Understanding chemical interaction between phosphonate-derivative molecules and a silver surface cluster in SERS: A combined experimental and computational approach

Article abstract of DOI:10.1039/c9cp01615e

The interaction between phosphonate functions and a silver surface cluster is investigated using Surface-Enhanced Raman Spectroscopy (SERS). Changing the functional group (methylphosphonic acid based molecule) by studying the effect of protonation, methylation and substitution of the side chain with amine and carboxylate functions enabled us to modulate the chemical interactions between the different functions and the metal cluster. We find that the adsorption energy of the methylphosphonic acid decreases with the protonation, the methylation processes and the substitution of the side chain. In all cases, only the deprotonated phosphonate forms are SERS active. To understand how the molecules interact with the nanoparticle, the electronic structure, adsorption energies and Raman spectra were computed for molecules adsorbed on a 20 atom silver cluster representing a nanoparticle surface. The qualitative agreement between computed static Raman spectra and experimental SERS spectra makes it possible to determine stable geometries of the analyte-silver cluster complexes and to characterize the adsorption modes. The findings presented here provide a framework for designing analytical developments based on SERS for simultaneous detection of phosphonated molecules, including pesticides such as glyphosate, creating practical opportunities in key areas such as environmental and water resource in situ monitoring.

Full text of DOI:10.1039/c9cp01615e

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H.-P. Loock et al.
Determination of the thermal, oxidative and photochemical
degradation rates of scintillator liquid by fluorescence EEM
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DOI: 10.1039/C9CP01615E
Journal Name
ARTICLE
Understanding chemical interaction between phosphonate-
derivative molecules and silver surface cluster in SERS: a
combined experimental and computational approach
Received 00th January 20xx,
Accepted 00th January 20xx
a,b,‖
c,‖
a
b
Gauthier Emonds-Alt,
Benoit Mignolet, Cédric Malherbe, Jean-Christophe M. Monbaliu,
c
a,
Françoise Remacle, and Gauthier Eppe *
DOI: 10.1039/x0xx00000x
The interaction between phosphonate functions and a silver surface cluster is investigated using Surface-Enhanced Raman
Spectroscopy (SERS). Changing the functional group (methylphosphonic acid based molecule) by studying the effect of the
protonation, the methylation and the substitution of the side chain with amine and a carboxylate functions enabled us to
modulate the chemical interactions between the different functions and the metal cluster. We find that the adsorption
energy of the methylphosphonic acid decreases with the protonation, the methylation processes and the substitution of
the side chain. In all cases, only the deprotonated phosphonate forms are SERS active. To understand how the molecule
interacts with the nanoparticle, the electronic structure, adsorption energies and Raman spectra were computed for
molecules adsorbed on a 20 atoms silver cluster representing a nanoparticle surface. The qualitative agreement between
computed static Raman spectra and experimental SERS spectra allows determining stable geometries of the analyte-silver
cluster complexes and to characterize adsorption modes. The findings presented here provide a framework for designing
analytical developments based on SERS for simultaneous detection of phosphonated molecules, including pesticides such
as glyphosate, creating practical opportunities in key areas such as environmental and water resources in situ monitoring.
www.rsc.org/
3
4
Functionalized nanoparticles are used in medicine , biology ,
5
6
Introduction
analytical chemistry or environmental chemistry . The adsorption
of small molecules on a metallic surface is affected by the chemical
environment and is therefore often studied under restricted
experimental conditions. However, the adsorption mechanisms are
often neglected in applied research.
Phosphonates, i.e. alkyl or aryl esters of phosphonic acids are
organophosphorous compounds that have found various
applications as pesticides, active pharmaceutical compounds -as
1
chelatants . As such, they help to stabilize and functionalize
nanoparticles. Liong M. et al developed nanoprobes based on iron
The adsorption of molecules on silver and gold surfaces can be
investigated by experimental techniques such as Atomic Force
Microscopy (AFM)7, Scanning Tunneling Microscopy (STM)8,
Infrared (IR)9 and X-Ray photoelectron spectroscopy (XPS)10.
Besides, Surface Enhanced Raman spectroscopy (SERS) is
particularly sensitive to the molecular structure in the close vicinity
nanoparticles stabilized with phosphonate molecules (acting as
2
stabilizing agent) for drug delivery . In 2015, Li R. et al developed
nanoparticle probes for imaging, where phosphonate prevented the
3
interferences of cellular phosphate. Understanding the orientation,
the structure and the bonding of small phosphonate-bearing
molecules at the surface of nanoparticle may therefore help to
control the stability of phosphonate-nanoparticle based system.
