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Surface-enhanced Raman spectroscopy (SERS) using Ag nanoparticle films produced by pulsed laser deposition
Applied Surface Science 264 (2013) 31–35
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface-enhanced Raman spectroscopy (SERS) using Ag nanoparticle films produced by pulsed laser deposition C.A. Smyth ∗ , I. Mirza, J.G. Lunney, E.M. McCabe School of Physics, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e
i n f o
Article history: Received 21 March 2012 Received in revised form 17 July 2012 Accepted 15 September 2012 Available online 18 October 2012 Keywords: Pulsed laser deposition Surface-enhanced Raman spectroscopy
a b s t r a c t Thin silver nanoparticle films, of thickness 7 nm, were deposited onto glass microslides using pulsed laser deposition (PLD). The films were then characterised using UV–vis spectroscopy and scanning transmission electron microscopy before Rhodamine 6G was deposited onto them for investigation using surface-enhanced Raman spectroscopy (SERS). The sensitivity obtained using SERS was compared to that obtained using a colloidal silver suspension and also to a commercial SERS substrate. The reproducibility of the films is also examined using statistical analysis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of surface-enhanced Raman spectroscopy (SERS) in the latter half of the 1970s [1,2] there has been much focus on the phenomenon, both in terms of explaining its origins and of maximising its applicability to chemical analysis. Possessing an ability to enhance Raman scattering by orders of 106 and higher, it is a technique of huge potential, though its progress has often been hampered with issues regarding reproducibility. The enhancement of the Raman signal is thought to be due to plasmonic enhancement of the incident radiation intensity in the vicinity of nanoscale features on a metallic surface, and also due to chemical enhancements arising from adsorption of the target material on the nanostructured metal substrate [3]. Nanoparticle metal films (nanoparticles deposited on solid substrates) with feature sizes in the range of 1–100 nm have attracted much interest due their novel magnetic, catalytic and optical properties [4]. Several physical deposition techniques such as sputtering, chemical vapour deposition, pulsed laser deposition (PLD) and thermal evaporation have been used for the fabrication of metal nanoparticle films. In particular, PLD is a relatively simple and effective nano-fabrication technique [5]. In PLD, a high power pulsed laser is focused on the target surface. For a sufficiently high laser fluence (for metals ∼1 J/cm2 ), each laser pulse vaporises, or ablates, a small amount of material which expands rapidly from the target surface in vacuum. This ablated material provides the deposition flux for thin film growth, and the morphology and size
∗ Corresponding author. E-mail address: [email protected] (C.A. Smyth). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.078
of nanoparticles can be controlled by the number of laser pulses. Nanostructured Ag substrates prepared using PLD in a background gas have proved to be a promising candidate for SERS [6], and have previously performed well in comparison with commercial substrates [7]. Raman enhancement can also be obtained by using colloidal particles in solution, where their nanoscale dimensions leads to the excitation of localised plasmon oscillations. In this paper we use Rhodamine 6G, a dye commonly used in SERS characterisation studies, and benzotriazole to investigate the relative SERS sensitivity of an Ag nanoparticle film prepared by ns-PLD, a solution of silver colloids and a commercial substrate. Statistical analysis was used to quantify the SERS performance of each of the three approaches. 2. Experimental methods 2.1. PLD of Ag nanoparticle films The Ag film was deposited on thin microscope glass slides and SiO2 coated scanning transmission electron microscopy (STEM) grids. The PLD was done in a high vacuum environment (5 × 10−5 mbar) at room temperature. Glass slides were pre-cleaned using acetone and isopropanol in an ultrasonic bath (10 min each) and were rinsed later with deionized water. An Ag target was ablated using a 248 nm, 25 ns KrF excimer laser operating at 10 Hz. The target was continuously rotated in order to avoid drilling a hole in the target surface. The laser was focused on the target to yield an average fluence of 0.8 J cm−2 on spot area of 0.0375 cm2 . A planar Langmuir ion probe with an area of 0.09 cm2 and biased at −25 V, to reject plasma electrons was placed 6.5 cm from the ablation spot. The ion probe was used to measure both the
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C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 1. (a) The ion time-of-flight signal and (b) the corresponding ion energy distribution for the ablation of Ag at 0.8 J cm−2 .
