Research ArticleFunctionalized-AgNPs for Long-Term Stability and ItsApplicability in the Detection of Manganese Ions

Van-Tuan Hoang ,1,2,3 Mai Mai,1 Le Thi Tam,1 Ngoc Phan Vu,2,3 Nguyen Tien Khi,2,3

Phuong Dinh Tam ,4 Tran Quang Huy,2,5 Anh-Tuan Le ,2,4 Ngo Xuan Dinh,6

and Vinh-Hoang Tran7

1Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST),1st Dai Co Viet Road, Hanoi, Vietnam2Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam3Faculty of Biotechnology, Chemistry and Environmental Engineering, Phenikaa University, Hanoi 12116, Vietnam4Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Vietnam5Faculty of Electric and Electronics, Phenikaa University, Hanoi 12116, Vietnam6University of Transport Technology, .anh Xuan District, Hanoi 12116, Vietnam7School of Chemical Engineering, Hanoi University of Science and Technology (HUST), 1st Dai Co Viet Road, Hanoi, Vietnam

Correspondence should be addressed to Van-TuanHoang; tuan.hoangvan@phenikaa-uni.edu.vn and Anh-Tuan Le; tuan.leanh@phenikaa-uni.edu.vn

Received 6 August 2019; Revised 2 December 2019; Accepted 3 January 2020; Published 7 February 2020

Academic Editor: Gyorgy Szekely

Copyright © 2020 Van-TuanHoang et al..is is an open access article distributed under theCreative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this study, silver nanoparticles (AgNPs) were functionalized by various molecules, including sodium borohydride (NaBH4),polyhexamethylene biguanide hydrochloride (PHMB), and Tween 80 to investigate the long-term stabilization of AgNPs in anaqueous dispersion. PHMB-functionalized silver nanoparticles (AgNPs/PHMB) exhibited better stability than others and could bestored at ambient temperature for at least 180 days. In addition to creating stabilization based on the electrostatic repulsion, theuse of PHMB helped to increase the degree of stability of the colloidal AgNPs for a long time owing to strong interactions betweenAg atoms on AgNPs with nitrogen (N) positions in PHMB molecules. .e formed bond led to improving maintenance ability ofthe electrostatic repulsion layer among independent nanoparticles. .e applicability of the as-prepared AgNPs/PHMB was alsoexamined for Mn2+ detection via a colorimetric approach. .e calibration curve was found to be linear over the range of0–100mM with a correlation coefficient of 0.97. .e amine groups of PHMB brought out a cooperative effect to form of ion-templated chelation with Mn2+, which caused the aggregation of AgNPs/PHMB. .is suggested that the AgNPs/PHMB could beused as a potential probe in the detection of Mn2+ ions. More importantly, the long-term stability of AgNPs/PHMB paved a greatpromising path to provide many further solutions for the producer in practical applications.

1. Introduction

Silver nanoparticles (AgNPs) possess many unique chemicaland physical properties; therefore, AgNPs have been widelystudied and exploited in various applications, includingmedical, home appliances, electronics, textiles, and cosmetics[1]. .e demand for AgNPs in the world is projected toincrease, with production estimated at around 320 tons peryear [1,2]. However, the properties of AgNPs strongly dependon their size, shape, and crystallinity. Moreover, large specific

surface area and high surface energy lead to the self-aggre-gation of AgNPs during storing time, which considerablydecreases their activity as well as applicability. .ese indicatethat the stabilization of colloidal AgNPs is one of the mostessential factors for developing practical applications [3–5].Furthermore, the use of suitable stabilizers not only helpsprevent the aggregation of AgNPs but also increases theiractivity. .erefore, many recent efforts have been performedto functionalize AgNPs by different stabilizers such aspolymer or functional molecules, which contain amine

HindawiAdvances in Polymer TechnologyVolume 2020, Article ID 9437108, 9 pageshttps://doi.org/10.1155/2020/9437108

groups/carboxylic acid groups/or thiol groups, etc., and due tothese, the functional groups can form coordination bondingand/or electrostatic interactions with AgNPs [6–8].

