Preparation and properties of organo-soluble tetraphenylethylene monolayer-protected gold nanorods

Article abstract of DOI:10.1016/j.dyepig.2015.08.028

Organo-soluble tetraphenylethylene derived monolayer-protected gold nanorods were prepared and characterized. The prepared gold nanorods covered with tetraphenylethylene thiol via the strong covalent Au-S linkage were found to be soluble and stable in org

Full text of DOI:10.1016/j.dyepig.2015.08.028

Accepted Manuscript
Preparation and Properties of Organo-soluble Tetraphenylethylene Monolayer-
Protected Gold Nanorods
Tiantian Cao, Dengfeng Li, Xuyang Yao, Yikai Xu, Xiang Ma
PII:
S0143-7208(15)00346-0
10.1016/j.dyepig.2015.08.028
DYPI 4910
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 June 2015
Revised Date: 28 August 2015
Accepted Date: 29 August 2015
Please cite this article as: Cao T, Li D, Yao X, Xu Y, Ma X, Preparation and Properties of Organo-soluble
Tetraphenylethylene Monolayer-Protected Gold Nanorods, Dyes and Pigments (2015), doi: 10.1016/
j.dyepig.2015.08.028.
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ACCEPTED MANUSCRIPT
Preparation
and
Properties
of
Organo-soluble
Tetraphenylethylene Monolayer-Protected Gold Nanorods
Tiantian Cao, Dengfeng Li, Xuyang Yao, Yikai Xu and Xiang Ma*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, P. R. China
Tel: (+86) 21-6425-2288, Fax: (+86) 21-6425-2288, E-mail: maxiang@ecust.edu.cn
Graphical abstract

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Preparation
and
Properties
of
Organo-soluble
Tetraphenylethylene Monolayer-Protected Gold Nanorods
Tiantian Cao, Dengfeng Li, Xuyang Yao, Yikai Xu and Xiang Ma*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, P. R. China
Tel: (+86) 21-6425-2288, Fax: (+86) 21-6425-2288, E-mail: maxiang@ecust.edu.cn
Abstract
Organo-soluble tetraphenylethylene derived monolayer-protected gold nanorods were
prepared and characterized. The prepared gold nanorods covered with
tetraphenylethylene thiol via the strong covalent Au-S linkage were found to be
soluble and stable in organic solvents. The successful thiol exchange and modification
of tetraphenylethylene derivative molecules on gold nanorods were confirmed from
UV-Vis, Raman and FT-IR spectra. The gold nanorods were found to have a
quenching effect on the aggregation-induced emission of the tetraphenylethylene thiol
molecule modified on their surfaces.
Keywords: Gold nanorod; Tetraphenylethylene; Monolayer; Aggregation-induced
emission (AIE); Hybrid

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1. Introduction
Nowadays, gold nanorods (GNRs) have attracted wide attention due to their
anisotropic property and distinguishing surface plasmon resonance (SPR) [1-11].
GNRs usually have more distinguished physical properties particularly for their
tunable absorption in the visible and near IR region, which are quite different from the
widely investigated spherical gold nanoparticles (GNPs) [12]. GNPs have many
promising applications in the areas of optics [13-15], sensors [16-18], biological
imaging [19-21] and anticancer agents [22, 23]. Until now, GNRs have mainly been
prepared in aqueous medium via the seed-mediated growth method [24-26], in which
a surfactant cetyltrimethylammonium bromide (CTAB) was used as a shape-directing
surfactant along their longitudinal axis to form a bilayer CTAB structure around the
sidewall of GNRs and with their two ends free from CTAB. The resultant GNRs are
water-soluble due to the protection of the surfactant CTAB. However, the GNRs
protected by the bilayer CTAB structure are not very stable due to the fact that the
dynamic CTAB layers can leave the GNR surface and result in the irreversible
aggregation of GNRs into gold clusters. In addition, the CTAB molecules are toxic to
proteins, tissues and biological cells, which limits their potential applications. It is
well established that thiol molecules can form strong covalent S-Au bonds with the
surface of GNRs and the resultant GNRs may readily disperse in organic solvents,
which would provide a superior method to tune the properties of GNRs and enrich the
application of GNRs. So far, only a few examples of thiol monolayer-protected
GNRs have been reported because the preparation of thiol-monolayer-protected

