We report on the fabrication of three-dimensional (3D) Au nanoporous films with pores the size of ~200nm by laser-induced modification of as-prepared Au flat films. The construction of honeycomb-like Au porous structures should be attributed to a model of subsurface micro-explosive boiling followed by laser irradiating of Au flat films. The 3D honeycomb-like Au nanoporous films with unique interconnected frameworks exhibit ultrahigh surface enhanced Raman scattering (SERS) activity with an enhancement factor up to ~109 and excellent stability/reusability even after 300 cycles of repeated SERS analyses. The project of wielding laser light as a versatile tool for sculpting stable nanoporous films will prompt the renewed interest in the multi-functional nanomaterials.
© 2017 Optical Society of America
Surface-enhanced Raman scattering (SERS) has been intensely explored as a powerful and mature vibrational spectroscopic technique to probe and identify molecules for molecular diagnostics and biomedical applications [1–11]. It is well established that the plasmonic metallic (Au, Ag and Cu) nanomaterials with increased surface roughness are able to produce strong and intense electromagnetic fields on their rugged structures, possessing pronounced SERS performances in molecular diagnostics [1–5]. Most recently, the single amino acid mutation in human breast cancer can be detected by disordered plasmonic self-similar chain . Among all available morphologies, such as core-shell nanoparticles [7,9], nanowire bundles , nanodumbbells , nanopillars , etc. porous-like nano-architectures have been well established as advanced prominent substrates for the SERS-based ultrasensitive probes [2,5,8,10]. Meanwhile, based on metallic nanostructure-substrates, there are several major issues need to be addressed to realize quantitative SERS measurements in an efficient and reliable way. First of all, to obtain highly sensitive and reproducible SERS measurements, the large scaled self-assembly of nanomaterial films with excellent uniformity (self-assembled monolayers) is necessary. Secondly, for a prominent substrate to be useful in SERS analyses, it should be stable and reusable under repeated applications. In fact, excellent stability/reusability of SERS substrate is an important factor for the recycling molecular diagnostics. For recycling applications, the original probe molecules should be removed from the substrate by rinsing in distilled water after each SERS analysis. Although a number of self-assembled monolayers have been rationally designed for substrate-based SERS analysis [13,14], the adhesive bounding/capacity of the self-assembled nanostructures is weak. After rinsing in distilled water, the layered nanomaterials will be seriously destroyed and separated from the substrate, which significantly reduce the SERS properties in the repeated applications. Therefore, based on metallic nanomaterials, non-traditional substrates with stable and reusable structures should be exploited for recycling applications in SERS analyses.
Recently, wrinkled nanoporous gold films (NPG) have been prepared by thermal contraction of the NPG/polymer substrate (PS) in the absence of colloidal metal nanoparticles . Different from the self-assembled films, the as-prepared Au films were physically attached to the pre-strained PS. The bonding between NPG and PS will be improved/enhanced by thermal treatment, which has a promising potential for repeated SERS applications. However, up until now the stability and reusability of the NPG-based SERS have not been previously reported.
In this paper, we report on the one-step synthesis of Au nanoporous films by laser irradiation of as-prepared Au flat films in vacuum (10−2Pa) condition. Without using any chemical etching, the novel honeycomb-like Au nanoporous structures with pores size of ~200nm can be easily formed by five pulses laser irradiation. Meanwhile, based on a model of subsurface micro-explosive boiling, a detail discussion of the physical mechanism is addressed. The three-dimensional (3D) honeycomb-like Au nanoporous films with unique rugged structures and interconnected frameworks exhibit ultrahigh SERS activity with an enhancement factor up to ~109 and excellent stability/reusability under 300 repeated applications. It is difficult to obtain by conventional chemical methods. Our results have open up a novel “green” paradigm to synthesize stable/reusable nanoporous metallic films, possessing high applicability in recycling application for practical SERS analysis.
