Nanoporous Silver Film Fabricated by Oxygen Plasma: A Facile

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

Nanoporous Silver Film Fabricated by Oxygen Plasma: a Facile Approach for SERS Substrates Chaoxiong Ma, Michael J Trujillo, and Jon P. Camden ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08191 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Nanoporous Silver Film Fabricated by Oxygen Plasma: a Facile Approach for SERS Substrates Chaoxiong Ma*, Michael J. Trujillo, and Jon P. Camden* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

ABSTRACT Nanoporous metal films are promising substrates for surfaced-enhanced Raman scattering (SERS) measurement, owing to their homogeneity, large surface area, and abundant hot-spots. Herein, a facile procedure was developed to fabricate nanoporous Ag film on various substrate surfaces. Thermally deposited Ag film was firstly treated with O2 plasma, resulting in porous Ag/AgxO film (AgxO-NF) with nanoscale feature. Sodium citrate was then used to reduce AgxO to Ag, forming nanoporous Ag film (AgNF) with similar morphology. The AgNF substrate demonstrates 30 folds higher Raman intensity than Ag film over polystyrene nanospheres (d=600 nm) using 4-mercaptobenzoic acid (4-MBA) as the sensing molecule. Comparing with ordinary Raman measurement on 4-MBA solution, an enhancement factor of ~ 6 x 106 was determined for AgNF. The AgNF substrate was evaluated for benzoic acid, 4nitrophenol, and 2-mercaptoethanesulfonate, showing high SERS sensitivity for chemicals that bind weakly to Ag surface and molecules with relatively small Raman cross-section at µM concentration. In addition to its simplicity, the procedure can be applied to various materials such as transparency film, filter paper, hard polystyrene film, and aluminum foil, revealing similar Raman sensitivity. By testing the durability of the substrate, we found that the AgxO films can be 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stored in ambient conditions for more than 90 days and still deliver the same SERS intensity if the films are treated with sodium citrate before use. These results demonstrate the advantage of the proposed approach for mass production of low-cost, sensitive, and durable SERS substrates. The transferable nature of these AgNF to different flexible surfaces also allows their easy integration with other sensing schemes. KEYWORDS: nanoporous silver film, surface-enhanced Raman spectroscopy, oxygen plasma, 4-mercaptobenzoic acid, flexible SERS substrate

INTRODUCTION Surfaced-enhanced Raman spectroscopy (SERS) demonstrates remarkable sensitivity for chemical and biochemical analysis, providing vibrational information for identification and quantitative determination of various analytes.1-5 Most SERS-based sensing schemes rely strongly on the interaction of the molecules with the plasmonic substrate,6-7 and this interaction is strongly correlated with the nanoscale geometry of the noble metal surface.8-9 The SERS enhancement factor (EF), a combined result of electromagnetic and chemical enhancements, produces reliable Raman signal for analytes in mM to nM concentration.2 Substantial efforts have been devoted to increase EF by developing substrates with nanometer (nm) to sub-nm features in order to maximize coupling of the plasmonic effect to the analyte molecules.10-12 While large enhancement factors are important, reproducibility, stability, and ease of fabrication are also key metrics for the design of SERS substrates. Silver and gold nanoparticles (NPs) are widely used as SERS substrates because of their simple preparation procedure and tunable plasmonic effect, which is achieved by varying the size and shape of the particles.13-15 In 2 ACS Paragon Plus Environment

Page 2 of 27

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


addition, the small size of NPs allows their uptake into cells for biosensing and bioimaging.16-17 A major drawback of using metallic NPs is their relatively low repeatability and stability, especially when aggregation agent is added to produce hot-spots8, 18 for improved sensitivity. In some cases, this problem has been circumvented by assembly of the NPs on solid substrates such as glass slides and silicon wafers,19-22 or even filter paper,23-27 while maintaining the SERS activity. Alternatively, plasmonic nanostructures are obtained by direct deposition of metal (Au or Ag) films on templates10 with nanoscale feature such as anodic aluminum oxide (AAO) films,28 track-etched membrane,29 block copolymers,30-31 or filter paper.32-33 Additionally, nanosphere lithography (NSL) has found wide-spread use in the creation of effective SERS substrates, yielding metal film over nanospheres (AgFON)11, 34 and highly ordered arrays of nanopyramids, nanopores, and nanorods.35-37 To produce nanostructures with precisely controlled shape and geometry, advanced lithographic techniques such as electron-beam lithography38 and focus ion beam milling39 have been employed. While lithographic methods generate high-resolution nanoscale patterns with flexibility and repeatability, these techniques are limited by low throughput and the expensive equipment required. Although EF of the substrate is important, the number of molecules adsorbed to or interacting with the substrate in the probe volume is also a critical parameter in a SERS assay.40 For this reason, nanoporous metal films could be advantageous as SERS substrates owning to their large surface area.41-42 Alloy metal (e.g. Au/Ag) films have been employed to produce nanoporous substrates by dealloying of the less-noble component after deposition42-44 or through galvanic replacement.45-46 Alternatively, nanoporous metal films have been created using selfassembled block copolymer micelles as templates.41 Although these nanoporous metal films

