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chemosensors
Article
Selective Enhancement of SERS Spectral Bands of Salicylic
Acid Adsorbate on 2D Ti3C2Tx-Based MXene Film
Sonata Adomaviˇci¯
ut˙
e-Grabusov˙
e1,*, Simonas Ramanaviˇcius 2,3, Anton Popov 4, Valdas Šablinskas 1,
Oleksiy Gogotsi 5and Ar¯
unas Ramanaviˇcius 2,*
Citation: Adomaviˇci¯
ut˙
e-Grabusov˙
e,
S.; Ramanaviˇcius, S.; Popov, A.;
Šablinskas, V.; Gogotsi, O.;
Ramanaviˇcius, A. Selective
Enhancement of SERS Spectral Bands
of Salicylic Acid Adsorbate on 2D
Ti3C2Tx-Based MXene Film.
Chemosensors 2021,9, 223. https://
doi.org/10.3390/chemosensors9080223
Academic Editor: Santiago
Sanchez-Cortés
Received: 7 July 2021
Accepted: 9 August 2021
Published: 13 August 2021
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4.0/).
1Institute of Chemical Physics, Vilnius University, Sauletekio Av. 3, LT-10257 Vilnius, Lithuania;
valdas.sablinskas@ff.vu.lt
2Department of Physical Chemistry, Faculty of Chemistry and Geosciences, Institute of Chemistry,
Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania; simonas.ramanavicius@ftmc.lt
3Center for Physical Sciences and Technology, Sauletekio Av. 3, LT-10257 Vilnius, Lithuania
4NanoTechnas—Center of Nanotechnology and Materials Science, Faculty of Chemistry and Geosciences,
Institute of Chemistry, Vilnius University, Naugarduko St. 24, LT-03225 Vilnius, Lithuania;
anton.popov@chgf.vu.lt
5Materials Research Center Ltd., Krzhyzhanovskogo Str. 3, 01001 Kiev, Ukraine; agogotsi@mrc.org.ua
*Correspondence: sonata.adomaviciute@ff.vu.lt (S.A.-G.); Arunas.Ramanavicius@chf.vu.lt (A.R.)
Abstract:
In this research, we have demonstrated that 2D Ti
3
C
2
X
n
-based MXene (MXene) films are
suitable for the design of surface-enhanced Raman spectroscopy (SERS)-based sensors. The enhanced
SERS signal was observed for a salicylic acid molecule on Ti
3
C
2
T
x
-based MXene film. Confirmation
of the adsorption of the salicylic acid molecule and the formation of a salicylic acid–MXene complex
were determined by experimental SERS-based spectral observations such as greatly enhanced out-of-
plane bending modes of salicylic acid at 896 cm
−1
and a band doublet at 681 cm
−1
and 654 cm
−1
.
Additionally, some other spectral features indicate the adsorption of salicylic acid on the MXene
surface, namely, a redshift of vibrational modes and the disappearance of the carboxyl deformation
spectral band at 771 cm
−1
. The determined enhancement factor indicates the value that can be
expected for the chemical enhancement mechanism in SERS of 220 for out-of-plane vibrational modes.
Theoretical modeling based on density functional theory (DFT) calculations using B3LYP/6311G++
functional were performed to assess the formation of the salicylic acid/MXene complex. Based
on the calculations, salicylic acid displays affinity of forming a chemical bond with titanium atom
of Ti
3
C
2
(OH)
2
crystal via oxygen atom in hydroxyl group of salicylic acid. The electron density
redistribution of the salicylic acid–MXene complex leads to a charge transfer effect with 2.2 eV
(428 nm) and 2.9 eV (564 nm) excitations. The experimentally evaluated enhancement factor can vary
from 220 to 60 when different excitation wavelengths are applied.
Keywords:
MXenes; SERS sensor; 2D materials; Ti
2
C
3
; salicylic acid; density functional theory (DFT)
1. Introduction
A recently discovered class of specific two-dimensional (2D) materials—MXenes—
promises a variety of applications with encouraging improvement in plasmonics, conduc-
tivity and catalysis compared to these properties of conventional non-metallic substances
such as graphene. Due to the high concentration of free charges, the conductivity of these
substances is metallic-like and higher than that of graphene [
1
,
2
]. Nevertheless, these
materials are quite new, and the most relevant publications considering the optical and
electric properties of MXenes have appeared just in the last decade.
MXenes were synthesized from MAX (M
n+1
AX
n
, where n= 1, 2 or 3) phase for the
first time in 2011 [
3
]. Here, M represents a III–VI group transition metal, A is typically an
element from IIIA or IVA group (e.g., Al, Si, Cd, Ga, etc.) and X usually depicts carbides,
nitrides or carbonitrides, though over 30 different types of MXenes have been reported so
far and even more are predicted to exist [
4
]. Henceforth, MXenes are generally produced
Chemosensors 2021,9, 223. https://doi.org/10.3390/chemosensors9080223 https://www.mdpi.com/journal/chemosensors
Chemosensors 2021,9, 223 2 of 18
by selectively etching the middle element of the MAX three-dimensional lattice A, thus
obtaining layers of MAX phase that ought to be further separated by various intercalant
agents or sonication yielding 2D MXenes. Recently, research of this material class expanded
rapidly. This is especially obvious for the Ti
3
C
2
T
x
MXene compounds (T
x
here denotes
the terminal functional group, usually -O, -OH or -F) formed from Ti
3
AlC
2
(MAX phase).
These MXenes are studied most extensively since their synthesis is widely established,
and they exhibit higher metallic conductivity in comparison to that of molybdenum and
nitride-based MXenes [
5
–
8
]. Nevertheless, further investigations are directed towards
the efficient synthesis of other MXene types that can yield more defect-less 2D MXene
layers [4,6].
Due to unique morphology and composition, these materials have the potential to
be used in sensor design [
9
,
10
] as catalysts [
11
–
13
]. High conductivity, transparency
and tunable work function enable the application of these materials in the design of
optoelectronic devices, including solar cells. Ti
3
C
2
T
x
MXene has the potential to be used as
an additive in a charge transport layer or as an electrode in perovskite and organic solar
cells [
14
]. In addition, intriguing metal-like properties (e.g., the decrease of conductivity by
the increase of temperature) and high conductivity determined by a high density of charge
carriers were observed for MXenes [
15
–
17
]. The reported concentration of free charge
carriers for Ti
3
C
2
T
x
MXene is 2
×
10
21
cm
−3
[
18
], while in noble metal nanoparticles, it is
approximately 6
×
10
22
cm
−3
[
19
] and for graphene, 3
×
10
13
cm
−2
[
1
,
2
]. The conductivity
of MXenes can be altered by their surface termination groups [
20
], leading to a possible
application as supercapacitors [
21
,
22
], resistive sensors [
23
–
25
] and other applications in
electrochemistry [
26
], electronics [
27
]. As a result of the relatively high concentration of free
electrons, the plasmonic effect of MXenes reclassifies these materials into a class of metals
because the plasmon frequency depends on the density of free electrons [
19
]. For this
reason, MXenes might be used as a substitute for metallic nanostructures with the ability
to enhance internal vibrations of molecules at near proximity as is in surface-enhanced
Raman spectroscopy (SERS) [28–30].
It is widely accepted that two mechanisms can cause the SERS effect: (i) electromag-
netic mechanism occurring due to localized plasmon resonance of noble metal nanostruc-
tures (such as silver, gold, platinum), which enhances the intensity of Raman spectral
bands up to six orders of magnitude and (ii) chemical mechanism, which occurs because of
interacting electronic energy levels between the substrate and the adsorbed molecule that
causes a shift in electronic energy levels of the molecule. The enhancement due to chemical
mechanism is lower and usually does not exceed two orders of magnitude [
31
]. However,
until now, only enhanced resonance Raman spectra of dye molecules (such as rhodamine
6G, crystal violet, methylene blue, malachite green) adsorbed on the MXene surface were
observed, and the enhancement was explained by the chemical mechanism [
28
–
30
]. The
chemical enhancement between the MXene materials and the adsorbed dye molecules
occurs because of the coupling between the dye and MXene energy levels. This coupling
can be considered as a chemical mechanism of SERS [
30
,
32
,
33
]. Nevertheless, the electro-
magnetic mechanism of enhancing the SERS signal due to the free electron oscillations in
MXene layers cannot be neglected as well [34,35].
The application of MXenes as SERS substrates is desirable in such cases when non-
metal SERS substrates with a different chemical affinity towards molecules are needed.