Especially, herein, the polymer, which has many func-tional groups and high viscosity, has been considered as apromising stabilizer for the synthesis of AgNPs. Of the sta-bilizers used, the ionic stabilizers showed better stability thannonionic stabilizers. When silver nanoparticles are modifiedby an anionic stabilizer, their surface will electrically becharged. .is leads to enhancing electrostatic repulsionamong them, and as a result, the aggregation of colloidalAgNPs can be prevented [8]..erefore, the cationic polymerssuch as polyhexamethylene biguanide hydrochloride(PHMB), chitosan (CS) [6,9], and polyethylenimine (PEI)[10], with molecule structures described in Figures 1(a)–1(c),respectively, are often used for stabilization of AgNPs. In suchpolymers, polyhexamethylene biguanide hydrochloride(PHMB), a surface-active agent, positively charged, highlybiocompatible, and has been used as a disinfectant in medical,textile, and food industries [11,12], can stabilize the size andincrease the dispersion of AgNPs. PHMB preserves the col-loidal AgNPs for better stability and uniform dispersibility inthe liquid media. Previous studies [11,13] showed that thePHMB-stabilized cationic silver nanoparticles displayed en-hanced antimicrobial activity and demonstrated the prom-ising potential for plant protection. However, the mechanismof durability and long-term stability of the colloidal AgNPs byionic stabilizers may have been not fully understood.

Manganese (Mn) element is an essential element ofliving organisms. .e existence of the Mn element plays acritical role in the metabolism of the body and other bio-logical processes. However, when using foods, especiallydrinking water with high Mn2+ ions content will cause harmto the human body, especially diseases related to the nervoussystem [14]. Some recent studies have shown that the col-loidal AgNP-based colorimetric sensors for Mn ion detec-tion have offered good sensitivity [15,16]. However, thesecolloidal AgNPs had short-term stability or have to usemanystabilizers, which hinder their widespread applications..erefore, the synthesis of the colloidal AgNPs with bothlong-term stability and flexibility is sustainable for practicalapplication.

In this work, the AgNPs were functionalized by PHMBfor long-term stability. Mechanism of the long-term stabi-lization of the AgNPs/PHMBwas discussed..e aggregationof AgNPs/PHMB can be prevented by the electrostatic re-pulsion among them and the formation of the Ag-N co-ordinate covalent bonds. Besides, the AgNPs/PHMB withlong-term stability have also been harnessed as a probe forthe detection of Mn2+ ions by the colorimetric assays. .eamine groups of PHMB brought out a cooperative effect toform ion-templated chelation with Mn2+, which caused theaggregation of AgNPs/PHMB.

2. Materials and Experimental

2.1. Materials. Silver nitrate (AgNO3, 99.9%), sodium bo-rohydride (NaBH4, 99%), polyhexamethylene biguanidehydrochloride (powder, PHMB 98%), Tween 80

(polysorbate 80, 99%), Mn(NO3)2·4H2O, Co(NO3)2·6H2O,and Ni(NO3)2·6H2O were purchased from ShanghaiChemical Reagent. Deionized water was used. All chemicalswere used without further purification.

2.2. Synthesis of the Functionalized AgNPs. .e AgNPs/PHMB, AgNPs/Tween 80, and AgNPs/NaBH4 were pre-pared by the chemical reduction method using NaBH4 as areducing agent along with PHMB or Tween 80 as stabilizersaccording to the following procedure: 60mL of AgNO3(1mM) solution was added dropwise into a 30mL ofaqueous solution containing each stabilizer (0.02mg/mL)under continuous stirring. After 2 h, 6mL of sodium bo-rohydride (NaBH4) solution (10mM, 1 :1 to the molar ratioof AgNO3) was added dropwise into the mixture. .e re-duction process was carried out for 3 h (optimized synthesistime) at room temperature (RT). Stirring was requiredduring the synthesis procedure. .e formation of AgNPswas proposed, according to [17]equation (1). Finally, the as-prepared functionalized-AgNPs were stored at room tem-perature in the dark until further use:

AgNO3 + NaBH4⟶ Ag +12H2 +

12B2H6 + NaNO3. (1)

2.3. Characterization. .e crystalline structure of AgNPswas examined using X-ray diffraction (XRD, Bruker D5005)using CuKα radiation (λ� 0.154 nm) at a step of 0.02° (2θ) atroom temperature. .e UV-Vis absorbance spectra of thecolloidal AgNPs were recorded by using a DR6000 spec-trophotometer, and 10mm path length quartz cuvettes wereused for the measurement of the spectra. .e surfacechemistry of the colloidal AgNPs was examined by usingFourier Transform Infrared Spectrophotometer (IRAffinity-1S, Shimadzu).

