Biocompatible Fluorescent Core-Shell Nanoconjugates Based on Chitosan/Bi2S3 Quantum Dots
© Ramanery et al. 2016
Received: 7 February 2016
Accepted: 4 April 2016
Published: 12 April 2016
Bismuth sulfide (Bi2S3) is a narrow-bandgap semiconductor that is an interesting candidate for fluorescent biomarkers, thermoelectrics, photocatalysts, and photovoltaics. This study reports the synthesis and characterization of novel Bi2S3 quantum dots (QDs) functionalized using chitosan (CHI) as the capping ligands via aqueous “green” route at room temperature and ambient pressure. Transmission electron microscopy (TEM), UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, dynamic light scattering (DLS), and zeta potential (ZP) analysis were used to characterize the hybrids made of biopolymer-functionalized Bi2S3 semiconductor nanocrystals. The results demonstrated that the CHI ligand was effective at nucleating and controlling the growth of water-soluble colloidal Bi2S3 nanoparticles. The average sizes of the Bi2S3 nanoparticles were significantly affected by the molar ratio of the precursors but less dependent on the pH of the aqueous media, leading to the formation of nanocrystals with average diameters varying from 4.2 to 6.7 nm. These surface-modified Bi2S3 nanocrystals with CHI exhibited photoluminescence in the visible spectral region. Moreover, the results of in vitro MTT (3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide) assay with human osteosarcoma cells (SAOS) cell line demonstrated no cytotoxic response of the nanoconjugates.
Furthermore, the results indicated that the Bi2S3 QD–CHI nanoconjugates showed HEK293T cell uptake; therefore, they can be potentially used as novel fluorescent nanoprobes for the in vitro bioimaging of cells in biomedical applications.
In recent years, the field of colloidal semiconductor nanocrystals, also referred to as colloidal quantum dots (QDs), has grown rapidly. The developments are the result of significant advances in nanoscience and nanotechnology that predominantly focus on biomedical and environmental applications . The interdisciplinary contributions from several areas such as materials science, chemistry, and physics, combined with biology, pharmaceutics, medicine, and environmental science have created a fascinating new class of hybrid nanomaterials or nanoconjugates. These nanomaterials can be designed and engineered with almost any property or to carry out almost any function . Basically, these nanosized conjugates combine the intrinsic functions of inorganic semiconductor nanomaterials and the versatile organic biointerfaces offered by polymers (e.g., chitosan, PVA, PEG) and biomolecules (e.g., amino acids, peptides, proteins, DNA) [3, 4]. In the realm of inorganic low-dimensional materials for producing nanoconjugates and nanostructures, QDs have been the major choice because of their unique combination of optical, electronic, magnetic, and chemical properties, which can be tuned via the modification of the nanoparticle size below the threshold value, named the Bohr radius [1, 5, 6]. In particular, the interest in narrow-bandgap materials such as bismuth chalcogenides (e.g., Bi2X3, X = S, Se, Te) nanocrystals has intensified in recent years [7, 8]. Studies from around the world suggest that these materials are realistic prospects for applications including solar cells, infrared optoelectronics (e.g., lasers, optical modulators, photodetectors, and photoimaging devices), low-cost/large-format microelectronics, and biological imaging and biosensor systems [7, 9].
However, due to their extremely low dimensions at the nanoscale and exceptionally high surface-area-to-volume ratio, these fluorescent nanocrystals must be stabilized by capping agents during their synthesis to restrict the growth of formed nuclei . Hence, QDs have been produced using numerous processes such as in the pioneer studies entrapped in glasses [11, 12] or molecular films , encapsulated in polymer nanoparticles , dispersion in organic solvents , and colloidal dispersions [16, 17].
Nevertheless, despite almost three decades of advances in QD synthesis, the majority of the reported methods rely on organometallic processing routes that employ toxic solvents at high temperature and that lead to the formation of nanocrystals with hydrophobic surfaces [1, 5, 18]. Thus, synthesis of semiconductor QDs using an aqueous colloidal process is an attractive alternative to organometallic routes, which have received increasing concern because of their use of chemical processes that can be harmful to humans and to the environment [17, 19, 20]. In support of this, new environmentally friendly processes for producing QDs have been reported recently in the literature. Most studies employ the use of water-based colloidal routes using environmentally friendly and biocompatible reagents and precursors at low temperatures [17, 21]. In addition, colloidal chemistry provides a flexible platform for the surface functionalization of the QDs by applying an appropriate capping ligand, which in turn can simultaneously stabilize the inorganic semiconductor core of the nanoparticles and form an organic shell with biochemical functionalities for further applications .
