High-Performance Real-Time SERS Detection with Recyclable Ag

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High-Performance Real-Time SERS Detection with Recyclable Ag Nanorods@HfO Substrates 2

Lingwei Ma, Hui Wu, Yu Huang, Sumeng Zou, Jianghao Li, and Zhengjun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10818 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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ACS Applied Materials & Interfaces

High-Performance Real-Time SERS Detection with Recyclable Ag Nanorods@HfO2 Substrates Lingwei Ma,† Hui Wu,† Yu Huang,† Sumeng Zou,† Jianghao Li,† and Zhengjun Zhang*, ‡ †

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and

Engineering, Tsinghua University, Beijing 100084, P.R. China ‡

Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering,

Tsinghua University, Beijing 100084, P.R. China * Author to whom all correspondence should be addressed. * E-mail: [email protected]

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ABSTRACT Ag nanorods coated with an ultrathin HfO2 shell (Ag NRs@HfO2) were prepared for the synthesis of a versatile, robust, and easily recyclable surface-enhanced Raman scattering (SERS) substrate. This substrate maximizes the high melting point of HfO2 shell and thus ensures the excellent plasmonic efficiency of Ag NRs. Therefore, it possesses extraordinary thermal stability and SERS activity, which could act as a reusable and cost-effective SERS detector. After SERS detection, the regeneration of Ag NRs@HfO2 was achieved by annealing the substrate within several seconds. This procedure led to the thermal release of adsorbed molecules and resulted in a refreshed substrate for subsequent measurements. The composite substrate maintained its SERS efficiency well during multiple “detection–heating” cycles, hence demonstrating the stability and recyclability of Ag NRs@HfO2. Furthermore, in addition to revealing the feasibility of SERS sensing in liquids, Ag NRs@HfO2 also provided continuous real-time monitoring of vapor-phase samples at ultralow concentrations. This work provides a robust and renewable SERS sensor with advantages of high sensitivity, stability, cost effectiveness, and easy operation, which can be implemented for both aqueous and gaseous analyte detection and is thus an intriguing candidate for practical applications in environmental, industrial, and homeland security sensing fields.

KEYWORDS SERS, core-shell structure, thermal stability, recyclability, aqueous and vapor-phase molecule sensing

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INTRODUCTION Surface enhanced Raman scattering (SERS) is a powerful spectroscopic technique for molecule detection under various conditions such as in liquids,1, 2 gases3, 4 and solid states.5, 6 Due to the strong electromagnetic (EM) enhancement by the localized surface plasmon resonance (LSPR)7, metallic nanostructures and the chemical (CM) enhancement,9,

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SERS technique offers

ultrasensitive and nondestructive characterization of target analytes with high specificity, which has elicited considerable attention in biomolecule sensing, environmental monitoring, and detection of hazardous substances,11-13 etc. Nevertheless, given that SERS substrates are generally made of noble metals (e.g., Au, Ag, and Cu) with high plasmonic efficiency,14–16 the costly preparation and disposable property of metallic substrates seriously obstruct the universality of SERS analysis.17–19 To address this issue, extensive efforts have been exerted in previous decades to fabricate cost-effective and recyclable SERS sensors for practical use. For example, Ag@Fe3O4 core-shell nanospheres possess both the magnetism of the Fe3O4 shell and the plasmonic property of the Ag core, so probe-adsorbed nanospheres can be collected by a magnet for sensitive SERS recognition and can subsequently be regenerated by washing the substrate in organic solutions.20 In addition, noble metal and TiO2/ZnO nanocomposites combine tremendous optical enhancement and photocatalytic degradation capability, enabling them to simultaneously detect and decompose target molecules.21–23 Such nanostructures serve as self-cleaning, recyclable, and multifunctional SERS platforms. Moreover, Ar+ plasma treatment has been utilized to clean gold nanowire forests after SERS sensing,24 thereby illustrating the morphological stability and renewability of Au substrates. Reusability has become an important evaluation index for the usefulness of SERS detection systems and is likely to promote the development of SERS technique as a routine analytical tool in many 3