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

represent substrates with enhanced sensitivity, uniformity, and reproducibility, the preparation is laborious and highly challenging for mass production. Herein, we report a simple but effective procedure based on O2 plasma for fabrication of nanoporous Ag films (AgNF) that exhibit strong SERS sensitivity. Oxygen plasma has been widely used in microfabrication for resist stripping and removal of organic residue.47 The process can also change the surface properties and enhance bonding strength between various substrates.48 Although commonly used for silicon, glass, and Au substrates, O2 plasma has been seldom applied to the treatment of Ag films, which could be significantly altered or even damaged by the oxidation process. Surprisingly, we found that by controlling the power and exposure time of O2 plasma on a thermally deposited Ag film, nanoporous Ag/AgxO can be created with which has feature sizes of ~100 nm. The resulting AgxO film can then be reduced to Ag via aqueous citrate solution, while the nanoscale features of the substrate are retained. Using 4-mercaptobenzoic acid (4-MBA) as the probe molecule, the AgNF produces ~30 folds higher Raman signal than the common Ag film over polystyrene nanosphere (AgFON). The significantly larger SERS signal when compared to measurements on AgFON is attributed to the combination of both increased surface area and enhancement factor. The applicability of the AgNF was also demonstrated for molecules that bind weakly to the Ag surface such as benzoic acid and 4-nitrophenol at µM concentration. As our method relies on a very simple fabrication procedure, the substrate for Ag deposition is not limited to more traditional glass supports and we demonstrate similar SERS sensitivity and reproducibility on various surfaces such as transparency films, polystyrene films, filter paper, and aluminum foil.


Page 4 of 27

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL SECTION Chemical and Materials. All reagents and materials were used as received without further purification. These include: sodium citrate tribasic, 4-mercaptobenzoic acid, benzoic acid, 4-nitrophenol, 2-mercaptoethanesulfonate, sodium perchlorate, methanol, and chloroform. Glass slides (VWR), filter paper (Whatman), polystyrene films (cut from petri dish, VWR), transparency films (Hewlett Packard) are used as substrates for silver deposition. Plain polystyrene nanospheres (600 nm, 10 w%) from Sigma Aldrich were diluted 1:1 by methanol prior to use. Chromium (Cr) chips (99.99%) and silver rods (99.999) from Alfa Aesar were used as the metal sources for thermal evaporation. Substrate Fabrication. Silver films of varying (50-600 nm) thickness were deposited by a thermal evaporator (308R, Ted Pella) using 5 to10 nm Cr as adhesion layer, which promotes the adhesion of the Ag to the glass slides or other substrate surfaces.49 The obtained Ag films were then treated with oxygen plasma (Plasma-Prep™ II, SPI) at 25 W and 100 mTorr. The optimized exposure time for the Ag film of 50 nm, 100 nm, and 200 nm or above, are 4, 8, and 10 min, respectively. This O2 plasma process yields a nanoporous Ag/AgxO film, which we denote as AgxO-NF. This film is either directly used for SERS measurements or immersed in 0.1 M sodium citrate tribasic (NaCitr) solution for 4 to 12 hours to reduce the silver oxide to silver. The chemical reduction of the AgxO by citrate is evidenced by the change of surface color from yellow-brown to white-grey, a process similar to the electrochemical roughening of Ag electrode using oxidation reduction cycle.50-51 The reduction of AgxO-NF produced AgNF, which is used for SERS measurements after thoroughly rinsing with DI water to remove the NaCitr. The AgFON substrates were prepared by evaporating a 300 nm Ag film on a monolayer of polystyrene nanospheres (600 nm) following previously established procedures.11 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Raman Measurement. Raman and SERS measurements were conducted on a homebuilt Raman spectrometer employing a HeNe laser (633 nm, Thorlabs). The laser beam (0.56 mW or 7.74 mW) was focused onto the substrate or bulk solution using an inverted microscope objective (Nikon, 20×, NA = 0.5). The scattered light was collected by the same objective and after passing through a Rayleigh rejection filter (Semrock) and dispersed in a spectrometer (PI Acton Research, f = 0.3 m, grating = 1200 g/mm). The light is then detected with a backilluminated deep depletion CCD (PIXIS, Spec-10, Princeton Instruments). Winspec 32 software (Princeton Instruments) was used to operate the spectrometer and CCD camera. For the ordinary Raman spectrum of the 4-MBA, a capillary (h=100 µm) filled with 10 mM 4-MBA in 0.1 M NaOH was used. For SERS measurements of 4-MBA, benzoic acid (BA), 4-nitrophenol (4-NP), 2-mercaptoethanesulfonate (2-MES), a 20 µL solution of given concentration was drop coated on the substrates. A rinsing step with methanol was used for 4-MBA but not BA, 4-NP, or 2-MES. Laser power of 0.56 mW and integration of 1 s were used for all SERS measurements. The reported error bars are obtained from the standard deviation of five replicates at randomly selected spots on the substrate. Electrochemical Measurement. Cyclic voltammetry (CV) experiments were conducted on a CHI bipotentiostat (842c, CH Instruments Inc.) using a platinum wire and a Ag/AgCl electrode as auxiliary and reference electrode, respectively. The surface area of the SERS substrates was specified by the open well (d=0.4 cm) of a PDMS chip. The CVs were scanned from 0 V to 0.16 V at scan rates of 10 mV/s to 200 mV/s.


Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Evaluation of Nanoporous Ag film. The thermally deposited Ag film (600 nm on glass slides) was treated with O2 plasma at 25 W for 10 mins, which results in a color change from silver to yellow-brown color (Fig. 1a inset). The SEM images reveal a homogenously nanoporous geometry with feature sizes ~100 nm on the nanoporous AgxO/Ag film (AgxO-NF) after exposure (Fig. 1a, d). The nanoscale feature formation is likely trigged by the uneven oxidation of the Ag in O2 plasma, since the Ag film obtained from thermal evaporation is not perfectly homogenous (see Fig. S1, Supporting Information, SI).52-53 The oxidation of the Ag to AgxO leads to the cracking of the surface,54 which further facilitates this uneven process due to the different density and permeability to oxidative species (O2+, O2−, O3, etc) between the Ag and AgxO. In order to produce a nanoporous Ag film (AgNF), the AgxO-NF substrate obtained after O2 plasma was treated with 0.1 M citrate solution, which has been widely used as reducing agent and stabilizing ligand in the synthesis of AgNPs. After treatment, the yellowish substrate turns to white-grey (Fig. 1b inset), indicating the reduction of AgxO to Ag. As Figs. 1b-c illustrate, the nanoporous features of the substrate was not destroyed by the reduction process. The oxidation and reduction of the Ag film was also characterized by X-ray diffraction (XRD) analysis and energy-dispersive X-ray spectroscopy (EDS), detailed in Fig. S2-S3, SI. The XRD spectra (Fig. S2) confirmed the oxidation of the Ag film to Ag2O/AgO after oxygen plasma, and complete reduction back to Ag by the citrate solution. It should be noted that the AgxO-NF spectra are dominated by peaks arising from Ag (111), (200), and (220), indicating that Ag was only partially oxidized by the oxygen plasma. The XRD spectra of the AgNF and AgxO-NF are similar with the exception of additional features in the AgxO-NF which are assigned to the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

presence of Ag2O/AgO. This interpretation is further supported by EDS analysis, which indicates the difference in abundance of oxygen in three surfaces (see Fig. S3, SI for detail).

Figure 1. (a) SEM image of the nanoporous Ag/AgxO film after oxygen plasma etching, (b) nanoporous Ag film after reduction with sodium citrate, the insets display the film as deposited on the 2 cm x 2 cm glass slide before and after reduction step, (c) tilt view (52º) of the reduced nanoporous Ag film, (d) cross section views showing multilayer structure of the nanoporous AgxO film (* the Pt layer is not part of the substrate, but only used as the protection layer in cross section milling).

Both of the resulting nanoporous AgxO-NF and AgNF were evaluated for their SERS responses by exposure to 4-MBA solution. Figure 2 compares the SERS spectra on the nanoporous films to a spectrum obtained on a AgFON under identical conditions. The most prominent features in the spectra correspond to the ring breathing modes of 4-MBA, observed at 1077 and 1590 cm-1, and are conserved for spectra obtained on all substrates.21, 55 The band centered at 1380 cm-1 (1360 cm-1 for AgxO-NF) is also characteristic of 4-MBA and is attributed to the symmetric stretch of the carboxylate group ν(COO-). The shift of the ν(COO-) band to 1380 cm-1 or 1360 cm-1 from the unbound carboxylate frequency55 of ~1410 cm-1 likely arises


Page 8 of 27

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from the COO-Ag interaction, ν(COO-Ag). This interpretation is in agreement with previous SERS spectra of carboxylate,56 benzoate,50 and 2-mercaptobenzoic acid51 on Ag substrates.

Figure 2. SERS spectra of 10 mM 4-MBA drop coated on nanoporous Ag film, AgNF (red), AgFON (black), and nanoporous AgxO film, AgxO-NF (blue) obtained at 0.56 mW for 1.0 s.

We hypothesize that this COO-Ag interaction results from the nanoporous geometry of the substrates, allowing both the thiol and COO- group of the 4-MBA to bind in a sandwich structure. Indeed, the shift of ν(COO-) to ~1360 cm-1 has been observed for 4-MBA on a sandwich architecture of the Au nanoparticles assembled on a smooth Au substrate.21 As indicated by intensity of ν(COO-Ag) in Fig. 2, the COO-Ag interaction is more prominent on AgxO-NF, which is likely facilitated by the high density of AgxO on the substrate surface. It is worthy to mention that ν(COO-Ag) at 1380 cm-1 was also seen but at a lower intensity for the SERS spectra of 4-MBA on AgFON, indicating intrinsic nanoscale feature of the substrate. Therefore, these results suggest that the carboxylate region of the spectra can serve as a proxy for nanoscale features in the evaluation of SERS substrates. While these results confirm the 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effectiveness of the nanoporous Ag film as SERS substrate, the spectral information obtained from Fig. 2 reveals the interaction of the probing molecules with nanoscale geometry of the substrates, which contributes largely to the Raman intensity and surface enhancement effect on the substrates. As shown in Figure 2, the Raman intensity obtained on AgxO-NF and AgNF are ~8 and ~30 times stronger than that on AgFONs under the same experimental conditions, respectively. Chemical reduction of AgxO-NF to AgNF by citrate results in 4-fold increase of SERS intensity. A previous study on Ag2O/Ag nanoparticles suggested contributions from the change of both chemical and electromagnetic effects to the observed Raman intensity on Ag2O and Ag surfaces.57 Additionally, this observation could be the result of more binding sites upon the reduction of AgxO to Ag, increasing the coverage of 4-MBA molecules on the substrate surface. The simplicity of our procedure compared to AgFON preparation coupled with the higher SERS response demonstrates the potential advantages of using AgNF as SERS substrates. While the above results demonstrate the advantage of AgNF for producing high SERS sensitivity, we also examine the effect of the film thickness (50 nm – 600 nm) on SERS intensity using the same fabrication procedure (Fig. S4, SI). We find that the SERS intensity increases up to a film thickness of 300 nm; however, strong SERS signal can still be obtained from Ag films as thin as 100 nm. Durability is another important criterion of the SERS substrate, especially for those fabricated from silver, which are known to degrade significantly when exposed to air. The oxidation degradation is largely avoided in our Ag substrate, since citrate reduction is employed as the last step. This stability is demonstrated by measuring the shelf life of our AgxO-NFs. Interestingly, we find that the AgxO-NF can be stored in ambient conditions for >90 days and, 10 ACS Paragon Plus Environment

