Journal of Agricultural and Food Chemistry
Article
sensitivity. Therefore, it is still a huge challenge for the accurate
and selective determination of formaldehyde with higher
sensitivity.
catalyzed by MnO2 nanosheets and reduce the fluorescence
intensity of the system. The inhibition of the oxidation of OPD
by the specific Schiff base reaction between OPD and
formaldehyde accompanied by a decline of the in situ-formed
fluorescence species can be utilized for the sensitive
determination of formaldehyde. The detection mechanism
and the selectivity were investigated in depth, which showed
that the in situ-formed fluorescence species enabled the
sensitive detection of formaldehyde and the specific chemical
reaction showed good selectivity. The detection of form-
aldehyde in air, beer, and some seafood samples was
successfully conducted. This work was different from those
reported in the literature which it involved the decline of the
nanozyme activity of MnO2 nanosheets or using the oxidation
property of MnO2. This work not only provides a universal
nanoplatform for formaldehyde determination, but also
expands the application of MnO2 nanosheets in food and
environmental areas.
The rapid development of nanomaterials has brought great
opportunities and progress to optical-related fields.11−14 Due
to the particularity of fluorescent nanoplatforms to transform
the chemical interaction between molecules into a fluorescence
signal, the selective recognition of specific molecules or ions
can be realized, which shows the advantages of high sensitivity,
simple operation, low cost, and has become a research hotspot
in the field of chemical and biological analysis.15,16 On this
respect, the utilization of two-dimensional nanomaterials as the
fluorescence quencher and molecular/fluorescence nanoma-
terial probes as the signal output is considered as a universal
strategy for the construction of a fluorescence-sensing
system.17,18 As one of the typical two-dimensional nanoma-
terials, MnO2 nanosheets have attracted much attention in
fluorescence sensing due to their surface properties, wide
absorption band, good redox properties, and good biocompat-
ibility.19−21 For example, MnO2-fluorescent polydopamine
nanoparticles have been used for the detection of human
serum alkaline phosphatase (ALP),22 and MnO2−AuNCs can
be used for H2O2 determination based on fluorescence
resonance energy transfer.23 In many of these MnO2-based
systems, luminescent probes were utilized, whose fluorescence
intensity may be greatly affected by the quencher of MnO2
nanosheets.24 Moreover, the strategy of using the nanoprobes
such as AuNCs and fluorescence polydopamine nanoparticles
was relatively complex and their fluorescence properties may
be greatly affected by the microenvironment.25 Another unique
property of MnO2 nanosheets or nanoflakes is their oxidase
(OXD)- or peroxidase-like activity, which has also attracted
much attention.26,27 The mimicking properties can be used in
construction of colorimetric sensors for the rapid qualitative
analysis of various substrates.28,29 Compared with the
colorimetric sensing, fluorescence analysis is more sensitive.
Meanwhile, in situ formation of fluorescence species as the
signal output based on the nanozyme activity of MnO2 may be
more convenient because there is no need to synthesize
fluorescent nanoprobes with higher purity. For instance, by
mixing nonfluorescent o-phenylenediamine (OPD) with MnO2
nanosheets that exhibit OXD-like activity, fluorescence species
of 2,3-diaminophenazine (DAP) resulting from the catalysis
oxidation of OPD by MnO2 can be obtained, which can be
used for the detection of ascorbic acid and ALP.30 The
detection mechanism was based on the fact that AA can reduce
MnO2 nanosheets, resulting in the loss of the OXD-like
activity. The in situ formation of fluorescence species is
attractive. However, the reductive targets will also cause
interference problem. Therefore, the in situ formation of
fluorescence species based on the OXD-like activity of MnO2
nanosheets31 combined with the utilization of a specific
chemical reaction for recognition of special molecules may be
employed to develop a nanoplatform with good sensitivity and
selectivity, which may be a promising work.
EXPERIMENTAL SECTION
■
Reagents and Materials. Manganese chloride tetrahydrate
(MnCl2·4H2O, 99.0%) and OPD (99.0%) were purchased from
Tianjin Commio chemical testing Co. Ltd; tetramethylammonium
hydroxide (25%) was obtained from Tianjin Guangfu fine chemical
research institute (Tianjin, China); bovine serum albumin (98.0%)
was obtained from Sigma-Aldrich; and dipotassium hydrogen
phosphate (K2HPO4, 99.5%), potassium dihydrogen phosphate
(KH2PO4, 99.5%), formaldehyde (HCHO, 37%), and hydrogen
peroxide (H2O2, 30 wt %) were provided by Shanghai Wokai
biotechnology Co. Ltd. All other chemicals were of analytical grade
and used without further purification. Ultrapure water was obtained
using a Milli-Q system.
Instruments. The morphology images and energy-dispersive
spectrum (EDS) of MnO2 nanosheets were recorded on a JEM-
2100 (JEOL, Japan) transmission electron microscope (TEM). X-ray
photoelectron spectra (XPS) were obtained on a K-Alpha 1063
(Thermo Fisher Scientific, British). Fourier transform infrared
(FTIR) spectra were collected on a Nexus-870 (Thermo Nicolet,
USA). X-ray diffraction (XRD) patterns were obtained using a Rigaku
2500 (Japan) X-ray diffractometer. Fluorescence and UV−vis spectra
were obtained on a F-7000 fluorescence spectrometer (Hitachi,
Japan) and UV-2450 (Shimadzu, Japan), respectively. The experi-
ments of electrospray ionization mass spectrometry (ESI-MS) were
performed on a liquid chromatography−mass spectrometer (LC−MS
8050, Shimadzu, Japan).
Fluorescence Assay of Formaldehyde. The synthesis of single-
layer MnO2 nanosheets was caried out using a procedure similar to
that of our previous work32 and the main process was shown in the
in the supernatant can be calculated according to the Beer−Lambert
Law with a molar extinction coefficient of 9.6 × 103 M−1 cm−1 at 380
nm.33
The stock solution of formaldehyde (0.2 mM) was prepared and
various concentrations of formaldehyde solution were obtained by
dilution of the stock solution. For the detection of formaldehyde, 100
μL of formaldehyde solution with different concentrations was added
sequentially to 100 μL of 4 mM OPD, and the mixture was incubated
for 50 min at 55 °C. Then, 700 μL of 0.1 M PBS (pH 6.5) and 100 μL
of MnO2 nanosheets aqueous solution (1.74 × 10−2 g/L) were added
sequentially with a total volume of 1.0 mL. After the solution was
thoroughly mixed and incubated at 55 °C for 70 min, the fluorescence
measurements were performed. All the solutions are not pumping of
N2 before detection excepted stated.
In this work, a nanoplatform with good sensitivity and
selectivity for formaldehyde detection was developed using
MnO2 nanosheets with good oxidase activity combined with
the in situ-formed fluorescent probes of DAP derived from
OPD. The MnO2 nanosheets can catalyze the oxidation of
OPD to DAP with yellow fluorescence in the presence of
oxygen; while formaldehyde can react with OPD, which can
cause a decrease of the concentration of free OPD that can be
Pretreatment of Actual Samples. The formaldehyde sample of
the air was obtained as follows: the sampled space was sealed for 24−
48 h, 10 L of air in the space was collected with a formaldehyde
7304
J. Agric. Food Chem. 2021, 69, 7303−7312