Superparamagnetic Maghemite-Based CdTe Quantum Dots as

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Superparamagnetic Maghemite-based CdTe Quantum Dots as Efficient Hybrid Nanoprobes for Water-bath Magnetic Particle Inspection Fernando Menegatti de Melo, Daniel Grasseschi, Bruno B. N. S. Brandão, Ying Fu, and Henrique Eisi Toma ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00502 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Nano Materials

Superparamagnetic

Maghemite-based

CdTe

Quantum Dots as Efficient Hybrid Nanoprobes for Water-bath Magnetic Particle Inspection Fernando Menegatti de Melo1*, Daniel Grasseschi‡2, Bruno B. N. S. Brandão‡1, Ying Fu3, Henrique E. Toma1,* 1

Supramolecular Nanotech Lab, Department of Chemistry, University of São Paulo, 05508-000,

São Paulo, SP, Brazil. 2

Machgraphe-Graphene and Nanomaterials Research Center, Mackenzie Presbyterian

University, 01303-907, São Paulo, SP, Brazil. 3

Science for Life Laboratory, Department of Applied Physics, Royal Institute of Technology,

SE-10691, Stockholm, Sweden

Keywords: Quantum Dots, Magnetic Nanoparticles, Hybrid Nanoparticles, Magneto-Fluorescent Nanoparticles, Nanoprobes, Magnetic Particle Inspection.

ACS Paragon Plus Environment

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ABSTRACT Fluorescent water-based cadmium telluride quantum dots (QDs) and citrate-functionalized maghemite nanoparticles (MghNPs) were synthesized and assembled together (MghNPs@QDs) through electrostatic interactions by using cetyltrimethylammonium bromide (CTAB) as a linker and steric spacer to minimize the Förster Resonance Energy Transfer (FRET) restriction. A whole family of hybrid and multifunctional nanoparticles has been successfully obtained, exhibiting good performance in non-destructive water-bath magnetic particle inspection (MPI) assays. INTRODUCTION: Magnetic particle inspection (MPI) is a conventional non-destructive testing (NDT) process for detecting structural flaws and shallow subsurface discontinuities in ferromagnetic materials in industry1. Most common MPI commercial products are based on micrometric particles in association with organic fluorophores. Their functioning is based on the induced magnetic attraction to the fracture borders, exposing their precise location for visual fluorescence detection. The use of nanoparticles can allow to detect the occurrence of nanometric flaws which is not accessible by the common MPI agents. However, despite the current exciting perspectives of applying fluorescent magnetic nanoparticles in industry and biomedicine, their development is quite challenging mostly because of the quantum restrictions imposed by FRET mechanism at the nanoscale.2 By allowing the detection of nanometric flaws, the use of QDs is particularly rewarding in MPI applications because they are highly luminescent and stable. In comparison to the traditional organic fluorophores, they convey many advantages for labeling3–10, including high brightness at


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a lower concentration (10-7 mol.L-1)11, broad optical absorption characteristics12 and narrow band emission spectra13 associated with the quantum size effects;14–17 as well as a great stability under harsh conditions12 with typical fluorescence lifetime varying from 10 to 40 ns.11,18 Basically, there are two main synthetic routes for the synthesis of QDs: the organometallic19–22 and the aqueous23 ones. Here we are focusing on aqueous route, which is becoming more attractive because it encompasses a greener strategy, both from the views of environmental and biological applications, and does not require high temperatures. However, it should be noticed that QDs nucleation and their growth mechanism in aqueous solution remain yet poorly understood. For this reason, in this work, we pursued the understanding and optimization of the quantum dots formation, by performing a real time study of the formation of 3mercaptopropionic acid-functionalized cadmium telluride quantum dots, aiming to develop a controlled synthesis under reproducible conditions. In this study, for convenience and practicability, we monitored the nanoparticles growth in the laboratory environment, using a conventional CCD camera, and then processing the red, green and blue (RGB) signals. It should be noted that the most common magnetic materials are based on iron oxide nanoparticles constituted by magnetite (Fe3O4) and maghemite (γ-Fe2O3). They exhibit strong superparamagnetic behavior in the presence of a magnetic field and their colloidal suspensions (also referred as ferrofluids24) play an important role in biotechnology, medicine25–27 as well as in the recent advances in mineral processing based on magnetic nanohydrometallurgy.28–31 Although maghemite has a slightly smaller (~ 25 %) magnetic moment as compared with magnetite, it is more stable in air and has the advantage of exhibiting a much lower optical absorption in the visible region. In fact, in magnetite there are O2- → Fe(III) charge-transfer and Fe(II) → Fe(III) intervalence transitions spanning all the visible region, imparting a typical black


