J. Phys. Chem. C 2007, 111, 8149-8152
8149
ARTICLES Pulsed Laser Deposited Au Nanoparticles as Substrates for Surface-Enhanced Vibrational Spectroscopy Concepcio´ n Domingo,† Vincenzo Resta,‡ Santiago Sanchez-Cortes,*,† Jose´ V. Garcı´a-Ramos,† and Jose´ Gonzalo‡ Instituto de Estructura de la Materia and Laser Processing Group, Instituto de O Ä ptica, CSIC, Serrano 121, 28006 Madrid, Spain ReceiVed: February 8, 2007; In Final Form: April 4, 2007
Gold nanoparticles with enhanced efficiency as substrates for surface-enhanced Raman and infrared spectroscopies have been grown by pulsed laser deposition on glass and CaF2 substrates. Transmission electron microscopy analysis has evidenced the presence of nanoparticles with characteristic sizes of several nanometers in diameter. The capabilities of the produced nanoparticles as enhancer substrates have been investigated using the dithiocarbamate fungicide thiram as a test molecule. The enhancement factors and features of SERS and SEIRA spectra are compared to those observed from large Au nanoparticles produced by evaporation at high temperatures. Slight changes in favor of the new substrates are discussed.
Introduction The increasing interest in fabricating nanostructured metallic surfaces with sensitivity, selectivity, and reproducibility good enough to be established as extended-use substrates for surfaceenhanced vibrational (Raman scattering and infrared absorption) spectroscopies (SERS and SEIRA) promotes the application of different methods for producing efficient enhancers such as metal nanoparticles (NPs). Moreover, robustness and stability of the substrates are considered to be important factors concerning the possibilities of SERS and SEIRA becoming important molecule-specific sensing technologies utilizing vibrational signatures. In fact, SERS-based sensors for quantitative biowarfare agents and glucose detection have been recently reported.1 The most efficient nanostructured SERS substrates seem to be Ag colloids, prepared by chemical reduction, dispersed in water. Nevertheless, an aqueous medium is neither appropriate for SEIRA measurements, due to the strong absorption of water in the mid-infrared region, nor appropriate for building lab-on-a-chip SERS-based sensors. For such purposes solid substrates are required, with metal evaporation2 and immobilization of colloids3-5 being the two methods most commonly used for fabricating NPs. Besides, different lithography techniques are progressively being used for constructing size- and shape-controlled metal NPs with tunable surface plasmon resonances.1,6 An alternative route to producing metal NPs on solid surfaces is pulsed laser deposition (PLD). This technique provides NPs with typical dimensions in the range of a few nanometers and reduced size dispersion,7 as well as excellent control of the NP dimensions. Recently, PLD combined with postdeposition excimer laser nanostructuring has been reported for producing supported arrays of Ag NPs on SiO2, * Corresponding author. E-mail:
[email protected]. † Instituto de Estructura de la Materia. ‡ Instituto de O Ä ptica.
