A novel and highly sensitive nanocatalytic surface plasmon resonance-scattering analytical platform for detection of trace Pb ions

Gold nanoparticles (AuNP) have catalysis on the reaction of HAuCl4-H2O2. The produced AuNP have strong resonance Rayleigh scattering (RRS) effect and surface-enhanced resonance Raman scattering (SERS) effect when Victoria blue B (VBB) and rhodamine S (RhS) were used as probes. The increased RRS/SERS intensity respond linearly with the concentration of gold nanoparticles (AuNPB) which synthesized by NaBH4 over 0.038–76 ng/mL, 19–285 ng/mL, 3.8–456 ng/mL respectively. Four kinds of tested nanoparticles have catalysis on the HAuCl4-H2O2 particles reaction. Thus, a novel nanocatalysis surface plasmon resonance-scattering (SPR-S) analytical platform was developed for AuNP. The DNAzyme strand hybridized with the substrate strand to form double-stranded DNA (dsDNA) which couldn’t protect AuNPc to aggregate to AuNPc aggregations, having strong RRS effect. Upon addition of Pb2+, dsDNA could be cracked by Pb2+ to produce single-stranded DNA (ssDNA) that adsorbed on the AuNPc surface to form AuNPc-ssDNA conjugates. The conjugates have strong catalysis on HAuCl4-H2O2 reaction. With increased Pb2+ concentration, the concentration of AuNPc-ssDNA increased and lead to the catalytic activity stronger. The increased RRS intensity responds linearly with Pb2+ concentration over 16.7–666.7 nmol/L. The SERS intensity responded linearly with the concentration of Pb2+ over 50–500 nmol/L.

The SPR-S techniques included the RRS and SERS, which the former is elastic and the later is inelastic scattering that both were based on the nanoparticle scattering. RRS is simple, sensitive spectral analysis method and has been used for protein, nucleic acid and metal ions analysis [19][20][21][22][23] . Lead is a harmful heavy metal, which has been listed as key detection project in food, drugs, environmental pollutants and supervision inspection. Based on the reaction of Pb 2+ reacting with excessive Ito form [PbI 4 ] 2− , and further associated with rhodamine 6 G (Rh6G) to produce particles with a strong RRS peak, Luo et al. 24 developed a RRS method for detection of Pb 2+ as low as 0.04 μg/mL. Luo et al. 25 reported a RRS method for detection Pb 2+ as low as 1.0 nmol/L, based on the binding of Pb(II) with thrombin and aptamer. Pb 4+ was reduced to PbH 4 gas by NaBH 4 and the gas trapped by Au 3+ to form nanogold that exhibited a RRS effect at 286 nm. This principle was used to detect Pb 2+ as low as 7.0 × 10 −8 mol/ L 26 . Based on the dsDNA cracked by Pb(II) to release a short single-stranded DNA that conjugated gold nanoparticles (AuNPs) to form a stable AuNPs-ssDNA complex, and its nanocatalysis of HAuCl 4 -vitamin C particle reaction, a sensitive RRS method was developed for detection of Pb(II) 27 . However, there are no reports about the HAuCl 4 -H 2 O 2 nanogold catalysis SPR-RS analytical platform being utilized to detect trace Pb 2+ , combing with the DNA enzymes cracked reaction. SERS is a sensitive and selective molecular spectrometry, based on the molecular probes adsorbed on rough surface of nanoparticles [28][29][30][31][32][33] . Although there are many SERS detection techniques, a few SERS quantitative methods have been reported, with good accuracy, simplicity and practicality. Liu et al. 34 proposed a SERS biosensor to detect lead ion, combining the DNAzyme cracking and nanocatalytic reaction. Zhang et al. 35 used the prepared tree-shape nanogold-DNA as a signal amplifier to fabricate a SERS biosensor for detection of 100 pmol/L Pb 2+ . A label-free rhodamine 6G SERS probe was reported for detection of trace Pb(II) in Au core Ag shell nanosol substrate, based on the Pb(II) cracking the DNAmyze 36 . However, there are no reports about aptamer combining with the nanocatalysis of H 2 O 2 -HAuCl 4 in SERS quantitative analysis of Pb 2+ . In this paper, we have considered the new nanocatalytic reaction of AuNP-HAuCl 4 -H 2 O 2 , and two new SPR methods were developed for detection of Pb(II), combining the analysis platform with the DNAzyme cracking.

