.5, 1.0, 2.0, 20.0, 100.0, 200.0, 300.0, 400.0, 600.0 and 800.0 nM. Inset: the interrupted PHT process by the detachment of Phen from the QDs surface and the formation of [Cd(Phen)2 (H2 O)2 ]2+ in solution; excitation: 290 nm.
(Scheme 1C), the complexation of a chelating reagent (in our case, Phen) with the shell Zn atoms generates “Zn2+ -imprinted” cavities on the surface of CdSe/ZnS QDs, and only added Zn2+ can repair the surface defects, restoring the QDs PL [30,31]. On contrary, a concentration of 20 M Zn2+ only slightly restores the PL of CdSe/ZnS QDs, while at the identical concentration, the presence of Cd2+ results in a full restoration of the QDs PL (the inset of Fig. 7). This observation is in accordance with the ligand displacement-induced PL switch strategy, where the detachment of ligand is highly dependent on its affinity with metal ions. Although the chelation of Phen with added Cd2+ as well as Zn2+ can break the Zn–Phen complexation at the surface of CdSe/ZnS QDs, Phen has much higher affinity toward added Cd2+ than Zn2+ .
3.5. Analytical performance of the CdTe QD–Phen sensor The analytical performance of QDs-based fluorescent sensors is dependent on the change of the surface states of QDs. In our strategy, the subtle change of surface states of QDs is a result of the ligand (receptor) displacement reaction, which is highly dependent on its relative affinity with metal ions (analyte) against surface metal atoms of QDs. On basis of this strategy, the present CdTe QD–Phen sensor has realized ultrasensitive detection of Cd2+ with wide linear ranges, and for the first time, excellent selectivity for Cd2+ over Zn2+ , as listed in Table 1.
Fig. 7. (A) Evolution of PL spectra of CdSe/ZnS QD–Phen with the Cd2+ concentration. The insets show the structure of CdSe/ZnS QD–Phen sensor and the relationship of relative PL intensity (I/I0 ) of CdSe/ZnS QD–Phen with the concentration of Cd2+ as well as Zn2+ . I and I0 are the PL intensities of CdSe/ZnS QD–Phen in the presence/absence of Cd2+ or Zn2+ , respectively.
Table 1 Comparison of chemical and electronic reactions as well as analytical parameters of the representative QDs-based sensors for Cd2+ detection. References
QDs
Receptor
Chemical reaction
Electronic process
Detection limit
Linear range
Selectivity
This study [24–28] [29] [30,31]
CdTe CdTe CdS:Mn/ZnS CdTe, CdTe/CdS
Phen S2− , SO3 2− 1,10-Diaza-18-Cr-6 EDTA, APDC
Ligand displacement Surface complexation Host–guest reaction Ion exchange
PHT Surface passivation PET Surface passivation
0.01 nM 5–12 nM N/A 6–10 nM
0.02 nM–0.6 M 0.1–15 M N/A 0.05–1 M
Cd2+ Cd2+ , Zn2+ Cd2+ , Zn2+ Cd2+ , Zn2+
Table 2 Real application of the proposed sensor in water samplesa . Samples
Added (nM)
By this sensor (nM)
By the AAS method (nM)
Tap water 1 Tap water 2 Spring water 1 Spring water 2
3.70 114.00 3.70 81.00
3.76 ± 0.08 114.55 ± 1.27 3.66 ± 0.05 82.13 ± 0.70
3.71 ± 0.03 114.32 ± 0.72 3.69 ± 0.02 81.80 ± 0.52
a
All values are reported as average of three determinations ± standard deviation.
198
X. Hu et al. / Analytica Chimica Acta 812 (2014) 191–198
The analytical performance of the present sensor is further evaluated in practical detection of drinking water. Tap water and spring water are used in our experiments. All samples are prepared by spiking with Cd2+ at diverse concentration levels, and analyzed by this fluorescent sensor and the standard AAS method independently. As summarized in Table 2, all results obtained by this sensor are in good agreement with those measured by the AAS method. As is known that many metal ions, as referred in Fig. 5, such as Ca2+ , Mg2+ , Mn2+ , Cu2+ , and Zn2+ , may be present in drinking water. For instance, the maximum allowed concentration levels are 15.7 M for Cu2+ and 15.3 M for Zn2+ , respectively. Our results of the determinations of drinking water samples show that the presence of the relative high concentration levels of such metal ions does not interfere with the detection of trace Cd2+ , owing to the ion-dependent affinity of Phen at the QDs surface.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusions [17]
In summary, this work has demonstrated a surface complexation-originated PHT mechanism of CdTe QDs through the attachment of Phen onto QDs by the formation of Cd–Phen complex at the surface, and a PL switch-on mechanism through the detachment of Phen by the formation of [Cd(Phen)2 (H2 O)2 ]2+ in the solution. This simple CdTe QDs-based fluorescent sensor has achieved ultrasensitive detection of Cd2+ , obtaining the lowest detection limit reported so far. Importantly, by utilizing the iondependent chelating ability of Phen (receptor), this sensor features a distinct discrimination of Cd2+ versus Zn2+ , and succeeds in real water samples. Moreover, this sensor is robust, and exhibits good reversibility and repeatability. Therefore, the QDs-based sensor demonstrated here provides a facile, simple and low cost approach for quantitative determination of Cd2+ residue in drinking water as well as in food.
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Acknowledgments
[34] [35] [36]
This work was funded by NSFC and MOE (Grants: 21005017, 21375015, and 20100092120037).
