Enhanced sensitivity and selectivity of chemosensor for malonate by anchoring on gold nanoparticles

Article abstract of DOI:10.1002/cjoc.201190118

A novel organic-inorganic nanohybrided receptor 4 functionalized with bis-thiourea arms and then assembled on gold nanoparticles was synthesized. The preorganized system possesses phenylthiourea units for the spectrophotometric sensing of dicarboxylates, especially malonate, based on changes in the surface plasmon absorption of the gold nanoparticles (GNPs). The intensity of the absorbance band increases gradually with the concentration of dicarboxylates increasing. But such an ion-selective change in the plasmon band was not observed in control tests carried out by mono-thiourea-modified GNPs, receptor 3 and the free receptor 2. As it is shown from the association constants, derived from quantitative titrations, receptor 4 can selectively recognize dicarboxylate anions of shorter carbon chain, and has the highest affinity to malonate. The interaction properties for anions of receptor 4 were evaluated by ¹H NMR and UV-vis spectroscopic methods.

Full text of DOI:10.1002/cjoc.201190118

FULL PAPER
Enhanced Sensitivity and Selectivity of Chemosensor for
Malonate by Anchoring on Gold Nanoparticles
Liu, Yanyun(刘彦运)
Li, Xiangyang(李向阳)
Liu, Yun(柳昀)
Liang, Zhongshi(梁重时)
Liu, Shunying*(刘顺英)
Yu, Jiahui*(余家会)
Institute for Advanced Interdisciplinary Research, East China Normal University, Shanghai 200062, China
A novel organic-inorganic nanohybrided receptor 4 functionalized with bis-thiourea arms and then assembled
on gold nanoparticles was synthesized. The preorganized system possesses phenylthiourea units for the spectropho-
tometric sensing of dicarboxylates, especially malonate, based on changes in the surface plasmon absorption of the
gold nanoparticles (GNPs). The intensity of the absorbance band increases gradually with the concentration of di-
carboxylates increasing. But such an ion-selective change in the plasmon band was not observed in control tests
carried out by mono-thiourea-modified GNPs, receptor 3 and the free receptor 2. As it is shown from the association
constants, derived from quantitative titrations, receptor 4 can selectively recognize dicarboxylate anions of shorter
carbon chain, and has the highest affinity to malonate. The interaction properties for anions of receptor 4 were
evaluated by 1H NMR and UV-vis spectroscopic methods.
Keywords thiourea, gold nanoparticle, assembly, manolate, sensor
Introduction
synthetic receptors for anions have been designed, rela-
tively few inorganic-organic nanohybrids optical sens-
ing systems, especially GNPs-based optical sensing,
have been developed for the selective sensing of
short-chain terminal dicarboxylate.
Anions play a fundamental role in a wide range of
chemical, biological, industrial, and environmental bio-
logical processes, and numerous efforts have been de-
voted to the development of a biotic receptor for anionic
species.1 Especially, given the important role of dicar-
boxylates in biology,2 such as their critical roles in nu-
merous metabolic processes, the need for their selective
recognition by synthetic sensors utilizing weak hydro-
gen bonding interactions is of great importance in mo-
lecular recognition research.3 Also, to assemble selec-
tive artificial ditopic receptors for terminal dicarboxy-
lates, based on trans-pyridylcinnamide,4 anthraquinone,5
and carbohydrate,6 some classical molecular skeletons
were rationally designed and exquisitely synthesized.
Also a number of receptors with a range of spacer
groups for neutral dicarboxylates of different chain
lengths are reported in the literatures.7 Ghosh and
Masanta8 have designed and synthesized two triphenyl-
amine-based PET chemosensors for the first time, for
size selective recognition of aliphatic dicarboxylic acids.
