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RETRACTED: Mercaptoacetic acid capped CdS quantum dots as fluorescence single shot probe for mercury(II)
Sensors and Actuators B 139 (2009) 91–96
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Mercaptoacetic acid capped CdS quantum dots as fluorescence single shot probe for mercury(II) Masilamany Koneswaran, Ramaier Narayanaswamy ∗ Centre for Instrumentation and Analytical Science, School of Chemical Engineering and Analytical Science, University of Manchester, PO Box 88, Sackville Street, Manchester M60 1QD, United Kingdom
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Article history: Available online 18 September 2008 Keywords: Cadmium sulfide Quantum dots Mercury ion Fluorescence Quenching
a b s t r a c t Water soluble CdS quantum dots (QDs) have been synthesised using mercaptoacetic acid (MAA) as surface modifying agent through a one step process by using safe and low cost materials. These MAA capped CdS QDs are highly stable in aqueous solution and have strong affinity for mercury ion as a result of interaction with the functional group present in the capping agent. This probe is based on the characteristic of fluorescence quenching of functionalised CdS QDs. Under the optimum conditions, the fluorescence intensity of CdS QDs is linearly proportional to the mercury ion concentration in the range 0.05 × 10−7 to 4.0 × 10−7 M with a detection limit of 4.2 × 10−9 M. There is no significant wavelength shift on the fluorescence-quenched signals in the presence of mercury ion at the pH 7.4. Compare to the organic dyes, these nanoparticles are brighter and more stable against photobleaching. The effect of common foreign cations on the fluorescence of the QDs was examined to evaluate the selectivity and the results showed a high selectivity of the MAA capped CdS QDs towards Hg(II) ions. The method presented here is simple, rapid, inexpensive, sensitive and suitable for practical application. © 2008 Published by Elsevier B.V.
1. Introduction The need for developing of selective and sensitive sensors to monitor in real-time the concentration of analytes of biological and environmental importance is of great interest. The determination of heavy metals in aquatic environment has tremendous importance due to their hazardous effect on the ecosystem and on the human health. Mercury is a highly toxic environmental pollutant arising from both natural and industrial sources. This metal causes damage to microorganisms and aquatic environment even at very low concentrations. When human consumes water contaminated with mercury, several serious disorders may result including sensory, motor, and neurological damage [1]. Therefore, it is important to develop a highly sensitive and selective luminescent probe to detect the mercury ion at low concentration level. There are several organic molecules being used to detect heavy metal ions [2–4]. These organofluorophores usually have some limitations such as photobleaching, and low fluorescence intensity. They also show narrow excitation and broad emission bands with red tailing. Inorganic nanoparticle called quantum dots (QDs) has good potential to overcome these problems.
∗ Corresponding author. Tel.: +44 161 306 4891; fax: +44 161 306 4399. E-mail address: [email protected] (R. Narayanaswamy). 0925-4005/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.snb.2008.09.011
QDs are semiconductor particles that have all three dimensions confined to the nanometer scale [5]. As a result of quantum confinement, they have unique optical and electronic properties such as broad excitation spectra, narrow symmetric and tunable emission spectra which can span from the ultraviolet (UV) to the infrared (IR) region [6]. Many studies have been focused on the development of new techniques to synthesise high-quality QDs with high luminescence quantum yields and with better photophysical properties in different media [7]. The fluorescence emission wavelength can be tuned by modifying the size of QDs particles and the type of the capping molecules. The surface modification of QDs may change their optical, chemical and photophysical properties such as enhancement of their excitation and emission intensities and improvement in the photostability of semiconductor nanoparticles. The generation of new traps on the surface of QDs lead to the appearance of new emission bands, enhancement of selectivity and stability of the QDs, etc. [8]. The surface-modified QDs are usually capped with some appropriate hydrophilic functional reagents used as stabilising agents including mercaptopropionic acid, l-cysteine, thioglycerol, etc. [9–11]. The introduction of organic ligands on their surfaces not only stabilise the nanoparticles in different solvents but also result in the desired surface functionality as well [12,13]. The QDs attracted considerable attention as novel fluorescence probes because they could be widely used in chemical analysis. So far very few reports of chemical sensing of small molecules and
