Article

Long-Standing Stability of SilverNanorod Array SubstratesFunctionalized Using a Series ofThiols for a SERS-Based SensingApplication

Ranjit De, Yong-Seok Shin, Chang-Lyoul Lee, andMyoung-Kyu Oh

Abstract

Silver nanorod (AgNR) array substrates were fabricated using an oblique angle thermal evaporation technique; their long-

term stability, surface uniformity and reproducibility, which are primary requirements for their widespread realistic appli-

cation and commercialization, were assessed using surface-enhanced Raman scattering (SERS) spectroscopy. The nanorod

surfaces were functionalized using a series of organic thiols, which range from hydrophilic to hydrophobic, to mimic various

conditions that often arise during detection of hydrophilic/phobic analytes in a realistic application field. A group of these

functionalized substrates was stored in ambient laboratory atmosphere; another in light minimized, moisture-free vacuum;

while another was stowed carefully and neatly in water to mimic realistic conditions. The effects of these storing con-

ditions were studied. A surfactant was added to the water to maintain consistent surface wetting in the third group. SERS

spectra of nanorod substrates prior to functionalization were also recorded to investigate the effect of adventitious

carbonaceous contaminants. A meticulous systematic study on the reproducibility of SERS signals was carried out:

spot-to-spot, substrate-to-substrate, batch-to-batch, day-to-day. The relative standard deviation (RSD) shown by the

SERS signals acquired from various spots of a single substrate was less than 3%, which is very similar to the only account

reported so far, in which RSD is reported as 2%. The wetting behavior of these thiol functionalized AgNR substrates are

investigated using static contact angle measurements. The functionalized substrates have exhibited excellent long-standing

stability over a period of six months when stored appropriately; hence, they are highly suitable for mass production

towards realistic application.

Keywords

Surface-enhance Raman scattering, SERS, nanorod arrays, stability, reproducibility, uniformity, relative standard deviation

Date received: 14 July 2015; accepted: 6 November 2015

Introduction

Since its inception in the 1970s, surface-enhanced Raman

scattering (SERS) spectroscopy has emerged as a widely

used powerful molecular spectroscopic technique for

ultra-sensitive detection of molecules in various chemical

environments, even when the analyte is present at trace or

single molecular level.1–2 SERS was first reported by

Fleischmann et al. in 1974, though the interpretation was

different to the present understanding.3 This was followed

by Jeanmaire and Van Duyne4 and Albrecht and Creighton,5

thereafter passing through a number of stages to result in

the present form, where enhancement factors of 106 to

1014 of Raman intensities are reported.2 Presently, it is

known that this enhancement is predominantly contributed

by long-range electromagnetic (EM) fields existing in nano-

gaps, called hotspots, between plasmonic metal nanostruc-

tures.6–9 The other mechanism that also contributes to the

total enhancement, though to a comparatively smaller

extent, is the short-range chemical effect (CM), which

Advanced Photonics Research Institute (APRI), Gwangju Institute of

Science and Technology (GIST), South Korea

Corresponding author:

Myoung-Kyu Oh, Advanced Photonics Research Institute (APRI),

Gwangju Institute of Science and Technology (GIST),

Gwangju 500 712, South Korea.

Email: omkyu@gist.ac.kr

Applied Spectroscopy

0(0) 1–13

! The Author(s) 2016

Reprints and permissions:

sagepub.co.uk/journalsPermissions.nav

DOI: 10.1177/0003702816652327

asp.sagepub.com

originates due to a charge transfer between a metallic sur-

face and the molecules attached to it or through the for-

mation of adsorbate electronic resonance over a localized

area.10–11 Of the noble metals, Au and Ag have been most

widely employed in SERS substrate fabrication: using these

as an upper limit of the SERS enhancement factor (EF) has

already been realized.2 However, no systematic studies have

been carried out to assess their surface uniformity, repro-

ducibility, and long-term stability, especially for nanorod

array substrates fabricated by the oblique thermal evapor-

ation technique.

Recently, feasibility of trace level as well as single

molecular level detection of a wide range of analytes has

become the driving force behind various attempts to fabri-

cate smart SERS substrates. There have been various stra-

tegies employed to fabricate SERS substrates with

considerable surface roughness to exploit the electromag-

netic behavior of metal nanostructures. Among the early

efforts, strategies like chemical etching, electrochemical

oxidation-reduction cycles, etc. were used but they have

been found to often suffer from poor uniformity, morph-

ology reproducibility, stability and also low surface enhance-

ment. Thereafter, most extensively studied SERS substrates

were designed using colloidal metal nanoparticles which

exhibited the desired advantages, such as biomolecule com-

patibility and high surface enhancement, but in many cases

the surface morphology was found to be vulnerable to the

laser field12 and interparticle spacing was challenging to

control; the colloidal stability was also often affected by

temperature, pH, and presence of adsorbates and reprodu-

cibility was often questioned. SERS signal intensity is highly

sensitive to the size of colloidal particles and varies from

sample to sample as reproducibility of monodisperse col-

loidal sol preparation is tedious and demands a great deal of

effort.13 Thus, high signal reproducibility could not be

achieved whereas realistic application requires it. Earlier,

it has been shown by theoretical as well as practical inves-

tigations that nanorods and nanowires have higher aspect

ratio and can cause greater surface enhancement.14 Thus,

researchers have fabricated such nanostructures using vari-

ous methods like seeding growth, electrochemical etching

and electroplating, wet chemical reaction, templating, etc.

Using Maxwell’s equations on adaptive meshes, Garcia-Vidal

and Pendry15 showed, through their theoretical investiga-

tion, that arrays of touching silver nanoparticles can result

in huge electromagnetic coupling while Zou and Schatz16

practically showed that the silver nanoparticle array struc-

tures can produce huge electromagnetic enhancement. A

combined understanding of these strategies to fabricate

nanostructures with high aspect ratios, along with arrayed

morphology, supports the present trend of fabricating

nanorod array structures to which high density has been

incorporated. Investigators have been using various meth-

ods like electron beam lithography (EBL), templates, chem-

ical or physical deposition, etc. to fabricate such

morphology. To the best of our knowledge, based on a

literature survey, there has only been one report where

Hankus et al.17 fabricated highly uniform and reproducible

nanostructure arrays by acid etching of an optical fiber

bundle and subsequently silver was deposited onto it at

less than 2% relative standard deviation (RSD) in SERS

intensity when a spot-to-spot scanning was carried out

on a single substrate. It is notable that, despite the ability

to produce metal nanostructures with a high aspect ratio

and sufficient surface roughness, most of the above-men-

tioned methods require complicated preparation protocols

and lack stability. EBL is also an expensive technique and

requires a special arrangement.

