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L-cysteine and 3-mercaptopropionic acid capped cadmium selenide quantum dots based metal ion probes
Journal of Luminescence 187 (2017) 126–132
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
L-cysteine and 3-mercaptopropionic acid capped cadmium selenide quantum dots based metal ion probes Reena K. Sajwan, Yana Bagbi, Purnima Sharma, Pratima R. Solanki n Special Centre for Nanoscience, Jawaharlal Nehru University, New Delhi 110067, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2016 Accepted 27 February 2017 Available online 28 February 2017
Herein, a report on the synthesis of 3-mercaptopropionic acid (MPA) and L cysteine (Cyst) capped luminescent cadmium selenide (CdSe) quantum dots (QDs) was studied. The MPA-CdSe and Cyst-CdSe QDs were conducted with different physiologically important metal ions such as Cu2 þ , Fe2 þ , Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ ions in aqueous solution using fluorescence spectroscopic technique. It was found that the MPA-CdSe QDs, categorically sensitive to only Cu2 þ ions only. However, other metal ions such as Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ , did not exhibit measurable quenching. On the other hand, Cyst-CdSe QDs were very selective and sensitive to Zn2 þ ions only, which showed fluorescence enhancement while for other ions no such behavior was noticed. Thus, MPA-CdSe and Cyst-CdSe QDs have successfully detected Cu2 þ and Zn2 þ ions in the linearity range from 4 to 160 mM, respectively. The effect of fluorescence response of QDs towards metals ions were described by Langmuir-type binding model or Stern–Volmer-type equation. & 2017 Elsevier B.V. All rights reserved.
Keywords: CdSe quantum dots Fluorescence Metal ions 3-mercaptopropionic acid L-cysteine
1. Introduction Photoluminescent semiconductor nanocrystals or quantum dots (QDs) have become one of the most attractive fields of current research because of their unique size-dependent optical properties [1]. Published data has proved that development of an optical sensor for detection of toxins, heavy metals, and other environmental pollutants is possible using this platform [2,3]. Over organic fluorescent dyes, these QDs are advantageous due to their interesting optical properties like size, tunable color, larger fluorescence quantum yields and narrow spectral bands and less photobleaching susceptibility [4]. In hot-injection protocol, it is possible to produce QDs in the size range of 2.5–6.3 nm, with each of these particles exhibiting their unique spectral properties [5]. Recently, selective detection of biologically important molecules and environmentally non-toxic metal cations is an active area of interest [6]. Most of the sensing systems reported in the literature have been found on organic molecules and inorganic complexes. Recently, research is focusing on designing QD-based hybrid systems for sensing applications, due to their fascinating optical properties, which are tunable with size and shape [7–9]. Surface modified QDs show a change in their optical, chemical and photo-catalytic properties leading to unusual optical effects such n
Corresponding author. E-mail addresses: [email protected], [email protected] (P.R. Solanki). http://dx.doi.org/10.1016/j.jlumin.2017.02.061 0022-2313/& 2017 Elsevier B.V. All rights reserved.
as enhancement of their excitonic, defect emission and improvement of the photo-stability of semiconductor nanoparticles. It is reported that, after functionalization, the QDs can create new emission bands that can enhance the selectivity and efficiency of light-induced reactions [10]. Most of the studies reported on CdSe QDs are capped with an organic layer, such as a trioctyl phosphine/ trioctylphosphine oxide mixture (TOP/TOPO). This layer is coordinated to Cd2 þ sites, and stabilizes the nanocrystals surface, thereby preventing an irreversible flocculation of the particulate matter [11]. Unfortunately, these protective ligands are hydrophobic, capped with organic molecules, and are not analytically compatible for aqueous assay. To overcome these problems, further modification of QDs should be done with hydrophilic molecules as capping agents. Production of water-soluble QDs are urgently required for analytical assay [12]. The most common binding groups such as thiol-containing molecules, amines, amino acids and peptides [13,14] are being utilized to make water soluble QDs, to replace the inherited organic ligand capped on the QD surface using electrostatic, hydrophobic interactions or host–guest interactions. The most common thiol-containing molecules used to make aqueous QDs are thiolated aliphatic carboxylic acids using mercaptopropionic acid (MPA) [15], mercaptoacetic acid (MAA) [16,17], 16-mercaptohexadecanoic acid (MHA) [18] and L-cysteine (Cyst) [9,19]. Among them, MPA and Cyst were more promising capping ligand which makes water soluble QDs. The water soluble QDs retain up to half of their quantum yields after transferring from organic solvent to aqueous solution [8]. The MPA is an organic
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molecule consisting two functional groups. The coordination of these groups on the surface of QDs has two important advantages: (i) dangling bonds act as quenchers for the emission in QDs, (ii) prevent the QDs from agglomeration. In addition, capping the QDs with such kind of molecules is very effective for binding the QDs with other molecules like metal ion polymer and forms a hybrid material, and can be used as biological labels [20]. Besides this, Cyst is an amino acid which combines catalytic activity with an extensive redox chemistry and has remarkable metal binding properties [21,22]. It is nontoxic, water-soluble, sulfur-containing amino acid having three functional groups (-SH, -NH2, -COOH), which has strong tendency to coordinate with any inorganic cations and heavy metals ions [22]. Thus, the surface modification of QDs with MPA and Cyst makes them more suitable for interaction with targeted materials [23], commonly used for QDs based metal ion sensing. In addition, MPA and Cyst capped QDs were cheap and more stable than the other surface modification. Chen and Rosenzweig have demonstrated the surface functionalized cadmium sulfide quantum dots as luminescence probes for detecting copper and zinc ions in aqueous solution. The nature of the ligand has dramatic effect on type and selectivity of the luminescence response [9]. The interaction between metal ions and QDs surface ligand is observed by quenching and enhancement of luminescence [8]. Before the metal ions can reach the QDs surface, they need to pass through the capping layer. It is accepted that the resulting chemical or physical interaction between metal ions and surface ligand of QDs could significantly change the photoluminescence of the QDs. Generally the optical sensing function of semiconductor QDs originates from the photoexciton of the QDs and transfer of electrons between the valance and conduction band. Sensing of small metal ions in aqueous solution using QDs could be demonstrated by the analyte-induced changes in photo luminescence or quenching the fluorescence luminescence. Quenching mechanism involves many effects including inner filter effects, ion induced generation of nonradiative recombination reaction, ion binding interaction (leads imperfection of the QD surface or the agglomeration of the QD particles) and electron-transfer processes [9,27,28]. Herein, we have synthesized the water soluble MPA and Cyst capped luminescent cadmium selenide (CdSe) quantum dots (QDs). The interaction of these MPA and Cyst capped CdSe QDs has been studied with different physiologically important metal ions such as Cu2 þ , Fe2 þ , Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ ions in aqueous solution. It is found that the MPA-QDs were categorically sensitive to only Cu2 þ and Fe2 þ ions as evident from their substantial fluorescence quenching data. On the other hand, CystCdSe QDs were very selective and sensitive to Zn2 þ ions only, which showed fluorescence enhancement while for other ions no such behavior was noticed. Therefore, the developed sensor is simple for detection of Cu2 þ and Zn2 þ ion as compared to previous reports [9,19].
