β-Cyclodextrin's orientation onto TiO2 and its paradoxical role in guest's photodegradation

Article abstract of DOI:10.1039/c2cc37244d

This work revealed that β-cyclodextrin was attached onto the surface of TiO₂ predominately by its secondary ring side, which caused paradoxical functions of β-cyclodextrin in the photodegradation of the four bisphenols. The equilibrium between the guest adsorbed through β-cyclodextrin onto TiO₂ and the one locked in β-CD in water could also change the role of β-cyclodextrin in the degradation of a certain guest.

Full text of DOI:10.1039/c2cc37244d

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b-Cyclodextrin’s orientation onto TiO2 and its
paradoxical role in guest’s photodegradation†
Xu Zhang,*a Zixin Yang,*b Xuankun Li,a Nansheng Denga and Shahua Qiana
Cite this: Chem. Commun., 2013,
49, 825
Received 4th October 2012,
Accepted 28th November 2012
DOI: 10.1039/c2cc37244d
www.rsc.org/chemcomm
This work revealed that b-cyclodextrin was attached onto the be a potential factor affecting the approach of the guest to the
surface of TiO2 predominately by its secondary ring side, which nanomaterial.
caused paradoxical functions of b-cyclodextrin in the photodegra-
Cyclodextrins/TiO2 (CDs/TiO2) nanomaterial is generally
dation of the four bisphenols. The equilibrium between the guest found to show more effective activity in the degradation of
adsorbed through b-cyclodextrin onto TiO2 and the one locked in pollutants compared to TiO2.13,18,20 In addition to facilitating
b-CD in water could also change the role of b-cyclodextrin in the photogenerated electron–hole separation,1,2,21 the cyclodextrins
degradation of a certain guest.
are hypothesized to recognize and transfer the size/shape-fitted
pollutants to TiO2.18,22 The b-CD/TiO2 nanomaterial can be easily
Cyclodextrin (CD) modified nanomaterial, a typical type of host- obtained by the direct adsorption or photo-induced self-assembly
modified nanomaterial, has attracted considerable interest because of b-CD onto TiO2,23 which is considered to be achieved by the
of its unique properties.1–6 As one of the most common host physisorption and/or H-bonding interactions between b-CD and
molecules, cyclodextrin does not only alter the external morphology TiO2.6,23 However, the orientation of cyclodextrin onto TiO2 is not
of the nanomaterial,7–11 but also the intrinsic properties including clear and has not attracted enough attention in the photodegra-
photoactivity, photoelectric activity, energy transfer, etc.2,4,12–15 dation of certain pollutants by CD/TiO2. Our adsorption experi-
More importantly, nanomaterial can be endowed with the recogni- ments of b-CD and its derivatives (Fig. S1, ESI†) onto TiO2 showed
tion ability for certain guest molecules after the modification with that their adsorption isotherms followed the Langmuir model
cyclodextrin,1,3,16,17 which can lead to more guest-targeting and (Fig. S2, ESI†) and the maximum adsorption capacities (qmax
)
efficient functions of the nanomaterial for cyclodextrin can play a onto TiO2 followed the order of b-CD (33 mmol gÀ1) > 2-O-methyl-
role as a ‘‘channel’’ or ‘‘bridge’’ between the guest molecules and b-CD (15.2 mmol gÀ1; DS is around 4) > 2,6-di-O-methyl-b-CD
the surface of the nanomaterial. The size/shape-fit relationship (0 mmol gÀ1; DS is 14). Those results confirmed that the hydroxyl
between cyclodextrin and the guest molecule, a general rule to group at ring sides of b-CD played a dominant role in its
predict the possibility of forming a complex in aqueous solution, is adsorption onto TiO2, though the results could not reveal the
usually adopted in the design of cyclodextrin modified nano- orientation of b-CD onto TiO2. Knowing that the specific surface
material.13,18,19 On the other hand, once cyclodextrin, whose area of TiO2 is approximately 250 m2 gÀ1 and the calculated
primary ring side has smaller dimensions than its secondary ring surface area of the 33 mmol b-CD molecule is 36.5 m2, it is also
side, is attached onto the surface of the nanomaterial, the relation- worth noting that the maximum coverage of TiO2’s surface by
ship between its orientation onto the nanomaterial (through which attached b-CD is calculated to be 14.6%, so the space between two
side cyclodextrin attaches) and its inclusion routes of the guest attached b-CD molecules is still large enough for the adsorption of
(from which side the guest can enter the cavity of cyclodextrin) can other guest molecules.
