1

FABRICATION AND CHARACTERIZATION OF

SURFACE-ENHANCED RAMAN SCATTERING SUBSTRATES

THROUGH PHOTO-DEPOSITION OF GOLD NANOPARTICLES

A dissertation presented

by

Kaaya Ismail

to

The department of Condensed Matter Physics

in partial fulfilment of the requirement

for the postgraduate Diploma

in condensed Matter Physics

The Abdus Salam International Center for Theoretical Physics

(ICTP)

Trieste Italy

August 2015.

Dissertation Advisors: Dr. Marco Lazzarino and Dr. Denys Naumenko.

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Abstract. The interparticle spacing of the surface-enhanced Raman scattering (SERS) substrate has a

strong relationship with its enhancement factor (EF). How to precisely adjust the interparticle

gap and produce SERS substrates with excellent quality and high reliability by a facile way is still

a challenge. Here, we explored a convenient and cheap method to fabricate gold nanoparticles

SERS substrates through photo-deposition of gold nanoparticles using one photon absorption.

We show that using one photon absorption with a continuous laser writing technique, it is

possible to immobilise and pattern the gold nanoparticle capable of generating SERS activity.

Preliminary, optical microscopy, SEM, UV-vis characterization show that our approach

represents a powerful alternative to the traditional fabrication of SERS substrates.

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CHAPTER ONE (introduction)

1.1. Introduction.

Surface-enhanced Raman scattering (SERS) has been extensively studied since Van Duyne and

Creighton’s discovery of Raman scattering enhancement of molecules adsorbed on roughened

metallic surfaces some 30 years ago. Subsequent experimental and theoretical studies have

attributed this observation to the light-induced surface Plasmon resonance (SPR) on the

metallic surfaces. The results of SPR is the creation of “hot” electric field spots on the curved

surfaces as well as the troughs between the nanoprotrusions. Consequently molecules that are

trapped within or situated in the vicinity of these “hot” zones would experience a strong field

and in turn emit amplified Raman intensities. Experimental and theoretical studies have shown

that the enhancement in the order of from 103 to 107 is attained in the SERS, giving a detection

limit up to the subpicogram range i.e single molecule detection. As such SERS has recently

become a promising tool in the medical field in general and has been used for the study of trace

analytes (e.g., urea, critic acid) and chemical changes in blood and urine in particular.

For all SERS-active metallic surfaces that have been developed and studied, it has been amply

demonstrated that surfaces consisting of closely packed but not aggregated colloidal arrays are

particularly enhancing, owing primary to the inter-particle Plasmon resonance. Thus many

attempts have been made to prepare closely packed colloidal films. Currently available

fabrication techniques encompass vapor deposition of nanoparticles on silica posts, fabrication

of periodic arrays of nanoparticles via nanosphere lithography, electron beam lithography, self-

assembly of nanoparticles on a chemically functionalised surface, and silver enhancements of

seed particles that have been predeposited on glass substrates. While these substrates have

been shown to exhibit large Raman enhancement and good SERS reproducibility, most are,

unfortunately, too laborious and expensive to fabricate in large quantities. As such we seek to

find a safer and cheaper alternative to fabricate SERS-active closely packed metal

nanostructures.

1.2. Objectives.

In this study we report the fabrication and characterisation of SERS substrates through photo-

deposition of gold nanoparticles using UV light absorption. We show that using one photon

absorption with a continuous laser writing technique, it is possible to immobilise and pattern

the gold nanoparticle capable in principle of generating SERS activity. Finally, optical

microscopy, UV-vis spectroscopy, SEM, EDX are performed.

This work is motivated by the previous work in which SERS substrates were produced using two

photon absorption, but creation as a side effect the carbonization of organic molecules. In this

project we substitute two photon absorption with UV light.

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1.3. The optical properties of Noble Metals

Metals such as Gold (Au), silver (Ag), copper (Cu), or aluminum (Al), have long been known to have different optical properties from standard dielectrics. They for example reflect light efficiently in the visible range, making them good materials for mirrors of various types. These particular optical properties along with many other physical properties (such as heat or electrical conductivity) all have the same physical origin that is the presence of free conduction electrons. The free electrons of a metal move in a background of fixed positive ions (the vibration of ions,

or phonons, are ignored in a first approximation), which ensures overall neutrality. This forms,

by definition, a plasma and can be called a free-electron plasma, or solid-state plasma. The

optical response of this free electron plasma will govern all the optical properties of the metals,

at least in the visible part of the spectrum where its characteristic resonant energies reside.

To model the optical response of a free electron plasma, one needs to determine the

constitutive equations relating the currents and charges in the plasma to the electromagnetic

fields. This is very difficult undertaking in general because of many possible complications,

including: the interactions with the underlying periodic structures of ions, the electron- electron

correlations and the fermionic nature of electrons, the interaction of electrons with impurities

and phonons, and the possible presence of surfaces. This response can be described by various

degrees of the simplified model, namely: the Drude model.

1.3.1. The Drude model of the free electron optical response.

One simple way to introduce the Drude model is by the Lorentz model for the atomic

polarizability. This model describes the optical response of an electron in an atom or molecule,

bound with a restoring force characterized by a resonant frequency 𝜔0.

1 +𝑛𝑒2/𝑚𝜖0

(𝜔20−𝜔2−𝑖Γ𝜔)

(1)

The conduction electrons in a metal are not bound and can therefore, in a first approximation,

be determined by the Lorentz model, without a restoring force (i.e.𝜔0~ 0). Moreover, because

the free electrons are distributed uniformly and randomly throughout the metal, there

contribution to the total optical susceptibility are simply the sum of their individual

polarizabilities, without any local field correction. This Drude relative dielectric function of a

metal can be obtained by taking 𝜔0 = 0 in the Lorentz model (1) , i.e;

𝜖(𝜔) = 1 −𝑛𝑒2

𝑚𝜖0

1

𝜔2+𝑖𝛾0𝜔, (2)

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Where n [𝑚−3] is the number of free electrons per unit volume and m[kg] the mass. The

damping term 𝛾0[rad 𝑠−1], corresponds to the collision rates of free electrons with the crystal

or impurities (which also leads to the electrical resistivity in this simple model). It is usually

small compared to 𝜔 in the regions of interest here. The optical response of the positive ions in

the crystal has so far been ignored. In a first approximation, these contribute to the crystal real

dielectric function 𝜖∞ ≥ 1. This affects the optical response of the crystal and the dynamics of

the free electrons. This can easily be incoperated in the Drude model and leads to a slightly

modified expression for 𝜖(𝜔), i.e:

𝜖(𝜔) = 𝜖∞ (1 −𝜔𝑝

2

𝜔2+𝑖𝛾0𝜔) (3)

Where 𝜔𝑝 [rad 𝑠−1] is being defined here as;

𝜔𝑝 = √𝑛𝑒2

𝑚𝜖0𝜖∞ (4)

In the absence of the external perturbation, the charge density of the plasma is uniform and

zero. It can be shown that 𝜔𝑝 is the natural oscillation frequency of the free electron plasma

charge density and it is therefore called the plasma frequency. One can also define the

corresponding wavelength 𝜆𝑝 = 2𝜋𝑐 𝜔𝑝⁄ . Taking the real and imaginery part of the previous

expression, we have:

𝑅𝑒(𝜖(𝜔)) = 𝜖∞ (1 −𝜔𝑝

2

𝜔2𝛾02) (5)

And

𝐼𝑚(𝜖(𝜔)) =𝜖∞𝜔𝑝

2𝛾0

𝜔(𝜔2+𝛾02)

(6)

As already been mentioned that 𝛾0 is small compared to 𝜔, we see that for a plasma described

by a Drude model, the plasma frequency can be obtained from the condition 𝑅𝑒 (𝜖(𝜔𝑝)) ~0.

