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Abstract
The fascinating nanocomposites of semiconductors coupled with noble metals have a promising potential to develop a low-cost substrate for surface-enhanced Raman scattering (SERS) applications. Herein, the controlled construction of Ag nano-islands on ZnO nanosheets (Ag@ZnO nanocomposites) was successfully achieved by a green and effective strategy based on ultraviolet light induced-photochemical reaction. It was found that the Ag content in Ag@ZnO nanocomposites linearly increases to 4.62% by simply increasing the irradiation time (0~10 min). More importantly, increasing the Ag content (0~4.62%) in the nanocomposites results in enhanced SERS activities with an enhancement factor up to 107 and a detection limit as low as 10−9 M. Compared with the complete noble metal substrate, the novel Ag@ZnO nanocomposites (<5% Ag compositions) were cost-effective and possessed high biocompatible properties; which can be established as an advanced inexpensive substrate for ultrasensitive SERS analysis, particularly for food safety and biomedical applications.
© 2017 Optical Society of America
1. Introduction
As a powerful spectroscopy analysis technique, surface-enhanced Raman scattering (SERS) has gained considerable attentions due to its wide applications in analytical, biophysical, biomedical, and environmental science [1–4]. Therefore, a large number of novel SERS substrates with different morphologies and microstructures have been successfully designed and synthesized, in order to get enhanced SERS activity [5–14]. Generally, the plasmonic metallic (Ag or Au) nanomaterials with enough rough surfaces could facilely produce unprecedented intense electromagnetic fields and then amplify the SERS intensity of the probe molecules, owing to their intrinsic stronger localized surface plasmon resonances (LSPR) effect and rugged structures [4,7–13]. Nevertheless, the noble metal nano-substrates have been severely plagued by their inherent high cost and poor biocompatibility. Hence, it becomes crucial to reduce the use of noble metal in SERS substrate.
Alternatively, semiconductor-based nanostructures as inexpensive functional materials have gradually attracted noticeable attentions owing to their extremely intriguing applications in various areas such as SERS analysis [14–16], photocatalytic devices [17-18] and photovoltaics [19], etc. In particular, if a few noble Ag or Au metals could be carefully grown on the semiconductor precursors, the novel nanocomposites as promising SERS substrates will be of great significance for ultrasensitive molecular analysis, owing to the unique LSPR of the plasmonic metallic composition coupled with low-cost of the support. In past years, various semiconductor-based nanostructures have been designed such as 3D chestnut-like Ag/WO3–x nanostructures [16], Ag@ZnO core-shell hybrids [18], multifunctional Au/TiO2 core-shell nanocomposites [20], Au-ZnO hybrid nanoparticles [21] and Cu2O superstructures [22]. Unfortunately, the fabrication process of aforementioned nanocomposites is extraordinarily complicated. In a typical nanostructure-fabricated procedure, specific compounds such as polymers, and other functional organic agents served as surfactants, coupling agents and stabilizing agents should be added to control reaction rate, prevent inter-particle aggregation and improve the stability of colloidal particles. In this way, the inevitable organic agents give rise to the formation of an organic layer contamination on the surface of final nanostructures. However, the subsequent cleaning of the contamination is very difficult and costly, which is a considerable challenge for standard chemical fabrication. The obstruction between nanostructures and probe molecules can unambiguously reduce the SERS activity of the initial nanocomposites [15]. In addition, extra interferential bands of the organic impurities would also seriously affect the SERS measurements of probe molecules. Up to now, although a number of nanocomposites based on semiconductor decorated with noble metal elements have been synthesized, it has not been extensively exploited to probe and identify molecules for molecular diagnostics. Therefore, the development of a green, facile, and efficient approach to obtain a kind of inexpensive nanocomposites substrate without any contamination is an urgent issue for the SERS-related analysis application [16].
Recently, light-induced photochemical reaction without using any complicated organic additives has emerged as a reliable alternative strategy to obtain a variety of nanomaterials [23–26]. Compared to the traditional chemical methods, the unique feature is that the generation of electron−hole pairs by light irradiation will lead to controlled oxidation-reduction reactions, and result in the moderation growth of nanocomposites. Lately, we reported on the successful synthesis of highly branched Ag@Au bimetallic nanocrystals by laser light-induced photochemical reaction [26].
