Extracellular facile biosynthesis, characterization and stability of gold nanoparticles by Bacillus licheniformis

Figures & data

Figure 1. Visual observation of B. licheniformis cell lysate supernatant after 24 h of incubation in test tubes. (a) Control sample having only bacterial cell lysate supernatant; (b) Cell lysate supernatant with aqueous solution of HAuCl₄ showing gold precipitation (color changed to pink).

Figure 2. The optical absorption spectra of gold nanoparticle at different time intervals (a) Absorbance of gold nanoparticles against wavelength (control, 12h, 24h, and 48h); (b) The graph shows relationship between optical absorbance of gold nanoparticles with incubation time; (c) Stability of gold nanoparticles at room temperature (1 month and 2 month).

Table I. Optical properties, size, and zeta potential of synthesized gold nanoparticles from cell lysate supernatant of B. licheniformis at different time interval.

Gold nanoparticles in suspension	Surface plasmon absorption wavelength (nm)	Average particle size (nm)	Zeta potential (mV)
As synthesized	540	38 ± 2.57	− 24.6 ± 1.76
After 1 month	545	45 ± 3.28	− 21.9 ± 2.45
After 2 month	548	47 ± 4.25	− 20 ± 3.56

Figure 3. Histogram showing particle size distribution of gold nanoparticles.

Figure 4. SEM and EDX analysis (a) representative SEM micrograph of gold nanoparticles synthesized by B. licheniformis cell lysate supernatant; (b) Spot EDX profile confirming the presence of gold in sample.

Figure 5. Antimicrobial activity of AuNPs against (a) E. coli, (b) P. aeruginosa, (c) B. Subtilis. Cup containing (I) sterile deionized water as control and (II) gold nanoparticles; concentration of AuNPs in each plate was 25 μg/mL.

Figure 6. A comparative flow chart of downstream processing steps to get purified gold nanoparticles from (a) Intracellular synthesis, (b) Extracellular synthesis (CLS mediated).