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Phospholipid detection by surface-enhanced Raman scattering using silvered porous silicon substrates: Phospholipid detection by surface-enhanced Raman scattering

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Silver particles, predominantly with the size of 40–80 nm, were immersion deposited onto porous silicon to form substrates appropriate for the detection of organic molecules by surface-enhanced Raman scattering technique. These substrates have been demonstrated for the first time to provide detection of phospholipid molecules represented by dipalmitoylphosphatidylcholine at concentrations as low as 10−12 M when a 532 nm laser wavelength is used and 10−11 M at 633 nm wavelength. Label-free detection is realized at these conditions.
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Phospholipid detection by
surface-enhanced Raman scattering
using silvered porous silicon substrates
Grigory Arzumanyan
1,2
, Nelya Doroshkevich
1
, Kahramon Mamatkulov
1
, Sergej Shashkov
3
, Kseniya Girel
4
,
Hanna Bandarenka*
,4
, and Victor Borisenko
4
1
Joint Institute for Nuclear Research, 6 Joliot-Curie Str., Dubna 141980, Russia
2
Dubna State University, 19 Universitetskaya Str., Dubna 141982, Russia
3
SOL Instruments Ltd., 58-10, Nezalezhnasti Av., Minsk 220005 Belarus
4
Belarusian State University of Informatics and Radioelectronics, 6 Browka Str., Minsk 220013, Belarus
Received 3 December 2016, revised 19 May 2017, accepted 31 May 2017
Published online 14 July 2017
Keywords nanoparticles, phospholipid, porous silicon, silver, surface enhanced Raman scattering
*
Corresponding author: e-mail h.bandarenka@bsuir.by, Phone: þþ375172932360, Fax: þþ375172929628
Silver particles, predominantly with the size of 4080 nm, were
immersion deposited onto porous silicon to form substrates
appropriate for the detection of organic molecules by surface-
enhanced Raman scattering technique. These substrates have
been demonstrated for the rst time to provide detection of
phospholipid molecules represented by dipalmitoylphospha-
tidylcholine at concentrations as low as 10
12
M when a
532 nm laser wavelength is used and 10
11
M at 633 nm
wavelength. Label-free detection is realized at these con-
ditions.
ß2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Phospholipids are ubiquitously pre-
sented in nature forming a bilayer of the cell membranes of
all living tissues. They are responsible for the elastic
properties of the membranes, stabilization of proteins
within the membrane and transportation of lipids and fatty
acids [1, 2]. Detection of their type, concentration, and
ratio in physiological liquids helps to recognize pulmo-
nary, hepatic, sclerotic, and many other diseases [1, 3].
Thus, the sensitivity of the detection technique plays an
important role in early diagnostics. Thin-layer chromatog-
raphy [4, 5] and single molecule uorescence spectroscopy
[6] have already demonstrated to meet this requirement.
However, their practical application is limited by a
complicated sample preparation, the necessity to use
specicmarkersanddifculties in the interpretation of the
obtained results.
Surface-enhanced Raman scattering (SERS) spectros-
copy of molecules of analyzed materials adsorbed on
metallic nanostructures [7, 8] provides an alternative
approach free from the above limitations. High selectivity
and sensitivity of this technique is determined by the optical
resonance observed when the frequency of an external
optical wave coincides with the frequency of the surface
plasmons, being a ngerprint of the covered metallic
nanostructures. The frequency shift reveals the composition
of the covering material.
SERS-active substrates are traditionally composed of
nanostructured noble metals due to their well-resolved
surface plasmon resonance (SPR) in the visible or near infra-
red (IR) ranges. Gold and silver nanoparticles were already
reported [911] to be suitable for the detection of
phospholipids. So far, the SERS-active layer of gold
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nanoparticles was used to study the Langmuir Blodgett
phospholipidic lms [9]. Gold nanoparticles are very
attractive as they have the SPR band in the near IR range
providing the benets of the transparency window for
biological tissues and a non-destructive effect on the organic
molecules. Meanwhile, silver nanoparticles have the
strongest plasmonic properties and can offer higher
sensitivity of detection than the gold ones. Moreover, the
SERS-active substrates with silver nanoparticles are
cheaper in comparison with those made of gold, which
makes them economically reasonable for a wide use as
expendable materials in the clinical practice. A limiting
factor is that SPR at silver nanoparticles occurs in the blue
band close to the ultra-violet (UV) range, in which many
organic molecules photodegrade. In particular, fatty acids,
which are the constituents of phospholipids, have been
reported to be destructed owing to the silver induced photo-
oxidation [10]. Thus, seeking ways to avoid such destructive
effect is important in the SERS-based phospholipid
detection. Recently, the dye-labeled phospholipids immo-
bilized in Langmuir Blodgett monolayer of the arachidic
acid have been detected on the silver SERS active substrate
due to the enhanced signal from the dye label [11].
However, in the routine work the SERS spectroscopy of
phospholipids should be realized avoiding prior ordering
and labeling of the molecules adsorbed on the SERS-active
substrates. More suitable approach has to be employed to
shift the SPR band of silver coated substrates to long
wavelengths of the visible range.