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color. This characteristic makes it unsuitable for MPI applications because of the optical absorption effect in addition to the FRET effect that significantly decreases the luminescence intensity. For this reason, the generation of hybrid magneto-fluorescent nanoparticles is not trivial. In this work we report on a new strategy to synthesize magneto-fluorescent nanoparticles, starting from the preparation of size-controlled cadmium telluride quantum dots, and their incorporation into MghNPs, previously treated with CTAB, that acts as an electrostatic binder and steric spacer to suppress the FRET effect. The MghNPs were produced by means of citratemagnetite nanoparticles (MgNPs) oxidation. This way, a new class of multifunctional nanoparticles has been successfully obtained, exhibiting a good performance in MPI assays. MATERIALS AND METHODS QDs synthesis: Briefly, in a three-neck round-bottom flask, a desired amount (100 µL – 160 µL) of 3-mercaptopropionic acid (Aldrich) was added to a 5 mmol.L-1 cadmium acetate solution (Labsynth) and the pH of that was adjusted to 9.0 using 1 mol.L-1 NaOH (Labsynth). This solution was deoxygenated for 30 minutes and heated up to the water refluxing temperature. In another flask, 2.5 mmol.L-1 sodium telluride solution was prepared, see supporting information, and added abruptly to the Cd(II)/ligand solution. As the reaction proceeded, aliquots were taken out to be analyzed. MgNPs synthesis: The synthesis was carried out starting from 15 mL of NH3 solution (Labsynth) in a 125 mL three-neck round-bottom flask equipped with mechanical stirring under N2 atmosphere. Then 35 mL of deaerated water were added, and concurrently, a N2 flow was passed at 0.5 L.min-1. Ferrous chloride tetrahydrate (0.63 mmol) and ferric chloride hexahydrate (1.26 mmol), both from Labsynth, were dissolved in 25 mL deaerated water, in a two-neck


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round-bottom flask (100 mL) equipped with magnetic stirring. Addition of iron salts to the ammonia solution was accomplished by applying a pressure difference from the N2 output, as connected to provide a transfer system. At the end of the addition, 10 mL of citric acid solution (0.5 mol.L-1) from Aldrich, was added, and the mechanical stirring was stopped after 1 h. The black magnetic nanoparticles were confined at the bottom by using an external magnet, and after removing the suspension, the solid was suspended in water and the same procedure repeated twice (stock suspension). After lyophilization, the isolated solid was kept dried in the desiccator, at room temperature, for subsequent characterization. MghNPs synthesis: The previous stock suspension (50 mL) was put in a three-neck roundbottom flask (125 mL) equipped with condenser and mechanical stirring under air bubbling. After starting the reflux, 5 mL of citric acid solution (0.5 mol.L-1) was added (1 mL per addition; 5 additions) and maintained for 150 minutes. The brown magnetic nanoparticles were confined at the bottom by using an external magnet, and after removing the suspension, the solid was dispersed in water and the same procedure was repeated twice. MghNPs@QDs synthesis: A desired amount of MghNPs was dispersed in water (0.4 g.L-1). After that, 2 mL of this suspension was added into 1 mL of CTAB solution (10 g.L-1, Aldrich) and ultrasonicated for 30 minutes. At the end, 2 mL of the QDs suspension was added and the mixture was kept under mechanical stirring for 30 minutes. To obtain the final material, the particles were precipitated with acetone (Labsynth), washed and stoked in acetone. Figure 1 shows a schematic representation of the whole process of the hybrid nanoprobes synthesis.


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Figure 1. Schematic representation of the whole process of the hybrid nanoprobes synthesis Real-time kinetics study: The formation kinetics of QDs were monitored in real-time using the MasterView software developed by Process Development Center and Chemical Analysis in Real Time at the Federal University of Rio de Janeiro,32 and a conventional webcam for collecting the transmitted and reflected lights from the reaction system. The signal generated by the CCD camera was sent to the graphics processing unit (GPU) of the computer. From these signals, the software reconstructs the image and stores the average data from the red, green and blue (RGB) channels, as functions of time in a predetermined spatial region.32,33 The data collection covered all the synthetic procedure, and the RGB spectral evolution was displayed in real-time. Figure 2 shows the experimental setup, encompassing the synthetic apparatus and the CCD camera placed in front of the three-neck round-bottom flask. More detailed experimental apparatus fabrication


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and the setup configuration for the video recording can be seen in Supporting Information, S1, S2 and S3.

