Materials Sciences and Applications, 2010, 1, 267-271
doi:10.4236/msa.2010.15039 Published Online November 2010 (http://www.SciRP.org/journal/msa)
267
Provitamin D Doped Silica and Polymeric Films:
New Materials for UV Biosensor*
T. N. Orlova1, I. P. Terenetskaya1, A. M. Eremenko2, N. I. Surovtseva2
1Institute of Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine; 2Chuiko Institute of Surface Chemistry, National
Academy of Sciences of Ukraine, Kiev, Ukraine.
Received August 25th, 2010; revised October 10th, 2010; accepted October 10th, 2010.
ABSTRACT
The original technologies of growing silica films, impregnated with 7-dehydrocholesterol (provitamin D3) on quartz
substrates and free transparent films on the basis of polyvinyl alcohol and polyvinyl butyral have been developed.
Provitamin D photoisomerization in the films under UVB irradiation was investigated by UV absorption spectroscopy.
Remarkable changes in the absorption spectrum of 7-DHC were observed in silica and polyvinyl alcohol films as com-
pared with ethanol solution, only in polyvinyl butyral film the spectrum was very nearly, while the spectral kinetics of
7-DHC photoisomerization in all the films was different from ethanol. We suggest that several films have potential as
UV dosimeters to measure accumulated ‘antirachitic’ UV dose in the same manner as erythemic UV dose is measured
by commonly used polysulphone film.
Keywords: Provitamin D Photoisomerization, Polymer Films, Silica Porous Film, UV Dosimetry
1. Introduction
The isomerization reactions of optically excited mole-
cules are of considerable current interest in photochemis-
try. Lately many efforts went into elaboration of new
materials with desirable optical properties that are based
on the inclusion of photosensitive labile molecules into
various organic and inorganic media (polymer films,
porous materials, etc.). These guest-host materials have
found wide application. They have been widely applied
to heterogeneous biocatalysis, to the development of
solid state optical materials [1-3]. In particular, poly-
mer-dye films are used as the photo-active layers in light
emitting diodes (LED), lasers and photovoltaic cells.
Besides, porous hybrid materials find use as highly spe-
cific chemical sensors.
To design and optimize a material whose optical be-
havior is based on properties induced by guest molecules,
it is necessary to understand how the microenvironment
affects their certain dynamic properties because such
processes as molecular rotations and conformation
changes determine the optical behavior of the final solid-
state material [4].
At a molecular level, inclusion, dissolution or adsorp-
tion processes involve direct chemical and physical in-
teractions between host and guest molecules, and elec-
tronic spectroscopy of specific probe molecules offers
sensitive methods of reporting the local environment of
the probe molecule. As an example, protein bioencapsu-
lation has been carried out in nanoporous silica-based
materials to study the biocompatibility and biological
activity of proteins [5,6].
In this paper we present the results of our investiga-
tions on 7-dehydrocholesterol (7-DHC, provitamin D3)
photoisomerization in “solid” media (polymer and po-
rous silica films). Being extension of our previous studies
on the reaction medium effect on provitamin D photo-
chemistry [7-10], this work is also directed on elabora-
tion of new UV sensitive materials, in particular, as UV
biosensors for direct monitoring of the vitamin D syn-
thetic capacity of solar/artificial UV radiation [11,12].
2. Materials and Methods
The main task was to grow the UV transparent optically
homogenous thin films impregnated with 7-DHC suitable
for investigation of provitamin D photoisomerization in
these films by means of UV absorption spectroscopy.
Three UV transparent media have been suggested as ap-
propriate candidates for the thin films, namely, porous
sol-gel silica, polymer polyvinyl alcohol and polymer
polyvinyl butyral.
*DOE/GIPP/LLNL/STCU

Provitamin D Doped Silica and Polymeric Films: New Materials for UV Biosensor 268
2.1. Sol-Gel Production of Porous Silica Films
Impregnated with Provitamin D
The challenge was in finding the preparation regime of
mechanically stable UV transparent homogeneous porous
silica films (glasses) with sufficient content of 7-DHC
entrapped within a silica matrix. First and foremost the
temperature, solution pH, particular alkoxide precursor
and solvent, and their relative concentrations, should be
determined to provide the optimum ratio of porosity,
thickness and UV/VIS absorbance/transparency of the
film. The pore size should be optimized with regard to
expected suppression of conformational mobility of
7-DHC (and its photoisomers) implied by their immobi-
lization on the OH-groups of silica surface. Furthermore,
the acidity regime of sol-gel process should be carefully
balanced in view of known unfavorable acid effect on
7-DHC photochemistry [13].
