Analytica Chimica Acta 994 (2017) 56e64
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Analytica Chimica Acta
journal homepage: www.elsevier.com/locate/aca
High reliable and robust ultrathin-layer gold coating porous silver
substrate via galvanic-free deposition for solid phase microextraction
coupled with surface enhanced Raman spectroscopy
Weiwei Bian a, b, Zhen Liu a, Gang Lian a, Le Wang c, Qilong Wang a, **, Jinhua Zhan a, *
a
b
c
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Department of Chemistry, Shandong University, Jinan 250100, China
Department of Pharmacy, Weifang Medical University, Weifang 261053, China
Center of Technology, Jinan Entry-Exit Inspection and Quarantine, Jinan 250014, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An uniform ultrathin-layer of Au was
deposited on porous Ag surface by
galvanic-free deposition.
This coating facilitates to have a high
oxidation resistance for the substrate
under
heating
in
atmosphere
condition.
A
high
enhancement
factor
(1.3 106) and low LOD (5.1 ppb) for
the extraction and identification of
nitrofurazone.
Rapid detection of prohibited antibiotic and its marker residue in a
complex matrix.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2017
Received in revised form
25 August 2017
Accepted 3 September 2017
Available online 13 September 2017
That intense demand for both high sensitivity and high reliability has been a key factor strengthening the
surface enhanced Raman spectroscopy (SERS) in the analytical application, particular in the hyphenation
with pre-concentration technique. Credible data acquisition and processing is very dependent on the
stable and uniform performance of SERS-active substrate. Here, a reliable and uniform ultrathin-layer Au
was proposed for protecting the porous Ag fiber (porous Ag@Au) and applied in the solid phase
microextraction coupled with SERS. The Au layer was carefully deposited on porous Ag surface to form
the uniform film by a galvanic-free displacement reaction. This coating endowed the substrate with high
oxidation-resistance under heating and good durability in the atmosphere condition. The extraction and
SERS performance of Nitrofurazone and Semicarbazide were investigated on this fiber, the bands at
1350 cm1 and 1387 cm1 were selected as the characteristic peaks for quantitative determination,
respectively. This robust and sensitive substrate provide the high enhancement factor of 1.3 106 and
low LOD of 5.1 ppb for the extraction and identification of Nitrofurazone compounds. Importantly, this
work develops a versatile strategy for rapid detection of prohibited antibiotic and its marker residue in a
complex matrix.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Surface enhanced Raman spectroscopy
Solid phase microextraction
Gold
Silver
Galvanic-free deposition
Nanostructures
* Corresponding author.
** Corresponding author.
E-mail address: jhzhan@sdu.edu.cn (J. Zhan).
http://dx.doi.org/10.1016/j.aca.2017.09.004
0003-2670/© 2017 Elsevier B.V. All rights reserved.
W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
1. Introduction
The goal of promoting surface enhanced Raman scattering from
an ultra-sensitive spectroscopic technique into a reliable quantitative analytical strategy has attracted the tremendous efforts for
many years [1e3]. The challenges are not only how to prepare the
sensitive enhancing substrate for the relevant identification, but
also the high demand for stability and reproducibility which can
provide a valuable vibratory and structural information about the
target [4e7]. On the other hand, the substrate capable of robust
performance and longer shelf-life may have greater available than
just ultra high sensitivity for many applications [8e10]. Currently,
the hyphenated technique which coupling solid phase microextraction (SPME) with SERS has been demonstrated as a powerful
tool for ultrasensitive analysis, which integrates the preconcentration and Identification in one step rapidly. The SPME-SERS
method can be performed using a portable kit of laser spectrometer, it has been successfully applied in the detection of pollutants
[11e13], pesticides [14e16] and additive [17,18]. As for this case, the
critical question of this technique mainly depends on the intrinsic
property of a difunctional substrate, which not only must process
the high-efficiency extraction but also the strong SERS response, in
particular the uniformity and long-term stability.
