Transmission electron microscope of AgNPs and [email protected]
The [email protected] and AgNPs were prepared according to the experimental method. Then the Thermofisher Talos F200s transmission electron microscope (TEM) was used to record the image and energy spectra of the sample. As shown in (Fig. 2A), AgNP has a spherical structure, and their average particle size was about 20 nm. Compared with AgNPs, [email protected] are composed of evenly distributed C, O, and Ag elements, that exhibit two peaks at 2.5 and 3 keV for Ag. It can be seen from its energy spectrum, the doping of AgNPs do not significantly change the structure of CB. And it can be clearly seen from the EDS mapping that AgNPs have been doped in CB (Fig. 2B). In addition, the TEM of AgNPs produced by the AptPb[email protected]3 reaction were recorded (Fig. 2C). When Pb2+ was no addition of Pb2+, the less AgNPs were formed, and its average particle size was 20 nm. When Pb2+ was added, a large number of AgNPs particles were generated by the reaction, and their average particle size was about 40 nm (Fig. 2D).
Molecular spectrum, Zeta potential and particle size distribution of CB, AgNPs and [email protected]
RRS spectroscopy is a simple and sensitive technique for studying the scattering properties of nanoparticles. Under the condition of Volt = 350 V and excitation slit = emission slit = 5 nm, the RRS spectra of CB, AgNPs and [email protected] at different temperatures can be obtained easily and quickly. Figure 3A shows that when temperature was less than 45 ℃, CB exhibits the strongest RRS peak at 340 nm and there are nanoparticles at room temperature. When temperature was high than 65 ℃,the strongest RRS peak changed to 370 nm. The both peak RRS values all enhanced greatly with increasing temperature, indicating that the formed CB nanoparticle numbers increased. In addition, the red-shift of strongest RRS peak was due to the size increasing. Figure 3B and C show the RRS intensity of [email protected] and AgNPs were decreased with increasing temperature. Figure 3D is the UV absorption spectrum of CB. The Abs peak at 226 nm enhanced with temperature increasing that was related to the changes of CBN number and size. Figure 3E and F are the UV absorption spectrum of [email protected] and AgNPs. The Abs signal value peak at 446 nm was decreased with temperature increasing.
Laser scattering was good and reliable technique to study the particle size distribution in solution because the conditions of the system were not destroyed. The particle size distribution of CB materials at different temperatures was studied by laser scattering nanoparticle analyzer. It can be clearly observed from Fig. 3G that in a certain temperature range (35–85℃), the diameter of CB increases from 101,116,123, 154 to 171 nm when the temperature increase. This was because the CB molecules gradually were aggregate into nanoparticles with larger size as the temperature increases. Figure 3J shows that the surface of CBN material is negatively charged, and the Zeta potential is -0.138 mV, indicated it was of good stability. The particle size distribution of [email protected] and AgNPs materials at different temperatures was studied by laser scattering nanoparticle analyzer. It can be clearly observed from Fig. 3H and I that in a certain temperature range (35–85℃), the diameter of [email protected] and AgNPs decreases when the temperature increase.
Using nanosilver as SERS base, we studied the Raman spectrum of CB solution. The results show that there is no obvious characteristic peak. After that, 0.1m NaCl solution was added as sensitizer, but there was still no obvious characteristic peak on the spectrum. Therefore, a small amount of solid CB powder was dried by rectangular glass plate, then the other glass sheet was rolled, the CB powder was dispersed, and then placed in a Raman measurement instrument to obtain CB normal Raman spectra. The normal Raman spectra of CB solid (Fig. 3K) show 8 peaks, of which 865 cm− 1 is ring "respiration", 1003 cm− 1is the triangular ring "breathing"; 1275 cm− 1 is the ring vibration of a para-disubstituted benzene ring; 1445 cm− 1 is a ring stretch; 1601 cm− 1 is caused by the expanded ring peak (double peak) of benzene derivatives; 854, 1154 and 1284 cm− 1 are attributed to ring vibration of the para-disubstituted benzene ring; 1608 cm− 1 is caused by the expanded ring peak (double peak) of benzene derivatives; 1713 cm− 1 is an extension of the C-O bond; 2864 cm− 1 and 3075 cm− 1 were attributed to CH3 stretching and aromatic C-H stretching, respectively. Using nanosilver as SERS base, and 0.1m NaCl solution was added as sensitizer, we studied the Raman spectrum of [email protected] solution. The results (Fig. 3K) show that 7 peaks, of which 436 cm− 1 and 679 cm− 1 are skeletal bending, which 878 cm− 1 is ring "respiration", 1485 cm− 1 is a ring stretch; 1615 cm− 1 is caused by the expanded ring peak (double peak) of benzene derivatives; 2933 cm− 1 is attributed to aromatic C-H stretching and 3441 cm− 1 is attributed to aromatic O-H stretching.
