The following instrumentation was used: a DXR SmartRaman spectrometer (Thermo Company, USA) with a 633-nm laser at 3 mW power, a Cary Eclipse fluorescence spectrophotometer (Varian Company, USA), a TU-1901 double-beam UV-Visible spectrophotometer (Beijing General Instrument Co., LTD, China), and a FEI Quanta 200 FEG field-emission scanning electron microscope (Field Electron and Ion Company, Holland).

Aptamer (Apt) sequence of 5′-3′ AGC TTG CTG CAG CGA TTC TTG ATC GCC ACA GAG CT [Sangon Biotech (Shanghai) Co., Ltd., China], 0.01 mol/L silver nitrate (Sinopharm Chemical Reagent Co. Ltd., China), 0.2 g/L fullerene(C60), 0.04 g/L fullerol(C₆₀OH), 0.1 g/L graphene oxide(GO) (Nanjing XFNANO Materials Tech Co., Ltd, China), 0.1 mol/L trisodium citrate (TSC) (Xilong Scientific Co., Ltd., China), 10.3 mol/L Victoria Blue B (VBB) solution, Victoria 4R (VB4R) solution, rhodamine S (RhS), rhodamine 6G (Rh6G, Sinopharm Chemical Reagent Co., Ltd., China), isocarbophos (Beijing Century OuKe Biological Technology Co., Ltd., China), profenofos (Sinopharm Chemical Reagent Co., Ltd., China), and glyphosate (J&K Scientific Ltd., China) were prepared. All reagents were analytically pure, and water was double-distilled.

To prepare fullerene (Andrievsky et al., 1995), 0.02 g fullerene was dissolved in 20 mL methylbenzene to give a bright purple solution. Double-distilled water (100 mL) was added, and the solution was sonicated until the toluene was completely volatilized. The solution changed to a dark-yellow suspension, and 0.2 g/L of fullerene sol was obtained.

Hydroxylated fullerene was prepared, referring to Li et al. (1998): 1 mL 0.2 g/L fullerene sol, 10 μL 30% H₂O₂ solution, and 100 μL 1 mol/L NaOH were mixed and reacted at room temperature. Then, 98 μL 1 mol/L hydrochloric acid was added to adjust pH to 7.5 and diluted 5 mL with water to obtain 0.04 g/L C₆₀OH.

Apt (20 μL of a 1.5 μmol/L solution), a certain amount of IPS, and 10 μL of 0.04 g/L fullerol solution were added to a 5-mL graduated tube, mixed well, and allowed to react for 10 min. Next, 200 μL 0.01 mol/L AgNO₃ and 70 μL 0.1 mol/L trisodium citrate were added and diluted to 1.5 mL. The mixture was heated for 21 min to 85°C in a water bath, then cooled with ice water. Next, 50 μL of 1.0 × 10⁻⁵ mol/L VBB and 40 μL of 1 mol/L NaCl were added and mixed well. SERS spectra were recorded using a Raman spectrometer. SERS intensity of the reaction solution at 1614 cm⁻¹ (I1614cm−1) and the blank solution without IPS (I₀) were recorded. The value of ΔI=I1614cm-1-I0 was calculated.

C₆₀(OH)_n is a good electron acceptor (Samal and Sahoo, 1997). It transfers electrons from a donor to an acceptor, thereby facilitating, or catalyzing, the reaction. The silver nitrate–trisodium citrate reaction does not occur in solution because of the effective collision between silver ions and citrate. When C₆₀(OH)_n is added, silver ions and citrate adsorb onto its surface, allowing electrons to transfer from citrate to silver ions, which leads to the generation of silver, 1,3-acetonedicarboxylic acid, and CO₂. Silver assembles as yellow Ag nanoparticles (Figure 1). Fullerol has abundant hydroxyl groups that can form hydrogen bonds with water; it combines better with silver ions and citrate for more efficient electron transfer. Thus, fullerol has enhanced catalytic ability compared to fullerenes lacking the hydroxyl group. An aptamer coating on the fullerol surface hinders the interaction between C₆₀(OH)_n and citrate and silver ions, such that C₆₀(OH)_n catalytic activity is inhibited. In the presence of IPS specific to the aptamer, C₆₀(OH)_n is once again exposed to the reaction system and its catalytic activity is recovered. The amount of Ag nanoparticles generated increases linearly with IPS concentration. Using this relationship, a method to detect IPS using the SPR absorption spectrum, RRS, and SERS was developed.

