Cadmium sulphide (CdS) nanoparticles (NPs) were synthesized through hydrothermal route and characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), Energy dispersive X-ray analysis, Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy and Thermo gravimetric analysis (TGA).The band gap of CdS nanoparticles was found to be 2.38 eV. CdS NPs are crystalline aggregates with hexagonal structure as shown by SEM and XRD analysis. TGA study revealed that the synthesized nanomaterials were very stable to temperature and only 6.54% total loss occurred during heating range (25 °C–600 °C).The CdS NPs were used for the first time against the degradation of Eosin B (EB) and Methyl green (MG) dyes in aqueous solution.The degradation of EB and MG over CdS nanocatalysts followed second order kinetics. The predicted activation energies for both the dyes' reactions were 61.1 kJ/mol and 32.11 kJ/mol, respectively. About 95% and 90% dye degradation was observed at the time interval of 160 minutes for EB and MG, respectively. High percent degradation of EB was observed at high pH (pH 0) while at low pH (pH 4) high percent degradation was found for MG dye. Maximum dye degradation was found at the optimal dose (0.03 g/L) of the catalyst and at low dye concentration. The rate of EB and MG dye degradation was found to increase with increase in temperature up to 45 °C. The recyclability study showed that CdS nanoparticles could be reused for the degradation of the given dyes. Good antibacterial activity against Staphylococcus aureus was shown by CdS NPs. From the biocompatibility it was confirmed that CdS NPS are bioincompatible compatible.

  • The band gap was 2.38 eV.

  • Dyes degradation over followed second order kinetics.

  • 61.1 kJ/mol and 32.11 kJ/mol were activation energies.

  • 95% and 90% dye degradation was observed for EB and MG, respectively.

  • Good antibacterial activity against Staphylococcus aureus.

  • CdS NPS were found bioincompatible compatible.

Industrial effluents containing dyes pose a serious environmental hazard since their dumping into natural water bodies frequently results in environmental contamination, functioning as a source of non-aesthetic pollution, eutrophication, and so causing harm to aquatic life. The removal or degradation of stubborn synthetic dyes is a major ecological issue that is difficult to solve. For the removal of synthetic colourants from wastewater, several methods such as electrolysis, adsorption, oxidation, coagulation, active sludge biochemical processes, membrane filtering, ozonization, and bio-degradation have been utilized (Menon et al. 2021). Advanced oxidation processes (AOPs), a promising technique, have recently been widely used for decolorization and degradation of textile dyes. The heterogeneous photocatalytic oxidation method is a technology that uses photocatalysts that is currently being investigated (Sarkar et al. 2020). Photocatalysts may degrade a variety of organic contaminants and pigments. To activate the photo-induced electrons, they need light energy. The charge carrier recombines to create H2O molecules when photo-generated electrons from the valence band (VB) are transported to conduction band (CB). Meanwhile, in the VB, H2O molecules are oxidized to enhance the quantity of •OH generated, which is used to destroy the synthetic colours. In photo oxidation, organic contaminants are completely oxidized in a short interval of time. Furthermore, no additional hazardous compounds are formed during this procedure (Lee et al. 2020).

Cadmium sulphide (CdS) nanoparticles have piqued interest among various nanoparticles due to the availability of discrete energy levels, size dependent optical properties, and a tunable bandgap, as well as a well-developed synthetic protocol, easy preparation technique, and good chemical stability (Desai et al. 2017). Due to the high photosensitivity of CdS, it is utilized in the detection of visible radiations, light emitting diodes, solar cells, photochemical catalysis, gas sensors, other luminescence devices, optoelectronic devices, and a range of biological applications (Blažeka et al. 2020; Chang et al. 2020; Mullamuri et al. 2021).

Pharmaceutical contaminants and bacteria have also been a source of concern, as they are absorbed by various water streams via wastewater effluents, industrial and agricultural operations (Munyai et al. 2021). For this purpose CdS nanoparticles were tested against Escherichia coli, Bacillus licheniformis, Pseudomonas aeruginosa, Staphylococcus aureus, and B. cereus at a dosage of 40 mg/mL CdS (Rajeshkumar et al. 2014), taking into account that CdS semiconductors can behave as photocatalysts in the photodegradation of the environmental organic pollutants and as antibacterial agents.

