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Organogel–quantum dots hybrid materials displaying fluorescence sensitivityand structural stability towards nitric oxide†‡

Prashant D. Wadhavane, M. Angeles Izquierdo,* Francisco Galindo, M. Isabel Burguete and Santiago V. Luis*

Received 14th November 2011, Accepted 26th January 2012

DOI: 10.1039/c2sm07175d

Nanoparticle doped hybrid organogels were prepared using the pseudopeptidic macrocycle 1 and CdSe/

ZnS quantum dots (QDs). The new semi-solid materials show the same thermal stability and excellent

optical transparency as compared to the parent organogel in the absence of the nanoparticles, but they

are fluorescent due to the presence of embedded semiconductor nanocrystals. The fluorescence lifetime

of a QD–organogel composite is reported for the first time, and it was found to be similar to that of the

QDs in solution in a related solvent, independent of the concentration of gelator. The chemical sensing

ability of the hybrid organogels towards gaseous nitric oxide was investigated by steady-state

fluorescence spectroscopy. The gels show fluorescence sensitivity towards NO ranging from 0.05 to

0.5 (vol%). The results reported herein constitute a proof of principle of the potential of the hybrid

supramolecular soft materials to develop nitric oxide sensor devices of practical application, especially

because the semi-solid state of the organogel is preserved after interaction with the analyte. This is

remarkable since, for the vast majority of organogels with analytical capabilities reported so far, the

signaling mechanism relies upon the disassembly of the supramolecular structure.

1 Introduction

Low molecular weight gelators are small molecules capable of

self-assembly into ordered supramolecular structures leading to

soft materials with physical properties in between those of solid

and liquid states.1 The resulting fibrillar networks impart a solid-

like macroscopic appearance to the solution and possess

a specific molecular organization with interesting properties for

the development of new supramolecular soft materials. In the

past decade, the development of fluorescent organogels has been

an important field of research, according to their potential

applications for the preparation of optoelectronic devices and

sensors.2 In most of the cases, well established organic fluo-

rophores, such as anthryl,3 pyrenyl,4 BODIPYS5 or linear p-

systems,2b,c are present in the initial structure of the organo-

gelator used for the preparation of those fluorescent organogels.

In other cases, however, photoactive organic molecules have

Universitat Jaume I, Departamento de Qu�ımica Inorg�anica y Org�anica, Av.Sos Baynat, s/n, E-12071 Castell�o de la Plana, Spain. E-mail: mizquier@uji.es; luiss@uji.es; Fax: +34964728214; Tel: +34964728236

† Electronic supplementary information (ESI) available: TEMmicrographs of the fibrillar structure of the hybrid organogels(Fig. S1), complementary absorption and emission spectra (Fig. S2, S3and S5), complementary fluorescence decay traces (Fig. S4), evolutionalong the time of the fluorescence intensity in the presence of NO(Fig. S6 and S7), comparative representation of r and t1/2 for differentconcentrations of NO in the QD–organogel medium (Fig. S8), QDssensitivity towards NO (Table S1). See DOI: 10.1039/c2sm07175d

‡ Article dedicated to the memory of Prof. Rafael Suau.

This journal is ª The Royal Society of Chemistry 2012

been entrapped in the fibrillar network of a known organo-

gelator.6 In all these systems, fluorescence techniques have been

shown to be very useful to characterize the resulting supramo-

lecular structures and, as a result, a number of interesting pho-

tophysical properties have been observed.7

An alternative strategy to obtain fluorescent organogels is the

preparation of organic/inorganic hybrid materials. In recent

years, different synthetic methodologies have been developed to

obtain nanoparticle–gel composites based on the interaction of

metal nanoparticles with self-assembled fibrillar networks, and

a great variety of new systems with novel properties have been

reported.8 Many examples involve the use of gold nanoparticles

(GNPs),9 and the resulting materials have optical properties that

can be fine-tuned through the interaction of the GNPs with

organic fluorophores.10 Nevertheless, the gold plasmon reso-

nance band is very broad and can overlap with the absorption of

the organic fluorophores, resulting in the shielding of the

absorbing fluorophore. Moreover, GNPs often quench the

fluorescence of organic chromophores in solution, a phenom-

enon that has also been observed in the organogel media.10,11

A different and less used approach for the preparation of

fluorescent hybrid materials is the use of intrinsically fluorescent

semiconductor quantum dots (QDs). Compared to organic flu-

orophores, these inorganic nanoparticles possess exceptional

features, such as size-dependent emission spectra along with

broad excitation bands, narrow fluorescence spectra and high

photoluminescence efficiencies.12 The synthesis and character-

ization of these nanoparticles have been the object of many

Soft Matter, 2012, 8, 4373–4381 | 4373

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studies,13 and different biological14 and analytical14b,c,15 applica-

tions have been described for them. One of the most interesting

applications of quantum dots is the development of sensors for

molecular species. Although the studies have been carried out

traditionally in solution,15a the utilisation of QDs supported on

an appropriate material is emerging as a practical alternative for

the implementation of the chemosensors in real apparatus or

devices.16

The incorporation of different quantum dots into self-assem-

bled organogel and hydrogel-based fibrillar networks has been

reported in recent years, although the resulting materials have

been mostly characterized from a qualitative point of view.17

Thus, for instance, Li and co-workers reported the immobiliza-

tion of CdSeS nanoparticles within the dipeptide (diphenylala-

nine)-based organogel network.17e Bardelang and co-workers

described the preparation of photoluminescent dipeptide gels

containing CdSe/ZnS QDs by means of ultrasound17f and qual-

itatively demonstrated that the fluorescence of the QD–gel

composite decreased in the presence of carbon-centered radicals.

These works are pioneering in this field, but the analytical

applicability of the quantum dots embedded within self-assem-

bled soft materials for important analytes of biological or envi-

ronmental interest remains to be explored and systematic studies

are needed to understand the fundamental processes determining

the sensing capability of the hybrid supramolecular soft

materials.

