Effect of CTAB on the surface resonance plasmon intensity of silver nanoparticles: Stability and oxidative dissolution

Journal of Molecular Liquids 302 (2020) 112565

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Effect of CTAB on the surface resonance plasmon intensity of silver nanoparticles: Stability and oxidative dissolution Alhanoof Bshara Albeladi, Shaeel Ahmed AL-Thabaiti, Zaheer Khan ⁎ Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 13 October 2019 Received in revised form 19 January 2020 Accepted 23 January 2020 Available online 25 January 2020 Keywords: Nucleation Solubilization Dissolution Silver nanoparticles

a b s t r a c t Stable silver nanoparticles (AgNPs) were prepared from the reduction of silver ions by sodium borohydride in presence of shape controlling cationic cetyltrimethylammonium bromide (CTAB) at room temperature. The intensity and position of surface plasmon resonance (SRP), morphology depends on the concentrations of CTAB. The complex forming tendency of CTAB with positive head group of surfactant and borohydride anion (BH− 4 ) was responsible for pre-, and post-micellar effect. The values of BH− 4 -CTAB binding constant (Ks = 33.6 × 104 mol−1 dm3) and rate constant in micellar pseudo phase (km = 5.5 × 10−5 s−1) were calculated. Catalytic role of CTAB discussed in term of pseudo-phase model. The silver soles were unstable at higher [BH− 4 ] (≥14.0 × 10−5 mol/L). Transmission electron microscopic images indicate that the AgNPs are roughly spherical and poly dispersed. The oxidative dissolution of silver nanoparticles was very slow. The [BH− 4 ] has significant impact on the sensing properties of AgNPs. The dissolution rate increases drastically by adding hydrogen peroxide. Scavengers quenched the oxygen radical reactive species (hydroxy radical and superoxide) and decreased the hydrogen peroxide catalyses rate of AgNPs. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Stabilizers play in important during the reduction of metal ions by strong and weak reducing agent in the preparation of advanced nonmaterial. Morphology depends on the capping action of stabilizer. Surfactants [1], proteins [2], polymers [3], enzymes [4], carbohydrates [5], lipids [6], ligands [7], DNA [8], bacteria [9] and an aqueous extract of living plants [10] used to block the unlimited growth of metal NPs and to find the desired shape and size. Out of these, surfactant behaves as an excellent stabilizer because they provide various regions for the incorporation and/or association of reactant as well as product [11]. It can easily be remove from the surface of nanomaterials by treatment with water. Surfactants are amphiphilic in nature and have water-avoiding (long non polar hydrophobic chain) and water-attracting (hydrophilic) groups within a molecule. They aggregate and cerate self-assembled molecular cluster (micelles) at a specific concentration. Use of surfactant to the preparation of nanoparticles has been the subject of various researchers from many decades [12–20]. For example, Lou and his coworkers used cationic surfactant, CTAB, for the multi-branched

⁎ Corresponding author. E-mail address: [email protected] (Z. Khan).

https://doi.org/10.1016/j.molliq.2020.112565 0167-7322/© 2020 Elsevier B.V. All rights reserved.

gold NPs at room temperature [15]. Xiao and Qi in his pioneering feature article suggested that the head group of micelles forming surfactants have significant impact on the morphology smart NPs of gold and silver in presence of single and mixed surfactant reaction system [17]. Recently, we reported the seedless synthesis of silver-nickel alloy in presence of sodiumdodecy sulphate (SDS) and CTAB [19] and suggested that the silver ion and SDS was not suitable for the preparation of perfect transparent silver-nickel sols due to the formation of white precipitate. Silver NPs were widely prepared in various laboratories due their applications in science, industries, medicine and removal of toxic materials. They have a well-defined sharp surface resonance plasmon (SRP) band in the visible region. Solomon et al. described a method to the preparation of yellow colloidal silver for the high school students by using sodium borohydride as reducing and capping agent [20]. They also pointed out that the time of breakdown of colloid depends on the ratio of sodium borohydride and silver ions. Wang et al. developed a silver NPs-hydrogen peroxide based senor for the colorimetric detection of iodide [21]. The SRP band intensity decreased with hydrogen peroxide as well as pH of the reaction media. The release of silver ion (Ag+) from the surface of silver NPs was the primary reason for the silver NPs toxicity against human pathogens [22]. Liu and his co-workers reported that the dissolution kinetics depend on the pH, temperature and size of the silver NPs [23]. Peretyazhko et al. proposed a mechanism

