Self-functionalizing polymer film surfaces assisted by specific polystyrene end-tagging

Article abstract of DOI:10.1021/cm903652x

Two simple approaches to build up micropatterned functionalized polymer films are reported here. Both of them are based on the formation of porous films consisting of two-dimensionally ordered void structures, produced by evaporation of a polymer solution in the presence of humidity. In the first approach, we utilize an amino-terminated linear polystyrene to fabricate honeycomb structured films in which the cavities are enriched with amino groups that preserve their chemical reactivity. This allows, in principle, to obtain films with different surface functionality by simply changing the chain-end of the polymer used. In the second approach, we synthesize a luminescent chain-ended polystyrene to show how the honeycomb structures can be easily transformed, by a thermal treatment, into flat micropatterned fluorescent films. Both the microporous and the flat micropatterned films resulting from this study are attractive materials since the chemical functions distributed on their surface can further react with other molecules to provide a more complex array suitable for biological tests.

Full text of DOI:10.1021/cm903652x

2764 Chem. Mater. 2010, 22, 2764–2769
DOI:10.1021/cm903652x
Self-Functionalizing Polymer Film Surfaces Assisted by Specific
Polystyrene End-Tagging
Francesco Galeotti,*,†, Valentina Calabrese,§, Marco Cavazzini,§, Silvio Quici,§,
Claude Poleunis,‡ Sami Yunus,‡ and Alberto Bolognesi†,
†CNR-Istituto per lo Studio delle Macromolecole (ISMAC), via E. Bassini 15, 20133 Milano,
‡
Italy, Universite catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium,
ꢀ
§CNR-Istituto di Scienze e Tecnologie Molecolari (ISTM), via C. Golgi 19, 20133 Milano, Italy, and
Polo Scientifico e Tecnologico (PTS), CNR, via Fantoli 16/15, 20138 Milano, Italy
Received December 3, 2009. Revised Manuscript Received March 5, 2010
Two simple approaches to build up micropatterned functionalized polymer films are reported here.
Both of them are based on the formation of porous films consisting of two-dimensionally ordered void
structures, produced by evaporation of a polymer solution in the presence of humidity. In the first
approach, we utilize an amino-terminated linear polystyrene to fabricate honeycomb structured films in
which the cavities are enriched with amino groups that preserve their chemical reactivity. This allows, in
principle, to obtain films with different surface functionality by simply changing the chain-end of the
polymer used. In the second approach, we synthesize a luminescent chain-ended polystyrene to show how
the honeycomb structures can be easily transformed, by a thermal treatment, into flat micropatterned
fluorescent films. Both the microporous and the flat micropatterned films resulting from this study are
attractive materials since the chemical functions distributed on their surface can further react with other
molecules to provide a more complex array suitable for biological tests.
Introduction
These features are commonly achieved by top-down strategies
such as microcontact printing or photolithographic techniques
combined with one or more functionalization steps. Although
multistep approaches are highly versatile and normally ensure
accurate results, self-organization techniques have the undis-
putable advantages of being fast, simple, and cheap.
In nature, specific functionalities of organisms are often
regulated by the precise micro or nanostructures on their
surfaces, more than by intrinsic properties of the materials.
In a similar way, the control over surface morphology is an
essential goal for those researchers that aim at the manipula-
tion and construction of new functional materials.1-6 Indeed
the possibility of obtaining in the laboratory materials having
not only definite surface morphology but also even a con-
trolled distribution of chemical functionalities is a challenge
that represents a fascinating field of research, one that could
possibly lead to applications in different areas like microelec-
tronics, microoptics, biotechnology, and smart packaging.7-11
In this view, the water drop templating method known as
the Breath Figure (BF) imprinting procedure comprises a
promising method for the preparation of polymer films
with a regular distribution of cavities on their surface. This
technique is based on the evaporative cooling of a polymer
solution in a wet environment, allowing water droplets to
produce ordered arrays of pores on the film surface.12 When
the process is adequately controlled, several parameters
can be adjusted (solvent, polymer concentration, evapora-
tion rate, humidity) to obtain different film morphologies.
