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Abstract
Multi-band surface-enhanced Raman spectroscopy (SERS) via multiple excitation wavelengths operated on broadband resonant substrate can empower a high-dimensional comprehensive molecular diagnosis of complex analytes. Herein, we demonstrate an extraordinary multi-resonant SERS active substrate that is composed of three-dimensional (3D) hierarchical plasmonic Au superstructures (SS) with both horizontal and vertical close-packed nano-polyhedron clusters on fluorine-doped tin oxide-coated (FTO) support. The stable solid-state Au SS can be simplistically developed by ultraviolet laser irradiation of FTO plate in HAuCl4 solution that facilitates photoexcited reduction of Au ions and then anisotropic nucleation of Au atoms. The 3D Au SS with ultrabroadband plasmonic resonance ranging from visible light to near-infrared region (400∼2000nm) provide remarkable enhanced multi-band SERS performances under 532, 633 and 785 nm excitation wavelengths, in comparison with the reference normal Au nanoparticles (NPs). Especially, the 785 nm NIR excitation of the generated SERS substrate enables the ultra-low detection limit of crystal violet (CV) molecules to be achieved as low as 10−16 M, which is obviously better than many previous works. The ultrahigh multi-band SERS activity is highly related the strong synergetic coupling effects of these interconnected Au nano-polyhedrons with hybridized multiple plasmonic modes. Besides, the ingenious Au configuration also possesses excellent SERS spatial uniformity, long-term stability and reproducibility, having more promising potentials for practical operation. Therefore, the versatile 3D plasmonic SS may grant attractive alternative pathway toward robust multi-band SERS analyses in the near future.
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1. Introduction
Surface-enhanced Raman spectroscopy (SERS) with highly sensitive vibrational fingerprint information has attracted immense attention for molecular analyses of ultralow concentration analytes in many important chemical and biological sensing applications including pollution monitoring, food safety and biomolecular diagnosis [1–5]. The enormous SERS enhancement is mainly attributed to the intense electromagnetic (EM) field derived from incident laser excitation of plasmonic (gold (Au), silver (Ag) or copper (Cu)) nano-substrates. The unique localized surface plasmon resonance (LSPR) via photoexcited collective oscillation of metallic conduction electrons can be greatly enhanced at resonance frequency in response to incident laser wavelength, giving rise to the strong EM for boosting SERS activity [6,7]. In this regard, extensive efforts have been deliberately focused on the construction of nano-architectures with desirable anisotropic features, optimal shape, composition or size [8–12]. For instance, hybrid branched Au/Ag nanodendrites were developed for improving LSPR efficiency at near-infrared (NIR) region and then enhancing 785 nm-excited SERS activity toward sensitive detection of biomolecules, dye molecules and polycyclic aromatic hydrocarbons [3,13,14]. On the other hand, the other previous report verified that the 532 nm, 633 nm or 785 nm excitation wavelength-excited SERS can be separately improved when the size of Ag nanoparticles (NPs) was carefully controlled at 150 nm, 175 nm or 225 nm [15]. However, most of these designed nano-substrates were merely composed of single-resonant plasmonic mode in each SERS operation, which can be only matched with a restricted individual excitation wavelength. In this way, the single resonance and narrowband SERS is still far from satisfactory toward the rapid and effective molecular diagnostics. Therefore, the emerging multi-band SERS that can empower integrated high-throughput molecular detection operated on broadband resonant substrate under multiple excitation laser wavelengths [2,16–18], should be highly desirable for in-situ SERS monitoring of complex analytes in real-world scenarios.
As for the promising multi-band SERS, the most important essence is the ability to construct broadband plasmonic resonant substrate that can sensitively response to almost all available laser excitation wavelengths from the visible to NIR region [16–19]. So, it should be composed of some unique arranged configurations with multiple plasmonic modes that have been realized in the three-dimensional (3D) hierarchical superstructures (SS) via effective assembly of close-packed plasmonic nanoparticle clusters [2,20–23]. As demonstrated in the extensive functional supraparticles [20–25], the plasmonic SS can enable the broad manipulation of their combined optical-electronic LSPR properties under multiple excitation wavelengths, due to the pronounced electromagnetic coupling effect of closely spaced NPs [21–23]. Consequently, tremendous efforts have been strongly encouraged to the rational construction of hierarchical plasmonic SS that could be more desirable for improving multi-band SERS performance. For instance, an ultrabroadband SERS chip based on Ag NPs/SiO2 spacer layer/thick Ag film was designed by high-vacuum sputtering/e-beam evaporation, which can be used for multiple excitation wavelengths within 450∼1100 nm [19]. Recently, an interesting multi-active SERS sensor was developed by assembling a monolayer of Ag NPs into Au/SiO2-deposited artificial plasmonic compound-eye (APC), resulting in broadband and omnidirectional SERS enhancements under laser excitations at 488, 633 or 785 nm [2]. Another recent report also demonstrated that the multi-band SERS with excellent enhancement factors (>106) under laser excitations at 532, 633 and 785 nm can be obtained by the construction of Ag/SiO2/Ag nanolaminate plasmonic nanocavities on vertical nanopillar arrays [16]. Despite these achievements, the practical implementation of multi-band SERS is still severely hampered by two main aspects that create huge barriers for widespread applications. Firstly, the great challenge is originated from the complicated design and tedious fabrication of plasmonic SS with limited programmable building blocks, including electron beam lithography (EBL) [19], nanoimprint [16], intricate colloid chemistry methods (aggregation of a colloidal solution with external agents, electrophoresis or gradient centrifugation) [20,21, 26,27], and vacuum thermal evaporation or magnetron sputtering/mechanical milling technique [28–30], etc. On the other hand, in order to broaden LSPR resonance, the intricacy of present complex metamaterials with many attractive building blocks is generally increased by inserting different semiconductor or insulator spacer layers into more than one plasmonic layers in vertical direction [16,19,28–30]. Compared to pure plasmonic structures with strong metallic bonding, the metal/spacer multilayers with intrinsic microstructural instability would be inevitably dissociated during long-term laser light irradiation in complex condition, which cannot be robust enough to withstand the practical repeated SERS operations. Therefore, it is absolutely imperative to develop a simplistic yet universal strategy for constructing broadband resonant plasmonic SS with stable building blocks to meet the ever-growing demand of multi-band SERS.
