Localized Surface Plasmon Resonance on Optical Fiber Surface

for Chemical Sensing

Amin Moslemi

, Lucia Sansone

, Flavio Esposito

, Stefania Campopiano

Michele Giordano

and Agostino Iadicicco

Department of Engineering, University of Naples “Parthenope”, 80143 Naples, Italy

Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, IPCB-CNR, 80055 Portici, Italy

Keywords: Fiber Optic Sensors, Localized Surface Plasmon Resonance, Gold Nanoparticles, Chemical Sensing.

Abstract: This study describes the basic principles of a fiber optic probe that exploits the localized surface plasmon

resonance (LSPR) effect achieved by depositing gold nanoparticles (AuNPs) onto the fiber optic transducer.

The idea is to read the absorbance spectra of AuNPs and its dependence on the environmental parameter, i.e.

the surrounding refractive index, using optical fiber. Basically, we selected a thin optical fiber to encourage

the presence of evanescent waves in the surrounding medium; furthermore, the fiber surface has been

functionalized allowing the grafting of AuNPs while a silver mirror on the fiber tip allows readout for

reflection configuration. The reflected spectra show absorbance characteristics related to single and

aggregated AuNPs. In this article, the peaks absorbance, i.e. the depths of the reflected signal, were studied

as a function of the surrounding refractive index for application in chemical sensing.

1 INTRODUCTION

The widespread adoption of optical fiber chemical

and biological sensors is notable, offering significant

potential to eventually supplant conventional and

unwieldy sensing devices. Their compact size,

flexibility, and resistance to electromagnetic

interference make them promising candidates for

future applications, facilitating remote

measurements. These sensors find utility across

diverse domains, including temperature, pressure,

and refractive index (RI) monitoring, along with the

quantification of biomarkers and chemical

compounds (Choudhary et al., 2023; Esposito et al.,

2021; Hernandez-Romano et al., 2015; Lu et al.,

2022; Soares et al., 2021a).

Various optical detection techniques exist,

necessitating diverse fiber geometries, materials, and

equipment for implementation. Plasmonic

approaches, such as surface plasmon resonance

(SPR), stand out for their swift, label-free, and real-

https://orcid.org/0000-0002-5032-6143

https://orcid.org/0000-0003-1187-5825

https://orcid.org/0000-0002-2987-9122

https://orcid.org/0000-0003-0872-5578

https://orcid.org/0000-0002-3540-7316

time sensitivity. In this mechanism, an interplay and

energy exchange transpire between photons from

incident light and electrons on a metal surface,

resulting in their collective oscillation. SPR manifests

at the interface between a dielectric and a metal

nanofilm, leading to the propagation of surface

plasmon polaritons (Esfahani Monfared, 2020; Mayer

& Hafner, 2011; Sansone et al., 2021).

Gold nanoparticles (AuNPs) are favored for

chemical and biosensor applications due to their

distinct features, including excellent compatibility,

intense light scattering/absorption, high surface area

to volume ratios, selective interoperability through

electrostatic interaction, stable structure, and non-

toxicity (Huong et al., 2021; Jain et al., 2006; Saha et

al., 2012; Zeng et al., 2011). Additionally, AuNPs are

employed for the excitation of a unique phenomenon

known as localized surface plasmon resonance

(LSPR), extensively explored in recent sensing

platforms due to its significant advantages (Chau et

al., 2006; Do et al., 2020; Guerreiro et al., 2014).

110

Moslemi, A., Sansone, L., Esposito, F., Campopiano, S., Giordano, M. and Iadicicco, A.

Localized Surface Plasmon Resonance on Optical Fiber Surface for Chemical Sensing.

DOI: 10.5220/0012593500003651

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 12th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2024), pages 110-117

ISBN: 978-989-758-686-6; ISSN: 2184-4364

LSPR offers a compact, label-free, highly sensitive,

and stable biosensing approach for detecting

biological molecules (Wang et al., 2017; Willets &

Van Duyne, 2007). This phenomenon in metallic

nanostructures is associated with the resonance of

free-electron waves in metal. Incident light can

resonate with the oscillations of surface electrons at

an excitation frequency, leading to the collective

oscillation of surface plasmons, referred to as an

LSPR mode (Unser et al., 2015).

In this paper, a fiber optic sensor is introduced for

monitoring changes in the concentration of chemicals

in a solution by measuring the refractive index. The

proposed sensor is based on the deposition of gold

nanoparticles around the fiber. The novelty in this

study lies in utilizing both single and aggregated

nanoparticle spectra to sense the surrounding

refractive index area, supported by numerical results.