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ions with QDs via analyte-induced changes in photoluminescence have been reported in the literature. Chen and Rosenzweig [14] first demonstrated the luminescent QDs probe for copper and zinc ions using functionalised CdS QDs capped with different organic ligands in aqueous medium. Subsequently, in recent years, most of the analytical chemists now concentrate on synthesising new types of functionalised QDs as selective chemosensors for metal ions. Asfura and Leblanc [15] reported the peptide-coated CdS QDs as the luminescence probe for Ag(I) and Cu(II) in aqueous medium. Functionalised CdSe QDs have been synthesised by Jin et al. [16] for the detection of CN− . Several research groups have synthesised different types of functionalised QDs as luminescent probe for Cu(II) [17–19], Ag(I) [20] and Pb(II) [21,22]. All these reports show the interaction between analyte and QDs, which alter their photophysical and photochemical properties. Only few workers have reported the measurement of mercury(II) ion using semiconductor QDs [23–27]. However, these methods show low sensitivity for the detection of mercury(II) ions. Synthesis of these surface-modified QDs requires extreme reaction conditions, with the process being costly and giving low yields. This work describes a one step synthesis of the mercaptoacetic acid (MAA) capped CdS QDs for use as fluorescence sensor for Hg(II). This method is simple, sensitive, of low cost and requires only room temperature conditions. The quenching effect of Hg(II) ion on the fluorescence of the MAA capped CdS QDs was used to develop the sensing probe for mercury ion. This quenching was found to be proportional to the concentration of mercury(II) ion that can be described by the Stern–Volmer relationship. 2. Experimental 2.1. Materials and reagents All chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared with double distilled deionised water. MAA (HS–CH2 –COOH), CdCl2 ·10H2 O, sodium sulphide nonahydrate (Na2 S·9H2 O), HgCl2 , CuCl2 ·2H2 O, Zn(CH3 COO)2 , NaCl, KCl, MgCl2 , CaCl2, AgNO3 , Co(NO3 )2 MnCl2 , Tris–(hydroxymethyl)-aminomethane, and other chemicals were purchased from Sigma–Aldrich. Ethanol was purchased from Fisher Scientific. 2.2. Instrumentation Absorption spectra of samples were acquired on a Shimadzu UV2401 PC spectrophotometer. All fluorescence measurements were recorded using a LS 55 PerkinElmer fluorescence spectrometer with both excitation and emission slits set at 5.0 nm. Dilute solutions of QDs in aqueous medium were placed in 1 cm quartz cuvettes to scan the spectrum. The transmission electron microscopy (TEM) images of the nanoparticles were acquired on a FEI–TECNAI, 300 kV transmission electron microscope. The colloidal solution of nanoparticles in water was dropped onto a 0.1 nm thick carboncoated copper grid with the excess solution immediately removed. FT-IR spectra were recorded by FT-IR-8400 Shimadzu Fourier Transform Spectroscopy. pH measurements were made with a Hanna Turtle pH meter. All optical measurements were performed at room temperature and under ambient conditions. 2.3. Procedure Functionalised MAA capped CdS QDs were synthesised via the procedures described by Winter et al. [28], with some modifications. 0.5 mmol portion of MAA and 0.5 mmol of CdCl2 ·10H2 O were dissolved into 250 ml double distilled deionised water in a round
Fig. 1. Absorption spectra (a) and effect of Hg(II) ion concentration on the fluorescence of MAA capped CdS QDs (b). Concentration of QDs is 1.0 mg l−1 and the concentrations of mercury ions (×10−7 M) from A to K: A: 0, B: 0.8, C: 1.6, D: 2.4, E: 3.2, F: 4.0, G: 4.8, H: 5.6, I: 6.4, J: 7.2, and K: 8.0.
bottom flask. The pH was adjusted by the dropwise addition of 0.1 M NaOH solution to raise the pH 6.0 with constant stirring. Then 0.5 mmol portion of Na2 S·9H2 O was dissolved in 20 ml of double distilled deionised water and the Na2 S solution was added into the flask dropwise with vigorous stirring under the nitrogen environment. Absolute ethanol was used to precipitate the MAA capped CdS QDs. The functionalised CdS QDs was treated with three repeated cycles of precipitation by ethanol, washing and redispersion to remove the contaminants. The luminescent MAA capped QDs was dissolved in double distilled deionised water and the pH adjusted to 7.4 by 0.05 M Tris–HCl buffer solution for further analysis. The stock solutions of metal ions were prepared by dissolving their respective salts in double distilled deionised water. 3. Results and discussion 3.1. Characterisation of MAA capped CdS quantum dots The optical properties of MAA capped CdS QDs were characterised by UV–visible absorption spectrometry and fluorescence spectroscopy. The characteristic absorption peak of the MAA capped CdS nanoparticles is located at 335 nm as shown in Fig. 1. The fluorescence emission maximum of the functionalised CdS QD is obtained at 460 nm when excited by the radiation of 335 nm. The morphology of the functionalised CdS nanoparticles was studied by the TEM. The TEM image of CdS QDs (Fig. 2) shows that the shape of the MAA capped particles are dispersed spherical with the diameter range from 5 to 7 nm. FT-IR spectra of free MAA and functionalised MAA capped CdS QDs are shown in Fig. 3. The most pronounced IR absorption bands occurred at 3034 cm−1 (OH, COOH), 2550–2670 cm−1 (S–H), 1710 cm−1 (C O), 1460 cm−1 (s COOH), 1250 cm−1 (C–O) for MAA and at 3053 cm−1 (OH, COOH), 1462, 1420–1580 cm−1 (s COOH), 1280 cm−1 (C–O) for MAA capped CdS QDs. IR spectra of both MAA capped CdS QDs and free MAA showed absorption peaks for carboxyl and carbonyl groups. This indicates carboxyl and carbonyl groups coexistence on the surface of the CdS QDs. While the peaks for S–H (2550–2670 cm−1 ) vibration were absent in MAA capped CdS QDs. The reason for the disappearance of S–H vibration in IR spectra of MAA capped CdS QDs was the result of the covalent bonding between thiols and Cd atom on the QDs surface. 3.2. Effect of pH on the fluorescence intensity The effect of pH in a range between 3 and 12 was studied in order to select the optimum conditions for the determination of Hg(II)
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Fig. 5. Effect of MAA/Cd molar ratio on the fluorescence intensity the QDs.
Fig. 2. TEM image of MAA capped CdS QDs.
