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Branched mercapto acid capped CdTe quantum dots as fluorescence probes for Hg2+ detection
Sensing and Bio-Sensing Research 23 (2019) 100278
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Branched mercapto acid capped CdTe quantum dots as fluorescence probes for Hg2+ detection
T
Yogesh S. Choudhary, Gomathi Nageswaran
⁎
Department of Chemistry, Indian Institute of Space Science and Technology, Thiruvananthapuram, Kerala 695547, India
ARTICLE INFO
ABSTRACT
Keywords: CdTe Quantum dots Mercury ion Optical sensor Fluorescence quenching
Herein, we report the synthesis of CdTe quantum dots prepared by colloidal synthesis route using the branched mercapto acid, 3 Mercaptoisobutyric acid (3-MIBA) as capping agent. The as synthesized 3-MIBA capped CdTe quantum dot (CdTe@3-MIBA) was used as fluorescent probe to detect Hg2+. Further, the performance was linear in the range 1.5–100 nM with a limit of detection of 1.5 ± 0.5 nM. The results demonstrated that fluorescence quenching of CdTe@3-MIBA by mercury follow different routes: (i) strong affinity of Hg towards S causes depassivation of capping agents thus resulting in aggregation of quantum dots (ii) excitonic electron transfer from the lowest unoccupied molecular orbital (LUMO) of CdTe@3-MIBA to the LUMO of Hg2+ thus preventing emission.
1. Introduction Mercury, a global pollutant, is released from number of natural processes and various anthropogenic sources. The bioaccumulative and persistent character of Hg2+ and its permeability through membrane cause undesirable effects including health problems in brain and kidney, hearing and vision loss, central nervous system of the foetus, etc. [1,2]. The growing environmental and health concerns necessitates the development of a simple, reliable, sensitive and selective detector for mercury. Conventional Hg2+ detection methods including inductively coupled plasma mass spectroscopy, atomic absorption spectroscopy, and atomic emission spectroscopy are time consuming and require expensive instrument and highly precise and tedious sample preparation. Recently, fluorescence nanosensors have received immense interest due to its rapid response, simplicity, high sensitivity and selectivity towards the detection of Hg2+. The detection limit and dynamic range of a fluorescence sensor is to a great extent determined by the physicochemical properties of the chromophore. Quantum dots, zero dimensional fluorescent materials, hold several advantages over the organic dye chromophores. Organic dyes suffer from the limitations such as low water solubility, strong hydration, low intensities, photo-bleaching, narrow excitation and broad emission spectra. This has driven an extensive research in the quantum dots based fluorescent probe in the last decade [3,4]. During the past few years, semiconductor quantum dot (QD) based fluorescence sensors for
⁎
detection of Hg2+ have been receiving more attention due to their high selectivity and sensitivity [5–8]. Cadmium Telluride (CdTe) quantum dots are gaining an increasing attention due to their high quantum yield, excellent photo stability, narrow emission wavelengths and size dependent fluorescence frequency [9]. The growth of core-shell supramolecular assembly of nano-sized CdTe core with surface ligand shell, is strongly governed by the surface ligands. The capping ligands influence the response of the CdTe fluorescence probe to various metal ions. Remarkable enhancement in the selectivity and sensitivity of CdTe fluorescence probes has been reported with various capping agents [10–12]. Mercapto salts paved their way on becoming one of the most commonly used capping ligands due to the strong affinity of the sulphur groups present in capping ligand towards the quantum dots. Though the most commonly used ligands, namely, thioglycolic acid (TGA) and mercaptopropionic acid (MPA) for the aqueous synthesis of CdTe, are linear mercapto acids, recent reports evidence the better fluorescence with branched mercapto acids [13,14]. The position and amount of methyl groups, influencing the properties such as growth rate, size distribution and solution stability, govern the fluorescence of QDs with branched mercapto acids. QDs surrounded by MPA derivatives with one methyl group are reported to show better properties in terms of narrow size distribution, enhanced fluorescence intensity and long term stability compared to MPA derivatives with two methyl side groups [14]. In the present work, CdTe quantum dots were prepared by using the branched mercapto acid with one methyl side group i.e. 3 Mercaptoisobutyric acid (3-MIBA) as a stabilizing agent. 3-MIBA
Corresponding author. E-mail address: [email protected] (G. Nageswaran).
https://doi.org/10.1016/j.sbsr.2019.100278 Received 3 February 2019; Received in revised form 15 March 2019; Accepted 20 March 2019 2214-1804/ © 2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Sensing and Bio-Sensing Research 23 (2019) 100278
Y.S. Choudhary and G. Nageswaran
capping ligand confines the growth of the quantum dots and limiting its size to maintain the quantum effects. A complete characterisation of CdTe quantum dots and CdTe-Hg2+ is carried out to confirm the formation of quantum dots and to understand the fluorescence based detection of Hg2+ ions using CdTe quantum dots, which perform as both signalling and analyte specific material. The signalling is through the fluorescence quenching during the interaction of quantum dots with the metal ions. Hence the sensor acts as a turn off sensor. The sensor responds to Hg2+ in the linear range of 1.5 nM to 100 nM with a limit of detection 1.5 ± 0.5 nM. The synthesized CdTe quantum dots are used as a probe for the sensing of mercury in the aqueous solutions.
dispersed in distilled water and used for the analysis by using various characterisation tools. 2.3. Characterisation
2. Experimental
The steady state and lifetime fluorescence spectroscopic measurements were performed by using the Fluoromax 4 spectrophotometer (Horiba Scientific, Japan). Ultra violet (UV) absorption spectra were obtained using Cary 100 Bio (Agilent Technologies). Fourier transform infrared (FTIR) analysis was performed using Spectrum 100 from Perkin Elmer. The transmission electron microscopy (TEM) analysis was carried out using JEM 2100, LaB6 from Jeol. The x-ray crystallography (XRD) was carried out using Ultima IV from Rigaku.