2. Experimental setup
The as-prepared Au flat films with depth of ~3.5μm have been successfully deposited on silicon (Si) substrate at 200°C temperature by radio frequency (RF) magnetron sputtering technology using FJL560 model (CAs, Shenyang). A 99.999% Au target with 150mm diameter and 5 mm thickness was used. Silicon substrate was carefully cleaned through ultrasonic cleaning with a sequence of acetone and alcohol (3:1) mixture solution. The distance between the Si substrate and the Au target was about 150mm, and the chamber were pumped to a pressure of ~10−5 Pa. the 500W RF power was used in this paper, and the sputtering time was 1 hour. The schematic plot of Au films on Si substrate is shown in Fig. 1(a). The morphology of the Au films was examined by field emission scanning electron microscope (SEM, Hitachi, S-4800). The representative SEM image in Fig. 1(b) reveals that the Au films with excellent uniformity is flat structures. Based on step profile measurement (Tencor 200 Alpha-step), the depth of the films is about 3.5μm. Then, the Au nanoporous films were constructed by laser irradiation of the as-prepared Au flat films located inside a vacuum (10−2 Pa) chamber with the pulses from a Nd:YAG(Yttrium Aluminum Garnet) laser (Quanta Ray, Spectra Physics) operating at wavelength of 1064nm with pulse width of 10ns. The average spot size of the laser beam at the Au films was measured to be 2mm. The used laser power densities varied from ~5GW/cm2 to ~12 GW/cm2, which were monitored by an energy meter (Molectron, EPM1000). After laser irradiation, the modified structures were analyzed via SEM equipped with energy-dispersive-X-ray spectroscopy (EDS). The crystallographic investigations of the products were acquired by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF). 4-Aminothiophenol (4-ATP) was chosen as probe molecules for SERS measurements. The substrates for SERS were then immersed into 1~10−9 M 4-ATP ethanol solution, and dried at room temperature. All of the SERS spectra were analyzed by a LabRAM HR800 spectrograph with 633 nm as an excitation wavelength with an output power of 70 mW. The acquisition time used for one spectrum was 20s.
3. Result and discussion
The physical method based on laser irradiation was used to fabricate Au nanoporous films, which is different from those complicated chemical strategies (chemical etching or galvanic replacement reaction, etc.). The distinctive feature is that the pulsed laser irradiation process allows one to control and manipulate well the modification process. The schematic plot of modification of Au films by pulsed laser irradiation is depicted in Fig. 2(a). After five pulses laser irradiation (laser power of ~8GW/cm2) of as-prepared Au flat films, the representative low- and enlarged morphologies of an typical modified area was characterized by scanning electron microscopy (SEM), as shown in Fig. 2(b-c). The low-magnification SEM image in Fig. 2(b) clearly shows that the numerous interconnected Au nanoporous with obvious interior cavities and raised sidewalls/borders are formed as three-dimensional (3D) honeycomb-like structures. It is significantly different from the original flat films. The EDS pattern of the Au nanoporous films (inset in Fig. 2(b)) demonstrates that the honeycomb-like structures are mainly composed of Au elements (few Si elements from Si substrate). The relative ratio of Au and Si is calculated about 49:1, supporting the dominant Au species in nanoporous films. The high-magnification SEM image in Fig. 2(c) provides a typical structural detail of the Au nanoporous. The average size of the pores is about 200 nm by measuring the diameters of more than 300 nanostructures in sight on the SEM images. Based on the step profile measurement (inset in Fig. 2(c)), the average depth of the sidewalls is about 3.43μm, which is slightly lower than the original Au flat films in Fig. 1(b). In addition, the crystallographic investigations of the Au films before and after laser irradiation were established by X-ray diffraction (XRD) in Fig. 2(d). The crystal structure of the original Au films (red line in Fig. 2(d)) shows that four distinct peaks are observable at 30.228°, 42.418°, 64.728° and 77.709° in the XRD spectrum, corresponding to (111), (200), (220) and (311) lattice planes of Au face-centered cubic structure (JCPDS, no.4-0784). Because the relative higher peak at 30.228° in XRD pattern, the preferential alignment of the Au(111) orientation should be formed in single-crystal Au flat films. After five pulses laser irradiation, the well single-crystal structure was damaged on the Au nanoporous films since the corresponding intensities of these diffraction peaks become weaker. The main reason is that the rough and rugged structures formed by laser-induced modification of as-prepared Au flat films.
