Investigation of chemically modified barium titanate beads as surface-enhanced Raman scattering (SERS) active substrates for the detection of benzene thiol, 1,2-benzene dithiol, and rhodamine 6G

Abstract

SERS active surfaces were prepared by depositing silver films using Tollen's reaction on to barium titanate beads. The SERS activity of the resulting surfaces was probed using two thiols (benzene thiol and 1,2-benzene dithiol) and rhodamine 6G. The intensity of the SERS signal for the three analytes was investigated as a function of silver deposition time. The results indicate that the SERS intensity increased with increasing thickness of the silver film until a maximum signal intensity was achieved; additional silver deposition resulted in a decrease in the SERS intensity for all of the studied molecules. SEM measurement of the Ag coated barium titanate beads, as a function of silver deposition time, indicate that maximum SERS intensity corresponded with the formation of atomic scale islands of silver nanoparticles. Complete silver coverage of the beads resulted in a decreased SERS signal and the most intense SERS signals were observed at deposition times of 30 min for the thiols and 20 min for rhodamine 6G.

Introduction

Since its discovery surface-enhanced Raman scattering (SERS) spectroscopy has proven to be a powerful spectroscopic tool in studying surface adsorbates. Its high sensitivity and low detection limit (concentrations of <10⁻⁸ M), along with reports of single molecule detection, enhances its applicability when compared to normal Raman spectroscopy [1], [2] and infra-red spectroscopy [3], [4], [5], [6]. SERS enhancements of up to 10⁶–10⁷ have been observed for molecules adsorbed onto electrochemically roughened surfaces [7], [8]. Single molecule detection by SERS was first reported by Nie and Emory [9], and Kneipp et al. [10]. Kneipp et al. observed single molecule SERS while investigating the molecular cross sections available for SERS using crystal violet (hexamethyl violet 10B dye) molecules in aqueous colloidal silver solution. Large enhancements, six orders of magnitude [14], were observed with near-infra-red excitations and crystal violet concentration of 3.3 × 10⁻¹⁴ M. Recently Ruan et al. [11] have reported single molecule detection of thionine on aggregated gold nanoparticles by SERS. The low detection limit of SERS along with the molecular specificity of vibrational spectroscopy makes it an ideal tool for probing adsorbate surface interactions. The enhanced applicability of SERS has led to an increased interest in the development of reproducible, stable and easily prepared SERS active substrates. Concurrent with this increase in SERS applicability is a renewed interest in deciphering the factors which are responsible for the SERS phenomenon.

Applicability of the SERS technique continues to be limited by the narrow range of metals which can serve as SERS substrates, mainly Ag, Au, and Cu, although SERS enhancement has been reported for other metals [12]. Studies in which SERS was observed on metals other than Ag, Au and Cu, have produced moderate enhancement in the region of 10⁴. Another impediment to expanded applicability of SERS is the requirement for atomic scale roughness and the need for reproducibility in substrate preparation specifically if the technique is used as a quantitative tool [13].

Among the metals used in substrate preparation silver exhibits the highest enhancement. Its dielectric constant near the Frohlich frequency gives rise to an intense surface plasmon absorption in the visible wavelength region [14]. Silver electrodes (roughened electrochemically) were the first substrates to produce enhanced Raman signals [15]. Since the discovery of SERS there have been several approaches in the development of SERS active substrates. These include colloidal dispersion of metal particles [16], [17], [18], [19], [20], vapor deposition of silver metal films [21], the use of Tollens reaction to deposit silver films on surfaces (a modification of which is reported in this study) [22], optical tweezers have been used to move silver nanoparticles into near-field contact with immobilized particles to form isolated SERS active Ag particle dimmers [23], silver-doped sol–gel films [24], fabrication of silver clusters on glassy carbon [5], electrocrystallisation of silver in a halide free electrolyte on electrodes [25], and photographic paper [26], silver nanodisks on silicon and silver nanocomposites of different shapes have recently been developed as SERS substrates [27], [28]. Also lithographically roughened silver surfaces have exhibited intense SERS signals [29].

Recently tip-enhanced Raman spectroscopy TERS has been reported with Ag or Au tips [30], [31], [32]. The TER technique involves the use of sharp metal tips with areas of approximately 100 nm² to create ‘hot spots’ to excite localized surface plasmons thereby enhancing Raman signals at the metal tips. TERS of brilliant cresyl blue (dye) molecules have been investigated using gold surface with a silver (substrate) tip of 1 nm gap and 2 nm height. An enhancement of 10⁶ was reported (this is lower than that observed for SERS). TERS has been reported for glycerin and rhodamine 6G dye as well as biological molecules such as acetaminophen and poly-l-lysine. The detection sensitivity for these molecules was shown to be 0.1 pg. TERS of CN⁻ ions and malachite green isothiocyanate (MGITC) molecules on Au and Pt using either Au or Ir tips showed an enhancements of 4 × 10⁵ for CN⁻ and 10⁶ for MGITC.

Ferroelectric barium titanate (BaTiO₃) is extensively used in microelectronics and integrated optics technology with specific applications in data storage and as capacitors [33], [34]. The high electromechanical coupling coefficients have led to the application of these ferroelectric materials as actuators, sensors, medical imaging devices, and ultrasonic transducers [35]. Nanoparticles (dots, rods and wires) of barium titanate have received attention because of their potential to increase ferroelectric nonvolatile-memory [36]. Recently Xia et al. [37] have reported a method for the fabrication of BaTiO₃ nanofibers. The resulting ribbon like nanofibers had dimensions of 200 nm width and 75 nm thickness. XRD analysis confirmed that the nanofibers consisted of amorphous BaTiO₃ without residual polymer. Because of the unique intrinsic physical properties of barium titanate and its potential applications we have explored the development of surface modified barium titanate beads as SERS substrates.

In this work, barium titanate spheres coated with nanometer sized silver nanoparticles were investigated for their ability to serve as SERS-active substrates. Silver film deposition was achieved by Tollens reaction. The effect of silver film thickness and morphology, as determined by Ag deposition time, on the intensity of the SERS signal was investigated for two thiols (benzene thiol and 1,2-benzene dithiol,) along with the fluorescent dye rhodamine 6G (Rh6G). The surface morphology of the silver coated beads was studied using SEM. The most intense SERS responses, for the thiols, were observed for 30 min deposition time while for Rh6G the maximum SERS intensity was observed for surfaces which had a silver deposition time of 20 min. SEM studies of the silver coated beads showed nanosize silver island deposits on the bead surface at 20 min deposition time. At longer deposition times the nanosize silver islands were replaced by larger diameter silver deposits whose sizes exceeded what is required for production of intense SERS spectra. The applicability of chemically modified barium titanate beads to act as a highly sensitive SERS surface is established.