Laser-processed Nanostructures of Metallic Substrates for Surface- Enhanced Raman Spectroscopy

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Laser-processed Nanostructures of Metallic Substrates for SurfaceEnhanced Raman Spectroscopy Shi Bai1, Weiping Zhou1, Chen Tao1, Ken D. Oakes2 and Anming Hu1* 1

Institute of Laser Engineering, Beijing University of Technology, 100 Pingle Yuan, Chaoyang District, Beijing 100022, P. R. China; 2Verschuren Centre, Department of Biology, Cape Breton University, P. O. Box 5300, 1250 Grand Lake Rd., Sydney, B1P 6L2 Canada Abstract: Surface-enhanced Raman spectroscopy (SERS) is rapidly emerging as a powerful analytical tool for trace analysis of metallic substrates. While innovative numerical applications of SERS have been developed, it is still a challenge to fabricate cost-effective and reproducible metallic substrates for SERS probes. Laser processing, especially using ultrafast pulsed laser, can address this issue owing to their high quality processing. Herein, we investigate critical technical requirements and the latest advancements in laser-processing substrates for SERS, while highlighting several diverse applications. In light of its powerful enhancement of Raman scattering, SERS will remain an exciting research area for highly sensitive, resolved analysis into the foreseeable future.

Keywords: Enhancement factor, laser interference, laser processing, metal nanostructure, Raman scattering, surface enhancement. INTRODUCTION TO SURFACE ENHANCED RAMAN SPECTROSCOPY (SERS) In their sentinel and widely cited paper, Fleischmann et al. in 1974 reported the phenomenon of a strong Raman spectra signal emitted from pyridine absorbed on roughened Ag electrodes [1]. Several years later, Jeanmaire and Van Duyne attributed an enhanced signal disproportionate to the concentration of the species under study to an electromagnetic effect on the Ag electrodes [2]. Currently, this phenomenon and that of chemical enhancement (or charge transfer) are the two primary theoretical mechanisms underlying surface-enhanced Raman scattering (SERS) [3, 4]. Electromagnetic enhancement (EM) can be understood as the excitement of Localized Surface Plasmon Resonances (LSPR) [5] using noble metals, such as Ag or Au, to produce the LSPR [6, 7], as the resonance frequencies of these materials are in the visible and near infrared band ranges, where most of Raman excitation occurs [8]. Currently, a majority of researchers consider EM as the main factor contributing to SERS [9, 10]. However, chemical enhancement (CM), generated by transferring charges between the metal substrate and adsorbed species, while a fundamentally different mechanism postulated to underlie SERS, explains signal enhancement under scenarios for which EM alone cannot be responsible for the observed enhancement. Theoretically, a 102 enhancement can be realized by CM [11, 12], while taking both mechanisms into account, the collective SERS enhancement can approximate 106-107 under experimental *Address correspondence to this author at the Institute of Laser Engineering, Beijing University of Technology, 100 Pingle Yuan, Chaoyang District, Beijing 100022, P. R. China; Tel: 86-10-67396559; Fax: 86-10-67392030; E-mail: [email protected]

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conditions [13-15]. Since such enhancement is largely contingent on the LSPR, the Raman enhancement is greatly depended on the particles size, shape and distribution [16]. There are two aspects of the EM enhancement worth noting: the first is the “hot-spot”, the 1-2 nanometer area where the electromagnetic field is focused [17] and where Raman activity can be highly amplified. Proper configuration of this hot spot can allow for SERS detection of individual molecules [18-20], although the nature of hot-spot distribution is dependent on the SERS substrate and individual fabrication methods. SERS substrates can be roughly divided into three groups: (a) Rough electrodes or rough metallic surfaces; this kind of substrate formed the electrochemical electrodes where SERS was first discovered [1]. (b) Nanostructures prepared by laser or electron beams [21, 22]; using femto or nanosecond laser, it is relatively easy to obtain the array of dots, circles fringes and other nanometerscale shapes [14, 23, 24], for which a high enhancement factor can be achieved. (c) Colloids of Au, Ag or other metals acting as the SERS probe [25, 26]; the colloid can be synthesized by a multitude of diverse means with conventional chemical methods used to synthesize the nanoparticles [27-30]. The second noteworthy aspect of EM enhancement is the ability to experimentally determine the enhancement factor (EF), thereby providing a standard to compare diverse substrates. As local field enhancement is proportional to the tetra-power of the ratio of the local-surface plasmonpolaritons field to the incident electromagnetic field [31], we can obtain 1012-1014 average enhancement of Raman activities due to EM. Experimentally, there are two common methods to measure EF [32- 34]. One is to find the ratio of © 2014 Bentham Science Publishers

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the measured SERS intensities to the non-enhanced Raman scattering using Equation 1 [15, 35]:

2. NANOSTRUCTURE ENGINEERING WITH LASER PROCESSING

EF= (ISERS/IOR)(NOR/NSERS),

Laser processing has been an effective method to fabricate structures at the micrometer or sub-micrometer scale [40], with ultrafast lasers capable of creating nanoscale structures [41].

