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Journal of Science: Advanced Materials and Devices (2017) 172e177 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Determination of low level nitrate/nitrite contamination using SERSactive Ag/ITO substrates coupled to a self-designed Raman spectroscopy system Chi T.K Tran**, Huyen T.T Tran*, Hien T.T Bui, Trung Q Dang, Liem Q Nguyen Institute of Materials Science (IMS), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet Road, CauGiay District, Hanoi, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 30 March 2017 Received in revised form May 2017 Accepted May 2017 Available online 12 May 2017 A portable and simple Raman scattering and photoluminescence spectroscopy system was set up for sensitive and rapid determination of nitrate/nitrite at low concentrations in water samples The SERS (Surface Enhanced Raman Scattering) e active Ag/ITO substrates were prepared and employed to obtain the enhanced Raman scattering light from the sample Concentrations as low as ppm and 0.1 ppm were detectable for nitrate and nitrite, respectively The obtained results confirmed the usefulness of the designed system in actual environmental measurements and analysis © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Raman spectrometer Detection of nitrite/nitrate SERS substrates Silver nanoparticles Limit of detection Introduction Intensive uses of chemical fertilizers and pesticides in agriculture as well as extreme utilization of preservatives have led to massive discharges of nitrates and nitrites into water and soil systems [1,2] Excessive concentrations of nitrates and nitrites are noticed to cause several adverse health effects such as decreased function of the thyroid gland, shortages of vitamin A by nitrate, etc Reactions of nitrites with hemoglobin in blood result in irreversible conversion of hemoglobin to meet hemoglobin with oxygen uptake and reduce the capacity of blood to carry oxygen [3,4] Nitrite within the acidic conditions of the stomach is converted to nitrous acid, which can act as a powerful nitro sating agent with a possible formation of carcinogenic nitrosamines known to be one of the most common causes of gastric cancer [4e7] In addition, nitrate (NO3 À ) can react with the enzyme in the stomach and produce NO2 À Besides, inputs of nitrate and nitrite to the environment can occur through industrial and domestic combustion processes with * Corresponding author ** Corresponding author E-mail addresses: chittk@ims.vast.vn (C.T.K Tran), huyenttt@ims.vast.ac.vn (H.T.T Tran) Peer review under responsibility of Vietnam National University, Hanoi gaseous NOx species to form NO3 À through photochemical conversion within the atmosphere The potential contamination of ground water presents the most immediate and extreme threat to human health and the environment [8] The fast in-situ determination of harmful chemical residues, particularly nitrites and nitrates, in the water and environment, therefore, has received increasing attention Various procedures covering major analytical methodologies have been developed to facilitate the detection, determination and monitoring of nitrates and nitrites [5,9,10] Spectroscopic methods are widely used, including UV/Vis, chemiluminescence, fluorimetry, Raman spectroscopy etc [5,11,12] Among these, Raman spectroscopy provides some superior advantages for microscopic analysis Raman spectra can be collected from a very small volume (95%), and (3-Mercaptopropyl) trimethoxysilane (MPTMS, >96%) were purchased from Tokyo Chemical Industry Co., Ltd Sulfosalicylic acid (95%), disulfophenic acid, sulfanilic acid (99%), 4-aminobenzene sulfonamide (98%), and silver nitrate (!99%) were commercial products of NacalaiTesque, Inc 2.2 Preparation of Ag-coated ITO substrates The Ag/ITO substrates were prepared by a hybrid method following the reported procedure [24,25] including the following main steps: (i) Cleaning of ITO (indium tin oxide) substrates under UV-ozone condition for h, and then drying in nitrogen condition; (ii) Functionalization of ITO surfaces by firstly immersing these substrates in 20 mL of 1% MPTMS for 34 h They were subsequently rinsed three times in methanol to remove all remained MPTMS Next, they were immersed in 20 mL of 1% HEXDT for 12 h and subsequently rinsed with IPA Finally, the as-functionalized ITO substrates were dried and preserved in pure nitrogen gas environment for further experiments; (iii) Functionalization of Ag NPs surfaces by using a mixed solution of C18 and C12 with a molar ratio of 1:6 These Ag NPs were rinsed and then dispersed in a mixed solution of n-hexane and acetone with a volume ration of 6:1; (iv) Deposition of functionalized Ag NPs on the functionalized ITO substrates by the electrochemical technique to obtain Ag colloidal films In this step, the ITO substrate worked as the cathode, while the carbon electrode worked as the anode The electrolyte 173 solution was a mixed solution of n-hexane and acetone containing the sulfanyl- (eSH) functional groups which were capped on Ag NPs surface