Fortunately, the continuous wave (CW) laser light-triggered solid-state protrusion of anisotropic plasmonic nanostructures [31] and induced coalescence of metallic dimers/clusters [32] in solutions have been successfully developed in recent years. The ingenious “green” photochemical fabrication with extremely simple procedure is radically different from the routine complicated techniques, offering an ideal alternative candidate for conveniently sculpting of stable plasmonic metallic SS toward high-performance multi-band SERS. Inspired by it, herein, we report an interesting broadband plasmonic resonant SERS active nano-substrate via laser beam-induced construction of 3D hierarchical Au SS with both horizontal (x-y) and vertical (z-) close-packed anisotropic nano-polyhedron clusters. The large-scale formation of high-yield 3D stacking Au SS can be simplistically achieved by ultraviolet (UV) laser irradiation of fluorine-doped tin oxide-coated (FTO) plate in HAuCl4 solution. Compared with normal Au NPs, the established 3D Au SS can provide ultrabroadband plasmonic response from whole visible to NIR region (400∼2000 nm), giving rise to ultrahigh multi-band SERS activities under 532, 633 and 785 nm conditions. Especially, the SERS signals of crystal violet (CV) probe molecules based on 785 nm excitation of the prepared 3D Au SS would be also clearly distinguishable even the concentration as low as 10−16M, enabling the limit of detection (LOD) to be achieved at 0.1 femtomole (fM) level. Moreover, the obtained broadband resonant plasmonic SS also show excellent SERS spatial uniformity, long-term stability and reproducibility, promoting reliable multi-band SERS quantitative analyses in various practical scenarios.
2. Experimental details
2.1 Materials
The chloroauric acid (HAuCl4) and FTO glasses were purchased from China Sinopharm International (Shanghai) Co., Ltd. Polyvinylpyrrolidone (PVP, Mw = 40000) and crystal violet (CV) dyes were obtained from Macklin Chemistry Co., Ltd. Ethanol and ascorbic acid (AA, 99%) were purchased from Tianjin Fuyu Chemical Co., Ltd. Deionized (DI) water used in the fabrication and measurement was prepared by using a Millipore purification system (18.2 M Ω cm). All chemical reagents were of analytical grade and used as received without further purification in all experiments.
2.2 Preparation of plasmonic Au SS on FTO supports
The formation of 3D hierarchical Au SS with close-packed nanoparticle clusters was based on UV laser (375 nm) beam irradiation of FTO glass in HAuCl4 solution. Firstly, 100 μL HAuCl4 (0.05M) and 0.5 mL PVP (0.05∼0.1M) were separately added into 10 mL solution (9 mL DI water + 1 mL ethanol) by 15 min ultrasonic vibration, resulting in the generation of homogenous mixed solution. The PVP used in this experiment is a stabilizing and capping agent for overgrowth of anisotropic building blocks during laser irradiation in liquid. Then, a well-polished FTO glass used as the solid target was placed on the bottom of a rotating beaker (∼200 rpm) filled with ∼1.0 cm depth of as-prepared HAuCl4 solution. The photochemical overgrowth of 3D Au SS was carried out by CW 375 nm laser light irradiation of FTO support with power of ∼600 mW and average diameter of beam ∼2 cm. After different irradiation times (0∼60 min), the obtained Au solid-state protrusions on FTO support were separately and carefully washed in DI water then naturally dried in the fume hood for 10 hours at room temperature(∼23°C). On the other hand, the reference normal Au NPs were simply produced by CW 375 nm laser light irradiation of 10mL HAuCl4 and PVP mixed solution that added with 0.05 mL AA (0.01M) weak reducing agent.
