Luminescent silole nanoparticles as chemoselective sensors for Cr(VI)

Article abstract of DOI:10.1021/ja052582w

Colloidal suspensions of 3-aminopropylmethyl(tetraphenyl)silole nanoparticles can be used as selective chemosensors for carcinogenic chromium(VI) analyte. Methylhydrosilole is functionalized by hydrosilation of allylamine, and the colloid is prepared by t

Full text of DOI:10.1021/ja052582w

Published on Web 07/29/2005
Luminescent Silole Nanoparticles as Chemoselective Sensors
for Cr(VI)
Sarah J. Toal, Kelsey A. Jones, Douglas Magde, and William C. Trogler*
Contribution from the UniVersity of California, San Diego, 9500 Gilman DriVe,
La Jolla, California 92093-0358
Received April 20, 2005; E-mail: wtrogler@ucsd.edu
Abstract: Colloidal suspensions of 3-aminopropylmethyl(tetraphenyl)silole nanoparticles can be used as
selective chemosensors for carcinogenic chromium(VI) analyte. Methylhydrosilole is functionalized by
hydrosilation of allylamine, and the colloid is prepared by the rapid addition of water to a THF solution of
the silole. The method of detection is through electron-transfer quenching of the fluorescence of the silole
colloid (λem ) 485 nm at 360 nm excitation) by the analytes, with hundred parts per billion detection limits.
Stern-Volmer plots are linear up to 10 ppm in the case of chromium, but exhibit saturation behavior near
5-10 ppm for arsenic. Dynamic light scattering experiments and AFM measurements show the particle
sizes to be around 100 nm in diameter and dependent on solvent composition, with a particle size dispersity
of (25%. The fluorescence lifetimes of the silole in solution and colloid are ∼31 ps and ∼4.3 ns, respectively,
while the silole has a lifetime of 6 ns in the bulk solid. A minimum volume fraction of 80% water is necessary
to precipitate the colloid from THF, and the luminescence continues to rise with higher water fractions.
Colloids in a pH 7 phosphate-buffered suspension show both higher sensitivity and greater selectivity (100-
2-
-
-
2-
-
fold) for CrO
4
detection than for other oxoanion interferents, NO
3
, NO
2
, SO
4
4
, and ClO .
1
1,12
electrochemical processes,13,14 and
Introduction
coupled plasma-MS,
electron capture GC.
1
5,16
Methods using luminescence quenching
Detection of chromium(VI) and arsenic(V) in drinking water
is important because both species pose serious health risks and
are regulated by the U.S. Environmental Protection Agency.
In aqueous solutions at pH 7, CrO4 , which is isostructural
with the sulfate ion, SO42 , is carried into mammalian cells by
for detecting chromate have also been used. These include
bacterial sensors with detection limits as low as 2.6 ppb.
Dynamic quenching of luminescence in lanthanide complexes
by chromate and nitrite using ion chromatography can detect
3 ppb of the anions. A synchronous absorbance and
fluorescence analysis of dynamic liquid drops at the end of a
flow injection capillary, monitoring the decrease in fluorescence
caused by the reaction between chromate and 3,3′,5,5′-tetra-
methylbenzidine dichloride, can detect chromate at the femto-
1
7
18
1
2
-
-
1
9
1
2-
sulfate transporters, where the CrO4 oxidatively damages
2
DNA. Similarly, the predominant environmental form of As-
3
-
(V) in oxygenated water, AsO4 , competes in cellular uptake
3- 3
with isoelectronic phosphate, PO4
arsenate may be introduced into drinking water through
industrial processes, as well as by environmental erosion. The
EPA has set the Maximum Contaminant Level Goals (MCLG)
for total chromium and arsenic concentrations in drinking water
. Both chromate and
2
0
molar level.