2.1. Materials
3. Results and discussion
Cadmium chloride hydrate (CdCl2.H2O, 99.99%) was purchased from Alfa Aesar. Tellurium dioxide (TeO2, 99.99%) powder was purchased from Sigma Aldrich. Sodium borohydride (NaBH4, 97%) and 3 Mercaptoisobutyric acid (3-MIBA) (97%) were purchased from Otto Chemicals pvt Ltd. and TCI chemicals respectively. All chemicals were used as received without any further purification.
3.1. Characterisation of CdTe QDs Size-dependent optical properties of QDs necessitate the determination of size and shape of the QDs by TEM. TEM images of as synthesized CdTe@3-MIBA QDs for the reaction times of 0.5, 1 and 2 h, presented in Fig. 1, reveals the spherical structure and uniform distribution of quantum dots. Typical sizes of the quantum dots are observed to be 1.07 nm, 2.29 nm and 2.93 nm for the reaction times of 0.5, 1 and 2 h respectively. Selected area electron diffraction (SAED) pattern confirms the quasi amorphous nature of the synthesized QDs which supports the data obtained from XRD. However, considering the more uniform distribution of size and shape of the quantum dots, CdTe QDs synthesized with a reaction of 0.5 h was focussed for further studies. Fig. 2 presents the IR spectra of CdTe@3-MIBA QD and 3-MIBA, which was employed to elucidate the binding of capping agent. The FTIR spectrum of CdTe@3-MIBA exhibits a broad band around 3320 cm−1 attributable to OH stretch. The peaks observed at 2938 and 2882 cm−1, and 1701 cm−1 in 3-MIBA spectrum are attributed to CH stretching and C]O stretching from the carboxyl group respectively.
2.2. Synthesis The quantum dots were prepared by the colloidal synthesis route. Briefly, 25 mM of 3-MIBA was added to 10 mM cadmium chloride monohydrate in 50 ml distilled water and was stirred on a hot plate. The pH of the solution was changed to 12 by adding 1 M NaOH solution. To this, 2 mM TeO2 and 4 mM NaBH4 were added sequentially followed by the vacuum. The reaction was carried out at 100 °C for half an hour. The resultant solution was removed from the hot plate and cooled using an ice bath. The solution was centrifuged to remove the unreacted chemicals using the ethanol followed by a dialysis for 12 h for the purification of the sample. The sample collected was then re-
Fig. 1. TEM images of CdTe@3-MIBA QDs with reaction time of (a) 0.5 h (b) 1 h and (c) 2 h and (d), (e) and (f) are the respective SAED pattern. 2
Sensing and Bio-Sensing Research 23 (2019) 100278
Y.S. Choudhary and G. Nageswaran
Fig. 2. FTIR spectra of 3-MIBA ligand and CdTe@3-MIBA QDs.
Fig. 4. Absorption spectra of 3-MIBA capping ligand and CdTe@3-MIBA QDs.
Characteristic thiol peaks of 3-MIBA were observed at 2567 cm−1 and 2645 cm−1 which disappear in CdTe@3-MIBA QDs confirming the binding of the capping agent with the QD. 3-MIBA co-ordinates with CdTe through breakage of SH bond and formation of new coordinate bond between thiols and the dangling bonds of Cd and Te on the surface of CdTe QDs due to the strong affinity of metal ions towards thiolate ions. The C-O-H peak at CdTe@3-MIBA QDs was observed at 1400 cm−1. The characteristic peak of C]O from the 3-MIBA is shifted to a lower wave number of 1551 cm−1 from 1701 cm−1 suggesting a distortion due to high density of the QDs capped with 3-MIBA ligand in the system along with its conformational changes. Also, the zeta potential of CdTe@3-MIBA QDs exhibits sufficiently negative value (−27 mV) as compared to the 3-MIBA (4.5 mV). This might be the reason for the huge shift in the characteristic peak of C]O during the formation of the quantum dots. Similarly, the peaks corresponding to CH stretching observed at 2938 and 2882 cm−1 are shifted to a slightly lower wave numbers of 2916 and 2851 cm−1 respectively. The typical XRD pattern of CdTe QDs is depicted in Fig. 3, exhibits three major diffraction peaks at 2θ= 24o, 38.8o and 44.4o attributable to the planes (111), (220) and (311) respectively, which matches with the JCPDS card number 15–0770; thus confirming the formation of the CdTe quantum dots. The interplanar spacing and full width half maximum of the XRD peaks of the planes (111), (220) and (311) were calculated as 3.7, 2.3 and 2 Å, and 0.42, 0.66 and 0.77 rad respectively
using Bragg's law and Scherrer's equation. The peaks with irregularity in baseline are observed to be broad due to the presence of both amorphous and crystalline contents in the synthesized quantum dots. UV–visible absorption and photoluminescence spectra were recorded to characterize the optical properties of CdTe@3-MIBA QDs. One can observe the CdTe QDs exhibiting a characteristic peak at 320 nm as shown in Fig. 4 and a wide absorption band at 490 nm (see inset of Fig. 4). The characteristic peak of 3-MIBA observed at 290 nm was observed at 275 nm in the case of CdTe@3-MIBA QDs. This result suggests the possible attachments or interaction of thiol ligands from capping agent with the QDs. The emission spectra of CdTe@3-MIBA QDs were recorded at different excitation wavelengths (λex). The maximum emission observed at λem/λex of 520/340 nm is observed to show no shift in the emission peak with different excitation wavelengths (Fig. 5). This result evidences the uniform distribution of the quantum dots in the solution. 3.2. Fluorescence quenching of CdTe@3-MIBA QDs by Hg2+ The fluorescence quenching process induced by quencher molecule enables the use of quantum dots as a probe to detect various analytes. Fluorescent sensor for Hg2+ is based on the fluorescence quenching of CdTe@3-MIBA QD probe by Hg2+. Decrease in the fluorescence
Fig. 5. PL Spectra exhibiting an excitation independent nature of the CdTe QDs, with an emission maxima at 520 nm for an excitation of 340 nm.
