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Abstract
This article aims to switch the surface-enhanced Raman spectrum (SERS) of a 4-Aminothiophenol (4-ATP) dye solution on a silver nanoparticle (Ag-NPs) film, where the SERS is largely influenced by the distance betweenss the 4-ATP and the Ag-NPs under applied an external field. The electrophoresis apparatus uses two ITO electrodes to build a cell in which one of the ITO electrodes is coated with 30-nm Ag-NPs. The SiO2 nanospheres form an insulating layer which effectively hinders 4-ATP adsorption on Ag-NPs when a + 3V bias voltage is applied. The 4-ATP dye molecules are carried by the SiO2 nanospheres to move off the Ag-NPs by applying a −3V bias voltage. The 0.4 wt% 4-ATP and less than 15 wt% SiO2 nanospheres (200 ~350 nm in diameter) are used to prepare samples for analysis. The results show that the 1.6 wt% SiO2 200-nm nanospheres achieved the best switching effect of SERS in this study.
© 2017 Optical Society of America
1. Introduction
Surface enhanced Raman Scattering (SERS) is one of the major topics of surface enhanced phenomena, which include surface enhanced absorption, fluorescence, and photochemistry in molecules near a rough surface. They also include enhanced second-harmonic generation, hyper-Raman scattering, and other nonlinear processes [1]. SERS has been studied for more than three decades and has become a very useful tool in many fields [2–5]. Compared to other kinds of surface enhanced techniques, one of the most important advantages of SERS is to provide a highly discriminating spectrum of the interesting analyte. Independent reports from two groups have shown that the SERS signals are comparable to or even stronger than molecular fluorescence to achieve the goal of single-molecule detection [6,7]. The combination of single-molecule detection [8–10] ability of SERS and extreme time resolution of ultrafast coherent spectroscopy makes it possible to observe the molecular vibration of a single molecule [11, 12]. The mechanism of Raman scattering is typically enhanced by the strong localized electromagnetic (EM) field from the surface of the metallic nanoparticles (NPs) [1, 4, 13, 14]. Collective excitations of the conduction electrons known as Localized Surface Plasmon Resonance (LSPR) occur. In addition to EM enhancement, SERS would be further enhanced by means of intermolecular or supramolecular charge transfer resonances, which is called as chemical enhancement [14–17].
The assembly of monodispersed plastic nanospheres into ordered arrays, called colloidal crystals, has extensive applications in optical devices such as photonic crystals [18], colloidal lithography [19], and wavelength selectors [20]. Additionally, the hybrid structures of plastic particles are also widely used in SERS [21–23]. The method incorporates alkane-thiols on metallic NPs and organosilanes on nanospheres to create a new composite. The carrier can be tracked via controlling the nanosphere location.
Colloidal electrophoresis is a method of separating the immersed molecules (or polar particles) in the colloidal solution by applying a bias voltage on the electrodes. The SiO2 nanospheres (SiO2-NSs) synthesized by a sol-gel method with negative polarity can assemble to a positive electrode under an external electric field [24]. The thiolate in 4-Aminothiophenol (4-ATP) dye ligand has an affinity for binding with metal NPs [25–27]. The colloidal crystals form an effective insulating layer to hinder 4-ATP from metal NPs. The switching process can be repeated several times and the coated metal NPs can be used in different samples to measure SERS signals.
2. Experimental details
Preparation of the samples: A 4 μm thick Ag film was first deposited on the ITO glass with an area of 2 × 3 cm2 by using thermal evaporation under a pressure of 10−5 torr at a deposition rate of 0.3 nm/s. After that, the sample was annealed at a temperature of 200 °C for 3 minutes to fabricate Ag nanoparticles. Figure 1(a) shows the SEM image of the Ag film, where the particles sizes were approximately 30 nm. SiO2-NSs were prepared by means of the Stӧber-Fink-Bohn method [28]. The SiO2 spheroids with diameters of 200 ~350 nm were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous ethanol solution containing ammonia. Figure 1(b) shows a typical 200-nm SiO2 sample with a relatively narrow particle size distribution. The cell is built by two ITO electrodes positioned opposite sides. One of the ITO electrodes is coated with 30-nm Ag-NPs and the other is not. The cell thickness, which is controlled by spacers, is 38 μm.
The solution injected into the cell is prepared with the weight proportions of 4-ATP: SiO2: DI water = 0.4: x: 99.6-x, the concentration of 4-ATP is fixed and the concentration of SiO2-NSs is less than 15 wt%. The added dye molecule of 4-ATP can form a self-assembled monolayer (SAM) on silver nanostructures through the formation of the Ag-S chemisorption bond or the weaker electrostatic force between dye molecules and polycrystalline electrode [25]. The SiO2 nanospheres can assemble to the positive ITO electrode as an insulating layer due to their negative polarity when a bias voltage is applied. The distance between the 4-ATP and Ag-NPs is largely dependent on the concentration and the particle sizes of SiO2 nanospheres.
