Effect of Cationic Surfactants with Different Counterions on the Growth

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Effect of Cationic Surfactants with Different Counterions on the Growth of Au Nanoclusters Yuanyuan Hu, Ling Wang, Aixin Song, and Jingcheng Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00603 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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TOC

ACS Paragon Plus Environment

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Effect of Cationic Surfactants with Different Counterions on the Growth of Au Nanoclusters Yuanyuan Hu, Ling Wang, Aixin Song, and Jingcheng Hao Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, China

7R ZKRP FRUUHVSRQGHQFH VKRXOG EH DGGUHVVHG E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86-531-8856-4750 1


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$%675$&7 The

influence

of

a

series

of

cationic

surfactants

composed

of

cetyltrimethylammonium (CTA+) with different counterions (Br-, Cl-, OH-, C7H8O3S-, [CeCl3Br]-, and NO3-) on the ageing process of gold nanoclusters (Au NCs) was studied. The finely different points of Au NCs treated by different surfactants were demonstrated by UV-Vis and fluorescence spectra, TEM images, etc. Due to the difference of counterions, these surfactants have diverse physicochemical properties in surface activity, specific conductivity, pH, and viscosity, which may account for the difference of Au NCs in the ageing process. In addition, the affinity of the counterions in surfactants to the surface of Au has also been demonstrated completely. This affinity may further guide the difference of the synthesized Au nanomaterials.

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1. ,1752'8&7,21 Gold nanoparticles (Au NPs) have attracted much attention because of their size and shape-dependent properties. Au NPs have been applied in different nanotechnology fields.1-3 Among the Au NPs with different shapes and sizes, the non-spherical Au NPs are usually termed the anisotropic Au NPs4-6 which give rise to superior optical and physicochemical features.7-9 A large number of literatures have been reported on the regulation factors in the shape and size of the anisotropic Au NPs, including WHPSHUDWXUH 10,11 VDOWV 12 SRO\PHUV 13,14 DQG VXUIDFWDQWV 15,16 HWF Among these factors, because surfactants have the tunable molecular architectures and the self-assembly behavior, they possess more remarkable ability to direct the shape of anisotropic Au NPs through seed-mediated process. Surfactants can adsorb on the different crystal facets of Au cores and assemble into stabilized layers on the Au surface.15 In addition, the interaction strength between the counterions of surfactants, e.g., Cl-, Br- and I- in hexadecyltrimethylammonium

bromide

(CTAB),

hexadecyltrimethylammonium

chloride (CTAC), and hexadecyltrimethylammonium iodide (CTAI), respectively, and the surface of Au NPs are in various ways. These ways can induce the different properties of the synthesized Au NPs.17,18 As example, the counterion, Br- of CTAB is critical in the formation of Au nanorods (Au NRs) by seed-mediated method.19 However, both CTAC and CTAI are fail to direct the formation of Au NRs. During the ageing process of Au seeds to AuNPs, the ultra-small seeds (~2 nm) are extremely critical to subsequent growth of Au NPs.20 This process is almost consistent with the Ostwald ripening. Unfortunately, it is difficult to understand the detailed changes of 3


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process because of the rapidity of the conventional seed-formation method and the characterization of quantifying size distribution of seeds. Au nanoclusters (Au NCs) are ultra-small nanoparticles with a core size less 2 nm,21,22 which are regarded as the Au seeds in the ageing process to Au NPs. Their unique structural and optical properties provide simple and effective ways to monitor the evolution of cluster size.23 Therefore, Au NCs are paramount to the initial formation of single crystal seeds and their subsequent growth. In this study, histidine stabilized Au NCs were used as the research model due to their facile synthetic method and distinguished optical properties. We aimed to understand the effect of a series of cationic surfactants (CTAX, X = Br-, Cl-, OH-, C7H8O3S-, [CeCl3Br]- and NO3-) on the Au NCs ageing process by measuring and analyzing the differences in their physicochemical properties. We found that Au NCs have significantly different changes when they were treated by CTAX surfactants. Due to the difference of counterions, these surfactants in solution show diverse physicochemical properties such as surface activity, specific conductivity, pH and viscosity. These properties may account for the difference of Au NCs in ageing process. The strength of affinity between counterions and the Au surface is another important factor that has impact on the changes of Au NCs.