In particular, patterned structures with regular size pores
ranging from 200 nm to 10 μm can be obtained, mono- and
multilayers can be formed, as well as deformation of the
spherical cavities can be achieved.13-15 Different models
have been proposed to explain BF patterns formation
*To whom correspondence should be addressed. E-mail: f.galeotti@
ismac.cnr.it. Fax: þ39 02 70636400. Phone: þ39 02 23699739.
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Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213–2217.
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Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894–897.
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involving thermocapillary forces, Bernard-Marangoni
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Licciardello, A.; Rampi, M. A. Nat. Mater. 2009, 8, 41–46.
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W.; Torres, C. M. S.; Ahopelto, J. Adv. Funct. Mater. 2009, 19,
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Mater. 2008, 18, 3706–3714.
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r
pubs.acs.org/cm
Published on Web 03/22/2010
2010 American Chemical Society

Article
Chem. Mater., Vol. 22, No. 9, 2010 2765
convection, and surface tension effects.13,16-18 Our group
previously described this phenomenon in detail.19 It was
pointed out how the presence of hydrophilic terminal
groups in linear polystyrenes promotes the stabilization
of the polymer-water interface during the BF formation,
thus increasing the regularity of the honeycomb structures.
Moreover we evidenced using the ToF-SIMS (time-of-
flight secondary-ion mass spectrometry) technique how
1H-pyrrole 1 (0.80 cm3, 5.94 mmol) and 4-[3-(1,3-dioxoiso-
indolin-2-yl)propoxy]benzaldehyde 2 (920 mg, 2.97 mmol) were
dissolved in dry CH2Cl2 (180 cm3) under nitrogen atmosphere.
One drop of trifluoroacetic acid was added, and the resulting
mixture was stirred at room temperature for 1 h. The solution
was diluted with CH2Cl2 (90 cm3), and 2,3-dichloro-5,6-dicyano-
benzoquinone (DDQ) (1.35 g, 5.94 mmol) was added. Stir-
ring was continued for 1 h, followed by the addition of DIPEA
(7.40 cm3, 44.55 mmol) and BF3 Et2O (7.50 cm3, 59.40 mmol).
3
these polar groups concentrate inside the film cavities.20
A
After 1 h the reaction mixture was treated with water (300 cm3),
and the two-phases mixture was passed through a Celite filter.
The organic phase was separated and washed again with water
(2 ꢀ 150 cm3), dried over Na2SO4, and evaporated to dryness.
The dark brown residue was purified by silica gel column
chromatography using dichloromethane as eluent to give 3
(810 mg, 47%) as an red-orange solid. δH(400 MHz; CDCl3)
7.85 (2 H, m), 7.73 (2 H, m), 7.11 (2 H, d, J 8.4), 6.88 (2 H, d,
J 8.4), 4.11 (2 H, t, J 6.4), 3.96 (2 H, t, J 6.4), 2.53 (6 H, s), 2.27
(6 H, m), 1.32 (6 H, s), 0.98 (6 H, t, J 7.5); δC(100 MHz; CDCl3)
168.4, 159.1, 153.5, 140.3, 138.5, 134.0, 132.6, 132.2, 131.2,
129.4, 128.0, 123.3, 114.9, 65.9, 35.5, 28.3, 17.1, 14.6, 12.5,
11.9. Anal. Calcd (%) for C34H36BF2N3O3 C 69.99, H 6.22, N
7.20; Found C 70.12, H 6.17, N 7.14.
similar behavior, observed by several authors, is explained
considering that water droplets act not only as template for
holes array but also can orient polar functions inside the
cavities.18,21-24
In this work we focus our attention on two different
approaches to build up micropatterned functionalized
films using the BF technique. In the first one we use an
amino-terminated polystyrene (PS) to produce thin por-
ous films having the cavities enriched in -NH2 residues.
Cavities are then selectively “painted” with a specific
fluorescent dye. In the second approach we utilize a
fluorescent labeled amino-polystyrene to show how the
honeycomb structures can easily be transformed into flat
micropatterned fluorescent films.
