Comparative adsorption of phenyl selenolate and selenocyanate on Au nanoparticle surfaces

doi:10.1016/j.apsusc.2006.10.041

Applied Surface Science

Volume 253, Issue 11, 30 March 2007, Pages 4830-4835

https://doi.org/10.1016/j.apsusc.2006.10.041 Get rights and content

Abstract

UV–vis absorbance spectra taken at different elapsed time for the surface plasmon band shift indicated that the self-assembly of PhSeCN on gold should be slower than that of phenyl selenolate (PhSeH). A surface-enhanced Raman scattering (SERS) study showed that a trace amount of CN species could remain on Au surfaces when aromatic selenocyanates are reduced to give CN and their selenium atoms bound to the surface. Our concentration dependent SERS spectra suggested that the CN adsorption should be more favorable at higher concentrations of PhSeCN as indicated from more prominent intensites of the CN stretching vibration at 2110–2150 cm⁻¹.

Introduction

Recently spontaneous assemblies of thiols on gold surfaces attracted much attention due to its potential applications in many areas including biomaterials, sensors and electronics [1]. Due to the problems of easy oxidations of thiols and the need for ancillary capping reagents, an alternative molecular assembly system has been suggested on gold surfaces. Thiocyanate-based self-assembly system has been suggested as a substitutive precursor for gold-thiolate assemblies [2]. Organic thiocyanates have been found to assemble as thiolates on gold via a surface mediated reduction. The cyanide group is expected to leave as an [Au(CN)₂]⁻ species, although some amount of CN species could remain on the surface [3]. Thiocyanate can bind to metal surfaces through the sulfur or the nitrogen atom. A recent Raman spectroscopy and density functional calculation study has exhibited that the thiocyanate ion may take a reorientation on Au electrodes depending on the applied potential [4]. Due to a large Raman cross-section of the C–N stretching vibration at 2100–2250 cm⁻¹, the SCN species adsorbed on metals could be used as a marker band in ultra sensitive chemical detections.

In contrast to many studies of self-assembled monolayers (SAMs) derived from organosulfur reagents [1], relatively little work has been done to fabricate organic SAMs on metal surfaces without sulfur as the anchoring groups. Selenium derived SAMs [5], [6] may be utilized as an attractive alternative to thiolates due to potentially similar adsorption characteristics as those of the sulfur compounds. It has been shown that the selenolate SAMs can be fabricated from either diselenide [5] or selenol [6] precursors. There has been no report on the adsorption behavior of the organic selenocyanates on Au surfaces yet.

Molecules adsorbed on noble metals can exhibit anomalously intense Raman signal known as surface enhanced Raman scattering (SERS) phenomena [7]. The analysis of spectral features has provided detailed information on interfacial structure, adsorption mechanism, and surface reactions [8], [9], [10]. The sensitivity could be achieved to monitor the adsorbates on metal substrates at submonolayer coverage limit.

Our recent UV–vis absorption spectroscopy and SERS study indicated that benzyl mercaptan (BM) was found to adsorb faster than thiophenol (TP) on gold nanoparticle surfaces [11]. Assuming the first order decay for the initial status of aggregation, the rate of flocculation for BM and TP, k_BM and k_TP could be approximately determined as 0.014 and 0.010 s⁻¹, respectively. The energy difference ΔG between BM and TP on Au nanoparticles could be roughly estimated from the measured rates at room temperature.

In this work the assemblies of organic selenocyanates have been examined by means of surface plasmon resonance spectroscopy and SERS. The main purpose of this study is to provide an alternative precursor to substitute thiol-based SAMs by better understanding the adsorption organic selenocyanates on Au. To the best of our knowledge, this is the first study to compare the adsorption behaviors of organic selenocyanate and selenolate on Au by means of SERS.

Section snippets

Experimental

PhSeH (97%), PhSeCN (96%), and 2-nitrophenyl selenocyanate (98%) were purchased from Aldrich and used without further purification. 1,4-Phenylenebis(methylene)selenocyanate was synthesized by the following procedure [12]. Citrate-stabilized gold nanoparticles were prepared by following recipes from the literature [13].

Raman spectra are obtained using a Renishaw confocal 1000 micro-Raman spectrometer using 632.8 nm radiation from a 35 mW air-cooled He–Ne laser (Melles Griot Model 25 LHP 928) with

UV–vis absorbance spectra

The aggregation of aromatic thiol-covered gold particles by a self-assembly process could be checked by monitoring the red shift in the UV–vis absorption spectrum of Au colloids due to a decrease of inter-particle distances. The λ_max value was found at 518 nm in agreement with the previous report as shown in Fig. 1(a) [16]. After the addition of a high bulk concentration of ∼10⁻³ M PhSeH or PhSeCN, the surface plasmon band was shifted to a longer wavelength region indicating the aggregation of

Summary and conclusions

The CN scission requiring higher energy should affect the adsorption characteristics as evidenced from the slow aggregation behaviors to raise surface plasmon shifts. The SERS results indicate that PhSeCN adsorb with the Se–CN bond cleaved by the gold nanoparticle surfaces during the self-assembly process. The CN bands could be observed at 2110–2150 cm⁻¹, indicating that a trace amount of CN species should remain on Au. The adsorption of the CN species appeared to be favorable at a high

Acknowledgments

SWJ thanks helpful guidance from Prof. Seong Keun Kim and Prof. Kwan Kim and appreciate kind comments on DFT calculations from Mr. Donghyung Lee and Prof. Seokmin Shin. This work was supported by the Soongsil University Research Fund.

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Self-assembled monolayers (SAMs) of n-dodecanethiol (C₁₂H₂₅SH) and n-dodecaneselenol (C₁₂H₂₅SeH) on polycrystalline copper have been elaborated with the purpose of achieving densely packed and crystalline-like assemblies. By combining the surface sensitivity of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), the effect of the self-assembly time (15 min, 30 min, 1 h, 2 h and 24 h) on the formation of n-dodecanethiol and n-dodecaneselenol monolayers on untreated and electrochemically reduced polycrystalline copper has been investigated. On electrochemically reduced copper, PM-IRRAS spectroscopy shows that both molecules are able to form well organized layers. SFG spectroscopy indicates that the C₁₂H₂₅SeH SAMs are slightly better ordered than those achieved with C₁₂H₂₅SH. On untreated copper, the two molecules lead to different film organizations. Both PM-IRRAS and SFG indicate that C₁₂H₂₅SH SAMs are of the same film quality as those obtained on electrochemically reduced copper. On the contrary, C₁₂H₂₅SeH monolayers are invariably poorly organized at the molecular level.

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Comparative adsorption of phenyl selenolate and selenocyanate on Au nanoparticle surfaces

Abstract

Introduction

Section snippets

Experimental

UV–vis absorbance spectra

Summary and conclusions

Acknowledgments

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Tetrahedron

Surf. Sci.

Vib. Spectrosc.

Chem. Rev.

J. Am. Chem. Soc.

Chem. Mater.

J. Am. Chem. Soc.

J. Phys. Chem. B

Langmuir

Surface Enhanced Raman Scattering

Chem. Lett.