2 NUMERICAL RESULTS

In the numerical analysis, the Finite Element Method

(FEM) is employed to simulate the interaction of light

with AuNPs. At the core of this simulation lies either

a single or an aggregate of nanoparticles, with a

diameter of 50 nm for the single NP. Surrounding this

central element is a 200 nm thick dielectric layer, and

beyond that, another 200 nm dielectric layer acts as a

Perfectly Match Layer (PML), as reported in Figure

1(a) whereas the zoom on the AuNP is highlighted in

Figure 1(b).

The purpose of the PML is to establish an infinite

dielectric environment around the gold structure. This

configuration ensures that incident light is not

reflected toward the gold structure, consequently

minimizing the impact on the absorption band. The

incoming light is planar in the z-direction, ranging

from -z to +z, and polarized in the x-direction. The

power of the illumination is maintained at 1 W/m².

Utilizing the FEM, the heat losses for each

wavelength in VIS range are computed and

subsequently plotted. This enables the plot of heat

losses curve versus wavelength, i.e., the absorption

spectrum. The red line in Figure 2 plots the absorption

spectrum of a single 50 nm AuNP in water.

According to (Lei et al., 2023; Litti & Meneghetti,

2019), 50 nm AuNP exhibits an absorption maximum

at around 𝝀 = 535 nm. Interestingly, increase in AuNP

size are manifested in slight redshift of the AuNPs

absorption band. In the initial phase, the absorption

spectrum of a single nanoparticle is investigated

across various surrounding refractive indices (SRI) to

determine sensitivity, as shown in Figure 2.

Figure 1: Schematic of the simulation environment: (a),

various layers, encompassing the PML, dielectric layers,

and the gold structure; (b) the electric field distribution

around the individual gold nanoparticle.

Figure 2: Absorption band of single AuNP in different

environments: air, water, and a dielectric environment with

refractive indices of 1.34 and 1.35.

Altering the refractive index of the surrounding

medium induces a redshift in the absorption band

peak. By tracking the resonance wavelength of this

peak, as reported in Figure 3, the sensitivity of the

system can be computed. This sensitivity value is

determined to be around 200 nm/RIU (Refractive

Index Unit, RIU).

Localized Surface Plasmon Resonance on Optical Fiber Surface for Chemical Sensing

111

Figure 3: Tracking of the resonance wavelength of the

absorption band peak of a single gold nanoparticle across

various surrounding refractive index values.

The aim of the subsequent phase of the numerical

simulation is the investigation of the effect of the

aggregations. A full understanding demands various

aggregation states to be explored. However, here,

groups of 3 nanoparticles, each with a 50 nm

diameter, are aggregated as reported in Figure 4, just

as an example. The absorption band for each structure

is then calculated. In a unique approach, each

structure undergoes a 90° rotation around the x, y, and

z axes during stimulation. Following this rotation, the

superposition of all absorption spectra is computed.

This step is essential, mirroring real-world scenarios

where aggregated nanoparticles exhibit rotational

dynamics.

Figure 4: Distribution of absorption energy from incident

light on the surface of three aggregated golden

nanoparticles.

The resulting output spectrum is a superposition

of numerous structures, incorporating different

numbers of nanoparticles and incident angles. In

Figure 5, the spectra of 3 nanoparticle group under

various incident light angles are depicted, along with

their superposition. The superposition spectrum

exhibits two mean resonance wavelengths at 560 nm

and 678 nm, respectively. Although this analysis is

not sufficient to investigate the state of all

aggregations, we can retrieve that single AuNPs show

an absorption peak around 520 nm whereas AuNPs

aggregations, due to the larger size, show an

absorption peak at a higher wavelength (Litti &

Meneghetti, 2019).

Figure 5: Absorption spectrum related to the superposition

of different configurations involving 3 single nanoparticles.

3 MATERIALS AND METHODS

3.1 Chemicals

Tetrachloroauric acid (HAuCl₄·3H₂O, 99%) and

trisodium citrate (99%) were purchased from Merck

Life Science (Milano, Italy), along with distilled

water. Piranha solution (mixture of H

and H

7:3 v/v). (3-Aminopropyl)triethoxysilane (APTES) in

acetone (5% w/w).

