nanomaterials

Article Green Synthesis and Catalytic Activity of Silver Nanoparticles Based on chaba Stem Extracts

Md. Mahiuddin 1,2,*, Prianka Saha 2 and Bungo Ochiai 1,*

1 Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan 2 Chemistry Discipline, Faculty of Science, Engineering and Technology, Khulna University, Khulna 9208, ; [email protected] * Correspondence: [email protected] (M.M.); [email protected] (B.O.); Tel.: +81-238-26-3092 (B.O.)  Received: 30 July 2020; Accepted: 6 September 2020; Published: 8 September 2020 

Abstract: A green synthesis of silver nanoparticles (AgNPs) was conducted using the stem extract of Piper chaba, which is a abundantly growing in South and Southeast . The synthesis was carried out at different reaction conditions, i.e., reaction temperature, concentrations of the extract and silver nitrate, reaction time, and pH. The synthesized AgNPs were characterized by visual observation, ultraviolet–visible (UV-vis) spectroscopy, dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), energy dispersive x-ray (EDX), and Fourier transform infrared (FTIR) spectroscopy. The characterization results revealed that AgNPs were uniformly dispersed and exhibited a moderate size distribution. They were mostly spherical crystals with face-centered cubic structures and an average size of 19 nm. The FTIR spectroscopy and DLS analysis indicated that the phytochemicals capping the surface of AgNPs stabilize the dispersion through anionic repulsion. The synthesized AgNPs effectively catalyzed the reduction of 4-nitrophenol (4-NP) and degradation of methylene blue (MB) in the presence of sodium borohydride.

Keywords: silver nanoparticles; Piper chaba; green synthesis; catalytic activity

1. Introduction In the last decade, green synthesis of metal nanoparticles has been exponentially increasing because of its various applications, such as electronics, catalysis, chemistry, energy, and medicine [1–3]. In particular, silver nanoparticles (AgNPs) have many important applications in several fields, such as high sensitivity biomolecular detection and diagnostics [4–6], antimicrobials [7–13], catalysis [14–18], optics [19], biomedicine [20], medicine [21,22], as well as food and cosmetic industries [23,24]. Conventional methods for manufacturing AgNPs, such as chemical reduction, electrochemical process, photochemical reduction, laser ablation, hydrothermal method, microemulsion method, radiation-induced, radiolytic reduction, sonochemical reduction, pyrolysis, and lithography [13,25–27] are expensive, environmentally toxic, and/or hazardous. For example, the most typical chemical synthesis of AgNPs requires reducing agents such as sodium borohydride, hydroxylamine, sodium citrate, and hydrazine with environmentally undesirable solvents in some cases [28]. In most cases, this produces byproducts that are harmful to the environment [2]. Thus, green synthesis methods for AgNPs are highly required. The green methods should reduce the need for high pressure, high temperature, and toxic chemicals. They should also be simple (including only one step), rapid, cost-effective, and reproducible [1]. Extracts from various , such as Solanum nigrum [8], Coleus aromaticus [9], Alpinia katsumadai [10], Acalypha indica [11], Clitoria ternatea,

Nanomaterials 2020, 10, 1777; doi:10.3390/nano10091777 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1777 2 of 15 Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 15

CinnamonBreynia rhamnoides zeylanicum [18],[12 ],AzadirachtaBreynia rhamnoides indica [29],[18 Lantana], Azadirachta camara indica [30], [29olive], Lantana (Olea europaea camara )[ 30leaf], olive[31], (ThymbraOlea europaea spicata) leaf[32],[ Gmelina31], Thymbra arborea spicata [33], Skimmia[32], Gmelina laureola arborea [34], Arbutus[33], Skimmia unedo [35], laureola Viburnum[34], opulusArbutus L. unedo[36], Pice[35], abiesViburnum L. [37], opulus greenL. coffee[36], Pice(Coffea abies arabica, L. [37 ],Coffea green canephora, coffee (Co andffea Coffea arabica, liberica Coffea) bean canephora, [13], and Cocloveffea liberica(Syzygium) bean aromaticum [13], and) clove [38] are (Syzygium promising aromaticum substances)[38 ]for are a promisinggreen and substancesfacile synthesis for a of green AgNPs. and facilePiper synthesis chaba of is AgNPs. a common plant known as chui jhal or choi jhal and belongs to the family.Piper It chabais a veryis a cheap common and plant edible known plant asfoun chuid in jhal abundance or choi jhal in South and belongs and Southeast to the Piperaceae Asia [39]. family.Beyond Itedible is a very purposes, cheap andit is edibleused plantin various found a inpplications, abundance such in South as in andanalgesic, Southeast antibacterial, Asia [39]. Beyondantioxidant, edible and purposes, hypotensive it is used functions in various [39–41]. applications, Piper suchchaba as contains in analgesic, various antibacterial, compounds, antioxidant, among andwhich hypotensive alkaloids and functions lignan [ 39are–41 predominant]. Piper chaba [42–44].contains Figure various 1 shows compounds, examples among of these which compounds. alkaloids andThe lignanpolyphenolic, are predominant conjugated, [42 –and44]. hemiacetal Figure1 shows struct examplesures of ofthese these compounds compounds. have The the polyphenolic, potential to + 0 conjugated,reduce Ag+ to and Ag hemiacetal0. The availability, structures cost-effectiveness of these compounds compared have to the other potential plant tosources, reduce edibility, Ag to Ag and. Thethe presence availability, of various cost-eff functionalectiveness comp comparedounds to made other us plant enthusiastic sources, to edibility, utilize Piper and chaba the presence as a source of variousof AgNP functional synthesis. compounds made us enthusiastic to utilize Piper chaba as a source of AgNP synthesis.