Adsorption processes of small molecules on the surface of
metallic nanostructures are important for various applications.
1
1
of the nanoparticle . SERS is based on the enhancement of the
Raman scattering, which gives structural information of the target
4
molecule. Usually, a Raman signal enhancement of the order of 10
6
14
15
to 10 is observed but the enhancement factor can reach 10 -10
1
2
allowing to detect single molecule . SERS effect can be explained
by two enhancement mechanisms: (i) the electromagnetic
enhancement and (ii) the chemical enhancement. The
electromagnetic mechanism explains the enhancement of the local
electromagnetic field near the metallic surface resulting from the
excitation of the surface plasmon of the nanoparticles. The
magnitude of the electric field |E| is than much higher near the
nanoparticles than far away from its surface. The electric field
decreases with the distance to the nanoparticle surface following a
a. Mass Spectrometry Laboratory, MolSys Research Unit, University of Liège, B4000
Liège, Belgium. * corresponding author : g.eppe@uliege.be
Center for Integrated Technology and Organic Synthesis, MolSys Research Unit,
b.
University of Liège, B4000 Liège, Belgium.
Theoretical Physical Chemistry, MolSys Research Unit, University of Liège, 4000
c.
Liege, Belgium.
†Electronic Supplementary Information (ESI) available: Explicit solvation, isomers
of the analyte-silver cluster complexes and full Raman spectra between 0 and
-1
2
000 cm are available in the ESI.
See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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d-3 relationship. The SERS signal, which is proportional to |E|4,
decreases therefore in d-12. On the other hand, the chemical synthesis of Creighton16 where the silver cDaOtiIo: n10s.1a0r3e9/rCe9dCuPc0e1d61b5Ey
Silver nanoparticles (Ag NPs) were prepared byVaiedwaAprtticinlegOntlhinee
enhancement, much weaker than the electromagnetic mechanism, NaBH
is due to the interaction between the metallic nanoparticle and the
target molecule forming a complex. Therefore, SERS spectroscopy is
4
according to Eq 1.
퐴푔 + + 퐵퐻 ― → 퐴푔 + 퐻2
1
1
2
+
퐵 퐻 (1)
2 6
4
2
Typically, an aqueous solution of AgNO
3
(2.5 mM, 50 mL, cooled at
an effective technique for investigating interfacial properties.
Studying the frequency shifts and the relative intensities of SERS
bands provides information on the average conformation and
relative proximity of different chemical functions of the adsorbed
molecule at the metallic surface. Electronic structure computations
provide additional information on the enhanced vibrational modes
and on the adsorption site of the molecule on the surface. Few
researches on the investigation of the adsorption of phosphonate
derivatives on silver surface have been carried out using SERS.
Podstawka E. et al have studied the interaction of phosphonate
derivatives of imidazole, thiazole and pyridine at the surface of
silver electrode in order to demonstrate the feasability of using
SERS spectroscopy to probe interactions that mimic the mechanism
of a substrate binding to its receptor.13 Costa J.C.S. et al correlated
experiment with theoretical study. They computed the electronic
structure of organophosphorus pesticides (such as (N-
phosphonomethyl)glycine commonly known as glyphosate) in close
contact with a single silver atom. Finally, they compared SERS
spectra of these molecules with computed compounds in order to
demonstrated how molecules interact with silver surface.5
Proniewicz L.M. at al. have also studied the adsorption geometry of
N-benzylamino(boronphenyl)-methylphosphonic acids and boron
derivatives of aminophosphonic acids on silver surface by
1
°C) was added dropwise to a vigorously stirred aqueous solution
4
of NaBH (2 mM, 150 mL) maintained at 1°C in an ice bath.
Diborane subsequently reacts with water to form boric acid that is
adsorbed at the surface of the Ag NPs, thereby stabilising them.
퐵2퐻6 + 6 퐻2푂 → 2 퐻3퐵푂3 + 6 퐻2 (2)
The colloidal suspension was then heated at 90°C for 45 minutes to
eliminate the excess of sodium borohydride. The residual solution
was then adjusted to 200 mL and stored at 4°C. The final solution
displayed a yellow color. Spherical Ag NPs were obtained with an
average diameter of 25 nm (as determined by Transmission
Electron Microscopy) and are characterized with a maximum of
Visible light absorption at 393 nm (as determined by UV-Visible
spectroscopy). The pH of the stored NP solution is 7.6.