Compound 4. Hydrazine monohydrate (0.6 cm3, 12.34 mmol)
was added to a solution of 3 (150 mg, 0.26 mmol) in EtOH
(20 cm3), and the resulting mixture was stirred at reflux,
monitoring the disappearance of starting compound by TLC
(silica, CH2Cl2). The reaction mixture was then cooled at room
temperature, and a white solid was filtered off. The deep red
solution was evaporated to dryness, and the obtained residue
was purified by silica gel column chromatography using chloro-
form/methanol as eluent (9:1) to give 4 (950 mg, 82%) as a red
solid. δH(400 MHz; CDCl3) 7.16 (2 H, d, J 8.4), 7.01 (2 H, d,
J 8.4), 4.14 (2 H, t, J 6.4), 3.06 (2 H, t, J 6.4), 2.09 (2 H, s. all.),
2.53 (6 H, s), 2.30 (4 H, quartet, J 7.6), 2.08 (2 H, quintet, J 6.4),
1.33 (6 H, s), 0.98 (6 H, t, J 7.6); δC(100 MHz; CDCl3) 159.2, 153.5,
140.2, 138.4, 132.7, 131.2, 129.5, 128.1, 116.1, 114.9, 65.9, 38.9,
31.5, 29.7, 17.1, 14.6, 12.5, 11.9. Anal. Calcd (%) for C26H34-
BF2N3O C 68.88, H 7.56, N 9.27; Found C 68.35, H 7.61, N 9.35.
Compound 5. Compound 4 (130 mg, 0.29 mmol) was dis-
solved in dry toluene (20 cm3) and Et3N (0.37 cm3, 2.87 mmol)
under a nitrogen atmosphere. Then thiophosgene (0.065 cm3,
0.86 mmol) was added to the solution, and the mixture was
refluxed under nitrogen until thin-layer chromatography (TLC)
showed complete consumption of the starting material (silica,
EtP/CH2Cl2 4:1). Then the clean solution was evaporate to
dryness to afford a brown residue, which was purified by silica
gel column chromatography using petroleum ether/dichloro-
methane as eluent (1:1) to give 5 (113 mg, 80%) as a red powder.
δH(400 MHz; CDCl3) 7.18 (2 H, d, J 8.4), 7.01 (2 H, d, J 8.4),
4.16 (2 H, t, J 5.6), 3.83 (2 H, t, J 6.0), 2.53 (6 H, s), 2.31 (4 H,
quartet, J 7.6), 2.22 (2 H, quintet, J 5.6), 1.34 (6 H, s), 0.99 (6 H, t,
J 7.6); δC(100 MHz; CDCl3) 158.9, 153.6, 140.0, 138.4, 132.7,
131.2, 129.6, 128.4, 115.0, 64.1, 42.1, 29.9, 17.1, 14.7, 12.5, 11.9.
Anal. Calcd (%) for C27H32BF2N3OS C 65.46, H 6.51, N 8.48;
Found C 66.03, H 6.57, N 8.41.
Experimental Section
€
Materials. Tetrahydrofuran (THF) (Riedel de Haen) was
distilled from Na/K alloy before use. Styrene (Aldrich) was purified
through the trap-by-trap technique before use. 2,2,6,6-Tetramethyl-
1-piperidyloxy radical (TEMPO) derivatives (4-amino-TEMPO,
4-hydroxy-TEMPO, and 4-carboxy-TEMPO), as well as all other
reagents and solvents mentioned in this section, were purchased
from Sigma-Aldrich and used without further purification.
GPC measurements were carried out on a Waters SEC system
equipped with a 2414 RI and a 490 UV detectors, 2 PL gel Mix C
columns, THF as solvent, and PS as reference.
Confocal and fluorescence images were collected with a
Nikon Eclipse TE2000-U inverted confocal microscope with a
long working distance and using a Plan Fluor objective (magnifi-
cation 40, N.A. 0.75). The confocal measurements were done
with an Arþ-ion laser at 488 nm.
Atomic force microscopy (AFM) investigations were per-
formed using a NT-MDT NTEGRA apparatus in tapping mode
under ambient conditions.
Scanning Electron Microscopy images were recorded using a
Zeiss Leo 982 apparatus at 3 kV tension.
Synthesis of Boron-Dipyrromethene TEMPO Derivative
(TEMPO-NH-bodipy) (6). Compound 3. 3-Ethyl-2,4-dimethyl-
(16) Pitois, O.; Franc-ois, B. Eur. Phys. J. B 1999, 8, 225–231.
