Sensors and Actuators B 251 (2017) 171–179
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
CdS quantum dots capped with hyperbranched graft copolymers: Role of hyperbranched shell in ﬂuorescence and selective mercury-sensing You Fan a , Ya-Qian Cai a , Hua-Ji Liu a,c,d , Yu Chen a,b,c,d,∗ a
Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300354 PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300354, PR China National Demonstration Center for Experimental Chemistry & Chemical Engineering Education, Tianjin 300354, PR China d National Virtual Simulation Experimental Teaching Center for Chemistry & Chemical Engineering Education, Tianjin University, Tianjin 300354, PR China b c
a r t i c l e
i n f o
Article history: Received 24 January 2017 Received in revised form 6 May 2017 Accepted 14 May 2017 Available online 15 May 2017 Keyword: Fluorescence Hyperbranched polymer Quantum dot Mercury Sensor
a b s t r a c t A series of hyperbranched graft copolymers with hyperbranched polyethylenimine (HPEI) as core and hyperbranched polyglycerol (HPG) as shell (HPEI-g-HPG) were used as nanoreactors and stabilizers for the synthesis of CdS quantum dots (QDs). The preparation conditions were optimized to obtain CdS QDs with a stronger ﬂuorescence. Stable CdS QDs could be prepared by using HPEI-g-HPGs or HPEI homopolymer, but not HPG homopolymer. The CdS QDs capped by HPEI-g-HPGs emitted stronger than those capped by HPEI. Moreover, the photoluminescence intensity increased pronouncedly with increasing the HPG shell content of HPEI-g-HPG stabilizer. The measurements of UV–vis spectroscopy, transmission electron microscopy and dynamic light scattering further veriﬁed the formation of CdS QDs. Compared with the CdS QDs capped by HPEI, those capped by HPEI-g-HPGs showed a better storing stability in solution at 4 ◦ C. The as-prepared CdS QDs were only stable in the pH range of 7.0–9.0. It was found that increasing the HPG shell content of HPEI-g-HPG stabilizer could effectively raise the selectivity of the as-prepared CdS QDs towards Hg2+ . As the ratiometric ﬂuorescence sensors, CdS QDs capped by HPEI-g-HPGs were most sensitive to Hg2+ and the limit of detection was ca. 1.5 × 10−8 M. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades ﬂuorescent semiconductor nanocrystals (also known as quantum dots, QDs) have attracted much interest because QDs possess many attractive features, such as size- and shape-dependent optical and electronic properties, broad excitation wavelength, high quantum yield, high photochemical stability against photobleaching [1–3]. These characters endow QDs enormous potential applications, such as in light-emitting devices , solar cells , nonlinear optical devices , sensors [7,8], and bioimaging . As industry’s development becomes faster and faster, the environmental pollution problems become much more serious. Due to its long-term and severe harm to ecological system and human health, mercury pollution has attracted the great attention of the people [10,11]. Therefore, there is a great need to detect
∗ Corresponding author at: Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300354, PR China. E-mail address: [email protected]
(Y. Chen). http://dx.doi.org/10.1016/j.snb.2017.05.070 0925-4005/© 2017 Elsevier B.V. All rights reserved.
mercury ion with high sensitivity and selectivity. Beyond the welldeveloped instrumental analytical methods, such as inductively coupled plasma mass spectroscopy , atomic absorption spectrometry , many kinds of optical sensors have been developed for the detection of Hg2+ ion [14–17]. Among these optical sensors, ﬂuorescence sensors based on QDs are also very efﬁcient to detect Hg2+ ion through special design [18–21]. Dendrimers and their less-deﬁned hyperbranched analogues are attracting tremendous interest due to their speciﬁc threedimensional topology, low viscosity, good solubility, and plenty of modiﬁable terminal groups, which are powerful motifs in the design of new molecular and supramolecular structures [22–25]. While the preparation of structurally perfect dendrimers suffers from its tedious multistep nature, hyperbranched polymers made in one step have emerged as excellent alternatives. So many merits render hyperbranched polymers as promising materials to prepare the multifunctional nanocomposites with QDs . The hyperbranched polymers for preparing QD nanocomposites normally contain thiol  or amine [28–34] groups that are good surface passivation ligands to eliminate surface traps of QDs, resulting in photoluminescence enhancement. These hyperbranched polymers can be categorized into two types. One is homopolymer or its
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
derivatives, such as hyperbranched polyethylenimine (HPEI) and thioether modiﬁed hyperbranched polyglycerol (HPG) [27–30]. The other is core-shell type polymer that is formed by a hyperbranched polymer core and a shell composed of many linear segments [31–34]. For example, palmitoyl chloride functionalized hyperbranched polyamidoamine (HPAMAM) was used as unimolecular nanoreactor to prepare CdS QDs through two-phase route . Zhu et al. prepared a double-hydrophilic multiarm hyperbranched polymer with an HPAMAM core and many poly(ethylene glycol) monomethyl ether arms, which was used as a nanoreactor for CdS QDs synthesis . Recently, hyperbranched polymers whose core and shell are both formed by hyperbranched moieties have been synthesized [35–39], however, to our best knowledge, such new core-shell type hyperbranched copolymers have never been applied for the synthesis of QDs. The aqueous synthesis of colloidal QDs remains attractive owing to the increasing requirements for environmental protection . Moreover, the water-borne QDs are applied more suitably in the ﬁeld of sensors and biological systems. Thus, in this work, a series of water-soluble hyperbranched grafting copolymers with HPEI as core and HPG moieties as shell (HPEI-g-HPG) were used as nanoreactor and stabilizer for the aqueous synthesis of CdS QDs. The HPEI core bearing a large number of amine groups has a strong interaction with the formed CdS QDs, whereas the HPG shell does not. However, it was observed that the HPG shell content had an obvious inﬂuence on the ﬂuorescence of CdS QDs and their selective detection on Hg2+ ion as ﬂuorescence sensor.