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GNRs was challenging [27-30].
Monolayer protected GNRs are fascinating hybrid materials composed of GNRs
protected by an organic molecule monolayer, which solves the problem limited by the
CTAB protecting layers. Great interest in the research in the GNR field continues to
be drawn to focus on building GNRs protected by functional organic molecules. It is
well known that modifying the chemical composition of GNR surfaces provides a
versatile means to expand their various functions. In these cases, thiol molecules may
exhibit superior stability compared with the CTAB molecules when they formed the
strong covalent S-Au linkage with GNRs. Moreover, the thiol monolayer-protected
GNRs may exhibit excellent solubility in organic solvents and distinguished surface
functions [31-34]. However, it is noteworthy to point out that the exchange process of
CTAB with lipophilic thiol molecules is not so easy since the CTAB molecules on
the surface of GNRs are densely packed, which would make the thiol molecules
difficult to access the gold surface to bind on the gold surface via a strong covalent
Au-S linkage [28-32]. Not only that, the thiol molecules are attached onto the ends of
GNRs primarily during the exchanging process [35, 36] since their two ends are free
from CTAB and the thiol molecules would replace the lateral CTAB molecules with
the extension of time. Without doubt, the preparation of these hybrid materials
composed of GNR and organic molecules and tuning their properties would open the
door to more fascinating applications of GNRs in the area of materials.
Aggregation-induced emission (AIE) is a rapidly growing, fascinating and
challenging scientific area and has been intensively studied by Tang since 2001

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[37-40]. Tetraphenylethylene (TPE) derivatives with their notable AIE performance
have many applications [41-47]. In this work, a TPE derivatives containing
compound
10-(4-(1,2,2-triphenylvinyl)phenoxy)decane-1-thiol
(TPPT)
was
synthesized through a series of chemical reactions including McMurry reaction,
Williamson reaction and hydrolysis reaction. TPPT was used to protect the entire
surface of Gold nanorod via strong covalent S-Au bond for the first time (Fig. 1). The
prepared GNR hybrids were stable in the organic solvent such as tetrahydrofuran
(THF) without aggregation or decomposition and the successful modification on the
surface of Gold nanorods was demonstrated via UV-Vis, Raman and FT-IR spectra.
The gold nanorods were found to exhibit a quenching effect on the AIE fluorescence
emission of the TPPT molecule modified on their surfaces.
Fig. 1 Schematic preparation of TPPT monolayer protected gold nanorods (TGNRs).
Fig. 2 Synthesis route of compound TPPT.
2. Experimental
2.1 General information
Benzophenone, 4-hydroxybenzophenone, 1,10-dibromodecane, CTAB and all the
1
solvents were commercial available and used without further purification. H and 13C
NMR spectra were measured on a Brüker AV-400 spectrometer. The electrospray
ionization (ESI) high-resolution mass spectra were tested on a HP 5958 mass

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spectrometer. The UV-Vis absorption spectra were obtained on a Varian Cary 100
spectrometer (1-cm quartz cell was used). Transmission electron microscopy (TEM)
experiments were done using JEOL JEM-2100 equipment. For the TEM observation,
the samples in aqueous solution were dispersed on Cu grids pre-coated with thin
carbon film (Cu-400 CN) and then completely dried for analysis. The FT-IR spectra
were obtained on Nicolet 380 FT-IR spectrometer, the Raman spectra were carried
out on inVia Reflex (Renishaw) spectrometer.
2.2 Synthesis and preparation
2.2.1 Synthesis of TPPT (Fig. 2)
4-(1,2,2-triphenylvinyl)phenol (TPP) was conveniently synthesized via a McMurry
reaction in one step [43]. Zinc dust (2.18 g, 33.00 mmol), 4-hydroxy diphenyl ketone
(1.65 g, 8.32 mmol), and benzophenone (1.50 g, 8.23 mmol) were added into a
round-bottom flask in that order. The flask was then evacuated of air under vacuum
and flushed with dry nitrogen several times. Anhydrous THF (80 mL) was then added
into the flask, then the resultant mixture was cooled to 0 °C and TiCl4 (1.88 mL, 16.50
mmol) was slowly added via syringe. The reaction proceeded at room temperature for
half an hour, and then heated under refluxed for 8 h. K2CO3 solution (10 % aq.) was
employed to quench the reaction and the solution extracted with diethyl ether several
times. The organic layer was combined and washed with brine twice. The mixture
was dried with anhydrous sodium sulfate and then the solvent was removed. The
resulting residue was subjected to column chromatography using
dichloromethane/petroleum ether (v/v 4:1) to give the 4-(1,2,2-triphenylvinyl)phenol