Over the last few years, we have also developed several
fluorescent sensors based on cholic acid which are
enable to sensitively detect long-chain terminal
dicarboxylate.7a,9 At the same time, a large number of
anion receptors have been reported which have thiourea
subunits acting as anion binding sites, owing to their
ability to act as H-bond donors. While a number of
In recent years, surface functionalization of GNPs
has attracted growing attention due to the singular
physical properties of gold nanoparticles, the possibility
of cooperativity between receptor sites at the surfaces
and the practicality of device fabrication. The optical
properties of gold nanoparticles are dominated by the
changes in the surface plasmon resonance (SPR) which
is dependent on the aggregation state, surface morphol-
ogy, and so on.10 The surface plasmon band (SP band) is
highly sensitive to changes in the environment near to
the nanoparticles surface, such as the composition of the
self-assembled monolayer (SAM) on the nanoparticles11
and the solution around the nanoparticles.12 This sensi-
tivity has been widely exploited in surface plasmon
resonance devices, which were produced by Biacore,13
and several types of anion sensing gold nanoparticles
have been reported that exploited the changes occuring
upon nanoparticle aggregation where the intensity of
this band decreases and the peak shifts to a longer
wavelength, resulting in a color change from red to pur-
ple or blue.14 For example, this method has been re-
cently used for the detection of various substances in-
cluding DNA,15 metal ions16 and proteins.17 However,
*
E-mail:syliu@sist.ecnu.edu.cn, jhyu@sist.ecnu.edu.cn; Tel./Fax: 0086-0021-62237226
Received May 19, 2010; revised October 19, 2010; accepted November 15, 2010.
Project supported by the National Natural Science Foundation of China (No. 20902026), the Shanghai Municipality Commission for Special Project
of Nanometer Science and Technology (No. 0952nm05300), the Shanghai Municipality Commission for Non-governmental International Corpora-
tion Project (No. 09540709000), and the East China Normal University (No. 78210032).
Chin. J. Chem. 2011, 29, 531— 538
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Liu et al.
FULL PAPER
there are few gold nanoparticles (AuNPs)-type optical
probes for dicarboxylate species in biology. As gold
nanoparticles have tremendously high molar absorption
in the visible region, they must have considerable po-
tential for optical detection of small anions and mole-
cules such as dicarboxylate.
7.3 Hz, 2H, ArH), 7.11 (d, J＝7.7 Hz, 2H, ArH), 3.79 (s,
4H, NHCH2), 2.96 (t, J＝6.7 Hz, 4H, CH2S); FT-IR
(KBr) v: 3378 (m), 3173 (m), 2934 (m), 1650 (s), 1525
＋
－1
(s), 1245 (w), 686 (w) cm ; MS m/z: 423 (M＋H) .
Anal. calcd for C18H22N4S4: C 51.15, N 13.26, S 30.35;
found C 51.21, N 13.33, S 30.34.
In our research, for selective and sensitive sensors of
biologically important anions, dicarboxylate, we re-
cently developed a structurally simple AuNPs-based
optical sensing systems functionalized with two thiourea
arms acting as a dual binding site. It is an ideal receptor
for short chain dicarboxylate anions that present two
proximate oxygen atoms.
Receptor 2 A solution of phenyl isothiocyanate
(8.1 g, 60 mmol) in 5 mL of absolute DMF was drop-
wise added to cystamine core PAMAM dendrimers of 1
generation (4.96 g, 10 mmol) in 20 mL of DMF and the
mixture was stirred in nitrogen atmosphere for 2 h at 60
℃. The mixture was evaporated to yield the crude
product as light yellow oil and the residue was sus-
pended in ethyl acetate, and the precipitate was filtered
off quickly. The crude product was dried in vacuo and
Experimental
1
used without further purification in the next step. H
Materials and general procedures
NMR (DMSO-d6, 500 MHz) δ: 9.58 (s, 4H, PhNHCS),
8.02 (s, 4H, CONHCH2), 7.71 (s, 4H, CH2CHCS), 7.38
(d, J＝7.9 Hz, 8H, ArH), 7.29 (t, J＝6.8 Hz, 8H, ArH),
7.09 (t, J＝6.8 Hz, 4H, ArH), 3.55—3.60 (m, 8H,
CH2CHCS), 3.21—3.24 (m, 8H, NHCH2CH2), 2.60—
2.67 (m, 8H, SCH2CH2N), 2.62 (t, J＝6.7 Hz, 8H,
NCH2CH2), 2.49, (t, J＝6.7 Hz, 8H, CH2CH2CO); IR
(KBr) v: 3260 (s), 3056 (m), 2934 (m), 2850 (m), 1660
Chemicals were of commercial grade and used
without further purification unless otherwise stated.
