Gallium/gold composite microspheres fixed on a silicon substrate for surface enhanced Raman scattering

Abstract

In this work, gallium (Ga) microspheres were successfully prepared on a silicon substrate by a chemical vapor deposition method and used as templates to fabricate Au nanoparticle-coated Ga (Ga/Au) composite microspheres using an oxidation–reduction reaction between Ga and HAuCl₄. The morphology and composition of the Ga microspheres were characterized by scanning electron microscopy and energy-dispersive X-ray. It was found that Ga microspheres were partly embedded in the Si substrate. The possible formation mechanism was discussed. The content of Au in the composites could be modulated by controlling the reaction time. SERS measurement shows that the content of Au in Ga/Au composite microspheres has a great effect on the SERS activity. The SERS signals collected by point-to-point and SERS mapping images showed that as-prepared composites exhibit good spatial uniformity and reproducibility. Detection of malachite green molecules with a low concentration (1.0 × 10⁻¹⁰ M) was used as an example to show the possible application of such a substrate.

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1. Introduction

Surface-enhanced Raman scattering (SERS) has gained extensive attention due to its potential use for sensitive characterization of molecules adsorbed on noble metal nanoparticles.^1–5 However, for the most part the SERS technique still remains limited to laboratory use. The main obstruction to the application of the SERS technique is the lack of a large-scale SERS-active substrate with high-density hot spots yielding huge enhancement in a uniform way. In order to broaden the real application fields of the SERS technique, the problem of sensitivity, reproducibility, stability, substrate cost, and ease of manufacturing need to be addressed. Up to now, various approaches have been developed to fabricate SERS substrates, but with limited successes. Structurally, most SERS substrates were made from pure metallic nanostructures, in particular Ag and Au nanoparticles with various morphologies, such as nanocubes, nanorods, nanotubes, nanoplates, nanowires, nanospheres, and core–shell nanoparticles.^6–14 The popularity of Au and Ag nanoparticles as SERS-active sites stems from the fact that they are simple to prepare, with good control of size and shape. However, the SERS substrates based on the Ag and Au nanoparticles exhibited a poor reproducibility because of the random aggregation of these nanoparticles on solid substrates (such as Si and SiO₂ substrates). To improve the reproducibility of the SERS substrates based on the Ag and Au nanoparticles, different methods using metal-affinity interactions (either amine or thiol as functional tail groups) have been used to attach the Au or Ag nanoparticles on glass or silica surfaces.^15–19 For example, (3-aminopropyl) trimethoxysilane, (3-mercaptopropyl) trimethoxysilane and (tert-butyldiphenylsilane-polyethyleneoxide)-S-(polystyrene-N-bromine) molecules are widely used.^15–17 These molecules are composed of short-chain silanes, however, which readily degrade in water because of the inherent hydrolytic instability of the surface Si–O bond. Poly(diallyldimethylammonium chloride) was also used to attach Ag nanoparticles by negatively charged mercaptoacetic acid.²⁰ However, the HSCH₂COO⁻ occupied the active sites on the surface of the Ag particles, thereby weakening the SERS effect. Recently, a type of hybrid nanostructure, which combines a semiconductor core with a layer of noble metal nanoparticles was used as SERS substrate and exhibited promising potentials.^21–33 The hybrid nanostructure has advantages over the conventional pure bimetallic noble metal nanostructures, such as higher SERS activity, batter reproducibility and lower cost when used as SERS substrate. Several kinds of nanomaterials including ZnO nanorod arrays,²¹ carbon nanosheets,²² germanium nanocap arrays,²³ ZnGa₂O₄ nanoarrays,²⁴ SiO₂ nanofibers or nanospheres,^25,26 silicon nanowires,²⁷ TiO₂ nanoarrays,²⁸ vanadate microspheres,^29,30 polymer spheres and arrays,^31,32 graphitic petal arrays³³ have been used as templates to fabricate such hybrid nanostructures. These hybrid nanostructures can combine the unique mechanical and electronic properties of the cores with the size- and shape-dependent optical properties of noble metal nanoparticles. Despite all these efforts, it still remains a challenge to develop facile and feasible methods to prepare SERS substrates based on noble metal nanoparticles and semiconductor nanostructures with high SERS activity, good reproducibility, and high chemical stability.