Next task was to assure adequate UV optical density
(no less than 0.2) of the silica film with entrapped mole-
cules 7-DHC, i.e., to find optimum film thickness and ade-
quate amount of 7-DHC inside. The point is that thick films
are liable to be destroyed due to collapse of the primary
particles at the stage of the film heat treatment, while in
thin film the amount of immobilized 7-DHC is insufficient
for accurate measurements of UV absorbance.
Several approaches have been suggested for the films
preparation: 1) under neutral рН, 2) with solubilization of
the 7-DHC molecules within -cyclodextrin to protect
molecule against the interaction with silica OH-groups
saving the conformation mobility, and 3) to introduce the
solubilized molecules within mesoporous silica films via
adsorption that is favorable for homogeneous distribution
of 7-DHC.
The mesoporous silica films have been prepared by the
sol-gel method using cationic surfactant cetyltrimethyl-
ammonium bromide (CTAB, Aldrich) as a template a-
gent to provide the homogeneous porosity. The precursor
sol was prepared by hydrolysis of TEOS in a mixture of
distilled water, ethanol (Fluka), and 1 M HCl solution.
Twenty-four hours after hydrolysis, the water solution of
CTAB was added to this solution. The total molar ratios
were 1 TEOS: 0.1 CTAB: 0.02 HCl: 10 H2O: 5 C2H5OH.
The material was deposited onto clean quartz substra-
tes by the dip-coating technique. The films were dried for
12 hours at ambient temperature, followed by heat treat-
ment at 400°C with a rate of 1°C /min and calcinated at
350°C for 6 h. The parameters of low-level cell of silica
film were as follows: the pores diameter 3 nm, the walls
thickness 1,7 nm, and the surface SBET = 550 m2/g.
X-ray diffraction (XRD) of the film demonstrated 3 sig-
nals: (2 theta): 2.7: 25: 8.6, it corresponded to the inter-
laminar distance 34.7 А; after 500 С the distance was
30.4 А, and after 600°C the collapse of the porous structure
occurred. Hence, the film was treated at 350°C during 1
hour before the adsorption of 7-DHC from hexane solution,
C7-DHC = 2*10-3 mol / l.
2.2. Preparation of Transparent Polymer Films
with Entrapped 7-Dehydrocholesterol
Two polymers - polyvinyl alcohol (PVA) and polyvinyl
butyral (PVB) – were checked for the films preparation
because of their rather low absorption in the spectral
range of interest (260 - 300 nm).
The different solubility of PVA and 7-DHC was for-
midable obstacle to the development of preparation tech-
nology of UV transparent optically homogenous free
(without quartz substrates) PVA film impregnated with
UV sensitive biomolecules (7-DHC). Since PVA is water-
soluble and 7-DHC is fat-soluble, at first PVA (MW 60000)
was dissolved in water to obtain 5% water solution of
polyvinyl alcohol. Further ethanol solution of 7-DHC was
slowly introduced into the polymer solution, which was
carefully mixed, putted on the glass substrate and slowly
evaporated. (The films of larger diameter (7.5 cm) were
prepared in the Petri’s vessels). Films drying procedure
was accomplished in the desiccators. The critical parame-
ters were as follows: the concentration of the initial solu-
tion of PVA in water, the drying conditions, and the ap-
propriate concentration of 7-DHC within PVA-based film
needed for the UV spectra registration. Obtained PVA
films of 40-200 mkm thickness were mechanically stable,
colorless, but a little scattering.
To obtain better optical homogeneity we used polyvi-
nyl butyral which can be dissolved in ethanol and thus
facilitates the film preparation.
UV irradiation of the samples was performed using
luminescent lamp EL-30 (280-330 nm). Spectral irradi-
ance of the UV lamp at the sample distance 8 cm was
determined using calibrated spectrometer EPP2000С-
UV + VIS (StellarNet, Inc), and the following data were
obtained: 0.25 mW/сm2 for the total spectral range
270-380 nm of El-30 lamp and 0.11 mW/cm2 in the UVB
spectral range.
