Oluwatosin Elizabeth Adeyemi

BioNanoScience https://doi.org/10.1007/s--x The In vitro Biological Activity of Biosynthesized Silver Nanoparticles Produced Using Mangifera indica Stem Bark Extract and Properties of Its Pharmaceutical Gel Formulation Oluwatosin E. Adeyemi1 · Omobolanle A. Omoteso2 · Tolulope O. Ajala1 Accepted: 19 April 2023 © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2023 Abstract This study reports the production of silver nanoparticles using Mangifera indica stem bark (aqueous and methanol) extracts as capping agents and formulation of pharmaceutical gel loaded with the nanoparticles. The extracts were prepared using standard procedures and utilized in biosynthesizing silver nanoparticles. Biosynthesis was ascertained through colour changes, UV–Visible and FTIR spectroscopy. Antioxidant activity of the extracts and biosynthesized nanoparticles were examined by DPPH method. The antimicrobial evaluation was carried out on Pseudomonas aeruginosa and Staphylococcus aureus. Pharmaceutical gels were produced (F1–F5), and loaded with the nanoparticles. Nanoparticles exhibited maximum absorption under UV–visible spectroscopy between 315 and 320 nm. FTIR spectrum showed that alkene and ester functional groups were conferred on the silver nanoparticles by the extracts used. The nanoparticles demonstrated antimicrobial activity against the organisms, which was significantly higher (p < 0.05) than for extracts and reference drug. The antioxidant capacity was in a concentration-dependent manner but significantly lower (p < 0.05) than that of the reference drug. Formulated gels had acceptable organoleptic profiles, pH range of 6.8–7.1, high viscosity, and pseudoplastic flow patterns. The in vitro release profiles of the gels showed was gradual, with t­ 90 higher than 2 h. The release seemed to be influenced by the viscosity of the gel systems. In addition, the release kinetics of the nanoparticle-loaded gel systems followed Higuchi model with r2 ranging from 0.9958 to 0.9980. Mangifera indica extracts were successfully used as bio-reducing agents in the synthesis of silver nanoparticles. The gel formulations had acceptable physical properties and release profiles. Keywords Mangifera indica extracts · Aqueous extract-mediated silver nanoparticles · Methanol extract-mediated silver nanoparticles · Antioxidant activity · Antibacterial activity · Pharmaceutical gel 1 Introduction The skin is the largest organ of the human body, and as a region of drug administration, it provides large contact surface to achieve both local and systemic effects. Topical formulations are designed to penetrate some extent into the skin layers where their effects are exerted for the treatment of skin diseases and microbial infections, pain and such like [1]. It is known that the stratum corneum usually poses a * Tolulope O. Ajala--1 Department of Pharmaceutics and Industrial Pharmacy, University of Ibadan, Ibadan, Nigeria 2 School of Pharmacy, University of the Western Cape, Bellville, Cape Town 7530, South Africa barrier for the absorption of drugs applied topically; however, new advances in the design of formulation have successfully achieved the administration of a large number of drugs through the skin. Drugs can be designed in such a way that improves the pharmacological and therapeutic properties of the drug, many of the pharmacological properties of conventional free drugs have been improved by the use of drug delivery systems which alter the pharmacokinetics and bio-distribution of drugs for better therapeutic outcome [2]. Nanotechnology is a term derived from the Greek word “nano” meaning dwarf. Albert Franks defined it as an area of Science and Technology where dimensions and tolerances are in the range of 0.1–100 nm [3]. Nanotechnology is applicable in gene therapy, drug delivery, imaging, and novel drug discovery techniques. In drug delivery, nano particulate systems such as nano suspensions, nano emulsions, and nano micelles have been able to achieve enhancement 13 Vol.:-) BioNanoScience in therapeutic effect by increasing dissolution rate and drug bioavailability [4]. In particular, various solubilityrelated problems of poorly soluble drugs have been solved [5]. Patient acceptance and compliance have also been improved. It can be generally stated that nano formulations have improved the physicochemical properties of the active pharmaceutical ingredients to achieve better effects. Nanoparticles include particles with a diameter of less than 1 µm consisting of various biodegradable materials such as natural or synthetic polymers, lipids, phospholipids, and even metals [3]. They have been used as drug carriers in which drug can be attached to the periphery or incorporated in the matrix for example, silver and carbon nanoparticles to mention a few. The potential areas of use for nanoparticles include medicine, catalysis, bio-sensors, chemical sensors, environmental remediation, semi-conductors, and photovoltaic devices [6]. Nanoparticles have been found to help in pro-drug absorption via the skin. Nano particles below 6–7 nm or 36 nm in size may be absorbed through the aqueous pores or lipidic transepidermal route, while those between 10 and 210 nm take the transfollicular route [7]. Although the mechanism of skin-particle penetration and interaction is yet to be fully understood, some of this pattern has been discovered and exploited as particles can interact with the skin on a cellular level as adjuvant. This nanoparticle skin interaction can be used to enhance immune reactivity for vaccine application. The synthesis of silver nanoparticles can be carried out using physiochemical techniques like gamma ray radiation, microemulsions, electrochemical techniques, chemical reduction, photochemical reduction, laser ablation, and microwave irradiation. However, green synthesis recently developed is mostly used [6]. In recent times, there has been a growing interest in the synthesis of metallic nanoparticles by green methods. For this purpose, biomass or extracts of different plants have been successfully tried as reducing and capping agents for such metals. For instance, in the literature, there are reports of the synthesis of silver, copper and gold nanoparticles using extracts of different plants [8]. This work is part of this new line of research. Green synthesis is a method of synthesizing nanoparticles using plant extracts; it also utilizes biological organisms, enzymes, algae, yeast, mold, and metabolites from arthropods [9]. This method helps to reduce the danger of toxicity of products from chemical methods and the high cost of physical methods. It has gained a lot of attention over the years because of its simplicity, for instance, the basic requirement for the green synthesis of silver nanoparticles is silver nitrate and a natural reducing agent [6]. Green synthesis has diverse advantages including, low energy consumption, use of moderate temperature and pressure, cost effectiveness, eco-friendliness, mass productivity, and therapeutic usefulness [10]. 