The benefits of the photocatalysis were kept in mind and a study was designed to synthesize and characterize CdS nanoparticles and to apply as a catalyst for the photo degradation of MG and EB dyes in aqueous medium. Investigation of order of reaction of photocatalytic degradation of selected dyes using CdS nanocatalyst was also the objective of the present study. Therapeutic applications of CdS NPs were among the aims of the study.

Apparatus

UV-visible spectrophotometer (Model Shimadzu UV-1800), Perkin Elmer Fourier transform infrared (FTIR) spectrometer version 10.4.00, TGA (Shimadzu TGA -50/50H) were used for characterization and analysis of the synthesized materials in Advance Research Lab (ARL) Department of Chemistry Bacha Khan University Chrasadda. Synthesized materials were also characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) in Quaid-i-Azam University Islamabad.

Synthesis of CdS nanoparticles

CdS NPs were synthesized through hydrothermal route. In this typical synthesis 15 mL (0.1 M) of each Cd (NO3)2.4H2O and Na2S.9H2O solutions were taken in a beaker. The mixed solution was stirred at room temperature for about 10 minutes. Then the mixture was transferred to 50 mL autoclave and was placed in a muffle furnace for 14 hours at 200 °C. After heating the reaction mixture, it was centrifuged and washed four times with water. The final product was dried at 50 °C for five hours (Supplementary Figure 1).

Preparation of the dye solution

Stock solutions (500 ppm) of EB and MG dyes were prepared separately in distilled water. Using dilution formula given in Equation (1), the working solutions of different concentration were prepared accordingly.
formula
(1)

Photocatalytic degradation of the dyes

The activity of CdS nanoparticles was evaluated while using them for the degradation of EB and MG dyes in aqueous solution. The degradation was carried out in UV-light. The λmax of EB and MG was found at 516 nm and 645 nm and was used as monitor wavelengths. Upon addition of an appropriate amount of photocatalyst (CdS, NPs) the dye solution was stirred for 20 minutes in the dark to establish adsorption/desorption equilibrium. Light source was placed 15 cm away from the surface of the solution during experiments in locally designed equipment. The catalyst was removed by centrifugation and the dye degradation was checked at various intervals of time using UV-Visible spectrophotometer. The same process was repeated in sun-light. The following relation (Equation (2)) was used to calculate percent degradation of the dye.
formula
(2)
where C0 and Ct represent concentrations of dye at time 0 min and t(s), respectively.

Anti bacterial assay using well diffusion method

The CdS nanoparticles were tested for antimicrobial activity using agar well diffusion method against pathogenic microbes such as Acinetobacter baumannii, S. aureus, and E. coli. The pure cultures of bacteria were subcultured on nutrient broth. Each strain was swapped homogeneously onto the individual plates using sterile cotton swabs. Wells of 8 mm were made on the nutrient agar plates. Then 25 μL, 50 μL, 75 μL and 100 μL concentration of CdS nanoparticles was poured on each well. Zone of inhibition was measured after 24 hours' incubation.

Haemolysis

CdS nanoparticles having concentration of 2 mg/mL, 4 mg/mL, 6 mg/mL, 8 mg/mL and 10 mg/mL and 1 mL erythrocyte/phosphate buffered saline (PBS) suspension (1 × 106 cells per mL) were incubated for 1 h at 37 °C. This was followed by centrifugation at 2,000 rpm for 10 min. The degree of haemolysis was determined by measuring the absorbance of the supernatant at 545 nm, as previously reported. Triton X-100 was used as positive control while blood/PBS solution was used as negative.

UV-visible studies

Using UV-Vis spectrophotometer CdS NPs showed maximum absorbance at 496 nm as shown in Figure 1(a) and the band gap was calculated using Tauc plot given in Equation (3).
formula
(3)
where ‘α’ represents the absorption coefficient, ‘hυ’ is the photon energy, ‘ Eg’ is the band gap and value of ‘n’ depends on transition involved, where n can have values 2, 1/2, 3, 3/2 related to indirect allowed, direct allowed, indirect forbidden and direct forbidden transitions, respectively. CdS nanoparticles show direct allowed transitions (Imran et al. 2018). To calculate the value of band gap, graph was plotted between (αhυ)2 and hυ, and the straight line obtained was extrapolated to zero absorption co-efficient so that it encountered the x-axis. The band gap of CdS was found to be 2.38 eV (Figure 1(b)).
Figure 1

(a) UV-Vis spectrum (b) Band gap (c) XRD pattern and (d) FTIR spectrum of CdS nanoparticles.

