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Effect of Cu doping on the optical property of green synthesised l-cystein-capped CdSe quantum dots

  • Oluwatobi Samuel Oluwafemi EMAIL logo , Sundararajan Parani , Thabang Calvin Lebepe and Rodney Maluleke
Published/Copyright: November 26, 2021
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 10 Issue 1

Abstract

In the present study, the synthesis of water-soluble copper-doped CdSe nanoparticles (NPs) via a low cost, facile, and environmentally benign method is reported. Simple reagents such as selenium powder, cadmium chloride, and copper sulphate were used as selenium, cadmium, and copper precursor, respectively, while l-cysteine was used as a capping ligand without the use of an additional stabiliser. The as-synthesised copper-doped CdSe NPs were characterised using ultraviolet (UV-Vis) absorption and photoluminescence (PL) spectroscopy, Fourier-transform infrared spectroscopy, and transmission electron microscopy. By varying the dopant concentration, the temporal evolution of the optical properties and the shape of the nanocrystals were investigated. The observation and the results showed that the colour of the solution changed rapidly from orange to black, and the PL shifted to a longer wavelength at the high dopant concentration. The micrographic images revealed that the as-synthesised materials are small and could be used for bio labelling.

Keywords: green synthesis; quantum dots; copper; doping; photoluminescence

1 Introduction

In the past two decades, semiconductor nanoparticles (NPs), also known as quantum dots (QDs), have been of considerable interest due to their fundamental as well as their theoretical properties, particularly the quantum confinement effect in which the electrical and optical properties can be changed simply by variation in its particle size [1,2,3]. The progress made in understanding the properties of semiconductor NPs has stimulated similar efforts in doping semiconductor nanocrystals to control the final properties of these materials [4,5,6]. Doping, the intentional introduction of impurities, is a common approach for tuning the electronic, optical, mechanical, and magnetic properties of materials. It plays a central role in many areas of modern semiconductor science and technology. This makes them useful for a variety of applications such as bio-theragnostic agent, sensors, photovoltaic cells, solar cells, etc. [7,8]. In previous studies, doped nanocrystals have been synthesised by the aqueous as well as the organometallic methods. Kłopotowski et al. [9] recently reported the synthesis of doped CdSe with Cu through an organometallic method. However, the limitation of this method is the use of expensive and hazardous chemicals. For green synthetic chemistry approach, organic solvents are discouraged, and water is considered as an ideal solvent. Compared with the organometallic route, the cheaper, more straightforward, and less toxic aqueous synthetic approach is an alternative strategy to directly prepare water-soluble nanocrystals, thus making the synthetic approach greener. In this regard, Wageh et al. [10] synthesised Cu-doped CdSe QDs in an aqueous method using water-soluble precursors. However, they have employed 3-mercaptopropionic acid, a highly reactive chemical that can pose potential environmental and biological risks as the capping agent. Therefore, complete synthesis of Cu-doped CdSe QDs in water with mild capping reagents is necessary. In this study, we report a facile green synthesis of water-soluble, Cu-doped CdSe NPs using l-cysteine as the capping agent. l-Cysteine is an essential amino acid naturally found in the human body and available in the pharmaceutical market as a dietary supplement. Hence, it is a biocompatible and environmentally suitable material for green synthesis [11,12]. In addition, we have investigated the effect of different copper ion concentrations on the optical and structural properties of the as-synthesised CdSe NPs. The as-synthesised l-cysteine-capped Cu-doped CdSe NPs could be a potential candidate for diverse QD applications.

2 Materials and methods

2.1 Materials

Sodium borohydride (NaBH4), deionised water (HPLC grade), cadmium chloride (CdCl2), selenium powder, copper sulphate (CuSO4), and l-cysteine ester hydrochloride were purchased from Sigma Aldrich, South Africa. All the chemicals were of analytical grade and used as purchased without further purification.