In this study, fullerol exhibited increased catalytic activity compared to C₆₀. Fullerol has –OH groups with excellent water solubility that increase its catalytic activity compared to C₆₀. Thus, fullerols were prepared by H₂O₂ oxidization using a previously published procedure (Li et al., 1998). Hydroxyl content in fullerol increases with increasing H₂O₂ (Figure S1), as does the catalytic action of silver nitrate–trisodium citrate; a 70-mmol/L H₂O₂ solution was selected to obtain highly catalytic fullerol (C₆₀OH). VBB, VB4R, RhS, Rh6G, and RhB were used as signal molecules; their strongest SERS peaks occurred at 1614, 1385, 1361, 1362, and 1508 cm⁻¹, respectively (Figure S2). The SERS intensities of VBB and VB4R were stronger than those of the others; VBB was chosen for further study. The catalytic activities of C₆₀OH, C₆₀, GO, and AgNP were investigated (Figure 2A and Figure S3). C₆₀OH exhibited the highest catalytic activity and was chosen for use. In the presence of Apt, which coats the fullerol surface and isolates it from the reactants, fullerol catalytic activity is suppressed and decreased SERS intensity is observed (Figure 2B). When added to the system, IPS conjugates to Apt, releasing fullerol and restoring its catalytic activity. As the IPS concentration increases, the amount of released fullerol increases and more AgNP is produced as well; thus, SERS intensity increases linearly with IPS concentration (Figure 2C and Figure S4).

In a water bath at 85°C, fullerol and other nanoparticles catalyze the reaction of silver nitrate and trisodium citrate to generate AgNP, which exhibits two strong RRS peaks at 360 and 550 nm (Figure 3 and Figure S5A) and a strong surface plasma resonance (SPR) absorption peak at 410 nm (Figure 4 and Figure S6A). The RRS peak at 550 nm is characteristic of AgNP, and the intensity of ΔI_550nm and ΔA_410nm increase linearly with the amount of nanocatalyst. When Apt coats the nanocatalyst surface, it isolates the nanocatalyst from the system and inhibits its catalytic activity, leading to decreased ΔI_550nm and ΔA_410nm values. In the presence of IPS, which conjugates specifically with Apt, fullerol is released and its catalytic activity recovers. As IPS concentration increases, the amount of released fullerol increases, as does the amount of AgNP generated; consequently, ΔI_550nm and ΔA_410nm intensities increase linearly with IPS concentration (Figures 3B, 4B and Figures S5B, S6B).

In the absence of the catalyst, AgNO₃ does not react readily with trisodium citrate. However, in the presence of a fullerol nanocatalyst, silver ions and citrate adsorb to the fullerol surface by interface free energy. As shown in Figure S7, the intensity of RRS for fullerol in aqueous solution is considerably lower than that of fullerene, suggesting that the fullerol particle size is less than that of fullerene. This may be responsible for the reduced catalytic activity of fullerenes compared to fullerol. In addition, silver ions and citrate adsorb to the surface more readily and electron transfer between the silver and citrate ions occurs more efficiently. That is, smaller particles demonstrate greater catalytic efficiency. As shown in Table 1, the slope of the C₆₀OH catalytic system is about 50 times that of C₆₀. In addition, the catalytic effect of AgNP on this reaction was studied. As shown in Figure S8 and Table 1, AgNP is an effective catalyst even with AgNP concentrations as low as 13.33 nmol/L. AgNP participates in autocatalysis, strengthening the catalytic effect. Furthermore, fullerene was hydroxylated using H₂O₂, according to a previously published procedure [42] and its catalytic activity was determined. Fullerol exhibited enhanced catalytic activity (compared to fullerene), and its catalytic activity increased with increasing hydroxylation (Table 1). This suggests that improved solubility of fullerol in water would increase its ability to bind with ions, thereby enhancing catalysis. When Apt is added, the intensity of RRS increases, as shown in Figure S7C, indicating that Apt coats the nanocatalyst surface and blocks the adsorption of silver and citrate ions to the nanocatalyst, inhibiting its catalytic activity. It is worth mentioning that the catalytic activity of fullerol is suppressed by Apt (Table 1). This is likely due to its smaller size; the hydroxyl group of fullerol produced a better combination of hydroxyl and –COOH, –NH₂.