[37]
Appendix A. Supplementary data
[38] [39]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.01.006.
[40] [41] [42]
References [1] R. Hernández-Martínez, I. Navarro-Blasco, Food Control 26 (2012) 6–14. [2] T. Schwerdtle, F. Ebert, C. Thuy, C. Richter, L.H.F. Mullenders, A. Hartwig, Chem. Res. Toxicol. 23 (2010) 432–442.
[43] [44]
G. Kazantzis, Am. J. Ind. Med. 20 (1991) 701–704. I.A. Darwish, D.A. Blake, Anal. Chem. 74 (2002) 52–58. Y. Wang, Y.H. Wang, Z.L. Fang, Anal. Chem. 77 (2005) 5396–5401. M. Suzuki, K. Ohta, Anal. Chem. 54 (1982) 1682–1689. R. Laocharoensuk, A. Bulbarello, S.B. Hocevar, S. Mannino, B. Ogorevc, J. Wang, J. Am. Chem. Soc. 129 (2007) 7774–7775. T. Shtoyko, S. Conklin, A.T. Maghasi, A. Piruska, J.N. Richardson, C.J. Seliskar, W.R. Heineman, Anal. Chem. 76 (2004) 1466–1473. S.P. Dolan, D.A. Nortrup, P.M. Bolger, S.G.J. Capar, Agric. Food Chem. 51 (2003) 1307–1312. K. Tsuji, K. Nakano, Anal. Chem. 84 (2012) 636–668. J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, Trends Anal. Chem. 25 (2006) 207–218. M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013–2015. W.C.W. Chan, S. Nie, Science 281 (1998) 2016–2018. I.L. Medintz, A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nat. Mater. 2 (2003) 630–638. Q.J. Sun, Y.A. Wang, L.S. Li, D.Y. Wang, T. Zhu, J. Xu, C.H. Yang, Y.F. Li, Nat. Photonics 1 (2007) 717–722. Q.J. Sun, S. Guru, L.M. Dai, C. Michael, C. Angela, N. Rajesh, G. James, Y.Q. Wang, ACS Nano 3 (2009) 737–743. X. Michalet, F. Pinaud, T.D. Lacoste, M. Dahan, M.P. Bruchez, A.P. Alivisatos, S. Weiss, Single Mol. 2 (2001) 261–276. D. Knopp, D.P. Tang, R. Niessner, Anal. Chim. Acta 647 (2009) 14–30. R. Freeman, T. Finder, I. Willner, Angew. Chem. Int. Ed. 48 (2009) 7818–7821. Y. Chen, Z. Rosenzweig, Anal. Chem. 74 (2002) 5132–5138. M.J. Ruedas-Rama, E.A.H. Hall, Anal. Chem. 80 (2008) 8260–8268. J. Völker, X. Zhou, X. Ma, S. Flessau, H. Lin, M. Schmittel, A. Mews, Angew. Chem. Int. Ed. 49 (2010) 6865–6868. E.M. Nolan, S.J. Lippard, Chem. Rev. 108 (2008) 3443–3480. M. Algarra, B.B. Campos, B. Alonso, M.S. Miranda, A.M. Martinez, C.M. Casado, J.C.G. Esteves da Silva, Talanta 88 (2012) 403–407. H.B. Li, Y. Zhang, X.Q. Wang, Sens. Actuators, B 127 (2007) 593–597. Y.L. Li, J. Zhou, C.L. Liu, H.B. Li, J. Mater. Chem. 22 (2012) 2507–2511. R. Gui, X. An, W. Huang, Anal. Chim. Acta 767 (2013) 134–140. H. Xu, R. Miao, Z. Fang, X. Zhong, Anal. Chim. Acta 687 (2011) 82–88. S. Banerjee, S. Kar, S. Santra, Chem. Commun. (2008) 3037–3039. P. Wu, X.P. Yan, Chem. Commun. 46 (2010) 7046–7048. R. Gui, X. An, H. Su, W. Shen, Z. Chen, X. Wang, Talanta 94 (2012) 257–262. S. Dobson, Cadmium-Environmental Aspects, World Health Organization, Geneva, 1992. L. Friberg, C.G. Elinger, T. Kjellstrom, Cadmium, World Health Organization, Geneva, 1992. P.G. Sammes, G. Yahioglu, Chem. Soc. Rev. 23 (1994) 327–334. A. Bencini, V. Lippolis, Coord. Chem. Rev. 254 (2010) 2096–2180. L. Zhang, K. Zhu, T. Ding, X.Y. Hu, Q.J. Sun, C.X. Xu, Analyst 138 (2013) 887–893. A.L. Rogach, T. Franzl, T.A. Klar, J. Feldmann, N. Gaponik, V. Lesnyak, A. Shavel, A. Eychmu1ller, Y.P. Rakovich, J.F. Donegan, J. Phys. Chem. C 111 (2007) 14628–14637. W.W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854–2860. J.J. Li, Y.A. Wang, W. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, J. Am. Chem. Soc. 125 (2003) 12567–12575. J.R. Lakwicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Press, New York, NY, 1999. I. Yildiz, M. Tomasulo, F.M. Raymo, PNAS 103 (2006) 11457–11460. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenburg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. L. Jing, C. Yang, R. Qiao, M. Niu, M. Du, D. Wang, M. Gao, Chem. Mater. 22 (2010) 420–427. Z.S. Yang, J.S. Yu, H.Y. Chen, Chin. J. Inorg. Chem. 4 (2002) 373–377.
