Molecular folding induced nanogold aggregation

Article abstract of DOI:10.1016/j.tetlet.2010.04.023

Bisacridinedione-functionalized gold nanoparticles (bisADD-GNPs) were prepared and characterized. BisADD is a flexible acyclic moiety and a specific Ca²⁺ sensor. Signaling of the binding events is achieved by the cation-induced folding of the bisADD-GNPs and the resultant fluorescence enhancement and visual color change are attributed to the suppression of photoinduced electron transfer (PET) through space and nano-Au aggregation. The selective binding of Ca²⁺ is clear from steady state fluorescence and TEM techniques.

Full text of DOI:10.1016/j.tetlet.2010.04.023

Tetrahedron Letters 51 (2010) 3102–3105
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Molecular folding induced nanogold aggregation
*
Ranganathan Velu, Pichandi Ashokkumar, Vayalakkavoor T. Ramakrishnan, Perumal Ramamurthy
National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai 600 113, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Bisacridinedione-functionalized gold nanoparticles (bisADD-GNPs) were prepared and characterized. Bis-
ADD is a flexible acyclic moiety and a specific Ca2+ sensor. Signaling of the binding events is achieved by
the cation-induced folding of the bisADD-GNPs and the resultant fluorescence enhancement and visual
color change are attributed to the suppression of photoinduced electron transfer (PET) through space
and nano-Au aggregation. The selective binding of Ca2+ is clear from steady state fluorescence and
TEM techniques.
Received 17 March 2010
Revised 6 April 2010
Accepted 8 April 2010
Available online 11 April 2010
Keywords:
Bisacridinedione
Ó 2010 Elsevier Ltd. All rights reserved.
Photoinduced electron transfer
Surface plasmon resonance (SPR)
Nano-Au aggregation
Fluorescence enhancement
In recent years, surface functionalization of gold nanoparticles
(AuNPs) has attracted growing attention owing to their tunable
photophysical properties and various potential applications,
including colorimetric nanosensors in the chemical and biological
sciences.1 The visual-sensing ability of AuNPs relies on changes
in the surface plasmon resonance (SPR), which is dependent on
the aggregation state, surface morphology, and so on.1–3 Generally,
the aggregation of AuNPs in solution through analyte-triggered
interparticle cross-linking leads to a change in color from red to
blue or purple, which is useful for observing molecular-recognition
events.4 This fundamental optical property of AuNPs has been
intensively used for the development of nanoparticle probes for
biological macromolecules.2,4AuNPs have also been successfully
used for the development of molecular probes for small molecules
of organic and inorganic analytes.
Colorimetric methods are convenient and attractive because the
color changes can be easily discerned with the naked eye. In a re-
cent effort, Kim et al. and De la Fuente et al. synthesized calseques-
trin and lactose-functionalized gold nanoparticles (GNPs) and used
for Ca2+ detection.25,26 We have already shown27 that PET suppres-
sion results in fluorescence enhancement when acyclic polyether
linked chromophore is folded with Ca2+. In the present work, we
have synthesized bisADD-GNP conjugate, which shows Ca2+-in-
duced folding of the bichromophore-GNPs resulting in the visual
color change due to nano-Au aggregation with fluorescence
enhancement due to the suppression of through space PET.
The synthetic procedure is shown in Scheme 1. Refluxing a mix-
ture of tetraketone 1 and thioctic-hydrazide in acetic acid afforded
the monoacridinedione derivative 2. A mixture of 2, pentaethylene
glycol ditosylate and NaH in DMF (30 ml) on refluxing, afforded
bisacridinedione derivative 3; the crude product was recrystallized
from a mixture (9:1) of CHCl3 and methanol to yield a yellow pow-
der of bisADD. In most of the literature, the synthesis of mono-
layer-protected clusters (MPCs) adopts modification of the
The synthesis of Ca2+-specific sensors using the principle of PET
was suggested by de Silva et al.5,6 A variety of crown-ether and re-
lated macrocyclic-based chemosensors are known, while the non
cyclic crown-ether-based chemosensors are relatively rare.7,8 In
the majority of chemosensors reported so far, the signaling of the
binding event is achieved by electron transfer,9–11energy or charge
transfer processes12,13, and chromophore (excimer and exciton)
interaction.14,15 Recently, gold nanoparticles-based colorimetric
detection has been devised for a variety of targets including metal
ions,16–20 DNA21,22 and bacterial toxins.23,24 The aggregation of
chromophore-functionalized gold nanoparticles upon binding to
a target results in a colorimetric response that is indicated by
broadening and shifting of surface plasmon resonance peak.
method reported by Brust et al.; this procedure involves a two-
À
phase (water-toluene) extraction of tetrachloro aurate (AuCl4
)
using a phase-transfer reagent (TOAB), followed by reduction.28
Addition of NaBH4 to the mixture of 3 and HAuCl4 in the presence
of TOAB reduces the disulfide to thiol as well as enables the forma-
tion of metallic gold nuclei, which leads to the formation of S–Au
bond29 to produce 4. (synthesis and characterization data are avail-
able in the SM).
The HR TEM image was recorded by drop-casting dilute suspen-
sion of 4 on a carbon-coated copper grid and the images are pre-
sented in Figure 1. Three-dimensional approach in TEM images
indicates that bisADD is anchored on the surface of the gold nano-
* Corresponding author. Tel.: +91 44 24540962; fax: +91 44 24546709.
E-mail address: prm60@hotmail.com (P. Ramamurthy).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.023

