DOI: 10.1002/chem.201304853
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& Electrochemistry
Graphene Nanosheets and Quantum Dots: A Smart Material for Electrochemical Applications Fernando Henrique Cincotto,* Fernando Cruz Moraes, and Sergio Antonio Spinola Machado[a]
and spectrophotometric measurements all showed good distribution of the quantum dots with a small particle size. The electrochemical measurements demonstrated the high performance of the composite response in the presence of a light source. Differential pulse voltammetry allowed the development of a method to determine 17b-estradiol levels in the range from 0.2 to 4.0 mmol L1 with a detection limit of 2.8 nmol L1 (0.76 mg L1).
Abstract: A novel material for the electrochemical determination of endocrine disruptors using a composite based on graphene oxide modified with cadmium telluride quantum dots has been evaluated. The morphology, structure and electrochemical performance of the composite electrodes were characterised by transmission electron microscopy, dynamic light scattering, UV-visible absorption spectra, fluorescence spectra, Raman spectra and cyclic voltammetry. The dynamic light scattering, transmission electronic microscopy
Introduction
deoxyribonucleic acid (DNA),[18] dopamine,[19] bisphenol A,[20] heroin and morphine.[21] Recently, several reports have focused on graphene-based hybrid nanomaterials for the development of electrochemical sensors.[22, 23] Although high loading and uniform distribution are demands for developing a graphene-based heterostructure, it is still a great challenge to establish new assembly strategies for functionalising graphene with other nanostructures. In this sense, Chen et al.[24] reported the use of a thermally reduced graphene oxide decorated with functionalised gold nanoparticles for the detection of HgII ions. Mao et al.[25] related the use of a thermally reduced graphene oxide sheet decorated with gold-nanoparticle–antibody conjugates to determine a specific protein. Quantum dots (QDs) are semiconductor nanocrystals that exhibit optical-electronic properties and dimensions in the range from 2 to 6 nm,[26] in which the bandgap value is dependent on the nanocrystal size. In QDs, the electron is in a quantum confinement, which results in a strong quantisation of the energy levels.[27] Thus, the material absorbs at a specific wavelength and energy equivalent to that of the confined electrons. For this reason, these materials are highly luminescent with narrow emission line widths, thus offering a range of potential applications including photovoltaic cells[28] and lasers.[29] The major application of QDs as analytical sensors comes about through the synergetic characteristics of the QDs and other nanomaterials. The hybrid materials exhibit a good distribution of particle sizes, allow for stable thin films and increase the intensity of the analytical signal. In this regard, Yu et al.[30] reported the use of reduced graphene modified with CdSe nanoparticles to understand photoinduced charge transfer. Other reports have described the use of composites based on graphene oxide (GO) and QDs of cadmium telurate (CdTe),
Graphene is a single layer of carbon atoms displayed in a closely packed two-dimensional lattice. This material has attracted extensive attention because of its unique nanostructure and intrinsic characteristics, including high thermal conductivity, low manufacturing cost[1] and excellent electrical, thermal, mechanical, electronic and optical properties.[2–4] Other remarkable properties are its high specific surface area,[5] excellent pliability, high chemical stability and high porosity and excellent electron-transport properties.[6] These unique properties render graphene a promising material for applications in several nanotechnology fields such as batteries,[7] supercapacitors,[8] field-effect transistors,[9] fuel cells[10] and electrochemical sensors.[11] Moreover, thin transparent graphene films are used as electrodes in solar cells[12] and in organic light-emitting devices.[13] In addition, this material has yielded promising results in bioscience owing to its high electrocatalytic activity and good biocompatibility.[14, 15] The most interesting use of graphene in electroanalysis is possible owing to its electronic properties such as rapid electron transport, electrocatalytic effect, the structural characteristics it possesses by virtue of possessing a huge specific surface area (2600 m2 g1), its 2D structure and the presence of functional groups that enable biocompatibility.[16] Thus, graphene has been used in the development of reliable and sensitive detection methods for several target analytes such as glucose,[17]
[a] Dr. F. H. Cincotto, Dr. F. C. Moraes, Prof. Dr. S. A. S. Machado Group of Electrochemical Materials and Electronalytical Methods, Institute of Chemistry of S¼o Carlos University of S¼o Paulo, S¼o Carlos—SP, CP 780 (Brazil) E-mail: [email protected] Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper such as a biosensor using graphene modified with CdTeCdS quantum dots, gold nanoparticles and glucose oxidase to determine glucose,[31] and an electrochemiluminescence immunosensor using an AuFe3O4 magnetic nanoparticle core–shell over graphene and a primary antibody for carbohydrate antigen-125 detection.[32] These studies suggest that the combination of graphene and QDs on electrochemical sensors presents a promising setup to detect low concentrations of analytes in different matrices. Nowadays, the development of methodologies for remediation, detection and quantification of endocrine disruptors is a target in several research fields such as clinical and environmental analysis. Endocrine disruptors are chemical compounds that interfere with the functioning of the endocrine systems of mammals, amphibians, birds and fish.[33] The hormone 17b-estradiol is an estrogenic hormone in natural sources that possesses a high level of endocrine interference activity.[34] Contamination of aquatic environments by natural estrogens is due to human and animal excretion, following the subsequent transport of effluents to sewage treatment stations. Thus, the bioaccumulation of 17b-estradiol in the environment is a problem to be solved. The analytical methods normally used for 17b-estradiol determination are based on chromatographic techniques such as high-performance liquid chromatography (HPLC)[35] and liquid chromatography (LC).[36] Electrochemical determination of 17bestradiol is based on the irreversible oxidation of the phenol group present in the molecule. However, it has been reported that this electrochemical process presents a low current response over surfaces[37] such as platinum, glassy carbon and boron-doped diamonds. As a result, several studies have related the use of surface modifiers to increase the 17b-estradiol electrochemical response. Among these electrocatalytic materials are CdSe quantum dots based on indirect competitive immunoassay,[38] a nickel-modified glassy carbon electrode[39] and a poly(l-serine) film-modified electrode.[34] Considering the work that has been described above, this study focuses on the synthesis, characterisation and application of a novel composite based on graphene oxide modified with CdTe quantum dots. This novel material yielded excellent sensitivity for 17b-estradiol electro-oxidation, as well as a high electrocatalytic activity and a low detection limit. A glassy carbon electrode modified with the composite GO/CdTe is shown to be promising for fast, simple and sensitive determination of 17b-estradiol in environmental samples.
Figure 1. DLS measurements of the distribution of particle sizes using a monochromatic light from a laser source with a wavelength of 250 nm.
on the time fluctuation in the scattering intensity, as shown in Figure 1. Figure 1 displays the distribution of particle sizes as a function of percent intensity. The observed values are the average of three measurements of QD samples. It was observed that the distribution of particle sizes varied over a range between 1.5 and 5.8 nm, with the greatest number at (3.2 0.2) nm. This distribution size is in agreement with the QD synthesis proposed by Algar et al.[24] in which the QDs have particle sizes that vary from 2.0 to 6.0 nm. The UV/Vis absorption and the photoluminescence properties of the QDs were determined using UV/Vis and fluorescence experiments and the spectra are shown in Figure 2.
Figure 2. UV/Vis absorption (solid line) and photoluminescence (open circles) spectra of CdTe quantum dots, with excitation wavelength at 400 nm.
