Composition-Tunable Optical Properties of Zn x Cd(1 − x)S Quantum Dot–Carboxymethylcellulose Conjugates: Towards One-Pot Green Synthesis of Multifunctional Nanoplatforms for Biomedical and Environmental Applications
© The Author(s). 2017
Received: 4 May 2017
Accepted: 25 June 2017
Published: 5 July 2017
Quantum dots (QDs) are colloidal semiconductor nanocrystals with unique properties that can be engineered by controlling the nanoparticle size and chemical composition by doping and alloying strategies. However, due to their potential toxicity, augmenting their biocompatibility is yet a challenge for expanding to several biomedical and environmentally friendly applications. Thus, the main goal of this study was to develop composition-tunable and biocompatible Zn x Cd1 − x S QDs using carboxymethylcellulose polysaccharide as direct capping ligand via green colloidal aqueous route at neutral pH and at room temperature for potential biomedical and environmental applications. The ternary alloyed QDs were extensively characterized using UV–vis spectroscopy, photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), and X-ray photoelectrons spectroscopy (XPS). The results indicated that Zn x Cd(1 − x)S QDs were surface stabilized by carboxymethylcellulose biopolymer with spherical morphology for all composition of alloys and narrow sizes distributions ranging from 4 to 5 nm. The XRD results indicated that monophasic ternary alloyed Zn x Cd1 − x S nanocrystals were produced with homogenous composition of the core as evidenced by EELS and XPS analyses. In addition, the absorption and emission optical properties of Zn x Cd1 − x S QDs were red shifted with increasing the amount of Cd2+ in the alloyed nanocrystals, which have also increased the quantum yield compared to pure CdS and ZnS nanoparticles. These properties of alloyed nanomaterials were interpreted based on empirical model of Vegard’s law and chemical bond model (CBM). As a proof of concept, these alloyed-QD conjugates were tested for biomedical and environmental applications. The results demonstrated that they were non-toxic and effective fluorophores for bioimaging live HEK293T cells (human embryonic kidney cells) using confocal laser scanning fluorescence microscopy. Moreover, these conjugates presented photocatalytic activity for photodegradation of methylene blue used as model organic industrial pollutant in water. Hence, composition-tunable optical properties of ternary Zn x Cd1 − x S (x = 0–1) fluorescent alloyed QDs was achieved using a facile eco-friendly aqueous processing route, which can offer promising alternatives for developing innovative nanomaterials for applications in nanomedicine and environmental science and technology.
Traditionally, alloys have been used for several centuries to create new materials with improved mechanical, structural, thermal, electrical properties leading to the development of high-performance materials, which cannot be achieved with each component separately. However, only very recently the interest in nanoalloys (also referred to as alloy nanoclusters or alloy nanoparticles) arises, as they constitute a new type of advanced nanoscale materials, which can have unique properties very distinct from those of individual atoms and molecules or original bulk matter [1–4]. One of the major reasons for interest in alloy nanoparticles is the fact that their chemical and physical properties may be tuned by varying the composition and atomic ordering as well as the dimension of the clusters [4, 5]. Besides the metal-based nanomaterial alloys, nano-sized semiconductor nanocrystals (referred to as quantum dots, QDs) have increasingly called the attention of the scientists, researchers, and manufacturers because of their broad range of potential applications in electronics, optics, magnetic, sensors, biosensors, and biomedical fields [6–11]. In that sense, quantum dots based on metal chalcogenide alloys, mostly of group II–VI semiconductors (type MX, M = Cd, Zn, Pb; X = Te, Se, S), are currently under intensive study in many research fields such as in optoelectronics, high-density memory, quantum-dot lasers, and lately for biosensing and biolabeling because they exhibit tunable optical properties (i.e., bandgap energy structure) by adjusting the chemical composition and size [5, 7, 12–15]. Since the bandgap engineering of ternary and quaternary alloyed QDs can be achieved via controlling their composition (relative constituent stoichiometry) in addition to their sizes and internal structures, it is therefore feasible to design and tune their optical properties, which are not readily viable to binary QDs [16–19]. This can be achieved by creating a solid solution (i.e., an alloy) of two semiconductors with different energy gaps, where an increase in the bandgap energy is generally observed with increasing the concentration of the wider bandgap semiconductor, either with cation (i.e., metal constituent) or anion (i.e., chalcogenide constituent) alloyed QDs [20, 21]. Nonetheless, the development of QDs based on Zn–Cd–S alloys using one-pot “greener” aqueous processes with biocompatible ligands are narrowly reported in the literature, where the large majority of studies report the production of QDs at high temperatures by organometallic routes, microwave-assisted synthesis, and using toxic organic solvents [15, 17, 18, 21–25]. High-temperature decomposition methods based on trioctylphosphine oxide (TOPO), commonly used as capping ligand for improved quantum yields, results in hydrophobic QDs insoluble in aqueous medium. However, biological and medical applications require QDs that are water-soluble at physiological conditions, biocompatible, and functionalized with biomolecules for targeting purposes. Hence, the development of novel strategies of surface functionalization and bioconjugation remains an important challenge. Yet, no report was found in the consulted literature addressing the synthesis of Zn x Cd1 − x S QDs functionalized with biopolymer ligands based on cellulose derivatives for multiple potential purposes, including cell bioimaging and photocatalytic activity for the degradation of organic dye pollutants.