R. Velu et al. / Tetrahedron Letters 51 (2010) 3102–3105
3103
OH
OH
OCH3
OCH3
O
O
O
N NH2
H
O
O
TsO
O
OTs
S S
4
AcOH /
NaH
N
O
O
N
2 atm /
NH
O
2
1
S
S
O
OCH3
O
O
H3CO
O
O
O
O
O
O
O
O
O
OCH3
H3CO
AuCl4- / TOAB
NaBH4, Toluene
O
O
N
O
O
O
O
O
N
O
N
N
HN
HN
NH
NH
O
O
O
O
S
S
S
S
S
S
S
S
Au
Au
3
4
Scheme 1. Schematic illustration of synthesis and assembly of bisADD-GNPs via the reaction of HAuCl4 with bisADD.
particles (GNPs). The size of GNPs was found to have an average
diameter of 4.5 nm. When the bisADD molecules were functional-
ized on gold nanoparticles, it was found that approximately 97 bis-
ADD molecules were bound to single nanoparticles if the foot print
area of the dithiol molecule is 0.657 nm2. Gold nanoparticles have
a total surface area of 63.58 nm2. When Ca2+ ions (0.096 mM) were
added to bisADD-GNPs conjugates, aggregation appeared due to
calcium-mediated aggregation (the TEM image of aggregation par-
ticle after addition of 0.081 mM Ca2+ is shown in image Fig. 1b).
This result suggests that the bisADD on the surface of GNPs folded
upon addition of Ca2+ with nano-Au aggregation.
The absorption and emission maxima of acridinedione dyes in
acetonitrile solution are centered around 380 and 430 nm, respec-
tively.30,31 The peak at 380 nm is attributed to the intramolecular
charge transfer (ICT) from nitrogen to carbonyl oxygen in the acrid-
inedione moiety, and the emission at 430 nm to that of the local
excited (LE) state. Absorption spectrum of bisADD 3 centered
around 360 nm as shown in Figure S1 (see in Supplementary data)
and the absorption spectrum of BisADD-GNPs 4 (Fig. S2) consist of
two bands: (i) the additive absorption spectrum of bisADD due to
ICT around 360 nm and (ii) a broad band in the visible region
around 529 nm, which is attributed to the surface plasmon reso-
Figure 1. HR-TEM micrographs of bisADD-functionalized gold nanoparticles (a) before and (b) after addition of (9.6 Â 10À5 M) Ca2+ ion to a bisADD-GNPs solution. (a)
Spherical shape gold nanoparticles attachment with bisADD. (b) Aggregation in response to addition of Ca2+
.