Cystamine and cystamine core PAMAM dendrimers of
1st generation were purchased (as methanol solutions or
solids) from Sigma-Aldrich. Dicarboxylate tetrabu-
tylammoniums were prepared from their respective ac-
ids by reacting with 25% tetrabutylammonium hydrox-
ide aqueous solution and other tetrabutylammonium
salts were purchased from Sigma-Aldrich. UV-vis ab-
sorption spectra were taken on a UV-1900 spectrometer.
Background correction was made with neat solvent (2
mL) in the reference cell and an empty sample cell.
HR-TEM images were recorded on a JEOL 2100 mi-
croscope operating at the working voltage of 100 keV.
Samples for TEM were prepared by spreading a drop of
aqueous solution of nanoparticles onto standard carbon
coated copper grids. 1H NMR (δ) were given relative to
trimethylsilyl (TMS) as internal standard and were re-
corded using a Bruker Avance 500 spectrometer at 500
MHz. IR spectra were obtained by using a instrument of
a Nicolet Nexus 670. Elemental analyses were per-
formed by an Elementer Varioel 3.
－1
(s), 1542 (s), 1247 (w), 704 (w) cm ; MS m/z: 1148
(M－H)＋. Anal. calcd for C52H72N14O4S6: C 54.33, N
17.16, S 16.74; found C 54.41, N 17.25, S 16.76.
Preparation of thiourea arms-modified GNPs
The thiourea arms-modified receptors 3 and 4 were
prepared according to a literature method for preparing
thiol-capped GNPs.18 0.44 g (1 mmol) of HAuCl4 solu-
tion was taken up in 25 mL of H2O. To this solution,
was added 1.29 g (3.2 mmol, 3.2 equiv.) of methyl
trioctylammonium chloride in toluene (80 mL). This
mixture was stirred for 2 h until the tetrachloroaurate
was transferred into the organic phase (the aqueous
phase became progressively colorless and the organic
phase became dark yellow). Whilst cooled with an ice
bath, 1 or 2 (0.25 mmol, 1 equiv.) was added to the so-
lution. NaBH4 (0.19 g, 5 mmol) in 100 mL of H2O was
added dropwise to this solution and the solution turned
inky black quickly. After stirring for 1 h, the organic
layer was separated and concentrated to 10 mL. 200 mL
of MeOH was added and the mixture solution was
cooled to －20 ℃ for 16 h. The solution was filtered
and the resulting solid was washed with 50 mL×2 of
MeOH and 50 mL×2 of acetone. The recrystallization
and washing were repeated to yield a waxy black solid
GNP 3 and 4.
Syntheses
Receptor 1 0.226 g (1 mmol) of cystamine dihy-
drochloride was dissolved in 25 mL of CH2Cl2. 0.5 g (5
mmol) of triethylamine was dropwise added. After the
addition of 1/4—1/2 triethylamine, the cloudy, white
suspension became clear. To this stirred solution was
added 0.27 g (2 mmol) of phenyl isothiocyanate. The
reaction mixture was allowed to stir at room tempera-
ture under a nitrogen atmosphere overnight. The organic
phase was washed with ultra pure water (50 mL×2),
and dried over anhydrous sodium sulfate followed by
filtration. The filtrate was evaporated in vacuo affording
the crude product as a light yellow power. The crude
product was purified via silica gel flash chromatography
using a solvent gradient (CH2Cl2→2% EtOAc/98%
CH2Cl2→10% EtOAc/90% CH2Cl2). 1H NMR (DMSO-
d6, 500 MHz) δ: 9.66 (s, 2H, PhNHCS), 7.88 (s, 2H,
CH2NHCS), 7.39 (d, J＝7.7 Hz, 2H, ArH), 7.32 (t, J＝
Receptor 3 1H NMR (DMSO-d6, 500 MHz) δ:
9.56 (s, 2H, PhNHCS), 7.81 (s, 2H, CH2NHCS) 7.39 (d,
J＝7.5 Hz, 2H, ArH), 7.28 (t, J＝7.3 Hz, 2H, ArH),
7.11 (t, J＝7.7 Hz, 1H, ArH), 3.72—3.75 (m, 4H,
NHCH2), 2.96 (t, J＝6.5 Hz, 4H, CH2S); FT-IR (KBr) v:
3378 (m), 3173 (m), 2923 (m), 1656 (s), 1530 (s), 1245
－1
(w), 706 (w) cm . Elemental founded for receptor 3: C
11.28, N 2.36, S 7.071.