2. Experimental 2.1. Materials Hyperbranched polyethylenimine (HPEI, Aldrich) with a number-average molecular weight (Mn ) of 1.0 × 104 g mol−1 and a polydispersity (Mw /Mn ) of 2.5, was dried under vacuum prior to use. Glycidol (99.9%, Alfa Aesar) and diglyme (99.9%, Alfa Aesar) were puriﬁed by distillation from CaH2 directly prior to use. Anhydrous methanol (A. R.), potassium methylate (A. R.) and acetone (A. R.) were purchased from Tianjin Jiangtian Chemical Company. Triethylamine (TEA, A. R.) was purchased from Tianjin Jiangtian Chemical Company and was puriﬁed by distillation from CaH2 directly prior to use. CdCl2 ·2.5H2 O (G. R.) and Na2 S·9H2 O (G. R.) were purchased from Tianjin Yuanli Chemical Company and used without further puriﬁcation. The aqueous solutions of CdCl2 and Na2 S were prepared just before use to prevent oxidation and hydrolysis. Other metal salts were of reagent grade or better. HPEI terminated with a large amount of hydroxyl groups (HPEI-OH) was prepared through the addition reaction between HPEI and glycidol at room temperature according to the literature . HPEI-g-HPGs were prepared through the anionic polymerization of glycidol with HPEI-OH as macroinitiator according to the literature .
2.2. Syntheses of CdS QDs The aqueous solution of CdCl2 (1 × 10−2 M, 0.1 mL) was added dropwise into 9.0 mL of the aqueous solution of polymer. The mixture was stirred for 1 h at certain temperature before the addition of 1.0 mL of the aqueous solution of Na2 S (1 × 10−3 M). After certain time for the growth of nanocrystals, supplemental CdCl2 solution was added and stirred for another 1 h. The as-prepared pale-yellow to colorless QDs solutions were either stored at 4 ◦ C or lyophilized before use.
2.3. Fluorescence measurement of CdS QDs in the presence of different metal ions The lyophilized polymer stabilized CdS QDs and different metal ions were dissolved in Tris-HCl buffer solution (0.01 mol/L, pH = 7.4). The ﬁnal concentrations of CdS QDs and metal ions were controlled to be 5 × 10−5 and 1 × 10−5 M, respectively. The mixture was stirred for 30 min before ﬂuorescence measurements. 2.4. Detection of Hg2+ by CdS QDs The lyophilized polymer stabilized CdS QDs were dissolved in Tris-HCl buffer solution (0.01 mol/L, pH = 7.4) containing a different concentration of Hg2+ ions. The ﬁnal concentration of CdS QDs was controlled to be 1 × 10−4 M. The mixture was stirred for 30 min before ﬂuorescence measurements. 2.5. Characterization 1 H NMR spectra were recorded on a Bruker 400 MHz spectrometer. The chemical shifts are given in parts per million (ppm). UV–vis spectra were obtained from a Purkinje General (China) T6 UV/Vis Spectrophotometer. The molecular weight and molecular weight distributions were determined by gel permeation chromatography (GPC) equipped with a Viscotek GPC270 system. Freshly distilled N,N-dimethyl formamide was used as an eluent at 35 ◦ C. The ﬂow rate was set to be 0.8 mL min−1 . Fluorescence spectra were recorded using a Varian Cary Eclipse photoluminescence spectrometer with a scan rate of 600 nm min−1 , and slit widths of excitation and emission are set to be 5 and 10 nm, respectively. Transmission electron microscope (TEM) photographs were taken from a FEI Tecnai G2 F20 transmission electron microscopy. Dynamic light scattering (DLS) was performed using a Malvern Nano ZS instrument at 25 ◦ C with 633 nm He–Ne laser light and light collection at 90◦ .
3. Result and discussion 3.1. Syntheses of CdS QDs According to our previous publication , a series of pure and water-soluble hyperbranched grafting copolymers, HPEI-g-HPGs, were prepared through the anionic polymerization of glycidol with HPEI-OH as macroinitiator. The obtained HPEI-g-HPGs have been well characterized by 1 H NMR and GPC . The HPEI-g-HPGs utilized here have the same HPEI core, but have a HPG shell with a different of degree of polymerizations (DP) of glycidol monomer. HPEI-g-HPG with a speciﬁc DP value of glycidol monomer is named as HPEI-g-HPGDP . The detailed structural information of these HPEIg-HPGs is shown in Table S1 in the Supporting Information. The synthetic procedure of CdS QDs with HPEI-g-HPGs as nanoreactor and stabilizer is depicted as Scheme 1. Firstly, Cd2+ ions are complexed with the amino groups of HPEI core in water. With the slow addition of S2− ions (the ratio of S2− ions to the preadded Cd2+ ions is 1:1), CdS QDs are gradually nucleated and grown within the HPEI core. Finally, extra Cd2+ ions are added to cap the formed CdS QDs. Fig. 1A shows the typical UV–vis spectra of CdS QDs prepared at different temperatures. The gentle absorption peak of QDs can be observed between 300 and 450 nm, by which the average size of CdS QDs is estimated with the Brus effective mass model [41,42]. It is clear that upon raising the preparation temperature from 5 to 40 ◦ C, the band edges only show a little red-shift from ca. 352 to 354 nm and the estimated diameters of CdS QDs are ca. 2.11 and 2.15 nm, respectively. Further raising the temperature to 60 and 80 ◦ C leads to an obvious red-shift of band edges to ca. 367 and
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
Scheme 1. Syntheses of CdS QDs assisted by HPEI-g-HPG.