ion fluence and energy distribution arising the plasma flow in the laser ablation plume. The target-to-substrate distance is 8 cm. 2.2. Deposition rate measurement The integral of ion probe signal may be used to obtain an estimate of the deposition per pulse since, at constant ablation conditions, the ratio of the ion fluence to atom fluence per pulse will be constant. The actual deposition rate was found by using grazing incidence X-ray reflectivity (XRR) to meaure the film thickness and density after a given number of laser shots. A 30 kV Zeiss-Ultra STEM was used to examine the morphology of the nanoparticle film on a SiO2 -coated TEM grid placed at the same position as the glass substrate. The UV–vis optical absorption of the nanoparticle films was measured using a Cary 50 single beam UV–vis spectrophotometer. 2.3. Colloidal nanoparticle synthesis The colloidal silver nanoparticles were synthesised according to a method developed by Leopold and Lendl [8]. Briefly, silver nitrate was reduced using an alkaline hydroxylamine hydrochloride solution, the alkalinity of which was obtained through addition of sodium hydroxide. A final pH of 7 is obtained from the resultant nanoparticle solution. 10 mL of a 10−2 M solution of silver nitrate was added dropwise to 90 mL of a 1.67 × 10−3 M hydroxylamine hydrochloride solution, containing 3.33 × 10−3 M sodium hydroxide. Immediately after preparation the nanoparticles in solution were aggregated with 1 × 10−2 M sodium chloride, which resulted
Fig. 2. (a) STEM image and (b) size histogram of Ag nanoparticle-film with equivalent thickness of 7 nm prepared at a laser fluence of 0.8 J cm−2 using ns-PLD. The line in figure (b) is the log-normal fitting to the experimental data.
in the eventual formation of the colloidal aggregates. All the chemicals were obtained from Sigma–Aldrich. 2.4. Klarite® substrate The commercial SERS substrate Klarite® substrate was obtained from Renishaw Diagnostics [9]. This silicon substrate is patterned with micron scale pyramidal indentations and is coated with a thin layer of gold. 2.5. SERS measurements The system used in the Raman analysis was a Horiba Jobin Yvon LamRAM HR using 633 nm laser excitation with a power of up to 12 mW, a diffraction grating of 600 lines/mm and a charge coupled device detector. SERS spectra were taken in solution and a 10× objective lens was used with an exposure time of 5 s, with the typical sample volume consisting of a 6 L droplet. 3. Results and discussion 3.1. Langmuir ion probe measurements Fig. 1(a) shows the ion time-of-flight signal of the laser ablation plume. The ion flux rises rapidly as the plume arrives at the probe and falls as the plume expands beyond the probe position. The
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 3. SEM images of the Klarite substrate showing (a) a low-magnification view of the array of surface features and (b) a high-magnification image of one of these features.
corresponding ion energy distribution (dN/dE) which is obtained from the ion current I(t) using Eq. (1), is plotted in Fig. 1(b). I(t)3 dN = dE Amed2
(1)
where I(t) is the ion current, t is the ion time of flight, A is the area of the probe, m is the ion mass and d is the target-probe distance. The average ion energy is ∼29 eV, but the distribution extends up to 100 eV, indicating that there is some self-sputtering during film growth. Integrating the ion signal yields an ion fluence of 1.5 × 1012 ions cm−2 per ablation pulse. Using a 1/r2 scaling to account for the difference in the position of the Langmuir ion probe and the substrate yields an ion fluence of 1.0 × 1012 cm−2 pulse at the substrate. The actual deposition rate was found by using XRR to measure the film thickness and density after a given number laser pulses. For 37,000 pulses the film thickness was 10.2 nm and the density was 6.8 g cm−3 which is 65% of solid density. Thus this film has an equivalent solid density thickness of 6.6 nm. The deposited atomic fluence per pulse is 1.0 × 1012 cm−2 , which is similar to ion fluence, indicating that the plume is nearly fully ionised.
33
Fig. 4. Absorption spectra of (a) the colloidal Ag nanoparticles and (b) the 7 nm Ag nanoparticle film.
diameter was obtained by fitting the size distribution to the lognormal distribution function. 3.3. Scanning electron microscopy of Klarite film Scanning electron microscopy (SEM) was used to characterise the commercial Klarite films. The images in Fig. 3 show details of the periodic distribution of the pyrimidical structures on the Klarite films and also a close-up of one of these structures. The images show the structures to measure about 1.2 m along all sides. 3.4. Optical absorption spectra Three different enhancing media were examined in this paper, and in Fig. 4 absorption plots of the silver colloidal solution, the Ag nanoparticle film and the Rhodamine 6G dye is shown. The silver colloid absorption in Fig. 4(a) shows a plasmonic absorption band centred at about 410 nm. The 7 nm PLD film shows a surface plasmon resonance (SPR) peak at the longer wavelength of about 550 nm (Fig. 4(b) and is much broader than that measured for the colloidal suspension.