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to the oxidative dissolution of silver NPs and suggested that the dissolution was initiated by the formation of O2 layer on the surface of NPs [24]. Adamczyk et al. developed a general model and described the oxidative dissolution of silver NPs suspension in aqueous media with varying pH [25]. It has been discussed on several occasions that the dissolved O2 was responsible for the release of Ag+ ions from the surface of silver NPs [26–30]. Although more than thousands of papers has published on the preparation of AgNPs using NaBH 4 as a reducing agent with and without capping agent. No efforts have made to describe the role of NaBH4 in the stability and/or dissolution of the resulting silver NPs in an aqueous solution. NaBH4 is a strong reducing agent and its aqueous solution is alkaline in nature. Our goal of this study was to investigate the formation and dissolution kinetics of AgNPs with CTAB using Ag+ ions and NaBH4. The batch experiments performed as a function of [reactant], [CTAB] and pH to assess the rate of NPs dissolution for the first time. In addition, the effect of hydrogen peroxide and radical scavengers also investigated on the dissolution of AgNPs.

2. Experimental

2.4. SRP sensing by oxidative dissolution The kinetic experiments were performed to the study the oxidative dissolution of silver NPs by varying the [NaBH4], [CTAB] and [Ag+] by using UV–visible spectrophotometer. In a typical experiment, the decay of AgNPs absorbance was monitor as a function of time. It was observe that the dissolution of AgNPs was slow at room temperature but accelerated drastically in presence of hydrogen peroxide. 3. Results and discussion 3.1. Stability and dissolution of silver NPs The as-prepared silver sols (resulting orange color) was not stable at room temperature for long time, i.e., one week. Surprisingly, the stability depends on the [Ag + ], [NaBH 4 ] as well as [CTAB]. Optical images of silver sols recorded at different time intervals, which clearly show that the both reactants as well as stabilizer have markedly impact on the dissolution of silver NPs (Figs. 1 to 3). For Ag+ ions, the orange color intensity increases with [Ag+]

2.1. Materials Stock aqueous silver nitrate (0.01 mol/L in 250 mL) solution was prepared in deionized double distilled water and stored in an amber glass container. Cetyltrimethylammonium bromide (0.01 mol/L in 250 mL; Sigma-Aldrich) used as a stabilizer and/or capping agent. Sodium borohydride (Sigma-Aldrich) aqueous solution is unstable due to the hydrolysis. Therefore, its solution prepared daily prior to use. Hydrogen peroxide (30% w/v; Sigma-Aldrich) used to determine the dissolution rate of silver NPs.

A Reaction time = 1h 104[Ag+] (mol/L) 4.0

8.0

12.0

16.0

20.0

2.2. Instruments UV–visible spectra provide preliminary information about the formation of silver NPs in an aqueous solution. Therefore, UV–visible spectra recorded on a Shimadzu UV–vis multi Spec-1501, spectrophotometer having 1 cm quartz cuvettes at different time intervals. Fisher Scientific pH meter used to monitor the pH at the beginning as well as end of the reaction. Transmission electron microscope (TEM) images recorded to determine the shape, size and the size distribution silver NPs.

B Reaction time = one week 104[Ag+] (mol/L) 4.0

8.0

12.0

16.0

20.0

2.3. Kinetics of silver sols preparation For the fabrication of transparent silver sol, the required solution of Ag+ ions (oxidant) and CTAB (stabilizer) were added in a conical flask and equilibrated at desired temperature for 20 min for attaining equilibrium. A fresh prepared NaBH 4 added into the reaction flask with constant stirring. The progress of the silver sol formation monitors by recording the UV–visible spectra at definite time intervals. The pseudo-first order rate constants calculated by using Eq. (1).

kobs ¼

  2:303 ðAα −A0 Þ log t ðAα −At Þ

C Reaction time = 2 week 104[Ag+] (mol/L) 4.0

8.0

12.0

16.0

20.0

ð1Þ

where kobs = pseudo-first order rate constant and all symbols have their usual significance. The values of kobs calculated from the initial slope of first order plot (log(Aα-A0/Aα-At) versus time) for a definite time interval.