379 nm, respectively, while the estimated diameters of CdS QDs increase to be ca. 2.33 and 2.76 nm, respectively. Fig. 1B shows the ﬂuorescence emission spectra. It is clear that the obtained nanocomposites can emit ﬂuorescence. The ﬂuorescence mainly originates from the quantum conﬁnement (1–10 nm) of the formed nano-sized CdS , while the HPEI core of HPEI-gHPGs as good surface passivation ligands to eliminate surface traps of CdS QDs also has a strong contribution to emission enhancement [28–30]. Increasing preparation temperature leads to the gradual red-shift of emission peak. As for the photoluminescent (PL) intensity, initially it increases pronouncedly when higher preparation temperature is adopted, whereas an obvious decrease appears when ca. 80 ◦ C is adopted. Increasing preparation temperature also results in an obvious red-shift of emission peak, which can be ascribed to the growth of CdS QDs, in accordance to results from the corresponding UV–vis spectra. Since the strongest PL intensity of CdS QDs is achieved at 60 ◦ C, preparation temperature for the following experiments are all performed at this temperature. The second step in Scheme 1 is the addition of S2− ions into the complex of HPEI-g-HPG with Cd2+ ions, in which the nucleation and growth of CdS nanocrystals start. Since this stage is timedependent, the inﬂuence of CdS nucleation and growth time on its PL intensity is studied (Fig. 2A). When the nucleation and growth time varies from 0.5 to 1 h, the PL intensity only shows a slight increase. Further prolonging the preparation time does not help to enhance the PL intensity. Therefore, the optimizing nucleation and growth time is set to be 1 h. The third step in Scheme 1 is to cap the formed CdS QDs with extra Cd2+ ions. Before the supplemental addition of Cd2+ ions, the total Cd/S ratio is 1:1. The supplemental addition of Cd2+ ions can remarkably enhance the PL intensity (Fig. 2B and Fig. S1 in the Supporting Information). For instance, the total Cd/S ratio increases
from 1:1 to 2:1, the PL intensity increases more than 50% while the maximum emission wavelength (em ) is still 514 nm. The further increase of Cd/S ratio only leads to a little increase of PL intensity, followed by a slight red-shift of emission peak. Such results can be tentatively explained as follows: When the feed Cd/S ratio is 1:1, a few Cd2+ ions fail to participate in the formation of CdS QDs because of the strong complexation interaction between HPEI core and Cd2+ ions, resulting in a slight excess of S2− ions. The excessive S2− ions are adsorbed at the surface of CdS QDs. However, the dangling bonds formed by the lone-pair electrons of excessive S2− ions constitute the non-radiative combination centers, which are bad to the ﬂuorescence emission . Thus, the fewer S2− ions at the surface of QDs are, the stronger PL intensity of QDs is. The addition of extra Cd2+ ions can effectively reduce the quantity of non-radiative combination centers constituted by S2− dangling bonds. Moreover, extra Cd2+ ions can also form Schottky barrier around QDs . These factors lead to the increase of PL efﬁciency of CdS with the supplemental addition of Cd2+ ions. According to the PL intensity, the optimal Cd/S ratio is set to be ca. 2.2. The inﬂuence of the ratio of polymer to CdS on ﬂuorescence was studied (Fig. 3 and Fig. S2 in the Supporting Information). Amine units of polymer are assumed to be the main groups to interact with CdS QDs and all the HPEI-g-HPGs used here have the same number of amine units, thus the N/S ratio is used to represent the ratio of polymer to CdS. From Fig. 3 it can be seen that PL intensity increases enormously as the N/S ratio changes gradually from 0.5:1 to 8:1, whereas it decreases obviously when the ratio reaches 16:1. Furthermore, increasing the N/S ratio leads to the obvious blue-shift of the maximum em . The PL property of QDs is strongly depended on their size and surroundings, which can all be related to the quantity of polymer templates. The larger the N/S ratio is, the less amount of Cd2+ ions would be complexed in each HPEI-g-HPG. Thus, the
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
Fig. 1. (A) Typical UV–vis spectra (inset: the estimated size of CdS QDs from UV–vis spectra) and (B) Typical ﬂuorescence spectra of CdS QDs prepared at different temperature (HPEI-g-HPG785 was used as the representative; [HPEIg-HPG785 ] = 0.235 mg/mL = 3.45 × 10−6 M; Optimal preparation conditions were adopted; ex = 370 nm).