2.3 Characterization of nano-products
The micro-morphologies and chemical composition investigations of as-prepared nano-products were performed by focused ion beam electron microscope (FIB, Helios G4 UC) equipped with energy-dispersive-x-ray spectroscopy (EDS). After ultrasonic treatment in solution, some plasmonic Au SS were separated from FTO support and dispersed into liquid. Then, the transmission electron microscope (TEM) measurements were carried out by using (JEOL-JEM-2100F) with an accelerating voltage of 200 kV. The TEM samples were prepared by dropping the product solution on the copper mash and completely drying in an oven. Additionally, the surface compositions and element valences were recorded by X-ray photoelectron spectra (XPS) by a PHI Quantera SXM. The crystallographic tests were analyzed by X-ray diffraction (XRD) patterns (Rigaku, Smart Lab 9 kW) using Cu Kα radiation (λ=0.15406 nm) in the range of 10∼80°. The absorption spectra of the nano-products were taken from UV-visible-NIR spectrophotometer (20/30 PVTM Micro-spectrophotometer). Meanwhile, the photoluminescence (PL) spectra were obtained by a high-efficiency integrated fluorescence spectrometer (Horiba, FluoroMax-4) with different selected excitation wavelengths (532, 633 and 785 nm).
2.4 SERS measurements
The SERS substrates were prepared by a series of as-prepared Au SS formed on FTO supports and the reference normal colloidal Au NPs dried on FTO glasses. In order to provide probe molecules, about 37.4 mg CV powders were dissolved in 10 mL DI water to form 0.01 M CV solution, which would be separately diluted with DI water to generate CV solutions with different concentrations of 10−6∼10−16M. Then, a typical Raman test was obtained by separately dropping a drop (10 μL) of CV solution on different SERS substrates, and each substrate was naturally dried in the fume hood for 6 hours at room temperature(∼23°C). All SERS signals were collected by a confocal microprobe Raman spectrometer (Renishaw, inVia Qontor) with different laser excitation wavelengths (532, 633 and 785 nm) at room temperature (∼23°C). The laser power in each excitation wavelength was fixed at 0.05 mW and the acquisition time of each Raman spectra was set at 1.0 s. Each of Raman spectra was processed (smoothed and baseline corrected) by using Wire 5.4 software (Renishaw). For all Raman measurements, the size of the laser spot on the sample was controlled at about 1.0 μm under a 50 × objective lens.
3. Results and discussion
The microstructures of initial FTO support were characterized by SEM images in Fig. 1(a)-1(b). The rugged FTO outer layer is composed of dense rock-shaped nano-architectures with plentiful pinnacles and sharp edges, offering uneven structured surface for subsequent laser-induced anisotropic growth of plasmonic Au SS in this work. It has been well known that the FTO glass is decorated with fluorine-doped tin dioxide (SnO2) semiconductor materials [33,34]. So, the photoexcited electrons generated by UV laser irradiation of the doping band-gape SnO2 semiconductors in FTO plate can efficiently facilitate multi-electron reduction of metallic ions and then promote the nucleation of metal atoms, which has been verified in previous works [35–37]. The laser-induced photochemical synthesis of anisotropic metallic nanostructures can be simply controlled by adding proper stabilizing and capping agent such as PVP in this work, which is very similar to the traditional wet-chemical techniques. After 30 min laser irradiation of FTO support in HAuCl4 solution mixed with PVP agent at low concentration of 0.05M, the SEM images of plasmonic Au nanostructures are illustrated in Fig. 1(c)–1(d). As expected, it can be found that many Au NPs are already assembled and organized into the stacking metallic clusters, giving rise to the formation of some isolative bud-shaped hierarchical plasmonic SS on FTO support (Fig. 1(d)). The average size of these metallic SS is calculated about ∼120 nm based on the measurements of more than 200 samples in sight on the SEM images. Then, the plasmonic Au nanoparticle clusters would be gradually grown up by further prolonging laser irradiation time to 60 min, as shown in Fig. 1(e)-1(f). Clearly, the nearly hemisphere-shaped Au SS with average size of ∼270 nm are formed on the FTO rugged surfaces (Fig. 1(f)), which are obviously larger than the initial bud-shaped small-sized Au SS in Fig. 1(d). On the other hand, we also found that the excess irradiation times (>60 min) was not applied to continually increase the vertical height of 3D hierarchical Au SS, which is mainly due to the insufficient PVP auxiliaries used in this experiment.
Fig. 1. (a-b) The low-magnification and enlarged SEM images of original FTO support. (c-d) The different enlargement SEM images of laser-induced protrusions of plasmonic Au nanoparticle clusters on FTO support by UV laser irradiation of 30 min. (e-f) The low-amplification and enlarged SEM views of plasmonic Au SS on FTO support after irradiation of 60 min. The reaction solution contains PVP agent with low concentration of 0.05 M. Scale bares: (a) 5μm; (c), (e) 1.5μm; (b), (d), (f) 250 nm, respectively.
