Polyphenylene dendrimers as sensitive and selective sensor layers

Article abstract of DOI:10.1002/1521-3773(20011105)40:21<4011::AID-ANIE4011>3.0.CO;2-C

Rigid, dendritic poly(paraphenylene)s (see picture) are excellently suited as sensitive and selective coatings for gravimetric sensors (quartz microbalances), since they incorporate guest molecules reversibly into their interior voids. The variety of thei

Full text of DOI:10.1002/1521-3773(20011105)40:21<4011::AID-ANIE4011>3.0.CO;2-C

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[3
LacNAc(CH
buffer system consisting of sodium cacodylate (100 mm; pH 6.5), 15 mm
H]galactose from the donor UDP-Gal (50 mm) to the acceptor molecule
CO CH (0.36 mm) by the pa(1-3)GalT (10 mm) in an assay
Wakaruchuk, S. G. Withers, N. C. J. Strynadka, Nat. Struct. Biol. 2001,
8, 166 ± 175. However, the key feature of the catalytic mechanism,
namely the catalytic nucleophile, could not be clarified.
2
)
8
2
3
�
1
MnCl
2
, and BSA (bovine serum albumin; 50 mgmL ) at 378C for 30 min.
[10] H. Streicher, A. Geyer, R. R. Schmidt, Chem. Eur. J. 1996, 2, 502 ± 510.
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[12] M. A. J. Ferguson, S. W. Homans, R. A. Dwek, T. W. Rademacher,
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Dwek, J. Biol. Chem. 1991, 266, 20244 ± 20261.
3
The radiolabeled product, [ H]Gala(1-3)LacNAc(CH
2
)
8
CO
2
CH
3
,
was
separated from unincorporated label by adsorption onto a SepPac C18
column[ . The ratio of incorporated to total radioactivity is proportional
to the activity of pa(1-3)GalT. We took precautions to ensure that the
enzyme was the limiting reagent and that all other cofactors were close to
the saturation point. Strictly speaking, the reported inhibition is an IC50
value because we have not carried out the assays with different concen-
trations of cofactors.
20]
[
15] S. Abele, Diplomarbeit, Universität Konstanz, 1996.
[16] B. Waldscheck, PhD Thesis, Universität Konstanz, 2000.
17] H. Kuzuhara, J. Fletcher, J. Org. Chem. 1967, 32, 2531 ± 2534.
Received: April 9, 2001 [Z16925]
[
[
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[19] a) For the synthesis of this compound, see: F. Kong, D. Lu, S. Zhou,
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Nagakura, J. Org. Chem. 1997, 62, 8932 ± 8936.
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1988, 5, 49 ± 63.
[
2] Y. T. Pan, A. D. Elbein in Glycoproteins (Eds.: J. Montreuil, J. F. G.
Vliegenthart, H. Schachter), Elsevier, Amsterdam, 1995, pp. 415 ± 454.
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[
1
391; c) C.-H. Wong, D. P. Dumas, Y. Ichikawa, K. Koseki, S. J.
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321 ± 7373; d) Y.-F. Wang, D. P. Dumas, C.-H. Wong, Tetrahedron
7
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Tokoyuni, J. Org. Chem. 1992, 57, 6693 ± 6666; g) Y. Kajihara, H.
Hashimoto, H. Kodama, Carbohydr. Res. 1992, 229, C5 ± C9; h) R. A.
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Chem. Lett. 1994, 4, 391 ± 394; i) S. H. Khan, O. Hindsgaul, Molecular
Glycobiology, Oxford University Press, Oxford, 1994, 206 ± 229; j) Y.
Jip, M. Ichikawa, Y. Ichikawa, J. Am. Chem. Soc. 1999, 121, 5829 ±
Polyphenylene Dendrimers as Sensitive and
Selective Sensor Layers**
Martin Schlupp,* Tanja Weil, Alexander J. Berresheim,
Uwe M. Wiesler, Joachim Bargon,* and Klaus Müllen*
5830; k) G. Dufner, R. Schwörer, B. Müller, R. R. Schmidt, Eur. J.
