Zinc metalloporphyrin-functionalised nanoparticle anion sensors

Article abstract of DOI:10.1039/b313658b

Disulfide-functionalised zinc metalloporphyrins self-assembled on gold nanoparticles exhibit remarkable, surface-enhanced, anion binding affinities as compared to the free metalloporphyrin.

Full text of DOI:10.1039/b313658b

Zinc metalloporphyrin-functionalised nanoparticle anion sensors†
Paul D. Beer,* David P. Cormode and Jason J. Davis*
Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford, UK. E-mail: Paul.Beer@chem.ox.ac.uk; Jason.davis@chem.ox.ac.uk; Fax: 44 (0)1865 275900;
Tel: 44 (0)1865 275914
Received (in Cambridge, UK) 29th October 2003, Accepted 12th December 2003
First published as an Advance Article on the web 20th January 2004
Disulfide-functionalised zinc metalloporphyrins self-assembled
on gold nanoparticles exhibit remarkable, surface-enhanced,
anion binding affinities as compared to the free metal-
loporphyrin.
surface pre-organisation of the neutral zinc metalloporphyrin
moiety significantly enhances anion coordination at the nano-
particle surface to such an extent that recognition in a mixed
aqueous–organic solvent system is possible.
The amide-disulfide functionalised porphyin 1 was prepared in
37% yield from the condensation reaction of the a,a,a,a-
atropisomer of 5,10,15,20-meso-tetrakis(o-aminophenyl)por-
phyrin9 and four equivalents of thioctic acid in the presence of
DCCI and HOBt (Scheme 1). Reaction of 1 with Zn(OAc)2
produced the metalloporphyrin derivative 2 in 75% yield. Gold
nanoparticles were synthesized via the Brust method,10 using
dodecanethiol as the protecting thiol. Porphyrin-modified nano-
particles 3 were subsequently obtained by ligand substitution.
Briefly, this was achieved by the addition of 2 to a chloroform
solution of gold nanoparticles, stirring for 20 hours, solvent
removal, then recrystallisation from chloroform and diethyl ether.
Non-covalently bound 2 was removed by washing the resulting
solid exhaustively with methanol and acetone until the liquor ran
The combined processes of molecular-scale recognition and
surface assembly offer a powerful route to the development of
refined sensing systems and, more generally, new technological
devices based on controlling and analysing interactions on the
nanometre scale. The design and construction of receptors which
can selectively recognise and sense anionic guest species via a
macroscopic physical response is a current area of chemical sensor
technology receiving considerable attention.1–3 We have recently
developed methods whereby redox-active anion receptors can be
assembled on planar gold electrodes in the formation of highly-
selective sensors.4
Metal nanoparticles have extraordinary size-dependent optical
properties, not present in the bulk metal and have, consequently,
been the subject of intense research during the past decade or so.5
Specifically, nanoparticles of silver, gold and copper show distinct
and well-defined plasmon absorption in the visible spectrum, an
absorption characterised by an extremely large molar absorption
coefficient.6 Attention has recently focused on functionalizing
colloidal nanoparticles with molecular recognition components for
potential sensing applications.5,7
1
colourless. Modified nanoparticles were characterized by H and
13C NMR, UV-Vis. spectroscopy, elemental analysis, mass spec-
troscopy and thin layer chromatography.11 The number of por-
phyrin receptor moieties per nanoparticle was estimated, using
TEM-derived nanoparticle diameters, an estimate of the receptor
“footprint” and elemental analysis, to be 30–80 (depending on
particle size 3–4 nm).
With the ultimate aim of producing optical and redox-active
nanoparticle anion sensors, we report here the synthesis of a new
amide-disulfide functionalised zinc metalloporphyrin.8 This com-
pound was self-assembled on to gold nanoparticles to produce a
novel anion-selective optical sensing system (Fig. 1). Importantly,
Comparative UV-Visible titrations of 2 and 3 with a variety of
anions in dichloromethane solution were undertaken by monitoring
the porphyrin chromophore characteristics (Fig. 2). The association
constants for 1 : 1 complexes were then calculated using the Specfit
program.12 The anion is, presumably, bound by a combination of
Scheme 1 Synthesis of the amide-disulfide porphyrin.
Fig. 1 Schematic of the anion-sensing porphyrin gold nanoparticle 3.
† Electronic supplementary information (ESI) available: synthetic proce-
dure for 1 and 2, titration experimental protocol and nanoparticle TEM. See
http://www.rsc.org/suppdata/cc/b3/b313658b/
Fig. 2 Titration of a 50 mM solution of 3 with [TBA] [Br] in
dichloromethane. Note: the nanoparticle plasmon band lies below the
metalloporphyrin chromophore absorption.
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C h e m . C o m m u n . , 2 0 0 4 , 4 1 4 – 4 1 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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interactions with the zinc Lewis acid centre and favourable anion–
with respect to that of the free metalloporphyrin. The surface pre-
amide hydrogen bonding effects. The data obtained in dichloro-
organisation of optical and/or electrochemical-group functionalised
host systems offers the opportunity to fabricate highly-sensitive
sensory devices.
methane solution (Table 1) are indicative of extremely strong
2
complex formation between 2 and 3 with both Cl2 and H2PO4
.
The relative binding affinities are consistent with a trend expected