2.1. Preparation of water soluble MPA capped CdSe QDs CdSe QDs were synthesized using CdO as a precursor according to a procedure described by Peng's group [24,25]. The CdSe QDs were prepared in trioctylphosphine oxide as a solvent. For the synthesis of MPA-capped CdSe QDs, a solution of MPA (108 mL) and NaOH (160 mg) in 25 mL carbinol was prepared under continuous stirring until a clear solution obtained. Typically, 10 mL CdSe QDs were dispersed in 25 mL of chloroform in a beaker and MPA solution were added dropwise under continuous stirring until flocculation appeared in the solution after addition of appropriate amount of water resulting in a two-phase separation; hydrophilic QDs (upper phase) and hydrophobic phase (lower phase) as shown in photograph in the inset (Scheme 1). Then MPA-capped QDs were separated from upper phase of the solution and were stored at room temperature in pre-cleaned borosilicate glass bottles for further use. Scheme 1 shows the accepted interaction between the CdSe QDs and Cyst or MPA. The general approach to make MPAcapped QDs is to exchange native TOPO ligands capped on the QD surface with thiol ligands. When MPA solution is added dropwise in TOPO CdSe QDs solution under continuous stirring, MPA slowly starts abolishing the TOPO ligands and thiol group taken place on QDs surface either by using electrostatic, hydrophobic or host guest interaction. The Cyst capped CdSe QDs were prepared using a solution of Cyst (1 mg) and NaOH (160 mg) in 25 mL carbinol under continuous stirring until a clear solution obtained. Next step was similar to the procedure used for the synthesis of MPA capped CdSe QDs.
2.2. Preparation of different metal ion stock solutions Freshly prepared aqueous solution of the chloride salts of metal ions (Zn2 þ , Cu2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ) with different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) in distilled water of pH 7 were prepared by stirring at room temperature.
2.3. Characterization techniques The CdSe QDs sizes were determined from high-resolution transmission electron microscope (HRTEM) images obtained from JEOL JEM-2200 FS (Japan) instrument operating at a voltage of 200 kV. Fourier transform infrared spectroscopy (FT-IR) spectra of QDs were recorded on a Varian 7000 FT. Fluorescence spectra of the CdSe QDs were analyzed using fluorescence spectrophotometer (Shimadzu model rf 5301pc Spectrofluorometer). The emission spectra (550–900 nm) was recorded under fixed excitation wavelength (490 nm).
2. Materials and methods Cadmium oxides (CdO, 99%), selenium powder (Se, 99%), trioctylphosphine (TOP, 90%), 1-octadecene (90%), Oleic acid (OA, 65%), 3-mercaptopropionic Acid (MPA; 90%) were purchased from sigma Aldrich (USA). Chloroform, methanol (Carbinol), L-cysteine (Cyst; 99%), sodium hydroxide pellets (NaOH), chloride salts of all metals (Zn2 þ , Cu2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ) and silicone oil/ paraffin oil, double distilled deionized water were purchased from a local chemical company supplier. All the chemical reagents were of analytical grade and used as received without further purification.
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Scheme 1. Exchange of ligand on surface of CdSe QDs.
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3. Results and discussion 3.1. Characterization studies Fig. 1 shows the TEM images of CdSe QDs. It is clearly visible that uniform dispersed QDs particle having the particles size in the range of 2–5 nm (image a). HRTEM images shows ultra-small fringes which implies that the structure of CdSe QDs was purely crystalline in nature (image b) and d-spacing value obtained as 3.18 nm. Fig. 2 shows the FTIR studies of (a) TOPO capped QDs; (b) pure MPA; (c) MPA capped QDs; (d) pure Cyst; (e) Cyst capped QDs. The OH- vibration peak at 3450 cm 1 (Fig. 2 curve c), indicates the presence of water molecules. Due to high surface-to-volume ratio, large numbers of water molecules were adsorbed on the surface of CdSe quantum dots. The band around 2928 cm 1 has C-C-H stretching that indicates the presence of TOPO (curve (a)) and there was no evidence of this peak after ligand exchange on surface of the CdSe QDs (curve (c and e)) indicates complete replacement of TOPO ligands. The peak around 1715 and 1643 cm 1 appears due to MPA capped CdSe QDs (curve (c)). However, the IR peaks around at 1715 and 1635 cm 1represents Cyst capped QDs (curve (e)) assigned to C¼C bending. In case of Cyst pure, primary amines contains the – NH2 group, and so have N–H bonds. The peak was observed around 3442 cm 1 which implied the presence of N–H bond on the surface of CdSe QDs (curve (e)). IR peaks within 2500–2750 cm 1 region, shows the presence of S–H bond, but here no any peak corresponding to S–H bond in capped CdSe QDs samples. This indicates that the sulfur bond replaced the thiol bond and formed a linkage as –S-Cd [26]. FTIR data clearly shows that the MPA and Cyst ligand attached on the surface of the CdSe QDs after replacing the TOPO ligand from the CdSe QDs surface. These results indicated that CdSe QDs were successfully capped to MPA and Cyst, respectively. 3.2. Effect of pH responses of QDs on luminescence and absorption Generally, pH is one of the major variables which influence the fluorescence of the QDs. To check the pH effect of MPA capped CdSe QDs on luminescence response on quenching, the experiment was conducted at different basic pH from 6–11. It was observed that the emission intensity of the MPA QDs were increases with the basic nature (pH 6–11) of the solution. The observed enhancement very likely results from reduced non-radiative recombination by minimizing the surface defects. It can be cleared from the Fig. S1 (Supplementary data) that optimal pH lies within 7–8, which can be selected for further fluorescence studies. In addition, the effect of pH on the absorption response of MPA-Capped CdSe QDs was performed and observed that the absorbance intensity of the watersoluble MPA-Capped CdSe QDs increases with the basicity of the environment (Fig. S2 data). This indicates, the absorbance intensity was found to be enhanced when the pH varies from 6 to 10. The
Fig. 2. FTIR spectra of (a) TOPO capped QDs; (b) Pure MPA; (c) MPA capped QDs; (d) pure L-Cyst; (e) L-Cyst capped QDs.
enhancement is results from the changes caused on the surface of CdSe QDs by variation in pH and red shift owing to increase in size. Due to size increment the energy gab decreased which shows that the quantum effects of QDs were eliminated when QDs particles tends towards the bulk size (Fig. S3 data). Similar results was obtained for Cyst capped CdSe QDs, thus, all the experiments were performed at pH 8.0. 3.3. Florescence studies The fluorescence emission spectra recorded at excitation wavelength 490 nm. The three intense peaks corresponding to TOPO CdSe QDs at 643 nm, MPA and Cyst capped CdSe QDs at 662 and 711 nm, respectively were observed (Fig. 3). The strong red shift of QDs was previously observed by Lakowicz et al. [27] discussed the potential of these particles as red luminescent probes in biological sample. Typically, the red shift was observed in the wavelength which implies increase in particle size. The particle size of the TOPO CdSe QDs was 3.1 nm. However, the size of CdSe QDs were changed after functionalized with MPA and Cyst as 3.5 and 4.4 nm, respectively. The spectra also gave clear indication for change in the fluorescence intensity after surface modification. 3.4. Quenching response studies of MPA capped Fig. 4 shows the response studies of MPA capped QDs as a function of different concentrations of Cu2 þ ion varies from 4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM. During the experiment, 12 samples having stoichiometric amount (2 mL) of MPA capped CdSe
Fig. 1. TEM images (a) and (b) HRTEM image of CdSe QDs.
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Scheme 2. Interaction of MPA Capped CdSe QDs with Cu2 þ ions.
Fig. 3. Emission spectra of TOPO, MPA and Cyst capped CdSe QDs.
Fig. 5. Quenching effect of copper ion concentration on the luminescence of MPA capped CdSe QDs.
Fig. 4. Quenching response studies of MPA capped CdSe QDs as a function of different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) of Cu2 þ ion.