In this work, we report on the use of the pseudopeptidic

macrocycle 1 to prepare organic/inorganic hybrid organogels

and the study of their morphology and photophysical properties

(Fig. 1). Cyclophane 1 is part of a family of synthetic macrocycles

prepared in our group,18 which is able to self-associate to form

fibrils leading to thermoreversible organogels in a number of

solvents.19 It can be easily prepared on a multigram scale and its

modular structure provides excellent motifs for further optimi-

zation. Due to the excellent optical transparency of the organo-

gels created by 1, we have used them in the past to investigate

photoinduced electron transfer processes in organogel media20

and to probe the interaction of fluorescent drugs with the fibrillar

network created by 1.21

Therefore, we selected macrocyclic compound 1 to prepare

soft supramolecular materials containing core–shell CdSe/ZnS

quantum dots. The new hybrid organogels present the same

thermal stability and excellent optical transparency as the parent

Fig. 1 (A) Chemical structure of organogelator 1. (B) Photograph of

cuvettes showing the fluorescence of CdSe/ZnS QDs in organogel media.

Samples containing: (a) CdSe/ZnS QDs (0.45 mM) in toluene, (b) orga-

nogel obtained frommacrocycle 1 (2.6 mgmL�1) in toluene, (c) CdSe/ZnS

QDs (0.45 mM) in organogel obtained from macrocycle 1 (2.6 mg mL�1)

in toluene.

4374 | Soft Matter, 2012, 8, 4373–4381

organogel, with the advantage of being fluorescent due to the

presence of embedded semiconductor nanocrystals. The photo-

physical properties of the QD–organogel hybrid material were

investigated by means of non-invasive fluorescence techniques.

In particular, time-resolved spectroscopy was used for the first

time to measure the lifetimes of QDs embedded in an organogel

matrix. We have also investigated their chemical sensing ability

towards a species of biomedical and environmental interest, such

as nitric oxide, by steady-state fluorescence spectroscopy. The

results herein reported constitute a proof of principle of the

potential of the developed QD–organogels for sensing nitric

oxide and set the basis for the development of analytical devices

based on this concept. It is worth mentioning that the utilisation

of QDs for NO sensing22 is a quite new field and, to our

knowledge, this application has never been attempted in the gel

state.

2 Experimental section

2.1 Materials

A toluene solution of core–shell CdSe/ZnS quantum dots (QDs),

surface-capped with hexadecylamine, displaying a maximum

emission at 480 nm, was purchased from Aldrich. Toluene

(spectroscopy grade, Scharlab) was used as received. Cyclophane

1 was synthesized according to the described experimental pro-

cedure.18a The spectral characterization of the prepared

compound 1 coincided with the reported data.

2.2 Photophysical characterization

UV-visible absorption measurements were made using a Hew-

lett-Packard 8453 spectrophotometer. Steady-state fluorescence

spectra were recorded in a Spex Fluorog 3-11 equipped with

a 450 W xenon lamp. Fluorescence spectra were recorded in the

front face mode. Time-resolved fluorescence measurements were

done with the technique of time correlated single photon

counting (TCSPC) in an IBH-5000U. Samples were excited with

an IBH 372 nm NanoLED with a FWHM of 1.3 ns and a repe-

tition rate of 100 kHz. Data were fitted to the appropriate

exponential model after deconvolution of the instrument

response function by an iterative deconvolution technique, using

the IBH DAS6 fluorescence decay analysis software. Reduced c2

values (<1.2) and weighted residuals served as parameters for

goodness of fit. All the samples were measured in aerated

conditions, except when otherwise stated.

2.3 Scanning electron microscopy

The scanning electron microscope (SEM) samples were prepared

by slow evaporation of a solution of the macrocycle 1

(2 mg mL�1) in toluene or in toluene containing QDs (0.56 mM)

directly onto the sample holder. The samples were convention-

ally coated prior to the measurements. Images were acquired on

a JEOL 7001F SEM at beam energy 5.0 kV.

2.4 Transmission electron microscopy

The transmission electron microscope (TEM) samples were

prepared by dissolving macrocycle 1 (7 mg mL�1) in toluene or in

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toluene containing QDs (0.65 mM). A drop of the solution was

deposited onto a holey carbon coated 300 mesh TEM copper grid

and was dried under air. The dried grid was loaded into a single-

tilt sample holder. The TEM samples were examined on a JEOL

2100 TEM equipped with a high resolution Gatan CCD camera

(11 Mpixels). The TEM was operated at 80 kV.

2.5 Formation of organogels

Gelation of toluene was accomplished inside a 1 � 1 � 4 cm

fluorescence cuvette (quartz) by dissolving the appropriate

amount of 1 in the boiling solvent, and allowing to cool to room

temperature, as previously described.20 For absorption, steady-

state-emission and single-photon counting measurements, the

quantum dots sample was diluted in toluene at the appropriate

concentration for the photophysical measurements (typically

0.45 mM), and such a solution was used to dissolve 1 at high

temperature. After cooling to room temperature inside the

fluorescence cuvette, transparent gels were obtained after 30 min,

which allowed the acquisition of the pertinent spectroscopic

data.

2.6 Nitric oxide sensing

Nitric oxide was synthesized by reacting NaNO2 (0.2 M) with

ascorbic acid (1 M), according to a method previously described

in the literature.22a,23 Different concentrations of nitric oxide

were obtained by diluting freshly prepared pure gaseous NO in

argon under anaerobic conditions. The samples of organogel

obtained from macrocycle 1 (2.6 mg mL�1) containing QDs

(0.45 mM) in toluene were prepared directly in the fluorescence

cuvette (1 � 1 � 4 cm) and they were purged with nitrogen for

30 minutes to accomplish an anaerobic atmosphere. Then, a fixed

volume of a specific concentration of NO (10 mL) was added

through the septum of the cuvette with a syringe. The evolution

of the fluorescence intensity of the QDs was measured immedi-

ately after addition of the nitric oxide. The melting temperatures

of the organogels treated with NO were measured 5 hours after

addition of the gas.

3 Results and discussion

3.1 Preparation of organogels and morphological

characterization

The preparation of the hybrid organogels with optical trans-

parency is not trivial and several practical issues must be

considered when designing the system. Thus, the inorganic

nanoparticles and the organogelator must be dispersed in

a compatible solvent in order to obtain a homogeneous solution

of the system, all the components must be stable under the

experimental conditions used to prepare the organogels and the

resulting soft supramolecular material must exhibit excellent

transparency suitable for the photophysical characterization. In

the course of the experiments carried out in our laboratories, we

have observed that macrocycle 1 is a readily available compound

that yields very transparent gels in toluene and therefore it could

be compatible with the colloidal solutions of the lipophilic CdSe/

ZnS QDs in toluene. Compound 1 has also been observed to

yield very transparent gels in the presence of other organic

This journal is ª The Royal Society of Chemistry 2012

compounds such as aromatic compounds and amines and, thus,

it could be used to investigate a number of photophysical

processes in the organogel media.20,21 In the present study, the

gelation process of toluene by cyclophane 1 in the presence of

CdSe/ZnS QDs capped by hexadecylamine as surface ligand was

investigated. Supramolecular organogels were prepared by dis-

solving a certain amount of 1 in toluene containing quantum

dots. The QDs suspension was prepared in toluene at the

appropriate concentration for the photophysical experiments

and such a solution was used to dissolve 1 at high temperature.