3104
R. Velu et al. / Tetrahedron Letters 51 (2010) 3102–3105
nance (SPR) for the stabilized gold nanoparticles. Addition of Ca2+
to the acetonitrile solution of 4 (2.5 Â 10À5 M) showed a red shift
in the absorption maximum (Fig. 2). The significant change in the
color visible to the naked eye (Fig. 3) demonstrates that the bis-
ADD-GNPs recognize Ca2+ and lead to the nano aggregation. The
absorption maximum of acridinedione remained unchanged on
Ca2+ addition. The above changes indicate that the added Ca2+
brings the aggregation of bisADD-GNPs without any direct interac-
tion with the chromophore. In contrast, the addition of other metal
ions like Na+, K+, and Mg2+ did not show any variation in the
absorption properties of 4 (Fig. S3).
Excitation of 4 at its absorption maximum, shows a weak emis-
sion with the quantum yield of 0.11 ( 0.01), which is attributed to
the PET process from the electron rich –OCH3 to the relatively elec-
tron deficient excited state of the acridinedione fluorophore. Addi-
tion of Ca2+ to 4 shows a fluorescence enhancement without any
spectral shift, as shown in Figure 4. An increase in the fluorescence
intensity of 4 on addition of Ca2+ is attributed to the suppression of
PET process. Whereas, the addition of other metal ions, such as Na+,
K+, and Mg2+ showed only marginal (Figs. S4–S6) changes when
compared to those with Ca2+. These observations reveal the unique
ability of 4 to detect Ca2+ ions selectively, which is evident from
Figure 5. Addition of Ca2+ induced folding of the bichromophore,
and the Ca2+ ions became nestled within the oxyethylene moiety
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
9.6x10-5M
0
400
450
500
Wavelength, nm
550
600
Figure 4. The emission (kex = 360 nm) spectra of 4 upon addition of Ca2+ (0–
9.6 Â 10À5 M) in acetonitrile.
and the OCH3 groups,27 as shown in Scheme 2. This binding sup-
presses the PET process and also results in the rigidification of
the host molecular framework, which in turn results in the fluores-
cence enhancement. Even though bisADD-GNPs and bisADD have
similar fluorescence properties, the former is useful in colorimetric
studies.
0.25
Ca2+
0.20
The complexation was established as 1:1 equilibrium from the
Benesi–Hildebrand plot (Fig. S7). Moreover, it is also reversible in
nature, which was confirmed by the addition of EDTA to bisADD-
GNPs-Ca2+ complex. Complete reversal of the plasmon absorption
band to the initial position was observed upon the addition of
0.15
0.10
0.05
0.00
95 lM EDTA to nanoparticle solution containing 85 l
M Ca2+
(Fig. S8). Fluorescence spectrum also shows a decrease in the fluo-
rescence intensity to the initial value of bisADD-GNPs upon the
addition of EDTA to bisADD-GNPs-Ca2+ (Fig. S9). The selectivity
300
400
500
600
700
Wavelength, nm
3.0
2.8
2.6
2.4
Figure 2. UV–visible spectra obtained of 4 upon addition of Ca2+ (0–9.6 Â 10À5 M)
in acetonitrile.
Ca2+
2.2
Na+
2.0
K+
Mg2+
1.8
1.6
1.4
1.2
1.0
0.8
0
2
4
6
8
10
[Mn+]/[4]
A
B
Figure 3. Photographs of (A) free bisADD-GNPs and (B) bisADD-GNPs with Ca2+
Figure 5. Plot of I/I0 versus the ratio of metal ion: 4, which illustrates the selectivity
(9.6 Â 10À5 M).
for Ca2+,over Na+, K+ and Mg2+ ions.