2. EXPERIMENTAL SECTION Chemicals and Materials.

+LVWLGLQH +LV

HAuCl4·3H2O, CeCl3·7H2O, FeCl3,

CTAC were purchased from J&K Chemical Company. CTAB, +&O +12 1D2+ 1D&O 1D%U DQG 1D12 were obtained from Sinopharm Chemical Reagent Co., Ltd. 4


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(Shanghai, China). Sodium p-toluenesulfonate and dodecytrimethylammonium bromide (DTAB) were purchased from Kermel Chemical Reagent Co., Ltd. (Tianjin, China).

Hexadecyltrimethylammonium

p-toluenesulfonate

(CTAS)

and

tetradecyltrimethylammonium bromide (TTAB) were bought from Sigma-Aldrich. In the experiments, tKH ZDWHU ZLWK D UHVLVWLYLW\ RI

0

FP ZDV produced from a

UPH-IV ultrapure water apparatus (China). Synthesis of Au NCs.

$X 1&V were synthesized in a facile method according to

previous report.24 %ULHIO\ the aqueous solution of His (100 mM, 3 mL) was mixed with +$X&O

P/

P0 DW URRP WHPSHUDWXUH 7KH FRORU RI WKH VROXWLRQ EHFDPH

\HOORZ LPPHGLDWHO\ DQG WKHQ WKH PL[WXUH ZDV LQFXEDWHG DW URRP WHPSHUDWXUH

°C

IRU K IRU IXUWKHU VWXG\ Synthesis

of

Surfactants,

CTAX

(X

=

OH-,

NO3-,

[CeCl3Br]- .

Cetyltrimethylammonium hydroxide (CTAOH) was obtained though ionic exchange method from CTAB.25 As a result, Br- was removed and replaced with OH- after crossing the ion-exchange resin. Finally, the powder of CTAOH was obtained by freeze-dried for 12 h. A certain amount of CTAOH was dissolved by dilute +12 . The finally obtained solution was hexadecyltrimethylammonium nitrate (CTAN) with a certain concentration due to the calibrated concentration of dilute +12 . Cetyltrimethylammonium trichloromonobromocerate (CTACe) was prepared by mixing equal amounts of CTAB and CeCl3·7H2O in methanol26 and stirring for 24 h at room temperature. Then, the products were evaporated to remove solvent and dried

5


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in vacuum at 50 °C for 24 h for further use. In addition, cetyltrimethylammonium trichloromonobromoferrate (CTAFe) was prepared by the same method.27 Characterizations.

TEM observations were performed on a JEOL JEM-1400 TEM

with an acceleration voltage of 120 kV and the images were recorded with Gatan multiscan CCD. UV-Vis spectra of Au NCs before and after treated by different surfactants were recorded on a Cary 60 UV-Vis spectrometer with the scanning speed 600 nm/min (Agilent, America). Fluorescence emission spectra were recorded on Hitachi F-7000 spectrometer (America). The excitation wavelength was 385 nm and the scanning speed was 600 nm/min. The emission data was measured within the range of 400 - 700 nm. The zeta potential and size of Au NCs were carried on the Malvern zetasizer ZS with a DTS1070 folded capillary cell. The viscosity of surfactants solutions was recorded by Malvern Zetasizer ZS with Microrheology mode. The critical micelle concentrations (cmcs) of different surfactants were determined by measuring the curves of surface tension and electrical conductivity of a serious of samples with different concentrations. In the electrical conductivity measurements, the DDSJ-308A analyzer was used. The cmc was determined according to the EUHDN SRLQW EHWZHHQ WKH KLJKHU >G G c @ DQG ORZHU >G G c)] linear curves at 25.0 f 0.1 °C. The surface tensions of surfactant aqueous solutions with various concentrations were measured using a platinum plate with Sigma700 tensiometer at 25.0 f 0.1 °C.