Results and Discussion
During all the spectrophotometric measurements, the QD suspension was loaded in a 2.5 mL disposable quartz cuvette and scanned at room temperature. The spectra of the CdTe QD suspensions were measured after dilution with hexane to provide optical densities appropriate for the measurements. Figure 2 shows typical UV/Vis absorption (solid line) and PL emission (open circles) spectra of CdTe nanocrystals. The results showed a well-resolved absorption maximum at the first electron transition at 585 nm, thus indicating a sufficiently
Characterisation of CdTe quantum dots The synthesis of QDs was evaluated using dynamic light scattering (DLS) measurements. The DLS experiments were carried out on the CdTe suspension by using a cell that contained hexane. For this, a monochromatic light from a laser source with a wavelength of 250 nm struck the QD particles in the suspension and the particle-size distribution was dependent &
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Full Paper narrow size distribution of the CdTe QDs. The size of the nanocrystal is related to the quantum confinement of the energy absorbed, which shifts to higher values with wavelength, thereby allowing an increase in the size of the nanocrystals. To evaluate the size distribution of the nanocrystals, a theoretical method was applied on the basis of the UV/Vis absorption maximum. This method was first reported by Yu et al.[53] and shows an estimated value of QD size on the basis of empirical fitting functions of the CdTe UV/Vis spectra. The relationship between the diameter of the particle (D) and the UV/Vis wavelength (l) is shown in the following equation [Eq. (1)]:
D ¼ ð9:8127 107 Þl3 ð1:7147 103 Þl2 þ ð1:0064Þl194:84 ð1Þ The average value of D obtained for measurements of the CdTe at l = 585 nm was 3.11 nm. This value is in good agreement with that obtained from the DLS results, thus showing the high efficiency of the synthesis method. Observation of the photoluminescence property of the QDs was carried out by using PL measurements. The PL emission spectrum (open circles) for CdTe is shown in Figure 2. It was clearly observed that the maximum emission wavelength was 660 nm for an excitation wavelength of 400 nm. The PL measurements indicate that the emission of the present CdTe QDs was due to trap-state emission rather than band-edge emission.[40] The quantum confinement emission is associated with electron transitions between the trap states and the conduction band. Additionally, as can be seen, the present CdTe QDs had an emission peak at 660 nm, whereas the absorption edge was at 585 nm. Note that the emission peak wavelength was close to 80 nm larger than the corresponding absorption edge wavelength. This indicates the emission of the present CdTe QDs was due to emissions of the quantum confinement state instead of band-edge emissions. Comparing the obtained results for the emission band, that is, 660 nm, and the corresponding energy 1.88 eV with other reports,[41, 42] these values are quite similar for a particle size around 3.0 nm. The size of the nanocrystals determines its optical and spectroscopic properties. Typically, as the particles become smaller, the luminescence energies are blueshifted to higher energies.[43] In addition, the PL properties vary sharply with QD size: small QDs (1.2 nm) emit UV light, medium-sized QDs (1.5–3.0 nm) emit visible light and large QDs (3.8 nm) have a near-infrared emission.[44]
Figure 3. A) TEM image of graphene oxide nanosheets. Inset: Raman spectra of graphene oxide nanosheets. B) AFM image of GO nanosheets. Inset: topopography plot.
vation is the dark lines with high contrast, which clearly illustrate that the GO nanosheets are wrinkled or folded. It is supposed that the wrinkled morphology can reduce the surface energy of GO nanosheets and thereby make the nanosheets stable. In this sense, the GO can act as a support for QD adsorption. Structural characterisation of the aggregations of the GO nanosheets was carried out using Raman spectroscopy. The inset in Figure 3A exhibits typical Raman spectra for the GO nanosheets. It was observed that the synthesised GO presented two characteristic absorption bands in its Raman spectrum: the G band at 1720 cm1 and the D band at 1635 cm1. The G band corresponds to bond stretching of sp2 carbon pairs. The D band is associated with the breathing mode of aromatic rings with dangling bond in-plane terminations. As shown in the Raman spectrum, the magnitude of the D band is due to the presence of defects introduced by functional groups such as carboxyl, hydroxyl and epoxy groups.[45] The relationship be-
Morphological characterisation of the GO/CdTe composite The synthesis of the GO was performed using a modified Hammers method and was evaluated using TEM microscopy, as displayed in Figure 3A. It was observed that the GO nanosheets are transparent. However, the differences in the degree of transparency suggest that the thickness of the GO nanosheets is very small. Also, the nanosheets are not entirely a single-layered structure and there are overlaps of several single layers. Another obserChem. Eur. J. 2014, 20, 1 – 9
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Full Paper Electrochemical behaviour of 17 b-estradiol on the GO/CdTe composite
tween the intensity of the D band (ID) and G band (IG) is usually employed to evaluate the size of the in-plane sp2 domains for carbon materials. The obtained value for the ID/IG ratio was 1.44 for obtained GO nanosheets. This value is consistent with other values that describe GO nanosheets.[46] To confirm the thickness of GO nanosheets, atomic force microscopy (AFM) measurements were performed; the results are displayed in Figure 3B. In this image, the presence of GO layers over the mica surface can be clearly observed. On the basis of the AFM topography analyses, the average of GO thickness was obtained in a range as shown by the arrow displayed in Figure 3B. The inset of this figure demonstrates that the GO layers have a width of 350.0 nm and an average height of 8.0 nm; each layer has a thickness close to 2.8 nm. Figure 4 displays TEM images of the CdTe QDs supported over GO nanosheets. It was observed that the QDs were uniform, well-dispersed and with a narrow particle-size distribution. The particles self-organise on the TEM grid in a close-
The electrochemical behaviour of 17b-estradiol over the GO/ CdTe surface was evaluated by using cyclic voltammetry. The CV experiments were carried out in phosphate buffer solution (PBS; 0.1 mol L1, pH 8.0) in the presence and absence of 17bestradiol (50 mmol L1) over a potential range from + 0.2 to + 1.0 V with a scan rate of 50 mV s1. In Figure 5 it can be ob-
Figure 5. CV scans of the GC/GO/CdTe electrode in the a) presence and b) absence of 17b-estradiol (50 mmol L1) with a scan rate of 50 mV s1. Inset: proposed electrochemical mechanism for 17b-estradiol oxidation.
served that the 17b-estradiol exhibited an oxidation peak at + 0.55 V over the GC electrode modified with a film of GO/ CdTe. This oxidation is attributed to irreversible oxidation of the hydroxyl group present in the aromatic ring of the 17b-estradiol moiety to form a ketone species,[47] as shown in the proposed electrochemical mechanism in the inset of Figure 5.
Figure 4. TEM image of CdTe quantum dots. Inset: histogram of QD particle size.
packed monolayer on the GO nanosheets, and local hexagonal symmetry is evident in the packing arrangement. The diameters of the particles were measured directly from TEM images of the composite by using Image Photo Pro Plus 6 computational software. This study was performed for three different TEM images, in which each microscopic image was mapped and scanned in four equal parts. The histogram presented in the inset of Figure 4 shows a QD size of 3.10 nm, a value that is the average of the studied images. The CdTe QD size was very close to the value estimated from the absorption spectra, which corroborates the results obtained by DLS and the theoretically estimated size on the basis of its UV/Vis spectrum.
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Figure 6. DPV voltammograms of GC/GO/CdTe electrode in 17b-estradiol (50 mmol L1) under different conditions: a) in the dark and b) in the presence of a source of visible light. Inset: photocurrent response of GC/GO/ CdTe electrode to on/off cycles of light illumination.
Figure 6 displays differential pulse voltammetry (DPV) voltammograms for 17b-estradiol electro-oxidation in the dark and in the presence of light. The voltammograms were collected using the GC/GO/CdTe electrode in 0.1 mol L1 PBS (pH 8.0) that contained 17b-estradiol (50 mmol L1) in the dark as well as in the presence of a source of visible light (Hg lamp, 20 W power). The electrochemical measurements were scanned over a potential range that varied from + 0.2 to + 0.8 V with an am4
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Full Paper 30 % at a pretreatment potential of + 0.2 V. Thus, 0.9 V was chosen as the pretreatment cleaning potential in further studies. The influence of the pretreatment time was also evaluated. In the range from 20 to 360 s, the current peak increased to 180 s and remained constant at longer time intervals. As such, 180 s was chosen as the time of pretreatment for cleaning the electrode surface.
plitude of 50 mV and step potential of 1 mV. It was evident that the current peak, attributed to 17b-estradiol oxidation, increased almost threefold in the presence of light when compared with the same measurement in the dark. To elucidate the increase in the current peak in the presence of the radiant light, an amperometric experiment was carried out to investigate the photoelectrochemical properties of the CdTe QDs. The inset in Figure 6 shows a typical result, as it exhibits a time-dependent response curve to light irradiation that corresponds to CdTe-QD-modified graphene oxide over a glassy carbon electrode. The measurement was repeated for three cycles. It can be seen that the GC/GO/CdTe electrode gives strong and rapid responses to light irradiation at a fixed applied potential of + 0.8 V. The current increases immediately when the light is turned on and decreases sharply when the light is turned off. The increase in the current by 12.1 mA is due to the induction of CdTe QDs in the gap. When the light is turned on, there is an excitation of the QDs that results in the transfer of electrons from the valence band to the conduction band, thus yielding electron–hole pairs.[48] When the light is turned off, there is a dramatic decrease in the current owing to the recombination of electrons and holes. This result confirmed that an electrochemical system based on graphene oxide modified with CdTe quantum dots becomes a suitable setup for use in environmental analysis in the presence of light.
