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Discrimination of nosiheptide sources with plasmonic filters Delong Wang, Haibin Ni, Zhongqiang Wang, Bing Liu, Hong-Yuan Chen, Zhongze Gu, and Xiang-Wei Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01335 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017
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Discrimination of nosiheptide sources with plasmonic fil filters Delong Wang, †, ‡ Haibin Ni, †, ‡ Zhongqiang Wang, $ Bing Liu, †, ‡ Hongyuan Chen, § Zhongze Gu, †, ‡ and Xiangwei Zhao *, †, ‡ †State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China ‡Suzhou key Laboratory of Environment and Biosafety, Suzhou Research Institute of Southeast University, Suzhou 215123, China $ SUNNY GROUP·SEL BIOCHEM, Paradise Software Park, Hangzhou 310012, China §State Key Laboratory of Coordination Chemistry, Department of Chemistry, Nanjing University, Nanjing 210093, China ABSTRACT: Bacteria identification plays a vital role in the field of clinical diagnosis, food industry, and environmental monitoring, which is in great demand of point of care detection methods. In this paper, in order to discriminate the source of nosiheptide product, a plasmonic filter was fabricated to filtrate, capture and identify Streptomycete spores with Surface enhanced Raman Scattering (SERS). Since the plasmonic filter was derived from self-assembled photonic crystal coated with silver, the plasmonic “hot spots” on the filter surface was distributed evenly in a fare good density and the SERS enhancement factor was 7.49 ×107. With this filter, a stain and PCR free detection was realized with only 5 µL sample solution and 5 minutes in a manner of “filtration and measure”. Comparison to traditional Gram stain method and silver-plated nylon filter membrane, the plasmonic filter showed good sensitivity and efficiency in the discrimination of nosiheptide prepared with chemical and biological methods. It is anticipated that this simple SERS detection method with plasmonic filter has promising potentials in food safety, environmental or clinical applications. KEYWORDS: photonic crystal, nosiheptide, plasmonic filters, SERS, spore detection Streptomyces actuosus from fermentation products. Hence the easiest way to discriminate nosiheptide sources is to detect Streptomyces spores. Common techniques for identification of Streptomyces spores include nucleic acid detection and staining method, which have been developed for decades. However, the former needs time-consuming procedures like DNA extraction, amplification, electrophoresis, chip hybridization or gene sequencing, which is obviously not applicable to point of care testing (POCT) in terms of time and cost. Up to now, the most widely used method is still Gram staining. Although the cost of staining is low, the staining and observation process is tedious with low sensitivity and requires large sample volumes. In recent decades, plasmonic materials or structures are more and more widely used in POCT applications 9. With plasmonics, not only label free biosensing could be realized by surface or localized surface plasmon resonance, but also SERS could be realized for quick and sensitive analysis 10-15. Especially, SERS could provide information about molecular vibrations with sensitivity down to single molecules, which can be utilized as fingerprint for sample identification or quantitation 16, 17. It has been proved to be a promising and powerful detection technique for bacteria or pathogen in the most recent reports 18-21, among which plasmonic nanoparticles were mostly employed to enhance the Raman signals of bacteria cell walls. Usually the plasmonic nanoparticles are absorbed on the bacteria cell wall by static electricity or on a substrate and then contact with bacteria dried on the substrate for SERS analysis. However,
INTRODUCTION Nosiheptide, a metabolite of Streptomyces actuosus, has been studied for more than fifty years since it was discovered in the culture broths of Streptomyces aetuosus 40037 1. It is not only an effective antibiotic for gram-positive bacteria but also a good feed additive for farm animals owing to the advantages of strong antibiotic activity, high safety, low toxicity and residue free 2-3. Recently, lots of papers have been published including the biosynthesis 4-6, chemical synthesis 7, molecular structure and configuration mechanism 8 of nosiheptide. In general, fermentation is a low-cost, mass production method of nosiheptide. However, the purity of nosiheptide by fermentation is worse than that prepared with chemical synthesis due to the existence of Streptomyces actuosus spore and other metabolites although the price of fermentation product is low. But the nosiheptide powder from these two sources are almost the same. Hence, some manufacturers fake high-purity synthesized nosiheptide with fermented product, which discounts the effect of nosiheptide on growth promotion and disease prevention. The problem becomes more and more serious since the global usage of nosiheptide is about 6,000 tons and increasing rapidly. Therefore, there is a great need for the exploration of detection methods that could discriminate nosiheptide from different sources or identify its purity. The main different ingredients between these two sources of nosiheptide are spores, culture medium and metabolites of
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Schematic of plasmonic filter fabrication and nosiheptide discrimination based on Streptomyces spores.