Fig. 3. XRD pattern of CdTe@3-MIBA. 3
Sensing and Bio-Sensing Research 23 (2019) 100278
Y.S. Choudhary and G. Nageswaran
relative quenching intensity was plotted and presented in Fig. 6b. CdTe@3-MIBA QD probe was observed to exhibit a linear quenching intensity with concentration of Hg2+ over a range of 1.5 nM to 100 nM. Table 1 presenting the comparison of the current work with the available CdTe based QDs with different capping ligands sheds a light on its significance. The fluorescence quenching is well described by the following Stern-Volmer equation
F0 = 1 + KSV [Q] F where, F0 and F are the respective actual and quenched fluorescence intensities of CdTe@3-MIBA QDs in the absence and presence of the quencher, Ksv is the Stern-Volmer constant, whereas [Q] is the concentration of the quencher, ie. Hg2+ concentration. 3.3. Fluorescence quenching mechanism Fluorescence quenching of quantum dots on addition of quencher molecules are caused by either static quenching or dynamic quenching. Quenching rate constant (kq) obtained from the relation kq = KSV/τ, using Stern-Volmer constant and lifetime of QDs can suggest the type of quenching mechanism. Dynamic quenching process plays a major role when the quenching rate constant is in the order of diffusion limited rate constant (1010 M−1 s−1) obtained from Smoluchowski equation kdiff = 8RT/3η [24]. The obtained kq value (3.48 × 1014 M−1 s−1) suggests that the static quenching plays a major role in the fluorescence quenching of CdTe by Hg2+. To investigate the quenching phenomena further, absorption spectra of CdTe@3-MIBA were recorded in absence and presence of Hg2+. As the absorption spectra of CdTe@3-MIBA did not show any appreciable shift in the absorption band with the addition of Hg2+ (Fig. 7), it clearly excludes the possibility of the formation of ground state complex between CdTe@3-MIBA and Hg2+. The stability of any colloidal solutions is governed by its surface charges. The higher the charges, the lower will be the possibility for the colloidal particles to come together and coalesce. Therefore, the zeta potential studies were performed to reveal the stability of CdTe@3MIBA in the presence of Hg2+ ions and to understand the mechanism of interaction between QDs and Hg2+ ions. It has been observed that upon addition of Hg2+, the zeta potential value of CdTe@3-MIBA found to be decreased substantially as shown in Table 2, and we hypothesize that due to the interaction with Hg2+ ions, which removes the ligands due to the affinity of the Hg towards sulphur in 3-MIBA capped CdTe, the surface charges of QDs change. As a result of the removal of the capping ligands by Hg2+ ions, the stability of the QDs is lost and thus leads to the aggregation of the QDs and their settling as depicted in TEM image (Fig. 8). This aggregation causes an increment in the overall size of the quantum dots.
Fig. 6. (a) Fluorescence emission spectra of CdTe@3-MIBA QDs with increasing concentration of Hg2+ ions, (b) F0/F vs concentration of Hg2+.
intensity of the probe induced by the quencher molecule usually changes with the concentration of the quencher molecule. Therefore a detailed study was performed on the fluorescence quenching of CdTe@ 3-MIBA QDs by Hg2+ ions by varying the concentration of Hg2+ to determine the linear range and limit of detection. Fig. 6a exhibits the fluorescence emission spectra of CdTe@3-MIBA QDs with λex = 340 nm after adding Hg2+ with different concentrations. Symmetric emission peak observed at 520 nm exhibits the decay in the peak intensity of CdTe@3-MIBA QDs with addition of Hg2+ ions of varying concentration. The concentration of the Hg2+ against the
Table 1 Comparison of performance of Hg2+ sensor using CdTe QDs with various capping ligands. CdTe QDs capped by different ligands
Linear range
LOD
Ref
BSA-coated CdTe QDs BSA coated CdTe QDs MPA-capped CdTe QDs dBSA-coated CdTe QDs MSA-capped CdTe QDs MPA-capped CdTe QDs MPA-capped CdTe QDs Cysteamine (CA)-capped CdTe QDs TGA-capped CdTe NCs N-acetyl-L-cysteine-(NAC-) capped CdTe QDs DMSA capped CdTe QDs immobilized on FTO electrode surface MPA-capped CdTe QDs MSA-capped CdTe QDs
0.001 μM - 1 μM 10 nM - 1μM 0 nM - 128 nM 12 nM-1.5μM 0.005 μM - 0.5 μM 8 nM - 3000 nM 2 nM - 14 nM 6 nM - 450 nM 3.33 nM–667 nM (in water) 20 nM - 430 nM 1 nM - 1 mM 8 nM - 2 μM 0.6 μM - 20 μM for GSH 2 μM - 20 μM for Cys 1.5 nM – 100 nM
1 nM 4.5 nM 0.5 nM 4 nM – 4.2 nM 1.55 nM 4.0 nM 3.33 nM (in water) 8 nM 0.3 nM 2.7 nM GSH - 0.1 μM Cys −0.6 μM
[9] [15] [16] [17] [18] [19] [20] [12] [10] [11] [21] [22] [23]
1.5 ± 0.5 nM
This work
CdTe@3-MIBA QDs
4
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Y.S. Choudhary and G. Nageswaran
Fig. 7. Absorbance spectra of CdTe@3-MIBA QDs before and after the addition of Hg2+ ions.
Fig. 8. TEM image of CdTe@3-MIBA + Hg2+ ions showing an aggregated structure.
