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CdTe quantum dots capped with different stabilizing agents for sensing of ochratoxin A
Author’s Accepted Manuscript CdTe Quantum Dots Capped with Different Stabilizing Agents for Sensing of Ochratoxin A Shaker Ebrahim, Mohamed Labeb, Tarek AbdelFattah, Moataz Soliman www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(16)30528-2 http://dx.doi.org/10.1016/j.jlumin.2016.09.038 LUMIN14267
To appear in: Journal of Luminescence Received date: 25 April 2016 Revised date: 16 September 2016 Accepted date: 19 September 2016 Cite this article as: Shaker Ebrahim, Mohamed Labeb, Tarek Abdel-Fattah and Moataz Soliman, CdTe Quantum Dots Capped with Different Stabilizing Agents for Sensing of Ochratoxin A, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.09.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CdTe Quantum Dots Capped with Different Stabilizing Agents for Sensing of Ochratoxin A
Shaker Ebrahima*, Mohamed Labeba, Tarek Abdel-Fattahb, Moataz Solimana a
Materials Science Department, Institute of Graduate Studies & Research, 21526, Alexandria
University, Egypt b
Applied Research Center, Thomas Jefferson National Accelerator Facility and Department of
Molecular Biology, and Chemistry, Christopher Newport University, Newport News, VA 23606, USA Abstract Cadmium telluride (CdTe) quantum dots (QDs) were prepared from aqueous solution with different capping agents of thioglycolic acid (TGA) and L-cysteine. Photoluminance spectra of CdTe QDs were used as a property of optical sensor for ochratoxin A (OTA) in the concentration range of ng/mL. It was found that L-cysteine capped CdTe QDs have linear response to OTA concentration in the range from 0.5 to 10 ng/mL. By applying Stern-Volmer relationship, it was found that a very good linearity (R2 = 0.99) is observed in the concentration range from 0.5 to 10 ng/ml and Ksv was calculated to be 3.48 ×104 Lg-1 and the minimum of detection of the method was 0.5 ng/ml. The mechanism of effect of pH on the fluorescence of CdTe QDs was proposed and the low fluorescence intensity of CdTe QDs in acidic media was attributed to protonation of the surface binding thiolates and consequently the dissociation of CdTe QDs. Keywords: CdTe; Quantum Dots; Optical Sensor; Photoluminescence; Ochratoxin A
1. Introduction
* Correspondence to Shaker Ebrahim, 163 Horreya Avenue, El-Shatby, Alexandria, Egypt Tel.: +201224879137, Fax: +2034285792, e-mail: [email protected]
Mycotoxins are toxic metabolises generated by fungi, mainly by saprophytic moulds growing on variety of foodstuffs including human foods and animal feeds [1-4]. Ochratoxins belong to the group of mycotoxins that are reproduced as secondary metabolites by fungi, in particular Aspergillus and Penicillium. These fungi flourish under certain conditions of temperature and humidity [5-6]. Ochratoxins types include ochratoxin A (OTA), ochratoxin B and ochratoxin C and OTA is considered the most toxic [7]. OTA consists of a chlorinated dihydroisocoumarin moiety linked through a 7 carboxyl group by amide bond to one molecule of L-ß-phenylalanine [8]. The detection of OTA in the environment and food industry is a urgently demand because OTA has been suggested to be responsible of Balkan Endemic Nephropathy disease. OTA A is rated by the International Agency for Research on Cancer as carcinogen of class 2B [9-10]. Different techniques such as ultra-performance liquid chromatography, high-performance liquid chromatography and gas chromatography can be used for detection of OTA. Though thier high sensitivity and accuracy, these techniques are laborious, expensive, time consuming and require high sophisticated instrumentation and/or sample preparation, limiting their practical applications [11-13]. Quantum dots (QDs) are class of nanocrystals composed of several atoms constrained in three dimensions to a somewhat spherical shape, with a diameter of 2-8 nm, containing approximately 200-10,000 atoms, surrounded by an organic outer layer as capping molecules [14-19]. They have unique optical and electronic properties, such as high quantum yield, narrow, tunable and symmetric emission spectra, and broad absorption spectra. QDs have the ability to emit different wavelengths over a broad range of the light spectrum from visible to infrared, depending on their sizes and chemical compositions. The unique optical properties enable QDs to exceed conventional organic dyes as fluorophores [20-27]. QDs-based chemical sensing systems employ fluorescence changes induced by the analyte presented at the surface or close to the surface of the QDs. Sensing via QDs is based on the interaction of the QDs with the analyte of interest, i.e. influencing the optical properties, which in turn are quite sensitive to the type of capping ligand used, indicating that surface phenomenon must be taken in the consideration. The analyte present close to the QDs leads to dramatic changes in the luminescent properties of these QDs, i.e. results either in fluorescence quenching or enhancement. This promotes QDs to be used and applied for analytical sensing [28-31]. Zhang et al used CdTe QDs as the fluorescent label to aflatoxin B1. CdTe QDs were linked to the monoclonal antibody against AFB1. Based on this conjugated complexes, a direct competitive fluorescence-linked immunosorbent assay developed for aflatoxin B1 detection. The method performance included the limit of detection of 0.016 ng/mL [32]. Gan et al developed an ultrasensitive electrochemiluminescent immunoassay for aflatoxins M1 in milk using magnetic Fe3O4-graphene oxides (Fe-GO) as the absorbent and antibody labeled CdTe QDs and indicated that it can adsorb aflatoxins M1 efficiently and selectively within a large extent of pH from 3.0 to 8.0. Adsorption processes reached 95% of the equilibrium within 10 min. The intensity of the electrochemiluminescent showed linear dependence of the logarithm of aflatoxins M1
concentration in the range from 1.0 pg/mL to 100 ng/mL, with a correlation coefficient of 0.9965. Limit of detection was 0.3 pg/mL [33]. Zekavati et al provided a competitive immunoassay for the determination of aflatoxin B1 using fluorescence resonance energy transfer from anti-aflatoxin B1 antibody (immobilized on the shell of CdTe quantum dots) to Rhodamine 123 (Rho 123-labeled aflatoxin B1 bound to albumin). The result was a linear relationship between the increased fluorescence intensity of Rho 123 with increasing concentration of aflatoxin B1 in spike human serum, over the range of 0.1–0.6 μmol·mL−1 and the limit of detection was 2×10−11 M [34]. For the one's best knowledge, there is no published data used the CdTe QDs to detect ochratoxin A. In this work water-soluble CdTe QDs with different capping agents of L-cysteine and thioglycolic acid were prepared and evaluated as fluorescence probes for OTA molecules in the range of ng/ml. Effect of pH of on the PL of CdTe QDs was also investigated and explained. The proposed mechanisms of the interactions between OTA and capped CdTe QDs were suggested.
2. Materials and Methods 2.1 Materials Sodium borohydride was purchased from Merck. Cadmium chloride (CdCl2) (20% hydrate) and L-cysteine was purchased from Sigma chemical CO. Telerium powder (Te) (99.9%) was purchased from Microphysic. Thioglycolic acid (TGA) was received from Merck Schuchardt OHG. Isopropanol and ethanol was purchased from Aldrich. Potassium chloride, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride and sodium hydroxide were purchased from Elnasr Pharmaceutical Chemical Co. Ochratoxin A (99%) was purchased from Supelco. Hydrochloric acid was purchased from Scharlab S.L. 2.2 Preparation of CdTe QDs with different stabilizing agents 2.2.1 Preparation of NaHTe solution NaHTe solution was prepared according to the following method. Simply, 40 mg of tellurium powder and 45 mg of NaBH4 were loaded in a three neck flask and then 5 mL deionized water was added. The mixture was kept reaction at 60°C for appropriate time under N2 flow to get a deep purple clear solution.
2.2.2 Preparation of TGA or L-cysteine capped CdTe QDs
CdTe QDs was prepared by the following procedure. Simply, 3.0 mmol stabilizing agent (TGA or L-cysteine) and 330 mg CdCl2 ( 20% hydrated ) were dissolved in 100 ml ultrapure water and the pH of mixture was adjusted to 12.0 by drop wise addition of 1.0 M NaOH solution with stirring in conical flask, then the fresh NaHTe solution was added through a syringe promptly under continually stirring. In case of L-cysteine the injection at 70 °C but in case of TGA the injection at 80°C. The final solution was refluxed at 90°C with condenser attached for 60 min. The CdTe QDs solution were purified by adding in isopropyl alcohol, then CdTe QDs were separated by centrifugation at 10,000 rpm for 6 min and were repeated several times to remove unreacted materials.