Fig. 1. Optical images of silver nanoparticles as a function of [Ag+]. Reaction conditions: [NaBH4] = 14.0 × 10−5 mol/L and [CTAB] = 10.0 × 10−4 mol/L.

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A

A Reaction time = 1h

Reaction time = 1h 104[CTAB] (mol/L)

105[NaBH4] (mol/L) 2.0

6.0

14.0

18.0

16.0

20.0

10.0

Reaction time = 1week

Reaction time = 1 week

104[CTAB] (mol/L)

105[NaBH4] (mol/L) 6.0

4.0

B

B

2.0

8.0

14.0

18.0

20.0

16.0

10.0

8.0

4.0

C

C

Reaction time = 2 week

Reaction time = 2 week 105[NaBH4] (mol/L) 2.0

6.0

14.0

18.0

104[CTAB] (mol/L)

20.0

4.0

Fig. 2. Optical images of silver nanoparticles as a function of [NaBH4]. Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L and [CTAB] = 10.0 × 10−4 mol/L.

(Fig. 1A) and became colorless (Fig. 1B) after 1 week for [Ag + ] (≤8.0 × 10 −4 mol/L). The pale yellow colors persist at higher [Ag +] even after 2 weeks (Fig. 1C). For [NaBH4], orange color was stable for 1 h with varying [NaBH4] (Fig. 2A). Surprisingly, all silver sols turn pale yellow with in 1and 2 weeks (Fig. 2B and C). For CTAB, the orange color completely disappeared at higher concentrations (≥16.0 × 10−4 mol/L) within a week (Fig. 3), whereas the silver sols show some yellow color at lower concentrations. At higher [CTAB] (≥20.0 × 10 −4 mol/L), the resulting color disappeared (Fig. 4). Inspection of optical images clearly indicates that the stable silver sols can be prepared by adjusting the molar ratios of silver ions and NaBH4 along with the suitable stabilizer concentration. The drastic effect of [CTAB] might be due to the formation of more and more micelles at higher concentrations, which incorporates all silver NPs into the reaction sites (Stern layer). On the other hand, pH of cationic micelles micellar pseudo-phase is ca. 2 unit higher than that of bulk aqueous solvent due to the solubilization of anionic species [31–33]. Therefore, pH of the reaction medium also plays an important role in inhibiting and acceleration the dissolution of silver NPs.

8.0

10.0

16.0

Fig. 3. Optical images of silver nanoparticles as a function of [CTAB]. Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L and [NaBH4] = 14.0 × 10−5 mol/L.

In order to determine the rate of silver NPs dissolution, the intensity of SRP band was monitored using UV–visible spectroscope at 414 nm for different [NaBH 4 ] (Fig. 5A) at different intervals. The dissolution rate constants were estimated from the initial slope of pseudo-first-order rate-law (Fig. 5B) and found to be 3.2, 2.3, 2.0 and 1.9 × 10 −6 s −1, respectively, for [NaBH4] = 2.0, 6.0, 14.0 and 20.0 × 10 −5 mol/L. To see insight into the generation of reactive oxygen species (hydroxyl radical; OH•), dissolution kinetic experiment were performed in methanol (5.0 mL) and tertiary butyl alcohol (5.0 mL). We did not observe any significant effect of these scavengers on the dissolution rates (Table 1), which ruled out the involvement of OH• during the dissolution of AgNPs. On the other hand, SRP intensity was very sensitive for the small [H2O2] (source of reactive radical species). Intensity of color decreased with increasing the [H2O2] used in the present studies (Fig. 6A). The kobs were calculated from the slope of initial part of straight lines of Fig. 6B (kobs = 9.9, 6.9, 5.2 and 4.6 × 10−4 s−1 for [H2O2] = 2.0, 3.0, 4.0 and 5.0 × 10−4 mol/L) at constant [NaBH4] = 14.0 × 10−5 mol/L.