Recently, we have found out that silver nanoparticles
deposited onto porous silicon allow for varying the SPR
band from blue to red wavelengths depending on the
fabrication regimes [12]. In this paper, the silvered porous
silicon is demonstrated to perform as a very sensitive SERS-
active substrate to detect phospholipids under green or red
light. Dipalmitoylphosphatidylcholine (DPPC) lipid mole-
cules, which are widely used as a model for phospholipids in
bilayer membranes [1315], were involved in the SERS
measurements.
2 Experimental
2.1 Fabrication of SERS-active substrates Porous
silicon layers were fabricated onto the surface of n
þ
-type
(100) silicon wafers of 100 mm in diameter by their
electrochemical anodic etching. Just before the anodization
the wafers were cleaned from chemical contaminants and
SiO
2
according to the procedure presented elsewhere [16].
The anodization was performed in the mechanically stirred
solution containing HF (45%), 1 propanol, H
2
O at their
volume ratio of 1:3:1. The anodizing current density was
xed to be 100 mA cm
2
for 85 s. The formed porous silicon
layer had the thickness of 5 mm and the porosity of 75%.
Silver nanoparticles were immersion deposited onto
porous silicon from the water solution of AgNO
3
and
ethanol following the slightly modied procedure described
in our previous papers [12, 17]. The front face of the wafer
was illuminated by an UV lamp (8 W). The immersion time
was chosen to be 5 and 25 min. After the silver deposition,
the wafers were mechanically divided into rectangular
samples with the surface area of about 0.75 cm
2
which were
then used as SERS-active substrates. Before the use, each
substrate was placed in a plastic zipper bag and kept in a
refrigerator at 5 8C. The air was sucked out of the bag before
zipping.
Rhodamine 6G (R6G) and DPPC were selected as
analytes for the SERS measurements. Distilled water and
2-propanol were used as solvents for R6G and DPPC,
respectively. A gradual decrease of their concentration in
the analyte solutions was achieved by a successive dilution
of the 10
2
M stock solutions. The analyte solutions were
thoroughly mixed in a CM-70M-09 centrifuge-mixer for
30 min. The samples for the SERS measurements were
prepared by an incubation of the silvered porous silicon in
the R6G solution for 2 h or by pipetting 1 mL drop of the
DPPC solution on the silvered porous silicon. The deposited
analyte solution was then dried for 1 h at room temperature.
To record the reference Raman spectra of the analytes, the
R6G and DPPC stock solutions were drop deposited on a
soda-lime glass and virgin silicon substrates, respectively.
2.2 Characterization of the substrates Surface
morphology of the substrates was analyzed by the scanning
electron microscopy (SEM, Hitachi S-4800, Japan) with
1 nm resolution. The sizes of the silver particles on porous
silicon were evaluated using their Ferets diameters from the
top view SEM images. The size of the analyzed SEM image
was 12.8 9.6 mm
2
.
The phase composition of the samples was determined
by X-ray diffractometry (XRD) using Cu K
a
radiation
(l¼0.15406 nm).
Reectance spectra of the silvered porous silicon were
recorded at room temperature with the MC122 spectropho-
tometer (Proscan Special Instruments, Belarus) in the range
of 2001000 nm.
2.3 Raman and SERS measurements A confocal
microspectroscopy set-up was used for Raman and SERS
measurements. It comprises a Confotec CARS scanning
laser spectrometer coupled to the NIKON TE2000-E
inverted microscope. The wavelengths of 633 and 532 nm
were provided by a He-Ne laser (Melles Griot 05-LHP-991)
and a diode laser with an adjustable output power (model
SLM-417-20), respectively. The laser power at the sample
was controlled by a variable neutral lter with the 03
optical density. For the Raman measurements, laser power
of 6 mW at the wavelength of 532 nm and 4.1 mW at the
wavelength of 633 nm was used. The SERS spectra were
recorded with the laser powers of 15 and 60 mW at the
wavelengths of 532 and 633 nm, respectively. The samples
were located at the motorized sample position adjustment
stage (Prior Scientic, H117TE). The laser light was
focused on the sample with a 40 objective (NA 0.6) in
the 1mm spot or a 100 (NA 0.95) objective in the
650 nm spot. All the Raman and SERS spectra were
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collected in the backscattering geometry and dispersed by
600 grooves/mm diffraction grating mounted in the MS520
monochromator-spectrograph. A Peltier-cooled CCD cam-
era (ProScan HS 101 H) was used for detection of the
spectra collected at different localizations of the analytes
with an integration time of 10 s for the spontaneous Raman
measurements and 1 s for recording SERS signals at
different analyte concentrations. We probed signals in the
7001700 cm
1
ngerprint range of vibrational frequencies
with the resolution of 1.8 cm
1
. Rayleigh scattering was
blocked using Semrock long-pass edge lters. All measure-
ments were performed at room temperature.
Detection limit is the smallest amount of analyte
concentration in the sample that can be reliably distin-
guished from zero [18]. Following this rule, the lowest
concentration of the analytes at which they are detectible
was estimated when their less intensive related peak
becomes unresolved at the noise level.
3 Results and discussion
3.1 Morphology of the silvered porous
silicon Figure 1 shows that the surfaces of the SERS
substrates fabricated by the silver immersion deposition on
porous silicon are uniformly covered with silver particles
enlarging according to an increase of the deposition time.