Fig. 3. Infra red transmission spectrum of free MAA and MAA capped CdS QDs.
with the MAA capped CdS QDs. As shown in Fig. 4, the pH of the solution had great effect on the fluorescence intensity of the system. The luminescence intensity increases with the pH with optimum at pH 8. Furthermore, the pH of the solution greatly affected the interaction between CdS QDs and mercury ion. The reason for the low fluorescence intensity in acidic medium is the result of the
Fig. 4. The effect of pH on the fluorescence intensity of MAA capped CdS QDs.
dissociation of Cd2+ –MAA–QDs due to the protonation of the surface binding thiolates [29]. When the pH increases up to certain level the fluorescence intensity also increases. This is due to the deprotonation of thiol group in the MAA molecule at higher pH. This deprotonation is expected to strengthen the covalent bond between thiol and Cd atom at the surface of QDs. Furthermore, increase of pH promotes negative charge of carboxylic acid group, which assist better dispersing of nanoparticles. The results show that the fluorescence intensity of CdS–QDs decreased when the pH was >8. This may be due to the formation of hydrated product, which decreases the fluorescence intensity. Furthermore, transition metals ions are precipitated as their hydroxides at higher pHs which also may reduce the fluorescence intensity. Therefore, a pH of 7.4 was chosen as optimum pH in this analytical work. In addition, pH plays a major role in the solubility of the QDs [30] as their solubility decreases with increasing pH. Increasing pH promotes the nucleation and growth, and this phenomenon generates more nanoparticles with improvement in the fluorescence intensity of QDs. Proper buffering and solution ionic strength are important when QDs are used as sensors. Therefore effects of various buffers and its ionic strengths on the fluorescence intensity of QDs were investigated. The experimental results showed that the maximum and constant synchronous fluorescence intensity occurred when the Tris–HCl buffer was in the concentration of 0.05 M. Hence, a concentration of 0.05 M Tris–HCl was used as a buffer to adjust the pH of aqueous medium. 3.3. Effect of MAA/Cd molar ratio As capping molecule, MAA stabilises the nanocrystals in the aqueous medium significantly. In order to study the MAA/Cd molar ratio on the fluorescence intensity of MAA capped CdS QDs, different batches of samples were synthesised containing different MAA/Cd molar ratios, and the fluorescence spectra of 1.0 mg l−1 of MAA capped CdS QDs were recorded from each batch of samples. The results show (Fig. 5) that the optimum fluorescence intensity was obtained at the molar ratio of MAA/Cd = 1:1. The fluorescence intensity were lower at the molar ratio of MAA/Cd = 0.5:1 and 0.25:1 than at MAA/Cd = 1:1. The reason for low fluorescence intensity is due to the fact that the CdS QDs were not completely capped by MAA. This leads the formation of clusters of QDs due to the poor dispersion. This results in low fluorescence intensity due to possible nonradiative energy transfer. The fluorescence intensity is also lower when the mole ratio of MAA/Cd was 1.5 and 2.0. This is the result of the concentration quenching effect on the fluorescent. Therefore, a molar ratio of MAA/Cd = 1 was adopted in this work.
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which is given by the following equation: Imax = 1 + KSV [S] I
Fig. 6. Responses of concentration of MAA capped CdS QDs on sensitivity of Hg(II). Concentrations of QDs (mg l−1 ), A: 1.0, B: 1.25, C: 1.50, and D: 1.75 at pH 7.4 (0.05 M Tris–HCl).
3.4. Effect of MAA capped CdS QDs concentration on fluorescence intensity and sensitivity The effect of concentration of functionalised MAA capped CdS QDs was studied to optimise the concentration of QDs to develop sensor probe for the determination of Hg(II). The fluorescence intensity increased dramatically with increase of the MAA capped CdS QDs concentration, with a maximum in the concentration range 0.9 and 1.1 mg l−1 and then decreased at higher concentrations. This decrease of fluorescence intensity at higher concentration may be due to self-quenching of fluorescence of the functionalised QDs. Furthermore, this phenomenon can also be explained that at higher concentrations the formation of clusters of nanoparticle is induced which results in a poor dispersion of QDs and thus causing a reduction in the fluorescence intensity of QDs. The QDs concentration affects not only the fluorescence intensity but also sensitivity of the assay. In order to obtain a good sensitivity for this sensor probe, different concentrations of MAA capped QDs was titrated with a series of different concentrations of Hg(II) ion and the results were plotted in the form of the Imax /I vs. concentration of Hg(II) ion as shown in Fig. 6. Here Imax and I are the fluorescence intensity of functionalised QDs in the absence and presence of Hg(II) ion, respectively. As the concentration of MAA capped CdS QDs is increased, the linear range of the calibration plot increased while the sensitivity decreased. The results show that the linear range and LOD obtained for Hg(II) ion determination were 0.05–4 × 10−7 and 4.2 × 10−9 M, respectively for a MAA capped CdS QDs at a concentration of 1.0 mg l−1 . Therefore, a concentration of 1.0 mg l−1 of MAA capped CdS QDs is recommended for the analytical studies. 3.5. Analytical performance of MAA capped CdS QDs The effect of metal ions adsorbed on the surface of the functionalised QDs may be different from that expected in bulk semiconductors [7]. The fluorescence spectra of MAA capped CdS QDs and their fluorescence titrations with analytes were recorded at optimum experimental conditions. The fluorescence intensity of MAA capped CdS QDs was significantly decreased with the addition of Hg(II) (Fig. 1). Considering this significant quenching of fluorescence intensity, the possibility of developing this into a sensitive method for Hg(II) ion was evaluated. It was found that Hg(II) quenches the fluorescence of QDs in a concentration-dependent manner that was best described by the Stern–Volmer relationship,