(1)

where ISERS and IOR (OR=ordinary) are the SERS and ordinary Raman intensities, and NSERS and NOR are the number of molecules excited by the laser to obtain the corresponding SERS and OR spectra, respectively. The EF can be readily determined using the SERS signal from a specially designed single molecule layer and normal Raman intensity. However, sometimes the number of adsorbent molecules is difficult to accurately measure [22, 36], and under such circumstances, Equation 1 can be substituted with Equation 2: EF= (ISERS/IOR)(COR/CSERS),

(2)

where CSERS and COR are the analytic concentrations for the SERS and normal Raman measurements, respectively [3739]. In this paper, we introduce laser processing methods and technologies to prepare substrates for SERS, including laser direct writing (LDW), laser interference lithography (LIL), laser ablation (LA) and pulsed laser deposition (PLD). We will focus on different technical features of laser processing based on various laser sources, including continuous laser, pulsed laser, visible and infrared wavelength laser. We will then discuss the application of SERS for environmental analyses and single molecule detection. Finally, we will summarize and discuss emerging future SERS developments and research trends.

2.1. Laser Direct Writing (LDW) In typical LDW applications, a program-controlled platform is used to change the substrate position [38, 42] using a setup schematically represented in Fig. (1) [38]. Relative to the nanosecond laser, the femtosecond laser (fs) has demonstrated superior nanostructure fabrication capabilities as the extremely short pulse durations markedly suppress adverse thermal effects [43, 44]. Further, as the peak power of fs laser is very high, due to the temporal compression of laser energy, almost any type of material can be processed [45, 46]. Finally, nanometer-scale machining beyond the diffraction limit of light is a possibility since non-linear optical interactions such as two- or multi- photon absorptions and selffocusing are attributes of fs laser pulses [47, 48]. In light of these intrinsic fs laser properties, a suitable combination of optical, thermal and chemical properties of materials should allow for reproducible fabrication with a resolution of tens of nanometers. Such precise processing is difficult using other types of lasers [49], resulting in the popularity of fs laser material processing to improve fabrication accuracy.

Fig. (1). Schematic of the experimental setup for femtosecond-laser direct writing via 2-photon polymerization. Taken from ref [38].

Laser-processed Nanostructures of Metallic Substrates

2.1.1. Femtosecond Laser Direct Writing Using Ti: sapphire fs laser, micro- or nano-structures could be written by line scanning gold films with 200 -300 nm fringe periods or hole sizes [50], which are otherwise unachievable using picosecond or longer duration pulsed laser. After synthesizing gold film on substrate by sol-gel and spin-coating, nanoparticles are induced on the film surface (writing) with a high-powered laser of 1 TW/cm2, then a non-destructive read-out can be performed by fs laser irradiation at 0.3 TW/cm2 (luminescence excitation, reading procedure). Notably, the 0.3 TW/cm2 laser does not modify the luminescent structures, so storage info (i.e. luminescence structures), can be read many times without significant bleaching. Consequently, this process is potentially very useful for high density optical memory applications, whereby writing, reading and deletion of luminescent gold nanostructures may find significant commercial applications, especially if 3D nano-patterns could be generated [32]. Alternatively, nanostructures can be first fabricated on the substrate, with the metallic film later coated on. For example, Diebold et al. [51] and Han et al. [34] fabricated nanostructures on silicon using the fs laser prior to preparing and adding the Ag-film onto the nanostructures by thermal deposition and chemical plating respectively. Both research teams enjoyed excellent Raman enhancement factors of approximately 107, with Diebold et al. reporting the EF was dependent on the deposited Ag film thickness. A further laser direct writing mechanism to generate nanostructure is photoreduction. By reducing silver nitrate and creating the nanoparticles on silicon, the detection limit of Rhodamin 6G (R6G) was estimated at about 10-15 M [52].

