Surface-enhanced Raman spectroscopy (SERS) technology has attracted widespread attention since its discovery in 1974 [1
]. Comparing to the normal Raman scattering process, SERS is capable of enhancing Raman scattering of analytes by up to a million times or more [2
]. It has the potential to provide a very fast and sensitive method of detecting chemicals and biomolecules, which is useful for applications that require fast and highly sensitive detection [3
]. For example, Caro [5
] et al. used a SERS probe for intracellular imaging, SERS signals were strong enough and could be detected even from inside cells. SERS can also be employed to study weak interaction between protein and alizarin [6
Many SERS substrates use colloidal clusters of noble metal nanoparticles or noble metals with a rough surface to enhance SERS signals. For example, An [7
] et al. used silver nanoparticles, tri-iron tetroxide, and carbon cores to form multilayered microsphere particles SERS substrate to detect pentachlorophenol (PCP), diethylhexyl phthalate (DEHP), and trinitrotoluene (TNT). In 2014, Au-Ag-S substrate developed by Cao et al. [8
] was used for surface-enhanced Raman detection and photocatalytic degradation of DEHP and DEHA. Liu et al. [9
] developed an alloy of gold and silver nanoparticles urchin shape (hollow Au-Ag alloy nanourchins, HAAA-NUs) as a SERS active substrate to detect 10−15
mol/L of DEHP. Most of these studies focus on achieving large enhancement factors, but fail to address the uniformity and reproducibility of these substrates. Due to the random distribution of nanoparticles on the substrate, the interparticle gap size is difficult to control. Therefore, it is difficult to uniformly control the generation of hot spots, resulting in large signal intensity variations, which is detrimental to quantitative analysis. Some researchers have tried to use linker molecules to control the distance between colloidal nanoparticles. For example, Anderson [10
] et al. made silver nanoparticle array tethered to a silver film using an appropriate tethering linker such as a dithiol or diamine. However, these linker molecules usually require a dedicated environment (some pH levels or temperatures) to serve the desired purpose, which limits their applications. In addition, linker molecules can block the analyte and prevent it from attaching to the plasmonic surface, which results in a low SERS signal of the analyte. Furthermore, it is possible for the linker molecules to introduce background noise, making the desired SERS signal difficult to measure.
The lack of uniform, reproducible and low-cost SERS active substrates suitable for quantitative analysis remains a major obstacle to the widespread use of SERS for routine analysis [11
]. Many research efforts have focused on the development of bottom-up and top-down approaches for the manufacture of controllable and reproducible metal nanostructures for SERS applications [12
]. These approaches include: anodic aluminum oxide (AAO) templates assist techniques, lithographic techniques, and oblique angle deposition (OAD) technique. For the metal nanoparticle arrays created using AAO templates, Mu et al. [13
] used electroless deposition and adjusted the pH and temperature of the gold plating bath to control the plating rate to achieve the desired particle size and interparticle gap. Lee [14
] used densely packed nanowires fabricated from AAO templates. Nevertheless, these approaches require sacrificing the AAO template each time to produce an ordered gold nanoparticle array, which results in increased time and cost expense.
For the lithographic techniques, Sánchez-Iglesias et al. [15
] uses block copolymer micellar nanolithography to create seeds for chemical growth of uniform Ag nanoparticle arrays. However, controlling seeds growth at the same rate is not easy, which makes it difficult to obtain a large-sized uniform Ag nanoparticle array. Another disadvantage of lithography is that if standard lithography is used, in order to create a few nanometer gap features, extreme ultraviolet (EUV) will be required, which is very expensive and disadvantageous for low-cost applications.
OAD technology is another method of making SERS active substrates. A number of studies have been conducted to fabricate nanostructured SERS substrates by OAD technology [16
]. It is a simple method of fabricating SERS substrates, and these fabricated substrates exhibit good sensitivity and uniformity. However, to fabricate SERS substrates by the OAD method, one will need a custom designed electron-beam/sputtering evaporation system. In addition, the vapor incident angle to the substrate normal is around 86°, which means that only a small portion of the material is deposited on the substrate, which results in increased manufacturing costs. Lastly, the anisotropic character of the optical properties, as well as the SERS responses of the substrates [4
], lead to the need of special angle coupling of the excitation laser which makes it not convenient to use.