3. RESULTS AND DISCUSSION

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The light yellow Au NCs dispersion samples were obtained through a facile procedure as previous reported.24 From TEM image in Figure 1a, one can clearly observe the monodispersed Au NCs with several nanometres in size. The analysis of size distribution, as shown in Figure 1b, exhibits the average size of ~2 nm, which demonstrate the small size of Au NCs. Figure 1c shows Whe chemical structures RI six cationic surfactants, CTAX (X = Br-, Cl-, OH-, C7H8O3S-, [CeCl3Br]- and NO3-) with different counterions. To investigate the effect of these surfactants with different counterions on Au NCs, a certain amount of surfactant solution was added to mix with the Au NCs dispersed sample. The equality of the final CTAX concentration, cCTAX = 5 mM, can be guaranteed. The samples were incubated for 20 h without shaking. As shown the top samples in Figure 1d, the macroscopic appearance of the samples shows the varying degrees of colour change. From left to right, the colour of the Au NCs dispersed samples changes from light yellow to reddish brown, and eventually to purple except the sample treated by CTACe. The sample treated by CTACe turns to be darker yellow. Correspondingly, under ultraviolet irradiation (365 nm), the blue-green fluorescence of the samples is gradually diminished from left to right (Figure 1d, the bottom samples). After two weeks, all samples, except the one treated by CTACe, turn to purple which looks like the samples treated by CTAB in Figure 1d. The colour of Au NCs dispersed sample treated by CTACe still shows the dark yellow without any colour change. The fluorescence intensity of all samples was very weak under ultraviolet irradiation (not shown). This indicates that Au NCs ageing and changes happened in all CTAX solutions but the ageing speed was various. 7


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The emission spectra of Au NCs dispersed samples treated by CTAX with different counterions are shown in Figure 2a, which consists with the order of photographs from the bottom samples in Figure 1d. Among these surfactants, CTAS shows the minimal effect on fluorescence intensity of Au NCs while the effect of CTACe is most significant. The order change of Au NCs dispersed samples in macroscopic appearance and fluorescence intensity after being mixed with CTAX was CTAS CTAN

CTAC

CTAOH

CTAB

CTACe. In Figure 2b, Au NCs sample treated

by 5 mM CTAB at different times were recorded. The fluorescence intensity of Au NCs gradually decreases with the increase of processing time. In addition, as shown in Figure 2c, the fluorescence intensity of Au NCs decreases dramatically with the increase of CTAB concentration. 0.4

b 0.3

*˜G *

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0.2

0.1

0.0 0.1

1

10

100

Rh / nm

Figure 1. A TEM image (a) and the corresponding size distribution of Au NCs (b). The chemical structures of six surfactants CTAX (X = Br-, Cl-, OH-, C7H8O3S-,

8


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[CeCl3Br]- and NO3-) (c), photos of Au NCs treated with different surfactants under visible light (top samples) and 365 nm XOWUDYLROHW light (bottom samples) (d).

The UV-Vis spectra of the above samples that treated by different surfactants are presented in Figure 2d. The absorption peaks of different samples shift to red with the following order: CTAS

CTAN

CTAC

CTAOH

CTAB, while Au NCs treated

by CTACe was an exception. From previous reports,13,23,28,29 the position of the signal peak is related to the size of the Au NPs. The intensity of Au NPs reflects the amount of the Au NPs in the given size of nanoparticles. The absorption peaks of Au NPs shift to red with the increase of size. In this system, the size of Au NCs becomes larger following a special order as the treated by different surfactants. Au NCs treated by CTAB are larger than the other samples, while Au NCs treated by CTAS are the smallest in size. The UV-Vis spectra provided further evidence that CTAS shows the minimal effect on Au NCs while the CTAB was the most significant effect except CTACe. Specifically, the Au NCs treated by CTACe exhibit the stronger absorption in the low wavelength than other samples, indicating the smallest changes of Au NCs in size. 8000

a

Control CTAS CTAN CTAC CTAOH CTAB CTACe

6000

4000

2000

0 400

500

600

Wavelength / nm

b

Time / h 0 3 6 9 12 24 30

6000

Intensity / a. u.

8000

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000

2000

0 400

700

500

600

Wavelength / nm

9


700

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3

c

cCTAB / mM 0 1 3 5 7 9

6000

4000

d

Control CTAS CTAN CTAC CTAOH CTAB CTACe

2

Abs / a. u.

8000

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

2000

0 400

500

600

700

0 300

400

500

600

700

800

Wavelength / nm

Wavelength / nm

Figure 2. Emission spectra of Au NCs dispersed sample treated by 5 mM CTAX (a) and treated by 5 mM CTAB recorded in different times (b). Emission spectra of Au NCs treated by CTAB with different concentrations (c), The UV-Vis spectra of Au NCs dispersed sample treated by 5 mM CTAX (d).