Thus, herein, we report the synthesis and comprehensive characterization of novel ternary alloyed Zn x Cd1 − x S QDs (x = 0 → 1) with composition-tunable optical properties using carboxymethylcellulose as a biocompatible and eco-friendly capping ligand produced directly by means of a single-step green colloidal process in aqueous media at room temperature. They proved to be suitable nanoplatforms for live cell bioimaging or heterogeneous photocatalysis of methylene blue organic dye.
All of the reagents and precursors, including zinc chloride (Sigma-Aldrich, USA, ≥98%, ZnCl2), cadmium perchlorate hydrate (Aldrich, USA, Cd(ClO4)2·6H2O), and sodium sulfide hydrate (Synth, Brazil, >98%, Na2S·9H2O) were used as received. Carboxymethylcellulose sodium salt (CMCel, Fluka Chemical, USA), degree of substitution (DS) 0.84 and medium viscosity (870 mPa s, 2% in H2O) was used as capping ligand. Deionized water (DI water, Millipore SimplicityTM) with a resistivity of 18 MΩ cm was used to prepare the solutions and the procedures were performed at room temperature (RT, 23 ± 2 °C), unless specified otherwise.
Synthesis of Quantum Dot Conjugates—Zn x Cd1 − x S/CMCel
CMCel solution (1% w/v) was prepared by adding sodium carboxymethylcellulose powder (0.5 g) to a 50 mL of water and stirring at room temperature until complete solubilization occurred. For controlling the molar ratios of cations, premixed Zn2+ and Cd2+ solutions were prepared from the individual stock solutions (ZnCl2 and Cd(ClO4)2·6H2O) at 1 × 10−3 mol L−1 total cation concentration (M) but with Zn2+:Cd2+ molar ratios of 100:0 (x = 1.0), 75:25 (x = 0.75), 50:50 (x = 0.50), 25:75 (x = 0.25), and 0:100 (x = 0).
Zn x Cd1 − x S nanoparticles were synthesized via an aqueous route at room temperature as follows: 2 mL of CMCel solution and 45 mL of deionized water were added to a flask. The pH was measured and it was close to neutral (pH∼7.0). Under magnetic stirring, 20.0 mL of the metal precursor solution (1 × 10−3 mol L−1) at the Zn2+:Cd2+ different molar ratios and 4.0 mL of the sulfide source solution (Na2S·9H2O, 1.0 × 10−2 mol L−1) were added to the flask (the S2−:M2+ molar ratio was kept at 2:1, i.e., excess of sulfide) and stirred for 10 min. The QDs colloidal dispersions produced were stable and homogeneous. ZnS dispersion was colorless while CdS dispersion was light yellow. The dispersions of ternary Zn x Cd1 − x S nanoparticles exhibit colors between clear and light yellow with continuous color gradient as a function of the Zn2+:Cd2+ molar ratio.
These colloidal dispersions were concentrated and purified using an Amicon® Ultra Filter (Millipore) with a 100,000 molecular mass (MW) cut-off cellulose membrane. Centrifugation was conducted and, after the first cycle, the QDs were washed 4 times with DI water. Centrifugal forces caused the removal of excess reagents through the membrane into a filtrate vial. After purification, the samples were stored at RT until further use.