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1
(1.40 g, 4.00 mmol, 50%, melting points: 225~226 °C). H NMR (400 MHz,
DMSO-d6) δ 9.36 (s, 1H), 7.10 (ddd, J = 14.1 Hz, 11.7 Hz, 6.8 Hz, 9H), 6.95 (dd, J =
13.5 Hz, 7.3 Hz, 6H), 6.74 (d, J = 7.6 Hz, 2H), 6.50 (d, J = 7.6 Hz, 2H).
TPP (1.00 g, 2.87 mmol), 1,10-dibromodecane (4.31 g, 14.4 mmol) and potassium
carbonate (0.59 g, 4.31 mmol) were added into a round-bottom flask, stirred and
refluxed in CH3CN (100 mL) under N2 atmosphere for 12 h, then cooled to room
temperature. The solution was filtered and the solvent was removed with rotavapor.
Afterwards the crude product was purified on a silica-gel column using petroleum
ether/dichloromethane
(v/v
500:1)
as
eluent
to
yield
(2-(4-((10-bromodecyl)oxy)phenyl)ethene-1,1,2-triyl)tribenzene (BDTB) as a white
1
solid (0.98 g, 1.73 mmol, 60.1%, melting point: 58~62 °C). H NMR (400 MHz,
CDCl3) δ 7.16 – 6.97 (m, 15H), 6.91 (d, J = 8.7 Hz, 2H), 6.62 (d, J = 8.7 Hz, 2H),
3.86 (t, J = 6.5 Hz, 2H), 3.41 (t, J = 6.9 Hz, 2 H), 1.91 – 1.80 (m, 2H), 1.78 – 1.66 (m,
2H), 1.42 (s, 4H), 1.30 (s, 8H). 13C NMR (101 MHz, CDCl3) δ 157.78 (s), 144.17 (d),
140.70 (s), 140.08 (s), 136.00 (s), 132.63 (s), 131.72 – 131.28 (m), 127.77 (d), 126.39
(d), 113.66 (s), 67.89 (s), 34.22 (s), 32.95 (s), 29.49 (t), 28.87 (s), 28.29 (s), 26.17 (s).
TOF-MS (Fig. S4): [M+H+]+=567.2263, found 567.2263. FT-IR (Fig. S5): The
asymmetrical stretching vibration of C-O-C was found at 1250 cm-1. The signal at
1610 cm-1 of BDTB was attributed to the stretching vibration of benzene ring. The
signals at 2926 cm-1 of CH2 stretching vibration was observed in spectra.
BDTB (0.82 g, 1.44 mmol) and potassium ethanethioate (0.75 g, 6.53 mmol) were
added to a flask containing CH3CN (100 mL). And then the mixture was cooled to

