Surfactant Selective Synthesis of Gold Nanowires by Using a DPPC

UniVersity of Western Ontario, 339 Windermere Road, London, ON, Canada N6A ... Nanotechnology, Edmonton, Alberta, Canada, and Department of Chemistry ...
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J. Phys. Chem. C 2007, 111, 5932-5940

Surfactant Selective Synthesis of Gold Nanowires by Using a DPPC-Surfactant Mixture as a Capping Agent at Ambient Conditions Mandeep Singh Bakshi,*,†,‡,§,| Gurpreet Kaur,| Pankaj Thakur,|,⊥ Tarlok Singh Banipal,⊥ Fred Possmayer,†,‡ and Nils O. Petersen‡,§,# Department of Obstetrics and Gynaecology, Department of Biochemistry, and Department of Chemistry, UniVersity of Western Ontario, 339 Windermere Road, London, ON, Canada N6A 5A5, National Institute for Nanotechnology, Edmonton, Alberta, Canada, and Department of Chemistry and Department of Applied Chemistry, Guru Nanak DeV UniVersity, Amritsar 143005, Punjab, India ReceiVed: January 29, 2007; In Final Form: February 26, 2007

Gold (Au) nanowires (NW) have been synthesized by using mixtures of L-R-dipalmitoylphosphatidylcholine (DPPC) and various conventional surfactants (CS) of different polarities. The capping ability of the surfactant mixture has been quantitatively evaluated by using UV-vis, TEM, XPS, FTIR, and XRD studies. The synthesis has been carried out by using a total [DPPC+CS] in the range of 0.125 to 0.5 mM. The results clearly demonstrate a surfactant selective synthesis that significantly depends on the polarity of the CS head group. An anionic surfactant like sodium dodecylsulfate (SDS) along with DPPC leads to a network of NW formation with ≈50 nm thickness at different total surfactant concentrations while cationic surfactant, cetyltrimethylammonium bromide (CTAB), predominantly controls the spherical Au nanoparticle (NP) formation at high concentration. At low [DPPC+CTAB], the NW is achieved by end-to-end fusion of spindle shaped Au NP with an aspect ratio of ≈2.15. The chemical composition of the adsorbed capping surfactant on the Au surface and the alignment of the molecules were confirmed from XPS and FTIR studies. XPS results indicate that a small amount of DPPC+SDS caps the Au NW in comparison to that of DPPC+CTAB. FTIR results further support this and indicate even a smaller amount of DPPC is adsorbed in comparison to that of SDS, which is attributed to a stable micellar phase formation in the case of the DPPC+SDS mixture in comparison to DPPC+CTAB. This leaves a small amount of surfactant for capping action in the former case and causes anisotropic growth of Au NP to NW formation. Interestingly, no Au NW formation is observed if no DPPC is used. All pure CS lead to the formation of spherical Au NP of different dimensions. This fact suggests that one can control the morphology of Au NP by using DPPC along with different CS and thus one can design biofriendly nanomaterials for bioengineering applications.

Introduction Recently numerous efforts have been made to prepare gold nanoparticles in biofriendly environments.1 The use of phospholipids provides new pathways for stabilizing metal nanoparticles. It serves two purposes: it provides a charge and steric stabilization essential for colloidal particle stability, and it produces biofriendly materials. Phospholipids act as models to study the biological membranes and their interactions with proteins.2 A combination of lipid and metal particles can be used as a biosensor and for drug delivery purposes.3 Suitably designed nanoparticles show effective DNA recognition behavior.4 The use of such systems is considered to be significantly important to synthesize and design various advance biomaterials. Phospholipids exist as vesicular assemblies2 in an aqueous phase. It is possible to coat metal nanoparticles with lipid bilayers and make them biofunctional materials. To coat a nanometallic surface with lipid bilayers, the liquid/solid inter* Corresponding author. E-mail: [email protected] † Department of Obstetrics and Gynaecology, University of Western Ontario. ‡ Department of Biochemistry, University of Western Ontario. § Department of Chemistry, University of Western Ontario. | Department of Chemistry, Guru Nanak Dev University. ⊥ Department of Applied Chemistry, Guru Nanak Dev University. # National Institute for Nanotechnology.