Page 10 of 27

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


after citrate reduction, the substrate still provides the same SERS signal intensity of freshly prepared films (Fig. S5, SI). Enhancement Factor of AgNF. In order to estimate the enhancement factor (EF) of the AgNF substrate, the SERS spectrum of 4-MBA in Fig. 2 was compared with the normal Raman spectrum of 10 mM 4-MBA in aqueous solution (Fig. S6, SI) and the EF is determined by,40 EF =

⁄ ⁄

(1)

where ISERS and IRS are the SERS and normal Raman intensity, respectively. Nsurf and Nbulk are the number of the molecules on the substrate surface and in the bulk solution in the probe volume, respectively. Nsurf can be estimated from the molecular density of a compact monolayer of 4MBA on the Ag surface (σ = 0.5 nm/cm2)21 and the area of the focus spot, A, = η

(2)

where NA is the Avogadro constant and η is a parameter accounting for the change of surface area due to the substrate morphology. It could be smaller or larger than 1.0 depending on the geometry of the substrate. For example, 7.4% has been used for the η of nanopyramid array fabricated from NSL.58 For nanoporous metal film, the capacitive charging current (ic) obtained from cyclic voltammetry has been employed for determining η by using, 59-60 = ν

(3)

where A and Cd are the surface area and capacitance of the electrode, respectively, and ν is the scan rate. Under the same experimental conditions, Cd is a constant. The plot of ic vs ν yields the slope k, which is directly proportional to surface area of the electrode. By ratioing the k obtained



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Cyclic voltammetry of 0.1 M NaClO4 on planar Ag film (a), and AgNF (c) substrate (600 nm Ag film on glass slide) with electrode diameter of 2 mm obtained at scan rate of 10 (pink), 20 (green), 50 (blue), 100 (red), and 200 (black) mV/s. The capacitive charging current changing with the scan rate and a linear fit of the data for substrate of planar Ag film (b), and AgNF (d).

on two surfaces, the parameter (η ) accounting for the increase of surface area due to nanoporous feature of the AgNF could be obtained. Figure 3 shows the ic obtained for AgNF and the planar Ag surface. The k obtained in Fig. 3 (b) and (d) are 0.0032 and 0.021 for the planar Ag film and AgNF, respectively, yielding a η of 6.5. Nbulk can be determined from, = !

(4)


Page 12 of 27

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where the effective height h=100 µm and the solution concentration of 4-MBA c=10 mM. The EF can be thus rewritten as, EF =

"#$" %

(5)

$" η&

ISERS and IRS were determined from the peak heights of the band at ~1590 cm-1 in Fig. 2 and Fig. S6, respectively, yielding an EF of 6 x 106 for AgNF using eqn. (5). This result confirms that the nanoporous Ag film, fabricated from a simple procedure, produces SERS enhancement similar to or stronger than many substrates obtained by previously reported procedures.22-23, 33, 38, 61

Although higher EFs have been reported for SERS substrates created by advanced fabrication

techniques,10, 19, 62-63 the SERS sensitivity obtained by AgNF can be comparable or even higher due to its large surface area inherent in nanoporous films. For example, surface area of AgNF is ~100 times larger than that of nanopyramid arrays fabricated by NSL.58 As a result, our AgNF can produce an order magnitude higher SERS response than the nanopyramid arrays, despite their lower EF when compared to those reported for nanopyramid substrates.2, 34, 58 General Application of AgNFs. Interaction of the molecules with the substrate plays an important role in the surface enhancement effect and resulting measurement sensitivity.2 In order to evaluate the applicability of the AgNF substrates for molecules with a weak affinity for Ag, SERS measurements were performed using µM concentration of benzoic acid (BA) and 4nitrophenol (4-NP). Figure 4 shows the SERS spectra obtained from 10 µM BA and 0.1 µM 4NP at 0.56 mW for 1 s, both of which demonstrate strong Raman intensity at µM concentration level. The SERS spectrum of BA on AgNF is similar to those reported previously on Ag nanoparticles and Ag electrode.50 In addition to typical characteristic modes of benzene and 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

Figure 4. SERS spectra of 10 µM benzoic acid (red), 0.1 µM 4-nitrophenol (blue), and 10 µM 2-mercaptoethanesulfonate (black) drop coated on AgNF (Ag = 500 nm) obtained at 0.56 mW for 1.0 s.

1600 cm-1, a strong band is observed at 1380 cm-1, which is attributed to ν(COO-). The ν(COO-) of BA is associated with COO- and Ag interaction,50 supporting the above assignment of ν(COOAg ) for 4-MBA on AgxO-NF and AgNF. Similarly, SERS spectrum of 4-nitrophenol displays vibrational modes at 813, 858, 1111, 1132, 1156, 1240, 1324, 1390, and 1596 cm-1, which are attributed to ring breathing, C-H bending (out of plane), and C-H bending (in plane) asymmetric and symmetric modes, and ring deformation respectively, in good agreement with previous studies.64 A molecule with a relatively small Raman cross-section, 2-mercaptoethanesulfonate (2-MES), was also tested on the AgNF substrate. As shown in Fig. 4, characterized bands of 2MES at 707, 799, 1067, and 1298 cm-1 of 2-MES match well with νT(C2-S1), νT(C3-S4) νs(SO3-), and νa(SO3-) observed in a previous study.65 The high SERS intensity of these analytes


Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


demonstrates the applicability of the AgNF for sensitive detection at low concentration, even for weakly interacting analytes. AgNF on Other Supporting Surfaces. The above results are obtained from Ag films deposited on glass slides, demonstrating the effectiveness of this simple fabrication procedure for producing sensitive SERS substrates. In some cases, it would be beneficial and even critical to fabricate SERS substrates on a flexible surface instead of silicon wafers or glass slides.66 For example, SERS substrates on filter paper24-26, 32-33 developed from nanocolloids or silver films allow separation and enrichment of the analyte,27 representing a promising candidate for lowcost, disposable SERS devices. On the other hand, polymer materials could be more easily cut and remodeled to integrate with other sensing schemes.66 Since the fabrication of AgNF requires only O2 plasma treatment and a chemical reduction step in aqueous solution, the procedure could be easily applied to other substrates such as transparency films, polystyrene films, filter paper, and aluminum foil. The SEM and optical images of nanoporous Ag films on filter paper and transparency films are given as examples in Figure 5, revealing uniformly nanoscale features. The SERS spectra of 4-MBA obtained on AgNF fabricated from these materials are similar to those obtained from glass slides (see Fig. S7, SI); however, the AgNFs prepared on Al foil and filter paper exhibit higher intensity at 1380 cm1

. We hypothesize that the stronger COO-Ag interaction results from the uneven morphology of

the Al and paper substrates but more exploration on this point is required. Interestingly, Figure 6 compares the ISERS of 4-MBA on AgNF deposited on different materials and a smaller SERS response obtained on the Al and paper supports, when compared to glass substrate, might be correlated with their larger but less homogenous nanostructure coated on the uneven surface (see Fig. S8, SI). 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. SEM images of the nanoporous Ag film (500 nm) fabricated on filter paper (a) and (b), and on transparency film (c) and (d) after O2 plasma and citrate reduction. The insets display the films as deposited on the 2 cm x 2 cm substrates after O2 plasma.

Among them, AgNF fabricated on the least favorable support, filter paper, exhibits the lowest SERS intensity, which is consistent with its obviously larger feature and rougher surface. Nevertheless, we point out that AgNFs on filter paper, still produces an order of magnitude larger SERS response than AgFON. Furthermore, our substrate produces a significantly higher response (~8 times larger) and much greater shelf life than Ag-films on filter paper as fabricated using a previously reported procedure32 (see Fig. S9, SI). These results demonstrate the versatility of the proposed procedure for fabricating AgNF substrates on various supported surfaces with strong SERS sensitivity. The transfer of AgNF to materials other than glass slides not only lowers the cost of the fabrication significantly but also provides an opportunity to choose a material better suited for a specific sensing purpose. For example, compared to paperbased substrates, the SERS substrates fabricated on polymer film or aluminum foil of the same size can offer much stronger endurance, which could be critical for the integration with other sensing schemes, especially those used for the in-situ measurements. 16 ACS Paragon Plus Environment

Page 16 of 27

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. The effect of substrate material to the SERS intensity (I1590) of 10 mM 4-MBA on AgNF (Ag film of 500 nm) obtained at 1 mW for 1.0 s. The error bars are reported as a standard deviations of five replicates.

CONCLUSIONS In this work, nanoporous Ag films (AgNF) fabricated by O2 plasma etching is shown to be a robust, simple, and cost-effective SERS substrate. The best AgNF developed here produces 30 fold stronger SERS intensity than AgFONs under the same conditions and yields an EF of 6 x 106. Despite this relatively modest EF compared to those substrates produced by advanced fabrication techniques, the remarkable SERS sensitivity observed on AgNF is attributed to its uniformly nanoporous geometry, which not only provides a large surface area but also strengthens the interaction of the sensing molecules with substrates. While the SERS sensitivity of AgNF exhibits a dependence on the Ag film thickness, Ag films as thin as 100 nm still produce excellent SERS response. Strong SERS intensities were also observed from µM



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

concentrations of benzoic acid, 4-nitrophenol, and 2-mercaptoethanesulfonate confirming the general applicability of AgNF to various analytes. Lastly, the simplicity of our procedure allows for easy extension to other substrates such as transparency films, filter paper, aluminum foil, and polystyrene films, demonstrating its promise for producing low-cost, flexible, sensitive, and durable SERS substrates.

ASSOCIATED CONTENT Supporting Information. SEM image of a planar Ag film, XRD and EDS analysis of three substrates surfaces, Effect of Ag film thickness, durability of the substrate, Raman spectrum of 4-MBA in aqueous solution, SERS spectra of 4-MBA and SEM images on other supported materials, and SERS spectra on Ag film on filter paper. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]

ACKNOWLEDGEMENTS


Page 18 of 27

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This work was supported by the University of Notre Dame and the U.S. National Science Foundation under CHE-1150687. Fabrication and structural characterization of the substrates was performed using equipment housed in the Bohn group, the Notre Dame Integrated Imaging Facility and Energy Materials Characterization Facility whose generous support is gratefully acknowledged. We gratefully acknowledge H. Turley for valuable discussions. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES

(1)

Camden, J. P.; Dieringer, J. A.; Wang, Y.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P., Probing the Structure of Single-Molecule Surface-Enhanced Raman Scattering Hot Spots. J. Am. Chem. Soc. 2008, 130 (38), 12616-12617.

(2)

Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Duyne, R. P. V., Surface-Enhanced Raman Spectroscopy. Ann. Rev. Anal. Chem. 2008, 1 (1), 601-626.

(3)

Lee, M.; Lee, K.; Kim, K. H.; Oh, K. W.; Choo, J., SERS-Based Immunoassay Using a Gold Array-Embedded Gradient Microfluidic Chip. Lab Chip 2012, 12 (19), 3720-3727.

(4)

Alvarez‐Puebla, R. A.; Liz‐Marzán, L. M., SERS Detection of Small Inorganic Molecules and Ions. Angew. Chem. Int. Ed. 2012, 51 (45), 11214-11223.