To measure the SERS spectra of 4-ATP, an incident light with a wavelength of 532 nm and an intensity of 15 mW is focused by an objective to irradiate the samples. The ITO glass of the cell coated with Ag-NPs which faces the incident beam is designated the front electrode. The other ITO glass is the rear electrode. An electric field pointing from a front to a rear electrode is defined as a forward bias field, whereas a field pointing from a rear to a front electrode is defined as a reverse bias field. The scattering light, at a 10° angle with respect to the incident beam, is dispersed by a monochromator (Triax 550, Jobin-Yvon) and detected by a liquid-nitrogen cooled CCD.
3. Results and discussion
This study aims to switch the SERS of the 4-ATP in the SiO2-NSs solution. The polarity of SiO2-NSs is identified by using electrophoresis without the 4-ATP. The SiO2-NSs are negative polarity because they are assembled around the positive electrode. We prepare 0.4 wt% 4-ATP solution without the SiO2-NSs for SERS measurement to identify the adsorption effect between 4-ATP and Ag-NPs. The result indicates that the duration needs to be more than 15 sec to detect the SERS signal after the solution has been injected. 4-ATP can be adsorbed on the Ag-NPs through the formation of metal-sulfide bonds. The EM enhancement effect leads to the observation of Raman peaks.
Figure 2 shows the illustration of the sample treatment. No electric source is applied onto the ITO glass in Fig. 2(a), then a + 3V forward voltage is applied in Fig. 2(b). The particle size of SiO2-NSs used in this experiment is around 200 nm. Initially, the SiO2-NSs and 4-ATP are uniformly distributed in the cell as shown in Fig. 2(a), and then the SiO2-NSs are transported to the front electrode due to the forward bias applied. The SERS signal can’t be measured in the beginning because the 4-ATP has not yet moved to near the front electrode. The intensities of SERS signals can be progressively increased after 15 minutes due to 4-ATP being in close proximity to Ag-NPs as shown in Fig. 2(b).
Fig. 2 Distribution of 4-ATP and SiO2-NSs in the solution (a) without applying voltage in the beginning, and (b) with applying + 3V voltage in forward bias for 15 min.
Figure 3 shows the measured SERS spectrum of 4-ATP with 1.5 wt% SiO2. Figure 3(a) shows the spectrum with forward bias measured at 30 minutes after completing the sample, and Fig. 3(b) shows the field alternative with reverse bias. These distinct Raman peaks in Fig. 3(a) shows that 4-ATP can migrate near the Ag-NPs to enhance the SERS signals. Afterwards, the SERS signal in Fig. 3(a) could be washed out when the reverse bias is applied. The result indicates that SiO2-NSs are functioned as an insulating layer to hinder the 4-ATP to be directly adsorbed on the Ag-NPs. This result in Fig. 3(b) indicates that the 4-ATP can be carried from the front electrode by following the movement of SiO2. It is worthwhile to note that the thickness of SiO2-NSs with 1.5 wt% is approximately 4 ~5 layers, which is equivalent to 1130 ~1415 nm, if all SiO2-NSs assemble in the hexagonal closed-packed (hcp) structure.
Fig. 3 Raman spectrum of 4-ATP with 1.5 wt% SiO2-NSs (a) measured with forward bias at 30 minutes after completing the sample, and then (b) measured with reverse bias.
The SERS enhancement primarily dominated by Förster resonance energy transfer is at distances of a few nanometers [1, 2, 29, 30]. We believe that some of 4-ATP molecules can penetrate through the gaps between the SiO2-NSs to approach Ag-NPs. However, the result that the SERS signals can be washed out when a reverse bias is applied indicates that most of 4-ATP, which are close to Ag-NPs, might not form a strong chemical bond on Ag-NPs. The switching process of alternating forward and reverse bias for measuring the SERS spectrum can achieve more than 7 cycles. Dye molecules are bleached by the charges accumulating on the electrode when the electric voltage is applied for a period of time. Changing the steady field into the continuous pulse will increase the number of times of cyclic processes. The spectrum shown in Fig. 3(a) has no peaks appeared at 1140, 1390, and 1432 cm−1. This observation indicates that no 4,4'-dimercaptoazobenzene (DMAB) is formed during the SERS measurement [27].