in-depth research shows that the quality of SERS signal is affected not only by the intensity, number and distribution of hot spots, but also the interaction of plasmonic surface and analytes 22-24. Therefore, the size distribution and surface charge of plasmonic nanoparticles will hinder the improvement of reproducibility and quantification in SERS detection of bacteria, as well as its capping reagent and the solution pH. The interaction of plasmonic “hot spots” and bacteria need to be designed for specific applications so that rapid, sensitive and reproducible discrimination will be realized. Photonic crystal (PC), with ordered and periodic dielectric micro/nanostructure, can control the propagation of light through photonic band gaps 25-26. It could not only be utilized as label-free platform27 for biosensing and bioimaging because it is sensitive to the change of refractive index 28, but also be used as substrate for SERS in combination with plasmonic materials 12, 29-31. Since PC could provide an ordered and rough plasmonic surface, distribution of plasmonic “hot spots” could be effectively controlled, which is in great demand for high reproducible detection32. Additionally, the low cost of colloidal PC fabricated by self-assembly of monodispersed colloids, which avoids high cost nanofabrication process in case of 2D PC composed of 2D cavity or pillar array, paves the way for its wide application in POCT. Furthermore, colloidal PC containing intrinsic 3D porous nanostructure with tunable pore sizes in the range of tens of nanometers can serve as a good filter candidate for bacteria capture. When the surface of colloidal PC is coated with plasmonic material, the contact of bacteria and plasmonic “hot spots” could also be easily controlled by filtration for effective SERS detection. Hence, in this paper, we proposed to use plasmonic filter in the SERS detection of Streptomyces spores for discrimination of nosiheptide source. The process is illustrated in Figure 1. First, we prepared colloidal PC by self-assembly method with the assistance of a filter membrane 31. Then, the PC was coated with silver through an electroless plating process. When the supernatant of nosiheptide was dropped on the plasmonic filter, the Streptomyces spores would be filtered onto the surface because the pore size of the plasmonic filter was much smaller than the spores. Finally, since the spores were in close contact to the plasmonic surface with “hot spots”, their enhanced Raman spectra were easily measured in few minutes. This simple “filtration and measure” method is amenable to point of care occasions where quick measurement is needed without professional laboratory operations. We think it may also be used for bacteria detection in air or water, which is promising in food safety, environmental and clinical applications.
Reagents and Materials. Chloroauric acid tetrahydrate (HAuCl4·4H2O), silver nitrate, sodium citrate, potassium sodium tartrate tetrahydrate, were purchased from Aladdin Reagent Company (Shanghai, China). All chemicals were of analytical grade and phosphate buffered saline (PBS, pH = 7.4) was used throughout the experiments. Silica nanoparticles used in this paper were 468 nm in diameter and purchased from Dongjian biological technology (Nanjing, China). Nylon filter membranes with 0.22 µm pore size and 50 mm diameter were obtained from Luqiao Taizhou, China. The filter holder was purchased from Wanqing (Nanjing, China). Preparation of samples. Gram staining Kit, silver-plated nylon membrane and silver-plated plasmonic filter were also adopted for the nosiheptide discrimination. For the preparation of samples, first, samples prepared with chemical synthesis and fermentation were dissolved into buffered saline (PBS, pH 7.4). The solution was then mixed with a vortex and maintained for 1 hour. Further, the supernatant pipetted into a new centrifuge tube and was stained or submitted to Raman spectroscopy. Preparation of plasmonic filter. AuNPs were synthesized following a modified procedure reported by Frens 33 and plasmonic filter was prepared with the method mentioned in previous paper 31. Then the silver-plated PC substrate was washed with deionized water for three times, dried at room temperature and cut into pieces (1×1 mm). 5 µL sample supernatant mentioned above was dropped onto the substrate and submitted to the Raman spectroscopy after 5 minutes. Surface Enhanced Raman Spectroscopy. All SERS signals were collected through a confocal Raman Microscope (inVia, Renishaw Inc) equipped with a 785 nm laser. SERS measurements were carried out with a laser power of 5.6 mW and sequential spectral collection of 10 s. Numerical simulation. Finite Difference Time Domain (FDTD) simulation was proceeded with Lumerical FDTD Solutions 8 for the calculation of electric field distribution, which was related to the enhancement of Raman signals. A plane wave at 785 nm was used as the excitation light whose polarization was along the x-axis and propagation is towards – z direction from the top of the structure. The diameter of the nanoparticle in the assembly was 468 nm and different thickness of silver layer was simulated to explore the distribution and the highest intensity of electric field.