Synthesis of the methyl hydrogen methylphosphonate
Methyl hydrogen methylphosphonate lithium salt was prepared
1
7
adapting the protocol of H. Krawczyk . A solution of dimethyl
methyl phosphonate (20 mmol) and LiBr (20 mmol) in isobutyl
methyl ketone (30 mL) was heated at 80 °C. After the appearance of
a white precipitate, the reaction mixture was kept at 80 °C for 30
minutes. The solvent was then removed under reduced pressure.
The hemisolid residue was taken up diethyl ether (40 mL) and
combining theoretical methods with experimental SERS filtrated. The residue was washed with diethyl ether (20 mL), and
measurements.14,15
dried for 1 h under vacuum. The purity of the methyl hydrogen
The main objective of this study is to understand the interaction methylphosphonate lithium salt was confirmed by P and H NMR
3
1
1
between the phosphonate function and a silver surface using SERS. in D
2
O.
Methylphosphonic acid, and four of its derivatives have been
SERS measurements
studied to highlight (i) the effect of the protonation, (ii) the effect of
the methylation of the phosphonate function and (iii) the effect of
the substitution of the side chain in order to evaluate the affinity of
the phosphonate function compared to an amine or a carboxylate
function. To determine the adsorption site of these targeted
molecules, adsorption energies and Raman spectra were computed
for all molecules adsorbed on a 20 atoms silver cluster that
represents the NP surface. The good agreement between computed
All samples were prepared by adding 50 µL of aqueous solution
of 1 mM of the target molecules (Methylphosphonic acid, Methyl
hydrogen methylphosphonate, dimethyl methyl phosphonate,
Aminomethylphosphonic acid or glyphosate) to 500 µL of Ag NPs
suspension. The target molecules and Ag NPs were incubated for 30
min, then 50 µL of sodium ascorbate buffer (0.5 M) and 200 µL of
aggregating agent (NaNO
3
, 3.75 M) were successively added into
disposable polystyrene cuvettes prior to SERS measurements. The
static Raman spectra and experimental SERS spectra provides use of an aggregating agent is usually necessary for SERS
insight on the adsorption mode of the phosphonate derivative measurements.18,19 The pH of the solution was determined with a
molecules on silver nanoparticles.
pH electrode HAMILTON double pore.
All SERS spectra were recorded with a Raman spectrometer
Labram 300 from Horiba interfaced with a confocal microscope
Olympus BX-40 and equipped with a 632.8 nm HeNe excitation
laser. Raman spectra of the samples were obtained directly into
polystyrene cuvettes using a 90° angled return mirror through an
achromatic lens (focal distance of 50 mm) and a laser power of
Experimental
Materials
All the reagents were obtained from commercial sources and
used without additional purification, except methyl hydrogen
methylphosphonate, which was prepared by desmethylation of
commercial dimethyl methylphosphonate. All solutions were
prepared with water distillated twice.
4
1
mW at the sample. Every spectrum was accumulated twice for
0s in the 650-1410 cm range of Raman shift.
-1
Computation details
Synthesis of silver nanoparticles
2
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All the computations on the bare Ag20 cluster (as a model to energy of a given targeted molecule (TM) is VcioewmAprtuictleedOnlianes
ETM=(EAg20+ETM)-EAg20-TM, where the nanoparticle is approximated by
DOI: 10.1039/C9CP01615E
represent the nanoparticle surface) and on the target molecules
adsorbed on the Ag20 cluster were carried out at the density a 20-atom silver cluster. If the adsorption energy of the targeted
functional theory (DFT) level with the polarizable continuum model molecule is larger than the adsorption energy of the stabilizing
2
0
(
PCM) as implemented in Gaussian 09. We used the long-range
agent, then the targeted molecule could displace the stabilizing
agent and lead to a SERS activity.
