Original ArticleDetermination of low level nitrate/nitrite contamination using SERS-active Ag/ITO substrates coupled to a self-designed Raman spectroscopy system Institute of Materials
Trang 1Original Article
Determination of low level nitrate/nitrite contamination using
SERS-active Ag/ITO substrates coupled to a self-designed Raman
spectroscopy system
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
Article history:
Received 30 March 2017
Received in revised form
5 May 2017
Accepted 5 May 2017
Available online 12 May 2017
Keywords:
Raman spectrometer
Detection of nitrite/nitrate
SERS substrates
Silver nanoparticles
Limit of detection
a b s t r a c t
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 1 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/)
1 Introduction
Intensive uses of chemical fertilizers and pesticides in
agricul-ture as well as extreme utilization of preservatives have led to
massive discharges of nitrates and nitrites into water and soil
sys-tems [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) can react with the enzyme in the stomach and produce
NO2 Besides, inputs of nitrate and nitrite to the environment can
occur through industrial and domestic combustion processes with
gaseous NOxspecies to form NO3 through photochemical con-version 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 determi-nation of harmful chemical residues, particularly nitrites and ni-trates, 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, chem-iluminescence, fluorimetry, Raman spectroscopy etc [5,11,12] Among these, Raman spectroscopy provides some superior ad-vantages for microscopic analysis Raman spectra can be collected from a very small volume (<1mm in diameter) of sample and allow for the identification of species present in that volume The sample can be both aqueous or having a high moisture content making this technique easy to be adapted tofield applications …[13] However, Raman signals are characteristically weak The effective cross sec-tion is only about 1029cm2, which is much smaller than that of fluorescence signals (1016cm2) Hence, a low number of scattered photons are available for detection To amplify the weak Raman signals, the surface-enhanced Raman scattering (SERS) technique is applied [2] This is a sensitive spectroscopic method for micro analysis and trace species detection in chemical and biomedical
* 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.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2017.05.002
2468-2179/© 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
Journal of Science: Advanced Materials and Devices 2 (2017) 172e177
Trang 2depending on the definition and measurement conditions[17e21].
However, an average factor of the order of 104has been mostly
reported By using Raman scattering spectroscopic technique
coupled with SERS substrates for nitrate determination in aqueous
environments, the detection limit ranging from 4.2 to 9.8 mg/L was
reported by Lieberman's group[22] More recently, Gajaraj's group
has reached a lower nitrate detection limit in water and wastewater
of 0.5 mg/L using a commercial gold SERS-active substrate and a
high-resolution Raman microscopic spectrometer[23] In our work,
as-synthesized Ag/ITO substrates and the self-designed Raman
spectroscopy system were applied to get a high signal-to-noise
ratio in the Raman scattering spectra of nitrate/nitrite under very
low conditions The experimental set up is believed to be
appro-priate for applications underfield conditions Furthermore,
fluo-rescence spectra are found to be easily recordable in the same
system
2 Experimental
2.1 Materials
For sample preparation, all chemicals were of analytical purity
grade and used without further purification, including: potassium
nitrate (KNO3, 99%, SigmaeAldrich), sodium nitrite (NaNO2,
99%, SigmaeAldrich),1,6-Hexanedithiol (HEXDT, 96%,
Sigma-eAldrich), isopropanol (IPA, 99,5%, Merck) Ethanol, acetone, and
methanol (99%, SigmaeAldrich), Octadecanethiol (C18, 97%),
1-dodecanethiol (C12, >95%), and (3-Mercaptopropyl)
trimethox-ysilane (MPTMS,>96%) were purchased from Tokyo Chemical
In-dustry 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 3 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
envi-ronment 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
colloidalfilms In this step, the ITO substrate worked as the cathode,
while the carbon electrode worked as the anode The electrolyte
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 solution In the case of NO
2 ,
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 1 mg/L The same amount of testing standards (4mL) 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 inFig 1 The dispersive type of operation mode was selected for the parallel recording of Raman scattering andfluorescent spectra The holo-graphic 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 cm1 with spectral resolution of ~28 cm1 This range allows displaying either the Raman scattering or thefluorescent spectrum in one equipment The selected detector was a scientific-grade, back-thinned, TE Cooled (TEC), 1044 64 elements CCD Array (Hama-matsu S7031-1006) The CCD can be cooled down 40 K below ambient, provides Signal-to-Noise ratio 1000:1 in a single acqui-sition 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 1 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/l4