Silver is the fascinating and essential material in surface plasmons field. Normally, incident light with compatible momentum
can excite surface plasmon polaritons on silver nano crystals
interface, which has stronger and sharper local electromagnetic
field intensity, and will enhance the inherently weak Raman scattering signal of molecules by many magnitudes [19]. Silver nanostructures substrates are largely preferred for their higher
enhancement factor, which promises numerous analytical applications including not only routine detection but also singlemolecule mapping and bio-imaging [20e22]. However, the relative activity of silver in the atmosphere leads to the drawback of
instability for SERS signals, the sophisticated manipulation is usually needed to be performed carefully to obtain the reliable data. An
alternative approach is to protect the Ag nanostructure by a
conformal and ultra thin shell from the oxidizing species, such as
the coating of Au [23,24], silica [25], alumina [26,27] or alkyl thiol
[28,29]. Importantly, Au element is an ideal shell material which
will prevent the substrate from oxidation or contamination over a
reasonably long period. The ultrathin Au layer exhibit comparable
plasmonic properties as silver (local field enhancements) in the
longer wavelength range (typically l > 600 nm), and significantly
improve the surface compatibility of Ag substrate [30,31]. Unfortunately, the deposition of uniform Au layer on the surface of Ag is
difficult in the aqueous solution containing Au3þ, the galvanic reaction will occur instantaneously between them and corrode the Ag
nanostructure [32,33]. In practice, the difference value of work
function between the depositing metal with the substrate object
dominates whether the monolayer deposition will happen (the
work function of Au is larger than Ag, so the Au atom will deposit on
Ag atom more difficult than it will deposit onto itself) [34,35].
Hence, the key issues for uniformly depositing is to restrain
galvanic replacement and decelerate the deposition rate [36,37].
Many methods have been developed to minimize the galvanic reaction by decreasing the redox potential of Au3þ through
complexation, such as halide ions [23], sulfite [38], cetyltrimethylammonium bromide [39]. On the other hand, when
selecting the appropriate complex agent with an efficient reductant, and controlling the reaction rate, it is demonstrated that the
conformal ultrathin layer of Au can be successfully deposited on Ag
substrate by chemical method.
Porous silver materials have the good porosity, large specific
surface area, excellent mechanical properties, make it very
57
attractive as the substrate materials. The porous silver layer which
prepared by conventional electrochemical synthesis can provide an
active and clean substrate for potential SPME-SERS application
[40,41]. Although the porous silver nanostructure has the greater
electromagnetic enhancement for adsorbate, the poor durability
still limit their stability and repeatability that is considered as the
critical factor for SERS measurement, especially in the complex
matrix. In this article, the ultrathin Au layer was deposition on
porous Ag surface by the galvanic-free displacement reaction. The
galvanic reaction of Ag/An3þ was inhibited by regulated the redox
potential of Au3þ in an alkaline solution containing I anion, then
Au3þ was slowly reduced by ascorbic acid to form the uniform film
on Ag surface. The porous Ag@Au substrate was characterized by
SEM, EDS, XPS and AFM methods. The stability and uniformity were
investigated by SERS using p-aminothiophenol (PATP) as the probe.
A robust, reliable and high sensitive Au protecting porous Ag SPMESERS was successfully fabricated. The SERS response of nitrofurazone (NFZ) and Semicarbazide (SCA) were investigated on this
substrate. The extraction capacity was subsequently optimized.
Finally, the proposed substrate was applied in the extraction and
identification of prohibited antibiotic and its marker residue in
seafood, the limit of detection was low to ppb level.
2. Materials and methods
2.1. Chemicals
Nitrofurazone and Semicarbazide hydrochloride as hydrochloride (analytical standards) were purchased from Aladdin chemicals
Co. Ltd. p-aminothiophenol (97%), HAuCl4$4H2O, potassium iodide,
and ascorbic acid were purchased from Sinopharm Chemical Reagent (China). Silver wire (ø0.4 mm, 99.9%) were obtained from
Beijing nonferrous metal research institute. The stock solution was
prepared by HPLC grade methanol (TEDIA®), and Milli-Q water
(18.2 MU) was used in all experiments.
