Fig. 1. Problems in SERS quantitative detection

Fig. 2. Schematic diagram of internal standard method

Fig. 3. Typical metallic nanostructures^[19,36-40]

Fig. 4. Shell core structure improves the performance of SERS substrate: (a)Shell acts as a protective layer to avoid the effects of chemical effects^[17]; (b) shell acts as a particle functionalization or molecular adsorption platform^[36]; (c) application of carbon nanomaterials wrapped metal nanoparticles in biological science[42-

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Fig. 5. Three internal standard addition modes. (a) A: External addition mode, including add internal standards directly or add labeling molecules; (b) B: core-internal standard-shell modes; (c) C: self-calibrating substrate

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Mode Structure IS Performance (LOD or EF) Reference External addition Silver Colloid IEIS:isotopicR6G 2×10-10 mol/L (R6G)10-9 mol/L (R6G) Zhang et al[52]Perera et al[53] AuNP ILC:Trp-d5 or13C 10-7 mol/L(Trp and caffeine) Subaihi et al[54] AuNP TMT 2.9×10-9 mol/L (Cd2+) Chen et al[50] Microfluidics withAg nanocolloids RhBp-thiocresol 5×10-7 mol/L (MG)5×10-8 mol/L (MG) Xia et al[49] Ag@AuNCs NTP EF:104, aldolcondensation reactionof acetone and MTBH Weatherston et al[33] Ag films Phosphatebackbone adenine and cytosine Freeman et al[8] Core-molecule-shell Au@CA+Mpy@AgNPs Mpy 5×10-10 mol/L (PDI) Shen et al[55] CISS NC pMBA+pATP 3×10-11 mol/L (R6G) Wu et al[56] Au@2-MB+PATP@Ag 2-MBPATP2-MB+PATP 3×10-7 mol/L (Phosmet) Zhang et al[57] Self-calibratingsubstrate FONTopUp@Ag SiSi 0.5 mg/L (E 122)2×10-8 mol/L (SMX) Peksa et al[61]Patze et al[32] AGNs Graphitic EF>106 (RhB) Zou et al[59] G-SERS GE 10-8 mol/L (CV and RhB) Tian et al[30] CNT/AgNPs CNT 10-9 mol/L (R6G) Zhang et al[60] GE/AgNHs GE 10-15 mol/L (R6G) Zhang et al[35] Note:Abbreviation:limits of detection (LOD), isotope edited internal standard (IEIS), isotopically labelled compound (ILC), enhancement factor(EF); 4-(methylthio) benzaldehyde(MTBH);Substrate:colloidal Ag-Au core-shell nanocubes (Ag@AuNCs), sulfamethoxazole (SMX), “film over nanospheres” (FON), Au@Ag nanocuboids (NCs),core-internal standard-shell (CISS);Material:Rhodamine6G (R6G), Tryptophan (Trp), malachite green (MG), 4-Nitrothiophenol (NTP), trithiocyanuric acid (TMT), 2-Mercaptobenzimidazole (2-MB),4-aminothiophenol (pATP), cysteamine (CA), 4-mercaptopyridine (Mpy), crystal violet (CV), 1,4-phenylene diisocyanide (PDI), basic red 9 (BR9) rhodamine B (RhB), food and drink colorant azorubine (E 122), 4-mercaptobenzoic acid (pMBA), p-Aminothiophenol(PATP).

Table 1. Structure, internal standard and performance in three internal standard modes