Figure 1

(a) UV-Vis spectrum (b) Band gap (c) XRD pattern and (d) FTIR spectrum of CdS nanoparticles.

Close modal

XRD studies

X-ray diffraction powder analysis was carried out to investigate the crystallinity, crystallite size, and phase of nanoparticles. There is no extra peak, which indicates the purity of the synthesized NPs (Devendran et al. 2013). XRD pattern shown in Figure 1(c) indicated hexagonal pattern of CdS NPs. The diffraction peaks were observed at 24.840°, 26.370°, 30.47°, 43.79°, 51.87°, 54.32°, 63.62°, 70.17°, and 72.36° at 2θ and indexed to be characteristic 100, 002, 101, 102, 110, 103, 112 and 211, respectively, for CdS NPs, similar to JCPDS card no # 00-041-1049.

FTIR studies

FTIR spectroscopy was used to confirm the purity and to investigate the functional groups of precursors or any other impurities. FTIR spectra were obtained in the range of 500–4,000 cm−1. In Figure 1(d), the peak at 400–700 cm−1 corresponds to the metal-sulphur bond. The peak at 630.33 cm−1 corresponds to Cd-S bonding mode and reveals the formation of CdS nanoparticles. The broad peak observed at 3,350.67 cm−1 was assigned to O-H (hydroxyl group) present because of the moisture absorbed by the CdS NPs. Symmetric C ≡ C bond mode was observed at 2,110.32 cm−1, and a peak at 1,633.54 cm−1 was due to asymmetric stretching of C = C bond representing the presence of traces of organic impurities (Mahdi et al. 2017).

SEM and EDX studies

The SEM image shown in Figure 2(a) obtained with 500 nm magnification indicates the formation and shape (morphology) of nanoparticles. The grains of particles have been aggregated to form clusters. EDX analysis was performed for finding the elemental composition of CdS NPs as shown in Figure 2(b). The EDX spectra reveal the presence of Cd and S as major elements in synthetic material and provide the quantitative analysis of weight percentage of compositional elements.

Figure 2

(a) Scanning electron micrograph, (b) Energy dispersive X-rays spectrum, (c) TGA curve of CdS weight loss in mg and (d) weight loss in percentage of CdS NPs.

Figure 2

(a) Scanning electron micrograph, (b) Energy dispersive X-rays spectrum, (c) TGA curve of CdS weight loss in mg and (d) weight loss in percentage of CdS NPs.

Close modal

Thermo gravimetric analysis (TGA) of CdS nanoparticles

TGA was used to study thermal behavior of the prepared CdS NPs. The utility of these NPs for various applications depends upon temperature and temperature-induced phase changes (Ayodhya & Veerabhadram 2019). The TGA was carried out under N2 atmosphere in the temperature range of 25–600 °C at a heating rate of 10 °C/min. The TGA curve of the sample shown in Figure 2(c) and 2(d) exhibited that the sample was quite stable to temperature. The weight loss of CdS was 4.89% from 40 to 250 °C, which is due to the presence of water and moisture content present in sample. The weight loss after 250 °C can mainly be assigned to the degradation of the nanomaterials and was only 1.65%. The TGA study indicated that the synthesized CdS nanomaterials were very stable to temperature and the total loss occurring was 6.54% only at 25 °C–600 °C.

Kinetics analysis

The kinetics of CdS catalyzed degradation of EB and MG dyes can be designated by Eley-Rideal (E-R) mechanism. According to this mechanism, oxygen molecules attack on the surface of catalyst in adsorbed form with the molecule of dye in fluid form (Ilyas & Saeed 2010). It has been examined that an electron-hole pair is formed between VB and CB of CdS nanocatalyst. Adsorbed oxygen present at the surface of CdS NPs takes up the electron and results in superoxide anion (O2) formation. The proton produced from superoxide anion generates energetic OH radicals. The hole created in VB of CdS moves towards the exterior, where it combines with a molecule of water and generates active OH radicals. These active species play a important role in the photodegradation of EB and MG dyes. The E-R mechanism can be described in terms of kinetics expression as
formula
(4)
where as θO2 and C represent concentration of EB and MG dye and concentration of oxygen at the surface respectively. By providing oxygen continuously make the reaction zero order with respect to oxygen, so expression of kinetic becomes as
formula
(5)
where kApp represents the rate constant of reaction and the above equation can be written in the integrated form as
formula
(6)
where as Co stands for initial concentration of dye and C for the concentration of dye after UV light illumination.
Equation (6) is realistic at various reaction temperatures (25 °C, 35 °C, 45 °C). The kinetic analysis for the photodegradation of EB and MG dyes as shown in Figure 3(a)–3(d) illustrates that the catalyst identity affects the speed of photocatalytic reaction. Tables 1 and 2 show the pseudo-first-order and pseudo-second-order photodegradation reaction parameters like kApp and their correlation coefficients. Kinetic parameters indicate how the catalytic performance is influenced by the interaction with CdS NPs. For the estimation of pseudo-second-order kinetics for the photodegradation of the EB and MG dye, the following mathematical expression was applied.
formula
(7)
Table 1