(5)

Choi, H.-K.; Park, W.-H.; Park, C.-G.; Shin, H.-H.; Lee, K. S.; Kim, Z. H., Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy. J. Am. Chem. Soc. 2016, 138 (13), 4673-4684. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(6)

Ko, H.; Singamaneni, S.; Tsukruk, V. V., Nanostructured Surfaces and Assemblies as SERS Media. Small 2008, 4 (10), 1576-1599.

(7)

Park, W.-H.; Kim, Z. H., Charge Transfer Enhancement in the SERS of a Single Molecule. Nano Lett. 2010, 10 (10), 4040-4048.

(8)

Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P., Controlled Plasmonic Nanostructures for Surface-Enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41 (12), 1653-1661.

(9)

Silva, W. R.; Keller, E. L.; Frontiera, R. R., Determination of Resonance Raman Cross-Sections for Use in Biological SERS Sensing with Femtosecond Stimulated Raman Spectroscopy. Anal. Chem. 2014, 86 (15), 7782-7787.

(10)

Oh, Y. J.; Jeong, K. H., Glass Nanopillar Arrays with Nanogap‐Rich Silver Nanoislands for Highly Intense Surface Enhanced Raman Scattering. Adv. Mater. 2012, 24 (17), 2234-2237.

(11)

Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P., Metal Film over Nanosphere (Mfon) Electrodes for Surface-Enhanced Raman Spectroscopy (SERS): Improvements in Surface Nanostructure Stability and Suppression of Irreversible Loss. J. Phys. Chem. B 2002, 106 (4), 853-860.

(12)

Frontiera, R. R.; Gruenke, N. L.; Van Duyne, R. P., Fano-Like Resonances Arising from LongLived Molecule-Plasmon Interactions in Colloidal Nanoantennas. Nano Lett. 2012, 12 (11), 59895994.

(13)

Guerrini, L.; Graham, D., Molecularly-Mediated Assemblies of Plasmonic Nanoparticles for Surface-Enhanced Raman Spectroscopy Applications. Chem. Soc. Rev. 2012, 41 (21), 7085-7107.

(14)

Gu, X.; Camden, J. P., Surface-Enhanced Raman Spectroscopy-Based Approach for Ultrasensitive and Selective Detection of Hydrazine. Anal. Chem. 2015, 87 (13), 6460-6464.

(15)

Pierre, M. C. S.; Haes, A. J., Purification Implications on SERS Activity of Silica Coated Gold Nanospheres. Anal. Chem. 2012, 84 (18), 7906-7911.


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16)

Liu, T.-Y.; Tsai, K.-T.; Wang, H.-H.; Chen, Y.; Chen, Y.-H.; Chao, Y.-C.; Chang, H.-H.; Lin, C.H.; Wang, J.-K.; Wang, Y.-L., Functionalized Arrays of Raman-Enhancing Nanoparticles for Capture and Culture-Free Analysis of Bacteria in Human Blood. Nat. Commun. 2011, 2, 538.

(17)

Premasiri, W. R.; Moir, D. T.; Klempner, M. S.; Krieger, N.; Jones, G.; Ziegler, L. D., Characterization of the Surface Enhanced Raman Scattering (SERS) of Bacteria. J. Phys. Chem. B 2005, 109 (1), 312-320.

(18)

Brus, L., Noble Metal Nanocrystals: Plasmon Electron Transfer Photochemistry and SingleMolecule Raman Spectroscopy. Acc. Chem. Res. 2008, 41 (12), 1742-1749.

(19)

Wells, S. M.; Retterer, S. D.; Oran, J. M.; Sepaniak, M. J., Controllable Nanofabrication of Aggregate-Like Nanoparticle Substrates and Evaluation for Surface-Enhanced Raman Spectroscopy. ACS Nano 2009, 3 (12), 3845-3853.

(20)

Wu, Y.; Dong, N.; Fu, S.; Fowlkes, J. D.; Kondic, L.; Vincenti, M. A.; de Ceglia, D.; Rack, P. D., Directed Liquid Phase Assembly of Highly Ordered Metallic Nanoparticle Arrays. ACS Appl. Mater. Interfaces 2014, 6 (8), 5835-5843.

(21)

Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J., Surface-Enhanced Raman Spectroscopy of Self-Assembled Monolayers: Sandwich Architecture and Nanoparticle Shape Dependence. Anal. Chem. 2005, 77 (10), 3261-3266.

(22)

Yang, L.; Yan, B.; Premasiri, W. R.; Ziegler, L. D.; Negro, L. D.; Reinhard, B. M., Engineering Nanoparticle Cluster Arrays for Bacterial Biosensing: The Role of the Building Block in Multiscale SERS Substrates. Adv. Funct. Mater. 2010, 20 (16), 2619-2628.

(23)

Qu, L.-L.; Li, D.-W.; Xue, J.-Q.; Zhai, W.-L.; Fossey, J. S.; Long, Y.-T., Batch Fabrication of Disposable Screen Printed SERS Arrays. Lab Chip 2012, 12 (5), 876-881.

(24)

Abbas, A.; Brimer, A.; Slocik, J. M.; Tian, L.; Naik, R. R.; Singamaneni, S., Multifunctional Analytical Platform on a Paper Strip: Separation, Preconcentration, and Subattomolar Detection. Anal. Chem. 2013, 85 (8), 3977-3983.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(25)

Lee, C. H.; Tian, L.; Singamaneni, S., Paper-Based SERS Swab for Rapid Trace Detection on Real-World Surfaces. ACS Appl. Mater. Interfaces 2010, 2 (12), 3429-3435.