We also see that in the region 𝜔 < 𝜔𝑝 (wavelength longer than 𝜆𝑝), we have 𝑅𝑒 (𝜖(𝜔𝑝)) < 0.

Moreover, if 𝜔 is not too small, the absorption, characterized by 𝐼𝑚(𝜖(𝜔)), is also small in this

region. It is these two conditions 𝑅𝑒 (𝜖(𝜔𝑝)) < 0 and small 𝐼𝑚(𝜖(𝜔)), that make possible a

whole range of interesting optical effects, including plasmon resonances. These conditions are

never fulfilled in ‘standard’ dielectrics where 𝑅𝑒(𝜖) is typically between 1 and~10. For many

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metals, the plasma frequency is in the UV-visible part of the electromagnetic spectrum, and the

region of interest is therefore in the visible (and close UV, or near infrared, depending on the

metal)

1.4. Plasmonics

Plasmonics is a flourishing new field of science and technology that exploits the unique optical

properties of metallic nanostructures to route and manipulate light at nanometre length scales.

The vivid optical properties of noble metal nanoparticles have been an object of fascination

since ancient times. The ruby red of stained glass windows arises from gold nanoparticles,

formed by the reduction of metallic ions in the glass-forming process. While these properties

have been known and used for centuries, our scientific understanding of these properties has

emerged far more recently, beginning with the development of classical electromagnetic

theory. Gustav Mie’s application of Maxwell’s equations to explain the strong absorption of

green light by a subwavelength gold sphere under plane wave illumination established the

rigorous scientific foundation for our understanding of this phenomenon.[10] The following

decades saw increasing interest in the properties of light scattering by small particles, and in

particular, the properties of small metallic nanoparticles, whose collective electronic

resonances, known as plasmons, give rise to the strong optical absorption properties of this

class of materials. [11-13]

An even more dramatic optical property of metallic nanoparticles is their color change when a

dilute suspension of nanoparticles aggregates. This can be seen when molecules or ions, such as

a solution of NaCl, or molecular linkers, such as hemoglobin or DNA, are added to a suspension

of noble metal nanoparticles. In the case of gold colloid, upon the onset of aggregation, a

dramatic red-to-blue color change can easily be seen. When gold or silver nanoparticles begin

to aggregate, they form pairs of nanoparticles and their optical spectrum acquires a new peak,

red-shifted significantly from the spectral peak of the absorption for the isolated nanoparticle.

For a fully aggregated solution, the red-shifted peak dominates the spectrum (Figure 3).[14]

This phenomenon, commonly witnessed by colloidal chemists, requires an understanding of the

electromagnetic properties of interacting metallic nanoparticles in close mutual proximity.[13-

14]

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Figure 3.Extinction spectra of dispersed and agglomerated of gold nanoparticles.

1.4.1. Plasmon coupling.

1.4.1.1. Nanoparticle Pairs and “Hot Spots”

An observation even more dramatic than the color change of colloidal aggregates is the ability

of gold or silver nanoparticles, under certain conditions, to give rise to extraordinarily large

enhancements of the Raman scattering spectra of adsorbed or adjacent molecules. The first

reports of Raman scattering greatly enhanced by the presence of a metal substrate appeared in

1978, an effect that became known as surface-enhanced Raman scattering (SERS).[15] The

million-fold enhancements initially reported by the discoverers of this effect were attributed

largely to the excitation of surface plasmons of the metallic substrate,[16] with further

enhancements by chemical effects. While the initial observations of SERS were indeed striking,

they were followed in 1997 by reports of single-molecule detection using SERS, that captured

the attention of the scientific world.[17,18] The initial report of single-molecule SERS was

obtained for silver colloid, “activated” by a low concentration of NaCl, with a dilute solution of

methyl violet consistent with the detection of individual molecules. The possibility of obtaining

chemical signatures of single molecules, with chemical spectroscopy at the sensitivity level of

fluorescence, has the potential to elicit breakthroughs in fundamental chemistry at the single-

molecule level, the study of biochemical pathways, and even the supramolecular chemical

dynamics within living cells. However, the mystery behind these enormous enhancements,

initially determined to be in the 1014 to 1015 range, was indeed complex. Based on analyses that

had been performed for molecules adsorbed to metallic nanoparticles, the origin of these

enhancements was unclear.

As researchers began to investigate this effect, it soon became apparent that the extremely

intense SERS signals were obtained from only certain specific nanoparticles. The optical

extinction spectra of individual colloidal silver particles obtained with a dark field microscope

revealed a remarkably heterogeneous distribution of particles, with widely varying spectral

properties[19]

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When these were correlated with single-molecule SERS spectra of the same particles, it was

discovered that virtually all the nanoparticles giving rise to SERS spectra themselves had

complex, red-shifted, typically multiply peaked spectra,[20] not the blue centered extinction

spectrum of an isolated individual silver nanoparticle. From these observations, it was inferred

that the extraordinary enhancements experienced by certain molecules were due to properties

of strongly interacting metallic nanoparticles. This hypothesis was soon substantiated by direct

measurements of single-molecule SERS at the individual nanoparticle or nanoparticle cluster

level:[21,22] in these studies, it was independently observed that colloidal clusters as simple as

two directly adjacent nanoparticles gave rise to a multiply peaked extinction spectrum and to a

single-molecule SERS signal. Dense, not sparse, clusters produced a far greater enhancement of

the anti-Stokes spectra of molecules [23,24]. From these pioneering experiments, it could be

inferred that the junction between adjacent nanoparticles, occurring for pairs, larger clusters,

or even aggregate films of nanoparticles, can give rise to highly intense and localized

electromagnetic fields when excited by incident light of the appropriate polarization. In a near

field optical microscopy interrogation of a metal island film, these highly localized, high-

intensity regions could be measured directly. From these studies, the inter-junction regions

were nicknamed “hot spots”. While these high-intensity regions were observed for many

complex nanocluster geometries, it became clear that the nearest-neighbor coupling between

two adjacent metallic nanoparticles, not long-range or radiative coupling, was responsible for

this electromagnetic focusing effect. In a combination single-particle Raman and AFM

experiment, bright SERS signals resulting directly from adjacent nanoparticle pairs were

identified.

1.4.2. Surface Plasmons (SPs)

Surface Plasmons are collective excitation of free conductive electrons on noble metal thin

films or nanoparticle surfaces excited by electromagnetic radiation at the metal-deilectric

interface.