Physicochemical Characterization of Quantum Dot Conjugates
Ultraviolet–visible (UV–vis) spectroscopy measurements were performed using Perkin-Elmer, Inc. (USA) equipment (Lambda EZ-210) in transmission mode with samples in a quartz cuvette over a wavelength range between 600 and 190 nm. All of the experiments were conducted in triplicate (n = 3) unless specifically noted, and the data were presented as the mean ± standard deviation.
The photoluminescence spectroscopy (PL) of the Zn x Cd1 − x S/CMCel conjugates was performed based on spectra acquired at RT using a violet diode laser module at λ exc = 405 nm (150-mW, Roithner LaserTechnik, Germany) coupled to a USB4000 VIS-NIR spectrophotometer (Ocean Optics, Inc., USA). All of the tests were performed using a minimum of four repetitions (n ≥ 4). Quantum yield (QY) was measured according to the procedure using Rhodamine 6G (Sigma, USA) in ethanol as the standard at λ excitation = 405 nm . Nanostructural characterization of the QDs was based on the images and electron diffraction patterns (ED) using Tecnai G2-20-FEI (FEI Company, USA) transmission electron microscope (TEM) at an accelerating voltage of 200 kV. In all of the TEM analyses, the samples were prepared by placing a drop of a dilute QD suspension onto carbon-coated copper grids (Electron Microscopy Sciences, USA) and allowing them to dry at room temperature overnight. The QD size and size-distribution data were obtained based on the TEM images by measuring at least 150 randomly selected nanoparticles using image processing program (ImageJ, version 1.50, public domain, National Institutes of Health) .
X-ray diffraction (XRD) patterns were recorded using PANalytical (UK) Empyrean diffractometer (Cu–Kα radiation with λ = 1.5406 Å). Measurements were performed in the 2θ range from 3° to 70° with steps of 0.06°. For the sample preparation, concentrated colloidal QD dispersions were dropped onto glass slides and oven dried at 40 ± 1 °C for 12 h.
Energy-filtered transmission electron microscopy (EFTEM) with low-loss electron energy-loss spectroscopy (EELS) was performed by using Tecnai G20 TEM operating at 200 kV accelerating voltage and equipped with GATAN GIF energy imaging filter. The cadmium composition map was obtained using the Cd M edge at 404 eV, and the Zn composition map was obtained using the Zn L edge at 1020 eV, using similar sample preparation procedure of TEM analysis.
X-ray photoelectron spectroscopy (XPS) analysis was performed using Mg–Kα as the excitation source (Amicus spectrometer, Shimadzu, Japan). All peak positions were corrected based on C 1s binding energy (284.6 eV). For sample preparation, concentrated QD colloidal medium was dropped onto glass slides and dried in a vacuum desiccator at RT for 48 h.
Dynamic light scattering (DLS) and zeta potential (ZP) analyses were performed using ZetaPlus instrument (Brookhaven Instruments Corporation, 35-mW red diode laser light, wavelength λ = 660 nm) with a minimum of ten replicates. The ZP measurements were performed at 25.0 °C ± 2 °C under the Smoluchowski approximation method. For the DLS measurements, the colloidal solutions of QDs were filtered three times through a 0.45-μm aqueous syringe filter (Millex LCR 25 mm, Millipore).
Biological Characterization of QD Conjugates
Evaluation of Cytotoxicity by MTT Cell Viability Assay
MTT (3-(4,5-dimethylthiazol-2yl) 2,5-diphenyl tetrazolium bromide) experiments were performed to evaluate the toxicity of QDs dispersions. MTT assays were conducted according to ISO 10993-5:2009 (Biological evaluation of medical devices: Tests for in vitro cytotoxicity) using kidney cell line of a human embryonic culture (HEK293T). HEK293T cells were kindly provided by Prof. M.F Leite (Department of Physiology and Biophysics, UFMG), and they were cultured in DMEM with 10% FBS, penicillin G sodium (10 units mL−1), streptomycin sulfate (10 mg mL−1), and amphotericin-b (0.025 mg mL−1), all from Gibco BRL (NY, USA), in a humidified atmosphere of 5% CO2 at 37 °C.
