Surface Modifi cation of Gold Nanorods
by Grafting Fluorene-Based Conjugated
Copolymers Containing Thiol-Pendants
Duryodhan Sahu,
Hsuan-Chih
Chu,
Po-Jen
Yang,
and
Hong-Cheu
Lin *
Two conjugated surfactants of monomer TF and polymer PTF with pendant thiol groups are
synthesized for surface modifi cations of gold nanorods (GNRs). Cetyltrimethylammonium
bro-mide (CTAB)-GNRs (without surface-modifi cation) in aqueous solutions are prepared, and the
original CTAB surfactant is replaced with TF and PTF due to
their thiol-gold grafting reactions. Surface-modifi ed TF-GNRs
and PTF-GNRs in THF are obtained. UV-visible spectra provide
the grafting evidence of TF and PTF. Compared with TF-GNRs,
the PTF-GNRs show better solubility and stability against
aggregations of nanorods in THF. TEM and XPS confi rm the
existence of binding bonds between the sulfur groups of PTF
and the gold surface in PTF-GNRs. Compared with polymer
surfactant (PTF), the faster PL emission decay in PTF-GNRs
match well with the PL quenching of PTF-GNRs. Surface
modi-fi cations are verimodi-fi ed by UV-visible, TEM, XPS, PL and lifetime
measurements.
D. Sahu , H.-C. Chu , P.-J. Yang , H.-C. Lin
Department of Materials Science and Engineering,
National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan 300, ROC
E-mail: [email protected]
resonance bands could be tuned from the visible to the near infrared (NIR) range. Previously, many research groups have reported wet chemical synthetic routes to GNRs, [ 11 , 12 ] in which the most commonly used method is a rod-shaped micelle growth via the surfactant cetyltrimethylammo-nium bromide (CTAB). Using CTAB as a bilayer surfactant on the surface of GNRs makes them become fairly soluble in the aqueous phase. However, the hydrophilic nature of CTAB surfactants [ 13 , 14 ] limits the further applications of GNRs. Normally, followed by centrifugation, GNRs are purifi ed by several cycles of suspension and then isolated in the precipitate, where the excess CTAB can be removed in the supernatant. However, if the CTAB surfactants could be fully removed from the solution, GNRs would lose their stability in colloidal dispersions and form aggregates. Therefore, considerable interest has arisen in developing techniques to facilitate surface modifi cations of GNRs to disperse GNRs well in nanocomposites for further applica-tions. Continuous efforts to prepare functional GNRs with novel surfactants, such as organo-soluble photoresponsive azo-thiol monolayer-surfactants, have been reported, with GNRs becoming very stable in both organic solvents and in the solid state without aggregation or decomposition. [ 15 ]
1. Introduction
Recently, nanocomposites containing metallic nanoparti-cles (or nanorods) and organic molecules have emerged as an exciting research area for their applications in optoelec-tronics, sensors and light energy conversions. [ 1–5 ] Hybrid materials (i.e. nanocomposites) composed of organics and nanostructural inorganics with better interactions could be produced by modifi cations of the functionalities on sur-factants. Modifi cation methods on the surfaces of metal nanostructures by organic molecules were signifi cantly developed for the design of biological sensors and opto-electronic devices. [ 6–8 ] To promote the application potential of nanocomposites, the specifi c surface plasmon resonance (SPR) bands of gold nanorods (GNRs) could be improved by increasing the aspect ratios of GNRs properly. [ 9 , 10 ] The lon-gitudinal plasmon band of two distinct surface plasmon