on the basis of relative anion basicities. Table 1 also displays the
striking result that the porphyrin-functionalised nanoparticle 3
The authors would like to thank the EPSRC for a studentship
(DPC) and The Royal Society (JJD) for financial support.
exhibits significantly larger magnitudes of association constant, in
2 13
comparison to 2, for Br2, I2 and NO3
.
This observation is
Notes and references
further reinforced by the results of anion complexation studies
carried out in the competitive solvent DMSO (Table 2). Particularly
striking are the respective chloride binding affinities, where the
surface-confined porphyrin-functionalised nanoparticle species 3
binds chloride more than two orders of magnitude more strongly
than the zinc metalloporphyrin 2. A remarkable enhancement in
anion complex stability is also seen with dihydrogen phosphate and
3. Analogous UV-Vis. titration experiments in 90% DMSO–10%
water solvent mixtures revealed that neutral 3 is even capable of
sensing Cl2 and H2PO42 in this aqueous solvent medium, though
Specfit was unable to quantitatively analyse the titration data.14
This increase in association constant for anion recognition
observed on confining the receptor to a surface is significant.15
Host–guest binding affinity is determined by factors controlling
enthalpy and entropy. By “pre-organising” receptors on surfaces,
and thereby reducing their conformational flexibility (a contribu-
tion which increases the energetic cost of association), entropic
contributions would be expected to be more favourable. Solvation
effects associated with close packing of receptors in a dominantly
hydrophobic self-assembled monolayer environment are also likely
to be significant.
1 P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486.
2 P. D. Beer, P. A. Gale and G. Z. Chen, J. Chem. Soc., Dalton Trans.,
1999, 1897.
3 C. Valerio, J. L. Fillaut, J. Ruiz, J. Guittard, J. C. Blais and D. Astruc,
J. Am. Chem. Soc., 1997, 119, 2580.
4 P. D. Beer, J. J. Davis, D. A. Drillsma-Milgrom and F. Szemes, Chem.
Commun., 2002, 1716; P. D. Beer, J. J. Davis, D. A. Drillsma-Milgrom,
K. E. N. Saunders and F. Szemes, manuscript in preparation.
5 A. N. Shipway, E. Katz and I. Willner, ChemPhysChem, 2000, 1, 18; C.
M. Neimeyer, Angew. Chem., Int. Ed., 2001, 40, 4123; A. C. Templeton,
W. P. Wuelfing and R. W. Murray, Acc. Chem. Res., 2000, 33, 27; S.
Chen, R. S. Ingram, M. J. Hostetler, J. J. Pietron, R. W. Murray, T. G.
Schaaff, J. T. Khoury, M. M. Alvarez and R. L. Whetten, Science, 1998,
280, 2098.
6 T. R. Jensen, M. D. Malinsky, C. L. Haynes and R. P. van Duyne, J.
Phys. Chem. B, 2000, 104, 10549.
7 A. Labande, J. Ruiz and D. Astruc, J. Am. Chem. Soc., 2002, 124, 8,
1782; M. Daniel, J. Ruiz, S. Nlate, J. Blais and D. Astruc, J. Am. Chem.
Soc., 2003, 125, 2617.
8 For examples of porphyrin-based anion receptors see: P. D. Beer, M. G.
B. Drew, D. Hesek and R. G. Jagessar, J. Chem. Soc., Chem. Commun.,
1995, 1187; P. D. Beer, M. G. B. Drew and R. C. Jagessar, J. Chem.
Soc., Dalton Trans., 1997, 881; R. C. Jagessar, M. Shang, W. R. Schendt
and D. H. Burns, J. Am. Chem. Soc., 1998, 120, 11684.
In summary, zinc metalloporphyrins self-assembled on gold
nanoparticles exhibit remarkable enhanced anion binding affinities
9 J. P. Collman, R. R. Gague, C. A. Reed, T. R. Halbert, H. Lang and W.
T. Robinson, J. Am. Chem. Soc., 1975, 97, 1429.
10 M. Brust, M. Walker, D. J. Schiffrin and R. Whyman, J. Chem. Soc.,
Chem. Commun., 1994, 801.
Table 1 Association constant (log K) data for 2 and 3 in dichloromethane
determined at 293 K, errors ±0.1
11 All data were as expected and observed previously for SAM modified
particles of the specified size range.
Anion
2
3a
12 R. A. Binstead and A. D. Zuberbuhler, Specfit Global Analysis, Version
2.90X. In a typical experiment aliquots of guest anion were added to
50–100 mM solutions of 3. Spectra and host–guest concentration
parameters were collected at each titration point and analysed by the
program. Parameters were refined by a global analysis that uses singular
value decomposition and nonlinear modelling by the Ivenberg–
Marquardt method. Calculated stability constants were then used in the
simulation of UV-Vis. spectra and accordingly cross-checked. Note that
these binding constants are based on spectroscopically-determined
porphyrin concentration and are independent of the number of receptor
units per nanoparticle.
Cl2
Br2
I2
NO3
H2PO4
> 6
4.1
3.2
2.4
> 6
0
> 6
5.0
4.0
3.2
> 6
2
2
2
ClO4
0
a Association constant values for the 1 : 1 porphyrin–anion complex on the
nanoparticle surface.
Table 2 Association constant (log K) data of 2 and 3 in DMSO determined
at 293 K, errors ±0.1
13 Control experiments confirmed that the presence of mM levels of anions
had a negligible effect on the UV-Vis. characteristics of dodecanethiol-
stabilised gold nanoparticles.
14 As expected with moieties confined to the surface of gold nanoparticles,
1H and 13C NMR spectra of 3 are broad; this precludes the possibility of
carrying out reliable NMR anion titration experiments.
15 In a recent study Labande et al. (reference 7) utilized hydrogen-bonding
amidoferrocene dendritic receptors on gold nanoparticles for voltam-
metric anion sensing where enhanced anion binding was referred to as
a “positive dendritic effect”.
Anion
2
3a
Cl2
H2PO4
< 2
2.5
4.3
4.1
2
a Association constant values for the 1 : 1 porphyrin–anion complex on the
nanoparticle surface.
C h e m . C o m m u n . , 2 0 0 4 , 4 1 4 – 4 1 5
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