Plasmon-Mediated Surface Engineering of Silver Nanowires for

Letter pubs.acs.org/JPCL

Plasmon-Mediated Surface Engineering of Silver Nanowires for Surface-Enhanced Raman Scattering Gang Lu,*,†,‡ Haifeng Yuan,‡ Liang Su,‡ Bart Kenens,‡ Yasuhiko Fujita,‡ Maha Chamtouri,‡ Maria Pszona,§ Eduard Fron,‡ Jacek Waluk,§ Johan Hofkens,‡,∥ and Hiroshi Uji-i*,‡,∥ †

Nanjing Tech University, Institute of Advanced Materials & Key Laboratory of Flexible Electronics (KLOFE), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), 30 South Puzhu Road, Nanjing 211816, Jiangsu, People’s Republic of China ‡ Departement Chemie, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium § Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland ∥ Research Institute for Electronic Science (RIES), Hokkaido University, N20W10, Sapporo City 001-0020, Japan S Supporting Information *

ABSTRACT: We reveal nanoscale morphological changes on the surface of a silver nanowire (AgNW) in the conventional surface-enhanced Raman scattering (SERS) measurement condition. The surface morphology changes are due to the surface plasmonmediated photochemical etching of silver in the presence of certain Raman probes, resulting in a dramatic increase of Raman scattering intensity. This observation indicates that the measured SERS enhancement does not always originate from the as-designed/ fabricated structures themselves, but sometimes with contribution from the morphological changes by plasmon-mediated photochemical reactions. Our work provides a guideline for more reliable SERS measurements and demonstrates a novel method for simple and sitespecific engineering of SERS substrate and AgNW probes for designing and fabricating new SERS systems, stable and efficient TERS mapping, and single-cell SERS endoscopy.

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and morphological changes of the metal nanostructures during measurements are, however, less investigated, even though it could cause a significant impact on the reproducibility of EF estimation. Photochemical reactions during SERS measurements, particularly, chemical changes of certain probe molecules, were recently reported.16−24 For instance, p-aminothiophenol (PATP) suffers from chemical conversion to 4,4′-dimercaptoazobenzene (DMAB) on silver surface upon laser irradiation facilitated by reactive oxygen species.16,19,25−28 However, change in metal nanostructures themselves upon light illumination and its impact on SERS signals have not yet been reported. In particular, silver is known to be chemically weak compared with gold. Nevertheless, silver has been widely used as material of SERS substrate due to the huge Raman enhancement in visible spectrum range.29,30 Therefore, the inherent vulnerability of silver in oxidative environments requires particular attention on structural changes of silvermade nanostructures during a SERS study. We reveal chemical and morphological changes of individual silver nanostructures under laser irradiation during SERS measurements. Among a wide variety of nanostructures, silver

urface plasmons (SPs), the collective oscillations of free electrons in conduction band of noble metals such as gold and silver, have become the subject of intensive study in fields of physics, chemistry, and biomedical applications.1−6 Using dedicated metallic nanostructures, SPs can lead to highly concentrated electromagnetic field in close vicinity of the nanostructures, resulting in tremendous enhancements of Raman scattering signals from the molecules at surfaces of metallic nanostructures. Therefore, this technique is commonly referred to as surface-enhanced Raman scattering (SERS).7,8 By designing nanostructures, such as nanoparticle clusters, nanogaps, and nanotips, SERS systems with strong Raman enhancements have been realized.9−13 Being noninvasive, selective, and sensitive down to a single-molecule level, SERS has attracted widespread attention, especially in the field of biosensing.2,14 To evaluate sensing sensitivity, SERS enhancement factors (EFs) are often used. The estimation of SERS EFs is, however, difficult to reproduce because it varies sample to sample and is heterogeneous even within the same sample structure. The difficulty of reproducing the exact EFs has been attributed to several factors: (1) low replicability of the nanostructures, (2) less control on the refractive indices in the microenvironment, which can influence the SP resonance, and (3) various calculation methods used for estimating EFs.15 Although these factors have been paid attention and improved in the corresponding research field, photochemical instability © 2017 American Chemical Society

Received: April 19, 2017 Accepted: June 6, 2017 Published: June 6, 2017 2774

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Figure 1. Raman spectra of PATP-functionalized AgNW measured at air−glass (a,b) and water−glass (c,d) interfaces. Measurements were carried out at the middle part of one bare AgNW with a laser power of 3000 kW cm−2.

