Two-dimensional Assembling of Au Nanoparticles Mediated by

Interparticle distances of ∼2 nm in the regular hexagons are in accord with the size of a H2TPyP molecule bridging two neighboring particles. UV-vis...
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NANO LETTERS

Two-dimensional Assembling of Au Nanoparticles Mediated by Tetrapyridylporphine Molecules

2002 Vol. 2, No. 2 121-125

Ivana Sˇ loufova´-Srnova´†,‡ and Blanka Vlcˇkova´*,† Department of Physical and Macromolecular Chemistry, Charles UniVersity, HlaVoVa 2030, 128 40 Prague 2, Czech Republic, and Institute of Macromolecular Chemistry, Academy of Sciences of Czech Republic, HeyroVsky Sq. 2, 162 00 Prague 2, Czech Republic Received July 31, 2001; Revised Manuscript Received November 12, 2001

ABSTRACT TEM images, UV-vis and SERRS (surface-enhanced resonance Raman scattering) spectra of two-dimensional assemblies of tetrapyridylporphinederivatized Au nanoparticles (Au colloid-H2TPyP films) are reported. The films are prepared by a spontaneous 2-D reassembling of multilayer interfacial films on water surface and a subsequent deposition, forming hexagonally packed arrays of Au nanoparticles with some lattice defects. Interparticle distances of ∼2 nm in the regular hexagons are in accord with the size of a H2TPyP molecule bridging two neighboring particles. UV-vis spectra indicate stabilization of the array by porphyrin−porphyrin hydrophobic interactions.

Bioinorganic nanocomposite materials based on Au nanoparticles and biomolecular species, namely DNA1 and proteins,2 have recently attracted a considerable interest due to a possibility to tune the optical, electrical, and even mechanical properties of these materials through factors such as the nanoparticle periodicity, interparticle distances, and size of ordered regions in the resulting arrays.1 Much progress in this direction has already been achieved in 2-D (2dimensional) and also 3-D assembling of Au nanoparticles mediated by a variety of molecular and ionic species, such as sulfonated triphenylphosphane,3,4 alkanethiols,5 bifunctional thiols (substituted thiolates),6 aryldithiols and diisonitriles,7 alkylamines,8 quarternary ammonium bromide,9 and C60.10 The strategy of assembling of metal nanoparticles into multilayer films at the interface between a metal hydrosol and a solution of adsorbate in a chlorinated solvent offered a possibility to incorporate various organic molecules and biomolecules into nanoparticle multilayers11 that could be subsequently deposited on supporting surfaces,12 providing, e.g., stable samples for SERS (surface-enhanced Raman scattering) spectroscopy. The hypothesis about formation of these films primarily as monolayers12 was confirmed by isolation, deposition, and TEM (transmission electron microscopy) imaging of monolayer Ag colloid-adsorbate films.13a Their morphology (described by the fractal dimension D) * Corresponding author. E-mail: [email protected] † Charles University. ‡ Academy of Sciences of Czech Republic. 10.1021/nl015590t CCC: $22.00 Published on Web 12/22/2001

© 2002 American Chemical Society

was found to be adsorbate dependent. Ag colloid-tetrapyridylporphine films were nonfractal (D ) 1.98); however, their ordering was rather poor.13a SERRS spectra of Ag colloid-H2TPyP films showed the spectral marker bands of both the free-base and the Ag-metalated form of the porphyrin.13b The presence of both forms of the porphyrin indicated a possibility of two types of bonding of H2TPyP to Ag nanoparticle surface. A question has thus arisen whether (i) introduction of a more elaborate procedure of the monolayer nanoparticulate film preparation, (ii) reduction of the polydispersity of the parent hydrosol, and (iii) employment of Au instead of Ag nanoparticles could lead to preparation of ordered 2-D nanoparticle assemblies. In this paper, we report the results of 2-D assembling of Au nanoparticles mediated by the tetrapyridylporphine molecules. Preparation of the nanoparticle assembly is based on formation of multilayer interfacial Au colloid-H2TPyP films, their transfer and a spontaneous 2-D reassembling on water surface, and a subsequent deposition. The morphology and internal structure of the deposited 2-D Au colloid-H2TPyP films are probed by TEM and by UV-vis and SERRS spectroscopy. Our goal is to learn about regularity of the 2-D Au nanoparticle array, about bonding of the porphyrin to Au nanoparticle surface, and about interactions that stabilize the array. Experimental Section. Au colloid was prepared by reduction of HAuCl4 by NaBH4. Briefly, 9 mL of 2.2 × 10-3 M solution of HAuCl4 was added dropwise, upon vigorous stirring, to 3.5 mg NaBH4 dissolved in 75 mL of deionized