Keeping the above-mentioned constraints in mind, to

fabricate smart substrates with large and uniform SERS

active surface area, high reproducibility and long-standing

stability, we employed oblique angle physical vapor depos-

ition, which provides the opportunity to control the aspect

ratio with arrayed morphology and the use of various

source materials, such as Ag, Au, Cu, etc., as long as

these can be evaporated and a simple standard thermal

evaporator can be used. A small change in the evaporation

chamber can provide the opportunity to fabricate various

nanostructures. Thus, we have chosen this method to fab-

ricate silver nanorod arrays as SERS substrate by integrating

oblique angle deposition (OAD) technique into a standard

thermal evaporation system. This is a simple, single-step

and chemical-free green process. In our earlier publication,

we presented the details of the fabrication procedure and

the effect of substrate temperature on the morphology of

OAD nanorod arrays.18 In the present work, we have con-

fined our interest to checking the long-standing stability, uni-

formity, and reproducibility of these OAD AgNR arrays for

SERS spectroscopy-based sensing application. Earlier it has

been reported that Ag nanorod arrays fabricated via oblique

angle vapor deposition can provide large SERS active surface

area but almost no works have been performed to assess

their uniformity, reproducibility and stability. There are vari-

ous SERS substrates that exhibit high surface enhancement

and can be used as promising candidates for ultra-sensitive

trace level molecular detection. However, commercializa-

tion of these substrates has often been challenged due to

poor stability, reproducibility of morphology, uniformity and

spectral intensity. In addition, Ag surfaces are susceptible to

contamination regardless of fabrication procedure, which

can interfere in trace level molecular recognition and

reduce stability, as well as reproducibility.

Here, we have presented SERS responses of OAD AgNR

arrays and demonstrated their large area morphological uni-

formity, spectral reproducibility and long-term stability by

studying those spot-to-spot, substrate-to-substrate, batch-

to-batch and day-to-day spectral responses, respectively.

Nanorod surfaces were functionalized using various organic

thiols: namely, carboxythiophenol (CTP), methoxythiophe-

nol (MTP), aminothiophenol (ATP), nitrothiophenol (NTP),

2 Applied Spectroscopy 0(0)

thiophenol (TP), and propanethiol (PT), that ranged from

hydrophilic to hydrophobic in nature to mimic the realistic

conditions faced during the sensing of a variety of hydrophilic

or hydrophobic analytes. Static contact angle measurement

was performed to investigate the wetting behavior of these

thiol functionalized AgNR substrates. It is known that these

thiol molecules get anchored on the metal surfaces through

a strong metal–sulfur bond formation which thereby con-

structs a self-assembled monolayer (SAM) and is stable

enough to withstand drastic conditions such as the variation

of pH over a long range. SAMs can also protect metal sur-

faces from various types of adventitious materials that are

often present in the immediate ambient atmosphere. A

group of all these functionalized substrates was stored in

an ambient laboratory atmosphere, another in a light-mini-

mized, moisture-free vacuum chamber, another in water to

mimic some realistic applications; a surfactant was used to

maintain uniform surface wettability.

Experimental

Fabrication of Silver Nanorod Arrays

Oblique angle deposited (OAD) silver-nanorod (AgNR)

arrays were fabricated on a silicon wafer using a bell-jar

type thermal evaporator (GVtech, Inc.). Prior to deposition,

the pressure in the vacuum chamber was brought down

below 10�6 Torr and maintained the same throughout the

whole fabrication process. Substrate cooling system

equipped with copper tube, through which water circu-

lated, was employed and the temperature was maintained

at 20 �C to reduce the heating due to the blackbody radi-

ation from the tungsten boat used as silver source holder.

The vapor deposition angle (flux angle), which is the angle

between the normal substrate surface and the vapor flux

direction, was maintained at 86� 1�. Appropriately sized

(1 cm� 1 cm) pieces of silicon (100) wafer were cleaned

in a solution of H2O:H2O2:NH4OH¼ 5:1:1 at 70 �C for

15 min followed by water rinsing. Highly pure Milli-Q

water was used wherever required. A schematic presenta-

tion of AgNR fabrication by OAD process is in Scheme 1.

A layer of silver film with thickness �50 nm, monitored by

quartz crystal microbalance (QCM) placed at normal inci-

dence to vapor flux, was first deposited on the cleaned Si-

wafer keeping the flux angle at 0� on which the OAD AgNR

arrays were fabricated. The metal source used was silver

grains of high purity (99.999%) procured from Alfa Aesar.

Evaporation rate was maintained at 5 A/s and the overall

thickness of silver deposited was constantly monitored by

QCM. Substrates were allowed to cool in vacuum before

removing from the chamber after the desired AgNR film

thickness of about 3 mm was achieved. These substrates

were stored in a clean vacuum chamber to protect from

adventitious materials prior to functionalization whenever

required.

Characterization of Silver Nanorod Arrays

The morphology of AgNRs fabricated via the above-men-

tioned OAD method was characterized by field emission

scanning electron microscopy (Hitachi, S-4700). To assess

crystallographic purity, X-ray diffraction (XRD) spectrum

was obtained using a computer controlled Rigaku diffract-

ometer with a Cu radiation (�¼ 0.15406 nm) running at

40 kV and 40 mA. Surface plasmon property was character-

ized by recording spectra in reflection mode using a

ultraviolet–(UV-Vis) spectrometer (Jasco, V-570).

Functionalization of Silver Nanorod Arrays

AgNR array substrates were functionalized in groups by six

thiols: namely, 4-aminothiophenol (97%), 4-nitrothiophenol

(80%), 4-carboxythiophenol (99%), thiophenol (99%), 4-

methoxythiophenol (97%), and 1-propanethiol (99%)

which were purchased from Aldrich. The substrates were

immersed in 10 mM ethanoic solution of thiols for about

12 h. Absolute ethanol, HPLC grade was purchased from

Fisher. Functionalized OAD AgNR array substrates were

rinsed with copious amount of ethanol to remove any phy-

sisorbed thiol molecules from the substrate to result in a

monolayer formation. These functionalized substrates were

then dried under a gentle stream of nitrogen gas. All func-

tionalization processes were carried out at room tempera-

ture. The morphology of nanorods remained intact after

functionalization. A group of the functionalized substrates

were stored in the ambient laboratory atmosphere;

another was stored at room temperature in moisture

Scheme 1. Schematic presentation of oblique angle deposition

technique used for AgNR array fabrication.

Oh et al. 3

free vacuum chamber wrapped with silver foil to minimize

effect of UV light while another group was stored in 5 mM

aqueous solution of sodium dodecylsulfate (�98.5%, Sigma).

Substrates fabricated and functionalized in the same batch

were considered in all these cases to compare the effect of

storing condition on substrate stability, uniformity, and spec-

tral reproducibility. Prior to the measurement of SERS spec-

tra, each time the substrates stored in aqueous SDS solution

were gently rinsed with water and dried.