2.2 Stock solution preparation

Initially, the selenium precursor stock solution was prepared by adding 0.16 mmol of selenium powder to 20 mL of deionised water in a three-necked flask. 0.81 mmol of sodium borohydride was carefully added to this mixture, and the flask was immediately purged with nitrogen gas. The mixture was then stirred for 2 h at room temperature. The entire selenium dissolves in water, giving rise to a colourless selenium solution. On the other hand, the cadmium precursor solution was prepared by adding 0.32 mmol of CdCl2 powder in 20 mL of deionised water. The capping solution was prepared by dissolving 3.2 mmol of l-cysteine ethyl ester hydrochloric acid powder in 20 mL of deionised water.

2.3 Synthesis of CdSe QDs and Cu-doped CdSe QDs

The synthetic approach is straightforward and does not require any special setup. In a typical room temperature reaction, 1.0 mL of cadmium stock solution was added to the ligand solution under constant magnetic stirring. The pH value of the solution was adjusted to the desired pH value by dropwise addition of ammonia solution. This was followed by the slow addition of 1.0 mL of Se stock solution under constant stirring. The reaction was refluxed and allowed to continue for 5 h. The same procedure was repeated for different pH values of 2.45, 4, 7, 9, and 11. For Cu-doped CdSe QDs, a certain amount of copper stock solution was mixed with cadmium solution followed by a similar process for the preparation of CdSe QDs. The as-synthesised QDs were isolated by precipitation with isopropanol followed by centrifugation. The resultant particles were dissolved in water to give a solution of nanocrystallite for further characterisations.

2.4 Characterisation

A Perkin Elmer Lamda UV-vis spectrophotometer was used to carry out absorption spectra in the 200–900 nm wavelength range. Photoluminescence (PL) spectra were recorded on a Perkin Elmer LS 55 luminescence spectrometer. Photoluminescence quantum yield (PLQY) of the samples was calculated using rhodamine 6 G as refernce. A JEOL 2100 TEM operating at 200 kV was used for transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Fourier-transform infrared spectroscopy (FTIR) analysis was performed using Perkin Elmer spectrum Two.

3 Results and discussion

3.1 Effect of pH on the optical properties of CdSe QDs

CdSe QDs were synthesised by simple wet chemical greener method in an aqueous medium using water-soluble precursors at different pH values from acid to alkaline range. Figure 1a and b represents the absorption and PL spectra of CdSe NPs synthesised at different pH values at room temperature after 24 h reaction time. The growth of the particles was observed by the change in the colour of the solution from colourless to slightly yellow. At the pH value between 2.25 and 9, the absorption spectra appear very broad, covering the whole ultraviolet-visible region indicating the particles obtained with low quality. At pH value of 11, the absorption spectra appeared clearly, showing an excitonic peak at 368 nm. The same trend was also observed in the PL spectra, where the QDs synthesised at pH value of 11 showed the highest intensity and narrow emission, with the peak centred at 435 nm. The syntheses at lower pH value produced broad and/or low-intensity emission peaks. This could be due to the nature of the capping agent l-cysteine at different pH values. At low pH value, the concentration of free thiols in l-cysteine would be inadequate, leading to the improper passivation of the QDs. This could lead to surface defects and aggregation, thus reducing the material quality and PL [13]. However, at high alkaline pH value of 11, the thiol and carboxylic acid group of the l-cysteine ligand are easily deprotonated. This will increase the concentration of the free thiols in the solution as well as the complexation of the surface of the NP by the thiol group. Thus, reducing the surface traps and hence increasing the luminescence intensity. In addition, the repulsion of the negative charge of the carboxylate group on the surface of the NPs by individual NP also prevents particle aggregation and thus, improves the stability of the as-synthesised material [14]. These two factors contribute to the increase in the PL intensity as the pH value increases. Hence, pH value of 11 was chosen for further reaction.