2.4. Long-Term Stability Measurements. .e as-preparedcolloidal AgNPs were stored in glass bottles under ambienttemperature in the absence of light and were characterizedby UV-Vis spectra in few hours and after 180 days of storage.In brief, 0.5ml of the as-prepared colloidal AgNPs solutionat pH 6.8 was diluted with 9.5ml of the distilled deionizedwater. .e UV-Vis absorbance spectra were recorded by aDR6000 spectrophotometer. In addition, the surface chargeand particle size distributions of the colloidal AgNPs werealso measured at 2 hours and 180 days of storage using aparticle size analyzer (Zetasizer Ver. 6.20, Malvern Instru-ments) to study the long-term stability of the colloidalAgNPs.

2.5. ColorimetricDetection ofMn2+ Ions. For the detection ofMn2+ ions, 1ml of the as-prepared colloidal AgNPs at pH 6.8was diluted 9 times by distilled deionized water. .en, 1mlof Mn2+ solution with various concentrations (0–100mM)was added into 9ml diluted colloidal AgNPs. .e pH of thesolution was adjusted to within the range of 3.5 to 9.0 usingacetic acid (0.05%) and NH4OH (30%). .e mixture was

2 Advances in Polymer Technology

kept in a shaker for 15 minutes at room temperature beforethe absorbance spectrum of this mixture was recorded. .elimit of detection was calculated by using the followingequation:

LOD �3S

b, (2)

where S is the standard deviation of the blank solution and bis the slope of the analytical curve.

3. Results and Discussion

3.1. Structural Analysis. Figure 2 shows the XRD spectra ofcolloidal AgNPs stabilized by PHMB (curve a), Tween 80(curve b), and NaBH4 (curve c), respectively. It can be seenthat four specific diffraction peaks of AgNPs at 2θ� 38.1°,44.3°, 64.4°, and 77.5° correspond to the crystals (111), (200),(220), and (311) of the silver metal crystal (JC PDS No. 04-0783)..ese results confirm the formation of the silver metalcrystal with a relative single-phase structure in colloidalAgNPs. However, the intensity and sharpness of diffractionpeaks in samples using various stabilizers are different. .isindicates the stabilizer effect on the particle size of AgNPs aswell as the quality of silver metal crystals. Line broadening inthe pattern can be quantitatively evaluated using theDebye–Scherrer equation (equation (3)), which gives a re-lationship between peak broadening in XRD and particlesize [18]:

D �k · λ

β · cos θ, (3)

where D is the thickness of the crystal, k is the Debye–Scherrer constant (0.9), λ is the X-ray wavelength(0.15406 nm), β is the line broadening in radian obtainedfrom the full width at half maximum, and θ is the Braggangle. According to the Debye–Scherrer equation, averageparticle sizes of AgNPs were estimated to be 25, 20, and15 nm when the used stabilizers were PHMB, Tween 80, andNaBH4, respectively.

3.2. Long-Term Stability of the Colloidal AgNPs. UV-Vis canbe used to observe the stabilization of AgNP colloids. StableAgNP colloid in the UV-Vis spectra was stable vs. storingtime, suggesting that particle size was stable and without anyaggregation. .e position of the plasmonic peak in the UV-