After cooling to room temperature inside the cell, transparent

gels were obtained (Fig. 1B). Formation of the organogel con-

taining quantum dots was investigated at different concentra-

tions of 1 in order to determine the optimal concentration of

cyclophane to form transparent organogels in the presence of the

nanoparticles. The concentrations of 1 used to prepare the

organogels are listed in Table 1.

Concentrations of cyclophane 1 lower than 1.6 mg mL�1 led to

formation of weak organogels, while concentrations higher than

3.2 mg mL�1 led to precipitation of 1 and subsequent collapse of

the fibrillar network. The gel-to-sol transition temperatures of

the different organogels were calculated by the inversion vial

method and are shown in Table 1. The results indicate that the

prepared gels start melting at 44–46 �C, irrespective of the gelatorconcentration and independent of the presence or absence of

QDs. Interestingly, it was possible to decrease the amount

of cyclophane needed to form stable organogels in the presence

of the nanoparticles (from 2.6 mg mL�1 in the absence of QDs to

1.6 mg mL�1 when QDs are present). This trend was confirmed

after several independent measurements. This apparent rein-

forcement of the self-assembled network at low concentrations of

gelator could be due to the presence of weak supramolecular

interactions of the quantum dots with the fibrillar network.8d It

must be noted that, although a physical reinforcement was

previously reported for some hybrid systems,8d,24 this was not

reflected in a significant modification of the gel-to-sol transition

temperatures of the gels in the presence of inorganic nano-

particles,10,25 as is also observed in the present case.

The organic–inorganic soft supramolecular materials main-

tained the same excellent thermal reversibility displayed by the

original organogels. Thus, the hybrid organogels were heated

until solutions were obtained, and they were cooled to room

temperature to reform the gels. The cycle could be repeated four

times in the same cuvette without any sign of fatigue. This result

indicates that the interactions between the nanoparticles and the

organogelator must be relatively weak. Therefore, it is possible to

preserve the reversibility of the self-assembled system in the

presence of the quantum dots.

The morphology of the assemblies was examined by scanning

electron microscopy (SEM) and transmission electron micros-

copy (TEM). The SEM images of the xerogel obtained from the

organogel containing QDs show an entangled 3D network con-

sisting of bundles of fibrous aggregates up to ca. 1 mm long

(Fig. 2B). The microscopic structure was similar to that of the

xerogel obtained in the absence of nanoparticles (Fig. 2A),

indicating that the quantum dots do not modify significantly the

microscopic organization of the supramolecular structure. The

TEM images allowed visualising the nanoparticles stacked to the

fibers of 1. Interestingly, the individual quantum dots are isolated

Soft Matter, 2012, 8, 4373–4381 | 4375

Table 1 Melting temperatures of organogels formed by 1 with and without CdSe/ZnS QDs and photophysical properties of the QDs in the differentsamples

Sample Gelator/mg mL�1 Tg/�C labs/nm lem

a/nm s1b/ns a1 (%) s2

b/ns a2 (%) s3b/ns a3 (%) sav/ns

1 + QDsc 1.6 (Gel) 45–47 457 478 10.5 21 36.6 48 137.0 30 103.41d 1.6 (Sol) — — — — — — — — —1 + QDsc 2.6 (Gel) 46–61 457 478 9.3 19 36.5 49 136.3 32 104.51d 2.6 (Gel) 45–54 — — — — — — — —1 + QDsc 3.2 (Gel) 44–57 457 476 10.2 19 35.7 48 137.5 32 106.6QDse 0 (Sol) — 457 477 11.1 24 38.6 52 139.9 24 97.8

a lexc ¼ 366 nm. b lexc ¼ 372 nm, lem ¼ 480 nm. c Hybrid QD–organogel in toluene. d Control organogel in toluene without CdSe/ZnS QDs. e ControlCdSe/ZnS QDs in toluene.

Fig. 2 (A) SEM micrograph of the fibrillar structure of cyclophane 1

xerogel. Scale bar is 100 nm. (B) SEM micrograph of the fibrillar struc-

ture of cyclophane 1 xerogel containing CdSe/ZnS QDs. Scale bar is

100 nm. (C) and (D) TEM micrographs of the fibrillar structure of

cyclophane 1 xerogel containing CdSe/ZnS QDs. Scale bars are: (C)

20 nm, (D) 10 nm.

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and distributed along the superstructures formed by the orga-

nogelator (Fig. 2C and D and ESI†, Fig. S1). The nanoparticles

could bind the fibrillar network due to weak molecular interac-

tions between the alkyl groups of the hexadecylamine ligand and

the hydrophobic residues of the peptidic fibers, analogously to

related systems.8c,10,11b,17f Although SEM and TEM techniques

are commonly used in the characterization of self-associated

systems,1a,1b,17a,c–f the experiments imply drying of the samples on

a sample holder, and hence the static images acquired under such

experimental conditions need to be handled with care, as they

could not represent the actual situation in the gel state,26 which is

better described by using dynamic and non-invasive techniques

such as steady-state and time-resolved fluorescence, as will be

shown later. In any case, SEM and TEMmeasurements provide,

again, strong evidence of the fact that the presence of the indi-

vidual QDs has little morphological effect on the fibrillar

network.

Fig. 3 Absorption spectra of samples prepared in toluene containing

CdSe/ZnS QDs (0.45 mM) and different concentrations of organogelator

1. The absorption spectrum of the control organogel obtained from

macrocycle 1 in toluene without QDs is also shown for comparison

(closed triangles, bottom spectrum).