Figure 1. TEM imaging and image analysis of the assembly of H2TPyP-derivatized Au nanoparticles (Au colloid-H2TPyP film): (A) TEM image, 73 000× instrumental magnification; (B) TEM image, 200 000× instrumental magnification [inset: hexagonal packing of Au nanoparticles in a regular region of the film (270 000×)]; (C) particle size distribution; (D) interparticle spacing distribution. The particle size and the interparticle spacing distributions were determined from the TEM image in Figure 1B.

distilled water and precooled to 2 °C. Stirring was continued for another 45 min. The resulting Au colloid was bright red, with the maximum of the surface plasmon extinction at 512 nm.14 Parent (multilayer) interfacial films of the porphyrincapped Au nanoparticles (Au colloid- H2TPyP films) were prepared according to the procedure analogous to that described in ref 12 for Ag colloid-bpy films. A two-phase system consisting of the Au hydrosol and a 2 × 10-4 M solution of H2TPyP in dichloromethane was vigorously shaken for 2 min. Au nanoparticles assembled on the interface into a nanoparticulate film of a gold-tinted luster in reflected light and of a violet color in the transmitted light. The loss of the bright red coloring of the aqueous phase indicated a complete transfer of the Au nanoparticles from the hydrosol into the nanoparticulate film. Reassembling of these multilayer nanoparticulate films into a monolayer was accomplished by a procedure employed earlier in Ag and (Ag)Au-2,2′-bipyridine monolayer film preparations.15 Briefly, the multilayer Au colloid-adsorbate film was collected from the dichloromethane-water interface together with small 122

portions of both water and the residual dichloromethane solution of the porphyrin by a 1 mL micropipet. The volume of the mixture was reduced by rejection of the dichloromethane solution, and the film of porphyrin-covered Au nanoparticles together with the residual water were transferred onto a glass slide. The film reassembled on the water drop into a nanoparticulate monolayer. The film was either deposited directly on the supporting slide by removal of water by a slip of filter paper (samples for optical measurements) or touched by the C-coated side of a Cu grid held in tweezers for transfer onto the C-coated grid, which was then placed on a filter paper to dry (samples for TEM). A JEOL-JEM 200 CX transmission electron microscope was employed for imaging of the grid-deposited films. The values of the instrumental magnifications were 100 000×, 200 000×, and 270 000×. SERRS spectra of the film were recorded with a modular Raman setup described in ref 15(a) using the 514.5 nm Ar ion laser excitation. UV-vis spectra were measured with a Hewlett-Packard diode array UV-vis spectrometer. Nano Lett., Vol. 2, No. 2, 2002