facial tension must be lowered. This can be achieved by adsorbing the vesicles at the interface in the form of lipid bilayer to form an interfacial film. This involves a transition from vesicular structure to fluid film formation as it happens at an air/liquid interface.2 The mechanism involves the rupturing of vesicular structures upon fusion of vesicles under dynamic collisions. It seems that the outer vesicular layer is pealed off and attains negative curvature to correctly align at the interface. However, it is not always true for all phospholipids since the degree of interfacial adsorption depends very much on the hydrophobicity of phospholipids. Strongly hydrophobic phospholipids such as DPPC produce very stable vesicles with little affinity for interfacial adsorption. But the interfacial adsorption of DPPC can be enhanced by converting the vesicles into mixed micelles with the help of conventional surfactants. This improves the fluidity of lipid molecules and hence increases their interfacial adsorption. Recently, lipids have been used as capping agents to synthesize Au NP of various anisotropic geometries.5 Fundamentally, it is important to understand the driving force behind the synergism between the phospholipids and nanomaterials at molecular scale in order to design biofriendly materials. Such materials at nanoscale are considered to be important to develop macromolecular machines for various biofunctions in the human body.6 Keeping this in view, an attempt has been made to

10.1021/jp070759y CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007

Surfactant Selective Synthesis of Gold Nanowires SCHEME 1

synthesize a network of lipid capped Au NW at room temperature and under ambient conditions. DPPC is a biological surfactant that exists in the form of a biomembrane and plays an important role in achieving high surface activity at air/ alveolar interfaces during the normal respiration process.7 It is an unsaturated phosphocholine molecule with a zwitterionic head group and acts as a wonderful capping agent as well as a template to synthesize anisotropic Au NP assemblies.5 Due to its poor solubility in the aqueous phase, we have used different CS (Scheme 1) such as SDS (anionic), CTAB (cationic), and 3-(N,N-dimethylhexadecylammoniopropane sulfonate) (HPS) (zwitterionic) to enhance its solubility8 and tried to evaluate how different kinds of conventional surfactants along with DPPC control the growth of Au NP because of their different capping mechanisms. The synthesis is carried out under environmentally friendly ambient conditions and characteristic features of DPPC+CS capped Au NW/NP have been discussed. Experimental Section Materials. Tetrachloroauric acid (HAuCl4), L-R-dipalmitoylphosphatidylcholine (DPPC, 99%), sodium dodecylsulfate (SDS, 99%), and sodium borohydride (NaBH4) were purchased from Aldrich. Ascorbic acid (99.7%), cetyltrimethylammonium bromide (CTAB, 98%), and 3-(N,N-dimethylhexadecylammoniopropane sulfonate) (HPS, 98%) were obtained from Fluka. Ultrapure water (18 MΩ cm) was used for all aqueous preparations. Synthesis of Au NP by Using DPPC+CS Mixtures. First of all, a stock solution of DPPC was prepared by dissolving 2 mg of DPPC in 10 mL of aqueous SDS ([SDS] ) 1 mM) solution. Different amounts of stock solution (i.e., 1, 2, and 4 mL) were added to three glass flasks. A total volume of 10 mL was achieved in each flask by adding the appropriate quantity (i.e., 9, 8, and 6 mL, respectively) of distilled water. Then, HAuCl4 was added to obtain a HAuCl4 concentration of 2 mM followed by the addition of NaBH4 to obtain a NaBH4 concentration of 5 mM. Finally, 0.1 mL of freshly prepared 0.1 M ascorbic acid was added to all three flasks, which gave an instant ruby red color in each case. Ascorbic acid is used to continue the reduction process under slow reducing conditions which facilitates the nanowire formation. The reaction mixture

