Waste Fiber Powder Functionalized with Silver Nanoprism for Enhanced Raman Scattering Analysis
© The Author(s). 2017
Received: 10 March 2017
Accepted: 1 May 2017
Published: 8 May 2017
Biomass disks based on fine powder produced from disposed wool fibers were prepared for surface-enhanced Raman scattering (SERS). The wool powders (WPs) were modified by silver nanoprisms via an assembly method and then pressed into disks using a hydraulic laboratory pellet press. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the WPs and disks before and after treatment with silver nanoparticles (AgNPs). The WPs retained porous structures after treatment with AgNPs. The silver nanoprisms on WPs were observed clearly and the localized surface plasmon resonance (LSPR) properties of silver nanoprisms led to blue color of wool powder (WP). The obtained WP disks with AgNPs were confirmed to enhance greatly the Raman signal of thiram. The SERS disks are low-cost and convenient to use, with high sensitivity. The characteristic SERS bands of 10−8 M thiram can be identified from WP disks containing silver nanoparticles.
The noble metal nanoparticles as common plasmonic materials possess a unique optical feature, i.e., localized surface plasmon resonance (LSPR) arising from interaction between electrons around nanoparticles and incident light [1–3]. The LSPR properties of noble metal nanoparticles are associated with their size, shape and surrounding environment [4, 5]. Among plasmonic materials, silver nanoparticles (AgNPs) have attracted extensive attentions, due to facile control synthesis and tenability of plasmonic features. Based on LSPR, AgNPs have been applied in many research fields such as surface enhanced spectroscopy [6, 7], biological sensing , and optoelectronic devices. Surface-enhanced Raman scattering (SERS) has become one of the most important modern analytical techniques as it can provide abundant molecular information, with high sensitivity and non-destructive detection [10–13]. SERS has been demonstrated to be a reliable platform for trace analysis of specific molecules, including food toxins , environmental pollutants , and bioanalysis . The enormous enhancement of Raman signal is originated predominantly from the electromagnetic amplification on noble metal nanostructures when the analytes are adsorbed on plasmonic metal nanoparticles [12, 17, 18]. Fabricating high SERS-active substrates with plasmonic nanoparticles is significant for development of Raman spectroscopy in practical applications.
A number of strategies have been attempted to prepare different types of SERS substrates including electrochemical or lithographical patterns, colloidal metal nanoparticle aggregates/assemblies and individual metal nanoparticles [19–23]. However, fabrication of eco-friendly and low-cost SERS substrates is still desirable for further application of SERS. Some SERS substrates have been developed through combining noble metal nanoparticles with biomaterials including cellulose and silk. Zhang et al. coated silver nanolayers on papers using physical vapor deposition (PVD) to obtain SERS test strips . Moreover, SERS substrates consisting of paper and gold nanoparticles (AuNPs) were developed by Ngo et al. . The papers with AuNPs were also applied for bio-diagnosis . Besides, Gong et al. treated cotton swabs with AgNPs to obtain SERS substrates . In addition to cellulose materials, protein materials were used to develop biomass SERS substrates. Silk film embedded with AuNPs was prepared via spin-coating mixture solution of trifluoroacetic acid (TFA) and AuNP–silk nanocomposite . Subsequently, the composite film was used as substrate for SERS.
Wool as a natural protein fiber has been widely used in the textile industry because of its unique properties. A large amount of fiber waste is also produced during fiber and textile processing . Converting fibers into microscaled ultra-fine particles is an alternate pathway to utilize disposed fibers. Milling has been applied to realize preparation of microstructural particles from fibers, avoiding long and costly chemical routes and use of harmful reagents . Wool powders (WPs) from milling retain most of microstructure and properties of wool fiber. It has been demonstrated that converting fibers into particles increases the material surface area, resulting in significant improvements in their reactivity and absorbency, which have promoted the applications of fibrous materials in both textile and non-textile fields [30–32]. For example, WPs could be used as a sorbent to remove metal ions from solution. It was suggested that WPs have potential for application in separation and recovery of metal ions from industrial effluents and environmental waterways [31, 32]. WP as a reducing agent in-situ synthesized AuNPs and the complex of WP and AuNPs showed remarkable catalytic activity to accelerate the reduction reaction of 4-nitrophenol by sodium borohydride (NaBH4) . Biomass fibers including wool, silk and cotton have been modified with nanoparticles to exhibit different functions such as antibacterial, UV-blocking and flame retarding features. In our previous work, anisotropic AgNPs were assembled on wool fibers to impart different colors and antibacterial properties to wool [34, 35]. The assembly of AgNPs on wool fibers was suggested to be due to the electrostatic attraction between the oppositely charged nanoparticles and fiber surface under acidic condition . Compared with wool fibers, WPs have a porous structure, a large surface area and high reactivity , which would enhance the adsorption ability of WPs to nanoparticles. It is significant to explore the combination of WPs with functional nanoparticles and potential applications of such a combination.
