Mechanism Studies. To understand the mechanism of
hydrophobic and hydrophilic conversion of the paper caused by
OTS coating and UV/O3-treatment, spectroscopic analyses
were conducted for the paper sheets before and after OTScoupling
and UV/O3-exposing. XPS analyses revealed that the
spectrum for native paper only contained C1s and O1s peaks
while the spectra for OTS-paper and UV-OTS-paper contained
Si1p and Si2p double peaks in addition to the C1s and O1s
peaks and that the C/O ratios of 2.2, 9.5, and 1 were observed
for native paper, OTS-paper, and the UV-OTS-paper,
respectively (see the Supporting Information, Figure S2).
This indicated that OTS was coupled to the fiber surfaces of the
paper after OTS-coating and that silicon atoms remained on
the paper and oxygen-rich moieties were formed after UV/O3
treatment of the OTS-paper. ATR-FT-IR spectra further
confirmed the deduction. Compared to the spectrum for native
paper, two new strong absorption bands at ∼2911/cm and
2844/cm, which could be assigned to the C−H stretching of
CH2 group in long hydrocarbon chain −C18H37, appeared in
the IR spectrum for the OTS-paper, indicating OTS was
attached to the paper after the silane coupling reaction (see the
Supporting Information, Figure S3). After the OTS-paper
subjected to UV/O3 exposure, relatively strong and overlapped
absorption bands centered at ∼1700/cm, which was the
characteristic absorption caused by CO stretching vibration
of the carbonyl groups, appeared in company of the
disappearance of ∼2911/cm and 2844/cm peaks. Thus, it can
be concluded that the long alkyl chain of OTS molecules
coupled to the paper fibers were decomposed to form polar
oxygen-containing moieties such as ketones, aldehydes, and
carboxylates. These analyses were similar to the reports for
UV/O3 degrading of OTS-SAM formed on the glass surface.32
Assay of Nitrite in Food Sample. To demonstrate the
feasibility of μPAD as a quantitative analysis device, a flowershaped
μPAD with eight detection zones was fabricated for
assay of nitrite in food samples. This design allowed the
quantitative determination of analyte concentrations via
measuring the surface color intensities in the detection zones,
where the designed color reaction between the penetrating-in
indicator reagent and pipetted standard solutions or sample
solutions occurred. In the present work, the Griess colorreaction
was used for determination of nitrite ions. Figure 4a
shows a scanned image of the color-developed μPAD where
seven detection zones were for standard solutions and the last
one was for the sample solution. On the basis of the color
intensities of the seven zones for standard solutions, a typical
calibration curve for nitrite ion was constructed as shown in
Figure 4b. Thus, the dynamic linear range was in the range of
0.156−2.50 mM NO2
−, within which a regression equation of y
= 3.815x + 0.2353 (x and y represent NO2
− concentration at
millimolar and color intensity at 103 arbitrary unit, respectively)
was obtained together with a correlation coefficient of 0.9965.
The proposed method was validated by determination of
NO2
− in the red cubilose sample. The NO2
− content in sample
determined by the proposed μPAD method was (0.5440 ±
0.0102 g/kg), which agreed well with the concentration
(0.5285 ± 0.0052 g/kg) (n = 3) determined by ion
chromatography. This result demonstrates that the designed
μPAD device can be applied for quick determination of NO2
−
content in food samples