1
Both chromate and arsenate are oxidants, a property that may
be exploited to detect their presence in water. Recently, it was
1
at 100 and 10 ppb, respectively. For chromium, it would be
(
7) Suresh Kumar, P.; Thomas Lee, S.; Vallabhan, C. P. G.; Nampoori, V. P.
N.; Radhakrishnan, P. Opt. Commun. 2002, 214, 25-30.
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advantageous to be able to distinguish harmful Cr(VI) from
benign Cr(III). There are currently several physical detection
methods used to identify the presence of chromium and arsenic
in aqueous media, including high performance liquid chroma-
tography coupled with atomic absorption spectrometry, fiber
optic wave sensors,7 ion chromatography,
(
113.
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Chrom. A 2003, 988, 151-159.
(
10) Butler, E. C. V. J. Chrom. 1988, 450, 353-360.
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58B, 1335-1349.
8-10
inductively
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(
(
(
(
(
(
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list
of
drinking
water
contaminants
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(
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10.1021/ja052582w CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 11661-11665
9
11661

A R T I C L E S
Toal et al.
Figure 1. Synthesis of siloleamine chemosensor.
reported that silole-containing polymers are selective sensors
for nitroaromatic oxidants, including TNT.21,22 Detection is
achieved though fluorescence quenching of the silole by the
electron-deficient analyte. The silole luminescence is due to a
σ*-π* LUMO stabilized though conjugation of the σ* orbital
23
of the silicon chain with the π* orbital of the butadiene moiety.
The selectivity of the sensor is due to the polymer’s helical
structure, which permits intercalation of planar nitroaromatics.
Figure 2. Fluorescence intensity for 2 in THF/H2O as a function of water
percentage.
22
The aim of the research reported herein is to make a luminescent
silole sensor for aqueous CrO42- and AsO4 by functionaliza-
tion of a silole monomer with anion binding groups. Recently,
it has been reported that colloidal suspensions of methylphe-
nylsilole may be prepared by the rapid addition of water to an
ethanolic silole solution, and that the colloid exhibits as much
as a 300-fold increase in fluorescence intensity as compared to
The UV-vis absorption at 360 nm (ꢀ ) 7900 L/mol‚cm),
assigned to the π-π* transition of the silole moiety, is typical
of tetraphenylsilole monomers. A powder sample of 2
3-
35
luminesces a bright yellow-green at 480 nm when excited at
3
60 nm; however, a THF solution of 2 is only weakly
luminescent at 475 nm. The fluorescence quantum yield of 2
36
in toluene, measured relative to 9,10-diphenylanthracene, is
24
that in the organic solution. Other silole colloids have since
-3
only (1.15 ( 0.35) × 10 , which is similar to the quantum
been prepared and characterized.2 As most nanoparticles
5
37
yields of other silole monomers. However, a dramatic increase
26
characterized in the literature are either purely inorganic, such
in luminescence is observed for colloids of 2, which are prepared
by adding water rapidly to a THF solution of 2. This rise in
luminescence is accompanied by a 10 nm red shift in emission
wavelength to 485 nm.
Luminescence of the colloid is highly dependent on solvent
composition, specifically the amount of water used to precipitate
27
as semiconductor quantum dots, or purely organic (e.g.,
2
8
29
carotenes and dendrimers ), the silole organometallic nano-
particles are of particular interest for their unique photophysical
and structural properties, as well as for their sensor applications.
Fluorescent inorganic quantum dots have been shown to be
widely useful in sensing applications.30,31 This paper describes
the potential utility of luminescent silole nanoparticles in redox
sensing applications. An attractive feature of these molecular-
based materials is their ease of functionalization for analyte
recognition.
2. Solutions containing 2, 4, and 6 mg/L of 2 in THF/H2O were
prepared at various water percentages. The concentration
represents the total mass of silole, not the mass of the
nanoparticles. A minimum volume fraction of 80% water is
necessary to effect a detectable increase in luminescence (Figure
2
9
). The luminescence continues to increase with 90, 95, and
9% water. For the 99% water samples, fluorescence rose by a
Results and Discussion
To make the silole nanoparticles bind oxoanions, such as
factor of 15× for 2 mg/L, 36× for 4 mg/L, and 46× for 6 mg/L
as compared with pure THF solutions of the respective
concentration.
