A highly sensitive and recyclable SERS substrate based on Ag-nanoparticle-decorated ZnO nanoflowers in ordered arrays.

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Highly sensitive and recyclable SERS substrate based on

DOI: 10.1039/C4DT03596H

Ag-nanoparticle-decorated ZnO nanoflowers in ordered arrays

Published on 07 January 2015. Downloaded by UNIVERSITY OF OTAGO on 08/01/2015 10:38:03.

Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, and School of Physics and Opto-electronic Technology, Dalian University of Technology, Dalian, 116024, China

Using a patterned sapphire substrate with hemisphere array, flower-like ZnO nanorods grouped in highly ordered array were fabricated in a wafer scale by a routine solution growth. By decorating with high-density Ag nanoparticles (NPs), the ZnO nanoflower arrays were used as the substrates for surface enhanced Raman scattering (SERS). Using Rhodamine 6G as the probe molecules with the concentration down to 10-10 M, the SERS substrates present an as high as 1010 Raman enhancement with good reproducibility. The influence of Ag NP decoration on SERS activity was explored and the sub-10 nm nanogaps between adjacent Ag NPs were proved to be the primary electromagnetic “hot spots” responsible for the significant Raman enhancement. The Ag-NP-decorated ZnO nanoflower arrays were demonstrated possessing self-cleaning function enabled by UV irradiation via photocatalytic degradation of the analyte molecules. In addition, the SERS substrate exhibited an extremely long service lifetime, possibly due to its superhydrophobicity and storage in dark and dry environment. Key words: ZnO nanorod, patterning, solution growth, SERS *

Author to whom correspondence should be addressed: [email protected] 1

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Qiang Tao, Shuai Li, Chunyu Ma, Kun Liu, and Qing-Yu Zhang*

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DOI: 10.1039/C4DT03596H

INTRODUCTION

Surface-enhanced Raman scattering (SERS) has been intensively studied as a


SERS enables detection of trace organic chemicals in a facile, low-cost, and highly sensitive way, thus has been applied in various fields including analytical chemistry, life science, and bio-sensing. It is commonly accepted that Raman signals of analyte molecules on SERS substrate are enhanced by the plasmonic “hot spots”, which are the points with strongly enhanced local electromagnetic fields induced by the excitation of the collective oscillation of free electrons in noble metals such as Au, Ag, and Cu, or in transition metals (4, 5). In most SERS procedures, the major hot spots are the nanogaps of plasmonic nanostructures. The enhancement factor (EF), i.e., the average signal enhancement from absorbents participated in SERS to that in Raman scattering, can be increased by reducing the nanogap spacing (6, 7) or by increasing the density of hot spots. Therefore, much effort has been put into the rational design and controlled fabrication of SERS substrates for high sensitivity detection (8-13). Three-dimensional (3D) SERS substrates have been demonstrated to be capable of improving the detection sensitivity (14-19). Because a 3D SERS substrate was generally fabricated by employing a substrate consisting of nanostructures as the support of noble metal nanoparticles (NPs), the nanostructure morphology is important in controlling the formation of plasmonic nanostructure and determines the performance of the SERS substrate. To date, various nanostructures such as Si nanowires (14, 15), ZnO nanorods (16-18), and glass nanopillar arrays (19) have been

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powerful analytical technique since the single molecule detection was reported (1-3).

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DOI: 10.1039/C4DT03596H used for the fabrication of 3D SERS substrates. However, it is generally difficult to

fabricate a periodic 3D SERS substrate with reproducible and controllable geometry,