Gallium (Ga), a main-group metallic element, has a wide range of applications in nanothermometers, semiconductor technology and surface-enhanced Raman scattering (SERS).^34–37 Many methods including chemical vapour deposition have been developed to fabricate Ga nano- or micro-structures with various morphologies such as nanoparticles, nanoribbons, nano- or micro-spheres.^38–41 In this work, Ga microspheres partly embedded in Si substrate were prepared by a chemical vapour deposition method and used as a template to fabricate Au nanoparticle-decorated Ga (Ga/Au) composite microspheres. As-prepared Ga/Au composites are suitable for applying in real applications because of the immobilization of Ga/Au microspheres on Si substrate. To evaluate the SERS activity of Ga/Au composite microspheres, Rhodamine 6G (R6G) molecules were used as the SERS probe molecules. The results show that the Ga/Au composite microspheres are of high SERS enhancement, reproducibility and stability due to its special microstructure. Their further applications in rapidly detecting malachite green (MG) relating to human health and safety are also demonstrated.

2. Experimental section

2.1 Synthesis of Ga microspheres

Ga microspheres were grown on silicon (Si) substrates by a chemical vapor deposition method. The Si substrate was cleaned and degreased prior to deposition. Then Ga microspheres were synthesized in a horizontal double-tube system. The details for the system were described in other report.⁴² In brief, a Si substrate was placed at 4–2 cm away from the open end of a small tube. After evacuating, the source materials (Ga₂O₃ and graphite powder, 1 : 1 by weight) and substrate were respectively heated to 950 and 450 °C at heating rates of 20–50 °C min⁻¹ and kept at these temperatures for 10–60 minutes. The chamber pressure was maintained constant (100 mbar) by a continuous flow of pure argon gas (99.995%). The system was then cooled down naturally to room temperature under the same gas flow and pressure.

2.2 Synthesis of Ga/Au composite microspheres

Ga/Au composites were synthesized by a solution method. Firstly, a suitable amount of HAuCl₄ was dissolved in 35 ml ethanol solution. The final concentration of HAuCl₄ was 0.5 mM. Then the solution was transferred to a stainless steel autoclave (40 ml in volume). The Ga microspheres together with the Si substrate were immersed in the above solution. After slow stirring for about 10 min, synthesis was carried out at 50 °C for different time in an electric oven without stirring. After cooling to room temperature naturally, samples were taken out from the solution, washed with deionized water several times to remove residual ions and molecules, and dried at 80 °C under vacuum.

2.3 Characterization and SERS measurement

Morphologies and compositions of the samples were characterized with scanning electron microscope (SEM, Philips XL 30 FEG), energy-dispersive X-ray (EDX), respectively. Raman measurements were conducted with a Renishaw 2000 laser Raman microscope equipped with a 633 nm laser of 2 μm spot size in diameter for excitation. All the spectra were acquired for 10 s with the laser power measured at the sample being 0.125 mW. Loading of probe molecules onto the substrate (∼5 × 5 mm) was accomplished by dropping 40 μL sample solution onto the substrate. The substrate was air-dried naturally before acquiring the SERS spectrum.

3. Result and discussion

Chemical vapor deposition method has been widely used to prepare metal or metal oxide nanostructures such as Si nanowires,⁴³ Ag nanowires,⁴⁴ ZnO nanoarrays,²¹ ZnGa₂O₄ nanorod arrays.^24,45 The detailed information for the mechanism of the chemical vapor deposition has been previously reported. Here, the formation of Ga metallic particles may result from the following process,^46–48 which is possible at high temperature. At the closed end of small quartz tube, at the given high temperature, Ga₂O₃ powder reacted with graphite to produce Ga₂O vapour, which is a volatile metal oxide. The formed Ga₂O vapour can be readily transported to the deposition zone by the carrier gas (Ar). Meanwhile, the Ga₂O vapor will also react with CO to form Ga at a desired temperature. Finally, Ga was deposited on Si to form Ga metallic particles.

Ga₂O₃ + 2C → Ga₂O + CO (1)

Ga₂O₃ → Ga₂O + O₂ (2)

O₂ + 2C →2CO (3)

Ga₂O + CO → 2Ga + CO₂ (4)

The morphology and composition of the products were studied using SEM and EDX. Fig. 1a shows the panoramic morphology of as-prepared products. It indicates that the products are composed of many microspheres in the size of 0.5–2 μm. The inset in Fig. 1a is a high-magnification SEM image of a single microsphere, revealing that the surface of the microsphere is not as smooth as observed in the low-magnification SEM image. There are many flower patterns in the surface of the microsphere, which was not observed in the surface of Ga microspheres prepared by other methods.³⁹ The flower patterns may be caused by the uneven surface of the microsphere. The EDX spectrum in Fig. 1b shows the presence of the Ga and Si elements. The Si element originated from the Si substrate. No other elements such as O, C, etc. were found, indicating the product only consist of Ga element.

