Fluorescent perylene derivative functionalized titanium oxide gel for sensitive and portable ascorbic acid detection

Article abstract of DOI:10.1039/c9ra01621j

An inorganic titanium oxide (TiO) gel sensor was demonstrated for convenient detection of ascorbic acid (AA). It is composed of TiO (PI-TiO) functionalized with a perylene diimide derivative containing carboxylic groups as a new soft dopant material. A tr

Full text of DOI:10.1039/c9ra01621j

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Fluorescent perylene derivative functionalized
titanium oxide gel for sensitive and portable
ascorbic acid detection†
Cite this: RSC Adv., 2019, 9, 24638
*
*
Feifei Xing
Binbin Zhang, Wenxia Liu, Yihao Liu, Zhiguang Suo, Lingyan Feng,
and Shourong Zhu
An inorganic titanium oxide (TiO) gel sensor was demonstrated for convenient detection of ascorbic acid
(AA). It is composed of TiO (PI–TiO) functionalized with a perylene diimide derivative containing
carboxylic groups as a new soft dopant material. A traditional solvothermal reaction is adopted to
prepare the PI–TiO composite, which exhibits a different spectrum according to the reaction time. The
final gel possesses a strong chelating affinity with AA molecules, in which phenol hydroxyl groups are
shown to compete with those already present in PI. We further utilize the functionalized gel to prepare
a series of films with a simple and portable AA response. A visual colour variation can be recognized by
the naked eye, together with obvious fluorescence changes for selective and sensitive AA detection.
Finally, the as-prepared gel film displays good stability and reproducibility for real sample responses with
satisfying results.
Received 4th March 2019
Accepted 30th June 2019
DOI: 10.1039/c9ra01621j
rsc.li/rsc-advances
perylene compounds at the imide or bay positions, increasing
the whole molecular solubility.12–15 Due to the presence of
1 Introduction
carboxyl substituents, interactions between PI and TiO are ex-
pected, making a hybrid composite with unique optical prop-
erties.8 However, few works have reported entrapping PI dyes in
TiO gels, and still no application has been discussed to utilize
the interesting phenomenon of a uorescent functionalized gel,
especially for sensor development.
Gels are widely used for biosensors, drug delivery carriers and
different stimuli-responsive operations, etc.1–3 Compared with
the well-researched polymer gels, inorganic gels have some
unique advantages, such as the low toxicity of precursor mate-
rials and ease of preparation. They are considered as a new
choice for hybrid gel materials in thin lm, nano- and porous
materials fabrication.4,5 To complement its optical function,
previous research has entrapped perylene dye in various inor-
ganic gels, such as silica matrix gels, to obtain sensitive pho-
touorochromic perylene doped gels.6,7 One novel perylene–TiO
gel has been recently reported with strong uorescence prop-
erties given by coordinative interactions between titanium
alkyloxide with the carboxylic groups (–COOH) of perylene tet-
racarboxylic acid (PTCA).8 In composite materials, TiO acts as
an efficient wet matrix and photoactive shelter for perylene dye.
Notably, PTCA molecule is insoluble in many solvents (e.g.,
ethanol and ethyl acetate) and is only soluble in strong basic
aqueous solution originating from perylene-3,4,9,10-
tetracarboxylic dianhydride.9 By comparison, perylene diimide
derivatives (PI) have excellent advantages of solubility,
chemical/photothermal stability and high quantum yield,
which are important for molecular probes and their applica-
tions.10,11 Carboxylic groups are also easily introduced on
Ascorbic acid (AA) is a necessary compound in maintaining
normal growth of the human body. It has important inuence
on many aspects of life activities, such as a lower risk of
cardiovascular disease, stroke and cancer, and increased
longevity.16 Traditional methods for AA detection include redox
titration, HPLC,17 electrochemical detection18 and so on.