Analysis of TEM images was performed by a commercial image processing and analysis computer program LUCIA 32G version 4.00, Laboratory Imaging Ltd. Fractal dimension was determined by a procedure based on the mass-radius relation. Briefly, the D value of the film was calculated as a slope of the dependence ln(S) ∝ Dln(xA), where A is the square frame area and S is the area occupied by colloidal particles within this frame. The frame was enlarged stepwise from a selected center of the micrograph. Distribution of interparticle distances in the overall image obtained by 270 000× instrumental magnification was determined as a distribution of average distances between the boundaries of a particle and those of its nearest neighbors. Width parameters representing halves of the average distances were determined from a differential image obtained by subtraction of the binary image of the particles from the zone of influence image. Interparticle distances in regions without lattice perturbations were determined as a single distance measurements by the above-mentioned image-processing program. The more detailed description of the image analysis is provided as a Supporting Information. Results and Discussion. Shown in Figure 1 are the TEM images of Au colloid-H2TPyP film depicting both the longrange (Figure 1A) and the short range (Figure 1B) ordering of Au nanoparticles, together with the particle-size distribution (Figure 1C) and the interparticle spacing distribution (Figure 1D). The film is a monolayer, nonfractal (D ) 2.05), hexagonally packed (see Figure 1B, inset) Au nanoparticle array with some lattice defects. The average value of the Au particle diameter is 4.9 ( 0.9 nm. The average value of the interparticle spacing (2.5 ( 0.6 nm) determined by the binary image subtraction procedure from the overall area of the micrograph shown in Figure 1B is somewhat biased by inclusion of the lattice defects into the image analysis. The histogram of interparticle spacings (Figure 1D) shows that the most frequent interparticle spacings are within 2.0-2.5 nm. The interparticle spacing parameter of ∼2 nm was determined by the single distance measurements (providing the values of the shortest interparticle distances) selectively for regions of the film with the regular packing of the nanoparticles (such as the hexagon marked in Figure 1 A-insert). The UV-vis spectrum of Au colloid-H2TPyP film (Figure 2) consists of two bands with maxima at 610 and 430 nm, respectively. The former is assigned to the surface plasmon extinction of the 2-D array of Au nanoparticles on the basis of the following argument. The radius-normalized centerto-center separation of the nanoparticles in the array R/a ) 3 (where R ) 7.5 nm is the average value of center-center separation and a ) 2.5 nm is the average radius). The obtained value of the radius-normalized center-to-center separation indicates that the electromagnetic coupling between the individual nanoparticle excitations should be within the dipolar limit.3,16 Hence, a single surface plasmon (SP) extinction band, red shifted and broadened with respect to that of the isolated nanoparticles, is expected to be observed in the SP extinction spectrum of the nanoparticle array. The 610 nm band (fwhm ) 138 nm), red shifted and broadened Nano Lett., Vol. 2, No. 2, 2002

Figure 2. UV-vis spectra of (A) the assembly of H2TPyPderivatized Au nanoparticles (Au colloid-H2TPyP film); (B) glassdeposited microcrystalline H2TPyP sample (spectrum a) and a solution of H2TPyP in dichloromethane (spectrum b); (C) parent Au hydrosol.

with respect to the 512 nm (fwhm ) 83 nm) SP extinction band of the parent hydrosol (Figure 2C)14 appears to meet this expectation. The position and the halfwidth of the second UV-vis spectral band (max. 430 nm, fwhm ) 45 nm) was found to match the Soret (B) electronic absorption band of a H2TPyP microcrystalline sample (max. 432 nm, fwhm ) 43 nm) obtained by crystalization from the dichloromethane solution onto a glass support (Figure 2B spectrum a). This band is thus attributed to the Soret band of the porphyrin molecules incorporated in the nanoparticle array. The Soret band of the porphyrin, both in the film (Figure 2A) and in the microcrystalline sample (Figure 2B, spectrum a) is markedly red shifted and broadened with respect to that in the UV-vis spectrum of H2TPyP a dichloromethane solution (max. 418 nm, fwhm ) 25 nm) containing noninteracting porphyrin monomers (Figure 2B spectrum b). A red shift and broadening of the Soret band in an assembly of porphyrin 123

Figure 3. (A) SERRS spectrum of H2TPyP obtained from the assembly of H2TPyP-derivatized Au nanoparticles (Au colloidH2TPyP film). (B) SERRS spectrum of H2TPyP adsorbed on Au film sputtered onto a glass slide. (C) RRS spectrum of H2TPyP crystallized on a glass slide.

