Optical Fiber Probe Based on Localized Surface Plasmon Resonance

of Gold Nanostructures for Chemical Sensing

Amin Moslemi

, Lucia Sansone

, Flavio Esposito

1 a

, Carlos Marques

3,4

Stefania Campopiano

1 b

, Michele Giordano

and Agostino Iadicicco

1 c

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

CICECO – Aveiro Institute of Materials, Physics Department, University of Aveiro, Aveiro 3810-193, Portugal

Department of Physics, VSB – Technical University of Ostrava, Ostrava 70800, Czech Republic

Keywords: Optical Fiber Sensors, Localized Surface Plasmon Resonance, Plasmonics, Biosensors, Chemical Sensors.

Abstract: In this study, we present an experimental investigation of highly sensitive optical fiber sensors utilizing

localized surface plasmon resonance (LSPR), achieved by depositing gold nanoparticles (NP) onto uncladded

silica multi-mode fiber. This setup takes advantage of the unique optical characteristics of optical fiber sensors

and plasmonic resonance provided by gold NPs. The experimental results demonstrated a maximum

sensitivity of about 130 nm/RIU in water solution, for an LSPR wavelength at 560 nm. As a study case, the

sensor was used to detect Thiram, a common agricultural pesticide, exhibiting a wide detection range from

10 nM to 100 µM, with a significant wavelength shift up to 4 nm. Moreover, a preliminary study involving

the use of nanostar-based optical fiber sensors is comparatively provided. The highest sensitivity makes this

approach highly promising for a range of applications, including environmental monitoring, biomedical

diagnostics, and chemical detection.

1 INTRODUCTION

Localized surface plasmon resonance (LSPR) refers

to the coherent oscillation of surface conduction

electrons in noble metal nanoparticles induced by

electromagnetic radiation at the nanoscale (Ma et al.,

2021; Mayer & Hafner, 2011). It is widely utilized for

sensing applications, with the LSPR absorption band

and its spectral position highly dependent on the

electrical properties of the noble metal (typically gold

or silver), the size and shape of the nanostructures,

and the dielectric properties of the surrounding

medium (Yaghubi et al., 2020). Consequently, these

devices are emerging as advanced sensors for

chemical and biological detection due to their

exceptional sensitivity to variations in the

surrounding medium refractive index (SRI) (Jeon et

al., 2019; Ma et al., 2021).

While early investigations of LSPR focused on

spherical gold nanoparticles (NPs), researchers are

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

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

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

now exploring a variety of sizes, shapes, and

materials. Nanostars (NSs) and nanorods (NRs), for

example, have different LSPR attenuation bands

compared to spherical NPs due to variations in their

shape and structural properties. These variations

allow for the generation of multiple resonances and

the ability to tune both the resonance wavelength and

sensitivity (He et al., 2020; Ringe et al., 2010;

Shabaninezhad & Ramakrishna, 2019; Ueno et al.,

2007). As a result, nanostructures with increased

aspect ratio offer high sensitivity for label-free

biosensing and chemical detection (Yuan et al.,

2012). For instance, Nguyen et al. introduced a

sensitive nanoplasmonic biosensor capable of

detecting two key epigenetic biomarkers using NSs:

methyl-CpG and MBD2, with detection limits of one

5-methylcytosine molecule and 125 fM MBD2,

respectively (Nguyen et al., 2015). Dondapati et al.

reported streptavidin binding to biotin-modified NSs

causing a plasmon resonance shift of 2.3 nm at

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

Optical Fiber Probe Based on Localized Surface Plasmon Resonance of Gold Nanostructures for Chemical Sensing.

DOI: 10.5220/0013395000003902

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

In Proceedings of the 13th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2025), pages 157-162

ISBN: 978-989-758-736-8; ISSN: 2184-4364

157

concentrations as low as 0.1 nM. (Dondapati et al.,

2010). Finally, Hashemi et al. developed a highly

sensitive electrochemical nanosensor to detect

monoclonal IgG antibodies against the SARS-CoV-2

S1 protein in blood within 1 minute. The sensor,

utilizing activated graphene and gold NSs, achieved

an ultra-low detection limit of 0.18∙10⁻

%V/V

(Alireza Hashemi et al., 2021).