Cellular Uptake of QD Conjugates by Laser Scanning Confocal Microscopy
The evaluation of the QD conjugates as fluorescent biological probes was performed using confocal laser scanning microscopy after exposing ZnS and Zn0.50Cd0.50S QDs to HEK293T cells. For the cellular internalization evaluation, HEK293T cells on passage 68 were plated (5 × 104 cells per well) in 24-well plate. The cells were incubated for 4 days in 5% CO2 at 37 °C and synchronized for 24 h, and the ZnS and Zn0.50Cd0.50S colloidal suspensions with the medium solution (DMEM) at final concentration of 500 nM were added to the cells. Next, the cells were incubated in 5% CO2 at 37 °C for 1 h and washed with phosphate buffered saline (PBS, Gibco, Brazil). In the sequence, the cells were fixed with paraformaldehyde (4%) for 30 min, washed three times with PBS, and cover slips were mounted with Hydromount (Fisher Scientific Ltd., UK) for posterior analysis in confocal laser scanning fluorescence microscopy (Zeiss LSM Meta 510, Carl Zeiss; excitation 488 nm argon laser; emission filter 505–530 nm). For the control, cells were incubated with only in the original medium with 10% FBS (autofluorescence).
Photocatalytic Activity of QD Conjugates for Environmental Applications
The photocatalytic activities of the ZnS and Zn0.50Cd0.50 conjugates were evaluated via the photocatalytic degradation of molecule methylene blue (MB) as model organic pollutant molecule under ultraviolet (UV) light irradiation.
Results and Discussion
Physicochemical Characterization of Quantum Dot Conjugates
Bandgap values for the prepared metal sulfide QDs (E QD) at different proportions of [Zn,Cd]:S were extracted from the UV–vis absorbance curves using the “TAUC relation” (Fig. 1B). The estimated E QD values are 3.90, 3.20, 2.85, 2.61, and 2.57 eV for ZnS, Zn0.75Cd0.25S, Zn0.50Cd0.50S, Zn0.25Cd0.75S, and CdS, respectively. Therefore, the tunable bandgap effect was indeed verified, as the excitonic transitions for the Zn x Cd1 − x S ternary solid solutions (Fig. 1A (b–d)) continuously red shifted (i.e., shifted to lower energy) with increasing the amount of Cd2+, which demonstrated the formation of Zn x Cd1 − x S nanoalloys.
Summary of types and energies of Zn x Cd1 − x S QD emissions
Defect activated emission (eV)
First, there is a non-radiative pathway from valence band and energy level of VS due to shallow trap states. In the sequence, the emission E1 can be associated with transitions involving sulfur vacancies (VS) and valence band (VB). Emission E2 can be assigned to the recombination of trapped electrons at Vs and holes trapped at interstitial metal (IS) point defects and E3 is attributed to the recombination between vacancy trap states (VS–VM). The minor emission bands at higher wavelengths can be assigned to surface defects [33, 35]. Despite the nature of the emission, peaks in the PL spectra shifted to lower energy with the increase of Cd molar ratio in the QDs demonstrated the composition-tunable emission effect of nanoalloys. These values of red shift observed in the PL spectroscopy support the results of previous sections, and they are further strong evidence of the formation of alloyed nanocrystals.
Nonetheless, a distinct behavior was observed for the physical properties (e.g., interplanar distances), where a linear correlation was observed by varying the ternary alloy composition in agreement with the literature . Figure 5b shows the lattice spacing for (111) and (220) planes calculated from ED patterns (TEM images). The interplanar distances exhibit a linear behavior upon changing the x values according to Vegard’s law evidencing that the Zn0.50Cd0.50S system is an alloyed QD instead of a mixture of CdS and ZnS nanocrystals.