Different methods have their strengths, but also some limita-
tions. For example, the redox titration method to determine the
chromogenic reaction is oen slow, unstable and lacks sensi-
tivity; the error of test results is relatively large.19 HPLC and
electrochemical methods are usually sensitive enough, but they
oen need expensive instruments, as well as tedious experi-
mental procedures.20–22 Compared with those methods, optical
detection has several advantages, which are high sensitivity and
convenience, and that direct colour changes in some systems
can be visually observed by the naked eye. It is also vital to
design portable AA sensors for potential point-of-care testing
(POCT) uses.23
Here we develop one novel PI–TiO gel sensor for highly
sensitive and selective AA detection, the principle for which is
based on the stronger interaction between TiO and contiguous
phenol hydroxyl groups in AA than with the –COOH groups,
Department of Chemistry, College of Science, Materials Genome Institute, Shanghai
University, Shanghai 200444, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01621j
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heated under a nitrogen stream of 20 mL minꢁ1, with a heating
rate of 20 ꢂC minꢁ1. X-ray photoelectron spectroscopy (XPS)
measurements were recorded using a Thermo Fisher Scientic
WSCALAB spectrometer.
2.2 Synthesis of perylene diimide derivative (PI)
The perylene diimide derivative with four carboxylic acid groups
(PI) was synthesized as reported previously.24 Briey, PTCDA
(0.50 g), 5-aminoisophthalic acid (0.51 g) and imidazole (5.00 g)
were heated at 130 ꢂC for 12 hours under a nitrogen atmo-
sphere. Then, the deep red solution was acidied with 2 M HCl
to about pH ¼ 2, and a red precipitate was obtained. Centrifu-
gation, water washing and ethanol washing were performed
three times, aer that the product was dried at 80 ꢂC in
a vacuum oven to obtain 0.916 g of deep red powder (yield:
91.8%).
Scheme 1 Schematic diagram of PI–TiO gel on glass for AA detection
using a portable UV lamp.
quenching PI uorescence efficiently. The inorganic gel-based
biosensor is also portable, prepared on simple glass substrate,
and so visual colour and uorescence changes can be obtained
conveniently. The detection process is shown in Scheme 1. The
prepared PI–TiO gel was easily spread on a clean glass substrate,
with high FL emission under a 365 nm ultraviolet lamp. When
AA solution was dropped onto the gel surface, the optical
intensity immediately decreased depending on the AA concen-
tration. The optical change was ascribed to the competitive
binding of AA with titanium which promotes the disassembly of
the PI–TiO complex. Controlled experiments demonstrate that
the established gel method has high sensitivity and selectivity
for AA, different from other analogous molecules. Since the
365 nm lamp is inexpensive compared with other complicated
instruments, the gel substrate is also stable enough for portable
applications.
2.3 Synthesis of the PI–TiO composite
The PI–TiO composite was prepared from titanium isoprop-
oxide and a minute quantity of PI in ethanol by a solvothermal
method. In a typical procedure, 2.0 mL of titanium tetraiso-
propanolate (Ti(OR)4) was dissolved in 4.0 mL of ethanol at
room temperature, 1.0 mg of PI was added, and then the
mixture was stirred then transferred into a 10 mL Teon-lined
autoclave at 40 ꢂC for 6 hours (alterable). The resulting solu-
tions were puried by ltration to remove tiny insoluble
impurities, giving a clear solution.
2.4 Synthesis of the PI–TiO gel
In order to control the reaction, the target product was prepared
by a typical sol–gel method, and a PI–TiO solution was used as
a precursor. Acetic acid (0.05 mL, 98%) was added to the PI–TiO
solution (5 mL) at a constant stirring rate at room temperature.
Deionized water (0.075 mL) was added drop-by-drop with
continuous stirring, and pH was adjusted to 3 by HCl. PI–TiO
gel was obtained aer letting it rest at room temperature over-
night. To prepare the PI–TiO lm on glass, fresh PI–TiO sol was
directly transferred to a glass dropper and dripped onto a pre-
treated clean glass sheet. Under normal barometric pressure,
2 Experimental
2.1 Chemicals and characterization
All analytically pure reagents, Ti{OCH(CH3)2}4 (99%), 3,4,9,10-
perylenetetracarboxylic dianhydride (PTCDA), 5-amino-
isophthalic acid (98%), imidazole (ACS reagent, 99%), etc., and
solvents were purchased from commercial sources and used
1
ꢂ
without further purication. H NMR spectra were carried out
ambient temperature 20 C and humidity ꢃ60%, the lm was
using a Bruker AVANCE 500 MHz spectrometer. Emission and
absorption spectra were collected using a PERSEE TU-1950 UV-
visible spectrophotometer and an Edinburgh Instruments FS5
uorescence spectrometer. The uorescence light source was
provided by 150 W xenon lamp and the uorescence signal was
evaporated.