Figure 2. (A) Apparatus used to accomplish the QD synthetic procedure as well as the real-time study. (B) Apparatus with lighting lightbulb. (C) Schematic representation of the light captured by the CCD camera. To ensure a good reproducibility of the RGB kinetic data, the system was mounted in a hood, sealed with a black film. To minimize the QDs excitation and the fluorescence interference in the RGB spectra, a lightbulb with maximum emission between 600-800 nm was used as illumination source. This way, only the scattered and transmission light is collected by the CCD and the RGB time evolution is related to changes in the QDs suspension color. So, during the formation of nanoparticles, light is absorbed and scattered at distinct intensities and wavelengths, depending on the particles size and composition. Consequently, the RGB counts start to increase or decrease depending on the contents of scattering and absorbing materials in the three-neck flask. In order to avoid electronic interferences in the RGB signals, the CCD camera automatic controls, such as shutter aperture, white balance, backlight compensation and gamma compensation, were all disabled (see Supporting Information, S2).


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Real MPI test: By using a Ball Joint and a Reference Block, both provided by Metal-Chek Co. (Brazilian company specialized in NDT and MPI) the MghNPs@QDs suspension (10-4 mol L-1) was dropped onto the surface and analyzed after acetone vaporization. The Reference Block was made of steel (Grade 90MnCrV8) hardened at 860 °C for 2 h and quenched in oil to give a surface hardness 63 HRC to 70 HRC. Characterization: Electronic absorption spectra were recorded on a diode-array HewelletPackard 8453A spectrophotometer. Photoluminescent emission was recorded on a Photon Technology equipment, using an InGaAs detector and FelixGX 1317 software. Infrared spectra were recorded on an ALPHA Bruker FTIR spectrometer (KBr pellets, 96 scans) and Raman spectra were recorded on a WITEC confocal microscope, with excitation wavelength of 532 nm, 0.1 mW of laser power, and integration time of 2 s. Scanning fluorescence was carried out on a WITec Raman confocal microscope, using 488 nm excitation and 2.9 mW of laser power (170 points per line, 180 lines per image, scan width of 170 µm and scan height of 180 µm). Sample irradiation and collection of the backscattered radiation was performed using the same microscope objective (Olympus, 100x). Total Reflection X-Ray Fluorescence analysis was taken out in a PicoFOX (XFlash Silicon Drift detector; Bruker). The fine particles morphology was analyzed by high resolution transmission electron microscopy (HRTEM) using a JEOL, model JEM 2100 equipment, operating with a LaB6 electron emitting filament, with a maximum acceleration voltage of 200 kV, using a drop casting technique (the electron diffraction, SAED, was made using the same JEM 2100), and by scanning probe


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microscopy (SPM) using a PicoSPM I equipment, with a PicoScan2100 controller, and a MACMode setup from Molecular Imaging, using a drop casting technique too. Magnetic hysteresis curves were obtained using a vibrating sample magnetometer manufactured by EG&G Princeton Applied Research, model 4500, using magnetic fields of up to 20 kOe (saturation field). The electromagnet used was produced by Walker Scientific - HR8 model. The Gauss meter was manufactured by Lake Shore - model 450. Differential Thermal Analysis/Thermogravimetric Analysis was carried out using a Shimadzu instrument, at a heating rate of 10 °C.s-1, under a synthetic air atmosphere. The thermal analyses coupled with mass spectroscopy were performed in TGA-DSC device model 490 PC Luxx, from Netzsch, coupled to a mass spectrometer QMS 403C Aeolos (synthetic air, 50 mL.min-1, and 10 ºC.s-1). The diffraction patterns were obtained on a Bruker D8 Discover facility. The measurement was obtained with primary Twin (0.6 mm; primary axial Soller) and secondary Twin (maximum aperture 5 mm; secondary axial Soller and nickel filter in 1D mode), with 0,05 as an increment and 1,5 as a step from 20 to 80 degrees. DFT calculus was carried out using the B3LYP functional with the 6-311G++(d,p) basis set for C, O, S and H atoms and LANL2DZ for the Cd atom. RESULTS AND DISCUSSION General RGB image analysis and QD size profiles QDs were initially synthesized according to the literature procedure34. However, for an accurate control of the process, we found out necessary to monitor, in real time, the intermediate species formed during the synthesis. In general, the biggest problem of the synthesis in aqueous medium is the competition between the cadmium hydroxide formation and the Cd complex precursor