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Chin. J. Chem. 2011, 29, 531— 538

Enhanced Sensitivity and Selectivity of Chemosensor for Malonate
Receptor 4 1H NMR (DMSO-d6, 500 MHz) δ:
9.60 (s, 4H, PhCSNH), 8.02 (s, 4H, CONHCH2), 7.72 (s,
4H, CH2CHCS), 7.38 (d, J＝7.8 Hz, 8H, ArH), 7.29 (t,
J＝6.8 Hz, 8H, ArH), 7.11 (t, J＝6.8 Hz, 4H, ArH),
3.50—3.53 (m, 8H, CH2CHCS), 3.21—3.24 (m, 8H,
NHCH2CH2), 2.69—2.73 (m, 8H, SCH2CH2N), 2.64 (t,
J＝6.4 Hz, 8H, NCH2CH2), 2.49 (t, J＝6.3 Hz, 8H,
CH2CH2CO); FT-IR (KBr) v: 3259 (s), 3049 (m), 2843
－1
(m), 1650 (s), 1515 (s), 1252 (s), 700 (m) cm . Ele-
mental founded for receptor 4: C 10.63, N 2.61, S 3.51.
Results and discussion
Syntheses
A simple method for one-step synthesis of receptors
1 and 2 has been developed and outlined in Scheme 1.
They were synthesized just via dropwise addition of a
dichloromethane solution of corresponding equivalents
of phenyl isothiocyanate to DMF of cystamine or cys-
tamine core PAMAM dendrimers of 1st generation,
which afforded the product as a light yellow powder in
Figure 1 HR-TEM images of receptor 4. The inset: the size
distribution of the receptor measured from DLS.
－5
－1
DMSO (c＝5.0×10 mol•L based on the free re-
ceptor), all have characteristic absorption spectra, owing
to its absorption of surface plasmons (SP), while sur-
prisingly the same concentration of thiourea receptors 1
and 2 do not show any absorption peak at the UV-vis
region of 200—900 nm in our test. It indicated that the
UV-vis absorption of the receptors 3 and 4 was much
stronger than the free receptors 1 and 2 in DMSO and
this maybe lead to an increased sensitivity of the sens-
ing of the nanohybided receptors to anions. In our re-
search, the GNPs, based on its surface plasmon absorp-
tion, act as a signaling subunit instead of organic fluo-
rescent groups most of which are not friendly to the
environment. This is consistent with previously reported
observations of enhanced anion binding in the GNPs
adsorbed porphyrin receptors.19
1
85% yield. The receptors were characterized by H
NMR, FT-IR and mass spectrometry.
The synthesis of thiourea-arm-functionalized gold
nanoparticles as illustrated schematically in Scheme 1
was carried out via the Brust method,18 which was re-
ported as a procedure for thiol-capped GNPs (Thio-
GNPs). Hydrogen tetrachloroaurate was transferred
from aqueous solution into toluene by use of Aliquat®
336, a phase transfer agent, to form a yellow solution.
And it was necessary to add sodium borohydride to the
reaction mixture to ensure that a significant number of
receptors 1 and 2 were adsorbed onto the surface of the
gold nanoparticles by the formation of Au—S band. The
receptors 3 and 4 were characterized by the following
The recognition behavior of receptor 4 towards ani-
ons was investigated by measuring the UV-vis spectra
upon the addition of several anions as their tetrabu-
tylammonium salts. The resulting absorption spectra
around the SP band of the thiourea-capped receptor 4 in
the presence and absence－of different anions (1 equiv.),
1
techniques: H NMR and mass spectrometry, high-
resolution transmission electron microscopy (HR-TEM),
UV-vis spectroscopy, elemental analysis, and FT-IR.