In this communication, a selective photodegradation phenom-
enon of a series of size/shape-fitted bisphenols by b-CD/TiO2 is
reported, and selectivity to guests was found be related to the
a School of Resources and Environmental Science, Hubei Biomass-Resource
Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University,
orientation of cyclodextrin onto TiO2 through the adsorption of the
bisphenols onto b-CD/TiO2 and their complex structures with b-CD
in aqueous solutions were investigated. Based on the type of
orientation of cyclodextrin onto TiO2, an explanation was further
proposed for the paradoxical role of b-CD in the b-CD/TiO2 system
under different conditions. Bisphenol is a typical kind of endocrine
Wuhan, 430079, P.R.China. E-mail: xuzhangwhu@gmail.com;
Fax: +86 2768778893; Tel: +86 2768778381
b Department of Chemistry, University of Victoria, Victoria, V8P5C2, Canada.
E-mail: zixinyangzxy@gmail.com; Fax: +1 2507217147; Tel: +1 2508859577
† Electronic supplementary information (ESI) available: Detailed experimental
description, a table showing the inclusion constants of four bisphenols with
cyclodextrins and the adsorption capacity of cyclodextrins onto TiO2, 1H ROESY
spectra of the b-CD–BPE and b-CD–BPF complexes. See DOI: 10.1039/c2cc37244d disrupting chemical.24 As listed in Table 1, bisphenol A (BPA),
c
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Table 1 Complex stability constant (Ks) of bisphenols with b-CD, observed
degradation constants (kobs) and maximum adsorption capacity (qmax) for
bisphenols onto 1.0 g LÀ1 TiO2 in the presence and the absence of 20 mM
b-CD, 25 1C
kobs  102/minÀ1
qmax/mmol gÀ1
With 20 mM
log Ks/
Without
With 20 mM Without
b-CD b-CD
Guest log MÀ1
b-CD
b-CD
BPA
BPE
BPF
4.9 Æ 0.3 2.1 Æ 0.1 3.9 Æ 0.1
4.6 Æ 0.4 2.3 Æ 0.1 4.2 Æ 0.3
4.2 Æ 0.2 1.7 Æ 0.1 2.9 Æ 0.3
5.8 Æ 0.5 15.7 Æ 1.6
4.3 Æ 0.6 21.8 Æ 2.2
5.0 Æ 2.0 22.2 Æ 2.0
BPAF 4.4 Æ 0.3 1.5 Æ 0.2 0.0 Æ 0.0
7.8 Æ 1.0
7.0 Æ 1.2
bisphenol E (BPE), bisphenol F (BPF) and bisphenol AF (BPAF) were
decomposed in TiO2 dispersion with similar degradation kinetic
constants (kobs). Stability constants (Ks) for 1 : 1 complexes of b-CD
with all these four bisphenols were very high (more than 104 MÀ1),
which suggested that b-CD strongly included all these bisphenols in
aqueous solutions. However, it is interesting to find that b-CD
showed distinguishable effects on the degradation of these bisphe-
nols by TiO2. The photodegradation of 5 mM BPA, BPE or BPF in
1.0 g LÀ1 TiO2 dispersion was highly promoted in the presence of
20 mM b-CD, while the photodegradation of 5 mM BPAF was
selectively totally inhibited in the presence of 20 mM b-CD (Fig. 1).
Since cyclodextrin is considered as a ‘‘channel’’ to recognize and
transfer the guest to TiO2, the adsorption of the bisphenols onto
b-CD/TiO2 could provide some useful information to reveal the
difference between BPAF and the other three bisphenols in the
degradation by b-CD/TiO2. As listed in Table 1, the adsorption of
BPAF on TiO2 shows no change in the presence and the absence of
20 mM b-CD, while the adsorption of other three bisphenols
considerably increased in the presence of 20 mM b-CD. The distinct
results of BPAF against the other bisphenols suggest that b-CD
cannot effectively transfer BPAF to TiO2, in other words, BPAF
actually cannot access the attached b-CD, even though it can be
easily included into b-CD in aqueous solution.