Immersion deposition of silver on porous silicon for
fabrication of SERS substrates is widely reported elsewhere
[12, 17, 19]. Here we optimized the immersion process by
additional UV excitation, which provided more conformal
distribution of the silver deposit on the external porous
silicon surface in contrast to a case of the deposition in the
absence of UV. Moreover, silver is localized exactly on
porous silicon without deposition on monocrystalline
silicon surrounding the porous silicon spot. We suppose
that silver ions from the solution are collected mostly on the
porous material due to intensive generation of electrons
promoted by UV excitation.
The packing density of the silver particles is estimated to
be 60% after 5 min immersion and 75% after 25 min
immersion.
The histograms presented in Fig. 2 show variation of the
particle size in the range from nano- to micrometers for
both deposition times. The range of each column in the
histograms is 40 nm. The particles formed for 5 min (Fig. 2a)
are characterized by the dominant Ferets diameters in the
range from a few nanometers to 40 nm, while those formed
for 25 min (Fig. 2b) mostly have twice-larger Ferets
diameters (4080 nm). Bimodal size distribution of silver
particles is observed in Fig. 2b. The rst size range is for
particles that are less than 120 nm, while the second one is
for larger particles. This is explained by the peculiarities of
Ag growth mechanism which takes place at the immersion
deposition and has been described in details before [12].
The content of the particles with the dominant Ferets
diameter does not range signicantly among the samples
examined, from about 13% for the 5 min immersed sample
to 9% for the 25 min immersed sample.
The XRD analysis performed showed the silver cover of
porous silicon is polycrystalline with the prevalent (111)
orientation of the grains (Fig. 3).
Estimating the morphology of silver deposited onto
porous silicon, one can conclude that the silver particles
formed are suitable to demonstrate surface plasmon effects.
The sample fabricated by 25 min immersion has higher
packing density of silver particles than the sample fabricated
Figure 1 Top view SEM images of the samples formed by the
immersion deposition of silver on porous silicon for (a) 5 and (b)
25 min.
Figure 2 Histograms of the Ferets diameter distribution of the
silver particles for the samples formed by the immersion deposition
of silver on porous silicon; total particle count was (a) 533 and (b)
348.
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for 5 min. This leads to minimization of gaps between the
particles, which should provide more intensive SERS signal
due to an overlap of electromagnetic elds of the nearest
particles under a laser light bringing about an abundance of
vacancies for so-called hot spots[20, 21].
3.2 Reectance of the silvered porous
silicon Reectance spectra of the silvered porous silicon
shown in Fig. 4 were recorded and analyzed in terms of SPR.
There is a well-resolved difference in the spectra for 5
and 25 min immersed samples. The rst one has a minimum
at 397 nm, while the second at 475 nm. The SPR minimum
from the 25 min immersed sample is shifted to longer
wavelengths in respect to the one belonging to the 5 min
immersed sample. It relates to the already mentioned
increased size of silver particles in that sample. Moreover,
its SPR band is two times broader than that of the 5 min
immersed sample. This effect takes place due to the greater
size deviation of the silver particles where multipolar SPRs
can be realized [22]. In addition, the spectrum from 5 min
immersed sample has a ngerprint of interference and an
intensity decrease of the reected light in the near-IR range,
which can be explained by a contribution of Si nanocrystals
in porous silicon [23].
3.3 Raman and SERS measurements It was
mentioned in the introduction that limiting factor of
SERS-spectroscopy of phospholipids with silver-based
substrates is in SPR of silver nanoparticles which is close
to UV range. This often leads to the photodegradation of
many organic molecules. Thus to study DPPC we used
silvered porous silicon formed for 25 min its due to the
attributed red shift of SPR.
In order to estimate their SERS activity, we rst tested it
with the extensively studied R6G which concentration in the
solution was 10
6
and 10
15
M. The laser wavelength was
532 nm. The lowest R6G concentration used was chosen as
it corresponds to the best reported detection limit of R6G
obtained with wavelength of 514.5 nm [19].
Figure 5 presents SERS spectra of R6G resolving
typical for this analyte peaks. Eight Raman signals are
observed at 613 cm
1
(CCC ring ip bend), 772 cm
1
(out
of plane bend), 1185 cm
1
(CH ip bend), 1311 cm
1
(C
OC stretch), 1363, 1510, 1575, and 1650 cm
1
(aromatic
CC stretch), which are consistent with already published
data [24, 25].
The strong scattering intensities of the R6G peaks allow
assuming that this analyte can be identied at lower
concentrations with the developed SERS-active substrates.
Reference Raman and SERS spectra corresponding to
DPPC molecules are presented in Figs. 6 and 7. The Raman
spectrum was recorded for the solution containing 10
2
M
of DPPC deposited onto SERS-inactive substrate. The
observed peaks at 718 cm
1
(choline CN stretch),
Figure 3 XRD patterns of the samples formed by the immersion
deposition of silver on porous silicon for (a) 5 and (b) 25 min.
Figure 4 Reectance spectra of the samples formed by the
immersion deposition of silver on porous silicon for (a) 5 and (b)
25 min.
Figure 5 SERS spectra of (a) 10
6
M and (b) 10
15
M R6G
collected at the 532 nm wavelength.
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769 cm
1
(OPO sym. stretch), 869 cm
1
(trans CC
stretch), 957 cm
1
(PO sym. stretch), 1065 cm
1
(acyl
chain trans CC stretch), 1093 cm
1
(acyl chain gauche
CC stretch), 1127 cm
1
(acyl chain trans CC stretch),
1297 cm
1
(acyl chain CH2 twist), and 1449 cm
1
(CH
2
bend) agree well with those earlier reported for this
compound [1315].