Then, the much taller Au vertical SS on FTO support would be further fostered by using enough PVP (0.1M) in the reaction solution that can promote the construction of higher anisotropic microarchitectures. The laser-induced remarkable coalescence of vertical plasmonic nanoparticle clusters was intuitively monitored by cross-sectional SEM measurements of a representative sample via laser irradiation at different times, as shown in Fig. 2(a)-2(d). After irradiation of 10 min, the elongated Au SS formed on the rugged FTO surface is supposed to look like a vertical treelet-shaped microstructure with a height of about ∼380 nm (marked with yellow circle in Fig. 2(a)). Then, the obtained results by prolonging irradiation times (20∼40 min in Fig. 2(b)-2(d)) reveal that the vertical plasmonic Au nano-protrusion can be gradually developed into a much taller hierarchical SS that are composed of dense close-packed Au nano-polyhedron clusters. The height of this vertical tree-shaped Au SS in Fig. 2(d) would be significantly increased to about 3 μm by prolonging irradiation time to 40 min, which is already ∼ 7.8 times taller than that of initial one in Fig. 2(a). It is also much higher than that at nanometer levels of bud-shaped or hemisphere-shaped Au nanostructures in Fig. 1(d) and Fig. 1(f). As shown in Fig. 2(d), the anisotropic building blocks of 3D hierarchical Au SS are mainly composed of many irregular tetrahedral, hexahedral or octahedral nano-architectures, which are further illustrated by TEM image in Fig. 2(e). It is demonstrated that the polyhedral Au NPs are definitely interconnected and accreted with each other, forming some necklace-like nanochains. Therefore, the 3D hierarchical Au SS in this work also possess abundant porous interspaces among these close-packed Au nano-polyhedron clusters. Meanwhile, the coupled connection between two Au nano-polyhedrons is then clearly verified by the typical high-resolution TEM (HRTEM) image in Fig. 2(f). The HRTEM result reveals the enlarged structural detail of the interconnected cross region with d-spacing lattice fringe of 0.204 nm (marked with blue square in Fig. 2(f)), which should be indexed as the Au (200) lattice plane. The photochemical overgrowth of these cross-linked Au nano-polyhedrons should be mainly attributed to the laser sintering of closely spaced metallic NPs, which has been well confirmed in many previous works [32,38–40]. It should be emphasized that the unique laser-induced combination of Au nano-polyhedron clusters into hierarchical plasmonic SS should be much stable configuration, which are remarkable different from the traditional self-assembly techniques by evaporating the suspension droplets [41–44].
Fig. 2. (a-d) Cross-sectional SEM images of vertical plasmonic Au SS on FTO support by UV laser irradiation at different times of 10, 20, 30 and 40 min. (c-d) TEM and HRTEM images of plasmonic Au nano-polyhedrons generated by laser irradiation of 40 min. The reaction solution contains enough PVP agent with relative high concentration of 0.1 M. Scale bares: (a)-(b) 500 nm, (c)-(d) 1μm, (e) 200 nm, (f) 5 nm, respectively.
Besides the above individual sample of plasmonic Au SS, the large-scale fabrication of high-yield 3D stacking hierarchical structures would be also achieved by laser-induced coalescence of close-packed Au nano-polyhedron clusters at both horizontal (x-y) and vertical (z-) directions on FTO support, as demonstrated in Fig. 3 (a). After prolonging irradiation time to 100 min, the top-view SEM image illustrates that the horizontal round plate with a large-size of ∼60 μm is completely covered by dense arranged vertical metallic clusters, forming the forest-like 3D plasmonic Au SS on the rugged FTO plate. Moreover, the 3D vertical Au configurations of close-packed nano-polyhedron clusters on the horizontal plate can be also undoubtedly verified by some typical enlarged cross-sectional SEM images of different representative regions marked in Fi.3(a). As shown in Fig. 3(b)-3(e), the hierarchical plasmonic SS composed of plentiful coalescent polyhedral Au NPs at vertical directions growth on the rough surfaces of FTO support. Moreover, some cross-linked roots between upper plasmonic clusters and underside FTO plate are intuitively observed in Fig. 3 (e). Meanwhile, the polyhedral Au NPs served as anisotropic building blocks of 3D plasmonic SS, especially some well-defined bipyramidal nanostructures are also clearly identified in these clusters, as the enlarged top-view SEM observed in Fig. 3(f). It reveals the efficient anisotropic nucleation of metallic atoms in the presence of enough PVP stabilizing and capping agent during laser-induced photochemical synthesis in this work. On the other hand, the corresponding elemental mapping images of high-yield plasmonic Au SS formed on FTO support are displayed at the bottom regions of Fig. 3. The obtained results explicitly confirm the excellent uniform spatial distribution of dense metallic Au elements at round region that can be clearly distinguishable from that of Sn and O elements originated from FTO support. It is convincingly demonstrated that the laser-induced coalescence of dense close-packed Au clusters would enable them to be effectively assembled into high-yield 3D stacking plasmonic SS at confined region on the FTO support.