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Ijiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 329,
854–856.
(19) Bolognesi, A.; Mercogliano, C.; Yunus, S.; Civardi, M.; Comoretto,
D.; Turturro, A. Langmuir 2005, 21, 3480–3485.
Compound 6. A solution of 4-amino-TEMPO (31 mg, 0.18
mmol) in dry toluene (3 cm3) was added dropwise to a solution
of 5 (100 mg, 0.22 mmol) in dry toluene (6 cm3) under nitrogen
atmosphere. The resulting mixture was stirred at room tempera-
ture for 5 h. After cooling, the organic solvent was evaporated
under reduced pressure, and the residue was purified by silica gel
column chromatography using toluene/ethyl acetate (1:1) as
eluent, to afford 6 (50 mg, 42%) as a dark red solid. m/z (ESI)
(20) Yunus, S.; Delcorte, A.; Poleunis, C.; Bertrand, P.; Bolognesi, A.;
Botta, C. Adv. Funct. Mater. 2007, 17, 1079–1084.
(21) Billon, L.; Manguian, M.; Pellerin, V.; Joubert, M.; Eterradossi,
O.; Garay, H. Macromolecules 2009, 42, 345–356.
(22) Boker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.;
Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick,
T.; Russell, T. P. Nat. Mater. 2004, 3, 302–306.
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3556.
(24) Zhang, Y.; Wang, C. Adv. Mater. 2007, 19, 913–916.

2766 Chem. Mater., Vol. 22, No. 9, 2010
Galeotti et al.
Scheme 1. Synthesis of TEMPO-NH-bodipy (6)
689.37070 (MþNaþ) C36H51BF2N5OS requires 666.38. Anal.
Calcd (%) for C36H51BF2N5O2S C 64.85, H 7.71, N 10.50;
Found C 65.32, H 7.77, N 10.41.
Scheme 2. Reaction of Fluorescamine with Amines to Form a
Fluorescent Compound
TEMPO Mediated Free Radical Polymerization of Styrene
(General Procedure). In a Schlenk flask 0.040 mmol of TEMPO
initiator were dissolved in 2 mL of styrene, flushed with
nitrogen, and polymerized at 125 °C under vigorous stirring
for expected times. After the polymerization was stopped, the
cooled mixture was solubilized in small amounts of THF and
then poured dropwise into methanol (150 mL) under stirring.
The resulting precipitate was filtered and washed with fresh
methanol. 4-Amino-TEMPO gave PST-NH2 (GPC: Mw 9.38
kDa, Mw/Mn 1.8). 4-Carboxy-TEMPO gave PST-COOH (Mw
3.30 kDa, Mw/Mn 1.14). 4-Hydroxy-TEMPO gave PST-OH
(GPC: Mw 5.15 kDa, Mw/Mn 1.2). TEMPO-NH-bodipy (6)
gave PST-NH-bodipy (Mw 84.7 kDa, Mw/Mn 1.7).
ToF-SIMS is a remarkably suitable technique to study the
distribution of different chemical groups inside and outside
the cavities of a honeycomb structured film prepared by the
BF procedure.20 In that study we used Tof-SIMS to char-
acterize microporous films prepared using a TEMPO termi-
nated PS in which piperydinyloxyl group was further con-
verted into a more polar p-toluensulfonate acid salt. Here we
have been able to demonstrate that even the less polar amino
group is hydrophilic enough to produce PS porous films in
which cavities are effectively enriched with -NH2 groups.
With this aim, an amino terminated PS (PST-NH2) hasbeen
synthesized by nitroxide-mediated living radical polymeri-
zation,25,26 and its carbon disulfide solution has been used to
prepare thin films by the BF procedure. For a preliminary
fast assay, these films have been treated with fluorescamine
(Scheme 2), a common reagent for amino acid revelation.
Fluorescamine is not fluorescent itself, but reacts with
primary amine groups giving a highly fluorescent adduct.27
This feature avoids undesirable fluorescent background
phenomena.