Herein, an assemble method based on electrostatic interaction was developed to fabricate a novel SERS substrate from wool powder (WP), a microstructural biomass material. silver nanoprisms (AgNPrs) combined with WPs via an assembly process in aqueous solution. Circular disks consisted of plasmonic material and biomass micron particles were obtained through pressing AgNPr treated WPs. The structures and components of modified WPs were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). UV-vis reflection absorption spectroscopy was also employed to investigate the optical properties of disklike WP samples. Significantly, the obtained WP disks with AgNPrs were used to adsorb analyte (thiram) in ethanol and enhance of Raman signals of analytes.
AgNO3 (>99%), trisodium citrate (≥99.0%) and sodium borohydride (>98%) were purchased from Sigma-Aldrich. Acetic acid (≥99.7%) and thiram were obtained from Sinopharm Chemical Regent Co., Ltd.. All chemicals were of analytical grade and used as received. WPs were prepared through ball milling wool fibers according to the procedure in an early report .
Extinction spectra of AgNP solutions were recorded using a Varian Cary 3E UV/vis spectrophotometer. The UV-vis diffuse reflection absorption spectra of samples were recorded by a Varian Cary 5000 UV-VIS-NIR spectrophotometer with a diffuse reflectance accessory (DRA-2500). SEM measurements were performed with a Supra 55 VP field emission SEM. XPS measurements were carried out on a Kratos XSAM800 XPS system with Kα source and a charge neutralizer. XRD patterns were collected using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation.
Photoinduced Synthesis of AgNPrs
AgNPrs were synthesized via a photoinduced growth process as described previously [36, 37]. First, silver seeds were prepared by dropwise addition of NaBH4 solution (8.0 mM, 1.0 mL) to an aqueous solution of AgNO3 (0.1 mM, 100 mL) in the presence of trisodium citrate (1.0 mM) under vigorous stirring. The yellow silver seed solutions were then placed under a sodium lamp (NAV-T 70 model from Osram China Lighting Co., Ltd.). Eventually, blue AgNPr solutions were obtained through conversion of the yellow silver seed solution during the irradiation of sodium lamp.
Fabrication of SERS Disks
Firstly, the pH value of the as-synthesized AgNP solution was adjusted to 4 using acetic acid. Subsequently, 0.15 g of WP was added to 750 mL of AgNP solution with pH = 4 under stirring. The weight ratio of AgNPr solution to WP was 5000. Secondly, the AgNPr solution with WPs was placed in a water bath and shaken for 1 h at 50 °C. The solution was placed at room temperature for 12 h to precipitate the blue WPs. And then the supernatant was poured out. The precipitate containing WPs was centrifuged at 6000 rpm for 6 min followed by 3 min of centrifugation at 10,000 rpm (Eppendorf, Centrifuge 5430). Finally, the blue WPs were dried using an Ecospin 3180C speed vacuum concentrator (BioTron Inc.). The dried blue WPs were ground to fine particles. Subsequently, 0.0130 g of WP was put in the pellet die and pressed at 28 MPa with a manual hydraulic press equipment (YP-2, Shanghai Shanyue Science instrument Co., Ltd.). Pure and treated WPs were pressed to circular disks through the compression process.
