Structural and optical properties of Mg doped ZnS quantum dots and biological applications

Accepted Manuscript Structural and optical properties of Mg doped ZnS quantum dots and biological applications M. Ashokkumar, A. Boopathyraja PII:

S0749-6036(17)32577-6

DOI:

10.1016/j.spmi.2017.11.005

Reference:

YSPMI 5340

To appear in:

Superlattices and Microstructures

Received Date: 28 October 2017 Revised Date:

2 November 2017

Accepted Date: 2 November 2017

Please cite this article as: M. Ashokkumar, A. Boopathyraja, Structural and optical properties of Mg doped ZnS quantum dots and biological applications, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.11.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Structural and optical properties of Mg doped ZnS quantum dots and biological applications M. Ashokkumara,*, A. Boopathyrajab a

Department of Physics, Nehru Memorial College (Autonomous), Puthanampatti, Trichy, Tamilnadu, India.

b

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Department of Zoology, Nehru Memorial College (Autonomous), Puthanampatti, Trichy, Tamilnadu, India. ABSTRACT

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Zn1-xMgxS (x = 0, 0.2 and 0.4) quantum dots (QDs) were prepared by co-precipitation method. The Mg dopant did not modify the cubic blende structure of ZnS QDs. The Mg

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related secondary phase was not detected even for 40% of Mg doping. The size mismatch between host Zn ion and dopant Mg ion created distortion around the dopant. The creation of distortion centres produced small changes in the lattice parameters and diffraction peak position. All the QDs showed small sulfur deficiency and the deficiency level were increased

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by Mg doping. Band gap of the QD was decreased due to the dominated quantum confinement effect over compositional effect at initial doping of Mg. But at higher doping the band gap was increased due to compositional effect, since there was no change in average

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crystallite size. The prepared QDs had three emission bands in the UV and Visible regions corresponding to near band edge emission and defect related emissions. The electron

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transport reaction chain which forms free radicals was broken by sulfur vacancy trap sites. Therefore, the ZnS QDs had better antioxidant activity and the antioxidant behaviour was enhanced by Mg doping. The enhanced UV absorption and emission of 20 % of Mg doped ZnS QDs let to maximize the zone of inhibition against E. Coli bacterial strain.

Keywords: ZnS quantum dot, X-ray diffraction, Optical property, antioxidant activity, coprecipitation, Photoluminescence

* Corresponding author. Tel.: + 91 9578082320

ACCEPTED MANUSCRIPT E-mail address: [email protected]

1.

Introduction

Zinc sulfide (ZnS) is an important II–VI compound semiconductor with wide direct

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band gap (3.72 eV at room temperature), high refractive index and wide wavelength pass band [1,2]. Recent days, ZnS quantum dots (QDs) have attracted considerable interest because of their broad applications in different technological areas, including fluorescent

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probes, light emitting diodes, electroluminescent devices, biomedical applications and lasers [3–7]. Properties of ZnS should be tuned for some specific needs and applications. The

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doping of certain transition metals (TM) largely modifies the structural, optical, magnetic and mechanical properties of ZnS QDs [8-11]. Among all other dopants Mg is an important dopant because the radius of Mg2+ (0.57Å) is very close to the radius of Zn2+ (0.60Å) [11]. Therefore the Mg2+ can be easily substituted in the place of Zn2+ in the ZnS lattice. Since

the ZnS QDs.

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MgS has large band (4.5 eV) than ZnS (3.72 eV), the Mg dopant can also widen the band gap

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In addition with the technological applications, ZnS nanoparticle is a promising candidate material for bio-medical applications such as ultra-violet rays blocking lotions and

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creams, anti-microbial agents, medicines and targeted drug delivery [5,12,13]. The oxidants and antioxidants in human body are maintained in balance for the optimal physiological conditions. Increased production of oxidants can cause an imbalance which ultimately leads to oxidative damage of large bio molecules such as lipids, DNA and proteins [14]. The increased number of free radicals is associated with many human diseases like atherosclerosis, arthritis, central nervous system injury, gastritis, ageing, diabetes mellitus and cancer [15]. Therefore, it is necessary to control the free radicals increase to prevent the damages in tissues. Even though, there were large numbers of researches based on the effect

ACCEPTED MANUSCRIPT of Mg dopant on the physical properties of ZnS, the correlation between physical and biological activity is still scanty. Therefore in the present investigation, the structural, optical properties, antibacterial and antioxidant activity of Mg doped ZnS QDs has been discussed in

2.