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stabilization. Both are directly linked with the medium pH. According to the 3-MPA pKa values of 10.21 (HS-) and 4.57 (COOH-), the optimal condition to stabilize the Cd precursor would be at pH higher than 10.2, where the totally deprotonated species prevail. However, Cd(II) precipitates as cadmium hydroxide above pH 9 and any QDs cannot be formed. During the synthesis, it is quite important to follow the pH in situ. The initial pH of Cd(II) solution was 7.2. After the addition of 3-MPA, the solution became turbid (Inter01, see Supporting Information, S4), probably due to the coordination reaction between Cd(II) and the MPA sulfur atom, while the pH decreased to 3.6 due to the dissociation of the thiol groups, releasing protons to the solution. In this way, it was necessary to add aliquots of alkaline solution just enough to deprotonate the carboxyl groups and solubilize Cd-MPA precursor species before the sodium telluride addition. However, if one performs a rapid addition of alkaline solution, (see Supporting Information, S4(C)), formation of Cd(OH)2, (Inter03) will take place. In contrast, if one performs a slow addition of alkaline solution, the Cd-MPA precursor can be generated and fully solubilized, (Inter02) thus acting as the platform for the quantum dots formation. Regarding this aspect, it is interesting to notice that our thermogravimetric analysis coupled with mass spectroscopy, and Raman spectroscopy analysis supported by DFT calculations (see Supporting Information, S5, S6, S7, S8 and S9), were consistent with the formation of Cd(II) complexes encompassing Cd-S-Cd bridges (Inter01) in the precursor species. After this step, we rapidly injected an aqueous sodium telluride solution into the Inter01 precursor, at 98 °C, to split up the nucleation from the growth steps. We proceeded with the synthesis starting from 1/0.5/2.3 proportion of Cd/Te/MPA and four aliquots were analyzed, (namely 1, 2, 3 and 4), in order to understand the QDs structural and luminescent properties and to compare with the real-time analysis, as shown in Figure 3. After that, we repeated the


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procedure, accessing the kinetics by monitoring the nanoparticles growth using the conventional CCD camera and a suitable software to process the red, green and blue (RGB) signals. For the online measurements at a time interval of 0.5 s, the RGB monitoring was found to be particularly convenient for probing the fine steps of the synthesis.

Figure 3. (A and B) QDs aliquots collected along the synthesis, with their optical images taken before and during the excitation at 365 nm, respectively. Four representative aliquots (1-4) are highlighted in (C and D), showing their emission spectra and the corresponding average size measured by transmission electron microscopy (see Supporting Information, S10, S11, S12 and S13), with the standard deviation, accordingly to the time reaction. Fluorescence images of the same aliquots can be seen in Supporting Information, S14.


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As can be seen in Figure 4, in a typical growth kinetics, in real-time, the system exhibits an induction period, presumably involving the reversible interaction between the precursor cadmium-MPA-telluride complexes, as shown in Figure 4D(1). Then, the initial clusters are generated, growing up to a critical size before beginning the nucleation of small nanoparticles. When the first nanoparticles are formed, they become responsible for the absorption of blue and green light (Figure 4B), as well as for the scattering of red light (Figure 4C). It is important to note that the growth of nanoparticles is modulated by their interaction with MPA. However, as the particles continue to grow, the size-dependent bandgap shifts the absorption spectrum from blue to red (see Supporting Information, S15). As the particles reach a certain size, the bandgap becomes more accessible, promoting the red light absorption while surpassing the size dependent light scattering effect (Figure 4C). This effect continues until the growth of nanoparticles is finished. Considering the parallel RGB monitoring processes, the changes in the green profile seem to provide the best general of view of the kinetics. It is curious that its exponential decay resembles a diffusion-controlled process, determining the size distribution along the time (Figure 3D).


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Figure 4. (A) Representative RGB spectrum in which the red, green and blue components are monitored as a function of the reaction time. (B) Correlation with electronic spectrophotometry, showing the absorbance of the aliquots collected along the reaction time, validating the RGB analysis. (C) Explanatory view encompassing the simultaneous behavior of the RGB components. (D) Zoom in the region between 0 and 10 minutes to highlight the induction period (1), the nucleation point (2) and the particle growing process (3). Based on the real-time analysis it was possible to correlate the intensities of the RGB channels with the particles emission properties and their sizes, as measured by TEM. In this way, a very useful correlation plot has been devised, as shown in Figure 3D. Accordingly, from the red and