Imax and I are the fluorescence intensities of QDs in the presence and absence of Hg(II), respectively. [S] is the Hg(II) concentration and KSV is the Stern–Volmer constant. Under the optimum condition, the linearity of the Stern–Volmer plot spans the range between 0.05 × 10−7 and 4 × 10−7 M with a correlation coefficient of 0.9982. The KSV was found to be 1.08 × 106 M−1 . The limit of detection (LOD), determined by using the equation 3/S (where is the standard deviation of blank measurements of 8 replicates and S is the slope of the calibration curve) is 4.2 × 10−9 M. The relative standard deviation was 3.2% obtained from five replicate measurements of a solution containing 2 × 10−7 M Hg(II) ion. Several quenching mechanisms have been proposed to explain how metal ions quench the fluorescence of functionalised QDs. Inner filter effect, non radiative recombination pathway, electron transfer process, and ion binding interaction are the possible mechanisms to explain the quenching phenomena [12,21]. Fluorescent quenching characteristics of this sensor show, that there was no significant shift in emission wavelength (emission band centred 460 nm with the excitation 335 nm) with increasing concentration of Hg(II) ion. Therefore, the quenching phenomenon in this system is possibly attributed to the effective electron transfer from MAA to Hg(II) ion. This implies that, these mercury ions effectively quench the fluorescence of functionalised QDs facilitating nonradiative recombination of excited electrons (e− ) in the conduction bands and holes (h+ ) in the valence band [30]. 3.6. Reaction time and temperature The reaction time influences the fluorescence intensity of the system. Fluorescence spectrum recorded at different time intervals after the addition of Hg(II) ion into the QDs solution. The result shows that all reactions were completed within 5 min and the fluorescence signal stabilised after 60 min. Therefore, fluorescence spectrum was recorded at 15 min after the addition of Hg(II) ion. It was noted that temperature, as expected had an effect on the fluorescence intensity of the MAA capped CdS QDs. Therefore all analytical studies were conducted at the room temperature (25 ◦ C). 3.7. The effect of foreign ions Physiologically important cations have the potential to quench or enhance the fluorescence intensity of functionalised QDs. Effect of different cations on the fluorescence intensity of MAA capped CdS QDs were studied (Fig. 7) and the results show that the fluorescence intensity of MAA capped CdS QDs was sensitive to Hg(II), Cu(II) and Ag(I). However, fluorescence intensity of CdS QDs was minimally affected by Cu(II) and Ag(I) and was significantly affected by the Hg(II). All other physiologically important cations shown little or no effect on the fluorescence intensity of this QDs even when their concentrations were 100 times higher than that of Hg(II). The effect of various coexisting ions on the fluorescence of MAA capped CdS QDs was studied by mixing a concentration of 4 × 10−7 M Hg(II) ion solution and interfering ions at concentration as listed in Table 1. The solution was mixed thoroughly and was added to MAA capped CdS QDs solution (1.0 mg l−1 ). The volumes of the interfering ion solutions and of the MAA capped CdS QDs solutions were identical (20 ml each).The fluorescence spectra of MAA–CdS QDs were recorded in the presence of interfering ions and the changes of fluorescence intensity obtained are shown in Table 1. The relative standard deviation for five replicate measurements of the solution containing 2 × 10−7 M Hg(II) was estimated
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Fig. 7. Effect of different metal ions on the fluorescence intensity of functionalised CdS QDs at pH 7.4 (0.05 M Tris–HCl buffer). Concentration of QDs is 1.0 mg l−1 and concentration of Hg2+ ion is 4 × 10−7 M and other ions are 4 × 10−5 M. Table 1 Interference studies of different metal ions on the fluorescence of MAA capped CdS QDs at pH 7.4 of 0.05 M Tris–HCl. Concentration of mercury = 4 × 10−7 M. Potential interferent
Concentration (10−7 M)
Change of fluorescence intensity (%)
Na+ K+ Ca2+ Al3+ Co2+ Mg2+ Ag+ Fe2+ Mn2+ Cu2+ Zn2+ Ni2+
300 300 300 300 300 300 100 300 300 100 300 300
−0.4 −0.5 −0.8 +1.2 +0.8 +0.7 +3.0 +1.6 −0.7 +4.5 +1.0 +1.1
as 3.2%. Thus, ions causing errors not more than 3.2% are considered interferents. According to Table 1, it can be noted that only Cu2+ ion appears to cause some interference. 4. Conclusion Water soluble functionalised MAA capped CdS QDs was synthesised in a one step process to develop a luminescent sensor for Hg(II) ion. This sensor is based on the fluorescence quenching of mercury ions, which interacts with functionalised CdS QDs. Under the optimum conditions, the calibration plot was linear in the range between 0.05 × 10−7 and 4 × 10−7 M and with a correlation coefficient of 0.9982. The detection limit of this sensor is 4.2 × 10−9 M. There is a little or no interference from many metal ions that normally coexist with Hg(II) ion. Therefore, this method can be used to detect the mercury ion at nanomolar levels. In addition to its good sensitivity, other advantages of this method include its simplicity, rapidity, highly resistant to chemical and metabolic degradation.
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Biographies M. Koneswaran is currently in the final year of his PhD in School of Chemical Engineering and Analytical Science at the University of Manchester. He obtained his BSc in chemistry in 1999 from Eastern University of Sri Lanka and completed his MSc in Analytical Chemistry in 2004 at University of Peradeniya, Sri Lanka. Since 2005, he has been a student in the OFCS group, with research interest on the development of quantum dots as luminescent sensors. R. Narayanaswamy is currently a Reader in School of Chemical Engineering and Analytical Science (CEAS) at the University of Manchester. He obtained his PhD in
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1972 (Imperial College, University of London, UK) in Analytical Chemistry and DSc in 1995 (University of London) in Analytical Science. Previously he was a lecturer in Chemistry at the University of Sri Lanka, Peradeniya, Sri Lanka (1967–1978), and a Postdoctoral Research Fellow at the University of Southampton, UK (1978–1981) and the University of Warwick, UK (1982). He joined DIAS, UMIST, UK in 1983 as
a senior postdoctoral research associate and became the manager of the Optical Sensors Research Unit (1984–1987) and lecturer in instrumentation and analytical science (October 1984 to July 1990). He leads the research group that deals with the fundamental and applied studies in molecular spectroscopy and in optical chemical sensors, biosensors and instrumentation.
