Surface-Enhanced Raman Spectroscopy-Based Approach for

Letter pubs.acs.org/ac

Surface-Enhanced Raman Spectroscopy-Based Approach for Ultrasensitive and Selective Detection of Hydrazine Xin Gu and Jon P. Camden* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: A probe mediated SERS-based strategy is developed to selectively detect hydrazine with superb sensitivity. Ortho-phthaldialdehyde, a simple probe, reacts specifically with hydrazine to form phthalazine, a molecule that possesses a larger Raman cross section and better affinity toward the SERS substrate. We observed a limit of detection of 8.5 × 10−11 M. Our method shows both qualitative and quantitative measurement of hydrazine with high sensitivity, low cost, and fast analysis time.

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analytes possessing a small Raman cross section is usually difficult when using a surface-modification strategy. Unfortunately, because of the intrinsically small Raman cross section of hydrazine,26 a SERS-based detection scheme for hydrazine has not been previously reported. Herein, we conceive a new SERS-based strategy that circumvents the need of functionalizing the Ag nanoparticle (AgNP) for robust and economical detection of hydrazine at concentrations 99% HPLC grade), hydrazine monohydrate (>98%), phthalazine (>98%), L-glutamine (>99%), cysteine (>97%), lysine (>99%), dimethylamine (>99%), triethylamine (>99%), hydroxylamine (>99%), thiourea (>99%), urea (>99.5%), AgNO3 (>99.99%), sodium citrate dehydrate (>99%), ammonium hydroxide solution (28%), BaCl2, CaCl2, Na2HPO4, CuSO4, FeCl3, FeSO4, KSCN, MgCl2, MnCl2, Na2SO3, Na2SO4, NaBr, NaCl, NaF, NaHCO3, NaI, NiCl2, Pb(OAc)2, ZnCl2 (All of them are ACS reagent grade.) were purchased from Sigma-Aldrich. Ultrapure water was obtained from a Millipore water system. Synthesis of Silver Nanoparticles. The Ag colloids were prepared according to Lee and Meisel’s method.27 Briefly, 36 mg of AgNO3 was dissolved in 200 mL water and boiled under continuous stirring. A volume of 4 mL of 1% (w/v) sodium citrate was added and the mixture was boiled with stirring for 30 min. The colloids were allowed to cool to room temperature and then diluted to 1 L. The solution was stored at 4 °C before use. The as synthesized Ag colloids were characterized by an Evolution 201 UV−vis spectrometer (Thermo Scientific) and imaged on a Carl Zeiss Libra 200MC transmission electron microscope equipped with a monochromator (Figures S1 and S2 in the Supporting Information). SERS Spectra. All SERS spectra were measured using a home-built Raman spectrometer employing a 532 nm Nd3+VAN laser (Spectra Physics) for 1 s exposure time and 60 acquisitions except for the phthalazine SERS spectra (yellow trace) in Figure 2, which was obtained by a 785 nm diode laser (Spectra physics). The laser beam (120 μW for 532 nm, 80 μW for 785 nm, measured at the sample) was focused onto aggregated Ag colloids using an inverted microscope objective (Nikon, 20×, NA = 0.5). The scattered light was collected by the same objective and after passing through a Rayleigh rejection filter (Semrock) was dispersed in a spectrometer (PI Acton Research, f = 0.3 m, grating = 1200 g/mm for 532 nm excited SERS spectra, grating = 600 g/mm for 785 nm excited SERS spectra). Light is detected with a back-illuminated deepdepletion CCD (PIXIS, Spec-10, Princeton Instruments). Winspec 32 software (Princeton Instruments) was used to operate the spectrometer and CCD camera. Each spectra was recorded 10 times using 10 different spots on the aggregated AgNP. Detection of Hydrazine at Various Concentrations. Water solutions of varying hydrazine concentrations (10−10− 10−5 M) were prepared as test solutions. A volume of 50 μL of 10 mM phthaldialdehyde was then added to 1 mL of the hydrazine test solutions. The mixture was stirred under 60 °C for an optimized time of 8 min (Figure S3, Supporting Information) and was then added into 4 mL of Ag colloids. After proper agitation, 200 μL of 1 M NaBr was added to aggregate the Ag colloids. The SERS spectrum of the resulting mixture was then recorded in situ after complete aggregation. The conversion rate from hydrazine to phthalazine was found to be 89%, which proved the high efficiency of our proposed scheme (Figure S4, Supporting Information).