Figure 3L shows the infrared spectrum of CB, in which the characteristic peaks at 994 cm− 1 are caused by the out-of-plane curvature of C-H, and 1109 cm− 1 is the C-O bond caused by expansion and contraction; 1271 cm− 1 is an in-plane flexion caused by a para-substituted phenyl group; 1369, 1449 and 1711 cm− 1 are extensions of C-O bonds; 2341 and 2361 cm− 1 are vibration peaks caused by triple bonds and cumulative double bonds; 2936 cm− 1 is the stretching vibration peak caused by CH. Figure 3L shows the infrared spectrum of [email protected], in which the characteristic peaks at 853 cm− 1 is caused by the out-of-plane curvature of C-H; 1337 is extensions of C-O bonds; 3226 and 3449 cm− 1 are the stretching vibration peak caused by CH.
To test the stability of AgNPs and [email protected], their stability in NaCl and over time were investigated respectively. 200 µL 1mmol/L AgNPs and [email protected] in a 10 mL plug calibrated tube, Different volumes of 1 mol/L NaCl solution were added, and the volume was fixed to 1.5 mL. After the mixture was evenly mixed, RRS signal was measured (Figure S1A, S1B). Figure S1A shows that with the increase of days, the RRS signal values of AgNPs and [email protected] were in a stable state. In addition, the stability of these catalytic materials was investigated in different concentrations of NaCl solutions, it was also found that the AgNPs materials were unstable in NaCl solutions with different concentrations, and the signal value would rise first and then increased slightly, which might be caused by AgNPs aggregation in salt solutions. The signal values of [email protected] in the low salt solution were very stable. With the increase of NaCl concentration, the signal values increased slightly, which might be caused by AgNPs aggregation in the solution. The results showed that [email protected] was relatively stable in salt solution and for a long time, while AgNPs had poor stability and tended to aggregate in salt solution. So when AgNPs were mixed with CB, it could be uniformly loaded in CB, which had a greater improvement in the stability of the salt than AgNPs.
SERS spectra of nanocatalytic system
Under experimental conditions, AgNO3 was reduced to AgNPs by Fo slowly, and fewer AgNPs were generated in the system. When the [email protected]/AgNPs/LC catalysts were added to the system, the reaction of the AgNPs were significantly accelerated, and the generated AgNPs were not only the indicator component but also the sol substrate. When VB4r was added as probe, a strong SERS peak appeared at 1618 cm− 1. When AptPb was present, it was adsorbed to the surface of [email protected] catalyst, the catalytic activity decreased, and the generated AgNPs decreased, which leaded to the decrease of SERS signal of the system. When Pb2+ was added, AptPb combined with Pb2+ to generate a stable G-tetrad structure, the catalytic activity of the [email protected] were restored, the generated AgNPs increased, and SERS signal was linearly enhanced. Within a certain concentration range, the SERS signal of the system was gradually enhanced with the increase of the concentration of the substance to be measured, and the SERS peak of the system at 1615 cm− 1 increased linearly (Fig. 4A). The other inorganic pollutants to be measured, namely As3+, Cd2+and Hg2+, were less sensitive than Pb2+ (Fig. 4C, 4D, 4E). From the slope of the linear curve of [email protected]/AgNPs/LC concentration and SERS peak, we could know that the slope of [email protected] system was the largest and its catalytic effect was the strongest (Fig. 4A, Figure S2A-E).