3.2 Photophysical characterization of organogels containing

quantum dots

The optical properties of the organogels containing CdSe/ZnS

QDs were measured in order to determine the effect of the

organogelator 1 on the photophysical properties of the

4376 | Soft Matter, 2012, 8, 4373–4381

fluorescent nanoparticles. The pertinent spectroscopic data are

summarized in Table 1 and indicate that the absorption and

emission properties of the QDs are not perturbed by their

entrapment in the gel. These results validate the methodology

used to prepare the fluorescent hybrid organogels based on

macrocycle 1 and CdSe/ZnS capped with hexadecylamine, which

involves heating the solution to the boiling point of toluene

under air in the presence of the gelator. Thus, the electronic

absorption spectra of the QDs embedded into the organogel

formed by different concentrations of 1 coincided in shape and

position with those of the QDs in solution at the same concen-

tration of nanoparticles (Fig. 3 and ESI†, Fig. S2). The only

appreciable difference consisted of the increased baseline recor-

ded in the gel state as a consequence of light scattering, a typical

phenomenon observed in gels.21,27

The steady-state fluorescence spectra of the CdSe/ZnS QDs

were measured for samples containing different concentrations

of cyclophane 1 and the data are summarized in Table 1. As can

be seen in Fig. 4A, the emission spectra of the QDs were found to

be similar in toluene and in organogelated toluene at different

concentrations of 1. This indicates that the nanoparticles are

stable and do not aggregate during the self-assembly process.

Noteworthy, the photoluminescence efficiency is preserved in the

organogel state, despite the possible interaction and subsequent

quenching of the QDs with the fibrillar network (Fig. 4A). It is

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 (A) Emission spectra of samples prepared in toluene containing

CdSe/ZnS QDs (0.45 mM) and different concentrations of organogelator

1 (lexc ¼ 366 nm). The emission spectrum of the organogel obtained from

macrocycle 1 in toluene without QDs is also shown (closed triangles,

bottom line). (B) Evolution over time of the emission intensity at 477 nm

of the samples prepared in toluene containing CdSe/ZnS QDs (0.45 mM)

and different concentrations of organogelator 1 (lexc ¼ 366 nm).

Fig. 5 Fluorescence decay traces of CdSe/ZnS QDs (0.45 mM) in toluene

and toluene organogel containing different concentrations of cyclophane

1 (lexc ¼ 372 nm, lem ¼ 480 nm). The incident light pulse is also shown

(open triangles).

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well known that the presence of organic compounds close to the

surface of semiconductor nanocrystals can influence their lumi-

nescence properties.28 In this case, the pseudopeptidic macro-

cycle 1 contains amide and amine groups in its chemical

structure. The interaction of amines with QDs has been studied

with simple model compounds and it can result in fluorescence

enhancement or decrease depending on the chain length and the

steric effect of the substituents or the electron donating ability of

the amine group.29 In this case, only a slight decrease in the

fluorescence of the QDs was observed in the hybrid organogels,

which was less than 13% for the higher concentrations of 1 (for

lower concentrations, the fluorescence decrease was below 10%).

These results indicate that the organogelator 1 does not quench

the fluorescence of the QDs in the organogel media to a signifi-

cant extent and constitutes a suitable organic molecular scaffold

to prepare fluorescent hybrid organogels based on QDs.

The long-term stability of the fluorescence signal after prepa-

ration of the hybrid materials was also investigated. In some

cases, it has been reported that a stabilization time is required to

observe slow quenching associated with ligand exchange proc-

esses.29a Therefore, the emission spectra of the QDs were recor-

ded over time in order to completely evaluate the effect of gelator

1 on the fluorescence of the QDs. As can be seen in Fig. 4B, the

This journal is ª The Royal Society of Chemistry 2012

fluorescence intensity of the CdSe/ZnS QDs in the hybrid orga-

nogels keeps constant with time, indicating that no ligand

exchange occurs in the hybrid materials which would modify the

photoluminescence of the QDs, at least for the time scale of our

experiments (5 h).

The fluorescence signal of the new materials was also found to

be very resistant to the application of several cycles of melting–

gelation. The organogels were heated to achieve a clear solution,

followed by cooling to room temperature to reform the gel. The

cycle was repeated four times and the emission spectra of the

samples were measured after formation of the organogel (see

ESI†, Fig. S3). No significant changes in the shape or position of

the band of the quantum dots were observed, indicating that the

photophysical properties of the material are preserved and the

CdSe/ZnS QDs are quite stable in this medium. This result

reinforces the idea that the interaction between the nanoparticles

and the organogelator must be weak and the QDs do not inter-

fere with the self-assembly process of the gelator.

In order to complete the photophysical characterization of the

new organic–inorganic materials, a time-resolved fluorescence

study was carried out using the technique of time correlated

single photon counting (TCSPC). Time resolved fluorescence

spectroscopy allows fine discrimination between fluorescent

species and it has been used to study dynamic quenching

processes and to probe different heterogeneous systems.21,30 The

fluorescence decay time of the QDs in toluene was measured by

excitation at 370 nm and monitoring the emission at 480 nm

(Fig. 5 and ESI† Fig. S4). The emission decay of QDs is still

a subject of study, but it is generally accepted that the occurrence

of multiphotonic processes leads to different excited states and

hence different decay processes.22c,31 Thus, the decays were fitted

to a multiexponential model following eqn (1),32 which is

commonly used in the case of QDs in solution.31

IðtÞ ¼Xi

ai exp ð�t=siÞ (1)

In this expression, si are the decay times and ai represents the

amplitudes of the components. In all the cases, the best fits of the

fluorescence decay traces required a sum of three exponential

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functions to obtain low c2 values as well as random distributions

of the weighted residuals, which are indicators of the goodness of

the fits. The individual fluorescence lifetime components and

their contributions to the total signal are shown in Table 1. A

pattern for the lifetime of the QDs in toluene was established,

which comprised a short lifetime of ca. 11 ns, an intermediate

lifetime of ca. 39 ns and a long lifetime of ca. 139 ns, contributing

24%, 52% and 24%, respectively, to the overall signal. A similar

pattern was observed for the QDs embedded in organogels

containing different concentrations of 1. To our knowledge, this

is the first time that a non-invasive time-resolved fluorescence

technique has been used to investigate the fluorescence lifetimes

of organogel–QD composites.