Figure 2. Temporal evolution of Raman spectra (image on the left part) and time trajectory of Raman intensity at peak of 1439 cm−1 (red curve on the right part) were obtained at the middle part of one AgNW (a) and the cross region of two AgNWs (b), respectively, at water−glass interface. (Excitation optical density was 8 kW cm−2.)

simple and cost-effective engineering of silver nanostructures, especially for TERS/SERS probes. AgNWs (100 ± 20 nm in diameter, shown in Figure S1) were functionalized with monolayers of PATP. Laser light (532 nm) was focused at a specific position of a single AgNW (e.g., at the middle part of the AgNW). The Raman spectra were collected from the focus position under two different conditions, namely, at the air−glass or the water−glass interfaces. First, at the air−glass interface, no resolvable Raman scattering was detected even at an intense excitation power of 3000 kW cm−2 (Figure 1a,b). However, after applying Milli-Q water on top of the NWs, a clear Raman signal from PATP/DMAB was detected immediately upon irradiation with the laser light (Figure 1c,d). The same behavior was obtained at a lower laser power of several kW cm−2 (Figure S2a,b). The peaks at 1073 and 1189 cm−1 are assigned to a1 mode vibration, highlighting the large increase in the localized electromagnetic field. Peaks at 1142, 1391, 1439, and 1577

nanowires (AgNWs) are of great interest in serving as SERS substrates31 and plasmonic waveguides, which allow the propagation of visible to near-infrared light along the nanowire with a diameter below the diffraction limit (down to 50 nm in diameter)32 and remote excitation of SERS using the plasmonic waveguiding in a live cell with minimum phototoxicity.33 Therefore, in this study, wet-chemically synthesized AgNWs34−36 were used as target structures for simplifying discussion and comparison because their physicochemical properties have been fully characterized by several research groups.31,37−39 We found that AgNWs were etched sitespecifically with the aid of SPs and certain Raman probe molecules, leading to a huge increase in the detected SERS signal. This surprising observation indicates that the reengineering of the metal nanostructures during the SERS measurement contributes significantly to further boost SERS signals, adding up to conventionally reported mechanisms.40 Moreover, because this etching is site-specific, it is useful for 2775

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Figure 3. Optical transmission image (a) and SEM image (b) of a silver nanowire after >10 min SERS measurement at 1 μM PATP aqueous solution and glass interface (excitation optical density was 3000 kW cm−2).

cm−1 are originating from the vibrational modes due to DMAB formation.41 To understand this behavior, we studied the temporal evolution of Raman spectra. At the air−glass interface, no resolvable Raman spectrum from PATP molecules was observed independently of irradiation time and optical density (Figure S3). At the very intense excitation power density of 3000 kW cm−2, broad Raman spectra from contaminated carbon were obtained instead of that from PATP molecules (Figure S4). This can be understood by the fact that the polyolsynthesized AgNWs are enclosed with extremely smooth surfaces; thereby surface plasmon polaritons (SPPs) cannot be efficiently excited and localized on AgNWs due to the momentum mismatching between them and free photons.42 This leads to a negligible electromagnetic field enhancement around the AgNW. The Raman signal from the contaminated carbon is most likely far-field Raman scattering of degraded PATP molecules or the remaining surfactant used for the synthesis of AgNWs. In contrast, once the AgNW was placed at the water−glass interface, the Raman scattering from PATP molecules was observed upon irradiation and the Raman intensity gradually increased as a function of time until 200 s and decreased afterward (Figure 2a). Such increase in SERS signals was not observed for as-synthesized AgNWs in the absence of PATP. We also demonstrate that no increase in Raman intensity was confirmed with a similar compound, for example, 4mercaptobenzoic acid and 4-methoxythiophenol. This implies that the presence of PATP molecules on the surface plays an important role. The observed increase in SERS signal in time must be attributed to the generation of SERS hot spots by surface morphological changes of nanowire upon light irradiation. In other words, AgNW surface was photochemically etched upon light irradiation; otherwise, no light coupling is possible on smooth surface of AgNW. However, any obvious morphological changes were not visible either in optical transmission image or SEM image. This is most likely because the change is below the spatial resolution of the microscopes. Because the amount of PATP on AgNW is limited, the morphological change would be also limited. To verify this point, further SERS measurement was conducted at the aqueous solution of PATP and glass interface. In such a way, PATP molecules could be infinitely supplied for reactions on a AgNW surface. Indeed, under such condition, the etching