Figure 4 shows the SERS spectrum of the 4-ATP with mass concentration of the SiO2 at (a) 0 wt% (0 layer), (b) 1 wt% (3 layers), (c) 3 wt% (9 layers), and (d) 7 wt% (21 layers) with a forward bias, where the 4-ATP is fixed at 0.4 wt%. The number of layers increased with the increase of SiO2-NSs concentration, which cause the intensity of Raman peaks to gradually decrease due to effectively blocking the 4-ATP in close proximity to the Ag-NPs. The intensities of the SERS spectrum are down to one fifth while the concentration of SiO2-NSs is increased from 0 to 3 wt%, and it is hardly measurable when the concentration of SiO2-NSs is higher than 7 wt%.
Fig. 4 Raman spectrum of 4-ATP measured with concentrations of SiO2-NSs (a) 0 wt%, (b) 1 wt%, (c) 3 wt%, and (d) 7 wt%.
The Raman peak of 4-ATP at 1081 cm−1 corresponding to the νSH + νNH vibrational bands is selected for the sample with an SiO2 concentration of less than 15% with a (i) forward and (ii) reverse bias. The results are shown in Fig. 5(a). The curve (i) in Fig. 5(a) is divided into three sections of the curve slope. The SiO2-NSs concentration less than 1.3 wt%, in between 1.3 and 3 wt%, and more than 3 wt% are designated sections I, II, and III, respectively. A considerable amount, but a gradually diminishing intensity of Raman peak can be found in section I. The assembling of SiO2-NSs less than 4 layers in this section does not entirely block the binding between the Ag-NPs and 4-ATP, so the Raman spectrum can be measured and cannot be washed out by applying a reverse bias as shown in curve (ii) of Fig. 5(a). In section II, the curve (i) with the number of SiO2-NSs layer between 4 to 9 layers drops in an approximately linear manner. The Raman peak with reverse bias is also linear and drastically decreases with the increase of the SiO2-NSs concentration from 1.3 to 1.6 wt%. It indicates that 4-ATP has an optimized distance from the Ag-NPs to enhance the SERS effect in section II, and it can be carried by the movement of SiO2-NSs. The average molecular distance between the Ag and NPs is proportionally increasing with the increase of SiO2-NSs concentration in section II, so the peak intensity in curve (i) of Fig. 5(a) drops down directly and linearly. In section III, the Raman peak in curve (i) of Fig. 5(a) quickly decreases, because the pumping light was scattered seriously and the 4-ATP was mostly blocked for the assembling of SiO2-NSs larger than 9 layers. This causes the switching process to be inefficiently operated.
Fig. 5 (a) Selected Raman peak of Ag-4ATP at 1081 cm−1 with various concentrations of SiO2-NSs (i) forward bias and (ii) reverse bias, and (b) the contrast ratio of the peak intensity values in curve (i) divided by the peak intensity values in curve (ii).
Figure 5(b) shows the contrast ratios versus the concentration of SiO2-NSs. The contrast ratio is the intensity of the Raman signal at 1080 cm−1 measured with a forward bias, divided by the peak intensity of the same peak measured with a reverse bias. It indicates that the optimized SiO2 concentration for the sample (in switching process) is between 1.6 and 3 wt%, and 1.6 wt% SiO2-NSs is the optimum operating concentration for the switching effect.
Figure 6 shows the Raman spectrum of 4-ATP solution mixed with a concentration of 0.4 wt% of 350 nm SiO2-NSs. The result indicates that the Raman peak corresponding to the characteristic vibration of 4-ATP is not distinct. The Raman peak of 1081 cm−1 diminishes, and the Raman peak of 1068 cm−1 appears as the SiO2 particle sizes increases from 200 to 350 nm. The 0.4 wt% SiO2-NSs is equivalent to 2 layers in thickness if the SiO2-NSs assemble in the hcp structure form. Although the 4-ATP can penetrate through the gaps to adhere on Ag-NPs since there are only 2 layers of SiO2-NSs, both the pumping source and Raman signals are scattered seriously by large nanospheres, and this causes the characteristic Raman peak to not be measurable. The optimum SiO2-NSs particle size for switching effect in this work is less than 300 nm.
4. Conclusion
In this work, a method has been developed to switch the Raman spectrum by applying alternating forward and reverse bias voltage to control the movement of SiO2-NSs on a cell. 200-nm particle sizes of SiO2-NSs with a concentration of 1.5 ~3.0 wt%, equivalent to 4 ~9 layers, can effectively switch the Raman spectrum. The switching process can be carried out many times. The results indicate that the 4-ATP does not adsorb on the Ag-NPs for SERS effect and can be taken off by following the SiO2 movement. The penetration distance between the 4-ATP and Ag-NPs can be adjusted by controlling the concentration of SiO2-NSs, and the results show the 1.6 wt% SiO2 200-nm nanospheres, equivalent to 4 layers, showed the best switching effect of SERS in this study. We will continue to study size effect of SiO2-NSs particles when the size decreases to less than 100 nm on SERS measurement.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of Taiwan for financially supporting this research under Contract No. MOST 105-2112-M-415-008.
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