Laser) 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 sub-strates 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 notchfilter 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 28which 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 cm1is cut (suppressed) by thisfilter The
Trang 3laser beam was focused by the lens (f¼ 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 neutralfilters 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
con-tained 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
win-dow with a cover was provided on the top of the equipment
ensuring easy replacements of the sample
3 Results and discussion
Fig 2a presents the change in the observed Raman scattering
signals of diazotized NO3 samples on SERS-active Ag/ITO
sub-strates In thefigure, 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 cm1and 1358 cm1, respectively, (Fig 2a-5), that are assigned to the vibration modes of NO3 ions The in-tensities of these bands, however, are several orders of magnitude weaker than that produced by the nitrate on the SERS-active sur-face (Fig 2a-1) At this concentration, the intense Raman band at
1007 cm1, followed by the ones at 1358 cm1and 1622 cm1 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
sig-nificance of the SERS substrate and its selectivity The Raman bands
at 1007 cm1and 1358 cm1still remain, but with decreased in-tensity when the NO3 concentration decreased from 1000 to
1 mg/L, whereas the band at 1062 cm1could only be observed in the spectrum corresponding to the highest nitrate concentration Thisfinding suggests a linear relationship between the SERS Raman signal intensity (peak height) and the nitrate concentration Furthermore, thefluorescent (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 4 103 for the
Fig 1 Compact Raman scattering and fluorescent spectroscopic system, (a) Upper stand: measuring arrangement, Lower stand: power supply units for the DPSS laser and mini-spectrometer (b) Top view of the system.
Fig 2 (a) SERS spectra at different concentrations (1e1000 mg/L) of diazotized NO 3 standards on (1e4) SERS-active Ag/ITO and (5) on ITO substrates (b) Corresponding PL
C.T.K Tran et al / Journal of Science: Advanced Materials and Devices 2 (2017) 172e177 174
Trang 4Raman band at 1007 cm1corresponding to the nitrate
concen-tration of 1 ppm (1 mg/L tested standard) The factor decreased
with the increase of nitrate concentration as the following:
6.7 102at 10 ppm, ~102at 100 ppm, and down to 17 at 1000 ppm
For the 1358 cm1Raman band, the REF is ~1.5 103, 4 102, 90,
and 12.5, corresponding to concentrations from 1 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
(inci-dent) 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 104mg/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
in-tensities measured by the SERS method The reason for thefinding
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 con-centration 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 SERS-active substrates was considered to obtain the clearer and stron-ger Raman signals of the nitrate/nitrite samples at lower concen-trations on such a self-designed measurement system For comparison, the SERS spectra of nitrate samples without diazoti-zation treatment were taken.Fig 3a shows a significant enhance-ment in the intensity of the 1358 cm1 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 1 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 cm1could be detected on the samples even with the highest concentration of testing stan-dard of 1000 mg/L However, the linear relationship between the
Fig 3 (a) Impact of diazotization treatment on Raman intensity (b) SERS spectra at different concentrations (1e1000 mg/L) of non-diazotized NO 3 standards on (1e4) SERS-active Ag/ITO and (5) ITO substrates Measurement conditions: exciting wavelength (532 nm), integration time (60 s), temperature (25C), laser power to the sample (20 mW).
Trang 5Raman 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 NO3from 300 nm to 420 nm and
those of NO2from 354 nm to 520 nm after diazotization treatment
(seeFig 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
under-taken The results are presented inFig 5
The results shown inFig 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 cm1and 1358 cm1, whereas only
the 1358 cm1 band is observed in the SERS spectra of the
NO3samples which were prepared in deionized water (Fig 3b)
Compared to the diazotized NO3samples prepared in deionized
water (Fig 2a), both characteristic Raman bands are observed in
this case However, thefirst main band is shifted from 1007 cm1to
1062 cm1and the intensity of the second band is higher than the
others In the natural water sample, the different anions as
chlo-rides, 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 1 mg/L is far strong enough for confirming the
significance of the analysis These results have proved the
appli-cability 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 NO2sample (seeFig 5b)
4 Conclusion
We have successfully built a compact spectrophotometer
sys-tem 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 103for ni-trate/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, thefluorescence 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 Labo-ratory for Electronic Materials and Devices (VAST/IMS) for the use
of facilities
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