To better compare the fluorescence lifetimes of all the sam-

ples,31b,c the weighted average lifetime (sav) was calculated by

using (eqn (2))32 and the values are given in Table 1. Although

a slight increase of ca. 6% was observed for the weighted average

lifetime of the QDs in the organogel media, such difference is

relatively small and the weighted average lifetimes of the QDs in

the organogels are thus comparable to that of the QDs in solu-

tion. Most importantly, the weighted average lifetime values

were found to be independent of the concentration of organo-

gelator 1.

sðavÞ ¼X

ais2iX

aisi

!(2)

Hence, the interaction between QDs and the fibrillar entan-

glement of 1must occur to a very low extent. As a matter of fact,

the nanocrystals could be in equilibrium between two states, one

interacting with the self-assembled fibers and the other as free

particles solubilised in the liquid phase (toluene) of the organo-

gel. All the spectroscopic similarities found between QDs in

solution and in the gel state point to the prevalence of the latter

state in the equilibrium, i.e. QDs freely dissolved in the liquid

fraction of the gel. Organogels have been described as soft

materials composed of a series of pools of solvent in which the

mobility of the molecules is identical to that in solution.20,33 This

model could also be compatible with our findings and it is

schematically represented in Fig. 6.

Overall, the optical characterization of the CdSe/ZnS QDs in

the hybrid organogel indicates that the pseudopeptidic macro-

cycle 1 can be used to prepare semi-solid materials with high

Fig. 6 Schematic representation showing the possible location of the

quantum dots (green circles) in the organogel.

4378 | Soft Matter, 2012, 8, 4373–4381

optical transparency and very stable fluorescence. In order to

check the generality of the method, organogels containing CdSe/

ZnS QDs emitting at 530 nm were prepared and the emission

properties of the nanoparticles followed the same behaviour as

observed in the case of the smaller QDs herein reported (data not

shown). Therefore, the excellent photophysical properties of

both CdSe/ZnS QDs emitting at 480 nm and at 530 nm are

preserved in the organogels, making them suitable for investi-

gating their potential use in the development of new formats for

supported optical sensors (see following section).

3.3 Nitric oxide sensing

Gel based systems have been proposed as interesting scaffolds for

the development of optical sensors because photoactive mole-

cules can be entrapped in the gel matrix with retention of the

sensing properties.34 Herein, we investigated the applicability of

the organogels containing quantum dots for the chemical sensing

of nitric oxide (NO), which is a gaseous species known to play

a wide variety of roles in biomedicine35 and environmental

chemistry.36 There are a great number of sensors for NO reported

in the literature, both based on molecular and macromolecular

systems, which highlights the importance of this species.37

Although some groups have developed sensors based on

quantum dots for detecting NO,22a,b as well as nitroxides,38 in

solution, and we have recently reported on a QD–poly-

methacrylate composite for the analysis of NOx,22c to the best of

our knowledge the sensing of nitric oxide in a gel matrix has not

been described.

The organogels containing CdSe/ZnS QDs were prepared as

described in the experimental section. Different concentrations

of gaseous NOwere injected in the cuvettes containing the hybrid

materials under anaerobic conditions. No changes in the

absorption spectra of the QDs were observed after the addition

of NO to the samples (ESI†, Fig. S5). The steady state fluores-

cence spectra were recorded over time after the addition of

variable concentrations of NO. A decrease in the fluorescence

of the CdSe/ZnS QDs was observed in all the cases. An example

of this decrease is shown in Fig. 7. Together with the fluorescence

quenching, a progressive though small red shift of 3 nm in the

emission maximum was observed in all the samples, in agreement

Fig. 7 Emission spectra monitoring (lexc ¼ 366 nm) of the CdSe/ZnS

QDs (0.45 mM) in the toluene organogel of cyclophane 1 (2.6 mg mL�1)

after addition of NO (1%).

This journal is ª The Royal Society of Chemistry 2012

Table 2 Quantum dots sensitivity towards nitric oxide in organogelmediaa

NO (vol%) t1/2/min �r � 10�4/s�1

0.05 269.5 1.10.15 99.5 1.80.5 30.0 3.31 22.5 4.31.5 26.5 4.02 31.0 3.9

a Samples containing CdSe/ZnS QDs (0.45 mM) in the toluene organogelof cyclophane 1 (2.6 mg mL�1).

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with previous observations on the interaction of QDs with

a number of analytes.38a This process has been attributed to the

differential quenching of the QDs: upon addition of a quencher,

small particles are quenched more effectively than the large ones,

resulting in an emitting population enriched in larger nano-

particles. This behaviour is observed for all the concentrations of

NO added to the hybrid organogel, indicating that size selective

quenching also occurs in the semi-solid materials as well as in

solution (see ESI†, Fig. S6 and S7).

The plots of the emission maxima of the QDs in the organogel

versus time for different concentrations of NO are shown in

Fig. 8. The fluorescence intensity has been translated into

normalized values to allow a comparative analysis of the effect of

the different concentrations of gas on the quenching process. A

control experiment for the hybrid organogel in the absence of

NO indicates that the fluorescence emission of the QDs was

stable over the time needed to carry out the experiments (black

triangles in Fig. 8, corresponding to 0% NO).

As can be seen in Fig. 8, the response of the hybrid organogels

to NO depends on the concentration of the gas. Two parameters

were calculated from these curves in Fig. 8 in order to evaluate

the sensing ability of the supramolecular gels containing

quantum dots. On the one hand, the time at which the emission

of the QDs at 477 nm was reduced 50% relative to the initial

intensity (t1/2) was determined. On the other hand, the slope of

the initial points of every curve (r (s�1) ¼ normalized emission/

time) was calculated in order to estimate the apparent rate of

quenching of the fluorescence of the QDs. The resulting

parameters are gathered in Table 2. For the comparative anal-

ysis, we will only use here the r values (Fig. 9), although similar

conclusions can be drawn using t1/2 (see ESI†, Fig. S8).

The results indicate that upon increasing the concentration of

the gas, the apparent rate of fluorescence quenching increases

until it reaches a plateau at a concentration of 1%. These

preliminary results indicate that the hybrid organogels are

sensitive to NO in concentrations ranging from 0.05 to 1%.

Further increase of the concentration of gas up to 2% did not

affect the apparent rate of quenching, suggesting that the

mechanism is controlled by the diffusion of NO through the

network of the organogel.

Fig. 8 Evolution over time of the fluorescence intensity at 477 nm of the

QDs (0.45 mM) in the toluene organogel of cyclophane 1 (2.6 mg mL�1) in

the presence of different concentrations of NO.