3 Results and discussion
3.1 Preparation of the PI–TiO solution and gel
amplied by a R982P photomultiplier tube. 360 nm was PI with four carboxylic acid groups was synthesized as described
selected as the excitation wavelength, and the scan slit was in the Experimental section.25 Scheme 2 shows the chemical
1.5 nm. Step was set at 2 nm and wavelength accuracy is structure of PI and the solvothermal synthesis of the PI–TiO
ꢀ0.5 nm. The detector sensitivity is >6000 : 1. FT-IR spectra solution. PI can be dissolved easily in the Ti(OR)4–ethanol
were observed as KBr pellets on a Thermo Fisher Nicolet iS50 solution by gentle thermal treatment, since the acidolysis
spectrometer. The morphologies of the resulting lms were reaction occurs as previously reported.26,27
investigated using scanning electron microscopy (SEM, ZEISS
At rst, the colour of the dissolved solution is light yellow.
Sigma500). Room temperature X-ray diffraction data were ob- Once the solvothermal reaction time has passed, the colour
tained using a Bruker D2 phaser desktop X-ray diffractometer. turns a dark yellow (Fig. 1a). Aer different lengths of time
Thermal analysis was conducted using a NETZSCH STA 2500 (1.5 h, 6.0 h and 24 h), the colour appears quite different
Regulus simultaneous thermal analyzer. The samples were without an obvious relationship. Time varies the colour of the
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emergence of a new peak at 580 nm is more obvious. Perylene
derivatives have colour changes and uorescence changes
ascribed to the continuous aggregation of molecules.29,30 The
aggregated state of molecules will form as the reaction time
increases due to the interaction between molecules, nally,
yellow uorescence appears aer 24 h reaction time.6,8
Scheme 2 (a) Synthesis and chemical structure of PI and (b) synthesis
of PI–TiO solution.
3.2 Characterization of the PI–TiO gel
We further conducted detailed characterization of the as-
prepared PI–TiO gel. Ti, C and O elements were found in the
gel powder via EDS analysis, and their content is shown in
Fig. 2a. Because hydrochloric acid is used during gelation, there
is also a very small amount of Cl element. The SEM diagram of
the gel powder shows the surface morphology of the aggregated
TiO nanoparticles (Fig. 2b–c). XRD results (Fig. S1†) for the PI–
TiO gel show that the gel is amorphous, and the two weak peaks
at (101) and (004) are ascribed to anatase peaks of TiO. Further
XPS results were obtained to demonstrate the valence bond
states, as shown in Fig. 2d–f. The abscissa is binding energy (eV)
and the ordinate is relative intensity. The peaks at 458.1 eV and
463.9 eV correspond to Ti 2p3/2 and Ti 2p1/2 in TiO, respectively
(Fig. 2d), while the peak at 529.6 eV corresponds to oxygen in
the TiO composite (Fig. 2e). The conjugated carbon of the per-
ylene was also clearly detected at 284.2 eV, and the carboxyl
carbon is displayed at 288.4 eV (Fig. 2f).31,32
solution due to the reaction of different quantities of PI and
Ti(OR)4. The corresponding uorescence colours of the PI–TiO
solution are obviously green, orange and yellow; the latter with
a 24 h reaction time possesses the brightest uorescence
performance (Fig. 1b). A transparent PI–TiO gel was further
obtained from the PI–TiO solution using a sol–gel method at
40 ꢂC.28 By carefully adjusting the pH value to 3, it became
a glue-like state at room temperature overnight. As shown in
Fig. 1c, we can obtain different PI–TiO gels at the corresponding
reaction times in solution. The whole gel is green aer 1.5 h
reaction time, a brown-yellow gel forms for the 6 h solution, and
nally a bright yellow PI–TiO gel was obtained for the 24 h
solution. On aging, the PI–TiO gel slowly becomes a dry, reddish
glue to the naked eye with the volatilization of the solvent
(Fig. 1d).