RESULTS AND DISCUSSION FDTD simulation. As is well known that the electromagnetic (EM) enhancement originated from the surface plasmon resonance plays a critical role in the process of SERS 22, 34 and
EXPERIMENTAL SECTION 2
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Figure 2. The FDTD simulated profiles of the electric field enhancement distribution of silver-plated plasmonic filter with different thickness of silver layer excited by 785 nm laser. (a), 10 nm silver layer; (b), 15 nm silver layer; (c), 20 nm silver layer; (d),
25 nm silver layer. The diameter of monodisperse silica nanoparticles are 468 nm. Figure 3. Photo of silver-plated plasmonic filter (a), SEM images of bare PC substrate (b), silver-plated plasmonic filter (c) and its cross-section (d). that no matter what the thickness was, the “hot spots” located in the gap between adjacent nanoparticles. Since the surface pattern of colloidal PC was hexagonal close packed (hcp), there were 12 “hot spots” around every nanoparticle of 468 nm and their distribution was very uniform. In addition, the strong enhanced electric field was localized mainly in the equator of the nanoparticles as in the x-y plane and the flank of the dome as in the x-z plane, almost forming a cone in three dimensions with tip downside and connected to the pore where the “hot spots” were also located in. When the spores were filtered onto the surface, they would cover several pores, which would guarantee the effective enhancement of their Raman signals. From the comparison of the strongest field intensity caused by different silver thickness we could see that the highest value (|E|2) occurred in thickness of 20 nm and the strongest enhancement of the “hot spots” is nearly 85 times according to
the resonance resultant “hot spots” together with their distribution accounts a lot for the enhanced signal. In the plasmonic filter, the pores, formed by adjacent monodispersed nanoparticles coated with silver, were used for filtration and SERS detection. In order to obtain the “hottest” surface, the pore size of the filter, determined by the diameter of monodispersed silica nanoparticles and the thickness of silver layer, should be optimized. The pore size of the colloidal PC was about 15% of the colloidal diameter. In this case, it was about 70 nm since the diameter of the silica nanoparticles was 468 nm. When the silver layer became thick, the pore would become small. Although thicker silver might have better SERS enhancement, the permeability would be destroyed. Therefore, we first simulated the electric field distribution of the plasmonic filter surface with different thickness of silver layer by FDTD. The results (|E|2) are shown in Figure 2, which only displays the top layer of the plasmonic filter. It could be seen
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Figure 4. Raman results of 10-3 M R6G from the bare PC substrate (a), 10-3 M~10-12 M R6G on the silver-plated plasmonic filter (b) and sensor response versus the logarithmic concentration of R6G (c).
Figure 5. The Gram stain results of pure Streptomycete spore (a), nosiheptide prepared with fermentation (b, c) and chemical synthesis (d). Scale bars are 10 µm. Figure S1, meaning that a largest enhancement factor at the level of 107 because the SERS signals are proportional to the biquadrate of the electric field (|E|4) 34. Furthermore, in comparison with gold nanoparticles, the “hot spots” of plasmonic filter were not only evenly distributed but also stronger in intensity34, 35. Therefore, we coated 20 nm silver on the surface of plasmonic filter in the later experiments.
the top of the first silica layer was coated with silver, after which the pore size was about 20 nm. When solution drop containing Streptomyces spores was dropped onto the plasmonic filter, the solvent was absorbed and the spores with size of 1~2 µm could be captured on the silver coated surface with ordered “hot spots” distribution. The structure of the plasmonic filter was not only used for the filtration and capture of spores but also for the enhancement of the Raman signal of the spores in close contact with the filter surface.