Herein, we further extend the novel synthetic route to fabrication of Ag@ZnO nanocomposites with controllable Ag composition. In the absence of any additive polymer reagents or complicated oxidation-reduction reactions, the distinctive advantage is that the moderate overgrowth of Ag species (0~4.62% Ag in Ag@ZnO nanocomposites) will be achieved by simply increasing the ultraviolet light irradiation time (0~10 min). Interestingly, the pure and low-cost Ag@ZnO nanocomposites (4.62% Ag) exhibit enhanced SERS performance for crystal violet (CV) molecules with an enhancement factor (EF) up to ~107 and a detection limit of 10−9 M, which is comparable to that of complete noble metal substrates. Without any residual reagents, the Ag@ZnO nanocomposites with pronounced SERS activity have a promising potential in food safety and biomedical application. Moreover, the convenient strategy by using light as a tool for sculpting pure semiconductor-metal nanocomposites can unambiguously possess high applicability in photo-catalysis, biomedical sensors, anatomical imaging, etc.
2. Experimental setup
The ZnO powers were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Compared with particle-shaped structure, as a type of representative two-dimensional nanostructure, the ZnO nanoplates with inherent larger surface area are more conducive to the effective growth of Ag species during the UV-light irradiation [27]. The overgrowth of Ag species on ZnO nanostructures was simply achieved by ultraviolet light irradiation. Briefly, a mixed solution containing 4 mg ZnO powders (99.9%), 5 mL distilled water and 1 mM/1 mL AgNO3 ethylene glycol solution was added in a glass dish by ultrasonic vibration (40 kHz). The Ag@ZnO nanocomposites were obtained via ultraviolet light irradiation at a wavelength of 365 nm and the power of 10 mW. After the irradiation of 0~10 min, the as-synthesized sediments were thoroughly washed with distilled water and centrifuged at 5000 rpm for 10 minutes in an ultracentrifuge. The obtained precipitates were carefully dropped on the copper mesh and completely dried in an oven for observation using a JEOL JEM-2100F transmission electron microscopy. Elemental mapping images were acquired by energy dispersive X-ray (EDX) spectroscopy using a JEOL-2100F electron microscope equipped with a STEM unit. Morphologies and chemical composition of the samples were characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDS). The absorption spectra were carried out via UV-Vis-IR spectrometer (UV-1800, Shimadzu). The crystallographic measurements of the products were collected by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500VHF) using Cu Kα radiation (λ = 0.15406 nm). The detailed sample composition analysis was performed on a PHI Quantera SXM with an Al Kα = 280.00 eV excitation source by X-ray photoelectron spectra (XPS). In a typical Raman spectroscopic analysis, the SERS substrate based on Ag@ZnO nanocomposites was performed by dropping 0.2 M/20 μL sediments on a clean silicon wafer. Then the substrates were dried at room temperature for 24 h. Subsequently, the as-prepared SERS substrates were totally immersed into certain concentration CV ethanol solution under lucifugal circumstance for 12 h and then dried spontaneously. All Raman spectra were obtained using a LabRAM HR 800 spectrograph with an excitation wavelength of 633 nm and output power of 50 mW at room temperature. The acquisition time used for each spectrum is 1 s. In order to ensure the test reproducibility, the Raman spectrums were recorded at different ten positions on each sample.
3. Result and discussion
Figure 1(a) shows the typical transmission electron microscopy (TEM) image of original ZnO nanosheets. The numerous ZnO nanoplates with an average diameter of 50nm are interconnected with each other, forming reticular-like structures. The ZnO nanosheets should be well crystalline according to the clear lattices fringes (inset in Fig. 1(a)). The enlarged region with a d-spacing of 0.280nm is indexed to the (100) ZnO structure. After 365 nm ultraviolet light irradiation for 10 min, the morphology of the product is depicted in Fig. 1(b). It can be clearly seen from the image that some nanoparticles with islands-like structures were formed on the original ZnO after the photochemical reaction, which is indicated by red arrows in Fig. 1(b). The enlarged TEM image in Fig. 1(c) demonstrates two different lattice fringes with d-spacing of 0.280 nm and 0.236 nm, which are separately indexed to the ZnO (100) and Ag (111), respectively. Moreover, the elemental mapping images in Fig. 1(d) also indicate the clear distributions of oxygen, zinc, and silver elementals in Ag@ZnO nanocomposites marked by the blue, green and red colours, respectively. The relative ratio of Zn and O is about 1.02:0.98, which is fit to stoichiometry ZnO chemical composition. The original ZnO nanostructures were well maintained after ultraviolet light irradiation. The relative ratio of the Ag species in the final nanocomposites is about 4.62%. Their spatial distribution of the three elements further confirms the isolated Ag island-like structure formed in Ag@ZnO nanocomposites. The formation of island-shaped Ag structures is attributed to the fact of inevitable large mismatch of crystal lattices between noble metal and semiconductor.
