s ray (nanotechnology 23 2012 495301) gold metamaterials

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 23 (2012) 495301 (8pp) doi:10.1088/0957-4484/23/49/495301

Amino-acid-based, lipid-directed, in situ

synthesis and fabrication of goldnanoparticles on silica: a metamaterialframework with pronounced catalyticactivity

Sudipta Ray1, Makoto Takafuji1,2 and Hirotaka Ihara1,2

1 Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 KurokamiChuo-ku, Kumamoto 860-8555, Japan2 Kumamoto Institute for Photo-Electro Organics (PHOENICS), 3-11-38 Higashimachi Higashiku,

Kumamoto 862-0901, Japan

E-mail: [email protected]

Received 30 July 2012, in final form 1 October 2012

Published 13 November 2012

Online at stacks.iop.org/Nano/23/495301

Abstract

We introduce a new example of the in situ preparation and fabrication of stable gold

nanoparticles on silica in an aqueous medium, by using only lipid-grafted silica particles inHAuCl4 solution without addition of any external reducing agent. The lipid-grafted silica

particles have been synthesized by graft-to methodology and characterized by elemental

analysis, thermogravimetric analysis and Fourier-transform infrared spectroscopy. The

metamaterial particles show high catalytic activity for the reduction of p-nitrophenol to

p-aminophenol.

S Online supplementary data available from stacks.iop.org/Nano/23/495301/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Metal composites have attracted a great deal of interestbecause of their various applications in biology, such as in

bioimaging [1], optical sensing [2], biomedicine [3], andcatalysis [4–9]. The use of gold-nanoparticle building blocksfor the creation of electrochemical sensing devices is alsopromising [10–17]. Although gold is a poor catalyst in its bulk form, nanometre-sized gold particles can exhibit excellentcatalytic activity owing to their large surface-to-volumeratio and different interface-dominated electronic propertiescompared to the corresponding bulk metal [18–21]. Despitetheir high reactivity and efficiency as homogeneous colloids,

large-scale applications of gold nanoparticles are limited

because of particle aggregation and their poor re-useprobability. These disadvantages have often been overcome

by immobilizing the catalytic nanoparticles on solid supports

such as silica [22–29], alumina [30], zeolite [31], and metaloxide [32]. In particular, metal-nanoparticle-coated silicacomposites are interesting because of their stability andinactivity with reacting molecules. There are some knownmethods available for synthesizing various kinds of silica-metal-based composite materials such as metal-core silicacells [33, 34], silica-core metal cells [35], and nanoparticlesembedded in porous silica [36]. Preparation of supported goldnanoparticles in a one-step manner and without utilizationof external reducing agents has been previously reported byShi et al [37]. However, there are very few references tothe synthesis of gold-nanoparticle–silica composite materialsusing only functionalized silica particles without the addition

of any external reducing agent [38]. Various multi-stepwet chemical methods have been developed; all these

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Nanotechnology 23 (2012) 495301 S Ray et al

synthetic methods involve the pre-treatment of silica, complex

processing, and special experimental conditions. Hence, it

is essential to have a simple, scalable protocol for the

synthesis of silica–gold composite materials. Taking this

into consideration, we designed one novel lipid-grafted silica

molecule (Sil-DYS) having two parts: a reductant part and

a stabilizer part. We selected the amino acid tyrosine asthe reductant part, as we have previously reported that

it can be used as a reducing and capping agent for the

synthesis of various nanostructures [39, 40]. We also selected

a long-chain alkyl group (stearyl group) to stabilize the

nanoparticles against multidimensional aggregation, and used

silica particles as the solid support for fabrication. Here,

we propose a new aqueous-phase method for the in situ

preparation of stable gold nanoparticles using silica–Tyr–lipid

as both the reductant and stabilizer.

2. Experimental section

2.1. Materials

Starting from D-tyrosine, all chemicals for the synthesis of

lipids were purchased from Wako Pure Organic Chemical

Industries, Sigma Aldrich, Tokyo Kasei Kogyo (Tokyo, Japan)

and Nacalai Tesque (Kyoto, Japan) and used as received.