Results
RRS spectra. The RRS signals of small particle size gold, silver, platinum and palladium nanosol are very weak. Different concentrations of AgNO 3 were added to preparation of AuNR 1 , AuNR 2 and AuNR 3 , with the diameter of 32 nm, 37 nm and 43 nm respectively that RRS values gradually reduced (Fig. S1). With the increase of AuNR concentration, the RRS peak linear increased at 370 nm (Fig. S1D). Nanoparticles can catalytic hydrogen peroxide reduction of HAuCl 4 under the condition of 0.67 mmol/L HCl, and with the increase of nanogold solution concentration, the RRS intensity of system linear increase at 370 nm ( Fig. 1, Fig. S2). The catalytic activity of AuNP B was better than that of AuNP c because the particle size of AuNP B was smaller than AuNPc, which lead to the surface energy more larger and the surface of AuNP B nanoparticles can absorb more HAuCl 4 . Different particle size of AuNR was added as catalyst, with the AuNR concentration increased , the RRS peak linear increased at 370 nm (Figs S3-S5). When AgNPs, PdNPs, PtNPs nanosol solution was used as catalyst, with the increase of nanoparticles concentration, the RRS peak linear increased at 370 nm (Fig. S6). It can be used to quantitative of HAuCl 4 and H 2 O 2 through this catalytic system, with the increase of HAuCl 4 concentration, the RRS peak of system linear increased and color from colorless gradually became red (Fig. S7), with the increase of H 2 O 2 concentration, the RRS peak of system linear increased (Fig. S8). When AuNPc-ssDNA solution was used as catalyst, AuNP c modified by aptamer catalytic activity is stronger than AuNP c solution, with the increase of AuNPc-ssDNA concentration, the RRS peak linear increased at 370 nm (Fig. S9).
DNAzyme catalytic strand hybridize with substrate strands to form double-stranded DNA (dsDNA). In pH 8.0 Tris-HCl buffer solution and 6.7 mmol/L NaCl, AuNP c were aggregated to the AuNP c aggregations which exhibited a strong RRS peak at 370 nm. Upon addition of Pb 2+ , the substrate chain of dsDNA could be cracked catalytically by Pb 2+ to produce a short single-stranded DNA (ssDNA) that adsorbed on the AuNPc surface to form stable AuNPc -ssDNA conjugate to prevent aggregation by NaCl, With the increase of Pb 2+ concentration, the RRS peak linear decreased at 370 nm (Fig. 2). The AuNPc-ssDNA probe of the apt-AuNP c -Pb 2+ system reaction solution has strong catalytic effect on the slow reaction between H 2 O 2 and HAuCl 4 , the products gold nanoparticles had a stronger RRS peak at 370 nm, with the increase of Pb 2+ concentration, the RRS peak linear increased at 370 nm (Fig. 3). SERS spectra. Au 3+ was reduced to Au and growing around the surface of nano-gold under the action of reducing agent H 2 O 2 , and the irregular shape, big particle size of nanoparticles was obtained. Upon addition of Rhs, it was adopted on the surface of gold nanoparticles which exhibited SERS peaks at 618 cm −1 , 732 cm −1 , 1199 cm −1 , 1277 cm −1 , 1356 cm −1 , 1507 cm −1 , 1527 cm −1 and 1645 cm −1 . Among them, the SERS peak at 1645 cm −1 is the biggest, and the SERS peak intensity linearly increased with the concentration of AuNP B increasing (Fig. S10). Upon addition of VBB, VBB molecular probes exhibited SERS peaks at 795 cm −1 , 1167 cm −1 , 1200 cm −1 , 1364 cm −1 , 1394 cm −1 and 1612 cm −1 . Among them, the SERS peak at 1612 cm −1 is the biggest, and the SERS peak intensity linearly increased with the concentration of AuNP B increasing (Fig. 4). Upon addition of Tibetan red T, Tibetan red T molecular probes exhibited SERS peaks at 349 cm −1 , 612 cm −1 , 1240 cm −1 ,  1372 cm −1 , 1551 cm −1 and 1639 cm −1 . Among them, the SERS peak at 1372 cm −1 is the biggest, and the SERS peak intensity linearly increased with the concentration of AuNP B increasing (Fig. S11), and we can know that the SERS signal strength of Tibetan red T is weaker than that of RhS and VBB. When Rh6G was used as SERS probe, the SERS signal is very weak. When PdNPs solution was used as catalyst and VBB was used as SERS probe, with the increase of PdNPs concentration, the SERS peak linearly increased at 1612 cm −1 (Fig. S12). For the apt-nanogold-Pb 2+ catalytic system, VBB and RhS was used as SERS probe respectively, the SERS peak intensity linearly increased with the concentration of AuNP c increasing (Fig. 5, Fig. S13).

