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Journal of Sol-Gel Science andTechnology ISSN 0928-0707Volume 70Number 2 J Sol-Gel Sci Technol (2014) 70:245-253DOI 10.1007/s10971-013-3155-x

Click approaches in sol–gel chemistry

Xavier Cattoën, Achraf Noureddine,Jonas Croissant, Nirmalya Moitra,Kristýna Bürglová, Jana Hodačová,Olivia de los Cobos, et al.

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ORIGINAL PAPER

Click approaches in sol–gel chemistry

Xavier Cattoen • Achraf Noureddine • Jonas Croissant • Nirmalya Moitra •

Kristyna Burglova • Jana Hodacova • Olivia de los Cobos • Martine Lejeune •

Fabrice Rossignol • Delphine Toulemon • Sylvie Begin-Colin • Benoıt P. Pichon •

Laurence Raehm • Jean-Olivier Durand • Michel Wong Chi Man

Received: 13 August 2013 / Accepted: 12 September 2013 / Published online: 19 September 2013

� Springer Science+Business Media New York 2013

Abstract The combination of the copper-catalyzed alkyne-

azide cycloaddition (CuAAC) reaction with sol–gel processing

enables the versatile preparation of sol–gel materials under

different shapes with targeted functionalities through a diver-

sity-oriented approach. In this account, the development of the

CuAAC reaction under anhydrous conditions for the synthesis

of sol–gel precursors and for the assembling of magnetic

nanoparticles on self-assembled monolayers is related, as well

as the use of the classical CuAAC methodologies for the

functionalization of mesoporous silica nanoparticles and mi-

crodots arrays. Coupling CuAAC and Sol–Gel will result in

simplified preparations of multifunctional materials with

controlled morphologies.

Keywords Sol–gel � Click chemistry � CuAAC �Organosilanes � Nanoparticles

1 Introduction

Introducing organic or biological functionalities into sol–gel

materials is now a commonly used technique to prepare

organic/inorganic hybrid materials with targeted properties.

Indeed, the sol–gel process allows forming inorganic matrices

with precise control on the particles’ size and morphology

(monoliths, nanoparticles, coatings, fibres, microdots…), but

also on the textural properties (meso- or macroporosity).

Furthermore, the moderate temperatures used (20–150 �C)

enable incorporating organic fragments within the inorganic

matrix during the synthesis, while preserving their integrity

[1]. The grafting of organo-alkoxysilanes on preformed silica

materials is typically used to incorporate organic functional-

ities on silica surfaces thanks to the strength and inertness of

most Si–C bonds. It displays the great advantage of preserving

the texture of the starting material, which synthesis is well-

controlled. However, the high temperature used (typically

110 �C) and the high spatial inhomogeneity of the resulting

material can constitute serious issues. These issues can be

circumvented by using the co-condensation synthesis between

a tetrafunctional silicon derivative and an organo-trialkox-

ysilane [2–4], known to afford materials with a low diversity

of environments for the organic moiety. However, in this case,

the texture and morphology of the materials may greatly

depend on the amount and nature of the organosilane used [5].

