Green Synthesis of Gold Nanoparticles from Garcinia kola Pulp Extract and Their Antibacterial Properties

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Green Synthesis, Characterization, and Antibacterial Investigation of Synthesized Gold Nanoparticles (AuNPs) from Garcinia kola Pulp Extract

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DOI: 10.1007/s11468-020-01274-9

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Green Synthesis, Characterization, and Antibacterial Investigation of Synthesized Gold Nanoparticles (AuNPs) from Garcinia kola Pulp Extract

Sunday Adewale Akintelu^1,2&Bo Yao¹&Aderonke Similoluwa Folorunso³

Received: 7 March 2020 / Accepted: 26 August 2020

#Springer Science+Business Media, LLC, part of Springer Nature 2020

Abstract

The properties of gold nanoparticles, such as conductivity, half-life, catalytic nature, improved solubility efficiency, chemical stability, and its mechanism in binding with bacterial cell wall to induce cell death, have increased its medicinal applications. This study investigated the antibacterial property of gold nanoparticles (AuNPs) synthesized fromGarcinia kolapulp extract. The ripe fruits ofGarcinia kolawere obtained, and the pulps were removed, air dried, pulverized, extracted, and mixed with gold chloride solution following standard procedure. The synthesized AuNPs were characterized using ultraviolet visible spectrophotometer (UV-visible), Fourier transform infrared (FTIR) spectrophotometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX). The antibacterial examination was carried out on four clinical isolate bacteria using agar diffusion technique. The UV-visible analysis confirmed the formation of stable AuNPs at pH 7 in the wavelength region of 564 nm. The FTIR analysis confirmed that the secondary metabolites with–OH functional groups serve as the reducing agent for the synthesis of AuNPs. The morphological assessment of the synthesized AuNPs revealed spherical- shaped particles. The zones of inhibition of the synthesized AuNPs against test bacteria ranges from 3 to 13 mm while that of the control was in the range of 14 to 22 mm. This study provided an environmentally friendly approach in the synthesis of AuNPs fromGarcinia kolapulps and also ascertained the medicinal application of the synthesized AuNPs as an antibacterial agent.

Keywords Garcinia kolapulp . Clinical isolates . Gold nanoparticles . Characterization and antibacterial activity

Introduction

Currently, the world at large is confronted with major challenge arising from the modern health sector as many antibiotics have lost their usefulness in preventing or curing infectious diseases caused by microbes [1]. Several decades ago, antimicrobial agents played a crucial role in curing infectious diseases that emanates from either bacteria or fungi, but the

existence of precarious and antibiotic-resistant bacteria is a major concern.

Drug resistance developed by pathogens can be imple- mented by many mechanisms; therefore, overcoming this kind of challenge is tasking [2]. Hence, the exploration of compounds with efficacy to inhibit the growth of these pathogenic organisms and their resistant to modern drugs is very crucial [3–6].

The application of gold nanoparticles as prospective antimicrobial agents in eradicating the growth of microbial infec- tions and in reduction of the alarming challenges of antimicrobial resistance to existing antibiotics has been reported [7–9]. However, most of the processes and methods adopted for the synthesis of nanoparticles are ineffective, toxic, and are of environmental concerns [10–12].

Therefore, the development of an environmentally friendly and non-toxic approaches for the synthesis of nanoparticles from plants extract is highly needed. Previous studies have reported a new method of synthesizing nanoparticles which entails the use of plant extracts as capping and reducing agents [13]. The synthesis of nanoparticles from plant materials has

* Aderonke Similoluwa Folorunso [email protected]

1 MOE Key Laboratory of Cluster Science, Beijing Key Laboratory of Photoelectronic Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, People’s Republic of China

2 Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso, Nigeria

3 Department of Chemistry, Louisiana State University, Louisiana, USA

https://doi.org/10.1007/s11468-020-01274-9

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been documented to be economical, easy, and environmentally harmless [14].

Garcinia kolasometimes called“wonder plant”has been reported to be a good medicinal source for curing some dis- orders [15–17]. Among other species of trees and plants in Africa,“Garcinia kola”has been enlisted for conservation in the Sub-Saharan Forest Genetic Resources Programme and as the sixth most preferred tree species for domestication in Africa by World Agroforestry Centre (ICRAF) as a result of its known medicinal properties [18–20].