Fiber optic sensors are increasingly popular due to

their advantages, including high sensitivity, improved

signal-to-noise ratio, cost-effectiveness, and the

ability to operate in harsh environments. These

characteristics make them suitable for a wide range of

chemical and biological sensing applications

(Choudhary et al., 2025; Gandhi et al., 2019;

Ricciardi et al., 2015).

While there is extensive literature on using

spherical NPs with optical fiber sensors, the

integration of other types of nanostructures with

optical fibers (Cennamo et al., 2013; Xiao et al.,

2025), however, is less explored, with only a few

studies available. For example, Cennamo et al.

introduced a novel optical chemical sensor that uses

molecularly imprinted polymer and NSs on plastic

optical fibers for the selective detection of

Trinitrotoluene in aqueous solutions. The device

achieves a sensitivity of 8.3∙10⁵ nm/M, 30 times

higher than previous gold layer SPR sensors

(Cennamo et al., 2015). In another study, Dos Santos

et al. utilized high aspect-ratio gold-silver nanorods,

which demonstrated a refractive index sensitivity of

1720 nm/RIU at 1350 nm (O-band) and 2325 nm/RIU

at 1550 nm (L-band). Using a side-polished optical

fiber, glyphosate detection they achieved a detection

limit improvement from 724 to 85 mg/L by shifting

to the C/L bands (dos Santos et al., 2025).

In this work, fiber optic probe based on spherical

gold nanoparticles properly grafted onto etched

multi-mode fiber (MMF) surface are evaluated for

chemical sensing applications. Specifically, its

performance has been assessed through the detection

of a dangerous compound like Thiram pesticide.

Moreover, a preliminary comparative study on the

use of gold-silver NSs in place of gold NPs is also

presented.

2 FABRICATION OF GOLD

NANOPARTICLES AND THEIR

CHARACTERIZATION

Gold NPs with spherical shapes were synthesized

using the well-assessed Turkevich method (Moslemi

et al., 2024). Figure 1(A) presents the UV-Vis spectra

recorded with an Agilent Cary 60 spectrophotometer

using quartz cuvettes, showing the results for

prepared NP samples. The absorbance peak centred at

531 nm is clearly visible. The morphological

characterization is provided by TEM images as in

Figure 1(B) where an average diameter of 40 nm can

be observed.

Figure 1: (A) UV-Vis spectra of gold NPs in solution; and

(B) TEM image of gold NP.

3 FABRICATION AND

CHARACTERIZATION OF THE

OPTICAL FIBER PROBE

3.1 Fabrication of the Probe

The basic sensor configuration is illustrated in Figure

2. It features an etched silica MMF with a mirrored

tip to enable reflected signal readout. The uncladded

etched region of the fiber is coated with metallic

nanostructures. The reflected signal from the probe

exhibits an attenuation band corresponding to the

excitation of LSPR.

(A)

(B)

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Figure 2: Schematic of the optical fiber probe.

For the fabrication, a few cm long piece of a

105/125 µm core/cladding MMF model Thorlabs

FG105LCA was etched for 25 minutes using 24%

hydrofluoric acid until the diameter was reduced to

95 μm, i.e. to remove the cladding and expose the

core. This allows the core modes to interact with

external media via the evanescent wave.

The silica surface of the etched fiber is then

silanized to facilitate the grafting of metallic

nanostructures. The process involves the following

steps. First, the fiber is immersed in piranha solution

for one hour to promote the formation of hydroxyl

groups, followed by thorough rinsing with water.

Next, it is immersed in an 5% w/w (3-

Aminopropyl)triethoxysilane (APTES) solution for

two hours to achieve surface silanization (Ben

Haddada et al., 2013). Afterward, the fiber is cleaned

with acetone and left to air-dry overnight.