13 Plant extracts contain abundant natural compounds such as alkaloids, flavonoids, saponins, steroids, tannins, and other nutritional compounds. These compounds are obtainable from various parts of plants such as leaves, stems, roots, shoots, flowers, barks, and seeds [11]. Plant parts such as fruit exocarp and latex are used for metallic nanoparticle synthesis because of the presence of active agents such as polyphenols and antioxidants [12]. Mangifera indica belongs to the family Anacardiaceae that consists of about sixty genera and six hundred species. It is a tropical fruit indigenous to south Asia and Central America [13] Mango varieties differ not only in fruit shapes, but also in flavor which is an important trait that makes the seed oil useful in cosmetics as a deodorizing agent [14]. This has been explored in the production of lotion and soaps. It's an herb commonly used in ayurvedic medicine. It is widely used as a source of food, medicine and timber. In Nigeria, different parts of mango tree are commonly used as herbal preparations in the treatment of toothache, gastrointestinal disorders, dysentery, and diarrhea [15]. The leaf extracts were investigated for antibacterial activity against Staph aureus, Escherichia coli, and Pseudomonas aeruginosa [16]. In addition, the antidiabetic property of the plant was investigated by [17]. 2 Materials and Methods Magnifera indica indica stem bark was collected at the University of Ibadan environment and authenticated at the Department of Botany, University of Ibadan. It was cleaned with distilled water, chopped into small pieces and dried. The dried bark was milled using a laboratory mill and stored in an airtight container. Cold extraction method was used, in which 100 g of the well dried bark was soaked in 1L each of the solvents (methanol and distilled water) using amber glass bottles. The mixture was periodically shaken to improve the extraction of phytochemicals. The aqueous mixture remained for 24 h while the methanol mixture was left for 48 h. The mixture was filtered thrice using a Whatmann filter paper. A clear solution was obtained as filtrate, this was transferred into an evaporating dish. The filtrate was concentrated using a water bath set at 40 °C. The dried extracts obtained were wrapped in aluminum foil and stored in amber glass containers for further investigations [18, 19]. 2.1 Qualitative Phytochemical Screening of Plant Extracts Phytochemical screening for tannins, saponins, alkaloids, flavonoids, glycosides, quinones, phenols, terpenoids, cardiac glycosides, coumarins, anthraquinone, steroids, and anthocyanins was conducted using standard methods as described by [20]. BioNanoScience 2.2 Preparation of 1 mM of Silver Nitrate Solution [ ] % Scavenging = (1 − ABS∕ABC) × 100 The molecular weight of silver nitrate (­ AgNO3) is 169.87 g/ mol. This means that 169.87 g in 1000 mL will yield 1 M solution and 0.16987 in 1000 mL will yield 1 mM solution [21, 22]. ­AgNO3 (0.16987 g) was dissolved in 1000 mL of distilled water to obtain 1 mM silver nitrate solution which was used in the biosynthesis of silver nanoparticles [23]. where “ABS” and “ABC” are absorbance of sample and control respectively. 2.3 Biosynthesis of Silver Nanoparticles The crude methanol and aqueous extracts of Mangifera indica stem barks were used for the biosynthesis of silver nanoparticles. Aqueous solution (1000 ml of 1 mM) of silver nitrate (AgNO3) was prepared in a volumetric flask, thereafter 40 mL (0.1 g/10 mL) and 20 mL (0.1 g/10 mL) of the plant extracts solutions were prepared and added each to 100 mL of silver nitrate for the bio-reduction of A ­ gNO3 to silver ions. The solutions were placed in an incubator (water bath DK-600A) for complete bio-reduction at 37 °C for 24–72 h. The silver nanoparticles produced were lyophilized using a freeze dryer (Lab kits -FD-10-MR) and stored in dark glass bottles for further analysis [24]. 2.4 Visual Observation of the Biosynthesized Silver Nanoparticles Visual observation of the plant extract and silver nitrate mixture was done to check for color changes at 24, 48, and 72 h of incubation. 2.5 UV–Visible spectroscopy of the biosynthesized SNPs The biosynthesized biosynthesized silver nanoparticles (SNPs) were characterized using UV–visible spectroscopy. Absorption spectra was taken between the ranges of 200–400 nm. 2.6 Determination of DPPH‑Free Radical Scavenging Ability The free radical scavenging ability of the extracts against 2,2-diphenyl-1-picrylhdrazyl was evaluated as described by Gyamfi et al. [25]. In brief, appropriate dilution of the samples (extracts and silver nanoparticles) (1 mL) was mixed with 1 mL of 0.4 mM methanol solution containing DPPH solution, the mixture was left in the dark for 30 min and the absorbance of the remaining DPPH was measured at 516 nm. The percentage DPPH scavenging ability of the extracts was subsequently calculated as percentage of control as shown in Eq. 1 below. Ascorbic acid, a potent antioxidant, was used as a reference drug in the study. (1) 2.7 Antimicrobial Evaluation of Extracts and Biosynthesized Silver Nanoparticles Agar-well diffusion method was used to evaluate the antimicrobial activities of the biosynthesized SNPs as well as the extracts. Pathogenic organisms that are implicated in skin infections (Staphylococcus aureus and Pseudomonas aeruginosa) were used