All above indicated that thiourea surface adsorption had
occurred successfully. The average diameter [about
(3.2±1.1) nm] of the GNP measured from its HR-TEM
image is given in Figure 1, and the inset shows the size
distribution of the GNP measured from its dynamic light
scattering (DLS), indicating that nearly mono-dispersed
GNP was successfully obtained. Receptors 3 and 4 can
stand stably more than one month with a very good
monodispersed distribution in DMSO, but can not be
stable in pure water which is possibly due to the hydro-
phobic phenyl group on the surface. Consequently, all
the following experiments were performed in DMSO.
The number of ligands per gold nanoparticle was calcu-
lated by its elemental analysis. From the sulphur content
and its average diameter, assuming the nanoparticles to
be perfect spheres, it can be estimated that there are
about 33 ligands per nanoparticle in this case.
－
－
－
－
such as F , Br , I , AcO , H2PO4 and dicarboxylates
anions [－OOC(CH2)n COO－, n＝1, 2, 3, 4, 5 and 8],
were depicted in Figure 2. As can be seen from Figure 2,
the absorption of receptor 4, especially treated with di-
carboxylates, is greater in magnitude than that of other
anions tested here. When receptor 4 was titrated with
the bis-anions such as malonate, succinate, glutarate,
and pimelate, the absorption band at 600 nm was sub-
stantially －enhanced, but upon addition of mono-anion
－
－
－
－
such as F , Br , I , AcO , and H2PO4 , a very slight
change of SP band (enhancement) was observed. This
demonstrated that the SP band changes of receptor 4
treated with dicarboxylates were attributed to the inter-
action of 2 on the gold particles with the guests, rather
than to the change of the ionic strength or the charge of
the environment. The enhancement of the SP band is
possibly due to the conformational change of the immo-
bilized 2 on the nanoparticle surfaces when reacted with
the bis-anions by two binding units. While for the
UV-vis anion binding studies
Thiourea-capped receptors 3 and 4, dispersed in
Chin. J. Chem. 2011, 29, 531— 538
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Liu et al.
FULL PAPER
Scheme 1 The synthesis of anion receptors 1, 2, 3, and 4
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Chin. J. Chem. 2011, 29, 531— 538

Enhanced Sensitivity and Selectivity of Chemosensor for Malonate
indicated that the anion-binding event, bis-thiourea ab-
sorbed on the GNPs, would cause some perturbation in
the SP band. However, no obvious change of the
UV-vis spectra of receptor 2 was observed when treated
with any anion in our research, which was probably at-
tributed to the strong solvation effect of DMSO on re-
ceptor 2. The results indicated that the sensitivity of the
sensing of the nanoparticles adsorbed thiourea receptor
4 was significantly enhanced as compared with the free
receptor 2 in DMSO. And as shown in the Figure 5, the
bis-thiourea receptor 4 showed a selective binding abil-
ity to dicarboxylates, which is unsurprising given that
the hydrogen bonding alignment for dicarboxylates and
bis-thiourea are complementary. A bis-thiourea subunit
can donate two hydrogen bonds according to a bifurcate
mode and is therefore an ideal receptor for Y-shaped
anions, just as dicarboxylate anions.20 Shao and her
co-workers21 pointed out that the selectivity for special
analyte of the host molecule would be rationalized on
the basis of not only the guest basicity but also shape
complementarity between the host and the anionic
guests.
－
Figure 2 UV-vis spectra of 4 (3×10－ mol•L ) in the pres-
5
1
ence and absence of different anions (3×10－ mol•L－ 1) in
5
DMSO.
mono-anions, only one binding unit between the host
and the guest has a very slight effect on the conforma-
tion of the immobilized 2. The results also implied that
receptor 4 would be served as a chemosensor for some
special bis-anions by the change of the SP band rather
than by the chromophore, which possibly affords a more
sensitive sensing pathway. Job plots showed that recep-
tor 4 formed 1∶1 complexes with dicarboxylate anions.
Figure 3 presented the typical job plot of receptor 4 with
malonate in DMSO.
Figure 4 UV-vis spectra of 4 with different amounts of malo-
nate in DMSO. The inset: the change of UV-vis spectra of 4 with
addition of malonate at 600 nm. Amounts of malonate: 1, 0; 2,
0.34; 3, 0.68; 4, 1.0; 5, 1.3; 6, 1.6; 7, 2.0; 8, 5.0 equiv.