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applications.25,

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However, most cleaning approaches are complicated, which require expensive

apparatus and specific operations (e.g., ultraviolet irradiation, magnet separation, or plasma treatment), and usually consume much time (from several minutes to several hours). As a consequence, it is meaningful to clean and regenerate SERS substrates in more convenient and efficient manners. Thermal annealing is a convenient and quick approach to detach molecules from the adsorbed surface, and accordingly it might be effective to clean and regenerate SERS substrates. One important prerequisite for this thermal cleaning means is the reliable thermal robustness of SERS substrates. For bare Ag nanorods (Ag NRs), their structures were damaged at a relatively low temperature of ~120 °C,27, 28 so they were not suitable for high-temperature sensing or thermal cleaning. In our previous studies, we have demonstrated the feasibility of using Al2O3 coating to enhance the high-temperature stability of Ag NRs.27, 28 However, the melting point of bulk alumina is ~2050 °C,29 which is correspondingly lower compared with many other oxides. Also, Al2O3 is not stable in acid or alkaline solution, which restricts the applicability of Ag NRs@Al2O3 substrates. To achieve SERS substrates with much stronger thermal robustness and better chemical stability, we selected HfO2 as a more reliable and promising material to protect the Ag NRs. First, the melting point of bulk hafnia is as high as ~2800 °C,30 so the Ag NRs@HfO2 substrate possesses better thermal stability and morphological robustness, which are significant for high-temperature SERS detection and thermal cleaning process. Secondly, because HfO2 is chemically inert, the stable HfO2 overlayers could further broaden the SERS applications of Ag NRs@HfO2 substrates in very erosive conditions. Furthermore, there is a strong interaction between oxide and functional groups such as -SH, -NH2,31 and -COOH,32,

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so the Ag NRs@HfO2 substrate has intensive absorbability and 4


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correspondingly improved SERS detection efficiency with molecules of these functional groups. In this study, a novel Ag NRs@HfO2 SERS platform with Ag NRs coated by an ultrathin and uniform HfO2 layer was developed. Ag NRs@HfO2 possesses superior SERS sensitivity and high-temperature robustness, and could thus function as a reusable and cost-effective SERS detector. After SERS measurement, the regeneration of Ag NRs@HfO2 was achieved by thermal annealing the substrate within several seconds, in which way the adsorbed molecules were removed completely and conveniently. Notably, the composite platform can not only be exploited for SERS determination in aqueous solutions, but also provides continuous real-time identification of vapor-phase samples at ultralow concentrations, showing great potential for in situ monitoring of air pollutants,34, 35 toxic chemicals,36, 37 and warfare agents,38, 39 etc.

EXPERIMENTAL SECTION Chemicals. Acridine, methylene blue (MB), crystal violet (CV), 2-naphthalenethiol (2-NAT) and 2-mercaptopyridine (2MPy) were purchased from J&K Scientific Ltd, Beijing, China. All chemicals were used without further purification. Synthesis of Ag NRs. Slanted Ag NRs were prepared on Si (001) films by glancing angle deposition (GLAD) technique in an electron-beam system (Thermionics Inc.) with a background vacuum level of 10-5 Pa. During deposition, the incident angle of the vapor flux was set at ~86° off the substrate normal, and the deposition rate was ~0.75 nm/s. The deposition finished at a thickness of 1000 nm read by a quartz crystal microbalance (QCM).27, 32 Synthesis of Ag NRs@HfO2. HfO2 layers were coated onto the as-prepared Ag NRs in an atomic layer deposition (ALD) reactor (MNT-100, Wuxi MNT Micro and Nanotech Co.) at 70 °C. 5



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The HfO2 precursors, i.e., tetrakis-(dimethylamido)-hafnium (TDMAH, maintained at 120 °C) and water (maintained at 40 °C), were alternatively pumped into the reaction chamber using high purity N2 (99.999%, 15 sccm) as the carrier and purge gas. Typically, one complete reaction cycle took ~40 s, and consisted of four steps: (1) TDMAH exposure, 80 ms; (2) N2 purging, 20 s; (3) water vapor exposure, 20 ms; and (4) N2 purging, 20 s. This reaction cycle was repeated for 1 to 4 times over Ag NRs. Characterization of Ag NRs@HfO2. The morphology and structure of the substrate were characterized by scanning electron microscope (SEM, JEOL-JMS-7001F) and high-resolution transmission electron microscope (HRTEM, JEOL-2011). The melting process of Ag NRs@HfO2 was monitored in situ via its reflectivity variations upon annealing, using Optical Power Thermal Analyzer (OPA-1200). SERS detections. MB and CV molecules were dissolved by deionized water with concentrations down to 10-6 M and 10-5 M, respectively; acridine, 2-NAT and 2MPy were dissolved by ethanol at different concentrations. SERS measurements were conducted with an optical fiber micro-Raman system (i-Raman Plus, B&W TEK Inc.). Raman spectra were obtained using a 785 nm laser as the excitation source, with its beam spot focused to ~80 µm in diameter. The excitation power was 150 mW, and the data collection time for each spectrum was set to be 10 s. On each template, the Raman spectrum was obtained by measuring and averaging the signals collected from 5 different spots, thus to minimize the errors brought about by measurement.