Analytical response On the basis of the optimised conditions, DPV was used to investigate the electrochemical response as a function of 17b-estradiol concentration. In this study using the GC/GO/CdTe electrode, analytical responses were performed under two different conditions, in the dark and in the presence of a light source, and their analytical responses were compared. Standard 17bestradiol concentrations were used to construct the calibration curves shown in Figure 7 with three measurements at each concentration.
Optimisation of experimental parameters on the GC/GO/ CdTe electrode To maximise the DPV analytical signals, the effects of experimental parameters were studied on the GC/GO/CdTe electrode in a 0.1 mol L1 solution of PBS at pH 8.0 that contained 17bestradiol (50 mmol L1). The amplitude was varied over a range from 10 to 100 mV with a fixed scan rate of 10 mV s1. The anodic current peak with the highest current signal was 50 mV, so this value was chosen as the differential pulse amplitude. The effect of potential step increments was studied over the range from 1 to 10 mV with a fixed scan rate of 10 mV s1. For potential steps greater than 2 mV, deformation of the voltammetric profile was observed and the anodic current peak decreased in height. Hence, 1 mV was chosen for the potential steps. Electrochemical detection of 17b-estradiol is accompanied by adsorption of its oxidation products. Thus, poisoning of the electrode surface occurs and there is a consequent decrease in the analytical signal. The influence of an applied potential pretreatment after each scan to remove adsorbed species on the electrode surface was investigated. Pretreatment potential values were tested in the range of 1.0 to + 0.2 V in a 0.1 mol L1 solution of PBS at pH 8.0 that contained of 17bestradiol (50 mmol L1). Complete recovery of the original response was obtained by applying 0.9 V. This potential value is sufficient to reduce the oxidation product of 17b-estradiol that is adsorbed onto the electrode surface. At more positive cleaning potentials, the peak current (Ipa) obtained during the following experiment begins to decrease and reaches only Chem. Eur. J. 2014, 20, 1 – 9
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Figure 7. Linear dependence of DPV peak current on the concentration of 17b-estradiol for the GC/GO/CdTe electrode in the dark (*) and in the presence of a light source (&). The concentrations of 17b-estradiol [mmol L1] were as follows: 0.25, 0.50, 1.00, 1.25, 1.50, 2.00, 2.50, 3.00, 4.00, 5.00, 10.00, 12.50, 15.00, 17.50 and 20.00.
The analytical signals obtained under dark conditions exhibited a linear range from 1.0 to 4.0 mmol L1 (R2 = 0.9824). The limit of detection (LOD) was calculated as 9.8 nmol L1. The detectability level was remarkably influenced by the presence of a light source. The photosensitised GC/GO/CdTe electrode exhibited a linear range from 0.2 to 4.0 mmol L1 (R2 = 0.997) and the LOD was calculated as 2.8 nmol L1. The LOD values were determined using the 3s/slope ratio (in which s is the standard deviation of the mean current for 10 DPV voltammograms of the blank, and the current values were collected at the 17b-estradiol oxidation peak potential. The calculations were performed according to IUPAC recommendations.[49] The effect of light on the proposed sensor can be further evaluated from the values of analytical sensitivity, which were 2.6-fold higher in the presence of a light source (1.61 mA/mmol L1) relative to their sensitivity in the dark (0.62 mA/mmol L1). Also, the LODs and sensitivity values were compared in a T-test at the 95 % confidence level by comparing the GC/GO/CdTe sensor in the presence and absence of light. 5
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Full Paper ty and repeatability and increasing the sensitivity. In essence, the GC/GO/CdTe electrode presents an alternative material for use in environmental analysis.
The reproducibility of the GC/GO/CdTe electrode was measured from seven experiments, in which each experiment consisted of five sequential DPV voltammograms. These experiments were performed on different days. Prior to each experiment, the electrode surfaces were rinsed thoroughly with double-distilled water. Thus, the DPV voltammograms were performed in PBS (0.1 mol L1, pH 8.0) that contained 17b-estradiol (50 mmol L1). The relative standard deviation (RSD) was calculated at 2.2 %. In addition, the intra-assay precision tests were performed from ten DPV voltammograms of that same solution. The RSD was calculated at 1.7 %. In addition, the composite can be stored for 2 months without losing electroactivity. Upon comparing the results at the GC/GO/CdTe electrode with other electroanalytical methods for the determination of 17b-estradiol, a higher detection limit was observed: 16 nmol L1 for a sensor based on carbon nanotubes modified with Pt nanoparticles[50] and 63 nmol L1 for a composite electrode based on carbon nanotubes modified with [Ni(Cyclam)] (Cyclam = 1,4,8,11-tetraazacyclotetradecane).[51] A low detection limit was reported by Wang et al.,[52] in which a flow injection chemiluminescence of CdTe nanocrystals treated with sodium hexametaphosphate and KMnO4, was used to determine 17bestradiol. The calculated LOD was 5.8 1011 mol L1. The lower detection limit and higher sensitivity observed at the GC/GO/CdTe electrode can be attributed to the efficiency of the electron transfer between the modified electrode and 17b-estradiol owing to excitation of electrons in the QDs in the presence of a light source, thereby allowing electron transfer from the valence band to the conduction band of energy and improving the electrochemical response. Other methods that use biosensors to detect 17b-estradiol have shown lower detection limits at nanomolar levels, that is, 0.18[53] and 0.036.[54] However, for the GC/GO/CdTe electrode, high sensitivity, good reproducibility and simple instrumentation, preparation and analytical procedure are important advantages. This method can be easily applied to the determination of disruptor endocrines in real samples.
Experimental Section Apparatus and procedures Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed using a model PGSTAT 302 Autolab electrochemical system (Eco Chemie, Netherlands) monitored with NOVA software. The electrochemical cell was assembled with a conventional three-electrode system: a glassy carbon electrode modified with a composite (GO/CdTe), an Ag/AgCl electrode in KCl (3.0 mol L1) as a reference electrode and Pt wire as an auxiliary electrode. All electrochemical measurements were performed in 0.1 mol L1 PBS (pH 8.0) at a controlled temperature (25 8C). CV measurements were carried out in a potential range from + 0.2 to + 0.8 V with a scan rate of 50 mV s1. DPV experiments were performed using amplitude of 50 mV and a step potential of 1 mV. The potential was varied from + 0.2 to + 0.8 V for 17b-estradiol calibration curves. The morphologies of the GO, CdTe and GO/CdTe composite were examined using a transmission electron microscope (Zeiss EM912 Omega, STEM). To prepare the samples, a small amount of the powder was dispersed in anhydrous dimethylformamide (DMF) in an ultrasound bath. A freshly prepared dispersion was then added dropwise onto a copper grid. The GO thickness was examined by using an atomic force microscope with a model Nanosurf EasyScan 2 AFM System (NanoSurf AG, Switzerland). The measurements were carried out with a non-contact/tapping mode long cantilever. The sample was prepared by adding the GO suspension dropwise over a previously cleaned mica surface. The CdTe particle-size distribution was determined by DLS using a Zetasizer Nano ZS90 (Malvern USA) instrument; analyses were performed in triplicate and reported data are the average of three runs in which the standard deviation was always below 5 %. UV/Vis absorption spectra were performed using a Jasco V-630 (Jasco Inc., USA) spectrometer configured with a slit width of 2 nm, a scanning speed of 4 nm per second and a data interval of 1 nm. The photoluminescence (PL) properties of the QDs were measured using a QM-4/2005 spectrofluorometer (Photon Technology International, Birmingham, NJ). The PL measurements were carried out using a xenon lamp at 75 W with both the excitation and emission slit width set at 2 nm, the voltage for the photomultiplier tube (PMT) detector set at 1050 V, an integration time of 0.25 s and a step size of 1 nm.