Preparation of Samples. Microporous films were prepared by
casting a CS2 solution of the polymer on a glass microscopy slide
or on a silicon wafer under a flow of moist nitrogen (60% R.H.
at 25 °C), obtained by flowing nitrogen through a two-necked
flask filled with water, at room temperature, at 2 L min-1 rate,
respectively. Best results were obtained using a concentration of
1 mg mL-1 for all polymers except for PST-NH-bodipy, for which
4 mg mL-1 has been used. Thermal treatments were carried out by
keeping the sample slide at 135 °C for 2 h under nitrogen, then slow
cooling (1 °C/min) to room temperature. For fluorescence revela-
tion of amino groups, the BF films of PST-NH2 were dipped in a
4 mM fluorescamine solution (3:1 methanol/water v/v) at room
temperature for 1 h, then dried at 50 °C for 2 h.
Results and Discussion
During the BF formation the condensation of water
droplets at the air/polymer solution interface allows super-
ficial polymer chains to orientate themselves by interaction
of the polar terminal groups with the water droplets, so
that porous films enriched with polar groups in the pores
are achievable in principle. We have already shown how
(25) Hawker, C. J.; Barclay, G. G.; Orellana, A.; Dao, J.; Devonport,
W. Macromolecules 1996, 29, 5245–5254.
(26) Georges, M. K.; Kee, R. A.; Veregin, R. P. N.; Hamer, G. K.;
Kazmaier, P. M. J. Phys. Org. Chem. 1995, 8, 301–305.
(27) Choi, W. S.; Noh, Y. Y.; Chae, K. H. Adv. Mater. 2005, 17,
833–837.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2767
Figure 1. Fluorescence microscopy image of a PST-NH2 honeycomb
structuredfilm after treatment with fluorescamine (scale bar is 5 μm long).
Figure 3. Micrographs of thin films prepared by casting 1 mg mL-1 CS2
solutions of (a) PST-NH2, (b) PST-COOH, (c) PST-OH, (d) PS-PAMS
(scale bar is 10 μm long).
distribution, together with a precise surface morphology,
is really possible. In fact, most of chemical functionalities
are reactive toward one of these three chemical groups,
which offers the possibility of anchoring bigger chemical
fragments, such as biomolecules, to specific surface sites.
To increase the amino group concentration inside the
cavities, we have also synthesized a linear aminomethyl-
polystyrene (PS-PAMS) starting from a commercially
available PS. Unfortunately the decrease of amphiphili-
city along the PS chain led to loss of regularity of the
hexagonal pattern. (Figure 3-d).
Once the goal of functionalizing the inside of the pores
with different chemical groups starting from a single ma-
terial has been reached, a derivative of boron-dipyrro-
methene (bodipy)30 has been synthesized and linked to the
amino-TEMPO initiator to prepare a fluorescent labeled
PS. As reported in Scheme 1, the bodipy derivative 3 has
been prepared in 47% yield by condensation of 2 equiv of
3-ethyl-2,4-dimethylpyrrole 1 and aldehyde 231,32 in the
presence of TFA and DDQ, followed by the addition of
Figure 2. Negative ToF-SIMS images (scale bars are 10 μm long). a-b)
PST-NH2 microporous film: (a) sum of all anions; (b) m/z: 26 (CN-).
(c-e) PST-NH-bodipy microporous film: (c) sum of all anions before
thermal treatment; (d) sum of all anions after 2 h of thermal treatment at
135 °C; (e) same film, m/z: 19 (F-).
The fluorescence microscopy image of PST-NH2 films
after this treatment, shown in Figure 1, evidences that
fluorescence response is higher inside the cavities, thus
indicating an elevated concentration of reactive amino
groups in those sites.
This result has been confirmed by ToF-SIMS analysis
of an analogous film (Figure 2-a/b). Negative ion images
show a large amount of CN- ions, related to PS amino
residues (i.e., TEMPO initiator) coming from the cavities.
The imaging of the NH- ion would have been more
characteristic of the amino function but unfortunately
Tof-SIMS sensitivity toward this ion is too weak to give a
sufficiently contrasted image.