Figure 1. Piper chaba Figure 1. Examples ofof compoundscompounds inin thethe stemstem extractextract ofof Piper chaba.. This study aimed to synthesize AgNPs using the stem extract of Piper chaba and to investigate This study aimed to synthesize AgNPs using the stem extract of Piper chaba and to investigate the effect of temperature, reaction time, concentration of AgNO3, quantity of plant extract, and pH the effect of temperature, reaction time, concentration of AgNO3, quantity of plant extract, and pH on the synthesis process. The synthesized AgNPs were characterized by ultraviolet–visible (UV-vis) on the synthesis process. The synthesized AgNPs were characterized by ultraviolet–visible (UV-vis) spectroscopy, dynamic light scattering (DLS), zeta potential measurements, scanning electron spectroscopy, dynamic light scattering (DLS), zeta potential measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), energy dispersive microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), energy x-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy. The catalytic activity dispersive x-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy. The of the synthesized AgNPs was examined during the reduction reaction of 4-nitrophenol (4-NP) and catalytic activity of the synthesized AgNPs was examined during the reduction reaction of 4- during the degradation of methylene blue (MB) in the presence of NaBH4. nitrophenol (4-NP) and during the degradation of methylene blue (MB) in the presence of NaBH4. 2. Materials and Methods 2. Materials and Methods 2.1. Collection of Plant Materials 2.1. Collection of Plant Materials The stem of Piper chaba was freshly collected from Khulna District, Bangladesh. The stem was thoroughlyThe stem of washedPiper chaba with was deionized freshly collected distilled from water, Khulna cut into District, small Bangladesh. pieces, and The air-dried stem was at roomthoroughly temperature. washed with deionized distilled water, cut into small pieces, and air-dried at room temperature. 2.2. Preparation of Plant Extract 2.2. Preparation of Plant Extract Finely cut stems (approximately 20 g) were placed in a round-bottom flask containing deionized distilledFinely water cut (200stems mL) (approximately and boiled for 20 40g) min.were placed The extract in a round-bottom was cooled down, flask filteredcontaining using deionized a filter paper,distilled and water stored (200 at 4mL)◦C forand further boiled use.for 40 min. The extract was cooled down, filtered using a filter paper, and stored at 4 °C for further use. 2.3. Materials 2.3. Materials AgNO3 and 4-NP were purchased from Merck Chemicals (Merck KGaA, Darmstadt, Germany) and TokyoAgNO3 Chemical and 4-NPIndustry were purchased Co. Ltd. from (Tokyo, Merck Japan),Chemicals respectively, (Merck KGaA, whereas Darmstadt, MB and Germany) sodium borohydrideand Tokyo Chemical were obtained Industry from KantoCo. Ltd. Chemical (Tokyo, Co. Japan), Inc. (Tokyo, respectively, Japan). whereas MB and sodium borohydride were obtained from Kanto Chemical Co. Inc. (Tokyo, Japan).

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2.4. Measurements UV-vis spectroscopic analysis was carried out on a JASCO (Tokyo, Japan) V-730 series and a UVD-3200 (LABOMED, CA, USA) spectrometer. Absorption spectra were recorded at a resolution of 1 nm within 200–800 nm. The hydrodynamic size and zeta potential were measured through DLS analysis conducted on a Malvern (Malvern, UK) Zetasizer Nano ZS instrument. FTIR spectra were recorded on a JASCO (Tokyo, Japan) FT/IR-460 plus spectrometer using a KBr pellet with a scan rate of 1 1 approximately 4 cm− s− at 25 ◦C. SEM measurements and EDX analysis were conducted on a Hitachi (Tokyo, Japan) SU-8000 microscope at accelerating voltages of 10 and 15 kV. TEM measurements were conducted on a TEM-2100F (JEOL, Tokyo, Japan) field emission electron microscope. XRD analysis was conducted on a Rigaku (Tokyo, Japan) Ultima IV RINT D/max-kA spectrometer with Cu-Kα radiation.

2.5. Green Synthesis of AgNPs

An aqueous solution of AgNO3 (1 mM, 100 mL) was prepared in a volumetric flask, and the flask was covered with carbon paper to prevent the autoxidation of silver. The aqueous stem extract of Piper chaba (4 mL) was placed in a conical flask. Then, the freshly prepared AgNO3 aq. (1 mM, 80 mL) was added to the conical flask. The mixture was heated in an oil bath at 60 ◦C for 30 min under constant stirring. After this, the color of the solution changed from colorless to reddish brown. The resulting suspension was stored at room temperature for 48 h. The reaction mixture containing AgNPs was then centrifuged at 13,000 rpm for 30 min, and the precipitate was thoroughly washed three times with sterile distilled water to remove impurities.

2.6. Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol The photo-catalytic reduction of 4-NP was performed in a quartz cuvette (height = 4 cm and optical path length = 1 cm). Aqueous solutions of NaBH4 and 4-NP at concentrations of 600 ppm and 10 ppm, respectively, were used in the process and they were stored at 4 ◦C before use. The photo-catalytic reduction was carried out by mixing the 4-NP solution (1 mL), NaBH4 solution (2 mL), and colloidal suspension of AgNPs (53.9 mg/L, 100 µL) in the cuvette. The time course of 4-NP consumption was monitored by optical absorbance at 401 nm. The control experiment without AgNPs was also carried out through an identical procedure.

2.7. Degradation of MB The degradation of MB was performed similarly to the reduction of 4-NP. The degradation reaction was studied by mixing an aqueous solution of MB (2 ppm, 2 mL), NaBH4 solution (600 ppm, 1.5 mL), and colloidal suspension of AgNPs (53.9 mg/L, 100-µL) in the cuvette. The time course of MB consumption was monitored by optical absorbance at 663 nm. The control experiment without AgNPs was also carried out through an identical procedure.

3. Results and Discussion

3.1. Formation of AgNPs AgNPs were synthesized by in situ reduction followed by a coating with the stem extract of Piper chaba. The initial mixture was colorless (Figure2a) and its color changed to reddish brown after the reaction (Figure2b). The formation of AgNPs in the medium was confirmed by a surface plasmon resonance (SPR) band and a UV-vis absorption peak corresponding to AgNPs at approximately 445 nm (Figure3). The resulting AgNPs dispersed stably over 12 weeks, while AgNPs prepared by conventional reduction with NaBH4 in the absence of stabilizing agent were precipitated within 12 h. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 15 Nanomaterials 2020, 10, 1777 4 of 15 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 15

Figure 2. Images of (a) the mixture of the Piper chaba stem extract and solution of AgNO3 (1 mM) and Figure 2. Images of (a) the mixture of the Piper chaba stem extract and solution of AgNO3 (1 mM) and (Figureb) the mixture2. Images after of ( a )30-min the mixture reaction. of the Piper chaba stem extract and solution of AgNO3 (1 mM) and (b) the mixture after a 30-min reaction. (b) the mixture after a 30-min reaction.