3.2 AuNPs Preparation and

Characterization

A solution containing 2.2 mM sodium citrate in

distilled water (150 mL) was heated using a heating

mantle in a 250 mL three-necked round-bottomed

flask for 15 minutes under vigorous stirring. A

condenser was employed to prevent solvent

evaporation. Once boiling started, 1 mL of HAuCl₄

(25 mM) was injected. The solution colour

transitioned from yellow to bluish-grey and then to a

soft pink within 10 minutes. Immediately following

the synthesis of the Au seeds and within the same

reaction vessel, the reaction was cooled until the

solution temperature reached 90 °C. Subsequently, 1

AOMatDev 2024 - Special Session on Advanced Optical Materials and Devices

112

mL of a HAuCl₄ solution (25 mM) was injected. After

30 minutes, the reaction was concluded. This process

was repeated twice. Subsequently, the sample was

diluted by extracting 55 mL of the solution and

adding 53 mL of MQ water and 2 mL of 60 mM

sodium citrate. This resulting solution served as the

seed solution, and the process was reiterated five

times (Figure 6).

Figure 6: Synthesis of gold nanoparticles.

To examine the morphology of AuNPs,

transmission electron microscopy (TEM)

experiments were carried out using a TEM FEI

Tecnai G12 Spirit Twin equipped with a LaB6

emission source (120 kV) and coupled with a CCD

camera Fei Eagle 4K (Japan). For TEM specimen

preparation, a drop of the diluted AuNPs suspension

was deposited onto a carbon-coated copper grid.

Subsequently, the sample was air-dried and loaded

into the electron microscope chamber.

The mean size and distribution of the

nanoparticles were determined through dynamic light

scattering (DLS) analysis. The measurements were

conducted at 25 °C using a Malvern DLS instrument

(model Zetasizer Nano ZS90, Malvern Instruments

Ltd., Worcestershire, UK), featuring a He-Ne laser

source at 632.8 nm and a detector set at a scattering

angle of 90°. Low-volume quartz batch cuvettes

(model ZEN2112, Malvern Instruments Ltd.,

Worcestershire, UK) served as the sample cells.

UV-Vis absorption spectra of AuNPs were

obtained using an Agilent Cary 60 UV/Vis

spectrophotometer, employing quartz cuvettes. To

prevent absorption saturation, dispersion samples

were appropriately diluted in deionized water at a

ratio of 1:8 v/v.

The two AuNPs water solutions were diluted at a

ratio of 1:10 v/v, and their size and polydispersity

index (PDI) were assessed through DLS analysis. The

presence of a single peak was observed for the AuNPs

solutions, corresponding to a size of 50.4 ± 5 nm (PDI

= 0.019). Additionally, the low values of PDI indicate

the high quality and monodisperses of the AuNPs

water solution. Figure 7 displays the visible (VIS)

spectrum of AuNPs, revealing a plasmon resonance

band at 535 nm. The narrow shape of the band

suggests a high uniformity in particle sizes.

According to (Bastús et al., 2011), the size of AuNPs

is approximately 50 nm.

Figure 7: UV spectrum and SEM images of AuNPs 50 nm.

3.3 Optical Transducer

The functioning principle of this sensor is grounded

in the response of 50 nm gold nanoparticles to

chemicals, probed by a fibre optics transducer, as

schematically reported in Figure 8.

The transducer incorporates a multimode fiber

(MMF) with core/cladding size of 105/125 µm

spliced with a short piece (less than 5 cm) of coating

free glass thin fiber with diameter of 80 µm.

To design a simple probe and capture the

reflection spectrum, the second end of the thin fiber

is sharply cut and mirrored to enhance reflection

power. Due to the fiber size mismatching, light in thin

fiber exhibits evanescent waves interacting with

surrounding medium.

Subsequently, nanoparticles are deposited around

the thin fiber, revealing an attenuation band in the

reflected spectrum of the device. Such attenuation

band is located at those wavelengths for which the

light is absorbed by the presence of nanoparticle, as

illustrated in the simulation section. This spectrum

changes in response to variations in the environment,

influencing the electrical and optical properties of

nanoparticles. Here, these changes are monitored to

sense alterations in the surrounding area.

Localized Surface Plasmon Resonance on Optical Fiber Surface for Chemical Sensing

113

Figure 8: Schematic of Optical Transducer.

3.4 Experimental Setup

The optoelectronic readout experimental arrangement

is shown in Figure 9 and comprises a broadband light

source (Avantes AvaLight-HAL-S-Mini). The light is

conveyed to the transducer using one branch of a

multimode fiber coupler, and the resulting reflection

spectrum is directed to a spectrometer (Ocean Optic

HR2000+) via the second channel of the coupler.