2
1
corrected functional wB97XD that includes empirical dispersion
Grimme’s D2 dispersion model), which are important to account
for the weak and long-range interactions between the molecule and
the Ag cluster. The basis sets are the LanL2DZ with Effective Core
(
In this study, the affinity of 5 phosphonic derivatives (Figure 1)
with a silver NP's surface were evaluated and the corresponding
adsorption mode on the silver cluster were determined by
comparing the computed static Raman spectrum with the
experimental SERS spectrum. In particular three molecular
parameters were investigated:
2
2
Potentials for Ag atoms and the cc-pVDZ basis set for the
adsorbed molecules. The vibrational frequencies at the equilibrium
geometries of the stable complexes reported below and in the SI
were all real. They were used to compute the Raman spectra that
were isotropically averaged over the orientations. The largest
component of the polarizability is that perpendicular to the surface
of the cluster. The initial geometry of the bare Ag20 cluster was
taken from Moore J.E. et al , where this cluster was used to
compute the chemical enhancement of pyridine in SERS. The Ag20
cluster is neutral and close shell. All the complexes investigated
below are close shell and computed in the singlet spin state.
(
a) The effect of the protonation state of the molecule. The
native methylphosphonic acid bears 2 acidic positions in the
direct vicinity of phosphorus.
2
3
(b) The effect of methylation.
c) The effect of side chain substitution.
(
To determine the lowest energy conformer of each of the 5
phosphonic derivatives investigated for different protonation
The SERS experiments are carried out in water. To determine the states, several isomers of the Ag20-analyte complexes exhibiting
equilibrium geometries and Raman spectra of a large number the different binding sites or rotational angles were investigated. We
isomers of the Ag20-analyte complexes, we used the PCM model report on the most stable isomer in the text and a selection of the
and only discuss the effect of explicit solvation by H O molecules in other stable isomers and conformers are given in the SI. The
2
2
-
the case of the MePO
3
analyte. The PCM solvation model
qualitative agreement between the computed experimental SERS
spectrum with the Raman spectrum of the Ag20-analyte complex
allows us determining how the analyte will likely bind to the silver
surface in the experiments.
describes the polarization of the silver-analyte complex by the
solvent. As discussed in the SI, neglecting explicit solvation might
lead to a red shift of the P-C vibrational band and could affect the
relative intensity of the symmetric and antisymmetric P-O
stretching bands. Using explicit solvation by water molecules may
lead to hydrogen bonds between the polar groups of the analyte
(phosphonate or amine functions) and the hydrogen atoms of the
water molecules. These interactions can also affect other bonds
2
-
that are close to the polar groups. For the MePO
3
anion, we find
that adding explicit solvation triggers a shortening of the P-C bond
and a blue shift of the P-C stretching frequency. Such blue shifts
2
4
have been reported for the 1, 4-dioxane .
Results and discussion
The SERS spectrum is representative of the molecular
composition in proximity of the NP surface. Therefore, SERS
spectroscopy is an interesting tool to study the adsorption mode of
molecules on the surface of noble metal NP (here silver NP). The
most intense Raman bands may be associated to chemical functions
which are in close vicinity with the NP surface.25 However,
nanoparticle surfaces are often coated by a stabilizing agent, either
added after the synthesis or generated in situ during the synthesis,
Figure 1. Phosphonate derivatives investigated in SERS in this study.
3 3
like here with H BO . Hence, the SERS spectrum of the subsequently
(
a) The effect of the protonation of the phosphonate.
added targeted molecules will only be observed if the targeted
molecule has a larger affinity with the metallic surface than the
stabilizing agent.
In order to determine whether a molecule will be SERS active or
not, we computed the adsorption energies of the stabilizing agent
and of the phosphonate-derivative molecules investigated in this
study (see Table 1 and computational details). The adsorption
Phosphonate is functional group with three possible
a
protonated forms, which can influence its adsorption on a NP's
surface. We investigated the effect of the protonation of the
phosphonate function by recording the SERS spectra of the
methylphosphonic acid (pKa = 2.12 and pKa = 7.29) at pH=4 and
1 2
pH=9, where the phosphonate function is respectively singly
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deprotonated (MePO
3 3
H-) and doubly deprotonated (MePO 2-). In phosphonate function is found to interact with threeVieswilAvretircleaOtonmlines
the SERS spectra shown in Figure 2, the major difference occurs that are at the edge of the Ag20 cluster. In thDeOeIx:p1e0r.1i0m3e9n/Cta9lCsPp0e1c6t1r5aE,
-
1
-1
between 700 and 800 cm . The totally deprotonated form (at one observes a broad pattern between 900 and 1200 cm , with
pH=9) has a single Raman band at 760 cm-1 while the singly multiple peaks. Ligands typically bind close to defects on the
deprotonated form (at pH=4) has two Raman bands located at 730 surface of the NP because those sites are more reactive. The broad
cm-1 and 765 cm . Between 800 and 1400 cm , the Raman bands peak reflects the variety of binding sites available on the NP surface
are located at identical Raman shifts, yet the relative intensities while there is only one available for the Ag20 cluster. In addition, the
vary with the pH. These variations observed on experimental SERS solvent can interact with the molecule and change the relative
spectra should be correlated to the presence or absence of protons intensity of the symmetric and antisymmetric P-O stretching bands
-1
-1
on the phosphonate function.