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Mercaptoacetic acid capped CdS quantum dots as fluorescence single shot probe for mercury(II) Masilamany Koneswaran, Ramaier Narayanaswamy ∗ Centre for Instrumentation and Analytical Science, School of Chemical Engineering and Analytical Science, University of Manchester, PO Box 88, Sackville Street, Manchester M60 1QD, United Kingdom
a r t i c l e
i n f o
Article history: Available online 18 September 2008 Keywords: Cadmium sulfide Quantum dots Mercury ion Fluorescence Quenching
a b s t r a c t Water soluble CdS quantum dots (QDs) have been synthesised using mercaptoacetic acid (MAA) as surface modifying agent through a one step process by using safe and low cost materials. These MAA capped CdS QDs are highly stable in aqueous solution and have strong affinity for mercury ion as a result of interaction with the functional group present in the capping agent. This probe is based on the characteristic of fluorescence quenching of functionalised CdS QDs. Under the optimum conditions, the fluorescence intensity of CdS QDs is linearly proportional to the mercury ion concentration in the range 0.05 × 10−7 to 4.0 × 10−7 M with a detection limit of 4.2 × 10−9 M. There is no significant wavelength shift on the fluorescence-quenched signals in the presence of mercury ion at the pH 7.4. Compare to the organic dyes, these nanoparticles are brighter and more stable against photobleaching. The effect of common foreign cations on the fluorescence of the QDs was examined to evaluate the selectivity and the results showed a high selectivity of the MAA capped CdS QDs towards Hg(II) ions. The method presented here is simple, rapid, inexpensive, sensitive and suitable for practical application. © 2008 Published by Elsevier B.V.
1. Introduction The need for developing of selective and sensitive sensors to monitor in real-time the concentration of analytes of biological and environmental importance is of great interest. The determination of heavy metals in aquatic environment has tremendous importance due to their hazardous effect on the ecosystem and on the human health. Mercury is a highly toxic environmental pollutant arising from both natural and industrial sources. This metal causes damage to microorganisms and aquatic environment even at very low concentrations. When human consumes water contaminated with mercury, several serious disorders may result including sensory, motor, and neurological damage [1]. Therefore, it is important to develop a highly sensitive and selective luminescent probe to detect the mercury ion at low concentration level. There are several organic molecules being used to detect heavy metal ions [2–4]. These organofluorophores usually have some limitations such as photobleaching, and low fluorescence intensity. They also show narrow excitation and broad emission bands with red tailing. Inorganic nanoparticle called quantum dots (QDs) has good potential to overcome these problems.
∗ Corresponding author. Tel.: +44 161 306 4891; fax: +44 161 306 4399. E-mail address: [email protected] (R. Narayanaswamy). 0925-4005/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.snb.2008.09.011
QDs are semiconductor particles that have all three dimensions confined to the nanometer scale [5]. As a result of quantum confinement, they have unique optical and electronic properties such as broad excitation spectra, narrow symmetric and tunable emission spectra which can span from the ultraviolet (UV) to the infrared (IR) region [6]. Many studies have been focused on the development of new techniques to synthesise high-quality QDs with high luminescence quantum yields and with better photophysical properties in different media [7]. The fluorescence emission wavelength can be tuned by modifying the size of QDs particles and the type of the capping molecules. The surface modification of QDs may change their optical, chemical and photophysical properties such as enhancement of their excitation and emission intensities and improvement in the photostability of semiconductor nanoparticles. The generation of new traps on the surface of QDs lead to the appearance of new emission bands, enhancement of selectivity and stability of the QDs, etc. [8]. The surface-modified QDs are usually capped with some appropriate hydrophilic functional reagents used as stabilising agents including mercaptopropionic acid, l-cysteine, thioglycerol, etc. [9–11]. The introduction of organic ligands on their surfaces not only stabilise the nanoparticles in different solvents but also result in the desired surface functionality as well [12,13]. The QDs attracted considerable attention as novel fluorescence probes because they could be widely used in chemical analysis. So far very few reports of chemical sensing of small molecules and