Three upper curves in Fig. 6 represent SERS signals
from the dried solutions containing DPPC in concentrations
between 10
6
and 10
12
M. The laser wavelength was
532 nm. The positions of the SERS peaks are evident to
correlate with the reference Raman peaks, while some small
deviations are observed. The other feature of the SERS
spectra at low concentrations is their weak reproducibility in
the 15271570 cm
1
wavenumber range associated with
COOC stretching. The lowest concentration at which of
DPPC was detected is estimated to be 10
12
M.
A similar result illustrated in Fig. 7 was obtained at the
wavelength of 633 nm. Even better peak-to-peak amplitude
correlation was observed. Meanwhile, the detection limit is
increased to 10
11
M. The higher detection limit for the red
laser in contrast to the green one can be caused by the
contribution of DPPC absorbance in the range from 350 to
550 nm [26].
The intensity of DPPC band at 1297 cm
1
was used
(Fig. 8) to nd calibration curves that visualize the linear
dependence of the SERS intensity on the analyte
concentration.
In general, the observed SERS activity under the red
light atypical for the silver nanostructures is supposed to
take a place through an effect of different modes of plasmon
oscillations in the silver particles on porous silicon. All the
above-mentioned peculiarities are intrinsic to the nature of
the SERS effect and could be ascribed to the different
molecular orientations and locations (distances) of the probe
molecules with respect to the hot spots. This mainly
concerns the concentrations lower than 10
6
M, when the
spectra would be reproducible only if the molecules are
oriented in a particular direction at all hot spots. Note that
variability of SERS spectra from one probing point to
another at the concentrations lower than 10
9
M compli-
cates their analysis.
Therefore, in this study, we restricted the detection limit
to the picomolar concentration of the DPPC molecule
solution dried onto our SERS substrates, although we
observed Raman signals at even lower concentrations.
We tested spot-to-spot and sample-to-sample reproduc-
ibility of the SERS signal from 10
6
M R6G on the silvered
porous silicon according to the rules reported in Ref. [27]. It
was found that deviation of the signal does not extend
beyond 5%.
Figure 6 (a, b, c) SERS and (d) Raman spectra of (a) 10
6
M, (b)
10
9
M, (c) 10
12
M, and (d) 10
2
M DPPC collected at the at the
532 nm wavelength.
Figure 7 (a, b, c) SERS and (d) Raman spectra of (a) 10
-6
M, (b)
10
9
M, (c) 10
12
M, and (d) 10
2
M DPPC collected at the
633 nm wavelength.
Figure 8 The calibration curves of normalized SERS intensity at
1297 cm
1
vs. DPPC concentration. The SERS measurements
were performed at (a) 532 nm and (b) 633 nm laser wavelengths.
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It should be also noted, that the fabrication of SERS
substrates based on porous silicon covered with Ag
nanoparticles by immersion deposition was reported in
Ref. [12]. However, the deposition process took several
times longer to reach SERS activity of the samples than in
the present work. The immersion time decreases due to the
acceleration of the silver deposition under the UV
illumination. We hypothesize that photon absorbance by
silicon leading to generation of electronhole pairs causes
more intensive reduction of Ag ions by electrons on the
surface of the silicon skeleton (especially in the vicinity of
its external surface), while electrons of impurity centres (in
highly doped n-type silicon) capture holes. The generation
of the electronphoton pairs predominantly occurs in the
area of porous silicon because of its extremely developed
surface full of localized states and conned defects [28].
Thus, silver is deposited exactly in the porous silicon spot on
the experimental sample in contrast to the lightless case, as
was described above.
4 Conclusions The developed silvered porous silicon
structures have been proved to be very efcient SERS-active
substrates providing detection of organic molecules in
solutions at their concentration from 10
6
to 10
15
M. Their
advantage is possibilities to apply the wavelengths of the
probing light both of green and red optical ranges using the
single substrate. Phospholipid detection at their concen-
trations in a solution above 10
12
M has been experimen-
tally demonstrated for the rst time with an example of
dipalmitoylphosphatidylcholine. We believe that the detec-
tion limit demonstrated in this paper for silvered porous
silicon SERS substrates can be further improved by their
technological optimization. The developed substrates may
be considered as a cost-effective and large-area platform for
the biosensors based on the SERS measurements.
Acknowledgements The authors would like to thank the
R&D center Belmicrosystems,JSC INTEGRAL for the SEM
analysis and Masters student Tatsiana Philippova for the support
with the UV illumination during silver deposition. This work has
been supported in parts by the Belarusian Republican Foundation
for Fundamental Research (Grant T16-099) and the Belarusian
State Program of Scientic Research Photonics opto- and
microelectronics(task 1.4.01).
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... Arzumanyan et al. have reported 40−80 nm AgNPs deposition on the electrochemically etched pSi with 5 μm long structure and 75% porosity. 308 Higher than the 10 −12 M concentration of phospholipid were detected in this article with the support of dipalmitoylphosphatidyl choline. The pSi prepared with highly resistive p-type Si of 0.005 Ω cm with applied DC current density of 16 mA/cm 2 for 45 s results in a 400 nm thick meso-pSi layer. ...