7KH HIIHFW RI &7$; RQ WKH $X 1&V ZDV DOVR LQYHVWLJDWHG E\ 7(0 REVHUYDWLRQV $V VKRZQ LQ WKH Figure 3 (a and b) DIWHU PL[LQJ Au NCs ZLWK &7$% VROXWLRQ without shaking DQG VWDQGLQJ IRU three hours WKH Au NCs grow WR ZRUPOLNH $X QDQRZLUHV ZKLFK GLVSHUVHG LQ WKH V\VWHP RU DJJUHJDWHG RQ VSKHUHV :H VXVSHFW WKDW WKH VSKHULFDO DJJUHJDWHV LQ V\VWHP PD\ EH RULJLQDWHG IURP WKH DVVHPEO\ RI +LV DQG &7$% DQG WKLV VSHFXODWLRQ was further proved as follow. Firstly, HCl was mixed with His to be instead of HAuCl4 and His for forming Au NCs. CTAB solution was added to make sure

the

component

is

HCl/His/CTAB,

achieving

the

replacement

of

HAuCl4/His/CTAB. Eventually, only these VSKHULFDO DJJUHJDWHV ZHUH IRXQG LQ 7(0 LPDJH Figure 3c ,QWHUHVWLQJO\ LI $X 1&V GLVSHUVHG VDPSOH DQG &7$% VROXWLRQ ZHUH PL[HG XQGHU vigorous and continuous stirring for three hours, the Au NCs can WXUQ WR VSKHULFDO Au NPs ZLWK ODUJHU VL]H Figure 3d It is known that CTAB can absorb on the surface of Au nanocrystals to form bilayer in the process of synthesizing Au NRs 10


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or Au nanowires.15,30,31 Br- ions as the counterion of CTAB play a crucial key in shape-directing of Au NPs. However, the growth process from Au nanocrystals to Au NRs can be disturbed by heating which induces the changes of nanoparticle shape.32 Similarly, vigorous stirring may affect the adsorption of CTAB on the surface of Au NCs which disturbed the direction of Au NCs growth eventually. Therefore, the difference of growth morphology of Au NCs between Figure 3b and Figure 3d could result from the vigorous and continuous stirring of samples. It is interesting that the other surfactants in this system failed to direct Au NCs growing into ZRUPOLNH $X QDQRZLUHV EXW WKH VSKHULFDO Au NPs. When Au NCs were treated by CTAOH, they become Au NPs with larger size that GLVSHUVHG LQ WKH V\VWHP RU DJJUHJDWHG RQ VSKHUHV DV VKRZQ LQ Figure 3e. Other surfactants in this work, CTAC, CTAN and CTAS, totally have the similar effect on the morphology (Figure S1). Like CTAOH, the Au NCs turn to be the larger VSKHULFDO Au NPs. The difference of Au NPs and Au nanowires is depended on the specific absorption of counterions on the surface of Au crystal. As reported in previous literatures,18,33 Cl- ions can bind to Au surface through weak attraction and NO3- ions have nonspecifically adsorption to Au surface. The interaction between Br- ions and Au surface is neither too strong, nor too weak15,28 which is needed in directing the anisotropic growth of Au NCs. As a result, compared with other surfactants in this system, CTAB FDQ GLUHFW WKH JURZWK RI anisotropic Au NCs while other CTAX (X = Cl-, OH-, C7H8O3S-, [CeCl3Br]- and NO3-) fail to achieve the shape-directing. However, as shown in Figure 3f, one can note that the size of Au NCs has little change in size when they were treated by CTACe. The TEM images 11


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indicate that the changes of Au NCs treated by different surfactants consist with the UV-Vis spectra data. It follows the order: CTAS

CTAN

CTAC

CTAOH

CTAB, while CTACe was an exception. The reason that CTACe act as an exception among these surfactants was further discussed.