QDs and an appropriate amount (0.5 mL) of different concentration of Cu2 þ added into them separately. The fluorescence response of each samples was recorded under fluorescence spectrophotometer at the excitation wavelength 490 nm. It was observed that the luminescence intensity decreases with increased in Cu2 þ ion concentrations. The Cu2 þ , bound to the surface of the QDs, effectively quenched the fluorescence luminescence of MPA capped QDs. These results indicate that the quenching in luminescence of MPA capped QDs occurs due to the interaction with Cu2 þ ion. There are two possibilities for interaction: First, due to the electrode potential of Cu2 þ /Cu þ made Cu2 þ most appropriate for the effective electron transfer from QDs to the metal ions, which accounted for the effective quenching of the QDs luminescence [9,29]. Second, Cu2 þ was brought to the CdSe QDs surface due to the intrinsic high affinity constant of the ligand-Cu2 þ complex, resulting in the chemical replacement of Cd2 þ ions by Cu2 þ ions and formed a partially soluble particle (CuSe) on the surface of CdSe QDs. These small particles could quiche the recombination luminescence of MPA capped QDs by facilitating nonradiative electron-hole decimation, acting as electron-hole recombination centre [28–30]. More over, the quenching mechanisms involve other effects such as inner filter effects, ion- induced generation of non-radiative recombination pathways, ion binding interaction and the photoinduced electron transfer process [8]. Scheme 2 shows the tentative interaction between the Cu2 þ
ion and hydroxyl group of MPA capped CdSe QDs. Fig. 5 shows the quenching effect of Cu2 þ concentration on MPA capped CdSe QDs. The fluorescence quenching is described by a Stern–Volmer -type Eq. (1) [31].
Io /I = 1 + Ksv⎡⎣ Q⎤⎦
(1)
Here, I and Io are the luminescence intensities of the MPA capped QDs samples at a given Cu2 þ concentration, and in ion free solution, respectively. Q is the Cu2 þ concentration. Ksv is the SternVolmer constant. Fig. 6 shows a Stern-Volmer quenching curve describing Io/I as a function of Cu2 þ concentration. The Stern –Volmer constant was obtained to be Ksv ¼9.3038 103 M 1. 3.5. Selectivity studies for MPA capped CdSe QDs The high intrinsic attraction between the metal ions and the surface ligand is the incisive factor for ions selectivity. For the selectivity of MPA capped QDs towards Cu2 þ , the response of other ionic species (interfering) such as Zn2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ions was monitored (Fig. 7). During this measurement, the fixed amount (0.5 mL) of interfering species of 1 mM concentration were separately added into stoichiometric amount (2 mL) of MPA capped CdSe QDs and the fluorescence emission were recorded under fluorescence spectrophotometer. It was observed that the highest quenching of MPA capped CdSe QDs occurred in the presence of Cu2 þ , that is not much affected in the presence of other ionic species except Fe þ . The quenching effect of Fe3 þ on the luminescence of MPA capped QDs is attributed to strong absorption of excitation wavelength by Fe3 þ . However, MPA capped QDs exhibit highest selectivity towards Cu2 þ .
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Fig. 6. Stern-Volmer plots describe Cu2 þ ions concentration dependence of the luminescence intensity of MPA capped CdSe QDs.
Fig. 8. Effect of the addition of different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) of Zn2 þ ions on Cyst capped QDs.
Scheme 3. Interaction of Cyst Capped QDs with Zn2 þ ions.
Fig. 7. Quenching effect of MPA capped CdSe QDs in the presence of other ionic species.
3.6. Quenching response studies of cyst capped CdSe QDs Fig. 8 shows the response studies of Cyst capped CdSe QDs as a function of different concentrations varies from 4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM. Here, 12 samples were taking having equal amount (2 mL) of Cyst capped CdSe QDs and a appropriate amount (0.5 mL) of different concentration of Zn2 þ ion were added separately into them. And after that the luminescence spectras of each samples were recorded by the fluorescence spectrophotometer under the excitation wavelength of 490 nm. It was observed that the luminescence intensity increases with increased in Zn2 þ ion concentrations (Fig. 8). This enhancement of the luminescence intensity is associated with the passivation of surface trap sites through formation of a surface passivation layer, due to binding of Zn2 þ ion, eliminating the non-radiative relaxation sites and preventing photochemical degradation and hence increased luminescence intensity of the Cyst capped CdSe QDs. Moreover, due to formation of complex of Zn2 þ ion onto QDs surface which would restrict ligand rotation that enhanced the conformational rigidity of the surface substituent, resulting in suppress the quenching of Cyst capped CdSe QDs [8]. The luminescence intensity of Cyst capped CdSe QDs was sensitized by 40% solution after addition of Zn2 þ ion of concentration as compared to the emission of these QDs from ion free
solution. Scheme 3 shows the tentative interaction between the Zn2 þ ion and carboxyl group of Cyst capped CdSe QDs. Fig. 9 shows the curve which explains the effect of Zn2 þ ion on emission intensity of Cyst capped CdSe QDs. Here the emission intensity continuously increased with Zn2 þ ion concentration. The concentration dependence of the luminescence intensity allowed the binding of Zn2 þ ions to the surface of these QDs, and could effectively describe Langmuir-type binding isotherm [32]. According to Langmuir, the surface of the QDs consists of a finite number of binding sites. Each of the binding sites could adsorb one ion from the solution. The fraction of occupied sites is defined as
Fig. 9. Sensitizing effect of Zn2 þ ions concentration on the luminescence of Cyst capped CdSe QDs.
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Θ. The rate of binding of ions to the surface is proportional to the ion concentration C in the analyte solution, and to the fraction of available binding sites (1-Θ). The fraction of occupied binding sites, Θ, is related to the ratio between the signal obtained at a given ion concentration I and the maximum intensity Io of ion free solution as shown in Eq. (2).
Θ ≈ I /Io
(2)
Accordingly, if the Langmuir description of the binding of Zn2 þ ions on the surface of Cyst capped QDs is valid, a plot of C/I as a function of C should be linear. Where C is the concentration of Zn2 þ ions, and I is the luminescence intensity of Cyst capped QDs at given Zn2 þ ions concentration is shown in Fig. 10. The correlation coefficient of the linear fit was obtained as 0.958. 3.7. Selectivity studies of Cyst capped CdSe QDs The luminescent intensity of Cyst capped CdSe were observed against different ions such as Zn2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ions (Fig. 11). The maximum emission intensity occurred in the presence of Zn2 þ ions. During this measurement, the fixed amount (0.5 mL) of interfering species of 1 mM concentration were separately added into stoichiometric amount (2 mL) of Cyst capped CdSe QDs and the fluorescence emission were recorded under fluorescence spectrophotometer. Thus, it was concluded
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that the Cyst capped CdSe QDs were sensitive towards the Zn2 þ ions concentration and shows maximum luminescence quenching in presence of Zn2 þ ions. Similar quenching trends were also observed with the Fe3 þ ions. The photostability of MPA and Cyst capped CdSe QDs was observed for about 25 days as stored in the daylight at room temperature (25 °C). During the initial period both the MPA and Cyst capped CdSe QDs does not show any change in their absorbance. Cyst capped CdSe QDs agglomerate and starts degradation over the period of 10–12 days and lost their emission properties, but MPA capped CdSe QDs found more stable at same condition.
4. Conclusions This work describes the quantitative application of luminescent semiconductors QDs in transition metal ions detection. The effect of two metal ions Zn2 þ and Cu2 þ on the luminescence response was investigated. Cyst-capped QDs showed a selective response to Zn2 þ , and showed minimal response to other cations. A Langmuir type binging model effectively describes the Zn2 þ concentration dependence of the luminescence intensity for Cyst-capped QDs. MPA-capped QDs showed preferential response, and sensitivity towards Cu2 þ over other cations. A Stern-Volmer equation described the Cu2 þ concentration dependence of the luminescence intensity of the MPAcapped QDs. With MPA-capped QDs, that could detect upto 4–160 mM concentration of Cu2 þ (safe limit of copper ion E10 mM, according to WHO guidelines). Similar, is the case with the Zn2 þ ions. The detection limit varies from 4 to 160 mM (safe limit of zinc ion E30 mM, according to WHO guidelines). This propose an alternative platform for the detection and sensing of metal ions (Zn2 þ and Cu2 þ ) in water. Transition metals are major pollutants and are commonly present in industrial affluent, thus, the study of their detection and remediation can hardly be stressed. Studies relating to pHs effect and energy responses on quenching of metal ions with CdSe QDs is open for future research.
