Investigation Of A Two-dimensional Material-sensitized Surface Plasmon Resonance Sensor In Optical Fiber

Surface plasmon resonance（SPR）sensing technology is a prominent approach in the field of micro-nano optics and biomedical research.Particularly,SPR sensors utilizing optical fibers as sensing substrates have garnered extensive attention due to their compact size,easy integration,real-time detection capability,and suitability for remote sensing applications.With the increasing prevalence of sensor-based disease detection in recent years,there is a growing demand for enhanced sensitivity,quality factor,and detection limit of fiber optic SPR sensing substrates.This thesis focuses on practical biomolecule detection as the research background and aims to improve the sensing performance of fiber optic substrates by enhancing fiber types,upgrading sensing substrates,introducing novel sensitizing layers,and refining modification techniques.Various designed and investigated sensing substrates are presented in this study.Among them,the thesis consistently emphasizes the concept of sensitization using two-dimensional（2D）materials,with monolayer graphene（G）and two-dimensional metal-organic frameworks（2D MOFs）serving as representative examples.As a sensitizing layer for optical fiber SPR sensors,2D materials can enhance electron mobility and light absorption on the sensing substrate surface,effectively improving surface electric field intensity and photon-plasma coupling efficiency.Moreover,these functional layers provide abundant binding sites for biomolecule attachment,facilitating low limit detection and specific identification of biomolecules by optical fiber SPR sensors.The research content encompasses:（1）By utilizing a D-shaped plastic optical fiber combined with a composite structure of hyperbolic metamaterials（Au/Al₂O₃）/monolayer graphene,we have successfully fabricated an optical fiber SPR sensor that is highly sensitive,cost-effective,and user-friendly.By utilizing hyperbolic metamaterial（HMMs）structures instead of pure metal structures,the gap plasmon polaritons（GPP）between layered structures induce the generation of bulk plasmon polaritons（BPP）with high wave vectors.The BPP mode can augment the surface confinement capability of the sensor and amplify the strength of the evanescent field.Through mathematical derivations,we have proven the existence of BPP within the hyperbolic metamaterials as well as its dispersion relationship with GPP and SPP.In terms of simulation,finite element method calculations were conducted on a scaled model using COMSOL software to demonstrate that a three-layer HMMs structure offers optimal sensitivity values and electric field intensity.Moreover,incorporating graphene as a sensitizing layer can further enhance sensing capabilities.The experimental results are in agreement with the simulation findings,and the incorporation of a monolayer graphene enhances the sensor’s detection sensitivity to 5166.7 nm/RIU,representing a 1.7-fold increase compared to its initial structural sensitivity value.In practical applications,by employing an optimized optical fiber sensing substrate,glucose solutions at various concentrations were detected to showcase excellent detection uniformity and repeatability.（2）Building upon the previous research on high-performance sensing properties of hyperbolic metamaterials/graphene composite structures,we upgraded the D-shaped plastic optical fiber to a single-mode quartz fiber,expanding the detection range from visible light to near-infrared region.In terms of simulation analysis,we established a model in COMSOL software and demonstrated that when the real part of effective refractive index for the core mode is equal to that of surface plasmon polariton（SPP）mode,phase matching between these two modes can be achieved.At this point,most of the energy from the core mode transfers to the sensing surface generating surface plasmon resonance.Additionally,formula calculations revealed a matching relationship between Bloch wave dispersion curve inside the metamaterial and momentum of plasma waves.In this study,we still employed monolayer graphene as a sensitizing layer to facilitate excitation of plasmons in metal structures and enhance surface electric field intensity.Moreover,monolayer graphene also served as a functional layer with honeycomb arrangement of carbon atoms capable of formingπ-stacking interactions with aromatic rings for linking with 1-pyrenebutyric acid succinimidyl ester（PBASE）molecules used for modification of probe DNA molecules.Ultimately,this enabled detection of target DNA molecules at different concentrations with a minimum detectable concentration being 10 p M.By conducting a thorough analysis of the DNA hybridization process using molecular dynamics,we have observed that the binding curve calculated from the formula aligns consistently with the trends observed in actual experimental data.（3）By introducing a new type of sensitization layer,the sensing parameters of the sensor are improved.The novel two-dimensional metal-organic framework（2D MOF）serves as a sensitization layer on the sensing substrate,effectively ensuring both sensitivity enhancement and quality factor preservation of the sensor.In this study,metal nanoparticles were combined with 2D MOF to generate localized surface plasmon resonance（LSPR）coupled with surface plasmon resonance（SPR）,thereby achieving secondary amplification of SPR signals.Compared to pure metal film structures,the sensitivity and quality factor of gold nanoparticle/metal-organic framework/gold film composite structure increased by 104.61%and 73.87%,respectively.In terms of simulation analysis,a proportional three-dimensional model was constructed to achieve a maximum sensitivity value of 10000 nm/RIU.Furthermore,the sensing mechanism of this composite structure is further explained through electron transfer and band transition theory.In terms of applications,probe DNA is used as a linker for specific detection of dopamine molecules by establishing multiple-mode hydrogen bonding between nucleobases in probe DNA and hydroxyl/amino groups in dopamine molecules,enabling low limit-of-detection for dopamine molecules(LOD=1.07±0.07×10^-14 M).