There are two types of surface Plasmons: (i) propagating and (ii) non propagating [20-21]

propagating surface Plasmons are called surface plasmon polaritons(SPPs) generated on

noble(such as Gold and silver) metallic thin films (10-200 nm in thickness). On the other hand,

Non-propagating surface plasmons are called localised surface plasmons polaritons(LSPPs) and

are generated on the surface of nanoparticles of 10-200 nm in size[21-22]. Light or electric

fields can excite those Plasmons, then resulting in surface and localised surface Plasmon

resonances (SPR and LSPR) in the case of planar and nanometric-sized metallic structures,

respectively. Plasmon oscillation is resonant with the light of a particular frequency. The electric

field intensity, the scattering and the adsorption cross-sections are then enhanced. Materials

exhibiting surface Plasmon properties are used to maximise surface sensitive spectroscopic

techniques, such as Raman scattering or fluorescence.[23].The resonance frequency strongly

depends on the size and the shape of the metal nanoparticles, as well as, on the metal complex

dielectric function and the surrounding medium. Noble metals such as silver, gold and copper

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exhibit strong visible-light Plasmon resonance, whereas other transition metals show only a

broad and poorly resolved absorptions band in the ultraviolet regions. [24].

1.5. Raman scattering.

Raman scattering or the Raman Effect is the inelastic scattering of a photon. It was

discovered by C. V. Raman and K. S. Krishnan in liquids,[3] and by G. Landsberg and L. I.

Mandelstam in crystals.[4] The effect had been predicted theoretically by Adolf Smekal in

1923.[5]

When light encounters an atom or molecule, the predominant mode of scattering is elastic

scattering, called Rayleigh scattering, such that the scattered photons have the same

energy (frequency and wavelength) as the incident photons. It increases with the fourth

power of the frequency and is more effective at short wavelengths. It is also possible for the

incident photons to interact with the molecules in such a way that energy is either gained or

lost so that the scattered photons are shifted in frequency. Such inelastic scattering is called

Raman scattering. Raman scattering produces scattered photons which differs in frequency

from the radiation that causes it and the difference is related to vibrational and or

rotational properties of the molecules from which the scattering occurs. [6]

We can use the quantum particle interpretation to better visualize the process and

determine additional information. In the quantum interpretation, the Raman Effect is

described as inelastic scattering of a photon off of a molecular bond. From the Jablonski

diagram shown in figure 1, we can see that this results from the incident photon exciting

the molecule into a virtual energy state.

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Figure 1. Jablonski Diagram of Quantum Energy Transitions for Rayleigh and Raman

Scattering.

When this occurs, there are three different potential outcomes. First, the molecule can

relax back down to the ground state and emit a photon of equal energy to that of the

incident photon; this is an elastic process and referred to as Rayleigh scattering. Second,

the molecule can relax to a real phonon state and emit a photon with less energy than the

incident photon; this is called Stokes shifted Raman scattering. The third potential outcome

is that the molecule is already in an excited phonon state, is excited to a higher virtual state,

and then relaxes back down to the ground state emitting a photon with more energy than

the incident photon; this is called Anti-Stokes Raman scattering. Due to the fact that most

molecules will be found in the ground state at room temperature, there is a much lower

probability that a photon will be Anti-Stokes scattered. As a result, most Raman

measurements are performed considering only the Stokes shifted light.

1.6. Surface-enhanced Raman scattering or SERS

Surface enhanced Raman scattering is based on the enhancement of Raman signals induced by

plasmonic metal surfaces on the nearby molecules [25]. The extent of enhancement depends

on the shape and size of the metal nanoparticles, as these factors influence the ratio of

absorption and scattering events. Large particles allow multipole excitation, which are

nonradiative modes, since only dipolar transitions contribute to Raman scattering, the overall

efficiency of the enhancement is then reduced. On the other side, too small particles lose their

electrical conductance and cannot enhance the field. When the size approaches a few atoms,

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the definition of Plasmon, which involve a large collection of electrons to oscillate together,

does not hold [26].

The enhancement factor is maximum for nano-structured metals (10-100 nm) [27], being thus

excellent materials for SERS. The exact mechanism accounting for the enhancement effect is

still a matter of debate. Although several models have been proposed in literature, nowadays,

two mechanisms are accepted [28] electromagnetic and chemical. The first one relies on the

excitation of the localized surface Plasmon on metal surfaces, whereas the second one

proposes changes of the molecule electronic structure [29].The chemical enhancement only

applies in specific cases that cannot be explained by the electromagnetic mechanism. The

chemical mechanism is short ranged (0.1-0.5 nm) and strongly dependent on the geometry of

bonding and on the molecule energy level. In any case, the contribution of this mechanism to

SERS is relatively small; its enhancement is estimated to be a factor of(10 − 103). However the

electromagnetic is the dominant contribution in SERS. It accounts for more situations, even for

molecules not adsorbed on the surface [30].

According to the electromagnetic model, (figure 4), the incident electromagnetic field 𝐸𝑖 excites

the surface Plasmons and induces oscillating dipoles given by 𝜇(𝑡) = ℒ𝐸𝑖 where ℒ here is the

Polarizability tensor[31]. The Plasmon oscillations must be perpendicular to the surface,

otherwise scattering does not occurs [32]. The induced polarization generates large fields on

the particle, it magnifies the incident field, thus increasing the Raman signal. On second step,

the emitted Raman field, 𝐸𝑅 , can also polarize the metal particles, which thereby acts as an

antenna to further intensify the Raman signal. The Raman intensity is proportional to the

square field. As a result of the first process, the intensity enhances(𝐸𝑖 + 𝐸𝑖,𝑠)2, whereas for the

second step, it increases as (𝐸𝑅+𝐸𝑅,𝑠)2 [33]. The frequency-shift between the incident light and

the Raman signal determines the extent of enhancement. For small shifts, both fields can be

nearly resonant with the surface plasmon, then establishing a total enhancement of 𝐸4.

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Figure 4. Schematic representation of the electromagnetic enhancement mechanism in SERS.

There are two processes: i) (blue) the incident field,𝐸𝑖, is enhanced due to the addition of the scattered field,𝐸𝑖,𝑠.

It arises from the particle polarization, yielding the field 𝐸𝑖 + 𝐸𝑖,𝑠 and exciting Raman modes on the probe

molecule; ii) (red) the emitted Raman field,𝐸𝑅, is also enhanced through the same mechanism. The improved field

𝐸𝑅 + 𝐸𝑅,𝑠 further enhances the Raman intensity, I. Raman signal is enhanced 𝐸2 in each process.

Typically SERs is observed with coinage metals such as gold, silver and copper due to the large

SERS enhancement they produce. Of these metals, silver provides the highest enhancement

factor due to its absorption properties. Optical absorption in metals increases by interband

transitions. The interband transition of silver is found in the ultraviolet range. As a result, there

will be less absorption in the visible or near-IR Raman wavelengths resulting in large SERS

intensities. In contrast gold and copper have interband transitions in the visible wavelength

range which result in a decrease in the maximum SERS intensity. Despite the greater

enhancement capability of silver, gold is often used as it is more stable Raman modes, namely

visible and near-infrared regions of the spectrum. SERS enhancement also depends on the

distance between the nanoparticle center and the target molecule, R; it falls off as ~(𝑟 𝑅)⁄ 12,

where r is the particle radius [34]. This expression indicates that molecule is not required to be

just in contact with the surface. Especially useful are the nanostructures capable of controlling

R, in order to tune the Raman enhancement in an accurate way.