Improved post-functionalization methods are thus needed,

with the aim of performing at low-temperature in short

X. Cattoen (&) � A. Noureddine � J. Croissant � N. Moitra �K. Burglova � L. Raehm � J.-O. Durand � M. Wong Chi Man

Institut Charles Gerhardt Montpellier (UMR 5253

CNRS-UM2-ENSCM-UM1), 8, rue de l’ecole normale,

34296 Montpellier, France

e-mail: xavier.cattoen@grenoble.cnrs.fr

X. Cattoen

Univ. Grenoble Alpes, Inst NEEL, 38042 Grenoble, France

X. Cattoen

CNRS, Inst NEEL, 38042 Grenoble, France

K. Burglova � J. Hodacova

Department of Organic Chemistry, Institute of Chemical

Technology, Technicka 5, 16628 Praha 6, Czech Republic

O. de los Cobos � M. Lejeune � F. Rossignol

Laboratoire de Science des Procedes Ceramiques et de

Traitements de Surface (SPCTS), UMR CNRS 7315, CEC,

12 rue Atlantis, 87068 Limoges, France

D. Toulemon � S. Begin-Colin � B. P. Pichon

Institut de Physique et Chimie des Materiaux de Strasbourg,

UMR 7504 (CNRS-UdS-ECPM), 23, rue du Loess, BP 43,

67034 Strasbourg Cedex 2, France

123

J Sol-Gel Sci Technol (2014) 70:245–253

DOI 10.1007/s10971-013-3155-x

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reaction times with a very wide scope, and without the need of

moisture-sensitive reagents such as organo-alkoxysilanes.

Recently, click reactions were defined as a set of wide-

scope, modular reactions occurring with high yield and

selectivity under mild and green conditions, using harmless

reactants and solvents for covalently coupling molecular

building blocks [6]. Initially developed for drug discovery,

this concept has been generalized, and is now commonly

applied in polymer and materials chemistry [7]. By far, the

most widely used (but not exclusive) click reaction is the

copper(I)-catalyzed azide-alkyne cycloaddition reaction

termed CuAAC that has known impressive popularity since its

discovery in 2002 [8–10]. Since 2008 the CuAAC reaction has

been applied to silica materials as a synthetic tool to covalently

support homogeneous catalysts on amorphous silica [11, 12],

to block the pore openings of mesoporous silica nanoparticles

(MSN) [13], or to bind proteins within large-pore SBA-15

[14]. Interestingly, the groups of Stack [15, 16] and Sen Gupta

[17] independently synthesized clickable SBA-15 materials

from a silica source and (3-azidopropyl)triethoxysilane

(AzPTES), that can be easily post-functionalized using the

CuAAC reaction. These authors showed that, owing to the co-

condensation strategy used, the azide groups were homoge-

neously distributed within the silica material. This enabled

wide-scope post-functionalization, occurring under mild

conditions with retention of the texture and morphology of the

starting material, thus featuring the advantages of both graft-

ing and co-condensation methods. Interesting applications for

grafting isolated catalysts could be exploited [18, 19]. In the

course of our investigations on organosilicas and bridged

silsesquioxanes for applications in catalysis [2, 20–22] and

optics [23–27], and as we faced difficulties in functionalizing

silica materials and in synthesizing organo-alkoxysilanes sol–

gel precursors for silica-based materials, we decided to

investigate the opportunities brought by the CuAAC reaction

in combination with the sol–gel process to readily obtain or-

ganosilane molecular derivatives as well as sol–gel materials

under various morphologies and textures (Fig. 1). In this

account, we will first describe the development of the CuAAC

reaction under anhydrous conditions to easily prepare organo-

alkoxysilanes, precursors of sol–gel materials (path A, Fig. 1).

We will then focus on the formation of clickable mesoporous

materials, opening the way to multifunctional materials with

controlled loadings (Path B, Fig. 1).

2 Discussion

2.1 CuAAC under anhydrous conditions to synthesize

sol–gel precursors

Despite the high number of examples of organo-alkoxy-

silanes existing in the literature, the introduction of

trialkoxysilyl groups on various organic fragments still

remains challenging. The main issues of the preexisting

methods are the nature of the linker between the silicon

atom and the functional organic moiety (urea or carbamate

groups may cause aggregation of organic fragments in the

final materials for instance [28, 29]), or the purity of the

final product, as the presence of the moisture-sensitive

alkoxysilyl groups strongly decreases the yields of chro-

matographic separation. Even if non moisture-sensitive

groups such as triallylsilanes [30] or tri(isopropoxy)silanes

[31] have been described, a general approach to easily

afford organo-alkoxysilanes in high yield, good purity and

with a very wide scope was needed.