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A Research time = 1h 104[CTAB] (mol/L) 40.0

30.0

24.0

20.0

B Research time = 1week 104[CTAB] (mol/L) 40.0

30.0

24.0

20.0

C Research time = 2 week 104[CTAB] (mol/L) 20.0

24.0

30.0

40.0 Fig. 5. Absorbance decay of silver NPs at different time intervals (A) and their first-order plots for different [NaBH4] (B).

Fig. 4. Optical images of silver nanoparticles as a function of [CTAB]. Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L and [NaBH4] = 14.0 × 10−5 mol/L.

3.2. Mechanism of silver NPs dissolution Before proposing a mechanism for the dissolution of silver NPs, it is critical to discuss the aqueous chemistry of NaBH4 (strong reducing agent and hydrolyzed by water to generate hydrogen gas). Its aqueous solution is alkaline in nature due to the formation of sodium hydroxide after hydrolysis (Eqs. (2) and (3)) [34]. NaBH 4 þ 2H2 O→NaBO2 þ 4H2

ð2Þ

NaBO2 þ 2H 2 O→NaOH þ H 3 BO3

ð3Þ

In this study, NaBH4 was used for the reduction of Ag+ ions into Ag in presence of CTAB. We observe the evolution of hydrogen gas in the form of bubbles during the synthesis of silver NPs, which provides an inert atmosphere automatically, and diminished the presence of dissolved molecular oxygen gas from the reaction mixture. The presence of OH− is responsible for the release of Ag+ ion from the surface of metallic Ag 0 (Scheme 1) under our experimental conditions. 0

In Scheme 1, Eq. (4) represents the generation of hydrogen gas and OH− from the hydrolysis of borohydride. The OH− adsorbed on the surface of silver NPs, leads to the formation of hydroxide layer around the NPs (Eq. (5)) [27–30]. The resulting silver sols mainly exist as (Ag)n1Ag-OH in solution under our experimental conditions. The (Ag)n-1AgOH under goes protonation at acidic pH (Eq. (6)) [35]. As a result, Ag\\O bond became weaker and surface Ag\\O broken and Ag+ ions released from the surface of NPs (Eq. (7)) [36]. The following rate-law is

Table 1 Effects of H2O2 and scavengers on the dissolution of AgNPs. 104[AgNPs]

105[BH− 4 ]

104[H2O2]

Scavenger

kobs

(mol/L)

(mol/L)

(mol/L)

(mL)

(s−1)

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

2.0 6.0 14.0 20.0 14.0 14.0 14.0 14.0 14.0 14.0 14.0

0.0 0.0 0.0 0.0 2.0 3.0 4.0 5.0 2.0 2.0 2.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0(BQ) 5.0(TBA) 5.0(methanol)

3.2 2.3 2.0 1.9 9.9 6.9 5.2 4.6 6.0 4.5 3.8

× × × × × × × × × × ×

10−6 10−6 10−6 10−6 10−4 10−4 10−4 10−4 10−4 10−4 10−4

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The Eq. (8) can be reduces to Eq. (9), if 1 N Kh [H+].   d½AgNPs ¼ k1 K h K ad Hþ ½OH− ½AgNPsTotal − dt