The colloids consist of particles on the order of 100 nm in
size, as determined by dynamic light scattering measurements.
Best fits to the data suggest a polydispersity of not more than
CrO42 and AsO4 , a hydrogen-bonding amine functionality
-
3-
was incorporated into the monomer via a chloroplatinic acid
32-34
(
H2PtCl6)-catalyzed hydrosilation
thylhydrosilole, 1, yielding the siloleamine, 2, in 90% yield
Figure 1).
of allylamine with me-
(
(
25%. AFM images of settled particles show similar particle
(
(
(
(
21) Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Angew. Chem.,
Int. Ed. 2001, 40, 2104-2105.
sizes. The colloid particles exhibit a minimum in size at 90%
water, with somewhat larger sizes at both greater and lesser
volume fractions of water. One possible explanation is that at
higher water concentrations the organic silole molecules ag-
gregate to a higher extent in the hydrophilic environment. The
larger colloid particles observed at lower water concentrations
may possibly be explained by THF absorbing into the particles
causing them to swell.
Fluorescent lifetime measurements were performed to com-
pare the dissolved silole and the silole in colloidal suspensions.
In toluene solution, the fluorescence lifetime of 2 is only 31 (
2 ps, which is somewhat longer than the lifetimes of both
dimethyl(tetraphenyl)silole at 17.5 ( 2.5 ps, and hexaphenyl-
22) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc.
2
003, 125, 3821-3830.
23) Rappoport, Z.; Apeloig, Y. The Chemistry of Organic Silicon Compounds;
John Wiley & Sons, LTD: New York, 2001; Vol. 3.
24) Luo, J.; Xie, Z.; Lam, J. W.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.;
Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740-
1
741.
(
25) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams,
I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535-1546.
(
26) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004,
1
04, 3893-3946.
(
(
(
27) Banin, U.; Millo, O. Annu. ReV. Phys. Chem. 2003, 54, 465-492.
28) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330-4361.
29) Lemcoff, N. G.; Spurlin, T. A.; Gewirth, A. A.; Zimmerman, S. C.; Beil,
J. B.; Elmer, S. L.; Vandeveer, H. G. J. Am. Chem. Soc. 2004, 126, 11420-
1
1421.
(
(
(
(
(
30) Lian, W.; Litherland, S. A.; Badrane, H.; Tan, W.; Wu, D.; Baker, H. V.;
Gulig, P. A.; Lim, D. V.; Jin, S. Anal. Biochem. 2004, 334, 135-144.
31) Lin, C. I.; Joseph, A. K.; Chang, C. K.; Lee, Y. D. J. Chrom. A 2004,
1
027, 259-262.
32) Benkeser, R. A.; Cunico, R. F.; Dunny, S.; Jones, P. R.; Nerlekar, P. G. J.
Org. Chem. 1967, 32, 2634-2636.
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969-974.
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A. L.; Trogler, W. C. Organometallics 2005, 24, 3081-3087.
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1
1662 J. AM. CHEM. SOC. VOL. 127, NO. 33, 2005
9

Luminescent Silole Nanoparticles as Sensors for Cr(VI)
A R T I C L E S
Figure 3. Semilog plots of fluorescence lifetime decay of 2, 6 mg/L, in
THF/water solutions for (a) 0%, (b) 80%, (c) 90%, (d) 95%, and (e) 99%
water. Amplitudes are normalized, so all curves begin at 1.
Figure 5. Photoluminescence quenching of 4 mg/L of 2 by CrO42-: (top)
quenching by, from top, 0, 0.1, 0.5, 1, 2.5, 5, 10, 20, 30, and 40 ppm
quencher in 90% water; (bottom) Stern-Volmer plots of photoluminescence
quenching in various water percentages.