Fig. 3. (a) Low magnification SEM image of high-yield 3D hierarchical Au SS with close-packed nano-polyhedron clusters on FTO support. (b-e) the enlarged cross-sectional SEM images and (f) top-view SEM image of different corresponding regions marked in (a). The bottom images show the elemental mapping results of as-prepared 3D stacking Au SS on FTO support. The laser irradiation time was increased to 100 min. Scale bares: (a) 20 μm, (b)-(c) 1 μm, (d)-(e)500 nm, (f) 150 nm.
In addition, the optical properties and crystallographic phases of the established 3D plasmonic Au SS formed on FTO support and the reference sample via colloidal normal Au NPs naturally dried on FTO were investigated by UV-visible-NIR absorption spectra and XRD tests. The insets in Fig. 4(a) show the SEM images of the above two targets, where the obtained 3D hierarchical configuration is clearly different from the dried film of normal Au NPs with relative smooth surface. As displayed in Fig. 4(a), the absorption spectra of the reference sample (red line) exhibit a strong peak at about ∼620 nm that should be attributed to LSPR position of the dried plasmonic Au NPs. It is obviously red-shifted in comparison with that derived from colloidal Au NPs in solution due to the inevitable aggregation of plasmonic nanoproducts after drying process [6]. As for the resultant 3D Au SS (violet line in Fig. 4(a)), it can be found that the ultrabroadband plasmonic resonance can be already ranged from the whole visible region to NIR (400∼2000 nm), providing a significantly stronger absorbance capacity of broad-range of lights. Compared to the normal Au NPs with narrowband resonance at visible region, the ultrabroadband resonant 3D hierarchical Au SS have a greater potential for improving diverse photo-electronic applications under multiple excitation wavelengths. Meanwhile, the XRD patterns in Fig. 4(b) also reveal the crystallographic differentiation between the above two samples. The XRD result of normal Au NPs shows a series of diffraction peaks observed at about 26.7°, 33.9°, 37.8°, 51.8°, 61.8° and 65.9° that are attributed to the (110), (101), (200), (211), (310) and (301) planes (JCPD#41-1445) of SnO2 doped in FTO. Then, the other diffraction peaks at about 38.1°, 44.1°, 64.4° and 77.3° are originated from the (111), (200), (220) and (311) lattice planes (JCPD#04-0784) of plasmonic metallic Au nanocrystals. As for the prepared 3D Au SS (violet line in Fig. 4(b)), the relative intensities of diffraction peaks derived from FTO support are significantly suppressed by overgrowth of vertical Au configuration, as compared to the reference sample covered with normal Au NPs. Meanwhile, the contrastive analysis of XRD patterns originated from 3D Au SS further reveals that the metallic higher-index facets such as (200), (220) and (311) lattice planes can dominate the main crystallographic phases because of their relative much higher diffraction intensities than that of low-index (111) fact. Especially, the preferential alignment of the (200) orientation should be convincingly verified in the 3D Au SS with close-packed Au nano-polyhedron clusters, owing to the strongest diffraction peak intensity at about 44.4° in Fig. 4(b). Compared with the common low-index (111) fact, the extraordinary metallic high-index facts with higher surface energies have been widely used to accelerate diverse catalytic reactions via enhanced absorption of molecules [45–47], which is also expected to provide an additional contribution for boosting SERS activity in this work. Moreover, the XPS results of Au4f originated from the above two samples in Fig. 4(c) can further reveal the enhanced synergistic coupling effect of 3D Au SS in comparison with the reference normal Au NPs. As illustrated in Fig. 4(c), the double features of Au4f at ∼84.27 eV and 87.97 eV originated from the reference sample are attributed to the binding energies of Au4f7/2 and Au4f5/2, which are consistent with previous work [48]. Then, the XPS peaks of Au4f7/2 and Au4f5/2 derived from as-prepared 3D Au SS are detected at ∼84.79 eV and 88.49 eV in Fig. 4(c), which are obviously positive-shift of about 0.52 eV as compared to that of normal Au NPs. The XPS shift of Au4f clearly manifests the modified electronic structures of 3D hierarchical Au SS through the effective electron-transfer among these interconnected and stacked plasmonic metallic clusters. In this way, the ultrabroadband plasmonic resonance of the established 3D Au SS under visible-NIR light excitation should be highly related to the considerable metallic coupling effect of close-packed clusters.
Fig. 4. (a)-(b) Absorption spectra and XRD patterns of as-prepared 3D plasmonic Au SS with close-packed nano-polyhedron clusters on FTO support and the reference normal Au NPs naturally dried on FTO. The insets show SEM images of the mentioned two targets. (c) High-resolution XPS spectra of Au4f originated from the above two samples.