To unfold the presence of any excited state phenomenon, we conducted detailed photo-physical studies as well. The fluorescence lifetime of quantum dots can get affected by energy transfer as well as electron transfer. The absence of spectral overlap of absorption spectra of the quencher (Hg2+ ions) and the emission profile of the fluorophore (CdTe@3-MIBA), readily circumvent the possibility for Forster resonance energy transfer, leaving a clear signal for the existence of the electron transfer (Fig. 9). To ascertain the possibility of quenching by electron transfer, time resolved emission spectra of CdTe@3-MIBA at different concentration of Hg2+ were collected, using a 451 nm pulsed diode laser with a pulse duration of 1.3 ns. If there is an excited state electron transfer between CdTe@3-MIBA and Hg2+, the lifetime of the former would decrease with quencher concentration owing to the emergence of new non-radiative routes. Here, after the addition of the mercury, when the sample is exposed to the UV light, the absorbed energy by the quantum dots was dissipated in the form of the non radiative recombination of the holes and electrons. With an increasing concentration of mercury, the number of non radiative recombination of the holes and electrons increases. Initially the lifetime is taken with the mercury free QDs sample followed by the sequential addition of mercury with increasing concentration and the lifetime is recorded against a common prompt. The response obtained from the fluorescence decay of CdTe@3-MIBA QDs in the presence of varying concentration of Hg2+ depicted in Fig. 10 was fitted with a biexponential decay (I (t) = A1exp(−t/τ1) + A2exp(−t/τ2)), where I(t) is the fluorescence intensity at time t, τ1 and τ2 are lifetime components, A1 and A2 are the corresponding amplitude. The lifetime results presented in Table 3 fitted with biexponential behaviour exhibiting short lived and long lived components clearly indicates that the fluorescence life time of CdTe@3-MIBA QDs decreases
Fig. 9. Spectral overlap of emission of CdTe@3-MIBA and absorption of Hg2+.
Table 2 Zeta potential values for CdTe@3-MIBA QDs with increasing Hg2+ ions. Sample
Zeta Value (mV)
CdTe CdTe CdTe CdTe CdTe CdTe
−27 −26 −25 −22 −21 −18
QDs QDs + 20 μM Hg2+ QDs + 40 μM Hg2+ QDs + 80 μM Hg2+ QDs + 160 μM Hg2+ QDs + 320 μM Hg2+
Fig. 10. Fluorescence decay of CdTe@3-MIBA QDs after addition of different concentrations of Hg2+. 5
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Table 3 Time-resolved fluorescence decay of CdTe@3-MIBA QDs with different concentration of Hg2+ ions.(τ1 and τ2 are lifetime components, A1 and A2 are corresponding amplitude, and τ is the average lifetime). System CdTe CdTe CdTe CdTe CdTe CdTe
QDs + 0.0 μM of Hg2+ QDs + 7.9 μM of Hg2+ QDs + 15.7 μM of Hg2+ QDs + 23.4 μM of Hg2+ QDs + 31.0 μM of Hg2+ QDs + 38.4 μM of Hg2+
2
τ1[ns]
A1 [%]
τ2[ns]
A2 [%]
(τ) = ∑(Ai τi2)/∑(Ai τi) [ns]
χ
6.66 4.70 4.00 3.22 2.51 1.90
20.34 27.30 28.29 34.51 45.17 57.57
23.04 19.13 17.34 15.24 12.65 9.80
79.66 72.70 71.71 65.49 54.83 42.43
21.91 17.91 16.23 14.04 11.23 8.15
1.08 1.59 1.18 1.27 1.19 1.18
Emission at 520nm
Excitation at 340nm
s
s
s
s
s s s
s s
s s s
s s
s
e
s
s
s
s
s s
s
2+
+ Hg s s
s
e
s s
HOMOs s
s
s
s s LUMO e
s
s s
s
HOMO h
s
s
CdTe@3MIBA QDs
LUMO
2+
Hg
s
h s
Aggregation
s
LUMO s
s
s
s
s
HgS
s
HOMOs s
+
s
s
h s
s
s
e
s
s
HOMOs
s
LUMO s
s
s
h
s s
s
s
LUMO s
s
s
HOMO
s
Electron Transfer
Scheme 1. Quenching mechanism of CdTe@3-MIBA QDs by Hg2+.
Thus we propose that, multiple mechanisms during the interaction of CdTe@3-MIBA with Hg2+ ions, including aggregation induced quenching at ground state and electron transfer quenching at excited state, cause fluorescence quenching of CdTe@3-MIBA. 3.4. Selectivity of CdTe@3-MIBA to Hg2+ In order to investigate the influence of various coexistence ions on selectivity, which influences the performance of fluorescent sensor for practical applications, the selective interaction of the fluorescence probe with Hg2+ ions was studied. To study the selectivity of CdTe@3MIBA QD probe towards Hg2+, luminescence of the QD was recorded in the presence of seventeen interferents including Co2+, Fe2+, Fe3+, K+, Na+, Ba2+, Ca2+, Mn2+, Mg2+, Zn2+, Cu2+, Ni2+, Al3+, Pb2+, Sn2+, Ag+ and Cd2+. From Fig. 11, it is observed that CdTe@3-MIBA QDs did not show significant fluorescent quenching by the interferent metal ions. This result suggests that the interferent metal ions only slightly quench the photoluminescent intensity due to the weak affinity of these interferent ions towards the thiol groups of the capping agent and have small influence on the interaction between the Hg2+ ions and the thiol ligands from 3-MIBA.
Fig. 11. Effect of different metal ions on the fluorescence intensity of CdTe@3MIBA QDs.
4. Conclusion
with the increasing concentration of mercury. Decrease in the lifetime of CdTe@3-MIBA QDs with increasing concentration of Hg2+ is attributed to the electron transfer from the lowest unoccupied molecular orbital of the donor (CdTe@3-MIBA QDs) to the lowest unoccupied molecular orbital of the acceptor (Hg2+) as shown in Scheme 1.
In this work, we have demonstrated the successful synthesis of branched mercapto acid capped CdTe QDs through colloidal route to obtain a stable QD. This QD was used as a turn off fluorescent sensor for 6
Sensing and Bio-Sensing Research 23 (2019) 100278
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mercury ions and was used for sensing the mercury ions at the nanomolar concentration with a linear range of 1.5–100 nM with limit of detection of 1.5 ± 0.5 nM. Time resolved fluorescence decay, UV–Vis absorption spectroscopy and TEM studies with addition of Hg2+ evidence the multiple quenching mechanisms namely aggregation induced quenching and electron transfer quenching involved in Hg2+ sensor using 3-MIBA capped CdTe QDs.