(26)

Betz, J. F.; Wei, W. Y.; Cheng, Y.; White, I. M.; Rubloff, G. W., Simple SERS Substrates: Powerful, Portable, and Full of Potential. Phys. Chem. Chem. Phys. 2014, 16 (6), 2224-2239.

(27)

Yu, W. W.; White, I. M., Chromatographic Separation and Detection of Target Analytes from Complex Samples Using Inkjet Printed SERS Substrates. Analyst 2013, 138 (13), 3679-3686.

(28)

Choi, D.; Choi, Y.; Hong, S.; Kang, T.; Lee, L. P., Self‐Organized Hexagonal‐Nanopore SERS Array. Small 2010, 6 (16), 1741-1744.

(29)

Wigginton, K. R.; Vikesland, P. J., Gold-Coated Polycarbonate Membrane Filter for Pathogen Concentration and SERS-Based Detection. Analyst 2010, 135 (6), 1320-1326.

(30)

Krishnamoorthy, S.; Krishnan, S.; Thoniyot, P.; Low, H. Y., Inherently Reproducible Fabrication of Plasmonic Nanoparticle Arrays for SERS by Combining Nanoimprint and Copolymer Lithography. ACS Appl. Mater. Interfaces 2011, 3 (4), 1033-1040.

(31)

Wang, Y.; Becker, M.; Wang, L.; Liu, J.; Scholz, R.; Peng, J.; Gösele, U.; Christiansen, S.; Kim, D. H.; Steinhart, M., Nanostructured Gold Films for SERS by Block Copolymer-Templated Galvanic Displacement Reactions. Nano Lett. 2009, 9 (6), 2384-2389.

(32)

Zhang, R.; Xu, B.-B.; Liu, X.-Q.; Zhang, Y.-L.; Xu, Y.; Chen, Q.-D.; Sun, H.-B., Highly Efficient SERS Test Strips. Chem. Commun. 2012, 48 (47), 5913-5915.

(33)

Yu, W. W.; White, I. M., Inkjet Printed Surface Enhanced Raman Spectroscopy Array on Cellulose Paper. Anal. Chem. 2010, 82 (23), 9626-9630.

(34)

Haes, A. J.; Haynes, C. L.; McFarland, A. D.; Schatz, G. C.; Van Duyne, R. P.; Zou, S., Plasmonic Materials for Surface-Enhanced Sensing and Spectroscopy. MRS Bull. 2005, 30 (05), 368-375.

(35)

Yang, S. M.; Jang, S. G.; Choi, D. G.; Kim, S.; Yu, H. K., Nanomachining by Colloidal Lithography. Small 2006, 2 (4), 458-475.


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36)

Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B., Hole–Mask Colloidal Lithography. Adv. Mater. 2007, 19 (23), 4297-4302.

(37)

Haes, A. J.; Zhao, J.; Zou, S.; Own, C. S.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P., Solution-Phase, Triangular Ag Nanotriangles Fabricated by Nanosphere Lithography. J. Phys. Chem. B 2005, 109 (22), 11158-11162.

(38)

Yan, B.; Thubagere, A.; Premasiri, W. R.; Ziegler, L. D.; Dal Negro, L.; Reinhard, B. M., Engineered SERS Substrates with Multiscale Signal Enhancement: Nanoparticle Cluster Arrays. ACS Nano 2009, 3 (5), 1190-1202.

(39)

Alexander, K. D.; Hampton, M. J.; Zhang, S.; Dhawan, A.; Xu, H.; Lopez, R., A High‐ Throughput Method for Controlled Hot‐Spot Fabrication in SERS‐Active Gold Nanoparticle Dimer Arrays. J. Raman Spectrosc. 2009, 40 (12), 2171-2175.

(40)

Le Ru, E.; Blackie, E.; Meyer, M.; Etchegoin, P. G., Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111 (37), 13794-13803.

(41)

Haupt, M.; Miller, S.; Glass, R.; Arnold, M.; Sauer, R.; Thonke, K.; Möller, M.; Spatz, J. P., Nanoporous Gold Films Created Using Templates Formed from Self‐Assembled Structures of Inorganic–Block Copolymer Micelles. Adv. Mater. 2003, 15 (10), 829-831.

(42)

Kucheyev, S.; Hayes, J.; Biener, J.; Huser, T.; Talley, C.; Hamza, A., Surface-Enhanced Raman Scattering on Nanoporous Au. Appl. Phys. Lett. 2006, 89 (5), 053102.

(43)

McCurry, D. A.; Kamundi, M.; Fayette, M.; Wafula, F.; Dimitrov, N., All Electrochemical Fabrication of a Platinized Nanoporous Au Thin-Film Catalyst. ACS Appl. Mater. Interfaces 2011, 3 (11), 4459-4468.

(44)

Huang, J. F.; Sun, I. W., Fabrication and Surface Functionalization of Nanoporous Gold by Electrochemical Alloying/Dealloying of Au–Zn in an Ionic Liquid, and the Self‐Assembly of L‐Cysteine Monolayers. Adv. Funct. Mater. 2005, 15 (6), 989-994.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(45)

Page 24 of 27

Gu, C. D.; Xu, X. J.; Tu, J. P., Fabrication and Wettability of Nanoporous Silver Film on Copper from Choline Chloride-Based Deep Eutectic Solvents. J. Phys. Chem. C 2010, 114 (32), 1361413619.

(46)

Gu, C.; Ren, H.; Tu, J.; Zhang, T.-Y., Micro/Nanobinary Structure of Silver Films on Copper Alloys with Stable Water-Repellent Property under Dynamic Conditions. Langmuir 2009, 25 (20), 12299-12307.

