Y. Zhao, X. Liu, Y. Jiang et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120003
an important indicator for water environment. In addition, a low
level of local dissolved oxygen concentration has been reported
as a sign for tumors in human body [2]. The sensitive and selective
determination for dissolved oxygen is thus an important issue in
life science, medical care and environmental science.
2. Experimental briefing
Scheme 1 shows a detailed synthetic procedure for this hybrid
structure, named as Ru@CD. A brief description about Ru@CD syn-
thesis and sensing operation is listed below.
There have been traditional analytical methods for dissolved
oxygen, such as iodimetric method, electrochemical method and
so on [3,4]. Although iodimetric method gives precise result, it
costs time and sample, which makes it inappropriate for on-line
detection. As for electrochemical method, such as Clark ampero-
metric electrode, its precision strongly depends on the surface sta-
tus of electrodes. Given these limitations, optical sensing has been
proposed as an attractive method for oxygen determination due to
its virtues of fast, effective and non-invasive detection [5]. Its sens-
2
2
.1. Synthesis of 1,10-phenanthrolin-5-amine (Phen-NH )
1
,10-phenanthrolin-5-amine (Phen-NH
lowing a two-step procedure. 1,10-phenanthroline (5 g) was
cooled by ice bath. Concentrated H SO (8 mL) was dropwise
added, then fuming nitric acid (30 mL) was dropwise added under
stirring. This mixture was stirred under ice bath for 30 min and
then at 90 °C for 30 h. The resulting mixture was poured into ice
water (100 mL) and then nitrilized with NaOH solution. Solid pro-
duct was collected, washed with water and recrystallized with
2
) was synthesized fol-
2
4
ing procedure is generally described as a dynamic collision
3
between excited probe and ground state O
probe is attacked by ground state O
2
( O
2
), where excited
2
and loses its energy, showing
ethanol to give 5-nitro-1,10-phenanthroline as yellow powder.
Yield = 63%. H NMR(CDCl
probe emission quenching effect. Since a long excited state lifetime
offers a high collision probability and consequently a high sensitiv-
ity, oxygen sensing probes are generally phosphorescent ones [6].
Luminescent Ru(II)-bpy (bpy = bipyridine) complexes have been
reported as promising probes owing to their long-lived phospho-
1
3
), d:7.76 (m, 2H), 8.76 (d, 1H), 8.77 (d,
1
H), 9.03 (s, 1H), 9.24 (d, 1H), 9.28 (d, 1H).
The above obtained 5-nitro-1,10-phenanthroline (1.5 g) was
dissolved in hot ethanol (20 mL). 5% Pd/C (0.3 g), hydrazine hydrate
50%, 15 mL) and ethanol (10 mL) were mixed together and heated
to 70 °C. These two solutions were mixed together and heated at
0 °C for 10 h. The resulting solution was filtered to remove Pd/
C. Yellow powder Phen-NH was obtained after extracting solvent
under reduced pressure. Yield = 78%. H NMR(CDCl
H), 6.86 (s, 1H), 7.53 (m, 1H), 7.74 (m, 1H), 8.06 (d, 1H), 8.65
m, 2H) , 9.04 (d, 1H).
(
rescent emission (~10
ls) based on metal-to-ligand-charge-
transfer (MLCT) [6,7]. A typical sensing procedure has been pro-
9
3
? Ru(II) + 1
posed as Ru(II)* +
state.