To determine the adsorption site of MePO
the NP, we computed the equilibrium geometry and frequencies of 1310 cm-1 in (Ag20-MePO
as reported in the SI (Fig. S1).
The band corresponding to the methyl breathing is localized at
H- and of MePO
2-
on
3
3
-
)2-
3
H) and (Ag20-MePO
3
, which is blue
the lowest energy isomers of the (Ag20-MePO
3
)2- and (Ag20
-
shifted by about 50 cm-1 compared to the experimental band. The
-
MePO
3
H) complexes and compared their computed Raman spectra computed spectra do not reproduce the fine structure that may
to the experimental SERS spectra. The computed Raman spectra arise from the relative orientation of the methyl with respect to the
take explicitly into account the interaction of the molecule with the phosphonate function (staggered or eclipse conformation, see SI).
-
)2-
silver cluster and the interaction with the solvent via the PCM At the equilibrium geometry of (Ag20-MePO
model. We compared the computed Raman spectra of (Ag20
MePO
H)- and of (Ag20-MePO )2- to the SERS spectra of the phosphonate function but the eclipse conformation is only 2.5
3
H) and (Ag20-MePO
3
-
complexes, the methyl is staggered with respect to the
3
3
methylphosphonic acid at pH = 4 and pH = 9 (Figure 2F and B), kcal/mol less stable so both conformers might be present in
respectively. The spectra exhibit three main regions (see Figure 2) solution.
corresponding to the P-C, P-O and methyl normal modes.
The qualitative agreement between the computed Raman
As in the experimental spectrum, the computed spectrum of the spectra and the experimental SERS spectra suggests that the target
two deprotonated forms exhibit 1 and 2 Raman bands in the region molecules are adsorbed on the NP at pH=9 and pH=4 as in the
between 700 and 800 cm-1 respectively. The doubly deprotonated lowest energy isomer of (Ag20-MePO
H) and (Ag20-MePO)
-
2-. In the
3
3
form (pH=9) only has one Raman band at 725 cm (Fig. 2C-D) which complex (Ag20-MePO
3
)2- , the three oxygen atoms of the totally
-1
corresponds to a delocalized normal mode composed mainly of the deprotonated molecule interact with three silver atoms of the
symmetric stretching of the P-C. The singly deprotonated form cluster in a top configuration. The Ag-O bond lengths are between
(
pH=4) has two Raman bands at 708 cm-1 and 763 cm-1 2.29 and 2.33 Å (tridentate mode, Figure 2A). The successive
corresponding respectively to the symmetric (HO)-P-C stretching protonation of the phosphonate group leads to bidentate and
normal mode and to the anti-symmetric (HO)-P-C one (Fig. 2F). monodentate adsorption mode (see Table 1 for the Ag-O bonds)
Therefore, the number of bands between 700 and 800 cm-1 is because the interaction of a silver NP with a negatively charged
-
clearly indicative of the protonation form of the phosphonate oxygen (R-O ) is stronger than with a protonated oxygen atom (R-
function. In the two protonated forms of methylphosphonate, the OH). Furthermore, the protonation of the phosphonate function
P-C Raman band is red shifted compared to the experimental adds steric hindrance, which also modifies the adsorption geometry
because the hydrogen bonding between the oxygen of the of the molecule on the silver surface and the Raman patterns
-1
phosphonate and the water molecules is not accounted for at the between 700 and 1200 cm as discussed above.
PCM level (see SI, see Figure 2D). Adding three water molecules
close to the phosphonate functional group for the (Ag20-MePO
complex induces a blue shift of the P-C band (figure 2C). In addition,
the Raman band computed for explicit solvation by H O molecules
is broader with a shoulder peak similar to the experimental one.