Figure 1 (a) Absorption spectra and (b) PL spectra of CdSe NPs at different pH values (i: pH = 2.25, ii: pH = 4, iii: pH = 7, iv: pH = 9, and v: pH = 11).
Figure 1

(a) Absorption spectra and (b) PL spectra of CdSe NPs at different pH values (i: pH = 2.25, ii: pH = 4, iii: pH = 7, iv: pH = 9, and v: pH = 11).

3.2 Optical and structural properties of water-soluble Cu-doped CdSe NPs

Figure 2a and b shows the absorption and emission spectra of 0.5% Cu ions-doped CdSe NPs at different refluxing times. All the samples show well defined excitonic peak and absorption band edge. The excitonic peak (368 nm) and absorption band edges (400 nm) appear at the same position without any significant shift throughout the reaction time. This signifies that the growth process was not affected by copper doping. The PL emission intensity increased as the reaction time increases and reached its maximum at 3 h, then decays at 5 h. The maximum emission was observed at 436 nm till 3 h, similar to undoped CdSe QDs and slightly blue-shifted at higher reaction times. The PLQY of the CdSe QDs at optimised conditions was calculated to be 20.7%.

Figure 2 (a) Absorption spectra and (b) PL spectra of 0.5% Cu ions-doped CdSe NPs at different refluxing time (i: 0 min, ii: 2 min, iii: 6 min, iv: 10 min, v: 1 h, vi: 3 h, and vii: 5 h).
Figure 2

(a) Absorption spectra and (b) PL spectra of 0.5% Cu ions-doped CdSe NPs at different refluxing time (i: 0 min, ii: 2 min, iii: 6 min, iv: 10 min, v: 1 h, vi: 3 h, and vii: 5 h).

3.3 Effect of dopant concentrations

Figure 3a and b shows the absorption and emission spectra of Cu-doped CdSe NPs prepared using different Cu ion concentrations. The different concentrations of the dopant used are shown in Table 1. The colour of the solution changed as the concentration of Cu ion increased from 0.0032% (orange) to 1% (black). The absorption spectra show distinct excitation at 0.0032%, 0.016%, 0.032%, and 0.5% Cu ions dopant level and a broad band-edge at 1%. The absence of excitonic shoulder and the presence of a broad band-edge at 1% Cu ions could be attributed to the defects caused by introducing a large number of Cu ions [15].

Figure 3 (a) Absorption spectra and (b) PL spectra of Cu-doped CdSe NPs at different % of Cu ions (i: 0.0032%, ii: 0.016%, iii: 0.032%, iv: 0.5%, and v: 1%).
Figure 3

(a) Absorption spectra and (b) PL spectra of Cu-doped CdSe NPs at different % of Cu ions (i: 0.0032%, ii: 0.016%, iii: 0.032%, iv: 0.5%, and v: 1%).

Table 1

The PL peak and colour of as-synthesised Cu-doped CdSe NPs

% Cu2+ PL peak Cu: CdSe QDs (mm) Colour of the solution
0.0032 436 Orange
0.016 436 Orange–brown
0.032 436 Brown
0.5 436 Black
1.0 488 Black

The emission spectra show that the PL intensity increases as the concentration of Cu ion increases and reaches a maximum at 0.032% dopant level without any further change at higher concentrations. The PLQY of the QDs at this Cu dopant concentration was calculated to 25.9%. It was noted that the PL peak position remains nearly the same at lower dopant concentrations, while a significant redshift (∼52 nm) was observed at higher dopant concentration (1% Cu ions). This increase in the PL intensity as the concentration of the dopant increases can be attributed to the defects caused by high Cu concentration. The Cu level dominates the recombination centre at higher dopant concentration, thus causing a significant redshift in the PL peak position [15].