(47)

Hartney, M.; Hess, D.; Soane, D., Oxygen Plasma Etching for Resist Stripping and Multilayer Lithography. J. Vac. Sci. Technol., B 1989, 7 (1), 1-13.

(48)

Bhattacharya, S.; Datta, A.; Berg, J. M.; Gangopadhyay, S., Studies on Surface Wettability of Poly (Dimethyl) Siloxane (Pdms) and Glass under Oxygen-Plasma Treatment and Correlation with Bond Strength. J. Microelectromech. Syst. 2005, 14 (3), 590-597.

(49)

Rossnagel, S.; Nichols, C.; Hamaguchi, S.; Ruzic, D.; Turkot, R., Thin, High Atomic Weight Refractory Film Deposition for Diffusion Barrier, Adhesion Layer, and Seed Layer Applications. J. Vac. Sci. Technol., B 1996, 14 (3), 1819-1827.

(50)

Kwon, Y. J.; Son, D. H.; Ahn, S. J.; Kim, M. S.; Kim, K., Vibrational Spectroscopic Investigation of Benzoic Acid Adsorbed on Silver. J. Phys. Chem. 1994, 98 (34), 8481-8487.

(51)

Ma, C.; Harris, J. M., Surface-Enhanced Raman Spectroscopy Investigation of the PotentialDependent Acid− Base Chemistry of Silver-Immobilized 2-Mercaptobenzoic Acid. Langmuir 2011, 27 (7), 3527-3533.

(52)

Seeger, K.; Palmer, R., Fabrication of Silicon Cones and Pillars Using Rough Metal Films as Plasma Etching Masks. Appl. Phys. Lett. 1999, 74 (11), 1627-1629.

(53)

Mao, H.; Zhang, Y.; Wu, W.; Xue, C. In Fabrication of Nanofiber-Based SERS-Active Substrates by Oxygen Plasma Removal of Su-8 Photoresist, Nano/Micro Engineered and Molecular Systems (NEMS), 2010 5th IEEE International Conference on, 20-23 Jan. 2010; 2010; pp 993-996.

(54)

Nguyen, P.; Zeng, Y.; Alford, T., Reactive Ion Etch of Patterned and Blanket Silver Thin Films in Cl2/O2 and O2 Glow Discharges. J. Vac. Sci. Technol., B 1999, 17 (5), 2204-2209. 24 ACS Paragon Plus Environment

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55)

Talley, C. E.; Jusinski, L.; Hollars, C. W.; Lane, S. M.; Huser, T., Intracellular Ph Sensors Based on Surface-Enhanced Raman Scattering. Anal. Chem. 2004, 76 (23), 7064-7068.

(56)

Ma, C.; Harris, J. M., Surface-Enhanced Raman Scattering Study of the Kinetics of SelfAssembly of Carboxylate-Terminated N-Alkanethiols on Silver. Langmuir 2012, 28 (5), 26282636.

(57)

Han, Y.; Lupitskyy, R.; Chou, T.-M.; Stafford, C. M.; Du, H.; Sukhishvili, S., Effect of Oxidation on Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles: A Quantitative Correlation. Anal. Chem. 2011, 83 (15), 5873-5880.

(58)

McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P., Wavelength-Scanned Surface-Enhanced Raman Excitation Spectroscopy. J. Phys. Chem. B 2005, 109 (22), 1127911285.

(59)

Lakshmanan, C.; Viswanath, R.; Polaki, S.; Rajaraman, R. In Determination of Surface Area of Nanoporous Metals: Insights from Double Layer Charging, AIP Conf Proc., 2015; p 140033.

(60)

Rouya, E.; Cattarin, S.; Reed, M.; Kelly, R.; Zangari, G., Electrochemical Characterization of the Surface Area of Nanoporous Gold Films. J. Electrochem. Soc. 2012, 159 (4), K97-K102.

(61)

Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P., Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS. J. Am. Chem. Soc. 2010, 132 (1), 268274.

(62)

Chen, B.; Meng, G.; Huang, Q.; Huang, Z.; Xu, Q.; Zhu, C.; Qian, Y.; Ding, Y., Green Synthesis of Large-Scale Highly Ordered Core@ Shell Nanoporous Au@ Ag Nanorod Arrays as Sensitive and Reproducible 3d SERS Substrates. ACS Appl. Mater. Interfaces 2014, 6 (18), 15667-15675.

(63)

Theiss, J.; Pavaskar, P.; Echternach, P. M.; Muller, R. E.; Cronin, S. B., Plasmonic Nanoparticle Arrays with Nanometer Separation for High-Performance SERS Substrates. Nano Lett. 2010, 10 (8), 2749-2754.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(64)

Perry, D. A.; Son, H. J.; Cordova, J. S.; Smith, L. G.; Biris, A. S., Adsorption Analysis of Nitrophenol Isomers on Silver Nanostructures by Surface-Enhanced Spectroscopy. J. Colloid Interface Sci. 2010, 342 (2), 311-319.

(65)

Kudelski, A., Raman and Electrochemical Characterization of 2-Mercaptoethanesulfonate Monolayers on Silver: A Comparison with Monolayers of 3-Mercaptopropionic Acid. Langmuir 2002, 18 (12), 4741-4747.

(66)

Polavarapu, L.; Liz-Marzán, L. M., Towards Low-Cost Flexible Substrates for Nanoplasmonic Sensing. Phys. Chem. Chem. Phys. 2013, 15 (15), 5288-5300.


Page 26 of 27

Page 27 of 27

O2 Plasma

Ag

Citrate Raman Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


1000

1200

1400

1600

-1

Raman Shift (cm )

ACS Paragon Plus Environment

AgNF

Nanoporous Silver Film Fabricated by Oxygen Plasma: A Facile

Recommend Documents