Acknowledgements
Fig. 10. Linear fit thought Zn2 þ ion concentration with Langmuir binding isotherm.
Authors are thankful to Advanced Instrumentation Research Facility JNU, New Delhi for providing the facilities of HRTEM and FT-IR characterizations. R.K.Sajwan is thankful to Dr. Kamla Rawat for discussion. This work has supported by a grant received from Department of Science and Technology Government of India (DST Purse, JNU) and UGC (UPE-II; Project 58).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.02.061.
References
Fig. 11. Effect of 1 mM of different ions on the luminescence of the Cyst capped QDs.
[1] C. Zhong, G.U. Zheng, J.L. Zeng, J.H. Liu, Q. Deng, J.B. Fan, J.N. Xiang, A novel fluorescent probe for copper ions based on polymer-modified CdSe/CdS core/ shell quantum dots, Anal. Sci. 27 (2011) 643. [2] H. Mattoussi, J.M. Mauro, E.R. Goldman, G.P. Anderson, V.C. Sundar, F. V. Mikulec, M.G. Bawendi, Self-assembly of CdSe ZnS quantum qot bioconjugates using an engineered recombinant protein, J. Am. Chem. Soc. 122 (2000) 12142–12150. [3] J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, The use of luminescent quantum dots for optical sensing, Trends Anal. Chem. 25 (2006) 3. [4] A.C. Quitaneg, G. Nanato, C. Santos, Cadmium selenide quantum qots synthesized by HVPC growth technique for sensing copper ion concentrations, J. Spec. Educ. Rehabil. 2 (12) (2011) 2229–5518.
132
R.K. Sajwan et al. / Journal of Luminescence 187 (2017) 126–132
[5] P. Wu, T. Zhao, S. Wang, X. Hou, Semiconductor quantum dots-based metal ion probes, Nanoscale 6 (2014) 43. [6] M.A. Ellis, G. Grandinetti, K.M. Fichter, Synthesis of Cd-free InP/ZnS quantum dots suitable for biomedical applications, J. Vis. Exp. 6 (2016) 108. [7] J. Li, D. Bao, X. Hong, D. Li, J. Li, Y. Bai, T. Li, Luminescent CdTe quantum dots and nanorods as metal ion probes, Eng. Asp. 257–258 (2005) 267–271. [8] Y.B. Lou, Y.X. Zhao, J.X. Chen, J.J. Zhu, Metal ions optical sensing by semiconductor quantum dots, J. Mater. Chem. C 2 (2014) 595–613. [9] Y. Chen, Z. Rosenzweig, Luminescent CdS quantum dots as selective ion probes, Anal. Chem. 74 (2002) 5132–5138. [10] A.V. Isarov, J. Chrysochoos, Optical and photochemical properties of nonstoichiometric cadmium sulfide nanoparticles: surface modification with copper (II) ions, Langmuir 13 (1997) 3142. [11] E. Rabani, Structure and electrostatic properties of passivated CdSe nanocrystals, J. Chem. Phys. 115 (2001) 1493. [12] H. Mattoussi, J.M. Mauro, E.R. Golman, G.P. Anderson, V.C. Sundar, F.V. Mikulec, 13 M.G. Bawendi, Self-assembly of CdSe ZnS quantum dot bioconjugates using an engineered recombinant protein, J. Am. Chem. Soc. 122 (2000) 12142. [13] F. Dubois, B. Mahler, B. Dubertret, E. Doris, C. Mioskowski, A versatile strategy for quantum dot ligand exchange, J. Am. Chem. Soc. 129 (2007) 482–483. [14] W.W. Yu, E. Chang, R. Drezek, V.L. Colvin, Water-soluble quantum dots for biomedical applications, Biochem. Biophys. Res. Commun. 348 (2006) 781–786. [15] D.R. Baker, P.V. Kamat, Tuning the emission of CdSe quantum dots by controlled trap enhancement, Langmuir 26 (2010) 11272–11276. [16] Y.S. Liu, Y.H. Sun, P.T. Vernier, C.H. Liang, S.Y.C. Chong, M.A. Gundersen, pHsensitive photoluminescence of CdSe/ZnSe/ZnS quantum dots in human ovarian cancer cells, J. Phys. Chem. C 111 (2007) 2872–2878. [17] J.A. Kloepfer, S.E. Bradforth, J.L. Nadeau, Photophysical properties of biologically compatible CdSe quantum dot structures, J. Phys. Chem. B 109 (2005) 9996–10003. [18] Y.H. Chan, J. Chen, Q. Liu, S.E. Wark, D.H. Son, J.D. Batteas, Ultrasensitive copper (II) detection using plasmon-enhanced and photo-brightened luminescence of CdSe quantum dots, Anal. Chem. 82 (2010) 3671–3678. [19] M. Koneswaran, R. Narayanaswamy, L-cysteine-capped ZnS quantum dots based fluorescence sensor for Cu2 þ ion, Sens. Actuators B 139 (2009) 104–109. [20] S. Wageh, Anwer A. Higazy, Ahmed S. Hassouna, Optical properties and
[21] [22]
[23]
[24] [25]
[26]
[27] [28]
[29] [30]
[31]
[32]
thermal degradation of CdSe capped with 3-mercaptopropionic acid, J. Mater. Sci.: Mater. Electron 24 (2013) 3049–3057. N.M. Giles, G.I. Giles, C. Jacob, Multiple roles of cysteine in biocatalysis, Biochem., Biophys. Res. Commun. 300 (2003) 1–4. J.A. Thomas, R. Mallis, H. Sies, Protein S-thiolation, S-nitrosylation, and irreversible sulfhydryl oxidation: rolesin redox regulation, in: C. Gitler, A. Danon (Eds.), Cellular Implications of Redox Signalling, World Scientific Publ, Singapore, 2003, pp. 141–174. R. Kho, C.L. Torres-Marteiz, R.K. Mehra, A simple colloidal synthesis for gramquantity production of water-soluble ZnS nanocrystal powders, J. Colloids Interface Sci. 227 (2000) 561–566. X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Shape control of CdSe nanocrystals, Nature 404 (2000) 59–61. W.W. Yu, L. Qu, W. Guo, X. Peng, Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals, Chem. Mater. 15 (14) (2003) 2854–2860. I.S. Liu, H.H. Lo, C.T. Chien, Y.Y. Lin, C.W. Chen, Y.F. Chen, W.F. Su, S.C. Liou, Enhancing photoluminescence quenching and photoelectric properties of CdSe quantum dots with hole accepting ligands, J. Mater. Chem. 18 (2008) 675–682. J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd edition, ISBN 9781-4757-3061-6. M.T. Fernández-Argüelles, W.J. Jin, J.M. Costa-Fernández, R. Pereiro, A. SanzMedel, Surface-modified CdSe quantum dots for the sensitive and selective determination of Cu(II) in aqueous solutions by luminescent measurements, Anal. Chim. Acta 549 (2005) 20–25. K.M. Gattás-Asfura, R.M. Leblanc, Peptide-coated CdS quantum dots for the optical detection of copper (II) and silver (I), Chem. Commun. (2003). H.S. Zhang, X.F. Guo, H. Wang, L-cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper (II) determination, Microchem. J. 89 (2008) 142–147. G. Dumitrascu, A. Kumbhar, W. Zhou, Z. Rosenzweig, The use of derivatized magneto liposomes for extraction of antibodies from aqueous solutions, Trans. Magn. 37 (4) (2001) 2932–2934. W. Brey, Physical Chemistry and Its Biological Applications, Academic Press, New York, 1978.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
L-cysteine and 3-mercaptopropionic acid capped cadmium selenide quantum dots based metal ion probes Reena K. Sajwan, Yana Bagbi, Purnima Sharma, Pratima R. Solanki n Special Centre for Nanoscience, Jawaharlal Nehru University, New Delhi 110067, India
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2016 Accepted 27 February 2017 Available online 28 February 2017
Herein, a report on the synthesis of 3-mercaptopropionic acid (MPA) and L cysteine (Cyst) capped luminescent cadmium selenide (CdSe) quantum dots (QDs) was studied. The MPA-CdSe and Cyst-CdSe QDs were conducted with different physiologically important metal ions such as Cu2 þ , Fe2 þ , Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ ions in aqueous solution using fluorescence spectroscopic technique. It was found that the MPA-CdSe QDs, categorically sensitive to only Cu2 þ ions only. However, other metal ions such as Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ , did not exhibit measurable quenching. On the other hand, Cyst-CdSe QDs were very selective and sensitive to Zn2 þ ions only, which showed fluorescence enhancement while for other ions no such behavior was noticed. Thus, MPA-CdSe and Cyst-CdSe QDs have successfully detected Cu2 þ and Zn2 þ ions in the linearity range from 4 to 160 mM, respectively. The effect of fluorescence response of QDs towards metals ions were described by Langmuir-type binding model or Stern–Volmer-type equation. & 2017 Elsevier B.V. All rights reserved.