SERS Spectra Measurement

Surface-enhanced Raman spectra of thiol-coated AgNR

array substrates were acquired using a home-built optical

fiber-coupled micro-Raman spectrometer employing a

computed tomography spectrometer (Spectro, Inc.) with

a cooled Si-CCD array detector (Andor, Inc., model

name: iVac). The focal length of the spectrometer is

20 cm and the resolution of the spectrometer is 0.3 nm

(10.6 cm�1) at 532 nm when an optical fiber bundle (inlet:

circular with 700 mm diameter, outlet: slit with 200 mm

width ) is used to couple the Raman signal to the spectrom-

eter. The wavelength of the excitation source used is

532 nm (continuous wave diode laser) and the linewidth

of the laser is smaller than 0.3 nm at full width half max-

imum. The spot size of the laser beam focused on Ag sur-

face was �30 mm in diameter. Rayleigh scattering was

reduced using a Notch filter (Semrock, Inc.) having the

bandwidth as 17 nm and optical density >6. The objective

used has the numerical aperture value of �0.5. Both the

incidence and collection angles of pump laser and Raman

signal were normal to the substrate. A motorized xy-trans-

lation stage was used to scan the substrates. Spectra were

collected over the range 3000 to 200 cm�1. For each spec-

trum, a minimum of three spectra were recorded and the

average was considered for analysis. The laser power was

10 mW and the acquisition time was one second for each

spectrum acquisition. It is worth mentioning that no pre-

processing has been used for correction of spectra; hence,

all the spectra used in this study are referred to as raw. The

AgNR substrate size used in this study was 1 cm� 1 cm and

each substrate was scanned over an approximate area of

60 000 mm2 (30� 2000 mm2) during spectral line scan with

spots considered randomly.

Contact Angle Measurements

The wetting behaviors of thiol functionalized nanorod sub-

strates were investigated through contact angle measure-

ments by gently dispensing 2 mL of molecular grade water

using a micropipette on the substrate surface, which was

mounted on a stage. Drop size was maintained the same in

all the substrates as the contact angle is also dependent on

the drop size.19 The pictures were taken using a common

camera.

Results and Discussion

Morphology of AgNR Arrays

SEM images (top and side view) of Ag thin film and AgNR

arrays fabricated on this thin film via thermal evaporation,

using the OAD technique, are shown in Figure 1a and b

respectively. According to these images, the average length

of these nanorods is 800� 50 nm with an average diameter

of 70� 10 nm. The density of these nanorods was found to

be 20� 1 rods/mm2 with an average distance of

120� 20 nm between two nanorods. The nanorods were

aligned at 70� 5� with respect to the substrate surface

normal. The individual rods were mostly cylindrical with

random nanoscale irregularities on their surfaces; which

often contributes to surface enhancement, as was also

observed by an earlier group.20 A detailed study of morph-

ology and SERS response of the AgNR array substrates

fabricated at various substrate temperatures are presented

elsewhere.18

The crystallinity of a fabricated AgNR array was studied

by investigating the obtained XRD spectrum presented in

Figure 2. The diffraction peaks observed at 38.10�, 44.22�,

64.50�, and 77.40� corresponding to (111), (200), (220),

and (311) planes suggests the AgNR to have a face-centered

cubic (fcc) lattice structure. The Ag is polycrystalline in

Figure 1. SEM images of (a) silver thin film and (b) silver nanorod arrays. The top (scale bar 2 mm) and cross-sectional (inset, scale bar

1mm) view.

4 Applied Spectroscopy 0(0)

nature with a stronger (111) phase suggesting dominant

out-of-plane growth along this crystal orientation.21 Thus,

these Ag nanorods fabricated via thermal evaporation tech-

nique are free from crystallographic contaminations as no

counterfeiting diffraction is observed.22

According to the UV-Vis reflectance spectrum presented

in Figure 3, the fabricated AgNR arrays have shown plasmon

resonance over a wide range of wavelengths which thereby

justifies the choice of pump laser at 532 nm (indicated by

vertical dashed line) during SERS line scan. This wide range

of wavelengths in the long tail of the plasmon band should be

the consequence of notable presence of nanorods with

larger average diameter and long lengths along the major

axis (high aspect ratio), as also seen in the SEM image

(Figure 1b), because the rate of radiative decay of surface

plasmon mode increases for larger sized particles.23

It had also been previously pointed out by various

researchers that SERS activity of nanorod array substrates

strongly depends on the optical response of their struc-

tures. Zhao et al.,24 for example, had shown that the optical

response of nanorods deposited on glass substrates

depends on the length of the nanorods; in another work,

Liu et al.25 have thoroughly investigated the dependence of

optical response on the nanorod length and deposition

angle. Thus, the reflectance spectrum is informative when

characterizing the optical properties of the nanorod array

substrates fabricated by thermal evaporation. Since the

nanorod arrays were deposited on a silver film which was

on a Si wafer, we could only characterize the optical reflect-

ance of the nanorod substrates. The shape of the reflect-

ance spectrum, shown in Figure 3, is observed to be

consistant with that shown by Liu et al., which is charac-

teristic of nanorods with such length, diameter and density,

as observed in our samples through SEM images (Figure

1b). Their group also investigated the reflectance spectra

of silver nanorods with different lengths, fabricated using a

fixed angle of deposition as well as a fixed length at different

deposition angles, and finding that reflectance depends on

both the length and deposition angle. The larger the depos-

ition angle used, the lower the reflectance was observed

and the SERS enhancement factor increases with the

decrease of reflectance at a particular excitation wave-

length. The optimized deposition angle maintained during

our fabrication process was the largest one that we could

use to produce nanorods with optimal morphology, which

is thus evident through the reflectance spectrum obtained

from the fabricated nanorod substrate.

Effect of Contamination

The effect of contamination of a nanorod on SERS spectra

are presented in Figure 4. These spectra show how con-

tamination by adventitious materials can affect the sub-

strate. Figure 4a presents the SERS spectrum of bare

(without functionalization) AgNR array substrate acquired

immediately after removal from the fabrication chamber;

Figure 4b represents a SERS spectrum which was acquired

from an AgNR array substrate functionalized by propa-

nethiol immediately after fabrication. The broad peak

observed at around 1618 cm�1 in Figure 4a may be attrib-

uted to the organic materials that might have outgassed

during fabrication in the chamber and this disappeared in

Figure 4b after functionalization, which thereby suggests

that those organic materials were substituted by propa-

nethiol molecules during the process of functionalization.

Another bare AgNR array substrate was kept in the ambi-

ent atmosphere for about a month prior to functionaliza-

tion: the SERS spectrum of that bare AgNR is presented in

Figure 4c, while Figure 4d represents the spectrum

acquired after functionalizing the same substrate by propa-

nethiol. A comparison of spectra in Figure 4c and d shows

that in this case the process of functionalization could not

substitute some of the contaminant materials completely as

Figure 3. UV-Vis reflectance spectrum of Ag nanorod arrays

with thin Ag film underneath. The excitation wavelength used for

SERS measurements is indicated by the dashed line.