ð9Þ

The complete derivation of Eq. (8) is given in the supporting information (Scheme S1). Eq. (9) clearly explains that the both [OH−] and [H+] are essential for the silver sols dissolution. The derived rate-law is in good agreement to the observations of Liu and Hurt [23], Ma et al. [27], Sotiriou et al. [36], Zhang et al. [37] and Kittler et al. [38] regarding the effects of pH on the silver NPs dissolution. The highest dissolution takes place at pH 8.0 in 1.0 M sodium bicarbonate solution. A schematic representation of a formation of hydroxide layer around the surface of AgNPs, protonation of layer and release of Ag + ions into the solution given in Scheme 2. Generally, free hydroxyl radical, bound hydroxyl radical with Ag metal and super oxide radical (•O=O−) and hydroperoxyl radical (•HO2) were the main reactive species in the H2O2 catalyzed dissolution of AgNPs [39,40]. Benzoquinone, methanol and tertiary butyl alcohol were used as scavengers to confirm the role of reactive oxygen radical species in the H2 O 2 catalyzed dissolution. As exhibited in Fig. 7, the dissolution efficiency of H2 O2 (=2.0 × 10 −4 mol/L) decreased from 9.9 × 10−4 s−1 to 6.0 × 10−4 s−1, 4.5 × 10−4 s−1 and 3.8 × 10−4 s−1, respectively, for H2O2, benzoquinone (BQ), tertiary butyl alcohol (TBA) and methanol indicating that various reactive oxygen radical formed as an intermediates (Table 1). Scheme 1 can be modifies for the hydrogen peroxide catalyzed dissolution of AgNPs (Scheme 3). 3.3. Role of Ag+ and BH− 4 concentrations

Fig. 6. Effects of hydrogen peroxide concentration on the dissolution of silver NPs (A) and their first-order plots (B).

deriving from Scheme 1.  þ − d½AgNPS k1 K h K ad H ½OH ½AgNPsTotal  þ 
¼ − dt 1 þ Kh H

ð8Þ

In order to establish the role of Ag+ ions on the nucleation path of silver sols formation, the effect of [Ag + ] (from 2.0 × 10 −4 to 20.0 × 10−4 mol/L) at constant [CTAB] (=10.0 × 10 −4 mol/L), [NaBH4] (=14.0 × 10−5 mol/L) and temperature (=23 °C). The SRP of silver sols increases with increasing [Ag +] from 2.0 × 10−4 to 10.0 × 10−4 mol/L at 414 nm (Fig. 8A). At higher [Ag+ ], intensity remains constant, indicating that the no further nucleation and growth processes occur and all [NaBH4] consumed during the course of reaction. The some kinetic experiments were performed with [NaBH4] from 2.0 × 10−5 to 24.0 × 10−5 mol/L at fixed [CTAB] (=10.0 × 10−4 mol/L), [Ag+] (=10.0 × 10−4 mol/L) and temperature (=23 °C). Interestingly, the SRP first increases with the [NaBH4 ], until reaches a maximum and then decreases (Fig. 8B). Out of these, only five concentrations of [NaBH4] were used to record the optical images of silver sols (Fig. 2A). Such type of behavior (hypochromic shift) might be due to the adsorption of BH− 4 ions onto the resulting silver NPs, which etch the adsorbed Ag+ ions from the surface of NPs. These results are in good agreement to the observations of Solomon et al. regarding the breakdown of silver NPs at higher [NaBH4] [20]. 3.4. Role of CTAB concentration in SRP sensing

BH4-

-

+ 4H2O

(Ags)n-Ag+ + OH(Ags)n-1-Ag-OH + H+ (Ags)n-1-Ag-OH2+

H3BO3 + OH + 4H2 Kad Kh k1

(Ags)n-1-Ag-OH (Ags)n-1-Ag-OH2+ (Ags)n-1 + Ag+ + H2O

Scheme 1. Mechanism to the dissolution of silver NPs.

Preliminary experiments show that the colorless reaction mixture containing silver nitrate and sodium borohydride became yellow-black turbid after 5 min of mixing. Therefore, CTAB was used as a stabilizer. To determine the CTAB role on nucleation and growth of silver NPs formation (appearance of prefect transparent and stable silver sol), a series of experiments were performed at various [CTAB] (from 0.0 to 40.0 × 10 −4 mol/L) with constant [Ag+] (=10.0 × 10−4 mol/L), [NaBH 4 ] (=14.0 × 10 −5 mol/L) and temperature (=23 °C). Fig. 9 shows the UV–visible spectra of silver NPs formation as a function of time with different [CTAB]. The peak intensity of

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Scheme 2. Schematic releases of Ag+ from the surface of Ag0.