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green data measured at a specific time, one can estimate the size, and consequently the emission wavelength of the corresponding quantum dot (see Supporting Information, S16 and S17). This feature makes the RGB plots very useful to follow the QDs growth without disturbing the reaction media. Moreover, it allows to understand how to obtain specific types of quantum dots by changing MPA and telluride concentration (see Supporting Information, S18, S19 and S20) enabling a better size control. In this way, by carefully controlling the variables, highly luminescent quantum dots could be successfully planned, and their properties precisely controlled. By changing the Cd:3-MPA ratio we were able to synthesize QDs with emission ranging from 514 to 650 nm (see Supporting Information, S17). This control is crucial to the efficiency of the hybrid MPI probe since we can choose the more suitable QDs to avoid energy transfer to the magnetic nanoparticles, as will be discussed. MgNPs and their assisted oxidation The synthesis of citrate-capped magnetite nanoparticles was based on the coprecipitation method.26,29 Although this method is suitable for large scale production, because of its great simplicity, the control of the reagents addition is quite important to define the nanoparticle structure. For instance, the slow addition of base can lead to the formation of iron oxide species, such as akaganeite, goethite and hematite. In the conventional coprecipitation procedure, when the Fe(III) and Fe(II) solutions at pH 1.5 and 11, respectively, are mixed, the large difference of pH generates a contact interface. In this transient interface, if the local pH remains relatively low, the hydroxide ions will preferentially react with Fe(III) ions. Under such condition, formation of Fe(OH)2 will not be favored, and the initial phase formed will be akaganeite (β-FeOOH)35. However, a rapid addition of base, or a rapid addition of iron salts to the base, will favor the formation of magnetite in relation to species such as lepdocrocite and hematite by generating


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Fe(OH)2 and FeOOH species from the abrupt increase of pH at the spot of mixture. The generated iron oxide particles in this case are constituted by magnetite, MgNPs, as we can see in Figure 5. This is confirmed by a comparison between the interplanar distance (d) measured by Selected Area Electron Diffraction (SAED) with the JCPDS 19:0629 data cards for Fe3O436–38 and the coating is confirmed by FTIR and AFM techniques, Figure 6. The contrast phase AFM images in Figure 6 show that the particles tend to form small agglomerates, exhibiting an average diameter of 33 nm, encompassing some magnetite cores, surrounded by the citrate coatings.

This result is very close to those obtained from DLS

measurements (see Supporting Information, S21(A)). Although the dynamic scattering measurements are influenced by the hydrodynamic radius of the species, there is a good agreement with the data obtained from AFM and DLS. The magnetic core size of the particles can be seen in TEM images (inset in Figure 5), revealing a crystalline structure with an average size of 14.0 ± 1.8 nm.


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Figure 5. TEM image of MgNPs with SAED analysis indicating a magnetite pattern and experimental d (exp) from SAED and d (Fe3O4) from JCPDS: 16-0629 data cards for Fe3O4.

Figure 6. AFM topography (above) and phase contrast (under) images of MgNPs confirming the size distribution and showing the interaction between iron oxide nanoparticles and the existence of external citrate layer at the nanoparticles surface confirmed by infrared spectroscopy in the left. (A) Inset from AFM topography image for small agglomerated nanoparticles clarifying its


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size. (B) Inset from AFM phase contrast image for the same above-mentioned small agglomerated nanoparticles showing different chemical information at the nanoparticle surface. In the IR spectra on the left panel of Figure 6, the intense and broad band around 3415 cm-1 can be ascribed to νO-H stretching.37 This band can be observed before the addition of citric acid, but after the addition of the citric acid there is a shoulder at 3178 cm-1 ascribed to hydrogen bonds between the water molecules and carboxylic groups. The peaks at 2925 cm-1 and 2856 cm1

appear only in the spectrum of the nanoparticle after the addition of citric acid and are

characteristic of C-H asymmetrical and symmetrical stretching vibrations. The strong peaks at 1625 and 1399 cm-1 correspond to the asymmetrical and symmetrical vibrations of the carboxylate group. The citrate layer on the nanoparticles surfaces is a key point to the hybrid material formation, since it provides a net of negative charge on the MgNPs that will interact with the CTAB polar head, as will be discussed later. The peak at 1625 cm-1 is also superimposed to the deformation vibrational peaks of water, also observed in the pure magnetite nanoparticle. The strong peak at 573 cm-1 is characteristic of νFe-O in the magnetite core and the peak at 1104 cm-1 can be ascribed to νSi-O stretching from alkaline reaction medium which could be worn the three-neck round-bottom flask38,39. After that, MgNPs were converted into MghNPs by a mild oxidation procedure through air bubbling during the synthesis. This conversion was monitored by powder X-ray diffraction and Raman spectroscopy by collecting the samples along the reaction, as we can see in Figure 7, and magnetic measurements (see Supporting Information, S21(B)). The magnetic behavior of the collected samples exhibited a saturation magnetization of 68.1 emu.g-1 Fe3O4 at room temperature, with negligible hysteresis, consistent with a superparamagnetic response. Its conversion into maghemite, leads to some decay of magnetism, corresponding to a saturation