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Branched mercapto acid capped CdTe quantum dots as fluorescence probes for Hg2+ detection
T
Yogesh S. Choudhary, Gomathi Nageswaran
⁎
Department of Chemistry, Indian Institute of Space Science and Technology, Thiruvananthapuram, Kerala 695547, India
ARTICLE INFO
ABSTRACT
Keywords: CdTe Quantum dots Mercury ion Optical sensor Fluorescence quenching
Herein, we report the synthesis of CdTe quantum dots prepared by colloidal synthesis route using the branched mercapto acid, 3 Mercaptoisobutyric acid (3-MIBA) as capping agent. The as synthesized 3-MIBA capped CdTe quantum dot (CdTe@3-MIBA) was used as fluorescent probe to detect Hg2+. Further, the performance was linear in the range 1.5–100 nM with a limit of detection of 1.5 ± 0.5 nM. The results demonstrated that fluorescence quenching of CdTe@3-MIBA by mercury follow different routes: (i) strong affinity of Hg towards S causes depassivation of capping agents thus resulting in aggregation of quantum dots (ii) excitonic electron transfer from the lowest unoccupied molecular orbital (LUMO) of CdTe@3-MIBA to the LUMO of Hg2+ thus preventing emission.
1. Introduction Mercury, a global pollutant, is released from number of natural processes and various anthropogenic sources. The bioaccumulative and persistent character of Hg2+ and its permeability through membrane cause undesirable effects including health problems in brain and kidney, hearing and vision loss, central nervous system of the foetus, etc. [1,2]. The growing environmental and health concerns necessitates the development of a simple, reliable, sensitive and selective detector for mercury. Conventional Hg2+ detection methods including inductively coupled plasma mass spectroscopy, atomic absorption spectroscopy, and atomic emission spectroscopy are time consuming and require expensive instrument and highly precise and tedious sample preparation. Recently, fluorescence nanosensors have received immense interest due to its rapid response, simplicity, high sensitivity and selectivity towards the detection of Hg2+. The detection limit and dynamic range of a fluorescence sensor is to a great extent determined by the physicochemical properties of the chromophore. Quantum dots, zero dimensional fluorescent materials, hold several advantages over the organic dye chromophores. Organic dyes suffer from the limitations such as low water solubility, strong hydration, low intensities, photo-bleaching, narrow excitation and broad emission spectra. This has driven an extensive research in the quantum dots based fluorescent probe in the last decade [3,4]. During the past few years, semiconductor quantum dot (QD) based fluorescence sensors for
⁎
detection of Hg2+ have been receiving more attention due to their high selectivity and sensitivity [5–8]. Cadmium Telluride (CdTe) quantum dots are gaining an increasing attention due to their high quantum yield, excellent photo stability, narrow emission wavelengths and size dependent fluorescence frequency [9]. The growth of core-shell supramolecular assembly of nano-sized CdTe core with surface ligand shell, is strongly governed by the surface ligands. The capping ligands influence the response of the CdTe fluorescence probe to various metal ions. Remarkable enhancement in the selectivity and sensitivity of CdTe fluorescence probes has been reported with various capping agents [10–12]. Mercapto salts paved their way on becoming one of the most commonly used capping ligands due to the strong affinity of the sulphur groups present in capping ligand towards the quantum dots. Though the most commonly used ligands, namely, thioglycolic acid (TGA) and mercaptopropionic acid (MPA) for the aqueous synthesis of CdTe, are linear mercapto acids, recent reports evidence the better fluorescence with branched mercapto acids [13,14]. The position and amount of methyl groups, influencing the properties such as growth rate, size distribution and solution stability, govern the fluorescence of QDs with branched mercapto acids. QDs surrounded by MPA derivatives with one methyl group are reported to show better properties in terms of narrow size distribution, enhanced fluorescence intensity and long term stability compared to MPA derivatives with two methyl side groups [14]. In the present work, CdTe quantum dots were prepared by using the branched mercapto acid with one methyl side group i.e. 3 Mercaptoisobutyric acid (3-MIBA) as a stabilizing agent. 3-MIBA
Corresponding author. E-mail address: [email protected] (G. Nageswaran).
https://doi.org/10.1016/j.sbsr.2019.100278 Received 3 February 2019; Received in revised form 15 March 2019; Accepted 20 March 2019 2214-1804/ © 2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Y.S. Choudhary and G. Nageswaran
capping ligand confines the growth of the quantum dots and limiting its size to maintain the quantum effects. A complete characterisation of CdTe quantum dots and CdTe-Hg2+ is carried out to confirm the formation of quantum dots and to understand the fluorescence based detection of Hg2+ ions using CdTe quantum dots, which perform as both signalling and analyte specific material. The signalling is through the fluorescence quenching during the interaction of quantum dots with the metal ions. Hence the sensor acts as a turn off sensor. The sensor responds to Hg2+ in the linear range of 1.5 nM to 100 nM with a limit of detection 1.5 ± 0.5 nM. The synthesized CdTe quantum dots are used as a probe for the sensing of mercury in the aqueous solutions.
dispersed in distilled water and used for the analysis by using various characterisation tools. 2.3. Characterisation
2. Experimental
The steady state and lifetime fluorescence spectroscopic measurements were performed by using the Fluoromax 4 spectrophotometer (Horiba Scientific, Japan). Ultra violet (UV) absorption spectra were obtained using Cary 100 Bio (Agilent Technologies). Fourier transform infrared (FTIR) analysis was performed using Spectrum 100 from Perkin Elmer. The transmission electron microscopy (TEM) analysis was carried out using JEM 2100, LaB6 from Jeol. The x-ray crystallography (XRD) was carried out using Ultima IV from Rigaku.