Moreover, the enhancement from 2D thin materials is beneficial in comparison to three-
dimensional surfaces due to the larger specific surface area of 2D materials. Additionally,
the localized plasmon resonance frequency in the metal nanoparticles depends on the
shape and size of these nanoparticles which are determined during the synthesis of the
nanoparticles, whereas for 2D materials such as graphene and MXenes, the surface plasmon
resonance frequency can be influenced by different functional groups and/or controlled
by an external electric field [
20
,
36
]. The possibility to use MXenes as a SERS substrate not
only for sensing molecules with electron-level energy similar to that of MXenes would
provide a wider field of application and, therefore, is highly desirable. SERS-based sensing
Chemosensors 2021,9, 223 3 of 18
is used for the detection of various molecules (or ions), usually at much lower limits of
detection than by conventional spectroscopic methods. The high sensitivity of the method
is directly related to the adsorption of the analyte molecules on the nanoparticles. Thus,
the adsorption of the molecules leads to enhanced intensity of the Raman spectrum.
Salicylic acid can act as a model molecule for studies of SERS enhancement on MXene
film. This molecule consists of a benzene ring and carboxyl group. The Carboxyl group
is involved in the adsorption of salicylic acid on the customary SERS substrates—silver
or gold nanoparticles. Salicylic acid is known to be SERS-active when silver and gold
nanoparticles are used, but this molecule does not exhibit electronic absorption in the
visible spectral range.
In this research, the adsorption of salicylic acid molecules and the formation of salicylic
acid–MXene complex were confirmed by experimental SERS-based spectral observations.
The enhanced SERS signal was observed for salicylic acid molecules on Ti
3
C
2
T
x
-based
MXene film. The goal of this work is to study the enhancement of Raman spectral bands of
salicylic acid when it is adsorbed on MXene film and to elucidate the enhancement features
of MXenes when they are used as SERS substrates.
2. Materials and Methods
2.1. Synthesis of Ti3C2TxMXene Substrates
In this study, MXenes were prepared by etching 0.1 g of Ti
2
AlC
3
MAX phase in 10 mL
of 5 wt.% hydrofluoric acid solution. Solution was stirred for 24 h at 25
◦
C temperature.
After this step, the solution was centrifuged in order to remove residue of hydrofluoric acid.
Centrifugation proceeded until the pH of solution became neutral. Finally, the MXene films
on a microscope glass slide were prepared. For this purpose, the glass slides were covered
with 0.5 mL of aqueous MXene solution (0.01 g/mL) and dried under nitrogen atmosphere.
2.2. Characterization of MXene Film and Its Interaction with Adsorbate
Sample images and elemental analysis were performed by scanning electron micro-
scope Helios Nanolab 650 (FEI, Eindhoven, Netherlands) equipped with an EDX spectrom-
eter X-Max (Oxford Instruments, Abingdon, UK). X-ray diffraction (XRD) analysis was
performed using Ni-filtered Cu K
α
radiation on MiniFlex II diffractometer (Rigaku, Tokyo,
Japan) working in Bragg–Brentano (
θ
/2
θ
) geometry. The diffractograms were recorded
within 2
θ
angle range from 5
◦
to 60
◦
at a step width of 0.02
◦
and speed of 2/min. Resistivity
measurements were performed on a thin-film gold interdigitated electrode ED-IDE3-Au
(Micrux Technologies, Oviedo, Spain).
Characteristic spectroscopic range of interband transition and plasmonic response
of MXenes were determined by ultraviolet, visible and near-infrared range (UV-Vis-NIR)
absorption spectroscopy. UV-Vis-NIR electronic absorption spectra were acquired with
dual-channel Lambda-1050 spectrometer (PerkinElmer, Boston, MA, USA). The spectra were
obtained in the range 350–2300 nm with 5 nm resolution.
Ti
3
C
2
T
x
-based MXene film and its interaction with salicylic acid (SA) were investigated
by means of Raman spectroscopy. MonoVista CRS+ Raman microscope system (S & I GmbH,
Warstein, Germany) equipped with four excitation lasers (457 nm, 532 nm, 633 nm and
785 nm) and a liquid-nitrogen-cooled CCD detector were used for acquiring the spectra.
Diameter of the focused laser beam on the sample was
≈
1
µ
m, and its power density
on the sample was
≈
20 kW/cm
2
(for 633 nm excitation) and
≈
45 kW/cm
2
(for 785 nm
excitation). Before the measurements, the spectrometer was calibrated to a fundamental
vibrational band at 520.7 cm
−1
of silicon wafer. Fourier transform MultiRAM spectrometer
(Bruker, Mannheim, Germany) equipped with liquid-nitrogen-cooled Ge diode detector
was used for the calculations of enhancement factor for salicylic acid–MXene complex with
1064 nm excitation wavelength.
Solution of salicylic acid in water with concentration equal to 2 mM was prepared.
Two drops (5
µ
L each) of the solution were dried on the glass substrate covered by the
MXene film. To ensure the most homogenous distribution of salicylic acid molecules, the
Chemosensors 2021,9, 223 4 of 18
drying was performed in a confined space when the saturated vapor diminishes the surface
tension in air–water interface and more uniform distribution can be achieved. The same
volume of salicylic acid solution was dried on the aluminum foil as a reference. Salicylic
acid solution spread to form a 2 mm spot on the film. After drying out, no crystallization
occurred on the MXene film, while on the reference glass substrate, crystals of salicylic
acid were formed. Presented SERS spectra of salicylic acid were recorded on 20 randomly
chosen positions on the MXene surface and averaged. The standard deviation for the
spectral intensity of salicylic acid on MXene substrate was calculated while applying 3 s
acquisition time.
2.3. Computational Methods
In order to make more detailed analysis of the interaction between salicylic acid
and MXene, theoretical calculations of structure and vibrational spectra of monomeric
salicylic acid and salicylic acid dimer were performed by means of quantum chemistry
calculations implemented in Gaussian 09W software package [
37
]. The salicylic acid dimer
was chosen to resemble the crystalline structure of solid-state salicylic acid. The density
functional theory (DFT) calculations using B3LYP/6311G
++
functional were performed. For
the investigation of the salicylic acid–MXene complex, the 2
×
2
×
1 supercell expansion
of Ti
3
C
2
(OH)
2
MXene crystal structure (of 20 atoms) was built and optimized. Initially,
geometry optimization was performed separately for salicylic acid (B3LYP/6311G
++
) and
Ti
3
C
2
(OH)
2
cluster (B3LYP/LanL2DZ) and for their complex afterward (B3LYP/LanL2DZ).
No virtual frequencies were present after the complex optimization. The excited-state
calculations were performed to assess the redistribution of the electronic energy levels.
3. Results
3.1. Characterization of MXene Films
The structure and morphology of the MAX phase and synthesized MXene-based
materials were evaluated using the scanning electron microscopy (SEM) imaging technique
(Figure 1). The compact and layered morphology, which is typical for ternary carbide [
38
],
was obtained in the case of the MAX phase. Etching with 5 wt.% HF solution allows
preparing Ti
3
C
2
T
x
, for which the structure is quite similar to that of the MAX phase.
However, MXene layers were slightly opened. The ‘accordion-like’ structure was not
observed. A possible explanation may be found in low hydrofluoric acid concentration,
which is insufficient for the formation of a sufficient amount of H
2
evolving during the
exothermic reaction of hydrofluoric acid with aluminum atoms. Successful etching of
aluminum atoms was confirmed by energy-dispersive X-ray spectroscopy (EDX) analysis
(Table 1). It was revealed that the number of aluminum atoms in MXenes decreased by five
times in comparison with the initial amount of these atoms in the MAX phase.
Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 18
Figure 1. SEM images of (A) MAX phase (Ti3AlC2) and (B) MXenes (Ti3C2Tx).
Table 1. EDX analysis results for aluminum (Al) and titanium (Ti) atomic ratio in MAX phase and
MXene samples.
Sample Atomic Ratio, %
Aluminum (Al) Titanium (Ti)
MAX phase (Ti3AlC2) 39.39 ± 0.96 60.61 ± 0.96
MXenes (Ti3C2Tx) 6.33 ± 1.97 93.67 ± 1.97
Comparing resistivity of MAX phase and MXenes etched in 5 wt.% HF, it was meas-
ured that the resistivity of samples at room temperature decreased from 36 Ω to 20.5 Ω. A
decrease in sample resistivity might be explained by a reduced amount of aluminum by
etching and the formation of semi-metallic Ti3C2 MXene structures.
The synthesis of MXenes was confirmed using XRD analysis (Figure 2). A shift of the
(002) peak of Ti3AlC2 at 9.5° to 7.3° for the Ti3C2Tx was observed. Such sufficiently large
shift is typical for wet multilayered MXene samples [39], whereas an intense and sharp
(002) peak is characteristic for MXenes etched by hydrofluoric acid [40]. Such results, to-
gether with the absence of other MAX phase characteristic peaks in MXene spectra, coin-
cide with EDX analysis results and confirm successful etching of Al out of Ti3AlC2.