Therefore, we decided to investigate the potential of the

CuAAC reaction between AzPTES and a functional alkyne

for the synthesis of such compounds. Although this reac-

tion is usually performed in aqueous medium (the typical

Sharpless procedure uses a copper (II) precatalyst with

sodium ascorbate as reducing agent in tert-butanol and

water [8]), the CuAAC reaction can also be performed in

non-aqueous, aprotic solvents [32]. In these cases, the

copper (I) sources that were used were copper(I) iodide,

Cu-NHC complexes, and the CuBr(PPh3)3 complex [33],

which has the advantage of being easily synthesized and

soluble in organic solvents. Based on previous work, we

decided to use a dry solvent mixture of THF and triethyl-

amine, this base being necessary to induce the formation of

the copper acetylide intermediate. From our first attempt,

this reaction fortunately occurs with quantitative yield and

selectivity, using a low amount of catalyst (Table 1, Entry

1) [34]. Indeed, the reaction between AzPTES and phen-

ylacetylene proceeds quantitatively at 20 �C overnight with

Fig. 1 Access to various functional hybrid silica materials using a

combination of sol–gel and CuAAC reactions

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only 0.5 mol % of catalyst (Table 1, Entry 2). After

evaporating the solvents, and extracting the residue with

pentane, the blue-green copper residue was discarded,

affording the new organo-alkoxysilane with only traces of

triphenylphosphine oxide as side-product. Interestingly,

this reaction can be easily thermally activated, either by

conventional heating (3 h at 50 �C under the previous

conditions) or by microwave irradiation at 100 �C

(Table 1, Entries 4 and 5), with a significantly reduced

reaction time of 5 min, while preserving the selectivity of

the reaction.

The scope of this reaction is very wide: arylacetylenes

featuring either electron donating or withdrawing groups

can be derivatized as well as simple alkynes with primary,

secondary or tertiary alkyl chains (Fig. 2). The reaction

also works for propargyl amine and propargyl alcohol. The

latter example is very interesting as trialkoxysilanes usu-

ally react with alcohol functions by transesterification

reactions at silicon, precluding their isolation in pure form.

Here, the reaction conditions allow isolating and fully

characterizing such species, before the oligomerization

takes place (within few days at room temperature). Com-

pounds bearing other challenging functional groups could

be derivatized, with functions such as thioethers, epoxides,

amino-alcohols and imidazoliums [35]. Organosilanes

bearing important functionalities for optics, electronics,

biological or catalytic applications could be obtained,

based on the pyrene, amino-naphthalimide, ferrocene,

thymine and tartrate moieties (Fig. 2). The CuAAC-sily-

lation reaction was also applied to obtain bridged organo-

silanes, which are the precursors of bridged silsesquioxanes

and periodic mesoporous organosilicas. It is worth men-

tioning that the scope of the reaction could be widened

after the synthesis of several alkyne-bearing organosilanes

(Fig. 3), which can be derivatized as previously, although

the thioether linker gave a weaker reactivity than the urea

or amino ones [35]. Thus, when performed under non-

aqueous conditions and in aprotic, dry solvents, the Cu-

AAC reactions can be used to provide a wide range of

functional and non previously accessible organo-alkoxy-

silanes, precursors of silica-based materials. It is note-

worthy that apart from clickable organosilanes, analogous

phosphonic esters and acids can be synthesized, providing

an easy way to functionalize transition-metal oxide mate-

rials, such as iron oxide nanoparticles.

Indeed, we used the above-described CuAAC method-

ology under non-aqueous conditions to covalently anchor

iron oxide nanoparticles functionalized with azide groups

to self-assembled monolayers (SAM) on gold containing

pending alkyne functions (Fig. 4) [36, 37]. The aprotic

conditions were essential to provide a good stability to the

nanoparticles colloidal suspension. However, the CuAAC

reaction, when performed at room temperature with a large

amount of catalyst, was slow, with a maximum coverage

occurring after 48 h of reaction [36]. We recently managed

to accelerate the CuAAC-driven assembly by using

microwave activation (50 W, 100 �C), which reasonably

decreased the reaction time, reaching maximum coverage

in less than 1 h [37]. Interestingly, by varying the reaction

time different densities of magnetic particles were

obtained, allowing a fine control over the collective mag-

netic properties (Fig. 4). Furthermore, the covalent nature

of the SAM–NP linkages formed under kinetic control

results in a good stability of the assemblies, whereas NP

assembled through weak interactions on the surface under

thermodynamic control may rearrange to the most stable

state as a result of inter-particle interactions [38, 39].

