Potassium ion recognition by facile dithiocarbamate assembly of benzo-15-crown-5-gold nanoparticles

Article abstract of DOI:10.1039/b822734a

Here we demonstrate a facile strategy of preparing gold nanoparticle (AuNP) crown ether assemblies by the generation of strongly binding dithiocarbamate (DTC) modified benzo-15-crown-5; we also report the efficient and selective ion sensing property of th

Full text of DOI:10.1039/b822734a

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Potassium ion recognition by facile dithiocarbamate assembly
of benzo-15-crown-5–gold nanoparticlesw
Gaurang Patel, Anish Kumar, Usha Pal and Shobhana Menon*
Received (in Cambridge, UK) 17th December 2008, Accepted 26th January 2009
First published as an Advance Article on the web 20th February 2009
DOI: 10.1039/b822734a
Here we demonstrate a facile strategy of preparing gold nano-
particle (AuNP) crown ether assemblies by the generation of
strongly binding dithiocarbamate (DTC) modified benzo-15-
crown-5; we also report the efficient and selective ion sensing
property of the novel assembly for K+.
were generated with ease by treating CS2 with secondary
amines in either aqueous or organic solvents, but was found
to be most efficient in ethanol.
No work has been reported for self assembled monolayer-
protected gold nanoclusters (MPC) of crown ether dithio-
carbamate (CE-DTC) in the literature. This report mainly
focuses on the synthesis of DTC assemblies of N-benzyl-4-
aminobenzo-15-crown-5 and gold nanoparticles. DTC ligands
were formed on mixing equimolar concentration of CS2 and
the corresponding crown ether secondary amine. Subsequently
an aqueous solution of Au nanoparticles was added to the
reaction mixture with continued shaking (see Scheme 1 and
ESIw) This one-pot procedure could be performed in either
aqueous or organic solvents.
Development of molecule-based ion sensors is a subject of
considerable current interest. A useful detection scheme
should meet two requirements: a sensing moiety with satis-
factory ionic selectivity and a measurable change in response
to the recognition event. Ionophores have become an impor-
tant class of ion-selective elements since Pedersen’s discovery
of crown ethers in 19671 and a large number of crown ethers
and related compounds have been extensively developed
because of their remarkable cation-binding properties that is
termed ‘‘host–guest complexation’’.2 For optical transduction
methods, ionophores are usually coupled with a molecular
chromophore whose extinction coefficient determines the
detection sensitivity. Most chromophores are nonpolar, and
thus, their applications in aqueous media are limited. Lately,
nanosized metal particles are emerging as an important type of
colorimetric reporter because their extinction coefficients are
several orders of magnitude larger than those of organic dyes3
and because the transition of the nanoparticles from a
dispersion to aggregation leads to a distinct change in color.4–8
The phenomenon is termed surface plasmon absorption, and
the color change upon aggregation is due to the coupling of
the plasmon absorbances as a result of their proximity to each
other.9,10 By taking advantage of the mechanism of color
transformation, we report here a strategy for potassium ion
recognition by crown ether attached nanoparticles; recogni-
tion of a given ion triggers particle aggregation via simple
host–guest interactions and induces an easy-to-measure color
change.
Fig. 1A, B and C depict typical transmission IR spectra
of benzylamino-4-benzo-15-crown-5, DTC-CE and Au
Np-DTC-CE respectively. The IR spectrum of Fig. 1(A)
exhibits the characteristic features of the crown moiety at
939, 1185, 1290 and 1444 cmꢀ1; (CH2) 2941 cmꢀ1; (aromatic
ring) 982 and 751 cmꢀ1; (N–H) 3586 cmꢀ1; (NCH3) 2848 cmꢀ1
which are also observed in Fig. 1B and C with little variation.
New bands are observed for DTC-CE (Fig. 1B): (S–H)
2583 cmꢀ1, (C–S) 1048 cmꢀ1 and (CS–NH) 1218 cmꢀ1
indicating a successful synthesis of the DTC-CE ligand. For
gold nanoparticles modified with benzo-15-crown-5, (Fig. 1C)
the close resemblance of the spectral band shapes implies a
successful attachment of the crown ether onto the gold nano-
particles. The significant feature in Fig. 1C is the absence of a
DTC gained substantial attention in nanotechnology due to
the ease with which they can be synthesized and the fact that
the small interatomic distance between the two sulfur atoms of
the ligand confer strong affinity between the ligands and a
metal surface. DTC conjugated with small organic entities
have previously been shown to bind strongly on Au nano-
particles and stabilize them.11 Recent reports have demon-
strated the utility of DTC ligands in stabilizing nanomaterials,
in particular quantum dots12a and nanorods.12b These ligands
Scheme 1 Schematic of the process to generate dithiocarbamate-
crown ether (DTC-CE) ligands and AuNp-DTC-CE assemblies.