3.4 Morphological analysis

The TEM images of the undoped and 0.5% Cu-doped CdSe QDs are shown in Figure 4a and b. In accordance with the optical analysis, the TEM image confirmed the well-dispersed nature of the undoped CdSe QDs produced under refluxing. The particles are in the range from 1.25 to 4.40 nm, with a mean particle diameter of 2.44 nm. On the other hand, the Cu-doped CdSe QDs are in the range from 2.0 to 6.0 nm with a mean particle diameter of 3.57 nm. The increase in size could be attributed to Cu doping. The HRTEM images (insets of Figure 4a and b) showed that nearly spherical particles with the clear lattice fringes indicate the crystallinity of QDs. The SAED pattern (Figure 4c and d) of both undoped and Cu-doped CdSe QDs shows sharp and clear singular reciprocal points indicating that the obtained NPs are highly crystalline.

Figure 4 TEM images of (a) CdSe QDs and (b) 0.5% Cu-doped CdSe QDs. SAED patterns of (c) CdSe QDs and (d) 0.5% Cu-doped CdSe QDs.
Figure 4

TEM images of (a) CdSe QDs and (b) 0.5% Cu-doped CdSe QDs. SAED patterns of (c) CdSe QDs and (d) 0.5% Cu-doped CdSe QDs.

3.5 FTIR analysis

The capping nature of QDs by l-cysteine was investigated using FTIR spectra. Figure 5 shows the FTIR spectra of l-cysteine ethyl ester hydrochloride which was used in the synthesis and l-cysteine-capped Cu-doped CdSe QDs. l-Cysteine ester reveals the characteristic FTIR peaks such as ʋS–H at 2,474 cm−1, ʋC═O at 1,745 cm−1, δNH 3 + at 1,478 cm−1, and ʋC(O)O at 1,225 cm−1. The FTIR spectrum of QDs shows significant differences in comparison with l-cysteine ester spectrum. The ester group peaks (ʋC═O and ʋC(O)O) disappeared indicating that the ester group was hydrolysed to the parent l-cysteine at the reaction conditions. In addition, importantly ʋS–H peak of l-cysteine ester also disappeared indicating that thiol was deprotonated, and the cysteine capped to QD surface via its sulphur as it has strong affinity towards metals like Cd. The FTIR results confirmed the capping of l-cysteine on the QDs.

Figure 5 FTIR spectra of l-cysteine ethyl ester and l-cysteine-capped QDs.
Figure 5

FTIR spectra of l-cysteine ethyl ester and l-cysteine-capped QDs.

4 Conclusion

We have reported the synthesis of Cu-doped NPs via a facile, “green”, and environmentally benign method using l-cysteine as a capping agent. By varying the pH and dopant concentration, the temporal evolution of the optical properties of the undoped and Cu-doped CdSe QDs were investigated. In comparison with other pH ranges, the CdSe QDs synthesised at pH value of 11 show the highest PL intensity. The PL emission intensity of Cu-doped CdSe NPs increased as the Cu doping increased and reached their maximum at 0.5%, and the emission peak position become red-shifted with further high Cu doping. The TEM images confirmed that the as-synthesised materials are small and retain their shape even after doping. The resultant NPs are non-air sensitive, highly water-soluble, and of good quality, which would be of immense utility for potential biological application.

Acknowledgments

The authors will like to thank Ms. Zukiswa Mbande for her contribution and the University of Johannesburg (URC) and the Faculty of Science (FRC) for support.

  1. Funding information: Authors state no funding involved.

  2. Author contributions: Oluwatobi Samuel Oluwafemi: conceptualisation, writing – original draft, writing – review and editing, methodology, resources; Sundararajan Parani: writing – editing, formal analysis; Thabang Calvin Lebepe: formal analysis; Rodney Maluleke: formal analysis.