Vis spectrum depends on the average particle sizes and shapes[19]. Figure 3 shows the optical absorption spectra of thecolloidal AgNPs withmodified surfaces by different stabilizersas prepared (black solid line) and after 6 months of storage(red dash line). Figure 3 shows that as prepared, all syn-thesized AgNPs samples illustrated the appearance of thecharacteristic AgNP peaks at a maximum adsorption ofλmax � 400 nm, 440 nm, and 420 nm, corresponding toNaBH4, PHMB, and Tween 80, respectively (black solid lines).However, after 180 days of storage, UV-Vis absorptionspectra of these samples were decreased for all tested samples..e strongest decrease can be observed in the case of Tween80 and NaBH4, respectively, and the peak absorption char-acteristic of silver nanoparticles was almost absent (red dashline in Figures 3(b) and 3(c)). In contrast, AgNP colloid wasstabilized well by PHMB, and it still exists as the specific peakof AgNPs at λmax � 455 nm with a small reduction in ab-sorption intensity. Clearly, this result indicated an excellentrole of PHMB as a stabilizer to AgNP colloid than that ofNaBH4 or Tween 80. Besides, in Figure 3(a), after 6 months,the specific peak of AgNPs is λmax � 455 nm, and there is asmall red shift, compared with λmax � 400 nm as-preparedsamples, this can be attributed to a small aggregation that wasformed in the AgNPs/PHMB system. .e small aggregationin the AgNPs/PHMB system can also be observed by the DLSspectra (Figure 4). Two zones of size distribution as-preparedAgNPs/PHMB and small shoulder can be seen in Figure 4(a)with a mean particles size around 60nm.

30 40 50 60 70 80

(b) Ag-Tween 80

(c) Ag-NaBH4

(a) Ag-PHMB

2-theta (deg.)

JC-PDS 04-0783

(111)

(311)(220)(200)

Figure 2: XRD diffraction pattern of the colloidal Ag-NPs stabi-lized by (a) PHMB, (b) Tween 80, and (c) NaBH4.

NH

NH

NH

HN

HN

n

×HCI

(a)

O OOH

HOOH

NH2n

(b)

n

H2NNH2N

H

(c)

Figure 1: Chemical structure and functional groups of (a) polyhexamethylene biguanide hydrochloride (PHMB); (b) chitosan (CS), and(c) polyethylenimine (PEI).

Advances in Polymer Technology 3

After 6 months of storage, a shoulder was disappeared(Figure 4(b)); however, two zones of size distribution werestill observed and themean size was shifted to 105 nm, whichis bigger than that as prepared. .ese analyst results arecompletely consistent with the above UV-Vis measure-ments, as well as zeta potential measurements (Table 1). .esurface charge of AgNP colloids with various used stabilizerswas analysed by zeta potential measurements. .e structureis formed by the surface interactions between AgNPs and

stabilizer molecules which changes over time. In particular,Table 1 shows that the surface of modified AgNPs by ionicstabilizers (including NaBH4 and PHMB) had less time-varying changes in zeta potential (±% vs. storage time) thanthat of the nonionic stabilizer (i.e., Tween 80). After 180days, AgNPs/PHMB showed better stability than the otherstabilizers (Table 1). Moreover, the long-term stability of thecolloidal AgNPs stabilized by PHMB when compared withother stabilizers or surfactants as shown in Table 2. In this

0.0

0.2

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rban

ce (a

.u)

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Ag-PHMB

400 500 600 700 800300Wavelength (nm)

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rban

ce (a

.u)

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Ag-Tween 80

Wavelength (nm)

2 hours180 days

(b)

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rban

ce (a

.u)

300 400 500 600 700 800

0.0

0.2

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0.6

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Ag-NaBH4

Wavelength (nm)

2 hours

180 days

(c)

Figure 3: UV-Vis absorbance spectrum of the colloidal Ag-NPs stabilized by (a) PHMB, (b) Tween 80, and (c) NaBH4.

10

8

6

4

2

0

Inte

nsity

(%)

0.1 1 10 100 1000 10000Size (d.nm)

(a)

10

8

6

4

2

0

Inte

nsity

(%)

0.1 1 10 100 1000 10000Size (d.nm)

(b)

Figure 4: Size distribution for long term of the colloidal Ag-NPs stabilized by PHMB ((a) 2 hours; (b) 180 days).

4 Advances in Polymer Technology

study, the colloidal AgNPs stabilized by PHMB for stabilityup to 180 days, which promises for many application fields,such as biomedical, agricultural, and environmental control.