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ions with QDs via analyte-induced changes in photoluminescence have been reported in the literature. Chen and Rosenzweig [14] first demonstrated the luminescent QDs probe for copper and zinc ions using functionalised CdS QDs capped with different organic ligands in aqueous medium. Subsequently, in recent years, most of the analytical chemists now concentrate on synthesising new types of functionalised QDs as selective chemosensors for metal ions. Asfura and Leblanc [15] reported the peptide-coated CdS QDs as the luminescence probe for Ag(I) and Cu(II) in aqueous medium. Functionalised CdSe QDs have been synthesised by Jin et al. [16] for the detection of CN− . Several research groups have synthesised different types of functionalised QDs as luminescent probe for Cu(II) [17–19], Ag(I) [20] and Pb(II) [21,22]. All these reports show the interaction between analyte and QDs, which alter their photophysical and photochemical properties. Only few workers have reported the measurement of mercury(II) ion using semiconductor QDs [23–27]. However, these methods show low sensitivity for the detection of mercury(II) ions. Synthesis of these surface-modified QDs requires extreme reaction conditions, with the process being costly and giving low yields. This work describes a one step synthesis of the mercaptoacetic acid (MAA) capped CdS QDs for use as fluorescence sensor for Hg(II). This method is simple, sensitive, of low cost and requires only room temperature conditions. The quenching effect of Hg(II) ion on the fluorescence of the MAA capped CdS QDs was used to develop the sensing probe for mercury ion. This quenching was found to be proportional to the concentration of mercury(II) ion that can be described by the Stern–Volmer relationship. 2. Experimental 2.1. Materials and reagents All chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared with double distilled deionised water. MAA (HS–CH2 –COOH), CdCl2 ·10H2 O, sodium sulphide nonahydrate (Na2 S·9H2 O), HgCl2 , CuCl2 ·2H2 O, Zn(CH3 COO)2 , NaCl, KCl, MgCl2 , CaCl2, AgNO3 , Co(NO3 )2 MnCl2 , Tris–(hydroxymethyl)-aminomethane, and other chemicals were purchased from Sigma–Aldrich. Ethanol was purchased from Fisher Scientific. 2.2. Instrumentation Absorption spectra of samples were acquired on a Shimadzu UV2401 PC spectrophotometer. All fluorescence measurements were recorded using a LS 55 PerkinElmer fluorescence spectrometer with both excitation and emission slits set at 5.0 nm. Dilute solutions of QDs in aqueous medium were placed in 1 cm quartz cuvettes to scan the spectrum. The transmission electron microscopy (TEM) images of the nanoparticles were acquired on a FEI–TECNAI, 300 kV transmission electron microscope. The colloidal solution of nanoparticles in water was dropped onto a 0.1 nm thick carboncoated copper grid with the excess solution immediately removed. FT-IR spectra were recorded by FT-IR-8400 Shimadzu Fourier Transform Spectroscopy. pH measurements were made with a Hanna Turtle pH meter. All optical measurements were performed at room temperature and under ambient conditions. 2.3. Procedure Functionalised MAA capped CdS QDs were synthesised via the procedures described by Winter et al. [28], with some modifications. 0.5 mmol portion of MAA and 0.5 mmol of CdCl2 ·10H2 O were dissolved into 250 ml double distilled deionised water in a round
Fig. 1. Absorption spectra (a) and effect of Hg(II) ion concentration on the fluorescence of MAA capped CdS QDs (b). Concentration of QDs is 1.0 mg l−1 and the concentrations of mercury ions (×10−7 M) from A to K: A: 0, B: 0.8, C: 1.6, D: 2.4, E: 3.2, F: 4.0, G: 4.8, H: 5.6, I: 6.4, J: 7.2, and K: 8.0.
bottom flask. The pH was adjusted by the dropwise addition of 0.1 M NaOH solution to raise the pH 6.0 with constant stirring. Then 0.5 mmol portion of Na2 S·9H2 O was dissolved in 20 ml of double distilled deionised water and the Na2 S solution was added into the flask dropwise with vigorous stirring under the nitrogen environment. Absolute ethanol was used to precipitate the MAA capped CdS QDs. The functionalised CdS QDs was treated with three repeated cycles of precipitation by ethanol, washing and redispersion to remove the contaminants. The luminescent MAA capped QDs was dissolved in double distilled deionised water and the pH adjusted to 7.4 by 0.05 M Tris–HCl buffer solution for further analysis. The stock solutions of metal ions were prepared by dissolving their respective salts in double distilled deionised water. 3. Results and discussion 3.1. Characterisation of MAA capped CdS quantum dots The optical properties of MAA capped CdS QDs were characterised by UV–visible absorption spectrometry and fluorescence spectroscopy. The characteristic absorption peak of the MAA capped CdS nanoparticles is located at 335 nm as shown in Fig. 1. The fluorescence emission maximum of the functionalised CdS QD is obtained at 460 nm when excited by the radiation of 335 nm. The morphology of the functionalised CdS nanoparticles was studied by the TEM. The TEM image of CdS QDs (Fig. 2) shows that the shape of the MAA capped particles are dispersed spherical with the diameter range from 5 to 7 nm. FT-IR spectra of free MAA and functionalised MAA capped CdS QDs are shown in Fig. 3. The most pronounced IR absorption bands occurred at 3034 cm−1 (OH, COOH), 2550–2670 cm−1 (S–H), 1710 cm−1 (C O), 1460 cm−1 (s COOH), 1250 cm−1 (C–O) for MAA and at 3053 cm−1 (OH, COOH), 1462, 1420–1580 cm−1 (s COOH), 1280 cm−1 (C–O) for MAA capped CdS QDs. IR spectra of both MAA capped CdS QDs and free MAA showed absorption peaks for carboxyl and carbonyl groups. This indicates carboxyl and carbonyl groups coexistence on the surface of the CdS QDs. While the peaks for S–H (2550–2670 cm−1 ) vibration were absent in MAA capped CdS QDs. The reason for the disappearance of S–H vibration in IR spectra of MAA capped CdS QDs was the result of the covalent bonding between thiols and Cd atom on the QDs surface. 3.2. Effect of pH on the fluorescence intensity The effect of pH in a range between 3 and 12 was studied in order to select the optimum conditions for the determination of Hg(II)
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Fig. 5. Effect of MAA/Cd molar ratio on the fluorescence intensity the QDs.
Fig. 2. TEM image of MAA capped CdS QDs.
