Qingye Liu‡
a,
Jinchao Dong‡a,
Yanghe Luoab,
Guiqing Wena,
Lin Weib,
Aihui Liang*a and
Zhiliang Jiang*a
aKey Laboratory of Ecology of Rare and Endangered Species and Environmental Protection of Ministry Education, Guangxi Normal University, Guilin 541004, China. E-mail: zljiang@mailbox.gxnu.edu.cn; ahliang2008@163.com
bHezhou University, Hezhou 542899, China. Fax: +086-0773-5846201; Tel: +086-0773-5846141
First published on 7th January 2014
Rhodamine 6G adsorbed onto a triangular plate-type nanosilver aggregate exhibits a strong SERS peak at 1508 cm−1. Based on NO2− catalysis of the bromate oxidization of Rh6G and SERS peak quenching, trace nitrogen oxides in air samples were analyzed by a catalytic amplification SERS method.
The preparation of TAgNPs was considered and polyvinylpyrrolidone was not added as it strongly inhibits the SERS effect. The recommended procedure is as follows; add 500 μL 10 mmol L−1 AgNO3, 1.5 mL 60 mmol L−1 trisodium citrate and 120 μL 30% H2O2, to a 100 mL round-bottom flask containing 47 mL water on a magnetic stirrer, at this point the mixture is colorless. After the addition of 200 μL 0.1 mol L−1 NaBH4 solution, the color changed quickly from pale yellow to blue with the appearance of lots of bubbles. After standing for 12 h, the mixture was stored in a refrigerator at 4 °C, the concentration of TAgNPs was 5.4 mg L−1 with an average side length of 40 nm and an absorption peak at 600 nm which remained constant for 20 days (see, Fig. 1S†).
The general detection procedure is as follows; 100 μL 1.0 × 10−5 mol L−1 Rh6G solution, 200 μL 10 mmol L−1 KBrO3 solution, 100 μL 1 mol L−1 H3PO4 and a certain amount of NO2− solution were added into a 5 mL marked test tube, diluted to 0.8 mL and mixed well. After 15 min at 50 °C, the reaction was stopped by cooling with tap water. 120 μL 1.0 mol L−1 NaCl solution and 1.1 mL 5.4 mg L−1 TAgNPs (or 300 μL 57 μg mL−1 AuNPs or 300 μL 19.9 μg mL−1 AgNPs) were then added, diluted to 2.0 mL and mixed well. The mixture was transferred into a 1 cm quartz cell. The SERS spectrum and the SERS intensity at 1507 cm−1 (I1507 cm−1) were recorded. Meanwhile, a blank reagent solution (I1507 cm−1)0 without NO2− was recorded, and a value of ΔI = (I1507 cm−1)0 − I1507 cm−1 was calculated.
Under the chosen conditions, the redox reaction of Rh6G–KBrO3 is slow. Trace NO2− has strong catalytic enhanced effects on the redox reaction (Fig. 1). When the NO2− concentration increased, the SERS peak at 1508 cm−1 decreased due to the decrease in Rh6G, and the decreased SERS intensity was directly proportional to the nitrite concentration (Fig. 2). As the NOx content in air is directly proportional to the nitrite concentration, a new catalytic SERS method was established to detect trace NOx in the air.
In 50 mmol L−1 H3PO4 solution, TAgNPs were stable, the color was blue and there were three surface plasmon resonance (SPR) absorption peaks at 330 nm, 395 nm and 700 nm. When NaCl was added, the TAgNPs aggregated into big particles, the color changed from blue to gray and the SPR absorption peak decreased greatly. Rh6G has a strong molecular absorption peak at 525 nm. The oxidation reaction between KBrO3 and Rh6G takes place when the NO2− catalyst was added, the absorption peak intensity at 525 nm decreased and shifted to 540 nm simultaneously (Fig. 4S†). In 50 mmol L−1 HCl solution, Rh6G has a strong fluorescence peak at 564 nm when λex is at 498 nm. When the concentration of NO2− increased, the catalytic effect was enhanced and the Rh6G concentration decreased by KBrO3 oxidization resulted in a fluorescence peak at 564 nm reducing the intensity significantly (Fig. 5S†).
As shown in the IR spectra (Fig. 6Sa†), the characteristic peak at 3226 cm−1 is due to the stretching vibration absorption of the N–H groups, 1716 cm−1 is the CO group, 1646 cm−1 is the C
N group, 1607 cm−1, 1529 cm−1, 1501 cm−1 and 1443 cm−1 are the benzene groups, 1312 cm−1 is the C–N group and 1179 cm−1 is the C–O group (in contact with the aromatic rings). Fig. 6Sb and 6Sc† are the FTIR spectra of ORh6G gained through different methods. The product of Fig. 6Sb† was achieved using NO2− as a catalyst and heating at 50 °C for 4 h, while the product of Fig. 6Sc† was achieved by not adding NO2− solution as a catalyst and heating at 50 °C for 7 h. By comparing with Fig. 6Sa†, we found that the characteristic peak shifts of N–H, C
O, C
N, benzene and C–O basically remain stable in the product of Fig. 6Sb† after the oxidation reaction, which indicates that these groups remained in ORh6G. Meanwhile, the characteristic absorption peak of the Ar–N(H)–R group at 1312 cm−1 disappeared and the characteristic absorption peak of the C–N
O group at 1287 cm−1 and C–NO2 group at 1339 cm−1 were present instead. In addition, although the oxidation product described in Fig. 6Sc† was prepared without a NO2− catalyst, it still had the characteristic absorption peaks from C–N
O groups and C–NO2 groups, which illustrate that the final product was a mixture of NRh6G and ORh6G.
