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Page 24 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

[179]
probe, forming the basis of a LSPR biosensor for the detection of Helicobacter pylori (H. pylori) . This
biosensor was achieved by coating the fiber surface with AuNPs and specific aptamers, as shown in
Figure 10E. CNTs have been selected as sensing materials for fiber optic LSPR sensors due to their unique
advantages, including high chemical stability and large surface area. An example is a fiber optic SPR sensor
based on multi-walled CNTs (MWCNTs)-PPy matrix for dopamine detection . In addition, nanoparticles
[180]
such as SiO 2 [181,182] , ZnO [183,184] , TiO 2 [185-187] , and MoS 2 [188] have been used in LSPR-based fiber-optic sensors for
detecting gas concentration, humidity, specific ion concentration, and other parameters. LSPR generates a
locally enhanced electromagnetic field, which makes LSPR particularly suitable for detecting localized
environmental changes or specific markers.

To further improve the detection accuracy and expand the application range of fiber optic SPR sensors,
multi-channel SPR sensors have been developed. These sensors compensate for non-specific binding and
allow for the simultaneous determination of multiple analytes [176,189] . Wang et al. proposed a dual-channel
[190]
self-compensating fiber optic SPR sensor for the detection of human immunoglobulin G (IgG) . One
sensing channel was coated with a bilayer of GO and gold to detect human IgG labeled with AuNPs. The
other sensing channel was coated with a silver film only, serving as a reference channel [Figure 10F]. The
sensor’s detection limit for human IgG was reduced to 15 ng/mL, which is superior to the detection limit of
conventional SPR sensors [Figure 10G]. Siyu et al. designed a dual-channel SPR sensor based on a hollow-
core fiber (HCF) . Coating the inner and outer surfaces of the HCF with silver and gold films,
[191]
respectively, enables the simultaneous measurement of seawater salinity and temperature. Recently, Zheng
et al. deposited AuNPs/β-cyclodextrin (β-CD) composites onto a gold-film-coated HCF and filled the inner
channel with glycerol as a temperature-sensitive material . This setup effectively induced both the SPR
[192]
effect and the multi-mode interference (MMI) effect, enabling simultaneous measurement of cholesterol
concentration and temperature.

The application of optical fiber sensors in environmental monitoring has highlighted their significant
advantages in high-sensitivity detection, particularly through SPR and LSPR technologies. However, current
techniques face challenges, such as crosstalk among multiple physical signals and distributed sensing.
Looking ahead, the ongoing development of new materials and advancements in multi-channel sensing
technology are expected to expand the applications of optical fiber sensors in areas such as bio-detection
and gas concentration monitoring, thereby facilitating the evolution of intelligent environmental
monitoring systems.

POF sensor with surface functionalization
Traditional silicon-based optical fibers have been developed over several decades and are renowned for their
high light conduction efficiency. However, their inherent brittleness constrains the range of potential
application scenarios. In contrast, POFs, made from polymers with superior bending flexibility, can
withstand larger strains, making them ideally suited for flexible sensors. Functional microstructures
processed on the POF surface enable single or multi-mode sensing [54,193] , including strain [194,195] ,
temperature , heart rate , blood monitoring [30,197] , etc. Specific approaches include increasing the surface
[111]
[196]
area to enhance inter-fiber friction , creating arrayed micro-nano structures for specialized optical
[198]
applications [199,200] , and even integrating electronic devices directly onto the POF surface [42,201] .
Nanoimprinting technology is well-suited for fabricating arrayed micro- and nanostructures on the surface
of POFs or other functional fibers, offering high throughput and high resolution. Mekaru et al. conducted
an extensive study using this process, preparing dot-array structures on the surface of 250-μm-diameter
[202]
POFs with a length of 1.6 m using roller nanoimprinting technology [Figure 11A and B]. Additionally,