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Yukun Han [53] adopted an Ag-ion reduction method to fabricate nanostructures, and found that the fs laser pulses not only reduced the silver ions, but also fused the silver particles onto the silicon substrate, potentially enhancing the stability and reliability of the SERS substrate. Han-Wei Chang [54] compared three laser processing methods (fs laser writing followed by chemical Ag plating; femtosecond laser-induced co-deposition of Ag nanoparticles; and femtosecond laser direct writing on Ag+-doped phosphate glass) by measuring the EF of R6G. Chang reported that the Ag surface EF with fs laser writing surpassed that of the two other SERS-active substrates, while remaining free from contamination. In Fig. (2), the ring-shaped nanostructure [55] created by single-pulsed-LDW is an Ag nanoparticle aggregate produced by photoreduction, while outside the ring, the AgOx layer remained intact. The ripple effect of fs laser can be used to fabricate sensitive nanostructures on Ag coated substrates [56], with up to 15 times higher SERS signal obtained through this processing than achievable with commercial SERS substrates. These results imply tunable plasmonic enhancement could be realized through fs laser irradiation [55]. Non-thermal surface melting induced by the fs laser is increasingly an important processing technique utilizing temperatures much lower than the particle melting temperature [57]. The resulting nanoweld between adjacent metal nanoparticles produced by the fs-induced surface melting forms neck areas with a narrow gap of a few nanometers [58]. These welded nanoparticles possess a ring-shaped hot spot area in the vicinity of the neck relative to a central point in simply adjacent nanoparticle pairs [59]; consequently, the Raman signal could be significantly enhanced [60].

Fig. (2). (a) Optical reflection and (b) AFM image of the Ag nanoparticle aggregates obtained on the laser illuminated AgOx thin film. (c) Magnified SEM image of an aggregate of Ag NPs. (d) TEM image and electron diffraction pattern of laser-generated Ag NPs. Taken from ref [55].

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2.1.2. Nanosecond Laser Direct Writing There are numerous investigations developing patterns on metallic films using nanosecond (ns) laser direct writing. One such study wrote nanostructures onto glass substrates based on the reduction of silver nanoparticles from solution [61]; after the laser melts the Ag films, metal spherical particles were formed that easily absorbed to the desired surface for SERS. It is important to note that unlike fs laser pulses, longer duration laser (such as ns laser) does induce a thermal effect, which sinters nanoparticles and forms 3D nanostructures [62]. 2.2. Preparation of Substrates by Laser Interference Lithography (LIL) Controlled laser interference can generate arrays of dots, fringes or rings on metallic films. Unlike direct laser writing, laser interference lithography utilizes parallel processing, which can markedly facilitate the development of periodic nanostructures over a large area. As shown in Fig. (3), there are two ways to create interference beams [63]; the first has an optical delay to adjust the light path of one beam relative to the reference beam while the second uses a group of lenses or gratings to generate the interference beams. 2.2.1. Laser Processing with Gratings Using gratings, one vertically incident laser beam can be split into an even number of absolutely coherent beams; interference dots or fringes can be obtained by overlapping these beams [64]. Further, the phase variation of coherent beams results in completely different structural shapes in the target plane. Bekesi et al. [65] reported a desired fringe pattern could be obtained by selecting appropriate gratings. Conversely, rather than relying on gratings, the 4f system contains two lenses capable of splitting beams as well as

creating interference patterns. Fig. (3b) illustrates four identical focal lengths (f) while Nakata [63] used two lenses of different focal lengths to yield a narrow coherent beam area. The latter system was successful in fabricating an array of dots and fringes with a distance of about 1 micrometer, with the fringe direction differing with the beam polarization. The ripple and dual periodic structures created by the polarization of the laser are shown in Fig. (4) [66]; it is obvious that the ripples and periodic structures are controlled by the polarized beam. However, a drawback of this kind of grating processing system is that it can be easily contaminated by metallic debris, due to the exceedingly small distance between the grating and target. To overcome this limitation, Kaakkunen [67] and his team utilized four separate gratings rather than a single one, increasing the distance between gratings and ablation region as the split beams are moved farther apart to obtain more focusing distance. 2.2.2. Laser Processing with the Optical Delay Optical delay is a popular method of temporally overlapping ultrafast pulses in the femtosecond laser system as it can more accurately control delay time. Planar submicron gratings with a period of 720 nm could be fabricated with 1030 nm 560 fs LIL on silicon with the ablated area occupying about 340 nm [68]. Guo [69] reported twostep processing with 800 nm, 120 fs laser on silica glass, producing doughnut-shaped structures due to the multiphoton absorption. It must be noted that relative to the gratings, the interference area generated by optical delay is smaller [65]. 2.2.3. Conventional Optical Interferences A simple interference system can be used for pattern fabrication [70, 71], in which one mirror and the sample were put together at an angle so that half of the expanded laser

Fig. (3). Beam correlators. (a) with optical delay and (b) with grating and lens. Adapted from ref. [63].