Scanning electron microscopy(SEM).
According to the procedure to get the aptamer reaction solution, a 1.0 mL the solution was taken into a 1.5 mL centrifuge tube, and centrifuged in 15000 r/min for 20 min before abandoned the supernatant. A 1.0 mL water was added into the centrifuge tube and dispersed by ultrasonic 30 min, and centrifuged again. The operation was repeated, and the dispersed sample solution was dropped onto a silicon wafers and dried naturally, then the scanning electron microscope (SEM) was recorded. The size of gold nanoparticles and silver nanoparticles are uniform and small (Fig. S14a,b). Different concentration of AgNO 3 was added to preparation of AuNR 1 , AuNR 2 , AuNR 3 , the diameter of them was 32 nm, 37 nm, 43 nm respectively ( Fig. S14c-e). For AuNP B -HAuCl 4 -H 2 O 2 system, in the absence of AuNP B , the reaction of H 2 O 2 and HAuCl 4 is slow under the condition of 0.67 mmol/L HCl medium and 60 °C, and the products of gold nanoparticles is less (Fig. 6a). Upon addition of the AuNP B , the reaction was accelerated by nano-catalyst of small gold nanoparticles (AuNP B ), and it would reacted to form a large number of gold nanoparticles which was irregular shape, big particle size. With the increase of AuNP B concentration, the amount of gold nanoparticles increased which had high SERS and RRS singals (Fig. 6b,c). When AuNPc-ssDNA was used as catalyst, the products gold nanoparticles were gathered into small clusters (Fig. 6d). Compared with the same concentration of nangold, the catalytic activity of nanogold modified by aptamer was better due to its size was smaller. For the apt-AuNPc-Pb 2+ system, with the increase of Pb 2+ concentration, the amount of reaction product gold nanoparticles increased (Fig. 6e,f).  Research of gold nanoparticle-HAuCl 4 -H 2 O 2 reaction. The effect of HCl concentration was examined.
It was found that the influence of hydrochloric acid concentration has a great influence on the formation of gold nanoparticles. The results showed that the ΔI value reached its maximum when the concentration was 0.5 mmol/L (Fig. S15). But under this condition, the color of the blank was pink and the RRS value was 3506 which indicated that the blank had formed a large number of gold nanoparticles. Thus, the 0.67 mmol/L was chosen for use which RRS value was 506 and colorless. The effect of HAuCl 4 and H 2 O 2 concentration was studied. The results showed that the ΔI value reached its maximum when the concentration was 4.48 μmol/L and 3.33 mmol/L H 2 O 2 respectively (Figs S16 and S17). Thus a 4.48 μmol/L of HAuCl 4 and 3.33 mmol/L H 2 O 2 solutions were chosen for use. The effect of the incubation temperature was considered, when the temperature was greater than 60 °C, with the increase of temperature, the RRS value and color of blank increased gradually. When the incubation temperature was 60 °C, the blank RRS value was 745 and the color was colorless, meanwhile the catalytic reaction was very slow below 60 °C. Therefore the best temperature was 60 °C (Fig. S18). The effect of incubation time on the catalytic reaction was considered, a fixed reaction time of 15 min was chosen for use, giving a good compromise between high sensitivity, short analytical time and low blank (Fig. S19). After quenching the catalytic reaction, cooling with ice water to quench the reaction, the scattering intensity was constant within 90 min (Fig. S20). The effect of Raman probe RhS, VBB and Tibetan red T concentration were examined, and the results showed that the ΔI value reached its maximum when their concentration were 7 μmol/L, 13.2 μmol/L, 6.7 μmol/L respectively (Figs S21-S23).