This journal is ª The Royal Society of Chemistry 2012

The quenching of the fluorescence of the CdSe/ZnS QDs by

different concentrations of nitric oxide was also measured in

solution. Although the decrease of the fluorescence of the QDs in

solution was faster (see ESI† Table S1), the fact that the new

hybrid materials display fluorescence sensitivity towards NO in

the same range of concentrations must be considered as a proof

of principle, opening the possibility for developing faster systems

in the future. Current studies are underway in our laboratories to

optimize the response of the hybrid organogels towards NO and

to establish the response of the system in the presence of other

potential quenchers.

It must be noted that the research on new methodologies to

support known chemosensors for gases onto different materials

is a topic of great interest. New materials with sensing capabil-

ities for gases are continuously emerging: polymer gels for the

detection of odors39 and chloride-containing gases,40 organogels

based on carbon tetrachloride for the analysis of nerve gas

stimulant41 or graphene–ionic liquid composites for the detection

of vapors of benzene42 or for the analysis of nitric oxide43 are

selected examples of soft systems useful for the analysis of

gaseous species.

Notably, the structural integrity of the hybrid organogels

herein described is maintained upon addition of NO since they

display identical strength as compared to the original ones before

the interaction with NO. It is important to note that, although

Fig. 9 Comparative representation of the apparent rate (r) of quenching

of the fluorescence of the CdSe/ZnS QDs for different concentrations

of NO in the toluene organogel–QD composite (cyclophane 1

(2.6 mg mL�1), CdSe/ZnS QDs (0.45 mM)).

Soft Matter, 2012, 8, 4373–4381 | 4379

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organogels have been proposed as interesting materials for the

development of sensors, in many cases the signalling mechanism

relies on the direct interaction of the analyte with the fibrillar

network, resulting in the complete collapse of the supramolecular

gel.41,44 This strategy has been widely applied in the naked eye

detection of anions,44a,b,e cations,44c pH44a–c and small mole-

cules.41,44b,d However, this could compromise the use of the

organogels for a number of practical applications, where the

structure of the support should be maintained.

4 Conclusions

We have described the preparation of a new type of soft material

that comprises an organogel and fluorescent CdSe/ZnS quantum

dots. The organogels have been characterized by means of

a number of techniques, including non-invasive fluorescence

techniques. In particular, time-resolved fluorescence has been

used for the first time to determine the fluorescence lifetimes of

QDs in organogel media, revealing a weak interaction of the

nanoparticles with the fibrillar network. Although a number of

fluorophores and materials have been developed as sensors of

nitric oxide in the past, to our knowledge, this is the first example

of a hybrid soft supramolecular organogel containing quantum

dots capable of sensing nitric oxide by fluorescence. The

preliminary results indicate that the CdSe/ZnS QDs used in these

experiments are sensitive to NO for concentrations ranging from

0.05 to 0.5 (vol%) and, remarkably, the organogel remains intact

after reaction with NO. Although the present systems is still far

from being able to be implemented for practical applications, the

present results can be considered as a clear proof of concept, and

open the way for the application of this kind of hybrid soft

materials in sensing devices. Future studies would allow us not

only to understand the mechanism involved in the fluorescence

quenching but also to modulate the properties of the semi-solid-

like sensors to overcome their current limitations for the devel-

opment of devices with practical utility in biomedicine and

environmental chemistry.

Acknowledgements

Financial support from the Spanish MICINN (CTQ2008-02907-

E, CTQ2009-09953 and CTQ2009-14366-C02-01), GV

(ACOMP/2010/258, ACOMP/2010/282) Fundaci�o Caixa

Castell�o-UJI (project P1 1B-2009-59, P1 1B2009-58) is

acknowledged. P.D.W. thanks the financial support from GV

(Grisolia Fellowship). We thank J. Javier Gomez (SCIC) for

technical assistance in SEM measurements, M. Carmen Peiro

(SCIC) for technical assistance in TEM measurements and

Victoria P�erez Belis for the artistic design in Fig. 6.

Notes and references

1 (a) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821;(b) M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489; (c)A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.Chem., Int. Ed., 2008, 47, 8002; (d) A. Dawn, T. Shiraki,S. Haraguchi, S. I. Tamaru and S. Shinkai, Chem.–Asian J., 2011,6, 266.

2 (a) T. Sagawa, S. Fukugawa, T. Yamada and H. Ihara, Langmuir,2002, 18, 7223; (b) S. S. Babu, K. K. Kartha and A. Ajayaghosh, J.Phys. Chem. Lett., 2010, 1, 3413; (c) A. Ajayaghosh andV. K. Praveen, Acc. Chem. Res., 2007, 40, 644; (d) Optical Sensors.

4380 | Soft Matter, 2012, 8, 4373–4381

Industrial, Environmental and Diagnostic Applications, ed. R.Narayanaswamy and O. S. Wolfbeis, Springer, 2004; (e) M. Ikeda,R. Ochi and I. Hamachi, Lab Chip, 2010, 10, 3325.

3 J.-P. Desvergne, T. Brotin, D. Meerschaut, G. Clavier, F. Placin,J.-L. Pozzo and H. Bouas-Laurent, New J. Chem., 2004, 28,234.

4 P. Babu, N. M. Sangeetha, P. Vijaykumar, U. Maitra, K. Rissanenand A. R. Raju, Chem.–Eur. J., 2003, 9, 1922.

5 F. Camerel, L. Bonardi, G. Ulrich, L. Charbonniere, B. Donnio,C. Bourgogne, D. Guillon, P. Retailleau and R. Ziessel, Chem.Mater., 2006, 18, 5009.

6 (a) S. H. Seo and J. Y. Chang,Chem.Mater., 2005, 17, 3249; (b) G. DePaoli, Z. Dzolic, F. Rizzo, L. De Cola, F. Vogtle, W. M. Muller,G. Richardt and M. Zinic, Adv. Funct. Mater., 2007, 17, 821; (c)S. Dutta, A. Shome, S. Debnath and P. K. Das, Soft Matter, 2009,5, 1607.