Figure 2. XRD spectrum of Ag nanorod array with stronger

(111) phase.

Oh et al. 5

the peaks of those materials interfered with the spectrum

of this propanethiol functionalized substrate (indicated by

dashed lines). The substrate stored in an ambient labora-

tory atmosphere had various peaks in its SERS spectrum at

480, 928, 1188, 1383, 1601 and 2852 cm�1. It is reported

that the main peaks (1383 and 1601 cm�1) are primarily due

to the graphitic carbonaceous contaminant materials.26

Graphite mainly has two characteristic peaks at

1340 cm�1 (D band) and 1580 cm�1 (G band) and it is

also known that G band shifts towards higher frequency

as the dimension of graphitic materials decreases.27 The

broad peak at 2852 cm�1 is due to n(C–H) modes of ali-

phatic alkanes.28 The other bands in the spectrum might be

attributed to the organic impurities which most likely out-

gassed in the vacuum chamber during fabrication, as men-

tioned earlier. Bare metal surfaces have a tendency to

adsorb adventitious materials as such adsorption can

reduce the free energy of the interface between the

metal surface and ambient environment.29 Thus, prior to

functionalization it is important to store substrates in a

controlled environment, whenever required, to protect

those from any undesired morphological changes that

may arise due to atmospheric humidity, temperature, con-

taminants or UV radiation. It is known that, once functio-

nalized, the metal surfaces can remain protected from such

adventitious materials as long as the surface is covered by

self-assembled monolayers (SAMs) formed during the func-

tionalization process.

Earlier, the presence of carbonaceous contaminants on

SERS-active substrates has also been observed by research-

ers and found to be present irrespective of fabrication

method.30 This often poses challenges by affecting the

reproducibility of measurements and limiting the sensitivity

of substrates in high sensitive analytical applications. Various

protocols, such as electrochemical cleaning,31 solid CO2

snow jet,32 Ar plasma33–35 or O3 treatment,36 to name

some, have been used to clean the surfaces but it is to be

mentioned that there have been none which can success-

fully remove the contaminants eliminating background con-

tamination on SERS substrates completely. Thus, while

these processes could be adopted it is to be noted that

during the cleaning process the SERS signal intensity can get

reduced33 and there is a risk of distortion in SERS-sub-

strates’ nanostructured morphology, if proper attention is

not paid, which is often effort demanding. A more detailed

study on this was beyond the present scope and might be

considered in an upcoming work.

SERS Spectra of AgNR Array SubstratesFunctionalized Using Various Thiols

The molecular structures of thiols used in this study are

presented in Scheme 2. Representative spectra of AgNR

array surfaces functionalized by aminothiophenol (ATP),

nitrothiophenol (NTP), carboxythiophenol (CTP), thiophe-

nol (TP), methoxythiophenol (MTP), and propanethiol (PT)

are shown in Figure 5. The absence of any spurious peak in

the spectra of functionalized substrates suggests that the

thiols are organically pure and if there was any adventitious

material adsorbed on the metal surface, it can be assumed

that such materials were substituted by thiols owing to

their high affinity towards silver, as discussed above. It has

been shown earlier that surface enhancement is greater in

nanorod substrates with underlying Ag thin film in compari-

son to the nanorod substrate without such film;37 hence,

the former is employed in all the experiments here to

Figure 4. (a) SERS spectrum of bare AgNR array acquired immediately after fabrication, (b) SERS spectrum of AgNR array functio-

nalized by propanethiol immediately after fabrication, (c) SERS spectrum of bare AgNR array acquired after keeping it at ambient

atmosphere and (d) SERS spectrum of AgNR array functionalized by propanethiol after keeping the bare AgNR at ambient atmosphere.

6 Applied Spectroscopy 0(0)

achieve maximized enhancements. The Raman enhance-

ment factor (EF) of fabricated AgNR substrates and func-

tionalized by different thiol probe molecules are calculated

using the following equation.

EF ¼ISERS=IbulkNSAM=Nbulk

ð1Þ

where ISERS and Ibulk are the intensities of Raman band

measured in the SERS-active medium and bulk state,

respectively. NSAM and Nbulk are the numbers of probe

molecules in the self-assembled monolayer (SAM) and

bulk state contributing to Raman signal, respectively.18

The ISERS=Ibulk values for each substrate were obtained

from their corresponding SERS spectrum by investigating

a particular peak intensity enhancement in comparison to

same peak intensity of the bulk probe molecules. The pos-

itions of the peaks involved in this calculation are men-

tioned in the parenthesis of each probe molecule. The

area occupied by each TP molecule is considered as

Figure 5. Representative SERS spectra of AgNR arrays functionalized by: (a) nitrothiophenol, (b) aminothiophenol, (c) carboxythio-

phenol, (d) thiophenol, (e) methoxythiophenol, and (f) propanethiol. Some strong peaks are specified in each spectrum.

Scheme 2. Molecular structures of thiols used in this investigation.

Oh et al. 7

0.2 nm2, which is the well known value, and is considered

the same for all the remaining molecules for simpli-

city.18,38–39 Though the morphology of the AgNR film sur-

face is highly rough, still we have considered the surface as

flat, which makes the calculation simple and expresses the

sensitivity or detection capability, explicitly. Thus, the EFs of

the substrates functionalized by NTP (at 1336 cm�1), ATP

(at 1470 cm�1), CTP (at 1586 cm�1), TP (at 1573 cm�1),

MTP (at 1591 cm�1) and PT (at 780 cm�1) are estimated

as 1.7� 108, 1.4� 108, 1.1� 108, 9.0� 107, 6� 107, and

2.1� 107 respectively. It is to mention that SERS enhance-

ment is caused by a joint contribution of CM and EM effect

where EM can contribute from 104 to 1012 of the total, the

prominent contributor, and CM can contribute from 100 to

102 of the total.39

Uniformity and Reproducibility of SERS Substratesand Signal Intensity

Spot-to-Spot. The uniformity of SERS signal intensities

and nanorod morphology were investigated. For this pur-

pose, the surface of a thiol-functionalized substrate was

thoroughly scanned out of which randomly selected spots

were considered to record SERS spectra. It is important to

mention that each spectrum is an average of a minimum of

three spectra and the spots were randomly selected in a

relatively large area; same process was repeated for all the

six types of thiol functionalized substrates. This spot-to-

spot signal intensity variation has shown the relative stand-

ard deviation (RSD) less than 3% which is very close to the

value observed by Hankus et al.17 and better than the

values shown by nanorod arrays fabricated using electron-

beam/sputtering evaporation method, where this RSD

value was found to vary in the range of 8 to 11% obtained

using trans-1,2-bis(4-pyridyl)-ethene as Raman probe mol-

ecules on the nanorods of comparable average length

868 nm.37 This wide range is attributed to the variations

in the length, which was calculated to be about 11%. In

our work, the variation of nanorod length, calculated via

similar way by investigating of AgNR SEM images (side view)

in different substrates,37 was found to vary less (�7%) in

comparison to the above-mentioned value, and hence pro-

duced nanorod arrays with better homogeneity in terms of

both the length and distribution along the silver thin film

surface on the silicon wafer. Thus, this high morphological

homogeneity of the nanorods is the reason for such good

spot-to-spot reproducibility. The results were verified using

various thiol molecules and are summarized in Figure 6 and

Table 1. A representative figure (Figure S1) can also be

found in the supporting information. This shows that the

disparity in SERS signal intensity at different spots across

the fabricated substrate is within the acceptable range and

the variation of nanorod morphology along a substrate is

very low, which is also visible in the side view of the SEM

image (Figure 1b, inset).