Aminobenzo-15-crown-5 and benzyl chloride were refluxed in DMF
to give the secondary amine of the crown ether. In the second step, CS2
and secondary amine-modified crown ether were reacted for 1 h to
generate the crown ether modified with a dithiocarbamate group. In
the third step, a colloidal solution of Au nanoparticles was added
leading to capping the Au nanoparticle surface with the DTC-crown
ether leading to stable AuNp-DTC-CE assemblies.
Department of Chemistry, School of Sciences, Gujarat University,
Ahmedabad, Gujarat, India. E-mail: shobhanamenon07@gmail.com;
Fax: +91 79 26308545; Tel: +91 79 26302286
w Electronic supplementary information (ESI) available: Experimental
methods and measurements. See DOI: 10.1039/b822734a
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Fig. 3 Photographs of (A) Au Np-DTC-CE and (B) after addition of
0.1 mM K+
.
co-workers10 with a range of techniques and electrodynamics
modeling. It is concluded that the change in solution color and
the degree of red shift are associated with the increase in the
size of aggregated nanoparticles10 (Fig. 4). Indeed, the strength
of the blue color decreases with time and sedimentation of a
Fig. 1 Transmission IR spectra of (A) N-benzyl-4-amino-15-crown-5,
(B) DTC-CE and (C) benzo-15-crown-5-modified gold nanoparticles
(note the disappearance of the S–H stretching in the Au Np-CTC-CE
as indicated by the arrow).
blue precipitate takes place concomitantly.13 To improve the
.
detection sensitivity, a lower concentration of crown capped
gold nanoparticles is thus employed. However, in the case
of high molar ratios of K+ to nanoparticles, the elevated
peak arising from aggregates overlaps with that at 522 nm
and makes the quantitative measurements of peak height
impossible. However the linear range for potassium, using
0.26 mM crown capped gold nanoparticles is found to be
0.05–0.95 mM (R = 0.995). The visual detection limit also lies
in the same range.
S–H (2583 cmꢀ1) stretch band, which unambiguously supports
the formation of gold–sulfur bonds.
The UV-visible spectrum of the crown capped gold nano-
particles solution shows an absorption maximum at 522 nm,
which shifts only slightly, within a few nanometers from that
of the gold nanoparticles (528 nm) (Fig. 2). After several
months storage the crown capped nanoparticles surface
plasmon band wavelengths remained the same. The crown-
capped gold nanoparticles are very miscible with water and
colloidal solutions appear red (Fig. 3A). However upon
addition of 0.1 mM K+, the solution immediately turns blue
(Fig. 3B). The significant change in color demonstrates that
the benzo-15-crown-5 capped gold nanoparticles recognize K+.
In Fig. 2, the surface plasmon absorption of a crown capped
gold nanoparticle solution is plotted as subjected to instanta-
neous exposure to increasing amounts of K+ from 0 to
0.35 mM. Na+ is added deliberately to serve as an interferent.
With the addition of 0.1 mM K+, the absorbance at 522 nm
decreases dramatically and a band around 637 nm rises. The
magnitude of this new band is small and not as striking as the
color change observed visually. This absorbance is attributed
to the coupled plasmon absorbance of nanoparticles in close
contact.10 Fig. 2 shows that, upon increasing the concentration
of K+, the intensity of the new band rises. The optical
properties have been elucidated in detail by Mirkin and
Control experiments of CS2 or secondary amine crown ether
alone with potassium salt showed no peak at 637 nm. These
results confirmed the role of the Au nanoparticles in the Au
Nps-DTC-crown ether assembly. In examining the specificity
of the crown modified gold nanoparticles it was found that the
color response is insensitive to addition of many physiologi-
+
cally important cations, such as Na+, Li+, Cs+, Rb+, NH4
,
Mg2+ and Ca2+ whereas upon addition of 0.5 mM K+ into
the solution, the color transforms immediately (Fig. S1,
ESIw). We also evaluated the stability of the AuNp-DTC-CE
assembly at different pH conditions (Table S1, ESIw). It
was observed that in solutions of low pH from 2 to 4, these
AuNp-DTC-CE exhibited lower stability and were aggregated
within a few minutes, however they were very stable in
solutions of pH 6–10 for weeks to months with no sign of
aggregation.14 Furthermore, the AuNp-DTC-crown ether
Fig. 4 TEM micrographs of benzo-15-crown-5 functionalized gold
nanoparticles (A) before and (B) after addition of 0.1 mM K+ into a
0.26 mM Au Np-DTC-CE solution. As shown in the inset to Fig. 4(A)
some shape variation occurred after round gold nanoparticle attach-
ment with crown ether. Fig. 4(B) shows the aggregation in response to
addition of K+. This state of aggregation is unaffected even after
keeping at 90 1C for 7–8 h.