The mastication of its seeds has been reported to prevent gastric problems, and the anti-inflammatory and antimicrobial activities of biflavonoid kolaviron isolated fromGarcinia kola kernel have been documented [21]. In addition, the wound healing power and inhibitory potency of kolaviron against malaria has been reported [22,23]. The laboratory investigation ofGarcinia kolaseed in inhibiting Ebola virus showed good activity against the growth of Ebola virus [24].

Despite the acclaimed health benefit of this plant, the application of gold nanoparticles synthesized from the pulp of this plant fruit as an antibacterial agent still remain a novel research area yet unexplored. This study was then aimed at the investigation of the antibacterial potency of AuNPs synthesized fromGarcinia kolapulp (Fig.1).

Materials and Methods

Ripe fruits ofGarcinia kolawere collected from a local farm in Irele, Ondo state in Nigeria. The pulps were removed, washed, air dried at room temperature in a dark room, and pulverized into fine particles. Twenty-three grams of the pulverized pulp was measured in a 200-ml Erlenmeyer flask, 100 ml of de-ionized water was added, and the mixture was boiled for 40 min. The mixture was cooled and filtered with

Whatman filter paper no. 1. The aqueous extract obtained was kept in an amber-colored sample bottle, and stored in the refrigerator for further analyses.

Synthesis of Gold Nanoparticles

One milliliter of Garcinia kola pulp extract was added to 1 mM gold chloride solution (HAuCl4) and was placed on a stirrer at room temperature for 5 h. The brown color of the Garcinia kola pulp extract changed to violet color which established the synthesis of AuNPs. The mixture was then incubated for 48 h to aid the complete reduction of gold chloride to AuNPs.

Characterization of Synthesized of AuNPs

UV-visible spectrophotometer (UV-1800 Shimadzu) was used for the investigation of the complete bioreduction of gold ion by scanning the synthesized AuNPs in the UV- visible region at wavelength 300–700 nm. The identifica- tion of functional groups found in the extract and synthesized AuNPs were examined by scanning the neat sample of Garcinia kola pulp extract and synthesized AuNPs at 500–4000 cm⁻¹wavelength on a Fourier transform infrared spectrophotometer (Nicolet iS50 (Thermo Fisher Scientific, Waltham, MA,USA). The morphological examination of the synthesized AuNPs were carried out using SEM (JEOL Japan, JSM5910) and TEM (JEM-1230, JEOL, Japan) following standard procedures [25]. The quantitative elemental evaluation of the AuNPs synthesized was ascertained by EDX analysis [26].

Bacterial Strains Used for the Antibacterial Screening

Four clinical isolate bacterial strains, namely Pseudomonas aeruginosa and Escherichia coli (Gram-negative bacteria) andStaphylococcus aureusandBacillus cereus(Gram-positive bacteria).

Preparation of Synthesized AuNPs for Antibacterial Screening

About 0.2 g of synthesized AuNPs that has been washed and concentrated on the rotary evaporator was quantitatively dis- solved with 2 mL of methanol to attain a concentration of 100 mg/ml which was further diluted to 75μL, 50 μL, and 25μL concentrations.

Fig. 1 Diagram showingGarcinia kolafruit, pulp, and seeds

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Antibacterial Activity

The bacterial activity of synthesized AuNPs was determined using agar diffusion technique. The pure cultures of the strains at 1 × 108 CFU/mL were homogenously swabbed onto the Mueller–Hinton agar (MHA) medium via sterile swabs. Six- millimeter holes of hollow block medium were cut from the MHA plates with 100μL sterile pipette edges. Sterilized mi- cropipette was used to add 25, 50, and 75μL concentrations of synthesized AuNPs and 25μL of amoxicillin serving as the control into the wells. The cultured inoculated plates treated with the synthesized AuNPs and amoxicillin were then incubated at 37 °C for 22 h. Their inhibition zones were then measured and recorded. This procedure was repeated for ad- ditional two times.