of the polymer layer, and higher stability than polyelectro-lyte materials were highly benefi cial. In the past decade, fl uorene-based conjugated polymers have been demon-strated as a very outstanding class of electron donors with high hole mobilities. In addition, polyfl uorenes were also synthesized and investigated for applications in the design of multifunctional chemical sensors and circuit compo-nents (such as single electron transistors). [ 21–24 ] The thiol end groups provide covalent attachment of the thiol deriv-ative of poly mers to gold surfaces. [ 25 ] Recently, the surface modifi cation of gold surfaces and nanospheres using well-defi ned homopolymers and copolymers prepared with thiol functional groups has been reported. [ 26 , 27 ] Herein, the surface modifi cation of GNRs by grafting a conjugated polymer, i.e. a thiol derivative of poly(fl uorene) (PTF), is reported. The fl uorene units (with and without thiol pen-dants) and their polymer were synthesized by following the synthetic steps in Scheme 1 . The pendant alky chains with In previous studies, using polymers to modify the surfaces
on gold nanorods were reported via a layer-by-layer (LbL) technique with poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) alternately. [ 16 ] The method of LbL assembly was a simple way to modify the surfaces of gold nanorods through the alternate adsorptions of cationic and anionic polymers in solutions. The bonding forces of the polymers and nano-rods on the interface between the two layers are non-covalent, which can lead to instabilities of the alternative polymeric surfactants. [ 17 ] The limitation of this LbL tech-nique is that anionic and cationic polyelectrolytes are required to modify the nanorods, which limits the types of functionalities and LbL systems that can be incorporated into the nanorods. As a result of this, several reports on the surface modifi cation of GNRs with thiol derivatives of polymers were reported, [ 18–20 ] in which stronger binding to the surface of nanorods, better control over the thickness
and 4 × 10 − 3 M silver nitrate) was gently
mixed with 5.0 mL of HAuCl 4 (1 × 10 − 3 M ). The
solution at this stage appeared bright brown-yellow, then became colorless upon addition of 70 μ L of ascorbic acid (0.1 M ). Finally, 12 μ L of a seed solution was injected, and the reac-tion mixture was gently stirred and kept at 25 ° C. Upon cooling, excess CTAB was crystal-lized and separated by fi ltration. The fi ltrate contained monodisperse Au nanorods and was used directly in subsequent studies. The formation of gold nanorods and their aspect ratios were confi rmed by UV-visible absorp-tion spectra and TEM analyses.
2.2. Measurements and Characterization
1 H NMR spectra were recorded on a Varian
Unity 300 MHz spectrometer using CDCl 3
solvent. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. The molecular weight and polydispersity data were determined by gel permea-tion chromatography (GPC) using polystyrene as a standard. UV-visible absorption spectra were recorded in water solution using a Jasco V-670 spectrophotometer, and photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectropho-tometer. Measurements were performed in quartz cells with a 10 mm optical path. The PL spectra were measured at the exci-tonic emission. Transmission electron microscopy (TEM) anal-yses were performed using a JEOL 2100 electron microscope with an acceleration voltage of 200 keV. A drop of a dilute solution of gold nanorods was allowed to dry on a carbon coated copper grid and then probed using TEM imaging. The samples were prepared from solutions collected on copper TEM grids (200 mesh/carbon fi lms) and allowing the solvent to evaporate. X-ray photoelectron spectra (XPS) were obtained using an ESCA ULVAC-PHI spectro-meter in constant analyzer energy mode with a pass energy of 50 eV and Al-K α (280 eV) radiation as the excitation source. XPS
analysis was performed at room temperature and at pressures under 5 × 10 − 10 Torr. The take-off angle used in XPS was 45 ° , and
the sputter ratio was 8.5 nm min − 1 . Time-resolved
photolumines-cence spectra were utilized to measure lifetime using a home-built single photon counting system. Excitation was performed using a 375 nm diode laser (Picoquant PDL-200, 50 ps fwhm, 2 MHz). The signals collected at the excitonic emissions of solu-tions ( λ = 480 nm) were connected to a time-correlated single photon counting card (TCSPC, Picoquant Timeharp 200). The emission decay data were analyzed with bi-exponential kinetics in which two decay components were derived.