Kinetic constant parameter values for the photodegradation of EB dye at various temperatures

Temperature (Degree Celsius)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2Activation Energy (kJ/mol)
CdS 25 0.0119 0.9672 0.0297 0.9954 61.1 
35 0.0144 0.8631 0.0573 0.6966 
45 0.0206 0.8557 0.1409 0.8991 
Temperature (Degree Celsius)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2Activation Energy (kJ/mol)
CdS 25 0.0119 0.9672 0.0297 0.9954 61.1 
35 0.0144 0.8631 0.0573 0.6966 
45 0.0206 0.8557 0.1409 0.8991 
Table 2

Kinetic constant parameter values for the photodegradation of MG dye at various temperatures

Temperature (Degree Celsius)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2Activation Energy (kJ/mol)
CdS 25 0.0129 0.9697 0.0379 0.9009 32.11 
35 0.0184 0.9786 0.0853 0.9296 
45 0.0115 0.968 0.071 0.9592 
Temperature (Degree Celsius)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2Activation Energy (kJ/mol)
CdS 25 0.0129 0.9697 0.0379 0.9009 32.11 
35 0.0184 0.9786 0.0853 0.9296 
45 0.0115 0.968 0.071 0.9592 
Figure 3

(a) Application of pseudo-first-order kinetics, (b) pseudo-second-order kinetics to EB dye, (c) Application of pseudo-first-order kinetics and (d) pseudo-second-order kinetics to MG dye degradation using CdS NPs at various temperatures.

Figure 3

(a) Application of pseudo-first-order kinetics, (b) pseudo-second-order kinetics to EB dye, (c) Application of pseudo-first-order kinetics and (d) pseudo-second-order kinetics to MG dye degradation using CdS NPs at various temperatures.

Close modal

Tables 1 and 2 exhibited that the correlation coefficient along with their kApp of the pseudo-first- and second-order photodegradation reaction of EB and MG dye over CdS NPs. However due to lower values obtain of KApp from pseudo-first-order, the data was best fitted to pseudo-second-order kinetics.

The Arrhenius equation was used to find out the activation energy of the given reaction and is given as
formula
(8)
where in Equation (8) the term Ea represents the activation energy and k represent rate constant of the reaction. The equation in logarithmic form is
formula
(9)

Figure 4(a) and 4(b) represented that a straight line is obtain by interpreting a graph between lnk and 1/T with a negative slope Ea/R. The activation energy that was calculated from the slope of graph for the degradation of EB and MG was found to be 61.1 kJ/mol and 32.11 kJ/mol, as revealed in Tables 1 and 2.

Figure 4

Application of Arrhenius equation to (a) EB and (b) MG dye degradation using CdS NPs.

Figure 4

Application of Arrhenius equation to (a) EB and (b) MG dye degradation using CdS NPs.

Close modal

The effect of concentration of the dyes on the rate of reaction using 0.03 g of CdS as photocatalyst was also studied. Different concentrations i.e. 10, 15, 25 and 35 mg L−1, of dyes were used. The experimental data was evaluated on first-order and second-order rate constant equations as shown in Figure 5(a)–5(c), to obtain the apparent rate constant and correlation coefficient values. Tables 3 and 4 depict the values of apparent rate constants along with their respective regression coefficients.