In this study, we used a hot embossing technique to create a uniform and closely packed nanocone arrays on a substrate. Furthermore, by utilizing the self-aggregating nature of nanoscale metal materials, we were able to sputter uniformly distributed silver nanoparticles at the tip of the nanocone array structure. Using this method, we can create a uniform silver nanosphere array with very small gaps between the nanoparticles to create hot spots evenly. We used this structure as our SERS active substrate for rapid detection of low concentrations of analytes. The analytes tested in this study included Rhodamine 6G (R6G) and DEHP. R6G is a highly fluorescent rhodamine family dye, and it is commonly used in SERS experiments as a probe molecule. DEHP is a PVC plasticizer commonly used in food-related containers, such as plastic packaging (PVC cling film), plastic bags, plastic bowls, and plastic cups. According to the Taiwan Ministry of Health, DEHP is defined as an environmental hormone and is defined as a Class 2B carcinogen. The ability to quickly detect low concentrations of DEHP helps prevent people from eating food contaminated with DEHP, which will benefit people’s health.
The developed method can be used to fabricate disposable, low cost and highly sensitive SERS substrates. The results show a strong and uniform SERS enhancement effect. It not only opens up possibilities for using SERS in routine food safety analytics but also helps SERS to be widespread in many other applications as well.
The method used for the fabrication of silver nanosphere arrays decorated on a polycarbonate substrate with self-organized, hexagonal close-packed nanocone is illustrated schematically in Figure 1
. First, a nanocone-shaped groove array structure of aluminum substrate was fabricated by using anodic aluminum self-assembly technique [18
]. To form a cone-shaped cavity, the anodization and pore widening processes were alternately repeated several times, which creates a top widened and bottom narrow holes, the diameter of these holes are reduced gradually from the top to the bottom of these holes. The fabrication process of the nanocone cavity array of the aluminum substrate including electrolytic polishing, anodic treatment, removal of anodic aluminum oxide. Briefly, a 99.999% purity aluminum substrate was polished in a solution mixed with perchloric acid and anhydrous alcohol in a volume ratio of 1:3.5 and applied with a voltage of 25 V for 2 min. The polished aluminum substrate was anodic oxidation treated in 0.3 M oxalic acid solution with 50 volts for two hours. The anodic aluminum oxide layer was removed by placing the substrate in a 5 wt% phosphoric acid solution at temperature 35 °C for 1 h. The anodization and anodic aluminum oxide removal processes were alternately repeated 5 times to obtain the desired nanocone cavity array structure.
Then a Nickel replica of the nanocone-shaped groove array was obtained by electroforming the nanostructured AAO layer. The nanocone-shaped groove array structured Nickel mold was used as a master mold. Figure 1
. illustrated the hot pressing process. The nanocone-shape groove structured Nickel mold was imprinted on a Polycarbonate(pc) plastic substrate using a hot-press molding machine for heating and pressing the nanostructured nickel mold on pc surface for several minutes. After cool down to room temperature, the pc substrate was released from the master mold. Using this method, we were able to produce many nanocone arrays (moth eye like structure as illustrated in Figure 1
) structured pc substrates with low-cost and same quality.
The silver nanoparticles were formed on top of nanocones using sputtering. A dc magnetron sputtering system (Cressington 108 Sputter Coater) was used. Due to the self-aggregating nature of nanoscale metallic materials, the sputtered silver nanoparticles tend to self-aggregate at the tip of the nanocones, and finally form silver nanospheres at the tip of each nanocone (as illustrated in Figure 1
). The diameter and interspacing of the nanospheres can be varied by adjusting the sputtering parameters. In order to obtain a uniform and a closely packed array of nanospheres, the appropriate sputtering parameter is needed. In this study, we varied the sputtering duration while the sputtering current and gas pressure were kept at constant (20 mA and 0.02 mbar respectively). The nanostructured polycarbonate substrates were sputtered for different durations to obtain nanospheres with various diameters, and at the same time, the interparticle gaps of the nanospheres were changed accordingly.