Fig. 1. Comparison of explosive hazards and detection methods. (a) Danger and detection necessity of explosives; (b) traditional detection methods and disadvantages; (c) fiber optic detection method and advantages

Fig. 2. Schematic diagrams of molecular orbital energy levels for different quenching mechanisms. (a) Fluorescent molecules absorb excitation light; (b) PET; (c) FRET; (d) EE

Fig. 3. Schematic diagrams of fluorescence-based optical fiber sensor detection methods. (a) Transmissive fiber optic sensor; (b) reflective fiber optic sensor

Fig. 4. Working principle of QDs interacting with TNT and fluorescence response spectra^[30]. (a) Working principle of gQDs@SiO₂@rQDs interacting with TNT; (b) fluorescence spectra and the corresponding photographs under different concentrations of TNT (from a to j: 0, 10, 50, 100, 500, 1200, 2100, 5000, 8000, 16000 nmol/L)

Fig. 5. Preparation process and sensing equipment for PPOF^[37]. (a) Preparation scheme of PPOF; (b) test setup for PPOF

Fig. 6. Optical fiber sensing platform for explosive detection and scanning electron microscopy (SEM) image of fiber end-face^[39]. (a) Suspended core microstructured optical fiber sensor platform; (b) SEM image of the inner wall for suspended core fiber internally coated with MEH-PPV

Fig. 7. Explosive detection equipments and PA test data analysis^[40]. (a) Reflective optical fiber sensor; (b) log plot of I₀/I^-1versus PA concentration of 0.1‒200 ppm, illustration is spectra of quenching process upon the addition of PA; (c) Stern-Volmer plots for PA concentration detection of aggregated polytriazole molecules and polytriazole-coated fiber optic sensor

Fig. 8. Structure of HC-PCF and interaction mechanism of AL-TPE molecules with explosives^[41]. (a) Structure of the HC-PCF trace explosive sensor based on the fluorescence quenching; (b) SEM image of the HC-PCF cross section; (c) electron transfer process between the fluorescence film and the TNT

Fig. 9. Schematic diagrams of SERS enhancement effects. (a) Electromagnetic enhancement theory; (b) chemical enhancement theory

Fig. 10. Detection principle of SERS fiber optic sensor^[49]

Fig. 11. UV-Vis absorption spectra and Raman signal spectra of AuNR@AgNCs for TNT detection^[57]. (a) Ultraviolet-visible absorption spectra of TNT, PATP, TNT-PATP complex in ethanol solution, and AuNR@PATP in aqueous solution, illustration is a schematic diagram of the formation for the Meisenheimer complex; (b) SERS spectra of TNT with different concentrations on SERS substrate

Fig. 12. Response mechanism and selectivity analysis of AuNPAs-TNT-AgNPs “sandwich” structure^[58]. (a) Schematic diagrams of TNT detection principle and interaction mechanism between PATP and TNT; (b) ratios of Raman intensities at 1073 cm^-1 for different substances

Fig. 13. Portable SERS sensor^[53]

Fig. 14. Explosive detection equipment based on elevated gold bowtie array and TNT detection data^[66]. (a) SEM image of elevated gold bowtie array; (b) schematic diagram of a portable Raman sensor coupled with a fiber-optic Raman probe and a miniaturized XYZ stage; (c) SERS spectra with the addition of different concentrations of TNT standard in polluted ground water samples (from a to j: 0, 1.6, 2.3, 5.5, 9.3, 14.0 mmol/L)

Fig. 15. SERS signal acquisition and spectral intensity variation^[62]. (a) Signal acquisition diagrams of high-NA lens a and low-NA optical fiber probe b; (b) SERS spectra of 2,4-DNT at different concentrations, a‒f are mass fractions of 100, 10, 1, 0.1, 0.01 mg/kg, and 0 mg/kg; (c) variation in SERS intensity with concentration of 2,4-DNT under 1352 cm^-1

Fig. 16. Detection of TNT based on SPR and LSPR. (a) SPR sensor system and experimental setup^[63]; (b) plasmon resonance wavelength curve with TNT concentration^[63]; (c) SPR-MIP sensor^[64]; (d) LSPR-MIP sensor^[64]

Fig. 17. Vapor-phase explosive detection equipment and TNT detection flowchart^[67]. (a) Optical experimental setup; (b) probe inserted into a glass capillary; (c) gas-phase detection

Fig. 18. Explosive detection experimental setup and internal geometric structure of the detector^[72]. (a) Schematic diagram of the sensor experimental setup; (b) geometric structure of the explosive detector