2.3 Studying the effect of pH on the PL of CdTe QDs To investigate the effect of pH, 1 ml of as-prepared luminescent CdTe solution was added to 3 ml of 1% PBS with different pH of 5, 6, 7 and 8 and incubated under darkness for 20 min at room temperature. 2.4 Detection of ochratoxin A by CdTe QDs A stock solution of 50 ppm and intermediate solution of 0.5 ppm of OTA were prepared in ethanol. To 5 ml volumetric flasks, 2 ml of as-prepared luminescent CdTe solution, 2 ml of PBS of pH 8 and various amounts of OTA were sequentially added, and then diluted to volume with ultrapure water and incubated under darkness for 20 min at room temperature.
2.5 Characterization techniques UV–visible characterization of the CdTe QDs solution was carried out using Evolution 600 spectrophotometer. High resolution transmission electron microscope (HRTEM) images of the CdTe QDs were obtained using JEOL (JEM-2100 LaB6) and by immersing copper grid in the CdTe QDs solution for 10 s followed by a drying stage. EDX of Cd to Te ratios for CdTe QDs capped with TGA or L-cysteine was performed using JEOL (JSM 5300). Photoluminance spectra were made with fluorescence spectrophotometer (Perkin Elmer LS-55) equipped with the computer and a 1 cm quartz cell with an excitation wavelength of 450 for CdTe QDs. The fluorescence spectra of the CdTe QDs were determined in range of 450-700 nm at excitation wavelengths of 535 nm for TGA capped CdTe QDs and 536 nm for L-cysteine capped CdTe QDs.
3. Results and Discussion 3.1 UV-Visible and PL Spectra of CdTe QDs Capped with Different Stabilizing Agents Figure (1) shows UV-visible and PL spectra of CdTe QDs capped with TGA in deionized water of pH 7.0. The appearance of the absorption shoulder in the range of 500-550 nm for the capped CdTe QDs with TGA indicates the formation of the QDs. The CdTe QDs colloids have symmetric emission peaks at about 551 nm. When a photon with an excitation wavelength exceeding the semiconductor band-gap energy is absorbed by CdTe QDs, electrons are promoted from the valence band to the conduction band. This excited electron then relaxes to its ground state by the emission of another photon with energy equal to the band-gap [35]. Insert of Figure (1) shows UV-visible spectra of TGA capped CdTe QDs prepared with TGA as a capping agent at different times. With the increase of the reaction duration, the absorption peak shows red shift due to the quantum size effect [20, 24]. With the increasing of reaction time, this excitonic absorption peak of CdTe QDs shifts to the longer wavelengths from 508 to 535 nm as the QDs grow to larger size. The size of the CdTe QDs can be calculated according to eq. (1) [35]. (
)
(
)
(
)
(1)
where D (nm) was the size of CdTe QDs, and λ (nm) is the wavelength of the first excitonic absorption peak. The size of the CdTe QDs capped with TGA is calculated to be ca. 2.55 nm at excitation wavelength of 508 nm to 3.05 nm at excitation wavelength of 535 nm. Figure (2) illustrates UV-visible and PL spectra of CdTe QDs capped with L-cysteine in deionized water of pH 7.0. The appearance of the well absorption peaks around 507 nm for the capped CdTe QDs with L-cysteine confirms the formation of the CdTe QDs. The CdTe QDs colloids have an emission peak at about 554 nm. Insert of Figure (2) presents UV-visible spectra of CdTe QDs prepared with L-cysteine as a capping agent at different times. With the increase of the reaction time, the absorption peak shows red shift to 536 nm after 30 min due to increase quantum size of CdTe QDs. After 30 min the size of CdTe QDs becomes fixed and the intensity of the peak increased. It is noted that after 60 min the CdTe QDs is precipitated due to degradation of L-cysteine. The size of the CdTe QDs capped with L-cysteine is calculated from
eq. (1) to be ca. 2.59 nm at excitation wavelength of 516 nm to 3.06 nm at excitation wavelength of 536 nm.
Figure (1)
Figure (2)
3.2 Morphological property and elemental analysis of CdTe QDs Figure (3) depicts HRTEM images of CdTe QDs capped with TGA and L-cysteine. CdTe QDs are dispersed with an average size about 3.1 nm for TGA-CdTe QDs which is close to the calculated value (3.05 nm) mentioned above and 3.2 nm for L-cysteine capped CdTe QDs which is close to the calculated value (3.06 nm). The existence of the lattice plan on the HRTEM images of the particles indicates that CdTe QDs are highly crystalline as shown in the circle indicated in Figure (3).
Ratios of Cd to Te for CdTe QDs capped with TGA and L-cysteine with different times are analyzed by using EDX as shown in Table (1). It is noted that Cd:Te ratio is 0.567 indicating CdTe QDs is need to start with double amount of CdCl2.
TGA capped CdTe QDs
L-cysteine capped CdTe QDs
Figure (3)
Table (1) EDX of CdTe QDs capped with TGA and L-cysteine with different times Composition
Cd
Te
Cd:Te ratio
CdTe QDs capped with TGA for 30 min
16.126
28.443
0.567
CdTe QDs capped with TGA for 60 min
16.126
28.443
0.567
CdTe QDs capped with L-cysteine for 30 min
16.126
28.443
0.567
CdTe QDs
3.3 Dependence of PL Spectra of CdTe QDs on pH The luminescence of quantum dots is sensitive to the changes in the synthesis conditions. The luminescence intensity and the lifetime of capped CdTe QDs are sensitive to the pH of the solution. In order to increase the luminescence intensity of CdTe QDs, we systematically investigate PL spectra of the CdTe QDs in PBS with different pH values. The fluorescence intensities of L-cysteine and TGA capped CdTe QDs are pH dependent as depicted in Figure (4).
The low fluorescence intensity in acidic media is attributed to protonation of the surface binding thiolates and consequently the aggregation and dissociation of CdTe QDs as illustrated in the schematic diagram of Figure (5). In addition, in the acidic media the surface ligands detach from CdTe QDs to create the defect states within the band gap and reduced the quantum yield by providing alternative pathways of excited-state relaxation. As the pH diminished, more surface defects are formed and resulted in a gradual quenching of the PL intensity. On the other hand, with the rising of the pH, the deprotonation of thiol groups of TGA and L-cysteine molecules is implemented. The deprotonation strengths the covalent bond between Cd and the capping agent molecules, which enhance the fluorescence intensity of CdTe QDs. These results are in agreement with the published data [36-38]. Alongside the emission peak of PL spectra of Lcysteine capped CdTe QDs is shifted to longer wavelength (red shift) by decreasing the pH. This confirmed that the size of the L-cysteine capped CdTe QDs increase with lower separation of the energy levels, therefore the band-gap of semiconductor QDs decrease [39]. This can also be explained by the formation of the collective electronic states due to electron overlap interactions results in the red spectral shifts compared to the separated QDs [40]. It is observed that at pH 8, the emission peak was out of the scale of the device. Consequently, we have used cut-off filter to decrease the emission intensity by factor equal to 100. Mandal. et al studied the effect of pH during the preparation of the CdTe QDs on the luminescence properties. The found that at low pH the absorbance of the CdTe QDs increases as significant scattering background is evidenced mainly due to the large aggregates of QDs. As the aggregates grow in size and the CdTe QDs continue to form colloidal suspension and the scattering increases [41]. They found that the PL was enhanced by decreasing the pH and this result is contradicted with our results. This because we fixed the pH during the preparation of the CdTe QDs then we studied the effect of pH on the PL after the preparation. Yun et al also studied the effect of pH on the PL spectra for TGA capped CdTe QDs and they noted that the decreasing of the pH leads to increase of emission peak. This may be elucidated by using of tricine buffer solution instead of PBS [42].
PL intensity (a.u.)
