Fig. 1. Molecular structures of morphine, heroin, and codeine

Fig. 2. Experimental results. (a) Process of rapid detecting morphine in urine by LLME-SERS; (b) Raman spectra of morphine with different mass concentrations in urine^[36]

Fig. 3. SERS spectra of morphine illegally added in chafing dish^[43]

Fig. 4. Experimental results. (a) Schematic diagram of morphine analysis based on immunochromatography-SERS; (b) relationship curve between B/B₀ value and morphine mass concentration (B/B₀: intensity of peak at 1078 cm^-1 of the sample solution/the intensity of the peak at 1078 cm^-1 of the blank solution. Inset: corresponding immunochromatographic detection assay stripes); (c) specificity of morphine detected by ICA-SERS^[44]

Fig. 5. Experimental results. (a) SERS spectrum obtained after wiping the surface containing 5 μg of heroin and performing lateral flow concentration; (b) heroin mass concentration curve obtained by measuring the peak intensity at 626 cm^-1 (inset: peak intensity at the low nanogram levels); SERS spectra of heroin and IR780 dye (c) before and (d) after chromatographic separation^[48-49]

Fig. 6. Experimental results. (a) Schematic diagram of NaCl crystal-induced SERS platform and the distribution of effective laser spots; (b) SERS spectra of heroin with different mass concentrations collected at of 2 μm from the edge of NaCl crystal^[50]

Fig. 7. Experimental results. (a) Photograph of the glass fiber sheet before (top) and after (bottom) loading Au nanoparticles, and a simple sampling rod (middle); (b) SERS spectra of codeine at 50, 25, 10, 5, 1, 0.5, 0.25, and 0.1 μg/mL; (c) photograph of the SERS flow-separation strip; SERS spectra of different mass concentrations of codeine in (d) saliva, (e) plasma, and (f) whole blood^[55-56]

Table 1. Characteristics of three methods for detecting morphine in complex matrices

Table 2. Summary of research on the detection of classical opioids based on SERS in recent years