2.2. Preparation and characterization of porous Ag@Au substrate
The porous Ag layer was synthesized by the electrochemical
method as the previous articles [14]. A silver wire (effective area
ø0.4 30 mm) was degreased and washed thoroughly, then
employed as a working electrode in the three-electrode system. The
porous nanostructure was prepared by cyclic voltammetry scanning from 0.2 V to þ0.2 V with the rate of 25 mV/s for 15 cycles at
Princeton® PARSTAT 4000 electrochemical workstation, then the
prepared fiber was rinsed and dried.
The deposition of ultrathin Au layer on porous Ag was performed by galvanic-free reduction [36]. In a standard synthesis,
50 mL of a mixed solution which has the concentration of
10 mmol L1 NaI and 10 mmol L1 ascorbic acid was added into a
beaker in tall form, then the PH value adjusted as 11.0 by 0.1 mL of
0.5 mol L1 NaOH solution. In the beginning, the porous Ag was
immersed into this solution via the hanging model, 0.5 mL of
0.1 mmol L1 HAuCl4 solutions was automatically into the system at
a rate of 0.05 mL/min under magnetic stirring (300 RMP). After
injection, the reaction keep continues for 20 min, then the prepared
fiber was rinsed thoroughly with methanol and water.
The crystallinity of porous Ag layer was determined by X-ray
diffraction (XRD, Bruker D8 Advance X-ray diffractometer), the
morphology of was characterized by scanning electron microscope
(JEOL JSM-6700F). The UV-Vis diffuse-reflectance spectra were
performed at Shimadzu UV-2550 with integrating sphere. The
deposition of Au coating was confirmed by X-ray photoelectron
spectroscopy (XPS, ThermoFisher SCIENTIFIC ESCALAB 250) and
EDS mapping (Oxford Instruments). The surface topography of Au
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W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
shell was imaged by Atomic force microscopy (Multimode, Veeco
Instruments & Bruker). The thermal conductivity of porous Ag@Au
fiber was visualized by IRS S6 portable thermal imager (IRS
Instruments).
The laser and thermal stability of substrate were investigated
using PATP as the probe. The porous Ag@Au fiber was immersed
into 0.1 mmol L1 PATP methanol solution for 12 h to form the selfassembly membrane. The Raman spectra were recorded at Ocean
Optics QE Pro Raman Spectrometer with the excitation wavelength
at 785 nm, the laser power was 500 mW and the integration time
was 1 s. The uniformity was evaluated by measuring spectra intensity along the axial and radial direction on the fiber surface. In
the axis direction, the sampling interval was 3.0 mm on the fiber
surface. In the radial direction, the sampling interval was set as
0.3 mm along the perimeter, the sampling sites were chosen at 3, 6,
9 and 12 o'clock, respectively. A number of sampling points for each
substrate were 40 sites.
2.3. Extraction and SERS analysis of Nitrofurazone and
Semicarbazide
The extraction was performed by DI-SPME mode, the SPME fiber
immersed into the 25 mL working solution for 120 min at 25 C.
After that, the SERS response of Nitrofurazone and Semicarbazide
on porous Ag@Au surface were investigated with the laser power of
100 mW. The electrostatic interaction determines the adsorption of
the amino compound on the Au surface. The influence of pH on
extraction process was optimized. The extraction equilibrium time
was also discussed by measuring kinetic curves with different
concentrations. The long time repeatability of SERS spectra on
porous Ag@Au substrate was compared at different storage periods.
The reproducibility of porous Ag fibers was also evaluated after the
extraction and elution.