2.3. Synthesis of Monomer TF
2.3.1. 2-Bromo-9,9′-bis(6,6″-bromohexyl)fl uorene (1)
2,7-Bromofl uorene (1 × 10 − 3 mol) was added to a mixture of
aqueous sodium hydroxide (50 mL, 50%), tetrabutylammonium bromide (1 × 10 − 3 mol) and 1,6-dibromohexane (10 × 10 − 3 mol)
at 75 ° C. After 4 h, the mixture was cooled to room temperature.
thiol functional groups could proceed the surfactant place exchange reaction. The thiol pendants of the polymeric surfactants could provide covalent attachments to the gold surfaces. As shown in Figure 1 , the binding of conjugated polymer PTF on the metal surface of GNRs resulted in the quenching effect of excited PTF by the process of energy transfers to GNRs, which are the main deactivation chan-nels for the excited molecules on the metal surfaces. [ 28 , 29 ] Easier surface modifi cations on gold nanorods, along with higher effi ciencies and reproducibility of the surfactant exchange reactions, make our approach promising. The generality of this approach opens up new nanomaterials for the conjunction of different organics (or polymers) and nanostructural inorganics into nanocomposites for dif-ferent applications in nanomaterials.
2. Experimental Section
2.1. Materials
All chemicals were purchased from Aldrich and used without further purifi cation. Ultrapure fi ltered water with a resistivity of 18.2 M Ω was used in all experiments. Compound 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene (3) was synthesized according to known literature procedures. [ 30 ]
The conjugated polymer with two side-chain thiol groups was also synthesized following previous a literature method. [ 31 ] The
polymerization was performed by a Suzuki coupling reaction. CTAB-GNRs were synthesized by the seed-mediated growth method (via HAuCl 4 ). [ 32 , 33 ] The gold nanorods were prepared by
a seed-mediated growth method with slight modifi cations. Spe-cifi cally, the spherical gold nanoparticles used as seeds were pre-pared fi rst. The solutions were mixed with 5.0 mL of an aqueous HAuCl 4 (0.5 × 10 − 3 M ) and 5.0 mL of CTAB (0.2 M ). 0.6 mL of an
aqueous ice-cold NaBH 4 solution (0.01 M ) was added and
vigor-ously stirred. Then, 5.0 mL of an aqueous solution (0.2 M CTAB
Figure 1 . Surface modifi cation processes of gold nanorods GNRs by monomer TF and
After extraction with CH 2 Cl 2 , the combined organic layers were
washed successively with water, aqueous HCl (0.5 M ), water and brine, and then dried over MgSO 4 . The excess 1,6-dibromohexane
solvent was removed by vacuum distillation, and the residue was purifi ed by silica gel column chromatography using hexane and dichloromethane (10:1) as the solvent. Compound 1 was obtained as a white solid in a yield of 59%. 1 H NMR (CDCl
3 , δ ): 7.52 (m, 2H),
7.45 (m, 2H), 7.43 (m, 2H), 3.26-3.28 (t, 4H), 1.91-1.99 (m, 4H), 1.54-1.71 (m, 4H), 1.07-1.21 (m, 8H), 0.60-0.65 (m, 4H).
2.3.2. Thioacetic Acid 6-[9-(6-acetylsulfanyl-hexyl)-2,7-dibromo-9H-fl uoren-9-yl]-hexyl ester (2)
The bromide (1) (1 × 10 − 3 mol) and potassium thioacetate (2.5 ×
10 − 3 mol) were stirred in 25 mL of acetone and 25 mL of ethanol to
refl ux for 24 h, and then the solvent of the mixture solution was removed on a rotary evaporator. After extraction with CH 2 Cl 2 ,
the organic layer was washed with water several times and dried over MgSO 4 . The resulting thioacetate was used without purifi
ca-tion. The yield was 92%. 1 H NMR (CDCl
3 , δ ): 7.52 (m, 2H), 7.45 (m,
2H), 7.43 (m, 2H), 2.85 (t, 4H), 2.28 (s, 6H), 1.91-1.98 (m, 4H), 1.54-1.65 (m, 4H), 1.07-1.22 (m, 8H), 0.61-0.65 (m, 4H).