process of the AgNW after a long-time light irradiation can be clearly identified in the optical transmission image as well as in the SEM image, as shown in Figure 3a,b, respectively. The dramatic structural change observed here clearly indicates that AgNW surface was indeed etched away in the presence of PATP molecules upon light irradiation. The above results unambiguously imply the increase in SERS enhancement (Figure 2) to correlate with surface photochemical reactions between AgNWs and probe molecules. Although the morphological changes are not visible under microscopes, they are sufficiently large enough to alter localizing SPs and to boost SERS intensity. The intensity drop after 200 s seen in Figure 2a is likely due to consumption of PATP molecules on surface or due to the roughness enlargement that can no longer couple to localized surface plasmon resonances (LSPRs) at visible-light frequency. The etching rate seems related to electromagnetic intensity. At a cross point of two AgNWs where SPs will be localized efficiently,43 the Raman intensity increased rapidly and reached maximum quickly at ∼20 s (Figure 2b), in contrast with the ∼200 s for single AgNW, implying that the etching process is accelerated by higher intensity of LSPRs. Note that the intensity trends for vibrational modes of DMAB molecules (1142, 1391, and 1439 cm−1) are similar to the trends for a1 vibration modes of PATP molecules (1073 and 1189 cm−1) (Figure S5). This means that PATP was oxidized to DMAB under laser illumination before/during the creation of SERS hot spots along the AgNW. We also found that SP heating is more dominant in this surface photochemical reaction on AgNW rather than the localized electromagnetic field, as robust etching process was observed at the gap between Au and AgNW, while slow etching was found at Ag NPs and AgNWs (Figures S9 and S10). Similar Raman intensity increase was not observed in air (Figure 1a and Figure S3) or in water-free solvents such as ethanol, isopropyl alcohol, toluene, and microscope immersion oil (data not shown). In contrast, the etching was observed at high humidity over 85%. Moreover, this etching process can be accelerated under acidic condition (quick increase in Raman intensity) and retarded under basic condition (no resolvable Raman signal observed). It was reported that under acidic condition PATP molecules are more easily oxidized to DMAB,6,44 implying that conversion to DMAB is actually the necessary step for this etching process. These results indicate 2776

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Figure 4. Temporal evolutions of spectra from the AgNWs immersed in 1 μM aqueous solutions of methylene blue (a) and crystal violet (b). Blue spectra on top are typical spectra from the white-line-labeled moments, and red curves on right part show the time trajectories of Raman/ fluorescence intensities at the peak of 1605 cm−1 (methylene blue) and between 650 and 715 nm (crystal violet), respectively.

Figure 5. Sensing with the etched AgNW. (a) Temporal evolution of the Raman spectra from PATP (left part) and water molecules (right part). (b) Time trajectory of Raman intensity at the peak of 1439 cm−1 for PATP and between 3000 and 3600 cm−1 for water molecules.

that water is crucial ingredient for this SP-mediated etching phenomenon because proton plays an important role, as it has been reported that water acts as a deprotonating agent during PATP transformation.45 Furthermore, the rate of the SP-mediated etching can be accelerated by adding trace amounts of sodium nitrate (NaNO3, 10 nM) to the PATP solution, but the etching rate does not change while increasing the concentration of NaNO3 from 10 nM to 100 μM, which implies the role of NaNO3 as molecule/ion transfer medium. It is also important to state that AgNW etching is prohibited by adding an oxygen scavenger (0.5 mg mL−1 glucose oxidase, 40 mg mL−1 catalase, 10% w/v glucose) to the solution. Hence, the etching likely originated from the photo-oxidative etching of silver, following an