Fig. 3. Infra red transmission spectrum of free MAA and MAA capped CdS QDs.
with the MAA capped CdS QDs. As shown in Fig. 4, the pH of the solution had great effect on the fluorescence intensity of the system. The luminescence intensity increases with the pH with optimum at pH 8. Furthermore, the pH of the solution greatly affected the interaction between CdS QDs and mercury ion. The reason for the low fluorescence intensity in acidic medium is the result of the
Fig. 4. The effect of pH on the fluorescence intensity of MAA capped CdS QDs.
dissociation of Cd2+ –MAA–QDs due to the protonation of the surface binding thiolates [29]. When the pH increases up to certain level the fluorescence intensity also increases. This is due to the deprotonation of thiol group in the MAA molecule at higher pH. This deprotonation is expected to strengthen the covalent bond between thiol and Cd atom at the surface of QDs. Furthermore, increase of pH promotes negative charge of carboxylic acid group, which assist better dispersing of nanoparticles. The results show that the fluorescence intensity of CdS–QDs decreased when the pH was >8. This may be due to the formation of hydrated product, which decreases the fluorescence intensity. Furthermore, transition metals ions are precipitated as their hydroxides at higher pHs which also may reduce the fluorescence intensity. Therefore, a pH of 7.4 was chosen as optimum pH in this analytical work. In addition, pH plays a major role in the solubility of the QDs [30] as their solubility decreases with increasing pH. Increasing pH promotes the nucleation and growth, and this phenomenon generates more nanoparticles with improvement in the fluorescence intensity of QDs. Proper buffering and solution ionic strength are important when QDs are used as sensors. Therefore effects of various buffers and its ionic strengths on the fluorescence intensity of QDs were investigated. The experimental results showed that the maximum and constant synchronous fluorescence intensity occurred when the Tris–HCl buffer was in the concentration of 0.05 M. Hence, a concentration of 0.05 M Tris–HCl was used as a buffer to adjust the pH of aqueous medium. 3.3. Effect of MAA/Cd molar ratio As capping molecule, MAA stabilises the nanocrystals in the aqueous medium significantly. In order to study the MAA/Cd molar ratio on the fluorescence intensity of MAA capped CdS QDs, different batches of samples were synthesised containing different MAA/Cd molar ratios, and the fluorescence spectra of 1.0 mg l−1 of MAA capped CdS QDs were recorded from each batch of samples. The results show (Fig. 5) that the optimum fluorescence intensity was obtained at the molar ratio of MAA/Cd = 1:1. The fluorescence intensity were lower at the molar ratio of MAA/Cd = 0.5:1 and 0.25:1 than at MAA/Cd = 1:1. The reason for low fluorescence intensity is due to the fact that the CdS QDs were not completely capped by MAA. This leads the formation of clusters of QDs due to the poor dispersion. This results in low fluorescence intensity due to possible nonradiative energy transfer. The fluorescence intensity is also lower when the mole ratio of MAA/Cd was 1.5 and 2.0. This is the result of the concentration quenching effect on the fluorescent. Therefore, a molar ratio of MAA/Cd = 1 was adopted in this work.
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which is given by the following equation: Imax = 1 + KSV [S] I
Fig. 6. Responses of concentration of MAA capped CdS QDs on sensitivity of Hg(II). Concentrations of QDs (mg l−1 ), A: 1.0, B: 1.25, C: 1.50, and D: 1.75 at pH 7.4 (0.05 M Tris–HCl).
3.4. Effect of MAA capped CdS QDs concentration on fluorescence intensity and sensitivity The effect of concentration of functionalised MAA capped CdS QDs was studied to optimise the concentration of QDs to develop sensor probe for the determination of Hg(II). The fluorescence intensity increased dramatically with increase of the MAA capped CdS QDs concentration, with a maximum in the concentration range 0.9 and 1.1 mg l−1 and then decreased at higher concentrations. This decrease of fluorescence intensity at higher concentration may be due to self-quenching of fluorescence of the functionalised QDs. Furthermore, this phenomenon can also be explained that at higher concentrations the formation of clusters of nanoparticle is induced which results in a poor dispersion of QDs and thus causing a reduction in the fluorescence intensity of QDs. The QDs concentration affects not only the fluorescence intensity but also sensitivity of the assay. In order to obtain a good sensitivity for this sensor probe, different concentrations of MAA capped QDs was titrated with a series of different concentrations of Hg(II) ion and the results were plotted in the form of the Imax /I vs. concentration of Hg(II) ion as shown in Fig. 6. Here Imax and I are the fluorescence intensity of functionalised QDs in the absence and presence of Hg(II) ion, respectively. As the concentration of MAA capped CdS QDs is increased, the linear range of the calibration plot increased while the sensitivity decreased. The results show that the linear range and LOD obtained for Hg(II) ion determination were 0.05–4 × 10−7 and 4.2 × 10−9 M, respectively for a MAA capped CdS QDs at a concentration of 1.0 mg l−1 . Therefore, a concentration of 1.0 mg l−1 of MAA capped CdS QDs is recommended for the analytical studies. 3.5. Analytical performance of MAA capped CdS QDs The effect of metal ions adsorbed on the surface of the functionalised QDs may be different from that expected in bulk semiconductors [7]. The fluorescence spectra of MAA capped CdS QDs and their fluorescence titrations with analytes were recorded at optimum experimental conditions. The fluorescence intensity of MAA capped CdS QDs was significantly decreased with the addition of Hg(II) (Fig. 1). Considering this significant quenching of fluorescence intensity, the possibility of developing this into a sensitive method for Hg(II) ion was evaluated. It was found that Hg(II) quenches the fluorescence of QDs in a concentration-dependent manner that was best described by the Stern–Volmer relationship,