Among several alternatives for capping agents, biopolymers, such as chitosan and its derivatives, have recently been proposed as a greener nanoplatform for producing water-soluble quantum dots. These polymers are intrinsically biocompatible, and they can be directly used as ligands for stabilizing QDs in aqueous media . Chitosan is a natural biopolymer that is commonly produced from the alkaline deacetylation of chitin, which is mostly extracted from the exoskeleton of marine crustaceans. As a biopolymer, it has been broadly used in numerous biomedical and environmental applications due to its biocompatibility, biodegradability, commercial availability, and worldwide abundance associated with its eco-friendly properties . Surprisingly, few studies have reported the preparation of nanomaterials based on bismuth sulfide (Bi2S3), such as quantum dots  and nanorods [9, 23], but no research investigating Bi2S3–chitosan nanoconjugates was found in the consulted literature.
Thus, in this study, new carbohydrate-based nanoconjugates combining chitosan with Bi2S3 semiconductor QDs were designed and synthesized via a single-step “green” aqueous colloidal process at room temperature. The results demonstrated that chitosan was an effective polymer ligand for nucleating and stabilizing ultra-small Bi2S3 QDs, forming colloidal core-shell nanostructures in aqueous dispersions. In addition, it was verified that variation of pH and molar ratio of precursors during the synthesis affected the physico-chemical properties and morphological aspects of the nanostructures. Moreover, these water-soluble nanoconjugates were photoluminescent under light irradiation and biocompatible toward SAOS cell culture, which can be potentially used as narrow-bandgap fluorophores in biomedical and pharmaceutical applications using an environmentally friendly process.
All of the reagents and precursors, including bismuth chloride (Aldrich, USA, ≥ 98%, BiCl3), sodium sulfide (Synth, Brazil, > 98 %, Na2S·9H2O), sodium hydroxide (Merck, USA, ≥ 99 %, NaOH), and acetic acid (Synth, Brazil, ≥ 99.7 %, CH3COOH) were used as received. Chitosan (Aldrich Chemical, USA, catalogue # 419419; high molecular weight, M W = 310 to > 395 kDa; degree of deacetylation DD ≥ 75.0 %; viscosity 800–2000 cP, 1 wt% in 1 % acetic acid) was used as the reference polysaccharide ligand. Unless otherwise indicated, deionized water (DI water, Millipore Simplicity™) with a resistivity of 18 MΩ cm was used to prepare the solutions, and the procedures were conducted at room temperature (RT, 23 ± 2 °C).
Synthesis of Bi2S3/Chitosan Nanoconjugates
Bi2S3 nanoparticles were synthesized via an aqueous colloidal route in a reaction flask at room temperature. Precursors with three different molar ratios, [Bi3+]/[S2−], were evaluated: 0.33 (excess of sulfur), 0.67 (stoichiometric), and 1.33 (excess of bismuth). The synthesis of the Bi2S3 nanoparticles was carried out as follows: 2 mL of chitosan (CHI) solution (1 % w/v in 2 % v/v aqueous solution of acetic acid) and 45 mL of DI water were added to the flask reaction vessel. Under moderate magnetic stirring, X mL (X = 6 mL for 0.33; X = 3 mL for 0.67 and 1.33) of S2− precursor solution (Na2S·9H2O, 1.0 × 10−2 mol L−1) and Y mL (Y = 2 mL for 0.33 and 0.67; Y = 4 mL for 1.33) of Bi3+ precursor solution (BiCl3, 1.0 × 10−2 mol L−1 in acetic acid) were added to the flask and stirred for 60 min. During the addition of Bi3+ solution, the pH was measured and adjusted to 2.5 ± 0.1 or 3.5 ± 0.1 with NaOH (1.0 mol L−1). The Bi2S3 QDs suspensions produced were referred to as QD_CHI[Bi3+]/[S2−]_pH, where [Bi3+]/[S2−] was 0.33, 0.67, or 1.33 and the pH was 2.5 or 3.5, as a function of the molar ratio of precursors and the pH of quantum dots synthesis.
Characterization of Bi2S3/Chitosan Nanoconjugates
UV-visible (UV-vis) spectroscopy measurements were conducted using Perkin-Elmer equipment (Lambda EZ-210) in transmission mode with a quartz cuvette. Measurements were taken over a wavelength range of 1100 to 190 nm.