2.2 CuAAC under classical conditions to functionalize

sol–gel materials

The control over the structure at different scales and over

the texture is a key element in sol–gel science. In the

past years, we were involved in the synthesis of functional

organosilica materials in the form of bulk powders, mes-

oporous powders, mesoporous nanoparticles and meso-

porous microdots arrays obtained by ink-jet printing. It is

well-known that the incorporation of organo-alkoxysilanes

during the co-condensation synthesis of mesoporous silica

leads to a change of the texture as the amount of orga-

nosilane increases, [4] but this effect becomes more

important and deleterious for the synthesis of organically

modified MSN and microdots arrays [40]. In the former

case, not only the mesopore organization is affected, but

also the shape of the nano-object can be altered depending

on the properties of the organosilane [41–43]. In the latter

Table 1 Optimization of the reaction conditions for the CuAAC of

AzPTES and phenylacetylene (reproduced from Ref. [34] by per-

mission of the Royal Society of Chemistry)

Entry Catalyst loading

(%)

Temperature Time Conversion

(%)a

1 1 rt 15 h 100

2 0.5 rt 24 h 100

3 0.3 rt 30 h 67

4 0.5 50 �C 3 h 100

5 0.5 100 �Cb 5 min 100

6 0 100 �Cb 5 min 0

Reaction conditions: AzPTES (2.0 mmol), phenylacetylene

(2.0 mmol), [CuBr(PPh3)3] in thf/Et3N 1:1 (1 mL) under nitrogen

atmospherea Determined by 1H NMR. bmicrowave heating

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case, the ejection of the sol can even be precluded owing to

changes in the rheological properties of the sol. Therefore,

in both cases the formulation of the sol has to be carefully

optimized for each organosilane added. In order to build up

a diversity-oriented approach, click chemistry revealed an

important concept to develop in combination with sol–gel

Fe

(EtO)3Si N3 + R[CuBr(PPh3)3]

anh THFanh Et3Nµw, 5 min100 °C

(EtO)3Si N NN

R

R =

F3C

CF3

NHO H2N

Ph OO O

N

O

O

H2N

90-98%

Fig. 2 Selected examples of organo-triethoxysilanes obtained by CuAAC from AzPTES [34, 35]

(EtO)3Si N3 (EtO)3Si N39

(EtO)3Si S (EtO)3Si N

(EtO)3Si NH

O

NH

Fig. 3 Easily synthesized clickable triethoxysilanes [35]

Fig. 4 Microwave-assisted

covalent anchoring of 20 nm

iron oxide nanoparticles on self-

assembled monolayers using the

CuAAC reaction. Adapted with

permission from Ref. [37].

Copyright 2013 American

Chemical Society

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processing. For both MSN and microdots arrays, sols

leading to well-defined structures with mercapto function-

alities were developed, thanks to the commercial avail-

ability of (3-mercaptopropyl)trialkoxysilanes [40, 42].

However, the functionalization of such materials by thiol-

alkene coupling was not undertaken. This reaction,

although classified as click reaction, has several drawbacks

such as the difficult monitoring of the reaction by FTIR or

the radical nature of the reaction, which might give rise to

side products. Therefore, we decided to synthesize azide or

alkyne-containing MSN and microdots arrays for further

CuAAC functionalization. Even if the starting organosi-

lanes are not commercially available, they can be easily

prepared. Furthermore, the azide groups display a very

strong absorption peak in IR at ca 2,100 cm-1, a region

where few other groups absorb, which enables easy mon-

itoring of the reaction.