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room temperature after stirred for 12 h under N2 atmosphere. After the solvent being
removed under vacuo, the crude product was extracted with ethyl acetate/H2O and
purified on a silica-gel column using petroleum ether/dichloromethane (v/v 20:1) as
eluent to (10-(4-(1,2,2-triphenylvinyl)phenoxy)decyl)ethanethioate (TPET) as a white
1
solid (0.524g, 0.931 mmol, 65%, melting point: 80~81 °C ). H NMR (400 MHz,
CDCl3) 7.14 – 6.98 (m, 15H), 6.91 (d, J = 8.6 Hz, 2H), 6.62 (d, J = 8.7 Hz, 2H), 3.86
(t, J = 6.5 Hz, 2H), 2.86 (t, J = 7.3 Hz, 2H), 2.32 (s, 3H), 1.77 – 1.66 (m, 2H), 1.28 (s,
13
14H). C NMR (101 MHz, CDCl3) δ 196.28 (s), 157.79 (s), 144.18 (d), 140.71 (s),
140.07 (s), 135.99 (s), 132.63 (s), 131.73 – 131.28 (m), 127.77 (d), 126.39 (d), 113.67
(s), 67.91 (s), 30.81 (s), 29.81 – 29.12 (m), 28.93 (s), 26.18 (s). TOF-MS (Fig. S8):
[M+Na+]+=585.2798, found 585.2802. FT-IR (Fig. 4): The signals at 1652 cm-1 of
TPET was attributed to the stretching vibration of -C=O-. The stretching vibration of
benzene ring was found at 1635 cm-1. The signal at 2921 cm-1 of CH2 stretching
vibration was observed in spectra.
TPET (0.25 g, 0.45 mmol), hydrazine monohydrochloride (0.12 g, 1.78 mmol) and
sodium acetate (0.29 g, 3.55 mmol) were stirred together and refluxed in DMF (30
mL) for 12 h under N2 atmosphere and then cooled to room temperature. After the
solvent was removed under vacuo, the crude product was extracted with ethyl
acetate/H2O and purified on
a
silica-gel column using petroleum
ether/dichloromethane (v/v 20:1) as eluent to yield compound TPPT (0.18 g, 0.346
mmol, 72%). 1H NMR (400 MHz, CDCl3) δ 7.12 – 6.98 (m, 15H), 6.91 (d, J = 8.7 Hz,
2H), 6.61 (d, J = 8.7 Hz, 2H), 3.86 (t, J = 6.5 Hz, 2H), 2.68 (t, J = 7.4 Hz, 2H), 1.77 –

8
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1.62 (m, 4H), 1.44 – 1.25 (m, 13H).
2.2.2 Preparation of CTAB-GNRs
The CTAB bilayer coated GNRs were freshly prepared via the seed-mediated
growth method [26]. The method can be divided into two steps. The preparation of
gold seed is as following. 0.01 M solution of HAuCl4 (0.250 mL) was added into 0.10
M CTAB solution (7.500 mL) in a vial. The solutions were gently mixed and the vial
was lightly inverted several times. At this time, the solution showed bright
brown-yellow. Then 0.01 M ice-cold NaBH4 (0.600 mL) solution was added to the
above solution all at once, followed by rapid inversion mixing for 2 min. Care should
be taken to allow the produced gas escape in the mixing process. The solution
appeared a pale brown-yellow color. Then the vial was kept in a water bath
maintained at 25-30 °C for 2 hours. After that time, the seed solution was prepared
and could be used for a week.
The preparation of CTAB-GNRs is as following. 0.10 M CTAB solution (9.500
mL) was added into an empty vial, then 0.01 M of HAuCl4 (0.400 mL) and 0.01 M
AgNO3 (0.060 mL) solution were added into the vial one by one in that order. Then
the solution was gently mixed by the inversion. The solution developed into bright
brown-yellow. After that, 0.10 M AA solution (0.064 mL) was added into the above
solution. Once the AA solution was added, the solution became colorless. In the end,
seed solution (0.020 mL) was added, and mixed gently for 10 s. Then the vial was left
undisturbed and kept in a water bath maintaining at 25-30 °C for 3 hours.
2.2.3 Synthesis of organic-soluble TPPT monolayer-protected TGNRs