3
)2-
2
2
-
Adding 25 H
Ag20 cluster) anion provides a complete solvation shell and
enhances the blue shift compared to explicit solvation with 3 H O,
2 3
O molecules around the isolated MePO (without the
2
leading to an agreement with the experimental spectrum for the
position of the band (Figure S1). We therefore expect that using a
2
-
complete solvation shell around the (Ag20-MePO
3
) would lead to a
good agreement with the experimental spectrum.
The computed Raman bands between 900 and 1200 cm-1
correspond to the symmetric and antisymmetric P-O stretching.
These vibrational modes are strongly influenced by the silver atoms
of the cluster nearby as well as by the solvent. Due to the small size
of the Ag20 cluster, there is only one binding pattern: the
4
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DOI: 10.1039/C9CP01615E
2
-
)2- and (Ag20-CH
-
Figure 2. (A) Equilibrium geometries of the adsorption mode of the analyte on the silver cluster for the (Ag20-CH
and for the trimer of stabilizing agent (Ag20-(H BO
phosphorous atom and silver for the silver atoms (the Van der Waals radii is shown). (B) Experimental SERS spectrum of methylphosphonic acid at pH = 9 where the doubly-
3
PO
3
) , (Ag20-CH
3
PO
3
-(H
2
O)
3
3
PO
3
H) complexes
3
3 3
)
)0. The color code is red for the oxygen atoms, white for the hydrogen atoms, grey for the carbon atoms, orange for the
3 3 3
deprotonated form (CH -PO
2-) dominates. (C) Lowest energy isomer of the (Ag20-MePO )2- complex that has been explicitly solvated by 3 water molecules. (D) Computed
Raman spectrum of lowest energy isomers of the (Ag20-MePO
3
)2- complex. (E) Experimental SERS spectrum of methylphosphonic acid at pH = 4 where the singly-deprotonated
-
-
form (CH
3
-PO
3
H ) dominates. (F) Computed Raman spectrum of lowest energy isomers of the (Ag20-MePO
3
H) complex.
In addition to the determination of the adsorption mode, we also into methyl hydrogen methylphosphonate and dimethyl
compared the adsorption energies of the different protonated methylphosphonate. The methylation of the phosphonate (Figure
forms of the methylphosphonic acid onto the silver cluster (Table 3) affects the adsorption modes in a way similar to that observed
1
). The doubly deprotonated acid has the largest binding energy for protonation. However, the steric effects are larger since the
due to the strong tripod interaction of the three oxygen atoms with hydrogen atom is replaced by a methyl group. As a consequence,
three silver atoms of the surface. The protonation of the the interaction of the methylated oxygen with the cluster is
phosphonate function leads to a significant decrease of the destabilized, which results in a longer Ag-OCH bond compared with
3
adsorption energy (Table 1). To be SERS active, the binding energy Ag-OH and lower binding energies (Table 1). No experimental
of the methylphosphonic acid should be larger than the one of the spectrum has been recorded because the methylated molecules
stabilizing agent (H
solution, a trimeric structure able to block one adsorption site adsorption energies that are lower (for CH
composed of three silver atoms can be formed. Therefore, three within numerical accuracy (for CH -PO (OCH
BO molecules must be displaced to accommodate the energy of the stabilizing agent (Table 1).
3
BO
3
). Since the stabilizing agent is in excess in were not SERS active, which is in agreement with the computed
) or equal
)-) to the adsorption
3
3 2
-PO (OCH )
3
2
3
H
3
3
adsorption of one methylphosphonic molecule. The doubly
deprotonated form, singly deprotonated form and the totally
protonated form respectively have an adsorption energy much
higher (+29.6 kcal/mol), similar (within the numerical accuracy) and
much lower (-8.3 kcal/mol) than the adsorption energy of the
trimeric stabilizing agent. Therefore, only the doubly and singly
deprotonated form should be active in SERS, as observed
experimentally.
3 2 3
-PO (OCH
)- and
Figure 3. Equilibrium geometries of the adsorption mode of CH
dimethyl methylphosphonate CH -PO(OCH on silver cluster.
3
3 2
)
(
b) The effect of methylation.
(
c) The effect of the side chain.