... Therefore, the Ag-DNFs grown for various synthesis times should present differences in light reflection and absorption. Also, a deeply etched Si nanopillar array (Arzumanyan et al., 2017;Fan et al., 2021;Cheng et al., 2021;Uddin et al., 2021;Omar et al., 2021), as shown in the side-view SEM images in Fig. 1, should result in high light absorption. Therefore, the combination of an Ag-DNF layer and an Si nanopillar array layer results in an oscillating reaction-time dependence in the reflected light spectra. ...
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The effects of synthesis time on the plasmonic properties of Ag dendritic nanoforests on Si substrate (Ag-DNF/Si) samples synthesized through the fluoride-assisted galvanic replacement reaction were investigated. The Ag-DNF/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction and surface-enhanced Raman spectroscopy (SERS). The prolonged reaction time led to the growth of an Ag-DNF layer and etched Si hole array. SEM images and variations in the fractal dimension index indicated that complex-structure, feather-like leaves became coral-like branches between 30 and 60 min of synthesis. The morphological variation during the growth of the Ag DNFs resulted in different optical responses to light illumination, especially those of light harvest and energy transformation. The sample achieved the most desirable light-to-heat conversion efficiency and SERS response with a 30 min growth time. A longer synthesis time or thicker Ag-DNF layer on the Si substrate did not have superior plasmonic properties.
... The Ag-coated por-Si was selected as the SERS substrate because it has been reported to provide ultrahigh sensitivity (up to a single molecule detection limit) [40][41][42][43] combined with an extremely long shelf life (up to 3 years) due to por-Si surface features improving chemical stability of AgPs. [43,44] In mercantile terms, the fabrication steps of the SERS substrates based on the por-Si are costeffective and completely compatible with conventional Si technology. ...
Article
Surface‐enhanced Raman scattering (SERS) by biochemically relevant organic reporter molecules, spread out over a promising localized surface plasmon resonance (LSPR) structure of randomly arranged silver nanoscale particles (AgPs) of various dimensions and shape, was induced using tightly focused continuous wave (CW) 785‐nm laser radiation, and the spectra were registered simultaneously in the anti‐Stokes and Stokes spectral ranges. The spectra were recorded as a function of the excitation laser power density in the range of their profile reproducibility. The power density dependences of the line strength ratio for three respective pairs of vibrational lines of a thiolate of 2‐nitrobenzoic acid (TNB) in SERS spectra were derived. Using these data, we specify and quantify the contributions responsible for the discrepancy between this ratio and that defined by the thermal equilibrium populations of the upper and lower vibrational levels corresponding to the Raman‐active transitions. These contributions are the following: (i) the spectral profile of an LSPR contour, (ii) local heating of the reporter molecule/AgP conjugates by the 785‐nm radiation, and (iii) optical (Raman) pumping of the upper vibrational levels of the transitions involved. The extraction of the latter contribution enabled us to estimate the cross‐sections of the TNB/AgP conjugates Raman vibrational pumping by the radiation for each of the three vibrational modes. The intensity dependences of the anti‐Stokes‐to‐Stokes line strength ratios for three pairs of vibrational lines in a CW 785‐nm laser excited SERS spectra of thionitrobenzoic acid molecules bound to randomly arranged silver nanoscale particles were derived. This allowed to specify and quantify the contributions from the surface plasmon resonance contour, laser heating, and Raman pumping of the upper vibrational levels to the discrepancy between the measured ratios and those defined by the thermal equilibrium populations at promising SERS‐active substrates.
... The other commercial product (BelSERS substrates [https://science.bsuir.by/en/microelectronics-and-nanotech nology/sers-active-substrates-for-increasing-sensitivity-of-raman-spec troscopy] that is based on the silvered meso-or macro-PS have demonstrated its applicability for detection of peptides [143] and proteins [134], meldonium [171], DNA [148], phospholipids [142], fullerenes [172] and other organic molecules [36,103,105]. ...
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Surface-enhanced Raman scattering (SERS) spectroscopy is one of the most prospective methods combining state-of-the-art nanomaterials and optical techniques for highly sensitive express-analysis and detection of organic and bioorganic objects in liquids and gases. Special programs have been recently started all over the world to bring the SERS-spectroscopy closer to wide implementation in medical diagnostics, forensics, security, monitoring sanitary conditions, etc. Despite outstanding features of SERS-spectroscopy, its effective practical use has been particularly slowed down by moderate reproducibility, non-versatility, and restrictions imposed by commercially available SERS-active substrates to measurement and storage regimes. The present review reports SERS-active substrates constituted by noble metals' nanoparticles (NPs) and porous silicon (PS), which potentially can be a tool to overcome the above-mentioned limitations. The PS template acts as a highly ordered host nanomaterial for the formation of a variety of metallic nanostructures, which morphological and optical properties can be easily tuned for the best performance to meet the customer requirements via managing PS synthesis regimes. An indubitable advantage of PS is the compatibility of its fabrication process with basic microelectronics operations and micro-electromechanical systems (MEMS) that make it possible to integrate SERS-active areas in a silicon chip. In contrast to the previously published reviews in the field, this one covers the most recent results on formation, characterization, and application of PS-based substrates demonstrating prominent SERS-activity that have been achieved for the last decade including modifications with graphene or Bragg structures, detection of molecules at amount down to attomolar concentration, bacteria recognition, etc.