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5933 was kept in the dark for 2 days without stirring. Similar reactions were carried out for CTAB ([CTAB] ) 1 mM) and HPS ([HPS] ) 1 mM) under identical reaction conditions. The purification of each sample was carried out as follows. In the first step, the prepared sample was washed with distilled water and then centrifuged at 5000 rpm for 5 min at least three times. This was followed by similar washing with dry MeOH, and finally the sample was dispersed in toluene. Methods. UV-visible spectra of colloidal Au solutions were taken with a UV spectrophotometer (Multiskan Spectrum, model no. 1500) in the wavelength range of 200-900 nm to determine the absorbance due to surface plasmon resonance (SPR) of nanometallic Au. The formation of Au NP was monitored in the visible absorption range of ∼540 nm. The hydrophobicity of DPPC+CS mixed surfactant systems was determined with the help of pyrene fluorescence measurements by using a Hitachi F2500 fluorescence spectrophotometer. The pyrene emission spectra were recorded by employing an excitation wavelength of 334 nm. Emission intensities I1 and I3 were measured at the wavelengths corresponding to the first and third vibronic bands located at ca. 373 and 384 nm. The shape and size of Au NP were characterized by transmission electron microscopy (TEM). Samples were prepared by mounting a drop of a solution on a carbon-coated Cu grid and allowed to dry in air. They were observed with the help of a Philips CM10 Transmission Electron Microscope operating at 100 kV. Infrared absorption measurements were recorded with a FTIR spectrometer (Shimadzu) in the range of 4000-400 cm-1 in the form of KBr pellets. Each spectrum was measured in transmission mode with 256 scans and 4 cm-1 resolution. The X-ray diffraction (XRD) patterns were characterized with graphite monochromatized Cu KR irradiation. The surface chemical composition was confirmed with the help of X-ray photoelectron spectroscopic (XPS) measurements. A portion of the NP solution in toluene was placed onto a clean silicon wafer and then it was put into the introduction chamber of the XPS instrument. The toluene was pumped away under high vacuum. The sample was analyzed by using a Kratos Axis Ultra X-ray photoelectron spectrometer. XPS can detect all elements except hydrogen and helium, and can probe the surface of the sample to a depth of 7-10 nm. Survey scan analyses were carried out with an analysis area of 300 × 700 µm. Results and Discussion The UV-vis absorbance due to SPR of Au colloids at different total [DPPC+CS] (0.125 to 0.5 mM) has been shown in Figure 1. Figure 1a shows the absorbances of Au NP synthesized in the presence of different amounts of DPPC+SDS. All curves of this system show broad absorbances spread from ca. 520 to 800 nm which demonstrate a blue shift with the increase in [DPPC+SDS]. The blue shift is very clear in the DPPC+CTAB mixture (Figure 1b). At [DPPC+CTAB] ) 0.125 mM, a broad absorbance is located around 600 nm that becomes sharp and shifts to ∼560 nm at [DPPC+CTAB] ) 0.25 mM. At [DPPC+CTAB] ) 0.5 mM, a relatively more sharp absorbance appears with a greater blue shift. Similar behavior is shown by the Au NP synthesized in the presence of different amounts of DPPC+HPS (Figure 1c). Figure 1d demonstrates a change in the amount of blue shift of colloidal Au NP with respect to an increase in [DPPC+CS]. Absorbance wavelength decreases linearly as the amount of DPPC+CS mixture increases. A plot for DPPC+SDS lies in the range of highest wavelength (755-650 nm) while that for DPPC+CTAB lies at the lowest (600-540 nm). The plot for DPPC+HPS lies in

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Figure 1. The UV-vis absorption spectra of Au colloidal solutions for different amounts of (a) DPPC+SDS, (b) DPPC+CTAB, and (c) DPPC+HPS. (d) A plot of the wavelength versus [DPPC+CS] showing a linear decrease with increase in the amount of each mixture.