2.4. Validation in seafood sample
The quantitative relationship was established between the intensity of fingerprint peak and the concentration in solution. The
RSD values were calculated from the data of six times determination. The LOD for each compound was calculated by extrapolating
to an S/N of 3. After that, the proposed method was validated in
seafood sample. The recovery was obtained by addition standard
solution in the blank sample. The spiked sample was prepared as
the following: 10.0 g surimi from fresh turbot mixed thoroughly
with 2.0 mL stocking solution of NFZ and SCA under stirring,
respectively. Then, the mixture was diluted with water pH 7.0
Fig. 1. (A) The morphology image of porous Ag@Au layer, (B) XPS spectra of the selected region for the binding peak of Au (4f7/2, 4f5/2) and Ag (3d5/2, 3d3/2), respectively.
W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
10 mmol L1 PBS buffer solution to give the final volume of 25.0 mL.
This prepared solution of the spiked sample was used for SPMESERS analysis immediately. The recovery ratio was calculated
from the ratio of the testing result to the theoretical value. Principal
component analysis (PCA) was also employed to discriminate the
coupling SERS spectra of the mixture. The spectral data were pretreated by smoothing, baseline subtraction using the software
LabSpec5 to optimize the data quality. The spectra were uploaded
to PCA procedure compiling with MATLAB R2014a.
3. Results and discussion
3.1. Galvanic-free deposition of Au shell on porous Ag surface
Porous Ag layer was prepared by cyclic voltammetry scanning in
0.1 mol L1 HCl electrolyte (Fig. S1). Galvanic replacement is a redox
reaction that exchanges the electron from sacrificial metal to the
metal cation with higher electrode potential in the electrolyte, the
template will be damaged and dissolved accordingly. In the Ag/
Au3þsystem, the electrode potential of Au3þ/Au can be reduced
from 0.93V to 0.56V by forming the complex of AuI
4 . The galvanic
reaction will be inhibited significantly, then Au was slowly deposited on porous Ag surface by ascorbic acid to form the ultrathin film.
The morphology and component of prepared porous Ag@Au substrate were characterized by electron microscope and XPS. As
shown in Fig. 1A, the substrate mainly consist of unconsolidated
porous structure, which was constructed by aggregated nanoparticles with the diameter of 100e150 nm. Compared to porous Ag
59
substrate, there is no defect or cavity damage was observed after
the deposition. This demonstrates that, with complexation, the
galvanic replacement was successfully limited, and the loss of
porous Ag layer was minimized simultaneously. The binding energy
of porous Ag @Au layer was measured by XPS (Fig. S2). The binding
peak at 368.4 eV and 374.5 eV was assigned to Ag (3d5/2) and Ag
(3d3/2), respectively. The weak binding peak of Au (4f7/2) and Au
(4f5/2) was also observed at 84.6 eV and 88.3 eV, which confirmed
the deposition of Au shell (Fig. 1B). Absorption spectra of porous Ag
and porous Ag@Au were compared in Fig. S3. The porous Ag has a
strong band at 322 nm in the ultraviolet region which is attributed
to the plasmon resonance of bulk electron (inter-band transition).
In contrast, the SPR band at 322 nm slightly shifted to longer
wavelengths, and a new absorption region appeared at 500 nm
after the Au coating. These results show that the porous Ag layer
remains intact during the disposition.
The elemental mapping of Au on porous Ag@Au which prepared
with a different injection volume of HAuCl4 was imaged by EDS
analysis. Fig. S4 confirm the presence and distribution of Au in the
sample. It demonstrates that Au is homogeneously distributed
within the whole fiber, and the content of Au increases as the injection volume. The difference value of work function between the
gold and silver determine the deposition of Au on itself is more
easily than it deposits on the Ag surface [42,43]. In this system, the
rate of deposition was mainly determined by the injection volume
of HAuCl4 in a redox reaction. The increase of injection volume
would generate excessive reduction products in a short time, which
can result in the island-like aggregation of Au on the substrate
Fig. 2. 2D and 3D topographic pattern of the Au shell deposited on porous Ag surface with different injection volume: (A) 0.5 mL, (B) 3.0 mL and (C) 5.0 mL. The RMS roughness of
local region was also measured as 2.7 nm, 36.4 nm and 72.9 nm, respectively.