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Fig. (4). Ripple structure in (a), and dual periodic structures in (b) to (g). The last red arrow is the polarization direction of the beam. Taken from ref [66].

beam fell directly onto the sample, and half fell onto the mirror. An interference pattern was then created by overlapping the reflecting beam with the initial beam from the UV laser as schematically depicted in Fig. (5). 2.3. Preparation of Substrates for Pulsed Laser Deposition (PLD) PLD is a simple and effective technique to produce nanostructured thin films, where various processing parameters such as laser wavelength, pulse duration, fluence, etc. can be modified to modulate nanostructure formation [72, 73]. The advantages of PLD are obvious, including the potential for fabrication of thinner metal films with a lower amount of metal, the greater effectiveness of the fabricated films, lower adsorbate degradation, and the

Fig. (5). Schematic diagram showing the two-beam interference apparatus. Taken from ref [71].

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Fig. (6). (a) Detail of the experimental Pulsed Laser Deposition configuration: S=silver plate; GS=glass slide; h=distance between silver target and substrate; lmax=crater depth; d=crater diameter; x=surface position. (b) Experimental set-up: S=silver plate; GS=glass slide; L=lens; M=mirror; SH=sensor head; TRM=thickness-rate monitor; VG=vacuum gauge; ODP=oil-diffusion pump. Taken from ref [75].

absence of surface impurities [74]. One typical experimental setup is shown in Fig. (6) [75].

2.4. Preparation of Substrates in Liquid by Laser Ablation (LA)

2.3.1. Femtosecond Laser Deposition

The preceding two sections have described the two main technologies used to prepare solid substrates for SERS; however, it is also possible to get metallic colloids in liquids, where they can be subjected to photoreduction in an ionic solution. Laser ablation is a synthesis method for chemically pure colloids that does not require expensive vacuum equipment [82-85].

Pulsed laser deposition incorporates several sequential steps; plume generation, cluster formation, island growth and the formation of nanostructures [76]. The silver nanoparticle film coated on the carbon film surface can probe the molecular structure of carbon films through SERS analysis [23]. In addition to the deposited film, the silver target could be also used for SERS substrate after the PLD [74]. Karmenyan et al. [77] deposited nanodiamonds on Ag substrate, which showed significant Raman spectra as well as less luminescence properties. 2.3.2. Nanosecond Laser Deposition Donnelly [78] and his team used the KrF laser (26 ns) to prepare ultra-thin Ag films by PLD under vacuum, and investigated film morphology and optical absorption spectrum as a function of equivalent thickness. In addition to enhancement dependent on the surface morphology, the highest efficiencies are observed when the sample surface is composed of clusters [79]. With proper gas pressure and pulse number, one can deposit films with high sensitivity, reproducibility, and repeatability [80]. Smyth [81] compared PLD substrates to commercial products, demonstrating the former are promising for SERS applications. In Fig. (7), two different trace molecules were detected using three different thin film substrates, nanoparticle colloids, and one commercial substrate. The SERS spectra, including two molecules using the colloidal nanoparticle suspension, yield better performance than the two other methods. The substrate prepared by PLD effectively differentiates analytes of similar structure, even at very low concentrations.

2.4.1. Synthesis of Ag Colloids by Laser Ablation Šmejkal et al. [86, 87] proposed a laser ablationnanoparticle procedure for Ag hydrosol preparation. Two lasers with wavelengths of 1064 nm and 532 nm are used for generation of homogenously distributed nanoparticles. Some researchers synthesized Ag colloids through photoreduction [88, 89], while Jiang [36] used a two-step fs laser reduction to generate an array of lines on silicon in silver nitrate. Lin [33] proposed one-step photoreduction of an aqueous silver nitrate solution by fs laser direct writing. In addition to silver nitrate, Dong [90] and Gan [91] reported a method of preparing a Ag colloid from a solution of AgCl using laser irradiation to synthesize AgCl cubes prior to ablating them to Ag nanoparticles (Fig. 8). It is clear that the intensities of the Raman peaks are dependent on concentration with a decrease in peak intensity recorded when R6G concentration decreases to 10-11M. 2.4.2. Synthesis of Au and other Metallic Colloids by Laser Ablation Using the two-step fs laser ablation-based method, gold nanoparticles were synthesized in water, with Au particle diameters increasing with increased irradiation time [92]. As with Ag, the photoreduction method can be used to prepare Au colloids as well [93]. While laser irradiation leads to

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Fig. (7). SERS spectra of (a) 6. 63×10−5M Rhodamine 6G and (b) 4.2×10−4M benzotriazole using three different enhancing media. Taken from ref [81].