Figure 2. XRD patterns of MAX phase (Ti3AlC2) and MXene (Ti3C2Tx) powders.
Usually, MXene films are composed of various sizes of Ti3C2Tx flakes (lateral sizes
vary from 0.1 to ~5 µm). In contrast, the thickness of monolayered MXene is supposed to
be about 0.95 nm [41]
10 20 30 40 50 60
(008)
(006)
(004)
(002)
Intensity
2-Theta, de
g
.
MAX phase
Mxene
(002)
Figure 1. SEM images of (A) MAX phase (Ti3AlC2) and (B) MXenes (Ti3C2Tx).
Chemosensors 2021,9, 223 5 of 18
Table 1.
EDX analysis results for aluminum (Al) and titanium (Ti) atomic ratio in MAX phase and
MXene samples.
Sample Atomic Ratio, %
Aluminum (Al) Titanium (Ti)
MAX phase (Ti3AlC2) 39.39 ±0.96 60.61 ±0.96
MXenes (Ti3C2Tx) 6.33 ±1.97 93.67 ±1.97
Comparing resistivity of MAX phase and MXenes etched in 5 wt.% HF, it was mea-
sured that the resistivity of samples at room temperature decreased from 36
Ω
to 20.5
Ω
. A
decrease in sample resistivity might be explained by a reduced amount of aluminum by
etching and the formation of semi-metallic Ti3C2MXene structures.
The synthesis of MXenes was confirmed using XRD analysis (Figure 2). A shift of
the (002) peak of Ti
3
AlC
2
at 9.5
◦
to 7.3
◦
for the Ti
3
C
2
T
x
was observed. Such sufficiently
large shift is typical for wet multilayered MXene samples [
39
], whereas an intense and
sharp (002) peak is characteristic for MXenes etched by hydrofluoric acid [
40
]. Such results,
together with the absence of other MAX phase characteristic peaks in MXene spectra,
coincide with EDX analysis results and confirm successful etching of Al out of Ti3AlC2.
Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 18
Figure 1. SEM images of (A) MAX phase (Ti3AlC2) and (B) MXenes (Ti3C2Tx).
Table 1. EDX analysis results for aluminum (Al) and titanium (Ti) atomic ratio in MAX phase and
MXene samples.
Sample Atomic Ratio, %
Aluminum (Al) Titanium (Ti)
MAX phase (Ti3AlC2) 39.39 ± 0.96 60.61 ± 0.96
MXenes (Ti3C2Tx) 6.33 ± 1.97 93.67 ± 1.97
Comparing resistivity of MAX phase and MXenes etched in 5 wt.% HF, it was meas-
ured that the resistivity of samples at room temperature decreased from 36 Ω to 20.5 Ω. A
decrease in sample resistivity might be explained by a reduced amount of aluminum by
etching and the formation of semi-metallic Ti3C2 MXene structures.
The synthesis of MXenes was confirmed using XRD analysis (Figure 2). A shift of the
(002) peak of Ti3AlC2 at 9.5° to 7.3° for the Ti3C2Tx was observed. Such sufficiently large
shift is typical for wet multilayered MXene samples [39], whereas an intense and sharp
(002) peak is characteristic for MXenes etched by hydrofluoric acid [40]. Such results, to-
gether with the absence of other MAX phase characteristic peaks in MXene spectra, coin-
cide with EDX analysis results and confirm successful etching of Al out of Ti3AlC2.
Figure 2. XRD patterns of MAX phase (Ti3AlC2) and MXene (Ti3C2Tx) powders.
Usually, MXene films are composed of various sizes of Ti3C2Tx flakes (lateral sizes
vary from 0.1 to ~5 µm). In contrast, the thickness of monolayered MXene is supposed to
be about 0.95 nm [41]
10 20 30 40 50 60
(008)
(006)
(004)
(002)
Intensity
2-Theta, de
g
.
MAX phase
Mxene
(002)
Figure 2. XRD patterns of MAX phase (Ti3AlC2) and MXene (Ti3C2Tx) powders.
Usually, MXene films are composed of various sizes of Ti
3
C
2
T
x
flakes (lateral sizes
vary from 0.1 to ~5
µ
m). In contrast, the thickness of monolayered MXene is supposed to
be about 0.95 nm [41]
Due to weak interaction between neighboring individual MXene flakes, the electric
and plasmonic properties of the film do not significantly depend on the thickness of MXene
structures, though the correlation between metallic behavior and the origin of the terminal
chemical groups of MXenes is well expressed [
17
,
21
,
34
,
42
]. MXenes terminated with -F
and -OH feature higher conductivity and plasmonic response to incident radiation in
comparison to the oxidized MXenes.
The plasmonic response and other optical properties of the MXene substrate were
assessed by the mean of UV-Vis-NIR absorption spectroscopy (Figure 3). Relying on
the correlation between the optical density and the thickness of the film [
17
,
21
,
43
], we
estimated the thickness to be approximately 70 nm.
Chemosensors 2021,9, 223 6 of 18
Chemosensors 2021, 9, x FOR PEER REVIEW 6 of 18
Due to weak interaction between neighboring individual MXene flakes, the electric
and plasmonic properties of the film do not significantly depend on the thickness of
MXene structures, though the correlation between metallic behavior and the origin of the
terminal chemical groups of MXenes is well expressed [17,21,34,42]. MXenes terminated
with -F and -OH feature higher conductivity and plasmonic response to incident radiation
in comparison to the oxidized MXenes.
The plasmonic response and other optical properties of the MXene substrate were
assessed by the mean of UV-Vis-NIR absorption spectroscopy (Figure 3). Relying on the
correlation between the optical density and the thickness of the film [17,21,43], we esti-
mated the thickness to be approximately 70 nm.
Figure 3. UV-Vis-NIR absorption spectrum of dried 70 nm thick Ti
3
C
2
T
x
MXene film on the glass
slide.
The UV-Vis-NIR absorption spectrum of 70 nm thick Ti
3
C
2
T
x
-based MXene film con-
tains a broad spectral band (full width at half maximum—195 nm) characteristic for de-
laminated MXenes. The center of this band is located at 750 nm (1.65 eV). The origin of the
spectral band is attributed to the interband transition. The plasmonic nature of this band
is controversial. Some researchers assign bands in this spectral range to transversal plas-
mon resonance [42,44], while other studies suggest that the plasmonic activity of MXenes
occurs as a consequence of the real part of the dielectric function, becoming negative only
in the near-infrared spectral range, implying that the plasma frequency of MXenes is also
in this range and excitation of surface plasmons can be expected at longer wavelengths.
During this research, we adhere to the assignment of this absorption band to the interband
transition because: (i) no plasmonic activities were observed in this spectral range and (ii)
the characteristics of enhanced SERS spectra of salicylic acid indicate a chemical enhance-
ment mechanism. It is important to note that the thickness of the MXene film or the size
and shape of the individual Ti
3
C
2
T
x
flakes have no influence on the interband gap. The
position of the spectral band can only be shifted by changing the concentration of free
charge carriers that can be altered with different terminal groups of Ti
3
C
2
T
x
MXene lattice
[17,21,34,42]. Therefore, we did not observe a shift in the interband transition energy when
a thicker (120 nm thick) MXene film was deposited.
Considering the origin of the absorption band, transversal and longitudinal plasmon
resonances are predicted to be closer to the middle infrared spectral range. It is notable
that the maximum of the spectral band related to the plasmon resonance is located at λ >
2200 nm (see Figure 2). According to the calculations, the width of the plasmon band
might be influenced by the high size dispersion of MXene flakes [45]. It should be noted
that in order to increase the contribution of the electromagnetic enhancement mechanism
into the amplification of the SERS spectrum together with the chemical enhancement
mechanism, the plasmon resonance should be observed in the visible region of the spec-
trum. Fortunately, the plasmon resonance wavelength can be tailored by changing the
flake dimensions [45,46].
Figure 3.
UV-Vis-NIR absorption spectrum of dried 70 nm thick Ti
3
C
2
T
x
MXene film on the glass slide.
The UV-Vis-NIR absorption spectrum of 70 nm thick Ti
3
C
2
T
x
-based MXene film
contains a broad spectral band (full width at half maximum—195 nm) characteristic for
delaminated MXenes. The center of this band is located at 750 nm (1.65 eV). The origin of
the spectral band is attributed to the interband transition. The plasmonic nature of this
band is controversial. Some researchers assign bands in this spectral range to transversal
plasmon resonance [
42
,
44
], while other studies suggest that the plasmonic activity of
MXenes occurs as a consequence of the real part of the dielectric function, becoming
negative only in the near-infrared spectral range, implying that the plasma frequency
of MXenes is also in this range and excitation of surface plasmons can be expected at
longer wavelengths. During this research, we adhere to the assignment of this absorption
band to the interband transition because: (i) no plasmonic activities were observed in
this spectral range and (ii) the characteristics of enhanced SERS spectra of salicylic acid
indicate a chemical enhancement mechanism. It is important to note that the thickness of
the MXene film or the size and shape of the individual Ti
3
C
2
T
x
flakes have no influence on
the interband gap. The position of the spectral band can only be shifted by changing the
concentration of free charge carriers that can be altered with different terminal groups of
Ti
3
C
2
T
x
MXene lattice [
17
,
21
,
34
,
42
]. Therefore, we did not observe a shift in the interband
transition energy when a thicker (120 nm thick) MXene film was deposited.