Figure 3. Ultraviolet–visible (UV-vis) absorption spectrum of reaction mixtures of AgNO (1 mM, Figure 3. Ultraviolet–visible (UV-vis) absorption spectrum of reaction mixtures of AgNO3 (13 mM, 40 40 mL) and Piper chaba extract (2 mL, 100 g/L) at 60 ◦C and pH = 7 after 30 min. mL)Figure and 3. Piper Ultraviolet–visible chaba extract (2 (UV-vis) mL, 100 absorptiong/L) at 60 °C spectrum and pH of= 7 reaction after 30 mixturesmin. of AgNO3 (1 mM, 40 3.2. EmL)ffect and of the Piper Synthesis chaba extract Conditions (2 mL, 100 g/L) at 60 °C and pH = 7 after 30 min. 3.2. Effect of the Synthesis Conditions 3.2. EffectThe keyof the factors Synthesis that Conditions affect the synthesis of AgNPs, i.e., temperature, concentration of AgNO3, The key factors that affect the synthesis of AgNPs, i.e., temperature, concentration of AgNO3, concentration of the plant extract, reaction time, and pH, were examined. concentrationThe key factorsof the plant that affectextract, the reaction synthesis time, of andAgNPs, pH, werei.e., temperature, examined. concentration of AgNO3, 3.2.1.concentration Effect of of Temperature the plant extract, reaction time, and pH, were examined. 3.2.1. Effect of Temperature Figure4 shows the absorption spectra of the reaction mixtures consisting of the stem extract 3.2.1.Figure Effect 4of shows Temperature the absorption spectra of the reaction mixtures consisting of the stem extract of of Piper chaba and AgNO3 after 1 h of reaction at different temperatures. The absorption of SPR Piperincreased Figurechaba withand 4 shows AgNO temperature. the3 after absorption 1 h Theof reaction changespectra at inof different the reacti color temperatures.on of mixtures the solutions Theconsisting absorption was acceleratedof the of stemSPR increased atextract higher of with temperature. The change in the color of the solutions+ was accelerated at higher temperatures, temperatures,Piper chaba and indicating AgNO3 after the 1 rapidh of reaction reduction at different of Ag to temperatures. form AgNPs. The Beyond absorption 60 ◦C, of the SPR absorption increased indicatingmaximawith temperature. slightly the rapid shifted The reduction change to longer of in Ag the wavelengths,+ tocolor form of AgNPs. the which solutions Beyond probably was 60 accelerated°C, originated the absorption fromat higher the maxima aggregation temperatures, slightly of shifted to longer wavelengths, which+ probably originated from the aggregation of AgNPs in AgNPsindicating inuncontrolled the rapid reduction reactions. of ThisAg to result form suggests AgNPs. that Beyond 60 ◦C 60 is °C, the the optimum absorption temperature maxima to slightly obtain uncontrolledhigh-qualityshifted to longer AgNPs.reactions. wavelengths, This result which suggests probably that 60 originated °C is the optimum from the temperature aggregation to of obtain AgNPs high- in qualityuncontrolled AgNPs. reactions. This result suggests that 60 °C is the optimum temperature to obtain high- quality AgNPs. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 15

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Figure 4. Absorption spectra of reaction mixtures of AgNO3 (1 mM, 40 mL) and Piper chaba extract (2 mL,Figure 100 4.g/L)Absorption at pH = 7 spectraafter 1 h of at reactiondifferent mixtures temperatures. of AgNO 3 (1 mM, 40 mL) and Piper chaba extract Figure 4. Absorption spectra of reaction mixtures of AgNO3 (1 mM, 40 mL) and Piper chaba extract (2 (2 mL, 100 g/L) at pH = 7 after 1 h at different temperatures. mL, 100 g/L) at pH = 7 after 1 h at different temperatures. 3.2.2. Effect of AgNO3 Concentration 3.2.2. Effect of AgNO3 Concentration 3.2.2.Figure Effect 5of shows AgNO the3 Concentration absorption spectra of the reaction mixtures consisting of the stem extract of Figure5 shows the absorption spectra of the reaction mixtures consisting of the stem extract of Piper Figurechaba and 5 shows AgNO the3 after absorption 1 h of reaction spectra at of 60 the °C reactiusingon different mixtures concentrations consisting of of the AgNO stem3 extractsolution. of Piper chaba and AgNO3 after 1 h of reaction at 60 ◦C using different concentrations of AgNO3 solution. The absorbance increased, and a blue shift occurred with the increase in the AgNO3 concentration. PiperThe absorbance chaba and AgNO increased,3 after and 1 h of a bluereaction shift at occurred 60 °C using with different the increase concentrations in the AgNO of AgNOconcentration.3 solution. Beyond 2.0 mM, the intensity stabilized due to the insufficient amount of the plant extract3 needed to TheBeyond absorbance 2.0 mM, increased, the intensity and stabilizeda blue shif duet occurred to the insu withffi thecient increase amount in of the the AgNO plant3 extractconcentration. needed react with Ag+. Thus, to obtain optimum quality and quantity of AgNPs, lower concentrations of Beyondto react with2.0 mM, Ag +the. Thus, intensity to obtain stabilized optimum due to quality the in andsufficient quantity amount of AgNPs, of the lowerplant extract concentrations needed to of AgNOreact with3 should Ag+ .be Thus, used. to obtain optimum quality and quantity of AgNPs, lower concentrations of AgNO3 should be used. AgNO3 should be used.

Figure 5. Absorption spectra of the reaction mixtures consisting of Piper chaba extract (100 g/L, 2 mL) Figure 5. Absorption spectra of the reaction mixtures consisting of Piper chaba extract (100 g/L, 2 mL) and AgNO3 (1 to 5 mM, 40 mL) at 60 ◦C and pH = 7 after 1 h of reaction. and AgNO3 (1 to 5 mM, 40 mL) at 60 °C and pH = 7 after 1 h of reaction. Figure 5. Absorption spectra of the reaction mixtures consisting of Piper chaba extract (100 g/L, 2 mL) 3.2.3. Effect of Plant Extract Concentration and AgNO3 (1 to 5 mM, 40 mL) at 60 °C and pH = 7 after 1 h of reaction. 3.2.3. Effect of Plant Extract Concentration Figure6 shows the absorption spectra of the reaction mixtures consisting of various concentrations 3.2.3. Effect of Plant Extract Concentration of theFigurePiper chaba6 showsstem the extract absorption and 1 mM spectra aqueous of solution the reaction of AgNO mixtures3 (40 mL) consisting after 1 h ofof reaction various at concentrations of the Piper chaba stem extract and 1 mM aqueous solution of AgNO3 (40 mL) after 1 h 60 ◦CFigure and neutral 6 shows conditions. the absorption The peak intensityspectra increased,of the reaction and a redmixtures shift occurred consisting with theof increasevarious of reaction at 60 °C and neutral conditions. The peak intensity increased, and a red shift occurred concentrationsin the plant extract of the concentration. Piper chaba stem The extract increased and 1 mM intensity aqueous indicates solution an of enhanced AgNO3 (40 production mL) after and 1 h withoffaster reaction the growth increase at 60 of AgNPs.°Cin theand plantneutral However, extract conditions. the concentratio red shiftThe peak originatedn. The intensity increased from increased, an intensity uncontrolled and indicates a red reduction shift an enhanced occurred by the productionwithplant the extract increase and excess. faster in Thus,the growth plant lower extractof concentrationsAgNPs. concentratio However, of then. theThePiper redincreased chaba shiftstem originated intensity extract indicatesfrom are suitable an uncontrolledan toenhanced obtain a reductionproductionquality product. by and the fasterplant extractgrowth excess. of AgNPs. Thus, However, lower concentrations the red shift of originated the Piper chabafrom steman uncontrolled extract are suitablereduction to byobtain the planta quality extract product. excess. Thus, lower concentrations of the Piper chaba stem extract are suitable to obtain a quality product. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 15