1000
TGA capped CdTe QDs
A
pH 5 pH 6 pH 7 pH 8 Deionized water
800
600
400
200
0 500
550
600
Wavelength (nm)
650
700
800
B
L-cysteine capped CdTe QDs
PL intensity (a.u.)
700 pH 5 pH 6 pH 7 pH 8 Deionized water
600 500 400 300 200 100 0 500
550
600
Wavelength (nm)
Figure (4)
650
700
Figure (5)
3.3 CdTe QDs Sensor for OTA The effect of OTA concentrations on PL spectra of synthesized TGA capped CdTe QDs were recorded in PBS of pH 8.0 as shown in Figure (6A). The luminescence property owing to thiol ligands on the surface of CdTe QDs suggests the possibility of considerable sensitivity in the fluorescence detection applications. There is no shift in the emission wavelength (λem = 554 nm) with addition of OTA that confirmed the fixed CdTe QDs size. The fluorescence quenching of TGA capped CdTe QDs may be attributed to facilitating nonradiative e-/h+ recombination annihilation on the surface of CdTe QDs through an effective electron transfer process between surface functional thiol groups and OTA molecule based on the affinity of OTA to sulfur atom
[43]. Figure (7) presents schematic illustration for quenching process of CdTe QDs by OTA molecules. The relationship between PL intensity of CdTe QDs and concentration of OTA can be described by Stern-Volmer equation as follows [44] :
[ ]
( )
where Io and I are the PL intensities of CdTe QDs in the absence and presence of quencher Q, [Q] is the OTA concentration, and Ksv is the Stern-Volmer constant. Figure (6B) describes a Stern-Volmer quenching curve with Io/I as a function of OTA concentration, and a fair linearity (R2 = 0.8929) is observed in the concentration range from 5-30 ng/mL. However a good linearity (R2 = 0.985) in OTA concentration in the range of 5-20 ng/mL and Ksv (sensitivity) is calculated to be 1.4×105 Lg-1. It was found that the minimum of detection of OTA was 5 ng/ml.
700
A 0 ng/ml 5 ng/ml 10 ng/ml 15 ng/ml 20 ng/ml 30 ng/ml
600
PL intensity (a.u.)
500 400 300 200 100 0 500
550
600
Wavelength (nm)
650
Figure (6)
Figure (7)
The effect of OTA concentrations on PL spectra of CdTe QDs capped with L-cysteine was also used to detect too small OTA concentrations in the range from 0.5 ng/ml to 10 ng/ml in of pH 8 as shown in Figure (8A). The fluorescence quenching of CdTe QDs by ochratoxin A may happen by energy transfer, charge diverting, and surface absorption, which could change the surface state of QDs. The CdTe QDs was stabilized by L-cysteine and after adding ochratoxin A to CdTe QDs solutions, the surface of CdTe QDs changed and caused the increase of surface defects. This may result in the aggregation of CdTe QDs and then quenched the fluorescence emission from CdTe QDs [45]. In the PL spectra of CdTe QDs stabilized with L-cysteine, there is a blue shift in the emission wavelength to shorter wavelength from 554 nm to 530 nm by increasing the concentrations of OTA. This confirmed that the size of the CdTe QDs decreases where larger separation of the energy levels and therefore the band-gap of the QDs are increased. This results can be assumed by considering OTA is a weak acid (electron donating group) when binding with L-cysteine. This is strengthened the bond between QDs and its capping agent (L-cysteine) and therefore QDs size is decreased [39]. By applying Stern-Volmer relationship shown in insert of Figure (8.B), it is found that a very good linearity (R2 = 0.992) is observed in the concentration range from 0.5 to 10 ng/ml and Ksv is calculated to be 3.48 ×104 Lg-1. It is concluded that the minimum of detection of OTA was 0.5 ng/ml using CdTe QDs capped with L-cysteine. We can be deduced that L-cysteine capped CdTe QDs is an optical sensor for OTA in range from 0.5 to 10 ng/ml. However, by increasing the concentration of OTA above 10 ng/ml, the response of this optical sensor decreases due to saturation which occurs in 30, 40 and 50 ng/ml as shown in Figure (8B).
100
A
PL Intensity (a.u.)
80
0 ng/ml 0.5 ng/ml 1 ng/ml 2 ng/ml 4 ng/ml 6 ng/ml 8 ng/ml 10 ng/ml 30 ng/ml 40 ng/ml 50 ng/ml
60
40
20
0 500
550
600
Wavelength (nm)
650
700
Figure (8)
4. Conclusions By studying the effect of pH on the optical properties of CdTe QDs we deduced that the PL of both TGA-CdTe QDs and L-cysteine-CdTe QDs was pH dependent. PL spectra of CdTe QDs prepared from aqueous solution with TGA or L-cysteine as capping agents were used as a property as optical sensor for OTA with low concentrations. In case of using TGA-CdTe QDs sensor a linear response to OTA in the concentration range from 5 to 30 ng/ml was observed in case of TGA as a capping agent and Ksv was calculated to be 6.58×105 Lg-1. The minimum of detection was 5 ng/ml. L-cysteine-CdTe QDs sensor was linear in the range from 0.5 to 10 ng/ml
and Ksv was calculated to be 3.48 ×104 Lg-1 and the minimum of detection of the method was 0.5 ng/ml.
References [1] N.W. Turner, S. Subrahmanyam, S.A. Piletsky, Analytical methods for determination of mycotoxins: a review, Analytica Chimica Acta, 632 (2009) 168-180. [2] A. Rhouati, C.Y. , A.H. , a.J.-L. Marty, Aptamers: A Promising Tool for Ochratoxin A Detection in Food Analysis : a review, Toxins, 5 (2013) 1988-2008. [3] T. Kőszegi, M. Poór, Ochratoxin A: Molecular Interactions, Mechanisms of Toxicity and Prevention at the Molecular Level, Toxins, 8 (2016) 111. [4] F. Berthiller, C. Crews, C. Dall'Asta, S.D. Saeger, G. Haesaert, P. Karlovsky, I.P. Oswald, W. Seefelder, G. Speijers, J. Stroka, Masked mycotoxins: a review, Molecular nutrition & food research, 57 (2013) 165-186. [5] M. Poor, S. Kunsagi-Mate, G. Matisz, Y. Li, Z. Czibulya, B. Peles-Lemli, T. Koszegi, Interaction of alkali and alkaline earth ions with Ochratoxin A, Journal of Luminescence 135 (2013) 276-280. [6] N. Didwania, M. Joshi, Mycotoxins: A critical review on occurrence and significance, International Journal of Pharmacy and Pharmaceutical Sciences, 5 (2013) 1005-1010. [7] E.P. Meulenberg, Immunochemical methods for ochratoxin A detection: a review, Toxins, 4 (2012) 244-266. [8] A. el Khoury, A. Atoui, Ochratoxin a: general overview and actual molecular status, Toxins, 2 (2010) 461-493. [9] G.d.C. Domenico Caputo, C.F. , A.N. , A. Ricelli, R.S. , Innovative Detection System of Ochratoxin A by Thin Film Photodiodes, Sensors, 7 (2007) 1317-1322. [10] A. Kaushik, S.K. Arya, A. Vasudev, S. Bhansali, Recent Advances in Detection of Ochratoxin-A, Open Journal of Applied Biosensor, 02 (2013) 1-11. [11] Y.-F. Li, C. Chen, B. Li, Q. Wang, J. Wang, Y. Gao, Y. Zhao, Z. Chai, Simultaneous speciation of selenium and mercury in human urine samples from long-term mercury-exposed populations with supplementation of selenium-enriched yeast by HPLC-ICP-MS, Journal of Analytical Atomic Spectrometry, 22 (2007) 925. [12] Z. Zhang, Y. Li, P. Li, Q. Zhang, W. Zhang, X. Hu, X. Ding, Monoclonal antibody-quantum dots CdTe conjugate-based fluoroimmunoassay for the determination of aflatoxin B1 in peanuts, Food chemistry, 146 (2014) 314-319. [13] S. Liu, X. Qin, J. Tian, L. Wang, X. Sun, Photochemical preparation of fluorescent 2,3diaminophenazine nanoparticles for sensitive and selective detection of Hg(II) ions, Sensors and Actuators B: Chemical, 171-172 (2012) 886-890. [14] L. Shao, Y. Gao, F. Yan, Semiconductor quantum dots for biomedicial applications, Sensors, 11 (2011) 11736-11751. [15] S.J. Rosenthal, J.C. Chang, O. Kovtun, J.R. McBride, I.D. Tomlinson, Biocompatible quantum dots for biological applications, Chemistry & biology, 18 (2011) 10-24.