subsequent addition of Na2 S leads to the formation of smaller CdS nanocrystals, which can be veriﬁed by the corresponding UV–vis spectra (Fig. S3 in the Supporting Information). The efﬁciencies of HPEI-g-HPGs in preparing CdS QDs were compared with the homopolymers, HPG and HPEI (Fig. S4 in the Supporting Information). It is found that the utilization of HPG homopolymer with plenty of hydroxyl and ether groups cannot prepare stable CdS QDs, no matter how many HPGs are added. Stable CdS QDs can be prepared with HPEI, which has been reported by several groups [28–30]. Under the optimal preparation conditions adopted here, the CdS QDs prepared from HPEI-g-HPGs emit stronger than those stabilized by HPEI homopolymers (Fig. 4 and Fig. S4 in the Supporting Information). Moreover, the PL intensity of CdS QDs increases pronouncedly with increasing the HPG shell content of HPEI-g-HPGs. The PL intensity of CdS QDs prepared from HPEI-g-HPG785 with the most HPG shell content is nearly 10 times more than those prepared from HPEI. It is also observed that the color of CdS QDs solution changes from bright yellow to almost colorless with increasing the HPG shell content. Furthermore, increasing the HPG shell content also leads to a slight blue-shift of the maximum em . For instance, CdS QDs prepared from HPEI have a maximum em of 536 nm, whereas those prepared from HPEI-g-HPG290 and HPEI-g-HPG785 have a maximum em of 529 and 514 nm, respectively. The blue-shift of maximum em also indicates that the CdS QDs prepared from HPEI-g-HPG with a more HPG shell content might have a smaller size, which can be
Fig. 2. Emission intensity of the formed CdS QDs inﬂuenced by (A) CdS nucleation and growth time and (B) the total ratio of Cd2+ ions to S2− ions (HPEI-g-HPG785 was used as the representative; [HPEI-g-HPG785 ] = 0.235 mg/mL = 3.45 × 10−6 M; Optimal preparation conditions were adopted; ex = 370 nm).
Fig. 3. Inﬂuence of N/S ratio on PL emission intensity and em,max of the formed CdS QDs (HPEI-g-HPG785 was used as the representative; [HPEIg-HPG785 ] = 0.235 mg/mL = 3.45 × 10−6 M; Optimal preparation conditions were adopted; ex = 370 nm).
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
Fig. 4. Inﬂuence of HPG shell content on the emission intensity of the polymer stabilized CdS QDs ([Polymer] = 3.45 × 10−6 M; Optimal preparation conditions were adopted; ex = 370 nm).
supported by the estimated size from the corresponding UV–vis spectra (Fig. S5 in the Supporting Information). The pronounced inﬂuence of HPG shell content on the PL properties and size of CdS QDs can be explained tentatively as follows: It is known that a higher generation dendrimer has a more rigid conformation . Increasing the HPG shell content not only makes the polymer more rigid, but also renders the shell more compact. It is so harder for CdS nuclei formed in the HPEI core to aggregate with each other that the formed CdS QDs have a smaller size when HPEI-g-HPG with a more HPG shell content is used as the template and stabilizer. Since the more compact shell is more effective to hamper the quencher molecules to access CdS QDs located in the inner core, the CdS QDs capped by HPEI-g-HPG with a more HPG shell content emit more intensely. 3.2. Characterization of CdS QDs The CdS QDs prepared from HPEI-g-HPG785 show the strongest ﬂuorescence emission, thus are further characterized. Fig. 5A shows the typical TEM image, and many small dark dots can be seen clearly. The corresponding high resolution TEM image (Fig. S6 in the Supporting Information) shows clear crystal lattice fringes, demonstrating the formation of well crystallized CdS QDs. The interplanar distance along the growth axis is 0.33 nm, which is consistent with the (002) wurtzite crystal lattice of CdS . The average diameter of CdS QDs calculated from TEM is 2.44 ± 0.32 nm, similar to that estimated from UV–vis spectrum (2.33 nm). Mono-modal signal exists in the DLS diagram (Fig. 5B), from which the average diameter of CdS QDs capped with HPEI-g-HPG785 is found to be ca. 5.23 nm, much bigger than the size calculated from TEM. In TEM, only the nano-sized CdS QDs can be visualized clearly. The larger size observed by DLS implies that the obtained CdS QDs are capped by HPEI-g-HPG785 polymers.
Fig. 5. (A) Typical TEM image and (B) typical DLS diagram of CdS QDs capped by HPEI-g-HPG785 .
3.3. Stabilities of CdS QDs Fluorescence stability is a key parameter to the various applications of CdS QDs. The storing stabilities of the aqueous solution of CdS QDs capped by HPEI-g-HPGs with different HPG shell content were compared at 4 ◦ C (Fig. 6). It is obvious that HPEI-g-HPG with a more HPG shell content shows a better stabilizing ability toward CdS QDs. For instance, the ﬂuorescence intensity of CdS QDs capped by HPEI reduces to ca. 30% after 30 days, whereas the ﬂuorescence
Fig. 6. Inﬂuence of storing time on the ﬂuorescence maintenance of CdS QDs capped by HPEI-g-HPGs with different HPG shell content (ex = 370 nm).
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
Fig. 7. Typical ﬂuorescence spectra of CdS QDs at different pH (HPEI-g-HPG785 is used as the representative; [Polymer] = 6.90 × 10−7 M; The CdS QDs prepared at the optimal conditions were diluted by 5 times; ex = 370 nm).
intensity of CdS QDs capped by HPEI-g-HPG785 only decreases to ca. 82%. It’s been widely reported that the PL intensity of water-soluble QDs is strongly depended on the solution pH . However, the tendencies of PL intensity change are always different [48,49], which is because different stabilizers always have different inﬂuence on PL intensity of QDs. As mentioned above, the amino-rich HPEI core is responsible for the complexation of Cd2+ ions. What’s more, the asprepared QDs are covered with extra Cd2+ ions that can still interact with amino groups. When pH of the QDs solution becomes acidic, more amino groups change into the N+ groups that have no ability to complex with Cd2+ ions and become repellent to them. The formation of N+ groups makes it easy for QDs to shake off the protection of polymers and aggregate with each other. So it can be seen from Fig. 7 that the QDs PL intensity shows a marked decrease as the pH changes from 7 to 6, and becomes almost negligible when pH is below 6. Once adjusting pH to basic, some N+ groups in the HPEI core change into tertiary amines. The decrease of N+ groups makes the interaction between polymers and QDs stronger, thus polymers cover onto QDs more tightly, which reduces the access of quenchers to QDs. Therefore, the PL intensity of QDs is 5 times stronger when pH is varied from 7.0 to 9.0. However, further raising pH results in the formation of Cd(OH)2 , and the PL intensity decreases due to the lack of extra Cd2+ ions.