To evaluate the multi-band SERS performances, the Raman tests of 10−9M CV probe molecules absorbed on the established 3D hierarchical Au SS and the reference normal Au NPs were separately carried out under three laser excitation wavelengths at 532, 633 and 785 nm, respectively, as displayed in Fig. 5(a)-5(c). The inset in Fig. 5(a) shows the optical microscope image of 3D Au SS-based round plate with a diameter of ∼60 μm. As for the normal Au NPs, the Raman spectral lines of CV molecules excited by 532 and 785 nm laser wavelengths are too weak to be effectively detected in Fig. 5(a) and 5(c), implying that this is no obvious SERS activity under these two excitation wavelengths. On the other hand, the dominating characteristic bands of CV molecules absorbed on this SERS substrate are clearly identified by selecting 633 nm laser excitation in Fig. 5(b). It is mainly composed of some well-defined Raman spectral lines of bending vibration of ring C-H bend at ∼807 cm-1, 917 cm-1, and 1176 cm-1, as well as N-phenyl stretching vibration and in-plane C-C stretching vibration of the ring at ∼ 1387 cm-1 and 1620 cm-1 [6]. So, the normal Au NPs with single-plasmonic resonance at ∼ 620 nm can only work for a restricted excitation wavelength at 633 nm that is close to the optical resonant condition of this sample, suffering from the limitation of narrowband SERS operation. Based on the resultant 3D Au SS in Fig. 5(a)-5(c), it can be found that the more noticeable SERS signals of CV molecules are explicitly detected by adopting 532, 633 and 785 nm multiple excitation wavelengths in the range of broad visible-NIR region, which are far better than that of the reference normal Au NPs. Even at the 633 nm excitation wavelength that matches with LSPR position of normal Au NPs, the SERS activity of the obtained 3D Au SS in Fig. 5(b) is also much higher than the reference sample. As displayed in Fig. 5(b), the Raman peak intensity of CV molecules at 1620 cm-1 is measured ∼ 3.4 × 104 a.u in the presence of 3D Au SS, which is already about 6.3 times higher than that of normal Au NPs. In regard to the generated 3D Au SS, some additional information of probe molecules can be also obtained from the Raman spectra under multiple excitation wavelengths, in comparison with that derived from a single excitation condition. For example, the characteristic band of CV molecules at ∼760 cm-1 (ring C-H bend) can be obviously detected by 633 nm and 785 nm irradiations, whereas a very weak signal is formed by 532 nm excitation. Besides the well-described plasmonic “hotspots” for improving SERS performance [2,19,26,30], the remarkable enhanced SERS activity of 3D structured plasmonic Au SS should be also highly related to the unique hierarchical close-packed nano-polyhedron clusters with interconnected and coupled building blocks at both horizontal and vertical directions (Fig. 2 and Fig. 3). It can provide strong synergetic coupling effect for boosting photo-induced charge transfer (PICT) efficiency under broadband light excitation. It has been well-known that the highly-efficient PICT of SERS plasmonic substrates is extremely beneficial to promote both LSPR-induced intense electromagnetic field and charge-transfer triggered chemical enhancement at metal-molecule resonance interface [3,49–51], which is also coincident with our case. Generally, the PICT efficiency can be simply evaluated by PL emission spectroscopy that is mainly originated from plasmonic resonance excitation [52–54]. So, the higher PL intensity of plasmonic metallic target usually implies the more photoexcited electrons that can participate into the transmission of energetic carriers. In order to evaluate PICT efficiency, the PL measurements of 3D Au SS and the normal Au NPs were carried out under multiple excitation wavelengths at 532, 633 and 785 nm, respectively, as illustrated in Fig. 5(d)-5(f). As expected, each PL intensity of 3D Au SS excited by either 532, 633 or 785 nm in Fig. 5(d)-5(f) is significantly higher than that of the reference Au NPs, although the enhancement degree from the two samples would be changed under different excitation wavelengths. For instance, the PL intensity of 3D Au SS is measured about 1.5 × 104 a.u under 785 nm excitation in Fig. 5(f), which is already ∼ 4.3 times higher than that of the normal Au NPs. Meanwhile, there are two PL peaks under 633nm excitation in Fig. 5(e) due to large bandwidth of 3D Au SS at visible region, which is also different from that of the normal Au NPs. The PL results in Fig. 5(d)-5(f) convincingly verify that the adopted 3D Au SS have a greater potential for boosting the PICT efficiency under multiple excitation wavelengths, giving rise to the enhanced multi-band SERS activities as compared to the normal Au NPs. On the other hand, the contrastive analysis of SERS performance on the same 3D Au SS under different laser wavelengths in Fig. 5(a)-5(c) was also carried out in this work. It reveals that the NIR excitation at 785 nm can offer a remarkable higher SERS activity in comparison with that of the visible 532 and 633 nm laser wavelengths. For example, the Raman peak intensity of CV molecules at 1176 cm-1 is measured about 8.7 × 104 a.u under 785 nm excitation, while about ∼2.6 × 104 a.u via 633 nm and 0.4 × 104 a.u via 532 nm, supporting that the obtained 3D Au SS should be particularly more suitable for boosting NIR-SERS activity. Therefore, the 785 nm excited SERS of this substrate was subsequent selected in the following tests.