3-aminopropyltrimethoxysilane (APS) was purchased from

Azmax (Chiba, Japan). Porous silica particles (YMC-GEL),

whose average diameter, pore size and surface area are 5 µm,

12.0 nm and 300 m2 g−1 respectively, were purchased from

YMC (Kyoto, Japan).

2.2. Synthesis of Sil-DYS particles

2.2.1. N-stearyl-D-tyrosinamide-methyl ester (1). 4.3 g

(15 mmol) of stearic acid in 10 ml of DMF was cooled in

an ice-water bath. H–D-Tyr–OMe was isolated from 5.85 g

(30 mmol) of the corresponding methyl ester hydrochloride

by neutralization, subsequent extraction with ethyl acetate

and concentration to 10 ml and it was added to the reaction

mixture, followed immediately by 3.09 g (15 mmol) of DCC

and 2.2 g (15 mmol) of HOBt. The reaction mixture was

stirred for two days. The residue was dissolved in ethyl

acetate (60 ml) and the DCU was filtered off. The organic

layer was washed with 2 M HCl (3 × 50 ml), brine (2 ×50 ml), 1 M sodium carbonate (3 × 50 ml) and brine (2 ×50 ml) again, then dried over anhydrous sodium sulfate and

evaporated in vacuo to yield 1 as a white solid. Yield =

5.6 g (12.1 mmol, 80%). C28H47NO4(461) requires C, 72.88;

H, 10.1; N, 3%. Found C, 72.62; H, 9.91; N, 2.97%. 1H

NMR (400 MHz, DMSO-d6): δ 8.25–8.23 (Tyr NH, 1H, d,

J = 8 Hz); 6.98–6.96 (ring Hs of Tyr, 2H, d, J = 8 Hz);

6.67–6.65 (ring Hs of Tyr, 2H, d, J = 8 Hz); 4.34 (CαH of

Tyr, 1 H, m); 3.57 (–OCH3, 3H, s); 2.89–2.76 (Cβ Hs of Tyr,

2H, dd); 2.06–2.02 (–CO–CH2–, 2H, t); 1.64 (–CO–C–CH2–,

2H, b); 1.39 (–(CH2)14–, 28 H, b); 0.85–0.83 (–CH3, 3H,

t). 13C NMR (100 MHz, DMSO-d6): δ 172.38, 171.85,

130.1, 127.33, 53.80, 51.62, 40,12, 39.7–38.87, 35.39, 33.54,31.24, 29.00, 28.88–28.44, 25.11, 22.05, 13.92. FT-IR data:

νmax(KBr) (cm−1) 3332, 2917, 2849, 1752, 1648, 1558, 1463,1229, 1165. ESI-HR-mass [ M +H]+ = 462.28, [ M +Na]+ =

484.12, [ M + K]+ = 500.05, M calc. = 461.

2.2.2. N-stearyl-D-tyrosinamide (SDY) (2). To 5.4 g(11.7 mmol) of 1, 20 ml of MeOH and 10 ml of 2 M NaOH

were added. The reaction mixture was stirred and the progressof saponification was monitored by thin layer chromatography(TLC). After 10 h, methanol was removed under vacuum andthe residue was dissolved in 50 ml of water and washed withdiethyl ether (2 × 50 ml). Then the pH of the aqueous layerwas adjusted to 2 using 1 M HCl and it was extracted withethyl acetate (3 × 50 ml). The extracts were pooled, driedover anhydrous sodium sulfate, and evaporated in vacuo toyield 2 as a solid compound. Yield = 2.3 g (5.14 mmol,44%). C27H45NO4(447) requires C, 72.48, H, 10.06, N,3.13%. Found C, 72.28, H, 9.87, N, 2.98%. m.p. 129–130 ◦C.1H NMR (400 MHz, DMSO-d6): δ 9.21 (Ph–OH, 1 H, b);8.02–8.00 (Tyr NH, 1H, d, J = 8 Hz); 7.00–6.98 (ring Hs