[16] A. Valizadeh, H. Mikaeili, M. Samiei, S.M. Farkhani, N. Zarghami, A. Akbarzadeh, S. Davaran, Quantum dots: synthesis, bioapplications, and toxicity, Nanoscale research letters, 7 (2012) 1-14. [17] G.P. Drummen, Quantum dots—from synthesis to applications in biomedicine and life sciences, International journal of molecular sciences, 11 (2010) 154-163. [18] T. Yang, Q. He, Y. Liu, C. Zhu, D. Zhao, Water-Soluble N-Acetyl-L-cysteine-Capped CdTe Quantum Dots Application for Hg(II) Detection, Journal of analytical methods in chemistry, 2013 (2013) 902951. [19] M. Feteha, S. Ebrahim, M. Soliman, W. Ramdan, M. Raoof, Effects of mercaptopropionic acid as a stabilizing agent and Cd:Te ion ratio on CdTe and CdHgTe quantum dots properties, Journal of Materials Science: Materials in Electronics, 23 (2012) 1938-1943. [20] T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A.M. Seifalian, Biological applications of quantum dots, Biomaterials, 28 (2007) 4717-4732. [21] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Quantum dots for live cells, in vivo imaging, and diagnostics, Science, 307 (2005) 538-544. [22] D. Bera, L. Qian, T.-K. Tseng, P.H. Holloway, Quantum Dots and Their Multimodal Applications: A Review, Materials, 3 (2010) 2260-2345. [23] J. Drbohlavova, V. Adam, R. Kizek, J. Hubalek, Quantum dots—characterization, preparation and usage in biological systems, International journal of molecular sciences, 10 (2009) 656-673. [24] S.J. Lim, M.U. Zahid, P. Le, L. Ma, D. Entenberg, A.S. Harney, J. Condeelis, A.M. Smith, Brightness-equalized quantum dots, Nature communications, 6 (2015) 8210. [25] S. Ebrahim, W. Ramadan, M. Ali, Structural, optical and ferromagnetic properties of cobalt doped CdTe quantum dots, Journal of Materials Science: Materials in Electronics, 27 (2015) 3826-3833. [26] D.Y. Guo, C.X. Shan, S.N. Qu, D.Z. Shen, Highly sensitive ultraviolet photodetectors fabricated from ZnO quantum dots/carbon nanodots hybrid films, Scientific reports, 4 (2014) 7469. [27] Y. Yin, A.P. Alivisatos, Colloidal nanocrystal synthesis and the organic-inorganic interface, Nature, 437 (2005) 664-670. [28] A.M. Smith, S. Nie, Chemical analysis and cellular imaging with quantum dots, The Analyst, 129 (2004) 672. [29] J. Jimenez-López, L. Molina-García, S.S.M. Rodrigues, J.L.M. Santos, P. Ortega-Barrales, A. Ruiz-Medina, Journal of Luminescence, 175 (2016) 158. [30] S. Ebrahim, M. Reda, A. Hussien, D. Zayed, CdTe quantum dots as a novel biosensor for Serratia marcescens and Lipopolysaccharide, Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 150 (2015) 212-219. [31] W.C. Chan, Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection, Science, 281 (1998) 2016-2018. [32] Z. Zhang, Y. Li, P. Li, Q. Zhang, W. Zhang, X. Hu, X. Ding, Monoclonal antibody-quantum dots CdTe conjugate-based fluoroimmunoassay for the determination of aflatoxin B 1 in peanuts, Food chemistry, 146 (2014) 314-319. [33] N. Gan, J. Zhou, P. Xiong, F. Hu, Y. Cao, T. Li, Q. Jiang, An ultrasensitive electrochemiluminescent immunoassay for aflatoxin M1 in milk, based on extraction by
magnetic graphene and detection by antibody-labeled CdTe quantumn dots-carbon nanotubes nanocomposite, Toxins, 5 (2013) 865-883. [34] R. Zekavati, S. Safi, S.J. Hashemi, T. Rahmani-Cherati, M. Tabatabaei, A. Mohsenifar, M. Bayat, Highly sensitive FRET-based fluorescence immunoassay for aflatoxin B1 using cadmium telluride quantum dots, Microchimica Acta, 180 (2013) 1217-1223. [35] M.R. Sh. Ebrahim , A. Hussien , D. Zayed CdTe quantum dots as a novel biosensor for Serratia marcescens and Lipopolysaccharide, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 150 (2015) 212–219. [36] T. Zhang, X. Sun, B. Liu, Synthesis of positively charged CdTe quantum dots and detection for uric acid, Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 79 (2011) 1566-1572. [37] N.L. Jose Aldana, Yunjun Wang, Xiaogang Peng, Size-Dependent Dissociation pH of Thiolate Ligands from Cadmium Chalcogenide Nanocrystals, American Chemical Society, 127 (2005) 2496-2504. [38] D.G. Wolfgang J Parak, Teresa Pellegrino, Daniela Zanchet, Christine Micheel, Shara CWilliams, Rosanne Boudreau, Mark A Le Gros, Carolyn A Larabell, A Paul Alivisatos, Biological applications of colloidal nanocrystals, Nanotechnology, 14 (2003) R15–R27. [39] C. Frigerio, D.S. Ribeiro, S.S.M. Rodrigues, V.L. Abreu, J.A. Barbosa, J.A. Prior, K.L. Marques, J.L. Santos, Application of quantum dots as analytical tools in automated chemical analysis: a review, Analytica chimica acta, 735 (2012) 9-22. [40] L.M. Yu Zhang, Pei-Nan Wang, Jiong Ma, Ji-Yao Chen, pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions, Journal of Luminescence 128 (2008) 1948– 1951. [41] N.T. Abhijit Mandal, Influence of Acid on Luminescence Properties of Thioglycolic AcidCapped CdTe Quantum Dots, J. Phys. Chem., 112 (2008) 8244–8250. [42] D.Z. Zhang Yun, Yue Jiachang, Tang Fangqiong, Wei Qun Using cadmium telluride quantum dots as a proton flux sensor and applying to detect H9 avian influenza virus, Analytical Biochemistry, 364 (2007) 122–127. [43] J. Chen, Y. Gao, Z. Xu, G. Wu, Y. Chen, C. Zhu, A novel fluorescent array for mercury (II) ion in aqueous solution with functionalized cadmium selenide nanoclusters, Analytica chimica acta, 577 (2006) 77-84. [44] J. Liang, Y. Cheng, H. Han, Study on the interaction between bovine serum albumin and CdTe quantum dots with spectroscopic techniques, Journal of Molecular Structure, 892 (2008) 116-120. [45] T. Zhang, X. Sun, B. Liu, Synthesis of positively charged CdTe quantum dots and detection for uric acid, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 79 (2011) 1566-1572.
Figure Captions Figure (1) UV-visible and PL spectra of CdTe QDs capped with TGA. Insert: UV-visible spectra of CdTe QDs prepared with different times. Figure (2) UV-visible and PL spectra of CdTe QDs capped with L-cysteine. Insert: UV-visible spectra of CdTe QDs prepared with different times. Figure (3) HRTEM images of CdTe QDs capped with TGA or L-cysteine capping agents. Figure (4) PL spectra of capped CdTe QDs with TGA (A) and L-cysteine (B) in deionized water and different pHs of PBS. Figure (5) Schematic diagram of the effect of pH on CdTe QDs. Figure (6) PL spectra of TGA capped CdTe QDs vs. OTA concentration in PBS of pH 8 (A). Stern-Volmer relationship between PL intensity of CdTe QDs and concentration of OTA (B). Figure (7) Schematic illustration for quenching process of CdTe QDs by OTA. Figure (8) Effect of OTA concentrations on PL spectra of L-cysteine capped CdTe QDs (A). OTA saturation curve, Insert: Stern-Volmer relationship between PL intensity of CdTe QDs and concentration of OTA. And the saturation curve (B).