as indicator strains. Mueller Hinton agar solution was prepared according to the manufacturer’s instruction using distilled water. The agar was autoclaved at 20 psi for 20 min. After autoclaving, it was allowed to cool to about 40 °C poured into petri dishes and allowed to solidify. The inoculum for isolates was prepared using those previously stored in slants, which were picked, sub-cultured on nutrient agar plates and incubated for 48 h at 37 °C. A colony of the freshly sub-cultured isolate was then picked into McCartney bottles containing 10 ml of sterile water and shaken to obtain a suspension of the organisms. Inoculum standardization was done using 0.5 McFarland standards. Broth culture of the test pathogen was prepared and streaked with the swab stick on labeled sterile Mueller Hinton Agar (MHA) plates. The inoculated plates were allowed to dry and wells were bored using 8 mm sterile cork-borer. About 200 μL of the fabricated silver nanoparticles, Streptomycin 10 mg/mL, and AgNO3 solution (1 mM) was placed into the wells. The plates were incubated and clear zones of inhibition around the wells was measured. 2.8 FTIR Spectroscopy A small amount of the sample was put in mortar, using a pestle, potassium bromide (KBr) was added to blend it. With the aid of a spatula, a small amount of sample was carefully taken out and put inside the sample holder to make a pellet; this was hung on to the machine (Buck scientific, model: M530, infrared spectrophotometer) to scan it and the spectra formed (at 40,000 to 1000 ­cm−) was observed. 2.9 Pharmaceutical Gel Formulation The biosynthesized nanoparticles were used as drug models in this study. The drug concentration was kept constant at 1% w/w in the gel formulations; propylene glycol was also kept constant at 2% w/w. The polymers hydroxypropylmethylcellulose (HPMC) and carboxymethylcellulose (CMC) were combined in the first formulation (F1), each of which was 13 BioNanoScience used at 1%w/w. Formulations 2 to 5 contained the drugs; triethanolamine was used to neutralize and adjust the pH of the gel systems. The preparations were made up to 100 g using distilled water as shown in Table 2. Finally, proper mixing was done with the aid of a magnetic stirrer (MS300 model) at room temperature [26]. Determination of the Physical Properties of Unloaded and Nanoparticle‑Loaded Pharmaceutical Gel Formulations All the formulated gels were visually inspected for color, consistency and homogeneity; this was done after they have been transferred into transparent containers where the presence of any aggregate could be easily detected. The pH of the gel formulation was determined using a pH meter at room temperature. An amount (2.5 g) of the gel was weighed and dispersed in 25 mL distilled water and the pH was read. The pH was done in triplicates for all the formulations [24]. The measurement of viscosity was done using a Brookfield Viscometer (DV-II + Pro model). The speed of rotation were 5, 10, 20, 50, and 100 rpm. The spindle size used was spindle 3. At each speed, the corresponding dial readings were noted in triplicates. In vitro Release Measurement for Nanoparticle‑Loaded Pharmaceutical Gel The release measurement in 900 mL buffer (pH 7.4) was conducted in a dissolution test apparatus using a seamless high retention dialysis tubing. The dialysis tubing was soaked in distilled water for 30 min before use; it was sealed at one end and the gel formulations (3 mL) was placed inside it while the other end was attached to the rotating paddle. The medium in the receptor compartment was maintained at 37 °C and rotated at 100 rpm. Samples (5 mL) of the receptor compartment fluid were withdrawn at various time interval (5, 10, 15, 30, 45, 60, 90, 120, and 150 min) and the percentage of the extract-mediated silver nanoparticles released was analyzed using UV–Visible Spectrophotometer at 315 nm. The volume withdrawn each time was replaced with drug-free receptor fluid. The cumulative percentage drug released at various time intervals was calculated and plotted against time. 3 Results 3.1 Phytochemicals Present in Extracts The phytochemical screening as presented in Table 1 shows that the methanol extract contained more phytochemicals than the aqueous. However, aqueous extracts revealed the presence of saponins which was not so in the methanol extracts. 13 Table 1  Phytochemical composition of Mangifera indica stem bark extract Phytochemical Aqueous extract Methanol extract Saponins Tannins Flavonoids Cardiac glycosides Anthraquinones Terpenoids Steroids Alkaloids Phenol Glycoside Quinones Coumarine Anthocyanin +  + +  + +  +  + − − − + + +  + − +  + +  + +  + − +  +  + +  + +  + − +  + + + +  +  + +  + +  +  + +  +  + +  + + present, +  + abundant, − absent 3.2 Visual Observation of Biosynthesis The color changes during the green synthesis of silver nanoparticles using Mangifera indica stem bark extracts are shown in Fig. 1. The change in color deepened as time increased. 3.3 UV–Visible Spectroscopy of Biosynthesized Silver Nanoparticles The UV–visible spectrum as presented in Fig. 2 showed that the wavelength of maximum absorption was at 315 nm. 3.4 FTIR Spectroscopy The results of FTIR as presented on Fig. 3 shows the presence of alkene and esters, which was conferred on the biosynthesized nanoparticles by the plant extracts. 3.5 Antimicrobial Evaluation of Extracts and Biosynthesized Nanoparticles In the antimicrobial evaluation, the results are shown in Fig. 4, using zone of inhibition of the extracts and biosynthesized SNPs. The results revealed that the extracts showed activity on the two microorganisms but lower than that of the biosynthesized nanoparticles. The nanoparticles labelled as 1–4 in plate E and F showed distinct zones of inhibition on both organisms tested. For Staphylococcus aureus, the nanoparticles showed less zone of inhibition than the positive control (Streptomycin) while for Pseudomonas aeruginosa, the biosynthesized SNPs showed higher inhibition zones than streptomycin. Silver nitrate on its own as seen in plates C and D had much less activity BioNanoScience Fig. 1  The color changes observed during the biosynthesis of silver nanoparticles using Mangifera indica aqueous and methanol extracts as reducing agent on the organisms. The activity of streptomycin was significantly higher on S. aureus while it was lower on P. aeruginosa. For Staphylococcus aureus, the ranking order was streptomycin > silver nanoparticles > extract > silver nitrate while for P. aeruginosa, it was silver nanoparticles > extract > silver nitrate > streptomycin. 