2.3.3. 6-[2,7-Dibromo-9-(6-mercapto-hexyl)-9H-fl uoren-9-yl]-hexane-1-thiol (TF)
The thioacetate (2) (1 × 10 − 3 mol) was dissolved in 50 mL of absolute
ethanol and the solution was degassed for 1 h with nitrogen. After 1 h, potassium carbonate (K 2 CO 3 ) was added to the fl ask and the
mixture was gently warmed to ∼ 35 ° C. After stirring under N 2 for
4 h, the solution in the fl ask was poured into 50 mL of an aqueous solution of saturated ammonium chloride and extracted with CH 2 Cl 2 . The organic layer was washed with water 5 times, dried
over MgSO 4 and concentrated. The residue was purifi ed by silica gel
column chromatography using hexane and dichloromethane (2:3) as the solvent to give a light gray solid in a yield of 68%. 1 H NMR
(CDCl 3 , δ ): 7.52 (m, 2H), 7.45 (m, 2H), 7.43 (m, 2H), 2.51 (quin, 4H),
1.91-1.98 (m, 4H), 1.02-1.65 (m, 14H), 0.61-0.65 (m, 4H); Anal. calcd for [C 25 H 32 Br 2 S 2 ]: C 53.96, H 5.80; found: C 54.45, H, 5.76.
2.4. Synthesis of Polymer PTF
The polymerization of compound 4 was carried out through pal-ladium (0)-catalyzed Suzuki coupling reactions. Into a 50 mL two-neck fl ask, 1 equiv. of monomer 2 and 1 equiv. of compound 3 were added in 10 mL of THF. The Pd(0) complex, Pd{P(p-tolyl)3}3 (1 mol%), was transferred into the mixture in a dry environment. Then, 2 M aqueous potassium carbonate and the phase transfer
catalyst, i.e. aliquat 336 (several drops), were subsequently trans-ferred via cannulas into the previous mixture under nitrogen. The reaction mixture was stirred at 75 ° C for 3 d. The reaction mixture was cooled to 50 ° C and added slowly into a vigorously stirred mixture of 300 mL methanol. The polymers were collected by fi l-tration and repreciptation from methanol. The poly mer with thio-acetate (2 × 10 − 3 mol) was dissolved in 50 mL of THF, and sodium
hydroxide solution (10 mL, 50 wt%) was added under nitrogen and stirred overnight. The reaction mixture was neutralized by adding acetic acid, and then poured into 25 mL of water. The organic layer was extracted with CH 2 Cl 2 and washed with water
5 times, dried over MgSO 4 and concentrated to give a light gray
solid. According to literature procedures, [ 30 ] the thiol group is one
of the functional groups that could not be tolerated in the Suzuki reaction. We had to use a protective group, such as thioacetate, to polymerize fi rst and then hydrolyze to PTF. The number-average molecular weight of PTF was 8420 and the polydispersity index (PDI) was 1.2. The yield was 49%. 1 H NMR (CDCl
3 , δ ): 7.62-7.38 (m,
12H), 2.51-2.45(m, 4H), 2.20-1.91 (m, 4H), 1.83-0.16 (m, 38H).