Figure 3 Job plots for the complexation of 4 with malonate in
－
DMSO. The total concentration is 3×10－ 5 mol•L .
1
Being due to the slight change of the UV-vis spectra
with treatment of mono-anion, the titrations of receptor
4 to these anions were not carried out. The binding
abilities of receptor 4 with dicarboxylates were assessed
by UV-vis spectroscopic titration. And the UV-vis titra-
tions were carried out in DMSO at a concentration level
－5
－1
of 5.0×10 mol•L based on the free receptor. The
result showed that the SP absorption band obviously
increased with incremental addition of 0.1 equiv. of
tetrabutylammonium dicarboxylates up to 20 equiv.
with a blue shift of about 10 nm. Figure 4 presented the
typical UV-vis spectra change of receptor 4 with the
addition of manolate in the range of 300—750 nm. This
Figure 5 UV-Vis titration curves for 4 with different anions in
DMSO (measured at 600 nm).
Chin. J. Chem. 2011, 29, 531— 538
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Liu et al.
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Association constants of receptor 4 for dicarboxylate
anions which is based on a 1∶1 (anion∶receptor)
stoichiometric model of bis-anions were calculated by
using EQNMR, a least squares fitting software package,
and were displayed in Table 1 with a good correlation
coefficient (R＞0.98).22 Job plot analysis (Figure 3)
showed that the dicarboxylate ∶ host binding stoi-
chiometry was 1∶1, as the maximum between each
anion and thiourea on the gold nanoparticle surfaces
closes to 0.5. The association constants determined by
UV-vis titration, illustrated that receptor 4 had a very
high selective binding ability for dicarboxylates over
mono-anions. Receptor 4 can bind dicarboxylate anions
in the order of malonate ＞ succinate ＞ glutarate ＞
adipate＞pimelate＞sebacate. In particular, the thio-
urea-modified receptor 4 complexed dicarboxylate ani-
ons with a high selectivity for malonate (Kass＝9.17×
dicarboxylate anions depends on the match of the chain
length of the anions and the distance between two arms
of receptors.23,27 So according to the chain length and
size, among these dicarboxylate anions, malonate is
found to be best fit for the incorporation into the cavity
of receptor 4. But based on the UV-vis spectroscopic
titration, free receptor 2 showed a too week binding
ability to any anion in DMSO to calculate the constants.
This suggested that the significantly enhanced sensitiv-
ity and selectivity of receptor 2 to malonate were due to
the effective assembly of 2 on the surface of the gold
nanoparticles. To further verify the geometrical factors,
we also synthesized one arm thiourea functionalized
receptor 3 derivatived from 1. There was no significant
change in the SP absorption treated with any anions in
the control tests carried out by mono-thiourea-modified
GNPs. This illustrated that the intramolecular rather
than intermolecular interactions happened between 2
immobilized on the gold nanoparticle surface and di-
carboxylates. This control test also further implied that
the UV-vis spectrum change (SP band) of receptor 4
treated with dicarboxylate was attributed to the confor-
mational change of receptor 2 on the surface of gold
nanoparticles.
－1
105 mol•L ) much over succinate, glutarate, adipate,
pimelate and sebacate, even in the presence of 5 equiv.
of AcO－ in DMSO (data not shown). Since the first
artificial receptor for dicarboxylate anions by Lehn and
Hosseini23 in 1982, numerous excellent receptors/sen-
sors for dicarboxylate anions have been reported.4-9 We
have also reported a cholic acid-based sensor for seba-
cate9 and adipate.24 But most of these artificial recep-
tors/sensors have a high selectivity for a relative
long-chain aliphatic or aromatic dicaroxylates, and few
literatures reported an artificial sensor for malonate,
which has a short chain between the two binding units.
