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A. Martí et al. / Tetrahedron Letters 55 (2014) 3093–3096
O
P
The corresponding triaryl carbonium ion, which was generated
O
P
by the addition of concd HCl to DMF or acetonitrile solutions of
the ligand, showed a broad UV–vis absorption band centered at
520 nm (5 ꢀ 10ꢁ5 M in DMF).
O
O
F
F
NC
NMe2
Sarin (GB)
O
Tabun (GA)
The functionalized gold nanoparticles were synthesized by a
two-step procedure. First citrate-stabilized nanoparticles were
prepared by reducing tetrachloroauric acid with trisodium citrate
in boiling water.11 Monodisperse citrate-stabilized nanoparticles
were obtained with an average size of 13 nm, as determined by
TEM, and with a Z-potential value of ꢁ72.6 mV. The surface plas-
mon peak appeared at 526 nm. The initial concentration of the cit-
rate capped AuNPs was calculated to be 3.4 ꢀ 10ꢁ9 M according to
the Lambert–Beer’s Law from an estimated molar extinction coef-
O
P
P
O
O
NC
O
O
DCNP
DFP
Figure 1. Chemical structures of nerve agents and the simulants used in this study.
ficient of
e
= 2.47 ꢀ 108 Mꢁ1 cmꢁ1 (obtained from the plot of
e vs
nanoparticle size previously reported).7 In a second step, in a
ligand-exchange reaction the citrate was replaced from the surface
of the nanoparticles by a mixture of thioctic acid and ligand L, in an
optimized ratio of 1:2. The pH of the aqueous solution was previ-
ously adjusted to 9.0 by addition of NaOH. The functionalized AuN-
Ps (NP1) were then centrifuged and re-dissolved in DMF. These
nanoparticles were neither stable in water nor in aqueous buffer
solutions probably due to a slow dehydration of the carbinols lead-
ing to aggregation processes. This item was confirmed by studying
the behavior of the free ligand L in buffer aqueous solution by UV–
vis spectroscopy. The appearance of a new band at 510 nm corre-
sponding to the triaryl carbonium ion could be observed within
minutes.
O
P
Ar
OH
Ar
X
OR
OR
X
O
P
OR
+
OR
HO
X = F or CN
R1
R2
R1
R2
Chromophore
Chromophore
DMF solutions of the triarylcarbinol functionalized AuNPs (NP1)
exhibited the characteristic surface plasmon resonance (SPR) band
at 526 nm in the UV–vis spectrum. As expected, the presence of
DCNP or DFP promoted a decrease in the intensity of this peak
and the appearance of a new peak at around 640 nm indicating
the formation of AuNP clusters. These results are consistent with
the phosphorylation of the tertiary alcohol followed by an elimina-
tion reaction to generate the corresponding carbocation as shown
in Figure 2.
Figure 3 shows the results of UV–vis titration studies performed
with DMF solutions of our material in the presence of increasing
amounts of DCNP and DFP. These changes in the spectra are con-
comitant with a change in the color of the solution from red to dark
blue.
The variation of the ratio of the absorbance intensities of NP1 at
640 nm and 526 (A640/A526) versus DCNP or DFP concentration is
also presented (insets of Fig. 3). A very significant increase in the
A640/A526 ratio was observed as the concentration of stimulant
was increased. The limits of detection (LODs) expressed in ppm
(v/v) obtained from these plots were 560 for DCNP and 465 for DFP.
To achieve a better understanding of the reaction, kinetic stud-
ies on the reactivity of the nanoparticles NP1 in the presence of
DFP were carried out in DMF solutions by using an excess of the
simulant. The changes in the absorbance intensity of the aggrega-
tion band (A640) in the UV–vis spectra versus the reaction time are
shown in Figure 4. As expected, an increase in the absorbance with
time is observed until a plateau is reached at about 9 min of
reaction.
Figure 2. Paradigm of the sensing mechanism using functionalized anionic AuNPs:
reaction of the terminal ligands with the simulant produces positive charges which
can compensate the negative charges of the nanoparticles inducing their aggrega-
tion (top). Colorimetric sensing using triarylcarbinols: The hydroxyl group under-
goes phosphorylation, followed by elimination to generate the corresponding
colored carbocation (bottom).
process.9 Therefore we believe that this is a field that requires dee-
per study.
Triarylcarbinol 1 (Scheme 1) was synthesized from 2-(4-bromo-
phenoxy)ethanol, 4-(dimethylamino)benzophenone, and BuLi in
THF. The carbinol was obtained in a 50% yield after column chro-
matography. In order to attach the triarylcarbinol to the surface
of the AuNPs, taking advantage of the strong affinity of gold for
disulfide groups, the less hindered primary alcohol was esterified
with thioctic acid in the presence of DCC and DMAP10 to yield
ligand L. The chemical structure and purity of L were confirmed
by spectroscopic techniques (see SI). The absence of absorption
bands in the visible region of the UV–vis spectrum is noteworthy,
indicating the absence of carbocation from dehydration reactions.
O
O
S
O
S
HO
N
It is well known that cyanide anions are capable of dissolving
metals such as Au and Ag in the presence of oxygen upon the for-
mation of soluble metal-cyanide complexes.12 In order to evaluate
if the cyanide which is released upon the reaction of our material
with DCNP has influence in the obtained results, we studied the
behavior of DMF solutions of NP1 in the presence of an excess of
KCN.
TEM studies proved that there had been a gradual dissolution of
the gold nanoparticles due to the etching by cyanide, reaching an
average size of ca. 4–6 nm, which was not observed for solutions
of NP1 exposed to DCNP (Fig. 5). Additionally, the UV–vis spectra
L
N
O
OLi
Li
,THF
HO
O
2) H3O+
OH
N
O
1
Scheme 1. Ligand L to be attached to the AuNPs and synthesis of triarylcarbinol 1.
Martí, Almudena