The formation of 3D honeycomb-like Au nanoporous films is attributed to a model of subsurface micro-explosive boiling followed by laser-induced heating and melting of Au films [15–19]. At the moment of pulsed laser arriving at Au flat films, the Au layers on the Si substrate absorb well the laser irradiation. Then, rapid boiling and vaporization of Au species will then occur, due to the pulsed laser-induced heating and melting processes. Immediately, the Au outmost surface layers are subject to cooling because of expiration of the pulse (10ns). Meanwhile, the subsurface regions of the Au films retain at a high temperature liquid state. Therefore, the pressure beneath the surface is even higher than that just at exterior surface, resulting in a type of micro-explosion. In this way, the micro-subsurface explosive boiling implies a dramatic rise of the pressure in the heated Au species followed by the releasing of trapped gasses. Finally, rapid condensation of the heated Au species will lead to the formation of Au nanoporous films with honeycomb-like structures. If such whole irradiation process at lower laser intensity, irradiated Au material is heated by the laser beam, followed by the normal vaporization from outside surface. Without enough laser energy, the micro-explosive boiling beneath the surface will become weaker, resulting in the formation of less obvious porous structures. As shown in Fig. 3(a), numerous raised Au nanomaterials with quasi-hemispherical-like structures and some incomplete holes were formed by laser irradiation at low intensity (~5GW/cm2). As for too high laser intensity adopted in laser irradiation, whereas the subsurface superheating will play a critical role in the bombarding process. Rapid and complete vaporization of layered Au species instead of micro-subsurface boiling will take place on the irradiated spot. As expected, the morphology of the irradiated area by high laser irradiation (~12 GW/cm2) in Fig. 3(b) shows that the re-deposition of superheated Au material with wave ridges-like morphology is formed on the bombarded area. Closer views of these modified structures indicate that numerous irregular solid droplets are formed on the ridges. According to the above results, the moderated laser irradiation plays a critical role in the formation of honeycomb-like Au nanoporous films. On the other hand, the unique porous structures will also be destroyed by subsequent pulses laser irradiation. As shown in Fig. 3(c), the honeycomb-like Au nanoporous have been damaged and irregular wrinkled fluctuation-like structures formed by excess pulses (ten pulses) laser irradiation. So, the appropriate pulsed laser irradiation (moderated power density and few pulses, etc.) should be recommended for the fabrication of Au nanoporous films.
Finally, the unique SERS properties of the honeycomb-like Au nanoporous films have been illustrated by using 4-ATP as the probe molecules under different concentrations, as shown in Fig. 4. Figure 4(a) shows that the dominating characteristic bands of the 4-ATP molecules at 464.8, 1090.2 and 1591.6 cm−1 are clearly detected in SERS spectra. The weak signals at 639.1, 824.3, 1008.2, 1176.0, 1294.1 and 1491.1 cm−1 can also been observed in Fig. 4(a). Moreover, even the concentration of 4-ATP was decreased to 10−9M, the substrate based on Au nanoporous films exhibits extremely intense Raman signals that are unambiguously higher than those from 1M 4-ATP on Si substrate and the 10−5M 4-ATP on the Au flat films.