urchin-like Ag NP/ZnO hollow nanosphere array has been fabricated employing nanosphere lithography and solution process (20). The urchin-like 3D SERS substrate was demonstrated to have high SERS sensitivity and good signal reproducibility. However, nanosphere lithography has some technique limitations with respect to the quality performance for massive fabrication at wafer scale. To obtain a periodic 3D SERS substrate at large area and high-throughput with relatively low cost (21, 22), an alternative is to use a patterned substrate fabricated by photolithography for the controlled growth of ZnO nanorod arrays. For example, using highly ordered Si nanopillars as the scaffolds for the ZnO nanorod growth, periodic Ag-NP-decorated Si/ZnO nanotrees have been successfully prepared in a large scale and served as SERS-active substrate (22), which exhibited good performance in terms of high sensitivity and good reproducibility. To improve the performance of periodic SERS substrate, there is a need to optimize the substrate pattern to control the growth of ZnO nanorod array further by paying respect to Ag NP decoration and excitation light trapping. In this study, we report a simple method to fabricate periodic Ag-NP-decorated ZnO nanoflower arrays as SERS substrates by a routine solution process and magnetron sputtering. A patterned sapphire substrate with highly ordered hemisphere array, which was fabricated by photolithography at wafer scale, was used to control

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which is desired in SERS-based sensor applications. SERS substrate based on the

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DOI: 10.1039/C4DT03596H the formation of flower-like ZnO nanorod arrays. The ordered ZnO nanoflower arrays

produce a radial nanostructure consisting of ZnO nanorods that provide an effective


plasmonic “hot spots” and absorbing target molecules. By optimizing Ag NP decoration, the SERS substrate presents an as high as ~1010 Raman enhancement. In addition, the SERS substrates exhibited excellent performance in terms of good reproducibility, long service lifetime, and self-cleaning function.

EXPERIMENTAL METHODS Fabrication of Ag-NP-decorated ZnO nanoflowers. The process to fabricate the Ag-NP-decorated ZnO nanoflower arrays is illustrated in Scheme 1. The template used for controlling ZnO nanorod growth was a patterned sapphire substrate with a highly ordered hemisphere array fabricated by photolithography. The hemisphere radius was ~ 1.6 m and the periodicity of the hemisphere array was ~ 4 m. Before the solution growth of ZnO nanorods, a ZnO seed layer was prepared by immersing the patterned substrate in 5 mM zinc acetate [Zn(CH3COO)2] ethanol solution for a few minutes and then heating the substrate at 500 oC for 1 h. Before heating, the redundant solution on the substrate was sipped up with filter paper and kept in a hotbox for drying. Subsequently, the highly ordered ZnO nanoflower arrays were grown by a routine solution method using 25 mM Zn(NO3)2 and C6H12N4 equimolar solution. The solution reaction was conducted at 95 oC for 5 h. After reaction, the substrates were removed from the solution, rinsed with deionized water, and dried by a N2 flow. Finally, Ag NP decoration was carried out at room temperature by

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large surface area for Ag NP deposition, thus allowing high possibility of forming

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10.1039/C4DT03596H sputtering an Ag target (99.99%) with a radio-frequency magnetron sputteringDOI: system

operated at 0.5 Pa in an ambient of Ar. The sputtering power was ~30 W, which led to


to 15, 30, 60, 120 and 180 s, respectively. Characterization. ZnO nanoflower arrays before and after Ag NP decoration were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and X-ray diffraction (XRD). SERS activity was evaluated using Rhodamine 6G (R6G) as probe molecules. Samples for SERS measurements were prepared by submerging Ag-decorated ZnO nanoflower arrays in R6G ethanol solutions for 24 h and then sipping up the excess solution on the substrate with filter paper. By checking the sample weight, the thickness of R6G solution layer on the substrates was estimated to be less than 0.01 mm. 10-4 to 10-10 M R6G solutions were used for the determination of SERS sensitivity. Raman measurement was performed at room temperature on Renishaw inVia plus Raman system with a 532 nm line laser as excitation. The laser spot was ~ 2 m in diameter and the excitation power was 2.2 mW. Raman enhancement factor was calculated using the Raman signals of 0.1 M R6G on the ZnO nanoflower array as the reference in the absence of Ag NPs. Using 10-5 M R6G solution, the self-cleaning function of the SERS substrate was demonstrated under ultraviolet (UV) irradiation in air by an 8 W low-pressure mercury lamp.