3.2. Morphology of silver films 3.5. SERS Fig. 2(a) shows a STEM image of a 7 nm Ag film prepared at a laser fluence of 0.8 J cm−2 . This image shows that the film contains well-separated nanoparticles with diameters in the range of 5–20 nm with a mean diameter of 6.5 ± 0.5 nm. The mean
Regarding the SERS analysis using the three enhancing media, a solution of 6.63 × 10−5 M Rhodamine 6G was measured in each of the three cases, with the results shown in Fig. 5(a). The Rhodamine
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C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 5. SERS spectra of (a) 6. 63 × 10−5 M Rhodamine 6G and (b) 4.2 × 10−4 M benzotriazole using three different enhancing media. Fig. 6. Peak intensities for the six principal peaks in (a) Rhodamine 6G and (b) benzotriazole, with error bars representing the standard deviation in the data.
6G dye shows some well-defined peaks within the region examined between 500 and 1625 cm−1 . By comparing the signals obtained from each of the three SERS-active media the Klarite substrate is seen to perform significantly worse in terms of the magnitude of the signal recorded. The PLD film and the nanoparticle suspension are seen to show a similar level of SERS response, with the colloidal nanoparticle suspension performing slightly better. A second compound was also investigated, so as to check for repeatability across the media prepared here. The response of each SERS media to 4.2 × 10−4 M benzotriazole was thus measured, with the spectra shown in Fig. 5(b). As can be observed from these spectra, the same relationship is evident, with the colloids proving the most efficient SERS medium and with both the colloids and the PLD film far outperforming the Klarite film. While the absolute signals quite clearly show greater enhancements using the PLD film and colloidal suspension, the three media were further examined regarding performance reproducibility. This was done by calculating the standard deviations of the six principal peaks of Rhodamine 6G, using ten spectra taken at separate areas within the deposited droplet. This data is displayed in Fig. 6(a), with each set of columns showing the relative peak heights for each of the enhancing media at the respective peak position. In the case of Rhodamine 6G in Fig. 6(a) it is immediately evident that the best-performing medium in terms of the obtained signal strength is the colloidal nanoparticle suspension, but it also has the worst reproducibility. While the Klarite film shows the best reproducibility with the lowest standard deviations, this is accompanied by a significantly lower signal than that obtained using the other two media. The PLD film displays a good trade-off in overall performance. It offers better reproducibility than the colloidal
nanoparticles, and only a slightly lower signal level. While the standard deviations for the PLD films are slightly worse than for Klarite, PLD films offer very significantly better SERS signal levels. Fig. 6(b) shows similar data for the case of benzotriazole, and, as was the case for the Rhodamine 6G, the silver colloidal nanoparticles give the best SERS signals. In this case the PLD film offers relatively lower values of SERS signals than the colloids and also higher standard deviations. Once again the standard deviations are the lowest for the Klarite film but again this is accompanied by a much lower SERS signal. The %RSD (percentage relative standard deviation) is also used to examine the reproducibility data, and is calculated according to the following formula: %RSD = 100
(2)
For Rhodamine 6G, using the 615 cm−1 peak as the reference, the %RSD is again lowest for the Klarite substrate, and in the case of the nanoparticle colloids used for this comparison is very high at 51%. This is a worst case extreme example for the colloids as this value has been observed to range, on average, at about 20% for different 10-value data sets. This can be observed in Fig. 7(b) for the %RSD values for the 792 cm−1 peak of benzotriazole. The colloidal nanoparticles used with benzotriazole offer a similar standard deviation to Klarite while the PLD film shows a less reproducible response. The %RSD values highlight the variability in reproducibility for both the 7 nm PLD nanoparticle film and the silver
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
35
4. Conclusions In conclusion the commercial Klarite substrate offers very small SERS signal levels, which may limit their application at low analyte concentrations. In the case of examining Rhodamine 6G the PLD substrate offers a very useful compromise of good distribution statistics alongside good SERS signal levels. Examining benzotriazole shows that further optimisation of the PLD substrates is necessary, but their good SERS performance when compared with silver colloids is promising. References
Fig. 7. %RSD values for the various enhancing media for (a) Rhodamine 6G and (b) benzotriazole.
colloids. There is scope to further investigate and optimise reproducibility issues for the PLD films with a view to continue to offer strong SERS signals compared to commercial substrates such as Klarite while offering improved reproducibility.