Considering the origin of the absorption band, transversal and longitudinal plasmon
resonances are predicted to be closer to the middle infrared spectral range. It is notable
that the maximum of the spectral band related to the plasmon resonance is located at
λ
> 2200 nm (see Figure 2). According to the calculations, the width of the plasmon band
might be influenced by the high size dispersion of MXene flakes [
45
]. It should be noted
that in order to increase the contribution of the electromagnetic enhancement mechanism
into the amplification of the SERS spectrum together with the chemical enhancement
mechanism, the plasmon resonance should be observed in the visible region of the spectrum.
Fortunately, the plasmon resonance wavelength can be tailored by changing the flake
dimensions [45,46].
3.2. Raman Spectra of MXene Films
Further assessment of MXene film composition can be performed using Raman spec-
troscopy data. The positions of Raman spectral bands of the MXene by itself are mostly
caused by various vibrations of Ti
3
C
2
T
x
lattice and the terminal groups. Consequently, the
presence of different terminal groups and even interactions with the target molecules can
be distinguished by examining the spectral changes.
The Raman spectra of an MXene film on a glass plate obtained with 532 nm, 633 nm
and 785 nm excitation are presented in Figure 4. The spectral bands of Ti
3
C
2
T
x
lattice
vibrations that interest us occur in the range of 100–850 cm
−1
. Excitation with a 532 nm
laser yields a Raman spectrum with a relatively low signal-to-noise ratio (S/N
≈
52),
whereas it is higher in the case of 633 nm and 785 nm excitations (S/N > 100).
Chemosensors 2021,9, 223 7 of 18
Chemosensors 2021, 9, x FOR PEER REVIEW 8 of 18
hancement of non-symmetric normal modes is possible when two excited states are cou-
pled by the normal mode vibration (B term). Hence, in both cases, the enhanced modes
correspond to the interaction between the molecule ground and excited states [52,53].
Figure 4. Raman spectra of MXene film with 532 nm (bottom), 633 nm (middle) and 785 nm (top)
excitation.
Furthermore, the lower energy vibrational spectral bands of Ti
3
C
2
T
x
MXenes can be
used for the determination of the changes in surface terminal groups (=O, –OH or –F)
[49,54,55]. The most stable MXene film is the one with carbonyl (=O) terminal group, e.g.,
Ti
3
C
2
O
2
[56] whilst, during the first step of synthesis of MXene—etching of Al—they are
terminated with the -F group. Usually, it is considered that MXenes are terminated with
all these functional groups to some degree
[57]. Very promising spectral bands for the
determination of terminal groups are the out-of-plane mode of C vibrations (ω
3
), which is
calculated to be located at 694 cm
−1
in Ti
3
C
2
(OH)
2
MXene and at 730 cm
−1
in Ti
3
C
2
O
2
and
the out-of-plane mode of mainly Ti and terminal atoms vibrations (ω
2
). It was experimen-
tally proved that ω
3
mode becomes red-shifted when the =O terminal group is reduced
into the –OH group [30], while ω
2
redshifts from 218 cm
−1
to 208 cm
−1
[49]. Other distinct
Raman spectral bands of Ti
3
C
2
O
2
are the out-of-plane mode of =O at 371 cm
−1
and Ti and
the =O in-plane vibrational mode at 589 cm
−1
. The bands at 281 cm
−1
–OH in-plane and 667
cm
−1
—C atom out-of-plane vibrational modes arise from Ti
3
C
2
(OH)
2
. Raman vibrational
frequencies observed for MXene films and assignments of the spectral bands are pre-
sented in Table 2.
Table 2. Vibrational frequencies of Ti
3
C
2
T
x
-based MXene films.
Vibrational Freq., cm
−1
Calculated Freq., cm
−1
[49] Assignments [49]
532 nm 633 nm 785 nm
W
129 120 128 (Ti,C)
ip
,
Ti
3
C
2
F
2
; ω
1
198 198 199 190 (Ti,F)
oop
, Ti
3
C
2
F
2
; ω
2
257 W 258 231 (F)
ip
, Ti
3
C
2
F
2
; ω
5
287 281 280 278 (OH)
ip
, Ti
3
C
2
(OH)
2
; ω
5
390 371 366 347 (O)
ip
, Ti
3
C
2
O
2
; ω
5
W
511
a
513
b
514 (OH)
oop
, Ti
3
C
2
(OH)
2
; ω
6
590
a
589
a
584
a
586 (O)
oop
, Ti
3
C
2
O
2
; ω
6
621 620 615 622 (C)
ip
,
Ti
3
C
2
(OH)
2
; ω
4
Figure 4.
Raman spectra of MXene film with 532 nm (bottom), 633 nm (middle) and 785 nm
(top) excitation.
It is worth mentioning that only a rather low power of 532 nm laser excitation could be
used for acquiring the spectra. Structural and chemical changes of MXene film have been
observed due to rapid oxidation when laser power exceeded 5 mW. In this case, the power
density on the sample exceeded 1.5 MW/cm
2
. Some degradation of the film was noticeable
starting from 17 kW/cm
2
excitation beam power density. In the case of rapid oxidation,
the intense G and D spectral bands arising from allotropic forms of carbon (formed from
Ti
3
C
2
T
x
lattice) emerge along with the Raman spectrum of TiO
2
in anatase form, exhibiting
the most intense spectral band at 143 cm
−1
. Rapid oxidation is known to disrupt the
structure of the MXene film [
47
]. Due to the disruption, Ti is oxidized into TiO
2
by the
formation of anatase nanoparticles and graphitic or amorphous carbon [
48
]. Raman spectra
acquired with 532 nm and 633 nm excitation fit very well with the literature data [
22
,
48
].
The lattice phonon modes of Ti
3
C
2
T
x
MXene terminated with different chemical groups
are observed as a combination of the broader spectral bands at 129 cm
−1
(
ω1
), 198 cm
−1
(
ω2
), 709 cm
−1
(
ω3
), 667 cm
−1
(
ω4
), 281 cm
−1
(
ω5
) and 371 cm
−1
(
ω7
) [
49
]. Higher energy
vibrational spectral bands presumably occur from non-uniform, defected MXene films and
the presence of free carbon materials [
24
,
48
,
50
]. G and D spectral bands distinctive for the
carbon materials are also present in the MXene spectra. The G band in MXene samples
was observed at 1581 cm
−1
, and the D band was almost imperceptible. Generally, the G
band in carbon materials occurs because of sp
2
hybridization caused by C-C stretching
being observed in both chain and ring structures, whereas the D band is observable only in
the defected ring structures [
51
]. In our observations, the appearance of graphene bands
indicates the disruption in MXene lattice structure and oxidation of MXenes. We will
further address this issue in our forthcoming research.
The spectrum obtained with 785 nm excitation is somewhat different from other
spectra. As can be seen in the UV-Vis-NIR absorption spectrum (Figure 3). MXenes in
this spectral region have a spectral band arising due to IBT (the corresponding absorption
band center, in our case, is at 750 nm). Thus, resonance effects are expected in the Raman
spectrum with a 785 nm excitation laser. As a consequence of resonant lattice vibrations,
the new spectral bands become observable. The new band at 120 cm
−1
arises from the
in-plane vibration mode of Ti and C atoms of the MXene lattice. In addition, the new
out-of-plane breathing mode (
ω6
) band at 513 cm
−1
becomes discernible. During this
vibration, terminal atoms are mainly moving in a transversal direction to the lattice plane.
The intensity increase for the band at 722 cm
−1
is observed as well. The spectral band
at 722 cm
−1
represents out-of-plane vibrations of C atoms perpendicular to the 2D plane
Chemosensors 2021,9, 223 8 of 18
of MXene film. The gradual increase in the intensity of these bands is observed when
excitation wavelength changes from 532 nm, 633 nm to 785 nm.
The appearance of new spectral bands can be explained by the resonant condition
of excitation [
44
,
45
,
49
]. The enhancement of certain vibrational modes occurs when the
exciting frequency comes into resonance with the lowest excited state. Usually, total
symmetric modes experience the greatest enhancement (through A term). In this case, the
symmetric normal modes, in which vibrations involve bonds that are affected by the change
in the electronic state of excited molecules, are enhanced. Nevertheless, the enhancement
of non-symmetric normal modes is possible when two excited states are coupled by the
normal mode vibration (B term). Hence, in both cases, the enhanced modes correspond to
the interaction between the molecule ground and excited states [52,53].