RESULTS AND DISCUSSION The substrate pattern geometry is important in determining the final morphology

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a ~6 nm/min deposition rate. To optimize Ag decoration, Ag deposition time was set

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DOI: 10.1039/C4DT03596H of ZnO nanorod array, which influences the sequent Ag NP decoration. Based on the

consideration in respects of Ag NP decoration and excitation light trapping, the


control the growth of ZnO nanorods. Figure 1 shows the SEM images of the patterned substrate in top and tilted views and the ordered ZnO nanoflower array in different magnifications. Unlike the compacted polystyrene nanosphere arrays prepared by self-assembly methods, a quite large spacing (~ 0.8 m) between the hemispheres is provided for the growth of ZnO nanorods on the hemispheres, as shown in Figures 1a and 1b. As a result, the obtained ZnO nanorods are primarily perpendicular to the hemisphere surface, forming the flower-like ZnO nanorods grouped in highly ordered array, as shown in Figures 1c to 1f. The specific hemisphere geometry produced a ZnO nanoflower consisting of ~700 nanorods with a large length-to-diameter aspect. The average length and diameter of ZnO nanorods are ~ 1 m and ~ 35 nm, respectively. Thus, the ZnO nanorods possess large surface area available for the formation of plasmonic hot spots by decorating Ag NPs. More importantly, the flower-like radial structures offered a better change of Ag NPs being deposited on the full surface of ZnO nanorods because shadowing effects during Ag deposition were considerably weakened. Shadowing effects generally cause unequal coverage of metallic NPs on the tops and side walls of ZnO nanorods in the case that the nanorods are compactly grown (23), leading to insufficient utility of the large surface-volume ratio of the nanostructure in the formation of plasmonic hot spots. The preparation of ZnO seed layer is a crucial step in the fabrication of the

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substrate pattern consisting of hemisphere array with a ~ 4 m periodicity was used to

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DOI: 10.1039/C4DT03596H highly ordered nanoflowers consisting of randomly oriented ZnO nanorods. Many

methods including pulsed laser deposition (PLD), magnetron sputtering, and solution


facile method to prepare a seed layer that produces uniformly distributed ZnO nanorods in each flower. The seed layer that was directly deposited on the patterned sapphire substrate by PLD or by magnetron sputtering generally produced different length-aspect ZnO nanorods grouped in the specific orientations due to the epitaxial growth, as shown in the supporting information Figure S1a. To avoid the epitaxial growth, a SiO2 layer was prepared prior to the deposition of ZnO seed layer, and then the nanoflowers with uniformly distributed ZnO nanorods were obtained, as shown in the supporting information Figure S1b. The results reveal a fact that the substrate pattern geometry, rather than the material, determines the formation of the ordered ZnO nanoflower array. Thus, patterned glass and quartz substrates or Si wafer can also be used for fabricating this kind of SERS substrates and is more cost-effective in SERS-based sensor application. In addition, the solution concentration and growth time are responsible for the length-to-diameter aspect and density of ZnO nanorods in the nanoflowers. In this study, 25 mM Zn(NO3)2 and C6H12N4 equimolar solution was found the best to obtain the ordered ZnO nanoflower arrays used as the SERS substrates. To optimize the SERS performance, Ag deposition was conducted at a ~ 6 nm/min for 15, 30, 60, 120, and 180 s, respectively. The SERS activity of different Ag-NP-decorated samples was evaluated using 10-6 M R6G as molecular probe. As

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reaction were tried to prepare the ZnO seed layer. The solution reaction was found a

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DOI: 10.1039/C4DT03596H shown in Figure 2, the characteristic peaks of R6G molecules are present at 612, 770,

1184, 1310, 1362 1510, 1572, and 1650 cm-1, and their intensities strongly depend on


signals of 10-6 M R6G under an excitation of 2.2 mW were detected at a level of ~ 103, which was about ten times of the reference signals (0.1 M R6G on the ZnO nanoflower array without Ag NP decoration). Therefore, the enhancement factor EFSERS was determined to be in the order of ~ 106 for the R6G peaks at 612, 1362, 1510, and 1650 cm-1 by using the formula EFSERS 