RRS spectra of catalytic amplification system
RRS was no addition of VB4r, and it is more simply than the SERS. Under experimental conditions, the AgNPs generated by AptPb[email protected]3-Pb2+ system had a strong RRS peak at 370 nm. Within a certain concentration range, the RRS signal of the system was gradually enhanced with the increase of the Pb2+ concentration (Fig. 5A), based on which a new RRS method for detecting Pb2+ could be established. In addition, the RRS spectra of [email protected]3-As3+/Cd2+/Hg2+ show in Fig. 5B-D. The AptPb- AgNPs/LC -Fo-AgNO3-Pb2+ system also has a RRS peak (Figure S3A-F).
Abs spectra of catalytic amplification system
Under the experimental conditions, the AgNPs produced by the AptPb[email protected]3-Pb2+ system had an absorption peak at 450 nm, which was corresponded to the surface plasmon resonance absorption peak of AgNPs. Within a certain concentration range, as the concentration of Pb2+ increased, the absorbance of the system gradually increased, and an absorption peak appeared at 450 nm (Fig. 6A). Similarly, the AgNPs/CB system has an absorption peak at 430 nm, the AgNPs, OA and HA systems have an absorption peak at 450 nm (Fig. 6B, 6C). In addition, the DB and DE nanocatalytic analytical systems have an absorption peak at 440 nm (Figure S4A-F). In which, the [email protected] catalytic analytical system was most sensitive.
Under the catalysis of [email protected], AgNO3was reduced by Fo to produce AgNPs. Within a certain concentration range, as the concentration of the catalyst increased, the catalytic ability increased, and the amount of AgNPs generated by catalysis increased, which had a strong RRS peak at 370 nm. Table S1 shows that the slope of the working curve of the [email protected] RRS system was 7 times that of CB and 2 times that of AgNPs respectively, it can be seen that the slope of the [email protected] system is the largest, and most AgNPs are generated in the system. When AptPbwas added to the system, it could be adsorbed on the surface of the catalyst due to static electricity and molecular forces, and inhibited its catalytic activity. With the increased of AptPb concentration, the concentration of free catalyst in the system decreased, and the catalytic activity weakened, thus the AgNPs generated in the system decreased, and the RRS signal of the system weakened (Table S1).
Because the hydrogen bond association force in the cholesterol molecule is so strong that when the solid reaches its melting point, the solid directly became an isotropic liquid, which means that cholesterol did not have an LC phase, and CB had an LC phase. The ring system in the molecule is another important factor that determines whether the molecule has an LC phase. Molecules with aromatic rings do not necessarily have an LC phase, but molecules with an LC phase almost all have more than one aromatic ring. Like the five LC molecules and [email protected] used in this experiment, when the solid melts into a liquid, the aromatic ring maintain the intermolecular relationship. The short-range attractive force prevents the molecule from turning into an isotropic liquid immediately. Common ring systems include saturated six rings and unsaturated benzene rings, or a combination of the two. The electrons in the saturated six-membered ring were σ electrons, which are not conjugated to each other, while the electrons in the benzene ring are π electrons, which have a conjugation effect. Conjugation of electrons have an important influence on the properties of LCs, especially the π electrons in the benzene ring help to promote electron transfer in redox reactions. The study finds that these five LCs and [email protected] materials can catalyze the reduction of AgNO3 by Fo to generate AgNPs. Among the five types of LC and [email protected], the slope of the linear relationship between [email protected] concentration and SERS/RRS/Abs is the largest, and it has the strongest catalytic effect on the AgNO3-Fo system, this is very related to molecular structure of CB (Fig. 7) with the most π electrons and without electron absorbing base F, and the higher LC phase temperature of 145–150℃that is easy to form CB nanoparticles. Nanoparticles such as metal nanozymes, semiconductor nanozymes, carbon dot nanozymes, metal organic framework nanozymes, and covalent organic framework nanozymes are rich in surface electrons, which are the driving force for promoting electron transfer in some redox reactions. The formate reduction of AgNO3 can slowly generate AgNPs and CO2. These five small molecules of LC and [email protected] can be arranged and assembled into rod-shaped nanoparticles in an aqueous solution at 95°C, and their surface is rich in surface electrons. Ag+ and HCOO− can be adsorbed on the surface of LC and [email protected] nanoparticles. This nano surface electron can accelerate the redox electron transfer between formic acid and silver ions, accelerate the formation of AgNPs, and show a strong catalytic effect (Fig. 7).