13 BioNanoScience Fig. 2  UV–visible spectra for biosynthesized silver-nanoparticles Key A-Extracts’ activity on Staph aureus. B- Extracts’ activity on Pseudomonas aeruginosa. C-Activity of Streptomycin and silver nitrate on Staph aureus. D-Activity of Streptomycin and silver nitrate on Pseudomonas aeruginosa. E- Nanoparticles’ activity on Pseudomonas aeruginosa. F-Nanoparticles’ activity on Staph aureus. 3.6 Antioxidant Activity of Extracts and Biosynthesized Silver Nanoparticles The DPPH scavenging ability of the extracts and nanoparticles is shown in Fig. 5; it was observed that aqueous extract-mediated silver nanoparticles (AEMSN) exhibited the highest antioxidant capacity among the formulations. In comparison with the reference; however, the responses of the silver nanoparticles were significantly (p < 0.05) lower. Aqueous extract-mediated silver nanoparticles exhibited higher antioxidant activities than Fig. 3  Fourier transform infrared spectra for extracts and biosynthesized silver-nanoparticles 13 methanol the one produced from methanol extract, this was also observed in the extracts. For the higher concentration, methanol extract-mediated silver nanoparticles showed 27.1% while aqueous extract-mediated nanoparticles showed 33%. For the extracts, aqueous extract highest scavenging activity was 36%, while for the methanol extract it was 33.9%. 3.7 Physicochemical Properties of Formulated gels The details of ingredients in the formulations are presented in Table 2. The gel formulations were smooth and homogeneous in appearance; the formulation without nanoparticles were white and transparent in appearance while those containing them were light brown and translucent. The pH and viscosity of the gel systems are presented in Table 3. The pH ranged between 6.8 and 7.1, while viscosity ranged from 983. 15 ± 21.40 to 1189.00 ± 18.40 cP. The flow pattern as revealed in Fig. 6 showed that as the shear rate increased, the viscosity decreased. The release profile of the nanoparticle-loaded pharmaceutical gels showed immediate BioNanoScience Fig. 4  Zones of inhibition of extracts and biosynthesized SNPs on the pathogens release which then became prolonged after the first 25 min; this is shown in Fig. 7. The release kinetics of the gel formulations are shown in Table 4 and the kinetics followed Higuchi pattern and n value > 0.5 but < 1.0. The release times of the gel formulations are presented in Table 4. 4 Discussion The extraction of Mangifera indica stem bark using water and methanol have shown that methanol gave a higher yield of 9.2% than water which gave 4.3% yield. This is in accordance with a study by Sanusi et al. [15] where methanol also gave a higher yield. This can partly be attributed to longer extraction time (48 h) of the methanol; this is in accordance with [24]. Water cannot be used for a longer period in extraction because of the possibility of contamination with microbes. The phytochemical screening of Mangifera indica stem bark aqueous and methanol extracts showed the presence of tannins, flavonoids, phenol, quinones, alkaloids, steroids, coumarins, and anthocyanins; this is in accordance with [27]. A study by Okwu et al. [28] showed the presence of flavonoids, alkaloids, saponins, tannins, and phenols in Mangifera indica stem bark. In addition to the aforementioned, El-Mahmood [29] reported the presence of cardiac glycosides while another study by Gitonga et al. [30] of methanol extracts of Mangifera indica shows presence of coumarins, terpenoids, and flavonoids. These phytochemicals are responsible for the activity of the plant and its use as medicine. The aqueous extracts showed the presence of Fig. 5  The antioxidant profiles of the extracts and extract-mediated silver nanoparticles 13 BioNanoScience Table 2  Details of ingredients in pharmaceutical gel formulations Ingredients F1 F2 F3 F4 F5 Hdroxypropylmethylcelulose Carboxyl methylcellulose Propylene glycol 0.4% MEMSN 0.2% MEMSN 0.4% AEMSN 0.2% AEMSN Triethanolamine Water to 1 1 2 − − − − Qs 100 g 1 1 2 1 − − − Qs 100 g 1 1 2 − 1 − − Qs 100 g 1 1 2 − − 1 − Qs 100 g 1 1 2 − − − 1 Qs 100 g MEMSN methanol extract-mediated silver nanoparticle, AEMSN aqueous extract-mediated silver nanoparticles saponins contrary to the methanol extracts; also, methanol extracts showed abundance of glycosides whereas the aqueous extract showed its absence. Knowledge of phytochemicals present in a plant assists scientists in the synthesis of more complex chemical substances; it also explains the reason behind other analytical results obtained during research on such plants [31]. Tannins are known to be useful in the treatment of inflamed or ulcerated tissues; they also have activity in cancer prevention [32]. Additionally, tannins are known to have antiviral and antibacterial properties, and certain tannins are able to inhibit HIV replication [33] Phenolic compounds are very important constituents because they exhibit antioxidant activity by inactivating free radicals, they also prevent decomposition of hydrogen peroxide into free radicals [34]. Cardiac glycoside are useful as natural drugs for the treatment of heart failure [32]. Coumarins are known for their anti-inflammatory, anticoagulant, bactericidal and fungicidal activities to mention a few [35]; these activities cut across the several types of coumarins. Anthocyanins belong to the flavonoid class, they are also found in foods and beverages like red wine and juices. They are highly water-soluble and are potent antioxidants [36]. In a study by Kalt et al. higher intake of anthocyanins was associated with reduction in hypertension risk [37]. Table 3  pH and viscosity of unloaded and nanoparticle-loaded pharmaceutical gel formulations Codes Description of contents pH Viscosity @ 50 rpm F1 F2 F3 F4 F5 Without nanoparticle 0.4% MEMSN 0.2% MEMSN 0.4% AEMSN 