The UV spectrum of the dry gel shows a broad absorption
around 480–540 nm, which proves the existence of perylene
(Fig. 3a). Infrared (IR) spectroscopy can help us better under-
stand the state of organic molecules doped in inorganic mate-
rials. Three different samples, pure TiO dry gel, PI and PI–TiO
dry gel, were characterized using IR (Fig. 3b). The carboxylic
The colour changes and different uorescent emissions were
due to the PI molecule assembly behavior along with the reac-
tion time, as shown in Fig. 1e. At the beginning of the reaction,
the peak is strong around 530 nm and the form of perylene
monomer is green. With an increase in reaction time, the
Fig. 1 Photographs of the PI–TiO solutions prepared over different times (left 1.5 h, center 6 h, right 24 h) with different colours at (a) the naked
eye and (b) fluorescence, (c) different fluorescence emissions of the corresponding gels, (d) the reddish dry glue after volatilization, and (e) time-
traced normalized fluorescence spectra of the PI–TiO solution after 1.5 h (black line), 6 h (red line) and 24 h (yellow line).
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Fig. 2 (a) EDS analysis of the precipitate confirms that it contains Ti, O, C and Cl. (b and c) SEM images of the gel powder at different scales, and
(d–f) XPS results for the PI–TiO dry gel; (d) Ti 2p, (e) O 1s and (f) C 1s.
AA titration were rstly carried out using PI–TiO solution in
quartz cuvettes. As illustrated in Fig. 4a, upon addition of AA
in increased amounts, the absorbance of PI–TiO at the 490 nm
and 525 nm peaks increased gradually. According to the
literature,34 perylene compounds are highly likely to aggregate
due to the p–p interactions of conjugated aromatic rings or
intermolecular forces. The ratio of the absorption at 525 and
490 nm (A0–0/A0–1) indicates different aggregation states of
perylene compounds. When A0–0/A0–1 # 0.7, it exists mainly in
Fig. 3 UV-vis absorption (a) and FTIR spectra (b) of the pure TiO dry
the form of aggregates; when A0–0/A0–1 is approximately 1.6,
gel, PI and the PI–TiO dry gel.
only the monomer form dominates. The ratio varies between
these two values according to the aggregation states. When the
acid peak of PI appears at 1700 cmꢁ1 (red) and the peaks shi to
about 1536 and 1438 cmꢁ1 in the PI–TiO gel for the perylene
ring nC]C and nC–N stretching vibrations, respectively (black).
The red shi is due to the coordination of the metal-carboxylic
group, meaning that the PI is chelated inside the PI–TiO gel.
The formed Ti–O–C bond is indicated by the vibration peak
around 1045 cmꢁ1, between the –COOH and Ti(IV).8,33 As
mentioned in Scheme 2b, PI reacts with the Ti(OR)4 agent,
forming a PI–TiO compound.
concentration of AA increases, A0–0/A0–1 of PI–TiO decreases
gradually from 0.91 to 0.58, indicating that AA concentration is
conducive to the formation of PI aggregates.15 Correspond-
ingly the uorescence intensities at 542 nm and 578 nm were
quenched linearly when AA was gradually added (Fig. 4b).
Relative changes in uorescence are more obvious than for UV
measurements, so we then took the peaks around 542 nm for
quantitative calculations. Fig. S3† presents the quantitative
behavior of the portable PI–TiO luminescence quenching at
542 nm. A detection limit of 18.0 mM was calculated at 3s/S,
with a linear range between 0 and 0.2 mmol Lꢁ1 AA (R2 ¼ 0.99).
With the addition of AA amount, it was noticed that the PI–TiO
colour gradually deepened from yellow to brown (Fig. 4c).
