ISBN : 978-602-17761-4-8
53
Proceedings of 2
nd
REPTech Crowne Plaza Hotel, Bandung, November 15-17, 2016
© 2016 Published by Center for Pulp and Paper through 2
nd
REPTech
In the luorescence detection, also referred to as quantitative detection by applying luorescence spectroscopy F-4500, Hitachi, Japan, the surface luorescence intensity of the paper-based sensor
was measured. The excitation and emission wavelengths were 490 nm and 567 nm, respectively. As anthraquinone derivative was quenched by Cu
2+
in solution, the surface luorescence intensity of paper- based sensors immersed in Cu
2+
solutions of various concentrations was determined. Paper-based sensors, fabricated using a 1 gL anthraquinone derivative acetone solution, were
immersed in 5-mL Cu
2+
solutions with concentrations of 1, 2, 3, 4, 5, and 6 ppm for 10 min. Additionally, to achieve higher sensitivity, paper-based sensors were fabricated using an anthraquinone derivative
acetone solution of lower concentration 0.4 gL. Subsequently, these fabricated sensors were immersed in 5-mL Cu
2+
solutions with concentrations of 200, 400, 600, and 800 ppb for 10 min. After immersion, excess water was removed with a paper wiper. Before drying, the surface luorescence intensity of the
paper-based sensors was measured, and the relationship between surface luorescence intensity and Cu
2+
concentration was determined. All aqueous samples in this research were adjusted to pH 7 using a buffer solution containing
4-2-hydroxyethyl-1-piperazineethanesulfonic acid HEPES and NaOH, which is widely used in research related to heavy metal solutions.
2.5 Interference
To determine the selectivity of the paper-based sensor, interference by other metal ions was studied in both visible and luorescence detections. Na
+
, K
+
, Ca
2+
, Fe
3+
, Co
2+
, Cd
2+
, Mn
2+
, Hg
2+
, Pb
2+
, Ni
2+
, Zn
2+
, and Ag
+
were tested. The paper-based sensors were immersed in a 5-mL 20-ppm aqueous solution of each metal ion. After 10 min immersion, the color of the sensor was observed by the naked eye and captured
using a digital camera. In luorescence detection, excess water on the paper-based sensors was removed and surface luorescence intensity was measured.
Results and Discussion 3.1 Fabrication and Characterization
A quick and easy fabrication method was developed using inkjet printing technology. The anthraquinone derivative was irmly adsorbed on cellulose iber surfaces through non-covalent bonds,
including hydrogen bonds, hydrophobic forces, and CH–π interactions.
16
The fabrication method developed in this research has the following advantages: i although acetone evaporated quickly, perhaps causing anthraquinone derivative to block the nozzle of the printer head,
the ink easily lowed in the nozzle, redissolving anthraquinone derivative and preventing the printer head nozzle from being blocked; ii acetone is non-destructive to the ilter paper iber network; and iii
inkjet printing technology makes lexible pattern design and homogeneous distribution of anthraquinone derivative possible. Furthermore, as shown in Fig. 1, anthraquinone derivative is only concentrated in
the top layer with a total thickness of 150 μm, suggesting that it was possible to easily control and reduce
the amount of anthraquinone derivative, and accelerate and accentuate the color reaction, compared with other fabrication methods, such as immersion.
Fig. 2 shows a CLSM image of the anthraquinone derivative distributed evenly on cellulose ibers by inkjet printing. Even distribution was important for detection, especially for luorescence detection,
and dificult to achieve using any other method. Consequently, inkjet printing appeared to be an ideal method for this fabrication regarding pattern design, operation, and cost.
3.2 Qualitative and Quantitative Detection
Fig. 3 shows a photograph of paper-based sensors immersed in Cu
2+
aqueous solutions. The color of the dye on the paper-based sensors changed from yellow to purple with increasing Cu
2+
concentration. This result conirmed that the paper-based sensor was able to detect Cu
2+
at concentration as low as 2 ppm, which is the maximum allowed amount in drinking water, according to the WHO. The entire
ISBN : 978-602-17761-4-8
54
Proceedings of 2
nd
REPTech Crowne Plaza Hotel, Bandung, November 15-17, 2016
© 2016 Published by Center for Pulp and Paper through 2
nd
REPTech
detection process took only 10 min and sensitive detection of Cu
2+
was successfully achieved. The 10-min immersion time was determined in the preliminary test, in which no additional obvious color
change was observed with immersion times longer than 10 min. This user-friendly detection provided the possibility for non-professionals to perform an on-site water safety check.
Fig. 4 shows the luorescence spectra of paper-based sensors after immersion in Cu
2+
aqueous solutions. As the Cu
2+
concentration increased, the luorescence intensity at 567 nm decreased. Fig. 5 shows a linear relationship between the surface luorescence intensity of the paper-based sensor and
Cu
2+
concentration, which provided the possibility for quantitative detection of Cu
2+
concentration using the paper-based sensor by simply combining with luorescence spectroscopy. In addition, low Cu
2+
Fig. 1. CLSM image representing the 3D structure and the anthraquinone derivative distribution of a paper-based sensor cross-sectioned at 45
o
on one side.
Fig. 2. CLSM image of the anthraquinone derivative distribution on cellulose ibers.
Fig. 3. Paper-based sensors after immersion in Cu2+ aqueous solutions at different concentrations for 10 min.
ISBN : 978-602-17761-4-8
55
Proceedings of 2
nd
REPTech Crowne Plaza Hotel, Bandung, November 15-17, 2016
© 2016 Published by Center for Pulp and Paper through 2
nd
REPTech
concentrations, at ppb levels, were detected accurately by applying a low-concentration anthraquinone derivative acetone solution in the fabrication, as shown in Fig. 6.
The detection mechanism can be explained by the quenching effect of Cu
2+
on the anthraquinone derivative. The complexation between Cu
2+
and the anthraquinone derivative results in electron or energy transfer from the anthraquinone derivative moiety to Cu
2+
, quenching the luorescence emission
17
. Regarding the kinetics of the chemical reaction at the solid–liquid interface, the amount of the
anthraquinone derivative per unit area and the Cu
2+
concentration were important factors in the detection reaction. After 10 min, the paper-based sensors printed with a certain amount of anthraquinone derivative
decreased the luorescence intensity because of an increasing quenching level, caused by the formation of more Cu
2+
and the anthraquinone derivative complexes with an increasing Cu
2+
concentration, within a certain range. In this research, the paper-based sensor printed with 5.7 × 10
–9
molcm
2
anthraquinone derivative was suitable for the visible detection of a 2 ppm Cu
2+
aqueous solution, while the paper- based sensors printed with 9.5 × 10
–9
and 3.8 × 10
–9
molcm
2
anthraquinone derivative were suitable for measuring Cu
2+
concentrations in the ranges 0–5 ppm and 0–600 ppb, respectively. This revealed the positive correlation between the amount of the anthraquinone derivative per unit area and the detection
range of Cu
2+
concentration. Based on this relationship, paper-based sensors for various detection ranges could be fabricated by controlling the amount of the anthraquinone derivative printed.
Fig. 5. Fluorescence intensity of paper-based sensor, fabricated with 1 gL anthraquinone
derivative solution, after immersion in Cu2+ aqueous solutions of various concentrations,
from 0 to 5 ppm. Fig. 6. Fluorescence intensity of paper-based
sensor fabricated with 0.4 gL anthraquinone derivative solution, after immersion in Cu2+
aqueous solutions at various concentrations, from 0 to 600 ppb.
3.3 Interference