3.4. The ion-probing properties of CdS QDs One of the most widely-used application of CdS QDs is as the ﬂuorescence sensors for metal ions . A desired ion sensor must possess excellent properties such as high sensitivity, precise ion-selectivity and relatively long linear range. Whether the as-prepared CdS QDs in this work have such merits were studied. Firstly, the CdS QDs solution was diluted gradually to certain concentration, and the corresponding ﬂuorescence spectra were measured (Fig. S7A in the Supporting Information). The PL intensity decreases gradually as the concentration goes down while the peak position and pattern remain the same. The peak intensity at the maximum em is proportional to the CdS concentration in the range of 3.0 × 10−5 −6.0 × 10−5 M, with a linear correlation coefﬁcient of 0.9943 (Fig. S7 B in the Supporting Information). Therefore, it’s suitable for the as-prepared QDs to be used as ﬂuorescence sensors within this concentration range. Finally, a concentration
of 5.0 × 10−5 M is chosen to carry on the following researches on metal ion detection. A series of typical metal ions including Na+ , K+ , Ca2+ , Ni2+ , Co2+ , Cr3+ , Fe3+ , Ag+ , Cu2+ , Hg2+ , Pb2+ , Cd2+ and Tl+ were used as the detecting targets and their ﬁnal concentrations were adjusted to be 1.0 × 10−5 M. Taking CdS QDs capped with HPEI-g-HPG785 as the representative sensor (Fig. S8 in the Supporting Information), it can be observed that among so many metal ions, Hg2+ shows the most efﬁcient quenching ability toward CdS QDs and its ﬂuorescence quenching efﬁciency is more than 90%. The strong quenching ability of Hg2+ toward CdS QDs is usually attributed to the formation of much more insoluble HgS and bridging S Hg S bonds at the surface . Cu2+ is another metal ion that has a marked inﬂuence on the ﬂuorescence spectrum. Although the addition of Cu2+ only leads to a PL attenuation by about 40%, it arouses a huge red-shift of emission peak (ca. 100 nm). The PL attenuation and red-shift of emission peak is normally attributed to the reduction of Cu2+ to Cu+ and the subsequent formation of CdS+ -Cu+ species on the surface of QDs . CdS+ -Cu+ has a lower energy level than pure CdS QDs, being responsible for the red-shift of emission peak. Moreover, Cu+ can also quenches the PL of CdS QDs by facilitating non-radiative recombination of excited electrons in the conduction band and holes in the valence band. The inﬂuence of other metal ions on the PL spectra is not so pronounced as Cu2+ and Hg2+ . The ion selectivity performances of CdS QDs capped by different polymers are compared (Fig. 8). As shown in the histograms, the CdS QDs capped by HPEI without HPG shell show obvious PL decrease in the response to Co2+ , Cr3+ , Fe3+ , Cu2+ , Hg2+ , Pb2+ and Tl+ ions. As the HPG shell content of HPEI-g-HPG increases, the sensitivity of CdS QDs towards Co2+ , Cr3+ , Fe3+ , Cu2+ , Pb2+ and Tl+ ions gradually decreases, thus realizing the ion selectivity towards Hg2+ . The increase of ion selectivity due to the increase of HPG shell content can be explained tentatively as follows: HPG shell contains a large number of hydroxyl and ether groups that can also complex with many metal ions, thus it is more difﬁcult for the metal ions to penetrate through the thicker HPG shell to reach CdS QDs located in the HPEI core. The thicker HPG shell has a better ability to differentiate the ability of metal ions accessing the core. Only those having weak interaction with HPG shell and strong interaction with CdS QDs can efﬁciently access CdS QDs, thus ion-selectivity is realized. The detection sensitivity to Hg2+ of CdS QDs capped by HPEI-gHPG785 was studied. As shown in Fig. 9A, an obvious PL emission decrease can be observed as the Hg2+ concentration changes from 1 × 10−8 to 1 × 10−4 M. More and more obvious blue-shifts due to the destruction of QDs structure can be also observed as the Hg2+ concentration increases. The change of PL intensity against Hg2+ concentration reading out from Fig. 9A is plotted in Fig. 9B. It’s notable that the PL intensity decreases linearly as the Hg2+ concentration varies from 5 × 10−8 to 1.6 × 10−6 M. The data are linear-ﬁtted to get an equation of y = 3.75x + 232.28 with a correlation coefﬁcient of 0.9948. The limit of detection (LOD) of Hg2+ is determined according to equation 1, which is based on the deﬁnition and classiﬁcation by IUPAC and American National Standards Institute. LOD = kS0
Where k stands for the conﬁdence factor (usually set to be 3), S0 ¯ represent the standard deviation and mean value of data and X respectively, C is the real concentration of standard sample. The detection for Hg2+ with an accurate concentration of 5 × 10−8 M was carried out for 20 times to accomplish the LOD determination and the measured concentrations were calculated from the ﬁtting equation. The LOD of Hg2+ was calculated to be 1.5 × 10−8 M with detailed data shown in Table S2.