Fig. (8). (a) Conceptual illustration figure of the AgCl preparation by pulsed laser ablation of Ag target in NaCl solution. (b) SERS spectra of R6G on AgCl particles and bare Si wafer. Adapted from ref. [90].

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reduction, it can also result in the aggregation or degradation of nanoparticles with different laser pulse numbers [94]. Zhang and Fang [87] proposed Cu and Pt as good substrate material options for SERS. Using a Pt nanoshell with a core of SiO2, the Raman spectra enhancement was remarkable [95], demonstrating that copper nanoparticles can play an important role in catalytic reactions within biological systems by interacting with nucleotides or other biomolecules [96]. 3. APPLICATIONS OF SERS SERS has been used in many different fields, including fast analysis in the semiconductor industry [97], environmental trace analyte detection [98], imaging [99], and tipenhanced Raman spectroscopy (TERS) [100]. Although some applications are still in their infancy, the effectiveness of SERS across a range of applications has been broadly confirmed, and undoubtedly, new applications will emerge. 3.1. Environmental Monitoring In recent years, the importance of environment monitoring and protection, from both a public and regulatory perspective, has increased markedly. Often, there is considerable overlap between environmental and human health concerns. For example, the dyes used in the leather and fabric industries while contaminating aquatic environments adjacent sites of use, are also of concern as they appear in food products and drinking water. Using the nickel particle substrate SERS method, the Sudan II (C18H16N2O) dye molecule can be easily detected even at low concentrations of 10-5M [101]. Further, this method can also be used in the analysis of other waterbome contaminants, such as arsenic, which is similarly of concern from both environmental and human health perspectives due to its lethality at trace concentrations. Ag nanocrystals of different shapes enable detection of low levels of arsenate in aqueous solutions, to the 1 ppb level [102] in water and wastewater [103]. The SERS method coupled with sensitized substrates demonstrates numerous advantages; such as relative ease of use, short time required for sample preparation, and its less destructive nature of measurement. 3.2. Single Molecule Detection by Tip-enhanced Raman Spectroscopy (TERS) As mention previously, the hot spot created by the close distance between adjacent nanoparticles greatly, enhances the Raman signal, although the distribution of these hotspots is usually random on laser-processed substrates. However, if we could spatially control the molecule of analyte with a probe, the enhanced signal could be used to detect single molecules [104]. Fortunately, tip-enhanced Raman spectroscopy has the potential to actually achieve this goal, with the distance between the tip and substrate of only a few nanometers controlled by Atomic Force Microscopy (AFM) or Scanning Tunneling Microscopy (STM) technologies [105-107]. Critical problems limiting TERS is how to make the tip cost-effectively, as well as preventing contamination during use. Often the Ag or Au surface of tip is contaminated by

organic species after measurements, but using the chemical method, the contaminant can be removed effectively [108], without sacrificing the intensity of the Raman signal. It is worth mentioning that the heat effect induced by the incident laser on the tip can decrease the signal intensity and even damage the tip [109]. The tip itself is more easily polluted in liquid than air milieus as the former increases the chance of absorbing contaminants. However, tip stability can be enhanced as demonstrated in water and acetone [110], by coating with Ag nanoparticles. While these nanoparticles will fall off the tip under extreme conditions, the tip itself is protected and can still provide excellent signals for analysis. SUMMARY AND OUTLOOK In this paper, we summarize the main fabrication methods for SERS laser-processed substrates,. The reviewed laser processing technologies include laser direct writing, laser interference lithography, laser ablation and pulsed laser deposition. All of these methods have the potential to manufacture relatively stable, high enhancement, cost-effective substrates for SERS. In evaluating the relative use of these methods, we found researchers were inclined to most commonly use ultrafast lasers, especially femtosecond laser, to fabricate or process the substrate. Considering the breadth and diversity of applications employing SERS, we believe this field will become increasingly attractive to researchers and commercial applications alike over the coming decades. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKONWLEDGEMENTS The work is partially supported by the high level overseas talent project of Beijing, and a strategic research project “Laser-induced nanostructures for surface-enhanced Raman scattering and environment application” (KZ201410005001) of Beijing natural science foundation, P. R. China. REFERENCES [1]

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Received: November 14, 2013

Revised: March 06, 2014

Accepted: April 01, 2014

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