Surface enhanced Raman scattering substrate with high-density hotspots fabricated by depositing Ag film on TiO2-catalyzed Ag nanoparticles
Graphical abstract
Introduction
Surface enhanced Raman scattering (SERS), which occurs on rough metallic (typically Au and Ag) surfaces or nanoparticles (NPs), has been intensely explored as a highly sensitive technique for detection of ultra-trace or single molecule [1], [2], [3]. For the purpose of SERS application in chemical analysis and biosensing, a key step is fabrication of high-quality SERS substrates, which needs to be highly sensitive, reproducible, stable over time, easy to be fabricated, and cost-effective [4]. In the past decades, many methods have been explored to fabricate various nanostructures for SERS substrates, such as cycling oxidation-reduction of Ag electrode surface [5], deposition of metallic island films [6] or Ag nanorods [7], lithography for ordered Ag nanostructures [8], [9] or three-dimensional metal nanostructures [10], [11]. Though these methods are potential for fabrication of SERS substrate, there is a necessity to be further improved in the viewpoint of practical applications. For example, lithography is capable of fabricating a SERS substrate with highly controlled hotspots, but its high cost is obstructive to commercial application. Extremely high SERS activity was claimed to achieve on Ag-decorated three-dimensional nanostructures [12], [13], [14], but the reproducibility needs to be improved. Therefore, it is necessary to explore the way for commercial applications.
Because of the excellent photocatalytic activity, TiO2 films have been widely used to grow Ag NPs on the surface of TiO2 films by reducing the Ag+ in AgNO3 aqueous solution under the irradiation of ultraviolet (UV) light [15], [16], [17], [18], [19]. The TiO2-catalyzed Ag NPs can be used as SERS substrate with the following advantages. First, photocatalytic reduction is a cost-effective method facilitating fabrication of TiO2-catalyzed Ag NPs, in which the semiconductor (TiO2) and Ag NPs synergically contribute to Raman enhancement. Second, the TiO2-catalyzed Ag NPs prepared by the photocatalytic reduction are surfactant free, thus preventing against the contamination in SERS measurement from the agents, which are usually necessary in synthesis of noble metallic NPs by other solution-phase reactions, such as the chemical synthesis of Ag NPs in colloids. Third, the TiO2-catalyzed Ag NPs are stable for long-time use (>1 year) [16]. More importantly, the TiO2-catalyzed Ag NPs is very uniform in size and controllable, thus leading to a high reproducibility in acquiring Raman signals. For example, Li et al. [17] found the spot-to-spot variance in Raman signals on the TiO2-catalyzed Ag NPs as small as ∼10%, which is a good performance among various SERS substrates. Despite of above advantages, the TiO2-catalyzed Ag NPs are still absent of enough attention, because the SERS activity needs to be improved when they are used as SERS substrates.
It is well known that the electromagnetic hotspots, which mainly occurred in the nanogaps less than 10 nm, make the most important contribution to the Raman enhancement in SERS substrates [20], [21], [22], [23]. By investigations into the TiO2-catalyzed Ag NPs, the spacing between adjacent Ag NPs was found too large to form hotspot, thus producing a relatively low SERS activity [17]. Systemic study revealed that the liquid environment is a major factor resulting in the wide gaps between the Ag NPs [24]. In the liquid environment, Ostwald ripening is the dominative mechanism controlling the photocatalytic growth of Ag NPs, as schematically shown in Fig. 1(a), and then leading to a wide distribution of Ag NPs in size, as shown in Fig. 1(c), and a lower density, thus obstructing the formation of high-density hotspots. This shortage cannot be overcome in the photocatalytic growth of TiO2-catalyzed Ag NPs. Therefore, we have to explore other methods for improving the SERS activity of the TiO2-catalyzed Ag NPs.
Differing from the photocatalytic growth of Ag NPs in liquid environment, OR-dominated growth of Ag NPs is limited at room temperature in physical vapor deposition (PVD), such as magnetron sputtering. After nucleation, the Ag NPs deposited by magnetron sputtering could uniformly grow up without obvious OR process, as schematically shown in Fig. 1(a) and the SEM image in Fig. 1(d). High-density hotspots have been obtained in deposition of Ag films at the percolation threshold [25], but the sizes of Ag NPs is at a level of 10–30 nm, which is too small to produce a stronger Raman resonance under excitation of visible light. Moreover, there is a difficulty in technology controlling the growth of Ag NPs at the percolation threshold. After the point of percolation threshold, coalescence of Ag NPs will take place, and then resulting in formation of Ag film, which exhibits a rather low SERS activity, as shown in Fig. 1(b).
The different growth modes between vapor and liquid environment make us think of a novel way to fabricate SERS substrates with high-density hotspots by combining the photocatalytic growth and magnetron sputtering. It can be expected that the limited OR growth of Ag NPs in vapor environment will increase the size of TiO2-catalyzed Ag NPs without decrease its density, leading to the formation of high-density hotspots.
In this work, we report our attempt to fabricate a new SERS substrate with high-density hotspots by depositing a layer of Ag film on the TiO2-catalyzed Ag NPs using magnetron sputtering method. The enhancement of Raman signals was studied as a function of deposition time of Ag films, and an enhancement factor as large as ∼1.2 × 107 was obtained by optimizing the deposition time of Ag films. This method was demonstrated easily to control the SERS activity of TiO2-catalyzed Ag NPs at a relatively high level, which is comparable with the SERS substrates fabricated by lithography or other methods. In addition, we studied the dependence of hotspots on the deposition time and gave a discussion on the enhancement mechanism in the SERS substrates.
Section snippets
Preparation of TiO2 films
Nanoparticulate TiO2 films for depositing Ag NPs were prepared by the conventional sol-gel method. Namely, 50 mL titanium isopropoxide was first mixed with 3 mL acetylacetone and stirred about 10 min (solution A). Then, 0.21 mL nitric acid, 1.4 mL deionized water and 150 mL ethyl alcohol were mixed and stirred for ∼10 min (solution B). The TiO2 sol was prepared by slowly adding solution B dropwise into the vigorously stirring solution A. After mixing, the TiO2 sol was kept stirring for 2 h at
Results and discussion
Fig. 2 (a–e) shows the typical SEM images of TiO2-catalyzed AgNPs before and after optimization by depositing a layer of Ag film with different deposition time. All the Ag NPs exhibit the similar shapes and uniformly distribute on the TiO2 surface, but their sizes vary with Ag deposition, as shown in the insets. As shown in Fig 2 (a), most of the as-grown Ag NPs on the TiO2 surface are in the sizes ranging 40–100 nm, leaving the spacing between two adjacent Ag NPs rather large, thus there are
Conclusions
In summary, we report a facile and cost-effective method to considerably improve the SERS activity of TiO2-catalyzed Ag NPs by depositing a layer of Ag film. The enhancement factor of optimized SERS substrate could be increased up to 1.2 × 107, which is high enough for practical applications in chemical analysis and biosensoring. The SERS activity is highly uniform and reproducible. More importantly, the SERS activity is easily optimized at a relatively high level by controlling Ag deposition
Acknowledgements
The work is supported by the National Natural Science Foundation of China (Grant Nos. 11364009).
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