involved in etching. It is also worth mentioning that the surface etching occurs not only on silver nanowires but also on silver nanoparticles systems prepared with different chemical processes (Figure S11). The photochemical etching process occurs not only with PATP molecules but also with some other widely used Raman probes, including methylene blue (MB), rose bengal (RB), crystal violet, and cresyl violet. Among these probes, MB and RB showed very rapid increase in Raman scattering upon light irradiation at water−glass interface (Figure 4a and Figure S12a). On the contrary, crystal violet and cresyl violet showed strong enhancements in fluorescence signals instead of SERS signals due to the surface-enhanced fluorescence (Figure 4b and Figure S12b). As aforementioned, there is no etching in solution of 4-mercaptobenzoic acid (4MBA). However, adding a trace amount of sodium chloride into the solution of 4MBA, AgNW was etched upon light irradiation. (Figure S13). In this case, the formation of AgCl on AgNW, known to be photosensitive, likely promotes the etching process. Therefore, chloride ions that are used as the counterions of the dye molecules may have contributed to the etching process.

hv

equation of 4Ag + 4H+ + O2 ⎯⎯⎯⎯⎯⎯⎯⎯→ 4Ag + + 2H 2O. Photomolecule

oxidization of silver with UV light has been reported,46 while oxidation with visible light is not known. Nevertheless, we have observed the etching of AgNWs at various visible wavelengths, that is, 488, 532, and 632.8 nm light in the presence of PATP molecules. This further indicates that some sort of photochemical processes between PATP molecules and silver were 2777

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(FWO) (G0B5514N, G081916N, G0259.2,N G045910N, G019711N, 1520915N), KU Leuven Research Fund (GOA 2011/03, OT/12/059, IDO/12/008, C14/15/053), funding from the Belgian Federal Science Policy Office (IAP-VI/27), and JSPS KAKENHI (JP17H03003) are gratefully acknowledged. G.L. and H.Y. acknowledge FWO for postdoctoral fellowships. L.S. acknowledges the support by ITN-SUPERIOR (PITN-GA-2009-238177). M.P. acknowledges the support from the International Ph.D. Projects Program of the Foundation for Polish Science (MPD/2009/1).

As demonstrated above, silver-based SERS enhancement can benefit from minute surface photochemical etching of silver by probe molecules (PATP, MB, RB, crystal violet, and cresyl violet) under certain experimental conditions (in the presence of light, water, oxygen, chloride ions, or nitrate ions). Therefore, extreme care needs to be taken when designing, fabricating, and measuring new SERS systems. Although the minor surface change during measurements leads to an unreliable estimation on the SERS EFs, SPpromoted etching can be used for simple, easy, and site-specific engineering of silver nanostructures, in particular, of AgNWs. The above-mentioned results already demonstrated that AgNW can be shortened or roughened site-specifically, which can be used for tip fabrication for tip-enhanced Raman scattering (TERS)47 or plasmonic waveguiding endoscopy.33 In addition to the PATP molecules, the Raman signal from water molecules also increased (Figure 5), although the increase is not as significant as that for PATP (Figure 5b) because only a small proportion of the water molecules in the excitation volume experiences an enhanced electromagnetic field. This finding demonstrates that the SP-engineered AgNW can be used for SERS enhancement on general molecules. In summary, our study revealed a surface-plasmon-mediated surface engineering of AgNW by photochemical etching reactions facilitated by certain Raman probe molecules in aqueous solutions. The morphology changes at silver surface can be generated at the laser focus position, yielding a large Raman enhancement. This site-specific etching to re-engineer the nanostructures can be probed simultaneously with the SERS measurements. This work paves the way for design, fabrication, and surface engineering of new SERS systems. Furthermore, this work also opens a possibility to engineer silver nanostructures for many other applications such as TERS,47 plasmonic waveguiding, and controllable single livecell SERS endoscopy.33

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and the supporting information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpclett.7b00958. Supporting figures and laser power dependency study. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.L.). *E-mail: [email protected] (H.U.). ORCID

Gang Lu: 0000-0003-1722-0176 Haifeng Yuan: 0000-0001-6652-3670 Yasuhiko Fujita: 0000-0003-1302-1436 Johan Hofkens: 0000-0002-9101-0567 Hiroshi Uji-i: 0000-0002-0463-9659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the JST PRESTO program and a European Research Council (ERC) grant (PLASMHACAT no. 280064). Support from the Research Foundation - Flanders 2778

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