The reaction solution was prepared and diluted 10 times. A 10 μL sample solution was dropped onto a silicon wafer and allowed to dry naturally, then scanning electron microscopy (SEM) was performed. As shown in Figure 5, in the absence of IPS, few AgNPs are detected in the reaction solution, with a mean grain size of 20 nm (Figure 5A). Upon addition of IPS, the catalyst recovered catalytic activity; a large amount of AgNP was generated and formed aggregates with a mean grain size of 40 nm (Figure 5B).

The effect of reagent concentration on the determination was studied. When AgNO₃ and TSC concentrations were 1.33 and 4.67 mmol/L, respectively, the SERS value was at its maximum. Thus, 1.33 mmol/L AgNO₃ and 4.67 mmol/L TSC were chosen as the optimal concentrations (Figures S9, S10). The effects of reaction temperature and time were tested as well; 85°C and 21 min resulted in the maximum value for ΔI (Figures S11, S12). The effects of VBB, VB4R, RhB, RhS, and Rh6G concentrations on ΔI were considered. Maximum ΔI was observed at VBB, VB4R, RhB, RhS, and Rh6G concentrations of 3.33 × 10⁻⁷ mol/L, 1 × 10⁻⁶ mol/L, 1 × 10⁻⁵ mol/L, 1.67 × 10⁻⁶ mol/L, and 1 × 10⁻⁶ mol/L, respectively. Among these, VBB was selected because of its lower detection limit (Figure S13). At an Apt concentration of 20.67 nmol/L, the ΔI value reached a maximum value and thus was chosen for use (Figure S14). Binding times for the aptamers with fullerol were tested. A maximum value of ΔI was reached and maintained at 8 min; thus, it was chosen as the optimal binding time (Figure S15).

Using the optimal conditions described in section Optimization of Catalysis Conditions, working curves were prepared for IPS concentration at the corresponding ΔI1614cm-1, ΔI_550nm, and ΔA_410nm values for SERS, RRS, and Abs, respectively (Figure 6 and Figures S16–S19). Analytical characteristics are listed in Table 2. SERS exhibited the best performance, with a maximum slope of 492.83, and a limit of detection of 8.2 ng/L; RRS was the next most effective method. However, the Abs method is inexpensive, convenient, and aligns with national standards such that it could be used for on-site tests. Fullerol, because of its small size, higher surface electronic density, and ability to bind with silver and citrate ions, displays enhanced catalytic activity and a greater sensitivity for IPS detection compared to fullerene. Organophosphate analogs glyphosate, profenofos, and tributylphosphine also were detected using this method, according to the linear equations I1614cm-1=0.93CIPS+93.71, I1614cm-1=0.31CIPS+75.53, and I1614cm-1=0.87CIPS+66.21, respectively (Figure S20). These detection ranges exceeded that of IPS, and the components did not interfere with the determination. Compared to previously reported methods for the determination of IPS, the SERS, RRS, and Abs method (Table S1) is simpler, requires an easily obtainable reagent, is highly sensitive, and exhibits good selectivity. It can be used to detect IPS residues in water and agricultural products.

Using fullerol as a catalyst, the influence of coexisting substances on the determination of 2 μg/L IPS was tested. The results indicate that common substances do not interfere with IPS determination (Table S2), with a relative error of ±10%.

Three water samples, taken from a pond, Lijiang, and cropland were collected using two 100 mL glass sampling bottles and were then filtered through a 150 nm filter membrane to obtain sample solutions, which were stored at 4°C. Food samples (200 g grape, 265 g orange (3), and 200 g Chinese cabbage) were purchased from farmer markets. The samples were immersed in 100 mL of acetone for 2 h. Extracts were air-dried, then dissolved with sonication in 100 mL water, and then stored at 4°C. Samples (50 μL) were then tested for IPS content. A known amount of IPS was added to each sample, and recoveries of 93–101.5% were obtained (Table 3).