between the two. Monodisperse small Au NP (3-5 mm) generally show sharp absorbance around 520 nm.9a-c As the size or polydisperse nature of Au NP increases, the absorbance shifts to higher wavelength. In addition, the nanorod or NW formation usually shows a marked red shift with broad absorbance.9 Since all plots of Figure 1d run almost parallel to each other, therefore, higher wavelength values in the case of DPPC+SDS mixtures indicate the formation of NW at all concentrations. A linear decrease in the absorbance with increase in the concentration suggests a decrease in the degree of NW formation. On the other hand, lowest values for DPPC+CTAB suggest the presence of mainly spherical NP especially close to 540 nm. The behavior of DPPC+HPS is considered to be an intermediate between that of DPPC+SDS and DPPC+CTAB since its plot lies in between the two. Because the higher and lower values of absorbance wavelength can be related to the nature of shape and structure of Au NP, therefore, different parallel plots in fact demonstrate the influence of different surfactants on the morphology of NP formation. It seems that SDS along with DPPC predominantly leads to NW formation while CTAB on the other hand favors the spherical NP formation. Due to the zwitterionic nature of HPS, its behavior lies in between that of SDS and CTAB. The results indicate that the shape and structure of Au NP predominantly depend on the capping ability of CS present along with that of DPPC in all cases. This has been confirmed by carrying out the identical parallel reactions in the absence of DPPC and in the presence of different CS. The UV-vis spectra have been shown in Figure S1 in the Supporting Information. All UV-vis spectra in the presence of different amounts of SDS, CTAB, and HPS show predominantly sharp absorbance around 530 nm and there is no significant concentration effect on the absorbance wavelength. The absorbance around 530 nm indicates the presence of small Au NP in each case (TEM images are not shown). This suggests that the presence of DPPC in combination with CS significantly affects the shape and structure of Au NP.

Figure 2 shows the TEM micrographs of Au NP synthesized in the presence of DPPC+SDS. Figure 2a shows a network structure of Au NW that is predominantly formed by the fusion of rod or spindle shaped Au NP. This sample also shows a lamellar phase10 of DPPC+SDS (as light background in Figure 2b) perforated with several vesicles (shown in circles). The network arrangement of Au NW in Figure 2b is very similar to the basic pattern of the DPPC+SDS lamellar phase. The presence of the lamellar phase can be easily evaluated from the pyrene fluorescence measurements. Pyrene is a hydrophobic probe and the I1/I3 ratio of the pyrene emission spectrum indicates the degree of polarity of a medium in which pyrene is solubilized.8 A variation in the I1/I3 value with respect to [DPPC] (Figure 2c) can show us a change in the aqueous environment from polar to nonpolar essential for the lamellar phase formation. Figure 2c shows that this ratio in the absence of DPPC especially in the case of SDS and CTAB indicates a polar aqueous environment with a value close to 1.7.8f,c But as the amount of DPPC increases, the ratio decreases dramatically and reaches around 1.2. The latter value indicates a complete hydrophobic environment in which pyrene is solubilized.8 Due to a much lower amount of SDS and CTAB (1 mM) than their cmc values (8.3 mM8f and 9.1 mM,8c respectively) used here, we do not expect the presence of micelles in the absence of DPPC. On the contrary, even the lowest concentration of DPPC (i.e., 0.027 mM) is already in the vesicular phase.2f Thus, a subsequent increase in the amount of DPPC will ultimately take the form of a lamellar phase in combination with CS as evident from a very low I1/I3 ≈ 1.2. Increase in the amount of DPPC+SDS (Figure 2d) does not essentially change the network arrangement of NW but it is much smoother with the thickness approximately close to 50 ( 15 nm. XRD patterns of Au NW (Figure 2e) indicate clear sharp peaks at 38.2°, 44.4°, 64.5°, and 77.5° corresponding to (111), (200), (220), and (311) planes, respectively, of a standard cubic phase. A highly significant (111) diffraction peak suggests a prominent growth of network

Surfactant Selective Synthesis of Gold Nanowires

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5935

Figure 2. TEM micrographs of Au NW in the presence of 0.125 (a), 0.25 (b), and 0.5 mM (d) DPPC+SDS. (c) Plot of I1/I3 intensity ratio of pyrene with respect to the amount of DPPC in DPPC+CS mixtures. (e) XRD patterns of Au NW formed in the presence of 0.125 mM DPPC+SDS (see details in the text).