Figure 4. Fluorescence intensity as a function of percent aggregated 2 for
a 6 mg/L colloid. The percentage of aggregated silole was determined from
the fluorescence decay curves.
lived nanoparticles observed have a fluorescence lifetime of 6
ns, which is also the lifetime of 2 in the solid state.
The unaggregated 2 is readily distinguished in Figure 3
because of its very short lifetime, as compared to the nanopar-
ticles. In an 80% water suspension, 92% of the silole remains
dissolved, as can be seen in Figure 3b with the sharp initial
decay. In 90% water, 83% silole remains dissolved, and in 95%
water, 65% of the silole remains dissolved. In a 99% water
silole, whose reported lifetime in acetone is 20 ps.38 Intramo-
lecular quenching of the π-π* excited state of the silole by
the donor amino moiety is, therefore, not responsible for the
short lifetime in the siloleamine. Indeed, the luminescence of
polysiloles is quenched by electron acceptors, rather than by
39
suspension, less than 10% of 2 is in solution. The luminescence
intensity is also directly proportional to the percent of aggregated
silole (Figure 4), as determined from the lifetime analysis. The
nonzero intercept in Figure 4 corresponds to the weak fluores-
cence contribution arising from the solution phase silole.
2
2
electron donors. The lifetime of 2 is about 20 times shorter
2
2
than that observed for poly(tetraphenyl)silole at 700 ps. The
polysilole adopts a helical structure, and solvent access to the
face of the silole π system is restricted. This may account for
the long fluorescence lifetime observed for the polymer and
for siloles in the solid state. Other workers have attributed the
2-
The ability of the siloleamine to detect the carcinogens CrO4
3-
and AsO4 was investigated by adding successive aliquots of
increased lifetime of silole monomers in the solid state to
aqueous stock solutions of the analytes to nanoparticle suspen-
sions of 2 and monitoring the decrease in fluorescence intensity
restricted rotation of the phenyl substituents.2
4,38
As water is
added and the nanoparticles begin to form, fluorescence emission
lifetimes become longer and are clearly nonexponential. Mul-
tiexponential fits of fluorescence decay curves show a fast decay
component from unaggregated species still present in the
colloidal suspension, along with additional longer decay time
components from the nanoparticle aggregates. Figure 3 shows
semilog plots of the fluorescence decay curves of 6 mg/L
solutions at various water concentrations. The nanoparticles have
mean lifetimes of 4.0, 4.3, and 4.6 ns in 90, 95, and 99% water
solutions, respectively. The lifetime reported is an average of
two exponentials needed to obtain an appropriate fit to the
nanoaggregate decay. This is not surprising, as the particles are
not uniform in size, but span a size distribution. The longest-
(
Figures 5 and 6 (top), respectively). Quenching of photolumi-
2-
nescence is observed at the EPA action level of 0.1 ppm CrO4
Not surprisingly, since AsO4 is a weaker oxidant than CrO42-,
it is a weaker quencher as well (standard reduction potentials
vs NHE are -0.68 and -0.13 V, respectively, and at pH 7 they
.
3-
40
are +0.15 and +0.56 V, respectively). This supports the
hypothesis that the quenching of photoluminescence of siloles
is due to electron transfer from the excited state of the silole to
the analyte.
(39) This assumes that the quantum yields of the aggregates and unaggregated
silole are proportional to τobs/τrad, where τobs is the observed lifetime and
τ
rad is the radiative lifetime. The radiative lifetimes are assumed to be the
same for both species because their absorption and emission spectra are
nearly identical.
(
40) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles
of Structure and ReactiVity, 4th ed; HarperCollins College Publishers: New
York, 1993.
(
38) Lee, M. H.; Kim, D.; Dong, Y.; Tang, B. Z. J. Korean Phys. Soc. 2004,
4
5, 329-332.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 33, 2005 11663

A R T I C L E S
Toal et al.