Org. Chem. 2000, 1467 ± 1482; l) S. Laferte, N. W. C. Chan, K. Sujino,
T. L. Lowary, M. M. Palcic, Eur. J. Biochem. 2000, 4840 ± 4849.
[
4] a) R. R. Schmidt, K. Frische, Bioorg. Med. Chem. Lett. 1993, 3, 1747 ±
Polyphenylene dendrimers (PDs) are monodisperse macro-
molecules, whichÐbecause of their rigid frameworkÐcontain
internal voids. This property differentiates them from other
dendrimers, which consist of flexible, aliphatic groups, and
makes them attractive as selective layers for gravimetric
1750; b) Liebigs Ann. Chem. 1994, 297 ± 303; c) C. Schaub, B. Müller,
R. R. Schmidt, Glycoconjugate J. 1998, 15, 345 ± 354; d) B. Müller, T. J.
Martin, C. Schaub, R. R. Schmidt, Tetrahedron Lett. 1998, 39, 509 ±
512; e) F. Amann, C. Schaub, B. Müller, R. R. Schmidt, Chem. Eur. J.
1998, 4, 1106 ± 1115; f) B. Müller, C. Schaub, R. R. Schmidt, Angew.
Chem. 1998, 110, 3021 ± 3024; Angew. Chem. Int. Ed. 1998, 37, 2893 ±
sensors. Such sensors based upon the quartz microbalance
2897; g) C. Schaub, B. Müller, R. R. Schmidt, Eur. J. Org. Chem. 2000,
[1]
(
QMB) are widely used to monitor the concentration of
1745 ± 1758.
various volatile organic compounds (VOCs) in different
environments. These types of sensors are of increasing
significance in many aspects of daily life, be it monitoring
the manufacture or storage of foodstuff,[ controlling chem-
[
[
5] a) L. M. Sinnott in Enzyme Mechanisms (Eds.: M. I. Page, A.
Williams), The Royal Society of Chemistry, London, 1987, pp. 259 ±
297; b) Chem. Rev. 1990, 90, 1171 ± 1202, and references therein.
2]
6] a) M. M. Palcic, L. D. Heerze, O. P. Srivastava, O. Hindsgaul, J. Biol.
Chem. 1989, 264, 17174 ± 17181; b) O. Hindsgaul, K. J. Kaur, G.
Srivastava, M. Blaszczyk-Thurin, S. C. Crawley, L. D. Heerze, M. M.
Palcic, J. Biol. Chem. 1991, 266, 17858 ± 17862.
7] A socalled bisubstrate analogue of a(1-2)-fucosyltransferase that
lacked the fucose moiety was synthesized (ref. [6]) and exhibited low
inhibitory activity.
8] Closely related to the disubstrate analogue 1b for glycosyltransferase
inhibition are the recently published ketoside-based a(1-3)-fucosyl-
transferase inhibitors, which were termed trisubstrate analogues, even
though glycosyltransferases employ only two substrates and catalyze
irreversible reactions: B. M. Heskamp, G. H. Veeneman, G. A.
van der Marel, C. A. A. van Boeckel, J. H. van Boom, Tetrahedron
[*] Dr. M. Schlupp, Prof. Dr. J. Bargon
Institut für Physikalische und Theoretische Chemie, Universität Bonn
Wegelerstrasse 12, 53115 Bonn (Germany)
Fax : (‡49)228-73-9424
[
[
E-mail: schlupp@thch.uni-bonn.de, bargon@uni-bonn.de
Prof. Dr. K. Müllen, T. Weil, Dr. A. J. Berresheim, Dr. U. M. Wiesler
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55129 Mainz (Germany)
Fax : (‡49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
1
995, 51, 8397 ± 8406; B. M. Heskamp, G. A. van der Marel, J. H.