[1] D.L. Jeanmarie, R.P. Van Duyne, Surface Raman spectroelectrochemistry, part 1: heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode, Journal of Electroanalytical Chemistry 84 (1977) p120. [2] M.G. Albrecht, J.A. Creighton, Anomalously intense Raman-spectra of pyridine at a silver electrode, Journal of the American Chemical Society 99 (15) (1977) 5215–5217. [3] E.C. Le Ru, P.G. Etchegoin, Principles of Surface-enhanced Raman Spectroscopy and Related Plasmonic Effects, Elsevier, Amsterdam, Boston, 2009. [4] W. Eberhardt, Clusters as new materials, Surface Science 500 (1–3) (2002) 242–270. [5] T. Donnelly, B. Doggett, J.G. Lunney, Pulsed laser deposition of nanostructured Ag films, Applied Surface Science 252 (13) (2006) 4445–4448. [6] C. D’Andrea, et al., The controlled pulsed laser deposition of Ag nanoparticle arrays for surface enhanced Raman scattering, Nanotechnology 20 (24) (2009). [7] N.R. Agarwal, et al., Au nanoparticle arrays produced by pulsed laser deposition for surface enhanced Raman spectroscopy, Applied Surface Science (2011), http://dx.doi.org/10.1016/j.apsusc.2011.12.030. [8] N. Leopold, B. Lendl, A new method for fast preparation of highly surfaceenhanced Raman scattering (SERS) active silver colloids at room temperature by reduction of silver nitrate with hydroxylamine hydrochloride, The Journal of Physical Chemistry B 107 (24) (2003) 5723–5727. [9] Klarite® SERS Detection Substrate, available from: http://www. renishawdiagnostics.com/en/klarite-sers-detection-substrates–12515
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface-enhanced Raman spectroscopy (SERS) using Ag nanoparticle films produced by pulsed laser deposition C.A. Smyth ∗ , I. Mirza, J.G. Lunney, E.M. McCabe School of Physics, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e
i n f o
Article history: Received 21 March 2012 Received in revised form 17 July 2012 Accepted 15 September 2012 Available online 18 October 2012 Keywords: Pulsed laser deposition Surface-enhanced Raman spectroscopy
a b s t r a c t Thin silver nanoparticle films, of thickness 7 nm, were deposited onto glass microslides using pulsed laser deposition (PLD). The films were then characterised using UV–vis spectroscopy and scanning transmission electron microscopy before Rhodamine 6G was deposited onto them for investigation using surface-enhanced Raman spectroscopy (SERS). The sensitivity obtained using SERS was compared to that obtained using a colloidal silver suspension and also to a commercial SERS substrate. The reproducibility of the films is also examined using statistical analysis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of surface-enhanced Raman spectroscopy (SERS) in the latter half of the 1970s [1,2] there has been much focus on the phenomenon, both in terms of explaining its origins and of maximising its applicability to chemical analysis. Possessing an ability to enhance Raman scattering by orders of 106 and higher, it is a technique of huge potential, though its progress has often been hampered with issues regarding reproducibility. The enhancement of the Raman signal is thought to be due to plasmonic enhancement of the incident radiation intensity in the vicinity of nanoscale features on a metallic surface, and also due to chemical enhancements arising from adsorption of the target material on the nanostructured metal substrate [3]. Nanoparticle metal films (nanoparticles deposited on solid substrates) with feature sizes in the range of 1–100 nm have attracted much interest due their novel magnetic, catalytic and optical properties [4]. Several physical deposition techniques such as sputtering, chemical vapour deposition, pulsed laser deposition (PLD) and thermal evaporation have been used for the fabrication of metal nanoparticle films. In particular, PLD is a relatively simple and effective nano-fabrication technique [5]. In PLD, a high power pulsed laser is focused on the target surface. For a sufficiently high laser fluence (for metals ∼1 J/cm2 ), each laser pulse vaporises, or ablates, a small amount of material which expands rapidly from the target surface in vacuum. This ablated material provides the deposition flux for thin film growth, and the morphology and size
∗ Corresponding author. E-mail address: [email protected] (C.A. Smyth). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.078