Table 1. Stern-Volmer Constants and Relative Sensitivities of a
pH 7 Phosphate-Buffered Colloid of 2 to Various Aqueous Anions
Stern
−
Volmer
relative
analyte
constant (M-1)
sensitivity
NO3-
87
140
170
780
1300
10900
1.0
1.6
2.0
9.0
15
-
ClO4
SO42
NO2
-
-
3
-
AsO4
CrO42
-
120
Figure 7. Stern-Volmer plots of luminescence quenching of 2, 4 mg/L,
in 90% water suspension, by chromate.
hydrogen bonding sites, the increased luminescence intensity
more than compensates for any sensitivity loss. Stern-Volmer
plots of Cr(VI) quenching in the buffered aqueous colloid are
linear only up to 1 ppm, after which there is a sharp rise in
sensitivity. This may be due to competition between the
chromate quencher and phosphate buffer for binding sites at
lower chromate concentrations. However, sensitivity to arsenate
decreased about 20-fold for the buffered colloid, which suggests
that phosphate effectively competes with arsenate. Much higher
Figure 6. Photoluminescence quenching of 4 mg/L of 2 by AsO43-: (top)
quenching by, from top, 0, 0.5, 1, 2.25, 5, 10, 20, 30, and 40 ppm quencher
in 95% water; (bottom) Stern-Volmer plots of photoluminescence quench-
ing in various water percentages.
Quenching efficiencies were fit to the Stern-Volmer equa-
tion, I0/I ) KSV [A] +1, which relates the fluorescence intensity,
I, at different concentrations of analyte quencher, [A], where I0
is the intensity at [A] ) 0, and KSV is the Stern-Volmer
constant. A plot of CrO42 quenching efficiency is linear up to
-
-
2-
concentrations of the interferent analytes (NO3 , NO2 , SO4
,
-
-
and ClO4 ) were needed to observe quenching. Comparison of
the slopes of Stern-Volmer plots gives relative sensitivities to
analyte detection. For instance, the buffered colloid is 120 times
more sensitive to chromate than to nitrate. A summary of the
relative sensitivities is shown in Table 1. The net result is that
preparation of the colloid of 2 with a pH 7 phosphate buffer
produces a highly selective chromate sensor.
1
0 ppm, at which point there is a rise in quenching efficiency
3
-
(Figure 5, bottom). However, a Stern-Volmer plot of AsO4
quenching saturates above 10 ppm (Figure 6, bottom). The
nonlinear relationship observed for arsenate may indicate
saturation of the surface-binding sites since the nanoparticle
12
concentration per liter is approximately 6 × 10 , which is less
than the analyte molecular concentration per liter of ap-
Quenching experiments were also carried out on environ-
mental water samples to determine the ability of the silole to
detect chromate in the presence of ambient interferents at actual
environmental concentrations. Quenching efficiencies for chro-
mate spiked in San Diego tap water were identical to those
measured in distilled water, and efficiencies for chromate spiked
in La Jolla seawater were only slightly higher (Figure 7). The
higher sensitivity to chromate in the saline seawater, with its
high ionic strength, is in agreement with the observation that
buffered suspensions also provide more sensitive chromate
detection.
1
8
3-
proximately 4 × 10 at 1 ppm AsO4 . It also suggests that
3
-
the lower driving force for electron transfer with AsO4 does
not permit complete quenching at saturation coverage of the
nanoparticles.
The selectivity of the colloid sensor was tested by performing
quenching experiments in pH 7 phosphate-buffered suspensions,
2
-
3-
using both CrO4 and AsO4 , as well as common aqueous
-
-
2-
-
interferents, NO3 , NO2 , SO4 , and ClO4 . Colloids prepared
in the buffer were almost twice as luminescent as those prepared
in distilled water since the presence of phosphate alone led to
an increase in nanoparticle luminescence. The buffer was used
in the interferent quenching studies in order to prevent variations
in the fluorescence intensity caused by changes in ionic strength
from quenchers at higher concentrations (>50 ppm). Sensitivity
to Cr(VI) detection more than doubled in the buffered aqueous
colloid as compared to that of the unbuffered suspensions. Even
though phosphate anions may compete with chromate for surface
Experimental Section
General. All synthetic manipulations were carried out under an
atmosphere of dry argon gas using standard Schlenk techniques.