0.2% AEMSN 6.90 ± 0.02 7.11 ± 0.09 6.91 ± 0.03 6.80 ± 0.05 6.82 ± 0.05 1029.00 ±- ±- ±- ±- ± 24.00 MEMSN methanol extract-mediated silver nanoparticle, AEMSN aqueous extract-mediated silver nanoparticles 13 Fig. 6  Rheological profiles of unloaded (F1) and nanoparticle-loaded (F2-F5) pharmaceutical gel formulations During the biosynthesis of the extract-mediated silver nanoparticles (AEMSN), there were notable color changes from light-yellow to brown within 24 h. This is presented in Fig. 1. The color change while using methanol extract was slower than that of aqueous extract. The delay in color change while using the methanol extract can be attributed to lack of free hydrogen which should react with the silver nitrate, this is in accordance with Odeniyi et al. [24]. The color change signifies the reduction of silver ions to silver nanoparticles [38]; it occurs as a result of excitation of surface plasmons vibration in the metal nanoparticle [21]. UV–visible spectroscopy was carried out to further confirm the formation of nanoparticles, maximum absorption occurred between 315 and 320 nm. Generally, absorbance reduces hours after reaction because the rate of nanoparticle formation has been reduced [39]. The UV–vis. Spectra of the plant extract showed peak at 315 nm; this is in agreement with Pratheesya et al. [40] UV spectroscopy gives information about concentration and structure of the nanoparticles. Plasmonic resonance frequency depends on shape, size, and dielectric environment of nanoparticles while shape and size of particles depends on molar ratio of the silver nitrate and the Nanoparticles [41]. The DPPH assay showed that higher concentration (0.4%w/v) of the biosynthesized silver nanoparticles had improved antioxidant activities than lower ones. The better antioxidant activity in the aqueous preparations can be attributed to their hydrogen donating ability; therefore, they can be used to reduce oxidative stress and associated complication [34]. The extracts and the aqueous nanoparticles showed less antioxidant activities than the standard, comparatively; however, they can be said to possess antioxidant activities. FTIR spectroscopy is very important to characterize the proteins binding with the silver Nanoparticles. It enables the investigation of surface adsorption of functional groups on Nanoparticles [42]. FTIR analysis of the biosynthesized nanoparticles showed the presence of Aromatics (699 nm), ester (1077 nm), alkene (1628 nm), alkane (2945 nm), and amine (3503 nm) functional groups in the nanoparticles. Alkene and BioNanoScience Fig. 7  Release profiles of nanoparticle-loaded pharmaceutical gel formulations ester functional groups that were originally absent from the silver nitrate were conferred on the aqueous nanoparticles (0.4%w/v); this is because of the presence of the extracts. In addition, amine and alkane peaks were improved as observed in the aqueous and methanol nanoparticles respectively. Generally, the ranking of the antimicrobial effect was silver nitrate < extracts < biosynthesized nanoparticles < streptomycin. The aqueous extract-mediated silver nanoparticles showed improved activity than the methanol extract-mediated. In addition, from the antimicrobial zone of inhibition of the extract and biosynthesized SNPs, it was observed that for Staph. aureus, the nanoparticles showed higher zone of inhibition than the silver nitrate and the positive control (streptomycin) while for Pseudomonas aeruginosa, the biosynthesized SNPs showed better inhibition zones than Streptomycin. It may that the strain of Pseudomonas aeruginosa used has developed resistance to streptomycin, this justifies the exploration of newer drugs to combat the challenge of antimicrobial resistance as earlier mentioned. Silver nanoparticles have been known to have inhibitory and bactericidal effects, the ones synthesized in this study have also demonstrated the inhibitory activity. The antibacterial activity of biosynthesized silver nanoparticles has been reported to be dependent on particle size and smaller sizes generally offer higher activity. In addition, interaction between AgNPs and biomolecules which contain silver and phosphorus in the bacterial cell may aid the entrance of the particles into the bacterial cell, leading to cell-killing through the attack of the respiratory chain and cell division [43]. For the formulated gels, the results of the pH showed that the gel systems were within acceptable pH range. The gels showed a pH range of 6.8–7.1 and this is in accordance with Shahtalebi et al. [44]. The effect of pH on skin infections have been studied and it has been observed that at low pH values of 4.7, Staphylococcus epidermidis shows growth, while Staphylococcus aureus is suppressed at a pH value of 7 [45]. From Table 3, or concentration of extract used in the biosynthesis did not have effect on the viscosity of the gel formulation. The viscosity was high as expected of semi solid dosage forms. In addition, a plot of viscosity against shea rate as observed in Fig. 4 showed that as shear rate increased, viscosity decreased. This implies a non-Newtonian (pseudo-plastic) flow, which is in accordance with a study by Yuan et al. [46]. Many polymer gels exhibit reversibility between the gel state and the sol, this is desirable for ease of drug administration [47]. 