I SERS / CSERS (23), where I and C I Ref / CRef

are the Raman intensity and R6G concentration, respectively, and subscripts represent the SERS and reference samples. With the extension of Ag deposition time to 30~180 s, the enhancement factors of all the SERS substrates were increased to be higher than 107. Compared to the Ag-NP-decorated ZnO nanorod array grown on a flat Si substrate, the SERS substrate based on Ag-NP-decorated ZnO nanoflower array exhibited an additional enhancement more than 5 times for 10−6 M R6G solution. The increased Raman intensity can be ascribed to the enhancement in excitation light trapping (24). The SERS substrate decorated with Ag NPs for 120 s possessed the maximum EFSERS value (~ 4107), and then was used for evaluating the SERS sensitivity. When decreasing the concentration of R6G solution down to 10−10 M, which produced a number of R6G molecules less than 5 within the detection area (~22 m2) in this study, the major Raman peaks of R6G molecules can be still resolved, as shown in Figure 3a, indicating that the SERS substrate is capable of probing single molecule. Using the 10−10 M R6G solution, the enhancement factor

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the time of Ag deposition. For the sample decorated with Ag NPs for 15 s, Raman

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DOI: 10.1039/C4DT03596H was determined to be ~ 1010, which is an indication that the SERS substrate is

ultrasensitive for the detection of trace chemicals. The plot of intensity (I) vs


log I  5.80  0.27 log C , as shown in Figure 3b. The enhancement of local electromagnetic field is known depending on the size, shape, and association of the metallic NPs, which determines the performance of SERS substrate. It has been confirmed, both experimentally and theoretically, that the local field due to the interparticle electromagnetic coupling can be significantly enhanced by reducing the spacing between the metallic NPs (6, 7). Figure 4 shows the typical SEM images of ZnO nanorods before and after Ag NP decoration. As the shadowing effects in Ag deposition were reduced, a sufficient coverage of Ag NPs was achieved on the tops and sidewalls of ZnO nanorods, even for Ag deposition at a longer time. Due to the Volmer-Weber (VW) growth mode, the Ag NPs deposited for 15 s was formed in the fashion of small Ag islands on the sidewalls. The average spacing between adjacent Ag islands on the nanorod sidewalls was ~ 10 nm. With an increase in Ag deposition time, the enlarged Ag nanoislands reduced the nanogap spacing to sub-10 nm. By further increasing Ag deposition, coalescence between Ag islands produced a decrease in the density of the sub-10 nm nanogaps and resulted in the formation of the knot-like structure. The SEM observation unambiguously revealed that the sub-10 nm nanogaps between adjacent Ag NPs sitting on the sidewall of same ZnO nanorod are the primary plasmonic hot spots contributing to the Raman enhancement. In addition, the plasmonic hot spots formed by Ag NPs on two

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concentration (C) in log-log scale was found having the linear dependence of

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DOI: 10.1039/C4DT03596H neighboring nanorods were increasing with Ag deposition, also making contributions

to the Raman enhancement. On the other hand, the significant Raman enhancement is


periodicity of ZnO nanoflower arrays was designed to be in the scale a little larger than the wavelength of excitation light. Using the ZnO nanorod arrays grown on the textured Si substrate, we have demonstrated that a micron-scale flower-like ZnO nanorod array can more effectively trap the incident light than an aligned ZnO nanorod array (24). Signal reproducibility is a concern for SERS substrates in practical application. Using the SERS spectra of 10-8 M R6G collected from 6 randomly selected positions, the relative standard deviations (RSD) were calculated. For the major characteristic peaks of R6G molecules, all the RSD values are in the range of 10% to 15%, depending on the R6G peaks used for calculation. To further evaluate the reproducibility of SERS substrates, Raman map was collected in a large area (5 m  5 m) with a ~ 1 m step at the Raman shift of 1362 cm-1. The comparison of Raman map with the SEM image of ZnO nanoflower array is presented in Figure 5. The Raman map was not found having obvious flower-like structure that observed in the SEM image, evidently demonstrating the good signal reproducibility. The uniformity and large Raman enhancement are primarily attributed to the advantages of 3D SERS substrate because it provides more electromagnetic hot spots and the probed molecules than a 2D SERS substrate. On the other hand, the uniformity can be ascribed to the flower-like ZnO nanorod structure that allows more Ag NPs being