Substrate-to-Substrate. The nanorod fabrication

chamber, where the silver source was evaporated, can

house several (1 cm� 1 cm) substrates, enabling mass pro-

duction in a single batch. Hence, it is important to confirm

whether all substrates can produce similar SERS signal

intensity and bear comparable AgNR morphology. To evalu-

ate this, six such (1 cm� 1 cm) substrates fabricated in the

first batch, were randomly selected and functionalized

maintaining same condition parameters. An average of

three SERS signals was accepted in each substrate and the

evaluation was carried out considering the bands at

1079 cm�1 and 1573 cm�1 for TP functionalized substrates

which showed RSD values of 4.88% and 4.94%, respectively

(Figure 7 and Table 1). A similar investigation was carried

out using the remaining five thiols. As we moved from sub-

strate to substrate functionalized by the thiols used in this

investigation, the repeatability of SERS signal intensity was

found to show RSD values of less than 5% (Table 1). This

demonstrates that the repeatability of SERS signal intensity,

and thereby the AgNR morphology, was satisfactory even

when randomly picked up substrates, prepared in a batch,

were considered. This also supports the great possibility of

mass production of AgNR array substrates using the ther-

mal evaporation technique.

Batch-to-Batch. In this approach, four substrates

were collected from four different fabrication batches and

every effort was made to keep condition parameters similar

during the AgNR array fabrication in each batch. The pur-

pose of this was to understand the reproducibility of the

SERS signal provided by substrates which were fabricated in

different batches, which is challenging. Here too, the same

SERS signal bands mentioned above were considered for

analysis. It was found that batch-to-batch reproducibility of

signal intensity for TP functionalized substrates has shown

the RSD value of 5.81% and 5.89% for the bands at

1079 cm�1 and 1573 cm�1, respectively (Figure 7).

Substrates fabricated in four different batches and functio-

nalized by ATP, NTP, CTP, MTP and PT were also

Figure 6. Spot-to-spot SERS signal intensity variation in thiol

functionalized AgNR substrates.

8 Applied Spectroscopy 0(0)

investigated and the highest RSD value was found to be

about 7% as listed in Table 1. It is notable that, in all the

above cases, spectra obtained from different substrates, or

various spots of a single substrate, varied only in intensity,

while the basic spectral shape remained unaltered. This also

shows that, to meet the practical need-based production,

fabrication of nanorod array substrates can be considered

from various batches as long as similar condition param-

eters during fabrication process are maintained.

Day-to-Day. One of the greatest challenges in the field

of widespread application of SERS substrates is lack of long-

term stability. There are various substrates in dispersion as

well as on solid dry support fabricated via various methods

and many of them can show extremely high surface

enhancement as well as trace-level sensitivity. But their

popularity in realistic application is always challenged due

to poor long-term stability. To evaluate long-standing sta-

bility and the effect of storing conditions, the functionalized

substrates were stored in three different environments: (i)

ambient laboratory atmosphere, (ii) moisture free, light

minimized vacuum chamber, and (iii) aqueous surfactant

solution. The substrates considered for these investigations

were fabricated in a single batch to ensure similar nanorod

morphology and their functionalization was also done in a

single batch to maintain same condition parameters. After

their SERS responses were recorded, one group of sub-

strates functionalized by different thiol molecules was

stored in ambient conditions, another was stored in a

vacuum chamber, with a third in aqueous surfactant solu-

tion of sodium dodecylsulfate (5 mM). This surfactant was a

judicious choice as it does not show any peak in the inves-

tigated spectral region and hence is not expected to affect

the intensity and sensitivity of the functionalized substrates.

The aqueous surfactant solution assists in preserving uni-

form wettability and mimics many realistic applications.40

SERS spectra were recorded using the substrates from allTab

le1.

The

RSD

valu

es

of

SER

Ssp

ect

raac

quir

ed

from

vari

ous

AgN

Rar

ray

subst

rate

sfu

nct

ional

ized

by

diff

ere

nt

thio

ls.

Thio

lsN

TP

AT

PC

TP

TP

MT

PPT

AgN

RSu

bst

rate

s

%R

SDva

lues

at

1336

cm�

11576

cm�

11188

cm�

11470

cm�

11144

cm�

11586

cm�

11079

cm�

11573

cm�

11081

cm�

11591

cm�

1780

cm�

11088

cm�

1

Spot-

to-s

pot

2.5

12.4

42.5

32.4

62.5

52.4

62.4

92.4

42.8

32.8

72.3

22.3

7

Subst

rate

-to-s

ubst

rate

4.1

54.2

04.3

14.3

64.6

44.5

94.8

84.9

43.9

33.8

84.3

34.2

3

Bat

ch-t

o-b

atch

6.7

76.7

85.8

95.9

76.9

06.9

65.8

15.8

96.0

96.1

56.1

16.0

6

Day

-to-d

ay11.6

911.5

411.2

411.3

610.8

910.7

79.9

410.0

19.1

29.0

58.6

98.7

3Figure 7. Relative intensity of thiophenol functionalized SERS

substrate fabricated in four different batches (RSD¼ 5.89%). Six

bars in Batch 1 represent relative intensities of six substrates

randomly selected from first batch (RSD¼ 4.94%).

Oh et al. 9

three groups at an interval of a few days and continued for

about three months for the first group stored at ambient

laboratory atmosphere, while it was continued for six

months to generate a lifetime plot. The substrates kept in

ambient atmosphere showed their stability for about three

months. One of the reasons for this comparatively low

stability could be due to the aerial oxidation of the sub-

strates, which often can occur in the presence of UV light.