[**] This work has been supported by the TMR European Research
Program, through the SISITOMAS Project, by the Volkswagen
Foundation, the German Science Foundation (DFG), the German
Federal Ministry for Science and Technology (BMBF), and the Fonds
of the German Chemical Industry, Frankfurt (Main); U.M.W. thanks
the latter for a graduate fellowship, we all thank C. Beer and S. Spang
for valuable support during the syntheses.
van Boom, J. Carbohydr. Chem. 1995, 14, 1265 ± 1277; a b(1-4)gal-
actosyltransferase inhibitor was reported which follows an entirely
different design: H. Hashimoto, T. Endo, Y. Kajihara, J. Org. Chem.
1997, 62, 1914 ± 1922.
[
9] The first crystal structure of a retaining galactosyltransferase was
recently reported: K. Persson, H. D. Ly, M. Dieckelmann, W. W.
Angew. Chem. Int. Ed. 2001, 40, No. 21
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ical reactions such as polymerization,[3] or safety standards in
the chemical industry, such as monitoring the maximum
allowed concentration of VOCs in the work environment.[
QMBs are mass-sensitive devices, which are easy to apply, yet
of low cost, and they provide both high selectivity and
excellent sensitivity down to concentrations of VOCs of only
type cycloadditions[11] leading to structurally quite dif-
ferent three-dimensional (3D) dendrimer architectures (Fig-
ure 1).
The influx of functional groups in the periphery of the PDs
on the sensor activity has also been investigated. For this
purpose, dendrimers of the second generation (G2) have been
synthesized carrying electron withdrawing (CN and COOH)
4]
[12]
[
5]
5
ppm. Both properties of these gravimetric sensors cru-
cially depend on the type of sensor-active layer coated on top
of the QMBs. For this purpose, certain types of polymers[ or
organic compoundsÐsuch as lactam macrocycles or rotax-
or electron donating (NˆCPh ) substituents by using the
2
6]
correspondingly functionalized cyclopentadienone units 7 ± 9
(Figure 2). This approach yields functionalized, monodisperse
[
7]
[13]
anes Ðhave been used successfully to date; even biomater-
dendrimers in excellent yields.
[
8]
ials have found application. However, polymers such as
polyvinylchloride (PVC) lack selectivity, and organic hosts as
sensor-active layers yield insufficiently reproducible results,
probably because of rearrangements or aging. Biological
sensor layers are frequently rather unstable both chemically
as well as thermally. For many analytes and applications
specific sensor layers have to be specially prepared. There-
fore, because of the variety of applications and the multitude
of simultaneous requirements, high-quality sensor-active
materials have to satisfy there is a high demand for durable
sensor-active layers with high selectivity for many types of
target guest molecules.
The coating of the QMBs with the PDs is by using a
procedure from mass spectrometry known as electrospray.[
The PDs are dissolved in THF and accelerated by a high DC
voltage through a thin capillary and sprayed onto the top
electrode of the QMB. This leads to the formation of
homogeneous, rigid layers, the thickness of which is controlled
in situ by simultaneously monitoring the frequency of the
QMBs to be coated. The resulting thickness is standardized
corresponding to a frequency reduction of 10 kHz, at which
point the coating is stopped. According to Sauerbrey this
7, 14]
�
2
offset is equal to a mass of 44 mgcm of the coating on a
10 MHz QMB.[ A standardized thickness (or mass) of the
host compound on the QMB is a prerequisite for comparing
the sensitivity and selectivity of the coatings and to assure
reproducible results. QMBs coated accordingly have been
sequentially exposed to different VOCs in a gas mixing
chamber at 508C and at a universal concentration of
1000 ppm. The associated lowering of the resonance frequen-
cies of the QMBs because of the reversible incorporation of
the VOCs into the host compound is termed ªsensor
responseº Dn. That the inclusion of the VOCs into the
coatings occurs into the volume and is not merely a surface
adsorption phenomenon follows from the observation that
doubling the thickness of a coating yields twice the sensor
response.