Fig. 1 (a) SEM images of Ga microspheres. The inset is the high-magnification SEM image of a single Ga microsphere. (b) EDX spectrum of a single Ga microsphere.

To understand the growth process of Ga microspheres, time-dependent experiments were also carried out. In this experiment, only reaction time was changed, keeping other reaction parameters constant. Fig. 2 shows the SEM images of the products prepared for various time intervals of 10, 20, 35, and 60 min. As shown in Fig. 2a, when a short reaction time of 10 min was used, many circular plates with rough surface were formed on the surface of Si substrate. In addition, some indistinct white spots with relatively small size can be found on the substrate. When the reaction time was prolonged to 20 min, some protuberances were formed on the surface of the circular plates, while the whole particle still remains circular profile (Fig. 2b). Moreover, the indistinct white spots were disappeared. As for the product formed after a reaction time of 35 min, the protuberances became more apparent and the surface became more rough and uneven (Fig. 2c). After 60 min of deposition time, well-defined microspheres with smooth surface were formed as shown in Fig. 2d. It should be noted that the average particle size increased with increasing the deposition time. To further study the growth process of Ga microspheres, Si substrate deposited with Ga microspheres was immersed in concentrated hydrochloric acid to remove Ga microspheres. Fig. 3a shows the SEM image of the Si substrate treated with hydrochloric acid. It was found there are many holes with a diameter of 0.5–2 μm in the surface of Si substrate. Interestingly, some particles were still remained in the holes. Based on the experimental results, a possible growth process demonstrating the formation of Ga microspheres can be described in Fig. 3b. Initially, Ga in the vapor phase was transported to the low temperature zone, where they condensed to form liquid droplets on the Si substrate as nuclei for Ga microspheres. The Si was dissolved by liquid Ga to form stable Ga–Si phases at interface of Si and Ga.⁴⁹ With increasing the deposition time, the diameter of the Ga droplets increased gradually. Solid Ga particles were finally obtained after cooling down. The detailed formation mechanism is not very clear and needs further investigation.

Fig. 2 SEM images of Ga products prepared for various time intervals of 10 (a), 20 (b), 35 (c), and 60 (d) min.

Fig. 3 (a) SEM image of Si substrate after reaction with concentrated hydrochloric acid. (b) Schematic illustration of the possible formation of the Ga microspheres. All the images are the cross-sectional view images.

The morphology and composition of Ga/Au composite microspheres prepared using Ga microspheres (as shown in Fig. 1) as template were also investigated using SEM and EDX. Fig. 4 presents the typical SEM images of the Ga/Au composite microspheres synthesized by reaction for various time intervals of 30, 45 and 60 min. As can be seen from Fig. 4a, after reaction with AuCl₄⁻ ions, the spherical morphology remained nearly unchanged when experiencing an in situ reduction approach, but the rough surface of the obtained microspheres indicates the successfully coating Au nanoparticles on the surface of Ga microspheres. Fig. 4b shows a high-magnification SEM image of Ga/Ag microspheres. It is clear that a lot of small nanoparticles have been deposited on the surface of Ga microspheres. When the reaction time was prolonged to 45 min, a layer of loose Au nanoparticles with a relatively large size was deposited on the surface of Ga microspheres (Fig. 4c). As the reaction time was further increased to 60 min, a much denser Au nanoshell formed on the surface of the microspheres (Fig. 4d). The EDX spectra (Fig. 5) of Ga/Au composite microspheres synthesized by reaction for various time were shown in Fig. 5, indicating the coexistence of the Au, Ga and Si elements. The Si element also originated from the Si substrate. The content of Ga in the composite microspheres decreased with increase the reaction time. Similar results could be observed using other Ga particles as templates (not shown here).

Fig. 4 SEM images of Ga/Au composite microspheres synthesized by reaction for (a and b) 30 min, (c) 45 min and (d) 60 min, respectively.

Fig. 5 EDX spectra of Ga/Au composite microspheres synthesized by reaction for (a) 30, (b) 45 and (c) 60 min, respectively.

The effect of the solvent on the final products was also studied. Fig. 6a shows the SEM image of the final products synthesized using deionized water as solvent. It was found that flower-like structures were obtained. High-magnification SEM image shows that the flower-like structures consist of nanoparticles and microrods, just like a pile of band inserted with many branches. The corresponding EDX spectrum indicated that the flower-like structures were composed of O, Au and Ga elements. The O element may be originated from the gallium oxide (Ga₂O₃ or GaOOH), which was formed when Ga was oxidized into Ga³⁺ by AuCl₄⁻.