Fig. 2 1H ROESY spectrum of the b-CD–BPAF complex saturated in D2O at 25 1C.
spectrum of the b-CD–BPAF complex displayed clear NOE cross-
peaks between the Hb and the H-3/H-5 (peaks A and B) and
between the Ha and the H-5 (peak C), which indicated that the
phenyl ring of BPAF was deeply included in the hydrophobic cavity
of b-CD. Much more important was the fact that no NOE cross-peak
between Ha and H-3 was observed, which revealed that Ha was not
close to the secondary ring side of b-CD in the structure of the
complex, that is, the phenyl ring of BPAF predominately penetrated
into the b-CD cavity from the secondary ring side (Scheme 1a). In
contrast, besides the same NOE cross-peaks observed in the ROESY
spectrum of b-CD–BPAF, the clear NOE cross-peaks between Ha
and H-3 were also observed in the ROESY spectra of b-CD–BPA,25
b-CD–BPE (Fig. S3, ESI†) or b-CD–BPF (Fig. S4, ESI†). They corre-
sponded to two equilibrium structures of those three complexes in
aqueous solution, where the phenyl ring of BPA, BPE or BPF could
be included into the b-CD cavity from both the primary ring side
and the secondary ring side (Scheme 1b).25
From the observation that BPAF can mainly access b-CD from
the secondary ring side of b-CD and cannot access the attached b-
CD on the surface of TiO2, it can be deduced that mostly the
secondary ring side of b-CD should face the TiO2 surface and
consequently be blocked when b-CD is attached onto the TiO2
surface (Scheme 2). So b-CD adsorbs onto TiO2 mainly through its
secondary ring side. Because of this orientation of b-CD onto TiO2,
guest molecules, such as BPA, BPE or BPF, that are able to enter
b-CD from the primary ring side can be ‘‘transferred’’ onto TiO2
more efficiently through b-CD and degraded. The other important
thing to note is that the photodegradation of BPAF by TiO2 was
Further complex structure studies revealed the difference
between BPAF and the other bisphenols in the interaction with
b-CD in aqueous solution. 2D ROESY is a powerful technique for
investigating the complex structures of b-CD. The NOE correlation
between the protons of the guest and the inner protons of the b-CD
cavity (H-3 at the secondary ring side and H-5 at the primary ring
side) is determined from the ROESY spectrum when the guest is
included in the b-CD cavity. As shown in Fig. 2, the ROESY
Fig. 1 Effect of 20 mM b-CD on the photodegradation of bisphenols in 1.0 g LÀ1
TiO2 suspension at 25 1C. Dkobs = kobs À kobs0, kobs and kobs0 are the degradation
kinetic constants for bisphenols in the presence and in the absence of b-CD,
respectively.
Scheme 1 Possible structure of the complexes BPAF–b-CD (a) and BPA–b-CD,
BPE–b-CD, BPF–b-CD (b) in aqueous solution.
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(Fig. 3). However, further increase of b-CD concentration led to
the decrease of the promotion extent. It was probably because
further increase of b-CD concentration could only result in the
increase of the bulk phase b-CD amount (b-CDfree) rather than the
attached b-CD amount (b-CDattached) (Scheme 2). b-CDfree is sup-
posed to play the same role as 2,6-di-O-methyl-b-CD does, which
could retard the diffusion onto TiO2 and subsequently slow down
the degradation of BPE.
In conclusion, it was uncovered that b-CD was attached onto
TiO2 mainly through its secondary ring side from the studies of the
adsorption and the complex structures of four bisphenols. The
orientation of b-CD onto TiO2 reveals that b-CD has a distinct
influence on the degradation kinetic results of BPAF from BPA, BPE
or BPF. On the other hand, the equilibrium between the guest
adsorbed on the surface of TiO2 through attached b-CD and the
one locked in b-CD in bulk solution is another factor that affects
the approach of the guest to TiO2 and consequently changes the
role of b-CD in the guest’s photodegradation.
Scheme 2 Mechanism for the adsorption of BPE and BPAF in b-CD/TiO2
suspension.
This work has been supported by National Natural Science
Foundation of China (Grant No. 21207104) and Natural Science
Foundation of Hubei Province (2011CDB274).
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 825--827 827