O
2
2
O *, where ‘‘*” means excited
2
1
3
), d:6.14 (s,
2
To ensure fast and fluent O diffusion, Ru(II)-bpy probes are
2
(
generally embedded into supporting matrix to form a hybrid struc-
ture, hoping to combine features of each component at nanometer
or molecular level. In this case, various optical oxygen sensing
structures have been developed and reported [8,9,10]. Many liter-
atures focusing on the development of dissolved oxygen have been
reported based on polymers, micelles and nanoparticles. For exam-
ple Vinogradov and coworkers reported their phosphorescent Pd-
probes for biological oximetry and tumor imaging [11]. Wang
and coworkers reported two-photon oxygen nanosensors based
on a conjugated fluorescent polymer doped with platinum por-
phyrins [12]. The ratiometric extracellular oxygen sensing has
2
.2. Synthesis of 4-((1,10-phenanthrolin-5-yl)amino)-4-oxobutanoic
acid (Phen-COOH)
4
-((1,10-phenanthrolin-5-yl)amino)-4-oxobutanoic acid (Phen-
COOH) was synthesized following below method. Butanedioic
anhydride (2.56 g) was dissolved in a mixture of acetonitrile/
dimethyl formamidine (10:1, 15 mL). Then Phen-NH (1 g) was
2
added. This mixture was heated to reflux and kept for 3 h. This
mixture was placed at 0 °C overnight. Solid product was collected,
been reported by Tian and coworkers using poly(e-caprolactone)-
washed with acetonitrile and water to give Phen-COOH. Yield = 67%.
containing graft copolymers [13]. Wolfbeis and coworkers have
summarized recent progress of optical methods for sensing and
imaging oxygen, including materials, spectroscopies and applica-
tions [14]. Never the less, there are still problems to be solved.
First, the linearity of working curve is yet to be satisfied. Second,
most sensing systems have only one dominant emission band for
sensing quantification. Considering that probe emission is affected
by various factors such as equipment condition, sample concentra-
tion and status, these optical sensing systems generally need aux-
iliary reference before giving previse quantification results.
1
6
H NMR (DMSO d ) d (ppm): 2.64 (t, 2H), 2.81 (t, 2H), 7.74 (dd, 1H),
7
1
.81 (dd, 1H), 8.16 (s, 1H), 8.45 (dd, 1H), 8.67 (dd, 1H), 9.04 (dd,
H), 9.13 (dd, 1H), 10.21 (s,1H), 12.21 (s, 1H).
2.3. Synthesis of Ru(bpy)
2
Cl
2 2
ꢁ2H O
Ru(bpy)
2
Cl
2
ꢁ2H
2
O was synthesized following a literature proce-
0
dure with RuCl
3
2
ꢁ6H O and 2,2 -bipyridine as starting compounds
[
5]. Yield = 45%.
To solve the above two problems, an ideal optical sensing sys-
tem for dissolved oxygen should be composed of a stable and
long-lived probe, a reference emission and a water-compatible
supporting matrix. In this effort, we intend to try the combination
of Ru(II)-bpy complex and carbon dots (CDs) via covalent bonds.
CDs have recently been nominated as an attractive material owing
to their strong and stable emission, good compatibility with water
2
.4. Synthesis of CDs
CDs were prepared as follows [16]. 0.84 g of citric acid and
.02 of ethylenediamine were dissolved in 20 mL of
1
g
double-distilled water, then the solution was decanted into a
Teflon-lined autoclave (30 mL) and heated at 200 °C for 5 h. After
the reactor was cooled to room temperature naturally, the disper-
sion was dialyzed [MWCO (molecular weight cut-off) = 1000]
against 500 mL of pure water for three times. Product was freeze
dried to obtain brown powder (Yield = 0.16 g) which was then dis-
persed in water again in a concentration of 10 mg/mL for further use.
[
15,16]. Upon the covalent bonding between CDs and Ru(II)-bpy,
their phase separation can be avoided. The good water compatibil-
ity of CDs and Ru(II)-bpy makes sure an effective collision with
analyte O
excited state lifetime (~several nanoseconds), which is too soon
for O attack, making CDs emission immune to O existence. In
2
molecules. The strong emission of CDs has a short
2
2
other words, CDs emission can be used as an inner reference for
Ru(II)-bpy optical sensing. In this case, a hybrid structure will be
constructed for the detection of dissolved oxygen, using Ru(II)-
bpy as sensing probe and CDs as supporting matrix.
2.5. Synthesis of Ru(II) probe
A mixture of Ru(bpy)
2
Cl
2
ꢁ2H
2
O (0.26 g), Phen-COOH (0.15 g)
and ethanol (20 mL) was heated at 80 °C for 12 h under N
2
2