5 In‑vitro Drug Release Drug release evaluation was done for two and half hours, and the extract-mediated silver nanoparticles showed controlled release pattern throughout the duration, this supports the advantage of gels in providing prolonged release of active ingredients [48]. Dissolution testing is being used to measure the quantity of active pharmaceutical ingredients bioavailable in the skin. It can also be employed as a quality control procedure because a dissolution test can reveal changes in a product's composition [49]. The nanoparticle release profile of the four formulations was compared with each other. The dissolution of F ­ 3 was the slowest, following this order F ­ 3 < ­F5 < ­F2 < ­F4. This release studies show that the lower the concentration of the nanoparticle in the gel for both MEMSN and AEMSN, the slower the dissolution and vice versa. There was slower dissolution of ­F3 (0.2% MEMSN) and F ­ 5 (0.2% AEMSN) from its gels because the 0.2% concentration of nanoparticles was interlocked and Table 4  Release kinetics of nanoparticle-loaded pharmaceutical gel formulations Code Description of content Zero order r F2 F3 F4 F5 0.4% MEMSN 0.2% MEMSN 0.4% AEMSN 0.2% AEMSN 2 - First order 2 Korsmeyer-Peppas 2 K0 r K0 N r - - - - - Hixson-Crowell 2 Higuchi K0 r K0 r2* K0 - - - - - MEMSN methanol extract-mediated silver nanoparticle, AEMSN aqueous extract-mediated silver nanoparticles * Highest correlation coefficients; the figures are highlighted to indicate the best fit of kinetic models for the formulations 13 BioNanoScience Table 5  Release times of nanoparticle-loaded pharmaceutical gel formulations Code Description of content T25% (min) T50% (min) T75% (min) T90% (min) F2 F3 F4 F5 0.4% MEMSN 0.2% MEMSN 0.4% AEMSN 0.2% AEMSN - - - - MEMSN methanol extract-mediated silver nanoparticle, AEMSN aqueous extract-mediated silver nanoparticles enclosed inside the matrix of the gel-slow dissolution profile, compared to the 0.4% concentration of nanoparticles ­(F2 (0.4% MEMSN) and ­F4 (0.4% AEMSN)) which were not totally enclosed inside the matrix of the gel-rapid dissolution profile. The time taken for 25 (­ t25%), 50 (­ t50%), 75 (­ t75%), and 90% (­ t90%) extract-mediated silver nanoparticles to be released from the gel is shown in Table 5 and were generated from the dissolution plots [Fig. 7]. The release of the nanoparticle was immediate because 25% of the extract-mediated silver nanoparticles were released in less than 15 min for all the four formulations. Formulation ­F3 had the longest duration of drug release, in which 90% of the nanoparticles were released after 150 min, which is the maximum duration for the dissolution study. A careful observation of the timing of each release of the extract-nanoparticles at ­t25%, ­t50%, ­t75%, and t­90% showed that the release profile of the four formulations is quite close. Different mathematical models such as zero-order, firstorder, Korsmeyer-Peppas, Hixson-Crowell, and Higuchi models were used to study the drug release kinetics. This study showed that the gel dissolution data is best fitted to the Higuchi equation for all four formulations [Table 4], which describes the quantity of drug release from a semi-solid is a linear function of the square root of time [49]. The Higuchi model had the highest correlation coefficient-r2 -). Therefore, the mechanism of extract-mediated nanoparticle release is diffusion controlled from the matrix of the gel, which is also its rate limiting step. The release rate of the nanoparticle from the gel matrix revealed the extract-mediated nanoparticle is hydrophilic or highly soluble and are partitioned more into the perimicellar microchannels and less in the micelles of the gel-matrix, thus available for release. Hydrophobic drugs molecules are poorly soluble and are usually more partitioned to the micelles and less partitioning into the perimicellar microchannels of the gelmatrix, thus they are not available for release [50, 51]. the extracts. The antioxidant capacity was in a concentrationdependent manner but significantly lower than that of the reference drug. The nanoparticle-loaded gel formulations were within acceptable pH range for topical preparations and high viscosity with pseudoplastic rheological patterns. The in-vitro release profiles showed that the release of the gel systems was gradual with t­90 > 2 h. In addition, the release kinetics of the nanoparticle-loaded gel systems followed Higuchi model with ­r2 ranging from- and n value > 0.5 but < 1.0. Acknowledgements The authors acknowledge the technical staff of the Department of Pharmaceutics and Industrial Pharmacy of the University of Ibadan, Ibadan, Nigeria. Author Contribution OEA (Oluwatosin Elizabeth Adeyemi) conducted some experiments and wrote the first draft of the manuscript, OAO (Omobolanle A. Omoteso) conducted the kinetic studies, the release times, wrote the concerned part in the manuscript and formatted the manuscript to Journal specification. TOA (Tolulope Omolola Ajala) designed the research concept, provided some of the materials for the experiments, supervised the conduct of all experiments, and reviewed the manuscript severally alongside other authors. Funding This research received no funding from any agency or other individual. Data Availability The data materials are with the authors as no data was obtained from outside sources. Declarations Research Involving Humans and Animals Statement This research did not involve human and animal participants. Informed Consent The informed consent was not needed in this study. Conflict of Interest The authors declare no competing interests. References 6 Conclusion The aqueous and methanol extracts of Mangifera indica stem bark were successfully used as bio-reducing agents in the synthesis of silver nanoparticles. The nanoparticles demonstrated significantly higher antimicrobial activity compared to 13 1. Ruela, A. L. M., Perissinato, A. G., Lino, M. E. D. S., Mudrik, P. S., & Pereira, G. R. (2016). Evaluation of skin absorption of drugs from topical and transdermal formulations. Brazilian Journal of Pharmaceutical Sciences, 52, 527–544. https://​doi.​org/​10.​1590/​ S1984-​82502​01600​03000​18 2. Tiwari, G., Tiwari, R., Sriwastawa, B., Bhati, L., Pandey, S., Pandey, P., & Bannerjee, S. K. (2012). Drug delivery systems: An BioNanoScience 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. updated review. International journal of pharmaceutical investigation, 2(1), 2. Zhang, J., Jiao, J., Niu, M., Gao, X., Zhang, G., Yu, H., Yang, X., & Liu, L. (2021). Ten years of knowledge of nano-carrier based drug delivery systems in ophthalmology: current evidence, challenges, and future prospective. International Journal of Nanomedicine, 16,-. https://​doi.​org/​10.​2147/​IJN.​S3298​31 Thakur, S. R., & Agrawal, R. (2015). Application of nanotechnology in pharmaceutical formulation design and development. Current Drug Therapy, 10(1), 20–34. https://​www.​ingen​tacon​nect.