2.5. Surface Functionalization of Gold Nanorods with Thiol-Terminated Monomer TF and Polymer PTF
Place-exchange methods have been widely employed to function-alize gold nanoparticles with thiol derivatives of organic mole-cules. [ 18 , 25 ] The surface of gold nanorods can be modifi ed with the
thiol-terminated monomer TF from the original CTAB surfactant. The phase exchange from the aqueous phase to organic solvent was followed by the following steps. First, 10 mL of aqueous gold nanorod solution and 10 mL of 25 × 10 − 3 M TF in toluene were
mixed with vigorous stirring. Then, 15 mL of acetone were added. The mixture was reacted at room temperature with constant stir-ring for 5 h. We extracted the upper layer solution of the mixture. The pink color was transferred from the lower aqueous phase to the upper toluene solvent, and the upper layer was collected. The formation of covalent bonds with thiol groups facilitated the phase transfer of gold nanorods. The modifi ed gold nanorods could be easily dispersed in THF. GNRs from the aqueous phase were directly extracted by thiol-group-ligands in a mixture of 1:1:2 (water:thiol:acetone) stirring for 3 h. The gold nanorods were transferred from water into toluene, and gold nanorods could be dispersed in THF. The same method was used for the surface modication of gold nanorods GNRs with polymer PTF.
3. Results and Discussion
3.1. GNRs Surface-Modifi ed with Monomer TF and Polymer PTF
As shown in Figure 2 , the UV-visible absorption spec-trum of CTAB-GNRs (without surface-modifi cation) in the aqueous solution exhibited transverse and longitu-dinal surface plasmon resonances at 512 nm and 778 nm, respectively. In addition, as revealed by the TEM image in Figure 3 a, the gold nanorods possessed an average length and diameter of 25.0 and 7.0 nm, respectively, featuring an average aspect ratio of 3.6. The thiol derivatives of TF and PTF were immobilized onto the surface of the gold nano-rods using a ligand-exchange technique, so the original CTAB surfactant on the gold surface could be replaced by thiol groups due to the thiol-gold grafting reactions of TF and PTF (see Figure 1 ). The binding of the thiol derivatives onto the gold nanorod surface was characterized by TEM and UV-visible measurements. As we know, it has previ-ously been proven that UV-visible spectroscopy is a pow-erful tool to monitor the coating of gold nanorods with polyelectrolyte multilayers. [ 15–17 ] The UV-visible absorption
due to the transverse surface plasmon band of the gold nanorods upon grafting of monomer TF and polymer PTF to the nanorod surface. Due to the modifi ca-tions on the surfaces of GNRs, these shifts in adsorption maxima were attributed to changes in the local refractive indexes at the surfaces of the rods, [ 32 ] resulting in increases in the local dielectric function values. Moreover, it shows that the red shifts in the absorption wavelengths for the transverse surface plasmon bands are associated with the surfactants adsorbed on the nanorod surfaces. In addition, the interactions between gold nanorods and TF and PTF molecules resulted in damp-ening and broaddamp-ening of the plasmon absorption bands of longitudinal absorp-tions, which are highly sensitive to the surfactant type (i.e. monomer TF and polymer PTF). For example, gold nano-rods surface-modifi ed with polymer PTF (i.e. PTF-GNRs) have narrower and higher absorbance in contrast to those surface-modifi ed with monomer TF (i.e. TF-GNRs). Compared with CTAB-GNRs, the longitudinal absorption of TF-GNRs (sur-face-modifi ed with monomer) possessed the largest red-shifted absorption from 778 nm to 818 nm. In addition, the longitudinal absorption of PTF-GNRs (surface-modifi ed with polymer) also showed a red-shifted absorption to a lesser extent at 803 nm. The characteristic red-shifted spectra in the transverse and longitudinal plasmon absorp-tions correspond well with the observaabsorp-tions of previous research groups, [ 33 ] who demonstrated that the increase in the wavelength of the surface plasmon band matched the nanometer thickness of different surfactants adsorbed on the surfaces of GNRs. It should also be mentioned that the longitudinal plasmon