Conclusion A novel nanomaterial for development of an electrochemical sensor to determine endocrine disruptors based on CdTe quantum dots supported on graphene oxide has been proposed in this paper. The GO/CdTe composite was successfully characterised by TEM, DLS, UV/Vis absorption, fluorescence, Raman and voltammetric techniques, which indicated that the CdTe was supported on the GO surface. Therefore, the electronic properties of graphene oxide in addition to the photoelectric catalytic ability of the QDs showed greater efficiency in the electro-oxidation of 17b-estradiol. A drawback in composite development is that the preparation is time consuming. However, this disadvantage is minimised by the composite synthesis yield, which allows the fabrication of more than 200 electrodes at a time. Finally, the electronic properties of the graphene in addition to the high catalytic activity of the CdTe yielded values for the LOD in the nanomolar range, thereby improving reproducibili&
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The structural characterisation was performed by XRD using a Rigaku Rotaflex diffractometer model RU200B at 50 kV and 100 mA, with a CuKa radiation wavelength of l = 1.542 . Raman spectra were collected using a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCDS detector and a holographic notch filter (Renishaw Ltd, Gloucestershire, UK). All chemicals were of analytical grade and were used without further purification. Analytical-grade 17 b-estradiol was obtained from Sigma–Aldrich (Germany). The supporting electrolyte was 0.1 mol L1 PBS. Graphite powder (< 20 micron), cadmium oxide (99 %), n-tetradecylphosphonic acid (TDPA, 98 %), 1-octadecene (ODE; tech. 90 %) and tellurium powder (200 mesh, 99.8 %) were purchased from Sigma–Aldrich (Germany). All organic solvents used for quantum-dot purification such as hexane, isopropanol, methanol and acetone were of analytical grade. All electrolytes
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Full Paper were prepared with nanopure water from a Barnested Nanopure System.
Keywords: electrochemistry · nanomaterials · quantum dots
graphene
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hormones
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Synthesis of graphene oxide [1] S. J. Li, Y. F. Shi, L. Liu, L. X. Song, H. Pang, J. M. Du, Electrochim. Acta 2012, 85, 628 – 635. [2] A. K. Geim, Science 2009, 324, 1530 – 1534. [3] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191. [4] K. S. Novoselov, ECS Trans. 2009, 19, 3 – 7. [5] M. D. Stoller, S. Park, Y. Zhu, R. S. Ruoff, Nano Lett. 2008, 8, 3498 – 3502. [6] M. Coros, F. Pogacean, A. R. Biris, A. S. Biris, S. Pruneanu, Micro and Nanosystems 2013, 5, 127 – 137. [7] T. Takamura, K. Endo, L. Fu, Y. Wu, K. J. Lee, T. Matsumoto, Electrochim. Acta 2007, 53, 1055 – 1061. [8] Y. Huang, J. Liang, Y. Chen, Small 2012, 8, 1805 – 1834. [9] S. Gilje, S. Han, M. Wang, K. L. Wang, R. B. Kaner, Nano Lett. 2007, 7, 3394 – 3398. [10] H. J. Choi, S. M. Jung, J. M. Seo, D. W. Chang, L. Dai, J. B. Baek, Nano Energy 2012, 1, 534 – 551. [11] Y. Fang, S. Guo, C. Zhu, Y. Zhai, E. Wang, Langmuir 2010, 26, 11277 – 11282. [12] X. Miao, S. Tongay, M. K. Petterson, K. Berke, A. G. Rinzler, B. R. Appleton, A. F. Hebard, Nano Lett. 2012, 12, 2745 – 2750. [13] T. Sun, Z. L. Wang, Z. J. Shi, G. Z. Ran, W. J. Xu, Z. Y. Wang, Y. Z. Li, L. Dai, G. G. Qin, Appl. Phys. Lett. 2010, 96, 133301. [14] S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature 2006, 442, 282 – 286. [15] J. R. Potts, D. R. Dreyer, C. W. Bielawski, R. S. Ruoff, Polymer 2011, 52, 5 – 25. [16] J. Dua, R. Yue, Z. Yao, F. Jiang, Y. Du, P. Yang, Z. Wang, Colloids Surf. A 2013, 419, 94 – 99. [17] J. Luo, H. Zhang, S. Jiang, J. Jiang, X. Liu, Microchim. Acta 2012, 177, 485 – 490. [18] X. Niu, W. Yang, J. Ren, H. Guo, S. Long, J. Chen, J. Gao, Electrochim. Acta 2012, 80, 346 – 353. [19] Q. Liu, X. Zhu, Z. Huo, X. He, Y. Liang, M. Xu, Talanta 2012, 97, 57 – 62. [20] Y. Zhang, L. Wang, D. Lu, X. Shi, C. Wang, X. Duan, Electrochim. Acta 2012, 80, 77 – 83. [21] A. Navaee, A. Salimi, H. Teymourian, Biosens. Bioelectron. 2012, 31, 205 – 211. [22] L. Cao, Y. Liu, B. Zhang, L. Lu, ACS Appl. Mater. Interfaces 2010, 2, 2339 – 2346. [23] Y. G. Zhou, J. J. Chen, F. B. Wang, Z. H. Sheng, X. H. Xia, Chem. Commun. 2010, 46, 5951 – 5953. [24] K. Chen, J. Chang, S. Mao, K. Yu, S. Cui, J. Chen, Anal. Chem. 2012, 84, 4057 – 4062. [25] S. Mao, G. Lu, K. Yu, Z. Bo, J. Chen, Adv. Mater. 2010, 22, 3521 – 3526. [26] W. R. Algar, A. J. Tavares, U. J. Krull, Anal. Chim. Acta 2010, 673, 1 – 25. [27] 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, Science 2005, 307, 538 – 544. [28] P. Reiss, E. Couderc, J. D. Girolamo, A. Pron, Nanoscale 2011, 3, 446 – 489. [29] H. Zhu, W. Zhang, S. F. Yu, Nanoscale 2013, 5, 1797 – 1802. [30] K. Yu, G. Lu, S. Mao, K. Chen, H. Kim, Z. Wen, J. Chen, ACS Appl. Mater. Interfaces 2011, 3, 2703 – 2709. [31] Z. G. Gu, S. P. Yang, Z. J. Li, X. L. Sun, G. L. Wang, Y. J. Fang, J. K. Liu, Electrochim. Acta 2011, 56, 9162 – 9167. [32] W. Liu, Y. Zhang, S. Ge, X. Song, J. Huang, M. Yan, J. Yu, Anal. Chim. Acta 2013, 770, 132 – 139. [33] D. M. Bila, M. Dezotti, Quim. Nova 2007, 30, 651 – 666. [34] J. H. Kang, F. Kondo, Y. Katayama, Toxicology 2006, 226, 79 – 89. [35] L. Novkov, P. Solich, L. Matysova, J. Sicha, Anal. Bioanal. Chem. 2004, 379, 781 – 787. [36] L. Havlkov, L. Novakova, L. Matysova, J. Sicha, P. Solich, J. Chromatogr. A 2006, 1119, 216 – 223. [37] J. Song, J. Yang, X. Hu, J. Appl. Electrochem. 2008, 38, 833 – 836. [38] E. Ortega, S. De Marcos, J. Galban, Biosens. Bioelectron. 2013, 41, 150 – 156.
The graphene oxide was prepared from graphite through a modified Hummers method.[55] A mixture of graphite (1.0 g) and sodium nitrate (1.0 g) were oxidised by using concentrated H2SO4 (46 mL) under magnetic stirring maintained at a temperature of 20 8C. Then, KMnO4 (6.0 g) was slowly added under vigorous stirring. The reaction vessel was kept in a refrigerator at 0 8C for 24 h. Afterward, the solution was stirred at 98 8C for 1 h and diluted with water (240 mL). Then a solution of H2O2 (85 mL, 30 % v/v) was added. The product was subjected to hot filtration, washed three times with HCl solution (5 % v/v) and dried in a vacuum oven at 50 8C for 48 h to obtain graphene oxide.