Table 3

At different initial concentrations, kinetic constant parameter values for the photocatalytic degradation of EB dye

Concentration (ppm)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2
CdS 10 0.0206 0.982 0.2154 0.8325 
15 0.0106 0.9457 0.0424 0.8102 
25 0.0072 0.9793 0.0055 0.9307 
35 0.007 0.9507 0.0039 0.9195 
Concentration (ppm)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2
CdS 10 0.0206 0.982 0.2154 0.8325 
15 0.0106 0.9457 0.0424 0.8102 
25 0.0072 0.9793 0.0055 0.9307 
35 0.007 0.9507 0.0039 0.9195 
Table 4

At different initial concentrations, kinetic constant parameter values for the photocatalytic degradation of MG dye

Concentration (ppm)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2
CdS 10 0.0165 0.9805 0.1235 0.9395 
15 0.0091 0.9724 0.0131 0.8994 
25 0.0087 0.9725 0.0071 0.934 
35 0.0081 0.9783 0.0082 0.9014 
Concentration (ppm)Pseudo-first-order kinetics
Pseudo-second-order kinetics
kAppR2kAppR2
CdS 10 0.0165 0.9805 0.1235 0.9395 
15 0.0091 0.9724 0.0131 0.8994 
25 0.0087 0.9725 0.0071 0.934 
35 0.0081 0.9783 0.0082 0.9014 
Figure 5

(a) Application of pseudo-first-order kinetics (b) pseudo-second-order kinetics to EB dye, (c) Application of pseudo-first-order kinetics and (d) pseudo-second-order kinetics to MG dye degradation using CdS NPs at various initial concentrations.

Figure 5

(a) Application of pseudo-first-order kinetics (b) pseudo-second-order kinetics to EB dye, (c) Application of pseudo-first-order kinetics and (d) pseudo-second-order kinetics to MG dye degradation using CdS NPs at various initial concentrations.

Close modal
Figure 6

Mechanism of possible photocatalytic degradation of EB and Mg dye by using CdS NPs.

Figure 6

Mechanism of possible photocatalytic degradation of EB and Mg dye by using CdS NPs.

Close modal

All the parameters indicated that the data best fitted pseudo second-order kinetics.

Mechanism of photocatalytic degradation of Methylene Blue dye

Photocatalysis usually involves photo-absorption and photo-excitation of electrons from VB to the CB of a semiconductor material as shown in Figure 6. Electron–hole formation (Equation (10)), their transfer across the valence, conduction and forbidden energy bands and their recombination have been reported in the photocatalysis on the basis of band gap theory. Upon absorption of a higher-energy photon, an electron is promoted from the VB to the CB (e) of CdS with simultaneous generation of a hole (h+) in the VB. The electrons and holes recombine in the bulk or surface of the particle in a few nanoseconds.Trapped energy in the surface sites can react with donor (D) or acceptor (A) species adsorbed or close to the surface of the particle. OH.− radicals are generated when a h+ in VB reacts with a water molecule (Equation (11)) and O2 radicals are formed by the reaction of an electron in CB with a dissolved O2 molecule (Equation (12)). Complete conversion of an organic substrate to CO2 and H2O is carried out by the oxidative pathway (Faisal et al. 2021).
formula
(10)
formula
(11)
formula
(12)
formula
formula

Factors affecting the degradation of dyes

Effect of time and concentration

The effect of irradiation time and initial dye concentration on the photocatalytic degradation of EB and MG with different initial dye concentration, i.e. 10, 15, 25 and 35 ppm, was evaluated by varying irradiation time. From Figure 7(a) and 7(b) it is evident that by increasing the initial concentration from 10 to 35 ppm, the percent degradation decreased from 95% to 65% and from 90% to 71% for EB and MG, respectively. Duration of irradiation directly affects the interaction between photons and photocatalysts. Greater contact is achieved at longer irradiation time, and consequently more OH radicals are produced (Jan et al. 2021). Due to the greater availability of active sites on the surface of the photocatalyst at lower dye concentrations, higher percent degradation can be observed. The degradation rate was found to decrease as the initial dye concentration was increased. This can be explained by the fact that increasing the dye concentrations reduces the amount of light that reaches the organic molecules of both colours (Kokilavani et al. 2021).

Figure 7

Effect of concentration on the degradation of (a) EB and (b) MG dye.

Figure 7

Effect of concentration on the degradation of (a) EB and (b) MG dye.

Close modal

Effect of temperature

The temperature effect on EB and MG dyes degradation using 10 ppm concentration of dyes at the time duration of 120 minutes in aqueous solutions in the presence of CdS was studied at various temperatures, i.e. 25, 35 and 45 °C, as shown in Figure 8(a) and 8(b). An increase in percent degradation of dye was observed through increase in temperature. Maximum degradation (94.4%) was observed for EB dye and 89.2% for MG dye at 45 °C in 120 minutes, respectively. The degradation of dyes increased as the temperature was increased. This is due to the fact that at high temperatures, the interaction of dye molecules with the photocatalyst surface increases, favouring dye photodegradation (Kumar & Pandey 2017).