RESULTS AND DISCUSSION Characterization of Ag NRs@HfO2. Figures 1a to 1b show the typical SEM and TEM images 6


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of Ag NRs coated with an HfO2 shell of two ALD cycles. Evidently, the slanted NRs were well-separated and approximately 620 nm in length. The HRTEM images were recorded after one to four ALD reaction cycles over Ag NRs and are displayed in Figures 1c to 1f. The HfO2 shells uniformly wrapped the Ag NRs with a growth rate of ~0.8 nm per ALD cycle, leading to robust coating layers with controllable thickness. Given that HfO2 pinholes significantly affect the thermal stability and chemical adsorption of SERS substrates, to explore the pinhole rate of HfO2 shells, we introduced acridine, a probing molecule that can directly interact with the Ag surface instead of HfO2.27, 28, 32 If pinholes exist in the HfO2 shells, acridine would be adsorbed on the Ag surface through the pinholes and generate Raman signals, paving a reliable strategy toward the characterization of HfO2 pinholes. One sees from Figure 2 that the Raman signals of 1×10−2 M acridine appeared not merely on uncoated Ag NRs but also on Ag NRs@HfO2 substrates with distinctive ALD cycles. This result indicates that the HfO2 layers coated over Ag NRs possessed several pinholes. Notably, the HfO2 pinhole rate in Ag NRs@2-4cycle HfO2 was negligible compared with that in the bare Ag NRs and was relatively lower than that in Ag NRs@1cycle HfO2. To ensure the stability of Ag NRs@HfO2, substrates with reduced pinholes are favorable.32 Meanwhile, it has been reported that the SERS enhancement of Ag NRs decreased by ~50% after coating 1-2 nm oxide shell, and it declined continuously with increasing the distance between adsorbed molecules and metal surface.22, 27 As a consequence, to guarantee both the stability and sensitivity of SERS substrates, Ag NRs@HfO2 with ~1.6 nm HfO2 shell of two ALD cycles was selected as the optimal one; it is denoted hereafter as Ag NRs@HfO2 for convenience.

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Figure 1. Typical (a) SEM and (b) TEM images of Ag NRs coated with an HfO2 shell of two ALD reaction cycles. HRTEM images of Ag NRs coated with HfO2 shells of (c) one, (d) two, (e) three and (f) four ALD reaction cycles.

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Figure 2. Raman spectra of 1×10−2 M acridine molecules adsorbed on uncoated Ag NRs and on Ag NRs covered with HfO2 shells of 1, 2, 3, and 4 ALD cycles.

Thermal stability of Ag NRs@HfO2. To achieve a “thermally cleanable” SERS sensor, its high-temperature stability must be assessed first. Given that the optical property of a nanostructure is highly sensitive to its morphology,40, 41 the thermal stability and melting process of Ag NRs@HfO2 can thus be characterized in situ through its reflectivity changes upon annealing.28 Figure 3a thoroughly investigates the melting procedure and morphological variations of Ag NRs@HfO2 during heating at 50 °C to 700 °C (red line). Considering that melting occurs continuously instead of instantaneously, we define the extreme point of the reflectivity’s derivative (blue line) as the melting point, at which temperature the nanostructure changes most drastically.28, 42, 43 Benefiting from the high melting point of HfO2, the coated substrate unexpectedly maintained its shape at temperatures as high as 400 °C, and the melting point was ~500 °C. This result reveals the superior protection of the HfO2 shell over Ag NRs. The thermal stability was also confirmed from the SEM images in Figures 3b to 3c. Specifically, the coated substrate preserved its morphology at 400 °C and exhibited several structural changes at the melting point. This stable Ag NRs@HfO2 substrate offers us a 9



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reliable means to thermally clean it even at high temperatures of 400 °C without visible structure destruction, endowing it with great potential as a renewable SERS detector.

Figure 3. (a) Melting procedure of Ag NRs@HfO2 at 50 °C to 700 °C determined by monitoring its reflectivity variations upon annealing (red line) and the corresponding reflectivity’s derivative (blue line). Morphological changes in Ag NRs@HfO2 at (b) 400 °C and (c) 500 °C.