Synthesis of CdTe quantum dots The synthesis of CdTe QDs was performed using a modified procedure reported by Yu et al.[56] For this, CdO (25.6 mg) was mixed with TDPA (114.0 mg) following the addition of ODE (7.8 g). The mixture was heated under vigorous stirring in an argon atmosphere. The solution appeared clear and colourless. Then, Te (12.5 mg) was dissolved in tributylphosphine (475.0 mg, 97 %) and ODE (1.5 g). The solution was prepared in an argon-filled glovebox and loaded into a syringe. Then the syringe was removed from the glovebox and the Te mixture was injected very quickly into the Cdcontaining mixture. The temperature decreased rapidly after injection and was held at 260 8C to allow growth of the QDs. The rapid introduction of the Te into the mixture produced a yellow/orange solution. An aliquot of the obtained CdTe QDs (10 mL) was added to anhydrous methanol (20 mL) to promote a reversible flocculation of the nanocrystals. The QDs were separated from the supernatant by centrifugation. Next the nanocrystals were dispersed in anhydrous 1-butanol (25 mL) followed by centrifugation to result in an optically clear solution of nanocrystals and a grey precipitate that was discarded. To remove the unreacted products, anhydrous methanol (25 mL) was added to the supernatant to produce another flocculation of nanocrystals. Then the flocculant material was washed with methanol (50 mL) followed by vacuum drying.
Preparation of the electrode Prior to modification, the surface of the glassy carbon (GC) electrode surface was polished with alumina slurries (0.3 mm), rinsed thoroughly with double-distilled water, sonicated for 5 min in ethanol and 5 min in water, and air-dried. Graphene oxide (1 mg) and CdTe (0.5 mg) were suspended in DMF (1.0 mL). The suspension was dispersed using ultrasonic stirring for 1 h. A 15 mL aliquot of the composite suspension was placed as a droplet on the GC electrode surface and the solvent was then evaporated at room temperature. DMF was used because it was found to be more compatible than other solvents (e.g., ethanol, acetone) given the hydrophobic nature of the GC surface and led to a more homogeneous film.
Acknowledgements The authors are thankful for the research support from a FAPESP scholarship (2012/18339-1 to F.H.C.) and CNPq. Chem. Eur. J. 2014, 20, 1 – 9
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Received: December 11, 2013 Revised: January 9, 2014 Published online on && &&, 0000
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FULL PAPER & Electrochemistry
Quantum leap: A novel material, based on a hybrid of cadmium telluride quantum dots and graphene oxide, and its use for the determination of 17b-estradiol levels is reported. No previous study has been reported using the proposed hybrid that correlates the effect of light with the remarkable improvement in sensitivity for endocrine disruptor determination (see figure).
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F. H. Cincotto,* F. C. Moraes, S. A. S. Machado && – && Graphene Nanosheets and Quantum Dots: A Smart Material for Electrochemical Applications
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Graphene Nanosheets and Quantum Dots: A Smart Material for Electrochemical Applications Fernando Henrique Cincotto,* Fernando Cruz Moraes, and Sergio Antonio Spinola Machado[a]
and spectrophotometric measurements all showed good distribution of the quantum dots with a small particle size. The electrochemical measurements demonstrated the high performance of the composite response in the presence of a light source. Differential pulse voltammetry allowed the development of a method to determine 17b-estradiol levels in the range from 0.2 to 4.0 mmol L1 with a detection limit of 2.8 nmol L1 (0.76 mg L1).
Abstract: A novel material for the electrochemical determination of endocrine disruptors using a composite based on graphene oxide modified with cadmium telluride quantum dots has been evaluated. The morphology, structure and electrochemical performance of the composite electrodes were characterised by transmission electron microscopy, dynamic light scattering, UV-visible absorption spectra, fluorescence spectra, Raman spectra and cyclic voltammetry. The dynamic light scattering, transmission electronic microscopy
Introduction
deoxyribonucleic acid (DNA),[18] dopamine,[19] bisphenol A,[20] heroin and morphine.[21] Recently, several reports have focused on graphene-based hybrid nanomaterials for the development of electrochemical sensors.[22, 23] Although high loading and uniform distribution are demands for developing a graphene-based heterostructure, it is still a great challenge to establish new assembly strategies for functionalising graphene with other nanostructures. In this sense, Chen et al.[24] reported the use of a thermally reduced graphene oxide decorated with functionalised gold nanoparticles for the detection of HgII ions. Mao et al.[25] related the use of a thermally reduced graphene oxide sheet decorated with gold-nanoparticle–antibody conjugates to determine a specific protein. Quantum dots (QDs) are semiconductor nanocrystals that exhibit optical-electronic properties and dimensions in the range from 2 to 6 nm,[26] in which the bandgap value is dependent on the nanocrystal size. In QDs, the electron is in a quantum confinement, which results in a strong quantisation of the energy levels.[27] Thus, the material absorbs at a specific wavelength and energy equivalent to that of the confined electrons. For this reason, these materials are highly luminescent with narrow emission line widths, thus offering a range of potential applications including photovoltaic cells[28] and lasers.[29] The major application of QDs as analytical sensors comes about through the synergetic characteristics of the QDs and other nanomaterials. The hybrid materials exhibit a good distribution of particle sizes, allow for stable thin films and increase the intensity of the analytical signal. In this regard, Yu et al.[30] reported the use of reduced graphene modified with CdSe nanoparticles to understand photoinduced charge transfer. Other reports have described the use of composites based on graphene oxide (GO) and QDs of cadmium telurate (CdTe),
Graphene is a single layer of carbon atoms displayed in a closely packed two-dimensional lattice. This material has attracted extensive attention because of its unique nanostructure and intrinsic characteristics, including high thermal conductivity, low manufacturing cost[1] and excellent electrical, thermal, mechanical, electronic and optical properties.[2–4] Other remarkable properties are its high specific surface area,[5] excellent pliability, high chemical stability and high porosity and excellent electron-transport properties.[6] These unique properties render graphene a promising material for applications in several nanotechnology fields such as batteries,[7] supercapacitors,[8] field-effect transistors,[9] fuel cells[10] and electrochemical sensors.[11] Moreover, thin transparent graphene films are used as electrodes in solar cells[12] and in organic light-emitting devices.[13] In addition, this material has yielded promising results in bioscience owing to its high electrocatalytic activity and good biocompatibility.[14, 15] The most interesting use of graphene in electroanalysis is possible owing to its electronic properties such as rapid electron transport, electrocatalytic effect, the structural characteristics it possesses by virtue of possessing a huge specific surface area (2600 m2 g1), its 2D structure and the presence of functional groups that enable biocompatibility.[16] Thus, graphene has been used in the development of reliable and sensitive detection methods for several target analytes such as glucose,[17]
[a] Dr. F. H. Cincotto, Dr. F. C. Moraes, Prof. Dr. S. A. S. Machado Group of Electrochemical Materials and Electronalytical Methods, Institute of Chemistry of S¼o Carlos University of S¼o Paulo, S¼o Carlos—SP, CP 780 (Brazil) E-mail: [email protected] Chem. Eur. J. 2014, 20, 1 – 9
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Full Paper such as a biosensor using graphene modified with CdTeCdS quantum dots, gold nanoparticles and glucose oxidase to determine glucose,[31] and an electrochemiluminescence immunosensor using an AuFe3O4 magnetic nanoparticle core–shell over graphene and a primary antibody for carbohydrate antigen-125 detection.[32] These studies suggest that the combination of graphene and QDs on electrochemical sensors presents a promising setup to detect low concentrations of analytes in different matrices. Nowadays, the development of methodologies for remediation, detection and quantification of endocrine disruptors is a target in several research fields such as clinical and environmental analysis. Endocrine disruptors are chemical compounds that interfere with the functioning of the endocrine systems of mammals, amphibians, birds and fish.[33] The hormone 17b-estradiol is an estrogenic hormone in natural sources that possesses a high level of endocrine interference activity.[34] Contamination of aquatic environments by natural estrogens is due to human and animal excretion, following the subsequent transport of effluents to sewage treatment stations. Thus, the bioaccumulation of 17b-estradiol in the environment is a problem to be solved. The analytical methods normally used for 17b-estradiol determination are based on chromatographic techniques such as high-performance liquid chromatography (HPLC)[35] and liquid chromatography (LC).[36] Electrochemical determination of 17bestradiol is based on the irreversible oxidation of the phenol group present in the molecule. However, it has been reported that this electrochemical process presents a low current response over surfaces[37] such as platinum, glassy carbon and boron-doped diamonds. As a result, several studies have related the use of surface modifiers to increase the 17b-estradiol electrochemical response. Among these electrocatalytic materials are CdSe quantum dots based on indirect competitive immunoassay,[38] a nickel-modified glassy carbon electrode[39] and a poly(l-serine) film-modified electrode.[34] Considering the work that has been described above, this study focuses on the synthesis, characterisation and application of a novel composite based on graphene oxide modified with CdTe quantum dots. This novel material yielded excellent sensitivity for 17b-estradiol electro-oxidation, as well as a high electrocatalytic activity and a low detection limit. A glassy carbon electrode modified with the composite GO/CdTe is shown to be promising for fast, simple and sensitive determination of 17b-estradiol in environmental samples.