Keywords: CdSe quantum dots Fluorescence Metal ions 3-mercaptopropionic acid L-cysteine
1. Introduction Photoluminescent semiconductor nanocrystals or quantum dots (QDs) have become one of the most attractive fields of current research because of their unique size-dependent optical properties [1]. Published data has proved that development of an optical sensor for detection of toxins, heavy metals, and other environmental pollutants is possible using this platform [2,3]. Over organic fluorescent dyes, these QDs are advantageous due to their interesting optical properties like size, tunable color, larger fluorescence quantum yields and narrow spectral bands and less photobleaching susceptibility [4]. In hot-injection protocol, it is possible to produce QDs in the size range of 2.5–6.3 nm, with each of these particles exhibiting their unique spectral properties [5]. Recently, selective detection of biologically important molecules and environmentally non-toxic metal cations is an active area of interest [6]. Most of the sensing systems reported in the literature have been found on organic molecules and inorganic complexes. Recently, research is focusing on designing QD-based hybrid systems for sensing applications, due to their fascinating optical properties, which are tunable with size and shape [7–9]. Surface modified QDs show a change in their optical, chemical and photo-catalytic properties leading to unusual optical effects such n
Corresponding author. E-mail addresses: [email protected], [email protected] (P.R. Solanki). http://dx.doi.org/10.1016/j.jlumin.2017.02.061 0022-2313/& 2017 Elsevier B.V. All rights reserved.
as enhancement of their excitonic, defect emission and improvement of the photo-stability of semiconductor nanoparticles. It is reported that, after functionalization, the QDs can create new emission bands that can enhance the selectivity and efficiency of light-induced reactions [10]. Most of the studies reported on CdSe QDs are capped with an organic layer, such as a trioctyl phosphine/ trioctylphosphine oxide mixture (TOP/TOPO). This layer is coordinated to Cd2 þ sites, and stabilizes the nanocrystals surface, thereby preventing an irreversible flocculation of the particulate matter [11]. Unfortunately, these protective ligands are hydrophobic, capped with organic molecules, and are not analytically compatible for aqueous assay. To overcome these problems, further modification of QDs should be done with hydrophilic molecules as capping agents. Production of water-soluble QDs are urgently required for analytical assay [12]. The most common binding groups such as thiol-containing molecules, amines, amino acids and peptides [13,14] are being utilized to make water soluble QDs, to replace the inherited organic ligand capped on the QD surface using electrostatic, hydrophobic interactions or host–guest interactions. The most common thiol-containing molecules used to make aqueous QDs are thiolated aliphatic carboxylic acids using mercaptopropionic acid (MPA) [15], mercaptoacetic acid (MAA) [16,17], 16-mercaptohexadecanoic acid (MHA) [18] and L-cysteine (Cyst) [9,19]. Among them, MPA and Cyst were more promising capping ligand which makes water soluble QDs. The water soluble QDs retain up to half of their quantum yields after transferring from organic solvent to aqueous solution [8]. The MPA is an organic
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molecule consisting two functional groups. The coordination of these groups on the surface of QDs has two important advantages: (i) dangling bonds act as quenchers for the emission in QDs, (ii) prevent the QDs from agglomeration. In addition, capping the QDs with such kind of molecules is very effective for binding the QDs with other molecules like metal ion polymer and forms a hybrid material, and can be used as biological labels [20]. Besides this, Cyst is an amino acid which combines catalytic activity with an extensive redox chemistry and has remarkable metal binding properties [21,22]. It is nontoxic, water-soluble, sulfur-containing amino acid having three functional groups (-SH, -NH2, -COOH), which has strong tendency to coordinate with any inorganic cations and heavy metals ions [22]. Thus, the surface modification of QDs with MPA and Cyst makes them more suitable for interaction with targeted materials [23], commonly used for QDs based metal ion sensing. In addition, MPA and Cyst capped QDs were cheap and more stable than the other surface modification. Chen and Rosenzweig have demonstrated the surface functionalized cadmium sulfide quantum dots as luminescence probes for detecting copper and zinc ions in aqueous solution. The nature of the ligand has dramatic effect on type and selectivity of the luminescence response [9]. The interaction between metal ions and QDs surface ligand is observed by quenching and enhancement of luminescence [8]. Before the metal ions can reach the QDs surface, they need to pass through the capping layer. It is accepted that the resulting chemical or physical interaction between metal ions and surface ligand of QDs could significantly change the photoluminescence of the QDs. Generally the optical sensing function of semiconductor QDs originates from the photoexciton of the QDs and transfer of electrons between the valance and conduction band. Sensing of small metal ions in aqueous solution using QDs could be demonstrated by the analyte-induced changes in photo luminescence or quenching the fluorescence luminescence. Quenching mechanism involves many effects including inner filter effects, ion induced generation of nonradiative recombination reaction, ion binding interaction (leads imperfection of the QD surface or the agglomeration of the QD particles) and electron-transfer processes [9,27,28]. Herein, we have synthesized the water soluble MPA and Cyst capped luminescent cadmium selenide (CdSe) quantum dots (QDs). The interaction of these MPA and Cyst capped CdSe QDs has been studied with different physiologically important metal ions such as Cu2 þ , Fe2 þ , Zn2 þ , Co2 þ , Pb2 þ , Mg2 þ , Ni2 þ and Ag þ ions in aqueous solution. It is found that the MPA-QDs were categorically sensitive to only Cu2 þ and Fe2 þ ions as evident from their substantial fluorescence quenching data. On the other hand, CystCdSe QDs were very selective and sensitive to Zn2 þ ions only, which showed fluorescence enhancement while for other ions no such behavior was noticed. Therefore, the developed sensor is simple for detection of Cu2 þ and Zn2 þ ion as compared to previous reports [9,19].
2.1. Preparation of water soluble MPA capped CdSe QDs CdSe QDs were synthesized using CdO as a precursor according to a procedure described by Peng's group [24,25]. The CdSe QDs were prepared in trioctylphosphine oxide as a solvent. For the synthesis of MPA-capped CdSe QDs, a solution of MPA (108 mL) and NaOH (160 mg) in 25 mL carbinol was prepared under continuous stirring until a clear solution obtained. Typically, 10 mL CdSe QDs were dispersed in 25 mL of chloroform in a beaker and MPA solution were added dropwise under continuous stirring until flocculation appeared in the solution after addition of appropriate amount of water resulting in a two-phase separation; hydrophilic QDs (upper phase) and hydrophobic phase (lower phase) as shown in photograph in the inset (Scheme 1). Then MPA-capped QDs were separated from upper phase of the solution and were stored at room temperature in pre-cleaned borosilicate glass bottles for further use. Scheme 1 shows the accepted interaction between the CdSe QDs and Cyst or MPA. The general approach to make MPAcapped QDs is to exchange native TOPO ligands capped on the QD surface with thiol ligands. When MPA solution is added dropwise in TOPO CdSe QDs solution under continuous stirring, MPA slowly starts abolishing the TOPO ligands and thiol group taken place on QDs surface either by using electrostatic, hydrophobic or host guest interaction. The Cyst capped CdSe QDs were prepared using a solution of Cyst (1 mg) and NaOH (160 mg) in 25 mL carbinol under continuous stirring until a clear solution obtained. Next step was similar to the procedure used for the synthesis of MPA capped CdSe QDs.