Furthermore, the lower energy vibrational spectral bands of Ti
3
C
2
T
x
MXenes can
be used for the determination of the changes in surface terminal groups (=O, –OH or
–F) [
49
,
54
,
55
]. The most stable MXene film is the one with carbonyl (=O) terminal group,
e.g., Ti
3
C
2
O
2
[
56
] whilst, during the first step of synthesis of MXene—etching of Al—they
are terminated with the -F group. Usually, it is considered that MXenes are terminated
with all these functional groups to some degree [
57
]. Very promising spectral bands for
the determination of terminal groups are the out-of-plane mode of C vibrations (
ω3
),
which is calculated to be located at 694 cm
−1
in Ti
3
C
2
(OH)
2
MXene and at 730 cm
−1
in
Ti
3
C
2
O
2
and the out-of-plane mode of mainly Ti and terminal atoms vibrations (
ω2
). It was
experimentally proved that
ω3
mode becomes red-shifted when the =O terminal group is
reduced into the –OH group [
30
], while
ω2
redshifts from 218 cm
−1
to 208 cm
−1
[
49
]. Other
distinct Raman spectral bands of Ti
3
C
2
O
2
are the out-of-plane mode of =O at 371 cm
−1
and Ti and the =O in-plane vibrational mode at 589 cm
−1
. The bands at 281 cm
−1
–OH
in-plane and 667 cm
−1
—C atom out-of-plane vibrational modes arise from Ti
3
C
2
(OH)
2
.
Raman vibrational frequencies observed for MXene films and assignments of the spectral
bands are presented in Table 2.
Table 2. Vibrational frequencies of Ti3C2Tx-based MXene films.
Vibrational Freq., cm−1Calculated Freq., cm−1[49] Assignments [49]
532 nm 633 nm 785 nm
W129 120 128 (Ti,C)ip, Ti3C2F2;ω1
198 198 199 190 (Ti,F)oop, Ti3C2F2;ω2
257 W 258 231 (F)ip, Ti3C2F2;ω5
287 281 280 278 (OH)ip , Ti3C2(OH)2;ω5
390 371 366 347 (O)ip, Ti3C2O2;ω5
W511 a513 b514 (OH)oop, Ti3C2(OH)2;ω6
590 a589 a584 a586 (O)oop, Ti3C2O2;ω6
621 620 615 622 (C)ip, Ti3C2(OH)2;ω4
673 a667 a655 684 (C)oop, Ti3C2(OH)2;ω3
715 709 a722 b730 (C)oop, Ti3C2O2;ω3
W W W D band
1550 1581 1520 aG band
a
Decreased intensity in spectral band compared to 785 nm excitation.
b
Increased intensity in spectral band compared to 785 nm excitation.
WNot prominent or weak band. ip In-plane phonon mode. oop Out-of-plane phonon mode.
3.3. Interaction between Salicylic Acid and MXenes
An interesting effect was observed when salicylic acid (SA) solution was dried on the
MXene film. The measured Raman spectrum of salicylic acid on MXene film clearly differs
from that of crystalline salicylic acid (Figure 5). The redshifts of the vibrational bands are
evident and indicate an interaction between salicylic acid molecules and the Ti
3
C
2
T
x
-based
MXene film. The appearance of prominent Raman spectral bands at 896 cm
−1
and a band
doublet at 681 cm
−1
and 654 cm
−1
confirms the interaction between salicylic acid and the
MXene. Based on our DFT calculations performed for monomeric salicylic acid molecule
and salicylic acid dimer (as in crystalline salicylic acid form), these newly emerged bands
Chemosensors 2021,9, 223 9 of 18
can be assigned to out-of-plane vibrations of CH groups and out-of-plane ring deformation,
respectively. The latter usually displays a low Raman signal intensity of the crystalline
form of SA. The increased intensity of out-of-plane vibrational bands was evaluated by
calculating enhancement factor for intensified bands as:
Enhancement f actor =(ISERS ×NRaman)/(IRaman ×NSERS )(1)
where ISERS and IRaman are SERS and Raman spectral band intensities, NRaman and NSERS—
number of excited molecules for Raman and SERS experiments.
Chemosensors 2021, 9, x FOR PEER REVIEW 9 of 18
673
a
667
a
655 684 (C)
oop
, Ti
3
C
2
(OH)
2
; ω
3
715 709
a
722
b
730 (C)
oop
, Ti
3
C
2
O
2
; ω
3
W W W
D band
1550 1581 1520
a
G band
a
Decreased intensity in spectral band compared to 785 nm excitation.
b
Increased intensity in spec-
tral band compared to 785 nm excitation.
W
Not prominent or weak band.
ip
In-plane phonon
mode.
oop
Out-of-plane phonon mode.
3.3. Interaction between Salicylic Acid and MXenes
An interesting effect was observed when salicylic acid (SA) solution was dried on the
MXene film. The measured Raman spectrum of salicylic acid on MXene film clearly differs
from that of crystalline salicylic acid (Figure 5). The redshifts of the vibrational bands are
evident and indicate an interaction between salicylic acid molecules and the Ti
3
C
2
T
x
-based
MXene film. The appearance of prominent Raman spectral bands at 896 cm
−1
and a band
doublet at 681 cm
−1
and 654 cm
−1
confirms the interaction between salicylic acid and the
MXene. Based on our DFT calculations performed for monomeric salicylic acid molecule
and salicylic acid dimer (as in crystalline salicylic acid form), these newly emerged bands
can be assigned to out-of-plane vibrations of CH groups and out-of-plane ring defor-
mation, respectively. The latter usually displays a low Raman signal intensity of the crys-
talline form of SA. The increased intensity of out-of-plane vibrational bands was evalu-
ated by calculating enhancement factor for intensified bands as:
/
(1)
where and are SERS and Raman spectral band intensities, and
—number of excited molecules for Raman and SERS experiments.
Comparing salicylic acid deposited directly on aluminum surface and on the MXene
film, enhancement factor reached 125, 110 and 220 for the band at 896 cm
−1
and band dou-
blet at 681 cm
−1
and 654 cm
−1
, respectively (532 nm excitation).
Figure 5. Raman spectra of MXene film (bottom) and salicylic acid: dried on MXene surface (middle)
and on aluminum foil (top). Excitation wavelength—633 nm.
Figure 5.
Raman spectra of MXene film (bottom) and salicylic acid: dried on MXene surface (middle) and on aluminum foil
(top). Excitation wavelength—633 nm.
Comparing salicylic acid deposited directly on aluminum surface and on the MXene
film, enhancement factor reached 125, 110 and 220 for the band at 896 cm
−1
and band
doublet at 681 cm−1and 654 cm−1, respectively (532 nm excitation).
Additionally, an increase in intensities and redshifts were observed for the other
salicylic acid spectral bands. The largest shift occurred for the C=O stretching vibrational
band of the carboxylic group at 1636 cm
−1
which shifted by 39 cm
−1
to 1597 cm
−1
; the other
bands experienced smaller redshifts—from 12 to 15 cm
−1
. For example, the vibrational
modes of the benzene ring observed at 1583 cm
−1
and 1473 cm
−1
were shifted to 1567 cm
−1
and 1467 cm
−1
, respectively. The C–O stretching band at 1325 cm
−1
is shifted down to
1311 cm−1; the spectral band of the C–O deformation of the hydroxyl group at 1244 cm−1
is shifted to 1232 cm
−1
. Indeed, the only spectral band that did not experience observable
shift is the benzene ring mode at 1031 cm
−1
. The assignments of the experimental spectral
bands for (i) salicylic acid crystals formed on the pure aluminum surface and (ii) vibrational
shifts for salicylic acid on MXene film are provided in Table 3.
Chemosensors 2021,9, 223 10 of 18
Table 3. Vibrational frequencies of crystalline salicylic acid and SA–MXene complex.
Crystalline Salicylic Acid, Freq.
cm−1Salicylic Acid on MXenes, Freq.
cm−1Assignments
180 - δoop(C-COOH)
258 - δip(C-COOH)
286 - δip(C-OH)
452 - δip(C-COOH)
- 473 δoop(ring) + δoop (OH)
534 - δoop(ring)
568 - δoop(O-H) a+δoop (ring)
- 595 δip(C-C, ring)+ δoop (OH)
657 654 ↑δip(-COOH) + δoop (ring)
705 681 ↑δoop(ring) + δoop (O-H) a
773 - δip(C-H) a+ 6 c
850 861 ν(C-OH) b+δip(ring)
876 896 ↑δoop(C-H); 17 c
1031 1031 18 c?