The electrochemical experiments were performed for h; (v) Annealing of Ag colloidal films on ITO substrates at 500 C for 12 h; (vi) Removing of nano-sized Ag multilayers to obtain an Ag monolayer 2.3 Preparation of testing standards Firstly, nitrate and nitrite solutions were prepared by dissolving the potassium nitrate salt (KNO3) and sodium nitrite salt (NaNO2) in distilled water, respectively to yield the desired concentrations Secondly, these solutions were diazotized, and then coupled with appropriate organic chemicals to form azo dyes solutions (stock solutions) which absorb the light in the wavelength range of 400e500 nm Sulfosalicylic acid or disulfophenic acid was used in the diazotization treatment of NO3 À solution In the case of NO2 À , sulfanilic acid or 4-aminobenzene sulfonamide was used The testing standards were prepared by diluting the stock solutions to obtain the concentration range from 1000 mg/L to mg/L The same amount of testing standards (4 mL) was dropped onto the SERS substrates, and then was dried in the air before each Raman scattering measurement 2.4 Set up and characterization of the portable Raman system The compact experimental spectroscopic system was set up based upon an OEM mini-spectrometer and shown in Fig The dispersive type of operation mode was selected for the parallel recording of Raman scattering and fluorescent spectra The holographic diffraction grating of 900 lines/mm in the base frame QE65Pro (OCEAN OPTICS) provides the visible spectral range of 400e640 nm corresponding to the Raman shift of 10e3200 cmÀ1 with spectral resolution of ~28 cmÀ1 This range allows displaying either the Raman scattering or the fluorescent spectrum in one equipment The selected detector was a scientific-grade, backthinned, TE Cooled (TEC), 1044  64 elements CCD Array (Hamamatsu S7031-1006) The CCD can be cooled down 40 K below ambient, provides Signal-to-Noise ratio 1000:1 in a single acquisition Combined with the total integration time of 15 min, the relatively weak signals were well detectable on this system The diode-pumped solid state (DPSS) Nd:YVO4 laser (Teem Photonics) in TEM00 single mode emitting at 532 nm was used as the exciting source Laser operates in the quasi-continuous regime at kHz repetition rate with a full width at half maximum (FWHM) of 400 ps, yielding an average power of 26 mW The selection of 532 nm wavelength for excitation significantly enhances the Raman intensity (IRaman ~ 1/l4Laser) compared to the 785 nm wavelength, typically used in Raman spectrometry On the other hand, it fits in the absorbance ranges of as-prepared SERS substrates used in this system, while the sample would be burned/ destructed by shorter wavelength excitation light (higher energy) Sample irradiating and Raman scattering collecting optics comprised of a fast objective f/0.95, 80 mm focal length lens, an aluminium coated mirror, and a notch filter The use of an objective dramatically reduced the dimension of the system The XENON 0.95/25 objective (Schneider Kreuznach Co.) was designed for the highest optical tolerances required in the scientific research With its 0.95 maximum numerical aperture, the objective has an acceptance angle of 28 which will capture a great amount of light improving the record ability of the low-intensity scattered light Undesirable elastic Rayleigh scattering was eliminated by the 532 nm Notch Filter (PNeZX000279, Iridian Spectral Technologies) with a blocking band of 17 nm (OD7@532) At the same time, Raman shift below 200 cmÀ1 is cut (suppressed) by this filter The 174 C.T.K Tran et al / Journal of Science: Advanced Materials and Devices (2017) 172e177 Fig Compact Raman scattering and fluorescent spectroscopic system, (a) Upper stand: measuring arrangement, Lower stand: power supply units for the DPSS laser and minispectrometer (b) Top view of the system laser beam was focused by the lens (f ¼ 80 mm) and directed onto the sample under an optimal angle of ~40 The maximum laser beam power reaching the sample of~ 20 mW could be attenuated by neutral filters The sample holder was constructed so that it may be shifted vertically and/or horizontally in the plane perpendicular to the optical axis to monitor the irradiated test spot All parts of the optical configuration including the DPSS laser and the mini-spectrometer were arranged in the upper stand of the unit forming the measuring compartment The lower stand contained the power supply units and the data cable connected to a personal computer (PC) The equipment was completely covered allowing measurements under the dark conditions A small window with a cover was provided on the top of the equipment ensuring easy replacements of the sample Results and discussion Fig 2a presents the change in the observed Raman scattering signals of diazotized NO3 À samples on SERS-active Ag/ITO substrates In the figure, Raman signals of the same sample on a simple ITO substrate is shown as a reference for all Raman scattering measurements for comparison as well The characterization data of SERS-active Ag/ITO substrates have been recently