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Figure 6. Absorption spectra of reaction mixtures of Piper chaba extract (100 g/L, 1–5 mL) and AgNO3 (1 mM, 40 mL) after 1 h of reaction at 60 °C and pH = 7. Figure 6. Absorption spectra of reaction mixtures of Piper chaba extract (100 g/L, 1–5 mL) and AgNO3 Figure 6. Absorption spectra of reaction mixtures of Piper chaba extract (100 g/L, 1–5 mL) and AgNO3 3.2.4. Effect(1 mM,of Reaction 40 mL) after Time 1 h of reaction at 60 ◦C and pH = 7. (1 mM, 40 mL) after 1 h of reaction at 60 °C and pH = 7. Figure3.2.4. E 7ff ectshows of Reaction the time Time course of the absorption spectra of the reaction mixtures consisting of 3.2.4. Effect of Reaction Time the Piper chabaFigure stem7 shows extract the time (2 coursemL) and of the AgNO absorption3 solution spectra (40 of mL, the reaction 1 mM) mixtures at 60 °C consisting and pH of = the7. The intensityFigurePiper of chaba 7the showsstem absorption extract the time (2 increased mL) course and AgNOof with the3 absorptionthesolution reaction (40 spectra mL, time, 1 mM) ofindicating the at 60 reaction◦C andthepH mixturesgradual= 7. The formationconsisting intensity ofof of the absorption increased with the reaction time, indicating the gradual formation of AgNPs. On the AgNPs.the Piper On chaba the stemother extract hand, (2the mL) absorption and AgNO maxima3 solution gradually (40 mL, shifted 1 mM) toward at 60 longer°C and wavelengthspH = 7. The afterintensity 6other h, whichof hand, the indicates theabsorption absorption the increased agglomeration maxima graduallywith ofthe AgNPs shiftedreaction to toward formtime, longerlargerindicating wavelengthsparticles. the gradualThus, after a 6reactionformation h, which time of indicates the agglomeration of AgNPs to form larger particles. Thus, a reaction time frame from 1 to frameAgNPs. from On 1 the to 6other h was hand, found the suitable absorption to produce maxima good gradually yields of shifted quality toward AgNPs. longer wavelengths 6 h was found suitable to produce good yields of quality AgNPs. after 6 h, which indicates the agglomeration of AgNPs to form larger particles. Thus, a reaction time frame from 1 to 6 h was found suitable to produce good yields of quality AgNPs.

Figure 7. Time-dependent absorption spectra of the reaction mixtures consisting of AgNO3 (1 mM, Figure 7. Time-dependent absorption spectra of the reaction mixtures consisting of AgNO3 (1 mM, 40 40 mL) and Piper chaba extract (100 g/L, 2 mL) at 60 ◦C and pH = 7. mL) and Piper chaba extract (100 g/L, 2 mL) at 60 °C and pH = 7. 3.2.5. Effect of pH on the Formation of AgNPs Figure 7. Time-dependent absorption spectra of the reaction mixtures consisting of AgNO3 (1 mM, 40 3.2.5. mL)Effect andThe of Piper pHpH of chabaon the the reactionextract Formation (100 medium g/L, of 2AgNPs playedmL) at 60 a pivotal °C and rolepH = during 7. the formation of AgNPs. The pH Thechange pH aofffects the the reaction shape andmedium size of played the nanoparticles a pivotal becauserole during the pH the alters formation the electrostatic of AgNPs. states The of pH the substances in the plant extract, which affects the stabilizing abilities and subsequently the growth of change3.2.5. Effect affects of thepH shapeon the and Formation size of theof AgNPs nanoparticle s because the pH alters the electrostatic states of the nanoparticles [31]. Figure8 illustrates the absorption spectra of the reaction mixtures consisting of the substancesThe pH of in the the reaction plant extract, medium which played affects a pivotal the stabilizing role during abilities the formationand subsequently of AgNPs. the Thegrowth pH the stem extract of Piper chaba (2 mL) and AgNO3 (40 mL, 1 mM) at different pH. The absorption of SPR ofchange the nanoparticles affects the shape [31]. Figureand size 8 illustratesof the nanoparticle the absorptions because spectra the ofpH the alters reaction the electrostatic mixtures consisting states of ofthe the substances stem extract in the of plantPiper extract,chaba (2 which mL) and affects AgNO the 3stabilizing (40 mL, 1 abilitiesmM) at differentand subsequently pH. The absorptionthe growth ofof theSPR nanoparticles increased and [31]. a blueFigure shift 8 illustrates occurred the with abso thrptione increase spectra in pH.of the This reaction suggests mixtures that lowerconsisting pH predominantlyof the stem extract induced of Piper the chabaaggregation (2 mL) andof AgNPs AgNO rather3 (40 mL, than 1 themM) nucleation at different to formpH. The new absorption particles. of SPR increased and a blue shift occurred with the increase in pH. This suggests that lower pH predominantly induced the aggregation of AgNPs rather than the nucleation to form new particles. Nanomaterials 2020, 10, 1777 7 of 15