Figure 9: Schematic of the optoelectronic readout setup.

4 FABRICATION OF THE FIBER

PROBE

The multimode fiber with 105/125 µm core/cladding

diameter (FG105LCA Thorlabs) was fused with a 5

cm length of 80 µm thin fiber (SM1500(4.2/80)

Fibercore). A microscope picture of the fibers is

reported in Figure 10(a) and Figure 10(b), before and

after the splicing, respectively. The free end of the

multimode fiber was linked to the coupler. Then the

second end of the thin fiber was sharply cleaved and

mirrored using Tollen’s reaction.

Figure 11 plots the spectra of the fiber device

during the fabrication steps using the cleaved

multimode fiber as reference. Consequently, the

reflectance after coating the mirror exceeds 100% due

to this factor. According to Figure 11, this resulted in

an approximately fivefold increase in the amplitude

of the reflection signal.

The subsequent step involved depositing

nanoparticles onto the fiber. Given that neutral glass

Figure 10: Microscope images: a) Multimode and thin fiber

before splicing; b) The spliced area of the multimode fiber

and thin fiber.

Figure 11: Reflection spectrum of the transducer before and

after mirror.

is not inherently attractive to gold nanoparticles, it

was essential to impart a positive charge to the glass

surface through a surface functionalization. To

achieve this, the thin fiber was immersed in a piranha

solution (mixture of H

and H

) for one hour,

subsequently washed with water. This procedure has

been carried before mirror to avoid any damage due

to the chemicals.

Following, the mirrored fiber was immersed in

APTES (5% w/w) for two hours, washed with

acetone, and allowed to air-dry overnight (Soares et

al., 2021b; Sypabekova et al., 2022).

Subsequently, the fiber was immersed in the

solution containing gold nanoparticles (50 nm, 1 mM)

for 2.5 hours and left to dry overnight. Figure 12

Multimode

Fiber

Thin fiber

Silver mirror

Au NP

evanescence wave

AOMatDev 2024 - Special Session on Advanced Optical Materials and Devices

114

illustrates the spectrum of the transducer after

depositing gold nanoparticles, taken in the air using

the mirrored fiber (red line of Figure 11) as reference.

Here, two minima are well visible; based on the

numerical section, we retrieve that the left one is

related to single AuNPs whereas the wide attenuation

peak at higher wavelength can be attributed to AuNPs

aggregations.

Figure 12: Reflection spectrum of the transducer in air after

depositing nanoparticles.

5 SENSING RESULTS

The sensor sensitivity was assessed through

experimentation with the surrounding refractive

index. In this evaluation, various solutions were

prepared by blending deionized water and glycerine

in different proportions. The precise refractive index

of each solution was then determined using an Abbe

refractometer.

Subsequently, the sensing area was fully

immersed in each solution. The spectrum was

recorded, and the fiber was extracted, sequentially

immersed in deionized water to rinse off glycerine

residues, air-dried briefly, and the process was

repeated for subsequent solutions.

As one can observe in Figure 13, both the first and

second peaks exhibited a redshift with an increase in

the refractive index, which agrees well with

numerical simulations.

For tracing the resonance wavelength of the

minima, a second-order polynomial was fitted to each

one, and then the minimum of each polynomial was

traced. Figure 14 illustrates the shift of each

minimum. The sensitivity of the first and second

peaks was found to be around 90 nm/RIU and 450

nm/RIU, respectively. It is noteworthy that the

sensitivity of the second peak surpasses that of the

first peak by more than fourfold.

Figure 13: Reflection spectrum of the transducer in

different surrounding area refractive indices.

Figure 14: Resonance wavelength shift data and linear fit

for the (a) first and (b) second minima, respectively.

Localized Surface Plasmon Resonance on Optical Fiber Surface for Chemical Sensing

115

6 CONCLUSIONS

In conclusion, this paper conducted simulations on

various configurations of nanoparticle aggregation,

calculating absorption bands for each scenario.

Subsequently, a novel fiber optics probe was

designed and fabricated, relying on the deposition of

nanoparticles on the thin fiber surface. The key

innovation lies in exploring the sensitivity of the first

and second peaks, corresponding to single and

aggregated nanoparticles, respectively. The

sensitivity to the surrounding refractive index was

found to be 90 nm/RIU for the first peak and 4 times

higher, at 450 nm/RIU, for the second peak. This

sensor holds promise for applications in sensing

chemical concentrations based on changes in the

surrounding refractive index around nanoparticles.

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