The gold nanoparticle reaction of HAuCl 4 -H 2 O 2 was slow in diluted HCl solution at 60 °C and was accelerated by nano-catalyst of small gold nanoparticles (AuNP B , AuNP c , AgNPs, PdNPs and PtNPs). Under the optimal conditions, the RRS intensity for different AuNP concentrations (C) was recorded and the working curves were drawn according the relationship between C and their corresponding ΔI values. We have investigated the influence of different kinds of AuNP on the working curve (Fig. 7, Fig. S24). Table 1 showed that the AuNP B system was the best, with the most wide linear range and lowest detection limit. We have investigated the influence of different size of AuNR on the working curve (Fig. S25), Table 1 showed that with the increase of AuNR particle size, the catalytic activity was weaker. As for AuNP B -HAuCl 4 -H 2 O 2 system, under the optimal conditions, RhS, VBB, and Tibetan red T was added as SERS probe respectively, the increased SERS intensity responded linearly with the concentration of AuNP B over 3.8-456, 19-285, 4-190 ng/mL respectively, with a linear regression equation of ΔI 1645 cm−1 = 2.28 C + 72.77, ΔI 1612 cm−1 = 5.94 C + 86, ΔI 1372 cm−1 = 1.47 C− 9.1 respectively (Fig. 8). Results showed that when VBB used as SERS probe was the most sensitive. When PdNPs was used as catalyst and VBB was used as SERS probe, the increased SERS intensity at 1612 cm −1 responded linearly with the concentration of PdNPs over 500-5950 ng/mL, with a linear regression equation of ΔI 1612 cm−1 = 14.71 C + 2.06 (Fig. S26). We have investigated the influence of different AuNP on the working curve, and the results (Table 1) showed that the AuNP B system was the best, with the lowest detection limit.
The gold nanoparticle reaction of HAuCl 4 -H 2 O 2 was slow in diluted HCl solution at 60 °C. Upon addition of the nanoparticles, HAuCl 4 would be adsorbed on the surface of nanoparticles catalyst. The surface energy was higher when the nanoparticles particle size was very small, and it can absorb a large number of HAuCl 4 in the surface of the nanoparticles. When Au 3+ was reduced to Au and growing around the nano-gold surface under the action of reducing agent H 2 O 2 , irregular shape, big particle size of nanoparticles were obtained. The products had  a strong RRS signal because of the particle size was very large. When VBB, RhS and Tibetan red T were added as SERS probe respectively, the products had strong SERS signal because of the shape of gold nanoparticles was random. From Table 1, we can know that the AuNP B RRS system was the best.
Optimization of aptamer detection of Pb 2+ system analysis conditions. The effect of Tris-HCl concentration and pH were examined, the results showed that the ΔI value reached its maximum when the concentration was 4 mmol/L and pH was 8.0 (Figs S27 and 28). The effect of AuNP c and NaCl concentration were examined, the results showed that the ΔI value reached its maximum when the concentration were 9.55 μg/mL and 10 mmol/L respectively (Figs S29 and 30), thus, 9.55 μg/mL of AuNP c and 10 mmol/L of NaCl solution were chosen for use.
Effect of foreign substances. According to the procedure, the effect of foreign substances on the determination of 0.167 μmol/L Pb 2+ was tested, with a relative error within ±10%. Results (Table S1) showed that common ions did not interfere with the determination, which indicated that this method had good selectivity.
Working Curve. Under the optimal conditions, the RRS intensity for different Pb 2+ concentrations (C) were recorded and the working curves were drawn according the relationship between C and their corresponding ΔI values. With the increase of Pb 2+ concentration, the RRS peak increased at 370 nm and the decreased RRS intensity responded linearly with the concentration of Pb 2+ over 125-425 nmol/L with a linear regression equation of ΔI 370 nm = 1.26C− 20.56, coefficient R 2 of 0.9836. For the apt-nanogold-Pb 2+ catalytic system, The increased RRS intensity at 370 nm responded linearly with the concentration of Pb 2+ over 16.7 -666.7 nmol/L, the linear regression equation is ΔI 370 nm = 9.85 C + 470, coefficient R 2 of 0.9856 (Fig. S31). VBB and RhS were added as SERS probe, the SERS intensity I 1612 cm−1 and I 1645 cm−1 responded linearly with the concentration of Pb 2+ over 17-250, 17-167 nmol/L respectively (Fig. S32).

Sample analysis.