reported [25] The results shown that nitrate dropped onto ITO substrate could be detected with the highest concentration of the testing standard (1000 mg/L) and this is confirmed by the appearance of the two Raman bands at 1007 cmÀ1 and 1358 cmÀ1, respectively, (Fig 2a-5), that are assigned to the vibration modes of NO3 À ions The intensities of these bands, however, are several orders of magnitude weaker than that produced by the nitrate on the SERS-active surface (Fig 2a-1) At this concentration, the intense Raman band at 1007 cmÀ1, followed by the ones at 1358 cmÀ1 and 1622 cmÀ1 appear on the spectra of the sample on the SERS-active Ag/ITO substrates (Fig 2a1e4) The observed bands are in good agreement with the reported studies [9,22,23] These results indicate the significance of the SERS substrate and its selectivity The Raman bands at 1007 cmÀ1 and 1358 cmÀ1 still remain, but with decreased intensity when the NO3 À concentration decreased from 1000 to mg/L, whereas the band at 1062 cmÀ1 could only be observed in the spectrum corresponding to the highest nitrate concentration This finding suggests a linear relationship between the SERS Raman signal intensity (peak height) and the nitrate concentration Furthermore, the fluorescent (PL) spectra of these samples could be recorded on the same system (Fig 2b) The results also show a similar tendency as observed with the recorded Raman spectra The PL intensities decrease as a function of nitrate concentration The Raman enhancement factor (REF) which characterizes the ability of a given SERS substrate to enhance the Raman signal is defined as the ratio of the SERS signal to the regular Raman signal [26] In this study, it has been found as large as  103 for the Fig (a) SERS spectra at different concentrations (1e1000 mg/L) of diazotized NO3 À standards on (1e4) SERS-active Ag/ITO and (5) on ITO substrates (b) Corresponding PL spectra Measurement conditions: exciting wavelength (532 nm), integration time (60 s), temperature (25 C), laser power to the sample (20 mW) C.T.K Tran et al / Journal of Science: Advanced Materials and Devices (2017) 172e177 175 Fig (a) Impact of diazotization treatment on Raman intensity (b) SERS spectra at different concentrations (1e1000 mg/L) of non-diazotized NO3 À standards on (1e4) SERS-active Ag/ITO and (5) ITO substrates Measurement conditions: exciting wavelength (532 nm), integration time (60 s), temperature (25 C), laser power to the sample (20 mW) Raman band at 1007 cmÀ1 corresponding to the nitrate concentration of ppm (1 mg/L tested standard) The factor decreased with the increase of nitrate concentration as the following: 6.7  102 at 10 ppm, ~102 at 100 ppm, and down to 17 at 1000 ppm For the 1358 cmÀ1 Raman band, the REF is ~1.5  103,  102, 90, and 12.5, corresponding to concentrations from ppm to 1000 ppm, respectively The nearly identical SERS behavior in these experiments may be partially caused by the screening effect when the molecules (e.g., the nitrate layers covering the SERS-active Ag/ ITO substrate as described in this study) block the incoming (incident) light reaching the surface of the metal nanoparticles As a result, the SERS effect and the REF value will be reduced However, this effect is expected at higher concentration levels of testing standard Gajaraj's group has identified this effect with a nitrate solution prepared in deionized water having a concentration greater than 104 mg/L [23] The lower limit of the screening effect (~100 mg/L) was observed in regular water samples due to the interference of dissolved solids (mineral salts) in the water Anions, such as chlorides, sulfates, or phosphates ubiquitously existing in water [9] will cause a significant interference in the nitrate and nitrite measurements leading to the decrease in Raman signal intensities measured by the SERS method The reason for the finding in our experiments is somewhat unclear The result for the low concentration levels cannot be explained by the screening effect alone The change in the enhancement factor presents a difficulty for quantitative analysis and strongly reduces the possible concentration range Nevertheless, the obtained Raman enhancement factor of 3e4  103 is sufficient for qualitative determination of nitrate content carried out on our setting system In this study, the diazotization treatment coupled with SERSactive substrates was considered to obtain the clearer and stronger Raman signals of the nitrate/nitrite samples at lower concentrations on such a self-designed measurement system For comparison, the SERS spectra of nitrate samples without diazotization treatment were taken Fig 3a shows a significant enhancement in the intensity of the 1358 cmÀ1 Raman band after diazotization treatment As it is clearly seen, the intensity of this band is increased by about 25e50 times as over the whole range concentrations from to 1000 mg/L Furthermore, the impact of diazotization treatment is also obviously reflected in the sensitivity As it can be clearly seen in Fig 3b, without the diazotization treatment, only one Raman band at 1358 