Fabrication of Plasmonic filter. filter. Cost is an important issue that should be considered in point of care detection. In this paper, we used low-cost self-assembly method to fabricate 3D PC with ordered nanoporous structure on a nylon filter membrane by filtration of silica colloids. In combination with electroless plating, which was also a convenient and low cost process, large area filter with plasmonic surface could be obtained (Figure 3a). Here the diameter of the plasmonic filter was 5 cm since we used a commercial 5 cm diameter nylon filter membrane as the supporting substrate for colloidal self-assembly. Then the filter was cut into pieces of 1×1 mm for use. However, fabrication of larger area was feasible. The SEM image shows that the PC surface is hexagonal close packed monodispersed silica nanoparticles (Figure 3b) and the electroless plated silver coating is uniform (Figure 3c and 3d). 10 nm AuNPs were used as the seeds for the growth of silver layer during the electroless plating 36. The thickness of the plated silver layer could be controlled by the plating time. In our experiment, 20 nm thickness was obtained when the plating time was 40 min. During this reaction time, most of the sliver reduction occurred on the surface of the colloidal PC. Therefore, only
SERS performance. Before the plasmonic filter was utilized for spore detection, R6G was utilized as a probe molecule to test its SERS performance. The feature peaks of R6G are at almost 614 cm-1, 1362 cm-1, 1508 cm-1 37, and then we can obtain the enhanced effect through making a comparison of the Raman intensity obtained from bare PC (Figure 4a) and that from plasmonic filter (Figure 4b). The results indicate that the silver-plated PC substrate can enhance the Raman intensity for 20 times compared with that obtained from bare PC. In combination with three-dimensional PC nanostructure, the rough metal surface can increase the Raman signals for several orders of magnitude due to the electromagnetic field enhancement 38. At the same time, the enhancement factor of the substrate was estimated to be 7.49×107. Additionally, the normalized response [i = I/I0] of the Raman intensity at 1362 cm−1 was plotted according to the R6G concentration on a logarithmic scale (Figure 4c). I0 and I were the intensities of the Raman peak of 10-3 M and dilute different concentrations of R6G, respectively. Standard deviations of characteristic
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Figure 6. Digital photo (a) and SEM image (b) of the silver-plated nylon filter membrane, SERS spectra of nosiheptide synthesized with chemical method (c) and nosiheptide prepared with fermentation (d) on the plasmonic filter. The concentrations of these nosiheptide are 0.1 g/mL. band of R6G at 1362 cm-1 were calculated with five Raman spectra taken at separate areas. The results showed that the silver-plated plasmonic filter had a good reproducibility. The limit of detection (LOD) was determined to be 2.02 pM.
concentration is 0.1 g/mL, the SERS spectra of nosiheptide prepared with chemical and fermentation method are shown in Figure 6c and 6d, which can not be distinguished from each other. The reason might lie in the structure of the filter. From Figure 6b it can be seen that the microscopic structure of nylon filter coated with silver is not ordered and there are nonuniform pores ranging from tens of nanometers to several microns. Since the filtration velocity in the larger pore is relatively high, the spores might enter them especially when the concentration is very low. In addition, the scattering of this kind of disordered structure is very strong. When the spores are trapped in the deep pores but not on the surface, not only the excitation light, but also their Raman signal lose a lot due to the scattering of the disordered structure, which greatly decreases the measurable Raman signal. Finally, silver-plated plasmonic filter was adopted for the discrimination of different nosiheptide sources. In POCT applications, minimal sample consumption and high sensitivity is needed. Hence, the sample volume was optimized according to the aqueous absorption feature of the filter. In principle, even single spore can be captured and detected using the plasmonic filter owing to its high sensitivity and uniform pore size. However, if the sample spread too much on the filter, some spores might not be detected due to the limited excitation laser spot size. In our experiments, we expanded the laser spot to 500 µm so that all the spores in the sample spot could be excited and detected. Raman spectra results of pure Streptomycete spore are shown in Figure 7a. It can be seen that the characteristic bands of Streptomycete spore were at 527 cm-1, 634 cm-1, 688 cm-1, 990 cm-1, 1342 cm-1 and 1536 cm-1. In order to confirm the feasibility for minimal sample detection, Raman signals of different concentrations of samples prepared with chemical
Discrimination of nosiheptide sources. In order to comprehensively demonstrate the discriminating performance of plasmonic filter on different nosiheptide sources, we tested it with nosiheptide prepared with chemical and fermentation method and compared the results with stain method and silver plated nylon filter. First, traditional Gram stain method was investigated as it was the most commonly used one, by user or administrator. Before staining, the nosiheptide powder was dissolved in PBS (pH 7.4) and the solution was then mixed with a vortex and maintained for 1 hour. Further, the supernatant was immobilized, and then stained with Gram staining kit. As shown in Figure 5a, the pure cultured Streptomycete observed with 500× microscope were rounded, oval or catenulate in purple with size around 1~5 µm. The staining images of concentrations at 0.1 g/mL and 0.05 g/mL are shown in Figure 5b and 5c. It could be seen that when the concentration was low, it was hard to find the spores in the field view and make a judgement. However, for chemical synthesized nosiheptide, it was also hard to find spores (Figure 5d). Therefore, we could not discriminate whether the chemical synthesized nosiheptide powder was doped with low concentration fermented one or not. Furthermore, it took about half an hour to complete the stain experiment and the stain results was prone to be affected by pollution of debris. Obviously, the staining method does not apply for POCT very well. Plasmonic materials coated nylon filters are also commonly reported as SERS substrate for bacteria detection 11, 20. Then, we prepared nylon filter with 20 nm silver (Figure 6a and 6b) and used it for the discrimination of nosiheptide. When the