Zeta potential (ζ) measurements evidenced the interactions of QDs with carboxylate groups at the nanoparticle–polymer interfaces and the role of these groups in the stabilization of the QDs. The measured ζ-potentials were all negative values, −52 ± 5 mV (ZnS), −53 ± 6 mV (ZnCdS), and −56 ± 8 mV (CdS), lower than −72 mV of CMCel solution, probably because of metal complexes remained at the nanocrystal–polymer interface (QD–CMCel). In addition, these highly negative values (i.e., ζ ≤ −50 mV) indicated that the nanoparticles were electrostatically stabilized by CMCel carboxylate functional groups avoiding the growth and agglomeration of the nanocrystals, which is crucial for quantum-size confinement effects. Moreover, the morphological and stability features of these colloidal ZnCdS conjugates in aqueous medium were assessed by DLS analysis. Thus, the DLS results showed that the systems were produced with hydrodynamic diameters (HD) of 12.5 ± 1.3 nm, 11.8 ± 0.8 nm, and 8.0 ± 0.5 nm for ZnS, ZnCdS, and CdS, respectively. The HD is assigned to the sum of contributions from the inorganic QDs “core” and the CMCel organic “shell” of the conjugates, including the effect of solvation layers and the lateral extension of the capping ligands. The DLS measurements clearly indicated a lower volume of solvation for the CdS that may be associated with the type, extension, and/or stability of the Cd2+ chelate complex with chemical groups of the carboxymethylcellulose compared to Zn2+, which probably caused the higher contraction of the polymeric shell around the Cd-based QD inorganic core.
Biological Characterization of Zn x Cd1 − x S QD Conjugates
Evaluation of Cytotoxicity by MTT Cell Viability Assay
Cellular Uptake of Zn x Cd1 − x S QD Conjugates by Laser Scanning Confocal Microscopy
In the present study, as a proof of concept, the uptake process of Zn0.50Cd0.50S and ZnS QD conjugates by live cells was performed by laser scanning confocal microscopy. CdS conjugates were not tested as it has already proved to reduce cell viability by the MTT assay results discussed in the previous section. On the other hand, ZnS QDs have been reported to be safe and non-toxic for in vitro endocytosis and cellular imaging assays. Thus, for the cellular internalization assay, HEK293T cells were incubated for 1 h with Zn0.50Cd0.50S and ZnS conjugates at final concentration of 500 nM. After this period of incubation, images were captured using laser confocal microscopy for the purpose of visualizing the internalization of the developed surface-biofunctionalized ZnS and Zn0.50Cd0.50S QDs. Figure 9B shows that the Zn0.50Cd0.50S (A) and ZnS (B) conjugates were effectively internalized by HEK293T cells. Based on these confocal microscopy images, it was verified that Zn0.50Cd0.50S and ZnS conjugates were cytocompatible as they penetrated through the cell membrane and were found homogeneously distributed inside the cellular cytoplasm. Therefore, these findings combined with MTT results highlight the suitability of using ZnS and alloyed Zn0.50Cd0.50S QDs capped by CMCel ligand as fluorescent nanoprobes for in vitro cellular imaging and labeling applications. Although it is a fascinating area of research, a more in-depth investigation is beyond the scope of this work and will certainly be subject of future studies.
Photocatalytic Activity of Zn x Cd1 − x S QD Conjugates for environmental Applications
Analogously, it can be observed the reduction of absorbance associated with the color of MB at λ = 664 nm (Fig. 10B) caused by the photo-oxidation of the MB molecule, shifting the absorption peaks to lower wavelength (blue shift). This photodegradation is related to chemical species generated by electron–hole pairs (QD + hυ → h+/e−) from irradiated QDs (i.e., ZnS or Zn0.50Cd0.50S, Fig. 10A, inset drawing). The oxidation mechanism is primarily governed by the valence band holes (h+), which are powerful oxidants, and they can react with water or surface-bound chemisorbed hydroxyl groups (OH−) producing hydroxyl radicals (OH•). Most organic photodegradation reactions utilize the strong oxidizing power of the holes directly or indirectly produced by excitation of nanomaterials [64–66]. It is reasonable to consider that this degradation of MB may have followed a double-stage process. Initially, the negatively charged CMCel polymer is likely to have attracted MB molecules (i.e., positively charged) leading to the adsorption process, which does not degrade or promote decolorization of MB. In the sequence, the photo-oxidation of the MB dye occurred by the photocatalytic process, which was enhanced by previous adsorption favoring the charge exchange at the conjugate-MB interfaces (i.e., h+/e− scavenging).