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 15 increased and a blue shift occurred with the increase in pH. This suggests that lower pH predominantly At a higherinduced pH, the many aggregation anionic of AgNPsfunctional rather groups than the can nucleation bind with to the form silver new ions particles. to form At aa higherlarge number pH, of nucleimany for anionic AgNPs functional which groups results can in bindAgNPs with with the silver a smaller ions to diameter. form a large Sathishkumar number of nuclei et al. for [12] reportedAgNPs a similar which results pH effect in AgNPs as they with found a smaller that diameter. silver species Sathishkumar were rapidly et al. [12 ]reduced reported at a similarelevated pH pH. Thus,e ffneutralect as they or basic found conditions that silver are species optimum were rapidly for the reduced reduction at elevatedreaction. pH. The Thus, pH of neutral the plant or basic extract and AgNOconditions3 mixture are optimum was seven, for the reductiontherewith reaction. neutral The conditions pH of the plantare advantageous extract and AgNO for3 mixturefacilitating was the reaction.seven, Thus, therewith the AgNPs neutral used conditions for further are advantageous analysis were for synthesized facilitating the using reaction. plantThus, extract the (2 AgNP mL)s and used for further analysis were synthesized using plant extract (2 mL) and AgNO3 (1 mM, 40 mL) at AgNO3 (1 mM, 40 mL) at 60 °C and pH = 7 for 1 h. 60 ◦C and pH = 7 for 1 h.

Figure 8. pH-dependent absorption spectra of the reaction mixtures of AgNO3 (1 mM, 40 mL) and Figure 8. pH-dependent absorption spectra of the reaction mixtures of AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) after the reaction for 1 h at 60 ◦C. Piper chaba extract (100 g/L, 2 mL) after the reaction for 1 h at 60 °C. 3.3. Structure of AgNPs 3.3. StructureThe structureof AgNPs of the organic components on the surface of AgNPs was investigated using FTIR Thespectroscopy structure (Figure of the9). organic The spectrum components AgNPs on obtained the surface using Piperof AgNPs chaba stem was extractsinvestigated shows using a broad FTIR 1 spectroscopyband at 3447 (Figure cm− 9).assigned The spectrum to the O–H AgNPs and obtained N–H stretching using vibrations.Piper chaba Thestem broad extracts shoulder shows at a the broad lower wavelength region implies that the O–H and N–H bonds form hydrogen bonds with the capping −1 band at 3447 cm assigned to the O–H and N–H stretching vibrations. The1 broad shoulder at the organic molecules and the surface of AgNPs. The band observed at 2925 cm− was attributed to the lower wavelength region implies that the O–H and N–H bonds form hydrogen1 bonds with1 the aliphatic C–H stretching. Signals were almost unobservable around 3000 cm− and 1700–1750 cm− . −1 cappingThese organic can be ascribedmolecules to negligibleand the surface contents of of AgNPs. aromatic The and band alkenyl observed protons as at well 2925 as cm ester was and ketoneattributed 1 −1 to themoieties. aliphatic The C–H broad stretching. band at1636 Signals cm− werecorresponded almost unobservable to the stretching around vibration 3000 of cm C=O and in amides, 1700–1750 cm−1.which These were can be found ascribed in Piper to chaba negligible[45–47 ].contents The bands of aromatic arising from and C–O–H alkenyl bending protons and as C–O well stretching as ester and 1 1 −1 ketonewere moieties. observed The at 1397 broad cm− bandand 1064at 1636 cm− cm, respectively, corresponded which agreeto the with stretching polysaccharides vibration commonly of C=O in amides,found which in plants. were This found FTIR in spectroscopic Piper chaba [45–47]. data confirm The thebands plausible arising structures from C–O–H for capping bending substances. and C–O stretchingThese characteristicwere observed FTIR at peaks 1397 werecm−1 notand observable 1064 cm− in1, respectively, the spectrum of which AgNPs agree synthesized with polysaccharides without any commonlystabilizing found agent in plants. [48], indicating This FTIR that spectroscopic the capping substances data confirm originate the plausibl from Pipere structures chaba stem extracts.for capping substances. These characteristic FTIR peaks were not observable in the spectrum of AgNPs synthesized without any stabilizing agent [48], indicating that the capping substances originate from Piper chaba stem extracts. NanomaterialsNanomaterials 20202020, ,1010, ,x 1777 FOR PEER REVIEW 8 8 of of 15 15 Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 15

Figure 9. Fourier transform infrared (FTIR) spectra of silver nanoparticles (AgNPs) synthesized using Figure 9. Fourier transform infrared (FTIR) spectra of silver nanoparticles (AgNPs) synthesized using AgNOFigure3 9.(1 Fourier mM, 40 transform mL) and infraredPiper chaba (FTIR) extract spectra (100 ofg/L, silver 2 mL) nanoparticles after the reaction (AgNPs) for synthesized 1 h at 60 °C using and AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) after the reaction for 1 h at 60 ◦C and pHAgNO = 7,3 and (1 mM, AgNPs 40 mL) synthesized and Piper with chabaout extract capping (100 agent g/L, reduced 2 mL) after by NaBH the reaction4. for 1 h at 60 °C and pH = 7,7, and and AgNPs AgNPs synthesized synthesized with withoutout capping capping agent reduced by NaBH 4. Figure 10 illustrates the EDX spectrum of the green-synthesized AgNPs. The spectrum shows Figure 10 illustrates the EDX spectrum of the green-synthesized AgNPs. The spectrum shows strongFigure signals 10 ofillustrates Ag. The thesignals EDX observed spectrum for of C,the N, green O, S,-synthesized and Cl can AgNPs. be attributed The spectrum to the organic shows strong signals of Ag. The signals observed for C, N, O, S, and Cl can be attributed to the organic substancesstrong signals found of inAg. the The plant signals extrac observedt. The weaker for C, intensities N, O, S, andof these Cl can signals be attributed imply the to presence the organic of a substances found in the plant extract. The weaker intensities of these signals imply the presence of a thinsubstances coating found layer onin theAgNPs. plant extract. The weaker intensities of these signals imply the presence of a thin coating layer on AgNPs.