Fig. 6 SEM image (a) and EDX spectrum (b) of the products synthesized using deionized water as solvent. The inset is the high-magnification SEM image of a single flower-like structure.

As SERS effect is very sensitive to the roughness of metal surface, Ga/Au composite microspheres prepared by reaction for different time were used as SERS substrates to examine their SERS activity. R6G was chosen as the probe molecule owing to its well-established vibrational features. Fig. 7 shows the SERS spectra of R6G solution (1.0 × 10⁻⁸ M) dispersed onto Ga/Au composite microspheres, while the spectra of the R6G solution (1.0 × 10⁻⁶ M) dispersed on Si, Ga microspheres, and Au nanoparticles were also included for comparison. It is clearly shown that no or very weak Raman peaks could be observed when the R6G solution of 1 × 10⁻⁶ M was dispersed on Si substrate, Ga microspheres, and Au nanoparticles. Careful observation shows that the spectrum profile of R6G solution on Ga microspheres (curve b in Fig. 7) is different from that of R6G solution on Au nanoparticles (curve c in Fig. 7). The intensities of the bands at 1510, 1361 cm⁻¹ are weaker than that of the band at 1648 cm⁻¹. This may be caused by the substrate effect to the adsorption model of R6G at the Ga surface.⁵⁰ On the Ga/Au composite microspheres prepared by reaction for different time, the Raman signals are significantly enhanced. A similar phenomenon were also found in the case of Ag nanoparticles, BiVO₄ microspheres and BiVO₄/Ag composite microspheres.³⁰ The improved SERS enhancement of Ga/Au composite microspheres may be attributed to their special hybrid microstructures.³⁰ An clear trend shows that the intensity of Raman signal at 1510 cm⁻¹ increased greatly with increasing the reaction time from 30 to 45 min. The change of SERS intensity may be attributed to the different coverage and particle size of Au nanoparticles on the surface of Ga microspheres. It can be seen from Fig. 4 that, the number density and particle size of the Au nanoparticles increased with increasing the reaction time. According to theoretical and experimental studies,^51,52 the large SERS enhancement occurs at the junction between two metal nanoparticles. The junction thus can be considered as an electromagnetic hot spot similar to those predicted to exist in Ag clusters.^53,54 As the number density and particle size of Au nanoparticles on the surface of Ga microsphere increases, the distance between the nanoparticles decreases, therefore the number of junction increases providing a larger SERS enhancement. Alternatively, the rough surface of the Ga/Au composite microspheres could be another cause of the dependency of the SERS activity on the amount of reaction time. With an increase in the reaction time, the surface of the Ga/Au composite microspheres becomes rougher (as shown in Fig. 4). This unique feature can result in a large enhancement in the electromagnetic field, which may also account for the observed enhancement in the Raman signals. When the reaction was further increased to 60 min, the intensity of Raman signal at 1510 cm⁻¹ decreased a little. A reasonable explanation for this observation may be the formation of a denser Au nanoshell on the surface of Ga microspheres.

Fig. 7 (A) SERS spectra of R6G solution adsorbed on Si (a), Ga (b), Au nanoparticles (c), and Ga/Au composite microspheres (d–f) prepared by reaction for 30 (d), 45 (e), and 60 (f) min. (B) SERS spectra of R6G (1.0 × 10⁻⁷ M) obtained from the freshly prepared substrate (a) and the substrate immersed in water for 10 days (b).

To investigate the stability of as-prepared substrates, we soak the Ga/Au composite microspheres in deionized water for 10 days and then examined their SERS activity. The collected SERS spectra of R6G (1.0 × 10⁻⁷ M) are compared with those obtained from the freshly prepared substrate (Fig. 7B). It is noted that neither a shift in the major Raman peaks nor a significant change in Raman intensity occurred for 10 days, suggesting that the Ga/Au composite microspheres is stable for at least a 10 day period. To evaluate the uniformity of the as-prepared substrates, we collected the SERS spectra of R6G by a 2D point by point mapping mode from a randomly selected area of the substrate (as shown in Fig. 8a). For mapping measurement, the substrates were dipped into 100 μL aqueous R6G solution (1.0 × 10⁻⁶ M) for 1 h and then dried naturally in air. The mapping areas were 42.0 × 48.0 μm² and the scan step was 3 μm. A base line-corrected peak intensity of a peak at 1510 cm⁻¹ is chosen for the acquisition of the SERS mapping. Red areas represent higher intensity of the SERS signal. Fig. 8b shows the mapping image of R6G on Ga/Au composite microspheres prepared by reaction for 45 min. It is clear that most of the color intensity was very close in the mapping image, which demonstrated the good reproducibility of the substrate.