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also ascribed to the enhanced interaction of excitation light with Ag NPs because the

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DOI: 10.1039/C4DT03596H deposited on the ZnO nanorods forming high-density plasmonic hot spots, and then

offering a better change of target molecules being placed near the hot spots.


received more and more attention (25-28). Figure 6a shows the evolution of Raman spectra of 10−5 M R6G on the SERS substrate under UV irradiation in air by an 8 W low-pressure mercury lamp. The Raman signals of R6G molecules were exponentially decreasing with the extension in the irradiation time, as shown in the inset of Figure 6, indicating that R6G molecules on the SERS substrate were photodegraded into small molecules under UV irradiation. UV photons and the photoinduced carriers in ZnO nanorods are suggested to be responsible for the degradation of analyte molecules. The photodegradation reaction of R6G molecules can be described by Langmuir-Hinshelwood mechanism, and the determined degradation rate was ~ 0.024 min-1. After 4 h irradiation, no obvious R6G Raman signal was detected, suggesting that the residual R6G molecules are lower than the detection limit, thus the SERS substrate can be repeatedly used. To demonstrate the recyclability for practical application, three self-cleaning cycles were performed by repeatedly loading 10-5 M R6G solution on the SERS substrate and then UV irradiating for 4 h. As shown in Figure 6b, the reproduced SERS substrate exhibited an EFSERS value close to that of the fresh one, whereas the background noises were considerably reduced. Ag is the most SERS-active metal with a broad spectral response from UV to near infrared band, thus has been widely used for the fabrication of SERS substrates. However, Ag NPs is instable against oxidation, which is the major deficiency of

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A self-cleaning substrate is significant for SERS-based sensor application and

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DOI: 10.1039/C4DT03596H Ag-NP-decorated SERS substrate leading to rapid decay in SERS performance in

atmosphere environment with aging time (29-31). We examined the SERS-activity of


surprised that the Raman spectrum of R6G molecules on the aged SERS substrate can be well resolved with an enhancement factor close to that of the fresh one. Using the same excitation power, the Raman intensities of 10-6 M R6G molecules collected from the 2-year aged SERS substrate were found only ~ 40% decrease, as shown in the supporting information Figure S2, evidencing the long service lifetime of the SERS substrate. We consider that the superhydrophobicity of the ordered ZnO nanoflower array is one of reasons responsible for the long service lifetime of the SERS substrate. As shown in Figure 7a, the contact angle of water on the Ag-NP-decorated ZnO nanoflower array is larger than 120º, indicating that the SERS substrate is superhydrophobic. However, we found that not all ZnO nanorod arrays are hydrophobic. For example, the Ag-NP-decorated aligned ZnO nanorod array on a flat Si substrate prepared by the same process is hydrophilic, as shown in the inset of Figure 7a. This discovery is very significant for understanding the stability of a SERS substrate based on Ag nanostructure. It revealed a fact that the hydrophobicity or hydrophilicity is a property associated with the morphological structure, which determines the service lifetime of a SERS substrate. In addition, it was reported that UV irradiation enables change of ZnO nanorod array from superhydrophobic to hydrophilic (32). As shown in Figure 7, the contanct angle of water on the Ag-NP-decorated ZnO nanoflower array was gradually decreased from ~120º to ~0º

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the Ag-NP-decorated ZnO nanoflower array that was fabricated ~ 2 years ago. It is

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10.1039/C4DT03596H after 18 min UV irradiation, suggesting that a SERS substrate exposed toDOI: light is

easier to absorb water molecules, and then resulting in rapid decay in SERS


the service lifetime of Ag-NP-decorated SERS substrate.