3.3. Understanding the Mechanism of the Colloidal AgNPsStabilizedbyPHMB. Amechanism for long-term stability ofAgNPs stabilized by PHMB can be proposed as described inFigure 5. Based on the electron configuration of silver (Ag0)is [Kr] 4d105s1, there are many vacant orbitals on the co-valent shell of silver atoms including 5p05 d0. Meanwhile,there are many free-electron pairs (unbonded covalenceelectron pairs) at nitrogen (N) positions on PHMB including-€NH− 7 and/or − €N� groups; therefore, coordination inter-action of vacant-covalent orbitals of Ag with free-electronpairs of nitrogen in PHMB molecules has been generated,and as a result, the PHMB molecules protect AgNPs asmodelled in Figure 5(a). More interestingly, the inside-bonding structure of Ag into AgNPs with PHMB moleculesare given in Figure 5(b), which suggested that AgNPs sta-bilized by PHMB were positively charged; therefore, in theAgNPs/PHMB, the stability of the electrostatic shieldaround AgNPs was enhanced by the formation of coordinatecovalent bonds between the atoms of silver metal and ni-trogen in the composition of PHMB (Figure 5(b)).According to this structure (Figure 5(b)), in the colloidalsolution, AgNPs were separated from each other by therepulsion layer because of an electrostatic shield that pro-tected the AgNPs surface in the form of positively chargedspheres (Figure 5(c)). .us, AgNPs tend to push each otherand survive independently. Electrostatic repulsion of sta-bilizing agents on the surface of AgNPs is considered animportant factor to improve the stability of the colloidalAgNPs [3].

.e coordination interaction of vacant-covalent orbitalsof Ag with free-electron pairs of nitrogen in PHMB mole-cules was further confirmed by the FTIR analysis. .e FTIRspectra of PHMB and AgNPs/PHMB are shown in Figure 6.Compared with PHMB, the FTIR spectrum of AgNPs/PHMB shows one new absorption peak at 1000 cm− 1 which

corresponds to C-N bond stretching [22] and the disap-pearance of the absorption peak at 1556 cm− 1 [11]. .eoccurrence of these changes indicates the interaction of theAgNPs with imine groups of PHMB through the formationof a coordination bond of Ag-N. .e FTIR results dem-onstrate that the strong interactions of AgNPs and PHMBmay efficiently prevent the aggregation of silver nano-particles [23].

For NaBH4-stabilized colloidal AgNPs, the AgNPs werealso stabilized by the charging shell covering the AgNPs.However, the electrostatic shield was created by the BH4

−

anion and was broken as given in the following reaction:

BH4−

+ 4H2O⟶ B(OH)4 + 4H2. (4)

.erefore, the degradation of BH4− to hydrogen gas is the

major cause of AgNPs to be aggregated [2,24]. With structureas shown in Figure 3(b) , Tween 80 is a nonionic stabilizer;therefore, the interaction between Tween 80 and the AgNPs isbased on the surface adsorption. .e AgNPs surface wascoated by a layer of Tween 80 to separate the nanoparticles andcreate great stability based on the steric repulsion mechanism[8,25]. Our finding results reveal that the long-term stability ofthe colloidal AgNPs was attributed to the formation of Ag-Ncoordinate covalent bonds and the electrostatic repulsionbetween PHMB-functionalized AgNPs.

3.4. Application of AgNPs/PHMB as a Probe for the DetectionofMn2+ Ions. As described in Section 3.3, PHMB coated theAgNPs, therefore, AgNPs/PHMB particles contain free-amino groups, which were from a terminal tail of PHMB(Figure 7). .e free -NH2 group can be used to recognizetransition metal ions via a colorimetric approach, such asMn2+ with the detection mechanism as shown in Figure 7.For Mn2+ detection, AgNP play the role to be a signal probealong with the free amino (-NH2) group on PHMB mole-cules plays the role as a chemical capture probe. .epresence of functional groups, such as amine (-NH2), on thesurface of AgNPs plays a critical role in the aggregationmechanism through metal-ligand interactions. .e amino

Table 1: Effect of stabilizers on zeta potential after 180 days.