Fig. 5. (a)-(c) Raman spectra of 10−9 M CV probe molecules absorbed on the established 3D Au SS and the reference normal Au NPs under laser excitation wavelengths of 532, 633 and 785 nm at the same power level of 0.05 mW, respectively. The inset in (a) shows the optical microscope image of the 3D Au SS-based round plate with a diameter of ∼60 μm. (d)-(f) The corresponding PL spectra of the above two substrates excited by three laser excitation wavelengths, respectively.
Based on the obtained 3D Au SS with ultrahigh NIR-SERS activity, the ultralow detection capability under 785 nm laser excitation was then illustrated by using CV probe molecules with different concentrations (10−7∼10−16M), as shown in Fig. 6(a)-6(b). Overall, the Raman peak intensities of CV molecules gradually decrease with the continuous decrease of molecular concentration in the broad range of 10−7∼10−16M. Surprisingly, the main characteristic bands of CV molecules absorbed on the excellent 3D Au SS are also distinguishable even with the molecular concentration down to as low as 10−16M in Fig. 6(a). The limited of detection (LOD) of the adopted 3D Au SS is already achieved at 0.1 femtomole (fM) level in this work, which is better than many previous SERS substrates, such as bimetallic Au/Ag self-assembled monolayers (SAMs) [6], hexagonal boron nitride (h-BN) decorated Au NPs [36], ZnO@Au nanorods [37], cross-nanoporous Au on polyethylene terephthalate (PET) [55], lotus leaf templated-GO wrapped Ag micro-islands [56], and also comparable with the Au NPs/h-BN grafted into 3D porous bacterial nanocelluloses (BNC) [57] (inset in Fig. 6(b)). Moreover, the detailed tendency of SERS signals with decreasing molecular concentration from 10−7M to 10−16M in Fig. 6(a) was then illustrated by plotting the variation of Raman peak intensity at 1387 cm-1 versus logarithmic concentrations of probe molecules. As displayed in Fig. 6(b), there are two linear response models that can fit well with experimental data at relatively high and low molecular concentration regions, respectively. As for the molecular concentrations in the relative high range of 10−7∼10−12M, the absolute value of slope coefficient of linear fitting equation in Fig. 6(b) is approximately calculated ∼ 1.4 × 10−4, while about 1.6 × 10−3 at the low concentrations of 10−12∼10−16M. It implies that the rapid-reduction and then slow-decrease behavior of SERS signals would be formed by decreasing molecular concentration from 10−7 M to 10−16M in this work. At the ultralow molecular condition, the relative slow-decrease of SERS signals should be mainly attributed to the fact that the probe molecules can be effectively concentrated in 3D Au SS with porous interspaces among interconnected nano-polyhedrons. The concentrated molecules in SERS substrate should be very suitable for improving ultralow detection capacity, which has been confirmed in many previous works [58,59] and also coincident with our case. Moreover, in order to further evaluate SERS activity of the generated 3D Au SS, the enhancement factor (EF) was calculated as EF = (ISERS/NSERS)/(IRaman/NRaman) [19,26,30,59,60], where ISERS and IRaman denote the Raman peak intensity originated SERS substrate and standard substrate without any nanostructures, respectively. Meanwhile, NSERS and NRaman represent the corresponding number of probe molecules in the incident laser spot on the SERS substrate and standard substrate. Then, it can be determined by N = (NA×C × V×Slas)/SSol, where NA is Avogadro constant, C and V indicate the concentration and volume of the added solution, while Slas and SSol are the laser area and solution area. In this experiment, the Raman peak intensity (∼1.2 × 104 a.u in Fig. 6(b)) of 10−12M CV molecules at 1387 cm-1 in the presence of 3D Au SS was selected to calculate EF. The standard Raman spectra of 10−4M CV molecules were recorded on the bare FTO glass without any plasmonic nanomaterials, and the peak intensity at 1387 cm-1was measured about ∼2100 a.u. In the mentioned experiments, 10 μL of CV solution was uniformly dispersed across the entire surface of each sample. The corresponding area of the solution on the 3D Au SS-based SERS substrate is similar to that formed on the FTO substrate due to the same size of each sample selected in the above experiments. So, the analytical EF value of the prepared 3D Au SS is roughly calculated about ∼5.7 × 108 in this work. On the other hand, the spatial SERS uniformity of the resultant 3D Au SS was then illustrated by monitoring the Raman spectra of CV molecules at different random points on this substrate, as presented in Fig. 6(c). It can be found that the dominating characteristic bands of probe molecules with similar Raman peak intensities can be well repeated at different spatial points on the planar surface. Moreover, the variations of some representative Raman peak intensities at 807, 917, 1176 and 1620 cm-1 versus random spatial points are further displayed in Fig. 6(d). The corresponding relative standard deviation (RSD) values of these four typical Raman peak intensities are then separately calculated about 7.45%, 6.16%, 5.15% and 7.54%, supporting the acceptable random point-to-point SERS uniformity. As for the adopted 3D Au SS on FTO support with area of 60 μm×60 μm, the spatial plane-scanning SERS tests were also performed on this square region. Then, the insets in Fig. 6(d) show the corresponding optical microscope image of the tested region and the mapping diagram of Raman peak intensity at 1176 cm-1. The experimental results manifest that the homogeneous distribution of SERS signals on the 3D Au SS can be clearly distinguished from background region, supporting the excellent spatial uniformity of the established SERS substrate.