Solvents were purchased from Aldrich Chemical Co. Inc. and distilled
from sodium/benzophenone ketyl. NMR data were collected with Varian
1
Unity 300 or 500 MHz spectrometers (300.1 MHz for H, 77.5 MHz
11664 J. AM. CHEM. SOC.
9
VOL. 127, NO. 33, 2005

Luminescent Silole Nanoparticles as Sensors for Cr(VI)
A R T I C L E S
for 13C, and 99.4 MHz for 29Si NMR). Fluorescence emission and
excitation spectra were recorded with the use of a Perkin-Elmer LS
H
PtCl
‚xH
O (2.5 mg, 5 µmol), and freshly distilled toluene (10 mL)
2
6
2
were placed under an Ar atmosphere and refluxed for 20 h. The solution
was cooled, passed through a sintered glass frit, and evaporated to
dryness. The product was dissolved in ether and precipitated with
hexanes and filtered. The precipitate, 2, was collected as a yellow
powder: (1.1 g, 90%); H NMR (300 MHz, CDCl
5
0B luminescence spectrometer. UV-vis spectra were obtained with
the use of a Hewlett-Packard 8452A diode array spectrometer. Atomic
force microscopy images of particles settled on glass slides were
obtained with the use of a Digital Instruments Nanoscope IIIa.
Fluorescence lifetimes were measured using time-correlated photon
counting, following excitation by the frequency-doubled output of a
1
3
(δ ) 7.26)) δ )
6.7-7.3 (m, 20H, Ph), 2.64 (t, 2H), 1.53 (m, 2H), 1.01 (m, 2H), 0.49
(s, 3H); 13C NMR (100 MHz, CDCl
(δ ) 77.00)) δ ) 154.638,
3
22
mode-locked Ti:sapphire laser, as described previously. Dynamic light
140.784, 139.691, 138.523, 129.802, 128.685, 127.838, 127.254,
126.079, 125.419, 44.847, 27.367, 10.593, 4.587; Si NMR (99.36
MHz, INEPT, CDCl
+ 1]; CHN (w/O purge) C32
2.94; found C 81.40, H 6.99, N 2.80.
4
1
29
scattering used an argon ion laser line at 514.5 nm, with detection at
0° in a solid angle of one or two coherence areas. Individual photons
9
3
, TMS (δ ) 0.0)) δ ) 9.5; MS(ES) m/z 458.8 [M
were detected and recorded in bin-widths of 10 µs and correlation
functions calculated by digital computer, assuming spherical particles.
Although the particle diffusion can be obtained through first principles,
the calibration was verified by measuring latex spheres of known
diameters.
2
H
31SiN‚H O, calcd C 80.79, H 6.99, N
2
Acknowledgment. We thank the Environmental Protection
Agency (Grant R829619) and the National Science Foundation
(Grant CHE-0111376) for financial support. S.J.T. also thanks
1
-Methyl-1-(3-amino)propyl-2,3,4,5-tetraphenylsilole (2): Meth-
the San Diego chapter of Achievement Rewards for Collegiate
Scientists for a fellowship. D.M. thanks the late Professor K.
Wilson.
4
2
ylhydrosilole (1.0 g, 2.5 mmol), allylamine (0.36 mL, 5.0 mmol),
(
41) Berne, B. J.; Pecora, R. Dynamic Light Scattering; John Wiley and Sons:
New York, 1976.
(42) Ruhlmann, K. Z. Chem. 1965, 5, 354.
JA052582W
J. AM. CHEM. SOC.
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