Three natural water samples were filtered to obtain water sample solutions, and were analyzed according to the procedures. Results (Table S2) showed that two of them had been detected out of Pb 2+ . A known amount of Pb 2+ was added into the water sample to obtain the recovery. The relative standard deviation was in the range of 4.4-5.5%, and the recovery was in the range of 98.0-102%.

Discussion
Analytical principle. Nanocatalytic reaction is an important way to amplify the signal of analysis method, so explore a new method to use nanocatalytic reaction is great significance. It was found that, the gold nanoparticle reaction of HAuCl 4 -H 2 O 2 is slow in diluted HCl solution at 60 °C, upon addition of nanoparticles such as AuNP B , AuNP c , AuNR, AgNPs, PdNPs and PtNPs, HAuCl 4 would be adsorbed on the surface of nanoparticles catalyst. The specific surface area is larger because of the nanoparticles particle size is very small, therefore it can absorb a large number of HAuCl 4 in the surface of the nanoparticles, owing to the fact that the small nanoparticles have a high surface energy. When Au 3+ was reduced to Au and growing around the nano-gold surface under the action of reducing agent H 2 O 2 , it can obtain irregular shape, big particle size of nanoparticles (Fig. 9), which have highly SERS signals and RRS signals. Thus the nanogold catalytic reaction can be used to build SPR-S analysis platform. The DNAzyme catalytic strand hybridized with substrate strands to form double-stranded DNA (dsDNA) which couldn't protect AuNP c in pH 8.0 Tris-HCl buffer solution containing 6.7 mmol/L NaCl, and were aggregated to AuNP c aggregations with a strong RRS peak at 370 nm. Upon addition of Pb 2+ , the substrate chain of dsDNA could be cracked catalytically by Pb 2+ to produce a short single-stranded DNA (ssDNA) that adsorbed on the AuNPc surface to form stable AuNPc-ssDNA conjugate to prevent aggregation by NaCl. Combining the nanocatalytic SPR-RRS analytical platform and the DNAzyme cracking reaction, the AuNPc-ssDNA conjugates have strong catalytic activity to HAuCl 4 -H 2 O 2 particles reaction, and its product gold nanoparticles had a stronger RRS peak at 370 nm. With the increase of Pb 2+ concentration, the concentration of AuNPc-ssDNA probe increase and lead to the catalytic activity stronger (Fig. 10). Based on this, the new sensitive RRS and SERS quantitative analysis methods were developed for detection of Pb 2+ .

Conclusion
In 0.67 mmol/L HCl medium at 60 °C, HAuCl 4 adsorbed on the surface of nanoparticles catalyst, Au 3+ was reduce to Au and growing around the nano-gold surface, the products have highly SERS signals and RRS signal, thus the AuNP-HAuCl 4 -H 2 O 2 nanogold catalytic reaction RSS and SERS analysis platform were built. The AuNPc-ssDNA probe of the apt-AuNP c -Pb 2+ system reaction solution has strong catalytic effect on the slow reaction between H 2 O 2 and HAuCl 4 . Combing the nanocatalysis and the DNAmyze reaction, a new nanocatalysis analysis platform was developed for the detection of Pb 2+ by the RRS and SERS, with advantages of high sensitivity, good selectivity, simplicity and rapidity. Compared with the reported methods, the methods are easier to operate and more sensitive. Further more, it is a rapid RRS and SERS quantitative method for Pb 2+ (Table S3).   0.01 mol/L HCl, 0.3% H 2 O 2 (0.1 mol/L), 50 mmol/L pH 7.4 Tris-HCl, 50 mmol/L pH 8.0 Tris-HCl, 5 × 10 −5 mol/L PdCl 2 and 1.45 × 10 −2 mol/L PdCl 2 , 2.9 × 10 −2 mol/L HPtCl 6 and 5.23 × 10 −5 mol/L RhS solution were prepared. A pH 7.0 Na 2 HPO 4 -citric acid buffer solution was prepared as follows, a 16.5 mL 0.2 mol/L Na 2 HPO 4 and 3.5 mL 0.1 mol/L citric acid solution were mixed together to obtain a concentrations 0.16 mol/L Na 2 HPO 4 . A 1.0 × 10 −3 mol/L VBB solution was prepared as follows, 0.0250 g VBB was dissolved in 5.0 mL ethanol, and diluted to 50 mL with water. The nanosols and ssDNA-AuNP were prepared as in the SI 37 .