PII: DOI: Reference:
S0022-2313(16)30528-2 http://dx.doi.org/10.1016/j.jlumin.2016.09.038 LUMIN14267
To appear in: Journal of Luminescence Received date: 25 April 2016 Revised date: 16 September 2016 Accepted date: 19 September 2016 Cite this article as: Shaker Ebrahim, Mohamed Labeb, Tarek Abdel-Fattah and Moataz Soliman, CdTe Quantum Dots Capped with Different Stabilizing Agents for Sensing of Ochratoxin A, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.09.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CdTe Quantum Dots Capped with Different Stabilizing Agents for Sensing of Ochratoxin A
Shaker Ebrahima*, Mohamed Labeba, Tarek Abdel-Fattahb, Moataz Solimana a
Materials Science Department, Institute of Graduate Studies & Research, 21526, Alexandria
University, Egypt b
Applied Research Center, Thomas Jefferson National Accelerator Facility and Department of
Molecular Biology, and Chemistry, Christopher Newport University, Newport News, VA 23606, USA Abstract Cadmium telluride (CdTe) quantum dots (QDs) were prepared from aqueous solution with different capping agents of thioglycolic acid (TGA) and L-cysteine. Photoluminance spectra of CdTe QDs were used as a property of optical sensor for ochratoxin A (OTA) in the concentration range of ng/mL. It was found that L-cysteine capped CdTe QDs have linear response to OTA concentration in the range from 0.5 to 10 ng/mL. By applying Stern-Volmer relationship, it was found that a very good linearity (R2 = 0.99) is observed in the concentration range from 0.5 to 10 ng/ml and Ksv was calculated to be 3.48 ×104 Lg-1 and the minimum of detection of the method was 0.5 ng/ml. The mechanism of effect of pH on the fluorescence of CdTe QDs was proposed and the low fluorescence intensity of CdTe QDs in acidic media was attributed to protonation of the surface binding thiolates and consequently the dissociation of CdTe QDs. Keywords: CdTe; Quantum Dots; Optical Sensor; Photoluminescence; Ochratoxin A
1. Introduction
* Correspondence to Shaker Ebrahim, 163 Horreya Avenue, El-Shatby, Alexandria, Egypt Tel.: +201224879137, Fax: +2034285792, e-mail: [email protected]
Mycotoxins are toxic metabolises generated by fungi, mainly by saprophytic moulds growing on variety of foodstuffs including human foods and animal feeds [1-4]. Ochratoxins belong to the group of mycotoxins that are reproduced as secondary metabolites by fungi, in particular Aspergillus and Penicillium. These fungi flourish under certain conditions of temperature and humidity [5-6]. Ochratoxins types include ochratoxin A (OTA), ochratoxin B and ochratoxin C and OTA is considered the most toxic [7]. OTA consists of a chlorinated dihydroisocoumarin moiety linked through a 7 carboxyl group by amide bond to one molecule of L-ß-phenylalanine [8]. The detection of OTA in the environment and food industry is a urgently demand because OTA has been suggested to be responsible of Balkan Endemic Nephropathy disease. OTA A is rated by the International Agency for Research on Cancer as carcinogen of class 2B [9-10]. Different techniques such as ultra-performance liquid chromatography, high-performance liquid chromatography and gas chromatography can be used for detection of OTA. Though thier high sensitivity and accuracy, these techniques are laborious, expensive, time consuming and require high sophisticated instrumentation and/or sample preparation, limiting their practical applications [11-13]. Quantum dots (QDs) are class of nanocrystals composed of several atoms constrained in three dimensions to a somewhat spherical shape, with a diameter of 2-8 nm, containing approximately 200-10,000 atoms, surrounded by an organic outer layer as capping molecules [14-19]. They have unique optical and electronic properties, such as high quantum yield, narrow, tunable and symmetric emission spectra, and broad absorption spectra. QDs have the ability to emit different wavelengths over a broad range of the light spectrum from visible to infrared, depending on their sizes and chemical compositions. The unique optical properties enable QDs to exceed conventional organic dyes as fluorophores [20-27]. QDs-based chemical sensing systems employ fluorescence changes induced by the analyte presented at the surface or close to the surface of the QDs. Sensing via QDs is based on the interaction of the QDs with the analyte of interest, i.e. influencing the optical properties, which in turn are quite sensitive to the type of capping ligand used, indicating that surface phenomenon must be taken in the consideration. The analyte present close to the QDs leads to dramatic changes in the luminescent properties of these QDs, i.e. results either in fluorescence quenching or enhancement. This promotes QDs to be used and applied for analytical sensing [28-31]. Zhang et al used CdTe QDs as the fluorescent label to aflatoxin B1. CdTe QDs were linked to the monoclonal antibody against AFB1. Based on this conjugated complexes, a direct competitive fluorescence-linked immunosorbent assay developed for aflatoxin B1 detection. The method performance included the limit of detection of 0.016 ng/mL [32]. Gan et al developed an ultrasensitive electrochemiluminescent immunoassay for aflatoxins M1 in milk using magnetic Fe3O4-graphene oxides (Fe-GO) as the absorbent and antibody labeled CdTe QDs and indicated that it can adsorb aflatoxins M1 efficiently and selectively within a large extent of pH from 3.0 to 8.0. Adsorption processes reached 95% of the equilibrium within 10 min. The intensity of the electrochemiluminescent showed linear dependence of the logarithm of aflatoxins M1
concentration in the range from 1.0 pg/mL to 100 ng/mL, with a correlation coefficient of 0.9965. Limit of detection was 0.3 pg/mL [33]. Zekavati et al provided a competitive immunoassay for the determination of aflatoxin B1 using fluorescence resonance energy transfer from anti-aflatoxin B1 antibody (immobilized on the shell of CdTe quantum dots) to Rhodamine 123 (Rho 123-labeled aflatoxin B1 bound to albumin). The result was a linear relationship between the increased fluorescence intensity of Rho 123 with increasing concentration of aflatoxin B1 in spike human serum, over the range of 0.1–0.6 μmol·mL−1 and the limit of detection was 2×10−11 M [34]. For the one's best knowledge, there is no published data used the CdTe QDs to detect ochratoxin A. In this work water-soluble CdTe QDs with different capping agents of L-cysteine and thioglycolic acid were prepared and evaluated as fluorescence probes for OTA molecules in the range of ng/ml. Effect of pH of on the PL of CdTe QDs was also investigated and explained. The proposed mechanisms of the interactions between OTA and capped CdTe QDs were suggested.
2. Materials and Methods 2.1 Materials Sodium borohydride was purchased from Merck. Cadmium chloride (CdCl2) (20% hydrate) and L-cysteine was purchased from Sigma chemical CO. Telerium powder (Te) (99.9%) was purchased from Microphysic. Thioglycolic acid (TGA) was received from Merck Schuchardt OHG. Isopropanol and ethanol was purchased from Aldrich. Potassium chloride, potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride and sodium hydroxide were purchased from Elnasr Pharmaceutical Chemical Co. Ochratoxin A (99%) was purchased from Supelco. Hydrochloric acid was purchased from Scharlab S.L. 2.2 Preparation of CdTe QDs with different stabilizing agents 2.2.1 Preparation of NaHTe solution NaHTe solution was prepared according to the following method. Simply, 40 mg of tellurium powder and 45 mg of NaBH4 were loaded in a three neck flask and then 5 mL deionized water was added. The mixture was kept reaction at 60°C for appropriate time under N2 flow to get a deep purple clear solution.
2.2.2 Preparation of TGA or L-cysteine capped CdTe QDs
CdTe QDs was prepared by the following procedure. Simply, 3.0 mmol stabilizing agent (TGA or L-cysteine) and 330 mg CdCl2 ( 20% hydrated ) were dissolved in 100 ml ultrapure water and the pH of mixture was adjusted to 12.0 by drop wise addition of 1.0 M NaOH solution with stirring in conical flask, then the fresh NaHTe solution was added through a syringe promptly under continually stirring. In case of L-cysteine the injection at 70 °C but in case of TGA the injection at 80°C. The final solution was refluxed at 90°C with condenser attached for 60 min. The CdTe QDs solution were purified by adding in isopropyl alcohol, then CdTe QDs were separated by centrifugation at 10,000 rpm for 6 min and were repeated several times to remove unreacted materials.