The morphological and structural features of the quantum dots were characterized using transmission electron microscopy (TEM, Tecnai G2-20-FEI microscope, 200 kV) coupled to an energy dispersive X-ray (EDX) microprobe and using selected area electron diffraction (SAED) analysis. QD sizes and distribution data were obtained based on the TEM images by measuring at least 100 randomly selected nanoparticles using an image processing freeware program (ImageJ, version 1.49, public domain, National Institutes of Health).
X-ray photoelectron spectra (XPS) analysis was performed on an Amicus spectrometer (Shimadzu, Japan) using Mg-Kα as the excitation source. All peaks positions were corrected based on C 1s binding energy (284.6 eV).
Dynamic light scattering (DLS) and zeta potential (ZP) measurements were performed in the QD colloidal dispersions using a ZetaPlus instrument with the laser light diffusion method (Brookhaven Instruments).
Photoluminescence (PL) characterization of the nanohybrids was conducted based on spectra acquired at room temperature using a violet diode laser module at λ exc = 405 nm (150 mW, Roithner LaserTechnik, GmbH) coupled to a USB4000 VIS-NIR spectrophotometer (Ocean Optics).
In addition, the QD colloidal solutions were placed inside a “darkroom-chamber” where they were illuminated by a UV radiation emission bulb (λ excitation = 365 nm, 6-W, Boitton Instruments). Digital color images were collected of the fluorescence of the QDs in the visible range of the light spectrum. Quantum yield (QY) was measured according to established procedure by using Rhodamine 6G (Sigma, USA) in ethanol as the standard at λ excitation = 405 nm .
Cytotoxicity Assay by MTT
Culture of Human Sarcoma Cell Line Culture (SAOS)
The immortalized human osteosarcoma-derived (SAOS) cells were provided by Prof. A. Goes of the Department of Immunology and Biochemistry, Universidade Federal de Minas Gerais (UFMG). SAOS cells are broadly accepted as a model cell line for the preliminary assessment of biocompatibility of materials and devices. The SAOS cells were cultured in DMEM (Dulbecco’s modified eagle medium) with 10 % fetal bovine serum (FBS), streptomycin sulfate (10 mg mL−1), penicillin G sodium (10 units mL−1), and amphotericin-b (0.025 mg mL−1), all of them were supplied by Gibco BRL (NY, USA), using a humidified atmosphere of 5 % CO2 at 37 °C. The cells were used for experiments on passage 23.
All of the biological tests were performed according to ISO standards 10993-5:1999 (Biological evaluation of medical devices; part 5: tests for in vitro cytotoxicity). All experiments were performed using the direct contact methodology.
Prism software (GraphPad Software, San Diego, CA, USA) was used for data analysis. Statistical significance was tested using one-way ANOVA followed by Bonferroni method, with p < 0.05 considered statistically significant. The experiments were performed in triplicate (n = 3).
Cellular Uptake of Bi2S3 Quantum Dot/Chitosan Nanoconjugates
Kidney Cell Line of a Human Embryo Culture (HEK293T Cells)
The human embryonic kidney cell line (HEK293T) was kindly provided by Prof. M.F. Leite of the Department of Physiology and Biophysics, UFMG. The cells 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), in a humidified atmosphere of 5 % CO2 at 37 °C. The HEK293T cells were used for the experiments on passage 7.
Confocal Laser Scanning Microscopy
The HEK293T cells were plated (5 × 104 cells/well) in 24-well plates. The cells were incubated for 4 days in 5 % CO2 at 37 °C and synchronized for 24 h. The QD_CHI0.67_2.5 sample containing 50 % of the medium solution was added to the HEK293T cells. Next, the cells were incubated in 5 % CO2 at 37 °C for 1 h and washed with phosphate-buffered saline (PBS, Gibco BRL, NY, USA). After washing, the cells were fixed with paraformaldehyde (4 %) for 30 min and washed three times with PBS, and cover slips were mounted with Hydromount (Fisher Scientific Ltd., Leicestershire, UK). Confocal laser scanning fluorescence microscopy (Zeiss LSM Meta 510, Carl Zeiss, Germany) was used to detect the fluorescence of the cells using a 488-nm argon laser irradiation to excite the QD–chitosan nanoconjugates. The emissions were collected in the range between 505 and 530 nm. For the control, HEK293T cells were incubated with only DMEM medium with 10 % FBS (immunofluorescence).