7 (a) L. Xue, H. Wu, Y. Shi, H. Liu, Y. Chen and X. Li, Soft Matter,2011, 7, 6213; (b) S. Banerjee, R. Kandanelli, S. Bhowmik andU. Maitra, Soft Matter, 2011, 7, 8207; (c) C. C. Tsou and S. S. Sun,Org. Lett., 2006, 8, 387; (d) B. K. An, D. S. Lee, J. S. Lee,Y. S. Park, H. S. Song and S. Y. Park, J. Am. Chem. Soc., 2004,126, 10232.

8 (a) J. Puigmarti-Luis, A. P. Del Pino, E. Laukhina, J. Esquena,V. Laukhin, C. Rovira, J. Vidal-Gancedo, A. G. Kanaras,R. J. Nichols, M. Brust and D. B. Amabilino, Angew. Chem., Int.Ed., 2008, 47, 1861; (b) L. S. Li and S. I. Stupp, Angew. Chem., Int.Ed., 2005, 44, 1833; (c) P. P. Bose, M. G. B. Drew and A. Banerjee,Org. Lett., 2007, 9, 2489; (d) J. Wu, Q. Tian, H. Hu, Q. Xia,Y. Zou, F. Li, T. Yi and C. Huang, Chem. Commun., 2009, 4100;(e) S. Roy and A. Banerjee, Soft Matter, 2011, 7, 5300.

9 (a) D. Das, S. Maiti, S. Brahmachari and P. K. Das, Soft Matter,2011, 7, 7291; (b) S. Bhattacharya, A. Srivastava and A. Pal,Angew. Chem., Int. Ed., 2006, 45, 2934; (c) I. A. Coates andD. K. Smith, J. Mater. Chem., 2010, 20, 6696; (d) M. Kimura,S. Kobayashi, T. Kuroda, K. Hanabusa and H. Shirai, Adv. Mater.,2004, 16, 335.

10 N. M. Sangeetha, S. Bhat, G. Raffy, C. Belin, A. Loppinet-Serani,C. Aymonier, P. Terech, U. Maitra, J. P. Desvergne and A. DelGuerzo, Chem. Mater., 2009, 21, 3424.

11 (a) J. van Herrikhuyzen, S. J. George, M. R. J. Vos, N. Sommerdijk,A. Ajayaghosh, S. C. J. Meskers and A. Schenning, Angew. Chem.,Int. Ed., 2007, 46, 1825; (b) V. R. R. Kumar, V. Sajini,T. S. Sreeprasad, V. K. Praveen, A. Ajayaghosh and T. Pradeep,Chem.–Asian J., 2009, 4, 840.

12 (a) U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschkeand T. Nann, Nat. Methods, 2008, 5, 763; (b) C. B. Murray,C. R. Kagan and M. G. Bawendi, Annu. Rev. Mater. Sci., 2000, 30,545; (c) R. E. Galian, M. De La Guardia and J. P�erez-Prieto, J.Am. Chem. Soc., 2009, 131, 892.

13 (a) P. D. Cozzoli, T. Pellegrino and L. Manna, Chem. Soc. Rev., 2006,35, 1195; (b) C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025.

14 (a) M. Bruchez, M. Moronne, P. Gin, S. Weiss and A. P. Alivisatos,Science, 1998, 281, 2013; (b) C. A. J. Lin, T. Liedl, R. A. Sperling,M. T. Fernandez-Arguelles, J. M. Costa-Fernandez, R. Pereiro,A. Sanz-Medel, W. H. Chang and W. J. Parak, J. Mater. Chem.,2007, 17, 1343; (c) A.M. Smith and S.M. Nie,Analyst, 2004, 129, 672.

15 (a) J. F. Callan, A. P. De Silva, R. C. Mulrooney and B. Mc Caughan,J. Inclusion Phenom. Macrocyclic Chem., 2007, 58, 257; (b)R. Mart�ınez-M�a~nez, F. Sancen�on, M. Hecht, M. Biyikal andK. Rurack, Anal. Bioanal. Chem., 2011, 399, 55.

16 (a) W. Wu, T. Zhou, J. Shen and S. Zhou, Chem. Commun., 2009,4390; (b) J. Li, X. Hong, Y. Liu, D. Li, Y. W. Wang, J. H. Li,Y. B. Bai and T. J. Li, Adv. Mater., 2005, 17, 163.

17 (a) E. D. Sone, E. R. Zubarev and S. I. Stupp, Angew. Chem., Int. Ed.,2002, 41, 1705; (b) B. Simmons, S. C. Li, V. T. John,G. L. McPherson, C. Taylor, D. K. Schwartz and K. Maskos, NanoLett., 2002, 2, 1037; (c) N. Gaponik, A. Wolf, R. Marx,V. Lesnyak, K. Schilling and A. Eychmuller, Adv. Mater., 2008, 20,4257; (d) G. Palui, J. Nanda, S. Ray and A. Banerjee, Chem.–Eur.J., 2009, 15, 6902; (e) X. H. Yan, Y. Cui, Q. He, K. W. Wang andJ. B. Li, Chem. Mater., 2008, 20, 1522; (f) D. Bardelang,M. B. Zaman, I. L. Moudrakovski, S. Pawsey, J. C. Margeson,D. S. Wang, X. H. Wu, J. A. Ripmeester, C. I. Ratcliffe and K. Yu,Adv. Mater., 2008, 20, 4517.

This journal is ª The Royal Society of Chemistry 2012

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

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Mar

ch 2

013

Publ

ishe

d on

02

Mar

ch 2

012

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C2S

M07

175D

View Article Online

18 (a) J. Becerril, M. Bolte, M. I. Burguete, F. Galindo, E. Garcia-Espana, S. V. Luis and J. F. Miravet, J. Am. Chem. Soc., 2003, 125,6677; (b) I. Alfonso, M. Bolte, M. Bru, M. I. Burguete, S. V. Luisand J. Rubio, J. Am. Chem. Soc., 2008, 130, 6137; (c) M. Bru,I. Alfonso, M. I. Burguete and S. V. Luis, Angew. Chem., Int. Ed.,2006, 45, 6155.

19 J. Becerril, M. I. Burguete, B. Escuder, F. Galindo, R. Gavara,J. F. Miravet, S. V. Luis and G. Peris, Chem.–Eur. J., 2004, 10, 3879.