Fig. 1 The typical TEM images of (a) original ZnO and (b) Ag@ZnO nanocomposites. (c) The HRTEM image of Ag@ZnO nanocomposites. (d) TEM image of enlarged area. The bottom pictures in (d) show the corresponding element mapping.
In order to further verify the structure of the Ag@ZnO nanocomposites, scanning electron microscopy (SEM) images of original ZnO and Ag@ZnO nanocomposites are illustrated in Fig. 2. As shown in Fig. 2(a), the original ZnO nanosheets were indeed the sheet-like structure, which are composed of O and Zn elements with relative atomic ratio of approximate 50.13:49.87. After irradiation for 10 minutes, numerous irregular quasi-sphere nanoparticles with mean size of ~80 nm can be obviously found in the products, which are indicated by yellow arrows in Fig. 2(b). The EDS result (inset in Fig. 2(b)) indicates the 4.62% Ag species formed in the Ag@ZnO nanocomposites, which is consistent with the chemical composition in Fig. 1. On the other hand, the crystallographic investigations of the nanostructures before and after irradiation were carried out by X-ray diffraction (XRD), as shown in Fig. 3. In addition to a series of diffraction peaks from the initial ZnO nanosheets (Zincite, JCPDS, no.36-1451), the four characteristic diffraction peaks originated from Ag (111), (200), (220) and (311) planes (face-centered-cubic structures, JCPDS, no.04-0783) were clearly observed in final product, supporting the formation of Ag@ZnO nanocrystals in this paper. Moreover, the single-crystalline nature has been also well kept during the overgrowth process. Meanwhile, it can be deduced that there is no byproduct formed in the final product, since the XRD pattern of the Ag@ZnO nanocomposites only contains diffraction peaks from ZnO and Ag.
Fig. 2 SEM images of (a) original ZnO and (b) ZnO@Ag nanoparticles, respectively. The insets show the corresponding EDS results, respectively.
Fig. 3 The XRD patterns of original ZnO and Ag@ZnO nanocomposites fabricated by 10 min ultraviolet light irradiation, respectively.
In addition, the elemental valence state and surface purity of obtained Ag@ZnO nanocomposites are further illustrated by the corresponding X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4. Generally, the binding energies were calibrated by using C 1s peak at 284.8 eV to reduce the sample charge effect. The survey spectrum in Fig. 4(a) confirms that the product is only composed of O, Zn and Ag elements, which is in agreement with EDX, EDS and XRD results in this paper. The enlarged spectra shown in Figs. 4(b)-4(d) were obtained from O, Zn, and Ag species, respectively. Figure 4(b) demonstrates the peak profile of O 1s is asymmetric and can be divided into two peaks at 530.2 eV and 531.8 eV. The existence of two kinds of O species should be separately attributed to the lattice oxygen of ZnO and surface oxygen after keeping them in an oven [28]. As seen from the Fig. 4(c), the two peaks of Zn 2p located at about 1021.3 eV (Zn 2p3/2) and 1044.7 eV (Zn 2p1/2) reveal that the oxidation state of Zn2+ formed in the product. Meanwhile, the two peaks in Fig. 4(d) located at 367.3 eV and 373.2 eV are ascribed to Ag 3d5/2 and Ag 3d3/2, respectively. Interestingly, compared with those of bulk Ag (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV) [29], the XPS patterns of Ag 3d in this paper slightly shift to lower binding energies. The change of Ag binding energy mainly derives from monovalent Ag that formed in the process of electron transferring from metallic Ag to ZnO crystals [18]. The above results clearly confirm the Ag@ZnO nanocomposites with isolated Ag island-shaped structure can be successfully constructed by ultraviolet light irradiation of ZnO powers in Ag ions solution.
Fig. 4 XPS spectra of Ag@ZnO nanocomposites: (a) survey-scan spectrum, (b) O 1s, (c)Zn 2p, (d) Ag 3d.
The moderate overgrowth process of Ag species was monitored by the UV-visible absorption spectra of the Ag@ZnO nanocomposites versus different irradiation times, as shown in Fig. 5(a). The dominating characteristic band of ZnO nanosheets at 365nm is observed in the absorption spectral line (red curve). During 365 nm ultraviolet light irradiation, the absorption peak at 485nm originated from the existence of Ag species significantly increases with the irradiation time prolonging from 0 to 10 min. Finally, the product possesses two obvious absorption peaks at about 365nm and 485nm, respectively, implying the formation of Ag@ZnO nanocomposites. Compared with original ZnO solution, the Ag@ZnO nanocomposites solution obviously changed from white to khaki color. Moreover, we further reveal the variation of Ag concentration in nanocomposites versus the irradiation time. The Ag concentrations were determined by EDS results of five samples via different irradiation times (2, 4, 6, 8, 10min). As shown in Fig. 5(b), the chart clearly illustrates that the Ag content in Ag@ZnO nanocomposites linearly increases to 4.62% as the irradiation time increases to 10 min. Further increasing irradiation time (>10min) will inevitably result in much higher Ag concentration in the nanocomposites. However, it will also improve the cost of SERS substrate, having no obvious advantages over the pure noble metal condition.