and their effectiveness as SERS substrates has been tested with thin films of tetrahedral amorphous carbon.8 In this work we have produced Au NPs supported on glass and CaF2 substrates by PLD, and their potential for providing enhanced Raman and infrared spectra has been investigated using one of the dithiocarbamate fungicides, thiram, as a test molecule. There is much interest in trace detection in soils, water, and food of such fungicides used in agriculture, owing to their environmental impact and their potential danger to human health. SERS and SEIRA spectroscopies could be applied for such analytical purposes not only because of their high sensitivity but also because, as molecular chemosensing techniques, they supply information about the chemical state at which the molecule to be detected is retained. Thiram renders dithiocarbamate groups in the presence of metal surfaces which remain strongly attached on the surface through one or two sulfur atoms.2,9,10 Moreover, the vibrational spectrum of dithiocarbamate group is also sensitive to the interaction strength with the surface,11 affording valuable information on the physicochemical properties of this surface. The characteristics and enhancement factors of the resulting spectra are similar to the previously reported vibrational enhanced spectra of thiram taken on nanostructured Au films obtained by evaporation at high temperatures,2 with the additional advantages of no degradation of substrates after illumination with the visible (633 nm) laser employed in SERS analysis. Experimental Section Au NPs were produced by PLD in vacuum (5 × 10-6 mbar) on glass and CaF2 (infrared transparent) substrates using an ArF excimer laser (λ ) 193 nm, τ ) 20 ns full width at halfmaximum (fwhm)) operating at 20 Hz. The laser beam was focused on a gold target at an angle of incidence of 45° to lead to a laser fluence of ∼2.7 J cm-2. The substrate was in all cases
10.1021/jp0710943 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
8150 J. Phys. Chem. C, Vol. 111, No. 23, 2007 held at room temperature and placed at a distance of 40 mm from the target surface. The deposition rate of Au in vacuum was determined by means of in situ reflectivity measurements prior to the production of the NPs.7 From this calibration and earlier results on PLD of Au NPs,11 we have chosen the number of laser pulses on the Au target equal to 500, leading to a mass equivalent thickness of 2.5 nm. The in-plane NP morphology has been determined by transmission electron microscopy (TEM) using a JEOL 2000FX microscope. For this purpose we prepared samples of Au NPs on special substrates consisting of 10 nm thick amorphous Al2O3 (a-Al2O3) layers grown by PLD on carbon coated mica (Al2O3/ C-mica) substrates. These substrates allow full imaging of the NPs, due to the reduced thickness of the a-Al2O3 layer while enabling the nucleation of the NPs on the surface of an oxide substrate similar to the case of the glass ones. After deposition, the samples were floated off in deionized water and picked up on copper TEM grids for observation. TEM images were digitally processed. In all cases an area of at least 200 × 200 nm2 was analyzed in order to obtain meaningful statistical data. Further details of the synthesis procedure, as well as their morphological analysis, can be found elsewhere.7,12 The optical extinction spectra of the samples deposited on glass and CaF2 substrates have been determined in the range 350-800 nm as ln(1/T), where T is the transmission measured at normal incidence using a WVASE J. A. Woollam ellipsometer. All the measurements were corrected for the spectral emission profile of the lamp, the wavelength-dependent transmission of the optical components, and the spectral response of the photodiodes. Bare substrates were used to rule out any contribution from them. Finally, in order to compare the effectiveness of the laser deposited Au NPs as SERS (at 633 nm) and SEIRA substrates with the enhancer properties of a well-proven system, we produced Au evaporated films similar to the ones previously used for SERS (at 1064 nm) and SEIRA detection of thiram.2 In brief, 10 nm (mass thickness) of gold was evaporated on CaF2 in a vacuum chamber held at a pressure of 10-6 mbar, at a deposition rate of 0.01 nm/s. The Au content of either pulsed laser deposited or evaporated samples was analyzed by Rutherford backscattering spectrometry (RBS) using a 3 MeV 4He2+ beam, and the experimental spectra were analyzed using the RUMP code.7 SERS and SEIRA samples were prepared by depositing an aliquot, 10 µL (0.5 × 0.5 cm2), of a solution of thiram (0.001% in methanol) on both laser deposited and evaporated Au NPs; the solvent was allowed to evaporate and afterward washed with ethanol. SERS spectra were recorded with a Raman microscope Renishaw RM2000, equipped with a Leica microscope (50× objective), a refrigerated CCD camera, and a He-Ne laser at 633 nm. Laser power was kept at 10 mW, and 100 s was the measuring time. Transmission mode SEIRA spectra (500 scans) using both Au/CaF2 substrates were measured on a FTIR Bruker IFS66 spectrometer provided with a DTGS detector. The nominal spectral resolution was 8 cm-1. Results and Discussion The Au content of the pulsed laser deposited samples was found to be [Au] ) (11 ( 1) × 1015 atoms cm-2 for all laser deposited samples independent of the substrate (glass, CaF2, or Al2O3/C-mica). Figure 1 shows a plan-view TEM image corresponding to Au NPs deposited on Al2O3/C-mica. The image shows a mixture of large and small NPs, with the latter having typical dimensions