CONCLUSIONS

In summary, we presented a facile strategy to fabricate wafer-scale SERS substrate with highly ordered ZnO nanoflower array. By optimizing Ag NP decoration, the SERS substrate exhibited an extremely high sensitivity and good reproducibility. The sub-10 nm nanogaps between adjacent Ag NPs were revealed to be the plasmonic hot spots leading to the significant Raman enhancement. In addition, the Raman enhancement can also be thanked for the radial ZnO nanoflower structure that allows high-coverage Ag NP deposition on the ZnO nanorods and enhances excitation light trapping. The SERS substrate is recyclable, as enabled by the self-cleaning function via photocatalytic degradation of the analyte molecules under UV irradiation, and has a long service lifetime, which is demonstrated to be possibly due to its superhydrophobicity and storage in dark and dry environment. The SERS substrate possesses the good performance for potential applications in chemical analysis and bio-sensing.

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performance. Therefore, storage in dark and dry environment is needed for prolonging

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DOI: 10.1039/C4DT03596H

ACKNOWLEDGEMENT

The research is supported by the National Natural Science Foundation of China


Liaoning province under Grant No. LZ2014006. One of authors, K. Liu, thanks the supports from Science and Technology Projects Funds of Liaoning Province under Grant No. 2013231005 and Fundamental Research Funds for the Central Universities of China under Grant No. DUT13LK21.

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under Grant No. 10774018, and Basic Research Project for Key Laboratory of

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DOI: 10.1039/C4DT03596H

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CAPTIONS OF SCHEME AND FIGURES

DOI: 10.1039/C4DT03596H

Scheme 1 Fabrication process of the 3D SERS substrate, (a) patterned substrate


growth of ZnO nanoflower array, (d) Ag NP decoration. Figure 1 (a) Top view and (b) titled view SEM images of the patterned sapphire substrate, (c-f) SEM images of ZnO nanoflowers in different magnifications. Figure 2 Raman spectra of 10-6 M R6G on the SERS substrates decorated with Ag NPs for deposition time of 15, 30, 60, 120, and 180 s. For comparison, the Raman spectrum of 0.1 M R6G on the highly ordered ZnO nanoflower array in the absence of Ag NPs is also presented in the figure. Figure 3 (a) Raman spectra of R6G molecules in the concentrations of 10-5-10-10 M on the SERS substrates, (b) log-log plot of Raman intensity peak at 1362 cm-1 versus R6G concentration. Figure 4 SEM images of ZnO nanorods (a) before Ag NP decoration, (b-f) after Ag NP decoration at a ~ 6 nm/min deposition rate for 15, 30, 60, 120, and 180 s, respectively. All scale bars in the figures are 60 nm. Figure 5 Raman mapping of 10-8 M R6G on SERS substrate in comparison with the corresponding SEM image of ZnO nanoflower array. Raman mapping was recorded using the Raman peak at 1362 cm-1 with a 1 m step. Figure 6 (a) Raman spectra of 10-5 M R6G on SERS substrate under UV irradiation for 0 to 4 h, (b) demonstration of Raman spectra in three self-cleaning cycles by repeatedly loading 10-5 M R6G on the SERS substrate and then UV

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fabricated by photolithography (b) preparation of ZnO seed layer, (c) solution

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irradiating for 4 h.

DOI: 10.1039/C4DT03596H

Figure 7 Evolution of water drop on the Ag-NP-decorated ZnO nanoflower array

photograph of water drop on the Ag-NP-decorated ZnO nanorod array on a flat Published on 07 January 2015. Downloaded by UNIVERSITY OF OTAGO on 08/01/2015 10:38:03.

Si substrate prepared by the same process.

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under UV irradiation at 0, 6, 12, and 18 min. The inset in figure (a) is the

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Scheme 1 Fabrication process of the 3D SERS substrate, (a) patterned substrate fabricated by photolithography (b) preparation of ZnO seed layer, (c) solution growth of ZnO nanoflower array, (d) Ag NP decoration. 99x62mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

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Dalton Transactions View Article Online

Figure 1 (a) Top view and (b) titled view SEM images of the patterned sapphire substrate, (c-f) SEM images of ZnO nanoflowers in different magnifications. 175x220mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