The corresponding variations of Raman intensities at 484.8, 1090.2 and 1591.6 cm−1 are illustrated in Fig. 4(b). Consequently, the quantitative results confirm that the obtained 3D honeycomb-like nanoporous films provides ultrahigh SRES signals with intensities reaching ~8 and ~2.5 fold of those from 1M 4-ATP on Si substrate and the 10−5M 4-ATP on the Au flat films, respectively. The improved feature should be attributed to the 3D honeycomb-like nanoporous structures with unique interconnected frameworks that contain abundant Raman-active hotspots produced by deformation and modification of single-crystal Au flat films via laser irradiation.1–10]. As shown in Fig. 4(a)-4(b), ISERS and IBULK are about 19122 a.u and 2598 a.u for the Au nanoporous films and normal Raman spectra of 4-ATP, respectively. In this way, the EF can be calculated to be about 7 × 109 for the obtained Au nanoporous films, approaching the requirement (~nM) for single molecule detection [6,10]. The enhanced SERS activities in this paper exceed many previous reports, especially those of periodic gold nano-cuboids by electron beam lithography (EBL) and ligand-free AuAg bimetallic nanoparticles by laser ablation approach [22,23]. Moreover, the SERS enhancement would likely be larger if the Au nanoporous sample is prepared on a quartz wafer instead of a Si wafer as the imaginary part of the dielectric function of Si in this spectral range is so large as to lower the quality of the surface plasmons. For a substrate to be useful in SERS analyses, it should be stable and reproducible under repeated applications. To test the stability and reusability of the SERS measurements by using Au nanoporous films as substrate, we further carried out the recycling SERS experiments repeatedly 300 times. After each SERS measurement, the 4-ATP molecules were separated from the Au nanporous films by carefully rinsing in distilled water. The same substrate will be dried in room temperature for the next SERS analysis. Figure 4(c) shows the recycling test of SERS spectroscopy based on Au nanoporous films. Surprisingly, the obtained Au nanoporous films provide excellent SERS-stability/reusability even after 300 repeated applications. The stable well-resolved Raman spectral lines of the 4-ATP could still be sustained for 300 cycles of repeated SERS analyses. Moreover, the variation of the SERS intensity (1591.6 cm−1) in Fig. 4(d) clearly illustrates that the measured peak intensities are nearly constant even up to 300 recycling tests. The SEM image of Au nanoporous films after 300 repeated SERS analyses (inset in Fig. 4(d)) is identical to that of the initial honeycomb-like porous structures, which was well maintained after the durability test. The excellent stability of the Au nanoporous films can be attributed to the ultra rapid cooling and freezing of heated Au species by pulsed laser irradiation [15–21]. The related physical mechanism has been confirmed in many reports, which is also coincident in our case. There is no doubt that the unique Au nanoporous films with ultrahigh SERS activity and excellent stability/reusability under repeated SERS analyses will have promising applications in various molecular diagnostics and biomedical fields. Finally, it should be noted that the obtained devices are well suited for detection of a single component even at low concentration but hardly mixtures of solutes due to the randomness of hot spot.
In summary, 3D honeycomb-like Au nanoporous films with pores size of ~200 nm have been constructed by simply laser irradiation of as-prepared Au flat films in vacuum condition. To reveal the related physical mechanism, subsurface micro-explosive boiling process originated by the moderated laser irradiation has been proposed. Benefiting from the unique interconnected frameworks, the honeycomb-like Au nanoporous films exhibit ultrahigh SERS activity (EF of ~109) at nM level molecule detection. Moreover, we found that the novel Au nanoporous films provide the excellent SERS-stability/reusability even after 300cycles of repeated tests, which would be applicable to recycling application in practical SERS analyses. The advantages make the obtained Au nanoporous films to become a promising SERS substrate. The simplistic, single-step and versatile strategy developed in this work will open a novel physical paradigm to obtained metallic nanoporous films.
National Natural Science Foundation of China (NSFC) (11405098, 11575102, 11105085); the Fundamental Research Funds of Shandong University (2015JC007).
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