Materials and methods

2.1. Synthesis of Zn1-xMgxS Quantum Dots

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detail.

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The Zn1-xMgxS QDs were synthesized by co- precipitation method. The high purity Zn(CH3COO)2.2H2O, Na2S and Mg(CH3COO)2.2H2O were used as the precursor for the

are similar with our earlier work [16].

2.2. Study of antioxidant activity

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preparation of Zn1-xMgxS QDs. The preparation procedure and characterization techniques

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The antioxidant activity of the Mg doped ZnS nanoparticles was assessed on the basis of the radical scavenging effect of the stable 1, 1-diphenyl-2-picrylhydrazyl (DPPH)-free radical activity method. The diluted working solutions of the test samples and the standard

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solution were prepared in ethanol, separately. In addition, 0.002% of DPPH was prepared in

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ethanol. 1 ml of DPPH solution was mixed with 3 ml of sample and standard solutions. The solution mixtures were kept in dark for 30 min and the optical density was measured using Spectrophotometer.

2.3. Study of antibacterial activity The antimicrobial assay of pure and Mg doped QDs was performed by Agar disc diffusion method towards clinically isolated Escherichia coli (E. Coli). The Mueller-Hinton Agar medium was prepared and sterilized in the autoclave at 120 0C for 30 minutes. They are then aseptically poured on to sterile petriplate and allowed to solidify. The bacteria culture

ACCEPTED MANUSCRIPT was swabbed on the each petriplate containing medium. The discs were immersed with the QDs of various concentrations (30, 40 and 50 mg/ml). The standard antibiotic, Erythromycin was used as a reference drug. The Mueller-Hinton Agar medium was incubated at 37 0C for

concentration were measured in millimetre (mm) scale. 3.

Results and discussion

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3.1. Structural analysis

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24 hours. After incubation, the zone of inhabitation around discs for each QDs and each

The structural properties of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs have been evaluated

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using X-ray diffraction patterns as shown in Fig. 1. The un-doped and Mg doped ZnS QDs have cubic blende structure with three main diffraction peaks corresponding to the (1 1 1), (2 2 0) and (3 1 1) planes (JCPDS card, No. 03-0570). Absence of Mg related secondary phases such as colloidal magnesium or MgS implies the phase purity of the system. The average

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crystallite size of the QDs is calculated using Debye Scherrer’s formula [10],
. λ

(1)

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Average crystallite size (D) = β  θ

where, λ is the wavelength of X-ray used (1.5406 Å), β is the angular peak width at half

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maximum of prominent diffraction peak and θ is Bragg’s diffraction angle. Peak position (2θ), full width at half maximum (FWHM, β) value, d-value, lattice parameter, volume and the average crystallite size (D) of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs are shown in table 1. The position of diffraction peaks are shifted towards higher 2θ side and the lattice parameter is slightly decreased, when 20% of Mg is doped with ZnS. Decrease of lattice parameter, dvalue and volume is due to the incorporation of comparatively smaller ionic radii Mg2+ into the position of larger ionic radii Zn2+[11]. Decrease in lattice parameter lead to shifts peak position towards higher value. But further increase of Mg2+doping concentration (40%)

ACCEPTED MANUSCRIPT increases the lattice parameter, d-value and volume. The ionic mismatch between dopant and host ion created distortion around the dopant site. Therefore at higher doping concentration the number of distortion site increases and hence the lattice get expand. Thus the increase of lattice parameter shifts back the peak position to the lower value. The slight increase in

the presence of Mg.