absorptions after surface-modifi ca-tion appear to have broader absorpca-tions with less absorp-tion intensities, which indicates that aggregaabsorp-tions of nanorods do occur upon grafting of thiol derivatives (such as monomer TF and polymer PTF) to the surfaces of nano-rods. To evaluate the contributions from the monomer and polymer surfactants, the UV-visible absorption spectra of TF-GNRs and PTF-GNRs were compared after surface-modifi cation. In TF-GNRs, the transverse surface plasmon band of the nanorods red shifted to 530 nm, and the lon-gitudinal surface plasmon band tranformed into a broader band absorption, which indicates a clear aggregation of nanorods. Because PTF is a fl uroene-based copolymer con-taining pendant thiol groups and alkyl side-chains, upon grafting PTF on the surfaces of GNRs, the resulting surface-modifi ed gold nanorods PTF-GNRs showed better solu-bility and stasolu-bility against their aggregations of nanorods spectrum of CTAB-GNRs illustrates that the absorption
maximum for the transverse surface plasmon band of the ungrafted nanorods occurred at a wavelength of 512 nm in Figure 2 . When gold nanorods (GNRs) are further
surface-modifi ed with monomer TF and polymer PTF, Figure 2
shows small red shifts in the transverse surface plasmon bands of 528 nm and 530 nm, respectively. These phe-nomena demonstrate that the red-shifted absorptions are
400 500 600 700 800 900 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Abso rba nce (a . u.) Wavelength (nm) CTAB-GNRs TF-GNRs PTF-GNRs
Figure 2 . UV-visible absorption spectra of CTAB-GNRs (in aqueous solution), TF-GNRs (in
THF solution) and PTF-GNRs (in THF solution).
Figure 3 . Photographs of GNRs dispersed in THF after place
exchange reaction: TF-GNRs (a); PTFs-GNRs in THF (b); CTAB-GNRs in aqueous solution (c).
in THF due to the stronger longitudinal surface plasmon band. Figure 3 a, 3 b and 3 c show photographs of TF-GNRs and PTF-GNRs dissolved in THF and CTAB-GNRs dispersed in aqueous solution, respectively. The colors of those solu-tions could be also observed clearly due to the surfactant exchanged reaction.
The red-shifted phenomena in the absorption wave-lengths upon the coating of nanorods with thiol deriva-tives (including monomer TF and polymer PTF) could also be investigated by transmission electron microscopy (TEM). The TEM micrograph of CTAB-GNRs in Figure 4 a indicates that the surfactants of CTAB attached to the sur-faces of the nanorods are too thin to be observed. How-ever, as shown in Figure 4 b, the TEM image of PTF-GNRs verifi es that polymer surfactants (PTF) with an average thickness of 1–3 nm were grafted to the surfaces of GNRs in layered forms. Since non-grafted surfactants (PTF) on the surfaces of GNRs could be removed by numerous THF washing and centrifugation cycles, the layered polymer (PTF) of PTF-GNRs detected by TEM should be covalently attached to the surface of the nanorods (GNRs), which was also confi rmed by the previous results of UV-visible spectra with red-shifted characteristic absorptions. TEM micrographs demonstrated that PTF as a new surfactant for GNRs also facilitated them to disperse well in THF solutions. The TEM results provide another source of evi-dence of PTF-GNRs in which the surfaces of gold nano-rods (GNRs) were modifi ed with PTF. Therefore, the TEM images confi rm the red-shifted UV-visible absorptions, which also provide direct evidence that the thiol deriva-tives are covalently attached to the surfaces of GNRs. To demonstrate the presence of thiol pendant groups in polymer PTF is vital for their covalent attachments to the gold nanorods. Hence, due to weaker dipole interactions
of polymer surfactants with gold sur-faces, it has previously been shown that there is a blue-shift in the transverse surface plasmon band of nanoparti-cles if polymer unwrapping occurs on gold surfaces. [ 34 ] Even after repeated washing of gold surfaces with a good solvent (such as THF) to remove the unreacted polymer surfactants on gold surfaces, there was no further change in the red-shifted absorptions on the surface-modifi ed gold nanorods, which illustrates that polymer unwrapping from the surface of the nanorods does not occur.