1093 δ(O-H) b
1154 1145 δ(O-H) a
1164 - 15 c
1244 1232 δ(O-H) a+ν(C-COOH)
1307 - δ(O-H) a
1325 1311 δ(C-O) b
1386 1397 14 c+δ(C-O) b
1473 1467 19 c
1583 1567 8 c
1636 1597 ↓ν(C=O) a
a
Vibration of atoms in –COOH functional group.
b
Vibration of atoms in –OH functional group.
↑
Increased
intensity.
↓
Decreased intensity. Stretching vibration.
δ
Deformation vibration.
oop
Out-of-plane.
ip
In-plane.
c
Modes derived from benzene [58].
Partly due to the porous MXene surface and high surface area, salicylic acid molecules
can spread easily and interact with the MXenes. Indication of such interaction is the change
in color from purple to yellowish when salicylic acid is dried on the MXene film (Figure 6
inset). Salicylic acid molecules are prone to form crystals when drying, though: after drying
on the MXene film, no crystals are visible.
Chemosensors 2021, 9, x FOR PEER REVIEW 10 of 18
Additionally, an increase in intensities and redshifts were observed for the other sal-
icylic acid spectral bands. The largest shift occurred for the C=O stretching vibrational
band of the carboxylic group at 1636 cm−1 which shifted by 39 cm−1 to 1597 cm−1; the other
bands experienced smaller redshifts—from 12 to 15 cm−1. For example, the vibrational
modes of the benzene ring observed at 1583 cm−1 and 1473 cm−1 were shifted to 1567 cm−1
and 1467 cm−1, respectively. The C–O stretching band at 1325 cm−1 is shifted down to 1311
cm−1; the spectral band of the C–O deformation of the hydroxyl group at 1244 cm−1 is
shifted to 1232 cm−1. Indeed, the only spectral band that did not experience observable
shift is the benzene ring mode at 1031 cm−1. The assignments of the experimental spectral
bands for (i) salicylic acid crystals formed on the pure aluminum surface and (ii) vibra-
tional shifts for salicylic acid on MXene film are provided in Table 3.
Partly due to the porous MXene surface and high surface area, salicylic acid mole-
cules can spread easily and interact with the MXenes. Indication of such interaction is the
change in color from purple to yellowish when salicylic acid is dried on the MXene film
(Figure 6 inset). Salicylic acid molecules are prone to form crystals when drying, though:
after drying on the MXene film, no crystals are visible.
Detailed investigation of the interaction between salicylic acid molecules and Ti3C2Tx
MXene can be completed by analyzing UV-Vis-NIR spectra. The change in UV-Vis-NIR
absorption spectrum of salicylic acid–MXene film is clearly seen in the differential spec-
trum where the MXene spectrum is subtracted from salicylic acid on the MXene spectrum
(Figure 6b). Salicylic acid water solution features absorption bands from π to π* and n to
π* transitions at ≈ 230 nm and ≈ 300 nm, whereas salicylic acid–MXene film possesses the
highest intensity band at 400 nm. Other, less distinct bands are at 495 nm, 595 nm and
1080 nm. The decrease in near-infrared absorption range from MXene film is probably
caused by redistribution of conduction band electrons—free charge carriers. These
changes in the absorption spectrum confirm that salicylic acid and MXene are forming
complex structures.
Figure 6. UV-Vis-NIR absorption spectra of the Ti3C2Tx-based MXene film (dotted line) and salicylic acid (straight line)
dried on the MXene (a); differential spectrum (b). Inset—optical microscope images of MXene film (left) and salicylic acid
dried on the MXene film (right).
Figure 6.
UV-Vis-NIR absorption spectra of the Ti
3
C
2
T
x
-based MXene film (dotted line) and salicylic acid (straight line)
dried on the MXene (a); differential spectrum (b). Inset—optical microscope images of MXene film (left) and salicylic acid
dried on the MXene film (right).
Chemosensors 2021,9, 223 11 of 18
Detailed investigation of the interaction between salicylic acid molecules and Ti
3
C
2
T
x
MXene can be completed by analyzing UV-Vis-NIR spectra. The change in UV-Vis-NIR absorp-
tion spectrum of salicylic acid–MXene film is clearly seen in the differential spectrum where
the MXene spectrum is subtracted from salicylic acid on the MXene spectrum
(Figure 6b).
Salicylic acid water solution features absorption bands from
π
to
π
* and nto
π
* transitions at
≈
230 nm and
≈
300 nm, whereas salicylic acid–MXene film possesses the highest intensity
band at 400 nm. Other, less distinct bands are at 495 nm, 595 nm and 1080 nm. The decrease
in near-infrared absorption range from MXene film is probably caused by redistribution of
conduction band electrons—free charge carriers. These changes in the absorption spectrum
confirm that salicylic acid and MXene are forming complex structures.
These spectral changes can be explained by the formation of the salicylic acid–MXene
complex and the charge-transfer effect. It is already known that certain molecules form
complexes with Ti
3
C
2
T
x
MXenes and, therefore, can undergo charge transfer [
30
,
32
,
33
].
For that matter, the SERS enhancement derived from the MXene films is explained using
this charge transfer mechanism and, naturally, by the chemical enhancement mechanism.
Briefly, the chemical enhancement is thought to be active because of one of these factors:
(i) the charge transfer mechanism between the adsorbed molecule and the substrate and
(ii) the influence of the substrate on the molecular polarizability tensor elements that
changes the efficiency of Raman scattering. In the case of charge transfer, the Raman signal
is enhanced because of the pre-resonance or resonance condition of the excitation to the
adsorbate–substrate complex. Thus, the charge transfer mechanism can be traced by: (i) the
greater enhancement of the antisymmetric vibrations compared to symmetric ones (due to
the B term excitation of resonance Raman); (ii) enhancement of the stretching vibrations
(in the resonant condition of excitation, the totally symmetric vibrations in accordance
with the excited state geometry of the molecule are enhanced (due to A term mechanism).
The bond length between atoms usually increases in the excited molecule state); (iii) high
dependence of the EF on the excitation wavelength.
The first indication of the charge transfer effect in the salicylic acid–MXene complex is
the greater enhancement of asymmetric vibrational modes. Since the salicylic acid molecule
is in the C
s
point symmetry group, the vibrational modes of salicylic acid can only be
classified as in-plane (A’) or out-of-plane (A”) vibrations. The latter is less symmetric and,
as seen in Figure 5, was enhanced via salicylic acid–MXene interaction. The dependence of
salicylic acid–MXene Raman spectra on the excitation wavelength is presented in Figure 7.
Chemosensors 2021, 9, x FOR PEER REVIEW 12 of 18
and, as seen in Figure 5, was enhanced via salicylic acid–MXene interaction. The depend-
ence of salicylic acid–MXene Raman spectra on the excitation wavelength is presented in
Figure 7.
Figure 7. Raman spectra of salicylic acid–MXene complex with different excitations. Excitations from top to bottom: 457
nm; 532 nm; 633 nm; 785 nm; 1064 nm.
Based on the spectra acquired at different excitations, the enhancement factor de-
pendence on the excitation can be evaluated (Figure 8). To minimize the influence of spec-
trometer response to different wavelength, the crystalline salicylic acid was acquired with
each excitation, and the salicylic acid–MXene complex spectrum was compared to crys-
talline salicylic acid spectrum acquired at the same conditions.
Indeed, the SERS enhancement factor for out-of-plane vibrations is as expected for
the chemical enhancement mechanism. The enhancement factor for the substrate covered
by the salicylic acid–MXene complex was calculated for the most enhanced vibrational
band at 654 cm
−1
(carboxyl deformation + out-of-plane C-C bending). As the enhancement
factor value dependency on the excitation wavelength indicates, the highest enhancement
of 220 is achieved with a 532 nm laser (Figure 8). Nevertheless, 457 nm and 633 nm exci-
tations also yield comparable enhancement of 165 and 178, respectively. The 785 nm exci-
tation fell into the absorption of MXenes; for this reason, the Raman spectral bands of
MXene phonon modes are still observable in the spectrum, and the enhancement factor
reaches 150. For the excitation profile of 1064 nm, an entirely different Fourier Transform-
Raman spectrometer was used. It was observed that MXene film without salicylic acid
highly absorbs this wavelength, generates heat and makes the registration of spectrum
with this excitation impossible. Nevertheless, when the salicylic acid–MXene complex is
excited, the characteristic spectrum is observed, which is indicating changes in the elec-
tron distribution of the MXene layer.
It is worth mentioning that based on our results, the chemical mechanism of enhance-
ment takes place between salicylic acid and MXenes. The observable hot spot effect of
SERS substrates arises when the intensity crucially depends on the different spots of the
Figure 7.
Raman spectra of salicylic acid–MXene complex with different excitations. Excitations from
top to bottom: 457 nm; 532 nm; 633 nm; 785 nm; 1064 nm.