Finally, to enable the reflection-based readout

approach, the fiber tip was cut and coated with a silver

layer to create a mirror.

Next, the functionalized fiber was immersed in the

metallic nanostructure solution prepared as detailed

in Section 2 for approximately four hours. The fiber

was then removed from the solution, left to dry

overnight, and washed five times with water to

remove loosely attached nanostructures.

Figure 3: Schematic of the optoelectronic readout setup.

An optoelectronic readout setup is necessary to

monitor the reflection spectrum of the so-prepared

sensing probe. The schematic of the setup is shown in

Figure 3: it consists of a broadband light source

(Avantes AvaLight-HAL-S-Mini) to provide incident

light. The light source is connected to a 50:50 MMF

coupler, while the transducer is connected to the

output of the coupler. The coupler then directs the

reflected signal to the spectrometer (Ocean Optics

HR2000+), and the data is displayed on a PC for

storage and analysis.

Figure 4 presents the reflected spectra of the

samples coated with the NPs, when immersed in

water environment. The spectra feature an attenuation

band indicative of LSPR. Specifically, the NP-based

sensor exhibits an attenuation band centred at 560 nm,

in agreement with nanostructure spectra taken in

solution in Figure 1. The slight difference is attributed

to the fact that now the gold NPs are deposited onto

optical fiber surface.

Figure 4: Reflection spectra of the transducer with NP

deposition when the surrounding medium is water.

3.2 Sensitivity to Bulk Surrounding

Refractive Index

For this study, the sensitivity to surrounding

refractive index (SRI) medium changes of the NP

sample was evaluated to compare their sensing

performance based on synthesis and morphology.

Solutions with varying refractive indices, ranging

from SRI = 1.33-1.40, were prepared by mixing

deionized water and glycerine.

Subsequently, the sensor was sequentially

immersed in the prepared solutions to monitor their

response. The corresponding wavelength shifts of the

attenuation band are depicted in Figure 5, where

experimental data points are shown as markers, and

linear fits are represented by solid lines. The

calculated SRI sensitivity is about 130 nm/RIU

(Zhang et al., 2023).

Mirror

Etched fiber

Nano particles

Core

Cladding

Multimode fiber

coupler

Light source

Spectrometer

Optical Fiber Probe Based on Localized Surface Plasmon Resonance of Gold Nanostructures for Chemical Sensing

159

Figure 5: Wavelength shift as a function of the SRI for NP-

based sensor.

4 STUDY CASE: DETECTION OF

PESTICIDE

In this section, NP-based sensor was analyzed for its

capability to detect Thiram pesticide, a compound

known to interact strongly with gold nanostructures

through its thiol groups. This binding event directly

affects the LSPR of the nanostructures, manifesting

as a wavelength shift in the attenuation band of the

sensor (Zhang et al., 2023).

Thiram solutions with concentrations ranging

from 10 nM to 100 µM were prepared in water to

perform this analysis. The sensor was initially

stabilized for the test by immersing them in deionized

water for 30 minutes. Following stabilization, it was

exposed sequentially to Thiram solutions of

increasing concentrations. Each immersion lasted

approximately 30 minutes, during which reflected

spectra were recorded every 30 seconds. Experiments

were performed under controlled temperature within

±0.5 °C.

Spectral results are reported in Figure 6(A), where

a progressive redshift in the LSPR attenuation band

was observed as the Thiram concentration increased

from 10 nM to 100 µM, confirming the binding

interaction between Thiram and the gold

nanostructures. Specifically, a maximum wavelength

shift of about 4 nm was achieved in the investigated

range of Thiram concentrations, allowing seamless

detection down to 10 nM.

5 PRELIMINARY RESULTS

BASED ON GOLD-SILVER

NANOSTARS

Recently, exotic noble metallic structures have been

widely investigated to enhance the plasmonic

interaction with target medium in order to reduce the

minimum detectable concentrations. On this line of

argument, in this section preliminary analysis on the

possibility to synthesize and investigate gold-based

NSs instead of gold NPs is reported.