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CHAPTER TWO: SERS SUBSTRATES– literature overview.

2.1. SERS substrates

Surface-enhanced Raman scattering (SERS) has been paid increasing attention for several

decades due to its marvelous enhancement and excellent sensitivity and selectivity among the

vibrational spectrum when the analyte is close to a nanoscale, rough metal surface. As the

unique local plasma resonance of noble metal nanomaterials, they have usually been employed

for producing substrates that can be used in SERS to fulfill the detection of chemical and

biological molecules. A great breakthrough in SERS was the achievement of single-molecule

detection in 1997. However, it is still a challenge to produce portable and cost-effective SERS

sensors so far, and one of the reasons is the difficulty in fabrication of reliable and reproducible

SERS substrates. Currently, SERS enhancement is generally considered to arise from the

electromagnetic effect and the chemical effect, the former of which is normally deemed to play

a vital role in enhancing Raman signal. The maximum enhancement occurs in the hot spots,

which refer to the regions usually less than 10 nm between metallic nanoparticle (NP) pairs,

where surface plasmon of the metal NPs can be coupled to each other to produce more intense

electromagnetic fields. Hence, how to prepare reliable SERS substrates with a large number of

hot spots is of great importance in both fundamental and practical senses.

The research emphasis on nanostructures of SERS is justified by the wide possibilities of

optimization parameters, since both the frequency and magnitude of the maximum field

enhancement are strongly dependent on the shape, size and arrangement of the metallic

nanofeatures[35].

Higher SERS enhancement factors are obtained when the wavelength of the LSPR of the

nanostructure (𝜆𝐿𝑆𝑃𝑅) is located between the excitation wavelength (𝜆𝑒𝑥𝑐) and the wavelength

of Raman signal (𝜆𝑅𝑆).Theoretical and experimental results demonstrated that the maximum

enhancement occurs when the 𝜆𝐿𝑆𝑃𝑅 is equal to the average of the 𝜆𝑒𝑥𝑐 and the 𝜆𝑅𝑆; that is

, 𝜆𝐿𝑆𝑃𝑅 = 12⁄ (𝜆𝑒𝑥𝑐 + 𝜆𝑅𝑆). Various advanced methods have been employed to fabricate 3D

well-defined nanostructures to control and manipulate the Plasmonic properties in order to

maximize the enhancement factor for SERS experiments [36].

Although the possibility of tuning the size and shape of nanostructures has quickly grown in the

last 20 years, there are arguments that they are still at rudimentary stage, and much greater

developments are to be expected in the years to come [35].

The SERS substrate can be classified arbitrarily into three categories: (1) Metal NPs (MNPs) in

suspension; (2) MNPs immobilised on the solid substrate and (3) Nanostructures fabricated

directly on solid substrates, which includes nanolithography and template synthesis of

nanostructures.

The variety of SERS substrates reported in the recent years is enormous and it is not possible to

review the whole field in a limited number of pages. For this reason allow me to summarise

briefly three general “types” of substrates classified according to their methods of fabrication:

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(1) MNPs immobilised in planar solid support; (2) metallic nanostructures fabricated using

nanolithographic methods; and (3) metallic nanostructures fabricated using template

techniques.

2.1.1.MNPs immobilized in planar solid supports.

As already been discussed above, MNPs have long been used in SERS experiments. MNPs are

easy to fabricate through regular wet chemistry, providing numerous choices in terms of size

and shape. SERS hot-spots (sub-wavelength regions of highly localized strong electromagnetic

field) can be easily achieved by aggregating MNPs from their suspensions using either salts or

the analyte of interest. However, the application of dispersed and aggregated MNPs as SERS

substrate in real analytical problems is limited due to the poor enhancement factor

reproducibility. The reproducibility problem can be mitigated by immobilizing the MNPs on

some kind of solid support [37] Since the first report of a SERS substrate consisting of MNPs

synthesized by wet chemistry and subsequently immobilized onto a solid support,[38] the

procedure gained popularity and several works have been published based on this approach

and its variations. Hence, in this section, I will try to focus on the more recent advances

fabrication methods using wet chemistry for usage as SERS substrates.

2.1.1.1. Self-assembly

2.1.1.1.1. Chemical attachment of MNPs to solid substrates.

The principle explored in this section is the use of bifunctional molecules for MNPs

immobilization. The idea is to anchor the molecule to the surface by one of its functional

groups, leaving the other functional group “free” to bind the MNPs [35].

2.1.1.1.2. Electrostatic interaction between MNPs and the solid substrate.

Self-assembly of MNPs onto solid surfaces based on electrostatic attraction using polymers and

biomolecules have also been widely reported. Poly(vinylpyridine) was used to immobilize Ag

NPs onto continuous Ag films. It was claimed that the Raman enhancement originated mostly

from the interaction between the MNPs and the metallic substrate, rather than from the inter-

particles SP-coupling. It was then suggested that, since the interaction with the surface does

not involve NPs aggregation, which is hard to control in terms of reproducibility, this kind of

substrate may provide a better performance in terms of sample-to-sample variation [35].

2.1.1.1.3. Capillary force as deriving mechanism for the deposition of MNPs on

solid surfaces.

Although chemical and electrostatic self-assembly (discussed above) are popular options for

fabricating SERS substrates, different approaches have also been explored. For example,

capillary forces, present during the evaporation of a liquid droplet, can be used to drive the

assembly of MNPs [39–40].The Halas group [39] used a drop-dry method to assemble a film of

CTAB-capped NPs on silicon wafers. It was found that this was an effective substrate for both

SERS and surface-enhanced IR spectroscopy. A combination of capillary assembly and soft

lithography was used to generate a potential addressable SERS biochip. A pre-fabricated PDMS

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stamp was placed underneath a glass slide. Au NPs solutions were injected, and the stamp was

pulled away relative to the glass slide, leading to a specific number of NPs being assembled on

the patterns of the stamp. The NPs pattern was transferred onto amine functionalized glass

slides. The SERS properties were shown to depend on the number of NPs in the aggregates.

2.1.1.1.4. Direct transfer of pre-assembled MNPs film to a solid support.

Immobilized MNPs can also be obtained by transferring MNPs monolayers from a liquid–liquid

interface to a solid support, usually glass or silicon wafer, using, for instance, the Langmuir-

Blodgett technique. This procedure is known to provide good single layer structures[41], and

the technique was also used to examine the effect of multiple layers of Au spherical NPs,

nanorods and their mixtures on the SERS efficiency of the substrate. The results indicated that

the SERS signal increased with increasing layers of NPs, and that nanorods films are more SERS

efficient than films of spherical NPs. Experiments with mixed layers of nanorods and spherical

MNPs showed that the ultimate enhancement was mainly decided by the top layer.