2.3 Studying the effect of pH on the PL of CdTe QDs To investigate the effect of pH, 1 ml of as-prepared luminescent CdTe solution was added to 3 ml of 1% PBS with different pH of 5, 6, 7 and 8 and incubated under darkness for 20 min at room temperature. 2.4 Detection of ochratoxin A by CdTe QDs A stock solution of 50 ppm and intermediate solution of 0.5 ppm of OTA were prepared in ethanol. To 5 ml volumetric flasks, 2 ml of as-prepared luminescent CdTe solution, 2 ml of PBS of pH 8 and various amounts of OTA were sequentially added, and then diluted to volume with ultrapure water and incubated under darkness for 20 min at room temperature.
2.5 Characterization techniques UV–visible characterization of the CdTe QDs solution was carried out using Evolution 600 spectrophotometer. High resolution transmission electron microscope (HRTEM) images of the CdTe QDs were obtained using JEOL (JEM-2100 LaB6) and by immersing copper grid in the CdTe QDs solution for 10 s followed by a drying stage. EDX of Cd to Te ratios for CdTe QDs capped with TGA or L-cysteine was performed using JEOL (JSM 5300). Photoluminance spectra were made with fluorescence spectrophotometer (Perkin Elmer LS-55) equipped with the computer and a 1 cm quartz cell with an excitation wavelength of 450 for CdTe QDs. The fluorescence spectra of the CdTe QDs were determined in range of 450-700 nm at excitation wavelengths of 535 nm for TGA capped CdTe QDs and 536 nm for L-cysteine capped CdTe QDs.
3. Results and Discussion 3.1 UV-Visible and PL Spectra of CdTe QDs Capped with Different Stabilizing Agents Figure (1) shows UV-visible and PL spectra of CdTe QDs capped with TGA in deionized water of pH 7.0. The appearance of the absorption shoulder in the range of 500-550 nm for the capped CdTe QDs with TGA indicates the formation of the QDs. The CdTe QDs colloids have symmetric emission peaks at about 551 nm. When a photon with an excitation wavelength exceeding the semiconductor band-gap energy is absorbed by CdTe QDs, electrons are promoted from the valence band to the conduction band. This excited electron then relaxes to its ground state by the emission of another photon with energy equal to the band-gap [35]. Insert of Figure (1) shows UV-visible spectra of TGA capped CdTe QDs prepared with TGA as a capping agent at different times. With the increase of the reaction duration, the absorption peak shows red shift due to the quantum size effect [20, 24]. With the increasing of reaction time, this excitonic absorption peak of CdTe QDs shifts to the longer wavelengths from 508 to 535 nm as the QDs grow to larger size. The size of the CdTe QDs can be calculated according to eq. (1) [35]. (
)
(
)
(
)
(1)
where D (nm) was the size of CdTe QDs, and λ (nm) is the wavelength of the first excitonic absorption peak. The size of the CdTe QDs capped with TGA is calculated to be ca. 2.55 nm at excitation wavelength of 508 nm to 3.05 nm at excitation wavelength of 535 nm. Figure (2) illustrates UV-visible and PL spectra of CdTe QDs capped with L-cysteine in deionized water of pH 7.0. The appearance of the well absorption peaks around 507 nm for the capped CdTe QDs with L-cysteine confirms the formation of the CdTe QDs. The CdTe QDs colloids have an emission peak at about 554 nm. Insert of Figure (2) presents UV-visible spectra of CdTe QDs prepared with L-cysteine as a capping agent at different times. With the increase of the reaction time, the absorption peak shows red shift to 536 nm after 30 min due to increase quantum size of CdTe QDs. After 30 min the size of CdTe QDs becomes fixed and the intensity of the peak increased. It is noted that after 60 min the CdTe QDs is precipitated due to degradation of L-cysteine. The size of the CdTe QDs capped with L-cysteine is calculated from
eq. (1) to be ca. 2.59 nm at excitation wavelength of 516 nm to 3.06 nm at excitation wavelength of 536 nm.
Figure (1)
Figure (2)
3.2 Morphological property and elemental analysis of CdTe QDs Figure (3) depicts HRTEM images of CdTe QDs capped with TGA and L-cysteine. CdTe QDs are dispersed with an average size about 3.1 nm for TGA-CdTe QDs which is close to the calculated value (3.05 nm) mentioned above and 3.2 nm for L-cysteine capped CdTe QDs which is close to the calculated value (3.06 nm). The existence of the lattice plan on the HRTEM images of the particles indicates that CdTe QDs are highly crystalline as shown in the circle indicated in Figure (3).
Ratios of Cd to Te for CdTe QDs capped with TGA and L-cysteine with different times are analyzed by using EDX as shown in Table (1). It is noted that Cd:Te ratio is 0.567 indicating CdTe QDs is need to start with double amount of CdCl2.
TGA capped CdTe QDs
L-cysteine capped CdTe QDs
Figure (3)
Table (1) EDX of CdTe QDs capped with TGA and L-cysteine with different times Composition
Cd
Te
Cd:Te ratio
CdTe QDs capped with TGA for 30 min
16.126
28.443
0.567
CdTe QDs capped with TGA for 60 min
16.126
28.443
0.567
CdTe QDs capped with L-cysteine for 30 min
16.126
28.443
0.567
CdTe QDs
3.3 Dependence of PL Spectra of CdTe QDs on pH The luminescence of quantum dots is sensitive to the changes in the synthesis conditions. The luminescence intensity and the lifetime of capped CdTe QDs are sensitive to the pH of the solution. In order to increase the luminescence intensity of CdTe QDs, we systematically investigate PL spectra of the CdTe QDs in PBS with different pH values. The fluorescence intensities of L-cysteine and TGA capped CdTe QDs are pH dependent as depicted in Figure (4).
The low fluorescence intensity in acidic media is attributed to protonation of the surface binding thiolates and consequently the aggregation and dissociation of CdTe QDs as illustrated in the schematic diagram of Figure (5). In addition, in the acidic media the surface ligands detach from CdTe QDs to create the defect states within the band gap and reduced the quantum yield by providing alternative pathways of excited-state relaxation. As the pH diminished, more surface defects are formed and resulted in a gradual quenching of the PL intensity. On the other hand, with the rising of the pH, the deprotonation of thiol groups of TGA and L-cysteine molecules is implemented. The deprotonation strengths the covalent bond between Cd and the capping agent molecules, which enhance the fluorescence intensity of CdTe QDs. These results are in agreement with the published data [36-38]. Alongside the emission peak of PL spectra of Lcysteine capped CdTe QDs is shifted to longer wavelength (red shift) by decreasing the pH. This confirmed that the size of the L-cysteine capped CdTe QDs increase with lower separation of the energy levels, therefore the band-gap of semiconductor QDs decrease [39]. This can also be explained by the formation of the collective electronic states due to electron overlap interactions results in the red spectral shifts compared to the separated QDs [40]. It is observed that at pH 8, the emission peak was out of the scale of the device. Consequently, we have used cut-off filter to decrease the emission intensity by factor equal to 100. Mandal. et al studied the effect of pH during the preparation of the CdTe QDs on the luminescence properties. The found that at low pH the absorbance of the CdTe QDs increases as significant scattering background is evidenced mainly due to the large aggregates of QDs. As the aggregates grow in size and the CdTe QDs continue to form colloidal suspension and the scattering increases [41]. They found that the PL was enhanced by decreasing the pH and this result is contradicted with our results. This because we fixed the pH during the preparation of the CdTe QDs then we studied the effect of pH on the PL after the preparation. Yun et al also studied the effect of pH on the PL spectra for TGA capped CdTe QDs and they noted that the decreasing of the pH leads to increase of emission peak. This may be elucidated by using of tricine buffer solution instead of PBS [42].
PL intensity (a.u.)
1000
TGA capped CdTe QDs
A
pH 5 pH 6 pH 7 pH 8 Deionized water
800
600
400
200
0 500
550
600
Wavelength (nm)
650
700
800
B
L-cysteine capped CdTe QDs
PL intensity (a.u.)