cmÀ1 could be detected on the samples even with the highest concentration of testing standard of 1000 mg/L However, the linear relationship between the Fig The absorption spectra of (a) NO3 À and (b) NO2 À before and after diazotization treatment 176 C.T.K Tran et al / Journal of Science: Advanced Materials and Devices (2017) 172e177 Fig (a) The change in Raman intensity at different concentrations (1e1000 mg/L) of non-diazotized NO3 À standards on (1e4) SERS-active Ag/ITO and (5) ITO substrates Inset: Corresponding SERS spectra (b) Raman spectrum of diazotized NO2 À standard Measurement conditions: exciting wavelength (532 nm), integration time (60 s), temperature (25 C), laser power to the sample (20 mW), NO3 À standard prepared in lake water sample Raman intensity and the nitrate concentration is still revealed The impact of the diazotization treatment could be attributed to the red shift of the absorption peaks of NO3 À from 300 nm to 420 nm and those of NO2 À from 354 nm to 520 nm after diazotization treatment (see Fig 4) The observed absorption positions of nitrate and nitrite in the samples with diazotization treatment are located closer to 532 nm revealing that the effective excitation is at the selected wavelength (532 nm) in Raman scattering measurements As a result, the SERS effect appears more obvious for the nitrate/nitrite determination at low concentrations To further confirm the sensitivity in the nitrate determination in our self-designed experimental system and with the synthesized SERS-active substrates, a study on the nitrate determination on natural water sample (for example water from a lake) was undertaken The results are presented in Fig The results shown in Fig 5a imply that it is possible to detect the nitrate content at the highest nitrate concentration (1000 mg/L) even with an ITO substrate This is revealed by the appearance of the two Raman bands at 1062 cmÀ1 and 1358 cmÀ1, whereas only the 1358 cmÀ1 band is observed in the SERS spectra of the NO3 À samples which were prepared in deionized water (Fig 3b) Compared to the diazotized NO3 À samples prepared in deionized water (Fig 2a), both characteristic Raman bands are observed in this case However, the first main band is shifted from 1007 cmÀ1 to 1062 cmÀ1 and the intensity of the second band is higher than the others In the natural water sample, the different anions as chlorides, sulphates, and phosphates or undefined ones may interfere with nitrate in the measurements resulting in the reduced sensitivity Interestingly, the signal intensity from sample with nitrate concentration of mg/L is far strong enough for confirming the significance of the analysis These results have proved the applicability of the system in practical conditions In addition, the experiment data of the nitrite determination under the same conditions have shown that it is possible to utilize our self-designed experimental system also for the detection of the Raman signal of the 0.1 mg/L NO2 À sample (see Fig 5b) Conclusion We have successfully built a compact spectrophotometer system coupled with synthesized SERS-active Ag/ITO substrates for simultaneous Raman scattering and fluorescent spectroscopy studies By using SERS substrates, the Raman signal intensity increased with the highest enhancement factor of ~4  103 for nitrate/nitrite determination in low detection limit conditions (1 ppm for nitrate concentration and 0.1 ppm for nitrite concentration) It is considerably much lower than the acceptable level of contaminants in drinking water With much longer integration time of the equipment, the real detection limits of the system should be still lower The system can also simultaneously detect other residual anions in water by the SERS method The short time required for each measurement makes the designed system suitable for rapid determination of nitrate and nitrite in the process of monitoring and control of environment Furthermore, the fluorescence spectra could be observed on this system Acknowledgments This work was supported by Vietnam Academy of Science and Technology (VAST.ÐL.06/13-14) We thank the National Key Laboratory for Electronic Materials and Devices (VAST/IMS) for the use of facilities References [1] K.B Mabrouk, T.H Kauffmann, M.D Fontana, Abilities of Raman sensor to probe pollutants in water, J Phys Conf Ser 450 (2013) 012014e012019 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Raman signals of the same sample on a simple ITO substrate is shown as a reference for all Raman scattering measurements for comparison as well The characterization data of SERS- active Ag/ ITO substrates. .. (a) Impact of diazotization treatment on Raman intensity (b) SERS spectra at different concentrations (1e1000 mg/L) of non-diazotized NO3 À standards on (1e4) SERS- active Ag/ ITO and (5) ITO substrates. .. diazotization treatment coupled with SERSactive substrates was considered to obtain the clearer and stronger Raman signals of the nitrate/ nitrite samples at lower concentrations on such a self- designed
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