Fig. 2 UV-visible spectra obtained from solutions of 0.26 mM
colloidal gold responding to various concentrations of K+. The
curves from top to bottom corresponded to progressively increasing
concentrations of K+: (A) 0 (B) 0.10 (C) 0.25 (D) 0.35 mM.
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assembly displays high stability at elevated temperatures, with
no sign of aggregation.
human serum sample where the typical concentration is
3.5–5.3 mM. The calibration curves are obtained by correla-
ting the concentration of analytes to the decrease in peak
intensity of the dispersive suface plasmon band (Fig. S2,
ESIw). The calibꢀra1tion curve shows good linearity with slope
The aggregation of crown-capped nanoparticles via
complexation with K+ is remarkably efficient, considering that
the formation constant of K+ with benzo-15-crown-5 in water
is very low (ꢀ0.05), four orders of magnitude lower than
measured in acetonitrile.15 In less polar environments, the
ethylene backbone exhibits a tendency to make contact with
the solvent, accompanied by the lone-pair electrons of oxygen
atoms being directing toward the cavity. Therefore, the crown
ether is in a partially preorganized configuration and shows a
high formation constant for cation complexation.15 In an
impedance spectroscopic study of 2-(6-mercaptohexyloxy)-
methyl-15-crown-5 self-assembled monolayers on a planar gold
surface, Reinhoudt et al.16 determined that the formation
constant of K+ with 15-crown-5 in aqueous solutions is
27100 Mꢀ1, 450 times that of Na+. The high selectivity
indicates the formation of 2 : 1 sandwich complexes with K+.
The unusually large formation constant is attributed to the less
polar environment inside the monolayers, which resembles the
aforementioned less polar solvents.17 Similarly, in the present
study, the benzo-15-crown-5 moiety lies at the interface of the
aqueous phase and dithiocarbamate bilayers. The curvature of
the nanoparticle renders a lower packing density further away
from the surface and thus creates space facilitating preorganiza-
tion of benzo-15-crown-5 for K+. Therefore, complexation of
K+ with crown modified nanoparticles is facile even in water.
In terms of the benzo-15-crown-5 capped gold nanoparticles,
two possible binding schemes for the sandwich complexation
are proposed, one of which incorporates two benzo-15-crown-5
molecules from neighboring arms of the same gold nano-
particles16 clipping a K+ ion (i.e. intraparticle association).
Alternatively, two benzo-15-crown-5 molecules may originate
from different nanoparticles, forming an intermolecular type of
association (see graphical abstract). On the one hand, the
mechanism of intraparticle K+ formation has the advantage
of possibly explaining the high association constant and Flink
et al.16 reported a high association constant for the intramono-
layer complex of K+ with 15-crown-5 self-assembled mono-
layers attached on an Au electrode. In our case, the high K+
selectivity would be attributable to the suitable configuration of
two benzo-15-crown-5 rings anchored on the nanoparticles, so
that the two adjacent crown ether rings act as a bis(benzo
15-crown-5) ether. The experimental results of aggregation of
nanoparticles by addition of K+ may possibly be explained by
the difference of the nature of the metal ions. According to the
Hofmeister series,17 the hydrophobicity of K+ is greater in
water than Na+. If almost all the benzo-15-crown-5 rings on
the nanoparticles bind the metal cations under excess addition
of them, K+ ion may lead to the aggregation of nanoparticles
more easily than Na+ ion due to van der Waals interaction in
aqueous media, resulting in precipitation. On the other hand, in
the case of interparticle association upon the addition of excess
K+, the gold nanoparticles and K+ may be networked via
sandwich complexes, followed by precipitation since the entire
network becomes water insoluble.13
of ꢀ4.4 ꢂ 10
and correlation coefficient of 0.995 for
determining K+. The serum samples were treated simply by
10-fold dilution to achieve the linearity range. The measured
concentration of K+, at 0.45 mM, was the same as by AAS
measurements. This result validates the use of crown capped
gold nanoparticles in the application of real samples.
In summary, we have demonstrated a method for enhancing
the stability of AuNp-DTC-CE assemblies by facile generation
of a DTC ligand by reacting CS2 with a secondary amine-
modified crown ether. This strategy not only increases the
robustness of the Au Np-crown ether assembly but also is
inexpensive and less tedious as compared to the procedure
reported by Chen and co-workers.13 This strategy may
contribute to improve Au nanoparticle based metal ion-
sensing systems, particularly of the physiologically important
potassium ion, found in serum.
Financial assistance from UGC, New Delhi is gratefully
acknowledged.
Notes and references
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The analytical application of crown modified gold nano-
particles is demonstrated by the quantification of K+ in
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Chem. Commun., 2009, 1849–1851 | 1851