15]
Herein the suitability of polyphenylene dendrimers (PDs)
as sensor layers for monitoring VOCs in the gas phase will be
demonstrated. PDs are ideally suited as host molecules, since
PET (Positron Emission Tomography) studies have shown
[
9]
that they contain stable cavities in their interior. Investiga-
tions of the crystal structure of different PDs of the first
generation (G1) have revealed that solvent molecules are
[
10]
readily trapped within these cavities. Accordingly, the most
important prerequisites are met for PDs to qualify as hosts,
namely sufficient free internal space, and the tendency to fill
these voids with small guest molecules.
The synthesis of PDs starts from core units of dif-
ferent structure and proceeds by a cascade of Diels ± Alder-
Figure 1. Various core units and resulting dendrimers. Tetraethynyl biphenyl (1; Biph), tetraethynyl tetraphenylmethane (2; Td), hexaethinyl
hexaphenylbenzene (3; HPB) and the G1 dendrimer 4 with 16 ethynyl functions based upon the biphenyl core 1.
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Figure 2. A) Functionalization of G2 PDs 6a ± c (Td-G2-(imine)16, Td-G2-(CN)16, and Td-G2-(COOH)16), by Diels ± Alder reaction of a G1 dendrimer 5,
which has eight ethynyl substituents, with functionalized cyclopentadienones. B) Syntheses of functionalized cyclopentadienones: 8 and 9 a) NHˆCPh
2
(
10 equiv), BINAP (12%), CsCO (15 equiv), [Pd(dba)
3
2
] (5mol%), toluene, 3 days, 808C, 86%); b) CuCN (2.5 equiv), DMF, 90%); BINAP ˆ 2,2'-
bis(diphenylphosphanyl)-1,1'-binaphthyl, dba ˆ dibenzylideneacetone C) 3D structure of a G2 PD (Td-G2-(NH
2
)16) 10 based upon the tetrahedral core 2 and
with surface amine functions.
The first series of measurements was conducted using
unsubstituted PDs of G2 derived from the four core units as
outlined in Figure 1. All the investigated dendrimers show a
similar behavior: all compounds react very selectively to polar
aromatic VOCs, such as acetophenone, anilin, benzaldehyde,
benzonitrile, fluorobenzene, nitrobenzene, and 2-methyl-ben-
zonitrile. It is remarkable that neither chlorinated nor
unsubstituted aliphatic hydrocarbons, alcohols, amines, alde-
hydes, or carbonyl compounds become included into these
hosts; therefore, members of this list of VOCs cannot be
monitored with dendrimer coatings of the type studied here.
However, this observation shows that PDs are very selective
host molecules (Figure 3). The sensor responses are totally
reversible and reproducible: after 4 min (adsorption plus
desorption time) these sensors are ready for the next
measurement. The response times are very short; t -times
dendrimer yielded practically identical results (deviations
below 5%).
The observed selectivity of PDs is attributed to their
exclusively aromatic skeleton, which may form pp-electron-
donor-acceptor complexes.[ Therefore, this type of den-
drimer shows a relatively weak affinity for benzene and
toluene (cf. Figure 3), since the p-electron densities of these
guests are virtually identical to that of the host molecule.
The host ± guest interaction between the dendrimers 1 ± 4
and the guest molecule acetophenone was investigated. The
concentration of the analyte acetophenone was altered
between 5, 10, 25, 50, 75, 100, 200, 300, and 400 ppm, at each
of five different temperatures (50, 55, 60, 65, 708C). The plot
of the resulting frequency shifts of the correspondingly coated
QMBs for any of these temperatures against the concentra-
tion of the acetophenone yields isotherms (Figure 4), termed
ªsensor characteristicsº. At low concentrations they resemble
the analogous Langmuir isotherms, whereas at higher con-
centrations they reach a linear domain. From the curvature
16]
90
(
ˆthe time until 90% of the response is reached) below
min have been measured. A comparison of the data
obtained using three different QMBs coated with the same
2
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dendrimer used significantly influen-
ces the energy of the host ± guest
interaction with the analyte. Since
the outer diameter of all four den-
drimers is almost the same (about
4
nm), and since the different binding
energies do not correlate with the
number of the phenyl rings, the four
dendrimers must contain voids of
individual size or shape, into which
the guests become incorporated by
physisorption.