700 pH 5 pH 6 pH 7 pH 8 Deionized water
600 500 400 300 200 100 0 500
550
600
Wavelength (nm)
Figure (4)
650
700
Figure (5)
3.3 CdTe QDs Sensor for OTA The effect of OTA concentrations on PL spectra of synthesized TGA capped CdTe QDs were recorded in PBS of pH 8.0 as shown in Figure (6A). The luminescence property owing to thiol ligands on the surface of CdTe QDs suggests the possibility of considerable sensitivity in the fluorescence detection applications. There is no shift in the emission wavelength (λem = 554 nm) with addition of OTA that confirmed the fixed CdTe QDs size. The fluorescence quenching of TGA capped CdTe QDs may be attributed to facilitating nonradiative e-/h+ recombination annihilation on the surface of CdTe QDs through an effective electron transfer process between surface functional thiol groups and OTA molecule based on the affinity of OTA to sulfur atom
[43]. Figure (7) presents schematic illustration for quenching process of CdTe QDs by OTA molecules. The relationship between PL intensity of CdTe QDs and concentration of OTA can be described by Stern-Volmer equation as follows [44] :
[ ]
( )
where Io and I are the PL intensities of CdTe QDs in the absence and presence of quencher Q, [Q] is the OTA concentration, and Ksv is the Stern-Volmer constant. Figure (6B) describes a Stern-Volmer quenching curve with Io/I as a function of OTA concentration, and a fair linearity (R2 = 0.8929) is observed in the concentration range from 5-30 ng/mL. However a good linearity (R2 = 0.985) in OTA concentration in the range of 5-20 ng/mL and Ksv (sensitivity) is calculated to be 1.4×105 Lg-1. It was found that the minimum of detection of OTA was 5 ng/ml.
700
A 0 ng/ml 5 ng/ml 10 ng/ml 15 ng/ml 20 ng/ml 30 ng/ml
600
PL intensity (a.u.)
500 400 300 200 100 0 500
550
600
Wavelength (nm)
650
Figure (6)
Figure (7)
The effect of OTA concentrations on PL spectra of CdTe QDs capped with L-cysteine was also used to detect too small OTA concentrations in the range from 0.5 ng/ml to 10 ng/ml in of pH 8 as shown in Figure (8A). The fluorescence quenching of CdTe QDs by ochratoxin A may happen by energy transfer, charge diverting, and surface absorption, which could change the surface state of QDs. The CdTe QDs was stabilized by L-cysteine and after adding ochratoxin A to CdTe QDs solutions, the surface of CdTe QDs changed and caused the increase of surface defects. This may result in the aggregation of CdTe QDs and then quenched the fluorescence emission from CdTe QDs [45]. In the PL spectra of CdTe QDs stabilized with L-cysteine, there is a blue shift in the emission wavelength to shorter wavelength from 554 nm to 530 nm by increasing the concentrations of OTA. This confirmed that the size of the CdTe QDs decreases where larger separation of the energy levels and therefore the band-gap of the QDs are increased. This results can be assumed by considering OTA is a weak acid (electron donating group) when binding with L-cysteine. This is strengthened the bond between QDs and its capping agent (L-cysteine) and therefore QDs size is decreased [39]. By applying Stern-Volmer relationship shown in insert of Figure (8.B), it is found that a very good linearity (R2 = 0.992) is observed in the concentration range from 0.5 to 10 ng/ml and Ksv is calculated to be 3.48 ×104 Lg-1. It is concluded that the minimum of detection of OTA was 0.5 ng/ml using CdTe QDs capped with L-cysteine. We can be deduced that L-cysteine capped CdTe QDs is an optical sensor for OTA in range from 0.5 to 10 ng/ml. However, by increasing the concentration of OTA above 10 ng/ml, the response of this optical sensor decreases due to saturation which occurs in 30, 40 and 50 ng/ml as shown in Figure (8B).
100
A
PL Intensity (a.u.)
80
0 ng/ml 0.5 ng/ml 1 ng/ml 2 ng/ml 4 ng/ml 6 ng/ml 8 ng/ml 10 ng/ml 30 ng/ml 40 ng/ml 50 ng/ml
60
40
20
0 500
550
600
Wavelength (nm)
650
700
Figure (8)
4. Conclusions By studying the effect of pH on the optical properties of CdTe QDs we deduced that the PL of both TGA-CdTe QDs and L-cysteine-CdTe QDs was pH dependent. PL spectra of CdTe QDs prepared from aqueous solution with TGA or L-cysteine as capping agents were used as a property as optical sensor for OTA with low concentrations. In case of using TGA-CdTe QDs sensor a linear response to OTA in the concentration range from 5 to 30 ng/ml was observed in case of TGA as a capping agent and Ksv was calculated to be 6.58×105 Lg-1. The minimum of detection was 5 ng/ml. L-cysteine-CdTe QDs sensor was linear in the range from 0.5 to 10 ng/ml
and Ksv was calculated to be 3.48 ×104 Lg-1 and the minimum of detection of the method was 0.5 ng/ml.
References [1] N.W. Turner, S. Subrahmanyam, S.A. Piletsky, Analytical methods for determination of mycotoxins: a review, Analytica Chimica Acta, 632 (2009) 168-180. [2] A. Rhouati, C.Y. , A.H. , a.J.-L. Marty, Aptamers: A Promising Tool for Ochratoxin A Detection in Food Analysis : a review, Toxins, 5 (2013) 1988-2008. [3] T. Kőszegi, M. Poór, Ochratoxin A: Molecular Interactions, Mechanisms of Toxicity and Prevention at the Molecular Level, Toxins, 8 (2016) 111. [4] F. Berthiller, C. Crews, C. Dall'Asta, S.D. Saeger, G. Haesaert, P. Karlovsky, I.P. Oswald, W. Seefelder, G. Speijers, J. Stroka, Masked mycotoxins: a review, Molecular nutrition & food research, 57 (2013) 165-186. [5] M. Poor, S. Kunsagi-Mate, G. Matisz, Y. Li, Z. Czibulya, B. Peles-Lemli, T. Koszegi, Interaction of alkali and alkaline earth ions with Ochratoxin A, Journal of Luminescence 135 (2013) 276-280. [6] N. Didwania, M. Joshi, Mycotoxins: A critical review on occurrence and significance, International Journal of Pharmacy and Pharmaceutical Sciences, 5 (2013) 1005-1010. [7] E.P. Meulenberg, Immunochemical methods for ochratoxin A detection: a review, Toxins, 4 (2012) 244-266. [8] A. el Khoury, A. Atoui, Ochratoxin a: general overview and actual molecular status, Toxins, 2 (2010) 461-493. [9] G.d.C. Domenico Caputo, C.F. , A.N. , A. Ricelli, R.S. , Innovative Detection System of Ochratoxin A by Thin Film Photodiodes, Sensors, 7 (2007) 1317-1322. [10] A. Kaushik, S.K. Arya, A. Vasudev, S. Bhansali, Recent Advances in Detection of Ochratoxin-A, Open Journal of Applied Biosensor, 02 (2013) 1-11. [11] Y.-F. Li, C. Chen, B. Li, Q. Wang, J. Wang, Y. Gao, Y. Zhao, Z. Chai, Simultaneous speciation of selenium and mercury in human urine samples from long-term mercury-exposed populations with supplementation of selenium-enriched yeast by HPLC-ICP-MS, Journal of Analytical Atomic Spectrometry, 22 (2007) 925. [12] Z. Zhang, Y. Li, P. Li, Q. Zhang, W. Zhang, X. Hu, X. Ding, Monoclonal antibody-quantum dots CdTe conjugate-based fluoroimmunoassay for the determination of aflatoxin B1 in peanuts, Food chemistry, 146 (2014) 314-319. [13] S. Liu, X. Qin, J. Tian, L. Wang, X. Sun, Photochemical preparation of fluorescent 2,3diaminophenazine nanoparticles for sensitive and selective detection of Hg(II) ions, Sensors and Actuators B: Chemical, 171-172 (2012) 886-890. [14] L. Shao, Y. Gao, F. Yan, Semiconductor quantum dots for biomedicial applications, Sensors, 11 (2011) 11736-11751. [15] S.J. Rosenthal, J.C. Chang, O. Kovtun, J.R. McBride, I.D. Tomlinson, Biocompatible quantum dots for biological applications, Chemistry & biology, 18 (2011) 10-24.