of nanoparticles can be controlled by the number of laser pulses. Nanostructured Ag substrates prepared using PLD in a background gas have proved to be a promising candidate for SERS [6], and have previously performed well in comparison with commercial substrates [7]. Raman enhancement can also be obtained by using colloidal particles in solution, where their nanoscale dimensions leads to the excitation of localised plasmon oscillations. In this paper we use Rhodamine 6G, a dye commonly used in SERS characterisation studies, and benzotriazole to investigate the relative SERS sensitivity of an Ag nanoparticle film prepared by ns-PLD, a solution of silver colloids and a commercial substrate. Statistical analysis was used to quantify the SERS performance of each of the three approaches. 2. Experimental methods 2.1. PLD of Ag nanoparticle films The Ag film was deposited on thin microscope glass slides and SiO2 coated scanning transmission electron microscopy (STEM) grids. The PLD was done in a high vacuum environment (5 × 10−5 mbar) at room temperature. Glass slides were pre-cleaned using acetone and isopropanol in an ultrasonic bath (10 min each) and were rinsed later with deionized water. An Ag target was ablated using a 248 nm, 25 ns KrF excimer laser operating at 10 Hz. The target was continuously rotated in order to avoid drilling a hole in the target surface. The laser was focused on the target to yield an average fluence of 0.8 J cm−2 on spot area of 0.0375 cm2 . A planar Langmuir ion probe with an area of 0.09 cm2 and biased at −25 V, to reject plasma electrons was placed 6.5 cm from the ablation spot. The ion probe was used to measure both the
32
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 1. (a) The ion time-of-flight signal and (b) the corresponding ion energy distribution for the ablation of Ag at 0.8 J cm−2 .
ion fluence and energy distribution arising the plasma flow in the laser ablation plume. The target-to-substrate distance is 8 cm. 2.2. Deposition rate measurement The integral of ion probe signal may be used to obtain an estimate of the deposition per pulse since, at constant ablation conditions, the ratio of the ion fluence to atom fluence per pulse will be constant. The actual deposition rate was found by using grazing incidence X-ray reflectivity (XRR) to meaure the film thickness and density after a given number of laser shots. A 30 kV Zeiss-Ultra STEM was used to examine the morphology of the nanoparticle film on a SiO2 -coated TEM grid placed at the same position as the glass substrate. The UV–vis optical absorption of the nanoparticle films was measured using a Cary 50 single beam UV–vis spectrophotometer. 2.3. Colloidal nanoparticle synthesis The colloidal silver nanoparticles were synthesised according to a method developed by Leopold and Lendl [8]. Briefly, silver nitrate was reduced using an alkaline hydroxylamine hydrochloride solution, the alkalinity of which was obtained through addition of sodium hydroxide. A final pH of 7 is obtained from the resultant nanoparticle solution. 10 mL of a 10−2 M solution of silver nitrate was added dropwise to 90 mL of a 1.67 × 10−3 M hydroxylamine hydrochloride solution, containing 3.33 × 10−3 M sodium hydroxide. Immediately after preparation the nanoparticles in solution were aggregated with 1 × 10−2 M sodium chloride, which resulted
Fig. 2. (a) STEM image and (b) size histogram of Ag nanoparticle-film with equivalent thickness of 7 nm prepared at a laser fluence of 0.8 J cm−2 using ns-PLD. The line in figure (b) is the log-normal fitting to the experimental data.
in the eventual formation of the colloidal aggregates. All the chemicals were obtained from Sigma–Aldrich. 2.4. Klarite® substrate The commercial SERS substrate Klarite® substrate was obtained from Renishaw Diagnostics [9]. This silicon substrate is patterned with micron scale pyramidal indentations and is coated with a thin layer of gold. 2.5. SERS measurements The system used in the Raman analysis was a Horiba Jobin Yvon LamRAM HR using 633 nm laser excitation with a power of up to 12 mW, a diffraction grating of 600 lines/mm and a charge coupled device detector. SERS spectra were taken in solution and a 10× objective lens was used with an exposure time of 5 s, with the typical sample volume consisting of a 6 L droplet. 3. Results and discussion 3.1. Langmuir ion probe measurements Fig. 1(a) shows the ion time-of-flight signal of the laser ablation plume. The ion flux rises rapidly as the plume arrives at the probe and falls as the plume expands beyond the probe position. The
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 3. SEM images of the Klarite substrate showing (a) a low-magnification view of the array of surface features and (b) a high-magnification image of one of these features.