2.1. Materials
3. Results and discussion
Cadmium chloride hydrate (CdCl2.H2O, 99.99%) was purchased from Alfa Aesar. Tellurium dioxide (TeO2, 99.99%) powder was purchased from Sigma Aldrich. Sodium borohydride (NaBH4, 97%) and 3 Mercaptoisobutyric acid (3-MIBA) (97%) were purchased from Otto Chemicals pvt Ltd. and TCI chemicals respectively. All chemicals were used as received without any further purification.
3.1. Characterisation of CdTe QDs Size-dependent optical properties of QDs necessitate the determination of size and shape of the QDs by TEM. TEM images of as synthesized CdTe@3-MIBA QDs for the reaction times of 0.5, 1 and 2 h, presented in Fig. 1, reveals the spherical structure and uniform distribution of quantum dots. Typical sizes of the quantum dots are observed to be 1.07 nm, 2.29 nm and 2.93 nm for the reaction times of 0.5, 1 and 2 h respectively. Selected area electron diffraction (SAED) pattern confirms the quasi amorphous nature of the synthesized QDs which supports the data obtained from XRD. However, considering the more uniform distribution of size and shape of the quantum dots, CdTe QDs synthesized with a reaction of 0.5 h was focussed for further studies. Fig. 2 presents the IR spectra of CdTe@3-MIBA QD and 3-MIBA, which was employed to elucidate the binding of capping agent. The FTIR spectrum of CdTe@3-MIBA exhibits a broad band around 3320 cm−1 attributable to OH stretch. The peaks observed at 2938 and 2882 cm−1, and 1701 cm−1 in 3-MIBA spectrum are attributed to CH stretching and C]O stretching from the carboxyl group respectively.
2.2. Synthesis The quantum dots were prepared by the colloidal synthesis route. Briefly, 25 mM of 3-MIBA was added to 10 mM cadmium chloride monohydrate in 50 ml distilled water and was stirred on a hot plate. The pH of the solution was changed to 12 by adding 1 M NaOH solution. To this, 2 mM TeO2 and 4 mM NaBH4 were added sequentially followed by the vacuum. The reaction was carried out at 100 °C for half an hour. The resultant solution was removed from the hot plate and cooled using an ice bath. The solution was centrifuged to remove the unreacted chemicals using the ethanol followed by a dialysis for 12 h for the purification of the sample. The sample collected was then re-
Fig. 1. TEM images of CdTe@3-MIBA QDs with reaction time of (a) 0.5 h (b) 1 h and (c) 2 h and (d), (e) and (f) are the respective SAED pattern. 2
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Y.S. Choudhary and G. Nageswaran
Fig. 2. FTIR spectra of 3-MIBA ligand and CdTe@3-MIBA QDs.
Fig. 4. Absorption spectra of 3-MIBA capping ligand and CdTe@3-MIBA QDs.
Characteristic thiol peaks of 3-MIBA were observed at 2567 cm−1 and 2645 cm−1 which disappear in CdTe@3-MIBA QDs confirming the binding of the capping agent with the QD. 3-MIBA co-ordinates with CdTe through breakage of SH bond and formation of new coordinate bond between thiols and the dangling bonds of Cd and Te on the surface of CdTe QDs due to the strong affinity of metal ions towards thiolate ions. The C-O-H peak at CdTe@3-MIBA QDs was observed at 1400 cm−1. The characteristic peak of C]O from the 3-MIBA is shifted to a lower wave number of 1551 cm−1 from 1701 cm−1 suggesting a distortion due to high density of the QDs capped with 3-MIBA ligand in the system along with its conformational changes. Also, the zeta potential of CdTe@3-MIBA QDs exhibits sufficiently negative value (−27 mV) as compared to the 3-MIBA (4.5 mV). This might be the reason for the huge shift in the characteristic peak of C]O during the formation of the quantum dots. Similarly, the peaks corresponding to CH stretching observed at 2938 and 2882 cm−1 are shifted to a slightly lower wave numbers of 2916 and 2851 cm−1 respectively. The typical XRD pattern of CdTe QDs is depicted in Fig. 3, exhibits three major diffraction peaks at 2θ= 24o, 38.8o and 44.4o attributable to the planes (111), (220) and (311) respectively, which matches with the JCPDS card number 15–0770; thus confirming the formation of the CdTe quantum dots. The interplanar spacing and full width half maximum of the XRD peaks of the planes (111), (220) and (311) were calculated as 3.7, 2.3 and 2 Å, and 0.42, 0.66 and 0.77 rad respectively
using Bragg's law and Scherrer's equation. The peaks with irregularity in baseline are observed to be broad due to the presence of both amorphous and crystalline contents in the synthesized quantum dots. UV–visible absorption and photoluminescence spectra were recorded to characterize the optical properties of CdTe@3-MIBA QDs. One can observe the CdTe QDs exhibiting a characteristic peak at 320 nm as shown in Fig. 4 and a wide absorption band at 490 nm (see inset of Fig. 4). The characteristic peak of 3-MIBA observed at 290 nm was observed at 275 nm in the case of CdTe@3-MIBA QDs. This result suggests the possible attachments or interaction of thiol ligands from capping agent with the QDs. The emission spectra of CdTe@3-MIBA QDs were recorded at different excitation wavelengths (λex). The maximum emission observed at λem/λex of 520/340 nm is observed to show no shift in the emission peak with different excitation wavelengths (Fig. 5). This result evidences the uniform distribution of the quantum dots in the solution. 3.2. Fluorescence quenching of CdTe@3-MIBA QDs by Hg2+ The fluorescence quenching process induced by quencher molecule enables the use of quantum dots as a probe to detect various analytes. Fluorescent sensor for Hg2+ is based on the fluorescence quenching of CdTe@3-MIBA QD probe by Hg2+. Decrease in the fluorescence
Fig. 5. PL Spectra exhibiting an excitation independent nature of the CdTe QDs, with an emission maxima at 520 nm for an excitation of 340 nm.