Fig. 7. Effects of scavenger on the hydrogen peroxide catalyzed dissolution of AgNPs.

silver sons increases with time and became constant after 20 min at lower [CTAB] (6.0 × 10−4 mol/L; Fig. 9A). The resulting sol exhibits a well-defined SRP band at 414 nm. The sols were unstable (absorbance increases and decreases with time) at [CTAB] =

(Ags)n + H2O2

20.0 × 10 −4 mol/L, and shows a broad band at ca. 450 nm (Fig. 9B). Interestingly, the intensity of band increases with time and a broad band appeared at ca. 450 nm. As the [CTAB] increases, the SPR band shifted from lower to higher wavelength (Fig. 9 C and D for [CTAB] = 30.0 × 10−4 and 40.0 × 10−4 mol/ L, respectively). TEM images also suggest that the shape and size distribution could be tune with [CTAB] (Fig. 9). The AgNPs are spherical along with some irregular shaped and polydispersed. The number of NPs depends on the [CTAB]. Each CTAB concentration shows different types of morphology. The size of the AgNPs also calculated from the corresponding TEM images and found to be 85, 15, 30, and 20 nm for 6.0 × 10 −4 , 20.0 × 10 −4 , 30.0 × 10 −4 and 40.0 × 10 −4 mol/L, respectively. Reaction-time profile shows that the [CTAB] has significant impact on the path of silver NPs during the reduction of Ag + ions by sodium borohydride. Nucleation and growth steps depend on the [CTAB] (Fig. 10A). The intensity and rate constant was found to increase with reaction time and CTAB concentrations (Fig. 10B). Sigmoidal behavior of CTAB on the kinetics of silver NPs formation can be rationalized by the sub-, and post-micellar effect of CTAB (Fig. 10B) [41]. Micelles forming surfactant behaves as an electrolyte at lower concentration and dissociates into corresponding anion and cation. As the concentration increases, they form aggregates (monomer, dimmer, trimer, etc.) but at a specific concentration aggregates come close together and formed micelles. This concentration called as critical micellar concentration (CMC), which is ca. 8.0 × 10 −4 mol/L at room temperature. The catalytic role of CTAB might be due to the formation of sub-micellar

(Ag)n-1-Ag-H-O-O-H

(Ags)n-1-Ag-H-O-OH

(Ags)n-1-Ag-OH+ + HO-

(Ags)n-1-Ag-OH+

(Ags)n-1 + Ag+ + HO

(Ags)n-1-Ag-OH+ + H2O2 + HO.

(Ags)n + HO + H+

.

.

(Ags)n-1 + Ag+ + O=O- + 2H2O (Ags)n-1 + Ag+ + H2O

Scheme 3. Role of reactive oxygen radicals in the silver NPs dissolution

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7

proposed by Menger and Protony [44] and modified by Bunton and his coworkers [45]. According to this model, micelle is a separate phase from aqueous one and that the reduction of Ag+ ions occurs in both the phases. Where kw and km are the first-order rate constants in aqueous phase and micellar pseudo phase, respectively, and Dn is the micellized surfactant concentration ([CTAB] - [CMC]). The head group of CTAB micelles is positive. Therefore, Ag+ ions cannot solubilized and/or incorporated into the Stern layer of micelles. In order to establish the AgBr formation, NaBr solution (5.0 mL of 0.001 mol/L) was added into the reaction mixture containing CTAB (10.0 × 10−4 mol/L), NaBH4 (14.0 × 10−5 mol/L) and Ag+ ions (10.0 × 10−4 mol/L) at the end of the reaction. We did not observe the appearance of yellow precipitate, which ruled out the possibility of AgBr formation under our conditions (Table 2). From Scheme 4, Eq. (18) can be deriving. kobs ¼

kw þ km K s ½Dn  ð1 þ K s ½Dn Þ

ð18Þ

The Eq. (18) was modified by neglecting the contribution of kw (first-order rate constant in water) [43]. The derivation of Eq. (18) is given in Scheme S2 (supporting information). kobs ¼

km K s ½Dn  ð1 þ K s ½Dn Þ

ð19Þ

On rearrangement, Eq. (19) can be written as Eq. (20). 1 1 1 ¼ þ kobs km K s km ½Dn 