of Tyr, 2H, d, J = 8 Hz); 6.64–6.62 (ring Hs of Tyr, 2H, d, J = 8 Hz); 4.32–4.31 (CαH of Tyr, 1H, m); 2.92–2.88 (Cβ Hsof Tyr, 2H, dd); 2.19–2.15 (–CO–CH2–, 2H, t); 1.40–1.36(–CO–C–CH2–, 2H, b); 1.23 (–(CH2)14–, 28 H, b); 0.86–0.83(–CH3, 3H, t). 13C NMR (100 MHz, DMSO-d6): δ 174.43,173.31, 172.09, 155.86, 129.90, 127.68, 114.86, 53.61, 40.12,39.91–38.87, 35.99, 35.0, 33.64, 31.27, 29.02–28.49, 25.17,24.47, 22.07, 13.92. FT-IR data: νmax(KBr) (cm−1) 3313,3240, 2917, 2849, 1708, 1645, 1542, 1516, 1462, 1235.ESI-HR-mass [ M +H]+ = 447.41, [ M +Na]+ = 470.27, [ M +

K]+ = 486.43, M calc. = 447.

2.2.3. Immobilization of lipid compound analogue on to

silica surface (Sil-DYS). 3-aminopropyltrimethoxysilane(APS)-grafted silica was prepared by refluxing porous silicagel (3.0 g) and APS (1.5 ml) in toluene for 24 h. Aftersuccessive washing with toluene, ethanol and diethyl etherthe particles were dried in vacuum. The dried particles werecharacterized by elemental analysis: H—2.05, C—4.96, N—1.65%. Silica-APS was then coupled with SDY. Silica-APS(3.0 g) and SDY (3.0 g) were dissolved in 100 ml dry THFand stirred. DEPC (1.5 g) and TEA (1.1 g) were added tothe solution and stirred at 60 ◦C. After being stirred for 1 daythe grafted particles (Sil-DYS) were washed with chloroformand methanol several times to remove the unreacted lipidmolecules and dried in vacuum. The grafting was confirmedby elemental analysis.

2.3. In situ preparation of gold nanoparticles and simultaneous fabrication

First we take 20 mg of Sil-DYS in a vial then dissolve it in8.5 ml of Millipore water. Then 0.5 ml of HAuCl4 (10 mmol)aqueous solution was added dropwise to the above solutionwith constant magnetic stirring. The pH of the solution wasadjusted to alkaline by addition of freshly prepared NaOHsolution. A colour change was observed from yellow topurple (supporting information figure S1 available at stacks.iop.org/Nano/23/495301/mmedia), indicating the formation

of silica–DYS–gold nanoparticle conjugate through reductionof the tyrosine residue of the lipid part [40].

2










































Scheme 1. Schematic representation of the synthesis of Sil-DYS particles.

2.4. Characterizations

2.4.1. NMR experiments. All NMR studies of the lipid

molecules in CDCl3

and DMSO-d6

at 25 ◦C were carried out

with JEOL JNM-LA 400 (Japan) spectrometers at 400 MHz.

Chemical shifts (δ) of 1H are expressed in parts per million

(ppm) with use of the internal standard Me4Si (δ = 0.00 ppm).

2.4.2. Mass spectrometry. Mass spectra were recorded

on a Bruker Daltonics mass spectrometer by positive mode

electrospray ionization.

2.4.3. DRIFT mode Fourier-transform infrared spectroscopy

and elemental analysis. FT-IR measurements were

conducted with a JASCO FT/IR-4100 (Japan). For DRIFT

measurement accessory DR PRO410-M (JASCO, Japan) wasused. Samples were prepared by mixing the corresponding

dried samples with KBr in a 1:100 (wt/wt) ratio. Elemental

analyses were carried out on a Yanaco CHN Corder MT-6

Apparatus (Japan).

2.4.4. Calculation relates to surface coverage. Surface

coverage of organic phase was calculated by using the

equation below and data given in table S1 (available in

supporting information at stacks.iop.org/Nano/23/495301/

mmedia).

The molar amount of organic phase per gram of silica ( M )

can be calculated as M (µmol g−1) = 106 (Pc/100)/12n (1)

where Pc is the percentage of carbon element according to

elemental analysis and n is the number of carbons present in

the grafted organic phases.