Fig. 3. XRD pattern of CdTe@3-MIBA. 3
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relative quenching intensity was plotted and presented in Fig. 6b. CdTe@3-MIBA QD probe was observed to exhibit a linear quenching intensity with concentration of Hg2+ over a range of 1.5 nM to 100 nM. Table 1 presenting the comparison of the current work with the available CdTe based QDs with different capping ligands sheds a light on its significance. The fluorescence quenching is well described by the following Stern-Volmer equation
F0 = 1 + KSV [Q] F where, F0 and F are the respective actual and quenched fluorescence intensities of CdTe@3-MIBA QDs in the absence and presence of the quencher, Ksv is the Stern-Volmer constant, whereas [Q] is the concentration of the quencher, ie. Hg2+ concentration. 3.3. Fluorescence quenching mechanism Fluorescence quenching of quantum dots on addition of quencher molecules are caused by either static quenching or dynamic quenching. Quenching rate constant (kq) obtained from the relation kq = KSV/τ, using Stern-Volmer constant and lifetime of QDs can suggest the type of quenching mechanism. Dynamic quenching process plays a major role when the quenching rate constant is in the order of diffusion limited rate constant (1010 M−1 s−1) obtained from Smoluchowski equation kdiff = 8RT/3η [24]. The obtained kq value (3.48 × 1014 M−1 s−1) suggests that the static quenching plays a major role in the fluorescence quenching of CdTe by Hg2+. To investigate the quenching phenomena further, absorption spectra of CdTe@3-MIBA were recorded in absence and presence of Hg2+. As the absorption spectra of CdTe@3-MIBA did not show any appreciable shift in the absorption band with the addition of Hg2+ (Fig. 7), it clearly excludes the possibility of the formation of ground state complex between CdTe@3-MIBA and Hg2+. The stability of any colloidal solutions is governed by its surface charges. The higher the charges, the lower will be the possibility for the colloidal particles to come together and coalesce. Therefore, the zeta potential studies were performed to reveal the stability of CdTe@3MIBA in the presence of Hg2+ ions and to understand the mechanism of interaction between QDs and Hg2+ ions. It has been observed that upon addition of Hg2+, the zeta potential value of CdTe@3-MIBA found to be decreased substantially as shown in Table 2, and we hypothesize that due to the interaction with Hg2+ ions, which removes the ligands due to the affinity of the Hg towards sulphur in 3-MIBA capped CdTe, the surface charges of QDs change. As a result of the removal of the capping ligands by Hg2+ ions, the stability of the QDs is lost and thus leads to the aggregation of the QDs and their settling as depicted in TEM image (Fig. 8). This aggregation causes an increment in the overall size of the quantum dots.
Fig. 6. (a) Fluorescence emission spectra of CdTe@3-MIBA QDs with increasing concentration of Hg2+ ions, (b) F0/F vs concentration of Hg2+.
intensity of the probe induced by the quencher molecule usually changes with the concentration of the quencher molecule. Therefore a detailed study was performed on the fluorescence quenching of CdTe@ 3-MIBA QDs by Hg2+ ions by varying the concentration of Hg2+ to determine the linear range and limit of detection. Fig. 6a exhibits the fluorescence emission spectra of CdTe@3-MIBA QDs with λex = 340 nm after adding Hg2+ with different concentrations. Symmetric emission peak observed at 520 nm exhibits the decay in the peak intensity of CdTe@3-MIBA QDs with addition of Hg2+ ions of varying concentration. The concentration of the Hg2+ against the
Table 1 Comparison of performance of Hg2+ sensor using CdTe QDs with various capping ligands. CdTe QDs capped by different ligands
Linear range
LOD
Ref
BSA-coated CdTe QDs BSA coated CdTe QDs MPA-capped CdTe QDs dBSA-coated CdTe QDs MSA-capped CdTe QDs MPA-capped CdTe QDs MPA-capped CdTe QDs Cysteamine (CA)-capped CdTe QDs TGA-capped CdTe NCs N-acetyl-L-cysteine-(NAC-) capped CdTe QDs DMSA capped CdTe QDs immobilized on FTO electrode surface MPA-capped CdTe QDs MSA-capped CdTe QDs
0.001 μM - 1 μM 10 nM - 1μM 0 nM - 128 nM 12 nM-1.5μM 0.005 μM - 0.5 μM 8 nM - 3000 nM 2 nM - 14 nM 6 nM - 450 nM 3.33 nM–667 nM (in water) 20 nM - 430 nM 1 nM - 1 mM 8 nM - 2 μM 0.6 μM - 20 μM for GSH 2 μM - 20 μM for Cys 1.5 nM – 100 nM
1 nM 4.5 nM 0.5 nM 4 nM – 4.2 nM 1.55 nM 4.0 nM 3.33 nM (in water) 8 nM 0.3 nM 2.7 nM GSH - 0.1 μM Cys −0.6 μM
[9] [15] [16] [17] [18] [19] [20] [12] [10] [11] [21] [22] [23]
1.5 ± 0.5 nM
This work
CdTe@3-MIBA QDs
4
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Fig. 7. Absorbance spectra of CdTe@3-MIBA QDs before and after the addition of Hg2+ ions.
Fig. 8. TEM image of CdTe@3-MIBA + Hg2+ ions showing an aggregated structure.