Upon irradiation with the 365 nm UV lamp, the bright yellow
uorescence was slowly quenched under ultraviolet light in
the presence of AA (Fig. 4d).
Fig. S2† shows the thermogravimetric analysis (TG) curves
for different degrees of solvent volatilization to form dry gel.
The gel begins to lose solvent at room temperature, the curve
drops sharply at 200 ꢂC, and then loses weight at about 380 ꢂC.
At this time, the dry gel gradually transforms into anatase.
Because only a small amount of PTCA is input, the TG curves do
not display an obvious perylene peak (benzene decomposition
ꢂ
temperature is close to 600 C).
We further conducted solid uorescence tests by xing the
gel on a solid holder to conrm the gel phenomenon changes
(Fig. S4†). The location of the two peaks of the PI–TiO gel is like
that of the previous solution peak. Once 2 mL of AA solution (300
mM) was dripped onto the gel with a pipette, the gel uorescence
intensity was quenched to less than half the original intensity. A
second 2 mL of the AA solution (300 mM) caused the whole gel
uorescence to be almost quenched. Later, the gel was cast on
3.3 Turn-off uorescent detection of AA
Although we can get three kinds of PI–TiO gels with different
colors by adjusting the reaction time, the uorescence of the
green and orange gels is comparably weak. Therefore, we
directly chose the yellow PI–TiO gel aer 24 h reaction time in
the subsequent AA tests. The absorption and uorescence of
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Fig. 4 UV/vis absorption spectra (a), fluorescence spectra (Ex ¼ 360 nm, scan slit ¼ 1.5 nm) (b) and photographs of PI–TiO solution with AA
solution gradually added (1.5 mM) under natural light (c) and under 365 nm light (d).
a glass plate to prepare a detection lm (around 1 cm2). To the
naked eye, the lm is white, and a bright uorescence emission
of the gel lm can be observed under a 365 UV lamp. Also, once
the AA aqueous solution is dropped onto the lm, the colour of
the gel lm changes from white to yellow, and the uorescence
emission is completely inhibited by the AA aqueous solution.
These optical change phenomena on the lm can be explained
by the strong ligand transition of metal charge transfer.35 It is
well known that Ti(IV) can easily form complexes with oxygen
atoms on various oxygen-containing ligands.36
3.4 Detection principle and AA selectivity
To fully prove the binding principle, we conducted various
control experiments. As shown in Fig. 5, AA could completely
inhibit the uorescence emission of the PI–TiO lms. Other
organic acids at the same concentration, such as citric acid (CA),
tartaric acid (TA) and oxalic acid (OA), have little effect on
uorescence emission. Also B12 vitamins and aromatic amines
cannot quench the uorescence emission of the PI–TiO lm, as
well as some other common amino acids. Interestingly,
although the uorescence of an aqueous solution of PI can be
quenched by Al3+, Cu2+ and Pb2+, the PI–TiO obtained by a sol–
gel method will not be affected by these, indicating a stronger Fig. 5 Fluorescence response of the film to aqueous solutions of
binding ability of Ti(IV) with PI.37 The glass substrate with PI–
different compounds at the same concentration (150 mM) (abbrevia-
tions: ascorbic acid, AA; benzoic acid, BA; ethylenediaminetetraacetic
acid, EDTA; para-phenylenediamine, PPD; melamine, MA; phenol,
POH; oxalic acid, OA; citric acid, CA; tartaric acid, TA; vitamin B12,
VB12; glucose, Glc; cysteine, Cys; aspartic acid, Asn; alanine, Ala;
TiO gel on surface is stable for transfer during the experimental
processes.