2.2. Preparation of different metal ion stock solutions Freshly prepared aqueous solution of the chloride salts of metal ions (Zn2 þ , Cu2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ) with different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) in distilled water of pH 7 were prepared by stirring at room temperature.
2.3. Characterization techniques The CdSe QDs sizes were determined from high-resolution transmission electron microscope (HRTEM) images obtained from JEOL JEM-2200 FS (Japan) instrument operating at a voltage of 200 kV. Fourier transform infrared spectroscopy (FT-IR) spectra of QDs were recorded on a Varian 7000 FT. Fluorescence spectra of the CdSe QDs were analyzed using fluorescence spectrophotometer (Shimadzu model rf 5301pc Spectrofluorometer). The emission spectra (550–900 nm) was recorded under fixed excitation wavelength (490 nm).
2. Materials and methods Cadmium oxides (CdO, 99%), selenium powder (Se, 99%), trioctylphosphine (TOP, 90%), 1-octadecene (90%), Oleic acid (OA, 65%), 3-mercaptopropionic Acid (MPA; 90%) were purchased from sigma Aldrich (USA). Chloroform, methanol (Carbinol), L-cysteine (Cyst; 99%), sodium hydroxide pellets (NaOH), chloride salts of all metals (Zn2 þ , Cu2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ) and silicone oil/ paraffin oil, double distilled deionized water were purchased from a local chemical company supplier. All the chemical reagents were of analytical grade and used as received without further purification.
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Scheme 1. Exchange of ligand on surface of CdSe QDs.
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3. Results and discussion 3.1. Characterization studies Fig. 1 shows the TEM images of CdSe QDs. It is clearly visible that uniform dispersed QDs particle having the particles size in the range of 2–5 nm (image a). HRTEM images shows ultra-small fringes which implies that the structure of CdSe QDs was purely crystalline in nature (image b) and d-spacing value obtained as 3.18 nm. Fig. 2 shows the FTIR studies of (a) TOPO capped QDs; (b) pure MPA; (c) MPA capped QDs; (d) pure Cyst; (e) Cyst capped QDs. The OH- vibration peak at 3450 cm 1 (Fig. 2 curve c), indicates the presence of water molecules. Due to high surface-to-volume ratio, large numbers of water molecules were adsorbed on the surface of CdSe quantum dots. The band around 2928 cm 1 has C-C-H stretching that indicates the presence of TOPO (curve (a)) and there was no evidence of this peak after ligand exchange on surface of the CdSe QDs (curve (c and e)) indicates complete replacement of TOPO ligands. The peak around 1715 and 1643 cm 1 appears due to MPA capped CdSe QDs (curve (c)). However, the IR peaks around at 1715 and 1635 cm 1represents Cyst capped QDs (curve (e)) assigned to C¼C bending. In case of Cyst pure, primary amines contains the – NH2 group, and so have N–H bonds. The peak was observed around 3442 cm 1 which implied the presence of N–H bond on the surface of CdSe QDs (curve (e)). IR peaks within 2500–2750 cm 1 region, shows the presence of S–H bond, but here no any peak corresponding to S–H bond in capped CdSe QDs samples. This indicates that the sulfur bond replaced the thiol bond and formed a linkage as –S-Cd [26]. FTIR data clearly shows that the MPA and Cyst ligand attached on the surface of the CdSe QDs after replacing the TOPO ligand from the CdSe QDs surface. These results indicated that CdSe QDs were successfully capped to MPA and Cyst, respectively. 3.2. Effect of pH responses of QDs on luminescence and absorption Generally, pH is one of the major variables which influence the fluorescence of the QDs. To check the pH effect of MPA capped CdSe QDs on luminescence response on quenching, the experiment was conducted at different basic pH from 6–11. It was observed that the emission intensity of the MPA QDs were increases with the basic nature (pH 6–11) of the solution. The observed enhancement very likely results from reduced non-radiative recombination by minimizing the surface defects. It can be cleared from the Fig. S1 (Supplementary data) that optimal pH lies within 7–8, which can be selected for further fluorescence studies. In addition, the effect of pH on the absorption response of MPA-Capped CdSe QDs was performed and observed that the absorbance intensity of the watersoluble MPA-Capped CdSe QDs increases with the basicity of the environment (Fig. S2 data). This indicates, the absorbance intensity was found to be enhanced when the pH varies from 6 to 10. The
Fig. 2. FTIR spectra of (a) TOPO capped QDs; (b) Pure MPA; (c) MPA capped QDs; (d) pure L-Cyst; (e) L-Cyst capped QDs.
enhancement is results from the changes caused on the surface of CdSe QDs by variation in pH and red shift owing to increase in size. Due to size increment the energy gab decreased which shows that the quantum effects of QDs were eliminated when QDs particles tends towards the bulk size (Fig. S3 data). Similar results was obtained for Cyst capped CdSe QDs, thus, all the experiments were performed at pH 8.0. 3.3. Florescence studies The fluorescence emission spectra recorded at excitation wavelength 490 nm. The three intense peaks corresponding to TOPO CdSe QDs at 643 nm, MPA and Cyst capped CdSe QDs at 662 and 711 nm, respectively were observed (Fig. 3). The strong red shift of QDs was previously observed by Lakowicz et al. [27] discussed the potential of these particles as red luminescent probes in biological sample. Typically, the red shift was observed in the wavelength which implies increase in particle size. The particle size of the TOPO CdSe QDs was 3.1 nm. However, the size of CdSe QDs were changed after functionalized with MPA and Cyst as 3.5 and 4.4 nm, respectively. The spectra also gave clear indication for change in the fluorescence intensity after surface modification. 3.4. Quenching response studies of MPA capped Fig. 4 shows the response studies of MPA capped QDs as a function of different concentrations of Cu2 þ ion varies from 4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM. During the experiment, 12 samples having stoichiometric amount (2 mL) of MPA capped CdSe
Fig. 1. TEM images (a) and (b) HRTEM image of CdSe QDs.
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Scheme 2. Interaction of MPA Capped CdSe QDs with Cu2 þ ions.
Fig. 3. Emission spectra of TOPO, MPA and Cyst capped CdSe QDs.
Fig. 5. Quenching effect of copper ion concentration on the luminescence of MPA capped CdSe QDs.
Fig. 4. Quenching response studies of MPA capped CdSe QDs as a function of different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) of Cu2 þ ion.