structure along (111) planes compare to (200). However, other prominent diffraction peaks at (220) and (311) point to the anisotropic (network) nature of NW.11 In the case of [DPPC+CTAB] ) 0.125 mM (Figure 3a), relatively smaller amount of continuous Au NW can be seen in comparison to that observed in the presence of DPPC+SDS at the same concentration (Figure 2a). A close inspection (Figure 3b) suggests that mainly spindle shaped NP with an average aspect ratio of 2.15 fuse together in an end-to-end pattern to give a NW. Increase in the concentration increases the average aspect ratio to 2.74 and reduces the regular NW formation ability of NP (Figure S2, Supporting Information). But all NP still tend to arrange in an NW-like arrangement. As the amount of DPPC+CTAB increases further, this arrangement completely vanishes and aggregates of spherical NP with an average size distribution of 23.0 nm are obtained (Figure 3c). XRD patterns of Au NP at [DPPC+CTAB] ) 0.125 mM are presented in Figure S3 in the Supporting Information and are very similar to that in the presence of DPPC+SDS. A similar kind of shape transitions in NP formation is observed when the DPPC+HPS

surfactant mixture is used. At low concentration, again a NWlike arrangement of mainly spindle shaped NP is found with few spherical NP (Figures S4a and S5, Supporting Information). An increase in the concentration seems to increase the number of spherical NP (Figure S4b, Supporting information), which become quite prominent with an average size distribution of 11.7 nm at higher concentration (Figure S4c, Supporting Information). The results indicate that clear NW formation is present at all concentrations of DPPC+SDS while this is not so for both DPPC+CTAB and DPPC+HPS mixtures. FTIR spectral studies of DPPC+CS mixtures in the absence and presence of Au NP were performed to determine the capping tendency of both DPPC and CS. Figure 4 shows the IR spectra of DPPC, SDS, and its mixture in the absence and presence of NP and various peaks have been compared in Table 1. Table 1 shows that the symmetric and antisymmetric CH2 stretching (νsym(C-H) and νasym(C-H)) vibrations of pure DPPC as well as SDS shift to much higher frequencies in the presence of NP, which suggests an increasing disorder of methylene chains12 with higher gauche/trans conformer ratio from their native state.

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Figure 3. TEM micrographs of Au NW in the presence of 0.125 (a), 0.25 (b), and 0.5 mM (c) DPPC+CTAB (see details in the text).

The information about the nature of surfactant chain packing on the gold surface can be extracted from the CH2 stretching and scissoring modes of vibration. The scissoring mode of DPPC (δs(C-H)) at 1375 cm-1 shifts to 1382 cm-1 in combination with SDS and then to 1384 cm-1 in the presence of Au NP. On the contrary similar peak for SDS at 1379 cm-1 show a drastic fall to 1352 cm-1. This behavior indicates a significant difference in the packing arrangement of hydrophobic tails of DPPC and SDS. A shift toward the higher frequencies in the former case suggests the unpacking of hydrophobic tails in comparison to its native state, which might point to little or no adsorption of DPPC on the Au surface, while a shift toward lower frequency in the latter case can be attributed to an enhanced packing of SDS tails on the Au surface. The absence of the rocking mode of the (-CH2-)n chain (Fr(CH2)n) in the presence of Au NP indicates that DPPC and SDS have lost their crystalline nature. The orientation of SDS molecules on the Au surface is also evident from a shift toward higher frequency of the symmetric SdO stretching vibrational mode (ν(SdO)) of sulfonic groups present in the head group. On the other hand, the peaks assigned to C-N+ stretching modes (1093, 1065, 970 cm-1) of pure DPPC show only one such peak when DPPC is mixed with SDS, and that too vanishes in the presence of Au