NSs were synthesized using a seed-mediated

growth method. To prepare the seed solution, 15 mL

of 1% citrate solution was added to 100 mL of a

boiling 1 mM tetrachloroauric acid HAuCl

solution

while stirring vigorously. After 15 minutes of boiling,

maintaining the volume, the mixture was cooled and

filtered through a 0.22 µm nitrocellulose membrane,

then stored at 4 °C for long-term use. For NS

synthesis, 100 µL of the citrate-stabilized seed

solution was added to 10 mL of 0.25 mM HAuCl

solution, along with 10 µL of 1 M HCl, in a 20 mL

glass vial at room temperature under moderate

stirring (700 rpm). Then, 100 µL of silver nitrate (1.5

mM) and 50 µL of ascorbic acid (100 mM) were

added simultaneously. The solution was stirred for 30

seconds, during which its colour changed rapidly

from light red to blue or greenish black. The

nucleation process was halted by centrifuging the

solution at 3000-5000 rcf for 15 minutes in a 15 mL

tube. The solution was then redispersed in deionized

water, filtered through a 0.22 µm nitrocellulose

membrane, and stored at 4 °C for long-term

preservation.

Once the NS solution is prepared, a new optical

fiber sample has been prepared by the same procedure

reported in Section 3.1. In NS-based sensors, the

resonance wavelength is strongly dependent on the

NS fabrication parameters. A deep study involving

numerical and experimental results is currently in

progress.

Figure 6(B) shows the performance of NS-based

device for the detection of Thiram. The experiment

was repeated in the same way as for NP-based sensor.

The sensor exhibited a significant increase of the

sensitivity with a maximum wavelength shift of 8 nm

in the range of concentrations 10 nM - 10 µM. These

concentrations are significantly lower than those

explored by UV-Vis-based methods and comparable

to those investigated by SERS (Moslemi et al., 2024),

holding promise for future application in bio-

chemical sensing domain.

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Figure 6: Spectra of LSPR fiber optic probes during the

detection of pesticide Thiram at different concentrations,

employing (A) NPs and (B) NSs.

6 CONCLUSIONS

In summary, this study introduces a highly sensitive

LSPR-based fiber optic sensors employing gold

nanostructures deposited on uncladded silica

multimode fiber. Results first revealed an SRI

sensitivity of approximately 130 nm/RIU (in a water

environment) for an LSPR wavelength near 560 nm

leveraging gold NPs.

The sensor performance was further evaluated for

detecting Thiram pesticide in water, achieving a wide

detection range (10 nM to 100 µM) with a maximum

wavelength shift of 4 nm for NP-based device.

Moreover, preliminary results on another device

employing NSs highlighted enhanced promising

performance. These straightforward and cost-

effective designs allow detection of very low analyte

concentrations compared to conventional SERS and

UV-Vis-based methods, making them an attractive

solution for applications in digital agriculture,

environmental monitoring, and diverse chemical

detection scenarios.

ACKNOWLEDGEMENTS

This work was performed within the project

“Monitoring of Honeybee Immunomodulation and

Resilience to Stress Factors by Fiber Optic

Technology” (MoBeeFO) CUP I53D23000390006

under PRIN 2022 grant financed by the European

Union – Next Generation EU, Mission 4 Component 1

and Italian Ministry of University and Research. This

work was also developed within the scope of the

projects CICECO (LA/P/0006/2020, UIDB/50011/

2020 & UIDP/50011/2020) and DigiAqua

(PTDC/EEI-EEE/0415/2021), financed by national

funds through the (Portuguese Science and

Technology Foundation/MCTES (FCT I.P.)). The

research was co-funded by the financial support of the

European Union under the REFRESH – Research

Excellence For REgion Sustainability and High-tech

Industries project number CZ.10.03.01/00/22_003/

0000048 via the Operational Programme Just

Transition.

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