Figure 1. DLS measurements of the distribution of particle sizes using a monochromatic light from a laser source with a wavelength of 250 nm.
on the time fluctuation in the scattering intensity, as shown in Figure 1. Figure 1 displays the distribution of particle sizes as a function of percent intensity. The observed values are the average of three measurements of QD samples. It was observed that the distribution of particle sizes varied over a range between 1.5 and 5.8 nm, with the greatest number at (3.2 0.2) nm. This distribution size is in agreement with the QD synthesis proposed by Algar et al.[24] in which the QDs have particle sizes that vary from 2.0 to 6.0 nm. The UV/Vis absorption and the photoluminescence properties of the QDs were determined using UV/Vis and fluorescence experiments and the spectra are shown in Figure 2.
Figure 2. UV/Vis absorption (solid line) and photoluminescence (open circles) spectra of CdTe quantum dots, with excitation wavelength at 400 nm.
Results and Discussion
During all the spectrophotometric measurements, the QD suspension was loaded in a 2.5 mL disposable quartz cuvette and scanned at room temperature. The spectra of the CdTe QD suspensions were measured after dilution with hexane to provide optical densities appropriate for the measurements. Figure 2 shows typical UV/Vis absorption (solid line) and PL emission (open circles) spectra of CdTe nanocrystals. The results showed a well-resolved absorption maximum at the first electron transition at 585 nm, thus indicating a sufficiently
Characterisation of CdTe quantum dots The synthesis of QDs was evaluated using dynamic light scattering (DLS) measurements. The DLS experiments were carried out on the CdTe suspension by using a cell that contained hexane. For this, a monochromatic light from a laser source with a wavelength of 250 nm struck the QD particles in the suspension and the particle-size distribution was dependent &
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Full Paper narrow size distribution of the CdTe QDs. The size of the nanocrystal is related to the quantum confinement of the energy absorbed, which shifts to higher values with wavelength, thereby allowing an increase in the size of the nanocrystals. To evaluate the size distribution of the nanocrystals, a theoretical method was applied on the basis of the UV/Vis absorption maximum. This method was first reported by Yu et al.[53] and shows an estimated value of QD size on the basis of empirical fitting functions of the CdTe UV/Vis spectra. The relationship between the diameter of the particle (D) and the UV/Vis wavelength (l) is shown in the following equation [Eq. (1)]:
D ¼ ð9:8127 107 Þl3 ð1:7147 103 Þl2 þ ð1:0064Þl194:84 ð1Þ The average value of D obtained for measurements of the CdTe at l = 585 nm was 3.11 nm. This value is in good agreement with that obtained from the DLS results, thus showing the high efficiency of the synthesis method. Observation of the photoluminescence property of the QDs was carried out by using PL measurements. The PL emission spectrum (open circles) for CdTe is shown in Figure 2. It was clearly observed that the maximum emission wavelength was 660 nm for an excitation wavelength of 400 nm. The PL measurements indicate that the emission of the present CdTe QDs was due to trap-state emission rather than band-edge emission.[40] The quantum confinement emission is associated with electron transitions between the trap states and the conduction band. Additionally, as can be seen, the present CdTe QDs had an emission peak at 660 nm, whereas the absorption edge was at 585 nm. Note that the emission peak wavelength was close to 80 nm larger than the corresponding absorption edge wavelength. This indicates the emission of the present CdTe QDs was due to emissions of the quantum confinement state instead of band-edge emissions. Comparing the obtained results for the emission band, that is, 660 nm, and the corresponding energy 1.88 eV with other reports,[41, 42] these values are quite similar for a particle size around 3.0 nm. The size of the nanocrystals determines its optical and spectroscopic properties. Typically, as the particles become smaller, the luminescence energies are blueshifted to higher energies.[43] In addition, the PL properties vary sharply with QD size: small QDs (1.2 nm) emit UV light, medium-sized QDs (1.5–3.0 nm) emit visible light and large QDs (3.8 nm) have a near-infrared emission.[44]
Figure 3. A) TEM image of graphene oxide nanosheets. Inset: Raman spectra of graphene oxide nanosheets. B) AFM image of GO nanosheets. Inset: topopography plot.
vation is the dark lines with high contrast, which clearly illustrate that the GO nanosheets are wrinkled or folded. It is supposed that the wrinkled morphology can reduce the surface energy of GO nanosheets and thereby make the nanosheets stable. In this sense, the GO can act as a support for QD adsorption. Structural characterisation of the aggregations of the GO nanosheets was carried out using Raman spectroscopy. The inset in Figure 3A exhibits typical Raman spectra for the GO nanosheets. It was observed that the synthesised GO presented two characteristic absorption bands in its Raman spectrum: the G band at 1720 cm1 and the D band at 1635 cm1. The G band corresponds to bond stretching of sp2 carbon pairs. The D band is associated with the breathing mode of aromatic rings with dangling bond in-plane terminations. As shown in the Raman spectrum, the magnitude of the D band is due to the presence of defects introduced by functional groups such as carboxyl, hydroxyl and epoxy groups.[45] The relationship be-
Morphological characterisation of the GO/CdTe composite The synthesis of the GO was performed using a modified Hammers method and was evaluated using TEM microscopy, as displayed in Figure 3A. It was observed that the GO nanosheets are transparent. However, the differences in the degree of transparency suggest that the thickness of the GO nanosheets is very small. Also, the nanosheets are not entirely a single-layered structure and there are overlaps of several single layers. Another obserChem. Eur. J. 2014, 20, 1 – 9
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tween the intensity of the D band (ID) and G band (IG) is usually employed to evaluate the size of the in-plane sp2 domains for carbon materials. The obtained value for the ID/IG ratio was 1.44 for obtained GO nanosheets. This value is consistent with other values that describe GO nanosheets.[46] To confirm the thickness of GO nanosheets, atomic force microscopy (AFM) measurements were performed; the results are displayed in Figure 3B. In this image, the presence of GO layers over the mica surface can be clearly observed. On the basis of the AFM topography analyses, the average of GO thickness was obtained in a range as shown by the arrow displayed in Figure 3B. The inset of this figure demonstrates that the GO layers have a width of 350.0 nm and an average height of 8.0 nm; each layer has a thickness close to 2.8 nm. Figure 4 displays TEM images of the CdTe QDs supported over GO nanosheets. It was observed that the QDs were uniform, well-dispersed and with a narrow particle-size distribution. The particles self-organise on the TEM grid in a close-
The electrochemical behaviour of 17b-estradiol over the GO/ CdTe surface was evaluated by using cyclic voltammetry. The CV experiments were carried out in phosphate buffer solution (PBS; 0.1 mol L1, pH 8.0) in the presence and absence of 17bestradiol (50 mmol L1) over a potential range from + 0.2 to + 1.0 V with a scan rate of 50 mV s1. In Figure 5 it can be ob-
Figure 5. CV scans of the GC/GO/CdTe electrode in the a) presence and b) absence of 17b-estradiol (50 mmol L1) with a scan rate of 50 mV s1. Inset: proposed electrochemical mechanism for 17b-estradiol oxidation.
served that the 17b-estradiol exhibited an oxidation peak at + 0.55 V over the GC electrode modified with a film of GO/ CdTe. This oxidation is attributed to irreversible oxidation of the hydroxyl group present in the aromatic ring of the 17b-estradiol moiety to form a ketone species,[47] as shown in the proposed electrochemical mechanism in the inset of Figure 5.