corresponding ion energy distribution (dN/dE) which is obtained from the ion current I(t) using Eq. (1), is plotted in Fig. 1(b). I(t)3 dN = dE Amed2
(1)
where I(t) is the ion current, t is the ion time of flight, A is the area of the probe, m is the ion mass and d is the target-probe distance. The average ion energy is ∼29 eV, but the distribution extends up to 100 eV, indicating that there is some self-sputtering during film growth. Integrating the ion signal yields an ion fluence of 1.5 × 1012 ions cm−2 per ablation pulse. Using a 1/r2 scaling to account for the difference in the position of the Langmuir ion probe and the substrate yields an ion fluence of 1.0 × 1012 cm−2 pulse at the substrate. The actual deposition rate was found by using XRR to measure the film thickness and density after a given number laser pulses. For 37,000 pulses the film thickness was 10.2 nm and the density was 6.8 g cm−3 which is 65% of solid density. Thus this film has an equivalent solid density thickness of 6.6 nm. The deposited atomic fluence per pulse is 1.0 × 1012 cm−2 , which is similar to ion fluence, indicating that the plume is nearly fully ionised.
33
Fig. 4. Absorption spectra of (a) the colloidal Ag nanoparticles and (b) the 7 nm Ag nanoparticle film.
diameter was obtained by fitting the size distribution to the lognormal distribution function. 3.3. Scanning electron microscopy of Klarite film Scanning electron microscopy (SEM) was used to characterise the commercial Klarite films. The images in Fig. 3 show details of the periodic distribution of the pyrimidical structures on the Klarite films and also a close-up of one of these structures. The images show the structures to measure about 1.2 m along all sides. 3.4. Optical absorption spectra Three different enhancing media were examined in this paper, and in Fig. 4 absorption plots of the silver colloidal solution, the Ag nanoparticle film and the Rhodamine 6G dye is shown. The silver colloid absorption in Fig. 4(a) shows a plasmonic absorption band centred at about 410 nm. The 7 nm PLD film shows a surface plasmon resonance (SPR) peak at the longer wavelength of about 550 nm (Fig. 4(b) and is much broader than that measured for the colloidal suspension.
3.2. Morphology of silver films 3.5. SERS Fig. 2(a) shows a STEM image of a 7 nm Ag film prepared at a laser fluence of 0.8 J cm−2 . This image shows that the film contains well-separated nanoparticles with diameters in the range of 5–20 nm with a mean diameter of 6.5 ± 0.5 nm. The mean
Regarding the SERS analysis using the three enhancing media, a solution of 6.63 × 10−5 M Rhodamine 6G was measured in each of the three cases, with the results shown in Fig. 5(a). The Rhodamine
34
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
Fig. 5. SERS spectra of (a) 6. 63 × 10−5 M Rhodamine 6G and (b) 4.2 × 10−4 M benzotriazole using three different enhancing media. Fig. 6. Peak intensities for the six principal peaks in (a) Rhodamine 6G and (b) benzotriazole, with error bars representing the standard deviation in the data.