[16] A. Valizadeh, H. Mikaeili, M. Samiei, S.M. Farkhani, N. Zarghami, A. Akbarzadeh, S. Davaran, Quantum dots: synthesis, bioapplications, and toxicity, Nanoscale research letters, 7 (2012) 1-14. [17] G.P. Drummen, Quantum dots—from synthesis to applications in biomedicine and life sciences, International journal of molecular sciences, 11 (2010) 154-163. [18] T. Yang, Q. He, Y. Liu, C. Zhu, D. Zhao, Water-Soluble N-Acetyl-L-cysteine-Capped CdTe Quantum Dots Application for Hg(II) Detection, Journal of analytical methods in chemistry, 2013 (2013) 902951. [19] M. Feteha, S. Ebrahim, M. Soliman, W. Ramdan, M. Raoof, Effects of mercaptopropionic acid as a stabilizing agent and Cd:Te ion ratio on CdTe and CdHgTe quantum dots properties, Journal of Materials Science: Materials in Electronics, 23 (2012) 1938-1943. [20] T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A.M. Seifalian, Biological applications of quantum dots, Biomaterials, 28 (2007) 4717-4732. [21] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Quantum dots for live cells, in vivo imaging, and diagnostics, Science, 307 (2005) 538-544. [22] D. Bera, L. Qian, T.-K. Tseng, P.H. Holloway, Quantum Dots and Their Multimodal Applications: A Review, Materials, 3 (2010) 2260-2345. [23] J. Drbohlavova, V. Adam, R. Kizek, J. Hubalek, Quantum dots—characterization, preparation and usage in biological systems, International journal of molecular sciences, 10 (2009) 656-673. [24] S.J. Lim, M.U. Zahid, P. Le, L. Ma, D. Entenberg, A.S. Harney, J. Condeelis, A.M. Smith, Brightness-equalized quantum dots, Nature communications, 6 (2015) 8210. [25] S. Ebrahim, W. Ramadan, M. Ali, Structural, optical and ferromagnetic properties of cobalt doped CdTe quantum dots, Journal of Materials Science: Materials in Electronics, 27 (2015) 3826-3833. [26] D.Y. Guo, C.X. Shan, S.N. Qu, D.Z. Shen, Highly sensitive ultraviolet photodetectors fabricated from ZnO quantum dots/carbon nanodots hybrid films, Scientific reports, 4 (2014) 7469. [27] Y. Yin, A.P. Alivisatos, Colloidal nanocrystal synthesis and the organic-inorganic interface, Nature, 437 (2005) 664-670. [28] A.M. Smith, S. Nie, Chemical analysis and cellular imaging with quantum dots, The Analyst, 129 (2004) 672. [29] J. Jimenez-López, L. Molina-García, S.S.M. Rodrigues, J.L.M. Santos, P. Ortega-Barrales, A. Ruiz-Medina, Journal of Luminescence, 175 (2016) 158. [30] S. Ebrahim, M. Reda, A. Hussien, D. Zayed, CdTe quantum dots as a novel biosensor for Serratia marcescens and Lipopolysaccharide, Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 150 (2015) 212-219. [31] W.C. Chan, Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection, Science, 281 (1998) 2016-2018. [32] Z. Zhang, Y. Li, P. Li, Q. Zhang, W. Zhang, X. Hu, X. Ding, Monoclonal antibody-quantum dots CdTe conjugate-based fluoroimmunoassay for the determination of aflatoxin B 1 in peanuts, Food chemistry, 146 (2014) 314-319. [33] N. Gan, J. Zhou, P. Xiong, F. Hu, Y. Cao, T. Li, Q. Jiang, An ultrasensitive electrochemiluminescent immunoassay for aflatoxin M1 in milk, based on extraction by
magnetic graphene and detection by antibody-labeled CdTe quantumn dots-carbon nanotubes nanocomposite, Toxins, 5 (2013) 865-883. [34] R. Zekavati, S. Safi, S.J. Hashemi, T. Rahmani-Cherati, M. Tabatabaei, A. Mohsenifar, M. Bayat, Highly sensitive FRET-based fluorescence immunoassay for aflatoxin B1 using cadmium telluride quantum dots, Microchimica Acta, 180 (2013) 1217-1223. [35] M.R. Sh. Ebrahim , A. Hussien , D. Zayed CdTe quantum dots as a novel biosensor for Serratia marcescens and Lipopolysaccharide, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 150 (2015) 212–219. [36] T. Zhang, X. Sun, B. Liu, Synthesis of positively charged CdTe quantum dots and detection for uric acid, Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 79 (2011) 1566-1572. [37] N.L. Jose Aldana, Yunjun Wang, Xiaogang Peng, Size-Dependent Dissociation pH of Thiolate Ligands from Cadmium Chalcogenide Nanocrystals, American Chemical Society, 127 (2005) 2496-2504. [38] D.G. Wolfgang J Parak, Teresa Pellegrino, Daniela Zanchet, Christine Micheel, Shara CWilliams, Rosanne Boudreau, Mark A Le Gros, Carolyn A Larabell, A Paul Alivisatos, Biological applications of colloidal nanocrystals, Nanotechnology, 14 (2003) R15–R27. [39] C. Frigerio, D.S. Ribeiro, S.S.M. Rodrigues, V.L. Abreu, J.A. Barbosa, J.A. Prior, K.L. Marques, J.L. Santos, Application of quantum dots as analytical tools in automated chemical analysis: a review, Analytica chimica acta, 735 (2012) 9-22. [40] L.M. Yu Zhang, Pei-Nan Wang, Jiong Ma, Ji-Yao Chen, pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions, Journal of Luminescence 128 (2008) 1948– 1951. [41] N.T. Abhijit Mandal, Influence of Acid on Luminescence Properties of Thioglycolic AcidCapped CdTe Quantum Dots, J. Phys. Chem., 112 (2008) 8244–8250. [42] D.Z. Zhang Yun, Yue Jiachang, Tang Fangqiong, Wei Qun Using cadmium telluride quantum dots as a proton flux sensor and applying to detect H9 avian influenza virus, Analytical Biochemistry, 364 (2007) 122–127. [43] J. Chen, Y. Gao, Z. Xu, G. Wu, Y. Chen, C. Zhu, A novel fluorescent array for mercury (II) ion in aqueous solution with functionalized cadmium selenide nanoclusters, Analytica chimica acta, 577 (2006) 77-84. [44] J. Liang, Y. Cheng, H. Han, Study on the interaction between bovine serum albumin and CdTe quantum dots with spectroscopic techniques, Journal of Molecular Structure, 892 (2008) 116-120. [45] T. Zhang, X. Sun, B. Liu, Synthesis of positively charged CdTe quantum dots and detection for uric acid, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 79 (2011) 1566-1572.
Figure Captions Figure (1) UV-visible and PL spectra of CdTe QDs capped with TGA. Insert: UV-visible spectra of CdTe QDs prepared with different times. Figure (2) UV-visible and PL spectra of CdTe QDs capped with L-cysteine. Insert: UV-visible spectra of CdTe QDs prepared with different times. Figure (3) HRTEM images of CdTe QDs capped with TGA or L-cysteine capping agents. Figure (4) PL spectra of capped CdTe QDs with TGA (A) and L-cysteine (B) in deionized water and different pHs of PBS. Figure (5) Schematic diagram of the effect of pH on CdTe QDs. Figure (6) PL spectra of TGA capped CdTe QDs vs. OTA concentration in PBS of pH 8 (A). Stern-Volmer relationship between PL intensity of CdTe QDs and concentration of OTA (B). Figure (7) Schematic illustration for quenching process of CdTe QDs by OTA. Figure (8) Effect of OTA concentrations on PL spectra of L-cysteine capped CdTe QDs (A). OTA saturation curve, Insert: Stern-Volmer relationship between PL intensity of CdTe QDs and concentration of OTA. And the saturation curve (B).