Novel green synthesis of gold nanoparticles using Citrullus lanatus rind and investigation of proteasome inhibitory activity, antibacterial, and antioxidant potential

Figures & data

Figure 1 (A,B) UV–vis spectra of gold nanoparticles synthesized using the aqueous extract of watermelon rind.

Note: (A) Change in color of the solution with respect to incubation time.

Abbreviations: UV–vis, ultraviolet-visible; min, minutes; h, hours.

Figure 2 FT-IR analysis of gold nanoparticles (ANPs) synthesized using the aqueous extract of watermelon rind.

Abbreviations: FT-IR, Fourier-transform infrared spectroscopy; WMP, watermelon outer peel.

Figure 3 SEM, EDX analysis, and size distribution of gold nanoparticles (ANPs).

Notes: (A and B) SEM images; (C) size distribution of ANPs; (D) EDX spectra of gold nanoparticles.

Abbreviations: EDX, energy-dispersive X-ray; SEM, scanning electron microscopy.

Figure 4 XRD spectra of gold nanoparticles.

Abbreviation: XRD, X-ray powder diffraction.

Figure 5 TGA analysis of gold nanoparticles.

Abbreviation: TGA, thermogravimetric analysis.

Figure 6 Proteasome inhibitory potential of gold nanoparticles and standard reference compound epoxomicin (E). ANP-1:100 µg/mL; ANP-2: 10 µg/mL; E-1: 5.56 µg/mL; and E-2: 0.56 µg/mL.

Abbreviation: ANPs, gold nanoparticles.

Table 1 Antibacterial activity of colloidal ANPs and standard antibiotics (kanamycin and rifampicin) against foodborne bacteria

Foodborne pathogens	ANPs (50 µg/disc)	WAQ extract (50 µg/disc)	Positive control		Negative control*	MIC (µg/mL)	MBC (µg/mL)
			Kanamycin (5 µg/disc)	Rifampicin (5 µg/disc)
Bacillus cereus ATCC 13061	11.58±0.65^a,#	0	0	0	0	50	100
Escherichia coli ATCC 43890	9.23±0.40^b	0	0	0	0	50	50
Listeria monocytogenes ATCC 19115	0±0^c	0	0	0	0	–	–
Staphylococcus aureus ATCC 49444	0±0^c	0	0	0	0	–	–
Salmonella typhimurium ATCC 43174	11.15±0.35^a	0	0	0	0	50	100

Notes: Values with different superscript letters in each column are significantly different at P<0.05.

# Data are expressed as the mean zone of inhibition in mm ± SD.

* 5% DMSO.

Abbreviations: ANPs, gold nanoparticles; MBC, minimum bactericidal concentration; MIC, minimum inhibitory concentration; WAQ, watermelon aqueous extract; ATCC, American Type Culture Collection; DMSO, dimethyl sulfoxide; SD, standard deviation.

Figure 7 Antibacterial potential of (A) aqueous extract of watermelon rind and (B) gold nanoparticles.

Abbreviations: B. cereus, Bacillus cereus; E. coli, Escherichia coli; L. monocytogenes, Listeria monocytogenes; S. aureus, Staphylococcus aureus; S. typhimurium, Salmonella typhimurium.

Table 2 Synergistic antibacterial potential of colloidal ANPs against foodborne bacteria

Foodborne pathogens	ANPs + kanamycin#	ANPs + rifampicin##
Bacillus cereus ATCC 13061	11.93±0.42^d,*	12.01±0.40^d
Escherichia coli ATCC 43890	21.08±0.94^b	10.32±0.30^d
Listeria monocytogenes ATCC 19115	12.16±0.36^c	10.56±0.25^d
Staphylococcus aureus ATCC 49444	12.04±0.23^c	24.84±1.86^a
Salmonella typhimurium ATCC 43174	12.74±0.70^c	12.00±0.40^d

Notes:

* Data are expressed as the mean zone of inhibition in mm ± SD. Values with different superscript letters in each column are significantly different at P<0.05.

# ANPs (25 µg) + kanamycin (5 µg).

## ANPs (25 µg) + rifampicin (5 µg).

Abbreviations: ANPs, gold nanoparticles; ATCC, American Type Culture Collection; SD, standard deviation.

Figure 8 Synergistic antibacterial potential of gold nanoparticles with kanamycin (A) and rifampicin (B).

Abbreviations: B. cereus, Bacillus cereus; E. coli, Escherichia coli; L. monocytogenes, Listeria monocytogenes; S. aureus, Staphylococcus aureus; S. typhimurium, Salmonella typhimurium.

Figure 9 DPPH free radical scavenging potential of ANPs.

Note: Different superscript letters indicate significant differences in the mean at P<0.05.

Abbreviations: ANPs, gold nanoparticles; BHT, butylated hydroxyl toluene; DPPH, 1,1-diphenyl-2-picrylhydraxyl; WAQ, watermelon aqueous extract.

Figure 10 Nitric oxide scavenging potential of ANPs.

Note: Different superscript letters indicate significant differences in the mean at P<0.05.

Abbreviations: ANPs, gold nanoparticles; BHT, butylated hydroxyl toluene; WAQ, watermelon aqueous extract.

Figure 11 ABTS radical scavenging potential of ANPs.

Note: Different superscript letters indicate significant differences in the mean at P<0.05.

Abbreviations: ABTS, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid); ANPs, gold nanoparticles; BHT, butylated hydroxyl toluene; WAQ, watermelon aqueous extract.

Figure 12 Reducing power potential of ANPs.

Note: Different superscript letters indicate significant differences in the mean at P<0.05.

Abbreviations: ANPs, gold nanoparticles; BHT, butylated hydroxyl toluene; OD, optical density; WAQ, watermelon aqueous extract.