R. Velu et al. / Tetrahedron Letters 51 (2010) 3102–3105
3105
Au
S
Folded BisADD-GNPs
S
O
O
PET
O
O
O
Ca2+
NH
O
N
O
O
O
OCH3
H3CO
O
PET
O
O
O
Free BisADD-GNPs
O
O
Ca2+
N
O
N
HN
O
NH
O
O
O
Enhanced fluorescence
Weak fluorescence
OCH3
O
O
PET
NH
S
S
S
Au
Au
S
O
N
Nano aggregation
O
Au
S
S
Scheme 2. The possible mechanism of the phenomenon of binding of the free bisADD-GNPs with Ca2+
.
of the Ca2+ sensing is proved by the addition of Ca2+ to bisADD-
GNPs in the presence of other interfering metal cations (such as
Na+, K+, Mg2+, Ba2+, and Sr2+), which shows similar red shift in
the absorption spectrum as observed with Ca2+ addition to bis-
ADD-GNPs in absence of above interfering metal ions (Fig. S10).
It is also found that the same fluorescence enhancement is ob-
served in both the cases (Fig. S11).
In summary, we have demonstrated that bisADD-functionalized
gold nanoparticles exhibit Ca2+-induced folding of the bichromo-
phore-GNPs, resulting in a color change from red to blue. This
red shifted surface plasmon resonance (SPR) is due to aggregation
of nanoparticles by the foldamer complexation, which was con-
firmed by TEM studies. Binding of Ca2+ at the oxyethylene moiety
and –OCH3 groups results in PET suppression, which leads to fluo-
rescence enhancement.
4. Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Lestinger, R. L.; Schatz,
G. C. J. Am. Chem. Soc. 2000, 122, 4640.
5. de Silva, A. P.; Gunaratne, H. Q. N. J. Chem. Soc., Chem. Commun. 1990, 186.
6. de Silva, A. P.; Gunaratne, H. Q. N.; Maguire, G. E. M. J. Chem. Soc., Chem.
Commun. 1994, 1213.
7. Suzuki, Y.; Morozumi, T.; Nakamura, H.; Shimomura, M.; Hayashita, T.; Bartsh,
R. A. J. Phys. Chem. B 1998, 102, 7910.
8. Mello, J. V.; Finney, N. S. Angew. Chem. 2001, 113, 1584.
9. de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. J. Am. Chem. Soc. 1997, 119,
7891.
10. He, H.; Mortellaro, M. A.; Leiner, M. J. P.; Fraatz, R. J.; Tusa, J. K. J. Am. Chem. Soc.
2003, 125, 1468.
11. He, H.; Mortellaro, M. A.; Leiner, M. J. P.; Young, S. T.; Fraatz, R. J.; Tusa, J. K.
Anal. Chem. 2003, 75, 549.
12. Rurack, K.; Rettig, W.; Resch-Genger, U. Chem. Commun. 2000, 407.
13. Witulski, B.; Weber, M.; Bergstrasser, U.; Desvergne, J. P.; Bassani, D. M.;
Laurent, H. B. Org. Lett. 2001, 3, 1467.
14. Sasaki, D. Y.; Shnak, D. R.; Pack, D. W.; Arnold, F. H. Angew. Chem., Int. Ed. 1995,
34, 905.
15. Hong, S. Y.; Czarnik, A. W. J. Am. Chem. Soc. 1993, 115, 3330.
16. Liu, J.; Lu, Y. Chem. Mater. 2004, 16, 3231.
17. Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165.
18. Huang, C. C.; Chang, H. T. Chem. Commun. 2007, 1215.
19. Slocik, J. M.; Zabinski, J. S.; Phillips, D. M.; Naik, R. R. Small 2008, 4, 548.
20. Aili, D.; Enander, K.; Rydberg, J.; Nesterenko, I.; Bjorefors, F.; Baltzer, L.;
Liedberg, B. J. Am. Chem. Soc. 2008, 130, 5780.
Acknowledgments
R.V. thanks the University of Madras for the University Research
Fellowship. NMR and TEM facilities provided by SAIF, IIT Madras
are gratefully acknowledged.
21. Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am.
Chem. Soc. 1998, 120, 1959.
22. Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz,
G. C. J. Am. Chem. Soc. 2000, 122, 4640.
23. Schofield, C. L.; Haines, A. H.; Field, R. A.; Russell, D. A. Langmuir 2006, 22, 6707.
24. Schofield, C. L.; Field, R. A.; Russell, D. A. Anal. Chem. 2007, 79, 1356.
25. de la Fuente, J. M.; Africa, G. B.; Teresa, C. R.; Javier, R.; Javier, C.; Asuncion, R.;
Soledad, P. Angew. Chem. 2001, 113, 2317.
26. Kim, S.; Won Park, J.; Kim, D.; Lee, I. H.; Jon, S. Angew. Chem., Int. Ed. 2009, 48,
4138.
27. Ashokkumar, P.; Ramakrishnan, V. T.; Ramamurthy, P. Eur. J. Org. Chem. 2009,
5941.
28. Brust, M.; Walker, M.; Bethell, D.; Schffrin, D. J.; Whyman, R. J. Chem. Soc., Chem.
Commun. 1994, 801.
29. Yan-Li, Z.; Chen, Y.; Wang, M.; Yu Liu, Y. Org. Lett. 2006, 8, 1267.
30. Srividya, N.; Ramamurthy, P.; Shanmugasundaram, P.; Ramakrishnan, V. T. J.
Org. Chem. 1996, 61, 5083.
Supplementary data
Supplementary data (detailed synthetic procedures, character-
ization, 1H NMR data, FT-IR, HR-TEM images and photophysical
studies of compound 4 with other metal ions) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.04.023.
References and notes
1. Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293.
2. Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547.
3. Mulvaney, P. Langmuir 1996, 12, 788.
31. Thiagarajan, V.; Ramamurthy, P.; Thirumalai, D.; Ramakrishnan, V. T. Org. Lett.
2005, 7, 657.