Next, the selectivity of the sensors
has been analyzed as a function of the
generation of the dendrimers. For this
purpose members of the G3, G3, and
G4 dendrimers have been used.
Thereby it has to be considered that
at constant thickness of sensor layer
with the increasing size (or genera-
tion) of the dendrimers the number of
the host molecules is cut in half
Figure 3. A selection of sensor responses of the unsubstituted G2 PDs to various volatile organic
compounds (VOCs); measurement temperature ˆ 508C, concentration ˆ 1000 ppm (0.1%). The nomen-
clature of the dendrimers follows from: 1) the core (Biph, Td, HPB, see Figure 1); 2) the generation;
3
) the end group. 2Cp corresponds to the cyclopentadienone building-unit 3,4-bis-[4-(tri-iso-propylsilyl-
ethinyl)phenyl]-2,5-diphenylcyclopenta-2,4-dienone (AB -building unit) and 4Cp corresponds to 2,3,4,5-
tetrakis-[4-(tri-iso-propylsilylethinyl)phenyl]-2,5-diphenylcyclopenta-2,4-dienone (AB -building unit)
2
4
[
12]
leading to a higher number of phenyl rings. Both building-units are already known.
whenever the molecular weight of the dendrimer is doubled.
However, whenever the size of the dendrimer is doubled,
twice as many guest can be incorporated.[ Table 2 outlines
the obtained results for acetophenone as the guest molecule
1000 ppm).
18]
(
Table 2. Influence of the generation n (and thus diameter d) of the PDs on
the inclusion of gaseous acetophenone (1000 ppm) and thus on Dn.
n
M
[
d
[nm]
Dn
Number of
guest molecules
gmol 1
�
]
[Hz]
[18]
1
1
1
5
5
5
2
3
4868
9544
20072
3.8
5.1
6.4
498
835
2175
1.31 Â 10
2.21 Â 10
5.75 Â 10
Figure 4. Plot of the sensor response Dn of Td-G2-2CP to different
concentrations of acetophenone at various temperatures (sensor character-
4
istics).
parameter of the Langmuir component the host ± guest
interaction can be determined using Equation (1) where b ˆ
Accordingly, PDs are capable of incorporating polar
aromatic guest molecules very selectively into their voids,
whereby the number of the included guest molecules depends
upon their size and shape. To determine the significance of
various factors for the sensor characteristic of the dendrimers
in more detail, the influence of terminal substituents on the
inclusion properties was investigated. It was found that the
PDs of G2 6a ± c (cf. Figure 2), namely those based upon the
tetrahedral core, each carrying 16 cyano-, carboxyl-, or imino-
substituents, respectively, yield the sensor responses outlined
in Figure 5. Depending on the substituent, either an enhance-
ment or an attenuation of the binding affinity has been found
using acetophenone, benzaldehyde, or 1-methyl-2-pyrrolidine
(NMP) as guests. Thereby, both the electron densities of the
host as well as of the guests change, as do the possibilities for
adsorption. In this fashion, even nonpolar, aromatic solvents
such as benzene or toluene can be detected. Remarkable is
the behavior of the acid-determined dendrimer 6c, whichÐ
for the first timeÐshows a striking affinity for inclusion of
guest molecules carrying amine substituents (NMP, diethyl-
amine). Therefore, dendrimer 6c can be used to detect even
Parameter of curvature of the Langmuir isotherm, k ˆ rate
A
constant of adsorption, k ˆ rate constant desorption, E ˆ
D
B
�
23
� 1
binding energy, k ˆ Boltzmann constant (1.381  10 JK ),
B
[
17]
Tˆ temperature.
E
B
k
A
bꢀT† ˆ
/ ek T
B
(1)
k
D
The interaction energy for the four unsubstituted PDs of
the G2 1 ± 4 (Figure 1) and acetophenone as the analyte gas
�
1
lies between 13.4 and 21.3 kJmol . From Table 1 it can be
concluded qualitatively that the identity of the specific
Table 1. Energy of interaction E between the PDs with acetophenone as
the guest.