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3.2. Compositional and Morphological analysis

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average crystallite size after Mg doping is due to the enhancement of grain surface growth by

Fig. 2 a-c shows the SEM micrograph of ZnS and Mg-doped ZnS QDs. The un-even

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distribution of grain in the cluster form is shown in micrograph. When size became low the agglomeration is the common nature of nanoparticles. In order to confirm the presence of Mg, Zn and S ions in the material with their nominal composition, the EDX spectra of undoped and Mg-doped ZnS samples were recorded as shown in Fig. 3 a–c. The atomic

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percentage of the compositional elements is shown in table 2. No traces of other elements were noticed in the spectra confirming the purity of the samples. All the samples show a small sulfur deficiency and the sulfur deficiency increases with the increase of Mg doping

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concentration. The amount of Mg calculated from EDX analysis is 21.49% and 42.56% for

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Mg = 20% and 40% doped ZnS, respectively. The EDX analysis confirms the purity of QDs and their nominal composition.

3.3. Optical study

The UV–visible absorption spectra of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs at room temperature is shown in Fig. 4a. Initial doping of Mg (20%) shifts the absorption peak towards higher wavelengths and increased the absorption peak intensity. But at Mg=40%, the absorption peak is shifted back to lower wavelength side and the absorption peak intensity is

ACCEPTED MANUSCRIPT decreased. The increase of absorption by Mg doping is due to the size effect. Fig. 4b shows the room temperature transmission spectra of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs. The suppression of transparency is due to the creation of distortion and other imperfection inside the Zn-S lattice by Mg2+ ion incorporation. The optical band gap is evaluated using the

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Tauc’s relation: hυ = A(hυ - Eg)n

(3.2)

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where, A as a constant, Eg is optical band gap of the material and the exponent n depends upon the type of transition. Tauc’s plot is plotted ((αhυ)2 versus hυ) to calculate the energy

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band gap (Eg) of the nanoparticles. The extension of linear portion of the curve towards hυ axis will give the band gap values as shown in Fig. 4c. The calculated values of band gaps are 3.94, 3.72 and 3.86 eV, respectively for Mg = 0, 20, and 40 % doped ZnS QDs. In the case of semiconducting nanomaterial, both composition and quantum confinement effects play major

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role on deciding band gap. When 20% of Mg is doped with ZnS, the band gap value is decreased from 3.94 to 3.72 eV (Red shift). But the band gap value is increased from 3.72 eV to 3.86 eV (Blue shift) when the doping concentration is increased from 20% to 40%. Since

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the average crystallite size is increases from ~1.7 nm to 2 nm at initial doping of Mg (20%),

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the quantum confinement effect dominated over composition effect. Thus the increase of crystallite size by Mg doping let to decrease the band gap of the QDs. When increasing the Mg doping concentration from 20 to 40%, there is no considerable change in the average crystallite size. Now the compositional effect can only influence the band gap of the QDs. Since MgS has larger band gap (4.65 eV) than ZnS, the doping of 40 % of Mg increased the band gap values. Similar blue shift in band gap by Mg doping was reported by Yousefi et. al. [17]. The band gap calculated by Tauc’s plot method is confirmed by plotting a curve, dT/dλ

ACCEPTED MANUSCRIPT versus hυ as in the inset of Fig. 3c. The measured band gaps by this method are 3.93, 3.74 and 3.87 eV respectively for Mg=0, 0.2 and 0.4 doped ZnS QDs.

3.4. Photoluminescence study

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The Photoluminescence spectra of Zn1−xMgxS (x = 0, 0.02 and 0.04) Quantum dots are carried out at room temperature as shown in Fig. 5a. All the samples exhibit dominant UV band and two additional peaks at blue and green regions. The UV emission of QDs is due to

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the near band edge emission (NBE) of free excitons [18]. Some of the conduction band electrons made non-radiactive or radiactive transition with deep levels instead of direct The conduction band excitons non

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recombination with the holes in the valence band.

radically transfer to shallow donor level (Zni) which lies below conduction band due to its larger life time than conduction band. These electrons directly recombine with the holes in the valance band by emitting the wave length slightly larger than UV emission (415-418 nm)

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[19]. The blue emission arises due to the sulfur vacancy in ZnS lattice and the green emission is due transitions between shallow donor levels (Zni) and shallow accepter levels (Vzn) [20].