... These materials are finding their application both independently and often as the 3D matrix media for complex multiphase compositions on their basis. The main areas of application of hierarchical porous materials and compositions based on them are power engineering (most often porous materials are used as electrodes of Li-ion batteries and fuel cells) [3,4,[7][8][9][10], sensorics and biosensorics [11][12][13][14][15][16][17][18], catalysis [19][20][21], biotechnology, targeted drug delivery, and theranostics [20][21][22][23][24][25]. Another very promising area of application of such materials is bioelectronics, the element base for flexible wearable electronics [26][27][28][29][30][31]. ...
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The spectra of dielectric relaxation of macroporous silicon with a mesoporous skin layer in the frequency range 1–106 Hz during cooling (up to 293–173 K) and heating (293–333 K) are presented. Macroporous silicon (pore diameter ≈ 2.2–2.7 μm) with a meso-macroporous skin layer was obtained by the method of electrochemical anodic dissolution of monocrystalline silicon in a Unno-Imai cell. A mesoporous skin layer with a thickness of about 100–200 nm in the form of cone-shaped nanostructures with pore diameters near 13–25 nm and sizes of skeletal part about 35–40 nm by ion-electron microscopy was observed. The temperature dependence of the relaxation of the most probable relaxation time is characterized by two linear sections with different slope values; the change in the slope character is observed at T ≈ 250 K. The features of the distribution of relaxation times in meso-macroporous silicon at temperatures of 223, 273, and 293 K are revealed. The Havriliak-Negami approach was used for approximation of the relaxation curves ε″ = f(ν). The existence of a symmetric distribution of relaxers for all temperatures was found (Cole-Cole model). A discussion of results is provided, taking into account the structure of the studied object.
... Another disadvantage of traditional substrates for SERS is the impossibility of registering the full spectrum of high molecular weight compounds due to the fact that not all bonds of these molecules fall into the places of localization of the plasmon excitation field. Large molecules (for example, protein globules) can be oriented in such a way that the bonds under study are not sufficiently close to the surface of the metal nanostructure, and the band corresponding to their vibrations cannot appear in the spectrum [20]. The above problems can be overcome using the so-called "plasmonic nanocavities or antinanoparticles" [21]. ...
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Using laterally ordered arrays of noble metal nanocavities as SERS substrates has been examined theoretically and experimentally. Simulation of the distribution of the electric field at the surface of nanostructures (nanocavities) has been carried out. The simulation results showed that cavities can be formed not only in a metal layer but in semiconductor or dielectric layers and then covered with a layer of a plasmon-supporting metal (silver or gold) 20…100-nm thick. In our work, chalcogenide glass has been used as a relief-forming layer. This paper presents the results of development and optimization of processes providing formation of SERS substrates as two-dimensional arrays of noble metal nanocavities by using interference photolithography based on a two-layer chalcogenide photoresist. Prototypes of SERS substrates were made as substrates with different spatial frequencies (from 1200 to 800 mm -1 ) and depths of nanocavities (from 250 up to 500 nm). It was shown that the use of such nanocavities with the sizes larger than 500 nm enables to efficiently analyze the structure of macromolecules by using surface- enhanced Raman light scattering spectroscopy, since these macromolecules completely overlap with the regions of enhanced electric field inside the nanocavities. Technology of interference lithography based on two-layer chalcogenide photoresists makes it possible to form effective SERS substrates in the form of laterally ordered arrays of nanocavities with specified morphological characteristics (spatial frequency, nanocavity sizes, composition and thickness of a conformal metal coating) for detecting high-molecular compounds.
... Ag density increased with synthesis time and reached 1.033 mg/cm 2 at 200 min. The XRD measurements in Figure 4c presented the XRD pattern of the Ag-DNFs/Si substrate, which was consistent with the standard data for crystal planes of cubic Ag (JCPDS-ICDD-04-0783) [34]. As shown in Figure 4d, the EDS results showed only signals of Ag as synthesis time approached 200 min. ...
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Silver dendritic nanoforests (Ag-DNFs) on silicon (Ag-DNFs/Si) were synthesized through the fluoride-assisted Galvanic replacement reaction (FAGRR) method. The synthesized Ag-DNFs/Si were characterized by scanning electron microscopy, energy-dispersive X-ray spectrometry, inductively coupled plasma mass spectrometry (ICP-MS), reflection absorbance spectrometry, surface-enhanced Raman scattering spectrometry, and X-ray diffractometry. The Ag+ concentration in ICP-MS measurements indicated 1.033 mg/cm2 of deposited Ag synthesized for 200 min on Si substrate. The optical absorbance spectra indicated the induced surface plasmon resonance of Ag DNFs increased with the thickness of the Ag DNFs layer. Surface-enhanced Raman scattering measurement and a light-to-heat energy conversion test presented the superior plasmonic response of Ag-DNFs/Si for advanced applications. The Ag-DNFs/Si substrate exhibited high antibacterial activity against Escherichia coli and Staphylococcus aureus. The large surface area of the dense crystal Ag DNFs layer resulted in high antibacterial efficiency. The plasmonic response in the metal–crystal Ag DNFs under external light illumination can supply energy to enhance bacterial inhibition. High-efficiency plasmonic heating by the dense Ag DNFs can lead to localized bacterial inhibition. Thus, the Ag-DNFs/Si substrate has excellent potential for antibacterial applications.