The weight percentage of the grafted phase Pw

in each

case can be calculated as

Pw = m × 10−4 M (n/n1) (2)

where m is the molecular mass and n1 is the number of carbons

in each molecule of the organic phases grafted onto the silica

surface.

N (µmol m−2) = M /[S {(100 − Pw)/100}]

= 106 Pc/[12nS (100 − Pw)] (3)

where S is the surface area of 1.00 g of nonmodified silica.

2.4.5. UV–visible absorption spectroscopy. UV–visibleabsorption spectra of lipid–Au conjugate and silica–lipid–Au

conjugate in water were recorded with a JASCO-V560

UV–visible spectrophotometer.

2.4.6. Field emission scanning electron microscopy. One

drop of as-prepared water suspension of silica–lipid–Au

nanoconjugate was placed on glass coverslip and allowed

to vacuum dry. It was then coated with osmium by using

Filgen OPC6ON. Then the micrographs were observed under

a Hitachi (S4800) scanning electron microscope.

2.4.7. Transmission electron microscopic study. The trans-mission electron microscopic studies of all the silica–lipid–Au

3










































Figure 1. UV–visible absorption spectra of DYS–Au solution andsilica–DYS–Au conjugate in water. The inset shows the

silica–DYS–Au conjugate in aqueous medium.

Figure 2. DRIFT mode FT-IR spectra of silica, silica–DYS andsilica–DYS–Au conjugate.

conjugate were done by placing one drop of water suspension

of the corresponding compounds on carbon-coated copper

grids and drying by slow evaporation. The grid was thenallowed to dry in vacuum for two days. Images were taken

at an accelerating voltage of 200 kV. TEM was carried out

with a JEOL JEM-2000 FX electron microscope.

2.4.8. Thermogravimetric analysis (TGA). Thermograms of

the dry powdered samples were recorded by using a Seiko

EXSTAR 6000 TG/DTA 6300 thermobalance in static air

from 40 to 800 ◦C at a heating rate of 10 ◦C min−1 under a

N2 atmosphere. Thermogravimetric curves are usually used to

determine the thermal stability and to confirm the amounts of

immobilized organic components. The weight loss observed

can be associated with the loss of organic groups attached tothe silica surface. As shown in figure S1 (available at stacks.

iop.org/Nano/23/495301/mmedia), Sil-APS presented a mass

loss of 7.99%, and after the lipid grafting the weight loss

observed is 12.9%, indicating that the organic content had

greatly increased.

2.4.9. Catalytic reduction of 4-nitrophenol. The reduction

of 4-nitrophenol was selected as a model reaction systemfor testing the catalytic activity of the in situ formed and

fabricated silica–lipid–Au conjugate. Aqueous solutions of

4-nitrophenol (1 ml, 0.001 M) and NaBH4 (5 ml, 0.1 M) were

added to 40 ml Millipore water in a beaker under constant

magnetic stirring. After adding silica–lipid–Au conjugated

catalyst particle (5 mg), the bright yellow solution gradually

faded as the reaction proceeded. The progress of the catalytic

reduction experiment was recorded at regular intervals using

UV–vis spectra.

3. Results and discussion

3.1. In situ formation and fabrication of gold nanoparticles

The synthetic routes for the lipid stearyl-D-tyrosinamide

(SDY) and the immobilization process of this lipid analogue

on silica are shown in scheme 1. The chemical structures

of all the final compounds were identified by melting-

point measurements, Fourier-transform infrared (FT-IR)

spectroscopy, 1H-NMR spectroscopy, thermal gravimetric

analysis (TGA) (supporting information figure S2 available

at stacks.iop.org/Nano/23/495301/mmedia) and elemental

analysis. The in situ preparation of gold nanoparticles and

the fabrication of conjugate particles on silica were achievedby the simple addition of HAuCl4 aqueous solution to the

silica–DYS aqueous solution and the adjustment of the pH

of the solution under stirring. First, silica–DYS (20 mg)

was put in a vial and dissolved in Millipore water (8.5 ml).