2.1.2. Metallic nanoparticles fabricated using nanolithographic methods.

I will briefly summarise here the SERS substrate obtained by directly fabricating metallic

nanostructures on a solid support. This approach is generally classified as “top down” in

contrast to self-assembled (“bottom up”) methods covered in the previous section.

2.1.2.1. Electron beam lithography.

One of the most used nanolithographic method is electron beam lithography (EBL). The EBL

technique consists of a 10–50 keV beam of electrons focused on a solid support, generally

electron resist covered SiOx/Si wafers. As in regular photolithography, EBL can be performed

using either positive or negative resists. The electron beam selectively etches off regions of the

positive resist from the surface in a predetermined form. In contrast, the region exposed

remains if a negative resistor is used. The most used electron resist is by far poly(methyl

methacrylate) (PMMA), which is a positive EBL resistor. Generally, after the EBL etching of

PMMA, there are two commonly follow-up procedures for the synthesis of SERS-active

nanostructures, both schematically depicted (see section 3.1 of [35]).

2.1.2.2. Nanoparticle arrays.

The preparation of nanoparticle arrays with controllable size and nanoparticle coverage are

advantageous as SERS substrates in microfluidic devices. The small area of the SERS active

surface allows for device miniaturization and the use of several sampling regions in the same

substrate, without interference between different arrays. The micro-contact printing technique

has been used to fabricate microarrays of Ag NPs. A chemically modified poly(dimethyl

siloxane) (PDMS) patterned substrate was exposed to Ag colloidal suspension, and the

adsorbed colloid where transferred to 4-mercaptopyridine or aminoethanethiol modified Au-

coated silicon substrate [42]. The resulting substrates presented regions with a high coverage

by Ag NPs, and regions deprived of them. The SERS mapping using R6G as probe molecule

16

presented the characteristic SERS bands of R6G only on the regions where the Ag NPs were

present.

2.1.3 Template techniques.

Control over the morphology of the nanoparticles is required for the SERS technique to reach a

new level of applicability in analytical sciences. Recent advances in both self-assembly and

nanofabrication have helped to overcome the most reproducibility issues for several types of

substrates. However the most reproducible structures prepared by EBL and FIB, cannot be mass

produced using those methods. The use of templates that allow the deposition of metals with

controlled geometry is one of the most promising approaches to overcome such challenges.

The template directed synthesis is a broad field of study, but the two most used template

deposition strategies are: electrochemical and vapour deposition (see section 4.1-4.2 of [35]).

2.1.4 Direct laser writing (DLW).

Among the strategies which can be used to produce SERS substrates, direct laser writing (DLW)

is one of the most attractive techniques for its low-cost and high throughput [43-45]. Using

DLW, plasmonic nanostructures have been formed by photoreduction of metal oxide films [44]

or metal salts in solution [43, 45]. In both cases the formed gold nanoparticles are randomly

shaped with a broad size distribution and require relatively high DLW power with a threshold of

about 10 mW or more. Recently, it was demonstrated [46] that micropatterns consisting of a 4

nm gold nanoparticles layer can be written on a glass surface using two photon DLW in the

range of powers from 0.2 mW to 4.0 mW. Briefly, gold nanoparticles (AuNP) were coated with

photo-caged dienes while the target surface was coated with enes. Under localized light

irradiation these two molecules undergo a rapid Diels–Alder cycloaddition and form a stable

covalent bond, resulting in the formation of a layer of AuNP in the exposed regions.

Fig. 2.1. Schematic description of the photo-induced surface assembly of Au NPs employing

DLW.

17

Box 2.1. The concept of two photon absorption and one photon absorption.

There are two widely used excitation methods for photo- induced fabrication of submicrometric structures, namely one photon absorption and two photon absorption. The two-photon idea proposes that if two photons of around half the wavelength (and therefore energy) are simultaneously absorbed by the fluorophore, for instance there should be enough energy to excite electrons into the higher energy level and therefore induce fluorescence (figure 1).

Figure 2.2. Diagram of photon and two absorption processes. However, the probability of near simultaneous absorption of the required two photons is very low, to get around this, a high photon flux is required and is provided by the output of a focused pulsed (femtosecond) tuneable infrared laser. An added benefit of this approach is that excitation of fluorophores outside of the focal plane is minimised primarily because of the reduced chance of simultaneous multiple photon absorption by the fluorophore. Two-photon absorption (TPA) is a third order nonlinear optical phenomenon in which a molecule absorbs two photons at the same time. The transition energy for this process is equal to the sum of the energies of the two photons absorbed. The probability that a molecules undergoes two-photon absorption depends on the square of the intensity of the incident light, if the two photons are from the same beam (in the case of two incident beams, the transition probability depends on the product of the beams intensities). This intensity dependence is at the origin of the interest in two-photon absorbing materials for use in microscopy and microfabrication applications. Two photon absorption excitation provide intrinsic 3D imaging due to local nonlinear absorption which occurs only for the very high local intensity provided by tightly focusing a femtosecond laser. As already mentioned TPA has many potential applications particularly in 3D imaging and 3D fabrication of submicrostructures[7-8]. However the technique requires expensive laser sources and delicate specialized optics Using a laser source operating at a wavelength within the absorption band of the target, the one photon absorption method has proven to be a very simple and low-cost technique. However, because of strong linear absorption, this approach is only convenient for thin films. In other words, the excitation is limited to the surface of the thick samples, achieving only a 1D or 2D structure

18

CHAPTER THREE (EXPERIMENTAL DETAILS).

3.1. APTES functionalised Si substrates

First, a Si (and alternatively glass) surface was modified with maileimide groups in a two-step

approach employing (3-aminopropyl) triethoxysilane (APTES) and 4- maleimidobutryol chloride.

APTES was used to functionalize the surface of plasma activated Si substrates. A glass petri dish

containing activated Si substrates was placed in a desiccator along with APTES in a separate vial.

The entire system was kept under vacuum for 2 h. The Si substrates were subsequently

annealed at 130 0C for 4 h before rinsing them with toluene, dichloromethane, acetone and

methanol. The resulting APTES functionalized Si substrates were dried with a stream of N2(g)

and stored in a desiccator under inert atmosphere.

The surface functionalization process on the silicon substrate was accomplished by the self-

assembly process of the APTES monolayer. It is triggered by the piranha treatment which

introduces abundant surface hydroxyl groups (-OH) on the silicon samples by hydroxylation

process. The subsequent silanization step proceeds with the hydrolysis of exthoxy (-𝐶2𝐻5)

groups from the APTES molecules, leading to the formation of silanols (Si-O-H). The APTES

silanols then start to condense with surface silanols, thereby, self-assembling into a monolayer

of APTES by a lateral siloxane (Si-O-Si) network as in figure 1.

Fig 3.1. Schematic diagram displaying the process of hydroxylation after piranha treatment and

silanization by 3-aminopropyl-triethoxysilane (APTES) treatment on silicon surface.

19

3.2. Maleimide functionalised substrates

The APTES functionalised Si substrate was immersed in 55mM THF solution of TEA. Equal

volume of a 50 mM solution of 4-maleimidobutyroryl chloride in dry THF was subsequently

added under inert atmosphere. The reaction was allowed to proceed for 4 h. The substrates

were subsequently rinsed with THF to remove any physisorbed maleimidobutyroryl chloride

and methanol to remove the quatemary ammonium salt, which formed during the reaction.