Fig. 8 (a) A typical optical microscopy image of Ga microspheres on Si substrate. (b) Mapping image R6G (1.0 × 10⁻⁶ M) on Ga/Au composite microspheres prepared by reaction for 45 min. The scale bar in (b) is 5 μm.

The SERS enhancement factor (EF) for R6G adsorbed on the Ga/Au composite microspheres prepared by reaction for 45 min are calculated according to the equation²¹ EF = (I_SERS/I_bulk) (N_bulk/N_surface), where I_SERS and I_bulk denote the integrated intensities for the 1510 cm⁻¹ band of the 100 nM R6G adsorbed on the surface of Ga/Au composite microspheres and 10 mM R6G on glass, respectively, whereas N_SERS and N_bulk represent the corresponding number of R6G molecules excited by the laser beam. The EF of the Ga/Au composite microspheres prepared by reaction for 45 min was evaluated to be on the order of 10⁶. This estimation is based on the assumption that molecules were uniformly dispersed in the region wet by the solution. Due to the number of adsorbed R6G molecules is poorly defined and measured, quantification of the EF is not trivial and subject to assumption.

Malachite green (MG) is widely used in the aquaculture industry because of its effectiveness against fungal and parasite infection in fish. In addition, it is mutagenic and teratogenic effects to humans, although it has been banned in some countries for use in aquaculture, it is still used by some people due to its low cost, availability, and high efficacy. Consequently, monitoring the level of MG in aquaculture products becomes very urgent for public health and food safety. SERS has been proved as a promising method for rapid and sensitive detection of chemicals and biochemicals. Herein, we used Ga/Ag composite microspheres prepared by reaction for 45 min as SERS substrate to detect MG. In the experiment, MG stock solutions were prepared by dissolving bulk MG into deionized water under vigorous stirring and diluted to various concentrations ranging from 1.0 × 10⁻¹⁰ to 1.0 × 10⁻⁷ M. Fig. 9 shows the SERS spectra of MG with various concentrations from 1.0 × 10⁻¹⁰ to 1.0 × 10⁻⁷ M, while the Raman spectrum of bulk MG is also included. It was found that the intensity of Raman signal at 1617 cm⁻¹ decreased strongly with decreasing concentration of MG. Moreover, the MG featured peak at 1617 cm⁻¹ persisted in the SERS spectra even if the concentration of MG solution decreased to 1.0 × 10⁻¹⁰ M. The detection limit of Ga/Au composite microspheres for MG is lower than those of fractal-like Au nanoparticles,⁵⁵ GO/Au composites,⁵⁶ Ag nanorod arrays,⁵⁷ but higher than that of TiO₂/Ag nanorods²⁸ and ZnO/Ag nanodome arrays.⁵⁸ Considering that the as-prepared nanostructure is of high enhancement effect, uniformity, and stability, it can be a potential SERS sensor to detect harmful materials in food.

Fig. 9 SERS spectra of MG of various concentrations (a) 1.0 × 10⁻¹⁰ M, (b) 5.0 × 10⁻¹⁰ M, (c) 1.0 × 10⁻⁹ M, (d) 5.0 × 10⁻⁹ M, (e) 1.0 × 10⁻⁸ M, (f) 1.0 × 10⁻⁷ M on Ga/Ag composite microspheres prepared by reaction for 45 min. The inset is a typical SERS spectrum of bulk MG on glass.

4. Conclusion

We have presented a simple method to prepare Ga/Au composite microspheres for SERS-based sensing. SEM and EDX spectra confirmed the formation of Ga/Au composite microspheres. The content of Au in the composites can be easily modulated by controlling the reaction time. SERS measurement shows that the content of Au element in Ga/Au composite microspheres has a great effect on SERS activity. Significantly, SERS substrate based on Ga/Au composite microspheres features a high EF (10⁶), good uniformity, high stability and reproducibility of the Raman signals, and is thus well suited as a high-performance biochemical and food safety sensor. MG, with a low concentration 1.0 × 10⁻¹⁰ M, was readily detected using such a substrate. Consequently, the Ga/Au composite microspheres, as a high performance SERS-active substrate, are highly promising for biological and food safety monitoring applications.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No. 21101172), the Central South University Postdoctoral International Exchange Program.

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