SERS Measurement of Thiram
The as-prepared WP disks without and with AgNPrs were immersed into 10 mL of thiram ethanol solution with different concentrations (10−8 M ~ 10−4 M) for 12 h. After that, the disks were taken out from solution and dried under ambient condition. SERS analysis was performed on a Renishaw inVia Raman microscope system (Renishaw plc, Wotton-under-Edge, UK). A 50×/N.A. 0.75 objective and a 785-nm near-IR diode laser excitation source (500 mW, 0.5%) were used in all measurements. The spectra within a Raman shift window between 200 and 1800 cm−1 were recorded using a mounted CCD camera with integration time of 10 s by single scan.
Results and Discussion
Assembly of Silver Nanoprisms on Wool Powders
Characterization and Analysis of SERS Disks
Figure 5e, f displays the surface SEM images of WP-Ag disk. The surface of disk was found to be smooth (Fig. 5e), due to the high pressure used to prepare the disks. Whereas, the porous structures can still be seen on the disks (Fig. 5f), which implies that the disks were porous with high reactivity. AgNPrs were observed clearly on WPs in disks, suggesting that the pressing process did not bring about the morphological variation of AgNPrs on WPs. The assembly of AgNPs led to the blue color of wool powder disks, which is consistent with conclusion from spectral characterization (Fig. 3d).
SERS Activity of WP-Ag Disks
Thiram (bis-(dimethyldithiocarbamoyl) disulfide) is a pesticide, widely used to protect crops from downy mildew, blight, anthracnose and cereal smut . The thiram residues on the surface of foods not only pollute the environment, but also are irritant for the eyes, skin and respiratory tract [49, 50]. It is necessary to evaluate the risk of thiram residues through detection of trace quantities of thiram. Some methods have been used for the determination of thiram, such as chromatography , enzyme linked immunosorbent assay  and chemiluminescence analysis . SERS is one effective method to determine the trace residue of thiram [48, 54, 55, 56]. LSPR properties of AgNPs influence remarkably the SERS activity, which has be widely investigated by researchers [40, 41]. The higher SERS enhancement would be obtained when the LSPR band of SERS substrate overlaps more with the wavelength of excitation laser during Raman testing. In the present study, the obtained SERS disk (WP-Ag disk) displays a broad LSPR band at a long wavelength (more than 730 nm as shown in Fig. 3d), due to assembly of silver nanoprisms. Therefore, 785-nm laser was chosen as excitation light source for SERS test in this work.
Detail of assignment of SERS spectra of thiram
δ (CH3NC), ν (C = S)
ν (CH3N), ν (C = S)
ρ (CH3), ν (CN)
δ (CH3), ν (CN)
ρ (CH3), ν (CN)
A novel surface-enhanced Raman scattering (SERS) substrate was fabricated through assembly of silver nanoprisms (AgNPrs) on wool powders (WPs) followed by hydraulic pellet press treatment. XRD and XPS demonstrated that AgNPrs have been bound to WPs. It was confirmed that the assembly process did not change visibly the morphologies of WPs and AgNPrs. Moreover, the WPs have great adsorption ability for AgNPrs. The properties of wool powder were retained in the disks with AgNPs (WP-Ag disks), which favors adsorption of analytes. The LSPR features of AgNPrs were transferred to wool powder after assembly process. The WP-Ag disks show strong enhancement activity for Raman signal of thiram. With the inherent ability to separate and concentrate AgNPrs and analytes, WPs are not only porous supports but also integral analytical platforms containing pretreatment and SERS detection, which may have potential applications in field studies and point-of-care testing combining a portable Raman spectrometer. The complex of microstructural particles from natural fibers and functional nanoparticles would expand the application scope of biomass materials.
This research was supported by the National Natural Science Foundation of China (No. 51403162, 51273153, 21003034) and the Educational Commission of Hubei Province of China (No. T201101, Q20131002). We also acknowledge support from the MoE Innovation Team Project in Biological Fibers Advanced Textile Processing and Clean Production (No. IRT13086).
BT, JZ, and XW conceived and designed the experiments; BT and TZ performed the preparation and characterization of the samples; BT and JL analyzed the data; BT wrote the paper. JZ, YY, and XW revised the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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