Dalton Transactions


Figure 2 Raman spectra of 10-6 M R6G on the SERS substrates decorated with Ag NPs for deposition time of 15, 30, 60, 120, and 180 s. For comparison, the Raman spectrum of 0.1 M R6G on the highly ordered ZnO nanoflower array in the absence of Ag NPs is also presented in the figure. 119x90mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

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Figure 3 (a) Raman spectra of R6G molecules in the concentrations of 10-5-10-10 M on the SERS substrates, (b) log-log plot of Raman intensity peak at 1362 cm-1 versus R6G concentration. 119x119mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

Dalton Transactions


Figure 4 SEM images of ZnO nanorods (a) without Ag NP decoration, (b-f) with Ag NP decoration at a ~ 6 nm/min deposition rate for 15, 30, 60, 120, and 180 s, respectively. All scale bars in the figures are 60 nm. 77x41mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

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Figure 5 Raman mapping of 10-8 M R6G on SERS substrate in comparison with the corresponding SEM image of ZnO nanoflower array. Raman mapping was recorded using the Raman peak at 1362 cm-1 with a 1 µm step. 80x56mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

Dalton Transactions


Figure 6 (a) Raman spectra of 10-5 M R6G on SERS substrate under UV irradiation for 0 to 4 h, (b) demonstration of Raman spectra in three self-cleaning cycles by loading 10-5 M R6G on the SERS substrate and then UV irradiating for 4 h. 240x87mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

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Figure 7 Evolution of water drop on the Ag-NP-decorated ZnO nanoflower array under UV irradiation at 0, 6, 12, and 18 min. The inset in figure (a) is the photograph of water drop on the Ag-NP-decorated ZnO nanorod array on a flat Si substrate prepared by the same process. 109x82mm (300 x 300 DPI)



DOI: 10.1039/C4DT03596H

Highly sensitive and simple SERS substrate based on photochemically generated carbon nanotubes-gold nanorods hybrids.

Hierarchical Nanoflowers on Nanograss Structure for a Non-wettable Surface and a SERS Substrate.

Highly sensitive immunoassay based on SERS using nano-Au immune probes and a nano-Ag immune substrate.

Au-ZnO hybrid nanoparticles exhibiting strong charge-transfer-induced SERS for recyclable SERS-active substrates.

Highly Ordered Arrays of Femtoliter Surface Droplets.

Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties.

Multifunctional Paper Strip Based on Self-Assembled Interfacial Plasmonic Nanoparticle Arrays for Sensitive SERS Detection.

Capillary force-induced glue-free printing of Ag nanoparticle arrays for highly sensitive SERS substrates.

Enhanced magnetism in highly ordered magnetite nanoparticle-filled nanohole arrays.

Highly reproducible SERS arrays directly written by inkjet printing.

Highly-Sensitive Surface-Enhanced Raman Spectroscopy (SERS)-based Chemical Sensor using 3D Graphene Foam Decorated with Silver Nanoparticles as SERS substrate.

Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate.

Highly ordered silicon pillar arrays as platforms for planar chromatography.

Directed liquid phase assembly of highly ordered metallic nanoparticle arrays.

Highly ordered 2D microgel arrays: compression versus self-assembly.

Facile Decoration of Polyaniline Fiber with Ag Nanoparticles for Recyclable SERS Substrate.

Highly sensitive hydrazine chemical sensor based on ZnO nanorods field-effect transistor.

Formation and dissolution of microbubbles on highly-ordered plasmonic nanopillar arrays.

A transition metal carbonyl probe for use in a highly specific and sensitive SERS-based assay for glucose.

Highly sensitive SERS-based immunoassay of aflatoxin B1 using silica-encapsulated hollow gold nanoparticles.

Hierarchical nanogaps within bioscaffold arrays as a high-performance SERS substrate for animal virus biosensing.

Plasmonic library based on substrate-supported gradiential plasmonic arrays.

Selective area growth of well-ordered ZnO nanowire arrays with controllable polarity.

Ordered array of Ag semishells on different diameter monolayer polystyrene colloidal crystals: An ultrasensitive and reproducible SERS substrate.