Fig. 5 (a) The UV-visible absorption spectra of Ag@ZnO nanocomposites via different ultraviolet irradiation times. (b) The curve of the Ag concentration in Ag@ZnO nanocomposites versus irradiation time.
The above findings confirm that the moderate overgrowth of Ag species on semiconductor can be achieved by simply using ultraviolet light irradiation. The ultraviolet light-induced electron-hole pairs dominate the overgrowth process. The growth mechanism of the Ag@ZnO nanocomposites will be described in the following section. Under ultraviolet light irradiation, the 365nm light ensures that the incident excitation wavelength is commensurate with the absorption peak of the ZnO precursors. The corresponding photon energy can be highly absorbed by the ZnO semiconductor, giving rise to the effective charge separation on the nanoparticle surface. In this way, numerous electron-hole pairs will be formed by the effective photo-excitation of ZnO (Eq. (1)). As shown in Eq. (2-3), then the ethylene glycol and Ag ions in the colloidal solution can be separately oxidized and reduced by these separated electrons and holes. Figure 6 sketches the typical schematic of overgrowth of Ag species on ZnO precursors. Once the nucleation sites of Ag nanoparticles were formed, the subsequent overgrowth of Ag species would preferentially locate at the initial nucleation sites, resulting in the formation of island-shaped Ag structures.
The formation of these electron-hole pairs on the semiconductor surface plays a critical role in the overgrowth process, which must be excited by light irradiation. So, the subsequent oxidation-reduction reaction can be effectively controlled via the photo-excitation process. In this way, the distinctive advantage is that the corresponding moderate overgrowth allows one to manipulate well the nucleation and growth rate. On the other hand, it should be noted that the moderate overgrowth process is highly related to the light wavelength. We found that other light sources with wavelengths of 532, 633, or 808 nm, cannot provide the effective overgrowth of noble metal on the ZnO semiconductor. The main reason is that the incident excitation wavelengths are not consistent with the absorption peak of ZnO semiconductor, resulting in insufficient electron-hole pairs formed on the surface. Meanwhile, if a higher-power (>100mW) light beam was adopted in this experiment, the original ZnO structures will be severely destroyed by the exceed photon energy. Therefore, the appropriate ultraviolet light irradiation should be recommended for the moderate overgrowth of Ag species on ZnO nanostructures.
As shown in Fig. 7, the excellent SERS performances of the as-synthesized Ag@ZnO nanocomposites were illustrated by using crystal violet (CV) (molecular structure in inset) as the probe molecules. As suggested in the literature [30], CV molecules show multiple Raman bands in the wavenumber range of 600-1800 cm−1 under laser excitation. Figure 7(a) shows the variation of the SERS intensities of CV molecules with the concentration of 10−5~10−9 M on the as-prepared Ag@ZnO nanocomposites (4.62% Ag). The dominating characteristic bands of the CV molecules in the range of 600~1700 cm−1 are clearly detected in SERS spectra. The normal Raman spectrum was originated from 1 M CV molecules on silicon substrate without any nanomaterials. The corresponding spectral lines at 1175, 1373, and 1617 cm−1 are about 108.2, 91.4, and 78.7a.u, respectively. When the concentration of CV molecules adsorbed on the Ag@ZnO nanocomposites substrate decreased to 10−5 M, the intensities of three main vibration peaks at 1175, 1373 and 1617 cm−1 can be dramatically improved to 15170, 13695 and 13637 a.u, respectively, which is about 100 times higher than that of the normal Raman spectrum. Moreover, as for Ag@ZnO nanocomposites substrate, the Raman signals (913, 1175, 1373 and 1617cm−1) are also clearly distinguishable even the concentration of CV solution was decreased to 10−9 M. Figure 7(b) explicitly shows the relationships between SERS intensities (1175, 1373 and 1617 cm−1) and logarithm of concentrations match relatively well with the GuassAmp nonlinear curve fitting model [14]. The enhancement factor (EF) of CV molecules adsorbed on the Ag@ZnO nanocomposites has been estimated quantitatively, as shown in Eq. (4) below [9,10,31–34]:
Where ISERS and INOR are the signal intensities of SERS and normal Raman spectrum of the same vibration peak for the CV molecule, respectively. And NSERS and NNOR represent the corresponding number of CV molecules exposed to the laser spot focused area. Since the same laser parameters were adopted in the SERS measurements, NSERS and NNOR can be approximately determined by the concentration of CV. Reasonable accuracy obtained from the above standard method has been verified in previous reports [31–35]. Therefore, the EFs of SERS for CV molecules at 1175, 1373 and 1617 cm−1 were estimated to be about 1.4 × 107, 1.5 × 107, and 1.7 × 107, respectively. In this way, the EF of the obtained Ag@ZnO (4.62% Ag) reaches up to ~107. The low-cost Ag@ZnO nanocomposites exhibit excellent SERS activates with low detection limit of 10−9 and noble-metal-comparable EF of ~107, which is better than many previous reports, especially those of pure semiconductor nanostructures, noble metal/semiconductor hybrid nanomaterials and monometallic nanoparticles [5,14,35–37]. The Ag composition plays an important role in the SERS analysis. To verify this assumption, based on six different Ag@ZnO nanocomposites substrates obtained by ultraviolet light irradiation for different time (0~10min), the variations of SERS spectra (10−5 M CV molecules) as function of Ag concentration are evaluated in Fig. 7(c). Without any Ag composition, the Raman signals of the CV molecules on the initial ZnO nanosheets were too weak to be detected in the wavenumber range of 600-1800 cm−1. Increasing Ag concentration in the nanocomposites via light irradiation results in much higher SERS activities. As for 4.62% Ag content, the Raman signals of CV molecules can be remarkably enhanced by a factor up to 100 orders of magnitude. Moreover, the quantitative results of the corresponding variations of three CV molecules peaks at 1175, 1373, and 1617cm−1 as a function of Ag concentrations are displayed in Fig. 7(d). When the Ag concentration gradually increases from 0 to 4.62%, it can be found that the SERS intensities of the CV molecules linearly increase from about dozens to tens of thousands. The enhanced SERS activity is highly related to the higher Ag content in nanocomposites. The numerous isolated Ag nano-islands structures provide much more active sites via strong localized surface plasmon resonance (LSPR) by laser excitation during SERS measurement, leading to significant enhancement of the Raman signals. On the other hand, we found that the Ag@ZnO with much more Ag composition (>5%) will slightly increase the SERS intensity since the formation of large-sized Ag islands by further increasing irradiation time (>10min). Meanwhile, it will also significantly raise the cost of SERS substrate. Therefore, the Ag@ZnO nanocomposites with Ag concertation (4.62%) can be illustrated as an advanced low-cost SERS substrate in this paper. More importantly, without any toxic reagents in ultraviolet light-induced fabrication, the excellent SERS activities enable the pure inexpensive Ag@ZnO nanocomposites to have promising applicability in practical biomedical molecular diagnostics.Fig. 7 (a) SERS spectra of different concentrations of CV molecules adsorbed on Ag@ZnO nanocomposites of 4.62% Ag and normal Raman spectrum of CV, respectively. Inset shows the CV molecular structure. (b) GuassAmp nonlinear curve fitting between CV 1175 cm−1, 1373 cm−1, 1617 cm−1 peak intensities and logarithm of concentrations, respectively. (c) SERS spectra via different Ag contents of Ag@ZnO nanoparticles in 10−5 M CV. (d) Linear curve fitting between CV 1175 cm−1, 1373 cm−1, 1617 cm−1 peak intensities and Ag concentration, respectively.
4. Conclusions
In summary, we have developed a green and effective strategy for convenient synthesis Ag@ZnO nanocomposites by simply using ultraviolet light irradiation of ZnO powders in Ag ions solution. The unique overgrowth of Ag species is highly related to the formation of electron-hole pairs on the precursors by photo-excitation process. The novel Ag@ZnO nanocomposites with trace amount of Ag (4.62%) have been illustrated as an advanced low-cost substrate for the ultrasensitive SERS analysis. In detail, an EF of the CV molecules on the Ag@ZnO nanocopistes reaches up to ~107 and a detection limit is decreased to ~10−9 M, approaching the requirement (~nM) for single molecule detection. More importantly, without using any complicated reductants or other functional organic additives, the pure Ag@ZnO nanocomposites with excellent SERS activities are expected to have high applicability for molecular diagnostics, especially in food safety and biomedical fields.
Funding
Natural Science Foundation of China (No.11575102 and 11105085); the Fundamental Research Funds of Shandong University (No.2015JC007); Technological Development Program in Shandong Province Education Department (Grant No. J14LJ03).
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