The binding bonds for sulfur groups of polymer PTF on the gold surface could be determined by X-ray photoelectron spectroscopy (XPS). Moreover, after surface-modifi cation of GNRs by polymer PTF, the presence of a sulfur signal detected by XPS meas-urements gives valuable information about the chemical bonding of the surfactant head (S) to the gold surface. The XPS spectrum of PTF-GNRs is shown in Figure 5 , which revealed the presence of sulfur from polymer surfactants (PTF) and the absence of nitrogen from CTAB surfactants on the surfaces of gold nanorods. The binding bond for the sulfur group on the gold surface could be determined by XPS. In fact, the S2p core level region for the alkanethiol chain on a variety of metals can be fi tted with different components. [ 35 , 36 ] The S2p
3/2 peak could be fi tted with two components, with average binding energy values of 162 and 163–164 eV, as observed in Figure 5 . The inset of Figure 5 , with a higher resolution of XPS spectrum, shows that each component consists of a doublet with spin–orbit
Figure 4 . TEM images of (a) CTAB-GNRs and (b) PTF-GNRs after the surface modifi cation.
Figure 5 . XPS spectrum of surface-modifi ed gold nanorods
PTF-GNRs. Inset: higher resolution of the sulfur component com-prising a doublet with spin–orbit splitting.
or electron transfer pathways dominate as the gold nano-structures are on the nanoscale. [ 16 , 31 ] In addition, the ability to accept and store electrons (i.e. electron transfer) in noble nanoparticles has been demonstrated. [ 40–42 ] The other pathway for the excited-state deactivation involves direct energy transfer to metal nanoparticles. The results of PL and lifetime analyses demonstrated that the deactivation of the excited chromophore (PTF) in surface-modifi ed gold nanorods PTF-GNRs could occur via energy transfer proc-esses to gold nanorods GNRs.
4. Conclusion
In conclusion, two conjugated surfactants of monomer TF and polymer PTF with pendant thiol groups were suc-cessfully synthesized for surface-modifi cations of gold nanorods GNRs. A seed-mediated process to produce gold splitting of 1.2 eV. The component at 162 eV is usually the
most important one, which is related to S chemisorbed on the Au surface through a thiolate bond. [ 37 ] Thus, the XPS result further confi rms the successful replacement of CTAB surfactants with polymer surfactants (PTF) on gold
nanorods. Although thiol-bearing conjugated poly mer
may function as a crosslinker (via disulfi de bonds among PTF) to crosslink the gold nanorods, as illustrated in Figure 3 b, the crosslinking density is not high enough to let PTF-GNRs precipitate out from the solution due to the major reaction of thio groups with GNRs. This can also be proven by the TEM image of PTF-GNRs in Figure 4 b, which demonstrated thin fi lm coatings of polymers on GNRs. However, this possible and minor crosslinking den-sity of PTF-GNRs (caused by disulfi de bonds among PTF) will not affect the solubility of PTF-GNRs in organic sol-vents as well as the miscibility of PTF-GNRs in polymer composites .