Selectivity is supposed to be due to the competitive coordi-
nation of PI and AA with TiO. Titanium in TiO can bind with AA tyrosine, Tyr).
to form ascorbic acid titanium (Ti(IV)–AA), which is yellow (or
reddish brown). The coordination between PI and titanium
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oxide decreases, and the uorescence of PI–TiO is destroyed by
the new complex Ti(IV)–AA. The bidentate binding of AA with
anionic oxygen as an electron donor via two ene-diolate oxygen
atoms is most likely because it results in a chelating ring
structure, which provides increased stability. Furthermore, the
ve-membered ring structure has an advantageous angle
conguration for atoms on the Ti surface, and the distortion of
bond angle and distance is very small.38 The binding affinity
increases with the phenolic hydroxyl groups in AA rather than
the –COOH groups in PI. Also, the glycolic side chain of AA does
not seem to be involved in the formation of complexes.35 An IR
spectrogram was obtained to compare the characteristic
Fig. 7 (a) Glass plates coated with the gel film. (b) Fluorescence image
of the glass plates coated with the gel film. (c) Film covered with AA
bovine serum solution. (d) Fluorescence image of the film covered
absorption peaks of the PI–TiO gel before and aer AA addition. with AA bovine serum solution.
As shown in Fig. S5† and 3b, the carboxylic acid peak of PI
returned from 1536 cmꢁ1 in PI–TiO to 1625 cmꢁ1 aer AA
addition, indicating that the coordination of carboxylic groups
Notably, a series of other perylene uorescent lms for the
with Ti(IV) was successfully destroyed. Another new peak at
580 cmꢁ1conrmed the existence of Ti–O bonds, due to the
coordination of Ti with hydroxyl groups in ascorbic acid.38 It is
said not all four of the carboxyl groups in PI were coordinated,
and that some of these may be esteried.8
detection of aniline vapor39,40 and phenol solutions41 have been
reported previously. However, these uorescent perylene lms
are rarely applied for sensitive and portable AA detection.
Considering these compounds with phenolic hydroxyl groups
do not usually co-exist with AA, this visualized detection
manner is also much more convenient for AA sensing, as well as
the present PI–TiO gel system being more economical and easy
to prepare.
In order to further prove the coordination effect between
phenolic hydroxyl groups and Ti(IV), other compounds con-
taining phenol hydroxyl groups, such as phenol, pyrocatechol
and 1,2,3-benzenetriol solutions, were checked for their optical
changes on PI–TiO gel lm. As shown in Fig. 6, the uorescence
is also quenched by pyrocatechol and 1,2,3-benzenetriol solu-
tions with a concentration higher than 300 mM, while phenol
can also signicantly weaken lm uorescence. When the
concentration of phenol is upped to 2 mM, lm uorescence
quenching occurs. In contrast, we dropped glucose and tartaric
acid solutions which contain –OH on the gel lm without any
change in uorescence. This is consistent with previous
discussion and proves that the strong coordination of the
hydroxyl group on the aromatic ring with Ti(IV) causes colour
change and uorescence quenching.38
3.5 Model biological sample analysis
The determinations of model biological samples with PI–TiO
gel were performed by adding AA inside bovine serum with
a standard addition technique. All serum samples were diluted
(10%) before the measurements. In order to ascertain the
correctness of the results, AA was detected in the diluted
samples at a nal concentration of 150 mM. The results are
displayed in Fig. 7. It was found that the PI–TiO lm may be
used for the direct determination of AA in real samples.
Fig. 6 Fluorescence responses of the PI–TiO film to different hydroxyl-containing compounds at 300 mM and 2 mM ((+) indicates fluorescence
on, (ꢁ) indicates fluorescence off).
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4 Conclusions
´
´
13 A. J. Jimenez, M. J. Lin, C. Burschka, J. Becker, V. Settels,
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tions, a strongly emitting PI–TiO gel can be obtained. During
the detection process, inorganic TiO gel is not only used as
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glass substrate can be applied as a simple visual colour and
uorescence double sensor for AA detection. Furthermore, the
sensing principle and selectivity are carefully discussed and
are ascribed to the strong coordination between phenolic
hydroxyl groups and Ti(IV). The portable ability of the sensor
also has potential for human application in future point-of-
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Conflicts of interest
The authors declare no competing nancial interest.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC 21705106), the Program for
Professor of Special Appointment (Eastern Scholar) at Shanghai
Institutions of Higher Learning (No. TP2016023), Shanghai
Sailing Program (17YF1405700) and Shanghai Natural Science
Foundation (No. 18ZR1415400).
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