FigureFigure 10. 10. EnergyEnergy dispersive x-rayx-ray (EDX)(EDX) spectrum spectrum of of AgNPs AgNPs synthesized synthesized using using AgNO AgNO3 (13 mM,(1 mM, 40 mL) 40 mL)Figureand andPiper 10. Piper chaba Energy chabaextract dispersive extract (100 (100 g/ L,x-ray g/L, 2 mL) (EDX)2 mL) at 60 spectrumat◦ C60 and °C and pH of =AgNPspH7 for= 7 1 forsynthesized h. 1 h. using AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) at 60 °C and pH = 7 for 1 h. 3.4.3.4. SEM SEM and and TEM TEM Results Results 3.4. SEM and TEM Results TheThe morphology morphology and and size size of of AgNPs AgNPs were were evaluated evaluated using using SEM SEM and and TEM. TEM. Figure Figure 11a 11showsa shows the SEMthe SEM Theimage morphology image of the of theAgNPs. and AgNPs. size The of The AgNPs AgNPs werewere were evaluated found found to using tobe be almost almostSEM andspherical spherical TEM. Figurewith with homogeneous homogeneous11a shows the morphology.SEMmorphology. image ofThe The the average average AgNPs. diameter diameter The AgNPs of of the the primary primarywere found pa particlesrticles to wasbe was almost 26 26 nm nm withspherical with a a standard standard with deviationhomogeneous deviation and and amorphology.a coefficient coefficient of of The variation variation average of of diameter 4.75%. 4.75%. of the primary particles was 26 nm with a standard deviation and a coefficient of variation of 4.75%. Nanomaterials 2020, 10, 1777 9 of 15 Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 15

Figure 11. (a) Scanning electron microscopy (SEM) and (b) transmission electron microscopy (TEM) Figure 11. (a) Scanning electron microscopy (SEM) and (b) transmission electron microscopy (TEM) images of the AgNPs synthesized from AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) at images of the AgNPs synthesized from AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) 60 ◦C and pH = 7 for 1 h. at 60 °C and pH = 7 for 1 h. Figure 11b shows the TEM image of the AgNPs. The particles were found to be spherical with an Figure 11b shows the TEM image of the AgNPs. The particles were found to be spherical with average diameter of 19 nm. Isolated primary particles were also observed. These results are in good an average diameter of 19 nm. Isolated primary particles were also observed. These results are in agreementgood agreement with the SEMwith the results. SEM results.

3.5. XRD3.5. XRD Analysis Analysis FigureFigure 12 shows12 shows the the XRD XRD patternpattern of of dried dried AgNPs. AgNPs. The five The distinct five distinct diffraction diff peaksraction with peaks 2𝜃 = with 2θ = 32.12°,32.12◦ 38.02°,, 38.02 46.1°,◦, 46.1 64.16°,◦, 64.16 and◦ ,75.36°, and 75.36which◦ ,can which be assigned can be to assigned the following to the lattice following planes: (101), lattice (111), planes: (101),(200), (111), (220), (200), and (220), (311), andrespectively, (311), respectively, in Ag(0) with in the Ag(0) face-centered with the cubic face-centered structure (JCPDS cubic file structure no. 84-0713 (JCPDS file no.and 84-071304-0783). andThe average 04-0783). size Theof nanocrystallites average size was of estimated nanocrystallites by the Scherrer was estimatedformula as follows: by the Scherrer formula as follows: 𝐷=𝑘𝜆/𝛽𝑐𝑜𝑠𝜃 D = kλ/βcosθ where D is the diameter of coherent diffraction domain, k is the shape constant (k = 1 for spherical wheredomains),D is the λ diameter is the wavelength of coherent of the di x-rayffraction source domain, (0.1541 knm),is the β is shape the full constant width at (halfk = maximum,1 for spherical domains),and 𝜃 λisis the the diffraction wavelength angle of corresponding the x-ray source to the (0.1541 lattice nm),planeβ (111).is the The full average width crystallite at half maximum, size calculated from the Scherrer equation is 20.9 nm, which is similar to the average size of AgNPs and θ is the diffraction angle corresponding to the lattice plane (111). The average crystallite size observed in the microscopic images. This suggests that the primary particles of AgNPs almost calculated from the Scherrer equation is 20.9 nm, which is similar to the average size of AgNPs observed consisted of single crystallites. in the microscopic images. This suggests that the primary particles of AgNPs almost consisted of single crystallites.Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 15

FigureFigure 12. X-ray 12. X-ray diffraction diffraction (XRD) (XRD) pattern pattern ofof AgNPs synthesized synthesized using using AgNO AgNO3 (1 mM,3 (1 40 mM, mL) 40and mL) and Piper chabaPiperextract chaba extract (100 (100 g/L, g/L, 2 mL) 2 mL) at at 60 60◦ C°C and and pH == 7 7for for 1 h. 1 h.

A few unassigned peaks (26.56°, 27.7°, 54.68°, and 56.42°) were also observed, which could be attributed to the biomass residue capping AgNPs. This suggests that the crystallization of the bio- organic phase occurs on the surface of the silver nanoparticles [9,22].

3.6. DLS Study and Zeta Potential Study DLS measurement was conducted to evaluate the hydrodynamic diameter of AgNPs (Figure 13). The average hydrodynamic diameter (Dh) of AgNPs was found to be 122 nm, which is larger than the sizes obtained from SEM and TEM measurements. A similar difference was reported for AgNPs prepared from Ekebergia capensis and Abelmoschus esculentus L. [49,50]. DLS measurements evaluate NPs with a hydrated layer consisting of swollen organic moieties on the surface of AgNPs in Brownian motion [51,52]. The hydrated layer and the Brownian motion resulted in the larger Dh. The polydispersity index was found to be 0.221. This moderate value probably originated from various components in the plant extract that was covering the nanosized AgNPs. The zeta potential was −22.2 mV. This sufficiently negative zeta potential confirms the good dispersibility of AgNPs caused by electrostatic repulsion.

Figure 13. Dynamic light scattering (DLS) curve of AgNPs synthesized using AgNO3 (1 mM, 40.0 mL) and Piper chaba extract (2 mL, 100 g/L) at 60 °C at pH = 7 for 1 h. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 15

Figure 12. X-ray diffraction (XRD) pattern of AgNPs synthesized using AgNO3 (1 mM, 40 mL) and Piper chaba extract (100 g/L, 2 mL) at 60 °C and pH = 7 for 1 h.

A few unassigned peaks (26.56°, 27.7°, 54.68°, and 56.42°) were also observed, which could be attributedNanomaterials to the2020 biomass, 10, 1777 residue capping AgNPs. This suggests that the crystallization of10 ofthe 15 bio- organic phase occurs on the surface of the silver nanoparticles [9,22].