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At first, the solution of CTAB bilayer coated GNRs was centrifuged for 20 min at
a speed of 12000 rpm for several times to remove the excessive CTAB and other
impurities in the supernatant. Then the CTAB-GNRs were dispersed in ultrapure
water (2 mL) and dispersed evenly by sonication. Next, the aqueous solution of
CTAB-GNRs was added dropwise to a solution of 60 mg TPPT in THF (50 mL)
with stirring under the protection of nitrogen. After that, the reaction flask was coated
with silver paper and the color of the solution was purple. The reaction proceeded at
room temperature for 48 hours. Then, the resultant TGNRs solution was centrifuged
for 20 min at a speed of 12000 rpm three times to remove the excessive CTAB and
the TPPT molecules. The resulting TGNRs were re-dispersed in THF (50 mL) with
addition of excess TPPT and stirred for another 24 h. Similarly, the reaction
proceeded under N2 atmosphere and centrifuged for 20 min at a speed of 12000 rpm.
The above procedure was repeated several times to ensure that the GNRs were well
encapsulated with the TPPT molecules over the entire surface via the strong covalent
S-Au bonds. At last, the mixture was centrifuged and washed with THF several times
until there was no free TPPT molecules in the system of TPPT monolayer protected
TGNRs, which could be confirmed from that there was no UV absorption band of
TPPT molecule in the top layer solution. Then the TPPT monolayer protected
TGNRs hybrid was successfully prepared. The resulting TGNRs can readily disperse
in THF solution and without obvious aggregation, which was evidenced by
transmission electron microscopy (TEM) observation (Fig. S10).

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3. Results and discussion
3.1 Characterization of TPPT monolayer-protected TGNRs
The UV-Vis absorption spectra of TPPT, GNRs and TGNRs were employed to
confirm the successful preparation of TPPT monolayer-protected TGNRs. Fig. 3
shows the two typical absorption peaks at 252 nm and 315 nm corresponding to
TPPT and the two absorption peaks at 517 nm and 761 nm corresponding to the two
SPR absorption bands of GNRs. After the TPPT monolayer protected TGNRs
hybrid was successfully prepared, the two SPR absorption bands of GNRs had
red-shifts by 54 nm and 39 nm to 571 nm and 800 nm, respectively.
Fig. 3 Absorption spectra of aqueous CTAB-GNRs (black), TPPT (red, 1.0 × 10-5 M) and
TGNRs (blue).
The FT-IR spectrum of TPET was compared with that for TGNRs owing to the
instability of thiol molecules (TPPT). The FT-IR spectra of TPPT monolayer
protected TGNRs and TPET are shown in Fig. 4. The similar signals at 2921 cm-1 of
CH2 stretching vibration were observed in both spectra. The signals at 1435 cm-1 and
1506 cm-1 of TGNRs were attributed to the stretching vibration of a benzene ring. At
the same time, benzene ring showed a characteristic out-of-plane bending vibration at
467 cm-1. The signals at 1089 cm-1 of TGNRs were attributed to the asymmetrical
stretching vibration of C-O-C. The FT-IR spectra also revealed the typical stretching
vibration of C-S at 871 cm-1. The comparison of TGNRs and TPET in the FT-IR

11
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spectra above helped to prove the successful modification of TPPT monolayer on the
surface of GNRs.
Fig. 4 FT-IR spectra of TPET (blue) and TGNRs (red).
Fig. 5 Normalized Raman spectra of CTAB-GNRs (blue) and TGNRs (pink).
The successful modification of TPPT molecules on the surface of the GNRs via
strong covalent S-Au linkages could also be evidenced from Raman spectroscopic
data. As shown in Fig. 5, the Raman band of CTAB-GNRs at 162 cm-1 was attributed
to Au-Br band. As expected, the characteristic Au-Br band disappeared in the Raman
spectrum of the TGNRs, accompanied with the appearance of new Raman peaks
especially corresponding to the Au-S band at around 126 cm-1 [30, 32]. The change of
the spectra indicated that the existence of TPPT molecular monolayer around the
surface of gold nanorods and no CTAB molecules remained on the surface of the
TGNRs.
Fig. 6 Fluorescence intensity changes by gradually changing the ratio of THF and H2O in the
solution of (a) TPPT and (b) TGNRs (Excited at 310nm).
It is well known to all that the tetraphenylethylene (TPE) derivatives have a
remarkable aggregation-induced emission (AIE) property. As seen in Fig. 6a, the