2.3 Clickable mesoporous silica nanoparticles

Mesoporous silica nanoparticles containing 1–10 % of

azide or alkyne groups were obtained from a concentrated

solution of TEOS, organosilane and CTAB, using a direct

microemulsion synthesis method in alkaline medium fol-

lowed by dilution then neutralization to quench the growth

of the particles [44]. Depending on the organosilane used,

and on the amount incorporated, different morphological

and textural features were observed: azide-containing

nanoparticles (50–80 nm) where obtained, though slightly

agglomerated, with disordered pores of 2.4–2.9 nm (Fig. 5)

[45]. Alternatively, the so-called Lin’s procedure to obtain

the nanoparticles at high dilution could be applied to afford

spherical particles with 2D-hexagonal pore assemblies, as

shown more recently by Sen Gupta and co-workers [46]. In

the case of alkyne-functionalized particles, individual

particles with well-defined morphology and 2D hexagonal

pore arrangements were characterized. Morphological dif-

ferences were evident depending on the amount of orga-

nosilane incorporated: nanorods (60 9 180 nm) and

spherical nanoparticles (70–80 nm) were obtained for or-

ganosilane loadings of 10 and 1 %, respectively. Interest-

ingly, the BJH pore diameter was also decreased when

increasing the organosilane loading, from 2.3 to 1.7 nm,

probably because of strong interactions occurring during

the materials formation between the lipophilic propargyl-

propyl thioether fragment and the core of the micelles.

The functionalization of these MSN was carried out using

simple pyrene derivatives in the presence of copper (II)

sulfate and sodium ascorbate. It is noteworthy that mostly all

azide groups could be functionalized, as evidenced by the

almost total disappearance of the band at 2,100 cm-1 in

FTIR, even for an initial azide loading of 10 %. This resulted

in a strong decrease of the surface area and adsorbed vol-

umes in N2-sorption experiments. As already observed for

SBA15-N3 powders, isolated pyrene groups were charac-

terized by fluorescence for organosilane loadings of 1 %,

whereas the excimer fluorescence was preeminent in the

case of loadings of 10 % after functionalization. This and

Fig. 5 Synthesis and electron micrographs of clickable MSN. Adapted from Ref. [45] by permission of The Royal Society of Chemistry

J Sol-Gel Sci Technol (2014) 70:245–253 249

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other ongoing studies show that a majority of isolated sites

can be obtained when keeping the organosilane loading

below 2 % [45]. These MCM41-N3 nanoparticles were

applied by Sen Gupta et al. to form hydrogen peroxidase

mimics by clicking Fe-biuret derivatives [46]. Furthermore,

bi-functional clickable MCM41-N3-C=C nanoparticles

were recently reported by Studer for sequential bi-func-

tionalization: alkene and azide groups can be individually

and orthogonally derivatized using thiol and alkynes groups,

respectively. This enabled an easy formation of nanoparti-

cles bearing both acidic and basic sites, useful for catalyzing

the Henry reaction through dual activation [47].

2.3.1 Clickable mesoporous silica microdots arrays

Applying ink jet-[48] or pin-printing [49] to pattern func-

tional silica on substrates is a promising technology for

miniaturizing biochips. Using a piezoelectric drop-on-

demand ink-jet printing device developed in the Limoges

laboratory by Ceradrop [50], previous studies showed that

it is possible to print arrays of microdots (100 lm in

diameter) formed of mesoporous silica [51], or even mer-

capto-functionalized mesoporous silica microdots that were

used to entrap gold nanoparticles within the mesopores

[40]. Aiming at incorporating a broad range of function-

alities within the microdots arrays, we first decided to

formulate a sol based on AzPTES. Following previous

studies, an acidic sol containing TEOS, TFTS, AzPTES

and the F127 surfactant was used, and printed on silicon

substrates [52]. This led to well-defined microdots arrays

with a worm-like pore organization, and accessible azide

functions. Indeed, after consolidation of the network and

removal of the surfactant, CuAAC reactions with simple

alkynes such as propargyl alcohol or methyl propynoate

were conducted at room temperature using Sharpless’

conditions. It is noteworthy that the functionalization

Fig. 6 Synthesis and

functionalization of bis-

clickable intercrossed

mesoporous microdots arrays.