Solid and nanosol substrates are used commonly in SERS detection. However, the reproducibility of solid substrates is not good, and they are difficult to utilize in routine quantitative analysis. Nanosols have good reproducibility and low-cost using modern nanosynthesis, and their shape can also be controlled accurately. In this article, stable TAgNPs with an average side length of 40 nm, AgNPsd with an average size of 10 nm (Fig. 3) and AuNPs with 10 nm sols were prepared and considered as SERS substrates. Results showed that the TAgNP substrate is the most sensitive, the preparation is simple and rapid, and the stability is very good. Thus, it was chosen for use.
H3PO4, H2SO4 and HCl were considered as the acidic medium. The results showed that H3PO4 gave the highest sensitivity, and it was therefore selected for use. We studied the effect of H3PO4 concentration on ΔI. The results indicated that the catalytic rate became fast when the concentration of H3PO4 was 50 mmol L−1 and the value of ΔI reached its maximum. When the concentration of H3PO4 exceeded 50 mmol L−1, the acidity was too high resulting in a decreased catalytic rate, and the value of ΔI tended to decline. We therefore chose 50 mmol L−1 as the final concentration of H3PO4. We have studied the effect of Rh6G concentration on ΔI. The results showed that when the concentration of Rh6G was 0.5 μmol L−1, the value of ΔI reached its maximum. When the concentration of Rh6G exceeded 0.5 μmol L−1, the concentration was too high resulting in an increase in the blank values, and the value of ΔI tended to decline (Fig. 7S†). We therefore chose to use 0.5 μmol L−1 Rh6G. The effect of KBrO3 concentration on ΔI was also studied. The results indicated that when the concentration of KBrO3 was 1.0 mmol L−1, ΔI reached maximum (Fig. 8S†). 1.0 mmol L−1 KBrO3 was therefore chosen for the assay. Reaction temperature had an important effect on the catalytic reaction. When the bath temperature was 50 °C, the ΔI values reached the maximum. When the bath temperature exceeded 50 °C, the non-catalyzed rate increased and the values of ΔI tended to decline. A bath temperature of 50 °C was therefore used for the reaction. In order to completely investigate the catalytic reaction, the effect of bath time on ΔI was also studied. The results showed that when the bath time was in the range of 15–30 min, the ΔI values reached the maximum and remained basically stable. 15 min was therefore chosen as the bath time.
The concentration of NaCl had an important effect on the aggregation extent of the TAgNPs, so the effect of NaCl concentration on ΔI was studied. The results indicate that when the concentration of NaCl was 60 mmol L−1, the value of ΔI reached its maximum. We therefore chose 60 mmol L−1 as the optimal concentration of NaCl. The effect of TAgNP concentration on ΔI was also studied. When the concentration of TAgNPs increased within a certain range, the SERS signal of the system increased. When the concentration of TAgNPs was 2.97 μg mL−1, the value of ΔI reached its maximum. However, when the TAgNP concentration exceeded 8.55 μg mL−1, the blank values of the system increased, and led to a decrease in ΔI (Fig. 9S†). We therefore chose to use 2.97 μg mL−1 TAgNPs.
According to the procedure, the influence of coexistent substances (CS) on the determination of 36.0 nmol L−1 NO2− was tested, with a relative error of ±5%. The results indicated that 500 times more SO32−, Br−, ClO4−, PO43−, SO42−, CO32−, NO3− and F−, 300 times more Co2+, Zn2+, Cd2+, Mg2+ and Ca2+, and 100 times more Fe3+, Ba2+ and Cu2+ do not interfere with the determination (Fig. 4). It was shown that this new SERS method had good selectivity.
Under the selected conditions, the TAgNP substrate system is most sensitive, and the AgNP is more sensitive than the AuNP (Table 1). Thus, the Rh6G–KBrO3–TAgNP system was chosen to determine NOx in air, with a linear range (LR) of 3.6–72 nmol L−1 NO2− and a detection limit (DL) of 1.2 nmol L−1 NO2− (Fig. 5). The accuracy was examined and found to be 8.6% and 6.0% for 7.2 nmol L−1 NO2− and 43.2 nmol L−1 NO2− respectively. Compared with the fluorescence32 and the standard method of N-(1-naphthyl)ethylene diamine dihydrochloride spectrophotometry, this catalytic SERS method is more sensitive, and a smaller air sampling volume was used.
Nanosol | Regression equation | LR (nmol L−1 NO2−) | Coefficient | DL (nmol L−1 NO2−) |
---|---|---|---|---|
AuNPs | ΔI = 0.46 C + 4.1 | 14.4–144 | 0.9917 | 7 |
AgNPs | ΔI = 3.60 C + 26.9 | 7.2–72 | 0.9782 | 3 |
TAgNPs | ΔI = 3.36 C + 5.4 | 3.6–72 | 0.9932 | 1.2 |
Air samples were collected using the air sampling device (Fig. 10S†) and analyzed for their NOx content. The results (Table 1S†) show that the SERS results were in agreement with those obtained by the fluorescence method and the standard method of N-(1-naphthyl)ethylene diamine dihydrochloride. The relative standard deviation was in the range of 5.5–8.4% and the recovery was in the range of 96.3–103.8%.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47279e |
‡ Author contributions, these authors (JC Dong and QY Liu) contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2014 |