20 F. Galindo, M. I. Burguete, R. Gavara and S. V. Luis, J. Photochem.Photobiol., A, 2006, 178, 57.

21 M. I. Burguete, M. A. Izquierdo, F. Galindo and S. V. Luis, Chem.Phys. Lett., 2008, 460, 503.

22 (a) S. H.Wang,M. Y. Han andD. J. Huang, J. Am. Chem. Soc., 2009,131, 11692; (b) X. Q. Yan, Z. B. Shang, Z. Zhang, Y. Wang andW. J. Jin, Luminescence, 2009, 24, 255; (c) V. Fabregat,M. A. Izquierdo, M. I. Burguete, F. Galindo and S. V. Luis, Inorg.Chim. Acta, 2012, 381, 212–217.

23 C. A. Bunton, H. Dahn and L. Loewe, Nature, 1959, 183, 163.24 S. Srinivasan, S. S. Babu, V. K. Praveen and A. Ajayaghosh, Angew.

Chem., Int. Ed., 2008, 47, 5746.25 R. K. Das, S. Bhat, S. Banerjee, C. Aymonier, A. Loppinet-Serani,

P. Terech, U. Maitra, G. Raffy, J.-P. Desvergne and A. DelGuerzo, J. Mater. Chem., 2011, 21, 2740.

26 I. Alfonso, M. Bru, M. Isabel Burguete, E. Garc�ıa-Verdugo andS. V. Luis, Chem.–Eur. J., 2010, 16, 1246.

27 K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata,T. Komori, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc.,1994, 116, 6664.

28 R. E. Galian and M. de la Guardia, TrAC, Trends Anal. Chem., 2009,28, 279.

29 (a) R. E. Galian and J. C. Scaiano, Photochem. Photobiol. Sci., 2009,8, 70; (b) D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase andH. Weller, Nano Lett., 2001, 1, 207; (c) C. F. Landes, M. Braun andM. A. El-Sayed, J. Phys. Chem. B, 2001, 105, 10554.

30 M. I. Burguete, F. Galindo, R. Gavara, M. A. Izquierdo, J. C. Lima,S. V. Luis, A. J. Parola and F. Pina, Langmuir, 2008, 24, 9795.

31 (a) X. Y. Wang, L. H. Qu, J. Y. Zhang, X. G. Peng and M. Xiao,Nano Lett., 2003, 3, 1103; (b) M. J. Ruedas-Rama, A. Orte,E. A. H. Hall, J. M. Alvarez-Pez and E. M. Talavera,ChemPhysChem, 2011, 12, 919; (c) I.-S. Liu, H.-H. Lo, C.-T. Chien,Y.-Y. Lin, C.-W. Chen, Y.-F. Chen, W.-F. Su and S.-C. Liou,

This journal is ª The Royal Society of Chemistry 2012

J. Mater. Chem., 2008, 18, 675; (d) J. Rubio, M. A. Izquierdo,M. I. Burguete, F. Galindo and S. V. Luis, Nanoscale, 2011, 3, 3613.

32 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer,2006.

33 (a) C. Geiger, M. Stanescu, L. H. Chen and D. G.Whitten, Langmuir,1999, 15, 2241; (b) K. Hanabusa, K. Hiratsuka, M. Kimura andH. Shirai, Chem. Mater., 1999, 11, 649.

34 I. Yoshimura, Y. Miyahara, N. Kasagi, H. Yamane, A. Ojida andI. Hamachi, J. Am. Chem. Soc., 2004, 126, 12204.

35 N. S. Bryan andM.B.Grisham,FreeRadical Biol.Med., 2007, 43, 645.36 D. D. Nelson, J. B. McManus, S. C. Herndon, J. H. Shorter,

M. S. Zahniser, S. Blaser, L. Hvozdara, A. Muller, M. Giovanniniand J. Faist, Opt. Lett., 2006, 31, 2012.

37 (a) T. Nagano and T. Yoshimura, Chem. Rev., 2002, 102, 1235; (b)L. E. McQuade and S. J. Lippard, Curr. Opin. Chem. Biol., 2010,14, 43; (c) Y. Yang, S. K. Seidlits, M. M. Adams, V. M. Lynch,C. E. Schmidt, E. V. Anslyn and J. B. Shear, J. Am. Chem. Soc.,2010, 132, 13114; (d) D. J. Blyth, J. W. Aylott, J. W. B. Moir,D. J. Richardson and D. A. Russell, Analyst, 1999, 124, 129; (e)M. Bru, M. I. Burguete, F. Galindo, S. V. Luis, M. J. Marin andL. Vigara, Tetrahedron Lett., 2006, 47, 1787.

38 (a) M. Laferriere, R. E. Galian, V. Maurel and J. C. Scaiano, Chem.Commun., 2006, 257; (b) C. Tansakul, E. Lilie, E. D. Walter,F. Rivera, A. Wolcott, J. Z. Zhang, G. L. Millhauser andR. Braslau, J. Phys. Chem. C, 2010, 114, 7793.

39 H. Kim and G. Kwak, Macromolecules, 2009, 42, 902.40 H. Lee, S. H. Jung, W. S. Han, J. H. Moon, S. Kang, J. Y. Lee,

J. H. Jung and S. Shinkai, Chem.–Eur. J., 2011, 17, 2823.41 T. H. Kim, D. G. Kim, M. Lee and T. S. Lee, Tetrahedron, 2010, 66,

1667.42 Q. Ji, I. Honma, S. M. Paek, M. Akada, J. P. Hill, A. Vinu and

K. Ariga, Angew. Chem., Int. Ed., 2010, 49, 9737.43 S. R. Ng, C. X. Guo and C. M. Li, Electroanalysis, 2011, 23, 442.44 (a) H. Yang, T. Yi, Z. G. Zhou, Y. F. Zhou, J. C.Wu,M. Xu, F. Y. Li

and C. H. Huang, Langmuir, 2007, 23, 8224; (b) Q. T. Liu,Y. L. Wang, W. Li and L. X. Wu, Langmuir, 2007, 23, 8217; (c)W. Deng and D. H. Thompson, Soft Matter, 2010, 6, 1884; (d)X. Chen, Z. Huang, S. Y. Chen, K. Li, X. Q. Yu and L. Pu, J. Am.Chem. Soc., 2010, 132, 7297; (e) P. Xue, Y. Zhang, J. Jia, D. Xu,X. Zhang, X. Liu, H. Zhou, P. Zhang, R. Lu, M. Takafuji andH. Ihara, Soft Matter, 2011, 7, 8296.

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