The photoreaction course was followed by UV absorp-
tion spectroscopy, i.e., absorption spectra of the samples
were recorded by Perkin&Elmer Lambda 25 UV/VIS
spectrophotometer within 230-330 nm before and after
fixed UV exposures. In parallel, simultaneously ethanol
solution of 7-DHC was irradiated in the same conditions.
3. Results and Discussion
Absorption spectra of silica and polymer films impregnated
with 7-DHC were recorded by Perkin&Elmer UV/VIS
spectrophotometer Lambda 25. These spectra are shown in
Figure 1 in comparison with the absorption spectrum of
7-DHC in ethanol.

Provitamin D Doped Silica and Polymeric Films: New Materials for UV Biosensor 269
As one can see in Figures 1 (c, d), inclusion of 7-DHC
into silica porous film and polyvinyl alcohol film sig-
nificantly affects its absorption spectrum (that differs
from well-known stability of 7-DHC spectrum in differ-
ent solvents [14]), only in polyvinyl butyral film the
7-DHC absorption spectrum more closely resembles the
spectrum in ethanol. The spectra in silica porous and
PVA films are red shifted (2 nm), and the redistribution
of the vibronic bands intensity slightly observed in silica
film significantly increases in PVA film indicating heavy
influence by the properties of the adjacent surrounding
media [15].
Transformations of initial absorption spectra of the
three films upon the UV lamp irradiation are shown in
Figure 2 in comparison with 7-DHC in ethanol. Surpris-
ingly, the spectral kinetics for all the films is signifi-
cantly distinct from each other and from the known ki-
netics of 7-DHC photoisomerization in ethanol [11].
From the above spectral transformations the absorb-
ance at 282 nm dependency versus UVB dose can be
derived (Figure 3) that can be used for UV dosimetry.
4. Conclusions
Biologically effective exposures can be determined in
two ways: 1) by measuring the spectral irradiance of a
UV source during the period of exposure and calculating
the weighted integral; or 2) by using a device whose rela-
tive response to different wavelengths resembles the ac-
tion spectrum for the particular photobiological effect.
Hence, strict conformity between spectral sensitivity of
radiation detectors and the biological action spectra that
they are designed to mimic is of primary importance for
adequate monitoring of a UV biologic dose. Most of
broad-band UV detectors are usually designed to have a
spectral sensitivity which is a close match to the CIE
erythema action spectrum, e.g. polysulphone film which
has a spectral response close to the erythemal response of
human skin is commonly employed as film dosimeter in
UV photobiology.
In view of significant difference between the CIE ery-
thema and ‘Vitamin D’ action spectra [11,12], polysul-
phone film is incapable to adequate evaluation of the
vitamin D synthetic capacity of sunlight and/or an artifi-
cial UV source. Since vitamin D synthesis is originated
from the UVB photon absorption by provitamin D mole-
cule, obviously, a UV transparent film impregnated with
provitamin D molecules is the most appropriate material
for the ‘antirachitic’ UV dosimetry.
This paper describes technologies of growing silica-
films, impregnated with 7-dehydrocholesterol (provita-
(a) (b)
(c) (d)
Figure 1. Absorption spectra of 7-DHC in ethanol (a), in PVB film (b), in silica porous film (c) and in PVA film (d). Dotted
lines in Figures 1(b), 1(c) and 1(d) show absorbance of the films without 7-DHC.

Provitamin D Doped Silica and Polymeric Films: New Materials for UV Biosensor 270
(a) (b)
(c) (d)
Figure 3. Transformation of the 7-DHC absorption spectrum in ethanol (a), PVB film (b), silica film (c) and PVA film (d)
upon irradiation with the EL-30 UV lamp.
Figure 3. The UVB dose dependence of the 7-DHC absorbance at 282 nm in PVB polymer film (a) and in silica film (b).
min D), on quartz substrates and free transparent films on
the basis of polyvinyl alcohol and polyvinyl butyral
doped with 7-DHC. Studies on the effects of UV expo-
sures showed that the complexity of the 7-DHC doped
PVA film preparation and, what is more important, non-
monotonic dependence of the absorbance at 282 nm ver-
sus UV exposure, preference should be given to the
PVB-based film for personal UV dosimetry.
5. Acknowledgements
This research is supported by the GIPP Program of the
US Department of Energy (the STCU Project P344).
The authors appreciate helpful advices from Prof.
A. Ischenko (Institute of Organic Chemistry, NAS
Ukraine).
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