QDs and an appropriate amount (0.5 mL) of different concentration of Cu2 þ added into them separately. The fluorescence response of each samples was recorded under fluorescence spectrophotometer at the excitation wavelength 490 nm. It was observed that the luminescence intensity decreases with increased in Cu2 þ ion concentrations. The Cu2 þ , bound to the surface of the QDs, effectively quenched the fluorescence luminescence of MPA capped QDs. These results indicate that the quenching in luminescence of MPA capped QDs occurs due to the interaction with Cu2 þ ion. There are two possibilities for interaction: First, due to the electrode potential of Cu2 þ /Cu þ made Cu2 þ most appropriate for the effective electron transfer from QDs to the metal ions, which accounted for the effective quenching of the QDs luminescence [9,29]. Second, Cu2 þ was brought to the CdSe QDs surface due to the intrinsic high affinity constant of the ligand-Cu2 þ complex, resulting in the chemical replacement of Cd2 þ ions by Cu2 þ ions and formed a partially soluble particle (CuSe) on the surface of CdSe QDs. These small particles could quiche the recombination luminescence of MPA capped QDs by facilitating nonradiative electron-hole decimation, acting as electron-hole recombination centre [28–30]. More over, the quenching mechanisms involve other effects such as inner filter effects, ion- induced generation of non-radiative recombination pathways, ion binding interaction and the photoinduced electron transfer process [8]. Scheme 2 shows the tentative interaction between the Cu2 þ
ion and hydroxyl group of MPA capped CdSe QDs. Fig. 5 shows the quenching effect of Cu2 þ concentration on MPA capped CdSe QDs. The fluorescence quenching is described by a Stern–Volmer -type Eq. (1) [31].
Io /I = 1 + Ksv⎡⎣ Q⎤⎦
(1)
Here, I and Io are the luminescence intensities of the MPA capped QDs samples at a given Cu2 þ concentration, and in ion free solution, respectively. Q is the Cu2 þ concentration. Ksv is the SternVolmer constant. Fig. 6 shows a Stern-Volmer quenching curve describing Io/I as a function of Cu2 þ concentration. The Stern –Volmer constant was obtained to be Ksv ¼9.3038 103 M 1. 3.5. Selectivity studies for MPA capped CdSe QDs The high intrinsic attraction between the metal ions and the surface ligand is the incisive factor for ions selectivity. For the selectivity of MPA capped QDs towards Cu2 þ , the response of other ionic species (interfering) such as Zn2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ions was monitored (Fig. 7). During this measurement, the fixed amount (0.5 mL) of interfering species of 1 mM concentration were separately added into stoichiometric amount (2 mL) of MPA capped CdSe QDs and the fluorescence emission were recorded under fluorescence spectrophotometer. It was observed that the highest quenching of MPA capped CdSe QDs occurred in the presence of Cu2 þ , that is not much affected in the presence of other ionic species except Fe þ . The quenching effect of Fe3 þ on the luminescence of MPA capped QDs is attributed to strong absorption of excitation wavelength by Fe3 þ . However, MPA capped QDs exhibit highest selectivity towards Cu2 þ .
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Fig. 6. Stern-Volmer plots describe Cu2 þ ions concentration dependence of the luminescence intensity of MPA capped CdSe QDs.
Fig. 8. Effect of the addition of different concentration (4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM) of Zn2 þ ions on Cyst capped QDs.
Scheme 3. Interaction of Cyst Capped QDs with Zn2 þ ions.
Fig. 7. Quenching effect of MPA capped CdSe QDs in the presence of other ionic species.
3.6. Quenching response studies of cyst capped CdSe QDs Fig. 8 shows the response studies of Cyst capped CdSe QDs as a function of different concentrations varies from 4, 6, 8, 10, 20, 40, 60, 80, 100, 120, 140, 160 mM. Here, 12 samples were taking having equal amount (2 mL) of Cyst capped CdSe QDs and a appropriate amount (0.5 mL) of different concentration of Zn2 þ ion were added separately into them. And after that the luminescence spectras of each samples were recorded by the fluorescence spectrophotometer under the excitation wavelength of 490 nm. It was observed that the luminescence intensity increases with increased in Zn2 þ ion concentrations (Fig. 8). This enhancement of the luminescence intensity is associated with the passivation of surface trap sites through formation of a surface passivation layer, due to binding of Zn2 þ ion, eliminating the non-radiative relaxation sites and preventing photochemical degradation and hence increased luminescence intensity of the Cyst capped CdSe QDs. Moreover, due to formation of complex of Zn2 þ ion onto QDs surface which would restrict ligand rotation that enhanced the conformational rigidity of the surface substituent, resulting in suppress the quenching of Cyst capped CdSe QDs [8]. The luminescence intensity of Cyst capped CdSe QDs was sensitized by 40% solution after addition of Zn2 þ ion of concentration as compared to the emission of these QDs from ion free
solution. Scheme 3 shows the tentative interaction between the Zn2 þ ion and carboxyl group of Cyst capped CdSe QDs. Fig. 9 shows the curve which explains the effect of Zn2 þ ion on emission intensity of Cyst capped CdSe QDs. Here the emission intensity continuously increased with Zn2 þ ion concentration. The concentration dependence of the luminescence intensity allowed the binding of Zn2 þ ions to the surface of these QDs, and could effectively describe Langmuir-type binding isotherm [32]. According to Langmuir, the surface of the QDs consists of a finite number of binding sites. Each of the binding sites could adsorb one ion from the solution. The fraction of occupied sites is defined as
Fig. 9. Sensitizing effect of Zn2 þ ions concentration on the luminescence of Cyst capped CdSe QDs.
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Θ. The rate of binding of ions to the surface is proportional to the ion concentration C in the analyte solution, and to the fraction of available binding sites (1-Θ). The fraction of occupied binding sites, Θ, is related to the ratio between the signal obtained at a given ion concentration I and the maximum intensity Io of ion free solution as shown in Eq. (2).
Θ ≈ I /Io
(2)
Accordingly, if the Langmuir description of the binding of Zn2 þ ions on the surface of Cyst capped QDs is valid, a plot of C/I as a function of C should be linear. Where C is the concentration of Zn2 þ ions, and I is the luminescence intensity of Cyst capped QDs at given Zn2 þ ions concentration is shown in Fig. 10. The correlation coefficient of the linear fit was obtained as 0.958. 3.7. Selectivity studies of Cyst capped CdSe QDs The luminescent intensity of Cyst capped CdSe were observed against different ions such as Zn2 þ , Fe3 þ , Co2 þ , Pb2 þ , Mn2 þ , Ni2 þ and Ag þ ions (Fig. 11). The maximum emission intensity occurred in the presence of Zn2 þ ions. During this measurement, the fixed amount (0.5 mL) of interfering species of 1 mM concentration were separately added into stoichiometric amount (2 mL) of Cyst capped CdSe QDs and the fluorescence emission were recorded under fluorescence spectrophotometer. Thus, it was concluded
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that the Cyst capped CdSe QDs were sensitive towards the Zn2 þ ions concentration and shows maximum luminescence quenching in presence of Zn2 þ ions. Similar quenching trends were also observed with the Fe3 þ ions. The photostability of MPA and Cyst capped CdSe QDs was observed for about 25 days as stored in the daylight at room temperature (25 °C). During the initial period both the MPA and Cyst capped CdSe QDs does not show any change in their absorbance. Cyst capped CdSe QDs agglomerate and starts degradation over the period of 10–12 days and lost their emission properties, but MPA capped CdSe QDs found more stable at same condition.
4. Conclusions This work describes the quantitative application of luminescent semiconductors QDs in transition metal ions detection. The effect of two metal ions Zn2 þ and Cu2 þ on the luminescence response was investigated. Cyst-capped QDs showed a selective response to Zn2 þ , and showed minimal response to other cations. A Langmuir type binging model effectively describes the Zn2 þ concentration dependence of the luminescence intensity for Cyst-capped QDs. MPA-capped QDs showed preferential response, and sensitivity towards Cu2 þ over other cations. A Stern-Volmer equation described the Cu2 þ concentration dependence of the luminescence intensity of the MPAcapped QDs. With MPA-capped QDs, that could detect upto 4–160 mM concentration of Cu2 þ (safe limit of copper ion E10 mM, according to WHO guidelines). Similar, is the case with the Zn2 þ ions. The detection limit varies from 4 to 160 mM (safe limit of zinc ion E30 mM, according to WHO guidelines). This propose an alternative platform for the detection and sensing of metal ions (Zn2 þ and Cu2 þ ) in water. Transition metals are major pollutants and are commonly present in industrial affluent, thus, the study of their detection and remediation can hardly be stressed. Studies relating to pHs effect and energy responses on quenching of metal ions with CdSe QDs is open for future research.