​ com/​conte​nt/​ben/​cdth/​2015/​00000​010/​00000​001/​art00​004. Göke, K., Lorenz, T., Repanas, A., Schneider, F., Steiner, D., Baumann, K., Bunjes, H., Dietzel, A., Finke, J. H., Glasmacher, B., & Kwade, A. (2018). Novel strategies for the formulation and processing of poorly water-soluble drugs. European Journal of Pharmaceutics and Biopharmaceutics, 126, 40–56. Tarannum, N., & Gautam, Y. K. (2019). Facile green synthesis and applications of silver nanoparticles: A state-of-the-art review. Royal Society of Chemistry Advances, 9(60),-. https://​doi.​org/​10.​1039/​C9RA0​4164H Hua, S. (2015). Lipid-based nano-delivery systems for skin delivery of drugs and bioactives. Frontiers in Pharmacology, 6, 219. https://​doi.​org/​10.​3389/​fphar.​2015.​00219 Awwad, A. M., Salem, N. M., & Abdeen, A. O. (2013). Green synthesis of silver nanoparticles using carob leaf extract and its antibacterial activity. International Journal of Industrial Chemistry, 4(1), 1–6. https://​doi.​org/​10.​1186/​2228-​5547-4-​29 Lateef, A., Oladejo, S. M., Akinola, P. O., Aina, D. A., Beukes, L. S., Folarin, B. I., & Gueguim-Kana, E. B. (2020). Facile synthesis of silver nanoparticles using leaf extract of Hyptis suaveolens (L.) Poit for environmental and biomedical applications. In IOP Conference Series: Materials Science and Engineering, 805(1),-. https://​doi.​org/​10.​1088/​1757-​899X/​805/1/​012042 Kareem, M. A., Bello, I. T., Shittu, H. A., Awodele, M. K., Adedokun, O., & Sanusi, Y. K. (2020). Green synthesis of silver nanoparticles (AgNPs) for optical and photocatalytic applications: A review. In IOP Conference Series: Materials Science and Engineering, 805(10), 012020. https://​doi.​org/​10.​1088/​1757-​899X/​ 805/1/​012020. IOP Publishing. Adelere, I. A., & Lateef, A. (2016). A novel approach to the green synthesis of metallic nanoparticles: The use of agro-wastes, enzymes, and pigments. Nanotechnology Reviews, 5(6), 567–587. https://​doi.​org/​10.​1515/​ntrev-​2016-​0024 Kharissova, O. V., Dias, H. R., Kharisov, B. I., Pérez, B. O., & Pérez, V. M. J. (2013). The greener synthesis of nanoparticles. Trends in Biotechnology, 31(4), 240–248. https://​doi.​org/​10.​ 1016/j.​tibte​ch.​2013.​01.​003 Chakraborty, T., & Das, M. K. (2017). De novo approach to utilize mango (Mangifera indica L.) seed kernel lipid in pharmaceutical lipid nanoformulation. Journal of Pharmaceutical Innovation, 12(3), 226–237. https://​doi.​org/​10.​1007/​s12247-​017-​9284-y Wu, S., Tokuda, M., Kashiwagi, A., Henmi, A., Okada, Y., Tachibana, S., & Nomura, M. (2015). Evaluation of the fatty acid composition of the seeds of Mangifera indica L. and their application. Journal of Oleo Science, 64(5), 479–484. https://d​ oi.o​ rg/1​ 0.5​ 650/​ jos.​ess14​238 Sanusi, B. M., Garba, A., Muhammad, A., Mohammed, A., & David, O. (2011). Phytochemical screening and antimicrobial efficacy of aqueous and methanolic extract of Mangifera indica (mango stem bark). World Journal Life-Science and Medical Research, 2(2), 81. Bbosa, G. S., Kyegombe, D. B., Ogwal-Okeng, J., BukenyaZiraba, R., Odyek, O., & Waako, P. (2007). Antibacterial activity of Mangifera indica (L.). African Journal of Ecology, 45, 13–16. https://​doi.​org/​10.​1111/j.​1365-​2028.​2007.​00731.x 17. Aderibigbe, A. O., Emudianughe, T. S., & Lawal, B. A. S. (2001). Evaluation of the antidiabetic action of Mangifera indica in mice. Phytotherapy Research, 15(5), 456–458. https://​doi.​org/​10.​1002/​ ptr.​859 18. Olasehinde, G. I., Sholotan, K. J., OPENIBO, J. O., Taiwo, O. S., Bello, O. A., Ajayi, J. B., Ayepola, O. O. and Ajayi, A. A. (2018). Phytochemical and antimicrobial properties of Mangifera indica Leaf Extracts. Covenant Journal of Physical & Life Sciences (CJPL), 6(1), 55–63. https://​core.​ac.​uk/​downl​oad/​pdf/​ 15912​9315.​pdf 19. Ashok, V. G., Priya, S. B., & Pranita, A. G. (2014). Evaluation of antibacterial and phytochemical analysis of Mangifera indica bark extracts. International Journal of Current Microbiology and Applied Sciences, 3(5), 567–580. https://w ​ ww.c​ abdir​ ect.o​ rg/g​ loba​ lheal​th/​abstr​act/​20143​-. Deyab, M., Elkatony, T., & Ward, F. (2016). Qualitative and quantitative analysis of phytochemical studies on brown seaweed, Dictyota dichotoma. International Journal of Engineering Development and Research, 4(2),-. Honary, S., Barabadi, H., Gharaei-Fathabad, E., & Naghibi, F. (2013). Green synthesis of silver nanoparticles induced by the fungus Penicillium citrinum. Tropical Journal of Pharmaceutical Research, 12(1), 7–11. https://​doi.​org/​10.​4314/​tjpr.​v12i1.2 22. Prasher, P., Sharma, M., Mudila, H., Gupta, G., Sharma, A. K., Kumar, D., Bakshi, H. A., Negi, P., Kapoor, D. N., Chellappan, D. K., Tambuwala, M. M. (2020). Emerging trends in clinical implications of bio-conjugated silver nanoparticles in drug delivery. Colloid and Interface Science Communications, 35,-. Krishnamoorthy, P., & Jayalakshmi, T. (2012). Preparation, characterization and synthesis of silver nanoparticles by using phyllanthusniruri for the antimicrobial activity and cytotoxic effects. Journal of Chemical and Pharmaceutical Research, 4(11),-. Quiñones, D., & Ghaly, E. S. (2008). Formulation and characterization of nystatin gel. Puerto Rico Health Sciences Journal, 27(1). 25. Odeniyi, M. A., Okumah, V. C., Adebayo-Tayo, B. C., & Odeniyi, O. A. (2020). Green synthesis and cream formulations of silver nanoparticles of Nauclea latifolia (African peach) fruit extracts and evaluation of antimicrobial and antioxidant activities. Sustainable Chemistry and Pharmacy, 15, 100197. https://​doi.​org/​ 10.​1016/j.​scp.​2019.​-. Gyamfi, M. A., Yonamine, M., & Aniya, Y. (1999). Free-radical scavenging action of medicinal herbs from Ghana: Thonningia sanguinea on experimentally-induced liver injuries. General Pharmacology: The Vascular System, 32(6), 661–667. https://​ doi.​org/​10.​1016/​S0306-​3623(98)​-. Ghorpade, V., Mali, K., Dias, R., & Karande, P. (2012). Carbopol and sodium carboxymethylcellulose based methylsulfonylmethane gels for treatment of osteoarthritis: in-vitro and in-vivo evaluation. Indian Journal of Pharmaceutical Education and Research, 46(3), 235–242. https://​ijper.​org/​sites/​defau​lt/​files/​ijper_​46_3_​7.