Chemosensors 2021,9, 223 12 of 18
Based on the spectra acquired at different excitations, the enhancement factor de-
pendence on the excitation can be evaluated (Figure 8). To minimize the influence of
spectrometer response to different wavelength, the crystalline salicylic acid was acquired
with each excitation, and the salicylic acid–MXene complex spectrum was compared to
crystalline salicylic acid spectrum acquired at the same conditions.
Chemosensors 2021, 9, x FOR PEER REVIEW 13 of 18
sample, because of the electromagnetic mechanism of enhancement when the analyte mol-
ecule is trapped between two nanoparticles [31]. The random distribution of these hot
spots varies the absolute intensity of SERS bands. In our case, the chemical enhancement
mechanism makes the substrate more uniform in the sense of changes in the intensity of
the Raman bands. For the calculations of the enhancement factor, the absolute intensity is
important, but for further studies, the normalization to a specific spectral band can be
considered (for salicylic acid, it was the only spectral band at 1031 cm
−1
that did not shift,
and for this reason, it is considered to be unaffected by chemical enhancement mecha-
nism).
Figure 8. Calculated SERS enhancement factor (EF) for 2 mM salicylic acid dried on the MXene film
with different excitation wavelengths.
However, the comparison of the achieved enhancement factor for salicylic acid with
that of conventional SERS materials such as silver and gold nanostructures is difficult.
Limited literature is available concerning the SERS enhancement factor of salicylic acid.
To the best of our knowledge, the evaluation of the analytical enhancement factor for sal-
icylic acid adsorbed on silver and gold nanoparticles with 1064 nm excitation is presented
only in one of our previous works [59]. The determined enhancement factors for gold and
silver nanoparticles prepared with different synthesis methods and stabilizing agents are
presented in Table 4.
Table 4. Comparison of enhancement factors for salicylic acid achieved with different materials.
Substrate Material Excitation Enhancement Factor
Citrate-stabilized AgNPs
1064 nm 2.5 × 10
4
Polymer-stabilized AgNPs 1064 nm 1.2 × 10
4
Citrate-stabilized AuNPs 1064 nm 2.5 × 10
3
Polymer-stabilized AuNPs 1064 nm 3.8 × 10
3
Ti
3
C
2
T
x
MXene 532 nm 2.2 × 10
2
AgNPs—silver nanoparticles. AuNPs—gold nanoparticles.
3.4. Computational Results of Salicylic Acid Interaction with MXene
In order to make a more detailed investigation of the salicylic acid and MXene inter-
action, the first-principle calculations based on DFT were performed. For salicylic acid–
MXene complex investigation, the cluster of Ti
3
C
2
(OH)
2
supercell was built and optimized
(B3LYP/LanL2DZ). According to the calculations, salicylic acid displays affinity of form-
ing a chemical bond with titanium atom of Ti
3
C
2
(OH)
2
crystal via oxygen atom in carboxyl
group of salicylic acid. The likeness of the complex formation is assured based on geomet-
rical parameters of optimized geometry of the salicylic acid–MXene complex. The length
Figure 8.
Calculated SERS enhancement factor (EF) for 2 mM salicylic acid dried on the MXene film
with different excitation wavelengths.
Indeed, the SERS enhancement factor for out-of-plane vibrations is as expected for the
chemical enhancement mechanism. The enhancement factor for the substrate covered by
the salicylic acid–MXene complex was calculated for the most enhanced vibrational band at
654 cm
−1
(carboxyl deformation + out-of-plane C-C bending). As the enhancement factor
value dependency on the excitation wavelength indicates, the highest enhancement of 220
is achieved with a 532 nm laser (Figure 8). Nevertheless, 457 nm and 633 nm excitations
also yield comparable enhancement of 165 and 178, respectively. The 785 nm excitation
fell into the absorption of MXenes; for this reason, the Raman spectral bands of MXene
phonon modes are still observable in the spectrum, and the enhancement factor reaches
150. For the excitation profile of 1064 nm, an entirely different Fourier Transform-Raman
spectrometer was used. It was observed that MXene film without salicylic acid highly
absorbs this wavelength, generates heat and makes the registration of spectrum with this
excitation impossible. Nevertheless, when the salicylic acid–MXene complex is excited, the
characteristic spectrum is observed, which is indicating changes in the electron distribution
of the MXene layer.
It is worth mentioning that based on our results, the chemical mechanism of enhance-
ment takes place between salicylic acid and MXenes. The observable hot spot effect of SERS
substrates arises when the intensity crucially depends on the different spots of the sample,
because of the electromagnetic mechanism of enhancement when the analyte molecule is
trapped between two nanoparticles [
31
]. The random distribution of these hot spots varies
the absolute intensity of SERS bands. In our case, the chemical enhancement mechanism
makes the substrate more uniform in the sense of changes in the intensity of the Raman
bands. For the calculations of the enhancement factor, the absolute intensity is important,
but for further studies, the normalization to a specific spectral band can be considered (for
salicylic acid, it was the only spectral band at 1031 cm
−1
that did not shift, and for this
reason, it is considered to be unaffected by chemical enhancement mechanism).
However, the comparison of the achieved enhancement factor for salicylic acid with
that of conventional SERS materials such as silver and gold nanostructures is difficult.
Limited literature is available concerning the SERS enhancement factor of salicylic acid. To
the best of our knowledge, the evaluation of the analytical enhancement factor for salicylic
acid adsorbed on silver and gold nanoparticles with 1064 nm excitation is presented only
in one of our previous works [
59
]. The determined enhancement factors for gold and
Chemosensors 2021,9, 223 13 of 18
silver nanoparticles prepared with different synthesis methods and stabilizing agents are
presented in Table 4.
Table 4. Comparison of enhancement factors for salicylic acid achieved with different materials.
Substrate Material Excitation Enhancement Factor
Citrate-stabilized AgNPs 1064 nm 2.5 ×104
Polymer-stabilized AgNPs 1064 nm 1.2 ×104
Citrate-stabilized AuNPs 1064 nm 2.5 ×103
Polymer-stabilized AuNPs 1064 nm 3.8 ×103
Ti3C2TxMXene 532 nm 2.2 ×102
AgNPs—silver nanoparticles. AuNPs—gold nanoparticles.
3.4. Computational Results of Salicylic Acid Interaction with MXene
In order to make a more detailed investigation of the salicylic acid and MXene in-
teraction, the first-principle calculations based on DFT were performed. For salicylic
acid–MXene complex investigation, the cluster of Ti
3
C
2
(OH)
2
supercell was built and
optimized (B3LYP/LanL2DZ). According to the calculations, salicylic acid displays affinity
of forming a chemical bond with titanium atom of Ti
3
C
2
(OH)
2
crystal via oxygen atom in
carboxyl group of salicylic acid. The likeness of the complex formation is assured based on
geometrical parameters of optimized geometry of the salicylic acid–MXene complex. The
length of the O-H interatomic bond in the salicylic acid carboxyl group increased from 0.976
to 1.536 Å, whereas the distance between the Ti
3
C
2
(OH)
2
MXene OH-H group was 1.034 Å,
indicating the possible proton transfer between SA and the MXene cluster. The C-O bond
length in the carboxyl group also increased from 1.232 to 1.339 Å. The bond formation
between salicylic acid and Ti
3
C
2
(OH)
2
MXene can explain the drastic experimental shift
of 40 cm
−1
for the C=O stretching vibrational band of the carboxylic group at 1636 cm
−1
and the disappearance of the salicylic acid band at 771 cm
−1
(benzene ring bending +
carboxyl deformation modes). The formed bond between salicylic acid and the MXene
cluster can become a channel for electron density redistribution around the salicylic acid
and MXene cluster. Moreover, the charge distribution on the atoms from the performed
Mulliken population analysis indicates a slight charge redistribution by
−
0.66 when from
−0.32 (monomeric salicylic acid) to −0.98 (salicylic acid–MXene complex).
As can be expected, the electron density in the highest occupied molecular orbital
minus 1 (HOMO-1) and lower molecular orbitals is focused on the donor Ti
3
C
2
(OH)
2
MXene. In the HOMO molecular orbital, the electron density redistribution is already
observed, also indicating the formation of a complex bond between the carboxyl group of
salicylic acid and MXene, whereas in the lowest unoccupied molecular orbital (LUMO), the
high electron density shifts from MXene to salicylic acid (1.45 eV). Based on the electron
density in molecular orbitals, other charge transfer excitations are observed at LUMO + 4
(2.2 eV) and LUMO + 10 (2.9 eV) (Figure 9). The calculated salicylic acid–MXene complex
excitations are in accordance with the observed UV-Vis absorption spectrum (Figure 6),
where the most intense absorption band is observed at 400 nm (calculated—428 nm) and
lower bands—at 495 nm and 595 nm (calculated—564 nm) and 1080 nm. By comparing the
HOMO-LUMO excitation of the monomeric salicylic acid molecule, the required salicylic
acid molecule excitation energy decreased from 4.5 eV to 2.9 eV.