6G dye shows some well-defined peaks within the region examined between 500 and 1625 cm−1 . By comparing the signals obtained from each of the three SERS-active media the Klarite substrate is seen to perform significantly worse in terms of the magnitude of the signal recorded. The PLD film and the nanoparticle suspension are seen to show a similar level of SERS response, with the colloidal nanoparticle suspension performing slightly better. A second compound was also investigated, so as to check for repeatability across the media prepared here. The response of each SERS media to 4.2 × 10−4 M benzotriazole was thus measured, with the spectra shown in Fig. 5(b). As can be observed from these spectra, the same relationship is evident, with the colloids proving the most efficient SERS medium and with both the colloids and the PLD film far outperforming the Klarite film. While the absolute signals quite clearly show greater enhancements using the PLD film and colloidal suspension, the three media were further examined regarding performance reproducibility. This was done by calculating the standard deviations of the six principal peaks of Rhodamine 6G, using ten spectra taken at separate areas within the deposited droplet. This data is displayed in Fig. 6(a), with each set of columns showing the relative peak heights for each of the enhancing media at the respective peak position. In the case of Rhodamine 6G in Fig. 6(a) it is immediately evident that the best-performing medium in terms of the obtained signal strength is the colloidal nanoparticle suspension, but it also has the worst reproducibility. While the Klarite film shows the best reproducibility with the lowest standard deviations, this is accompanied by a significantly lower signal than that obtained using the other two media. The PLD film displays a good trade-off in overall performance. It offers better reproducibility than the colloidal
nanoparticles, and only a slightly lower signal level. While the standard deviations for the PLD films are slightly worse than for Klarite, PLD films offer very significantly better SERS signal levels. Fig. 6(b) shows similar data for the case of benzotriazole, and, as was the case for the Rhodamine 6G, the silver colloidal nanoparticles give the best SERS signals. In this case the PLD film offers relatively lower values of SERS signals than the colloids and also higher standard deviations. Once again the standard deviations are the lowest for the Klarite film but again this is accompanied by a much lower SERS signal. The %RSD (percentage relative standard deviation) is also used to examine the reproducibility data, and is calculated according to the following formula: %RSD = 100
(2)
For Rhodamine 6G, using the 615 cm−1 peak as the reference, the %RSD is again lowest for the Klarite substrate, and in the case of the nanoparticle colloids used for this comparison is very high at 51%. This is a worst case extreme example for the colloids as this value has been observed to range, on average, at about 20% for different 10-value data sets. This can be observed in Fig. 7(b) for the %RSD values for the 792 cm−1 peak of benzotriazole. The colloidal nanoparticles used with benzotriazole offer a similar standard deviation to Klarite while the PLD film shows a less reproducible response. The %RSD values highlight the variability in reproducibility for both the 7 nm PLD nanoparticle film and the silver
C.A. Smyth et al. / Applied Surface Science 264 (2013) 31–35
35
4. Conclusions In conclusion the commercial Klarite substrate offers very small SERS signal levels, which may limit their application at low analyte concentrations. In the case of examining Rhodamine 6G the PLD substrate offers a very useful compromise of good distribution statistics alongside good SERS signal levels. Examining benzotriazole shows that further optimisation of the PLD substrates is necessary, but their good SERS performance when compared with silver colloids is promising. References
Fig. 7. %RSD values for the various enhancing media for (a) Rhodamine 6G and (b) benzotriazole.
colloids. There is scope to further investigate and optimise reproducibility issues for the PLD films with a view to continue to offer strong SERS signals compared to commercial substrates such as Klarite while offering improved reproducibility.
[1] D.L. Jeanmarie, R.P. Van Duyne, Surface Raman spectroelectrochemistry, part 1: heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode, Journal of Electroanalytical Chemistry 84 (1977) p120. [2] M.G. Albrecht, J.A. Creighton, Anomalously intense Raman-spectra of pyridine at a silver electrode, Journal of the American Chemical Society 99 (15) (1977) 5215–5217. [3] E.C. Le Ru, P.G. Etchegoin, Principles of Surface-enhanced Raman Spectroscopy and Related Plasmonic Effects, Elsevier, Amsterdam, Boston, 2009. [4] W. Eberhardt, Clusters as new materials, Surface Science 500 (1–3) (2002) 242–270. [5] T. Donnelly, B. Doggett, J.G. Lunney, Pulsed laser deposition of nanostructured Ag films, Applied Surface Science 252 (13) (2006) 4445–4448. [6] C. D’Andrea, et al., The controlled pulsed laser deposition of Ag nanoparticle arrays for surface enhanced Raman scattering, Nanotechnology 20 (24) (2009). [7] N.R. Agarwal, et al., Au nanoparticle arrays produced by pulsed laser deposition for surface enhanced Raman spectroscopy, Applied Surface Science (2011), http://dx.doi.org/10.1016/j.apsusc.2011.12.030. [8] N. Leopold, B. Lendl, A new method for fast preparation of highly surfaceenhanced Raman scattering (SERS) active silver colloids at room temperature by reduction of silver nitrate with hydroxylamine hydrochloride, The Journal of Physical Chemistry B 107 (24) (2003) 5723–5727. [9] Klarite® SERS Detection Substrate, available from: http://www. renishawdiagnostics.com/en/klarite-sers-detection-substrates–12515