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fluorescence emission intensity of TPPT at 365 nm declined gradually when
increasing the proportion of water in the solution of THF. While a new fluorescence
emission band at 476 nm appeared when dissolving the TPPT in pure water. This was
a typical AIE emission band of the TPPB, which was induced via a restricted
intramolecular rotation (RIR) mechanism of TPPT molecules. Specifically, the
hydrophobic TPPT molecule aggregated in water, restricting the rotation of the
phenyl units of TPPT thus inducing the strong fluorescence emission with a red-shift
by about 111nm [37-40]. The emission band of TGNR (Fig. 6b) at 365 nm
corresponding to TPPT showed a blue-shift to 352 nm, as a result of the formation of
TGNR. The same experimental methods were conducted to the TGNR, the
fluorescence intensity of TPPT at 352 nm declined gradually when increasing the
proportion of water in the solution of THF, which was similar with the situation of
TPPT. The only difference was that there was no any obvious AIE fluorescence
emission band in the range of longer wavelength of the fluorescence spectrum when
dissolving the TGNR into the pure water, the reason of which might be that the GNR
quenched the AIE (emission band at 476 nm) of TPPT fluorophore connected on its
surface due to the electron transfer interaction from the Gold nanorods to the TPPT
molecules very close on their surface [48].
4. Conclusion
In summary, we have successfully fabricated novel tetraphenylethylene (TPE)
derivative monolayer-protected Gold nanorods TGNRs on the entire surface of GNRs
via S-Au covalent linkages. The hybrid TGNRs were stable in organic solvents and

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without obvious aggregation. Besides, this hybrid showed quenching the AIE
fluorescence emission of the tetraphenylethylene (TPE) derivatives. Construction of
hybrid materials involving Gold nanorods remains challenging and employing these
hybrid materials as smart materials are in progress.
Acknowledgments
This work is financially supported by NSFC (21421004, 21476075 and 21272072),
and Shanghai Pujiang Program (13PJD011). T. Cao and D. Li contribute equally to
this work.
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Figure Captions:
Fig. 1 Schematic preparation of TPPT monolayer protected gold nanorods (TGNRs).
Fig. 2 Synthesis route of compound TPPT.
Fig. 3 Absorption spectra of aqueous CTAB-GNRs (black), TPPT (red, 1.0 × 10-5 M) and
TGNRs (blue).
Fig. 4 FT-IR spectra of TPET (blue) and TGNRs (red).
Fig. 5 Normalized Raman spectra of CTAB-GNRs (blue) and TGNRs (pink).
Fig. 6 Fluorescence intensity changes by gradually changing the ratio of THF and H2O in the
solution of (a) TPPT and (b) TGNRs (Excited at 310 nm).

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Fig. 1 Schematic preparation of TPPT monolayer protected gold nanorods (TGNRs).
Fig. 2 Synthesis route of compound TPPT.

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Fig. 3 Absorption spectra of aqueous CTAB-GNRs (black), TPPT (red, 1.0 × 10-5 M) and
TGNRs (blue).
Fig. 4 FT-IR spectra of TPET (blue) and TGNRs (red).

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Fig. 5 Normalized Raman spectra of CTAB-GNRs (blue) and TGNRs (pink).
Fig. 6 Fluorescence intensity changes by gradually changing the ratio of THF and H2O in the
solution of (a) TPPT and (b) TGNRs (Excited at 310 nm).

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Supplementary Information
Preparation
and
Properties
of
Organo-soluble
Tetraphenylethylene Monolayer-Protected Gold Nanorods
Tiantian Cao, Dengfeng Li, Xuyang Yao, Yikai Xu and Xiang Ma*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, P. R. China
Tel: (+86) 21-6425-2288, Fax: (+86) 21-6425-2288, E-mail: maxiang@ecust.edu.cn
Fig. S1 1H NMR spectrum of TPP in DMSO-d6.

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Fig. S2 1H NMR spectrum of BDTB in CDCl3.
Fig. S3 13C NMR spectrum of BDTB in CDCl3.

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Fig. S4 TOF-MS of BDTB：[M+H+]+=567.2263, found 567.2263.
Fig. S5 FT-IR spectra of BDTB.

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Fig. S6 1H NMR spectrum of TPET in CDCl3.
Fig. S7 13C NMR spectrum of TPET in CDCl3.

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Fig. S8 TOF-MS of TPET: [M+Na+]+=585.2803, found 585.2802.
Fig. S9 1H NMR spectrum of compound TPPT in CDCl3.