�
Host compound
E [kJmol 1]
Number of phenyl rings
Biph-G2-2CP[
a]
19.1
21.3
13.4
14.4
62
102
64
[a]
Biph-G2-4CP
Td-G2-2CP[
a]
HPB-G2-2CP[
a]
97
[
a] unsubstituted.
4014
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ensures a long-term stability of 12 to
24 months, and it guarantees reprodu-
cible results.
Received: May 25, 2001 [Z17175]
[1] a) A. Janshoff, H. J. Galla, C. Steinem,
Angew. Chem. 2000, 112, 4164 ± 4195;
Angew. Chem. Int. Ed. 2000, 39, 4004 ±
4
032; b) C. K. OꢁSullivan, G. G. Guil-
bault, Biosens. Bioelectron. 1999, 14,
63 ± 670; c) F. L. Dickert, H. Stathopu-
los, M. Reif, Adv. Mater. 1996, 8, 525 ±
29; d) R. Schumacher, Chem. Unserer
6
5
Zeit 1999, 33, 268 ± 278.
[
2] a) W. Grosch, Chem. Unserer Zeit 1996,
Figure 5. Change of the sensor characteristic upon terminal functionalization of the PDs; concentration
of the gaseous guest compounds ˆ 1000 ppm, measurement temperature ˆ 508C.
3
0, 126 ± 133; b) M. Rapp, J. Reibel,
Nachr. Chem. Tech. Lab. 1996, 44,
088 ± 1092.
1
the non-aromatic solvents acetone, acetonitrile, isopropyl
methyl ketone, and nitromethane with very good sensitivity.
To demonstrate the uniqueness of theseÐadmittedly diffi-
cult to synthesizeÐmonodisperse macromolecules as highly
selective sensor layers, series of measurements have also been
performed using the hyperbranched polyphenylenes for
[3] K. Cammann, U. Lemke, A. Rohen, J. Sander, H. Wilken, B. Winter,
Angew. Chem. 1991, 103, 519 ± 541; Angew. Chem. Int. Ed. Engl. 1991,
3
0, 516 ± 538.
[
4] H.-G. Neumann, H. W. Thielmann, J. G. Filser, H.-P. Gelbke, H.
Greim, H. Kappus, H. Norpoth, U. Reuter, S. Vamvakas, P. Warden-
bach, H.-E. Wichmann, J. Cancer Res. Clin. Oncol. 1998, 124, 661 ±
669.
[
19, 20]
[5] F. L. Dickert, P. A. Bauer, Adv. Mater. 1991, 3, 436 ± 438.
comparison.
The results show that these compounds
[6] A. Hierlemann, U. Weimar, G. Kraus, M. Schweizer-Berberich, W.
Göpel, Sens. Actuators B 1995, 26 ± 27, 126 ± 134.
yield sensor characteristics similar to those of the PDs, that is,
a high selectivity for substituted aromatic solvents. However,
by contrast to the dendritic analogues, the sensor responses
using the hyperbranched relatives of the PDs are not
reproducible, as different measurements using one and the
same host yield considerable deviations of up to 80%.[20]
Apparently, this is the consequence of the lack of a defined
structure of the hyperbranched polyphenylenes; therefore,
any individual coating of the QMBs results in different three-
dimensional orientations of the macromolecules on the
surface of the QMB electrodes, which gives rise to intrinsically
different voids. Similar shortcomings have been observed
[
7] C. Heil, G. R. Windscheif, S. Braschos, J. Flöhrke, J. Gläser, M. Lopez,
J. Müller-Albrecht, U. Schramm, J. Bargon, F. Vögtle, Sens. Actuators
B 1999, 61, 51 ± 58.
[8] M. Kaspar, H. Stadler, T. Weiss, C. Ziegler, Fresenius J. Anal. Chem.
2
000, 366, 602 ± 610.