Recyclable SERS detection in liquids. We then investigated the SERS performance of the Ag

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NRs@HfO2 substrate using MB as the Raman probe. Figure 4a presents the SERS spectra of 10-6 M MB on Ag NRs@HfO2 with increasing the immersion time. All spectra with distinct intensity clearly reveal the specific Raman bands of MB.22, 27 Due to the outstanding plasmonic activity of silver, Ag NRs@HfO2 gave rise to considerable SERS enhancement, as evidenced by the apparent Raman signals of trace MB. Herein, the 1623 cm-1 Raman peak with strong intensity was selected to analyze the adsorption kinetics of MB molecules to Ag NRs@HfO2. Figure 4b shows that the peak intensity ascended continuously and saturated finally with increasing the immersion time. The enhancement factor (EF) of Ag NRs@HfO2 is approximately 6.1×107, and the relative standard deviation (RSD) value is about 5.14% (see Supporting Information). Therefore, the substrate with superior sensitivity and reproducibility was capable to extend the practicability of SERS technique. Given that Ag NRs@HfO2 has high thermal stability, to achieve its reusability, thermal annealing was adopted to detach the adsorbed dye molecules from the substrate surface by heating the substrate on a hot plate at 250 °C for 30 s. Regeneration efficiency was confirmed by measuring MB Raman signals before and after annealing the substrate (Figure 4c). It is evident that after heating, all the Raman peaks of MB disappeared, resulting in a substrate free of analytes. Afterward, the substrate was re-soaked in 10-6 M MB solution for 30 min, and Raman measurement was implemented again. This “detection– heating” cycle was repeated 30 times on the same substrate, and the 1623 cm-1 Raman peak intensity was exploited to test the substrate robustness in response to multiple cycles. Figure 4d shows that despite several small fluctuations of the MB signals, Ag NRs@HfO2 sustained its SERS efficiency within these “sensing–cleaning” cycles, illustrating ultrahigh stability and intriguing renewability. Moreover, reusability was also assessed by exposing the same substrate to different probing molecules, i.e., MB and CV (see Figures 4e to 4f). After MB sensing and detachment, the substrate 11



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was utilized to adsorb and distinguish CV. Then a second thermal cleaning treatment was applied to remove CV, and MB was again allowed to be absorbed onto the substrate. This cyclic detection was repeated 30 times, and the SERS activity variations of Ag NRs@HfO2 were monitored by tracking the Raman intensity at 1623 cm-1 band for MB and 1176 cm-1 band for CV. Remarkably, the substrate demonstrated nearly constant sensitivity in the 30 “detection–heating” cycles and contained no dye signs after each thermal cleaning. The Ag NRs@HfO2 composite not only exhibits similar SERS enhancement in comparison with several outstanding SERS sensors,44-46 but also illustrates its unique recyclability for SERS determination of multiple analytes. It will open up a promising avenue to reduce the costs of real-life applications.

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Figure 4. (a) SERS spectra and (b) the 1623 cm-1 peak intensity of 10-6 M MB on Ag NRs@HfO2 with different immersion times. (c) Raman spectra of 10-6 M MB on Ag NRs@HfO2 measured in multiple cyclic detections and (d) the 1623 cm-1 peak intensity variations in the 30 “detection– heating” cycles. (e) Raman spectra of 10-6 M MB and 10-5 M CV alternately adsorbed onto Ag NRs@HfO2 in multiple cyclic detections and (f) Raman intensity variations at 1623 cm-1 band for MB and 1176 cm-1 band for CV in the 30 “detection–heating” cycles.

