S. Kai et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 155 (2016) 81–87
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Third, RS-NH2 was reacted with oxalaldehyde to give RS-CHO. RS-
NH2 (5 mmol), oxalaldehyde (10 mmol) and absolute ethanol (50 mL)
were mixed together and heated at 80° for 10 h. Solvent was removed
by rotary evaporation under reduced pressure. Crude product was puri-
fied on a silica gel column with CH2Cl2 as eluent to give RS-CHO. 1H NMR
(CDCl3), δ (ppm): 1.19 (t, 12H, NCH2CH3), 3.25 (q, 8H, NCH2CH3), 6.25
(dd, 2H, xanthene-H), 6.40 (d, 2H, xanthene-H), 6.47 (d, 2H,
xanthene-H), 7.13 (dd, 1H, Ar-H), 7.32 (d, _CHC), 7.47 (dd, 2H, Ar-
H), 8.13 (dd, 1H, Ar-H), 9.45 (d, 1H, −CHO). 13C NMR (CDCl3), δ
(ppm): 12.44, 44.40, 72.38, 98.59, 104.89 108.85, 123.89, 126.45,
127.79, 128.67, 149.35, 152.77, 152.87, 192.65, 200.9. ESI-MS m/e:
calc. for C30H32N4O2S, 512.2; found, 512.0 [m]+. Anal. Calcd. for
C30H32N4O2S: C, 70.28; H, 6.29; N, 10.93. Found: C, 70.33; H, 6.35; N,
11.01.
an improved water solubility. There are C, O, Na, F, Y, Yb and Er elements
in these UCNCs, as suggested by their EDX spectra (inset of Fig. 2). The
first two elements are assigned to α-CD and oleic acid on UCNCs surface,
while the latter five ones are consistent with elemental composition of
NaYF4:Yb3+/Er3+ nanocrystals. These UCNCs are further characterized
by powder XRD pattern. As shown in Fig. 2, there are 17 detectable dif-
fraction peaks which are nearly identical to those of pure hexagonal
NaYF4 nanocrystals (JCPDS card No. 28–1192) [13]. There are no diffrac-
tion peaks from mixed phases or impurities, suggesting that Yb and Er
have been successfully doped into NaYF4 lattice, forming an up-
conversion host of NaYF4:Yb3+/Er3+
.
The successful α-CD modification on UCNCs is tentatively investigat-
ed by IR spectra of UCNCs covered by oleic acid and UCNCs modified by
α-CD. As shown in Fig. 3, there is stretching vibration of _C–H group at
2923 cm−1 which is originated from oleic acid [11]. The weak peak
around 1162 cm−1 is assigned to coupled stretch vibration of C–C
bond. These bands suggest that our NaYF4:Yb3+/Er3+ crystals are in-
deed covered by oleic acid. After being modified by α-CD, these peaks
are well preserved. There is a new band peaking at 2974 cm−1 which
corresponds to anti-symmetric glycosidic vibration of C–O–C from α-
CD, as shown in Fig. 3 [11]. The multiple bands around 1086 cm−1 can
be attributed to coupled stretch vibrations of C–C and C–O bands from
α-CD. It is thus tentatively confirmed that NaYF4:Yb3+/Er3+ UCNCs
have been modified by α-CD. Correspondingly, their hydrophobic is
transformed as hydrophilic, showing a good dispersibility in aqueous
solution. As shown by the inset of Fig. 3, before α-CD coating, all
nanocrystals are dispersed in hexane layer, indicating their hydrophobic
nature. After α-CD coating, these nanocrystals are dispersed in water
layer, showing hydrophilic nature. This result finally confirms the suc-
cessful coating of α-CD.
2.3. Construction of UCNCs
Excitation host β-NaYF4:Yb3+/Er3+ nanocrystals were constructed
following a literature method [11]. The mixture of ErCl3·6H2O (0.02
mmol), YbCl3·6H2O (0.20 mmol), YCl3·6H2O (0.78 mmol), oleic acid
(6 mL) and 1-octadecene (15 mL) was kept at 160° under N2 protection
for 30 min and then naturally cooled. NH4F (4 mmol) and NaOH
(2.5 mmol) were dispersed in methanol (10 mL) and slowly added.