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magnetization of 47.1 emu.g-1 γ-Fe2O3, keeping the same superparamagnetic behavior but now exhibiting a lighter color.

Figure 7. (Left) Series of powder diffraction for all the samples collected at different bubbling times of air, 1) 0 min; 2) 30 min; 3) 60 min; 4) 120 min respectively, and after nitric acid addition, 5). (Medium) The (511) shift confirms the transition of magnetite into maghemite40. (Right) Raman spectra of magnetic nanoparticles for the samples collected at different bubbling times of air, and after the addition of nitric acid, for comparison purposes. Magnetite crystal belongs to cubic space group Fd3m, thus, five Raman active modes (one A1g band, one Eg and three T2g) are predicted. In the Raman spectra of magnetite, only the Td sites occupied by Fe(III) ions contribute to Raman activity while Fe(II) is not directly involved. As shown in Figure 7, the most intense band of magnetite is observed at 668 cm-1, assigned to the A1g mode. Other three phonon modes have been reported for magnetite at 194 cm-1 (T2g), 303 cm-1 (Eg) and 528 cm-1 (T2g), leading to weak bands usually masked by the poor signal/noise ratio in this region. The broad bands at 371, 498, 719 and 1400 cm-1 in the Raman spectra are


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consistent with the formation of maghemite. It is important to note that after the addition of nitric acid, the Raman spectrum also undergoes a drastic change showing that the material is completed oxidized. As a matter of fact, hematite has a R3c crystal symmetry, giving rise to 2A1g and 5E1g Raman active bands, and a primary evidence is given by the presence of a strong and thin peak at 1320 cm-1 in the Raman spectra, associated with twice the longitudinal frequency of phonons of the crystal.41,42 In addition, there is a slightly difference between 60 min and 120 min of bubbling air that can be just seen in the Raman spectra. It is quite relevant to note that almost the whole material turns into maghemite as we could see through the last characterizations. In this way, it was not necessary to do further Mössbauer measurement since the powder X-ray diffraction, showed (511) shifts confirming the transition of magnetite into maghemite and Raman spectra was totally clear. As will be discussed later, the presence of maghemite is fundamental to reduce FRET and the QDs fluoresce quenching in the hybrid material. Hybrid nanoprobe formation and CTAB function Finally, the hybridization of QD nanoparticles with MghNPs was performed by using CTAB as an effective mediator. As shown in Figure 8, CTAB provides a convenient linker due to its long aliphatic carbon chain which decreases extensively the FRET mechanism between quantum dots and citrate-capped maghemite. It is important to say that the Förster critical distance, from 4 nm to 8 nm18, has to be imposed to preventing charge-transfer interactions. As we can see in Figure 8(B) and Figure 8(C) the CTAB provides distances varying between 3 and 5 nm.


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Figure 8. (A) Schematic representation of the composite and its average size as well as the distance between particles guaranteeing the Förster critical distance. (B) Original image obtained by High Resolution Transmission Electron Microscopy (HRTEM) of the hybrid magneto-