The substrate were dried by employing a stream of N2(g) and stored at 4 0C under inert

atmosphere.

Figure 3.2. The scheme of maleimide modification of silicon substrate.

3.3. Synthesis of Mercaptoundecanol-capped Gold Nanoparticles (Au-MUD)

Gold nanoparticles were prepared by using the synthesis procedure of Raula et al.[4] HAuCl4

(354 mg, n(Au)) 0.93 mmol) was dissolved in dry THF (10 mL) and stirred for 15 min. MUD(184

mg, 0.90 mmol) was added gradually to the mixture with stirring. After that the mixture was

stirred for 30 min until the color of the mixture changed from brown to orange. Reduction was

conducted at room temperature by the Super-Hydride (6 mL) added slowly to the reaction

mixture with a fast stirring rate. In the beginning of the addition of the reductant, the solution

changed to a black suspension. Thereby, the temperature of the mixture slightly increased. The

stirring was continued until the temperature of the mixture decreased back to ambient

temperature, ca. 1 h. Purification of the gold nanoparticles was conducted by centrifugation

(13200 rpm) in an ethanol/THF mixture.[9]

3.4. Synthesis of Photoenol-modified Gold Nanoparticles (Au-MUD-PE)

MPC-MUD (155 mg), which contained 28.4 µmol of ligands, DCC (3.53 mg, 17.1 µmol), and 4-

DMAP (0.17 mg, 1.39 µmol) were dissolved in 1.3 mL of DMF and flushed with dry nitrogen for

15 min. 4-((2-Formyl-3-methylphenoxy)methyl)benzoic acid (1.58 mg, 5.85 µmol) dissolved in

1.3 mL of DMF was added dropwise to the mixture with stirring at ambient temperature.

Afterwards, that the temperature of the reaction mixture was raised to 30 °C, and kept at that

temperature for 48 h. The purification of the product was accomplished by using a 2000 kDa

dialysis bag in water. In the course of the exchange of DMF and water in the dialysis tube the

Au NPs precipitated. The success of the purification was ascertained by 1H-NMR [chem comm]

20

3.5. Spatially Resolved Immobilization of Au NP

A droplet of suspension containing Au-MUD-PE in DMF (approximately 40 µM) was placed onto

the surface of a maleimide containing glass or Si substrate. Two optical configurations with top

or bottom illumination were used. The selection of configuration is related to the substrates

used in experiment, namely a top illumination for Si and a bottom illumination for glass. A 375

nm continuous wave laser beam (Oxxius) was focused onto the sample surface by 50x water

immersion objective (0.9 NA) or by 100x air one (0.8 NA) for top and bottom illumination

respectively. In spite the fact that dimethylformamide is a solvent with relatively low

evaporation rate, additional glass coverslip was placed onto the drop to reduce evaporation

and so induced defocusing. The samples were produced positioning the sample by piezodrives

in XY plane with nm precision. A variable scanning speed was used with a purpose to vary an

exposure dose for entire structures in the range of 0.005 J – 5 J.

The squares of 20 × 20 µm2 or 50 × 50 µm2 dimension were usually written. The distance of

each scanning line was 200 nm.

After patterning, each sample was immersed into DMF for 20 min. Then, the sample was

shortly immersed into acetone and water, subsequently and dried under flow of nitrogen.

Fig 3.3. Immobilised photo-generated o-quinodimethane located on the Au NPs with surface

anchored maleimides.

3.6. Optical microscopy.

BXFM Olympus microscope was used for optical characterization of the produced structures in

both bright and dark-field regimes with proper 20x and 50x air objectives.

3.7. Scanning electron microscopy.

SEM characterization was performed using a Zeiss SUPRA40 scanning electron microscope

equipped with EDX module.

21

3.8. UV-vis characterisation.

UV-Vis characterization of the immobilized Au NPs was performed on an inverted optical

microscope (Axiovert 200, Zeiss) in transmitted light illumination (HAL 100 illuminator, Zeiss)

coupling a microscope with a 750 mm long spectrometer (Shamrock SR-750, Andor Technology

plc). The transmitted through the sample light was directed into a spectrometer, dispersed by a

diffractive grating of 600 lines mm-1, and analyzed using TE-cooled EMCCD (Newton DU971-

UVB, Andor Technology plc). The position of the grating was controlled by Andor SOLIS

software and changed automatically to cover a wavelength range of 450-900 nm. The XY

sample holder allows move the sample focusing on the locations of interest, i.e. on the

produced patterns (to measure the intensity of transmitted light I) or clean glass surface (to

measure the reference intensity I0).

22

CHAPTER FOUR: RESULTS

4.1. Functionalization Analysis.

This was done using contact angle goniometer. Contact angle measurements are useful for

determining the surface properties of a material. This can include a surface's wettability and its

surface free energy. The contact angle was carried out to determine the cleanliness of a

substrate surface and to verify that a particular chemical modification has taken place.

4.1.1. APTES functionalised Si substrates.

Measurement from contact angle goniometer gave an average contact angle of 64.40 which is

comparable with the values reported in the literature [47] . Which proves the presence

monolayer of APTES.

Fig 4.1.Snapshot of Contact angle measurement in progress

Table 4.1. Measurements of contact angle for APTES functionalised Si substrate.

.

20_05_15

Big Sample

No. Age [h:m:s:ms]Theta(M)[deg]IFT [mN/m]Vol [ul] Area [mm*2]BD [mm] System Theta(L)[deg]Theta(R)[deg]Method

0-0 00:02:41:02864.6 ± 0.39 10.66 19.87 4.294 Water (Ström) 64.2 65 T-1

0-1 00:04:33:27164.3 ± 0.29 10.42 19.6 4.272 Water (Ström) 64 64.6 T-1

0-2 00:05:54:58965.0 ± 0.33 9.61 18.51 4.136 Water (Ström) 64.6 65.3 T-1

0-M 64.6 ± 0.33 10.23 ± 0.5519.33 ± 0.724.234 ± 0.086Water (Ström)64.3 ± 0.3165.0 ± 0.35T-1

23

4.1.2. Maleimide functionalised substrates

Using the same procedure of finding the contact angle, results showed that the contact angle

increased from 64.40 to 72.40 which is evidence that another functional group has been

attached to APTES.

Fig 4.2.Snapshot of Contact angle measurement in progress.

Table 4.1. Measurements of contact angle for maleimide substrate.

mailemide

No. Age [h:m:s:ms]Theta(L)[deg]Theta(R)[deg]Theta(M)[deg]System

0M 144.277 72.9 72.7 72.8 Water (Ström)

0 144.277 72.4 72.5 72.4 Water (Ström)

1 303.337 71.4 71.4 71.4 Water (Ström)

2 325.733 75.2 74.5 74.9 Water (Ström)

3 371.4 72.7 72.3 72.5 Water (Ström)

24

4.2. Patterning

This reflects the different methods applied in photo-deposition. Here two approaches were

used i.e. top and bottom illumination for NP deposition (setups shown below). In both

procedures EDX, UV SEM characterisation was carried out.