Imax and I are the fluorescence intensities of QDs in the presence and absence of Hg(II), respectively. [S] is the Hg(II) concentration and KSV is the Stern–Volmer constant. Under the optimum condition, the linearity of the Stern–Volmer plot spans the range between 0.05 × 10−7 and 4 × 10−7 M with a correlation coefficient of 0.9982. The KSV was found to be 1.08 × 106 M−1 . The limit of detection (LOD), determined by using the equation 3/S (where is the standard deviation of blank measurements of 8 replicates and S is the slope of the calibration curve) is 4.2 × 10−9 M. The relative standard deviation was 3.2% obtained from five replicate measurements of a solution containing 2 × 10−7 M Hg(II) ion. Several quenching mechanisms have been proposed to explain how metal ions quench the fluorescence of functionalised QDs. Inner filter effect, non radiative recombination pathway, electron transfer process, and ion binding interaction are the possible mechanisms to explain the quenching phenomena [12,21]. Fluorescent quenching characteristics of this sensor show, that there was no significant shift in emission wavelength (emission band centred 460 nm with the excitation 335 nm) with increasing concentration of Hg(II) ion. Therefore, the quenching phenomenon in this system is possibly attributed to the effective electron transfer from MAA to Hg(II) ion. This implies that, these mercury ions effectively quench the fluorescence of functionalised QDs facilitating nonradiative recombination of excited electrons (e− ) in the conduction bands and holes (h+ ) in the valence band [30]. 3.6. Reaction time and temperature The reaction time influences the fluorescence intensity of the system. Fluorescence spectrum recorded at different time intervals after the addition of Hg(II) ion into the QDs solution. The result shows that all reactions were completed within 5 min and the fluorescence signal stabilised after 60 min. Therefore, fluorescence spectrum was recorded at 15 min after the addition of Hg(II) ion. It was noted that temperature, as expected had an effect on the fluorescence intensity of the MAA capped CdS QDs. Therefore all analytical studies were conducted at the room temperature (25 ◦ C). 3.7. The effect of foreign ions Physiologically important cations have the potential to quench or enhance the fluorescence intensity of functionalised QDs. Effect of different cations on the fluorescence intensity of MAA capped CdS QDs were studied (Fig. 7) and the results show that the fluorescence intensity of MAA capped CdS QDs was sensitive to Hg(II), Cu(II) and Ag(I). However, fluorescence intensity of CdS QDs was minimally affected by Cu(II) and Ag(I) and was significantly affected by the Hg(II). All other physiologically important cations shown little or no effect on the fluorescence intensity of this QDs even when their concentrations were 100 times higher than that of Hg(II). The effect of various coexisting ions on the fluorescence of MAA capped CdS QDs was studied by mixing a concentration of 4 × 10−7 M Hg(II) ion solution and interfering ions at concentration as listed in Table 1. The solution was mixed thoroughly and was added to MAA capped CdS QDs solution (1.0 mg l−1 ). The volumes of the interfering ion solutions and of the MAA capped CdS QDs solutions were identical (20 ml each).The fluorescence spectra of MAA–CdS QDs were recorded in the presence of interfering ions and the changes of fluorescence intensity obtained are shown in Table 1. The relative standard deviation for five replicate measurements of the solution containing 2 × 10−7 M Hg(II) was estimated
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Fig. 7. Effect of different metal ions on the fluorescence intensity of functionalised CdS QDs at pH 7.4 (0.05 M Tris–HCl buffer). Concentration of QDs is 1.0 mg l−1 and concentration of Hg2+ ion is 4 × 10−7 M and other ions are 4 × 10−5 M. Table 1 Interference studies of different metal ions on the fluorescence of MAA capped CdS QDs at pH 7.4 of 0.05 M Tris–HCl. Concentration of mercury = 4 × 10−7 M. Potential interferent
Concentration (10−7 M)
Change of fluorescence intensity (%)
Na+ K+ Ca2+ Al3+ Co2+ Mg2+ Ag+ Fe2+ Mn2+ Cu2+ Zn2+ Ni2+
300 300 300 300 300 300 100 300 300 100 300 300
−0.4 −0.5 −0.8 +1.2 +0.8 +0.7 +3.0 +1.6 −0.7 +4.5 +1.0 +1.1
as 3.2%. Thus, ions causing errors not more than 3.2% are considered interferents. According to Table 1, it can be noted that only Cu2+ ion appears to cause some interference. 4. Conclusion Water soluble functionalised MAA capped CdS QDs was synthesised in a one step process to develop a luminescent sensor for Hg(II) ion. This sensor is based on the fluorescence quenching of mercury ions, which interacts with functionalised CdS QDs. Under the optimum conditions, the calibration plot was linear in the range between 0.05 × 10−7 and 4 × 10−7 M and with a correlation coefficient of 0.9982. The detection limit of this sensor is 4.2 × 10−9 M. There is a little or no interference from many metal ions that normally coexist with Hg(II) ion. Therefore, this method can be used to detect the mercury ion at nanomolar levels. In addition to its good sensitivity, other advantages of this method include its simplicity, rapidity, highly resistant to chemical and metabolic degradation.
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Biographies M. Koneswaran is currently in the final year of his PhD in School of Chemical Engineering and Analytical Science at the University of Manchester. He obtained his BSc in chemistry in 1999 from Eastern University of Sri Lanka and completed his MSc in Analytical Chemistry in 2004 at University of Peradeniya, Sri Lanka. Since 2005, he has been a student in the OFCS group, with research interest on the development of quantum dots as luminescent sensors. R. Narayanaswamy is currently a Reader in School of Chemical Engineering and Analytical Science (CEAS) at the University of Manchester. He obtained his PhD in
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1972 (Imperial College, University of London, UK) in Analytical Chemistry and DSc in 1995 (University of London) in Analytical Science. Previously he was a lecturer in Chemistry at the University of Sri Lanka, Peradeniya, Sri Lanka (1967–1978), and a Postdoctoral Research Fellow at the University of Southampton, UK (1978–1981) and the University of Warwick, UK (1982). He joined DIAS, UMIST, UK in 1983 as
a senior postdoctoral research associate and became the manager of the Optical Sensors Research Unit (1984–1987) and lecturer in instrumentation and analytical science (October 1984 to July 1990). He leads the research group that deals with the fundamental and applied studies in molecular spectroscopy and in optical chemical sensors, biosensors and instrumentation.