Results and Discussion
Physico-chemical Characterization of Chitosan/Bi2S3 Nanoconjugates
The direct bandgap value of Bi2Si3 QDs was calculated from the plots of (αhν)2 versus hν extrapolating the straight portion of the graph (inset of Fig. 1a) to αhν = 0. The obtained E QD values (Fig. 1b) were higher than the corresponding bulk value (1.33 eV) for bismuth sulfite  as a consequence of the size quantization characteristics of the nanoparticles produced. The “blue-shift” (ΔE = E QD − E g) determined for the QDs compared to the bulk value is presented in Fig. 1b.
DLS results (Fig. 3e) revealed that the hydrodynamic diameter (H D) of the core-shell (Bi2S3–chitosan) nanoconjugates were approximately 22 and 26 nm for the systems QD_CHI0.67_2.5 and QD_CHI0.67_3.5, respectively. These values are associated with the interactions between chitosan and aqueous medium (Fig. 3f). As expected, the H D values are higher than the sizes evaluated by TEM technique, which were only related to dimension of the inorganic core of Bi2S3 nanocrystals.
Zeta potential (ζ) measurements for QD_CHI0.67_2.5 and QD_CHI0.67_3.5 were ζ = +56 ± 1 mV and ζ = +37 ± 3 mV, respectively. Under the highly acidic conditions of both pH values investigated, all amine groups of chitosan are protonated, but at pH 2.5, there are more H+ ions in solution repelling and exposing more –NH3 + groups at the nanohybrid surface (i.e., the outermost surface of the organic shell). At pH 3.5, the molar ratio of the precursors also influenced the ZP values. At a molar ratio [Bi3+]/[S2−] = 0.33, ZP was measured to be ζ = +31 ± 3 mV, which is lower than the value obtained for the stoichiometric system where [Bi3+]/[S2−] = 0.67 (i.e., Bi2S3, ratio 2/3 = 0.67). This difference may be due to the excess S2− ions forming complexes with protonated amines reducing the positive charge at the double layer. For the system QD_CHI1.33_3.5, ZP was ζ = +33 ± 11 mV. The relatively higher statistical standard deviation may be attributed to the excess of Bi3+ species that increases the repulsion between the –NH3 + groups in the chitosan chains, thus disrupting the balanced core-shell nanostructure. In addition, ZP results indicated that the QDs were mostly electrostatically stabilized (ζ > +30 mV), preventing close contact between nanoparticles, compatible with the homogenously dispersed features of nanoparticles observed in TEM images. As a general trend, irrespective to the molar ratio of Bi:S used in the synthesis, all nanoconjugates presented positive ζ-potential values, which was attributed to the protonation of amine groups of the chitosan polymeric shell under the acidic conditions at pH = 2.5 and pH = 3.5. Clearly, it is worth mentioning that, as far as the potential applications of these nanoconjugates in biomedical and environmental fields are concerned, these colloidal systems need to be previously buffered to raise the pH of the media to the range of 6.0 to 8.0, to render them biocompatible.
Cytotoxicity Assay by MTT of Chitosan/Bi2S3 Nanoconjugates
The biocompatibility of Bi2S3 nanomaterials has been reported in the literature [33–35]. In this study, the assessment of the cytotoxicity of core-shell Bi2S3/chitosan nanoconjugates was performed using the enzymatic-based MTT assay, with samples synthesized at pH = 3.5, (QD_CHI0.67_3.5) using SAOS cell line. This method is considered superior to other similar methods because it is safe, easy to use, has a high reproducibility, and is broadly performed for both cell viability and cytotoxicity tests.
Cell Uptake of Bi2S3/Chitosan Nanoconjugates Biomarkers
In summary, nanoconjugates were designed and synthesized with chitosan as the biopolymer shell and Bi2S3 semiconductor quantum dot as the fluorescent inorganic core. These nanohybrids were produced using a single-step eco-friendly aqueous colloidal processing route at room temperature. The results demonstrated that chitosan behaved as an effective ligand for nucleating and stabilizing ultra-small Bi2S3 QDs, leading to the formation of colloidal core-shell nanostructures in aqueous dispersions. These nanoconjugates were cytocompatible evaluated by the MTT assay and exhibited photoluminescence under light excitation. Furthermore, the results of cell studies show that the QD–chitosan conjugates display good HEK293T cell uptake in the absence of non-specific binding to the cell membrane and, therefore, they can be potentially used as fluorescent nanosized bioprobes to label cells in vitro for cell bioimaging applications.
The authors acknowledge financial support from the following Brazilian research agencies: CAPES—Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (PROEX-433/2010;PNPD), 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). The authors also express their gratitude to the staff of the Microscopy Center, UFMG, for their assistance on the TEM analysis.
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