Then, an aqueous solution of HAuCl4 (0.5 ml, 10 mmol) was

added dropwise to the above solution with constant magnetic

stirring. The pH of the solution was adjusted to be just

alkaline by the addition of freshly prepared NaOH solution.

Within 5 min, the colour of the reaction mixture changed

distinctly from pale yellow to purple. This purple colour

arises because of the appearance of gold-nanoparticle–silica

conjugate particles, and occurs as a result of the excitation

of the surface plasmons of the gold nanoparticles. Thelipid (DYS) can be used to synthesize gold nanoparticles

by the same procedure, but it coagulates readily with time,

whereas the conjugate particles are colloidally stable in water

for a long time and are purple in colour, as shown in

the inset of figure 1. It seems that the synthesis of gold

nanoparticles and the fabrication of the conjugate particles

occurred simultaneously. These conjugate particles can be

collected easily by simple centrifugation.

3.2. UV analysis

The UV–vis spectrum (figure 1) of the lipid–DYS–goldsolution shows a sharp peak at λmax = 535 nm. These data

4















































































Figure 3. (a) Scheme of the procedure used to produce gold nanoparticles on silica and (b) SEM image of a silica particle covered withgold nanoparticles.

Figure 4. (a) TEM picture of gold nanoparticles synthesized by lipid–SDY in water and (b) corresponding histogram showing the particlesize distributions.

indicate the presence of well dispersed gold nanoparticles less

than 20 nm in size [41]. In the case of the silica–DYS–Au

conjugate the peak is observed at λmax = 552 nm. This slight

shift in the surface plasmon band for gold nanoparticles might

be due to the change of their size after conjugation withsilica–DYS [42, 43].

3.3. DRIFT mode FT-IR analysis

The DRIFT mode FT-IR spectra (figure 2) show bands for

silica–DYS at 2980 and 2899 cm−1, occurring as a result

of the asymmetric and symmetric stretching of the graftedCH2 groups, respectively. Bands in the same region are also

5




Figure 5. TEM image of (a) silica–DYS–Au conjugate in water and (b) layer of Au nanoparticles on silica surface. (c) High resolutionTEM image of silica–DYS–Au conjugate and (d) corresponding histogram showing the particle size distribution.

observed for the silica–DYS–Au conjugate, but with slightchanges in peak positions (3057 and 2907 cm−1, respectively)

due to the change in conformation of the grafted phase after

nanoparticle fabrication and conjugation. Both the silica–DYS

conjugate and silica–DYS–Au conjugate show amide I (due

to stretching vibration of CO group) and amide II (due

to bending vibration of NH group) bands. In the case of

silica–DYS the amide I and amide II bands appear at 1643

and 1540 cm−1, respectively, whereas for the silica–DYS–Au

conjugate they appear at 1602 and 1507 cm−1, respectively;

the peak intensity also decreases. These data indicate that

this amide functionality has some role in stabilizing the

gold nanoparticles for their facile fabrication over the silicasurface. Moreover, the peak for the phenolic OH group at

1447 cm−1 is clearly present in the silica–lipid conjugate, butis barely visible in the silica–lipid–Au conjugate. This clearly

suggests that the phenolic OH group of tyrosine in the lipid

acts as the main reducing agent for the in situ formation of

gold nanoparticles. This result proves that the DYS molecule

plays the key role in the formation and fabrication of gold

nanoparticles over silica particles.

3.4. SEM morphological characterizations

The FE-SEM study showed that gold nanoparticles were

selectively fabricated on the surface of the silica particles.

Figure 3(a) gives a schematic presentation of the depositionprocedure of gold nanoparticles over silica, and figure 3(b)

6




shows that spherical gold nanoparticles are dispersed evenly

over the silica surface. An SEM picture of Sil-DYS particles

and a high resolution one are also given in supporting figures

S3(a) and (b) (available in supporting information at stacks.

iop.org/Nano/23/495301/mmedia).