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
Fig. 8. Fluorescence intensity ratio F/F0 of CdS QDs capped by different polymers after the contact with 1 × 10−5 M of different metal ions ([Polymer] = 1.73 × 10−6 M, [CdS] = 5 × 10−5 M; ex = 370 nm).
The interference of other metal ions to Hg2+ detection was studied. Hg2+ ions as well as other interfering ions were added to QDs solutions. The concentration of Hg2+ and interfering ions were set to be 1 × 10−5 M and 1 × 10−4 M, respectively. The PL intensity comparison is shown in Fig. 10, from which it can be seen that the addition of other ions cannot interfere with the Hg2+ detection.
4. Conclusions Core-shell type hyperbranched grafting copolymers, HPEI-gHPGs, were good nanoreactors and stabilizers for the aqueous synthesis of CdS QDs. Compared with the HPG and HPEI homopolymers, HPEI-g-HPGs were better stabilizers to render CdS QDs stronger emission. Moreover, increasing HPG shell content could
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179
bility in solution at 4 ◦ C. The as-prepared CdS QDs were stable in the pH range of 7.0–9.0. As the ratiometric ﬂuorescence sensors, CdS QDs capped by HPEI-g-HPGs were most sensitive to Hg2+ and the LOD value was 1.5 × 10−8 M. Their selectivity towards Hg2+ could be raised effectively through increasing the HPG shell content. Overall, it can be deduced that HPEI-g-HPGs are superior to the homopolymers to prepare robust and functional CdS QDs in water. Moreover, the CdS QDs capped by HPEI-g-HPG785 can be used as the ratiometric ﬂuorescence sensor selectively detecting Hg2+ . Acknowledgment This work was ﬁnancially supported by the National Natural Science Foundation of China (21074088). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.05.070. References
Fig. 9. (A) PL change of HPEI-g-HPG785 stabilized CdS QDs against different Hg2+ concentration, and (B) the linear ﬁtting of PL change against Hg2+ concentration ([Polymer] = 3.45 × 10−6 M, [CdS] = 1 × 10−4 M; ex = 370 nm).
Fig. 10. The interference of other metal ions with the Hg2+ detection by HPEI-g-HPG785 stabilized CdS QDs ([Polymer] = 1.73 × 10−6 M, [CdS] = 5 × 10−5 M; ex = 370 nm).
increase pronouncedly the PL intensity of CdS QDs. The average diameter of CdS QDs with the strongest emission was ca. 2.44 nm, while the average diameter of the composite of CdS QDs and HPEIg-HPG was ca. 5.23 nm. Compared with the HPEI-stabilized CdS QDs, the HPEI-g-HPG-stabilized CdS QDs had a better storing sta-
 C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chem. Rev. 105 (2005) 1025–1102.  B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Recent advances in the liquid-phase syntheses of inorganic nanoparticles, Chem. Rev. 104 (2004) 3893–3946.  C.J. Murphy, Optical sensing with quantum dots, Anal. Chem. 41 (2002) 520A–526A.  A.H. Mueller, M.A. Petruska, M. Achermann, D.J. Werder, E.A. Akhadov, D.D. Koleske, M.A. Hoffbauer, V.I. Klimov, Multicolor light-emitting diodes based on semiconductor nanocrystals encapsulated in gan charge injection layers, Nano Lett. 5 (2005) 1039–1044.  A.J. Nozik, M.C. Beard, J.M. Luther, M. Law, R.J. Ellingson, J.C. Johnson, Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells, Chem. Rev. 110 (2010) 6873–6890.  V.C. Sundar, H.J. Eisler, M.G. Bawendi, Room-temperature, tunable gain media from novel II–VI nanocrystal-titania composite matrices, Adv. Mater. 14 (2002) 739–743.  R.E. Galian, M. de la Guardia, The use of quantum dots in organic chemistry, Trends Anal. Chem. 28 (2009) 279–291.  J.M. Costa-Fernández, R. Pereiro, A. Sanz-Medel, The use of luminescent quantum dots for optical sensing, Trends Anal. Chem. 25 (2006) 207–218.  X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Quantum dots for live cells, in vivo imaging, and diagnostics, Science 307 (2005) 538–544.  I. Onyido, A.R. Norris, E. Buncel, Biomolecule-mercury interactions: modalities of DNA base-mercury binding mechanisms. Remediation strategies, Chem. Rev. 104 (2004) 5911–5929.  W.F. Fitzgerald, C.H. Lamborg, C.R. Hammerschmidt, Marine biogeochemical cycling of mercury, Chem. Rev. 107 (2007) 641–662.  J.L. Gomez-Ariza, F. Lorenzo, T. Garcia-Barrera, Comparative study of atomic ﬂuorescence spectroscopy and inductively coupled plasma mass spectrometry for mercury and arsenic multispeciation, Anal. Bioanal. Chem. 382 (2005) 485–492.  S. Gil, I. Lavilla, C. Bendicho, Ultrasound-promoted cold vapor generation in the presence of formic acid for determination of mercury by atomic absorption spectrometry, Anal. Chem. 78 (2006) 6260–6264.  H.N. Kim, W.X. Ren, J.S. Kim, J. Yoon, Fluorescent and colorimetric sensors for detection of lead, cadmium, and mercury ions, Chem. Soc. Rev. 41 (2012) 3210–3244.  Z. Ansari, S.S. Singha, A. Saha, K. Sen, Hassle free synthesis of nanodimensional Ni, Cu and Zn sulﬁdes for spectral sensing of Hg, Cd and Pb: a comparative study Spectrochim, Acta A, Mol. Biomol. Spectrosc. 176 (2017) 67–78.  Y. Liu, L. Xu, J. Liu, X. Liu, Simultaneous enrichment, separation and detection of mercury(II) ions using cloud point extraction and colorimetric sensor based on thermoresponsive hyperbranched polymer-gold nanocomposite, Anal. Methods 7 (2015) 10151–10161.  Y. Ding, S. Wang, J. Li, L. Chen, Nanomaterial-based optical sensors for mercury ions, Trends Anal. Chem. 82 (2016) 175–190.  T. Gong, J. Liu, X. Liu, J. Liu, J. Xiang, Y. Wu, A sensitive and selective sensing platform based on cdte qds in the presence of L-cysteine for detection of silver, mercury and copper ions in water and various drinks, Food Chem. 213 (2016) 306–312.  J. Zhu, Z.J. Zhao, J.J. Li, J.W. Zhao, Cdte quantum dot-based ﬂuorescent probes for selective detection of Hg (II): The effect of particle size Spectrochim, Acta A, Mol. Biomol. Spectrosc. 177 (2017) 140–146.