Fig. 6. (a) The Raman spectra of CV probe molecules with different concentrations (10−7∼10−16 M) absorbed on the resultant 3D Au SS under 785 nm excitation. (b) The relationship between Raman peak intensity at 1387 cm-1 and logarithmic concentrations of CV molecules. The inset shows the comparison of LOD values. (c) The Raman spectra of 10−9 M CV molecules at different 22 random points on the SERS substrate. (d) Variations of the Raman peak intensities at 807, 917, 1176 and 1620 cm-1 versus random spatial points. The insets show the optical microscope image of the adopted 3D Au SS on FTO support (60 μm×60 μm) and the corresponding spatial mapping diagram of Raman peak intensity at 1176 cm-1 performed on this region.
Finally, the established 3D Au SS served as a competitive SERS nano-substrate should be also long-term stable and reproducible system that can be robust enough to withstand the practical operation in widespread application. To verify the SERS stability, the time-dependence Raman tests of 10−12M CV probe molecules absorbed on the same 3D Au SS were carefully recorded after being exposed under ambient condition for a long-term duration of 32 days. As displayed in Fig. 7(a), the SERS signals with similar Raman peak intensities in comparison with the fresh ones can be well maintained after keeping the same target for more than one month. Meanwhile, the corresponding temporal variation of Raman peak intensity at 1176 cm-1 versus different delay times is further illustrated in Fig. 7(b). In detail, it can be found that the Raman peak intensity is only dropped from about 2.0 × 104 a.u at initial state to ∼ 1.93 × 104 a.u after more than one month. The slight drop of SERS signals with negligible ∼ 3.5% lower than the initial one can explicitly confirm the excellent long-term SERS stability of the resultant 3D Au SS. Moreover, we also carried out the substrate-to-substrate Raman measurements of CV molecules with the same concentration of 10−12M separately absorbed on different 20 batches, in order to demonstrate the reproducibility of 3D Au SS-based SERS substrates. As shown in Fig. 7(c), the repeated experimental results convincingly verify that the SERS signals of probe molecules performed on different batches are very similar to each other. Then, the corresponding variations of Raman peak intensity at 1176 cm-1 versus different batches are subsequently illustrated in Fig. 7(d). In this way, the RSD value of these batch-to-batch Raman signal variation is calculated about 4.9%, supporting the reliable SERS reproducibility of the obtained 3D Au SS. Overall, the stable 3D plasmonic metallic SS generated by laser light-induced coalescence of close-packed Au clusters possess excellent multiple SERS performances toward widespread molecular analyses.
Fig. 7. (a) The time-dependence Raman spectra of 10−12 M CV molecules absorbed on the same 3D Au SS after being exposed under ambient conditions for a long-term duration of 32 days. (b) The corresponding temporal variation of Raman peak intensity at 1176 cm-1 versus different delay times. (c) the batch-to-batch SERS tests of CV molecules originated from different 20 substrates. (d) the corresponding variations of Raman peak intensity at 1176 cm-1 versus different samples.
4. Conclusions
In summary, an exceptional 3D plasmonic Au SS with close-packed metallic nano-polyhedron clusters can be successfully and simplistically fabricated by CW laser light irradiation of FTO support in HAuCl4 solution mixed with PVP stabilizing and capping agent. The coalesced Au hierarchical nano-polyhedron clusters with strong synergistic coupling effect exhibit ultrabroadband plasmonic response ranging from visible light to NIR region (400∼2000 nm), which is much better than the reference normal Au NPs with single resonance peak at visible region. As excepted, the obtained 3D Au SS provide remarkable enhanced multi-band SERS activities under 532, 633 and 785 nm excitation wavelengths, as compared to the reference normal Au NPs. Based on 785 nm excitation of the resultant 3D Au SS, the ultralow detection of CV probe molecules can be down to as low as 10−16M, which is better than many complicated Au or Ag nanocomposites in previous works. Moreover, the excellent SERS spatial uniformity, long-term stability, and reproducibility were also convincingly verified in this work. Therefore, the laser light-induced coalescence of unique 3D plasmonic SS should be expected to provide a feasible alternative avenue for developing robust multi-band SERS in widespread applications.
Funding
National Natural Science Foundation of China (11905115); Shandong Jianzhu University XNBS Foundation (1608).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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