Antibacterial Activities of Syzygium aromaticum Leaves Assisted Gold Nanoparticles

Nanotechnology has attracted great interest in recent years due to its expected impact on many areas such as energy, medicine and electronics [1]. Currently, there is a growing need to develop an environmentally benign colloidal nanoparticle synthesis process that does not involve any toxic chemicals in the synthesis protocol, which raise great concern for environmental reasons [2]. Among the nanoparticles, gold nanoparticles (AuNPs) have received major attention due to their unique and tunable surface plasmon resonance (SPR) [3]. They have many effective applications in the biomedical sciences, including drug delivery, photothermal therapy and immunochromatographic identification of pathogens in clinical specimens [4]. Biosynthesis of AuNPs using microorganisms such as bacteria, fungi and yeast [5] has already been exploited. However, biosynthesis of nanoparticles by plant extracts is currently under exploitation. Syzygium aromaticum (clove) is a traditional spice that has been used for food preservation and possesses various pharmacological activities. Syzygium aromaticum is rich in many phytochemicals, such as sesquiterpenes, monoterpenes, hydrocarbons and phenolic compounds. It is a medicinal plant with various therapeutic applications, including antimicrobial, anticancer and anti-inflammatory activities.

For the synthesis of AuNPs, 5 mL of Syzygium aromaticum leaf (SAL) extract was added to an aqueous solution of HAuCl₄ (1 mM) and stirred continuously for 15 min; the solution turned pink after 6 h, giving SAL-AuNPs. The samples were then dried in a muffle furnace at 100 °C for 2 h. The obtained powder was stored for further characterization by XRD, UV–vis and TEM.

The UV–vis absorption spectrum of SAL-AuNPs is shown in Fig. 1(a). SPR is a collective excitation of electrons in the conduction band around the nanoparticle surface. The colour of the HAuCl₄ solution changed from transparent to light or ruby red, indicating the formation of SAL-AuNPs. The visible red colour is due to the reduction of Au³⁺ to Au⁰. The characteristic absorption peak centred at 535 nm indicates the formation of nanoparticles, which are polydispersed (Fig. 1(a)). This result is in accordance with the results obtained from the bioreduction of gold nanoparticles using SAL extract.

Fig. 1(b) shows the XRD pattern of the AuNPs. The diffraction peaks at 2θ = 37.8°, 44.6°, 64.1° and 77.4° were indexed to the (111), (200), (220) and (311) planes, respectively, for the resultant particles with a face-centred cubic (FCC) phase. The XRD peaks of the AuNPs synthesized with the extract (JCPDS card no. 04-0784) match previously reported results. The average crystallite size of the SAL-AuNPs, calculated using the Debye–Scherrer formula, was found to be 22 nm. The SAL extract completely bioreduced the gold with high purity. This result reveals that the green synthesis produces AuNPs in a highly stable form of Au.

The TEM image of AuNPs derived from a higher concentration of SAL is shown in Fig. 2(a). TEM analysis elucidates the size and morphology of the obtained particles. It reveals that the AuNPs are spherical in shape. The particle sizes are in the range of 15 nm. The selected area electron diffraction (SAED) pattern of SAL-AuNPs shown in Fig. 2(b) confirms their crystalline nature.

The antibacterial activity of the synthesized AuNPs was evaluated by the agar well diffusion method. The antibacterial activity of SAL-AuNPs against Gram-positive (S. aureus) and Gram-negative (P. aeruginosa) bacteria is shown in Fig. 3. The highest zone of inhibition was observed in the range of 23 mm. Based on Fig. 3, it can be concluded that the SAL-AuNPs were more active against P. aeruginosa, with the greatest antibacterial effect. In our results, the Gram-positive bacteria showed a lower zone of inhibition, while the Gram-negative bacteria showed better results. This characteristic enhances the biological and chemical activity of the nanoparticles, with high antibacterial efficacy.

The green synthesis method is eco-friendly and capable of producing AuNPs at room temperature. The UV–vis absorption spectrum shows the SPR band at 535 nm, which is the characteristic peak of AuNPs. The XRD pattern demonstrated the polycrystalline nature of the AuNPs. The average crystallite size of the AuNPs is 22 nm. The HR-TEM image showed a size distribution in the range of 15 nm and a spherical morphology. Stronger antibacterial inhibition was observed against Gram-negative bacteria than against Gram-positive bacteria.