The adsorption energies were calculated to assess the steric
effect induced by the methylation of the methylphosphonic acid
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The relative affinity of the phosphonate function associated with band around 800 cm-1 while the spectrum of the θ
form only has
3
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competitive amino and carboxylate functions of the side chain was one, indicating that one of the oxygen atomDisOpI:r1o0t.1o0n3a9t/eCd9CinP0th16e15θE2
studied by investigating the adsorption of the form.
aminomethylphosphonic acid (AMPA) and the N-phosphonomethyl The AMPA molecule can be adsorbed perpendicularly to the
glycine (glyphosate). The computation of the electronic structure of cluster surface by the phosphonate or by the amine functional
AMPA and glyphosate have been performed for different group, or it can also be adsorbed by both the amine and
protonation forms of each acidic function. The adsorption energies phosphonate groups lying parallel to the surface (see SI). Electronic
are reported in Table 1.
structure computations show that the tripod adsorption by the
phosphonate is the most stable isomer (Fig. 4A). The Raman
spectrum of Ag20-AMPA(θ ) and Ag20-AMPA(θ ) where the
2 3
adsorption occurs through the phosphonate function are overall in
good agreement with the experimental one (Figure 4). The main
difference between the two charged species lies in the lowest band
Table 1. Computed Adsorption energies and selected Ag-O bond lengths of the
different protonation forms of the phosphonate-Ag20 complexes as well as of the
complexes with boric acid (DFT/wB97XD level see computational details). The
adsorption energy E
a
is defined as E
a
=(EAg20+Eanalyte)-Ecomplex
Adsorptio
Bond
length
(Å)
that is composed of a double peak for Ag20-AMPA(θ
peak for Ag20AMPA(θ ) as observed in the SERS spectra of MePO
and MePO
2-, respectively, see Figure 2 above. In the case of AMPA
2
) and a single
Molecule
n
Energy
Bond
H-
(kcal/mol)
3
3
2
.29
3
(CH
(CH
(CH
3
3
3
-PO
-PO
-PO
3
3
3
)2-
51.5
AgO
AgO
2.32
too, the computed P-POC band is red shifted because explicit
solvation effects are not taken into account and the broad Raman
2
2
.33
.39
-1
H)-
pattern between 900 and 1200 cm in the experimental spectrum
22.6
13.6
2.42
2.64
2.40
3.48
AgO-H
AgO
reflects the multiple sites of similar binding energies that are
possible at the surface of the NP.
H
2
)0
AgO-H
AgO
3
2
2
.60
.33
.41
(
CH
3
3
-PO
2
(OCH
3
))-
28.5
Ag
H
3
C-
C-
3
.25
O
O
AgO
Ag
2.50
2.86
3.62
(
(
(
(
(
CH
-PO (OCH
3
)
2
)0
17.0
49.3
21.6
37.7
19.2
H
3
2
.29
NH
2
3
-CH
-CH
2
2
-PO
3
3
)2- (θ
3
)
AgO
AgO
2.32
2
2
.33
.38
NH
-PO
)- (θ
2
)
2.41
3.21
AgO-H
AgO
2
.31
OOC-CH
2
2
-NH
2
-CH
-CH
2
2
-PO
3
3
)2- (θ
3
)
2.35
2
2
.40
.37
AgO
-
OOC-CH
-NH
2
-PO
H) (θ
2
)
2.46
2.73
AgO-H
H
3
BO
3
8.6
(
H
H
3
BO
BO
3
)
)
2
3
13.6
21.9
(
3
3
We first discuss the adsorption of the AMPA molecule. As for the
methylphosphonic acid, SERS spectra were measured at two values
of pH, at pH =7.5 and pH =10.5. However, the molecule can be
protonated not only at the phosphonate function but also on the
amine, which results in three pKa (pKa
and four protonated forms that are labeled θ
1
= 1.8, pKa
2
= 5.4, pKa
where θ
3
= 10)
is the
0
to θ
3
x
protonation form of the target molecule in which x acidic protons
are lost by the molecule. At pH=7.5 and pH=10.5, the θ and θ
2
3
forms respectively dominate in solution. The SERS spectra of AMPA
at pH=7.5 and pH=10.5 exhibit the same number of peaks at similar
Raman shifts as the methylphosphonic acid's spectra at pH = 4 and
pH = 9, which indicates that the adsorption occurs through the
2
phosphonate function. The spectrum of the θ form has a double
6
| J. Name., 2012, 00, 1-3
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Page 7 of 8
Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Journal Name
ARTICLE
Figure 4. A) Equilibrium geometries of the (Ag20-AMPA(θ
complexes. (B) Measured SERS spectrum of AMPA measured at pH=7.5, where there is
0% of the monoanionic form of AMPA θ . (C) Computed Raman spectrum of lowest
energy isomer of the (Ag20-AMPA(θ
))- complex shown in panel A. (D) Experimental
SERS spectrum of AMPA measured at pH=10.5 where the AMPA θ form dominates. (E)
))2- and (Ag20-AMPA(θ ))-
3 2
View Article Online
DOI: 10.1039/C9CP01615E
8
2
2
3
Computed Raman spectrum of the lowest energy isomer of (Ag20-AMPA (θ
3
))- complex
shown in panel A.