Just to mention, the relative intensities in all these cases

were produced by comparing the signal intensities of differ-

ent days with respect to that of the first day. The life-time

plots for the second and third groups produced in the same

way are presented in Figure 8. This figure illustrates the

long-term stability of all six thiol-functionalized substrates

stored in the remaining two conditions: one in a moisture

free, external light minimized vacuum chamber (Figure S2);

the other in aqueous surfactant solution (Figure 8). The

scattering of the points originated around the straight line

could be due to variation in positioning the substrate during

the record of SERS spectra: it is not possible to maintain

the same spot when spectra is recorded on day-to-day

basis. To minimize error, thorough scanning of the substrate

was performed during the record of spectra and an average

was considered. The slopes for each of these substrates

provide a measure of their long-term stability. It is also

observed that the reproducibility of the SERS intensity pro-

duced by the SERS substrates stored in surfactant solution

provided similar longevity to the substrates stored in a

chamber, which suggests that such AgNR array substrates,

fabricated by oblique angle thermal evaporation, can be

used successfully for realistic application purposes. It also

shows that storing of such substrates in aqueous solution of

SDS could also be a decent option, as long as the water

quality is good with low dissolved oxygen and it can protect

Figure 8. Lifetime plot showing long-term stability of AgNR array substrates functionalized by (a) NTP, (b) ATP, (c) CTP, (d) TP,

(e) MTP, and (f) PT. Functionalized substrates were stored in surfactant solution.

Table 2. Adjusted R2, slope and % RSD for the lifetime plot of thiol functionalized AgNR array substrates.

Thiols

(Peak position)

Surfactant solution Vacuum Ambient atmosphere

Adjusted

R2Slope

(Rel. Int.) % RSD

Adjusted

R2Slope

(Rel. Int.) % RSD

Adjusted

R2Slope

(Rel. Int.) % RSD

NTP (1576 cm�1) 0.685 –0.001 10.46 0.922 –0.002 10.56 0.881 –0.012 9.89

ATP (1470 cm�1) 0.864 –0.002 10.24 0.862 –0.001 10.57 0.861 –0.013 10.01

CTP (1144 cm�1) 0.874 –0.001 10.57 0.887 –0.001 10.97 0.884 –0.011 10.61

TP (1573 cm�1) 0.876 –0.002 10.33 0.898 –0.002 11.06 0.816 –0.012 10.49

MTP (1591 cm�1) 0.750 –0.001 10.98 0.832 –0.002 10.79 0.849 –0.014 10.71

PT (1088 cm�1) 0.932 –0.004 11.31 0.971 –0.003 11.74 0.892 –0.017 11.51

10 Applied Spectroscopy 0(0)

the substrate from aerial oxidation, which is often acceler-

ated by light. The Adjusted R2, slope and % RSD values of

the lifetime plots for all the substrates are summarized in

Table 2. As SERS signal intensity is concerned, these slope

values for the second and third groups showed that the

substrates are stable for more than six months, though

the slope shown by the propanethiol functionalized sub-

strate was found to be slightly lower; which shows that

the spectral reproducibility is slightly lower in the case of

propanethiol functionalized substrates in comparison to the

other thiol molecules, though overall stability was found to

be long enough.

Hydrophilic/Hydrophobic Probe Moleculesand Wetting Behavior

In addition to all the above-mentioned factors, the choice

of different probe molecules with variable hydrophilic as

well as hydrophobic characters represents the various real-

istic conditions often faced during detection of numerous

analytes; hence it is important to know the stability and

reproducibility of SERS substrates functionalized by such

molecules. As these analytes could be hydrophilic as well

as hydrophobic, during the choice of analyte specific probe

molecules for functionalization of our SERS substrates, a

series of thiol molecules (Scheme 2), with variable hydro-

philic and hydrophobic characters, were chosen and their

long-term stability and reproducibility investigated. The

static contact angle measurements have shown the

change in wetting behavior of the AgNR substrates functio-

nalized by different thiols (Figure 9). The AgNR substrates

functionalized by PT, and TP have shown very high contact

angles—close to 180�, showing their hydrophobic nature.

On the other hand, AgNR substrates functionalized by CTP,

MTP, ATP, NTP showed relatively lower contact angles,

especially with a CTP functionalized substrate, where it

was lower than 30� – showing their hydrophilic surfaces

and the spreading of the water drop to be comparatively

faster. By the definition of contact angle, it is known that

with an increase in hydrophobicity the contact angle

increases but decreases with the increase in

hydrophilicity.41 It is also well known that the wetting

behaviors of these types of pristine nanostructured metallic

surfaces are highly dependent on the aspect ratio of the

nano-array.33,42 According to the aforementioned analysis,

where the aspect ratios of all AgNR array substrates used

here are almost same, it is clear that the variation in hydro-

phobicity and hydrophilicity is the result of surface functio-

nalization caused by different thiols; this type of strategy

can be successfully used for the sensing of a number of

analytes with variable hydrophobic and hydrophilic behav-

ior. Thus, we could correlate the wetting behavior of dif-

ferent thiols on AgNR substrates using contact angle

measurements.

On the whole, this study has shown that in all these

cases the substrates were similarly stable over the span

of six months and produced reproducible spectra

(Table 2) which present these substrates to be highly suit-

able for various analyte detections.

It is also to be noted that the change in distance between

source and substrate inside the evaporation chamber can

bring variation in signal intensity, and this primarily could

be attributed to the variation in nanorod length and morph-

ology.43–44 Effective distance between the source and sub-

strate in our fabrication chamber was maintained at�35 cm.

The effect of source to substrate distance is beyond the

scope of this work and will be pursued in a forthcoming

communication.

These results suggest that the nanorod array substrates

fabricated by oblique-angle thermal deposition have shown

large surface uniformity, high reproducibility, and long-term

stability in terms of nanorod morphology, as well as SERS

signal intensity. These findings also comply with the results

found in earlier investigations.37,45 Thus, these substrates

are highly reliable and analytically sensitive to be used as

SERS substrates with good performances. A further study is

underway to apply these nanorod substrates in trace level

hazardous gas sensing as a part of our future work. This

investigation also proposes that these nanorod array sub-

strates, fabricated by oblique-angle thermal evaporation,

have excellent potential to be used for various SERS

based practical application purposes in large scale.

Figure 9. Contact angle pictures of 2 mL molecular grade water dispensed on the surface of AgNR substrates functionalized using

(a) CTP, (b) MTP, (c) ATP, (d) NTP, (e) TP, and (f) PT showing the wetting behavior.