3.6. DLS StudyA few and unassigned Zeta Potential peaks Study (26.56 ◦, 27.7◦, 54.68◦, and 56.42◦) were also observed, which could be attributed to the biomass residue capping AgNPs. This suggests that the crystallization of the bio-organicDLS measurement phase occurs was onconducted the surface to of evaluate the silver the nanoparticles hydrodynamic [9,22 ].diameter of AgNPs (Figure 13). The average hydrodynamic diameter (Dh) of AgNPs was found to be 122 nm, which is larger than the sizes3.6. obtained DLS Study from and SEM Zeta Potentialand TEM Study measurements. A similar difference was reported for AgNPs preparedDLS from measurement Ekebergia capensis was conducted and Abelmoschus to evaluate esculentus the hydrodynamic L. [49,50]. diameter DLS measurements of AgNPs (Figure evaluate 13). NPs Thewith average a hydrated hydrodynamic layer consisting diameter of (D hswolle) of AgNPsn organic was foundmoieties to beon 122 the nm, surface which of is AgNPs larger in Brownianthan the motion sizes obtained[51,52]. The from hydrated SEM and layer TEM and measurements. the Brownian A similarmotion di resultedfference wasin the reported larger D forh. The polydispersityAgNPs prepared index fromwas Ekebergiafound to capensisbe 0.221.and ThisAbelmoschus moderate esculentus value probablyL. [49,50]. originated DLS measurements from various componentsevaluate in NPs the with plant a hydrated extract that layer was consisting covering of swollenthe nanosized organic moietiesAgNPs. on the surface of AgNPs in Brownian motion [51,52]. The hydrated layer and the Brownian motion resulted in the larger D . The zeta potential was −22.2 mV. This sufficiently negative zeta potential confirms the hgood The polydispersity index was found to be 0.221. This moderate value probably originated from various dispersibility of AgNPs caused by electrostatic repulsion. components in the plant extract that was covering the nanosized AgNPs.

Figure 13. Figure 13. DynamicDynamic light light scattering scattering (DLS) (DLS) curv curvee of of AgNPs synthesizedsynthesized using using AgNO AgNO3 (13 mM, (1 mM, 40.0 40.0 mL) mL) and Piperand Piper chaba chaba extractextract (2 (2mL, mL, 100 100 g/L) g/L) at at 60 60 °C◦C at at pH == 7 forfor 11 h. h. The zeta potential was 22.2 mV. This sufficiently negative zeta potential confirms the good − dispersibility of AgNPs caused by electrostatic repulsion.

3.7. Application of the Synthesized AgNPs in the Reduction of 4-Nitrophenol (4-NP) The ability of the synthesized AgNPs to act as catalysts was examined. The reduction of 4-NP to 4-aminophenol by NaBH4 was selected as a model for the catalytic reduction. The catalytic process was monitored using UV-vis spectroscopy. The original absorption peak of 4-NP was observed at 316 nm. Upon addition of freshly prepared aqueous NaBH4 solution, the absorption maxima instantaneously shifted from 316 to 401 nm in the presence of AgNPs, and the color of the solution changed from light yellow to yellowish green, indicating the formation of the 4-nitrophenolate ion. When no AgNPs were used, the color and optical absorption did not change until after 1 h of reaction. The reduction of 4-NP in the presence of NaBH4 is a thermodynamically feasible process, but it is kinetically restricted in the absence of catalysts [14]. The kinetics of the reduction reaction was monitored using UV-vis spectroscopy. In the presence of AgNPs, 4-NP was almost completely reduced in approximately 9 min (Figure 14), which is comparable to reported fine AgNPs with identical sizes [13,38]. As the reduction reaction of 4-NP progressed, the intensity of the peak at 401 nm decreased, and a new peak corresponding to the 4-AP formation simultaneously occurred at 296 nm [13,16–18]. Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 15

3.7. Application of the Synthesized AgNPs in the Reduction of 4-Nitrophenol (4-NP) The ability of the synthesized AgNPs to act as catalysts was examined. The reduction of 4-NP to 4-aminophenol by NaBH4 was selected as a model for the catalytic reduction. The catalytic process was monitored using UV-vis spectroscopy. The original absorption peak of 4-NP was observed at 316 nm. Upon addition of freshly prepared aqueous NaBH4 solution, the absorption maxima instantaneously shifted from 316 to 401 nm in the presence of AgNPs, and the color of the solution changed from light yellow to yellowish green, indicating the formation of the 4-nitrophenolate ion. When no AgNPs were used, the color and optical absorption did not change until after 1 h of reaction. The reduction of 4-NP in the presence of NaBH4 is a thermodynamically feasible process, but it is kinetically restricted in the absence of catalysts [14]. The kinetics of the reduction reaction was monitored using UV-vis spectroscopy. In the presence of AgNPs, 4-NP was almost completely reduced in approximately 9 min (Figure 14), which is comparable to reported fine AgNPs with identical sizes [13,38]. As the reduction reaction of 4-NP progressed, the intensity of the peak at 401 nm decreased, and a new peak corresponding to the 4-AP formation simultaneously occurred at 296 nm [13,16–18].Nanomaterials 2020, 10, 1777 11 of 15

Figure 14. Optical images and time-dependence of the absorbance at 401 nm for the catalytic reduction Figure 14. Optical images and time-dependence of the absorbance at 401 nm for the catalytic reduction of 4-NP by NaBH4 in the presence of AgNPs. Conditions: 4-NP = 10 ppm; catalyst = 53.9 mg/L; of 4-NP by NaBH4 in the presence of AgNPs. Conditions: 4-NP = 10 ppm; catalyst = 53.9 mg/L; NaBH4 NaBH4 = 600 ppm; temperature = 25 ◦C. = 600 ppm; temperature = 25 °C. 3.8. Application of the Synthesized AgNPs in the Catalytic Degradation of MB

3.8. ApplicationThe catalytic of the Synthesized degradation AgNPs of dyes in in the the Catalytic presence Degradation of NaBH4 wasof MB examined as another model reaction to confirm the catalytic activity of AgNPs using MB. The catalytic process was monitored The catalytic degradation of dyes in the presence of NaBH4 was examined as another model by UV-vis spectroscopy. Similar to 4-NP, NaBH needs a catalyst to react with MB. Thus, the color of reaction to confirm the catalytic activity of AgNP4s using MB. The catalytic process was monitored by the solution did not change for more than 1 h without AgNPs. With the addition of as-synthesized UV-visAgNPs spectroscopy. to the reaction Similar mixture, to 4-NP, the NaBH catalytic4 needs reduction a catalyst of dyes to immediatelyreact with MB. occurred. Thus, the During color the of the solutiondegradation did not ofchange MB, the for color more of thethan solution 1 h withou changedt AgNPs. from blue With to colorlessthe additi afteron 8of min, as-synthesized which is AgNPsalso to comparable the reaction to previouslymixture, the reported catalytic fine AgNPsreduction [17, 33of]. dyes The originalimmediately absorption occurred. peak at During 663 nm the degradationprogressively of MB, decreased the color with of the time, solution which changed confirms thefrom catalytic blue to activity colorless of the after synthesized 8 min, which AgNPs is also comparable(Figure 15to). previously Figure 16 schematically reported illustratesfine AgNPs the mechanism[17,33]. The of theoriginal catalytic absorption processes for peak degradation at 663 nm Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 15 progressivelyof both 4-NP decreased and MB usingwith time, NaBH which4 as the confirms reductant andthe catalytic AgNPs as activity the catalyst of the [16]. synthesized AgNPs (Figure 15). Figure 16 schematically illustrates the mechanism of the catalytic processes for degradation of both 4-NP and MB using NaBH4 as the reductant and AgNPs as the catalyst [16].