NP. Apart from this, there is practically no shift in P-O stretching vibrations. This perhaps indicates a stronger adsorption of SDS molecules on the Au surface in comparison to DPPC and the orientation of the surfactant molecules can be depicted in Scheme 2. Similarly in the case of DPPC+CTAB (Table 2), the νsym(C-H) and νasym(C-H) vibrations of pure components significantly shift to higher frequencies. A shift in 1473 to 1465 cm-1 and 1464 to 1457 cm-1 of δs(C-H) in the presence of Au NP indicates that alkyl chains of CTAB have attained a more ordered and more hydrophobic environment.13 A prominent shift in the N+-CH3 band toward higher frequency shows strong interactions of the alkylammonium head group of both DPPC and CTAB with Au surface. On the other hand, a variation in the C-N+ stretching modes further supports this conclusion. The absence of 912 and 937 cm-1 bands and further appearance of new bands at 1107 and 894 cm-1 in the presence of Au NP suggests that head groups of both DPPC and CTAB are directed toward the Au surface14 (Scheme 2). Again, the absence of Fr(CH2)n in the presence of Au NP indicates the absence of the crystalline nature of DPPC and CTAB. In the case of DPPC+HPS (Table 3), a little change has been observed in νsym(C-H) and νasym(C-H) vibrations of pure components in the absence and

Surfactant Selective Synthesis of Gold Nanowires

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5937 TABLE 2: Mode Assignments of DPPC and CTAB in the Absence and Presence of Au NP peak assignmenta DPPC CTAB DPPC+CTAB DPPC+CTAB+Au NP νsym(C-H) νasym(C-H) δs(C-H)

2848 2910 1375

νasym(N+-CH3) ν(C-N+)

1468 1093 1065 970

Fr(CH2)n

721

ν(P-O)

2361

2849 2918 1473 1464 1487 1063 1045 1038 964 937 912 876 729 719

2852 2924 1465 1496 1087 970

2877 2989 1465 1457 1500 1107 1086 962 939 894

719 2361

2359

ν ) stretching, sym ) symmetric, asym ) antisymmetric, δs ) methylene scissoring, Fr ) rocking. a

TABLE 3: Mode Assignments of DPPC and HPS in the Absence and Presence of Au NP peak assignmenta DPPC HPS DPPC+HPS DPPC+HPS+Au NP νsym(C-H) νasym(C-H) νasym(N+-CH3) νsym(S-O) ν(C-N+)

δs(C-H) ν(P-O) Fr(CH2)n

2848 2910 1468 1186 1065 1093 1065 970

2850 2918 1487

1186 1049 966 881 1375 1384 1352 2361 721 718

2850 2920 1465 1049 1045 972

2851 2920 1464 1186 1045 1045 852

1383 1352 2363 719

1355 2360

a ν ) stretching, sym ) symmetric, asym ) antisymmetric, δs ) methylene scissoring, Fr ) rocking.

Figure 4. FTIR spectra of DPPC (I), SDS (II), DPPC+SDS (III), and DPPC+SDS+Au NP (IV) (see details in the text).

TABLE 1: Mode Assignments of DPPC and SDS in the Absence and Presence of Au NW peak assignmenta DPPC SDS DPPC+SDS DPPC+SDS+Au NW νsym(C-H) νasym(C-H) δs(C-H)

2848 2910 1375

Fr(CH2)n ν(SdO)

721 1213 1250 1093 1065 970 2361

ν(C-N+) ν(P-O)

2849 2916 1467 1379 721

2852 2924 1382

2879 2926 1384 1352

720 1215 1252 1062 2362

2361

ν ) stretching, sym ) symmetric, asym ) antisymmetric, δs ) methylene scissoring, Fr ) rocking. a

presence of Au NP which prefer to maintain the structural order of methylene chains. The antisymmetric mode of the methylene moiety (N+-CH3) of DPPC and S-O symmetric stretching of HPS shift to lower frequencies indicating a less mobile environment. It is difficult to determine an appropriate orientation of DPPC and HPS zwitterionic head groups toward the Au surface from these results. However, one can speculate on the basis of an arrangement that would allow the PO4- group of DPPC to interact with alkylammonium group of HPS even