4.2.1. Top Laser illumination.

This was done as described in section 3.5 using the setup below.

Fig 4.3. (a) Set up used for top Laser illumination and (b) magnified image of the stage.

The sample was placed on the stage and laser beam was focused on to the sample from the

top. Using different output powers of 15mW, 5mW, 1mW and 0.5mW, patterns of 100 ×

100𝜇𝑚 with 100 lines each and the same exposure time of 10ms. The scanning was done in

the x-y direction using the piezodrives controlled by a program designed in LabView software.

25

4.2.1.1. Optical microscopy.

The optical microscopy image of the produced structure at about 10 µW laser power on the

sample surface in dark-field regime with 20x air objective is depicted in figure 4.4. The size of

the pattern was 100𝜇𝑚 × 100𝜇𝑚.

The optical microscopy image of the fabricated pattern indicates the immobilization of Au

nanoparticles in the illuminated region at 10 µW and no patterns were seen at other lower

power illumination. The change of the shape can be explained by the defocusing during

illumination of the sample or it’s due to the diffusion of activated Au NPs during the patterning

process.

Fig 4.4. Nanoparticles structure with dark-field regime with 20x air objective.

26

4.2.1.2. UV-vis characterisation.

UV-vis spectrum of the prepared pattern was recorded, the spectrum was plotted together with

that already produced using two photon absorption (red curve). The extinction spectrum of the

prepared pattern indicates low LSPR intensity or alternatively significant broadening of LSPR

peak which might be due to the result of essential NP agglomeration.

Fig 4.3 UV-vis spectra for the produced pattern displayed in fig 4.2 and for TPA results [46].

4.2.2. Bottom laser illumination.

This was done as described in section 3.5 using the setup below.

Fig 4.5. (a) Set up used for bottom laser illumination and (b) magnified image of x-y piezostage.

27

4.2.2.1. Optical microscopy.

The optical microscopy image of the produced structures in dark-field regimes with 20x air

objectives is depicted in figure 4.6.(b)

During this experiment the patterning was performed in the scheme shown in figure 4.6 (a).

The same size (20 × 20𝜇𝑚) patterns A, B, C, D and E were scanned with 64 lines each and the

same power of 10 µW. Each region was scanned with different exposure time of 2.5, 3.21, 4.3,

6.4 and 8.5 min respectively resulting in different exposure doses. The image depicts clusters of

NP in only the regions were the exposure time was high i.e. around D and E.

The dark lines in figure 4.6 (b) can be due to the fact that the laser was illuminating the sample

during the change of location for the next scanning process.

Fig 4.6 . (a) Scheme of laser illumination with different exposure doses. (b) Nanoparticles

structure with dark-field regime with 20x air objective.

28

4.2.2.2. Scanning electron microscopy (SEM)

SEM characterization was performed using a Zeiss SUPRA40 scanning electron microscope

equipped with EDX module. The scanning electron microscopic image of the illuminated region

depicted in Figure 4.7 revealed that immobilization of NPs was achieved in the illuminated

region.

Fig 4.7 SEM image of the produced patterns.

In order to analyse the elemental composition of the patterned region, Energy Dispersive X-ray

Analysis (EDX) was performed which is presented in figure 4.8.

The peaks as shown in 4.8 (a) denote the elements signals on glass surface in the patterned

region, and the corresponding signal maps of the elements are also shown in figures 4.8 (c) ,

4.8(d), 4.8(e) and 4.8(f). The map depicts high amount of Au (4.8 (c)), with corresponding low

silicon (4.8(d)) and oxygen (4.8(e)) signals in this region which also explains the presence of Au

NP, i.e. AuNPs cover the surface of Si wafer with native oxide layer. The corresponding high

carbon signal (4.8(f)) is due to the fact that the Au NP were functionalised by organic

compound, MUD in particular.

29

From these results we can say that Au NP were anchored in regions where high exposure time

was used since no evidence of Au NP presence is depicted in areas where low exposure time

illumination was used.

Fig 4.8 EDX signal spectra and corresponding elements maps.

30

EDX spectrum from the non-illuminated area was also carried out and the result depicted low

signals of Au NP in this region as shown in figure 4.9(b) compared to 4.9(a).

(a)

(b)

Fig 4.9 (a) EDX spectra of the illuminated region. (b) EDX spectra of the

non-illuminated region

31

4.2.2.3. Uv-vis characterisation.

UV-vis spectra of the produced square patterns based on different exposure dose were

recorded in regions B and D (see figure 4.4), indicating that Au NP aggregation increases with

exposure dose (i.e. there is a red-shift of localised plasmon resonance).

Fig x. (a) Raw extinction and (b) normalised extinction spectra showing LSPR red-shift with

higher exposure dose.

32

CHAPTER FIVE

5.1. Discussion.

Starting from the results obtained the top illumination, the optical microscopy image figure 4.4,

of the fabricated pattern indicates the immobilization of Au nanoparticles in the illuminated

region at 10 mW and no patterns were seen at other lower power illumination. In order to

characterize plasmonic properties of the patterns, UV-vis measurements were performed,

indicating broadening of LSPR peak which might be due to the result of strong NP

agglomeration. The NP agglomeration might be due to heating effect of the NPs since with top

illumination a laser excitation is passing through the colloidal solution and therefore higher

power and high exposure dose was used. The change of the patterned shape can be explained

by the defocusing during illumination of the sample or it’s due to the diffusion of activated Au

NPs during the patterning process.

Secondly from the bottom illumination, both optical and SEM images depict immobilization of

NPs in the illuminated area. In order to confirm the presence of Au-particles in these areas EDX

spectra was carried out in both the irradiated and unilluminated regions which confirmed high

signals of Au NPs in the patterned regions (see figure 4.8 and 4.9).

In addition to further confirm the results UV-vis characterisation was performed. The

corresponding raw and normalised extinction spectra of the pattern from regions B and D (see

figure 4.6) are displayed in figure 4.10 (a and b) respectively. Both patterns show a

characteristic extinction peak in the wavelength range of 500-600nm caused by an excitation of

LSPR on individual NPs and plasmonic coupling effect on closely located NPs. The position of

LSPR peak is red-shifted with increased exposure dose, and since the size of Au NPs is fixed, the

dominant effect in observed LSPR red shift is caused by the decrease of the distance between

NPs, i.e. NP agglomeration, which becomes more significant with longer exposure time.

5.2. Conclusion.

We have demonstrated that, using one photon absorption with a continuous wave laser source,

it is possible to immobilise and pattern the Au NPs on chemically modified glass or Si

substrates, which can be used to produce the patterns of various sizes capable in principle of

generating SERS activity. We have characterized plasmonic properties of the structures

produced with different exposure doses and demonstrated that broadband plasmonic

structures with increased number of active hot-spots can be reproducibly achieved. The

position of LSPR peak and therefore SERS efficiency can be tuned in broad spectral range and

thus many analytes with characteristic spectral response can be potentially detected and

characterised with higher sensitivity. .