Figure 4. TEM image of CdTe quantum dots. Inset: histogram of QD particle size.
packed monolayer on the GO nanosheets, and local hexagonal symmetry is evident in the packing arrangement. The diameters of the particles were measured directly from TEM images of the composite by using Image Photo Pro Plus 6 computational software. This study was performed for three different TEM images, in which each microscopic image was mapped and scanned in four equal parts. The histogram presented in the inset of Figure 4 shows a QD size of 3.10 nm, a value that is the average of the studied images. The CdTe QD size was very close to the value estimated from the absorption spectra, which corroborates the results obtained by DLS and the theoretically estimated size on the basis of its UV/Vis spectrum.
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Figure 6. DPV voltammograms of GC/GO/CdTe electrode in 17b-estradiol (50 mmol L1) under different conditions: a) in the dark and b) in the presence of a source of visible light. Inset: photocurrent response of GC/GO/ CdTe electrode to on/off cycles of light illumination.
Figure 6 displays differential pulse voltammetry (DPV) voltammograms for 17b-estradiol electro-oxidation in the dark and in the presence of light. The voltammograms were collected using the GC/GO/CdTe electrode in 0.1 mol L1 PBS (pH 8.0) that contained 17b-estradiol (50 mmol L1) in the dark as well as in the presence of a source of visible light (Hg lamp, 20 W power). The electrochemical measurements were scanned over a potential range that varied from + 0.2 to + 0.8 V with an am4
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Full Paper 30 % at a pretreatment potential of + 0.2 V. Thus, 0.9 V was chosen as the pretreatment cleaning potential in further studies. The influence of the pretreatment time was also evaluated. In the range from 20 to 360 s, the current peak increased to 180 s and remained constant at longer time intervals. As such, 180 s was chosen as the time of pretreatment for cleaning the electrode surface.
plitude of 50 mV and step potential of 1 mV. It was evident that the current peak, attributed to 17b-estradiol oxidation, increased almost threefold in the presence of light when compared with the same measurement in the dark. To elucidate the increase in the current peak in the presence of the radiant light, an amperometric experiment was carried out to investigate the photoelectrochemical properties of the CdTe QDs. The inset in Figure 6 shows a typical result, as it exhibits a time-dependent response curve to light irradiation that corresponds to CdTe-QD-modified graphene oxide over a glassy carbon electrode. The measurement was repeated for three cycles. It can be seen that the GC/GO/CdTe electrode gives strong and rapid responses to light irradiation at a fixed applied potential of + 0.8 V. The current increases immediately when the light is turned on and decreases sharply when the light is turned off. The increase in the current by 12.1 mA is due to the induction of CdTe QDs in the gap. When the light is turned on, there is an excitation of the QDs that results in the transfer of electrons from the valence band to the conduction band, thus yielding electron–hole pairs.[48] When the light is turned off, there is a dramatic decrease in the current owing to the recombination of electrons and holes. This result confirmed that an electrochemical system based on graphene oxide modified with CdTe quantum dots becomes a suitable setup for use in environmental analysis in the presence of light.
Analytical response On the basis of the optimised conditions, DPV was used to investigate the electrochemical response as a function of 17b-estradiol concentration. In this study using the GC/GO/CdTe electrode, analytical responses were performed under two different conditions, in the dark and in the presence of a light source, and their analytical responses were compared. Standard 17bestradiol concentrations were used to construct the calibration curves shown in Figure 7 with three measurements at each concentration.
Optimisation of experimental parameters on the GC/GO/ CdTe electrode To maximise the DPV analytical signals, the effects of experimental parameters were studied on the GC/GO/CdTe electrode in a 0.1 mol L1 solution of PBS at pH 8.0 that contained 17bestradiol (50 mmol L1). The amplitude was varied over a range from 10 to 100 mV with a fixed scan rate of 10 mV s1. The anodic current peak with the highest current signal was 50 mV, so this value was chosen as the differential pulse amplitude. The effect of potential step increments was studied over the range from 1 to 10 mV with a fixed scan rate of 10 mV s1. For potential steps greater than 2 mV, deformation of the voltammetric profile was observed and the anodic current peak decreased in height. Hence, 1 mV was chosen for the potential steps. Electrochemical detection of 17b-estradiol is accompanied by adsorption of its oxidation products. Thus, poisoning of the electrode surface occurs and there is a consequent decrease in the analytical signal. The influence of an applied potential pretreatment after each scan to remove adsorbed species on the electrode surface was investigated. Pretreatment potential values were tested in the range of 1.0 to + 0.2 V in a 0.1 mol L1 solution of PBS at pH 8.0 that contained of 17bestradiol (50 mmol L1). Complete recovery of the original response was obtained by applying 0.9 V. This potential value is sufficient to reduce the oxidation product of 17b-estradiol that is adsorbed onto the electrode surface. At more positive cleaning potentials, the peak current (Ipa) obtained during the following experiment begins to decrease and reaches only Chem. Eur. J. 2014, 20, 1 – 9
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Figure 7. Linear dependence of DPV peak current on the concentration of 17b-estradiol for the GC/GO/CdTe electrode in the dark (*) and in the presence of a light source (&). The concentrations of 17b-estradiol [mmol L1] were as follows: 0.25, 0.50, 1.00, 1.25, 1.50, 2.00, 2.50, 3.00, 4.00, 5.00, 10.00, 12.50, 15.00, 17.50 and 20.00.
The analytical signals obtained under dark conditions exhibited a linear range from 1.0 to 4.0 mmol L1 (R2 = 0.9824). The limit of detection (LOD) was calculated as 9.8 nmol L1. The detectability level was remarkably influenced by the presence of a light source. The photosensitised GC/GO/CdTe electrode exhibited a linear range from 0.2 to 4.0 mmol L1 (R2 = 0.997) and the LOD was calculated as 2.8 nmol L1. The LOD values were determined using the 3s/slope ratio (in which s is the standard deviation of the mean current for 10 DPV voltammograms of the blank, and the current values were collected at the 17b-estradiol oxidation peak potential. The calculations were performed according to IUPAC recommendations.[49] The effect of light on the proposed sensor can be further evaluated from the values of analytical sensitivity, which were 2.6-fold higher in the presence of a light source (1.61 mA/mmol L1) relative to their sensitivity in the dark (0.62 mA/mmol L1). Also, the LODs and sensitivity values were compared in a T-test at the 95 % confidence level by comparing the GC/GO/CdTe sensor in the presence and absence of light. 5
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Full Paper ty and repeatability and increasing the sensitivity. In essence, the GC/GO/CdTe electrode presents an alternative material for use in environmental analysis.
The reproducibility of the GC/GO/CdTe electrode was measured from seven experiments, in which each experiment consisted of five sequential DPV voltammograms. These experiments were performed on different days. Prior to each experiment, the electrode surfaces were rinsed thoroughly with double-distilled water. Thus, the DPV voltammograms were performed in PBS (0.1 mol L1, pH 8.0) that contained 17b-estradiol (50 mmol L1). The relative standard deviation (RSD) was calculated at 2.2 %. In addition, the intra-assay precision tests were performed from ten DPV voltammograms of that same solution. The RSD was calculated at 1.7 %. In addition, the composite can be stored for 2 months without losing electroactivity. Upon comparing the results at the GC/GO/CdTe electrode with other electroanalytical methods for the determination of 17b-estradiol, a higher detection limit was observed: 16 nmol L1 for a sensor based on carbon nanotubes modified with Pt nanoparticles[50] and 63 nmol L1 for a composite electrode based on carbon nanotubes modified with [Ni(Cyclam)] (Cyclam = 1,4,8,11-tetraazacyclotetradecane).[51] A low detection limit was reported by Wang et al.,[52] in which a flow injection chemiluminescence of CdTe nanocrystals treated with sodium hexametaphosphate and KMnO4, was used to determine 17bestradiol. The calculated LOD was 5.8 1011 mol L1. The lower detection limit and higher sensitivity observed at the GC/GO/CdTe electrode can be attributed to the efficiency of the electron transfer between the modified electrode and 17b-estradiol owing to excitation of electrons in the QDs in the presence of a light source, thereby allowing electron transfer from the valence band to the conduction band of energy and improving the electrochemical response. Other methods that use biosensors to detect 17b-estradiol have shown lower detection limits at nanomolar levels, that is, 0.18[53] and 0.036.[54] However, for the GC/GO/CdTe electrode, high sensitivity, good reproducibility and simple instrumentation, preparation and analytical procedure are important advantages. This method can be easily applied to the determination of disruptor endocrines in real samples.
