Direct Patterning of Silver Colloids by Microcontact Printing: Possibility As SERS Substrate Array

Vibrational Spectroscopy 29 (2002) 79±82

Direct patterning of silver colloids by microcontact printing: possibility as SERS substrate array Hyeon Suk Shin, Hyun Jung Yang, Young Mee Jung, Seung Bin Kim* Laboratory for Vibrational Spectroscopy and Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Pohang 790-784, Republic of Korea Received 31 August 2001; received in revised form 19 November 2001; accepted 30 November 2001

Abstract

The present study describes an easy and clean method that makes the pattern of silver colloids by means of microcontact printing. Hydrophilic silver colloids adsorbed on a hydrophobic poly(dimethyl siloxane) (PDMS) stamp are transferred directly to the self-assembled monolayer (SAM) of 4-mercaptopyridine or aminoethanethiol (AET) on an Au-coated silicon substrate. To adsorb silver colloids on the PDMS stamp successfully, adhesion layers such as poly(vinyl pyrrolidone) and stearic acid were pre-treated onto the PDMS stamp. Finally, we obtained a surface-enhanced Raman scattering (SERS) spectrum and image of rhodamine 6G (R6G) on the patterned silver colloids. # 2002 Elsevier Science B.V. All rights reserved.

Keywords: Microcontact printing; Surface-enhanced Raman scattering (SERS); Silver colloids; Self-assembled monolayer (SAM)

1. Introduction of the colloid. These problems may be solved if, in microcontact printing for patterning colloids, they are Colloidal nanoparticles should be ®xed to speci®c directly transferred to a substrate. Such method can positions for the application of electrical or optical fast and clearly obtain many patterns of the colloids. devices [1]. Several methods for patterning colloidal So far, microcontact printing has used hydrophobic nanoparticles have been developed [1±5]. For exam- ink because a poly(dimethyl siloxane) (PDMS) stamp ple, Whitesides and coworkers reported the pattern of is hydrophobic, exception for some methods such as Pd colloids stabilized with tetraoctadecylammonium the treatment of a PDMS stamp with oxygen plasma bromide as a new technique for the activation of sub- [6±8]. Hydrophilic colloidal ink has not been used for strates for electroless deposition [2,3]. Methods using the purpose of patterning the colloids with a PDMS the self-assembled monolayer (SAM) as a template to stamp. form the pattern of colloids have also been reported In this contribution, we propose a new method for [1,4,5]. Although these methods were successfully patterning of hydrophilic silver colloids by microcon- applied, there have been some problems such as the tact printing. The hydrophilic colloidal ink is adsorbed reproducibility of the colloidal pattern and washing on a PDMS stamp by an adhesion layer and the treated PDMS stamp is contacted to an Au-coated silicon substrate, on which SAM is already formed. Then, the * Corresponding author. Tel.: 82-54-279-2106; fax: 82-54-279-3399. silver colloids are transferred to the substrate with a E-mail address: [email protected] (S.B. Kim). pattern without a washing step. This provides a clean

0924-2031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0924-2031(01)00186-2 80 H.S. Shin et al. / Vibrational Spectroscopy 29 (2002) 79±82 pattern of silver colloids. We also demonstrate the possibility of a surface-enhanced Raman scattering (SERS) substrate array to obtain Raman images and spectra.