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Recyclable SERS detection of vapor-phase molecules. Aside from serving as a recyclable substrate for dye identification in liquids, Ag NRs@HfO2 may also be used in gas sensing. To capture and sense target gases, Ag NRs@HfO2 was placed in a gas detection chamber of a homemade bubbling system, a schematic of which is presented in Figure 5a. N2 gas (99.999%) was utilized as the carrier gas, and the flow rate was controlled with a flow controller, which was fixed at 6 L/min in the experiments. 2-NAT47, 48 and 2MPy49, 50 were selected as model gases, and the vapor concentrations were regulated by varying their proportions in the solutions. Approximately 0.5 mL ethanol containing the probing molecules was allowed to evaporate per minute for the calculation of the concentrations of 2-NAT and 2MPy vapors (see Supporting Information). During detection, N2 was injected to analyte solutions, and target molecules in the vapor phase were carried out together with the N2 flow. A certain concentration of analytes passed through the detection chamber and were captured by Ag NRs@HfO2. SERS spectra were recorded simultaneously during the gas flow. The inset in Figure 5b shows the representative Raman spectra of 600 ppb (V) 2-NAT molecules collected from the Ag NRs@HfO2 substrate with a gas flowing time of 100 min. The adsorption of 2-NAT was monitored by registering its peak intensity at 1379 cm-1 as a function of time (Figure 5b). As expected, the 2-NAT signals exhibited a continuous increase with the gas flow until the saturation of the substrate surface. After 40 min of gas flow, the limit of detection (LOD) of 2-NAT was down to 20 ppb (Figure 5c), thus verifying the effectiveness of the Ag NRs@HfO2 substrate as a molecular trap and detector for vapor-phase targets. Given that thermal cleaning could detach dye molecules from the substrate without significant alteration of its morphology and SERS response, the reusability of Ag NRs@HfO2 for gas recognition was explored. Vapor flowing time of 10 min between each SERS measurement was allowed to achieve sufficient gas diffusion over the 14


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substrate and, after each 40 min, the substrate was heated on the hot plate at 250 °C for 30 s. Figure 5d shows the Raman intensity variations of 600 ppb 2-NAT at 1379 cm-1 peak during the repetition of “vapor exposure–thermal cleaning” cycles on the substrate. Ag NRs@HfO2 exhibited almost no degradation in SERS sensitivity after multiple measurements, and the composite could regenerate rapidly and completely after annealing. Eventually, the gas sensing availability and renewability of the substrate were validated through the recognition of 600 ppb 2-NAT and 600 ppb 2MPy mixtures in the vapor phase. As shown in Figure 5e, the Raman spectral features of both analytes were clearly visible after 10 min of gas flow and increased gradually within 40 min. The substrate was then readily cleaned, and its SERS response was maintained well in the subsequent sensing cycles. We thus suggest that this highly robust and versatile SERS platform act as a recyclable sensor for in situ monitoring of complex gas samples from realistic environments, such as explosives, volatile organic compounds, and chemical warfare agents.

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Figure 5. (a) Schematic of the gas sensing device. (b) SERS spectra and the 1379 cm-1 peak intensity of 600 ppb 2-NAT on the Ag NRs@HfO2 substrate as a function of gas flow time of 100 min. (c) Raman spectra of 600, 120, 60, and 20 ppb 2-NAT collected on the Ag NRs@HfO2 substrate after 16


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40 min of gas flow. (d) 1379 cm-1 peak intensity variations of 600 ppb 2-NAT during the repetition of “vapor exposure–thermal cleaning” cycles on the substrate. (e) Raman spectral features of 600 ppb 2-NAT and 600 ppb 2MPy mixtures in the vapor phase during the repetition of “vapor exposure– thermal cleaning” cycles on the substrate.

CONCLUSIONS In summary, we fabricated a sensitive and recyclable Ag NRs@HfO2 substrate and successfully applied it for SERS detection of both aqueous and vapor-phase molecules. The ultrathin (~1.6 nm) but uniform HfO2 shell ensured the SERS sensitivity and thermal stability of Ag NRs@HfO2. Accordingly, target probes adsorbed on the substrate surface could be effectively detected and then be readily removed by annealing the substrate. The composite maintained its SERS response during multiple “detection–heating” cycles, thereby demonstrating the feasibility of Ag NRs@HfO2 as a reusable SERS platform. Notably, Ag NRs@HfO2 could not only distinguish probe molecules in liquids but also extend its SERS sensing capability towards in situ determination of vapor-phase targets. This work presents an efficient, robust, and recyclable SERS sensor with advantages of high sensitivity, stability, cost effectiveness, and easy operation, which holds great potential for environmental, industrial, and homeland security applications.

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ASSOCIATED CONTENT Supporting Information Additional SERS enhancement factor calculation, the uniformity evaluation, and the calculation of vapor concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are very grateful to the financial support by the National Basic Research Program of China (973 Program, Grant No. 2013CB934301), the National Natural Science Foundation of China (Grant No. 51531006 and No. 51572148), the Research Project of Chinese Ministry of Education (Grant No. 113007A), and the Tsinghua University Initiative Scientific Research Program.

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High-Performance Real-Time SERS Detection with Recyclable Ag

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