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fluorescent nanocomposite (scale bar: 10 nm). (C) Differentiation of the QD and MghNPs based on the crystallographic spacing distances (scale bar: 10 nm). (D) (1) and (2) are the MghNPs and MgNPs absorption spectra respectively. In green, comparison between the emission spectra of QD and MghNPs@QD emitting in the green region, MghNPs@QDG. In red, comparison between the emission spectra of QD and MghNPs@QD emitting in the red region, MghNPs@QDR. (E) Magnetization curves before, 1), and after, 2), the assisted oxidation of iron oxides nanoparticles (MgNPs and MghNPs respectively) and the hybrids nanoprobes magnetization, 3) and 4) (MghNP@QDR and MghNP@QDG respectively). To evaluate FRET mechanism in the hybrid material two different QDs were used, with emission maxima at 556 nm (QDG) and 657 nm (QDR). The emission spectra of the QDs and their magneto-fluorescent association are being compared in Figure 8(D). In the case of MghNPs@QDG the emission at 556 nm is shifted to 566 nm, with a small decrease of intensity. In contrast, in the MghNPs@QDR species the emission is practically preserved, showing only a small shift and decay. These results show that even in the case of maghemite, certain degree of FRET effect is occurring because of the small overlap between the emission spectra of QDs and the absorption spectra of maghemite nanoparticles, showed in the gray area in Figure 7D, and this effect is minimized in the MghNPs@QDR. In Figure S22 (see Supporting Information) we show that the decrease of intensity is due to de interaction between iron oxide nanoparticles and quantum dots, since de presence of CTAB does not causes any considerable changes in the QDs emission spectra. Through the magnetization characteristics, it is possible to note the preserved properties, like the superparamagnetic behavior, even after the hybrid nanoprobes formation as we can see in Figure 8E. Figure S23, (see Supporting Information), depicts time lapsed images of MghNPs@QDs


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nanocomposites illustrating the preserved magnetic properties after the hybrid formation. In this way, a better magnetic nanocomposite with higher fluorescence quantum yield can be achieved, in relation to the use of magnetite. Water-bath magnetic particle inspection and the use of hybrid materials in industry Water-bath MPI is a simple technique to identify failures in metallic materials in industry and the most common commercial MPIs are based on micrometric particles in association with organic species. Their functioning is based on the induced magnetic attraction to the fracture borders, exposing their precise location for visual fluorescence detection (See Supporting Information, S24). For this, the use of nanoparticles can greatly improve the spatial resolution of such inspection baths, allowing to detect the occurrence of nanometric flaws. Another motivation of their uses is related to the lowest concentration that is necessary to identify the flaw (at around 10-7 mol.L-1). In this way it is easier to purify and discard any type of industrial waste. To assess the performance of MghNPs@QD as MPI agents, firstly, we used an official high coercivity magnetic stripe employed for MPI Bath Evaluation with inter stripe separation of 700 µm43 as shown in Figure 9. In a typical test we dropped a solution containing MaghNPs@QDs on the card and waited a few seconds or minutes until most of the acetone solvent evaporated. Since our agent has a superparamagnetic behavior it does not retain any magnetic information and can be easily washed away after removing the applied field, thus avoiding any background interference. It is important to point out that the hybrid nanoprobe must be purified before the test. After citrate-functionalized iron oxide nanoparticles synthesis and further oxidation, the material was sedimented by a magnetic field and washed with water 3 (three) times and, after the quantum dots synthesis, the material was precipitated in acetone 3


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(three) times and centrifuged por 10 minutes in 10000 rpm. During the hybrid nanoprobe formation, just to guarantee that there is no free CTAB, it was centrifuged per 5 min in 10000 rpm. At the end of this process, we used a magnetic gradient to separate the unreacted iron oxide nanoparticles (bigger magnetic response) from the hybrid nanoprobe (lower magnetic response) using a magnetophoretic technique30

Figure 9. Reading the information of an official high coercivity magnetic stripe for MPI Bath Evaluation, by applying the MghNPs@QDs particles and examining under UV light. (A) High coercivity magnetic stripe for MPI bath evaluation as a body proof. (B) Same body proof after applying the hybrid nanoprobes. (C) Revealing the code bar by the MghNPs@QDG. (D) Revealing the code bar by the MghNPs@QDR. (E) and (G) Optical images of one magnetic line where the quantum dots are expected to stay. (F) and (H) Same region of (E) and (G) under 488nm excitation proving the specificity of the hybrid multifunctional materials. In the first part of the Figure 9, the magnetic stripes in the original card, used as a proof body (A), is not visible under any light, but after applying a drop of the hybrid nanoprobe onto the magnetic stripe (B), the code bar patterns are clearly revealed, both using MghNPs@QDG, (C), and MghNPs@QDR, (D). This test corroborates the potential use of the magneto-fluorescence nanoparticles in water-bath MPI tests, as well as for capturing magnetic information, processing