Apparatus.
Procedure of HAuCl 4 -nanoparticles -H 2 O 2 system. A 80 μL 0.1% HAuCl 4 (84 μmol/L), 100 μL 0.01 mol/L HCl, a certain amount of nanoparticles including AuNP B , AuNP c , AuNR, AgNPs, PdNPs and PtNPs, and 50 μL 0.3% (0.1 mol/L) H 2 O 2 were added into a 5 mL marked test tube and mixed well, and diluted to 1.5 mL. The mixture was heated at 60 °C for 15 min, cooling with ice water to quench the reaction. A part of the solution was transferred into a 1 cm quartz cell. The RRS spectra were recorded by synchronous scanning excited wavelength λ ex and emission wavelength λ em (λ ex − λ em = Δλ = 0), a PMT voltage of 400 v, both excited and emission slit width of 5 nm, emission filter of 1%T attenuator on fluorescence spectrophotometer. The RRS intensity at 370 nm (I 370 nm ) and the blank value (I 370 nm ) 0 without nanoparticles were recorded. The value of ΔI 370 nm = I 370 nm − (I 370 nm ) 0 was calculated. 200 μL 1.0 × 10 −5 mol/L VBB, 20 μL 5.23 × 10 −5 mol/L RhS or 100 μL 1 × 10 −4 mol/L tibetan red T was added in the mixture respectively, The SERS intensity corresponding at 1612 cm −1 , 1645 cm −1 , 1370 cm −1 and the blank value I 0 without nanoparticles were recorded. The value of ΔI = I − I 0 was obtained.
Procedure of apt-AuNP c -Pb 2+ system. A 500 μL 2 μmol/L Substrate strand, 500 μL 1 μmol/L DNAzyme catalytic strand, 1 mL 50 mM pH 7.4 Tris-Hcl buffer solution and 50 μL 1 mol/L NaCl were mixed well, incubated at 65 °C water bath for 10 min, then gradually cooled to room temperature over 2 h, and hybrid solution was obtained. In a 5 mL marked test tube 120 μL 50 mM pH 8.0 Tris-Hcl buffer solution, 50 μL hybrid solution, a certain amount of Pb 2+ was added respectively, mixed well and diluted to 1.5 mL. Then the tube was placed at 37 °C water bath for reaction 60 min before cooling with ice water to quench the reaction. After that 250 μL AuNP c and 30 μL 0.5 mol/L NaCl were added in the mixture and mixed well to obtain Pb 2+ aptamer reaction solution, then a part of the solution was transferred into a 1 cm quartz cell. The RRS spectra were recorded by synchronous scanning excited wavelength λ ex and emission wavelength λ em (λ ex − λ em = Δλ = 0), a PMT voltage of 450 v, both excited and emission slit width of 5 nm, The RRS intensity at 370 nm (I 370 nm ) and the blank value (I 370 nm ) 0 without Pb 2+ were recorded. The value of ΔI 370 nm = (I 370 nm ) 0 − I 370 nm was calculated.
Procedure of apt-nanogold-Pb 2+ catalytic system. A 80 μL 0.1% HAuCl 4 (84 μmol/L),100 μL 0.01 mol/L HCl, 100 μL Pb 2+ aptamer reaction solution and 50 μL 0.3% (0.1 mol/L) H 2 O 2 were added into a 5 mL marked test tube and mixed well, and diluted to 1.5 mL. The mixture was heated at 60 °C for 15 min, cooling with ice water to quench the reaction. A part of the solution was transferred into a 1 cm quartz cell. The RRS spectra were recorded by synchronous scanning excited wavelength λ ex and emission wavelength λ em (λ ex − λ em = Δλ = 0), a PMT voltage of 400 v, both excited and emission slit width of 5 nm, emission filter of 1%T attenuator on fluorescence spectrophotometer. The RRS intensity at 370 nm (I 370 nm ) and the blank value (I 370 nm ) 0 without Pb 2+ were recorded. The value of ΔI 370 nm = I 370 nm − (I 370 nm ) 0 was calculated. 200 μL 1.0 × 10 −5 mol/L VBB 20 μL or 5.23 × 10 −5 mol/L RhS was added in the mixture, The SERS intensity at 1612 cm −1 and the blank value I 0 without Pb 2+ were recorded. The value of ΔI = I -I 0 was obtained.