33

34

References:

1. Anirban Sarkar and Theda Daniels- Race. Electrophoretic Deposition of Carbon

Nanotubes on 3-Amino-Propyl-Triethoxysilane (APTES) surface Functionalized Silicon

Substrates. Nanomaterials 2013,3,272-288;doi:10.3390/nano3020272

2. A. Hermann, G. Mihov, G. W.M. Vandermeulen, H.-A. Klok and K Mullen, Tetrahedron,

2003,59,3925

3. Raman, C. V. (1928). "A new radiation". Indian J. Phys. 2: 387–398.

4. Landsberg, G.; Mandelstam, L. (1928). "Eine neue Erscheinung bei der Lichtzerstreuung

in Krystallen". Naturwissenschaften 16 (28): 557

5. Smekal, A. (1923). "Zur Quantentheorie der Dispersion". Naturwissenschaften 11 (43):

873–875.

6. Harris and Bertolucci (1989). Symmetry and Spectroscopy. Dover Publications. ISBN 0-

486-66144-X.

7. B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A.

Heikal, et al., Two-photon polymerization initiators for three dimensional optical data

storage and microfabrication, Nature 398, pp. 51–54, 1999.

8. M. Farsari and B. N. Chichkov, Materials processing: two-photon fabrication, Nat.

Photon. 3, pp. 450–452, 2009.

9. Supporting Information For Photo-Induced Surface Encoding of Gold Nanoparticles.

Lukas Stolzer, Alexander S. Quick, Doris Abt, Alexander Welle, Denys Naumenko, Marco

Lazzarino, Martin Wegener, Christopher Barner-Kowollik* and Ljiljana Fruk*

10. Mie, G. Ann. Phys. 1908, 25, 377.

11. Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: New York, 1995.

12. Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John

Wiley and Sons, Inc.: New York, 1983.

13. Maier, S. A. Plasmonics: Fundamentals and Applications; Springer: New York, 2007.

14. Dusemund, B.; Hoffmann, A.; Salzmann, T.; Kreibig, U.; Schmid, G. Z. Phys. D: At., Mol.

Clusters 1991, 20, 305

15. Jeanmaire, D. L.; Van duyne, R. P. J. Electroanal. Chem. 1977, 84, 1.

16. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783.

17. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.;

18. Dasari, R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, 1667.

19. Nie, S. M.; Emery, S. R. Science 1997, 275, 1102.

20. Stewart, M. E. et al. Nanostructured Plasmonic Sensors. Chem. Rev. 108, 494–521

(2008).

21. Willets, K. A. & Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy and

Sensing. Annu. Rev. Phys. Chem. 58, 267–297 (2007).

22. Haes, A. J. et al. Plasmonic Materials for Surface-Enhanced Sensing and Spectroscopy.

MRS Bulletin 30, 368–375 (2005).

35

23. Hutter and Fendler 2004. Exploitation of localised surface plasmon resonace. Advanced

Materials 16(19):1685-1706

24. Link and El-Sayed 1999. Spectral properties and relaxation dynamics of suface plasmon

electronic oscillations in gold and silver nanodots and nanorods. Jounal of physics and

chemistry B 103(40):8410-8426.

25. Otto, A., I. Mrozek, et al. (1992). "Surface-Enhanced Raman-Scattering." Journal of

Physics-Condensed Matter 4(5): 1143-1212.

26. Moskovits, M. (2006). Surface-enhanced Raman spectroscopy: a brief perspective.

Surface-Enhanced Raman Scattering: Physics and Applications. 103: 1-17.

27. Tian, Z. Q., B. Ren, et al. (2002). "Surface-enhanced Raman scattering: From noble to

transition metals and from rough surfaces to ordered nanostructures." Journal of

Physical Chemistry B 106(37): 9463-9483

28. Campion, A. and P. Kambhampati (1998). "Surface-enhanced Raman scattering."

Chemical Society Reviews 27(4): 241-250.

29. Vo-Dinh, T. (1998). "Surface-enhanced Raman spectroscopy using metallic

nanostructures."Trac-Trends in Analytical Chemistry 17(8-9): 557-582.

30. Xu, H. X., J. Aizpurua, et al. (2000). "Electromagnetic contributions to single-molecule

sensitivity in surface-enhanced Raman scattering." Physical Review E 62(3): 4318-4324.

31. Stevenson, C. L. and T. Vo-Dinh (1996). Modern Techniques in Raman Spectroscopy New

York, Wiley.

32. Smith, E. and G. Dent (2005). Modern Raman Spectroscopy: A Practical Approach. John

Wiley and Sons.

33. Schlucker, S. (2009). "SERS Microscopy: Nanoparticle Probes and Biomedical

Applications."Chemphyschem 10(9-10): 1344-1354.

34. Stiles, P. L., J. A. Dieringer, et al. (2008). "Surface-Enhanced Raman Spectroscopy."

Annual Review of Analytical Chemistry 1: 601-626.

35. Meikun Fan, Gustavo F.S.Andrade, (2011). “a review on the fabrication of substrates for

enhanced Raman spectroscopy and their applications in analytical chemistry.” Analytica

chimica Acta 693(2011) 7-25.

36. Mehmet Kahraman, Pallavi Daggumati, Ozge Kurtulus (2013). “ Fabrication and characterization of flexible and tunable plasmonic nanostructures.

37. E.C. Le Ru, P.G. Etchegoin, Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects, Elsevier, Amsterdam; Boston, 2009.

38. R.G. Freeman, K.C. Grabar, K.J. Allison, R.M. Bright, J.A. Davis, A.P. Guthrie, M.B. Hommer, M.A. Jackson, P.C. Smith, D.G. Walter, M.J. Natan, Science 267 (1995)1629.

39. H. Wang, J. Kundu, N.J. Halas, Angewandte Chemie-International Edition 46 (2007) 9040. 40. M. Kahraman, M.M. Yazici, F. Sahin, M. Culha, Langmuir 24 (2008) 894. 41. J. Ma, J.J. Lu, L.H. Lu, J.W. Hu, J.G. Pan, W.Q. Xu, Chemical Journal of Chinese

Universities-Chinese 30 (2009) 2288. 42. H.S. Shin, H.J. Yang, Y.M. Jung, S. Bin Kim, Vibrational Spectroscopy 29 (2002)79.

36

43. Xu B-B, Ma Z-C, Wang L, Zhang R, Niu L-G, Yang Z, Zhang Y-L, Zheng W-H, Zhao B, Xu Y, Chen Q-D, Xia H and Sun H-B 2011 Lab Chip 11 3347

44. Tseng ML, Huang Y-W, Hsiao M-K, Huang HW, Chen HM, Chen YL, Chu CH, Chu N-N, He YJ, Chang CM, Lin WC, Huang D-W, Chiang H-P, Liu R-S, Sun G and Tsai DP 2012 ACS Nano 6 5190

45. Izquierdo-Lorenzo I, Jradi S and Adam P-M 2014 RSC Adv. 4 4128 46. Stolzer L, Quick AS, Abt D, Welle A, Naumenko D, Lazzarino M, Wegener M, Barner

Kowollik C and Fruk L 2015 Chem. Commun. 51 3363 47. Howarter JA, Youngblood JP. Langmuir. 2006;22:11142. [PubMed].