Experimental Section Apparatus and procedures Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed using a model PGSTAT 302 Autolab electrochemical system (Eco Chemie, Netherlands) monitored with NOVA software. The electrochemical cell was assembled with a conventional three-electrode system: a glassy carbon electrode modified with a composite (GO/CdTe), an Ag/AgCl electrode in KCl (3.0 mol L1) as a reference electrode and Pt wire as an auxiliary electrode. All electrochemical measurements were performed in 0.1 mol L1 PBS (pH 8.0) at a controlled temperature (25 8C). CV measurements were carried out in a potential range from + 0.2 to + 0.8 V with a scan rate of 50 mV s1. DPV experiments were performed using amplitude of 50 mV and a step potential of 1 mV. The potential was varied from + 0.2 to + 0.8 V for 17b-estradiol calibration curves. The morphologies of the GO, CdTe and GO/CdTe composite were examined using a transmission electron microscope (Zeiss EM912 Omega, STEM). To prepare the samples, a small amount of the powder was dispersed in anhydrous dimethylformamide (DMF) in an ultrasound bath. A freshly prepared dispersion was then added dropwise onto a copper grid. The GO thickness was examined by using an atomic force microscope with a model Nanosurf EasyScan 2 AFM System (NanoSurf AG, Switzerland). The measurements were carried out with a non-contact/tapping mode long cantilever. The sample was prepared by adding the GO suspension dropwise over a previously cleaned mica surface. The CdTe particle-size distribution was determined by DLS using a Zetasizer Nano ZS90 (Malvern USA) instrument; analyses were performed in triplicate and reported data are the average of three runs in which the standard deviation was always below 5 %. UV/Vis absorption spectra were performed using a Jasco V-630 (Jasco Inc., USA) spectrometer configured with a slit width of 2 nm, a scanning speed of 4 nm per second and a data interval of 1 nm. The photoluminescence (PL) properties of the QDs were measured using a QM-4/2005 spectrofluorometer (Photon Technology International, Birmingham, NJ). The PL measurements were carried out using a xenon lamp at 75 W with both the excitation and emission slit width set at 2 nm, the voltage for the photomultiplier tube (PMT) detector set at 1050 V, an integration time of 0.25 s and a step size of 1 nm.
Conclusion A novel nanomaterial for development of an electrochemical sensor to determine endocrine disruptors based on CdTe quantum dots supported on graphene oxide has been proposed in this paper. The GO/CdTe composite was successfully characterised by TEM, DLS, UV/Vis absorption, fluorescence, Raman and voltammetric techniques, which indicated that the CdTe was supported on the GO surface. Therefore, the electronic properties of graphene oxide in addition to the photoelectric catalytic ability of the QDs showed greater efficiency in the electro-oxidation of 17b-estradiol. A drawback in composite development is that the preparation is time consuming. However, this disadvantage is minimised by the composite synthesis yield, which allows the fabrication of more than 200 electrodes at a time. Finally, the electronic properties of the graphene in addition to the high catalytic activity of the CdTe yielded values for the LOD in the nanomolar range, thereby improving reproducibili&
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The structural characterisation was performed by XRD using a Rigaku Rotaflex diffractometer model RU200B at 50 kV and 100 mA, with a CuKa radiation wavelength of l = 1.542 . Raman spectra were collected using a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCDS detector and a holographic notch filter (Renishaw Ltd, Gloucestershire, UK). All chemicals were of analytical grade and were used without further purification. Analytical-grade 17 b-estradiol was obtained from Sigma–Aldrich (Germany). The supporting electrolyte was 0.1 mol L1 PBS. Graphite powder (< 20 micron), cadmium oxide (99 %), n-tetradecylphosphonic acid (TDPA, 98 %), 1-octadecene (ODE; tech. 90 %) and tellurium powder (200 mesh, 99.8 %) were purchased from Sigma–Aldrich (Germany). All organic solvents used for quantum-dot purification such as hexane, isopropanol, methanol and acetone were of analytical grade. All electrolytes
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Keywords: electrochemistry · nanomaterials · quantum dots
graphene
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hormones
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The graphene oxide was prepared from graphite through a modified Hummers method.[55] A mixture of graphite (1.0 g) and sodium nitrate (1.0 g) were oxidised by using concentrated H2SO4 (46 mL) under magnetic stirring maintained at a temperature of 20 8C. Then, KMnO4 (6.0 g) was slowly added under vigorous stirring. The reaction vessel was kept in a refrigerator at 0 8C for 24 h. Afterward, the solution was stirred at 98 8C for 1 h and diluted with water (240 mL). Then a solution of H2O2 (85 mL, 30 % v/v) was added. The product was subjected to hot filtration, washed three times with HCl solution (5 % v/v) and dried in a vacuum oven at 50 8C for 48 h to obtain graphene oxide.
Synthesis of CdTe quantum dots The synthesis of CdTe QDs was performed using a modified procedure reported by Yu et al.[56] For this, CdO (25.6 mg) was mixed with TDPA (114.0 mg) following the addition of ODE (7.8 g). The mixture was heated under vigorous stirring in an argon atmosphere. The solution appeared clear and colourless. Then, Te (12.5 mg) was dissolved in tributylphosphine (475.0 mg, 97 %) and ODE (1.5 g). The solution was prepared in an argon-filled glovebox and loaded into a syringe. Then the syringe was removed from the glovebox and the Te mixture was injected very quickly into the Cdcontaining mixture. The temperature decreased rapidly after injection and was held at 260 8C to allow growth of the QDs. The rapid introduction of the Te into the mixture produced a yellow/orange solution. An aliquot of the obtained CdTe QDs (10 mL) was added to anhydrous methanol (20 mL) to promote a reversible flocculation of the nanocrystals. The QDs were separated from the supernatant by centrifugation. Next the nanocrystals were dispersed in anhydrous 1-butanol (25 mL) followed by centrifugation to result in an optically clear solution of nanocrystals and a grey precipitate that was discarded. To remove the unreacted products, anhydrous methanol (25 mL) was added to the supernatant to produce another flocculation of nanocrystals. Then the flocculant material was washed with methanol (50 mL) followed by vacuum drying.
Preparation of the electrode Prior to modification, the surface of the glassy carbon (GC) electrode surface was polished with alumina slurries (0.3 mm), rinsed thoroughly with double-distilled water, sonicated for 5 min in ethanol and 5 min in water, and air-dried. Graphene oxide (1 mg) and CdTe (0.5 mg) were suspended in DMF (1.0 mL). The suspension was dispersed using ultrasonic stirring for 1 h. A 15 mL aliquot of the composite suspension was placed as a droplet on the GC electrode surface and the solvent was then evaporated at room temperature. DMF was used because it was found to be more compatible than other solvents (e.g., ethanol, acetone) given the hydrophobic nature of the GC surface and led to a more homogeneous film.
Acknowledgements The authors are thankful for the research support from a FAPESP scholarship (2012/18339-1 to F.H.C.) and CNPq. Chem. Eur. J. 2014, 20, 1 – 9
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Received: December 11, 2013 Revised: January 9, 2014 Published online on && &&, 0000
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FULL PAPER & Electrochemistry
Quantum leap: A novel material, based on a hybrid of cadmium telluride quantum dots and graphene oxide, and its use for the determination of 17b-estradiol levels is reported. No previous study has been reported using the proposed hybrid that correlates the effect of light with the remarkable improvement in sensitivity for endocrine disruptor determination (see figure).
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F. H. Cincotto,* F. C. Moraes, S. A. S. Machado && – && Graphene Nanosheets and Quantum Dots: A Smart Material for Electrochemical Applications
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