Regarding to toxicity, some researchers have raised concerns about the environmental safety related to the use of nanomaterials composed with toxic elements such as Cd-containing QDs motivated by the possibility of photo-oxidation with eventual releasing of hazardous elements in the medium [67, 68]. However, despite the hypothetical possibility, it is not likely to occur in these alloyed conjugates because of the biocompatible organic shell encapsulating the inorganic core, the extremely low concentration of the metallic elements (i.e., from μM to nM), associated with the very low water solubility of their compounds (e.g., oxides, sulfates, hydroxides) minimizing eventual risk of contamination.
Thus, these results demonstrated that ZnS and Zn x Cd1 − x S semiconductor conjugates were surface functionalized by carboxymethylcellulose using a novel green sustainable process, which can offer promising alternatives as nanoplatforms in environmental and biomedical applications.
In this study, it is presented the synthesis and characterization of new ternary alloyed Zn x Cd1-x S semiconductor QDs using carboxymethylcellulose as a biocompatible polymer capping ligand directly produced via eco-friendly colloidal process in aqueous media and at room temperature. XPS results evidenced the mechanism of stabilization of the Zn x Cd1 − x S nanoalloys dominated by the chemical interactions of metal-rich surface (Zn2+ > Cd2+ > S2−) with the carboxylates and hydroxyls groups of the polymer ligands. The EELS, TEM, XRD, and selected area electron diffraction analyses ruled out the formation of core-shell or multiphasic systems, demonstrating that homogenous Zn x Cd1 − x S nanoalloys were produced with optical absorption and emission dependent on the concentration of Zn2+. In addition, it was observed that the composition of ternary Zn0.5Cd0.5S alloys improved the luminescence quantum yield compared to the pure binary systems (ZnS and CdS QDs). These optical properties of Zn x Cd1 − x S nanoalloys were studied based on empirical model of Vegard’s law and chemical bond model (CBM), where the differences observed of the values were assigned to the bandgap structure of each system. Moreover, in order to provide insights about potential applications of these alloyed-QD conjugates, they were tested for live cell imaging and for photocatalysis of organic molecules. The results demonstrated that ZnS and Zn0.5Cd0.5S conjugates were non-toxic and behaved as effective fluorophores for in vitro imaging live human embryonic kidney cells (HEK293T) with confocal fluorescence microscopy. Additionally, these alloyed-QD conjugates presented photocatalytic activity for photodegradation of methylene blue, which was used as model organic industrial pollutant in water. Hence, composition-tunable optical properties of ternary Zn x Cd1 − x S (x = 0–1.0) fluorescent alloyed QDs was verified based on a facile eco-friendly process. It is foreseen that this class of alloyed fluorescent semiconductor nanocrystals offers several possibilities of bandgap engineering for multiple applications such as biolabeling and bioimaging in nanomedicine or as nano-photocatalyst for environmental purposes in water treatment.
The authors acknowledge the financial support from the following Brazilian research agencies: CAPES—Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (PROEX-433/2010;PNPD;PROINFRA2010-2014), FAPEMIG—Fundação de Amparo à Pesquisa do Estado de Minas Gerais (PPM-00202-13;BCN-TEC 30030/12), CNPq—Conselho Nacional de Pesquisa (PQ1B–306306/2014-0; UNIVERSAL-457537/2014-0), and FINEP—Financiadora de Estudos e Projetos (CTINFRA-PROINFRA 2008/2010/2011). The authors express their gratitude to the staff at the Microscopy Center at UFMG for their assistance with TEM–EFTEM–EELS analysis and Prof. A. Bicalho for XRD experiments. The authors thank the staff at the Center of Nanoscience, Nanotechnology and Innovation-CeNano2I/CEMUCASI/UFMG for the spectroscopy analyses. Finally, the authors thank Universidade Federal de Minas Gerais—UFMG/PRPq for the financial support for the publication of this study (PRPq—02/2017).
HSM carried out the experimental design and analysis and drafted the manuscript. AAPM carried out the synthesis, physicochemical characterization, and analysis of conjugates and drafted the manuscript. RLM carried out the synthesis, physicochemical characterization, and analysis of conjugates. AJC designed, performed, and analyzed TEM, SAED, EFTEM-EELS experiments. LCO designed, performed, and analyzed the photocatalysis experiments. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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