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Figure 7. SERS spectra of Streptomycete spore (a), nosiheptide synthesized with chemical method (b) and nosiheptide prepared with fermentation (c) on the silver-plated plasmonic filter. Raman intensities of feature peaks (990 cm-1) versus different concentrations of nosiheptide prepared with fermentation and chemical synthesis (d). SEM images of supernatants of nosiheptide synthesized with chemical method (e) and nosiheptide prepared with fermentation (f). The yellow arrow of (e) is nosiheptide synthesized with chemical method, while that of (f) is Streptomycete spore. Scale bars are 1 µm. synthesis (Figure 7b) were investigated from 0.005 g/mL to 0.1 g/mL while those of the same concentration gradient of samples prepared with fermentation were obtained (Figure 7c). The characteristic band of Streptomycete spore at 990 cm-1 is also found in Figure 7c as in Figure 7a, while not in Figure 7b, which indicates that we can discriminate these two kinds of nosiheptides through characteristic Raman band of the spore. Moreover, the yellow arrows of SEM results in Figure 7e (nosiheptide) and Figure 7f (spore) indicate that the Raman signals obtained (Figure 7b and 7c) are the characteristic bands of nosiheptide prepared with chemical method and fermentation, respectively. In addition, the results in Figure 7d show that from the concentration of 0.1 g/mL to 0.005 g/mL, Raman intensities at 990 cm-1 of nosiheptide prepared with fermentation decrease gradually, while there is no remarkable change at 990 cm-1 about those of nosiheptide prepared with chemical synthesis. Hence, the plasmonic filter can be utilized for the discrimination of nosiheptide prepared with fermentation and chemical synthesis due to the existence of feature peak at 990 cm-1. Yet, expect for the Raman feature peak at 990 cm-1, the other bands of spore could not be recognized in Figure 7c, which might be due to the effect of other components of medium or bacterial metabolites in samples prepared with fermentation. In other words, the Raman signals of other components in sample prepared with fermentation interfered and overlapped those of spores. Even though, the SERS signal at 990 cm-1 could be selected as a characteristic peak for the identification of Streptomycete spore and discrimination of nosiheptide. More importantly, with the silver-plated plasmonic filter, the detection process could be completed in several minutes with minimal sample, which means the silver-plated plasmonic filter is very appropriate for the applications in POCT.
CONCLUSIONS In summary, a sensitive, staining and PCR free silver-plated plasmonic filter was developed for the sensitive discrimination of nosiheptide prepared with chemical synthesis and biological fermentation. First, the FDTD results indicated that the silver thickness of 20 nm could remarkably enhance the SERS intensity around the silica nanoparticles compared with SERS substrate reported before. Second, the SERS sensor exhibited high sensitivity toward R6G molecule with a LOD of 2.02 pM. The enhancement factor of the substrate was estimated at 7.49 ×107. Third, the SERS sensor could be utilized for Raman signals identification of Streptomycete spore with a low sample concentration of 0.005 g/mL and a sample volume of only 5 µL in less than 5 min. Compared with traditional Gram stain method and frequently-used silver-plated nylon filter membrane, the biosensor showed a good sensitivity and efficiency. Furthermore, the SERS sensor provided a good performance and stability in various batches of experiments. The staining-free, highly sensitive, silver-plated plasmonic filter will be a promising tool in food safety, environmental and clinical applications.
AUTHOR INFORMATION Supporting Information The histogram of the highest electrical field intensity of silver-plated plasmonic filter with different thickness of silver layer excited by 785 nm laser.
AUTHOR INFORMATION Corresponding Author
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ACS Applied Materials & Interfaces (16) Mosier-Boss, P.; Sorensen, K.; George, R.; Obraztsova, A., Sers Substrates Fabricated Using Ceramic Filters for the Detection of Bacteria. Spectrochim. Acta, Part A 2016, 153, 591-598.
*E-mail:
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Author Contributions
(17) Çulha, M.; Kahraman, M.; Şahin, F.; Kocagöz, S., SurfaceEnhanced Raman Scattering of Bacteria in Microwells Constructed from Silver Nanoparticles. J. Nanotechnol. 2012, 2012, Article ID 297560.
All authors have given approval to the final version of this manuscript.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (Grants 21373046, 21073033and 21327902), Program Sponsored for Scientific Innovation Research of College Graduate in Jiangsu Province (KYLX16_0284), Jiangsu Science and Technology Department (Grant No. BE2014707), the Program for New Century Excellent Talents in University, Fundamental Research Funds for the Central Universities.
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