Figure 3. (a and b) TEM images of Au NCs samples treated by CTAB at cHis = 75 mM, cHAuCl4 = 2.5 mM, and cCTAB = 5 mM. (c) VSKHULFDO DJJUHJDWHV formed in control sample His/HCl/CTAB solution at cHis = 75 mM, cHCl = 2.5 mM, and cCTAB = 5 mM. (d) TEM image of Au NCs sample treated by CTAB under vigorous stirring. (e) TEM images of Au NCs samples treated by CTAOH at cHis = 75 mM, cHAuCl4 = 2.5 mM, cCTAOH = 5 mM and (f) CTACe at cHis = 75 mM, cHAuCl4 = 2.5 mM, and cCTACe = 5 mM.

Because both CTAFe and CTACe are magnetic surfactants. Metal complex ions are the counterions,3,34,35 both similar surfactants are selected for further investigation of 12


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the specificity of CTACe. The mixed sample of CTAFe and Au NCs shows the similar results with CTACe in emission spectra and TEM image (Figure 4a and Figure S1d). The fluorescence of the Au NCs can be quenched by heavy metal ions such as Fe3+,36,37 Hg2+,38 Pb2+,39 Cu2+,40 and Ag+,41 there may be the specific interaction between these metal ions and Au NCs. In our work, this phenomenon may be the interaction between the metal ions of counterions in CTACe with the surface of Au NCs, which leads to its fluorescence quenching. The counterions tightly bonded on the surface of Au NCs, which hindered the further deposition of Au atoms on the surface of Au NCs, and thereby limiting its further growth. We also treated the Au NCs with Fe3+ and Ce3+ separately to prove our conjecture that the quenching of the fluorescence and keeping shape of Au NCs. In order to be the control group of 5 mM CTACe, the Ce3+ solution containing 5 mM of CeCl3 and 5 mM of NaBr was prepared to make sure the same components except FHW\OWULPHWK\ODPPRQLXP FDWLRQV &7$ . Similarly, the solution of Fe3+ which contained 5 mM of FeCl3 and 5 mM of NaBr was prepared. As shown in Figure 4a, Fe3+ and Ce3+ can induce fluorescence quench of Au NCs. The effect degree on the fluorescence quenching of Au NCs is equal to that of CTAFe and CTACe in spite of the forms of [FeCl3Br]- and [CeCl3Br]-, respectively. Thus, the effect of &7$ in CTACe or CTAFe on Au NCs is negligible when both Ce3+ and Fe3+ metal ions are presented. It indicates that there is special interaction between Au NCs and the two kinds of heavy ions in their counterions. As it was reported, Fe3+ ions have the special interaction with Au seeds.42 Therefore, the metal ions in counterions of CTACe can 13


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account for the effect of CTACe on Au NCs which is clearly different from other CTAX surfactants in this work. In addition, we also investigated the effect of other sodium salts (1D; corresponding to CTAX on Au NCs. Comparing Figures (1d and 2a) and Figures (S2 and 4b), respectively, one can note that &7$ SOD\V D YLWDO UROH LQ fluorescence quenching of Au NCs. Among these 1D; VDOWV, the different effect of C7H8O3S-, Br-, Cl- and NO3- on the fluorescence intensity of Au NCs is not obvious, while the effect of OH- can be found more obvious than others. As previous reports that the fluorescence intensity of Au NCs decreases with increasing pH,24,21 this phenomenon results from that the fluorescence intensity of Au NCs varies at different pH. ,Q RUGHU WR GHPRQVWUDWH WKH DERYH K\SRWKHVLV WKH S+ RI GLIIHUHQW VDPSOHV ZDV PHDVXUHG DQG WKH FRUUHVSRQGLQJ UHVXOWV ZHUH OLVWHG LQ 7DEOH

:KHQ WUHDWHG E\

&7$2+ DQG 1D2+ WKH S+ RI $X 1&V GLVSHUVHG VDPSOH DUH

DQG

UHVSHFWLYHO\ ZKLFK ZHUH KLJKHU WKDQ RWKHU VDPSOHV 7KXV WR VRPH H[WHQW WKH UHVXOWV DOVR H[SODLQ WKDW the fluorescence intensity of Au NCs mixed with CTAOH is lower than those of CTAS, CTAN and CTAC. 8000

8000

a

b

Control CTAFe FeCl3+NaBr

6000

CTACe CeCl3+NaBr

Intensity / a. u.