StabilizersZeta potential (mV)

2 hours 180 days ±(%) zeta potential change after 180 daysPHMB 31.5 26.5 − 15.87NaBH4 − 25.5 − 6.62 − 74.04Tween 80 − 7.9 − 0.02 − 99.75

Table 2: Long-term stability of the colloidal AgNPs.

Methods Precursors Reducing agent Stabilizer or surfactant Stability time Ref.Chemical synthesis AgNO3 Citrate Citrate/gelatin mixture 118 days [3]Chemical synthesis AgNO3 NaBH4 Aminosilanes 30 days [9]Chemical synthesis AgNO3 NaBH4 β-Cyclodextrin, adamantine 14 days [16]Chemical synthesis AgNO3 NaBH4 Dodecanethiol 60 days [7]Green synthesis AgNO3 NaBH4 L-Cysteine 60 days [20]Biological synthesis AgNO3 Bacterial biomass Bacterial biomass 90 days [21]Chemical synthesis AgNO3 NaBH4 PHMB 180 days .is study

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groups of PHMB have a cooperative effect to form ion-templated chelation with Mn2+, which causes the aggrega-tion of AgNPs [15]. On the contrary, the AgNP aggregationis induced by interparticle bond formation (cross-linkingaggregation). .e chelation form changes the interparticledistances of AgNPs, leading to the shift of the absorptionpeak of AgNPs to a longer wavelength. .erefore, cross-

linking aggregation has been used for the detection of metalions [26].

For that, control testes have been done based on adding10mM of various ions including Ni2+, Co2+, and Mn2+ intothe AgNPs/PHMB solution. .e UV-Vis spectra of theAgNPs/PHMB solution before and after adding a solutioncontaining the above ions are shown in Figure 8(a). It can

Ag+ + PHMBNaBH4 reduction

Ag0nuclei Ag clustersSilver nanoparticle

stabilized by PHMB

Ag

Ag

Ag

+ N N

: PHMB

Lone electron pairVacant orbitals

Coordinate covalent bond

Ag-PHMB

(a)

(b)

Electrostatic shield

Repulsion layer= PHMB = silver nanoparticles

(c)

N(+)

(+)

(+)

(+)

(+)

(+)

(+)

(+)

(+)

N

N

NN

N

(+)

(+)

AgNPs

AgNPs

AgNPs

AgNPs

(+)

Figure 5: Schematic representation of AgNPs/PHMB: (a) NaBH4 reduction Ag+ to AgNPs in the presence of PHMB; (b) the formation ofthe coordinate covalent bond between silver metal atoms (Ag0) and free-covalence electron pairs of the nitrogen (N) atom in PHMBmolecules; and (c) mechanism stabilization of the AgNPs/PHMB via repulsion force AgNPs/PHMB particles.

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitt

ance

(%)

Wavenumber (cm–1)

(a) PHMB (b) Ag-PHMB

(a)

(b)

Figure 6: FTIR spectra (a) PHMB and (b) AgNPs stabilized by PHMB (AgNPs/PHMB).

6 Advances in Polymer Technology

Mn2+

= silver nanoparticles

= PHMBHN N

HNH

NH NH

NH2

n

(+)NH

NH

HN

NH NH

NH2

n

(+)

NH

NH

HN

NH NH

NH2

n

(+)

Figure 7: Mechanism of Mn2+-induced AgNPs/PHMB aggregation.

300 400 500 600 700 8000.0

0.5

1.0

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(iv) AgNPs/PHMB-Mn2+

(iii) AgNPs/PHMB-Co2+(ii) AgNPs/PHMB-Ni2+

Abso

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ce (a

.u)

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(i) AgNPs/PHMB

(a)

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ce (a

.u)

300 400 500 600 700 8000.0

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pH 5

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(g)(f)(e)(d)

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(e) 30 mM(f) 50 mM(g) 100 mM

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CMn2+ (mM)

Equation y = 0.0281x + 1.2054Residual sum ofsquares 0.11059

Pearson’s r 0.98889Adj. R-square 0.97238

Value Standard error

A0/AIntercept 1.20537 0.10156

Slope 0.02805 0.00211

(d)

Figure 8: Colorimetric detection of Mn2+ ions using the colloidal Ag-NPs: (a) UV-Vis spectra of AgNPs/PHMB without and with addingCo2+, Ni2+, and Mn2+ solution; (b) the UV-Vis spectra of AgNPs/PHMB solutions with different pH ranging from pH 3.5 to pH 9.0 afteradding 10mM Mn2+ solution; (c) UV-Vis spectra of AgNPs/PHMB in the presence of Mn2+ with different concentrations from 0 to100mM; and (d) a calibration curve for Mn2+ detection.