  3. Conflict of interest: Authors state no conflict of interest.

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Received: 2021-06-10
Revised: 2021-10-16
Accepted: 2021-11-02
Published Online: 2021-11-26

© 2021 Oluwatobi Samuel Oluwafemi et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/gps-2021-0075
Keywords for this article
green synthesis; quantum dots; copper; doping; photoluminescence
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  20. Continuous sulfonation of hexadecylbenzene in a microreactor
  21. Synthesis, characterization, biological activities, and catalytic applications of alcoholic extract of saffron (Crocus sativus) flower stigma-based gold nanoparticles
  22. Foliar applications of plant-based titanium dioxide nanoparticles to improve agronomic and physiological attributes of wheat (Triticum aestivum L.) plants under salinity stress
  23. Simultaneous leaching of rare earth elements and phosphorus from a Chinese phosphate ore using H3PO4
  24. Silica extraction from bauxite reaction residue and synthesis water glass
  25. Metal–organic framework-derived nanoporous titanium dioxide–heteropoly acid composites and its application in esterification
  26. Highly Cr(vi)-tolerant Staphylococcus simulans assisting chromate evacuation from tannery effluent
  27. A green method for the preparation of phoxim based on high-boiling nitrite
  28. Silver nanoparticles elicited physiological, biochemical, and antioxidant modifications in rice plants to control Aspergillus flavus
  29. Mixed gel electrolytes: Synthesis, characterization, and gas release on PbSb electrode
  30. Supported on mesoporous silica nanospheres, molecularly imprinted polymer for selective adsorption of dichlorophen
  31. Synthesis of zeolite from fly ash and its adsorption of phosphorus in wastewater
  32. Development of a continuous PET depolymerization process as a basis for a back-to-monomer recycling method
  33. Green synthesis of ZnS nanoparticles and fabrication of ZnS–chitosan nanocomposites for the removal of Cr(vi) ion from wastewater
  34. Synthesis, surface modification, and characterization of Fe3O4@SiO2 core@shell nanostructure
  35. Antioxidant potential of bulk and nanoparticles of naringenin against cadmium-induced oxidative stress in Nile tilapia, Oreochromis niloticus
  36. Variability and improvement of optical and antimicrobial performances for CQDs/mesoporous SiO2/Ag NPs composites via in situ synthesis
  37. Green synthesis of silver nanoparticles: Characterization and its potential biomedical applications
  38. Green synthesis, characterization, and antimicrobial activity of silver nanoparticles prepared using Trigonella foenum-graecum L. leaves grown in Saudi Arabia
  39. Intensification process in thyme essential oil nanoemulsion preparation based on subcritical water as green solvent and six different emulsifiers
  40. Synthesis and biological activities of alcohol extract of black cumin seeds (Bunium persicum)-based gold nanoparticles and their catalytic applications
  41. Digera muricata (L.) Mart. mediated synthesis of antimicrobial and enzymatic inhibitory zinc oxide bionanoparticles
  42. Aqueous synthesis of Nb-modified SnO2 quantum dots for efficient photocatalytic degradation of polyethylene for in situ agricultural waste treatment
  43. Study on the effect of microwave roasting pretreatment on nickel extraction from nickel-containing residue using sulfuric acid
  44. Green nanotechnology synthesized silver nanoparticles: Characterization and testing its antibacterial activity
  45. Phyto-fabrication of selenium nanorods using extract of pomegranate rind wastes and their potentialities for inhibiting fish-borne pathogens
  46. Hydrophilic modification of PVDF membranes by in situ synthesis of nano-Ag with nano-ZrO2
  47. Paracrine study of adipose tissue-derived mesenchymal stem cells (ADMSCs) in a self-assembling nano-polypeptide hydrogel environment
  48. Study of the corrosion-inhibiting activity of the green materials of the Posidonia oceanica leaves’ ethanolic extract based on PVP in corrosive media (1 M of HCl)
  49. Callus-mediated biosynthesis of Ag and ZnO nanoparticles using aqueous callus extract of Cannabis sativa: Their cytotoxic potential and clinical potential against human pathogenic bacteria and fungi