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000

2000

0 400

500

600

6000

NaNO3

4000

NaBr NaCl NaOH

2000

0 400

700

Wavelength / nm

Control C7H8O3S˜Na

500

600

700

Wavelength / nm

Figure 4. (a) Emission spectra of Au NCs sample treated by CTAFe and CTACe and their corresponding 1D; solution at cCTAFe, cCTACe, cFeCl3+NaBr and cCeCl3+NaBr are 5 mM. (b) Emission spectra of Au NCs sample treated 1D; solution at cNaX = 5 mM. 14


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7DEOH

Page 16 of 29

S+ RI $X 1&V WUHDWHG E\ GLIIHUHQW &7$; DQG WKH FRUUHVSRQGLQJ 1D;. $X 1&V &7$;

S+

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7KH V\QWKHVL]HG $X 1&V LQ WKLV ZRUN ZHUH QHJDWLYH FKDUJHG RQ WKH VXUIDFH ZKLFK LV SURYHG LQ )LJXUH D $V WKH FRQWURO JURXS WKH ]HWD SRWHQWLDO RI $X 1&V LV P9 ,WV ]HWD SRWHQWLDO KDV VOLJKW FKDQJHV EXW VWLOO NHHSV QHJDWLYH LQ C7H8O3SNa, NaOH, NaBr, NaCl, and NaNO3 solution, respectively. As an exception, CeCl3 has an obvious influence on the ]HWD SRWHQWLDO of Au NCs that becomes to be 21.63 mV. This result should be ascribed to the strong attraction or electrostatic attraction between Ce3+ and Au NCs. It also further verifies the above speculation that Ce3+ ions can bond on the surface of Au NCs. In contrast, as no significant changes in zeta potential of other samples after mixing with 1D; solution, we can conclude that there are neither obvious special interaction between Au NCs and Na+ nor strong attraction between anions such as NO3- and Cl- and negative Au NCs, as illustrated in the scheme in Figure 5c (left). Besides, it also consists of the emission spectra that no obvious changes in the intensity of fluorescence. However, from Figure 5b, the zeta potential of all samples in different CTAX surfactants has undergone very significant 15


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changes which are totally above 30 mV. There are several possible forms of particles existence in solution illustrated in Figure 5c (right). Firstly, &7$; FDQ self-assemble WR IRUP micelles or assemble with His to form the spherical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degree of fluorescence quenching more obvious among different counterions.

a

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Zeta potential / mV

30

Zeta potential / mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

b

45

15

30

0

Control

C7H8O3S˜Na

NaNO3

NaCl

NaOH

CeCl3+NaBr

15

-15

-30

NaBr

0

Control CTAS

CTAN

CTAC

CTAOH

CTAB

CTACe

-15

AuNCs/CTAX

AuNCs/NaX

Figure 5. Zeta potential of Au NCs treated by different 1D; solution (a) and 16


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

surfactants CTAX solution (b). (c) Schematic illustration of interaction between Au NCs and anions in different aqueous solutions. In order to investigate the effect of surface activity of surfactants on the Au NCs, surfactants with various lengths of the hydrocarbon tails but the same counterion were used in this work. The effect of the FDWLRQLF VXUIDFWDQWV on the fluorescence intensity of Au NCs was examined while kept the counterions (Br-) and concentration constant (5 mM). Even though the headgroup and counterions are the same in these three surfactants, as shown in Figure 6a, the fluorescence intensity of Au NCs decreases more significantly with the increase of the length of the hydrocarbon tail. It is known that the specific conductivity N changes linearly with the surfactants concentration and there is a break at the cmc when the slopes are different due to the binding of some counterions to micelles.45 As shown in Figure 6b of the specific conductivity vs. concentration plots, the cmcs of CTAB, TTAB and DTAB are obtained at the intersection points in curves. They are 0.93 mM, 3.8 mM and 14.5 mM, respectively. These data consist with the results in previous report.46 The shorter the hydrophobic chains of the surfactants, the less easily aggregation in solution. For DTAB, it does not reach its cmc when the concentration is 5 mM. Most of molecules tend to absorb at the gas-liquid interface rather than in solution to form micelles by interaction with His. As a result, fewer surfactant molecules interact with Au NCs in solution (Figure S3). Thus, the surfactants which have the lower cmc and the higher surface activity can have the more significant its effect on Au NCs. We investigated surface tension J DQG specific conductivity of CTAX DW GLIIHUHQW FRQFHQWUDWLRQV WR PDNH VXUH WKHLU 17