​pdf. 28. Quintana, S. E., Salas, S., & García-Zapateiro, L. A. (2021). Bioactive compounds of mango (Mangifera indica): A review of extraction technologies and chemical constituents. Journal of the Science of Food and Agriculture, 101(15),-. https://d​ oi.​ org/​10.​1002/​jsfa.​-. Okwu, D. E., & Ezenagu, V. I. T. U. S. (2008). Evaluation of the phytochemical composition of mango (Mangifera indica Linn) stem bark and leaves. International Journal of Chemical Science, 6(2),-. El-Mahmood, M. A. (2009). Antibacterial efficacy of stem bark extracts of Mangifera indica against some bacteria associated with respiratory tract infections. Scientific Research and Essays, 4(10),-. https://a​ cadem ​ icjou​ rnals.o​ rg/j​ ourna​ l/S ​ RE/a​ rticl​ e-f​ ull-​ text-​pdf/​11254​E9184​82. 13 BioNanoScience 31. Gitonga, G. M., Ngeranwa, J., Machocho, A. K. O., & Kiruki, S. (2020). Qualitative phytochemical screening and antipyretic activity of methanolic stem bark extract of Mangifera indica in wistar rats. The Journal of Phytopharmacology, 9(4), 218–223. https://​ doi.​org/​10.​31254/​phyto.​2020.​9401 32. Kumari, P., Kumari, C., & Singh, P. S. (2017). Phytochemical screening of selected medicinal plants for secondary metabolites. International Journal of Life-Sciences Scientific. Research, 3(4),-. https://​doi.​org/​10.​21276/​ijlssr.​2017.3.​4.9 33. Usunomena, U., & Ngozi, O. P. (2016). Phytochemical analysis and proximate composition of Vernonia amygdalina. International Journal of Scientific World, 4, 11–14. 34. Oke, D. G. (2014). Proximate and phytochemical analysis of Cajanus cajan (Pigeon pea) leaves. Chemical Science Transactions, 3(3),-. https://​doi.​org/​10.​7598/​cst20​14.​785 35. Noghogne, L. R., Gatsing, D., Kodjio, N., Sokoudjou, J. B., & Kuiate, J. R. (2015). In vitro antisalmonellal and antioxidant properties of Mangifera indica L. stem bark crude extracts and fractions. British Journal of Pharmaceutical Research, 5(1), 29. https://​doi.​org/​10.​9734/​BJPR/​2015/​-. Venugopala, K.N., Rashmi, V., & Odhav, B. (2013). Review on natural coumarin lead compounds for their pharmacological activity. BioMed research international, 2013. https://d​ oi.o​ rg/1​ 0.1​ 155/​ 2013/​-. Smeriglio, A., Barreca, D., Bellocco, E., & Trombetta, D. (2016). Chemistry, pharmacology and health benefits of anthocyanins. Phytotherapy Research, 30(8),-. https://​doi.​org/​10.​ 1002/​ptr.​5642 38. Kalt, W., Cassidy, A., Howard, L. R., Krikorian, R., Stull, A. J., Tremblay, F., & Zamora-Ros, R. (2020). Recent research on the health benefits of blueberries and their anthocyanins. Advances in Nutrition, 11(2), 224–236. https://​doi.​org/​10.​1093/​advan​ces/​nmz065 39. Moodley, J. S., Krishna, S. B. N., Pillay, K., & Govender, P. (2018). Green synthesis of silver nanoparticles from Moringa oleifera leaf extracts and its antimicrobial potential. Advances in Natural Sciences: Nanoscience and Nanotechnology, 9(1), 015011. https://​doi.​org/​10.​1088/​2043-​6254/​aaabb2 40. Malarkodi, C., Rajeshkumar, S., Paulkumar, K., Vanaja, M., Jobitha, G. D. G., & Annadurai, G. (2013). Bactericidal activity of bio mediated silver nanoparticles synthesized by Serratia nematodiphila. Drug Invention Today, 5(2), 119–125. https://​doi.​org/​ 10.​1016/j.​dit.​2013.​05.​005 41. Pratheesya, T., Harish, S., Navaneethan, M., Sohila, S., & Ramesh, R. (2019). Enhanced antibacterial and photocatalytic activities of silver nanoparticles anchored reduced graphene oxide nanostructure. Materials Research Express, 6(7), 074003. https://​doi.​org/​ 10.​1088/​2053-​1591/​ab1567 42. Alim-Al-Razy, Md., AsikBayazid, G. M., Rahman, Raihan Ur, Bosu, Rajib, & Shamma, ShahlaSamim. (2020). Silver nanoparticle synthesis, UV-Vis spectroscopy to find particle size and measure resistance of colloidal solution.". In Journal of Physics: 13 43. 44. 45. 46. 47. 48. 49. 50. 51. Conference Series, 1706(1), 012020. https://​doi.​org/​10.​1088/​ 1742-​6596/​1706/1/​012020. IOP Publishing, 2020. Elamawi, R. M., Al-Harbi, R. E., & Hendi, A. A. (2018). Biosynthesis and characterization of silver nanoparticles using Trichoderma longibrachiatum and their effect on phytopathogenic fungi. Egyptian Journal of Biological Pest Control, 28(1), 1–11. https://​ doi.​org/​10.​1186/​s41938-​018-​0028-1 Shahtalebi, M. A., Asghari, G. R., Rahmani, F., Shafiee, F., & Jahanian-Najafabadi, A. (2018). Formulation of herbal gel of antirrhinum majus extract and evaluation of its anti-propionibacterium acne effects. Advanced Biomedical Research, 7. https://d​ oi.​ org/​10.​4103/​abr.​abr_​99_​17 Koudounas, S., Bader, D. L., & Voegeli, D. (2022). An exploratory study of the effects of the pH of synthetic urine on skin integrity in healthy participants. Skin Pharmacology and Physiology, 35(3), 166–173. Yuan, C., Zou, Y., Cui, B., Fang, Y., Lu, L., & Xu, D. (2021). Influence of cyclodextrins on the gelation behavior of κ-carrageenan/ konjac glucomannan composite gel. Food Hydrocolloids, 120, 106927. https://​doi.​org/​10.​1016/j.​foodh​yd.​2021.​106927 Rathod, H. J., & Mehta, D. P. (2015). A review on pharmaceutical gel. International Journal of Pharmaceutical Science, 1(1), 33–47. https://​www.​resea​rchga​te.​net/​profi​le/​Hemen​drasi​nh-​ Rathod/​publi​cation/​28645​1492_A_​Review_​on_​Pharm​aceut​ical_​ Gel/l​ inks/5​ 66a47​ 3308a​ e430a​ b4f77​ 4b5/A-R ​ eview-o​ n-P ​ harma​ ceut​ ical-​Gel.​pdf. Huang, G., Gao, J., Hu, Z., John, J. V. S., Ponder, B. C., & Moro, D. (2004). Controlled drug release from hydrogel nanoparticle networks. Journal of Controlled Release, 94(2–3), 303–311. https://​doi.​org/​10.​1016/j.​jconr​el.​2003.​10.​007 Kovács, A., Petró, É., Erős, I., & Csóka, I. (2014). The role of dissolution testing in quality control. Periodica Polytechnica Chemical Engineering, 17–20. https://​doi.​org/​10.​3311/​PPch.​7301 Paul, D. R. (2011). Elaborations on the Higuchi model for drug delivery. International Journal of Pharmaceutics, 418(1), 13–17. https://​doi.​org/​10.​1016/j.​ijpha​rm.​2010.​10.​037 Paavola, A., Bernards, C. M., & Rosenberg, P. H. (2016). Controlled release ibuprofen-poloxamer gel for epidural use–a pharmacokinetic study using microdialysis in pigs. European Journal of Pharmaceutics and Biopharmaceutics, 108, 180–186. https://​ doi.​org/​10.​1016/j.​ejpb.​2016.​09.​006 Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.