  50. Ionic liquids as capping agents of silver nanoparticles. Part II: Antimicrobial and cytotoxic study
  51. CO2 hydrogenation to dimethyl ether over In2O3 catalysts supported on aluminosilicate halloysite nanotubes
  52. Corylus avellana leaf extract-mediated green synthesis of antifungal silver nanoparticles using microwave irradiation and assessment of their properties
  53. Novel design and combination strategy of minocycline and OECs-loaded CeO2 nanoparticles with SF for the treatment of spinal cord injury: In vitro and in vivo evaluations
  54. Fe3+ and Ce3+ modified nano-TiO2 for degradation of exhaust gas in tunnels
  55. Analysis of enzyme activity and microbial community structure changes in the anaerobic digestion process of cattle manure at sub-mesophilic temperatures
  56. Synthesis of greener silver nanoparticle-based chitosan nanocomposites and their potential antimicrobial activity against oral pathogens
  57. Baeyer–Villiger co-oxidation of cyclohexanone with Fe–Sn–O catalysts in an O2/benzaldehyde system
  58. Increased flexibility to improve the catalytic performance of carbon-based solid acid catalysts
  59. Study on titanium dioxide nanoparticles as MALDI MS matrix for the determination of lipids in the brain
  60. Green-synthesized silver nanoparticles with aqueous extract of green algae Chaetomorpha ligustica and its anticancer potential
  61. Curcumin-removed turmeric oleoresin nano-emulsion as a novel botanical fungicide to control anthracnose (Colletotrichum gloeosporioides) in litchi
  62. Antibacterial greener silver nanoparticles synthesized using Marsilea quadrifolia extract and their eco-friendly evaluation against Zika virus vector, Aedes aegypti
  63. Optimization for simultaneous removal of NH3-N and COD from coking wastewater via a three-dimensional electrode system with coal-based electrode materials by RSM method
  64. Effect of Cu doping on the optical property of green synthesised l-cystein-capped CdSe quantum dots
  65. Anticandidal potentiality of biosynthesized and decorated nanometals with fucoidan
  66. Biosynthesis of silver nanoparticles using leaves of Mentha pulegium, their characterization, and antifungal properties
  67. A study on the coordination of cyclohexanocucurbit[6]uril with copper, zinc, and magnesium ions
  68. Ultrasound-assisted l-cysteine whole-cell bioconversion by recombinant Escherichia coli with tryptophan synthase
  69. Green synthesis of silver nanoparticles using aqueous extract of Citrus sinensis peels and evaluation of their antibacterial efficacy
  70. Preparation and characterization of sodium alginate/acrylic acid composite hydrogels conjugated to silver nanoparticles as an antibiotic delivery system
  71. Synthesis of tert-amylbenzene for side-chain alkylation of cumene catalyzed by a solid superbase
  72. Punica granatum peel extracts mediated the green synthesis of gold nanoparticles and their detailed in vivo biological activities
  73. Simulation and improvement of the separation process of synthesizing vinyl acetate by acetylene gas-phase method
  74. Review Articles
  75. Carbon dots: Discovery, structure, fluorescent properties, and applications
  76. Potential applications of biogenic selenium nanoparticles in alleviating biotic and abiotic stresses in plants: A comprehensive insight on the mechanistic approach and future perspectives
  77. Review on functionalized magnetic nanoparticles for the pretreatment of organophosphorus pesticides
  78. Extraction and modification of hemicellulose from lignocellulosic biomass: A review
  79. Topical Issue: Recent advances in deep eutectic solvents: Fundamentals and applications (Guest Editors: Santiago Aparicio and Mert Atilhan)
  80. Delignification of unbleached pulp by ternary deep eutectic solvents
  81. Removal of thiophene from model oil by polyethylene glycol via forming deep eutectic solvents
  82. Valorization of birch bark using a low transition temperature mixture composed of choline chloride and lactic acid
  83. Topical Issue: Flow chemistry and microreaction technologies for circular processes (Guest Editor: Gianvito Vilé)
  84. Stille, Heck, and Sonogashira coupling and hydrogenation catalyzed by porous-silica-gel-supported palladium in batch and flow
  85. In-flow enantioselective homogeneous organic synthesis
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