Page 18 of 29

Page 19 of 29

UHVSHFWLYH FPFV 7KH UHVXOWV DUH VKRZQ LQ )LJXUHV

F DQG 6

WKH FPFV GHWHUPLQHG

E\ N RI &7$; ZLWK WKH different counterions DUH YDULRXV UDQJLQJ IURP

WR

mM. According to the measurements of surface tensions, the FPFV GHWHUPLQHG E\ J DOVR KDYH D GLVWULEXWLRQ IURP

WR

mM. The differences of cmcs obtained by

these two methods are acceptable as other reports.47 The cmc order of CTAX is CTAS CTACe

CTAB

CTAN

CTAC, despite the difference of the FPFV

CTAOH

WKH\ ZHUH GHWHUPLQHG E\ J DQG N GDWD UHVSHFWLYHO\ 1600

a

1200

N / mS˜cm

6000

Intensity / a. u.

b

Control CTAB TTAB DTAB

-1

8000

4000

2000

CTAB TTAB DTAB

800

400

0

0 400

500

600

0

700

Wavelength / nm

2.0

-1

60

50

40

30

Complex Viscosity / cP

c

CTAS CTACe CTAB CTAN CTAOH CTAC

5

10

15

20

c XTAB / mM

70

J mN·m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

d

cCTAX = 5 mM CTAS CTACe CTAB CTAN CTAOH CTAC H2O

1.6

1.2

0.8

0.1

1

c CTAX / mM

1000

10

Angular Frequency / rad˜s

-1

10000

Figure 6. (a) Emission spectra of Au NCs solution treated by CTAB, TTAB and DTAB at c = 5 mM. (b) Specific conductivity vs. concentration plots for CTAB, TTAB and DTAB. (c) Surface tensions vs. concentration plots, and (d) complex YLVFRVLW\ YV DQJXODU IUHTXHQF\ of CTAX at 25.0 f 0.1 °C. 18


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7DEOH

Page 20 of 29

7KH K\GUDWHG UDGLXV RI FRXQWHULRQV FPFV DQG UHODWLYH YLVFRVLWLHV RI &7$; +\GUDWHG UDGLXV QP

&7$; &RXQWHULRQV &7$&

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12

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FPF N P0

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The FPFV RI &7$; UHFRUGHG E\ surface tensions J and specific conductivities N DUH UHFRUGHG at 25.0 f 0.1 °C, and the rHODWLYH YLVFRVLWLHV RI &7$; DUH UHFRUGHG DW F&7$;

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7KH K\GUDWHG UDGLXV RI FRXQWHULRQV ZDV DOVR WDNHQ LQWR DFFRXQW ZKLFK PD\ LQIOXHQFH WKH FKDUDFWHULVWLFV RI VXUIDFWDQWV $OWKRXJK WKH K\GUDWLRQ UDGLXV RI [CeCl3Br]- and & + 2 6- ZHUH IDLOHG WR EH IRXQG, they should be larger than other FRXQWHULRQV LQ 7DEOH

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Langmuir

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4. CONCLUSIONS ,Q VXPPDU\ Au NCs have significantly different changes when they were treated by different surfactants, CTAX, with different FRXQWHULRQV. )RXU DVSHFWV LQGXFH WKH FKDQJHV RI $X 1&V ZKHQ WKH\ ZHUH WUHDWHG E\ &7$; L %\ FRPSDULQJ WKH HIIHFW RI GLIIHUHQW 1D; DQG FRUUHVSRQGLQJ FDWLRQLF VXUIDFWDQWV RQ $X 1&V LW FDQ EH FRQFOXGHG WKDW WKH FDWLRQLF SDUW RI WKH VXUIDFWDQWV (CTA+) KDV D YHU\ VLJQLILFDQW HIIHFW RQ WKH IOXRUHVFHQFH SURSHUWLHV RI $X 1&V

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CTAN

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CTAC

CTAOH

CTAB

Page 22 of 29

CTACe

ASSOCIATED CONTENT The supporting information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: (+86) 531-8856-4750 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the financial aids supported by the NSFC (Grant No. 21420102006).

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Effect of Cationic Surfactants with Different Counterions on the Growth

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