Acknowledgements
Fig. 10. Linear fit thought Zn2 þ ion concentration with Langmuir binding isotherm.
Authors are thankful to Advanced Instrumentation Research Facility JNU, New Delhi for providing the facilities of HRTEM and FT-IR characterizations. R.K.Sajwan is thankful to Dr. Kamla Rawat for discussion. This work has supported by a grant received from Department of Science and Technology Government of India (DST Purse, JNU) and UGC (UPE-II; Project 58).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.02.061.
References
Fig. 11. Effect of 1 mM of different ions on the luminescence of the Cyst capped QDs.
[1] C. Zhong, G.U. Zheng, J.L. Zeng, J.H. Liu, Q. Deng, J.B. Fan, J.N. Xiang, A novel fluorescent probe for copper ions based on polymer-modified CdSe/CdS core/ shell quantum dots, Anal. Sci. 27 (2011) 643. [2] H. Mattoussi, J.M. Mauro, E.R. Goldman, G.P. Anderson, V.C. Sundar, F. V. Mikulec, M.G. Bawendi, Self-assembly of CdSe ZnS quantum qot bioconjugates using an engineered recombinant protein, J. Am. Chem. Soc. 122 (2000) 12142–12150. [3] J.M. Costa-Fernandez, R. Pereiro, A. Sanz-Medel, The use of luminescent quantum dots for optical sensing, Trends Anal. Chem. 25 (2006) 3. [4] A.C. Quitaneg, G. Nanato, C. Santos, Cadmium selenide quantum qots synthesized by HVPC growth technique for sensing copper ion concentrations, J. Spec. Educ. Rehabil. 2 (12) (2011) 2229–5518.
132
R.K. Sajwan et al. / Journal of Luminescence 187 (2017) 126–132
[5] P. Wu, T. Zhao, S. Wang, X. Hou, Semiconductor quantum dots-based metal ion probes, Nanoscale 6 (2014) 43. [6] M.A. Ellis, G. Grandinetti, K.M. Fichter, Synthesis of Cd-free InP/ZnS quantum dots suitable for biomedical applications, J. Vis. Exp. 6 (2016) 108. [7] J. Li, D. Bao, X. Hong, D. Li, J. Li, Y. Bai, T. Li, Luminescent CdTe quantum dots and nanorods as metal ion probes, Eng. Asp. 257–258 (2005) 267–271. [8] Y.B. Lou, Y.X. Zhao, J.X. Chen, J.J. Zhu, Metal ions optical sensing by semiconductor quantum dots, J. Mater. Chem. C 2 (2014) 595–613. [9] Y. Chen, Z. Rosenzweig, Luminescent CdS quantum dots as selective ion probes, Anal. Chem. 74 (2002) 5132–5138. [10] A.V. Isarov, J. Chrysochoos, Optical and photochemical properties of nonstoichiometric cadmium sulfide nanoparticles: surface modification with copper (II) ions, Langmuir 13 (1997) 3142. [11] E. Rabani, Structure and electrostatic properties of passivated CdSe nanocrystals, J. Chem. Phys. 115 (2001) 1493. [12] H. Mattoussi, J.M. Mauro, E.R. Golman, G.P. Anderson, V.C. Sundar, F.V. Mikulec, 13 M.G. Bawendi, Self-assembly of CdSe ZnS quantum dot bioconjugates using an engineered recombinant protein, J. Am. Chem. Soc. 122 (2000) 12142. [13] F. Dubois, B. Mahler, B. Dubertret, E. Doris, C. Mioskowski, A versatile strategy for quantum dot ligand exchange, J. Am. Chem. Soc. 129 (2007) 482–483. [14] W.W. Yu, E. Chang, R. Drezek, V.L. Colvin, Water-soluble quantum dots for biomedical applications, Biochem. Biophys. Res. Commun. 348 (2006) 781–786. [15] D.R. Baker, P.V. Kamat, Tuning the emission of CdSe quantum dots by controlled trap enhancement, Langmuir 26 (2010) 11272–11276. [16] Y.S. Liu, Y.H. Sun, P.T. Vernier, C.H. Liang, S.Y.C. Chong, M.A. Gundersen, pHsensitive photoluminescence of CdSe/ZnSe/ZnS quantum dots in human ovarian cancer cells, J. Phys. Chem. C 111 (2007) 2872–2878. [17] J.A. Kloepfer, S.E. Bradforth, J.L. Nadeau, Photophysical properties of biologically compatible CdSe quantum dot structures, J. Phys. Chem. B 109 (2005) 9996–10003. [18] Y.H. Chan, J. Chen, Q. Liu, S.E. Wark, D.H. Son, J.D. Batteas, Ultrasensitive copper (II) detection using plasmon-enhanced and photo-brightened luminescence of CdSe quantum dots, Anal. Chem. 82 (2010) 3671–3678. [19] M. Koneswaran, R. Narayanaswamy, L-cysteine-capped ZnS quantum dots based fluorescence sensor for Cu2 þ ion, Sens. Actuators B 139 (2009) 104–109. [20] S. Wageh, Anwer A. Higazy, Ahmed S. Hassouna, Optical properties and
[21] [22]
[23]
[24] [25]
[26]
[27] [28]
[29] [30]
[31]
[32]
thermal degradation of CdSe capped with 3-mercaptopropionic acid, J. Mater. Sci.: Mater. Electron 24 (2013) 3049–3057. N.M. Giles, G.I. Giles, C. Jacob, Multiple roles of cysteine in biocatalysis, Biochem., Biophys. Res. Commun. 300 (2003) 1–4. J.A. Thomas, R. Mallis, H. Sies, Protein S-thiolation, S-nitrosylation, and irreversible sulfhydryl oxidation: rolesin redox regulation, in: C. Gitler, A. Danon (Eds.), Cellular Implications of Redox Signalling, World Scientific Publ, Singapore, 2003, pp. 141–174. R. Kho, C.L. Torres-Marteiz, R.K. Mehra, A simple colloidal synthesis for gramquantity production of water-soluble ZnS nanocrystal powders, J. Colloids Interface Sci. 227 (2000) 561–566. X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Shape control of CdSe nanocrystals, Nature 404 (2000) 59–61. W.W. Yu, L. Qu, W. Guo, X. Peng, Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals, Chem. Mater. 15 (14) (2003) 2854–2860. I.S. Liu, H.H. Lo, C.T. Chien, Y.Y. Lin, C.W. Chen, Y.F. Chen, W.F. Su, S.C. Liou, Enhancing photoluminescence quenching and photoelectric properties of CdSe quantum dots with hole accepting ligands, J. Mater. Chem. 18 (2008) 675–682. J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd edition, ISBN 9781-4757-3061-6. M.T. Fernández-Argüelles, W.J. Jin, J.M. Costa-Fernández, R. Pereiro, A. SanzMedel, Surface-modified CdSe quantum dots for the sensitive and selective determination of Cu(II) in aqueous solutions by luminescent measurements, Anal. Chim. Acta 549 (2005) 20–25. K.M. Gattás-Asfura, R.M. Leblanc, Peptide-coated CdS quantum dots for the optical detection of copper (II) and silver (I), Chem. Commun. (2003). H.S. Zhang, X.F. Guo, H. Wang, L-cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper (II) determination, Microchem. J. 89 (2008) 142–147. G. Dumitrascu, A. Kumbhar, W. Zhou, Z. Rosenzweig, The use of derivatized magneto liposomes for extraction of antibodies from aqueous solutions, Trans. Magn. 37 (4) (2001) 2932–2934. W. Brey, Physical Chemistry and Its Biological Applications, Academic Press, New York, 1978.