molecules indicates porphyrin-porphyrin interactions analogous to formation of J-type aggregates and/or dimeric structures with either a linear (side-by-side) or a card-deck arrangement of monomeric porphyrin molecules.17 Both the electronic absorption of the molecular component (interacting porphyrin molecules) and SP extinction of the electromagnetically coupled metal nanoparticle thus manifest themselves in optical spectrum of the array. A SERRS spectrum of the porphyrin obtained from a glass-deposited sample of the film upon 514.5 nm excitation (Figure 3A) consists of spectral bands at 966, 1002, 1141, 1246, 1331, 1363, 1500, and 1557 cm-1. All the observed bands have their counterparts in the RRS (resonance Raman scattering) spectrum of H2TPyP microcrystals on a glass support (Figure 3C). The spectral bands are, in accord with ref 18, attributed to the resonance enhanced vibrational modes of the free-base porphyrin macrocycle. The SERRS spectrum of Au colloid-H2TPyP film thus contains entirely the characteristic spectral bands of the native free base form of the porphyrin, which indicates that no incorporation of Au ion into the center of the porphyrin macrocycle occurred upon adsorption of the free base porphyrin on the Au nanoparticle surface. SERRS spectra of the porphyrin obtained from the Au colloid-H2TPyP film are also in accord with SERRS of the porphyrin adsorbed on a ca. 30 nm Au layer sputtered on a glass slide (Figure 3B). The TEM image analysis and spectral investigations provide some insight into the internal structure of the film, which is depicted schematically in Figure 4. Based on the SERRS spectral evidence that no incorporation of Au ion into the center of porphyrin macrocycle occurred upon adsorption, bonding of porphyrin molecules to the surface via coordination of a pyridine side group to Au in a edge124

Figure 4. Molecular structure of H2TPyP and schematic outline of the proposed internal structure of the Au colloid-H2TPyP film. The particle layout was obtained by magnification of the TEM image in Figure 1A inset.

on orientation appears to be more probable than bonding via the π-electron system of the porphyrin macrocycle with a flat orientation of the macrocycle with respect to the surface. UV-vis spectra indicate mutual interactions of the adsorbed porphyrin molecules analogous to the solid-state interactions.19 Analysis of the interparticle distances within the overall image of the film shows that the distances with the highest frequencies of occurrence are in the 2.0-2.5 nm range. Considering the ∼1.7 nm size of an edge-on bonded porphyrin molecule [calculated as the sum of the diagonal dimension of the porphyrin molecule (1.5 nm) and one Au-N (pyridine) bond (0.2 nm)] and the range of the most frequent interparticle distances, we find that the layers of the adsorbed porphyrin molecules on the adjacent nanoparticles in the 2-D array interpenetrate, resulting in ∼1.90.9 nm overlaps of porphyrin molecules attached to the adjacent nanoparticles (Figure 4 A). Interpenetration of the adsorbed porphyrin layers is considered to be responsible for the observed manifestations of porphyrin-porphyrin interactions. Nano Lett., Vol. 2, No. 2, 2002

In the regular hexagons (such as that depicted in Figure 1A), the average interparticle distances of ∼2 nm were determined as the shortest possible interconnects of the adjacent particles. This value is in accord with the size of a porphyrin molecule cross-linking two adjacent nanoparticles [1.9 nm, calculated as the sum of the diagonal dimension of the porphyrin molecule (1.5 nm) and two Au-N (pyridine) bond lengths (0.2 nm each)], as outlined in Figure 4B. In conclusion, we have demonstrated that adsorption of porphyrin molecules with pyridine peripheral substituents mediates assembling of Au nanoparticles into hexagonally packed arrays. Furthermore, we show that the assembling can be accomplished by a simple preparation procedure based on capping of Au hydrosol nanoparticles by the porphyrin molecules in a two phase (liquid-liquid) system and spontaneous reassembling of the multilayer interfacial film of porphyrin-capped Au nanoparticles into a nanoparticulate monolayer on a water-air interface. We propose that the nanoparticles are assembled primarily via hydrophobic interactions between the adsorbed porphyrin macrocycles and that the regular regions of the array are further stabilized by cross-linking of Au nanoparticles by the pyridine-substituted porphyrin molecules. Acknowledgment. Financial support of this work by 203/ 01/1013 grant awarded by the Grant Agency of Czech Republic is gratefully acknowledged. The study is a part of the long-term Research Program of the Faculty of Science, Charles University Prague, Grant No. MSM 113 100001. Supporting Information Available: TEM image analysis data for the deposited nanoparticles. This information is available free of charge via the Internet at http://pubs.acs.org. References (1) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P. J.; Schultz, P. G. Nature 1996, 382, 609. (c) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (2) Conolly, S.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 4765. (3) Dusemund, B.; Hoffmann, A.; Salzmann, T.; Kreibig, U.; Schmid, G. Z. Phys. D 1991, 20, 305. (4) Giersig, M.; Mulvaney, P. J. Phys. Chem. 1993, 97, 6334.

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