Oh et al. 11

Conclusion

Oblique angle deposited silver nanorod array substrates

with large SERS active surface areas were fabricated using

a cost-effective and environmentally friendly single-step

thermal evaporation technique, which has been shown to

be an excellent SERS substrate. These nanorods were

found to be free from crystallographic and chemical con-

tamination, with an average length of 800� 50 nm and

density 20� 1 rods/mm2. The effect of contamination on

the nanorod surface was discussed. Various probe mol-

ecules, namely aminothiophenol, nitrothiophenol, carbox-

ythiophenol, thiophenol, methoxythiophenol and

propanethiol, with variable hydrophilic–hydrophobic prop-

erties, have been used to investigate long-term stability,

surface uniformity, and reproducibility of the functionalized

nanorod array films, which also mimics many realistic appli-

cations. In most cases, the functionalization process was

also shown to be successful in substituting the contamin-

ants, if present, on the silver surface. The spot-to-spot

signal intensity variation was found to vary less than RSD

value of 3% for the functionalized substrates showing high

uniformity of nanorod morphology with large SERS active

surface area. The effect of storing conditions on long-term

stability of functionalized substrates was studied. It has been

found that the substrates properly stored in dry, as well as

wet, conditions has shown high SERS signal reproducibility

during the investigated period of six months, as long as the

substrate morphology was not disturbed and the substrates

are protected from extensive contamination and deterior-

ation. This study demonstrated that silver nanorod arrays

fabricated by the oblique-angle thermal evaporation tech-

nique possess a uniformly distributed large SERS active sur-

face area, a highly reproducible morphology and thereby

signal intensity, and are stable over a period of six

months. Thus, we propose that these reliable and reprodu-

cible substrates are suitable for mass production and util-

ization in SERS-based sensor technology for a wide range of

analyte detection often present in ultra-trace level rather

than meagre identification.

Conflict of Interest

None declared.

Funding

The authors gratefully acknowledge support from the grant (grant

number 2009-0077245) through the National Research

Foundation of Korea, funded by the Ministry of Science, ICT and

Future Planning, Korea and the grant of Gwangju Institute of

Science and Technology under the ‘‘Ultrashort Quantum Beam

Facility Program’’ in 2016.

Supplemental Material

All supplemental material mentioned in the text, consisting of two

figures, is available in the online version of the journal, at http://

www.s-a-s.org

References

1. R. Kodiyath, S.T. Malak, Z.A. Combs, T. Koenig, A.M. Mahmoud, A. El-

Sayed, V.V. Tsukruk. ‘‘Assemblies of Silver Nanocubes for Highly

Sensitive SERS Chemical Vapor Detection’’. J. Mater. Chem. A. 2013.

1(8): 2777–2788.

2. S. Nei, S.R. Emory. ‘‘Probing Single Molecules and Single Nanoparticles

by Surface-Enhanced Raman Scattering’’. Science. 1997. 275(5303):

1102–1106.

3. M. Fleischmann, P.J. Hendra, A.J. McQuillan. ‘‘Raman Spectra of

Pyridine Adsorbed at a Silver Electrode’’. Chem. Phys. Lett. 1974.

26(2): 163–166.

4. D.L. Jeanmaire, R.P. Van Duyne. ‘‘Surface Raman

Spectroelectrochemistry. Part I. Heterocyclic, Aromatic, and

Aliphatic Amines Adsorbed on the Anodized Silver Electrode’’.

J. Electroanal. Chem. 1977. 84(1): 1–20.

5. M.G. Albrecht, J.A. Creighton. ‘‘Anomalously Intense Raman Spectra

of Pyridine at a Silver Electrode’’. J. Am. Chem. Soc. 1977. 99(15):

5215–5217.

6. L. Tong, H. Xu, M. Kall. ‘‘Nanogaps for SERS Applications’’. MRS

Bulletin. 2014. 39(02): 163–168.

7. J.L. Abell, J.D. Driskell, Y. Zhao. ‘‘Controllable and Reversible Hot Spot

Formation on Silver Nanorod Arrays’’. Chem. Commun. 2014. 50(1):

106–108.

8. H. Im, K.C. Bantz, N.C. Lindquist, C.L. Haynes, S.-H. Oh. ‘‘Vertically

Oriented Sub-10-nm Plasmonic Nanogap Arrays’’. Nano Lett. 2010.

10(6): 2231–2236.

9. A. Shiohara, Y. Wang, L.M. Liz-Marzan. ‘‘Recent Approaches towards

Creation of Hot Spots for SERS Detection’’. J. Photochem. Photobiol

C. 2014. 21: 2–25.

10. L. Guerrini, D. Graham. ‘‘Molecularly-mediated Assemblies of

Plasmonic Nanoparticles for Surface-Enhanced Raman

Spectroscopy’’. Chem. Soc. Rev. 2012. 41(21): 7085–7107.

11. P. Negri, R.A. Dluhy. ‘‘Ag Nanorod Based Surface-enhanced Raman

Spectroscopy Applied to Bioanalytical Sensing’’. J. Biophotonics.

2013. 6(1): 20–35.

12. R.P. Cooney, M.R. Mahoney, M.W. Howard. ‘‘Auger and Raman

Spectroscopic Studies of Visible Laser Microzones in the

Electrochemical SERS System: Silver/2,20-Bipyridine’’. Langmuir. 1985.

1(3): 273–277.

13. N. Shirtcliffe, U. Nickel, S. Schneider. ‘‘Reproducible Preparation of

silver Sols with Small Particle Size Using Borohydride Reduction: for

Use as Nuclei for Preparation of Larger Particles’’. J. Colloid Interface

Sci. 1999. 211(1): 122–129.

14. C.J. Orendorff, L. Gearheart, N.R. Jana, C.J. Murphy. ‘‘Aspect Ratio

Dependence on surface Enhanced Raman Scattering Using silver and

Gold Nanorod Substrates’’. Phys. Chem. Chem. Phys. 2006. 8(1): 165–170.

15. F.J. Garcia-Vidal, J.B. Pendry. ‘‘Collective Theory for Surface Enhanced

Raman Scattering’’. Phys. Rev. Lett. 1996. 77(6): 1163–1166.

16. S. Zou, G.C. Schatz. ‘‘Silver Nanoparticle Array Structures that

Produce Giant Enhancements in Electromagnetic Fields’’. Chem.

Phys. Letts. 2005. 403(1–3): 62–67.

17. M.E. Hankus, H. Li, G.J. Gibson, B.M. Cullum. ‘‘Surface-Enhanced

Raman Scattering-Based Nanoprobe for High Resolution, Non-

Scanning Chemical Imaging’’. Anal. Chem. 2006. 78(21): 7535–7546.

18. M.-K. Oh, Y.-S. Shin, C.-L. Lee, R. De, H. Kang, N.E. Yu, B.H. Kim,

J.H. Kim, J.-K. Yang. ‘‘Morphological and SERS Properties of Silver

nanorod Array Films Fabricated by Oblique Thermal Evaporation at

Various Substrate Temperatures’’. Nanoscale Res. Lett. 2015. 10(1): 259.

19. W.J. Herzberg, J.E. Marian. ‘‘Relationship between Contact Angle and

Drop Size’’. J. Colloid. Interface Sci. 1970. 33(1): 161–163.

20. J.A. Sanchez-Gil, J.V. Garcia-Ramos. ‘‘Local and Average

Electromagnetic Enhancement in Surface-enhanced Raman Scattering

from Self-affine Fractal Metal Substrates with Nanoscale

Irregularities’’. Chem. Phys. Lett. 2003. 367(3–4): 361–366.