Results and Discussion

Visual Observation

The color of the mixture changes immediatelyGarcinia kola pulp extract was added to the solution of HAuCl4. The color changed from purple to complete violet color during the incu- bation period in the dark. This color change indicated the formation of gold nanoparticles. This color change was due to excitation of surface plasmon vibration of AuNPs [27].

UV Analysis of Synthesized AuNPs

The spectra obtained from the combination of various ratios of Garcinia kolapulp extract solution and 1 mM gold chloride

solution analyzed by UV spectrophotometer are shown in Fig. 2. The combination ofGarcinia kolapulp extract and 1 mM gold chloride solution at ratio 1:4 gave the maximum absorption band at 568 nm. The occurrence of strong absorption in the ultraviolet visible region of 500 to 600 nm has been reported as gold surface plasmon resonance [28]. Therefore, the observation of a strong absorption peak at 568 nm in the UV visible region in this study confirmed the formation of AuNPs from pulp extract.

Stability and Optimization Study

pH measurement was used to monitor the optimization and stability of the synthesized AuNPs. The UV spectrum of synthesized AuNPs at various pH values is shown in Figs.4and 5. The absorption intensity increase as the pH values increase from acidic to basic values (3–7) which indicated the bioreduction process of the synthesis of AuNPs. The most stable and optimal absorption intensity at pH 7 in the wavelength region of 564 nm confirmed the neutralization of the reaction mixture and the stability of the synthesized AuNPs.

This finding is supported by previous investigation which started that enhanced AuNP synthesis is observed at pH of 6–8 [29,30] (Fig.3).

Mechanism for the Synthesis of AuNPs

The mechanism for the synthesized AuNPs entails the reduction of gold salt to gold nanoparticles (Au³⁺→Au⁰) by Garcinia kolapulp extract. The secondary metabolites such as terpenoids, steroids isoflavonoids, and neoflavonoids in the Garcinia kolapulp extract are responsible for the reduction

Fig. 2 UV spectrum of synthesized AuNPs

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and stabilization of the metal nanoparticles because such secondary metabolites have ability to donate electrons or hydro- gen atoms required for the conversion of (Au³⁺→Au⁰). The presence of hydroxyl functional groups in the secondary metabolites ofGarcinia kolapulp extract has indicated on the IR spectrum at wavelength 3318.67 cm⁻¹in Fig.4 predict the possibility of the–OH group to bind with Au³⁺ions to form gold complexes which will be reduced to Au⁰seed particles.

This then confirmed that the synthesis of AuNPs was from the interaction of AuCl⁴⁻withGarcinia kola pulp extract. This finding is in accordance with the report of previous re- searchers [31–33].

FTIR Analysis

The functional groups responsible for the interactions between theGarcinia kolapulp extract and the synthesized AuNPs are shown in FTIR spectra in Figs.4and5. The FTIR spectrum of Garcinia kola pulp extract displayed absorption peaks at 3318.67 cm⁻¹, 2028 cm⁻¹, 2912.15 cm⁻¹, and 1642.14 cm⁻¹ that correspond to–OH,–NCS,–C–H, and–C=C–functional groups, respectively, which indicated that theGarcinia kola pulp extract contained certain secondary metabolites. Also, the FTIR spectrum of synthesized AuNPs showed peaks at 3340.43, 2880.23, 1630, 1250.11, and 750.34 cm⁻¹that are

Fig. 4 FTIR spectrum of Garcinia kolapulp extract Fig. 3 UV spectrum of AuNPs at different pH values

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linked to–OH, –C–H, –C=C–, and fatty acid, respectively.

The disappearance of some of peaks and variation in the absorption wavelength of the functional groups indicated that Garcinia kolapulp extract contains essential secondary metabolites that can function as capping and reducing agent for the synthesis of AuNPs. This finding is in agreement with the report of Bakur et al. and Bakur et al. [34,35].

Morphological Assessment of the Synthesized AuNPs

SEM micrograph of synthesized AuNPs shown in Fig.6un- veiled an agglomeration of spherical shape particles. The shape of the AuNPs is similar to the findings of Mendoza

et al. [36]. Consequently upon this, the TEM micrograph of synthesized AuNPs shown in Fig.7also established that the synthesized AuNPs are spherical. The measurement of the size confirmed that the particle size is in the range of 18–

38 nm with an average size of 28 nm. This finding is in line with the study of Lei et al. [37].