Figure 15. Optical images and time-dependence of the absorbance at 663 nm for the catalytic Figure 15. Optical images and time-dependence of the absorbance at 663 nm for the catalytic degradation of methylene blue (MB) dye by NaBH4 in the presence of AgNPs. Conditions: MB = 2 ppm; degradationcatalyst =of53.9 methylene mg/L; NaBH blue4 = (MB)600 ppm; dye temperatureby NaBH4 in= 25the◦C. presence of AgNPs. Conditions: MB = 2 ppm; catalyst = 53.9 mg/L; NaBH4 = 600 ppm; temperature = 25 °C.

Figure 16. Schematic illustration of the catalytic process occurring on AgNPs.

4. Conclusions In this study, a successful, rapid, and green synthesis of AgNPs was achieved using Piper chaba stem extract as an effective reducing and capping agent. The parameters affecting the synthesis of AgNPs (i.e., temperature, concentration of AgNO3, quantity of plant extract, reaction time, and pH) were studied. The formation of AgNPs was confirmed by the SPR peak at 445 nm and XRD pattern. The obtained AgNPs were spherical and monocrystalline, ranging in size from 20 nm to 26 nm. The capping of the obtained AgNPs was confirmed using FTIR spectroscopy. The DLS measurement showed that AgNPs were uniformly dispersed and exhibited a moderate size distribution. The catalytic activity of the green-synthesized AgNPs was confirmed when used for a rapid reduction of 4-NP and degradation of MB in the presence of NaBH4. This successful synthesis and catalytic applications of the green-synthesized AgNPs suggests the potential of using this method as an alternative to the chemical methods in large-scale synthesis, other applications such as electronics, and fabrication of other nanoparticles. The availability, cost-effectiveness compared to other plant sources, and edible properties of Piper chaba are the significant advancements of this work.

Author Contributions: Conceptualization, M.M. and B.O.; methodology, M.M. and P.S.; data investigation, M.M. and P.S.; supervision, B.O.; writing-original draft, M.M. and P.S.; writing-review and editing, M.M. and B.O. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding. Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 15

Figure 15. Optical images and time-dependence of the absorbance at 663 nm for the catalytic Nanomaterialsdegradation2020, 10 of, 1777methylene blue (MB) dye by NaBH4 in the presence of AgNPs. Conditions: MB = 122 of 15 ppm; catalyst = 53.9 mg/L; NaBH4 = 600 ppm; temperature = 25 °C.

Figure 16. Schematic illustration of the catalyticcatalytic process occurring on AgNPs.

4. Conclusions InIn thisthis study,study, aa successful,successful, rapid,rapid, andand greengreen synthesissynthesis ofof AgNPsAgNPs waswas achievedachieved using Piper chaba stem extract as an an effective effective reducing reducing and and capping capping agent. agent. The The parameters parameters affecting affecting the the synthesis synthesis of of AgNPs (i.e., temperature, concentration of AgNO , quantity of plant extract, reaction time, and AgNPs (i.e., temperature, concentration of AgNO3, quantity3 of plant extract, reaction time, and pH) pH)were were studied. studied. The formation The formation of AgNPs of AgNPs was confirmed was confirmed by the bySPR the peak SPR at peak445 nm at 445and nmXRD and pattern. XRD pattern.The obtained The obtained AgNPs AgNPswere spherical were spherical and monocrystalline, and monocrystalline, ranging ranging in size in from size 20 from nm 20 to nm 26 tonm. 26 The nm. Thecapping capping of the of theobtained obtained AgNPs AgNPs was was confirmed confirmed using using FTIR FTIR spectroscopy. spectroscopy. The The DLS DLS measurement showed thatthat AgNPs AgNPs were were uniformly uniformly dispersed dispersed and exhibitedand exhibited a moderate a moderate size distribution. size distribution. The catalytic The activitycatalytic of activity the green-synthesized of the green-synthesized AgNPs was AgNPs confirmed was confirmed when used when for a used rapid for reduction a rapid reduction of 4-NP and of degradation of MB in the presence of NaBH . This successful synthesis and catalytic applications 4-NP and degradation of MB in the presence4 of NaBH4. This successful synthesis and catalytic ofapplications the green-synthesized of the green-synthesized AgNPs suggests AgNPs the potential suggests of the using potential this method of using as an this alternative method toas thean chemicalalternative methods to the chemical in large-scale methods synthesis, in large-scale other applications synthesis, other such asapplications electronics, such and as fabrication electronics, of otherand fabrication nanoparticles. of other The nanoparticles. availability, cost-e Theff availability,ectiveness compared cost-effectiveness to other plantcompared sources, to other and edible plant propertiessources, and of ediblePiper chaba propertiesare the of significant Piper chaba advancements are the significant of this advancements work. of this work. Author Contributions: Conceptualization, M.M. and B.O.; methodology, M.M. and P.S.; data investigation, M.M. andAuthor P.S.; Contributions: supervision, B.O.; Conceptualization, writing-original M.M. draft, and M.M. B.O.; and methodol P.S.; writing-reviewogy, M.M. and and P.S.; editing, data M.M. investigation, and B.O. AllM.M. authors and P.S.; have supervision, read and agreed B.O.; towriting-original the published versiondraft, M.M. of the and manuscript. P.S.; writing-review and editing, M.M. and B.O. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments:Funding: This researchM.M. received gratefully no external acknowledge funding. the Ministry of Education, Culture, Sports, Science and Technology, Japan, for providing Japanese Government (Monbukagakusho: MEXT) Scholarship. Conflicts of Interest: The authors declare no conflict of interest.

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