[9] ªStable Voids in Polyphenylene Dendrimersº: K. Süvegh, T. Marek,
A. V e rtes, Macromolecules, submitted.
[10] a) A. J. Berresheim, PhD thesis, Johannes Gutenberg Universität
Mainz, 2000; b) U.-M. Wiesler, PhD thesis, Johannes Gutenberg
Universität Mainz, 2001.
11] a) F. Morgenroth, K. Müllen, Tetrahedron 1997, 53, 15349 ± 15366;
b) A. J. Berresheim, M. Müller, K. Müllen, Chem. Rev. 1999, 99,
1747 ± 1785; c) U. Wiesler, K. Müllen, Chem. Commun. 1999, 22,
[
2
293 ± 2294; d) F. Morgenroth, C. Kübel, K. Müllen, J. Mater. Chem.
when using commercially available, aliphatic Starburst-(PA-
MAM)-dendrimers.[
20]
1997, 7, 1207 ± 1211; e) F. Morgenroth, E. Reuther, K. Müllen, Angew.
Chem. 1997, 109, 647 ± 649; Angew. Chem. Int. Ed. Engl. 1997, 36, 631 ±
The poor reproducibility of coatings
consisting of the latter is probably because of their lack of
structural stability. If namely the end groups of the dendrim-
ers dive back into the interior, this causes a change in the
density distribution, upon which the structures collapse.[ By
contrast, the very rigid molecular framework of the PDs
assures a rather narrow variation of the voids in their interior,
which in turn guarantees a high degree of reproducibility and
hence assures accurate recognition and quantification of the
guest molecules.
6
34; f) F. Morgenroth, A. J. Berresheim, M. Wagner, K. Müllen,
Chem. Commun. 1998, 10, 1139 ± 1140.
[12] U.-M. Wiesler, A. J. Berresheim, F. Morgenroth, G. Lieser, K. Müllen,
21]
Macromolecules 2001, 34, 187 ± 199.
[
[
13] U.-M. Wiesler, T. Weil, K. Müllen, Top. Curr. Chem. 2001, 212, 1 ± 40.
14] M. Schlupp, C. Heil, A. Koch, J. Müller-Albrecht, U. Schramm, J.
Bargon, Sens. Actuators B 2000, 71, 9 ± 12.
[15] G. Sauerbrey, Z. Phys. 1959, 155, 206 ± 222.
16] a) H. A. Staab, S. Nikolic, C. Krieger, Eur. J. Org. Chem. 1999, 1459 ±
[
1
2
470; b) H. A. Staab, A. Feurer, R. Hauck, Angew. Chem. 1994, 106,
542 ± 2545; Angew. Chem. Int. Ed. Engl. 1994, 33, 2428 ± 2431.
Accordingly, PDs allow the selective detection with high
sensitivity and accuracy of polar aromatic target molecules
[
17] P. W. Atkins, Physikalische Chemie, 2nd ed., VCH, Weinheim, 1996,
pp. 931 ± 937.
(
ˆªanalytesº). The number of included guest molecules
[18] The calculation of the number of included guest molecules Nguest is
based upon the linear correlation between the lowering of the
depends both on the structure and the size of the dendrimer
and may be predicted, that is, a structure-property relation-
ship applies. Incorporating functional groups enables fine
tuning of the sensor properties, which makes it possible to
detect even aliphatic solvents. The sensitivity of dendrimer-
coated QMB sensors is remarkably high: for aniline and
acetophenone it is around 5 ppm. In addition, the PDs are
very stable both chemically and temperature-wise, which
�
2
frequency and the included mass mguest (1 Hz ˆ^ 4.4 ngcm ) according
� 1
� 1
to the following relation: Nguest ˆ mguest
M
guest
N
A
; where N
A
ˆ
Avogadros number and Mguest is the mass of one guest molecule.
[19] F. Morgenroth, K. Müllen, Tetrahedron 1997, 53, 15349 ± 15366.
[
[
20] M. Schlupp, PhD thesis, Universität Bonn, 2001.
21] R. Hesse, PhD thesis, Universität Bonn, 1999.
Angew. Chem. Int. Ed. 2001, 40, No. 21
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