12 Applied Spectroscopy 0(0)

21. C. Khare, C. Patizig, J.W. Gerlach, B. Rauschenbach. ‘‘Influence of

Substrate Temperature on Glancing angle Deposited Ag Nanorods’’.

J. Vac. Sci. Technol. A. 2010. 28(4): 1002–1009.

22. R. Varshney, S. Bhadauria, M.S. Gaur. ‘‘Biogenic Synthesis of Silver

Nanocubes and Nanorods Using Sundried Stevia rebaudiana Leaves’’.

Adv. Mat. Lett. 2010. 1(3): 232–237.

23. J.B. Khurgin, G. Sun. ‘‘Plasmonic Enhancement of the Third Order

Nonlinear Optical Phenomena: Figures of Merit’’. Opt. Express.

2013. 21(22): 27460–27480.

24. Y.-P. Zhao, S.B. Chaney, Z.Y. Zhang. ‘‘Absorbance Spectra of Aligned

Ag Nanorod Arrays Prepared by Oblique Angle Deposition’’. J. Appl.

Phys. 2006. 100(6): 063527.

25. Y.-J. Liu, H.Y. Chu, Y.-P. Zhao. ‘‘Silver Nanorod Array Substrates

Fabricated by Oblique Angle Deposition: Morphological, Optical, and

SERS Characterization’’. J. Phys. Chem. C. 2010. 114(18): 8176–8183.

26. C.E. Taylor, S.D. Garvey, J.E. Pemberton. ‘‘Carbon Contamination at

silver Surfaces: Surface Preparation Procedures Evaluated by Raman

Spectroscopy and X-ray Photoelectron Spectroscopy’’. Anal. Chem.

1996. 68(14): 2401–2408.

27. W. Lu, C.L. Feldman, Y. Song, S. Dhar, W.E. Collins, W.C. Mitchel,

J.R. Williams. ‘‘Graphitic Features on SiC Surface Following

Oxidation and Etching Using Surface Enhanced Raman

Spectroscopy’’. Appl. Phys. Lett. 2004. 85(16): 3495–3497.

28. D. Lin-Vien, N. Colthup, W. Fately, J. Grasselli. The Handbook of

Infrared and Raman Characteristic Frequencies of Organic

Molecules. New York: Academic Press, 1991.

29. A.W. Adamson, A.P. Gast. ‘‘Physical Chemistry of Surfaces’’.

New York: Wiley-Interscience, 1997. 6th ed.

30. C.E. Taylor, S.D. Garvey, J.E. Pemberton. ‘‘Carbon Contamination at

Silver Surfaces: Surface Preparation Procedures Evaluated by Raman

Spectroscopy and X-ray Photoelectron Spectroscopy’’. Anal. Chem.

1996. 68(14): 2401–2408.

31. R.P. Cooney, M.R. Mahoney, M.W. Howard. ‘‘Intense Raman Spectra of

Surface Carbon and Hydrocarbons on silver Electrodes’’. Chem. Phys.

Lett. 1980. 76(3): 448–452.

32. J.T. Hugall, J.J. Baumberg, S. Mahajan. ‘‘Disentangling the Peak and

Background Signals in Surface-Enhanced Raman Scattering’’. J. Phys.

Chem. C. 2012. 116(10): 6184–6190.

33. P. Negri, N.E. Marotta, L.A. Bottomley, R.A. Dluhy. ‘‘Removal of

Surface Contamination and Self-Assembled Monolayers (SAMs) from

Silver (Ag) Nanorod Substrates by Plasma Cleaning with Argon’’. Appl.

Spectrosc. 2011. 65(1): 66–74.

34. N. Nuntawong, P. Eiamchai, B. Wong-ek, M. Horprathum,

K. Limwichean, V. Patthanasettakul, P. Chindaudom. ‘‘Shelf Time

Effect on SERS Effectiveness of Silver Nanorod Prepared by OAD

Technique’’. Vacuum. 2013. 88(1): 23–27.

35. K.L. Norrod, K.L. Rowlen. ‘‘Removal of Carbonaceous Contamination

from SERS–Active Silver by Self-Assembly of Decanethiol’’. Anal.

Chem. 1998. 70(19): 4218–4221.

36. K.L. Norrod, K.L. Rowlen. ‘‘Ozone-Induced Oxidation of Self-

Assembled Decanethiol: Contributing Mechanism for

‘Photooxidation’’’? J. Am. Chem. Soc. 1998. 120(11): 2656–2657.

37. J.D. Driskell, S. Shanmukh, Y. Liu, S.B. Chaney, X.-J. Tang, Y.P. Zhao,

R.A. Dluhy. ‘‘The Use of Aligned Silver Nanorod Arrays Prepared by

Oblique Angle Deposition as Surface Enhanced Raman Scattering

Substrates’’. J. Phys. Chem. C. 2008. 112(4): 895–901.

38. H.G. Kang, H.W. Lee, Y.J. Kang, M. Hara, J.G. Noh. ‘‘Two-dimensional

Ordering of Benzenethiol Self-assembled Monolayers Guided by

Displacement of Cyclohexanethiols on Au(111)’’. Chem. Commun.

2008. 41: 5197–5199.

39. A. Martin, A. Pescaglini, C. Schopf, V. Scardaci, R. Coull, L. Byrne,

D. Iacopino. ‘‘Surface-Enhanced Raman Scattering of 4-

Aminobenzenethiol on Au Nanorod Ordered Arrays’’. J. Phys.

Chem. C. 2014. 118(24): 13260–13267.

40. T.O. Deschaines, K.T. Carron. ‘‘Stability and Surface Uniformity of

Selected Thiol-coated SERS Surfaces’’. Appl. Spectrosc. 1997. 51(9):

1355–1359.

41. T. Smith. ‘‘The Hydrophilic Nature of a Clean Gold Surface’’. J. Colloid.

Interface Sci. 1980. 75(1): 51–55.

42. J. Bico, C. Marzolin, D. Quere. ‘‘Pearl Drop’’. Europhys. Lett. 1999.

47(2): 220–226.

43. J. Wang, J. Shao, K. Yi, Z. Fan. ‘‘Layer uniformity of Glancing Angle

Deposition’’. Vacuum. 2005. 78(1): 107–111.

44. W.N.W. Mustapha, S.A.R.S.A. Hamid, S.D. Hutagalung, N.V. Hieu,

K. Mohamed, C.K. You. ‘‘Synthesis and Gas Sensing Properties of

SnO2 Nanostructures by Thermal Evaporation’’. Adv. Mater. Res.

2013. 620: 350–355.

45. M.A. De Jesus, K.S. Giesfeldt, J.M. Oran, N.A. Abu-Hatab, N.V. Lavrik,

M.J. Sepaniak. ‘‘Nanofabrication of Densely Packed Metal–Polymer

Arrays for Surface-Enhanced Raman Spectrometry’’. Appl.

Spectrosc. 2005. 59(12): 1501–1508.

Oh et al. 13