The elemental composition of the synthesized AuNPs obtained from EDX spectrum in Fig.8shows three major peaks.

The prominent peak at 2.3 keV is traceable to the absorption signal of gold. This confirmed the synthesis of gold nanoparticles. Furthermore, the signals of carbon and oxygen are also detected. Carbon and oxygen signal might have resulted during sample preparation or exposure to the atmosphere. The

Fig. 6 SEM micrograph of synthesized AuNPs Fig. 5 FTIR spectrum of synthesized AuNPs

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data generated from the EDX report is synonymous to that which was reported by Khan et al. [38].

Antibacterial Activity of Synthesized AuNPs

The antimicrobial activity of the synthesized AuNPs is shown in Figs.9and10The synthesized AuNPs showed broad spectrum of inhibition against the growth of the tested bacteria.

Maximum inhibition zone of synthesized AuNPs was observed againstEscherichia coli(13 mm) at 75μL, and the

least inhibition zone was observed against Staphylococcus aureus(3 mm) at 25μL, while the maximum inhibition of the control was observed against Pseudomonas aeruginosa and was least inhibition was againstStaphylococcus aureus.

The order of susceptibility of the tested bacterial strain to the synthesized AuNPs was Pseudomonas aeruginosa >

Escherichia coli > Bacillus cereus > Staphylococcus aureus.

This showed that the synthesized AuNPs is most potent against the growth of Gram-negative bacteria (Pseudomonas aeruginosa) at all concentrations.

Fig. 7 TEM micrograph of synthesized AuNPs

Fig. 8 EDX spectrum of synthesized AuNPs

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The antibacterial mechanism demonstrated by the synthesized AuNPs is subjected to the degree of susceptibility of microbes. When the synthesized AuNPs come in contact with the bacteria, it binds with the bacterial surface through elec- trostatic interaction. The distinct smaller size and proton mo- tive force of the synthesized AuNPs enable their penetration into the bacterial cell through the membrane proteins

especially the sulfur groups present in proteins to form thiols due to its greater affinity toward sulfur groups and also on the phosphates forming complexes, thereby leading to their DNA damage [39–42].

It has also been documented that the interaction of metal nanoparticles with cysteine residues results in ROS generation by inhibiting electrons at terminal oxidase, thereby prompting Fig. 9 Antibacterial activity of

synthesized AuNPs against test bacterial strain

0 5 10 15 20 25

25 μL Garcinia kola pulp

extract

50 μL 75 μL 25 μL

Amoxicillin

Pseudomonas aeruginosa Escherichia coli Staphylococcus aureus Bacillus cereus

Inhibion Zones

Bacterial Strains Fig. 10 Antibacterial activity of

synthesized AuNPs

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bacterial cell death [27]. The variation in the susceptibility pattern of Gram-positive and Gram-negative strains toward AuNPs can be linked to the cell wall. This finding is supported with previous studies which stated that Gram-positive bacteria possess a thick cell wall that prevent intrusion of foreign attack, while Gram-negative bacteria possess tiny cell wall which makes them susceptible to antibiotics [28,29].

Conclusions

An efficient, cheap, and environmentally friendly method of synthesizing AuNPs from Garcinia kola pulps was established. This makes the green approach of synthesizing AuNPs the most desirable method in terms of economic sus- tainability. The synthesized AuNPs demonstrated a promising antibacterial activity against test bacteria. Thus,Garcinia kola pulps could serve as a potential ingredient in production of novel effective antibiotic against bacteria attack. Study on the evaluation of the antimicrobial potency of the synthesized AuNPs fromGarcinia kolapulp extracts against extensive range of human pathogenic bacteria and fungi should be carried out. Further research on the isolation and elucidation of compounds responsible for the antibacterial potency of Garcinia kolapulps should be encouraged. Also, the domestication ofGarcinia kolaplants should be encouraged.

Authors’Contribution All the authors contributed equally.

Compliance with Ethical Standards

Conflict of Interest The authors declare that they have no conflict of interest.

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