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hyde yield an appreciable SERS signal (Figure 2) even at high concentrations (1 mM). The result not only proved the difficulty of sensing hydrazine directly, due to its intrinsic small Raman cross section, but also aligned with our expectation that phthaldialdehyde had limited surface accessibility due to the lack of affinity toward AgNP. In agreement with previous studies,28 we observed instant aggregation of colloids and a strong SERS signal upon addition of 10−6 M phthalazine solution (Figure 2). The strong peaks at 1450 and 1381 cm−1 are due to C−H bending and aromatic ring stretching coupled with NN stretching, respectively; 1223 cm−1 is due to C−H bending; 1016 cm−1 is attributed to skeletal breathing; and the peaks at 801 and 532 cm−1 arise from ring distortion. These results confirmed that phthalazine binds strongly to AgNP by replacing the citrate capping agent. Unlike previous studies,29 which employed higher powers and shorter wavelengths, we did not observe products from photochemistry in our measurements. We attribute this difference to the longer wavelength (532 nm), shorter acquisition time (60 s), and lower powers (120 μW) employed herein. As many commercial Raman spectrometers utilize 785 nm excitation, we also display the SERS spectra of phthalazine (Figure 2) using 785 nm excitation to illustrate the generality of our scheme. Qualitative and Quantitative Detection of Hydrazine. We found that the SERS spectra arising from 10−10 M hydrazine and phthaldialdehyde is identical to that of pure phthalazine, confirming the generation of phthalazine using the Schiff-base scheme (Figures 1 and 2). The optimal reaction

Figure 1. Scheme for SERS-based hydrazine detection using a phthaldialdehyde probe.

time was found to be 8 min (Figure S3, Supporting Information). As discussed above, the strong peak at 1381 cm−1 is associated with ring stretching coupled with NN stretching and the intensity of this peak is used to quantify the concentration of hydrazine. As Figure 3a shows, the SERS intensity has a wide range dynamic range, reporting on concentrations of hydrazine from 10−5 to 10−10 M with a linear range of detection from 10−10 to 10−9 M (Figure 3b). The error bars represent the relative standard deviations (RSD) from 3 replicate samples each of which was measured at 10 different spots. The LOD is calculated to be 8.5 × 10−11 M determined by 3σ/k, where σ is the standard deviation of the background and k is the slope of the calibration curve (Figure 3b). At higher concentrations of hydrazine (10 μM) the SERS signal stabilizes, suggesting the surface is saturated by phthalazine, and the signal is determined by the enhancement factor (EF) of aggregated Ag colloids and the number of hot spots measured each time rather than the total number of phthalazine molecules on the AgNP surface.31 Because of

RESULTS AND DISCUSSION

SERS Measurement of Hydrazine, Phthaldialdehyde, and Phthalazine. Prior to taking the SERS spectra of the reaction product between hydrazine and phthaldialdehyde, we confirmed that neither hydrazine nor the probe phthaldialdeB

DOI: 10.1021/acs.analchem.5b01566 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

the probe (Figure 4a). Therefore, while the primary amines tested might undergo the Schiff-base type reaction with the

Figure 2. SERS spectra of 1 mM phthaldialdehyde, 1 mM hydrazine, 10−6 M phthalazine, and 10 −10 M hydrazine with 10−5 M phthaldialdehyde in Ag colloids solution. All of the Ag colloid samples were aggregated by 1 mM NaBr solution before measurement (λex = 532 nm, Plaser = 120 μW, t = 60 s; yellow trace, λex = 785 nm, Plaser = 80 μW, t = 60 s). The asterisk-labeled (∗) peak is due to the SERS background caused by citrate.30

Figure 4. (a) SERS spectra in the presence of 10−6 M glutamine, cysteine, lysine, dimethylamine, trimethylamine, hydroxylamine, ammonia, thiourea, urea (red traces) and 10−9 M hydrazine (blue trace) with 10−5 M phthaldialdehyde in Ag colloids solution. (b) The Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, K+, Na+, F−, Cl−, Br−, I−, HPO42−, SO42−, SO32−, SCN−, HCO3−, OAc− (purple trace) and 10−9 M hydrazine mixed with the same concentration of above ions with 10−5 M phthaldialdehyde in Ag colloids solution. All of the Ag colloids samples were aggregated by 1 mM NaBr solution before measurement. The blank (black trace in both graphs) is the SERS spectra of Ag colloids aggregated by 1 mM NaBr solution (λex = 532 nm, Plaser = 120 μW, t = 60 s).