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W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
surface. The surface topography of Au shell was under different
volume was characterized by AFM. As illustrated in Fig. 2, A
500 nm 500 nm area was imaged by the 2D and 3D pattern. A
smooth and rounded cover was observed on Ag nanoparticle surface when the injection volume was set at 0.5 mL, and the RMS
roughness of local region was also measured as 2.7 nm, which
demonstrates that the Au film deposited uniformly. As injection
volume increased to 3.0 mL, many Au nanoparticles with the
diameter about 50 nm begin to spread over the Ag surface, and the
RMS roughness increased to the value of 36.4 nm. Finally, when the
5.0 mL Au3þ was injected, the heterogeneous nucleation and
growth initiated rapidly, which will lead to the pomegranate-like
polycrystal and cavitation erosion in local places. The highdensity distribution of Au nanoparticles make the uniform of
porous layer become worse, the RMS roughness also has the value
of 72.9 nm. While, the outmost layer of Au may not be complete, if
we strip the porous Ag@Au layer from the wires. It is extremely
difficult to directly identify and measure the thickness of conformal
Au layer on a solid silver wire. An alternative way, the AFM was
employed to indirectly provide the information for the thickness of
Au layer. Fig. S5 shown the local topography of Au coating, the
thickness of Au layer was estimated as 5.0e6.0 nm by section
analysis.
The SERS response of PATP was investigated on the porous
Au@Ag substrate prepared with different HAuCl4 volume. As shown
in Fig. S6, the SERS enhancement recede as the injection volume
ranging from 0.5 mL to 5.0 mL, the intensity of characteristic band
(1077 cm1) gets the maximum at the 0.5 mL, then it decreases
gradually. The enhancement factor was also calculated from the
characteristic SERS intensity, the change of enhancement effect can
be attributed to the weak local field intensity of Au coating and
destruction of porous Ag layer. Consequently, the volume of 0.5 mL
was selected in the following experiments for deposition.
Gold is the high stability metal which can resist to oxidation and
corrosion even in the heating. The temporal stability of substrate
was evaluated at high energy laser irradiation and after the
annealing. The continuous SERS spectra of PATP were recorded for
600s (sampling interval 2 s) with laser power at 500 mW on porous
Ag substrate before and after deposition of Au shell, respectively. As
shown in Fig. 3, there is a slight change in SERS intensity under
500 mW continuous radiation, the RSD of variations in peak intensity at 1077 cm1 is calculated as 2.27%. In contrast, the SERS
Fig. 3. Stability of the substrate probed with PATP under 600s continuous laser radiations, (A) the porous Ag@Au fiber and (B) the porous Ag fiber. The inset plot showed the
intensity variation of the characteristic peak at 1077 cm1 with time.
W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
intensity decreases significantly after 4.5 min without Au protection. For more evidence, the surface temperature was measured
and visualized by infrared imagery (Fig. S7). The surface temperature of porous Ag@Au substrate increase rapidly (the photothermal
effect of gold nanostructure) within 2 min, and reach a maximum
with good stability. In addition, the resistance of long-term heating
was also assessed. The porous Ag @Au substrate was annealed at
different temperature for 30min, then that was modified by PATP
for measurement. The results clearly show that the substrate can
provide the good SERS response range from 60 C to 180 C, but
when the annealing exceed 240 C, the peak intensity fades away
subsequently (Fig. S8). These results indicate that the ultrathin Au
coating can significantly improve the anti-oxidation capacity of the
substrate for SERS analysis.
3.2. SERS response of NFZ and SCA
The SERS performance of NFZ and SCA was investigated by
immersing the porous Ag @Au fiber into the 1.0 mmol L1 NFZ and
1.0 mmol L1 SCA working solution for 120 min at 25 C, respectively. After extraction, Raman spectra were measured and shown
Fig. 4. SERS spectra on porous Ag@Au fiber after extraction. (A) The solution of
1.0 mmol L1 NFZ, (B) the solution of 1.0 mmol L1 SCA. Raman spectra were measured
with laser power 100 mW, the integration time was 1 s, the spectra of solid powder
was shown on the bottom of each graphic.