on the Au surface. This would result in a shift to lower frequencies of the C-N+ stretching mode from 1049 to 1045 cm-1 and 881 to 852 cm-1. We will come back to the mechanistic point of view of surfactant adsorption on the Au surface after incorporating the XPS results in the following section. The surface composition of Au NW has been studied by XPS measurements. Figure 5 shows the XPS spectra of some samples and Table 4 lists the atomic percent values. Each spectrum has been divided into two ranges of binding energies for the sake of clarity. Figure 5a shows strong emission peaks due to Au 4f and Au 4d electrons for all samples while weak peaks due to Au 4p and Au 4s are present in Figure 5b. Apart from this, other strong peaks due to C 1s and O 1s are also visible. As mentioned in the previous section, since the nature of CS significantly influences the morphology of Au NP, we expect the presence of both DPPC and CS as capping agents in each case. Strong C 1s emission can therefore be due to the presence of hydrocarbon tails of adsorbed DPPC as well as CS in each case. A ratio of the atomic percent of C/Au can provide a qualitative comparison about the amount of C available on the nanometallic Au surface. This ratio is maximum for DPPC+CTAB and minimum for DPPC+SDS mixtures. It suggests a relatively smaller amount of the latter is present for capping the Au surface in comparison to the former. Based on the fact that the poor capping ability of a surfactant generally leads to anisotropic growth,11 the formation of Au NW at all concentrations of the DPPC+SDS mixture can be attributed to this. Even the IR spectra and corresponding data (Table 1) demonstrate a rela-

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SCHEME 2

tively smaller amount of DPPC in comparison to SDS adsorbed on the Au surface. A strong emission of O 1s is predominantly due to SO4-, SO3-, and PO4- groups of SDS, HPS, and DPPC, respectively.15 In our samples, the emission due to oxidized Au species is presumed to be minimum since its binding energy is mainly located around 529.2 eV16 though many studies have also reported this value close to 530.1 eV.17-19 In our case, the binding energy of O 1s is at least more than 2 eV higher than this value. Moreover, the intensity of emission from the oxidized Au surface usually decreases when a capping layer is adsorbed by a long chain amphiphilic species such as alkanethiols.16,20 In our results, the intensity of O 1s is so strong that it is

Figure 5. XPS spectra of Au NW/NP prepared in the presence of 0.125 mM DPPC+CS.

comparable to that of C 1s in all cases. Therefore, we can reasonably assign this emission due to SO4-, SO3-, and PO4functional groups. Assuming that 6.3% O 1s is only due to PO4of DPPC in the case of the DPPC+CTAB mixture, the O/Au ratio will give a qualitative amount of DPPC adsorbed on the Au NP surface (Table 4). This ratio should be the sum of oxygen contents of both components in the case of DPPC+SDS and DPPC+HPS mixtures. Interestingly, this ratio for DPPC (in the case of the DPPC+CTAB mixture) is more than 3 and 13 times than that of DPPC+HPS and DPPC+SDS, respectively. The minimum O/Au ratio for DPPC+SDS further confirms the presence of the least amount of capping surfactant and could be the cause of anisotropic growth in the form of Au NW. Thus, the XPS results fully support the FTIR results and clearly indicate that the head groups of SDS, HPS, DPPC (as evident from O 1s binding energies), and CTAB (as evident from N 1s binding energy) interact with the Au surface and the difference in their adsorbed amount along with DPPC affects their capping ability. Thus, a combined evaluation of FTIR and XPS results can provide valuable information about the orientation of adsorbed surfactant molecules on the Au surface that consequently controls the morphology of Au NP. Soluble surfactants indeed interact with the Au surface due to van der Waals forces or Coulombic interactions with hydrophobic or charged portions of the surface.21-23 As the size of the Au surface decreases to the nanometric range, the surface properties become significantly prominent in comparison to that of bulk-phase due to the presence of free d electrons. Under the effect of the surrounding conditions, changing the dielectric constant of the surrounding medium (especially in the presence of ionic surfactants) induces polarization in the charge density of the conduction band electrons.24,25 That triggers the short-range Coulombic interactions between the soluble ionic species and Au surface, and is considered to be the driving force26,27 for the capping ability of the present DPPC+CS mixtures. Furthermore, the ionic species are expected to have strong interactions with the Au surface in comparison to zwitterion. Therefore, both DPPC+SDS and DPPC+CTAB mixtures should have almost equal capping ability as one can find sharp SPR of Au NP in the absence of DPPC for both SDS and CTAB (Figure S1, Supporting Information). But a drastic difference between the capping ability of both mixtures is mainly due to the presence of the DPPC component in both cases and that consequently leads to a marked difference in the morphology of Au NP. In our recent study,8d we have reported that 1,2-diheptanoyl-sn-glycero-3phosphocholine (another lower homologue with phosphocholine