Fig. 8. Effect of [NaBH4] (A) and [Ag+] (B) on the intensity of silver nanoparticles at 414 nm in presence of [CTAB] = 10.0 × 10−4 mol/L. Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L, [NaBH4] = 14.0 × 10−4 mol/L (A).

aggregates at lower [CTAB] [42]. The formation of aggregates controlled by the distribution of surfactant (D 1 ) among the various association-dissociation equilibria (Eqs. (15)–(17)). D1 þ D2 ↔D2

ð15Þ

D1 þ D2 ↔D3

ð16Þ

Dn−1 þ D1 ↔Dn

ð17Þ

+ These aggregates formed active species with BH − 4 and Ag through physical interaction. As a result, both reactants come close together and nucleation step takes place [43]. The role of CTAB micelles can be explained with the pseudo-phase model (Scheme 4)

ð20Þ

The plot of 1/kobs versus 1/Dn should be linear and this was found to be so for higher [CTAB]. Interestingly, Fig. 10C deviates from the linearity at lower [CTAB]. The value of km (=5.5 × 10−5 s−1, first-order rate constant in micellar media) and Ks (=33.6 × 104 mol−1 dm3, binding constant between micellized surfactant (Dn) and BH− 4 ) was calculated from the intercept and slope of the double-reciprocal plot of 1/k obs versus 1/D n (where D n = [CTAB] - CMC) from the initial part of the linear line. The high value of K s strongly suggests the formation of stable complex between positive head group of CTA (ionized CTAB) and BH − 4 through electrostatic interactions. Scheme 5 proposed to the reduction of Ag + ions by CTA-BH − 4 complex. In Scheme 5, Eqs. (21) and (22) represents the ionization of CTAB [10] and NaBH4. In the next step (Eq. (23)), ionized CTA and BH− 4 formed a complex. Eq. (24) shows an interaction of Ag+ ions with CTA-BH− 4 , which leads to the formation of a ternary complex (CTABH4-Ag+). Finally, one-step oxidation-reduction occurred (rate determining step). As a result, Ag0 and other products were formed (Eq. (25)). The Ag+ adsorbed on the surface of Ag0 and stable species of silver nanoparticles (Ag2+ 4 ) formed after complexation and dimerization (Eqs. (26) and (27)) [46–48], which was capped with CTAB (yellow-orange color with λmax = 414 nm). Morphology of metal NPs depends on the nature of the reducing agent (strong and weak), presence of stabilizer, pH and method of preparation [12,13,21]. Generally, ascorbic acid, sodium borohydride, hydrazine, ethylene glycol, cysteine, dimethylformamide, Tollens reagent and sodium citrate were used for the reduction of Ag+ in the aqueous or non-aqueous solutions. Table 3 shows that the desired morphology of AgNPs can be achieved by using a suitable reducing agent, stabilizer and method of preparation (seedless and seed growth) [49–56]. Spherical AgNPs prepared by using NaBH4 , ascorbic acid and hydrazine through chemical reduction method in presence of stabilizer, which stabilized the NPs growth and protect NPs from agglomeration.

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Fig. 9. Effect of [CTAB] on the SRP and morphology of AgNPs Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L, [NaBH4] = 14.0 × 10−5 mol/L, [CTAB] = 6.0 (A), 20.0 (B), 30.0 (C) and 40.0 × 10−4 mol/L (D).

A.B. Albeladi et al. / Journal of Molecular Liquids 302 (2020) 112565

(BH4-)w + Dn kw

9

Ks

(BH4-)m km

Product

Product

Scheme 4. Pseudo-phase model of observed micellar effects.