Advances in Polymer Technology 7

be seen that the maximal absorption peak of the AgNPs/PHMB solution after adding the Mn2+ ion (curve iv) wasstrongly diminished than the original AgNPs/PHMB so-lution (curve i), as well as this decrease was stronger thanadding other ions such as Ni2+ (curve ii) or Co2+ (curve iii)..ese indicated the specific Mn2+ detection of the proposedmethod. .e PHMB polymer can be used as a strongchelating agent to Mn(II) ions due to the amino groupswhich can form stable complexes with Mn(II) ions. .e Natom and amino groups of the PHMB structure can formcoordinated bonds with manganese ions causing the ag-gregation of large particles, which is in good agreementwith similar previous reports [15,16,26] in which the sul-fonic group, hydroxyl group, and/or carboxyl groups havebeen considered as an important chelator in specific rec-ognition with Mn2+ ions.

To investigate the effect of pH on the sensitivity of thiscolorimetric method to Mn2+, the AgNPs/PHMB solu-tion was added with 10mMMn2+ solution at different pH(from pH 3.5 to pH 9.0) and UV-Vis spectra are shown inFigure 8(b). It can be seen that when the pH value was lessthan 6.5, the UV-Vis spectra of the AgNPs/PHMB so-lution were unchanged after adding Mn2+ because the-NH2 group of PHMB is changed to -NH3

+, a positivelycharged group; therefore, Mn2+ cannot interact to thesepositively charged groups. As the pH value was higherthan pH 8, it is noticed that the aggregation of AgNPs/PHMB, as well as Mn2+, would be precipitated as theMn(OH)2 form. At pH 6.5 to pH 7.5, the intensity andposition of the maximum adsorption peaks changed moreclearly; therefore, pH in the range 6.5 to 7.5 was selectedfor further experiments.

In the UV-Vis spectra of AgNPs/PHMB added withdifferent Mn2+ concentrations (Figure 8(c)), we notice thatthe intensity of the maximal absorption peak (atλmax � 403 nm) was decreased with increasing Mn2+ ionconcentrations. In addition, there was a red shift of this peak,which could be attributed to the cause of the AgNPs/PHMBaggregation via Mn2+ as linkers. .e linear relationshipbetween the absorbance intensity of the AgNPs/PHMBsolution with Mn2+ concentration is shown in Figure 8(d)with the Mn2+ range from 0mM to 100mM. .e limit ofdetection (LOD) was calculated at approximately 0.22mM..ese finding results suggest that the AgNPs/PHMB alsomay be used as a useful probe for the detection of Mn2+ ions.

4. Conclusions

.e long-term stability of colloidal AgNPs functionalizedby PHMB, BH4

-, and Tween 80 has been studied. .eresults indicated that the stability of AgNPs/PHMB (up to180 days) was the best. .e long-term stability of colloidalAgNPs is created by the formation of the coordinate co-valent bonding as well as the enhancement of electrostaticrepulsion between functionalized AgNPs. Moreover, theAgNPs/PHMB has been successfully tested for Mn(II) iondetection using the colorimetric assay. .e AgNPs/PHMBaggregate in the presence of Mn2+, resulting in a colorchange of the solution as well as a red shift of the

absorbance peak. .e proposed method is capable of de-termining Mn2+ ions over a range of 0–100mM. .e assayis simple in design, fast in operation, low-cost, and can beobserved with the naked eye. With long-term stability, thePHMB-functionalized AgNPs will become a promisingmaterial for many fields such as textile industry, agricul-tural, and environmental control.

Data Availability

.e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

.e authors declare that they have no conflicts of interest.

Acknowledgments

.is research was funded by the Vietnam National Foun-dation for Science and Technology Development (NAFOS-TED) under grant no. 103.99-2017.340.

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