Surfactant Selective Synthesis of Gold Nanowires

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5939

TABLE 4: Binding Energies/eV (I) and Results of XPS Analysis in Atomic Percent (II) of Au NW/NP Synthesized in the Presence of DPPC+CS Mixtures with Total Concentration ) 0.125 mM Au 4f

C 1s

O 1s

N 1s

mixtures

(I)

(II)

(I)

(II)

C/Au

(I)

(II)

O/Au

(I)

(II)

DPPC+SDS DPPC+HPS DPPC+CTAB

84.0 84.5 83.8

7.5 2.4 0.2

284.5 284.5 284.0

59.4 42.0 85.1

7.9 17.5 425

532.0 532.5 532.0

17.4 23.6 6.3

2.3 9.8 31.5

401.5

0.2

moiety) undergoes stable mixed micelle formation with SDS in comparison to CTAB. A stable micellar phase surfactant would therefore prove to be a weak capping agent for the NP surface in comparison to a less stable micellar phase. Thus, a stable micellar phase would leave a smaller amount of monomeric surfactant for NP capping and at the same time micelles as a whole entity are not expected to adsorb significantly at the NP surface, as well as in the case of NW. Thus, the former would leave some of the crystal planes uncapped for secondary nucleation and lead to the formation of NW. That is why a network of NW is observed at all concentrations of DPPC+SDS (Figure 2) while it is only observed for low amount of DPPC+CTAB. At a high concentration of DPPC+CTAB, mainly spherical NP are obtained because a greater amount of monomeric surfactant is available in the bulk for an effective capping and that reduces the chances of anisotropic growth. A stable micellar phase in the case of the DPPC+SDS mixture exists in the form of a lamellar phase, which is clearly evident from Figure 2b. In comparison to the less stable micellar phase of the DPPC+CTAB mixture, the former is considered to be less fluid and best suited for a soft template. During the nucleation process, the small NP are accommodated in certain size dependent soft templates and become part of it. The growth starts by diffusing the gold atoms into the template and the overall structure essentially attains the shape of a soft template.14 Therefore, a stable lamellar phase of the DPPC+SDS mixture predominantly acts as a soft template and thus leads to the formation of a fine network of Au NW. On the other hand, a less stable micellar phase in the case of DPPC+CTAB would perform a better capping ability and consequently lead to the formation of spherical Au NP. The present study, therefore, demonstrates how one can synthesize the biofunctional materials by choosing an appropriate combination of lipid+surfactant mixtures under ambient conditions. Apart from this, a systematic change in the amount of surfactant can give us the desired results whether to synthesize NW or nanospheres for bioengineering applications. Conclusions A network of Au NW formation has been achieved by using a surfactant selective synthesis under ambient conditions. The head group polarity of CS can significantly control the morphology of Au NP. A mixture of DPPC+SDS leads to the formation of Au NW of ∼50 nm thickness at different concentrations whereas mainly spindle shaped or spherical Au NP are obtained at the same concentrations of the DPPC+CTAB mixture. Spindle shaped NP demonstrate a tendency for NW formation by arranging themselves in an end-to-end fashion. The morphology of Au NP in the presence of DPPC+HPS remains in between the former two mixtures due to the zwitterionic nature of HPS. The driving force behind the achievement of NW formation originates from the stable micellar phase of the DPPC+SDS mixture, which leaves a small amount of surfactant for capping the Au NP surface. That causes some of the crystal planes available for secondary nucleation. Apart from this, the lamellar phase of the DPPC+SDS mixture acts as a soft template

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