3.2. PL and Lifetime Analyses of Surface-Modifi ed Gold Nanorods (PTF-GNRs)
To explore the excited-state interactions of polymer PTF with gold nanorods GNRs, the emission behavior of surface-modifi ed gold nanorods PTF-GNRs was monitored by PL spectra and lifetime analyses. As shown in Figure 6 a, where the PL spectra are excited at concentrations of the fl uorene moieties with the same absorption intensity in both PTF and PTF-GNRs, in contrast to a stronger PL sion of polymer surfactant (PTF) at 423 nm, the PL emis-sion of surface-modifi ed gold nanorods PTF-GNRs was quenched due to the energy transfer of polymer surfactant (PTF) to gold nanorods GNRs. Previous studies also reported that chromophores bound to gold nanoparticles under-went excited-state deactivation by energy or electron transfer processes. [ 38 , 39 ] The excited polymer PTF emission at 423 nm was further analyzed by comparing the PL emis-sion decay (lifetime) of polymer surfactant (PTF) with that of surface-modifi ed gold nanorods PTF-GNRs. As shown in Figure 6 b, polymer PTF shows a mono-exponential fl uo-rescence decay with a lifetime of 3.2 ns in THF. The emis-sion decay of surface-modifi ed gold nanorods PTF-GNRs exhibited an additional fast deactivation pathway to have a much shorter lifetime of 0.42 ns. The fl uorescence decay of PTF-GNRs was analyzed by a bi-exponential kinetic fi t, and the slower component had a lifetime (2.7 ns) similar to that of unbound polymer PTF. The faster PL emission decay component of surface-modifi ed gold nanorods PTF-GNRs matched well with the excited-state quenching in its PL emission spectrum. On the other hand, the slower PL decay component of PTF-GNRs represented the unquenched PL component of the bound polymer PTF. As discussed in earlier studies, smaller metal particles cannot quench all the bound chromophores. It was reported that the energy
Figure 6 . (a) PL spectra (excited at the maximum absorption 379 nm) of polymer PTF and surface-modifi ed gold nanorods PTF-GNRs in THF, which are excited at the concentrations of the fl uorene moieties with the same absorption intensity in both PTF and PTF-GNRs. (b) PL lifetime spectra of polymer PTF and surface-modifi ed gold nanorods PTF-GNRs.
350 400 450 500 550 600 0 1000 2000 3000 4000 5000 6000 7000 (a) PL In te ns ity (a . u. ) Wavelength (nm) PTF PTF-GNRs 0 1 2 3 4 5 6 7 8 100 1000 10000 PTF PTF-GNRs Cou n ts ( a . u.) Delay time (ns) (b)
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nanorods CTAB-GNRs (without surface-modifi cation) in aqueous solutions was employed, in which the original CTAB surfactant on the gold surface could be replaced with TF and PTF due to their thiol-gold grafting reactions. Then, gold nanorods surface-modifi ed with monomer TF and polymer PTF (i.e. TF-GNRs and PTF-GNRs, respectively) were obtained and well dispersed in THF. After surface modifi cation, due to consistent red-shifts in the absorp-tion maxima of surface plasmon bands, both UV-visible spectra of TF-GNRs and PTF-GNRs provided evidence of the grafting of TF and PTF to the surfaces of gold nanorods GNRs. Compared with TF-GNRs, the resulting PTF-GNRs (gold nanorods surface-modifi ed with polymer) showed better solubility and stability against the aggregations of nanorods in THF due to the stronger longitudinal surface plasmon band. Moreover, both TEM and XPS analyses con-fi rmed the binding bonds for sulfur groups of polymer PTF on the gold surface in surface-modifi ed gold nanorods PTF-GNRs. The PL emission of surface-modifi ed gold nano-rods PTF-GNRs was quenched due to the energy transfer of polymer surfactant (PTF) to gold nanorods GNRs. Compared with polymer surfactant (PTF), the faster PL emission decay (with a shorter lifetime of 0.42 ns) in surface-modifi ed gold nanorods PTF-GNRs matched well with the excited-state quenching in its PL emission spectrum. Finally, surface modifi cations of gold nanorods by grafting fl uorene-based conjugated copolymers (containing thiol-pendants to react with gold surfaces) were newly developed and verifi ed by UV-visible spectroscopy, TEM, XPS, PL and lifetime meas-urements in this study.
Acknowledgements : We acknowledge the National Center for High-performance Computing for computer time and facilities. Financial support for this project was provided by the National Science Council of Taiwan (ROC) through NSC 99-2113-M-009-006-MY2 and National Chiao Tung University through 97W807.
Received: October 3, 2011 ; Revised: December 6, 2012; Published online: July 13, 2012; DOI: 10.1002/macp.201100550
Keywords: gold nanorod; surface modifi cation; conjugated polymer; fl uorine; life-time
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