We now turn to the adsorption of the glyphosate, a molecule
characterized by a longer side chain than AMPA. Unlike AMPA,
glyphosate has a carboxylate function at one of its extremity, which
does not significantly alter the adsorption mode but strongly lowers
the binding energies (Table 1). The molecule has 4 acidic protons
1 2 3 4
pKa = 0.78, pKa = 2.29, pKa = 5.96, pKa
= 10.98)26 that could
(
play a role in the interaction with the silver surface. SERS spectra of
glyphosate have therefore been recorded at different pH values
ranging from 4 to 10, where silver nanoparticles are stable in
solution. Within that pH range, the intensity of the SERS signal
perfectly matches the fractional composition curve of the θ
of the glyphosate (Figure 5C), which are correlated to the
concentration of the θ form of the glyphosate in solution. It is
therefore possible to consider a quantification of glyphosate at
3
form
3
3
pH = 8 where the majority of the molecule are in the θ form in
solution.
Figure 5. (A) Equilibrium geometries of the isomers of the Ag20-Glyphosate (θ
complexes. (B) Experimental SERS spectrum of glyphosate measured at pH=8. (C),
computed Raman spectra of the complex (Ag20-glyphosate)2- (θ
). (D) fractional
2 3
and θ )
3
composition curves of the 5 forms of glyphosate in aqueous solution as a function of
pH. The red dots represent the intensity of the SERS signal.
The computed binding energies of the lowest energy isomers of
)2- and
θ
2
(OOC-CH
3
2
-NH
2
-CH
2
-PO
3
2 2 2 3
-NH -CH -PO
H)-
θ
(OOC-CH
complexes show that only the θ
3
complex should be SERS active, in
agreement with the experimental measurements. For the lowest
energy isomer, the adsorption occurs through the phosphonate
function in a tripod configuration as for the other studied
molecules. All the other adsorption modes (through the amine or
carboxylate) have significantly lower binding energies and are not
likely present in solution. The computed Raman spectrum is in good
agreement with the SERS spectrum, supporting that the adsorption
occurs through the phosphonate function. The assignment of the
vibrational modes is provided in Figure 5.
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 8 of 8
ARTICLE
Journal Name
The SERS spectra of AMPA and glyphosate are relatively similar
but differ by Raman bands specific to the side chain. For example, a
Raman band at 1293 cm-1 (NH
wagging) is observed in the SERS
spectrum of both molecules, while a new Raman band appears at
379 cm-1 (CH
wagging) only for the glyphosate. Therefore, this
family of molecules which possess similar chemical structure and
exhibit identical types of interaction with the silver cluster be could
be differentiated by their SERS spectrum and quantified in specific
pH conditions.
2
3
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Optimized SERS developments for analytical purposes are often
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number of dependent experimental parameters that can affect the
SERS signal. Although this can be effective in some simple cases, it
2
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nanoparticle substrate is in contact with aqueous media. We have 10
seen the role played by the protonation and the methylation of the
1
1
1
1
1
2
3
4
phosphonate function but also the effect of the substitution of its
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7
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a
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There are no conflicts to declare.
18
Author Contributions
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‖
G. Emonds-Alt and B. Mignolet contributed equally to this
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The authors acknowledge the support of the RU MolSys at the
University of Liège. F.R. and B.M. acknowledge support from the
Fonds National de la Recherche Scientifique, Belgium.
Computational resources have been provided by the Consortium
des Equipements de Calcul Intensif (CéCI), funded by the Fonds de
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Notes and references
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8
| J. Name., 2012, 00, 1-3
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