This enrichment of the cavity internal wall of the BF
films with polymer chain end-groups, already reported in
the literature,14,21-23,28,29 transforms the perspectives of
the BF approach from a mere building technique to a
powerful procedure able to functionalize detailed areas of
a polymeric surface with specific chemical groups. Similar
results are also achievable with -OH and -COOH
terminated linear PSs, which suggests that using this
approach as an effective control over the chemical
BF3 OEt2. The deprotection of the phtalimido group has
3
been readily performed by using hydrazine monohydrate in
refluxing ethanol, affording derivative 4 in 82% yield. The
amino group was converted into isothiocianate by reaction
with thiophosgene in refluxing toluene and Et3N as base,
giving compound 5 in 80% yield. Finally the addition of 5 to
4-amino-TEMPO, carried out in toluene at room tempera-
ture, afforded luminescent labeled radical 6 in 42% yield.
This modified TEMPO radical was employed as initiator
for the living radical polymerization of styrene. The intense
and narrow emission peak of bodipy centered at 540 nm
(30) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932.
(31) Dick, D. L.; Rao, T. V. S.; Sukumaran, D.; Lawrence, D. S. J. Am.
Chem. Soc. 1992, 114, 2664–2669.
(32) Baell, J. B.; Duggan, P. J.; Forsyth, S. A.; Lewis, R. J.; Lok, Y. P.;
Schroeder, C. I. Bioorg. Med. Chem. 2004, 12, 4025–4037.
(28) Sun, H.; Li, W.; Wu, L. X. Langmuir 2009, 25, 10466–10472.
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A. H. E.; Stenzel, M. H. J. Mater. Chem. 2008, 18, 4718–4730.

2768 Chem. Mater., Vol. 22, No. 9, 2010
Galeotti et al.
Figure 5. PST-NH-bodipy microporous film after heating to 135 °C for
2 h; (a) scanning confocal laser microscopy image; (b, c) 3-D reconstruction
of z-scans done via confocal laser microscopy taken at two different
magnifications; (d) roughness analysis measured by AFM along the
white line (inset).
Figure 4. (a) PST-NH-bodipy honeycomb structured film; 330 μm2
fluorescence microscopy image with few defects. (b) Bigger magnification
of the same film. (c) SEM micrograph of same kind of film. (d) Films
prepared by drop-casting a CS2 solution of PST-NH-bodipy on glass
microscopy coverslips and on silicon wafers.
and a PS coil, an ordered array of fluorescent spots is
formed on the film surface in the same position of the
cavities left by water microdroplets.33
makes the thin films obtained from this polymer suitable to
be studied by fluorescence microscopy techniques. By cast-
ing a solution of this polymer under BF conditions, highly
ordered porous films are easily obtained (Figure 4).
In these films, relatively wide areas (up to 0.5-0.6 mm2)
with none or few defects of the hexagonal order are achiev-
able. Along these areas the diameter size distribution of
pores is quite homogeneous, while for longer distances, a
variation of ∼40% has been recorded throughout a sample
of 1 cm2, so that pore size ranges from 1.5 to 2.8 μm.
The presence of diverse slightly polar groups at the end
of the PS chain, its Mw = 84 KDa, and a polydispersity
index of 1.7 probably account for the neat aptitude
showed by this polymer to give ordered BFs in different
casting conditions. What we experienced may appear
partially in contrast with what recently was observed by
Billon et al., who found that formation of highly struc-
tured films is promoted by ionization of the low molecular
weight PS carboxylic end group, which stabilizes the
water/CS2 interface.21 Qiao and co-workers, on the other
hand, demonstrated on star polymers that increasing
the number of hydrophilic groups over a certain degree
brings a decrease of regularity through the honeycomb
structure.14 The behavior showed by our polymer seems
to confirm that a small difference of polarity between the
terminal groups and the rest of the polymer chains is
sufficient to ensure a perfect hexagonal arranged pore
formation.
With the much simpler structure of PST-NH-bodipy,
we have not only observed a similar phenomenon but also
been able to optimize the procedure to produce ordered
spotted surface films. In fact, because of the high degree
of order obtainable in the hexagonal array of the pristine
film, when the thermal annealing over the polymer glass
transition temperature (Tg) causes the collapse of the BF
structures and the material is brought from the internal
surface of the spherical cavities to the outer surface of
film, an array of fluorescent spots periodically arranged is
obtained. AFM analysis confirmed that film roughness
decreases (from micrometric porosity) to less than 3 nm,
as shown in Figure 5-d.