Figure 8

Effect of temperature on degradation of (a) EB and (b) MG dye.

Figure 8

Effect of temperature on degradation of (a) EB and (b) MG dye.

Close modal

Effects of catalyst dosage

The effect of catalyst dosage on EB and MG degradation was investigated by changing the catalyst's mass from 0.01 g/L to 0.05 g/L with illumination under UV light for 140 minutes as shown in Table 5 and supplementary Figures 2 and 3. An increase in dye degradation was observed with an increase in CdS photocatalyst mass from 0.01 g/L to 0.03 g/L. This can be attributed to the fact that an increase in catalyst mass possibly contributes an increased surface, resulting in more photons received at the surface of the catalyst (Christopher et al. 2011). Shielding of the photons by the suspension also occurs at increased catalyst dosage. An optimum amount of catalyst should be used for dye degradation in wastewater, above which the rate of degradation will eventually decrease. Maximum catalyst dosage for maximum activity has been reported to be 3–4 g/L of dye solution (Mohammadzadeh et al. 2015).

Table 5

Percent degradation of EB dye and MG dye using different dosage of CdS NPs

Catalyst dosage (g/L)0.010.020.030.040.05
Eosin B (%) degradation 48 55.8 61.5 54.1 49.6 
Methyl Green (%) degradation 43 74.4 81.7 56.4 50.9 
Catalyst dosage (g/L)0.010.020.030.040.05
Eosin B (%) degradation 48 55.8 61.5 54.1 49.6 
Methyl Green (%) degradation 43 74.4 81.7 56.4 50.9 

Effect of pH

Degradation of the dyes is strongly affected by the pH of the medium, as the ionization of the surface functional groups is influenced by the solution pH. The effect of pH on the degradation of EB and MG was studied by varying the pH from 4 to 10. As is evident from Table 6 and supplementary Figure 4, with increase in pH, degradation ability of EB also increased. This can be attributed to the fact that at high pH a greater number of the negatively charged sites are available on the adsorbent surface. Strong electrostatic attraction between the cationic dye (EB) and OH ions present in the catalyst leads to utmost degradation of dyes from the medium (NirmalaDevi et al. 2018).

Table 6

Percent degradation of CdS NPs on EB dye and MG dye degradation at different pH

pH45678910
Eosin B (%) degradation 33.1 43.4 47.3 62.5 64.9 77.9 79.2 
Methyl Green (%) degradation 88.3 67.8 62.8 53.1 43.4 38.7 35.4 
pH45678910
Eosin B (%) degradation 33.1 43.4 47.3 62.5 64.9 77.9 79.2 
Methyl Green (%) degradation 88.3 67.8 62.8 53.1 43.4 38.7 35.4 

Conversely, MG is anionic dye and behaves differently as compared to EB dye. Maximum degradation was observed at low pH (pH = 4). This can be attributed to the fact that a larger amount of MG can adsorb on the catalyst surface at low solution pH, as evident from Table 6 and supplementary Figure 5. The adsorption of anionic adsorbate species on the negatively charged surfaces of the catalyst is opposed at higher pH. The surface tends to acquire positive charge at low pH, thereby resulting in an increased adsorption of dyes, thus increasing electrostatic attraction between the negatively charged dye and the positively charged catalyst (Haque et al. 2011). Our results are in agreement with previously reported results.

Effect of reusability of catalyst

The reusability of photocatalyst is important in large-scale processes; that's why the recyclability and stability of the CdS photocatalyst was investigated through the degradation of EB and MG dye under UV light illumination. The catalyst was centrifuged and then recycled without further treatment by washing with ethanol, twice deionized water, and drying at 50 °C for 30 minutes. The maximum photocatalytic efficiency of CdS for EB was 65.3% and for MG dye was 64.4% in 140 minutes' time interval, as shown in Table 7 and supplementary Figures 6 and 7 respectively. From the figure it is evident that a slight decline in the recovery occurs as compared to the original. This can be explained by the catalyst's excellent stability and durability, as evidenced by its low loss and deactivation during the cycling experiment (Chen et al. 2017).