3.5. TEM morphological characterizations

The morphology and distribution of the gold nanoparticles on

the silica surface can be directly observed by transmission

electron microscopy (TEM). Figures 4(a) and (b) show that

the DYS–gold conjugate can form well dispersed spherical

gold nanoparticles with an average diameter of 7.9 nm, in

the range 3–10 nm. From figure 5(a), it can be observed that

highly dispersed gold nanoparticles, which appear as dark

dots, are nicely arranged on the silica surface. Actually, the

gold nanoparticles form a layer-like array on the silica surface,

as observed in figure 5(b). High resolution TEM pictures of

Sil–DYS–Au conjugate particles are also given in figure 5(c),

and figure 5(d) shows the corresponding histogram. A slight

increase in particle size has been observed, with an average

diameter of 14 nm, in the range of 8–20 nm, after fabrication

on the silica surface.

3.6. Catalytic efficiency

The obtained silica–DYS–Au conjugate particles were applied

for the catalytic reduction of p-nitrophenol by NaBH4. It is

well known that this reaction is simple and fast in the presence

of metallic surfaces [44, 45]. It was confirmed that this

reaction did not occur without the silica–DYS–Au conjugate

catalysts, even after a period of four days. The kinetics of p-nitrophenol reduction in the presence of the silica–DYS–Au

conjugate was studied by UV–vis spectroscopy. The reaction

progress was monitored by taking small portions of the

reaction mixture at regular time intervals. Figure 6 shows

the typical UV–vis absorption change of the reaction mixture

upon addition of silica–DYS–Au conjugate particles at regular

intervals. After the addition of the silica–DYS–Au conjugate

in the reaction medium, it was observed that the intensity

of the peak at 400 nm (characteristic for p-nitrophenol)

decreased, and a new peak at 300 nm (characteristic

for p-aminophenol) increased gradually with time. The

successive decrease in the intensity of the 400 nm peak withtime was considered to obtain the rate constant. The ratio of

C /C 0, where C and C 0 are the p-nitrophenol concentrations

at times t and 0, respectively, was measured from the relative

intensity of the respective absorbance, A/ A0, at 400 nm. A

linear relationship of ln(C /C 0) versus time was observed,

indicating that the reactions followed first-order kinetics. The

observed rate constant for the catalyst was 5.95 × 10−4 s−1,

as calculated directly from the slope of the straight line shown

in the inset of figure 6. The conversion yield of p-nitrophenol

to p-aminophenol was almost 60% within 5 min and almost

90% within 1 h. The advantages of these silica–DYS–Au

nanocomposite catalyst particles are their easy preparation,

dispersion, and separation in the reaction mixture. Thesecomposite catalyst particles can be collected easily from the

Figure 6. Time-dependent UV–visible spectral changes of the p-NP reaction mixture catalysed by silica–DYS–Au conjugate. Theinset shows the plot of ln(C /C 0) versus time for the p-NP to p-APreaction mixture.

reaction mixture by simple centrifugation, and can be used

repeatedly by re-dispersion in water through a brief sonication

procedure.

4. Conclusion

In conclusion, we have reported a facile method for the

environment friendly, in situ preparation and fabrication

of noble-metal nanoparticles over a silica support. In this

synthesis, the silica–lipid conjugate acts as the reducing agent,

stabilizing agent, and template in the aqueous medium. Thismetal–silica framework can be applied successfully in the

catalytic reduction of p-nitrophenol as a model reaction, and

may find numerous applications in the field of heterogeneous

catalysis. Because of the simple synthetic approach, we

believe that this new smart Au-nanoparticle-fabricated silica

support material is promising for a range of applications for

which a thin film of metal is required, such as in electrically

conductive metamaterials.

Acknowledgments

SR (ID-P11034) gratefully acknowledges the Japan Societyfor the Promotion of Science (JSPS) for providing financial

support to carry out this research.

References

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Chen H-R 2005 Adv. Mater. 17 557[3] Hirsch L R, Stafford R J, Bankson J A, Sershen S R, Rivera B,

Price R E, Hazle J D, Halas N J and West J L 2003 Proc. Natl Acad. Sci. 100 13549

[4] Visser T, Nijhuis T A, Vander Eerden Ad M J, Jenken K, Ji Y,Bras W, Nikitenko S, Ikeda Y, Lepage M andWeckhuysen B M 2005 J. Phys. Chem. B 109 3822

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