Optimization of analysis conditions
According to the experimental method, the SERS analysis system conditions were examined. When the experimental conditions were 0.667 nmol/L AptPb (Figure S5A), 73.98 µmol/L NaAc-HAc (Figure S5B), 1.33 mmol/L AgNO3 (Figure S5C), 0.1 mol/L Fo (Figure S5D), reaction temperature was 95 ℃ (Figure S5E), reaction time was 20 min (Figure S5F), 1.33 µmol/L [email protected] (Figure S5G), standing time was 15min (Figure S5H), 0.67 µmol/L VB4r (Figure S5I), 0.067 mol/L NaCl solution (Figure S5J), the SERS signal of the analysis system was the largest, so these conditions were selected as the SERS experimental conditions.
According to the experimental method, the RRS analysis system conditions were considered. When the experimental conditions were 0.667 nmol/L AptPb (Figure S6A), 73.98 µmol/L NaAc-HAc (Figure S6B), 1 mmol/L AgNO3 (Figure S6C), 0.1 mol/L Fo (Figure S6D), reaction temperature was 85 ℃ (Figure S6E), reaction time was 15 min (Figure S6F), 1.33 µmol/L [email protected] (Figure S6G), standing time was 15min (Figure S6H), the ΔI of the analysis system was the largest, so the above conditions were selected for use. The conditions of the Abs method were the same as those of the RRS method.
3.8. Working curve
Under the optimal experimental conditions, the different concentrations of Pb2+ were plot its corresponding ΔI/ΔA to obtain the working curve, it could be seen from the Table S2 and 3 show that the slope of the working curve of the [email protected] system was 5 times that of CB and 1.5 times that of AgNPs respectively, indicating that it was the most sensitive. The linear equation wasΔI1618cm − 1= 76201C + 414.6, and the linear range was 4.47×10− 3-0.201 nmol/L. Although RRS/Abs had a short reaction time and low cost, its sensitivity was not as good as SERS (Table S2). In addition, other inorganic pollutants such as As3+, Cd2+ and the Hg2+ can be also determined by [email protected] nanocatalytic-SERS/RRS assay platform (Table S3). Comparing with the reported methods for measuring lead ions (Table S4), this method is a more sensitive molecular spectral method.
Influence of interfering ions
According to the experimental method, the interference of coexisting ions on the AptPb[email protected]3-Pb2+ SERS determination of 0.1 nmol/L Pb2+ was investigated. The experimental results showed that the relative error is within ± 10%, 1000 times Mg2+, Ba2+, Mn2+, Cr6+, Fe2+, Cr3+, Ca2+, BSA; 500 times Zn2+, NH4+, Co2+, serum protein, ascorbic acid, Al3+; 100 times Fe3+, Hg2+, Cu2+, NO2−, HSA, PO43− do not interfere with the measurement (see Table S5). This method has good selectivity.
Tap water and mineral water were 100mL each, and a water sample was taken from pond water of Yanshan Campus of Guangxi Normal University and wastewater of Bokang Building Laboratory of Guangxi Normal University. The sample pretreatment: orange peels and preserved eggs were all purchased from local supermarkets. Different orange peels and preserved eggs were respectively weighed and cut into small pieces and dried 10h in an oven at 105°C. The dried sample was ground with an agate mortar. 2.0 g (accurate to 0.1 mg) of sample was weighed and put in a 25 mL conical flask with 10 mL mixed acid of HNO3 and HClO4 (V1:V2 = 4:1) and placed overnight about 12h. The flask was placed on a power adjustable heating plate to digest until the white smoke appeared. When the solution became colorless and transparent or slightly yellow, it took down and cooled. Then the volume was adjusted to 10 mL.Finally, 100 µL samples were collected after filtration with 0.45 µm microporous membrane. According to the experimental method, the Pb2+was measured, and the known concentration of Pb(II) was added. The detection results were shown in Table S6. The content of Pb2+ in water samples was 0.033–0.3310 nmol/L, and the content of Pb2+ in food was 4.986–16.653 ng/g. The relative standard deviation (RSD) was 1.3–7.1%, and the recovery rate was 92.53–109.4%, showed good recovery and reproducibility.