2. Experimental

The PDMS stamp was pre-treated with a 2 mM stearic acid or a 1% (w/w) poly(vinyl pyrrolidone) solution in chloroform, which plays a role of an adhesion layer, to attach a hydrophilic silver colloidal ink. Then silver colloids were adsorbed on the stamp and the resulting stamp was dried under a vacuum. Silver colloids, prepared by the Lee and Meisel method [9], are hydrophilic because of the adsorption of citrate ion on the silver surface. The stamp treated with silver colloids was placed on the Au-coated silicon substrate on which a SAM of 4-mercaptopyridine or aminoethanethiol (AET) had been formed already and pressed with a proper pressure for 20 min. It is well understood that the adsorption of colloids is due to the electrostatic attractive force between colloids and SAM. The patterns of silver colloids were identi®ed by a scanning electron microscopy (SEM). SERS spectra were measured with a Renishaw± Raman system 3000 equipped with a peltier-cooled CCD detector (400 Â 600 pixels). The radiation (514.5 nm, 20 mW) from an air-cooled argon-ion laser (Omnichrome Model 170-A) was used as the excita- tion source. The laser light was focused onto the pattern using a 20Â objective lens mounted on an Olympus BH-2 microscope. The exposure time of the laser light for obtaining spectra was 10 s. Raman Fig. 1. SEM images of the patterns of silver colloids with stearic image was obtained with a narrow-band multiplier acid (a) and poly(vinyl pyrrolidone) (b), as an adhesion layer, and À1 without an adhesion layer (c), dark squares correspond to the dielectric ®lter with a 20 cm band-pass. patterns of silver colloids. Furthermore, we have been investigating the stabi- lity of patterns of silver colloids prepared by g-irradia- tion [10]. Up to 70 days, the shape of patterns has been treated with stearic acid and poly(vinyl pyrrolidone), remained well and the patterns still play a role as the respectively, to form an adhesion layer of silver col- SERS substrate because Raman scattering of analytes loids, whereas Fig. 1c shows the pattern of silver dropped on the patterns is enhanced. colloids without any adhesion layer. It can be seen from the SEM images that patterns of silver colloids with stearic acid show well-de®ned shapes of patterns 3. Results and discussion and a uniform distribution and that with poly(vinyl pyrrolidone) shows further aggregated silver colloids, Fig. 1a and b shows SEM images of the patterns of as indicated by the dark areas in Fig. 1b. However, silver colloids prepared with the PDMS stamp pre- patterns of silver colloids without any adhesion layer H.S. Shin et al. / Vibrational Spectroscopy 29 (2002) 79±82 81 results in many defects (see Fig. 1c). These observa- (R6G) adsorbed on the pattern of silver colloids, the tions suggest that a suitable adhesion layer is very SAM of 4-mercaptopyridine was replaced with that of important for forming a clean and distinct pattern of AET. The pattern of silver colloids by the use of stearic silver colloids without any defects. Poly(vinyl pyrro- acid as an adhesion layer for the colloids on the PDMS lidone) is not proper for forming the pattern because a stamp and the AET SAM on the Au-coated silicon considerable aggregation of silver colloids is induced. substrate shows a ®ne image as shown in Fig. 1a, Thus, a surfactant, a fatty acid, or a polymer with both without any aggregation and defects. SERS spectra of hydrophobicity and hydrophilicity can be good can- R6G were taken after dipping the resulting stamp into didates for an adhesion layer. a1Â 10À5 M R6G solution in a methanol and drying To characterize the pattern of silver colloids, we it in air. Fig. 3a shows the SERS spectrum for R6G in measured SERS spectra of a SAM of 4-mercaptopyr- the region of an AET SAM with no patterned silver idine below the pattern of silver colloids. Zhu et al. colloids, whereas Fig. 3b shows the SERS spectrum reported that in the Au colloid/4-mercaptopyridine/Au for R6G in the region of an AET SAM with patterned substrate, the SAM of 4-mercaptopyridine on an Au silver colloids. The SERS spectrum of R6G in Fig. 3b substrate was detected in the SERS spectrum because is consistent with that reported previously [12]. of the roughness of the Au colloids adsorbed on the Fig. 4 shows the SERS image of the pattern, which SAM [11]. The same reason for observing the SERS was measured with a band at 1363 cmÀ1 in Fig. 3b. spectrum was suspected in this study with silver Lighter area in the SERS image indicates higher SERS colloids. Fig. 2 shows SERS spectra of 4-mercapto- intensity of the band at 1363 cmÀ1. Thus, dark area pyridine, which were measured at the region of the corresponds to the region without silver colloids. The Au-coated silicon substrate without silver colloids width of the dark region is 20 mm, which is consistent (Fig. 2a) and the region with patterned silver colloids with the size of TEM grid used to prepare the PDMS (Fig. 2b). Of particular note in Fig. 2 is that Raman stamp. scattering of 4-mercaptopyridine with silver colloids This result indicates that patterns of silver colloids is greatly enhanced, whereas 4-mercaptopyridine are a powerful substrate array for the SERS effect. It is without silver colloids is not detected. possible to analyze a lot of samples fast if the tech- To remove the spectral interference of 4-mercapto- nique to drop an aliquot of analyte solution on a pyridine and to obtain SERS spectra of rhodamine 6G pattern is developed.

Fig. 2. SERS spectra of 4-mercaptopyridine measured at the regions of the Au substrate without silver colloids (a) and with patterned silver colloids (b). 82 H.S. Shin et al. / Vibrational Spectroscopy 29 (2002) 79±82

Fig. 3. SERS spectra for R6G in the regions of an AET SAM on the Au-coated silicon substrate without (a) and with (b) the patterned silver colloids.

Acknowledgements

This study was supported by the Korea Research Foundation (BK21 project).

References

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