In Figure 6 is shown a schematic representation of our
explanation of this phenomenon. During the thermal
treatment, the polymer starts moving from the bulk to
fill the empty spaces. Consequently, film thickness
decreases from about 3 to less than 1 μm, and the holes
disappear. This implies that the fluorescent heads, which
were preorganized around the cavity internal surface,
compact themselves in a smaller surface (from spherical
surface to disk) giving rise to a spot which is seen more
fluorescent than the surrounding material.
ToF-SIMS analysis of the spotted flat film reveals a
higher concentration of fragments related to bodipy
residues coming from the fluorescent spots, thus indi-
cating that really we are handling a functionalized sur-
face (Figure 2-e). In some parts of the film, however, the
signals from bodipy residues disappear, probably because
in those regions the fluorescent spots are slightly dipped
under the PS film surface during the closure of the pore.
As for PST-NH2, ToF-SIMS analysis of PST-NH-
bodipy porous films revealed that the internal walls of
the cavities are enriched with the bodipy group with
respect to the bulk of the film (data not shown).
We have recently observed that after the thermal treat-
ment of BF structures obtained from an amphiphilic
block copolymer formed by a polyfluorene polar rod
(33) Bolognesi, A.; Galeotti, F.; Giovanella, U.; Bertini, F.; Yunus, S.
Langmuir 2009, 25, 5333–5338.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2769
functionalized PS, can be evidenced using fluorescence
microscopy after reaction with fluorescamine.
The second approach is based on the BF film formation
starting with a PS already functionalized with a specific
molecule. In this regard, we have used a bodipy-functio-
nalized PS to get the BF film with cavities enriched by
bodipy group. The thermal treatment of the film at a
temperature over the PS Tg leads to a flat film with spots
of fluorescent dye arranged in the same position where the
cavities were in the pristine film.
Our group recently reported on zeolite-loaded BF holes
in polymeric films.34 Their formation is promoted when
hydrophilic/hydrophobic properties of the crystals are
enhanced through a chemical modification of the zeolites’
external surface. In the light of the current study, a proper
treatment of those hybrid BF films may give rise to
interesting spotted hybrid surfaces.
We are currently expanding these approaches to biolo-
gical molecules with the aim of obtaining ordered pat-
terns suitable for biological tests.
Figure 6. Transformationofa PST-NH-bodipy microporous film during
thermal treatment (from left to the right). (a) Cross-section of the film
placed on silicon substrate. (b) Sketched drawing of the cross-section of a
pore. Green triangles stand for the bodipy fluorescent heads of the
polymer. (c) 3-D representation of the same phenomenon.
Conclusions
Acknowledgment. We thank Dr. Guido Scavia (ISMAC-
CNR) for the AFM analysis included in this paper. This
work was partially supported by Cariplo-PRESTO pro-
ject and by Regione Lombardia, agreement Regione/CNR,
project 4: “Nanoscienze per materiali e applicazioni bio-
mediche”. S.Y. is grateful to the Walloon Region of Belgium
for its financial support in the frame of the NANOTIC
research project.
We have shown that the BF approach is a suitable
method to obtain films with cavities which can be func-
tionalized. Two approaches have been reported and are
based on the use of PS as a suitable material. In the first
method, cavities with specific chemical functions are
created. These can be easily obtained by using PS func-
tionalized with polar groups. In this respect TEMPO
modified with carboxylic, amino, and alcohol groups is
a proper initiator to get functionalized PS suitable for the
BF film formation. Because of the high concentration of
specific chemical functions on the wall of the cavities,
these are more prone than the film surface to react with
other molecules. In this work we have shown that the
cavities of a microporous film, obtained using an amino
Supporting Information Available: Absorption and emission
spectra, detailed AFM analysis after film thermal treatment,
and GPC results for PST-NH-bodipy. This material is available
free of charge via the Internet at http://pubs.acs.org.
(34) Vohra, V.; Bolognesi, A.; Calzaferri, G.; Botta, C. Langmuir 2009,
25, 12019–12023.