Chemosensors 2021,9, 223 14 of 18
Chemosensors 2021, 9, x FOR PEER REVIEW 14 of 18
of the O-H interatomic bond in the salicylic acid carboxyl group increased from 0.976 to
1.536 Å, whereas the distance between the Ti
3
C
2
(OH)
2
MXene OH-H group was 1.034 Å,
indicating the possible proton transfer between SA and the MXene cluster. The C-O bond
length in the carboxyl group also increased from 1.232 to 1.339 Å. The bond formation
between salicylic acid and Ti
3
C
2
(OH)
2
MXene can explain the drastic experimental shift of
40 cm
−1
for the C=O stretching vibrational band of the carboxylic group at 1636 cm
−1
and
the disappearance of the salicylic acid band at 771 cm
−1
(benzene ring bending + carboxyl
deformation modes). The formed bond between salicylic acid and the MXene cluster can
become a channel for electron density redistribution around the salicylic acid and MXene
cluster. Moreover, the charge distribution on the atoms from the performed Mulliken pop-
ulation analysis indicates a slight charge redistribution by −0.66 when from −0.32 (mono-
meric salicylic acid) to −0.98 (salicylic acid–MXene complex).
As can be expected, the electron density in the highest occupied molecular orbital
minus 1 (HOMO-1) and lower molecular orbitals is focused on the donor Ti
3
C
2
(OH)
2
MXene. In the HOMO molecular orbital, the electron density redistribution is already ob-
served, also indicating the formation of a complex bond between the carboxyl group of
salicylic acid and MXene, whereas in the lowest unoccupied molecular orbital (LUMO),
the high electron density shifts from MXene to salicylic acid (1.45 eV). Based on the elec-
tron density in molecular orbitals, other charge transfer excitations are observed at LUMO
+ 4 (2.2 eV) and LUMO + 10 (2.9 eV) (Figure 9). The calculated salicylic acid–MXene com-
plex excitations are in accordance with the observed UV-Vis absorption spectrum (Figure
6), where the most intense absorption band is observed at 400 nm (calculated—428 nm)
and lower bands—at 495 nm and 595 nm (calculated—564 nm) and 1080 nm. By compar-
ing the HOMO-LUMO excitation of the monomeric salicylic acid molecule, the required
salicylic acid molecule excitation energy decreased from 4.5 eV to 2.9 eV.
Figure 9. Calculated (B3LYP/LanL2DZ) molecular orbitals of salicylic acid–MXene complex and re-
quired transition energy.
4. Discussion
In the search for non-metallic SERS substrates, a very big role is placed on the very
high enhancement of vibrations. Briefly, the chemical enhancement mechanism of SERS
Figure 9.
Calculated (B3LYP/LanL2DZ) molecular orbitals of salicylic acid–MXene complex and required transition energy.
4. Discussion
In the search for non-metallic SERS substrates, a very big role is placed on the very
high enhancement of vibrations. Briefly, the chemical enhancement mechanism of SERS in
widely used metal nanoparticles contributes to the total enhancement effect only on the
order of 10–100, whereas the electromagnetic enhancement mechanism is thought to be
responsible for spectral enhancement as high as 10
6
times [
31
]. Thus, the electromagnetic
enhancement should enhance the molecular spectra by a huge amount. Nevertheless,
the electromagnetic enhancement mechanism depends strongly on the charge carrier
concentration in materials and thus their metallic properties.
During the last decade, the metallic properties of MXenes were investigated rather
comprehensively. Similar to other 2D semimetals [
16
], MXenes show plasmonic behavior
and the negative real part of the dielectric function in the near-infrared range [
17
,
34
,
42
,
46
].
Experimental findings for highly oriented MXene films reveal that at wavelengths longer
than 1000 nm, the real part of the dielectric function becomes negative, indicating the
onset of free-electron plasma oscillations [
17
,
42
,
46
]. The wavelength threshold at which
MXene films become metallic depends on the film thickness [
34
,
46
], and thus can be slightly
shifted. Furthermore, SEM [
22
] and atomic force microscopy [
55
] images clearly reveal the
structure of MXenes as the flakes are packed in the form of multilayer sheets. After the
delamination, these sheets part out. Extended studies of optical properties of the monolayer
flake revealed that the free-electron plasma oscillations occur in two spectral regions:
(I) at wavelengths longer than 885 nm and (II) in the narrow 615–740 nm region [
42
].
The surface plasmon in the latter region was assigned to transversal plasmon resonance
(1.7 eV). The transversal plasmon resonance is insensitive to the size and shape of the flake,
but it depends on the concentration of free charge carriers that can be altered with the
different terminal groups [17,34,42,46].
In performed studies, it was reported that some molecules could interact with multi-
layered MXenes. Intercalated molecules increase the distance between individual Ti
3
C
2
T
x
mono-sheets and, due to mechanical agitation, cause the delamination of multilayered MX-
ene flakes. Usually, such intercalants are dimethyl sulfoxide DMSO, tetraalkylammonium
hydroxide, isopropylamine, hydrazine, urea, Li
+
ions [
60
,
61
] or even water molecules [
62
].
Chemosensors 2021,9, 223 15 of 18
As a result of this process, a large surface area of MXene layers becomes available for
interaction, which is desirable for the SERS enhancement. In this work, SERS spectra of the
salicylic acid–MXene complex was observed for both the multilayered and delaminated
MXenes (it should be noted that for the latter, SERS intensity and reproducibility were
higher). This supports our prediction that salicylic acid was also responsible for partial
delamination of MXene layers.
MXenes have a mixture of oxygen, hydroxyl and fluoride terminal groups that can
be protonated/deprotonated at different pHs [
23
]. At higher hydrogen ion concentration
(lower pH), the oxidation of MXenes is slowed down; therefore, a higher surface concen-
tration of hydroxyl terminal groups is favored. Due to pronation at low pH, the hydroxyl
terminal groups provide a slightly positive charge on the surface of the MXene layer. This
fact should be considered while investigating the mechanism of interaction between MX-
ene and salicylic acid. Moreover, salicylic acid is deprotonated in deionized water (loses
hydrogen ions from the carboxylic group); thus, it has a negative charge localized at the
carboxylic group that also favors the formation of hydrogen bonds with terminal hydroxyl
groups. Therefore, salicylic acid adsorbs well to the hydroxyl-terminated MXene layer.
Out-of-plane vibrations of salicylic acid are enhanced when the molecule interacts
with MXene. These spectral changes indicate the molecule of salicylic acid is lying flat on
the MXene film. It is known that for dye molecules, charge transfer between the electronic
dye level and MXene electronic level takes place, and as a consequence, the dye molecules
experience enhancement. Nevertheless, the electromagnetic enhancement mechanism from
MXenes, which is expected, was still not observed, neither in our study nor to the best of
our knowledge in any other studies.
5. Conclusions
The surface-enhanced effect of spectral Raman bands of salicylic acid adsorbed on
Ti
3
C
2
T
x
-based MXene film was observed for the first time. The adsorption of the salicylic
acid molecule and the formation of a salicylic acid–MXene complex was confirmed by ex-
perimental spectral observations such as substantial enhancement of out-of-plane bending
modes of salicylic acid at 896 cm
−1
, 681 cm
−1
and 654 cm
−1
. Additionally, other spectral
features indicate the adsorption of salicylic acids, such as the redshift of some vibrational
frequencies as well as the disappearance of the carboxyl deformation spectral band at
771 cm
−1
. The values of calculated experimental enhancement factors indicate that chemi-
cal enhancement mechanisms are dominant in SERS spectra of salicylic acid adsorbed on
the MXene surface. For the deformation out-of-plane vibrational modes, this factor varies
from 220 (at λ= 532 nm) to 60 (at λ= 1064 nm).
Author Contributions:
Conceptualization: S.A.-G. and A.R.; Methodology: S.A.-G., A.P., S.R. and
O.G.; Software: S.A.-G. and V.Š., formal analysis: S.A.-G., A.P., S.R. and O.G.; Writing—original draft
preparation: S.A.-G.; Writing—review and editing: V.Š. A.P., S.R. and A.R.; Supervision: V.Š. and A.R.
All authors have read and agreed to the published version of the manuscript.
Funding:
This project has received funding from H2020 Marie Skłodowska-Curie Actions (CanBioSe
778157, SALSETH 872370).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
Computations were performed on resources at the High Performance Comput-
ing Center, ‘HPC Sauletekis’, at the Vilnius University Faculty of Physics.
Conflicts of Interest: The authors declare no conflict of interest.
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