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magnetic codes and images for many industrial applications. The characterization was carried out by Raman confocal image in a scanning fluorescence technique, as we can see in Figure 9(F) and Figure 9(H), proving the hybrid multifunctional material selectivity to the magnetic striped and their potential as a nanoprobe in the water-bath magnetic particle inspection (MPI). To evaluate the real applicability of the new hybrid nanoprobes synthesized in this work, we performed a standard test according to the NDT procedure, as shown in Figure 10. For this purpose, two distinct probes have been employed: a Ball Joint (spherical bearings that connect the control arms to the steering knuckles, used on virtually every automobile by working similarly to the ball-and-socket design of the human hip joint) and a Reference Block (disc with coarse and fine cracks in the surface, produced by grinding and stress corrosion). MghNPs@QDR shows an excellent capability to recognize the flaws. In addition, the concentration of the fluorescent species in the new hybrid nanoprobe is much lower in relation to the conventional MPI agents (e.g. typically around 1.0 g.L-1). As shown in the Supporting Information, S25, a good visual comparison can be obtained for the conventional MPI agent and MghNPs@QDR but using much less material in our case.


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Figure 10. (A) A real Ball Join and Reference Block (B) used in the MPI test. (C) Zoom in the Ball Joint showing the location of the tiny flaw. (D) Reference Block under the 365 nm excitation in the presence of the MghNPs@QDR revealing the tine fractures. (E) Ball Joint under 365 nm excitation in the presence of the MghNPs@QDR, revealing the tine flaw as indicated by the white arrow. (F) Comparing with (D), if we increase the concentration of the MghNPs@QDR solution until 10-4 mol.L-1 the visual signal is intensified without any kind of visual background. CONCLUSION In summary, kinetic controlled synthesis, monitored by a real-time image analysis, allowed to obtain nanocrystalline 3-mercaptopropionic acid-capped cadmium telluride quantum dots of specific sizes, displaying selective emission in the visible. These QDs were successfully hybridized with nanocrystalline citrate-functionalized maghemite nanoparticles by using CTAB


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as a linker and spacer to maintain QD’s strong emission properties and overcoming the critical limitations from the FRET mechanisms. As showed using real metallic parts, the hybridized fluorescent magnetic nanoparticles provide suitable agents for magnetic sensing in ferrous materials for MPI bath evaluation technologies as well as many other future interesting applications including bioimaging. For that application, it is quite relevant to know that quantum dots can be cytotoxic. However, it will depend on their physicochemical properties and environmental factors. Their sizes, charges, concentration, outer coating bioactivity (capping material and functional groups), and oxidative, photolytic, and mechanical stability will have been implicated as determining factors in QD toxicity allowing us to plan specific ways to apply this hybrid nanoprobes in bioimaging44. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available on the ACS Publications website at DOI: Sodium telluride synthesis; Detailed experimental setup for real-time analyses; Excitation spectrum of the quantum dots; TXRF, ICP-OES, CHN, TG, TG-MS, FTIR, Raman and DFT calculation to study the synthetic precursors of QD synthesis; Average nanoparticles size evaluated from TEM images; Electronic absorption spectrums of QD in the ratio 1:2.3 during the nanoparticles growth; RGB correlation, a compilation of absorption and emission spectrum of all the QDs and RGB study of different synthetic conditions; TXRF of QD confirming the presence of S, Cd and Te, DLS and VSM measurements. Timelapse of the hybrid nanoprobes in the presence of a magnetic field and real MPI agents acting as revealing probes in comparison with the new nanoprobes presented here. (PDF) Sodium telluride synthesis; Detailed experimental setup for real-time analyses; Excitation spectrum of the quantum dots; TXRF, ICP-OES, CHN, TG, TG-MS, FTIR, Raman and DFT calculation to study the synthetic precursors of QD synthesis; Average nanoparticles size evaluated from TEM images; Electronic absorption spectrums of QD in the ratio 1:2.3 during the nanoparticles growth; RGB correlation, a compilation of absorption and emission spectrum of all the QDs and RGB study of different synthetic conditions; TXRF of QD confirming the presence of S, Cd and Te, DLS and VSM measurements. Time-


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lapse of the hybrid nanoprobes in the presence of a magnetic field and real MPI agents acting as revealing probes in comparison with the new nanoprobes presented here. (DOCS) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; Notes The authors declare no competing financial interest Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We greatly acknowledge the support from São Paulo Research Foundation (FAPESP) (Grant 2013/24725-4) and National Council for Scientific and Technological Development (CNPq) (Grant 482383/2013-5). ACKNOWLEDGMENT The authors acknowledge Marcelo Nakamura for assistance with AFM images, National Centre of Research in Energy and Materials (CNPEM), and Fernando Dias from Metal-Chek that provided us the real samples and the usual images of conventional MPI agents for comparison purposes. REFERENCES (1)

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Superparamagnetic Maghemite-Based CdTe Quantum Dots as

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