The resulting mixture was heated to 75° for 20 min and then to 100 de-
grees for 20 min to remove methanol. This mixture was then kept at
300° for 1 h under N2 protection. After cooling, ethanol was added to
yield precipitate which was washed with ethanol and dispersed in cy-
clohexane for further operation.
2.4. Construction of UCNCs covered by α-CD
UCNCs covered by α-CD were prepared as follows. The above ob-
tained UCNCs (covered by oleic acid) were dispersed in hexane
(5 mg in 10 mL). Α-CD was dissolved in water (5 mM) and mixed
with UCNCs solution (V:V = 1:1). This mixture was stirred at room
temperature for 24 h. Upper hexane layer became transparent and
was discarded. Lower water layer was centrifuged and washed
with deionized water for 3 times. The obtained UCNCs covered by
α-CD were dispersed in water for further operation.
3.3. Photophysical analysis on UCNCs and chemosensor
3.3.1. Emission and absorption
Absorption and emission spectra of UCNCs (modified with α-CD)
and RS-CHO are firstly discussed so that the possibility of energy trans-
fer between them can be evaluated. Considering that free rhodamine
molecules tend to take a non-emissive spirolactam structure, CYS
(1 eq.) is added to make sure that they take an emissive xanthene struc-
ture and thus are open for energy transfer. As shown in Fig. 4, RS-CHO
shows a broad absorption band ranging from 450 nm to 600 nm with
a main peak of 562 nm and a shoulder peak of 524 nm, respectively.
Its emission peaks at 574 nm with no vibronic progressions, which is
similar to literature case [11]. As for the up-conversion emission of our
UCNCs (modified with α-CD), there are three bands peaking at
519 nm, 539 nm and 653 nm, respectively. These transitions are
3. Results and discussion
3.1. Explanation on design strategy of chemosensor and up-conversion host
It has been suggested that rhodamine and its derivatives are good
chemosensors for various target molecules [18–19]. Such molecules fin-
ish their sensing procedure through a structural transformation be-
tween spirolactam structure (non-emissive) and delocalized xanthene
structure (emissive), showing sensing signals. Aiming at an improved
selectivity towards CYS, rhodamine hydrazine was reacted with cyclo-
hexane and its O atom was replaced by a S atom. Excitation window
of rhodamine and its derivatives usually localizes at ~550 nm which
overlaps well with NaYF4 up-conversion emission [11,18,19]. Thus,
UCNCs were prepared and used as excitation host to minimize
chemosensor photobleaching. To make these hydrophobic UCNCs
water-dispersible, they were modified by an amphiphilic nonionic sur-
factant of α-CD, as above mentioned.
assigned to 2H11∕2
→ , → , →
4I15∕2 4S3∕2 4I15∕2 4F9∕2 4I15∕2 transitions
of Er ion, confirming that this up-conversion excitation host has been
successfully prepared [11,13]. There is a good spectral overlap between
RS-CHO absorption and the first two up-conversion emission bands,
which guarantees a possible energy transfer between them.
3.3.2. Energy transfer radius
Energy transfer radius between our UCNCs (modified with α-CD)
and RS-CHO, which is known as Forster radius (R0), is calculated
through their absorption and emission spectra, as depicted by Formula
1 and Formula 2, where Q0, J, κ2, nd, NA, λ, fd(λ), εA(λ) stand for
UCNCs emission yield, overlap integral between UCNCs emission and
RS-CHO absorption spectra, mutual molecular orientation, solvent re-
fraction index, Avogadro number, wavelength, UCNCs emission intensi-
ty and RS-CHO molar extinction efficient, respectively [20]. Q0 has been
reported as 3% by Boyer [21]. With their absorption and emission spec-
tra on hand, R0 is calculated as 20 Å which is slightly longer than litera-
ture values owing to our good spectral overlap [11,13,18]. This long
energy transfer radius suggests that an efficient energy transfer be-
tween our UCNCs and RS-CHO can be accomplished in a dilute solution.
3.2. Characterization on UCNCs modified by α-CD
Our UCNCs modified by α-CD are firstly analyzed by their scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM) images shown in Fig. 1. Round-liked nanocrystals are observed
with mean diameter of ~20 nm. These nanocrystals have uniform mor-
phology and smooth surface, suggesting that α-CD can well modify
their surface without changing their shape or aggregation, leading to