Figure 3. (a) Intensity of the 1381 cm−1 band versus log concentration of hydrazine. (b) Intensity of the 1381 cm−1 band versus the concentration of hydrazine ranging from 10−10 M to 10−9 M (λex = 532 nm, Plaser = 120 μW, t = 60 s).

variations in the EF of the aggregated AgNPs differ from 106 to 109 and the variation in the number of the hotspots in each measurement,32 it is understandable that the standard deviation of 10−5 M is greater than that in the lower concentrations. The results proved our scheme has great sensitivity toward hydrazine at low concentrations as well as a wide detection range at higher concentrations. Selective Detection of Hydrazine in the Presence of Interferences. In order to test the selectivity of our method, we checked primary amines including glutamine, cysteine, lysine, dimethylamine, trimethylamine, hydroxylamine, ammonia, thiourea, and urea which might potentially interfere with the detection. A relatively high concentration of 10−6 M of above amines were mixed with the phthaldialdehyde probe and no SERS signal was detected from the samples, while 10−9 M of hydrazine solution, 1000 times more dilute than those tested primary amine solutions, yielded strong signal when mixed with

probe phthaldialdehyde, none of them formed a ligand group strong enough to displace the citrate on the AgNP surface. However, the formation of NN from the reaction between hydrazine and phthaldialdehyde displaced the capping agent citrate and anchored the molecule on the AgNP surface firmly. We further explored the detection of 10−9 M hydrazine in the presence of 10−5 M Ba2+, Ca2+, Cu2+, Fe3+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, K+, Na+, F−, Cl−, Br−, I−, HPO42−, SO42−, SO32−, SCN−, HCO3−, and OAc−. We observed strong SERS signal from 10−9 M hydrazine using our probe mediated scheme even in the presence these interfering ions (Figure 4b). This result is expected since ion interferences themselves do C

DOI: 10.1021/acs.analchem.5b01566 Anal. Chem. XXXX, XXX, XXX−XXX

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not generally yield strong SERS signals nor do they react with the probe to form citrate displacing ligands. Quantitative Detection of Hydrazine in Real Lake Water Sample. We further tested the applicability of this method to real lake water. Water was collected from a lake in Notre Dame, Indiana. Before measurement, the sample was filtered with a 0.45 μm syringe filter to remove any particulate suspension. No SERS signal was obtained from the lake water upon addition of the probe (Figure 5) indicating no detectable

Letter

ASSOCIATED CONTENT

S Supporting Information *

Procedures to optimize the reaction time and the determination of the conversion rate between phthaldialdehyde and hydrazine, TEM image, and UV−vis spectra of synthesized Ag colloids. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01566.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Advanced Diagnostics and Therapeutics (XG) at the University of Notre Dame and U.S. National Science Foundation under Grant CHE-1150687. J.P.C. thanks Zachary Schultz for a critical reading of the manuscript. X.G. thanks Vighter Iberi for the TEM images of the synthesized colloids and Wenqi Liu for using the UV−vis spectrometer.

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Figure 5. SERS spectra of lake water and 180 pM hydrazine spiked lake water (λex = 532 nm, Plaser = 120 μW, t = 60 s).

hydrazine is present in the lake water sample. A sample of hydrazine spiked (1.8 × 10−10 M) lake water was analyzed using our SERS method and yielded a concentration of 1.7 × 10−10 M (10 measurements with 7% RSD and 92.2 ± 8.5% recovery) indicating quantative detection. From start to finish, the detection process requires less than 15 min and the cost of materials for analysis of a suspected sample is less than $0.01. These features, which can potentially be coupled to a low-cost, portable Raman spectrometer, illustrate the suitability of SERS-based analysis for on-site detection. Moreover, we expect that further optimization is capable of reducing reagent amounts to the microliter range without sacrificing sensitivity, further enhancing the flexibility in detection.

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REFERENCES

CONCLUSION

We developed a strategy utilizing SERS-based nanoprobe to selectively and quantitatively detect hydrazine with a LOD 8.5 × 10−11 M which is competitive with GC/MS methods and 4 orders of magnitude lower than the EPA TLV of 3.1 × 10−7 M. Quantitative detection of sub-nanomolar concentration of hydrazine in lake water samples was realized. This approach offers a new opportunity for on-site quick detection of hydrazine in environmental and biological samples with low cost and superb sensitivity. We further emphasize that carefully designed chemical derivation followed by SERS detection opens a new window in developing SERS-based sensors toward analytes that possess a small Raman cross section or little affinity to the SERS substrate. D

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DOI: 10.1021/acs.analchem.5b01566 Anal. Chem. XXXX, XXX, XXX−XXX