Receptor 4 presents an optically linear response to
1H NMR titration experiments
1H NMR spectroscopy is of immense value in the
understanding of receptor-substrate interaction and is
capable of providing a revealing picture of the details of
the interaction between receptor and dicarboxylate ani-
－5
1
ons.28 So H NMR experiments were also carried out to
manolate in the range of 0—1.0 equiv. (5.0×10
－1
1
mol•L , the inset in Figure 4), which is quite more
monitor the anion recognition process. And the H
sensitive than electrochemical sensing. Robinson et al.25
have studied the physicochemical properties of sodium
malonate by using electrochemical method, and Hefter
et al.26 further reported a relatively sensitive electro-
chemical method, dielectric relaxation spectra (DRS), to
detect the concentration of sod－ium malonate (the detec-
NMR spectra of receptor 4 were recorded using
DMSO-d6 in the presence and absence of malonate to
investigate the interaction between the host and the
1
guest. The H NMR spectra of 4 reacted with 1 and 2
equiv. of TBA malonate were shown in Figure 6.
1
tion limit is up to 0.025 mol•L ). Compared with these
electrochemical sensing, the UV-vis optical sensing
based on receptor 4 gives a much more sensitive selec-
tivity for manolate.
Table 1 The associate constants (Kass) of receptor 4 towards
anions in DMSO
Receptor 4
Anion
－
1
Kass/(L•mol )
R
－
－
－
－
－
F , Br , I , AcO , H2PO4
not be calculated
malonate
(9.17±0.070)×105
(6.07±0.035)×105
(5.32±0.064)×105
(3.49±0.083)×105
(9.09±0.085)×104
(4.00±0.060)×104
0.9889
0.9829
0.9922
0.9964
0.9857
0.9956
－
1
Figure 6 Plots of H NMR spectra of 4 ([4]＝2×10 3 mol/L)
on addition of malonate in DMSO-d6. (a) 4 only; (b) [malo-
nate]/[4]＝1∶1; (c) [malonate]/[4]＝2∶1.
succinate
glutarate
adipate
pimelate
As expected, large downfield shifts in the resonance
sebacate
for the two N—H thiourea protons were observed. In
1
the H NMR spectrum of 4, the proton chemical shifts
of thiourea (Ha, Hb) shifted from δ 9.60 to 10.22 (∆δ＝
As we know, the recognition ability of receptor 4 for
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Chin. J. Chem. 2011, 29, 531— 538

Enhanced Sensitivity and Selectivity of Chemosensor for Malonate
0.62), and from 7.72 to 7.93 (∆δ＝0.21) respectively
upon addition of 2 equiv. of malonate. These larger
downfield shifts indicated that the thiourea moiety was
involved in the receptor-anion binding event, suggesting
the formation of complexes in which both thiourea arms
were involved. However, the proton signal of Hc did not
shift notably proving that Hc did not participate in the
formation of hydrogen-bonding. All of these results in-
dicated that the immobilized 2 on the particle surfaces
reacted with dicarboxylate anions by two binding units
and formed 1∶1 complexes. Above all, a probable
structure for the complex of 4 with manolate is shown in
Scheme 2.
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In conclusion, we have synthesized two GNP-based
receptors 3 and 4 and investigated their binding proper-
ties towards mono-anions and bis-anions, such as di-
carboxylates of various chain lengths. Receptor 4 func-
tionalized with two arm thioureas has a higher selectiv-
ity for dicarboxylate a－nions than－other simple mono-
－
－
－
anions, such as F , Br , I , AcO , and H2PO4 . And
for geometrical reasons after being assembled on gold
nanoparticles, among the six aliphatic dicarboxylic ac-
ids (malonate, succinate, glutarate, suberate, pimelate,
sebacate), the maximum association constant was ob-
served for malonate. It is noteworthy that receptor 4
(two-arm thiourea 2 immobilized on the nanoparticle
surface) exhibited significantly enhanced anion sensing
sensitivity as compared with the free receptor 2 and
one-arm thiourea-functionalized 3 in DMSO. The en-
hancing sensitivity and selectivity were due to the SP
band increase of the nanoparticles which was induced
by the conformational change of the immobilized 2 on
the surface. The sensitivity and selectivity of or-
ganic-inorganic hybided nanoparticle 4 for malonate
have been notably improved versus that of most elec-
trochemical sensors and the free receptor. This also pro-
vided an insight into the design of chemosensors for
anions based on the assembly of organic receptor on the
inorganic nanoparticles.
Chin. J. Chem. 2011, 29, 531— 538
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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FULL PAPER
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 531— 538