... At present, many types of metal nanostructures have been reported as SERs substrates in applications of biological, chemical and other kinds of molecular detections [10], for example, a rough metal surface [11], multi-metal nanostructures [12], porous nanometer films [13], rose petals films [9,14], Silicon silver grating substrates [15], and so on. Many prepared technologies have been also applied for processing metal SERs substrates, such as electron beam lithography (EBL) [16], optical lithography [17], ion beam engraving [18] and nano-imprinting lithography [19], etc. Although the above technologies and products have a well performance in the monitoring analytes, most of them have disadvantages of expensive costs and complex operation procedures. ...
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We studied a Surface-enhanced Raman scattering (SERs) film by self-assembled deposition of Ag nanoparticles on the surface of PPy@PEDOT:PSS films to detect melamine molecules. The PPy@PEDOT:PSS film was prepared using an electrochemical polymerization method in a mixture solution of PPy and PEDOT:PSS. Then, Ag nanoparticles were directly deposited on the surface of PPy@PEDOT:PSS films by a chemical self-assembly method as an active SERs substrate to detect melamine molecules. The results showed that the density of Ag nanoparticles on the surface of PPy@PEDOT:PSS film was gradually increased due to the increase of depositional time of Ag nanoparticles. Accordingly, The SERs effect was also substantially enhanced. The signal of melamine was increased up to 440 times. The melamine molecules at 6.4 ng/ml concentration could be clearly identified. This preparing method of SERs film and detecting method of melamine molecules provide a new strategy for the design and application of chemical sensors and bio-sensors.
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The aim of this work was to determine the sensitivity threshold and enhancement factor of a planar SERS substrate based on an array of composite Ag - Cu nanoparticles. The nanoparticles were deposited using the method of vacuum-thermal evaporation followed by annealing at 300 ° C. Methylene blue was chosen as the analyte. The possibility of detecting the micro- and nanomolar concentration of methylene blue using a SERS-substrate with an active Ag-Cu layer at a laser wavelength of 632.8 nm is shown. The use of such arrays of nanoparticles as an active layer makes it possible to achieve an analytical enhancement factor of the SERS substrate of the order of 6×10 ⁵ .
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Discovery of surface‐enhanced Raman scattering (SERS) followed by evolution of optical systems and nanoengineering approaches has paved a path to detection of essential organic molecules on solid SERS‐active substrates from solutions at concentrations attributed to single‐molecule ones, i.e. below 10 ‐15 M. However, in practical terms confident SERS‐imaging of single molecules is still quite a challenge. In present work, we fabricated and comprehensively characterized tightly‐packed 3D silver dendrites with prevalent chevron morphology that demonstrated ultrahigh sensitivity as SERS‐active substrates resulted in 10 ‐18 M detection limit. Using these substrates we achieved SERS‐imaging of single 5‐thio‐2‐nitrobenzoic acid (TNB) molecule released from the attomolar‐concentrated solution of of 5,5'‐dithio‐bis‐[2‐nitrobenzoic acid] (DTNB), which is vital compound for chemical and biomedical analysis. In contrast to generally accepted belief about adsorption of only uniform monomolecular TNB layer on surface of silver nanostructures, we showed formation of a coating constituted by TNB layer and DTNB nanoclusters on the dendrites’ surface at 10 ‑6 –10 ‑12 M DTNB concentrations confirmed by presence/absence of disulfide bonds signature in the SERS‐spectra and by scanning electron microscopy. DTNB concentrations below 10 ‐14 M resulted in adsorption of TNB molecules in separated spots on the surface of silver nanostructures.
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Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.
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Microporous and macro-mesoporous silicon templates for surface- enhanced Raman scatter (SERS) substrates were produced by anodization of low doped p-type silicon wafers. By immersion plating in AgNO3, the templates were covered with silver metallic film consisting of different silver nanostructures. Scanning electron microscopy (SEM) micrographs of these SERS substrates showed diverse morphology with significant difference in an average size and size distribution of silver nanoparticles. Ultraviolet-visible-near-infrared (UV-Vis-NIR) reflection spectroscopy showed plasmonic absorption at 398 and 469 nm, which is in accordance with the SEM findings. The activity of the SERS substrates was tested using rhodamine 6G (R6G) dye molecules and 514.5 nm laser excitation. Contrary to the microporous silicon template, the SERS substrate prepared from macro-mesoporous silicon template showed significantly broader size distribution of irregular silver nanoparticles as well as localized surface plasmon resonance closer to excitation laser wavelength. Such silver morphology has high SERS sensitivity that enables ultralow concentration detection of R6G dye molecules up to 10(-15) M. To our knowledge, this is the lowest concentration detected of R6G dye molecules on porous silicon-based SERS substrates, which might even indicate possible single molecule detection.
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The quality of an analytical method developed is always appraised in terms of suitability for its intended purpose, recovery, requirement for standardization, sensitivity, analyte stability, ease of analysis, skill subset required, time and cost in that order. It is highly imperative to establish through a systematic process that the analytical method under question is acceptable for its intended purpose. Limit of detection (LOD) and limit of quantification (LOQ) are two important performance characteristics in method validation. LOD and LOQ are terms used to describe the smallest concentration of an analyte that can be reliably measured by an analytical procedure. There has often been a lack of agreement within the clinical laboratory field as to the terminology best suited to describe this parameter. Likewise, there have been various methods for estimating it. The presented review provides information relating to the calculation of the limit of detection and limit of quantitation. Brief information about differences in various regulatory agencies about these parameters is also presented here.