The morphology (from spherical, ellipsoidal, rod to worm like) of ionic micelles strongly depends on polar head group, hydrocarbon tail, nature of counter ion, pH, temperature, concentrations of additives as well as surfactant [57–60]. Kumar and his coworkers reported that the low CTAB concentration (25 mM) is not sufficient to produce structural changes in the CTAB micelles in presence of aromatic salts [58]. The worm-like micelles formed at very high CTAB concentrations ranging from 100 to 200 mM with a constant concentration of additives. The size of the micelles increases and decreases with CTAB concentration and temperature monotonically, respectively [59]. Patel et al. reported the morphological structural changes (from ellipsoidal micelles to elongated) aggregates in presence of aromatic salts with 25 mM CTAB solution [60]. The spherical CTAB micelles are formed under our experimental conditions ([CTAB] = 10.0 × 10−4 to 40.0 × 10−4 mol/L). No morphological changes occurred at the lower CTAB concentrations without aromatic additives.

4. Conclusion We descried a simple method to determine the stability of transparent silver sols with cationic CTAB at 23 °C. A specific ratio − of Ag + and BH − 4 is essential for the stability of AgNPs. The BH 4 + acted as a source of hydrogen to the reduction of Ag ions into Ag0 , provide an inert atmosphere due to the evaluation of hydrogen and stabilize the NPs simultaneously. The resulting sols are not stable for ca. 1 week for entire concentrations of Ag + , BH − 4 and CTAB. The complete oxidative dissolution of AgNPs was occurred at higher [CTAB] within 2 weeks. The OH− generated in solution by hydrolysis of BH − 4 with varying concentration, formed a layer of hydroxide around the positive surface of AgNPs, and would release of Ag + ions into solution. The dissolution rate increases with adding hydrogen peroxide in the similar reaction conditions. The different oxygen radical scavengers show remarkable rate inhibition. The present studies would be helpful provide to understand the stability and dissolution of AgNPs based sensors in a aqueous solution [61]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2020.112565.

Fig. 10. Reaction time profile (A) and effect of [CTAB] to the intensity of silver nanoparticles formation (B). Reaction conditions: [Ag+] = 10.0 × 10−4 mol/L (A and B), [NaBH4] = 14.0 × 10−5 mol/L (A).

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A.B. Albeladi et al. / Journal of Molecular Liquids 302 (2020) 112565

Table 2 Effects of [Ag+], [BH− 4 ], [CTAB] on the sensing properties of AgNPs. 104[Ag+]

105[BH− 4 ]

104[CTAB]

(mol/L)

(mol/L)

(mol/L)

4.0 2.0 4.0 8.0 12.0 16.0 20.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

14.0 14.0 14.0 14.0 14.0 14.0 14.0 2.0 4.0 6.0 10.0 18.0 20.0 24.0 14.0 14.0 14.0 14.0 14.0 14.0 10.0

0.0 0.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 4.0 8.0 16.0 20.0 24.0 30.0 40.0

Appearance of AgNPs after 1 h

White turbidity Pale orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Pale yellow; transparent; stable Pale yellow; transparent; stable Yellow; transparent; stable Yellow; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Pale yellow; transparent; stable Pale; transparent; stable Pale yellow; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable Orange; transparent; stable

Dissolution of AgNPs One week

Two week

– – Colorless Colorless Yellow Yellow Yellow Colorless Colorless Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Yellow Pale yellow Colorless Colorless Colorless Colorless Colorless

– – Yellow Yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Colorless Colorless Colorless Colorless Colorless

Scheme 5. Mechanism to the reduction of silver ions into silver NPs by sodium borohydride.

References Table 3 Effects of reducing and capping agents on the morphology of AgNPs. Reducing agent

Capping agent

Morphology

Ref.

Ascorbic acid Hydrazine Polyethylene glycol NaBH4/ascorbic acid Ethylene glycol Ethylene glycol NaBH4/citrate/H2O2 Camellia sinensis NaBH4

CTAB CTAB PVP CTAB PVP PVP PVP CTAB CTAB

Spherical; mono-dispersed Spherical; mono-dispersed Spherical; mono-dispersed Nano rods; well-dispersed Nanowires; poly-dispersed Nanocubes; poly-dispersed Nanoprisms; poly-dispersed Triangular; poly-dispersed Spherical; poly-dispersed

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