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Superior photoluminescence intensity at initial doping (Mg=20%) is mainly due to size effect of ZnS QDs. But at higher doping concentration (Mg=40%), the PL intensity get

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diminished due to the increase of non-radiative recombination process. Auger process would be the main reason for non-radiative recombination in semiconductors, the defect centres can acted as a non-radiative recombination centres [21]. As discussed in the earlier section, the distortions and defects are created inside the lattice due to the ionic mismatch between dopant and host ion. Hence, the conduction band electrons are directly absorbed by defect centres. The ratio of intensity of blue emission with the intensity of UV emission is increases with the Mg concentration (Fig.5b), which implies that the sulfur deficiency in the ZnS QDs is increases with Mg doping and the result is well agreement with the EDX analysis.

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The results of DPPH radical scavenging activity is shown in Fig. 6. All the samples show antioxidant activity and the antioxidant activity increases with the increase of Mg

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doping concentration. The sulfur deficiency which created by Mg doping is responsible for better antioxidant activity of the ZnS QDs. The highly reactive free radicals are formed as an intermediate during electron transport chain reaction [22]. The sulfur vacancy sites are act as

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a trap sites for electrons and can break the reaction chain by absorbing either electron or super oxide anion. Seung et. al also suggested that nanoparticles have the ability to absorb

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free radicals [23]. Thus the Mg doped ZnS QDs control the formation of oxidants by controlling free radicals generation. Since the antioxidant activity of Zn0.6Mg0.4S QDs is very close to standard drug, the Zn0.6Mg0.4S QDs can be used as an alternating drug to prevent the tissues from oxidative damage. The formulation of Mg doped ZnS is cost effective and can be

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useful for economically weaker section of people.

3.4. Antibacterial activity

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The antibacterial assay of prepared QDs towards E. Coli bacterial strains was performed agar disc diffusion method. The zone of inhibition (ZOI) of ZnS and Mg doped ZnS

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QDs are shown in Fig. 7 for various concentrations. Even though all the prepared QDs shows antibacterial activity against E. Coli, the 20 % Mg doped ZnS QDs show better antibacterial activity against E. Coli among all other QDs for all concentration. The ZOI of Zn0.8Mg0.2S QDs (22 mm) is close to the standard drug, Erythromycin (26 mm). There are several mechanisms proposed for the antibacterial resistant of nanoparticles. Due to smaller sizes of QDs (~200 times smaller than a bacterium size) they can easily adhere with cell wall of the bacterium and causing destruction, which will lead to the death of the cell of bacterium [24]. The electro static interaction of nanomaterials with cell wall and photocatalytical light activation are also

ACCEPTED MANUSCRIPT the common possible reason behind the antibacterial activity of nanomaterials [25]. Thus the QDs can inhibit the bacteria around the discs which immersed with QDs. The larger ZOI of Zn0.8Mg0.2S QDs then other QDs is also because of its larger UV absorption and emission nature. Even though most strains of the E. Coli bacteria are harmless, some strains of the

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bacteria can cause bloody diarrhea, severe anemia, kidney failure, urinary tract infections and other infections, which can lead to death. Therefore, the 40% of Mg doped ZnS QDs is proposed to be a suitable antibacterial agent for E. Coli bacteria and can prevent from these deadly

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diseases.

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3.5. Conclusion

Mg doped ZnS quantum dots were prepared by co-precipitation method. The variation in the lattice parameter and diffraction peak position was due to ionic mismatch between Zn and Mg ion. Red shift in band gap at initial doping was due to the quantum confinement effect.