To unfold the presence of any excited state phenomenon, we conducted detailed photo-physical studies as well. The fluorescence lifetime of quantum dots can get affected by energy transfer as well as electron transfer. The absence of spectral overlap of absorption spectra of the quencher (Hg2+ ions) and the emission profile of the fluorophore (CdTe@3-MIBA), readily circumvent the possibility for Forster resonance energy transfer, leaving a clear signal for the existence of the electron transfer (Fig. 9). To ascertain the possibility of quenching by electron transfer, time resolved emission spectra of CdTe@3-MIBA at different concentration of Hg2+ were collected, using a 451 nm pulsed diode laser with a pulse duration of 1.3 ns. If there is an excited state electron transfer between CdTe@3-MIBA and Hg2+, the lifetime of the former would decrease with quencher concentration owing to the emergence of new non-radiative routes. Here, after the addition of the mercury, when the sample is exposed to the UV light, the absorbed energy by the quantum dots was dissipated in the form of the non radiative recombination of the holes and electrons. With an increasing concentration of mercury, the number of non radiative recombination of the holes and electrons increases. Initially the lifetime is taken with the mercury free QDs sample followed by the sequential addition of mercury with increasing concentration and the lifetime is recorded against a common prompt. The response obtained from the fluorescence decay of CdTe@3-MIBA QDs in the presence of varying concentration of Hg2+ depicted in Fig. 10 was fitted with a biexponential decay (I (t) = A1exp(−t/τ1) + A2exp(−t/τ2)), where I(t) is the fluorescence intensity at time t, τ1 and τ2 are lifetime components, A1 and A2 are the corresponding amplitude. The lifetime results presented in Table 3 fitted with biexponential behaviour exhibiting short lived and long lived components clearly indicates that the fluorescence life time of CdTe@3-MIBA QDs decreases
Fig. 9. Spectral overlap of emission of CdTe@3-MIBA and absorption of Hg2+.
Table 2 Zeta potential values for CdTe@3-MIBA QDs with increasing Hg2+ ions. Sample
Zeta Value (mV)
CdTe CdTe CdTe CdTe CdTe CdTe
−27 −26 −25 −22 −21 −18
QDs QDs + 20 μM Hg2+ QDs + 40 μM Hg2+ QDs + 80 μM Hg2+ QDs + 160 μM Hg2+ QDs + 320 μM Hg2+
Fig. 10. Fluorescence decay of CdTe@3-MIBA QDs after addition of different concentrations of Hg2+. 5
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Table 3 Time-resolved fluorescence decay of CdTe@3-MIBA QDs with different concentration of Hg2+ ions.(τ1 and τ2 are lifetime components, A1 and A2 are corresponding amplitude, and τ is the average lifetime). System CdTe CdTe CdTe CdTe CdTe CdTe
QDs + 0.0 μM of Hg2+ QDs + 7.9 μM of Hg2+ QDs + 15.7 μM of Hg2+ QDs + 23.4 μM of Hg2+ QDs + 31.0 μM of Hg2+ QDs + 38.4 μM of Hg2+
2
τ1[ns]
A1 [%]
τ2[ns]
A2 [%]
(τ) = ∑(Ai τi2)/∑(Ai τi) [ns]
χ
6.66 4.70 4.00 3.22 2.51 1.90
20.34 27.30 28.29 34.51 45.17 57.57
23.04 19.13 17.34 15.24 12.65 9.80
79.66 72.70 71.71 65.49 54.83 42.43
21.91 17.91 16.23 14.04 11.23 8.15
1.08 1.59 1.18 1.27 1.19 1.18
Emission at 520nm
Excitation at 340nm
s
s
s
s
s s s
s s
s s s
s s
s
e
s
s
s
s
s s
s
2+
+ Hg s s
s
e
s s
HOMOs s
s
s
s s LUMO e
s
s s
s
HOMO h
s
s
CdTe@3MIBA QDs
LUMO
2+
Hg
s
h s
Aggregation
s
LUMO s
s
s
s
s
HgS
s
HOMOs s
+
s
s
h s
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HOMOs
s
LUMO s
s
s
h
s s
s
s
LUMO s
s
s
HOMO
s
Electron Transfer
Scheme 1. Quenching mechanism of CdTe@3-MIBA QDs by Hg2+.
Thus we propose that, multiple mechanisms during the interaction of CdTe@3-MIBA with Hg2+ ions, including aggregation induced quenching at ground state and electron transfer quenching at excited state, cause fluorescence quenching of CdTe@3-MIBA. 3.4. Selectivity of CdTe@3-MIBA to Hg2+ In order to investigate the influence of various coexistence ions on selectivity, which influences the performance of fluorescent sensor for practical applications, the selective interaction of the fluorescence probe with Hg2+ ions was studied. To study the selectivity of CdTe@3MIBA QD probe towards Hg2+, luminescence of the QD was recorded in the presence of seventeen interferents including Co2+, Fe2+, Fe3+, K+, Na+, Ba2+, Ca2+, Mn2+, Mg2+, Zn2+, Cu2+, Ni2+, Al3+, Pb2+, Sn2+, Ag+ and Cd2+. From Fig. 11, it is observed that CdTe@3-MIBA QDs did not show significant fluorescent quenching by the interferent metal ions. This result suggests that the interferent metal ions only slightly quench the photoluminescent intensity due to the weak affinity of these interferent ions towards the thiol groups of the capping agent and have small influence on the interaction between the Hg2+ ions and the thiol ligands from 3-MIBA.
Fig. 11. Effect of different metal ions on the fluorescence intensity of CdTe@3MIBA QDs.
4. Conclusion
with the increasing concentration of mercury. Decrease in the lifetime of CdTe@3-MIBA QDs with increasing concentration of Hg2+ is attributed to the electron transfer from the lowest unoccupied molecular orbital of the donor (CdTe@3-MIBA QDs) to the lowest unoccupied molecular orbital of the acceptor (Hg2+) as shown in Scheme 1.
In this work, we have demonstrated the successful synthesis of branched mercapto acid capped CdTe QDs through colloidal route to obtain a stable QD. This QD was used as a turn off fluorescent sensor for 6
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mercury ions and was used for sensing the mercury ions at the nanomolar concentration with a linear range of 1.5–100 nM with limit of detection of 1.5 ± 0.5 nM. Time resolved fluorescence decay, UV–Vis absorption spectroscopy and TEM studies with addition of Hg2+ evidence the multiple quenching mechanisms namely aggregation induced quenching and electron transfer quenching involved in Hg2+ sensor using 3-MIBA capped CdTe QDs.
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