Adapted with permission from

Ref. [52]. Copyright 2012

American Chemical Society

Fig. 7 CuAAC covalent

functionalization of alkyne-

derivatized silica microdots

with azidopropyl-adenine and

subsequent recognition with

labeled thymine. The central

column consists of azide-silica

and serves as control. It can be

further functionalized by alkyne

nucleosides in the same fashion

to obtain a multi-functional

network

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reaction is again very easy to monitor by following the

disappearance of the strong N3 band at 2,100 cm-1.

Alkyne-containing microdots arrays were also successfully

deposited on silicon by ink-jet printing, using (3-(propar-

gylthio)propyl)triethoxysilane (Fig. 6). However, in this

case, the monitoring of the reaction is much more difficult,

owing to the weak mCC and mC-H vibrations at ca 2,100 and

3,300 cm-1 in IR. Raman experiments are therefore nee-

ded to follow the disappearance of the mCC band [45].

Using clickable dyes in the CuAAC reaction allowed us to

investigate the selectivity of the functionalization (Fig. 6).

Indeed, bi-functional arrays consisting of intercrossed

networks of azide- and alkyne-functionalized microdots

were produced. When the alkyne-derivatized naphthali-

mide was first clicked, the fluorescence was detected only

on half of the microdots, which clearly evidences that the

alkyne functions reacted only on the azide microdots, and

that the functionalization was indeed covalent, with neg-

ligible unreacted adsorbed species remaining. Moreover, a

second reaction, this time with an azide-containing hemi-

cyanine (red fluorophore) allowed obtaining a bi-functional

material with well-defined alternate positions for each

fluorophore. Notably, the order of the reactions could be

reversed, without modification of the final result (Fig. 6)

[52]. From this principle, we then envisioned to build

multifunctional biosensors. As a proof of principle, 9-(3-

azidopropyl)adenine was selectively grafted on the alkyne

functions of intercrossed arrays of alkyne- and azide-con-

taining mesoporous silica microdots (Fig. 7). After a

chloroform solution of the complementary base thymine

labeled with a hemicyanine was added, then rinsing, con-

focal fluorescence microscopy revealed the successful

selective recognition on the functionalized microdots. This

work paves the way to the development of efficient bio-

sensors, thanks to the high density of available sites in the

mesoporous structure.

3 Conclusion

Combined with the richness of sol–gel processing which

allows forming materials at low temperature while con-

trolling morphology and texture, the CuAAC reaction

appears to be an excellent post-functionalization method to

introduce a wide variety of functionalities into well-defined

silica-based materials. Recent progress includes the for-

mation of new organosilane precursors in a selective,

straightforward and time-effective manner, the formation

of clickable mesoporous nanoparticles that can be further

functionalized while keeping intact the morphology and

texture, and the coupling of ink-jet printing and CuAAC

chemistry to afford a new generation of platforms for

biosensing. Further work is in progress to implement

multiple functionalities on single nano- or micro-objects,

aiming at developing new catalysts, nanomachines, bio-

sensors and imaging and therapeutic agents in a diversity-

oriented approach.

Acknowledgments The CNRS, the Agence Nationale pour la

Recherche (ANR P2N 2010 MECHANANO and ANR P2N 2012

NanoptPDT), the Direction Generale de l’Armement (PhD Grant to

DT), the Czech Science Foundation (P108/12/1356), the BAR-

RANDE project (MSMT 7AMB12FR004), the French embassy in

Prague (Grant to KB), the Limousin region through Theranostic

program and the Partner University Fund, a program of FACE, are

gratefully acknowledged for financial support.

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