61
in Fig. 4. The characteristic Raman bands of NFZ appear in the region from 400 cm1 to 1600 cm1, the characteristic peaks of NFZ at
811 cm1, 968 cm1, 1020 cm1, 1247 cm1, 1350 cm1, 1385 cm1,
1479 cm1,1560 cm1, 1606 cm1 can be clearly identified with the
Raman bands of solid powder at 807 cm1, 966 cm1, 1023 cm1,
1249 cm1, 1349 cm1, 1389 cm1, 1472 cm1, 1563 cm1 and
1596 cm1 (Fig. 4A). The characteristic bands at 1247 cm1,
1350 cm1 and 1606 cm1 were assigned to r (C-H) rocking, d(NC¼O, N-H) and n(C¼O), respectively [44e46]. For SCA, the bands at
471 cm1, 670 cm1, 1035 cm1, 1136 cm1, 1218 cm1, 1309 cm1,
1387 cm1, 1440 cm1 and 1608 cm1 were identified, the corresponding Raman bands of solid powder was at 466 cm1,
1139 cm1, 1221 cm1 and 1389 cm11 (Fig. 4B). The characteristic
bands at 1035 cm1 and 1218 cm1 were assigned to r (NHþ
3 ),
1387 cm1belong to n(C-N), d(N-C¼O) and d(N-H) [47,48]. The
enhancement factors were also calculated as 1.3 106 and 3.5 106
for the NFZ and SCA, respectively.
The uniformity, reusability and long-term repeatability are the
crucial parameters of the substrate for quantitative detection. The
uniformity was evaluated by recording the Raman response along
with the axial and radial direction on the fiber surface, then the
intensity of characteristic peaks was mapped in the 3D pattern
(Fig. S9). The RSD of SERS bands intensity was 3.6% (NFZ) and 2.7%
(SCA), revealing the good uniformity of substrate. The reusability
was also evaluated by comparing the multi-cycle of extraction/
elution process. The elution was performed in 5% NaBH4/methanol
mixed solution with ultrasonic cleaning for 1min. After that, the
Fig. 5. SERS performance of porous Ag@Au fiber after long-term storage in the atmosphere. The porous Ag@Au substrate was stored on the 1 day, 10 days and 30 days,
then were used for extraction and detection, respectively.
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W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
SERS spectra of extracted and blank fiber were subsequently
recorded. As shown in Fig. S10, the RSD of peak intensity are 4.4%
(NFZ) and 5.3% (SCA) after four cycles, these results indicate the
acceptable performance for analysis. Finally, the shelf life of this
SPME fiber was investigated after long-term storage in the atmosphere. The porous Ag@Au substrates of the different stored time
were used for extraction and detection simultaneously. The results
show that the fiber has good SERS performance even after a
month of storage, the shape and intensity of peaks keep in stability
(Fig. 5).
3.3. Optimization of extraction
The electrostatic force arising from amino plays an essential role
in the adsorption behavior of amino compound on the charged Au
surface (surface-bound AuCl2, AuCl
4 ions) [49]. In acidic conditions, the terminal amino group will be protonated to take the
positive charge on nitrogen atoms, this positive species can be
extracted by the porous Ag@Au fiber. The influence of pH on the
extraction has been investigated ranged from 3.5 to 10.0 which was
regulated by HCl or NaOH solution. Fig. S11A shows that the band
intensity of NFZ remains relatively stable as the pH increase from
5.0 to 7.0. But when the pH value is between 7.0 and 9.0, the band
intensity decreases gradually. In particular, the deprotonation of
NFZ occurs at the pH value exceed 10.0, which lowers the SERS
response to the minimum. Similarly, the band intensity of SCA
decreases rapidly following the pH value increases from 5.0 to 10.0
(Fig. S11B). These processes can be attributed to the deprotonation
of amino which will affect the affinity of the positive amino compound to the substrate, when the pH value below their pKa (the
pKa of NFZ [50] and SCA [51] was 10.0 and 3.6, respectively) in
solution. Consequently, the pH 7.0 10 mM PBS buffer solution was
selected as working solution for the extraction of the two compounds simultaneously.