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The anionic conjugated polyelectrolyte, poly[3-(6-sulfothioatehexyl)thiophene] (), functions as a highly sensitive probe of membrane order, uniquely capable of sequentially detecting the three key phase transitions occurring within model phospholipid bilayers. The observed sensitivity is the result of charge-mediated, selective localisation of within the head-groups of the phospholipid bilayer.
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Surface-Enhanced Raman Scattering (SERS) was discovered in the 1970s and has since grown enormously in breadth, depth, and understanding. One of the major characteristics of SERS is its interdisciplinary nature: it lies at the boundary between physics, chemistry, colloid science, plasmonics, nanotechnology, and biology. By their very nature, it is impossible to find a textbook that will summarize the principles needed for SERS of these rather dissimilar and disconnected topics. Although a basic understanding of these topics is necessary for research projects in SERS with all its many aspects and applications, they are seldom touched upon as a coherent unit during most undergraduate studies in physics or chemistry. This book intends to fill this existing gap in the literature. It provides an overview of the underlying principles of SERS, from the fundamental understanding of the effect to its potential applications. It is aimed primarily at newcomers to the field, graduate student, researcher or scientist, attracted by the many applications of SERS and plasmonics or its basic science. The emphasis is on concepts and background material for SERS, such as Raman spectroscopy, the physics of plasmons, or colloid science, all of them introduced within the context of SERS, and from where the more specialised literature can be followed. * Represents one of very few books fully dedicated to the topic of surface-enhanced Raman spectroscopy (SERS) * Gives a comprehensive summary of the underlying physical concepts around SERS * Provides a detailed analysis of plasmons and plasmonics. "Besides an overview of current promising research topics, this book is a self-contained introduction to Raman spectroscopy and fluorescence that summarises the main concepts and ideas needed for SERS. It is also a self-contained introduction to the physics of plasmon resonances within the broader scope of plasmonics. A detailed presentation of the SERS electromagnetic model and its extension to surface-enhanced fluorescence is included." "Aimed primarily at newcomers to the field, graduate students, and other researchers or scientists attracted by the many possible applications of SERS and plasmonics, or their basic science."--BOOK JACKET.
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A rapid and simple approach to fabricate large-area surface-enhanced Raman scattering-active (SERS-active) substrates is reported. The substrates are fabricated by using femtosecond laser (fs-laser) direct writing on Silicon wafers, followed by thin-film coating of metal such as gold. The substrates are demonstrated to exhibit signal homogeneity and good enhancement ability for SERS. The maximum enhancement factor (EF) up to 3×107 of such SERS substrates for rhodamine 6G (R6G) at 785 nm excitation wavelength was measured. This technique could demonstrate a functional microchip with SERS capability of signal homogeneity, high sensitivity and chemical stability.
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Porous silicon (PS) represents the prospective template for the fabrication of metal/PS nanostructures. However n+-type Sb doped Si Czochralski wafers widely used to fabricate PS may contain resistivity striations and asgrown microdefects with specific spiral distribution which influence Si dissolution during the formation of PS by anodizing. This may strongly affect on the PS microstructure and repeatability of metal/PS properties causing an increase of risk of a device failure. In the present research we studied the morphology and SERS-activity of Ag/PS nanostructures formed by Ag immersion deposition on PS. SEM investigation revealed unequal distribution of pore diameters in the surface regions of PS which corresponds to spiral swirl-like distribution of resistivity revealed in Si wafer by anodizing. Morphological changes of Ag deposit on spiral areas were found to cause decreasing of intensity of SERS signal. Additional implantation of Sb ions followed by high temperature annealing has shown to be a technique to improve homogeneity of Ag/PS SERS substrates.
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Large-scale two-dimensional (2D) arrays of metallic nanostructures formed by thin-film evaporation over hexagonally close-packed polystyrene spheres are established substrates for surface-enhanced Raman spectroscopy (SERS). By using these substrates with an integrated atomic force microscopy (AFM) and inverted Raman spectroscopy system, simultaneous topographical imaging and high-sensitivity chemical mapping can be performed. In this paper, we have used this technique to investigate supported bilayers of long-chain fatty acids and phospholipids deposited by the Langmuir-Blodgett (LB) and spin-coating techniques. Nanosphere lithography (NSL) substrates created from 384 and 1002 nm polystyrene spheres and silver (Ag) deposition on glass and sapphire substrates were characterized for SERS in terms of their structure, distribution, and level of enhancement. SERS mappings of rhodamine 6G (R6G) and p-aminothiophenol (p-ATP) monolayers on the 384 nm substrates demonstrate high and uniform enhancement at a micrometer scale. The enhancement was sufficiently high to enable measurement of SERS spectra for arachidic acid (AA) and dipalmitoylphosphatidylcholine (DPPC) layers on sapphire/Ag substrates. The roughness of these substrates (<2 nm) was lower than for glass/Ag (∼5 nm); therefore, simultaneous to SERS it was possible to measure the topography of the samples by AFM and determine the number of layers of AA and DPPC. This study shows the potential of the combined AFM/SERS technique for spectral and topographical characterization of phospholipid bilayers. This may prove to be an interesting approach for further studies with more complex heterogeneous lipid mixtures aiming to measure spatially resolved features such as microdomains.