Recently a simple process referred to as the “reprecipitation method” was proposed for the preparation of aqueous dispersion of organic nanoparticles [ 15 , 16 ]. The typical preparative process involves injection of a water-miscible organic solution of a dye compound into water under vigorous stirring. Disper- sion of nano-crystalline dye compounds is formed immediately upon combining the solution [ 15 , 17 ]. A wide variety of analytical dye compounds can be dispersed
Figure 28.1 Apparatus for enrichment of a target metal ion and removal of interfering ions. A piece of detection membrane was sandwiched between the separable holder, and the sample solution was fi ltered by extrusion with a pump from the top of the reservoir or by suction from the bottom. Reproduced from [14]. Copyright, Royal Society of Chemistry.
Detection membrane
Suction by pump Sample solution
containing a masking reagent Reservoir
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in aqueous solution by this method [ 13 ]. The morphology of the nanoparticles varies from particle to fi brous depending on the molecular structure ( Fig. 28.2 ).
Nanoparticles are uniformly and fi rmly coated onto a membrane fi lter (cellulose ester) simply by fi ltration of the dispersion. We demonstrated the validity of the present fabrication method of dye nanoparticle-coated membrane for popular indicator dyes ( Fig. 28.2 ). The amount of metal can be directly determined on the membrane surface by colorimetric analysis.
The nano-dye-coated membranes are remarkably sensitive (ppb level) for the detection of metal ions. For example, membranes coated with 1-(2-pyridylazo)- 2-naphthol (PAN) nanofi bers were applied as test strips for the detection of Zn 2+ in a test solution (pH 8.4). Figure 28.3(a) shows the color changes of PAN-coated strips in dip test. Notably, 65 ppb of Zn 2+ was detected by a naked-eye color test. Sub-ppb concentrations of Zn 2+ were successfully detected by fi ltration enrichment of 100 ml of the sample solution ( Fig. 28.3(b) ). This fi ltration-enrichment procedure amplifi es the signal intensity capable of eye detection of ppb level metal ions. Interference from foreign ions was avoided by the addition of masking reagents into the sample solution. Leaking of reagents out of the sample solution was negligible during dip test and sample fi ltration procedures. We also monitored the color change and the relative color intensity ( Fig. 28.4 ) with refl ectance-absorption spectrometry. The peak at l max = 550 nm indicates that neutral [Zn(PAN) 2 ] is formed on the membrane fi lter as the major species. The increasing peak intensity with increasing Zn 2+
concentration enables quantitative determination of test samples by comparison with the calibration curve ( Fig. 28.4 inset).
Figure 28.2 Photographs of dye-nanoparticle/fi ber-loaded membranes and the scanning electron microscope images. Reproduced from [13]. Copyright Wiley-VCH Verlag GmbH
& Co. KGaA. Reproduced with permission.
Dith
Nanofiber
5 μm 1 μm
Nanoparticle
H
HO HO
N N
N
N N N
N N
HN NH N
S N N
N N
N SH
TAN PAN TPP Bathophen
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Figure 28.3 Detection of Zn(II) by PAN-loaded membrane via (a) dip method where a piece of test strip was dipped into 10 mL of aqueous Zn(II) solution at pH 8.4 for 15 minutes; and via (b) fi ltration method where 100 mL of aqueous Zn(II) solution at pH 8.4 was fi ltrated through the PAN membrane under a fl ow rate of approximately 6.9 ml/min.Reproduced from [13]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission.
(a)
(b)
0 0.654 3.27
Zn(II) / ppb Zn(II) / ppb
6.54 32.7 65.4
0 6.54 3.27 65.4 130.8 780.5 1308
2 cm 2 cm
Figure 28.4 Change in refl ectance absorption spectra of 1-(2-pyridylazo)-2-naphthol (PAN) membrane upon dip into Zn(II) solution of various concentration (pH 8.4).
Inset shows the calibration curve obtained by plot the intensity at 550 nm. Reproduced from [13]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with
permission.
0300 0.2 0.4 0.6 A
0.8 1.2 1
400 λ / nm
500 600
0 3.27 6.54 32.7 65.4 130.8 550 nm [Zn(II)] / ppb
[Zn'] / ppb 0
0 0.2 0.4 0.6
Intensity at 550 nm
0.8 1 1.2
200 400 600 800 1000 1200 1400
700
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We also examined a Dithizone-coated membrane for Hg 2+ detection. By simple fi ltration of the sample solution (100 ml, pH 2.0), detection down to 10 ppb was successfully achieved. The addition of ethylenediamine- N,N,N ′,N ′ -tetraacetic acid (EDTA) to the sample solution eff ectively masked interferences from Ca 2+
(400 ppm), Fe 2+ (5.4 ppm), Cu 2+ (6.4 ppm), Zn 2+ (6.5 ppm), and Pb 2+ (0.5 ppm).
It is noteworthy that the present membranes were prepared without any additives such as coating polymers or modifi ers. Yet the reagent is not removed from the support by rubbing with a fi nger or by immersion into water. A cross- sectional scanning electron microscope image ( Fig. 28.5 ) indicates that the thickness of the dye layer is less than 1 μ m. Thus it provides a remarkably concentrated signaling surface composed of 100 percent pure indicator dye ( Fig. 28.5 ). In contrast, the surface concentration of dye is rather low in conventional test strips prepared by soaking in dye solution ( Fig. 28.5 ).