The equilibrium time of extraction was also evaluated, the kinetic curve of different concentration of NFZ and SCA were determined. Fig. S12 shows that the lower the concentration of the
sample, the longer the time require reaching equilibrium. Whatever the concentration is, the systems can reach equilibration as the
extraction time is more than 120 min. Hence, the extraction time of
120 min was selected to ensure the performance of porous Ag@Au
fibers.
3.4. Quantitative detection
Under the optimization, the extraction was operated in the solution of NFZ (1.0 mmol L1-0.1 nmol L1) and SCA (1.0 mmol L11.0 nmol L1), then the SERS spectra were recorded respectively.
The SERS spectra and absorption curves were shown in Fig. S13. The
intensity of characteristic bands increases as the concentration and
reaches its maximum due to the saturation adsorption on active
hotspots. Meanwhile, the Raman intensity of NFZ at 1350 cm1
exhibits a good linear relationship with the concentration from
0.1 mmol L1 to 5.0 nmol L1, the limit of detection (LOD) is calculated as 2.7 nmol L1 (5.1 ppb). The linear relationship of SCA at
1387 cm1 is ranging from 0.1 mmol L1 to 7.0 nmol L1, the LOD is
6.4 nmol L1 (7.3 ppb). This method was validated in seafood
sample which was spiked to give the final concentration of
0.05 mmol C for both of these compounds. The recovery of samples
with this method was listed in Table S1. The comparison with
existing methods was also listed in the Table S2.
Fig. 6A shows the capacity of porous Ag @Au fiber to identify the
mixtures, the bands at 805 cm1, 1240 cm1, 1347 cm1 and
1600 cm1 (blue square) are assigned to the characteristic peak of
NFZ, the band at 1384 cm1 (green star) is assigned to the
Fig. 6. (A) SERS spectra on porous Ag@Au fiber extracted in the mixture containing
0.05 mmol L1 NFZ and SCA. The spectrum of the mixture is shown as violet color, the
single NFZ and SCA is blue and green color for identification. (B) PCA plots for the
characteristic peaks for the mixture. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
characteristic peak of SCA. These results reveal that the compounds
can be extracted and detected on this fiber simultaneously. PCA
process also demonstrates that the component of compounds can
be accurately identified by reducing the dimension of SERS characteristic bands (Fig. 6B).
4. Conclusions
In conclusion, ultrathin-layer gold coating porous silver was
successfully prepared as the reliable substrate for SPME-SERS hyphenated method. The porous Ag@Au substrate has good stability
under 200 C heating and long lifetime for 10 days in the atmosphere. NFZ and SCA were extracted in aqueous solution, the SERS
performance was investigated, and the high SERS-active substrate
provided the enhancement factors of 1.3 106 and 3.5 106 for
them, respectively. After that, the extraction conditions were subsequently optimized, the good stability and sensitivity ensure the
quantitative detection by SPME-SERS methods. The low LOD of
5.1 ppb and 7.3 ppb was obtained for NFZ and SCA, respectively.
Finally, the reliable SMPE fiber was applied in the mixture sample
with satisfied results.
W. Bian et al. / Analytica Chimica Acta 994 (2017) 56e64
Acknowledgements
We are grateful for financial support from National Basic
Research Program of China (973 Program 2013CB934301) and National Natural Science Foundation of China (NSFC21377068,
21575077). Weiwei Bian acknowledges financial support from
Shandong Provincial Natural Science Foundation (ZR2015BL020),
National Natural Science Foundation of China (21705120). Le Wang
acknowledges support from the general administration of Quality
Supervision, Inspection and Quarantine of the PRC (2015IK212,
SK201616), the National Major Research Program of China
(2016YFF0203704).
[20]
[21]
[22]
[23]
[24]
[25]
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.aca.2017.09.004.
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