Y. Fan et al. / Sensors and Actuators B 251 (2017) 171–179  Y.S. Xia, C.Q. Zhu, Use of surface-modiﬁed cdte quantum dots as ﬂuorescent probes in sensing mercury (II), Talanta 75 (2008) 215–221.  I. Costas-Mora, V. Romero, I. Lavilla, C. Bendicho, An overview of recent advances in the application of quantum dots as luminescent probes to inorganic-trace analysis, Trends Anal. Chem. 57 (2014) 64–72.  J.M.J. Fréchet, Dendrimers and other dendritic macromolecules: from building blocks to functional assemblies in nanoscience and nanotechnology, J. Polym. Sci. Part A: Polym. Chem. 41 (2003) 3713–3725.  D. Astruc, E. Boisselier, C. Ornelas, Dendrimers designed for functions: from physical, photophysical, and supramolecular properties to applications in sensing, catalysis, molecular electronics, photonics, and nanomedicine, Chem. Rev. 110 (2010) 1857–1959.  C. Gao, D. Yan, Hyperbranched polymers: from synthesis to applications, Prog. Polym. Sci. 29 (2004) 183–275.  B. Voit, Hyperbranched polymers—all problems solved after 15 years of research? J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 2679–2699.  X. Hu, L. Zhou, C. Gao, Hyperbranched polymers meet colloid nanocrystals: a promising avenue to multifunctional robust nanohybrids, Colloid Polym. Sci. 289 (2011) 1299–1320.  D. Wan, Q. Fu, J. Huang, Synthesis of a thioether modiﬁed hyperbranched polyglycerol and its template effect on fabrication of CdS and CdSe nanoparticles, J. Appl. Polym. Sci. 102 (2006) 3679–3684.  J. Mao, J.N. Yao, L.N. Wang, W.S. Liu, Easily prepared high-quantum-yield CdS quantum dots in water using hyperbranched polyethylenimine as modiﬁer, J. Colloid Interface Sci. 319 (2008) 353–356.  M.L. Hassan, A.F. Ali, Synthesis of nanostructured cadmium and zinc sulﬁdes in aqueous solutions of hyperbranched polyethyleneimine, J. Crystal Growth 310 (2008) 5252–5258.  S. Kosmella, J. Venus, J. Hahn, C. Prietzel, J. Koetz, Low-temperature synthesis of polyethyleneimine-entrapped cds quantum dots, Chem. Phys. Lett. 592 (2014) 114–119.  Y. Shi, C. Tu, R. Wang, J. Wu, X. Zhu, D. Yan, Preparation of cds nanocrystals within supramolecular self-assembled nanoreactors and their phase transfer behavior, Langmuir 24 (2008) 11955–11958.  L. Zhu, Y. Shi, C. Tu, R. Wang, Y. Pang, F. Qiu, X. Zhu, D. Yan, L. He, C. Jin, B. Zhu, Construction and application of a pH-sensitive nanoreactor via a double-hydrophilic multiarm hyperbranched polymer, Langmuir 26 (2010) 8875–8881.  S. Saliba, C.V. Serrano, J. Keilitz, M.L. Kahn, C. Mingotaud, R. Haag, J.D. Marty, Hyperbranched polymers for the formation and stabilization of ZnO nanoparticles, Chem. Mater. 22 (2010) 6301–6309.  Y. Shi, J. Liang, L. Liu, L. He, C. Tu, X. Guo, B. Zhu, C. Jin, D. Yan, T. Han, X. Zhu, A new two-phase route to cadmium sulﬁde quantum dots using amphiphilic hyperbranched polymers as unimolecular nanoreactors, J. Appl. Polym. Sci. 120 (2011) 991–997.  C.S. Popeney, M.C. Lukowiak, C. Böttcher, B. Schade, P. Welker, D. Mangoldt, G. Gunkel, Z. Guan, R. Haag, Tandem coordination, ring-opening, hyperbranched polymerization for the synthesis of water-soluble core-shell unimolecular transporters, ACS Macro Lett. 1 (2012) 564–567.  Y. Liu, Y. Fan, Y. Yuan, Y. Chen, F. Cheng, S.C. Jiang, Amphiphilic hyperbranched copolymers bearing a hyperbranched core and a dendritic shell as novel stabilizers rendering gold nanoparticles with an unprecedentedly long lifetime in the catalytic reduction of 4-nitrophenol, J. Mater. Chem. 22 (2012) 21173–21182.  Y. Liu, Y. Fan, X.Y. Liu, S.Z. Jiang, Y. Yuan, Y. Chen, F. Cheng, S.C. Jiang, Amphiphilic hyperbranched copolymers bearing a hyperbranched core and dendritic shell: synthesis, characterization and guest encapsulation performance, Soft Matter 8 (2012) 8361–8369.  Y. Xu, C. Gao, H. Kong, D. Yan, P. Luo, W. Li, Y. Mai, One-pot synthesis of amphiphilic core-shell suprabranched macromolecules, Macromolecules 37 (2004) 6264–6267.  Y. Fan, Y.Q. Cai, X.B. Fu, Y. Yao, Y. Chen, Core-shell type hyperbranched grafting copolymers: preparation, characterization and investigation on their intrinsic ﬂuorescence properties, Polymer 107 (2016) 154–162.  L. Jing, S.V. Kershaw, Y. Li, X. Huang, Y. Li, A.L. Rogach, M. Gao, Aqueous based semiconductor nanocrystals, Chem. Rev. 116 (2016) 10623–10730.