Table 7

Percent degradation of CdS NPs at recovered catalyst on EB dye and MG dye degradation

Time (min)20406080100120140
Re-used CdS Eosin B (%) degradation 25.5 35.3 39.4 42.2 50.5 58.8 65.3 
Re-used CdS Methyl Green (%) degradation 14.4 16.1 35.7 51.3 56.5 59.3 64.4 
Time (min)20406080100120140
Re-used CdS Eosin B (%) degradation 25.5 35.3 39.4 42.2 50.5 58.8 65.3 
Re-used CdS Methyl Green (%) degradation 14.4 16.1 35.7 51.3 56.5 59.3 64.4 

Antibacterial activity by well diffusion method

Figure 9 displays antibacterial activity of CdS nanoparticles. Pathogenic bacterial strains such as A. baumannii, S. aureus and E. coli were used for this study. The results showed that chemically synthesized CdS were energetically involved in the antibacterial activity against pathogenic bacteria. Good antibacterial activity against S. aureus was shown by CdS NPs. It can be explained that CdS NPs might have better penetration through the agar and consequently into cell bacteria than the plant extract due to the small size of nanoparticles. Hypothetical theories reveal that the inhibition is caused by ionic adsorption of nanoparticles on the bacteria's surface, resulting in a high intensity of the proton motive force. However, the actual mechanism of the bacteria's inhibition is yet unknown. One idea claims that NPs infiltrate the bacterial cell and bind to essential enzymes containing thiol groups (Prasad & Swamy 2013).

Figure 9

(a) and (b) Anti-bacterial activity of CdS NPs.

Figure 9

(a) and (b) Anti-bacterial activity of CdS NPs.

Close modal

Human RBC haemolysis assay

Table 8 and supplementary Figure 8 depict the degree of haemolysis caused by CdS nanoparticles exposed to human red blood cells for 1 h of incubation. With increase in the concentration of nanoparticles in the PBS/RBC mixture, the percent haemolysis also increased. For the particle concentrations at 2 mg/mL, CdS NPs caused significantly greater levels of haemolysis, exceeding 29%. The ASTM E2524–08 standard criterion states that haemolysis of more than 5% indicates that the test material damages RBCs, but this criterion was exceeded in the pour study at a particle concentration of 2 mg/mL for CdS nanoparticles. Because 5% haemolysis is considered a permissible limit for nanomaterials, up to 2 mg/mL of CdS nanoparticles can be used for haemolysis activity. As a result, the generated CdS nanoparticles can be said to be biocompatible in nature at very low concentrations (Singh et al. 2019).

Table 8

Biocompatibility study of CdS NPs

Concentration of CdS NPsPBS2 mg/mL4 mg/mL6 mg/mL8 mg/mL10 mg/mLTriton X-100
Hemolysis (%) 1.00 29.17 35.40 48.30 62.30 77.70 99.54 
Concentration of CdS NPsPBS2 mg/mL4 mg/mL6 mg/mL8 mg/mL10 mg/mLTriton X-100
Hemolysis (%) 1.00 29.17 35.40 48.30 62.30 77.70 99.54 

The CdS successfully synthesized through the hydrothermal route have a band gap of 2.38 eV. The particles were aggregated to form clusters with crystalline hexagonal structures. CdS nanoparticles were found to be very stable to temperature as a small loss (6.54%) in weight occurred during heating from 25 °C to 600 °C. More than 89% dye degradation was noticed at 160 minutes' time duration for both the dyes. The dye degradation was found to decrease with increase in the initial concentration of dyes. Increasing the temperature enhanced % degradation, while with varying pH the dyes behaved differently. High % degradation of EB was observed at high pH (pH 10), while at low pH (pH 4) high % degradation was found for MG dye. Up to pH 9 of the medium the degradation was found to increase. The degradation of EB and MG over CdS catalyst surface follows second-order kinetics. The recyclability study showed that the CdS are stable and durable nanoparticles. At lower concentration, CdS NPs are found to be biocompatible, while good antibacterial activity was shown against S. aureus.

All relevant data are included in the paper or its Supplementary Information.

Ayodhya
D.
&
Veerabhadram
G.
2019
Facile fabrication, characterization and efficient photocatalytic activity of surfactant free ZnS, CdS and CuS nanoparticles
.
Journal of Science: Advanced Materials and Devices
4
(
3
),
381
391
.
Blažeka
D.
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