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But at higher doping, compositional effect produced the blue shift in band gap. An UV emission corresponding to NBE emission, blue and green emission corresponding to sulfur vacancy and Zn defect were noted in emission spectra. The antioxidant activity of ZnS QDs

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had been enhanced by sulfur defects. The sulfur vacancy sites broke the free radical reaction

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chain by absorbed the electron as a trap. The antioxidant activity of ZnS QDs had been enhanced by Mg doping and the Zn0.4Mg0.6S QDs showed maximum antioxidant activity. The 20% of Mg doped nanoparticles showed better antibacterial behaviour against to E. Coli bacterial strain.

Acknowledgment: The authors are thankful to the SERB, DST, New Dehli, India, for financial support (File No. – SR/FT/LS-124/2011).

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Figure captions

Fig. 1. Powder X-ray diffraction pattern of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs

Fig. 2. Scanning electron microscope (SEM) images of a) ZnS b) Zn0.8Mg0.2S c) Zn0.6Mg0.4S QDs

ACCEPTED MANUSCRIPT Fig. 3. Energy dispersive X-ray (EDX) spectra of a) ZnS b) Zn0.8Mg0.2S c) Zn0.6Mg0.4S QDs

Fig. 4a. UV-Visible absorption spectra of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs at room temperature

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Fig. 4b. UV-Visible transmission spectra of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs at room temperature

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Fig. 4c. Tauc’s plot for Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs and the inset shows dT/dλ versus hυ curves

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Fig. 5a. Photoluminescence spectra of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs

Fig. 5b. Intensity ratio between blue peak and UV peak of photoluminescence spectra for different concentrations of Mg

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Fig. 6. Antioxidant activity of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs

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Fig. 7. Antibacterial activity of Zn1-xMgxS (x = 0, 0.2 and 0.4) QDs

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D

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Table 1

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The variation of Peak position (2θ), full width at half maximum (FWHM, β) value, d-value, Lattice parameter, Volume and average crystallite size (D) of Zn1-xMgxS (x = 0, 0.2 and 0.4) quantum dots.

ZnS

28.51

4.6833

3.1285

5.4186

159.09

1.7

Zn0.8Mg0.2S

28.54

3.8750

3.1253

5.4130

158.60

2.1

Zn0.6Mg0.4S

28.50

4.1127

159.23

2.0

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5.4201

(nm)

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Table 2

(degrees)

d-value (Å)

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FWHM, β

Samples

Lattice Volume parameter (nm) (Å)

Average crystallite size, D

Peak position, 2θ (˚)

The quantitative analysis of the compositional elements present in the Zn1-xMgxS (x = 0, 0.2 and 0.4) quantum dots using EDX analysis Atomic percentage of the elements (%)

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Samples

S

Zn

Mg

Mg/(Zn+Mg) ratio

42.25

57.75

-

-

Zn0.6Mg0.4S

40.17

46.97

12.86

21.49

Zn0.2Mg0.8S

39.92

34.51

25.57

42.56

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ZnS

ACCEPTED MANUSCRIPT HIGHLIGHTS In this paper, we report the structural, optical and photoluminescence properties Mg doped ZnS QDs and it biomedical applications such as antioxidant activity and antibacterial activity of Mg doped ZnS QDs. The paper should be of interest to readers in the areas of optical material, biomedical application, semiconductor nanoparticles, etc. The following are the highlights our work

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1. ZnS QDs have been found broad applications in many different technological areas, including fluorescent probes, light emitting diodes, electroluminescent devices, biomedical applications and lasers. 2. Even though, there were few research works on the effect of Mg dopent on the physical properties of ZnS, the correlation between physical and biological activity is still scanty. 3. The Mg dopant did not alter the cubic blende structure of ZnS. 4. Band gap of the QD decreased due to the dominated quantum confinement effect over compositional effect at initial doping of Mg. But at higher doping the band gap increased due to compositional effect. 5. Mg dopant created sulfur deficiency and the deficiency was increased with the increase of dopant concentration. 6. The sulfur deficiency enhanced the antioxidant activity. 7. 40% Mg doped ZnS QDs show maximum antioxidant activity and is very close to standard drug. 8. 20% of Mg doped ZnS QDs show maximum antibacterial behaviour against E. Coli bacterial strain.