 L.E. Brus, Electron–electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state, J. Chem. Phys. 80 (1984) 4403–4409.  W.W. Yu, L. Qu, W. Guo, X. Peng, Experimental determination of the extinction coefﬁcient of CdTe, CdSe, and CdS nanocrystals, Chem. Mater. 15 (2003) 2854–2860.  B.C. Fitzmorris, J.K. Cooper, J. Edberg, S. Gul, J. Guo, J.Z. Zhang, Synthesis and structural, optical, and dynamic properties of core/shell/shell CdSe/ZnSe/ZnS quantum dots, J. Phys. Chem. C 116 (2012) 25065–25073.  D.E. Moore, K. Patel, Q-cds photoluminescence activation on Zn2+ and Cd2+ salt introduction, Langmuir 17 (2001) 2541–2544.  G.R. Newkome, C.N. Mooreﬁeld, F. Vögtle, Dendritic Molecules: Concepts, Synthesis, Perspectives, VCH, Weinheim, 2001.  W. Cai, Z. Li, J. Sui, A facile single-source route to CdS nanorods, Nanotechnology 19 (2008) 465606.  M. Tomasulo, I. Yildiz, F.M. Raymo, pH-sensitive quantum dots, J. Phys. Chem. B 110 (2006) 3853–3855.  Y. Wang, Z. Tang, M.A. Correa-Duarte, I. Pastoriza-Santos, M. Giersig, N.A. Kotov, L.M. Liz-Marzán, Mechanism of strong luminescence photoactivation of citrate-stabilized water-soluble nanoparticles with CdSe cores, J. Phys. Chem. B 108 (2004) 15461–15469.  Y.S. Liu, Y. Sun, P.T. Vernier, C.H. Liang, S.Y.C. Chong, M.A. Gundersen, pH-sensitive photoluminescence of CdSe/ZnSe/ZnS quantum dots in human ovarian cancer cells, J. Phys. Chem. C 111 (2007) 2872–2878.  Y. Chen, Z. Rosenzweig, Luminescent cds quantum dots as selective ion probes, Anal. Chem. 74 (2002) 5132–5138.  Y. Long, D. Jiang, X. Zhu, J. Wang, F. Zhou, Trace Hg2+ analysis via quenching of the ﬂuorescence of a CdS-encapsulated DNA nanocomposite, Anal. Chem. 81 (2009) 2652–2657.  A.V. Isarov, J. Chrysochoos, Optical and photochemical properties of nonstoichiometric cadmium sulﬁde nanoparticles: surface modiﬁcation with copper(II) ions, Langmuir 13 (1997) 3142–3149.
Biographies You Fan graduated in Applied Chemistry in 2012 from Tianjin University, Tianjin, China. Currently, he is a PhD student in the Department of Chemistry of Tianjin University. His work focuses on the syntheses of functional hyperbranched copolymers and their composites with inorganic nanoparticles. Ya-Qian Cai graduated in Applied Chemistry in 2016 from Tianjin University, Tianjin, China. Currently, she is a master student in the College of Chemistry of Nankai University, Tianjin, China. Her current work focuses on the syntheses and applications of functional comb copolymers. Hua-Ji Liu graduated in Chemical Education in 1998 and received her Master degree in Physical Chemistry in 2001 from Qufu Normal University, Shandong, China. She obtained her PhD in Physical Chemistry in 2004 from Zhengjiang University, Zhejiang, China. Then she worked in Department of Chemistry of Tianjin University as lecturer. Since July 2007, she became associate professor in Department of Chemistry of Tianjin University. Her research focuses on preparation and application of organic/inorganic hybrid materials. Yu Chen graduated in Chemistry in 1996 and received his PhD in Polymer Chemistry & Physics in 2001 from Peking University, Beijing, China. Later he worked as postdoctoral fellow at Albert Ludwigs-Universität Freiburg, Germany (Feb., 2002–Sept., 2002) and Johannes Gutenberg-Universität Mainz, Germany (Oct., 2002–Aug., 2005), respectively. Then he returned to China and worked in Department of Chemistry of Tianjin University as associate professor. Since July 2010, he became full professor in Department of Chemistry of Tianjin University. His research focuses on (i) syntheses of intelligent dendritic polymers or supramolecules; (ii) synthesis and application of branched polymers; (iii) preparation and application of composites of branched polymers and inorganic particles.